Sedimentology, volcanism, structure and metamorphism of the northern margin of a Lower Palaeozoic accretionary complex; Bail Hill-Abington area of the Southern Uplands of Scotland

Abstract

NE-SW faults in the Bail Hill-Abington area of the Northern Belt of the Southern Uplands define blocks up to 3.2 km wide. The strata, folded and locally overturned, young predominantly to the NW but successive blocks to the SW contain progressively younger sequences. Analogous configurations occur in modern accretionary margins. The oldest rocks are Arenig basalts, dolerites, cherts and brown mudstones underlying red shales, possibly Llanvirn, and black fossiliferous shales and cherts of Llandeilo and Caradoc age.Trench sediments overlying pelagic sequences represent a range of depositional mechanisms. Rudites and associated fine-grained lithologies of lateral origin relate to a lower trench slope canyon system, whilst axially transported sands, originating on the lower trench slope, were deposited by turbidity currents and related flows.The Bail Hill Volcanic Group (Upper Llandeilo) represents a mildly alkaline seamount in the Iapetus Ocean, with volcanic activity spanning the transition from pelagic plate to trench sedimentation before accretion.Faults, initially low-angle thrusts, and bedding were rotated through the vertical within the accretionary complex, pre-dating or accompanying slaty cleavage development. Soft sediment deformation, two fold phases and a kink-band set are recognized. Imbricate fault zones located in incompetent pelagic sequences are tentatively equated with tectonic m61anges of other accretionary complexes.Index minerals, illite crystallinity and 'vitrinite' reflectance establish metamorphic grade as a zeolite to prehnite-pumpellyite facies.

Grade and time of metamorphism in the Caledonide Orogen of Britain and Ireland

“…In the Abington area of the northern belt graptolites give Ro max 8.2-3.9% (Hepworth et al 1982). The northern belt values indicate a slightly higher temperature than the central belt.…”

“…A strip of zeolite-facies rocks has been identified adjacent to the Southern Upland fault based on the presence of kaolinite in siltstones (Hepworth et al 1982) and fresh calcic plagioclase in volcaniclastic rocks. Associated lavas contain amygdales with mixtures of carbonate, thomsonite, prehnite, pumpellyite and albite and are cut by veins of carbonate and analcite.…”

“…The recognition of the prehnite-pumpellyite facies is based on several factors: 1 Sandstones in the northern and central belts contain secondary albitic feldspar and consistently show post cleavage veins of prehnite + calcite + quartz + pumpellyite (Kelling 1962, Leggett et al 1979a, 1979b, Oliver & Leggett 1980, Hepworth et al 1982 (Watson 1976). In the Abington area of the northern belt graptolites give Ro max 8.2-3.9% (Hepworth et al 1982).…”

“…The zeolite facies occurs in the Crawick Block, which has a faulted boundary with the prehnite-pumpellyite-facies metamorphosed blocks, situated to the south (Hepworth et al 1982). The zeolite-facies metamorphic minerals may have formed in three different ways: a) by hydrothermal-sea water alteration of lava immediately after extrusion of the volcanics onto black shales of N gracilis age (Llandeilo-Caradoc); b) they may have formed during burial of the volcano by trench sediments (i.e.…”

“…Burial metamorphism would have occurred in the trench where up to 3 km of sediment accumulated (Hepworth et al 1982). It is unlikely under the low heat flow found in trenches that prehnite-pumpellyite facies metamorphism occurred in the sediments; zeolite-facies metamorphism would be more likely.…”