Late Ordovician brachiopod biofacies of the Girvan district, SW Scotland

Abstract

ABSTRACTDiverse and abundant brachiopod faunas, associated with unstable outer shelf and slope environments, occur through the Upper Ardmillan Group (upper Caradoc–upper Ashgill) in the Girvan district of SW Scotland. Representatives of the deep-waterFoliomenafauna occur intermittently throughout the group, appearing in both the Whitehouse and Drummuck subgroups. This distinctive assemblage of small, thin-shelled brachiopods, includingDedzetina, Christiania, CyclospiraandFoliomenaitself, first appeared in South China during the early Caradoc but had colonised the Laurentian margins by the late Caradoc. Within the upper Caradoc–lower Ashgill Whitehouse Subgroup, theFoliomenafauna is interbedded with a variety of other less cosmopolitan deep-water assemblages including theOnniella–SkenidioidesandLingulella–Trimurellinaassociations. Shallower-water environments in the middle Ashgill Lower Drummuck Subgroup hosted theFardenia–Eopholidostrophiaassociation in sands, and theChristiania-Leptaenaassociation in muds and silts. The remarkable Lady Burn Starfish Beds in the upper part of the group contain a variety of brachiopod-dominated assemblages including theEochonetesandPlaesiomys-Schizophorellaassociations, transported from various shelf locations, within a very diverse mid-Ashgill biota. Nevertheless, elements of theFoliomenafauna persisted to near the top of the Drummuck Subgroup, occurring as rare assemblages in more muddy and silty facies. The upper Ashgill High Mains Formation contains abundant elements of the terminal OrdovicianHirnantiafauna includingEostropheodonta, HindellaandHirnantiaitself, but also some taxa more typical of the Laurentian Edgewood Province. As a whole, the changing brachiopod biofacies monitor environmental fluctuations, on part of the Laurentian margin, driven by mainly eustatic and tectonic events.