Journal of the Geological Society, London, Vol. 147, 1990, pp. 885-891, 4 figs. Printed in Northern Ireland

Discussion on the palaeocurrent evidence of a northern structural high to the Welsh Basin during the Late Llandovery

Journal, Vol. 146, 1989, 211-l2

D. K. Loydell writes: In recenta paper, McCann & Lithostratigraphy. The coastalexposure theauthors ex- Pickering (1989) describe a series of current ripples in the amined lies somewhere between Wallog and and the Wallog-Borth region in the northern part of the outcrop of formationstudied is citedas the AberystwythGrits. Two the Telychian (Upper Llandovery) Grits formations crop out across this coastline, the Aberystwyth Formation. Whilst theircurrent data provideevidence of Grits in the south and the Borth Mudstones in the north derivationfrom the NNW (perhapsfrom the IrishSea (British Geological Survey 1:5OOOO map Sheet 163) and Platform), their stratigraphical conclusions are at variance from the description it is unclear as to which formation the with the BGS mapping of the Aberystwyth Sheet (see Cave 30 m of strata belong. The rocks are described (p. 211, col. & Hains 1986) and the author’s own palaeontological data. 2, para. 2) as ‘mainly pelagic and hemipelagic mudstones; McCann & Pickering statethat the overallnorthward with some sandstones and siltstones, (though fig. 2 refers to younging of the Lower Palaeozoic rocks in Central them as turbidites). Thirty metres of such strata do not fit suggest thatthe strata in the Wallog-Borth region are easily intothe Aberystwyth Grits, yet neither doesthe younger than thoseto the south. However, the overall description match the Borth Mudstones which are mainly structure of the Aberystwyth Grits is that of a periclinal mudstones of the T, interval of the Bouma turbidite cycle. In syncline with thosebeds lying around Wallog andBorth bothformations hemipelagites are of insignificant propor- dipping to the south. This is borne out by the fact that, in tions. The locality also needs to be clearly identified. northern Dyfed, the underlying Borth Mudstones Formation Sole marks. The authors report (p. 211, col. 2, para. 2) that crops out immediately to the north and east of the base of sole marks are absent. Sole marks, particularly flute casts, the Aberystwyth Grits. Palaeontological data provide are common in the Aberystwyth Grits and they consistently confirmation of the southward younging here; collections indicate a northward flow. Flute casts are uncommon in the madefrom south of Harp Rock (SN595872), which lies Borth Mudstones, butwhere seen near Borth (Cave & between Borthand Wallog,include Rastrites linnaei, Hains 1986, pp 52 and 95) they indicate a palaeocurrent flow Pristiograptus renaudi and Petalograptus conicus indicating a to the east. McCann and Pickering’s model is based upon horizon within the lower half of the Monograptus current ripples, butnevertheless when constructing their turriculatus Zone, whilst atthe northern end of Allt-wen model such evidence must surely have been considered. (SN576796), 6 km south of Wallog, the presence of Petalograptus tenuis, Monograptus proteus and M. rickardsi ‘Northward younging’ of the outcrop. Theauthors claim minor is indicative of a level high in the turriculatus Zone. thatthe Aberystwyth Grits young northwardsfrom I agree with McCann & Pickering that it ‘is not realistic Aberystwyth whereas earlier views, except possibly those of to have totally opposing flow directions (140” apart) of the Keeping (1878, p. 536), are in almost diametric opposition. same age separated by a lateral distance of about 1 km’ and The new assertion is so fundamental that full data upon I also agree that the rocks to the north and southof Wallog which it is founded should be supplied. For instance, Cave (which show these different flow directions) are indeed of & Hains (1986, pp 51-52) reported that ‘there can be no different ages. However,the rocks north of Wallog are doubt of the stratiformsuperposition of the Aberystwyth clearly older(and not younger,as McCann & Pickering Grits above the Borth Mudstones on the foreshore’ and that suggest) than those to the south. ‘there was no fauna1 substance for the view that the base of the Aberystwyth Grits is youngerin the north‘ (atHarp R. Cave & B. A. Hains write: The recent Short Paper by Rock) ‘than it is to the south’ (near Grogal). These views McCann & Pickering (1989) proposed that SSE-directed should notbe discounted before contrary evidence is submarine currentscontributed turbidites tothe northern supplied. part of the Aberystwyth Grits Formation, Llandovery Series (Telychian Stage). The proposal was based on 28 current Biostratigraphy. The authors place the Aberystwyth Grits in ripples in a sequence 30 m thick and it was suggested that the upper part of the turriculatus Zone and in the crispus the sedimentforming the turbidites was shed from a Zone without giving their evidence. Because of the similar basin-floor high to the north of the present outcrop. It was statement by Wood & Smith (1959) evidence of the crispus claimed that this basin-floor high was of long standing and Zone was sought by BGS from the Aberystwyth district but had caused shallow marineor emergent conditions in not found (Cave & Hains 1986, p. 57), whilst within the Caradoctimes. Such aproposal arrests the momentum of turriculatus Zone the graptolite assemblages were too poorly current geological research in the area. It prompts a request understood to refine the sequence with precision. For for more of the data which must underpin the paper and for example, recognition of the maximus Subzone required the greater discussion of the models which these data appear to presence of R. maximus while itsabsence implied higher make implausible. Will theauthors comment onthe horizons in the zone. On this basis the lower parts of the following points? Borth Mudstones were placed in the maximus Subzone, i.e. 885

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low turriculatus Zone and the upper parts of the formation This discussion is published with the permission of the Director of were thought to rise above the subzone. The Aberystwyth the British Geological Survey and with the authors’ thanks for the Grits were believed to occupy a position in the middle of the helpful comments of colleagues in Aberystwyth. zone(Cave 1979, p. 519). The discovery of M. halli (Barrande) in the northernmost parts of the outcrop led to D. M. D. James writes: rather than the structural inversion the consideration that the lowest parts of the formation also of thenorthern Welsh Basin having commenced in the belonged to the base of the zone (Cave & Hains 1986, p. Llandovery, with attendant difficulties forthe Wenlock- 57). Southward, towardsAllt-wen, the assemblages Ludlow evolution of the Denbigh Trough, cannot the indicated younger ages, though it was thought that nowhere intriguing new data set out by McCann & Pickering (1989) in this section do the Aberystwyth Grits rise to the topmost be more simply explained by the presence of the more local parts of the zone (Cave & Hains 1986, table 2, p. 53). These Derwen Horst (Fitches & Campbell 1987)? The authors do collections are being considered by D. K. Loydell (U.C.W. not refer to this feature, but a structural high in this position Aberystwyth) in an intensive analysis of the graptolite is certainly crediblein the Llandoveryand also existed assemblages of the turriculatus Zone and his conclusions are earlieras SW-SSW directedpalaeocurrents arenot awaited. Any evidence the authors have for the presence of uncommon amidst the more general NNE-NW palaeocur- the biozone of M. crispus is of great importance as it would rents in the late Ordovician strata of the Machynlleth district indicatealmost totaloverlap of horizonsbetween the (James 1972, fig. 8; 1973, fig. 4). Intra-basinalhorst and Aberystwyth Grits and the Rhuddnant Grits to the east. graben activity is typical of much of the Welsh Basin and its local impact on sedimenttransport is not necessarily a pointer to the timing of regional deformation. The degree to Bathyrnetry. The model of a basinal rise supplying sediment which there was ever a regional SW facing palaeoslope in to the Aberystwyth Grits from north of their outcrop needs and tothe NE of the Borth-Machynlleth area is more sedimentological data. A basinal rise with a shallow questionable, the SSE transport recorded by the authors in marine environment in the late Caradoc and Ashgill of the primary turbidites is itself not easily reconciled with such