Late Pleistocene mixing zone dolomitization, southeastern Barbados, West Indies

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.     

Primary dolomite in the Late Triassic Travenanzes Formation,Dolomites, Northern Italy: Facies control and possible bacterial influence

In the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ¹³C values range between ca ?6‰ and +4‰, with the most ¹³C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most ¹³C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ¹⁸O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ¹³C as low as ?6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ¹³C values of ca +3‰. Major shifts of δ¹³C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.     

Formation,diagenesis and palaeoenvironmental significance of upper Ediacaran fibrous dolomite cements

Neoproterozoic marine dolomite cements represent reliable, albeit complex, archives of their palaeoenvironment. Petrological and high-resolution geochemical data from well-preserved fibrous dolomite and pyrite in the upper Ediacaran (ca 551·1 to 548·0 Ma) Dengying Formation in south-west China are presented and discussed here. The aim of this research is to reconstruct the redox state of late Ediacaran shallow seawater and porewater in the Sichuan Basin using early marine diagenetic fabrics. Based on crystalline texture and axis, four basic types of fibrous dolomite cements formed penecontemporaneously in a microbialite reef setting at the platform margin: (i) bladed dolomites (replacement from a high-Mg calcite precursor); (ii) fascicular fast dolomites (replacement from an aragonitic precursor); (iii) fascicular slow dolomites; and (iv) radial slow dolomites. The latter two fabrics are considered direct marine porewater precipitates due to their length-slow character, cathodoluminescent zonation, and enriched copper and cobalt concentrations. Marine cements yield rare earth element and yttrium patterns comparable to modern seawater and represent a refined set of archive data relative to previously published bulk dolostones. Redox-sensitive elements and cathodoluminescence indicate that the fascicular fast dolomites formed in suboxic seawater, while fascicular slow and radial slow dolomites formed in euxinic marine porewaters. Microbial sulphate reduction during the formation of fascicular slow and radial slow dolomites is recognized by nanometre-scale spheroidal ankerite and sulphur-containing dolomite, and intergrown pyrite grains with U-shaped δ³⁴S transects. Data shown here suggest predominantly suboxic shallow late Ediacaran seawater and euxinic marine porewaters, with microbial activity promoting the direct precipitation of dolomite.     

The role and impact of freshwater–seawater mixing zones in the maturation of regional dolomite bodies within the proto Floridan Aquifer, USA

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.     

Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk,Switzerland

The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ¹⁸O, high ⁸⁷Sr/⁸⁶Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ¹⁸O_VPDB decrease from ?1·0‰ at the top to ?6·7‰ at the base and an ⁸⁷Sr/⁸⁶Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ¹⁸O_VSMOW = 0‰; ⁸⁷Sr/⁸⁶Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ¹⁸O_VSMOW = ?4·0‰; ⁸⁷Sr/⁸⁶Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ¹⁸O and high ⁸⁷Sr/⁸⁶Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps.     

Early and pervasive dolomitization by near-normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

The upper Palaeocene–lower Eocene Umm er Radhuma Formation in the subsurface of Qatar is dominated by subtidal carbonate depositional packages overlain by bedded evaporites. In Saudi Arabia and Kuwait, peritidal carbonate depositional sequences with intercalated evaporites and carbonates in Umm er Radhuma have been previously interpreted to have been dolomitized via downward reflux of hypersaline brines. Here, textural, mineralogical and geochemical data from three research cores in Qatar are presented which, in contrast, are more consistent with dolomitization by near-normal marine fluids. Petrographic relationships support a paragenetic sequence whereby dolomitization occurred prior to the formation of all other diagenetic mineral phases, including chert, pyrite, palygorskite, gypsum, calcite and chalcedony, which suggests that dolomitization occurred very early. The dolomites occur as finely crystalline mimetic dolomites, relatively coarse planar-e dolomites, and coarser nonplanar dolomites, all of which are near-stoichiometric (50.3 mol% MgCO₃) and well-ordered (0.73). The dolomite stable isotope values (range −2.5‰ to +1‰; mean δ¹⁸O = −0.52‰) and trace element concentrations (Sr = 40 to 150 ppm and Na = 100 to 600 ppm) are compatible with dolomitization by near-normal seawater or mesohaline fluids. Comparisons between δ¹⁸O values from Umm er Radhuma dolomite and the overlying Rus Formation gypsum further suggest that dolomitization did not occur in fluids related to Rus evaporites. This study provides an example of early dolomitization of evaporite-related carbonates by near-normal seawater rather than by refluxing hypersaline brines from overlying bedded evaporites. Further, it adds to recent work suggesting that dolomitization by near-normal marine fluids in evaporite-associated settings may be more widespread than previously recognized.     

