Geochemistry and provenance of stream sediments of the Ganga River and its major tributaries in the Himalayan region,India

Major, trace and REE compositions of sediments from the upper Ganga River and its tributaries in the Himalaya have been examined to study the weathering in the Himalayan catchment region and to determine the dominant source rocks to the sediments in the Plains. The Ganga River rises in the Higher Himalaya from the Higher Himalayan Crystalline Series (HHCS) bedrocks and traverses over the Lesser Himalayan Series (LHS) and the Himalayan foreland basin (Siwaliks) rocks before entering into the Gangetic Plains. The major element compositions of sediments, reflected in their low CIA values (45.0–54.7), indicate that silicate weathering has not been an important process in the Himalayan catchment region of the Ganga River. Along the entire traverse, from the HHCS through LHS and the Siwaliks, the sediments from the tributaries and the mainstream Ganga River show higher Na₂O, K₂O, CaO and silica. This, and the higher ratios of La/Sc, Th/Sc and lower ratios of Co/Th, suggest that the source rocks are felsic. The fractionated REE patterns and the significant negative Eu anomalies (Eu/Eu? = 0.27–0.53) indicate highly differentiated source. Moreover, the comparison of the sediments with different source rock lithologies from the HHCS and the LHS for their major elements clearly suggests that the HHCS rocks were the dominant source. Further, comparison of their UCC (upper continental crust) normalized REE patterns suggests that, among the various HHCS rocks, the metasediments (para-gneiss and schist) and Cambro-Ordovician granites have formed the major source rocks. The Bhagirathi and Alaknanda River sediments are dominantly derived from metasediments and those in the Mandakini River from Cambro-Ordovician granites. The resulting composition of the sediments of the Ganga River is due to the mixing of sediments supplied by these tributaries after their confluence at Devprayag. No further change in major, trace and rare earth element compositions of the sediments of the Ganga River after Devprayag up to its exit point to the Plains at Haridwar, suggests little contribution of the Lesser Himalayan and Siwalik rocks to the Ganga River sediments.     

Geochemistry and provenance of bed sediments of the large rivers in the Tibetan Plateau and Himalayan region

We investigated the geochemical characteristics of major, trace and rare earth elements and Sr–Nd isotope patterns of bed sediments from the headwaters and upper reaches of the six large rivers draining the Tibetan Plateau (the Jinsha River—Yangtze, Lancang River—Mekong, Nujiang River—Salween, Huang He—Yellow, Indus, and Yarlung Tsangpo—Brahmaputra). By using Ca/Al versus Mg/Al, La/Sc versus Co/Th, and ⁸⁷Sr/⁸⁶Sr versus ε_Nd (0) binary differentiation diagrams of provenance, some typical contributors to the different catchment sediments can be identified. In the Three-River (the Jinsha, Lancang, and Nujiang Rivers) tectonomagmatic belt, acidic–intermediate-acidic volcanic rocks are very important provenance of sediments. Carbonate rocks and Permian Emeishan basalts are dominant in the Jinsha River. The Yellow River sediments have similar geochemical characteristics with loess in catchments. The Indus and Yarlung Tsangpo Rivers sediments are mainly from ultra-K volcanic rocks and Cenozoic granitoids widely distributed in the Indus–Yarlung suture. The intensity of chemical weathering in these river catchments is evaluated by calculating the chemical indices of alteration (CIA) of sediments and comparing them with bedrocks. The CIA values of the six river sediments are from 46.5 to 69.6, closing to those of bedrocks in the corresponding catchment, which indicates relatively weak chemical weathering intensity. Lithology, climate, and topography affect the chemical weathering intensity in these river catchments.     

Sr isotopic signature of the Ganga Alluvial Plain and its implication to Sr flux of the Ganga River System

