Tectonic and climatic controls on a Mesozoic forearc basin succession, Alexander Island, Antarctica

Abstract

The Cretaceous Fossil Bluff Group on Alexander Island, on the west side of the Antarctic Peninsula, contains a remarkably complete record of the evolution of a forearc basin. The latest (Aptian–Albian) stages in the basin history are recorded in a well-exposed succession at the southern end of the island, where a series of nunataks provide exposure of over a thousand metres of shallow marine and continental deposits. An abrupt facies shift from upper shoreface marine facies to braided fluvial deposits is interpreted as the record of regional uplift in the volcanic arc. This event coincides with the Palmer Land deformation event which may be related to a mid-Cretaceous mantle plume. A gradual reduction in depositional gradient and a return to shallow marine conditions towards the top of the exposed section is interpreted as a consequence of erosion of the arc and subsidence within the basin. Palaeocurrent data and facies distributions indicate that the continental deposits formed a fan-shaped wedge at least 30 km in diameter in the southern part of the forearc basin. Fossil plants indicate that the palaeoclimate was warm and humid throughout the period of deposition. Mapping and facies analysis of the upper part of the Fossil Bluff Group in southern Alexander Island has resulted in a revision of the stratigraphic terminology for the area. The Triton Point Member, formerly part of the Neptune Glacier Formation, has been raised to formation status and two members (the Citadel Bastion Member and the Coal Nunatak Member) and a Bed (the Upper Coal Nunatak Sandstone Bed) are defined here within the formation.