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IOURNAl OF QlJAllRNARY SCIENCE (2()()O) 15 (5) 501-501!.Copyril\ht «) 20(KJ lohn Wilcy & Sons, Ltd.

Chronology of Late Pleistocene environmental

changes in the lower Mahi basin, western

India .

NAVIN JUYAL1, RACHNA RAJ2, DEEPAK M. MAURYA2, LAXMAN s. CHAMYAL2 and ASHOK K. SINGHVI1*

IPhysical Research Laboratory, Ahmedabad, 380 009, India

lDepartment of Geology, M. S. University of Baroda, Vadodara 390 001, India

luyal. N., Raj. R., Maurya, D. M., Chamyal, l. s. and singhvi, A. K. Chronology of late Pleistocene environmental changes in the lower Mahi basin, westernIndia. I. Qualernary Sci., Vol. 15, pp. 501-508. IssN 0267-8179.

Received 23 September 1998; Revised 12 October 1999; Accepted 19 October 1999

ABSTRACT: Sediments exposed in the lower Mahi basin at the southern fringe oi the Thar DDesert, Rajasthan, India, provide evidence of three distinct depositional environments, namelymarine, aeolian and fluvial. These have been used to reconstruct Late Pleistocene environmentaland tectonic history of the region. Infrared stimulated luminescence (IRSL) chronology of thefluvial and aeolian litho-units provides evidence of two major fluvial aggradation phases in theregion corresponding to Oxygen Isotopic Stages 5 and 3. The basal marine clay is inferred torepresent the last interglacial stage and its present elevation at +20 m a.s.l. is attributed to post- journal of Quaternary Sciencedepositional tectonism. Comparison of fluvial records from other regions indicates interhemi-spherically documented wetter phases during Oxygen Isotope Stages 5 and 3. Copyright @ 2000John Wiley & Sons, Ltd.

KEYWORDS: Late Pleistocene; fluvial stratigraphy; luminescence dating; palaeoenviromental change;Mahi valley.

I ntroduction and Chamyal, 1997). The study was based on a near com-plete sequence at Rayka (Merh and Chamyal, 1997), andthe results show that a direct comparison of the sedimentary

Ephemeral rivers in arid zones provide an important archive succession to Oxygen Isotope Stages without any firm chron-of climate. It has been recognised nonetheless that rivers in ology (e.g. Khadikar et al., 1996) can sometimes lead toarid zones respond in a complex manner to climatic change. erroneous palaeoenvironmental reconstruction.Changes in erosion potential owing to precipitation varia-bility, changes in sediment supply, tectonism and extremeevents, all control the sedimentation patterns and associatedlandforms. Consequently, a proper elucidation of the genesis Geology and tectonic frameworkof sediments associated with such river systems is a prerequi-site for palaeoenvironmental reconstruction (Reid, 1994;Thornes, 1994; Vandenberghe, 1995; Knighton and Nanson, The Mahi river basin is the third largest in Gujarat (320001997; Reid and Frostick, 1997; Nanson and Tooth, 1999). km2) after the Rivers Narmada and Tapi. The Mahi river

In .India, only a limited effort has been expended towards emerges from the Arravalli mountains and flows along thean understanding of semi-arid/arid fluvial systems and eastern flank of the Cambay graben, which is an intracratonictowards establishing their chronology ()ain et al., 1998a; rift graben (Biswas, 1987). It then crosses the alluvial plainsTandon et al., 1996, 1999). We report here an attempt to of Gujarat before debouching into the Gulf of Cambay (Fig.reconstruct the chronology of the exposed sediments of the 1 ). The East Cambay Basin Margin Fault (ECBMF) cuts acrossMahi River, for which a preliminary stratigraph!c framework the Mahl river, as indicated by a change in orientation ofhas been proposed recently (Khadkikar et al., 1996; Merh the river (Fig. 2, Maurya et al., 1997). Several N-S and

NNW-SSE trending fractures that developed parallel to theECBNIF have strongly influenced the drainage pattern of this

.Correspondence to. A. K. singhvi, Earth Science Division, Physical Research river. Thus whenever the river approaches a lineament itlaboratory, Ahmedabad, )80009, India. deviates from its NE-SW path and takes a NNE-SSW course.E-mail: singhvi@prl.ernet.in Subsurface data indicate the presence of pre-existing step

C C ., f Sc.. f . d I d I R h faults parallel to the ECBMF that acted as the depocentreontract grant sponsor: ouncl o lentl IC an n ustrla esearc ...

Contract grant sponsor: Department of Science and Technology, India for Late Pleistocene sedimentation (Maurya et al., 1995).

Contract grant number: EsS/CA/A3-(08)95 Figure 3 shows a cross-section through the Quaternary sedi-