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.     

鄂尔多斯盆地西缘中奥陶统地球化学特征及古环境意义

鄂尔多斯盆地西缘位于中国东西部构造接合区,其奥陶纪末期构造体制的改变引起了广泛关注。选取老石旦剖面中奥陶统碳酸盐岩作为研究对象,通过地球化学分析,恢复其古环境特征并探索盆地西缘中奥陶世构造体制改变在沉积学与地球化学方面的响应。结果显示:(1)三道坎组和桌子山组碳酸盐岩δ¹⁸O为-5.2‰~-8.9‰,平均值为-7.0‰;δ¹³C为1.3‰~-1.8‰,平均值为-0.1‰。δ¹⁸O自下而上不断增加,并在顶部出现小幅降低。δ¹³C与δ¹⁸O变化趋势相似。(2)古环境特征表现为从三道坎组到桌子山组,水温经历了初期震荡,从32.7 ℃缓慢降至15.7 ℃。古盐度Z值呈不断增加的趋势,反映出该沉积期水体加深、盐度不断升高。(3)主量、微量元素指标反映出研究区中奥陶世沉积水体不断加深,从三道坎组到桌子山组陆源碎屑物质含量逐渐降低,指示沉积环境由局限台地向开阔台地转变。结合研究区中奥陶统构造背景,认为中奥陶世鄂尔多斯盆地西缘已经处于前陆盆地的构造体制下,逐渐受到阿拉善地块和华北板块碰撞造山影响,盆地呈现西北高、东南低的古地貌特征,从而导致蒸发浓缩的富钾卤水向东南部汇聚;因此,陕北盐盆东南部次级凹陷是海相钾盐矿床的重点勘探靶区。     

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.     

青藏高原那曲河流域降水及河流水体中氧稳定同位素研究

根据青藏高原中部那曲河流域1998年夏季测得的上下游中稳定同位素的日变化,并与同期观测的流域降水中稳定同位素比较,分析了河水中δ18O的变化特征,初步研究了该流域的稳定同位素水文循环过程.河水中δ18O的变化幅度远小于降水,它是降水中δ18O、降水量以及地表蒸发过程共同作用的结果.研究发现湖水对于稳定同位素变化起着显著的调节作用.河水中δ18O与流域降水中δ18O的差异可能反映了该流域强烈的地表和湖面蒸发作用.     

湖北江陵凹陷古新统沙市组四段硫酸盐硫同位素组成及其地质意义

江汉盆地江陵凹陷古新统沙市组四段硬石膏和钙芒硝中硫同位素组成为25.2‰~32.6‰,远远高于同时期海水硫同位素(20‰)。结合研究区古新统沙四段蒸发岩⁸⁷Sr/⁸⁶Sr研究,借助Mg、Ca、Sr、Ba等元素及其Mg/Ca、Sr/Ba,认为江陵凹陷沙四段时期湖盆为封闭的陆相盐湖。根据典型暖相盐类矿物(原生钙芒硝)的广泛发育和指示气候干旱程度的Fe₂O₃/FeO特征,可以推断研究区在沙市组四段沉积晚期的古气候属于暖旱型气候。在温暖干旱的气候环境下,硫酸盐沉积物供给率较低,因此当时的沉积环境对硫酸盐是封闭的。通过Fe、Mn、Al、 Mg、Ca、Ni、V 、Co、Cu等主微量元素以及Fe/Mn、(Fe+Al)/(Ca+Mg)、w(V+Ni+Mn)、V/(V + Ni)、Ni /Co、Cu/Zn的研究表明,研究区沙市组四段环境为湖盆浪基面之下的缺氧还原环境,为常年性较深水分层湖泊。上述缺氧沉积环境下存在大量的厌氧细菌,表生（≤50 ℃）条件下,厌氧细菌使硫酸盐SO^2-₄还原成H₂S,后者与金属离子结合形成硫化物或直接从体系中逸出,再加上当时的内陆环境对硫酸盐是相对封闭的,这种硫循环过程是造成研究区硬石膏和钙芒硝硫同位素值较高的原因,也是研究区最重要的硫同位素分馏过程。     

