Provenance study of the Late Triassic - Early Jurassic Elliot Formation, main Karoo Basin, South Africa

Abstract

Based on regional palaeocurrent pattern analysis and petrographic studies, the Late Triassic-Early Jurassic Elliot Formation (Karoo Supergroup, South Africa) is divisible into a Lower and an Upper Elliot Formation. Laterally, along a north-south section, the lower unit displays both changes in the sediment supply pattern and petrological composition, whereas the upper unit lacks significant petrological variations. A break in sedimentation took place after the deposition of the Lower Elliot Formation, and a regional subaerial unconformity is detected between the Lower and Upper Elliot formations. This unconformity is emphasized by marked differences in the sedimentological and palaeontological composition of the deposits below and above the surface. Prevailing northerly sediment supply patterns of the Lower Elliot Formation are replaced by mainly easterly-northeasterly transportation directions in the Upper Elliot Formation, trends which were mentioned, but not accounted for by earlier researchers of the Late Triassic-Early Jurassic Elliot Formation. Simultaneously with the reorganization of the sediment supply patterns, the origin of the sandstones also shifted from recycled-orogen provenance, to a transitional continental source situated between craton interior and basement uplift provenances. This paper presents the contrast in palaeocurrent trends and sandstone composition across this surface, and reports, for the first time, the basin-wide presence of outsized quartzite pebbles and boulders, sourced from the Cape Fold Belt.