An Early Cambrian Assignment for the Caerfai Group of South Wales

Journal ofrhe Geological Society, London, Vol. 152, 1995, pp. 221-224, 4 figs. Printed in Northern Ireland

An early Cambrian assignment for the Caerfai Group of South Wales

D. J. SIVETER & M. WILLIAMS Department of Geology, University of Leicester, Leicester LE1 7RH, UK

The existence of .the earlyCambrian in South Wales is demonstrated.The ostracode Indiana lenfifonnis occurs in the CaerfaiBay Shales, South Walesand the Olenellid Zone Red Callavia Sandstone, Comley, Shropshire. The Caerfai Group (basal Conglomerate, St Non’s Sandstone, Caerfai Bay Shales, Caerbwdy Sandstone) of South Wales is therefore correlatable with the Lower Caerfai Group ComleyGroup of Shropshire. The Conglomerateand St Non’s Sandstoneare inferred to correlate with the ‘Non-trilobite’ Zone and eady part of the Olenellid Zone of Shropshire. The early or Fig. 1. Outcrop of the Cambrian in Wales and England (inset map; mid-Cambrian age of the Caerhwdy Sandstone and the definition of black) and of the Caerfai Group in Pembrokeshire. the Protolenid-Strenuellid Zone Southin Wales remain undetermined. (Ac2-Ad; c. 2 m), extends over three fossil zones (Cowie et Keywords:Cambrian, Ostracoda, biostratigraphy, South Wales, al. 1972). The so-called ‘Non-trilobite’ Zone, which includes Shropshire. the Wrekin Quartzite and lower 60 m of the Lower Comley Sandstone, is characterized by burrows, small shelly fossils, The Precambrian-Cambrianboundary is defined in brachiopodsand problematica. The succeeding Olenellid Newfoundland (Landing 1993; Brasier et al. in press) but the Zone spans the upper part of the Lower Comley Sandstone international chronostratigraphy of the Cambrian is not yet andlower part (Ac2, Red Callavia Sandstone + Ac3, formalized. The early, mid-and late Cambrian in Britain Serrodiscusbellimarginatus Limestone) of the Lower are based onthe here-named Lower Comley Group of Comley Limestones. The Protolenid-StrenuellidZone, Shropshire (= Comley ‘Series’ of Cowie et al. 1972), the St signified by the loss of olenellids and an increased diversity David’s ‘Series’ (Solvaand Menevian groups) of South of eodiscidand opisthoparian trilobites, characterisesthe Walesand theMerioneth ‘Series’ of NorthWales Strenuella (Ac~),Protolenus (Ac5) and Lapworthella (Ad) respectively. divisions of the Lower Comley Limestones (Rushton 1974). The biostratigraphy of the British Cambrian is based Microfossils (Brasier 1986) and trilobites allow correlation mainly on trilobites (see Thomas et al. 1984). Within each with southeastern Newfoundland (e.g. Brasier 1989~). of the British Cambrian regions lithological correlation of formations is often possible, but detailed correlation of such TRILOBITEZONES sequences with any formof standard chronostratigraphy and SHROPSHIRE PEMBROKESHIRE v)8 p particularly betweenWelsh and the typically more t, g ANDSUBZONES fossiliferous Englishsequences is oftenunsatisfactory (Rushton 1974). For example, the Caerfai Group of Pembrokeshire (Fig. l), SouthWales has been only tentatively correlated with theLower Comley Group of Shropshire(Cowie et al. 1972; Rushton 1974). Basedon ostracode distributions we demonstratethat most of the Caerfai Group is early Cambrian. Rushton & Siveter (in press)summarize knowledge of British Cambrianostracodes. Siveter et al. (1993) and Williams et al. (in press) demonstrate the biostratigraphical potential of certainmid- and late Cambrian British ostracodes.

Stratigraphy of the Lower Comley Group and the Caerfai Group. The LowerComley Group. The LowerComley Group of Shropshire (Cobbold 1921, 1927, 1933; Cobbold- Pocock 1934;Fig. 2) is unconformable onthe late Precambrian Uriconian igneous suite (Patchett et al. 1980). The Group, comprising the Wrekin Quartzite (division Aa of Cobbold’s notation; 7-50m), Lower Comley Sandstone Fig. 2. Stratigraphy of the Cambrian of Pembrokeshire and (Abl-4 and Ad; c. 150 m) and Lower COmley Limestones Shropshire. Alphanumerics: Cobbold 1921. 221

