Deposition of turbidites and agitated-water siltstones : A study of the Upper Carboniferous Westward Ho! Formation, North Devon

Abstract

The Westward Ho! Formation consists dominantly of mudstones and siltstones. Two distinct groups of facies can be contrasted. The first contains very fine grained black mudstones and parallel bedded graded siltstones and fine sandstones which show all the sedimentary structures now associated with turbidites. By contrast, the second group of facies contains irregularly rippled and cross-bedded siltstones and sandstones, the sedimentary structures indicating agitated water. The sequence of facies shows that the turbidites occur only in the lower parts of coarsening-upward (shallowing) sequences, or are associated with black mudstones in very quiet water. The upper parts of the regressive sequences contain only agitated-water facies. Two groups of siltstone-filled scours occur. In the lower group, the scours are of wide extent and the sequence above each scour fines-upward. In the upper group, similar fining-upward sequences above scoured surfaces occur, but the upper parts of the sequences contain many small channels which have a fill finer than, or identical with, the surrounding sediment. The fine nature of the fill suggests a slump-scar origin rather than channelling. The Westward Ho! Formation is unique in containing three extremely rapid vertical transitions from turbidite to agitated-water facies. Progressive increase in agitation in the area of turbidite accumulation is suggested, rather than regional progradation of a slope and hence more gradual transition into agitated-water facies.