Depositional and microfaunal response to Pliocene climate change and tectonics in the eastern Gulf of Alaska

ELSEVIER Marine Micropaleontology 27 (1996) 121-140

Depositional and microfaunal response to Pliocene climate change and tectonics in the eastern Gulf of Alaska

Martin B. Lagoe, Sarah D. Zellers Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712, LISA

Received 1 October 1994; accepted 1 December 1994

Abstract

The Yakataga Formation provides an excellent record of Pliocene paleoclimate in the Gulf of Alaska and northeastern Pacific Ocean. Evidence for glaciomarine influence and paleoclimatic change includes the distribution of ice-rafted debris and diamictite. planktic and benthic foraminiferal biofacies, and variable sediment accumulation rates. Three paleoclimatically defined intervals within the Pliocene Yakataga Formation are recognized in onshore and offshore sections. Interval Pl (5.35 to 4.2 Ma) contains the first evidence of Neogene glaciomarine deposition in the form of ice rafted debris and rare diamictite. lnterval P2 (4.2 to 3.0-3.5 Ma) exhibits a reduction in glaciomarine influence and warming to cool temperate conditions. lnterval P3 (3.0-3.5 to I .8 Ma) contains massive amounts of glaciomarine diamictite and correlates with other evidence indicating the onset of major northern hemisphere glaciation. Sediment accumulation rate and foraminiferal based paleobathymetric curves show some correlation to the above paleoclimatic subdivision, but also significant independent relationships, indicating a strong tectonic influence on those parameters. Comparison of the Yakataga record to ODP sites in the North Pacific shows that there is good agreement in the ice-proximal (Yakataga) and deep-sea records of Pliocene paleoclimate change.

1. Introduction

Late Cenozoic paleoclimatic changes in the Gulf of

Alaska are reflected in the stratigraphy of the Miocene

to Pleistocene Yakataga Formation. This formation has

a maximum thickness of 7 km and is well exposed in the coastal mountains around the northeastern Gulf of

Alaska, as well as underlying the Yakataga continental margin (Fig. 1) . Recent work on these rocks has iden- tified a rich geological and paleontological record of

glaciomarine and normal marine environments (e.g.

Lagoe, 1978, 1983; Lagoe et al., 1989, 1993, 1994; Eyles et al., 1991; Eyles and Lagoe, 1990, 1994; Zell- ers, 1990). These rocks are of global significance because they represent the longest and most complete record of Late Cenozoic glaciation in the northern hem-

isphere (Plafker and Addicott, 1976: Armentrout.

1983; Eyles et al., 1991). High latitude environments are particularly sensitive

to changes in global temperature, but reconstruction of

northernmost Pacific paleoenvironments has been ham- pered by a sparse deep-sea drilling database (Kulm et

al., 1973; Creager et al., 1973), recently supplemented by Ocean Drilling Program Leg 145 ( Rea et al., 1993). A detailed paleoclimatic and stratigraphic analysis of the Yakataga Formation is useful in providing an ice- proximal continental margin record to compare to these

deep-sea records. In addition to its paleoclimatic significance, the Yak-

ataga Formation also reflects deposition on a tectoni- cally active continental margin, at the juncture of major NW-SE oriented strike slip-faults and the trend of the Aleutian Trench. Tectonic events have a profound

0377-8398/96/$15.00 Q 1996 Elsevier Science B.V. All rights reserved SSDIO377-8398(95)00055-O

122 M. B. Lugoe. S. D. Zellers /Marine Micropaleontology 27 (I 996) 121-140

I I

143”W AC 141” w

Middleton Island -.

‘. 59” N ‘.

%

Okm 50 km *...

I.. . -. . 1 Fig. 1. General location map for the northeastern Gulf of Alaska showing the outcrop and offshore subsurface sections of the YakatagaFormation mentioned in this study. Also shown is the distribution of multichannel reflection seismic data used to correlate offshore sequences. Onshore distribution of Yakataga Fm. is shown in black. Outcrop sections: YR=Yakataga Reef; KM=Kultieth Mountain; P=Peak 2170; AC= “Armentrout’s Channel”; L/M=Lawrence Creek/Munday Creek; LG=Lam Glacier; IB= Icy Bay. Offshore sections: 1 =Exxon OCS Y-0080 No, 1; 2=Exxon OCS Y-00.50 No. 1; 3=TexacoOCS Y-0046No. 1; 4=AmoGCS Y-0007 No. 1; S=ArcoOCS Y-0211 No. 1; The locations of additional exploration wells are shown as black dots. Dashed line indicates 200 m bathymetric contour. Seismic lines displayed in other figures are labeled. _

influence on the stratigraphy of the Yakataga Forma- tion, so it is necessary to try to separate climatic from tectonic effects. Our ability to do this will determine just how important the Gulf of Alaska record will be to reconstructing a detailed record of Pliocene paleoclimate for the far northeastern Pacific Ocean.

2. Objectives

The objective of this study is to investigate the influence of climate and tectonics on the stratigraphy of the Pliocene portion of Yakataga Formation. From this analysis a history of Pliocene paleoclimatic change and its timing in the northeast Pacific Ocean will be reconstructed.

Specific objectives include: A review of sedimentologic and microfaunal evidence for paleoclimate and tidewater glaciation during the Pliocene in the eastern Gulf of Alaska. Information from both onshore exposures and offshore subsurface well sections is used to reconstruct paleoclimatic history; A review of the chronostratigraphy of the Yakataga Formation and the timing of paleoclimatic events;

Documentation of sediment accumulation rates and relative sea level during the Pliocene and their rela- tionship to both paleoclimate and tectonic events; Comparison of the Yakataga Formation record to deep-sea records obtained from ODP Leg 145; and Evaluation of the relative roles of paleoclimate and tectonics in controlling stratigraphic patterns within the Yakataga Formation.

