HR: 1340h
AN: V13A-0514    [Abstracts]
TI: 238U-234U-230Th disequilibria and the sediment transit time in alluvial plains: examples from the Gangetic plain rivers
AU: * Chabaux, F J
EM: fchabaux@illite.u-strasbg.fr
AF: CGS-EOST universitAc Strasbourg 1, 1 rue Blessig, Starsbourg, Fr 67084 France
AU: Granet, M
V13A-0514 AF: CGS-EOST universitAc Strasbourg 1, 1 rue Blessig, Starsbourg, Fr 67084 France
AU: Pelt, E
V13A-0514 AF: CGS-EOST universitAc Strasbourg 1, 1 rue Blessig, Starsbourg, Fr 67084 France
AU: Franc-Lanord, C
V13A-0514 AF: CRPG-CNRS, rue ND des Pauvres, Nancy, 54501 France
AU: Galy, V
V13A-0514 AF: CRPG-CNRS, rue ND des Pauvres, Nancy, 54501 France
AB: The mechanisms and timescales of processes controlling the sediment formation and their transfers from continent to ocean are a prerequisite for correctly quantifying the rates of continental erosion but also for precisely evaluating the response of the erosion and sedimentation processes to tectonic and climatic forcing. Here we propose to illustrate, in the case of the Gangetic plain river system, the interest of 238U-234U-230Th disequilibria in the bank-river sediments for estimating the transfer time of sediments in the alluvial plains. The studied rivers involve (1) mountain-fed rivers, i.e. river transferring a great quantity of sediments from their source of high relief and forming large depositional areas in the plain, and (2) a foothill-fed river, that derives its sediments from foothills and from within the plains, and for which a large proportion of this material is redeposited after local reworking. The main result displayed by the U-Th data is a regular evolution of the 238U-230Th-232Th systematics along the rivers with a systematic decrease of both (238U/232Th) and (230Th/232Th) ratios in river-bank sediments from upstream to downstream. The variations can be related to mineralogical and chemical sediment evolution during their transfer (and storage) in the alluvial plains and valleys. The choice of a realistic model for the U and Th evolution model during the transfer of alluvions in the valleys and plains allows us to estimate the transit time of sediments in the Gangetic plain : about 100 ka for rivers taking their source in the himalayan chain, and longer transit-time of about 160-250 ka for foothill-fed rivers. This difference is probably related to the difference in the sediment transport dynamics of these two types of rivers. These results highlight the potential of the 238U-234U-230Th disequilibria to constrain the dynamic of the sedimentary transfers in the alluvial plains.
DE: 0744 Rivers (0483, 1856)
DE: 1040 Radiogenic isotope geochemistry
DE: 1105 Quaternary geochronology
DE: 1862 Sediment transport (4558)
DE: 3675 Sedimentary petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005