Oncoids As Environmental Indicators in the Much Wenlock Limestone Formation of the English Midlands

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 thearea, 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 tothe 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 NodularMember (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 seenat surfaceexposure, of which dominantly of in situ colonies of Halysites, Palaeofauosites 117

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% 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 discretegrains 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 isless 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

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Fig. 3. Lithological composition of the Lower Quarried Limestone Member. Left-hand column 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, cement; Mat, matrix; Cl, allochems. Final column shows the microfacies to which the specimens are assigned. Percentage allochems obtained by counting 500 grains. Percentage allochems, matrix and cement based on point counts where n = 2000.

isolated algal tubules associated with clottedtextures. 1984), but the taxonomic positions of Rothpletzella and Peloids are diffuse and often grade into clotted zones. This Wetheredella are less certain. Riding (1984) regarded microfacies is, with the included beds of M.F. 5, the lateral Rothpletzella as probably a cyanophyte and Wetheredella a equivalent of the reef lithofacies (Fig. 2). cyanophyte or chlorophyte. In the Lower Quarried Limestone Member Rothpletzella and Giruanella occur bothin the reef andbedded Algae in the Lower Quarried Limestone lithofacies. In the reefs they encrust colonial organisms and Algae in the Much Wenlock Limestone Formation were first occur as isolated patches in the mottled micrite. In the described by Wethered (1893), who thought thatthe bedded lithofacies the algae most commonly encrust variously sized tubulesin a particular oncoid represented allochems to form oncoids. different species of Girvanella. Rothpletz (1913), who assigned Wethered's oncoids to Sphaerocodium, onthe Oncoids in the Lower Quarried Limestone other handconsidered different types of tubule to be vegetative and reproductive cells of the same species. This The oncoids were studied in sectioned cores so that only approach of assigning morphologically different albeit two-dimensional representations of three-dimensionalob- intertwining filaments tothe same genus hasled to jectswere seen.To guard against bias, the core sections confusion in classification. Wood (1948) re-examined were oriented randomly and a large population was studied. Rothpletz'smaterial, recognizing Girvanella and naming The Lower Quarried LimestoneMember oncoids are two new genera, Rothpletzella and Wetheredella. These composed of micrite and spar which form spongiostromate three genera are the dominant forms in the Much Wenlock fabrics, and cyanobacterial tubules which form porostrom- LimestoneFormation in the West Midlands. The present ate fabrics (Monty 1981). The tubules always exhibit examples of Giruanella and Rothpletzella (Figs 4a & b) encrusting growth morphologies,never forming upright closely resemble the forms described by Wood (1948). bushes. Despite this, the most obvious feature of the Wetheredella specimens, however, sometimes differ from the oncoids is their morphological variation, forming a material described by him, which have an average diameter continuous series from small (5-10 mm) subspherical forms of 0.1 mm and wall thickness of 0.03 mm. In addition to to large (up to 70mm) branchedforms. For descriptive typical Wetheredella there are large forms which have l mm purposes the oncoids are divided into three morphological external diameters and 0.1 mm thick walls (Fig. 4d). There types, A, B and C (Figs 4 & 5). is a complete gradation in size from these large forms into others identical to those described by Wood (1948). This, Type A oncoids are small, smooth-surfaced, spherical to combined with the similarity inradial porestructure, subspherical and composeddominantly of concentrically warrants classification of the larger forms as Wetheredella. laminatedspongiostromate fabric (Fig. 4d).Discontinuous Spheres 0.2-1.0mm in diameter commonly occur in the lenses of porostromate fabric occur but are not abundant sediment surrounding Wetheredella-rich oncoids. These have (Fig. 6). When the ratio of maximum cortex thickness to a wall thickness and structure identical to the Wetheredella minimum cortex thickness is calculated,Type A oncoids occurring as encrusting forms in the oncoids, suggesting that have a low value (Fig. 5). Type A oncoids dominate in the the spheres are non-encrusting forms of the same genus. peloidal packstones of M.F.4 atthe top of the Lower Giruanella is now accepted as acyanophyte (Riding Quarried Limestone Member (Fig. 7).

