Structure and Sedimentology of the Beardmore Group, Antarctica: Latest Neoproterozoic to Early Paleozoic Tectonic Evolution of the East Antarctic Margin

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The Neoproterozoic to early Paleozoic transition (700-500 million years ago (Ma)) is a critical period in Earth history. During this period one supercontinent known as Rodinia was coalesced, then fragmented, prior to amalgamation of a second supercontinent referred to as Gondwanaland. These events were manifested by major mountain building, continental erosion, species diversification, sea-level fluctuations, and changes in sea- water composition. Continental-margin sedimentary sequences from this era (notably in western and eastern North America, northeastern Asia, southern Africa, eastern Australia, and the Ross margin of Antarctica) provide detailed records of sea-level fluctuations, faunal distributions, and post- depositional tectonism. One of these sequences, the Beardmore Group in Antarctica, represents a significant element in the evolution of the East Antarctic craton, yet very little is known about its depositional or tectonic history. The Beardmore Group is a thick sequence of graywacke and carbonate that is traditionally correlated with similar rocks elsewhere in the Transantarctic Mountains. Beardmore rocks are largely unfossiliferous, lithologically monotonous, and lie in uncertain depositional relation with both crystalline basement rocks and fossiliferous Lower Cambrian carbonates. Previous workers have suggested that the entire Beardmore Group is Neoproterozoic in age and that it records two distinct deformations, the well-known Ross orogeny (~500 Ma) and an earlier cryptic "Beardmore" orogeny. Reconnaissance field and geochronologic studies indicate that portions of the Beardmore Group are significantly younger (600 Ma) than previously thought, and that it only shows evidence for Ross deformation. These preliminary data reflect uncertainty in the geologic relations of these rocks, and they indicate that the Neoproterozoic tectonic history for the region must be revised. This project will investigate the Beardmore Group in the ce ntral Transantarctic Mountains in order to better understand its depositional history and its role in orogenic events shaping the outer margin of Gondwanaland. The study will include: (1) detailed field study of the stratigraphy, sedimentology, and structure; (2) provenance (source) study of graywackes to constrain the depositional setting of the turbidites; (3) determination of carbonate isotopic compositions for comparison with the global C-isotopic record; (4) refinement of depositional age(s) through U-Pb dating of detrital and igneous zircons; and (5) constraint on the age(s) of deformation(s) with Ar-isotope thermochronology. Field work will be completed during the 1998-99 and 1999-2000 austral summers. This work will test a recently proposed model for Beardmore Group deposition and deformation that involves latest Neoproterozoic-early Paleozoic(?) rift-margin sedimentation and structural inversion as an early expression of Ross activity. The results will help to directly resolve long- standing uncertainties in the geologic history of the Antarctica and they will improve our understanding of global paleogeographic and plate-tectonic relations during supercontinent transformation at the close of the Proterozoic.