THE SEQUENCE STRATIGRAPHY OF THE

COMMANCHEAN - GULFIAN INTERVAL,

BIG BEND NATIONAL PARK,

WEST TEXAS

A THESIS SUBMITTED TO THE GRADUATE SCHOOL

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE

MASTER OF SCIENCE

BY

NICHOLAS S. TIEDEMANN

DR. RICHARD FLUEGEMAN

BALL STATE UNIVERSITY

MUNCIE, INDIANA

MAY 2010

ii

Chapter Page

AKNOWLEDGEMENTS iii

ABSTRACT iv

LIST OF TABLES v

LIST OF FIGURES vi

1. INTRODUCTION 1

2. SYSTEMATIC PALEONTOLOGY 24

3. BIOSTRATIGRAPHY OF THE BUDA LIMESTONE 59

4. BIOSTRATIGRAPHY OF THE LOWERMOST BOQUILLAS FORMATION 73

5. STABLE ISOTOPE GEOCHEMISTRY 83

6. DISCUSSION AND CONCLUSION 108

PLATES vii

REFERENCES xxviii

iii

AKNOWLEDGEMENTS

I would like to thank the National Park Service for encouraging research in such a beautiful and

fascinating place as Big Bend National Park. Their work preserving America's National Parks is

deserving of much recognition. I am grateful for my advisor, Dr. Richard Fluegeman, whose

wealth of knowledge and guidance are seemingly limitless. This research was made possible by

internal grants from the Ball State Department of Geological Sciences and from Dr. Richard

Fluegeman. I would like to thank Brenda Rathel and Mike Kutis for assisting me with everything

from filling out grants and making copies to making thin sections and using assorted pieces of

software. I owe thanks to Drs. Scott Rice-Snow, Kirsten Nicholson, and Jeff Grigsby for their

valuable guidance and support. I would like to extend my wholehearted gratitude to Dr. Brian

Lock and Drs. Roger and Dee-Anne Cooper for providing me with valuable field information,

supplementary documentation, advice, and genuine goodwill. Recognition is also due to

National Petrographic Services, Inc. for their expedient thin sectioning services. My thanks go

out to Dr. Greg Ludvigson, Gregory Cane, and the entire Keck Paleoenvironmental &

Environmental Stable Isotope Laboratory at the University of Kansas for their services and

assistance with stable isotope data collection. A special thanks goes out to Bob Liska for

guidance with foraminifera resources. I owe so much to my loving parents, Craig and Lisa

Tiedemann, and my grandfather, Jack Tiedemann, without whose moral and financial support I

would never have accomplished this work. Finally, this research would not have been possible

without my field assistant, proof-reader, lunch-packer, and companion, Erica Evans.

iv

ABSTRACT

Within Big Bend National Park, the unconformable contact between the Buda Limestone and

the overlying Boquillas Formation represents the Commanchean-Gulfian boundary. Previous

studies of the geochronology of this interval have relied primarily on provincial ammonite

faunas rather than foraminifera, and place the Buda and basal Boquillas in the Lower

Cenomanian. Because of its indurated nature, a comprehensive foraminiferal biozonation has

not been acquired for the Buda Limestone. Recent revisions to Cretaceous foraminiferal

biozonations and taxonomies necessitates a new biostratigraphic study of the Buda - Boquillas

interval. The overlapping ranges of F. washitensis, G. bentonensis, G. caseyi, P. appenninica, P.

delrioensis, P. stephani, and R. montsalvensis place the Buda within the upper portion of the

Early to Middle Cenomanian Th. globotruncanoides Zone. Microkarst found on the surface of

the Buda Limestone has been interpreted as representing a subaerial exposure and sequence

boundary. However, microkarst-like features can result from subaqueous or intrastratal

processes. Carbon and oxygen stable isotope analysis of the lower and middle Buda has

indicated a mean δ13C value of 1.73‰ VPDB, which is in line with other values reported from the

Lower Cenomanian. The top 2.6m of Buda contains a 0.62‰ negative δ13C shift from 1.88‰

VPDB to 1.26‰ VDPB in a 40 cm interval, expected if subaerial exposure occurred. Higher

variation in measured carbon isotope values beneath the contact also lend evidence for

meteoric alteration. The standard deviation in δ13C values from the top 2.8 m of the Buda is

0.207, which is 2.16 times larger than the rest of the studied section at 0.096. The Buda

contains a shallow pelagic-dominated fauna of heterohelicids (45-90%), globigerinellids (3-37%),

and hedbergellids (4-22%). Intermediate-depth globigerinellids display an initial increase

followed by a marked decrease in abundance upsection, interpreted as sea level transgression

and regression, respectively. The lower contact of the Buda with the Del Rio Clay has been

previously interpreted as a subaerial exposure, and a P:B break from ~0% planktonics in the

upper Del Rio to ~80% in the Buda supports this claim. This study therefore interprets both the

upper and lower contacts of the Buda as sequence boundaries. The overlying 1.2 m Boquillas is

nearly devoid of benthics and represents a deeper assemblage including the double-keeled

Dicarinella sp., as well as several Upper Cenomanian (D. algeriana Subzone) species. Based on

foraminiferal data, the duration of the Buda - Boquillas unconformity is roughly equivalent to

the missing Th. reicheli and Th. greenhornensis Biozones, or a sizable portion of the Middle

Cenomanian.

v

LIST OF TABLES

Table Page

1. Distribution of identifiable foraminifera at Dog Canyon locality 63

2. Distribution of identifiable foraminifera at Highway localities 64

3. Distribution of identifiable foraminifera at Dagger Flat DS1 locality 65

4. Absolute and relative abundances of main genera at Dagger Flat DS1 67

5. Foraminiferal distribution and abundances at Dagger Flat DS2 80

6. Stable isotope data at Dog Canyon locality 100

7. Stable isotope data at Dagger Flat DS1 locality 101

8. Stable isotope data at Dagger Flat DS2 locality 102

vi

LIST OF FIGURES

Figure Page

1. Stratigraphic column of Big Bend National Park 3

2. Dog Canyon sampling locality map 16

3. Photograph of Buda Limestone outcrop at Dog Canyon locality 17

4. Photograph of Buda - Boquillas contact at Dog Canyon locality 18

5. Dagger Flat sampling localities map 19

6. Photograph of Buda Limestone outcrop at Dagger Flat DS1 locality 20

7. Photograph of Buda - Boquillas outcrop at Dagger Flat DS2 locality 21

8. Highway section sampling location map 22

9. Photograph of Buda Limestone exposure at Highway locality 23

10. Geochronologic distribution of selected foraminifera from the Buda Limestone 66

11. Relative abundances of main genera from the Dagger Flat DS1 68

12. Absolute abundances of heterohelicids and globigerinellids per thin section from Dagger Flat DS1 69

13. Geochronologic distribution of selected foraminifera from the Boquillas Formation 81

14. Geochronologic positions of the Buda Limestone and Boquillas Formation according to different authors 82

15. Idealized C-isotope profile of a meteorically-altered paleoexposure surface 91

16. Geochronologic distribution of Oceanic Anoxia Events 92

17. Stable isotope profile for the Dog Canyon 103

18. Stable isotope profile for the Dagger Flat DS1 104

19. Stable isotope profile for Dagger Flat DS2 105

20. Dog Canyon δ13C and δ18O cross-plot (PLCT) 106

21. Dagger Flat DS1 δ13C and δ18O cross-plot (PLCT) 107

CHAPTER 1 - INTRODUCTION

General Overview

Big Bend National Park (BBNP)

Big Bend National Park is located in Brewster County, TX, on the US – Mexico border, and

represents an area larger than 800,000 acres. The park was founded in 1935 and was the first

National Park in Texas. It occupies the southern tip of "Big Bend Country," the area of West

Texas bounded on three sides by the Rio Grande as it dips south toward the Coahuila Desert.

Big Bend Country contains thick sequences of mixed clastic and carbonate strata from the

southernmost portion of the Cretaceous Western Interior Seaway unconformably overlying

older Paleozoic strata (Frush and Eicher, 1975).

Stratigraphy

The Cretaceous of Big Bend National Park is represented, from youngest to oldest, by the Glen

Rose Limestone, Telephone Canyon Formation, Del Carmen Limestone, Sue Peaks Formation,

Santa Elena Limestone, Del Rio Clay, Buda Limestone, Boquillas Formation, Pen Formation,

Aguja Formation, and Javelina Formation (Fig. 1). These formations record two large-scale

2

transgressive-regressive cycles, the Commanchean Series and Gulfian Series, separated from

each other by the unconformity between the Buda Limestone and the Boquillas Formation.

The Commanchean Series of BBNP represents about 25 m.y. of earth history and

records mostly carbonate deposition with periodic clastic influxes. It begins with deposition of

the Glen Rose Limestone in approximately the middle Aptian and continues through the Albian

to the Lower Cenomanian deposition of the Del Rio and Buda Formations (Scott and Kidson,

1977; Scott and Mancini, 2005). The Commanchean Series contains several higher-order

sequences as outlined by Scott and Kidson (1977): The Trinity sequence is Glen Rose; the

Fredricksburg sequence is Telephone Canyon - Del Carmen; the lower Washita sequence is Sue

Peaks - Santa Elena; and the upper Washita sequence is Del Rio - Buda. More recent studies in

West Texas have shown that the stratigraphic relationships between the Commanchean

Formations may be more complex than the interpretation given by Scott and Kidson (1977),

especially within the Washita Group. Mancini and Scott (2006) interpret the Buda and Del Rio as

a single transgressive - regressive sequence, while Lock et al. (2007) interpret each unit as

unconformity bound within their own depositional sequences.

The Gulfian Series of BBNP begins with deposition of the Ernst Member of the Boquillas

Formation in the Middle to Upper Cenomanian. The Boquillas is a pelagic marine limestone and

shale unit representing deposition in deep water (Lock and Peschier, 2006). Progressively

younger strata records a generally regressive package and a shift from a mostly carbonate to a

mostly clastic regime. The Santonian to early Campanian Pen Formation represents a marine

shale, while the middle to late Campanian Aguja and Maastrichtian Javelina Formations

represent paralic and fluvial deposition, respectively (Ashmore, 2003).

3

Figure 1 - BBNP stratigraphic column. Stratigraphy from USGS.gov modified after: Scheubel, R.F. and D.H. Mruk, eds., 1994, Road Log, Day 2, Laramide and late Tertiary tectonics and structures: Structure and Tectonics of the Big Bend and Southern Permian Basin, Texas: West Texas Geologcial Society 1994 Field Trip Guidebook, p. 23-69. URL: http://3dparks.wr.usgs.gov/bibe/html/bb_strat.htm.

4

Del Rio Clay (Washita Group)

Originally called the Exogyra arietina Marl (Shumard, 1860), the name Del Rio Clay was coined

by Hill and Vaughan (1898). Its type locality is 2 mi south of Del Rio, Val Verde Co., TX (Adkins,

1933). The Del Rio Clay marks a period in the Lower Cenomanian of relatively low sea level and

terrigenous influx between the deposition of the Albian Santa Elena Limestone and late Lower

Cenomanian Buda Limestone. The Del Rio's thickness in Big Bend National Park is variable,

ranging from around 30 m in the Dagger Flat area to less than 3 m near Rio Grande Village. This

variability in the thickness of the Del Rio Clay is likely due to deposition on a subaerial exposure

surface atop the Santa Elena Limestone followed by erosion of Del Rio sediment before the

Buda Limestone was deposited. The Del Rio consists of laminated, hummocky, and cross-

bedded claystones, carbonate muds, and very fine sandstones. It contains oyster packstones

with abundant Ilymatogyra arietina that are interpreted as resulting from storm-winnowing

events. Other fossils include echinoids, bivalves, and planktonic and benthic foraminifera,

including the large arenaceous Cribratina texana (Lock et al., 2006).

Lock et al. (2006) interpret the Del Rio Clay as a shallow deposit above storm wave base

that is bounded by subaerial unconformities. This interpretation is supported by a foraminiferal

P:B curve created by Mauldin and Cornell (1986). Their data show an initial increase in

planktonics during transgression from about 10-20% to over 60%, then a decrease to 0% during

regression. According to their research, the planktonic foraminifera Heterohelix moremani

represents 0-49% of total foraminiferal assemblage, but H. delrioensis, H. planispira, and F.

washitensis are all present in small quantities of <1% each.

5

The Del Rio Clay correlates to the Grayson Formation of north-central Texas and

Oklahoma (Scott and Kidson, 1977; Young, 1977) and the Cuesta del Cura Formation of northern

Mexico (Young, 1977; Lehmann et al., 2000). The Santa Elena Limestone - Del Rio Clay

unconformity of BBNP likely correlates to the unconformity beneath the Grayson Formation (the

base of the WA 6 cycle) assigned by Scott et al. (2002) in northern Texas. The base of the WA 6

cycle in turn may correlate to the CSB 5 unconformity at the top of the Aurora Formation (top of

sequence Co 5) in northern Mexico (Lehmann et al., 2000), and the SB4 sequence boundary

(base of sequence 4) in the Cretaceous Western Interior Seaway (Scott et al., 2001).

Buda Limestone (Washita Group)

Vaughan (1900) named the Buda Limestone after the town of Buda, Hays Co., TX. It is a dense,

porcelaneous foraminifer-calcisphere (?Calcisphaerula innominata Bonet) biomicritic

wackestone with little to no clastic material. Small (±2 cm) gastropods are by far the most

common macrofossils, but ammonites, oysters, and echinoids are also relatively common. The

unit weathers white to buff in color, but fresh exposures are generally light gray in color. The

lower members are generally nodular and the upper member consists of massive beds

occasionally separated by both nodular and thin fissile layers. Distinct bedding planes are

absent in outcrop, presumably due to extensive bioturbation. Pore spaces from freshly-cut

hand samples often contain a sticky, black petroleum-like substance that is presumably "dead

oil." Within Big Bend National Park, the thickness of observed Buda outcrops are roughly 7-9 m,

but the thickness most likely varies due to erosion.

In thin section, the Buda Limestone is a wackestone throughout the studied sections.

Micrite forms the matrix, but spar is present in primary void spaces and fractures. Microfossils

6

include both benthic and planktonic foraminifera, calcispheres, dinoflagellate cysts, and

echinoid spines. Small (0.5 mm) veins with sparry calcite infill are common throughout the unit.

Dolomitization varies from 0% to as high as 10-20% locally and normally consists of patchy

concentrations of dolomite rhombs. However, the unit is not generally dolomitic. Several (~2-3)

possible grainflows were observed in thin section, comprised of thin layers of brecciated

foraminifer, calcisphere and gastropod test material. Ooids also occur very rarely, and were

probably transported from shallower shelfal environments.

The Buda Limestone is an extensive formation spanning much of Texas, through Big

Bend National Park, and southward through Coahuila, Mexico. The middle of the Buda

Limestone of Travis County in central Texas reportedly contains a disconformity (Martin, 1967).

Documentation of a disconformity within the Buda Limestone in Big Bend region is, however,

unknown to the author. The Buda Limestone correlates with a portion of the upper Grayson

Formation in the far north of Texas (Hancock et al., 1993). It may also correlate

geochronologically with portions of the upper Graneros Shale of New Mexico, Colorado, and

Kansas (Scott et al., 2001), and with the uppermost portion of the Cuesta del Cura Formation of

northern Mexico (Young, 1977; Ice and McNulty, 1980). The Buda Limestone of the south-

central Texas Cretaceous trend, east of Big Bend Country, has represented a proven reservoir

rock for over 80 years. Hydrocarbon migration into fractures at the top of the Buda Limestone,

likely from the Eagle Ford Group, has historically produced substantial amounts of oil (Dawson,

1986).

7

Boquillas Formation (Terlingua Group)

The Boquillas Formation does not have an official type section. It was named in the vicinity of

Boquillas Post Office, along the west flank of the Sierra del Carmen Range, Brewster Co., TX by

Udden (1907). Directly overlying the Buda Limestone, the Ernst Member of the Boquillas

Formation is a 40 to 100 m thick reddish brown to tan colored flaggy argillaceous limestone and

shale. Decimeter thick calcareous foraminiferal-calcisphere wackestone and packstone beds are

often laterally discontinuous, and are vertically separated from each other by thin, shaley layers.

Slump folding, debris flows, and even possible turbidites have been reported in the lowest

member of the Boquillas in road cuts on Highway 90, north of BBNP (Lock and Peschier, 2006).

Above the Ernst, the San Vicente Member of the Boquillas Formation consists of resistant thinly

bedded gray argillaceous limestone (Ashmore, 2003). Fossils of the Boquillas Formation include

planktonic foraminfera, calcispheres, echinoderm fragments, fish teeth, gastropods, bivalves,

and ammonites. Benthic foraminifera are virtually absent from the basal Boquillas, indicating

deep water or dysoxia, or both (Frush and Eicher, 1975).

The Ernst Member of the Boquillas Formation is equivalent to the Eagle Ford Group of

eastern Texas (Pessagno, 1969; Lock and Peschier, 2006). North of Texas, this member also

correlates with the Greenhorn Formation of the Cretaceous Western Interior Seaway (Eicher

and Worstell, 1970). The San Vicente Member of the Boquillas Formation and the Pen

Formation of BBNP correlate with the Austin Chalk and Taylor Marl of the Gulf Coast region

farther east, and with Niobrara Chalk and Pierre Shale farther north in the Cretaceous Interior

Seaway (Ashmore, 2003). To the west of BBNP, the Boquillas thickens basinward to over 700 m

and becomes the Ojinaga Formation. Northwest of BBNP, Boquillas-equivalent rock is named

after nearby Chispa Summit, Texas, becoming the Chispa Summit Formation.

8

Geochronologically correlative sections also include the Morelos and Mexcala Formations of

southern Mexico (Aguilera-Franco et al., 2001), and the Calera Limestone of California (Sliter,

1999).

The Boquillas Formation, Eagle Ford Group, and correlative strata have attracted

attention recently as self-sourced reservoir rocks. Organic-rich black shales are present

throughout the lower and middle Boquillas, with TOC contents in surface exposures around 1%

on average (Trevino, 1988). Subsurface TOC values, however, are likely significantly higher (Lock

and Peschier, 2006). Modern shale fracturing techniques may allow for further exploitation of

these unconventional plays.

Purpose

Foraminiferal biostratigraphy and paleoenvironmental interpretation

The only foraminiferal biostratigraphic study of the Ernst Member of the Boquillas Formation in

BBNP was performed by Frush and Eicher (1975). Revisions to the Cretaceous foraminiferal

geochronology since 1975 (as outlined in the Systematic Paleontology) warrant a revisit to the

foraminiferal biozonation of the basal Boquillas Formation. While this biostratigraphic work in

the Cenomanian of BBNP has focused on the Boquillas Formation, a foraminiferal biozone for

the Buda Limestone in the region has not previously been acquired. Until the foraminiferal

biozonation of the Buda Limestone is established completely, the Commanchean - Gulfian

interval in BBNP remains poorly bracketed by foraminiferal data. This lack of data is likely a

result of the highly indurated nature of the Buda Limestone and the necessity of studying

foraminifera in thin section.

9

Additionally, phylogenetic trends of planktonic foraminifera within the Buda Limestone

can lend insight to the Cenomanian sequence stratigraphy of the region. For instance, recent

research has interpreted the Del Rio Clay and Buda Limestone occupying multiple sequences

(Lock et al., 2006), as opposed to a single transgressive-regressive sequence (Scott and Kidson,

1977; Scott et al., 2002; Mancini and Scott, 2006). This study will use foraminiferal phylogentic

trends to help answer whether the Buda Limestone occupies its own depositional sequence.

