Journal of rhe Geological Society, London, Vol. 148, 1991, pp. 781-784, 1 fig. Printed in Northern Ireland

Discussion on palynofacies in a Late Silurian regressive sequence in the Welsh Borderland and Wales Journal, Vol. 147, 1990,pp. 675-686

R. B. Ainsworth writes: Richardson & Rasul (1990) attempt longer and more intense storms would result in the transport to compare the depositionalenvironments inferred from of relatively large quantities of palynomorphs relatively their palynofacies analyses of theUpper Whitcliffe longer distances offshore,alongshore or onshore. The Formation and Downton Castle Sandstone Formation ‘with ‘storm event’ in the Downton Castle Sandstone Formation thoseproposed previously fromsedimentological data’. identified by Richardson & Rasul(l990, fig. 2; Fig. la) on the However, more recent sedimentological data than that used basis of theirsample 16C may representa storm event by Richardson & Rasul (1990), and regional geological resulting in the onshoretransport of sediment. However evidence, can be utilized to explain more fully the patterns their samples 16D and 16F, which weretaken from strata and trends of inshore and marine influence indices shown by directly above sample 16C, may represent examples of more the palynofacies curves in their paper. intense, long duration storms which resulted in the transport of sediment in an offshore direction. Similarly, the A3 event Sedimentology of the Upper Whitcliffe and Downton Castle identified by Richardson & Rasul (1990, fig. 2) in the Upper SandstoneFormations. Richardson & Rasul (1990) cor- Whitcliffe Formation may also be explained by these simple rectly point out that work by Muller (1959) indicates that autocyclical processes without the need to invoke the oceanic currents, the prevailing wind, and land and marine allocyclical controls proposed by them. The fact that the A3 physiography may modify the general pattern of decreased shale sample contains a higher percentage of spores than the abundance of land-derivedpalynomorphs in an offshore A4, 165 and A5 samples (all taken from a few centimetres direction.However, they fail to considersome of these above theA3 sample) is not necessarily evidence of potential modifications in their study. The storm-dominated regression followed by transgression (Richardson & Rasul nature of the Downton Castle Sandstone Formation in the 1990, p. 679), but can simply be explained by the derivation Ludlow area has been documented by Smith & Ainsworth of the mud from differentsource areason the coast (1989; published afterthe acceptance of Richardson & containing varying amounts of spores(Muller, in 1959, Rasul 1990), who describedamalgamated hummocky illustrated the uneven distribution of sporomorphs along the cross-stratification, thinly bedded fining-upwards siltstone coastline of the modernOrinoco Delta). This is not beds with basal lags of skeletal sand, wave ripple surprising given the variable intensity, duration, track and cross-lamination and gutter casts fromthe Ludford Lane points of landfall of successive storms on any given coastline locality. Allen (19856, p. 90) described ‘storm-related planar (Marsaglia & Klein 1983; Brenchley 1985; Morton 1988) to hummocky lamination, cross-lamination and wave- resulting in varying quantitiesand directions of offshore current ripples’ from theUpper Whitcliffe Formation. sediment transport. Watkins (1979) also discussed the storm signature of Ludlow strata in the Welsh borders.Therefore, due tothe Origin of the Ludlow BoneBed. Richardson & Rasul storm-dominated nature of the whole succession, extreme (1990, fig. 2) suggest adeepening towards the top of the cautionshould be exercised when interpreting any Upper Whitcliffe Formation, followed by a sudden shoaling palynological data.Net sediment transport duringstorms (represented by a high inshore index anda low marine (and thereforetransport of palynomorphs within the influence index) through the Ludlow BoneBed Member silt-sized fraction of the sediment) can beoffshore, (Bassett et al. 1982) of the DowntonCastle Sandstone alongshore, or onshore (Komar1983 and references therein; Formation (Fig. 1). They then propose a further deepening Niedorada et al. 1985). Land-derived spores and nearshore and then a gradual shoaling through the Platyschimsa Shale microfossils may be transportedand deposited long Member and the Sandstone Member (Bassett et al. 1982) of distances offshore and alongshore by storm-generated shelf the DowntonCastle Sandstone Formation. The regional turbidity currents, storm surge-ebb currents and geostrophic context suggests a relative lowstand in sea level at the Upper flows. These processes could produce anomalously high Whitcliffe Formation-Downton Castle Sandstone Formation nearshore indices and low marine influence indices in boundary, associated with localized tectonic uplift onthe offshore environments.Conversely, storms resultingfrom southern margin of the Welsh Basin (Allen19856). offshore-directed winds may result in onshore transport of Subsequent transgression (relative sea-level rise), evidenced sediment and marine microfossils (Niedorada et al. 1985), by the onlap of DowntonCastle Sandstone Formation therefore producing low nearshore indices and high marine equivalents onto pre-Upper Whitcliffe Formation strata in influence indices in nearshore environments. In all SW Wales, resulted in sedimentstarvation and repeated eventualities theend result is one of overall mixing and winnowing of the shelf area by storm waves (for an reworking of terrestrial and marine assemblages. unknown but probably substantial period of time) producing The varying durations, intensities, tracks and points of condensed, time-rich horizons of wave-rippled skeletal sands landfall of subsequent storms may explain the irregular (Ludlow Bone Bed Member; Anderton et al. 1979; Smith & fluctuations in the overall trends of the palynofacies curves. Ainsworth 1989). These skeletal sands do not occur as one These fluctuations are even more obvious if all, rather than discrete bed but as a number of wave-rippled lenticles within selected, data points are plotted on the curves (Fig. lb). The grey mudstones. Further evidencefor winnowing on a 78 1

