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1.
The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran.Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and δ18O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28 °C.Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The δ18O and δ13C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments. 相似文献
2.
Relatively few studies have so far addressed diagenetic processes in Heterozoan carbonates and the role that sediment composition and depositional facies exert over diagenetic pathways. This paper presents a study of Oligocene shallow-water, Heterozoan carbonates from the Maltese Islands. We investigate stratigraphic distribution, abundance and timing of diagenetic features and their relationship to sediment composition and depositional facies. The studied carbonate rocks comprise rud- to packstones of the Heterozoan association predominantly containing coralline red algae, bryozoans, echinoids and benthic foraminifers. XRD analyses show that all high-Mg calcite has been transformed to low-Mg calcite and that no aragonite is preserved. Diagenetic processes include dissolution of aragonitic biota, neomorphism of high-Mg calcitic biota to low-Mg calcite and cementation by fibrous, bladed, epitaxial and blocky cements. Stable isotopes on bulk rock integrated with petrographic data suggest that the study interval was not exposed to significant meteoric diagenesis. We interpret early cementation to have taken place in the marine and marine burial environment. The distribution and abundance of early diagenetic features, determining the diagenetic pathway, can be related to the primary sediment composition and depositional texture. Sorting and micrite content are important controls over the abundance of diagenetic features. 相似文献
3.
Theodore M. Present Melissa Gutierrez Guillaume Paris Charles Kerans John P. Grotzinger Jess F. Adkins 《Sedimentology》2019,66(7):2605-2626
Late Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with a sulphur isotopic composition distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition. 相似文献
4.
基于西沙永兴岛上最新钻孔（SSZK1）取得的55.92 m岩芯的 U?Th定年、矿物、薄片、主微量元素及碳氧稳定同位素等资料,开展了西沙群岛晚第四纪碳酸盐岩沉积相和淡水成岩作用的研究。根据不同的矿物组成特征,可将SSZK1钻孔岩芯分为上、中、下三段: 下 段（33.89~55.92 m,主要为低镁方解石）、中 段（18.39~33.89 m,主要为文石和低镁方解石）、上段（0~18.39 m,主要为文石、高镁方解石和低镁方解石）。由于下段碳酸盐岩几乎全为稳定的低镁方解石组成,碳氧同位素值的严重负偏和小幅度变化,可推断其经历了程度较大的淡水成岩作用。中段和上段还存在不稳定的文石和高镁方解石,碳氧同位素值相对下段正偏和高幅高频变化,推测其淡水成岩作用的程度比下段要小。中段碳氧同位素值高幅高频变化同时也说明该段的矿物纵向变化较复杂。这种矿物组成的复杂变化可能是由于晚第四纪海平面频繁变化,该段被大气水渗流带和潜流带交替占据引起的。主微量元素的变化同时受到淡水成岩作用和沉积环境的影响。在中段、下段中可识别出sq1、sq2、sq3、sq4四个完整的相旋回。Na2O,S,Sr 和碳氧同位素受到的淡水成岩作用而被消耗和负偏,且由于老一期的旋回经历了更长时间的淡水成岩作用,新、老旋回间的 Na2O,S,Sr含量值和碳氧同位素值有明显差异。利用新、老时期形成的旋回间淡水成岩作用剩余Na2O,S,Sr含量和碳氧同位素值的差别可以将新、老两个旋回区分开来。 相似文献
5.
阿姆河盆地卡洛夫-牛津阶碳酸盐岩储层地球化学特征和成岩流体分析 总被引：2，自引：1，他引：1
卡洛夫-牛津阶碳酸盐岩为土库曼斯坦阿姆河盆地主力天然气产层,以储层岩石学分析为基础,结合Fe、Sr、Mn微量元素和C、O、Sr同位素及流体包裹体地球化学特征,分析了储层成因和成岩流体性质,得出以下几点认识:(1)由厚壳蛤壳体测定的87Sr/86Sr比值在全球锶同位素地层曲线上可标定的年龄为157.2Ma,储层发育层位属于卡洛夫-牛津阶;(2)卡洛夫-牛津阶为一持续海侵-海退旋回,持续海侵期是礁、滩相储层形成期,而持续海退期为致密盖层发育期;(3)较高的Fe和Sr及较低的Mn含量,以及伴随成岩强度加大δ13C变化不大而δ18O向负值方向偏移的演化趋势,证明成岩作用发生在缺乏大陆淡水影响和以温度为主控因素的还原性封闭系统中;(4)综合同位素地球化学与流体包裹体和镜质体反射率特征,可确定洛夫-牛津阶碳酸盐岩仍处在中成岩阶段;(5)礁、滩相灰岩的溶蚀、埋藏白云化和充填缝、洞的方解石是不同成岩阶段的水-岩反应产物,成岩流体主要来自于深部富Sr的地层热卤水。 相似文献
6.
