King George 25 de Mayo Island South Shetland Islands

The South Shetland Islands contain a different story of early glacial events within a fore-arc/outer-arc terrestrial setting.

King George Island and neighbouring Nelson Island consist of several tectonic blocks bounded by two systems of strike-slip faults of Tertiary (54-21 Ma) age (Birkenmajer, 1989). Thus, considerable differences in stratigraphic succession, age and character of the rocks occur between particular blocks. The stratigraphic sequence includes mainly Late Cretaceous to Early Miocene island-arc extrusive and intrusive rocks comprising mainly terrestrial lavas, pyroclastic and volcaniclastic sediments often with terrestrial plant fossils. Hypabyssal dykes and plutons intrude the latter. Fossiliferous marine and glaciomarine sediments are also represented that provide clues to palaeoclimates.

Several sequences of tillites crop out within these complicated sequences, representing glacial and interglacial events. Reports of supposed Eocene-age tillites at Magda Nunatak (Birkenmajer, 1980a,b), named the Krakow Glaciation and dated at 49 Ma, have been disproved by Sr dating (Dingle and Lavelle, 1998). However, Birkenmajer et al. (2005) describe valley-type tillites between lava sequences in the E/O Point Thomas Formation in Admiralty Bay. K-Ar dating gives ages of 41-45 Ma for the lavas below the tillites and 45-28 Ma in the lavas above. Birkenmajer et al. thus propose that a glacial period occurred at 45-41 Ma during the Middle Eocene, being the oldest record of alpine glaciers in West Antarctica.

A clear record of glacial activity is present as diamictites and ice-rafted deposits within the Polonez Cove Formation, of mid-Oligocene age (26-30 Ma) (Troedson and Smellie, 2002). This is called the Krakowiak Glacial Member. At its maximum extent, ice was grounded on a shallow marine shelf. Interestingly, exotic clasts within this sequence may represent ice-rafted debris that was derived from as far away as the Transantarctic Mountains, suggesting marine-based glaciation in the Weddell Sea region. Non-glacial sediments that overlie the Polonez Cove Formation signal an interglacial period in the Late Oligocene (26-24.5 Ma), before another glacial phase in the Miocene. The only other terrestrial evidence for Oligocene ice, possibly representing local alpine glaciation, is from Mount Petras in Marie Byrd Land, West Antarctica, where deposits indicate volcanic eruptions beneath ice (Wilch and McIntosh, 2000).

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