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.     

Meganodular anhydritization: a new mechanism of gypsum to anhydrite conversion (Palaeogene–Neogene,Ebro Basin,North‐east Spain)

A number of Palaeogene to Early Neogene gypsum units are located along the southern margins of the Ebro Basin (North‐east Spain). These marginal units, of Eocene to Lower Miocene age, formed and accumulated deposits of Ca sulphates (gypsum and anhydrite) in small, shallow saline lakes of low ionic concentration. The lakes were fed mainly by ground water from deep regional aquifers whose recharge areas were located in the mountain chains bounding the basin, and these aquifers recycled and delivered Ca sulphate and Na chloride from Mesozoic evaporites (Triassic and Lower Jurassic). In outcrop, the marginal sulphate units are largely secondary gypsum after anhydrite and exhibit meganodules (from 0·5 to >5 m across) and large irregular masses. In the sub‐surface these meganodules and masses are mostly made of anhydrite, which replaced the original primary gypsum. The isotopic composition (11·1 to 17·4‰ for δ¹⁸O_VSMOW; 10·7 to 15·3‰ for δ³⁴S_VCDT) of secondary gypsum in this meganodular facies indicates that the precursor anhydrite derived from in situ replacement of an initial primary gypsum. As a result of ascending circulation of deep regional fluid flows through the gypsum units near the basin margins, the gypsum was partly altered to anhydrite within burial conditions from shallow to moderate depths (from some metres to a few hundred metres?). At such depths, the temperatures and solute contents of these regional flows exceeded those of the ground water today. These palaeoflows became anhydritizing solutions and partly altered the subsiding gypsum units before they became totally transformed by deep burial anhydritization. The characteristics of the meganodular anhydritization (for example, size and geometry of the meganodules and irregular masses, spatial arrangement, relations with the associated lithologies and the depositional cycles, presence of an enterolithic vein complex and palaeogeographic distribution) are compared with those of the anhydritization generated both in a sabkha setting or under deep burial conditions, and a number of fundamental differences are highlighted.     

Sedimentology of the Cretaceous Maha Sarakham evaporites in the Khorat Plateau of northeastern Thailand

Evaporites of the Cretaceous to early Tertiary Maha Sarakham Formation on the Khorat Plateau of southeast Asia (Thailand and Laos) are composed of three depositional members that each include evaporitic successions, each overlain by non-marine clastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins are presently separated by the northwest-trending Phu Phan anticline. The thickness of the formation averages 250 m but is up to 1.1 km thick in some areas. In both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation. The stratigraphy, lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition. Evaporites include thick successions of halite, anhydrite and a considerable accumulation of potassic minerals (sylvite and carnallite) but contain some tachyhydrite, and minor amounts of borates. During the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline-pan environment. Many halite beds, however, contain a curious `sieve-like' fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow-water gypsum. The δ³⁴S isotopic values obtained from different types of anhydrite interbedded with halite range from 14.3‰ to 17.0‰ (CDT), suggesting a marine origin for this sulphate. Bromine concentration in the halite of the Lower Member begins around 70 ppm and systematically increases upward to 400 ppm below the potash-rich zone, also suggesting evaporation of largely marine waters. In the Middle Member the initial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromine concentration in the Upper Member exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence of tachyhydrite in association with the potassic salts was probably the result of: (1) the large volumes of halite replacement of gypsum, on a bed by bed basis, releasing calcium back into the restricted waters of the basin; and (2) early hydrothermal input of calcium chloride-rich waters. The borates associated with potash-rich beds likely resulted from erosion and influx of water from surrounding granitic terrains; however, hydrothermal influx is also possible. Interbedded with the evaporites are non-marine red beds that are also evaporative, with displacive anhydrite nodules and beds and considerable amounts of displacive halite. The δ³⁴S isotopic values of this anhydrite have non-marine values, ranging from 6.4‰ to 10.9‰ (CDT). These data indicate that the Khorat and Sakhon Nakhon basins underwent periods of marine influx due to relative world sea-level rise but were sporadically isolated from the world ocean.     

Strontium distribution and celestite occurrence in Zechstein (Upper Permian) anhydrites of West Poland

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.     

