首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 548 毫秒
1.
塔里木盆地北部丘里塔格群(寒武系至奥陶系)白云岩的成因   总被引：17，自引：2，他引：17  
叶德胜 《沉积学报》1992,10(4):77-86
本文通过详细岩石学及地球化学研究,探讨了塔里木盆地丘里塔格群(寒武至奥陶系)白云岩的成因。研究表明藻纹层白云岩、微晶白云岩及颗粒白云岩中的颗粒为近地表准同生白云化的产物。结晶白云岩(细晶以上)及颗粒白云岩中的中粗晶白云岩胶结物是深埋藏成岩环境的产物。并对埋藏白云化的镁离子来源及搬运机理进行了探讨。  相似文献   

2.
In the long continental history of the Provence Basin which extended from Santonian to Oligocene times, a major period of palaeosol development occurred in the Danian. Dolocretes developed within floodplain silts, and partly from palustrine limestones. Dolocretes are overlain by pedogenic facies: calcretes and palustrine limestones. Gradational lower and upper limits of profiles, succession of nodular, coalescent and massive horizons, the epigenesis of quartz by dolomite, the unimodal crystal size and the euhedral dolomite fabric, as well as the absence of biogenic structures and vadose cements show that dolocretes formed in the phreatic zone. The exclusive occurrence of dolocretes around the palaeolake or playa suggests that dolocrete formed by the mixing of groundwaters and lake brines, which infiltrated the phreatic zone during periods of strong evaporation and lake level lowering. The term halo dolocrete is proposed to describe this type of dolocrete deposit. Subsequent alteration of the dolocrete includes leaching of the central core of dolomite crystals and calcitization. Calcitization was either fabric-destructive (type I) or fabric-preserving (type II) and took place during very early diagenesis, i.e. concomitant with calcrete formation or palustrine limestone deposition. Fabric-destructive calcitization is attributed to a drop in the lake level, when the upper part of the dolocrete was subjected to vadose zone processes, whereas the fabric-preserving calcitization resulted from reactions with dilute lake and groundwaters during rise in lake level.  相似文献   

3.
The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ18O and δ13C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ18O=–2·2, δ13C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ18O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.  相似文献   

4.
Dark grey, bituminous dolostones interbedded with marine-derived anhydrite horizons occur in the Triassic Reichenhall Formation of western Austria. Fossils are rare and indicate a hostile, hypersaline depositional environment. The dolomites are finely crystalline, fairly stoichiometric, well ordered and non-ferroan. Closely spaced samples (94 in total) of individual dolomite units have been analysed for their carbon and oxygen isotopic composition. The data indicate surprisingly low δ18O values (-5.7 to -2.1%0 PDB), whereas the δ13C values are comparable to the contemporary Triassic seawater (+0.2 to +2.6%0 PDB). Sedimentological evidence, including (i) lack of any evidence for extensive dissolution, (ii) distinct oxygen and carbon isotope ratios of individual dolomite units, (iii) covariance of carbon and oxygen isotopes within some dolomite layers and (iv) inclusions of celestite in dolomite, indicates a nearly closed system after early diagenesis. Combining this information with water-rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation. A secondary factor may be dolomite recrystallization at elevated temperatures during burial.  相似文献   

5.
Dolomitization of the Zechstein (Late Permian) Main Dolomite carbonates of northern Poland was penecontemporaneous and/or very early diagenetic. Well-ordered, stoichiometric dolomites are associated with the basinal facies. The platform dolomites are relatively poorly ordered and usually non-stoichiometric. Most samples are highly enriched in 13C, as in other Zechstein carbonates. δ18O values show large variations from -5·1%0 to + 7·4%. There is an isotope zonation of the examined dolomites. The isotope signature indicates that dolomites formed from variable solutions of meteoric water, seawater, and evaporitic brines of possible marine or continental origin. Once initiated, dolomitization proceeded despite the evolution of dolomitizing brines. This evolution explains the occurrence of lagoonal dolomites with common evidence for dissolution in the lower part of sections compared with well-developed rhombohedra in the upper part. Crystal zoning suggests the initiation of dolomite growth in hypersaline water and progressive dilution by fresh water. There is isotopic evidence for migration of continental waters into the basin, presumably following sea-level fall at the end of the deposition of the Main Dolomite. Influence of fresh water on syndepositional dolomitization, well established in the Main Dolomite, strongly suggests that similar relationships may be characteristic for other evaporite-associated dolomites as well.  相似文献   

