The sedimentology of a Middle Jurassic lagoonal delta system: Elgol Formation (Great Estuarine Group), NW Scotland

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Abstract
Summary: Deposition of N-S prograding lagoonal deltas took place in two separate extensional basins (Sea of the Hebrides and Inner Hebrides Basins) of different size and hence wave energy. Salinity of water in the basins varied spatially and temporally from marine to fresh-brackish. The reduced salinities were controlled by diminished connections with the open sea together with copious run-off from a hilly hinterland with a subtropical seasonal climate. Sections in N Trotternish, S Trotternish, Raasay (Sea of the Hebrides Basin) and Strathaird (Inner Hebrides Basin) represent three different styles of delta sedimentation corresponding to different salinities and basinal energy regimes (waves and tides). In N Trotternish, marine macrofaunas and microfloras occur in a wave-tide interaction delta system in which tidal currents controlled distributary-channel and mouthbar hydrodynamics and high wave energies generated beach-ridge sediments at the delta shoreline. In S Trotternish and Raasay, probable brackish-marine salinities and a deeper basin are reflected in buoyant mouth-bar hydrodynamics. In Strathaird (Inner Hebrides Basin) probable fresh-brackish salinities correspond to a fluvial-dominated delta recording friction-dominated mouth-bar hydrodynamics. This lobate delta is characterized by low angle offshore inclined bar-front sandstones. In the Inner Hebrides Basin, the Elgol Formation delta reaches a depositional limit between Strathaird and Eigg. The N-S transition from marine to fresh-brackish salinities records the gradual establishment of a non-marine depositional system (which is maintained throughout the rest of the Great Estuarine Group) during progradation of the Elgol Formation delta.