The Marl Slate: A Taphonomic and Petrographic Analysis.

The Marl Slate is the basal unit of the Upper Permian (Zechstein) in northeast England. A finely laminated, organic-rich, dolomitic siltstone, equivalent to the Kupferschiefer of the Southern North Sea Basin and northern Germany and Poland. Several lines of evidence indicate the Marl Slate/Kupferschiefer accumulated in the anoxic bottom waters of a shallow (<200m), stratified, epicontinental sea during the first Zechstein marine Transgression (Z1). The sea was formed virtually instantaneously when the Boreal Ocean broke into and flooded a chain of sub-sea-level inland drainage basins that were dominated by aeolian dune facies (Rotleigend) before flooding. Detailed investigation, undertaken as part of this study, of the sedimentology and petrography (1), focussing on and around three sites in a transect through the edge of the Zechstein Z1 deposits across County Durham (Middridge Quarry, Crime Rigg Quarry and Claxheugh Rock) revealed that a cyclic pattern of freshwater influx and phytoplanktonic blooms initiated the formation of a stratified water column in the early stages of the Zechstein Sea. The induced anoxia in the bottom waters, alongside mass mortalities of the present fish fauna, resulted in the abundance of Marl Slate fossils (2). SEM-EDS, X-CT and XRD were used on Permian fish specimens reposited in the Durham University collection to develop and further the understanding of the Permian fish taphonomy and overall preservation. A combination of apatite and pyrite mineralized and replaced the organic tissue, initiated by thin biofilms of sulphur reducing bacteria adhering the carcasses to the anoxic sea floor in low-energy conditions. Furthermore, micrometre thick, illite clay masks have been observed to have coated the fossil material. Biofilms of sulphur reducing bacteria, combined with clay nucleation on the outer surfaces of the films, had a central role in the decay and fossilization of the Marl Slate fishes. These results shed new light on how the sedimentology, petrography and palaeontology of the Marl Slate all factor into driving the taphonomy and the unique preservational pathway undergone by the fish fauna within the Marl Slate of northeast England. This research uses the Marl Slate to bridge the gap between taphonomy and the environment of preservation of early Permian fish fauna, it takes the first steps to generate visualized models representing the depositional and taphonomic environments of the Marl Slate involved in producing a high standard of fossilization.