Dinantian, Namurian and Westphalian rocks of the district south-west of Barnstaple, North Devon

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Abstract
Summary: A stratigraphical succession in the lower part of the Culm Measures has been established in an area between the rivers Taw and Torridge, south-west of Barnstaple, North Devon. Higher Culm strata rest, possibly with unconformity, on the following sequence:— (d) I nstow B eds : A greywacke-shale sequence including the Instow Fish-bed. (1500+feet.) (c) L imekiln B eds : Bedded siliceous siltstones and black shales. (100–120 feet.) (b) C hert B eds : Recognized in two facies—a central or Codden Hill facies, consisting almost entirely of bedded chert, and a northern or Fremington facies, containing in addition much limestone and shale. At the top of the Chert Beds in each facies are a Posidonia becheri and a “ Goniatites spiralis ” bed. (150–300+ feet.) (a) P ilton B eds (thickness unknown). The evidence of goniatites, trilobites and lamellibranchs is used to correlate the succession with the Carboniferous of North Germany and northern England. The presence of parts of zones II, IIIα and IIIβ of the German sequence is recognized in successive levels in the Codden Hill facies of the Chert Beds, whilst the Fremington facies is correlated with P 1 of northern England. The Chert Beds in both facies are terminated by the “ G. spiralis ” bed of P 1 d-P 2 a ( =IIIβ 7 or IIIγ 1 ) age. The lower and upper Reticuloceras stages are represented in the Limekiln Beds, whilst G 2 goniatites are found in the Fish-bed, which lies near the top of the Instow Beds. The lower part of the sequence is involved in a number of southward-dipping thrust-sheets; the upper part is steeply folded but rarely inverted. The general trend of fold-axes is E.–W.; there is a general westerly plunge of around 12°. Faults are both north-westerly and north-easterly, and appear to have succeeded the main thrusting, whilst minor E.–W. faulting has been observed. The cleavage and jointing can in most cases be related to the major tectonic pattern.