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Caerfai Group. The Caerfai Group (Harkness & Hicks 1871; notionthat the Caerfai Group is possibly earlyCambrian Hicks 1881; see Fig. 2), consisting of clastic rocks with (e.g. Cox et al. 1930; Crimes 1970; Williams & Stead 1982) is occasional beds of volcanic ash, unconformably overlies the based on such general stratigraphic grounds but is hitherto late Precambrian Dimetian and Pebidian igneous sequences unproven. (Green 1911; Patchett & Jocelyn 1979) of the St David’s and Haycastleareas, Pembrokeshire. The basal Conglomerate Newevidence. Re-investigatingPringle’s locality at Cwm of the Group (c. 18 m) is overlain in turn by the St Non’s Bach [SM 841 2301, Pembrokeshire, we collected some 50, Sandstone (c. 120m), Caerfai Bay Shales (c. 15m) and mostly fragmentary, specimens of an ostracode species, CaerbwdySandstone (c. 130m), representinga shallow largely froma single horizon in the Caerfai Bay Shales marine, transgressive sequence (Rushton 1974). (University Museum, Oxford nos. OUM A.1545-1592; Figs 1 The succeedingLower Solva Beds(Hicks 1881)yield & 3). Associates consist of aprobable trilobite fragment what have hitherto been considered as the earliest definitive anda specimen of Coleoloides? sp. (OUM A.1543 and Cambrian fossils in South Wales: trilobites, which indicate A.1544; Fig. 4M-P). We regard this ostracode as conspecific an early mid-Cambrian age for the base of the Solva Group. with 22 specimens collected by Pringle from the Caerfai Bay The contactbetween the Caerbwdy Sandstones and the Shales at Cwm Bach and nearby Crow Cwm [SM 864 2371 Lower Solva bedshas been regarded as unconformable (British GeologicalSurvey, Keyworth,Nottingham, Eng- (Jones 1940) or conformable and ‘transitional’ (Williams & land, nos.Pg3870, 3871, 3873-76, 3878-83, 3885-88, Stead 1982). 3892-3900). We assign all theostracodes to Leperditia (?) lentiformis Cobbold, 1921, here referred to Indiana. The age of the Caerfai Group. Conventional notion. Fossils Characteristicfeatures of I. lentiformis include: a large of the Caerfai Groupare rare and supposedlybiostratig- (upto 1.33cm long),thin, moderately postplete, gently raphically undiagnostic(see Cowie et al. 1972, 26;p. convex,calcium phosphaticcarapace (X-raydispersive Rushton 1974, p. 54). The basalconglomerate and the St analysis), with a simple dorsalcontact margin; a gently Non’s Sandstonecontain only Skolithus burrows;the curved dorsaloutline in lateral view; a shallow, flattened CaerbwdySandstone also has onlypossible trace fossils region paralleland immediatelyadjacent to thean- (Crimes 1970; Turner 1979). The distinctively reddish, teroventral margin; and fine, concentrically arranged, bioturbated, Caerfai Bay Shales (Turner 1979) contains the earliest shelly fossils of the Cambrian of South Wales: Hicks Comley Quarry Cwm Bach (1871) described the brachiopod Lingulella primaeva and Shropshire Pembrokeshire the ostracode Leperditia ? cambrensis from Porth Clais [SM 742 2421 and recorded trilobite fragments from Southeast of Rhosson (atthe Iron Age fort [SM724 2461) in nearby Ramsey Sound; he also figured an unlocalized, and as yet unlocated,trilobite cephalon from the base of the ‘purple rocks’. Jones (in Cox et al. 1930, p. 250) thought that this ‘cephalon’ was from the Caerfai Bay Shales; based also on evidencein Hicks (1892) andfrom a fragment collected from the vicinity of Newgale by Pringle in 1908 (specimen not identified in BGS collections), Jones considered that the unitcontained fragments of olenellids. Pringle & George (1948,p. 19) laterregarded the fragment collected near Newgale as an ostracode. The Caerfai Group resembles other earlyCambrian Welsh Borderlandsequences Shropshirein and the Malverns and inthe English Midlands, wherebasal conglomerate also overlies a Precambrian igneous suite and Fig. 3. Stratigraphy at Comley Quarry, Shropshire (after Cobbold passes up into burrowed sandstones and then sediments with and log of authors) and on west side of Cwm Bach (authors’ log), shelly fossils (Rushton 1974; Brasier et al. 1978; Brasier Pembrokeshire. The fault produces minor repetition of the Lower 19896). InSouth Wales the succeedingSolva Group has Comley Limestones. ‘Upper’ and ‘lower’ ridge refer to mid-Cambrian Paradoxides oelandicus Zone trilobites. The topographical features at Cwm Bach.

Fig. 4. Light photographs of specimens on slabs from: Red Culluuia Sandstone (Ac~),Olenellid Zone, Comley ‘Series’, Comley Quarry, Shropshire (A-F); and Caerfai Bay Shales, Caerfai Group, W of stream at Cwm Bach (H, J, M-P), lkm NW of Newgale and at Crow Cwm (G, I, K, L), 2 km NE of Newgale, Pembrokeshire. A-L, Indiana lentiformis (Cobbold, 1921). A-D, BU 51, holotype, carapace: left valve lateral (stereo-pair), right valve lateral, dorsal and ventral views, X4. E, F, SM A9522, partly exfoliated, incomplete large valve: E, granulose ornament (stereo-pair), X10; F, granulose ornament, ~5.5.G, BGS Pg3892, lateral view of anteriorly incomplete right valve, X4. H, OUM A.1545, valve fragment showing concentric granulose ornament (stereo-pair), x15. I, K, BGS Pg3897, partly exfoliated, incomplete small valve: I, granulose ornament, X10 K, granulose ornament, X5.5. J, BGS Pg3876, lateral view of posteriorly and anterodorsally incomplete left valve (stereo-pair), X4. L, BGS Pg3894, lateral view of open carapace, left valve incomplete posteriorly and right valve slightly incomplete posterodorsally and posteroventrally (stereo-pair), X4. M, OUM A.1543, pleural segment (?) of trilobite, X14. N-P, OUM A.1544, Coleoloides? sp.: N, P, part and counterpart, X8; 0, detail, X12. BGS, British Geological Survey, Keyworth; BU, Lapworth Museum, Birmingham University; OUM, University Museum, Oxford; SM, Sedgwick Museum, Cambridge.

