Evaporite Basin Research Articles

Distribution of prokaryotic genetic diversity in athalassohaline lakes of the Atacama Desert, Northern Chile

Athalassohaline lakes are inland saline aquatic environments with ionic proportions quite different from the dissolved salts in seawater. Prokaryotes inhabiting athalassohaline environments are poorly known and very few of such places have been surveyed for microbial diversity studies around the world. We analyzed the planktonic bacterial and archaeal assemblages inhabiting several of these evaporitic basins in a remote and vast area in northern Chile by PCR-denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene fragments. Most systems were springs and athalassohaline ponds in different saltflats of the Atacama Desert region, including Salar de Llamará (in the Central Depression), Salar de Atacama (in the Pre-Andean Depression) and Salar de Ascotán (in the Altiplano). Overall, we analyzed more than 25 samples from 19 different environments with strong gradients of altitude, qualitative ionic compositions and UV influence. Between 4 and 25 well-defined DGGE bands were detected for Bacteria in each sample, whereas Archaea ranged between 1 and 5. Predominant DGGE bands (defined by intensity and frequency of appearance) were excised from the gel and sequenced. Bacterial assemblages were dominated by the Cytophaga–Flavobacterium–Bacteroides (CFB) phylum and a few Proteobacteria. There was a tendency for increasing contribution of CFB with higher salinities and altitude. Thus, CFB accounted for the major fraction of band intensity in the Ascotán samples and for lower percentages in Atacama and Llamará. When the distribution of particular CFB sequences was examined, there were several relatives of Psychroflexus torquis substituting each other as salinity changed in Ascotán. Another set of CFB sequences, very distantly related to Cytophaga marinovorus, was abundant in both Llamará and Atacama at salinities lower than 7%. Archaeal assemblages were dominated by uncultured haloarchaea distantly related to cultured strains mostly obtained from thalassohaline environments. Most of the archaeal sequences did not have a close match with environmental 16S rRNA genes deposited in the database either. Therefore, athalassohaline environments are excellent sources of new microorganisms different from their counterparts in thalassohaline sites and useful tools to relate microbial genetic diversity and environmental characteristics such as changes in salinity (both qualitative and quantitative) and altitude.

EXTENSIONAL EVOLUTION OF THE GULF OF MEXICO BASIN AND THE DEPOSITION OF TERTIARY EVAPORITES

Current interpretations of the evolution of the Gulf of Mexico Basin are guided by the critical assumption that all the salt in this huge basin is Jurassic and time‐equivalent to the Louann salt of the interior evaporite basins of Texas, Louisiana and Mississippi. This assumption forces the interpretation that extensive salt sheets that now reside within Eocene to Plio‐Pleistocene stratigraphic levels are allochthonous, having been emplaced by several episodes of Jurassic salt piercement and horizontal flow as shallow salt sills or sub‐aqueous salt glaciers during Tertiary sedimentation.This interpretation is contested from several geological viewpoints. The uncontaminated Louann salt differs substantially from salt of the coastal, offshore northern Gulf and Mexican Saline Basin, in all of which the salt carries multiple shaley layers and exotic material. The intra‐salt deformation of offshore sheets appears from cores and seismic profiles to be much less severe than would be expected if it had been emplaced via several generations of deep piercement diapirs. The intra‐salt shale laminations in the Belle Isle Salt Mine, Southern Louisiana, carry Oligocene fauna and are so closely conformable with primary salt layering that their contemporaneity with the salt is a valid conclusion. Because the Belle Isle Shale Zone is so similar to shaley zones encountered in offshore sheets, a contemporaneous origin for them is also implied. The mixed Tertiary fauna found within, and below salt sheets is so typical of sedimentary reworking that its origin from evaporite basin margins is a logical conclusion.The Gulf of Mexico Basin is unique as the presumed depository of multiple allochthonous salt sheets. Salt piercements in the remote Kavir Basin of Iran have been proposed as an analogue for the emplacement of salt canopies, but there is no sub‐surface control in the Kavir Basin and the canopy concept is not unanimously accepted. Furthuremore, the popular view that salt sheets were emplaced by submarine salt glaciers during successive episodes of piercement diapirism is doubted because of the complete absence of glaciers in todays halokinetically active basin. The preservation from solution of purported submarine salt extrusions lacks both experimental and credible geological support.Angular sub‐salt contacts of many offshore salt sheets in the northern continental slope have been interpreted as sheer or thrust contacts. However, thrusting seems unlikely in an overall extensional environment and it is here concluded that the angular contacts represent sub‐salt unconformities and that the Cretaceous and older exotic blocks found in some piercement domes have been plucked from a base‐salt unconformity surface.There is much circum‐Gulf evidence for extensional fracturing of the basement during the Tertiary period which, it is thought, supports the expectation of basement block faults, now hidden from view below thick Tertiary sediments; these faults governed sedimentation into depocenters as the basin expanded.It is concluded that salt sheets at Eocene, Oligocene, Mio‐Pliocene and Plio‐Pleistocene levels are autochthonous, a conclusion that has evolved from critical geological observations in the Belle Isle Salt Mine.

