GONDWANA GLACIATION IN THE DARWIN AND CENTRAL TRANSANTARCTIC MOUNTAINS, ANTARCTICA: EVIDENCE AGAINST AN IMMENSE LATE PALEOZOIC ICE SHEET CENTERED OVER ANTARCTICA

Gondwana glaciation is arguably the most significant environmental event of the late Paleozoic. Despite the glaciation’s importance to Earth system evolution, essential questions concerning its initiation, extent, duration, style, and final demise remain unanswered.

The history of the glaciation is often inferred indirectly from Northen Hemisphere proxy records. Because cyclothem models invoke high frequency and high amplitude changes in base level, cyclothems are interpreted to have resulted from the waxing and waning of an immense ice sheet that lasted from the Mississippian until the Early Permian, and which was forced on Milankovitch time-scales. Most glacial reconstructions hypothesize that a single massive ice sheet covered Antarctica and extended outward over much of Gondwana. In these models, Carboniferous and Permian ice flowed out of a glacial spreading center located over Victoria Land, covered the Darwin and central Transantarctic Mountains (D-CTM), and extended to glacimarine margins in the Ellsworth Mountains and in South Africa.

D-CTM contains some of the best exposed upper Paleozoic glacial strata in Antarctica. Although early workers identified D-CTM diamictites as subglacial units deposited during advance and retreat cycles of the ice sheet, reexamination of the strata indicates that the deposits are Permian, and that glacial basinal (glacilacustrine or glacimarine) rather than glacial terrestrial conditions prevailed during deposition. Within the elongate Transantarctic basin, the occurrence of massive and stratified diamictites resting on sharp and gradational contacts, dropstone-bearing mudstones, slumped sandstones, and cm- to tens of m-scale soft-sediment deformation indicate glacial basinal deposition from rain-out, ice-rafting, sediment gravity flows, and deposition on water saturated substrates. Subglacial diamictites are rare, but become locally more abundant along basin margins where they are interstratified with glacial basinal units. Although these findings do not negate Carboniferous glaciation in Antarctica, they do show that areas formerly hypothesized as covered by km of ice were instead glacial basinal and that glaciation was less widespread (temporally and spatially) during the late Paleozoic than previously hypothesized.