Abstract

Two different successions of Middle and early Upper Devonian rocks are described from the north coast of Cornwall around Padstow. These have been named the Trevone and Pentire successions. The former is seen especially south and west of Camel Estuary and is characterised by a tectonic style of overturned or recumbent folds facing north. The Pentire succession seen north and east of the outer part of the Camel Estuary shows greater tectonic complexity with a southward facing recumbent fold style. A correlation of these successions with each other has been attempted and faunas enable correlation with the international scale. Some interpretation of the depositional environments is given and estimates of the thickness of units have been made. The axis of the St. Minver synclinorium of the literature approximately separates the two successions.

The Trevone Succession in order of deposition comprises Trevose Slates (upper Eifelian and Givetian and at least 490 m thick), Marble Cliff Beds of turbiditic limestones (lowest Frasnian and at least 67 m thick), Longcarrow Cove Beds of grey slates with agglomerates and tufts (probably low Frasnian and perhaps as much as 300 m thick), Merope Island Beds of goniatite-bearing grey slate (mid Frasnian and some 60 m in thickness), Harbour Cove Slates of grey slate with purple bands and many goniatite beds (mid Frasnian of about 580 m thickness), Polzeath Slates of colourful purple and green slate (presumed late Frasnian and earliest Famen-nian seen to 120 m).

The Pentire Succession shows closely similar Trevose Slates and Polzeath Slates, the latter seen to some 200 m. Marked differences are shown by the intervening sequence which appears complete and comprises the Pentire Pillow Lava Group some 450 m thick, comprising slates, pillow lavas, volcanic tilloids and tufts, the Pentire Slates some 340 m thick, the Gravel Caverns Conglomerate, a remarkable group some 90 m in thickness of slates, mud flow conglomerates and volcanics with mid Frasnian goniates. The latter is overlain by the Polzeath Slates.

Tectonic juxtaposition of two essentially deep water argillite successions is invoked to explain the present proximity of the two differing successions. Some comparisons are made with equivalent successions in South Cornwall and South Devon.

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