a areanorth of the Aberystwyth Gritsoutcrop was nota palaeoslope unless the turbidites are overspill deposits.I consideration of Pugh (1923) in theCorns area, of Jehu prefer tosee a local SW-facing slopeas an interference (1926) in the Towyn area,nor was it suggested by Cave effect between the regional WNW-facing slope which (1965). They all interpretedthe sequence to reflect extends far to the south and the SE-facing slope relating to comparatively deep water environments at these times and the southern margin of the Derwen Horst parallel to the this interpretation was supported by the palaeoecological Bala fault. The Aberystwyth Gritsare in general axially studies of Lockley (1980) and Price & Magor (1984). transported with respect to the WNW-facing regional slope. Recently the surveys of BGS around Aberdovey, and by M. On the evidence presented in the paper it seems to me that Leng and W. Pratt (U.C.W. Aberystwyth) between Towyn the ripple data could still be compatible with the sole mark andCorns, have also revealed no evidence of shoaling. data reported by Crimes & Crossley (1980). The measured Therefore it would be of value if the authors would indicate ripples could indicate local SSE-directed overspill froma more clearly the source and route of the clastic sediment meandering channel/lobe system with variable transport which fed into the Aberystwyth Grits from the NNW for it direction between NNE and ESE as indicated by the sole would have to travel throughareas of outcrop already marks. Would the authors comment on this possibility? It examined. In the Aberdovey area the turriculatus Zone is would also be helpful if grid references to the key localities composed almost entirely of turbiditic mudstones, possibly could be given for the benefit of future work. the deposits of afan-fringe or basin-plain. The coarser sedimentfraction (Bouma zd) wanes rapidly north- R. D. A. Smith writes: The observations by McCann & northwestward; thereare no condensedsequences, pene- Pickering (1989) of south-directedpalaeocurrents in the contemporaneously disturbed beds, laminites, or any other Wallog-Borth area of western mid-Wales are interesting indications of slope-depositionassociated specifically with and important. However, in theirpaper they fail to cite this area. Further to the NNW the same zone in the Lleyn relevant recent work on the Lower Silurian fill of the basin Peninsula comprises even finer turbiditicmudstone and and in particular they fail to interpret their observation in black anoxic hemipelagic mudstone in aboutequal the context of current understanding of the tectonic proportions (Baker 1981), while the low and middle parts of framework for sedimentation in the Welsh Basin. the Llandoverysequence are condensed compared with Recentstudies of Silurian depositional systems in the those of Central Wales. Evenhad theserocks been the Welsh Basin (Smith 1987a,b, references in Woodcock et al. deposits of abathymetric high, they could not haveshed 1988) emphasize the importance of structurally-induced much sand-grade detritus. basin floor topography on sedimentationpatterns. Such The reference to Cave (1979, p. 523) in support of topography was produced in response to reactivation of an current diversion against the basin high is amisquotation array of long-lived basement discontinuities (Fig. la). The which distorts the meaning. It was not a statement about south-directedpalaeocurrents recorded by McCann & SW-directed flow, but evidence from the turriculatus Zone Pickering are most simply explainedasproduct a of (now augmented by thedata of M. Scott,U.C.W. low-volume, low-density turbidity currents flowing south- Aberystwyth) of current-flow to ‘south of west’ north of the wards down a gentle slope localized by the Bala Lineament Dovey Estuary ‘rather than northof it’ south of the estuary. which is known to have downthrown to the south in early Whetheror not thesecurrent differences were dueto Silurian time (Smith 1987b, Fig. lb). Differential subsidence bathymetricunevenness they do reveal thatdetritus was across this structure was inferred from the dramatic thinning carriedfrom an easterly quadrant directly intothe area of Telychian (Upper Llandovery) strata northwards towards postulated as a basinal high, not away from it. the Bala Fault and is supported by reported palaeocurrent

