Marine Dolomite Research Articles

Testing clumped isotopes as a reservoir characterization tool: a comparison with fluid inclusions in a dolomitized sedimentary carbonate reservoir buried to 2–4 km

Abstract Constraining basin thermal history is a key part of reservoir characterization in carbonate rocks. Conventional palaeothermometric approaches cannot always be used: fluid inclusions may be reset or not present, while δ 18 O palaeothermometry requires an assumption on the parent fluid composition. The clumped isotope palaeothermometer, however, is a promising technique for constraining the thermal history of basins. In this study, we test if clumped isotopes record temperatures of recrystallization in deeply-buried dolomitic reservoirs, through comparison with fluid-inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitized sedimentary carbonate hydrocarbon reservoir. It provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of c. 2000–4000 m below seafloor (mbsf) and the constituent dolomites are relatively homogeneous. Across this depth range, fluid-inclusion homogenization temperatures for the Pinda Formation record a range of temperatures from c. 110 to 170°C, increasing with depth. These closely match present-day ambient well temperatures, indicating recent resetting of the fluid inclusions. Clumped isotopes, however, record temperatures significantly ( c. 20–60°C) below fluid-inclusion and well temperatures for the seven samples analysed. The deepest five samples ( c. 2800–3700 mbsf) record clumped isotope temperatures of around 100–120°C, interpreted to represent a deep burial recrystallization event responsible for a massive (re)dolomitization of the reservoir. The lower clumped isotope temperatures (65 and 82°C) of the shallower (2055 and 2740 mbsf) samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallization of an early shallow dolomite. Determination of temperature through clumped isotopes allows calculation of the parent fluid δ 18 O values. In the five deepest samples, the fluid δ 18 O values of 3.7–6.5‰ cluster around the modern-day porewater composition (5‰), suggesting that burial dolomitization occurred in the presence of evolved brine. Mineral δ 18 O values of c . −7 to −4.5‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallization. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallization events. This study shows that clumped isotopes are a valuable tool in characterizing the thermal history of deeply-buried (>2000 m) carbonate hydrocarbon reservoirs.

Neoproterozoic marine carbonates and their paleoceanographic significance

Abstract The primary mineralogy of marine carbonate precipitates has been a crucial factor in constraining the major element composition of ancient oceans. Secular changes in Phanerozoic marine chemistry, including Mg/Ca, have been well-documented using the original carbonate mineralogy of ooids, marine cements and biominerals. However, the history of Precambrian seawater chemistry is not as well constrained, partially due to the prevalence of dolomitisation in the Precambrian geological record. The Neoproterozoic (~ 1000 Ma to ~ 541 Ma) record of primary carbonate mineralogy is documented here using a combination of literature data and new analysis of marine carbonate precipitates from the Otavi Fold Belt, Namibia, the Death Valley succession, USA and the Adelaide Fold Belt, Australia. These data suggest that the last ~ 460 million years of the Proterozoic were dominated by aragonite and high-Mg calcite precipitation in shallow marine settings. In contrast, low-Mg calcite has only been recognised in a small number of formations. In addition to aragonite and calcite precipitation, marine dolomite precipitation was widespread in Neoproterozoic oceans, including mimetic (syn-sedimentary) dolomitisation and primary dolomite marine cementation. The combination of marine aragonite, high Mg-calcite and dolomite precipitation during the Neoproterozoic suggests extremely high seawater Mg/Ca conditions relative to Phanerozoic oceans. Marine dolomite precipitation may also be linked to widespread marine anoxia during this time.

Dolomitization by penesaline sea water in Early Cambrian Longwangmiao Formation, central Sichuan Basin, China

The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum's precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitiza-tion may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high- and low-frequency sea-level changes. This kind of dolomiti-zation can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity (normal, penesaline to hypersaline).

A review and new descriptions of Elk Point Group outcrops in the Athabasca Oil Sands mining region

Abstract Elk Point Group outcrops in the Athabasca Oil Sands mining region (AOSMR) and adjacent areas include exposures of the La Loche, Contact Rapids, Keg River, and Prairie Evaporite formations. Here, we review prior investigations of these formations in outcrop, followed by new descriptions of some outcrops, including those previously unpublished or newly discovered. The fluvial to marginal marine sandstone and conglomerate of the La Loche Formation, informally known as the granite wash, outcrops along the Clearwater River in Saskatchewan, where it is sandwiched between the Precambrian basement and the Contact Rapids Formation. In Alberta, the La Loche Formation is exposed at a locality along Whitemud Falls, where it directly underlies the Keg River Formation as a lithic sandstone and fills paleokarst crevices in the Keg River dolostone. From these two outcrops we recognize three facies in the La Loche Formation: regolith, lithic conglomerate, and lithic to arkosic sandstone. The marginal marine shale, silt, and dolomite of the Contact Rapids Formation outcrops in Saskatchewan at its namesake Contact Rapids, but is exposed only as a slumped bank of grey to greenish mud. We examined Keg River Formation dolostone from three outcrops along the Clearwater River at Contact Rapids in Saskatchewan and Whitemud Falls and Cascade Rapids in Alberta. We also describe an outcrop on the Firebag River in Alberta. From these outcrops, we recognize three general facies: bedded to laminated cryptalgal dolomitized bindstone (originating from an intertidal paleoenvironment), coral and stromatoporoid-bearing dolomitized floatstone to rudstone (originally a reef), and crinoid and brachiopod dolomitized floatstone (from off-reef or inter-reef areas). A newly recognized outcrop of the collapse residue from the dissolved Prairie Evaporite Formation occurs along the Clearwater River, where cobbles and boulders of breccia and other less soluble Prairie Evaporite rock weather out of the river bank between several sulfur springs.

Stable Sm isotopic analysis of terrestrial rock samples by double-spike thermal ionization mass spectrometry

This study presents the first report on mass-dependent stable Sm isotopic variability in geological materials using the double-spike thermal ionization mass spectrometry (DS-TIMS). The precision of the stable Sm-isotopic-composition DS-TIMS analysis was ca. 0.5ε/amu. The accuracy of the analysis was demonstrated by the consistent Sm isotope fractionation behavior observed during cation-exchange column chromatography. The Sm isotope fractionation factor between the cation-exchange resin (AG 50W X8) and alpha-hydroxyisobutyric acid (HIBA) at 22.6°C was obtained from the experimental results to be α147=(147Sm/150Sm)AG50WX8/(147Sm/150Sm)HIBA=1.000076±0.000011, which corresponds to a Sm isotopic difference of 0.25ε/amu between the resin and HIBA under equilibrium condition. Mass-dependent Sm stable isotopic variations on the order of epsilon unit were observed among the analyzed geological rock samples. Six igneous rocks from various rock types showed no resolvable variation in their Sm isotopic compositions, whereas a Mesozoic seamount-type marine dolomite and an Archean banded ferruginous chert showed stable Sm isotopic compositions significantly enriched in heavy isotopes as compared to the igneous rocks. The range of the Sm isotopic variation among the geological materials herein studied was 0.9ε/amu. Our results suggest that significant stable Sm isotope fractionation occurs in the marine environment at low temperatures and the stable Sm isotopic composition may thus be used to trace the Sm cycling in the ocean and earth surface environment.

Sequence stratigraphic framework for mixed aeolian, peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

AbstractDue to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross‐bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. Highstand systems tract deposits are lacking, either due to non‐deposition or post‐depositional erosion. The magnitude of inferred relative sea‐level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments.