Sequence stratigraphy of a carbonate-evaporite succession (Zechstein 1, Hessian Basin, Germany)

The evaporitic Hessian Zechstein Basin is a sub‐basin of the Southern Zechstein Basin, situated at its southern margin. Twelve facies groups were identified in the Zechstein Limestone and Lower Werra Anhydrite in order to better understand the sequence‐stratigraphic evolution of this sub‐basin, which contains economically important potassium salts. Four different paleogeographic depositional areas were recognized based on the regional distribution of facies. Siliciclastic‐carbonate, carbonate, carbonate‐evaporite and evaporite shallowing‐upward successions are developed. These allow the establishment of parasequences and sequences, as well as correlation throughout the Hessian Basin and into the Southern Zechstein Basin. Two depositional sequences are distinguished, Zechstein sequence 1 and Zechstein sequence 2. The former comprises the succession from the Variscan basement up to the lowermost part of the Werra Anhydrite, including the Kupferschiefer as part of the transgressive systems tract. The highstand systems tract is defined by the Zechstein Limestone, in which two parasequences are developed. In large parts of the Hessian Basin, Zechstein sequence 1 is capped by a karstic, subaerial exposure surface, interpreted as recording a type‐1 sequence boundary that formed during a distinct brine level fall. Low‐lying central areas (Central Hessian Sub‐basin, Werra Sub‐basin), however, were not exposed and show a correlative conformity. Topography was minimal at the end of sequence 1. Widely developed perilittoral, sabkha and salina shallowing‐upward successions indicate a renewed rise of brine level (interpreted as a transgressive systems tract), because of inflow of preconcentrated brines from the Southern Zechstein Basin to the north. This marks the initiation of Zechstein sequence 2, which comprises most of the Lower Werra Anhydrite. In the Central Hessian Sub‐basin, situated proximal to the brine inflow and on the ridges within the Hessian Basin, physico‐chemical conditions were well suited for sulphate precipitation to form a thick cyclic succession. It consists of four parasequences that completely filled the increased accommodation space. In contrast, only minor sulphate accumulation occurred in the Werra Sub‐basin, situated further southwards and distal to the inflow. As a result of substantially different sulphate precipitation rates during increased accommodation, water depth in the region became more variable. The Werra Sub‐basin, characterized by very low sedimentation rates, became increasingly deeper through time, trapping dense halite brines and precipitating rock salt deposits (Werra Halite). This ‘self‐organization’ model for an evaporitic basin, in which depositional relief evolves with sedimentation and relief is filled by evaporite thereafter, contradicts earlier interpretations, that call upon the existence of a tectonic depression in the Werra area, which controlled sedimentation from the beginning of the Zechstein.     

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.     

Controls on the evolution and demise of Lower Carboniferous carbonate platforms,northern margin of the Dublin Basin,Ireland

Shallow water platform limestones of the Chadian–Asbian Milverton Group are restricted to the north-eastern part of the Lower Carboniferous (Dinantian) Dublin Basin. Here, they are confined to two granite-cored fault blocks, the Kentstown and Balbriggan Blocks, known to have been active during the late Dinantian. Three areas of platform sedimentation are delimited (the Kentstown, Drogheda and Milverton areas), although in reality they probably formed part of a single carbonate platform. Resedimented submarine breccias and calciturbidites (Fingal Group) composed of shallow water allochems and intraclasts sourced from the platform accumulated, along with terrigenous muds, in the surrounding basinal areas. Sedimentological evidence suggests that the Kentstown and Balbriggan Blocks possessed tilt-block geometries and developed during an episode of basin-wide extensional faulting in late Chadian time. Rotation of the blocks during extension resulted in the erosion of previously deposited sequences in footwall areas and concomitant drowning of distal hangingwall sequences. Antithetic faults on the northern part of the Balbriggan Block aided the preferential subsidence of the Drogheda area and accounts for the anomously thick sequence of late Chadian platform sediments present there. Continued subsidence and/or sea-level rise in the late Chadian–early Arundian resulted in transgression of the Kentstown and Balbriggan Blocks; carbonate ramps developed on the hangingwall dip slopes and transgressed southward with time. Subsequent progradation and aggradation of shallow water sediments throughout the Arundian to Asbian led to the development of carbonate shelves. Several coarse conglomeratic intervals within the contemporaneous basinal sequences of the Fingal Group attest to periodic increases of sediment influx associated with the development of the shelves. Sedimentological processes controlled the development of the carbonate platforms on the hangingwall dip slopes of the Kentstown and Balbriggan Blocks, though periodic increases of sediment flux into the basinal areas may have been triggered by eustatic falls in sea level. In contrast, differential subsidence along the bounding faults of these blocks exerted a strong control on the margins of the late Dinantian shelves, maintaining relatively steep slopes and inhibiting the progradation of the shelves into the adjacent basins. Tectonically induced collapse and retreat of the platform margins occurred in the late Asbian–early Brigantian. Platform sediments are overlain by coarse-grained proximal basinal facies which fine upwards before passing into a thick shale sequence, indicating that by the late Brigantian carbonate production had almost stopped as the platforms were drowned.     

