An early to middle Miocene record of ice-sheet and landscape evolution from the Friis Hills, Antarctica

Abstract

The Friis Hills (77°45′S, 161°30′E), in the McMurdo Dry Valleys region, Antarctica, is a 1-km-high inselberg with early to middle Miocene–aged glacial deposits preserved on its upper surface. Lithology, sedimentology, and ice-flow direction define three units within a 34-m-thick sequence: the Friis I, Friis II, and Cavendish drifts. Tills in Friis I drift were deposited from alpine glaciers flowing to the SW. An extensive tephra bed in the drift is 19.76 ± 0.11 Ma in age based on 40 Ar/ 39 Ar dating. Tills in Friis II drift were deposited from alpine glaciers flowing to the SE. Fossiliferous fluvial and lacustrine beds in the drift indicate that vegetation and insects repeatedly recolonized the Friis Hills during periods of reduced ice. The geographic distribution of modern descendants of the fossils specifies mean summer temperatures of 6–7 °C and precipitation as much as 3000 mm during these interglacial periods. The presence of fossils also indicates deposition of Friis II drift occurred before the mid-Miocene climate transition ca. 14.1 Ma. After this, eastward-flowing ice partially eroded the Friis I and II drifts. The ice also cut parallel troughs to the N and S, topographically isolating the Friis Hills. The Cavendish drift represents renewed deposition but from eastward-flowing outlet glaciers within the newly established troughs. Together, the Friis Hills drifts register multiple stages in the evolution of the East Antarctic cryosphere, including the general growth of the system, reorganizations of glacial drainage patterns, and periods of major downcutting. Importantly, they also provide constraints on terrestrial climate conditions during these stages. The Friis Hills record is the first that can be directly compared with shallow-marine records from the Ross Sea. The record also provides new constraints for paleoglaciological models of the early East Antarctic Ice Sheet. Based on lithologic and geomorphic arguments, the Friis Hills drifts are younger than nearby Sirius Group tillites, which predicates that the Sirius Group in the Dry Valleys region is at least early Miocene, or more probably, Oligocene in age.