Revision of the age of magnetization of the Montmartin red beds, Normandy, France

Abstract

Summary. A new roadcut has enabled us to sample the south-dipping limb of the Montmartin syncline for a palaeomagnetic reevaluation of an earlier result published by Jones, Van der Voo & Bonhommet. In combination with the results previously published in 1979 for the north-dipping beds of the syncline, a conclusively negative fold test is obtained. The resulting magnetization (declination/inclination =206°/-3°, α95= 12°, palaeopole at 38°S, 325°E) is interpreted to be of Late Carboniferous age, not Late Devonian as thought earlier. Simultaneously, we have re-evaluated the age of the rocks, previously thought to be Late Devonian on the basis of Acritarchs, Chitinozoans and spores. It has not been possible to reconfirm these fossils, not even in the same samples as studied originally; in contrast, the regional presence of Early Palaeozoic fossils suggests to us an age similar to that of other red beds in the Arrnorican Massif, which have been dated as Early Ordovician. The geodynamic implications of our finding that the Montmartin rocks are completely remagnetized, however, are of no great consequence for the geodynamics of the Hercynian belt. Pre-folding magnetization obtained from Silurian and Devonian rocks in Spain and Germany argue for the same conclusion as reached erroneously in our earlier study, namely that the Armorican Massif and adjacent parts of Hercynian Europe were adjointed to North America, Great Britain, the Baltic Shield and the Russian Platform since at least Late Devonian time. If a Medio-European ocean existed during the Palaeozoic, it was virtually closed before the mid-Devonian and of insignificant width during Culm deposition in Early Carboniferoirs time.