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Discussion on Palynofacies in a Late Silurian Regressive Sequence in the Welsh Borderland and Wales Journal, Vol

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Discussion on Palynofacies in a Late Silurian Regressive Sequence in the Welsh Borderland and Wales Journal, Vol 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 were taken 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/4/781/4891658/gsjgs.148.4.0781.pdf by guest on 29 September 2021 782 DISCUSSION 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 that the 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
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