Recent sediments and sedimentation in the Inner Hebrides

Abstract

SynopsisLittle land-derived material currently reaches the sea. Inland deposits are dominantly organic, peat succeeding lacustrine diatomite. Marine sediments result from the wave and tidal current reworking of Pleistocene glacial debris, with an admixture of Holocene skeletal carbonate. Muddy sands are accumulating in glacially overdeepened and tidally scoured hollows, reworked from moraine and outwash. Lag gravels occur on the shallows.Beach sands have a local origin, with mixtures of shelly fauna and rock fragments eroded from adjacent cliffs. Beaches are less carbonate-rich than in the Outer Hebrides, except on Coll and Tiree. They are also finer, which encourages well-developed dune systems. Calcareous dunes are locally cemented on Coll, the cementation picking out ‘fossilised’ hoof-prints.There is no transport of coastal bedload to the shelf, so that offshore sediments are more calcareous. Bivalves, barnacles, gastropods, worm tubes and sea urchins make significant contributions to the shellsand. Underwater television surveys show a diverse suite of sedimentary facies: algal gravels in shallow, sheltered tidal channels; carbonate-rich sand ribbons and sandwaves along the main tidal streams; highly burrowed muds in the deeper, lower energy areas, with many examples of penecontemporaneous nodule formation.