The Lower Devonian Dartmouth Group records some of the earliest stages in the evolution of the Variscan basins of southwest England. This thick, mudstone-dominated group has previously been interpreted as distal fluvial sediments, deposited on a coastal mudplain.
The sequence at Bigbury Bay is made up of two large-scale sedimentary cycles. The base of each cycle consists of pebbly mudstones, graded siltstones and sandstones, massive mudstones, and heterolithic sediments with sporadic phosphatic nodules. These facies were deposited in substantial lakes; they were modified by phases of syn-sedimentary deformation which generated subaqueous debris flows and turbidites. The overlying parts of the cycle, making up the bulk of the succession, consist of mudstone and thin, sheet-like, sandstones. These are fluvial and were probably deposited in mud-dominated terminal fans. Each cycle represents a phase of rapid, probably fault-controlled, subsidence followed by relative tectonic quiescence. This behaviour may reflect pulses of rifting or extension in strike-slip basins, followed by thermal relaxation.