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1.
Robin W. Renaut R. Bernhart Owen Tim K. Lowenstein Gijs De Cort Emma Mcnulty Jennifer J. Scott Anthony Mbuthia 《Sedimentology》2021,68(1):108-134
Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO3) forms bladed crystals that nucleate on the lake floor. Trona (Na2CO3·NaHCO3·2H2O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere. 相似文献
2.
Depositional environments and facies of the Late Triassic Abu Ruweis Formation, Jordan 总被引：1，自引：0，他引：1
The Abu Ruweis Formation is composed of carbonates, evaporites, and mudstones, with some locally developed pelletic, oolitic and stromatolitic limestones. The lateral persistence of bedding, the purity of the evaporite rocks, the alternating arrangement of marine carbonates and evaporites indicates periodic deposition in subaqueous conditions (salina). Petrographic investigations, X-ray diffraction analysis as well as chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum, with rare anhydrite relics. Five microfacies of gypsum were recognized according to their fabrics: porphyroblastic and granoblastic gypsum showing polarization texture, gypsum pseudomorph after anhydrite laths, and satin spar gypsum. The textures they display indicate a hydration origin of precursor anhydrite, which is in turn rehydrated from primary gypsum. Some of these anhydrites were formed as a result of replacement processes of the carbonate sediments associated with the evaporites, as evidenced from the textural relationships of the carbonate and sulfate minerals. The O18 content ranges from 1.45 to 8.38% PDB and the C13 content ranges from −1.52 to 4.73% PDB. Trace elements analysis has shown that the Abu Ruweis dolomites are rich in strontium (up to 600 ppm), and sodium (up to 835 ppm). The isotope composition and trace elements content, as well as the petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the Abu Ruweis dolomites. The evaporites were deposited during a regressive lowstand systems tract, whereas the carbonates were deposited under shallow water marine conditions during a highstand systems tract. The Abu Ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin. Restricted conditions were provided by the development of beach barriers. 相似文献
3.
N.J. Tosca S.M. McLennan J.P. Grotzinger A.H. Knoll S.W. Squyres 《Earth and Planetary Science Letters》2005,240(1):122-148
New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe2+ and Fe3+ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop. 相似文献
4.
SONG Chunhui ZENG Yongyao YAN Maodu FANG Xiaomin FENG Ying PAN Jiaqiu LIU Xifang MENG Qingquan HU Chunhu ZHONG Sirui 《《地质学报》英文版》2017,91(1):156-174
The Qiangtang Basin (QB), located in the central Tibetan Plateau, is a Jurassic marine basin and one of the most important prospective salt resource belts in China. In recent decades, many outcrops of gypsiferous?bed have been found in the Jurassic marine strata in the basin. Salt springs with abnormally high sodium (Na+) contents had been identified in the Late Jurassic Xiali Formation (Fm.) in the basin in the last years. However, to date, no potash or halite deposits have been identified in the QB. Gypsum outcrops and salt springs are very important?signs in the investigation of halite and potash deposits. Therefore, the Xiali Fm. is?a potentially valuable layer to evaluate for the possible presence of halite and potash deposits in the basin. However,?few studies?have explored the formation?conditions of evaporites in the unit. Here, we present detailed geochemical records from the Yanshiping section related to the study of the formation?conditions of evaporites in the Xiali Fm. of the QB. Climate proxies based on the obviously increased anion concentrations of SO42? and Cl? and the significant correlation?coefficients of Ca2+-SO42? (R = 0.985) and Na+-Cl? (R = 0.8974) reveal that the upper member of the Xiali Fm. (the upper Xiali Fm.) formed under an arid climate and evolved into the sulfate phase or early chloride phase. Provenance proxies based on the obviously increased K+ and Na+ ion concentrations and the significant correlation?coefficient of Na+-Cl? (R = 0.8974) suggest that the upper Xiali Fm. featured optimal provenance conditions for the possible formation of halite deposits. The regression and the semi-closed tidal?flat environment in the upper Xiali Fm. were favorable for the formation of potash and halite deposits. The low Mg2+ /Ca2+ values (mean value = 1.82) and significant Na+-Cl? correlation?coefficient (R = 0.8974) also suggest that the upper Xiali Fm. is the layer most likely to contain potential halite deposits. In addition, the macroscopic correlations of tectonism, provenance, paleoclimate, saliferous strata and sedimentary?environment between the QB and the adjoining Amu Darya Basin in Central Asia reveal that the two basins shared similar geologic settings that were favorable for the formation of evaporites during the Late Jurassic. Therefore, the upper Xiali Fm. is a valuable layer to explore for halite deposit and may be potentially valuable in the future exploration for potash deposits in the QB. 相似文献
5.
