LATE CRETACEOUS STRATIGRAPHY OF THE ADMIRALTY SOUND REGION, JAMES-ROSS BASIN, ANTARCTICA

Key exposures through the Late Cretaceous (Campanian-Maastrichtian) Ma rambio Group are located in the Admiralty Sound region, James Ross Isl and group, Antarctica. On southern James Ross Island, an extensive seq uence of bioturbated silty mudstones, muddy sandstones, fine-grained s andstones, ash layers and rare conglomerates has been subdivided into two component members of the Santa Marta Formation: the Rabot Member a nd the overlying, newly defined, Hamilton Point Member. Both members a re fossiliferous, and have yielded a variety of both macro- and microb iotas indicating relatively shallow (i.e., shelf depth) marine conditi ons. In particular the Rabot Member contains an assemblage of both het eromorph and regularly coiled ammonites, giant inoceramid bivalves, an d other benthos. A combination of both ammonite and palynomorph eviden ce suggests that both the Rabot and Hamilton Point members are early t o late Campanian in age. The Santa Marta Formation is believed to pass directly up into the newly defined Snow Hill Island Formation, which forms the majority of the exposure on Snow Hill Island and the south-w esternmost tip of Seymour Island. This unit comprises poorly lithified grey sandy mudstones, lithified fine-grained sandstones, and dark mud stones. It contains numerous concretion horizons and is typified by th e late Campanian-early Maastrichtian Gunnarites antarcticus molluscan assemblage. The Snow Hill Island Formation is in turn unconformably ov erlain by the Lopez de Bertodano Formation, which, as redefined herein , is restricted to the northern tip of Snow Hill Island, Seymour Islan d, and one small exposure on Vega Island. An informal lithostratigraph ical unit characterised by distinctive, pale grey weathering mudstones is identified at the base of the Lopez de Bertodano Formation, and on the basis of palynological studies may be of mid- to late Maastrichti an age. The stratigraphical scheme presented here has enabled us to en hance regional correlation of the Late Cretaceous strata within the Ja mes Boss Basin. Based on these new correlations, we can prove that the Campanian-Maastrichtian sequence is between 2500 and 2900 m thick. Th is is one of the thickest onshore Late Cretaceous successions in the S outhern Hemisphere, and has the potential to become a key reference se ction. In addition, given its high palaeolatitude location, it is a cr ucial locality to examine Late Cretaceous palaeoenvironmental change. (C) 1997 Academic Press Limited.