Sludge weathering and mobility of contaminants in soil affected by the Aznalcollar tailing dam spill (SW Spain)

As a result of the collapse of the Aznalcollar mine tailings dam, a large extension of the Guadiamar valley in SW Spain was covered with a layer of pyrite sludge. Although most of the sludge was removed, a small amount remains in the soil, constituting a potential source of water pollution. A column experiment was carried out in order to determine the rate of sludge oxidation in the soil, and the existence of metal retention processes. The column was filled with a mixture of sludge and a sandy soil common in the region. At different time intervals, the column was leached with water and the resulting solution analysed.

The pH of the water dropped to values around 2 after 260 days and then remained constant due to the buffering role of silicate dissolution. The concentration of Zn, Cd and Co in the leachates matched the expected values from flow-through experiments at atmospheric oxygen pressure. This indicates that oxygen diffusion in the pores was complete. Moreover, no efficient processes existed for retaining Zn, Cd and Co, which formed soluble salts and were entirely incorporated into the infiltrating water. During the first 2 months of the experiment, when pH was higher than 4.5, the concentrations of Fe and Al were very low. Saturation indices showed that the solution was in equilibrium with amorphous Fe(OH)₃ and Al(OH)₃. Subsequently, at lower pH values, jarosite [(Na,K)Fe₃(SO₄)₂(OH)₆] formed. This solid phase was identified by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Jarosite was also responsible for the depletion of As, Pb, Sb and probably Tl in the water resulting from infiltration.     

Trace elements contamination of agricultural soils affected by sulphide exploitation (Iberian Pyrite Belt,Sw Spain)

Agricultural soils of the Riotinto mining area (Iberian Pyrite Belt) have been studied to assess the degree of pollution by trace elements as a consequence of the extraction and treatment of sulphides. Fifteen soil samples were collected and analysed by ICP-OES and INAA for 51 elements. Chemical analyses showed an As–Cu–Pb–Zn association related with the mineralisation of the Iberian Pyrite Belt. Concentrations were 19–994 mg kg⁻¹ for As, 41–4,890 mg kg⁻¹ for Pb, 95–897 mg kg⁻¹ for Zn and of 27–1,160 mg kg⁻¹ for Cu. Most of the samples displayed concentrations of these elements higher than the 90th percentile of the corresponding geological dominium, which suggests an anthropogenic input besides the bedrock influence. Samples collected from sediments were more contaminated than leptosols because they were polluted by leachates or by mining spills coming from the waste rock piles. The weathering of the bedrock is responsible for high concentrations in Co, Cr and Ni, but an anthropogenic input, such as wind-blown dust, seems to be indicative of the high content of As, Cu, Pb and Zn in leptosols. The metal partitioning patterns show that most trace elements are associated with Fe amorphous oxy-hydroxides, or take part of the residual fraction. According to the results obtained, the following mobility sequence is proposed for major and minor elements: Mn, Pb, Cd, > Zn, Cu > Ni > As > Fe > Cr. The high mobility of Pb, Cu and Zn involve an environmental risk in this area, even in soils where the concentrations are not so high.     

Iron isotopes in acid mine waters and iron-rich solids from the Tinto–Odiel Basin (Iberian Pyrite Belt, Southwest Spain)

