Stratigraphy and sedimentation of the Namurian strata in the Coalcleugh - Rookhope district, Northern Pennines

Abstract

Carboniferous Upper Limestone group strata of Namurian age have been investigates in an area of 45 square miles in the Northern Pennines, with a view to establishing a rational stratigraphy and determining their depositional environment. Several cyclothems, comprising a coarsening upward sequence of clastic sediments, have been recognised and named by the prominent marine bands taken as the base' of each. The marine band may either be a widely distributed limestone or a more impersistent ironstone, A complex series of channel sandstones, shell beds and fine clastics, which does not conform to the cyclothemic concept, is grouped together as the Slate Sills Formation. Strata above the grindstone Sill, the topmost member of the Upper Limestone Group, are poorly exposed, and are grouped together as the Rowton Well Formation. By comparative correlations, largely outside the research area, it is known that the Upper Limestone Group is wholly of Eumorphoceras age, and it is suggested that the E(_1)/E(_2) boundary lies in the vicinity of the Upper Rookhope Ironstone. The Rowton well Sill (or "First Grit") is known to be of R(_1) age, but no evidence of H zone strata has yet been found. The petrography of the sandstones in the area is discussed and details of the main types are given. The petrography and mineralogy of the Knucton Ironstone, a sandy, sideritic chamosite oolite, are also presented. The chamosite has a 14 Å orthohexagonal structure and is diagenetic, at least in part. Evidence is presented which shows that the Upper Limestone Group cyclothems possess all the sedimentary features of a deltaic sequence. The rocks are subdivided into eight lithological facies, and the sedimentological properties of these are given and discussed in terms of possible depositional environment. The detailed study strongly supports the general Impression that the Upper Limestone Group is essentially a marine sequence in which the dominant sedimentologlcal control was of deltaic type, with occasional episodes of dominantly fluvial sedimentation.