The Upper Permian Reef Complex (EZ1) of North East England: Diagenesis in a Marine to Evaporitic Setting

Abstract

The Zechstein of North East England is a complex sequence of carbonate evaporite cycles of Late Permian age. It includes the Permian Magnesian Limestone of North East England which was deposited around the margin of the Zechstein Sea which covered much of northwest Europe some 230–240 million years ago. The Magnesian Limestone includes a 100-m-thick reef complex (part of the Ford Formation) which trended north-south in County Durham and Tyne and Wear (Fig. 1). The growth of the reef (Tunstall Reef Member) resulted in the development of an extensive lagoon to the west, towards the shoreline. There has been much discussion on how the reef was formed and on the roles played by organisms, especially bryozoans and algae, but pervasive dolomitization has obliterated much of the evidence and preservation of the fauna is generally poor. Dolomitization has also destroyed many of the early diagenetic textures of these rocks. However, recent discoveries of undolomitized reef rock have provided beautifully preserved fossils and allowed the recognition of submarine cements. Evidence for the latter is now being found in dolomitized parts of the reef too. This paper documents the diagenesis of the Zechstein reef of NE England in the light of data revealed in the reefal limestones, and against a background of major changes in sea level, exposure and evaporite precipitation.