The Devonian sedimentation and tectonism of a rapidly subsiding, semi-arid fluvial basin in the Midland Valley of Scotland

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Abstract
Synopsis: Early Devonian fluvial sedimentation took place in a large, late-orogenic basin bounded by actively-subsiding high-angled faults. Deposition of gravels and sands took place on mountain-fringing bajadas from sheetfloods and braided streams; sands, silts and clays were deposited in adjacent flood-plains. Basin-margin faults, notably the Highland Boundary Fault, controlled subsidence of an elongate depositional basin and when movement on these was sufficiently rapid, coalescing bajadas were constructed. When the rate of source area uplift/depositional basin subsidence decreased, the uplands were reduced and floodplains initially encroached on to the inactive bajadas before extending over the former source areas. The Devonian sedimentary rocks of the Midland Valley which accumulated under semi-arid conditions, indicate that varying rates of source uplift/depositional basin subsidence were the dominant controls influencing the nature of the depositional processes. Moreover the various vertical sequences of genetically-different deposits found in the study area also occur in other similar alluvial basin deposits and a model is formulated to demonstrate that these successions reflect a limited number of distinct evolutionary tectono-sedimentary sequences.