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1.
Based on many new fossil discoveries mainly conodonts and ostracodes an attempt has been made to determine the stratigraphic range of the high pressure/low temperature metamorphic Phyllite Group in West Crete (Greece). The numerous conodont findings indicate — in spite of strong supply of clastic material — a fully marine environment from the Upper Carboniferous up to the end of the Lower Triassic. The prevailing part of the Middle Triassic cannot yet covered by fossils. The upper Triassic shows in the lower (invers) part of the Phyllite Group a shallow water facies with ostracodes, bivalves and at the Triassic/Lias boundary a saliniferous facies, however in the upper (in original position lying) part a conodonts-foraminiferes bearing littoral facies. The present biostratigraphic data point out that the lower part of the Phyllite Group lies in inverted order and the upper part in original position. Thus the Phyllite Group on the whole seems (in West Crete) to form a huge recumbent isoclinal fold analogous to the isoclinal folding on a smaller scale as frequently exposed. The carbonate Trypali Group can be interpreted as either the recumbent limb of this fold structure or as part of the underlying Talea Ori Group. The Trypali Group seems not to be a particular unit. For the reconstruction of the paleogeographic and geodynamic evolution of the South Aegean region the following evidence may be important:
  1. the marine sedimentation in the Upper Palaeozoic and the Lower Triassic, especially the marine Upper Permian (Dorashamian), an isolated occurrence 2000 km away from comparable sediments further east and
  2. the obvious termination of deposition at the Triassic/Lias boundary. A future interpretation of these results may be the key to a better understanding of the geodynamic process which led to the high pressure/low temperature metamorphism of the Phyllite Group.
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2.
中下扬子区广泛分布着三叠纪滩相、潮坪相和岩溶带三类碳酸盐岩。根据成岩作用类型、成岩组构、孔隙类型和成岩环境等特征对三类岩石进行比较。滩相碳酸盐岩主要成岩作用是在海水渗流、潜流和淡水渗流、潜流成岩环境下产生的胶结、云化和溶解作用。潮坪相交代白云岩和富含石膏的蒸发岩则以高盐度的海水渗流带的准同生毛细管蒸发浓缩云化作用为主。在大气淡水渗流或潜流成岩环境下则以去云化、去膏化和溶解作用为主。岩溶带碳酸盐岩-蒸发盐岩及其岩溶堆积角砾岩主要经历早期海水强烈蒸发作用下准同生云化作用和晚期表生淡水环境下的岩溶作用。上述主要成岩作用使三类岩石次生孔隙率增加。因此,成岩作用对其成为有希望的储层起到了主导作用。  相似文献   

3.
Evaporites, including rock salt (halite) and gypsum (or anhydrite), are the most soluble among common rocks; they dissolve readily to form the same types of karst features that commonly are found in limestones and dolomites. Evaporites are present in 32 of the 48 contiguous states in USA, and they underlie about 40% of the land area. Typical evaporite-karst features observed in outcrops include sinkholes, caves, disappearing streams, and springs, whereas other evidence of active evaporite karst includes surface-collapse structures and saline springs or saline plumes that result from salt dissolution. Many evaporites also contain evidence of paleokarst, such as dissolution breccias, breccia pipes, slumped beds, and collapse structures. All these natural karst phenomena can be sources of engineering or environmental problems. Dangerous sinkholes and caves can form rapidly in evaporite rocks, or pre-existing karst features can be reactivated and open up (collapse) under certain hydrologic conditions or when the land is put to new uses. Many karst features also propagate upward through overlying surficial deposits. Human activities also have caused development of evaporite karst, primarily in salt deposits. Boreholes (petroleum tests or solution-mining operations) or underground mines may enable unsaturated water to flow through or against salt deposits, either intentionally or accidentally, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence and/or catastrophic collapse. Evaporite karst, natural and human-induced, is far more prevalent than is commonly believed.  相似文献   

4.
Abstract Interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ18O = 0·9 ± 1·0‰, N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ18O = 2·1 ± 0·7‰, N = 48) than the hypersaline backreef dolomite (δ18O = 3·6 ± 0·9‰, N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.  相似文献   

5.
四川盆地三叠纪蒸发岩系普遍含碳酸盐组分 ,除方解石、白云石外 ,常出现大量菱镁矿。三种碳酸盐矿物的分布 ,可反映出盆地海水浓缩程度 ,进而分析成盐盆地浓缩淡化发展进程 ;可作为纵、横向蒸发岩系剖面成盐相的对比标志  相似文献   