Basement influences on dolomite-hosted vertical sedimentary intrusions in marine erg-margin deposits from the Pennsylvanian of Northern Wyoming (USA)

Understanding soft-sediment deformation structures and their triggers can help in assessing the influence of tectonics, climate, and diagenesis on the stratigraphic record. Such features commonly record processes that would not otherwise be preserved. The description of soft-sediment deformation in Pennsylvanian deposits of the western United States, characterized by orbitally driven alternations between eolian sandstones, marine dolomites, and shales, has been limited to contorted cross-beds. We document discordant, sheet-like sedimentary intrusions in three marine intervals over a 45-km-wide area. Intrusions consist of very well to moderately cemented, very fine to fine-grained quartz sandstone. Body widths range from 5 to 50cm, and heights up to 2m. The orientations of 103 vertical bodies were measured. Based on upward- and downward-tapering, and the presence of deformed, microfractured fragments of host rocks, these intrusions are interpreted to result from seismically induced fluidization of water-saturated sands. Their sheet-like morphology indicates injection through fractures. Two predominant directions (WNW-ESE and N-S) were recognized and interpreted as pre-injection fracture sets. Folding of surrounding layers around the intrusions suggests negligible compaction prior to injection, indicating penecontemporaneous or shallow burial fluidization. The intraplate location of Wyoming implies that seismicity did not originate at a plate boundary. The area within which intrusions are found is crossed by a zone characterized by localized development of thick eolian stories at the top of the formation, interpreted to reflect the rejuvenation of a basement lineament. The seismically active character of lineaments may explain overpressure and fluidization, substantiating the notion that Precambrian structures repeatedly affected Phanerozoic sedimentation. Similar intrusive features may be wrongly identified or overlooked in deposits of arid environments, but their recognition provides insights into structural context during deposition and burial.

Magnesium isotopic compositions of altered oceanic basalts and gabbros from IODP site 1256 at the East Pacific Rise

To investigate the behaviour of Mg isotopes during alteration of oceanic crust and constrain the Mg isotopic compositions of the altered oceanic crust (AOC), high-precision Mg isotope analyses have been conducted on forty-four altered basalts and gabbros recovered from IODP site 1256, which represent the carbonate-barren AOC formed at the East Pacific Rise (EPR). These samples were altered by interaction of seawater-derived fluids with oceanic crust at different temperatures and water/rock ratios. With the exception of one sample that has a slightly heavier Mg isotopic composition (δ26Mg=0.01±0.08‰), all the other samples have relatively homogenous and mantle-like Mg isotopic compositions, with δ26Mg ranging from −0.36 to −0.14‰ (an average value of −0.25±0.11‰, 2SD, n=43). This suggests that limited Mg isotope fractionation occurred during alteration of oceanic crust at the EPR at bulk rock scale, irrespective of highly variable alteration temperatures and variable water/rock ratios. Thus, our study suggests that the offset of δ26Mg values between seawater and global runoff dominantly results from the formation of marine dolomite as a sink for Mg. The mantle-like Mg isotopic composition further indicates that recycling of carbonate-barren AOC would not result in Mg isotope heterogeneity of the mantle at global scale. Consequently, the light Mg isotopic compositions of the mantle at local scale must result from incorporation of recycled Mg isotopically light carbonates.

Enigmatic carbonates of the Ombombo Subgroup, Otavi Fold Belt, Namibia: A prelude to extreme Cryogenian anoxia?

