8. SEISMIC STRATIGRAPHIC C 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 Ocean Drilling Program Sites 1166 and 742 are interpreted to link acoustic features to lithologic units at the two drill sites. New findings include: (1) Site 1166 drilled a deeper (older) section than Site 742; (2) Paleogene units mostly do not extend between the two sites, except the deformed sand unit (Units III [1166] and VI [742]); (3) the preglacial to glacial unconformity sampled at Site 1166 lies ~50 m below Site 742; (4) the Paleogene flooding surface at Site 1166 lies on top of Unit III, not within, as previously reported; and (5) Pliocene diatomaceous horizons correlated between the sites based on downhole logging cannot be traced between the two sites in seismic data.

For the study region, we infer a progression from a preglacial setting on a low-relief alluvial plain to glaciomarine and subglacial settings. Late Cretaceous alluvial plain and lagoonal environments evolved to a late Eocene broad fluvial channel system or outwash plain. Marine transgression infilled and buried the channel system with glacial deposits that were extensively eroded during the Oligocene to late Miocene. Late Neogene environments were mostly subglacial with episodes of reduced 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: . [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: akcooper@pangea.stanford.edu 3Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Room 719, Salt Lake City UT 84105, USA.

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

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INTRODUCTION

Prydz Bay lies at the mouth of the Amery Ice Shelf?Lambert Glacier System and drains ~20% of the East Antarctic Ice Sheet (Fig. F1). The continental shelf there contains a record of early Cenozoic and late Neogene glaciation. Ocean Drilling Program (ODP) Legs 119 and 188 drilled at five sites on the Prydz Bay continental shelf to study the proximal Cenozoic record of Antarctic glaciation (Barron, Larsen, et al., 1989, 1991; O'Brien, Cooper, Richter, et al., 2001) (Fig. F1). The Leg 119 drill sites (Sites 739?742) lie along a cross-shelf transect that was traversed by a composite seismic reflection profile recorded to assist correlation 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 Leg 119 transect to sample an older Cenozoic section, but was not located on a seismic line that could be directly tied to the Leg 119 sites. The only correlation possible between Sites 742 and 1166 at the time of drilling was for the late Neogene section and was based on similar glacial lithologies and similar shapes in resistivity and velocity profiles from 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 (Leg 119), to correlate stratigraphic units between the drill sites. The profile is shown at both page size and foldout size (Fig. F3). We use the downhole logging information and seismic-source signatures to create synthetic seismic traces at the two drill sites. The synthetic traces are used to link the drill core information to the new seismic profile and thereby to regional geologic models. Our new integrated seismic and drilling results more clearly define regional subsurface geometries of acoustic units near and between the drill sites, and establish a tie between the seismic 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 Lambert Graben, which is filled with >5 km of late Paleozoic to Quaternary sediments (Stagg, 1985; Cooper et al., 1991a). In addition to drill cores, downhole logging measurements that included velocity, density, resistivity, porosity, natural gamma radiation, and others were recovered at Sites 742 and 1166 to determine and infer rock lithologies (Tables T1, T2). A diverse suite of facies characterizing depositional environments that include preglacial alluvial plain and lagoonal, early glacial fluvial outwash plain and shallow marine, and glacial marine and subglacial were drilled at the two sites (Barron, Larsen, et al., 1989, 1991; Barron et al., 1991; Shipboard Scientific Party, 2001).

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

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

1165

Prydz Trough-Mouth Fan

1167

70?E

743 Continenta7l 3sh9elf

break

Four Ladies Bank

Prydz Bay

742

1166

741

Fram Bank

Prydz

740

Channel

70?S Amery

Ice Shelf

Lambert Graben

70?S Prince Ch

arles Mountains

330?

0?

30?

300?

Weddell

Sea

West M Antarctica

Transanta rctic

Lambert drainage basin

East

Prydz Amery Bay Ice Shelf

80?

