Western Antarctic Peninsula And Western Ellsworth Land

Few data exist over the continental margin of Antarctica between the Ross Sea and Thurston Island. Further to the east, geophysical data over the upper continental rise and shelf are confined to detailed studies in the northern Antarctic Peninsula region (e.g., Ashcroft, 1972) and to widely spaced profiles (Houtz, 1974; Tucholke and Houtz, 1976) including multichannel seismic data (Kimura, 1982) across the continental margin between Anvers Island and Thurston Island (Fig. 6.7).

The continental shelf varies in width, reaching a maximum of 500 km in the Bellingshausen Sea and 400 km in the Amundsen Sea (Davey, this volume). Long narrow depressions on the shelf lying sub-parallel to the shelf edge have been interpreted as marking the boundary between old continental rock and more recent sedimentary wedges forming the outer continental shelf, slope and rise for the region west of the Antarctic Peninsula (Vanney and Johnson, 1976). A well-developed continental rise exists along the central and western sector of the margin. Seismic data show sediments under the rise to reach about 3 km thickening under the continental slope. Data coverage is poor for the shelf region apart from in the north where Ashcroft (1972) measured a sedimentary thickness of 5 km (seismic velocity less than 4.2 km sec.'1) to the northwest of the South Shetland Islands.

In the Jurassic to Early Cretaceous, the Antarctic Peninsula was the site of a volcanic arc associated with subduction along the Pacific margin (Dalziel and Elliot, 1973). During this time, sediments of the Fossil Bluff Formation in Alexander Island were laid down in a fore-arc basin. Over 5,000 m of sediments of dominantly shallow water origin (mudstone, sandstone, and conglomerate) occur with lavas interbedded in the lower part and plant-bearing sequences, possibly terrestrial, containing local coal measures in the upper part. Marine deposits, similar to the lower part of the section, occur farther north at South Shetland Islands (Thomson, 1982). Uplift and erosion took place in middle Late Cretaceous followed by the extrusion of mid-Late Cenozoic basic volcanics and pyroclastics in Marie Byrd Land and Late Cenozoic volcanics and conglomerates in northern Antarctic Peninsula. The structural development of the region throughout the Mesozoic and Tertiary has been controlled by the development of the active plate margin which is now active only north of the Hero Fracture Zone (Fig. 6.7, Davey, this volume).

Hydrocarbon maturation models have been produced for the upper continental rise sediments off Thurston Island and for the outer shelf sediments near the South Shetland Islands (Davey, 1985). The age of the sediments for the Thurston Island model was based on the results of DSDP sites 323 and 324. An average heat flow was assumed using a heat flow age relationship for oceanic crust. The sediment conductivity was based on the lithologies sampled at the drill sites and a geothermal temperature gradient derived. The computed maturation value was well below the onset of oil generation. The region, however, lies where a spreading ridge passed through the region such that the sediments experienced higher temperatures. However, based on the assumptions made, the margin does

Fig. 6.7. Antarctic Peninsula. The western Antarctic Peninsula-Bellingshausen Sea margin. HFZ = Hero Fracture Zone, TFZ = Tula Fracture Zone. Bathymetry contours at 500 m and 3,000 m delineate continental slope and shelf (Polar Stereographic projection).

not appear to show good hydrocarbon maturation potential.

The age of the sediments, lithology and heat flow for the South Shetland Islands model were very poorly constrained. Although a significant sedimentary section exists, the model served mainly to point out the paucity of useful data in the Antarctic region. On the assumptions made, Davey (1985) derived a very low maturation value.

The geology of the fore-arc basin sediments indicates possible source rocks (coal measures) and possible reservoir rocks (sandstone) in the sequence. The information is, however, too sparse to justify further discussion.

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