Structure of the Lower Palaeozoic rocks around Cairnsmore of Fleet, Galloway

Abstract

Synopsis The Lower Palaeozoic rocks around the Cairnsmore of Fleet pluton comprise representatives of the Moffat Shale Group, ranging from the Glenkiln or Lower Hartfell Formation (Llandeilo or Lower Caradoc) to the Upper Birkhill Formation (Fronian), conformably though diachronously overlain by greywackes of later Llandovery age. The Main folds ( f 1 ) display two contrasting styles. Two tracts of northwestward dips, mainly steep and locally inverted, are separated by a tract of concentric folds, upright or overturned to the NW, with wavelengths mostly less than 500 m. The first phase concluded with the generation of complementary wrench fault sets, certain of which were subsequently reactivated. An episode of compressional faulting supervened, generating the southeastward-translating and strike-parallel listric faults of the Central Belt, of which four lie within the area. The tracts of steep NW dips were simultaneously modified by SE-verging open folds having axial planes with moderate SE dips ( f 2 ). Later Caledonian movements, producing crossfolds associated with oblique-slip faulting ( f 3 ) and kink bands ( f 4 ), are of minor significance.

The stratigraphical setting of the Cairnsmore of Fleet Pluton, Galloway

“…A southeastward descent of the tectonic profile is also reflected in corresponding narrowing of line widths in successive lines in this direction. The southernmost line, the Pibble Line, is nowhere more than 200 m wide within the present area (Cook 1976;Cook and Weir, 1979). The Pibble Line is considered to represent the Ettrick Valley Fault (Toghill 1970b), on account of exposing the southernmost development of Moffat Shales within the area, and demarcates the southernmost segment.…”

“…1), which are orientated parallel to the regional strike, are designated in N W -S E order (Table 1) The Lower Palaeozoic succession is divided into three units based on lithological evidence, which was found (Cook 1976) to reinforce and complement observed geochemical contrasts (especially within the flysch successions), and scarce palaeontological control ( Fig. 1 (Toghill 1968(Toghill , 1970a.…”

Tectonic contrasts in the Southern Uplands

“…This variability is most clearly expressed in the Talnotry and Blauirbuies Lines (Cook and Weir 1979), both of which display a WSW-ENE decrease in width. Relationships within a fault zone are most clearly expressed in the Talnotry Line.…”

“…com. ); several fault zones in West Nithsdale (Floyd 1975); Clanyard Bay (Watson 1976); and Cairnsmore of Fleet (Cook 1976;Cook and Weir 1980). Indeed, it was to avoid tedious repetition of a cumbrous term such as 'outcrops of Moffat Shales in association with greywackes and other lithologies' that Floyd (1975) employed the term 'Line', implying thereby restriction of Moffat Shales (and stratigraphically associated rock types including cherts, lavas and pyroclastics) to those impressively linear outcrops.…”

“…The immediately impressive feature of the fault zones, in addition to their considerable lateral extent, is their variability in width, from more than 1 km to about 30 m (Cook 1976;). This variability is most clearly expressed in the Talnotry and Blauirbuies Lines (Cook and Weir 1979), both of which display a WSW-ENE decrease in width.…”