The Messinian Sicilian stratigraphy revisited: new insights for the Messinian salinity crisis

Controversies around the Messinian salinity crisis (MSC) are because of the difficulties in establishing genetic and stratigraphic relationships between its deep and shallow‐water record. Actually, the Sicilian foreland basin shows both shallow and deep‐water Messinian records, thus offering the chance to reconstruct comprehensive MSC scenarios. The Lower Gypsum of Sicily comprises primary and resedimented evaporites separated in space and time by the intra‐Messinian unconformity. A composite unit including halite, resedimented gypsum and Calcare di Base accumulated between 5.6 and 5.55 Ma in the main depocentres; it records the acme of the Messinian Salinity Crisis during a tectonic phase coupled with sea‐level falls at glacials TG14‐TG12. These deposits fully post‐date primary gypsum, which precipitated in shallow‐water wedge‐top and foreland ramp basins between 5.96 and 5.6 Ma. This new stratigraphic framework results in a three‐stage MSC scenario characterized by different primary evaporite associations: selenite in the first and third stages, carbonate, halite and potash salt in the second one associated with hybrid resedimented evaporites.     

Depositional environments of Upper Miocene (Messinian) evaporite deposits of the Sicilian Basin*

In Sicily, Messinian evaporitic sedimentary deposits are developed under a wide variety of hypersaline conditions and in environments ranging from continental margin (subaerial), to basin-margin supratidal, to intertidal, to subtidal and out into the hypersaline basin proper. The actual water depth at the time of deposition is indeterminate; however, relative terms such as ‘wave base’ and ‘photic zone’ are utilized. The inter-fingering relationships of specific evaporitic facies having clear and recognizable physical characteristics are presented. These include sub-aerial deposits of nodular calcium sulphate formed displacively within clastic sediments; gypsiferous rudites, arenites and arenitic marls, all of which are reworked sediments and are mixed in varying degrees with other clastic materials (subaerial, supratidal, and intertidal to deep basinal deposits). Laminated calcium sulphate alternating with very thin carbonate interlaminae and having two different aspects; one being even and continuous and the other of a wavy, irregular appearance (subtidal, intertidal, and supratidal deposits). Nodular calcium sulphate beds, usually associated with wavy, irregular laminated beds (supratidal, sabkha deposits); very coarsely crystalline gypsum beds (selenite), associated with more even, laminated beds (subaqueous, intertidal to subtidal deposits); wavy anastomozing gypsum beds, composed of very fine, often broken crystals (subaqueous, current-swept deposits); halite having hopper and chevron structures (supratidal to intertidal); and halite, potash salts, etc. having continuous laminated structure (subaqueous, possibly basinal). Evidence for diagenetic changes is observed in the calcium sulphate deposits which apparently formed by tectonic stress and also by migrating hypersaline waters. These observations suggest that the common, massive form of alabastrine gypsum (or anhydrite, in the subsurface) may not always be ascribed to original depositional features, to syndiagenesis or to early diagenesis but may be the result of late diagenesis.     