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deflections towards theeast in the Wallog-Borth area (Crimes & Crossley 1980) and the observations of McCann & Pickering. Fitches & Campbell (1987) consider The Tal-y-llyn and Bala Faults of the Bala Lineament to have beenearly Palaeozoic normal faults, buttheir subsurface attitudes are not known. The Bala Fault downthrew to the south from at least early Ashgill to lateLlandovery times, as the southern margin of the persistently positive Derwen Horst(James & James 1969; Fitches & Campbell 1987; Smith 19876). Therefore the existence of a structural high to the north of the Bala Lineament clearly does not indicate the commencement of ‘structural inversion of the northern part of the Welsh Basin. . . in the Llandovery’ as suggested by McCann & Pickering. Relatively deep water to the north of the Bala Lineament in early Silurian times is indicated by ichnofacies and diagenetic mineral assemblages which reflect deposition in oxygen-deficient bottom waters, also a feature of the southern part of the basin (Smith 1987c, 1988). A further, more generalpoint worth addressing is the origin of other anomalous palaeocurrent directions cited by McCann & Pickering and Crimes & Crossley (1980). McCann & Pickering imply that west-directed palaeocur- rents documented from the Lower and Middle Llandovery -Rhuddnant Grits limited to fill of the basin are ‘anomalous’. However, the change from abundant west-directed palaeocurrents to dominantly SE by CentralWales Lineament Smith(1987a) northeast to north-directedpalaeocurrents seen in the Telychian deposits is simply dueto abandonment of the Local,small volume turbidity currents eastern basin margin as a major supply margin during traveldown slope localized by Bala the Telychian flooding of the Midland Platform (Smith Lineament (MC Cann and Pickering 1989) 19876). Controls on palaeocurrent patternsare not only \ allocyclic (e.g. governed by bounding slopes and location of hlaeocurrent deflection in response inputpoints) and autocyclic controlsshould also be to slope localized by Bala Lineament fl considered. East-directed palaeocurrents recorded from the (Crimesand Crossley 1980) Aberystwyth Grits further south and inferred to have been produced by channelized flows (Crimes & Crossley 1980) probably do not reflect tectonically-induced intra-basin topography (such as that caused by the BalaLineament further north) or a different (westerly) source margin from that which supplied the bulk of the Aberystwyth Grits, as suggested by Crimes & Crossley (in fact the presence of Llandovery age graptolitic shales on Anglesey suggests that there probably was noemergent IrishSea High inearly Silurian times). An alternativeexplanation is thatthese clockwise deflections may result from the Coriolis effect on levee development, which in the southern hemisphere would Sediment supply fromhigh- relief have resulted in development of lower eastern levees and southernlandmass consequent channel avulsion to the east (see Smith 19876). Palaeocurrents in the deposits of channelized flows might be expected to show clockwise deviationsfrom mean Fig. 1. (a) Compilation of structures which predated Acadian palaeocurrentdirections in associated deposits of unchan- deformation and influenced early Palaeozoicpatterns of sedimenta- nelized flows. Unconfined flows would have been deflected tion and volcanism (modified after Smith 19876, 1988,see these tothe west to varying degrees dependingontheir references for names of structures and sources). Structures thickness/velocity characteristics (Hill 1984; Smith 1988). A particularly important in early Silurian times includethe Pontesford further possible cause of spread in local palaeocurrent Lineament (PL), Tywi Lineament (TL), Central Wales Lineament directions (which might, in small datasets, suggest significant (CWL), Bala Lineament (BL), Benton Fault (BF)and Ritec Fault trends) is the meandering of individual turbidity currents (W).(b) Palaeogeographic sketch for early Telychian times, (Parkash & Middleton 1970). illustrating the influence of long-lived structures. The orientations of the Benton and Ritec faults are approximately corrected for T. McCaon 81 K. T. Pickering reply: Many of the points post-Acadian clockwise rotation indicated by palaeomagnetic raised by Cave & Hains in their discussion of ourpaper evidence (McClelland-Brown 1983). Both were reverse faults in early Telychian times. refer to lithological differences between the Aberystwyth Grits Formation and the slightly older to contemporaneous Borth Mudstones Formation. Our study was situated in their

