seismic stratigraphic correlations between odp sites 742 and 1166

Cooper, A.K., O’Brien, P.E., and Richter, C. (Eds.)Proceedings of the Ocean Drilling Program, Scientific Results Volume 188

8. SEISMIC STRATIGRAPHIC CORRELATIONS BETWEEN ODP SITES 742 AND 1166: IMPLICATIONS FOR DEPOSITIONAL PALEOENVIRONMENTS IN PRYDZ BAY, ANTARCTICA1

Tzvetina Erohina,2 Alan Cooper,2 David Handwerger,3 and Robert Dunbar2

ABSTRACT

New high-resolution seismic reflection data recorded between OceanDrilling Program Sites 1166 and 742 are interpreted to link acoustic fea-tures to lithologic units at the two drill sites. New findings include: (1)Site 1166 drilled a deeper (older) section than Site 742; (2) Paleogeneunits mostly do not extend between the two sites, except the deformedsand unit (Units III [1166] and VI [742]); (3) the preglacial to glacial un-conformity sampled at Site 1166 lies ~50 m below Site 742; (4) thePaleogene flooding surface at Site 1166 lies on top of Unit III, notwithin, as previously reported; and (5) Pliocene diatomaceous horizonscorrelated between the sites based on downhole logging cannot betraced between the two sites in seismic data.

For the study region, we infer a progression from a preglacial settingon a low-relief alluvial plain to glaciomarine and subglacial settings.Late Cretaceous alluvial plain and lagoonal environments evolved to alate Eocene broad fluvial channel system or outwash plain. Marinetransgression infilled and buried the channel system with glacial depos-its that were extensively eroded during the Oligocene to late Miocene.Late Neogene environments were mostly subglacial with episodes of re-duced ice and biogenic deposition.

1Erohina, T., Cooper, A., Handwerger, D., and Dunbar, R., 2004. Seismic stratigraphic correlations between ODP Sites 742 and 1166: implications for depositional paleoenvironments in Prydz Bay, Antarctica. In Cooper, A.K., O’Brien, P.E., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 188, 1–21 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/VOLUME/CHAPTERS/011.PDF>. [Cited YYYY-MM-DD]2Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Room 118, Stanford CA 94305, USA. Correspondence author: [email protected] of Geology and Geophysics, University of Utah, 135 South 1460 East, Room 719, Salt Lake City UT 84105, USA.

Initial receipt: 12 August 2002Acceptance: 15 August 2003Web publication: 12 February 2004Ms 188SR-011

mailto:[email protected]

T. EROHINA ET AL.SEISMIC STRATIGRAPHIC CORRELATIONS 2

INTRODUCTION

Prydz Bay lies at the mouth of the Amery Ice Shelf–Lambert GlacierSystem and drains ~20% of the East Antarctic Ice Sheet (Fig. F1). Thecontinental shelf there contains a record of early Cenozoic and lateNeogene glaciation. Ocean Drilling Program (ODP) Legs 119 and 188drilled at five sites on the Prydz Bay continental shelf to study the prox-imal Cenozoic record of Antarctic glaciation (Barron, Larsen, et al.,1989, 1991; O’Brien, Cooper, Richter, et al., 2001) (Fig. F1). The Leg 119drill sites (Sites 739–742) lie along a cross-shelf transect that was tra-versed by a composite seismic reflection profile recorded to assist corre-lation of geologic sections at the drill sites (Fig. F2) (Cooper et al.,1991a). Site 1166, drilled during Leg 188, was sited ~40 km from the Leg119 transect to sample an older Cenozoic section, but was not locatedon a seismic line that could be directly tied to the Leg 119 sites. Theonly correlation possible between Sites 742 and 1166 at the time ofdrilling was for the late Neogene section and was based on similar gla-cial lithologies and similar shapes in resistivity and velocity profilesfrom downhole logging (Shipboard Scientific Party, 2001).

This paper presents a new high-resolution seismic reflection profile,Palmer line 01-1-04, which crosses Sites 1166 (Leg 188) and 742 (Leg119), to correlate stratigraphic units between the drill sites. The profileis shown at both page size and foldout size (Fig. F3). We use the down-hole logging information and seismic-source signatures to create syn-thetic seismic traces at the two drill sites. The synthetic traces are usedto link the drill core information to the new seismic profile and therebyto regional geologic models. Our new integrated seismic and drilling re-sults more clearly define regional subsurface geometries of acousticunits near and between the drill sites, and establish a tie between theseismic and lithostratigraphic units at the drill sites.

REGIONAL SETTING

Sites 742 and 1166 were drilled within the northeast-southwest–trending Prydz Bay Basin that lies at the oceanward end of the LambertGraben, which is filled with >5 km of late Paleozoic to Quaternary sedi-ments (Stagg, 1985; Cooper et al., 1991a). In addition to drill cores,downhole logging measurements that included velocity, density, resis-tivity, porosity, natural gamma radiation, and others were recovered atSites 742 and 1166 to determine and infer rock lithologies (Tables T1,T2). A diverse suite of facies characterizing depositional environmentsthat include preglacial alluvial plain and lagoonal, early glacial fluvialoutwash plain and shallow marine, and glacial marine and subglacialwere drilled at the two sites (Barron, Larsen, et al., 1989, 1991; Barron etal., 1991; Shipboard Scientific Party, 2001).

The seismic stratigraphy of the Prydz Bay region was first defined byStagg (1985) and was later modified by Cooper et al. (1991a), based onhigher-resolution seismic data and drilling (Fig. F2). Cooper et al.(1991a) defined seven acoustic units, five preglacial and two glacialunits. Units PS.5 and 6 are Precambrian metamorphic and intrusivebasement rocks. Units PS.3 and PS.4 represent nonmarine, alluvial Me-sozoic rift sediments. Unit PS.2B is a nonmarine alluvial deposit of Cre-taceous age, the top of which was not sampled at Site 742 and wasinferred to be the preglacial to glacial unconformity (Cooper et al.,1991a). Units PS.2A and PS.1 are glacial deposits separated by a regional

1165

Prydz Trough-MouthFan

70°E1167

743

739742

1166

740

741

Four LadiesBank

PrydzChannel

Continental shelf break

Prydz Bay

FramBank

AmeryIce Shelf

Lambert G

rabenLam

bert Glacier

Rock outcrops

ODP drill sites

Ice flow unit boundaries

Ice flow direction

Groundingline

Prince Charles Mountains

70°S

70°S

km

0 100 200

330° 0° 30°

60°

PrydzBayAmery

Ice Shelf

120°

65°

70°

75°

80°

85°

150°180°

210°

240°270° 80°

Transantarctic

Mountains

WestAntarctica

Antarctica

East

Lambertdrainage basin

Ross Sea

Weddell Sea

300°

F1. Map of the Prydz Bay region, p. 13.

