Oncoids As Environmental Indicators in the Much Wenlock Limestone Formation of the English Midlands
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Journal of rhe Geological Sociefy, London, Vol. 145, 1988, pp. 117-124, 9 figs. Printed in Northern Ireland Oncoids as environmental indicators in the Much Wenlock Limestone Formation of the English Midlands K. T. RATCLIFFE Department of Geological Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK Abstract: The Much Wenlock LimestoneFormation of the West Midlands was deposited in a mid-shelf setting and is divisible intothree members; theUpper and Lower QuarriedLimestone members being separated by the more argillaceous Nodular Limestone Member. Oncoids, composed predominantly of micritic fabrics with Rothplerzella and Giruanella, occur commonly in the Lower Quarried Limestone Member. These oncoids vary from subspherical bodies up to 5 mm in diameter to forms with a highly irregular and branched upper surface which reach 70mm across. Each form is indicative of a different depositional environment, which is also reflected in the sediment enclosing the oncoid. Equidimensional oncoids in peloidal packstones were formed by continuous rolling, whereas the larger, branched forms enclosed in loosely packed wackestones developed in quieter conditions below wave base. The distribution of oncoid morphotypes in the Lower Quarried Limestone Member shows that small variations in relative sea level weresuperimposed on the overall middle to late Wenlock regressive episode during which the Much Wenlock Limestone Formation was deposited. The uniformity of the formationthroughout the West Midlands indicatesthat the sea floor was essentially planarover a large area. Vertical variation of oncoid morphology within the Lower QuarriedLimestone Member can be tracedthroughout the area, allowing accuratecorrelation of relative sea-level variations. In latest Wenlock times,carbonate-dominated deposition Wren’s Nest Hill (SO 936920; National Grid References are took place to the east of the Welsh Basin producing the prefixed SO, SP or SIC) andDaw End Railway Cutting rocks of the Much Wenlock LimestoneFormation. This (SK 034003) are the only extensive examples in the region formation crops out in a roughly triangular area defined by (Fig. 1). Dueto quarrying at Wren’s Nest the Lower Much Wenlock (Shropshire), Walsall (West Midlands) and Quarried LimestoneMember is beststudied at Daw End Usk (South Wales) (Fig. 1).To the southand west the railway cutting, which is the only section where unbedded lateralequivalents of the limestone are shallow water masses of limestone in this member are accessible. sandstones and basinal shales. The subsurface extent of the The sedimentology of the Much Wenlock Limestone formation to the north and east is unknown. Formation in the West Midlands indicates an increase in In the Much Wenlock LimestoneFormation of the energy of depositionalenvironment reflecting the overall English Midlands three memberswere recognized by mid to late Wenlock regression evidencedthroughout the Doming (1983) based on Butler’s (1939) original subdivi- Welsh Borders by both sedimentology (Scoffin1971) and sion. The basal Lower Quarried LimestoneMember fauna1 variations (Hurst 1975). Oncoiddistribution and (6-10 m thick), is composed of 20-100mm thick peloidal microfacies variationindicate that minor sea-level fluctua- packstonesand skeletal wackestones which are rich in tions are superimposed on the overall regressive pattern. oncoids, separated by 1-5 mm thick shaley mudstone partings. The overlying Nodular Member (18-20 m thick) Sedimentology of the Lower Quarried Limestone comprises pale grey nodules of carbonate mudstoneand loosely packed skeletal wackestones embedded in dark grey Oncoids are restricted almost entirely tothe Lower to black shaley mudstones. Thin butcontinuous, sharp- Quarried Limestone Member, in which their morphology based beds of skeletal crinoidal packstones also occur in this vanes between different microfacies. Two broad lithofacies member, particularly towards the top. The highest member, groups are distinguished: the reef and the bedded lithofacies the Upper Quarried Limestone (5-7 m thick), is bedded on (Fig. 2). a similar scale tothe lowest member, but thedominant lithology is peloidal packstone, which is cross-laminated and wave-rippled. Crinoidalgrainstones occur as 20-500mm Reef lithofacies thick beds within the peloidal packstone. Unbeddedto Theterm ‘reef‘ is used hereto refer to mound-like irregularly bedded limestone mounds up to 6 m thick occur structures, either layered or massive, which stood above the within both the Lower Quarried Limestone and upper part surroundingcontemporaneously deposited sediment and of the Nodular Member. wereformed by sedentarycalcareous organisms (Bates & The vertical