Abstract

The Bude Formation consists of at least 1 km of interbedded mudstone, silt-stone and very fine sandstone, showing no obvious cyclicity, deposited in a foreland-basin epeiric sea that embraced much of southwest England. Many of the sandstones are thin (< 30 cm), with sharp bases, grading, and other features indicative of deposition during a single, waning-energy event. Other sandstones are composite units, up to 10 m thick, consisting of amalgamated event deposits. Marine fossils are very scarce, suggesting poor oceanic connections. From the abundance of graded beds showing the vertical sequence “sharp base ± massive texture ± parallel lamination ± asymmetrical ripple cross-lamination”, and the lack of evidence for wave activity or subaerial exposure, several workers have concluded that the sandstones were deposited as turbidites beneath storm-wave base. However, recent field observations by the author cast doubt, firstly, upon the authenticity of many of the supposed turbidites, and, secondly, upon the validity of the deep-water model, by revealing that most of the ripple cross-lamination in the Bude Formation is of an asymmetrical, wave-influenced variety. Furthermore, mud-filled scours and hummocky cross-stratification, typical of wave-dominated offshore successions, seem to be common.

It is suggested, therefore, that deposition took place largely above storm-wave base. Those event deposits containing hummocky cross-stratification and/or wave-influenced ripple cross-lamination are interpreted as tempestites, deposited during storms under the joint influence of sediment-supplying unidirectional currents and wave-induced oscillatory flow. Event deposits showing only massive texture and/or parallel lamination could be either tempestites or turbidites, since both of these sedimentary structures can form under unidirectional, oscillatory, and (presumably) combined flows.