Structure and sequence of the Ingleton Group, basement to the central Pennines of northern England

Abstract

SUMMARY The Ingleton Group is a sequence of steeply dipping turbiditic greywackes that crops out in two of the Craven inliers at the southern margin of the Askrigg Block, at Ingleton and in Ribblesdale. The structure has previously been interpreted as a series of concertina-like isoclinal folds, much more intense than the deformation recorded in Late Ordovician–Silurian strata of the overlying Windermere Supergroup. For this reason, the Ingleton Group was long regarded as late Precambrian in age. In 1982 the British Geological Survey (then Institute of Geological Sciences) assigned it to the early Ordovician on the basis of an acritarch assemblage recovered from a single borehole sample. This paper reports a resurvey based on the ample wayup evidence provided by turbidite bedforms. The greywackes are disposed in a large isoclinal fold pair, the previously recognized Skirwith Syncline in the Ingleton Inlier and the Horton Anticline mapped in Ribblesdale. Some 3 km of strata are exposed on the common limb, and assigned to five lithofacies. A start has been made on lithostratigraphical subdivision, three formations being recognized in the uppermost part of the succession. Isoclinal folding of the Ingleton Group predates the Acadian (Devonian) deformation of the overlying Silurian strata; the Acadian cleavage is superimposed obliquely across the isoclines and faces down on their inverted limbs. The group records a prelate Ordovician history comprising the deposition of penecontemporaneous continental volcanic arc detritus in a substantial turbidite basin, intrusion of mafic dykes, major folding and low-grade metamorphism, followed by deep erosion. Uncertainty about the depositional age of the Ingleton Group presents problems as to its affinities. Lithofacies and provenance characteristics match late Neoproterozoic Avalonian interarc basin deposits in the English Midlands and Welsh Borderland, but not the apparently contemporaneous Ordovician Skiddaw and Manx groups. Confirmation of an early Ordovician age would suggest that the deformation history of the Ingleton Group matches that of the Monian Supergroup of Anglesey. The problem is discussed in the context of Ordovician terrane assembly.