Mechanisms of Sandstone Deposition in a Late Proterozoic Submarine Canyon, Adelaide Geosyncline, South Australia

Abstract

Late Proterozoic submarine canyon fills in the Adelaide Supergroup (Flinders Range, South Australia) are asymmetrical in terms of their facies. Coarse breccia units, commonly associated with coarse-grained channelized turbidite sandstone units, generally occur adjacent to north walls of all the east-west-trending canyon incisions. In contrast, fine-grained sandstones and mudstones within the canyon fill are generally associated with south walls. In one canyon (Patsy Springs canyon), an additional element of asymmetry is associated with the prevalence of northward-climbing sets of climbing ripples (southward-dipping stoss sides) within channelized turbidites, in what are interpreted to be major thalweg channels and their associated levees. Flute casts at the bases of indiv dual turbidite sandstones invariably indicate initial turbidity current flow to the west throughout the vertical sequences of the channel fills. Parallel laminations above the flute casts in each flow pass upward into climbing ripples with south-dipping stoss sides, implying southward lateral accretion across the channel of a levee or bar as each turbidity current decelerated. The asymmetries outlined could be explained by: (1) Coriolis force acting on the turbidity currents, or (2) the existence on a steep slope of a meandering canyon gorge, confining a thalweg channel developed within subsequent canyon fill. In such a situation, roller-coastering turbidity currents would seek outer bends of the meandering primary gorge. The single accessible cross section of Patsy Springs canyon does not provide a unique solution; however, studies still in progress on a related series of canyons to the north suggest that a strongly meandering gorgelike canyon with an internal thalweg channel complex, situated in an upper fan or basin slope, provides the best explanation for all observations. A significant Coriolis force influence appears to be ruled out by observations on sandstone-body geometries and associated paleocurrents. The preferred model is one in which high-energy turbidity currents were confined within and influenced by the steep walls of the primary canyon gorge during aggradation of canyon fill.