Mesozoic sedimentation and sedimentary rocks in the Inner Hebrides

Abstract

SynopsisIn the Inner Hebrides Mesozoic sediments rest unconformably on much older, harder and more deformed rocks: they are overlain by Tertiary lavas and cut by sub-volcanic intrusions. The Triassic rocks are unfossiliferous conglomerates and sandstones, deposited on alluvial fans and river flood plains in a semiarid climate. Active faults controlled sedimentation. In the early Jurassic the sea transgressed across the region; a basin of net sediment accumulation lay between uplifted regions now represented by the Highlands and the Outer Hebrides. The lower and lower-middle Jurassic is composed of marine shalesandstone alternations comparable to those providing the sources and reservoirs for North Sea oil. Ammonites provide good correlation with the European succession. In the Great Estuarine Group, environments ranged from lagoonal to deltaic to dolomitic mud-flat. Molluscan assemblages can be compared to modern brackish-water faunas. In the upper Jurassic there is a return to marine shales and minor sandstones; ammonites help correlation between Europe and the Boreal province. The lower Cretaceous is absent; thin upper Cretaceous rests disconformably on older rocks. The rocks are not metamorphosed except slightly near the Tertiary plutonic centres. Their softness makes them susceptible to erosion, and, especially where shales are overlain by basalts, extensive landslips occur.