The influx of Sr responsible for increase in marine Sr has been attributed to rise of Himalaya and weathering of the Himalayan rocks. The rivers draining Himalaya to the ocean by the northern part of the Indian sub-continent comprising the Ganga Alluvial Plain (GAP) along with Central parts of the Himalaya and the northern part of the Indian Craton are held responsible for the transformation of Sr isotopic signature. The GAP is basically formed by the Himalayan-derived sediments and serves as transient zone between the source (Himalaya) and the sink (Bay of Bengal). The Gomati River, an important alluvial tributary of the Ganga River, draining nearly 30,500 km² area of GAP is the only river which is originating from the GAP. The river recycles the Himalayan-derived sediments and transport its weathering products into the Ganga River and finally to Bay of Bengal. 11 water samples were collected from the Gomati River and its intrabasinal lakes for measurement of Sr isotopic composition. Sr concentration of Gomati River water is about 335 μg/l, which is about five times higher than the world’s average of river water (70 μg/l) and nearly three times higher than the Ganga River water in the Himalaya (130 μg/l) The Sr isotopic ratios reported are also higher than global average runoff (0.7119) and to modern seawater (0.7092) values. Strong geochemical sediment–water interaction appearing on surface is responsible for the dissolved Sr isotopic ratios in the River water. Higher Sr isotopic rations found during post-monsoon than in pre-monsoon season indicate the importance of fluxes due to monsoonal erosion of the GAP into the Gomati River. Monsoon precipitation and its interaction with alluvium appear to be major vehicle for the addition of dissolved Sr load into the alluvial plain rivers. This study establishes that elevated ⁸⁷Sr/⁸⁶Sr ratios of the Gomati River are due to input of chemical weathering of alluvial material present in the Ganga Alluvial Plain.     

Geochemical evolution of the Tertiary succession of the NW shelf,Bengal basin,Bangladesh: Implications for provenance,paleoweathering and Himalayan erosion

Whole rock major, trace and rare earth element (REE) compositions of Paleogene to Neogene sedimentary rocks of the NW shelf succession (Province 1) of Bangladesh contain a record of interaction of the India and Asian plates, Himalayan tectonism, and climatic development. Analyses of 66 sandstones and mudrocks from the Tertiary succession of Bangladesh were made to examine provenance, source weathering, and the influence of paleoclimate and tectonism. The sediments display linear geochemical trends due to quartz dilution, and varying quartz–clay ratios produced by hydrodynamic sorting. Chondrite-normalized REE patterns for both sandstones and mudrocks from different groups are similar to upper continental crust, with moderate to high LREE enrichment (lithotypes within formations average La_N/Yb_N 5.31–11.41) and marked negative Eu anomalies (Eu/Eu^* 0.51–0.69). Based on geochemical criteria the succession can be divided into three parts (Jaintia; Barail–Surma; and Dupi Tila). Very high silica contents in Jaintia Tura sandstones and high Chemical Index of Alteration (CIA) indices in Kopili mudrocks (Fe-shales) suggest derivation from a deeply weathered and stable cratonic source (India). The Tura sandstones are interpreted as first-cycle quartz arenites, produced while the Indian plate drifted across equatorial regions during the Paleocene–Eocene Thermal maximum (PETM). The Barail–Surma and Dupi Tila sediments were derived from a felsic orogen (the Himalaya). The Barail–Surma sediments were mainly derived from the Trans Himalayan Batholith and associated granitoids, with significant contribution from the Lesser Himalaya. Mafic input is also evident, probably from intraoceanic arc material within the Himalaya. Barail mudrocks have uniformly high CIA values (92–95), suggesting intense steady-state weathering of their proto-Himalayan source, and warm and humid climate. In contrast, CIA values of Surma mudrocks range from 66 to 93 (average 84), suggesting non-steady state weathering related to active uplift in the Himalaya. The Dupi Tila sediments were derived from a more felsic Lesser Himalaya source, with significant contribution from the Trans-Himalaya and very little or no ophiolitic or arc material. Dupi Tila mudrocks have CIA ratios of 62–99 (average 72), also indicating non-steady state weathering in the rising Himalayan source. Geochemical compositions of the NW shelf sediments are comparable to coeval successions in the Surma basin (Province 2) of Bangladesh and the Siwaliks (India), indicating similar source. Evolution of the Indian monsoon and associated high precipitation caused intense chemical weathering of the Surma and Dupi Tila source, despite rapid uplift. The Surma Group thus bears the signature of evolution of the Asian monsoon in the Bengal basin at 21 Ma, simultaneous with the development of the East Asian monsoon. This supports proposals that both monsoon systems developed at the same time.     

