Carbonate facies on a Lower Carboniferous storm influenced ramp in SW Britain
Faulkner, Thomas Joseph
PublisherUniversity of Bristol
MetadataShow full item record
The Lower Carboniferous ('mid'-Courceyan to early Chadian) Black Rock Limestone ramp succession of SW Britain, accumulated along the southern margin of the Welsh-Brabant Massif during the early stage of development of the SW European foreland basin. Two depositional cycles are recognized which culminate in the subaerial exposure of those sequences in a more landward setting. The lower depositional cycle includes the Barry Harbour Limestone Formation, which contains a variety of storm-related sedimentary structures including hummocky cross-stratification. This is conformably overlain by the tidally influenced, cross-stratified Brofiscin Oolite Formation which locally displays evidence of subaerial exposure. Seven sedimentary lithofacies are identified in these two formations, which record a distal to proximal, shallowing-upward trend that passes from beneath mean wave-base to above fairweather wave-base. The upper depositional cycle of the Friars Point Limestone Formation includes the volcanics at Middle Hope, the Waulsortian facies at Castlemartin and Cannington Park, and is capped by the (herein named) Portishead Paleosol Beds (near Bristol). Above the basal transgressive sequence, the Friars Point Limestone consists of two distinct offshore ramp facies belts. The more proximal inner ramp facies belt was more strongly influenced by storm-related currents and exhibits tubular tempestites (storm sediment-filled burrows). The finer-grained, outer ramp facies belt is extensively bioturbated and preserves few features indicative of deposition from storm-related currents. The effects and relative timing of differential subsidence was elucidated by using the well documented biostratigraphy to separate the succession into isochronous units. The onset of pronounced differential subsidence was signified by volcanic activity at Middle Hope (Weston-super-Mare). The volcanic tuffs form a coarsening- and shallowing-upward sequence which records the development of a localised, probably non-emergent, offshore-volcanic high. Recognition of a major eustatic sea level fall at the end of the Courceyan and during the early part of the Chadian, is revealed by studying the decline of the Waulsortian reefs in an offshore ramp setting and evaluating the regional significance of the Portishead Paleosol Beds. Overall, the development of the Black Rock Limestone ramp was the result of the interplay between eustacy and tectonism. A full petrographic and geochemical analysis of the Waulsortian dolomites at Castlemartin, including the use of stable isotopes revealed that dolomitization occurred during burial, most probably as a result of basin dewatering.