Diagenesis of a mixed siliciclastic/evaporitic sequence of the Middle Muschelkalk (Middle Triassic), the Catalan Coastal Range, NE Spain

The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north-east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north-eastern parts of the basin are terminal alluvial fan deposits. South-westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K-feldspar and mixed-layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed-layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ¹⁸O_SMOW values for microdolomite (+23.7 to +28.4%) and K-feldspar overgrowths (+17.3 to +17.7%) suggest either low-temperature, isotopic disequilibrium or precipitation from low-¹⁸O porewaters. Low-¹⁸O waters might have developed, at least in part, during low-temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ¹⁸O_SMOW=+21.5 to +25.6%o; δ¹³C= 7.7 to - 5.6%o) and presence of haematite indicate infiltration of meteoric waters.     

Geochemische Untersuchungen an Sinterkalken (Travertinen)

Formation of travertines from continental waters requires very special conditions. Their porous, cellular and concretionary appearance is often due to calcite incrustations on plants. Therefore the high porosity beside a very low concentration of detrital silicates must be related to a high rate of calcite accumulation (0.1 to 1 mm/year). A high rate of sedimentation from high Ca-concentrations (>0.01%) does not occur in normal continental waters equilibrated with the atmospheric carbon dioxide.Formation of travertines is probably not an indicator of special climatic conditions as often assumed but is related to a special chemical composition of fresh waters. Creeks with continuous travertine deposition, investigated by us, contain about 200 ppm Ca. This is more than ten times the normal concentration of average continental waters. Bicarbonate concentration in these waters is five times that of equilibrium with atmospheric carbondioxide. Many of the productive waters are characterized by high sulfate concentrations and Sr/Ca-ratios (1000 Sr/Ca ranges from 9 to 22; seawater: 20).Except one from the Schwäbische Alb 15 samples (Leine-valley, Teutoburger Wald, Thüringen) are high in sulfate and Sr/Ca-ratio to be accumulated in waters with remarkable concentrations in the respective elements. The Sr/Ca-ratios cannot be explained from dissolved limestones underlying the exposed areas. The assumption of dissolution of gypsum beds very well explains our observations and data. From the S³²/S³⁴ ratio an origin from Zechstein (Upper Permian), Muschelkalk (Middle Triassic) or Keuper (Upper Triassic) deposits respectively can be assumed. Like other freshwater limestones the travertines under investigation differ from marine carbonates by their light carbon (mean C¹³ –8,1) from decomposed organic matter and their light oxygen (mean O¹⁸ –6,7) from exchange with freshwaters. The discrimination freshwater- and marine limestones in oxygen isotopes is only valid for those of low age (Upper Mesozoic to Recent).Hence the special conditions of travertine formation are related to bicarbonate spring waters which have circulated on faults where they could dissolve gypsum (and/or calcite) from subsurface beds.Additional examples of travertine deposits from Czechoslovakia (Zyka, 1958), Lüneburger Heide (NW Germany), Italy (Tivoli near Rom) and Jugoslavia can be explained as formations of either sulfate-bicarbonate or of bicarbonate waters.

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Wir danken der Deutschen Forschungsgemeinschaft für Beihilfen zu dieser Arbeit.     

Carbon and oxygen isotopic composition and foraminifers of condensed basal Zechstein (Upper Permian) strata in western Poland: environmental and stratigraphic implications