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. 相似文献
6.
Alfonso F. Davila Christoph Gross Alberto G. Fairén Thomas Kneissl James M. Dohm 《Icarus》2011,212(2):579-589
Different lines of evidence point to hydrological cycling in the martian past. The extent, duration, and magnitude of this cycling remains unclear, as well as the magnitude of aqueous processes on the surface. Here, we provide geomorphic and mineralogic evidence of a large inter-crater sedimentary basin located in the Terra Sirenum region, which was once covered by a body of liquid water with an areal extent of at least 30,000 km2 and a depth of approximately 200 m. The topographic basin, which is modified by a number of large impact craters, is partly controlled by ancient impact and tectonic structures. As a result of evaporation of the large body of water, salt flats formed in the lowest topographic reaches of the basin. Hydrated phyllosilicates occur in close proximity to the salt flats and in the ejecta and rim materials of small impact craters with stratigraphic relations that suggest that they underlie the evaporite deposits. Crater statistics place the maximum age of aqueous activity during the Late Noachian epoch. The relatively pristine mineral deposits in the basin have a high potential to yield information of the geochemistry and water activity during the ancient Noachian Period when conditions were seemingly more conducive to life. 相似文献
7.
湖北金山店矽卡岩型铁矿田硫同位素特征及其地质意义 总被引：2，自引：1，他引：1
矿集区或矿田尺度硫同位素的空间分布特征研究不仅具有重要的理论意义,而且对找矿实践具有重要的指示作用。本文对金山店矿田范围内的矽卡岩型铁矿床开展了系统的硫同位素研究工作,发现该矿田内矽卡岩型铁矿床中的黄铁矿(+15. 1‰~+25. 6‰)、硬石膏(+24. 9‰~+31. 5‰)和石膏(+27. 5‰~+30. 4‰)均具有富集重硫同位素的特征,明显不同于岩浆热液矿床中这些矿物的δ~(34)S值组成特征,且硬石膏和石膏的δ~(34)S值与三叠系嘉陵江组地层中沉积石膏的δ34S值较为接近,暗示金山店矿田内矽卡岩型铁矿中的硫可能主要来自于含膏盐地层。含膏盐地层广泛参与了金山店矿田中矽卡岩型铁矿的成矿作用,对成矿作用产生了重要的影响:大量硫酸根(SO_4~(2-))的还原过程可以将成矿体系中的Fe~(2+)氧化成Fe~(3+),导致大量磁铁矿的形成;含膏盐地层与流体作用形成大量的盐溶角砾岩,有利于加速成矿流体与围岩之间的水岩作用,并提供容矿空间。系统的对比研究发现,大冶地区的矽卡岩型铁矿和矽卡岩型铁铜矿的赋矿地层、热液硬石膏/石膏规模和硫同位素值组成均存在明显的差异,暗示这些矿床的成矿围岩存在显著的差异。鄂东矿集区尺度的硫同位素等值线所揭示的空间变化规律具有重要的找矿指示作用:在天青石矿区(如狮子立山)或附近可能具有寻找大冶式矽卡岩型铁矿床或铁铜矿床的潜力,而在硬石膏/石膏发育的矽卡岩型铁矿区(如金山店铁矿田和程潮铁矿)或附近则具有寻找狮子立山式热液天青石矿床的潜力。 相似文献
8.