The isotopic composition of Fe was determined in water, Fe-oxides and sulfides from the Tinto and Odiel Basins (South West Spain). As a consequence of sulfide oxidation in mine tailings both rivers are acidic (1.45 < pH < 3.85) and display high concentrations of dissolved Fe (up to 420 mmol l^− 1) and sulphates (up to 1190 mmol l^− 1).The δ⁵⁶Fe of pyrite-rich samples from the Rio Tinto and from the Tharsis mine ranged from − 0.56 ± 0.08‰ to + 0.25 ± 0.1‰. δ⁵⁶Fe values for Fe-oxides precipitates that currently form in the riverbed varied from − 1.98 ± 0.10‰ to 1.57 ± 0.08‰. Comparatively narrower ranges of values (− 0.18 ± 0.08‰ and + 0.21 ± 0.14‰) were observed in their fossil analogues from the Pliocene–Pleistocene and in samples from the Gossan (the oxidized layer that formed through exposure to oxygen of the massive sulfide deposits) (− 0.36 ± 0.12‰ to 0.82 ± 0.07‰). In water, δ⁵⁶Fe values ranged from − 1.76 ± 0.10‰ to + 0.43 ± 0.05‰.At the source of the Tinto River, fractionation between aqueous Fe(III) and pyrite from the tailings was less than would be expected from a simple pyrite oxidation process. Similarly, the isotopic composition of Gossan oxides and that of pyrite was different from what would be expected from pyrite oxidation. In rivers, the precipitation of Fe-oxides (mainly jarosite and schwertmannite and lesser amounts of goethite) from water containing mainly (more than 99%) Fe(III) with concentrations up to 372 mmol l^− 1 causes variable fractionation between the solid and the aqueous phase (− 0.98‰ < Δ⁵⁶Fe_{solid–water} < 2.25‰). The significant magnitude of the positive fractionation factor observed in several Fe(III) dominated water may be related to the precipitation of Fe(III) sulphates containing phases.     

Hydrological and geomorphological criteria to evaluate the dispersion risk of waste sludge generated by the Aznalcollar mine spill (SW Spain)

Following the Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5?hm³ of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge. However, despite the efficiency of cleaning activities, there is still evidence of a fine superficial layer of sludge and some soil contamination, with the consequent risk of remobilisation of the pollutants by water erosion. There is much concern that these contaminated sediments may affect the precious ecosystems of the Doñana National Park and the Guadalquivir marshlands. This report describes the evaluation of the risk of mobilisation of the waste sludge through (1) detailed geomorphological analysis, indicating potential areas of erosion-sedimentation on the floodplains of the rivers Agrio and Guadiamar, and predicted dynamics of the waste sludge, and (2) evaluation of the potential dispersion of the waste sludge provoked by future flood events, including hydraulic calculations to model channel flow and the analysis of the texture of the sludge to obtain critical transport and sedimentation values. Findings suggest that the waste sludge is likely to be transported and deposited within the Doñana National Park during future flood events.     

Geochemistry and mineralogy of surface pyritic tailings impoundments at two mining sites of the Iberian Pyrite Belt (SW Spain)

Two pyritic tailings impoundments located in two mining areas of the Iberian Pyrite Belt (Cueva de la Mora and Minas de Ríotinto-Zarandas) were selected to asses their potential environmental impact. Mineralogical (XRD diffraction study), physico-chemical characterization (colour, particle size, pH, acid–base account, total Fe, As, Cu, Pb and Zn) as well as a speciation study (by means of a seven-step sequential extraction procedure) were performed in superficial (0–20 cm) tailings samples. Arsenic and metal contents in soils around the tailings impoundments were also studied. Zarandas dam, a reclaimed impoundment, which has been limed, partially topsoiled and planted, has supported and allowed the growth of pine trees and other plants for many years. The surface of this impoundment can be considered very acid but nonacid forming. Although total As and metal concentrations were relatively high, it is not possible to conclude that the Zarandas tailings have polluted the surrounding soils. Tailings in Cueva de la Mora showed high total and easily mobilizable concentration of toxic elements. The net neutralization potential was strongly negative as a consequence of the acid generation caused by the sulphide oxidation, the presence of secondary acid-generating minerals and the absence of neutralizing materials. Coquimbite and rhomboclase efflorescences formed during the Mediterranean dry summers on the surface of this impoundment contained very high levels of soluble As, Cu and Zn that were easily dissolved and released to the running water in the first rains of autumn.     

