Dark mottles are a prominent and widespread feature of the regressive, cyclic, shallow marine limestones which form the late Asbian succession in many parts of Britain.
The colour difference which defines mottles in outcrop is caused by distinct but often subtle petrographic differences in the limestone fabric. Specifically, mottles contain a light brown coloured opaque inclusion-rich calcite spar with characteristic dull brown luminescence; ‘mottle spar’. Outside mottle margins this calcite spar is absent, with clear inequant blocky cements forming the pore filling phase. ‘Mottle spar’ comprises a fabric of irregular crystals predominantly 5–40 μm in diameter, with more regular crystals up to 100 μm diameter often occurring in intraparticle and large interparticle pores. Under cathodoluminescence, ‘mottle spar’ displays crystal morphologies and growth patterns which indicate that both localized neomorphism and patchy cementation contributed to mottle formation.
Cathodoluminescence cement stratigraphy shows that ‘mottle spar’ pre-dates all other major pore filling cements in the local Asbian succession, but post-dates marine micritization. ‘Mottle spar’ sharply defines the moulds of former aragonitic allochems which are now filled by the later clear, inequant spar cements. This shows that aragonite dissolution occurred after the formation of ‘mottle spar’. Mottles in calcretes contain unaltered allochems which have been protected from the effects of subaerial micritization by ‘mottle spar’, although mottles are often affected by subaerial brecciation. This evidence shows that mottles formed during early diagenesis; after marine micritization, but before dissolution of aragonite, subaerial exposure and meteoritic phreatic cementation.
Mottles represented lithified patches of very low porosity which are interpreted to have formed in the marine/freshwater mixing zone, during the repeated phases of regression and emergence in the late Asbian.