Stratigraphy of the Late Paleozoic Ice Age glacial sequences in Tasmania (Australia): implications for the glaciation in southern Gondwana

The Late Paleozoic Ice Age (LPIA) in one of the coldest periods in the Earth history which led to the development of ice covers across the entire Gondwana from Carboniferous to Permian. The LPIA view is changing from a single ice sheet covering the entire Gondwana to a series of small and diachronous ice caps widespread through the supercontinent. Stratigraphic studies and facies analysis are key tools for the evaluation of the paleo-environmental depositional setting and, consequently, of the style of glaciation.

Tasmania is a key region because it was settled between northern Victoria Land (Antarctica) and Australia and the LPIA deposits could help to provide links between the two sectors of Gondwana. Tasmania constituted a sedimentary basin in the late Paleozoic and Mesozoic and thick sedimentary sequence, both marine and terrestrial, known as Parmeener Supergroup, crops out. The lowermost part of the Lower Parmeener Supergroup, consisting in the Wynyard Tillite and its correlative throughout the region, recorded glacial sedimentation linked with ice caps that developed in the region.

Here, we provide a detailed sedimentological analysis of two drillcores which recovered glacial sequences deposited in the Tasman Basin. The cores were placed into two separate sub-basins: the first hole, named Ross 1, is located in the central part of Tasmania and recorded ca. 60m of glaciogenic rocks of the Stockers Tillite; the second, named Cygnet 3, is located in the southern part of the island and recorded ca. 200m of glaciogenic rocks belonging to the Truro Tillite. The centimetric scale sedimentological analyses allow the identification of 14 lithofacies which were grouped into 6 facies association on the basis of depositional genesis. Facies associations vary from possibly sub-glacial or ice contact to ice distal. Deposition is dominated both by gravity and sediment remobilization processes and suspension settling with ice rafted debris accumulation. All of them are indicative of subaqueous deposition, likely glacimarine. Moreover, along the succession the glacial sequence stratigraphy approach was applied and glacial system tracts and bounding surfaces which define glacial sequences were identified. The stacking pattern of the facies associations allow to demonstrate that the glacial sequences record phases of advance and retreat of the glacial front into the basin within the main end of the main glacial phase. The facies associations, mainly interpreted as gravity driven deposits, together with the thin thickness, show that Ross 1 core was located in a basin margin position and that possibly recorded sub-glacial erosion, while Cygnet 3 core, which have greater thickness, shows facies associations mainly related with suspension settling, indicating a more basinal position. Petrographic analysis of the gravel size fraction constituting the diamictite and the ice rafted debris shows difference in the lithological composition of the two formations, sustaining the hypothesis that the sub-basins were fed by different ice caps.