海相钾盐矿床溶滤卤水找钾指标体系: 以川东北天星桥构造寒武系深部地下卤水为例

长期工作成果显示我国现阶段常用的找钾指标Br×103/Cl值偏低.创新性地应用"以古验古"的溶滤实验与地质统计法厘清了海相蒸发盐盆地找钾指标体系,充分考虑了不同地质年代海水成分的变化,也可克服"将今论古"应用于现代海水在等温等压条件下实验数据的不足.通过对世界上典型钾盐矿床的石盐、含钾石盐及钾盐(含光卤石)进行溶滤实验...     

四川盆地宣汉渡口和重庆北碚下三叠统海相碳酸盐碳同位素组成与演化

早三叠世海水的碳同位素组成与演化是二叠纪-三叠纪生物群体灭绝事件之后全球生态萧条与重建时期地球碳循环的重要记录,为地学界长期高度关注。文中测试了四川盆地宣汉渡口和重庆北碚下三叠统及相邻地层258个海相碳酸盐样品的碳、氧同位素组成,以便探讨早三叠世海水的碳同位素组成与演化。这些样品对海水碳同位素组成具有不同的保存性,飞仙关组及嘉陵江组第一、三段保存较好,而嘉陵江组第二、四段保存较差。基于氧同位素可以更为敏感地反映海相碳酸盐成岩蚀变性的原理,在碳、氧同位素相关性分析的基础上,以δ¹⁸O<-7.5‰为剔除标准,筛选出近200个有效数据建立了碳同位素演化曲线,该曲线与前人在贵州建立的同期曲线具有很好的可对比性。曲线显示出海水碳同位素组成的巨大波动,变化幅度达8.55‰,包括2个完整的上升-下降旋回,第1个旋回由飞仙关组和嘉陵江组第一段组成,第2个旋回由嘉陵江组第二段、第三段和部分第四段组成,同时还包括若干短周期的次级旋回。利用曲线形态与前人建立的同期曲线进行了岩石地层与年代地层单位的对比,飞仙关组可大致与Induan阶对应,嘉一段可大致与Smithian亚阶对应,嘉二段、嘉三段和嘉四段下部可大致Spathian亚阶对应;绿豆岩作为下-中三叠统界线其位置可能偏高,下-中三叠统界线应该位于绿豆岩之下的嘉四段内部。早三叠世海水碳同位素组成高频率大幅度的波动指示了生物大灭绝后环境的不稳定性,其中菌藻类等微生物灾后过度繁盛,可能导致了有机碳在短时间内的快速埋藏和海水碳同位素的正漂移;而碳同位素大规模的负漂移可能与甲烷水合物释放带来的轻碳有关。研究认为,白云岩所在的地层段往往具有更高的δ¹³C值,这可能说明微生物活动、白云岩形成和海水δ¹³C值升高之间的有机联系。下三叠统的白云岩、尤其是嘉二段和嘉四段的白云岩可能是微生物成因的,嘉陵江组有机碳的生产和埋藏可能主要是与白云岩形成有关的微生物的贡献,具最高δ¹³C值的嘉二段的烃类在一定程度上可能具有自生自储的特点。     

Lithologic transition and bed thickness periodicities in turbidite successions of the Antola Formation, Northern Apennines, Italy

