Tidal erosion, solution cavities and exhalative mineralization associated with the Jurassic unconformity at Ogmore, South Glamorgan

The Glamorgan coast between Ogmore and Southerndown displays a Mesozoic shoreline that developed during a prolonged marine transgression. The Jurassic unconformity has a stepped topography, is extensively bored, and its subhorizontal platforms are channelled. These features are considered to be the result of tidal erosion and cliff-line retreat during still-stands, and periodic rapid rises in relative sea-level. The tidal range during the Lower Lias is estimated to be approximately 2 m. The sediments deposited adjacent to the shoreline include breccias formed beneath overhangs, coquinas, calcarenites and conglomeratic lags. Post-depositional interreaction between meteoric and marine pore fluids led to the formation of solution cavities within the shoreface sediments, which were subsequently filled by geopetal silts and cavity cements. Hydrothermal fluids were exhaled into the semilithified sediments from fractures within the underlying Dinantian limestones, which resulted in replacement of the geopetal sediments and skeletal forms, and in-filling of open spaces by barite, calcite and galena.