Formation of bed‐scale spatial patterns in dolomite abundance during early dolomitization: Part I. Mechanisms and feedbacks revealed by reaction–transport modelling

Dolomites of varied ages exhibit metre‐scale nested patterns of lateral periodic variation in permeability and porosity and, by inference, dolomite abundance as most examples are 100% dolomite. Two‐dimensional reaction–transport modelling simulations of bed‐scale dolomitization were used to assess whether those patterns in dolomite abundance could form during near‐surface replacement dolomitization. Simulations used a 2 m high and 18 m long model domain, a low‐Mg calcite grainstone precursor and an evaporated Mississippian seawater brine (430 parts per thousand salinity) as the dolomitizing fluid. The domain was initially populated with random variations in porosity and/or grain size. Results reveal that spatial patterns in dolomite abundance emerge when there is as little as 1% dolomite formed, with similarities between the modelled patterns and outcrop‐documented patterns. The nested patterns include a near‐random component that constitutes ≤40% of the total variance, short‐range correlation ranging from 1·5 to 3·3 m and a longer‐range periodic trend with a wavelength up to 6·5 m. The emergence of pattern in dolomite abundance is the result of an autogenic self‐organizing phenomenon. It is triggered by variation in initial calcite reactive surface area that occurs due to the random heterogeneities in initial porosity and/or grain sizes. The pattern develops due to a combination of kinetic disequilibrium reactions (dolomite precipitation and calcite dissolution) and positive feedbacks between dolomite growth, calcite dissolution and fluid flow. Flow is around loci of higher dolomite, lower porosity and higher reactive surface areas, but through loci of lower dolomite, higher porosity and lower reactive surface areas. The resulting less porous/more dolomite and more porous/less dolomite structures at the metre‐scale arise from those localized interactions. This self‐organizing mechanism for pattern formation constitutes a new model for geochemical self‐organization during dolomitization and is the only self‐organization model that is proven applicable to the formation of metre‐scale patterns during early, near‐surface dolomitization.     

Reactive transport modelling of reflux dolomitization in the Arab‐D reservoir,Ghawar field,Saudi Arabia

It has long been recognized that the Arab‐D reservoir in Ghawar field has been significantly dolomitized and that the distribution of dolomites is highly heterogeneous across this reservoir. Previous studies indicated that dolomite occurs with either a stratigraphic or non‐stratigraphic distribution; when mapped, dolomite tends to form several parallel linear trends across the field. Although stratigraphic dolomite was suggested to be formed early from highly evaporated pore fluids sourced from overlying evaporite deposits, non‐stratigraphic dolomite was thought to be generated primarily from hydrothermal fluids sourced from below. This study focuses primarily on these non‐stratigraphic dolomites, and proposes that: (i) these dolomites initially formed via seepage reflux, but were reinforced by late stage hydrothermal dolomitization; and (ii) reflux is also responsible for the formation of parallel, linear trends of dolomite. The reflux model hypothesizes that an evaporative lagoon (which is the source of dolomitizing fluids) formed during the falling stage systems tract of a depositional sequence, and that with continuing sea‐level fall this lagoon migrated progressively towards deeper parts of an intrashelf basin adjacent to the Ghawar field, leaving behind lines of dolomite bodies along a series of temporary coastlines. Two‐dimensional reactive transport models have been built to test this hypothesis, and have resulted in a predicted pattern of dolomite bodies that agrees with both the observed vertical distribution of non‐stratigraphic dolomite, as well as the mapped lateral distribution of the dolomite trends. In addition, the major ion compositions of Late Jurassic seawater are calculated based on fluid inclusion data in the literature. Using Jurassic seawater in current models leads to the absence of anhydrite cements and less potential of over‐dolomitization than using modern seawater.     

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

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

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

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

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

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

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

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ¹⁸Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ¹³C values together with ⁸⁷Sr/⁸⁶Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ¹⁸Ο and δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ¹⁸Ο, δ¹³C and ⁸⁷Sr/⁸⁶Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.     

