Aspects of sedimentary facies and diagenesis in limestone-shale formations of the (Middle Jurassic) Great Estuarine Group, Inner Hebrides
The upper part of the Great Estuarine Group (Middle Jurassic) comprises three lithostratigraphic formations, the Duntulm, Kilmaluag and Skudiburgh Formations. They record a series of paralic palaeoenvironments, marine-brackish lagoons. low-salinity, ephemeral lagoons and alluvial plains respectively. This general transition of environments was produced by late Bathonian regression. Algal limestones from the Duntulm Fm. are interpreted as lagoon-marginal, algal marshes which underwent synsedimentary calcification by magnesium-calcite. Stable-isotope ratios of oxygen are light, delta18O = -4%, which suggests this calcification occurred in an environment with significant freshwater input. Stable-isotope ratios of carbon are also light and imply a pore-water system with abundant light organic carbon. Early diagenetic carbonate microfabrics were probably influenced by microbial systems. The algal marsh environment was hostile to the usual lagoonal fauna, mainly through exposure. The Kilmaluag Fm. is dominated by a low-salinity fauna. Accumulation of muddy detrital sediment was common, while fine-grained carbonates were probably primary precipitates of calcite and magnesium-calcite. During evaporative periods concentration of Mg allowed synsedimentary dolomitization of these primary carbonates. Stable-isotope ratios of oxygen for dolomite are the heaviest reported from the Great Estuarine Group and corroborate the theory formation of dolomite from evaporated waters of low salinity. Depositional clay mineral suites from mudrocks are largely unaffected by burial diagenesis. The assemblages contain significant amounts of smectite, and one bentonite horizon in the Kilmaluag Fm. suggests that smectite was mainly of volcanic origin. Immediately adjacent to Tertiary igneous centres, mild metamorphism caused destruction of the smectite phase. The formation of calcified, freshwater-influenced algal marshes, probable seasonal evaporites, early diagenetic dolomites interspersed with calcitic limestones in ephemeral lagoons and nodular calcretes in alluvial sediments, all suggest a warm palaeoclimate with marked rainy and dry seasons.