Eocene sedimentary facies in volcanogenic succession on King George Island, South Shetland Islands : a record of pre-ice sheet terrestrial environments in West Antarctica

About 34 Ma ago there was a radical change of climate that led to the formation of Antarctic ice sheet. King George Island, located in the South Shetland Islands volcanic arc (northern Antarctic Peninsula region), is one of a few places in West Antarctica which shows a geological record of sedimentary environments preceding development of the ice sheet. The Eocene sedimentary facies occur in the dominantly volcanogenic succession of King George Island. They have been recognized in the Arctowski Cove and Point Thomas formations (Ezcurra Inlet Group) and in the Mount Wawel Formation (Point Hennequin Group) in Admiralty Bay, and in the Mazurek Point Formation (Chopin Ridge Group) and Lions Cove Formation (Polonia Glacier group) in King George Bay. They record a cooling trend in terrestrial environments that began at termination of the Early/Middle Eocene Climatic Optimum, and was followed by a significant deterioration of climate during Late Eocene and earliest Oligocene, directly preceding glacial conditions in the northern Antarctic Peninsula region. The ongoing research confirms the existence of three preglacial climatic stages (PGS-1 – PGS-3) during Eocene – earliest Oligocene, from humid, warm to moderate climate (PGS-1), through cool and dry climate (PGS-2), up to cold and humid conditions (PGS-3). Studies were carried out on usually fine-grained volcanoclastic sediments, containing Podocarpaceae–Araucaria–Nothofagus plant fossils assembly. Beds of reworked pyroclastic material alternate with lava flows or volcanic agglomerates, as well as ex situ blocks of Eocene volcanogenic sediments on a moraine. Calculated geochemical indices of weathering (CIA, PIA and CIW) confirm moderate to high chemical weathering under warm and humid climate conditions at the beginning and deterioration of conditions in the end of Eocene.