Tectonically driven dolomitization of Cambrian to Lower Ordovician carbonates of the Quruqtagh area,north‐eastern flank of Tarim Basin,north‐west China

Dolomites occur extensively in Cambrian to Lower Ordovician carbonates in the Tienshan orogen of the Quruqtagh area, north‐east Tarim Basin, where thick (up to 1 km), dark grey lenticular limestones of semi‐pelagic to pelagic origin are prominent. The dolomites generally occur as beige, anastomosed geobodies that cross‐cut well‐stratified limestones. Based on detailed field investigations and petrographic examination, three types of matrix dolomite are identified: fine crystalline planar‐e (Md1), fine to medium crystalline planar‐s (e) (Md2) and fine to coarse crystalline non‐planar‐a (Md3) dolomites. One type of cement dolomite, the non‐planar saddle dolomite (Cd), is also common. The preferential occurrence of Md1 along low‐amplitude stylolites points to a causal link to pressure dissolution by which minor Mg ions were probably released for replacive dolomitization during shallow burial compaction. Type Md2, Md3 and Cd dolomites, commonly co‐occurring within the fractured zones, have large overlaps in isotopic composition with that of host limestone, implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water–rock interaction. However, the lower δ¹⁸O and higher ⁸⁷Sr/⁸⁶Sr ratios of these dolomites also suggest more intense fluid–rock interaction at elevated temperature and inputs of Mg and radiogenic Sr from the host limestones with more argillaceous matter and possibly underlying Neoproterozoic siliciclastic strata. Secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher‐temperature Mg‐rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting, forming irregular networks of dolomitized bodies enclosed within the host limestones. This scenario probably took place during the Late Hercynian orogeny, as the Tarim block collided with Tienshan island arc system to the north and north‐east. Subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the South Tienshan Ocean (or Palaeo‐Asian Ocean) and significant tectonic uplift of the Tienshan orogen. This study demonstrates the constructive role of notably tensional (or transtensional) faulting/fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin; however, a relatively short‐lived, low fluid flux may have limited the dolomitization exclusively within the fractured/faulted limestones in the overthrust sheets.     

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

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

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

Palaeogene dolostones from the sub‐surface of Florida are ideal for the study of dolomite maturation because they record the early stages of a secondary dolomite overprint without destruction by later diagenetic overprints. Two distinct dolomite textures occur in the dolostones of the Upper Eocene Ocala and Lower Oligocene Suwannee limestones in west‐central Florida: a porous and permeable sucrosic dolomite and a less porous and relatively impermeable indurated non‐sucrosic dolomite. In both textures, the initial matrix dolomite is dully luminescent, whereas the secondary overprint is dominantly luminescent cement in the Suwannee and only neomorphic luminescent dolomite in the Ocala. The abundance of luminescent dolomite ranges from 2% to 38%, which translates to 1·6 km³ of material in the Suwannee and 13·5 km³ in the Ocala. Extrapolated trace‐element contents (Sr and Na) and δ¹⁸O values for the matrix and luminescent end‐members indicate a marine origin for the matrix dolomite in both units, and a freshwater–seawater mixing‐zone origin for the secondary luminescent dolomites. The δ¹⁸O values indicate that a saline, middle mixing‐zone environment overprinted the Suwannee but a more dilute mixing zone affected the Ocala. Fluid–fluid mixing models constrained by modern Floridan aquifer hydrochemistry and extrapolated ⁸⁷Sr/⁸⁶Sr values of the luminescent phases indicate that the mixing zones operated during the Late Miocene to Pliocene in the Ocala and affected the Suwannee in the Pliocene. The luminescent Suwannee mixing‐zone cement reduced porosity up to threefold and permeability up to 100‐fold, which converted many sucrosic dolomites to indurated dolomites. By contrast, the neomorphic luminescent Ocala dolomite did not have an appreciable impact on the maturations. Although freshwater–seawater mixing zones were not the sites of the initial dolomitization, the mixing‐zone environment did dramatically overprint and mature the regionally widespread dolomites of the Ocala and Suwannee limestones. This maturation occurred shortly after formation of the proto‐Floridan aquifer; the timing suggests the matrix dolomites were ‘ripe’ for alteration and that the only prerequisite for mixing‐zone dolomite is pre‐existing dolomite substrates to reduce kinetic barriers. In contrast to recent claims, the results of this study demonstrate that mixing zones can be effective in forming regionally significant amounts of secondary dolomite and influencing the petrophysical maturation of dolomite bodies.     