V. N. Kuleshov 《Lithology and Mineral Resources》2001,36(5):429-444
Isotope-geochemical features of the formation of carbon and oxygen isotopic compositions in sedimentary and diagenetic carbonates are considered. Isotopic criteria for the identification of early diagenesis zone are proposed. The transition from sedimentogenesis to diagenesis (upper boundary of the early diagenesis zone) is accompanied by the alteration of carbon isotopic composition in the HCO–
3(hydrosphere)–HCO–
3(pore water of sediment)system. The lower boundary of early diagenesis zone is registered by the alteration of oxygen isotopic composition in the pore water of sediments and authigenic carbonates. 相似文献
7.
P. P. Chakraborty A. Sarkar S. K. Bhattacharya P. Sanyal 《Journal of Earth System Science》2002,111(4):379-390
Enriched13C/12C ratios with δ13C ∼3%0 (w.r.t PDB) of two Late Riphean (∼ 700-610 Ma) intracratonic carbonate successions viz., Bhander Limestone of Vindhyan
Basin and Raipur Limestone of Chattisgarh Basin suggest higher organic productivity during this period. This view is supported
by sedimentological evidence of higher biohermal growth and consequent increase in depositional relief in the low gradient
ramp settings inferred for these basins. Oxygen isotope analysis of these carbonates show distinct segregation between enriched
deeper water carbonate mudstone and depleted shallow water stromatolite facies that received fresh water influx. This shows
that facies-specific analyses can be useful in understanding the depositional setting of these sediments. 相似文献
8.
The diagenesis of carbonate platform sediments is controlled by the original facies and mineralogy, climate, sea-level changes and burial history; these controls are clearly seen in the diagenesis of the Urgonian platform carbonates of SE France. Early diagenesis in the Urgonian platform included the precipitation of marine cements, dissolution of rudist shells and minor karstification. Diagenetic features produced during this phase were controlled by several falls in relative sea-level during the Barremian to mid-Aptian punctuating platform sedimentation, the original mineralogy of the sediment and the prevailing semi-arid/arid climate in the region at this time. Following a relative sea-level rise and further sedimentation, progressive burial of the platform led to minor compaction, followed by precipitation of coarse, equant, zoned to non-luminescent, calcite cement. This cement was cut by later stylolites, suggesting a relatively shallow-burial origin. Stable isotope (mean values - 7.94%δ18O and 0.36%δ13C) and trace element (mean values of Fe 334 ppm, Mn 92 ppm and Sr 213 ppm) data suggest that these cements precipitated from meteoric fluids at temperatures slightly elevated relative to depositional temperatures. A variable thickness of replacive dolomite which occurs preferentially within the shelf-margin facies of the lower part of the Urgonian post-dates mechanical fracturing and chemical compaction, but pre-dates the main phase of stylolitization. It is probable that the dolomitizing fluid was sourced by the early compaction-driven release of connate fluids held within the underlying muddy units. The burial history of these rocks suggests that calcite cementation and dolomitization took place at relatively shallow burial depths (1–1.5 km). The overall diagenetic history of the Urgonian Limestone Formation is a reflection of the pre-conditioning of the platform limestones by climate, sea level, tectonics and the shallow burial depths experienced by the platform during the later Mesozoic. 相似文献
9.
