TAPHONOMY OF THE ORDOVICIAN SOOM SHALE LAGERSTATTE - AN EXAMPLE OF SOFT-TISSUE PRESERVATION IN CLAY-MINERALS

The late Ordovician Soom Shale of South Africa contains exceptionally preserved fossils of several taxa, the soft tissues of which are uniqu ely composed of clay and alunite group minerals. In addition, original ly phosphatic brachiopod shells and conodont elements have been replac ed by clays. Sub-cellular structural details of conodont muscle tissue s are faithfully replicated by the clay minerals. Geochemical analyses have constrained interpretation of the conditions in the sediment and bottom waters of the Soom Shale basin during deposition and early dia genesis. Anoxic-euxinic conditions prevailed with low carbonate and ir on concentrations in the sediment; hence there was no mechanism to buf fer or fix H2S produced by organic matter decomposition. Under low pH conditions and in the presence of cations, organic substrates would ha ve had an affinity for colloidal clay minerals and may have acted as t emplates, controlling the absorption of clay minerals which eventually completely replaced them. An initial phase of mineralization involvin g phosphate, followed by its replacement by clay minerals, is unlikely because the low pH conditions in the sediment would have been inimica l to phosphate concentration, and the high fidelity of some soft tissu e replication militates against two phases of replacement.