The geology of the lower paleozoic rocks in the southern Rhinns of Galloway, S. W. Scotland: studies in an imbricate thrust terrane

Abstract

This thesis is the first detailed, modem account of the geology of the southern Rhinns of Galloway, SW Scotland subsequent to work by the Geological Survey last century. It integrates the geology described with the rest of the Southern Uplands-Down-Longford terrane and offers a plate-tectonic synthesis for its development. The southern Rhinns consists of eleven NE-SW trending, sub-vertical tectonic blocks resulting from the thrust imbrication of a c. 35 million year Caradoc to Llandovery age sequence of chert, bentonite, shale, mudstone, greywacke and conglomerate. The pelagic/hemipelagic Moffat Shale Group acted as a locus for thrust development. The greywackes have a siliceous petrography with sporadic input of volcaniclastic material and a major influx of detrital carbonate in the youngest formation. A recycled orogen provenance is indicated. Facies analysis indicates the catastrophic progradation of clastic, inboard deposits southeastwards over contemporaneous pelagic/hemipelagic, outboard deposits. Most of the formations were deposited in a highly confined basin, whereas the Port Logan Formation and Mull of Galloway Formation were deposited in large, unconfined submarine fans. Palaeocurrent flow was dominantly from the NE or NW, though the two youngest formations provide the first unequivocal evidence of major southeasterly derivation in the Southern Uplands. The style and vergence of D1 deformation changes across the Port Logan Bay Fault, the area to the SE constituting a 12 km zone of opposing fold and thrust geometry to that dominant in the terrane. A model of sequential Silurian simple shear and pure shear deformation in a steady-state trench environment above a NW-dipping subduction zone is proposed. A set of post-D₂ NW-vergent recumbent folds and thrusts related to end-Caledonian terminal collision of Cadomia and Laurentia provide evidence of major sinistral strike-slip along the Cairngarroch Fault prior to collision.