Environmental and sequence stratigraphic implications of anhydrite textures: A case from the Lower Triassic of the Central Persian Gulf

The Lower Triassic Kangan Formation in the Persian Gulf (South Pars Gas Field) and its adjacent areas are composed of carbonate–evaporite sequences. These sediments were deposited in a shallow marine homoclinal ramp. Study of the anhydrite-bearing intervals shows various structures and textures. The anhydrite structures are mainly bedded, massive, chicken-wire and nodular type and the main textures are felted, sparse crystal, needle shape, lath shape, equant and fibrous. Pervasive and poikilotopic cement together with replacement and porphyroblastic gypsum are accounted as the most common diagenetic features in anhydrite. Evaluation of anhydrite occurrences and features support both primary and secondary formations. The nodular to chicken-wire anhydrite formed under synsedimentary sabkha conditions, whereas anhydrite cements occurred during the late stages of diagenesis (shallow burial stage). Massive to bedded anhydrite could have been formed under subaqueous conditions or originated by coalescing and continued growth of anhydrite nodules in the sabkha zone. Anhydrite fabrics impose a significant control on the reservoir quality of the Kangan carbonates at the South Pars Gas Field. Thick massive and bedded anhydrite could have been formed as an intraformational seals and anhydrite cements occluded pore spaces and reduced the poroperm values. The sequence stratigraphic analysis revealed two depositional sequences in the studied intervals, which are composed of TST and HST. Investigation of anhydrite throughout depositional sequences indicates a change in the content and style of anhydrite texture. Anhydrite content (volume) decreases upward through transgressive system tract (sea-level rise) whereas, it enhances during highstand system tract (sea-level fall). Pervasive and poikilotopic anhydrite cements together with replacement by anhydrite are prevalent features during transgressive and early highstand system tract. At the late HST, with a progradational stacking pattern, anhydrite value increases and felted, radial, equant, crystalline and mosaic texture are the most common anhydrite fabrics. Sequence boundaries that indicate maximum sea level fall and exposure of successions are marked by the broad anhydrite deposits with massive to bedded and chicken-wire structures and various textures that located in late HST package. There is an unambiguous relationship between the microfacies associations, the evaporite textures, and the sea-level fluctuations. This relationship could lead to a predictable pattern that can be of use as a general guide for the sequence stratigraphic interpretations in the area.     

靖边潜台西侧奥陶系马五4亚段岩相古地理特征

鄂尔多斯盆地中奥陶统马五₄亚段是靖边气田重要的天然气勘探层段,目前相关的岩相古地理图多以段和亚段为单位、以盆地为尺度,作图精度不能满足油气勘探开发的需要。以靖边潜台西侧为研究区,根据该区的古地貌特征、标志性矿物（硬）石膏的类型及其环境意义,分马五₄3、马五₄2和马五₄1三个小层开展岩相古地理工作。根据沉积背景和地层等厚图将该区中部厚度较大、地形略陡的区域解释为洼地,将洼地周边厚度较小、地形平缓的区域解释为坪。目的层段（硬）石膏有块状、球状结核和晶体等三种类型,块状硬石膏与暗色泥质藻纹层白云岩互层,代表了一种浅水水下蒸发、间或遭海水漫侵的潮上环境;球状硬石膏结核分散于浅黄色泥粉晶白云岩中,为准同生成因,代表了蒸发、偏氧化、变盐度的潮上环境;石膏晶体相对较少,多为柱状,或与球状硬石膏结核混生,代表的沉积环境与球状硬石膏结核基本相同。块状硬石膏主要分布于洼地中,为潮上带硬石膏洼地,根据硬石膏的含量进一步细分为含块状硬石膏白云岩洼地、块状硬石膏质白云岩洼地和白云质块状硬石膏洼地;球状硬石膏结核及石膏晶体主要分布于古地貌的坪中,结合其局限蒸发潮上带的背景,将其命名为球状硬石膏结核白云岩潮坪。马五₄3、马五₄2和马五₄1的岩相古地理格局基本一致,但从下向上,硬石膏洼地范围逐渐收缩,块状硬石膏的含量也逐渐下降,反映了沉积过程中水体逐渐蒸发变浅的过程。     

