Deccan Basalts Research Articles

The geological site characterisation of the Mandla region, Eastern Deccan Volcanic Province, Central India

Detailed geological studies were carried out on the basaltic sequences along the Jabalpur–Niwas, Jabalpur–Chutka and Jabalpur–Mandla traverses covering an area of about $$12\hbox { km} \times 15\hbox { km}$$ to characterise various basaltic lava flows and their behaviour on seismotectonics and geodynamic setting of their formation in the Mandla region of the Eastern Deccan Volcanic Province (EDVP). The studies involve an analysis of the satellite images for the identification of lineaments/faults and field geological studies consisting of geological controls such as ground check, thickness of fractures and orientation along the acknowledged lineaments/faults. The results of the present research comprising 65 lineaments/faults mainly belonging to two geometric groups, minor and major dominantly in the NW–SE and the NE–SW and altered strata varying lithology (weathered to compact basalts) are recognised in the study area. Based on their extent, 57 lineaments have been classified as minor ( 300 km) lineaments are noticed in the study region. The field geological investigations have facilitated the recognition of 10 flows with different characteristic features and a variety of volcanic structures such as columnar, vesicular, amygdaloidal, inflated pahoehoe lava flows and red bole interflow horizons have been documented. Basement rocks of these Deccan basalt lavas are represented by Tirodi Biotite gneisses, quartzite, quartz–mica schists and crystalline limestone in the SE part of the study area of the Mandla region. The present study will help evaluate the localised site characterisation for urban planning and setting up major civil structures.

Geochemical investigations in bulk and clay size fractions from lower Krishna river sediments, southern India: implications of elemental fractionation during weathering, transportation and deposition

Major and trace elements, including rare earth elements (REEs) geochemistry of bed sediments from the lower reaches of Krishna river suggest their origin mainly from weathering of Deccan basalts and Precambrian rocks of the southern India. The elemental compositions indicate evidence of sedimentary sorting during weathering, transportation and deposition. Highly depleted Na2O and high values of Chemical Index of Alteration (CIA) are suggestive of moderate to strong chemical weathering environment in lower reaches of Krishna river. Major, trace and REE concentrations of bulk sediments suggest marginal variations in relative contribution from mafic and felsic minerals. The bed sediments contain significantly low concentrations of trace elements like Ba, Sr, La, Ce and Nd and moderately low concentrations of V, Zr, Rb, Zn, Ni and Cr relative to upper continental crust (UCC) composition. Most of the other trace elements and REEs are almost similar to UCC indicating their origin from weathering of mixed rock types in catchment areas of Krishna river. Strong relationships of ΣREEs with Ti and Zr in bed sediments suggest that REEs are mainly controlled by heavy minerals such as titanite and zircon. A marked increase in REEs in clay fractions of sediments indicates their adsorption and/or substitution on clay lattice.

Clay Minerals and Associated Metals in Mudflat Core Sediments of the Vaghotan Estuary, India: Implications of Metal Sorption

ABSTRACT Mudflat core sediments representing sub (V-1) and main (V-2) channels of the Vaghotan estuary were investigated for clay mineralogy and their chemistry. The depositional environment has regulated differential flocculation of clay minerals in sub and main channels of the estuary. The concentration of kaolinite and smectite was higher than illite and chlorite in sediments which was attributed to the weathering of laterites and Deccan basalts in the drainage basin, and river-sea water mixing condition. Further, the concentration of Al, Fe, Mn, Co, Cu, Zn and Cr in the clay fraction was higher in the sub-channel than the main channel of the estuary. The variation in clay minerals composition, source of metals and depositional conditions regulated the distribution of metals in sub and main channels of the estuary. The significant correlation of smectite with Zn, and kaolinite with Al, Fe, Mn, Ni, Co and Cr in core V-2 suggests their role in sorption of metals. However, clay minerals (except smectite with Zn) showed insignificant correlation with metals in the core V-1. In the sub-channel adsorbents, other than the clay minerals seemed to have regulated the sorption of metals.

