Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean   总被引：6，自引：0，他引：6  

Hairuo Qing  Daniel W. J. Bosence  & Edward P. F. Rose 《Sedimentology》2001,48(1):153-163

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   

白云岩化研究的新进展   总被引：5，自引：0，他引：5  

覃建雄 《地质科技情报》1992,11(2):23-27

近年来,随着沉积学理论的发展,白云岩化的研究取得了重大进展,包括白云岩化的物质来源、白云岩化环境、白云岩化阶段和白云岩化机理等。其中以白云岩化机理中与区域构造运动有关的构造热液白云岩化、与火山活动有关的火山热液白云岩化和与变质作用有关的变质热液白云岩化最为重要。  相似文献   

Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China   总被引：5，自引：0，他引：5  

Daizhao Chen  Hairuo Qing†  Chao Yang† 《Sedimentology》2004,51(5):1029-1051

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 (T_h) 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 T_h values (171–209 °C), which correspond to trapping temperatures (T_r) 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 T_h 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   

加拿大阿尔伯达省西部上泥盆统凯恩组白云岩的成因   总被引：1，自引：0，他引：1  

金振奎 Spen.  RJ 《地质论评》1993,39(6):529-534

加拿大阿尔伯达省西部上泥盆统凯恩组主要由粉晶白云岩和细晶白云岩组成，另外还常见一种充填溶蚀孔洞的亮晶白云石。粉晶白云岩纹理、泥裂和鸟眼等构造发育，其δ＾１３Ｃ为＋１．０‰－＋３．０‰，δ＾１８Ｏ为－６．０‰－－４．３‰，为潮坪上混合水白云化形成。细晶白云岩分布于假整合面之下，其δ＾１３Ｃ为＋０．２‰－＋２．９‰，δ＾１８Ｏ为－６．９‰－－４．６‰，也是混合水白云化的产物。亮晶白云石洁净明亮、晶粒粗  相似文献   

地质记录中的海水白云岩化作用          下载免费PDF全文

朱同兴  黄志英 《沉积与特提斯地质》1993,(5):16-26

地质记录中的海水白云岩化作用是广泛存在的。它不仅发育于时代较新的始新世地层内(如太平洋中的 Enewetak 珊瑚岛地区),而且也发育于时代较老的震旦纪灯影期地层内(如云南东北部地区)。在以前的白云岩成因研究过程中,海水白云岩化作用的普遍性和重要性被大大地低估了。自从 Saller(1984)首次报道了正常海水白云岩化的实例以来,海水白云岩化作用的研究已取得了长足的进展,如 Kasner(1984);Land(1985);Smart(1988);Tucker(1990);朱同兴、罗安屏(1993)等。近年来,作者通过对云南东北部地区震旦系灯影组潮缘沉积物的岩石学、沉积相、古地理格局和沉积地球化学标志等综合研究,认为该套巨厚的白云岩层(400—1000m)的成因不是原生的,而是成岩交代形成的,其主要的形成机制之一就是成岩早期的海水白云岩化作用。海水白云岩化作用的发育主要受海平面变化、沉积作用类型以及古气候等条件的限制。造成滇东北地区灯影期海水白云岩化作用的主要机制可能是位于其西北部的青藏大洋内的冷洋流向东部沉积物内泵吸,和位于其东南缘的扬子边缘海的海水向西部沉积物内循环对流(柯特对流)。上述的泵吸作用与对流作用的相互叠加可使巨大体积的海水通过扬子海域,尤其是扬子台地西南边缘(滇东北地区)沉积物,从而使其发生巨大规模的海水白云岩化作用。由海水白云岩化作用形成的白云岩的结构有序度中等,I=0.68—0.82,其碳、氧同位素组成均显示海水碳酸盐来源,δ~(13)=+1.8—+2.6‰(PDB),δ~(18)O=+2.1—+2.7‰(PDB)(冷洋流来源,Enewetak Atoll,始新统)或-5.1—-6.5‰(受地热和大气淡水双重影响而偏负值,滇东北地区;灯影组)。灯影期海水白云岩化作用模式合理地解释了区域巨厚沉积物的块状白云岩化成因、充足的镁离子来源以及多余的钙离子的搬运等白云岩成因研究过程中的关键性问题。  相似文献   

