Synopsis Revised lithostratigraphical terminology for the Great Estuarine Group in Skye, Raasay, Eigg and Muck is proposed. The group comprises the Cullaidh Shale Formation, Elgol Sandstone Formation with Kildonnan Member and Lonfearn Member, Valtos Sandstone Formation, Duntulm Formation, Kilmaluag Formation and Skudiburgh Formation. The type sections of these formations are defined, new measured sections of 5 of them are illustrated and their lateral variations described. The Group is predominantly Bathonian in age, having its base in the Garantiana Zone (Upper Bajocian) and its top in the Macrocephalus Zone (Lower Callovian). The mainly argillaceous formations show wide lateral continuity indicating that a single depositional basin must have occupied the Inner Hebrides—Minch area during the Bathonian.

In the Inner Hebrides the facies distribution patterns and mineralogy of outcropping Great Estuarine Group (Bathonian) sandbodies indicate SSW progradation of copiously supplied lagoonai deltas in two sub-basins (Sea of the Hebrides Basin and Inner Hebrides Basin). These were both westerly tilted half graben and were probably separated by a slowly subsiding basement ridge known as the mid-Skye palacohigh. In the Bathonian these basins had an intermittent connection with the open sea and were supplied with clastic sediment from mineralogically distinct source areas in the Scottish Landmass to the east and an Outer Hebrides Landmass to the west. Fluvial-dominated and fluvial-wave interaction deltas supplied by low sinuosity rivers form successive sandbodies intercalated with lagoonal mudstones. The facies distribution patterns and mappable depositional limits of these sandbodies are dependent on the style and rate of fluvial supply, the configuration of the basins, variations in wave energy and salinity fluctuations. This demonstrates concepts which are probably applicable to the prediction of reservoir thickness and facies patterns in related sedimentary basins.

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.

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