Palynofacies in a Late Silurian Regressive Sequence in the Welsh Borderland and Wales

Journal of the Geological Society, London, Vol. 147, 1990, pp. 675-686, 10 figs. Printed in Northern Ireland

Palynofacies in a Late Silurian regressive sequence in the Welsh Borderland and Wales

J. B. RICHARDSON’ & S. M. RASUL2 Department of Palaeontology, The Natural History Museum, London SW75BD, UK 2Paleo Services, Unit 15, Paramount Industrial Estate, Sandown Road, Watjord WD24XA, UK

Abstract: TheUpper Whitcliffe and lower part of theDownton Castle Sandstone Formations (Ludlow and Pridoli Series) have been sampled from sequences distributed over an area roughly 4800 square km in west-central England and Wales. Ludfordian age rocks have been interpreted as the deposits of shelf and basin environments, and those of the lower Downton Group as near-shore to beach. Sections sampled from the ‘shelf area are in the Ludlow type area, and in the Usk, May Hill and Woolhope inliers. ‘Basin’ sediments were sampledat Knighton, the Long Mountain, Capel Horeb and Kington. The samples were analysed for their palynological content, and all the organic-walled microfossils were studied together with the fragmentary remainsof larger plants and animals. Using a series of ratiosbetween different groups of microfossils,it is apparent that there are consistent changes through time in both the palynomorph assemblages and in the structured and unstructured organicmaterial. Dominance of somemicroplanktic groups/taxa usedis to definenewa microphytoplankticphase concept and to compare the sequence of palynoevents.Depositional environmentsinferred from palynofacies interpretation of theLudlow type area are closely comparable with those proposed previously from sedimentological data. Palynofacies sequences differ for theshelf and basin areas but show similar trends. We have also recognized a dramatic shift in palynofacies at a level within the Downton Castle Sandstone Formation and its correlatives in both shelf and basin areas; this change is tentatively interpreted as a ‘storm’ event.

One hundredand sixty foursamples from the Upper gramme within about 4 km from the shore. Muller’s maps Whitcliffe andlower Downton Castle Sandstone Forma- also show that river mouths are a major influence and may tions, and their equivalents in shelf and basin sequences in bedetectable in ancient sediments by the abundance and the Welsh Borderland area of England and in Wales, have nature of plant fragments preserved there. Coastal and sea been processed forpalynomorphs. At least 200 organic- floor physiography, ocean currents and areas of upwelling walled microfossils werecounted from each sample. also influence phytoplanktondistribution, together with Microfossils are usually abundantand preservation is temperature, salinity, depth of water, and latitude. excellent in many of the Welsh Borderland Silurian inliers, butin the ‘basin’ thermalmaturation is high, and counts have not been made of the assemblages from one locality Methods (Capel Horeb) thisin region. Two basictypes of Samples were processed using the standard palynological technique palynomorph occur: land-derived sporomorphs (miospores of digestion first in hydrochloric, then hydrofluoric acid. Residues andcryptospores), and marine organic-walled microfossils were washed and mounted on glass slides for optical study. In the (acritarchs, prasinophycean ‘cysts’, chitinozoans and scolec- early stages of the work residues were mounted in glycerine jellyor odonts). Acritarchs and prasinophycean ‘cysts’ are regarded Cellosizebut neither of thesemedia is suitable for fluorescence as phytoplankton (unicellular algae), scolecodonts represent studies and most of the samples were thus remounted in Elvacite or the jaws of worms,and chitinozoans are of uncertain Elvacite and Cellosize. After initial tests comparing counts of 500, affinities. Some of the acritarchs have been reworked from 250,and 200 specimens using normal light microscopy and oldersediments and are of Ordovician(commonly Nomarski differential interference contrast microscopy (NDIC), we Tremadoc)age. Atsome horizons recycled Silurian concludedthat there was no significant difference between the cryptosporeshave been detected, derived probably from higher and lower counts, and using the Nomarski system increased theproportions ofthin-walled phytoplankton detected. Fluores- Llandovery strata. cence microscopy in combination with normal light microscopy also Land-derivedpalynomorphs are distributed widely in made counting much easier and showed differences in fluorescence continental and marine sediments but generally decrease in of otherwiseidentical microfossils. These differences indicate a abundance offshore. Oceanic currents, the prevailing wind, variety of preservationalhistories and are probably due to and land and marine physiography may modify this general reworking from older, more indurated material. Subsequently 200 pattern considerably (Muller 1959). Muller’s study of Recent specimens were counted from each sample using both the Nomarski sediments of the Orinoco delta shows that currents parallel and fluorescence/normal light methods. All the microfossils were tothe shore have modified thepalynomorph distribution counted and the prasinophycean ‘cysts’ and acritarchs divided into pattern so muchthat there is anabrupt decrease in the several categories interpreted as belonging mainly to a particular abundance of spores/pollen close tothe shore in some environment(Table 1). Forthe purpose of thecounts acritarchs areas. For example, on the coast opposite eastern Trinidad weresubdivided into subgroups following Downie et al. (1963; there is a steep decline in the spore/pollen amount from acanthomorphs,herkomorphs, netromorphs, andpolygono- over 4000 grainsper gramme to less than 500 grains per morphs). In addition, we found it useful to identify separately the 675

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Table 1. Comparative palynomorph statistics (%) of samples from the Upper Whitcliffe Formation and the Ludlow Bone Bed Member of the Downton Castle Sandstone Formation and their equivalents, in shelf and basin areas. The changes indicate a major shift through time to inshore environments throughout the area, with the exception of the results for the Knighton area

Ludford Lane Downton Gorsley LonghopeMountainLong Knighton

Upr. Lwr. Lwr.Upr. Upr. Lwr. Upr.Lwr.Upr. Lwr. Upr. Lwr. Whit.Dwnt. Whit.Dwnt. Whit. Dwnt. Whit.Dwnt.Whit. Dwnt. Whit. Dwnt.