鄂尔多斯盆地东部下奥陶统白云岩地球化学研究   总被引：10，自引：0，他引：10  

覃建雄  曾允孚 《矿物学报》1994,14(1):22-31

在岩石学研究的基础上，运用稳定同位素，微量元素，Ｘ射线衍射，阴极发光，扫描电镜及流体包裹体等技术和方法，对鄂尔多斯盆地东部下奥陶统白云岩进行综合研究表明，区内白云岩化具多期叠加的特点，白云岩储集性与白去岩组构，白云岩化程度及白云岩成因密切相关，按成岩阶段可划分为准同生白云岩，成岩（早期和晚期）白云岩和后生（早期及晚期）白云岩（石）其中，准同生白云岩属蒸发泵－渗透回流白云化成因：成岩白云岩早期和晚期  相似文献   

Late Pliocene to Holocene platform evolution in northern Belize, and comparison with coeval deposits in southern Belize and the Bahamas     

S.J. MAZZULLO 《Sedimentology》2006,53(5):1015-1047

Lithostratigraphy, depositional facies architecture, and diagenesis of upper Pliocene to Holocene carbonates in northern Belize are evaluated based on a ca 290 m, continuous section of samples from a well drilled on Ambergris Caye that can be linked directly to outcrops of Pleistocene limestone, and of overlying Holocene sediments. Upper Pliocene outer‐ramp deposits are overlain unconformably by Pleistocene and Holocene reef‐rimmed platforms devoid of lowstand siliciclastics. Tectonism controlled the location of the oldest Pleistocene platform margin and coralgal barrier reef, and periodically affected deposition in the Holocene. A shallow, flat‐topped, mostly aggradational platform was maintained in the Holocene by alternating periods of highstand barrier‐reef growth and lowstand karstification, differential subsidence, and the low magnitude of accommodation space increases during highstands. Facies in Pleistocene rocks to the lee of the barrier reef include: (i) outer‐shelf coralgal sands with scattered coral patch reefs; (ii) a shoal–water transition zone comprising nearshore skeletal and oolitic sands amidst scattered islands and tidal flats; and (iii) micritic inner‐shelf deposits. Four glacio‐eustatically forced sequences are recognized in the Pleistocene section, and component subtidal cycles probably include forced cycles and autocycles. Excluding oolites, Holocene facies are similar to those in the Pleistocene and include mud‐mounds, foraminiferal sand shoals in the inner shelf, and within the interiors of Ambergris and surrounding cayes, mangrove swamps, shallow lagoons, and tidal and sea‐marginal flats. Meteoric diagenesis of Pliocene and Pleistocene rocks is indicated by variable degrees of mineralogic stabilization, generally depleted whole‐rock δ¹⁸O and δ¹³C values, and meniscus and whisker‐crystal cements. Differences in the mineralogy and geochemistry of the Pliocene and Pleistocene rocks are attributed to variable extent of meteoric alteration. Dolomitization in the Pliocene carbonates may have begun syndepositionally and continued into the marine shallow‐burial environment. Positive dolomite δ¹⁸O and δ¹³C values suggest precipitation from circulating, near‐normal marine fluids that probably were modified somewhat by methanogenesis. Sedimentologic and diagenetic attributes of the Pliocene–Pleistocene rocks in the study area are similar to those in the Bahamas with which they share a common history of sea‐level fluctuations and climate change.  相似文献   

Genesis of island dolostones     

《Sedimentology》2018,65(6):2003-2033

Cenozoic ‘island dolostones’ are found on islands throughout the oceans of the world. Due to their geological youth and lack of deep burial, these dolostones provide an opportunity to resolve some of the mysteries surrounding the dolomite problem. In island dolostone bodies, which are of variable size and variable dolomitization, the petrographic and geochemical properties of the dolostones are characterized by geographic and stratigraphic variations. In the larger island‐wide dolostone bodies, like those found on Grand Cayman, there are progressive increases in mole %Ca (%Ca_mean: 53·9 to 57·6%), depletion of the heavier ¹⁸O and ¹³C isotopes (δ¹⁸O_mean: 3·6 to 2·1‰ _VPDB; δ¹³C_mean: 3·1 to 1·4‰ _VPDB), and changes from fabric‐retentive to fabric‐destructive fabrics and a decrease in the amount of dolomite cement from the coastal areas towards the centres of the islands, similar to the Little Bahama Bank. These changes define geographically concentric zones that parallel the coastlines and reflect geochemical modification of the dolomitizing fluid through water–rock interactions, mixing with meteoric water and the changes in the rate and flux of seawater as it flowed from coasts to island interiors. The pattern of dolomitization, however, is not consistent from island to island because geographic and stratigraphic variations, specific to each island, influenced groundwater flow pattern (for example, geometry and size of the islands; the porosity and permeability of the precursor limestone), the duration of the dolomitization reaction, and other factors. The geographic extent of dolomitization and variation in dolomite stoichiometry of island dolostones may be comparable to the reaction stages established in high‐temperature laboratory experiments.  相似文献   

