See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/302902544 Quaternary Geomorphic Processes and Landform Development in the Thar Desert of Rajasthan Chapter · January 2011 CITATIONS READS 6 4,293 1 author: Amal Kar Central Arid Zone Research Institute (CAZRI) 90 PUBLICATIONS 1,166 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Thar Desert Natural resources and their management View project Late Quaternary paleoclimate of the Thar Desert View project All content following this page was uploaded by Amal Kar on 11 May 2016. The user has requested enhancement of the downloaded file. acb publications Landforms Processes & Environment Management Kolkata, India Editor: S. Bandyopadhyay et al. [email protected] ISBN 81-87500-58-1 2011 (223-254) Quaternary Geomorphic Processes and Landform Development in the Thar Desert of Rajasthan Amal Kar1 Abstract: Evolution of landforms in the Thar Desert of Rajasthan is very much influenced by the exogenic and endogenic processes operating in the region during the Quaternary period. Studies have revealed that several fluctuations in climate between drier and wetter phases and periodic earth movements decided the type and intensity of geomorphic processes. The paper describes the broad sedimentation pattern in the desert, known facets of Quaternary climate and landform characteristics. It also discusses the influence of Quaternary climate change, neotectonism and human activities on landform evolution. Introduction The Thar, or the Great Indian Sand Desert, is situated in the arid western part of Rajasthan state in India and the adjoining sandy terrain of Pakistan. It forms a distinctive, but integral part of the arid lands of western India that runs through the states of Punjab, Haryana, Rajasthan and Gujarat. The eastern limit of the desert can be marked along the calculated moisture availability index (also called the aridity index) of –66.6, which roughly passes through the foothill zone of the degraded, NNE to SSW-trending Aravalli mountain ranges. In the west, the desert extends up to the fertile alluvial plains of the Indus in Pakistan. The Aravalli hill ranges, which partially control the spatial pattern of present-day rainfall in the region, and through it the efficiency of different geomorphic processes, were formed more than 2500 million years ago. It underwent at least three cycles of orogenesis and planation since the Proterozoic, and is now one of the oldest hill ranges in the world. The Aravalli orogenic cycles and the attendant widespread igneous activities were responsible for the construction of much of the basement for subsequent sediment accumulation. The basement in much of the desert area today is made up of granite and rhyolite and the gneissic complex. From the upper Proterozoic period onward sedimentation here took place in several basins, under continental and marine conditions. The identified basins are: (i) Marwar basin, (ii) Lathi basin, (iii) Jaisalmer basin, (iv) Barmer basin, (v) Palana-Ganganagar Shelf, and (vi) Sanchor basin. These 1Central Arid Zone Research Institute, Jodhpur 342 003 224 Landforms Processes and Environment Management basins, separated from each other by major tectonic features, including faults, formed together a shelf region that merged with the Indus geosyncline further west. Excellent reviews on the geological and structural frameworks of the Aravalli and the desert area to the west of it are available in Heron (1953), Narayanan (1964), Sen (1970), Chatterji (1977), Dasgupta and Chandra (1978), Pareek (1984), Roy (1988) and Sinha Roy et al. (1998). Broadly, the landforms in the Thar are fluvial, aeolian and lacustrine in nature (Fig. 1; Table 1). Fluctuating climate since the beginning of the Quaternary period about 2 million years ago played a major role in their evolution. There is also growing evidence of periodic tectonic activities during the Quaternary, which accelerated the subaerial processes and left their imprints on the landforms. The basement for Figure 1. Landforms of Thar desert, Rajasthan Quaternary sedimentation in Rajasthan part of the Thar was essentially a vast pediplaned surface which was composed largely of the Pre-Cambrian metasediments and igneous in the east and a gradually thickening sedimentary deposits of Mesozoic and Tertiary periods in the west. We propose to discuss here the present understanding of landform development in the region, the related geomorphic processes, as well as the possible forcing mechanisms. Earlier reviews on the geomorphology of the desert 226 Landforms Processes and Environment Management are available in Ghose et al. (1977a), Allchin et al. (1978), Kar (1992, 1995), Singhvi and Kar (1992), and Singh et