Sporomorphs 25.5 14.5 2.5 50.5 8.5 48.5 4.0 65.015.5 37.0 2.5 20.0 Prasinophycae & Acritarchs Inshore 35.07.5 38.5 45.0 19.0 37.5 35.0 20.0 12.5 25.5 18.5 16.5 Inner neriticInner 11.0 10.0 32.5 0 50.0. 5.5 7.0 1.0 5.5 6.5 12.0 11.0 Outer neriticOuter 56.522.5 35.5 4.5 17.0 3.5 43.0 0.5 73.5 29.0 50.5 50.5

genera Micrhystridium, ‘Dictyotidium’, and Visbysphaera. Many of Inshore Index: thesecategories have been shown to increase in abundanceand sphaeromorphs + tasmanites + micrhystridia varietyoffshore in neritic environments, while others such as X 100 Visbysphaera and some types of Micrhystridium are most abundant (above) + outer neritic forms in inshore shelf environments (AI-Amen 1980, 1983a, b; Dorning (netro-, acantho- and polygonomorphs) 1981). Sphaeromorphsand tasmanites closely resemble the cysts or phycomata of modern prasinophycean algae (see Tappan 1980), Palynomorphs classified for use in the above equations and andwere also counted separately. Although sphaeromorphs and usedas in the followingpostulated environmental tasmanites (prasinophyceae) occurin sediments of the open sea and occurrences are as follows: deep sea basin (where acritarchs are generally rare or absent), they Land derived: sporomorphs (cryptospores, miospores). also increase in relation to acritarchs in nearshore environments. In Inshore:Prasinophyceae (sphaeromorphs, tasmanites) certain circumstances, therefore, their abundance maybe used to dominantalong with some forms of micrhystridia indicate inshore depositional environments. (acritarchs). Inner neritic: Visbysphaera and ‘Dictyotidiurn’. Outer neritic: polygonomorphs,acanthomorphs including Counts some micrhystridia, and netromorphs. In the proximal part Two hundred or 250 organic-walled microfossils were counted from of theouter neritic area three-spined Veryhachium are twoslides of eachsample. Initially a comparisonwas made of dominant; in the distal partfour-spined Veryhachium are oxidized and unoxidized residues. The fossils were counted from the dominantand netromorphs aremore abundant and same series of samplesusing normal transmitted light, Nomarski Leoniella carminae and pterospermopsids become consistent interference, and fluorescence microscopy. It was found that more components. acritarchs and sphaeromorphs were observed using the two latter Thereappears to be agradual increase in acritarch methods. In the early stages of the work one of us counted using diversity offshore but species diversity studies on this group fluorescencemicroscopy whereas theother usedNomarski have not been made for the present work. Lister (1970, fig. interference. There was no significant difference between the counts 13) showed thatcounts of 200-250 usuallyprovided andsubsequent counts were allconducted using a Zeiss Photomicroscope with Nomarski interference contrast and fluores- underestimates of acritarchdiversity butthat at certain cence.One advantage in usingfluorescence is in distinguishing horizons, for example the Whitcliffe Formation, the values between thick-walled sphaeromorphs and alete spores. Many of the of diversity as revealed in the counts and the totalobserved indeterminate bodies may be in this category. values, approach one another. These occurrences tend to be fromhorizons where particulara genus or species is abundantand so true diversitymay be masked by the Analysis of results enormous productivity of some microplanktic taxa. Two ratios have been found to be useful for environmental interpretation: (1) a MarineInfluence Index, which is the ratiobetween land-derived spores and marine organic- Pdynofacies walledmicrofossils, used aas generalindicator of Sections have been sampled (see Fig. 1) in the shelf area: environment and in particular proximity to the shore;(2) an the Ludlow-Downtontype area,the Silurian inliers of InshoreIndex, based onthe ratio between marine Gorsley, May Hill, Woolhope (Perton district) and Usk; and palynomorphsthought to be more abundant in nearshore in the basin area: Long Mountain, Knighton, Capel Horeb environments and those forms regarded as more open seaor andKington. Countswere made of all thesesequences outer neritic. These ratios aremodifications of those used by except for Capel Horeb and Kington. Traverse (1978) for palynomorphs from Recent Sediments. Each index is determined as follows: Marine Influence Index: Microplankton phases Microphytoplankton(sphaeromorphs and acritarchs) of a acritarchs + chitinozoa + scolecodonts X 100 single group,genus or speciesmay dominatethe (above) + total sporomorphs assemblages andtherefore it is possible to characterize

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Formation (Figs 4 and 5) and acritarchs of a single group, genus or species may dominate (see also Lister 1970). In the Upper WhitcliffeFormation two types of acritarchare abundant,polygonomorphs (particularly Veryhachium but also Onondagella) and Visbysphaera; acanthomorphs (excluding Micrhystridium) andnetromorphs may be 81N importantaccessories, and Diexallophasis is arelatively t common acanthomorphic acritarch. The lower samples (Al, A2,A3A; Fig. 5) fromWeir Quarry belong tothe Downton Group and Lower Devonian polygonomorphphase; this is overlain by a Visbysphaera phase(samples A3, A4, 165 andA5) and followedby a return to apolygonomorph phase as indicated by the youngest Upper Whitcliffe Formation sample (A6). Above the Ludlow BoneBed the dominant microphytoplankton are sphaeromorphs and all the assemblages from the lower Downton Group can be referred to a sphaeromorph phase. Sphaeromorphsare important accessories in the Upper Whitcliffe Formation with maxima in samples A3A and A5 of around lO%, whereas in samples from the Ludlow Bone Bedand above their relative abundance varies between 8.5% and 38.5%. Sporesshow little variation in theUpper Whiitcliffe Formationand mainlyconsist of varieties of Archaeozonotriletes chulus, especially A. chulus var. inframurinatus. The MarineInfluence Index is high,often over 80% and the Inshore Index and spore percentage are Fig. 1. Main sampling locations in relation to the Silurian outcrop. W., Weir Quarry; LL., Ludford Lane.

WEIR QUARRY Sample no particular planktic events by their dominant microplankton - 16H type. Thedominant planktic type is used to designate assemblagesreferred to belowas phases, for example the Visbysphaera phase. These phases are found to be useful in Sandstonc comparing sequences of palynofacies events in the sections Member studied. Colbath (1980), in his studies of upper Ordovician microplankton, also showed fluctuating patterns of acritarch abundances. He identified two phytoplankton communities based on associations of species andhe recognized the Platy- potential of suchstudies for identifying fluctuations in - 16B schisma ancient water masses. - 16A Shale deepening Member Shelf area L.B.B.Mbr. Two sections were examined in the Ludlow-Downton area (seeAppendix for locality details): thatat Weir Quarry along the River Teme in the Downton area (Table 1, Figs Upper 1-5), and that at Ludford Lane, Ludlow (Table 1, Fig. 1).

Weir Quarry Whitcliffe Twenty seven samples were collected through a 7 m section from theUpper WhitcliffeFormation (upperLudfordian Dsandstone E Bone Bed Formation Stage, Ludlow Series) and lower Downton Castle Sandstone 300 Formation(Pridoli Series). Initial reconnaissance sampling msiltstone P Plants was followed by moredetailed sampling of theUpper B:;:by 2T2ated@I Platyschisma Brachiopodlshell Whitcliffe Formation,and the Ludlow Bone Bed and 400 mshale debris PlatyschismaShale Members of theDownton Castle SandstoneFormation. Distinctchanges in palynomorphs, . , InshoreIndex ._..._.... the palynofaciesindexes, andthe type and nature of the ' 9'0 8b 7'0 6'0 50 40 30 20 10 0 r..,...,.. MarineInfluence organic matter take place in the section, especially at the 0 10 20 30 40 50 60 70 80 90 'Index Ludlow Bone Bed level. Fig. 2. Palynofacies curves showing sporomorph percentageand Microphytoplankton. Organic-walled phytoplankton is Inshore Index, Upper Whitcliffe Formation and lower Downton usually abundantbut not varied in the Upper Whitcliffe Group, Weir Quarry section, near Downton.