Data sets

3. I. Onshore sections

Seven outcrop sections provide most of the data for our synthesis of Pliocene paleoclimatic trends from the onshore area ofthe Yakataga District: Kultieth Moun- tain, Yakataga Reef, Peak 2170, Lawrence Creek/ Munday Peak, ‘ ‘Armentrout’s Channel”, Lare Glacier and Icy Bay (Fig. 1). Useful information on these sections is contained in Ariey (1978a, b), Armentrout (1983,1994), Eyleset al. (1991), Eyleset al. (1992), Lagoe ( 1978, 1983)) Lagoe et al. ( 1993, 1994)) Plaf- ker and Addicott ( 1976)) Rau et al. ( 1983), and Zellers ( 1990).

M.B. L.ugoe, S.D. Zellers/Marine Micropaleontology 27 (1996) 121-140 123

3.2. Offshore sections

Four petroleum exploration wells provide most of the data (Zellers, 1995; Zellers et al., 1992; Zellers and Lagoe, 1994) for our synthesis of Pliocene paleocli-

matic and paleobathymetric trends from the offshore Yakataga continental shelf. From east to west (Fig. 1) ,

these wells are: Exxon OCS Y-0080 No. 1, Exxon OCS

Y-0050 No. 1, Texaco OCS Y-0046 No. 1, and Arco OCS Y-0007 No. 1. In addition, lithostratigraphic and

biostratigraphic data from Turner et al. ( 1992) is used to calculate accumulation rates for the Arco OCS Y-

0211 No. 1 well (Fig. 1) . Biostratigraphic information

is integrated with multichannel reflection seismic data (Bruns and Schwab, 1983) to correlate the offshore wells and to establish sequence stratigraphic relation-

ships along the margin (Zellers, 1993, 1995; Zellers and Lagoe, 1994).

4. Methods

4. I. Evidence of paleoclimate/glacption _

The physical sedimentology of the Yakataga For- mation provides evidence of tidewater glaciation. Onshore sections were logged at a meter scale using

techniques described by Eyles et al. ( 1985). Primary evidence includes the presence of dropstones/ice-

rafted debris and the occurrence of diamictites attrib- uted to glaciomarine processes (see for example, Eyles

et al., 1991; Eyles and Lagoe, 1990). Foraminiferal faunas provide some evidence of

paleoclimate. Planktic foraminiferal faunas in the Yak-

ataga Formation are low diversity and generally dom-

inated by Neogloboquadrina pachyderma and Globigerina bulloides (Lagoe, 1983; Zellers, 1990;

Lagoe et al., 1993). Coiling ratios (right vs. left) for N. pachyderma have long been known to be paleoclimatically significant (Bandy, 1960) ; left-coiling forms representing subarctic surface water temperatures and right-coiling forms reflecting cool temperate water

temperatures. Ingle ( 1973, 1977a, 1977b) has related coiling shifts in N. pachydenna to northward and south- ward migration of subarctic and temperate surface waters associated with the Alaska Gyre and California Current. Entirely left-coiling populations of N. pachyderma are interpreted by Ingle (1973, 1977a, 1977b)

as representing surface water temperatures below 10°C.

Entirely right-coiling populations represent surface water temperatures of 15” to 25°C. Thompson and Shackleton ( 1980) and Thompson ( 1981). in studies of the distribution of left-and right-coiling N. pa&v-

dermu in the modem northwestern Pacific Ocean, cor- roborate the interpretations of Ingle ( 1973), Ingle

( 1977a), Ingle ( 1977b). These studies show that coiling variations in N. pachyderma can be related to the position of the polar front. North of the polar front, left-

coiling populations dominate, with seasonal ranges in surface water temperatures from 3” to 12°C. South of

the polar front, right-coiling populations dominate, with seasonal temperatureranges of 16”-26°C. The low diversity of assemblages containing right-coiling N.

pachyderma in the Yakataga Formation would argue for the lower end of the temperate temperature ranges

cited above. Benthic foraminifera, particularly shallow water

(neritic) faunas, also provide paleoclimatic data. Dis- tributional data for western North America (e.g., Cul-

ver and Buzas, 1985,1986) and the Arctic Ocean (e.g.,

Lagoe, 1979a, b, 1980) define temperature-significant

biogeographic provinces. For the Yakataga Formation the initial occurrence and subsequent predominance of arctic/subarctic faunas, dominated by Elphidium excavatum clavatum, Elphidium bartletti, and Buccella frigida, is particularly significant (Lagoe et al., 1993). In addition, benthic foraminiferal faunas are used 10

reconstruct paleobathymetry (e.g., Ingle, 1980). thus yielding a record of relative sea level for individual sections. Paleoenvironmental zones established from

analyses of modem foraminiferal data (Lagoe et al., 1989; Zellers, 1989) collected by Bergen and O’Neil

(1979) and Echols and Armentrout ( 1980) for the

Gulf of Alaska are shown in Fig. 2.

4.2. Chronostratigraphy

The Yakataga Formation contains subarctic to cool temperate microfaunas, generally of low diversity. Biostratigraphic subdivision of the formation is ham- pered somewhat by there being fewer biostratigraphic

events on which to base correlations than there are at low latitudes. The chronostratigraphic framework used

here (Fig. 3) follows that in Lagoe et al. ( 1993). It is based on planktic foraminiferal datums, paleoclimati- tally significant coiling shifts in Neogloboquadrina

124 MB. Lagoe, SD. Zellers /Marine Micropaleontology 27 (1996) 121-140

GULF OF ALASKA PALEOENVIRONMENTAL MODEL

INNER NERITIC (IN) - 10 TO 66 M OUTER NERITIC (ON) - 66 TO 150 M UPPER BATHYAL I (UB I) - 150 TO 300 M UPPER BATHYAL II (UB II) - 300 TO 500 M MIDDLE BATHYAL (MB) - 500 TO 1600 M LOWER BATHYAL (LB) - > 1600 M

NOT TO SCALE

1600m’ 7

Fig. 2. Paleoenvironmental model for the Gulf of Alaska based on studies of modem foraminiferal assemblages done by Bergen and O’Neil (1979) and Echols and Armentrout (1980).

pachydemza, a limited amount of paleomagnetic stra-

tigraphy near the base of the formation, and regional

stratigraphic correlation to key sections throughout the

far North Pacific Ocean (see Lagoe et al., 1993). Dia-

tom biostratigraphy from offshore sections (Anderson,

Warren and Associates, 1975; Larson, 1992) provides

some additional chronostratigraphic control in the

younger part of the Yakataga section but is not useful

in the Pliocene.

