OR/18/139 Abstract

Melchin, M J1, Davies, J R2, De Weirdt, J3, Russell, C4, Vandenbroucke, T R A3, Zalasiewicz, J A. 2018. Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report. British Geological Survey Internal Report, OR/18/139. 1  Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada 2  Department of Geography & Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK (also British Geological Survey, Cardiff Office, Cardiff University, Main Building, Park Place,Cardiff, CF10 3AT, Wales, UK) 3  Department of Geology, Ghent University, Krijgslaan 281 S8, BE-9000, Ghent, Belgium 4  School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 5  Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK

Rheidol Gorge, approximately 17 km west of Aberystwyth, mid Wales, exposes a continuous succession of strata from the middle part of the upper Rhuddanian Coronograptus cyphus Biozone through the lower Aeronian Demirastrites triangulatus (= Monograptus triangulatus) Biozone. Parts of the Aeronian succession are well known for their beautiful lower Aeronian graptolites preserved as pyrite internal moulds. We measured this section and sampled for graptolites, palynomorphs, and for lithological and geochemical analyses. One of our objectives was to assess the section for its suitability as a candidate for a new Global Stratotype Section and Point for the base of the mid-Llandovery Aeronian Stage.

The succession alternates between bioturbated grey mudstones lacking in graptolites and laminated, graptolitic black shales. The black shales commonly show thin, interbedded siltstones. The grey mudstones are interpreted to represent deposition under oxic to dysoxic conditions, the black shales an anoxic seafloor environment. The strata have undergone low-grade metamorphism, commonly with a weakly to moderately developed cleavage, and the graptolites often show ductile and/or brittle deformation.

Strata of the middle to upper cyphus Biozone are c.10.2 m thick and yield graptolite faunas of varying diversity and preservation quality. 0.8 m below the base of the D. triangulatus Biozone there is a change from predominantly organic-poor mudrocks with interbeds of darker shales with sparse graptolites to an interval of predominantly black shales with a relatively rich graptolite fauna. The graptolites in the black shale interval, which spans the zonal boundary, are flattened or in partial relief, commonly deformed, and the strata tend to break along cleavage rather than bedding planes. Nevertheless, a distinctive graptolite fauna occurs through the boundary interval that allows good correlation with successions in other parts of the world. The base of the D. triangulatus Biozone is marked by the first appearance of D. triangulatus. Other species first appearing just below the base of the D. triangulatus Biozone that are useful for international correlation include Pristiograptus concinnus and Pseudorthograptus finneyi. Strata rich in well-preserved, pyritic graptolites become common about 2.3 m above the base of the triangulatus Biozone.

Chitinozoans are poorly to moderately well preserved in the section and indicate the Spinachitina maennili Biozone through the boundary interval, without any significant faunal changes, as is the case in many other parts of the world.