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occasionalfragments of cutilce andbanded tubes. cm WEIR QUARRY Scolecodonts and chitinozoans are consistently present. Two Enlargementof section around categories of acritarch dominate the marine phytoplankton, Ludlow BoneBed in the lower part of the sequence sampled, polygonomorphs Sample no. are dominant but in sample A3A and above Visbysphaera - 16M - 16L becomesincreasingly importantand, apart from in the uppermostsample (A6), it exceeds thepercentage of polygonomorphs (Table 3).

Interpretation. Whitcliffe Formation. Using theAl-Ameri - All - A10 model(1980, 1983a, b) theparameters outlined above - A9 indicatethat this formationwas formed in an offshore - 16K(1! location, but in ashallow shelf sea, and probablyin an inner --+”’ :G- ...... ,...._...... - -1y6‘ ’ neritic environment.A more open shelf sea would be indicated by greater taxonomic diversity of acritarchs, with morevaried acanthomorphs, and a greater percentage of 7-netromorphs, pteromorphs and herkomorphs. The low proportion of spores,particularly the miospore Archaeozonotrifetes, and the small amount of land-derived structuredplant fragments also indicates anoffshore - A5 environment.Spores are much moreabundant in two ‘‘’‘‘‘16 - 16J samples from the UpperWhitcliffe Formation: sample A3A, - A4 acalcareous siltstone, andsample A3 fromthe overlying shale. The percentage of sporesin A3 (56.5%) is very similar tothat in the Ludlow BoneBed (50.5%) and consequently theA3 horizonmay representan earlier regressive event, but in the Ludlow Bone Bed the Inshore . InshoreIndex < 90 80 70 60 50 40 30 2010 0 ’.’.....’ . Marine lnfluenc 0 10 20 30 40 50 60 70 80 90 >

WEIR QUARRY 3. Fig. Sporomorph and Inshore Index curves through the Micrhystridium uppermost part of the Upper Whitcliffe Formation, and the Ludlow P Bone Bed Memberof the Downton Castle Sandstone Formation, showing the major regression at the presumed Ludlow-Pridoli Vlsbysphaera boundary, Weir Quarry section, near Downton.For legend of section see Fig. 2. U Polygonomorph

Acanthomorphl Netromorph generallylow. Occasionally thereare indications of rapid shallowing. At 108.5-100.5 cmbelow the LudlowBone Bed,from the top of calcareousa siltstone rich in brachiopods and bryozoans, the spore ratio rises to 31.0%, Sphaeromorph Phas andin the overlying3 cm shaleintercalation it reaches 56.5%.Samples above and below this level have much lower sporomorph percentages (Table 2). Micrhystridia have been used in this study as indicators of nearshoreenvironments but there is evidence of a bimodal distribution (Fig. 4). Thus, though many of them lived inshore,there were some that had a similar L.B.E-- distribution to the main forms of acanthomorphic acritarchs and livedin moreoffshore locations. Dorning (1981) 2 /iy Visbysphaer a Phas consideredthat offshore micrhystridia have longer spines than inshore specimens but did not quantify the differences. In general, the abundance of long spined forms of acritarch and dinoflagellatecysts increases offshorein neritic environments. In deeper waters acritarchs and dinoflagellate cysts are usually absent and Prasinophyceae are frequently 10 3020 40

abundant. Acritarch % Analysis of samples from the Upper Whitcliffe Forma- tion shows that there is a major oscillation in the Marine Fig. 4. Variation in acritarch groups across the presumed Index and spore abundance but organic matteris still domin- Ludlow-Pridoli boundary, Weir Quarry section, near Downton. ated by amorphous,pale-brown to yellowkerogen with Samples levels are not to scale.

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from sample A3 have a low Inshore Index (44.4%) but it is WEIR QUARRY the highest found in the Upper Whitcliffe Formation in this section (Fig. 2). Shales in the sequence tend to have more sporesthan the calcareous silts but this lowestshale intercalation(sample A3) hasan exceptionally high sporomorph percentage compared with other samples from the Upper WhitcliffeFormation. Shales indicate increased silt and clay, usually from the land, and one would expect them to have an increase in land-derived palynomorphs but three samples (A4, 165 & A5) examined from a shale only 9cm thick higher in the Upper Whitcliffe Formation have sporomorph percentages of 17%, 18.5% and, in the upper 6cm, only 3.5%. Thusthe lowershale event may reflect local uplift causing regresion and erosion to produce a flood of finer material from the land, and possibly represents the distal part of a turbid flow, whereas the higher shale was deposited farther offshore. The highest part of theUpper WhitcliffeFormation (sample A6) palynofacieshas a belonging theto

Table 2. The relative proportion of sporomorphs, and the Inshore and Marine Influence Indexes, Weir Quarry

Sample Marine Sample I. numberSporesInshoreLevelIndexIndex

Sandstone Mmbr 16H 87.5% 92.0%87.5% 16H 12.5% +446.5-455.5 cm 0 85.5 72.5% 16G 27.5 r +249.5-256.5 cm 16F 84.5 80.6 84.5 16F 15.5 r +224-226.5 cm

