Continental Sequence Stratigraphy of the Upper Triassic Chinle Strata, Northern New Mexico, USA - JSR, 2007

Journal of Sedimentary Research, 2007, v. 77, 909–924

Research Article

DOI: 10.2110/jsr.2007.082

CONTINENTAL SEQUENCE STRATIGRAPHY OF THE UPPER TRIASSIC (NORIAN–RHAETIAN) CHINLESTRATA, NORTHERN NEW MEXICO, U.S.A.: ALLOCYCLIC AND AUTOCYCLIC ORIGINS OF

PALEOSOL-BEARING ALLUVIAL SUCCESSIONS

DAVID M. CLEVELAND, STACY C. ATCHLEY, AND LEE C. NORDTDepartment of Geology, Baylor University, 1 Bear Place, Waco, Texas 78798-7354, U.S.A.

e-mail: [email protected]

ABSTRACT: Two age-equivalent Upper Triassic fluvial successions deposited on the continental interior of the southwesternUnited States were evaluated using an adapted marine stacking-pattern analysis methodology. A three-tier cyclic hierarchy ispresent in the strata at both study areas. Meter-scale fining-upward fluvial aggradation cycles (FACs) comprise fluvialaggradational cycle sets (FACSETs) 4–15 m thick (avg. 8.4 m). FACSETs in turn stack into four fluvial sequences 26–48 mthick (avg. 41 m). Within these sequences, transgressive-systems-tract equivalents (TE) are characterized by channel sands andassociated minor overbank deposits and relatively immature paleosols (i.e., high rates of deposition), whereas highstand- tofalling-stage-systems-tract equivalents (HFE) are dominated by overbank muds and relatively well-developed paleosols (i.e.,lower rates of deposition). These two fluvial successions, which are 200 km apart, contain age-equivalent fluvial sequences thatrecord similar histories of deposition and pedogenesis: Sequence 1 contains only an incomplete HFE; Sequence 2 includes boththe TE and HFE; Sequence 3 is an HFE; and Sequence 4 contains only a TE. Fluvial sequences likely accumulated in responseto pulses of source area uplift and/or basin subsidence, which resulted in changes in accommodation. Conversely, higher-frequency FACs and FACSETs that occur within sequences do not correlate between study areas and are likely the products ofautocyclic processes, such as channel avulsion, floodplain aggradation, and channel migration. These results suggest thatregionally significant tectonic episodes may be discernible in suspended-load fluvial deposits that accumulated over a broadarea.

INTRODUCTION

It has been suggested that paleosol-bearing alluvial successions containa hierarchical record of cyclic sediment accumulation produced inresponse to the combined effects of autogenic and allogenic processes(e.g., Beerbower 1964; Bridge and Leeder 1979; Bridge 1984; Kraus 1987,1999; Kraus and Aslan 1999; Shanley and McCabe 1994; McCarthy andPlint 1998; Kraus 2002; Atchley et al. 2004). Alluvial cycles tens tohundreds of meters thick have typically been regarded as the product ofallogenic processes such as tectonic activity, eustatic sea-level changes,and climate changes (Allen 1978; Read and Dean 1982; Blakey andGubitosa 1984; Posamentier and Allen 1993; Wright and Marriott 1993;Kraus 2002; Atchley et al. 2004). In contrast, smaller-scale cycles arethought to be a product of autogenic processes such as channel avulsionand migration (Kraus and Aslan 1999; Atchley et al. 2004).

Previous stratigraphic work on the Upper Triassic Chinle Group of thesouthwestern United States has focused on lithostratigraphic, biostrati-graphic, and magnetostratigraphic regional correlations (Stewart et al.1972; Blakey and Gubitosa 1984; Blakey 1989; Lucas 1993; Lucas 1997;Lucas et al. 1997; Tanner 2003a). Although this work has been essentialto understanding the paleogeographic evolution of the southwestern U.S.(Blakey and Gubitosa 1983; Blakey 1989), a detailed sequence-stratigraphic interpretation is notably lacking. Lucas (1997) and Lucaset al. (1997) suggest that the Chinle strata in New Mexico contains threethird-order fluvial depositional cycles (Carnian-, Norian-, and Rhaetian-age strata) separated by regional-scale unconformities (Fig. 1). The lower

two unconformities bounding these large-scale cycles (e.g., Fig. 1, 228.0and 216.5 Ma) have been correlated with marine sequences in Nevada(Lucas et al. 1997). Higher-frequency cyclicity in the Chinle Group hasnot been evaluated previously.

Atchley et al. (2004) demonstrated the usefulness of marine ‘‘stacking-pattern’’ techniques in the identification of cyclic fluvial successions ofCretaceous–Paleocene age that were influenced by eustatic sea-levelchanges. Atchley et al. (2004) also documented a three-tier, cyclic, stratalhierarchy composed of meter-scale fining-upward fluvial aggradationalcycles (FACs), decameter-scale fluvial aggradational cycle sets (FAC-SETs), and hectometer-scale fluvial sequences. Prochnow et al. (2006a)and Boucher (2004) applied this methodology within the Chinle strata insoutheastern Utah and observed a similar three-tier cyclic hierarchy;however, they attribute their fluvial sequences to long-term variations inaccommodation related to halokinesis (i.e., salt tectonics).

The purpose of this study is to examine two age-equivalent successionsof Upper Triassic strata in New Mexico in order to evaluate the high-frequency fluvial cyclicity of the Chinle Group where deposition was notinfluenced by eustatic changes or salt tectonism. Succession 1 is composedof the Bull Canyon and Redonda formations and succession 2 iscomposed of the Painted Desert Member of the Petrified ForestFormation and the Rock Point Formation (Fig. 1). These two age-equivalent stratal successions, approximately 200 km apart, are com-pared to discern between locally and regionally significant trends ofsedimentation and pedogenesis (Fig. 2).

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-909/$03.00

STRATIGRAPHIC OVERVIEW AND PALEOGEOGRAPHY

The Chinle Group was deposited in fluvial and lacustrine settings duringthe Late Triassic (Carnian to Rhaetian) in the southwestern United States(Stewart et al. 1972; Blakey and Gubitosa 1983, 1984; Dubiel 1987, 1989;Dubiel et al. 1991; Lucas 1993, 1997; Lucas et al. 1997; Therrien andFastovsky 2000). Paleogeographic reconstructions of the Late TriassicWestern Interior suggest that the Ancestral Rocky Mountains in Colorado,the Mogollon Highlands to the south, and a more distant volcanic-arccomplex (Cordilleran Arc) to the west served as the sediment source areasfor these northwest-trending fluvial systems (Fig. 2). Kraus and Middleton(1987) and Tanner (2003a) suggest that the dominant control on Chinledeposition is tectonism. Climate may also have influenced Chinledeposition, but the relationship between climatic changes and thechronology of Chinle depositional style is unknown. Climatic changesthat occurred during Chinle deposition show a general shift froma subhumid, seasonal climate in the Carnian to a more arid climate by

the late Norian to Rhaetian (Stewart et al. 1972; Blakey and Gubitosa1983, 1984; Dubiel 1987, 1989; Blodgett 1988; Dubiel et al. 1991; Therrienand Fastovsky 2000; Tanner 2003b; Prochnow et al. 2006b).

The two study locations are approximately 200 km apart and over750 km inland from the paleocoastline (Fig. 2). Location 1 (35u 009 480 N,104u 059 170 W), is in Captivas Canyon on Louisiana Mesa directly southof the town of Montoya, New Mexico (Fig. 2). The exposure includesalternating sandstones and mudstones of the Bull Canyon and Redondaformations that contain abundant paleosols, trace fossils, and minor bodyfossils. These formations were historically considered part of the Dockumdepositional area, which included the eastern part of the Late Triassicfluvial system extending from Texas into New Mexico (Stewart et al.1972; Blakey and Gubitosa 1983, 1984; Dubiel 1987, 1994). Newell (1993)correlated these formations with the Chinle Formation and suggesteda common depositional control on the basis of similarities in lithology,alluvial architecture, and paleocurrent indicators. Lucas (1993) proposedthat the Chinle Formation be elevated to the group level and suggested

FIG. 1.—Stratigraphic correlation chart of theChinle strata in New Mexico (modified fromLucas 1993 and Lucas et al. 1997). Dominantfluvial style is based on the previous stratigraphicstudies of Stewart et al. (1972), Blakey andGubitosa (1984), Blakey (1989), Lucas (1993),Dubiel (1994), Lucas (1997), Lucas et al. (1997),and Tanner (2003a). Third-order cycles are afterLucas (1997) and Lucas et al. (1997), sea-levelreconstruction is after Haq et al. (1988), andabsolute ages from Gradstein et al. (2004).

FIG. 2.— Paleogeographic reconstruction ofthe Late Triassic within the southwestern U.S.showing the two study locations (modified fromBlakey and Gubitosa 1983, 1984; Blakey 1989;Dubiel 1994). Paleolatitude approximation isfrom Van der Voo et al. (1976), Habicht (1979),and Zeigler et al. (1983).

910 D.M. CLEVELAND ET AL. J S R

that the Chinle Group encompass all of the nonmarine strata of the LateTriassic Western Interior, including the Redonda and Bull Canyonformations of this study. This study follows the nomenclature used byLucas (1993), although many previous studies refer to the Chinle Groupas a formation, and the associated formations as members.

