Piecing together the Ganges-Brahmaputra-Meghna River delta: Use of sediment provenance to reconstruct the history and interaction of multiple fl uvial systems during Holocene delta evolution Steven L. Goodbred Jr.1,†, Penny M. Paolo2, Mohammad Shahid Ullah1, Russell D. Pate1, Sirajur R. Khan3, Steven A. Kuehl4, Sunil K. Singh5, and Waliur Rahaman5 1Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, Tennessee 37240, USA 2School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York 11794, USA 3Geological Survey of Bangladesh, Ministry of Power, Energy and Mineral Resources, Segunbagicha, Dhaka 1000, Bangladesh 4Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062, USA 5Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad, Gujarat 380009, India ABSTRACT ity developed in the mid-Holocene despite least two thirds of the riverine sediment load declining discharge and sea-level rise, sug- has been trapped within the delta through the Three main rivers—the Ganges, Brahma- gesting that earlier channel behavior had Holocene, constructing 50–90 m of stratigraphy putra, and Meghna—coalesce in the Ben- been strongly infl uenced by antecedent to- that preserves a high-resolution record of fl uvio- gal basin to form the world’s largest delta pography of the lowstand valleys. It is only deltaic processes and regional environmental system, which serves as fi lter and gateway after the delta had aggraded above the valley change (Goodbred and Kuehl, 1999; Good- between the Himalayan collision and vast margins that the fl uvial systems were able to bred, 2003). Bengal fan repository. New insights into avulse freely, resulting in numerous channel Over the Holocene, the three main Ganges- the Holocene construction of the Ganges- reorganizations from mid-Holocene to pres- Brahmaputra-Meghna delta rivers have depos- Brahmaputra-Meghna delta, with a focus ent. These channel-system behaviors and ited ~8500 km3 of sediment within the Bengal on river sedimentation, channel migration, their role in delta evolution remain coarsely basin (Goodbred and Kuehl, 2000b), but the and avulsion history, are presented here us- defi ned based only on this initial application sediment load of each river is also geochemi- ing the Sr geochemistry of bulk sediments as of Sr-based provenance tools, but the ap- cally distinct, making this an ideal location to a provenance tracer. The sediment load of proach is promising and suggests that a more unravel the history of multiple fl uvial systems each river transmits a distinct Sr signature complete understanding can be achieved interacting within a tectonically active basin owing to differences in source rocks from the with continued study. to construct a major deltaic sequence. Such Himalaya, Tibet, and local regions, allowing information is directly relevant to the 150 mil- for effective tracking of river channels and INTRODUCTION lion people living on the Ganges-Brahmaputra- stratigraphic development within the delta. Meghna delta, where understanding patterns and In the early Holocene, vigorous delta aggra- Most sediment eroded from continental controls of river behavior and delta sedimenta- dation occurred under rapid sea-level rise basins is dispersed to the oceans through narrow tion is essential to anticipating impacts of rela- and high river discharge and supported the fl uviodeltaic gateways, where sediment loads tive sea-level rise, climate variability, and shifts construction of sand-dominated stratigraphy are fi ltered and transformed, and 20%–80% in land use and water management (e.g., Nich- by laterally mobile, braided-stream chan- of the sediment load is generally sequestered olls and Goodbred, 2005; Woodroffe et al., 2006; nels. However, the vertically (i.e., tempo- within the delta before reaching the continen- Brammer, 2014). Furthermore, the source, depo- rally) uniform, but geographically distinct, tal slope (e.g., Goodbred and Kuehl, 1999; sitional history, and stratigraphic architecture of Sr signatures from these deposits indicate Nittrouer et al., 2007; Walsh et al., 2014). Thus, Ganges-Brahma putra-Meghna delta sediments that the Ganges, Brahmaputra, and Meghna deltas play a major role in defi ning the mass are known to be key controls on the behavior fl uvial systems remained isolated from one and character of terrigenous sediment that is of groundwater arsenic, which is widespread another and apparently constrained within discharged to the ocean and that constitutes the but heterogeneously distributed within Holo- their lowstand valleys. By the mid-Holocene, long-term geologic record. Among the world’s cene aquifers of the delta (e.g., Aziz et al., 2008; though, delta stratigraphy records spatially largest sediment dispersal systems, the Ganges McArthur et al., 2008; Weinman et al., 2008). and temporally nonuniform Sr signatures and Brahmaputra Rivers of South Asia drain Within the Himalayan orogen and surround- that hallmark the onset of avulsions and approximately two thirds of the Himalayan oro- ing geologic terranes, strontium (Sr) isotopes