distribution of thethree members is best Jackson 1980). In the Lower Quarried Limestone Member studiedin six boreholes (Fig. l), two each from Walsall the reefs, which are here massive limestones extending 6m railway station (R2 and R3), fromjunction 10 of the M6 vertically and 30 mlaterally, are composed of three motorway (M1and M6), and from Dudley sports ground microfacies labelled M.F. 1-3 (Fig. 2). (D104 andD102). Detailed lateral field relationships, Microfacies 1 comprises coralline framestones composed however, can only be seen at surfaceexposure, of which dominantly of in situ colonies of Halysites, Palaeofauosites 117 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/1/117/4900564/gsjgs.145.1.0117.pdf by guest on 25 September 2021 118 K. T. RATCLIFFE % Limestone 0 KEY UPPER QUARRIED L/ Skeletal fragment LIMESTON e Crinoidossicle MEMBER . Peloid Q Oncoid P Cross-lamination IC- Wave ripple Md Mudstone NODULAR Wk Wackestone LIMESTONE PkPackstone MEMBER Gr Grainstone I00 M6 95‘ KEY ’\ Fig. 1. Location map and generalized stratigraphy. , e MuchWenlockLimestoneFormation Lower left insetshows the outcrop of the Wenlock LOWER A Fault / Series in England and Wales; the Much Wenlock QUARRIED Wk Boreholelocality Limestone Formation occurs to the east of the km LIMESTONE dashed line. Main map shows outcrop of the MEMBER Wk / formation in the West Midlands and the position 0 1 2km ”:, ,97 of boreholes: Walsall Railway Station (W,SP GENERALISEDSTRATIGRAPHY 7 0 1 Miles / 011984; R3 SP 012985), Junction 10 of the M6 motorway (Ml, SO 993984; M6, SO 990980) and Dudley sports ground (D104, SO 952908; D102, / SO 954903). Upper left inset shows generalized stratigraphy of the Much Wenlock Limestone Wrens Nest Hill Formation in the West Midlands. The solid line to F the right of the lithological column represents the D102 F DUDLEY percentage of carbonate nodules and beds . S$ (0-100%) as opposed tosilty mudstones. and Heliolites, with branched andlor domalmorphology. grumose texture (sensu Pettijohn 1975), fenestral fabric and They are embedded in micrite. occasional algal tubules, suggesting that much is of algal or Microfacies 2 is tightly packed crinoidal grainstones, in cyanobacterialorigin. Within the micrite, in situ laminar which articulated stems up to 100 mm long are found. colonies of Labechia and Stelliporella sit directly above Microfacies 3 contains mainly mottled micrite with 1-2 mm thick shaley mudstone partings. Bedded lithofacies - 0 - 0 The bedded lithofacies of the Lower Quarried Limestone 0 0 Member are divided into three microfacies, labelled M.F. - 0 o NODULAR MEMBER 4-6 (Figs 2 & 3). 0 0 0 Microfacies 4 comprises oncolitic peloidal packstones, in which the peloids are discrete grains which do notmerge into clotted textures. The peloids do, however, grade into micritized fragments recognizable skeletalas debris. Articulated brachiopods and unbroken grains are rare, most allochems consisting of fragmentaryskeletal debris. The matrix is composed of silt-grade fragments and micrite. This microfacies dominates the upper few metres of the member, and overlies both reefs and M.F. 5 and 6 (Fig. 2). Microfacies 5 is composed of skeletalpackstones and wackestones, commonly containing articulated brachiopods and abundant complete valves. Micrite is less abundant in this microfacies than in any other Lower Quarried Limestone microfacies (Fig. 3). Brachiopods are dominantly rhynchonellids and spiriferids forming an assemblage which bENE approximates to Hurst’s (1975) high energy subassemblage I of the Sphaerirhynchia Community. Oncoids are relatively rare in this microfacies. Microfacies 5 occurs as sharp-based Fig. 2. Microfacies distribution in the Lower Quarried Limestone beds 20-50 mm thick within M.F. 6. Member. M.F. 1, Coralline framestones; M.F. 2, crinoidal Microfacies 6 is loosely packed, poorly sorted oncolitic grainstones; M.F. 3, algal micrites; M.F. 4, peloidal packstones; skeletalwackestones. Articulated brachiopods and other M.F. 5, skeletal packstones and wackestones; M.F. 6, loosely whole fossils are common, but comminuteddebris is also packed skeletal wackestones. abundant. The grains float in micritic matrix which contains Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/1/117/4900564/gsjgs.145.1.0117.pdf by guest on 25 September 2021 ENVIRONMENT OF THE WENLOCK LlMESTONE 119 --=H3 I I1 4 l Fig. 3. Lithological composition of the Lower Quarried Limestone Member. Left-hand col- umn is height in metres of specimen above base. Cr, crinoid ossicle; Bra, brachiopod I =!?! II -11 fragment; W. Bra, whole brachiopod; Tril, trilobite fragment; Ost, ostracode fragment; W. Ost, whole ostracodes; Gast, gastropods; Bry, bryozoans; Pel, peloids; A.F., algal fragment; Weth, Wetheredelk sphere; Calc, calcisphere; M.F., micritized fragment; Onc, oncoid; Cem,