6.
Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3?/CO32? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.  相似文献   

7.
HAIRUO Qing 《Sedimentology》1998,45(2):433-446
The petrography and geochemistry of fine- and medium-crystalline dolomites of the Middle Devonian Presqu’ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse-crystalline and saddle dolomites that are associated with late-stage hydrothermal fluids. Fine-crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 μm (mean 8 μm). The dolomite interbedded with evaporitic anhydrites that occur in the back-barrier facies in the Elk Point Basin. Fine-crystalline dolomite has δ18Ο values between ?1·6 to –3·8‰ PDB and 87Sr/86Sr ratios from 0·7079–0·7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55–225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this fine-crystalline dolomite formed from Middle Devonian seawater at or just below the sea floor. Medium-crystalline dolomite in the Presqu’ile barrier is composed of anhedral to subhedral crystals (150–250 μm, mean 200 μm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium-crystalline dolomite has δ18O values between ?3·7 to ?9·4‰ PDB (mean ?5·9‰ PDB) and 87Sr/86Sr ratios from 0·7081–0·7087 (mean 0·7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium-crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35–40 °C) from fluids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin. These results indicate that dolomitization in the Middle Devonian Presqu’ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.  相似文献   

8.
Palaeogene dolostones from the sub‐surface of Florida are ideal for the study of dolomite maturation because they record the early stages of a secondary dolomite overprint without destruction by later diagenetic overprints. Two distinct dolomite textures occur in the dolostones of the Upper Eocene Ocala and Lower Oligocene Suwannee limestones in west‐central Florida: a porous and permeable sucrosic dolomite and a less porous and relatively impermeable indurated non‐sucrosic dolomite. In both textures, the initial matrix dolomite is dully luminescent, whereas the secondary overprint is dominantly luminescent cement in the Suwannee and only neomorphic luminescent dolomite in the Ocala. The abundance of luminescent dolomite ranges from 2% to 38%, which translates to 1·6 km3 of material in the Suwannee and 13·5 km3 in the Ocala. Extrapolated trace‐element contents (Sr and Na) and δ18O values for the matrix and luminescent end‐members indicate a marine origin for the matrix dolomite in both units, and a freshwater–seawater mixing‐zone origin for the secondary luminescent dolomites. The δ18O values indicate that a saline, middle mixing‐zone environment overprinted the Suwannee but a more dilute mixing zone affected the Ocala. Fluid–fluid mixing models constrained by modern Floridan aquifer hydrochemistry and extrapolated 87Sr/86Sr values of the luminescent phases indicate that the mixing zones operated during the Late Miocene to Pliocene in the Ocala and affected the Suwannee in the Pliocene. The luminescent Suwannee mixing‐zone cement reduced porosity up to threefold and permeability up to 100‐fold, which converted many sucrosic dolomites to indurated dolomites. By contrast, the neomorphic luminescent Ocala dolomite did not have an appreciable impact on the maturations. Although freshwater–seawater mixing zones were not the sites of the initial dolomitization, the mixing‐zone environment did dramatically overprint and mature the regionally widespread dolomites of the Ocala and Suwannee limestones. This maturation occurred shortly after formation of the proto‐Floridan aquifer; the timing suggests the matrix dolomites were ‘ripe’ for alteration and that the only prerequisite for mixing‐zone dolomite is pre‐existing dolomite substrates to reduce kinetic barriers. In contrast to recent claims, the results of this study demonstrate that mixing zones can be effective in forming regionally significant amounts of secondary dolomite and influencing the petrophysical maturation of dolomite bodies.  相似文献   

9.
ALI M'RABET 《Sedimentology》1981,28(3):331-352
Combined field, sedimentological, mineralogical, isotopic and geochemical study of the Lower Cretaceous dolomites of Central Tunisia has demonstrated considerable diversity in origin. Environments of dolomite formation include deep phreatic, karst, lacustrine and evaporitic sabkha. All four groups of dolomite are composed of non-stoichiometric and/or disordered crystals which are more or less rich in calcium and in iron. Petrographic fabrics are of three types: replacement, recrystallization and cementation. These three fabrics are proposed among the various criteria for the different environments of dolomitization. Average isotope (δ18 O and δ13 C) contents for these four dolomite groups range from ?10·5 to +0·4%o (PDB) and ?3·9 to + 3·7%o respectively. The distribution of strontium is related both to the degree of recrystallization and to the palaeosalinity. Ferrous iron, also very common, is regarded as an indicator of relatively deep reducing conditions, mainly in meteoric groundwaters. Sodium distribution is related to inclusions within the dolomite, its distribution being relatively constant in all four groups; it cannot be regarded as a reliable criterion for palaeosalinity of dolomitizing fluids. This study confirms that dolomitization may occur under widely different palaeoenvironments, either at the surface or during burial. With the exception of the sabkha environment, dolomitizing fluids seem to have been essentially meteoric.  相似文献   

