Devonian Stratigraphy and Paleogeography of the Armorican Massif (Western France)

Abstract Since the first synthesis of Devonian stratigraphic history in the Armorican Massif (1973) much new lithological and paleontological information has contributed to a greater understanding of geodynamic evolution in this crustal segment. The entire Devonian succession is fossiliferous, showing abundant benthic faunas and microfaunas (mainly Chitinozoans and Conodonts) which have provided biostratigraphic control. Platform deposits are developed throughout the Armorican Massif from the base of the Devonian to the upper Famennian (Fa2a in the NW and FaV in the SE). The sedimentation is characterized by a strong terrigenous influence with deposits showing locally great vertical and lateral variations. The sedimentary succession is marked by two remarkable phenomena: a very high rate of sedimentation during the Early Devonian; and a Late Devonian regression related to the bretonic phase. In Central Brittany, from Lower Devonian time onwards an important terrigenous input interrupts the generally homogenous succession observed towards the eastern and western extremities of the Central Synclinorium. Further towards the southeast Devonian rocks are found within Lower Carboniferous olistolite bearing deposits. These Devonian rocks display a particular facies not seen elsewhere. Continuous sedimentation was periodically interrupted by several volcanic episodes and synsedimentary tectonic movements. The stratigraphic position of different alkaline or acid volcanic episodes is determinable. This volcanism was restricted to Central Brittany and the Rade de Brest. The Strunian is related to the history of the Carboniferous basin and is characterized by the development of olistolites in a setting only slightly affected by marine influence in the Morlaix and Chateaulin basins. The Armorican Massif belongs to the Northern Gondwana realm, like the Iberian peninsula and Northwest Africa. It was separated from Laurussia by the Rheic ocean which progressively shrank during the Devonian. The very early development of reefs in the Rade de Brest (Lower Pragian) continued sporadically up to the Givetian. It implies a south tropical paleogeographic position for the Armorican Massif.