Location 2 (36u 159 420 N, 102u 249 560 W) is in the Chama Basin ofnorth-central New Mexico on USDA Forest Service land north ofAbiquiu (Fig. 2). The upper portion of the Painted Desert Member of thePetrified Forest Formation and the Rock Point Formation are exposed atthis location (Fig. 1; Lucas 1993, 1997). Age-equivalence of theseformations with those at location 1 has been established by regionallithostratigraphy, sequence stratigraphy, and biostratigraphy (Stewartet al. 1972; Blakey and Gubitosa 1983, 1984; Dubiel 1987, 1989; Dubielet al. 1991; Lucas 1993, 1997; Lucas et al. 1997; Therrien and Fastovsky2000). The three third-order fluvial depositional cycles of Lucas (1997)and Lucas et al. (1997) are Carnian, Norian, and Rhaetian strataseparated by regional unconformities (Fig. 1). The bases of the lower twocycles are dominated by braided-river deposits, whereas the overlyingintervals are dominated by meandering-river deposits.

Lucas (1997) reviewed the biostratigraphic work on the Chinle Groupand correlated the strata based upon palynomorphs, plant megafossils,ostracodes, bivalves and gastropods, vertebrate coprolites, fish, andtetrapods. Fish and tetrapods have been most useful at separatingCarnian, Early Norian, Late Norian, and Rhaetian strata. Though nobody fossils were identified in this study, fish and tetrapod fossils used forbiostratigraphic correlation are found proximal to the two study areas(Zeigler et al. 2002; Johnson et al. 2002). The study intervals in this paperare equivalent to the late Norian through Rhaetian strata (Fig. 1).

METHODS

Sequence-Stratigraphic Methods

The ‘‘stacking-pattern’’ methodology associated with the sequence-stratigraphic interpretations in this paper follow Atchley et al. (2004).Composite measured sections were constructed over a very small lateraldistance (, 150 meters) in order to show changes through time fora single location. Data collected from outcrops includes stratal thickness,grain size, physical and biogenic sedimentary structures, lithostrati-graphic contacts, and the occurrence of paleosols. Strata at both locationsare nearly horizontal (, 5u dip) and were dip-corrected to true thickness.Fluvial facies (Table 1) are defined following the classification scheme ofMiall (1978), from which channel and overbank facies associations areassigned. Fining-upward meter-scale fluvial aggradation cycles (FAC),consisting of a fining-upward succession of stratal bodies, were identifiedas sections were measured. FAC boundaries are typically overlain bycoarser-grained sediment and/or have a paleosol top.

To assist in the identification of cyclic depositional trends, FAC stackingpatterns from each locality were analyzed via plots of cumulative deviationfrom mean FAC thickness and mean grain size (Sadler et al. 1993;Drummond and Wilkinson 1993; Lehrmann and Goldhammer 1999;Atchley et al. 2004). To determine the cumulative deviation of grain size,the following relative values were assigned to the standard Udden–Wentworth grain-size classes: 1 5 mud, 2 5 silt, 3 5 fine sand, 4 5 mediumsand, 5 5 coarse sand, 6 5 very coarse sand, 7 5 gravel. Mean grain sizefor each FAC was determined from graphical measured sections. Cyclethickness plots were constructed using dip-corrected thicknesses foreach FAC. Histograms documenting the lithologic occurrences and

TABLE 1.—Depositional facies present in strata of the study areas, criteria for recognition, and environmental interpretations (based upon the classificationscheme of Miall 1978).

Facies Lithology Features Interpretation Association

Gm Conglomerate unstratified; poorly sorted gravel to fine sand;matrix supported

channel-filling debris Channel or Crevasse Splay

Gt Conglomerate trough cross beds; poorly sorted gravel tomedium sand; clast supported

minor channel fills Channel or Crevasse Splay

Sl Sandstone low-angle (, 10u) crossbeds; medium tovery coarse sand; may be pebbly

scour fill; washed out dunes Channel or Crevasse Splay

Ss Sandstone broad, shallow scours within fine to coarsesand; occasional carbonaceous materialand pedogenic nodules

upper-flow-regime scour fill Channel or Crevasse Splay

Sm Sandstone unstratified; very fine to medium sand; wellsorted; massive to bioturbated

channel-filling sands and/or primarybedform destruction by secondarybiological activity

Channel or Crevasse Splay

Sh Sandstone horizontal lamination; fine to coarse sand;moderately sorted; parting or streaminglineation

planar upper-flow-regime Channel or Crevasse Splay

St Sandstone trough cross beds; fine to coarse sand;moderately sorted

lower-flow-regime 3-D dunes Channel or Crevasse Splay

Sr Sandstone ripple lamination; fine to medium sand;well sorted

lower-flow-regime current ripples Channel or Crevasse Splay

Fl Mudstone laminated mudstone overbank or abandoned channeldeposits; little exposure time

Overbank

Fr Mudstone massive to bioturbated mudrock, or paleosolswith a maturity , 1; roots and insectburrows common

overbank or abandoned channeldeposits; significant exposure time

Overbank

Facies AssociationChannel or Crevasse Splay Fine sand or coarser grain size with either preserved primary sedimentary structures or signs of bioturbation. Facies include Gm, Gt,

Sl, Ss, Sm, Sh, Sr, and St. These rocks occur at the base of fining-upward cycles (lateral accretion or vertical bar aggradation), oras isolated sand bodies within mudstone (channel or splay deposits).

Overbank Fine-grained rocks that are either bioturbated (most common) or laminated (rare), or sandy paleosols with a maturity rating of 1 ormore. Facies include Fr and Fl. These rocks occur at the top of fining-upward cycles (lateral accretion or vertical aggradation) orin a succession of mudrocks and paleosols (floodplain aggradation).

CONTINENTAL SEQUENCE STRATIGRAPHY OF UPPER TRIASSIC CHINLE STRATA, NM 911J S R

decompacted thicknesses within each FAC were also constructed toquantitatively assess vertical trends in lithofacies proportions.

Because this stacking-pattern methodology uses thickness trends,burial compaction has been accounted for on the basis of the regressionalgorithm of Sheldon and Retallack (2001) for alluvial deposits andpaleosols. Stratigraphic approximations of overburden strata for theMontoya section (location 1) range from 850 to 1024 meters (COSUNA,1983), and approximations of overburden for location 2 range from 1550to 2150 meters (Muehlberger et al. 1960). Therefore, the measuredthicknesses of facies are between 4% and 14% of the depositionalthickness, depending on facies.

Paleosol Maturity

In order to support the stacking-pattern methodology and trends insedimentation, trends in paleosol maturity were also evaluated. Paleosolmaturity is used to reflect the amount of sediment exposure time andamount of weathering. Assessment of relative paleosol development isbased upon a modification of the Retallack (1988) categorization ofpaleosol maturity. The criteria by which paleosols are categorized in thisstudy are horizon development and ped structure, thickness of thepaleosol, and pedogenic carbonate content (Tables 2, 3).

Horizon designations (Table 2) are based upon field-observablefeatures in the paleosols after Soil Taxonomy (Soil Survey Staff 1999).AB horizons are those that are likely A horizons but may be Bw horizons.This uncertainty is due to the unknown amount of removal of the upperpart of the paleosol and ability to preserve an A horizon in the rock

record. All other paleosol horizons (i.e., Bw, Bk, Bkm, Bssk, Bssg, andBC) are used consistently with definitions in Soil Taxonomy (Soil SurveyStaff 1999). The most mature paleosols are stage 4 and have an AB—oneor more Bw or Bk–Bkm–BC horizon successions. Intervals that showevidence of rooting but have no evidence of ped development,horizonation, or pedogenic carbonate are assigned a paleosol maturityof 0. Although the presence of weakly to moderately developedslickensides does not change the maturity rating according to thisscheme, paleosols with slickensides have been identified by adding a ‘‘b’’to the maturity rating (Tables 3, 4, Fig. 3).

RESULTS

Depositional Facies

Facies (Table 1; Fig. 4) are characterized and differentiated on thebasis of grain size and mechanical and biogenic sedimentary structuresmodified from Miall (1978, 1992). Strata are dominated by interbeddedmudstone and sandstone, with the two most abundant facies being Fr(, 60% of the strata; Fig. 4A) and Sm (, 15% of the strata; Fig. 4B, C).Thus, most of the strata (. 70%) do not contain preserved primarysedimentary structures. Facies with preserved bedding (Gt, Sl, Ss, Sh, St,Sr, and Fl) are not usually associated with Sm or Fr facies and are morecommon at location 1 (Fig. 5).

Facies were grouped into two facies associations, which are overbankdeposits and channel and crevasse-splay deposits. The overbankassociation is composed of the Fr and Fl facies and accounts for

TABLE 2.—Paleosol horizons and recognition criteria.

Horizon Designation Features Observed

AB When preserved, has darker colors than other horizons. Ped structure is fine to medium, subangular blocky to blocky. Root traces arerare to common. Must be the uppermost horizon in a profile.

Bw Has brighter colors than an overlying or underlying horizon. Ped structure is medium to coarse, subangular blocky to blocky, orprismatic. Root traces are rare to common.

Bk Contains 1 to 15% pedogenic carbonate nodules that are typically pitted, white to grayish brown to reddish brown, and 0.5 to 4 cm indiameter. Ped structure is medium to coarse subangular blocky to blocky. Root traces are rare to few.