6.
《Sedimentology》2018,65(6):1827-1858
Dedolomitization is a common diagenetic process in shallow burial environments and is often associated with sulphates in mixed carbonate‐evaporite successions. In these settings, elevated Ca2+/Mg2+ ratios necessary for dedolomitization result from the dissolution of sulphate phases by the incursion of undersaturated groundwater. Reported dedolomite textures from other studies are varied, but the most prevalent is a rhombic texture interpreted to result from the partial to complete pseudomorphic replacement of secondary dolomite rhombs formed in the burial diagenetic realm. In this study of primary cryptocrystalline to finely crystalline dolomicrites in the Prairie Evaporite Formation of north‐eastern Alberta, dedolomitization has resulted in sutured to loosely packed mosaics of dedolomite that range from subhedral to distinctly euhedral (rhombic) crystal fabrics; however, no prior aggrading neomorphism producing dolomite rhombs is evident in the precursor dolomicrites. Non‐pseudomorphic dedolomitization of the dolomicrites results in textures that include rhombic dedolomite crystals with cloudy cores comprising remnant dolomicrite and clear rims. These textures are similar to those observed in the pseudomorphic dedolomitization of secondary dolomite rhombs. The Prairie Evaporite Formation of north‐eastern Alberta has experienced extensive karstification near the erosional margin of the sedimentary succession. Dedolomitization of dolomicrites occurs in marker beds within the Prairie Evaporite succession associated with evaporite karstification. Along with stratigraphic and petrographic considerations, stable isotope results support the interpretation of a shallow dedolomitization event influenced by meteoric waters derived from the basin margin. Negative δ 18O and low δ 13C values (averages of −13·6‰VPDB and 0·5‰VPDB, respectively) of the dedolomite, compared with those of the primary dolomicrite (averages of −6·0‰VPDB and 1·2‰VPDB, respectively), point to isotopically light diagenetic fluids. These results show that rhombic dedolomite textures can form through shallow, non‐pseudomorphic dedolomitization of dolomicrites by meteoric fluids in the presence of sulphates, with resulting textures that are similar to the pseudomorphic dedolomitization of secondary dolomite rhombs.  相似文献   

7.
The Eifelian sedimentary record in the epeiric Baltic Basin contains an extensive brecciated interval known as the Narva Breccia. During recent decades, the origin of this breccia has been attributed to (1) palaeoseismicity, (2) extraterrestrial impact event or (3) diagenetic collapse. Our studies show the presence of two different breccia types that form 11 stratigraphic horizons and contain several laterally and stratigraphically unrelated breccias. The occurrence of Type‐1 and Type‐2 breccias, together with carbonate sabkha and carbonate inter‐ to supratidal tidal flat deposits on the basin margins, indicates synsedimentary brecciation formed in a sabkha environment. Type‐2 breccia could also originate from wave‐cycling loading.  相似文献   

8.
The lower Visean Belle Roche breccia (east Belgium) displays a number of features that indicate brecciation by evaporite dissolution collapse: the sharp lower contact of the breccia, the gradual transition into the overlying strata, the presence of semi-continuous beds within the breccia giving it a crude ‘stratification’, and the existence of several types of (calcite, dolomite and silica) evaporite pseudomorphs. Furthermore, the majority of the breccia fragments indicates hypersaline to lagoonal sedimentation conditions. Most of these fragments display an interlocking fabric. The interpretation is also supported by the existence of continuous evaporite beds replaced by carbonates (calcite and dolomite) both under and overlying the breccia. The brecciation history is characterized by gradual subsidence. Multiple brecciation episodes are recognized, and are best seen in the lower breccia which underwent at least two major brecciation episodes. Here, brecciated and veined breccia fragments occur within a microsparite (neomorphosed mud) matrix. Brecciation of these strata was due to the dissolution of interlayered evaporites. The second brecciation event relates to infiltration of meteoric water and to the dissolution of the remaining evaporites. This infiltration was probably triggered by the orogenic event at the end of the Visean (Sudetic orogenic phase). The whole breccia was finally cemented by a blocky calcite. The different lithologies and cements were characterized by their trace element (Mg, Sr, Na, Fe, Mn, K), insoluble residue and organic matter content. Carbon/oxygen isotope data of the cements and replaced evaporite layers helped to place the multiple collapse episodes within a general diagenetic model. Solution-reprecipitation processes within the original aragonite-dominated mud fragments, as well as in the early diagenetic dolomite fragments, have been recognized. The geochemical data show that these transformation processes occurred in equilibrium with the same fluid. These processes may have occurred within a freshwater lens very early in the diagenetic evolution or under shallow burial conditions. Cementation of blocky calcite occurred in a meteoric realm under burial conditions.  相似文献   

