Continental tillites document Late Oligocene change from chemical to physical weathering under a wet-based east Antarctic ice sheet
|Location||International Geological Congress,oslo 2008|
|Author||Baroni, Carlo۱; Fasano, Francesco۲; Giorgetti, Giovanna۲; Salvatore, Maria Cristina۳|
|Holding Date||07 October 2008|
Antarctic ice sheets play a relevant role in conditioning the sea level, the oceanic circulation, and the climate on a global scale. Reactions of the Antarctic cryosphere to past climatic-environmental variations are complex and poorly known. For this reason, Antarctic glacial history must be better known to have confidence in understanding the mechanisms forcing global changes.
There is an ongoing dispute on Antarctic glacial history, not only regarding the East Antarctic Ice Sheet (EAIS) establishment but also mainly focusing on 1) the ice sheet shifting from temperate to polar dynamics, and 2) its stability since that switch.
Although continental environment is a difficult task because continuous continental sequences are absent and paleontological documentation is scattered, glacial deposits and glacially shaped bedrock preserved in Antarctic ice-free areas offer valuable opportunities for investigating the history and behaviour of the complex Antarctic glacial system. On the continent, the Sirius Group includes the most spread and studied glacial deposits, widely outcropping in southern Victoria Land. The northernmost outcrop of lithified continental glacial deposits in Victoria Land has recently referred as Ricker Hills Tillite (RHT).
Micromorphological analysis of tillite samples from the Sirius Group and from the RHT shows a similar mechanism of deposition. Soft-bed condition with ductile deformation is well represented in the RHT and also commonly recognizable in samples from the Sirius Group. All the studied samples testify deposition in wet conditions under a wet-based ice sheet, although ductile features more deeply imprinted samples from the RHT.
X-ray diffraction and SEM-EDS analyses of clay minerals show a high content of smectite and caolinite in RHT samples, indicating continental chemical weathering under wet conditions also after the RHT deposition. Illite and chlorite represent the prevailing if not exclusive clay minerals in samples from the Sirius Group, suggesting that only physical weathering was active during and after their deposition.
As documented by clay mineral assemblage variation in CRP drillholes, the progressive cooling of the Antarctic continent allowed chemical weathering in "warm" conditions until the Late Oligocene period in southern Victoria Land, leading to a chronological constrain for RHT deposition. For the same reason, the tillite from the Sirius Group would be younger than the RHT and deposited after Late Oligocene.
Although RHT and Sirius tillites deposited under an enduring wet-based glacial environment, different clay mineral assemblages document a change in weathering processes. Late Oligocene period is hence confirmed to represent a threshold in weathering processes marking the shift toward cooler climatic condition in Antarctica. Nevertheless, it is demonstrated that Late Oligocene does not represent a significant threshold in glacier dynamics, since wet-based conditions endured also later.