Geomorphological evolution of the Armorican massif (Western France): a base Upper Cretaceous wave-cut platform degraded in a tropical setting until Late Miocene
Résumé
The Armorican massif is one of the four main variscan basements outcropping in France. Its geomorphological study has been performed since at least one century with the classical works of E. de Martonne, R. Musset, A. Guilcher, A. Meynier among the other ones. This relief is characterized by numerous planation surfaces, sometimes associated with different types of wethering as laterites, silcretes, calcretes or clays with flints. Surimposed on these surfaces, thin sediments deposits of four different ranges of ages are preserved: Ypresian to Lutetian marine deposits, Bartonian to Rupelian marine to continental sediments preserved in several grabens, Langhian to Serravalian marine deposits and Tortonian to Gelasian marine to continental sediments. These last ones are contemporaneous of a major change of climate, from tropical to temperate, during Late Miocene to Early Pliocene period. We performed a geomorphological study of those surfaces and associated pediments, weatherings and sedimentary deposits. DEM (SRTM, 3 arc-seconds resolution) analysis and field controls were used to produce a macro-geomorphological features map. At least, three types of planation surfaces were defined prior the Late Miocene climate change. They record an overall base level change, which could be due to both tectonic uplift in response to Africa-Iberia-Eurasia convergence and eustasy, and are thereafter listed from high to low levels: i) The Cenomanian high surface, a wave-cut platform surimposed on Lower Cretaceous alterites; ii) The Paleogene pediplains, composed of pediments with some preserved laterites, silcretes and calcretes, which is sealed by Bartonian deposits; iii) The Neogene pediplains and pediments, less weathered and sealed by Langhian to Serravalian deposits. Theses planation surfaces and pediments are later incised, first by the "Red sands" system of Tortonian to Gelasian age, and second by the present-day river system since its incision at the end of the Early Pleistocene.