Fluvial systems of the drylands of western India: a synthesis of Late Quaternary environmental and tectonic changes
Introduction
Fluvial systems provide vital evidence of geomorphic evolution of an area. Since rivers are extremely sensitive to tectonic and environmental changes, they are ideally suited for investigating the interaction between these two geological factors. This is well evidenced in regions having a long history of tectonic movements and sea level changes. Fluvial systems of semi-arid and arid areas of Gujarat show a strong structural control on the drainage architecture and the landscape of the area, which has evolved due to neotectonism and palaeoenvironmental changes (Pant and Juyal, 1993; Rachna 1998, Rachna 1999a, Rachna 1999b; Maurya 1995, Maurya 2000).
The state of Gujarat is located astride the Tropic of Cancer and forms an important part of the drylands of western India bordering the Thar Desert of the Indian sub-continent (Fig. 1) The climatic conditions over the state are mainly influenced by the southwest monsoon and the topography. Most part of the state falls in a semi-arid climatic zone merging with the arid zone to the north and northwest (Fig. 1). The monsoon rains are restricted to the period from June to September; the rest of the months are dry. The spatial distribution of rainfall is uneven, allowing distinct categorisation of the state into sub-humid, semi-arid, arid and extremely arid zones.
In the present article, we synthesise the available data on Late Quaternary environmental and tectonic changes and their implications on the fluvial systems and landscape of the drylands of Gujarat. In the process, several interesting features of drainage unique to this part of the Indian sub-continent are highlighted. The significance of the sedimentary records is emphasised in understanding the Late Quaternary geomorphic evolution of the region.
Section snippets
Regional geology and structure
The geological environment of Gujarat is the result of complex interactions between tectonism and sea level changes during the Cenozoic. The basic framework was formed due to sequential fragmentation of the western continental margin of the Indian plate during the Late Mesozoic as it collided with the Eurasian plate in the north (Biswas, 1987) (Fig. 2). The break up of the margin resulted in the formation of the Kachchh, Cambay and Narmada rift basins along the Delhi, Dharwar and Satpura trends
Geomorphology and drainage systems
Physiographically, Gujarat State is divisible into three well-defined areas: Mainland Gujarat, Saurashtra, and the Kachchh peninsula (Fig. 1). The three divisions correspond well with the three distinct tectonic provinces of Western India (Fig. 2).
Mainland Gujarat
The fluvial history of the various river basins of Mainland Gujarat has been studied in detail by Zeuner (1950), Pant and Chamyal (1990), Sridhar et al. (1994), Chamyal et al. (1997), Merh and Chamyal (1997), Tandon et al. (1997), Rachna et al. (1998), Maurya et al. (2000). These researchers have described the sedimentary records of the various river basins of Mainland Gujarat including the Luni basin in some detail. The exposed sedimentary record of these river valleys consists of
Quaternary environmental effects
Data on Early and Middle Pleistocene environmental changes in western India are completely lacking. The only area where evidence of such changes could be preserved is in the subsurface sediments of Mainland Gujarat. Some data on the Late Pleistocene–Holocene period have been generated from the exposed sediment successions of the Sabarmati, Mahi, and Narmada valleys and the Saurashtra coast. Data from Kachchh region on these aspects are virtually non-existent. According to Jain et al. (1999),
Tectonic impacts
The remarkable convergence of the geomorphic configuration of Gujarat with the various fault systems points to the overwhelming influence of tectonics as the genetic factor in the evolution of its landscape. However, this factor has not been understood well partly because of (1) the complicating factor of sea level change, (2) the different geological effects in different regions, and (3) there have been few well-documented studies. However, recent studies have provided unequivocal evidence of
Discussion
A simple argument put forward to emphasise the influence of tectonics on drainage basins is that tectonics creates new areas of continental surface characterised by discontinuities in surface slope (Leeder, 1993) and rivers subsequently adjust to these causes by changes in sinuosity, drainage length, avulsion, drainage basin area changes and other depositional and erosional changes in and around them. The drainage system of Gujarat is fundamentally influenced by the tectonic framework and
Acknowledgements
The inspiration to write this article came from Prof. A.K. Singhvi. We are grateful to Prof. Nicholas Lancaster and Prof. R.J. Wasson, and an anonymous referee for constructive reviews and useful suggestions, which improved the quality of the paper. This is a contribution to UNESCO-IGCP-413 on Drylands. Generous financial support from the Department of Science and Technology, New Delhi, under the research projects ESS/CA/A1-21/94 (LSC and DMM), ESS/23/VES/008/98 (LSC and DMM) and
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