The base of the Yakataga Formation represents the

onset of tidewater glaciation in the northeast Pacific

Ocean. Dating the age of this event is critical to under-

standing the paleoclimatic evolution of this area. The

age of the basal Yakataga Formation has previously

been controversial.

Marincovich ( 1990) argues that molluscan biostra-

tigraphy places this onset of glacial activity in the early

middle Miocene. Lagoe et al. (1993) cite phtnktic

foraminiferal, paleomagnetic and regional paleocli-

matic evidence that this event is latest Miocene. The

latter interpretation is confirmed by drilling results from

ODP Leg 145 (Rea et al., 1993; Krissek, 1994)) which

are summarized below.

4.3. Sequence stratigraphy

Eight seismic stratigraphic sequences were defined in the offshore Yakataga Formation by tying foraminiferal biostratigraphic age and paleobathymetric inter-

pretations with multichannel seismic data (see Zellers

and Lagoe, 1994; Zellers, 1995). These sequences are: Brown (early Pliocene) ; Red (late Pliocene) ; Orange (late Pliocene) ; Yellow (early Pleistocene) ; Green

(middle Pleistocene) ; Aqua (late Pliocene) ; Blue (late

Pleistocene) ; and Violet (latest Pleistocene to Recent). The sequences are bounded by unconformities (Yak/ PC, R, 0, Y, G, A, B, V) marking truncation of older, often uplifted, strata. The three Pliocene sequences (Brown, Red, and Orange) will be related to the climatic history presented below.

4.4. Accumulation ratedbackstripping

Sediment accumulation rates reported here for outcrop sections represent bulk accumulation rates that are calculated using the present thickness of each section (uncorrected for compaction). For offshore subsurface sections, accumulation rates are calculated using orig-

M.B. Lagoe, S.D. Zellers/Marine Micropaleontology 27 (1996) 121-140 125

4

5

N. humerosa

N. kugaensis N. eggeri = N. dutertrei G. truncatulinoides N. asanoi G. injlata (modem)

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$ $ 0 -V G. nepenthes

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5 Pn (d .- aa

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E

P

i3 -A = First occurrence (FO) 7 = Last occurrence (LO)

CD 1-7

CD6

CD9

CD10

CD11

CD 12

CD 13

CD14

CD15

CD 16

CD = COILING DOMINANCE

m Zn9

ISSi c”;:

Fig. 3. Chronostratigraphic framework for the northeastern Pacific Ocean relating magnetostratigmphy, planktic foraminifeml evolutionary events, coiling curve for N. pachyderma, and coiling dominance zones (modified from Lagoc and Thompson, 1988).

inal sediment thicknesses (corrected for compaction). Original sediment thicknesses are determined by back-

stripping the sections using a modification of the pro-

gram developed by Bond and Kominz (1984).

Determination of backstripping units for each section

is based on age information, position and duration of

unconformities, and differences in lithology. Age con-

trol is entered for datable backstripping unit bounda-

ries; for other unit boundaries the backstripping

program interpolates the ages. Accumulation rates for corrected offshore sections are then calculated by divid-

ing the decompacted thickness of each backstripping unit by the duration of each unit. A more detailed description of the backstripping analyses done on the

offshore Yakataga petroleum exploration wells is pro- vided elsewhere (Zellers, 1995).

5. Pliocene paleoclimate history

Analysis of onshore and offshore sections shows that the Pliocene part of the Yakataga Formation can be

126 M.B. Lagoe, S.D. Zellers / Marine Micropaleontology 27 (1996) 121-140

,,<, c4 co co u5 ~ . . . . . . N P A C F IC I ~ o~ co ~- ~- - ~ G O A G . O . A . < - ,,, tu ,,, co O < O X Y G E N " " T E C T O N I C . &

I b U / U l ~ l = [ _ ~ l z z ~ ~ ~- tu ~_ ~ C L I M A T E D E P O S I T I O N A L ~ ___ ~ ~ ~ ~ --- u4, ~< R E C O R D

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Z 0 . O ~ oar _ , ~ OPTIMA I ~< -~ / ~ = O ~ ~ O - - ~ i ;~::':~ ~ BERINGIAN ~ I I ~ ~ - - T ~ ~0_ o~ ==~=

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I -J .~GLACIAL Pl - i O ~ to 4 a ~ ~ i : : : : : ] ;:::-::"~:' ~ : ' : : - " " I ,o R A T ~ ~< °

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16 " I I . . . I AONA' : ~ i i ~ ! i i $ ~ ! ! I J I " MIGRATION OF OFIGANIC RICH SHALE (EASTERN G.O.A)

18 ~ i~- : " : ' I ? . ~ . TEMPERATESPP. "~" -~ TO G.O.A. REGIONAL CHALK

DEPOSITION (DEEP SEA)

Fig. 4. Stratigraphic, depositional, and climatic framework for the Gulf of Alaska and adjacent regions. Lithologic columns are based on references discussed in Lagoe et al. (1993). Data for ODP Site 887 is from Reaet al. (1993). Lithologic symbols: stippled, fine-and coarse grained elastics; horizontal lines, diatomaceous sediments; brick pattern, chalk; solid triangles, ice-rafted debris; g = glauconite; blank, missing section due to unconformities; dark shaded pattern, intervals not represented by individual sections. Oxygen isotopic record is for the south- western, mid-latitude Pacific Ocean and is after Kennett (1986), Climatic optima CO1 to CO3 are after Barron and Baldauf ( 1990); MPW is from Lagoe et al. (1993); Climate intervals P1 to 1)3 are from this study. Gulf of Alaska (G.O.A.)/North Pacific climate events: Oldest Yakataga IRD and glacial intervals A and B are from Lagoe et al. (1993); oldest glacineritic diatoms, periods of upwelling, and early middle Miocene warm period based on diatom biofacies from Oreshkina (1986); increase in biosiliceous sedimentation based on Barron and Baldauf (1990); faunal migration of temperate species into the G.O.A. during the early Miocene based on Lagoe (1984). G.O.A. tectonic events based mainly on Lagoe (1983), Armentrout (1983), Eyles and Lagoe (1990), Eyles et al. (1991), and Lagoe et al. (1993). Modified from Lagoe et al. (1993).

subdivided into three main intervals, based on biostra-

t igraphy and pa leoc l imat ic indicators. These intervals

are informal ly g iven the fo l lowing designations: P1

(5.35 to 4.2 M a ) , P2 (4.2 to 3 .0-3 .5 M a ) and P3 ( 3 . 0 -

3.5 to 1.8 M a ) (Fig. 4) . Pa leoenvi ronmenta l , paleo-

bathymetr ic and pa leoc l imat ic interpretat ions o f these

three intervals are summar ized below.

