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

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Abstract
Summary: 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 mélanges of other accretionary complexes. Index minerals, illite crystallinity and ‘vitrinite’ reflectance establish metamorphic grade as a zeolite to prehnite-pumpellyite facies.