Seismically-induced load structures in the basal Coal Measures, South Wales

Abstract

SummaryWithin the basal Coal Measures between Ystradgynlais and Abercraf a number of liquefaction layers are seen displaying ball and pillow structures. These layers consist of balled-up, ovoid masses of sandstone some 0.25–0.3 m thick and 0.5–0.7 m across. It is considered that they were formed by loading, resulting from the lique-faction of underlying silts, produced by the passage of earthquake shock waves associated with the main fault zone of the Swansea Valley Disturbance. Elsewhere in South Wales, horizons displaying load and slump structures in rocks of Upper Palaeozoic age may have resulted from seismic shock waves caused by contemporary movements along major Variscan fractures.