Lower Palar River Sediments,Southern Peninsular,India: Geochemistry,Source-Area Weathering,Provenance and Tectonic Setting

This present study describes the geochemistry of fluvial sediments of the Palar river (lower reaches), Southern Peninsular India, with an aim to evaluate their provenance, weathering and tectonic setting. The bulk sediment chemistry is influenced by grain size. The river sediments are enriched with SiO₂ and depleted in Al₂O₃, K₂O, CaO, Na₂O, MgO, P₂O₅, MnO, Fe₂O₃ as compared with UCC values. Geochemical classification indicate that the sediments are mainly arkose, wacke and shale in composition. Discriminant diagrams together with immobile element ratio plots reveal that, the Palar river sediments are mostly derived from rocks formed in an active continental margin. Additionally, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate felsic rock sources. The chemical indices of alteration suggest that Palar river sediments are chemically immature and have experienced low chemical weathering effects. This is further supported by the Th/U Rb/Sr ratio and A-CN-K ternary diagram, with most of the sample data points falling close to the plagioclase-smectite line. The bivariate plot of Th/Sc versus Zr/Sc suggest a moderate recycled origin of the sediments.     

Chemical erosion in the eastern Himalaya: Major ion composition of the Brahmaputra and δC of dissolved inorganic carbon

Major ion composition of waters, δ¹³C of its DIC (dissolved inorganic carbon), and the clay mineral composition of bank sediments in the Brahmaputra River System (draining India and Bangladesh) have been measured to understand chemical weathering and erosion and the factors controlling these processes in the eastern Himalaya. The time-series samples, collected biweekly at Guwahati, from the Brahmaputra mainstream, were also analyzed for the major ion composition. Clay mineralogy and chemical index of alteration (CIA) of sediments suggest that weathering intensity is relatively poor in comparison to that in the Ganga basin. This is attributed to higher runoff and associated physical erosion occurring in the Brahmaputra basin. The results of this study show, for the first time, spatial and temporal variations in chemical and silicate erosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude. The Eastern Syntaxis basin dominates the erosion with a rate of ∼300 t km⁻² y⁻¹, one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains. In contrast, the flat, cold, and relatively more arid Tibetan basin undergoes much slower chemical erosion (∼40 t km⁻² y⁻¹). The abundance of total dissolved solids (TDS, 102-203 mg/L) in the time-series samples collected over a period of one year shows variations in accordance with the annual discharge, except one of them, cause for which is attributable to flash floods. Na* (Na corrected for cyclic component) shows a strong positive correlation with Si, indicating their common source: silicate weathering. Estimates of silicate cations (Na_sil+K_sil+Ca_sil+Mg_sil) suggest that about half of the dissolved cations in the Brahmaputra are derived from silicates, a proportion higher than that for the Ganga system. The CO₂ consumption rate due to silicate weathering in the Brahmaputra watershed is ∼6 × 10⁵ moles km⁻² y⁻¹; whereas that in the Eastern Syntaxis subbasin is ∼19 × 10⁵ moles km⁻² y⁻¹, similar to the estimates for some of the basaltic terrains. This study suggests that the Eastern Syntaxis basin of the Brahmaputra is one of most intensely chemically eroding regions of the globe; and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern Himalaya.     

Major ion chemistry of the Ganga source waters: Weathering in the high altitude Himalaya

A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO₃ and SO₄ are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations. The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km²/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.     

Weathering of the Ganga alluvial plain,northern India: implications from fluvial geochemistry of the Gomati River