0

1

2

Dep

th (

km)

PS.5

PS.4

PS.2B

740

7417421166

PS.2AEarly glacial

PreglacialPS.2B PS.2A

739PS.1

743

NS

0 50km Leg 188 drill site

Leg 119 drill sites

F2. Seismic sequences drilled dur-ing Leg 119, p. 14.

2500Site 1166

Crosses line BMR 33-233000 3500 4000 4500 4900 5200

Crosses line AGSO 149/1301

Site 742

0.6

0.8

1.0

1.2

1.4

0.6

0.8

1.0

1.2

1.4

Shotpoint

Two-

way

trav

eltim

e (s

)Tw

o-w

ay tr

avel

time

(s)

GlacialPreglacial

Multiple

PS.2A2

PS.1

PS.2A1

PS.2B Top of layered unit

3 km

3000

2500

2000

BM

R 33-23

2000

2500

3000

3500

4000

45005000

2000

2900

Site 1165

~15 km

75°E67.5°S

75.5°E

Site742

Palmer 01-1-4

AG

SO

149/1301

BM

R 33-22

Site 1166

SW NE

Site 742

F3. Palmer line 01-1-04 and reflec-tions, p. 15.

T1. Lithostratigraphic units, Site 742, p. 19.

T2. Lithostratigraphic units, Site 1166, p. 20.


seismic unconformity. Unit PS.2A contains late Eocene–early Oligocenemarine massive and friable diamictite at Site 742 (Cooper et al., 1991a)and massive alluvial sands at Site 1166 (Shipboard Scientific Party,2001). Unit PS.1 is a late Miocene to Holocene marine diamictite withthin layers of diatomaceous sediment inferred to be glacial till deposit(Cooper et al., 1991a). Holes 1166A and 742A in total penetratedthrough Units PS.1 and PS.2A and into Unit PS.2B (Fig. F2).

NEW SEISMIC DATA AND SEISMIC UNITS

The new single-channel high-resolution seismic data, Palmer line 01-1-04, were acquired in February 2001 using a single 90-in³ generator in-jector air gun fired every 6 s at a ship’s speed of ~ 5 kt and distance be-tween shotpoints of ~15 m. The offset between navigation shotpoint(i.e., ship’s satellite antenna) and seismic midpoint (i.e., between airgun and streamer) is ~60 m. We incorporate the offset in the figuresthat show correlations of seismic traces to drill cores. The shotpointnumbers on the “foldout” seismic section and shotpoint map (Fig. F3)are, however, satellite antenna positions, and do not include the offset.

Synthetic seismic traces were calculated for Sites 742 and 1166 fromimpedance profiles derived from downhole logging velocity data andfrom seismic pulses extracted from seafloor reflections in the Palmerdata (Handwerger et al., this volume). We compared the synthetictraces with the Palmer seismic reflection profile to establish the positionof the drill core lithologic boundaries on the seismic reflection profile atboth drill sites (Fig. F4). We then traced reflections and acoustic unitsbetween the drill sites, initially from Site 1166 to Site 742 and then inthe opposite direction (Fig. F3). We used two out of three existing seis-mic cross lines near the drill sites (Figs. F5, F6) to refine and verify theacoustic unit boundaries. The third existing line (BMR 33-22) was notused for this study because it was of lower resolution and, although itwas used in earlier correlations (Cooper et al., 1991a), it did not addnew information to the present study. The reflection times to the sea-floor in each of the seismic lines differs, due to the different source-receiver geometries and recording parameters; however, the relativesubsurface reflection times can be accurately compared because of thelarge water depths (relative to the source-receiver separations). Table T3gives data on seismic line crossings and closest approach to drill sites.

We describe acoustic units in the Palmer data using the nomenclatureof Cooper et al. (1991a) and seek to trace lithologic units identified atdrill sites as acoustic units in the seismic data. The large apparent lateraland vertical subsurface variations commonly lead to variable acousticfacies within each acoustic unit. However, over the distance betweenSites 742 and 1166, a reasonable and coherent correlation of acousticunits and lithologic units (Fig. F4; Tables T1, T2) can be made. In thefollowing description of units from deepest to shallowest, we give thesite number for each lithologic unit (e.g., Unit IV [1166]) to help avoidconfusion.

Unit PS.2B

Unit PS.2B in the Palmer data is principally a seismically transparentunit with intervals of subparallel layered reflections. Cooper et al.(1991a) interpreted the upper boundary of Unit PS.2B to be the pregla-cial to glacial unconformity, which at Site 1166 lies at the top of litho-

NE

0.6

0.7

0.8

0.9

1.0

1.1

2.0

2.5

VP (km/s)ss

?

?

PS.1

G3

PS.2A1

PS.2A2

G2

G1

?

?

CT

A

L

A A?

A?

Site 742

Site 1166

Crosses AGSO line 149-1301Crosses

BMR line 33-23

SW

Precursor

1 km 1 km

?

L LagoonalA Alluvial plain CT Channel-fill or marine

transgressiveG1 Proximal glaciomarine

G2 Subglacial/proglacialG3 Mixed glacial (subglacialand glaciomarine)

100

300

1.9

2.3

2.7

I

II

III

IV

V

TD = 381 mbsf

Lithology VP (km/s)ss

0.6

0.7

0.8

0.9

1.0

1.1

1.2

PS.2B

~28 km

I

II

III

IV

V

VI

Lithology

100

200

300

e. Plio.-Quat.

I. QuaternaryQuaternary -

I. Pliocene Homogeneous diamictite with

pebbles

Homogeneous diamictite

Oligocene-Eocene

calcareous diamictite

Silt,sand, clay?

TD = 316 mbsf

Top of layered unit

Seafloor

200

??