Depositional processes and basin analysis of Messinian evaporites in Cyprus

Messinian evaporites in Cyprus resulted from the interplay of Mediterranean-wide and eustatic sea-level changes and local tectonics, in an inferred above-subduction zone setting. Distinctive Tortonian-early Messinian pre-evaporitic facies include diatomaceous marls and microbial carbonates, overlain by a variety of gypsum facies and then by lagoonal-lacustrine deposits and local palaeosols. Facies analysis and comparisons allow construction of a simple model, in which evaporites formed in semi-isolated small basins not far below global eustatic sea-level. Coarsely crystalline gypsum formedin situ along the margins of small basins and within shallow-water lagoons (< 10 m deep); this comprised common banded-stacked (i.e. layered) selenite, swallowtail selenite, botryoidal selenite and sugary-bedded selenite. Fine-grained gypsum precipitated widely and was reworked into basinal areas (< 70 m deep) by weak traction currents and low-density turbidity currents. Shallow-water derived selenite was also reworked basinwards by high-density turbidity currents and debris flows. Slumps indicate tectonic instability. More detailed basin analysis can be achieved by study of individual sub-basins. In the Polemi sub-basin in the west, a Lower Unit (up to 60 m thick) comprises basinal gypsum, interbedded with gypsum turbidites and mass flow deposits, with slumps. This is overlain by an extensive mega-rudite (up to 20 m thick) including up to metre-sized clasts of marginal gypsum facies. Above, the Upper Unit (up to 70 m thick) includes shallow-water gypsum (e.g. swallowtails), marl and minor microbial carbonates. The Pissouri sub-basin in the south-west exposes marginal facies of the Upper Unit, including deltaic elastics and palaeosols. The Maroni sub-basin in the south exhibits a basinal lower gypsum unit, with laterally equivalent marginal facies (up to 50 m thick), overlain by an extensive mega-rudite (up to 20 m thick). Finally, the Mesaoria subbasin in the north exposes relatively marginal gypsum facies in an unstable tectonic setting. Formation of the Polemi, Pissouri and Mesaoria gypsum sub-basins relates to coeval extensional faulting and graben development. Evaporites in south Cyprus (Maroni sub-basin) formed in elongate basins between former compressional lineaments created by localized Early Miocene thrusting. In the sub-basins of west, south-west and south Cyprus, large-scale slumping of marginal gypsum facies took place towards depocentres (to form megarudite debris flows), triggered by one or several phases of extensional faulting.     

Geochemical variations within a laminated evaporite deposit: evidence for brine composition during formation of the Permian Castile Formation, Texas and New Mexico, USA

The Upper Permian Castile Formation of the Delaware Basin in northwest Texas and New Mexico consists of up to 600 m of evaporites and is subdivided into units of anhydrite overlain by halite. The Castile Formation has commonly been interpreted as a deep-water, deep-basin deposit in which sediments were laid down in several hundred metres of water or brine. Recent textural observations within anhydrite units, in which the thick-bedded anhydrite horizons have been interpreted as being of shallow-water origin, have challenged this assumption. This geochemical study of the oldest anhydrite unit in the Castile Formation (the Anhydrite 1 Member) attempts to resolve some of the problems regarding brine depth and evolution in the basin. The Anhydrite 1 Member has been subdivided into five major cycles on the basis of the distribution of stratigraphic units of thick-bedded anhydrite.

Stable isotopic analyses of sulphur from anhydrite, and oxygen and carbon from calcite show that the basin waters were chemically homogeneous during precipitation of anhydrite, and do not indicate any significant input of meteoric, continental-derived waters. Throughout the section studied progressive enrichment of ¹⁸O upwards within cored intervals indicates continuous evaporation of the water body. Carbon isotopes appear to indicate fluctuations in organic activity within the cycles. Trace elemental analyses of Fe, Mg, Sr, Mn, Al, Ba, Zn, Pb and Cu from the sulphate fraction of the samples show a very high variability. There is a distinct increase in trace elemental abundances at the tops of cycles which may indicate variations in precipitation kinetics. Analyses of texturally defined cycles show that up-core trends for many of the trace elements correlate with changes in δ¹⁸O, indicating a progressive increase in the influence of evaporation. In addition, cyclical variations in trace elemental composition indicate changes in basin conditions with around a 350-year cyclicity. These changes are independent of δ¹⁸O values. The geochemical data do not provide conclusive proof of water depth during deposition of the Castile Formation. The data are interpreted as reflecting small-scale changes in conditions of deposition, despite the fact that water input remained essentially constant in terms of chemical composition.     