Paleoexposure of the Buda Limestone

The contact between the Buda and the overlying Boquillas Formation (Commanchean - Gulfian)

is interpreted as a subaerially exposed unconformity based on the presence of a pitted

microkarst surface at the top of the Buda Limestone (Lock and Peschier, 2006; Lock et al., 2007).

However, submarine processes can sometimes create sedimentary features resembling surface

karst. If there was no subaerial exposure during the Buda - Boquillas hiatus, then the Buda

Limestone and Boquillas Formation are related in terms of sequence stratigraphy, since eustatic

sea level need not have produced the unconformity surface. If there was subaerial exposure,

then carbon and oxygen isotope evidence for meteoric alteration should be present at the top

of the Buda Limestone (Allen, and Matthews, 1982).

This study seeks to determine whether the Buda - Boquillas unconformity arose from

subaerial or submarine processes, and how long the depositional hiatus lasted. A foraminiferal

biostratigraphic framework of strata adjacent to the unconformity will allow for precise dating

of time lost during the Commanchean - Gulfian hiatus and will assist with future regional

stratigraphic interpretations. Planktonic foraminifera are particularly useful for this kind of

study, because they are less provincial than faunas such as ammonites, and have a more

10

complete deep-sea sedimentary record than other fossils. Furthermore, regional-scale

paleoceanographic understanding of the Commanchean - Gulfian boundary, and of Cenomanian

and Turonian anoxia, depends upon local sequence- and biostratigraphic interpretations.

Oceanic Anoxia Events (OAEs)

Several Oceanic Anoxia Events have been recorded in Cretaceous sediments worldwide (Fig. 16).

During these periods, global oceanic oxygen levels are reduced, and large global shifts in the

isotopic composition of seawater can be produced (Schlanger and Jenkyns, 1976). These events

lead to the widespread deposition of organic carbon, and the subsequent potential for

hydrocarbon generation (Irving et al., 1974; Jenkyns, 1980). This study will attempt to place

these events in the stratigraphic context of West Texas.

Synopsis

The purpose of this study is to evaluate the Buda - Boquillas stratigraphic interval by: 1)

Identification of the planktonic foraminiferal biozones present within the Buda Limestone and

lowermost Boquillas Formation, 2) Correlation of these units with the global geochronologic

scale of Gradstein et al. (2004); 3) Determination of phylogenetic trends and the establishment

of paleoenvironmental and sequence-stratigraphic interpretations; 4) Isotopic evaluation of the

Buda - Boquillas to determine whether the unconformity originated from subaerial or submarine

processes; 5) Isotopic evaluation of the Buda - Boquillas to determine evidence for Oceanic

Anoxia Events (OAEs).

11

Localities

Dog Canyon (DC-DS1)

This site is located roughly 3 km east of the Main Park Road towards Dog Canyon, on the left

side of the canyon when facing east (Fig. 2). The structural geology of the Dog Canyon area is

complex, with thrusted and overturned Glen Rose and Buda Formations forming the crest of the

ridge. These overturned strata rest upon non-overturned Buda Limestone and Boquillas

Formation. This lower outcrop of cliff-forming Buda Limestone was sampled at Dog Canyon,

about halfway up the ridge. The sampling site consists of a 2.5 m exposure of the massive

member at the top of the Buda Limestone, and an exposure of overlying Boquillas Formation

(Fig. 3, 4). The Del Rio Clay and the lower nodular facies of the Buda Limestone are completely

covered with talus at this location. A total of 15 samples were taken from the top of the Buda

Limestone. Additionally, a large grapefruit-sized gastropod float was recovered sitting on top of

the Buda Limestone (on the Buda - Boquillas contact) at this locality.

Dagger Flat Dataset 1 (DC-DS1)

This sampling site is located on Dagger Flat Auto Trail approximately 7.9 km from the Main Park

Road and 3.8 km before the loop at the end of Dagger Flat Auto Trail, on the northwest side of

the road (Fig. 5). An arroyo crosses Dagger Flat Auto Trail in a small valley at this location,

flowing west-southwest (from left to right across the road as one faces toward the loop at the

end of Dagger Flat Auto Trail). If one continues farther northeast up Dagger Flat Auto Trail

toward the loop, exposures of Buda Limestone and Del Rio Clay can be seen to the on the high

cliff to the left.

12

A 6.5m section near the base of the Buda Limestone was sampled at this locality (Fig. 6).

The base of the outcrop consists of the nodular lower member of the Buda Limestone. A well-

preserved echinoid was recovered in talus that likely derived from the lower portion of the

outcrop. Other macrofossils observed include only 1-3 cm long gastropod shells. This portion of

the section consists of nodules of gray porcelaneous limestone that weathers white to tan in

color. Two-thirds of the way up on the sampled outcrop, bedding becomes massive, with dm to

m thick beds separated by thin nodule and fissile layers, much like the section exposed at Dog

Canyon. This portion of the section weathers slightly darker than the nodular portion beneath,

but fresh exposures are lighter gray-buff in color. The lower nodular member tends to form a

steep slope, as opposed to the massive upper member, which tends to form vertical cliffs. This

difference in erosion must stem from the difference in nodular vs. massive characteristics

between upper and lower members, since all hand samples taken are well indurated. The Buda

- Boquillas contact was not observed at the top of the outcrop, presumably due to erosion. This

section likely represents very nearly the entire section of Buda Limestone, however the bottom

±0.5-1 m and top of the section were inaccessible. A total of 31 samples were taken at this

location.

Dagger Flat Dataset 2 (DC-DS2)

This sampling site is located along Dagger Flat Auto Trail approximately 7.3 km from the Main

Park Road and 4.5 km from the loop at the end of the trail (Fig. 5). Looking south across about

200 m of flat plain, one can see hills consisting of westward dipping Boquillas Formation. An

arroyo runs westward along the top of the Buda Limestone at the base of these hills, and 7

samples of indurated carbonate were taken there in a 1.5 m outcrop where the Buda and

Boquillas contact is exposed (Fig. 7).

13

An anomalous dm-thick layer of very dark gray-black foraminiferal porcelaneous

limestone at the contact between the Buda and Boquillas was observed at this locality. The bed

weathers light gray and is not noticeable unless broken with a rock hammer, where after it

displays its black coloration on conchoidal broken surfaces. This layer has been found only in

one other location south of Dog Canyon (Cooper, 2008). Thin section analysis reveals that this

layer contains a foraminiferal assemblage more akin to the Boquillas Formation than the Buda

Limestone, since it contains abundant keeled species. Black opaque material is present in the

matrix surrounding sparry foraminifer and calcisphere tests, but does not cross-cut them. This

bed may be a local Corg-rich layer, may represent an outlier of some unit deposited during the

Buda - Boquillas hiatus, or may have resulted from some kind of hydrothermal alteration of the

lowermost bed of the Boquillas Formation. This location is relatively close to several igneous

intrusions, so the alteration hypothesis seems likely. It is noteworthy that similar outcrop

evidence for alteration was not observed in either the Buda Limestone directly below, or the

Boquillas Formation directly above, the black limestone.

The Buda Limestone at this locality is buff-light gray in color and well indurated. It forms

a resistant "spillway" for discharge to flow through the arroyo at this location. The Boquillas

here consists of resistant flaggy brown-gray carbonate beds 1-10 cm in thickness separated by

thinner, more friable shaley layers. Within 30 cm of the top of the Buda Limestone, and 20 cm

of the anomalous black carbonate bed, a 2 cm thick black shale layer was observed. A total of 7

samples were taken at this location.

14

Highway Section Datasets 1 and 2 (HW-DS1/2)

The highway section is located on Park Route 12 near the southern entrance to the Old Ore

Road, west of the Park Route 12 tunnel (Fig. 8). A weathered exposure of Buda Formation was

sampled on a hill on the south side of Park Route 12 (Fig. 9). A series of normal faults run

roughly north-south here, and the sampled outcrop is a topographic high formed by a horst.

The Del Rio Clay at this locality was not observed, and is only about 5 ft thick this far south in

BBNP (Lock et al., 2007). Boquillas Formation crops out to the west of the sampled hill, but the

Buda - Boquillas contact was not observed at this locality. The Buda Limestone weathers

brownish tan at this location, and exposed surfaces are light gray to buff in color. Two vertical

transects of Buda Limestone hand samples were taken at this locality, comprising datasets 1 and

2. Dataset 1 is the lower transect, and dataset 2 is a vertical continuation of dataset 1 in a

slightly different horizontal location on the hill. A total of 16 samples were taken, representing

approximately 3.2 m of section.

Methods

Field Methods

All research was conducted during March, 2008 with a permit granted from the National Park

Service. Field work was conducted on foot using the geologic map of Maxwell (1968), Garmin

handheld GPS device, and Brunton compass. At each outcrop, hand samples were collected

using a rock hammer at a 20 cm interval and notes were recorded in a Rite in the Rain field

book. Any macrofossils were noted and/or collected. Samples were then placed in labeled

plastic bags and transported back to the vehicle.

15

Laboratory Methods

All rough hand samples were cut into billets at Ball State University Department of Geological

Sciences, and approximately half were thin sectioned to 25µm in-house. The rest of the billets

were thin sectioned to 25µm by Applied Petrographic Services, Inc. Thin sections were then

examined under a petrographic microscope for identification of foraminifera. Foraminifera

recognizable to at least genus level were counted, photographed, and charted in Microsoft

Excel. Pessagno (1969), Silva and Sliter (2002), and others as outlined in the Systematic

Paleontology section were used to aid in identifications. A carbon and oxygen stable isotope

analysis for Dog Canyon and Dagger Flat samples was performed at the University of Kansas W.

M. Keck Paleoenvironmental and Environmental Stable Isotope Laboratory. Billets were

polished to 1000 grit and bulk-rock micrite was drilled with a dental drill to obtain 20-80 µg of

calcite powder for isotope analysis. Areas particularaly high in dolomite and/or secondary

calcite (veins) were avoided or removed from analysis completely. Samples were baked at

200:C for one hour to release volatile compounds, then transferred to glass vials and reacted

under vacuum with 3 drops of 100% prepared phosphoric acid (ρ=1.8913 g/cm3) at 75:C. CO2

was released and trapped cryogenically, then transfered online to an IRMS instrument where it

was measured 8 times versus a calibrated CO2 reference tank for δ ratio analysis and reported

versus the VPDB scale. Analysis was performed using a Kiel Carbonate Device III + Finnigan

MAT253 isotope ratio mass spectrometer. NBS-18 Carbonatite (NIST Ref. Mat. 8543) & NBS-19

Limestone (NIST Ref. Mat. 8544) were used as quality control standards. Analytical precision

can be reported as better than 0.02‰ for δ13C, and as better than 0.05‰ for δ18O. Microsoft

Excel was used for isotope data analysis.

16

Figure 2 - Dog Canyon sampling locality. Image courtesy of the USGS and Microsoft Research Maps. URL: http://www.MSRMaps.com.

17

Figure 3 - Upper portion of the Buda Limestone exposed at the Dog Canyon locality. Looking E. Note flags of Boquillas Formation above. Pencil provides scale.

18

Figure 4 - Buda Limestone - Boquillas Formation contact at the Dog Canyon locality. Looking NE. Rock hammer provides scale.

19

Figure 5 - Dagger Flat sampling localities. DF-DS1 is the eastern location and DF-DS2 is the western location. Image courtesy of the USGS and Microsoft Research Maps. URL: http://www.MSRMaps.com.

20

Figure 6 - Buda Limestone outcrop at Dagger Flat DS1 locality. Looking NW. Author Nicholas Tiedemann provides scale.

21

Figure 7 - Boquillas Formation at Dagger Flat DS2 locality. Looking W. Rock hammer provides scale and is resting upon anomalous black limestone layer.

22

Figure 8 - Highway sampling locality. Image courtesy of the USGS and Microsoft Research Maps. URL: http://www.MSRMaps.com.

23

Figure 9 - Buda Limestone at Highway locality. Looking W. Scale card is 10 cm.

CHAPTER 2 - SYSTEMATIC PALEONTOLOGY

Genus: DICARINELLA (Porthault, 1970)

Remarks: Members of the genus Dicarinella are the first planktonic foraminifera to evolve

double keels (Premoli Silva and Sliter, 2002). The genus is easily distinguished by the recognition

of double keels in cross-sectional view.

Paleoceanographic significance: Isotopic evidence suggests that dicarinellids inhabited deep,

sub-thermocline waters, similar to the rotaliporids (Gebhardt et al, 2004); however the genus

might have inhabited more intermediate depths at the thermocline along with the

globigerinellids and praeglobotruncanids (Keller et al., 2001). In either case, the genus is here

interpreted as inhabiting open marine environments.

Dicarinella algeriana (Caron)

Praeglobotruncana algeriana Caron, 1966, Revue de Micropaléontologie, v. 9, p. 74-75.

25

Remarks: D. algeriana is biconvex with a low trochopire, and is of small to medium size. This

species possesses a shallow, wide umbilicus and double keel (Premoli Silva and Sliter, 2002;

CHRONOS, 2009).

Range: Dicarinella algeriana, the first member of its genus, first appears in the R. cushmani

Zone in the lower part of the Upper Cenomanian (Premoli Silva and Sliter, 2002). The FAD of

this species defines the beginning of the Upper Cenomanian D. algeriana Subzone of the R.

cushmani Zone. Multiple specimens of the genus Dicarinella (including D. algeriana) were

recovered from the basal Boquillas Formation in this study, based on the recognition of a double

keel.

CHRONOS Range:

FO stage: Cenomanian LO stage: Turonian FO zone: R. cushmani LO zone: M. schneegansi FO age (Ma): 93.9 LO age (Ma): 90.3

26

Genus: FAVUSELLA (Michael, 1972)

Paleoceanographic significance: Favusellids are interpreted as inhabiting shallow inner to

middle neritic waters, however large favusellids (350-500 µm), as observed in the Buda

Limestone, are restricted to water greater than 30-50m deep (Koutsoukos et al., 1989). Leckie

(1987) tentatively places F. washitensis in his Epicontinental Sea Fauna, and notes that the

species is characteristic of relatively warm, shallow epeiric or marginal seas.

Favusella washitensis (Carsey)

Hedbergella washitensis (Carsey) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 278, pl. 4,

fig. 9, 10a-c, 11a-c; Pessagno, 1967, Palaeontographica Americana, v. 5, p. 245-445.

Globigerina washitensis Carsey, 1926, Univ. Texas Bulletin 2612, p. 44, pl. 7, fig. 10.

Favusella washitensis (Carsey) Michael, 1972, Journ. Foram. Research, v. 2(4), p. 215, pl. 5, fig. 1-

3.

Remarks: Favusella washitensis is problematic for thin section biostratigraphy, since it has been

divided into several species with similar taxonomic ranges, with some disagreement over which

is the genotypic species (Michael, 1972; Longoria and Gamper, 1977; Ross and McNulty, 1981).

High morphological variation in style of coiling, number of chambers, degree of inflation, and

the degree of test ornamentation further complicate identification. However, the genus does

have extensive global distribution (Hart, 1983; Koutsoukos et al., 1989). Favusella is distinctive

for ornamental ridges that form a polygonal, honeycomb-like pattern on the test surface, which

usually display as “spikes” in cross section, although in certain cross sections the polygonal

pattern is visible. Recognition of Favusella washitensis in thin section is made possible due to

27

the species’ relatively large size, trochospiral and umbilicate form, and thick double layered wall.

Favusella occurs commonly throughout the Buda Limestone as a predominately large and

globular form. The genus Favusella has enigmatic morphological relationship with Hedbergella

and Whiteinella, and may be regarded as a subgenus of Hedbergella (Hart, 1983; Koutsoukos et

al., 1989). Some specimens in this study bear resemblance to reference photomicrographs of

Hedbergella and Whiteinella, especially W. brittonensis, W. baltica, and W. paradubia. The

difficult task of differentiating these species in thin section was based upon some morphological

assertions: 1) Favusella differs from Hedbergella by the presence of the ornamental ridges on

the test, and by having a higher trochospire. 2) Whiteinella differs from Favusella by having

more offset chambers and by having a pustulose wall not ornamented in a polygonal fashion.

The latter category is difficult to distinguish in thin section; however some thin section

specimens of Favusella display the polygonal ornamentation. Whiteinella brittonensis and

Whiteinella paradubia may also have a larger size and higher trochospire than Favusella

washitensis, although size and trochospire height is not considered an absolute means of

differentiation, given the morphological variability within Favusella.

Range: F. washitensis was identified by Pessagno (1969) in the Buda Limestone only in the

vicinity of Austin, TX. The exact stratigraphic location within the Buda is not specified, nor is

whether F. washitensis was recovered in any other locations. Pessagno (1969) reports that no

specimens of F. washitensis have been positively identified in sediments attributable to the

Rotalipora cushmani Zone, yet specifies in a range chart that F. washitensis may extend into the

Rotalipora cushmani Zone in the Trans-Pecos region. Silva and Sliter (2002) indicate the range of

F. washitensis from the late Aptian Ticinella bejaouaensis Zone through the early Cenomanian

Rotalipora brotzeni Zone. Stewart and Pearson (2002) place the first appearance of F.

28

washitensis at 116.6 Ma (Late Aptian) and the last appearance at 96.2 Ma (Early to Middle

Cenomanian). Hart (1983) reports Favusella washitensis occurring within the Early to Middle

Cenomanian Thalmanninella reicheli Zone alongside Praeglobotruncana stephani,

Praeglobotruncana delrioensis, Parathalmanninella appenninica, Thalmanninella reicheli, and

Whiteinella brittonensis. Similarly, favusellids have been recovered from the top of the

Thalmanninella reicheli Zone in the Anglo-Paris Basin (Koutsoukos et al., 1989). While many

workers consider Favusella extinct by the Middle Cenomanian, there are scattered reports of

the genus occurring within the Early to Middle Cenomanian Rotalipora greenhornensis Zone and

even into the Upper Cenomanian (McNeely, 1973; Koch, 1977; Rösler et al., 1979).

CHRONOS Range:

FO stage: Aptian LO stage: Cenomanian FO zone: T. bejaouaensis LO zone: R. globotruncanoides FO age (Ma): 116.6 LO age (Ma): 96.2

29

Genus: GLOBIGERINELLOIDES (Cushman and Ten Dam, 1948)

Paleoceanographic significance: The genus Globigerinelloides inhabited intermediate depths

with normal salinity, below the surface mixed layer, and above keeled forms such as rotaliporids

(Leckie, 1987; Keller et al., 2001; Stewart and Pearson, 2002; Keller and Pardo, 2003).

Globigerinelloides and Hedbergella characterize the Shallow Water Fauna (>100m) of Leckie

(1987), and are also present in his shallower Epicontinental Sea Fauna (>100m), which is

dominated by genera such as Heterohelix, Guebelitria, and Gubkinella. Leckie (1987) reports

that members of the Epicontinental Sea Fauna increase relative to members of the open marine

Shallow Water Fauna with decreasing water depth. The abundance trends of Globigerinelloides

may be a proxy for water depth, with a decrease in Globigerinelloides shoreward (Sliter, 1972).

The number of Globigerinelloides sp. per thin section was counted for the Buda, which shows a

significant decrease up section. The data are interpreted as showing a lower deepening trend,

followed by an upper shoaling trend that persists to the top of the section.

Globigerinelloides bentonensis (Morrow)

Anomalina bentonensis Morrow, 1934, Jour. Paleont., v. 8, p. 201, pl. 30, figs. a-b.

Planomalina caseyi Bolli, Loeblich and Tappan, 1957, p. 24. pl. 1, figs. 4-5.

Globigerinelloides bentonensis (Morrow), Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 267-

278, pl. 2, figs. 8-10.

