Pervasive Dolomitization Research Articles

The effects of pervasive dolomitization on the geochemical records of Cambrian series-3 evaporative carbonates in Bachu Uplift, China

The effects of pervasive dolomitization on the geochemical records of Cambrian series-3 evaporative carbonates in Bachu Uplift, China

Diagenetic history and porosity evolution of the Middle Permian clastic-carbonate mixed system, Indus Basin, Pakistan: Implications for reservoir development

This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin, Pakistan. Multiple data sets including outcrop, petrography, cathodoluminescence, scanning electron microscopy (SEM), mineralogy, and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis. The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend. The low values of δ18O of the target strata, relative to average values of the Permian carbonate, hints to diagenetic alteration in the strata. The data sets used in this study reveal modification of the strata in four environments, that is, i) early marine diagenesis indicated by micritization, pervasive dolomitization and isopachous fibrous cements, followed by ii) meteoric dissolution, and iii) shallow burial diagenetic processes including the precipitation of blocky cement, compaction of skeletal and non-skeletal allochems, and stylolites, and iv) a deep burial environment, characterized by pressure solution, and micro-fractures. The clastic intervals host subangular to subrounded quartz grains, floating textures, and almost complete absence of deleterious clay minerals, consequently resulting in the preservation of primary porosity. The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity. The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution. The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction, precipitating sulfides within the pores. The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit, and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies. However, both intervals show very low permeability values probably due to isolated moldic pores and intense micritization. Therefore, clastic intervals may provide an opportunity to serve as a moderate reservoir; however, the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential.

Temporal variations in Rare Earth Element distributions in the Cenozoic succession and modern sediments of the Cayman Islands

The Rare Earth Elements (REEs) in limestones and dolostones that develop on isolated oceanic islands, like the Cayman Islands, were acquired from the seawater in which they formed and various diagenetic processes that led to their lithification and, in some cases, pervasive dolomitization. Modern sediments from North Sound, Grand Cayman, contain up to 30.93 ppm ∑REEs and 45.23 ppm ∑REEs + Y, whereas modern corals contain up to 2.30 ppm ∑REEs and 3.65 ppm ∑REEs + Y. The limestones and dolostones in the Brac Formation (late Oligocene), the Cayman Formation (Miocene), and Ironshore Formation (Pleistocene) generally contain <7 ppm and <10 ppm, ∑REEs and ∑REEs + Y, respectively. In contrast, the Pedro Castle Formation (Pliocene) contains up to 14.18 ppm ∑REEs and 20.81 ppm ∑REEs + Y. The Distribution Coefficients for the REEs from the modern Cayman corals are similar to those determined for corals elsewhere in the world. In the modern sediments from North Sound there is a high correlation between the ∑REEs + Y and Fe and Al but a low correlation with Na and Mn. In contrast, the correlation between the ∑REEs + Y and other elements in the limestones of the Ironshore Formation and dolostones/limestones of the Brac Formation and Cayman Formation is low. The light REEs were >Limit of Quantification (LoQ) in most samples, whereas the medium REEs and heavy REEs were commonly <LoQ. There is no recognizable pattern to the presence/absence of the light, medium, and heavy REEs. Collectively, these data show that (1) the REEs in the limestone/dolostone succession on Grand Cayman are similar to those from Cayman Brac, (2) dolomitization does not seem to have modified the REEs of the original limestones, and (3) the content of the REEs of the limestones and dolostones of the Pedro Castle Formation is higher than those in the Brac Formation and Cayman Formation. This may indicate that the concentration of the REEs in the seawater during the Pliocene highstand was higher than those when the original sediments of the Brac Formation and Cayman Formation were deposited.

Diagenetic Study of Marrón Emperador Ornamental Stone (Upper Cretaceous, SE Spain)

The “Marrón Emperador” ornamental stone is known for its characteristic deep brown colour filled with white spots and veins. It consists of a brecciated dolostone with different generations of calcite/dolomite veins and veinlets that represent repeated episodes of fracture opening and partial or complete cementation, which likely corresponds to individual stages of fluid expulsion. Mineralogical, petrographic and geochemical studies point to the formation of these rocks through brecciation, dolomitization and rapid cementation processes in an active tectonic regimen. The composition and textural features of the stratiform dolomite geobody point to a structurally controlled dolomitization model. The overall breccia geometry, breccia texture and vein characteristics are all consistent with a brecciation origin driven by hydraulic fracturing, with subsequent calcite precipitation in open space and partial solution replacement of clasts. A paragenetic sequence includes: (1) marine sedimentation of original tidal carbonate sediments; (2) early lithification and marine cementation; (3) burial diagenesis with early fracturation of limestones; (4) entrance of dolomitizing fluids through fractures causing pervasive dolomitization (brown dolostones) and dolomite cements (fracture-lining and saddle dolomites); (5) fracturation by hydraulic overpressure under an active tectonic regime; (6) calcite cementation (white veins and veinlets); and (7) uplift and meteoric diagenesis producing dedolomitization, karstification and local brecciation.

Types, Petrophysical Properties and Pore Evolution of Late Ediacaran Microbial Carbonates, Tarim Basin, NW China

AbstractThe Upper Ediacaran microbial carbonates of the Tarim Basin are potential reservoir geobodies for future hydrocarbon exploration with rising interest in exploration for deeply‐buried reserves. However, little knowledge has been acquired on the types of microbial carbonates that are present, the properties of the reservoir and the pore evolution, hampering predictions of high‐quality reservoirs in these carbonates. Integrated with petrography and in‐situ U‐Pb dating geochronology analyses, this study aims to clarify the types of microbial carbonates present and to reconstruct the pore evolution processes of the potential reservoir rocks. The Upper Ediacaran microbial carbonates of the Tarim Basin can be divided into four types, based on their features in terms of different scales (macro‐ to micro‐): microbial laminite, stromatolite, spongiomicrobialite and microbial‐peloidal wackestone/mudstone. Petrophysical properties show that all these microbial carbonates have low porosity and very low permeability with poor connectivity. These carbonates were subject to long‐term and complex diagenetic processes, mainly consisting of dissolution, compaction, pervasive dolomitization, cementation and fracturing. The most important reservoir spaces are contributed by vugs and dissolution‐enlarged pores, which are likely to have been associated with the widespread uplift of the Aksu area in the terminal Ediacaran. In contrast, the cementation of the fine‐to‐medium crystalline dolomite greatly reduced the pre‐existing pores. Pore types are closely related to different microbial fabrics, which played an important role in the pore evolution of the microbial carbonates.

Diagenesis and pervasive dolomitization of pre- and post evaporite shallow marine carbonates, Ras Banas Peninsula, Red Sea Coast, Egypt

The present study deals with diagenesis and dolomitization modeling of Middle Miocene – Quaternary sedimentary sequence outcropped at Ras Banas Peninsula, Egyptian Red Sea Coast. This sequence is comprising successively overlapped clastics, carbonates, and evaporites attaining 254 m total thickness resting on the Pre-Cambrian basement rocks in the Peninsula. Diagenetic history and dolomitization were carefully investigated using several techniques including petrographic microscope, XRD, SEM, cathodoluminescence, atomic absorption, and isotopes of oxygen and carbon. The conspicuous phenomena is the pervasive dolomitization of directly pre- and post evaporite limestone components. These dolomites are mostly nonstochiometric, poorly to moderately ordered crystals. Based on all the results obtained from this study, the dolomitization processes of Ras Banas Peninsula are the result of complex geologic setting, paleo-climate changes, tectonic activity, and sea-level fluctuations. The proposed graphic model describes dolomitization mechanism which refers to mixed meteoric-hypersaline water environments.

Dolomitization of the Middle Jurassic limestones at the Vajont Canyon (Southern Alps, Italy): Fault-controlled dolomitization by hypo-to mesosaline fluids

The Middle Jurassic limestones of the Vajont Formation, which constitute major parts of the flanks of the Vajont Canyon (Southern Alps, Italy), have been subjected to pervasive dolomitization. The dolomites are localized within fault damage zones. Based on field and petrographic characteristics, two dolostone groups can be differentiated. Group 1, being the most prevailing, corresponds to replacive matrix dolostones. They form non-stratabound plume-like geobodies. They also occur as infill in infrequent veins in the host limestones that occur adjacent to these replacive plume-like dolostone bodies. Group 2, that is less common, consists of saddle dolomite cements. It occurs in breccia and veins or makes up zebra dolostones. The latter cements frequently affect the replacive group 1 dolostones. Fluid inclusion analyses of both dolostone groups indicate comparable salinity values of 1.2–5.3 eq. wt.% NaCl with homogenization temperatures of 70–108 °C, which in addition of overlapping 87Sr/86Sr ratios (0.70811–0.70846) reflect similar parental fluids. The estimated salinities, in combination with δ18O values (−5.5 to −14.8‰ V-PDB), indicate significant contributions of 18O-depleted fluids postulated to be modified meteoric waters. This origin is supported by the low trace element contents measured in the studied dolostones. These data support that the fault-controlled dolostones in the studied Vajont area precipitated from hot hypo- to mesohaline fluids. These fluids could have circulated along inherited Mesozoic syn-rift fault and fracture corridors. However, another possibility is that fluids circulated along fault and fracture systems that developed during the Neo-Alpine Orogeny (Miocene). Both could have served as efficient deep-seated fluid migration conduits as well as relatively shallow ones facilitating the downward flow of topography-driven meteoric waters.

Development and dolomitization of Anisian isolated carbonate platforms in the Transdanubian Range, Hungary

Abstract In the Middle Anisian, extensional tectonic movements led to the development of isolated carbonate platforms in the area of the southwestern part of the Transdanubian Range. The platforms are made up of meter-scale peritidal–lagoonal cycles bounded by subaerial exposure surfaces. One of the platform successions (Tagyon Platform) consists predominantly of limestone that contains partially and completely dolomitized intervals, whereas the other one (Kádárta Platform) is completely dolomitized. Drowning of the platforms took place in the latest Pelsonian to the early Illyrian interval when submarine highs came into existence and then condensed pelagic carbonate successions with volcanic tuff interbeds were deposited on the top of the drowned platforms from the late Illyrian up to the late Ladinian. The comparative study of dolomitization of the coeval platforms, affected by different diagenetic histories, is discussed in the current paper. Traces of probably microbially-mediated early dolomitization were preserved in the slightly dolomitized successions of the Tagyon Platform. This might also have been present in the successions of the Kádárta Platform, but was overprinted by geothermal dolomitization along the basinward platform margin and by pervasive reflux dolomitization in the internal parts of the platform. The Carnian evolution of the two submarine highs was different, and this may have significantly influenced the grade of the shallow to deeper burial dolomitization.

Integrated petrophysical and microfacies analyses for a reservoir quality assessment of the Asmari Dolostone sequence in the Khesht Field, SW Iran

