Sumit K. Ghosh

Tectonic impact on the fluvial deposits of Plio-Pleistocene Himalayan foreland basin, India

Abstract The Himalayan foreland basin comprises a number of subbasins developed in various sedimentary environments during the Plio-Pleistocene phases of Himalayan orogeny. This paper describes the evolutionary processes that led to the development of the Himalayan foreland basin in the Dehra Dun and Subathu subbasins. The Dehra Dun subbasin essentially consists of conglomerate facies whereas Subathu subbasin is primarily composed of sandstone–mudstone intercalated with conglomerate. The two subbasins were sites of braided river systems. Evidence from chronostratigraphy, provenance and tectonic and climatic conditions suggests that differential tectonic movement, sedimentation rate and basin subsidence, coupled with climatic variation, were responsible for the contrasting fluvial facies in these subbasins. Following the merger of the two subbasins at about 5.23 Ma, the conglomerate facies assemblage of the Dehra Dun subbasin passes laterally into a sandstone–mudstone–conglomerate facies assemblage of the Subathu subbasin. Thickly bedded, amalgamated sheet conglomerate of Dehra Dun subbasin has a high sedimentation rate (0.70 mm/year) between 5.23 and 4.8 Ma, with a slow rate of basin subsidence. In contrast, a slow sedimentation rate (0.39 mm/year) in coeval sediments of the Subathu subbasin, with relatively fast subsidence, produced a succession of thick multistorey sandstone body with mature paleosols. Petrographic and clast composition data from Dehra Dun subbasin suggest that the detritus was derived from the pre-Tertiary Lesser Himalayan sequence lying in the north, during an active tectonic phase of the Main Boundary Thrust. Between 4.8 and 1.77 Ma, progradation of posttectonic conglomerate of the Dehra Dun subbasin continued during a quiescent phase of the Main Boundary Thrust. However, as a consequence of partitioning of Subathu subbasin along the Intra-Foreland Thrust, interfingering of buff (piedmont drainage) and grey (trunk drainage) sandstones took place. In contrast, the syntectonic conglomerate of Subathu subbasin originated from the uplifted proximal part of the foreland basin (Tertiary belt) during the active phase of Intra-Foreland Thrust around 1.77 Ma.

A multistorey sandstone complex in the Himalayan Foreland Basin, NW Himalaya, India

Abstract Ten parallel stratigraphic sections (1500–1800 m thick) spread over an area of >400 km2 in Dehra Dun sub-basin (DSB) of the Himalayan Foreland Belt (HFB) were studied to understand the anatomy of one of the largest (900–1200 m thick) fluviatile Multistorey Sandstone Complexes (MSC) of the world using fluvial geometry, compositional data and magnetic fabrics over a magnetostratigraphically controlled master section. The multistorey sandstone complex, between 10–5 Ma representing the Middle Siwalik sub-Group, comprises of grey, medium- to fine-grained lithic arenites to lithic greywacke and records tectonic and/or climatic episodes. Three main facies associations are recognised: sandstone–mudstone, sandstone, and conglomerate-sandstone that represent fluvial fan deposit. Palaeocurrent data show radial palaeoflow pattern with major palaeodrainage towards the southern quadrant. The magnetic fabric studies suggest three major tectonic pulses. The first pulse at ∼8.7 Ma resulted in the development of major depocenter for the MSC, the second pulse at ∼7.65 Ma enhanced the sedimentation and progradation, while the third pulse at ∼6.5 Ma records overlapping earlier fluvial fan by another coarse grained piedmont alluvial fan. Thrust movement in the northern fold belt, basement lineaments and rate of basin subsidence controlled the lateral and vertical facies distribution and palaeodrainage. The sedimentation pattern of the multistorey complex is characterised by mainly sheet flood deposits of laterally avulsing unconfined braided rivers and resembles to the modern megafan sedimentation in the Ganga Basin to the south.

Regressive-transgressive sedimentation in the Ordovician sequence of the Spiti (Tethys) basin, Himachal Pradesh, India

Abstract The Spiti basin comprises about 10,000 m of more or less fossiliferous Cambrian to Cretaceous sedimentary succession in the Tethyan-Tibetan Himalaya. The Ordovician strata in this basin consist of dominant sandstone with minor mudstone-siltstone facies and locally developed basal conglomerate and persistent topmost minor carbonates. This sequence has been regarded variously as continental and/or marine. To stretch out the environment of deposition, a detailed study on field characteristics, palaeocurrents, petrography and reported marine fauna and trace fossils was carried out, and suggests three major depositional environments, viz., fluvial, transitional and marine. The fluvial facies assemblage is characterised by fine- to coarse-grained multistoried sheets of sublithic arenite with abundant planar cross-stratification and low mudstone content and is interpreted as having formed in a braided river environment. The generalised palaeoslope is towards NE and NW. The transitional facies comprises fine- to medium-grained sublithic to subarkosic arenites having sheet and lenticular sandstones with small- to medium-scale trough and planar cross-stratification and subordinate herringbone cross-stratification, sigmoidal reactivation surfaces, flaser and tidal bedding, etc. These features indicate the influence of both non-marine (fluvial) and marine (tidal flat) environments. The marine facies consists of fine- to medium-grained subarkose, sublithic arenite and quartz wacke as well as sandy dolomicrite in the upper part. The sandstone facies are generally multistoried, having a sheet geometry, and are interstratified with mudstone. The important sedimentary structures include trough, planar and hummocky cross-stratifications and parallel lamination. The palaeoflow direction is generally towards NE and NW with some minor fluctuations toward the south and west (landward side). These characteristic features reveal deposition in the nearshore-shore zone. Three different stages of regression and transgression are recognised. Stage I resulted in the deposition of fluvial facies on the earlier marine domain (Cambrian) representing a regressive phase caused by differential uplift and subsidence probably triggered by magmatic intrusion and/or regional tectonics. Stage II represents channelising of the fluvial system on tidal flats marking an initiation of transgression. Finally Stage III, denoting the development of shore zone facies caused by basin subsidence and a change in sea level, is interpreted as a transgressive phase.

