OR/18/139 Graptolite biostratigraphy

Melchin, M J1, Davies, J R2, De Weirdt, J3, Russell, C4, Vandenbroucke, T R A3, Zalasiewicz, J A. 2018. Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report. British Geological Survey Internal Report, OR/18/139. 1  Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada 2  Department of Geography & Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK (also British Geological Survey, Cardiff Office, Cardiff University, Main Building, Park Place,Cardiff, CF10 3AT, Wales, UK) 3  Department of Geology, Ghent University, Krijgslaan 281 S8, BE-9000, Ghent, Belgium 4  School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 5  Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK

The general graptolite zonation of the Rheidol Gorge succession was documented by Jones (1909). 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) 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.

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Figure 3    Lithologic log of the studied section showing key graptolite horizons recognized by this study, marker horizons identified by Jones (1909) 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) 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). 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). 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
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.

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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).

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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.

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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).

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Figure 4d    Strongly burrow-mottled mudstone (RO = 3; see Table 1) at c.8.50 m (see Figure 3). Scale bar is 21 mm.
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).

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) 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
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) 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) 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) can be recognized by the appearance of Pernerograptus austerus sequens. The base of the upper triangulatus Biozone of Zalasiewicz et al. (2009) 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) 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).

Neodiplograptus magnus Biozone
The interval identified by Sudbury (1958) 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.

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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).

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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.