NEOGENE LAKE SEDIMENTS IN THE OLYMPUS RANGE, SOUTHERN VICTORIA LAND, ANTARCTICA

Unconsolidated diatom-bearing lake sediments occur at 1400 masl in the Olympus Range, southern Victoria Land, Antarctica. Diatoms range from very abundant to rare and often occur in unbroken chains suggesting deposition in quiet shallow water. . This observation is also supported by the presence of woody material containing epiphytic diatoms. The lake sediments form an isolated patch <400 m2 in area and are situated on a gently sloping bedrock bench that lacks enclosed topographic depressions suggesting that the lake must have been impounded against thick ice filling the adjacent valley. Because main trunk valleys were graded to near sea level more than 15 million years ago (Sugden et al., 1995), uplift is assumed to have been minimal since deposition. The lake sediments are unconsolidated, thinly bedded fine sand, silt, and clay. Rip-up clasts of finely laminated mud occur in some of the lowermost beds. These delicate clasts, some as much as 15 cm long and 7 cm thick, contain the diatoms described here. Shifting glacier margins likely caused subaqueous sediment flows to redistribute these laminated-mud clasts. These diatom-bearing sediments are at least middle Miocene in age. Field evidence indicates that, in some locations, lake deposits were partially modified when thick ice overtopped the Olympus Range. In the adjacent Asgard Range, this overriding event occurred between 15 Ma and 13 Ma (Marchant et al., 1993).