10.
Dolomites from the upper calcareous-siliceous member of the Miocene Monterey Formation exposed west of Santa Barbara, California, were analysed for geochemical, isotopic and crystallographic variation. The data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids. Recrystallization is recognized by the following compositional and crystallographic variations. Dolomites have decreasing δ18O and δ13C compositions, decreasing Sr contents and increasing Mg contents with increasing burial depths and temperatures from east to west in the study area. δ18O values vary from 5·3‰ in the east to − 5·5‰ PDB in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west. δ13C values correlate with δ18O and decrease from 13·6‰ in the east to − 8·7‰ PDB in the west. Sr concentrations correlate positively with δ18O values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west. Mol% MgCO3 values inversely correlate with δ18O values and increase from a minimum of 41·0 in the east to a maximum of 51·4 in the west. Rietveld refinements of powder X-ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering. The fraction of the Ca sites in the dolomites that are occupied by Ca atoms increases slightly with the approach to stoichiometry. The fraction of the Mg sites occupied by Mg atoms strongly correlates with mol% MgCO3. Even in early diagenetic, non-stoichiometric dolomites, there is little substitution of Mg in Ca sites. During recrystallization, the amount of Mg substituting for Ca in Ca sites decreases even further. Most of the disorder in the least recrystallized, non-stoichiometric dolomites is related to substitution of excess Ca on Mg sites.  相似文献   

11.
Discordant zebra dolomite bodies occur locally in the Middle Cambrian Cathedral and Eldon Formations of the Main Ranges of the Canadian Rocky Mountains Fold and Thrust Belt. They are characterized by alternating dark grey (a) and white (b) bands, forming an ‘abba’ diagenetic cyclicity. These bands developed parallel to both bedding and cleavage. Dark grey (a) bands consist of fine (< 300 μm) non-planar crystalline impure dolomite. The white (b) bands are composed of coarse (up to several millimetres) milky-white pure saddle dolomites (b1) which are often covered by pore-lining zoned dolomite (b2). The b phases often possess a saddle-shaped morphology. In contrast to the replacement origin of the a dolomite, the zoned b2 dolomite rims are interpreted as a cement formed in open cavities. The b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non-carbonate components. All the zebra dolomites studied are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ depending on the sample locality. Fluid inclusion data indicate that the dolomites formed from relatively hot (TH = 130–200 °C), saline (20–23 wt% CaCl2 eq.) fluids. A diagenetic high temperature origin is also supported by depleted δ18O values (−20 to −14‰ VPDB). A contribution of 87Sr-enriched fluids is reflected in the 87Sr/86Sr values (0·7091–0·7123). Zebra dolomite development is explained by focused fluid flow, which exploited areas of structural weaknesses (e.g. basin-platform, rim areas, faults, etc.). Expulsion of hot basinal brines in a tectonically active regime generated overpressures, which explains the development of secondary porosity during zebra dolomitization as well as the intra-zebra fracturing at decimetre to micrometre scale.  相似文献   

12.
Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ18O values from ?13·7 to ?9·7‰ VPDB, δ13C values from ?2·7 to + 1·5‰ VPDB and 87Sr/86Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (Th) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ18O values (?14·2 to ?9·3‰ VPDB), lower δ13C values (?3·0 to ?0·7‰ VPDB), higher 87Sr/86Sr ratios (≈ 0·7100) and higher Th values (171–209 °C), which correspond to trapping temperatures (Tr) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in 87Sr/86Sr and δ18O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ18O values (?18·5 to ?14·3‰ VPDB), variable δ13C values (?11·3 to ?1·2‰ VPDB) and high Th values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).  相似文献   

