U. K. Shukla

Upland interfluve (Doab) deposition: Alternative model to muddy overbank deposits

SummaryMajor alluvial plains contain large tracts of fine-grained muddy sediments, deposited away from the main river channels, which are mostly classed as overbank or floodplain deposits. Systematic study of the Ganga plain shows that such large tracts of deposition of muddy sediments are located several metres above the major channels, and are not flooded by overtopping of the major river channels. These surfaces are here designated as upland interfluve areas (Doab) where deposition of fine-grained sediments takes place independent of the processes operating in the main channels. The surfaces show distinct depositional domains with characteristic deposits. These include higher sloping surfaces (mottled silt), lower flat surfaces (variegated clayey silt), gulleys (sandy silt), small channels (mottled silty sand), ponds (shelly sandy clayey silt), lakes (shelly clayey silt). These deposits are prone to diagenetic changes, especially the development of calcrete horizons. Redistribution of these domains through time produces characteristic mud-dominant alluvial stratigraphy as observed in the Late Quaternary deposits of the Ganga plain. This succession shows similarity to mud-dominant deposits of the Siwalik succession. These Doab deposits are distinct from the overbank deposits formed close to the river channels affected by channel processes. It is argued that many of the thick mud-dominant fluvial deposits of the ancient fluvial record are products of deposition in upland interfluve areas.

Late Pleistocene¿Holocene hydrologic changes in the interfluve areas of the central Ganga Plain, India

Abstract Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region. Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to ∼8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds.

Sedimentation pattern in a trans-Himalayan Quaternary lake at Lamayuru (Ladakh), India

Abstract Located on the Indo-Tsangpo Suture Zone, Lamayuru Lake in western Ladakh was created by neotectonic instability around 35–40 ka. The valley fill sequence, about 110 m thick, reveals an interplay of lacustrine to fluvio-deltaic to colluvial processes operating in response to changing climate and tectonic conditions. Based on integrated approach of sedimentology and palaeontological analysis, we propose a sedimentation model for Lamayuru Lake with reference to changing climatic conditions and neotectonics. The coarsening upward succession was divided into five lithofacies associations (Associations A–E). Each association, comprising one or more lithofacies, represents a specific depositional environment, and marks an independent episode of sedimentation. A dominantly muddy sequence of Facies Association A, a product of a lacustrine environment, is followed by coarsening upward cycles (Association B) composed of silt to sand that implies laterally shifting deltas. Thereafter, thick sand and gravel deposits constituting fining upward (FU) unit cycles (Associations C and D) represent deposition in a fluvial domain. The upper part of Association D, composed of matrix-supported gravel horizons, was deposited as debris flows and marks the termination of sedimentation in the valley. The clastic lithofacies (Facies Associations A to D) with plant fragments and charcoal presumably indicate warm–humid climatic conditions. In contrast, Association E, comprising marl lithofacies occurring at four different levels is biogenic and indicates arid to semi-arid climatic phases. Five main types of trace fossils (including rhizoliths) seem to be representatives of Scoyenia ichnofacies and animal burrows indicating a thin population of soft-bodied annelid and arthropod communities living under low energy condition. Nine fossiliferous horizons have yielded characteristic ostracods, charophytes and gastropods indicating shallow, low energy and cold-water ecological conditions. Coarsening up lake succession, extensive synsedimentary deformation, tilting of lake beds and the entrenchment of the Lamayuru drainage indicate basin evolution under an effective role of neotectonics in the area.

Nagthat Formation: An example of a progradational, tide-dominated Proterozoic succession in Kumaun Lesser Himalaya, India

