Aspects of the micropalaeontology of Devonian strata of North Devon

Abstract

An integrated micropalaeontological investigation has been made of a ca.3km thick continuous sequence of Devonian strata in north Devon (S.W. England), encompassing the Lynton Formation, Hangman Sandstone Group, Ilfracombe Slates and Morte Slates. The sequence comprises interdigitated marine and non-marine sediments of low greenschist metamorphic facies: fossils have previously been considered rare in this succession. Low abundance, icriodid-dominated conodont faunas have been recovered from the Lynton Formation and the Ilfracombe Slates: in total, 33 pectiniform conodont taxa are reported. Polygnathids are confined in their recovery to the Ilfracombe Slates. Despite the uniformly low total organic carbon content of the rocks (0.1-0.3%), diverse palynofloras have been isolated from the marine, or marine-influenced sediments. Initial problems of the oxidation and temporary stability of opaque phytoclasts were overcome, and assemblages found to be dominated by well preserved, terrestrially-derived organic matter, comprising miospores, tetrads, and wood. Acritarchs, chitinozoa and scholecodonts are infrequent. Over 130 species of miospore are described, in addition to 29 acritarch taxa, 11 forms of chitinozoa, and a diverse suite of scolecodont and wood morphotypes. The influence of reworking proved difficult to assess, due to the low yields and generally good preservation of the material. Age determinations are based largely on the miospore assemblages, although conodonts provide independent corroborative evidence. The palynostratigraphy established for north Devon is compared with the zonal schemes of Richardson & McGregor (1986) and Streel et al., (1987). Based on miospore data, the Lynton Formation is interpreted as no older than latest Emsian in age, with the uppermost horizons and overlying Hollowbrook Formation of possibly earliest Eifelian age. Conodont faunas recovered from the isolated, inland section at Watersmeet (of uncertain stratigraphic position, but ?located within the lower third of the exposed Lynton Formation), are of ealy Eifelian aspect. The nonmarine Trentishoe Formation and the majority of the Rawns Formation proved barren. The sediments of the upper Hangman Sandstone Group yielded comparatively abundant, diverse miospore assemblages of ?mid-late Eifelian age. The overlying lower Ilfracombe Slates are interpreted as earliest Givetian in age. Conodont faunas recovered from the Rillage, Holey, Combe Martin Beach and David's Stone Limestone Members are indicative of the Lower/Middle varcus conodont Subzones (≈mid Givetian). The upper Combe Martin Slates are considered late Givetian in age, with the conformably overlying Kentisbury Slates attributable to the Frasnian Stage. A marked depletion in the miospore assemblages approximates to the Kentisbury/Morte Slates boundary. The Morte Slates yielded miospores and acritarchs typical of the Frasnian Stage, although the topmost horizons may possibly be of earliest Famennian age. The conodont faunas are interpreted as pectiniform lags. The sole recovery or predominance of Icriodus over narrow-platformed polygnathids suggests that a shallow-water marine environment prevailed during the deposition of parts of the Ilfracombe Slates. A detailed palynofacies study of the sequence was limited by the paucity of indigenous marine palynomorphs. However, a distinct dependency between scolecodont occurrence and the more proximal, shallow-marine facies has been observed. An extensive geothermometry study integrated conodont C.A.I. values and vitrinite reflectance data. Vitrinite reflectivity is uniformly high throughout the studied sequence, and the lack of dependency of Rm on stratigraphic position suggests that coalification was syn-orogenic. By applying the time independent conversion formula of Barker & Pawlewicz (1986) to this vitrinite data, the sediments are interpreted as having been influenced by temperatures in excess of 300^oC. This study demonstrates the value of integrating both palynological and conodont data in a regional biostratigraphical study, and furthermore identifies the Devonian sequence of north Devon as an important, continuous reference section for Devonian miospore palynostratigraphy.

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