Unfolding the Ingletonian Rocks of North Yorkshire

(a lithostratigraphic and structural study).

by P N Hildreth

Abstract

Sedimentary structures providing way-up evidence have enabled the detailed mapping of folding within a sequence of low-grade metamorphosed turbidite sediments. This, together with lithological observations, has provided the author to present a stratigraphic sequence for the Ingletonian rocks, a structural model of the Ingleton Inlier and an age correlation with the Manx and Skiddaw Slate groups. A thickness of 1056 metres is proposed for the sequence in which several of the seven beds recognised have been repeated by isoclinal folding. A Cambro-Ordovician (Late Tremadocian-Lower Llanvirnian) age is proposed for the accumulation of the turbidite sequence which has then been affected by Lower Ordovician and end-Silurian tectonic deformation.

Introduction

The Ingleton Inlier occupies an area of some 8.5 square kilometres on the south-western flanks of the Pennines below Whernside and Ingleborough Hill, North Yorkshire where the Rivers Doe and Greta have cut down into a limestone plateau to expose older beds of differing age and attitude. The inlier is bounded on its southern margin by the North Craven Fault, Great Scar Limestone (Lower Carboniferous) being downfaulted to the south. Elsewhere the Great Scar Limestone rests on the Ingletonian rocks with spectacular angular unconformity. The major part of the inlier is occupied by a series of low grade metamorphic rocks attributed to the Ingletonian Series. The rocks are devoid of fossils and are of uncertain, and therefore controversial, age.

Early workers such as Nicholson (1869) and Hughes (in Dakyns and others 1890) believed the Ingletonian Series to be of Silurian age and the latter assumed that the beds dipped regularly south-west giving an estimated thickness of about 3050 metres. Nicholson however believed in the possibility of the beds being folded. Rastall (1906) assumed a continuous sequence as he did not observe folding but never entirely dismissed the possibility. Later work by Leedal and Walker (1950) proved isoclinal folding implying that the sequence would have a considerably reduced thickness because of the repetition of beds. They also proposed a Precambrian age. Simpson (1968) has tentatively correlated the Ingletonian Series with a part of the Manx Series of the Isle of Man on structural observations thereby suggesting a late Cambrian to early Ordovician age. Recent work by Soper, yet to be published, argues for a Precambrian age though Kabrna (2003) reports that an Arenigian (Lower Ordovician) acritarch was recovered from a borehole penetrating Ingletonian rocks.

Lithostratigraphy

In Chapel-le-Dale and Kingsdale below Thornton Force (SD694753), rocks of pre-Carboniferous age crop out. These rocks, the Ingletonian Series, consist of alternating greenish grey and grey slates and greywackes with subordinate bands of siltstone, arkosic conglomerate and mud-pellet conglomerate. They retain sedimentary structures such as graded bedding, cross laminations, slump bedding and rarer sole markings. These have proved to be invaluable pieces of way up evidence. Kabrna (2003), on a website for the Craven and Pendle Geological Society, has made photographs available of sedimentary structures in the Ingletonian rocks.

The sequence was originally mapped on the six-inch-to-one-mile scale as twenty one individual beds striking north-west to south-east with a steep, sometimes vertical, dip to the south-west. Bed 1 was the first bed encountered after crossing the North Craven Fault and upstream of Manor Bridge (SD 695745) on the River Doe. Bed 21 was the last and most north westerly bed seen, in Chapel-le-Dale north-west of Atkinson's Hull (SD 725763). Later observations recognized isoclinal folding within the sequence and this led to a mapping of the structure of the rocks through recordings of the attitude of sedimentary structures and, where visible, exposures of fold hinges (see also photographs available through Kabrna (2003)). This latter work revealed evidence of nine north-west to south-east striking folds which, when plotted and cross sections drawn, reduce the number of beds from twenty one to seven. These were correlated and named after locations from within the Ingleton Inlier. The resulting stratigraphical correlation of mapped beds is shown in Figure 1.

Figure 1. Correlation of the mapped beds of the Ingletonian Series.

A model of the folding of the Ingletonian Series is shown in Figure 2. The succession of beds from the Ingleton Granite Quarry (SD 719752) north-eastwards to God's Bridge (SD 733764) is the full sequence of the Ingletonian Series, the beds to the south-west of the quarry being repetitions of these same beds.

Figure 2. Folding of the Ingletonian Series.

Length of section: 4 km

This cross section has been used to calculate the approximate thickness of each stratigraphic unit recognized. Figure 3 shows a scaled Ingletonian succession with brief lithological descriptions. A conservative estimate of the total thickness of the series exposed within the Ingleton inlier is 1056 metres, considerably less than the 3050 metres quoted by Hughes (in Dakyns and others 1890).

