‘Instantaneous’ sedimentation, early microbial sediment strengthening and a lengthy record of chemical diagenesis preserved in Lower Jurassic ammonitiferous concretions from Dorset

Ammonites from different localities and horizons within the Lower Lias of Dorset record a complex diagenetic history preserved in carbonate cements within their body chambers. The studied ammonites are commonly preserved uncrushed in thinly bedded (0.001 m) peloidal sediments associated with small‐scale upward fining couplets. The body chambers of the studied ammonites are partially infilled by uncompacted peloidal material with the remaining porosity, both here and in the other chambers, being occluded by a complex succession of carbonate cements. These comprise early, microdolomite/pyrite inclusion‐rich radiaxial Mn‐rich calcite cements which are enclosed by later inclusion‐free sparry cements. The assemblages of ammonites found are ‘life’ assemblages, often preserving the remains of jaw structures and many rest oblique to the plane of sedimentation. One likely explanation for these observations is that the ammonite assemblages were rapidly engulfed by a fine sediment drape resulting from event resuspension processes. Alternative explanations, such as mass mortality due to bottom water anoxia ‘events’ are more difficult to sustain. Following burial, the package of sediment enclosing the ammonites rapidly became anoxic and early radiaxial calcite precipitated in response to microbial organic matter mineralisation (Mn‐, Fe‐ and sulphate reduction) reactions. This early cement prevented compaction. Following precipitation of these early fringing cements, cavities were progressively filled with zoned, inclusion‐poor sparry carbonate cements. These represent deeper, slower, equilibrium crystallization wherein pore water solute access was severely limited by grain boundary diffusion processes. Finally, remaining porosity was infilled by a distinctive ferroan calcite, the iron most probably sourced in deep, thermally induced redox reactions.