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Fig. 4. Algae and oncoids in the Much Wenlock Limestone Formation. (a)-(c) Photomicrographs of three alga-types: (a) Rorhplerzella;(b) Girvanella; (c) Wetheredella.(d)-(f) Oncoid types in the Much Wenlock Limestone Formation: (a) Type A, negative print from stained peel; (e) type B, polished slab; (0 type C, polished slab.

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Fig. 5. Ratio of cortex to circumference of oncoids from borehole R3, Walsall Railway Station (SP 012985). Typical forms of each oncoid type, A, B and C, are shown, taken from tracings of the oncoids on polished slabs. Cortex ratio obtained by dividing maximum cortex thickness (X) by minimum cortex thickness (y), see inset of schematic oncoid. n = 140. (0)Type A, (0)type B, (A)type C.

Type B oncoids have a greater size range than Type A. Their cortex is continuous and composed of irregular and % oftendiscontinuous laminae of spongiostromate and 00- ONCOIDS porostromatefabrics (Fig. 4e). Successive laminae have nTYPEC their maximum thicknesses at different places within the 90- U cortex.Type B oncoids contain a higher proportion of B spongiostromatefabric thanType A oncoids buta lower 80- TYPE proportion than Type C. Rothpletzella is the dominant taxon TYPE A (Fig. 6). 70- Type C oncoids are composeddominantly of porostromate fabric which coats only the upper surfaces of \ \\ \\ allochems (Fig. 4f). Hence, Type C oncoids plot at infinity \ \\ \\ \ on the cortex.ratio axis of Fig. 5, Rothpletzella again being \\ \\ the most abundant cyanobacterium. The cortex is irregular \\\ \\ \ and often branched, eventhough it is composed of \\ \\ \ encrusting, not upright, tubule growth forms. Types B and \\ \\ \\\ C occur in the skeletal packstones and wackestones of M.F. \\\ \\ \ 5 and 6. Type C dominates in the basal parts of the Lower \ \\ \ Quarried LimestoneMember andType B in the \ \\ # \ intermediate parts (Fig. 7). \ \ \ \ 4 \ \ Oncoids as environmental indicators \ hi1 b By definition, oncoids are constructed by algae and will 2 therefore only form in conditionssuited to the growth of

Fig. 7. Sequential variation of oncoid morphotypes in the Lower Quarried Limestone Member. Each column represents the percentage of different oncoid types per metreof core, where 0 is the base of the member and 1-11 are metres above the base, Poor core recovery accounts for the lack of oncoids at 6-7 m. n = 140.

b 5'0% 101 those organisms. Algae are generally accepted to be indicative of shallow waterdeposits (Swinchatt 1969; Porstrgmate Spongiostromate ?ab rlc fabric ?abrlc Lauritzen & Worsley 1974), although they have now been found at depths of 1000m in the Indian Ocean (Bernard & RRothgletzellaHMicrite Lkcal 1960). Caution must thus be taken when using algae HGirvanella j-J Spar as depth indicators.Despite this, optimum growth conditions for algae are within the photoic zone, generally mwetheredella less than 50m depth (Riding 1975; Wray 1977) on marine carbonate shelves (Riding 1984). Fig. 6. Composition of the three oncoid morphotypes. Based on In order to determine the environmental significance of point counts of 20 oncoids, each count consisted of loo0 points. oncoids, Peryt (1981) considered the sediments in a variety

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of oncoid-bearing sequences. He concluded that all oncoids packstones)indicative of periodic high energyconditions, are associated with slow sedimentation rates,and that supports theinterpretation of intermittentturbulence. oncoidscomposed of spongiostromatefabrics typify Additional evidence is seen in the laterally equivalent reefs, high-energy conditionswhereas those with porostromate wherehorizons of the opportunisticstromatoporiod fabrics are generally associated with quiet water conditions. Labechia reflect periods of recolonization and stabilization It has also been noted that due to abrasion, the percentage (Powell 1980) following turbulent events. of porostromate fabricpreserved oncoidin cortices Type A oncoids have concentric laminae suggesting they decreasesas depositional energy increases (Dahanayake were frequently, if not continuously rolled during formation 1978; Peryt 1983; Wright 1983). (Wright 1983). Supporting this is the rarity of algal tubules, Theabundance of algae and oncoids in the Lower common a feature of oncoids formed in relatively Quarried LimestoneMember, and their close association high-energy conditions (Dahanayake 1977; Peryt 1983; with a shelf fauna including a relatively high energy Wright 1983). Type oncoidsA are very similar to brachiopodassemblage, indicates thatthe member was Dahanayake’s Type I1 oncoids and Wright’s ‘dense oncoids’, deposited in a shallow marineenvironment, within the both interpreted by thoseauthors to be relatively photic zone. The depth to which the photic zone extended high-energy forms.Surrounding Type A oncoids, the locally cannot be estimated accurately, but the abundance of peloidal packstones of M.F. 4contain very few whole micrite in this member suggests the water may have been fossils, suggesting an environment in which the sediment cloudy, restricting the photic zone to a few tens of metres. was agitated. However, they also contain micrite, indicating Within this general framework, morphological variations in thatthe environment was not sufficiently energetic to the oncoids andthe relative abundance of variousforms remove all the fine-grained sediment. The peloids in this within themember enable more detailed analysis of microfacies donot resemble peloidal cements (Macintyre environments. 1984). They do grade into micritized but recognizable Type C oncoids could not have formed in an skeletal fragments, suggesting the peloids are abraded and environment where rolling occurred, as their branches micritized skeletal debris. would have been broken. Since Rothpletzella and Giruanella In the Lower Quarried Limestone Member the change of are probably cyanobacteria,optimum growthconditions microfacies from base totop (Figs 2 & 3) indicates an would havebeen in well-lit areas, particularly theupper increasein energy of depositionalenvironment. This is surfaces of allochems. Therefore the absence of cortex from matched by the changein relative abundance of different the undersides of Type C oncoidsindicates that only one oncoid types (Fig. 7). Initially the environment was low side of a grain was exposed at the sediment/water interface energy with little turbulence, producing dominantly Type C during its growth, confirming that Type C oncoids formed in oncoids, but as deposition proceeded intermittent periods of a non-turbulentenvironment. This interpretation is turbulence became more frequent, favouring the formation supported by the close similarity of Type C oncoids to of Type B oncoids. At the top of the member, constant but ‘tender’oncoids (Fuchtbauer 1968) and Sphaerocodium gentle rolling produced Type A oncoids. Such changes on a (Rothpletz 1890), boththought to haveformed in low shallow carbonate shelf are best explained by upward energyconditions (Peryt 1977). Morphologically, Type C shallowing. The lowest parts of the member were deposited oncoidsresemble Wright’s (1983) ‘irregular Garwoodia’ in an environment within the photic zone, but below wave oncoids, interpreted as a low energy growth form.The base. Shallowing took the sea bed above storm wave base abundance of micritein the surrounding microfacies resulting insediments which show evidence of sporadic supports the low energy depositional environment suggested turbulencein an otherwise quiet environment. As by oncoidmorphology. As the upper surfaces of Type C depositioncontinued theturbulent eventsbecame more oncoids often coincide with thebase of a silty mudstone common, probablyas aresult of continued shallowing parting, their growth termination is thought to be due to an taking the environment abovethe wave base of progres- increase in sedimentation rate. sively weakerstorms. At the top of the member the sea Type B oncoidshave continuous cortices. Jones & floor was close to wave base resulting in constant agitation. Wilkinson (1978) statedthat in someconditions a Whilst the peloidal packstones of M.F. 4 show evidence of continuouscortex may grow aroundan allochem with no agitation,they also contain micrite indicating thatthe rolling. However,they also considered that greatest algal agitation was gentle and implying that wave energy was low. growthoccurs in well-lit waters andthat, when present, cortices are thin on theundersides of oncoids. The change in position of the maximum thickness of successive laminae in Correlation and palaeogeography Type B oncoids indicates that different parts of each oncoid Lithological correlation of the Much Wenlock Limestone were exposed to optimum growth conditions at the sediment Formation in the West Midlands is facilitated by the lack of surface at different times. This means that the oncoids must lateralvariation (Fig. 8).The regression reflected in the have beenintermittently rolledduring their growth. The sedimentology of the formation can be correlated across the presence of both whole fossils and comminuted debris in the areaand related to the overall middle tolate Wenlock skeletal wackestones of M.F. 6 suggests that periods of quiet regressive event. The sedimentology of eachmember is deposition were occasionally interrupted by periods ‘of almost the same inevery section, and even the relatively turbulence. Thisinterpretation also explains the co- small variations reflected by oncoids in the Lower Quarried existence of Type B and Type C oncoids within M.F. 6. As Limestone Member can be traced laterally for 10 km. This deposition proceededType B oncoidsbecame more allows more detailed lithological correlation than has been abundant,due to the increasingfrequency of turbulent previously possible (Fig. 9). The lack of lithological events. Theoccurrence within M.F. 6 of sharp-based variationacross thearea indicates that environmental brachiopod-rich beds of M.F. 5 (skeletalwackestones and changes affected the sea floor uniformly, suggesting the sea

