# Flexural deformation controls on Late Quaternary sediment dispersal in the Garo-Rajmahal Gap, NW Bengal Basin

Jean-Louis Grimaud, C. Grall, S. Goodbred, M.S. Steckler, R. Sincavage, J.L.

Pickering, C. Paola, N. Seeber, M.S. Hossain

► To cite this version:

Jean-Louis Grimaud, C. Grall, S. Goodbred, M.S. Steckler, R. Sincavage, et al. Flexural deformation controls on Late Quaternary sediment dispersal in the Garo-Rajmahal Gap, NW Bengal Basin. Basin Research, 2020, 10.1111/bre.12425. hal-02404480

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DR JEAN-LOUIS GRIMAUD (Orcid ID: 0000-0001-8857-8269)

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Article type: Original Article

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¹ St Anthony Falls Laboratory, University of Minnesota, 2 Third Avenue SE, Minneapolis, & Department of Earth Sciences, University of Minnesota, MN 55414, USA

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² now at MINES ParisTech, PSL Research University, Centre de Géosciences, 35 rue St Honoré, 77305 Fontainebleau Cedex, France

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³ Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9w, Palisades, NY 10964, USA

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⁴ Vanderbilt University Earth & Environmental Sciences Department, 5726 Stevenson Center; 7th floor, Nashville, TN 37240

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⁵ now at Radford University Department of Geology, Radford, VA 24060, USA

# Flexural deformation controls on Late Quaternary sediment dispersal in the Garo-Rajmahal Gap, NW Bengal Basin

J.-L. Grimaud¹,²*, C. Grall³, S. Goodbred⁴, M.S. Steckler³, R. Sincavage⁴,⁵, J. L. Pickering⁴,⁶, C. Paola¹, N. Seeber³, M. S. Hossain⁷

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/BRE.12425

This article is protected by copyright. All rights reserved
⁶ now at Shell International Exploration and Production, Inc., Houston, TX 77082, USA

⁷ Department of Geology, University of Dhaka, Nilkhet Road, 1000, Dhaka, Bangladesh

* Corresponding author: jean-louis.grimaud@mines-paristech.fr

Revised version, 2nd December 2019

**Key words:** River, Tectonics, Sediment Routing, Weathering, Geophysics

**Abstract**

Subsurface deformation is a driver for river path selection when deformation rates become comparable to the autogenic mobility rate of rivers. Here we combine geomorphology, soil and sediment facies analyses, and geophysical data of the Late Quaternary sediments of the central Garo-Rajmahal Gap in Northwest Bengal to link subsurface deformation with surface processes. We show variable sedimentation characteristics, from slow rates (< 0.8 mm yr⁻¹) in the Tista megafan at the foot of the Himalaya to non-deposition at the exposed surface of the Barind Tract to the south, enabling the development of mature soils. Combined subsidence in the Tista fan and uplift of the Barind Tract are consistent with a N-S flexural response of the Indian plate to loading of the Himalaya Mountains given a low value of elastic thickness (15-25 km).

Provenance analysis based on bulk strontium concentration suggests a dispersal of sediment consistent with this flexural deformation – in particular the abandonment of the Barind Tract by a Pleistocene Brahmaputra River and the current extents of the Tista megafan lobes. Overall, these results highlight the control by deeply rooted deformation patterns on the routing of sediment by large rivers in foreland settings.

# 1. Introduction
Characterizing surface dynamics at the apex of deltas is key for understanding the behavior of rivers as well as the large-scale distribution of their water and sediments, which in turn affects delta-surface evolution and stratigraphic architecture. The associated sediment heterogeneities affect ground permeability and are therefore studied as water / hydrocarbon reservoir analogs (e.g., Blum et al., 2013; van Dijk et al., 2016). A classic method used to constrain sediment architecture is sequence stratigraphy (e.g., Vail et al., 1977; Catuneanu et al., 2010), which focuses on downstream (i.e., sea level) forcings that longitudinally displace the loci of depocenters. In the case of sea level fall, this displacement would be associated with the incision of a paleovalley system bounded by wide interfluves capped by paleosols. The sequence stratigraphy method is under scrutiny, particularly for fluvial sections (e.g., Wright & Marriott, 1993; Amorosi et al., 2017; Pattison, 2018). One reason sequence stratigraphy may not adequately model fluvial systems is that boundaries in the sedimentary record may also be triggered by lateral displacement of depositional lobes due to switching (avulsion) of river course (e.g., Slingerland & Smith, 2004).

The locations of recurring avulsions (i.e., avulsion nodes) control the distribution and size of sedimentary lobes within basins. This is the case in physical and numerical models, where lobe size and surface occupation dynamics are dominated by the location of the delta apex (e.g., Reitz et al., 2010). But how can avulsion nodes persist at a single location in natural systems? Recent studies show that the backwater length influences avulsion node position in the coastal zone (e.g., Jerolmack, 2009; Chatanantavet et al., 2012; Ganti et al., 2016). However, sea level influence on river hydrodynamics is reduced upstream when moving towards the piedmont and bedrock regions (Blum et al., 2013; Hartley et al., 2016; Pickering et al., 2019).

At the piedmont of mountain ranges, tectonic deformation has a strong influence on river behavior. Hence, these upstream areas represent targets for identifying external controls on delta evolution, as opposed to more commonly studied downstream forcings such as sea level. The growth of faults and fold structures has been shown to affect the path of rivers and associated sediment load through diversion, drainage reorganization or temporary sediment storage (Gupta, 1997; Holbrook & Schumm, 1999; Humphrey & Konrad, 2000; Densmore et al., 2016). However, directly linking river dynamics to substrate deformation in depositional environments over short timescales (10²⁻¹⁰⁵ yr) is challenging because: (i) topographic and sediment
thickness variations are often subtle and (ii) processes related to millennial climatic variations or internal river dynamics may overprint tectonic forcings (e.g., Hajek & Straub, 2017; Amorosi et al., 2017; Sincavage et al., 2018).

In this study, we combine analyses of sedimentological, geomorphic, and geophysical data to constrain the Late Quaternary surface dynamics and stratigraphic architecture of the Garo-Rajmahal Gap in northwest Bengal (Fig. 1). We show that river avulsion dynamics and sediment dispersal are influenced by a deeply rooted warping to the south and associated subsidence deep to the north, which are likely flexural features related to loading by the Himalaya.

