Stratigraphic evolution of the late Holocene Ganges–Brahmaputra lower delta plain

Abstract

Sediment cores from the Ganges–Brahmaputra delta in Bangladesh were examined for sedimentological character, clay mineralogy, elemental trends (C, N, S), and ¹⁴C geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late Holocene. A widespread facies succession from Muddy Sand to Interbedded Mud records progradation of shoal–island complexes and the transition from subtidal to intertidal conditions. Mangrove-vegetated islands and peninsulas represent the final phase of progradation; a Mottled Mud that is deposited by penetration of turbid coastal water into the mangroves during high water events. Organic matter preservation is generally low (<1% TOC) in most of these well-drained deposits that are characterized by a permeable, silt-dominated granulometry. Clay mineralogy in the cores records the relative influence of smectite and kaolinite-rich Ganges sediments and illite and chlorite-rich Brahmaputra material. The lower delta plain west of the modern river mouths was deposited as a Ganges-dominated delta in three phases since 5000 cal years BP, with Brahmaputra influence confined to the Meghna estuary area and to the supratidal section of western delta deposits. Evolution of the lower delta plain in the late Holocene was influenced by regional subsidence patterns in the tectonically active Bengal Basin, which controlled distributary channel avulsion and migration, and the creation of accommodation space.

Introduction

The classification of deltas has been based on the geomorphic expression of the balance between riverine sediment supply and destructive wave and tidal processes Wright and Coleman, 1973, Galloway, 1975, Coleman and Wright, 1975, grain size of the sediment load (Orton and Reading, 1993), or plume dynamics in the river mouth (Wright, 1977). In the first two classifications, and to a certain extent the third, subaerial geomorphology defines the deltaic subtypes. The subaerial delta is typically subdivided into a saline lower delta plain colonized by saltmarsh and/or mangroves grading landward with increased elevation into freshwater wetlands of the upper delta plain (Wright, 1985). These classification schemes demonstrate that subaerial delta morphology, particularly the lower delta plain, is sensitive to changes in the balance between hydrodynamics and riverine input, and to changes in relative sea level.

High-energy examples of the deltaic spectrum, defined here as those with tidal range >3 m in the river mouth and mean significant wave heights of >1 m on the adjacent continental shelf, compose 9 of the 13 riverine sediment inputs in excess of 100 million tons annually to the world ocean (e.g., Ganges–Brahmaputra, Amazon, Changjiang, Irrawaddy, Fly, Mekong, Godavari, Narmada, Hungho; Milliman and Syvitski, 1992). Despite their hydrodynamic similarities, these rivers exhibit a diverse subaerial morphology including examples with no modern lower delta plain except for river mouth islands (e.g., Amazon, Changjiang, Fly), and a number with prograding subaqueous mud clinoforms on the continental shelf (e.g., Amazon, Ganges–Brahmaputra, Fly). The Ganges–Brahmaputra delta (Fig. 1) has recently been shown to be undergoing both shoreline accretion Martin and Hart, 1997, Allison, 1998 and aggradation of the lower delta plain surface up to 70 km inland due to onshore advection of riverine sediment (Allison and Kepple, 2001). The high sediment yield of the Ganges–Brahmaputra has caused the river–ocean interface to prograde seaward 100–300 km over a wide (250 km) front since maximum sea level transgression Banerjee and Sen, 1987, Umitsu, 1987, Goodbred and Kuehl, 2000. However, little examination has been made of these extensive late Holocene lower delta plain deposits to determine how they were formed in the presence of energetic marine conditions and a tectonically active basin. The present study utilizes vibracore and augur core surveys to reconstruct the shallow (<7 m) stratigraphy of the Ganges–Brahmaputra lower delta plain deposits and to identify sedimentary facies relationships indicative of environments of deposition. Facies distributions and radiocarbon dating for these Ganges–Brahmaputra lower delta plain sample sites are utilized to develop a model for late Holocene evolution of the subaerial delta.