Marine geology of the northwestern Weddell Sea and

This scientific party consisted of John B. Anderson (chief scientist), Lou Bartek, Tom Griffith, Margaret Herron, Doug Kennedy, Jill Singer, and Mike Smith of Rice University, and Tom Kellogg, Davida Kellogg, and Terry Hughes of the University of Maine at Orono. We are indebted to the men and women of the U.S. Coast Guard icebreaker Glacier for their enthusiastic support during our cruise. We could not have expected any greater support than we received. Peter Barker kindly provided seismic data from the South Orkney Plateau, which proved invaluable in the planning of our cruise. John West of the U.S. Geological Survey, Corpus Christie, assisted us in preparing for our seismic work. Steve Montgomery also helped in this regard. This reasearch is funded by National Science Foundation grants DPP 83-15555 (to John B. Anderson) and DPP 80-20000 (to Thomas B. Kellogg).

Marine geology of the northwestern Weddell Sea and adjacent coastal fjords and bays: Implications for glacial history M.J. SMITH*

References

Denton, G.H., and T.J. Hughes. 1981. The last great ice sheets. New York: John Wiley and Sons. Haase, H. 1984. Marine geology, Sea Beam and 3.5 kHz measurements during the German Antarktis II expedition. Berichte zur Polarforschung, 129 - 136. Harrington, P.K., P.F. Barker, and D.H. Griffiths. 1972. Crustal structure of the South Orkney Islands area from seismic refraction and magnetic measurements. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. Sugden, D.E., and B.S. John. 1973. The ages of glacier fluctuations in the South Shetland Islands, Antarctica. [Reprinted from E.M. van Zinderen Bakker, Sr. (Ed.), Paleoecology of Africa and the Surrounding Islands and Antarctica. VIII Balkema, Cape Town, pp. 141 - 159.1

tional glacial marine units, which suggests that a floating ice shelf was not present in this region (figure 3). Surface sediments on the shelf show that marine currents are redistributing terrigenous material derived mainly from local islands, because there is textural grading in an offshore direction. 56W

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Department of Geology Rice University Houston, Texas 77251

During austral summer 1981 - 1982, scientists on board the U.S. Coast Guard icebreaker Glacier collected 53 piston cores and grab samples from the northwest Weddell Sea continental shelf and from fjords and bays of the James Ross Island area (figure 1). The purpose of this study was to examine these cores n light of the postulated expansion of the late Pleistocene Antrctic Peninsula ice sheet into this region. Detailed sedimentologic, compositional, and clast-shape nalyses reveal that piston cores penetrated compound glacial anne sediments north of 64°30'S, residual glacial marine sedients adjacent to Seymour Island, and transitional glacial marne sediments south of 64°30'S (figure 2). Basal tills are not resent in these cores. If the deposits penetrated in these cores a e of late Wisconsin age, these data are inconsistent with prevously proposed models for the late Wisconsin extent of g ounded ice in the Antarctic Peninsula region (Stuiver et al. 1 81). The presence of transitional glacial marine sediments on t continental shelf does, however, imply expansion of a floatirg ice shelf over this area at some time during the past. Petrologic analyses of pebbles and coarse sand from the transitional glacial marine units has led to the identification of disti petrologic provinces and indicates eastward expansion of th Larsen Ice Shelf over the continental shelf. Compound glicial marine sediments are located to the north of the transi* Fresent address: Pecten International Company, Houston, Texas 77001.

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Figure 1. Locations of piston cores and grab samples.

Sediments obtained in fjords and bays of the northeastern Antarctic Peninsula were examined to characterize sedimentary processes active in these areas and to relate sedimentation to land-based studies of local glacial history. Sediments obtained in the Prince Gustav Channel consist of sand and gravel, suggesting deposition by floating ice under the influence of rela85

tively strong currents. Lithologic analyses indicate that sediment distribution in the southern portion of the channel is related to proximity to source. Sedimentation in Croft Bay and Herbert Sound is characterized by (1) a minor coarse-grained ice-rafted component, (2) a very fine-grained terrigenous sand component transported into the marine environment via suspension in meltwater streams or possibly by wind, and (3) high biogenic productivity which masks the input of ice-rafted debris except adjacent to actively calving glaciers or ice-free areas characterized by high ablation rates and meltwater runoff in summer.

This research was supported by National Science Foundation grant DPP 81-16623. This report is a result of my Master's thesis research at Rice University in Houston, Texas.

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Figure 2. Distribution of glacial marine sediments on the northwest Weddell Sea continental shelf. "TGM" denotes "transitional glacial marine sediment"; "CGM" denotes "compound glacial marine sediment"; "RGM" denotes "residual glacial marine sediment." Petrographic, sedimentologic, and shape analyses imply that TGM units were deposited from the base of a floating ice shelf. This suggests eastward expansion of the Larsen Ice Shelf in the study area. CGM sediments represent deposition from floating ice within a low-energy marine environment.

Marine currents are sufficiently strong in Hope Bay to remove most fine-grained sediment, resulting in sandy deposits. In contrast, the fine-grained sediments in Herbert Sound suggest that currents in the inlet are weak, allowing fine-grained material to settle. The overall trend in sedimentation in fjords and bays of the northeastern Antarctic Peninsula is related to glacial retreat and climatic warming. While fluvially transported glacial sediment comprises the greatest volume of sediment delivered to temperate and subarctic fjords, biogenic sediments (primarily diatomaceous mud) comprise the bulk of sediment accumulating in fjords and bays of the northeastern Antarctic Peninsula. Similar to arctic fjords, ice-rafted debris in antarctic fjords is an important component of sediments only adjacent to glacier termini. Fluvial or eolian transported fine-grained sediment typically comprises less than 30 percent of the total volume of fjord sediment.

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Figure 3. Two petrologic provinces were established on the northwestern Weddell Sea continental shelf, shown here with a generalized outcrop map of the study area. Province 1 is characterized by nearly equal amounts of material derived from James Ross Island (JRI) and the Antarctic Peninsula. Province 2 contains a very high percentage of rocks derived from the James Ross Island region. The province lines extend perpendicular to the coastline. The dashed southern boundary of province 1 is Inferred because of the lack of Upper Mesozoic Volcanics and very limited amounts of Andean intrusive material in the cores, although these rocks outcrop extensively south of Cape Longing. ("KIT" denotes "Cretaceous/Tertiary." "MV denotes Mesozoic." "TRIN PENIN" denotes "Trinity Peninsula:')

Reference Stuiver, M., G.H. Denton, T.J. Hughes, and J.L. Fastook. 1981. Histry of the marine ice sheet in West Antarctica during the last glaciatior: A working hypothesis. In G.H. Denton and T.J. Hughes (Eds.), The ast great ice sheets. New York: John Wiley and Sons.

ANTARCTIC JOURNAL