## 2. Geological setting

This study focused on the northwest margin of the Bengal basin and covers ca. 90 x 10³ km² in northwest Bangladesh at the border with India (Fig. 1). The Bengal basin has been active since the early Cretaceous rifting of India from Antarctica, and has been influenced by the convergence between the Indian and Eurasian-Sunda plates since the Eocene (Steckler et al., 2008). The margin is located north of the Hinge Zone (Fig.1), which marks the transition between the Indian craton to the northwest and thinned, rifted lithosphere to the southeast, where about 18-20 km of sediment accumulated since the Early Cretaceous (Alam et al., 2003; Singh et al., 2018; Mitra et al., 2018). The Hinge Zone today marks also the transition between the basin margin and the Ganges-Brahmaputra-Meghna (GBM) delta plain (Grall et al., 2018).

The Brahmaputra catchment originates in the Tibetan Plateau north of the main Himalayan divide where it collects sediments from erosion of trans-Himalayan batholiths and Tethys Himalaya (Fig. 1). In contrast, the Ganges River drains a large portion of the Indian craton, although its sediment load derives primarily from the Himalaya (Singh et al., 2008). The Ganges and Brahmaputra Rivers each flow at the foot of the Himalayan orogen where they collect sediments from transverse sources varying from individual bedrock streams to mega-alluvial fans (Brammer, 2012). In the Bengal basin, the two rivers merge. The Tista River, located in northwest Bengal (Figs. 1 and 2), is one of the largest local sediment sources to the Bengal basin (~ 5% of the total load transported by the Ganges-Brahmaputra). Over the Late Quaternary, total sediment delivery to the Bengal basin has varied an order of magnitude with
orbital-scale climatic variations (Wiedicke et al., 1999; Goodbred, 2003; Pickering et al., 2018), thus affecting the sediment-loading component of subsidence. Holocene sedimentation rates adjusted for subsidence range from 0.2 – 4.5 mm.yr⁻¹ (Grall et al., 2018).

The Late Quaternary record of the Bengal basin is generally marked by well-developed Pleistocene paleosols that are overlain by largely unweathered Holocene sediments (Morgan & McIntire, 1959; McArthur et al., 2010; Hoque et al., 2014; Pickering et al., 2014). Where buried by Holocene sediments, the Pleistocene boundary is often referred to as the “LGM surface” after the Last Glacial Maximum when a ca. 130 m sea-level fall triggered the development of large lowstand valleys and broad interfluves were exposed to enhanced weathering (e.g., Goodbred & Kuehl, 2000). Such distinction between Pleistocene and Holocene sediments has been validated by over a hundred radiocarbon ages in the GBM delta (Fig. 1) (Sincavage et al., 2018; Pickering et al., 2014, 2018, 2019 and references therein). However, it is now increasingly recognized that the “LGM” surface is really a composite surface of multiple Pleistocene episodes of deposition and incision (Pickering et al., 2014, 2018, 2019) in NW Bangladesh. River mobility is well constrained during the Holocene for the Brahmaputra and Ganges Rivers; major avulsion periodicity is about 1500 – 2500 years (Goodbred et al., 2014; Pickering et al., 2014; Reitz et al., 2015; Sincavage et al., 2018). The course of the Tista River has also changed in past centuries, switching from the Atrai and Karatoa river channels to its present location along the eastern boundary of its fan (Chakraborty & Gosh, 2010; Fig. 2). The evolution of the Tista megafan is however less constrained prior to historical times, although the degree of soil development suggests an abandonment of the western “old piedmont” fan and the more recent occupation of the eastern active fan during the Holocene (Chakraborty & Gosh, 2010; Brammer, 2012; Abramahi et al., 2018).

South of the Tista megafan, the Barind Tract, a low-gradient topographic high divided into Lower (LBT) and Upper (UBT) parts (Fig. 2), is composed of Quaternary sediments currently located above the influence of sea level-related backwater effects (sensu Wilson & Goodbred, 2015). The nearby avulsion node between the Jamuna and the Old Brahmaputra valleys (Bristow, 1999; Sincavage et al., 2018) therefore lies beyond influence from sea level, at least since the mid-Holocene, and it is not directly influenced by bedrock due to Holocene alluvial cover (Hartley et al., 2016; Ganti et al., 2016); these circumstances suggest that there may be additional controls on avulsion dynamics, such as substrate deformation. Tectonic uplift
has been invoked as controlling the Plio-Quaternary path of the Brahamputra River, particular
along the Shillong massif and its bounding Dauki fault (e.g., Pickering et al., 2014; Govin et al.,
2015; Najman et al., 2016). In the Barind Tract, evidence for faulting is more speculative
(Morgan & McIntire, 1959). In the following, we evaluate the hypothesis that, similar to the
Mississippi River valley (Blum et al., 2012), flexural deformation explains rivers patterns and
sediment routing in the Barind Tract and Tista fan.

### 3. Methods

#### 3.1 Geomorphic and subsurface data

Mapping of the present-day surface geomorphology (Fig. 2) is based on field campaigns, topographic DEM analyses, as well as existing physical, geological, and soil maps of the area (Morgan & McIntire, 1959; Alam et al., 1990; Chakraborty & Gosh, 2010; Brammer, 2012; McArthur et al., 2010; Pickering et al., 2014, 2018). In addition, sediment deposits were sampled along three east-west borehole transects (T1, T2 and T3 transects) in fall 2015 and an additional borehole (TXBD01) in spring 2016 (Figs. 1 and 2). The unconsolidated sediments were percussion drilled through 5-cm diameter PVC tube sections by hand-generating a reverse-circulation flow (Fig. 3a). During the drilling process, the sediments were incorporated into water in suspension and expelled with drilling fluids (water and sediment) that were collected at 1.5 m intervals, or finer with notice of lithological changes. The drilling method extended to 70 m below the surface (Figs. 3 and 4). Because of the unconsolidated nature of the sediments, sedimentary structures were not preserved in the coarser samples; however, structure was preserved in cohesive mud intervals.

