The Lower Carboniferous reef limestones of Cracoe, Yorkshire

Abstract

Summary The limestones forming the reef-knolls in the Cracoe district of Yorkshire lie at the eastern end of a belt of similar limestones which stretches westwards across the Central Province of the Carboniferous Limestone Series. This reef belt separates areas of contrasted sedimentation, the strata to the north being of massif (Great Scar) facies, while to the south lies the much thicker basin facies, of impure limestone with much interbedded shale. The Cracoe knolls have attracted much attention in the past, owing to their peculiar topography and the abundance of their fossils. Theories on their origin were based either upon supposed quaquaversal dips of original deposition or upon more normal deposition, followed by tectonic deformation and subsequent erosion. It is now shown that the limestones in the Cracoe area are divisible into local faunal and lithological subdivisions, which can be mapped by the usual methods. These subdivisions are correlated with the coral-brachiopod zones and goniatite succession of the Visean, and extend from S2 to D2. The peculiar facies is due to the beds having been laid down against a long narrow ridge which kept the basin and massif areas of sedimentation apart until it was finally submerged in D2 times, when sedimentation became continuous across it. The reef limestones are, in fact, the marginal facies of the Great Scar Limestone, and the lateral change of each member of the local subdivision is traced through the marginal facies into the normal Great Scar Limestone. The structure consists of a series of anticlines and broader synclines, arranged in echelon, which pass obliquely across the facies belts and die out north-eastwards. They were formed by pressure from the south impinging obliquely on a ridge of older rocks. The folding took place under exceptionally light cover and was completed before the Bowland Shales were deposited. The knoll topography was blocked out by faults which cut across the fold axes. The knolls themselves were eroded to very much their present shapes before the Bowland Shales, which lie with strong unconformity on the limestones, were laid down. The removal of these shales is now proceeding, and is re-exposing the buried topography of the knolls.