5.1. Interval P l (5.35 to 4.2 Ma)

This interval actually spans the latest M i o c e n e to

earliest Pl iocene. It contains the earl iest indicat ions o f

t idewater glaciat ion in the North Pacific. Onshore sections used to character ize this interval include Yakataga

Reef , Kultieth Mountain , Peak 2170, Lawrence Creek,

M.B. hgoe, S.D. Zellers /Marine Micropaleontology 27 (19%) 121-140 127

a N. Paleo-

:: 2 pachydmna bathymetry

ifi p Planktic Foram. Coiling Cutve _ lenthic Forum. z Events

MAGNETOSTRATIGRAPHY

4 YAKATAGA REEF

Y A K A T A G A

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INCLINATION

No

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4.24

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4.57

4.77

5.35

Fig. 5. Foraminiferal events, paleobathymetry, magnetostratigraphy (inclination only shown), and generalized lithology for the uppermost Paul

Creek Formation and lowermost YakatagaFormationexposed at YakatagaReef. The position of climatic intervals Pl and P2 are shown. Planktic

foraminiferal events: A =FAD of Neogioboquadrina pachyderma; B= Globororafia cf. G. suterae; C= Gfoborotalia scitula. Benthic forami-

niferal events: 1 = LAD Anomalina glabrata. reduction in temperate fauna; 2 = FAD Elphidium excac~atum clauatum, initial appearance of

subarctic fauna. Paleobathymetric abbreviations are the same as those in Fig. 2. Interpreted magnetic polarity bar shows periods of normal

(black) and reversed (white) polarity. Ages of polarity transitions from Berggren et al. ( 1985). K/Ar date from Armentrout et al. ( 19’78).

Modilied from Lagoe et al. ( 1993).

and Lare Glacier (Fig. 1) . The best studied section for beds of graded sandstone and mudstone, interpreted as

this interval is at Yakataga Reef (Fig. 5). The Poul turbidites (Eyles et al., 199 1; Lagoe et al., 1993). Ice-

Creek Formation/Yakataga Formation contact is rafted debris in the lowermost Yakataga Formation

defined by the highest occurrence of glauconite in the occurs scattered in alternating sandstones and mud-

Poul Creek Formation and the lowest occurrence of stones, also interpreted as turbidites. Clasts up to 20 cm

ice-rafted debris in the Yakataga Formation (Fig. 6). in diameter are noted. In addition, several clast-rich

The uppermost Poul Creek Formation consists of nor- beds associated with bioturbated muddy sandstone

mal marine, glauconitic sandstones and alternating seem to represent periods of reduced turbidite accu-

128 M.B. L.ugoe, S.D. Zellers/Marine Micropaleontology 27 (1996) 121-140

Fig. 6. Dropstone and diamictite cluster deforming laminated turbidite bed from the upper part of the Yakataga Reef section.

mulation but continued ice-rafted debris (Lagoe et al.,

1993). The most distinctive evidence of a tidewater glacial influence is a 22 m thick diamictite occurring

80 m above the Poul Creek/Yakatagacontact (Fig. 5). The uppermost part of the Yakataga Reef section consists of alternating sandstones and mudstones, inter-

preted as turbidites, with little evidence of a

glaciomarine influence. Foraminiferal faunas within the lowermost Yakataga

Formation (Lagoe, 1983; Lagoe et al., 1993) consist

of subarctic, low diversity planktic assemblages dominated by left-coiling populations of Neogloboquad- rina pachyderma and mainly outer neritic to upper

bathyal benthic assemblages that contain the earliest occurrences of subarctic species (e.g., Elphidium exca- uatum clavatum, Buccella frigida). These subarctic

elements become more predominant upsection. In the

upper part of the Yakataga Reef section the planktic assemblages are higher diversity, containing right-coiling N. pachyderma, Globorotalia scitula and Globor- otalia cf. G. suterae (Fig. 5). This would indicate a warming to cool temperate surface water temperatures and coincides with the diminution in the amount of ice- rafted debris within the section.

The timing of events within this interval is based on foraminiferal biostratigraphy, paleomagnetic stratig-

raphy and radiometric dates on glauconites. The left- coiling N. pachyderma in the lowermost Yakataga

Formation, and uppermost Poul Creek Formation as

well, indicate an age no older than late Miocene (Lagoe et al., 1993). The shift to right-coiling faunas in the

upper part of the section is correlated to the CD15/ CD16 shift of Lagoe and Thompson (1988), which is assigned at age of 4.2 Ma based on correlation to paleomagnetic stratigraphy. Glauconites in the uppermost

Poul Creek Formation yield K/Ar dates of 6.4 _+ 0.4

and 5.6 4 0.5 Ma (Armentrout, 1983). Paleomagnetic

stratigraphy of the lowermost Yakataga Formation (Lagoe et al., 1993) indicates that the section falls

within the Gilbert polarity chron (Fig. 5) consistent

with the foraminiferal biostratigraphy and glauconite

dates. The evidence described above is corroborated by ,less

detailed analyses from the Kultieth Mountain, Peak 2170, Lawrence Creek and Lare Glacier sections (Lagoe, 1983; Lagoe et al., 1993). Interval Pl is characterized by initiation of tidewater glaciation in the northeastern Pacific Ocean, subarctic ( < 10°C) surface water temperatures and cool, neritic bottom waters, which show evidence of further cooling throughout the interval. The paleobathymetry of the Pl interval in the onshore sections studied is dominantly