A fraction of sediment (~200 g) was kept at each interval and analyzed later. Dominant sediment grain sizes and facies were described in the field (Figs. 3 and 4; Appendix 1) (Table 1). Seven wood and organic sediment samples were identified and collected for radiocarbon dating, performed at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) and at Beta Analytic (Miami, FL) (Fig. 4; Appendix 2). Sediment weathering was described according to the paleosols classification of Mack et al. (1993) combined with reference to the US soil order for the modern soil equivalent (Soil Survey Staff, 1999) (Figs. 3 and 4). In parallel, we
also described the degree of sediment oxidation according to coloration (i.e., increasing from brown to red; Figs. 3 and 4).

Sediment geochemistry and provenance were investigated by measuring Sr concentrations of the bulk sediments of selected borehole samples using a Niton XL3t X-Ray Fluorescence (XRF) handheld device mounted on a base (see results in Fig. 5 and Appendix 2) (Pickering et al., 2014; Goodbred et al., 2014). We performed analyses over 120 s with calibrations every 20 measurements. Sr concentration was specifically verified using NIST 2709a (239±6 ppm Sr) and CCRMP Till-4 (109 ppm Sr) standards. We analyzed 68 boreholes with a 6 km average horizontal spacing (Fig. 5). Since UBT sediments were very intensely weathered (see below), we focus on the LBT and Tista fan.

Within the Ganges-Brahmaputra-Meghna river delta, Brahmaputra sediments have a distinctively higher Sr concentration due to the erosion of mafic minerals -mainly plagioclase where Sr substitutes for Ca- from the trans-Himalayan batholiths (Garzanti et al., 2010; Galy & France-Lanord, 2001; Goodbred et al., 2014; Figs. 1, 5 and 6). In Holocene sediments, Sr concentrations of Brahmaputra sediment are typically >140 ppm, compared with 80-110 ppm for the Ganges. A lower limit of 120 ppm Sr may be used for Brahmaputra-derived sediment (Sincavage et al., 2018) when considering Pleistocene deposits with potential Sr leaching by weathering and mixing with other sources (i.e., Tista in the present case). In this study, an even lower limit of 100 ppm was considered for potential Brahmaputra-derived sediment, given that published Sr values for Tista river-bank samples are < 95 ppm (Singh & France-Lanord, 2002) (Figs 5 and 6).

## 3.2 Geophysics data and modeling

We estimated the position of the peripheral bulge and the depression (foredeep) associated with the tectonic load of the Himalaya using continuous elastic plate models (e.g., Watts et al., 2013) (Fig. 7), consistent with recent models for the Himalaya (Berthet et al., 2013; Hammer et al., 2013). We ran models using both the entire Himalaya as a load to produce the foredeep and flexural bulge and using an advance rate of the Himalaya topography at 25 mm yr⁻¹ to estimate approximate rates of Holocene uplift.
For a 2D beam over a weak substratum, the flexural equation is

$$ \frac{d^2}{dx^2} \left( D(x) \frac{d^2 \omega}{dx^2} \right) + \Delta \rho g \omega = V_0 \quad (1) $$

where $x$ is distance, $D(x)$ is the flexural rigidity, $\omega$ is the deflection of the lithosphere, $\Delta\rho$ is density contrast between the mantle and the material infilling the deflection, $g$ is the acceleration of gravity and $V_0$ is the weight of the load.

In the case of a line load on a 2D unbroken plate of constant flexural rigidity

$$ \omega = \omega_0 \cdot e^{-x/\alpha} \cdot (\cos x/\alpha + \sin x/\alpha), \quad \text{with } \omega_0 = \frac{V_0 \cdot \alpha^3}{8D} \text{ and } \alpha = \left[ \frac{4D}{\Delta\rho g} \right]^{1/4} \quad (2) $$

where $\alpha$ is the flexural parameter, $\omega_0$ the maximum deflection at the loading location. $D$ is a function of the effective elastic thickness $Te$ with $D = \frac{ETe}{12(1 - \sigma^2)}$, where $E$ is Young's modulus and $\sigma$ is Poisson's ratio. For this simple case, the forebulge starts where $\omega = 0$ at $x = \frac{3\pi}{4}\alpha$. The maximum height of the bulge is at $x = \pi\alpha$. For a distributed load, such as the Himalaya and Tibet, the deflection is the convolution of the line load with the response. For calculating the deflection for a spatially variable flexural rigidity, the flexure equation is solved numerically using finite differences.

The position of the forebulge is a function of the lithospheric elastic thickness. $Te$ is high in the Indian craton (75-100 km; Jordan & Watts, 2005), although it decreases northward towards the Main Frontal Thrust (MFT) of the Himalaya due to thermal and flexural weakening (Watts et al., 2013; Berthet et al., 2013) and eastward toward the passive margin beneath the Bengal basin. In the study area, $Te$ estimates are of 30 km across the Garo-Rajmahal Gap (Jordan & Watts, 2005), from 25 km to about 15 km towards the MFT beneath Sikkim and the Bhutan Himalaya on either side of the Garo-Rajmahal Gap (Hammer et al., 2013) and of 5-20 km across the overthrust of the Shillong Massif (Mitra et al., 2018) over the Sylhet Basin (Fig. 1). This suggests that the best estimate may be obtained for an intermediate (10-30 km) plate model. Thus we used a 15 - 25 km range (Fig. 7; Appendix 2). For the infill density, we use 2300 kg m⁻³ in the foreland and 2700 kg m⁻³ beneath the Himalayas. The Bouguer gravity anomaly derived from
satellite data (Bonvalot et al., 2012) is used as an independent constraint on the flexural models (Fig. 8).

