Using sedimentology to address the marine or continental origin of the Permian Hutchinson Salt Member of Kansas

The Permian Hutchinson Salt Member of the Wellington Formation of the Sumner Group of Kansas (USA) has multiple scientific and industrial uses. Although this member is highly utilized, there has not been a sedimentological study on these rocks in over 50 years, and no study has investigated the full thickness of this member. Past publications have inferred a marine origin as the depositional environment. Here, this marine interpretation is challenged. The goals of this study are to fully document sedimentological and stratigraphic characteristics of the Permian Hutchinson Salt Member in the Atomic Energy Commission Test Hole 2 core from Rice County, Kansas. This study documents colour, mineralogy, sedimentary textures, sedimentary structures, diagenetic features and stratigraphic contacts in core slab and thin sections. The Hutchinson Salt Member is composed of five lithologies: bedded halite, siliciclastic mudstone, displacive halite, bedded gypsum/anhydrite and displacive gypsum/anhydrite. These lithologies formed in shallow surface brines and mudflats that underwent periods of flooding, evapoconcentration and desiccation. Of note are the paucity of carbonates, lack of marine-diagnostic fossils, absence of characteristic marine minerals and lithofacies, and the stratigraphic context of the Hutchinson with associated continental deposits. The Hutchinson Salt Member was most likely deposited in an arid continental setting. This new interpretation offers a refined view of Pangaea during the middle Permian time.     

Depositional environments and facies of the Late Triassic Abu Ruweis Formation, Jordan

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.     

Neogene evaporites in desert volcanic environments: Atacama Desert, northern Chile

The Upper Miocene and Pliocene evaporite deposits of the Atacama Desert of northern Chile (Hilaricos and Soledad Formations) are among the few non‐marine evaporites in which aridity not only formed the deposits, but has also preserved them almost unaltered under near‐surface conditions. These deposits are largely composed of displacive Ca sulphate and halite together with minor amounts of glauberite, thenardite and polyhalite. However, at the base and top of these deposits, there are also beds of gypsum crystal pseudomorphs that originally formed as free‐growth forms within shallow brine bodies, rather than as displacive sediments. The halite is present as interstitial cement, displacive cubes and shallow‐water, bottom‐growth chevron crusts. Most of the calcium sulphate is presently anhydrite, pseudomorphous after gypsum, that was the primary depositional sulphate mineral. The secondary anhydrite formed under early diagenetic conditions after slight burial (some metres) resulting from the effect of strongly evolved pore brines. The anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the Atacama Desert. Both the Hilaricos and the Soledad Formations contain geochemical markers indicating that these Neogene evaporites had a largely non‐marine origin. Bromine content in the halite is very low (few p.p.m.), indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites. Moreover, the δ³⁴S of sulphates (+4·5‰ to +9‰) also reflects a non‐marine origin, with a strong volcanic influence, although some recycling of Mesozoic marine sulphates cannot be ruled out. δ³⁴S of dissolved sulphate from hot springs and streams in the area commonly displays positive values (+2‰ to +10‰). Leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies, very common in the associated drainage areas, have been the main contribution to the accumulation of evaporites. The sedimentary and diagenetic evolution of the Hilaricos and Soledad evaporites (based on lithofacies analysis) provides information about the palaeohydrological conditions in the Central Depression of northern Chile during the Neogene. In addition, the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from Late Miocene until the present. A final phase of tectonism took place permitting the internal drainage to change and open to the sea, resulting in dissolution and removal of a significant portion of these deposits. Despite the extensive dissolution, the remaining evaporites have undergone little late exhumational hydration.     

Origin of the late Cretaceous potash-bearing evaporites in the Vientiane Basin of Laos: δ11B evidence from borates

The evaporites on the Khorat Plateau comprise one of the largest potash deposits in the world, and their origin has long been a controversial problem. Based on boron isotope measurements from borate as a good indicator to distinguish marine and nonmarine evaporites, the borates of potash layers were identified and the results were used to indicate the source of evaporites in the study area as representative of the whole Khorat Plateau. The results show that the main borates include boracite and hilgardite. The boracites occur as crystals and ooids with minor amounts of hilgardite aggregates. The range of δ¹¹B values is from +21.30‰ to +32.94‰ (averaging +29.74‰) and falls in the range of marine borates. The δ¹¹B values in the potash layer are seemingly variable because of the influence of fluvial influx when salts were precipitating. Although the composition of seawater has possibly been modified, the evidence from boron isotopic composition implies that the evaporites in the study area or in the whole Khorat Plateau are marine deposits.     