Bkm Subsurface horizon that is . 10 cm thick and is cemented or indurated by pedogenic carbonate. Ped structure is medium to coarsesubangular blocky to blocky. Root traces are rare to few.

Bss Fine to coarse, wedge-shaped aggregates with few to many slickensides. Rare to few root traces.Bssk Contains 1 to 15% pedogenic carbonate nodules that are typically pitted, white to grayish brown to reddish brown, and 0.5 to 4 cm in

diameter. Fine to coarse, wedge-shaped aggregates with few to many slickensides. Root traces are rare to few.Bssg Dominated by gray colors and Fe remains in a reduced state. Interpreted to be a result of saturated conditions lacking free oxygen for

an unknown period of time. Fine to coarse, wedge-shaped aggregates with few to many slickensides. Root traces are rare to few.BC Transition of B and C horizons with weak or no ped development. Occasionally contains preserved bedding overprinted by root traces.C Parent material with no signs of pedogenic alteration.

TABLE 3.— Relative paleosol maturity classification.

Relative Maturity Index Horizon Sequence Other Comments

0 N/A evidence of rooting and/or initial pedogenesis; relatively low color values and chromas1 AB-BC development of AB horizon; relatively low color values and chromas1.5 AB-Bw-BC development of a subsurface horizon2 AB-Bw1-Bw2-BC thicker solum than less mature paleosols2b AB-Bss1-Bss2-BC thicker solum than less mature paleosols; weak slickensides observed2.5 AB-Bw1-Bw2-BC thicker solum than less mature paleosols; rare (, 1%) carbonate nodules present2.5b AB-Bss1-Bss2-BC thicker solum than less mature paleosols; rare (, 1%) carbonate nodules; weak to moderate

slickensides present3 AB-Bw-Bk-BC thicker solum than less mature paleosols; common carbonate nodules.3b AB-Bss-Bssk-BC thicker solum than less mature paleosols; common carbonate nodules; weak to distinct slickensides

present3.5 AB-Bk1-Bk2-BC thicker solum than less mature paleosols; abundant carbonate nodules4 AB-Bk1-Bk2-Bkm-BC thicker solum than less mature paleosols; presence of a horizon . 10 cm thick that is cemented or

indurated by pedogenic carbonate


, 75% of the strata exposed at the two locations (Table 1). The channeland crevasse-splay association includes Gm, Gt, Sl, Ss, Sh, Sm, St, and Sr.The channel/crevasse-splay association was not subdivided because nearlyhalf of the channel/crevasse-splay association is composed of massive orbioturbated sandstone (Sm) and does not have preserved depositionalbedding. Additionally, the majority of facies that do contain preservedbedding may be deposited in a number of possible subdivisions of thisassociation, such as downstream-accretion and lateral accretion macro-forms, gravel bars, or splay deposits (Miall 1992). The predominance ofoverbank fines and paleosols with a lesser volume of channel andcrevasse-splay deposits is consistent with previous interpretations ofa suspended-load fluvial system (Newell 1993; Lucas 1997; Lucas et al.1997; Miall, 1992). There is no dramatic change in depositional style inthe two sections, although there is variation in the abundance of channeland crevasse-splay deposits in a given stratigraphic interval.

Paleosols and Maturity Trends

In contrast to other studies of Late Triassic paleosols from thesouthwestern United States (Retallack 1997; Therrien and Fastovsky2000; Prochnow et al. 2006a), no strongly leached horizons (i.e., an Ehorizon of Soil Taxonomy) nor horizons showing elevated levels of

pedogenic clay relative to the adjacent horizons (i.e., a Bt horizon of SoilTaxonomy) were identified in the strata at either location. The Bw and Bkhorizons are the most common horizons in the paleosols (Fig. 3).

Excluding individual peds, the most abundant pedogenic features in thepaleosols of this study are root traces and pedogenic carbonate nodules.The root traces range in size from 0.2 to 2 cm wide, some of which can betraced up to 1 meter vertically and display downward bifurcation. Therooting size and type is uniform throughout the strata and shows nodiscernible changes with variations in paleosol maturity or betweenlocations. The abundance of pedogenic carbonate in the paleosols istypically highest between 40 and 70 cm below the top of the paleosol(after correction for burial compaction) in profiles that do not haveevidence of extensive erosion. These soils can typically be traced laterallyfor a hundred meters or more and appear to have been developed ona relatively flat surface rather than on a slope.

Small burrows (0.2–1.0 cm wide) with meniscate backfilling arepresent in all of the strata, excluding the intervals at location 1 thatlack paleosols (Fig. 5; FACs 16–31, 78–89). The burrows are more easilyidentified in poorly developed paleosols (maturity of 1 or less), becausethere is less pedogenic overprinting. The burrow morphology betweenlocations (spatially) and vertically in the sections (temporally) does notchange.

TABLE 4.— List of paleosols and their respective maturity assignments. A ‘‘b’’ indicates a paleosol with slickensides (compare with Fig. 3).

Location 1 Location 1 Location 2 Location 2

Paleosol FAC Maturity Paleosol FAC Maturity Paleosol FAC Maturity Paleosol FAC Maturity

BC1 1 1.5 RED1 44 2 PF1 1 1.5 RP1 45 1BC2 2 2 RED2 45 2.5 PF2 2 1.5 RP2 46 2BC3 3 3 RED3 46 2.5 PF3 3 2b RP3 47 2.5BC4 4 2 RED4 47 2.5 PF4 4 2b RP4 48 1.5BC5 5 2 RED5 48 1.5 PF5 5 3b RP5 49 3BC6 6 2 RED6 49 1.5 PF6 7 2.5b RP6 50 3.5BC7 7 2 RED7 50 2 PF7 8 2b RP7 51 4BC8 8 3 RED8 51 2 PF8 9 2b RP8 52 2BC9 9 1.5 RED9 52 2 PF9 10 2b RP9 53 1.5BC10 10 2.5 RED10 53 2 --------SB1-------- RP10 54 2.5BC11 11 2 RED11 54 3 PF10 14 3b RP11 55 3BC12 12 2 RED12 55 2.5 PF11 15 2.5b RP12 56 1BC13 13 1 RED13 56 3 PF12 16 2.5b RP13 57 3BC14 14 2 RED14 57 1.5 PF13 18 2.5b RP14 58 4BC15 15 3 RED15 58 2.5 PF14 19 2b RP15 59 3

--------SB1-------- RED16 59 3 PF15 20 2b RP16 64 2.5BC16 31 2.5 RED17 60 3 PF16 21 2b RP17 65 3.5BC17 32 3 RED18 61 3 PF17 23 2b RP18 66 3.5BC18 33 3 RED19 62 3 PF18 24 2.5b RP19 67 3BC19 34 2 RED20 63 2 PF19 25 3b --------SB3--------BC20 35 2 RED21 64 2.5 PF20 26 1.5 RP20 71 1.5BC21 36 3 RED22 65 2.5 PF21 28 2b RP21 74 2.5BC22 37 2 RED23 66 3.5 PF22 29 2.5b RP22 75 3BC23 38 3 RED24 67 3 PF23 30 2.5b RP23 77 3BC24 39 4 RED25 68 2.5 PF24 31 1.5 RP24 78 3BC25 40 4 RED26 70 2 PF25 32 2.5b RP25 79 3BC26 41 4 RED27 71 2.5 PF26 33 1.5 RP26 80 3BC27 42 2 RED28 72 2 PF27 34 2b --------SB4/J0--------BC28 43 3 RED29 73 3 PF28 35 2.5b

--------SB2/TR5-------- RED30 74 2 PF29 36 1.5RED31 75 3 PF30 37 2bRED32 76 3 PF31 38 3bRED33 77 2.5 PF32 39 3b

--------SB3-------- PF33 40 2.5bRED34 90 1 PF34 41 3bRED35 91 1.5 PF35 42 2bRED36 92 1 PF36 43 3bRED37 93 2 --------SB2/TR5--------RED38 94 1.5

--------SB4/J0--------


Trends in paleosol maturity (Figs. 5, 6) show thick intervals at location1 that contain no paleosols and have preserved sedimentary structures(FACs 16–31, 78–89). Location 2 does not have any thick interval lackingpaleosols and is only 121 meters thick, whereas location 1 is 156 metersthick (Fig. 5). These differences could be due to a lower sedimentationrate at location 2; however, the methodology of this study evaluatesdepositional trends relative to an individual location.

Sequence Stratigraphy

Meter-scale fluvial aggradational cycles (FACs) are grouped intodecameter-scale fluvial aggradational cycle sets (FACSETs) on the basisof an overall upward trend of decreasing grain size, the presence oferosional bounding surfaces (with meter-scale erosional relief), andpedogenic evidence for decreasing sedimentation rates and increasingexposure represented by successive paleosols with increasing maturity(Figs. 5, 6, 7). FACSETs are grouped into fluvial sequences based uponan overall upward trend of decreasing grain size, pedogenic evidence fordecreasing sedimentation rates and prolonged exposure, and the presenceof sequence boundaries (Figs. 5, 6, 7). Sequence boundaries are identifiedon the basis of: (1) erosional surfaces, (2) evidence of a dramatic change insedimentation rates (i.e., a succession of several mature paleosols overlainby thick channel and overbank deposits with preserved sedimentarystructures), (3) inflection points between segments of the cumulativedeviation plots of grain size and cycle thickness that thin and fine-upwardand segments that thicken and coarsen-upward (Fig. 8), (4) changes inlithofacies proportions (Fig. 9), and (5) known formational contactswhere an unconformity has been established previously by lithostrati-graphic and biostratigraphic methods.