Fig. 5. Variation in palynomorph abundances(%), Weir Quarry Platyschisma Shale Mmbr section, near Downton. Sample levels are notto scale. 91.6 Sporo., 86.5 16D 123 + 182-187.0 cm sporomorph; Sph./Lo./Mi., sphaeromorphs 86.9 51.5 16C 49.7t +M-182.0 cm Lophosphaeridium/Micrhystridium; VisJDi., 96.0 62.0 16B 38.0 +127-133.0cm Visbysphaera/'Dictyotidium'; Poly., polygonomorphs; Ac./Net.,57.5 16A 80.0 42.5 +89-97.0 cm acanthomorphs/netromorphs;Hg./Hk./C./Sc., A1396.0 56.5 40.5 +48-53.0 cm hoegispheres/herkomorphs/chitinozoans/scolecodonts. A12A 60.0 85.5 37.5 +34-36.0 cm A12B71.0 55.0 42.4 +31-34.0 cm A1282.1 63.5 34.2 +26-31.0 cm 16M 46.0 83.7 46.0 16M 54.0 r +24-25.0 cm Index is high, 90.9%, whereas in A3 it is only 44.4%.88.4 This,57.0 16L 43.0 * r +21-24.0 cm together with the fact that the sediments and the organic matterat the two horizons are totally dissimilar,may Ludlow Bone Bed Mmbr indicatethat they represent two different phenomena, A1 1 79.4 59.0 37.2 +9-13.0 cm namely a regressive-transgressive event at the Ludlow Bone A10 64.5 80.7 33.0% +8-9.0 cm Bed and a storm event represented by sample A3 from the A9 66.0 85.2 31.65 +7-8.0 cm Upper Whitcliffe Formation.On the other hand, the 53.0 A8 80.0 42.1 +2-3.0 cm increase in spores in the Upper Whitcliffe event, although 16K1 81.5 67.6 18.5*r +0-2.0 cm relatively abrupt(from 31.0% to 56.5%), crosses a 16K 50.5 90.9 49.5*r +0-3.0 cm lithological boundary. The samples involved (A3A and A3) are from the top 3 cm of a massive calcareous siltstone with Whitcliffe Fm brachiopods and the immediately overlying11.7 thin (5 cm)2.5 silty A6 97.5 -0-5.0 cm shale intercalation. Since there is an increase in sporomorph 32.5A5 3.5 96.5 -19-25.5 cm 81.4 -23-26.0 cm percentagecrossing a bedding plane between two distinct 16515.9 18.5 A4 17.0 16.1 17.0 A4 82.6 -23.5-28.0 cm lithologies, the likely explanation is thatit is caused by A3 56.5' 44.4 56.5' A3 37.6 -101.5-106.5 cm regression rather than by a storm event. It would be42.5 a more 31.0' A3A 68.0 -106.5-109.5 cm distal effect of regression than that seenat 27.6the horizon 9.5 of the A2 90.4 -187.0-193.0cm Ludlow Bone Bed where there is actual disconformity.32.0 17.5Thus A1 82.0 -209.0-213.0 cm wemay beseeing the effects of alocal regression some distanceoffshore, beyond thecoastal concentrations of * Possible penecontemporaneous reworking. land-derived banded tube fragments or abundance of large t Possible storm event. pieces of cuticle concentrated near the mouths of rivers and Probable regression event. theirdistributary complexes. Furthermore, assemblages r, Reworking.

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Table 3. Percentages of the principal acritarch subgroupslgenera sedimentinterface, (from 51.5% to 86.5%),thereafter and sporomorphs, andthe occurrence of scolecodonts (sc.) and remaininghigh tothe top of thesection (87.5%). The chitinozoans (ch.), Weir Quarry InshoreIndex for the Downton Group is between67.6% and 96%, although only 3 of the 19 samples are below 80%; Sample Polygono- Sphaero- Visby- Micrhys- Sporo- this contrasts with much lower values, usually below 33%, number morphs morphs sphaera tridia morphs for the underlying Upper Whitcliffe Formation. Sporomorph Sandrtone Mmbr percentagesrange between 50.5% and 87.5%whereas 16H - 11.5 - - 87.5 - - percentages of polygonomorph and acanthomorph acritarchs 1% 0.5 20.5 - 3.0 72.5 - - are always in single figures. Micrhystridia are the only 16F 1.5 12.5 - - 84.5 - - relatively common acritarchs in the Ludlow Bone Bed and PlatyschismaShale Members andeven their occurrence Platyschisma Shale Mmbr becomes erratic in the Sandstone Member. Prasinophycean 16D 0.5 10.5 - 0.5 86.5 - - 16C 3.5 33.0 1.0 9.0 51.5 - - ‘cysts’ are the only common marine palynomorphs, and in 16B - 34.0 - 2.5 62.0 - - the samples studied from this formation they range between 16A 5.0 28.0 - 6.0 57.5 - - 8.5% and 38.5% of the total palynomorph flora. A further A13 1.0 22.5 1.0 13.5 56.5 - - feature of the lowerDownton Group is thefrequent - - A12A15.0 1.5 6.0 8.5 60.0 presence,concentrated at certain horizons, of reworked 5.5 13.5 55.0 - - A12B 6.0 11.0 acritarchs andsporomorphs, which are of Tremadoc, A12 3.5 14.0 3.0 9.0 63.5 - - 16M 3.5 30.5 2.0 13.0 46.0 - - undifferentiated Ordovician, or lower Silurian age. 16L 1.o 31.0 - 7.0 57.0 - - LudlowBone Bed Member. The lowermostsample (16K) of the Ludlow Bone Bed Member lies directly on the Ludlow Bone Bed Mmbr uppermostsample (A6) recovered from the Upper All 3 .O 12.5 3.0 12.5 59.0 - - Whitcliffe Formation (Table 2). Both the Marine Influence A10 11.5 2.5 3.5 11.0 64.5 - - Indexand theInshore Index change markedlybetween A9 2.0 10.0 2.5 13.0 66.0 - - A8 -2.5 19.5 8.5 53.0 - - thesetwo samples, reflecting profound changes in the 16K1 2.5 8.5 - 4.0 81.5 - - depositionalenvironment. Reworking occurs commonly in 16K 3.0 38.6 - 6.5 50.1 - - the Ludlow Bone Bed and the Marine Influence Index for this member may havebeen inflated by undetected Whitclife Fm reworking of acritarchs from older Silurian strata. The fine A6 5.5 32.5 2.0 2.5 - - 50.0 siltstone(sample 16K(1)) which drapes over the ripple- A5 34.0 10.5 39.0 8.0 3.5 - - 165 16.0 3.5 59.0 - 18.5 X. - bedded Bone Bed (sample 16K) shows a marked increaseto A4 32.0 3.0 37.0 4.0 17.0 - - that of the Bone Bed in the percentage of sporomorphs it A3 6.5 7.5 14.0 0.5 56.5 - ch. contains,and its Marine Influence Index is verylow, but A3A 28.0 9.5 9.5 14.5 31.0 sc. - both indexes increase in the overlying samples. - 17.5 9.5 - ch. A2 53.0 7.5 Platyschisma Shale Member. Analysis of samplesfrom - 19.0 17.5 sc. - A1 51.0 6.5 this member shows a variable but strong marine influencein the lower metre of sediment, with a dramatic decreasein the Marine Influence Index from 49.7% to 12.2% between two polygonomorph phase and using our model indicates a shift adjacentsamples higher in the section(16C, 16D), which offshore,although theacritarch assemblages arenot as showa marked increase in sporomorphs from 51.5% to variedas thosefrom the lower part of thesection. 86.5%. The Inshore index fluctuates but has a high average Alternativelythe increase in polygonomorphs mayhave of 85.6%. Reworked acritarchs are relatively common in the been caused by a bloom of Veryhachium in more inshore lower 36cmbut penecontemporaneous reworking is not waters. Land influence is minimal with spores less than 3%, thought to be the major influence in the shift seen between and the Inshore Index, at 11.7%, is the lowest amongst our samples16C and16D because Visbysphaera wasnot samplesfrom theUpper Whitcliffe Formation.Variation observed in the counts, even though it is abundant in the in theUpper Whitcliffeassemblages indicates a potential underlying samples from the Upper Whitcliffe Formation. area for future research, as modifications of the Al-Ameri SandstoneMember. Onlythree samples have been modelusing aphytoplanktic taxa diversity index would analysed from the Sandstone Member and these show high probablyhelp to distinguishseveral types of offshore spore percentages, a high Inshore Index and a low Marine environments.Such modela would have totake into Influence Index. Prasinophyceae dominate the marine forms accountthe ‘swamping’ effect caused by phytoplanktic andacritarchs are absentfrom the counts of the highest blooms in inner neritic environments, but it could be done sample.There are much greateramounts of cuticle in by makingtwo counts, thesecond ignoring the dominant samples from this member than in samples from all other forms. lithostratigraphic units in the section; reworked Ordovician DowntonCastle Sandstone Formation. Palynomorphs acritarchs have also been detected. andthe nature of theorganic fragments in palynological residues from the Downton Castle Sandstone samples show The Platyschismaevent. A major shift in theMarine adifferent series of quantitativeand qualitative changes InfluenceIndex (sporomorph abundance) occurs between from those seen in the samples from the Upper Whitcliffe the contiguous samples 16C and 16D across a sandstone-silt Formation. The spore percentage (Table 2) rises from 2.5% interface. The index reaches its second maximum in sample to 50.5% across the Upper Whitcliffe Formation/Downton 16 C and is at its minimum in 16D. Throughout the lower Group boundary, increases to over 80% and then declines Downton Group spores, both miospores and cryptospores, beforeincreasing rapidly across a sandstone/siltstone are diverse.Prasinophycean ‘cysts’ arethe commonest