GlacialPreglacial

Depth(mbsf)

Depth(mbsf)

Tw

o-w

ay tr

a ve l

t ime

(s)

Tw

o -w

a y t r

a ve l

t ime

(s)

F4. Seismic reflections in Palmer line 01-1-04 and lithology at Sites 1166 and 742, p. 16.

e. Plio.-Quat.



pebbles


Oligocene-Eocene

Calcareous diamictite

Silt,sand, clay

0

100

200

300

TD = 316

Top of Unit PS.2

Top of layered unit

1 km

NW SE~2200 m from Site 742

Age andlithology

Dep

th (

mbs

f)

PrecursorSeafloor

0.6

0.8

1.0

1.2

0.6

0.8

1.0

1.2

Tw

o-w

ay tr

avel

time

(s)

Tw

o-w

ay tr

avel

time

(s)

F5. Seismic line AGSO 149/1301 recorded near Site 742, p. 17.

NW SE

0.6

0.7

0.8

0.9

1.0

Tw

o-w

ay tr

avel

time

(s)

~780 m from Site 1166

Lihologic unit ageI-early Pliocene to HoloceneII-late Eocene to earlyOligoceneIII-late EoceneIV-Late CretaceousV-Cretaceous

IB-DiamictIC-Sandy/clayeysiltID-DiamictII-ClaystoneIII-Sand

TD = 381mbsf

Lithologic unit descriptions

1 km

Lithostratigraphicunit

IB GlacialIC GlacialID Glacial

II Proglacial

III Fluvial/deltaic

IV Preglaciallagoonal

V Preglacial

F6. Seismic line BMR 33-23 re-corded near Site 1166, p. 18.

T3. Location information for seis-mic lines, p. 21.


stratigraphic Unit IV (Shipboard Scientific Party, 2001). In theuppermost part of Unit PS.2B reflections are weak and contorted, andthese correspond to the restricted marine or lagoonal deposits of litho-stratigraphic Unit IV (1166). Layered reflections in the deeper part ofPS.2B correspond to the poorly sampled alluvial plain claystone oflithostratigraphic Unit V (1166). The layered deeper part of Unit PS.2Bthickens to the northeast toward Site 742 (Fig. F4). Within Unit PS.2B,some layered reflections truncate to the southwest against the con-torted layer at the top of Unit PS.2B.

The preglacial to glacial unconformity was not reached at Site 742.Here, the unconformity is not marked by a prominent layered reflec-tion. We establish the unconformity’s position by (1) projection of thelayered reflection at the top of Unit PS.2B along the Palmer line to andbeyond Site 742 and (2) comparison of the reflection geometries of thelayered and upper contorted units in PS.2B on the Australian GeologicalSurvey Organization (AGSO) cross line (Fig. F5) with the Palmer linenear and at Site 742.

Unit PS.2A

We divide Unit PS.2A into two seismic subunits in the Palmer data,Subunits PS.2A1 and PS.2A2, (Figs. F3, F4). Subunit PS.2A2, the deepersubunit, unconformably overlays Unit PS.2B and is characterized byweak distorted reflections. Subunit PS.2A2 has an undulating upper sur-face, and the unit thins to the northeast away from Site 1166. At Site1166, Subunit PS.2A2 corresponds to the massive sands of lithostrati-graphic Unit III (156–268 meters below sea floor [mbsf]) (Table T2).Subunit PS.2A2 was either not sampled at Site 742 or was sampled atthe bottom of the hole (i.e., possibly the bottommost core of the hole,304–307 mbsf in Unit VI).

Subunit PS.2A1 is divided into lower, middle, and upper reflectionpackages with different seismic characteristics. The three packages to-gether correspond to lithostratigraphic Units II (1166), V (742, lowerhalf), and VI (742), which are glacial units with glaciomarine, waterlaintill and possible subglacial sediments (Fig. F4; Tables T1, T2). The lowerreflection package is composed of continuous strong layered reflectionsthat drape the undulating top surface of Subunit PS.2A2 and that fill de-pressions in this surface (Figs. F3, F4). At Site 1166 this package corre-sponds to glaciomarine interlayered sands and clays (Fig. F4). Themiddle reflection package has homogeneous to disrupted low-ampli-tude reflections that correspond at Site 742 to variable-compositiondiamictites (Fig. F4). This package thins toward Site 1166 and can betraced to within 1 km of the drill site, at which point it cannot be re-solved. The upper reflection package has interlayered high-amplitudeand chaotic reflections that also correspond at Site 742 to variable com-position diamictities (Table T1). Reflections in the upper package aretruncated by the overlying unconformity, and the package pinches outmidway between Sites 742 and 1166 (Fig. F4).

Unit PS.1

Unit PS.1 lies between the regional Paleogene/Neogene unconfor-mity at the top of Subunit PS.2A1 (i.e., tops of lithostratigraphic Units II[1166] and V [742]) and the seafloor (Figs. F3, F4). The unit is character-ized principally by chaotic reflections, but some continuous layered re-flections are also observed. The bottom of the unit is in places denoted


by a continuous high-amplitude banded reflection and in other placesby the base of a thin chaotic unit below the banded reflection (Fig. F3).The interlayering of high-velocity diamictites with low-velocity sedi-ments above (diatom bearing) and below (interbedded sand/silts) closeto the unconformity leads to a complex seismic response (e.g., syn-thetic traces at Sites 1166 and 742) in the thin and laterally variablelithologic units (Figs. F3, F4). Unit PS.1 comprises stratified to massivediamictites with at least one interbedded layer of diatom-bearing glacio-marine sediments (Fig. F4; Table T2). Recovery was low at Sites 1166and 742, and the origin of layered reflections within Unit PS.1, otherthan the strong reflection from the base of lithostratigraphic Unit I(742), is unknown from the drill cores.

