Overview and biostratigraphy

Abstract Excellent coastal exposures of strongly folded and faulted Cambrian rocks occur around the St David9s Peninsula of SW Wales. The best sections occur in the St David9s Anticline, in the vicinity of St David9s itself, and these have been the subject of scientific study since the mid-Nineteenth Century. Based on this early work, a quadripartite lithostratigraphical scheme emerged, comprising the Caerfai, Solva and Menevian groups and the Lingula Flags Formation. However, this scheme is unnecessarily complex, and sometimes unworkable in practice. The Caerfai Group is here redefined to embrace much of the Solva Group of earlier authors, while the upper part of the Solva Group, the Menevian Group and Lingula Flags are combined in the new Porth-y-rhaw Group. Traditionally, all these Cambrian rocks have been thought of as being deposited on a stable cratonic platform, but there is substantial evidence of tectonic influences on sedimentation, particularly in the older part of the succession. The Cambrian was originally recognized and defined in North Wales. There, as in many other areas, it became common practice to divide the period into three parts, but more recent work favours a four-fold chronostratigraphical subdivision. The oldest part of the Cambrian is missing in South Wales, but rocks of Terreneuvian, C2, C3 and Furongian age are all represented. Age-diagnostic fossils are few in the older part of the local succession. Trace fossils low in the Caerfai Group indicate the avalonensis Ichnozone or Teichichnus Interval of the Terreneuvian, while a radiometric date of early C2 (CS3) age has been obtained higher up. Trilobite faunas occur more commonly in the higher parts of the Caerfai Group and in the Porth-y-rhaw Group, and these facilitate international correlation of the C3 (CS5) to Furongian (Paibian) parts of the succession. Trilobites characteristic of the oelandicus Biosuperzone and the gibbus , fissus , parvifrons , punctuosus , pisiformis and Olenus biozones all occur.