The Juxtaposition of Turbidite and Shallow-Water Sediments Study of a Regressive Sequence in the Pennsylvanian of North Devon, England

The cyclothem discussed in this paper is 150 m. thick and is assigned to the Upper Carboniferous Northam Formation, north Devon, England. A regressive sequence of facies, including turbidites, is particularly well developed. The lower part of the cyclothem consists of quiet-water black mudstones. One hundred forty graded siltstone-turbidites represent the first incoming of coarser clastic material into the basin. Above the turbidites, the facies gradually show more and more evidence of basin agitation, and siltstones give way to cross-laminated sandstones. Near the top is a 15-m.-deep channel partially filled with turbidites-the channel presumably acted as a feeder for turbidity currents flowing farther out into the basin. Above the channel are more cross-laminated siltstones and sandstones, and the regressive sequence is topped by a 20-m.-thick large-scale cross-bedded sandstone body. The top of the sandstone is bioturbate and passes very abruptly into black mudstones of the next cyclothem. The sequence is therefore asymmetrical, with almost no depositional record of the transgressive phase. Finally, the nature of the transition from turbidites to shallow-water sediments is reviewed, recognizing small-scale transitions without regional slope deposits; large-scale transitions in which several tens of meters of regional slope deposits are preserved; and the transitions involving unconformity between "flysch" and "molasse" facies.