In the northern part of the Indian sub-continent, the Ganga alluvial plain (GAP) feeds its weathering products to the Ganga–Brahmaputra River system, one of the world’s largest fluvial systems. The authors present a geochemical study of the GAP weathering products transported by the Gomati River (the Ganga River tributary) to understand weathering processes of an alluvial plain in a humid sub-tropical climate. A total of 28 sediment samples were collected during the monsoon season and were analysed by X-ray fluorescence spectrometry for 25 major and trace elements. Bulk chemistry of the channel, flood and suspended sediments mostly consists (>90%, >80% and >75%, respectively) of three elements; Al, Si and Fe. Major element concentrations normalised with respect to upper continental crust (UCC) show strong depletion of highly mobile elements (Na, Ca) and enrichment of immobile elements (Ti, Si). Silica enrichment in the sand fraction is probably caused by chemical weathering of feldspar. Mineral sorting during fluvial transportation acts as the single important factor that controls the geochemistry of these weathering products and also strongly influences major and trace element distribution in the individual sediment samples. Trace element (Ba, Cr, Cu, Nb, Ni, Pb, V and Zn) concentrations were strongly correlated with major element (Si, Al, Fe, Mn and K) concentrations indicating that the abundance of trace elements is controlled by the same processes that control the major element distribution in these sediments.The GAP weathering products were geochemically distinguished as arkose to litharenite in rock classification. Chemical mobility, normalised with respect to TiO₂ in UCC, indicates that Si, Na, Zr, Ba and Sr, mainly derived from feldspar, muscovite and biotite, are lost during weathering. Iron and Zn remained immobile during weathering and were strongly adsorbed by phyllosilicates and concentrated in fine-grained sediment fractions. The chemical index of alteration indicates that the GAP has experienced chemical weathering of incipient to moderate intensity. The GAP weathering products also demonstrated a progressive incomplete alteration in the alluvial sequence made-up of the Himalayan-derived sediments. A model has been proposed to better understand weathering processes and products of the GAP in temporary storage of ∼50 ka in a humid sub-tropical climate.     

Geochemical characteristics of modern river sediments in Myanmar and Thailand: Implications for provenance and weathering

The elemental composition of organic matter and the major and trace element compositions of stream sediments from Myanmar (Ayeyarwady and Sittaung rivers) and Thailand (Mekong and Chao Phraya rivers, and their tributaries) were determined to examine their distributions, provenance, and chemical weathering processes. Higher total organic carbon (TOC) and total nitrogen (TN) contents in the finer grained sediments indicate hydrodynamic energy may control their distributions. TOC/TN ratios indicate inputs of both aquatic macrophyte and higher vascular plant material to the river sediments. The major element abundances of the sediments are characterized by predominance of SiO₂ in coarser fractions and a marked negative correlation with Al₂O₃, representing primary grain size primarily control on SiO₂ content. Marked depletion of most labile elements (Na₂O, CaO, K₂O, Ba and Sr) relative to UCC (upper continental crust), indicate destruction of feldspar during chemical weathering in the source area or during transport. However, enrichment of some high field strength elements (Zr, Th, Ce and Y) relative to UCC and higher Zr/Sc ratios indicate moderate concentration of resistant heavy minerals in finer-grained samples. Discriminant diagrams and immobile trace element characteristics indicate that the Mekong, and Chao Phraya river sediments were largely derived from felsic sources with compositions close to typical rhyolite, dacite/granodiorite, UCC, I- and S-type granites. Relative enrichment of ferromagnesian elements (e.g. MgO, Cr, Ni) and high Cr/V and low Y/Ni ratios in Ayeyarwady and Sittaung sediments indicate the presence of a mafic or ultramafic component in their sources. The ICV (Index of Compositional Variability), CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), α^Al, Rb/Sr and K₂O/Rb ratios indicate that the Ayeyarwady and Sittaung sediments record low to moderate degrees of chemical weathering in their source, compared to moderate to intense chemical weathering in the Mekong and Chao Phraya river basins. These results are compatible with existing major ion data for river waters collected at the same locations.     

Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere

Extensive measurements of dissolved Re and major ion abundances in the Yamuna River System (YRS), a major tributary of the Ganga, have been performed along its entire stretch in the Himalaya, from its source near the Yamunotri Glacier to its outflow at the foothills of the Himalaya at Saharanpur. In addition, Re analysis has been made in granites and Precambrian carbonates, some of the major lithologies of the drainage basin. These data, coupled with those available for black shales in the Lesser Himalaya, allow an assessment of these lithologies’ contributions to the Re budget of the YRS.The Re concentrations in the YRS range from 0.5 to 35.7 pM with a mean of 9.4 pM, a factor of ∼4 higher than that reported for its global average concentration in rivers. Dissolved Re and ΣCations∗ (= Na∗+K+Ca+Mg) are strongly correlated in the YRS, indicating that they are released to these waters in roughly the same proportion throughout their course. The Re/ΣCations∗ in most of these rivers are one to two orders of magnitude higher than the (Re/Na+K+Mg+Ca) measured in granites of the Yamuna basin. This leads to the conclusion that, on average, granites/crystallines make only minor contributions to the dissolved Re budget of the YRS on a basin-wide scale, though they may be important for rivers with low dissolved Re. Similarly, Precambrian carbonates of the Lesser Himalaya do not seem to be a major contributor to dissolved Re in these rivers, as their Re/(Ca+Mg) is much less than those in the rivers. The observation that Re concentrations in rivers flowing through black shales and in groundwaters percolating through phosphorite-black shale-carbonate layers in phosphorite mines are high, and that Re and SO₄ are significantly correlated in YRS, seems to suggest that the bulk of the dissolved Re is derived from black shale/carbonaceous sediments. Material balance considerations, based on average Re of 30 ng g⁻¹ in black shales from the Lesser Himalaya, require that its abundance in the drainage basin of the YRS needs to be a few percent to yield average Re of 9.4 pM. Furthermore, the positive correlation between Re and ΣCations∗ would require that these Re-rich sediments (e.g., black shales) and Re-poor lithologies (e.g., crystallines, Precambrian carbonates) contribute Re and cations in roughly the same proportion throughout the drainage basin. The available data on the abundance and distribution of black shales in the basin are not adequate to test if these requirements can be met.The annual fluxes of dissolved Re at the base of the Himalaya from the Yamuna are ∼150 mol at Batamandi and ∼100 mol at Saharanpur, compared to ∼120 mol from the Ganga at Rishikesh. The total flux from the Yamuna and the Ganga account for ∼0.4% of the global riverine Re flux, much higher than their contribution to global water discharge. This is also borne out from the mobilization rate of Re: ∼1 to 3 g km⁻² y⁻¹ in the Ganga and Yamuna basins in the Himalaya, compared to the global average of ∼0.1 g km⁻² y⁻¹.Black shale weathering can also significantly influence the budgets of Os and U in rivers and CO₂ in rivers and the atmosphere. Using dissolved Re in rivers as a proxy, it is estimated that ∼(6-9) × 10⁸ kg y⁻¹ of black shales are being weathered in the Ganga and Yamuna basins in the Himalaya. Weathering of such amounts of black shales can account for the reported concentrations of Os and U in these rivers. Furthermore, if the weathering results in the conversion of organic carbon in the black shales to CO₂, it would release ∼2 × 10⁵ mol of CO₂ km⁻² y⁻¹ in the Yamuna and Ganga basins in the Himalaya, comparable to the CO₂ consumption from silicate weathering.     

Geochemistry of Silurian–Carboniferous sedimentary rocks of the Ulaanbaatar terrane,Hangay–Hentey belt,central Mongolia: Provenance,paleoweathering, tectonic setting,and relationship with the neighbouring Tsetserleg terrane

The Hangay–Hentey belt is situated in the central Northern Mongolia, and forms part of the Central Asian Orogenic Belt (CAOB). It is internally subdivided into seven terranes, the largest of which are the neighbouring Ulaanbaatar and Tsetserleg terranes. These coeval terranes are mainly composed of Silurian–Devonian accretionary complexes and Carboniferous turbidites. Proposals for their depositional setting range from passive margin through to island arc. A suite of 19 Ulaanbaatar terrane sandstones and mudrocks (Gorkhi and Altanovoo Formations) were collected with the aim of constraining their provenance, source weathering, and depositional setting based on whole-rock major and trace element data, and for comparison with the neighbouring Tsetserleg terrane. New REE analyses were also made of 35 samples from the Ulaanbaatar and Tsetserleg terranes. Geochemically the Ulaanbaatar sandstones are classed as wackes, and most of the mudstones as shales. Geochemical parameters suggest an immature source, similar to that of the Tsetserleg terrane. Geochemical contrasts between sandstones and mudrocks in the Ulaanbaatar sediments are small, and trends on element – Al₂O₃ variation diagrams are weak. Comparison with average upper continental crust (UCC), major element discriminant scores, and immobile element ratios (Th/Sc, Zr/Sc, Ce/Sc, Ti/Zr) indicate a uniform average source composition between dacite and rhyolite. Maximum Chemical Index of Alteration value in the Ulaanbaatar terrane is ∼65 after correction for K-metasomatism, indicating minimal weathering in a tectonically active source, similar to that of the Tsetserleg terrane. REE data in both terranes show moderate LREE enrichment and flat HREE segments, with negative Eu anomalies somewhat less than those in UCC and PAAS. Chondrite-normalized patterns are very similar to that for average Paleozoic felsic volcanic rock, supporting the relatively felsic source indicated by immobile trace element ratios. Tectonic setting discriminants (K₂O/Na₂O–SiO₂/Al₂O₃, La–Th–Sc, Th–Sc–Zr) indicate an evolved continental island arc (CIA; A2) environment for both terranes, similar to several other CAOB suites of similar age. This common arc source was situated within the Mongol-Okhotsk Ocean during Silurian–Lower Carboniferous time. The present-day Aleutian arc is a possible modern analogue of the depositional setting.     