Mud dispersal across a Cretaceous prodelta: Storm‐generated,wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.     

Origin of Dinantian zebra dolomites south of the Brabant-Wales Massif, Belgium

Zebra dolomites, characterized by a repetition of dark grey (a) and light (b) coloured dolomite sheets building up abbabba-sequences, occur in Dinantian strata from deep boreholes (> 2000 m) south of the Brabant-Wales Massif in Belgium. These zebra dolomite sequences are several tens of metres thick. The dark grey dolomite sheets (a) consist of non-planar crystals, 80–150 μm in diameter. These crystals display a mottled red–orange luminescence and are interpreted to be replacive in origin. The white dolomite sheets (b) consist of coarse crystalline nonplanar b₁ dolomite, which evolves outwards into transparent saddle shaped b₂ dolomite. The b₁ dolomites possess a mottled red–orange luminescence similar to the a dolomites, while the saddle shaped b₂ rims display red to dark brown luminescent-zones. The b₁ dolomites are possibly partly replacive and partly cavity filling. Their b₂ rims display criteria typical for a cement origin. Locally, cavities exist between two succeeding white dolomite sheets. These cavities make up ≈5% of the zebra rocks and are locally filled by saddle shaped ankerite and/or xenomorphic ferroan calcite. Geochemical and fluid inclusion data (T_h ≈ 120 °C) indicate a burial diagenetic origin for these zebra dolomites. The a and b₁ dolomites are characterized by similar geochemical compositions and fluid inclusion data pointing toward a related origin. To explain the development of the zebra textures, suprahydrostatic pressures in conjunction with late Variscan tectonic compression are invoked. A model involving dolomitizing fluids expelled during the Variscan orogeny is proposed. An overpressured system is also invoked to explain the important porosity development, the creation of centimetre-scale subvertical displacements of the zebra pattern and the microfractures affecting the b₁–b₂ dolomite crystals.     

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

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

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, central Spain)

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO₃), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO₂ and K contents are highly correlated. δ¹⁸O‐ and δ¹³C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in ¹⁸O and ¹³C between the two minerals. The correlation coefficient between δ¹⁸O and δ¹³C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO₃^?/CO₃^2? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.     

Multiple dolomitization events along the Pozalagua Fault (Pozalagua Quarry,Basque–Cantabrian Basin,Northern Spain)

The Pozalagua Quarry in the Basque–Cantabrian Basin of northern Spain exposes a unique set of fault‐associated dolomites that can be studied on a decametre scale. The dolomites developed along the Pozalagua Fault system in slope‐deposited limestones of Albian age. Following marine phreatic diagenesis, the limestones were subject to meteoric karst formation. The resulting cavities were filled either by angular limestone fragments in a black clay‐rich matrix, or by cave floor/pond (now dolomitized) sediments. The subsequent diagenetic history reflects repeated periods of fracturing, fluid expulsion, dissolution and cementation. Contrasting fluid pulses resulted in the formation of a network of hydrothermal karst and the subsequent development of coarse‐crystalline calcite cement, zebra dolomite, recrystallized coarse‐crystalline dolomite, elongated blue–grey coarse‐crystalline dolomite cement in the open fault and, finally, coarse‐crystalline saddle dolomite. Decimetre‐size reworked host‐rock fragments present in the latter two dolomite phases probably reflect roof collapse fragments of a cave system that developed along the Pozalagua Fault system. However, there are also metre‐scale host‐rock fragments that apparently ‘float’ in the coarse‐crystalline saddle dolomites, implying that either fragment assimilation was a widespread process or violent expulsion of fluids occurred along the Pozalagua Fault system. The presence of pre‐dolomite and post‐dolomite stylolites, parallel to bedding, supports a linkage between the diagenetic events and the Late Albian tectonism that affected the region.     

