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1.
Two oolites in the Dinantian (Mississippian/Lower Carboniferous) of Glamorgan, SW Britain, were deposited in similar depositional environments but have contrasting diagenetic histories. The Brofiscin and Gully Oolites occur in the upper parts of shallowing-upward sequences, formed through strandplain progradation and sand shoal and barrier growth upon a southward-dipping carbonate ramp. The Brofiscin Oolite is characterized by a first-generation cement of equant calcite spar, preferentially located at grain-contacts and forming non-isopachous fringes around grains, interpreted as meteoric vadose and phreatic in origin. Isopachous fibrous calcite fringes of marine origin are rather rare and occur only at a few horizons. Burial compaction was not important and porosity was occluded by poikilotopic calcite spar. Fitted grain-grain contacts locally occur and could be the result of near-surface vadose dissolution-compaction. Syntaxial overgrowths on echinoderm debris are common. Pre-compaction overgrowths are cloudy (inclusion-rich) and probably of meteoric origin, and post-compaction overgrowths are inclusion-free. By contrast, the Gully Oolite has little first-generation cement. However, marine fibrous calcite is common in oolitic intraclasts, as isopachous fringes of acicular calcite crystals closely associated with peloidal internal sediment; and early equant, drusy calcite spar occurs in the uppermost part of the Gully, beneath a prominent palaeokarst where pedogenic cements also occur. The major feature of Gully diagenesis is burial compaction, resulting in extensive grain-grain dissolution and microstylolitic grain contacts, and post-compaction poikilotopic spar occluded remaining porosity. The Brofiscin Oolite is pervasively dolomitized up-dip but the Gully Oolite for the most part only contains scattered pre-compaction dolomite rhombs and late veins of baroque dolomite, with less pervasive dolomitization. The difference in diagenetic style of the two Dinantian oolites is attributed to prevailing climate. The paucity of early meteoric cements in the Gully is a result of an arid climate, and this is supported by the nature of the capping palaeokarst. The abundant meteoric cements in the Brofiscin reflect a more humid climate, and effective meteoric recharge also resulted in up-dip pervasive mixing-zone dolomitization. The style of early diagenesis in these two oolites exerted a major control on the later burial diagenesis: in the Brofiscin, the early cements inhibited grain-grain dissolution and pressure solution, while these processes operated extensively in the Gully Oolite. Thus, prevailing climate can influence a limestone's diagenetic history from near-surface through into deep burial.  相似文献   

2.
The partly dolomitized Swan Hills Formation (Middle‐Upper Devonian) in the Simonette oil field of west‐central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post‐date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (Th=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (Tpeak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon‐bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more 87Sr‐rich fluid. Fabric‐destructive and fabric‐preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume‐like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on 87Sr/86Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and 206Pb/204Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity.  相似文献   

3.
The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate grains. Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filled with widespread, very coarse, blocky calcite spar. These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement and pore-filling calcites have δ13C values ranging from 0·5 to -25·3%, and δ18O values of -16·1 to -24·30 (PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid-Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with the in situ hydrocarbon-rich pore fluids and evaporite minerals, and precipitated calcite cements. The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non-ferroan), and hydrocarbon emplacement. The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.  相似文献   

