Transactions of the Royal Society of Edinburgh: Earth Sciences

Research Article

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

The Southern Uplands Terrance, Tectonics and Biostratigraphy within the Caledonian Orogen, Our Dynamic Earth, Edinburgh.

R. A. Smitha2, E. R. Phillipsa1, J. D. Floyda1, H. F. Barrona1 and E. A. Picketta1

a1 E. R. Phillips, J. D. Floyd, H. F. Barron and E. A. Pickett, British Geological Survey, Murchison House, Edinburgh EH9 3LA

a2 R. A. Smith, British Geological Survey, Murchison House, Edinburgh EH9 3LA e-mail: r.a.smith@bgs.ac.uk

Abstract

ABSTRACT 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 sparse 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 succession was punctuated by the input of fresh volcanic detritus shed from an oceanic/continental island-arc situated to the W/NW of the Northern Belt basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Northern Belt 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 terrane broadly corresponds to uplift and erosion of earlier obducted ophiolite in both Scotland and Ireland, possibly in response to collision of Cambrian–early Ordovician island-arc systems 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 oceanic/continental island-arc complex within an overall transpressional regime.

(Received November 15 1999)

(Accepted July 11 2000)

KEY WORDS

  • provenance;
  • sinistral strike-slip;
  • Southern Uplands;
  • structure References