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Platy- Platy- - 168 schisma "+ :+ - 16~ Shale

deepenmg Member +: ', , Member

. . Inshore Index .___._...... Inshore Index ._._..... ' 9b 8'0 70 6'0 5'0 do 30 20 10 0 ' 9b 8.0 f0 6'0 50 40 30 20 10 0 I..,..,,,.~ar~ne Influence > Marme Influence 0 10 20 30 40 50 60 70 80 90 'Index 0' 10 20 3b 40 50 60 70 80 90 Index a b Fig. 1. Partial sections from Fig.2 of Richardson & Rasul (1990). (a) unaltered section, (b) modified section encompassing all data points given in table 2 of Richardson & Rasul(l990). This illustrates irregular fluctuationsin the palynofacies curves above and below the Ludlow Bone Bed Member which are probably associated with storm mixing and reworking of terrestrial and marine assemblages. Also note that the apparent deepening above the Ludlow Bone Bed Member shownin (a) is not observed in (b) when all data points are plotted. Vertical scale is in centimetres. See original figurefor explantion of lithologies.

sedimentstarved shelf is provided by the presence of Smith & Ainsworth (1989), is supported by the general thick-walled tasmanitid prasinophytes, reworked Llandovery increasing inshore and decreasing marine influence indices cryptospores(Richardson & Rasul 1990), and phosphatic trends shown by the palynofacies curves of Richardson & shells and pellets in the Ludlow Bone Bed (Bassett et al. Rasul (1990). 1982). Galloway (1989) haspointed outthat marine- The new sedimentological and regional geological condensedsections oftenform on the shelf during basin evidence suggests that the Ludlow Bone Bed Member was margin transgression and flooding. Unfortunatelycon- generated by repeated storm reworking during a period of straints on precise water depths are unavailable. However, reduced sediment supply, probably associated with a raised the shelf must have been within storm wave base. Loutit et sea level. This interpretation is atodds with the sudden al. (1988) contains further examples of marine-condensed shoaling through the Ludlow Bone Bed Member proposed sections.This scenario suggests thatthe Upper Whitcliffe by Richardson & Rasul (1990), using palynological Formation-Downton Castle Sandstone Formation boundary evidence. It is suggested that this anomalously high inshore represents both a regressive and a transgressive surface as index and low marine influence index in the Ludlow Bone opposed to justa regressive surface(Richardson & Rasul Bed Member (Richardson & Rasul 1990) can be explained 1990). Figures 4 and 5 of Richardson & Rasul (1990) by acombination of the same winnowing processes that demonstrate a fundamental change in acritarch groups and produced the skeletalsands, and a possible externally palynomorph abundances across theUpper Whitcliffe controlled microfaunal change across the Upper Whitcliffe Formation-DowntonCastle Sandstone Formation bound- Formation-Downton CastleSandstone Formation bound- ary;for example there is a notabledecrease in the ary. During intense winnowing of the shelf by storm waves, percentage of acritarch groups. This change is not unlike the the more delicate marine microfossils would theoretically be macrofaunal patterns demonstrated by Bassett et al. (1982, preferentially removed and the morerobust spores p. 13) across thesame boundary, which marks a distinct concentrated (as stated in a different context by Richardson basin-wide fauna1 change and many species of brachiopods & Rasul 1990, p. 681). If marine microfossils were also (notably the articulate) and molluscs do not persist across it. present in less abundance due to environmental (salinity?) This pattern cannot be explained by simply shifting changes, this winnowing effect would be amplified. This environments landwards or seawards. Could the same cause scenario would produce the observed effect of a high inshore that precipitated the macrofaunal change, possibly have index anda low marine influence indexina shelf affected the marinemicrofauna as well? Salinity changes depositional environment. related to tectonic isolation of the Welsh Basin have been proposedas a possible cause (Smith & Ainsworth 1989). Conclusions. (1) Extreme caution should be exercised when Wightman et al. (1987) have documented restricted acritarch interpreting palynological analyses from storm-dominated assemblages associated with salinity changes in the environments.Overall trends in palynofacies curves are Cretaceous Mannville Group of western Canada. If a more likely to reflect sea-level histories than spikes in the microfaunal decimation did occur at this level, it would help data which can oftenbe explained by storm mixing and to explain thegreater percentages of sporomorphs and reworking of terrestrial and marine assemblages. smaller percentages of marine microfossils found above the (2) The palynological evidence presented by Richardson & boundary.Marine microfossils would have been less Rasul (1990) can be taken to support the hypothesis that the abundantat this level and would thereforerepresent a Ludlow BoneBed Member was generated by repeated smaller percentage of the total palynomorph population. storm reworking during a period of reduced sediment supply The ensuing progradation of the shoreline (through the associated with a raised sea level. Platyschisma Shale Member and the Sandstone Member) (3) AsRichardson & Rasul (1990) state in their associated with a highstand systems tract, as suggested by conclusions, it is essential that palynological studies are

