Depositional Sequence Analysis Applied to Late Proterozoic Wilpena Group, Adelaide Geosyncline, South Australia

Abstract

The initial application of depositional sequence analysis to selected stratigraphic sections through outcropping Late Proterozoic strata of the Adelaide Geosyncline in South Australia has identified major depositional sequences within the several‐kilometre‐thick Wilpena Group. Sharp facies shifts in vertical stratigraphic sections are proposed as actual sequence boundaries which, provided they are the result of eustatic sea level variations, may be key elements for future attempts at inter‐regional chronostratigraphic correlation. Two major sequence boundaries are identified, one at the base of the Nuccaleena Formation (boundary A) and a second at the top of the Brachina Subgroup (boundary B). These are attributed to significant basinward shifts in coastal onlap resulting in subaerial exposure and at least localized erosion, followed in each case by establishment of relatively deepwater environments. A somewhat different boundary (boundary C) is associated with an interval of diagenetic dolostone interbeds and is interpreted either as a downlap surface within a sequence, or as a combined deepwater sequence boundary and downlap surface. It may have developed during an episode of reduced sediment input in response to a period of maximum transgression. Alternatively it may represent a hiatus at the termination of a depositional sequence, prior to subsequent downlap or onlap of the succeeding sequence. Boundary C lies a few metres below the stratigraphic level from which kilometre‐deep canyons have incised underlying sequences. These canyons, which are infilled by a complex succession of carbonate breccias, conglomerates, sandstone and mudstone, may have been eroded in a submarine setting by turbidity currents. Such a model requires a significant increase in rate of eustatic sea level fall or a decrease in the rate of tectonic subsidence, in order to move the locus of coastal onlap to the vicinity of the shelf edge. If the cause was eustatic, evidence for it should be found at an equivalent sequence boundary in Late Proterozoic basins remote from the Adelaide Geosyncline. Alternatively, the canyons may have been eroded in a subaerial setting and infilled by coastal sediments during an ensuing period of relative sea level rise. In this model a considerably greater drop in relative sea level is required, most likely related to localized tectonic uplift