# 4. Results and interpretations

## 4.1. Sediment facies and weathering

### a) Lithofacies and soil types distribution

Five facies are found based on sediment lithology, thickness, and lateral continuity (Table 1). Large braidbelt deposits (LBf) consist of well-sorted fine to medium sand alternating with coarse to very coarse sand; gravels are often present. LBf are typically over 50 m thick and over 20-30 km wide. Small braidbelt deposits (SBf) have roughly similar grainsize and may be differentiated from LBf by lesser thickness and decreased lateral continuity (i.e., ca. 10-40 m and less than 10 km, respectively). Sinuous stream deposits (SSf) are typically less than 10-m thick intercalated sand and clay-silt deposits with limited (< 6 km) lateral extent. Overbank Mud facies (TOf and MOf) are clay-silts whose thickness (ca. 5-40 m) vary depending on the location. Tista overbank muds (TOf) may be distinguished from mixed provenance overbank muds (MOf) using Sr concentration (see section 4.2). TOf and MOf facies were very often weathered. Two general soil types were observed (Fig. 4): inceptisols and ultisols; together with their paleosol equivalents: protosols and argillisols, respectively. *Inceptisols* are slightly developed soils where many weatherable minerals are still retained. They are usually of brownish color near the surface and grayish below the water table (1-4 m depth). *Ultisols* correspond to more intensely weathered soils. They contain more translocated clays, which increases their cohesiveness compared to inceptisols, and they may have a red color that extends many meters deep due to the significant mobilization and oxidation of iron.

These facies and soil types can be found in specific locations across the study area (Figs. 2 and 4) (Brammer, 2012). In the Tista megafan north of Dinajpur and Rangpur (T1 transect and borehole TXBD01; Fig. 4a and d), deposits consist of SBf and TOf facies, with local SSf facies. Argillisols are observed from 20 to 60 m deep along T1 transect, except in borehole T1118 (10-15 m). All soils at the surface are inceptisols. They are abundant between boreholes T1007 and T1082, i.e., the Western Tista lobe (Fig. 4a). The UBT area (boreholes T2001 to T2018 and
T3027 to T3052; Fig. 4) is dominated by thick (~20-40 m) MOf facies associated with ultisol at the surface. In UBT deeper layers, SBf and SSf facies are observed. The LBT area (i.e., boreholes T2024 to T2080, T3058 to T3124 and A-2 to A-5; Figs. 4b and 4c) is dominated by thin (~5-15 m) MOf facies with varied degrees of weathering at the surface and LBf facies at depth. Ultisols are dominant at the surface of the LBT along T2 transect while they are restricted to the west (i.e., from boreholes T3064 to T3088) along T3 transect. To the East, inceptisols are the most common (Fig. 4c). Finally, along T2 and T3 transects, the Eastern Tista lobe (i.e., boreholes T2086 to T2118) and the Ganges floodplain (i.e., boreholes T3001 to T3013) are both dominated by LBf facies with MOf at the limit with the Barind Tract (Figs. 4b and c). Oxidation of these deposits is low as attested by the presence of only few inceptisols and protosols.

b) Interpretation

The main difference between SBf and LBf is their lateral extent, which is typically <10 km for SBf (i.e., compared to 20-30 km for Brahmaputra deposits; Sincavage et al., 2018) (Table 1). LBf is therefore similar to Brahmaputra River facies. In the Barind Tract, facies are dominantly LBf, supporting deposition by a large river near the scale of the Brahmaputra. In contrast, Tista deposited facies are dominantly SBf (Fig. 4) with the occurrence of SSf that may be interpreted as low-energy streams (Chakraborty & Ghosh, 2010) and TOf corresponding to distal floodplain deposits and associated crevasse splays. These rapid lateral facies changes would be consistent with the deposition of smaller fan lobes associated with the Tista.

At the surface of the Barind Tract, paleosols are widespread and laterally continuous with an overall eastward decrease of weathering intensity, i.e., from the UBT area to the Karatoya and Brahmaputra Rivers (Figs. 4b and c). The high (30 m) elevation above main rivers and enhanced weathering of the UBT compared to the LBT suggests that this area has been abandoned for the longest period. The western Tista lobe associated with inceptisols at the surface roughly corresponds to the old, inactive piedmont fan described by Brammer (2012). Mature soils (argillisols), which are typical of Barind Tract and Madhupur terrace, are found below the surface of the Tista megafan.
## 4.2. Geochemistry and age of the sediments

The distribution of Sr concentrations along boreholes is shown in Fig. 5 (see Fig. 4 for comparison with lithology). Along the T1 transect, the concentration of Sr is < 100 ppm between boreholes T1001 and T1088 and 50-140 ppm between boreholes T1094 and T1152 (Fig. 5a). Along the T2 and T3 transects, high and low Sr populations were delimited at a threshold of 100 ppm in the LBT. This limit is sharp and marks a boundary between overall low-strontium sediments in the western and shallow deposits (mostly MOf), and the high-strontium sediments in the deeper units and toward the east (Figs. 5b and 5c).

Sr populations were considered in the light of published analyses (Singh & France-Lanord, 2002; Pickering et al., 2014) to evaluate potential effects of weathering and mixing between sources on the measured Sr concentrations (Fig. 6). The eastern, deeper sediments along transects T2 and T3 have a median Sr concentration of 135 ± 25 ppm (i.e., using standard deviation). These values are indicative of Brahmaputra provenance although they are lower than (i) that of Pickering et al. (2014) in Holocene sediments of Transect A (149 ± 44 ppm) and (ii) that of Singh and France-Lanord (2002) along the bank or in monsoonal suspended sediment of the modern Brahmaputra channel (183 ± 41 ppm) (Fig. 6). Between T1001 and T1088, sediment of the Tista fan has a Sr concentrations median value of ca. 61 ± 13 ppm. This is lower than the ca. 90 ppm values reported by Singh & France-Lanord (2001) for modern Tista sediment. T3001 to T3020 that lie in the Ganges floodplain yield Sr concentrations of 80-110 ppm, which is typical for unweathered Ganges sediments (Fig. 5e; Lupker et al., 2013; Goodbred et al., 2014). The western, shallow deposits along the T2 and T3 transects (excluding UBT and Ganges flood plain sediments) represent another definable set of samples that have Sr concentrations of 61 ± 17 ppm (Figs. 5 and 6). This last set has a range comparable to that of Tista sediment but is not distributed normally, unlike T1. This suggests that the sediment here has not only a Tista provenance but incorporates other material such as reworked Brahmaputra sediment.

Corrected radiocarbon ages of sediments deposited in the vicinity of the Ganges are 4,130 ± 20 yrs BP at 15 m (Fig. 4c). Between the Karatoya and Brahmaputra rivers, ages of shallow sediments are 1,750 - 4,000 yrs BP (Fig. 4b). Along borehole T2092, the age at 20 m depth of 4,000 yrs BP is associated with changes of grain size (from coarse to medium sand) and Sr concentration (from 105 to 80 ppm). In the LBT, sediments are older than 48,000 yrs BP (the
time limit of the radiocarbon technique) at 43-53 m deep (borehole T3064; Fig. 4c). One
Pleistocene age (35,680 ± 260 yrs BP) is reported at 29 m deep in the Tista megafan (TXBD01
borehole; Fig. 4d).

