Biostratigraphy of the Currarie Formation on the northwestern edge of the Southern Uplands: Implications for the cessation of basic volcanism

The Currarie Formation forms part of the deep water succession near the northern margin of the Southern Uplands. It comprises red and green mudstones (distal turbidites), chert and interbedded lava and lava breccia and overlies basalt lavas. The age of the Currarie Formation is critical in providing a minimum age for the end of basic volcanism in the northernmost part of the Southern Uplands and a maximum age for the onset of northerly derived turbidite flysch. It also allows biostratigraphical correlations to be made across the Southern Upland Fault. Graptolites indicative of the lower Nemagraptus gracilis Biozone and the first record of the conodont species Protopanderodus varicostatus from the formation indicate an earliest Caradoc (early Aurelucian) age for the Currarie Formation. This dates the cessation of basic volcanism in the northernmost part of the Southern Uplands as earliest Caradoc. However, the waning of volcanism may have been diachronous, continuing into the upper N. gracilis Biozone in the area between the Southern Upland-Glen App and Carcow faults in a succession thought to have been developed further to the south than the Currarie Formation. The onset of coarse turbidite deposition followed within the same graptolite zone marking the onset of a major phase in uplift of the Midland Valley terrane and subsidence in the Southern Uplands basin.