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

5.
《Sedimentology》2018,65(3):745-774
This paper explores little investigated diagenesis of spicule‐dominated sediments, based on Permian spiculites and cool‐water carbonates of the Tempelfjorden Group in central Spitsbergen. Field observations, petrography, stable isotope geochemistry, and mineralogical and chemical analyses reveal that the strata have been subjected to multistage diagenesis as the result of silica phase transitions at medium burial depths and deep‐burial overprinting. The growth of silica concretions occurred during the opal‐A/opal‐CT conversion and was controlled by the content and distribution of clay and spicules in the sediment, resulting in a variety of megascopic silica fabrics. Opal‐CT was subsequently dissolved, and all silica is now in a stable quartz stage. Petrographically, the rocks are characterized by a variety of chalcedony and quartz cements which perfectly preserve precursor textures. Most cements precipitated from silica‐oversaturated fluids, and their shapes reflect the silica saturation state and geometry of the pore space. Some microquartz and cryptoquartz also formed by a solid–solid inversion (recrystallization) of chalcedony. The cements have δ 18O values between +30‰ and +20‰ Standard Mean Ocean Water and display a systematic depletion in 18O from the first to the last crystallized, interpreted to reflect a gradual increase in temperature during burial. The precipitation of quartz cements started in the Middle Triassic when the strata passed the 19°C isotherm at burial depths of ca 600 m, and was completed in the mid‐Cretaceous, 2·3 km beneath the sea floor at temperatures of 75°C. Late diagenetic overprinting of the chert includes fracturing, brecciation and cementation with carbonate cements having δ 18O values between +2‰ and −30‰ Pee Dee Belemnite and δ 13C values between +4‰ and −14‰ Pee Dee Belemnite; they are linked to hot solutions introduced during Cretaceous volcanism or Palaeogene tectonism. This study illustrates the diagenetic pathway during burial of spicule‐rich sediments in a closed system and thereby provides a baseline for studies of more complexly altered chert deposits.  相似文献   

6.
Early marine diagenetic dolomite is a rather thermodynamically-stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post-depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ13C and δ18O isotopes, 87Sr/86Sr ratios, and U-Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike-slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric-preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto-Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high-grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late-stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years. Evidence for this preservation includes preserved Late Triassic seawater δ13CDIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically-complex dolomite archives must be considered with caution.  相似文献   

7.
The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ18O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ13C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.  相似文献   

8.
An integrated approach consisting of fracture analysis, petrography, carbon, oxygen and strontium‐isotope analyses, as well as fluid‐inclusion micro‐thermometry, led to a better understanding of the evolution of fluid–rock interactions and diagenesis of the Upper Permian to Upper Triassic carbonates of the United Arab Emirates. The deposited carbonates were first marked by extensive early dolomitization. During progressive burial, the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture‐filling dolomite, quartz and calcite cements. After considerable burial during the Middle Cretaceous, sub‐vertical north–south oriented fractures (F1) were cemented by dolomite derived from mesosaline to hypersaline fluids. Upon the Late Cretaceous maximum burial and ophiolite obduction, sub‐vertical east–west fractures (F2) were cemented by dolomite (Dc2) and saddle dolomite (Ds) derived from hot, highly saline fluids. Then, minor quartz cement has precipitated in fractures from hydrothermal brines. Fluid‐inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot (200°C) saline brines (20 to 23% NaCl eq.). Through one‐dimensional burial history numerical modelling, the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200°C. Tectonically‐driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the Late Cretaceous maximum burial and lasted until the Oligocene–Miocene compressional tectonics and related uplift. The circulation of such hydrothermal brines led to partial dissolution of dolomites (Dc2 and Ds) and to precipitation of hydrothermal calcite C1 in new (mainly oriented north–south; F3) and pre‐existing, reactivated fractures. The integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history. Hence, this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes, which may subsequently affect reservoir properties.  相似文献   

9.
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   

10.
Petrographic and geochemical evidence shows convincingly that the diagenetic environment changes from marine phreatic through freshwater phreatic to vadose during lithification of the oolites and the overlying Coral Rag of the Osmington Oolite Formation in the Oxford—Berkshire area. This change is thought to be a consequence of tectonic uplift following deposition of the Coral Rag. Extensive neomorphic replacement of the corals took place in a freshwater phreatic environment created by tectonic uplift. Retention of Sr in the Coral Rag is related to differing diagenesis: pore-fluids remained for protracted periods within the Coral Rag as the downward flow was greatly retarded because of obliteration of porosity in the oolites by marine phreatic cementation before deposition of the Coral Rag.  相似文献   

