Ordovician faunas in mass-flow deposits, Southern Scotland

In the Ordovician Northern Belt of the Southern Uplands, basal volcanics (Arenig) are followed by cherts (Llanvirn-?Llandeilo), then by graptolite shales (Llandelio-Lower Caradoc), and finally by Caradoc greywackes. Within the greywackes (Kirkcolm Formation) are a number of occurrences of fossiliferous conglomerate and overlying mudstone; these can be traced along-strike for some 30 km. The conglomerates, and especially the mudstones, yield rich mid- Caradoc shelly assemblages; brachiopods (20 spp.)/ trilobites (14 spp.), gastropods, bivalves, bryozoans, and the first known Palaeozoic scleractiniamorph coral. These occurrences are interpreted as mass-flow deposits derived by downslope movement from a now-vanished shelf to the North, and may belong either to a single gigantic debris flow event, or to a series of smaller, but roughly contemporaneous flows. Strong faunal similarities to faunas at Girvan (western Scotland), and Tyrone (Northern Ireland), lying North of the Southern Upland Fault may suggest sinistral strike-slip movement of no more than a few hundred kilometres.