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 Borth and the Wallog-Borth region in the northern part of the outcrop of formationstudied is citedas the AberystwythGrits. Two the Telychian (Upper Llandovery)Aberystwyth 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 Wales 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‘ (at Harp 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 Caradoc times. Such a proposal 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/5/885/4890513/gsjgs.147.5.0885.pdf by guest on 27 September 2021 886 DISCUSSION 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 the Corns 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.