9.
Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high‐Mg calcite and/or Ca dolomite formed by an iso‐oriented assemblage of sub‐micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as sub‐micron flat and filamentous mucus‐like structures inside the interspaces of the high‐Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0·2 to 4 μm in diameter, sub‐spherical, rod‐like and filamentous forms, isolated or in colony‐like clusters. The co‐existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub‐micron polyhedral crystal formation of high‐Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub‐micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro‐environmental water conditions around the peloids change.  相似文献   

10.
Analysis of a 275 m‐thick section in the Milford Borehole, GSI‐91‐25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?‐late Chadian to late Holkerian (Viséan, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68 m‐thick, well‐developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian–Holkerian strata, transgressive–regressive carbonate units are recognized. These comprise high‐energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite‐bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif. The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base‐metal sulphide deposits. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

11.
Comparison of Upper Guadalupian fore-reef, reef and back-reef strata from outcrops in the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occurred in both areas. In both surface and subsurface sections, the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite. These evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solution-enlarged fractures and karstic sink holes into the underlying strata. Outcrop areas today, however, contain no preserved evaporites in reef and fore-reef sections and only partial remnants of evaporites are retained in back-reef settings. In their place, these rocks contain minor silica, very large volumes of coarse sparry calcite and some secondary porosity. The replacement minerals locally form pseudomorphs of their evaporite precursors and, less commonly, contain solid anhydrite inclusions. Some silicification, dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km; however, no calcite replacements were noted from any subsurface core samples. Subsurface alteration has also led to the widespread, late-stage development of large- and small-scale dissolution breccias. The restriction of calcite cements to very near-surface sections, petrographic evidence that the calcites post-date hydrocarbon emplacement, and the highly variable but generally ‘light’carbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic (telogenetic) phenomenon. Evaporite dissolution and calcitization reactions have only taken place where Permian strata were flushed with meteoric fluids as a consequence of Tertiary uplift, tilting and breaching of regional hydrological seals. A typical sequence of alteration involves initial corrosion of anhydrite, one or more stages of hydration/dehydration during conversion to gypsum, dissolution of gypsum and precipitation of sparry calcite. Such evaporite dissolution and replacement processes are probably continuing today in near-outcrop as well as deeper settings. This study emphasizes the potential importance of telogenetic processes in evaporite diagenesis and in the precipitation of carbonate cements. The extensive mineralogical and petrophysical transformations which these strata have undergone during their uplift indicates that considerable caution must be exercised in using surface exposures to interpret subsurface reservoir parameters in evaporitic carbonate rocks.  相似文献   

12.
四川盆地早三叠世飞仙关早期的环境基本上继承了晚二叠世长兴末期的格局。在川东北地区演化为碳酸盐蒸发台地。蒸发台地飞仙关组为一套含硫酸盐的富白云岩的蒸发潮坪沉积层系。下部为薄层的含石膏、硬石膏晶体、团块及肠状石膏层的泥晶灰岩、微晶白云岩组成的小潮差蒸发潮坪层序。中部为层状鲕粒白云岩、鲕粒灰岩的潮下鲕粒滩相与萨布哈环境的富层状膏岩的膏坪相、含石膏岩的膏云坪相组成的大潮差蒸发潮坪层序。上部为中薄层状富泥的含石膏质夹层的泥质泥晶灰岩、微晶白云岩组成的旋回性清楚的似蒸发潮坪层序。这三种层序的纵向组合在蒸发台地相区可以很好对比,尽管各层序厚度有明显变化。强烈白云石化的鲕粒白云岩是四川盆地飞仙关组大中型气藏的主要储层,依据鲕粒岩的相序特征可将其分为有障壁性质的台缘鲕粒坝和台内鲕粒滩两类。蒸发台地西缘的台缘鲕粒坝在飞仙关期基本稳定。东缘的台缘鲕粒坝在飞仙关中后期大幅度向盆地方向迁移。  相似文献   