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Borth Mudstones Formation, although a number of other depositional history for the present-day coastal section from authors (notablyPrice 1962; Crimes & Crossley 1980) south of toBorth in Middle andUpper consider thearea to be part of the Aberystwyth Grits Llandovery times. Deposition of the Derwenlas Formation Formation. Identical facies associations to the one in which from point sources to the east during the Middle Llandovery we made our palaeocurrent determinations occur all along (Fig. 3a) was followed by a change in provenance area from the coastal section between Borth (SN605888) and Clarach east to south for the deposition of the Aberystwyth Grits Bay (SN 587836), north of Aberystwyth. The only significant Formation (Fig. 3b). Coeval with the initial progradation of sedimentological difference between the Aberystwyth Grits the sandstoneturbidite system of the Aberystwyth Grits and Borth Mudstones formations is the possible provenance Formation, there was deposition of the muddier Borth directionas determinedfrom the ripplelamination in Mudstones Formation by sediments probably derived from a fine-grained sandstoneand siltstonebeds. The muddy structural high situated to the north and controlled by the sections to the northof Wallog (SN 590859) show derivation Bala Lineament. Parts of the Borth Mudstones Formation from the north while those closer to Clarach (SN 587836) farther eastand northeast may havebeen derived from are southerly derived. dilute turbidity currents sourced from theAberystwyth Grits The coastal section from Clarach to Borth is structurally Formationfan system. The final stage (Fig. 3c) involved . complex and is dominated by upright, tight, macroscopic progradation of the Aberystwyth Grits Formation system folds with axes trending N-S to about 020" (not NW-SSE over the muddier Borth Mudstones Formation, the slewing as shown on the BGS map (Cave & Hains 1986). We have round of turbidity currents towards the NW and NE as a also recordedthrusts with westerly transport directions result of deflection against the submarine slope parallel to cutting these strata and therefore doubt the reliability of a the Bala Lineament. simple layer-cake stratigraphy as inferred by Cave & Hains Before answering the comment of substance in Cave & in their discussion of our paper. The northernmost section, Hains' discussion,. wewish to addresspoints which they in the Borth area, does indeed appear to young southwards raised that are a misinterpretation of our original paper. overall suggesting that thisparticular section (defined by Cave & Hains (1986) as Borth Mudstones Formation) may (1) Cave & Hains state that 'this basin floor high was of beolder thanthe sectionimmediately tothe south. Our long standing and had caused shallow marine or emergent impression, however, of the facies-associations and structure conditions in Caradoc times'. This, however, was not stated is thatthere is a gradual increasein the percentage of by us but by Woodcock (1984) to whom the comment was mudstone-facies fromAberystwyth to north of Wallog. attributed.We are perplexed by the comment that this Apart from the northernmost sectionbeing slightly older, interpretation 'arrests the momentum of current geological we envisage the stratigraphy from immediately north of research in the area'! Wallog to Aberystwythas essentially contemporaneous. (2) Theymisinterpret our lithological description. If Parts of the Aberystwyth GritsFormation, however, pass they read our description carefully then it would be clear gradationally into the Borth Mudstones Formation (Fig. 2) that we recognize not only pelagic/hemipelagic mudstones as shown by Cave & Hains (1986). but also turbidites, i.e. 