| Summary: | Detailed facies mapping and microfacies study were employed to improve upon sedimentological models for the margins of a carbonate platform of late Visean (Asbian - Brigantian) age in north Derbyshire, UK. The early Asbian upper slope was built of automicrite stiffened with early marine cements. The system also included marginal bioclastic sand shoals, situated c.500m from the platform break, which was in slightly deeper water, basinward from the shoals. Despite this bevelled configuration the upper slope received only low volumes of shallow water allochems until late in the early Asbian third order cycle. This early phase was characterised by nearly vertical aggradation of the margin. The later increase in basinward export resulted in a pulse of progradation and more mixed lithologies on the slope. In the late Asbian to Brigantian, the northern margin experienced an episode of local tectonic subsidence, resulting in back-stepping of c. 1km. However, high productivity by the benthic community enabled the margin to recover and build back to the previous platform break within three, fourth order, cycles. During this phase little automicrite was produced, or preserved, on the upper slope, production of automicrite moved to bioherms on the outer platform. Export of coarse bioclastic material to the mid and lower slope resulted in accumulation below a largely by-passed upper slope area. The southern margin also subsided in the Brigantian, but low productivity by a slightly stressed community meant the margin did not recover fully and remained as a low-angle slope, often dominated by the deposition of mud and silt exported from the platform interior. This, and the occurrence of ooids only on the southern margin, suggests the south to be a leeward margin. The facies architecture and the geometry of the margins were controlled by the interaction between eustatic sealevel changes and local tectonics.
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