Onset of Tertiary continental glaciations in the Antarctic Peninsula sector (West Antarctica)

Abstract

At the close of the Cretaceous, the Antarctic Peninsula sector had a rather warm and dry climate, differentiated into summer and winter seasons, as indicated by annual growth-rings in petrified logs. Vegetation cover was probably patchy due to low amount of precipitation. -There is no indication of contemporaneous continental glaciation, however small ice-caps may have grown on tops of stratovolcanoes and in high mountain groups.

The Early Tertiary saw climatic and environmental conditions initially similar to the precedent ones. Increase in amount of rainfall with time resulted in wide spreading of vegetation cover, with Nothofagus forests rich in fern undergrowth, including tree ferns, and with Araucaria, during Palaeocene and Eocene, followed by Nothofagus-podocarp forests poorer in fern undergrowth during Oligocene. Climatic seasonality is well marked in petrified wood logs as annual growth-rings. Terrestrial animaI life (marsupials, large birds) is recorded at the beginning of PaIaeogene. There is no indication of continental glaciation in the Antarctic Peninsula sector during the whole Palaeogene. The uppermost Oligocene plant-bearing beds (dated at about 24.5 Ma) still evidence a non-glacial -climate. There are, however, evidences from lahar-type debris-flow agglomerates of existence of local ice-caps on tops of stratovolcanoes.

The onset of continental glaciation (ice-sheet at sea level) in the Antarctic Peninsula sector, :slightly post-dates the Oligocene/Miocene boundary. Early Miocene brachiopod-bearing shallow-marine sediments contain pieces of carbonized wood, and are still devoid of convincing glacial climate indicators. The succeeding Early Miocene highly fossiliferous glacio-marine strata are ;crowded with iceberg-rafted debris, often of large dimensions, of Antarctic continent provenance. Andesite dykes which cut through these strata have been K-Ar dated at about 20 Ma. The K-Ar dating of the geological events leaves a narrow bracket for the onset of continental glaciation in :the Antarctic Peninsula sector at between 24 and 20 Ma.