Abstract
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.
Highlights
Sieve analysis demonstrates that the fine mode in all of our samples is represented by very fine silt and clay (Figure 3A) rather than by medium silt (Figure 3B), testifying how laser granulometry markedly underestimates the clay content of sediment samples [90,91]
We developed developed aa detailed detailed procedure procedure for for the the high-resolution, high-resolution, multiple-window multiple-window analysis analysis of of the the mineralogical and geochemical composition of mud-rich sediments
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Summary
Silt represents represents a very very major major part part of of fluvial fluvial detrital detrital fluxes fluxes and and 50%. Silt on Earth thethe diverse difficulties involved in sample treatment and analysis, on. Becauseof of diverse difficulties involved in sample treatment and analysis, mud and mud mudrocks are infrequently considered in provenance studies studies (e.g., [5,6]). Silt is Silt a main and mudrocks are infrequently considered in provenance is a component of offshore shelf deposits, which represent the trait-d’union between fluvial sediments main component of offshore shelf deposits, which represent the trait-d’union between fluvial largely carried in suspension and deep-sea-fan turbidites. Learning to Learning extract provenance information sediments largely carried in suspension and deep-sea-fan turbidites.
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