Extensive and rapid changes in seawater chemistry during the Phanerozoic: evidence from Br contents in basal halite

Abstract Variation of seawater chemical composition during the Phanerozoic is still a contentious subject. Br concentration in primary marine basal halites was used to indicate variations in seawater composition during the past 550 Ma. Evaporation experiments and thermodynamic modelling suggests that the partition coefficient of Br in halite depends on the composition of the seawater. A combination of this correlation with the time‐dependent models for ocean chemistry of Hardie leads to a prediction of Br in basal halite during the Phanerozoic. This prediction is in good agreement with data from different ancient evaporite basins. The Br data give evidence for rapid and massive changes in seawater chemistry during the Phanerozoic and are thus a first independent test for the Hardie model.

The geochemical evolution of the Catalan potash subbasin, South Pyrenean foreland basin (Spain)

Over the last decade, our studies in ancient evaporitic basins have been based on a detailed study of a single borehole record. The detailed findings in medium- to large-sized evaporitic basins were shadowed with a relevant question: can interpretations from a representative evaporitic record in a single borehole be extended to the whole evaporitic basin? This paper addresses that question; the results obtained are compared with results from another distant point within the basin. The general methodology not only proves its reliability in interpreting the evolution of evaporitic basins from a single borehole but reveals its capability to obtain detailed palaeoenvironmental interpretations. The chemical evolution of an Upper Eocene evaporitic sequence from the South Pyrenean foreland basin (Spain) has been investigated along the Súria-19 borehole record. Detailed petrographic and mineralogical study, X-ray microanalysis of frozen primary inclusions trapped in halite (Cryo-SEM-EDS), systematic isotopic analysis ( δ 34S and δ 18O in sulphates) and computer-based evaporation models have been integrated in a multi-proxy methodology. This study revealed that a variable amount of Ca excess is required throughout different parts of the marine Lower Halite Unit (LHU) for sylvite, instead of K–Mg sulphates, to form. This Ca excess is in turn different from that required for the western sector of the same evaporitic basin (Navarrese subbasin). Quick and variable changes in Ca-rich brines or equivalent dolomitization required are explained as internal processes within the basin rather than secular variations in seawater chemistry. The general hydrological evolution of the Catalan subbasin is explained as a restricted subbasin with a first marine stage in which continental input (up to 50% of total input) had an important control on the geochemistry of the subbasin. A second stage was determined during potash precipitation, in which the subbasin was cut from any seawater input to end up in its last stage as a purely continental evaporitic basin. Coupling evaporation models and analytical results we have obtained the proportions of recycling and their sources, estimated to change from a 100% (total mass of sulphate) Eocene source to 20% Eocene and 80% Triassic (Keuper) towards the latest stage of potash precipitation. The results obtained have been compared with results from the Navarrese subbasin allowing an integrated interpretation of the hydrological evolution of the whole Upper Eocene South Pyrenean basin. Local geochemical variations within the Upper Eocene south Pyrenean basin are explained by the differences in paleogeographical setting of the Navarrese and Catalan subbasins.