The basinal facies of the Lopingian Zechstein Limestone in SW Poland consists of thin (often less than 1 m thick) limestones and/or dolomites, often containing the Kupferschiefer (few tens of centimetres thick) at their base, and local thick (up to 90 m) reefal carbonates. The δ¹³C curve of these basal Zechstein deposits strongly suggests that even when the Kupferschiefer is lacking, the thin (condensed) sequences record the entire interval of the Zechstein prior to the onset of evaporite deposition, in contrast to the thick reef sequences which lack the characteristic δ¹³C curve for the lowermost part of the Zechstein. The calcite samples show considerable ranges of δ¹⁸O values. If the maximum δ¹⁸O values are considered to be the closest to the pristine original ones and if δ¹⁸O_water value = 0 is assumed, then the calculated range of palaeotemperatures for the Kupferschiefer and Zechstein Limestone calcite ranges from 19 to 34 °C. The faunal restriction, common dwarf foraminifers and the predominance of lagenids in the foraminiferal assemblage indicate continual dysaerobic conditions and possibly elevated salinity of seawater during deposition of thin basinal Zechstein Limestone deposits. The mixing of shallow and deeper waters in the stratified Zechstein Basin caused by upwelling could result in prolific carbonate precipitation in reefs located at the slope of the marginal carbonate platform of the Zechstein Limestone and in isolated reefs related to palaeohighs within the basin; however, there is no isotopic record of eventual upwelling. Copyright © 2014 John Wiley & Sons, Ltd.     

Sedimentology and geochemistry of fluvio-lacustrine tufa deposits controlled by evaporite solution subsidence in the central Ebro Depression, NE Spain

The Urrea de Jalón tufa deposits constitute the 20‐ to 50‐m‐thick caprock (0·3 km²) of an isolated mesa. They disconformably overlie horizontal strata of the Tertiary Ebro Basin (NE Spain), which contains a thick succession of lacustrine gypsum and marls, followed by limestones, marls and, locally, fluvial sandstones and mudstones. The tufa deposits show a complex, large‐scale framework of basin‐like structures with centripetal dips that decrease progressively from the base to the top of the tufa succession, and beds that thicken towards the centre of the structure (cumulative wedge‐out systems). These geometries reveal that the tufa deposits were affected by differential synsedimentary subsidence. Distinct onlapping depressions reflect time migration of the subsiding areas. The studied carbonates are composed mostly of low‐Mg calcite, with minor quartz. Some samples have anomalously high contents of Fe, Mn and Ba that may exceed 1% (goethite, haematite and barite are present). Carbonate facies are: (a) macrophyte encrustation deposits; (b) bryophyte build‐ups; (c) oncolite and coated grain rudstones; (d) non‐concentric stromatolite‐like structures; (e) massive or bioturbated biomicrites; and (f) green and grey marls. Facies a and c show a great variety of microbial‐related forms. These facies can be arranged in dm‐ to 2‐m‐thick vertical associations representing: (i) fluvial–paludal sequences with bryophyte growths; (ii) pond‐influenced fluvial sequences; and (iii) lacustrine–palustrine sequences. The Urrea de Jalón tufa deposits formed in a fluvio‐lacustrine environment that received little alluvial sediment supply. Isotope compositions (δ¹³C and δ¹⁸O) reveal meteoric signatures and accord with such a hydrologically open system of fresh waters. The Fe, Mn and Ba contents suggest an additional supply of mineralized waters that could be related to springs. These would have been discharge points in the Ebro Depression of a regional aquifer of the Iberian Ranges. Rising groundwater caused the solution of the underlying evaporites and the synsedimentary subsidence of the tufa deposits.     

Syntectonic hydrothermal calcite in a faulted carbonate platform margin (Albian of Jorrios, northern Spain)

ABSTRACT A calcite mass more than 1·5 km long and 20 m wide crops outs along the faulted margin of the Albian carbonate platform of Jorrios in northern Spain. The mass contains abundant dissolution cavities up to 7 m long and 1 m high, filled with cross‐stratified quartz sandstone and alternating sandstone–calcite laminae. Similar cavities are also present in a 50‐m‐wide zone of platform limestones adjacent to the calcite mass that are filled with limestone breccias and sandstone. The calcite mass has mean δ¹⁸O values of 19·6‰ (SMOW), whereas platform limestones have mean δ¹⁸O values of 24·4‰ (SMOW). Synsedimentary faulting of the carbonate margin and circulation of heated fault‐related waters resulted in replacement of a band of limestone by calcite. Soon after this replacement, dissolution by undersaturated fluids affected both the calcite mass and the adjacent limestones. Percolating marine quartz sand filled all dissolution cavities, sometimes alternating with precipitating calcite. The resulting cavities and fills, which recall products of meteoric diagenesis, are attributed to a hydrothermal origin based on their geometry, occurrence along the profile and synsedimentary tectonic relationships. The early faulting and diagenesis are related to local extensional tectonism in a large‐scale strike‐slip setting. Movements occurred during the early dispar/appenninica zone of the Late Albian.     