The Ombombo Subgroup of the Otavi Fold Belt, Kaokoveld, Namibia preserves a succession of clastic and carbonate sediments with unusual sedimentary features. The stratigraphy of these units is discussed here in detail for the first time since their initial definition, with particular emphasis on the sedimentology of carbonate units. Early Neoproterozoic shales of the Beesvlakte Formation, equivalent to the Zambian Katangan Copperbelt's Lower Roan Formation, host evaporitic lithologies and minor copper mineralisation. The overlying, dolomitic ~760Ma Devede Formation contains carbonate platformal lithologies which are in many ways dissimilar to Phanerozoic shallow-water carbonates. This includes unusual “curl breccias”, sheet cavities, carbonate shrubs, and tepee carbonate lithologies which contain large quantities of fibrous cements. “Curl breccias” are defined here as distinctive, curled intraclasts of laminated dolomite that often have shrinkage cracks in their margins, and are cemented by fibrous dolomite cements. Fibrous cements take on two forms: an early, length-fast fascicular-optic dolomite and a later length-slow phase with unit extinction. The presence of overlying internal sediments, the fibrous habit of these first-generation cements, as well as their preserved cathodoluminescent and optical character, suggests that these cements originally precipitated as calcite and dolomite marine cements respectively. After this initial marine calcite precipitation, all components of Devede Formation carbonates have been mimetically dolomitised, preserving original depositional fabrics. Combined with the presence of marine dolomite cements, this style of dolomitisation is suggested to be syn-sedimentary, similar to that of some Cryogenian dolomites, suggesting unusual ocean conditions during the Early Neoproterozoic. In particular, the presence of dolomite marine cements, which have been linked to ocean anoxia and high seawater Mg/Ca conditions, suggests that the onset of marine anoxia in the Neoproterozoic may have occurred during the growth of the Devede Formation carbonate platforms, prior to the Sturtian glaciation. This implies that glaciation may not be the sole cause for the development of marine anoxia during the Neoproterozoic. It is possible that marine anoxia in this southern African ocean basin may have contributed to a build-up of metals in seawater, perhaps pre-enriching basinal fluids for large-scale stratabound copper mineralisation.

Extreme ocean anoxia during the Late Cryogenian recorded in reefal carbonates of Southern Australia

The Neoproterozoic was a time of great change in the Earth's surface and marine environments, including extensive climate variability, the widespread oxygenation of the oceans and the accompanying rise of animal life. However, the timing of ocean oxygenation remains uncertain, particularly in regard to Cryogenian seas, which were disrupted by large periods of global glaciation. Interglacial Cryogenian reef complexes in the Northern Adelaide Fold Belt of Australia contain abundant primary marine dolomite cements. These cements have well-preserved textural and growth zonation, indicating they preserve their original marine chemistry and can be used as geochemical proxies for Late Cryogenian paleooceanography. Analysis of marine cements from peritidal nearshore facies, shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo-depth. While nearshore cements have low Fe contents and commonly contain iron-oxide inclusions, the shallow and deep cements have very high Fe concentrations. Chalcophile elements (Cu, Cd, Pb, Zn, etc.) are most abundant in the nearshore cements, whereas rare earth elements are found in highest concentrations in the deeper facies cements. Rare earth element profiles are unusual, with shallow and deep facies having convex profiles with negligible Ce/Ce* anomalies and positive Eu/Eu* anomalies.Being constrained by sedimentology, this carbonate geochemistry provides a window into interglacial Cryogenian ocean chemistry and structure. The marine cements reveal pronounced chemical stratification in this Late Cryogenian ocean. A thin veneer of oxic surface waters existed at the ocean surface, in peritidal facies, with increasingly anoxic and Fe-rich seawater at depth. The distribution of strongly chalcophile elements like Cd and Cu across the chemocline suggests that although ferruginous, deeper anoxic waters probably contained some dissolved sulphide. These conditions describe a ferro-sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late Precambrian. A return to Archean-like ocean conditions at this time suggests large-scale disruption of the ocean system during the Neoproterozoic.

