The succession and structure east of the Glaslyn River, North Wales

Abstract

The succession and structure of some mid-Ordovician sedimentary and volcanic rocks are described. Marine slates and grits are succeeded by welded and bedded rhyolitic pyroclastic rocks which are overlain by basic tuffs, lavas, and intercalated slates. A local intravolcanic unconformity cuts out over 5000 ft of strata below part of the Snowdon Volcanic Series. The fragmented debris of the missing strata forms a mixed deposit of breccias which overlies the unconformity in places. Normal faulting of Lower Caradoc age is postulated, and it is suggested that the unconformity is part of a buried volcanic collapse-structure. Timerelations suggest that the collapse was initiated by concealed intrusive activity near Aberglaslyn. The intrusion produced local thermal metamorphism and tourmalinization of the early Caradoc sediments in post-Harnagian and pre-Longvillian times. The main structure is the south-westward continuation of the Dolwyddelan syncline, flanked to the west by a complementary anticline.

A Fissure‐Controlled Acidic Volcanic Centre (Ordovician) at Yr Arddu, North Wales

“…The correlation of the Yr Arddu Tuffs with the Pitts Head Tuff Formation (Beavon 1963) is refuted on the palaeontological, lithological, and geochemical evidence presented in this paper.…”

“…The subdivision of the Yr Arddu Tuffs as distinguished by Beavon (1963) Figure 5h The respective ranges of these analyses (e.g. Nb(x), 25.4 and 27.1 ppm; Th(x), 15-6 and 16.9 ppm; Zr(x), 315-2 and 290.8 ppm) and various element ratios, however, contrast markedly (Tables 2 and 3, Figure 5 ) with those of the Yr Arddu Tuffs/rhyolite association.…”

“…The 1851 Geological Survey map (Old Series 75NE) designated the outlier as 'contemporaneous felsite' although only brief reference, to 'the feldspar porphyry of Yr Arddu', was made in the accompanying memoir (Ramsay 1881, p. 125). Beavon (1963) divided the tuff sequence of the Yr Arddu outlier into the Lower, Middle, and Upper Lapilli tuffs. They were correlated respectively with the Lower and Upper Pitts Head 'Rhyolites' (Pitts Head Tuff Formation) and the Lower Rhyolitic 'Tuffs' (Lower Rhyolitic Tuff Formation) of Moel Hebog (Shackleton 1959) and consequently with horizons in the Snowdon Volcanic 'Series' (Snowdon Volcanic Group) on Snowdon (Williams 1927) and at Dolwyddelan (Williams and Bulman 1931).…”

Longvillian Shelly Faunas From the Dolwyddelan Area, North Wales

“…According to Williams & Bulman (1931, p. 434) the nearest recorded occurrence of the Pitt's Head Rhyolite is at Cerrig Cochion (Fig. 1) in the extreme west of the Dolwyddelan area but Beavon (1963) showed that the nearest outcrops of the lateral equivalent of the Pitt's Head Rhyolite (the Lower and Middle Lapilli Tuffs) crop out around Yr Arddu, approximately 4 km south-west of Cerrig Cochion.…”

Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales

“…2) comprising acid ash-flow tuff, extrusive rhyolite, sedimentary slide-breccias and slumped and reworked tuffs with associated marine sediments. Shackleton (in discussion of Beavon, 1963) proposed that the tuffs were associated with a subaerial volcanotectonic structure about the Snowdon massif. This idea was developed by Rast (1969) and Bromley (1969) into a volcano with a central caldera, rim syncline and caldera fault.…”