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combined with modernsedimentological studies when sample 16K1), and also those of the Platyschisma Shale attempting to refine environmental analyses and identify Member, show a shift offshore but the erratic nature of the sea-level fluctuations in the rock record. acanthomorph/netromorph record perhaps indicates tur- bulent inshoreenvironments where penecontemporaneous Thanksare due to R. D. A. Smith, D. R. Braman and R. G. reworking was common (Richardson & Rasul 1990, fig. 5, p. Walker for many useful comments on earlier versions of this 681). This broad offshore-onshore pattern, from the Upper discussion. Whitcliffe Formation to the lower part of the Downton Castle Sandstone Formation, is distinct and repeated 5 November 1990 (Richardson & Rasul 1990, table 1, figs 2, 6, 7), with some variation, in sections from both shelf and basin areas, and it J. B. Richardson & S. M. Rasul reply: It is most compares very well with theinterpretations of Allen & encouraging to receive the above comments on our paper, Tarlo (1963; see also Allen 1974, 198%). especially afterour conclusion that combinedsedimen- The regressive-transgressive eventat the Ludlow Bone tologicial and studies offer palaeontological-palynological Bed horizon (Richardson & Rasul 1990, p. 679) andthe greatpotential for refinement of palaeoenvironmental Platyschisma event, are the only major palynofacies events analyses in these upper Silurian strata. Before the we have discovered in both shelf and basin sequences, but publication of our paper, andindeed from the late 1970s, we moredetailed work is needed onthe Lower andUpper gave progress reports on our palynofacies studies at several Whitcliffe Formations and their equivalents throughout the internationalmeetings, and to a variety of geological and area. Smith & Ainsworth (1989) described storm-related palynological audiences. At none of these meetings did we sedimentation from one section only, that at Ludford Lane. receive eitherdisagreement with, orsupport for, our We have not studied the Ludford Lane section in the same tentative view thatthe Platyschisma eventrepresents a detail as that at Weir Quarry but, as indicated in our paper storm event. While we accept the possibility of storm (1990, p. 681), thereare interesting differences in modification to palynofacies in other parts of theUpper palynofacies between samples from the Weir Quarry section Whitcliffe Formation-Downton Castle Formation sequence, andthose from Ludford Lane. Allen (1974) reported that and have alreadypointed (1990, p. 681, 684) tothe his Facies C was seen only at the Ludford Lane outcrop and possibility of high turbulence in some late Silurian inshore hypothesized that this facies represents barriera inlet environments, we have not yet detected any other experiencing large energy changes. Because the Ludford comparableevent in thelate Silurian shelf and basin Lane section is, according to Allen and our less complete environments of the Welsh Borderland. Nevertheless, this is palynofacies data, different from the rest of the Downton an exciting challenge andfurther collecting andstudy, outcrop, it is perhaps premature of Ainsworth to attempt to ideally in