Evolution of late Triassic rift basin evaporites (Passaic Formation): Newark Basin, Eastern North America

The Passaic Formation of the late Triassic Newark Supergroup is 2700 m thick and was deposited in series of wide, deep to shallow lacustrine environments in the Newark rift basin (eastern North America). The Passaic Formation can be divided into lower, middle, and upper sections based on depositional structures, composition and the distribution and morphology of its evaporites. Evaporites formed as a result of syndiagenetic cementation and/or displacive processes. Evaporitive minerals now include gypsum and anhydrite, although other mineral species, such as glauberite, may have originally existed. Most of the evaporites of the Passaic Formation occur within massive red mudstone and siltstone lithologies in the form of diffuse cements, void-fillings, euhedral crystals, crystal clusters and nodules. These evaporites grew displacively within the fine siliciclastic matrix as a result of changes in the hydrochemical regimes of the rift basin. A well-developed upward increase in the amount of evaporite material is present in the Passaic Formation. This resulted from: (1) long-term, progressive increase in aridity, and (2) significant increase in evaporation surface area of the basin during its tectonic evolution. A nonmarine source for the evaporites is evident from the isotopic data. Sulphate δ³⁴S ranges from 11%. to 3.3%. CDT, while δ¹⁸O ranges from + 15.1%. to + 20.9%. SMOW, indicating derivation from early diagenetic oxidation of organic sulphur and pyrite within the organic-rich, lacustrine deposits. The ⁸⁷Sr/⁸⁶Sr ratios in sulphate are radiogenic (average 0.71211), showing the interaction of basin waters with detrital components and that the Newark Basin was isolated from the world ocean. Most of the original evaporites show evidence of diagenetic change to polycrystalline and polymineralic pseudomorphs now filled with recrystallized coarse-grained anhydrite (1–3 mm size) and low-temperature albite. Homogenization temperatures of fluid inclusions within the coarse-grained anhydrite indicate crystallization temperatures for anhydrite in the range of 150° to 280°C. Such elevated temperatures resulted from circulation of hot water in the basin. Later exhumation of these rocks caused partial to total replacement of anhydrite by gypsum in the upper part of the section. The resulting increase in volume due to hydration of anhydrite at shallow depths also emplaced non-evaporative satin-spar veins (fibrous gypsum) along bedding planes and in fractures. While the local geology of the Newark rift basin controlled the distribution of facies, the sedimentological development of the Passaic Formation evaporites resulted from the world-wide climatic aridity that prevailed during the late Triassic. because the Newark Basin sequence was only covered with about 3 km of sedimentary overburden that correspond to about 100°C and hence suggests that evaporites have experienced alteration by hot fluids. 5 As the Triassic marks the greatest evaporite formation world-wide and profound sense of parched continentality throughout the world existed before the final break-up of the Pangea, the Passaic Formation evaporites are an example of the influence of these palaeoclimatic conditions at the eastern margin of North America.     

The characteristics,formation and exploration progress of the potash deposits on the Khorat Plateau,Thailand and Laos,Southeast Asia

The giant potash deposit on the Khorat Plateau is one of the most promising targets for exploitation of potassium salts. So far, many researches and geologic survey have been conducted on the giant potash deposits. Hence, it is necessary to make an overall review on the potash deposits. The potash deposit on the Khorat Plateau was formed during the Middle to Late Cretaceous, during which seawater was enriched in Ca²⁺ and depleted in SO₄^2- compared with those of modem seawater. In addition to seawater, continental water and hydrothermal fluids could have affected the evaporite basins. The seawater was probably derived from Tethys ocean, and the brine should have evaporated to some extent before entering into the basin systems based on the evidence of absence of carbonates and unproportionate sulphate compared with chloride salts. The paleo-climate during Middle to Late Cretaceous was characterized as high temperature and extremely arid environment, which is favourable for deposition of potassium-magnesium saline minerals. The major saline minerals are of anhydrite, halite, camallite, sylvite and, tachyhydrite, with trace amounts of borates. The resources of the potash deposit on the Khorat Plateau could be approximately as much as 400×10⁹ t of camallite and 7×10⁹ t of sylvite. The evaporite sequences have been deformed and altered by postdepositinal processes, including tectonic movements and chemical alteration. Salt domes were formed in the postdepositional processes. Based on the analyses of geophysical surveys and drilling projects, high-quality sylvinite ores are commonly found at the flanks of those salt domes due to incongruent dissolution of camallite. The future potential prospecting areas for the high-quality sylvinite ores would be on the edges of the Khorat Plateau.     