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130 MB. hgoe, S.D. Zellers /Marine Micropaleontology 27 (1996) 121-140

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M.B. Lagoe, S.D. Zellers / Marine Micropaleontology 27 (1996) 121-140 131

TEXACO OCS

Y-0046 No. 1

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13-early 14000 ~ 3rown -4 s i ~ I 4500 i . . . . . . . . . I /

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Fig. 9. Summary of data and interpretations for the Texaco OCS Y-0046 No. 1 well: (A) spindle diagram for the well showing lithology and relative abundance of planktic foraminifera and key benthic foraminifera. Benthic foraminifera are arranged in order of paleoenvironmental significance from shallow (left) to deep (right); (B) summary of age, paleobathymetric, paleoclimatic, sequence, and seismic stratigraphic interpretations. Key to abbreviations given in caption for Fig. 7. See Fig. 3 for coiling zones.

132 M.B. Lagoe, SD. 2eller.s /Marine Micropaleontology 27 (1996) 121-140

Fig. 10. Photo of core section from the Arco OCS Y-0007 No. 1 well in the interval from 1938 to 1941 m (6360 to 6371 ft) showing mudstone with minor amounts of ice-rafted debris.

outer neritic (70-150 m) to upper bathyal (150-500 m), with a few intervals deposited at inner neritic (O-

70 m) water depths (Fig. 5). The Poul Creek/Yakataga boundary is not pene-

trated in many offshore well sections in the Yakataga District. Where the boundary is present (e.g., Arco OCS Y-0007 No. 1, Exxon OCS 0080 No. l), the lowermost Yakataga Formation offshore is younger than it is onshore (Fig. 7). It does not appear that the PI interval is present offshore in the Yakataga District.

5.2. Interval P2 (4.2 to 3.0-3.5 Ma)

The lower boundary of this interval is marked by the left to right coiling shift recorded in the Yakataga Reef

section (Fig. 5 and Fig. 8). This represents a change from subarctic to cool temperate surface waters and

was coincident with a great reduction in the amount of ice-rafted debris in the Yakataga Formation. Onshore

sections characterizing this interval include Yakataga

Reef, Lawrence CreeklMunday Peak and Icy Bay (Fig. 1) . Offshore, the P2 interval corresponds to the Brown sequence, which is recognized in all four of the

exploration wells (e.g., Fig. 7 and Fig. 9). Rocks within this interval onshore consist of alter-

nating sandstones and mudstones, interpreted as turbidites, and massive to cross stratified/hummocky cross

stratified sandstones, probably representing shelf deposition (Armentrout, 1983; Eyles et al., 1991)

Fig. Il. Photo of core section from the Arco OCS Y-0007 No. I well in the interval from 1481 to 1485 m (4861 to 4873 ft) showing a massive glaciomarine diamictite.

M.B. Lagoe, S.D. Zellers/Marine Micropaleontology 27 (19%) 121-140 133

A) LINE 452 Exxon Exxon OCS Y-0080 No. 1 OCS Y-0050 No. 1

10km w - + Q E

6) LINE 409

NW 0

1 z2

z3 4

5

5 km

Texaco OCS Y-0046 No. 1

+ SE

C) LINE 406 Arco OCS Y-0007 No. 1

4 5km

SE I - A _ \_ \I ~~~~~

1 ‘G .Y

Fig. 12. Line tracings of selected multichannel seismic lines showing relationships of sequences and the unconformities that define them: (A)

strike line 452 showing the Exxon OCS Y-0050 No. 1 and Exxon OCS Y-0080 No. 1 wells; (B) central dip line showing the Texaco OCS Y-

0046 No. 1 well; and (E) eastern dip line across the Arco OCS Y-0007 No. 1 well. Thinning of seismic sequences at the well locations illustrates

that thrusting, which deformed the lower part of the offshore Yakataga Formation, began in the late Pliocene (late P3). Vertical scale is two-

way travel time in seconds.

134 M.B. Lagoe, S.D. Zellers/Marine Micropaleontology 27 (1996) 121-140

(Fig. 8). In general, ice-rafted debris is rare, though

scattered dropstones are found. Mud logs and wireline

logs indicate similar lithologies offshore. This interval

is overlain by a thick (at least 2 km in onshore sections)

succession dominated by glaciomarine diamictite

(Fig. 8).

Foraminiferal faunas in this interval onshore include

cool temperate planktic assemblages dominated by

Globigerina bulloides with subordinate right-coiling

Neogloboquadrinapachyderma, Globigerina quinque- loba, Globorotalia cf. G. suterae, Globorotalia scitula and Globorotalia puncticulata (Fig. 8). Planktic

foraminiferal assemblages offshore are similar, but

they also contain Neogloboquadrina asanoi and more

abundant right-coiling N. pachyderma. Benthic faunas

onshore range from inner neritic to upper bathyal

assemblages but still contain only rare subarctic spe-

cies. Offshore, benthic faunas indicate slightly deeper environments ranging from outer neritic through the

lower portion of the upper bathyal zone (UB II, Fig. 2).

Elphidium excavatum clavatum is more common in

offshore than in onshore sections.

Although cold bottom waters are indicated by off-

shore assemblages, lithologic evidence (little ice-rafted

material) and planktic foraminiferal data suggest

warmer conditions. This evidence for cool temperate

conditions and reduced glaciomarine influence led

Lagoe et al. ( 1993) to call this interval the mid-Plio-

cene warm (MPW) interval.

The chronostratigraphy of this interval includes the

CD15/CD16 coiling shift (4.2 Ma) of Lagoe and

Thompson ( 1988) at its base and the influx of massive

amounts of glaciomarine diamictite at its top. Lagoe et

al. ( 1993) correlated this return of glaciomarine influ-

ence to several DSDP sections in the far North Pacific,

which indicated an age of 3.0-3.5 Ma for this event.

The base of this interval is not present in any of the

offshore wells examined. Offshore, the top of this inter-

val is marked by an unconformity that is dated at

approximately 3.5 Ma (Zellers, 1995).