《Precambrian Research》2002,113(1-2):43-63
Carbon, oxygen and strontium isotope compositions of carbonate rocks of the Proterozoic Vindhyan Supergroup, central India suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter Proterozoic. The carbonate rocks of the Lower Vindhyan Supergroup from eastern Son Valley and central Vindhyan sections show δ13C values of ∼0‰ (V-PDB) and those from Rajasthan section are enriched up to +2.8‰. In contrast, the carbonate rocks of the Upper Vindhyan succession record both positive and negative shifts in δ13C compositions. In the central Vindhyan section, the carbonates exhibit positive δ13C values up to +5.7‰ and those from Rajasthan show negative values down to –5.2‰. The δ18O values of most of the carbonate rocks from the Vindhyan Supergroup show a narrow range between –10 and –5‰ (V-PDB) and are similar to the ‘best preserved’ 18O compositions of the Proterozoic carbonate rocks. In the central Vindhyan and eastern Son Valley sections, carbonates from the Lower Vindhyan exhibit best-preserved 87Sr/86Sr compositions of 0.7059±6, which are lower compared to those from Rajasthan (0.7068±4). The carbonates with positive δ13C values from Upper Vindhyan are characterized by lower 87Sr/86Sr values (0.7068±2) than those with negative δ13C values (0.7082±6). A comparison of C and Sr isotope data of carbonate rocks of the Vindhyan Supergroup with isotope evolution curves of the latter Proterozoic along with available geochronological data suggest that the Lower Vindhyan sediments were deposited during the Mesoproterozoic Eon and those from the Upper Vindhyan represent a Neoproterozoic interval of deposition. 相似文献
10.
碳酸盐胶结物中氧碳同位素组成研究是分析成岩过程中流体-岩石相互作用的重要技术方法.综合运用岩石学、矿物学和地球化学方法，对辽河盆地西部凹陷沙河街组砂岩中碳酸盐胶结物的化学组成和碳酸盐胶结物及成岩流体同位素组成特征进行系统分析.研究表明，研究区碳酸盐岩主要为方解石和白云石，胶结物主要类型为嵌晶式胶结、孔隙式胶结、斑块状胶结和星点状胶结.碳、氧稳定同位素组成能有效地反映成岩-成矿流体及其他物质的来源，碳酸盐胶结物与现今浅层地下水氧同位素组成差异巨大而与变质水同位素组成具有相似性，反映了盆地演化过程中活动热流体对成岩作用的影响.包裹体的氢、氧同位素组成可表征成矿溶液的演化特征，砂岩碳酸盐胶结物包裹体更富集氢的轻同位素和氧的重同位素，表明发生了明显的“氧-18漂移”.碳酸盐胶结的成矿溶液表现出“受热雨水”的同位素组成特征，反映了深源活动热流体对成岩作用的影响. 相似文献
11.
JOHN D. HUMPHREY 《Sedimentology》1988,35(2):327-348
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
12.
A. A. Makhnach B. G. Pokrovsky O. V. Murashko O. L. Petrov 《Lithology and Mineral Resources》2020,55(1):24-35
The behavior of stable carbon and oxygen isotopes in carbonates during the deposition and diagenesis of sediments in the bioproductive Upper Famennian Pripyat Trough (southern Belarus) is discussed. Limestones and clayey limestones (Corg 0.92 ± 0.11%) are characterized by very low δ13C values (–9.6 ± 0.3‰). Parental sediments of these rocks were deposited in the shallow-water zone during slow downwarping episodes of the seafloor. Lithification of the sediments took place in oxidative conditions of the diagenesis zone. Organic matter was actively oxidized by free oxygen. Carbon dioxide with isotopically light organic carbon formed in this process was used during the crystallization of diagenetic carbonates that are visually indiscernible from the sedimentational variety. Marls, clayey marls, and carbonate-bearing clays (Corg 6.02 ± 0.80%) are characterized by δ13C values as high as –3.5 ± 0.6‰. In combustible shales (Corg >10%), δ13C value is ‒1.2 ± 0.6‰). The clayey rocks mark the episodes of sedimentation in relatively deep-water conditions that appear during the uncompensated sagging of the basin floor. The diagenetic zone with free oxygen was significantly decreased or absent at all. Here, oxygen of marine sulfates was the main or single oxidizer of organic matter (sulfate reduction). The sulfate oxygen is a weaker oxidizing agent than free oxygen. Therefore, much more organic matter was retained and fossilized in clayey rocks than in carbonate rocks. Organic carbon released during the sulfate reduction and mobilized later for the diagenetic carbonate formation was insufficient for the significant decrease of δ13C values relative to the marine carbonate standard. Isotopic composition of carbonate oxygen in the studied rocks is invariable and does not depend on the content of clay and organic matter. In these rocks, δ18O values are at the level (approximately –5‰) shown for the Famennian in the Global Chemostratigraphic Chart. This fact is consistent with the supposition that oxygen isotope composition of atmospheric precipitates, which influenced the rock formation via continental flow, is close to that of sea water in low latitudes where the territory of Belarus was located in the Late Devonian. 相似文献
13.
