The sedimentology of the Cambrian clastic sediments of Northwest Scotland
The Cambrian clastic sediments of northwest Scotland crop-out along the line of the Moine Thrust Zone between Skye in the south and Loch Eriboll in the north and form the basal 250m of a broadly transgressive Cambro-Ordovician sequence of clastic and carbonate sediments. These sediments were deposited on the passive western margin of the lapetus Ocean. The clastic stratigraphy consists of four members; the Lower Member, Pipe Rock, Fucoid Beds and Salterella Grit. The Lower Member consists of 100-125m of mature, cross-bedded quartzarenites which have been subdivided into three facies associations. The lowest association is a 10m thick series of cross-bedded channel sands interpreted as mesotidal barrier inlet deposits. This association is erosively overlain by 10m of thinly bedded, cross-bedded and parallel laminated sands interpreted as lower shoreface sediments. The remainder of the Lower Member comprises compound cross-bedded cosets 1-10m thick interpreted as tidal sandwave deposits. The sudden appearance of numerous Skolithos burrows at the Lower Member-Pipe Rock boundary is interpreted as an evolutionary event representing the colonisation of the Cambrian shelf by suspension feeding annelids. The Pipe Rock is an 85-100m thick sequence of mature, highly burrowed quartzarenites considered to have been deposited in a tidal shelf to outer shelf tempestite setting. The Fucoid Beds consists of 20m of a mixed clastic-carbonate sequence of thinly bedded wave rippled tempestites interbedded with fairweather echinoderm grainstones. The Salterella Grit is a 0-15m thick coarsening upwards sequence of muds and quartzarenites interpreted as having been deposited as tidal sandridges which went through active and moribund stages of development before being buried under carbonate platform sediments. The dominant controls on the facies developed in this sequence were thermal subsidence, eustatic sea level rise and tidal resonant effects. Two rapid shallowing events, in the middle of the Pipe Rock and at the top of the Fucoid Beds, may have been produced by variations in the spreading rate of the lapetus Ocean.