Abstract The Proterozoic Nagthat Formation of the Krol-belt succession, in the Nainital area, is composed mainly of fine- to coarse-grained quartzarenite with a subordinate amount of purple to grey sandstone, siltstone-shale and conglomerate horizons. The association with spilitic lava flows, variable palaeocurrent trends and the restricted lateral extent of the Nagthat Formation within the Krol-belt succession imply an active role for tectonism in the basin of deposition. In the upward coarsening succession of the Nagthat Formation, six major lithofacies have been identified: medium- to coarse-grained gravelly quartzarenite (Lithofacies A), planar cross-bedded, medium-grained quartzarenite (Lithofacies B), horizontally laminated, fine-grained quartzarenite (Lithofacies D), interbedded sandstone-shale (Lithofacies E) and matrix-supported conglomerate (Lithofacies F). The constituent lithofacies are repetitive in nature, forming upward fining unit cycles and interpreted to reflect deposition as upper shore-face, shoals and bars, barrier-beachface, tidal channels (inlets), intertidal–sandflat–mixedflat environments and, occasionally, in the form of gravity flows in subtidal channels. The general upward coarsening succession of the Nagthat Formation represents deposition in a progradational (regressive) barrier island system. The palaeocurrent pattern in the Nagthat Formation is distinctly polymodal and indicates sediment distribution across the roughly NW–SE trending shoreline, in response to a dominating flood tidal current system. The palaeocurrent pattern shows higher variability in the upper shore-face deposits than in the tidalflat domain. A recycled metasedimentary terrain served as the source for the Nagthat Formation, probably supplying the sediments from E, NE and S directions.

Climate control on erosion distribution over the Himalaya during past ~100 ka: COMMENT

[Rahaman et al. (2009)][1] inferred a climate control on Himalayan erosion using a sedimentary core from the Ganga Plain. Using prior sedimentological and chronological investigations done on the same core by their co-workers ([Sinha et al., 2007][2]), their study extends the work using Sr and Nd

Discovery of Elephas cf. namadicus from the late Pleistocene strata of Marginal Ganga Plain

We describe an elephant skull recovered from a cliff section of Dhasan river of Marginal Ganga Plain. The dental morphology and cranial features of the skull have been compared with the known species of Elephas from the Indian subcontinent. Although it shows very near resemblance to Elephas namadicus, but being an isolated specimen its specific identity cannot be proclaimed with certainty. As such, the specimen is provisionally referred as E. cf. namadicus. The Optically Stimulated Luminescence ages place this find at ~56 ka BP. This is the first chronologically well constrained report of E. cf. namadicus from the Ganga Plain.

Lithofacies of transgressive–regressive sequence on a carbonate ramp in Vindhyan basin (Proterozoic): a case of tidal-flat origin from central India

Palaeoenvironment of late Proterozoic carbonate deposits of Vindhyan basin of central India has been studied using lithofacies analysis, petrographical character and sediment dispersal patterns. Based on field parameters, 12 lithofacies were identified from five sections. For environmental interpretation, these lithofacies have been grouped into two major lithofacies associations namely a lower association A comprising reefal lithofacies, algal lithofacies, oolitic-intraclastic lithofacies, stromatolitic limestone lithofacies and an upper lithofacies association B, which includes nodular limestone lithofacies, large-scale planar cross-bedded lithofacies, parallel and ripple cross-laminated lithofacies, lenticular and flaser bedded lithofacies, shaly limestone lithofacies, synaeresis cracked lithofacies, deformed limestone lithofacies and finally, channelized and sandy limestone lithofacies. Reefal lithofacies and oolitic-intraclastic lithofacies were deposited under subtidal conditions. Synaeresis cracked lithofacies, lenticular and flaser bedded lithofacies, channelized and sandy limestone lithofacies were deposited under intertidal conditions whereas stromatolitic limestone lithofacies deposition ranged between subtidal to intertidal domains. Lithofacies character of the rock succession as well as the microfacies associations recognized petrographically indicate that the depositional environment belongs to platform interior with restricted circulation to marginal sand shoals in a carbonate ramp setting. Due to calcium-carbonate-saturated water, the diagenesis took place in stagnant marine phreatic environment where little compaction occurred, with abundant micrite as primary groundmass, and rapid cementation by calcite took place. With palaeocurrent current direction towards south and azimuthal dispersion of 270°, a pattern akin to a marine system is corroborated.

