Stylolite development at sites of primary and diagenetic fabric contrast within the Sutton Stone (Lower Lias), Ogmore-by-Sea, Glamorgan, UK

Pressure dissolution within the Sutton Stone, a Lower Jurassic littoral succession at Ogmore-by-Sea, Glamorgan, resulted in the stylolitization of bedding planes between units of contrasting lithological fabric. The stylolites are divisible into ‘major’ stylolites, nucleated upon persistent primary bedding planes traceable across the outcrop, and ‘minor’ stylolites, developed upon less favourable, localized surfaces. Two suture morphologies are recognized, a highly angular and an undulose form. Morphological variation is considered a consequence of two positive correlations; that observed between the magnitude of lithological contrast at the plane and the fundamental magnitude of the suture waveform amplitude, and that observed between host rock grain-size and the fundamental magnitude of suture frequency. Mesogenetic stylolitization was aided by eogenetic cementation with a marine-phreatic fringe cement. This cement pre-dates the dissolution of aragonitic bioclasts within grainstones, and the degree of early cementation is thus not proportional to primary aragonite availability. However, subsequent aragonite dissolution increased the magnitude of fabric contrast at bedding planes, elevating porosity, solubility and subsequent pressure-dissolution during mesogenesis. Burial cements associated with stylolites are considered to represent the reprecipitation of pressure-dissolved carbonate. The occlusion of porosity protected such areas from extensive dissolution by highly aggressive basin waters during deep burial.