Tidal sedimentation in Inner Hebrides half grabens, Scotland: the Mid-Jurassic Bearreraig Sandstone Formation

Abstract The Bearreraig Sandstone Formation (Late Toarcian-Bajocian), on the islands of Skye and Raasay, in NW Scotland, forms part of the marine infill of one of the Hebridean rift basins that developed during the early stages of the opening of the North Atlantic. The Formation, up to 250 m thick in south Skye, and from 150 m to 180 m thick in north Skye and Raasay, is a spectacular succession of medium to coarse-grained, tidally generated, cross-bedded sandstones, and subordinate shales. The sandstones sharply overlie the Dun Caan Shales and Raasay Ironstone and are abruptly blanketed by the Garantiana Clay Member. The succession was deposited under the influence of a series of significant sea-level fluctuations during a 12 Ma interval. During regressive phases of the basin coastline, sedimentation occurred largely at the mouth of a tidal-dominated delta which evolved into a macrotidal estuary during each transgressive phase. Five major facies associations have been recognized: (1) shale/siltstone-dominated successions containing sharply based, strongly bioturbated sandstone beds interpreted as prodelta deposits; (2) medium to coarse sandy successions of thickening-upward, small to very large-scale planar cross-stratified sets, representing dune fields in the tidally dominated delta-front environment; (3) delta-plain, very fine sandstones with roots and fluvial channels; (4) estuarine, tidal channel-fill deposits which overlie erosion surfaces up to 10 m deep and consist of both bioturbated and non-bioturbated, large-scale, tabular or trough cross-bedded medium/coarse sandstones (sets 2–10 m thick) often deformed by water-escape structures and slumping; (5) bioturbated, thinning-upward tabular sandstone beds, interpreted as transgressive shelf deposits. Palaeocurrent data indicate that during the early stage of deposition of the Bearreraig Formation, tidal-dominated sedimentation occurred separately in the north Skye/Raasay and in the south Skye sub-basins. During a late stage of deposition, the sub-basins merged, and sedimentation occurred uniformly throughout the region. This change in palaeogeographical configuration is associated with a regionally extensive unconformity. Tectonic activity, particularly at an early stage in the form of tilting and block rotation, is believed to have enhanced tidal circulation within the fault-constrained sub-basins.