The Asmari Formation is one of the most important prolific reservoirs along both sides of the Persian Gulf. The present study aims to characterize the fractured dolomite of the Asmari reservoir sequence in SW Iran. Such characterization can be achieved through a petrophysical reservoir characterization using core data integrated with a detailed microfacies analysis and a detailed study on the dominant diagenetic features. The detailed microfacies analysis indicates that the Asmari reservoir can be discriminated into seven non-clastic microfacies (laminated to massive anhydrite, sandy dolomudstone, peloidal bioclastic dolowackestone, peloidal foraminiferal dolowackestone/dolopackstone, peloidal bioclastic dolopackstone/dolograinstone, framestone, and dolograinstone microfacies). These microfacies have been primarily deposited in tidal flat, lagoonal, and carbonate shoal depositional environments. Petrophysically, some X-Y plots were established to present the available petrophysical core data (porosity ∅He, horizontal and vertical permeabilities, kH and kV, respectively) to enable a mathematical reservoir modeling. Besides, the reservoir quality has been defined using the flow zone indicator (FZI), reservoir quality index (RQI), and the effective pore radius (R35). Based on these petrophysical core data and quality parameters, the studied samples can be subdivided into five reservoir rock types (RRTs). Also, reservoir zonation on the borehole scale using the improved stratigraphic modified Lorenz technique (ISML) indicated that the characterized reservoir can be subdivided into four hydraulic flow units (HFU-1, HFU-2, HFU-3, and HFU-4). The HFU-2 (dolograinstone, dolopackstone, and framestone samples) is characterized by the best reservoir quality (average ∅He = 7.8%, kH = 95.2 md, RQI = 0.84 µm, FZI = 15.9 µm, and R35 = 10.5 µm); it is composed of RRT1 and RRT2 samples. On the other side, the lowest reservoir quality is assigned to the HFU-3 which is composed of the RRT5 (dolomudstone and evaporates, average ∅He = 11.5%, kH = 0.24 md, RQI = 0.04 µm, FZI = 0.22 µm, and average R35 = 0.21 µm). The prevailed reservoir characteristics of the Asmari dolostone sequence in SW Iran are mostly controlled by replacive and pervasive dolomitization, fracturing, and cementation by anhydrite.

Control of climate, sea-level fluctuations and tectonics on the pervasive dolomitization and porosity evolution of the Oligo-Miocene Asmari Formation (Dezful Embayment, SW Iran)

The Asmari reservoir properties in various hydrocarbon fields across the Dezful Embayment (SW Iran) are modified by dolomitization. Petrographic and geochemical analyses of the dolomitized Asmari Formation units highlight their origin in the Bibi Hakimeh and Gachsaran oilfields. The petrographic studies revealed that dolomite in the Asmari Formation can be classified texturally (based on crystal sizes) into three main groups: (i) very fine crystalline (D1, <20 μm), (ii) fine to medium crystalline (D2, 20–150 μm) and (iii) coarse crystalline dolomites (D3, >150 μm). Petrography, fluid inclusions, stable isotopes (δ13C, δ18O), radiogenic isotopes (86Sr/88Sr) and major and trace elemental analyses were used to interpret the origin and paragenetic sequence of each dolomite group. Pervasive dolomitization prevails in the upper part of the Asmari Formation where very fine and medium crystalline dolomites with partially and fully fabric-retentive and fabric destructive textures, replaced micrite and allochems of the platform-top facies, respectively. The parent fluids of the very fine dolomites originated from contemporaneous Oligo-Miocene seawaters produced in a sabkha environment. The fine to medium dolomites (D2) most likely shared the same seawater origin and relate to refluxing, sabkha-derived fluids. The medium to coarse crystalline dolomite has more depleted oxygen isotopes than the former, very fine to fine crystalline, dolomites suggesting a burial dolomitization origin. The estimated high salinities (≈19 to 20 wt% NaCl) together with compatibility of oxygen and strontium isotope data with evaporative seawater values, suggest Oligo-Miocene seawater was the parent fluid for precipitation of coarse and cement phase of the medium crystalline dolomites. This study reveals that reactivated basement faults with associated fractures have probably played an important role in convecting and heating evaporative seawater along their planes. This heated seawater provided the required fluid for recrystallization of coarser matrix and cement phases of medium (D2) and coarse (D3) dolomites at depth. Therefore, the interaction of climate, sea-level changes, and tectonics played a critical role in formation and development of pervasive dolomitization in the Asmari Formation. Additionally, dissolution (partly as a consequence of dolomitization) was one of the key processes during early diagenesis in the formation of different types of porosity such as biomolds, vugs and intraparticles porosity. This study shows that the reservoir quality improves from finely crystalline dolomite towards samples dominated by coarsely crystalline dolomites.

