An underwater television survey of facies variation on the inner Scottish shelf between Colonsay, Islay and Jura

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Abstract
Synopsis: Using underwater television, eleven recurrent bottom facies have been identified and mapped over an area of about 250 km 2 . Bare rock areas are rare: Turritella -rich fine sands with trails, and crustacean-burrowed muds widespread. Coarse sands, fining northwards from megaripple fields in the Sound of Islay and the Passage of Oronsay are dominantly of biogenic origin. The kelp Laminaria saccharina was seen rooted in gravelly parts of the megaripple field in the former area (one of strong tidal currents but reduced wave action) where the calcareous alga Lithothamnium is an important contributor. Barnacles and molluscs dominate in the latter region, which is more exposed to Atlantic swell. Extensive spreads of Modiolus seem to rest on a tough clay surface, possibly of eroded late-Glacial material which also probably underlies Tarbert Bank, since it crops out along its eastern margin. The rippled sand body located on the Bank could represent residual out-wash. Facies analysis by combined underwater television and grab/dredge/box coring is optimal for shelf waters of intermediate depth (10–100 m): it reveals particularly well vagile epifauna, normally missed by conventional grabbing. In addition, it permits sediment/faunal samples to be studied in a truer perspective. Differentiation of facies by television scores over bottom sampling in higher energy, coarser sediments; but in lower energy muds and fine sands of the level bottom, where infauna dominate, biofacies analysis by faunal clustering and resin impregnation of internal structures will probably achieve greater refinement