Origin,characteristics and distribution of fault‐related and fracture‐related dolomitization: Insights from Mississippian carbonates,Isle of Man          下载免费PDF全文

James P. Hendry  Jay M. Gregg  Kevin L. Shelton  Ian D. Somerville  Stephen F. Crowley 《Sedimentology》2015,62(3):717-752

Viséan limestones on the Isle of Man host numerous examples of fault‐controlled and fracture‐controlled dolomitization, which have been investigated to determine their macro‐scale to micro‐scale characteristics, geofluid origin, timing and relation to basin evolution. Geobodies composed of fabric destructive, ferroan, non‐planar dolomite range from several centimetres to >300 m wide and tens to hundreds of metres long parallel to faults and/or fractures; they have sharply defined margins, cross‐cut stratigraphy and locally finger out along beds or bed boundaries for tens of metres. Larger geobodies accompany NNE–SSW extensional faults with substantial breccia zones. One of these bodies hosts a sphalerite‐rich breccia deposit cemented by dolomite. Saddle dolomite lines or fills vugs and fractures within dolomite geobodies, and is a minor late diagenetic phase in undolomitized limestones. Replacive dolomite has low matrix porosity owing to non‐planar texture and associated cementation, and there is no evidence for subsequent leaching. Three dolomite stages are discriminated by texture, cathodoluminescence petrography and electron microscopy. Disseminated ‘Dolomite 1’ is substantially replaced and may be residual early diagenetic dolomite. Pervasive ‘Dolomite 2’ and ‘Dolomite 3’ have overlapping carbon–oxygen–strontium isotopic and fluid‐inclusion characteristics that indicate precipitation from allochthonous, high‐temperature (98 to 223°C) and high‐salinity (15 to 24 wt% NaCl eq.) brines. These variably equilibrated with host limestones and mixed with resident pore fluids. Overlying mudrocks formed a seal for ascending fluids. Integration of data from the mineral deposit suggests that fault‐fracture systems tapped different deep‐seated fluid reservoirs at different temperatures, and implies fluid interactions with both metamorphic basement and sedimentary cover in large‐scale circulation systems. This phenomenon probably took place during Mesozoic rifting, although an earlier event at the end of the Early Carboniferous cannot be discounted. In either case, a transient heat flow anomaly, previously unrecognized in the Irish Sea region, may be required to account for the hottest fluids.  相似文献   

四川盆地三叠系飞仙关组气藏储层成岩作用研究拾零   总被引：33，自引：0，他引：33  

王一刚  文应初  洪海涛  夏茂龙  何颋婷  宋蜀筠 《沉积学报》2007,25(6):831-839

近年来在四川盆地三叠系飞仙关组鲕粒白云岩储层中发现的高含硫天然气三级储量已近1×1012 m3。鲕粒白云岩储层集中分布在含蒸发岩的层序中,储层中有残余石膏、硬石膏及它们的模孔、方解石铸模（假象）等,表明白云石化流体与蒸发海水有关。白云石化由层间古暴露面向下增强的成岩层序说明这种白云石化是成岩早期发生的,并可能有大气降水与蒸发海水混合的影响。白云岩样品各结构组分氧、碳同位素微区测试数据差别显著,这说明采用岩石混合样做研究白云石化成因的地球化学分析可能导致误解。飞仙关组的白云岩中多数保存了原岩结构幻影、结构残余或原岩的全部结构说明白云石化过程是等体积交代。白云岩储层中的孔隙是各种溶蚀孔而非白云石化等摩尔交代的体积收缩孔。飞仙关组高含硫气藏储层中沥青和溶蚀孔的关系表明深埋晚期溶蚀孔最发育。在溶液中碳酸的电离常数大大高于氢硫酸,飞仙关组高含硫气藏天然气组分中的CO2相对于H2S的质量亏损和储层中有富轻碳的高温方解石充填晚期溶孔、裂缝表明储层中的晚期溶解作用是由硫酸盐热还原过程（TSR）中生成的CO2引起的。  相似文献