The Antola Formation of Upper Cretaceous age crops out extensively in the Northern Apennines and consists of graded units of calcareous sandstones, sandstones, marlstones, and shales. It can be subdivided into the Cerreto, Antola Marlstone, Bruggi, and S. Donato Members on the basis of bed thicknesses and percentage of shales. Although the whole formation is interpreted as a deep-sea basin plain deposit, the members constitute lateral facies subdivisions which range from proximal, thick-bedded turbidities that show a prevalence of thinning upward cycles in bed thicknesses to distal turbidites that show predominantly thickening upward cycles and have a high percentage of shale. Repetitive patterns in the lithological sequence of the turbidite association are generally distinctive and are satisfactorily described as first order Markov chains. Only the Antola Marlstone Member has an additional second order Markov property. Imaginary eigenvalues of the transition probability matrices of all but the Bruggi Member demonstrate a strong cyclic character in the lithologic ordering within the formation. The behaviour of the Antola Marlstone and of the Bruggi may reflect the influence of a secondary ophiolitic intra-basinal source of clastics that contributed sandy turbidites and olistostromes. Systematic long-term variations in the sequence of bed thickness development in some sections of the Antola Formation are often subtle and equivocal, and pose special problems in interpretation. Fourier analysis was applied to the task of partitioning fundamental wavelengths from “background noise” introduced by essentially random depositional processes. In all members there is (1) strong short-term wavelength of two to three beds indicative of alternating thin and thick beds and judged to be typical of turbidite sequences; (2) an intermediate wavelength ranging from about five beds (proximal facies), eight beds (distal) to nine beds (very distal), which have both thinning and thickening upward trends, interpreted respectively as valley fill due to shifting talwegs of low density turbidity currents, and to progradational, flat turbidite lobes; (3) a poorly defined long-term wave-length of from thirty to greater than sixty beds that may be related to an unspecified trend in the evolution of the sedimentary basin. Phase angles associated with the coniputed power spectra give indications as to the asymmetry (thickening or thinning upward) or symmetry of the representative units.     

Stable isotopic signature of a palaeoaquifer, Mississippian Alamogordo Member limestones, Sacramento Mountains, New Mexico, USA

Nine stratigraphic sections, each ≈5 m thick, were sampled from the Alamogordo Member limestones of the Lake Valley Formation, Sacramento Mountains, New Mexico, USA. Four stratigraphic sections consist entirely of lime mudstone and wackestone, whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base. Stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in δ¹⁸O and constant δ¹³C values, whereas those in the sections solely composed of lime mudstone and wackestone have, in general, relatively uniform δ¹⁸O and δ¹³C values. The diagenesis of the Alamogordo Member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre-Pennsylvanian exposure surface ≈100 m above this unit. However, the uniform δ¹³C and downward decrease in δ¹⁸O values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the Alamogordo Member, rather than descended from the overlying exposure surface. This indicates that the basal packstones were probably a conduit for meteoric water. This is further supported indirectly by the relatively uniform δ¹⁸O and δ¹³C values of the lime mud in the sections without basal packstones. The implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers, flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer.     

Ilmenite, magnetite, and peraluminous Mesoproterozoic anorogenic granites of Laurentia and Baltica

Emplacement of 1.6 to 1.3 Ga Mesoproterozoic plutons in Baltica and Laurentia formed an immense belt of A-type granite batholiths that include (1) low-fO₂, ilmenite-series granite intrusions from the Baltic region to Wyoming, (2) high-fO₂, magnetite-series granite intrusions of the central to southwestern U.S., and (3) peraluminous, two-mica granite intrusions from Colorado to central Arizona. These mineralogic divisions are mirrored by substantial elemental and oxygen isotopic differences. The ilmenite-series granites, which often contain classic rapakivi textures, have the highest Fe/Mg ratios and are highest in LIL element enrichment. They also have the lowest whole-rock δ¹⁸O values at 5.7‰ to 7.7‰. The magnetite-series granites are less potassic, less LILE-enriched, and have higher whole-rock δ¹⁸O values, ranging from 7.6‰ to 10.8‰. Although they retain A-type characteristics, the peraluminous granites are the least LILE-enriched and have the lowest Fe/Mg ratios. They also have the highest whole-rock δ¹⁸O values ranging from 8.8‰ to 12.0‰. Feldspar, where strongly reddened, can exhibit elevated δ¹⁸O values, which is interpreted to indicate subsolidus exchange with surface-derived aqueous fluids. Quartz δ¹⁸O values are interpreted to generally retain their magmatic values. The transcontinental mineralogic, chemical, and oxygen isotopic variations are interpreted as indicative of broad changes in the composition of a lower crustal source, which is compatible with a reduced mantle-derived crustal source for the ilmenite-series granites and a more oxidized crustal source for the others, including a metasedimentary component in the source for the two-mica granite subprovince.

