PRESERVED TILL SEQUENCES AND THE REORGANIZATION OF GLACIAL PATTERNS DURING MIOCENE TIME – FRIIS HILLS, ANTARCTICA

The Friis Hills is an isolated plateau standing as much as 600 m above surrounding topography in the Dry Valleys region of Antarctica. Preserved on the plateau surface is a sequence of early to middle Miocene-aged tills. At the eastern edge of the plateau, these tills fill a shallow paleovalley to a depth of at least 34 m. The tills are exposed in a natural cross-section where modern topography crosscuts the paleovalley. Fossil-rich paleosols form interbeds between tills and mark times when ice was absent from the Friis Hills. These well-preserved fossil assemblages can be used to provide a direct estimate of paleoclimate for a time when little is known about Antarctic terrestrial temperatures.

Five tills fill the ancient paleovalley in the eastern Friis Hills. Each is a horizontal bed that laps on paleovalley sidewalls. The lowest, till 5, is a compact diamicton that is overlain by an in-situ volcanic ash bed. Based on ⁴⁰Ar/³⁹Ar dating, the ash is 19.76 ± 0.07 Ma old. A second diamicton, till 4, conformably blankets till 5 and the volcanic ash. Neither of these tills contains erratic clasts; both are basal tills deposited from small, locally derived, alpine glaciers. Bedrock striations show ice flow from 025 to 032°, parallel to the trend of the paleovalley. Above these, tills 3-1 contain erratic clasts of sandstone and granite and all three include proglacial fluvial deposits as much as four meters thick. Where they lap onto paleovalley sidewalls, bedrock striations show ice flow from 077 to 107°, which is parallel to modern ice flow directions. This younger group of tills was deposited from thick ice flowing across and around the Friis Hills sometime after 19.8 Ma. Downcutting eventually isolated the Friis Hills plateau, resulting in the preservation of the till sequence and the crosscutting of the paleovalley. Based on the ⁴⁰Ar/³⁹Ar-dated glacial record from the nearby Olympus Range and on oceanic d¹⁸O records, downcutting was most likely associated with growth of the East Antarctic Ice Sheet 14 Ma ago. This age constraint means that the fossils and tills preserved in the Friis Hills date from a time just before the East Antarctic expanded and became a permanent feature. Together they indicate a dynamic climate with phases warm enough to support communities of plants and animals alternating with cold glacial phases.