Iceberg rafting and scouring in the Early Permian Shoalhaven Group of New South Wales, Australia: Evidence of Heinrich-like events?

This paper describes multiple layers of ice-rafted debris and an ice-scour structure in Early Permian marine strata within the Shoalhaven Group of the Sydney Basin, New South Wales, Australia. Strata were deposited on a high-latitude, glacially influenced continental margin during the final stages of Late Palaeozoic glaciation between 277 and 260 Ma. Poorly sorted ice-rafted debris, forming layers up to 1.25 m thick, is common within inner to outer shelf and continental slope facies of the Shoalhaven Group. More than 120 layers can be identified. These are argued to record the enhanced delivery of debris by icebergs drifting north from marine-based Antarctic ice margins. Deformation structures in associated sediments are interpreted as iceberg scours and ice keel turbates. Associated glendonites record early diagenesis of organic-rich sediment and bottom water temperatures close to freezing. A speculative hypothesis relates layers of ice-rafted debris within the Shoalhaven Group to multiple episodes of accelerated calving at the margins of a marine-based Early Permian ice sheet in East Antarctica. These events may be analogous to late Pleistocene Heinrich events, when huge armadas of icebergs were released from the marine-based Laurentide Ice Sheet and triggered abrupt changes in ocean circulation and climate. Similar forcing may have characterized the Early Permian ice-ocean system. Regardless of the climatic significance of ice-rafted layers, this is the first time such facies have been documented in the pre-Pleistocene literature; to date, few ice scour structures, and no ice keel turbates, have been reported.