Étude sédimentologique et palynologique des dépots lacustres tardiglaciaires et holocènes du Gour de Tazenat (Massif Central, France) [Sedimentological and palynological investigations of late glacial and holocene lacustrine deposits of the Gour de Tazenat (Massif Central, France)

Abstract

Cores were taken below 66 m of water from lacustrine deposits of the extinct basaltic maar named "Gour de Tazenat", Chaîne des Puys, Massif Central, France. Those cores allowed us to describe the environmental changes in the northern Chaîne des Puys (France) from the early Late Glacial to the present time. Geomorphological evolution of the catchment, along with the sedimentological investigations of lacustrine deposits are described. This shows that the only inlet of the lake has formed a delta; the volume of deltaic sediment is almost equal to the volume of the dell eroded by the post-eruptive backward erosion of the inlet. Since the delta is little developped, the lake (and the end of the eruption) should not be much older than Upper Weichselian in spite of TL-dates obtained on quartz grains from the adjacent tufiQ-ing and going back te about 120 ka. This paper deals also with sedimentological analyses of a 10 m long core that was taken in the muddy deposits of the flat bottom of the lake. That sequence corresponds to a period running from the early Late Glacial to the present time. The overall variations of clay-, water, organic matter contents and percentage of tree pollen throughout the core are correlated with each other. A thin tephra bed (volcanic ash-falls) was also found and linked with the Pavin volcano based upon trachytic composition of glass shards, along with the stratigraphical position within the Atlantic. That finding allowed us to enlarge the northern fan of the Pavin tephra over the entire Chaîne des Puys. New results of pollen analyses of the Atlantic to present time period shows the obvious development of Fagus (up to 50% of pollen sum). Later on, two phases of human deforestation are clearly recorded and attributed respectively to Neolithic then Celtic and subsequent populations; those two phases are separeted by a period of natural reforestation. Five sandy layers are interbedded in the Subboreal to Subatlantic mud; they should have anthropogene origine. The pollen analysis of the Late glacial to Boreal sequence was published elsewhere (Juvigné and Bastin, 199S). Nevertheless the main relevant results are summarized in this paper: Middle Dryas is particularly well recorded and the Late Dryas can be subdivided in three cold sub-oscillations. The same sequence contains two tephra beds: one is mugearitic within the Altered palynozone and dated at 10,280 yr BP and ; the other is trachytic within the Boreal and dated at about 8220 yr BP.