The rocks of the Ingletonian Series are in the main typical turbidite sediments as can be seen from their lithologies and sedimentary structures (Bouma and Brouwer 1964). From observations of the cross lamination structures within the series, and by returning the beds to horizontal, it would appear that the major current direction was from the south-east quadrant. The lower beds of the Granite Quarry Beds represent the base of a turbidite unit in which rip-off clasts and the unsorted nature of the sediments may well have been produced by gravitational sliding.

Figure 3. The Ingletonian Succession.

Vertical scale shown is in 50m blocks

Structure and Geological History

The whole of the Ingletonian Series has undergone low grade, chlorite-muscovite grade, regional metamorphism which must be related to the severe F1 deformation. The chlorite, which is finely disseminated throughout the beds, imparts the characteristic greenish colouration to the rocks. The similarities in structure between the Ingletonian Series and the Manx and Skiddaw Series (Simpson 1968) would suggest that the Ingletonian Series are probably of similar age to the lower beds of the Manx Series, i.e. Tremadocian to Arenigian. The occurrence of detrital glauconite within some of the Ingletonian rocks would also suggest a Lower Palaeozoic age, the author knowing of no record of glauconite in Precambrian rocks.

The beds were laid down in a tectonic basin environment, probably close to the south-eastern edge of the North Wales-Manx-Lake District trough. Such a situation would agree with the suggested south-easterly derivation of sediment, possibly from an extension of the Baltic Shield, comprising the Ingletonian rocks.

Rocks of the Ingletonian Series display three distinct axial plane cleavages (S1, S2 and S3) which suggest polyphase folding with three distinct phases of tectonic activity (F1, F2 and F3). F1 and F2 are present only in rocks of the Ingletonian Series whilst F3 is also present in the Coniston Limestone Series, which oversteps the Ingletonian rocks in the extreme south-west of the inlier. This represents the earliest phase of activity to have affected post-Ingletonian rocks. None of the phases are present in the Carboniferous rocks.

The S1 cleavage is almost invariably aligned more or less parallel to the bedding (S) of the Ingletonian rocks and is related to the mesoscopic isoclinal folding, the fold hinges of which trend north-west to south-east. S and S1 are often vertical but generally dip steeply to south-west. The stress field reflected in the Ingleborough district would therefore appear to have been a horizontal north-east to south-west compression. The severe F1 deformation was accompanied by syntectonic basic intrusions and synkinematic chlorite-muscovite grade regional metamorphism.

S2 cleavage is seen only at one or two localities owing to the severe nature of F1 deformation. It is related to a flat-lying F2 fold recognised in Skirwith Quarry (SD 707738) and the axial planes are generally slightly inclined from the horizontal, dipping north-east. The stress field would therefore have been vertical compression.

Within the Ingletonian Series, S3 cleavage is only seen in the softer slates of the Dale Barn Slates just north-east of the Ingleton Granite Quarry but it is well-developed locally in the Coniston Limestone Series. It is clearly related to the mesoscopic asymmetrical folds which trend north-west to south-east and which plunge gently to south-east. The minor folding in the Coniston Limestone Series reflects the major structure which, in the neighbouring Horton-in-Ribblesdale area, is a syncline trending north-west to south-east and which plunges south-east (Dunham and others 1953).

Simpson (1968) has recorded three phases of tectonic deformation of similar disposition to those at Ingleton within rocks of the Manx Group of the Isle of Man and the Skiddaw Group of the Lake District. He tentatively correlates the Ingletonian Series with the lower part of the very thick Manx-Skiddaw Group and relates F1 deformation at Ingleton with the F1 deformation in the Isle of Man and in the Lake District. It is suggested therefore that the F2 and F3 of the Ingleton inlier can be correlated with F2 and F3 of the Manx-Skiddaw Group (Figure 4).

Figure 4. Correlation of F1, F2 and F3 of the Manx, Skiddaw and Ingletonian rocks (after Simpson (1968).

References

Bouma A.H. and A. Brouwer 1964. Turbidites. Elsevier, Amsterdam.

Dakyns J.R., Tiddeman R,H., Gunn W. and A. Strahan 1890. The Geology of the Country around Ingleborough. Memoir of the Geological Survey.

Dunham K.C., Hemingway J.E., Versey H.C. and W.H. Wilcockson 1953. A Guide to the Geology of the District round Ingleborough. Proceedings of the Yorkshire Geological Society.

Kabrna P. 2003. http://www.kabrna.com/cpgs/countryside/ingleton

Leedal G.P. and G.P.L. Walker 1950. A restudy of the Ingletonian Series of Yorkshire. Geological Magazine 87, 57-66.

Nicholson H.A. 1869. Notes on the Green Slates and Porphyries of the neighbourhood of Ingleton. Geological Magazine 6, 213-5.

Rastall R.H. 1906. The Ingletonian Series of west Yorkshire. Proceedings of the Yorkshire Geological Society 16, 87-100.

Simpson A. 1968. The Caledonian history of the north-eastern Irish Sea region and its relation to surrounding areas. Scottish Journal of Geology 4(2), 135-63.

(c) Hull Geological Society 1999 + 2007

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