Dolomitization of Late Norian carbonate deposits of restricted basin facies in the Keszthely Mts., Transdanubian Range, Hungary

In the Transdanubian Range (Hungary), a wide spectrum of Triassic dolomites is known. Mechanism of dolomitization of the platform carbonate successions was subject of a number of studies but the study of dolomitization of basinal carbonates is very limited. Petrographical and isotope–geochemical characteristics of the Upper Triassic dolomitized carbonate deposits, formed in a fault-controlled intraplatform basin, and interpretation of the dolomite-forming processes are presented in the current paper. From the latest Carnian to Middle Norian under semi-arid climatic conditions density-driven flux of seawater derived mesohaline fluids was the dominant mechanism of the near-surface pervasive dolomitization of the thick platform carbonate succession. In the late Middle Norian incipient rifting of the Alpine Tethys led to establishment of an extensional structural regime and onset of the formation of the Kössen Basin. In the study area, above the dolomitized platform carbonate succession, platform-derived carbonate sediments were accumulated in a fault-bounded, semi-restricted sub-basin of the Kössen Basin, whereas talus breccias and debrites were deposited near a basin-bounding master fault. The basin deposits (lower and upper members of the Rezi Dolomite Formation) were subject of early diagenetic dolomitization in shallow burial setting. Enhanced salinity seawater was the dolomitizing fluid; the synsedimentary fracturing may have promoted the fluid flow and thereby the dolomitization. Thick-bedded carbonates (middle member of the Rezi Dolomite Formation) representing a progradational tongue of the ambient platform are intercalated between the basin deposits. The dolomitization of this unit can be explained by the reflux model.

Late Cretaceous volcanism and fluid circulation in the South Atlantic: Insights from continental carbonates in the onshore Namibe Basin (Angola)

The Namibe Basin developed in the Early Cretaceous as part of the South Atlantic rift system between Africa and South America. Magmatic activity occurred during the syn-rift (Valanginian-Hauterivian) and post-rift (Coniacian-Early Campanian) phases, with the latter triggering fluid circulation events that had significant impacts on reservoir quality and hydrocarbon prospectivity. The continental carbonates of the Mariquita Member (Mb.) developed during the Late Cretaceous magmatic pulse and record the associated fluid circulation. Thus, study of these carbonates provides insights into paleo-fluid sources and circulation pathways, which can ultimately help to constrain diagenetic phases, their effects on reservoir quality, and hydrocarbon re-mobilisation.This study analyses the tectonostratigraphic setting, facies and diagenesis of the Mariquita Mb. carbonates by integrating field, petrographic and geochemical data. The carbonates are stratigraphically bound by Upper Cretaceous magmatic rocks, and constitute a series of fault-associated spring mound systems that pass laterally into lacustrine sediments. The main diagenetic events include pervasive dolomitisation, silicification and multi-stage fracturing. Fluid temperatures during dolomitisation were between ∼32 and 56 °C, while silicification occurred between ∼70 and > 260 °C. Fluids likely followed fault-controlled convective circulation pathways. The fluid compositions comprised infiltrated/modified meteoric and marine waters, as suggested by negative carbon isotopes, enriched oxygen and strontium isotopes, and REE data. Fluid modification occurred via interaction with subsurface rock units, such as basement, volcanic/siliciclastic aquifers and evaporites. However, higher temperatures during silicification could suggest late contributions from magmatic fluids too. Bitumen within porosity possibly reflects regional hydrocarbon mobilisation processes driven by magmatic fluids.

