Abstract

The Coal Measures succession in the Little Haven-Amroth Coalfield, Pembrokeshire, has a minimum thickness of 1500 feet. Fifteen facies and facies variants have been recognized, most of which can be grouped into orderly or disorderly successions. Orderly successions include coarsening-upwards, fining-upwards, and sphaerosiderite bearing coarsening-upwards sequences. Coarsening-upwards sequences depending on scalar and sedimentary properties are interpreted as the offlap succession of a prograding shoreline, and the fill of freshwater lakes (floodbasins); fining-upwards sequences as alluvial floodplain successions with substratum and topstratum sediment accretion; and sphaerosiderite bearing coarsening-upwards sequences as the fill of restricted, ferruginous, freshwater lakes. The sedimentary history of the Coal Measures succession is complex. The principal phases in order of occurrence include: Gastrioceras subcrenatum --M1 horizon, southerly and easterly advance of a prograding shoreline; M1-M4 horizon, marginal continental-marine environment with periodic minor transgressions, variable paleocurrent pattern; M4-Amman Marine Band, major regressive period with continued alluvial floodplain sedimentation followed by a widespread marine transgression, variable paleocurrent pattern: Amman Marine Band-Picton Point Marine Band, a regressive period with coastal plain and alluvial floodplain sedimentation followed by a further extensive marine transgression, more consistent paleocurrent pattern with northerly and westerly flows predominating.

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