东濮盆地蒸发岩的成因

东濮盆地第三纪蒸发岩含大量芒硝、钙芒硝等矿物；石盐岩具有低的溴含量，都低于30ppm，部分接近或低于10ppm；硬石膏岩硫同位素的δ34S的范围从+3.31到+37.5,大大超过现代及第三纪海洋硫酸盐的数值。上述资料表明，东濮盆地第三纪蒸发岩可能为非海相成因。     

Diagenetic origin of Basal Anhydrite in the Cretaceous Maha Sarakham salt: Khorat Plateau, NE Thailand

Development of a diagenetic anhydrite bed at the base of the Cretaceous Maha Sarakham Saline Formation (the `Basal Anhydrite' member) of the Khorat Plateau in north-eastern Thailand took place due to leaching and/or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite-dominated evaporite sequence. Basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing, with anhydrite produced as the insoluble residue. The result is a 1·1 m thick interval of nodular anhydrite displaying unique, basin-wide continuity. Observed textures, petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the Basal Anhydrite Member as an accumulation residue from the dissolution of the Maha Sarakham salts. Petrographically, the anhydrite in this unit is made up of crystals that are blocky and recrystallized, sheared, generally elongated and broken, and is bounded at the bottom by organic-rich stylolite surfaces. Authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite. Traces of pyrite, galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial. The δ<sup>34</sup>S of sulphate in the Basal Anhydrite averages 15 ‰ (CDT) and falls within the isotopic composition of the anhydrite in the Cretaceous Maha Sarakham Formation proper and the Cretaceous values of marine evaporites. Measured δ<sup>18</sup>O in dolomite range from ?4·37 to ?14·26‰ (PDB) suggesting a re-equilibration of dolomite with basinal water depleted in <sup>18</sup>O and possible recrystallization of dolomite under relatively elevated temperatures. The δ<sup>13</sup>C, however, varies from +1·57 to ?2·53‰ (PDB) suggesting a contribution of carbon from oxidation of organic matter. This basal anhydrite bed, similar to basinwide beds found at the bottom of many giant evaporite sequences, has always been considered to be depositional. Here, at the base of the Maha Sarakham Formation, we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer: it represents an `upside-down' caprock.     

云南兰坪-思茅盆地和老挝钾盐矿床物质来源新认识

云南兰坪-思茅盆地勐野井钾盐矿与泰国—老挝的钾盐矿的成矿关系被认为可能具有同源性,尤其深部热液可能是其重要的物质来源之一。针对该观点氢氧同位素证据缺乏、且深部热液到底是哪种热液尚不清楚的问题,文中根据老挝钻孔ZK2893中石盐包裹体水的氢氧同位素分析结果:δ~(18)O为-2.3‰~9.5‰,平均值为2.9‰,δD的范围为-78‰~-150‰,平均值为-108.6‰,大部分小于-90‰;在δD-δ~(18)O关系图上,数据点均在交代热液范围内,因此推断,老挝钾盐成矿的深部热液为大气降水与围岩形成的交代热液,围岩提供了重要的成矿物质。此项分析还表明老挝的交代热液温度主要集中在150℃左右,即包裹体形成的温度可能也在150℃左右,云南兰坪—思茅石盐包裹体的捕获温度为145℃左右,最高达170℃,二者比较接近,这为两地钾盐矿同源的可能性提供了新的证据。     

An explanation for the varves of the Castile evaporites (Upper Permian), Texas and New Mexico, USA

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.     

Sedimentology and evaporite genesis in a Holocene continental-sabkha playa basin—Bristol Dry Lake, California

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.     

Lithofacies and cyclicity of Mohilla evaporite basins on the rifted margin of the Levant in the Late Triassic,Makhtesh Ramon,southern Israel

Marine‐connected basins with evaporites occur beneath most extensional continental margins that originated at low‐latitudes and often are of major economic significance. Cyclicity in the evaporite lithofacies reflects the degree of restriction of the basin, overprinted by sea‐level changes, and caused by structural movements in the barrier region, whether by fault‐block rotation, footwall uplift or hanging wall subsidence, in both extensional and compressional basins. The Upper Triassic evaporites of the Ramon section in southern Israel model cyclic sedimentation in such environments. The Mohilla Formation is a carbonate–evaporate–siliciclastic succession of Carnian age that fills a chain of basins extending along the Levant margin from southern Israel to Jordan and Syria. The basins developed in half‐grabens adjacent to normal faults that formed during a period of regional extension. Evaporites of this formation are well‐exposed in outcrops at Makhtesh Ramon, the southernmost of these basins. The M2 Member of the Mohilla Formation is composed of 42 sub‐metre cycles of alternating dolostone, gypsum and calcareous shales. Field and microfacies analysis showed these cycles to conform mostly to restricted shallow and marginal marine environments, spatially limited by the uplifted shoulders of the half‐graben systems. A total of 10 facies types belonging to six depositional environments have been identified. From stacking patterns and analysis of bed to bed change, cycles can be categorized into three groupings: (i) low frequency exposure to exposure cycles that developed under eustatic or climate control; (ii) high frequency deepening/shallowing‐upward cycles, characterized by gradual transitions due to short‐term sea‐level or runoff‐event oscillations possibly referable to orbital forcing; and (iii) high frequency shallowing‐upward cycles, characterized by abrupt transitions, attributable to sporadic tectonic events affecting accommodation space or barrier effectiveness. The way facies and cycling of the sedimentary environments was deciphered in the Mohilla evaporite basin can be used to unravel the genesis of many other evaporite basins with barriers of tectonic origin.     

Palaeogeographic and burial controls on anhydrite genesis: the Badenian basin in the Carpathian Foredeep (southern Poland, western Ukraine)

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.     

Origin of polyhalite deposits in the Zechstein (Upper Permian) Zdrada platform (northern Poland)

Polyhalite deposits in the Zechstein (Upper Permian) of northern Poland occur in the Lower Werra Anhydrite. In the Zdrada Sulphate Platform, the polyhalite appears to be a very early replacement of anhydrite. The replacement was caused by the halite-precipitating brines which contained potassium and magnesium ions. The formation of polyhalite was preceded by the syndepositional anhydritization of the original gypsum deposit which has often preserved its primary textures. This anhydritization on the platform and its slopes was a reaction of the precipitated gypsum in a hydrologically open evaporite basin, with brines of salt basins adjacent to the sulphate platform. These brines, when nearly saturated with respect to halite, and potassium and magnesium rich, reacted with anhydrite to precipitate polyhalite along the slopes of the Zdrada Platform. The oxygen and sulphur isotopic compositions of sulphate evaporites indicate that marine solutions were the only source of sulphate ions supplied to the Zechstein basin, and that anhydrite was transformed to polyhalite by reaction with marine brines more concentrated than those that precipitated precursor calcium sulphate minerals.     

The anatomy of a sulphate platform and adjacent basin system in the Leba sub-basin of the Lower Werra Anhydrite (Zechstein, Upper Permian), northern Poland

The Lower Werra Anhydrite (Zechstein, Upper Permian) deposits of the teba area originated in a deep basin setting, in shallow to deep water conditions. Facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms. This pattern of local platforms and adjacent basins developed during deposition. In basinal areas, the sequence is clearly transgressive, whereas on platforms accumulation kept pace with subsidence after an initial transgression. Nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition. Massive anhydrite with pseudomorphs after upright-growth gypsum crystals suggest rapid precipitation in a subaqueous environment and/or fluctuating, but generally high, salinity conditions. Massive clastic sulphate originated due to periodic high energy events and resedimentation, or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite. Massive, textureless anhydrite is locally porous and passes upward into breccia, indicating a strongly saline environment. Bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water. Bedded anhydrites contain portions which are graded. Intercalations of sulphate turbidites and upright-growth gypsum suggest fluctuating water depths, with comparatively deep water during turbidite deposition, but shallower conditions during upright-growth gypsum deposition. The sequence observed in slope zones at platform-basin margins, detrital (parautochthonous) sulphate sand to graded beds to basinal laminites, indicates that redistribution processes were important. At the onset of the Lower Werra Anhydrite deposition bathymetric relief existed between the central part of the basin and its margins, where carbonate platforms remained subaerially exposed. Formation of local platforms and adjacent basins required a relatively high subsidence rate, as pre-existing relief cannot account for the total accumulated thickness of the Lower Werra Anhydrite deposits. One implication of this is that the main argument against ‘the shallow water - shallow basin’ evaporite basin model, i.e.,a very fast rate of subsidence, may not be valid for the Łeba Lower Werra Anhydrite basin.     

老挝东泰钾盐矿床地球化学及其沉积后变化初步研究

呵叻高原是世界上最大的钾盐沉积矿床分布区之一,研究区位于呵叻高原北部的沙空那空盆地东部边缘,钻探资料揭示该地区钾盐资源相当可观。该含盐建造包含上、中、下3个盐段,每个盐段都由一套蒸发岩_碎屑沉积旋回组成,钾盐层主要赋存于下盐段上部。剖面和地层学分析表明,研究区盐构造类型为低结构低成熟度的盐背斜构造,盐体几何形态为一低结构低成熟度的盐背斜,盐背斜轴部矿层厚度较两翼矿层厚得多。岩盐中微量元素Br、K含量及Br×1 000/Cl系数的变化指示成盐卤水经过了阶段性的浓缩和淡化过程,并且成盐作用越来越弱。ZK04孔较低的Br含量〔w(Br)=18×10〈sup>-6〈/sup>〕表明下膏盐层在成盐过程中受到了非海相流体的影响。中膏盐层基底石盐中高K低Br特征表明,下膏盐层残余高浓度卤水影响了其成盐过程,并且该基底石盐很可能又经历了溶解和重结晶过程。     

Multiepisodic evaporite sedimentation as an indicator of palaeogeographical evolution in foreland basins (South‐eastern Pyrenean basin,Early–Middle Eocene)

Extensive deposition of marine evaporites occurred during the Early–Middle Eocene in the South‐eastern Pyrenean basin (north‐east Spain). This study integrates stratigraphic and geochemical analyses of subsurface data (oil wells, seismic profiles and gravity data) together with field surveys to characterize this sedimentation in the foredeep and adjacent platform. Four major evaporite units were identified. The oldest was the Serrat Evaporites unit, with a platform‐slope‐basin configuration. Thick salina and sabkha sulphates accumulated on the platform, whereas resedimented and gravity‐derived sulphates were deposited on the slope, and salt and sulphates were deposited in the deep basin. In the subsequent unit (Vallfogona evaporites), thin sulphates formed on the platform, whereas very thick siliciclastic turbidites accumulated in the foredeep. However, some clastic gypsum coming from the platform (gypsarenites and gypsum olistoliths) was intercalated in these turbidites. The following unit, the Beuda Gypsum Formation developed in a sulphate platform‐basin configuration, where the topography of the depositional surface had become smooth. The youngest unit, the Besalú Gypsum, formed in a shallow setting. This small unit provides the last evidence of marine influence in a residual basin. Sulphur and oxygen isotope compositions are consistent with a marine origin for all evaporites. However, δ³⁴S and δ¹⁸O values also suggest that, except for the oldest unit (Serrat Evaporites), there was some sulphate recycling from the older into the younger units. The South‐eastern Pyrenean basin constitutes a fine example of a foreland basin that underwent multiepisodic evaporitic sedimentation. In the basin, depositional factors evolved with time under a structural control. Decreasing complexity is observed in the lithofacies, as well as in the depositional models, together with a diminishing thickness of the evaporite units.     

Origin and geochemistry of Miocene marine evaporites associated with red beds: Great Kavir Basin,Central Iran