Remarks: Globigerinellids are identified in thin section by their distinctive planispiral test. G.

bentonensis is identified in thin section by its thin, smooth wall, rapidly increasing globular

chambers, and small umbilicus (Pessagno, 1967; Premoli Silva and Sliter, 2002). G. bentonensis is

30

very similar to G. caseyi, but is distinguished from G. caseyi by being less compressed in cross

section (Ice and McNulty, 1980), by being smaller and more evolute, and by having less rapidly

expanding and less inflated chambers (Eicher and Worstell, 1970). It is distinguished from G.

ultramicrus by having a larger size,and by lacking a pustulose wall (Petrizzo and Huber, 2006).

While characterizing the three species in thin section can be difficult, their complete taxonomic

range overlap in the Cenomanian mitigates problems caused by misidentification.

Range: G. bentonensis ranges from the Albian T. primula Zone through the Upper Cenomanian

D. algeriana Zone (Premoli Silva and Sliter, 2002). Ice and McNulty (1980) consider the species

an Early and Middle Cenomanian form. G. bentonensis is also reported from the Upper

Cenomanian Greenhorn Formation of Kansas and the Lower Cenomanian Grayson (Del Rio)

Formation of Texas (Pessagno, 1967).

CHRONOS Range:

FO stage: Albian LO stage: Cenomanian FO zone: T. primula LO zone: R. cushmani FO age (Ma): 107.3 LO age (Ma): 94

31

Globigerinelloides caseyi (Bolli, Loeblich and Tappan)

Planomalina caseyi Bolli, Loeblich and Tappan, 1957, U.S. National Museum Bulletin, v. 215, p.

24, pl. 1, figs. 4-5.

Globigerinelloides eaglefordensis (Moreman), Loeblich and Tappan, 1961, p. 268, pl. 2, figs. 3-7.

Globigerinelloides caseyi (Bolli, Loeblich and Tappan), Low, 1964, Contr. Cushman Found. Foram.

Res., v. 15, p. 122-123.

Globigerinelloides escheri (Kaufman), Eicher, 1969b, p. 167 (list only).

Remarks: Globigerinelloides caseyi is distinguished by its planispiral test and smooth wall. It has

less inflated chambers that G. bentonensis, and further distinguishing features are discussed in

the entry for G. bentonensis.

Range: G. caseyi ranges from the Early Cenomanian Th. globotruncanoides Zone through the

Santonian Dicarinella asymetrica Zone (Premoli Silva and Sliter, 2002). This species has been

reported from the Del Rio Clay and from the Eagle Ford Group, including the Boquillas Formation

(Pessagno, 1967).

CHRONOS Range:

FO stage: Albian LO stage: Cenomanian FO zone: P. appenninica LO zone: R. cushmani FO age (Ma): 100.6 (Approximation) LO age (Ma): 94 (Approximation)

32

Globigerinelloides ultramicrus (Subbotina)

Globigerinella ultramicra Subbotina, 1949, Leningrad, Vsesoyuznogo Neftyanogo

Nauchnoissledovatel'skogo Geologorazvedochnogo Instituta (VNIGRI), Vypusk, v. 34, p. 33.

Remarks: Globigerinelloides ultramicrus is characterized by its small size, spinose wall, and

slowly enlarging chambers (Premoli Silva and Sliter, 2002). It is the smallest species of

globigerinellid observed in the Buda Limestone.

Range: This species ranges from the Albian Rotalipora subticinensis Zone through the

Campanian-Maastrichtian Gansserina gansseri Zone (Premoli Silva and Sliter, 2002).

CHRONOS Range:

FO stage: Albian LO stage: Maastrichtian FO zone: R. subticinensis LO zone: G. gansseri FO age (Ma): 101.6 LO age (Ma): 70.4

33

Genus: HEDBERGELLA (Bronnimann and Brown, 1958)

Paleoceanographic significance: Hedbergellids are interpreted as inhabiting near-surface

waters based on stable isotope rankings and paleogeographic distribution (Sliter, 1971; Leckie,

1987; Price et al., 1998; Keller et al., 2001; Keller and Pardo, 2004; Bornemann and Norris,

2007). The δ13C values of Hedbergella delrioensis are heavier than keeled morphotypes,

indicating a shallower depth ecology, and δ18O values are highly variable, possibly indicating a

relatively wide range of depth habitats and an opportunistic strategy (Price et al., 1998;

Bornemann and Norris, 2007).

Hedbergella delrioensis (Carsey)

Globigerina cretacea d’Orbigny var. delrioensis Carsey, 1926, Texas Univ. Bulletin, p. 43.

Hedbergella delrioensis (Carsey) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 275, fig. 11-

13.

Remarks: Hedbergella delrioensis is distinguished by its small size, low trochospiral test,

macroperforate wall, coarsely rugose first chambers, deep and small umbilicus, and smooth final

chambers (Pessagno 1967). It differs from H. planispira by having a deeper umbilicus and thicker

test, and by having more rapidly increasing chambers (Pessagno, 1967; Petrizzo and Huber,

2006).

Range: Hedbergella delrioensis was originally described from the Grayson Formation (Del Rio

Clay equivalent). This species has a large biostratigraphic range and makes a poor biomarker. It

34

is reported from the Early Cretaceous through the Coniacian (Silva and Sliter, 2002). This

species occurs sporadically throughout the Buda Limestone.

CHRONOS Range:

FO stage: Albian LO stage: Santonian FO zone: P. appenninica LO zone: D. concavata FO age (Ma): 100.6 LO age (Ma): 86 (Approximation)

35

Hedbergella planispira (Tappan)

Globigerina planispira, Tappan, 1940, Journal of Paleontology, v. 14, p. 12, pl. 19, fig. 12.

Remarks: Hedbergella planispira differs from other members of the genus be having a low

trochospiral to planispiral test (Pessagno, 1967). The species is recognized in thin sections of the

Buda Limestone by its relatively small size, comparatively smooth wall, and by its flat or slightly

depressed trochospire (Eicher and Worstell, 1970). This species is differentiated from the genus

Globigerinelloides by lacking a true planispiral test, the early chambers being slightly offset

above the horizontal axis.

Range: H. planispira has a seemingly large biostratigraphic range and is reported from the

Graneros Shale, Mowry Shale, and overlying Greenhorn-Fairport sequence (Eicher and Worstell,

1970). Its total range extends from the Aptian through the Campanian (Silva and Sliter, 2002),

but its early origins may be problematic (Petrizzo and Huber, 2006). This species is, however,

known from the Cenomanian (Petrizzo and Huber, 2006). It occurs frequently yet in low

abundance throughout the Buda Limestone.

Paleoceanographic significance: Keller and Pardo (2004) observed that low H. planispira

percentages correlate with 2-4‰ negative δ18O excursions. They suggest that H. planispira had

a lower tolerance for normal marine salinity than H. delrioensis and C. simplex, and report H.

planispira abundances are a good proxy for surface salinity.

CHRONOS Range:

FO stage: Aptian LO stage: Coniacian FO zone: G. blowi LO zone: D. concavata FO age (Ma): 118.1 LO age (Ma): 88.6

36

Clavihedbergella simplex (Morrow)

Hastergerinella simplex Morrow, 1934, Journal of Paleontology, v. 8, p.198, pl. 30, fig. 6 .

Clavihedbergella simplex (Morrow) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 279, 280,

pl. 3, fig. 12a-14b.

Remarks: Clavihedbergella simplex is a medium sized species with a low trochospire. It is

characterized in thin section by its large globular and elongate final chamber (Silva and Sliter,

2002).

Range: Clavihedbergella simplex has been reported from the Cenomanian and Turonian

portions of the Eagle Ford Group from the Rio Grande area to Dallas (Pessagno, 1967). The

species has a long biostratigraphic range, from the Albian Ticinella primula Zone through the

Coniacian Dicarinella concavata Zone (Silva and Sliter, 2002). It occurs very rarely and

sporadically in the Buda Limestone.

CHRONOS Range:

FO stage: Albian LO stage: Coniacian FO zone: H. planispira LO zone: D. concavata FO age (Ma): 112.2 LO age (Ma): 85.8

37

Genus: HETEROHELIX (Ehrenberg, 1843)

Paleoceanographic significance: Heterohelicids are interpreted as being opportunistic surface

dwellers, able to tolerate adverse environments (Sliter, 1971; Leckie, 1987; Keller et al., 2001;

Gebhardt et al., 2004). Leckie (1987) uses the genus Heterohelix, along with Guebelitria, and

Gubkinella, to define his shallow Epicontinental Sea Fauna (<100m), reporting that these genera

occur in drastically reduced abundances in open marine sections. Isotope work done by

Bornemann and Norris (2007) supports a shallow depth ecology. High abundances of

Heterohelix have been attributed to expansion of the oxygen minimum zone, oxygen

fluctuation, salinity fluctuation, and high nutrient supply (Eicher and Worstell, 1970; Sliter, 1971;

Leckie, 1987; Nederbragt et al., 1998; Keller et al., 2001; Gebhardt et al., 2004). Nederbragt et

al. (1998) showed that increased relative abundances of heterohelicids may correlate with

positive δ13C excursions due to global anoxia events, and demonstrated strong correlations

across the Cenomanian/Turonian boundary. Because heterohelicids were opportunists capable

of living in a variety of unfavorable conditions, an exact cause for high H. moremani abundance

in the Buda Limestone is difficult to determine. However, because benthic foraminifera and

gastropods are consistently present throughout the entire Buda Limestone, it is unlikely that the

abundance of H. moremani indicates low oxygen conditions or oxygen minimum zone

expansion. It may be that the Buda was deposited in shallow enough water to have created a

hostile planktonic environment with significant elimination of open ocean planktonic habitats,

yet still deep enough to permit a mostly planktonic assemblage. Variable or increased nutrient

supply and/or upwelling may also have played a role (Nederbragt, 1998; Gebhartd et al., 2004).

Further analysis is given in the Biostratigraphy and Stable Isotopes sections of this work.

38

Heterohelix moremani (Cushman)

Guembelina moremani Cushman, 1938, Contr. Cushman Lab. Foram. Res., v. 14, p. 10, pl. 2, figs.

1-3.

Guembelina washitensis Tappan, 1940, Jour. Paleont., v. 14, p. 115, pl. 19, fig. 1.

Heterohelix sp. Ayala, 1962, Soc. Geol. Mexicana, Bol., v. 25, p. 11, pl. 1, figs. 1a-c; pl. 6, figs. 1a-c

Remarks: Heterohelix moremani is distinguished in thin section by its small size, biserial test,

and smooth wall. The globular chambers enlarge slowly until they become parallel-sided and

equi-dimensional (Premoli Silva and Sliter, 2002). This species is distinguished from H. globulosa

by having more slowly enlarging chambers, lacking well-developed striae, and by being generally

smaller, although a range of transitions between the two species has been observed

(Nederbragt, 1991; Premoli Silva and Sliter, 2002). Heterohelix moremani dominates the

planktonic foraminiferal assemblage of the Buda Limestone, and in most samples represents

approximately half of all planktonic forms.

Range: Heterohelix moremani ranges from the Albian Pseudothalmanninella ticinensis Zone

through the Santonian Dicarinella asymetrica Zone (Premoli silva and Sliter, 2002). It is the

earliest known species from the Heterohelix genus and is ancestral to all later species

(Nederbragt, 1991). This species is known from Texas in the Grayson Formation (Del Rio Clay

equivalent) and Eagle Ford Group of Texas, and from the San Felipe Formation of Mexico

(Pessagno, 1967). It has also been recognized in the Morelos, Cuautla, and Mexcala Formations

of southern Mexico (Aquilera-Franco, 2003). Heterohelix moremani occurs ubiquitously and

consistently throughout the Buda Limestone, each thin section containing over 20 recognizable

specimens. Heterohelix moremani dominates the planktonic assemblage except in the lower-

39

middle portion of the Buda Limestone where globigerinellids reach their very greatest

abundance of about 30 – 40 recognizable specimens per thin section.

CHRONOS Range:

FO stage: Albian LO stage: Turonian FO zone: R. ticinensis LO zone: M. schneegansi FO age (Ma): 101.6 LO age (Ma): 90.7

40

Heterohelix globulosa (Cushman)

Textularia globulosa Ehrenberg, 1840, p. 135, pl. 4, figs. 2b, 4b, 5b, 7b, 8b.

Textularia striata Ehrenberg 1840, p. 135, pl. 4, figs. 1a, 2a, 3a, (? 9a).

Guembelina reussi Cushman, 1938, Cushman Laboratory for Foraminiferal Research,

Contributions, v. 14, p. 11., pl. 2, figs. 6-9.

Remarks: H. globulosa differs from H. moremani by having more rapidly increasing chambers,

and by possessing better-developed striae (Premoli Silva and Sliter, 2002). This species evolved

from H. moremani, and is thus very similar (Pessagno 1967, Nederbragt, 1991). This study

follows the conclusions drawn by Nederbragt (1991), which considers the species H. reussi and

H. striata as morphotypes of H. globulosa. The morphotype H. reussi is characterized by

triangular depressions along its median sutures, whereas the morphotype H. striata possesses

coarser costae.

Range: This species ranges from the Late Cenomanian D. algeriana Subzone of the R. cushmani

Zone through the end of the Cretaceous (Nederbragt, 1991). Variable cross sectional

orientation in thin section complicates identification of striae and chamber enlargement, making

H. globulosa difficult to distinguish from H. moremani (Nederbragt et al., 1998). Nevertheless,

certain specimens from the basal Boquillas are likely H. globulosa, since they display more

rapidly increasing chambers than the heterohelicids observed in the Buda. This species has

been reported from the basal 10-20 feet of the Boquillas in BBNP (Frush and Eicher, 1975).

CHRONOS Range:

FO stage: Cenomanian LO stage: Maastrichtian FO zone: R. cushmani LO zone: A. mayaroensis FO age (Ma): 93.6 LO age (Ma): 65

41

Genus: PARATHALMANNINELLA Lipson-Benitah, 2008

Paleoceanographic significance: (See genus ROTALIPORA).

Parathalmanninella appenninica (Renz)

Globotruncana appenninica Renz, 1936, Eclogae Geologicae Helvetiae, v. 29, p. 20.

Rotalipora appenninica (Renz) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 299-301, figs.

11a-c, 12a-c.

Rotalipora appenninica appenninica (Renz), Luterbacher and Premoli Silva, 1966, p. 266–268, pl.

19, figs. 1, 2.

Rotalipora appenninica (Renz), Robaszynski, Caron, and others, 1979, p. 60, pl. 4, figs. 1a-c.

Thalmanninella appenninica (Renz) González-Donoso, Linares, and Robaszynski, 2007, Journal of

Foraminiferal Research, v. 37, p. 183.

Parathalmanninella appenninica (Renz) Lipson-Benitah, S., 2008, Journal of Foraminiferal

Research, v. 38, p. 185, fig. 2.

Remarks: Parathalmanninella appenninica is characterized in thin section by its small to

medium size, low trochospiral test, smooth and macroperforate wall, single-keeled angular

peripheral margin, rapidly enlarging chambers, and oblate outline (Premoli Silva and Sliter,

2002; Petrizzo and Huber, 2006). The species is known for considerable taxonomic variability

(Lipson-Benitah, 2008).

Range: P. appenninica ranges from the base of the Upper Albian P. appenninica Zone to the

lower R. cushmani Zone in the Middle Cenomanian (González-Donoso, 2007; Lipson-Benitah

42

2008). P. appenninica is reported from the Maness Formation overlying the Buda Limestone in

Tyler County, TX (Barret and Goodson, 2006). Rare specimens have been identified from the

Buda Limestone by this study.

CHRONOS Range:

FO stage: Albian LO stage: Cenomanian FO zone: P. appenninica LO zone: R. cushmani FO age (Ma): 101.3 LO age (Ma): 94.3

43

Parathalmanninella micheli (Sacal and Debourle)

Globotruncana (Rotalipora) micheli Sacal and Debourle, 1957, Eclogae Geologicae Helvetiae, v.

78, p. 58.

Thalmanninella micheli (Sacal and Debourle) González-Donoso, J.M., Linares, D., and

Robaszynski, F., 2007, Journal of Foraminiferal Research, v. 37, p. 183.

Parathalmanninella micheli (Sacal and Debourle) Lipson-Benitah, S., 2008, Journal of

Foraminiferal Research, v. 38, p. 185, fig. 2.

Remarks: P. micheli is characterized in thin section by its medium size, asymmetrical profile,

and compact test. It appears similar to Th. brotzeni in cross-section, but is distinguished by its

curved final chamber (Premoli Silva and Sliter, 2002). Lipson-Benitah (2008) contends that this

species descended from P. appenninica rather than P. balernaensis, as proposed by González-

Donoso et al. (2007).

Range: P. micheli ranges from the upper portion of the Th. globotruncanoides Zone in the late

Lower Cenomanian to the base of the Th. reicheli Zone in the Middle Cenomanian (González-

Donoso et al., 2007; Lipson-Benitah, 2008). A single specimen was identified from the Buda

Limestone in this study.

CHRONOS Range:

FO stage: Albian LO stage: Cenomanian FO zone: P. appenninica LO zone: Th. reicheli FO age (Ma): 99 LO age (Ma): 96.8

44

Genus: PRAEGLOBOTRUNCA (Bermudez, 1952)

Paleoceanographic significance: Praeglobotruncanids are interpreted as inhabiting

intermediate depths, within the lower photic zone (Keller et al., 2001; Gebhardt et al., 2004;

Keller and Pardo, 2004). Leckie (1987) places the genus in his Deep Water Fauna (>100m), but

indicates that praeglobotruncanids likely lived above rotaliporids in the water column.

Praeglobotruncana delrioensis (Plummer)

Globorotalia delrioensis Plummer, 1931, Univ. Texas Bulletin, p. 199, pl. 13, figs. 2a-c.

Praeglobotruncana delrioensis (Plummer) Robaszynski, Caron, and others, 1979, p.29, pl. 43,

figs. 1a-c.

Remarks: Praeglobotruncana delrioensis is distinguishable in thin section by its small to

medium size, trochospiral test, biconvex and lobulate outline, truncate peripheral band, and

pustulose umbilical surface (Premoli Silva and Sliter, 2002; Petrizzo and Huber, 2006). The

species exhibits a large range of morphological variability in the number and shape of the

chambers, and in the degree of keel development (Petrizzo and Huber, 2006).

Range: Premoli Silva and Sliter (2002) indicate the first appearance of P. delrioensis in the

Albian Rotalipora subticinensis Zone and ranges into the lowermost portion of the Upper

Cenomanian Dicarinella algeriana Subzone of the Rotalipora cushmani Zone. Petrizzo and

Huber (2006) indicate that P. delrioensis first appears in the Albian Parathalmanninella

appenninica Zone and ranges into the upper part of the Middle-Upper Cenomanian Rotalipora

cushmani Zone. P. delrioensis is known from the Grayson Formation (Del Rio Clay) of Texas, and

from the upper Cuesta del Cura Formation of Mexico (Pessagno, 1967; Ice and McNulty, 1980).

45

CHRONOS Range:

FO stage: Albian LO stage: Cenomanian FO zone: P. appenninica LO zone: H. helvetica FO age (Ma): 99.9 LO age (Ma): 92 (Approximation)

46

Praeglobotruncana stephani (Gandolfi)

Globotruncana stephani Gandolfi, 1942, Rivista Italiana di Paleontologia, v. 4, p. 130, pl. 3, figs.

4a-c.

Praeglobotruncana stephani (Gandolfi) Robaszynski, Caron, and others, 1979, p. 47, pl. 48, figs.

1a-c.

Remarks: Praeglobotruncana stephani is characterized in thin section by its medium size,

moderately high trochospire, biconvex outline, umbilical-extraumbilical aperture (Premoli Silva

and Sliter, 2002; Petrizzo and Huber, 2006). The species differs from P. derioensis by having a

more convex spiral side, a higher trochospire, and by possesing a more highly developed line of

pustules on the peripheral margin forming a keel (Petrizzo and Huber, 2006).

