Palaeoecology

Sedimentary environment

During the late Ordovician, the southern tip of Africa was at a latitude of about 60°S, and the Soom Shale was deposited under the influence of the glacial episode that widely affected the southern Gondwanan continents at that time.  The shale is directly underlain by tillites of the Pakhuis Formation, and there are occasional dropstones, particularly near the base of the shale, providing evidence of at least some continued ice cover.  The depositional environment has been interpreted as within a quiet water basin close to the retreating ice front.  The mud and silt laminae are normally less than 1 mm thick, rarely up to 10 mm;  they are laterally persistent and undisturbed by any current activity or penetrative bioturbation.  They appear to have been deposited by the settling of fine, perhaps wind-blown, detritus from suspension.  Many of the bedding planes are covered with algal ribbons, and there is a possibility that the sediment surface was bound by microbial mats.  Above the Soom Shale, the siltstones and fine sandstones of the Disa Siltstone Member display sedimentary structures that testify to an increasing importance of wave and current activity within the basin.
Several lines of evidence, most notably the parallel , non-bioturbated sediment, the preponderance of nektonic and necktobenthic organims and the sedimentary geochemistry, indicate that the bottom waters in which the Soom Shale accumulated were dominantly anoxic or euxinic with brief periods of  oxygenation.

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