NIGEL H. PLATT 《Sedimentology》1989,36(4):665-684
The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non-marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline. Carbon and oxygen stable isotope analyses from the carbonates show non-marine values with ranges of δ13 from ? 7 to ? 11‰and δ18 from ? 3 to ? 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil-derived CO2. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic modification of lake carbonates. 相似文献
14.
Cyclostratigraphy and high-frequency carbon isotope fluctuations in Upper Cretaceous shallow-water carbonates, southern Italy 总被引：2，自引：0，他引：2
Francesco P. Buonocunto Mario Sprovieri† Adriana Bellanca† Bruno D'Argenio‡ Vittoria Ferreri‡ Rodolfo Neri† Gaetano Ferruzza† 《Sedimentology》2002,49(6):1321-1337
Abstract A detailed carbon isotope study has been carried out on a Santonian (Upper Cretaceous) carbonate platform succession that crops out at Monte Sant'Erasmo (southern Italy). Previous centimetre‐scale studies on this succession have shown that high‐frequency eustatic changes, resulting from the Earth's orbital fluctuations, controlled the hierarchical organization of the depositional and early diagenetic features in elementary cycles, bundles (groups of three to five elementary cycles) and superbundles (groups of three or four bundles). The elementary cycles, which correspond to single beds, suggest a control caused by Earth's precession; the bundles and superbundles record the short (≈ 100 kyr) and long (≈ 400 kyr) eccentricity periodicity respectively. The δ13C signal of the Monte Sant'Erasmo succession is cyclic in nature and may be considered to be a reliable proxy for the sedimentary evolution (and related sea‐level history) of the analysed sequence. The carbon isotope cyclicity is recorded at bundle and superbundle level, but it is not evident at the scale of the elementary cycles, at least with the sampling interval used in this study. Spectral analysis of the δ13C record shows two main peaks corresponding to the short‐ and long‐eccentricity periodicity, whereas the precession signal is not evident in the power spectrum. In addition, lithofacies analysis shows that, in each bundle (and superbundle), higher C isotope values occur in sediments characterized by marine cements, whereas lower values are normally found in more restricted deposits overprinted by early meteoric diagenesis. Early diagenesis, driven by periodic sea‐level fluctuations, developed in either shallow‐subtidal (marine diagenesis) or subaerial‐exposed (meteoric overprint) sedimentary environments and directly influenced the carbon isotope signature. As a consequence, the δ13C record at Monte Sant'Erasmo reflects high‐frequency climatic oscillations controlling both environmental and early diagenetic changes. The long‐term isotopic record is similar to that of contemporaneous pelagic sections in England and elsewhere in Italy. It is concluded that the δ13C signature of shallow‐water carbonates, such as those of Monte Sant'Erasmo, offers great potential for correlation with coeval sections, including those of the pelagic realm. 相似文献
15.
土壤次生碳酸盐碳氧稳定同位素古环境意义及应用 总被引：9，自引：1，他引：9
土壤次生碳酸盐碳氧稳定同位素特征是反映古气候与古环境的重要代用指标,其碳氧稳定同位素组成分别受土壤CO2中C同位素组成和大气水的O同位素控制。在一定深度的土壤中,土壤次生碳酸盐δ^13C就主要受当地植物类型(C3植物和C4植物等)控制。土壤次生碳酸盐样预处理中剔除土壤中原生碳酸盐以及有机物污染尤为重要。土壤中次生碳酸盐C、O稳定同位素地球化学在土壤发生学、古气候恢复、古生态重建以及全球变化研究中应用日益广泛,但解译时可能受应用年代范围、成岩作用、原生和次生碳酸盐混杂、土壤碳酸盐多元发生等因素影响,其应用机理和范围还需进一步探讨。 相似文献
16.