Thermodynamic Modelling and Experimental Validation of CO2 Mineral Sequestration in Mandla Basalt of the Eastern Deccan Volcanic Province, India

Abstract Deccan basalt contains primary silicates rich in Ca, Mg, and Fe ions, suitable for CO2 sequestration. Previous, basalt-water-CO2 interaction studies were focused on other than Deccan basalt types. However, such studies on the Deccan basalt are rare. Thus, present basalt-CO2 water saturated interaction modelling under hydrothermal-like conditions was carried out on the Deccan basalt of the Mandla area to understand apposite pCO2 and time parameters. Modelling results were further validated by experiments run in a laboratory time framework. Present results show negative entropy (ΔS) and enthalpy (ΔfH) that suggest feasibility of plagioclase, pyroxene and magnetite dissolution. Obtained negative Gibb’s free energy (ΔfG), ΔfH and ΔS values for calcite, dolomite and magnesite indicate spontaneous reaction, whereas, positive ΔfG and negative ΔfH and ΔS values of the siderite suggest non-spontaneous and opposing reactions. Calcite is the first carbonate mineral to form, but, at a faster rate. Magnetite dissolution begins after a time lag (not initiated along with the plagioclase and pyroxene). X-ray Powder Diffraction results of post-experiment residues revealed formation of calcite, aragonite, ankerite, huntite, siderite, smectite, chlorite, smectite/chlorite mixed layers and chabazite. Scanning electron microscopic images show tiny calcite crystal growth over the larger calcite crystal and incipient-disordered calcite with imperfections on crystal faces. Thus, basalt carbonation is mainly controlled by time, but temperature, pCO2 and pH played sub-ordinate role. Largely, thermodynamic models do not agree well with the experimental results as numerical models indicate larger carbonate growth. Additionally, transition state theory based models work well to predict dissolution rates for most of the minerals, but, they overpredict growth of the secondary minerals.

Tracing source–sink processes and productivity from trace metals (Ba, Zn, Pb, Cd) of the surface sediments off Mahanadi to Pennar, western Bay of Bengal

In this paper, an attempt is made to understand the provenance of sediments and the role of dissolved oxygen in the preservation of metals and the productivity variations. Sixty-one surface sediment samples collected along eight transects from western Bay of Bengal off Mahanadi, Vamsadhara, Godavari, Krishna and Pennar Rivers were analyzed for spatial variations in selected metals (Ba, Zn, Pb, Cd). A change in source of sediment from north to south was very well reflected from the distribution of metals. Ba and Pb were dominant in the northern transects off Mahanadi and Vamsadhara indicating a felsic source for the sediments as Ba and Pb are more associated with rocks rich in feldspar. Conversely, lower Ba and Pb off Godavari, Krishna and Pennar indicated mafic source for the sediments as these metals are less associated with Deccan basalts. Furthermore, the preservation and distribution pattern of the redox-sensitive metals Zn and Ba was largely controlled by the oxygen minimum zone prevalent in the region at intermediate water depths. Negative biogenic Ba values and a poor correlation of Cd with Chl-a and P indicated low Ba and Cd input from plankton productivity, further enhancing the existing knowledge regarding low productivity in western Bay of Bengal.

Mineralogy, geochemistry and40Ar–39Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps: early silicic magmas derived by flood basalt fractionation

AbstractMost continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.

The Late Cretaceous-Paleogene Deccan Traps: a Potential Global Heritage Stone Province from India

The Late Cretaceous/Palaeogene Deccan Traps, spread over half a million square kilometres in west-central parts of the Indian Peninsula, comprises predominantly basalts and subordinate felsic rocks (rhyolite, trachyte, tinguaites and spilites). The use of Deccan basalts and trachyte in the range of Indian architectonic heritage, from antiquity to current times, is quite fascinating. The abundance and ease of workability of Deccan Trap rocks render them the most preferred building material in all the architectonic heritage structures that document various cultural and ethnic habitations in the west central India. The Buddhist, Hindu and Jain temples of Ajanta, Ellora and Elephanta are the oldest in-situ basalt cave temples, dating back to second century. A contemporary iconic architectonic heritage ‘The Gateway of India’ personifies a majestic edifice built in buff coloured trachyte. In the present times, the Deccan basalts are being quarried for a variety of purposes such as masonry stone for buildings, road metal and as raw material for corrosion-resistant basaltic pipes and basalt wool that are also in great demand internationally. The trachyte quarries are almost extinct and relict trachyte outcrops are preserved in some places in and around Mumbai. The Deccan Trap rocks basalt and trachyte bear testimony to India’s cultural and architectonic heritage, are therefore, the most suitable candidates for according the status of Global Heritage Stone Resource (GHSR) as they fulfil all the requisite criteria specified by the IUGS subcommission on Hertaiage stones referred to as the Heritage Stone Subcommission (HSS). It is worthwhile to mention that both basalt and trachyte of the Deccan Traps are in geographic and geological proximity with each other and this qualifies Deccan Trap province to be recognised as a Global Heritage Stone Province (GHSP) from this subcontinent in consonance with the criteria for GHSP mentioned in the terms of reference given by HSS.

