Fluvial landforms and sediments in the north-central Gangetic Plain, India
Shobhit Chandra
Darwin College
Abstract
The Indo-Gangetic plains are part of the active foreland basin coupled to the Himalayan orogen. This geographical area forms a unique setting for the study of large scale river system morphology and alluvial sedimentation patterns in both the short-term and long-term. Also, study of the large scale basin filling process gives a unique modern example of the consequent alluvial architecture, comparable to ancient foreland sequences.
This research project is focussed on the interfan area between the Gandhak and Ghaghra fans in the north central Gangetic plains, India. This region is drained by the mountain-fed river Rapti and its tributaries. The study has involved the mapping of areal surfaces in the interfan area, using multi-date remotely-sensed Landsat and Indian Remote Sensing satellite data. This has distinguished the landforms of the study area from the distinct terrace surfaces present in the central and southern parts of the Gangetic basin. Linear features such as the rivers, "palaeochannels", and associated fluvial landforms are intricately interrelated, and digital image processing has allowed their delimitation. Interpretation of the relationship of the present day channels to abandoned channels is crucial for the interpretation of avulsive channel system behaviour. A Geographic Information System (G.I.S.) has allowed the building of digital terrain model of the area using ARC /INFO software, so that the channels may be related to the topography.
The River Rapti and its associated channels provides an example of the interdependency of channels in an avulsive system. An anastomosing-avulsive system continuum is proposed which parallels the meandering-braiding channel- continuum, and reflects different scales and rates of various depositional processes. The channel pattern characteristics of present day channels and abandoned channels is studied using a series analysis approach. The interpretation of avulsive channel systems is seen to require a different scale of analysis from that conventionally adopted in fluvial geomorphology and sedimentology, particularly in terms of palaeohydrology and Quaternary climate-controlled river adjustments.
The sediment transport dynamics in the basin area are analysed using grain size analysis of point bar sediments, hydrological data and suspended sediment load data. This has shown the aggrading nature of the present day river channel which makes it susceptible to future avulsions. Optical luminescence dating of sediments is found to be a useful alternative to 14 C dating, given the paucity of organic dateable material and problems of interpreting dates of calcretes. This has proved useful in establishing a chronological framework of deposition in the Late Quaternary.
Three-dimensional GeoScientific Information System (G.S.I.S.) tools, utilising data from resistivity surveys and borehole records, are used to decipher the regional subsurface fluvial architecture incorporating local and regional sand body geometry. Variation in the architecture between avulsive channel systems and laterally-migrating channel systems is seen in different parts of the basin, providing information about the fluvial processes, sedimentation behaviour, and sand body architecture in a highly differentiated, subsiding basin. This may ultimately lead to a comparison with the Siwalik Group sediments, and a clearer picture of their depositional environments.