Widespread thermal metamorphism at 1.4 Ga is present throughout much of the magmatic province and is viewed as a consequence of this immense event. Compressional deformation associated with several western 1.4 Ga Laurentia granite batholiths, alternatively interpreted as the distal expressions of a presumed 1.4 Ga orogeny, have at least in part been shown to be localized on preexisting Paleoproterozoic zones of deformation. Thus, we do not find compelling evidence for a 1.4 Ga orogeny related to the formation of most of these granites. Renewed intrusions at 1.0–1.1 Ga between and immediately following phases of the Grenville orogeny indicate that situations leading to their formation need to be more broadly considered.

The origin of this red granite-forming event in Laurentia and Baltica is considered as part of a global magmatic event that was coeval with intrusion of massif anorthosites and associated charnockites. Most are viewed as anorogenic, but it is recognized that the same conditions leading to their formation may have occurred during extensional phases of orogens. The immense volumes of red granites produced are also essentially unique to the Mesoproterozoic and appear to be tied to the stabilization and eventual break up of supercontinents of both Paleoproterozoic and Mesoproterozoic age.     

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.     

Overview on the middle to upper Jurassic sedimentary succession of Gangta Bet in the Kachchh basin,western India,with special emphasis on its lithostratigraphy,biostratigraphy, and palaeoenvironment

The Jurassic succession of Gangta Bet in the Kachchh basin of western India comprises around 130 m of mostly siliciclastic rocks. The strata belong to the Gangta Member of the Gadhada Formation and are herein sub-divided into four units: the Gangta sandstone beds, the lower silty sandstone beds, the upper silty sandstone beds, and the Gangta ammonite beds. These units can be separated by three marker horizons: the Brachiopod bed, the Gangta Conglomerate Bed, and the Gervillella Bed. Ammonites indicate an Oxfordian age for the upper half of the succession, but the scarcity of identifiable fossils in its basal part so far prevented precise biostratigraphic assignments. The shallow-water sediments can be interpreted as parasequences as a result of minor sea-level changes. Deposition took place close to the palaeo-coastline at water depths around the fair-weather wave-base.     

山西高阳煤矿奥陶系峰峰组含水层赋存特征

高阳煤矿主采太原组9-10-11#煤层,矿区内断层、陷落柱发育,奥陶系灰岩溶裂隙水是威胁矿井安全开采的主要水源。据井下突水资料及地面水文地质勘探成果,总结出奥陶系峰峰组岩溶裂隙含水层的赋存特征:①从垂向上看,一段以泥灰岩、石膏、硬石膏为主,为相对隔水层,二段由青灰色及黑色厚层状泥晶、粉晶灰岩组成,钻探可见岩溶裂隙,富水性中等;②从水平方向上看,西部埋藏较浅区富水性明显强于中、东部深埋区,且在向斜轴部出现滞流区;③水位西高东低,相差近300m;④顶部存在隔水层段,隔水层厚度一般3.09~34.01m;⑤水位和水化学特征对比发现,峰峰组与马家沟组含水层没有水力联系;⑥地下水总体自西向东,自北向南径流,侧向径流排出井田外。该研究为煤矿底板水的防治提供了依据。     

