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1.
Bristol Dry Lake, a 155 km2 continental-sabkha playa basin in the Mojave Desert of south eastern California, is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. Evaporite facies conform approximately to a bull's eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan to the centre of the playa, reveal: (1) crudely-bedded, alluvial fan clastics interfingering with (2) playa-margin sand flat and wadi sand and silt, followed by (3) gypsum, anhydrite, chaotic mud halite, and clay of the saline mud flat, and (4) salt-pan halite beds. Terrigenous clastics were deposited in Bristol Dry Lake by sheetflow and by suspension settling from ponded floodwater. Some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin. Thin nodular-like beds of anhydrite and several types of gypsum occur across most of the playa. Giant hopper-shaped halite cubes are suspended in saline mud flat facies, suggesting that they grew displacively in brine soaked sediment just below the surface. Thick beds (4 m) of halite, in the playa centre, may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods, intense evaporation and brine-level lowering, and capillary discharge of brines. The stratigraphy in the playa centre is cyclic. An ideal cycle consists of: (1) chaotic mud halite at the base overlain by (2) green to red clay with abundant, giant hoppers, and at the top (3) red clay, gypsum, and anhydrite with flaser- to wavy-bedded sand and silt. This type of cycle probably records a gradual progradation of mud-flat facies over salt pans. Bristol Dry Lake sediments are nearly identical to some of the Permian evaporites of the Permian Basin region, U.S.A. and they can serve as modern analogues for ancient-sabkha facies analysis.  相似文献   

2.
Salt mounds are commonly distributed along playa margins and typically comprise alternating layers of loose fine sand and slightly hard halite-rich sediments as a result of long-term underground spring activity. A model of salt mound development was constructed for this study. It suggests that wind-blown sand supply and upward recharge of underground springs are two important factors in salt mound construction. Furthermore, it proposes that salt mound height is mainly controlled by the vertical transport range of underground springs and the thickness of the capillary fringe. A 1.5 m representative profile dug from the center of salt mound LP1 in the Lop Nor playa revealed a fairly complicated mineral assemblage including halite, gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. This matches closely with the assemblage predicted by the EQL/EVP model. The groundwater in the area is highly concentrated brine rich in Cl− and Na+ and poor in Ca2+, displaying low alkalinity, and containing considerable amounts of SO42−, Mg2+ and K+. Chemical analysis of groundwater revealed considerable variation in the salinity and chemical composition of groundwater over time. The Cs-137 technique was used to measure the accumulated ages of the salt mounds. This method may prove useful in the research of relatively young playa environments where carbon dating techniques are unworkable because of an absence of carbon-rich materials in recent saline sediments.  相似文献   

3.
The Badenian (Middle Miocene) Ca-sulphate deposits of the fore-Carpathian basin – including the shelf and adjacent salt depocentre – have undergone varying degrees of diagenetic change: they are preserved mainly as primary gypsum in the peripheral part of the platform, whereas toward the centre of the basin, where great subsidence occurred during the Miocene, they have been totally transformed into anhydrite. The facies variation and sequence of Badenian anhydrites reflect different genetic patterns of two members of the Ca-sulphate formation. In the lower member (restricted to the platform), anhydrite formed mainly by synsedimentary anhydritization (via nodule formation), whereas in the upper member (distributed throughout the platform and depocentre) the various gypsum/anhydrite lithofacies display a continuum of distinctive anhydrite type-fabrics. These fabrics are based on petrographic features and show from the centre to the margin: (1) syndepositional, interstitial growth of displacive anhydrite; (2) early diagenetic, displacive to replacive (by replacement of former gypsum) anhydrite formation near the depositional surface; (3) early diagenetic, displacive to replacive anhydrite formation during shallow burial; and (4) late-diagenetic (and only partial) replacement of gypsum at deeper burial. The cross-shelf lateral relations of anhydrite lithofacies and fabrics suggest that the diagenesis developed as a diachronous process. These fabrics of the upper member reflect both palaeogeographic (linked to different parts of the basin) and burial controls. Anhydrite growth started very early in the basin centre, presumably related to high-salinity pore fluids; anhydritization prograded updip toward the shelf (landward in a generalized cross-section through the basin). The intensity of gypsum replacement by anhydrite was progressively attenuated landward by a decrease in the salinity of the pore fluids. In each part of the basin, the anhydrite fabric was also controlled by the texture and degree of lithification of the fine-grained primary gypsum lithofacies. Recrystallization of these anhydrite fabrics during late diagenesis, linked to deeper burial conditions, is insignificant, allowing reconstruction of the original anhydritization pattern.  相似文献   

4.
Sabkhas are ubiquitous geomorphic features in eastern Saudi Arabia. Seven brine samples were taken from Sabkha Jayb Uwayyid in eastern Saudi Arabia. Brine chemistry, saturation state with respect to carbonate and evaporate minerals, and evaporation-driven geochemical reaction paths were investigated to delineate the origin of brines and the evolution of both brine chemistry and sabkha mineralogy. The average total dissolved solids in the sabkha brines is 243 g/l. The order of cation dominance is Na+   >>  Mg2+ >>  Ca2+>K+, while anion dominance is Cl− >> SO4 2− >> HCO3 −. Based on the chemical divide principle and observed ion ratios, it was concluded that sabkha brines have evolved from deep groundwater rather than from direct rainfall, runoff from the surroundings, or inflow of shallow groundwater. Aqueous speciation simulations show that: (1) all seven brines are supersaturated with respect to calcite, dolomite, and magnesite and undersaturated with respect to halite; (2) three brines are undersaturated with respect to both gypsum and anhydrite, while three brines are supersaturated with respect to both minerals; (3) anhydrite is a more stable solid phase than gypsum in four brines. Evaporation factors required to bring the brines to the halite phase boundary ranged from 1.016 to 4.53. All reaction paths to the halite phase boundary follow the neutral path as CO2 is degassed and dolomite precipitates from the brines. On average, a sabkha brine containing 1 kg of H2O precipitates 7.6 g of minerals along the reaction path to the halite phase boundary, of which 52% is anhydrite, 35.3% is gypsum, and 12.7% is dolomite. Bicarbonate is the limiting factor of dolomite precipitation, and sulfate is the limiting factor of gypsum and anhydrite precipitation from sabkha brines.  相似文献   

5.
Lake Tyrrell is a large ephemeral salt lake, the level of which is controlled by climate and groundwater. Up to a metre of water fills the basin during the wetter and cooler winter season, but evaporates during the summer, precipitating up to 10 cm of halite. Each year essentially the same pool of ions is redissolved by this annual freshening. The small percentage of gypsum precipated (< 2%) in the surface salt crust reflects the low calcium content of the brine which, in turn, is a function of the negligible net discharge of calcium from the groundwater system. The small influx of fine‐grained clastic sediment to the lake floor comes from surface runoff, wind, and reworking of older sediment from the shoreline.

The Lake Tyrrell basin lies in a setting in which three different groundwater types, identified by distinct salinities, interact with surface waters. A refluxing cycle that goes from discharging groundwater at the basin margin, to surface evaporation on the lake floor, to recharge through the floor of the lake, controls the major chemical characteristics of the basin. In this process, salts are leached downward from the lake floor to join a brine pool below the lake. This provides an outlet from the lake, especially under conditions that have been both drier and wetter than those of today. Enhanced discharge occurs under drier conditions, when the enclosing regional groundwater divide is lowered, whereas a rise in lake level increases the hydraulic head over that of the sub‐surface brine and promotes an increase in brine loss from the lake.

Sulphate‐reducing bacteria in a zone of black sulphide‐rich mud beneath the salt crust help prevent gypsum from being incorporated into the recent sedimentary record. However, below the upper 5 to 10 cm zone of bacterial activity, discoidal gypsum is being precipitated within the mud from the groundwater. These crystals have grown by displacing the mud and typically “float” in a clay matrix; in some zones, they form concentrations exceeding 50% of the sediment. The occasional laminae of more prismatic gypsum that occur within the upper metre of mud have crystallised from surface brines. The scarcity of these comparatively pure prismatic‐crystal concentrations probably is a function of unfavourable chemical conditions in the lake brine and of the role that sulphate‐reducing bacteria have played.  相似文献   

6.
Kahak salt playa in South Khorasan province of Iran, have special geomorphological characteristics by the presence of ephemeral saline lakes, wetlands, salt crusts, surface accumulations of salt and zones of patterned ground. Salt crusts in the soil surface are unique in the region and have laminated horizons in the playa soil. Soil-surface salt accumulations are dominated by NaCl and gypsum. It has been found that distribution of chemical soluble is not uniform across the playa landscape, and this result influences on the variety form of patterned ground. In this study, the percent changes in some of the chemical elements such as NaCl, gypsum and also brine extent have been calculated in the playa. Indicating changes in Kahak salt playa is the main aim of this study by using remote sensing and GIS techniques. In this paper, techniques such as spectral un-mixing, maximum likelihood classification, band rationing, fuzzy classification and correlation relationships are discussed. This contribution presents modeling of temporal and spatial changes of salinity and playa developing using combined approaches that incorporate different data-fusion and data-integration techniques for two periods of date. Furthermore, percent changes in the surface-patterned ground of the playa have been calculated using texture and pattern analysis of the PCA1. Results have revealed that, in the playa developing, chemical materials such as sodium, NaCl, gypsum and also brine extent are positively correlated with each other and the most increased changes are related to gypsum and the most decreased changes are related to the NaCl. Also changes in the amount of agricultural area in the playa-lakes margin, show low effects in the desertification process.  相似文献   

7.
Abstract Extraordinary sequences of conspicuous, pervasive and laterally persistent varves characterize the Castile evaporites. They occur as singlets (calcite laminae), couplets (calcite laminae interstratified with anhydrite laminae), thick couplets (calcite laminae interstratified with thin anhydrite beds) and triplets (calcite and anhydrite laminae interstratified with thin halite beds). The varves accumulated in a deep (initially ≈ 550 m), persistently stratified, saline lake surrounded by an extinct reef. The lake had formed when the reef grew across a channel between an embayment and the ocean. Although located virtually on the palaeo-equator, the lake experienced negligible meteoric influx and extreme seasonality. During the season of high relative humidity, more marine groundwater entered the lake through the permeable reef barrier than exited as reflux and, secondarily, as evaporation. Consequently, the lake level rose by up to several metres to sea level. The ‘refreshening’ decreased salinity and replenished dissolved CO2– the critical nutrient limiting growth of indigenous phytoplankton. Algae proliferated, pH increased and CaCO3 precipitated. It mixed with organic matter to form a thin, dark lamina. During the season of low relative humidity, tens of cubic kilometres of water evaporated from and, secondarily, leaked out through the surrounding reef. More water exited than entered, brine level fell below sea level, and salinity of the upper brine layer increased. Gypsum usually precipitated and rained onto the basin floor forming a couplet; infrequently, halite also precipitated forming a triplet. Every few thousand years, for <50 to several hundred years, the lake became unstratified during the dry season, and wind-induced overturn allowed a layer of gypsum crystals up to ≈ 2 cm high to precipitate on the basin floor. Each layer, now thin beds of anhydrite nodules and anhydrite pseudomorphs after gypsum, and an underlying lamina of CaCO3 and admixed organic matter formed a thick couplet. The different varve types recur with a period of 1800–3000 years reflecting climatic changes on a millennial time scale.  相似文献   

8.
This study aims at understanding the physico-chemical interactions between the saturated brine and the rocks enclosing the underground salt workings in Lorraine (eastern France). These anhydrite-rich and argillaceous rocks were characterized in terms of mineralogy, micro-texture and connected porosity. Then, the two main lithofacies, massive anhydrite and anhydrite-rich argillite, were immersed in brine during more than 1 year. During this batch experiment, the argillites were affected by macroscopic splitting, contrarily to the massive anhydrite. Micro-texture and brine chemical analyses clearly show the swelling due to the hydration of anhydrite into gypsum inside the argillites, whereas hydration occurs superficially on the massive anhydrite, due to its very low permeability. Anhydrite–gypsum transformation is promoted by the presence of dissolved strontium and potassium in saturated brine. The low activity of water in saturated brine does not allow the clay fraction to swell significantly during the experiment. Thus, the expansion resulting from the hydration of anhydrite into gypsum might be responsible of the splitting of argillite in a saturated brine environment. The superficial anhydrite hydration on massive anhydrite can be explained by the low amount of connected porosity (less than 1%).  相似文献   

9.
The Didwana playa, the second largest playa in the eastern part of the Thar desert, is 5.6 km long and 2.4 km wide and supports commercial salt production. The thickness of lake sediment package is reported to be 20 m and comprises fine grained clays and silts, with abundant calcite, gypsum, and halite, associated with hypersaline water. Isolated hills of graphitic phyllite and quartzite are seen on the western side of the lake. During the course of investigations for uranium in surficial environment of semi-arid terrain of Rajasthan, ground water sampling defined a NE-SW trending uranium halo encompassing the Didwana playa. Subsequent sampling of unlined dug wells, up to water table in central part of the playa, indicated uranium values up to 190 ppm and 2072 ppb in lake sediments and brine respectively. These values are of the order of 21 ppm and 192 ppb towards the southwestern periphery of the lake. The average uranium content, as inferred from 12 samples in the central part of the lake, is around 60 ppm over a thickness of 5 m. It appears that the uranium is loosely bonded to the sediments in amorphous form and is, hence, easily leachable. Samples of brine (n=10), from both the central and southwestern portions of the lake, analysed high (1,67,500–3,00,000 mg/l) TDS, HCO3− (1128–8395 mg/l), and SO4 (30,536–88,000 mg/l). These are of alkaline (pH: 7.2–9.3) and reducing (Eh: −200 to −340 mV) nature. Under these Eh-pH conditions below the groundwater table, and for such uranium bearing groundwater, precipitation of primary uranium is expected. It is, therefore, modelled that uranium in lake sediment package above water table is concentrated by evaporation process and by chemical reduction below the water table.  相似文献   

10.
Diagenetic-hydrothermal brines, here called “hydrothermal Ca–Cl brines,” have compositions that reflect interactions between groundwaters and rocks or sediments at elevated temperatures. Hydrothermal Ca–Cl brines reach the surface by convection-driven or topographically driven circulation, and discharge as springs or seeps along fault zones to become important inflow waters in many tectonically active closed basins. Case studies from (1) Qaidam Basin, China, (2) Death Valley, California, (3) Salar de Atacama, Chile, and (4) Bristol Dry Lake, California illustrate that hydrothermal Ca–Cl inflow waters have influenced brine evolution in terms of major ion chemistries and mineral precipitation sequences. All four basins are tectonically active; three (Death Valley, Salar de Atacama, and Qaidam Basin) have well-documented Ca–Cl spring inflow and Holocene faulting. Bristol Dry Lake has young volcanic deposits and Salar de Atacama has an active stratovolcano on its eastern margin, indicating subsurface magma bodies. A midcrustal magma chamber has been identified in southern Death Valley. Volcanism and faulting in these closed basins provides the heat source for hydrothermal-diagenetic processes and the energy and pathways to deliver these waters to the surface.  相似文献   

11.
Playas are shallow ephemeral lakes that form in arid and semi-arid regions. Iran has a large number of playas such as Meyghan Playa, which is located in the northeast of Arak city that borders the central Iran and Sanandaj-Sirjan zones. This study aims to investigate the mineralogical, sedimentological, and geochemical characteristics of the playa sediments. In order to determine the palaeoenvironment, we carried out X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) studies. Meyghan Playa sediments consist of very fine-grained sediments and contain both evaporite and clastic minerals. The evaporite minerals include calcite, gypsum, halite, glauberite, and thenardite, whereas clastic minerals are quartz and clay. The calcite abundance decreases from the margin to the central portion of the playa but gypsum and halite abundances show an increasing trend from the margin to the center. This observation is consistent with the general zonation of other playas. Variations of calcite and gypsum concentration profiles present increasing and decreasing trends with depth, which could be ascribed to the changes in climatic factors. These factors include brine chemical modifications owing to changes in evaporation and precipitation rates and variations in relative abundance of anions-cations or in the rate of clastic and evaporite minerals due to variations in the freshwater influx (climatic changes) with time. A decrease in calcite and increase in sulfate minerals (especially gypsum) with depth is probably due to the higher water level and rainfall, a more humid climate, and salinity variations.  相似文献   

12.
Carbonates often accompany lake and lake‐margin deposits in both modern and ancient geological settings. If these carbonates are formed in standing water, their stable isotope values reflect the aquatic chemistry at the time of precipitation and may provide a proxy for determining regional hydrologic conditions. Carbonate rhizoliths and water samples were collected from a playa lake in eastern Nevada. Pilot Valley (～43°N) is a closed‐basin, remnant playa from the Quaternary desiccation of palaeo‐Lake Bonneville. Water is added to the playa margin by free convection of dense brines to the east and forced convection of freshwater off the alluvial fan to the west. Both freshwater and saline springs dot the playa margin at the base of an alluvial fan. Water samples collected from seven springs show a range from ?16 to ?0·2‰ (Vienna Standard Mean Ocean Water), and are consistent with published values. The δ18Ocalcite values from rhizolith samples range from ?18·3 to ?6·7‰ (Vienna Pee Dee Belemnite), and the average is ?12‰ V‐PDB (1 ? σ SD 2‰). With the exception of samples from Little Salt Spring, the range in the δ18Ocalcite values collected from the rhizoliths confirms that they form in equilibrium with ambient water conditions on the playa. The initial geochemical conditions for the spring waters are dictated by local hydrology: freshwater springs emerge in the northern part of the basin to the east of a broad alluvial fan, and more saline springs emerge to the south where the influence of the alluvial fan diminishes. Rhizoliths are only found near the southern saline springs and their δ13Ccalcite values, along with their morphology, indicate that they only form around saltgrass (Distichlis sp.). As the residence time of water on the playa increases, evaporation, temperature change and biological processes alter the aquatic chemistry and initiate calcite precipitation around the plant stems. The range in δ18Ocalcite values from each location reflects environmental controls (e.g. evaporation and temperature change). These rhizoliths faithfully record ambient aquatic conditions during formation (e.g. geochemistry and water depth), but only record a partial annual signal that is constrained by saltgrass growth and the presence of standing water on the playa margin.  相似文献   

13.
The Abu Ruweis Formation is composed of carbonates, evaporites, and mudstones, with some locally developed pelletic, oolitic and stromatolitic limestones. The lateral persistence of bedding, the purity of the evaporite rocks, the alternating arrangement of marine carbonates and evaporites indicates periodic deposition in subaqueous conditions (salina). Petrographic investigations, X-ray diffraction analysis as well as chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum, with rare anhydrite relics. Five microfacies of gypsum were recognized according to their fabrics: porphyroblastic and granoblastic gypsum showing polarization texture, gypsum pseudomorph after anhydrite laths, and satin spar gypsum. The textures they display indicate a hydration origin of precursor anhydrite, which is in turn rehydrated from primary gypsum. Some of these anhydrites were formed as a result of replacement processes of the carbonate sediments associated with the evaporites, as evidenced from the textural relationships of the carbonate and sulfate minerals. The O18 content ranges from 1.45 to 8.38% PDB and the C13 content ranges from −1.52 to 4.73% PDB. Trace elements analysis has shown that the Abu Ruweis dolomites are rich in strontium (up to 600 ppm), and sodium (up to 835 ppm). The isotope composition and trace elements content, as well as the petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the Abu Ruweis dolomites. The evaporites were deposited during a regressive lowstand systems tract, whereas the carbonates were deposited under shallow water marine conditions during a highstand systems tract. The Abu Ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin. Restricted conditions were provided by the development of beach barriers.  相似文献   

14.
Groundwater constitutes the main source of water supply in the High Mekerra watershed of northwestern Algeria. This resource is currently under heavy pressures to meet the growing needs of drinking water and irrigation. This study assesses the geochemical characteristics of groundwater of the High Mekerra watershed at 21 points distributed across the two main aquifers (Ras El Ma and Mouley Slissen) in the region. Hydrochemical facies of Ras El Ma groundwater are dominantly MgCl and CaCl type, while those of Mouley Slissen groundwater are of CaHCO3 type. Principal component analysis shows a strong correlation between groundwater mineralization and Ca2+, Na+, Cl? and SO4 2? ions stemming from the dissolution of carbonates, gypsum and anhydrite. Groundwater mineralization evolves from south to north. Geochemical modeling shows that the High Mekerra groundwater is saturated with respect to calcite and dolomite and undersaturated with respect to gypsum and anhydrite. Nitrate concentrations that exceed the WHO standard (50 mg L?1) at several points are linked to the agro-pastoral activities in this region.  相似文献   

15.
Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L−1 and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and δD suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite [As(III)] was the dominant form of inorganic arsenic at Eh values less than about −170 mV while arsenate [As(V)] was predominant at higher Eh values. Organic arsenic was negligible (<0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg−1 and a maximum concentration range (20 to 40 mg kg−1) was reached as solution concentration increased up to 80 mg L−1, indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As2S3) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake.  相似文献   

16.
The coprecipitation of Sr, Mg, Na and K with anhydrite during the dehydration of gypsum was studied in laboratory experiments. The partition coefficients of Mg and Sr between anhydrite and solution decrease with increasing temperature. The partition coefficients of the alkali-ions do not depend upon temperature, but are affected by the brine composition.The mechanism of the phase transformation gypsum → anhydrite occurs via dissolution and precipitation, when the coprecipitated-ions are repartitioned between the new phase and the solution. The partition coefficients established in this study are applicable also for primary anhydrite.During the dehydration of gypsum at elevated temperatures metastable bassanite may form as an intermediate stage. The amount of cations coprecipitated with bassanite is much larger than the amount coprecipitated with anhydrite or gypsum. This phenomenon may have an influence on the partitioning of cations during the dehydration of gypsum, particularly on Sr.The partition coefficients of seawater cations between anhydrite and the brine are similar to those between gypsum and the brine. For this reason the coprecipitated-ions are not expected to be good indicators to distinguish between primary and secondary calcium sulfate minerals.The temperature effect on the coprecipitation of Mg and Sr with anhydrite makes these ions possible indicators for the temperature at which the phase transformation occurred. This temperature corresponds to the depth of burial of the gypsum at the stage of dehydration.The coprecipitation of seawater cations with anhydrite in the natural environment was studied in two systems: A small Pleistocene evaporite lens from the Sinai Peninsula and the Triassic anhydrite of the Mohilla Formation, Israel. The coprecipitated-ion composition of these samples was used to derive the conditions under which the anhydrite was formed.  相似文献   

17.
An exceptional flood in January 1990 led to the formation of a large ephemeral lake on the Chott el Djerid, a salt playa in southern Tunisia. Repeated observations made during 1990 show that the ephemeral lake underwent four evolutionary stages: (1) initial flooding, (2) evaporative concentration of lake waters, (3) the movement of concentrated brine pools over the playa surface as a result of wind action, and (4) total desiccation of the lake by September 1990. During all four stages the brine chemistry of the lake was monitored. Water inflow into the Chott el Djerid basin was found to have a consistent Ca-SO4-Cl-rich and HCO3-CO3-poor chemistry, reflecting the recycling of homogeneous assemblages of Cretaceous, Mio—Pliocene and Quaternary evaporites within the catchment. As the ephemeral lake shrank, these waters produced an Na-Mg-K-Cl-SO4 brine which was similar to modern sea water. Mineral saturation data show that, during the desiccation of the lake, saturation with respect to both gypsum and halite was achieved and that the most concentrated brines were ultimately saturated with respect to potash phases. After the desiccation of the lake the main mineral phases found on the Chott included gypsum and halite. In addition, ephemeral deposits of carnallite (observed as carnallitite,3KMgCl3·6H20 + NaCl) were found. This assemblage is that which would be expected to form if the waters had undergone salt norm evaporation at 1 bar pressure at 25°C (SNORM) in the evaporation model proposed by Jones and Bodine (1987). The nature of both the brine chemistry and evaporite mineralogy provides a new and rare example of marine-like potash-bearing evaporites being formed in a contemporary continental playa.  相似文献   

18.
The previous study showed that the Zechstein (Upper Permian) anhydrites have about 0.2% strontium with a remarkably small sample scatter. Our study of three lower Zechstein anhydrite units (Lower Anhydrite, Upper Anhydrite and Basal Anhydrite) from West Poland indicate that although often the Sr content is 0.1–0.2%, there are common deviations. In particular, a considerable part (28%) of the studied samples is characterized by lower values (<0.1%), and on the other hand ca. 15% of samples are Sr-enriched, and in those samples celestite was recorded. Particular anhydrite levels differ especially in the frequency of samples showing great Sr content. The greatest variation was found in the Lower Anhydrite. This agrees well with the conclusion derived from the sedimentological studies indicating that there was the greatest differentiation of depositional environments during the deposition of the Lower Anhydrite. The Sr content is a good indicator of brine concentration during the gypsum precipitation and it seems that the subsequent gypsum–anhydrite transformation itself does not affect the strontium distribution. The histograms of Sr content in the Basal Anhydrite indicate a slightly higher brine concentration than it was during the Lower Anhydrite deposition, and the latter in turn was higher than brine concentration during the Upper Anhydrite sedimentation. Celestite veins are clearly diagenetic in origin. The form of celestite occurrence and the increased strontium content (1% or more) indicate an additional source of ions that occurred outside the anhydrite series. In the case of the Lower Anhydrite, the supposed additional source of Sr was related to aragonite-to-calcite transition and squeezing of CaCl2 brines from reefs into anhydrite series due to increased pressure. For the Basal Anhydrite this source could be related to brines derived from the Older Halite deposits.  相似文献   

19.
Groundwater, surface water, and soil in the Goose Lake oil field in northeastern Montana have been affected by Cl−-rich oil-field brines during long-term petroleum production. Ongoing multidisciplinary geochemical and geophysical studies have identified the degree and local extent of interaction between brine and groundwater. Fourteen samples representing groundwater, surface water, and brine were collected for Sr isotope analyses to evaluate the usefulness of 87Sr/86Sr in detecting small amounts of brine. Differences in Sr concentrations and 87Sr/86Sr are optimal at this site for the experiment. Strontium concentrations range from 0.13 to 36.9 mg/L, and corresponding 87Sr/86Sr values range from 0.71097 to 0.70828. The local brine has 168 mg/L Sr and a 87Sr/86Sr value of 0.70802. Mixing relationships are evident in the data set and illustrate the sensitivity of Sr in detecting small amounts of brine in groundwater. The location of data points on a Sr isotope-concentration plot is readily explained by an evaporation-mixing model. The model is supported by the variation in concentrations of most of the other solutes.  相似文献   

20.
Limestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the Winnipegosis Formation on the east‐central margin of the Elk Point Basin where the overlying Prairie Evaporite deposits have been removed. This type of crystalline limestone is interpreted as dedolomite, based on petrographic observations. The δ18O and δ13C values of the Winnipegosis dedolomite vary from ?12·8‰ to ?11·9‰ VPDB (Vienna Pee Dee Belemnite) and from ?0·5‰ to +1·7‰ VPDB, respectively; both values are significantly lower than those for the corresponding dolomite. The 87Sr/86Sr ratios of the dedolomite are significantly higher, between 0·7082 and 0·7087. The spatial distribution and geochemical data of the Winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the Prairie Evaporite anhydrite during the latest Mississippian to the Early Cretaceous when the basin was subjected to uplift and erosion. The Winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite, including the occurrence of dedolomite restricted along fractures and adjacent areas, dolomite patches ‘floating’ in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts. However, the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the Winnipegosis dedolomite. Coarsely to very coarsely crystalline, subhedral to euhedral calcite cement is restricted in the dedolomite. The petrographic features, isotopic compositions and homogenization temperatures, coupled with the burial history of the Winnipegosis Formation, constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during Late Cretaceous to Neogene time. The more negative C‐isotopic signatures of the calcite cement (?5·3‰ to ?2·3‰ VPDB) probably reflect a hydrocarbon‐derived carbon.  相似文献   

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