Surface-piercing salt domes of interior North Oman, and their significance for the Ara carbonate ‘stringer’ hydrocarbon play

ABSTRACT The six surface-piercing salt domes of interior North Oman form prominent topographic and geological features in an otherwise flat, rocky desert environment. These domes in the central part of the Ghaba Salt Basin have been known since the 1950s but very little data has been published on them. Our geological survey in 2001 provided significant new lithological, stratigraphic, and sedimentological information on the rocks exposed in the domes. This paper provides a comprehensive overview of the morphology, geometry, structural geology and geological evolution of the salt domes. Furthermore, it incorporates relevant information from unpublished subsurface studies to place the new geological field data in the context of ongoing exploration for deep hydrocarbon plays in Oman. A wide variety of rocks is exposed in the salt domes: carbonates, clastics (conglomerates, sandstones, siltstones and clays), volcanics, evaporites and ‘caprocks’. Constituent rocks and structural style vary considerably from one dome to another, but at the surface the main lithological elements of the diapirs are carbonates and evaporites of the ‘Infracambrian’ (Late Precambrian to Early Cambrian) Ara Group, the uppermost unit of the Huqf Supergroup. Very large exotic blocks of bedded Ara carbonates––commonly hundreds of meters long––are well-exposed and form distinctive hills and ridges, thus allowing detailed field observations on intra-salt carbonate ‘stringers’ that have been carried up by rising diapiric salt. A close correlation exists between the facies of the carbonate exotics in the salt domes and Ara ‘stringer’ carbonates penetrated and extensively cored in recent deep exploration wells in the South Oman Salt Basin. This demonstrates the regional significance of the salt domes for the intra-salt ‘stringer’ hydrocarbon play in Oman. Our work has implications for the prospectivity of other ‘Infracambrian’ evaporite basins in Oman, and possibly also for time-equivalent (‘Hormuz’) salt basins elsewhere in the Middle East.

Neoproterozoic allochthonous salt tectonics during the Lufilian orogeny in the Katangan Copperbelt, central Africa

The Neoproterozoic Katangan rocks in the Copperbelt of Shaba (Democratic Republic of the Congo) and Zambia contain the world's largest concentration of stratiform copper-cobalt ores. We propose that during the Lufilian orogeny, the Katangan basin was radically transformed first by extrusion of allochthonous evaporites, then by orogenic shortening. Salt tectonics plausibly explains spectacular breccias underlying 25,000 km2 and containing gigaclasts up to 10 km long. The brecciated Shaban Roan Supergroup—the lowermost Katangan unit—forms regional detachments and diapirs. The former existence of Roan evaporites is indicated by sabkha facies, crystals, and pseudomorphs of gypsum and anhydrite, stratigraphic gaps underlain by collapse breccias, chloride inclusions in ores, hydrothermally propylitized ore hosts, and saline springs. Salt tectonics began during deposition of the Roan Supergroup between 1050 and 950 Ma. In mid-Roan time, small walls and extrusions of evaporite-gigabreccia began to be emplaced. In early Kundelungu time (940–850 Ma), evaporitic diapirs enlarged. Lufilian deformation began between 850 and 650 Ma by laterally squeezing diapirs to form salt welds. Then, a large sheet of commingled Roan evaporite and carbonate-dominated sediments and ores was extruded northward by ∼65 km. Extrusion was fast enough to blanket a uniform preorogenic footwall unit without overriding any synorogenic deposits. Continued shortening then emplaced large thrust sheets, lubricated by the preexisting salt-sediment extrusive sheet. Restoration suggests that the Lufilian foreland in this area shortened from 193 km to 85 km. The depositional northern edge of the evaporite basin controlled the shape of the Outer Lufilian Arc. We infer three original salt provinces. The large-scale salt tectonics inferred here has implications for ore genesis and may yet be recognized in other Precambrian basins.

Chemical composition of brine inclusions in halite fromclayey salt (zuber) facies from the Upper Tertiary (Miocene) evaporite basin (Poland)

The chemistry of the primary brines for halites from the upper part of the Wieliczka deposit and the clayey salt (zuber) unit of the Bochnia deposit has been investigated. The results evidenced that the brines belong to the Na K Mg Cl SO 4 type and the proportion of the ions was similar tothe concentration in recent seawaters but depleted in S04 ion.

