Fluvial response to Late Quaternary sea level changes along the Mahanadi delta, east coast of India

Abstract

This study presents the fluvial response to late Quaternary sea level changes in the Mahanadi delta. Channel morphological adjustments indicative of sea level changes, such asanastomosing-meandering transition, dendritic channels, and river mouth shift, have been used to reconstruct the paleo-coastlines. Major rivers in the Mahanadi delta (e.g., Mahanadi, Devi, Brahmani, and Baitarani) show alteration of anastomosing and meandering fluvial systems at the late Quaternary strandlines. Two generations of dendritic channels, i.e., Early to Mid- (>5 ka) and Mid-to Late (<5 ka) Holocene, have been identified based on OSL ages and affinity to the paleostrandlines. Lateral shift of river mouths in response to sea level changes have been observed in the lower deltaic plain. The delta distributaries such as Bhargavi, Kushabhadra, Brahmani and Baitarani rivers shifted their mouths due to Early to Mid-Holocene (6–7 ka) marine transgression. Different generations of paleochannels in the Mahanadi delta indicate episodes of the delta progradation since Mio-Pliocene. The distributary paleochannels along the Mio-Pliocene strandline indicate the initial stage of Mahanadi delta development. The river morphological response to the changing coastline of the Mahanadi delta is similar to paleo-fluvial morphological adjustments observed along the paleostrandlines of major deltas around the world.

Introduction

The concept of the base level has been critically important to understand the fluvio-geomorphic response to external forcings. The base level is the lowest limit to which a river can flow and erode its bed, the ultimate base level being the sea level (Powell, 1875; Schumm, 1993). The sea level has fluctuated many times in the earth's history (Banerjee, 1993, 2000; Murray-Wallace and Woodroffe, 2014). The Quaternary has witnessed sea level changes up to 100 m, resulting from fluctuation of ice volumes during the glacial and interglacial cycles (Schumann et al., 2016; Martinson et al., 1987; Hope, 2005; Murray-Wallace and Woodroffe, 2014). Tectonically driven upliftment and subsidence are the other major contributing factors to sea level change. Fluvial channels respond to sea level change by adjusting their length, gradient, width, sinuosity, and pattern, to adjust their flow velocity and sediment discharge. Previous studies concerning fluvial response to sea level change have focused on paleochannel morphology/pattern, such as anastomosing-meandering transition (Smith et al., 1989; Tornqvist, 1993; Makaske, 2001), river mouth shift (Dominguez et al., 1987; Schumm, 1993; Kapsimalis et al., 2005; Hijma and Cohen, 2011; Wang et al., 2018; Xue, 1993; Ren and Shunan, 1990), and paleo-dendritic channels (Gammisch et al., 1988; Schumm, 1993; Eisma, 1998; Kong et al., 2011), to reconstruct paleo-coastline positions. Anastomosing channels are formed due to aggradations during transgression and are readjusted to straight/meandering channels at a lower slope during regression (David Knighton and Nanson, 1993; Tornqvist, 1993; Berendsen and Berendsen, 1995; Makaske, 2001). Channel avulsion associated with sea level rise shifts the river mouth (Schumm, 1993; Hori et al., 2002; Tanabe et al., 2006; Hori and Saito, 2007; Tamura et al., 2009; Tjallingii et al., 2010, 2014; Hijma and Cohen, 2011; Stouthamer et al., 2011; Zong et al., 2012; Song et al., 2013). Flow accumulation generates dendritic drainage networks in low-lying areas near the coast (Schumm, 1993). These near coastal dendritic channels have low base level erosion capacity and are abandoned when sea level retreats. The paleo-dendritic channels have been used to demarcate the paleo-coastline positions in stratigraphic record (Fan et al., 1990; Nordfjord et al., 2005). The coastal zone channel morphological changes are preserved as paleochannels in the sedimentary record and provide evidence of paleo-coastline position. Correlation among the morphology of coastal zone paleochannels and sea level changes has been well documented in previous studies (Colman and Mixon, 1988; Fan et al., 1990; Koss et al., 1994; Donoghue and White, 1995; Sloss et al., 2005; Weschenfelder et al., 2010; Ishihara et al., 2012; Bae et al., 2018; Bufarale et al., 2019).

Instead of the fact that these channel morphological studies have played a key role in reconstructing past sea level changes, these studies have been limited to a few deltaic areas. Unlike the tectonically active regions of the world, where structure control on drainage morphological changes is well established, empirical evidence supporting fluvial response to sea level changes lacks due to a limited number of studies. Moreover, our ability to interpret fluvio-geomorphic changes in dynamic environmental settings is confounded by the complexity of river response to external forcings, as different allogenic (climate, tectonics, and eustasy) and autogenic (river processes) forcings can produce similar river morphological changes in the fluvial realm (Schumm, 1993). Therefore, more insight into the nature of the controls on fluvial channel morphological changes is needed to facilitate a better understanding of fluvial response to external forcings.

This study aims to investigate the paleo-fluvial morphological changes in the Mahanadi delta and correlate with the paleo-sea level changes. Mahalik (2000, 2006) reconstructed the Late Quaternary strandline positions along the Mahanadi delta (Fig. 1), using facies variations and fossil assemblages from borehole cuttings. The Mio-Pliocene, Late-Upper Pleistocene, and Holocene periods witnessed marine transgression, punctuated by intervals of regressional phases. With this high-resolution paleo-sea level record, Mahanadi delta is an ideal site to study fluvial response to sea level changes, as (i) the delta is situated in a passive continental margin (Subrahmanyam and Chand, 2006); hence tectonic control on channel morphologic changes can be ruled out; (ii) the distributary systems of the Mahanadi delta have evolved during successive stages of progradation from Mio-Pliocene to Holocene (Mahalik, 2006), suggesting that the Late Quaternary sea level changes modified the fluvio-geomorphic architecture of the delta; (iii) the delta experiences a uniform climate, suggesting that the major allogenic forcings, i.e., climate and sea level changes can be assumed to have acted uniformly and simultaneously on all the drainage systems of the delta.