Environmental isotopes in determining groundwater flow systems, northern part of Epirus, Greece

On the basis of the isotopic composition of water in the northern part of Epirus, Greece, from springs at different altitudes with well-defined recharge areas, the altitude effect on the δ¹⁸O value of groundwater is –0.142±0.003ö (100?m)^–1 and is uniform over the entire study area. Using the δ¹⁸O composition of surface water and groundwaters, the contribution of Ioannina Lake and the channel draining the lake water to the Kalamas River to the recharge of springs and boreholes was confirmed and quantitatively defined. In contrast, the Voidomatis and Vikos Rivers are not sources for recharge of the big springs along their banks. However, water from the Aoos River does replenish the aquifer in the unconsolidated deposits underlying the plain of Konitsa. In addition, limestones of Senonian–Late Eocene ages, dolomites, and limestones of the "Vigles" facies are hydraulically interconnected, and the limestones of the "Pantokrator" facies are hydraulically isolated from the other carbonate formations.     

Sedimentology and isotope geochemistry of lacustrine carbonates of the Oligocene Campins Basin, north-east Spain

The non-marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre-thick carbonate- and mudstone-dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate. The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ¹³C and δ¹⁸O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ¹³C and δ¹⁸O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend. The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.     

Isotopic and morphological features of fracture calcite from granitic rocks of the Tono area,Japan: a promising palaeohydrogeological tool

This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ¹⁸O_SMOW from −3 to ca. 10‰, and with δ¹³C_PDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ¹³C_PDB of ca. 0‰ and δ¹⁸O_SMOW > ca. 20‰; (3) fresh water, with a variable δ¹³C_PDB composition, but which precipitated calcite distinguished by δ¹³C_PDB that was significantly < 0‰ and as low as ca. −29‰ and δ¹⁸O_SMOW > ca. 17‰. Data for ¹⁴C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.     

Formation of the Dongmozhazhua Pb–Zn Deposit in the Thrust‐Fold Setting of the Tibetan Plateau,China: Evidence from Fluid Inclusion and Stable Isotope Data

The Dongmozhazhua deposit, the largest Pb–Zn deposit in south Qinghai, China, is stratabound, carbonate‐hosted and associated with epigenetic dolomitization and silicification of Lower–Middle Permian—Upper Triassic limestones in the hanging walls of a Cenozoic thrust fault system. The mineralization is localized in a Cenozoic thrust‐folded belt along the northeastern edge of the Tibetan plateau, which was formed due to the India–Asia plate collision during the early Tertiary. The deposit comprises 16 orebodies with variable thicknesses (1.5–26.3 m) and lengths (160–1820 m). The ores occur as dissemination, vein, and breccia cement. The main sulfide assemblage is sphalerite + galena + pyrite + marcasite ± chalcopyrite ± tetrahedrite, and gangue minerals consist mainly of calcite, dolomite, barite, and quartz. Samples of pre‐ to post‐ore stages calcite yielded δ¹³C and δ¹⁸O values that are, respectively, similar to and lower than those yielded by the host limestones, suggesting that the calcite formed from fluids derived from carbonate dissolution. Fluid inclusions in calcite and sphalerite in the polymetallic sulfidization stage mostly comprise liquid and gas phases at room temperature, with moderate homogenization temperatures (100–140°C) and high salinities (21–28 wt% NaCl eq.). Micro‐thermometric fluid inclusion data point to polysaline brines as ore‐forming fluids. The δD and δ¹⁸O values of ore fluids, cation compositions of fluid inclusions, and geological information suggest two main possible fluid sources, namely basinal brines and evaporated seawater. The fluid inclusion data and regional geology suggest that basinal brines derived from Tertiary basins located southeast of the Dongmozhazhua deposit migrated along deep detachment zones of the regional thrust system, leached substantial base metals from country rocks, and finally ascended along thrust faults at Dongmozhazhua. There, the base‐metal‐rich basinal brines mixed with bacterially‐reduced H₂S‐bearing fluids derived from evaporated seawater preserved in the Permo–Triassic carbonate strata. The mixing of the two fluids resulted in Pb–Zn mineralization. The Dongmozhazhua Pb–Zn deposit has many characteristics that are similar to MVT Pb–Zn deposits worldwide.     