Regional isostatic response to Messinian Salinity Crisis events

The salinity crisis of the Mediterranean during Messinian time was one of the most dramatic episodes of oceanic change of the past 20 or so million years, resulting in the deposition of kilometer thick evaporitic sequences. A large and rapid drawdown of the Mediterranean water level caused erosion and deposition of non-marine sediments in a large ‘Lago Mare’ basin. Both the surface loading by the Lower Messinian evaporites, and the removal of the water load resulted in isostatic/flexural rebound that significantly affected river canyons and topographic slopes. We use flexure models to quantitatively predict possible signatures of these events, and verify these expectations at well-studied margins. The highly irregular shape of the reconstructed basin calls for a three-dimensional model. Near basin margins, plate-bending effects are most pronounced which is why flexure is particularly important for a relatively narrow basin like the Mediterranean. We focus on one specific sea level scenario for the Messinian Salinity Crisis, where most of the evaporite load was deposited during a sea level highstand, followed by a rapid desiccation. Evaporite loading at current sea level is expected to cause subsidence of the deep basins by hundreds of meters and simultaneous uplift of continental parts of the margins. Differential uplift may lead to significant slope angle changes and thus gravity flows. The relative scarcity of Lower Evaporite sequences along the margins may be a result of these phenomena. Normal faulting of Lower Evaporite and older sediments and rocks is expected on the margins. Desiccation enhances erosion of the freshly exposed continental shelf and slope. Subsidence and riverbed sedimentation occurs on the continental margins, and significant uplift towards the basin center. Reverse faulting is predicted at the margins. Finally, regional isostatic uplift following Zanclean flooding is predicted to destabilize margin slope deposits, and to cause marginal uplift, river down-cutting, and normal faulting.     

板块运动对中国近海新生代盆地沉降及充填的控制作用

太平洋板块、印度板块和欧亚板块的演化对中国近海沉积盆地的沉降及充填具有控制作用。根据地幔对流及地壳拉伸特征可将中国近海沉积盆地沉降类型划分为被动、主动和组合热沉降型3种。不同沉降类型分别具有不同的盆地结构,其中被动热沉降型以断陷为主,主动热沉降型以坳陷为主,组合热沉降型则是两种盆地结构的叠加或侧加。中国近海北部板内沉积盆地沉降类型以被动热沉降为主,远离海洋,受海侵影响较小,以陆相沉积体系为主;中部板缘沉积盆地沉降类型为被动侧加主动热沉降,水体整体较浅,坡折及三角洲发育规模小;南部板缘沉积盆地沉降类型也为被动侧加主动热沉降,水体整体较深,坡折及三角洲发育规模大。     

Uppermost Cretaceous–Lower Tertiary Ulukışla Basin,south-central Turkey: sedimentary evolution of part of a unified basin complex within an evolving Neotethyan suture zone