Immobilization of toxic elements in mine residues derived from mining activities in the Iberian Pyrite Belt (SW Spain): Laboratory experiments

In the mining environments of the Iberian Pyrite Belt (IPB), the oxidation of sulphide wastes generates acid drainage with high concentrations of SO₄, metals and metalloids (Acid Mine Drainage, AMD). These acid and extremely contaminated discharges are drained by the fluvial courses of the Huelva province (SW Spain) which deliver high concentrations of potentially toxic elements into the Gulf of Cádiz. In this work, the oxidation process of mine tailings in the IPB, the generation of AMD and the potential use of coal combustion fly ash as a possible alkaline treatment for neutralization of and metal removal from AMD, was studied in non-saturated column experiments. The laboratory column tests were conducted on a mine residue (71.6 wt% pyrite) with artificial rainfall or irrigation. A non-saturated column filled solely with the pyrite residue leached solutions with an acid pH (approx. 2) and high concentrations of SO₄ and metals. These leachates have the same composition as typical AMD, and the oxidation process can be compared with the natural oxidation of mine tailings in the IPB. However, the application of fly ash to the same amount of mine residue in another two non-saturated columns significantly increased the pH and decreased the SO₄ and metal concentrations in the leaching solutions. The improvement in the quality of leachates by fly ash addition in the laboratory was so effective that the leachate reached the pre-potability requirements of water for human consumption under EU regulations. The extrapolation of these experiments to the field is a promising solution for the decontamination of the fluvial courses of the IPB, and therefore, the decrease of pollutant loads discharging to the Gulf of Cádiz.     

The Salamón gold deposit (León, Spain)

Located 55 km NE of the provincial capital León, Salamón deposit, discovered in 1985, is located on the southern slope of the Cantabrian Mountains, in the north of the Iberian Peninsula. The deposit is located on the León fault, which is a late-Variscan, E–W trending, deep structure extending for more than 100 km. The León fault has a complex history, and many mines and occurrences are located near it. The deposit is also close to small stocks and dykes of igneous rocks with intermediate to basic composition to which the mineralisation is related. The mineralisation is hosted mainly by the limestones and bituminous shales of the Lena Group (Namurian–Westphalian). There is also some mineralisation in other stratigraphic units of the Upper Carboniferous, such as the Maraña Group or the Stephanian B sediments.Apart from local and regional exploration, a detailed mineralogical and metallogenic research has been carried out. The epithermal mineralisation of Salamón was developed in two phases: an early dominant and extensive stage, with very fine crystalline gold-bearing sulphides, mainly pyrite, arsenic-bearing pyrite and arsenopyrite, in a matrix of quartz–chalcedony (jasperoid) and dolomite, and a later stage, of a larger crystal size, which occurs replacing the early stage or in pockets and veins, with greater mineralogical variety. Last of all there is a stage of supergene mineralisation, a product of the oxidant action of meteoric waters over the previous minerals. The hydrothermal alterations of the host rocks related to the orebodies are fundamentally decarbonatisation–dolomitisation, silicification and argillitisation. The early stages of mineralisation were produced in a temperature of 148–241°C, while that in the later stages occurred at 86–123°C. The early stage has been dated as 269±5 Ma, and this agrees with the ages of the other deposits of the district, which lay between 292 and 263 Ma, and the igneous rocks of the Peña Prieta stock (277±1 Ma), all which are of Permian age.The results of the studies carried out until now lead to the conclusion that Salamón is a Carlin-type gold deposit.     

Collapse dolines in miocene gypsum: an example from SW Sivas (Turkey)

 This study investigated the formation mechanism of dolines. Dolines are caused by dissolution of gypsum in the study area, 50–220 m in diameter. Two systems were applied to determine the support necessary for natural underground opening of given span in a given rock mass. From the characteristics of the rock mass, a 5 m-wide unsupported span in the gypsum can be expected to stand up for over 9 years. The designed span of an artificial opening would normally be considerably less than the span of a collapsed natural opening in the same rock. Dolines in the study area play an important role in the groundwater flow system. Received: 10 November 1995 · Accepted: 4 January 1996     

Palaeoecology of Late Triassic marine assemblages from the proto-Atlantic Basin (Ayamonte,SW Spain)