11.
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.  相似文献   

12.
In order to understand the post-depositional history of carbonate rocks of Guri Member (Lower to Middle Miocene), three stratigraphic sections were selected in north Bandar-Abbas in southeast of Iran. Sampling was carried out, analyzed for selective parameters such as oxygen and carbon isotopic compositions (δ18O and δ13C) and interpreted in the present study. We recognized several diagenetic processes including micritization, cementation, neomorphism, compaction, dissolution, silicification, dolomitization, fracturing and vein filling. Some of the diagenetic processes occurred at different conditions, so in order to achieve precise interpretation, samples from different carbonate components such as, micrite, fracture cement, solution pore cement, intergranular cement, and some biotic allochems were analyzed. In this study micrite samples were subdivided into two groups including micro-spary and micrite. They were recognized under Cathodoluminescence microscope. In addition, micrite samples were classified into five groups based on their depositional environments: supratidal, lagoon, coral bar, open sea, and open basin. There were minor changes in stable isotope ratios based on the sedimentary environments, stratigraphy successions, and micro-spary or micrite properties. In this study, similar calcite cements in petrography studies were differentiated by stable isotope data. Those calcite cements have formed in different diagenetic environments such as meteoric and burial cements. Paragenetic sequence of carbonate rocks were interpreted by integration of petrographic and isotopic studies. We have reconstructed diagenetic models of Guri Member into four stages including marine, meteoric, burial, and uplifting.  相似文献   

13.
The Wilde Kirche reef complex (Early-Late Rhaetian) grew as an isolated carbonate structure within the shallow Kössen Basin. At the Triassic/Jurassic boundary a single brief (c. 10–50 ka) period of subaerial exposure occurred. The preserved karst profile (70 m thick) displays a vadose zone, enhanced dissolution at a possible palaeo-watertable (5–15 m below the exposure surface), and a freshwater phreatic zone. Karst porosity was predominantly biomouldic. Primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone; cavity networks developed preferentially in patch reef facies. Resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and Fe-Mn crust deposition on a sediment-starved passive margin. Fibrous calcite (FC). radiaxial fibrous calcite (RFC) and fascicular optic calcite (FOC) cements preserved as low Mg calcite (LMC) are abundant in primary and karst dissolution cavities. FC cement is restricted to primary porosity, particularly as a synsedimentary cement at the windward reef margin. FC, RFC and FOC contain microdolomite inclusions and show patchy non-/bright cathodoluminescence. δ18O values of non-luminescent portions (interpreted as near original) are − 1.16 to − 1.82%0 (close to the inferred δ18O of calcite precipitated from Late Triassic sea water). δ13C values are constant (+3 to + 2.2%0). These observations suggest FC, RFC and FOC were originally marine high Mg calcite (HMC) precipitates, and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression. The HMC to LMC transition may have occurred in contact with meteoric water only in the case of FC cement. The most altered (brightly luminescent) portions of RFC/FOC cements yield δ18O=−2.44 to − 5.8%0, suggesting HMC to LMC alteration at up to 34°C. in the shallow burial environment at depths of 180–250 m. Abundant equant cements with δ18O =−4·1 to −7.1%0 show crisp, uniform or zoned dull luminescence. They are interpreted as unaltered cements precipitated at 33–36°C at 200–290 m burial depth, from marine-derived fluids under a slightly enhanced geothermal gradient. Fluids carrying the equant cements may have induced the HMC to LMC transition in the fibrous cements.  相似文献   