collaboration with a sedimentologist, are needed generalize from his study of this one outcrop to those that to test this hypothesis. we have studied.Analyses similar to those of Smith & Palynofacies and the Upper Whitcliffe Formation. On Ainsworth are needed for other upper Silurian outcrops in balance we see no reason to change our conclusion (1990, p. the Anglo-Welsh Basin, and in particular the sections that 678-679), based on palynofacies analysis, thatthe Upper we have studied, in order to establish the pattern of storm Whitcliffe event is more !ikely to be regressive in nature related sediments and any regional variations. Nevertheless, rather than storm-related. We considered the possibility of and in spite of interpretative differences, we find it storm control, but rejected it partly because the increase in fascinating that palynologists and sedimentologists have spore percentages occurred across a bedding plane between independently concluded thatthere were storm-tossed two samples of distinct lithology: a calcareous silt (A3a, with shallow seas in the late Silurian. brachiopods and bryozoans) and an immediately overlying Conclusions. A model is needed combining both palaeon- shale (A3). Allen (19856, pp. 88, 90) considered that in the tological and sedimentological data for the interpretation of ‘Ludlow Beds’ there are horizons of active shoaling, possibly the affects of storm-influenced sequences on land-derived regressive-transgressive events,and that locally thereare material and phytoplankton. Allen (1985a, pp. 264-265, fig. storm-relatedsedimentary features. So bothphenomena 13.34) illustrated a ‘model for storm sedimentation’ in which may have affected sediments in theUpper Whitcliffe an onshorestorm provides net transport offshore. Under Formation and we cannot completely discount the possibility these conditions, sporomorphs, sphaeromorphs and banded that a storm event occurred at the horizons represented by tubes, the abundance of which would normally indicate our A3a and A3 samples. Whatever the cause of this event, proximity toshore, could becarried furtherout to sea, it mayit have influenced the local marineenvironment presumably undergoingsome sorting. These conditions because sample A3a marks the beginning of an increase in could produce theabrupt kind of change in palynofacies specimens of the genus Visbysphaera andthe start of a seen in our Platyschisma event. Visbysphaera phytoplankticphase (Richardson & Rasul 1990, fig. 4). 20 February 1990

Palynofacies and the Ludlow Bone Bed horizon and Lower Downton Group. Onthe basis of thedata fromWeir References Quarry, we (1990) suggested a shift offshore towards the top ALLEN, J. R. L. 1974. Sedimentology of the Old RedSandstone of the Upper Whitcliffe Formation (sample A6), followed by (Siluro-Devonian) in theClee Hills area,Shropshire, England. Sedimentary Geology, U,73-167. asudden shoaling atthe base of the Ludlow Bone Bed -1985~. Principles of Physical Sedimentology. Allen and Unwin, London. Member (sample 16K, level of the main Ludlow Bone Bed). -19856. Marineto fresh water:the sedimentology of theinterrupted Higher samples in the Ludlow Bone Bed Member (above environmentaltransition (Ludlow-Siegenian) in the Anglo-Welsh