4.3. Flexural deformation

In the study area, estimates of *Te* suggest a decrease from 25 km to 15 km towards the MFT (Hammer et al., 2013). We present models using these two values as well as a model in which the *Te* decreases linearly from 25 km (160 km from MFT) to 15 km (at MFT) (Appendix 2). Results show that the MFT to zero crossing (i.e., the limit between subsiding foreland basin and uplifted flexural bulge) distance is between 90 and 170 km (Fig. 7). The maximum elevation of the flexural bulge is found respectively ~175, 200, and 275 km away from the MFT for the three models. These estimates yield a flexural bulge that fall within the Barind Tract area (Figs. 7, 9 and 10). The fit could be improved if the lower rigidity close to the MFT was increased to 20 km or varying from 20 to 30 km yielding 130-135 km for MFT to zero crossing distance and 225-255 km from MFT to maximum elevation distance.

To estimate rates of subsidence and uplift, we created a differential load by advancing the topography at 25 mm yr⁻¹. This load is centered over the front face of the Himalaya. These models show a similar pattern to the overall load, but the zero crossing and flexural bulge are farther from the MFT (Fig. 7e). In fact, the node is close to the position of the maximum overall bulge. Thus, the differential loading indicates the southern part of the Barind tract is uplifting, while the northern part is subsiding to eventually be buried by the Tista Fan. The models yield maximum uplift rates from the Himalayan load of 0.03-0.11 m kyr⁻¹. This suggests 4-14 m of uplift since the OIS stage 5e highstand.

The wavelength of gravity anomalies derived from satellite data is consistent with the flexural deformation model (Figs. 7 and 8). South of the MFT, a ca. 100-150 km wavelength negative anomaly is observed (Fig. 8a), while even further south, the Bouguer anomalies become positive within the Barind Tract. The anomaly contrast between the Barind Tract and Tista megafan is around 200 mGal, i.e., largely exceeding variability due to the density of sediment
fill. Hence, the Bouguer anomalies observed are best explained by a deeper density contrast resulting from lithospheric warping below the Garo-Rajmahal Gap consistent with flexural deformation.

# 5. Discussion

## 5.1. Distribution and timing of sediment accumulations

The present study contrasts surface dynamics and sediment infill between the Barind Tract and Tista megafan. Tista megafan stratigraphy is characterized by rapid lateral facies changes (~10 km) (Fig. 4; Table 1). We interpret this as an indication of deposition by fan lobes of limited lateral extent. The preservation of overbank muds further indicates that fine deposits were not completely reworked by the Tista River system. Radiocarbon dating of borehole TXBD01 (Fig. 4) gives a sedimentation rate of 0.8 mm.yr⁻¹. Published sediment ages based on OSL and cosmogenic nuclide dating techniques at the surface of the Tista megafan (ca. 30 to 1 kyr BP) suggest sedimentation rates that may be even lower locally (Abrahami et al., 2018). Pleistocene to early-Holocene ages were sampled in the western Tista lobe, which is capped by inceptisols (Fig. 4). This supports previous interpretations that this lobe is older than the eastern one (Chakraborty & Gosh, 2010). Additionally, the ages indicate that the mature argillisols below the Tista megafan are older than 30 kyr BP and therefore formed prior to the LGM.

The Barind Tract -especially the LBT- preserves limited clay-silt deposits and soils at depth; rather, most fines are found at the surface (Fig. 4). Apart from these shallow fine deposits, the stratigraphy of the LBT is sand-dominated and characteristic of a big river able to efficiently rework fine deposits (Goodbred et al., 2003). Strontium data indicate a Brahmaputra provenance for these sediments, in agreement with facies interpretations (Fig. 6) (Table 1). Recent OSL dating of the Bogra Terrace, the easternmost part of LBT, returned an age of 106 ± 10 kyr of the Brahmaputra sediments at 6 m deep (Pickering et al, 2017; Fig. 4c). As also suggested by two radiocarbon ages along borehole T3064 (> 48,000 yrs BP), LBT sediments are mostly Pleistocene and older than the LGM. This suggests that the Barind Tract remained exposed to weathering throughout the past 100,000 years as attested by the presence of mature soils at the
surface (i.e., ultisols) (Kraus, 1999). Hence, the Barind Tract was occupied by the Brahmaputra River system in the Late Pleistocene but has since been largely abandoned.

The UBT is capped by the thickest ultisol of the study area, with the local presence of iron nodules that resemble those of lateritic profiles on bedrock (Grimaud et al., 2015) and suggest very long term exposure (10⁵⁻⁶ yr; Targulian & Krasilnikov, 2007). The UBT is therefore likely the oldest landform of the Garo-Rajmahal Gap area. The UBT is separated from the Ganges River by a scarp, suggesting that the river is actively eroding the landform before entering the GBM delta (Figs. 1, 2 and 9a). Similarly, rivers from the Tista system moderately incise the LBT surface. This is confirmed by field observations along the Atrai River (Fig. 2), which locally flows over and loosely incises stiff ultisols in the LBT area. These observations suggest active uplift in the Barind Tract.