Sedimentary evolution and environmental history of Lake Van (Turkey) over the past 600 000 years

The lithostratigraphic framework of Lake Van, eastern Turkey, has been systematically analysed to document the sedimentary evolution and the environmental history of the lake during the past ca 600 000 years. The lithostratigraphy and chemostratigraphy of a 219 m long drill core from Lake Van serve to separate global climate oscillations from local factors caused by tectonic and volcanic activity. An age model was established based on the climatostratigraphic alignment of chemical and lithological signatures, validated by ⁴⁰Ar/³⁹Ar ages. The drilled sequence consists of ca 76% lacustrine carbonaceous clayey silt, ca 2% fluvial deposits, ca 17% volcaniclastic deposits and 5% gaps. Six lacustrine lithotypes were separated from the fluvial and event deposits, such as volcaniclastics (ca 300 layers) and graded beds (ca 375 layers), and their depositional environments are documented. These lithotypes are: (i) graded beds frequently intercalated with varved clayey silts reflecting rising lake levels during the terminations; (ii) varved clayey silts reflecting strong seasonality and an intralake oxic–anoxic boundary, for example, lake‐level highstands during interglacials/interstadials; (iii) CaCO₃‐rich banded sediments which are representative of a lowering of the oxic–anoxic boundary, for example, lake level decreases during glacial inceptions; (iv) CaCO₃‐poor banded and mottled clayey silts reflecting an oxic–anoxic boundary close to the sediment–water interface, for example, lake‐level lowstands during glacials/stadials; (v) diatomaceous muds were deposited during the early beginning of the lake as a fresh water system; and (vi) fluvial sands and gravels indicating the initial flooding of the lake basin. The recurrence of lithologies (i) to (iv) follows the past five glacial/interglacial cycles. A 20 m thick disturbed unit reflects an interval of major tectonic activity in Lake Van at ca 414 ka bp . Although local environmental processes such as tectonic and volcanic activity influenced sedimentation, the lithostratigraphic pattern and organic matter content clearly reflect past global climate changes, making Lake Van an outstanding terrestrial archive of unprecedented sensitivity for the reconstruction of the regional climate over the last 600 000 years.     

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

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

Direct physical evidence of dolomite recrystallization

The possibility of recrystallization is a long‐standing barrier to deciphering the genetic origin of dolomites. There is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event. Results from 65 new high‐temperature dolomite synthesis experiments (1 m , 1·0 Mg/Ca ratio solutions at 218°C) demonstrate dolomite recrystallization affecting stoichiometry, cation ordering and nanometre‐scale surface texture. The data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization, which occur by various dissolution–precipitation reactions. During the first phase (induction period), no dolomite forms despite favourable conditions. The second phase (replacement period) occurs when Ca‐rich dolomite products, with a low degree of cation ordering, rapidly replace calcite reactants. During the replacement period, dolomite stoichiometry and the degree of cation ordering remain constant, and all dolomite crystal surfaces are covered by nanometre‐scale growth mounds. The third phase (primary recrystallization period), which occurs in the experiments between 97% and 100% dolomite, is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering. The end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat, laterally extensive layers. The fourth phase of the reaction (secondary recrystallization period) occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time. Inferences of recrystallization, in natural dolomite, based on cation order or stoichiometry of dolomite, usually depend on assumptions about the precursor dolomite subjected to recrystallization. If it is assumed that the experimental evidence presented here is applicable to natural, low‐temperature dolomites, then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization.     