Rare earth element geochemistry of dolomites in the Middle Devonian Presqu'ile barrier, Western Canada Sedimentary Basin: implications for fluid-rock ratios during dolomitization

Rare earth elements (REE) were determined in fine, medium and coarse crystalline replacement dolomites, and for saddle dolomite cements from the Middle Devonian Presqu'ile barrier from Pine Point and the subsurface of the Northwest Territories and north-eastern British Columbia. REE patterns of the fine crystalline dolomite are similar to those of Middle Devonian limestones from the Presqu'ile barrier. Fine crystalline dolomite occurs in the back-barrier facies and may represent penecontemporaneous dolomitization at, or just below, the sea floor. Medium crystalline dolomite is widespread in the lower southern and lower central barrier. Medium crystalline dolomite is slightly depleted in heavy REE compared with Devonian marine limestones and fine crystalline dolomite, and has negative Ce and Eu anomalies. Medium crystalline dolomites replaced pre-existing limestones or were recrystallized from earlier fine crystalline dolomites. During these processes, the REE patterns of their precursors were modified. Late stage, coarse crystalline replacement dolomite and saddle dolomite cements occur together in the upper barrier and have similar geochemical signatures. Coarse crystalline dolomites have negative Eu anomalies, and those from the Pine Point area also have positive La anomalies. Saddle dolomites are enriched in light REE and have positive La anomalies. The REE patterns of coarse crystalline dolomite and saddle dolomite differ from those of marine limestones and fine and medium crystalline dolomites, suggesting that different diagenetic fluids were responsible for these later dolomites. Although massive dolomitization requires relatively large volumes of fluids in order to provide the necessary amounts of Mg^2-. dolomitization and subsequent recrystallization may not necessarily modify the REE signatures of the precursor limestones because of the low concentrations of REE in most natural fluids. Thus, relative fluid-rock ratios during diagenesis may be estimated from REE patterns in the diagenetic and precursor minerals. Fine crystalline dolomites retain the REE patterns of their limestone precursors. In the medium and coarse crystalline dolomites the precursor REE patterns were apparently altered by large volumes of fluids involved during dolomitization. This study suggests that REE compositions of dolomites and their limestone precursors may provide important information about the relative amounts of fluids involved during diagenetic processes, such as dolomitization.     

中下扬子区三叠纪海相碳酸盐岩成岩作用和成岩环境

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。     

Evidence for high temperature and 18O‐enriched fluids in the Arab‐D of the Ghawar Field,Saudi Arabia

Using the clumped isotope method, the temperature of dolomite and calcite formation and the oxygen isotopic composition (δ¹⁸O_w) of the diagenetic fluids have been determined in a core taken from the Arab‐D of the Ghawar field, the largest oil reservoir in the world. These analyses show that while the dolomites and limestones throughout the major zones of the reservoir recrystallized at temperatures between ca 80°C and 100°C, the carbonates near the top of the reservoir formed at significantly lower temperatures (20 to 30°C). Although the δ¹⁸O values of the diagenetic fluids show large variations ranging from ca <0‰ to ca +8‰, the variations exhibit consistent downhole changes, with the highest values being associated with the portion of the reservoir with the highest permeability and porosity. Within the limestones, dolomites and dolomites associated with the zone of high permeability, there are statistically significant different trends between the δ¹⁸O_w values and recrystallization temperature. These relationships have different intercepts suggesting that fluids with varying δ¹⁸O_w values were involved in the formation of dolomite and limestone compared to the formation of dolomite associated with the zone of high permeability. These new data obtained using the clumped isotope technique show how dolomitization and recrystallization by deep‐seated brines with elevated δ¹⁸O_w values influence the δ¹⁸O values of carbonates, possibly leading to erroneous interpretations unless temperatures can be adequately constrained.     

Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China

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).     

川北元坝地区长兴组白云石化作用机制及其对储层形成的影响

通过岩心观察和薄片鉴定,在岩石学和矿物学特征分析基础上,结合白云石有序度测定,碳、氧同位素、锶同位素和稀土元素组成及配分模式分析,详细研究了川北元坝地区长兴组白云石化作用的特征、机制及模式,结果表明长兴组发育微晶白云石(岩)、粉-中晶他形白云石、粉-中晶自形白云石和异形白云石等四种类型,它们的有序度由低变高;白云石的碳、氧同位素、锶同位素和稀土元素组成及配分模式特征表明,长兴组微晶白云石(岩)、粉-中晶他形白云石、粉-中晶自形白云石主要形成于浓缩海水环境、正常海水环境、或者与正常海水相似的地层水环境中,并遭受过热液地质作用的改造,从微晶白云石(岩)→粉-中晶他形白云石→粉-中晶自形白云石是一个沉积埋藏过程中多阶段白云石化作用的产物,异形白云石则由热液作用形成;根据长兴组白云石的矿物学和地球化学特征及白云石化作用与层序和沉积相之间的关系,分别可以用准同生期蒸发泵白云石化作用模式、准同生期渗透回流白云石化作用模式、成岩早期浅埋藏状态下地层水白云石化作用模式和成岩晚期热液白云石化作用模式来解释微晶白云石(岩),粉-中晶他形白云石、粉-中晶自形白云石和异形白云石的形成。白云石化作用是有利于长兴组储层形成的建设性成岩作用。     

Genesis of evaporite-associated platform dolomites: case study of the Main Dolomite (Zechstein, Upper Permian), Leba elevation, northern Poland

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.     

Magnesian calcite cements and their diagenesis: dissolution and dolomitization, Mururoa Atoll

The most ubiquitous syn-sedimentary cements affecting Mururoa atoll are composed of magnesian calcite. Three main types are distinguished: fibrous, bladed and sparitic on the basis of petrography, morphology and MgCO₃ concentration of the constituting crystals, while peloid infills, a particular form of HMC chemical precipitation, also exist. Petrographic evidence and isotopic signatures are compatible with marine precipitation. Mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of Mg-calcite cements. These alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology. The first stage of the partial dissolution of Mg-calcite induces the development of chalky, white friable zones within the initially crystalline, hard cement layers. At ultrascale, this is due to the creation of micro-voids along the elongate cement fibres. Advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and/or between superposed cement layers. Mg-calcite dissolution is related to meteoric diagenesis during periods of Quaternary exposure. The creation of voids within Mg-calcite layers is due to the mechanical removal of previously altered calcium carbonate, a process suggesting marine or non-marine water flow, probably in the vadose environment. Selective dolomitization of Mururoa cements involves alternations of calcite and dolomite which form successive cement-like rinds within primary cavities. At Mururoa, these alternations are the result of selective dolomitization of the pre-existing Mg-calcite cements rather than successive precipitation of calcite and dolomite. Selective dolomitization of Mg-calcite cements at Mururoa indicates that a given cement succession is not necessarily a simple chronological sequence. Oxygen isotope values of dolomites are enriched in δ¹⁸6 by about 3‰ PDB within calcite-dolomite pseudo-alternations. The dolomitizing fluid at Mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization.     

Zebra dolomitization as a result of focused fluid flow in the Rocky Mountains Fold and Thrust Belt, Canada

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 (T_H = 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.     

