Depositional history of pre-Devonian strata and timing of Ross orogenic tectonism in the central Transantarctic Mountains, Antarctica

Abstract

Abstract A combination of field mapping, detailed sedimentology, carbon isotope chemostratigraphy, and new paleontological finds provides a significantly improved understanding of the depositional and tectonic history of uppermost Neoproterozoic and lower Paleozoic strata of the central Trans ant arc tic Mountains. On the basis of these data, we suggest revision of the existing stratigraphy, including introduction of new formations, as follows. The oldest rocks appear to record late Neoproterozoic deposition across a narrow marine margin underlain by Precambrian basement. Siliciclastic deposits of the Neoproterozoic Beardmore Group—here restricted to the Cobham Formation and those rocks of the Goldie Formation that contain no detrital components younger than ca. 600 Ma—occupied an inboard zone to the west. They consist of shallow-marine deposits of an uncertain tectonic setting, although it was likely a rift to passive margin. Most rocks previously mapped as Goldie Formation are in fact Cambrian in age or younger, and we reassign them to the Starshot Formation of the Byrd Group; this change reduces the exposed area of the Goldie Formation to a small fraction of its previous extent. The basal unit of the Byrd Group—the predominantly carbonate ramp deposits of the Shackleton Limestone—rest with presumed unconformity on the restricted Goldie Formation. Paleontological data and carbon isotope stratigraphy indicate that the Lower Cambrian Shackleton Limestone ranges from lower Atdabanian through upper Botomian. This study presents the first description of a depositional contact between the Shackleton Limestone and overlying clastic units of the upper Byrd Group. This carbonate-to-clastic transition is of critical importance because it records a profound shift in the tectonic and depositional history of the region, namely from relatively passive sedimentation to active uplift and erosion associated with the Ross orogeny. The uppermost Shackleton Limestone is capped by a set of archaeocyathan bioherms with up to 40 m of relief above the seafloor. A widespread phosphatic crust on the bio herms records the onset of orogenesis and drowning of the carbonate ramp. A newly defined transitional unit, the Holyoake Formation, rests above this surface. It consists of black shale followed by mixed nodular carbonate and shale that fill in between, and just barely above, the tallest of the bioherms. This formation grades upward into trilobite- and hyolithid-bearing calcareous siltstone of the Starshot Formation and alluvial-fan deposits of the Douglas Conglomerate. Trilobite fauna from the lowermost siltstone deposits of the Starshot Formation date the onset of this transition as being late Botomian. The abrupt transition from the Shackleton Limestone to a large-scale, upward-coarsening siliciclastic succession records deepening of the outer platform and then deposition of an eastward-prograding molassic wedge. The various formations of the upper Byrd Group show general stratigraphic and age equivalence, such that coarse-grained alluvial-fan deposits of the Douglas Conglomerate are proximal equivalents of the marginal-marine to shelf deposits of the Starshot Formation. Paleocurrents and facies distributions from these units indicate consistent west (or southwest) to east (or northeast) transport of sediment. Although the exact structural geometry is unknown, development of imbricate thrust sheets in the west likely caused depression of the inner margin and rapid drowning of the Shackleton Formation carbonate ramp. This tectonic activity also caused uplift of the inboard units and their underlying basement, unroofing, and widespread deposition of a thick, coarse clastic wedge. Continued deformation in the Early Ordovician (younger than 480 Ma) in turn affected these synorogenic deposits, causing folding and thrust repetition of all pre- Devonian units.