The magnesium isotope (δ26Mg) signature of dolomites

Dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments. Using well-established isotope systems (δ18O, δ13C, 87Sr/86Sr), fluid inclusions and elemental data, as well as a detailed sedimentological and petrographic data set, we established the precipitation environment and subsequent diagenetic pathways of a series of Proterozoic to Pleistocene syn-depositional marine evaporative (sabkha) dolomites, syn-depositional non-marine evaporative (lacustrine and palustrine) dolomites, altered marine (“mixing zone”) dolomites and late diagenetic hydrothermal dolomites. These data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios (δ26Mgdol). Dolomite δ26Mg ratios documented here range, from −2.49‰ to −0.45‰ (δ26Mgmean=−1.75±1.08‰, n=42). The isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites (−2.11±0.54‰ 2σ, n=14). In comparing ancient compositions to modern ones, some of the variation is probably due to alteration. Altered marine (−1.41±0.64‰ 2σ, n=4), and earliest diagenetic lacustrine and palustrine dolomites (−1.25±0.86‰ 2σ, n=14) are less negative than sabkha dolomites but not distinct in composition. Various hydrothermal dolomites are characterized by a comparatively wide range of δ26Mg ratios, with values of −1.44±1.33‰ (2σ, n=10). By using fluid inclusion data and clumped isotope thermometry (Δ47) to represent temperature of precipitation for hydrothermal dolomites, there is no correlation between fluid temperature (∼100 to 180°C) and dolomite Mg isotope signature (R2=0.14); nor is there a correlation between δ26Mgdol and δ18Odol. Magnesium-isotope values of different dolomite types are affected by a complex array of different Mg sources and sinks, dissolution/precipitation and non-equilibrium fractionation processes and overprinted during diagenetic resetting. Further progress on the use of δ26Mgdol as a proxy will require new theoretical and experimental data for Δ26Mgfluid-dol that includes dehydration effects of the free Mg aquo ion versus fluid temperature. In ancient diagenetic systems, complex variables must be considered. These include fluid chemistry and physical properties, Mg sources and sinks, temporal changes during precipitation and post-precipitation processes including open and closed system geochemical exchange with ambient fluids. All of these factors complicate the application of δ26Mgdol as proxy for their depositional or diagenetic environments. Nevertheless, the data shown here also indicate that δ26Mgdol can in principle be interpreted within a detailed framework of understanding.

The Kipushi Cu–Zn deposit (DR Congo) and its host rocks: A petrographical, stable isotope (O, C) and radiogenic isotope (Sr, Nd) study

Near the city of Kipushi, located in the southern part of the Central African Copperbelt, a major vein-type Cu–Zn ore deposit occurs. A combination of petrographic techniques and both stable (O, C) and radiogenic (Sr, Nd) isotope analysis is used to investigate the influence of the mineralisation on the Neoproterozoic dolomite host rocks. A quantification of the abundance and size of the different host rock constituents (dolomite types, quartz, phyllosilicates) revealed a lithostratigraphical controlled variation, without trends towards the ore body. The bulk oxygen isotopic composition of the host rock varies between −2.54‰ and −9.64‰ V-PDB, with most values within the range of Neoproterozoic marine dolomite. Samples with more positive δ18O all originate from the same stratigraphic interval and are interpreted as the result of reflux dolomitisation by an evaporated brine. Few samples with depleted δ18O signatures could indicate the influence of a depleted or high temperature fluid, but are not related to the ore deposit. Moreover, the presence of the ore body cannot be traced through the host rock oxygen isotopic composition. δ18O of gangue dolomite is significantly depleted in comparison with the host rocks and ranges between −7.67‰ and −12.46‰ V-PDB. For an estimated mineralisation temperature of 310°C, this implies a δ18Ofluid between 10.7‰ and 15.6‰ V-SMOW. This is a significant enrichment compared to Neoproterozoic seawater, indicating that the mineralising fluid underwent significant fluid–rock interactions. δ13C of both host rock and gangue dolomite are in range of Neoproterozoic marine dolomites. However, a limited stratigraphic interval has clearly more negative δ13C signatures, due to in situ maturation of carbonaceous material. At the time of mineralisation (450Ma), the host rock dolomite has a strontium isotopic composition partly more radiogenic than Neoproterozoic marine carbonates (0.70793<87Sr/86Sr<0.71167). Nevertheless, the signatures show no relation to the ore body. The gangue dolomite is significantly more radiogenic (0.71061<87Sr/86Sr<0.71332) than the host rock. The radiogenic signature may be due to the interaction of formational and mineralising fluids with Neoproterozoic siliciclastic-rich dolomites, e.g. the top the Kakontwe Supérieur which has 87Sr/86Sr values up to 0.72575 at 450Ma. Alternatively, the mineralising fluid could have interacted with basement rocks. According to εNd450 values between −5.4 and −0.9 for gangue dolomites, this basement was mafic in nature. However, mafic rocks also occur in the Roan Group near the Kipushi deposit. Taken into account previous fluid inclusion data, the mineralising fluid most likely derived metals both from mafic and felsic basement units and possibly interacted with Roan rocks.