DISCUSSION

Regional Stratigraphy

The high-resolution seismic data from the Palmer line between Sites1166 and 742 (Fig. F3) and AGSO line across Site 742 (Fig. F5) providesubsurface images, more detailed than existing seismic data, to aug-ment prior interpretations of Prydz Bay seismic stratigraphy (e.g., Coo-per et al., 1991a) and inferred depositional paleoenvironments (e.g.,Hambrey et al., 1991; Shipboard Scientific Party, 2001). Shipboard Sci-entific Party (2001) interpreted drill cores at Site 1166 as depicting achange in depositional paleoenvironments from alluvial plain and la-goonal settings in preglacial times (Units V and IV [1166]) to the devel-opment of fluvial outwash plains in early glacial times (Unit III [1166];Unit VI base [742]) to marine transgression also in early glacial time(Unit II [1166]), and finally deposition of proximal and subglacial diam-ictites in late glacial times (Unit I [1166]; Units II and IV [742]). Theirinterpretations were built partly upon Site 742 results (e.g., Barron,Larsen, et al., 1989, 1991; Barron et al., 1991; Hambrey et al., 1991) ofproximal to subglacial marine environments in early glacial times(Units VI and V [742]) followed by waterlain tills and subglacial diamic-tites in late glacial times (Units IV through I [742]). Our interpretationsof the new seismic data are generally consistent with the prior ones, butwe differ in several important details.

The layered section imaged below the two drill sites (Unit PS.2B)shows little deformation and was interpreted, based on a small clay-stone sample from the top of the unit and on seismic and lithologiccorrelation with Site 741 (Leg 119), as being possibly deposited on alow-relief alluvial plain (Shipboard Scientific Party, 2001). A chaotic tolayered unit that includes similar lithology strata but more induratedand older age (Early Cretaceous) was sampled at Site 741 (Leg 119), andwas also interpreted (Barron, Larsen, et al., 1989) as being deposited ona low-relief alluvial plain. Prior regional seismic profiles show that thelayered section of Unit PS.2B is extensive across the inner shelf (Cooperet al., 1991a) and thickens to the northeast, suggesting that these stratawere deposited on the flanks of a slowly subsiding basin (i.e., Prydz Baybasin [Masolov et al., 1981] that trends northeast–southwest across thecentral part of Prydz Bay [Cooper et al., 1991a]). Unit PS.2B reflectionstruncate to the southwest near Site 1166, indicating that these stratahave been eroded, likely during sea level lowstands (Fig. F3). The over-lying Late Cretaceous lagoonal or restricted marine deposits (i.e., UnitIV [1166]) likely reflect a time when Prydz Bay Basin was low-lying and


relatively close to the coast. The reflections from the top and base ofUnit IV, between Site 1166 and Site 742, are strong in places but aremostly disrupted and weak. We infer the base of the lagoonal Unit IV(1166) to be an unconformity based on the truncation of underlying re-flections and the strong basal reflection from Unit IV (1166). However,the unconformity is not evident in the drilling record because coreswere not recovered in this interval (Fig. F4). We believe that these la-goonal deposits extend at least between the drill sites and likely fartherto cover a large part of the central Prydz Bay Basin rather than being alocal deposit to the area around Site 1166.

Preglacial to Glacial Transition

We cannot determine depositional environments during the long hi-atus (i.e., Late Cretaceous to Eocene time) at the preglacial to glacial un-conformity. From Prydz Bay Basin history of Cooper et al. (1991a), weinfer that this was a period of relative stability (i.e., reduced basin sub-sidence) with likely subaerial depositional/erosional environments inan area of relatively low relief. The large lithologic change and resultinghigh-amplitude reflection across the preglacial to glacial unconformityat Site 1166, between the underlying lagoonal silts/sands of Unit IV andoverlying massive alluvial sands of Unit III, signal a large change in de-positional environments. Because lithostratigraphic units are relativelythin, compared to seismic wavelengths, the high-amplitude reflectionscaused by the large acoustic impedance changes at the unconformityand within the underlying interlayered sand/silt beds result in a com-plex waveform that is difficult to trace uniquely away from the drillsite.

Previously, the depth to the preglacial to glacial unconformity be-neath Site 742 has been debated. Cooper et al. (1991a) suggested thatthe unconformity was ~100 m below the drill site based on lower-reso-lution seismic data. But, recovery of deformed late–middle Eocenesandy units with carbonaceous material containing Late Cretaceouspollen and spores (Truswell, 1991) in the lowermost core from Site 742led the Shipboard Scientific Party (2001) to speculate that these rocksmay be similar to the ones recovered from the lowermost part of themassive sands of Unit III at Site 1166 and would imply the unconfor-mity was shallower than 100 m below Site 742. In the new Palmer seis-mic data, we can confidently trace the preglacial to glacialunconformity from Site 1166 to an area a few kilometers southwest ofSite 742, where the underlying layered Cretaceous section thickens rap-idly. Here, the reflections become weaker and more contorted within azone that extends to near where Site 742 was drilled, and there isgreater uncertainty in the identification of the preglacial to glacial un-conformity. However, a seismic cross line ~2 km from Site 742 (Fig. F5)and within the zone of uncertainty shows a strong reflection, from theunconformity, at ~70 ms below the bottom of the drill site projected tothe cross line (Fig. F4). Based on the Palmer data and the cross line, webelieve that the preglacial to glacial unconformity lies ~45–50 ms (~50m) below the bottom of the drill hole at Site 742.

The alluvial plain, delta, or proglacial outwash sands above thepreglacial to glacial unconformity signal a widespread change to rapidsupply (relative to rate of basin subsidence) of coarse terrigenous sedi-ments to Prydz Bay Basin. Between Sites 1166 and 742, the upperboundary of the sand unit undulates with up to ~50 m of relief, and therelative widths of depressions are ~1–5 km, but the bottom of the unit is


nearly flat. The top of the unit deepens to the northeast and at Site 742lies close to the base of the hole (Fig. F4; Table T1). The broadly undu-lating subsurface geometry, with an internal weak and contorted acous-tic character, and coarse sand lithology of the massive unit suggests tous a morphology of a broad fluvial channel system, possibly part of analluvial plain along the edge of the Prydz Bay Basin. The plain mayhave been deposited in the early stages of glaciation, possibly as an out-wash system from upstream glaciers within the Lambert Graben (Ship-board Scientific Party, 2001; Strand et al., 2003). We believe that thedeformed sands recovered at the bottom of Site 742 are part of the flu-vial channel system and are from the same lithostratigraphic unit thathas the massive and deformed sands at Site 1166 (i.e., Unit III). Our cor-relation agrees with the inferences of Shipboard Scientific Party (2001)from drilling data alone.

