Primary versus diagenetic bedding in the limestone-marl/shale alternations of the epeiric seas, an example from the Lower Lias (early Jurassic) of SW Britain

Abstract

Micritic limestones form part of cyclic alternations of limestones- marls/shales in many offshore facies of epeiric seas and their origin has been, and still is, subject to controversy. As an example from the offshore ramp facies, the origin of Lower Lias micritic limestones of SW Britain has been investigated. The very continuous limestone beds show many petrographic and geochemical similarities to isolated nodules, which occur within the marls and are interpreted as being of diagenetic origin. Displacive growth of the microspar crystals was the most important process that occurred during the near-surface formation of the central part of limestone beds, but their edges formed during later burial. The carbon isotopic values (+1.1‰ to −3.4‰ (PDB)) of the central part of the limestone beds and nodules suggest a predominantly marine source for the carbonate, whereas bacterial sulphate reduction played a minor part in their cementation. The oxygen isotopic values (−1.5‰ to −7.2‰ (PDB)) of the characteristic limestone beds, and nodules suggest their formation (cementation/recrystallization) commenced at shallow depth but continued during progressive burial. The microspar crystals of the matrix of the limestones show rare aragonitic relics or pitted surface. Aragonitic fossil remains are rare in these offshore facies and this is in contrast to their abundance in the laterally equivalent shallow marine facies (Sutton Stone). It is suggested that early submarine dissolution of aragonitic components (lime mud and bioclasts) was important in providing carbonate for cementation of the limestones. Early submarine dissolution may have been a common process in epicontinental epeiric settings with low deposition rates, and as the result long residence time in the early diagenetic zones.