Maneesh Sharma

Luminescence chronometry and Late Quaternary geomorphic history of the Ganga Plain, India

Abstract Flexing of the Indian lithosphere due to the continent–continent collision and thrust fold loading produced the Gangetic Foreland Basin during the Middle Miocene. The basin attained its present configuration during the Late Quaternary. Neotectonics and climate change since the last interglacial influenced the evolution of the present landscape of the Gangetic (Ganga) plains. The regional geomorphic surfaces in these plains are: (1) Upland Interfluve Surface (T2); (2) Marginal Plain Upland Surface (MP); (3) Megafan Surface (MF); (4) Piedmont Fan Surface (PF); (5) River Valley Terrace Surface (T1); (6) Active Flood Plain Surface (T0). Minor geomorphic features comprise ponds, alluvial ridges, and small channels, which occur on the Upland Interfluve Surfaces. The present contribution reviews geomorphic processes that sculpted the basin landscape and, then, using the blue green-stimulated luminescence (BGSL) and infrared-stimulated luminescence (IRSL) ages provides a first order chronological framework for the relationship of major and minor geomorphic features to climate and tectonics. Luminescence dating of the Upland Interfluve Surface (T2) shows that formation of this surface started before 51 ka and continued at least up to 7 ka. Deposition in the Marginal Plain (MP) began at least at 76 ka and continued after 32 ka bracketing the evidence of a more humid climate during 50–40 ka. Sedimentation on the T2 and MP surfaces was synchronous. The top of these surfaces gives variable ages ranging from 48 to 7 ka, due to differential erosion of an undulatory topography caused by tectonic activity. Age upper bound for the initiation and termination of the final building phase of Ganga Megafan can be placed at ∼26 ka and ∼22 ka respectively. Deposition on Piedmont Fan Surface continued between 8 and 3 ka. The River Valley Terrace Surface (T1) of Ganga River shows the activity of Yazoo type channels during 3–1.1 ka. The last phase of active accretion on the narrow, poorly developed youngest Active Flood Plain Surface (T0) was after 1.5 ka. The ponds of T2 surface formed due to cessation of fluvial activity sometime during 8–6 ka. A prominent tectonic activity at 7–5 ka and dry climate at 5 ka produced undulatory topography and high siltation rates in the lakes and ponds. The peripheral bulge, southern Ganga Plain has been tectonically active causing deep incision of rivers and a 40-ka seismic event, and Mid–Late Holocene tectonic activity can be inferred.

Upland interfluve (Doab) deposition: Alternative model to muddy overbank deposits

SummaryMajor alluvial plains contain large tracts of fine-grained muddy sediments, deposited away from the main river channels, which are mostly classed as overbank or floodplain deposits. Systematic study of the Ganga plain shows that such large tracts of deposition of muddy sediments are located several metres above the major channels, and are not flooded by overtopping of the major river channels. These surfaces are here designated as upland interfluve areas (Doab) where deposition of fine-grained sediments takes place independent of the processes operating in the main channels. The surfaces show distinct depositional domains with characteristic deposits. These include higher sloping surfaces (mottled silt), lower flat surfaces (variegated clayey silt), gulleys (sandy silt), small channels (mottled silty sand), ponds (shelly sandy clayey silt), lakes (shelly clayey silt). These deposits are prone to diagenetic changes, especially the development of calcrete horizons. Redistribution of these domains through time produces characteristic mud-dominant alluvial stratigraphy as observed in the Late Quaternary deposits of the Ganga plain. This succession shows similarity to mud-dominant deposits of the Siwalik succession. These Doab deposits are distinct from the overbank deposits formed close to the river channels affected by channel processes. It is argued that many of the thick mud-dominant fluvial deposits of the ancient fluvial record are products of deposition in upland interfluve areas.

Luminescence chronology of incision and channel pattern changes in the River Ganga, India

The River Ganga in the central Gangetic plain shows the incision of ∼20 m of Late Quaternary sediments that form a vast upland terrace (T2). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T1) and the present-day flood plain (terrace-T0). Terrace-T1 shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T0 and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided. Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T2 show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.

Facies characteristics and cyclicity of Lower Siwalik sediments, Jammu area: a new perspective

The Lower Siwalik succession of the Jammu area has been distinguished into three major lithofacies associations: a sand-dominant association, a sandy-mud-dominant association, and a siltyheterolithic association. The sand-dominant association is made up of three lithofacies: cross-bedded sandstone, rippled silty sandstone and bioturbated sandy siltstone, which are organized in multistoreyed sandbodies representing deposition in major river channels. The sandy-mud-dominant association is made up of two lithofacies, mottled clayey siltstone and interbedded sandstone, siltstone and mudstone, representing deposition in overbank areas of flood-plain and natural levee-crevasse splays. The sand-dominant association and sandy-mud-dominant association are grouped together as a channel-related succession and are products of processes in the river channel. The siltyheterolithic association consists of four lithofacies: mottled siltstone, mottled silty sandstone, bedded calcrete and mottled mudstone. They are considered to be deposits of Doab (upland interfluve) areas operating independently of present-day major river channels. These deposits have been formed in minor channels, sloping surfaces, and lakes and ponds of the interfluve regions. The cyclicity of both successions (channel-related and Doab-related) has been determined using a partial-independence statistical model.