L. S. Chamyal

The character and genesis of calcrete in Late Quaternary alluvial deposits, Gujarat, western India, and its bearing on the interpretation of ancient climates

Late Quaternary deposits in Mainland Gujarat contain sediments deposited in subhumid and semi-arid climates. The 30–35 m succession shows the presence of Vertisols at the base and a red-bed horizon (ferric Calcisol when pedogenic in origin) that roughly bisects the succession. A widespread development of calcretes is observed throughout the succession. The various varieties of calcrete include pedogenic calcrete, groundwater calcrete, calcrete conglomerate (transported calcrete) and rhizogenic calcrete. Pedogenic calcrete nodules associated with Vertisols and the red-soil show marked differences in morphology, dimensions and the distribution of microscopic features. These differences arise due to contrasting climate-controlled physicochemical environs under which they formed. Pedogenic calcrete nodules associated with Vertisols acquire large (5–10 cm) dimensions and are characterised by either showing the presence of a nucleus of soil-matter or showing a dense micritic groundmass cut by thick sparitic veins. In contrast, calcrete nodules that formed in the red-soil are <3 cm in size and do not exhibit dense networks of sparitic veins. Calcretes associated with Vertisols and the ferric Calcisol also exhibit differences in the morphology of rhizoliths. These differences also show up in the distribution of microfabrics of calcretes. Grain coats are present in rhizogenic and pedogenic calcretes, but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes. Clotted micrite is present in all types except groundwater calcretes. Sparitic veins, however, are observed in each type, but are relatively less developed in groundwater calcretes. A similar distribution of displacive and replacive textures is also seen, although some grains in groundwater calcretes showed signs of corrosion. The Vertisol-associated calcretes represent a subhumid (500–700 mm) climate, whereas the red-soil calcretes suggest a semi-arid (100–500 mm) climate. Calcretes from the Vertisol association show a range in δ13C composition constrained between −9‰ and −5‰, whereas the red-soil calcretes exhibit the whole spectrum of values from −9‰ to −1‰. Based on mineralogical associations, calcretes, usually taken to reflect semi-arid episodes in the Earths history, may be classified further. Calcretes, when associated with sepiolite/palygorskite, suggest an arid climate (mean annual rainfall=50–100 mm), when associated with smectite, haematite and the absence of hydromorphism, a semi-arid climate (mean annual rainfall=100–500 mm), and when found in smectitic Vertisols, subhumid climates (mean annual rainfall=500–700 mm).

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

Holocene paleoenvironmental changes in the Lower Mahi Basin, Western India

Evidence of paleoenvironmental changes during the Holocene from the Lower Mahi basin of Western India have been documented. The unpaired S (sub 2) surface all along the estuarine zone of the Mahi basin has been identified as an uplifted marine terrace. The terraces have preserved in their lithosections fairly distinct horizons of grayish brown clays rich in marine microfauna. The intervening silty-sand horizons are indicative of freshwater origin. The sedimentary structure and faunal assemblage indicate that these units have been deposited in a marginal marine environment. The (super 14) C ages obtained on these marine mud horizons show that between 3600 and 1700 BP the sea level was higher than at present. The geomorphic evidence suggests that a late Holocene uplift has played a significant role in lowering the relative sea level to its present position.

Bedrock gorges in the central mainland Kachchh : Implications for landscape evolution

Kachchh possesses a fault-controlled first-order topography and several geomorphic features indicative of active tectonics. Though coseismic neotectonic activity is believed to be the major factor in the evolution of the landscape, detailed documentation and analysis of vital landscape features like drainage characteristics, bedrock gorges and terraces are lacking. The present study is a site-specific documentation of gorges developed in the central part of the mainland Kachchh. We analyzed and interpreted four gorges occurring on either side of Katrol Hill Fault (KHF). The Khari river gorge is endowed with six levels of bedrock terraces, some of which are studded with large potholes and flutings. Since no active development of potholes is observed along the rivers in the present day hyper-arid conditions, we infer an obvious linkage of gorges to the humid phases, which provided high energy runoff for the formation of gorges and distinct bedrock terraces and associated erosional features. Development of gorges within the miliolites and incision in the fluvial deposits to the south of the KHF indicates that the gorges were formed during Early Holocene. However, ubiquitous occurrence of gorges along the streams to the south of KHF, the uniformly N40‡ E trend of the gorges, their close association with transverse faults and the short length of the exceptionally well developed Khari river gorge in the low-relief rocky plain to the north of KHF suggests an important role of neotectonic movements

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.

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.

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.