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phytoplankton.Acritarchs are generally rare or absent Orinoco, but in contrast to the tracheids, marginal marine except for Micrhystridium; Visbysphaera is rarely found and assemblagesdominated by fragments of large cuticle polygonomorphs andacanthomorphs are persistently indicate proximity to distributary channel complexes (Muller present.Reworked Tremadoc andother Ordovician 1959, fig.20) andprobably had similar environmental acritarchs occur and how much of the acritarch assemblage distribution patterns in the Palaeozoic. We think that this representsundetectable late Silurian andmore or less observationnecessitates a change in theAllen & Tarlo penecontemporaneousreworking is difficult todetermine. (1963) interpretation of thedepositional environment for However,the nature of theacanthomorph/netromorph the Sandstone Member of the Downton Castle Sandstone record (Fig. 5) perhapsindicates, by itserratic nature, Formationand for the Temeside Shale Formation, to turbulentinshore environments where penecontemporan- emphasize the proximity of rivers, or distributary channel eousreworking was common. The Platyschisma event is complexes, in the Downton area at this time (Table 4). regardedas a storm event simulating an offshore shift by bringing in material from the open sea. The real shift was Lane inshore, as the underlying three samples indicate that more Ludford marine conditions prevailed (Fig. 2) whereas higher samples Acomparison between the results from Weir Quarry and generally show increasing land influence. thosefrom the section atLudford Lane (Table 1) shows broad similarities but also some interesting differences. The Ludford Lane counts show that thereis a marked increasein Dispersed organic matter. The palynologicalresidues (Fig. thepercentage of land-derivedsporomorphs in the basal 9) show a major change at the Ludlow Bone Bed Member part of the Downton Group, but this increaseis much lower compared with the underlying Upper Whitcliffe Formation. than in the equivalent samples from Weir Quarry c. 5.5 km There is a large increase in humic kerogen. The material is tothe west in theDownton area. Atthe same time the largely black and dark brown but also includes grey-brown, samples from the Upper Whitcliffe Formation and the lower brown and yellow-brownmaterial. It consists of irregular part of the Downton Castle Sandstone Formation at Weir or lath-shaped unstructured fragments and a large number Quarryhad a lesser proportion of acritarchsubgroups of unstructuredfilaments of variousdiameters, the small regarded primarily as outer neritic forms. So the differences ones sometimes brown; also minor amounts of amorphous in the results from the lowermost Downton Group samples kerogenand some structured brown kerogen; small between the Ludlow and the Downton areas may be related fragments of dark-brown,thinunstructured cuticle tothe pattern of distributarychannels delivering high (probably animal); and structured animal and plant cuticle concentrations of land-derivedsporomorphs in non-a andbanded tubes. The shaleimmediately overlying the uniformfashion along an irregularly prograding shoreline. Bone Bed has a similar range of material but there is more In contrast, the uppermost Whitcliffe Formation at Ludford lath-shapedmaterial and larger fragments of unstructured Lane has yieldedassemblages which indicate greater land cuticle (probablyanimal); structured animal and plant influence anda higher proportion of inshoremicrofossils cuticle and largefragments of bandedtubes; branched, than those from Weir Quarry. Winnowing may also account unstructured tubes and more complex aggregations of tubes forsome of the differences but many of theinshore and filamentsand including fragments of Tortotubus acritarchs andprasinophytes are as small as and more (BM/NERC/16K1/4 043 0976). Some polygonomorphs have delicatethan thespores, so theoreticallythey should be pyrite growths, perhaps indicating that they are derived, but preferentiallyremoved and themore robust spores this feature was not noted in the counts. In the silty, lower concentrated.There are, in fact, signs of turbulentand part of the Downton Castle Sandstone Formation (samples erosiveenvironments aslarge, thick-walled tasmanitid 16L, 16M and A12) large, black, lath-shaped kerogen and prasinophytes and reworked, probably Llandovery, crypto- filaments aredominant, and occur withsmall, brown spores(dyads and‘permanent’ tetrads) occur in the basal filaments, small cuticular pieces, and fragments (sometimes Downton group and are common at this horizon at Ludford large) of bandedtubes. Mats of fine filaments are also Lane. A further possibility is that the abrupt changes across present(e.g. in sample A12). In the higher parts of the the Upper Whitcliffe Formation/Downton Group boundary sequence, the Sandstone Member, fragmentsof large cuticle at Weir Quarryare caused by thepresence of a become increasingly common. disconformity, with part of the Upper Whitcliffe Formation beingmissing, whereas the palynofaciespercentages at Interpretation. Byanalogy with datafrom the Orinoco LudfordLane aremore transitional and indicate a more Delta (Muller 1959, fig. 21), the assemblages dominated by complete sequence. lath-shaped fragments and unbanded and banded tubes (in the Ludlow BoneBed and Platyschisma ShaleMembers) Additional sections on the shelf area wouldcorrespond tothe sediments ’ containingabundant tracheidsdeposited in inshore waters. Although banded Analysis of samplescollected across the(presumed) tubesare not tracheids, they are resistant, probably Ludlow/Pridoli boundary from sections in the Gorsley, May hydrodynamically similar and probably would be sorted in Hill, andUsk inliers shows thatthey all havea similar similarways to modern woodymaterial. Tracheids are pattern of palynofacies changes to those sections studied in frequent (up to622 fragments per gramme) in a moreor less the Ludlow andDownton areas. In horizons correlated evenly distributed area along the delta front and adjacent with theUpper WhitcliffeFormation aninner shelf coast,and concentrations, of upto 423fragments per environment is indicated.Inshore elements such as gramme, were found up to 25 miles offshore. Beyond this Visbysphaera and Micrhystridium arefrequently abundant distance they were largely absent. Large fragmentsof cuticle and spores are low. At May Hill and Gorsley ‘Dictyotidium’ wereagain abundantup to 25 milesoffshore from the and Visbysphaera are the most abundant forms in the Upper

