The Northern Belt 100 years on : a revised model of the Ordovician tracts near Leadhills, Scotland

A new model for the provenance, depositional environment and tectonic setting of the Northern Belt of the Southern Uplands is presented. This turbiditic sandstone dominated sequence was deposited in a sand-rich submarine fan environment, overlying hemipelagic mudstones. The oldest sandstones are rich in juvenile ophiolitic material and record the first clastic input into the Southern Uplands basin. The bulk of the Northern Belt sedimentary sequence, however, is dominated by relatively quartzose sandstones derived from a Proterozoic continental/metamorphic source represented by the Midland Valley terrane of Scotland and Ireland. The quartzose dominated sequence was punctuated by the input of fresh volcanic detritus shed from a oceanic/continental island-arc situated to the W/NW of the Southern Uplands basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Southern Upland basin remains uncertain. The complex provenance of the sandstones and recognition of major olistostrome units within the Northern Belt succession suggest that it was tectonically active. The onset of clastic deposition within the Southern Uplands broadly corresponds to ophiolite obduction in both Scotland and Ireland, possibly in response to collision of a Cambrian-early Ordovician island-arc system with the Laurentian continental margin. If this interpretation is correct then the possibility arises that the Southern Uplands-Midland Valley terranes record the dismembering of this island-arc complex within an overall transpressional regime.