Chitinozoans and the age of the Soom Shale, an Ordovician black shale Lagerstätte, South Africa

Thijs R A Vandenbroucke,Sarah E Gabbott,Richard J Aldridge,Johannes N Theron,Florentin Paris

doi:10.1144/jm.28.1.53

Abstract

Abstract. Isolated chitinozoans from the Soom Shale Member of the Cedarberg Formation, SW South Africa are described and provide a date of the latest Hirnantian–earliest Rhuddanian. The recovered chitinozoans are typical of the latest Ordovician Spinachitina oulebsiri Biozone, although an earliest Silurian age is possible. They indicate a very short time span (less than 1 Ma) across the Ordovician–Silurian boundary. This is currently the highest biostratigraphical resolution attainable for the Soom Shale Lagerstätte. Correlation of the Soom Shale chitinozoans with identical assemblages in post-glacial, transgressive deposits of Northern Africa is possible; both faunas occur in shales that overlie glacial diamictites of the Hirnantian glaciation. A new species, Spinachitina verniersi n. sp. is described.

Highlights

Location, sedimentology and importance of the Soom Shale The Soom Shale Member and the overlying Disa Siltstone Member comprise the Cedarberg Formation of the Table Mountain Group, which crops out in the Cape Province of South Africa
Correlation of the Soom Shale chitinozoans with identical assemblages in post-glacial, transgressive deposits of Northern Africa is possible; both faunas occur in shales that overlie glacial diamictites of the Hirnantian glaciation
The Soom Shale Member is famous as it hosts exceptionally well-preserved fossils, including the soft part remains of conodonts (Aldridge & Theron, 1993; Gabbott et al, 1995), many arthropod groups (e.g. Braddy et al, 1995, 1999; Gabbott et al, 2003; Whittle et al, 2007) and enigmatic taxa (Aldridge et al, 2001), as well as a shelly fauna dominated by brachiopods, but most notably including orthocones with well-preserved radulae (Gabbott, 1999)

Summary

Introduction

Sedimentology and importance of the Soom Shale The Soom Shale Member and the overlying Disa Siltstone Member comprise the Cedarberg Formation of the Table Mountain Group, which crops out in the Cape Province of South Africa. Both members of the Cedarberg Formation contain the only dateable fossils known in the whole Table Mountain Group, a sequence of sediments of latest Cambrian to Early Devonian age. The Soom Shale Member is famous as it hosts exceptionally well-preserved fossils, including the soft part remains of conodonts (Aldridge & Theron, 1993; Gabbott et al, 1995), many arthropod groups The Soom Shale Member is famous as it hosts exceptionally well-preserved fossils, including the soft part remains of conodonts (Aldridge & Theron, 1993; Gabbott et al, 1995), many arthropod groups (e.g. Braddy et al, 1995, 1999; Gabbott et al, 2003; Whittle et al, 2007) and enigmatic taxa (Aldridge et al, 2001), as well as a shelly fauna dominated by brachiopods, but most notably including orthocones with well-preserved radulae (Gabbott, 1999)

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