The Origin of Cyclothems in the Late Dinantian Platform Carbonates at Crich, Derbyshire

SUMMARY The Monsal Dale Limestones at Crich, Derbyshire represent a carbonate platform succession of early Brigantian (D 2 ) age, The exposed succession (36m) contains five cyclothems each terminated by a limestone surface providing evidence of subaerial exposure, overlain by a thin clay. The lower part of each cyclothem is dominated by a skeletal-peloidal grainstone/packstone which was probably deposited in an open bay or lagoon, but the upper part of each cyclothem is dominated by fine-grained peloidal grainstone/packstone. Both facies show evidence of bioturbation, but in addition the peloidal facies contains fenestrae indicative of periodic emergence which suggests that it may have formed in shallow subtidal or lower intertidal conditions. Each cyclothem therefore records the progressive shoaling which occurred prior to emergence, but there is a negligible sedimentary record of each subsequent transgressive event. Four of the cyclothems are terminated by surfaces displaying minor karstic solution features, but the fifth is coated by a subaerial crust and is associated with early diagenetic rhizoliths, pseudomorphs of gypsum and minor silicification.