Sr concentrations show that T1 is dominated by low Sr, Tista sediment and that T2 and T3 are dominated to the east by high Sr, Brahmaputra sediment. Within a population, Sr concentration variability may be related to grain size differences, sediment reworking/mixing and chemical weathering (Lupker et al., 2013; Sincavage et al., 2018). Overall, Quaternary Sr concentrations are 30% lower than modern Tista and Brahmaputra sediments (Fig. 6). In the case of Tista, the fact that the source area is reduced -and that mixing with sediment from catchments other than the Tista would have essentially very similar lithology - supports the suggestion that this 30% reduction is mostly related to weathering and less so to sediment mixing. Sr depletion by weathering for Brahmaputra sediment is consistent with their age: the Pleistocene sediment of the Garo-Rajmahal Gap are more depleted than the Holocene sediment of the Jamuna valley (Pickering et al., 2014), which are more depleted than the modern sediment (Singh & France-Lanord, 2001) (Fig. 6). Singh & France-Lanord (2001) however reported an increase in lower Sr sources in the modern Brahmaputra sediment towards Bangladesh. Hence the effects of sediments mixing and weathering may not always be isolated with certainty without further investigation using other tracers. The easternmost boreholes along T2 (T2098 to T2118; Fig. 5b) have very high Sr concentrations and are not capped by paleosols. Thus they are interpreted as part of the recent to modern Brahmaputra alluvial plain (Fig. 2). The western shallow (MOf) deposits of the Barind Tract remain the most difficult to interpret (Fig. 5). Because their transition with underlying Brahmaputra deposits is sharp, they may be interpreted as resulting
from distal channels of the Tista that slightly rework underlying Brahmaputra sediments - i.e., similarly to the modern Atrai River. This is consistent with a low Sr concentration valley visible below the Karatoya River (Fig. 5b). Alternatively, Mof deposits may be related to extremely intense and long-lasting weathering that depleted Sr of Brahmaputra sediment by more than 30%. This further supports the suggestion that the Barind Tract remained exposed to reworking and weathering through the Late Quaternary.

The mature soils exposed at the surface of the Barind Tract and paleosols buried in the Tista megafan are not dated with certainty. We do, however, know that they are older than 35 kyr BP, and potentially may be older than 100 kyr BP. This further supports the view that the “LGM” surface is a composite surface of multiple Pleistocene episodes of deposition and incision (Pickering et al., 2018, 2019). Hence it cannot be simply used as a geologically instantaneous marker for sequence stratigraphy-based correlation, questioning how paleovalleys may form in response to a single sea level fall event or incision (Pickering et al., 2019). In Fig. 7b, the geometry of the top of this composite Pleistocene weathering surface is interpolated, using a basic GIS algorithm. A broad N-S upwarp is observed (see also Fig. 9e). Further investigation on the associated weathering horizons and their ages would be promising for studying pre-Holocene climatic variations, and more generally, the stratigraphy of the GBM. This would require careful assessment of initial sediment composition depending on provenance (Garzanti et al., 2010; Brammer, 2012; Pickering et al., 2017; this study).

## 5.2 Regional deformation pattern

Gravity anomalies of the Garo-Rajmahal Gap are consistent with a lithosphere depressed towards the MFT (Figs. 7 and 8). The geometry of the top of intensely weathered Pleistocene sediments also shows a deflection north of Dinajpur (Fig. 8b). The Barind Tract has therefore remained a topographic (and basement) high since 100 ka while the Tista megafan has slowly subsided. The upward movement of the basement below the Barind Tract surface is referred to as the “Rangpur saddle” in earlier geophysical studies (e.g., Mukhopadhyay & Dasgupta, 1988; Alam, 1989). Archean rock of the Indian craton is found at about 500 m depth at Barakupuria in the Barind Tract (Norman, 1992). The tectonic activity in the Barind Tract could be explained by fault-related tectonics (e.g., Rashid et al., 2015), but there is little direct evidence for it. The

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uplifting Barind Tract has been considered as the western termination of the Shillong anticlinorium (e.g., Clark & Bilham, 2008) controlled by the continuation of the Dauki Fault, the Dapsi Fault, the proposed Dhubri-Chungthang Faults (Diehl et al., 2017) to the north, or unnamed fault to the south (Grall et al., 2018). However, the trend of seismicity and 3 crossings of the Dauki Fault by high-resolution seismic lines (Spiess et al., 2012) suggest that the boundary of the Shillong block bends to the NW (Vernant et al., 2014; Diehl et al., 2017; Hetenyi et al., 2019) and does not underlie the Barind Tract. As we argue below, the broad deformation pattern in the Garo-Rajmahal Gap is more compatible with flexure.

Flexure in response to loading by the Himalaya can explain both the subsidence in the Tista fan and the uplift of the Barind Tract, i.e., respectively behaving as a foredeep and a forebulge (Fig. 10), without the necessity of invoking additional faulting. The range of 2D estimates for bulge distances from the Himalayan front on a roughly N-S profile (90-275 km; Fig. 7) supports a location in the Barind Tract given the estimates of elastic thicknesses in the area (15-30 km; Jordan & Watts, 2005; Hammer et al., 2013). The wavelength of the deformed Barind Pleistocene weathering surface (ca. 200-300 km; Fig. 9e), which is broader than that of the fault-related Shillong topography along an N-S section (ca. 100 km), further supports deformation due to flexure in the Garo-Rajmahal Gap as the origin of the Barind Tract. In 3D, sedimentary loading of the GBM Delta and Shillong plateau, probably also contributes to the flexural pattern. This is a promising research direction, particularly because the Barind and Madhupur Tracts may be part of a single sinuous composite flexural bulge (Steckler et al. 2018). Modeled forward motion of the Himalaya suggests 10-30 m elevation of the Barind Tract relative to the surrounding region over one to several hundred thousand years (Figs. 8e and 8d). The total (i.e., at the scale of the Cenozoic sediments thickness) negative deflection observed at the MFT is ca. 5 km for Te = 15-25 km (Fig. 7c), i.e., consistent with Himalayan foreland strata thickness at the same location (4 - 6 km; Burbank, 1992). At the MFT, estimates subsidence rates are thus ca. 1.5-2.5 mm yr⁻¹ for the Cenozoic and 0.4-1.4 mm yr⁻¹ for the Late Quaternary - consistent with the sediment accumulation rate in the Tista fan.

In Pakistan, Duroy et al. (1989) also showed a bulge, located on the other side of the Indian craton, that they called the Sargoda ridge. Similar to the Rangpur saddle, the Sargoda ridge is narrow (i.e., 100-150 km) when compared to the forebulge interpreted from GRACE
geoid data in the Indian craton (i.e., > 500 km; see Bilham et al., 2003; DeCelles, 2012; Fig. 1a). Duroy et al. (1989) further suggested such a narrow bulge to be related to an elastic thickness that is lower in this area (ca. 30 km) than the rest of the Himalayan and Indian lithospheres. In the study area, shift of the Himalayan flexural bulge close to the MFT is consistent with the rigidity of the Indian plate decreasing eastward as it approaches the Hinge Zone and edge of the Indian craton (Hammer et al., 2013). In any case, it seems that lower plate rigidity at either end of the Indian craton supports the emergence of structural highs in the Himalayan foreland, whether related to faulting or flexure. With the forward migration of the mountain belt, the Garo-Rajmahal Gap forebulge is expected to propagate southward across India. The region north of the peak of the forebulge toward the Himalaya will undergo subsidence, while the frontside of the advancing forebulge undergoes uplift.