Dolomitization of the Miocene carbonates in Gebel Abu Shaar El Qiblie and Salum area, Egypt: a petrographical and geochemical comparative study

The shallow marine Miocene carbonates of Gebel Abu Shaar El Qiblie and Salum escarpment, Egypt have been extensively dolomitized. Different dolomite textures are identified in the two areas: mimic replacive dolomite with uni- to polymodal size distribution and high preservation of the grain fabrics, non-mimic replacive crystalline dolomite with relatively uniform crystal size distribution, and partial dolomitization with polymodal size distribution and planar crystal boundaries. Minor cavity-filling dolomite cement is also observed. The development of non-planar crystal boundaries, stoichiometric characteristic, Sr content (92–245 ppm), negative δ18O values (−9.34 to 1.59 ‰) of Abu Shaar dolomites and their association with Fe–Mn ore deposits suggest that these dolomites formed from partial recrystallization and modification of early formed normal marine dolomites by the influence of hydrothermal fluid evolved during rifting of Red Sea. However, recrystallization by freshwater-dominated fluid during meteoric water diagenesis can be supposed. Alternatively, the non-stoichiometric composition, δ18O values (2.15–3.19 ‰) of marine signature and Sr content (73–204 ppm) of Salum dolomites indicate that dolomitization was probably mediated by seawater or slightly modified seawater within the subtidal zone during early stage of diagenesis. The δ13C values (–1.03 to 0.7 ‰ for Abu Shaar, and –0.23 to 1.69 ‰ for Salum dolomites) reflect normal marine-derived carbon source and are probably inherited from the precursor marine limestones. The negative values of some Abu Shaar dolomites may be attributed to minimal contribution of carbon derived from soil-derived CO2 or from oxidation of methane derived from deep hydrocarbon reservoir via fractures during rifting of Red Sea.

Neoproterozoic aragonite-dolomite seas? Widespread marine dolomite precipitation in Cryogenian reef complexes

The extraordinary abundance of dolomite in the Proterozoic challenges our understanding of Precambrian marine environments. Here we show that synsedimentary marine dolomite precipitation was pervasive within Cryogenian reef complexes from the Adelaide Fold Belt, South Australia. Although these reefs are composed of dolomite, textural evidence indicates an originally aragonitic mineralogy for depositional components, in common with many other Neoproterozoic carbonates. Allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts, indicating synsedimentary dolomitization in the reefs. We describe several new forms of fibrous marine dolomite cement from the reefs that have a length-slow optical character. These fascicular slow, radial slow, and rhombic dolomite cements have finely preserved cathodoluminescent growth zones, and optical characteristics that indicate they originally precipitated as dolomite, rather than replacing calcite or aragonite cements. Abundant early marine dolomite precipitation implies a radically different seawater chemistry for the Cryogenian. Perhaps these aragonite-dolomite seas are associated with extreme Neoproterozoic glacial events and/or ocean anoxia.