Range: P. stephani has a moderately long biostratigraphic range and first appears in the Albian

R. appenninica Zone and ranges into the upper part of the Turonian H. helvetica Zone (Premoli

Silva and Sliter, 2002). Rare specimens are observed throughout the Buda Limestone and in the

basal Boquillas Formation in this study.

CHRONOS Range:

FO stage: Albian LO stage: Turonian FO zone: P. appenninica LO zone: H. helvetica FO age (Ma): 101.3 LO age (Ma): 91

47

Praeglobotruncana gibba Klaus

Praeglobotruncana stephani Gandolfi var. gibba Klaus, 1960, Eclogae Geologicae Helvetiae, v.

53, p. 304-305.

Praeglobotruncana stephani (Gandolfi) Loeblich and Tappan, 1961, p. 280-284, pl. 6, figs. 4a-b,

5a-c, 6, 7a-c.

Praeglobotruncana gibba Klaus, Robaszynski and Caron, 1979, p. 33-38, pl. 44, figs. 1a-c, 2a-c; pl.

45, figs. 1a-c, 2a-c.

Remarks: Praeglobotruncana gibba is distinguished in thin section from other members of the

genus by its medium to large size and very distinct high trochospire. The species is also marked

by its pustulose umbilical surface (Hasegawa, 1999; Premoli Silva and Sliter, 2002).

Range: P. gibba first appears just below the FAD of Th. greenhornensis in the Early Cenomanian

(Hasegawa, 1999) and ranges through the Turonian H. helvetica Zone (Premoli Silva and Sliter,

2002). A single specimen was identified with high confidence from the lower Boquillas in this

study. Possible fragments of P. gibba were also observed.

CHRONOS Range:

FO stage: Cenomanian LO stage: Turonian FO zone: P. appenninica LO zone: H. helvetica FO age (Ma): 96.5 (Approximation) LO age (Ma): 90.3

48

Genus: ROTALIPORA Brotzen, 1942

Paleoceanographic significance: Rotaliporids are interpreted as inhabiting deep waters below

the thermocline, with stable isotope values close to those of benthic foraminifera (Price et al.,

1998; Keller et al., 2001; Gebhardt et al., 2004; Keller and Pardo, 2004). Leckie (1987) uses

rotaliporids, along with the genera Planomalina and Praeglobotruncana, to define his Deep

Water Fauna (>100m). Paleogeographic faunal distributions indicate that rotaliporids were

adapted for stable and well-stratified water masses, and were especially sensitive to

environmental changes (Leckie, 1987; Keller and Pardo, 2004).

Rotalipora montsalvensis (Mornod)

Globotruncana (Rotalipora) montsalvensis Mornod, 1949, Eclogae Geologicae Helvetiae, v. 42, p.

584-585, fig. 4 (Ia-c).

Rotalipora montsalvensis Mornod; Caron, 1976, p. 329-330, figs. 1a-c.

Remarks: Rotalipora montsalvensis is distinguished in thin section by its medium size,

biconvexity, weakly developed peripheral keel, and slightly inflated chambers (Caron and

Spezzaferi, 2005; González-Donoso et al., 2007). The species differs from R. cushmani by lacking

a spiral depression and fully-developed keel (Keller et al., 2001).

Range: R. montsalvensis ranges from the middle of the Lower to Middle Cenomanian Th.

globotruncanoides Zone through the Middle to Upper Cenomanian R. cushmani Zone (Caron and

Spezzaferri, 2006; González-Donoso et al., 2007). Rare specimens of R. montsalvensis were

49

recognized from the Buda Limestone, and represent important biostratigraphic indices. A single

specimen of R. montsalvensis was recognized from the basal Boquillas Formation.

CHRONOS Range:

FO stage: Cenomanian LO stage: Cenomanian FO zone: Th. globotruncanoides LO zone: R. cushmani FO age (Ma): 97.2 LO age (Ma): 93.8

50

Rotalipora cushmani (Morrow)

Globorotalia cushmani Morrow, 1934, Jour. Paleont., v. 8, p. 199, pl. 31, figs. 2, 4.

Rotalipora turonica Brotzen, 1942, Sver. Geol. Unders., Ser. C, No. 451, Ars. 36, n. 8, p. 32, figs.

10, 11.

Globotruncana alpina Bolli, 1945, Eclogae Geol. Helv., v. 37, p. 224, 225, pl. 9, figs. 3, 4.

Rotalipora montsalvensis Mornod, Hagn, and Zeil, 1954, Eclogae Geol. Helv., v. 47, p. 29, pl. 1,

fig. 4; pl. 5, fig. 2.

Rotalipora cushmani (Morrow) Loeblich and Tappan, 1964, Treatise on Invert. Paleont., v. 2, p.

659-661 fig. 528, 1a-c, 2a-c.

Rotalipora cushmani (Morrow) Desmares, Grosheny, and Beaudoin, 2008, Marine

Micropaleont., v. 69, p. 96, pl. 1, figs. 3a-c.

Remarks: Rotalipora cushmani is characterized by its medium to large size, both spirally and

umbilically inflated angular chambers, and strongly developed single keel (Premoli Silva and

Sliter, 2002; González-Donoso et al., 2007). It is differentiated in thin section from the similar R.

montsalvensis by the presence of a spiral depression and fully-developed keel (Keller et al.,

2001). Rotalipora montsalvensis may be the ancestor of Rotalipora cushmani; however the two

species may also have derived from a common ancestor (Desmares et al., 2008). In thin section,

R. cushmani is distinguished from Th. greenhornensis by having more inflated and less sharply

angular chambers, and by posessing a spiral depression.

Range: The Middle-Upper Cenomanian R. cushmani Zone is defined by the total range of the

nominate species. The extinction of Rotalipora cushmani at the onset of the OAE2 event

51

corresponds to the extinction of all rotaliporids except Anaticinella multiloculata, which likely

persists for a short time into the Upper Cenomanian W. archaeocretacea Zone (Caron et al.,

2006; González-Donoso et al., 2007). Rotalipora cushmani was not identified in the Buda

Limestone in this study. However, a small number of relatively poorly preserved specimens

were identified from the basal Boquillas Formation.

CHRONOS Range:

FO stage: Cenomanian LO stage: Cenomanian FO zone: R. cushmani LO zone: R. cushmani FO age (Ma): 96.2 LO age (Ma): 93.5

52

Genus: THALMANNINELLA Sigal, 1948

Paleoceanographic significance: (See genus ROTALIPORA).

Thalmanninella globotruncanoides (Sigal)

Thalmanninella brotzeni Sigal, 1948, Revue de l’Institut Francais du Pêtrole et Annales des

Combustile Liquides, v. 3, p. 102.

Thalmanninella globotruncanoides Sigal, 1948, Revue de l’Institut Francais du Pêtrole et Annales

des Combustile Liquides, pl. 1, fig. 4.

Thalmanninella brotzeni (Sigal) Premoli-Silva and Sliter, 2002, Practical Manual of Cretaceous

Planktonic Foraminifera, p. 406, pl. 163, figs. 9, 12.

Thalmanninella globotruncanoides (Sigal) González-Donoso, Linares, and Robaszynski, 2007,

Journal of Foraminiferal Research, v. 37, p. 184.

Remarks: There is some dispute whether Thalmanninella globotruncanoides and

Thalmanninella brotzeni represent a single species. The species were originally distinguished by

the presence of umbilical secondary apertures in Th. brotzeni, and the presence of umbilical

(first chambers of the last whorl) and sutural (last chambers) supplementary apertures in Th.

globotruncanoides (González-Donoso et al., 2007). Th. brotzeni is also smaller and more

biconvex. González-Donoso et al. (2007) consider Th. brotzeni a junior synonym of Th.

globotruncanoides, while Lipson-Benitah (2008) considers the two species separate. Lipson-

Benitah (2008) differentiates the two species based on their sizes, growth rates, chamber

morphologies, and position of the last-formed supplementary apertures, and states that Th.

brotzeni appears before Th. globotruncanoides in the Cenomanian of Israel. This study does not

53

differentiate between the two species, since the most distinguishing characteristics are not

observable in thin section.

Range: Thalmanninella globotruncanoides ranges from the base of the Lower Cenomanian to

the upper part of the Middle-Late Cenomanian R. cushmani Zone (González-Donoso et al., 2007;

Lipson-Benitah, 2008). The first appearance of Th. globotruncanoides through the first

appearance of Th. reicheli defines the Lower-Middle Cenomanian Th. globotruncanoides Zone.

A single specimen of this species was tentatively identified from the Buda Limestone. This

species is included here due to its importance for Lower Cenomanian biostratigraphy.

CHRONOS Range:

FO stage: Cenomanian LO stage: Cenomanian FO zone: Th. globotruncanoides LO zone: R. cushmani FO age (Ma): 98.9 LO age (Ma): 93

54

Thalmanninella greenhornensis (Morrow)

Globorotalia greenhornensis Morrow, 1934, Journal of Paleontology, v. 8, p. 199.

Rotalipora greenhornensis (Morrow) Loeblich and Tappan, 1961, Micropaleont., v. 7, p. 299-301,

pl. 7, figs. 7a-9c.

Thalmanninella greenhornensis (Morrow) González-Donoso, Linares, and Robaszynski, 2007,

Journal of Foraminiferal Research, v. 37, p. 184.

Remarks: Thalmanninella greenhornensis is distinguished in thin section by being nearly plano-

convex and by its large size. The species is marked by its smooth spiral surface, broad umbilicus,

and angular chambers on the umbilical side (Premoli Silva and Sliter, 2002). Th. greenhornensis

displays reversed features with respect to the evolutionary trends between other members of

the genus. The Th. praebalernaensis to Th. globotruncanoides lineage is marked by further

development of the keel, decreased inflation of the chambers, and an increase of the umbilical

ornamentation (González-Donoso et al., 2007). In thin section, Th. greenhornensis may have a

weaker keel and slightly more inflated chambers compared to other members of the genus. A

tentatively recognized form is that of Anaticinella sp. cf. A. multiloculata. This species evolved

from Th. greenhornensis by the reduction of the keel and by the inflation of the chambers, likely

an adaptation to living higher in the water column as a response to expanded global oxygen

minimum zones in deeper water (Caron et al., 2006; Keller and Pardo, 2004). Differentiation of

A. multiloculata and Th. greenhornensis is difficult due to transitional morphotypes between the

two (Keller and Pardo, 2004), however several unidentified specimens in this study likely

represent A. multiloculata.

55

Range: Th. greenhornensis first appears in the Th. globotruncanoides Zone in the Lower

Cenomanian and ranges through the end of the R. cushmani Zone in the Upper Cenomanian

(González-Donoso et al., 2007; Lipson-Benitah, 2008). The species was reported from the basal

110 feet of the Boquillas Formation at the Hot Springs locality in Big Bend National Park by Frush

and Eicher (1975). Numerous specimens were identified in the lower 1.5 meters of the Boquillas

Formation by this study.

CHRONOS Range:

FO stage: Cenomanian LO stage: Cenomanian FO zone: R. cushmani LO zone: R. cushmani FO age (Ma): 96.2 LO age (Ma): 93.7

56

Genus: WHITEINELLA (Pessagno, 1967)

Remarks: The globular whiteinellids are distinguished from each other in thin section primarily

by trochospire height, width of umbilicus, and symmetry (Premoli Silva and Sliter, 2002). This

genus is likely present from samples taken from the lower Boquillas Formation; however all

specimens are poorly preserved and infilled with calcite spar. Sparry infill complicates the

recognition of the cross-sectional orientation of globular foraminifera, therefore making

identification to species level difficult. Pessagno (1967) reports the presence of whiteinellids

from the Eagle Ford Group in Texas, including W. brittonensis (Loeblich and Tappan) from

Cenomanian and Turonian portions of the sections.

Paleoceanographic significance: Whiteinellids likely occupied the lower part of the surface

mixed layer and preferred normal marine salinity (Keller and Pardo, 2004). They share

ecological affinities with hedbergellids and heterohelicids, and are interpreted as shallow to

intermediate depth opportunists. However, the genus is also commonly found in open ocean

deposits (Keller et al., 2001; Gebhardt et al., 2004).

57

Whiteinella aprica (Loeblich and Tappan)

Ticinella aprica Loeblich and Tappan, 1961, Micropaleontology, v. 7, p. 292, pl. 4, figs. 14-16.

Remarks: Whiteinella aprica possesses a particularly low trochospire, wide umbilicus, and

pustulose surface. Chambers are globular and enlarge slowly, making the species is nearly

symmetrical in cross-section (Premoli Silva and Sliter, 2002). Well-preserved specimens may

display thin, delicate umbilical plates (Eicher and Worstell, 1970).

Range: This species was identified by Frush and Eicher (1975) in the lowest sampled intervals of

Boquillas Formation at Chispa Summit and in the Jeff Davis Mountains. Whiteinella aprica

ranges from the Upper Cenomanian D. algeriana Subzone of the R. cushmani Zone to nearly the

end of the Turonian Helvetoglobotruncana helvetica Zone (Premoli Silva and Sliter, 2002).

CHRONOS Range:

FO stage: Cenomanian LO stage: Coniacian FO zone: W. archaeocretacea LO zone: M. schneegansi FO age (Ma): 94 LO age (Ma): 88.8

58

Whiteinella baltica Douglas and Rankin, 1969, Lethaia, v. 2, p. 197.

Remarks: This species is characterized by its medium to large size, by posessesing a low

trochospire, and by having a nearly symmetrical profile and narrow umbilicus (Premoli Silva and

Sliter, 2002).

Range: Whiteinella baltica ranges from the Middle Cenomanian R. cushmani Zone to the base

of the Santonian D. asymetrica Zone (Premoli Silva and Sliter, 2002). Two sepcimens were

identified from the basal Boquillas Formation in this study. Additional specimens of Whiteinella

sp. may also represent Whiteinella baltica.

CHRONOS Range:

FO stage: Cenomanian LO stage: Santonian FO zone: R. cushmani LO zone: D. concavata FO age (Ma): 94.7 LO age (Ma): 85.2

CHAPTER 3 - BIOSTRATIGRAPHY OF THE BUDA LIMESTONE

Previous work

Ammonites

The ammonite Neophlycticeras (Neophlycticeras) texanum (Shattuck 1903) is reported from the

lowermost Buda Limestone, while Neophlyicticeras (Budaiceras) hyatti is abundant in the middle

and upper portions of the Buda Limestone (Cobban et al., 2008). These species form individual

ammonite Zones in the late Lower Cenomanian that likely correlate to the Lower Cenomanian

Mantelliceras saxbii Subzone through the late Lower Cenomanian Mantelliceras dixoni Zone of

Europe (Hancock, 2004). Other ammonites from the Buda Limestone in this region include:

Budaiceras elegantior (Lasswitz 1904), B. alticarinatum sp., Faraudiella franciscoensis (Kellum

and Mintz 1962), F. roemeri (Lasswitz 1904), F. barachoensis sp., F. archerae sp., Mariella

wysogorskii, and Sharpeiceras tlahualilense (Kellum and Mintz 1962) reported by Young (1979);

as well as various species of the genus Mantelliceras Hyatt 1903 reported by several workers. In

addition, Cobban et al. (2008) produce a detailed list of ammonite species and references.

60

Foraminifera

A previous comprehensive thin section biostratigraphic survey of the foraminifera in the Buda

Limestone in the Big Bend region is unknown to the author. However, Pessagno (1969)

examined thin sections of the Buda from Chispa Summit, Jeff Davis County and Austin, Travis

County. At Chispa Summit, he reports Rotalipora appenninica (Renz), Hedbergella delrioensis

(Carsey), Praeglobotruncana delrioensis (Plummer), and Heterohelix sp. At Austin, he reports

Rotalipora appenninica (Renz), Hedbergella washitensis (Carsey), and Praeglobotruncana

delrioensis (Plummer). All of the species noted by Pessagno (1969) were also recovered from

the Buda Limestone in this study.

Other taxa

Cornell (1997) examined the Buda Limestone at Cerro de Cristo Rey, New Mexico for

dinoflagellate cysts. He reports an assemblage dominated by Spiniferites, with smaller numbers

of Tanyosphaeridium, Hystrichodinium, and others. A single genus of achritarch, Michrystridium,

was also reported. Cornell (1997) notes that the dinoflagellate cyst assemblage agrees with the

established Cenomanian age of the Buda Limestone.

Other age dating

In northern and central Texas, Denison et al. (2003) analyzed strontium isotopes from 10 oyster

shells collected from the base of the Buda at three locations. They report a Δsw mean of -174.2

± 1.1., and dated their youngest sample of Buda at approximately 97 Ma (latter half of the Early

Cenomanian).

61

Species used in the foraminiferal biozonation

Key species of foraminifera used in the biozonation of the Buda Limestone include Favusella

washitensis (Carsey), Globigerinelloides bentonensis (Morrow), G. caseyi (Bolli), Loeblich and

Tappan), Parathalmanninella appenninica (Renz), Praeglobotruncana delrioensis (Plummer), P.

stephani (Gandolfi), and Rotalipora montsalvensis (Mornod) (Tables. 1, 2, 3; Fig. 10). A single

noteworthy specimen of Parathalmanninella micheli (Sacal and Debourle) was also recovered

from the Buda Limestone. This species occupies a narrow biostratigraphic range in the upper

Th. globotruncanoides Zone to the lowermost Th. reicheli Zone.

The upper boundary of the Th. globotruncanoides Zone with the overlying Th. reicheli

Zone is defined by the first appearance of Th. reicheli. Similarly, the upper boundary of the Th.

reicheli Zone with the overlying R. cushmani Zone is defined by the first appearance of R.

cushmani. Neither Th. reicheli nor R. cushmani was recovered from the Buda Limestone,

constraining the unit below the Th. reicheli Zone. The Buda is constrained above the base of the

Th. globotruncanoides Zone by the first appearances of Globigerinelloides caseyi, Rotalipora

montsalvensis, and Thalmanninella micheli.

Foraminiferal biozonation discussion

The overlapping ranges of all species allow for placement of the Buda within the upper portion

of the Early to Middle Cenomanian Th. globotruncanoides Zone (Fig. 10). No definitive first or

last appearances of any species of foraminifera occur within the studied sections. Species

common in the Buda are present throughout, and species uncommon in the Buda occur too

sporadically to infer FAD or LAD occurrences. The scarcity of biostratigraphically useful

62

rotaliporid foraminifera in the Buda is likely a function of the water depth during the time of

deposition. However, enough rotaliporid specimens were recovered to infer a biostratigraphic

zonation. The precise placement of the Buda Limestone in the late Early Cenomanian using

foraminiferal data is in agreement with the ammonite zonation of Cobban et al. (2007). Finally,

the strontium isotope age assignment of 97 Ma given by Denison et al. (2003) falls within the Th.

globotruncanoides Zone and is in agreement with this study.

63

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l

DC-DS1-1 x 1 1 1 1 4

DC-DS1-2 x 1 1 5 7

DC-DS1-3 x 6 1 7

DC-DS1-4 x 1 5 1 7

DC-DS1-5 x 1 1 1 3

DC-DS1-6 x 1 1 1 3

DC-DS1-7 x 0

DC-DS1-8 x 1 2 6 9

DC-DS1-9 x 2 7 1 10

DC-DS1-10 x 3 1 1 1 6

DC-DS1-11 x 1 1 1 3 6

DC-DS1-12 x 2 1 2 1 6

DC-DS1-13 x 1 1 2

DC-DS1-14 x 0

DC-DS1-15 x 3 2 1 6

Table 1 - This chart shows the distribution of recognizable foraminifera per thin section in the Dog Canyon section (Buda Limestone). Shaded areas represent an uncertain identification. Areas marked with an 'x' indicate present, but not counted.