Evolution of Permian seawater: evidence from fluid inclusions in halite

The examination of primary fluid inclusions in sedimentary halite from several Permian evaporite basins of East and Central Europe (Asselian and Sakmarian Dnipro-Donets Basin of Ukraine, Sakmarian Dvina-Sukhona Basin of Russia, Kungurian Solikamsk Basin of Russia, Zechstein Basin of Poland and Peri-Baltic Russia) revealed that the brines were of Na-K-Mg-Cl-SO 4 (SO 4 -rich) chemical type. They are characterized by minor content of SO 4 ions, when compared with modern seawater concentrated to the corresponding stage. The lowest SO 4 content was recorded in the Kungurian; a higher SO 4 content was found in the Asselian and Sakmarian; and the highest content in the Late Permian. The bromine content in halite of the Permian salts of East and Central Europe (mostly 30-160 ppm) correlates with ion concentration in brine inclusions. The marine origin of salt in most of the studied sections is supported by isotopic composition of sulphate sulphur. During salt deposition in every basin, local factors - such as the inflow of surface or subsurface waters, sulphate reduction, and water-rock interaction - have exerted important influences on the K, Mg, and SO 4 ion ratios. However, these changes did not lead to the change of the chemical, SO 4 -rich type that is characteristic for the Permian evaporite basins.

Stratigraphy and sedimentary petrology of an Archean volcanic–sedimentary succession at Mt. Goldsworthy in the Pilbara Block, Western Australia: implications of evaporite (nahcolite) and barite deposition

Abstract A approximately 3.3 Ga sedimentary succession at Mt. Goldsworthy in the northeastern Pilbara Block, Australia contains beds composed of silicified pseudomorphs of nahcolite (NaHCO3) or barite (BaSO4). This sedimentary succession correlates with the Corboy Formation and consists of lower, middle and upper sedimentary units, which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks. The lower and middle units are predominantly siliciclastic, whereas the upper unit is characterized by ferruginous cherts and banded iron-formations with minor proportions of intercalated mature sandstone. The succession defines an overall upward fining and deepening trend. The detrital materials were derived from older greenstone successions, volcaniclastics and reworked precipitative beds. Silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit. The formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from Na+-HCO3− brines in a closed or semi-closed evaporitic basin. Bladed barite deposits occur mainly in the middle unit, which was deposited in a sub-aerial to shallow marine environment. Barite may have precipitated as a result of mixing of SO42−-rich seawater and Ba2+-rich hydrothermal fluids. The Mt. Goldsworthy sedimentary succession records an Archean shallow to sub-aerial sedimentary environment that probably developed in a continental margin setting.

Active faulting and folding without topographic expression in an evaporite basin, Chile

The Salar de Atacama in northern Chile accumulated halite during the Pliocene and Quaternary under conditions that alternated between a saline lake and a dry salt flat. Hidden beneath its uninterrupted flat surface is the Salar fault system. The halite deposits provide a high-resolution history of deformation. Contours on the ca. 5 Ma base of the halite unit in the southern salar, defined in a deep oil-exploration borehole and traced through reflection seismic lines, reveal that net reverse offset across the Salar fault system is down-to-the-east by ∼900 m; of that total, 200 m occurred during the Quaternary and 700 m during the Pliocene. Distributions, thicknesses, and geometries of eight stratigraphic sequences within the halite unit reveal a history of episodic faulting and demonstrate that faulting during approximately half the 5 m.y. interval, including the Holocene, did not generate fault scarps. We suggest that deposition caused by evaporation of ground water brines in the salt flat, rather than either dissolution or deflation, has smoothed the topography of the dry lake bed across the active fault zone. For the case of west-directed groundwater flow, we propose that the east-facing Salar fault system focuses groundwater flux along the eastern flank of the fault, which enhances halite precipitation in the downthrown block during desiccated stages and counteracts fault-driven topographic relief. The realization that a seismic risk may exist even though the Salar fault system is hidden leads to the appreciation that similar unrecognized seismic risks may exist in more populous arid basins.