In summary, interval P2 is characterized by a reduc-

tion in glaciomarine deposition and cool temperate surface water temperatures. Paleobathymetry varied between inner neritic and upper bathyal. Several paleobathymetric cycles occur within this interval, ranging from upper bathyal mudstones and turbidite sandstones

to inner neritic sandstones which may be massive, bio-

turbated or hummocky cross stratified (Armentrout, 1983; Eyles et al., 1991).

5.3. Interval P3 (3.0-3.5 to 1.8 Ma)

This interval is very thick onshore, consisting mainly

of massive to stratified, glaciomarine diamictite and interbedded mudstones and sandstones, interpreted as turbidites (Armentrout, 1983; Eyles et al., 199 1) . Off- shore sequences contain similar lithologies. Conven-

tional cores, mudlogs, and wireline logs indicate an increase in the amount of coarse material up section. A core from the Arco OCS Y-0007 No. 1 well from 1938 to 1941 m (6360 to 6371 ft) shows a mudstone with

some ice-rafted debris (dropstones up to 1 cm) (Fig. 10). Core from the same well in the interval from

1481 to 1485 m (4861 to 4873 ft), illustrates the mas-

sive diamictites present in the offshore sections

(Fig. 11) . The dominance of glaciomarine facies in the interval has been correlated to the onset of major northern hemisphere glaciation (Lagoe et al., 1993).

Another feature of this interval is the occurrence of

megachannels with up to 500 m of relief ( Armentrout, 1983, 1994; Eyles et al., 1991; Lagoe et al., 1994).

These have been variously interpreted as fjords

( Armentrout, 1983) and glacially influenced sea val-

leys (Lagoe et al., 1994). A dramatic change in depo-

sitional style is apparent from the underlying interval

P2. Planktic foraminiferal faunas in this interval onshore

are scattered and often rare. They are dominated by

Globigerina bulloides and Neogloboquadrina pachy- demur. N.pachydetma exhibits mixed coiling (Zellers,

1990; Lagoe et al., 1994). Neogloboquadrina asanoi is also found in this interval. Planktic foraminiferal faunas in offshore sections within this interval are more

abundant and contain similar taxa, however, right-coil-

ing N. pachyderma are more common. Benthic fora-

minifera in onshore sections range from inner neritic to upper bathyal assemblages, differing from earlier assemblages in that subarctic species (e.g., Elphidium excavatum clavatum) are a predominant element. Off- shore, benthic faunas indicate somewhat deeper paleoenvironments which range from mainly outer neritic to upper bathyal (UB II). Subarctic species are also common to abundant in the offshore sections. The widespread abundance E. excavatum clavatum is coin-

M.B. Lagoe, S.D. Zellers/Marine Micropaleontology 27(1996) 121-140 135

cident with the return of abundant glaciomarine facies in the section.

Onshore the base of this interval is marked by the return of common ice-rafted debris and glaciomarine diamictites. The upper limits of the interval are not well constrained onshore, where planktic foraminiferal faunas are absent or rare in the uppermost Yakataga For- mation examined to date. Offshore sections contain more abundant planktic foraminiferal faunas and the top of the interval is approximately marked by the last appearance datum (LAD) of Neogloboquadrina asa-

noi, which was assigned an age of 1.85 Ma by Lagoe and Thompson ( 1988). In places the top of interval P3 is thinned or missing due to erosion at the tops of syndepositionally formed anticlines in the center of the study area (Fig. 7 and Fig. 9 and Fig. 12). These anticlines were formed by a series of NE&SW trending thrust faults that developed during the late Pliocene (Bruns and Schwab, 1983; Zellers, 1993; Zellers and Lagoe, 1994).

Because the upper limits of interval P3 are difficult to constrain onshore, it is not possible to calculate accumulation rates for this interval from onshore sections. The types of sediment dominating this interval, thick diamictites and interbedded turbidites, would suggest an increase in accumulation rates over the Pl and P2 intervals. Strata from interval P3, which are best pre- served in the offshore sections, show two distinct trends in sediment accumulation rates (Fig. 13). In the early part of interval P3 (3.5 to 2.6 Ma), accumulation rates were much higher than in interval P2, with rates exceeding 6000 m/Myr and averaging about 4000 m/ Myr (Fig. 13). In the later part of P3 (2.6 to 1.6 Ma), accumulation rates are much lower, with rates averaging less than 2000 m/Myr (Fig. 13). Possible controls on accumulation rates of the Yakataga Formation were presented by Zellers (1993) and are further examined below.

The P3 interval is characterized by abundant evidence for glaciomarine activity marked by an increase in the amount of diamictites and ice-rafted debris. Sur- face water temperatures probably remained cool, while benthic foraminifera indicate subarctic conditions. Paleobathymetry was predominantly upper bathyal, with a few excursions into deeper (UB II) and shallower (inner to outer neritic) water depths.

7. Pliocene relative sea level

6. Pliocene accumulation rates

Calculations of sediment accumulation rates in the onshore and offshore sections show significant changes in deposition from the latest Miocene through the Pli- ocene. Accumulation rates in the uppermost part of the Poul Creek Formation (Fig. 5) average 12.5 m/Myr (Zellers, 1993). Average sediment accumulation rates within interval Pl, calculated from the Yakataga Reef section, are 175 m/Myr, showing a 10 fold increase in accumulation rates across the Yakataga/Poul Creek Boundary.

Benthic foraminiferal assemblages from both the onshore and offshore sections indicate that relative sea level along the Yakataga margin during the Pliocene fluctuated from inner neritic to upper bathyal environments. Fig. 14 provides a summary of relative sea-level changes recorded in the offshore sections. Pliocene relative sea level in the present offshore area is generally shallower than during the late Miocene (Lattanzi, 1981) and during the Pleistocene (Fig. 14) when middle bathyal conditions were dominant. Just above the Yakataga/Poul Creek boundary onshore, there is a small relative sea-level fall (Fig. 5). Offshore, lower Pliocene rocks of the Yakataga representing neritic through upper bathyal environments overlie Oligocene (Fig. 7) to Miocene Poul Creek rocks deposited in a middle bathyal environment. The magnitude of relative sea-level change during Pliocene averaged less than 100 m and did not exceed 400 m. Pleistocene relative sea-level changes along the Yakataga margin were higher in magnitude, exceeding 400 m.

