Fluvial systems of the drylands of western India: a synthesis of Late Quaternary environmental and tectonic changes

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Abstract

The fluvial systems and landscape of the drylands of western India have preserved evidence of Late Quaternary environmental and tectonic changes. Data on the fluvial systems of Mainland Gujarat, Saurashtra and Kachchh have been synthesised to evaluate the roles of geological factors in the evolution of these drylands.

Mainland Gujarat is largely underlain by the flat alluvial plain, and has a structurally controlled fluvial system that originates in the eastern uplands. The fluvial system of Mainland Gujarat shows deeply incised valleys, entrenched meanders, extensively developed ravines and uplifted terraces, which have preserved a Late Quaternary succession of continental (fluvial and aeolian) and marine sediments dating back to ∼125ka. Marine sediments correspond to the transgressive phases of the last interglacial (∼125ka) and post-glacial maximum (6ka). The overlying alluvial sediments suggest deposition by ephemeral rivers in varied fluvial depositional environments under a semi-arid to sub-humid climatic regime with periods of intense pedogenesis. The sedimentation can be correlated with marine isotopic stages (MIS) 5 and 3. The fluvial sediments are buried under a thick blanket of aeolian sediments, which are indicative of enhanced dune building activity in the Thar Desert and are correlatable to the global arid phase of the last glacial maximum. The post-aeolian tectonic uplift triggered severe erosion of Late Pleistocene sediments and 40–50m deep incised fluvial valleys were formed. This erosional phase suggests a resumption of fluvial activity, which coincided with the rapidly rising sea level on the west coast during the Early Holocene. The Holocene marine and fluvial aggradation was initiated within the incised fluvial valleys around 6ka and continued uninterrupted until ∼2ka. These sediments now occur as raised valley fill terraces suggesting a Late Holocene uplift further corroborated by low to moderate seismic activity during historical times.

The landscape of Saurashtra is marked by flat-topped basaltic (trappean) ridges and a highly varied coastline where a narrow belt of low ridges and cliffs of miliolite limestones and other shore deposits are found. The fluvial system of Saurashtra shows a radial drainage pattern. The channels have steep banks in the hilly regions and show significant deflections before meeting the sea. Evidence of last interglacial high sea levels (∼125ka) are found on the coastal cliffs of southern Saurashtra in the form of oyster reefs and notches of various types which now occur at higher levels. A net sea level rise of +7m consistent with the global sea level estimates at 125ka has been obtained by recent studies after decoupling the tectonic component. Dating of Late Pleistocene and Holocene sea levels on this coast suggest continued uplift of Saurashtra since 125ka even though the sea level continued to fluctuate. The Holocene high sea submerged a considerable stretch of land including the Okha Rann on the northern Saurashtra coast and isolated patches in and around the river mouths on the southern coast. The short, straight and parallel courses of rivers in the direction of tectonic slope, incised and confined channel belts also suggest a strong component of tectonic uplift. The continental sediments exposed in these river valleys have however remained uninvestigated leading to lack of palaeoclimatic data.

A remarkable control of structure on landscape evolution is depicted by the Kachchh peninsula. The fluvial system exhibits the characteristics of drainage flowing along tectonically provided slopes. The overall drainage pattern of Kachchh shows excellent correlation with N–S trending transverse fault patterns. The transverse fault system has brought out changes in the landscape of Kachchh though the Kachchh rift basin evolved along E–W latitudinal faults. The Quaternary deposits occur in the form of miliolite limestones, colluvial and alluvial fans, fluvial sands and silts, and Rann sediments. Significant sedimentologic details on these sediments are lacking. However, the marine incursions seem to correlate with the adjacent Saurashtra and Mainland Gujarat.

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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