Abstract

The Soom Shale Member of the Cedarberg Formation (South Africa) is a key early Paleozoic Lagerstätte that directly overlies glacigenic deposits of the Late Ordovician glacial maximum; it was deposited on the Gondwanan craton during deglacial transgression. We show that it contains a substantial coarse silt to fine sand component that occurs in discrete laminae intimately associated with plankton-derived organic material. We interpret this component as loess, sourced from glacially derived debris and blown into the sea either directly or across seasonal sea ice, as happens today on the McMurdo Ice Shelf, Antarctica. Falling through the water column, this material likely stimulated production of phytoplankton, which then sank as sediment-loaded aggregates. In marked sedimentary partitioning, these silt and/or organic laminae alternate with fine, organic-poor mud laminae that likely represent (river-derived?) nepheloid plumes and that sporadically thicken into centimeter-scale mud turbidites. Sustained eolian input directly into the surface water may have been key to maintaining the high productivity of the Soom sea and then, via eutrophication and anoxia, to the exceptional preservation of its biota.

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