Fluvial style variations in the Late Triassic?Early Jurassic Elliot formation, main Karoo Basin, South Africa

Abstract

Architectural element analysis of the Late Triassic–Early Jurassic Elliot Formation (South Africa) reveals two contrasting sandstone body geometries resulting from different fluvial depositional styles. In the lower part of the formation, the sandstones are multi-storey, asymmetrical channel-fills. They are interpreted as deposits of perennial, moderately meandering fluvial systems characterised by trough and planar cross-stratification, massive beds and less commonly low-angle cross-stratification. In the upper part of the formation, the mostly tabular, multi-storey sheet sandstones are internally structured by massive beds, horizontal lamination, ripple cross-lamination, and rare trough cross-stratification. These sandstone bodies are interpreted to have been deposited by ephemeral fluvial processes. The change in fluvial style is accompanied by changes in sandstone petrography and palaeocurrent patterns, suggesting that this shift in the depositional style is predominantly controlled by tectonism rather than climate.