塔里木盆地塔河地区中下奥陶统鹰山组碳酸盐岩层系内幕不整合识别特征*

为论证T⁶₇界面性质并探索碳酸盐岩层系不整合的识别方法,文章综合利用柯坪—巴楚地区露头和塔河地区钻井、测井和地震资料,研究了塔里木盆地塔河地区中下奥陶统鹰山组内幕不整合面T⁶₇。在露头上, T⁶₇界面表现为岩相转换面,但在不同剖面的暴露强度存在差异。在地化数据上, T⁶₇不整合面之下碳酸盐岩地层碳氧同位素值出现明显偏负;Sc,Co,Ni,Ba,U,Sr等微量元素表现出同样的特点,而Li和V在T⁶₇界面之下富集。在自然伽马测井与自然伽马能谱测井曲线上, T⁶₇不整合面之下的鹰山组下段自然伽马测井曲线明显较界面之上的鹰山组上段齿化剧烈,经过滤波处理的Th/U曲线在T⁶₇界面处呈现出明显的低值。在地震剖面上,难以从地震同相轴反射终止现象中识别出T⁶₇界面,但“串珠”状反射沿T⁶₇不整合面发育,这些串珠状反射被解释为与喀斯特作用相关的岩溶洞穴。T⁶₇界面属于海平面相对下降形成的短暂暴露面(<1,Ma),其主要受控于海平面升降和古地貌,在塔河不同区域的暴露、剥蚀强度不同,强暴露区主要位于中东部,弱暴露区域主要位于西北部。T⁶₇不整合面对塔河地区鹰山组内幕碳酸盐岩储集层发育具有重要意义,既可为层间岩溶的发育提供条件,亦可与断裂构成连通体系为大气淡水提供通道,从而对塔河地区深部碳酸盐岩储集层起到积极的改造作用。     

塔里木盆地中央隆起区上寒武统—下奥陶统白云岩地球化学特征及白云石化流体演化规律*

以详细的岩石学研究为基础,综合利用碳、氧、锶同位素等地球化学资料,深入分析了塔里木盆地中央隆起区上寒武统—下奥陶统白云石化流体演化规律以及白云岩成因机制。结果表明,上寒武统白云岩主要由泥晶—粉晶白云岩、微生物白云岩和(残余)颗粒白云岩等原始结构保留较好的白云岩构成,其C、Sr同位素与同期海水相近,O同位素值偏正,属于同生/准同生期与轻微蒸发海水有关的白云石化的产物;下奥陶统白云岩以细晶自形—半自形白云石为主,原始结构保留差,其C、Sr同位素与同期海水近似,但O同位素值略微偏负,主要为浅埋藏期白云石化的产物。部分早期白云岩在中—深埋藏过程中受埋藏重结晶和构造—热液白云石化的影响,形成细晶—粗晶他形白云岩和缝洞鞍形白云石充填物,该阶段白云石化流体主要来自于地层内封存的海源流体、深部热液以及蒸发岩层间热卤水,多期多源流体的共同作用导致该类白云岩具有较宽的Sr同位素组成和明显负偏的O同位素值。总体上,研究区白云岩具有早期形成(近地表到浅埋藏期大规模交代)、中期加强(中—深埋藏期部分重结晶)、晚期改造(热液局部调整)的整体演化趋势。     

黑河流域中上游地区降水δ18O变化特征

水资源短缺和合理利用是黑河流域面临的一个严峻问题,解决问题的关键就是要深入了解水循环过程.降水作为水循环中一个重要环节,分析其环境同位素变化特征是应用同位素示踪技术研究水循环过程所必须的前提.根据黑河流域中上游地区取得的降水水样和降水气象资料,分析了该区域降水中δ¹⁸O的变化特征.结果表明:降水δ¹⁸O受降水量、季节、高度和温度等众多因素综合影响,其中温度占主导地位;由于位于干旱内陆地区,降水中δ¹⁸O的季节变化幅度较大,可达20‰以上;降水δ¹⁸O与降水前后平均气温的相关性显著,分别高于δ¹⁸O与降水前气温或降水后气温的相关性.从空间上来看,山区或上游地区降水δ¹⁸O与气温之间的相关性显著,分别高于δ¹⁸O与山前或山前盆地的相关性.     

热分层效应控制的水库水体氢氧同位素特征

为掌握水库热分层与氢氧同位素空间分布的关系,研究了广西兴安县五里峡水库夏季氢氧同位素空间分布特征及影响因素。研究表明:①五里峡水库夏季出现明显的热分层,库表层至-5 m为表水层,-5 m至-20 m为温跃层,-20 m以下为底温层;②五里峡水库δ18O、δD值沿大气降水线分布,但均落在桂林市大气降水线右下方,其线性方程为δD=4.66δ18O-10.85（R2=0.76）,其斜率与全球大气降雨线（GMWL）和桂林市降雨线（CLMWL）的斜率和截距差异明显,表明五里峡水库主要由降水补给,但在补给五里峡水库前已经过了强烈的蒸发作用和水岩作用;③ δ18O、δD随水深的增加逐渐偏负,具有表水层 > 入库水体 > 温跃层 > 底温层 > 出库水体（>表示偏正）的垂向分布特征。分析认为入库水体氢氧同位素的季节变化和蒸发作用随深度增加而降低是五里峡水库夏季分层期间水体氢氧同位素垂向变化的主要影响因素。     