Periods of increasing (transgressive-equivalent) and decreasing(highstand-equivalent) accommodation space, and subsequent erosion(falling-stage equivalent), are summarized using modified systems tractnomenclature (sensu Atchley et al. 2004). These terms refer to the trends inaccommodation and sedimentation only, and in this paper do not have anyimplications with respect to sea level, as is the case with marine successions.Sand-prone facies and immature paleosols (maturity of 2 or lower) areassociated with the transgressive-equivalent (TE) part of a sequence. These

intervals reflect an increase in accommodation space and relatively highrates of deposition, and often result in strata containing preservedsedimentary structures. TE strata are characterized by upward-thickening,thicker-than-average FACs and FACSETs. Mudrock-dominated over-bank facies and related paleosols are associated with the highstand tofalling-stage equivalent (HFE). These intervals reflect a decrease inaccommodation and lower rates of deposition that result in strata withnumerous mature (maturity 3 or higher) paleosols and heavily bioturbatedintervals. HFE strata are dominated by upward-thinning, thinner-than-average FACs and FACSETS. The criteria by which the TE and HFEstrata are identified are relative to the individual location. Maximumflooding equivalents (MFE) mark the change from increasing accommo-dation space to decreasing accommodation space and are identified bythe inflection points on the cumulative deviation plots (Fig. 8). Thus, it isthe change in the FAC stacking pattern, rather than specific observablefeatures at a specific surface, that identifies the MFE.

In the Bull Canyon and Redonda formations at locality 1, 95 FACs aregrouped into 19 FACSETs and four fluvial sequences (Fig. 5). A total of66 paleosols were described from the two formations: 28 from the BullCanyon Fm. and 38 from the Redonda Fm. In the upper part of thePetrified Forest and the Rock Point formations at locality 2, 80 FACs aregrouped into 14 FACSETs and 4 fluvial sequences (Fig. 5). A total of 62paleosols were described at location 2: 36 from the Petrified Forest Fm.and 26 from the Rock Point Fm. (Table 5).

Sequence 1 at location 1 (Bull Canyon Fm.) and location 2 (PetrifiedForest Fm.) is dominated by overbank muds and moderately to well-developed paleosols (Figs. 5, 8). The basal contacts of these formationsare not exposed in either study area, and the first sequence is thereforeincomplete. Within this sequence, location 1 has 15 FACs grouped into 3FACSETs, and location 2 has 10 FACs grouped into 2 FACSETs. TheFACs exposed in Sequence 1 at both localities are thinner than average,as indicated by the negative slope on Figure 7. The features observed atboth locations within the exposed part of this sequence are consistent withan HFE.

Sequence boundary 1 (SB1) is an intraformational erosional surfacethat has not been previously recognized. This surface separates abundant

FIG. 3.— Diagrammatic paleosol profiles,field-recognizable characteristics, and relativematurity assignments.


FIG. 4.—Photographs representative of various facies. A) Fr facies from FAC 1 in the Rock Point Fm. (Location 2). B) Sm facies from FAC 49 in the Redonda Fm.(Location 1). C) Sm facies from FAC 69 in the Rock Point Fm. (Location 2). D) Fr and Sr facies from FAC 6 and 7, respectively, in the Bull Canyon Fm. (Location 1). E)Sh and Fr facies from FAC 28 in the Petrified Forest Fm. (Location 2). F) Sr facies from FAC 13 in the Bull Canyon Fm. (Location 1). G) St facies from FAC 84 in theRedonda Fm. (Location 1).


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mature paleosols of the HFE of Sequence 1 from the channel sandsassociated with the TE of Sequence 2. This erosional surface coincideswith the inflection points in Figure 7 indicating a change from thinner-and finer-than-average FACs to thicker- and coarser-than-average FACs.SB1 coincides with a transition from abundant mudstones and paleosolsto a more sand-rich interval (Figs. 8, 9, 10).

Sequence 2 at location 1 (Bull Canyon Fm.) contains 28 FACs and 5FACSETs. The lower 3 FACSETs are TE and dominated by channelsands, minor overbank muds, and preserved sedimentary structures withlittle or no biogenic or pedogenic modification (Figs. 5, 8). The FACs forthis part of the sequence are thicker than average, as shown by thepositive slope in Figure 8. The upper two FACSETs of Sequence 2 aredominated by overbank muds containing abundant mature paleosols(Figs. 5, 8) with FACs that are thinner and finer than average (Fig. 8).These two FACSETs are interpreted as HFE. The inflection point onFigure 7 at FAC 31 is the MFE that separates the underlying TE from theoverlying HFE.

Sequence 2 at location 2 (Petrified Forest Fm.) contains 33 FACs and 4FACSETs that have overall trends similar to Sequence 2 at location 1.The lower two FACSETs are interpreted as TE strata with channel sands,preserved sedimentary structures, and interbedded paleosols (Figs. 5, 8).FACs in this part of the sequence are coarser and thicker than average(Fig. 8). The upper two FACSETs of Sequence 2 are dominated byoverbank mudrocks, lack primary sedimentary structures, and containabundant mature paleosols (Figs. 5, 8). These upper two FACSETs havethinner- and finer-than-average FACs (Fig. 8) and make up an HFE. TheMFE for this sequence is at the top of FAC 28.

The sequence boundary separating Sequence 2 and Sequence 3 (TR-5/SB2) is a regional unconformity surface (Lucas 1997; Lucas et al. 1997).At location 1 this surface is at the contact between the Bull Canyon and

FIG. 6.— Trends in relative paleosol maturityfor locations 1 (upper) and 2 (lower). Paleosolmaturity tends to increase approaching a fluvialsequence boundary, and the most maturepaleosols are associated with HFE strata.

FIG. 7.— Conceptual model of the idealized three-tier fluvial cyclic hierarchyshowing relative vertical and lateral scales (based upon Atchley et al. 2004, andProchnow et al. 2006a).


FIG. 8.— Cumulative deviation plots of grainsize and cycle thickness for FACs from location1 (upper) and location 2 (lower). Note that thetrends of FAC thickness and grain size betweenlocations are strikingly similar at thesequence scale.

FIG. 9.— Histograms showing proportions oflithologic components within FACs for locations1 (upper) and 2 (lower). Note the higherproportions of the sandstone lithology at thebases of the sequences.


Redonda formations. TR-5/SB2 is clearly identified by an abrupt colorchange from red-brown to orange-red that is coincident with a changefrom the overbank fines and paleosols of Sequence 2 of the Bull CanyonFm. to alternating sandstone ledges and recessive mudstones of Sequence3 of the Redonda Fm. Although the formational contact is easilyidentified on the basis of clear lithologic and color changes at locality 1,there appears to be only minor erosional relief. Within the ten metersbelow the unconformity at both locations is a succession of several well-developed paleosols (with a maturity of 3 or more; Fig. 5).

The TR-5/SB2 surface at location 2 is clearly identified at the abruptcolor change from purplish-red mudstones and gray, poorly sortedsandstones of the Petrified Forest Fm. to brown mudstones of the RockPoint Fm. At this surface there is also a change in weathering patternmarked by the disappearance of the ‘‘popcorn’’ type badlands weatheringassociated with abundant bentonite in the Petrified Forest Fm. (Dubiel1994). As with locality 1, there is no evidence of extensive erosion.

Sequence 3 at location 1 (Redonda Fm.) is characterized by bioturbatedfine-grained sandstones at the base of FACSETs, which are overlain bythicker overbank mudrocks that include abundant paleosols of varyingmaturity (Figs. 5, 8). No primary sedimentary structures are identified inthis sequence because of the extensive biogenic and pedogenic overprinting.This sequence contains 34 FACs and 7 FACSETs (Fig. 5). FACs thinupward and are finer and thinner than average (Fig. 8). As with location 1,Sequence 3 at location 2 (Rock Point Fm.) is dominated by overbankmudrocks including paleosols of varying maturity (Figs. 5, 8). Sequence 3contains 24 FACs that thin upward, are finer and thinner than average(Fig. 8), and stack into 5 FACSETs. There are few sand bodies within it,and there are no preserved sedimentary structures. Sequence 3 at bothlocalities contains only HFE strata.

Sequence boundary 3 (SB3) is another intraformational erosionalsurface that has not been recognized previously. This surface separatesabundant mature paleosols of the HFE of Sequence 3 from channel sandsassociated with the TE of Sequence 4. SB 3 coincides with inflectionpoints in Figure 7 that separate thinner- and finer-than-average FACsbelow from thicker- and coarser-than-average FACs above. Thisboundary also coincides with a change from abundant mudstones andpaleosols to a more sand-rich interval (Figs. 8, 9, 10).

Sequence 4 at location 1 (Redonda Fm.) contains 18 FACs and 4FACSETS. Both FACs and FACSETs thicken upward and are thickerthan average (Figs. 5, 8). The basal part of the sequence contains thick,

coarser-grained channel sands with well-preserved sedimentary structuresthat include trough cross-bedding, ripple lamination, and planar bedding(Fig. 5). There are only a few immature paleosols in the uppermost partof sequence 4. Thus, all of Sequence 4 at this locality is interpreted as TE.At both locations, the sequence boundary at the top of Sequence 4 (SB4)is the Triassic–Jurassic contact (i.e., J-0 unconformity) that separates theRhaetian formations from the overlying Jurassic Entrada Formation(Blakey 1989; Dubiel 1994; Lucas et al. 1997). Both localities showevidence of extensive erosion at this surface.

