SEDIMENTOLOGY AND STRUCTURE OF THE TRENCH-SLOPE TO FORE-ARC BASIN TRANSITION IN THE MESOZOIC OF ALEXANDER ISLAND, ANTARCTICA

The Mesozoic forearc of Alexander Island, Antarctica, is one of the fe w places in the world where the original stratigraphic relationship be tween a forearc basin and an accretionary complex is exposed. Newly di scovered sedimentary rocks exposed at the western edge of the forearc basin fill (the Kimmeridgian-Albian Fossil Bluff Group) record the eve nts associated with the basin formation. These strata are assigned to the newly defined Selene Nunatak Formation (?Bathonian) and Atoll Nuna taks Formation (?Bathonian-Tithonian) within the Fossil Bluff Group. T he Selene Nunatak Formation contains variable thicknesses of conglomer ates and sandstones, predominantly derived from the LeMay Group accret ionary complex upon which it is unconformable. The formation marks eme rgence and subsequent erosion of the inner forearc area. It is conform ably overlain by the 1 km thick Atoll Nunataks Formation, characterize d by thinly-bedded mudstones and silty mudstones representing a marine transgression followed by trench-slope deposition. The Atoll Nunataks Formation marks a phase of subsidence, possibly in response to tecton ic events in the accretionary prism that are known to have occurred at about the same time. The Atoll Nunataks Formation is conformably over lain by the Himalia Ridge Formation, a thick sequence of basin-wide ar c-derived conglomerates. This transition from fine- to coarse-grained deposition suggests that a well-developed depositional trough (and hen ce trench-slope break) had formed by that time. The Atoll Nunataks For mation therefore spans the formation of the forearc basin, and marks t he transition from trench-slope to forearc basin deposition.