Lower palaeozoic geology of the Gala area borders region, Scotland

Abstract

The Lower Palaeozoic succession of the Gala area, Southern Uplands, comprises two contrasting facies, a pelagic/hemipelagic sequence, the Moffat Shales, and overlying turbidite sandstone (greywacke) successions variously of Ordovician and Silurian age. The area is divided into fault blocks bounded by a series of major strike faults, leading to contrasting sequences variously successive fault blocks; pelagic/hemipelagic facies are replaced by turbidites progressively later south-eastwards. Turbidites in the two northernmost blocks are of Ordovician age, and include two formations. Blocks to the south are formed of three successively younger formations of the Gala Group (Upper Llandovery), with Moffat Shales spanning the Upper Llandeilo - Upper Llandovery interval, whilst the southernmost block includes one formation representing the Hawick Group (Wenlock). Two formations within the Gala Group have been further subdivided into members. In most instances, formations crop out within discrete fault blocks, though in one instance successively higher levels within a single formation form three separate blocks, whilst another block includes an interdigitating intraformational contact, attributed to the overlapping of two turbidite fans of contrasting source areas. Whereas blocks display successively younger flysch sequences to the SE, strata within each block young dominantly to MW. The structural style is comparable to areas elsewhere in the Southern Uplands; showing evidence of prolonged and continuous deformation. Though these observations would support formation of an accretionary prism on an active continental-oceanic margin in response to a north-westward subduction of an oceanic plate, lack of consistent variation in dip attitudes and axial overturning may dispute this proposition. Palaeocurrents suggest a combination of axial and lateral derivation of rudites and associated fine-grained lithologies with contrasting zones of facies associations, representing environments ranging from inner- to outer-fan (fringe) and basin plain, and suggesting at least three cycles of progression and regression of turbidite fans. Greywacke petrography suggests mainly a magmatic-arc and ophiolitic derivation for 'basic-clast' greywackes and a dominantly Highland type derivation for 'silicic' greywackes, with recycling becoming increasingly significant in the higher levels, and indicating elevations of parts of the succession already accreted. Mineral assemblages, illite crystallinity and values of illites establishes the anchizone of metamorphism without a significant areal variation increase in grade.