## 5.3 Sediment dispersal by deep structure in foreland basins

Sediment routing related to tectonics is a key issue in foreland basins that have been extensively studied based on fieldwork, theory, and experiments (e.g., Heller et al., 1988; Gupta, 1997; Posamentier & Allen, 1998; Humphrey & Konrad, 2000; Roddaz et al., 2005; DeCelles 2010; Densmore et al., 2016; Grimaud et al., 2017). Rivers are sensitive to substrate deformation if their intrinsic mobility timescale is long compared to the tectonic deformation timescale. We estimated this intrinsic mobility for the Tista River based on existing theory (e.g., Reitz et al., 2015; Grimaud et al., 2017). The visitation timescale of an alluvial fan surface by a river ($T_c$) scales with the intrinsic timescale necessary to fill a unit depositional lobe ($T_L$) divided by the fan surface fraction $f_A$ occupied by this lobe. $T_L$ scales with the volume of the lobe (i.e., lobe width $B$ times fan radius $r$ times channel depth $h$) divided by sedimentary flux $Q_s$.

$$T_c = \frac{T_L}{f_A} = \frac{hBr}{Q_s f_A} \quad (3)$$

Parameters of equation (3) were estimated from this study and the literature. We used $h = 2$ m based on modern hydrologic surveys (Khan, 2001) and thickness of channel fills in the megafan (Chakraborty & Ghosh, 2010). Millennial denudation rates in the Tista catchment (1.31 mm yr$^{-1}$; Abrahami et al., 2016) yielded 27 Mt yr$^{-1}$ for sediment flux at the apex of the fan. Sinha and
Friend (1994), however, demonstrated that up to 90% of sediment is transferred to big rivers in the case of the Ganges, leaving only 10% to megafans (i.e., 2.7 Mt yr⁻¹). We use r = 150 km and B = 10 km, based on LBf facies extent (Fig. 4) (Table1), which returns fA ~ 0.08. Using these parameters with 20-40% porosity in deposits, we found TL ~2 kyrs and Tc ~25 kyrs. The Tc estimate is roughly consistent with OSL ages up to 30 kyrs found at the surface of the Western Tista lobe (Abrahami et al., 2018).

In comparison, TL is 550 years for the Brahmaputra River (Reitz et al., 2015). This indicates that the Brahmaputra River is intrinsically 3-4 times more laterally mobile than the Tista River. Therefore, the latter will be more sensitive to tectonic uplift (Bufe et al., 2016; Grimaud et al., 2017). This is in agreement with (1) the Tista River being steered to the east, (2) the Tista fan not extending further than the Barind Tract, and (3) the Brahmaputra River cutting its way across the bulge. The Brahmaputra river system once flowed over the Barind Tract sometime in the Late Pleistocene, but has been confined to the Jamuna Valley since then (Figs. 4 and 8). Carving of the younger Jamuna valley is related to sea level drop during LGM times, augmented by Late Pleistocene and early Holocene megaflood outbursts (Pickering et al., 2019). In addition, our study demonstrates flexural rock uplift across the Barind Tract. This uplift reduces channel lateral mobility (Bufe et al., 2016) and likely contributed to the entrenchment of the Brahmaputra River in the Jamuna valley. We predict a similar fate for the Ganges River, which is currently confined in its valley flanked by the UBT to the East (Figs. 1, 2 and 9). This flexural response of the lithosphere to loading also controls the downstream extent of depositional lobes of piedmont megafans. In accordance with the distribution of accommodation rates, it favors the entrenchment of big rivers that cross upwarps as well, which leads to long-lasting avulsion nodes. In conclusion, the Garo-Rajmahal example shows that deep structures are key features for sediment dispersal in foreland basins.

## 6. Conclusion

Late Quaternary sediment dispersal was studied in the Garo-Rajmahal Gap by combining geomorphology, soils and sediment facies analyses, and geophysical data. We found that:
513 (1) Surface dynamics vary from subsidence and sediment infilling (rate ~0.8-2 mm yr⁻¹) in the Tista megafan, at the foot of the Himalaya, to uplift (rate ~ 0.03-0.11 mm yr⁻¹) and prolonged weathering in the Barind Tract to the south.
514
515

516 (2) Combined subsidence in the Tista fan and uplift of the Barind Tract are consistent with a N-S flexural response of the Indian plate to loading of the Himalaya Mountains, but with a flexural rigidity that is less than cratonic India farther west.
517
518

519 (3) Provenance analysis shows a dispersal of sediment consistent with this flexural deformation, particularly the abandonment of the Barind Tract by the Pleistocene Brahmaputra River and the current extent of the Tista megafan lobes.
520
521

522
523
7. Acknowledgments
524
525
526
527
528
This work was supported by the SAFL Industrial Consortium for Experimental Stratigraphy and the BanglaPIRE project, NSF Partnerships for International Research and Education grant OISE 09-68354. We are grateful to Neils Hovius and his colleagues at GFZ Postdam, and Christian France-Lanord for discussions and comments on this work. We thank the Bureau Gravimétrique International (BGI) / International Association of Geodesy for access to data.

529
Data availability statement

530
The data that support the findings of this study are provided in the supplementary material.

531
532
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759 **Figure captions**

**Figure 1:** (a) Simplified regional map showing the Brahmaputra and Ganges rivers' main channel. The estimated location of the forebulge in the Indian craton is after DeCelles (2012). (b) Physiographic map of the Ganges Brahmaputra Meghna (GBM) delta. The study area is shown by the dashed rectangle. The grey dashed stripe corresponds to the hinge zone (after Goodbred et
al., 2014). MFT stands for Main Frontal Thrust, GH for the Garo Hills of western Shillong Massif, SB for the Sylhet Basin, BD for the city of Boda (where borehole TXBD01 was drilled) and RT for the Rajmahal traps at the edge of the Indian craton.