Dolomite Controls on Phanerozoic Seawater Chemistry

We investigate the potential role of dolomite as a long-term buffer on Phanerozoic seawater composition. Using a comprehensive model of Phanerozoic geochemical cycling, we show how variations in the formation rate of sedimentary marine dolomite have buffered seawater saturation state. The total inventory of inorganic carbon reflects the sum of fluxes derived from continental weathering, basalt-seawater exchange, alumino-silicate diagenesis (reverse weathering), and global deposition of calcium carbonate. Although these fluxes are approximately balanced, model results indicate that seawater saturation state is sensitive to the marine dolomite depositional flux. This conclusion is consistent with and constrained by independent proxy data for seawater ion ratios, paleo-atmospheric CO2 concentrations, and paleo-pH data, and dolomite mass-age distribution through Phanerozoic time. Abundant research indicates that dolomite’s occurrence in marine sediments is sensitive to many factors: temperature, seawater composition, paleogeographic setting, continental organization, etc. Although the complexity of the process of dolomite formation prevents a complete understanding of the relative role of these factors, our model results clearly underscore the importance of this mineral in the chemical history of Phanerozoic seawater.

Silurian dolostones of eastern Lithuania

Silurian dolostones representing the Jaani (Verknė and Jocionys formations (Fms)) and Minija (Pabradė Formation (Fm.)) regional stages were studied in the Jocionys 299 borehole located in eastern Lithuania. In addition to petrological studies, dolostones were subjected to XRD, XRF and ICP-MS analyses. X-ray diffraction analysis revealed that dolomite crystals in dolostones were very close to stoichiometric and well ordered and could be interpreted as early diagenetic. Strontium in dolostone of the Pabradė Fm. is comparable to that of other ancient dolomites, and is much lower than Sr concentrations in typical modern marine dolomites. Slight enrichment in Sr and S in the Verknė and Jocionys Fms is due to the presence of celestine (SrSO4) and gypsum. Evaporative (sabkha), seepage-reflux, mixing-zone, burial and seawater dolomitization models of modern and ancient examples from literature were considered. For the Jocionys Fm. we suggest seepage-reflux and burial (?) models. Evaporative (sabkha) and mixing-zone dolomitization models may be applied to the Verknė Fm. and the Pabradė Fm., respectively.

ドロマイト化作用と貯留岩性状

Dolomite (dolostone) is a common carbonate rock in the geological records and is also a very important carbonate reservoir rock, which stock about 40% of global oil reserves. Most of the dolomites in the geological record are considered to be of replacement origin, although some were precipitated directly from pore-fluids. A variety of dolomitization models in different diagenetic environments are currently proposed for interpreting ancient dolomites: (1) evaporative dolomitization including sabkha and seepage-reflux models, (2) mixed-water dolomitization, (3) marine dolomitization, (4) burial dolomitization, (5) hydrothermal dolomitization. The dolomites formed by each dolomitization model have different geological, petrographical, mineralogical and geochemical features. Based on these features, we can, therefore, identify dolomitization models and diagenetic environments of ancient dolomites. A dolomite reservoir is often of as good quality as a limestone reservoir. Dolomitization affects carbonate reservoir characteristics, such as porosity and permeability, and, as a result, reservoir characteristics are significantly changed from those of primary carbonate rocks. The important factors associated with dolomitization and controlling the characteristics of carbonate reservoir rocks are: (1) increasing crystal size (2) decreasing porosity due to a net addition of dolomite, (3) developing moldic pores, (4) increasing resistance to compaction, and (5) increasing fractures. Dolomitization and diagenetic history of individual carbonate reservoirs differ from each other and result in the complexity of reservoir characteristics. It is, therefore, indispensable to understand the processes that formed each dolomite reservoir.