The Triassic represents a critical period that attests to the diversification of marine ecosystems. This paper analyses the stratigraphy of the easternmost Mesozoic series of the Algarve Basin in the Ayamonte sector (SW Spain). Two sets forming this series are described. The first, mainly siliciclastic, was deposited in continental contexts represented by fluvial and lacustrine environments with bioturbation associated with swamp-like environments (Taenidium) or with plain alluvial sediments (Labyrintichnus terrerensis, Planolites). This ichnoassemblage corresponds to the ichnofacies of Scoyenia. The second set, is carbonate-dominated and shows a remarkable palaeontological content comprising bivalves and gastropods, which made it possible to review our current knowledge of the regional stratigraphy of the Triassic in this area. Trigonodus-Isocyprina form this association for bivalves and Coeslostylina and Coeslostylina-Spirostylus-Mathilda for gastropods allowing to infer a Late Triassic (Carnian-Rhaetian) age for the Ayamonte area. The characteristic trace fossils of the carbonate sections are represented by Thalassinoides Treptichnus pollardi and Helminthoidichnites tenuis, which belong to the proximal expression of the Cruziana ichnofacies. Shallow marginal, low energy, and brackish and locally lacustrine conditions are inferred for this carbonate-dominated set, alternating with episodes of normal salinity, and towards the top, with punctuated episodes of higher energy. This palaeoenvironmental context is congruent with the palaeogeographical scenario conditioned by the existence of a threshold of major order that would indicate the initial breaking of Pangea and the opening of the Tethys in the eastern Mediterranean.     

Gold-bearing hedenbergite skarns from the SW contact of the Andorra granite (Central Pyrenees, Spain)

Several varieties of skarn outcrop have been found to develop along the contact between the Andorra granite and the Devonian limestones. The skarns contain variable amounts of gold ranging up to 5 g/t, always associated with sulphides. The sulphides in the skarn include arsenopyrite and pyrrhotite with lesser amounts of chalcopyrite, galena, sphalerite and Bi-minerals. Geothermometric and geobarometric data indicate the skarns formed at about 2 kbar and temperatures ranging from 500 ° to 350 °C from CO₂-free polysaline brines at a slightly acidic pH and oxygen fugacity which decreases with time from the pyrite-pyrrhotite-magnetite towards the QFM buffer. Available data on gold solubility suggest that sufficient quantities of gold to form an ore deposit could have been transported as AuCl ₂ ^- at the high temperatures and salinities under which the skarns formed. Both gold deposition and sulphide precipitation could have occurred due to a decrease in temperature and/or oxygen fugacity.     

Use of mine spoils from Teruel coal mining district (NE,Spain)

Coal mine restoration projects increase public perception of mining companies. Spain has relatively few examples of completed opencast mine rehabilitation projects. This study seeks to obtain the use of mine spoils from coal mines in Teruel. The studied mine spoils may be used as raw material for the Spanish ceramic industry, located in the provinces of Castellon, Valencia and Teruel. This study specifically discusses the mineralogical and geochemical characteristics of the Lower Cretaceous materials from coal deposits in Estercuel basin, around Teruel and Castellon in Spain. These provinces have a large ceramic industry. A characterization of mine spoils from coal deposits has been carried out. This characterization is based on the data of both mineral and chemical analysis. The mineralogical characterization was complemented with SEM/EDAX. The study of the chemical composition allows the evaluation of the applicability of the studied mine spoils to the following industries: refractory, fine clay, red clay for pavement and coating, and heavy clay. The clay used for refractory industry has the highest compositional demand. The positive results obtained on this set of preliminary tests lead us to envisage new research programs, focusing on testing these mine spoils on a semi-industrial scale.     