During interval P2, accumulation rates onshore In general, the magnitude of change in paleobathy- exceed 300 m/Myr (Fig. 8) Accumulation rates in off- metry (i.e., fluctuating between neritic and upper bath- shore sections are much greater, with values ranging yal paleoenvironments) is the same for each of the three up to 3000 m/Myr (Fig. 13). Accumulation rates in climatic intervals (Pl to P3) (Fig. 14). However the offshore wells during interval P2 vary from 100’s of frequency of relative sea-level changes varies across m/Myr to 3000 m/Myr, and average 1800 m/Myr. the margin. In some areas (e.g., at Arco OCS Y-0007

136 M.B. Lagoe. S.D. Zellers/Marine Micropaleontology 27 (1996) 121-140

10000

8000

8000 E”

4000 -z

2000

l-3 6 5 4 2 1 0”

Arco OCS Y-0007 No. 1 10000

6000

6000 E”

4000 2

2000

0 6 5 4 2 1 0

Arco OCS Y-021 1 No. 1 T 10000 All wells r10000

8000

6000 E”

4000 2

2000

0 6 5 4 2 1 0‘

Exxon OCS

Not Drilled

6 5

10000

8000

6000 E”

4000 2

2000

0

Texaco OCS

Not Drilled

Y-0046 No. 1

8 5 4 lGE3(ml)

2 1

Pl P2 P3 -4

6 5 4 2 1 0‘

FiE. 13. Sediment accumulation rates calculated for offshore Yakataga sections. See text for discussion of accumulation rate calculations. Unconformities are shown with diagonal lines.

No. 1 well, Figs. 7 and 14), numerous ( > 20) changes in relative sea level are recorded for the Pliocene, whereas in other areas, such as Yakataga Reef and Icy

Bay (Fig. 8) and the central part of the study area

offshore (Fig. 14), changes in relative sea level are

fewer (ca. 10). There are even fewer fluctuations in

relative sea level in the offshore region area east of the Yakataga margin as recorded by the Arco OCS Y-02 11 No. 1 well (Fig. 14).

Not only does the frequency of relative sea-level

change across the area, but the timing of sea-level fluctuations can vary from section to section (Fig. 14).

This is most evident in the Pleistocene part of the section, but is also apparent in the Pliocene. Offshore sections in particular often have distinctively different paleobathymetric histories (Fig. 14).

8. Yakataga Formation and ODP Leg 145 records

Recent drilling by the Ocean Drilling Program (ODP) in the North Pacific has produced important reference sections for studying late Cenozoic paleo-

ceanography and paleoclimate (Rea et al., 1993). In particular, ODP Site 887, drilled in the eastern part of the Patton-Murray Seamount group, Gulf of Alaska, provides the opportunity to compare the Yakataga continental margin record with an adjacent, carbonate and diatom-bearing, deep-sea record. Evidence relating to periods of glaciomarine activity includes the occurrence of ice-rafted debris and sediment accumulation rates. Rea et al. ( 1993) report dropstones as deep as 130 m in Hole 887C, corresponding to an age of about 4.6 Ma (using depth vs. age plots to interpolate age).

M.B. Lagoe, SD. Zellers/ Marine Micropaleontology 27 (1996) 121-140 137

PAL .EOBATHYMETRY(M)

I NO Samples I

NO

Samples I

Exxon Exxon Texaco Arco Arco OCS Y-0080 No. 1 OCS Y-0050 No. 1 OCS Y-0046 No. 1 OCS Y-0007 No. 1 OCS Y-021 1 No. 1

Fig. 14. Late Miocene through Pleistocene paleobathymetric history of offshore petroleum exploration wells. Unconformities are shown by

wavy lines or diagonal striped pattern. Paleobathymetric interpretation for the Arco OCS Y-021 1 No. 1 is from Larson ( 1992).

The oldest abundant dropstones are found at 90 m

(below sea floor) and correspond to an age of approximately 3.2 Ma. Rea et al. ( 1993) summarize sediment accumulation rate data using different age intervals than this study. The interval 5.7 to 11.4 Ma has a sed-

iment accumulation rate of 11.7 m/Myr, with diatoms the dominant component. The interval 2.6 to 5.7 Ma exhibits a rate of 26.3 m/Myr with diatoms again the predominant component. The interval from 1.05 to 2.6

Ma has a sediment accumulation rate of 20 m/Myr with clay the predominant component.

Krissek ( 1994) provides an additional analysis of the ice-rafting record from ODP Site 887. Sporadic ice- rafted debris is reported at this site from 5.5 to 2.6 Ma. At 2.6 Ma a marked increase in ice-rafted debris is

noted both at Site 887 and at sites in the northwestern Pacific Ocean (ODP Sites 881 and 883).

These deep-sea records match the Yakataga record well. Initiation of glaciomarine influence is noted in the latest Miocene to earliest Pliocene, with a marked increase in glaciomarine activity in the late Pliocene.

9. Discussion: tectonics and climate change

Yakataga depositional sequences that formed during

the three intervals discussed above are the products of

the interplay of the following interrelated processes: tectonics (basement uplift and subsidence, lateral movements), eustatic sea-level changes, climate

change, glacioisostatic responses, glaciomarine and normal marine sedimentation. Characteristics of the three intervals (Pl-P3), including accumulation rate

patterns and relative sea-level changes, can be described in the context of some of the above processes.

The Yakataga/Poul Creek boundary marks a change

in depositional style related to both local tectonic processes and regional climate change. The following

changes occur across this boundary: ( 1) decrease in sea surface temperatures (Figs. 5 and 8); (2) initial appearance of subarctic/arctic benthic foraminifera (Figs. 5 arid 8) ; (3) cessation of glauconite formation (Fig. 5) ; ten-fold increase in accumulation rates; (4) decrease (as much as 400 m) in relative sea level, and

138 M.B. Lagoe, SD. Zellers /Marine Micropaleontology 27 (1996) 121-140

(5) oldest appearance of ice-rafted debris throughout

the Gulf of Alaska, including ODP Site 887. Interval

Pl is characterized by a change in depositional style

and the initial development of tidewater glaciation in

this area; these changes are due to both continued uplift of the Alaskan coastal ranges and regional cooling

(Lagoe et al., 1993).