Channelized CO2-Rich Fluid Activity along a Subduction Interface in the Paleoproterozoic Wutai Complex, North China Craton

Greenschist facies metabasite (chlorite schist) and metasediments (banded iron formation (BIF)) in the Wutai Complex, North China Craton recorded extensive fluid activities during subduction-related metamorphism. The pervasive dolomitization in the chlorite schist and significant dolomite enrichment at the BIF–chlorite schist interface support the existence of highly channelized updip transportation of CO2-rich hydrothermal fluids. Xenotime from the chlorite schist has U concentrations of 39–254 ppm and Th concentrations of 121–2367 ppm, with U/Th ratios of 0.11–0.62, which is typical of xenotime precipitated from circulating hydrothermal fluids. SHRIMP U–Th–Pb dating of xenotime determines a fluid activity age of 1.85 ± 0.07 Ga. The metasomatic dolomite has δ13CV-PDB from −4.17‰ to −3.10‰, which is significantly lower than that of carbonates from greenschists, but similar to the fluid originated from Rayleigh fractionating decarbonation at amphibolite facies metamorphism along the regional geotherm (~15 °C/km) of the Wutai Complex. The δ18OV-SMOW values of the dolomite (12.08–13.85‰) can also correspond to this process, considering the contribution of dehydration. Based on phase equilibrium modelling, we ascertained that the hydrothermal fluid was rich in CO2, alkalis, and silica, with X(CO2) in the range of 0.24–0.28. All of these constraints suggest a channelized CO2-rich fluid activity along the sediment–basite interface in a warm Paleoproterozoic subduction zone, which allowed extensive migration and sequestration of volatiles (especially carbon species) beneath the forearc.

Application of fluorescent light microscopy to enhanced lithofacies analysis of Jurassic carbonate rocks, SE Bohemian Massif

Upper Jurassic limestones and dolomites of the Vranovice Formation occur on the southeastern slopes of the Bohemian Massif below the Western Carpathian thrust belt. They represent a special type of reservoir rocks in the potential geothermal systems and exhibit highly variable porosity, permeability, and flow mechanisms due to different type of diagenesis and pervasive dolomitization. Fluorescence and translucent light microscopy was used for more detailed petrographic analysis of Vranovice limestones and dolomites. Isopachous, syntaxial and dolomitic cements were distinguished along with fossils recrystallization. Two generations of dolomite were identified – replacement dolomite and zonal cementation dolomite, what suggests a number of changes in the formation water chemistry during the dolomitization process. Extent of the observed dolomitization is inversely related to the porosity of the Jurassic rocks. Postdiagenetic generation of fissures cutting the strata due to tectonic deformation is documented by bright fluorescent color. In the studied samples of Vranovice limestones and dolomites the fissures represent the most important migration pathways, more than pore throats themselves. Further genetic information on petoleum migration is interpreted from intercrystalline porosity filled with precipitated solid bitumen. Present results provide basis for further investigations in pore-water-rock interactions and changes in chemical conditions during diagenesis.

Open thermal convection dolomitization: an example from East Yunnan (China)

AbstractDolostones are widely developed in the middle Permian rocks of East Yunnan, China, mainly in the shoal-facies Maokou Formation. The previously reported dolostone formation mechanisms cannot explain the distribution and geochemical characteristics of these dolostones, in particular their strontium, magnesium and oxygen isotope signatures. To help predict the distribution of dolostone reservoirs and reduce the exploration risk and cost, this study proposes a new model of dolomitization: open thermal convection dolomitization. In this new dolomitization model, Mg2+ in dolomitizing fluids originates mostly from seawater, with a minor component coming from deep hydrothermal fluids. Elevated heat flux (in this case due to the nearby Emei mantle plume) causes spatial temperature variations in the fluid along the circulation flow pathways, resulting in fast and pervasive dolomitization of limestone. The proposed model not only explains the characteristics and distribution of dolostones in the study area but also serves as a reference for predicting the distribution of dolostones in other areas subjected to thermal convection.

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

AbstractThe 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.

