Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones

The Coinstone is a well known hiatus‐concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay‐hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea‐floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack‐lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV‐fluorescent and inclusion‐rich, and is similar to the first calcite generations seen in many other clay‐hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post‐exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today. The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.