Palaeoclimate implications from deep drilling of Neoproterozoic strata in the Officer Basin and Adelaide Rift Complex of Australia; a marine record of wet-based glaciers

Abstract

Two prominent episodes of Neoproterozoic glaciation in Australia are recorded in the Cryogenian (Sturtian < 725 Ma) and during the Ediacaran (Elatina < c. 580 Ma). Deep stratigraphic drilling and coring of Sturtian and Elatina glacial strata within the Adelaide Rift Complex (Blinman 2), the eastern Officer Basin (Nicholson 2) and the western Officer Basin (Vines 1 and Empress 1/1A) permits detailed sedimentary logging of lithofacies in more than 3000 m of continuous drill core. Glacial intervals in widely spaced drill sites show a commonality of facies types consisting of diamictites intimately interbedded with graded conglomerates, sandstones and siltstones that were deposited by subaqueous sediment gravity flows below wave base. Direct evidence for a glacial source of sediment is provided by striated and outsized clasts in fine-grained laminated facies. Soft-sediment deformation structures produced by downslope slumping, together with intraclasts of reworked sediment, are common. Facies suggest a depositional setting such as that found on submarine fans within Pleistocene glaciated basins strongly influenced by floating ice and the delivery of poorly sorted debris by the rainout of ice rafted debris, fall out of fine-grained suspended sediment and episodic downslope resedimentation. The large volume and great thickness of these subaqueous glacial facies within the Adelaide Rift Complex and Officer Basin indicates that Neoproterozoic glaciers in Australia delivered large fluxes of meltwater and sediment to offshore basins in similar fashion to wet-based glaciers typical of younger Phanerozoic glaciations. Evidence of meltwaters as a key agent of transportation is difficult to reconcile with climate models of a fully permafrozen planet where the hydrological cycle was shut down by extreme cold and normal sedimentary processes were suspended.