Shallow-Water Sedimentation as illustrated in the Upper Devonian Baggy Beds

Abstract

The Baggy Beds (440 m thick) form a transgressive sequence of shallow-water marine and delta-like sediments of Upper Famennian age between the continental Pickwell Down Sandstone and the overlying neritic Pilton Beds (which straddle the Devonian-Carboniferous boundary). The Baggy Beds are divided into lower and upper divisions (the lower division containing the bulk of the sandstones) and a number of members. There is a strong diversity of facies, and nine main facies are recognized; they show little evidence of cyclicity but their stratigraphical relationships are important for their interpretation. Penecontemporaneous erosion, principally attributed to wave-action, was often intense, and has contributed more than any other factor to the overall aspect of the marine facies. The bulk of the sediment comprises the shales, siltstones, and very fine sandstones of the Diplocraterion yoyo facies group, which display all gradations from non-life (letal) graded siltstones and shales to intensely bioturbated sandstones, in which the trace-fossil D. yoyo is generally conspicuous. The Reynard facies comprises graded sets of fine to very fine sandstones, the upper portions of which commonly show oscillation ripple or are bioturbated. The sets are intercalated singly in the D. yoyo facies, or form cosets (11 per cent of Baggy Beds). These two facies, which yield a small neritic fauna, are considered to represent deposits of the delta-front platform, or a like off-shore environment, with the cosets of Reynard facies for the most part representing a nearer-shore environment. The other facies (with an interpretation of their depositional environment) are as follows: