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Le Devonien du Massif Armoricain
In the middle Armorican synclinorium the Lower, Middle and Upper Devonian are present in very variable thicknesses. The Lower Devonian can be recognized throughout the median synclinorium; the Middle Devonian is well developed in the west, while north of Rennes and in the east of the synclinorium it is very much reduced. The Upper Devonian crops out only in the Finistere region.
In the St. Julien-de-Vouvantes fold, the Upper Devonian, the youngest representative of the stratigraphic column present, is represented only by shaly sand and limestone of Famennian age which is known, among other places, at la Derouere, la Vallee and la Briantiere. At the last outcrop, at least, it is transgressive and lies discordantly on the Lower Devonian, and includes a horizon of lydian stones and phosphatic nodules.
In the Acenis fold only the Frasnian, consisting of argillaceous limestones with lydian stones, has been determined palaeontologically; its distribution on the two flanks of the syncline is more extensive than was previously thought. It represents the transgressive base of the “Culm graywacke” which starts in marine facies and then becomes lagoonal, shaly-arenitic and conglomeratic, commonly red and green, a truly comprehensive sequence which perhaps embraces in addition to the Frasnian, the Famennian, Dinantian and even lower Namurian.
Devonian shelly Bivalve faunas are relatively poorly developed in the Armorican region, and, for a variety of reasons, it is difficult to prepare an adequate account of the Bivalves not only of this but also of other European regions.
The Devonian Bivalves can be accomodated in a revision of L. R. Cox’s (1960) classification.
Distribution and nature of the Bivalves in the Devonian rocks of the Armorican region present a few characteristic features. During different epochs, there was a relative scarcity of the Bivalves, strong environmental control upon their distribution, which is possibly the explanation of the appearance of some stocks later in the Armorican region than in others, and the development of endemic species.
However, general distribution of the Bivalves cannot readily be explained only in terms of variations of facies: Rhenish, Bohemian, and intermediate. There is a suggestion of east to west migration of the Bivalve stocks with, at times, an “exhaustion” of some of them.
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