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{{OR/18/139}}
The general graptolite zonation of the Rheidol Gorge succession was documented by Jones (1909)<ref name="Jones 1909"></ref>. For the interval of interest in this study, Jones recognized the Zone of ''Monograptus cyphus'' overlain by the Zone of ''Monograptus communis''. The latter was subdivided into a ''triangulatus''-var band, ''triangulatus'' band, ''magnus'' band, and ''leptotheca'' band, in ascending order. Jones (1909)<ref name="Jones 1909">Jones, O T. 1909. The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd (north Cardiganshire). Quarterly Journal of the Geological Society, 65, 463–537, pls 24, 25. </ref> regarded the base of the ''M. communis'' Zone as occurring at the horizon of the large calcareous concretions, which, in our lithological log (Figure 3), occurs at the 7.3–7.4 m level.

[[Image:OR18139fig3.jpg|thumb|center|600px| '''Figure 3'''    Lithologic log of the studied section showing key graptolite horizons recognized by this study, marker horizons identified by Jones (1909)<ref name="Jones 1909"></ref> and sample intervals of Sudbury (1958), as well as features indicative of relative oxicity (see Table 1), estimated % turbidite beds, and intervals of occurrence of pelagic faunas. ]]
Sudbury (1958)<ref name="Sudbury 1958">Sudbury, M. 1958. Triangulate monograptids from the Monograptus gregarius Zone (Lower Llandovery) of the Rheidol Gorge (Cardiganshire). Philosophical Transactions of the Royal Society of London, 241, 485–555. </ref> employed essentially the same zonation in her detailed study of triangulate monograptids from this section, although she referred the strata above the ''cyphus'' Zone as belonging to the ''gregarius'' Zone, rather than the ''communis'' Zone. Sudbury also recognized that the stratigraphically lowest graptolites indicative of the ''triangulatus''-var band occur approximately 2 m above the top of the level of the large concretions. Sudbury documented only one graptolite-bearing horizon between the concretion layer and the first appearance of triangulate monograptids (her horizon T), and this horizon immediately overlies the prominent level of change from light grey mudstones to black shales in our log at 9.37 m.

Our study employs the graptolite zonation of Zalasiewicz ''et al''. (2009)<ref name="Zalasiewicz 2009">Zalasiewicz, J A, Taylor, L, Rushton, A W A, Loydell, D K, Rickards, R B, and Williams, M. 2009. Graptolites in British stratigraphy. Geological Magazine, 146, 785–850. </ref>. Graptolites were collected mainly by MJM, JAZ and Professor Mark Williams (Leicester University), with assistance from CR, Rosemary Melchin, and James Wilkinson. For this report we focus mainly on the faunas of the ''revolutus'' Biozone (≈ ''cyphus'' Biozone) and the lower part of the ''triangulatus'' Biozone, which we sampled for graptolites in detail (Figure 5). Most of our current interpretation of the graptolite biostratigraphy of the overlying strata is based on the work of Sudbury (1958)<ref name="Sudbury 1958"></ref>. To date we have relatively few new collections from these higher strata. The ranges of the most common and biostratigraphically significant graptolites from the ''revolutus'' and lower ''triangulatus'' biozones are shown in Figure 5 and specimens of these taxa are illustrated in Figure 6.

''Pernerograptus revolutus Biozone''<br>
The lower 10.13 m of our measured section contains graptolite faunas indicative of the ''Pernerograptus revolutus'' Biozone. Some of the indicative species of this biozone are ''P. revolutus, Normalograptus? wyensis'', as well as ''Pernerograptus sudburiae'' and ''Metaclimacograptus undulatus'', which extend into the overlying strata. This biozone also contains the highest occurrences of ''Coronograptus cyphus'' and ''Cystograptus penna''.