Sequence 4 at location 2 (Rock Point Fm.) contains 13 FACs and 3FACSETs. This interval is composed of FACs that are coarser andthicker than average (Fig. 8). The sand bodies in the basal part of bothFACs and FACSETs have abundant insect burrows and root traces andlack primary sedimentary structures (Fig. 5). Sequence 4 is classified asa TE because of the stacking pattern, lack of mature paleosols, andrelatively high sedimentation rates.

DISCUSSION

Accommodation Trends and Locality Comparisons

When comparing the two Upper Triassic sections, a few differences arenotable. Based on preserved sedimentary structures, grain size, andoverall thickness, location 2 appears to have had lower rates of sedimentaccumulation (Fig. 5). Also, the intervals designated as TE at location 1lack paleosols, whereas TE intervals at location 2 have many paleosols.However, at location 2 the TE strata contain more facies of the channeland crevasse-splay association, have a greater abundance of preserveddepositional features, and have thicker and coarser than average FACtrends than the HFE strata (Figs. 5, 8). Because facies and sedimentationrates can vary within a given systems tract depending on the positionwithin the depositional system (Posamentier and Allen 1993), varyingfeatures within TE strata of different locations do not precludecorrelation.

Another difference between locations is that the Petrified Forest Fm.contains the only paleosols with slickensides (Tables 3, 4). This may bea function of parent material, inasmuch as this is the only formation inthis study that has abundant bentonite derived from volcanic sources(Dubiel 1994). Other than the presence of slickensides, the paleosols ofthe Petrified Forest Fm. have features similar to the other paleosols ofthis study (Fig. 3).

The differences between the two study locations appear to be lessnoteworthy than the similarities in their accommodation histories, whichare demonstrated by sequence-scale stacking patterns, changes inlithology, number and distribution of sequence boundaries, and trendsof intrasequence systems-tract equivalents (Figs. 5, 8, 9, 10). The twointraformational sequence boundaries identified at each location (SB 1and 3) are easily recognized by lithologic changes and inflections oncumulative deviation plots (Figs. 8, 9, 10). Oddly, the regional un-conformity in the middle of the sections (TR-5) does not show evidence ofextensive erosion of the Norian formations, nor is it apparent on thecumulative deviation plots (Fig. 8). Instead, the pronounced physicalchanges at the formational contact (described previously) are diagnosticof the SB2/TR-5 surface. While both locations have several well-developed paleosols in the 10 meters below the SB2/TR-5 surface(Fig. 5), there are other intervals at both locations with many maturesoils in a small stratigraphic interval. Therefore, care must be taken whenidentifying sequence boundaries lacking significant erosional relief.

Autocyclic Mechanisms

Paleosol-bearing alluvial successions commonly contain a hierarchy ofdepositional cyclicity. Recent conceptual models of alluvial sequencestratigraphy hold that recurring episodes of deposition are produced in

TABLE 5.— Quantitative comparisons of stratigraphic units for the twostudy areas.

Stratigraphic Feature

Norian Rhaetian

Loc. 1 Loc. 2 Loc. 1 Loc. 2

Formation(s) BC PF Red RPFACs 43.0 43.0 52.0 37.0FACSETs 8.0 6.0 11.0 8.0Fluv. Seq. (complete or partial) 2.0 2.0 2.0 2.0Paleosols 28.0 36.0 38.0 26.0Thickness of strata exposed (m) 71.0 60.0 85.0 61.0Avg. Paleosols/Fluv. Seq.* 13.0 25.0 19.0 13.0Avg FAC thickness (m) 1.7 1.4 1.6 1.6Average FACSET thickness (m) 8.9 10.0 7.7 7.6Average Fluv. Seq. thickness (m)* 45.0 48.0 43.0 31.0Average # of FACs/FACSET 5.4 7.2 4.7 4.6Average # of FACSETs/Fluv. Seq.* 5.0 4.0 5.5 4.0

BC 5 Bull Canyon FormationPF5 Upper portion of Painted desert memb. of Petrified Forest FM.Red 5 Redonda FormationRP 5 Rock Point Formation* only complete sequences are considered



response to the interaction of allogenic and autogenic controls atvariable scales (e.g., Beerbower 1964; Bridge and Leeder 1979; Bridge1984; Kraus 1987, 1999; Kraus and Aslan 1999; Shanley and McCabe1994; Kraus 2002; Atchley et al. 2004). In the Upper Triassic strata ofthis study, a three-tier hierarchy is present that includes meter-scale fluvial aggradational cycles (FACs) which stack into decameter-scale fluvial aggradational cycle sets (FACSETs) which, in turn, stackinto fluvial sequences (sensu Atchley et al. 2004). Prochnow et al. (2006a)attributed the three-tier hierarchy of FACs, FACSETs, and fluvialsequences in the Late Triassic Western Interior to channel avulsion andfloodplain aggradation, successive episodes of channel avulsion, andtectonic changes, respectively.

The nature of meter-scale FACs and decameter-scale FACSETs in thisstudy suggests autogenic depositional mechanisms consistent with themodels for similar-scale fluvial cycles proposed by Bridge (1984), Kraus(1987), Kraus and Aslan (1999), and Atchley et al. (2004). FACs areinterpreted to record individual events of channel avulsion and sub-sequent periods of channel stability (sensu Kraus and Aslan 1999; Atchleyet al. 2004). During avulsion episodes, sedimentation rates were likelyelevated (due to the abundant crevassing that occurs) and resulted inweak paleosol development (Kraus and Aslan 1999). Coarser-grainedfacies in the channel and crevasse-splay association would have beendeposited as downstream and lateral accretion macroforms, gravel bars,or progradational splay deposits.

Although, the primary causes of channel avulsion are still poorlyunderstood, avulsion events are thought to be related to the combinationof channel gradient, channel distributions, and substrate compositions(Tornqvist and Bridge, 2002; Aslan et al. 2005a, 2005b; Tornqvist andBridge 2006; Blum and Aslan 2006). After an avulsion event, primarydepositional bedforms may be destroyed by bioturbation and pedogen-esis. Between avulsion events, flooding episodes would likely account formost floodplain deposition and aggradation. Flood deposits ona centimeter scale are rare in this study and were likely reworked andincorporated into soils during pedogenesis or preserved as the Fl facies.Mature paleosols are interpreted to be the product of periods of channelstabilization and reduced rates of sedimentation on the floodplain.

FACSETs are thought to be produced by successive episodes ofavulsion as a channel drifts away from or toward a reference position inthe alluvial valley (Kraus 1987; Kraus and Aslan 1999; Atchley et al.2004). The upward change within FACSETs from sand- to mud-dominated textures and corresponding variations in paleosol maturitylikely reflect a progressive change in the location of the overbank sedimentsource (sensu Atchley et al. 2004). Channel migration away froma reference position causes a reduction in grain size and floodplaindeposition (Bown and Kraus 1987; Kraus and Aslan 1999; Atchley et al.2004) and is the likely depositional process responsible for FACs that fineand thin upward within FACSETs (Fig. 5). Conversely, channel migrationtoward a reference point results in an increase in grain size and floodplain

r

FIG. 10.—Photographs of the intraformational fluvial sequence boundaries identified in this study. Black numbered arrows indicate FACSETs (compare with Fig. 5).A) Sequence boundary 3 in the Redonda Fm. at location 1. B) Sequence boundary 3 within the Rock Point Fm. at location 2. C) Sequence boundary 1 in the Bull CanyonFm. at location 1. D) Sequence boundary 1 in the Petrified Forest Fm. at location 2.

FIG. 11.—Conceptual sequence-stratigraphicmodel for Chinle strata at the two locations. A)Summarized sequence stratigraphy of the twomeasured intervals in this study. B) Conceptualmodel of composite base-level changes over timerelated to systems tracts (modified from Atchleyet al. 2004). C) Theoretical model of theaccommodation history in the study interval.


deposition (Bown and Kraus 1987; Kraus and Aslan 1999; Atchley et al.2004) and is the likely depositional process responsible for FACs thatcoarsen and thicken upward within FACSETs (Fig. 5). While the lateralmigration of a fluvial channel can generate the same stratal pattern as TEor HFE strata, it is on a smaller scale (Fig. 7; Atchely et al. 2004).

Given the inferred autogenic origin of FACs and FACSETS, it is notsurprising that FACs and FACSETs do not correlate between locations200 km apart, inasmuch as each should record independent histories offloodplain aggradation and channel avulsion and migration. Thesemechanisms are also consistent with the fact that thinner and finer-grained cycles typically contain thicker, welded, and more maturepaleosols (Fig. 5).