64

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a

Sha

koin

a s

p.

Sha

cko

ina

cen

om

an

a

Pla

no

ma

lin

a s

p.

Loeb

lich

ella

co

arc

tata

A. p

lan

oco

nve

xa

Tota

l

HW-DS1-1 1 1 1 3

HW-DS1-2 1 1 2

HW-DS1-3 2 1 3

HW-DS1-4 1 1 2 4

HW-DS1-5/6 0

HW-DS1-7 0

HW-DS1-8 1 1

HW-DS1-9 1 1 2

HW-DS1-10 4 4

Het

ero

hel

ix s

p. (

H. m

ore

ma

ni)

Het

ero

hel

ix r

euss

i

Glo

big

erin

ello

ides

sp

.

Glo

big

erin

ello

ides

ben

ton

ensi

s

Glo

big

erin

ello

ides

ult

ram

icru

s

Glo

big

erel

lin

oid

es c

ase

yi

Hed

ber

gel

la s

p.

Hed

ber

gel

la d

elri

oen

sis

Hed

ber

gel

la p

lan

isp

ira

Hed

ber

gel

la s

imp

lex

Co

stel

lag

erin

a li

byc

a

Favu

sell

a w

ash

iten

sis

Ro

tali

po

ra s

p.

Ro

tali

po

ra m

on

tsa

ven

sis

Ro

tali

po

ra c

ush

ma

ni

Tha

lma

nn

inel

la g

reen

ho

rnen

sis

Tha

lma

nin

nel

la b

rotz

eni/

glo

bo

tru

nca

no

ides

Pa

rath

alm

an

nin

ella

ap

pen

inn

ica

Pa

rath

alm

an

nin

ella

mic

hel

i

Pra

eglo

bo

tru

nca

na

sp

.

Pra

eglo

bo

tru

nca

na

del

rio

ensi

s

Pra

eglo

bo

tru

nca

na

ste

ph

an

i

Pra

eglo

bo

tru

nca

na

gib

ba

Wh

itei

nel

la s

p.

Wh

itei

nel

la b

alt

ica

Wh

itei

nel

la a

pri

ca

Wh

itei

nel

la p

raeh

elve

tica

Dic

ari

nel

la s

p.

Dic

ari

nel

la a

lger

ian

a

Sha

koin

a s

p.

Sha

cko

ina

cen

om

an

a

Pla

no

ma

lin

a s

p.

Loeb

lich

ella

co

arc

tata

A. p

lan

oco

nve

xa

Tota

l

HW-DS2-1 1 4 1 6

HW-DS2-2 1 1 3 5

HW-DS2-3 1 2 1 4

HW-DS2-4 0

HW-DS2-5 1 5 6

HW-DS2-6 5 1 6 12

Table 2 - These charts show the distribution of identified foraminifera from the Highway Datasets (Buda Limestone). Shaded areas represent an uncertain identification.

65

Het

ero

hel

ix s

p. (

H. m

ore

ma

ni)

Het

ero

hel

ix r

euss

i

Glo

big

erin

ello

ides

sp

.

Glo

big

erin

ello

ides

ben

ton

ensi

s

Glo

big

erin

ello

ides

ult

ram

icru

s

Glo

big

erel

lin

oid

es c

ase

yi

Hed

ber

gel

la s

p.

Hed

ber

gel

la d

elri

oen

sis

Hed

ber

gel

la p

lan

isp

ira

Hed

ber

gel

la s

imp

lex

Co

stel

lag

erin

a li

byc

a

Favu

sell

a w

ash

iten

sis

Ro

tali

po

ra s

p.

Ro

tali

po

ra m

on

tsa

ven

sis

Ro

tali

po

ra c

ush

ma

ni

Tha

lma

nn

inel

la g

reen

ho

rnen

sis

Tha

lma

nin

nel

la b

rotz

eni/

glo

bo

tru

nca

no

ides

Pa

rath

alm

an

nin

ella

ap

pen

inn

ica

Pa

rath

alm

an

nin

ella

mic

hel

i

Pra

eglo

bo

tru

nca

na

sp

.

Pra

eglo

bo

tru

nca

na

del

rio

ensi

s

Pra

eglo

bo

tru

nca

na

ste

ph

an

i

Pra

eglo

bo

tru

nca

na

gib

ba

Wh

itei

nel

la s

p.

Wh

itei

nel

la b

alt

ica

Wh

itei

nel

la a

pri

ca

Wh

itei

nel

la p

raeh

elve

tica

Dic

ari

nel

la s

p.

Dic

ari

nel

la a

lger

ian

a

Sha

koin

a s

p.

Sha

cko

ina

cen

om

an

a

Pla

no

ma

lin

a s

p.

Loeb

lich

ella

co

arc

tata

A. p

lan

oco

nve

xa

Be

nth

ics

Tota

l all

pla

nkt

on

ics

DF-DS1-31 66 2 1 2 3 74

DF-DS1-30 x 3 4 1 8 12

DF-DS1-29 70 2 4 3 1 80

DF-DS1-28 x 3 3 2 5 1

DF-DS1-27 40 2 1 1 1 4 1 50

DF-DS1-26 x 2 1 2 7 1

DF-DS1-25 33 5 1 7 1 1 48

DF-DS1-24 x 5 1 2 1 3

DF-DS1-23 56 3 1 1 1 62

DF-DS1-22 51 5 2 1 59

DF-DS1-21 x 6 1 1 4 1

DF-DS1-20 76 6 2 2 1 87

DF-DS1-19 x 3 2 2 5 1 1 16

DF-DS1-18 77 10 3 1 91

DF-DS1-17 x 3 2 3 2 1 1 1

DF-DS1-16 55 2 1 4 62

DF-DS1-15 x 11 2 2 2

DF-DS1-14 62 4 4 1 3 1 1 1 77

DF-DS1-13 x 3 3 1 3 1

DF-DS1-12 52 7 1 2 1 2 1 1 67

DF-DS1-11 x 15 1 1 2 1

DF-DS1-10 66 11 1 3 1 1 1 84

DF-DS1-9 x 28 3 1 3 3

DF-DS1-8 42 28 1 1 1 7 1 81

DF-DS1-7 x 42 2 1 3 5 1 23

DF-DS1-6 47 27 4 2 1 3 7 1 1 1 94

DF-DS1-5 x 19 2 3 1 1 2 1

DF-DS1-4 32 15 1 1 3 1 1 54

DF-DS1-3 38 10 1 1 2 2 54

DF-DS1-2 31 17 5 3 1 57

DF-DS1-1 27 16 2 4 1 2 6 1 59

Table 3 - This chart shows the distribution of identified foraminifera from the Dagger Flat Dataset 1 section (Buda Limestone). Shaded areas represent an uncertain identification. Areas marked with an 'x' indicate present, but not counted.

66

Figure 10 - This graph shows the geochronologic distribution of selected foraminifera from the Buda Limestone. Note that the taxon range overlap indicates deposition in the Th. globotruncanoides Zone. Please refer to the Systematic Paleontology for detailed references. Chart created with TimeScale Creator.

67

Sam

ple

Tota

l glo

big

eri

ne

llid

s

Tota

l he

tero

he

lici

ds

Tota

l he

db

erg

ell

ids

Tota

l ro

tali

po

rid

s

Tota

l pra

egl

ob

otr

un

can

ids

Tota

l, a

ll o

the

rs

% g

lob

ige

rin

ell

ids

% h

ete

roh

eli

cid

s

% h

ed

be

rge

llid

s

% r

ota

lip

ori

ds

% p

rae

glo

bo

tru

nca

nid

s

% o

the

rs

Sam

ple

He

igh

t (m

)

DF-DS1-31 3 66 5 0 0 0 4.1% 89.2% 6.8% 0.0% 0.0% 0.0% DF-DS1-31 6.0

DF-DS1-30 3 x 13 0 0 0 DF-DS1-30 5.8

DF-DS1-29 2 70 7 0 1 0 2.5% 87.5% 8.8% 0.0% 1.3% 0.0% DF-DS1-29 5.6

DF-DS1-28 3 x 10 0 1 0 DF-DS1-28 5.4

DF-DS1-27 2 40 7 0 1 0 4.0% 80.0% 14.0% 0.0% 2.0% 0.0% DF-DS1-27 5.2

DF-DS1-26 2 x 10 0 1 0 DF-DS1-26 5.0

DF-DS1-25 5 33 8 1 0 1 10.4% 68.8% 16.7% 2.1% 0.0% 2.1% DF-DS1-25 4.8

DF-DS1-24 6 x 6 0 0 0 DF-DS1-24 4.6

DF-DS1-23 3 56 3 0 0 0 4.8% 90.3% 4.8% 0.0% 0.0% 0.0% DF-DS1-23 4.4

DF-DS1-22 5 51 3 0 0 0 8.5% 86.4% 5.1% 0.0% 0.0% 0.0% DF-DS1-22 4.2

DF-DS1-21 7 x 5 0 1 0 DF-DS1-21 4.0

DF-DS1-20 6 76 4 0 1 0 6.9% 87.4% 4.6% 0.0% 1.1% 0.0% DF-DS1-20 3.8

DF-DS1-19 5 x 7 0 1 1 DF-DS1-19 3.6

DF-DS1-18 10 77 4 0 0 0 11.0% 84.6% 4.4% 0.0% 0.0% 0.0% DF-DS1-18 3.4

DF-DS1-17 5 x 5 2 0 1 DF-DS1-17 3.2

DF-DS1-16 3 55 4 0 0 0 4.8% 88.7% 6.5% 0.0% 0.0% 0.0% DF-DS1-16 3.0

DF-DS1-15 13 x 4 0 0 0 DF-DS1-15 2.8

DF-DS1-14 4 62 8 1 1 1 5.2% 80.5% 10.4% 1.3% 1.3% 1.3% DF-DS1-14 2.6

DF-DS1-13 6 x 4 0 0 1 DF-DS1-13 2.4

DF-DS1-12 8 52 5 0 0 2 11.9% 77.6% 7.5% 0.0% 0.0% 3.0% DF-DS1-12 2.2

DF-DS1-11 15 x 4 1 0 0 DF-DS1-11 2.0

DF-DS1-10 12 66 5 0 0 1 14.3% 78.6% 6.0% 0.0% 0.0% 1.2% DF-DS1-10 1.8

DF-DS1-9 32 x 6 0 0 0 DF-DS1-9 1.6

DF-DS1-8 30 42 8 0 1 0 37.0% 51.9% 9.9% 0.0% 1.2% 0.0% DF-DS1-8 1.4

DF-DS1-7 44 x 9 0 0 1 DF-DS1-7 1.2

DF-DS1-6 34 47 10 1 1 1 36.2% 50.0% 10.6% 1.1% 1.1% 1.1% DF-DS1-6 1.0

DF-DS1-5 25 x 3 0 1 0 DF-DS1-5 0.8

DF-DS1-4 16 32 5 0 1 0 29.6% 59.3% 9.3% 0.0% 1.9% 0.0% DF-DS1-4 0.6

DF-DS1-3 12 38 4 0 0 0 22.2% 70.4% 7.4% 0.0% 0.0% 0.0% DF-DS1-3 0.4

DF-DS1-2 17 31 8 0 1 0 29.8% 54.4% 14.0% 0.0% 1.8% 0.0% DF-DS1-2 0.2

DF-DS1-1 18 27 13 0 0 1 30.5% 45.8% 22.0% 0.0% 0.0% 1.7% DF-DS1-1 0.0

Table 4 - This chart shows the absolute and relative abundances of main genera in the Dagger Flat data set 1 section (Buda Limestone). Note Favusella is included as a hedbergellid.

68

Figure 11 - Relative abundances of the main genera from the Dagger Flat Dataset 1 section. Upsection is from right to left. Note the decline in globigerinellids and increase in heterohelicids upsection.

m

m

m

69

Figure 12 - Absolute abundances of heterohelicids and globigerinellids per thin section from the Dagger Flat Dataset 1 section. The deepest water (Maximum Flooding Surface) is interpreted as the maximum in the globigerinellid : heterohelicid ratio at approximately 1.2 m.

m

# individuals per thin section

70

Foraminiferal abundances at Dagger Flat locality

The Dagger Flat DS-1 locality (DF-DS1) represents the most complete section of Buda sampled in

this study. Every recognizable foraminifer was counted in thin section from each of the 31 DF-

DS1 samples, except for the ubiquitous heterohelicids, which were counted in every other

sample. Some problems arise when attempting to analyze relative abundance trends of small

numbers of individual species of foraminifera studied in thin section. Because the absolute

numbers of foraminifera per thin section are relatively low in this study (~50-80 specimens),

abundance data for individual species is unreliable. However, analysis of larger-scale trends on

the genus level mitigates sources of error in low abundance assemblages, without eliminating

significant paleoceanographic data. The Buda Limestone contains a shallow water pelagic

foraminiferal assemblage roughly consisting of 46-90% heterohelicids, 3-37% globigerinellids,

and 4-22% hedbergellids (incl. favusellids) (Table 4; Fig. 11). Deeper water forms such as

praeglobotruncanids and rotaliporids are present in low quantities of 0-3 specimens per thin

section throughout the section, but are not abundant enough to warrant statistical analyses.

Quantitative analysis of samples DF-DS1-1, -7, -19, and -30 yielded 27, 23, 16, and 12

recognizable benthic specimens, respectively. Benthic foraminifera are consistently present in

every thin section, and make up roughly 45-15% of the total foraminiferal assemblage. Low

absolute numbers of benthic specimens per thin section make statistical analysis problematic.

The number of benthic specimens per thin section is highly dependent upon the thin section’s

orientation, as well as the presence or absence of larger bioclasts in the particular thin section.

Due to their thick, high-Mg calcite test walls, benthic foraminifera are more easily identified in

71

thin section than planktonic foraminifera. Hence, benthic counts in thin section may be inflated

due to preservational bias.

A more reliable method of thin section analysis uses shallow heterohelicid and deep

dwelling globigerinellid planktonic foraminifera. An increase in the numbers of globigerinellids

from 20-30% to 37% is recorded upsection from the base of the outcrop, followed by a decrease

to 14% at about 2m above the base of the measured section (Fig. 11). A declining trend in the

relative abundance of globigerinellids continues slightly until the top of the section. Absolute

numbers of globigerinellids per thin section undergo an order of magnitude decrease from the

bottom of the section to the top of the section (Fig. 12). Heterohelicids are abundant

throughout the section and show an overall increase in relative abundance from 46% at the

base of the section to 90% near the top. Two notable troughs in relative abundance at 1.6m and

5m above the base of the section correspond to relative peaks in the globigerinellid and

hedbergellid populations, respectively.

Foraminiferal abundances discussion

Foraminiferal assemblages dominated by relatively shallow-dwelling pelagic faunas imply water

depths between about 50 to 100 m (Baker, 1976). This interpretation agrees with Scott and

Kidson (1977) and Lock et al. (2007), who assigned an open shelf paleodepth for the Buda

Limestone in Big Bend National Park. Intermediate-depth, open marine globigerinellids show an

initial increase followed by overall decrease in abundance upsection. The accompanied increase

in opportunistic shallow-depth heterohelicids upsection indicates the development of

conditions more favorable to opportunistic species than to other species (Nederbragt, 1991).

72

Furthermore, open marine assemblages are expected to decrease in abundance shoreward

(Sliter, 1972; Leckie, 1987). Hence, these abundance data are interpreted as showing initial sea

level transgression followed by regression. The maximum flooding surface (MFS) likely occurs

near the first 1.2 m of Buda. Other effects of eustacy are apparently small, however, since the

entire Buda Limestone remains deep enough for a pelagic-dominated facies even in the

uppermost sections studied. It is likely that the extant Buda Limestone was deposited entirely

within the 50-100 m depth interval discussed above. Further paleoceanographic implications of

heterohelicid dominance are discussed in the Systematic Paleontology section of this work.

CHAPTER 4 - BIOSTRATIGRAPHY OF THE LOWERMOST BOQUILLAS FORMATION

Previous work

Ammonites

The basal ±1 m of the Boquillas Formation in Big Bend National Park is placed within the

Acompsoceras inconstans Zone based on the presence of Moremanoceras bravoense (Cobban

and Kennedy, 1989) and Euhystrichoceras adkinsi (Powell, 1963), and is assigned a late Early

Cenomanian age (Cobban et al., 2008; Cooper, 2008). Acompsoceras inconstans (Schlüchter)

and two possible specimens of A. renevieri (Sharpe 1857) were reported from east of Big Bend

National Park by Daugherty and Powell (1963). Acompsoceras inconstans is reported only from

the late Lower Cenomanian Mantelliceras dixoni Zone in northwest Europe (Cobban and

Kennedy, 1989; Hancock et al., 1993). All specimens assigned an Early Cenomanian age are

restricted to the lowest ±1 m of the Boquillas in Big Bend National Park (Cooper, 2008). Several

ammonites of the genus Inoceramus are reported from the within first ±11 m of the Boquillas,

and are assigned a Middle Cenomanian age (Cooper, 2008). The Middle Cenomanian ammonite

Zones Conlinoceras tarantense - Plesiacanthoceras wyomingsense of the Western Interior

Seaway, however, are not recognized in Trans-Pecos Texas (Cobban et al., 2008).

74

Foraminifera

Pessagno (1967, 1969) assigned the lowermost Boquillas to the Rotalipora cushmani-Rotalipora

greenhornensis Zone in the upper half of the Cenomanian. He specifies a significant amount of

time missing during the Buda - Boquillas unconformity during the middle portion of the

Cenomanian. Frush and Eicher (1975) sampled the first ±10-20 ft of Boquillas at the Hot Springs

locality in Big Bend National Park, and report Hedbergella delrioensis (Carsey), Heterohelix

globulosa (Cushman), Hedbergella amabilis (Loeblich and Tappan), Hedbergella planispira

(Tappan), Globigerinelloides bentonensis (Morrow), and Rotalipora greenhornensis (Morrow).

They report Rotalipora cushmani (Morrow) and Praeglobotruncana stephani (Gandolfi) ±40 ft

above the basal contact of the Boquillas with the Buda. According to their study, Whiteinella

aprica (Loeblich and Tappan) first occurs at ±60 ft, while Whiteinella archaeocretacea (Pessagno)

first occurs at about 100 ft above the basal contact. Frush and Eicher (1975) assign the base of

the Boquillas to the Rotalipora greenhornensis-Rotalipora cushmani Zone based on this

assemblage.

Other age dating

Denison et al. (2003) estimates the age of the base of the Gulfian Series (Woodbine Formation)

immediately above the Buda Limestone in northern and central Texas at 94.4 Ma, based on

strontium isotopes. This age determination corresponds to the Upper Cenomanian portion of

the R. cushmani Zone.

75

Species used in the foraminiferal biozonation

Key species recovered from the lowermost 1.2 m Boquillas Formation include Dicarinella sp.,

Dicarinella algeriana (Caron), Rotalipora sp., Rotalipora cushmani (Morrow), Thalmanninella sp.

cf. Th. greenhornensis (Morrow ), and Whiteinella baltica Douglas and Rankin (Table 5a).

Additionally, single (or very rare) specimens of Helvetoglobotruncana sp. cf. H. praehelvetica

(Trujillo), Heterohelix globulosa (Cushman), Praeglobotruncana gibba Klaus, Praeglobotruncana

stephani (Gandolfi), Shackoina cenomana (Shacko), Rotalipora montsalvensis (Mornod), and

Whiteinella sp. cf. W. aprica (Loeblich and Tappan) were also recovered.