Antarctica

60?

Grounding line

270?

85? 80? 75? 70? 65? 120?

ountains

240?

Ross Sea

210?

180?

150?

km

0

100

200

Ice flow unit boundaries

Ice flow direction

Lambert Glacier

Rock outcrops ODP drill sites

F2. Seismic sequences drilled during Leg 119, p. 14.

Depth (km)

S 0 1

N

PS.2A

Early glacial

1166 742

739 PS.1

741

740

743

PS.2B PS.4

Preglacial

PS.2B

PS.2A

PS.5 2

0

km

50

Leg 188 drill site Leg 119 drill sites

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

Two-way traveltime (s)

SW

Site 1166

2500

3000

Crosses line BMR 33-23

0.6

0.8

1.0

1.2

1.4 Site 1166

0.6

0.8 1.0 1.2 1.4

2500 2000

Glacial PS.2A2 Preglacial

5000 AGSO 149/1301 4500

3000

75?E 67.5?S

2000

BMR 33-23

4000

2500 Site 1165

Palmer 01-1-4 2000

3500 3000

~15 km

BMR 33-22

75.5?E

Site 742

2900

3500

Shotpoint 4000

Site 742

4500

4900

Crosses line AGSO 149/1301

NE 5200

PS.1

PS.2A1

Multiple

Site 742

PS.2B

Top of layered unit

3 km

Two-way traveltime (s)

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

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

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seismic unconformity. Unit PS.2A contains late Eocene?early Oligocene marine 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 with thin layers of diatomaceous sediment inferred to be glacial till deposit (Cooper et al., 1991a). Holes 1166A and 742A in total penetrated through 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 011-04, were acquired in February 2001 using a single 90-in? generator injector air gun fired every 6 s at a ship's speed of ~ 5 kt and distance between shotpoints of ~15 m. The offset between navigation shotpoint (i.e., ship's satellite antenna) and seismic midpoint (i.e., between air gun and streamer) is ~60 m. We incorporate the offset in the figures that show correlations of seismic traces to drill cores. The shotpoint numbers 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 from impedance profiles derived from downhole logging velocity data and from seismic pulses extracted from seafloor reflections in the Palmer data (Handwerger et al., this volume). We compared the synthetic traces with the Palmer seismic reflection profile to establish the position of the drill core lithologic boundaries on the seismic reflection profile at both drill sites (Fig. F4). We then traced reflections and acoustic units between the drill sites, initially from Site 1166 to Site 742 and then in the opposite direction (Fig. F3). We used two out of three existing seismic cross lines near the drill sites (Figs. F5, F6) to refine and verify the acoustic unit boundaries. The third existing line (BMR 33-22) was not used for this study because it was of lower resolution and, although it was used in earlier correlations (Cooper et al., 1991a), it did not add new information to the present study. The reflection times to the seafloor in each of the seismic lines differs, due to the different sourcereceiver geometries and recording parameters; however, the relative subsurface reflection times can be accurately compared because of the large water depths (relative to the source-receiver separations). Table T3 gives data on seismic line crossings and closest approach to drill sites.

We describe acoustic units in the Palmer data using the nomenclature of Cooper et al. (1991a) and seek to trace lithologic units identified at drill sites as acoustic units in the seismic data. The large apparent lateral and vertical subsurface variations commonly lead to variable acoustic facies within each acoustic unit. However, over the distance between Sites 742 and 1166, a reasonable and coherent correlation of acoustic units and lithologic units (Fig. F4; Tables T1, T2) can be made. In the following description of units from deepest to shallowest, we give the site number for each lithologic unit (e.g., Unit IV [1166]) to help avoid confusion.

Unit PS.2B

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

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

Two-way traveltime (s) Two-way traveltime (s)

2.5 2.0

2.7 2.3 1.9

SW 0.6 0.7 0.8 0.9 1.0

Site 1166

Depth Lithology VP (km/s)

(mbsf)

ss

I 100

II 200

III

300IV V TD = 381 mbsf

Crosses BMR line

33-23

Crosses AGSO line 149-1301

Precursor Seafloor ?