Allocyclic Mechanisms

In contrast to the FACs and FACSETs, many similarities have beenidentified in the sequence-scale trends via the modified stacking-patternanalysis of Atchley et al. (2004). Not only do the age-equivalent sectionshave approximately the same relative distribution of sequence boundaries,they also have identical patterns of systems tracts, remarkably similartrends of grain size and FAC thickness, and comparable trends of lithologicdistributions (Figs. 5, 8, 9). At the two study locations, only Sequence 2includes both TE and HFE strata (Fig. 11). Sequence 1 is incomplete andmay have TE strata beneath it. Sequence 4 is truncated by the J-0 unconformity, and therefore HFE strata could have been deposited andsubsequently eroded. However, Sequence 3 is fully exposed and boundedby unconformity surfaces, yet contains no TE strata. Therefore, Sequence 3may record a reduced rate of accommodation gain, as evidenced by stratawith numerous mature paleosols and heavily bioturbated intervals (Fig. 5).Given the remarkably similar accommodation trends, these age-equivalentsections are inferred to record similar histories of deposition andpedogenesis that were likely produced in response to an allocyclicmechanism having a 1–2 Myr frequency (depending upon the timeconsumed by the unconformity surfaces; Fig. 1).

Hectometer-scale fluvial sequences have been attributed to allogenicmechanisms such as fluctuations in eustatic sea level, regional to globalclimate changes, and/or tectonics (uplift and/or subsidence) (e.g.,Posamentier and Allen 1993; Schumm 1993; Shanley and McCabe1994; Schwans 1995; Kraus and Aslan 1999; Atchley et al. 2004). Becausethe two locations of this study are over 750 km inland from the equivalentpaleocoastline (Fig. 2; Blakey and Gubitosa 1983, 1984; Blakey 1989;Dubiel 1994), they are likely beyond the influence of eustatic sea-levelchanges. A modern example is the Mississippi River, which is a low-gradient, high-sediment-supply fluvial system and has responded topostglacial Quaternary sea-level rise along the Gulf Coast of the U.S.A.to an up-dip limit of 300 km inland (Aslan and Autin 1999; Blum andTornqvist 2000; Blum and Aslan 2006).

It is less clear what (if any) role climate played in the cyclic sedimentationhistory of the study interval. Although climatic changes have beendocumented elsewhere in the Chinle Group (Carnian through mid-Norian), no dramatic climatic variations have been identified within themid-Norian through Rhaetian time interval (Stewart et al. 1972; Blakeyand Gubitosa 1983, 1984; Dubiel 1987; 1989; Blodgett 1988; Dubiel et al.1991; Therrien and Fastovsky 2000; Tanner 2003b; Prochnow et al. 2006b).Tanner (2003b) suggests that the climate was relatively static from the lateNorian into the earliest part of the Jurassic. At both locations in this study,the most mature soils are characterized by an abundance of pedogeniccarbonate. Paleosol maturity trends (Fig. 3) are similar to those observedin modern semiarid to arid soils developed from alluvial deposits (SoilSurvey Staff online database). In modern soils, petrocalcic horizons aremost common in soils older than the Holocene and suggest advanced soildevelopment in arid to semiarid regions (Soil Survey Staff 1999). This is

consistent with previous interpretations of a semiarid to arid climate duringthe deposition of upper Norian to Rhaetian strata in the Late TriassicWestern Interior (Stewart et al. 1972; Blakey and Gubitosa 1983, 1984;Dubiel 1987, 1989; Blodgett 1988; Dubiel 1991; Therrien and Fastovsky2000; Tanner 2003b; Prochnow et al. 2006b).

Based upon the similarity of soil features at the two locations, there waslikely little climate variance between the areas and throughout eachstratigraphic interval. The variations in paleosols are more easilyexplained as a function of the duration of pedogenesis (Table 3; Fig. 3).For example, the feature that varies the most between paleosols is thepedogenic carbonate content. However, the depth at which the pedogeniccarbonate content increases in the paleosols (maturity 2.5 to 4) is typicallybetween 40 and 70 cm below the top of paleosol profiles that do not haveevidence of extensive erosion, regardless of the placement withina sequence or the stratigraphic section. While pedogenic carbonate canbe present in subhumid environments, the depth to carbonate is normallygreater than one meter, unless the soil developed on a slope (Soil SurveyStaff, online database). If there had been strong climatic variations, therewould likely be depth-to-carbonate variations throughout the strata, as ispresent in other portions of the Chinle (e.g., Prochnow et al. 2006b).

Additionally, there is no observed change in rooting style or burrowmorphology, and only little variation of the soil horizons present in thepaleosols. No horizons were present that were leached (E horizon of SoilTaxonomy) or had abundant pedogenic clay accumulation (Bt horizon ofSoil Taxonomy), though they are present in paleosols in older ChinleGroup strata (Retallack 1997; Therrien and Fastovsky 2000; Prochnowet al. 2006b). If there had been climate changes within the strata, theywere not such that the physical attributes of the soils were alteredsignificantly throughout the section.

As with most fluvial sequences dominated by overbank mudrocks,sequence-scale (or larger) depositional cycles in the Chinle are generallyconsidered to be the result of various tectonic mechanisms (Kraus andMiddleton 1987; Kraus and Aslan 1999; Tanner 2003a; Prochnow et al.2006a). Tectonism during Chinle deposition is thought to have resulted inpulses of source-area uplift that caused progradational fluvial depositioncharacterized by symmetrical cycles that coarsen and then fine upwardacross the Chinle Basin (e.g., Steel et al. 1977; Steel and Aasheim 1978).Lucas (1997) and Lucas et al. (1997) describe three third-order‘‘sequences’’ in the Chinle strata in New Mexico and Arizona that areattributed to tectonic pulses (Fig. 1). The basal parts of these cycles arecharacterized by coarse, bedload deposits of multistory channelcomplexes (e.g., Shinarump Formation of southern Utah and the age-equivalent Agua Zarca Formation of northern New Mexico; see Fig. 1).The upper parts of these cycles are characterized by finer-grainedoverbank deposits and interbedded single-story channel sandstones ofsuspended-load fluvial systems (e.g., Petrified Forest Fm. and equiva-lents). Between these cycles are the regional TR-4 and TR-5 unconformitysurfaces (Lucas 1997; Lucas et al. 1997).

The sequence-scale cyclicity in this study has a higher frequency thanthat of the ‘‘third-order sequences’’ of Lucas (1997) and Lucas et al.(1997). The ‘‘third-order sequences’’ have a frequency of approximately4–10 Myr, whereas the fluvial sequences identified herein likely havea frequency closer to 1–2 Myr (Fig. 1). Because eustatic and climaticchanges are unlikely, higher-frequency pulses of source area uplift and/orsubsidence are the most likely mechanism for sequence-scale deposition ofthis study. Regional-scale tectonism can cause changes in deposition ratesand avulsion frequency on a 1 Myr scale, which can result in changes inmean grain size and the proportion of channel deposits in strata on the100 m scale (Bridge 2003). It should also be noted that fluvial response touplift or loading and erosion or unloading can have inverse relationshipsin various parts of a basin (Posamentier and Allen 1993; Shanley andMcCabe 1994; Schwans 1995). Therefore, although tectonic uplift and/orsubsidence events produced similar sedimentation responses in the study


areas, other parts of the Chinle Basin could have responded differently tothese high-frequency tectonic events.

Overall, this study suggests that for some areas of the Late TriassicWestern Interior there is a four-tier hierarchy of depositional cyclicitycomposed of FACs, FACSETs, fluvial sequences, and larger compositesequences (i.e., third-order cycles of Lucas 1997 and Lucas et al. 1997).This hierarchy is thought to be produced in response to channel avulsionand floodplain aggradation, channel migration, high-frequency pulses ofsource area uplift and/or basin subsidence, and lower-frequency pulses ofsource area uplift and/or basin subsidence, respectively.

CONCLUSIONS

(1) The Upper Norian to Rhaetian strata at the two locations in thisstudy were deposited by a suspended-load fluvial system withina semiarid to arid environment. Outcrop sections are dominatedby overbank mudrocks that contain abundant paleosols at variousstages of maturity. The most notable paleosol features in this studyare roots, pedogenic carbonate nodules, and calcic horizons. Basedupon the similarity of soil features at the two locations, there is noevidence of sequence-scale climate variance between the areas orthroughout each stratigraphic interval.

(2) The study interval at both locations shows a three-tier hierarchy ofdepositional cyclicity whereby meter-scale fluvial aggradationalcycles (FACs) stack into decameter-scale fluvial aggradationalcycle sets (FACSETs), which in turn stack into fluvial sequences.By comparing the two sections, it appears that FACs andFACSETs are non-correlative over distances of , 200 km,whereas one partial and three complete fluvial sequences appearto be correlative over the same distance.

(3) FACs, FACSETs, and fluvial sequences are thought to have beendeposited in response to channel avulsion events, channel avulsionand migration episodes, and tectonic pulses, respectively. Thecorrelation of fluvial sequences suggests that the sequence-scaledeposition was the product of tectonism, inasmuch as both sealevel and climate seem unlikely. Sea level is ruled out due to thedistance from the age-equivalent shoreline, and the relativelyconsistent features of the paleosols provide no evidence forsequence-scale changes in climate.

(4) Two significant episodes of increased accommodation (i.e., TEstrata of Sequence 2 and Sequence 4) resulted in high rates ofdeposition and abundant preserved sedimentary structures. Threeepisodes of decreased accommodation (i.e., HFE strata ofSequence 1, Sequence 2, and Sequence 3) produced lower ratesof deposition, a more stable landscape, and abundant mature soils.

