Controls on alluvial architecture by synsedimentary faults in the Coal Measures of South Wales

Abstract

AbstractA series of small‐scale gravity induced synsedimentary faults are described from the Middle Coal Measures (Westphalian B) of the South Wales Coalfield. The synsedimentary nature of the faults is indicated by abrupt changes in sediment thickness across faults, eroded fault scarps and the restriction of faulting to 8·75 m of sediment bounded by laterally persistent black shales above and a seatearth below. Evidence for the non‐tectonic origin of the faults is given by the presence of passive footwalls.Channel sandstone bodies occurring within the hanging walls of individual faults were deposited by discrete overbank flood events. Drainage patterns during flood events were controlled by topographic lows adjacent to the faults such that palaeocurrent data within the channel sandstone bodies indicate a S/SW transport direction parallel to fault strike.Synsedimentary fault movements resulted in the diagonal stacking of successively younger sandstone bodies adjacent to the faults, and an increase in tilt of the bodies with depth from 0 → 26°. The architecture of channel sandstone bodies was controlled by (1) the amount of fault movement and (2) fault block dimensions (or fault spacing). The interaction of these controlling factors resulted in deposition of symmetrical channel sandstone bodies over narrow fault blocks (12 m wide) with large fault displacements (7 m), asymmetrical sandstone bodies over wide fault blocks (30 m) with large fault displacement (7.5 m), and limited channel sandstone body development over fault blocks with small fault displacements (1.5 m).Synsedimentary faulting is thought to have been initiated by either seismic shock and/or overpressuring of pore fluids within buried mudstones.