ABSTRACT

Carbonate shelf sequences in the Lower Carboniferous of southwest Britain contain thin peritidal intercalations within open shelf facies. Analysis of one such unit, the Caswell Bay Mudstone, at Miskin, South Wales, shows it to contain a lower paleosol and an upper tidal-flat/lagoon sequence. These are separated and bounded by erosion surfaces directly related to the origin of the units they underlie. The complete sequence, using letters for rock units and numbers for disconformities (with interpretation in brackets), is: a) oolite [inner shelf shoal], 1) irregular disconformity [subaerial/subsoil surface], b) oolite breccia-conglomerate in clay matrix [regolith with clay introduced as windblown or overbank material], c) mottled micrites [calcrete], 2) planar disconformity [back-barrie shoreface], d) peloid micrites, e) laminated micrites, f) paper shales and thin limestones [all shallow subtidal lagoon-low intertidal flat sediments], 3) planar disconformity [barrier shoreface], g) bioclastic limestone [inner shelf open marine sands and gravels].

Units b-f total 6 m thickness and constitute the Caswell Bay Mudstone. They comprise a couplet of terrestrial and back-barrier sediments preserved during a transgression, probably because of the resistance of the calcrete and a moderate subsidence rate. Breaks 1-3 represent a subaerial surface plus two barrier-associated erosion surfaces. These three breaks are here termed a triple disconformity. The subaerial surface is the major break of the three and the back-barrier shorezone surface marks the base of the transgressive sequence. The uppermost break, the barrier shoreface, is a relatively minor intra-sequential erosion plane.

Recognition of this sequence of deposition and erosional events clarifies the origin of the units involved, provides an environmental framework for further biofacies and lithofacies analysis, and indicates the relative significance of breaks that may be used for stratigraphic subdivision. Paleosol-tidal flat/lagoon couplets and triple disconformities should be widespread features of transgressive barrier-island carbonate shoreline sequences.