Diagenesis of the Gully Oolite (Lower Carboniferous), South Wales

Deposition of the Gully Oolite was locally interrupted by emergence and a regionally extensive palaeosol is present at the top of the unit. Early diagenetic phases include isopachous, fibrous submarine cements, nonluminescent vadose cements, and mixing zone dolomite. Subsequent nonferroan phreatic cements are non- to dully luminescent and in restricted vertical intervals predate significant compaction. More usually, however, phreatic cements postdate extensive overpacking of allochems. Ooid isotopic composition (δ¹⁸O=-7·80° to -3·10° and δ¹³C = -2·38° to +3·28°) is similar to that of associated phreatic cements and the data suggest that the bulk of ooid stabilization and cementation occurred within meteoric groundwaters. The extensive allochem overpacking appears to have occurred during the first few tens of metres of burial and intergranular macroporosity was eliminated prior to deep burial. Fracturing of the Gully Oolite during the Hercynian Orogeny and subsequent post-orogenic uplift led to localized dolomitization, several generations of calcite veins, and the restricted occurrence of ¹⁸O depleted cements in inter- and intragranular microporosity. Some of the veins clearly relate to Triassic exhumation of the Carboniferous Limestone, but others may be related to post-Mesozoic uplift and erosion of South Wales. Fracture-associated dolomitization may have occurred within a large-scale post-orogenic groundwater system, with Mg²⁺ being supplied through the release of deeply buried diagenetic brines.     

Epigenetic dolomitization of the Přaídolí formation (Upper Silurian), the Barrandian basin,Czech Republic: implications for burial history of Lower Paleozoic strata

Stratabound epigenetic dolomite occurs in carbonate facies of the Barrandian basin (Silurian and Devonian), Czech Republic. The most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated Přídolí and Lochkov formations deposited on a carbonate slope. Medium-crystalline (100–400 μm), inclusion-rich, xenotopic matrix dolomite (δ ¹⁸O=−4.64 to −3.40‰ PDB;δ ¹³C=+1.05 to +1.85‰ PDB) which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type. Coarse crystalline saddle dolomite (δ ¹⁸O=−8.04 to −5.14‰ PDB;δ ¹⁸C=+0.49 to +1.49 PDB) which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event. In some vugs, saddle dolomite coprecipitated with petroleum inclusion-rich authigenic quartz crystals and minor sulfides which, in turn, were post-dated by semisolid asphaltic bitumen. The interpretation of the dolomitization remains equivocal. Massive xenotopic dolomite, although generally characteristic of a deeper burial setting, may have been formed by a recrystallization of an earlier, possibly shallow burial dolomite. Deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the Přídolí sequence appears as a viable explanation for this dolomitization overprint. Saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone. The presence of saddle dolomite, the fluid inclusion composition of associated quartz crystals, and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160°C. Although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures, burial heating beneath approximately 3-km-thick sedimentary overburden of presumably post-Givetian strata, no longer preserved in the basin, appears to be the most likely interpretation. This interpretaion may imply that the magnitude of post-Variscan erosion in the Barrandian area was substantially greater than previously thought.     

Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

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.     

Dolomite formation in breccias at the Musandam Platform border, Northern Oman Mountains, United Arab Emirates

The presence of dolomite breccia patches along Wadi Batha Mahani suggests large-scale fluid flow causing dolomite formation. The controls on dolomitization have been studied, using petrography and geochemistry. Dolomitization was mainly controlled by brecciation and the nearby Hagab thrust. Breccias formed as subaerial scree deposits, with clay infill from dissolved platform limestones, during Early Cretaceous emergence. Cathodoluminescence of the dolostones indicates dolomitization took place in two phases. First, fine-crystalline planar-s dolomite replaced the breccias. Later, these dolomites were recrystallized by larger non-planar dolomites. The stable isotope trend towards depleted values (δ¹⁸O: − 2.7‰ to − 10.2‰ VPDB and δ¹³C: − 0.6‰ to − 8.9‰ VPDB), caused by mixing dolomite types during sampling, indicates type 2 dolomites were formed by hot fluids. Microthermometry of quartz cements and karst veins, post-dating dolomites, also yielded high temperatures. Hot formation waters which ascended along the Hagab thrust are invoked to explain type 2 dolomitization, silicification and hydrothermal karstification.     

