The Nod Glas sediments of Caradoc age in north Wales

Abstract

Twenty feet to thirty feet of black shale, phosphorite and limestone exposed near Welshpool, Montgomeryshire,exhibit a facies which enables a correlation to be made between the Welsh graptolitic Nod Glas shales of Caradoc age and the shelly Onnia Beds of south Shropshire.Graptolitic evidence suggests that the visible thickness of the Nod Glas near Welshpool belongs to the top of the Dicranograptus clingani zone, and the shelly fauna indicates the zone of Onnia gracilis.At the base of the Nod Glas throughout Montgomeryshire is a phosphatic horizon of variable development .

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

“…The phosphatic horizon at the base of the Nod Glas at Welshpool has been shown to be Onnian in age by Cave (1965) and it seems likely that this distinctive horizon is synchronous throughout its development. The Onnian age for the phosphorite suggests there is a considerable hiatus in deposition at its base where the Marshbrookian and Actonian are missing.…”

“…As Whittington (1968 When the Pen-y-garnedd Formation and Gelli Grin Calcareous Ashes are considered as a single regional lithostratigraphic unit it appears likely that the base of this unit is diachronous provided that the appearance of E. alifrons is synchronous throughout the area. Estoniops alifrons appears above the Cymerig Limestone at Bala (nearly 50 m above the base of the Formation) 10 m above the base of the Pen-y-garnedd Formation in the west Berwyns, but at the base of the formation at Meifod and Welshpool (Cave 1965;Pickerill 1974). …”

“…The Pen-y-garnedd Formation has been restricted in this account to the calcareous beds below the shales and both outcrops are included in the Blaen-ytwm Formation. It has long been the practice to refer to the black shales, which are widespread in north Wales at this horizon, as the Nod Glas (Cave 1965) and this informal stratigraphic term is retained when discussing their regional development.…”

“…However the lithology of these two lithostratigraphic units is sufficiently different to warrant the establishment of two separate formations, particularly as they are separated by a significant disconformity (Cave 1965). The Pen-y-garnedd Formation has been here restricted to the Pen-y-garnedd Limestone, and the Pen-y-garnedd Shales have been referred to as the Blaen-ycwm Formation.…”

Arenig to Llandovery faunal distributions in the Caledonides

“…This situation continued until almost the end of the Ashgill when most of the shallow-water Richmondian brachiopods and trilobites of America are different from their Rawtheyan contemporaries in the Anglo-Baltic area even though many of the sedimentary facies, which included large bioherms with all their attendant diversity of faunas, are similar on both sides of the Iapetus Ocean. Caradoc tectonic activity in N Wales was sufficiently intensive to obliterate the effects of the transgression locally (Fortey & Cocks 1986), but deeper-water conditions, represented by the condensed Nod Glas deposits (Cave 1965), were quickly re-established. These local variations in topography are also reflected in the distribution of the subsequent lower Ashgill trilobite communities (Price & Magor 1984).…”