Contribution à la connaissance du précambrien récent en Europe Occidentale et développement géochronologique du Briovérien en Bretagne (France)

The study of Precambrian formations in central and western Europe has interested geologists for a long time. Most of these presumed Precambrian rocks are actually gneisses, whose last metamorphism occurred in the Hercynian orogeny, so that chronostratigraphic investigations (that is, investigations of the typical stratigraphic evolution) are not possible. However, there are several regions where Precambrian rocks have not been transformed by the later Hercynian orogeny; these are situated in Bohemia (Czechoslovakia), in Thuringia (Eastern Germany), in the Cantabrian Mountains (Spain), and in several regions of France, especially in Brittany. All of the rocks in these regions are petrographically rather monotonous and fossils are rare and of untypical species. Only the combination of classical geological methods with isotopic analyses (K-Ar and Rb-Sr) of suitable rocks has permitted the development of a chronological and stratigraphical scheme in Brittany.The oldest rocks, highly metamorphosed, belong to the Pentevrian, with ages of 900–1200 m.y. The Pentevrian is an orogenic unit and not a stratigraphic one. After a period of 150–200 m.y. for which no record of sedimentation and erosion remains, the upper Precambrian (Brioverian) stratigraphie system began. Older ages than the Pentevrian are presently known only in northern Europe.The stratigraphic subdivisions previously proposed for Normandy and Brittany can be confirmed by radioactive datings. The system began with arkoses and basaltic volcanic ashes (700 m.y.), followed by important submarine diabases (670 m.y.). Following them, the middle Brioverian began without any tectonic movement, with some thin horizons of black silica schists of about 630 m.y. and continued with very fine-clastic schists and shales several hundred meters thick. After a tectonic phase, the upper Brioverian period commenced, with important phyllitic (flysch) series, which show in the upper parts tillites and other glacial phenomena. This period was followed by important tectonic movements, folding, and granitic intrusions, including the great granitic pluton in western Normandy at 560 m.y. After an evidently short period of erosion the Georgian (Cambrian) period commenced, with a transgression conglomerate.These results are briefly compared with the evolution of the younger Precambrian in the other European regions. Except for northern Europe and the British Isles the same stratigraphic and tectonic scheme appears in each region.