The Ulukışla Basin, the southerly and best exposed of the Lower Tertiary Central Anatolian Basins, sheds light on one of the outstanding problems of the tectonic assembly of suture zones: how large deep-water basins can form within a zone of regional plate convergence. The oldest Ulukışla Basin sediments, of Maastrichtian age, transgressively overlie mélange and ophiolitic rocks that were emplaced southwards onto the Tauride microcontinent during the latest Cretaceous time. The Niğde-Kirşehir Massif forming the northern basin margin probably represents another rifted continental fragment that was surrounded by oceanic crust during Mesozoic time. The stratigraphic succession of the Ulukışla Basin begins with the deposition of shallow-marine carbonates of Maastrichtian–Early Palaeocene age, then passes upwards into slope-facies carbonates, with localised sedimentary breccias and channelised units, followed by deep-water clastic turbidites of Middle Palaeocene–Early Eocene age. This was followed by the extrusion of c. 2000 m of basic volcanic rocks during Early to Mid Eocene time. After volcanism ended, coral-bearing neritic carbonates and nummulitic shelf sediments accumulated along the northern and southern margins of the basin, respectively. Deposition of the Ulukışla Basin ended with gypsum deposits including turbidites, debris flows, and sabkhas, followed by a regional Oligocene unconformity.The Ulukışla Basin is interpreted as the result of extension (or transtension) coupled with subsidence and basic volcanism. After post-volcanic subsidence, the basin was terminated by regional convergence, culminating in thrusting and folding in Late Eocene time. Comparisons of the Ulukışla Basin with the adjacent central Anatolian basins (e.g. Tuzgölü, Sivas and Şarkişla) support the view that these basins formed parts of a regional transtensional (to extensional) basin system. In our preferred hypothesis, the Ulukışla Basin developed during an intermediate stage of continental collision, after steady-state subduction of oceanic crust had more or less ended (“soft collision”), but before the opposing Tauride and Eurasian continental units forcefully collided (“hard collision”). Late Eocene forceful collision terminated the basinal evolution and initiated uplift of the Taurus Mountains.     

A depositional model for the terminal Neoproterozoic–Early Cambrian Ara Group evaporites in south Oman

Abstract Six evaporite–carbonate sequences are recognized in the terminal Neoproterozoic–Early Cambrian Ara Group in the subsurface of Oman. Individual sequences consist of a lower, evaporitic part that formed mainly during a lowstand systems tract. Overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts. Detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment, source of brines and basin evolution. At the beginning of the evaporative phase (prograding succession), a shallow-water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas. Minor amounts of highly soluble salts locally record the last stage of basin desiccation. This gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow-water halite from shallow-water sulphate, and shallow-water sulphate from relatively deeper water carbonate respectively. These surfaces record repeated flooding of the basin, dissolution of evaporites and stepwise reduction in salinity. Final flooding led to submergence of the basin and the establishment of an open-water carbonate ramp. Marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines. The Ara Group sequences represent a time of relatively stable arid climate in a tectonically active basin. Strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres, while tectonic barriers simultaneously provided the required restricted conditions. Subsidence allowed evaporites to blanket basinal and platform areas. The study suggests a deep-basin/shallow-water model for the evaporites.     

Evaporative systems and diagenetic patterns in the Calatayud Basin (Miocene,central Spain)

This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (>200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession.     

坳陷湖盆浅水三角洲的沉积特征--以松辽盆地南部姚一段为例

坳陷湖盆浅水三角洲的形成古地形背景、水动力学特征、平面形态、微相类型及三角洲内部结构等多个方面与正常三角洲有着极大的差异。姚一段沉积时期,松辽盆地古地形平坦并不具备明显的坡折带,水体浅、面积小并且湖泊能量较弱,湖岸线不稳定并常常发生大范围的迁移,为典型的浅水沉积湖盆。浅水湖盆的沉积充填主要以浅水三角洲和滨浅湖沉积为主,深湖、半深湖相对不发育,浅水三角洲砂体是湖泊砂体的主要类型。根据供源水系的不同,松南的浅水三角洲可以分为浅水型扇三角洲、浅水型辫状河三角洲和浅水曲流河三角洲等。浅水型三角洲主要有以下特点:1.具高建设性河控三角洲的沉积特点,平面上呈鸟足状,前缘相带延伸较远,形成了大规模的湖盆中心砂体;2.浅水三角洲前缘主要以分流河道为主,河口坝极不发育;3.前缘分流河道呈网状展布,河道遭受不同程度的席状砂化,依次划分为弱席状化、中等席状化、强席状化等3种类型;4.不具备典型三角洲的三层式沉积结构及向上变粗的反序列特征。松南姚一段沉积时期,总体表现为基准面上升的湖侵过程,I、II砂组低位和湖侵域的砂体为成藏提供了的有利储集空间,III砂组的湖侵为区域性成藏提供了良好的盖层。     

Deep-water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt

The Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault-bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north-western Red Sea has cored, below a massive salt unit, an anhydrite-bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca-sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine-grained laminated sediments to coarse-grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken-wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom-rich oozes formed in marine, shallow-water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca-sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope-foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine-grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.