Another tentatively recognized and potentially problematic form is that of Anaticinella

sp. cf. A. multiloculata, as outlined in the Systematic Paleontology section of this work. High

abundances of this genus are associated with the Late Cenomanian in the Hartland Shale at

Pueblo, CO. Though likely derived from rotaliporid stock, the genus Anaticinella apparently

survived the extinction of all other rotaliporids and persisted into the W. archaeocretacea Zone,

vanishing at the Cenomanian-Turonian boundary (Caron et al., 2006). Differentiation of

Anaticinella sp. and Th. greenhornensis is difficult due to transitional morphotypes between the

two (Keller and Pardo, 2004), and also morphological similarities with R. cushmani (Leckie,

1985). Given these potential morphological similarities, it must be noted that mis-identification

of Anaticinella as a “classic” rotaliporid may lead to the inclusion of the bottom of the W.

archaeocretacea Zone into the underlying D. algeriana Subzone, since the bottom of the W.

archaeocretacea Zone is defined by the disappearance of rotaliporids (specifically Rotalipora

cushmani).

76

Double-keeled dicarinellids are an especially important group of foraminifera for the

biozonation of the basal Boquillas Formation. The genus is first represented by the species D.

algeriana, which appears in the latter portion of the R. cushmani Zone, in the lower part of the

Upper Cenomanian. D. algeriana is the first species of foraminifera to evolve tests with a

double keel (Premoli Silva and Sliter, 2002). While the author is confident in the presence of

double-keeled dicarinellids in the sampled sections, it must be noted that Frush and Eicher

(1975) reported no members of the genus Dicarinella in the entire Boquillas Formation in any of

their localities. Most foraminifera recovered from the basal Boquillas by this study are in-filled

with calcite spar, and are not perfectly preserved. It may be that the recognition of a double

keel is not possible in washed samples, as prepared by Frush and Eicher (1975), but may instead

only be identified in thin section.

Foraminiferal biozonation discussion

Based on the overlapping ranges of recovered foraminifera, this study places the basal Boquillas

Formation in the Upper Cenomanian D. algeriana Subzone of the R. cushmani Zone (Fig. 13).

The lower constraint is given by the presence of Dicarinella algeriana, the first species of

foraminifera to evolve a double keel. The upper constraint (the base of the overlying W.

archaeocretacea Zone) is defined by the extinction of rotaliporids (Premoli Silva and Sliter, 2002;

González-Donoso et al., 2007). The presence of rotaliporids in all studied samples of Boquillas

therefore restricts the zonation below the R. cushmani Zone - W. archeocretacea Zone border.

This age designation is in agreement with the strontium isotope age given by Denison et al.

(2003) discussed above.

77

The original zonation given by Frush and Eicher (1975) is herein considered obsolete

(Fig. 14). For example, it is now known that the first appearance of Heterohelix globulosa

(Cushman) occurs in the Upper Cenomanian D. algeriana Subzone of the R. cushmani Zone

(Nederbragt, 1991). The first appearance of Whiteinella aprica (Loeblich and Tappan) also

occurs in the D. algeriana Subzone (Premoli Silva and Sliter, 2002). Therefore it now appears

that the lower 0-60 feet of the Boquillas is constrained to the D. algeriana Subzone given both

the data of Frush and Eicher (1975) and of this study. This re-zonation is supported by samples

gathered by Frush and Eicher (1975) at Chispa Summit and in the Jeff Davis Mountains, where

both Heterohelix globulosa (Cushman) and Whiteinella aprica (Loeblich and Tappan) are

reported in the first sample from the lowermost 5 ft of Chispa Summit Fm. (Boquillas Fm.) above

the Buda Limestone.

The late Early Cenomanian - Middle Cenomanian age designation for the basal Boquillas

given by ammonites is in disagreement with the Upper Cenomanian D. algeriana Subzone

designation given by this study (Fig. 14). There are likely several reasons for this discrepancy.

First, it must be pointed out that the duration of the Middle Cenomanian in comparison with the

Lower Cenomanian is relatively short, at approximately 1.2 million years. The duration of the

Middle Cenomanian means that an age discrepancy between ammonites and foraminifera is not

overly egregious, since the latest Lower Cenomanian and earliest Upper Cenomanian are

relatively close in absolute age. Also, ammonite faunas from the Western Interior and Europe

are generally not found in the Trans-Pecos region which limits correlation. A single ammonite

species found in the immediate vicinity of Big Bend National Park, A. inconstans, has been

correlated to European sections (Hancock et al., 1993). Reworking of faunal assemblages that

lived during times of non-deposition must also be considered, since the oldest ammonites are

78

found only in a limited stratigraphic interval at the very basal ±1 m of the Boquillas (Cooper,

2008). Young (1958) reports a silicified and abraded Lower Cenomanian fauna occurring along

with an Upper Cenomanian fauna in the basal Boquillas in the Jeff Davis Mountains, suggesting

that faunal reworking has occurred in this region. Finally, paleoenvironmental changes and/or

paraconformities in the rock record may artificially truncate the ranges of Trans-Pecos

ammonites. Ergo, these faunas may extend for longer intervals of time than their known

occurrences in the rock record might suggest.

Slump folding and signs of soft-sediment deformation have been reported in the park by

Maxwell (1969) and along US-90 and Lozier Canyon by Lock and Peschier (2006). It may be

suggested that contamination from sediment deposited later in time may have transported

younger foraminifera to a lower horizon. However, no clear signs of large-scale soft-sediment

deformation were observed at the Dagger Flat DS-2 locality, and laminar beds of Boquillas rest

upon the Buda Limestone. Since ammonite data suggests that the Upper Cenomanian occurs

approximately 11 meters above the contact with the Buda (Cooper, 2008), any model invoking

soft sediment deformation to explain transported foraminifera must account for a full 11 m of

section. Hypothetically, a structural process such as a detachment fault or a recent mass

wasting process such as slumping could have transported higher Boquillas Formation atop Buda

Limestone at this locality, but these explanations seem unlikely given that Frush and Eicher

(1975) also reported Heterohelix globulosa (Cushman) and Whiteinella aprica (Loeblich and

Tappan) in the lowermost Boquillas at their Hot Springs locality. These species, when

interpreted using the modern zonation, independently support placement of the basal Boquillas

Formation in the D. algeriana Subzone.

79

Foraminiferal abundances

The total number of foraminifera identified from the Boquillas Formation is too small to allow a

detailed statistical analysis. A composite count of all thin sections made from the lowermost 1.2

m of Boquillas yields 22% dicarinellids, 44% rotaliporids, 16% whiteinellids, 13%

praeglobotruncanids, and 22% all others (hedbergellids, etc.) (Table 5b). Only a single, broken

benthic foraminifer was recognized out of all thin sections analyzed from the Boquillas. While

these abundance data represent approximations, the Boquillas Formation is clearly dominated

by keeled forma. These forms include deep-dwelling rotaliporids, and intermediate-dwelling

dicarinellids and praeglobotruncanids (see Systematic Paleontology section for more detail).

Hence, data from this study supports a deep, open marine depositional setting for the

lowermost Boquillas.

80

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DF-DS2-m20 1 1 1 4 1 1 9

DF-DS2-m30 4 4

Table 5a - This chart shows the distribution of identified foraminifera from the Dagger Flat Dataset 2 section in the Boquillas Formation and topmost Buda Limestone (DF-DS2-m30). Samples were measured in centimeters plus (p) or minus (m) relative to a black shale horizon. Sample DF-DS2-m20 represents the anomalous black limestone layer. Shaded areas on the graph represent an uncertain identification. Areas marked with an 'x' indicate present, but not counted.

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Rotaliporids 44%

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Table 5b - This graph shows the relative abundances of genera from all samples taken from the Boquillas Formation at the Dagger Flat Dataset 2 location.

81

Figure 13 - This graph shows the geochronologic distribution of selected foraminifera from the Boquillas Formation. Note that the taxon range overlap indicates deposition in the D. algeriana Subzone. Please refer to the Systematic Paleontology for detailed references. Chart created with TimeScale Creator.

82

Figure 14 - This graph shows the geochronologic positions of the Buda Limestone and Boquillas Formation according to different authors. Frush and Eicher (1975) report W. aprica and H. globulosa, both with FADs in the Upper Cenomanian, but place the Boquillas in the Middle Cenomanian. Their work with washed samples likely prevented the recognition of double keels (Dicarinella). The oldest ammonites are found only in a limited stratigraphic interval at the very basal ±1 m of the Boquillas (Cooper, 2008).

CHAPTER 5 - STABLE ISOTOPE GEOCHEMISTRY

Previous work

Paleoexposure

Carbon isotope δ13C values in a carbonate are generally unaffected by many diagenetic

processes, except when a carbonate is subjected to subaerial exposure (Heydari et al., 2001).

During times of subaerial exposure, meteoric waters percolating through Corg-rich soils become

enriched in 12C. Organic respiration and decomposition can increase soil PCO2 levels at least two

orders of magnitude higher than atmospheric levels (James and Choquette, 1983). Elevated soil

PCO2 can further enhance the dissolution of the rock in the meteoric environment. Dissolution of

metastable carbonates in this meteoric environment results in the re-precipitation of calcite

with comparatively low δ13C values beneath the exposure surface (Fig. 15). Oxygen isotope

values are subject to a more diverse range of diagenetic processes than carbon isotopes

(Heydari et al., 2001), so whether they represent primary values, or values retained from

subaerial exposure, or values from some other diagenetic event is difficult to determine.

Nevertheless, where δ18O values altered by meteoric diagenesis are preserved, a positive δ18O

shit at a subaerial exposure may be present, due to preferential depletion of 16O through

evaporation (Allen and Matthews, 1982; James and Choquette, 1983). Furthermore, if sea level

84

drop is caused by the growth of continental glaciers, rainwater isotopic compositions will shift

positive during lowstand and subaerial exposure. Although unlikely under Cretaceous

greenhouse earth conditions, this process may also account for observed positive δ18O shifts at

surfaces of subaerial exposure (Allen and Matthews, 1982). Hence, recognition of

paleoexposure surfaces in a wide range of carbonates is possible using both C- and O-isotope

profile data. For this study, the minimum magnitude of negative δ13C shift required for

definitive recognition of a subaerial paleoexposure surface is approximately 0.5‰ over a 1-2

dm-scale vertical transect (Goldstein, 1991; Railsback et al., 2003). Since the zone of

supersaturation with respect to CaCO3 may lie beneath the zone of dissolution, precipitation of

δ13C-poor cements may occur in a zone displaced approximately 0-3 m lower than the

paleoexposure surface (Allen and Matthews, 1982; Railsback et al., 2003).

Identification of subaerial exposure surfaces by isotopic analysis of bulk carbonate is not

without caveats. Isotopic expression of subaerial exposure is dependent on the duration of the

exposure, levels of CO2, amount of soil and vegetation cover, and climate. Evidence for

subaerial exposure is most pronounced in warm, humid regions but can often be completely

absent, especially in arid regions (James and Choquette, 1983). Exposure surfaces that did

devolop under hot, arid conditions may display both surface karst and caliche, but such features

often form slowly and in a thinner profile than wetter climates. Likewise, paleoexposures which

were devoid of substantial soil or vegetation cover may not generate an isotopic signature.

Furthermore, isotopic signatures associated with meteoric diagenesis can occur with substantial

heterogeneity in bulk rock samples due to localized abundance of meteorically altered phases.

Local variations in solubility, porosity, permeability, soil cover, and fracture pattern can all affect

the distribution of altered cements (Theiling et al, 2007). Some subaerially exposed marine

85

limestones may not show a δ13C shift by using bulk rock analysis in a single vertical transect, due

to initially scattered distributions of meteorically altered calcite cements (Goldstein 1991,

Theiling et al., 2007), or because the altered material was removed during subsequent

transgressive ravinement (Heydari et al., 2001). Furthermore, a positive shit in δ18O data may or

may not be present at the exposure surface. If similar diagenetic processes affected the rocks

above and below the exposure surface, δ18O values may not vary significantly between them

(Goldstein, 1991).

According to Theiling et al. (2007) the maximum shift observed between single adjacent

samples may or may not accurately represent the average shift over the given vertical transect.

In order to overcome lateral variation in the geochemical signature and the "patchiness" of

diagenesis, multiple vertical transects across the sequence boundary should be evaluated.

Comparison of the mean δ13C and δ18O values from each sampled horizon above and below the

unconformity has a much greater chance of showing any isotopic signatures. Because only a

single vertical transect of rock was sampled in this study, a mean isotopic value per horizon

cannot be established. However, averaging the isotopic values from both above and below the

maximum observed δ13C shift in the top of the Buda Limestone may dampen any effects of

statistical anomaly.

Theiling et al. (2007) showed that minima in mean δ13C values may either lie beneath

the exposure surface or above the exposure surface, presumably if transgressive lag (including

micritic components) was reworked into sediment overlying the exposure surface. Theiling et

al. (2007) also noted that the range of measured δ13C and δ18O values increases markedly in

sediments both immediately above and below a surface of subaerial exposure. Conversely, as

one traces away from this zone, variation between measured values decreases (Fig. 15). They

86

propose that the identification of increased variation in isotope values can be another way to

indentify subaerial exposure surfaces if the negative δ13C excursion is not present. A detailed

statistical analysis of isotopic data for the Buda - Boquillas contact is not within the scope of this

study, however the conclusions drawn by Theiling et al. (2007) are important when considering

data presented herein.

Hardgrounds, submarine unconformities, and other surfaces

Distinguishing between surfaces of subaerial exposure and other surfaces in carbonates can be

extremely difficult. Intrastratal or subjacent corrosion between lithological units in the

subsurface can create features that superficially resemble surface karst, complicating

stratigraphic surface identification (James and Choquette, 1983). Moreover, breakdown of

organic matter in sediments immediately overlying a carbonate can increase acidity enough to

cause subaqueous karstification, without the unit ever seeing meteoric processes (Froede and

Reed, 2007). Several diagnostic attributes for hardgrounds have been recognized. Because the

surface becomes partially to fully lithified beneath the surface of the water, hardgrounds are

often bored into or encrusted upon by organisms during their formation. Other features

associated with hardgound surfaces are pyritization, Fe-oxidization, phosphatization, and rip-ups

(Nicolaides and Wallace, 1997; Railsback et al., 2003; Nalin and Massari, 2009). The

geochemical expression of hardground surfaces is variable. Since lithification at hardground

surfaces presumably occurs in equilibrium with seawater, they may contain heavy δ13C and δ18O

values (Marshall and Ashton, 1980). Conversely, if the surface has a high influx of organic

matter (Mutti and Bernouli, 2003), or if sulfate reduction occurs at the surface (Dickson et al.,

2008), a negative δ13C shift may be present. However, any surface that did not form through

subaerial processes would not be expected to display a negative carbon isotope excursion in a

87

zone displaced below the surface, because one would not expect there to be significant

downward advection of water (James and Choquette, 1983; Railsback et al., 2003).

Positive Linear Covariance Trends (PLCTs)

Positive covariance in 13C and 18O cross-plots are interpreted as mixing lines between two

phases of carbonate, the diagenetic component (micrite) and the primary component (Allen and

Matthews, 1982; Mitchell et al., 1997). This phenomenon is interpreted as being caused by

"infiltrating meteoric water and evaporative losses to the atmosphere" (Suarez et al., 2009) or

by phreatic-marine mixing zone interactions (Allen and Matthews, 1982). Positive linear

covariant trends in Permian-Triassic boundary sections have been used to infer surfaces of

subaerial exposure and subsequent meteoric diagenesis (Heydari et al., 2001). Isotope values

from confirmed subaerial exposure surfaces in the Ordovician Nashville Dome and Creaceous

Tlayua Formation of Mexico show positive covariance as well (Railsback et al., 2003; Suarez et

al., 2009). Allen and Matthews (1982) report an increase in covariance with increasing depth

beneath a surface of subaerial exposure in the Upper Mississippian Newman Limestone of

northern Kentucky. They interpret the positive linear covariance in the Newman Limestone as

resulting from marine-meteoric mixing, and interpret the thin overlying 12C-enriched subaerial

exposure zone as representing meteoric-only alteration with little covariance. Hence, while the

meteoric-only zone displays little positive linear covariance, covariance increases deeper in the

formation due to alteration in a marine-meteoric mixing zone environment. Regardless of the

mechanism, positive linear covariance is interpreted herein as representing diagenetic

alteration.

88

Oceanic Anoxia Events (OAEs)

Schlanger and Jenkyns (1976) proposed a model to explain the widely distributed black organic-

rich Late Cretaceous sediments observed from a large range of paleoenvironments. These

organic-rich deposits have important implications for the formation of oil, and may account for

about 60% of the world’s oil supply (Irving et al., 1974; Jenkyns, 1980). OAEs have been

associated with positive δ13C excursions in both bulk carbonate and organic carbon due to the

deep ocean burial of 12C-rich organic carbon and subsequent global enrichment of 13C in total

dissolved carbon (TDC) (Scholle and Arthur, 1980; Arthur et al., 1987; Coccioni and Galeotti,

2003; Bowman and Bralower, 2005; Jarvis et al., 2006). The magnitude of the excursions are

+0.5‰ to +0.8‰ in bulk carbonate during the Middle Cenomanian Event (MCE) (Coccioni and

Galeotti, 2003; Jarvis et al., 2006), and roughly +2.5‰ to +4‰ in bulk carbonate and organic

carbon, respectively, during Oceanic Anoxic Event 2 (OAE2) (Pratt and Threlkeld, 1984; Tsikos et

al., 2004; Bowman and Bralower, 2005; Jarvis et al., 2006; Parente et al., 2007).

Schlanger and Jenkyns (1976) note that the Late Cretaceous experienced marked

increases in seafloor spreading rates. Volumetric increase of mid-ocean ridges combined with

the warm, tropical Cretaceous climate to produce widespread transgression. This transgression

greatly increased the surface area of epicontinental seas, and provided much more volume to

the photic environment. Increased organic production in the oceans would have resulted from

this expansion of the photic zone, which would have increased the absolute amount of organic

matter escaping to deeper water. The warm high latitude seas during the Late Cretaceous

would not have had the oxygen-carrying capacity of today’s seas, which would have limited the

recharge of oxygen-rich water to the deep ocean. These effects combined to create widespread

expansion of the oxygen minimum layer to within 300 m of the ocean surface during times of

89

increased organic productivity. Thus, the widespread preservation of black organic-rich

sediments from a broad range of paleoceanographic environments during the Late Cretaceous is

at least partly attributable to major transgressive episodes along with a very warm climate.

Arthur et al. (1987) point out that increased freshwater influx to relatively restricted

waterways such as the Cretaceous Western Interior Seaway may have created low-salinity

surface “cap” layers which prevented oxygen interchange between the atmosphere and the

lower ocean layers. Oxidation of pelagic organic material would have gradually decreased

dissolved oxygen levels in these lower ocean layers, leading to dysoxia or anoxia. However,

Arthur et al. (1987) stress that the common factor in the deposition of organic material is marine

transgression and high productivity.

Recently, OAE2 has been linked to eruptions of ~107 km3 of Caribbean oceanic plateau

basalts, which depleted already low oceanic O2 levels through oxidation of reduced metals

(Sinton and Duncan, 1997; Kerr, 2005; Elrick et al., 2009). These eruptions may have released

large quantities of bio-limiting elements, enhancing primary productivity. Intriguing evidence

for this phenomenon has been recovered in the Guerrero-Morelos platform of southern Mexico

by Elrick et al. (2009). They showed that the isotopic onset of OAE2 in the Morelos Formation is

also associated with concentration anomalies of trace metals 5-20X those of background levels,

thought to have been derived from hydrothermal effluents. In their study, two mysterious

negative δ13C excursions, where the carbon isotope value can drop below 0‰ VPDB, were

measured the Morelos Formation beneath the onset of OAE2. If this signal is indeed regionally

correlable, it could be used to identify pre-OAE2 Upper Cenomanian sediment.