Origin and metamorphic evolution of rocks with braunite and pyrophanite from the Iberian Massif (SW Spain)

Summary ?Rocks containing braunite from the Ossa-Morena central belt (Iberian Massif, SW Spain) have been studied; these include nodules and layers of braunite (association I), Mn-slates (association II) and Mn-metatuffs (associations III and IV). Geochemical features of braunite nodules such as Mn/Fe ratios around 2, positive Ce-anomalies and good correlations among Mn, Fe, Co, Cu and REE contents indicate that the protolith of the braunite-nodules was precipitated from oxidising sea water. Greenschist facies Hercynian metamorphism reduced initial Mn⁴⁺ to Mn³⁺ and Mn²⁺. High initial fO₂ of oxide beds (association I) limited reduction to the formation of braunite. Reduction continued until the formation of garnet + piemontite (associations II and III), and pyroxmangite + pyrophanite (association IV). Ti-rich braunites (up to 6.8% of TiO₂) occur in slates and metatuffs in which the (Mn + Fe)/Ti ratio of the whole rock is lower than 30, while braunites have lower Ti contents in slates and metatuffs with (Mn + Fe)/Ti ratios around 90. Fe-rich braunite crystallized in rocks with Mn²⁺ oxide and silicate where low Mn³⁺/Mn²⁺ in the whole rock facilitated substitution of Fe³⁺ for Mn³⁺. Received January 30, 2002; revised version accepted May 7, 2002 Published online November 22, 2002     

Mercury mobility in mine waste from Hg-mining areas in Almería,Andalusia (Se Spain)

Mining wastes and calcines from two abandoned mining areas (Valle del Azogue and Bayarque in Almería) have been characterized. In the mining wastes, the dominant mercury phases are cinnabar and elemental mercury in the matrix. In the calcines, however, the dominant mercury phase is elemental mercury bounded to the matrix. Water-leaching experiments were conducted on low-grade stockpiles and calcines in order to simulate the mobilization of mercury by runoff under environmental conditions. The laboratory column-leaching experiments show a possible mobilization of mercury from Hg⁰ dissolution, colloid transport and a possible dissolution of calomel and other soluble phases in the mine wastes from the Valle del Azogue and Bayarque mines. Equilibrium speciation modeling of Hg, conducted using the numerical code MINTEQ, showed that the theoretical dominant mercury species in the calcine and mining wastes samples are Hg(OH)₂, HgCl₂, HgClOH and Hg⁰. In some leachates obtained from the Valle del Azogue mining wastes (sample A06), the high Hg concentrations may indicate the possible dissolution of mineral phases such as calomel and other soluble phases, which are subsaturated. The environmental results indicate a great environmental mobility of mercury, especially during wet episodes associated with intense precipitation events, when there are significative amounts of secondary soluble minerals.     

Metal extractability in acidic and neutral mine tailings from the Cartagena-La Unión Mining District (SE Spain)

Mine tailings are ubiquitous in the landscapes of mined areas. Metal solubilities were compared in two chemically distinct mine tailings from the old Mining District of Cartagena-La Unión (SE Spain). One of the tailings was acidic (pH 3.0) with 5400 mg/kg Zn, 1900 mg/kg As and 7000 mg/kg Pb. The other was neutral (pH 7.4) with 9100 mg/kg Zn, 5200 mg/kg Pb and 350 mg/kg As. In samples from the acidic tailings, more than 15% of the Zn and 55% of the Cd were extractable with 0.1 M NaNO₃, and distilled water. In the neutral tailings, using the same reagents, less than 1% of the metals were extractable. A sequential extraction procedure revealed that the sum of the residual and the Fe oxide fractions of Cu, Zn and Pb comprised 80–95% in the acidic tailings and 70–90% in the neutral tailings. The acidic mine tailings had a higher metal solubility, resulting in more metal leaching in the short-term, but also a higher fraction of inert metal. In contrast, in the neutral tailings, the metals were evenly distributed between, oxides and the residual fraction. This implies lower metal mobility in the short-term, but that metal mobility may increase in the long-term. When applied to mine tailings, sequential extractions may provide misleading results because the strong cation exchange capacity of some extractants may induce pH changes and thereby significantly change metal solubility.     

