The Role of Eustasy in the Development of a Regional Shallowing Event in a Tectonically Active Basin: Fossil Bluff Group (Jurassic—Cretaceous), Alexander Island, Antarctica

Summary

A 7 km thick, Kimmeridgian–Albian fore-arc basin succession, unconformably overlying an accretionary complex, is exposed on the east coast of Alexander Island adjacent to the Antarctic Peninsula. The basin fill represents a single regressive megasequence, strongly controlled by active faulting along the arc-basin margin. This regressive megasequence comprises a large-scale (minimum 440 m thick) slope-collapse deposit overlain by a 2.2 km thick Tithonian–Berriasian sequence of proximal submarine fan deposits that record three tectonic pulses of coarse clastic sediment input. Intra-basinal synsedimentary tectonism was the primary control on the development of the overlying, 1 km thick, slope mudstone sequence (Valanginian – Aptian). The basin fill is capped by an Aptian – Albian shallow-marine and terrestrial sandstone sequence, 3.5 km thick, which is probably diachronous across the basin. A clearly defined petrographic arc-unroofing trend is documented by a shift in conglomerate clast composition and sandstone petrofacies, and indicates a substantial episode of arc uplift and dissection during Tithonian and Berriasian times.

This overall, 7 km thick, regressive megasequence is punctuated during the late Berriasian by a well-exposed, abrupt facies from inner fan channel, deep marine conglomerates to a 70–90 m thick, shallow marine, storm-dominated shelf sandstone unit. This synchronous, basin-wide event is interpreted as reflecting a sedimentary response, at least in part, to a short-term eustatically controlled sea-level fall. This event correlates with a third-order short-term sea-level fall from the Exxon coastal onlap cycle chart.