Late Carboniferous palaeobotany of the upper Bideford Formation, north Devon: a coastal setting for a Coal Measures flora

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  • Late Carboniferous palaeobotany of the upper Bideford Formation,north Devon: a coastal setting for a Coal Measures flora

    Christopher J. Cleal1 & Barry A. Thomas"

    CLEAL, C. J. & THOMAS, B. A. 2004. Late Carboniferous palaeobotany of the upperBideford Formation, north Devon: a coastal setting for a Coal Measures flora. Proceedingsof the Geologists' Association, 115, 267-281. The Culm Seams are thin coals in the UpperCarboniferous upper Bideford Formation of north Devon. Clastic sedimentary rocks associ-ated with the coals have yielded a fossil macroflora dominated by the remains of theCalamostachyales and Medullosales, together with subsidiary lycophytes, sphenophylls, ferns,lagenostomaleans, rare cordaites, and a possible early cycad. The flora is probably earlyLangsettian in age, which is in agreement with the evidence of the non-marine bivalves andmarine bands. It is broadly similar in composition to contemporaneous macrofloras from SouthWales. It is unlikely to represent an assemblage formed from plant remains subject to long-distance transportation. Rather, it was probably preserved in the lower reaches of a 'bird-foot'delta that had temporarily transgressed into the Culm Basin. The distal margin of this deltawould represent a comparable habitat to the levees of the rivers further inland, and thus wouldhave supported vegetation similar to that which generated the more usual Coal Measuresmacrofloras.

    Key words: Carboniferous, palaeobotany, biostratigraphy, palaeoecology

    'Departmen: of Biodiversity and Systematic Biology, National Museums and Galleries ofWales, Cathays Park, Cardiff CF10 3NP, UK (e-mail: chris.cleal@nmgw.ac.uk)2Jnstitute of Rural Sciences, University of Wales Aberystwyth, Llanbadarn Fawr, AberystwythSY23 2EX, UK

    1. INTRODUCTION

    The palaeobotany of the mainly non-marine West-phalian (Late Carboniferous) 'Coal Measures' ofBritain has been extensively studied and there havebeen many monographs and reviews (e.g. Kidston,1923-1925; Crookall, 1955-1976; Cleal & Thomas,1994, 1995). However, the Westphalian of southwestEngland is in a different, mainly marine facies andconsequently has a much poorer palaeobotanicalrecord. The main exception are the beds associatedwith thin coals known as the Culm Seams, whichoccur in a narrow strip of land between the coast atGreencliff (Bideford Bay) and Hawkridge Wood nearChittlehampton (Edmonds et al., 1979). Even here, theplant fossils are generally sporadic and fragmentary,but are nevertheless of interest for the light they throwon the biostratigraphy and palaeoecology of these beds.

    The first description of these macrofloras was byLindley (in De la Beche, 1834, 1839; and in Sedgwick &Murchison, 1840). The definitive study on them was byE. A N. Arber (1904a, b, 1907, 1911a, b), who alsoreviewed the earlier history of research. Rogers (1910)gave a detailed account of the different plant-bearinglocalities but no specimens were illustrated. In somecases, the published description of the localities ispoor, but M. A. Arber (1982) has managed to identifyProceedings of the Geologists' Association, 115, 267-281.

    several of them from Rogers' field-maps and notes.Since E. A N. Arber's work, the only publishedpalaeobotanical work on these beds has been a critiqueof his biostratigraphical conclusions by Crookall(1930), and the reclassification by Leary (1980) ofArber's Megalopteris. Hofmann (1992) reported ad-ditional plant remains from beds associated with theCulm Seams, but neither illustrated them nor recordedthem beyond 'axes and leaves of calamites, with rarerfern pinnules'. In view of the recent reawakening ofinterest in these beds (e.g. Xu Li, 1990; Hofmann,1992; Eagar & Xu Li, 1993) and the considerablechanges that have occurred in the taxonomy of LateCarboniferous fossil plants, a review of the evidence istimely. An attempt by one of us (RAT.) to collectplant fossils from these beds exposed along the coastyielded only a little additional material. The presentstudy is therefore based on existing collections, princi-pally in the Natural History Museum, London(BMNH), the Sedgwick Museum, Cambridge (CSM),the Royal Albert Memorial Museum, Exeter(RAMM), and the Museum of Barnstaple and NorthDevon (MBND; this includes the collections formerlyin the Athenaeum Museum of Barnstaple).

    The etymology of 'culm' has been the subject ofsome debate, with the consensus being that it is derivedfrom 'coal' and that the Culm Basin and Culm Group

    0016-7878/04 $15.00 2004 Geologists' Association

  • 268 C. J. CLEAL & B. A. THOMAS

    N

    1lkm

    1-----1

    ~L.:.J

    E:ilD

    Budc Fm

    Bidcford I'm(showing Comborough Sst)

    Crackington Fill

    Fig. 1. Simplifiedgeological map of the area near Bideford, North Devon, showing the location of the main localities that haveyielded fossils of the Culm Seams flora. The number of each locality is given in the text and in Table 1. Based upon BGS1:50000 Geological Map Sheets 292, 293 and 309, by permission of the British Geological Survey.

    were so named because they contained the Culm Seam.However, this seems unlikely, as the coal-bearinginterval is not a typical part of the Culm Group orBasin. Edmonds et al. (1979) has instead suggested thatit was derived from the Welsh word cwlwm, meaningknot, and referring to the contorted nature of the beds.Hence, the Culm Seam was named after the interval ofrocks in which it occurs, and not the other way around.

