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Case Studies |
Faculty of Applied Sciences, Luton College of Higher Education, Park Square, Luton, Beds. LU1 3JU, UK
This guide to the mid-Ordovician oolitic ironstones of North Wales describes the more readily accessible localities, illustrating the main ironstone types and showing their depositional and post-depositional features. The ironstones are exposed entirely in abandoned mines and adits (Fig. 1). Other ironstones localities are mentioned where pertinent and are located by a grid reference number only.
Within the Lower Palaeozoic North Wales Basin (Kokelaar et al. 1984) oolitic ironstones occur as discontinuous lenses within fine-grained shelf siliciclastic sediments. These ironstones formed the study of a PhD thesis (Trythall 1988), and have been previously studied by Trythall et al. (1987), Weinberg (1973), Pulfrey (1933) and Strahan et al. (1920). There are two phases of ironstone deposition in the North Wales Basin (Trythall et al. 1987), a minor Upper Arenig phase (Beckley 1987) and, the subject of this field guide, a mid-Ordovician (upper Llanvirn to basal Caradoc) phase. The majority of the mid-Ordovician ironstones were formed during the eretiusculus Biozone regression or subsequent gracilis Biozone transgression. They were deposited on shallow water shoals formed by synsedimentary faulting, above a stratigraphic hiatus (Trythall et al. 1987).
During the closure of the Iapetus Ocean the Welsh Basin was uplifted and deformed, forming major north-east to south-west trending folds with an axial planar cleavage (Coward & Siddans 1979). Faulting predominantly follows the same trend. Maximum metamorphic grades attained in North Wales were low greenschist facies in Central Snowdonia and prehnite—pumpellyite facies elsewhere (Bevins & Rowbotham 1983; Roberts & Merriman 1985).
Mining of the mid-Ordovician oolitic
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