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

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.

The Evolution of Lacustrine Environments on Mars: Is Mars Only Hydrologically Dormant?

“…Finally, if the channel water density is less than the lake density, the sediment load is transported further away. Because of the tendency of these sediments to be carried as a buoyant plume, the resulting deltas are also called buoyancy-dominated mouth bars (Harris 1989, Pulham 1989, Martinsen 1990.…”

High resolution zonation within a tide-dominated deltaic reservoir: the Middle Jurassic Beryl Formation, Beryl Field, UKCS

“…Outcrop examples of this type of salinity variation developed in similar facies are found in the Middle Jurassic of Skye (e.g. Elgol, Cuillaidh and Garantiana Clay formations ;Harris 1989;Hudson et al 1995). The sandstones and siltstones are interpreted to represent sand sheets developed during transgressive reworking of tidal dune and channel deposits.…”

Mafic intrusions, hydrothermal venting, and the basalt-sediment transition: Linking onshore and offshore examples from the North Atlantic igneous province

“…The contact is obscured by approximately 10 m of dead ground at Neist Point, but the first exposed eruptive deposits comprise subaerial sheet lavas. The sedimentary rocks below the subaerial lavas are dominated by Middle Jurassic shallow marine siliciclastic sediments (Searl, 1994;Morton, 1999;Hesselbo and Coe, 2000) and estuarine strata with many coarse-grained facies, crossbedding sedimentary structures, and nonmarine shales (Morton, 1983;Harris, 1989). The subbasalt sediments have locally been extensively intruded by dolerite sheet intrusions.…”