13.
The Kelçyra area is emplaced in a foreland fold-and-thrust belt (FFTB), characterized by a westward thrusting with the Triassic evaporites as the major décollement level. Several secondary features related with this evolution, like backthrusting, folding, duplex structures, evaporite diapirism are present. During the FFTB evolution, the study area has been subjected to several fracturing events with associated stages of fluid migration. During the pre-deformational stage, complex textures such as crack-and-seal features most likely reflect expulsion of overpressured fluids. These fluids were dominantly host-rock buffered. Within the post-deformational stage, a meteoric fluid caused cementation and development of a karst network during a period of emergence after the thrust emplacement. Subsequently, Mg calcite reprecipitated in the more stable carbonate phase calcite and dolomite, which filled part of the karts network. The latter is finally dedolomitized and locally partially dissolved by a second meteoric fluid flow, which greatly increased the secondary porosity.  相似文献   

14.
Chemical sediments are common and diverse in the c. 3500 Myr old North Pole chert-barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi-permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea-shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect.  相似文献   

15.
ABSTRACT A number of non‐marine evaporite units composed of primary gypsum were deposited in saline lakes that developed in the southern Teruel Basin (NE Spain) during the Miocene. In the basin depocentre, a continuum of lacustrine evaporite lithofacies influenced by the activity of organisms is displayed. The Libros Gypsum was deposited in a deep lake, in which water stratification became unstable with progressive shoaling. Rhythmites, composed of laminae of pelletal gypsum and laminae of very fine lenticular gypsum crystals mixed with siliceous microorganisms, formed in addition to gypsum turbidites, intraformational gypsum breccias and slump structures. The pelletal laminae originated from the faecal activity of animals (crustaceans?) ingesting gypsum crystallites in the lake water during episodes of maximum evaporation, whereas the laminae of very fine lenticular gypsum mixed with microorganisms accumulated during episodes of relative dilution. In the wide marginal zones of the basin, the Libros Gypsum unit consists of massive to thin‐bedded bioturbated gypsum and thin‐bedded clotted gypsum, which formed in intermediate to very shallow (palustrine) water depths. The bioturbated gypsum lithofacies were produced by the action of diverse organisms, presumably worms and coleopterans, and chironomid larvae to a lesser extent; the massive lithofacies precipitated in very shallow water; and the thin‐bedded lithofacies formed in shallow to deeper settings. The thin‐bedded clotted gypsum is a relatively deep facies that may have diverse origins (e.g. bioturbation, compaction, disruption of soft sediments and early diagenesis). There is a well‐developed metre‐scale cyclicity in the marginal lake sequences, which is not observed in the inner lake deposits. This suggests a depth control in the various lacustrine subenvironments to record cyclic evaporitic processes. The isotopic composition of the gypsum indicates early sulphate‐reducing bacterial activity in the bottom of the lake and suggests that the sulphate was derived from the chemical recycling of Triassic evaporites of the country rocks.  相似文献   

16.
The Passaic Formation of the late Triassic Newark Supergroup is 2700 m thick and was deposited in series of wide, deep to shallow lacustrine environments in the Newark rift basin (eastern North America). The Passaic Formation can be divided into lower, middle, and upper sections based on depositional structures, composition and the distribution and morphology of its evaporites. Evaporites formed as a result of syndiagenetic cementation and/or displacive processes. Evaporitive minerals now include gypsum and anhydrite, although other mineral species, such as glauberite, may have originally existed. Most of the evaporites of the Passaic Formation occur within massive red mudstone and siltstone lithologies in the form of diffuse cements, void-fillings, euhedral crystals, crystal clusters and nodules. These evaporites grew displacively within the fine siliciclastic matrix as a result of changes in the hydrochemical regimes of the rift basin. A well-developed upward increase in the amount of evaporite material is present in the Passaic Formation. This resulted from: (1) long-term, progressive increase in aridity, and (2) significant increase in evaporation surface area of the basin during its tectonic evolution. A nonmarine source for the evaporites is evident from the isotopic data. Sulphate δ34S ranges from 11%. to 3.3%. CDT, while δ18O ranges from + 15.1%. to + 20.9%. SMOW, indicating derivation from early diagenetic oxidation of organic sulphur and pyrite within the organic-rich, lacustrine deposits. The 87Sr/86Sr ratios in sulphate are radiogenic (average 0.71211), showing the interaction of basin waters with detrital components and that the Newark Basin was isolated from the world ocean. Most of the original evaporites show evidence of diagenetic change to polycrystalline and polymineralic pseudomorphs now filled with recrystallized coarse-grained anhydrite (1–3 mm size) and low-temperature albite. Homogenization temperatures of fluid inclusions within the coarse-grained anhydrite indicate crystallization temperatures for anhydrite in the range of 150° to 280°C. Such elevated temperatures resulted from circulation of hot water in the basin. Later exhumation of these rocks caused partial to total replacement of anhydrite by gypsum in the upper part of the section. The resulting increase in volume due to hydration of anhydrite at shallow depths also emplaced non-evaporative satin-spar veins (fibrous gypsum) along bedding planes and in fractures. While the local geology of the Newark rift basin controlled the distribution of facies, the sedimentological development of the Passaic Formation evaporites resulted from the world-wide climatic aridity that prevailed during the late Triassic. because the Newark Basin sequence was only covered with about 3 km of sedimentary overburden that correspond to about 100°C and hence suggests that evaporites have experienced alteration by hot fluids. 5 As the Triassic marks the greatest evaporite formation world-wide and profound sense of parched continentality throughout the world existed before the final break-up of the Pangea, the Passaic Formation evaporites are an example of the influence of these palaeoclimatic conditions at the eastern margin of North America.  相似文献   