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WALSALL RAILWAY WRENSNEST DUDLEY STATION (D 102) (R31

KEY \ \ €83 Reef \ Limestone beds \ -0 Limestonenodules - Shaky mudstone U Skeletalfragment

Q Crinoidossicle - Peloid Oncoid Crossiambation Wave ripple R Reef JUNCTION 10 OF M6 (M61

DAW END

\ \ \

Fig. 8. Lithological i correlation of the Much Wenlock Limestone Formation from Dudleyto Walsall. The solid lineto the right of the lithological columns represents the percentage of carbonate nodules and beds as opposed to silty mudstone.For locations see Fig. 1.

bed was essentially planar.Supporting this is the relative Shropshire; only a generalized representation of the West uniformity in thickness of the formation in the West Midlands was shown. The data presented here allow a more Midlands, which also suggests that the area was tectonically detailedpalaeogeographic reconstruction for the West stable. Hence,the WestMidlands was part of astable Midlands atthat time.When Lower Quarried Limestone carbonate shelf, whose known extent covers anarea of Member deposition was initiated, the West Midlands lay in 8 X 10 km. Its extent outside this area is uncertain, but as a shallow marine environment below wave base. The planar the Much WenlockLimestone Formation is diachronous, sea bed was muddy and alga-rich. The only break in the becoming younger to the W (Bassett 1974), the outer shelf monotonous sea floor would have been upright, in situ coral was dominated by fine grained siliciclastics during deposition colonies. By the time the sediment was being affected by of at least the lower parts of the formation in the West storm waves the sea floor still lacked significant topography, Midlands. When carbonate sedimentation did begin in the although reefs had developed. Although estimation on the Welsh Borders it was marked by rapid lateral variations in relief of the reefs is outside the scope of this paper, both sedimentary facies and formation thickness, indicating correlation of the oncoids above and below the reefs, with that this area differed from the stable mid-shelf environment oncoids in the surrounding sediments, reveals that 4-5 m of which existed in the West Midlands. unbeddedlimestone corresponds to1-2m of bedded A previous attemptto re-create the palaeogeography limestone. This, combined with the local 3-4 m increase in duringdeposition of the Much Wenlock Limestone thickness of the member where reefs occur implies that the Formation (Scoffin1971) concentrated on Wenlock Edge, relief was in the order of 2-4 m. This figure is probably an