Depositional mechanisms as revealed from grain-size measures of the palaeoproterozoic kolhan siliciclastics, Keonjhar District, Orissa, India

Abstract The depositional mechanisms of the Palaeoproterozoic Kolhan Siliciclastics have been examined using grain-size analytical approaches. Grain-size distributions of different lithofacies reveal a somewhat diversified paralic depositional environment including nearshore, beach, macrotidal estuary and inland dunes. These inferences arise from different ways of analysing grain-size data, such as cumulative curves, bivariant plots, linear and multigroup discriminant functions and sediment trend matrix analyses. Sediment trend matrix analysis of various lithofacies taken together appears to be most effective for synthesizing the relationship between facies and sub-environments, taking into account the regional and local variations of lithofacies distribution and primary sedimentary structures. Grain-size distributions and sediment transport paths as revealed from the above statistical parameters suggest overall hydraulic control of the grain-size distribution of the Kolhan Siliciclastics in a fluvio-beach-shallow marine depositional setup.

Palaeoenvironmental analysis of the Late Proterozoic Nagthat Formation, NW Kumaun Lesser Himalaya, India

Abstract The Late Proterozoic Nagthat Formation (∼ 750 m) of the Jaunsar Group exhibits extensive development of multistoried medium- to thick-bedded sandstone, impersistent conglomerate (pebbly beds) and siltstone-shale. It represents a shoreface to a proximal inner-shelf deposit, developed under the dominant influence of tidal and occasional storm conditions. The major lithofacies recognised are the following. (a) The coarse-grained siliciclastic facies (CGS), represented by discontinuous beds of coarse-grained sandstone with lenses of conglomerates. The important sedimentary features observed in this facies are medium- to large-scale trough, tabular cross-stratifications and crudely graded conglomerates etc. The coarse-grained sandstone is frequently punctuated by numerous broadly undulatory or locally more or less flat erosional surfaces. A proximal shoreface environmental set-up has been assigned to this facies. (b) The interbedded medium- to fine-grained siliciclastic facies (IMFS), consisting of decimetre- to metre-thick alternations of medium- to fine-grained subarkosic to sublithic arenite with subordinate quartz arenite and quartz wacke. The striking sedimentary features are the herringbone cross-stratification and rare hummocky cross-stratification (HCS). The rareness of the HCS suggests a frequent reworking of storm-laden sediments by fair-weather processes in the intermediate shoreface zone. (c) The fine-grained siliciclastic facies (FGS) represented by parallel to cross-laminated silty sandstone and shale, is interpreted as the distal shoreface to proximal inner-shelf deposit. The study of these facies reveals the three prominent associations, viz., fining upward CGS-IMFS-FGS and CGS-FGS, and coarsening upward FGS-IMFS. The first two facies associations have resulted mainly from tidal activity, whereas the last one provides evidence of storm activity. The overall coarsening upward stratigraphic sequence suggests gradual shallowing of the basin and thus indicates a prograding behaviour of the Nagthat Formation.

Depositional environment and tectono-provenance of Upper Kaimur Group sandstones, Son Valley, Central India

The Mesoproterozoic Upper Kaimur Group consists of Bijaigarh Shale, Scarp Sandstone, and Dhandraul Sandstone. Based on the lithofacies data set, two major facies associations were identified, namely—tidal sand flat/sand bar facies association (TSFA) and tidally influenced fluvial channel facies/tidal channel facies association (TIFCFA). The Dhandraul Sandstone has been interpreted as a product of TIFCFA and the underlying Scarp Sandstone in TSFA which endorses a tidal dominated estuarine setting. Detrital modes of the Dhandraul and Scarp Sandstones fall in the quartz arenite to sub-litharenite types. Petrographical data suggest that the deposition of the Upper Kaimur Group sandstones took place in humid climate and was derived from mixed provenances. The sandstone composition suggests detritus from igneous rocks, metamorphic rocks, and recycled sedimentary rocks. The sandstone tectonic discrimination diagrams suggest that the provenances of the Upper Kaimur Group sandstones were continental block, recycled orogen, rifted continental margin to quartzose recycled tectonic regimes. It is envisaged that the Paleo- and Mesoproterozoic granite, granodiorite, gneiss, and metasedimentary rocks of Mahakoshal Group and Chotanagpur granite–gneiss present in the western and northwestern direction are the possible source rocks for the Upper Kaimur Group in the Son Valley.