Malcolm K. Jenyon 《Marine and Petroleum Geology》1984,1(1):27-36
The presence of probable salt dissolution-associated collapse structures in the North Sea based on the evidence of contemporary seismic data was discussed by Lohmann (1972). Since then, little attention has been paid to them in North Sea literature, although many oil companies will have studies on file. This Paper describes and illustrates a selection of such features in the area from recent seismic data, and suggest a broad classification for them as they are seen on the seismic section, based on mode of origin. Reference will also be made to certain types of carbonate dissolution and collapse effects. 相似文献
9.
Loss on ignition: a qualitative or quantitative method for organic matter and carbonate mineral content in sediments? 总被引：5，自引：7，他引：5
Juan I. Santisteban Rosa Mediavilla Enrique López-Pamo Cristino J. Dabrio M. Blanca Ruiz Zapata M. José Gil García Silvino Castaño Pedro E. Martínez-Alfaro 《Journal of Paleolimnology》2004,32(3):287-299
Since the publication of the paper of , loss on ignition (LOI) has been widely used as a method to estimate the amount of organic matter and carbonate mineral content (and indirectly of organic and inorganic carbon) in sediments. The relationships between LOI at 550 °C (LOI550) and organic carbon (OC) content and between LOI at 950 °C (LOI950) and inorganic carbon (IC) content are currently accepted as a standard. However, the comparison of 150 analyses of samples of diverse lithologies, collected from a single core, reveals that these relationships are affected by sediment composition (presence of clays, salts, and the variable content of organic carbon). This results in an incremental error on the estimation of carbon content from LOI values that invalidates the use of LOI values as a quantitative method for estimating carbon content. Conversely, the general trends of LOI550 and LOI950 show a good correlation with carbon content (both organic and inorganic) allowing use of LOI as a qualitative test for carbon content. Similarly, in our case, LOI at 105 °C (LOI105) is a good qualitative proxy for the trends in gypsum content. 相似文献
10.
The Mediterranean margin of Libya can be divided into three physiographic provinces, the Pelagian Shelf, the Sirt Embayment, and Offshore Cyrenaica. The petroleum potential of the Pelagian Shelf has been investigated but the Sirt Embayment and Offshore Cyrenaica are almost unexplored. During 2004–2005, new 2D pre-stack time-migrated seismic data were acquired and used to examine the large-scale structural, depositional, and salt tectonic features of the Libyan shelf and slope. The data cover approximately 38,000 line kilometers in water depths ranging from under 50 to over 3000 m.Cenozoic strata along much of the Libyan margin have a demonstrable progradational character punctuated by surfaces of erosion and margin failure. Within the Sirt Embayment the most visible retrogradational surface becomes seismically coincident with the top of Messinian unconformity. This retrogradational surface extends for over 700 km along strike and cuts both sides of the Sirt Embayment. Over 5000 cubic kilometers of material are missing from above this surface. There are two ideas for how retrogradation occurred. One idea relates the retrogradation to margin erosion during the Messinian salinity crisis. The other idea suggests that a large part of the Libyan margin experienced a cataclysmic failure during the late Miocene.Some existing models for offshore Libya have interpreted a widespread layer of halite lying within the Messinian which thickens basinward. This interpretation was probably based on the fact seismic reflection continuity was lost over much of the Sirt Embayment on older data beneath the top Messinian unconformity. The loss of good reflection character adversely affected exploration efforts by obscuring deep structures. Recent seismic data and a current understanding of salt behavior do not support the interpretation of thick halite within the Messinian section. Regional observations do not indicate any mobile halite present in the Sirt Embayment. There is a relatively thin, high amplitude and high velocity layer of non-halite evaporites (mainly anhydrite) which caps the Messinian section. Where this high amplitude and high velocity layer is absent or eroded, seismic continuity within the Messinian interval is restored. Limited available well data support this interpretation.True mobile halite is interpreted to exist in offshore Libya only in the far west of the Pelagian Shelf near the Tunisian border. Beneath the Pelagian Shelf are a series of tight contractional folds that are interpreted to be salt cored. Basins adjacent to the folds display geometries characteristic of salt withdrawal. The fold crests formed bathymetric highs which served as nucleation sites for nummulitic shoal development. These shoals are the principal reservoirs of the Pelagian Shelf. 相似文献