Sedimentology and Isotopic Chemistry of the Bengal Fan Sediments: The Denudation of the Himalaya
Résumé
Mineralogical and H, O, Sr, and Nd isotope compositions have been analyzed on a set of representative samples from the 17-m.y. section in ODP Leg 116 Holes 717C and 718C. Based on the mineralogical composition of the fraction <2 µm together with the lithogenic-biogenic composition of the fraction >63 µm, the whole section can be subdivided into three major periods of sedimentation. Between 17.1 and 6 m.y., and between 0.8 m.y. to present, the sediments are characterized by sandy and silty turbiditic inputs with a high proportion of minerals derived from a gneissic source without alteration. In the fraction <2 µm, illite and chlorite are dominant over smectite and kaolinite. The granulometric fraction >63 µm contains quartz, muscovite, biotite, chlorite, and feldspars. The 6-to 0.8-m.y. period is represented by an alternation of sandy/silty horizons, muds, and calcareous muds rich in smectite, and kaolinite (50% to 85% of the fraction <2 µm) and bioclastic material. The presence of smectite and kaolinite, as well as the I8 O/ 16 O and the 87 Sr/ 86 Sr ratios of the fraction <2 µm, imply an evolution in a soil environment and exchanges with meteoric ground water. The ranges of isotopic compositions are limited throughout the section: δ 18 θ quartz = 11.7 to 13.3% c , 87 Sr/ 86 Sr = 0.733 to 0.760 and eNd 0 =-17.4 to-13.8. These values are within those of the High Himalaya Crystalline series, and they are considered to reflect this source region. The data imply that, since 17 Ma, this formation has supplied the major part of the eroded material.
Domaines
Sciences de la Terre
Origine : Accord explicite pour ce dépôt
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