paleoweathering in southern French Massif Central ...

1 downloads 0 Views 17KB Size Report
(1) Bureau de Recherches Géologiques et Minières, 3, av. Claude Guillemin – BP 36009, 45060 Orléans – Cedex. 2, FRANCE. (2) UMR CNRS 6118, Université ...
Paleomagnetic datings of "Siderolithic" paleoweathering profile in southern French Massif Central (Puy en Velay basin): stratigraphic and geodynamic implications. Lenoir, F. (1,2), Ricordel-Prognon C. (1*), Lagroix F. (3), Cortial C. (4), Thiry M. (5) (1) Bureau de Recherches Géologiques et Minières, 3, av. Claude Guillemin – BP 36009, 45060 Orléans – Cedex 2, FRANCE (2) UMR CNRS 6118, Université Rennes 1, Campus de Beaulieu, 263 av. du Général Leclerc, CS 74205, 35042 Rennes (3) Groupe Géologique de la Haute-Loire (G.G.H.L.), Centre Culturel Pierre CARDINAL, rue Jules Vallès, 43000 Le PUY-EN-VELAY (4) Institut de Physique du Globe de Paris, 4, place Jussieu -Case 89, 75252 Paris Cedex 05, France (5) Mines ParisTech, Géosciences, 35 rue St Honoré, 77305 Fontainebleau, FRANCE * Corresponding author : E-mail address: [email protected]

The dating of land surfaces has been for long the major problem in correlating continental paleosurfaces with marine basin deposits. Basement rocks on the southern Massif Central are extensively covered by siderolithic kaolinic paleoweathering horizons. This is the case in the Puy en Velay basin in “la Collange” where these formations are apparently interbedded in Tertiary deposits (“Sables de la Laussonne”) attributed to the Oligocene (Larqué et Weber, 1978). Paleomagnetic data from the in situ paleosols of “la Collange” give a Late Jurassic age (160 Ma ±10 Ma). This is in good agreement with datings previously obtained for other siderolithic formations (Ricordel-Prognon et al., in press). Several points have to be highlighted: 1) From a regional point of view, the obtained dating reveals stratigraphic imprecisions emphasizing the lack of age constraints on sandy deposits underlying the profile. Attributed to the Tertiary, these sands correspond to distinct deposits. The sands below the "Siderolithic" red formation are older than Upper Jurassic. It should be related to residual sand deposits in paleomorphologies dating back to the Trias! The sands above the «Siderolithic» formation correspond to the Oligocene “Sables de la Laussonne”. 2) The data have major implications for the geodynamic evolution of the French Massif Central. Our results show a lack of sedimentary cover at the Jurassic-Cretaceous boundary. This implies that at that time basement rocks were cropping out and devoid of any Mesozoic cover, or of residual formations resulting from the weathering of such a cover. In the future, more accurate and higher resolution age determinations will facilitate correlations between the continental records and the diverse processes involved in their development. It will also allow to develop a "continental stratigraphy" of climatic and geomorphological events. Larqué et Weber, 1978. Sci. Geol., Bull., 31, 4, 151-155. Ricordel-Prognon et al., in press. JGR, Solid Earth.