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, Alverdiscott; 12, Somers, Hiscott; 13, Ford Quarry.

    numbering of the localities is the same as that used onthe map (Fig. I) and Table I.

    (I) Greencliff (or Cornborough Cliff), Bideford BayISS 405271 j. The cliffs west of Bideford provide thebest available outcrops of the Bideford Formationand are a Site of Special Scientific Interest (Cleal &Thomas, 1996). Edmonds et al. (1979) published adetailed measured stratigraphical section. Rogers(1909) identified the main fossil horizons as occurringin shales within what are now called Cycles 8 and 9 inthe upper Bideford Formation, with soft black shalesbelow the coal of Cycle 9 yielding the best material.

    (2) Pit Quarry, Abbotsham [SS 421270 j. This is theclassic site from which Sedgwick & Murchison (1840)obtained plant fossils. Rogers (1910) stated that thefossils occur in the thick sandstone (now known as theCornborough Sandstone) associated with the CulmSeams. Locality details were not given but M. A. Arber(1982) has traced it to a disused and overgrown quarrynorthwest of Abbotsham.

    (3) Bideford Wharf Neither Arber (l904a) norRogers (1910) mentioned this locality, but there is asingle specimen from here in the Rogers Collection atthe Sedgwick Museum. However, it does not given anyfurther locality details other than that the fossil waspreserved in shales exposed on the east side of the river.

    (4) Bideford Railway Station, East-the-Water ISS457263 j. Rogers (1910) recorded plants from vertical

    beds exposed alongside the road from the stationleading to Chudleigh Fort. These lie immediatelyabove the Cornborough Sandstone.

    (5) Robert's Quarry, East-the- Water ISS 461265 or462265J. M. A. Arber (1982) listed this as one of twopossible sites immediately east of Chudleigh House,East-the-Water. Exposed here were bluish 'ovoidallysplitting' shales that lie a short distance above theCornborough Sandstone. As well as yielding plantfossils, Rogers (1910) reported that the shales containnon-marine bivalves. However, from evidence pro-vided by Eagar & Xu Li (1993) it seems more likelythat the shells originated from below the sandstone(these shells were also discussed by Simpson (1933).

    (6) Broadstone Quarry, East-the-Water. The plantfossils from here were collected from a spoil heap ofshale and impure coal, which had resulted from adrainage adit being driven through the north side ofthe quarry (Rogers, 1910). The coal is probably thatwhich overlies the Corn borough Sandstone, as thereare several old workings of that seam in the vicinity(Arber, 1904a). The exact position of the quarry isunknown but according to M. A. Arber (1982) wasnear Pollard's Quarry (see next).

    (7) Pollard's Quarry, East-the- Water ISS 469264].Arber (1904a) reported only poorly preserved speci-mens from here but Rogers (1910) gave a list of speciescollected from one of the coals exposed here in 1910.

  • 270 C. J . C L E A L & B . A . THOMAS

    Ta ble 2. Percentage composition of the Culm Seams macroflora, compared with tha t o f two Lan gsettian-aged macroflor as fromSouth Wales (based on data from Davies (1929 .

    Culm macroflora Gellideg Seam Five Feet Seam

    Lepidocarpales 0.74(2.30)5.33 1.48(1.64)1.82 0.03(0.16)0.47Sigillar iostrobales 0.49(1.83)4.67 0.03(0.06)0.10 0.00(0.06)0.31Sphenophyllales 3.60(6.52)10.72 0.01(0.02)0.05 0.00(0.00)0.20Calamostachyales 24.33(30.43)37.09 38.96(39.61)40.26 28.58(30.69)32.86Ferns 5.05(8.40)12.97 0.00(0.00)0.02 0.00(0.00)0.20Lagenostomales 9.66(14.03)19.43 0.00(0.00)0.02 1.57(2.19)2.97Medullosalcs 28.30(34.67)41.48 53.79(54.45)55.11 20.65(22.55)24.53?Cycadales 0.01(0.47)2.59 0.00(0.00)0.02 0.00(0.00)0.20Co rdaitantha les 0.27(1.35)3.97 3.96(4.22)4.49 42.06(44.35)46.66No. of specimens 213 22056 1831

    Values are given with their 95% two-sided confidence bound s, calculated as recommended by Howarth (1998).

    M . A. Arber (1982) identified the exact position of thissite, which she reported was now a rubbish dump. Th estratigraphical position was not stated but the localityis immed iately south of the Cornborough Sandstone,and so was presumably the seam that immediatelyoverlies that sand stone.

    (8) Pillhead Copse. Rogers (1910) stated that hecollected fossils from the eastern end of this wood,three-quarters of a mile (1.2 km) east of Bideford. Thefossils were from spoil tips near old coa l workings, butno such tips or workings can be identified on theGeol ogical Surveyor Ordnance Survey maps (1:50 000).

    (9) Warmin gton Farm i S S 478260] . Rogers (1910)repo rted finding abundant plant fragments in a hedgebank 300 yards (280 m) north of the farm .

    ( 10) Webbery Wood iSS 501260j. Rogers (1910)sta ted that he collected fossils from one of the old spoiltips at a disused coal working, about 270 m west ofWebbery House. It is very near a thick sandstone thatis probably the Cornborough Sandstone (Edmondset al., 1985).

    (II) Alverdiscott. Arber (1904a) noted that fossi lsfrom near a coal seam exposed at this village, 7 kmESE of Bideford, were collected by Townshend Hall,but further locality details were not given .

    (12) Somers, Hiscott Down iSS 551255]. Rogers(1910) collected from another old spoil tip from adisu sed coa l working, about 450 m west of SomersHouse. It is, again , near the Cornborough Sandstone.

    (13) Ford Quarry , Umberleigh iSS 605237]. Shalesassociated with a 0-45...

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