The development of the Betton Farm Coral Bed within the Malton Oolite Member (Upper Jurassic, Middle Oxfordian) of the Scarborough District, North Yorkshire, UK

摘要

Betton Farm quarries contain, within a small area, a remarkable range of facies associated with the development of a Jurassic coral reef. A NW-SE-trending barrier reef protected the whole area containing the two quarries. Betton Farm North Quarry displays Thamnasteria patch reefs. These consist of scattered, 1 m high masses of colonial coral surrounded by reef debris that were situated behind the barrier within a large reef platform area. In Betton Farm South Quarry, bioclastic sand, consisting largely of coral and bivalve debris, interdigitates with the development of a small ribbon reef made up of the massive colonial coral Thamnasteria. This ribbon reef, only 3 to 4 m wide, sheltered a lagoonal area that accumulated lime mud and was colonized by large gastropods and burrowing bivalves. A fauna of reef-specializing bivalves and echinoids occupied these areas of patch/ribbon reef development, which were periodically swept by nutrient-bearing currents. Competition between the growth of the ribbon of Thamnasteria and the accumulation of fringing sediments saw periods when the Thamnasteria expanded over the coral-shell sand, and periods when the encroaching sediment cut back the growth of the Thamnasteria considerably. Considered initially to represent the Coral Rag Member of the Coralline Oolite Formation, the Betton Farm Coral Bed is now seen to represent localized coral reef developments situated near the base of the Malton Oolite Member, and growing upon antecedent bedrock features such as hard-ground surfaces that formed during pauses in sedimentation of the Malton Oolite.