Effects of the Permian–Triassic boundary on reservoir characteristics of the South Pars gas field,Persian Gulf

The Permian–Triassic boundary (PTB) is a world‐wide event characterized by the most extensive mass extinction in the history of life. In the Persian Gulf, the rock record of this time interval host one of the most important hydrocarbon reserves in the world: the South Pars Gas Field and its southern extension, the North Dome (or North Field). These carbonate and evaporite successions were sampled in eight wells for petrographic, geochemical and porosity–permeability studies. An important characteristic of the Dalan and Kangan formations is the centimetre‐scale lithological heterogeneities caused by facies changes and diagenetic imprints that led to the compartmentalization of these reservoirs. These Permian–Triassic (P‐T) sediments were deposited in a shallow marine homoclinal ramp. The PTB in this hydrocarbon field is represented by a reworked coarse‐grained intraclastic/bioclastic grainstone facies deposited during a marine transgression. Prolonged subaerial exposure in the P‐T transition caused hypersaline and meteoric diagenesis, including extensive cementation, dolomitization and some dissolution, influencing reservoir characteristics of bordering units. Both δ¹⁸O and δ¹³C values in this succession mirror worldwide excursions typical of other P‐T sections, with some variations due to diagenetic alterations. A pronounced decline in ⁸⁷Sr/⁸⁶Sr values, reflective of global seawater geochemistry for most of the Permian is evident in our data. Reservoir quality declines through the late Permian, as a result of facies change and diagenesis. The Late Permian is succeeded by a Triassic transgressive facies and decline in reservoir quality. Copyright © 2009 John Wiley & Sons, Ltd.     

川东三叠系飞仙关组白云岩锶含量、锶同位素组成与白云石化流体

川东飞仙关组白云岩包括微晶白云岩、具原始结构的粒屑白云岩和结晶白云岩等3种主要的结构类型,其中结晶白云岩是重要的天然气储集岩。根据白云岩的锶舍量和锶同位素组成,结合相关岩石学和地球化学资料,本文研究了白云石化过程中锶的迁移和相应的同位素变化。三种类型白云岩具有类似的锶含量,但其(87)~Sr/(86)~Sr 比值均不同程度地高于同期海水值,锶在白云石和方解石(或文石)中分配系数的差别是影响白云石化过程中白云石锶含量最为重要的因素。不同流体中锶的混合方程计算表明:如果白云石化流体是大气淡水和海水的混合流体,则结晶白云岩的白云石化流体相当于94%的大气淡水和6%的海水混合,具原始结构的粒屑白云岩的白云石化流体相当于81%的大气淡水和19%的海水混合,微晶白云岩的白云石化流体相当于92%的大气淡水和8%的海水混合。但极低的锰含量、相对较高的锶含量和包裹体均一化温度等资料均不支持有94%的大气淡水参与了结晶白云岩的白云石化作用,其白云石化流体应是埋藏成岩过程中囚禁的近同期、但是非同期的海源流体,其来源可能与埋藏成岩过程中该时间段蒸发盐的溶解有关。锰含量、锶含量、(87)~Sr/(86)~Sr 比值都表明具原始结构的粒屑白云岩和微晶白云岩的白云石化过程都存在大气淡水的介入,微晶白云岩的白云石化可能与潮坪环境的蒸发泵机理有关;具原始结构的粒屑白云岩的白云石化可能与混合水白云石化作用有关,这类白云岩分布于向上浅滩化旋回滩体的顶部,这是埋藏前或浅埋藏阶段大气淡水与海水混合白云石化作用最容易发生地带。