ACKNOWLEDGMENTS

Generous financial support for this study was provided by the PetroleumResearch Fund administered by the American Chemical Society via grantPRF#45548-AC8 awarded to S.A. and L.N. Student support was also providedby the Geological Society of America Grants-in-Aid Program, the Robert J.Weimer Student Grant of the SEPM Foundation, the Fort Worth GeologicalSociety Scholarship, and Baylor University Department of Geology. Addition-ally, we would like to thank Kate Ziegler for assisting with outcrop selection andJimmy and Dorothy Randall, who kindly allowed field studies to be conductedon their ranch. The quality of this manuscript improved thanks to the insightfulreviews of Paul McCarthy, Neil Tabor, and an anonymous reviewer.

REFERENCES

ALLEN, J.R.L., 1978, Studies in fluviatile sedimentation: an exploratory quantitativemodel for the architecture of avulsion-controlled alluvial suites: SedimentaryGeology, v. 21, p. 129–147.

ASLAN, A., AND AUTIN, W.J., 1999, Evolution of the Holocene Mississippi Riverfloodplain, Ferriday, Louisiana: insights on the origin of fine-grained floodplains:Journal of Sedimentary Research, v. 69, p. 800–815.

ASLAN, A., AUTIN, W.J., AND BLUM, M.D., 2005a, Causes of river avulsion: insights fromthe late Holocene avulsion history of the Mississippi River, U.S.A.: Journal ofSedimentary Research, v. 75, p. 648–662.

ASLAN, A., AUTIN, W.J., AND BLUM, M.D., 2005b, Causes of river avulsion: insights fromthe late Holocene avulsion history of the Mississippi River, U.S.A.—Reply: Journal ofSedimentary Research, v. 76, 960 p.

ATCHLEY, S.C., NORDT, L.C., AND DWORKIN, S.I., 2004, Eustatic control on alluvialsequence stratigraphy: a possible example from the Cretaceous–Tertiary transition ofthe Tornillo Basin, Big Bend National Park, West Texas: Journal of SedimentaryResearch, v. 74, p. 391–404.

BEERBOWER, J.R., 1964, Cyclothems and cyclic depositional mechanisms in alluvial plainsedimentation, in Merriam, D.F., ed., Symposium on Cyclic Sedimentation: StateGeological Survey of Kansas, Bulletin 169, v. II, p. 31–42.

BLAKEY, R.C., 1989, Triassic and Jurassic geology of the southern Colorado Plateau, inJenney, J.P., and Reynolds, S.J., eds., Geologic Evolution of Arizona: ArizonaGeological Society, Digest 17, p. 369–396.

BLAKEY, R.C., AND GUBITOSA, R., 1983, Late Triassic paleogeography and depositionalhistory of the Chinle Formation, southern Utah and northern Arizona, in Reynolds,M.W., and Dolby, E.D., eds., Mesozoic Paleogeography of West-Central UnitedStates: SEPM, Rocky Mountain Section, p. 57–76.

BLAKEY, R.C., AND GUBITOSA, R., 1984, Controls on sandstone body geometry andarchitecture in the Chinle Formation (Upper Triassic), Colorado Plateau: Sedimen-tary Geology, v. 38, p. 51–86.

BLODGETT, R.H., 1988, Calcareous paleosols in the Triassic Dolores Formation,southwestern Colorado, in Reinhardt, J., and Sigleo, W.R., eds., Paleosols andWeathering through Geologic Time—Principles and Applications: Geological Societyof America, Special Paper 216, p. 103–122.

BLUM, M.D., AND ASLAN, A., 2006, Signatures of climate vs. sea-level change withinincised valley-fill successions: Quaternary examples from the Texas Gulf Coast:Sedimentary Geology, v. 190, p. 177–211.

BLUM, M.D., AND TORNQVIST, T.E., 2000, Fluvial responses to climate and sea-levelchange: a review and look foreword: Sedimentology, v. 47, p. 2–48.

BOUCHER, T.E., 2004, Influence of halokinesis on fluvial architecture and paleosoldevelopment: the Late Triassic Chinle Formation at Castle Valley (Paradox Basin),southeastern Utah [Master’s thesis]:, Baylor University: Waco, Texas, 44 p.

BRIDGE, J.S., 1984, Large-scale facies sequences in alluvial overbank environments:Journal of Sedimentary Petrology, v. 54, p. 583–588.

BRIDGE, J.S., 2003, Rivers and Floodplains: Oxford, U.K., Blackwell Science, 491 p.BRIDGE, J.S., AND LEEDER, M.R., 1979, A simulation model of alluvial stratigraphy:

Sedimentology, v. 26, p. 617–644.COSUNA, 1983, Southwest/southwest mid-continent region stratigraphic correlation

chart, in Hills, J.M., and Kottlowski, F.E., regional coordinators., eds., Correlation ofStratigraphic Units of North America (COSUNA) Project: American Association ofPetroleum Geologists: Tulsa, Oklahoma, chart.

DRUMMOND, C.N., AND WILKINSON, B.H., 1993, On the use of cycle thickness diagramsas records of long-term sea level change during accumulation of carbonate sequences:Journal of Geology, v. 101, p. 687–702.

DUBIEL, R.F., 1987, Sedimentology of the Upper Triassic Chinle Formation,southeastern Utah: paleoclimatic implications: Arizona–Nevada Academy of Science,Journal, v. 22, p. 35–45.

DUBIEL, R.F., 1989, Depositional and climatic setting of the Upper Triassic ChinleFormation, Colorado Plateau, in Lucas, S.G., and Hunt, A.P., eds., Dawn of the Ageof Dinosaurs in the American Southwest, New Mexico Museum of Natural History,Spring Field Conference Guidebook, p. 171–187.

DUBIEL, R.F., 1991, Architectural-facies analysis of nonmarine depositional systems inthe Upper Triassic Chinle Formation, southeastern Utah, in Miall, A.D., and Tyler,N., eds., The Three-Dimensional Facies Architecture of Terrigenous ClasticSediments and Its Implications for Hydrocarbon Discovery and Recovery: SEPM,Concepts in Sedimentology and Paleontology 3, p. 103–110.

DUBIEL, R.F., 1994, Triassic deposystems, paleogeography, and paleoclimate of thewestern interior: SEPM Theme Meeting, Mesozoic Systems of the Rocky MountainRegion, p. 133–168.

DUBIEL, R.F., PARRISH, J.T., PARRISH, J.M., AND GOOD, S.C., 1991, The Pangeanmegamonsoon—evidence from the Upper Triassic Chinle Formation, ColoradoPlateau: Palaios, v. 6, p. 347–370.

GRADSTEIN, F.M., OGG, J.G., AND SMITH, A.G., 2004, The Geologic Time Scale:Cambridge University Press.

HABICHT, J.K.A., 1979, Paleoclimate, Paleomagnetism, and Continental Drift: AmericanAssociation of Petroleum Geologist, Studies in Geology, v. 9, 31 p.

HAQ, B.U., HARDENBOL, J., AND VAIL, P.R., 1988, Mesozoic and Cenozoic chrono-stratigraphy and cycles of sea-level change, in Wilgus, C.K., Hastings, B.S., Kendall,C.G.St.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C., eds., Sea-LevelChanges: An Integrated Approach: SEPM, Special Publication 42, p. 71–108.

JOHNSON, S.C., LUCAS, S.C., AND HUNT, A.P., 2002, Macro-fish fauna of the UpperTriassic (Appachean) Redonda Formation, eastern New Mexico, in Heckert, A.B.,and Lucas, S.C., eds., Upper Triassic Stratigraphy and Paleontology, New MexicoMuseum of Natural History Bulletin, v. 21, p. 107–114.

KRAUS, M.J., 1987, Integration of channel and floodplain suites, II. Vertical relations ofalluvial paleosols: Journal of Sedimentary Petrology, v. 57, p. 602–612.


KRAUS, M.J., 1999, Paleosols in clastic sedimentary rocks: their geologic applications:Earth-Science Reviews, v. 47, p. 41–70.

KRAUS, M.J., 2002, Basin-scale changes in floodplain paleosols: implications forinterpreting alluvial architecture: Journal of Sedimentary Research, v. 72, p. 500–509.

KRAUS, M.J., AND ASLAN, A., 1999, Palaeosol sequences in floodplain environments:a hierarchical approach, in Thiry, M., and Simon-Coincon, R., eds., Palaeoweather-ing, Palaeosurfaces and Related Continental Deposits: International Association ofSedimentologists, Special Publication 27, p. 303–321.

KRAUS, M.J., AND MIDDLETON, L.T., 1987, Dissected paleotopography and base-levelchange in a Triassic fluvial sequence: Geology, v. 15, p. 18–21.

LEHRMANN, D.J., AND GOLDHAMMER, R.K., 1999, Secular variation in parasequence andfacies stacking patterns of platform carbonates: a guide to application of stacking-pattern analysis in strata of diverse ages and settings, in Harris, P.M., Saller, A.H.,and Simo, J.A., eds., Advances in Carbonate Sequence Stratigraphy: Application toReservoirs, Outcrops and Models: SEPM, Special Publication 63, p. 187–225.

LUCAS, S.G., 1993, The Chinle Group: Revised stratigraphy and biochronology of theUpper Triassic nonmarine Strata in the Western United States, in Dickins, J.M., Yin,H., and Lucas, S.G., eds., Permo-Triassic of the Circum-Pacific: Cambridge, U.K.,Cambridge University Press, p. 200–228.

