Environments of Deposition of Middle Jurassic Sandstones in the Great Estuarine Group, N. W. Scotland

The lithostratigraphy of the Great Estuarine Group is revised by reference to defined type sections of the eight formations (joint publication with J.D. Hudson). Within this framework a series of lagoonal delta and lagoon shoreline depositional models are proposed for the two sandstone dominated formations (Elgol and Valtos). Sedimentation was controlled by brackish, widely fluctuating and locally marine salinities in two elongate basins (Inner Hebrides and Sea of the Hebrides) separated by a slowly subsiding ridge termed the mid-Skye palaeohigh. Low sinuosity rivers of rapidly fluctuating discharge supplied the deltas with sand sourced from the Moinian and Dalradian (probably via the ORS) of the Scottish landmass to the east and tram the Lewisian of the Outer Hebrides landmass to the west. The Elgol Formation represents a fluvial-wave-tide interaction delta in the north of the Sea of the Hebrides Basin Where salinities were marine and shows a southwards transition to a fluvial dominated delta with buoyant mouth bar dynamics. In the Inner Hebrides Basin it represents a fluvially dominated lobate delta system with fresh-brackish salinities controlling friction dominated mouth bar hydrodynamics. The Valtos Formation in the Sea of the Hebrides Basin represents 8 phases of fluvial-wave interaction delta progradation separated by transgressive shell debris beds. The deltas pass laterally into interdeltaic lagoon shorelines and part of the formation represents offshore shell debris sheets and shoals. In the smaller Inner Hebrides Basin the formation represents 6 fluvially dominated lobate delta progradation phases with only minor wave reworking. The identification of genetic relationships between facies allows correlation and the prediction of sand body geometry while the recognition of the overall control of contemporary subsidence rates allows palaeogeographic reconstruction.