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Petrography of alluvial sands as a past and present environmental indicator:Case of the Loire River(France)

International Journal of Sediment Research 2013年第3期28卷 285-303页

作者： Jean-Jacques MACAIRE Isabelle GAY-OVEJERO Michel BACCHI Constantin COCIRTA Luc PATRYL Stéphane RODRIGUES University Franqois-Rabelais of Tours University Francois-Rabelais of Tours SARL RIVE 4. Commissariat a l'Energie Atomique-CEA DAMDIF

Four sample sets of the Upper and Middle Loire river sands were analyzed in order to study the impact of natural and anthropogenic factors on their petrographic composition in space(on an 800 km stretch) and *** was determined by modal analysis of three sand-size fractions using a polarizing optical microscope and calculated for each sample("standard sand" = Sst).The watershed is composed mainly of endogenic(Massif Central) and sedimentary(southern Parisian Basin) rocks.B-set sands collected in channels for different water flows in 1996 show that Sst compositions vary by only 5%.Present-day sands in the Upper Loire and Middle Loire have very high petrographic immaturity comparing to others worldwide fluvial sands,although bio-climatic conditions favor sand maturation by source-rock weathering in the *** shows the strong impact of the Massif Central on sediment yield due to relief rejuvenation as a consequence of the formation of the Alps during the *** sands stored during the Weichselian and the Holocene in the Middle Loire floodplain,although partly weathered since their deposition,show higher inputs from the endogenic rocks of the Massif Central than present-day *** can be explained by Weichselian periglacial conditions and the development of crop farming since the Neolithic,which favored mechanical erosion,particularly in the Massif Central which is characterized by a cold,humid climate and steep *** upstream-downstream change in the composition of presently deposited sand is low in the diked *** shows however that basalt and some heavy mineral grains are vulnerable to abrasion during transport and indicates a marked sediment yield from ancient sediment stored in the *** is in line with the high incision of the river bed over the last 150 years partly due to dam construction and aggregate mining.

关键词： Fluvial sand Sand petrography Sediment source tectonics impact Human impact Loire river

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