South Australian record of a Rodinian epicontinental basin and its mid-neoproterozoic breakup (∼700 Ma) to form the Palaeo-Pacific Ocean

Abstract

The Neoproterozoic Adelaidean System and overlying Cambrian rocks are a thick succession of terrestrial and shallow-marine sediments that were deposited on older continental crust near the eastern edge of the exposed Precambrian rocks in Australia. The Adelaidean System records four major tectonic regimes: (1) early Neoproterozoix formation of an epicontinental, partly rifted basin; (2) mid-Neoproterozoic (∼700 Ma) continental breakup and accumulation of a passive margin wedge; (3) latest Neoproterozoic/Early Cambrian renewed rifting, and (4) Middle to Late Cambrian conversion of the passive margin to a convergence zone during the development of the Delamerian Orogen. Deposition of the Adelaidean System commenced by ∼830 Ma, when Australia was contiguous with Laurentia in the Rodinia supercontinent. Sandstone and evaporitic carbonate of an initial epicontinental basin are preserved as the lowest succession of the Adelaide Geosyncline and the Amadeus, georgina, Ngalia, Officer and Savory-Yeneena basins. Mafic volcanics were extruded under NE-SW continental extension during the Willouran, which was overprinted by a more widespread E-W Torrensian extension. Continental breakup occurred around 700 Ma, during accumulation of the Sturtian glaciogenic deposits in rift valleys. The succeeding siltstone, sandstone and carbonate were deposited on a passive continental margin that faced the newly formed Palaeo-Pacific Ocean as Laurentia rotated clockwise away from Australia. A wide Palaeo-Pacific Ocean existed at the time of the younger Ice Brook (Laurentia) and Marinoan (Australian) glaciations. Dextral shear between northern and southern Australia along the Paterson-Petermann Ranges Orogen in the 600-550-Ma interval, broke up the continuuity of the central Australian basins through uplift of the Musgrave Block, and led to renewed rifting, reflecting the latest Neoproterozoic continental breakup that formed the eastern margin of Laurentia. The South Australian passive margin persisted until the Middle Cambrian when convergence along the Pacific Ocean margin led to development of the Delamerian Orogebn. Deposition of the marine siliciclastic Kanmantoo Group commenced in the late Early Cambrian in a rapidlly subsiding trough, for which both extensional (rift-basin) and compressional (foredeep) origins gave been proposed. Reversal of the eastward-dipping Early Cambrian palaeoslope in the Middle Cambrian shed siliciclastic sediments back onto the continent from an initial uplift in advance of Delamerian deformation.