Sedimentologic attributes of the Proterozoic siliciclastic packages of the Garhwal–Kumaun Lesser Himalaya, India: Implication for their relationship and palaeobasinal conditions

The present work addresses the long-standing issues on the characterization aspect of the Proterozoic siliciclastic successions exposed in the central part of the Lesser Himalaya, restricted between the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT). Geologic, sedimentologic, and petrographic study divides the Lesser Himalaya in two zones- northern Palaeo- Mesoproterozoic Inner Lesser Himalayan (ILH) and southern Neoproterozoic Outer Lesser Himalayan (OLH) zones. The major lithofacies recognized from the zones are - (i) coarse grained siliciclastic (CGS), (ii) interbedded medium and fine-grained siliciclastic (IMFS), (iii) argillite (ARG), and (iv) siliciclastic–argillite rhythmites (SAR). Amongst all these facies, the nearshore IMFS facies shows consistent presence in both OLH and ILH zones. From the facies distribution pattern, a northwest–southeasterly trending palaeo- shoreline has been envisaged. The CGS facies in the ILH hints towards an alluvial fan setting during 1.8 Ga rifting phase associated with penecontemporaneous basic magmatism. Compositionally, the siliciclastics of both the zones (ILH and OLH) are arenite and wacke types with minimal variation in their detrital proportions, derived from the early Proterozoic (between 2.4-1.6Ga) Aravalli-Delhi Supergroup provenance. Nearly matching types and content of detrital modes and the lithofacies pattern of the ILH and OLH siliciclastics probably conclude the derivation from the rising (nearby) Aravalli-Delhi orogen and deposition in a foreland like situation.

Petrography and Diagenetic Evolution of the Proterozoic Kaimur Group Sandstones, Son Valley, India: Implication Towards Reservoir Quality

In Central India the Upper Kaimur Subgroup of Vindhyan Supergroup, primarily consists of three lithounits-Dhandraul Sandstone, Scarp Sandstone and Bijaigarh Shale. The framework grains, mineralogy, matrix, pore properties and cements were identified. Average framework composition of the texturally super-mature Dhandraul Sandstone is Qt99 F0.1L0.8 and texturally less mature, Scarp Sandstone is Qt99 F0.2L0.8. The important diagenetic components identified based on the framework grain–cement relationships are mechanical compaction, cements, authigenic clays and dissolution and alteration of unstable clastic grains and tectonically induced grain fracturing. The early to intermediate stage of the diagnostic realm e.g., mechanical compaction, cementation, dissolution, and authigenesis of clays (dominantly kaolinite, mixed illite-smectite and minor illite). Mixed illite-smectite and illite occur as pore-filling and or lining during authigenic phases. Kaolinite and silica (quartz) overgrowth occur as pore-filling and lining cements. Compaction played an added role than the cementation in modifying the primary porosity. Cementation drastically reduced the porosity and permeability. Kaolinite fills pore spaces and caused reduction in the porosity and permeability of the sandstone. Secondary porosity development occurred due to partial to complete dissolution of feldspar. The diagenetic signatures observed in the Upper Kaimur Subgroup Sandstones are suggestive of intermediate burial (2–3 km depth). The reservoir quality of the studied sandstones is reduced by authigenic clay minerals (kaolinite, mixed illite-smectite and minor illite), cementations, and on other hand, it is increased by alteration and dissolution of unstable grains.

Petromineralogic and rock magnetic aspects of clastic sedimentation in the Surma basin, Mizoram

We present here comprehensive petromineralogic, mineral magnetic and lithologic observations from five stratigraphic sections representing the Barail Group, Middle and Upper units of Bhuban Formation, Bokabil Formation and Tipam Group in the Mizoram area. These stratigraphic units mainly display interplay of the grey and buff colored sandstones of the clast compositions varying from sub-lithic to lithic arenites (Q79F4L17 to Q55F3L42) including sedimentary, meta-sedimentary and subordinate igneous rock fragments. The buff sandstones with higher lithic fragments [Q69F4L27 (Ls61Lm38Lv1)] are dominated by recycled components with higher clast angularity (VA1A15SA51SR24R8WR0.4) relative to the grey sandstones. Mineral magnetic studies decipher bimodal (ferri- and antiferromagnetic) mineralogy with higher concentration in buff sandstones relative to the unimodal ferrimagnetic nature of the grey sandstones.The study infers that the buff sandstones mark the regressive phases driven by hinterland uplifts; whereas the growth of the grey sandstone facies is marked by transgressive basinal processes. Gradual increase in the frequency and appearance of the buff sandstones in the Surma stratigraphy, therefore, can be related to the evolution of the Indo-Burmese ranges.