TABLE NUNATAK - A KEY OUTCROP OF UPPER CRETACEOUS SHALLOW-MARINE STRATA IN THE SOUTHERN LARSEN BASIN, ANTARCTIC PENINSULA

The northern, James Boss Island region of the Larsen Basin, on the eas tern, back-are margin of the Antarctic Peninsula magmatic are, include s one of the thickest and most complete Upper Cretaceous sedimentary s uccessions exposed in the Southern Hemisphere. However, the southern p art of the basin remains poorly known, mainly owing to inaccessibility and lack of exposure. Table Nunatak, an isolated, l-km-long, 400-m-wi de outcrop at the tip of Kenyon Peninsula, is the only known exposure of Upper Cretaceous or younger strata in this region. The 62-m-thick s uccession exposed there is assigned to the newly defined Table Nunatak Formation. It consists mainly of sharp-based, amalgamated beds of fin e-grained sandstone up to 2.8 m thick, with subordinate intervals of i ntensely bioturbated mudstone. Wave ripples are present at some levels , and locally developed swaley cross-stratification provides evidence for storm-generated combined-flow deposition. However, most sandstone beds appear to be internally structureless apart from normal grading, and are interpreted as the direct suspension deposits of highly sedime nt-charged storm- and/or flood-related flows. The succession represent s relatively nearshore deposition, probably at the mouth of a river or deltaic distributary channel. Charcoalified plant debris, abundant at the tops of some sandstone beds, suggests a periodically wildfire-swe pt hinterland forested largely by coniferous trees. Dinoflagellate cys t assemblages indicate a late Santonian age, and suggest correlation w ith the basal part of the Lachman Crags Member of the Santa Marta Form ation (Marambio Group) on James Ross Island. Palaeocurrents, sandstone petrography and the high sediment supply rate proposed for the Table Nunatak Formation, suggest a relatively high-relief source area to the west, with large-scale erosion of granitoid plutons and metamorphic r ocks, possibly related to are uplift during a mid-Cretaceous compressi onal episode. The formation is evidence of a major southward extension of the Upper Cretaceous strata exposed in the northern Larsen Basin, and suggests lateral continuity of shallow-marine deposition for at le ast 500-600 km along the Weddell Sea margin of the Antarctic Peninsula in Santonian times.