[[Image:OR18139fig4a.jpg|thumb|center|400px| '''Figure 4a'''    Delicately laminated, anoxic hemipelagic mudstones (dark grey-black) with fine-grained sandstone and siltstone turbidite beds and laminae (pale grey/brown) (RO = 0; see Table 1) at c.1.32 m (see Figure 3). Lens cap is 5 cm in diameter. The thickest turbidite bed is ‘flag G’ of Jones (1909; see also Cullum & Loydell, 2011<ref name="Cullum 2011">Cullum, A A, and Loydell, D K. 2011. The Rhuddanian-Aeronian transition in the Rheidol Gorge, mid Wales. ''Proceedings of the Yorkshire Geological Society'', '''58''', 261–266. </ref>). ]]

[[Image:OR18139fig4b.jpg|thumb|center|400px| '''Figure 4b'''    Enhanced close-up of hemipelagic lamination, location as Figure 4a. Note well preserved varve-like alternation of dark, organic-rich and lighter silt-rich laminae. Scale bar is 5 cm. ]]

[[Image:OR18139fig4c.jpg|thumb|center|400px| '''Figure 4c'''    Upwards transition from dark grey, diffusely laminated hemipelagic mustones with paler turbidite beds and laminae and sparse burrows (RO = 1) into increasingly burrow-mottled equivalents (RO = 2 to 3), at c.9.15 m (see Figure 3). Scale bar is 3 cm. Note the more diffuse nature of the lithological contacts and more poorly preserved hemipelagic lamination in the lower facies the compared with Figs 4A and 4B and the increasing levels of disruption and loss of definition upwards (see Table 1). ]]

[[Image:OR18139fig4d.jpg|thumb|center|400px| '''Figure 4d'''    Strongly burrow-mottled mudstone (RO = 3; see Table 1) at c.8.50 m (see Figure 3). Scale bar is 21 mm.<br>Note range of burrow sizes including large diameter varieties and presence of branching burrow systems. ]]

The occurrence of ''Glypograptus tamariscus linearis'' in our lowest sample indicates that level is within the middle ''revolutus'' Biozone, as recognized by Zalasiewicz et al. (2009)<ref name="Zalasiewicz 2009"></ref>.

Graptolite samples are sparse and rather poorly preserved in the ~5.5–~8.75 m interval. Between 8.75 and 10.05 m there is a relatively rich and, in some samples, well preserved fauna indicative of the upper ''revolutus'' Biozone. This subzone is marked by the incoming of ''Pernerograptus austerus bicornis'' and ''Glyptograptus tamariscus angulatus''. Taxa that appear in our collections in this interval and extend into the overlying ''triangulatus'' Biozone include ''Coronograptus gregarius, Pristiograptus concinnus'', and ''Pseudorthograptus finneyi''. The last species is characteristic of the Rhuddanian-Aeronian boundary interval in the Prague Basin (Štorch, 2015<ref name="Štorch 2015">Štorch, P. 2015. Graptolites from the Rhuddanian–Aeronian boundary interval (Silurian), Prague Synform, Czech Republic. Bulletin of Geosciences, 90, 841–891. </ref>) and this is the first report of this species in the UK. In addition, fragmentary specimens that we questionably identify as ''Demirastrites brevis'' first appear in the upper part of the revolutus Biozone. This is significant because it suggests that this species, which is characterized by having a few axially elongate proximal thecae, may represent a transitional form between pernerograptids with strongly triangulate proximal-mesial thecae and other species of ''Demitrastrites''.

''Demirastrites triangulatus Biozone''<br>
Our lowest sample containing ''D. triangulatus'' occurs 0.8–0.85 m above the level of the prominent lithological change at 9.37 m. Our correlation with the sample level records of Sudbury (1958)<ref name="Sudbury 1958"></ref> suggests that this sample occurs immediately below her horizon S, which is also her lowest level yielding ''D. triangulatus'' (specimens she assigned to ''Monograptus separatus separatus'', which has more recently been referred to as ''Demirastrites triangulatus separatus''). The data presented by Sudbury showed that ''D. t. separatus'' and ''D. t. triangulatus'' co-occur in many samples and are also intergradational in form. We, therefore, have not regarded these as distinct subspecies for the purpose of this study and note that more quantitative morphometric work is required on this species group.