Mineralogy,major and trace element geochemistry of riverbed sediments in the headwaters of the Yangtze,Tongtian River and Jinsha River

We collected riverbed sediments of the headwaters of the Yangtze River (Chumaer River, Tuotuo River, Gaerqu River and Buqu River), Tongtian River and Jinsha River (HTJR) flowing on the eastern Tibetan Plateau and analyzed their mineralogical features, major and trace element contents. The results show: (i) very poor correlations of Na₂O, K₂O, CaO, Ba, and Sr to SiO₂, LREE to Th, HREE to Hf, and Ta/La to Ti, and characteristics of Eu anomaly (the ratios of (Eu/Eu^*)_N range from 0.60 to 0.83 with an average value of 0.71) all indicate that the Jinsha River sediments have not undergone much mineralogical sorting; (ii) illite and chlorite are predominant clay minerals, and quartz, calcite, dolomite, albite, and K-feldspar are prevailing non-clay minerals. The characteristics of mineral assemblage indicate relatively weak chemical weathering degree in these river basins; (iii) very high contents of Fe₂O₃, MgO, TiO₂, Sc, V, Cr, Co, and Ni at Panzhihua mainly result from the huge-sized V–Ti magnetite deposits occurred in layered gabbroic intrusion; and (iv) the chemical alteration index (CIA) in the HTJR ranges from 46.5 to 69.2 and with an average value of 60.5 which indicates relatively weak weathering degree.     

Petrography and geochemistry of Eocene sandstones from eastern Pontides (NE TURKEY): Implications for source area weathering,provenance and tectonic setting

Subaerial weathering level, source area and tectonic environments were interpreted by using petrographic and geochemical characteristics of Eocene age sandstones found in the eastern Pontides. The thickness of Eocene age clastic rocks in the eastern Pontides ranges from 195 to 400 m. Mineralogical components of sandstones were mainly quartz, feldspar, rock fragments, and opaque and accessory minerals. Depending on their matrix and mineralogical content, Eocene age sandstones are identified as arkosic arenite-lithic arenite and feldspathic wacke-lithic wacke. CIA (Chemical Index of Alteration) values observed in the Eocene age sandstones (43–55) suggest that the source terrain of the sandstones was not affected by intense chemical weathering. Low CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values of the sandstones studied here suggest only slightly decomposed material and having undergone little transport until final deposition. Zr/Hf, Th/Sc, La/Sc and CIA ratios are low and demonstrate a mafic source; on the other hand, high LREE/HREE ratios and a slightly negative Eu anomaly indicate a subordinate fclsic source. Modal mineralogical and SiO₂/Al₂O₃ and K₂O/Na₂O and Th, Zr, Co, Sc of Eocene age sandstone contents indicate that they are probably magmatic arc originated and deposited in the back arc basin.     

Petrography and geochemistry of Jurassic sandstones from the Jhuran Formation of Jara dome,Kachchh basin,India: Implications for provenance and tectonic setting

Sandstones of Jhuran Formation from Jara dome, western Kachchh, Gujarat, India were studied for major, trace and rare earth element (REE) geochemistry to deduce their paleo-weathering, tectonic setting, source rock characteristics and provenance. Petrographic analysis shows that sandstones are having quartz grains with minor amount of K-feldspar and lithic fragments in the modal ratio of Q ₈₉:F ₇:L ₄. On the basis of geochemical results, sandstones are classified into arkose, sub-litharenite, wacke and quartz arenite. The corrected CIA values indicate that the weathering at source region was moderate to intense. The distribution of major and REE elements in the samples normalized to upper continental crust (UCC) and chondrite values indicate similar pattern of UCC. The tectonic discrimination diagram based on the elemental concentrations and elemental ratios of Fe₂O₃ + MgOvs. TiO₂, SiO₂ vs. log(K₂O/Na₂O), Sc/Cr vs. La/Y, Th–Sc–Zr/10, La–Th–Sc plots Jhuran Formation samples in continental rift and collision settings. The plots of Ni against TiO₂, La/Sc vs. Th/Co and V–Ni–Th ?10 reveals that the sediments of Jhuran Formation were derived from felsic rock sources. Additionally, the diagram of (Gd/Yb) _N against Eu/Eu ^? suggest the post-Archean provenance as source possibly Nagar Parkar complex for the studied samples.     