Exploration of deep terrestrial subsurface microbiome in Late Cretaceous Deccan traps and underlying Archean basement, India

Scientific deep drilling at Koyna, western India provides a unique opportunity to explore microbial life within deep biosphere hosted by ~65 Myr old Deccan basalt and Archaean granitic basement. Characteristic low organic carbon content, mafic/felsic nature but distinct trend in sulfate and nitrate concentrations demarcates the basaltic and granitic zones as distinct ecological habitats. Quantitative PCR indicates a depth independent distribution of microorganisms predominated by bacteria. Abundance of dsrB and mcrA genes are relatively higher (at least one order of magnitude) in basalt compared to granite. Bacterial communities are dominated by Alpha-, Beta-, Gammaproteobacteria, Actinobacteria and Firmicutes, whereas Euryarchaeota is the major archaeal group. Strong correlation among the abundance of autotrophic and heterotrophic taxa is noted. Bacteria known for nitrite, sulfur and hydrogen oxidation represent the autotrophs. Fermentative, nitrate/sulfate reducing and methane metabolising microorganisms represent the heterotrophs. Lack of shared operational taxonomic units and distinct clustering of major taxa indicate possible community isolation. Shotgun metagenomics corroborate that chemolithoautotrophic assimilation of carbon coupled with fermentation and anaerobic respiration drive this deep biosphere. This first report on the geomicrobiology of the subsurface of Deccan traps provides an unprecedented opportunity to understand microbial composition and function in the terrestrial, igneous rock-hosted, deep biosphere.

Geochemical Evidences for Possible Absence of Cu-Sulfide Deposits in the Deccan Volcanic Province, India

Continental flood basalt provinces (CFBs) are important hosts for large-scale Cu-sulfide deposits. However, sulfide mineralization is yet to be discovered, if any, in the end-Cretaceous Deccan volcanic province, India. In the present study, geochemical evidences for the possible absence of Cu-sulfide deposits associated with the Deccan basalts by analyzing and comparing the geochemistries of the Deccan and Siberian CFBs are provided. The Fe-rich nature and high fO2 conditions did not favour sulfide saturation at any stage of magma evolution in the Deccan province. Crustal contamination of the Deccan magmas also did not increase the sulfur budget. The most contaminated basalts of Bushe and Poladpur formations of the Deccan province do not show any depletion in the copper contents compared to other formations. In the absence of sulfide saturation, copper behaved as an incompatible element in the Deccan magmas in contrast to the Siberian basalts, in which copper behaved as a compatible element during magma evolution due to sulfide saturation consequently formed world-class copper sulfide deposits. It is demonstrated that the lithosphere- and asthenosphere-derived Deccan magmas have similar Cu abundances thereby suggesting that the Cu-sulfide deposits associated with the CFBs are process-controlled rather than source-controlled. Although Cu-sulfide deposits may not have formed, the geochemical patterns suggest favourable conditions for native copper mineralization in the Deccan volcanic province. In the present study, a set of geochemical proxies that can be utilized as preliminary exploration tools for Cu-sulfide mineralization in the CFBs is proposed.