Stable isotope geochemistry of ground and surface waters associated with undisturbed massive sulfide deposits; constraints on origin of waters and water-rock reactions

Stable isotopes (H, O, C) were determined for ground and surface waters collected from two relatively undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp (BMC), New Brunswick, Canada. Additional waters from active and inactive mines in the BMC were also collected. Oxygen and hydrogen isotopes of surface and shallow groundwaters from both the Halfmile Lake and Restigouche deposits are remarkably uniform (− 13 to − 14‰ and − 85 to − 95‰ for δ¹⁸O_VSMOW and δ²H_VSMOW, respectively). These values are lighter than predicted for northern New Brunswick and, combined with elevated deuterium excess values, suggest that recharge waters are dominated by winter precipitation, recharged during spring melting. Deeper groundwaters from the Restigouche deposit, and from active and inactive mines have heavier δ¹⁸O_VSMOW ratios (up to − 10.8‰) than shallow groundwaters suggesting recharge under warmer climate or mixing with Shield-type brines. Some of the co-variation in Cl concentrations and δ¹⁸O_VSMOW ratios can be explained by mixing between saline and shallow recharge water end-members. Carbon isotopic compositions of dissolved inorganic carbon (DIC) are variable, ranging from − 15 to − 5‰ δ¹³C_VPDB for most ground and surface waters. Much of the variation in the carbon isotopes is consistent with closed system groundwater evolution involving soil zone CO₂ and fracture zone carbonate minerals (calcite, dolomite and siderite; average = − 6.5‰ δ¹³C_VPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ¹³C_VPDB), indicating carbon isotopic fractionation from methanogenesis via CO₂ reduction, consistent with the lack of dissolved sulfate in these waters and the observation of CH₄-degassing during sampling.     

Unique Origin of Skarn at the Ohori Base Metal Deposit,Yamagata Prefecture,NE Japan: C,O and S Isotopic Study

The Ohori ore deposit is one of the Cu–Pb–Zn deposits in the Green Tuff region, NE Japan, and consists of skarn‐type (Kaninomata) and vein‐type (Nakanomata) orebodies. The former has a unique origin because its original calcareous rocks were made by hydrothermal precipitation during Miocene submarine volcanism. Carbon and oxygen isotope ratios of skarn calcite and sulfur isotope ratios of sulfides were measured in and around the deposit. Carbon and oxygen isotope ratios of the skarn calcite are δ¹³C = ?15.51 to ?5.1‰, δ¹⁸O = +3.6 to +22.5‰. δ¹³C values are slightly lower than those of the Cretaceous skarn deposits in Japan. These isotope ratios of the Kaninomata skarn show that the original calcareous rocks resemble the present submarine hydrothermal carbonates at the CLAM Site, Okinawa Trough, than Cenozoic limestones, even though some isotopic shifts had occurred during later skarnization. δ³⁴S ratios of the sulfide minerals from the Kaninomata and Nakanomata orebodies are mostly in a narrow range of +4.0 to +7.0‰ and they resemble each other, suggesting the same sulfur origin for the both deposits. The magnetite‐series Tertiary Kaninomatasawa granite is distributed just beneath the skarn layer and has δ³⁴S ratios of +7.5 to 8.1‰. The heavy sulfur isotope ratio of the skarn sulfides may have been affected by the Kaninomatasawa granite.     

