Subaqueous Outwash Fan and Esker Sandstones in the Permo-Carboniferous Dwyka Formation of South Africa

Abstract

ABSTRACT Four sandstone facies were defined in a 300-m-thick diamictite member of the glacigene Permo-Carboniferous Dwyka Formation. The laterally persistent type-A sandstones comprising quartz arenite are conformable to the bedding of the diamictite and are unreformed. They are interpreted as subaqueous outwash-fan deposits. Sandstone facies B (also quartz arenites) occurs intermittently along bedding planes in the diamictite and shows soft-sediment deformation. It represents subaqueous, segmented outwash-fan deposits which were primarily deformed by density loading of the soft diamicton. The highly irregular type-C sandstones comprise quartz and lithic arenites and wackes, are randomly distributed in the diamictite, and are deformed. They ale interpreted as esker and esker-delta deposits and rafted lumps of frozen meltwater sands deformed by overriding ice or density loading of the diamicton. Facies D consists of quartz and lithic arenites and represents clastic dikes and diapiric injections formed by density loading of waterlogged meltwater sand. The three diamictite subfacies associated with the sandstones were deposited in open water by iceberg rafting, suspension settling, and debris flow, by a combination of lodgement and melt-out processes below grounded ice, and below an ice shelf by debris rain, suspension settling, and slope failure. The outwash-fan deposits, esker deltas, clastic dikes, and diapiric injections are confined to those parts of the basin covered by open water and the ice shelf. The esker deposits formed in areas overlain by grounded ice. Episodic advance of the ice lobes and oscillation of the ice-grounding line eroded the fan sediments and also contributed to the deformation of the sand bodies.