Short Paper: Palynological evidence from the Porthleven area, south Cornwall: implications for Devonian stratigraphy and Hercynian structural evolution

Abstract

Detailed structural and lithological mapping combined with a paly-nological study of Devonian sediments in the Porthleven area, have revealed that previous structural and stratigraphic interpretations are seriously in error. Palynomorphs recovered from sediments classically assigned to the Gramscatho Group and generally regarded as pre-Famennian indicate a late Famennian age. A reinterpretation of the section is proposed which emphasizes the importance of imbricate thrusting and local facies variations. Interpretations of the Devonian geology of south Cornwall have been hindered by the lack of palaeontological control on the ages of the main lithostratigraphic units. This paper details new palynomorph assemblages which necessitate a reinterpretation of the structure and stratigraphy of the south Cornish nappe and thrust terrain (Rattey & Sanderson 1984; Barnes & Andrews 1986; Holder & Leveridge 1986). Palynology. A suite of 14 samples was collected from the parautochthonous Mylor Slate Formation and allochthonous Portscatho Formation (Holder & Leveridge 1986), on the west coast of the Lizard Peninsula (Fig. 1). Palynomorphs were recovered from three samples representative of the sequence exposed immediately southeast of Loe Bar [SW646237-SW647234]. Within this section, traditionally referred to the Gramscatho Group, sediments of both Mylor and Gramscatho affinity are intercalated. The finer grained (Mylor-type) lithologies were preferentially sampled and yielded comparatively diverse, rich and remarkably well-preserved (although opaque) palynomorph assemblages. Yields range from 22–146 specimens per gramme of sediment. The kerogens were oxidized using potassium chlorate and fuming nitric acid, and in transmitted light were found to be composed almost exclusively of miospores, with occasional acritarchs