Neogene shallow-marine and fluvial sediment dispersal, burial, and exhumation in the ancestral Brahmaputra delta: Indo-Burman Ranges, India

Abstract

The stratigraphic record of Cenozoic uplift and denudation of the Himalayas is distributed across its peripheral foreland basins, as well as in the sediments of the Ganges–Brahmaputra Delta (GBD) and the Bengal–Nicobar Fan (BNF). Recent interrogation of Miocene–Quaternary sediments of the GBD and BNF advance our knowledge of Himalayan sediment dispersal and its relationship to regional tectonics and climate, but these studies are limited to IODP boreholes from the BNF (IODP 354 and 362, 2015-16) and Quaternary sediment cores from the GBD (NSF-PIRE: Life on a tectonically active delta, 2010-18). We examine a complementary yet understudied stratigraphic record of the Miocene–Pliocene ancestral Brahmaputra Delta in outcrops of the Indo-Burman Ranges fold–thrust belt (IBR) of eastern India. We present detailed lithofacies assemblages of Neogene delta plain (Tipam Group) and intertidal to upper-shelf (Surma Group) deposits of the IBR based on two ∼ 500 m stratigraphic sections. New detrital-apatite fission-track (dAFT) and (U-Th)/He (dAHe) dates from the Surma Group in the IBR help to constrain maximum depositional ages (MDA), thermal histories, and sediment accumulation rates. Three fluvial facies (F1–F3) and four shallow marine to intertidal facies (M1–M4) are delineated based on analog depositional environments of the Holocene–modern GBD. Unreset dAFT and dAHe ages constrain MDA to ∼ 9–11 Ma for the Surma Group, which is bracketed by intensification of turbidite deposition on the eastern BNF (∼ 13.5–6.8 Ma). Two dAHe samples yielded younger (∼ 3 Ma) reset ages that we interpret to record cooling from denudation following burial resetting due to a thicker (∼ 2.2–3.2 km) accumulation of sediments near the depocenter. Thermal modeling of the dAFT and dAHe results using QTQt and HeFTy suggest that late Miocene marginal marine sediment accumulation rates may have ranged from ∼ 0.9 to 1.1 mm/yr near the center of the paleodelta. Thermal modeling results imply postdepositional cooling beginning at ∼ 8–6.5 Ma, interpreted to record onset of exhumation associated with the advancing IBR fold belt. The timing of post-burial exhumation of the IBR strata is consistent with previously published constraints for the avulsion of the paleo-Brahmaputra to the west and a westward shift of turbidite deposition on the BNF that started at ∼ 6.8 Ma. Our results contextualize tectonic controls on basin history, creating a pathway for future investigations into autogenic and climatic drivers of behavior of fluvial systems that can be extracted from the stratigraphic record.

Provenance Shifts During Neogene Brahmaputra Delta Progradation Tied to Coupled Climate and Tectonic Change in the Eastern Himalaya

“…Active subduction accretion (Y. Wang et al, 2014) deforms the ∼16-20 km thick pile of Paleogene-Quaternary sedimentary deposits (Curray, 1991;Mitra et al, 2018;Singh et al, 2016), forming the outer-and inner-foldthrust-belts of the IBR (Betka et al, 2018;Maurin & Rangin, 2009;Sincavage et al, 2020). The deformation front of the IBR accretionary prism is buried by Pliocene-Holocene sediments of the Ganges-Brahmaputra Delta (Figure 1; Alam et al, 2003;Najman et al, 2012;Steckler et al, 2008Steckler et al, , 2016Uddin and Lundberg, 1999).…”

“…The IBR contain early Eocene to Pliocene fluvial-deltaic sediments derived from the eastern Himalaya and Tibet (Alam et al, 2003;Bracciali et al, 2015;Gani & Alam, 1999;Johnson & Alam, 1991;Sincavage et al, 2020;Vadlamani et al, 2015;Yang et al, 2018). The Neogene stratigraphy of the IBR has recently been described in terms of facies representative of modern environments on the Ganges-Brahmaputra Delta (Sincavage et al, 2020;Figures 2 and 3). The Neogene facies succession generally shows a coarsening upwards trend illustrative of the transition from shallow marine to fluvial sedimentation reflecting progradation of the ancestral Brahmaputra delta (Figure 3).…”

“…The Barail Group represents a late Eocene to early Miocene deep water to deltaic succession upon which the younger delta prograded from the early Miocene to Pliocene (Bezbaruah & Muzamil, 2013;Biswas & Mukhopadhyay, 2011;Sincavage et al, 2020). The Barail Group consists of interbedded silts and fine-medium sands, with bed thicknesses generally on the order of tens of centimeters up to one meter thick (Figure 3).…”

“…M1 grades upwards into the M2 facies, which contains alternating cyclical beds (15-20 cm thick) of cross-bedded fine sands and silts. This unit is interpreted to represent the prograding foresets of the subaqueous delta clinoform (Sincavage et al, 2020). The M3 facies is a bioturbated silty shale with cm-scale bedding exhibiting flaser, lenticular, and wavy bedding associated with sub-to intertidal depositional settings.…”

“…However, mixing between these two disparate sources means they tend to homogenize discrete records of tectonic, climate, and geomorphic interactions related to development of the Himalayan orogen (e.g., Blum et al, 2018). In contrast, exhumed Neogene sedimentary rocks within the IBR primarily record the late Cenozoic progradation of the ancestral Brahmaputra delta only (Govin, Najman, Copley, et al, 2018;Sincavage et al, 2020), and thus preserve a more direct record of changes in upstream allogenic forcings related to development of the eastern Himalaya and Tibet (e.g., Lang et al, 2016;Najman et al, 2008). Over the past decade, multiple studies of IBR stratigraphy have renewed interest in the regional stratigraphic correlation of IBR sedimentary units to the more proximal Himalayan foreland sequences of the Siwalik Group (Cina et al, 2009;Chirouze et al, 2013;Govin, Najman, Copley, et al, 2018, Govin, Najman, Dupont-Nivet, et al, 2018Lang et al, 2016) as well as the Bengal and Nicobar fans (Bergmann et al, 2020;Blum et al, 2018;Pickering, Carter, et al, 2020;Pickering, Pouderoux, et al, 2020) to reconstruct late Cenozoic changes in tectonic and climate signals within the Brahmaputra watershed.…”