Microbially mediated carbonates in the Mesoproterozoic Stoer Group of NW Scotland; earliest evidence of life in Britain?

Lake margin and associated facies in the Mesoproterozoic Stoer Group of NW Scotland contain laminated carbonate structures and sediment surface textures. The former are reinterpreted as microbial stromatolites based on the combined evidence of constructional depositional relief, binding of sediment to support subvertical and overhanging layers and the presence of microbially mediated carbonate fabrics. Stromatolites range from domical structures a few centimetres across with a depositional relief of a few centimetres deposited in very shallow water in lake margin settings to biohermal structures several metres thick and several tens of metres in lateral extent that occur in lake margin settings and drape fan-delta lobes that prograded into lakes. Sediment surface textures are represented by laterally linked domes on mudstone bedding planes, desiccated carbonate sheets and mudstone coatings on fluvial surfaces and fan-delta lobes in conglomeratic facies. The biological involvement in the formation of sediment surface textures is not proven. The formation of sediment surface textures as opposed to stromatolites may reflect variations in the degree of carbonate saturation of the groundwater or differing communities of microbial species. Dissolved sulfate may have been supplied by weathering of sulfides, whereas carbonate may have been supplied by semi-arid weathering of Scourian dykes. Marine–freshwater mixing zones are another possible source of sulfates and carbonates.