Subsurface imaging of brown coal bearing Tertiary sedimentaries - Deccan Trap interface using microtremor method

We propose an application of the microtremor (ambient noise) H/V spectral ratio technique to identify significant rheological boundaries at shallow depths, estimate thickness of both lignite bearing Tertiary sedimentary sequence and late Cretaceous Deccan basalt flows and comprehend basinal geometry of Umarsar Basin (Babia syncline). Forty-six stations were gauged in a grid format at ~250 m resolution during the microtremor survey. The microtremor H/V spectral ratio technique takes into account the frequency of the ratio between the horizontal (NS + EW) and vertical components of persistent Rayleigh waves in the area. Depth estimates are made using Mean_Vs (433.69 m/s) from three borehole records (MMA_Vs (431.6 m/s), MMB_Vs (406.86 m/s) and MMC_Vs (462.6 m/s) using standard relationship between depth and velocity. In the present study, we recognize three rheological interfaces viz., L1 interface (0.2328 Hz to 0.3862 Hz), L2 interface (0.7843 Hz to 2.5123 Hz) and L3 interface (6.2477 Hz to 27.1119 Hz). The geology and stratigraphic records supplement correlation for L1, L2 and L3 interfaces with Mesozoic–Deccan Trap (M-DT) boundary, Deccan Trap-Tertiary (DT-T) boundary, and boundaries between shale‑carbonate hardpans within sediment sequence belonging to Naredi Formation respectively. The estimated depth range for M-DT boundary (L1) is 281–466 m and for DT-T boundary (L2) is 43–138 m. The subsurface image acquired from the frequency records advocates for a palaeo high in SE portion that retains its entity over the present landscape. The frequency records advocate for 369 m to 206 m thick Deccan basalt and 85 m to 18 m thick lignite bearing Naredi Formation. The ambient noise seismic study further suggests NW-SE trending basin geometry of Babia syncline comprising three distinct depressions having six local depocenters. Finally, we propose the microtremor H/V spectral ratio technique as a tool to develop economical borehole plan with realistic reserve estimate and a step forward towards rapid economical assessment covering large mining lease areas complementary to local geological studies.

Lineaments in Deccan Basalts: The Basement Connection in the Koyna–Warna RTS Region

Lineaments in Deccan Basalts: The Basement Connection in the Koyna–Warna RTS Region

Electrical resistivity cross-section across northern part of Saurashtra region: An insight to crystallized magma and fluids

Saurashtra region forms the Western part of the Deccan volcanic province (DVP) in India and is bounded by the Cambay rift basin (CRB) in the east, Narmada rift basin (NRB) in the South and Kachchh rift basin (KRB) in the north. This region is uplifted due to different stages of inter-continental breakup (Madagascar-India & Seychelles-India), different stages of rifting, reactivation of fault zones and Deccan volcanism. The major portion of Saurashtra region is covered by Deccan basalts underlain by Mesozoic sediments. Detailed magnetotelluric (MT) and long period magnetotelluric (LMT) soundings have been carried out in the northern part of Saurashtra with an aim to bring out the geoelectrical configuration of different basins (Jamnagar, Jasdan, and Cambay) along the profile. 2D inversion carried out for different modes: transverse electric (TE), transverse magnetic mode (TM) and TE + TM independently. The inversion of different modes shows distinct electrical signatures. Due to the 3D nature of the data, we have considered the TM mode of interpretation because it is less prone to 3D distortions. The final model represents large-scale heterogeneities in the crust due to the presence of different resistive and conductive blocks. High conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust. Exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and/or dehydration of minerals resulting in high conductivity anomalies. A prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic/ultra-mafic intrusions related to the Reunion plume activity. Different resistive blocks in conjunction with seismic tomography studies represent recrystallized older Precambrian crust that forms the uplifted blocks of Jamnagar, Jasdan and western part of Cambay basins.

LREE–Nb Mineralization in the South Western Part of Ambadongar Carbonatite Complex, Chhota Udepur District, Gujarat, India

The Ambadongar sub-volcanic carbonatite alkali complex is located about 140 km east of Vadodara, Chhota Udepur district, Gujarat, India and falls in Survey of India Toposheet No. 46 K/1. The complex intrudes Bagh sandstone (Cretaceous) and overlying Deccan basalts (Eocene) and is situated in the Narmada rift zone. The carbonatites came into prominence in the early 1960s with significant discoveries of fluorite and conspicuous radioactivity located near Chhota Udepur, at Ambadongar, the erstwhile Baroda district, Gujarat. Earlier workers have clarified many aspects of the carbonatite complex.

