La séquence sédimentaire des gravanches / gerzat enregistrement d'événements «catastrophiques» À valeur chronologique en limagne d'auvergne (massif central, France)

Abstract

A sedimentary type-sequence of the Grand Limagne marsh has been established after numerous operations of preventive archaeology in the eastern suburbs of Clermont-Ferrand. When complete, this sequence records volcano-sedimentary processes from the Late-Glacial up to the Subatlantic period. Abrupt, brief and catastrophic events mark out to this succession: seven tephras are markers of the Chaîne des Puys volcanic activity; a possible cosmic event (the "Red Layer") is recorded; alluvial deposits testify of major floods and seismites attest of local instability. Besides archaeological data, the direct dating of tephras (CFla / CFlb: ca. 13,500 cal. yr BP; CF3: 12,064 ±316 cal. yr BP; CF4: 11,400 cal. yr BP; CF7: 8,468 ± 147 cal. yr BP) and of the "Red Layer" (7,741 ± 74 cal. yr BP) bring absolute markers for the Gravanches / Gerzat sequence and widely for other paludal sequences in Grande Limagne. Black, humic and polycyclic deposits, namely the famous « black earth of Limagne », thus find a good stratigraphie position between the Boreal period (with CF7 tephra) and the fourth/fifth centuries (regional humid crisis).