Sedimentological and diagenetic patterns of anhydrite deposits in the Badenian evaporite basin of the Carpathian Foredeep, southern Poland

Anhydrite deposits are widely distributed in the Middle Miocene Badenian evaporite basin of Poland, including the marginal sulphate platform and adjacent salt depocenter. Particular sedimentological, petrographic and geochemical characteristics of these anhydrite deposits and especially common pseudomorphic features, inherited from the precursor gypsum deposits, allow the interpretation of the original sedimentary facies. The observed facies distribution and succession (lower and upper members) reveal three distinct facies associations that record a range of depositional environments from nearshore to deeper basinal settings. Platform sulphates were deposited in subaerial and shallow-marine environments (shoreline and inner platform-lagoon system) mainly as autochthonous selenitic gypsum. This was reworked and redistributed into deeper waters (outer platform-lagoon, slope and the proximal basin floor system) to form resedimented facies composed mostly of allochthonous clastic gypsum and minor anhydrite. The general variation in petrographic and geochemical compositions of anhydrite lithofacies of the lower and upper members reflects the brine evolution, as the result of interactions between seawater, meteoric runoff and highly saline, residual pore fluids. The results indicate the importance of synsedimentary and diagenetic anhydritisation processes in formation of the Badenian anhydrite lithofacies, all of which preserve the original depositional features of the former gypsum. This also applies to the basinal anhydrite previously interpreted to have a depositional genesis. Two different genetic patterns of anhydrite have been reinforced by this study: (1) synsedimentary anhydritisation of gypsum deposits by highly concentrated brines or elevated temperatures in surficial to shallow-burial environments (lower member), and (2) successive phases (syndepositional de novo growth, early diagenetic to late diagenetic replacement of former gypsum) of anhydrite formation during progressive burial (upper member).

Seismic investigation of thick evaporite deposits on the central and inner unit of the Mediterranean Ridge accretionary complex

Seismic reflection observations from the IMERSE cruise have confirmed the presence of at least two localised thick evaporite basins on the crest and Inner Plateau of the Mediterranean Ridge accretionary complex. The discovery is supported by interval velocity results from both near-normal incidence reflection data and wide-angle seismic data. These evaporite basins are located north of a deep trough known as the Cleft and at a currently active thrust zone south of the Cleft, on the crest of the Mediterranean Ridge. The polarity of the seismic reflections associated with the base of the evaporite shows a phase reversal relative to the seafloor. This high amplitude phase reversed reflection indicates a decrease in velocity between the evaporite and the underlying sediment. The existence of these thick evaporite deposits suggests that the distribution of Messinian salt is much thicker (∼1.8–2 km) on parts of the Ridge crest than initially perceived by previous investigations. We propose that these thick evaporite deposits were deposited in localised basins on the Ridge, formed in response to pre- and syn-Messinian tectonics. The evaporites on the Inner Plateau basin most likely represent the infill of a Messinian forearc basin. The existence of local deep evaporite basins on the crest of the Ridge at depths of 2.7–3.3 km below sea level at the present day supports the hypothesis that the Mediterranean Sea level must have dropped by around 3 km below its present sea level during the Messinian salinity crisis.

The structure and evolution of the western Mediterranean Ridge

The Mediterranean Ridge is a unique accretionary complex, consisting of five key elements: the frontal slope, the upper slope, the crest of the Ridge, the Cleft area, and the Inner Plateau. The IMERSE data show that these correspond roughly to the locus of frontal accretion, of underplating, of a pre-Messinian wedge, of complex faulting and possible strike-slip tectonics, and of a backstop of Hellenic nappes covered by a Messinian forearc basin. The frontal portion of the complex is a post-Messinian accretionary wedge (composed of Messinian evaporites and overlying tightly folded Plio–Quaternary sediments), underlain by pre-Messinian sequences attached to the African Plate. The basal detachment at the front of the wedge occurs at the base of the evaporites. Moving further to the northeast (the upper slope), the basal detachment cuts to deeper levels leading to the development of duplex structures where pre-Messinian units are subcreted beneath the Messinian evaporites. Just behind the subcreted units, the evaporites thin and may be absent on the crest of the Ridge. This region we interpret as the site of a pre-Messinian accretionary wedge: we suggest that following the deposition of thick evaporites in the Messinian, the pre-Messinian accretionary tectonics continued as subsurface accretion (subcretion) beneath the evaporites. Although the crest of the Ridge is largely devoid of evaporites, local deep evaporite basins observed here formed as local closed basins on top of the pre-Messinian wedge. We infer that the Messinian sealevel was at about the level of the Ridge crest, that is 3000 m below present. Allowing for isostatic adjustment to the removal of the water load, this would imply a sealevel drop of at least 2000 m. The Cleft basins mark the northeast limit of the accretionary complex. Thick evaporite deposits to the northeast (beneath the Inner Plateau) may have been deposited in a Messinian forearc basin. The evaporites of the Inner Plateau are underlain by a thin pre-Messinian sequence and by crystalline basement of the Hellenic nappes. This basement forms the backstop to the accretionary complex.

