Sedimentology of part of the Lower Oil-Shale Group (Dinantian) sequence at Granton, Edinburgh, including the Granton “shrimp-bed”

Abstract

ABSTRACTThe Dinantian Lower Oil-Shale Group of the Edinburgh area records the evolution of a SW–NE trending basin within an area of fluvial-dominated deltaic sedimentation, with rare marine incursions from the E. The sequence at Granton records the abandonment of a delta lobe, with deltaic sandstones overlain by lagoonal mud-shales. These thinly-laminated non-bioturbated mud-shales record low-energy, stagnant conditions. Fish, ostracodes and conchostracans inhabited the lagoon at this time.Within the mud-shale sequence is the dolomitic Granton “shrimp-bed” which contains fenestrae, possible gypsum pseudomorphs andin situshrinkage breccias suggesting periods of emergence. The “shrimp-bed” contains a number of laminae packed with soft-bodied fossils including marine forms, which resulted from brief marine incursions into the stagnant lagoon. The fluctuating salinity conditions seem to have favoured the development of an unusual fauna of shrimp-like crustaceans, fish, conchostracans, ostracodes and bellerophontid gastropods. This fauna appears to have suffered periodic mass mortalities during marine incursions which brought orthocone cephalopods, polychaete worms and conodonts into the lagoon, according to our current knowledge of the distribution of these marine taxa in the sequence.The Granton sequence displays a variety of ductile and brittle deformation features most of which can be attributed to local SW-directed extension related to regional dextral strike-slip across the Midland Valley of Scotland.