Ultrasonic P- and S-Wave Attenuation and Petrophysical Properties of Deccan Flood Basalts, India, as Revealed by Borehole Studies

Petrophysical properties and ultrasonic P- and S-wave attenuation measurements on 35 Deccan basalt core specimens, recovered from Killari borehole site in western India, provide unique reference data-sets for a lesser studied Deccan Volcanic Province. These samples represent 338-m-thick basaltic column, consisting four lava flows each of Ambenali and Poladpur Formations, belonging to Wai Subgroup of the Deccan volcanic sequence. These basalt samples are found to be iron-rich (average FeOT: 13.4 wt%), but relatively poor in silica content (average SiO2: 47.8 wt%). The saturated massive basalt cores are characterized by a mean density of 2.91 g/cm3 (range 2.80–3.01 g/cm3) and mean P- and S-wave velocities of 5.89 km/s (range 5.01–6.50 km/s) and 3.43 km/s (range 2.84–3.69 km/s), respectively. In comparison, saturated vesicular basalt cores show a wide range in density (2.40–2.79 g/cm3) as well as P-wave (3.28–4.78 km/s) and S-wave (1.70–2.95 km/s) velocities. Based on the present study, the Deccan volcanic sequence can be assigned a weighted mean density of 2.74 g/cm3 and a low Vp and Vs of 5.00 and 3.00 km/s, respectively. Such low velocities in Deccan basalts can be attributed mainly to the presence of fine-grained glassy material, high iron contents, and hydrothermally altered secondary mineral products, besides higher porosity in vesicular samples. The measured Q values in saturated massive basalt cores vary enormously (Qp: 33–1960 and Qs: 35–506), while saturated vesicular basalt samples exhibit somewhat lesser variation in Qp (6–46) as well as Qs (5–49). In general, high-porosity rocks exhibit high attenuation, but we observed the high value of attenuation in some of the massive basalt core samples also. In such cases, energy loss is mainly due to the presence of fine-grained glassy material as well as secondary alteration products like chlorophaeite, that could contribute to intrinsic attenuation. Dominance of weekly bound secondary minerals might also be responsible for the generation of microcracks, which may generate squirt flow in saturated samples. Hence, we argue that the Deccan basalts attenuate seismic energy significantly, where its composition plays a major role.

Geothermal energy potential of Tulsishyam thermal springs of Gujarat, India

Tulsishyam thermal springs are located in the Saurashtra region of Gujarat, India with discharge temperatures varying from 39 to 42 °C. The pH of these thermal springs varies from 7.1 to 7.4, indicating neutral character. Though these thermal springs propagate through the near surface layer of Deccan basalt, detailed geochemical analysis of the thermal waters using Piper diagram suggests that the water is interacting with the granitic basement rock. Silica and cation geothermometry estimates have reservoir temperature in the range of 138 to 207 °C categorizing it into a low to moderate enthalpy geothermal system. Furthermore, the area has high heat flow values of 53–90 mW/m2 because of shallow Moho depth. The prevailing conditions suggest that the geothermal energy can potentially be exploited through an enhanced geothermal system (EGS). The study also indicates different mineral phases that may precipitate out of water during exploitation of geothermal energy and it should be taken into account while designing an EGS for the area.

Geochemistry of sediments of the Peninsular rivers of the Ganga basin and its implication to weathering, sedimentary processes and provenance

