New evidence of Eocene preglacial environments has been found on the southern coast of Ezcurra Inlet on King George Island, South Shetland Islands, West Antarctica. Plant remains (trunks, leaves, detritus) and carbonaceous seams and beds occur in sedimen− tary strata in a 4 km long Cytadela outcrop of the Point Thomas Formation. They are an evi− dence for the presence and diversity of terrestrial vegetation in the northern Antarctic Penin− sula region. The forests were composed mostly of Podocarpaceae-Araucaria-Nothofagus, with an undergrowth of hygrophilous and thermophilous ferns, and grew on volcanic slopes and surrounding lowland areas of King George Island during breaks in volcanic activity. The succession that crops out at Cytadela provides a record of changing climatic conditions from a warm and wet climate with extensive vegetation to a much drier climate with limited vegetation and ubiquitous weathering of volcanic bedrock. The geochemical indices of weathering (CIA, PIA and CIW) have narrow and relatively high value ranges (76-88), sug− gesting moderate to high chemical weathering under warm and humid climate conditions. The decrease in humidity and the decline in plant life through the succession can be related to the gradually cooling climate preceding development of the Oligocene ice cover across the Antarctic continent.

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Abstract:Radiometric and geochemical studies were carried out at Red Hill in the southern part of King George Island (South Shetland Islands, northern Antarctic Peninsula) on the Bransfield Strait coast. The rock succession at Red Hill has been determined to represent the Baranowski Glacier Group that was previously assigned a Late Cretaceous age. Two forma− tions were distinguished within this succession: the lower Llano Point Formation and the up− per Zamek Formation. These formations have stratotypes defined further to the north on the western coast of Admiralty Bay. On Red Hill the Llano Point Formation consists of terrestrial lavas and pyroclastic breccia; the Zamek Formation consist predominantly of fine to coarse tuff, pyroclastic breccia, lavas, tuffaceous mud−, silt−, and sandstone, locally conglomeratic. The lower part of the Zamek Formation contains plant detritus (Nothofagus, dicotyledonous, thermophilous ferns) and numerous coal seams (vitrinitic composition) that confirm the abundance of vegetation on stratovolcanic slopes and surrounding lowlands at that time. Se− lected basic to intermediate igneous rocks from the succession have been analysed for the whole−rock K−Ar age determination. The obtained results indicate that the Red Hill succes− sion was formed in two stages: (1) from about 51-50 Ma; and (2) 46-42 Ma, i.e. during the Early to Middle Eocene. This, in combination with other data obtained from other Bara− nowski Glacier Group exposures on western coast of Admiralty Bay, confirms the recently defined position of the volcano−clastic succession in the stratigraphic scheme of King George Island. The new stratigraphic position and lithofacies development of the Red Hill succession strongly suggest its correlation with other Eocene formations containing fossil plants and coal seams that commonly occur on King George Island.

Pyrite framboids occur in loose blocks of plant−bearing clastic rocks related to volcano−sedimentary succession of the Mount Wawel Formation (Eocene) in the Dragon and Wanda glaciers area at Admiralty Bay, King George Island, West Antarctica. They were investigated by means of optical and scanning electron microscopy, energy−dispersive spectroscopy, X−ray diffraction, and isotopic analysis of pyritic sulphur. The results suggest that the pyrite formed as a result of production of hydrogen sulphide by sulphate reducing bacteria in near surface sedimentary environments. Strongly negative d 34 S VCDT values of pyrite (−30 -−25 ‰) support its bacterial origin. Perfect shapes of framboids resulted from their growth in the open pore space of clastic sediments. The abundance of framboids at cer− tain sedimentary levels and the lack or negligible content of euhedral pyrite suggest pulses of high supersaturation with respect to iron monosulphides. The dominance of framboids of small sizes (8-16 μm) and their homogeneous distribution at these levels point to recurrent development of a laterally continuous anoxic sulphidic zone below the sediment surface. Sedimentary environments of the Mount Wawel Formation developed on islands of the young magmatic arc in the northern Antarctic Peninsula region. They embraced stagnant and flowing water masses and swamps located in valleys, depressions, and coastal areas that were covered by dense vegetation. Extensive deposition and diagenesis of plant detritus in these environments promoted anoxic conditions in the sediments, and a supply of marine and/or volcanogenic sulphate enabled its bacterial reduction, precipitation of iron mono− sulphides, and their transformation to pyrite framboids.