Syn-sedimentary gravity slides (growth faults) in the Coal Measures of South Wales

Abstract

Growth faults are an important class of syn-sedimentary fault which have been widely recognized in sub-surface studies of hydrocarbon-bearing clastic basin successions^1–4. They are listric normal faults which are active for a limited period during sediment deposition and therefore affect a discrete sedimentary interval. Whilst active, the faults substantially influence sedimentation and a thicker, generally sand-dominated succession accumulates on the downthrown side which is consistently towards the basin. The faults are not directly related to basement tectonics, and instead are thought to originate principally by gravity sliding within the sediment pile⁵. Over-pressured/undercompacted conditions in buried muds aid development of the faults by reducing the slope required to initiate sliding⁵, and in some cases associated mud diapirism also contributes by creating steeper slopes⁶. Despite their widespread recognition in sub-surface studies, relatively few examples have been described from exposed successions^7–10, and none from the economically important Upper Carboniferous Coal Measures of Europe. Here we describe examples of growth faults recently discovered in an opencast site of the Coal Measures in South Wales and comment on their possible significance.