Hydrothermal dolomitization in a complex geodynamic setting (Lower Palaeozoic,northern Spain)

Burial hydrothermal dolomitization is a common diagenetic modification in sedimentary basins with implications for oil and gas reservoir performance. Outcrop analogues represent an easily accessible source of data to refine the genetic models and assess risk in hydrocarbon exploration and production. The Palaeozoic succession of northern Spain contains numerous excellent exposures of epigenetically dolomitized limestones, particularly in the Carboniferous and Cambrian. The epigenetic dolomites in the Cambrian carbonates of the Láncara Formation are volumetrically small, but have a large aerial distribution across different tectonic units of the Variscan fold and thrust belt. Coarse crystals, abundant saddle dolomite cement, negative δ¹⁸O and fluid inclusion homogenization temperatures between 80°C and 120°C characterize these dolomites, which are petrographically and geochemically similar to the tens of kilometre‐sized hydrothermal dolomites replacing the Upper Carboniferous succession in the same area. In both cases, the dolomitizing fluids are derived from highly evaporated sea water, modified to a limited degree through fluid‐rock interaction. The dolomitization events affecting both Cambrian and Carboniferous strata are probably related to the same post‐orogenic hydrothermal fluid flow. The formation of the post‐collisional (latest Carboniferous) Cantabrian arc fostered dolomitization: the extension related to bending of the arc generated deep‐reaching faults and strike‐slip movements, which favoured the circulation of hot dolomitizing fluids in the outer parts of this orocline. A similar dolomitization process affected other areas of Europe after the main stages of the Variscan orogeny. Dolomitization was a continuous, uninterrupted, isochemical process. Limestone replacement resulted in a major porosity redistribution and focused the fluid flow into the newly created porous zones. Replacement was followed immediately by partial to complete cementation of the pores (including zebra fabrics and vugs) with saddle dolomite. The amount of porosity left depends on the volume of cement and therefore on the volume of fluids available.     

Massive dolomitization of a late Miocene carbonate platform: a case of mixed evaporative brines with meteoric water,Nijar, Spain

Late Miocene platform carbonates from Nijar, Spain, have been extensively dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically highest outcrops, suggesting that dolomitizing fluids were derived from the adjacent Nijar Basin. The dolomite crystals range from <10 to ≈100 μm existing as both replacements and cements. Na, Cl and SO₄ concentrations in the dolomites range from 200 to 1700 p.p.m., 250–650 p.p.m., and 600–7000 p.p.m., respectively, comparable with other Tertiary and modern brine dolomite values, and also overlapping values from mixing-zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 7× seawater brine to freshwater ratios. The δ¹⁸O and δ¹³C of the dolomites range from ?1·0 to +4·2‰ PDB, and ?4·0 to +2·0‰ PDB, respectively. ⁸⁷Sr/⁸⁶Sr values (0·70899–0·70928) of the dolomites range from late Miocene seawater to values greater than modern seawater. Mixtures of freshwater with seawater and evaporative brines probably precipitated the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. δ¹⁸O and Na/Ca vs. δ¹⁸O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, nonstoichiometry and quantitative water–rock interaction modelling of covariations of Na vs. Sr and δ¹⁸O vs. δ¹³C. The possibility of multiple dolomitization events induced by evaporative brines, seawater and freshwater, respectively, is consistent with mineral-mineral mixing modelling. The basin-derived dolomitizing brines probably mixed with freshwater in the Nijar Basin or mixed with fresh groundwater in the platform, and were genetically related either to deposition of the Yesares gypsum or the Feos gypsum. Dolomitization occurred during either the middle Messinian or the early upper Messinian. Nijar dolomitization models may be applicable to dolomitization of other late Miocene platform carbonates of the western Mediterranean. Moreover, the Nijar models may offer an analogue for more ancient evaporite-absent platform carbonates fringing evaporite basins.     

Fault-controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west-central Alberta, Canada

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 (T_h=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (T_peak=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.