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Table 4. Organicmatter (based on the present study) and Whitcliffe. ‘Dictyotidium’ (included by someauthors in environment of deposition (after AIlen & Tarlo 1963) of the Upper Prasinophyceae Tappan, 1980) comprises 24% of one of the Whitcliffe Formation and the lower Downton Group Gorsely assemblages (G4 sample), and twice this amount in asample from May Hill where Visbysphaera is alsohigh m atter EnvironmentOrganic matter (39%), but it is notably rare or absent in the other sections Temeside ShaleFormation Subtidal & Intertidal:Sands of examined.palaeoenvironmental our In model Verylarge cuticle fragments. subtidal shoals and lower ‘Dictyotidium’ occurs most abundantly in the inner neritic intertidal flats.Silts of upper area along with Visbysphaera. The assemblages in all these intertidal flats. Strong waves upperLudfordian (Upper Whitcliffecorrelative) sections and currents. Brackishand indicate inner neritic environments. Monospecific blooms of turbid water. prasinophytes are known in modem coastal waters enriched Downton Castle Sandstone Near-shore to beach: Strong in nitrogenand phosphates. Perhaps similar blooms Formation waves, turbid, often brackish. occurred in the past and the high numbers of Visbysphaera Sandstone Member Transgressive sand shoals and and ‘Dictyotidium’ indicateshallow marine waters where Increase in large fragmentsof beaches. plant cuticle; lath-shaped high turbulence provided increased mineral nutriments. (some long) and irregular vitrinite fragments; some banded tubes; brown and Basin area black unstructured ‘filaments’ sometimes in mats; sporangial masses. Long Mountain Platyschimsa Shale Member Silts formed offshorewithin The upperLudlow-lower Pridoli sequence of the Long Lath-shaped vitrinite fragments, wavebase. Mountain (Figs 6 & 7) was sampled at Wallop Hall Quarry and banded, ‘scrolled’ & (seeAppendix for locality details). The sequence of smooth tubes become dominant; many large palynofacies variations shows a similar pattern in both the irregularly-shaped fragments Long Mountain section and in that at Weir Quarry from the of humic kerogen especially in Downton area, but in the former the microphytoplankton is the lower partof the member; morevaried, and the netromorphs are much more mats of fine tubes or abundant. The sequence of theevents in ‘filaments’; plant cuticle more spore/microplanktonand Inshore Index curves are ne- abundant in some samples from the upper partof the vertheless basically the same in both areas. The lowest Long member (16A & 16C); Mountainsample (WH3A) contains relativelya high sporangial masses. Large proportion of spores (28%), comparable tothe number fragments of animal cuticle, obtained from sample A3 in Weir Quarry and indicating a some perforate. similar influx of small-sizedterrigeneous material. This is Ludlow Bone Bed Member Near-shore to offshore: followed firstly by a movement to more open-sea conditions Black vitrinite and brown humic laminated silts and Somethin and the proportion of spores drops to 1% (compared with material; lath-shaped sands. 2.5% in the Downton area), then by a sudden movement to fragments and smooth and inshoreenvironments between the upper Ludlow and the banded tubes common, tubes variable in size, up to400 pm lower Pridoli, andthen by agradual movement offshore. long; narrow,3-5 pm wide, Thereafterboth the Weir Quarry and LongMountain yellow to brown, smooth graphs (Figs 2, 3, 6 and 7) show a similar sharp movement tubes; black and brown inshore, succeeded by a gradual increase in the percentage irregular fragments; sheets, of spores (from 63% to 87%) indicating increasing proximity some perforate, mainly black, to the coastline. some brown; small fragments of plant cuticle; thin brown (probable animal) cuticle. Phytomicroplankton. Polygonomorphsrecovered from the Ludlow Bone Bed Advance of strand after retreat: upper Ludfordian (basin) sequence of Wallop Hall Quarry Irregular organic fragments beach and littoral sands and (humic material) abundant; silts. include some four-spined species of Veryhachium where the banded tubes and simple and fourth spine is more or less at right angles to the plane of branched filaments. the other three. Such four-suined forms are considered, in boththe Al-Amen (1980, i983a, b) andDoming (1981) Upper Whitclitre Formation models, to havelived farther offshorethan three-spined (upper part of) Amorphouskerogen dominant, Chiefly open sea: clays and forms. Boththe three- and four-spined Veryhachium forms rare smallfragments of plant/limestones formed chiefly continue into the basal Downton Group and after a small animalcuticle and banded within wavebase. Clear sea, initial decreasethey show a slight increaseindicating a tubes. often turbid, sometimesturbid, often tubes. decreasesteadilyforms both then deepeningphase; brackish.Periodic strand throughoutthe rest of section.the Although the retreat and advance. phytoplankton is dramaticallyreduced in the WallopHall section it continuesin more variety and greater numbers into the lower Downton Group than in sections examined from shelf areas. At Wallop Hall sphaeromorphs are only slightly dominant in the basal Downton Group sample (WH

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the polygonomorphphase (Fig. 8) with Visbysphaera and sphaeromorphsboth having maxima of 12%.Other LONG MOUNTAIN planktonphases have not been detected but there is a gradual decline in netromorphs which, however, are more Sporomorphs- common here than in the other sections sampled and reach Inshore Index ._...... 10.5% in the lowestsample (KIA). Curiously, the Veryhachium included sporomorphpercentage is approximatelythe same as in theLudford Lane section (Upper Whitcliffe andbasal Downton strata), but as the plankton indicate outer neritic environmentspresumably the source of thesporomorphs may havebeen a local island or islands, surrounded by deeper water, which may have been located along theline of theChurch Stretton fault complex. Fewer samples have been examined from Knighton than from Weir Quarry but a Sample no. broadly similar trend in palynofacies occurs (Fig. 8), which - LMl1 indicatesa similar series of environmental changes in the - LMlO two areas though Knighton is in a more offshore position. - LM9 These changes are, in ascending order: (1) open sea, outer - WH3J neritic, (2) an increase in temgenous material and a slightly - WH3H more inshore position, (3) a deepening to the outer part of theinner neritic area, (4) regression andan increase in sporomorphs. The overall pattern also follows that seen in p,... theLong Mountain and is broadly one of regression- ,..A - WH3F.G deepening-regression. If the ‘storm’ eventthought to be ------present in bothWeir and Wallop Hall quarries is also -WHBD present at Knighton then it should be detectable in the1.2 m -WH3C of strata between samples K1C and K1D. The presence of reworkedTremadoc acritarchs and ‘permanent’ tetrads of .....’ - WH3A g- -90 80 70 60 50 40 30 20 10 0 -WH4B Fig. 6. Palynofacies curves showing sporomorph percentageand LONGMOUNTAIN Inshore Index, upper Ludlowand probable lower Pridoli strata, Wallop Hall Quarry section, LongMountain. For legend of section %Sporomorphs - see Fig. 2. InshoreIndex...... Veryhachium excluded

