D. M. Maurya

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

Application of GPR for delineating the neotectonic setting and shallow subsurface nature of the seismically active Gedi fault, Kachchh, western India

The present field and GPR based investigations were carried out along the E–W trending Gedi Fault to precisely constrain the field location and shallow subsurface nature of the fault. The field investigations revealed the presence of thin Quaternary sedimentary cover, especially in the central and western part. Field examination of the scanty exposures showed that the fault trace marks the lithotectonic contact between Mesozoic rocks in the north and the Tertiary (Miocene) sediments to the south. Five sites were selected after field studies for GPR investigations of the Gedi Fault. The well-compacted Mesozoic rocks showed high amplitude returns while the softer and finer grained Tertiary sediments yielded low amplitude returns. The Quaternary sediments are reflected as consistent with wavy reflections in the upper parts of the profiles. The GPR data indicate that the Gedi Fault is a steep north dipping reverse fault which becomes vertical at depth. Since the fault does not displace the Quaternary deposits, we infer that the Gedi Fault has been characterized by low to moderate seismic activity under a compressive stress regime during the late Quaternary period.

Microbial Activity and Culturable Bacterial Diversity in Sediments of the Great Rann of Kachchh, Western India

Abstract The Great Rann of Kachchh, a vast expanse of salt desert in Western India is a unique hostile ecosystem posing an extreme environment to life forms due to high salt content, hyper-arid climate, seasonal water logging and extremes of temperature. In the virtual absence of natural vegetation, soils and sediments of Rann of Kachchh are microbially dominated ecosystems. In the present study microbial activity and the diversity of cultivated heterotrophic bacteria were investigated in the sediments collected along a 5-m exposed section at Khadir Island in the Great Rann of Kachchh. Microbial activity (as an index of sediment enzymes) was found to be high in the middle of the section (200-280 cm). Dehydrogenase (DHA), substrate-induced DHA and alkaline phosphatase activities revealed the oligotrophic nature of the basal portion (320-480 cm). Abundant bacterial isolates obtained from different depths were found to be clustered in 12 different phylogenetic groups by amplified ribosomal DNA restriction analysis. 16S rRNA gene sequencing revealed the dominant bacterial ribotypes to be affiliated to Firmicutes (Families Bacillaceae and Staphylococcaceae ) and Actinobacteria (Family Brevibacteriaceae ) with minor contribution of Proteobacteria (Families Phyllobacteriaceae and Bartonellaceae ), pointing their endurance and adaptability to environmental stresses. Statistical analysis indicated that sediment organic carbon, salinity, total available nitrogen and total available phosphorous are most likely critical determinants of microbial activity in the Khadir Island sediments.

A review and new data on neotectonic evolution of active faults in the Kachchh Basin, Western India: legacy of post-Deccan Trap tectonic inversion

Abstract The Kachchh Basin, located in Gujarat (India) at the western trailing edge of the Indian plate, comprises several east–west trending seismically active faults. The Kachchh Basin evolved in two major stages. The first is the rift stage, which correlates with the break-up and separation of the Indian plate in the Late Triassic–Early Cretaceous and synrift sedimentation. The second is the post-Deccan Trap inversion stage, when the basin was periodically uplifted along the existing east–west trending intrabasinal master faults: the Katrol Hill Fault, the Kachchh Mainland Fault, the South Wagad Fault, the Gedi Fault and the Island Belt Fault. The inversion of basin was initiated by the onset of a compressive stress regime in response to the collision of the Indian plate with the Eurasian plate in the far north during the Early Eocene. This is shown by the tilting of the Deccan Trap lava flows along with the underlying Mesozoic sequence and the associated intrusive bodies that occupy the core portions of domal and anticlinal flexures bounded by major fault lines. Seismotectonic data on the prolonged aftershock sequence after the 2001 Bhuj earthquake (Mw 7.7) reveal continuous low-to-moderate seismic activity along multiple faults covering a large area, now identified as the Kachchh Seismic Zone. This article reviews the neotectonic perspective of the active faults with the prime objective of delineating the post-Deccan Trap inversion phase of the Kachchh Basin, with an emphasis on neotectonism with regard to modern seismic activity. The datasets presented are primarily field-based neotectonic data from active fault zones that cover aspects of the tectonic geomorphology, Quaternary stratigraphy, near-surface fault traces and the nature of the fault in the shallow subsurface based on ground-penetrating radar studies. We also attempt a comparative neotectonic evaluation of each active fault in the Kachchh Basin and discuss the constraints for evolving a viable neotectonic model of the basin.

