Jonathan Radley writes: The Penarth Group (of Late Triassic and possibly ranging to Early Jurassic age) of the southern UK marks a marine transgression and the establishment of a shallow epicontinental seaway (Hallam & El Shaarawy 1982; Warrington & Ivimey-Cook 1992), influenced by regressive–transgressive pulses and characterized by rapid facies changes (Hallam & Wignall 2004; Hesselbo et al. 2004). A well-documented invertebrate macrofauna includes corals, brachiopods, molluscs and echinoderms (Swift & Martill 1999). Facies developments range from storm-influenced, shallow-marine mudrocks (Westbury Formation) upwards into calcareous mudstones, siltstones and essentially fine-grained carbonates (Lilstock Formation) demonstrating extremely shallow-water conditions: wave ripples, storm beds, desiccation and omission surfaces (Swift 1999a; Wignall 2001; Hallam & Wignall 2004). The low-diversity macrofauna of the Lilstock Formation has frequently been attributed to salinities deviating from that of normal seawater within shallow-water, partly landlocked settings, isolated from the open sea (Hallam & El Shaarawy 1982; Swift 1995; Allison & Wright 2005). However, widespread occurrences of stenohaline marine macrofossils such as corals and echinoderms (Swift & Martill 1999) suggest that salinity fluctuations were at most slight (Wignall 2001).
Mander et al. (2008) have presented high-resolution benthic macrofaunal data, principally from molluscan macrofossils, for significant palaeoecological change within the Penarth Group of two key sections in the SW UK. Their detailed results demonstrate loss of taxonomic richness within the uppermost Westbury Formation–lower Lilstock Formation (Cotham Member) interval (latest Rhaetian), followed by a poorly fossiliferous interval represented by the latest Rhaetian or early Hettangian upper Lilstock Formation (upper Cotham Member–lower Langport Member). They attributed this pattern to an extinction event followed by a post-extinction ‘Dead Zone’; the latter coinciding with the onset of a negative organic carbon-isotope excursion, and an extinction event recorded in the terrestrial palynoflora. The relatively diverse shelly …
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