Fluvial and aeolian deposition in the Siluro-Devonian Swanshaw Sandstone Formation, SW Scotland

Synopsis The recognition of fluvial and aeolian facies within the Siluro-Devonian Swanshaw Sandstone Formation of SW Scotland allows a more detailed depositional history of the Lanark basin in the southern part of the Midland Valley of Scotland to be inferred. The formation is well exposed in Ayrshire and has four main sedimentary facies associations: Channel, Aeolian, Floodplain and Mudflat. Each combines several sedimentary facies: eight facies are recognized in the channel association, three in the aeolian, four in the floodplain and two in the mudflat. These facies associations co-existed in seven different depositional settings in the western Lanark basin. Deposition occurred on a semi-arid proximal alluvial plain with an aeolian component described for the first time in the Lanark basin. Sedimentation was pulsed, largely driven by periods of source area uplift that resulted in the deposition of coarse-grained conglomeratic and sandstone-dominated facies. The principal transport was via river channels, commonly in shallow braided systems. Periods of low sediment flux are marked by fewer channels and by the preservation of floodplain successions including aeolian deposits. Abandonment of the fluvial system towards the top of the formation in Ayrshire is marked by a progression through sandflats and mudflats to the overlying volcanic formation. The relationship of the fluvial and aeolian deposits is analysed. The palaeo-wind direction is compatible with that recorded in sandstones of similar age in the Orcadian basin.