90

Mitchell et al. (2008) have tied OAE 1d, the Mid-Cenomanian Event (MCE), and OAE2 to

Milankovitch cyclicity in the Umbria-Marche Succession of Italy. They propose that OAEs were

relatively extreme events which were superimposed on a much smaller-scale background cycle

of oxic/anoxic conditions driven primarily by seasonality in the Cretaceous climate. They assert

that these OAEs, and likely others, are orbitally driven, and may represent 2.45 Myr cycles of

"extremely weak insolation variation brought about by coeval nodes in precession, obliquity,

and eccentricity.” The 2.45 Myr cycle, caused by interaction between Earth - Mars orbital

cycles, creates especially reduced seasonality which ultimately leads to stagnation of the global

oceanic oxygen cycle. They point out that this cyclicity explains why the MCE occurred

approximately 2.4 Myr before OAE2.

Timing of the Middle Cenomanian Event (MCE) and Oceanic Anoxia Event 2 (OAE2)

In the Scaglia Bianca Formation in the Umbria-marche basin, the MCE occurs in sediment

containing H. reussi, Dicarinella sp., W. praehelvetica, and W. brittonensis. The Middle

Cenomanian Event is placed within the D. algeriana subzone of the R. cushmani Zone (Coccioni

and Galeotti, 2003). It is worthy to note that the D. algeriana Subzone is herein interpreted as

limited to the Upper Cenomanian rather than Middle Cenomanian. The discrepancy is very

small, however, as the MCE may actually begin at the uppermost Middle Cenomanian, very

close (within 0.08 Myr) to the base of the Upper Cenomanian (Lugowski et al., 2009). This

discrepancy may lead to speculation that the FAD of D. algeriana slightly predates the Middle

Cenomanian - Upper Cenomanian boundary or that the MCE really occurs in the Upper

Cenomanian (Fig. 16). OAE2 spans the Cenomanian - Turonian boundary during the Whiteinella

archeaocretacea Partial Range Zone (Lugowski et al., 2009). It is most notably associated with

the extinction of the rotaliporid group of planktonic foraminifera (Arthur et al., 1987).

91

Figure 15 – Idealized C-isotope profile of a meteorically-altered paleoexposure surface. Adapted from Theiling et al. (2007).

92

Figure 16 - Chart showing the geochronologic distribution of Oceanic Anoxia Events. Created with TimeScale Creator.

93

Results - Paleoexposure

Dagger Flat Dataset 1

At Dagger Flat DS1, the largest transect of Buda Limestone was analyzed. The δ13C curve is

relatively stable over the entire vertical transect (Table 7a; Fig. 6, 18). The largest magnitude

δ13C shift between any two adjacent samples is only 0.23‰ and occurs between samples DF-

DS1-24 (4.6 m above the base of the sampled outcrop) at 1.54‰ VPDB and DF-DS1- 26 (5 m) at

1.77‰ VPDB in a 40 cm interval. The absolute range of values varies from a maximum of 1.97‰

VPDB at the very lowest sample, DF-DS1-1 (0 m), to a minimum of 1.54‰ VPDB at DF-DS1-24

(4.6 m), representing a maximum range of 0.43‰. Another notable minimum of 1.58‰ VPDB

occurs at DF-DS1-18 (3.4 m). Neither minimum occurs with enough magnitude to warrant direct

evidence for meteoric effects. This is unsurprising given the likely paleo-depth of these samples.

If there exists a disconformity in the middle of the Buda of BBNP, then isotopic evidence for

subaerial exposure is lacking. Furthermore, any hiatus within the Buda of BBNP is apparently

too short to show up biostratigraphically.

The δ18O signature at Dagger Flat varies roughly between -5‰ and -7‰ VPDB (Table 7a;

Fig. 6, 18). Like the carbon signature, the heaviest values are found at the bottom of the

sampled interval. Interestingly, the only two samples lighter than -7‰ VPDB are the same

samples which gave the lightest δ13C values, DF-DS1- 18 (3.4 m) and DF-DS1-27 (5.2 m). This

covariance will be further discussed below, in the Positive Linear Covariance section. It is worth

noting that oxygen isotope values are more susceptible late diagenesis than carbon isotopes

(Heydari et al., 2001), so this signature likely represents diagenetic values, either from subaerial

exposure or some other process.

94

Dog Canyon

At Dog Canyon, the top 2.6m of Buda was sampled and analyzed (Table 6a; Fig. 3, 17). A slight

negative shift in δ13C values is observed. The largest magnitude δ13C shift is 0.62‰ and occurs

from samples DC-DS1-8 (1.2 m above the base of the sampled outcrop) at 1.88‰ VPDB to DC-

DS1-6 (1.6 m) at 1.26‰ VDPB in a 40 cm interval. The magnitude of this shift exceeds the

minimum requirement set by this study of 0.5‰ to infer subaerial exposure, but does so over a

slightly larger vertical distance than previous authors have established (Goldstein, 1991;

Railsback et al., 2003).

The average δ13C value of all samples taken from above 1.4 m (DC-DS1-1 to DC-DS1-7) is

1.46‰ VPDB (Table 6b). The average δ13C value from taken from below 1.4 m (DC-DS1-8 to DC-

DS1-14) is 1.78‰ VPDB. The magnitude of the shift of 0.325‰ falls short of the required

minimum shift. However, the fact that the average δ13C shift is less than the maximum δ13C shift

is to be expected. Areas of the rock that lie above or below the horizon of the maximum δ13C

shift were less affected by presumed meteoric processes. Alone, the observed negative δ13C

shift in the top 2.6 m of the Buda Limestone does not conclusively confirm the existence of

paleoexposure, since the average magnitude is not large. Nonetheless, the presence of any

negative δ13C shift in the top of the Buda Limestone does lend evidence for subaerial exposure.

If the Buda Limestone were exposed to subaerial effects at its top, one would expect the

standard deviation between all samples taken from the top of the Buda (i.e. the 2.6 m section at

Dog Canyon) to be higher than the standard deviation from samples taken from the rest of the

Buda (i.e. the Dagger Flat section). The standard deviation of all δ13C and δ18O values taken

from the top of the Buda are 0.207 and 0.822, respectively (Table 6b). The entire Dagger Flat

95

section has a standard deviation in δ13C and δ18O values of 0.096 and 0.709, respectively (Table

7b). The standard deviation in the measured oxygen isotope values from the top of the Buda at

Dog Canyon is 1.16 times larger than those from Dagger Flat. The standard deviation in the

measured carbon isotope values from the top of the Buda at Dog Canyon is 1.93 times larger

than those from Dagger Flat. This result lends supporting evidence for a subaerial exposure

surface between the Buda and Boquillas Formations, since there is higher variation in measured

carbon isotope values closer to the unconformity.

The oxygen isotope signature from Dog Canyon shows two negative shifts, one at the

base of the sampled section at -6.85‰ VDPB, and one at 1.4 m at -6.83‰ VDPB, which

correlates with the negative δ13C shift. This lends further support in favor of a meteoric

diagenetic origin for the shifts, because meteoric waters are enriched in the lighter isotopes of

both carbon and oxygen. Assuming the horizon at 1.4 m containing the negative shifts was the

primary zone of re-cementation, one would expect both oxygen and carbon to follow the same

negative trend as observed. Above 1.4 m, the δ18O signature oscillates, and then shifts positive

to -4.06‰ VDPB. This positive δ18O shift may indicate evaporation and 16O depletion during

subaerial exposure. Again, the influences of subsequent diagenesis here are unknown.

The micrite infill of a large gastropod floatstone found resting on the Buda - Boquillas

contact at Dog Canyon was also analyzed. The δ13C value is 1.39‰ VPDB and the δ18O value is -

4.06‰ VPDB, which are not appreciably different from the sample taken directly beneath, in the

topmost Buda Limestone. That material above the subaerial exposure surface yields similarly

light isotopic vales as below the exposure is expected if there has been transgressive reworking

of material underlying the exposure. This result suggests that chemical components derived

from the Buda Limestone may have been incorporated into the Boquillas Formation.

96

Positive Linear Covariance Trends (PLCTs)

Isotope data from Dagger Flat DS1 shows marked positive covariance when plotted on a δ13C

and δ18O cross-plot (Table 7b; Fig. 21). Covariance is 0.053 and R2 is 0.669. Isotope data taken

from Dog Canyon yields different results. In the top 1.4 m of Buda Limestone, where meteoric

alteration is interpreted to be the greatest, there is no positive linear covariance trend.

Covariance in this interval is -0.011 and R2 is 0.014 (Table 6b; Fig. 20). Beneath 1.4 m, positive

linear covariance is observed. Covariance in the interval below 1.4 m is 0.034 and R2 is 0.154.

These results show that positive covariance in the Buda Limestone increases beneath

the zone of apparent meteoric alteration. Thus, the Buda may have experienced diagenesis in a

marine-meteoric mixing zone overlain by a thin zone of meteoric-only alteration much like the

Newman Limestone of Allen and Matthews (1982). Because data for this analysis was

composited from two separate sections, Dog Canyon and Dagger Flat DS1, local diagenesis or

lateral variation on covariance may have influenced the results. The section at Dagger Flat may

simply have stronger positive covariance due to local diagenetic effects rather than being lower

in the Buda section. However, there is still demonstrable covariance at the Dog Canyon locality

below 1.4 m. Regardless of uncertainties, positive linear covariance from Dog Canyon and

Dagger Flat shows that the data is derived from more than one phase of carbonate with a

significant diagenetic component, possibly associated with a mixing zone environment and/or

subaerial exposure.

Dagger Flat Dataset 2

The Dagger Flat DS2 section is somewhat unusual because two samples from above the Buda

give anomalously negative δ13C and δ18O values (Table 8, Fig. 19). One of these samples, DF-

97

DS2-m10 (0.1 m), was taken from the anomalous black carbonate immediately overlying the

Buda Limestone in this area, and yielded δ13C and δ18O values of -1.22‰ VPDB and -6.74‰

VPDB, respectively. The highest sample, DF-DS2-p120, was taken from "normal" Boquillas flaggy

carbonate, and yielded δ13C and δ18O values of -1.27‰ VPDB and -9.67‰ VPDB, respectively.

Between these two negative samples lies another sample, DF-DS2-p80, again taken from

"normal" Boquillas flags, which yielded more expected δ13C and δ18O values of 1.82‰ VPDB and

-5.50‰ VPDB, respectively. Since the Dagger Flat Dataset 2 locality is within a roughly a

kilometer of several small igneous intrusions, it is possible that some form of hydrothermal

alteration has occurred in this area. It is also possible that contamination from organic

components in the rocks have artificially driven down bulk-rock isotopic compositions. Finally,

the reworking of meteorically altered material which eroded before or during transgression can

lead to negative isotopic shifts in basal transgressive sediments. Each explanation for the low

δ13C values seems plausible. For example, black shale was present in this sampling interval. The

reworking of meteorically-altered hiatal chemical components is also likely, presuming the basal

Boquillas Formation was deposited during transgression. Furthermore, the very negative δ18O

values, especially in sample DF-DS2-p120, may very well indicate some form of alteration.

Overall, it appears that diagenetic overprinting or contamination from organic matter has

affected the isotopic results in at least two of the four samples at this locality, complicating

interpretation. If the signal measured here is not significantly altered, then there is a possibility

that this negative δ13C and δ18O peaks may correspond to one or both negative isotopic trends

measured in the Upper Cenomanian of the Morelos Formation of southern Mexico by Elrick et

al. (2009). However, too little data has been gathered to make formal conclusions as of the time

of this writing.

98

Interestingly, the topmost Buda Limestone sampled in this locality, DF-DS2-m30, yielded

δ13C and δ18O values of 1.72‰ VPDB and -6.54‰ VPDB, respectively. The δ13C value is 0.27‰

higher than the highest sample from the Buda taken at Dog Canyon, and the δ18O value is

2.48‰ lower. A δ13C value of 1.72‰ VPDB is more in line with samples taken beneath the

negative shift in the topmost Buda observed at Dog Canyon. Still, these values fall within the

normal range of values taken from the Buda Limestone. Since the rest of Buda Limestone is

beneath the surface at this location, no lower samples could be taken. It is unknown if the

values grow heavier with increasing depth at this location, as would be expected if the contact

underwent subaerial exposure.

Results - Oceanic Anoxia Events

Dog Canyon and Dagger Flat Data Set 1 (Buda)

Given the Lower Cenomanian age of the Buda Limestone, the unit was probably deposited

before the onset of the MCE or OAE2 (Fig. 16). The Boquillas Formation, however, was likely

deposited during one or both events. This study affirms the conclusion drawn by Frush and

Eicher (1975) that the basal Boquillas was deposited in an oxygen-poor environment, due to the

presence of black shale layers and absence of benthic foraminifera. Given the reported

presence of dicarinellids in sediments bearing evidence of the MCE (Coccioni and Galeotti,

2003), it is possible that the base of the Boquillas was deposited at or near time of the MCE.

OAE2 peaks in the W. archaeocretacea Zone, which is defined by the LAD of R. cushmani (and all

rotaliporids). The presence of rotaliporids in the sampled interval of Boquillas sediment

suggests that OAE2 may manifest higher in the section than analyzed by this study.

99

Since the Dog Canyon and Dagger Flat sections are probably highly influenced by

diagenesis, it is unlikely that a reliable primary isotope signal is preserved. However, carbon

isotope values slightly less than 2.0‰ VPDB, comparable to values from the Buda Limestone in

this study, have been reported from other formations of similar Cenomanian age (Price et al.,

1998; Coccioni and Galeotti, 2002; Bowman and Bralower, 2005; Jarvis et al., 2006). At the very

base of the Dagger Flat DS1 section, the δ13C value is 1.97‰ VPDB, the most positive value

recorded. At three higher locations in the section (DF-DS1-12, DF-DS1-18, DF-DS1-24), three

minima are observed, each of which fall below is 1.6‰ VPDB. When comparing the maximum

and minimum values measured in this section, a ~-0.4‰ shift δ13C is observed. Excluding the

effects of diagenesis discussed above, this negative isotopic trend could correlate to several

negative δ13C excursions noted in the late Lower Cenomanian of England by Jarvis et al. (2006).

However, given the strong evidence of diagenetic alteration in the Buda and the distance

separating these sections, isotope correlation based on this dataset is tenuous at best.

100

Dog Canyon Sample Height (m) δ13

C VPDB σ δ18

O VPDB σ

DC-DS1-16 2.8 1.39 0.01 -4.06 0.02

DC-DS1-1 2.6 1.45 0.04 -4.06 0.03

DC-DS1-2 2.4 1.42 0.03 -5.07 0.01

DC-DS1-3 2.2 1.49 0.01 -4.86 0.04

DC-DS1-4 2 1.43 0.01 -5.49 0.03

DC-DS1-5 1.8 1.68 0.01 -4.93 0.02

DC-DS1-6 1.6 1.26 0.01 -4.89 0.03

DC-DS1-7 1.4 1.47 0.01 -6.83 0.02

DC-DS1-8 1.2 1.88 0.01 -5.16 0.04

DC-DS1-9 1 1.76 0.01 -4.61 0.04

DC-DS1-10 0.8 1.80 0.02 -5.26 0.04

DC-DS1-11 0.6 1.80 0.02 -4.53 0.03

DC-DS1-12 0.4 1.56 0.01 -5.24 0.03

DC-DS1-13 0.2 1.95 0.02 -4.53 0.01

DC-DS1-14 0 1.71 0.01 -6.85 0.02

Table 6a - Sample number, height abiove base of outcrop, and isotope data for the Dog Canyon DS1 data set. DC-DS1-16 is gastropod floatstone found resting on Buda - Boquillas contact.

Avg. δ13C ‰ VPDB top 1.4 m Dog Canyon Std. Dev 13C

1.46 0.208

Avg. δ13C ‰ VPDB below 1.4 m Dog Canyon Std. Dev. 18O

1.78 0.822

Difference (δ13C ‰ VPDB shift) Covariance top 1.4 m

0.325 -0.011

Avg. δ18O ‰ VPDB Covariance below 1.4 m

-5.16 0.034

Table 6b - Average carbon isotope values, standard deviations, and covariance from above and below the zone of inferred meteoric alteration, and average oxygen isotpe value.

101

Dagger Flat DS1 Sample Height (m) δ13

C VPDB σ δ18

O VPDB σ

DF-DS1-31 6.0 1.74 0.01 -5.65 0.02

DF-DS1-30 5.8 1.82 0.01 -5.43 0.01

DF-DS1-29 5.6 1.73 0.01 -6.20 0.03

DF-DS1-28 5.4 1.70 0.02 -6.70 0.01

DF-DS1-27 5.2 1.77 0.01 -6.13 0.03

DF-DS1-26 5.0 1.77 0.02 -6.29 0.02

DF-DS1-24 4.6 1.54 0.02 -7.28 0.01

DF-DS1-23 4.4 1.67 0.01 -6.14 0.02

DF-DS1-22 4.2 1.71 0.02 -5.61 0.02

DF-DS1-21 4.0 1.68 0.01 -5.62 0.02

DF-DS1-19 3.6 1.70 0.01 -5.84 0.02

DF-DS1-18 3.4 1.58 0.01 -6.94 0.03

DF-DS1-16 3.0 1.65 0.02 -6.52 0.03

DF-DS1-15 2.8 1.71 0.01 -6.36 0.02

DF-DS1-14 2.6 1.67 0.01 -6.09 0.02

DF-DS1-13 2.4 1.79 0.01 -6.31 0.02

DF-DS1-12 2.2 1.58 0.01 -7.11 0.02

DF-DS1-10 1.8 1.73 0.01 -6.06 0.02

DF-DS1-8 1.4 1.82 0.01 -5.38 0.00

DF-DS1-6 1.0 1.73 0.02 -5.88 0.03

DF-DS1-4 0.6 1.71 0.02 -5.14 0.01

DF-DS1-3 0.4 1.76 0.01 -5.04 0.02

DF-DS1-2 0.2 1.91 0.03 -4.95 0.03

DF-DS1-1 0.0 1.97 0.01 -4.34 0.02

Table 7a - Sample number, height abiove base of outcrop, and isotope data for the Dagger Flat DS1 data set.

Dagger Flat DS1 Avg. δ13C ‰ VPDB Dagger Flat DS1 Std. Dev. 13C

1.73 0.108

Dagger Flat DS1 Avg. 18O Dagger Flat DS1 Std. Dev. 18O

-5.96 0.799

Covariance

0.053

Table 7b – Average isotpe values, standard deviation, and covariance for the Dagger Flat DS1 data set.

102

Dagger Flat DS2 Sample Height (m) δ13

C VPDB σ δ18

O VPDB σ

DF-DS2-p100 1.2 -1.27 0.01 -9.67 0.04

DF-DS2-p60 0.8 1.82 0.01 -5.50 0.02

DF-DS2-m10 0.1 -1.22 0.01 -6.74 0.01

DF-DS2-m20 0.0 1.72 0.01 -6.54 0.02

Table 8 - Sample number, height abiove base of outcrop, and isotope data for the Dagger Flat DS2 data set.

103

Figure 17 – Stable isotope profile for Dog Canyon. Top most point is from gastropod floatsone found resting on the Buda - Boquillas contact.

104

Figure 18 – Stable isotope profile for Dagger Flat data set 1.

105

Figure 19 – Stable isotope profile for Dagger Flat data set 2. Bottom most point is Buda Limestone, while higher points are from basal Boquillas Formation.

