Mid-Westphalian A to late Westphalian B sediments of the South Wales Coalfield comprise mainly lacustrine, raised mire and flood deposits developed in an upper coastal plain environment. Occasional high sinuosity, fine-grained channel systems drained the subdued topography of the Wales-Brabant Massif to the north and east of the basin. Low sinuosity channels with lithic, coarse-grained fills drained a rising source to the south. Direct marine influence was restricted to a single marine band.

Facies distributions were controlled by (1) local scale (102-103 m2) autocyclic sedimentary processes, (2) regional scale (10 km2) synsedimentary fault activity and differential subsidence and 3) basin-wide scale (102 km2) response to changes in relative sea-level. Base-level changes are recorded by basin-wide bivalve horizons (up-dip or landward extension of marine bands) developed in lacustrine mudstones above basin-wide coal seams. Peat accumulation was terminated by a water table rise related to base-level rise. Following flooding, re-establishment of the clastic supply resulted in the gradual infilling of the basin-wide lake to produce coarsening- upwards cyclothems or parasequences. A eustatic origin for parasequence development is supported by the development of identical facies associations and parasequences in coeval sediments from the Pennine and Midland Valley Coalfields.

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