Interval P2 contains evidence for regional warming

and the reduction of glaciomarine activity in the area.

The paleobathymetric shifts recorded in this interval are within the range of eustatic sea-level change and

may have a large eustatic component. A dominant eus-

tatic component would yield correlative paleobathy-

metric shifts throughout the margin. However, the

difficulty of correlating many paleobathymetric shifts

between sections, particularly offshore, implies a significant tectonic component to the variations. The large increase in accumulation rates from Pl to P2 is a func-

tion of increased sediment supply probably due to both

continued uplift and erosion of the coastal ranges and cool temperate conditions.

The large increase in accumulation rates from interval P2 to P3 and the increase in the amount ice-rafted debris offshore and increase in glaciomarine diamicti-

tes onshore reflect the development of major tidewater glaciers in the northeastern Gulf of Alaska. The amount

of glaciomarine material and the very high sediment

accumulation rates indicate that these glaciers were more extensive than those in Pl . The development of

these tidewater glaciers is coincident with the onset of major northern hemisphere glaciation (Lagoe et al.,

1993; Krissek, 1994). The reduction in accumulation rates (Fig. 13)

recorded by upper P3 sediments is mainly due to structural deformation of the offshore sections (Fig. 12). A

series of NE-SW trending thrust faults formed along the margin during the late Pliocene (Bruns and Schwab, 1983; Zellers and Lagoe, 1994; Zellers, 1995)

which syndepositionally deformed upper Pliocene sed-

iments. Thinning and absence of the Orange sequence (e.g., Figs. 9 and 12) is due to either non-deposition or erosion on the tops of anticlines that formed during this deformation.

Examination of the paleobathymetric histories of the offshore sections shows that there is much variability in the timing, magnitude and frequency of relative sea- level changes among the wells. These differences in relative sea level are primarily a function of differential

uplift and subsidence along the margin (Zellers, 1993, 1995; Zellers and Lagoe, 1994). Variations in sediment

supply due to shifting of sediment sources (e.g., posi-

tions of rivers, dominant glacial streams, etc.) may also affect loading subsidence and resultant paleobathymetry (Zellers, 1995).

10. Concluding remarks

The best indications of glaciomarine history on the

Yakataga continental margin are dropstones and dis-

tinctive glaciomarine diamictites. The occurrence of

these lithologies during both subarctic and cool temperate surface water intervals shows that tidewater gla-

ciation has occurred within a range of climatic conditions. The occurrence of tidewater glacier systems during both climatically different conditions (subarctic vs. cool temperate) suggests that the glaciomarine rec-

ord is somewhat insensitive to some paleoclimatic changes, at least in surface water temperatures. The

growth of glaciers during Yakataga time is a function of both regional climate change and responses to

changes in topographic relief and local climate changes

affecting the alpine glacial systems that fed the tide-

water ice margins. Accumulation rates and relative sea level are

undoubtedly sensitive to regional paleoclimatic changes and global eustasy. The accumulation rate record on the Yakataga continental margin shows some

correlation to paleoclimatic indicators (abundance of glaciomarine indicators). Rates increase across the

Yakataga Formation/Paul Creek Formation boundary and again at the base of interval P3. However, other trends, like the increase at the base of P2 and the

decrease in the upper part of P3 offshore, can only be explained as tectonic in nature.

In general, the history of relative sea level on the

Yakataga margin exhibits a strong tectonic influence.

This is apparent both in the variability of paleobathymetric fluctuations and in the difficulty of correlating individual shifts, especially in the offshore area. This suggests that paleobathymetric history is closely tied to the tectonic history of individual structural elements.

The history of glaciomarine activity in the Yakataga Formation is consistent with regional paleoclimatic trends in the far North Pacific (Lagoe et al., 1993; Rea et al., 1993; Krissek, 1994). Initiation of glaciomarine

M.B. Lagoe, S.D. Zellers/Marine Micropaleontology 27 (1996) 121-140 139

conditions occurs in the latest Miocene and extends into the early Pliocene. A reduction in glaciomarine influence is noted in the middle part of the Pliocene, along with indications of a modest climatic warming ( “mid-Pliocene warm interval” of Lagoe et al., 1993). Massive glaciomarine deposition marks the onset of major northern hemisphere ice sheets during the late Pliocene.

Barron, J.A. and Baldauf, J.G., 1990. Development of biosiliceous sedimentation in the North Pacific during the Miocene and early Pliocene. In: R. Tsuchi (Editor), Pacific Neogene Events. Univ. Tokyo Press, Tokyo, pp. 63-64.

Bergen, F.W. and G’Neil, P., 1979. Distribution of Holocene foraminifera in the Gulf of Alaska. J. Paleontol., S3: 1267-l 292.

Bond, G.C. and Kominz, M.A., 1984. Construction of tectonic sub-

sidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for subsidence mech- anisms, age of breakup and crustal thinning. Geol. Sot. Am

Bull., 95: 155-173.

Acknowledgements

This work has been partly funded by National Sci- ence Foundation Grants EAR8720823 and EAR9017680 to M.B. Lagoe. We are happy to acknowledge our collaborators on many Yakataga studies cited here, Nicholas Eyles and Carolyn Eyles. We thank Jim Ingle and Peter Thompson for helpful reviews of this paper. Partial support for this paper was obtained from the Owen Coates fund, Geology Foun- dation, University of Texas at Austin.

Bruns, T.R. and Schwab, W.C., 1983. Structure maps and seismic stratigraphy of the Yakataga segment of the continental margin. northern Gulf of Alaska. U.S. Geol. Surv. Misc. Field Stud. Map MF-1424,4 sheets and 20 pp., scale: 1:250.000.

Creager, J.S.. Scholl, D.W. et al., 1973. Init. Rep. DSDP, Leg 19. 913 pp.

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Documents

Depositional and microfaunal response to Pliocene climate change and tectonics in the eastern Gulf of Alaska