Geology of the Sirius Group at Mount Feather and Table Mountain, South Victoria Land, Antarctica

The Sirius Group comprises of wet based glacial and related deposits found at high elevations throughout the Transantarctic Mountains. The discovery of marine Pliocene diatoms from within glacial till by Harwood (1983) led Webb et al. (1984) to propose that they were sourced from diatom bearing sediment eroded by glaciers from middle Pliocene marine basins inland of the Transantarctic Mountains. Others consider that temperatures during middle Pliocene times were not high enough to melt back the Antarctic ice sheet and expose these inland basins. They support the long held view of a stable Antarctic ice sheet since middle Miocene times, and insist that the Sirius Group is much older, explaining the diatoms as wind blown. This study was undertaken in order to determine whether the diatoms were incorporated into Sirius Group tills during or after their deposition. Sites were sampled at Mount Feather and Table Mountain in South Victoria Land. The distribution of diatoms through the upper 37 cm of the till were documented. Samples were also taken in snow and from other non Sirius Group surfaces (regolith) for comparison purposes. The geomorphic setting of the Sirius Group tills at Mt. Feather and Table Mountain suggests that their deposition predated the deep valleys that now run through the Transantarctic Mountains. Diatom abundances from within the tills are low (averaging about 1 diatom diatom per gram) and highly variable from site to site. Initially 184 diatoms were recovered from 10 samples at Mt. Feather and less than 7 diatoms were found from 4 samples of till at Table Mountain. At Mount Feather diatoms are concentrated in the surface few centimetres of the till and numbers generally decrease with depth. The pore size within the tills is highly variable but is on average ten times the size of the average diatom (10-15 microns) from within the deposits, allowing at least some diatoms to work their way into the tills from the surface. Diatoms from the snow and regolith from other rock surfaces have a similar diatom assemblage to the Sirius tills, containing many of the same common forms. Some non Sirius Group regolith samples have much larger concentrations of diatoms suggesting they have a much better trapping ability than the Sirius Group tills. These data indicate that most diatoms from the Sirius Group tills have been introduced from the atmosphere and have worked their way into the till. Thus the Sirius diatoms record not Pliocene marine basins of the Antarctic interior and subsequent extensive over riding glaciation, but the atmospheric transport and collection of both modern and ancient diatom bearing dust from within and beyond the continent. The Sirius Group tills do however have a phytolith (siliceous clasts from the cells of plant tissue) flora of glaciogenic origin, indicated by the lack of a vertical trend in abundance and very low levels of phytoliths in nearby snow and regolith samples