14.
Partially dolomitized carbonate successions provide a good opportunity to understand the commonly multistage process of dolomitization. Petrographic methods, fluid inclusion microthermometry and stable isotope measurements were applied to reconstruct the diagenetic evolution and dolomitization of a partially dolomitized Carnian reef limestone from the Transdanubian Range, Hungary. The diagenetic history began with reef diagenesis and formation of dolomite micro‐aggregates in microbial fabric elements; this was followed by the development of euhedral porphyrotopic dolomite crystals through overgrowths around the previously formed dolomite micro‐aggregates during the earliest burial stage. Increasing burial resulted in the extension of the dolomite patches via formation of finely crystalline replacement dolomite. From the Late Norian, when the Carnian reef carbonates reached the depth of 1·0 to 1·8 km, the diagenetic evolution continued in an intermediate to deep‐burial setting. Contemporaneously, an extensional regime was established, leading to fracturing. The progressive burial resulted in the recrystallization of the pre‐existing dolomite with increasing temperature, while saddle dolomite cement was precipitated in fractures. In connection with the Alpine Orogeny, intense denudation took place during the Late Cretaceous, accompanied by fracturing. Similar tectonically controlled denudation and fracturing occurred in several stages during the Cenozoic. As a result of these processes, the studied Carnian carbonates were raised to a near‐surface position or became subaerially exposed, leading to dedolomitization of the last dolomite phase and precipitation of calcite cement in cavities and fractures. This study revealed that by investigating partially and selectively dolomitized rock types, it is possible to document and understand those stages of the multiple dolomitization process which can barely be detected in the completely dolomitized rock bodies. Recognition of the dolomitization phases could provide the basis for the analysis of their relations with the depositional, diagenetic and tectonic processes, and stages of basin evolution.  相似文献   

15.
ABSTRACT This paper examines the diagenetic history of dual (i.e. matrix and fracture) porosity reservoir lithologies in Cretaceous to Eocene carbonate turbidites of the Ionian fold and thrust belt, close to the oil‐producing centre of Fier–Ballsh (central Albania). The first major diagenetic event controlling reservoir quality was early cementation by isopachous and syntaxial low‐Mg calcite. These cements formed primarily around crinoid and rudist fragments, which acted as nucleation sites. In sediments in which these bioclasts are the major rock constituent, this cement can make up 30% of the rock volume, resulting in low effective porosity. In strata in which these bioclasts are mixed with reworkedmicrite, isopachous/syntaxial cements stabilized the framework, and matrixporosity is around 15%. The volumetric importance of these cements, their optical and luminescence character (distribution and dull orange luminescence) and stable isotopic signal (δ18O and δ13C averaging respectively; ?0·5‰ VPDB and +2‰ VPDB) all support a marine phreatic origin. Within these turbidites and debris flows, several generations of fractures alternated with episodes of cementation. A detailed reconstruction of this history was based on cross‐cutting relationships of fractures and compactional and layer‐parallel shortening (LPS) stylolites. The prefolding calcite veins possess orange cathodoluminescence similar to that of the host rock. Their stable isotope signatures (δ18O of ?3·86 to ?0·85‰ VPDB and δ13C of – 0·14 to + 2·98‰ VPDB) support a closed diagenetic rock‐buffered system. A similar closed system accounts for the selectively reopened and subsequently calcite‐cemented LPS stylolites (δ18O of ?1·81 to ?1·14‰ VPDB and δ13C of +1·52 to +2·56‰ VPDB). Within the prefolding veins, brecciated host rock fragments and complex textures such as crack and seal features resulted from hydraulic fracturing. They reflect expulsion of overpressured fluids within the footwall of the frontal thrusts. After folding and thrust sheet emplacement, some calcite veins are still rock buffered (δ18O of ?0·96 to +0·2‰ VPDB and δ13C of +0·79 to +1·37‰ VPDB), whereas others reflect external (i.e. extraformational) and thus large‐scale fluid fluxes. Some of these veins are linked to basement‐derived fluid circulation or originated from fluid flow along evaporitic décollement horizons (δ18O around +3·0‰ VPDB and δ13C around +1·5‰ VPDB). Others are related to the maturation of hydrocarbons in the system (δ18O around ?7·1‰ VPDB and δ13C around +9·3‰ VPDB). An open joint system reflecting an extensional stress regime developed during or after the final folding stage. This joint system enhanced vertical connectivity. This open joint network can be explained by the high palaeotopographical position and the folding of the reservoir analogue within the deformational front. The joint system is pre‐Burdigalian in age based upon a dated karstified discordance contact. Sediment‐filled karst cavity development is linked to meteoric water infiltration during emergence of some of the structures. Despite its sediment fill, the karst network is locally an important contributor to reservoir matrix porosity in otherwise tight lithologies. Development of secondary porosity along bed‐parallel and bed‐perpendicular (i.e. layer‐parallel shortening) stylolites is interpreted as a late‐stage diagenetic event associated with migration of acidic fluids during hydrocarbon maturation. Development of porosity along the LPS system enhanced the vertical reservoir connectivity.  相似文献   