13.
The Tepearasi Formation of the autochthonous Geyikdagi Group in the Central Tauride Belt, SE of Beysehir, is Dogger in age and consists dominantly of massive limestones and greyish dolomites occurring within the middle to upper sections. The total thickness of the dolomitic levels ranges from 100-300 m and laterally extends 500-700 m. Three types of dolomite were distinguished through petrographic analyses: homogeneous, mottled (saddle-crystalline) and joint-filling dolomite, which were interpreted to have formed in two different stages, early diagenetic and late diagenetic. The homogeneous dolomite of the early diagenetic stage is light-coloured and monotonous-textured and shows the form of a dolosparite mosaic. The mottled dolomite formed in the late diagenetic stage is light- to dark-coloured and coarsely granular idiomorphic. The other type of late diagenetic dolomite, described as the joint-filling type, presents a crystal growth pattern from the joint walls towards the centre of the joint space. I  相似文献   

14.
作为近年来新兴的实验技术,二元同位素(D47)测温技术已被应用于碳酸盐岩成岩环境的研究中。简要介绍了二元同位素测温技术的原理及应用方法,并以塔里木盆地中下寒武统白云岩为例,优选11块样品,测试其D47值和白云石的碳氧同位素,并计算出样品的成岩温度和古流体的δ~(18)O值。综合分析认为:样品中,颗粒白云岩形成于低温准同生—浅埋藏环境,成岩流体为海水;细晶白云岩为深埋藏成岩环境中原岩受到了高温重结晶作用的改造,成岩流体为地下热卤水;孔缝中的白云石胶结物是深埋藏成岩环境富镁热卤水沉淀作用的产物。研究证明二元同位素测温技术可以较好地恢复白云岩的成岩温度,减少储层成因的多解性,它为今后储层成因研究提供了一种新的手段和依据。  相似文献   

15.
ABSTRACT Field, geochemical, and petrographic data for late Pleistocene dolomites from southeastern Barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters. The dolomite is localized in packstones and wackestones from the algalAmphistegina fore-reef calcarenite facies. Stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization. Carbon isotopes in pure dolomite phases average about -15%0 PDB. This light carbon is attributed to the influence of soil gas CO2, and precludes substantial mixing with seawater. A narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint. Dolomitization likely occurred with as little as a five per cent admixture of seawater. Strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy. The dolomite is characterized by low sodium values. Low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization. A sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix. Limpid, euhedral dolomite cements precipitated in primary intra- and interparticle porosity subsequent to replacement dolomitization. As waters became progressively less saline, dolomite cements alternated with thin bands of syntaxial calcite cement. The final diagenetic phase precipitated was a blocky calcite spar cement, representing diagenesis in a fresh-water lens. This sequence of diagenetic features arose as the result of a single fall in eustatic sea-level following deposition. A stratigraphic-eustatic-diagenetic model constrains both the timing and rate of dolomitization in southeastern Barbados. Dolomitization initiated as sea-level began to fall immediately following the oxygen isotope stage 7–3 high stand, some 216 000 yr bp . Due to the rapidity of late Pleistocene glacio-eustasy, dolomitization (locally complete) is constrained to have occurred within about 5000 yr.  相似文献   

16.
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 δ34S 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 δ18O in dolomite range from ?4·37 to ?14·26‰ (PDB) suggesting a re-equilibration of dolomite with basinal water depleted in 18O and possible recrystallization of dolomite under relatively elevated temperatures. The δ13C, 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.  相似文献   

17.
Late Miocene platform carbonates from Nijar, Spain, have been extensively dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically highest outcrops, suggesting that dolomitizing fluids were derived from the adjacent Nijar Basin. The dolomite crystals range from <10 to ≈100 μm existing as both replacements and cements. Na, Cl and SO4 concentrations in the dolomites range from 200 to 1700 p.p.m., 250–650 p.p.m., and 600–7000 p.p.m., respectively, comparable with other Tertiary and modern brine dolomite values, and also overlapping values from mixing-zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 7× seawater brine to freshwater ratios. The δ18O and δ13C of the dolomites range from ?1·0 to +4·2‰ PDB, and ?4·0 to +2·0‰ PDB, respectively. 87Sr/86Sr values (0·70899–0·70928) of the dolomites range from late Miocene seawater to values greater than modern seawater. Mixtures of freshwater with seawater and evaporative brines probably precipitated the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. δ18O and Na/Ca vs. δ18O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, nonstoichiometry and quantitative water–rock interaction modelling of covariations of Na vs. Sr and δ18O vs. δ13C. The possibility of multiple dolomitization events induced by evaporative brines, seawater and freshwater, respectively, is consistent with mineral-mineral mixing modelling. The basin-derived dolomitizing brines probably mixed with freshwater in the Nijar Basin or mixed with fresh groundwater in the platform, and were genetically related either to deposition of the Yesares gypsum or the Feos gypsum. Dolomitization occurred during either the middle Messinian or the early upper Messinian. Nijar dolomitization models may be applicable to dolomitization of other late Miocene platform carbonates of the western Mediterranean. Moreover, the Nijar models may offer an analogue for more ancient evaporite-absent platform carbonates fringing evaporite basins.  相似文献   

