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Lower Palaeozoic |
School of Environmental Sciences, Ulster Polytechnic, Newtownabbey, Co. Antrim BT37 0QB, U.K.
Wales Geological Survey Unit, British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY, U.K.
Department of Geology, National Museum of Wales, Cathay’s Park, Cardiff CF1 3NP, U.K.
Department of Geology, University of Keele, Keele, Staffordshire ST5 5BG, U.K.
This is a modified version of the guide which was prepared for a field discussion meeting, 1–8 September 1982, prior to the Ordinary General Meeting of the Geological Society with the theme ‘Volcanic Processes in Marginal Basins’. The guide describes some of the localities in the Ordovician of Wales which have recently provided much information on processes in an environment interpreted (Kokelaar et al., this volume) as being a marginal basin on the SE side of the Iapetus Ocean. In particular, the volcanic and associated sedimentary and tectonic processes are emphasized.
General geological context of the Lower Palaeozoic volcanism
The geology of Wales is dominated by the Lower Palaeozoic (Fig. 1) and it was here in the 19th century that Murchison, Sedgwick and Lapworth established the Cambrian, Ordovician and Silurian Systems, with their various series. The nomenclature of Lower Palaeozoic stratigraphy (Harland et al. 1982) reveals the influence of these Welsh sequences.
During Lower Palaeozoic times a major sedimentary basin, the Welsh Basin (equivalent to the geosyncline of Jones 1938), was established on a basement of late Precambrian rocks. Its NE-SW orientation was largely determined by faults along its NW and SE margins (Fig. 1). Sedimentation at the edges of the basin was discontinuous and generally of shallow-water clastics or carbonates whilst towards the centre there was slow accumulation of pelagic muds with periodic incursions of turbiditic sands and silts. An estimated 13 km of sediment accumulated towards the centre of the basin and a maximum of 5 km at
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This 250-word extract was created in the absence of an abstract.
* Almost invariably, little can be gained by using a hammer on the rocks described here, whereas a great deal can be lost. Please conserve the outcrops.
