A Clastic Fluvial-Deltaic Highstand System From the Neoproterozoic of South Australia: An Excellent Outcrop Analog for Marginal Marine Deposits in the Subsurface

Abstract

Ancient depositional systems that are well-exposed over a wide area are critical in understanding the stratigraphic architecture of similar deposits in the subsurface. As marginal marine deposits form a significant portion of the world's hydrocarbon bearing strata, new outcrop studies on these environments are particularly important. The late Ediacaran (~565 Ma) Bonney Sandstone is part of a proximal, marginal marine, fluvial-deltaic system that formed on the passive margin of the Australia-East Antarctica subcontinent just prior to the assembly of Gondwana. Although extensive outcrops are present in the Flinders Ranges of South Australia, few studies have been conducted on the formation. Measured sections reveal a series of coarsening-upward parasequences, composed primarily of clastic sediments deposited as part of a highstand systems tract. The formation shows significant thickening over ~130 kilometers laterally, indicating the presence of a depocenter, deepening, and increasing accommodation to the northwest. Parasequences, even in more distal settings, are characterized by shaly bases overlain by amalgamated sands containing trough cross-stratification, wave and current ripples. Scours, lateral accretion, and granule-sized grains are locally common. The combination of these features suggests deposition above wave base and occasionally in fluvial environments. It is therefore unlikely that the basin reached significant depths during Bonney time, even during flooding events. Lithofacies and lithofacies associations indicate a wave-dominated succession at the base, with both wave and fluvial processes in the middle, and increasing fluvial influence toward the top. Detrital zircon geochronology suggests that sediments were ultimately sourced from the distant Musgrave Province, rather than the adjacent Gawler Craton, implying that sediments may have been transported up to 600 kilometers, possibly through the tectonically controlled Willouran Trough. This study therefore provides valuable insight into the paleogeography and depositional environments of Precambrian Australia, and improves our understanding of the source, seal, and reservoir potential of these types of environments.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015