ALBERT

Description: The Jurassic Mount Poster Formation of eastern Ellsworth Land, southern Antarctic Peninsula, comprises silicic ignimbrites related to intracontinental rifting of Gondwana. The identification of less voluminous basaltic and sedimentary facies marginal to the silicic deposits has led to a reclassification of the volcanic units into the Ellsworth Land Volcanic Group. This is formally subdivided into two formations: the Mount Poster Formation (silicic ignimbrites), and the Sweeney Formation (basaltic and sedimentary facies). The Mount Poster Formation rhyolites are an intracaldera sequence greater than 1 km in thickness. The basaltic and sedimentary facies of the Sweeney Formation are consistent with deposition in a terrestrial setting into, or close to, water. The geochemistry of the Mount Poster Formation is consistent with derivation of the intracaldera rhyolites from a long-lived, upper crustal magma chamber. The basalts of the Sweeney Formation are intermediate between asthenosphere- and lithosphere-derived magmas, with little or no subduction-modified component. The basalt could represent a rare erupted part of the basaltic underplate that acted as the heat source for local generation of the rhyolites. U–Pb ion microprobe zircon geochronology of samples from the Mount Poster Formation yield an average eruption age of 183.4±1.4 Ma. Analysis of detrital zircons from a Sweeney Formation sandstone suggest a maximum age of deposition of 183±4 Ma and the two formations are considered coeval. In addition, these ages are coincident with eruption of the Karoo-Ferrar Igneous Province in southern Africa and East Antarctica. Our interpretation of the Ellsworth Land Volcanic Group is consistent with the model that the Jurassic volcanism of Patagonia and the Antarctic Peninsula took place in response to intracontinental extension driven by arrival of a plume in that area.

Description: Recent detailed mapping, section logging and an improved understanding of the geological evolution of the Antarctic Peninsula provide a robust framework for an improved lithostratigraphic subdivision of the Latady Basin, eastern Ellsworth Land. Within the Latady Basin we recognize two main groups: Ellsworth Land Volcanic Group and Latady Group. The focus of this paper is the Latady Group, which is formally subdivided into five formations: Anderson Formation, Witte Formation, Hauberg Mountains Formation, Cape Zumberge Formation and Nordsim Formation. Middle Jurassic, shallow marine deposits of the Anderson Formation are overlain by quiet anoxic deposits assigned to the Witte Formation. The start of the Late Jurassic is marked by the deposition of higher energy deposits of the Hauberg Mountains Formation, subdivided into three members (Long Ridge, Mount Hirman and Novocin members) that reflect varying lithological and environmental characteristics. Thermal subsidence during the latest Jurassic led to deposition of the basinal Cape Zumberge Formation, while uplift of an active continental arc along the Antarctic Peninsula led to deposition of the terrestrial Nordsim Formation in the latest Jurassic to earliest Cretaceous. The evolution of the Latady Basin reflects early extension during Gondwana break-up, from the Early Jurassic to earliest Cretaceous, and is consistent with a shift in the underlying forces driving extension in the Weddell Sea area from intracontinental rifting related to a mantle plume, to active margin forces in response to subduction.