18.
川东三叠系飞仙关组碳酸盐岩的阴极发光特征与成岩作用   总被引：16，自引：0，他引：16  
四川盆地东部三叠系飞仙关组是近年来我国发现的重要天然气储层, 高孔隙度、高渗透率的碳酸盐储层都分布于白云岩地层中, 因而碳酸盐的成岩作用, 尤其是白云岩化作用和白云岩的成因为石油地质学家和沉积学家高度关注.对四川盆地东部罗家寨构造三叠系飞仙关组42个碳酸盐岩样品进行了阴极发光分析, 结合与之有关的Mn、Fe、Mg元素分析和岩石学研究, 讨论了包括白云岩化作用在内的碳酸盐岩成岩过程中可能的成岩流体性质及来源.四川盆地东部三叠系飞仙关组碳酸盐岩普遍具有很弱的阴极发光性, 这与其很低的Mn、Fe含量有关, 说明沉积期后非海相流体对飞仙关组碳酸盐岩的影响非常有限, 海源流体在成岩过程中发挥了主导作用; 不同石灰岩类型和不同白云岩类型仍然具有不同的阴极发光性, 成岩组分含量越高的碳酸盐岩, 或者说与沉积期后流体(主要是孔隙流体) 关系越密切的碳酸盐岩的阴极发光强度越低, 说明随着埋藏成岩作用的进行, 四川盆地东部三叠系碳酸盐岩孔隙流体受海源流体的影响是逐渐增强的; 阴极发光分析结果表明, 作为四川盆地东部主要储集岩的结晶白云岩形成机制与埋藏过程中的深循环流体有关, 这种深循环流体没有或很少穿越铝硅酸盐地层, 但穿越了三叠系内部的某些海相地层, 这些海相地层可能是广泛存在于四川盆地三叠系的蒸发盐地层, 由蒸发盐成岩过程提供的海源流体参与了结晶白云岩的白云岩化作用.   相似文献   

19.
Carbon isotope measurements carried out on 67 dolomite samples from the Middle Precambrian Lomagundi Group (Rhodesia) have yielded a δ13C mean of +8.2 ± 2.6%. vs PDB. With the outcrop of these dolomites extending over a distance of almost 300 km, the Lomagundi dolomite faces is likely to represent the largest isotopically anomalous sedimentary carbonate province ever recorded. It is concluded that the anomalous carbonates formed in a closed basin whose δ13C level had been substantially increased as a result of a preferential removal (within sedimentary organics) of the light carbon isotope.  相似文献   

20.
Microbial dolomite crusts from the carbonate platform off western India   总被引：1，自引：1，他引：1  
Abstract The occurrence of Late Pleistocene dolomite crusts that occur at 64 m depth on the carbonate platform off western India is documented. Dolomite is the most predominant mineral in the crusts. In thin section, the crust consists of dolomitized microlaminae interspersed with detrital particles. Under scanning electron microscopy, these laminae are made up of tubular filaments or cellular structures of probable cyanobacterial origin. Dolomite crystals encrust or overgrow the surfaces of the microbial filaments and/or cells; progressive mineralization obliterates their morphology. Well-preserved microbial mats, sulphide minerals (pyrrhotite and marcasite) and the stable isotope composition of dolomite in the crusts indicate hypersaline and anoxic conditions during dolomite formation. The crusts are similar to dolomite stromatolites, and biogeochemical processes related to decaying microbial mats under anoxic conditions probably played an important role in dolomite precipitation. The dolomite is therefore primary and/or very early diagenetic in origin. The dolomite crusts are interpreted to be a composite of microbial dolomite overprinted by early burial organic dolomite. The results of this study suggest that a microbial model for dolomite formation may be relevant for the origin of ancient massive dolomites in marine successions characterized by cryptalgal laminites. The age of the crusts further suggests that the platform was situated at shallow subtidal depths during the Last Glacial Maximum.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号