'the deposits are. ..primary T,,, During deposition of the Aberystwyth Grits Formation, T, divisions of turbidity currents' (p. 212, col. 1, para. 1). a sandy turbidite system sourced from the southern Welsh Such a description certainly is consistent with parts of the Basin,accumulated in the axis of the basin and passed Aberystwyth Grits Formation farthersouth, for example laterally andgradationally northwards intothe Borth in the muddiersections around Wallog, Clarach and Mudstones Formation.The younger parts of the Abery- Aberystwyth. stwyth Grits Formation, however, prograded over the Borth (3) Our observation that solemarks, particularly flute Mudstones Formation,and both formationswere subject casts, are absent clearly relates to thesection in question not tolater Caledonian/Acadian deformation (folding and the entire Aberystwyth Grits Formation. Indeed, immedi- bedding-parallel thrusting).This createdthe stratigraphic ately north of Clarach, we have observed bedding surfaces relationshipsseen today. Figure 3 summarizes the full of flute casts. Cave & Hains' commentsconcerning palaeocurrent data are muddled and we are not entirely sure of the significance of their statementthat palaeocurrents 'consistently indicate a northward flow'. We are naturally aware of this for much of the Aberystwyth Grits Formation IE and the purpose of our paper was to discuss a section where I Uwchygarreg I this is not the case. Again, based on current ripples in the IRuddnmtUd I Aberystwyth GritsFormation, we observevariable palae- ..... ocurrent flow betweenClarach tonorth of Wallog ...... suggesting deflection of turbidity currents bothtoward the ...... NE and NW (Fig. 4). -Ddwenmu&-. .- : : . .~...... F-ah ...... (4) Cave & Hains state that 'the authors claimed that ...... the Aberystwyth Grits young northwardsfrom Abery- ...... ,DeviCrBridpe,...... Fmah ...... stwyth'. Nowhere in ourpaper do we say this and, ...... "i-9 ...... therefore, since their initial premise is incorrect, the rest of ...... Lower ...... their points arising from this are irrelevant.We did, w--ljililBrei!*p=a :,=:=E=*;; ...... Cwmsymkg kmah ...... however, statethat overall the Aberystwyth Grits Formation youngs northwards, and we maintainthis Fig. 2. Lithological units within part of the Upper Llandovery of position asdoes all previously published work onthe west Wales (after Cave & Hains 1986). formation (see references in our paper).

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H%?HiL/MHHKHY

Fig. 3. Block diagram showing a facies reconstruction for the study area during the Middle-Late Llandovery.

(5) Cave & Hains citereferences which favour (6) Cave & Hains statethat: ‘Thereference to Cave ‘comparatively deep-waterenvironments’ north of the (1979, p. 523) in support of current diversion against the Aberystwyth GritsFormation, at the time of deposition basin high is a misquotation which distorts the meaning. It (Cave 1965; Jehu 1926; Pugh 1923). We, however,cite was not a statementabout ‘SW-directed’ flow, but. . of Woodcock (1984) who suggested that there may have been current-flow to ‘south of west. . .’. The relevant quote from shallow marine or emergent conditions tothe north, Cave (1979)is as follows: ‘In thenorth, around something that would be consistent with our palaeocurrent Machynlleth . . . there are indications that current flow was data. We do, however, envisage a submarine topographic south of west rather than north of it. This difference from high north of Borth against which turbidity currents were more southerly areas may not be indicative of another source deflected and probably derived as dilute flows. or fan, but may be dueto current deflection against a bathymetric slope tothe north.’(Cave 1979, p. 523, our emphasis). Cave & Hains appearto consider the difference in meaning between the terms ‘south-west’ and ‘south of west’ as of supreme importance. Their caveat is simply a matter of semantics. As far as we were concerned the suggestion of palaeocurrents directed to a southerly direction was of great importance since it was at variance with the majority of previously reported occurrences(see references in paper) andsupported our observations of anomalouspalaeocur- rents in the Wallog area. The only point of substance in Cave & Hains’ lengthy critique is theirpoint concerning biostratigraphy which is based upon Loydell’s graptolite data and his comments. We will, therefore, deal with the biostratigraphic arguments in answering Loydell’s critique and direct Cave & Hains toour reply. Part of the problem when looking at the rock succession in the Borth-Aberystwyth area is the lack of good palaeontological datato supplement the lithostratigraphy. Cave & Hains (1986) noted that there was little evidence as Fig. 4. Map of the area between Borth and Clarach Bay showing to whether theBorth MudstonesFormation was truly the variation in palaeocurrent directions. older than, or merely coevalwith, the Aberystwyth Grits