Isotopic geochemistry of burial-metamorphosed volcanogenic sediments,Great Valley sequence,northern California

Isotopic and mineralogic data from an 8500-m thick section of the Great Valley sequence, northern California, indicate that changes in the δ¹⁸O values of authigenic minerals resulted from the conversion of smectite to a 10 Å clay-mineral as temperature increased with burial in the Jurassic- Cretaceous outer-arc basin. The clay-mineral assemblage in mudstone is characterized by a proportional increase of the 10 Å clay-mineral with increasing stratigraphic depth, and by a depletion in the δ¹⁸O value of the mixed-layer smectite/10 Å clay-mineral with descending stratigraphic position from +21.9 to + 15.5%. SMOW. Modeling of the oxygen isotopic data from authigenic phases, based on equilibrium fractionation during clay-mineral diagenesis, indicates that δ¹⁸O values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25°C/km during maximum burial to about 6–7 km. δD values of the mixed-layer smectite/10 Å clay-mineral range between ?69 to ?44%. SMOW. Using temperatures calculated from the oxygen isotopic data, the deuterium and oxygen isotopic data indicate that the smectite underwent late-stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of δ¹⁸O ≈ +8%. SMOW and δD ≈ ?25%. SMOW. The δ¹³C values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved HCO^t-₃ was continuously enriched in ¹³C as temperature increased with burial. At the base of the sequence and immediately overlying the ophiolitic basement rocks, several hundred meters of strata were altered by more oxygen-depleted (δ¹⁸O ? +4 to +5%.) hydrothermal fluids emanating from the ophiolitic rocks, probably at maximum burial depth.     

Solution-collapse breccias of the Minkinfjellet and Wordiekammen Formations, Central Spitsbergen, Svalbard: a large gypsum palaeokarst system

Large volumes of carbonate breccia occur in the late syn-rift and early post-rift deposits of the Billefjorden Trough, Central Spitsbergen. Breccias are developed throughout the Moscovian Minkinfjellet Formation and in basal parts of the Kazimovian Wordiekammen Formation. Breccias can be divided into two categories: (i) thick, cross-cutting breccia-bodies up to 200 m thick that are associated with breccia pipes and large V-structures, and (ii) horizontal stratabound breccia beds interbedded with undeformed carbonate and siliciclastic rocks. The thick breccias occur in the central part of the basin, whereas the stratabound breccia beds have a much wider areal extent towards the basin margins. The breccias were formed by gravitational collapse into cavities formed by dissolution of gypsum and anhydrite beds in the Minkinfjellet Formation. Several dissolution fronts have been discovered, demonstrating the genetic relationship between dissolution of gypsum and brecciation. Textures and structures typical of collapse breccias such as inverse grading, a sharp flat base, breccia pipes (collapse dolines) and V-structures (cave roof collapse) are also observed. The breccias are cemented by calcite cements of pre-compaction, shallow burial origin. Primary fluid inclusions in the calcite are dominantly single phase containing fresh water (final melting points are ca 0 °C), suggesting that breccia diagenesis occurred in meteoric waters. Cathodoluminescence (CL) zoning of the cements shows a consistent pattern of three cement stages, but the abundance of each stage varies stratigraphically and laterally. δ¹⁸O values of breccia cements are more negative relative to marine limestones and meteoric cements developed in unbrecciated Minkinfjellet limestones. There is a clear relationship between δ¹⁸O values and the abundance of the different cement generations detected by CL. Paragenetically, later cements have lower δ¹⁸O values recording increased temperatures during their precipitation. Carbon isotope values of the cements are primarily rock-buffered although a weak trend towards more negative values with increasing burial depth is observed. The timing of gypsum dissolution and brecciation was most likely related to major intervals of exposure of the carbonate platform during Gzhelian and/or Asselian/Sakmarian times. These intervals of exposure occurred shortly after deposition of the brecciated units and before deep burial of the sediments.     