Mineralogy and environmental relevance of AMD-precipitates from the Tharsis mines,Iberian Pyrite Belt (SW,Spain)

This paper documents the solid phases associated to acid mine drainage (AMD) at the Tharsis mines (SW Spain). It provides an inventory of the AMD-precipitates, describing their main modes of occurrence and mineral assemblages. Results indicate that iron, aluminum and magnesium sulfates predominate in the assemblages. They occur as efflorescences composed of complex mixtures of metallic salts, and as ochres (jarosite combined with goethite). Also, their distributions illustrate two hydrochemical environments: the open pits, which reflect a proximal secondary paragenesis; and the downstream river banks (Meca River), which represent a more evolved paragenesis, resulting from the evolution of AMD produced throughout the system. These environments can be differentiated by composition and variety of minerals, which is considerably lower along the Meca River.The newly-formed minerals have monitoring significance and proved capable of participating in cycles of retention–liberation of hydrogen ions, sulfate, and metals. In a semi-arid climate, the importance of the AMD-precipitates as environmental indicators is stressed. They may help to understand the response of the system to the episodic rainfall events that occur after prolonged dry periods.     

Environmental problems and geological implications derived from evaporite dissolution in the Barbastro salt anticline (NE Spain)

The halite-bearing Barbastro Formation crops out in the core of the Barbastro Anticline (Ebro Tertiary Basin). This anticline is traversed perpendicularly by some of the most important Pyrenean drainages such as the Cinca and Noguera-Ribagorzana Rivers. The terrace sequences of these fluvial systems have been used as markers to identify and assess dissolution-induced subsidence and salt tectonics. In the limbs of the anticline, terrace deposits underlain by detrital bedrock do not show any evidence of deformation and have a consistent thickness of less than 10 m. The deposits of certain terrace levels of the Noguera-Ribagorzana River and its tributary, the Lo Reguer Creek, are locally thickened filling basins generated by dissolution-induced synsedimentary subsidence up to several kilometers long and more than 100 m deep. Conversely, terraces of the Cinca River do not show anomalously high thicknesses, but local uplifts related to differential upward flow of the halite-bearing bedrock. Locally, a minimum uplift rate of 0.3 mm/year has been estimated from a 64-ka terrace tilted away from the valley. The subsidence hazards occur chiefly in areas where the ground receives artificial water recharge. Serviceability of some canals has been notoriously affected by evaporite karstification. The problem has been mitigated to acceptable levels by grouting. Numerous buildings of Ivars de Noguera are severely damaged by dissolution subsidence, and possibly, by hydrocompaction of gypsiferous silts. The pipe network has been replaced to ameliorate the subsidence risk. In the Cinca River valley, cavities with a total volume of about 180,500 m³ have been created by solution mining at depths greater than 500 m. No investigation methods are applied in the brine field to monitor the distribution and evolution of artificial voids. Substantial increase in salinity of the Cinca River is another evidence of subjacent evaporite dissolution.     

Pollution characteristics of polycyclic aromatic hydrocarbons （PAHs） in oily sludge from the Zhongyuan Oilfield and its peripheral soils

The purpose of this study is to determine the degree of contamination caused by polycyclic aromatic hydrocarbons (PAHs) in oily sludge and soils around it in the Zhongyuan Oilfield. The contents of polycyclic aromatic hydrocarbons in oily sludge samples were determined with HPLC. The contents of PAHs of oily sludge from three different oil production plants vary from high to low in the order of the Wenming oily sludge dumping site of No. 3 Oil Production Plant (3W)>the Mazhai oily sludge dumping site of No. 3 Oil Production Plant (3M)>the Wen’er oily sludge dumping site of No. 4 Oil Production Plant (4W). Naphthalene, acenaphthylene, acenaphthene, fluorine and phenanthrene are the major pollutants of PAHs in oily sludge. The contents of PAHs in soil samples around the oily sludge dumping sites vary widely from 434.49 to 2408.8 ng/g. Naphthalene, acenaphthene, fluorine, phenanthrene and pyrene are the characteristic factors of PAHs in soil samples of 3M and 3W, and naphthalene, acenaphthene, fluorine and phenanthrene are the characteristic factors of PAHs in soil samples of 4W. According to these data and the ratios of Fl/Py, PAHs in oily sludge samples come mainly from petrogenic sources, and soil samples are divided into petrogenic soil samples and mixed-source soil samples, and both petrogenic and pyrogenic soil samples in terms of the sources of PAHs. The classification by Nemero index P indicates that soils around the oily sludge dumping sites have been seriously polluted.     

