The caradocian rocks of the north and west berwyn hills, north wales

Abstract

The Caradoc of the Berwyn Hills is a thick sequence of marine mudstones and siltstones with interstratified sub-aerial tuff formations. Near the top of the succession there is a sharp facies change to graptolitic shales. The sequence has been remapped and two new formations (the Pen-plaenau Siltstone Formation and Cwm Rhiwarth Siltstone Formation) and one new member are proposed. Assemblage zones can be traced round the outcrop in the lower part of the sequence (Soudleyan) but higher in the succession (Longvillian) there are differences between the faunal assemblages of the north and west. The northern faunas are referred to Dulmunellu community which is believed to have occupied a shallower environment than the Nicolellu community to the west. In spite of the presumed environmental control of the faunas, the sequence can be fairly precisely correlated with the classic sections in Shropshire and Bala.The Caradoc sedimentary sequence represents a mainly shallow sub-tidal environment with periodic episodes of local emergence. The area deepened during the Longvillian, particularly in the west. There followed a period of nondeposition with probably some erosion at the base o f the Nod Glas which itself represents a period of reduced sediment.There is a well-known stratigraphic break at the base of the Ashgill which rests on rocks of Onnian age and on different horizons of the Longvillian.

Palaeomagnetism of Late Ordovician igneous intrusions from the northern Welsh Borderlands: implications to motion of Eastern Avalonia and regional rotations

“…1). Discordant intrusive contacts occur with host volcanoclastic sediments of Caradocian age (Wedd et al 1929;Brenchley, 1978). Although considered to be Devonian in age by Wedd et al (1929), Dixon (1990) has drawn attention to the regional tectonic setting and the strong similarities with the Moel-y-Golfa Andesite.…”

A new species ofSiluraster(echinodermata: Asteroidea) from the Late Ordovician of North Wales

“…Groom and Lake (1908) record Palaeaster obtusus (now Stenaster obtusus ) from the Dolhir Formation (Ashgill), with no exact locality given. A ‘starfish’ was also mentioned by Brenchley (1978) in his paper on the Caradoc of the Berwyn Hills, but no details of species, locality or formation were given, and we have been unable to trace the specimen. Here we describe two superbly preserved specimens of a demonstrably new species of Siluraster , most probably from the Caradoc Teirw Siltstone or Bryn Siltstone formations, as discussed below.…”

“…Two sites have yielded Stenaster obtusus (Forbes, 1848) and four sites Protaster salteri Salter, 1857; there are two individuals of the former and perhaps a dozen of the latter species (see Table 1). The Frondderw Tuff is equivalent to the Pen Bwlch Tuff of the west Berwyns and corresponds to a horizon in the middle of the Teirw Siltstone Formation (Figure 1) in the Ceiriog Valley (Brenchley, 1978; Fortey et al ., 2000).…”

“… Composite section showing the local geological succession, based on Groom and Lake (1908), Wills and Smith (1922), Brenchley (1978) and Hillier (1981). …”

“…Later Caradoc and the earliest Ashgill Series are not identified in, or to the east of, the Ceiriog river valley, the presumed unconformity being followed by further shallow-water sedimentary rocks of the Cautleyan to Hirnantian Stages of the Ashgill. Brenchley (1978) While the underlying formation at the collection site is recorded as Bryn Siltstone (Hains and Davies, 1994;Schofield and Waters, 2000), and there is extensive outcrop nearby, in a glaciated area, this may not necessarily be the source of the slab. However, the block was associated with numerous other fossiliferous blocks that overwhelmingly indicate an Ordovician age.…”