Name: The Soom Shale
Location: Cape Province, South Africa
Age: Late Ordovician (Ashgill)

Geological Setting and Age

The Cederberg Mountains are comprised of an arenaceous (sandstone-dominated) sedimentary sequence, known as the Table Mountain Group. The bottom half of the Table Mountain Group, the Graafwater and Peninsular formations, form Table Mountain at Cape Town.

Stratigraphic log of the Cederberg Formation. Image used with permission of the Palaeontological Association.

The Soom Shale occurs in the top half of the Table Mountain Group, at the base of the Cederberg Formation. The Cederberg Formation is approx. 120 m thick and consists of two members, the upper Disa Siltstone, and the lower Soom Shale. The Soom Shale is 10-15 m thick at outcrop and has been dated as late Rawtheyan / Early Hirnantian (Ashgill, Ordovician), approx. 435 Mya. It consists of very dark grey laminated silts and muds, with laminations from >1mm to 10mm in thickness. These laminations are laterally persistent, and show no other sedimentary structures.

Below: Pictures of the Soom Shale site at Keurbos, Cape Province. All photos courtesy of Prof. Richard Aldridge & Dr Sarah Gabbott, University of Leicester, U.K.

Mechanical excavator removing scree to expose fresh surface. Prof. Richard Aldridge and Dr Sarah Gabbott looking for fossils at the newly exposed surface. Prof. Richard Aldridge and Dr Sarah Gabbott using power tools to remove specimens. Exposure of Soom Shale showing most of the lithology.

Depositional environment

The depositional environment of the Soom Shale has been interpreted as a shallow marine basin, close to a retreating ice front. The Soom Shale overlies the Packhuis Tillite (a glacial ice deposit), and is in turn overlain by the Disa Siltstone (a shallow marine sediment). The base of the Soom Shale contains drop-stones, rocks that were picked up by ice and then released upon melting. The minimum depth of the water at the time of deposition has been estimated to be 100 m or more. This is because the Soom Shale itself shows no indication of storm-wave induced rippling, which affects sediments less than 100 m deep. The presence of the drop stones at the base of the Soom Shale suggests some ice coverage, but their absence through the rest of the sequence indicates ice was generally absent.

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