The origin of the deformed sands above the preglacial to glacial un-conformity is less clear. From the regional seismic morphology of theinferred massive sand unit (i.e., Unit III [1166]), we believe that the de-formation resulted from fluvial rather than subglacial processes, al-though the source of the sands may have been from nearby glaciers(Strand et al., 2003). This differs from the interpretation of Hambrey etal. (1991), who suggested that the deformed sands at the bottom of Site742 resulted from glaciotectonic deformation (Table T1). In the newseismic data, the widths of the depressions are relatively narrow and aremore consistent with those of fluvial channels or alluvial plain valleysthan subglacial troughs (e.g., Prydz trough). If the sands had been de-posited subglacially, we would expect to see evidence of overcompac-tion in the sands and upper part of the underlying lagoonal deposits,but we do not see such evidence at Site 1166. The massive sands at Site1166 contained clayey layers and organic debris including pieces ofwood typical of fluvial systems. Subglacial channels in temperate sys-tems might be a possible explanation, yet we did not recover coarse de-bris such as found in these systems (Eyles and Eyles, 1992) and we donot see seismic evidence of large lenticular features that might be eskersand end moraines of a subglacial system. Additionally, elsewhere on theshelf, there is seismic evidence for remnants of buried fluvial channelsystems in equivalent Paleogene sections (Cooper et al., 1991a, 2001).Although we cannot eliminate local subglacial deformation at Site 742,the regional evidence is more suggestive to us of fluvial rather than sub-glacial processes for depositing the early glacial sands of Unit III (1166)across the shelf.

Transition to a Marine Glacial Environment

Drill cores indicate that the Prydz Bay Basin region shifted perma-nently from a subaerial-deltaic to marine depositional environment inearly glacial times (Shipboard Scientific Party, 2001). The massive flu-vial sands at Site 1166 grade upward into alternating sand–clay layerswith increasing percentage of marine dinoflagellate cysts (Macphailand Truswell, this volume). This sand–clay sequence of Unit II was de-posited during a marine transgression (Shipboard Scientific Party, 2001)and filled inferred fluvial channels with layered reflective units that weinterpret as being the glaciomarine sediments of Units II (1166) and VI(742). Subaerial units are not found in the drill cores above this level(Tables T1, T2) (Shipboard Scientific Party, 2001; Hambrey et al., 1991).Shipboard Scientific Party (2001) also interpret that a strong regionalunconformity reflection appears to correlate with a boundary within


the upper part of Unit III above which downhole measurements indi-cate prominent sand–clay alternations suggestive of the marine trans-gression. However, the new Palmer data illustrate that this strongregional reflection is from the top of the massive sand Unit III, ratherthan from within the sand unit (Fig. F4). The discrepancy results from achange in bed thickness in the 800-m offset interval between the seis-mic line they used (i.e., BMR-33-23) (Fig. F6) and the drill site (Hand-werger et al., this volume). Seismic line BMR-33-23 crosses the sandunit where the unit is more deeply buried, and, hence, the strong reflec-tion appears to be originating from deeper in the drilled section (Fig.F6). The alternating sand and clay layers near the top of Unit III resultfrom fluvial and tidal deposition (Macphail and Truswell, this vol-ume) within alternating high- and low-energy environments of a broadproglacial fluvio-deltaic system where Site 1166 was drilled.

Following the abrupt marine transgression (i.e., directly above UnitIII [1166]) (Macphail and Truswell, this volume), the new continentalshelf of Prydz Bay continued to subside slowly and receive greater thick-nesses of glaciomarine deposits near the center of the Prydz Bay Basin.This can be seen in the northeast thickening and deepening of glacio-marine and subglacial units between Sites 1166 (Unit II) and 742 (UnitsVI and V) (Fig. F3). Site 1166 samples an older section than Site 742.The pinchout and truncation of reflections within this depth interval ofthe section against the overlying Paleogene to Neogene unconformitymark a period of regional erosion during which up to several hundredmeters of the sedimentary section may have been removed at Site 742(Solheim et al., 1991) and Site 1166 (Forsberg et al., 2001), based onconsolidation analyses. However, Solheim et al. (1991) also acknowl-edge that the strongest erosion may have occurred during the time ofthe missing rock record (i.e., early Oligocene to latest Miocene time)and in Pliocene time where other loading/erosion events are also noted.Regardless, the existing drill cores from the Prydz Bay shelf show thatthe last subaerial deposits are in late Eocene time (Shipboard ScientificParty, 2001).

Paleogene to Neogene Transition

The identification of the Paleogene to Neogene unconformity re-corded in drill cores cannot be uniquely identified in seismic data. Al-though there is a ubiquitous and nearly continuous high-amplitudereflection that is clearly seen in seismic data, the reflection lies close to(i.e., within 20 m of) but not always at the unconformity. The strongcomplex reflection is caused by the composite effects of thin and vari-able thickness interlayered soft and hard layers above (e.g., Pliocene di-atomaceous and diamictites) and below (e.g., early Oligocene sand/siltsand diamictites) the unconformity. The complexity of the reflectionalso makes it difficult to uniquely resolve and trace the thin Pliocene di-atomaceous units that were recovered at Sites 1166 and 742 and werecorrelated based on biostratigraphy and on the similarity of shapes ofdownhole logging curves (Shipboard Scientific Party, 2001). We cannotverify the downhole logging correlation with the new high-resolutionseismic data and suspect that this will not be possible due to the limita-tions of the seismic technique (i.e., seismic frequency vs. depth penetra-tion) and the large lateral variability in lithostratigraphic units over the40 km between the drill sites.

The seismic record of regional depositional events in Unit PS.1 sinceearly Pliocene time is poorly constrained because of the generally cha-


otic reflection character of the unit and the poor recovery in drill cores.We note that there is a strong continuous reflection that lies ~20–40 msbelow the seafloor and can be traced almost between the drill sites.However, in this interval core recovery was poor and there are nodownhole logs; hence, we cannot explain this reflection. We suspectthat, like elsewhere around the Antarctic margin where similar strongreflections are noted near the seafloor in seismic reflection profiles (e.g.,Cooper et al., 1991b, Anderson and Bartek, 1992), this reflection is a re-gional unconformity marking a latest Neogene advance of grounded iceacross the continental shelf.

CONCLUSIONS

We use new high-resolution seismic data to map acoustic units andunconformities between Sites 1166 and 742 and link seismic features tolithostratigraphic units at the two drill sites. The principal new findingsfrom seismic data include:

1. Site 1166 drilled an older section than Site 742, and most unitssampled either do not extend between the two sites or are verythin (few meters) at the sites.

2. The preglacial to glacial unconformity (top of Unit PS.2B) thatlies between lithostratigraphic Units III and IV at Site 1166 lies~50 m below Site 742.

3. The deformed sands in the bottommost core at Site 742 are fromthe same acoustic unit as sampled by lithostratigraphic Unit III(1166).

4. The flooding surface reported by Shipboard Scientific Party(2001) as originating within Unit III (1166) actually occurs di-rectly above Unit III and regionally marks the transition fromthe high-velocity unit consisting of fluvial sands below to thelow-velocity unit consisting of marine sediments above.

5. The new seismic data do not have adequate resolution to con-firm or deny that early Pliocene diatomaceous horizons sampledat Sites 1166 and 742 are the same, as has been inferred fromdownhole logging and biostratigraphy (Shipboard ScientificParty, 2001).

From the seismic and drilling results and prior interpretations of Ship-board Scientific Party (2001), we infer a progression from preglacial de-posits on a low-relief alluvial plain to glaciomarine and subglacial depos-its within the study area of the Prydz Bay Basin. The low-relief alluvialplain (Units PS.2B and V [1166]) and lagoonal (Units PS.2B and IV[1166]) environments during Late Cretaceous time evolved to a broadfluvial channel system or outwash plain (Units PS.2A2 and III [1166]) inEocene time. Marine transgression in late Eocene to early Oligocene timeresulted in filling (Units PS.2A1 and II [1166]) and burying (Units PS.2A1and VI [742] and V [742]) the channel systems beneath a cover of glacio-marine and waterlain till deposits. This transgressive event marked thepermanent flooding and deepening of the Prydz Bay region and creationof a marine continental shelf. The shelf was extensively glacially erodedin Oligocene to late Miocene times, and massive proximal glaciomarineand subglacial diamictons (Units PS.1 and I [1166] and I through IV[742]) accumulated in late Neogene time with episodes of high biogenicproductivity and reduced glacial cover (Units PS.1 and III [742]).


ACKNOWLEDGMENTS

We wish to thank Amy Leventer and Dave Mucciarone for helping tocollect the seismic data. The manuscript was enhanced by discussionswith Phil O’Brien, Kevin Theissen, Peter Barrett, and members of theshipboard party, and a review by Carlota Escutia. This research useddata and samples provided by the Ocean Drilling Program (ODP). ODPis sponsored by the U.S. National Science Foundation and participatingcountries under the management of Joint Oceanographic Institutions,Inc. This work was supported in part by funds to T. Erohina from theStanford University Department of Geological and Environmental Sci-ences and by a grant to A. Cooper by the U.S. Science Support Program.


REFERENCES

Anderson, J.B., and Bartek, L.R., 1992. Cenozoic glacial history of the Ross Searevealed by intermediate resolution seismic reflection data combined with drill siteinformation. In Kennett J.P., and Warnke D. (Eds.), A Perspective on Global Change.Am. Geophys. Union, 56:231–263.

Barron, J., Larsen, B., and Baldauf, J.G., 1991. Evidence for late Eocene to early Oli-gocene Antarctic glaciation and observations on late Neogene glacial history ofAntarctica: results from Leg 119. In Barron, J., Larsen, B., et al., Proc. ODP, Sci.Results, 119: College Station, TX (Ocean Drilling Program), 869–891.

Barron, J., Larsen, B., et al., 1989. Proc. ODP, Init. Repts., 119: College Station, TX(Ocean Drilling Program).

————, 1991. Proc. ODP, Sci. Results, 119: College Station, TX (Ocean Drilling Pro-gram).

Cooper, A., Stagg, H., and Geist, E., 1991a. Seismic stratigraphy and structure of PrydzBay, Antarctica: implications from Leg 119 drilling. In Barron, J., Larsen, B., et al.,Proc. ODP, Sci. Results, 119: College Station, TX (Ocean Drilling Program), 5–26.

Cooper, A.K., Barrett, P.J., Hinz, K., Traube, V., Leitchenkov, G., and Stagg, H.M.J.,1991b. Cenozoic prograding sequences of the Antarctic continental margin: arecord of glacio-eustatic and tectonic events. Mar. Geol., 102:175–213.

Cooper, A.K., O’Brien, P.E., and ODP Leg 188 Shipboard Scientific Party, 2001. Earlystages of East Antarctic glaciation—insights from drilling and seismic reflectiondata in the Prydz Bay region. In Florindo, F., and Cooper, A.K., (Eds.), The GeologicRecord of the Antarctic Ice Sheet from Drilling, Coring and Seismic Studies. Quad.Geofis., Inst. Naz. Geofis. Vulcanol., 16:41–42. (Abstract)

Eyles, N., and Eyles, C.H., 1992. Glacial depositional systems. In Walker, R.G., andJames, N.P. (Eds.), Facies Models: Response to Sea-level Change. Geol. Assoc. Can., 73–100.

Forsberg, C.F., Solheim, A., Gruetzner, J., Taylor, B., and Strand, K., 2001. Glacialdevelopment in the Prydz Bay region as witnessed by geotechnical and mineralog-ical properties of Leg 188 Sites 1166 and 1167. In Florindo, F., and Cooper, A.K.(Eds.), The Geologic Record of the Antarctic Ice Sheet from Drilling, Coring and SeismicStudies. Quad. Geofis., Inst. Naz. Geofis. Vulcanol., 16:71–72. (Abstract)

Hambrey, M.J., Ehrmann, W.U., and Larsen, B., 1991. Cenozoic glacial record of thePrydz Bay continental shelf, East Antarctica. In Barron, J., Larsen, B., et al., Proc.ODP, Sci. Results, 119: College Station, TX (Ocean Drilling Program), 77–132.

Hambrey, M.J., and McKelvey, B., 2000. Neogene sedimentation on the western mar-gin of the Lambert Graben, East Antarctica. Sedimentology, 47:577–607.