16.
The Ulster White Limestone Formation is an unusual chalk because it underwent a period of post-depositional emergence and erosion, followed by burial under 1·5–2·0 km of Tertiary basalts. A high degree of pressure solution and cementation produced a well-lithified limestone with low porosities (2·3–10·4%). The Ulster White Limestone shows no evidence of thermally induced textural alteration, except for thin (<0·5 m) pseudospar contact recrystallization zones adjoining basalt dykes. Whole-rock δ18O values of samples not associated with basalt dykes range from - 3·26%o to - 6·50%o (PDB). The δ18O values of macropore cements range from - 4·96%o to - 11·52%o (mean=-8·27%o). Modelling of the diagenesis of the Ulster White Limestone using trace element concentrations and carbon, oxygen and strontium isotopic ratios of whole rock and cement samples suggests a low water-rock ratio and either marine or mixed marine-meteoric pore water environment during the main episode of recrystallization. The maximum possible burial temperature was modelled to be ? 105°C. The diagenetic history of the Ulster White Limestone is similar to that of North Sea chalks that are at comparable burial depths as the Ulster White Limestone after basalt deposition. The geochemical data show no indication of hydrothermal alteration associated with the overlying basalts. The degree of alteration of fine-grained limestones composed predominantly of low-magnesium calcite, such as the Cretaceous/Tertiary chalks, appears to be controlled largely by the burial (effective stress) history of the limestone.  相似文献   

17.
《Sedimentology》2018,65(6):1973-2002
Dolomitization is commonly associated with crustal‐scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass‐imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1·43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface‐breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0·7 m year−1 into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal‐scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization.  相似文献   

18.
Petrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO3, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low- to intermediate-Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO3 content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light-brown turbid fibrous calcite has low MgCO3 (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ13C and δ18O values of all fibrous calcite form a tight field (δ13C=1·7 to 3·1‰ PDB, δ18O= ? 2·6 to ? 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ18O values are larger than adjacent meteoric or burial cements, which have δ18O δ ? 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First-generation equant and bladed calcite cements, which pre-date fibrous calcite, are interpreted as unaltered, low-Mg calcite marine cements based on δ13C and δ18O data (δ13C = 2·3 to 2·7‰ PDB, δ18O= ? 2·8 to ? 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ18O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ18O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.  相似文献   