3F, G) butthe sphaeromorph phase becomes established abovesample LMlO for the rest of thesection. Between these two samples polygonomorphs are dominant. Interms of thenature and composition of itsupper Ludfordian-Lower Pridoli palynofacies, the Long Mountain is transitional between those of the shelf sequences and that of the Knighton sequence,where in the latter area the Sample no. polygonomorphphase occurs throughout the section from - LM11 theuppermost Llan-wen Beds,through the localLudlow - LMlO BoneBed correlative into the Platyschisma Beds. The - LM9 phasesidentified in the LongMountain section vary -WH3J -WH3H sequentially in a similar way to those on the shelf In the upperLudfordian there are Visbysphaera and polygono- morphphases, and in the lowerPridoli there is a sphaeromorph-polygonomorph phase(where the percentages of the two groups are almost equal at 23% and 21% -WH3F.G respectively),followed finally by a sphaeromorphphase -WH3D though this latter phase includes more polygonomorphs than -WH3C the upper parts of the Platyschisma Shale Member and the -WH3B Sandstone Member, Downton Castle Sandstone Formation, of Weir Quarry. -WH3A !U I(. . , , , , . . , 4 90 80 70 60 50 40 30 20 10 0 Knighton i Our analyses for the Knighton sequence (see Appendix for Fie. 7. The same profile as in Fig. 6 but with Veryhachium excluded locality details) indicate that the whole section belongs to from the calculations. For legendof section see Fig.2.

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~ cm possible Llandoveryage in the highest Knighton sample

l DE- (KlD) is a feature also seenin the lower Downton Groupin mostsections. This reworking may bea result of high KNIGHTON turbulence in inshore environments or of the proximity of - river distributary channels. 300 \ Polygonomorph Capel Horeb Quarry, Powys - 250 Beds In thissection (Ordnance Survey mapreference IC- P ?i SN84443238) Ludlowage strata(Roman Camp Beds; probablylower Whitcliffe correlatives)are unconformably 200 - overlain by rocks (the Long Quarry Formation) equivalent in age, at least in part, to the lower part of the Downton Group. Thus thissequence, with its unconformity, is not 150 - comparable to thoseseen in the other areas studied. 1 B- However, like otherbasin areas the microphytoplankton are -- morediverse in the Pridoli-age rocksthan those seen in I A- either the shelf sequences or in the Long Mountain 'basinal' 100 - , deposits,butsporomorphs increase upwards in the succession throughthe Long Quarry Formation as in the DowntonCastle Sandstone Formation inother areas. Organic maturation in the Capel Horeb samples is high and no detailed counts have been attempted.

Palynomorph phases Most of thesequences studied show a similar pattern of phases and may represent events on a basinalscale, at least. <'"""." Inshore...... Index 100 90 80 7060 50 40 30 20 10 0 In Weir Quarry the sequence is: (1) polygonomorph phase, Marine (2) Visbysphaeru phase (c. 1m below the LudlowBone b.....,.. . - Bed), (3) polygonomorphphase (just below the Ludlow BoneBed), (4) sphaeromorphphase (from the Ludlow Fig. 8. Palynofacies curves showing sporomorph percentage and Bone Bed and above). At Gorsley there is a Visbysphueru Inshore Index, uppermost Llan-wen Beds (Ludlow Series), and phasein a sample 0.7 m below theUpper Phosphatic Platyschisma Beds (lower Downton Group), old quarry, Gwernaffel NoduleBed (an equivalent horizon to the Ludlow Bone Estate, Knighton. Bed),and above thisbed all thesamples belong to the

l@. 9. Photomicrographs of palynological preparations from WeirQuarry, takenwith Nomarski interference contrast. All specimens are X 250. (P & b) Upper Whitcliffe Formation, sample165. Palynofacies from the Visbysphaeruphase, showing abundantVisbyspheru associated withVeryhchiwn trispinosum (polygonomorph), rare Diexullopharis sp., rare cryptospores and miospores. Note the rare kerogen fragments (humic material) including possible animal cuticle(a) and scolecodont(b). (c & d) Downton Castle Sandstone Formation. Palynofacies from the sphaeromorph phase, showing abundance of land-derived kerogen. (c) Ludlow Bone Bed, sample16K. Sphaeromorph phase, showing abundanceof lath-shaped, irregular organic fragments and tubes, some banded, black,to dark-brown, kerogen and miospores and sphaeromorphs. (d) Platyschisma Shale Member, sample 16B. Sphaeromorph phase, showing tubes and large lath-shaped and irregular organic fragments, and small fragmentsof banded tubes. Spores common, with sphaeromorphs and rareMicrhystridium.

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Fig. 10. SEM photomicrographs of palynomorphs from theUpper Whitcliffe Formation, sample 16J. BM numbers referto the negative archives of the SEM unit in the Natural History Museum, London, and FS, FC, and FA numbers referto the type and figured collection in the Palynology Section there. (a) Scolecodont, ~188,FS1, BM 080065. (b) Chitinozoan, Eisenuckitinu burrundeiParis, ~525,FC1, BM 079297. (c) Acritarch, Visbysphueru oligofurcutu, X1275, FAl, BM 080064. (a) Acritarch, Micrhystridium, X3000, FA2, BM 079290. (e) ?Prasinophycean ‘cyst’, Dictyotidium, X3000, FA3, BM080063. (0Acritarch, Diexullophmis sp., ~1125,FA4, BM079303.