G3 PS.1

?

?

G2

PS.2A1 G1

CT

PS.2A2 A ? Glacial

Preglacial L A?

A

A?

PS.2B

1.1 1 km

1.2

~28 km

A Alluvial plain L Lagoonal

CT Channel-fill or marine transgressive G1 Proximal glaciomarine

G2 Subglacial/proglacial G3 Mixed glacial (subglacial and glaciomarine)

Site 742

Depth

Lithology VP (km/s)

(mbsf)

ss

?

I I. Quaternary Quaternary -

?

I. Pliocene II Homogeneous

100

diamictite with pebbles

III e. Plio.-Quat.

IV

Homogeneous diamictite

200

Oligocene-

V Eocene

calcareous

diamictite

300 VI

Silt,

?

sand, clay

?

TD = 316 mbsf

Top of layered unit

1 km

NE 0.6 0.7 0.8 0.9 1.0 1.1

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

Two-way traveltime (s) Depth (mbsf)

Two-way traveltime (s)

NW 0.6 Precursor Seafloor

0.8

1.0

~2200 m from Site 742

Age and

lithology 0

I. Quaternary

Quaternary -

I. Pliocene

Homogeneous

diamictite with

100

pebbles

e. Plio.-Quat. Homogeneous

diamictite

200

OligoceneEocene

Calcareous

diamictite

Silt,

300

sand, clay

TD = 316

Top of Unit PS.2

Top of layered unit

SE 0.6

0.8

1.0

1 km

1.2

1.2

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

Two-way traveltime (s)

NW 0.6 0.7 0.8 0.9 1.0

~780 m from Site 1166

SE

Lihologic unit age I-early Pliocene to Holocene II-late Eocene to early Oligocene III-late Eocene IV-Late Cretaceous V-Cretaceous

Lithostratigraphic unit

IB Glacial IC Glacial ID Glacial II Proglacial

III Fluvial/ deltaic

IV Preglacial lagoonal

V Preglacial

Lithologic unit descriptions IB-Diamict IC-Sandy/clayey silt ID-Diamict II-Claystone III-Sand

TD = 381 mbsf

1 km

T3. Location information for seismic lines, p. 21.

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stratigraphic Unit IV (Shipboard Scientific Party, 2001). In the uppermost part of Unit PS.2B reflections are weak and contorted, and these correspond to the restricted marine or lagoonal deposits of lithostratigraphic Unit IV (1166). Layered reflections in the deeper part of PS.2B correspond to the poorly sampled alluvial plain claystone of lithostratigraphic Unit V (1166). The layered deeper part of Unit PS.2B thickens to the northeast toward Site 742 (Fig. F4). Within Unit PS.2B, some layered reflections truncate to the southwest against the contorted 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 reflection. We establish the unconformity's position by (1) projection of the layered reflection at the top of Unit PS.2B along the Palmer line to and beyond Site 742 and (2) comparison of the reflection geometries of the layered and upper contorted units in PS.2B on the Australian Geological Survey Organization (AGSO) cross line (Fig. F5) with the Palmer line near 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 deeper subunit, unconformably overlays Unit PS.2B and is characterized by weak distorted reflections. Subunit PS.2A2 has an undulating upper surface, and the unit thins to the northeast away from Site 1166. At Site 1166, Subunit PS.2A2 corresponds to the massive sands of lithostratigraphic Unit III (156?268 meters below sea floor [mbsf]) (Table T2). Subunit PS.2A2 was either not sampled at Site 742 or was sampled at the 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 reflection packages with different seismic characteristics. The three packages together correspond to lithostratigraphic Units II (1166), V (742, lower half), and VI (742), which are glacial units with glaciomarine, waterlain till and possible subglacial sediments (Fig. F4; Tables T1, T2). The lower reflection package is composed of continuous strong layered reflections that drape the undulating top surface of Subunit PS.2A2 and that fill depressions in this surface (Figs. F3, F4). At Site 1166 this package corresponds to glaciomarine interlayered sands and clays (Fig. F4). The middle reflection package has homogeneous to disrupted low-amplitude reflections that correspond at Site 742 to variable-composition diamictites (Fig. F4). This package thins toward Site 1166 and can be traced to within 1 km of the drill site, at which point it cannot be resolved. The upper reflection package has interlayered high-amplitude and chaotic reflections that also correspond at Site 742 to variable composition diamictities (Table T1). Reflections in the upper package are truncated by the overlying unconformity, and the package pinches out midway between Sites 742 and 1166 (Fig. F4).