Abstract Geochemistry of the Himalayan rivers has been a major point of contention in sedimentary geochemistry because of its global impact on ocean chemistry and climate. Among the Himalayan rivers, the Ganga and its Himalayan catchment have been studied extensively. However, the Peninsular rivers constituting a significant part of the Ganga basin have not received much attention, heretofore. In the present geochemical study of the Peninsular river's sediments, we have found that the initial chemical weathering of rocks in the catchment produces texturally, mineralogically and geochemically diverse sediments and fluvial processes further segregate them into the geochemically contrasting coarse channel and fine-grained suspended sediments. The overbank sediments possess an intermediate texture and geochemistry, but more towards the suspended sediments. The sediment geochemistry data indicates that the Peninsular river's sediments show an increase in the weathering intensity from the west (Banas river) to the east (Son river) along the climate gradient, i.e., from the arid-semiarid to the sub-tropical condition. The higher chemical index of alteration (CIA) values of the Peninsular river's sediments than the Yamuna and Ganga river sediments reflect the control of weatherable lithology in the tectonically stable, low relief and subtropical climate condition. The Yamuna and Ganga river sediments with lower CIA values show more physical weathering in the tectonically active Himalaya. The suspended and overbank sediments of the Yamuna and Ganga rivers show an increase in their CIA values after the confluence of the Peninsular rivers, indicating the contribution of more weathered materials by the Peninsular rivers. It has been found that there is a geochemical split between the channel and suspended sediments of the Peninsular rivers for sharing different sources, i.e., the dominant contribution of felsic sources to the channel and mafic sources to the suspended sediments. The weathering geochemistry, trace element systematics, and REE patterns suggest that the dominant sources of the channel sediments of the Peninsular rivers are the felsic crystallines and sedimentary lithologies of the Aravalli range, Bundelkhand and Chhotanagpur granite and gneisses, and Vindhyan sandstones. Whereas, the mafic lithologies such as the Deccan traps and mafic components of the felsic lithologies appear to be the major sources of the suspended sediments. Within the range of intermediate composition, the overbank sediment chemistry reflects more contribution of the mafic sources except for the Ken which shows a higher contribution of the felsic sources. The geochemical split between the channel and suspended sediments of the Peninsular river's sediments for the different provenances indicates differential weathering of the Deccan basalts, Bundelkhand crystalline, and Vindhyan sedimentary rocks; and also the hydrodynamic control of the Peninsular rivers during erosion, transport, and deposition. However, the Himalayan rivers do not show such contrast between the channel and suspended sediments because of the higher physical weathering and sediment mixing in the high gradient topography of the Himalayan catchment.

Th/U and U series systematics of saprolite: importance for the oceanic 234U excess

The presence of excess U-234 in seawater is a compelling argument for active delivery of solutes from the continents to the oceans. Previous studies found, however, that the complementary U-234 deficit on the continents is surprisingly modest, which would require protracted U loss from a large continental weathering pool. Our new compilation and statistical analysis of the published data, coupled with a mass balance calculation demonstrates that the apparent small U-234 deficit in the continental weathering pool implied by previous studies is insufficient to balance the observed oceanic excess. Our new data for a saprolite weathering profile developed on Deccan basalt reveal a very strong overall loss of U (elevated Th/U) with a strong U-234 deficit attributable to chemical weathering. The U and U-234 deficits reported here from a geologically recent saprolite confirm the importance of the early stages of chemical weathering at the weathering front in the supply of nutrients to the oceans. Thus, as much as half the oceanic U-234 inventory is likely sourced from a thin active saprolite zone.

Mixing processes in modern estuarine sediments from the Gulf of Khambhat, western India

Sandy and muddy detritus from two distinctive end-member provenance terrains, basaltic and granitic, are transported to the estuaries of the Gulf of Khambhat by fluvial discharge and opposing tidal currents making this embayment ideal for investigating the effects of mineralogical and geochemical mixing on sediment composition in estuarine depositional systems. Rivers flowing predominantly from the north over Precambrian metasediments and granites supply a felsic plutoniclastic sand to the gulf, whilst a mafic volcaniclastic sand, sourced by Deccan basalts, is transported by rivers draining the Western Ghats and from the local coastal erosion in the southern part of the gulf.Sediments from the Tapti estuary are rich in basaltic rock fragments (∼50%) and are characterised by a distinctly basaltic source composition (La/Sc∼0.25–0.86, Th/Sc∼0.12–0.40, Cr/Th∼13–121, Eu/Eu*∼0.93–1.68 and LREE/HREE∼2.96–6.57). By contrast, the sediments of the Sabarmati inner estuary are rich in quartz grains (80%) as evident from the major and trace element compositions (La/Sc∼1.38–6.63, Th/Sc∼0.58–2.98, Cr/Th∼0.58–19.39, Eu/Eu*∼0.46–0.86 and LREE/HREE∼2.90–11.40). The detrital compositions of the remaining major estuaries, the Narmada, Dhadhar and Mahi, scatter across a broad range in a QFR diagram and contain varying amounts of quartz, feldspar and rock fragments. In the Narmada Estuary the mixed signature characterised by REE values including Eu/Eu* (∼0.42–2.28) and LREE/HREE (∼4.37–14.80) is caused by the presence of diverse rock types that crop out in the source area. In the Mahi estuary, which largely flows across felsic source terrains the compositional signature is controlled by mixing of felsic detritus from riverine input with mafic sediments from seaward side in the estuary.The two end-member detrital compositions, plutoniclastic and volcaniclastic, are mixed in the macro-tidal estuaries. Mixing of sediment takes place where fluvial currents and tidal flows interact and the proportion of mixing varies systematically from outer to central to inner estuary. A high degree of mixing of felsic sediments with Deccan basalt derived sediments takes place in the outer estuary resulting in felsic:mafic ratios of ∼40:60. Sediments in the central estuaries (∼65:35) and in the fluvial-dominated inner estuaries (∼90:10) are characterised by moderate to minor degrees of mixing. The mixing in estuaries is systematic and is reflected in both the mineralogy and geochemistry of the sediments. Provenances of ancient estuarine successions should therefore be interpreted with great care incorporating awareness of the mixing processes which influence sediment compositions to different degrees in estuaries, especially those characterised by strong tidal currents.