《Journal of Asian Earth Sciences》1999,17(1-2):183-201
Modern and Tertiary carbonate production is, and was, extensive and diverse in the seas surrounding Borneo, and mirrors the variety of carbonate depositional systems seen in SE Asia. The availability of favourable conditions for carbonate sedimentation around Borneo was related to a combination of factors, including tectonic setting, the formation of large basinal areas, differential subsidence providing shallow marine areas, a tropical climate and a range of local factors, such as currents or limited clastic input. A detailed sedimentological and diagenetic study was undertaken of middle Eocene to Plio–Pleistocene carbonates which developed in the north Kutai Basin and the Mangkalihat Peninsula, northeast Kalimantan. Carbonate sedimentation in this area occurred in a range of depositional environments, from mixed carbonate clastic shelves, localised and transient shoals or reefs, a variety of platform top settings to deep water redeposited carbonates. An understanding of carbonate depositional environments, spatial facies relationships, and diagenesis is essential in order to develop models for these carbonates which can be used as predictive tools in the subsurface. This study also helps to evaluate tropical carbonate development in SE Asia and the evolution of sedimentary environments in Borneo during the Cenozoic. 相似文献
17.
The Zechstein Basin of Poland was an area of widespread cyclical deposition of carbonates and evaporites during Late Permian time. The Zechstein shelves, along both the northern and the southern margins of the basin, were sites of shallow-water sedimentation during the formation of the Main Dolomite and Platy Dolomite, two widespread carbonate units. These units consist of oolitic, peloidal, skeletal, micritic and evaporitic carbonates formed in depositional settings ranging from open marine to coastal (lagoonal, sabkha and salina). Although originally deposited as limestones, the Main Dolomite and Platy Dolomite are inferred to have been completely replaced by dolomite through very early stage (essentially penecontemporaneous) reflux of hypersaline brines. The dolomites of the two basin margins, however, have very different petrographic and isotopic characteristics. Many northern shelf dolomites show early stage calcitization (dedolomitization) and even, in some cases, evidence of a subsequent redolomitization event. These northern shelf samples also have a broad range of carbon and oxygen isotopic ratios (up to 12%0 for oxygen). Samples from the southern shelf, on the other hand, are petrographically much simpler; they do not show complex calcitization and redolomitization patterns. Likewise, their isotopic values are much more tightly clustered, with only about a 5%0 range of oxygen isotopic ratios. The differences between dolomites of the same age from the northern and southern margins are best explained by regional variations in river water influx during episodic exposure events associated with regional or global sea-level fluctuations. The distribution of clastic terrigenous materials and palaeokarstic features indicate that areas of the northern shelf had extensive river input, an influx largely lacking on the southern shelf. Early formed dolomites appear to have been calcitized during sea-level lowstands through the infiltration of meteoric fluids into the evaporitic dolomites created during the previous highstand. In some cases, redolomitization occurred when meteoric fluids were again replaced by hypersaline brines during subsequent sea-level highstands. Although repeated sea-level fluctuations are clearly evident in these strata, it is likely that associated climatic changes (rainfall variations) also played a role in forming these complex diagenetic patterns. Age-equivalent strata from Texas and New Mexico (from sites at much lower palaeolatitudes) show no such alteration patterns; samples from Greenland (slightly higher palaeolatitudes) show even more intense diagenetic alteration during depositional cycles. Thus, the examination of patterns of diagenesis may be useful in interpreting ancient, palaeolatitudinally sensitive climate patterns. 相似文献
18.
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The Early Cretaceous Fahliyan Formation (middle part of the Khami Group), is one of the important reservoir rocks in the Zagros Fold-Thrust Belt. The Zagros Fold-Thrust Belt is located on the boundary between the Arabian and Eurasian lithospheric plates and formed from collision between Eurasia and advancing Arabia during the Cenozoic. In this study area, the Fahliyan Formation with a thickness of 325 m, consists of carbonate rocks (limestone and dolomite). This formation overlies the Late Jurassic Surmeh Formation unconformably and underlies the Early Cretaceous Gadvan Formation conformably at Gadvan Anticline. The formation was investigated by a detailed petrographic analysis to clarify the depositional facies, sedimentary environments and diagenetic features in the Gadvan Anticline. Petrographic studies led to recognition of the 12 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environments. The absence of turbidite deposits, reefal facies, and gradual facies changes show that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. The diagenetic settings favored productioning a variety of features which include cements from early to late marine cements, micritization, dolomitization, compaction features, dissolution fabric, and pores. The diagenetic sequence can be roughly divided into three stages: (1) eugenic stage: marine diagenetic environment, (2) mesogenic stage: burial environment, and (3) telogenic stage: meteoric diagenetic environment. 相似文献