17.
特提斯域的密西西比河谷型(MVT)铅锌矿床   总被引：3，自引：0，他引：3       下载免费PDF全文
特提斯域是全球最重要的密西西比河谷型(MVT)铅锌矿床富集区,矿床广泛地分布在陆-陆碰撞造山带两侧的陆块上,就位于碰撞形成的褶皱-逆冲带和前陆带内,成矿通常发生在碰撞挤压作用晚期或之后、区域处于走滑或伸展的阶段。矿床、矿体的就位与张性断裂密切相关,主要控矿因素有蒸发盐底辟构造、碳酸盐岩溶蚀垮塌构造、蒸发盐溶蚀垮塌构造、高孔隙度白云岩、含重晶石地层等。油气流体在许多矿床和矿集区出现,其主要通过与硫酸盐发生反应为铅锌成矿提供还原硫。陆-陆碰撞的大地构造环境、大量的蒸发盐、丰富的油气流体是特提斯域富集MVT矿床的重要原因,域内寻找MVT矿床的潜力巨大。  相似文献   

18.
Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ18Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ13C values together with 87Sr/86Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ18Ο and δ13C values and 87Sr/86Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ18Ο, δ13C and 87Sr/86Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.  相似文献   

19.
Bedding‐parallel dolomite seams occur in a clay‐poor carbonate succession of the Apennines. The seams are composed of a high concentration of dolomite crystals compared to the hosting dolomitic limestone. SEM images document microcracking, and in many cases even crushing and fragmentation, of dolomite crystals and accumulation of non‐carbonate insoluble material both along micro‐stylolites within the seams and around dolomite crystals of the seams. We interpret the seams as hybrid structures between pressure‐solution seams and compaction bands, which formed during burial. The euhedral dolomite crystals scattered in the micritic matrix represent the insoluble residue produced by the progressive dissolution of calcite. As calcite dissolution proceeds, the concentration of dolomite crystals increases, eventually resulting in a dolomite seam in which locally a dolomite crystal‐supported texture is attained. At this stage, the dolomite crystals within the seam start to collide, crush and fragment, so that the dolomite seam behaves like a compaction band.  相似文献   

20.
This paper describes the occurrence of dolomite and the mechanism of dolomitization of the Upper Triassic-Lower Jurassic K?z?loren Formation in the autochthonous Bolkardag? unit of the middle Taurus Mountains in south western Turkey. Dolomites were analyzed for geochemical, isotopic and crystallographic variation. Dolomites occur as a replacement of precursor carbonate and cement. The dolomite crystals range from <10 to ~1000 μm existing as both replacements and cements. Sr concentrations range between 84 and 156 ppm, and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 0.0066 to 0.013 ratios. Dolomites are Ca-rich (with average CaCO3 and MgCO3 equal to 56.43 and 43.57 mol%, respectively) and they are non-stoichiometric, with an average Sr=116 ppm, Na=286 ppm, Mn=81 ppm, Fe=1329 ppm, and δ18O and δ13C ranges from –0.6‰ to –6.1‰ Pee Dee Belemnite [PDB], and +1.2 to +3.9‰ PDB. The North American Shale Composition [NASC]-normalized rare earth element (REE) values of the both limestone and dolomite sample groups show very similar REE patterns characterized by small positive Eu (mean=1.32 and mean=1.42, respectively) and slightly or considerably negative Ce (mean=0.61 and mean=0.72, respectively) anomalies and a clear depletion in all REE species. The K?z?loren Formation dolomites have been formed as early diagenetic from mixing zone fluids at the tidal-subtidal environment and at the late diagenetic from basinal brines at the shallow-deep burial depths.  相似文献   

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