'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 D orset record a complex diagenetic history preserved in carbonate cements wi thin their body chambers. The studied ammonites are commonly preserved uncr ushed in thinly bedded (0.001 m) peloidal sediments associated with small-s cale upward fining couplets. The body chambers of the studied ammonites are partially infilled by uncompacted peloidal material with the remaining por osity, both here and in the other chambers, being occluded by a complex suc cession of carbonate cements. These comprise early, microdolomite/pyrite in clusion-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 ob servations is that the ammonite assemblages were rapidly engulfed by a fine sediment drape resulting from event resuspension processes. Alternative ex planations, such as mass mortality due to bottom water anoxia 'events' are more difficult to sustain. Following burial, the package of sediment enclos ing the ammonites rapidly became anoxic and early radiaxial calcite precipi tated in response to microbial organic mailer mineralisation (Mn-, Fe- and sulphate reduction) reactions. This early cement prevented compaction. Foll owing precipitation of these early fringing cements, cavities were progress ively filled with zoned, inclusion-poor sparry carbonate cements. These rep resent deeper, slower, equilibrium crystallization wherein pore water solut e access was severely limited by grain boundary diffusion processes. Finall y, remaining porosity was infilled by a distinctive ferroan calcite, the ir on most probably sourced in deep, thermally induced redox reactions.