Tidal signatures in sand prone, tectonically generated Jurassic straits, Scotland

Blackwood, Stuart, 1972-
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The Mid-Jurassic Bearreraig Formation is superbly well exposed on the Isles of Skye and Raasay in NW Scotland. In southern Skye the Bearreraig displays a spectacular, cyclical hierarchy of sand rich, tide-generated bedforms: 1. Upward thickening to thinning compound and compound-compound trough cross-sets (3D dunes) up to 2.5m thick 2. Compound trough cross-sets (3D dunes) up to 10m thick 3. Upward thinning, simple and compound planer cross-sets (2D dunes) up to 50cm thick Tectonically-generated sequences (up to 140m thick) consisting of these three dune types can be recognized within the formation. Sedimentation occurred in NNE-SSW-aligned, North Skye/Raasay and South Skye half-grabens, defined by Aalenian-Bajocian faulting associated with the initial opening of the North Atlantic. SSW oriented (S. Skye) and NNE oriented (N. Skye/Raasay) fault-parallel paleocurrent directions (indicative of basin separation) suggest that the faults were active and had enough topographic expression to enhance the tidal regime in these very narrow, block-tilted zones. Previous depositional interpretation of the Bearreraig is one of alternating tide-dominated deltaic and macro-tidal estuarine cycles with high-frequency regressions and transgressions. Facies cropping out at Trotternish in N. Skye, and Raasay display trends consistent with this regional interpretation. However, in southern Skye the overall thickness of the cross-stratified succession, anomalously large size of the bedforms (implying relatively deep water), intensely sand-rich character of the succession, together with the high bioclastic content of broken shell debris is indicative of a more complex, higher-energy depositional environment. The character and scale of the bedforms, and the absence of pro-delta, lagoonal or delta-plain deposits suggests a narrow seaway or tidal strait generated by active tilting of narrow fault blocks