The Marl Slate: A Taphonomic and
Petrographic Analysis.

Abstract

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.