Masolov, V.N., Kurinin, R.G., and Grikurov, G.E., 1981. Crustal structures and tec-tonic significance of Antarctic rift zones (from geophysical evidence). In Cresswell,M.M., and Vella, P. (Eds.), Gondwana Five: Rotterdam (A. A. Balkema), 303–309.

O’Brien, P.E., Cooper, A.K., Richter, C., et al., 2001. Proc. ODP, Init. Repts., 188 [CD-ROM]. Available from: Ocean Drilling Program, Texas A&M University, College Sta-tion TX 77845-9547, USA.

Shipboard Scientific Party, 2001. Site 1166. In O’Brien, P.E., Cooper, A.K., Richter, C.,et al., Proc. ODP, Init. Repts., 188, 1–110 [CD-ROM]. Available from: Ocean DrillingProgram, Texas A&M University, College Station TX 77845-9547, USA.

Solheim, A., Forsberg, C.F., and Pittenger, A., 1991. Stepwise consolidation of gla-cigenic sediments related to the glacial history of Prydz Bay, East Antarctica. In Bar-ron, J., Larsen, B., et al., Proc. ODP, Sci. Results, 119: College Station, TX (OceanDrilling Program), 169–184.

Stagg, H.M.J., 1985. The structure and origin of Prydz Bay and MacRobertson shelf,East Antarctica. Tectonophysics, 114:315–340.


Strand, K., Passchier, S., and Näsi, J., 2003. Implications of quartz grain microtexturesfor onset of Eocene/Oligocene glaciation in Prydz Bay, ODP Site 1166, Antarctica.Palaeogeogr., Palaeoclimatol., Palaeoecol., 198:101–112.

Truswell, E.M., 1991. Palynology of sediments from Leg 119 drill sites in Prydz Bay,East Antarctica. In Barron, J., Larsen, B., et al., Proc. ODP, Sci. Results, 119: CollegeStation, TX (Ocean Drilling Program), 941–948.


Figure F1. Index map of the Prydz Bay region showing locations of ice drainages, Ocean Drilling Program(ODP) drill sites, and the location of the tie line between Sites 742 and 1166 (Modified from Hambrey andMcKelvey, 2000).

1165

Prydz Trough-MouthFan

70°E1167

743

739742

1166

740

741

Four LadiesBank

PrydzChannel

Continental shelf break

Prydz Bay

FramBank

AmeryIce Shelf

Lambert G

rabenLam

bert Glacier

Rock outcrops

ODP drill sites

Ice flow unit boundaries

Ice flow direction

Groundingline

Prince Charles Mountains

70°S

70°S

km

0 100 200

330° 0° 30°

60°

PrydzBayAmery

Ice Shelf

120°

65°

70°

75°

80°

85°

150°180°

210°

240°270° 80°

Transantarctic

Mountains

WestAntarctica

Antarctica

East

Lambertdrainage basin

Ross Sea

Weddell Sea

300°


Figure F2. Profile of seismic sequences drilled during Leg 119 with the projected location of Site 1166 (mod-ified from Cooper et al., 1991a).

0

1

2

Dep

th (

km)

PS.5

PS.4

PS.2B

740

7417421166

PS.2AEarly glacial

PreglacialPS.2B PS.2A

739PS.1

743

NS

0 50km Leg 188 drill site

Leg 119 drill sites

T. E

RO

HIN

A E

T AL.

SE

ISMIC S

TR

AT

IGR

AP

HIC C

OR

RE

LA

TIO

NS

15

Figure same as those in Figures F4, p. 16, F5, p. 17, andF6, p.

4500 4900 5200Crosses line AGSO 149/1301

Site 742

0.6

0.8

1.0

1.2

1.4

0.6

0.8

1.0

1.2

1.4

Two-

way

trav

eltim

e (s

)Tw

o-w

ay tr

avel

time

(s)

PS.2B Top of layered unit

3 km

NE

Site 742

F3. Palmer line 01-1-04 and interpretive tracing of reflections. Tracings depicted are the18. See text for descriptions. (This figure is also available in an oversized format.)

2500Site 1166

Crosses line BMR 33-233000 3500 4000

Shotpoint

GlacialPreglacial

Multiple

PS.2A2

PS.1

PS.2A1

3000

2500

2000

BM

R 33-23

2000

2500

3000

3500

4000

45005000

2000

2900

Site 1165

~15 km

75°E67.5°S

75.5°E

Site742

Palmer 01-1-4

AG

SO

149/1301

BM

R 33-22

Site 1166

SW

T. E

RO

HIN

A E

T AL.

SE

ISMIC S

TR

AT

IGR

AP

HIC C

OR

RE

LA

TIO

NS

16

Figure Thick lines are lithostratigraphic unitbound ctions. Lithologic logs are from Ship-board s and synthetic traces are from Hand-werge PS.1) are modified from Cooper et al.(1991a

NE

0.6

0.7

0.8

0.9

1.0

1.1

ss

?

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Tw

o-w

ay tr

avel

time

(s)

Tw

o-w

ay tr

avel

time

(s)

F4. Correlation of seismic reflections in Palmer line 01-1-04 and lithology at Sites 1166 and 742. aries at Site 1166 projected to Site 742. Thin dashed lines are acoustic boundaries or prominent refle

Scientific Party (2001) for Site 1166 and from Barron, Larsen, et al. (1989) for Site 742. Impedance logr et al. (this volume). VP = compressional velocity, ss = synthetic seismic. Acoustic unit names (e.g., ). Seismic data are continuous across the annotation gaps at the drill sites. TD = total depth.

2.0

2.5

VP (km/s)

?

PS.1

G3

PS.2A1

PS.2A2

G2

G1

?

?

CT

A

L

A A?

A?

Site 742

Site 1166

Crosses AGSO line 149-1301Crosses

BMR line 33-23

SW

Precursor

1 km 1 km

?

L LagoonalA Alluvial plain CT Channel-fill or marine

transgressiveG1 Proximal glaciomarine

G2 Subglacial/proglacialG3 Mixed glacial (subglacialand glaciomarine)

100

300

1.9

2.3

2.7

I

II

III

IV

V

TD = 381 mbsf

Lithology VP (km/s)ss

PS.2B

~28 km

I

II

III

IV

V

VI

Lithology

100

200

300

e. Plio.-Quat.



pebbles


Oligocene-Eocene

calcareous diamictite

Silt,sand, clay?