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region. Philosophical Transactions of the Royal Society, London, 8309, MORTON, R. A.1988. Nearshore responses to great storms. In: CLIFTON,H. 85-104. E. (ed.) Sedimentologic consequences of convulsivestorm events. -& TARLO,L. B. H. 1963. The Downtonian and Dittonian facies of the Geological Society of America, Special Paper, 229, 7-21. Welsh Borderland. Geological Magazine, 100, 129-155. MULLER, J.1959. Palynology of Recent Orinoco delta and shelf sediments: ANDERTON, R. H., BRIDGES,P. H., LEEDER. M.R. & SELLWOOD, B. W.1979. Reports of the Orinoco Shelf Expedition; Volume 5. Micropaleontology, A Dynamic Stratigraphy of the British Isles. GeorgeAllen & Unwin, 5, 1-32. London. NIEDORADA,A. W., SWIFT, D. J. P. & HOPKINS,T. S. 1985. The Shoreface. BASSETT,M. G., LAWSON, J.D. & WIIITE,D. E. 1982. The Downton Series In: DAVIS, R. A.Jr.(ed.) Coastal Sedimentary Environments. as the fourth Series of the Silurian System. Lethaia, 15, 1-24. Springer-Verlag, New York, USA, 533-624. BRENCHLEY, P. J.1985. Storm influenced sandstone beds. Modern Geology, RICHARDSON,J. B. & RASUL,S. M. 1990. Palynofaeies in aLate Silurian 9, 369-396. regressive sequence in the Welsh Borderland and Wales. Journal of the GALLOWAY,W. E. 1989. Genetic stratigraphic sequences in basin analysis I: Geological Society, London, 141, 675-686. Architecture and genesis of flooding-surface bounded depositional units. SMITH,R. D. A. & AINSWORTH,R. B. 1989. Hummocky cross-stratification in American Association of Petroleum Geologists Bulletin, 13, 125-142. the Downton of the Welsh Borderland. Journal of the Geological Society, KOMAR,P. D. 1983. Handbook of Coastal Processes and Erosion. CRC Press, London, 146,897-900. Inc., Boca Raton, USA. WATKINS,R. 1979. Benthic community organisation in the Ludlow Series of LOUTIT,T. S., HARDENBOL,J., VAIL,P. R. & BAUM,G. R. 1988. Condensed the Welsh Borderlands. Bulletin of the Britkh Museum (Geology), 31, sections:the key to agedetermination and correlation of continental 175-280. margin sequences. In: WILGUS,C. K. et al. (eds) Sea Level Changes: An WIGHTMAN,D. M.,PEMBERTON, S. G. & SINGH,C. 1987. Depositional Integrated Approach. Society of EconomicPaleontologists and modelling of theUpper Mannville (LowerCretaceous), east central Mineralogists, Special Publication, 42, 183-213. Alberta: Implications for the recognition of brackish water deposits. In: MARSAGLIA,K. M. & KLEIN,G. deV. 1983. The paleogeography of Palcozoic TILLMAN,R. W. & WEBER,K. J. (eds) Reservoir Sedimentology. Society and Mesozoic stormdepositional systems. Journal of Geology, 91, of Economic Paleontologists and Mineralogists, Special Publication, 40, 117-142. 189-220.

R. B. AINSWORTH, Departmentof Geology, McMaster University, Hamilton, Ontario, L8S4M1, Canada. .I. B. RICHARDSON, Departmentof Palaeontology, Thc Natural History Museum, London SW7 5BD, UK. S. M. RASUL,Paleo Services, Unit 15, Paramount Industrial Estate, Sandown Road, Watford WD2 4XA, UK.

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