Sedimentology of Lower Cretaceous Subtidal Sand Complex, Woburn Sands, Southern England: ABSTRACT

Abstract

The Lower Cretaceous Woburn sands have long been considered a transgressive and partly open marine tidal sand deposit. They form a 60-m thick, northward-wedging sandstone body between Jurassic shales, which they unconformably overlie, and a transgressive marine clay which oversteps them northward. Our studies identify three southward-imbricated, erosionally bounded sand units as follows. Orange Sands (oldest) comprise alternations of cross-bedded, channel-fill sands and thinner bedded, bioturbated, heterolithic sands. Bidirectional paleocurrents show a dominant northeasterly flood direction. Silver Sands are characterized by tabular cross-bed sets (up to 3 m thick) which overlie subhorizontal, low-angle (4 to 8°) or concave-upward erosion surfaces. Bidirectional paleocurrents reflect a slight dominance of the southwesterly ebb direction. Red Sands (the youngest) are structurally similar to the Silver Sands but are distinctive on the basis of abundant detrital ferric oxide and strong horizontal burrowing. Northeasterly dipping cross-bedding is relatively uncommon, producing an overwhelmingly dominant southwesterly, ebb-directed paleocurrent mode. The interfingering of high-energy tidal channel-fill deposits and heterolithic beds is typical of subtidal estuarine deposits. Furthermore, the upward decrease in the proportion of heterolithic facies and burrowing intensity, the upward increase in the proportion of large-scale cross-bedding and channel width/depth ratios, End_Page 728------------------------------ and the upward transition from flood to ebb dominance are interpreted as reflecting the onlap of estuary-mouth sands over inner estuarine channel/shoal deposits. The closest modern analogs to this type of transgressive sequence are the estuarine retreat sand complexes on the eastern United States shelf. End_of_Article - Last_Page 729------------