Swaley cross-stratification produced by unidirectional flows, Bencliff Grit (Upper Jurassic), Dorset, UK

Abstract

Unusual bedforms occur in the Bencliff Grit at Osmington Mills, Dorset. The geometry of cross-strata in stacked, predominantly fine-grained sandstone units is dominated by low-angle troughs with (locally) perfectly-preserved antiforms which wedge-out onto climbing low-angle erosion surfaces. Many of the antiforms (hummocks) and troughs (swales) appear to have grown at similar rates of sediment fall-out. Others show draped antiforms and thickening into swales. In both cases laminae are concordant with the lower bounding surfaces of the cross-sets, demonstrating that vertical accretion was the dominant process. This geometry together with the conspicuous absence of angle-of-repose cross-strata give the bedforms a resemblance to amalgamated hummocky cross-stratification or swaley cross-stratification. Many workers consider amalgamated hummocky cross-stratification and swaley cross-stratification to be identified with storm sedimentation on the shelf to shoreface, thereby bracketting the structures both in terms of process and environment. However, the structures in the Bencliff Grit suggest that such a precise interpretation of amalgamated hummocky cross-stratification or swaley cross-stratification is inadvisable. The geometry of the cross-stratification suggests deposition under a highly sediment charged but essentially unidirectional flow. This study therefore supports recent views that hummocky and swaley styles of cross-stratification are in reality a form of low-angle trough cross-stratification produced primarily by unidirectional flows. Clearly, since undulatory forms of stratification are substrate- and process-dependent and not environmentally-specific, great care must be taken in the interpretation of palaeoenvironmental settings, especially where exposures are limited, as for instance in borehole cores. It is unlikely that core data can be used to identify bedforms analogous to hummocky cross-stratification or swaley cross-stratification unless boreholes are highly deviated.