TEPHROSTRATIGRAPHIE DE SEDIMENTS LACUSTRES SITUES EN CONTEXTE GEODYNAMIQUE ACTIF : EXEMPLE DES SEDIMENTS DU LAC ICALMA (CHILI, ZONE VOLCANIQUE SUD, 38°S)

摘要

In this paper, we document the mineralogical and geochemical composition of tephra layers identified in two long sediment cores (ICA-1 and ICA-IIJ'collected in Lake Icalma (Southern Volcanic Zone of the Andes, Chile) The sediment cores have been dated by radiocarbon and cover the last 8000 years Due to the frequent regional volcanic activity and prevailing westerly winds, the sediments of Lake Icalma contain a large number of tephra deposits (134 in ICA-I and 98 in ICA-II) However, some of these tephras occur in reworked deposits and have therefore been excluded from the present tephrostratigraphic study. Based on the mineralogical composition of grains in the 105-420 p.m fraction, it has been demonstrated that the tephra layeis are highly contaminated by the host sediment- We theiefoie separated and analyzed the coarse-giained scoriae and (or) pumices (> 420 |Xm) to constrain the mineralogical and geochemical composition of the corresponding juvenile magmas Our results demonstrate that only one tephra, the Alpehue pumice, clearly differs from the rest of the record by its original geochemical (rhyolitic glass) and mineralogical (essentially hyperstene) composition This tephra was emitted by the Sollipulli volcano in 3000 cal yr BP In addition, our results demonstrate that the mineralogy and geochemistry of successive tephra layers are generally very similar and don't permit correlations of individual tephra layers, A method of correlation by group has therefore been used, This'method clearly shows two distinct mineralogical and geochemical tephrostratigraphical units, in addition to the Alpehue tephra Tephras occurring in the upper unit are composed of olivine-rich basaltic andesite, whereas those of the lower unit have a pyroxene-rich andesitic composition Using this technique, we have been able to correlate the two sediment cores with each other, as well as with more distant peat and soil deposits In each case the Alpehue tephra is the more reliable tephrostratigraphic marker This paper demonstrates that the quality of lacustrine tephrostratigtaphic records is affected by several parameters such as the direction of prevailing winds, the frequency of volcanic eruptions and the seismic activity Using tephras deposited in such sedimentary environments for stratigraphic correlations requires the adjustment of classical methods of tephrostratigraphic investigation.