Unit PS.1

Unit PS.1 lies between the regional Paleogene/Neogene unconformity 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 characterized principally by chaotic reflections, but some continuous layered reflections are also observed. The bottom of the unit is in places denoted

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by a continuous high-amplitude banded reflection and in other places by the base of a thin chaotic unit below the banded reflection (Fig. F3). The interlayering of high-velocity diamictites with low-velocity sediments above (diatom bearing) and below (interbedded sand/silts) close to the unconformity leads to a complex seismic response (e.g., synthetic traces at Sites 1166 and 742) in the thin and laterally variable lithologic units (Figs. F3, F4). Unit PS.1 comprises stratified to massive diamictites with at least one interbedded layer of diatom-bearing glaciomarine sediments (Fig. F4; Table T2). Recovery was low at Sites 1166 and 742, and the origin of layered reflections within Unit PS.1, other than 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 Sites 1166 and 742 (Fig. F3) and AGSO line across Site 742 (Fig. F5) provide subsurface images, more detailed than existing seismic data, to augment prior interpretations of Prydz Bay seismic stratigraphy (e.g., Cooper et al., 1991a) and inferred depositional paleoenvironments (e.g., Hambrey et al., 1991; Shipboard Scientific Party, 2001). Shipboard Scientific Party (2001) interpreted drill cores at Site 1166 as depicting a change in depositional paleoenvironments from alluvial plain and lagoonal settings in preglacial times (Units V and IV [1166]) to the development 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 diamictites in late glacial times (Unit I [1166]; Units II and IV [742]). Their interpretations were built partly upon Site 742 results (e.g., Barron, Larsen, et al., 1989, 1991; Barron et al., 1991; Hambrey et al., 1991) of proximal to subglacial marine environments in early glacial times (Units VI and V [742]) followed by waterlain tills and subglacial diamictites in late glacial times (Units IV through I [742]). Our interpretations of the new seismic data are generally consistent with the prior ones, but we 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 claystone sample from the top of the unit and on seismic and lithologic correlation with Site 741 (Leg 119), as being possibly deposited on a low-relief alluvial plain (Shipboard Scientific Party, 2001). A chaotic to layered unit that includes similar lithology strata but more indurated and older age (Early Cretaceous) was sampled at Site 741 (Leg 119), and was also interpreted (Barron, Larsen, et al., 1989) as being deposited on a low-relief alluvial plain. Prior regional seismic profiles show that the layered section of Unit PS.2B is extensive across the inner shelf (Cooper et al., 1991a) and thickens to the northeast, suggesting that these strata were deposited on the flanks of a slowly subsiding basin (i.e., Prydz Bay basin [Masolov et al., 1981] that trends northeast?southwest across the central part of Prydz Bay [Cooper et al., 1991a]). Unit PS.2B reflections truncate to the southwest near Site 1166, indicating that these strata have been eroded, likely during sea level lowstands (Fig. F3). The overlying Late Cretaceous lagoonal or restricted marine deposits (i.e., Unit IV [1166]) likely reflect a time when Prydz Bay Basin was low-lying and

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