Carbonate pond deposits related to semi-arid alluvial systems: examples from the Tertiary Madrid Basin, Spain

Carbonate pond deposits occur associated with alluvial sediments in Miocene sequences of the Madrid Basin, central Spain. The ponds developed near the basin margins, either in floodplain environments (north) or mud-flat settings (south). Three main facies assemblages are recognized: (1) floodplain/mud-flat, (2) palaeosols and (3) pond deposits. In the northern part of the basin, ponds developed on the floodplain of terminal fluvial systems. The floodplain facies are typically red mudstones with interbedded sandstones and siltstones. Palaeosols associated with the ponds show a pedofacies relationship, the maturity of soils increasing with distance from the main channel. Carbonate pond deposits consist mainly of limestones, which display typical ‘palustrine’features. The formation and further accumulation of carbonate in the ponds took place in periods of reduced clastic sediment input and it is suggested that recharge into the pond areas was mainly from groundwater. In the south, ponds developed on mud-flats located between sheet-flood-dominated alluvial fans and evaporite lakes. Mud-flat facies consist of red mudstone that exhibits evidence of progressive soil development near both edges and beneath the carbonate pond lenses. Carbonate in the ponds is mainly dolomite and comprises two subfacies, mottled and laminated dolomicrites. This mineralogy, together with the presence of gypsum crusts below and in the lower part of the carbonate body, suggests higher evaporation rates and/or more saline waters filling the ponds in this part of the basin. In spite of differences in depositional setting and, to some extent, climatic conditions between the two areas of the basin, both facies associations and the sequential arrangement of the ponds show strong similarities that allow the proposal of a facies model for carbonate pond deposits related to semi-arid alluvial systems. The sequences recognized from the pond deposits record a set of facies clearly different to those forming in swampy lakes associated with many permanent fluvial systems developed in more humid climates.     

Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico

Oxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co-precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non-ferroan calcite cements indicates that both are original precipitates that formed during pre-Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ¹⁸Os of megaquartz (mean =+27.0^0/00 SMOW; range =+ 24.8 to + 28.9^0/00) and calcite (mean =+ 28.0^0/00 SMOW; range =+ 27.3 to + 28.4^0/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre-Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre-Pennsylvanian opal-CT precursors, and therefore probably re-equilibrated during recrystallization in late or post-Mississippian time. We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ¹⁸Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground-water system, the seaward terminus of which moved southward during lowering of pre-Pennsylvanian sea level. The calcite cements and some of the opal-CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ¹⁸Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.     

Hydrogeochemical investigation of surface and groundwater composition in an irrigated land in Central Tunisia

Major element concentrations, stable (δ¹⁸O and δ²H) and radiogenic (³H, ¹⁴C) isotopes determined in groundwater provided useful initial tracers for understanding the processes that control groundwater mineralization and identifying recharge sources in semi-arid Cherichira basin (central Tunisia).Chemical data based on the chemistry of several major ions has revealed that the main sources of salinity in the groundwaters are related to the water–rock interaction such as the dissolution of evaporitic and carbonate minerals and some reactions with silicate and feldspar minerals.The stable isotope compositions provide evidence that groundwaters are derived from recent recharge. The δ¹⁸O and δ²H relationships implied rapid infiltration during recharge to both the Oligocene and Quaternary aquifers, with only limited evaporation occurring in the Quaternary aquifer.Chemical and isotopic signatures of the reservoir waters show large seasonal evolution and differ clearly from those of groundwaters.Tritium data support the existence of recent recharge in Quaternary groundwaters. But, the low tritium values in Oligocene groundwaters are justified by the existence of clay lenses which limit the infiltration of meteoric water in the unsaturated zone and prolong the groundwater residence time.Carbon-14 activities confirm that groundwaters are recharged from the surface runoff coming from precipitation.     

