Conodont-distributions in some British Dinantian shelf sediments : the Gayle limestone (Brigantian) of North Yorkshire and selected sections (Courceyan-Brigantian) in the South Wales - Mendip area

Davies, Roger Brett (1981) Conodont-distributions in some British Dinantian shelf sediments : the Gayle limestone (Brigantian) of North Yorkshire and selected sections (Courceyan-Brigantian) in the South Wales - Mendip area. University of Southampton, Doctoral Thesis.

Abstract

New conodont faunas are reported from the Dinantian shelf sediments of the South-West Province of the British Islas and the Brigantian Gayle Limestone of North Yorkshire. 2740 conodont elements are referred to 1 multi-element species, 27 form-genera, 79 form-species and 6 form-subspecies. In the South-West Province, Courceyan rocks have yielded conodont faunas dominated in one or more of the following species: Patroanathus variabilis, Polvgnathuornatus, P.comnunis communist Bispathodus aculeatus aculeatus, B.stab list Pseudopolysnathus dentilineatus and Ps.multistriatue. Basal Courceyan faunas, dominated by g.variabilis, have been reworked into a lagoon environment by storm surges. Succeeding conodont faunas are interpreted as recording biofacies rather than evolutionary trends. Chadian to Asbian rocks have yielded, at best, spares conodont faunas; many are barren. Extremely shallow water environments characterise the rocks of these Stages, particularly those of Holkerian and Asbian age, and resulted in the general exclusion of conodonts. Mestognathus beckmanni and Apatognathus inhabited shallow, shelf environments but were less tolerant of euryhaline conditions than Cavusgnathus. In Brigantian times, generally deeper water covered the shelf of the area and resulted in more diverse conodont faunae dominated by G athodus is rtyi, G_.homopunctatus and Paragnathodus conmutatus. These species inhabited near-surface waters in shelf and offshore environments. The Gayle Limestone formation of the Yoredale Group has been mapped across its entire outcrop area on the Askrigg Block. The depositional environment has been interpreted by palaeoenvironmental analysis of the fossil groups present and from the distribution of rock-types. The diagenetic history of the carbonates has been studied by means of staining techniques. A shelf edge, crinoid shoal separated the Pennine Basin from a 'normal marine' shelf lagoon of lime mud deposition. The lagoon deepened towards the southwest corner of the Askrigg Block. Crinoid bioherms were common in the lagoon environment. Well sorted calcarenites and fine sandstones are restricted to the western half of the Askrigg Block, and were supplied by reworking of previously deposited sediments. Offshore and distal prodelta shales occasionally prograded into the shelf lagoon.The conodont faunas of the Gayle Limestone are dominated by G, iR rtyi, G.homopunctatus, P.commutatus and Apatognathus. Cnathodus bilineatus bilineatue is uncommon end was confined to the deeper part of the lagoon. The proximity of shelf to basin, availability of nutrients, and interactions of wave energy, water-depth and shelf topography strongly influenced conodont distribution, whereas salinity variations, temperature, water turbidity and local substrate had no discernible influence. The conodont faunas display some lateral segregation and a crude depth-stratification. Neither the depth-stratification model of Seddon & Sweet (1971) nor the nektobenthonic model of Barnes & Fahraeus (1975) account for conodont distribution in the Cayle Limestone. It is suggested that a free-swimming habit accounts for the distribution of the majority of conodonts in Dinantian seas. Few species were tolerant of agitated, euryhaline conditions. Only those species inhabiting the shallowest environments may have followed a nektobenthonic mode of life. Conodont zonations of international application should only be erected on the basis of rapidly evolving lineages present in offshore environments.

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