Evolution of Silurian Depositional Systems in the Southern Uplands, Scotland

Abstract

Throughout the early Silurian, south-west-flowing palaeocurrents in the turbidite sequences of the Southern Uplands record major axial flow and sediment dispersal parallel to the margin consistent with the presence of a topographic trench. Mid-Silurian sequences, however, record a major change in the character of sedimentation at the north-west margin of the Iapetus ocean. Early Wenlock depositional trends follow those of the Upper Llandovery. In the central Southern Uplands thick sequences dominated by massive sandstones are characterized by south- east-flowing palaeocurrents indicating transverse input to the trench via high gradient sand-rich fans. In the south-west Southern Uplands, however, relatively thin sequences of thin-bedded turbidites were deposited by south-west-flowing palaeocurrents suggesting deviation of transverse currents to flow axially along the trench. Mid-Wenlock facies represent a profound change from the monotonous classical turbidites of the late Llandovery and early Wenlock sequences. Diverse facies associations, which can be correlated both across and along strike, record the development of extensive channel/overbank complexes and coeval channel-mouth, depositional lobe and fan-fringe facies. These sedimentary associations represent the outbuilding of large passive-margin style fans which probably extended at least 200 km from the inner trench slope and infilled the topographic trench. The existence of such large fan bodies suggests a pause or slowing in subduction during the mid-Wenlock prior to closure in the Southern Uplands sector in the late Wenlock/early Ludlow interval.