The latest Post-Variscan fluids in the Spanish Central System: evidence from fluid inclusion and stable isotope data

The Spanish Central System has been subjected to repeated fluid incursions, which were responsible for a variety of mineralizing episodes including W–Sn, Cu–Zn–Pb–As–(Ag), F–Ba and barren quartz veins. These hydrothermal fluids occurred over a 200Ma time period and the latest hydrothermal event is recorded in barren quartz veins. This study is a multidisciplinary approach leading to the characterization of the hydrothermal fluids preserved in barren quartz veins, which are spatially but not temporally related to Hercynian upper crustal granites. The veins were dated by the 39Ar/40Ar method, and the fluids were examined using petrographic, microthermometric, chemical and isotopic methods. Fluid inclusions in barren quartz veins indicate that two fluids were related to this hydrothermal event. The main part of the quartz veins were formed from an early low salinity (<1wt% NaCl) H2O–NaCl fluid. This fluid was trapped at around 270±25°C and 0.1–1kbar under sublithostatic to hydrostatic conditions. δ18O (−9 to 2‰) and δD (−70 to −34.5‰) values indicate a meteoric origin for water, with significant water/rock interactions. The latest H2O–NaCl–CaCl2 fluid is found in two types of fluid inclusions: a primary liquid–vapour type (16–24wt% NaCl and 1–12wt% CaCl2) and secondary hypersaline type (7–15wt% NaCl and 21–27wt% CaCl2). Significant Li concentrations in this fluid were confirmed. This late Ca-bearing fluid formed quartz crystals in the central part of the veins, and was trapped at 70–140°C, at a maximum pressure of 0.5kbar. The low δ18O (−20 to −6‰) and δD (−137 to −116‰) values suggest a meteoric origin for this fluid, however its high salinity probably requires a source from Triassic evaporite basins located in the NE tip of the Spanish Central System. Anomalously low isotopic values have been previously reported from kaolinites of Lower Cretaceous age. Anomalous climatic conditions during the Cretaceous appear to be the main reason to explain this very negative meteoric water. Strong isotopic depletion in meteoric water has been observed in modern areas with monsoonal climates. The hydrothermal evolution of barren quartz veins in the Spanish Central System is comparable to other hydrothermal Post-Variscan events in central and south-western Europe related to the opening of the North-Atlantic during Cretaceous time.

Geochemistry of Early Triassic seawater as indicated by study of the Röt halite in the Netherlands, Germany, and Poland

Study of three halite sections in the Röt Basin from the eastern Netherlands (Twente-Rijn 382 borehole), central Germany (Mardorf 6 borehole) and western Poland (Ośno IG2 borehole) shows that the δ 34S values in Röt sulphates of studied samples (27.14–31.98‰) are characteristic of marine sulphates of late Early Triassic. These values fit in the Röt event shift described in the literature. The bromine content in halite is 32–158 ppm. In the Dutch and Polish sections, relics of sedimentary structures—chevron and hopper—are recorded in halite grains. These structures contain fluid inclusions that were analysed using the ultramicrochemical method of Petrichenko (Petrichenko, O.I. 1973. Metody doslidzhennya vkluchen' v mineralakh galogennykh porid. Kiev, Naukova Dumka, 92 pp.). In the German section, no sedimentary halite structures occur. Because the pressure in the inclusions in recrystallized halite is only slightly greater than atmospheric pressure, these inclusions are presumed to be early diagenetic in origin. The analysis of individual brine inclusions, both in sedimentary and diagenetic forms of halite, shows that the Röt brines correspond to the Na-K-Mg-Cl-SO 4 (SO 4-rich) chemical type, i.e. the same type that characterises modern seawater. However, the Röt brines differ from the composition characteristic of modern concentrated seawater by a decrease in the content of sulphate ion and an increase in the content of potassium ion. On a Jänecke diagram, the points of the brine composition during deposition of Röt halite in Poland, Germany and the Netherlands only show a small range. This suggests a stable composition of basinal brines during the deposition of the halite. At the same time, the relative content of sulphate ion in the Lower Triassic Röt Basin was slightly lower than that in Upper Permian evaporite basins, but higher than that in Lower and Middle Permian and Middle and Upper Triassic evaporite basins. The implication is that the content of sulphate ion in seawater was subject to changes in Permian and Triassic times.

Seawater composition during deposition of Viséan evaporites in the Moncton Subbasin of New Brunswick as inferred from the fluid inclusion study of halite

The Viséan evaporites of the Windsor Group in the Moncton Subbasin at the western limit of the Maritimes Basin of eastern Canada represent an important stratigraphic sequence to study in determining changes in the chemistry of marine brines. Seventeen samples of halite have been selected for fluid inclusion study. The chemical composition of inclusion brines was examined using the Petrichenko method of glass capillaries with applied ultramicroanalytical techniques. The results of the study indicate that the conditions of evaporite deposition in the Windsor Basin differed from those recorded in other Phanerozoic basins and that the history of the Windsor Basin brines was characterized by changes in the chemical composition of the brines. It was also found that there existed continual unfavourable conditions for the day-and-night zonality in chevron halite crystals and for the origin of relatively large (&gt;50 µm) fluid inclusions and that the gas content was low not only during the deposition but also during the diagenesis of potash deposits. The chemical composition of the Viséan waters of the Moncton Subbasin at the beginning and end of deposition of the Windsor Group evaporite sequence was controlled by the predominating waters of nonmarine origin; during the potash deposition the controlling factor was chloride-type marine waters. In general, the geochemical data support the concept of Carboniferous evaporite basins with Na–K–Mg–Ca–Cl-type brines.

Latest Guadalupian (Middle Permian) conodonts and foraminifers from West Texas

Clarkina, which characterizes Upper Permian (Lopingian Series) strata, evolved from Jinogondolella altudaensis in the Delaware basin of West Texas as demonstrated by transitional continuity. The West Texas section is significantly more complete in the uppermost Guadalupian interval than that of the probable GSSP reference section in South China, and clarifies the phylogenetic relationships among other conodont taxa as well. Jinogondolella granti clearly evolved into J. artafrons new species, both characterized by Pa elements with a distinctive fused carina. Representatives of Jinogondolella crofti are limited to the uppermost part of the altudaensis zone, and are interpreted as terminal paedomorphs. The associated foraminifer (non-fusulinid) fauna has some species in common with Zechstein faunas, possibly presaging the evaporitic basin that would develop following this latest Guadalupian marine deposition in West Texas.

Latest Guadalupian (Middle Permian) conodonts and foraminifers from West Texas

Clarkina, which characterizes Upper Permian (Lopingian Series) strata, evolved from Jinogondolella altudaensis in the Delaware basin of West Texas as demonstrated by transitional continuity. The West Texas section is significantly more complete in the uppermost Guadalupian interval than that of the probable GSSP reference section in South China, and clarifies the phylogenetic relation- ships among other conodont taxa as well. Jinogondolella granti clearly evolved into J. artafrons new species, both characterized by Pa elements with a distinctive fused carina. Representatives of Jinogondolella crofti are limited to the uppermost part of the altudaensis zone, and are interpreted as terminal paedomorphs. The associated foraminifer (non-fusulinid) fauna has some species in common with Zechstein faunas, possibly presaging the evaporitic basin that would develop following this latest Guadalupian marine deposition in West Texas.

Sodium sulphate deposits of Neogene age: the Kirmir Formation, Beypazari Basin, Turkey

The Evaporite Member of the Kirmir Formation was deposited in shallow lacustrine environments during the upper Miocene. The most soluble minerals of this member can be currently observed in the Çayirhan mine. The Evaporite Member, which is composed of secondary gypsum at outcrop, can be subdivided into a bedded lower unit and a massive upper unit. In the bedded lower unit, most of the gypsum throughout the basin can be identified as having been transformed from glauberite. In the glauberite layers of the Çayirhan mine, some glauberite textures (‘clear glauberite’) suggest a primary, subaqueous, free precipitation on a depositional floor. More common, however, are the glauberite textures indicating an interstitial growth within a clayey-magnesitic matrix. In the thenardite layers accompanying the glauberite in the Çayirhan mine, some disruption structures can be assigned to synsedimentary dissolution. These structures together with the textures of the thenardite suggest that the original sodium sulphate was mirabilite, thenardite being a secondary phase, which formed during early to moderate burial diagenesis. The massive upper unit, in which evidence of sodium-bearing minerals is absent, is characterized by laminated to banded gypsum and nodular gypsum in the marginal areas of the evaporitic basin, whereas thick, clast-supported gypsum breccias prevail in the northern, deeper part of the basin. The brecciation of these calcium sulphate layers occurred as a result of synsedimentary, gravitative slumping under tectonic control. Although the sulphur isotopic values (δ 34 S) of the sulphates of the Kirmir Formation suggest a marine-derived brine supply, the oxygen isotopic values (δ 18 O) and the strontium ratios ( 87 Sr/ 86 Sr) do not support such a supply. The origin of the mother brines, the glauberite genesis, the depositional model of the sodium sulphates, and the salinity evolution are discussed.

Gypsum facies transitions in basin‐marginal evaporites: middle Miocene (Badenian) of west Ukraine

In the middle Miocene Badenian gypsum basin of the Carpathian Foredeep, west Ukraine, three main zones of gypsum development occur in the peripheral parts of the basin. Zone I consists entirely of stromatolitic gypsum formed in a nearshore zone. Zone II is located more basinward and is characterized by stromatolitic gypsum in the lower part of the section, overlain by a sabre gypsum unit. Zone III occurs in still more basinward areas and is characterized by giant gypsum intergrowths (or secondary nodular gypsum pseudomorphs of these) in the lowermost part, overlain by stromatolitic gypsum, sabre gypsum and then by clastic gypsum units. Correlation between these facies and zones has been achieved using lithological marker beds and surfaces. Of particular importance for correlation is a characteristic marker bed (usually 20–40 cm thick) of cryptocrystalline massive gypsum occurring in zones II and III. The marker was not distinguished in zone I, possibly because this bed is older than the entire gypsum section of that zone. These new results strongly suggest that the deposition of giant gypsum intergrowth facies and stromatolitic gypsum facies was coeval. In some sections of zones I and II, limestone intercalations have been recorded within the upper part of the gypsum sections. Considerable scatter of the δ18O and δ13C values of these limestones indicates variable diagenetic overprints of marine carbonates, but a marine provenance of the limestones is confirmed by microfacies analysis. Some of the limestones are coeval with an intercalation of gypsarenitic, mostly laminated gypsum occurring in the sabre gypsum unit of zones II and III. Badenian gypsum formed in extremely shallow‐water to subaerial environments on broad, very low relief areas of negligible brine depth, which could be affected by rapid transgressions. Stable isotope (δ34S, δ18O) studies of the gypsum demonstrate that the sulphate was of sea‐water origin or was derived from dissolution of Miocene marine evaporites. Investigations of individual inclusions in the gypsum indicate decreased water salinity when compared with modern marine‐derived, calcium sulphate‐saturated water. Groundwater influences are indicated by high calcium sulphate contents of the brines in the evaporite basin. The chemical composition of Badenian waters was thus a mixture of relic sea water (depleted in NaCl), groundwater (enriched in calcium sulphate) and surface run‐off.