Sedimentology and palaeoecology of lonestone-bearing mixed clastic rocks and cold-water carbonates of the Lower Permian Basal Beds at Fossil Cliffs, Maria Island, Tasmania (Australia): Insight into the initial decline of the late Palaeozoic ice age

Abstract

AbstractThe middle Sakmarian Basal Beds on Maria Island were deposited during the initial decline of the Late Palaeozoic Ice Age following Late Pennsylvanian–Early Sakmarian maximum glaciation. At that time, Tasmania was located within the South Polar Circle between an apparent ice-free pole (Antarctica) and the mid- to high-latitude Sydney-Bowen-Gunnedah glacigenic basins in eastern Australia. The dropstone-bearing Basal Beds consist of: interstratified siltstone, conglomerate and cold-water limestones of the Lower Erratic Zone; siltstone andEurydesma-rich cold-water carbonates of the Darlington Limestone; and siltstone and conglomerate of the Upper Erratic Zone. Interstratification of the coarse-clastic strata, siltstones and limestones within these units were previously attributed to glacial/non-glacial cycles. However, the interfingering of beds within each of these units and the occurrence of large, fossil bryozoans crossing and abutting lithological boundaries indicate that cyclicity was of shorter duration than that of Milankovitch-driven cycles. Within these intertidal and subtidal deposits, the occurrence of rounded dropstones derived from local basement rocks exposed along a rocky coastline and an absence of glacial indicators other than dropstones, along with other evidence, suggest that ice rafting was by sea ice rather than by icebergs. Study results confirm the spatial restriction of Middle Sakmarian to earliest Wuchiapingian glaciation.