The stratigraphy and structure of part of west Finistère, France

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Abstract
The area described includes the Plougastel and Crozon peninsulas and the south-west Pays de Léon. There are two distinct rock sequences: Pre-Cambrian Brioverian greywackes and their metamorphic equivalents, overlain unconformably by Lower Palaeozoic and Devonian shelf sediments. The Brioverian sediments were folded and metamorphosed during the late Pre-Cambrian Cadomian orogeny. The main folding occurred before the emplacement of the Gneiss de Brest granodiorite (previously regarded as granitized Brioverian) and impressed on the area an ene–wsw axial-plane cleavage. There followed regional metamorphism that reached almandine amphibolite grade. After peneplanation, Lower Ordovician seas transgressed northwards across the area and subsequently sedimentation continued, though with breaks, until Middle Devonian times. Posthumous movement on ene–wsw Cadomian structures influenced sedimentation from time to time. A shelf sea stretched southwards to Iberia and was bounded immediately to the north of the area studied by a land mass whose northern limit is unknown. Variscan movements threw the Palaeozoic sediments into asymmetrical folds with trends parallel to the ene–wsw Cadomian structures whilst the underlying Brioverian rocks deformed largely by shearing on pre-existing planes. Along the Elorn Valley Variscan folding and faulting combined to produce a major dislocation zone that separates the dominantly crystalline Pays de Léon from the Palaeozoic sedimentary area to the south. The ene–wsw structural trend of western Brittany is of Pre-Cambrian initiation. Structures of similar trend in the English Channel may well have been initiated at the same time.