Key exposures through the Late Cretaceous (Campanian-Maastrichtian) Marambio Group are located in the Admiralty Sound region, James Ross Island group, Antarctica. On southern James Ross Island, an extensive sequence of bioturbated silty mudstones, muddy sandstones, fine-grained sandstones, ash layers and rare conglomerates has been subdivided into two component members of the Santa Marta Formation: the Rabot Member and the overlying, newly defined, Hamilton Point Member. Both members are fossiliferous, and have yielded a variety of both macro- and microbiotas indicating relatively shallow (i.e., shelf depth) marine conditions. In particular the Rabot Member contains an assemblage of both heteromorph and regularly coiled ammonites, giant inoceramid bivalves, and other benthos. A combination of both ammonite and palynomorph evidence suggests that both the Rabot and Hamilton Point members are early to 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-westernmost tip of Seymour Island. This unit comprises poorly lithified grey sandy mudstones, lithified fine-grained sandstones, and dark mudstones. It contains numerous concretion horizons and is typified by the late Campanian-early Maastrichtian Gunnarites antarcticus molluscan assemblage. The Snow Hill Island Formation is in turn unconformably overlain by the Lopez de Bertodano Formation, which, as redefined herein, is restricted to the northern tip of Snow Hill Island, Seymour Island, and one small exposure on Vega Island. An informal lithostratigraphical 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 Maastrichtian age. The stratigraphical scheme presented here has enabled us to enhance regional correlation of the Late Cretaceous strata within the James Boss Basin. Based on these new correlations, we can prove that the Campanian-Maastrichtian sequence is between 2500 and 2900 m thick. This is one of the thickest onshore Late Cretaceous successions in the Southern Hemisphere, and has the potential to become a key reference section. In addition, given its high palaeolatitude location, it is a crucial locality to examine Late Cretaceous palaeoenvironmental change. (C) 1997 Academic Press Limited.