Abstract

The basal part of the thick mudrock sequence of the Ecca Group overlying the glacigenic Dwyka Group (late Carboniferous to Artinskian in age) shows evidence of periglacial conditions during deposition. The dark-coloured to black mudrocks which attain a thickness of up to 250 m, contain organic-rich horizons, siltstone and fine-grained sandstone interbeds, marine fossils, and carbonate, siliceous and phosphate concretions, lenses and beds. Deposition was by suspension settling of mud, turbidity current activity and minor fall-out of airborne volcanic ash in a large sea with episodic anoxic bottom conditions. The climate probably varied from subpolar to temperate. The syn- to post-glacial mudrock sedimentation in the foreland basin was controlled by the rapid collapse of the marine ice sheet, basin tectonics (subsidence and rising foreland arc), oceanic circulation, relative sea-level change and a cool to temperate climate.

Introduction

The present Karoo Basin formed when the Gondwana supercontinent broke up during the Late Jurassic to Early Cretaceous. The beds of the Karoo Supergroup consist of remnants of upper Carboniferous to lower Jurassic strata which covered extensive parts of southwestern Gondwana. In southern Africa, Karoo beds also occur in several smaller, mostly fault-controlled, basins towards the north, but this paper deals exclusively with the postglacial sequence in the main Karoo Basin, which covers an area of just over 600000 km2 (Fig. 14.1). The glacigenic Dwyka Group which occurs at the base of the Karoo Supergroup, is conformably overlain by a predominantly mudrock sequence (Ecca Group) up to 3000 m thick (Tankard et al., 1982).