Sedimentology and Stratigraphy of Miocene-Age Glacial Deposits, Friis Hills, Antarctica
Smith, Alexander Ryan
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The Friis Hills is an isolated plateau standing as much as 600 m above surrounding topography in the McMurdo Dry Valleys region or Antarctica.Preserved on the plateau surface is a sequence of early to middle Miocene-aged dritis. At the eastern edge of the plateau, these drifts fill a shallow paleovalley to a depth of at least 35 m. The drills are exposed in a natural cross-section where modern topography crosscuts the paleovalley. Establishing an age and an environmental interpretation for these deposits is important because Antarctic paleoclimate records are lacking from the Mid-Miocene Climate Optimum. Two drifts fill the ancient paleovalley in the eastern Friis Hills. The upper drift is here named Cavendish drift: the lower is here named Friis drift. Cavendish can be subdivided into three units, whereas Friis drift can be subdivided into two units. Each of these units is a horizontal bed that laps on paleovalley sidewalls. The lowest, Friis II, is a compact diamicton that is overlain by a nearly in-situ bedded volvanic ash. Based on Ar/Ar dating, the ash is 19.76 [plus/minus] 0.07 Ma old. A second diamicton, Friis I, conformably blankets Friis II and was discovered to hold fossileferous interbeds. Both Friis I and II contain erratic clasts and both are lodgemont tills deposited from small, locally derived, alpine glaciers. Bedrock striations show ice flow to the northeast at azimuths between 025° to 032°, parallel to the trend of the paleovalley axis. Above these, Cavendish I. II. and III were deposited when thick ice covered the Friis Hills. Where the Cavendish drift laps onto paleovalley sidewalls, bedrock striations show ice flow from 077° to 150°. Cavendish drift was deposited sometime alter 19.8 Ma but before 14 Ma. when the Dry Valleys glacial records show that regional glaciers became cold-based. Downcutting eventually isolated the Friis Hills plateau, resulting in the preservation of the drift sequence. This event was most likely associated with growth or the East Antarctic Ice Sheet 14 Ma ago. This age constraint means that the tills preserved in the Friis Hills date from a time just before the East Antarctic Ice Sheet expanded and became a permanent feature. Based on the age-dated stratigraphy presented in this thesis, future work focusing on fossiliferious interbeds could provide unique and important constraints on Miocene climate change.