Neoproterozoic rocks of the Garvellachs, Scotland

Glacial features in basement rocks were first interpreted in 1871, located on the Scottish island of Islay. It was a century later that a detailed memoir of the glacial geology of Islay and the Garvellachs was published, which remains an essential source to this day. The Garvellachs are islands to the north–east of Islay, and they provide a showcase for what became known as the Port Askaig Formation. These particular rocks are of Neoproterozoic age and have been used to correlate diamictite-like sediments from localities ranging from Connemara in Ireland to the north–east of Scotland. Different views, however, have emerged about the interpretation of depositional environments. This work has three aims: to review geological evidences and disputed interpretations; to highlight issues deserving greater attention; and to re-appraise the significance of the geology of the Garvellachs. There are three main models of diamictite formation, each reflecting different depositional processes: (1) movement of grounded ice, which physically transports and deposits unsorted material; (2) glacial rainout, where debris is released from melting ice into water; and (3) mass-flow deposits, resulting from gravity-driven sediment flows that may occur independently of glacial influence. Published discussions of these models are summarised to show that the evidence has ambiguities. This paper contributes to the discussion by focusing on four broader contextual issues. First, the global tectonic framework indicates that tectonics not only accounts for basinal subsidence exceeding 1000 meters, but also actively drives processes of erosion and sediment deposition. Second, this dynamic tectonic regime can generate a wide range of sedimentary features, including those characteristic of diamictites. Third, the quartz-rich sandstones deserve a status greater than interbeds signalling interglacial phases, and they point to an active depositional system driven by tectonics. Fourth, the presence of dolostones poses a challenge for proponents of glaciation. Evidence suggests that freshly precipitated dolomite formed substantial layers of relatively pure sediment. Traditionally, dolomite is associated with warm, tropical waters, yet the Garvellachs’ dolostones represent a prominent component of the succession. The reappraisal presented here is that the Garvellachs are a witness to active tectonism in Neoproterozoic times, not glaciation.