Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

Eduardo Garzanti,Mara Limonta,Sergio Andò,Giovanni Vezzoli,Laura Borromeo,Christian France-Lanord

doi:10.3390/min9100642

Abstract

The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata.

Highlights

The Ganga and Brahmaputra rivers drain the Himalayan orogen and join in Bangladesh to form the Meghna estuary, which represents the largest single entry point of detritus in the world oceans ([1,2])
This study adds new data and observations that, combined with extensive previous work, contributes to a better understanding and quantification of the enormous sediment flux that throughout the Neogene has transited across the Bengal basin and accumulated on Bengal Sea floors
Detritus mostly generated from rapid erosion of the active Himalayan orogen and entrained by the Ganga and Brahmaputra rivers, which joined two centuries ago to form the Meghna estuary, is partly stored in the subaqueous delta prograding onto the Bengal shelf, as in the subaerial delta formed by the Ganga

Summary

Introduction

The Ganga and Brahmaputra rivers drain the Himalayan orogen and join in Bangladesh to form the Meghna estuary, which represents the largest single entry point of detritus in the world oceans ([1,2]). The Bengal sediment system, which represents the focus of this study, is composed of an abandoned subaerial delta in the west, drained by tidal distributary channels and partially subject to erosion, and an actively prograding subaqueous delta that connects the estuary mouth with the Swatch of No. Ground, the deep canyon through which detritus is conveyed toward the submarine fan ([5,6,7]). The thorough evaluation of the mineralogical variability associated with hydrodynamic sorting and other physical and chemical processes across deltaic and shallow-marine environments represents the fundamental pre-requisite to link the orogenic provenance signatures of Ganga and Brahmaputra sediments with the deep-sea sedimentary record ([20,21,22,23])

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Conclusion