sphaeromorph phase. In the Long Mountain area the same areas traditionally regarded as either belonging to the shelf sequence of phases is repeatedalthough there are or the basinduring thistime. The sequence of differences in the microplankton assemblages. The sequence palynomorphsidentified is interpretedas indicating three sampled in Wallop Hall Quarry starts with a sample c. 1 m majorevents, two regression events and a third, ‘storm’, below the local equivalent of the Ludlow Bone Bed in the event. The environmental model used in this study is based Vkbysphueru phase, followed by samples up to the top of onthat first proposedby Al-Ameri (1980,1983u, b) for the Ludlow Series which are in the polygonomorph phase moreopen marine environments and modifiedherein for (though including more four-spined varieties than in shelf more inshore environments. In this paper theoriginal model assemblages), and these are overlain by beds of the lower is expanded to includea new phase concept. The phases DowntonGroup belonging sphaeromorph-the to occur in the same sequence throughout much of the area polygonomorph, polygonomorph and finally sphaeromorph and may prove a useful tool for basin analysis. It is possible phases. also that the sequence of phases, in a modified form, may proveuseful in fine-scaleintrabasinal correlation. For example the Vkbysphaeru phase c. 1m below the base of Conclusions the Downton Group may represent a marker band more or A combination of palynological and plant and animal tissue less equivalent in durationto modem ephemeral eventssuch information has been used to determine fluctuating patterns as a planktic bloom (often seasonal) or a volcanic ashfall, of marineand marginal marine environments in a late both of which in terms of geological time are instantaneous. Siluriansequence in Wales and the Welsh Borderland, in However,at this stage caution should be exercised in

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attempting to use this particular phase as a biostratigraphical We are most grateful to NERC who financed the initial phaseof the markerbecause patterns of organic-walled planktonin research, to C.Bell for duplicating some of thecounts and for modem oceans are complex (Wall et al. 1977). In strata of preparing the diagrams and the plate, to P. York for photography uppermost Whitcliffe Formation age the Visbysphueru phase and to F. Paris who identified the chitinozoan in Fig. lob. is widespread except in what is regarded the most basinal area at Knighton. Tentatively, the polygonomorph phase is divided intotwo subphases based upon the relative abundance of three-spined to three-dimensional,four- References spined Veryhachium species. The present study shows that assemblageswith larger numbers of suchfour-spined AL-AMEERI,T. K. 1980. Palynology, biostratigraphy and palaeoecology of subsurface Mid-Palaeozoic strata from the Ghadames Basin, Libya. polygonomorphsinclude greaternumbers of netromorphs Ph.D. thesis, University of London, Kings College. andother acritarchs thought to indicatemore offshore, - 1983a.Acid-resistant microfossils used in the determination of probably deeper water, environments. Palaeozoic palaeoenvironmentsLibya. in Palaeogeography, Palynofacies eventscan be correlated throughout the Palaeoclimatology, Palaeoecology, 44, 103-116. -19836. Palynological application for the palaeogeography of Tripolitania areaand show that the base of the Downton Group and duringSilurian time. Proceedings of the Indian National Science several events above and belowthis level are recognizable in Academy, 49, A(6), 637-646. a variety of shelf and basin facies. Other major regressive ALLEN,J. R. L. & TARM,L. B. H. 1963. The Downtonian and Dittonian eventsand possibly the ‘storm’ event may also be facies of the Welsh Borderland. Geological Magazine, 100, 129-155. recognizable throughoutthe basin andhave potential as CQLBATH,G. K. 1980. Abundance fluctuationsin Upper Ordovician organic-walledmicroplankton from Indiana. Micropaleontology, 26, regionalcorrelation markers. There would appear great 97-102. potentialfor refinement of environmentalanalyses based DORNING,K. J. 1981. Silurian acritarch distribution in the Ludlovian shelf sea uponcombined sedimentological, macropalaeontological of South Wales and the Welsh Borderland In: NEALE,J. W. & BRASIER, and palynological studies. Unfortunately none of the studies M. D. (eds) Microfossik from Recent and fossil shelf seas. Ellis Horwood Ltd., Chichester, England, 31-36. undertaken by OilCompanies have been published in DOWNIE,C., Em,W. R. & SARJEANT,W. A. S. 1963.Dinoflagellates, detail and Muller’s (1959) work, although dating from the hystrichospheresand the classification of the acritarchs. Stanford timewhen all organic-walledphytoplankton were lumped Uniuersity Publications, Geological Sciences, 7, 1-16. togetheras ‘Hystrix’, remainsthe chiefreference to HOLLAND,C. H. 1959. The Ludlovian and Downtonian rocks of the Knighton district, Radnorshire. Quarterly Journal of the Geological Society of palynologicaldistribution patterns of a moderndeltaic London, L14,449-482. complex. LAWSON,J.D. 1954. The Silurian successionat Gorsley (Herefordshire). Geological Magazine, 91, 227-237. -& WHITE,D. E. 1989. The PWdoli Series in the Welsh Borderland and Appendix south-central Wales. In: HOLLAND,C. H. & BASSEIT,M. G. (e&) A global standard for the Silurian System. NationalMuseum of Wales, Locality data for sections covered in Table 1. Geological Series No. 9, Cardiff, 131-141. (1) Weir Quarry, near Downton. Approximately 275 m northeast of LISTER,T. R. 1970. The acritarchs and Chitinozoa from the Wenlock and BringewoodForgeBridge,Hereford Worcester and Ludlow Series of the Ludlow and Mllichope areas, Shropshire, Part I. (SO 4560 7525). Monograph of the Palneontographical Society, London 1-100 (Publ. No. (2) Ludford Lane, Ludlow. Roadcut at junction between Ludford 528, part of Vol. 124 for 1970). Lane(Whitcliffe Road) and the A 49,Ludlow, Shropshire MULLER,J. 1959. Palynology of Recent Orinoco delta and shelf sediments: Reports of the Orinoco Shelf Expedition, volume 5. Micropaleontology, (SO 5123 7413). See Lawson & White (1989, pp, 137-138). 5, 1-32, (3) Linton Quarry, near Ross-on-Wye. Old quarry, 0.92 km west of TAPPAN,H. 1980. The paleobiology of plant protists. W. H. Freeman & Co., GorsleySchool, Gorsley Common, Hereford and Worcester San Francisco. (SO 6775 2571). Locality F of Lawson 1954. TRAVERSE,A. 1978. Supplementary palynologicalinformation from site (4) Longhope bypass, May Hill. Roadsidesection on the A 136, reports for DSDP leg 42B. 3. Site 379. In: Ross, D. A., NEPROCHNOV,Y. P. ET AL. (eds) Initial Reports of the Deep Sea Drilling Project, 42 (2). c. of (SO 550 m southeast Longhope, Gloucestershire, 6940 1851). Washington; 29, p. 41. (5) Wallop Hall Quarry,Long Mountain. Old quarry, 2.7km WALL,D., DALE, B., LOHMANN, G. P. and SMITH, W. K. 1977. The southwest of Westbury, Shropshire, (SJ 3150 0725). environmental and climatic distribution of dinoflagellate cysts in modern (6) Knighton. Old quarry, Gwernaffel Estate, Powys, (SO 273 706). marine sediments from regions in the north and south Atlantic Oceans See Holland (1959, p. 462). and adjacent seas. Marine Micropaleontology, 2, 121-200.

Received 19 July 1989; revised typescript accepted 30 October 1989.

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