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Formation. Lack of systematicwork on thegraptolitic Basin, during the late Llandovery, situate the area close to faunascoupled with observeddiachroneities led to such the equator (within 20 degrees), it follows that the Coriolis conclusions (Cave & Hains 1986). Our suggestion that the effect would be neglible or at best minimal. rocks to the north of the Wallog area might be younger in Shallow marineor emergent conditions are widely age was based largely on the overall structure together with reported from the northern parts of the Welsh Basin during the lack of reported chronostratigraphic data. After careful the Cambrian (Crimes 1970~)and Ordovician (Brenchley & mapping, we do accept that the northernmost section (Borth Pickerill 1980; Crimes 1970b). The presence, therefore, of a Mudstones Formation of Cave & Hains (1986)) from Borth probabledeep-marine, structurally-controlled high tothe to north of Wallog, is older than the section immediately to north of the Borth area during theLate Llandovery is a the south (see our comments above). reasonableassumption in the context of known earlier If, as suggested by Loydell (and emphasized by Cave & structures. Such a feature would best explain the observed Hains in their discussion), the Borth Mudstones Formationis anomalouspalaeocurrents. The presence of a S-dipping older by half a graptolite zone than we surmised, then our structurally-controlledslope, associated with the Bala inferrednorthern SW-dipping submarineslope furnishing Lineament, wouldhave locally enhanced this effect. The thin-bedded, fine-grained silty turbidites to the Welsh Basin postulatedDerwen Horst is probablythe most likely would haveexisted at a slightly earlierdate than we contender for a structuralhigh. suggested. Thesegraptolite data do not conflict with our basic interpretation;they merely provide tighter a constraint, for which we are grateful. References The area between Borth and Clarach was re-examined BAKER,S. J. 1981. The graptolite biostratigraphy of a Llandovery outlier near by us determineto whether there is any sign of Llanystumdwy, Gwynedd, North Wales, GeologicalMagazine, lUI, interdigitationbetween the mudstone-dominated deposits 355-365. BRENCHLEY,P. J. & PICKERILL,R. K. 1980.Shallow subtidal sediments of (i.e. BorthMudstones Formation) showing northerly- Soudleyan (Caradoc) age in the Berwyn Hills, North Wales, and their derived palaeocurrentdirections, with thesandstone- palaeogeographic context. Proceedings of the Geologists’Association, 91, dominateddeposits (i.e. Aberystwyth Grits Formation) 177-194. which show a southerly provenance (Fig. 4). While the data CAVE,R. 1965. The NodGlas sediments of Caradoc agein North Wales. Geological Journal, 4, 279-298. clearly show thepresence of twototally opposing flows - 1979.Sedimentary environments of the basinalLlandovery of within thesame area there is littleevidence of true mid-Wales. h:HARRIS, A. L., HOLLAND,C. H. & LEAKE,B. E. (eds) interdigitation. The observation could be due to: (a) a subtle TheCaledonides of theBritish Isles-reviewed. GeologicalSociety, onlap of the northernmost and youngest Aberystwyth Grits London, Special Publication, 8, 517-526. Formation on to the Borth Mudstones Formation; andtor -& HAINS,B. A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the BritishGeological Survey, Sheet 163 (b)bedding parallel thrusting of theAberystwyth Grits (England & Wales). Formation on to the Borth Mudstones Formation from the CRIMES,T. P.19700. A facies analysis of the Cambrian Rocks inWales. east. The latter is supported by the presence of many early Palaeogeography, Palaeoclimatology, Palaeoecology, l, 