Geochemistry and mineralogy of the Al-rich shale from Baluti formation,Iraqi Kurdistan region: implications for weathering and provenance

The mineralogical and geochemical characteristics of the Upper Triassic Baluti shale from the Northern Thrust Zone (Sararu section) and High Folded Zone (Sarki section) Kurdistan Region, Iraq, have been investigated to constrain their paleoweathering, provenance, tectonic setting, and depositional redox conditions. The clay mineral assemblages are dominated by kaolinite, illite, mixed layers illite/smectite at Sararu section, and illite > smectite with traces of kaolinite at Sarki. Illite, to be noted, is within the zone of diagenesis. The non-clay minerals are dominated by calcite with minor amounts of quartz and muscovite in Sararu shale; and are dominated by dolomite with amounts of calcite and quartz in Sarki shale. Baluti shale is classified as Al-rich based on major and minor elements. The chemical index of alteration (CIA) is significantly higher in the Sararu than the Sarki shales, suggesting more intense weathering of the Sararu than the Sarki shales. The index of compositional variability (ICV) of the Sararu shale is less than 1 (suggesting it is compositionally mature and was deposited in a tectonically quiescent setting). More than 1 for Sarki shales (suggest it is less mature and deposited in a tectonically active setting). Most shale of the Baluti plot parallel and along the A-K line in A-CN-K plots suggest intense chemical weathering (high CIA) without any clear-cut evidence of K-metasomatism. Clay mineral data, Al enrichment, CIA values, and A-CN-K plot suggest that the source area experienced high degree of chemical weathering under warm and humid conditions, especially in Sararu. Elemental ratios critical of provenance (La/Sc, Th/Sc, Th/Cr, Th/Co, Ce/Ce*_PN, Eu/Eu*_PN, and Eu/Eu*_CN) shows slight difference between the Sararu and Sarki shales; and the ratios are similar to fine fractions derived from the weathering of mostly felsic rocks. The Eu/Eu^* _CN, Th/Sc, and low K₂O/Al₂O₃ ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Discrimination diagrams based on major and trace element content point to a role of the felsic-intermediate sources for the deposition of Baluti Formation, and probably mixed with mafic source rocks at Sararu section. The chondrite-normalized rare earth elements (REE) patterns are similar to those of PAAS, with light REE enrichment, a negative Eu anomaly, and almost flat heavy REE pattern similar to those of a source rock with felsic components. The source of sediments for the Baluti Formation was likely the Rutba Uplift and/or the plutonic-metamorphic complexes of the Arabian Shield located to the southwest of the basin; whereas the Sararu shale was affected by the mafic rocks of the Bitlis-Avroman-Bisitoun Ridge to the northeast of Arabian Plate. The tectonic discrimination diagrams, as well as critical trace and REE characteristic parameters imply rift and active setting for the depositional basin of the shale of Baluti Formation. The geochemical parameters such as U/Th, V/Cr, V/Sc, and Cu/Zn ratios indicate that these shales were deposited under oxic environment and also show that Sarki shale was deposited under more oxic environment than Sararu.     

Role of Quaternary stratigraphy on arsenic-contaminated groundwater from parts of Middle Ganga Plain,UP–Bihar,India