19.
《Sedimentary Geology》2001,139(3-4):285-317
Petrographic and geochemical data from five localities in the Ziz Valley of Morocco indicate that Jurassic limestones have undergone early diagenesis that varied with location from shelf to basinal settings, burial diagenesis that was most pronounced in basinal settings, and late diagenesis caused by compression and uplift of the High Atlas Mountains.Marine cements occur at all five localities from shelf-to-basin center, although cement types vary from peloidal microcrystalline cements updip on the shelf-to-equant calcite in basinal settings. Presence of moldic grains and/or Mg-poor, Fe-poor blocky cements suggest that meteoric waters influenced early diagenesis at all shelf localities and on an upturned fault block in the basinal region, leaving only one locality unaffected by early meteoric processes. 87Sr/86Sr ratios of 0.70810–0.70895 (greater than 87Sr/86Sr of coeval limestones), Mg contents that decrease upward from 47.5 to 43.0 mol% MgCO3, presence of dolomitized marine cements, and dolomite cements that postdate marine cements but predate meteoric-to-burial cements suggest that dolomitization and dolomite cementation at two shelf localities took place in mixed meteoric and marine waters early in diagenesis. However, poorer preservation of depositional fabrics, lower δ18O values, and larger and more anhedral crystals suggest that dolostones downdip underwent later modification during burial, whereas those updip did not.Compaction during diagenesis generated numerous concavo–convex and sutured intergranular contacts at updip shelf, downdip shelf, and basinal localities where earlier meteoric cementation was not extensive. Compaction was insignificant in more extensively cemented mid-shelf settings. High Sr (1200–3800 ppm) and Fe (1000–2300 ppm) contents in brachiopod grains suggest that LMC components underwent some modification during burial in basinal settings in Sr-rich reducing waters. Fe contents of late intergranular cements increase from 2000 ppm at the basin's edge to as much as 6000 ppm in the basin's center. Bedding-parallel stylolites occur at all localities.The most negative δ18O values of sparry dolomites near the Tizi n'Firest fault (−6.2‰ vs. PDB) imply diagenetic temperatures of 65–85°C assuming water δ18O values of 0.0–2.0‰ vs. SMOW. Those temperatures are much less than previous estimates of burial temperatures in the High Atlas basin. An isotopic gradient extrapolating to roughly 5‰/km in diagenetically modified dolostones likewise suggests a geothermal gradient less than gradients previously proposed for at least parts of the area.Comparison of morphologies of transverse stylolites, which are found at all localities, with morphologies of bedding parallel stylolites suggests that transverse stylolites formed due to compression during late diagenesis. Uplift accompanying that compression allowed influx of low-Mg waters that, along with other factors, caused calcitization of dolomites. The Fe concentration of calcite that fills late fractures increases from less than 2000 ppm at the basin center to values in excess of 3000 ppm at the basin edge, opposite trends in earlier cements and reflecting uplift of the High Atlas Mountains and resultant changes in patterns of groundwater flow.  相似文献   

20.
《Sedimentology》2018,65(1):123-150
The reconstruction of past diagenetic conditions in sedimentary basins is often under‐constrained. This results from both the analytical challenge of performing the required analyses on the minute sample amounts available from diagenetic mineral phases and the lack of tracers for some of the diagenetic parameters. The carbonate clumped isotope thermometry (Δ47) opens new perspectives for unravelling the temperatures of diagenetic phases together with the source of their parent fluids, two parameters that are otherwise impossible to constrain in the absence of exploitable fluid inclusions. Here is reported the study of a large number of sedimentary and diagenetic carbonate phases (from Middle Jurassic reservoirs of the Paris Basin depocentre) by combining detailed petrographic observations with a large number of Δ47 data (n  > 45) on a well‐documented paragenetic sequence, including calcite and dolomite burial cements. The data reveal carbonate crystallization at temperatures between 29°C and 98°C from fluids with δ 18Owater values between −7‰ and +2‰, in response to the progressive burial and uplift of the Paris Basin, throughout 165 Myr of basin evolution. Coupled with the time–temperature evolution previously estimated from thermal maturity modelling, these temperatures allow determining the timing of four successive cementation episodes. The overall data set indicates a history of complex water mixing with a significant contribution of hypersaline waters from the Triassic aquifers migrated upward along faults during the Cretaceous subsidence of the basin. Subsequent large‐scale infiltrations of meteoric waters induced a dilution of these pre‐existing brines in response to the Paris Basin uplift in the Tertiary. Overall, the data presented here allow proposing an integrated approach to characterize the cementation events affecting the studied carbonate reservoir units, based on temperature, oxygen isotope composition and salinity of the parent fluids as well as on petrographic grounds.  相似文献   

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