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granulostriate ornament in which many of the granules are Clarendon Press, Oxford. polygonal as seen in both shell and exfoliated valves (Fig. - 1989b. Sections in England and their correlation. In: COWIE, J. W. & 4A-L). BRASIER,M. D. (eds) The Precambrian-Cambrian Boundary. Oxford Monographs on Geology and Geophysics, 12, 82-104. Clarendon Press, Leperditia ? cambrensis Hicks, 1871, from the Caerfai Oxford. Bay Shales, is probably a senior synonym of I. lentiformis; -, HEWITT, R. A.& BRASIER,C. J. 1978. On the Late Precambrian-Early confirmation awaits the location and study of Hicks’ Cambrian Hartshill Formation of Warwickshire. Geological Magazine, material. Furthermore, despite extensive effort we have not 115,21-36. -, COWIE,J. W. & TAYLOR,M. E. Decision on the Precambrian-Cambrian mananged to collect additional material of L. ? cambrensis Boundary Stratotype. Episodes, in press. at Caerfai Bay itself, the most complete section of the COBBOLD,E. S. 1921. The Cambrian horizons of Comley (Shropshire) and Caerfai Bay Shales. their Brachiopoda, Pteropoda, Gasteropoda,etc. Quarterly Journal of the I. lentiformis was described originally only fromthe Geological Society of London, 76, (for 1920), 325-386. c. 75 cm thick, Red Callavia Sandstone (division Ac2 of -1927. The stratigraphy and geological structure of the Cambrian area of Comley (Shropshire). Quarterly Journal of the Geological Society of Cobbold; see Figs 2 & 3), late Olenellid Zone, at Comley London, 83,551-573. Quarry, Shropshire [SO 484 9651 (see Cobbold 1933), thus - 1933. Notes on Comley Quarry,near Church Stretton, Shropshire. affording a point of correlation between the Lower Comley Report of the British Association (for 1933), 473-476. Group and the Caerfai Group. The Red Callavia Sandstone -, POCOCK,R. W. 1934. TheCambrian area of Rushton(Shropshire). Philosophical Transactions of the Royal Society, B223, 305-409. alsocontains trilobites, brachiopods and conoidal fossils COWIE,J. W., RUSHTON,A. W. A. & STUBBLEFIELD,C. J. 1972. A correlation (Rushton 1974; Brasier 1986; Hinz 1987). of Cambrian rocks inthe British Isles. Geological Society, London, Assuming that the Precambrian-Cambrian boundary is Special Reports, 2. correctly drawn in Shropshire and Pembrokeshire, the Cox, A. H., GREEN,J. F. N., JONES,0. T. & PRINGLE,J. 1930. The geology of the St. David’s district, Pembrokeshire. Proceedings of the Geologists’ stratigraphic implications of the dating using ostracodes is Association, London, 41,241-273. thatthe Caerfai Group basal conglomerateand St Non’s CRIMES,T. 1970.P. A facies analysis of theCambrian of Wales. Sandstonecorrelate with the Non-trilobite Zoneand/or Palaeogeography, Palaeoclimatology, Palaeoecology, ‘7,113-170. early part of the Olenellid Zone of Shropshire. Above the GREEN,J. F. N.1911. 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Sedimentary Geology, 24,269-281. References WILLIAMS,B. P. J. & STEAD,J. T. G. 1982. The Cambrian rocks of the BRASIER,M. D. 1986. The succession of small shelly fossils (especially Newgale-St. David’s area. In: BASSETT,M. G. (ed.) Geological conoidal microfossils). from English Precambrian-CambrianBoundary Excursions in Dyfed, south-west Wales. National Museum of Wales, Beds. Geological Magazine, 123, 237-256. Cardiff, 27-49. - 1989a. Towards a biostratigraphy of the earliest skeletal biotas. In: WILLIAMS,M., SIVETER,D. J., RUSHTON, A.W. A. & BERG-MADSEN,V. The COWIE,J. W. & BRASIER,M. D. The Precambrian-Cambrian Boundary. UpperCambrian bradoriid ostracod Cyclotron lapworthi is a OxfordMonographs onGeology and Geophysics, 12, 117-165. hesslandonid. Transactions of the Royal Society of Edinburgh, (in press). Received 30 September 1994; revised typescript accepted 9 October 1994.

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