Orientated Calcareous Concretions — Indicators of Palaeo-Fluid Flow Direction in Deepwater Sands — Outcrop Example from the Permian Karoo Basin, South Africa

Johann Conradie and De Ville Wickens
Geology, Geography and Environmental Studies, University of Stellenbosch, Stellenbosch, South Africa

Large-scale, teardrop-shaped calcareous concretions, evenly distributed in the upper part of thick-bedded, fine-grained turbidite sandstones, are interpreted to reflect palaeo-fluid flow directions. These concretions occur in massive, amalgamated, sheet-like sandstone beds of the Slypsteenkop turbidite succession in the northwestern Karoo Basin. They are sideritic, resistant to weathering, and exhibit a unique elongated shape with a large rounded part, smoothing into a flat, elongated tongue. Their physical dimension is variable but 75% of them have lengths of 1-4 m, widths of 0.5-3m, and heights of up to 2m. The host sandstone is classified as compositionally immature feldspathic arenites with quartz, potassium and plagioclase feldspar, and lithic fragments as the main framework minerals. The majority of authigenic minerals, such as chlorite, sericite and kaolinite, formed due to alteration of mostly feldspar.

The general orientation of the concretions is roughly parallel to the palaeotransport direction with the tongue pointing in an up-dip direction. Orientated concretionary growth in basin shale-confined reservoirs during early diagenesis, when extensive cementation occurs, seems to reflect not only the depositional fabric of the host aquifer, but likely also regional aspects such as sand body geometries and perhaps overall basin configuration.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery