Le cône détritique de la Drôme : une contribution à la connaissance de l'Holocène du Sud-Est de la France

Abstract

Two cores were drilled in the river Drômes's vast and thick (10-12 m) holocène alluvial cone, where it flows into the river Rhône. The presence of alterning layers of terrigenous silty-clayey sediments and organic peaty loam made it possible - thanks to sedimentology and radiocarbon dating - to identify the different evolving phases of the river. The period of post-maxi wurmian river-bed formation ends before 11 850 B.P. It is followed until 7000 B.P. by a period of low accumulation (average rate: 10 centimeters per 1000 years) and then between 7000 B.P. and 3000 B.P., by a stage of accelerated accumulation (2 meters per 1000 years). Subsequently and up to the present the average rate drops markedly (50 centimeters per 1000 years), although this doesn't necessarily indicate a decrease in erosion phenomena nor a slowdown in downstream accumulation : increase in the cone's surface area and retention of recent upstream accumulation. We would suggest an attempted correlation between on the one hand the detrital accumulation crisis from 7000 B.P., and on the other, the early and large anthropisation of the mountain basin during the Middle and perhaps even Early Neolithic periods equally demonstrated by pollen analysis. Therefore could be explained the importance of both upstream erosion phenomena and downstream accumulation.

How natural are Alpine mountain rivers? Evidence from the Italian Alps

“…Nonetheless, Brochier et al . () identify the start of human‐derived erosion in southern French Alps as early as 7000 yr bp . During the Iron Age (since about 2500 yr bp ), deforestation for expanding agriculture increased substantially (Tinner et al ., ), and intense trading activities from the Alps to the plains were already being carried out along the major rivers (Furlanetto, ).…”

History and evolution of Mesolithic landscapes in the Haut-Quercy (Lot, France): New charcoal data from archaeological contexts

“…A palynological study on core levels dated between 13,175 and 10,977 cal. bc (11,850±400 BP, Ly-4450) shows the dominance of Quercus / Corylus and a very discreet presence of Pinus (Brochier et al, 1991). Similarly, Delhon et al (2010) note the early abundance of hardwoods in this region on the basis of the study of charcoal and phytoliths from natural contexts dated between 13,670–13,310 cal.…”

Tectonique et dynamique fluviale du Würm à l’Holocène à la confluence Saône-Rhône (France)

“…Dans la moyenne vallée du Rhône, les deux tiers du creusement avaient été acquis dès 15 000 BP. Brochier et al (1991) ont, depuis, montré que le creusement s'est poursuivi, jusqu'à une cote inférieure au niveau actuel de la plaine alluviale. Au confluent de la Drôme et du Rhône, à 120 km au sud de Lyon, le creusement était totalement acquis dès avant 11 850 ± 400 BP (Ly -849) jusqu'à la cote 87 m alors que le lit majeur actuel est à une altitude de 95,5 m. La nouveauté de cette contribution est de proposer que l'incision tardiglaciaire du Rhône est un phénomène qui a touché l'ensemble de la vallée à l'aval du confluent de l'Ain.…”