A case of normal regression with sea level transgression: Example from the Ganurgarh shale, Vindhyan basin, Maihar area, M.P., India

The Ganurgarh shale, a formation belonging to the Bhander Group of Vindhyan basin is investigated using field based detailed lithofacies and petrofacies analyses in order to interpret the depositional environment in a sequence stratigraphic context. Five major lithofacies have been recognized consisting of calcareous sandstones, laminated mudstones, rippled siltstones, red-grey shales and sandy limestones characterized by small to large-scale cross-bedding, ripple cross-lamination of wave and current origin, parallel lamination, low-angle horizontal bedding, flaser and lenticular bedding, mud-cracks, salt pseudomorphs, convolute bedding and load structures. The constituent lithofacies are recurring and grouped into three lithofacies associations where, the association A is composed of fining upwards and B with coarsening upwards cycles at the lower and middle levels of the succession respectively, are dominantly arenaceous whereas, the association C occurring at upper levels is fining upwards (FU) and becomes calcareous with meager representation of clastics. Petrographically, the section offers three main petrofacies viz., (a) sandstone- (b) siltstone- (c) sandy limestone-petrofacies. Lithofacies characters complimented with petrography show that deposition occurred within the shoreface (subtidal) to foreshore intertidal domain involving tidal flats with sub-environments ranging from intertidal to supratidal. However, lithofacies associations within the Ganurgarh shale of Maihar area represent a case of normal regression during sea level transgression. In the beginning, probably because of excessive sediment supply the sea level had a falling trend during an overall transgressive phase ultimately culminating into limestone sedimentation.

Palaeoenvironment and provenance of the Early Eocene arenaceous sequence of Neyshaboor, Binalud region, Iran

The Early Eocene sequence of Neyshaboor, Binalud region of Iran is predominantly composed of arenaceous deposits. Two stratigraphically important sections from the Damanjan and Taghan areas have been investigated based on field work, petrographic and geochemical analyses. Eight lithofacies were identified and have been grouped in to conglomerate, sandstone, and mudstone facies association. Petrographic and geochemical data show that the Early Eocene sandstones are mainly composed of arkoses and litharenites classes. Provenance analysis indicates that sediments were supplied from a nearby andesitic–granitic source with minor contribution of metamorphic and sedimentary sources. The presence of predominance of monocrystalline over polycrystalline quartz and abundance of K-feldspars; Cu and Pb contents also support this interpretation. However, subordinate representation of polycrystalline quartz grains, chert, volcanic rock fragments, biotite, zircon, as well as higher percentage of MgO and Fe2O3, suggest some contribution from high-grade metamorphic gneissic rocks and, to a lesser degree, from intermediate to basic volcanics. Climate varied from humid in the beginning of the sedimentation to sub-humid and arid during the later phases. Sedimentation was also influenced by prominent tectonic activity in the source when coarser clastic sediments were deposited as multistoried conglomerates. Lithofacies characters of the rock succession suggest sedimentation took place in a piedmont fan environment, adjacent to a rising orogeny in an active foreland basin setting. Clay to sand and gravel-sized sediments were laid down by meandering and braided rivers and by debris flows under changing conditions of climate and tectonics.

Ganga: The Arterial River of India

Ganga alluvial plain is one of the most densely populated regions of the world. It is drained by large snow-fed and small groundwater-fed rivers having different sources of water and sediment. Ganga, the main river of this plain, is formed by the confluence of Bhagirathi River with the Alaknanda River at Devprayag in Uttarakhand Himalaya and after travelling a distance of about 2525 km, drains into Bay of Bengal where it forms a huge Sunderban Delta. It is the most sacred river of India. The important tributaries are Ramganga, Yamuna, Gomati, Ghaghara, Son, Great Gandak, Burhi Gandak and Kosi. It exhibits braid bar, lateral bar, natural levee and various river terraces and is characterized by narrow incised channel confined within very wide valley in direct response to the climate, tectonics and sea level changes. The sediments and water of this river supports the culture and agriculture of the Ganga Plain as it is a perennial source of water. However, from last few decades the interference of man in the natural cycle of the river has polluted the river, increased the sediment load, reduced the natural recharge area, reduced the water holding capacity and floodplain of the river and completely disturbed its biodiversity and thus increased the river-borne hazard by ignoring the law of nature and scientific facts.