al. (1997). Climate change during the Quaternary and sedimentation pattern The discovery of the wide, dry bed of the Ghaggar in the northern fringe of the Thar, its suggested link with the legendary Saraswati river from the Himalayas and establishment of the contribution of the Sutlej in its survival (Oldham, 1893), provoked the earth scientists to search for clues on climate change and tectonism. Ghose (1964) reconstructed from aerial photographs the early integrated drainage systems in the south-central part of the desert between Pali and Jalor, and concluded that some time in the past the climate in the desert was wetter. Several multi-disciplinary studies since then have established that the Quaternary climate fluctuated many time between wetter and drier phases (Bryson and Baerries, 1967; Singh et al. 1974; Ghose et al., 1977b; Allchin et al., 1978; Wasson et al., 1983; Singhvi and Kar, 1992, 2004; Kar et al., 2001, 2004). Observation of several shallow and deep Quaternary sequences confirms that the Quaternary lithofacies in the desert comprise essentially of the alternate fluvial and aeolian sedimentary deposits. Lacustrine and fluvio-lacustrine deposits are noticed in some favoured locations. Correct assessment of land forming processes and chronology of events during the early part of the Quaternary is difficult, especially because of a sharp erosional contact between the Quaternary and the underlying Tertiary/pre-Tertiary deposits, shallowness of the Quaternary sediment thickness at many places, paucity of good exposures in the areas of deep sequences, and problems of dating the old sediments through currently available techniques. Reliable chronology of events, on the basis of radiocarbon dating of organic materials and luminescence dating of quartz and feldspar in the sediments (Singhvi and Krbetschek, 1996), is available for the late Quaternary period only. The number of sites explored systematically for such purposes is, however, limited. Therefore, the spatio-temporal resolution of the derived chronology of events is coarse, and needs improvement. So far the majority of older surviving Quaternary deposits have been found to be a mixture of sand and gravel. Signatures like cross-bedded gravel-and-pebble-rich sediments over many of the Tertiary/ pre-Tertiary sequences in the western part of the desert suggest a wetter climate during the transition from Tertiary to Quaternary period, with high intensity rainfall that was responsible for such high energy fluvial deposition. Wadhawan (1988), Raghav (1992), Dhir et al. (1994), Wadhawan and Kumar (1996), and Rakshit and Sundaram (1998) have described typical Quaternary litho-stratigraphy from different parts of the desert and its eastern fringe. A summary is provided in Singhvi and Kar (1992). At many places the lowermost conglomerates consist of boulders and gravels of quartzite and other metamorphics of Aravalli provenance, as also granites, gneisses and sedimentaries from within the desert. However, considerable discordance was found in the sedimentation pattern, which provoked Wadhawan and Sural (1992) to suggest that the sedimentation took place Quaternary Geomorphic Processes and Landform Development in the Thar Desert of Rajasthan 227 in four different neotectonically disturbed linear sub-basins, named as ‘Shahgarh- Kishangarh’, ‘Sanchor-Shergarh-Dechu’, ‘Merta-Degana-Jayal-Didwana’ and ‘Bikaner-Churu-Ganganagar’. They opined that a series of NE-SW trending horst and graben structures was responsible for controlling the origin, configuration and development of these sub-basins of Quaternary deposition. These supposed sub-basins are, however, different from those which controlled the pre-Quaternary depositional pattern in the region, and which were described by Dasgupta and Chandra (1978). Although the early Quaternary deposits at many places were found to contain fine to medium sand of possible aeolian provenance, the paucity of purely aeolian strata within or below the conglomerate deposits, and a sharp contact with the underlying Pre-Cambrian or Tertiary beds still intrigue researchers about the subaerial processes which preceded the early Quaternary fluvial activity. Only on rare occasions the contact with basement is in the form of aeolian sand deposits, as for example near Osian (60 km N of Jodhpur) where the Pre-Cambrian Jodhpur Sandstone of Marwar Supergroup is followed upwards by a slightly lithified, fine aeolian sand, and then by the calcreted sand and gravel. It is possible that aeolian deposits of earlier dry phases in the desert were mostly reworked and mixed with fluvial sand and gravel during a prolonged and intense wet phase sometime during the early part of the Quaternary, which resulted