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Open AccessArticle

Provenance of Bengal Shelf Sediments: 1. Mineralogy and Geochemistry of Silt

1
Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano-Bicocca, 20126 Milano, Italy
2
Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine-CNRS, BP 20, 54501 Vandoeuvre-lès-Nancy, France
3
MARUM Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany
*
Authors to whom correspondence should be addressed.
Minerals 2019, 9(10), 640; https://doi.org/10.3390/min9100640
Received: 30 May 2019 / Revised: 23 September 2019 / Accepted: 1 October 2019 / Published: 18 October 2019
(This article belongs to the Special Issue Heavy Minerals)
This article illustrates a multi-technique frontier approach for the provenance study of silt-size sediments. The mineralogical composition of low-density and heavy-mineral fractions of four samples of fine to very coarse silt deposited on the Bengal shelf was analyzed separately for six different grain-size classes by combining grain counting under an optical microscope, Raman spectroscopy, and X-ray diffraction. The geochemical composition was determined on both bulk-sediment samples and on their <5-μm classes. Such a “multiple-window” approach allowed capturing the full mineralogical information contained in each sample, as well as the size-dependent intra-sample variability of all compositional parameters. The comparison between grain-size distributions obtained by different methods highlighted a notable fallacy of laser granulometry, which markedly overestimated the size of the finest mode represented by fine silt and clay. As a test case, we chose to investigate sediments of the Bengal shelf, where detritus is fed from the Meghna estuary, formed by the joint Ganga and Brahmaputra Rivers and representing the largest single entry point of sediment in the world’s oceans. The studied samples show the typical fingerprint of orogenic detritus produced by focused erosion of collision orogens. Bengal shelf silt is characterized by a feldspatho-quartzose (F-Q) composition with a Q/F ratio decreasing from 3.0 to 1.7 with increasing grain size, plagioclase prevailing over K-feldspar, and rich transparent-heavy-mineral assemblages including mainly amphibole with epidote, and minor garnet and pyroxene. Such a detrital signature compares very closely with Brahmaputra suspended load, but mineralogical and geochemical parameters, including the anomalous decrease of the Q/F ratio with increasing grain size, consistently indicate more significant Ganga contribution for cohesive fine silt. The accurate quantitative characterization of different size fractions of Bengal shelf sediments represents an essential step to allow comparison of compositional signatures characterizing different segments of this huge source-to-sink system, from fluvial and deltaic sediments of the Himalayan foreland basin and Bengal shelf to the Bengal Fan. View Full-Text
Keywords: provenance analysis; sieving of fine silt; fallacy of laser granulometry; benthic foraminifera; heavy minerals; Raman spectroscopy; Ganga–Brahmaputra river system; Bay of Bengal; Himalayan orogen provenance analysis; sieving of fine silt; fallacy of laser granulometry; benthic foraminifera; heavy minerals; Raman spectroscopy; Ganga–Brahmaputra river system; Bay of Bengal; Himalayan orogen
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Borromeo, L.; Andò, S.; France-Lanord, C.; Coletti, G.; Hahn, A.; Garzanti, E. Provenance of Bengal Shelf Sediments: 1. Mineralogy and Geochemistry of Silt. Minerals 2019, 9, 640.

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