TD = 316 mbsf

Top of layered unit

Seafloor

200

??

GlacialPreglacial

Depth(mbsf)

Depth(mbsf)


Figure F5. A continuous segment of seismic line AGSO 149/1301 recorded near Site 742. The lithology sec-tion is from Site 742 (Fig. F4, p. 16) and is shown for approximate comparisons of seismic to lithology. Thefirst notable seismic reflection is a precursor and not the seafloor.

e. Plio.-Quat.



pebbles


Oligocene-Eocene

Calcareous diamictite

Silt,sand, clay

0

100

200

300

TD = 316

Top of Unit PS.2

Top of layered unit

1 km

NW SE~2200 m from Site 742

Age andlithology

Dep

th (

mbs

f)

PrecursorSeafloor

0.6

0.8

1.0

1.2

0.6

0.8

1.0

1.2

Tw

o-w

ay tr

avel

time

(s)

Tw

o-w

a y tr

avel

time

(s)


Figure F6. A continuous segment of seismic line BMR 33-23 recorded near Site 1166. The figure is repro-duced from Shipboard Scientific Party (2001), with addition of our correlation lines to show proper positionof reflections at the crossing of this line and Palmer line 01-1-04. See text for explanation. TD = total depth.

NW SE

0.6

0.7

0.8

0.9

1.0

Tw

o-w

ay tr

avel

time

(s)

~780 m from Site 1166

Lihologic unit ageI-early Pliocene to HoloceneII-late Eocene to earlyOligoceneIII-late EoceneIV-Late CretaceousV-Cretaceous

IB-DiamictIC-Sandy/clayeysiltID-DiamictII-ClaystoneIII-Sand

TD = 381mbsf

Lithologic unit descriptions

1 km

Lithostratigraphicunit

IB GlacialIC GlacialID Glacial

II Proglacial

III Fluvial/deltaic

IV Preglaciallagoonal

V Preglacial


Table T1. Lithologic units, Leg 119, Site 742.

Note: Lithology and interpretation are from Hambrey et al. (1991); lithology and age are from Barron et al. (1991).

UnitDepth (mbsf) Lithology Age Interpretation

IA 0–0.07 Diatom sand-silt late Quaternary Marine sediment with glacial influences; distal glaciomarine, outer shelf, interglacial

IB 0.07–5.4 Massive pebbly diamictons, up to 15% gravel Waterlain till to proximal glaciomarine sediment

II 5.4–115.2 Homogeneous massive diamictite, up to 15% gravel

Quaternary–late Pliocene Lodgement till (subglacial) in upper part, mainly waterlain till below

IIIA 115.2–127.7 Stratified diamictite, average 5% gravel Waterlain till to distal glaciomarine sediment with much resedimentation

IIIB 127.7–128.3 Diatomite with minor sand or silt (of terrigenous origin)

late Pliocene Distal glaciomarine

IIIC 128.3–133.7 Stratified diamictite, sandstone, and siltstone Waterlain till to distal glaciomarineIIID 133.7–134.4 Single boulder of gneiss Ice-rafted boulder

IV 134.4–172.5 Diamictite with diffuse layering 1%–5% gravel Inferred early Pliocene–late Miocene

Waterlain till and lodgement till (subglacial) deposited on shelf

V 172.5–304.3 Pale diamictite with minor carbonate-rich layers; 1%–5% gravel

?Oligocene–Eocene Waterlain till forming the bulk of the prograding sequence (?)

VIA 304.3–307.5 Interbedded sand-silt claystone and silty claystones Proximal glaciolacustrine or protected marine with minor ice rafting

VIB 307.5–313.3 Pale massive diamictite, 1%–5% gravel Waterlain tillVIC 313.3–316.0 Carbonaceous well-sorted sand and siltstone with

interbedded diamictite, deformed sands at the bottom, older lignite fragments

early Oligocene–Eocene Proximal glaciolacustrine or glaciomarine, with waterlain till and turbiditic components; glaciotectonic deformation at base


Table T2. Lithologic units, Leg 188, Site 1166.

Note: Table is adapted from Shipboard Scientific Party (2001).

UnitDepth (mbsf) Lithology Age Interpretation

IA 0.0–2.74 Biogenic clay Holocene Iceberg turbated hemipelagic sediment; outer continental shelf, interglacial conditions

IB 2.79–106.36 Diamictons, clayey silt Subglacial and proximal glaciomarine sedimentation; homogenization by subglacial deformation or redeposition of ice-proximal sediments

IC 113.30–117.22 Biogenic clayey silt (diatom rich) Pleistocene–late Pliocene Soft-sediment deformation—loading and rapid dewatering of the underlying sediments during abrupt fluctuation in environmental setting. Glacial retreat? increased biogenic sedimentation

ID 123.0–135.42 Diamictons, interbeds of dark gray clast poor and clast rich

(Pleistocene) late Pliocene None cited

II 135.42–156.62 Diatom-bearing claystone; interlayered sands and clays

early Oligocene–late Eocene Glaciomarine, proglacial sedimentation during marine transgression; uphole transition from shallow-water glaciomarine facies to a dominance of diatom-bearing claystones deposited in open marine environment; flooding surface at base of unit

III 156.62–267.17 Massive sands, ~20 m bottom deformed

late Eocene Alluvial plain or delta or proglacial outwash; top 30 m probably part of channel system deposits

IV 276.44–314.91 Carbonaceous clay and fine sandy silt (high organic content, iron sulfides, preserved laminae)

Late Cretaceous (Turonian) Restricted marine or lagoonal; slow sedimentation rates under reducing conditions

V 342.80–342.98 recovered Claystone Late Cretaceous (Turonian) Low-relief alluvial plain


Table T3. Location information for seismic linesshown.

Notes: SP = shotpoint. NA = not applicable.

FeaturePalmer line

1–4BMR line 33–23

AGSO line 149/1301

Seismic lines cross ~780 m northeast of Site 1166.

SP 2697 SP 5790 NA

Seismic lines cross ~1910 m southwest of Site 742.

SP 4858 NA SP 2526

Palmer line crosses ~27 m southeast of Site 1166.

SP 2637 NA NA

Palmer line crosses ~19 m southeast of Site 742.

SP 4999 NA NA

seismic stratigraphic correlations between odp sites 742 and 1166

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