LUCAS, S.G., 1997, The Upper Triassic Chinle Group: western United States:a nonmarine standard for Late Triassic time, in Morales, M., ed., Aspects ofMesozoic Geology and Paleontology of the Colorado Plateau, Museum of NorthernArizona Bulletin 59, p. 27–50.

LUCAS, S.G., HECKERT, A.B., ESTEP, J.W., AND ANDERSON, O., 1997, Stratigraphy of theUpper Triassic Chinle Group, Four Corners region, in Anderson, O.J., Kues, B., andLucas, S.G., eds., Mesozoic Geology and Paleontology of the Four Corners Region:New Mexico Geological Society, Guidebook, p. 81–108.

MCCARTHY, P.J., AND PLINT, A.G., 1998, Recognition of interfluve sequence boundaries:integrating paleopedology and sequence stratigraphy: Geology, v. 26, p. 387–390.

MIALL, A.D., 1978, Lithofacies types and vertical profile models in braided riverdeposits: a summary, in Miall, A.D., ed., Fluvial Sedimentology: Canadian Society ofPetroleum Geologists, Memoir 5, p. 598–604.

MIALL, A.D., 1992, Alluvial Deposits, in Walker, R.G., and James, N.P., eds., FaciesModels: Response to Sea Level Change: Geological Association of Canada, p.119–142.

MIALL, A.D., AND ARUSH, M., 2001, Cryptic sequence boundaries in braided fluvialsuccessions: Sedimentology, v. 48, p. 971–985.

MUEHLBERGER, W.R., ADAMS, G.E., LONGGOOD, T.E, JR, AND ST. JOHN, B.E., 1960,Stratigraphy of the Chama Quadrangle, northern Rio Arriba County: New Mexico,New Mexico Geological Society, Guidebook, p. 93–102.

NEWELL, A.J., 1993, Depositional environments of the Late Triassic Bull CanyonFormation (New Mexico): Implications for ‘‘Dockum Formation’’ paleogeography, inLucas, S.G., and Morales, M., eds., The Nonmarine Triassic: New Mexico Museumof Natural History and Science, Bulletin 3, p. 359–368.

POSAMENTIER, H.W., AND ALLEN, G.P., 1993, Siliciclastic sequence stratigraphic patternsin foreland ramp-type basins: Geology, v. 21, p. 455–458.

PROCHNOW, S.J., ATCHLEY, S.C., BOUCHER, T., NORDT, L.C., AND HUDEC, M.R., 2006a,The influence of salt withdrawl subsidence on paleosol maturity, sedimentation rates,and cyclic fluvial deposition in the Triassic Chinle Formation: The Big BendMinibasin near Castle Valley, Utah: Sedimentology, v. 53, p. 1319–1345.

PROCHNOW, S.J., NORDT, L.C., ATCHLEY, S.C., AND HUDEC, M.R., 2006b, Multi-proxypaleosol evidence for Middle and Late Triassic Climate Trends in eastern Utah:Palaeogeography, Palaeoclimatology, Palaeoecology, v. 232, p. 53–72.

READ, W.A., AND DEAN, J.M., 1982, Quantitative relationships between numbers offluvial cycles, bulk lithological composition and net subsidence in a ScottishNamurian basin: Sedimentology, v. 29, p. 181–200.

REEVE, S.C., AND HELSLEY, C.E., 1972, Magnetic reversal sequence in the upper portionof the Chinle Formation, Montoya, New Mexico: Geological Society of America,Bulletin, v. 83, p. 3795–3812.

RETALLACK, G.J., 1988, Field recognition of paleosols, in Reinhardt, J., and Sigleo,W.R., eds., Paleosols and Weathering Through Geologic Time: Techniques andApplications, Geological Society of America, Special Paper 216, p. 1–20.

RETALLACK, G.J., 1994, The environmental factor approach to the interpretation ofpaleosols, factors in soil formation—a fiftieth anniversary perspective, in Luxmoore,R.J., ed., Factors of Soil Formation: Soil Science Society of America, SpecialPublication 33, p. 31–64.

RETALLACK, G.J., 1997, Dinosaurs and dirt, in Wolberg, D., and Stump, E., eds.,Dinofest: Academy of Natural Sciences: Philadelphia, Pennsylvania, p. 345–359.

RETALLACK, G.J., 2001, Soils of the Past, Second Edition: Oxford, U.K., BlackwellScience, 404 p.

RETALLACK, G.J., 2005, Pedogenic carbonate proxies for amount and seasonality ofprecipitation in paleosols: Geology, v. 33, p. 333–336.

SADLER, P.M., OSLEGER, D.A., AND MONTANEZ, I.P., 1993, On the labeling, length, andobjective basis of Fischer plots: Journal of Sedimentary Petrology, v. 63, p. 360–368.

SCHUMM, S.A., 1993, response to baselevel change: implications for sequencestratigraphy: Journal of Geology, v. 101, p. 279–294.

SCHWANS, P., 1995, Controls on sequence stacking and fluvial to shallow marinearchitecture, in Van Wagoner, J.C., and Bertram, G.T., eds., Sequence Stratigraphy ofForeland Basin Deposits; Outcrop and Subsurface Examples from the Cretaceous ofNorth America: American Association of Petroleum Geologists, Memoir 64, p.55–102.

SHANLEY, K.W., AND MCCABE, P.J., 1994, Perspectives on the sequence stratigraphy ofcontinental strata: American Association of Petroleum Geologists, Bulletin, v. 78, p.544–568.

SHELDON, N.D., AND RETALLACK, G.J., 2001, Equation for compaction of paleosols dueto burial: Geology, v. 29, p. 247–250.

SOIL SURVEY STAFF, 1999,Soil Taxonomy: A Basic System for Soil Classification forMaking and Interpreting Soil Surveys: Washington, D.C., U.S. Government PrintingOffice, 831 p.

SOIL SURVEY STAFF, online database,Official Soil Series Descriptions Natural ResourcesConservation Service, U.S. Department of Agriculture, URL: http://soils.usda.gov/technical/classification/osd/index.html. Accessed 10 January 2007.

STEEL, R., AND AASHEIM, S.M., 1978, Alluvial sand deposition in a rapidly subsidingbasin (Devonian, Norway), in Miall, A.D., ed., Fluvial Sedimentology: CanadianSociety of Petroleum Geologists, Memoir 5, p. 385–412.

STEEL, R., MAEHLE, S., NILSEN, H., ROE, S.L., AND SPINNANGR, A., 1977, Coarsening-upward cycles in the alluvium of Hornelen Basin (Devonian), Norway—Sedimentaryresponse to tectonic events: Geological Society of America, Bulletin, v. 88, p.1124–1134.

STEWART, J.H., POOLE, F.G., AND WILSON, R.F., 1972, Stratigraphy and origin of theChinle Formation and related upper Triassic strata in the Colorado Plateau region:U.S. Geological Survey, Professional Paper 690, 336 p.

TANNER, L.H., 2003a, Possible tectonic controls on Late Triassic sedimentation in theChinle Basin, Colorado Plateau, in Lucas, S.G., ed., Geology of the Zuni Plateau:New Mexico Geological Society, Guidebook, 54th Field Conference, p. 263–267.

TANNER, L.H., 2003b, Pedogenic evidence of Late Triassic Chinle Group, Four Cornersregion: evidence of Late Triassic aridification, in Lucas, S.G., ed., Geology of the ZuniPlateau: New Mexico Geological Society, Guidebook, 54th Field Conference, p.269–280.

THERRIEN, F., AND FASTOVSKY, D.E., 2000, Paleoenvironments of Early Theropods,Chinle Formation (Late Triassic), Petrified Forest National Park, Arizona: Palaios,v. 15, p. 194–211.

TORNQVIST, T.E., AND BRIDGE, J.S., 2002, Spatial variation of overbank aggradation rateand its influence on avulsion frequency: Sedimentology, v. 49, p. 891–905.

TORNQVIST, T.E., AND BRIDGE, J.S., 2006, Causes of river avulsion: insights from the lateHolocene avulsion history of the Mississippi River, U.S.A.—Discussion: Journal ofSedimentary Research, v. 76, 959 p.

VAN DER VOO, R., MAUK, F.J., AND FRENCH, R.B., 1976, Permian–Triassic continentalconfigurations and the origin of the Gulf of Mexico: Geology, v. 4, p. 177–180.

WRIGHT, V.P., AND MARRIOTT, S.B., 1993, The sequence stratigraphy of fluvialdepositional systems: the role of floodplain sediment storage: Sedimentary Geology,v. 86, p. 203–210.

ZIEGLER, A.M., SCOTESE, C.R., AND BARRETT, S.F., 1983, Mesozoic and Cenozoicpaleogeographic maps, in Brosche, P., and Sundermann, J., eds., Tidal Friction andthe Earth’s Rotation, II: Berlin, Springer-Verlag, p. 240–252.

ZEIGLER, K.E., LUCAS, S.C., AND HECKERT, A.B., 2002, A Phytosaur skull from theUpper Triassic Snyder Quarry (Petrified Forest Formation, Chinle Group) of northcentral New Mexico, in Heckert, A.B., and Lucas, S.C., eds., Upper TriassicStratigraphy and Paleontology: New Mexico Museum of Natural History, Bulletin 21,p. 171–178.

Received 8 November 2006; accepted 20 April 2007.


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Continental Sequence Stratigraphy of the Upper Triassic Chinle Strata, Northern New Mexico, USA - JSR, 2007