113-170. thrustsand associated, slightly overturned, tight folds as -19706. A facies analysis of the Arenig of western Lleyn, North Wales. observed in the sections north of Clarach. Proceedings of the Geologists’ Association, 81,221-239. -& CROSSLEY,J. D. 1980. Inter-turbidite bottom current orientation from When we wroteour paper, we wereaware of the trace fossils with an example from the Silurian Flysch of Wales. Journal proposedexistence of theDerwen Horst (Fitches & of Sedimentary Petrology, 50, 821-830. Campbell 1987) as a possible structural high at the time of F~Es,W. R. & CAMPBELL,S. D. G. 1987. Tectonic evolution of the Bala Lineament in the Welsh Basin. Geological Journal, 22, 131-153. deposition of the Aberystwyth Grits Formation, but did not HILL,P. R. 1984. Facies and sequence analysis of Nova Scotian slope muds. wish to suggest a specific site for our postulated basin high. In: STOW,D. A. V. & PIPER, D.J. W. (eds) Fine-GrainedSediments: We would, however, concur with both James’s and Smith’s Deep-Water Processes and Facies, Geological Society Special Publication preference for a local SW-facing slope parallel to the Bala 15, 311-318. Fault (see below). JAMES,D. M. D. 1972. Sedimentation across an intra-basinal slope: the Garnedd-wen Formation (Ashgillian), West Central Wales. Sedimentary Both Smith and James in their discussions suggest that Geology, l, 291-307. thecurrent ripplescould indicate local SSE-directed - 1973. The Garnedd-wen Formation (Ashgillian) of the Towyn- overspillfrom a meandering channel/lobe system or even Abergynolwyn district, Merionethshire. GeologicalMagazine. 110, the Corioliseffect. While thesereasons may be used to 145-152. - & JAWS, J. 1969. The influence of deep fractures on some areas of interpretother locations, there is no evidencefrom the Ashgillian-Llandoverian sedimentation in Wales. Geological Magazine, outcropthat the deposits were in any way relatedto 106,562-582. overbank deposits. There is no evidence of channelling in JEHU, R. M.1926. The geology of the district around Towyn and the immediate area although this does not totally preclude Abergynolwyn (Merioneth). Quarterly Journal of the Geological Society the possibility of channels having existed nearby to supply of London, 82,465-489. MCCANN,T. & PICKERING,K. P. 1989. Palaeocurrent evidence of a northern fine-grained overbanksediments. Alternatively, the sedi- structural high to the Welsh Basin during the Late Llandovery. Journal ments could haveresulted from marginal deposition of the Geological Society, London, 146,211-212. on lobes. MCCLELLAND-BROWN,E. 1983. Palaeomagnetic studies of fold development The Coriolis effect can be expressed such that: and propagation in the Pembrokeshire Old Red Sandstone. Tectonophysicr, 98, 131-149. Coriolis effect = 2mPu sin @ KEEPING,W.1878. Notes on the geology of the neighbourhood of Aberystwyth. Geological Magazine, 5, 532-547. where: m = mass of body; C2 = angular rotational velocity of LOCKLEY, M. G. 1980. Caradoc Faunal Associations of the area between Bala Earth (= 7.29 X 10-5 rad S-’); @ = latitude and U = current and Dinas Mawddwy, North Wales. Bulletin of theBritish Museum of velocity. In lower latitudes,therefore, where @ would Natural History (Geology), 33, 165-235. PARKASH,B. & MIDDLETON,G. V. 1970. Downcurrent textural changesin approach zero, the Coriolis effect would be minimal. Given Ordovician turbidite greywackes. Sedimenrology, 14, 259-292. thatpalaeoenvironmental reconstructions for the Welsh PRICE, D.Lk MAGOR,P. M. 1984. The ecological significance of variation in

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D.K. LOYDELL, Institute of EarthStuules, University College of Wales, Aberystwyth, Dyfed SY23 3DB, UK. R. CAVE & B. A. HAINS,British Geological Survey, Bryn Eithyn Hall, , Aberystwyth, Dyfed SY23 4BY, UK. D. M. D. JAMES, ShellOffshore Inc., 701 Poydras, New Orleans, Louisiana 70139, USA. R. D. A. SW. Department of Earth Sciences, The University, PO Box 147, Brownlow Street,Liverpool L693BX, UK. Present address: KSEPL, Shell Research, Volmerlaan 6, 2288 GD, Rijswijk, ZH, The Netherlands. T. MCCANN& K. T. PICKERING,Department of Geology,The University, Leicester LE1 7RH, UK.

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