Regional evolution of a fluviodeltaic cyclic succession in the Marsdenian (late Namurian Stage, Pennsylvanian) of the Central Pennine Basin, UK

Abstract

SUMMARY Basin-wide analysis of sedimentary facies, isopachytes and palaeocurrents for two late Marsdenian (Pennsylvanian) sedimentary cycles within part of the Millstone Grit Group, has led to a new sequence-stratigraphic interpretation of the relationships between its constituent sandstone units (currently named the Huddersfleld White Rock, Chatsworth Grit, Brooksbottoms Grit, Holcombe Brook Grit, Brown Edge Flags and Redmires Flags). The Bilinguites superbilinguis (R 2c 1) and Verneulites sigma (R 2c 2) marine bands are related to fourth-order marine highstands, and show faunal variations that possibly reflect fifth-order sea-level fluctuations. The lower R 2c 1 cycle consists entirely of relatively deep water (basin floor) mudstone. The overlying R 2c 2 cycle shows an upward regressive passage from similar mudstone through pro-delta and delta-slope deposits to mouthbar and channel sandstones. The latter comprises an ‘eastern inflow’ of northerly provenance, the distribution of which was not influenced by the underlying basement configuration, and a ‘southern inflow’ sourced from the Wales-Brabant High. Falling sea-levels resulted in progressive narrowing of the fluvial pathway within the ‘main sandstone body’ of the ‘eastern inflow’, with a concomitant increase in flow velocities and grainsize. At lowstand, this culminated in the Chatsworth palaeovalley, 25 km wide, the basal surface of which can be correlated into the interfluve areas as a leached palaeosol. ‘Higher sandstone bodies’, where developed, are of two kinds: an earlier set present outside the palaeovalley was formed during regression, and a later set within the palaeovalley was formed as sea level rose. As part of this transgressive systems tract, a Lingula band developed across the flooded Chatsworth palaeovalley and its interfluve margin. The transgression culminated in the highstand of the Cancelloceras cancellatum ( G 1a 1) Marine Band.