Tracing the Vedic Saraswati River in the Great Rann of Kachchh

The lost Saraswati River mentioned in the ancient Indian tradition is postulated to have flown independently of the Indus River into the Arabian Sea, perhaps along courses of now defunct rivers such as Ghaggar, Hakra and Nara. The persistence of such a river during the Harappan Bronze Age and the Iron Age Vedic period is strongly debated. We drilled in the Great Rann of Kachchh (Kutch), an infilled gulf of the Arabian Sea, which must have received input from the Saraswati, if active. Nd and Sr isotopic measurements suggest that a distinct source may have been present before 10 ka. Later in Holocene, under a drying climate, sediments from the Thar Desert probably choked the signature of an independent Saraswati-like river. Alternatively, without excluding a Saraswati-like secondary source, the Indus and the Thar were the dominant sources throughout the post-glacial history of the GRK. Indus-derived sediment accelerated the infilling of GRK after ~6 ka when the Indus delta started to grow. Until its complete infilling few centuries ago, freshwater input from the Indus, and perhaps from the Ghaggar-Hakra-Nara, probably sustained a productive marine environment as well as navigability toward old coastal Harappan and historic towns in the region.

Internal Sedimentary Architecture and Coastal Dynamics as Revealed by Ground Penetrating Radar, Kachchh coast, Western India

The coastline constitutes a very sensitive geomorphic domain which is constantly subjected to dynamic coastal processes and stores vital information regarding past sea level fluctuations. A ground-penetrating radar (GPR) survey was carried out along the northern coast of the Gulf of Kachchh which is one of the largest macrotidal inlets of the Arabian Sea, Western India. Our studies have delineated several radar surfaces and radar facies which reflect the internal architecture and sediment body geometry, which can be related to the processes acting along this coastline. Various radar facies, namely, beach ridge (Br), washover (Wo), coastal dune (Cd), swale (Sw), berm plain (Bp), and sandsheet facies (Ss) have been identified. The GPR studies successfully documented the subsurface presence of ancient beach ridge system towards the sea, and the coastal dunes towards the land side. The results are suggestive of signatures of changes in sea level and the coastline being prone to high energy events in the recent past. The GPR has been found to be an important non-invasive geophysical tool in the study of past coastal dynamics.

Morphotectonic control on Quaternary sedimentation and landscape evolution, Pachham Island, Kachchh, Western India

Pachham Island is the westernmost island in a chain of four rocky islands within the Great Rann of Kachchh that are bounded in the north by the Island Belt Fault (IBF). The island is characterized by two parallel and structurally controlled Kaladungar hill range and Goradungar hill range separated by a synclinal central valley. The central valley has acted as a sink for the Quaternary sediments. The island is transversely traversed by a N-S trending basement high termed as Median high that shows pronounced geomorphic and drainage anomaly. The Quaternary sediments comprise coarse-grained colluvio-fluvial deposits, miliolite, and sandy alluvium. The coarse-grained Quaternary deposits were primarily derived from the colluvium generated due to the uplift of the Kaladungar hill range and Goradungar hill range along the IBF and Goradunagar Fault, respectively. The colluvial debris were subsequently reworked, transported down slope and deposited in the central valley. The presence of the deeply incised valleys in Quaternary sediments, two levels of terrace surfaces over the Median high, and the raised Rann floor surrounding the island suggest a dominant role of neotectonics in the landscape evolution and Quaternary sedimentation.

Internal Geometry of Reactivated and Non-reactivated Sandblow Craters Related to 2001 Bhuj Earthquake, India: A Modern Analogue for Interpreting Paleosandblow Craters

The liquefaction attributes and crater geometry related to 2001 Bhuj earthquake has been reconstructed by trenching along large known craters formed near Umedpar in Kachchh. The study characterises the liquefied sediments in a large reactivated crater and distinguishes it from a non-reactivated crater located nearby. These characteristics can help in the interpretation of large paleocraters formed as a result of earthquake induced liquefaction.