准噶尔盆地玛湖凹陷下二叠统风城组含碱层段韵律特征及成因*

准噶尔盆地玛湖凹陷早二叠世风城组沉积时期为典型的碳酸盐型咸化湖盆,湖盆咸化过程中析出大量的碱矿(钠碳酸盐类矿物)。通过含碱层段主量元素、碳氧同位素分析,结合岩心宏观及微观矿物岩石特征,探讨了含碱层段韵律特征及形成机理。研究层段碱矿与沉凝灰岩互层,形成明暗相间的咸化韵律。浅色碱矿层质地较纯,主要为苏打和小苏打,主量元素表现为 Na₂O含量高,CaO、MgO、SiO₂和Al₂O₃含量低;暗色层段主要为含斑点状碳钠钙石的沉凝灰岩,主量元素表现为Na₂O含量低,CaO、MgO、SiO₂和Al₂O₃含量高。浅色碱矿层和暗色沉凝灰岩段碳氧同位素值均为正,表明其沉积水体具高盐度特征。浅色含碱层段具有更高的 δ¹⁸O 值,反映了碱矿层沉积时期强烈蒸发的气候条件。根据δ¹⁸O/δ¹³C值的波动将研究层段划分为炎热干旱和相对湿润的气候条件,并据此解释不同韵律层盐类矿物形成机理。暗色的沉凝灰岩沉积代表地表流水注入量大于蒸发量,沉凝灰岩中盐类矿物含量少或者呈斑点状从颗粒间的卤水中析出;随着蒸发量的增高以及古气候由温暖潮湿转变为干旱炎热,地表流水注入量急剧减少,强烈蒸发控制了碱矿的沉积,形成浅色质地较纯的碱矿层。气候的频繁交替形成了风二段碱矿与沉凝灰岩频繁互层的特征。     

An evaporitic facies in Neoproterozoic post-glacial carbonates: The Gifberg Group, South Africa

A geological study of the hitherto poorly described Neoproterozoic Gifberg Group, with emphasis on lithogeochemistry and O, C and Sr isotopic composition of the carbonate-dominated Widouw Formation (Vredendal Outlier, westernmost South Africa) revealed that the entire group is an equivalent of the relatively well constrained Port Nolloth Group in the external, paraautochthonous part of the Pan-African Gariep Belt further north. Thus, the Vredendal Outlier can be regarded as the southern extension of the Port Nolloth Zone. Two diamictite units are recognised in the Vredendal Outlier, which can be correlated respectively with the c. 750 Ma Kaigas Formation diamictite and the 583 Ma, syn-Gaskiers Numees Formation diamictite in the Gariep Belt proper. The dominating carbonate unit in the studied area is post-glacial with respect to the older of the two diamictite units. The combined textural, structural and geochemical evidence suggests that parts of the variably dolomitised limestone succession represent former evaporite beds. Sedimentation in a restricted, very shallow and proximal basin led to a wide range in C isotope ratios (δ¹³C_PDB from − 4.2 to + 4.8‰), very high Sr concentrations (derived from original anhydrite) and initial ⁸⁷Sr/⁸⁶Sr ratios that are significantly higher (0.70785) than those of coeval seawater. As C and Sr isotopes are commonly used for chemostratigraphic correlation, and high Sr concentrations in Neoproterozic carbonates are often interpreted as evidence of former aragonite, the findings of this study should be used as warning against uncritical use of geochemical and isotopic parameters for describing ancient seawater composition. Thus C and Sr isotope ratios alone in Neoproterozoic carbonates may be less powerful proxies of ancient seawater composition, and high Sr contents are not necessarily indicative of an “aragonite sea”, as previously inferred.     

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.