Tidal sedimentation in Inner Hebrides half grabens, Scotland: the Mid-Jurassic Bearreraig Sandstone Formation D. MELLERE 1 & R. J. STEEL 2 1Statoil Research Centre, Arkitekt Ebbels veg 10, Rotvoll, N-7005 Trondheim, Norway 2 Department of Geology and Geophysics, University of Wyoming, Laramie, USA 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 12Ma 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 10m deep and consist of both bioturbated and non- bioturbated, large-scale, tabular or trough cross-bedded medium/coarse sandstones (sets 2- 10m 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. The Mesozoic basins of the Hebrides form a series of linked extensional, westerly tilted half grabens (Steel 1971, 1977; Brewer & Smithe 1984; Stein 1988; Earle et al. 1989; Stein & Blundell 1990; Morton 1992a) (Fig. 1), con- trolled in their position and extent by the extensional reactivation of Caledonian thrusts. The Aalenian-Bajocian Bearreraig Sandstone Formation was deposited across depositional 'blocks' which were tilting slightly to the west- northwest during sedimentation. The northern block was bounded to the east by the North Raasay Fault, whereas three blocks further to the southeast were bounded successively by the Beinn na Leac and South Scalpay/Camasunary fault system (Fig. 1). Three of the tilted blocks probably had exposed basement 'highs' support- ing local drainage, whereas the sub-basin ,bounded by the North Raasay and Beinn na Leac faults probably had only subdued up- tilting of Triassic/Jurassic strata along its crest. The striking differences in thickness, in facies and in sequential organization of the Bearreraig Sandstone Formation within and between the four sub-basins is probably attributable to the differential subsidence history of these linked extensional basins. The purpose of this paper is to record and emphasize the differing styles and evolution of the alternating deltaic and estuarine sedimentation during high-frequency regressions and transgressions across the province. We have constructed a sedimentological and stratigraphi- cal model for the Bearreraig Sandstone Forma- tion. Details of the tectonic effects on the succession within and between the tilted blocks will be presented elsewhere. Stratigraphy The Bearreraig Sandstone Formation, a promi- nent part of the Jurassic succession in the From De Batist, M. & Jacobs, P. (eds), 1996, Geologyof Siliciclastic Shelf Seas, Geological Society Special Publication No. 117, pp. 49-79.