The interaction of tectonics, climate and eustasy in controlling dolomitization: A case study of Cenomanian–Turonian, shallow marine carbonates of the Iberian Basin

abstractDuring the Cretaceous, high global sea‐level and low latitudinal temperature variations led to the growth of epeiric carbonate platforms. Platform‐scale dolomitization of these platforms is not common, reflecting the low Mg/Ca ratio of seawater and a humid climate. This study describes the processes governing pervasive dolomitization of a land‐attached carbonate platform within the Iberian Basin. Dolomite is planar to sub‐planar with a geochemical signature consistent with dolomitization from penesaline seawater. Dolomitization was most pervasive during a 1 Myr period in the middle Cenomanian, by repeated reflux of seawater from brine pools formed on the top of a southward‐prograding carbonate platform. Tilting and structural reorganization in the Upper Cenomanian led to a reversal in polarity of the platform, and dolomitization was restarted by the northward reflux of seawater. Rising relative sea‐level and oceanic acidification led to back‐stepping of the platform such that the supply of dolomitizing fluids was cut off. In the Lower Turonian, pervasively dolomitized rudist rudstone facies in the south of the study area indicate that dolomitization restarted, either penecontemporaneously or later, from highly evaporated Campanian–Maastrichtian seawater. A systematic increase in dolomite crystal size up‐section ties broadly, but not entirely, to stratigraphy. It is possible that these textural differences reflect changes in fluid chemistry, limestone permeability or precursor rock texture. However, the lack of stratigraphic conformance, and the preservation of the earliest‐formed dolomite only in the oldest sediments, could indicate a progressive recrystallization of early‐formed dolomite through repeated reflux of brines. As such, the succession appears to preserve a fossilized record of dolomite recrystallization through time during the Cenomanian–Turonian. The results of this study therefore provide a record of the progressive dolomitization of a carbonate platform and demonstrate the important interplay of climate and basin‐scale tectonics on dolomite distribution and crystallinity.

Hydrothermal influence on barite precipitates in the basal Ediacaran Sete Lagoas cap dolostone, São Francisco Craton, central Brazil

Most Precambrian sediment-hosted barites have been interpreted to be diagenetic, hydrothermal exhalation, or methane-seepage in origin. Seafloor barite precipitates and void-filling barite cements in basal Ediacaran cap dolostones have been interpreted as sedimentary and early diagenetic in origin, and they have been used to infer ocean geochemistry in the aftermath of the terminal Cryogenian (or Marinoan) Snowball Earth glaciation. Barite crystals precipitated syndepositionally from seawater or authigenically from marine porewaters can potentially offer insights into Neoproterozoic ocean geochemistry. In this study, we analyzed void-filling barite cements from the basal Ediacaran Sete Lagoas cap dolostone to determine whether the barite records seawater geochemical signatures and whether it experienced post-depositional hydrothermal alteration. Sete Lagoas barite occurs as veins and major void-filling cement in multiple horizons, sometimes interbedded with carbonate fans. Barite crystals, commonly forming rosettes, grew upwards and downwards as radiating bladed crystals, isolated crystals in the matrix, and inclusions within carbonate fan crystals. Sulfur isotope compositions of coexisting barite and carbonate-associated sulfate (CAS) show similar values, suggesting a similar sulfate source, potentially seawater sulfate from a shallow marine environment. Assuming that CAS was sourced from seawater sulfate, we infer that the Sete Lagoas barite was precipitated from marine porewaters with seawater as a source of the sulfate. Carbonaceous material (CM) and fluid inclusions in barite crystals share a similar thermal history with a peak temperature in the range of 206–257 °C. CM was not detected in the host rock, possibly due to pervasive dolomitization, thermodegradation of CM or the fine-grained nature of the host rock. Estimated temperatures and salinity based on fluid inclusions are both higher than seawater, suggesting that hydrothermal activity, perhaps those related to Mississippi-Valley type mineralization in the São Francisco Craton, may have influenced the Sete Lagoas barite. Thus, although the Sete Lagoas barite may have originally precipitated from seawater or marine porewaters in communication with seawater, it may have been subsequently influenced by hydrothermal activity and therefore a more thorough diagenetic assessment must be done before the Sete Lagoas barites can be used to infer seawater geochemistry and environment in the aftermath of Marinoan Snowball Earth.