106

Figure 20 - δ13

C and δ18

O cross-plot showing positive linear covariance trend (PLCT) in the top 1.4 m of Buda Limestone at the Dog Canyon DS1 locality. Below 1.4 m, there is no PLCT. Also note the lighter average δ

13C values

above 1.4 m.

107

Figure 21 - δ13

C and δ18

O cross-plot showing positive linear covariance trend (PLCT) in the Buda Limestone at the Dagger Flat DS1 locality.

CHAPTER 6 - DISCUSSION AND CONCLUSION

Interpretation of the Commanchean - Gulfian of Big Bend National Park

Foraminiferal biostratigraphy and paleoenvironment

The Buda Limestone was deposited during the late Lower to early Middle Cenomanian Th.

globotruncanoides Zone. The sea that deposited the Buda Limestone was virtually devoid of

clastic influx, representing a temporary hiatus in terrigenous deposition between the Del Rio

Clay and Boquillas Formation. This sea was deep enough for the foraminiferal assemblage to be

dominated by shallow to intermediate depth planktonics, with only about 20% of the

assemblage represented by benthics. It was also deep enough to remain beneath normal wave

base, as evidenced by the lack of ripples and high-energy deposits. On the other hand, this sea

was still shallow enough to truncate the habitat of deep-dwelling planktonic foraminifera such

as rotaliporids. This open shelf environment was likely between about 50 to 100 m in depth.

The deposition of the Boquillas Formation in Big Bend National Park began with mixed

carbonate clastic sedimentation during a major transgression. Water depth was greater than

during the deposition of the Buda Limestone, and could accommodate the deep-dwelling

rotaliporid species. Bottom waters during Boquillas time were likely oxygen-deficient, which

109

severely reduced benthic habitats (Frush and Eicher, 1975). This study places the basal ±1 m of

the Boquillas in the Late Cenomanian D. algeriana Subzone of the Rotalipora cushmani Zone.

The duration of the Buda - Boquillas unconformity is therefore roughly equivalent to the

duration of the missing Th. reicheli and Th. greenhornensis Biozones, which represent a sizable

portion of the Middle Cenomanian. The foraminiferal age determination of the basal Boquillas

Formation presented herein is in disagreement with the late Early Cenomanian - Middle

Cenomanian age given by ammonites (Cobban et al., 2008; Cooper, 2008). Whether this

discrepancy results from misidentification, reworking, or errors in the biozonations of either

foraminifera or ammonites is uncertain. This study does suggest that because the basal

Boquillas is likely a transgressive deposit, the incorporation of previously deposited or hiatal

ammonite fauna in a transgressive lag is possible. The fact that ammonites restricted to the

basal Boquillas are often silicified and abraded may support this interpretation (Young, 1958).

Sequence stratigraphy and paleoexposure of the Buda Limestone

The Buda - Boquillas unconformity is interpreted as resulting from subaerial processes by: 1)

The maximum negative δ13C isotope shift in the 1.4 m beneath the unconformity; 2) The average

negative δ13C isotope shift in the 1.4 m beneath the unconformity; 3) The increase in variability

between measured δ13C values in the 1.4 m below the unconformity; 4) The positive linear

covariance trend observed in δ 13C and δ 18O cross-plots beneath the top 1.4 m of Buda; and 5)

The presence of a dm-scale microkarst unconformity surface (Lock and Peschier, 2006).

This study is in agreement with conclusions drawn by Lock et al. (2007), who interpreted

the Buda Limestone as occupying its own stratigraphic sequence. In Big Bend National Park, the

lower contact of the Buda Limestone with the Del Rio Clay is likely a subaerial exposure. This

110

interpretation is based on the presence rip-ups of Del Rio Clay incorporated into the basal Buda

(Lock et al., 2007) and an upsection decrease in the P:B ratio in the Del Rio Clay to ~0-5%

planktonics at its top, indicating significant regression (Mauldin and Cornell, 1986). Since the

foraminifera have been previously studied in the Del Rio Clay (Mauldin and Cornell, 1986) and

shown to be benthic-dominated, it was not sampled by this study. However, some inferences

can be drawn from the foraminiferal assemblage observed in the lowermost samples of Buda

Limestone. Based on the high percentage of planktonic foraminifera near the base of the Buda

Limestone, the contact of the Buda Limestone with the Del Rio Clay is presumably a P:B break

from roughly 0% planktonics in the uppermost Del Rio Clay to roughly 55-75% planktonics in the

basal Buda Limestone. This data seems to suggest that the lowermost portion of the Buda

Limestone was deposited during transgression following subaerial exposure of the Del Rio Clay.

The rest of the Buda Limestone represents a highstand to regressive package that culminates

with the subaerial unconformity at its top. It is unclear whether Buda sediment records all of

the time during which sea level rose and subsequently fell, or if the Buda is a mostly highstand

deposit with little depositional record existing of the flanking transgression and subsequent

regression. Since the whole Buda Limestone represents an entirely pelagic-dominated

assemblage below storm wave base, it is unlikely that sea level changes during deposition of

existing Buda sediment were of enough magnitude to result in significant environmental or

oceanographic changes.

Oceanic Anoxia Events (OAEs)

The Buda Limestone was deposited in an isotopically quiet period between the major Oceanic

Anoxia Events OAE1d and OAE2, and before the Middle Cenomanian Event. Furthermore,

distinguishing subtle isotopic signals from diagenetic effects is very difficult in the absence of

111

large-scale isotopic trends. However, the observed isotopic values from the Buda Limestone do

compare with other reported values from the late Lower Cenomanian (Price et al., 1998;

Coccioni and Galeotti, 2002; Bowman and Bralower, 2005; Jarvis et al., 2006). The Boquillas

Formation, on the other hand, was likely deposited during the Middle Cenomanian Event (MCE)

and/or Oceanic Anoxia Event 2 (OAE2). If such is the case, then the MCE is likely near the base

of the Boquillas Formation, while OAE2 likely manifests higher in the section than analyzed in

this study, in the W. archaeocretacea Zone (Fig. 14, 16).

Conclusions

1. The overlapping ranges of Favusella washitensis (Carsey), Globigerinelloides bentonensis

(Morrow), G. caseyi (Bolli), Loeblich and Tappan), Parathalmanninella appenninica

(Renz), Praeglobotruncana delrioensis (Plummer), P. stephani (Gandolfi), and Rotalipora

montsalvensis (Mornod) allow for placement of the Buda within the upper portion of

the Early to Middle Cenomanian Th. globotruncanoides Zone.

2. Given the age of the Buda Limestone, the unit was probably deposited in a relatively

isotopically stable time before the onset of the MCE or OAE2.

3. The Buda Limestone contains a shallow water pelagic foraminiferal assemblage roughly

consisting of 46-90% heterohelicids, 3-37% globigerinellids, and 4-22% hedbergellids

(incl. favusellids). Deeper water forms such as praeglobotruncanids and rotaliporids are

present in very low quantities of 0-3 specimens per thin section throughout the section.

112

Benthic foraminifera make up roughly 15-45% of the total foraminiferal assemblage.

This assemblage implies water depths between about 50-100 m (Baker, 1976), which

agrees with Scott and Kidson (1977) and Lock et al. (2007), who assigned an open shelf

paleodepth for the Buda Limestone in Big Bend National Park.

4. Intermediate-depth, open marine globigerinellids show an initial increase followed by

overall decrease in abundance upsection. The accompanied increase in opportunistic

shallow-depth heterohelicids upsection indicates the development of conditions more

favorable to opportunistic species than to other species (Nederbragt, 1991). These

abundance data are interpreted as showing initial sea level transgression followed by

regression, since open marine assemblages would be expected to decrease in

abundance shoreward (Sliter, 1972; Leckie, 1987). Since the Buda is entirely pelagic-

dominated, it is likely that the unit was deposited entirely within the 50-100 m depth

interval. The peak abundance of globigerinellids is interpreted as a proxy for the

maximum flooding surface (MFS).

5. The largest magnitude δ13C shift values in the top 2.6 m of Buda observed at Dog

Canyon is 0.62‰ and occurs from samples DC-DS1-8 (1.2 m above the base of the

sampled outcrop) at 1.88‰ VPDB to DC-DS1-6 (1.6 m) at 1.26‰ VDPB in a 40 cm

interval. The magnitude of this shift exceeds the minimum requirement set by this

study of 0.5‰ to infer subaerial exposure, but does so over a larger vertical distance

than previous authors have established (Goldstein, 1991; Railsback et al., 2003).

113

6. The average δ13C value of all samples taken from the top 1.4 m of Buda at Dog Canyon

(DC-DS1-1 to DC-DS1-7) is 1.46‰ VPDB. The average δ13C value from taken from below

1.4 m (DC-DS1-8 to DC-DS1-14) is 1.78‰ VPDB. The magnitude of the shift of 0.325‰

falls short of the required minimum shift. However, the presence of any negative δ13C

shift in the top of the Buda Limestone does lend evidence for subaerial exposure.

7. The standard deviation of all δ13C and δ18O values taken from the top of the Buda are

0.207 and 0.822, respectively. The entire Dagger Flat section has a standard deviation in

δ13C and δ18O values of 0.096 and 0.709, respectively. The standard deviation in the

measured oxygen isotope values from the top of the Buda at Dog Canyon is 1.16 times

larger than those from Dagger Flat. The standard deviation in the measured carbon

isotope values from the top of the Buda at Dog Canyon is 1.93 times larger than those

from Dagger Flat. This result lends supporting evidence for a subaerial exposure surface

between the Buda and Boquillas Formations, since there is higher variation in measured

carbon isotope values at the top of the Buda Limestone.

8. The oxygen isotope signature from Dog Canyon shows two negative shifts, one at the

base of the sampled section at -6.85‰ VDPB, and one at 1.4 m at -6.83‰ VDPB, which

correlates with the negative δ13C shift. This lends further support in favor of a meteoric

diagenetic origin for the shifts, because meteoric waters are enriched in the lighter

isotopes of both carbon and oxygen. Assuming the horizon at 1.4 m containing the

negative shifts was the primary zone of re-cementation, one would expect both oxygen

and carbon to follow the same negative trend as observed. Above 1.4 m, the δ18O

signature oscillates, and then shifts positive to -4.06‰ VDPB. Positive δ18O shifts (often

114

overlying negative shifts) have been reported from the surfaces of subaerial exposure

that experience intense evaporation, which preferentially removes 16O in a thin zone

near the surface (Allen and Matthews, 1982; James and Choquette, 1983).

9. The top 1.4 m of Buda Limestone, where meteoric alteration is interpreted to be the

greatest, contains no positive linear covariance trend. Covariance in this interval is -

0.011 and R2 is 0.014. Beneath 1.4 m, positive linear covariance is observed. Covariance

in the interval below 1.4 m is 0.034 and R2 is 0.154. A δ13C and δ18O cross-plot of Dagger

Flat DS1 data from lower in the Buda shows a distinct positive linear covariance trend,

with covariance of 0.053 and R2 value of 0.669. These results show that positive

covariance in the Buda Limestone increases beneath the zone of apparent meteoric

alteration. Thus, the Buda may have experienced diagenesis in a marine-meteoric

mixing zone overlain by a thin zone of meteoric-only alteration much like the Newman

Limestone of Allen and Matthews (1982). Because data for this analysis was composited

from two separate sections, local diagenesis or lateral variation on covariance may have

influenced the results. The section at Dagger Flat may have stronger positive covariance

due to local diagenetic effects rather than being lower in the Buda section. However,

positive linear covariance from Dog Canyon and Dagger Flat does show that the data is

derived from more than one phase of carbonate with a significant diagenetic

component, possibly associated with a mixing zone environment and/or subaerial

exposure.

10. In Big Bend National Park, the lower contact of the Buda Limestone with the Del Rio Clay

is likely a subaerial exposure (Lock et al., 2007). Based on the high percentage of

115

planktonic foraminifera near the base of the Buda Limestone, the contact of the Buda

Limestone with the Del Rio Clay is presumably a marked P:B break from near 0%

planktonics in the uppermost Del Rio Clay (Mauldin and Cornell, 1986) to approximately

80% planktonics in the basal Buda Limestone . The lowermost portion of the Buda

Limestone was likely deposited during transgression following subaerial exposure of the

Del Rio Clay. The rest of the Buda Limestone probably represents a highstand to

regressive package that culminates with a subaerial microkarstic unconformity at its top.

Based on this interpretation, the Buda Limestone it bounded by subaerial exposure

surfaces and represents its own sequence.

11. This study places the basal Boquillas Formation in the Upper Cenomanian D. algeriana

Subzone of the R. cushmani Zone based on the overlapping ranges of Dicarinella sp.,

Dicarinella algeriana (Caron), Rotalipora sp., Rotalipora cushmani (Morrow),

Thalmanninella sp. cf. Th. greenhornensis (Morrow ), and Whiteinella baltica Douglas

and Rankin. The duration of the unconformity is therefore roughly equivalent to the

duration of the missing Th. reicheli and Th. greenhornensis Biozones, which represent a

sizable portion of the Middle Cenomanian.

12. The foraminiferal age determination of the basal Boquillas Formation presented herein

is in slight disagreement with the late Early Cenomanian - Middle Cenomanian age given

by ammonites (Cobban et al., 2008; Cooper, 2008). Whether this discrepancy results

from misidentification, reworking of fauna, or errors in the biozonations of either

foraminifera or ammonites is uncertain.

116

13. The δ13C and δ18O values of the micrite infill of a large gastropod floatstone found

resting on the Buda - Boquillas contact at Dog Canyon is 1.39‰ VPDB and -4.06‰ VPDB,

respectively. These values are not significantly different from the sample taken from

topmost Buda Limestone. This result suggests that chemical components derived from

the Buda Limestone may have been incorporated into the Boquillas Formation (Theiling

et al., 2007). The incorporation of previously deposited or hiatal macrofossil fauna in a

transgressive lag is possible. The fact that ammonites restricted to the basal Boquillas

are often silicified and abraded supports this interpretation (Young, 1958).

14. The original zonation of the Boquillas Formation given by Frush and Eicher (1975) is

herein considered obsolete. It is now known that the first appearance of Heterohelix

globulosa (Cushman) occurs in the Upper Cenomanian D. algeriana Subzone of the R.

cushmani Zone (Nederbragt, 1991). The first appearance of Whiteinella aprica (Loeblich

and Tappan) also occurs in the D. algeriana Subzone (Premoli Silva and Sliter, 2002).

Therefore it appears that the lower 0-60 feet of the Boquillas is constrained to the D.

algeriana Subzone given both the data of Frush and Eicher (1975) and of this study.

15. A composite count of all thin sections made from the lowermost 1.2 m of Boquillas

yields 22% dicarinellids, 44% rotaliporids, 16% whiteinellids, 13% praeglobotruncanids,

and 22% all others (hedbergellids, etc.). Just one broken benthic foraminifer was

recognized out of all thin sections. The Boquillas Formation is dominated by deep-

dwelling rotaliporids and intermediate-dwelling dicarinellids and praeglobotruncanids.

117

The Boquillas Formation was almost certainly deposited in deeper water than the Buda

Limestone. It likely represents deep, open marine, oxygen-poor transgressive

depositional conditions following subaerial exposure of the Buda Limestone.

16. Given the reported presence of dicarinellids in sediments bearing evidence of the MCE

(Coccioni and Galeotti, 2003), it is possible that the base of the Boquillas was deposited

at or near the time of the MCE. Furthermore, OAE2 probably occurs higher in the

section than analyzed by this study, in the overlying W. archaeocretacea Zone.

17. Two samples from the Boquillas Formation at the Dagger Flat DS2 section give

anomalously negative δ13C and δ18O values, including the anomalous black limestone

layer. Since the Dagger Flat Dataset 2 locality is within a kilometer of several small

igneous intrusions, it is possible that some form of hydrothermal alteration has occurred

in this area. It is also possible that contamination from organic components in the rocks

have artificially driven down bulk-rock isotopic compositions. If the signal measured

here is not significantly altered, then there is a possibility that this negative δ13C and

δ18O peaks may correspond to one or both negative isotopic trends measured in the

Upper Cenomanian of the Morelos Formation of southern Mexico by Elrick et al. (2009).

The reworking of meteorically altered material which eroded before or during

transgression can also lead to negative isotopic shifts in basal transgressive sediments

(Theiling et al., 2007).

vii

PLATES - BUDA LIMESTONE

Favusella

Pl. 1 - F. washitensis (Carsey)

Pl. 2 - F. washitensis (Carsey)

viii

Pl. 3 - F. washitensis (Carsey)

Globigerinelloides

Pl. 4 - G. bentonensis (Morrow)

Pl. 5 - G. bentonensis (Morrow)

ix

Pl. 6 - G. caseyi (Bolli, Loeblich and Tappan)

Pl. 7 - G. ultramicrus (Subbotina)

Pl. 8 - G. caseyi (Bolli, Loeblich and Tappan)

x

Hedbergella

Pl. 9 - Clavihedbergella simplex (Morrow)

Pl. 10 - Hedbergella planispira (Tappan)

Pl. 11 - Hedbergella sp. cf. H. delrioensis (Carsey)

xi

Heterohelix

Pl. 12 - H. moremani (Cushman)

Pl. 13 - H. moremani (Cushman)

xii

Parathalmanninella

Pl. 14 - P. appenninica (Renz)

Pl. 15 - P. appenninica (Renz)

Pl. 16 - P. micheli (Sacal and Debourle)

xiii

Praeglobotruncana

Pl. 17 - P. delrioensis (Plummer)

Pl. 18 - Praeglobotruncana sp. cf. P. stephani (Gandolfi)

Pl. 19 - Praeglobotruncana sp.

xiv

Rotalipora

Pl. 20 - R. montsalvensis (Mornod)

Pl. 21 - R. montsalvensis (Mornod)

Pl. 22 - ?R. montsalvensis (Mornod)

xv

Thalmanninella

Pl. 23 - ?Thalmanninella globotruncanoides (Sigal)

Miscellaneous

Pl. 24 - Large benthic foraminifer, planktonic foraminifera, calispheres, echinoderm spine (lower right)

xvi

Pl. 25 - Benthic foraminifer

Pl. 26 - Uncertain classification (rare, single specimen recovered)

xvii

Pl. 27 - Benthic foraminifer

Pl. 28 - Benthic foraminifer

Pl. 29 - Benthic foraminifer

xviii

PLATES - BOQUILLAS FORMATION

Dicarinella

Pl. 30 - Dicarinella sp.

Pl. 31 - D. algeriana (Caron)

xix

Pl. 32 - Dicarinella sp.

Pl. 33 - Dicarinella sp. cf. D. algeriana (Caron)

xx

Pl. 34 - Dicarinella sp.

Hedbergella

Pl. 35 - Hedbergella sp.

xxi

Heterohelix

Pl. 36 - H. globulosa (Cushman)

Pl. 37 - H. globulosa (Cushman)

xxii

Praeglobotruncana

Pl. 38 - P. stephani (Gandolfi)

Pl. 39 - P. gibba Klaus

xxiii

Rotalipora

Pl. 40 - ?R. cushmani (Morrow) (top), ?Anaticinella sp. (bottom)

Pl. 41 - Rotalipords (single keel)

xxiv

Pl. 42 - Rotalipord (single keel)

Pl. 43 - ?R. cushmani (Morrow)

xxv

Shackoina

Pl. 44 - Shackoina cenomana (Shacko)

Thalmanninella

Pl. 45 - Broken Thalmanninella sp.

xxvi

Whiteinella

Pl. 46 - W. baltica Douglas and Rankin

Pl. 47 - ?W. aprica (Loeblich and Tappan)

xxvii

Pl. 48 - Whiteinella sp.

Miscellaneous

Pl. 49 - Photomicrograph of DF-DS2-m20 (anomalous black limestone layer) showing calcispheres and ?rotaliporid foraminfer.

xxviii

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