unconstrained channel migration, like those gen and together deliver >1 Gt/yr of sediment have become a well-established provenance that characterize the modern Ganges and to the Bengal margin, where they construct and tracer for source rocks, and they are often used Brahmaputra fl uvial systems. Such mobil- sustain the world’s largest delta and marine fan in conjunction with Nd isotopes to unravel pat- system (Fig. 1; Curray and Moore, 1971; Kuehl terns of erosion and sediment dispersal within †E-mail: [email protected]. et al., 2005). Along this transport pathway, at the region (e.g., Derry and France-Lanord, GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–16; doi: 10.1130/B30965.1; 9 fi gures; Data Repository item 2014225. For permission to copy, contact [email protected] 1 © 2014 Geological Society of America Goodbred et al. 76°E 80°E 84°E 88°E 92°E 96°E Namchaa E. Himalayan Figure 1. Map of major tribu- Barwaa syntaxis taries and source rocks contrib- THB uting sediment to the Bengal basin and Ganges-Brahma- LH putra-Meghna River delta, in- TSS Subansiri 28°N cluding the Trans-Himalayan HHC batholiths (THB), Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Shillong Sequence (HHC), and Lesser Massif Himalayas (LH). The Deccan Bengal Surma Traps and Indian Shield con- basin 24°N Tripura c tribute relatively little sedi- r Fold belt A ment. The Shillong Massif and n Tripura fold belt are locally a important sediment sources to the eastern Bengal basin. ITS— Fig. 2 Indian Shield 200 km Indus-Tsangpo suture. (Figure Deccan Indo-Burman Arc redrawn after Galy et al., 2010.) Traps Bay of Bengal 20°N 1996; Galy et al., 1996; Galy and France- ment sources that have contributed to construc- 2001; Singh and France-Lanord, 2002; Wasson , Lanord, 2001; Singh and France-Lanord, 2002; tion of the Ganges-Brahmaputra-Meghna delta 2003; Garzanti et al., 2004; Sinha et al., 2005; Najman, 2006; Singh et al., 2006, 2008; Raha- (Pickering et al., 2013). Singh et al., 2007). The principal source rocks man et al., 2009). Here, we present the fi rst Sr Within the Bengal basin itself, elevated Pleisto- include the Trans-Himalayan batholiths, isotope measurements, and some Nd isotope cene sediments, notably the Madhupur terrace Tethyan Sedimentary Series, High Himalayan measurements, for Holocene stratigraphy of the and Barind tracts, serve as topographic barriers Crystalline Sequence, and Lesser Himalayas Ganges-Brahmaputra-Meghna delta. In addi- that infl uence river migration and sediment dis- (Fig. 1). The Tethyan Sedimentary Series and tion, measurements of bulk-sediment Sr con- persal (Fig. 2; Goodbred et al., 2003; Pickering High Hima layan Crystalline Sequence comprise centrations are also introduced as an effective et al., 2013). This partitioning of Bengal basin the dominant rock units exposed along the peaks fi rst-order provenance marker and are shown and its underlying tectonic controls support and drainage divide of the Himalaya, with the to be a valuable proxy in conjunction with Sr varying rates of subsidence across the Ganges- Tethyan Sedimentary Series exposed primar- and Nd isotopes. Together, results from these Brahmaputra-Meghna delta, from millennial- ily along the back slope, and the High Hima- provenance indicators provide a more thorough scale rates of 1–3 mm/yr in the southern regions laya Crystalline Sequence along the wetter, understanding of Holocene delta evolution and to 4 mm/yr or more in the northeast Sylhet basin more rapidly eroding front slope. As such, High river course changes than have been otherwise (Goodbred and Kuehl, 2000a; Hanebuth et al., Himalayan exposures mostly drain southward possible using grain size and sedimentological 2013). During the Holocene, accommodation to the Ganges watershed, whereas the Tethyan measurements alone. produced through subsidence and glacio eustatic rocks principally drain north into the Brahma- sea-level rise has led to development of thick putra catchment (Fig. 1). Unlike the Tethyan BACKGROUND fl uvial and coastal marine deposits that reach and High Himalayan series, the monzonitic 70–90 m depth within the lowstand valleys and Trans-Himalayan batholiths are exclusive to the Bengal Basin central Sylhet basin (Umitsu, 1993; Goodbred Brahmaputra watershed and are widely exposed and Kuehl, 2000a). Broad interfl uves that sepa- along the Indus-Tsangpo suture and around the The Ganges-Brahmaputra-Meghna delta rate the lowstand valleys are defi ned by a well- eastern Himalayan syntaxis (Fig. 1). Low-grade has formed within the Bengal basin along the developed paleosol that dips southward from sedimentary rocks of the Lesser Himalaya are trailing-edge margin of the Indian plate at its outcrop exposures to ~50 m below surface, well exposed within the Ganges’ watershed, but juncture with the Eurasian and Sunda plates buried by Holocene delta sediments (Goodbred in the Brahmaputra catchment, they outcrop (Steckler et al., 2008). Along the basin’s north- and Kuehl, 2000a; McArthur et al., 2008; Sarkar only locally along Assam Valley tributaries.