Interaction of acid mine drainage with waters and sediments at the Corona stream, Lousal mine (Iberian Pyrite Belt, Southern Portugal)

This study investigates the geochemical characteristics of the acid mine drainage discharged from the abandoned mine adits and tailing piles in the vicinity of the Lousal mine and evaluates the extent of pollution on water and on the stream sediments of the Corona stream. Atmospheric precipitation interacting with sulphide minerals in exposed tailings produces runoff water with pH values as low as 1.9–2.9 and high concentrations of (9,249–20,700 mg l⁻¹), Fe (959–4,830 mg l⁻¹) and Al (136–624 mg l⁻¹). The acidic effluents and mixed stream water carry elevated Cu, Pb, Zn, Cd and As concentrations that exceed the water quality standards. However, the severity of contamination generally decreases 4 km downstream of the source due to mixing with fresh waters, which causes the dilution of dissolved toxic metals and neutralization of acidity. Some natural attenuation of the contaminants also occurs due to the general reduced solubility of most trace metals, which may be removed from solution, by either co-precipitation or adsorption to the iron and aluminium precipitates.     

Facies, dissolution seams and stable isotope compositions of the Rohtas Limestone (Vindhyan Supergroup) in the Son valley area, central India

The early Mesoproterozoic Rohtas Limestone in the Son valley area of central India represents an overall shallowing-upward carbonate succession. Detailed facies analysis of the limestone reveals outer- to inner-shelf deposition in an open marine setting. Wave-ripples, hummocky cross stratifications and edgewise conglomerates argue against a deep marine depositional model for the Rohtas Limestone proposed earlier. Stable isotope analysis of the limestone shows that δ¹³C and δ¹⁸O values are compatible with the early Mesoproterozoic open seawater composition. The ribbon limestone facies in the Rohtas Limestone is characterized by micritic beds, each decoupled in a lower band enriched and an upper band depleted in dissolution seams. Band-wise isotopic analysis reveals systematic short-term variations. Comparative enrichment of the heavier isotopes in the upper bands is attributed to early cementation from sea water and water derived from the lower band undergoing dissolution because of lowering of pH at depth. The short-term positive shifts in isotopic compositions in almost every upward gradational transition from a seamed band to a non-seamed band support the contention that dissolution seams here are of early diagenetic origin, although their formation was accentuated under overburden pressure.     

3D gravity modeling of the Triassic salt diapirs of the Cubeta Alavesa (northern Spain)

Up to now subsurface information permitted the delineation of the top of the Triassic salt, all throughout the Cantabro–Navarro domain, although little was known on the location and geometry of its base and thus on the estimation of the total salt thickness. A 3D-gravity inversion scheme combined with a 3D analytic method has been conducted to map out the geometry of the main salt structures of the basin. The gravity modeling results have been constrained by well log information and available geological and reflection seismic data. The combined 3D scheme integrated with available geological and geophysical data has allowed us to obtain the geometry of the main diapirs that characterize the central and marginal regions of the basin.From our interpretation, the Salinas de Añana diapir has almost vertical flanks and can be divided into two different parts, one of them forming a lateral overhang of the main body.The Salinas de Oro diapir has near vertical flanks and a main axis in the N–S direction. Also, the anomaly is rather more extensive than the outcrop of the diapir, which implies an important expansion of non-outcropping salt in this area. Like the Hoz-Sobrón diapir, the Salinas de Ollo diapir is long and narrow. stretching in the NW–SE direction, which includes three important highs, plus an intense zone of salt migration.The Estella and Alloz diapirs crop out individually in spite of being connected at depth. Also two non-outcropping salt domes have been detected to the south of Atauri that, like the Estella diapir, are related to the thrust front. We point out the gravity signature of the Murguia diapir, which shows an intense gravity high probably due to the presence of high-density rocks in the cap rock or more probably due to the existence of Triassic volcanites of ophitic texture pinched-off into the diapir.