The distribution of strontium in limestones on Barbuda, West Indies

The Sr²⁺ content of Barbudan limestones is proportional to the aragonite content. The Sr²⁺ content of calcite which has replaced aragonite varies from 3000 p.p.m. in the youngest limestones to 50 p.p.m., average values are approximately 300 p.p.m. The (mSr²⁺/mCa²⁺)_L ratios from well waters within the limestone fall within the predicted values assuming K^Sr_C～0·1. The high (mSr²⁺/mCa)²⁺_L ratios expected by a lower value of K^Sr_C do not occur. The build up of (mSr²⁺/mCa²⁺)_L ratios seen in waters from Barbados is not seen in the well waters from Barbuda. The Sr²⁺ content of calcite is generally lower than is seen in Barbados, but similar to that seen in parts of the Miami Oolite. Diagenesis of aragonite in Barbudan limestones is probably occurring in a more open system than in Barbados.     

Diagenesis of the Newark Rift Basin, Eastern North America

Late Triassic nonmarine strata in the Newark Rift Basin, eastern North America, accumulated in a subsiding half-graben prior to opening of the Atlantic Ocean. These strata consist, in ascending order, of the Stockton, Lockatong, and Passaic formations. Although different in specific lithology, these formations all exhibit diagenetic fabrics dominated by authigenic albite and analcime. These same minerals have a similar presence in Late Triassic (Newark Supergroup) strata of other rift-related basins to the north, suggesting that related authigenesis is not simply a result of local diagenetic factors. The basal deposit, the Stockton Formation, is composed of fluvial sandstones and overbank mudstones, with nodular pedogenic calcite (calcrete). During burial, original micrite was first recrystallized into sparry calcite and then later subjected to partial replacement by authigenic albite, which is also present as overgrowths and void fillings in overbank mudstones. The Lockatong Formation contains organic-rich shales, carbonates, and evaporative mudstones deposited under cyclic conditions in laterally extensive lacustrine environments. Analcime comprises up to 40% of these strata by volume, occurring within the matrix, as a replacement of original carbonates and evaporites, and as fillings in macrovoids. The overlying Passaic Formation is made up of massive red mudstones, evaporites, and local calcareous lacustrine sequences. Evaporites are replaced by coarse-grained anhydrite together with some authigenic albite. A central question concerns the source for the Na, Si, and Al required for albite and analcime authigenesis. It is suggested that, in addition to alteration of primary siliciclastic material, sodium in particular was supplied in two ways: (1) from high concentrations in original evaporative brines and groundwaters (Lockatong and Passaic formations); and (2) from dissolution of associated sodium-bearing evaporites (Lockatong and Passaic Formation) during diagenesis. It is proposed here that basin-sourced, Na-enriched brines circulated through the section over time. As albite is more stable at elevated temperatures relative to analcime, it developed in the lowermost strata of the basin (Stockton Formation). Analcime is more prevalent in the overlying Lockatong Formation.     

Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints

Chemical (major and trace elements) and isotopic compositions (δD and δ¹⁸O in waters and δ¹³C in CO₂ and ³He/⁴He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO₃, B-depleted groundwaters and rising convective Ca-SO₄(HCO₃), CO₂-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N₂, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N₂, Ne, He-depleted hot rising waters. Boron in the liquid phase and N₂ and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured ³He/⁴He ratios, suggesting a contribution of mantle ³He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of ⁴He in the deeper gases. Paradoxically, there is more relative concentration of ⁴He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ¹³C of CO₂ in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil-¹³C depleted organic C, have δ¹³C in CO₂ more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO₂ should generate CO₂ with more positive values of δ¹³C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed.     

西秦岭早三叠世沉积特征及其构造控制作用

西秦岭早三叠世沉积由深水浊积岩、角砾岩、砾岩、滑塌堆积、深水灰岩和钙质泥岩六个岩相组成，在垂向上表现为受构造控制而形成的四个沉积旋回。浊积岩及滑塌堆积的指向表明，大陆边缘的斜坡倾向西南，古水流方向２００-２６０°、说明沿碌曲－成县以北的北方板块边缘在早三叠世存在一引张构造背景，这与扬子板块向北俯冲引起的拖拉滚动有关。