Late Quaternary stratigraphy and sedimentation in the Middle Ganga Plain (MGP) (Uttar Pradesh–Bihar) have influenced groundwater arsenic contamination. Arsenic contaminated aquifers are pervasive within narrow entrenched channels and flood plains (T₀-Surface) of fine-grained grey to black coloured argillaceous organic rich Holocene sediments (Newer Alluvium). Contaminated aquifers are often located close to distribution of abandoned or existing channels and swamps. The Pleistocene Older Alluvium upland terraces (T₂-Surface) made up of oxidized yellowish brown sediments with calcareous and ferruginous concretions and the aquifers within it are free of arsenic contamination. MGP sediments are mainly derived from the Himalaya with minor inputs from the Peninsular India. The potential source of arsenic in MGP is mainly from the Himalaya. The contaminated aquifers in the Terai belt of Nepal are closely comparable in nature and age to those of the MGP. Arsenic was transported from disseminated sources as adsorbed on dispersed phases of hydrated-iron-oxidea and later on released to groundwater mainly by reductive dissolution of hydrated-iron-oxide and corresponding oxidation of organic matter in aquifer. Strong reducing nature of groundwater is indicated by high concentration of dissolved iron (11.06 mg/l). Even within the arsenic-affected areas, dugwells are found to be arsenic safe due to oxyginated nature.     

Major ion chemistry of Renuka Lake and weathering processes, Sirmaur District, Himachal Pradesh, India

Renuka Lake in the Lesser Himalaya, Himachal Pradesh is in a valley surrounded by mountains comprised of highly crumpled, shattered, crushed, folded and dislocated rocks consisting of carbonaceous shales-slates often pyrite-ferrous, limestone, quartzites, boulder beds, etc. A detailed and systematic study of the major ion chemistry of the lake, clay mineral composition of the bed and core sediments and the Pb²¹⁰ isotope estimation in the latter was conducted. The chemistry is dominated by carbonate weathering and (Ca + Mg) and (HCO₃ + SO₄) accounts for about 90% of the cations and anions. The SO₄ content is almost the same as the HCO₃. The low contribution of (Na + K) to the total cations and the (Ca + Mg) and HCO₃ data tends to indicate that silicate weathering has not been the potential source of major ions to the lake waters. This difference may be related to the increasing susceptibility to weathering of carbonate over silicate rocks exposed in the catchment and also seepage of water at the bottom. The high sulphate content in waters is derived from dissolution of pyrite-ferrous reduced black shales, which constitute a significant lithology in the catchment. The chemical index of alteration (CIA) value in core sediments is on an average 76, which is comparable with average shale (70-75) and the rate of sedimentation 3.3 mm/year based on measurement of Pb²¹⁰, indicating a fairly high weathering rate in the catchment. Illite is the dominant clay mineral (52-90%) in the bed and core sediments, chlorite constitutes 7-48% and the kaolinite-chlorite mixed silicate layer is l-2%. This is consistent with the shale-slate, sandstone lithology in the valley.     

Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra–Jamuna River, Bangladesh: Inference from geochemistry

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj–Tangail (downstream) section of the Brahmaputra–Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO₂, Fe₂O₃, MgO, K₂O, P₂O₅, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO₂/Al₂O₃ and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na₂O, Sr and Pb increase with increasing SiO₂/Al₂O₃ and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra–Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra–Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra–Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A–CN–K ternary diagram, most of the samples close to the plagioclase–K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra–Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz.     

Late Cretaceous redbeds from the Gan-Hang Belt in Southeast China: petrography and geochemistry implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in Southeast China were generated by crustal extension associated with subduction of the Palaeo-Pacific plate during the late Mesozoic. Compared with adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstones from the Late Cretaceous Guifeng Group in the Yongchong Basin, Southeast China. Detrital grains are subangular to subrounded, poorly sorted, and rich in lithic fragments. Variable Chemical Index of Alternation values (59.55–79.82, avg. 66.79) and high Index of Compositional Variability (ICV) values (0.67–3.08, avg. 1.40) indicate an overall low degree of chemical weathering and rapid physical erosion of source rocks. Such features are consistent with an active extension tectonic setting. Other chemical indices (e.g. Al₂O₃/TiO₂, Th/U, Cr/Th, Th/Sc, Zr/Sc) also suggest significant first-cycle sediment input to the basin and a dominant felsic source nature. Thus, the Guifeng Group possibly underwent moderate to low degrees of weathering upwards. Sandstone framework models and geochemical characteristics suggest the provenance was likely a combination of passive margin (PM) and active continental margin (ACM) with minor continental island arc (CIA) tectonic settings. Sediment derivation from Neoproterozoic metamorphic rocks and Cambrian to Triassic granitoids indicates PM provenance, whereas sediments derived from Jurassic to Cretaceous granitoids suggest ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben basin under the back-arc extension regime when Southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.