Sedimentology and reservoir quality of a Messinian mixed siliciclastic-carbonate succession, onshore Nile Delta, Egypt

Based on seismic and well data, this study reports for the first time on the Messinian mixed siliciclastic-carbonate succession in the onshore Nile Delta, Egypt. It records a regressive-transgressive dominated siliciclastic succession followed by post-transgressive highstand carbonates accumulated prior to the Mediterranean forced regression phase of the Messinian salinity crisis (MSC). On seismic, the studied succession comprises two different seismic units. The lower corresponds to the prograding siliciclastic facies; whereas the upper unit represents the mounded carbonate build-up. The prograding siliciclastic succession encompasses the coarsening-upward facies of basal prodelta followed by delta-front and delta-plain sediments. The cored interval constitutes the upper part of the prograding cycle consisting of floodplain mudstones, argillaceous interdistributary bay sandstones and coarse-grained distributary channel-fill sandstones. These are followed by a transgressive 3.5 m thick interval of calcareous tidal sand bars deposited after the abandonment of the delta due to the pre-MSC transgressive phase. Post-transgressive highstand carbonate build-up is characterized by subtropical-temperate rhodalgal and foramol facies. The abundant rhodoliths assemblages dominated by melobesioids (>90%) suggesting a shallow-water depositional conditions (~10–20 m). Pervasive dolomitization of carbonates suggests upward shallowing of the carbonate platform and a transition into evaporative supratidal sabkha conditions subsequent to the onset of the MSC. The carbonate platform was subaerially exposed, and meteoric water incursion led to formation of karst depressions (>0.5 km wide) locally infilled by continental argillaceous sediments. Offshore transition wackestone/mudstone and mudstone deposits of the post-MSC marine flooding (Zanclean) effectively sealed the karstified carbonates forming excellent stratigraphic-type hydrocarbon traps. Besides the economic significance of the present results, this study provides a new onshore evidence of the Late Messinian sea-level drawdown and extends our knowledge of basin evolution of the Mediterranean realm around the MSC.

DOLOMITE FORMATION BY NANOCRYSTAL AGGREGATION IN THE DOLOMIA PRINCIPALE OF THE BRENTA DOLOMITES (NORTHERN ITALY)

Upper intra- to supratidal laminites in the Norian Dolomia Principale of the Brenta Dolomites (Northern Italy) commonly consist of aphanitic and partially peloidal dolomite previously interpreted as very early diagenetic or penecontemporaneous. Re-examination of a sample prepared by focused ion beam milling through high-resolution transmission electron microscopy (TEM) revealed that dolomicrite crystals may consist of ca. 5-nm-sized nanocrystals characterized by imperfectly aligned orientation. A similar type of dolomite was found in clay-rich Carnian laminites of the Travenanzes Formation (Venetian Alps, Northern Italy) and was interpreted as indication of primary precipitation. The observation of nanocrystals in both the Dolomia Principale and the Travenanzes Formation allows hypothesizing that dolomite mud formed directly from solution via a non-classical pathway involving nucleation and aggregation of nano-particles. Domains of nanocrystalline dolomite in the Dolomia Principale are embedded within, or cemented by, calcian dolomite showing coherent lattice at the micrometre scale and a modulated structure under the TEM. This new finding provides the first evidence that one of the largest dolomite bodies occurring in the geological record, the Dolomia Principale, commenced with deposition of sediment, consisting of dolomite mud formed from solution via non-classical crystallization, and was partially affected by later pervasive diagenetic dolomitization. Based on these new observations we propose that further nano-scale studies are necessary to substantiate the hypothesis that the formation of large dolomite bodies characterized by abundant dolomicrite may have commenced by non-classical nucleation and growth processes. This approach may provide insight on non-actualistic conditions in ancient environments that may have differed, in their boundary conditions, from their modern analogues.