Abstract

Horderley Sandstone with hummocky cross-stratified sandstone, succeeded by the Cheney Longville Flags consisting of interbedded mudstones, siltstones, and sandstones, some of which also exhibit hummocky bedding. Siltstone and sandstone beds are of two types: type 1 beds are massive or faintly planar-laminated, and have good lateral continuity, type 2 beds are lenticular, coarser, well laminated, and have hummocky bedding. The former have been deposited from suspension, and the latter from powerful bottom traction currents flowing offshore during storm surges. Two varieties of hummocky bedding have been recognized. In the Cheney Longville Flags, the type 1 beds increase in proportion upwards whereas the type 2 beds show two strong maxima. We interpret the two developments of type 2 beds as reflecting two localized lobes of sand carried offshore into an inner-shelf environment by strong laterally restricted offshore currents, analogous to, but on a larger scale and more powerful than, rip currents. The independent upward increase in type 1 beds might reflect regional changes in paleogeography which caused a progressive exposure of the shoreface to an oceanic wave climate.--Modified journal abstract.

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