Rhobell-y-Big–Foel Gron

[NY 7859 2845] and [NY 7882 2827]

Introduction

Localities in the Dolgellau Formation between Rhobell-y-big and Foel Gron yield the best faunas of the Peltura scarabaeoides Zone in Wales. These are of value for correlation with those from the measured section at Ogof Ddû and the Scandinavian standard succession.

The Peltura scarabaeoides Zone is recognized on both east and west sides of the Harlech Dome, but whereas the material collected to the west (from Penmorfa and Garreg-wen (Salter, 1866b, p. 250) and from Ogof Ddû (see site report)) is commonly strongly deformed, the localities between Foel Gron and Rhobell-y-big yield relatively well-preserved specimens. Various localities hereabouts were evidently known to early collectors and appear to include the type localities of some trilobite species described by Belt (1868), but the present localities were described more exactly by Wells (1925, p. 467) and Allen et al. (1981).

Description

Two main streams drain the moorland between Rhobell-y-big and Foel Gron. The lower reaches of the western stream [NY 7869 2907] to [NY 7880 2921] cross the lower part of the Dolgellau Formation (spinulosa Zone), but the upper part reveals sporadic exposures of the upper Dolgellau Formation, which consists of soft, black, pyritous mudstone that is finely laminated and weakly cleaved. Exposures are not extensive and the folding and faulting preclude assessment of the superposition or thickness of these beds. Locally [NY 7859 2845] trilobites occur plentifully. Allen et al. (1981, p. 314) recorded a fauna representing the bisulcata Subzone of the scarabaeoides Zone: Ctenopyge (Ct.) bisulcata (Phillips), Ct. (Ct.) directa Lake, Ct. (Ct.) falcifera Lake, Ct. (Ct.) pecten (Salter), Lotagnostus trisectus (Salter), Parabolinella aff. caesa Lake, Parabolinites? williamsonii (Belt), Peltura scarabaeoides scarabaeoides (Wahlenberg) (Figure 3.8)d and Sphaerophthalmus humilis (Phillips). They are flattened but fairly well preserved (Allen et al., 1981, pl. 2, figs 1, 2, 4, text-fig. 5). The next stream to the east yields a different assemblage representing a slightly higher horizon, possibly the linnarssoni or lobata Subzone, including L. trisectus, Micragnostus sp., Parabolinites? longispinus (Belt), P. scarabaeoides westergardi Henningsmoen, Plicatolina cf. quadrata Pokrovskaya and Sphaerophthalmus major Lake [NY 7882 2823] and [NY 7882 2827]. Earlier collectors also found Parabolinites longispinus (Belt) and Hedinaspis? expansa (Salter) hereabouts. The lower stretch of the eastern stream crosses higher beds of the Dolgellau Formation and yields good faunas of the Acerocare Zone (locality A of Rushton, 1982, p. 44).

Interpretation

The faunas consist entirely of trilobites and include only agnostids and olenids, forms that were adapted to dysaerobic environments typical of the Dolgellau Formation, also evinced here by the lack of bioturbation. The species of Ctenopyge reported from the bisulcata Subzone allow correlation with the bisulcata Subzone of the Scandinavian sequences (Henningsmoen, 1957) and are the same as those recorded from the White-leaved Oak Shale of the Malvern area (Worssam et al., 1989). None of the Ctenopyge species is known from the measured section at Ogof Ddû, and the bisulcata Subzone has not yet been identified there with certainty. The fauna of the linnarssoni–lobata Subzone has species in common with the corresponding fauna from Ogof Ddû (Howells and Smith, 1997), but at Rhobell it includes the olenid Plicatolina (known from a single specimen) and lacks both brachiopods and the remopleuroid trilobite Ricbardsonella? invita (Salter) known from Ogof Ddû.

Belt's specimens are generally poorly localized and his publications state only that various of his species, such as Parabolinites? longispinus, williamsonii and Pseudagnostus obtusus, are from 'near Rhiw-felyn' [SH 781 292] (Belt, 1868, pp. 9–11); but the Geological Survey investigations (Allen and Jackson, 1985) suggest that the streams draining Rhobell-y-big and Foel Gron are probably the type localities of some of Belt's species.

Conclusions

These localities reveal good faunas of trilobites of the Peltura scarabaeoides Zone, that were adapted to living in poorly oxygenated sea water such as characterized many areas during the later Cambrian. These faunas compare closely with others from parts of England, Wales and Scandinavia and allow an exact correlation.

References