Origin of dolomites in the Cretaceous Maha Sarakham evaporites of the Khorat Plateau, northeast Thailand

The Khorat Plateau of northeast Thailand and Laos was an area of widespread deposition of evaporites and siliciclastics (Maha Sarakham Formation) during the Cretaceous and three types of dolomites are associated with this formation: (1) limpid dolomite, (2) coarse, subhedral and euhedral dolomite, and (3) saddle dolomite. Limpid dolomite is present as isolated crystals in recrystallized, clear halite beds while the other two types are only present as authigenic clusters and individual large crystals within the widespread Basal Anhydrite Unit (BAU) (1.1 m), which defines the lowest member of the formation. The δ 18O PDB isotopic values and the 87Sr/ 86Sr ratios of the limpid dolomite are within the range of Cretaceous seawater. Petrography and carbon and oxygen isotopes suggest that limpid dolomite was formed under shallow burial during times of freshening in a mostly marine-fed aggrading halite salina. Dolomite formation was possibly influenced by bacterial metabolism. The coarse crystalline and saddle dolomite types float within a matrix of coarse, recrystallized anhydrite in the Basal Anhydrite. The δ 18O isotopic values of the coarse, subhedral and euhedral dolomite types are more negative than the limpid dolomite, and the 87Sr/ 86Sr ratios are more radiogenic than typical marine dolomite of the Cretaceous. Saddle dolomite has even more negative δ 18O isotopic values than the coarse euhedral dolomite. The 87Sr/ 86Sr ratios of the saddle dolomite are also radiogenic. The isotopic and petrographic data indicate that the dolomites in the Basal Anhydrite Unit are late and authigenic but do not appear to replace a precursor carbonate. Rather, they appear to have precipitated within the deep burial environment along a 1–2-m-thick subsurface mixing interface between warm upwelling basinal brines and dense highly saline brines created by dissolving the underside of the overlying impervious halite bed. The upwelling basinal waters presumably rose through the underlying siliciclastics of the Khorat Group and then moved laterally along the base of the Maha Sarakham salt. The result of this inferred hydrologic process apparently is an accumulation of burial dolomite and recrystallized residual anhydrite in a petrographically complex unit that ultimately appear to be a solution residue, the Basal Anhydrite Unit.

ABSTRACT: Platform-Wide Dolomitization in Northern Belize: Modern Analog of Widespread, Early Platform Dolomitization

Holocene dolomite is ubiquitous in northern Belize, apparently more so than in any other area of the world. Average amount in peritidal and shallow-marine deposits is 70% (range 35- 100%) and 12% (range 1-65%), respectively, and it is present throughout such sediment sections. Dolomites are poorly ordered and calcic (39.5-47 mole% MgCO3). Peritidal dolomites occur in both cement and replacive modes; average crystal size is 1.5 microns. Marine dolomites occur dominantly as cement; average crystal size is 7 microns. Despite pronounced differences in pore- water salinities of dolomitic peritidal and marine sediments, mean dolomite del 18O values are identical -- 2.1 o/oo PDB with low standard deviation. These data are enigmatic in that they may suggest a limited salinity range of dolomite formation, or post-formative alteration in normal- salinity pore fluids. Depleted del 13C values of peritidal dolomites (-0.6 to -5.2 o/oo PDB) suggest precipitation facilitated only by sulfate reduction, whereas the range of values of marine dolomites (-5 to +12 o/oo PDB) suggest precipitation facilitated by both sulfate reduction and methanogenesis. Reasons for these differences remain unclear. Dolomite is found on most of the peritidal flats along the length (35 km) of Ambergris Caye, where it is present in Holocene sediments as much as 1 m thick. It is also present within individually extensive areas (20-50 km2) of organic-rich, shallow-marine mud bank and associated sediments (3-8.5 m thick) both on the outer and inner shelves. Accordingly, the Belize dolomite factory is a viable modern analog of widespread early dolomitization in some ancient platforms.

ABSTRACT: Syndepositional Marine Dolomitization at Bulkhead Shoals Mudbank, Northern Belize

ABSTRACT: Syndepositional Marine Dolomitization at Bulkhead Shoals Mudbank, Northern Belize