    2. GEOLOGICAL BACKGROUND

    The Upper Carboniferous deposits of Devon wereformed in the Culm Basin, on the foreland that devel-oped between the cratonic Wales-Brabant Massif andthe northwards migrating Variscan Orogen (Hartley &Warr, 1990). The relationship between the Culm Basinand the contemporaneous South Wales Basin has beenthe subject of some speculation (Hartley, 1993). Cur-rent opinion is that there has been significant crustalshortening between the two basins, as a result ofthrusting along the Bristol Channel Fault Zone. How-ever, the amount of crustal shortening is unknown andso the original distance between the two basins duringCarboniferous times is uncertain.

    Edmonds (1974) classified the Late Carboniferousdeposits of the Culm Basin into three formations, inthe following ascending sequence.

    (I) Crackington Formation. Turbidites of mainlyNamurian age deposited in an east-west-trendingbasin (Edmonds et al., 1979), but which according toThomas (1988) were ultimately derived from a sourceto the north of the basin.

    (2) Bideford Formation. A localized sequence, con-fined mainly to an elongate area between Bidefordand Chittlehampton, where it overlies the CrackingtonFormation. It consists of nine coarsening-upwardscycles probably formed in an elongate 'bird-foot'-type

    delta (De Raaf et al., 1965; Walker, 1966, 1969;Elliott,1976). The upper eight cycles were formed by a pro-grading delta front (Xu Li, 1990), culminating inrooted beds and, in cycles 5 and 9, a coal. The cycle 5coal is only thin; but that at the top of cycle 9 is moresubstantial (sometimes more than 4 m thick; Edmondset al., 1985, p. 116) and is underlain by a prominentsandstone (the Comborough Sandstone). The sedi-ment was derived from the north, probably from anuplifted area that had resulted from the inversion ofthe Bristol Channel Fault Zone (Hartley, 1993).

    (3) Rude Formation. Shallow-water deposits, formedin either lacustrine or near-shore marine conditions(Higgs, 1983, 1984, 1986; Melvin, 1986;Hartley, 1993).The base of the Bude Formation is at the first appear-ance of massive sandstone beds, although the contactmay be visually gradational (British Geological SurveyLexicon of Named Rock Units). Near Bideford, theBude Formation overlies the Bideford Formation,but further south it lies directly on the CrackingtonFormation.

    Plant fragments occur throughout the UpperCarboniferous of the Culm Basin, but the bulk ofthe identifiable remains are in the upper BidefordFormation, in the area between Westward Ho! andUmberleigh (Fig. 1). The succession here is generallysouth dipping. However, a major east-west-trendingfault about 1 km south of Bideford brings theCrackington Formation back to the surface. On thesouth side of this fault, the Bideford Formation isabsent and the Bude Formation immediately overliesthe Crackington Formation.

    3. PROVENANCE OF SPECIMENS

    Significant assemblages of plant fossils were found(mainly by Rogers) at the following localities. The

  • PALAEOBOTANY OF UPPER BIDEFORD FORMATION

    Table 1. Distribution of species in the main plant localities in the upper Bideford Formation.

    269

    2 3 4 5 6 7 8 9 10 II 12 13

    Lepidodendron aculeatum Sternberg xLepidodendron dichotomum Sternberg ?Lepidodendron fusiforme Corda xLepidophloios acerosus Lindley and Hutton x xSigillaria scutellata Brongniart xSigillaria tessellata Brongniart xSphenophyllum cuneifolium (Sternberg) Zeiller x x x xCalamites undulatus Sternberg x x xCalamites suckowii Brongniart x x x xCalamites cistii Brongniart xCalamites sp. x x xAsterophyllites charaeformis (Sternberg) G6ppert x ? xRenaultia crepinii (Stur) Zeiller x ? xCorynepteris angustissima (Sternberg) Nernejc ?Karinopteris acuta (Brongniart) Boersma x x x x x xLyginopteris hoeninghausii (Brongniart) Gropp x xEusphenopteris sp.Neuralethopteris schlehanii (Stur) Laveine x x ?Neuralethopteris rectinervis (Kidston) Laveine x ? x x x xNeuralethopteris jongmansii Laveine ?Alethopteris lonchitica Sternberg ? ? x ?Alethopteris decurrens (Artis) ZeillerNeuropteris obliqua (Brongniart) Zeiller x xTrigonocarpus sp. xLesleya cf. cheimarosa Leary & Pfefferkorn xArtisia sp. x

    1, Greenc1iff;2, Pit Quarry; 3, Bideford Wharf; 4, Bideford Station; 5, Robert's Quarry; 6, Broadstone Quarry; 7, Pollard's Quarry; 8, PillheadCopse; 9, Warmington Farm; 10, Webbery Wood; 11, Alverd