Deccan Basalts in and around Koyna — Warna Region, Maharashtra: Some Reflections

ABSTRACT In this paper, an attempt is made to review the current status of chemo-and magnetostratigraphy of the Deccan Traps, particularly in the Western Province, and to interpret geomorphological and borehole data in relation to the earlier known structural data in the whole region, to that of the Koyna-Warna region, covering Deccan Traps and part the Archean basement. Koyna and Warna region comprises of chemically defined younger Formations of Wai Subgroup and Lonawala Subgroup displaying over-stepping arrangement from older Formations further north in the Nasik area and younger Formations down south. This is further corroborated by a single polarity change from reversed to normal, all the way to Belgaum. This is considered as a time marker around 65Ma (Chron C29r/C29n) matching with the K-T boundary. The Deccan Trap contact with the Archean basement in the 7 boreholes shows a plateau around 200m in the western section but a variation ranging up to 500m in the eastern side of the Warna region. These variations may be due to faulting or upwrap in this region. It is to be noted that the Koyna –Warna volcanic pile is similar to the thick sequences of the Mahabaleswar Ghat section further north in terms of their chemistry and magnetostratigraphy. A combination of surface and sub-surface data of the Koyna-Warna region may not show the presence of older volcanic formations such as Kalsubai Subgroup nor ages older than 65Ma. A few points are raised in this paper which may necessitate a detailed field, chemical, magnetic and geochronological study of both surface and drill cores in tandem with the available huge data base of the Deccan Traps, which may reveal surprises and perhaps provide clues to the origin of seismicity in this region and provide possible clues to a better understanding of the study area.

Influence of Permeability in Modeling of Reservoir Triggered Seismicity in Koyna Region, Western India

ABSTRACT The Koyna region located in the west coast of India is a classic example of reservoir triggered seismicity (RTS) that started soon after the impoundment of the Koyna reservoir in 1962. Previous studies have shown that RTS can be explained in terms of stress and pore pressure changes due to poroelastic response of the rock matrix. The permeability of rock matrix is a key parameter for pore pressure diffusion which is mainly responsible for generation of stress perturbation related to seismicity. Based on the poroelastic theory, we employ 2-D finite element models to simulate the evolution of pore pressure up to 5 years after the reservoir impoundment in 1962, using a range in permeability, 10−16-10−14 m2. Constraints on material properties of Deccan basalt and granitic rocks were taken from available studies. The results show the formation of pore pressure front and its propagation with depth and time since the reservoir impoundment as a function of permeability. While a permeability of 10−16 m2 does not produce any significant change in pore pressure, a ten-fold increase in permeability produces significant changes up to a depth of 2 km only beneath the reservoir after 5 years of impoundment. Permeability values between 10−15 m2 and 10−14 m2 are required to induce critical pore pressure changes in the range 0.1-1 MPa up to depth of 10 km, capable of triggering earthquakes in a critically stressed region. Studies on core samples of granitic basement rock down to a depth of 1522 m in the Koyna region provide evidences of fracture zones that may contribute to water channelization. Direct measurements of material properties through the ongoing deep drilling programme would help to develop more realistic models of RTS.