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References Walsall Railway StationJunction 10 Dudley BASSETT,M. G. 1974. Review of the stratigraphy of the Wenlock Series in the (R 2) (R31 J, Welsh Borderland and South Wales. Palaeontology, 17, 145-77. ,,iOf M6 L iD104 BATES, R.L. & JACKSON,J. A. 1980. Glossary of Geology. American Geological Institute, Falls Church, Va. BERNARD,F. & LECAL,J. 1960. Plancton Unicellulaire rtcolte dans I'Odan Indienpar le Chacot (1950) etle Norse1 (1955-60). Bulletin Institute Oceanographic, Monaco, 1166, 1-59. BUTLER,A. J. 1939. The stratigraphy of the Wenlock Limestone of Dudley. Quarterfy Journaf of the Geological Sociery of London, 95, 37-74. DAHANAYAKE,K. 1977. Classification of oncoidsfrom Upper Jurassic carbonates of the French Jura. Sedimentary Geology, U,337-53. - 1978. Sequentialposition and environmental significance of different types of oncoid. Sedimentary Geology, 20, 301-16. I DORNING,K. J. 1983. Palynology andstratigraphy of theMuch Wenlock Limestone Formation Dudley, Central England. 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Refined lithological correlation of the Lower Quarried MONTY,C. L. 1981. Spongiostromate vs. porostromatestromatolites and Limestone made available by studying changes in oncoid oncolites. In: MONTY, C. (ed.)L. Phanerozoic Stromatolites. Springer-Verlag, Berlin, 273-275. morphology. For locations see Fig. 1. PERYT,T. M. 1977. Environmental significance of Foraminiferal oncolites. In: FLUGEL,E. (ed.) Fossil Algae. Springer-Verlag. 61-5. -1981 Phanerozoic oncoids-an overview. Facies, 4, 197-214. - 1983. Oncoids: a comment to recent developments. In: PERYT,T. M. (ed.) Coated Grains. Springer-Verlag, Berlin, 273-5. overestimate dueto compaction of the argillaceous PETTIJOHN,F. J. 1975. Sedimentary Rocks. Harper & Row, New York. component of the non-reef facies. During the period that POWELL, J. 1980. Palaeocology and taxonomy of some Wenlock tabulate corab the sea bed was being sporadically affected by storm waves and stromatoporoids. D. Phil. thesis, Univ. Newcastle upon Tyne. the sediments would generallyhave been alga-rich RIDING,R. 1975. Giruanella and other algae as depth indicators. Lethaia, 8, containing relatively few other organisms. Succeeding a 173-9. - 1984. Sea-level changes and the evolution of benthic marine calcareous stormevent, however, the sea floor was colonized by algae during the Palaeozoic. Journal of the Geological Scoiety, London, brachiopods and the reefs by stromatoporoids. Such periods 141, 547-53. are, respectively, represented by the skeletal packstones and RomPLETL, A. 1890. Uber Sphaerocodium Bournemanni, eineneue fossile wackestones of M.F. 5, andthe thinlayers of mudstone kalkalgeausdem Raibler Schichlen desOstalpen. Botanisches Centralblatt, 52, 9. overlain by colonies of in situ Labechia in the reefs. By the - 1913. Uber die Kalkalgen, Spongiostromenund einige andere fossilien time the upper parts of the member were being deposited aus dem obersilur Gottlands. Sueriges geologiska undersokning, 10. the reefs hadbeen covered andthe sea floor was again SCOFFIN,T. P. 1971. The conditions of growth of theWenlock reefs of essentially planar, and with water perhaps only a few metres Shropshire. Sedimentology, 17, 173-219. deep over the whole of the West Midlands area, any strong SWINCHATT,J. P. 1969. Algal borings: a possible depth indicator in carbonate rocks. Bulletin of the Geological Society of America, 80, 1391-6. wave action would have been rapidly expended, leaving only WETHERED, E. 1893. On the microscopic structure of the Wenlock agentle rolling effect todisturb sediment close to wave Limestone. Quarterly Journal of the Geological Society of London, 49, base. 236-48. WOOD, A. 1948. "Sphaerocodium", a misinterpreted fossil from the Wenlock Limestone. Proceedings of theGeo1ogist.s' Association,London, 59, 9-22. I thank Aston University for supplying the grant for this work, and WRAY,J. L. 1977. Calcareous Algae. Deuelopments in Palaeontology and A. T. Thomasand P. Turnerfor helpful suggestionsduring the Stratigraphy. 4. Elsevier, Amsterdam. preparation of the manuscript. I am grateful to two referees who WRIGHT,V. P. 1983. Morphogenesis of oncoids in the Lower Carboniferous Llanelly Formation of South Wales. In: PERYT,T. M.(ed.) Coated made considerable improvements to the text. Grains. Springer-Verlag, Berlin.

Received 6 November 1986; revised typescript accepted 3 April 1987.

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