Fferam-uchaf

[SH 3617 8668], [SH 3618 8655] and [SH 3650 8657]

Introduction

An exposure south-west of Fferam-uchaf is of significance because it is one of only two localities on Anglesey from which the murchisoni Zone has been recognized. It was reported by Greenly (1919, p. 452), who also noted two outcrops nearby that yielded graptolite faunas indicative of the underlying D. artus Zone. All three exposures are within the topmost part of the Nantannog Formation and lie at the northwesterly extremity of its outcrop.

Description

West of the road, at the bend 180 m west of Fferam-uchaf farm [SH 3617 8668], are shales, fine gritty beds with fragments of grey mudstone and thin ribs of fine sandstone that dip north at 30–50°. Bates (1972, p. 40) redetermined Greenly's collection from here as follows:

Didymograptus 'bifidus'(Hall) (probably = D. artus Elles and Wood), D. (Expansograptus) hirundo Salter, Orthambonites? sp. and Paterula? balclatchiensis (Davidson). More or less along strike, 170 m south-east of the farm [SH 3650 8657], similar beds yielded a shelly fauna including brachiopods (Dactylogonia sp., Ptychopleurella sp., and Skenidioides sp.), bryozoans and pelmatozoan columnals. To the south-west of the farm [SH 3618 8655], shales and fine sandstones yielded the graptolite Didymograptus murchisoni (Beck), trilobites (Bergamia? sp., Cyclopyge? sp., Placoparia sp., and Illaenus (sensu lato) sp., an orthid brachiopod and a bivalve).

Interpretation

The mixed shelly–graptolitic faunas from the top of the Nantannog Formation here are probably indicative of fairly shallow-water conditions. The presence of rock fragments within the matrix implies redistribution of an already deposited sediment (Bates, 1972, p. 56). Because beds assigned to the murchisoni Zone crop out to the south of those of the artus Zone in a north-dipping sequence, the former are either downfaulted or folded into their present positions (Bates, 1972, p. 40). At Fferam-uchaf itself, the Fferam ironstone and overlying graptolite shales to the north belong to the Llanbabo Formation (basal Caradoc, gracilis Zone). Although the contact with the Llanvirn is not exposed, it is presumably unconformable or faulted, and there is no evidence for the intervening teretiusculus Zone.

Conclusions

The outcrops at Fferam-uchaf are important in providing fossiliferous exposures of both the Abereiddian (Llanvirn) graptolite zones, artus and murchisoni, in a mixed shelly–graptolite facies. They are complementary to the larger exposures in Nantannog Ravine (see site report).

References