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.

Polyphase low-grade metamorphism of the Ingleton Group, northern England, UK: a case study of metamorphic inversion in a mudrock succession

“…This biostratigraphical age is in close agreement with a Rb-Sr radiometric age (recalculated with the 1977 decay constant, with a 95 % confidence limit) of 494 ± 18 Ma obtained by O'Nions et al (1973). Soper & Dunning (2005) recorded five lithofacies in the Ingleton Group, including medium-to thick-bedded sandstone, thin-bedded sandstone-siltstone-mudstone, laminated siltstone-mudstone, gravel-grade greywacke and massive sand-grade greywacke. They formally (Leedal & Walker, 1950).…”

“…This study of higher-grade slates from Ingleton, rather than the lower-grade samples examined by O'Nions et al (1973), produced an isochron age of 465 ± 10 Ma (95 % confidence level), which may relate to either a metamorphic or diagenetic interpretation. If this represents a metamorphic age, then it indicates an early Arenig deformation of the Ingleton Group, a possibility also considered by Soper & Dunning (2005). However, until further palynological research is completed, a pre-Ordovician age for the Ingleton Group cannot be discounted.…”

“…A thin metadolerite dyke, < 1 m wide, cuts the Pecca Formation in the old slate workings [SD 7034 7443] and then appears to split into two dykes, each less than 0.5 m thick, that cross the gorge of the River Doe [SD 7043 7435]. Based on their degree of alteration and fabric development, discussed below, Soper & Dunning (2005) suggested that these intrusions were emplaced prior to the folding of the Ingleton Group. Two lamprophyre (kersantite) dykes are recorded by Soper & Dunning (2005).…”

“…The Ingleton Group is a tightly folded sequence of mudstone, wacke and sporadic conglomerate of possible Precambrian or Early Ordovician age that crops out within the Craven inliers at Ingleton and in Ribblesdale, northwest Yorkshire (Soper & Dunning, 2005;Williamson, 2005). The group is unconformably overlain by strata assigned to the Windermere Supergroup, a series of late Ordovician shelf deposits succeeded by lower Silurian hemipelagic rocks and a foreland basin sequence of thick, Upper Silurian turbidites (Kneller, King & Bell, 1993;Kneller et al 1994).…”

“…Despite the limited outcrop of the Ingleton Group, extensive research has been focused on these 'ancient rocks', stretching from the first description of the classic unconformity at Thornton Force (Playfair, 1802) to the recent details of their structure and sequence (Soper & Dunning, 2005, and references therein) and age (Dodson & Robinson, 2006). This is * Author for correspondence: sjk@bgs.ac.uk the first detailed study of their burial and metamorphic history.…”