During the Cenozoic numerous shallow epicontinental evaporite basins formed due to tectonic movements in the Northern Province of the Central Iran Tectonic Zone (the Great Kavir Basin). During the Miocene, due to sea‐level fluctuations, thick sequences of evaporites and carbonates accumulated in these basins that subsequently were overlain by continental red beds. Development of halite evaporites with substantial thickness in this area implies inflow of seawater along the narrow continental rift axis. The early ocean basin development was initiated in Early Eocene time and continued up to the Middle Miocene in the isolated failed rift arms. Competition between marine and non‐marine environments, at the edge of the encroaching sea, produced several sequences of both abrupt and gradual transition from continental wadi sediments to marginal marine evaporites in the studied area. These evaporites show well‐preserved textures indicative of relatively shallow‐brine pools. The high Br content of these evaporites indicates marine‐derived parent brines that were under the sporadic influence of freshening by meteoric water or replenishing seawater. However, the association of hopper and cornet textures denotes stratified brine that filled a relatively large pool and prevented rapid variations in the Br profile. Unstable basin conditions that triggered modification of parent brine chemistry prevailed in this basin and caused variable distribution patterns for different elements in the chloride units. The presence of sylvite and the absence of Mg‐sulphate/chlorides in the paragenetic sequence indicate SO₄−depleted parent brine in the studied sequence. Petrographic examinations along with geochemical analyses on these potash‐bearing halites reveal parental brines which were a mixture of seawater and CaCl₂‐rich brines. The source of CaCl₂‐rich brines is ascribed to the presence of local rift systems in the Great Kavir Basin up to the end of the Early Miocene. Copyright © 2007 John Wiley & Sons, Ltd.     

老挝他曲地区石盐流体包裹体特征、氢氧同位素组成及成盐物质补给方式

对老挝他曲地区钻孔中石盐流体包裹体特征、类型以及氢氧同位素组成进行了研究,用以重塑盆地成盐过程中的古环境,并在此基础上对盆地成盐物质的补给方式进行了探讨。结果显示,钻孔中塔贡组下盐段原生石盐岩主要包括人字形和漏斗形晶体两种类型。其中人字形石盐晶体在下盐段中广泛发育,漏斗形晶体数量较少,但通常与人字形石盐晶体产于同一层位,指示了下盐段沉积时盆地处于一种极浅水环境。石盐原生流体包裹体氢氧同位素组成明显偏离全球大气降水线,集中分布于其右下方,反映盆地成盐过程中处于强烈蒸发条件之下。氢氧同位素组成在纵向上的变化,可能揭示了在盐湖演化至钾盐沉积阶段大气温度具有升高趋势。白垩纪中期全球海平面达到极大值,而在呵叻盆地内部,持续的坳陷和强烈的蒸发作用将导致湖平面快速下降。这样,外海与盐盆之间则会产生水位差,由此产生的水力梯度将为外海海水通过障壁向盆地方向进行渗透提供潜在的动力。此外,早白垩世呵叻高原广泛发育的沙漠沉积,则为外海海水大规模的侧向渗透进入盐盆提供了可能的通道。因此,结合现有的资料,本文提出,除海侵补给外,外海通过障壁侧向渗透补给对老挝钾盐盆地成盐物质的供给也具有重要作用。     

Mineralogy,diagenesis and hydrochemical evolution in a probertite–glauberite–halite saline lake (Miocene,Emet Basin,Turkey)

The Emet basin is one of the Neogene basins in western Turkey containing significant amounts of borate minerals, mainly colemanite. The petrologic study of core samples from two exploratory wells in the Do?anlar sector, under optic and electron microscopy, reveals a complex mineral association in which probertite, glauberite, and halite constitute the major primary phases (without mineral precursors) precipitated in a saline lake placed in a volcano-sedimentary context. Other sulfates (anhydrite, gypsum, thenardite, celestite and kalistrontite), borates (colemanite, ulexite, hydroboracite, tunellite, kaliborite and aristarainite), and sulfides (arsenopyrite, realgar and orpiment) are attributed to early diagenesis. So far, the Do?anlar deposit is the most important deposit of probertite known up to now.Chemical changes in the groundwater inflow led to the precipitation of Ca-bearing borates (colemanite) in the tuff-flat environment surrounding the lake, while Na–Ca sulfates and borates (glauberite and probertite) precipitated in the center of the lake. Fluid inclusion compositions in halite indicate that the advanced brines correspond to the Na-K-Cl-SO₄ type. During restricted stages of the saline lake, the residual brines seeped through the tuff-flat sediments, leading to transformations of previous precipitates that resulted in the formation of K-bearing minerals.The abundance of coccoid-like biogenic dolomite, colloidal arsenopyrite and the isotopic composition of sulfates are indicative of bacterial sulfate reduction. In contrast, arsenic sulfides are attributed to acidophilic micro-organisms in oxidizing conditions. Fluctuations of redox conditions in both free and interstitial brines control the biological influence in some diagenetic transformations.