Sudbury’s (1958)<ref name="Sudbury 1958"></ref> data also indicate that two other species assigned ''Demirastrites, D. praedecipiens'' and ''D? walkerae'' (named ''Monograptus toernquisti toernquisti'' by Sudbury) also occur in the lower part of the ''triangulatus'' Biozone. We have also found the first occurrence of ''Petalolithus'' sp. and ''Glyptograptus enodis enodis'' as taxa first appearing in the lower ''triangulatus'' Biozone.

In the data presented by Sudbury, the base of the middle ''triangulatus'' Biozone of Zalasiewicz ''et al''. (2009)<ref name="Zalasiewicz 2009"></ref> can be recognized by the appearance of ''Pernerograptus austerus sequens''. The base of the upper ''triangulatus'' Biozone of Zalasiewicz ''et al''. (2009)<ref name="Zalasiewicz 2009"></ref> can be identified at the ~16.35 m level in our Rheidol Gorge section by the first appearance of a species most commonly known in Britain as ''Demirastrites triangulatus fimbriatus''. Many authors have regarded this form as being a junior synonym of ''Demirastrites pectinatus''. If this interpretation is correct, then the base of the upper triangulatus Biozone of Zalasiewicz et al. (2009)<ref name="Zalasiewicz 2009"></ref> can be regarded as correlative with the base of the pectinatus Biozone as recognized in some other parts of Europe (e.g. Štorch et al., 2017<ref name="Štorch 2017">Štorch, P, Manda, Š, Tasáryová, Z, Frýda, J, Chadimová, L, and Melchin, M J. 2017. A proposed new global stratotype for Aeronian Stage of the Silurian System: Hlásná Třebaň section, Czech Republic. Lethaia. DOI 10.1111/let.12250. </ref>).

''Neodiplograptus magnus Biozone''<br>
The interval identified by Sudbury (1958)<ref name="Sudbury 1958"></ref> as representing the ''magnus'' Biozone (her horizon X) occurs at a level of approximately 18.65–18.85 m in our measured section. Another level of black shales below this, at 17.45–17.65 m, which is laterally terminated by a fault/slide plane, may also yield specimens of ''Neodiplograptus magnus''. This material is currently under study.

[[Image:OR18139fig5.jpg|thumb|center|500px| '''Figure 5'''    Graptolite biozonation and ranges of selected graptolite taxa for the lower-mid portion of the Rheidol Gorge section sampled by the authors. Black boxes — samples from this study; white boxes — samples from Sudbury (1958)<ref name="Sudbury 1958"></ref>. ]]
[[Image:OR18139fig6.jpg|thumb|center|400px| '''Figure 6'''    Biostratigraphically important and common graptolites from the ''revolutus'' and lower ''triangulatus'' Zones. A – ''Rhaphidograptus toernquisti''; B – ''Pribylograptus sandersoni/incommodus''; C – ''Metaclimacograptus slalom''; D – ''Metaclimacograptus undulatus''; E – ''Pernerograptus sudburiae''; F – ''Pernerograptus revolutus''; G – ''Glyptograptus tamariscus linearis''; H – ''Cystograptus penna''; I – ''Pernerograptus austerus bicornis''; J – ''Coronograptus cyphus''; K – ''Glyptograptus tamariscus tamariscus''; L – ''Normalograptus? wyensis''; M – ''Pseudorthograptus obuti''; N – ''Pseudorthograptus finneyi''; O, P – ''Pristiograptus concinnus''; Q – ''Glyptograptus tamariscus angulatus''; R, S – ''Demirastrites brevis?''; T – ''Coronograptus gregarius''; U – ''Petalolithus sp.''; V – ''Glyptograptus enodis enodis''; W – ''Demirastrites? walkerae''; X, Y, Z – ''Demirastrites triangulatus''. ]]
==References==
{{reflist}}
[[Category:OR/18/139 Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report | 05]]

OR/18/139 Graptolite biostratigraphy - Revision history

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Geosource>Ajhil at 13:51, 4 October 2018