**Figure 2:** Mapping of present-day surface geomorphology in the study area based on our field observations as well as existing physical, geological and soil maps of the area (Morgan & McIntire, 1959; Alam et al., 1990; Chakraborty & Gosh, 2010; Brammer, 2012; McArthur et al., 2010; Pickering et al., 2014). Location is shown in Fig. 1b.

**Figure 3:** Description of the sample and description procedure for borehole data based on borehole T1-56 km (see Fig. 2). (a) Picture showing the local drilling team at work, the driller is the person at the right holding the plastic tube (see Methods section for details). (b) Picture of the sampled sediments showing variations of grain size, coloration related to oxidation/weathering and stiffness/cohesion. (c) Interpretation of the lithological succession from (b) based on grain size and weathering.

**Figure 4:** Borehole lithological successions along T1 (a), T2 (b), T3 (c) transects and TXBD01 borehole (d). Note that the last 3 digits of borehole names correspond to distance along transect in km. For borehole description method, see Figure 3. Borehole locations are shown in Figs 1 and 2. Boreholes A-2 and A-5, shown for comparison, are from Pickering et al. (2014, 2018).

**Figure 5:** Variations of Strontium (Sr) of concentration (ppm) along transects T1 (a), T2 (b) and T3 (c). The dashed line draws an approximate boundary between Brahmaputra and Tista sediments. TM: Tista Megafan, LBT: Lower Barind Tract, UBT: Upper Barind Tract, GF: Ganges Floodplain.
**Figure 6:** Distribution of Sr concentration showed for the different populations from Fig. 5. Data in (A) and (B) are respectively from Pickering et al. (2014) and Singh & France-Lanord (2001).

**Figure 7:** Prediction of forebulge position based on gravity data and various elastic models. (a) Sketch explaining the flexural deformation associated with a tectonic load. (b) Regional N-S gravity profile crossing the Himalaya, the Tista Fan and the Barind Tract (see Fig. 7a for location). Note the 100 km width negative gravity anomaly observed at the Tista Fan in light red that is interpreted as a flexural depression and the positive anomalies observed at the Barind Tract. Topography and simplified topography used for modeling shown in grey and thin blue lines (c) Predicted deflection under the Himalaya and Tibetan loads including the forebulge geometry for probable elastic thicknesses (Hammer et al, 2013). The variable elastic thickness model has a linear increase in elastic thicknesses from 15 km at the MFT to 25 km at 160 km farther south. (d) Closeup of the same models as in (c) showing the foreland basin and forebulge with deflection related to the total load by the Himalaya. (e) Closeup of the same models as in (c) with deflection related to an advancing Himalayan front at a rate of 25 mm.yr⁻¹ since OIS5e. The red dashed line represents the transition from positive to negative Bouguer gravity anomaly and transition from foreland basin to forebulge.

**Figure 8:** (a) Map of Bouguer residual anomalies. The black dashed stripe corresponds to the hinge zone (after Goodbred et al., 2014) that marks the northern boundary of the subsiding delta (Grall et al., 2018). (b) Map of the top of intensely weathered Pleistocene sediments (see section 5.2). Such boundary has been identified on the field, and along boreholes (e.g., Figs. 4 and 5). The interpolation is based on a simple natural neighbor's procedure using the ArcGis software.

**Figure 9:** Interpretative sections of observations reported in Figures 2, 4 and 5. (a) Topographic map of the study area with location of the sections. Crossed circles correspond to boreholes described by Goodbred et al. (2014). (b)(c)(d) Interpretations of transects T1, T2 and T3,
respectively. (e) Interpretation of a roughly N-S longitudinal profile across the Tista megafan and Barind Tract.

**Figure 10:** Geological model explaining patterns of sediment provenance and weathering in the study area.

<table><caption>**Table 1:** Description of main sedimentary facies in the vicinity of the Garo-Rajmahal Gap</caption><thead><tr><th rowspan="2">Facies Name</th><th colspan="5">Description</th><th rowspan="2">Interpretation</th></tr><tr><th>Lithology</th><th>Thickness</th><th>Lateral continuity</th><th>Sr concentration</th><th>Locations</th></tr></thead><tbody><tr><td>Large braidbelt deposits (LBf)</td><td>Well-sorted fine-medium sand alternating with coarse-very coarse sand; gravels often present; basal pebble layer may be found</td><td>Typically &gt; 50 m</td><td>&gt; 20-30 km</td><td>Typically &gt; 120 ppm; locally 90-120 ppm</td><td>Jamuna Valley and typically below 10 m in the Barind Tract</td><td>Alluvial deposits of the Brahamputra River system; local mixing with Tista material</td></tr><tr><td>Small braidbelt deposits (SBf)</td><td>Moderately-sorted coarse-very coarse sand alternating with fine-medium sand; gravels often present</td><td>Typically 10-40 m; locally up to 60 m</td><td>Typically &gt; 10 km; up to 15 km</td><td>Typically 30-80 ppm; locally 90 ppm</td><td>Tista fan lobes and locally along the Atrai River</td><td>Alluvial deposits of the Tista megafan</td></tr><tr><td>Sinuous stream deposits (SSf)</td><td>Intercalated sand deposits and clay-silts deposits</td><td>Typically &lt; 10 m</td><td>&lt; 6 km</td><td>Typically 30-80 ppm; locally 90 ppm</td><td>Tista fan lobes and locally along the Atrai River</td><td>Alluvial deposits of the Tista megafan</td></tr><tr><td>Tista Overbank Muds (TOf)</td><td>Clay-silts layers, various degree of weathering observed</td><td>Typically 10-20 m</td><td>&lt; 6 km</td><td>Typically 30-80 ppm; locally 90 ppm</td><td>Tista fan lobes</td><td>Distal floodplain deposits of the Tista River system</td></tr><tr><td>Mixed Overbank Muds (MOf)</td><td></td><td>Typically 5-15 m; locally up to 40 m</td><td>&gt; 20-30 km</td><td>Variable (30-100 ppm)</td><td>Barind Tract</td><td>Distal floodplain deposits; Mixed provenance (Tista-Brahmaputra), interpretation complicated by weathering</td></tr></tbody></table>
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