Storm-Influenced Inner-Shelf Sand Lobes in the Caradoc (Ordovician) of Shropshire, England

Abstract

ABSTRACT A sequence through the middle Caradoc (Ordovician) of Shropshire shows the Horderley Sandstone with thick beds of hummocky cross-stratified sandstone, succeeded by the Cheney Longville Flags consisting of interbedded mudstones, siltstones, and sandstones, some of which also exhibit hummocky bedding. In the upper formation the mudstones are commonly bioturbated but also have many wave-rippled silty horizons. The interbedded thick siltstones and sandstones all lack current ripples and appear to have been rapidly deposited as a result of storm episodes. The siltstone and sandstone beds can be separated into 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 f rmer are believed to 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 within the sequence. The more usual type has laminae mantling a hummocky erosional topography. A second variety confined to the Cheney Longville Flags has laminae that accreted upwards from a planar base to produce undulating bedforms (wavelength up to 3 m; wave height up to 15 cm) that have symmetrical internal laminae that are form-concordant. The bedforms resemble wave ripples in cross section but are about an order of magnitude larger than usual wave ripples. In the Cheney Longville Flags, the argillaceous sediments and their fauna show virtually no change through 80 m of the succession. The type 1 beds, however, increase in proportion upwards whereas the type 2 beds show two strong maxima. Because the overall environment apparently remained nearly constant, and there is no evidence for progradation, 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.