The sedimentology and geotectonic significance of upper Ordovician and lower Silurian sand-bodies in the Rhinns of Galloway and adjacent areas, southwest Scotland

Abstract

The study reported here involved detailed mapping, logging, stratigraphical, structural, sedimentological and petrographical analyses of sequences exposed in two areas: the Rhinns of Galloway, between Killantrlngan Bay (NW 9820 5700) and Clanyard Bay (NX 1010 3800) and the strike-wise equivalent sector on the east side of Luce Bay.
The upper Ordovician and lower Silurian sediments exposed within these areas represent an aggregate thickness in excess of lOkm of greywackes, conglomerates and shales displaying the typical Southern Uplands structural style of broad steeply inclined northwest-younging belts abruptly terminated by steep strike-faults. Ten of these tectonostratigraphical tracts have been recognized within the study areas.
Sedimentological, petrographical and biostratigraphical data have enabled 16 stratigraphical units (formations and members) to be defined in the Rhinns tracts and 8 units in the Glenluce area. Recognition of the diachronous development of some of these units in the two areas (both along- and across-strike) provides important evidence bearing on palaeoenvironmental and geotectonic evolution of this part of the Southern Uplands.
Sedimentoloqical data from these sequences confirm their exclusively deep-water nature, while lithofacies analysis permits the recognition of a variety of depositional systems, the evolutionary trends of which have been determined.
Two phases of tectono-sedimentary development are recognized: (1) During the Caradoc-Ashgill interval north- and northwest-prograding clastic slope aprons and axially diverted. Immature sand-rich fans were formed on the northern flanks of a contemporaneously active volcanic terrain. Petrographical and stratigraphical data suggest that this may have been a sinistrally displaced, tectonically migrating continental margin arc, briefly juxtaposed within a fore-arc and trench region: (2) During the Llandovery the depositional systems evolved from an
axial wedge, to axially diverted sand-rich fans and, eventually, to mud-rich fans. Small local debris cones were also formed. These systems were probably also deposited in a fore-arc and were derived from sources to the north and northeast which initially included remnants of the arc terrain. The evolutionary trends displayed by the early Silurian systems indicate a general increase in sediment supply and/or a decrease In convergence rate during the Llandovery.