Rachna Raj

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

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 ∼125 ka . Marine sediments correspond to the transgressive phases of the last interglacial (∼125 ka ) and post-glacial maximum (6 ka ) . 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– 50 m 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 6 ka and continued uninterrupted until ∼2 ka . 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 (∼125 ka ) 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 +7 m consistent with the global sea level estimates at 125 ka 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 125 ka 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.

Late Quaternary geomorphic evolution of the lower Narmada valley, Western India: implications for neotectonic activity along the Narmada–Son Fault

Geomorphic data combined with stratigraphic studies provide significant information to constrain timing and amount of fault movement. The lower Narmada valley lies astride the Narmada–Son Fault (NSF), an important ENE–WSW-trending tectonic element responsible for the current intraplate seismicity being experienced in the central part of the Indian plate. Varying nature and degree of tectonic movements along the NSF during Late Pleistocene and Holocene have produced four geomorphic surfaces in the lower Narmada valley: the alluvial plain (S1), ravine surface (S2), a gravelly fan surface (S3) and the valley fill terrace surface (S4). Two major phases of tectonic movements in a compressive stress regime are recorded along the NSF: slow synsedimentary subsidence of the basin during Late Pleistocene due to differential movement, followed by inversion of the basin during the Holocene marked by differential uplift along the NSF. The study suggests that the inversion of the basin is in response to the significant increase in the intensity of compressive stresses in the Indian plate mainly during the Early Holocene. The present incisive drainage and recent seismic activity indicate that the compressive stresses continue to accumulate along the NSF due to continued northward movement of the Indian plate.

Chronology of Late Pleistocene environmental changes in the lower Mahi basin, western India

Sediments exposed in the lower Mahi basin at the southern fringe of the Thar Desert, Rajasthan, India, provide evidence of three distinct depositional environments, namely marine, aeolian and fluvial. These have been used to reconstruct Late Pleistocene environmental and tectonic history of the region. Infrared stimulated luminescence (IRSL) chronology of the fluvial and aeolian litho-units provides evidence of two major fluvial aggradation phases in the region corresponding to Oxygen Isotopic Stages 5 and 3. The basal marine clay is inferred to represent the last interglacial stage and its present elevation at +20 m a.s.l. is attributed to post-depositional tectonism. Comparison of fluvial records from other regions indicates interhemispherically documented wetter phases during Oxygen Isotope Stages 5 and 3. Copyright

“Fluvial systems of the drylands of western India: a synthesis of Late Quaternary environmental and tectonic changes”: Quaternary International 104 (2003) 69–86

1. Page 73–‘After Maurya et al., 2000 and references therein’ to be added after Mainland Gujarat in caption of Fig. 4. 2. Page 74 column 2 para 1 line 14: the word ‘similar’ is to be replaced by ‘basal’ after Luni 3. Page 74 column 2 para 1 line 22: the words ‘Mahi and Narmada’ are to be deleted. 4. Page 74 column 2 para 1 line 23–26: The sentence ‘This correlation y and Narmada basins’ be read as ‘The Type-1 deposits of the Luni basin are not exposed in the Sabarmati, Mahi and Narmada basins.’ 5. The reference of Baskaran et al., 1986 in the reference list be read as ‘Baskaran, M., Rajgopalan, G. Somayajulu, B.L.K., 1989. Th/U and C dating of the Quaternary carbonate deposits of Saurashtra, India. Isotope Geoscience 79, 65–82. In the text ‘Baskaran et al., 1986’ be read as ‘Baskaran et al., 1989’.