Llanrhychwyn Slates and Black Slates of Dolwyddelan - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75
|From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)
Chapter 4 Llanrhychwyn Slates and Black Slates of Dolwyddelan
The Llanrhychwyn Slates are correlated with the Black Slates of Dolwyddelan on the basis of their lithologies and their relationship to the underlying volcanic rocks. The slates are graptolitic and all the faunas collected during the survey have been assigned to the Diplograptus multidens Zone (p. 70) which Skevington (1969) advocated to replace the Climacograptus peltifer and C. wilsoni zones–individually unrecognisable in North Wales. Limitations to the usefulness of the determinations are the predominance of biserial graptoloids, the poor state of preservation and the uncertainty as to the limit of the D. multidens Zone, particularly as this zone may overlap with the overlying Dicranograptus clingani Zone (Skevington, personal communication, 1971). Thus the present determination of the D. multidens Zone and the earlier recognition of the D. clingani Zone in the Black Slates of Dolwyddelan (Bulman and Williams, 1931) should not be read together as meaning, necessarily, that both zones are present either wholly or in part within the district.
The problem is further accentuated by a lack of agreement as to the general relationship of the graptolite zones to the stages based on shelly faunas. Both faunas occur within black argillaceous sediments with phosphatic nodules (the Nod Glas horizon) in the Welshpool area, where Cave (1965) assigned the graptolites to the D. clingani Zone and the shelly forms to the Actonian and Onnian stages, the Actonian resting non-sequentially on the Longvillian. On that basis, Cave's further correlation of the Nod Glas with the Black Slates of Dolwyddelan would imply that the latter also represent the Actonian and Onnian stages. Apart from the questionable precision of the D. clingani faunas, however, there are no known non-sequences or shelly faunas in the Black Slates of the Capel Curig and Betws-y-Coed district.
The junction between the Llanrhychwyn Slates and the underlying Crafnant Volcanic Group is not exposed in the two areas where the slates crop out. The largest area is elongated from NE to SW along the core of a syncline extending from Ty'n-y-Groes to Llyn Goddionduon. Exposures there are numerous, but they are generally small, except in old quarries, as on the south side of Coed Tal-y-Llyn, where about 35 m of thickly bedded, well cleaved bluish black mudstone with hard iron-rich bands are exposed.
In the other area of outcrop, east of the Llyn-y-Parc Fault, exposures in a stream section near Aber-llyn Mine show the slates to be brecciated and mineralised with associated sulphurous weathering. Farther north the outcrop is largely obscured by scree, but a roadside exposure [SH 8000 5854], 0.5 km N of Cwmlanerch, yielded graptolites indicating a D. multidens Zone age (p. 70).
Black Slates of Dolwyddelan
The Black Slates are black, 'sooty' and graptolitic with pyrite along bedding and cleavage. They overlie the Snowdon Volcanic Group forming the core of the Dolwyddelan Syncline. Within the district they are exposed between Tan-y-Castell in the east and Ty'n-y-Ddol Quarry in the west, and have been quarried extensively in the past at Chwarel Ddu and Ty'n-y-Ddol. The junction with the underlying volcanic rocks is exposed only in the north side of Chwarel Ddu where it shows evidence of slipping or thrusting.
Bulman and Williams (1931) assigned graptolites from the Black Slates to the D. clingani Zone. Faunas collected during the present survey indicate the D. multidens Zone, but after examining Bulman's material Professor Skevington and Dr R. B. Rickards confirmed that it represents a clingani Zone assemblage.
Williams and Bulman (1931) assumed that the Black Slates of Dolwyddelan were deposited in shallow water because they conformably overlie the supposed shallow-water admixed lithologies of the 'Upper Rhyolite Series'. They accepted the graptolites as evidence of water circulation, taking the restricted fauna, the occurrence of pyrite and the black colour of the slates to indicate a lack of oxygen and circulation where the muds were deposited. Cave (1965) assumed that the Nod Glas sediments, the presumed correlatives of the Black Slates, were similarly deposited in shallow water covering a rising shelf. He thus explained the non-sequences between the shallow-water Caradoc sediments below and Ashgill sediments above the Nod Glas as well as the phosphorite that forms a conspicuous part of a condensed Nod Glas sequence in the Bala–north Montgomeryshire area. Such an uplifted shelf implies gradients not only to the deeper water basin which lay to the east and which was receiving elastics from the east, but also to deeper water in the west and north, in the area of the Capel Curig and Betws-y-Coed district.
Although contrary to the view of Bulman and Williams (1931), this deeper water interpretation for the Black Slates of Dolwyddelan is here preferred. It explains the upward passage from the coarse terrigenes of the Carneddau Group to the black slates of Middle Crafnant and subsequent formations. Moreover, it is improbable that such an uplifted landmass to the north would have been exhausted as a source of coarse sediments in a volcanically active area and that a wide area of North Wales was fortuitously shielded from that source. It is more likely that the landmass was progressively submerged and that the depositional basin was covered by deeper water than hitherto in the Caradoc.