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Abstract

Considerable controversy exists concerning the stability of the Antarctic Ice Sheet during the Neogene Period. The northern Antarctic Peninsula is in a critical position in this debate as it represents a peripheral area of the ice sheet and is therefore likely to have been sensitive to climatic changes. This paper is concerned with Neogene glacial deposits that occur on James Ross and Vega islands. They occur between a thick volcanic sequence, the James Ross Island Volcanic Group, and Upper Cretaceous sedimentary rocks; they also occur within the volcanic sequence itself. The glacial deposits, where dated, give a series of snapshots of glacial conditions in Neogene time. The deposits are characterized by diamictite and sandy mudstone. Published 87Sr/86Sr ages on shelly fossils in some deposits range from 9.9 to 4.7 Ma, although additional 40Ar/39Ar ages on interbedded volcanic rocks suggest that younger sedimentary deposits are also present. On James Ross Island the basal diamictite is interpreted as glaciomarine sediment that has undergone subaqueous mass movement, and on Vega Island as basal till originating from the west. Provenance studies indicate that the Antarctic Peninsula Ice Sheet expanded sufficiently to deposit these sediments. These diamictites, in places, are overlain by waterlain tuffaceous rocks that include a minor ice-rafted component. Complex deformation of sedimentary and volcanic deposits and contact-metamorphism relationships confirm that volcanism was contemporaneous with glaciation. Later glacial events (within the volcanic sequence) are characterized by glacial erosion of basalt followed by basal till and, possibly, glaciofluvial deposition. The clasts in the latter are almost exclusively local, hence later glaciation was as a small ice cap constructed on the growing volcanic complex of James Ross Island.

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