History of the Ross Sea Sector of Antarctica as Recorded by the Beacon Supergroup in Southern Victoria Land and the Darwin Mountains

Abstract

Flat-lying sedimentary strata of the Beacon Supergroup (Devonian - Triassic) crop out extensively along the higher portions of the Transantarctic Mountains, reaching a thickness of 2.5 km. This thesis examines the history of the Ross Sea sector of Antarctica as recorded by these strata exposed along a 500 km segment of the Transantarctic Mountains in southern Victoria Land and the Darwin Glacier area. Over recent years both the paleoenvironmental and paleotectonic settings of the Beacon Supergroup have been the subject of much debate. These questions are addressed with new field evidence and a reinterpretation of existing published and unpublished data. The Beacon Supergroup is divided into the Taylor Group (Devonian) and the Victoria Group (Carboniferous to Triassic). The Taylor Group rests unconformably on Ordovician and older basement rocks of the East Antarctic Craton, and contains varied and abundant trace fossils, which have been interpreted by palaeontologists as indicating marine deposition. However, sedimentologists have largely favoured nonmarine (mostly braided river and ephemeral lake) deposition on the basis of desiccation polygons, fining upward cycles, unidirectional paleocurrents, small channels, red beds and paleosols. The debate is reconciled by a reinterpretation of the trace fossil assemblage. Widespread and well preserved Cruziana, Diplichnites, Rusophycus and Skolithos which have previously been used as marine indicators, are shown to comprise an ichnofacies (Scoyenia ichnofacies) that is entirely typical of an alluvial plain setting. The basal strata of the Victoria Group record several episodes of continental glaciation. By the time this ended in the Early Permian land plants had evolved to a stage that they were able to rapidly colonise former outwash fans and tills, resulting in a dramatic change in both the style of sediment transport and deposition. Coal seams and carbonaceous horizons, widespread fine grained beds and diverse paleocurrent patterns indicate deposition by meandering rivers and widespread coal swamps, in contrast to the extensive braided systems of the Devonian plains. The Beacon Supergroup and the underlying Kukri Erosion Surface are largely undeformed, showing that no compressive strain has occurred in the Darwin Mountains and southern Victoria Land region since at least the Early Devonian. This absence of compression suggests that the suturing of the East Antarctic Craton and the crust presently beneath the Ross Embayment occurred during the Ross Orogeny (Ordovician). Following this accretion of several hundred kilometres of continental crust the active plate margin was displaced so far to the east that southern Victoria Land (and the Darwin Mountains) no longer experienced the direct effects of subduction along the paleoPacific margin of Gondwana. The absence of deformation, extremely low accumulation rates and sustained continental freeboard show the Beacon Supergroup in southern Victoria Land and the Darwin Mountains to have been deposited on thick continental crust. Sedimentation rates for the Beacon Supergroup are very slow (12.5 m/Ma), suggesting deposition in a long-lived (about 200 Ma) but slowly subsiding intracratonic basin. Accumulation was probably initiated by subsidence caused by thermal decay of the deep seated Ross Orogen, but continuation of basin formation was more likely due to weak intracratonic stresses (extension) or adjacent intracratonic uplift associated with deep seated mantle processes. During the Triassic a change from the previously quiescent tectonic setting is recorded by the incoming of volcanic detritus - a change recorded in Permian strata in the Beardmore Glacier area 300 km south of the study area. It is inferred that this influx of volcanic detritus records a period of intraplate silicic volcanism caused by crustal melting associated with the initial mobilisation or accumulation of tholeiitic magma prior to initiation of the Ferrar magmatic event in the Middle Jurassic (179 Ma), which marked the end of Beacon Supergroup deposition.