Point-source submarine groundwater discharge (PSGD) delivers mass and solutes and locally reduces salinity along karstic coasts, yet the geometry of conduits conveying meteoric and mixed waters is rarely imaged. Here we present the first marine electrical resistivity tomography (ERT) characterization of PSGD along the northwestern Yucatán Peninsula and evaluate marine ERT across contrasting settings to detect and characterize PSGD conduits and the confining coastal aquitard under field conditions. Four dipole-dipole ERT profiles span an intact coastal aquitard on land (Sisal), a coastal lagoon perforated by PSGD (Dzul-Ha), and their offshore equivalents near the Xbuya-Ha vent. On land, a laterally continuous resistive horizon at -5 to -9 masl overlies low-resistivity units that host saline groundwater and is interpreted as a cemented coastal aquitard that confines the aquifer. Beneath Dzul-Ha, the inversion images an elongated conductive chimney that cross-cuts this horizon and coincides with the mapped PSGD. Seabed profiles near Xbuya-Ha resolve discrete resistive bodies embedded in a conductive matrix at the main and secondary PSGDs. Forward modeling shows that conduits remain detectable even when the salinity contrast generated by freshwater discharge is weak; however, anomaly amplitudes decrease with increasing sea-floor depth, following an exponential decay with a characteristic depth of ~8 m. Together, these results provide a resistivity-based framework for combining tracers and flow models to quantify PSGD fluxes. Because the study targets vigorous PSGD under favorable electrical contrasts, these performance estimates represent an upper bound and clarify the limits of marine ERT for detecting PSGD in comparable karstic coasts.

Early marine carbonate cements generally form in CaCO3-supersaturated seawater at the seabed or shallow burial depths, resulting in syn-sedimentary cemented firmgrounds and hardgrounds. The processes controlling early marine diagenesis are complex, particularly in coastal environments where geochemistry is influenced by different water sources and local biogenic activity. Investigating the modern firmgrounds and hardgrounds in the intertidal zones of the U.A.E. reveals how physical, chemical and biological factors influence early marine precipitation. In these environments, early marine diagenesis is governed by interacting hydrogeochemical, microbial and physical processes, whilst bioturbation modifies solute transport dynamics. In situ porewater and sediment analyses show that the intensely bioturbated lower intertidal zone experiences enhanced sediment permeability, persistent seawater flushing and sediment reworking, producing geochemically uniform porewaters and fewer firmground features. In the middle intertidal zone, reduced seawater exchange, shallower bioturbation and longer porewater residence times promote geochemical conditions favourable for early marine cement precipitation. Additionally, mixing between upwelling continental brines and intertidal seawaters affects porewater redox, salinity and ion concentrations, leading to localised diagenesis. Spatial variations in porewater and sediment chemistry can develop over short distances (<1 km) because of complex hydrogeochemical interactions, in which bioturbation acts as an important modifier. This study highlights the importance of integrating ichnological, geochemical and hydrogeological approaches.

Microplastic (MP; 63 μm–5 mm) transport, deposition, and burial is largely governed by well understood sedimentological processes, reflected by a growing human imprint on the sedimentary record. Microplastics are highly-mobile, and longer-lived than natural particles of similar density; as contaminants and vectors they pose ecological risks, so understanding their accumulation is crucial. However, most sedimentological research has focused on siliciclastic settings, while carbonate environments—typically shallow, low-energy, and biologically influenced—offer a contrasting context to assess MP behavior. By comparing MP abundance and characteristics across diverse carbonate sedimentary environments on several carbonate platforms, this study examines the controls on their transport and retention in carbonate-dominated systems. Here we investigate the factors which influence the occurrence, transport and deposition of MPs in near-shore environments. Carbonate sediment cores (> 50 cm), from two low-energy, restricted, heavily bioturbated tidal flats (The U.A.E. and Bahamas), are assessed to compare MP abundance to bioturbation intensity and environmental location (lower intertidal vs. upper intertidal). Additionally, surface sediments (upper 5 cm) from carbonate beaches in six different countries are compared to evaluate MP abundance across contrasting climatic and hydrodynamic environments. Results reveal that MPs are sequestered at all depths within the sediment column (0–~50 cm), indicating some sedimentological mechanism(s) for their penetration and retention within reworked carbonate sands. Surface MP concentrations vary with hydrological energy and proximity to waste sources, while sediment properties (grain size, sorting, roundness) appear less significant. These findings highlight the key role of biological and hydrodynamic processes, rather than purely sedimentary attributes, in controlling MP distribution and burial, providing new insights into the transport pathways and long-term sinks of MPs in global near-shore carbonate systems. In addition, this context will support ongoing research of the organismal (microbial through infaunal) response to MPs in carbonate environments.

Fault-controlled, hydrothermal dolomitization typically involves the interaction of high pressure (P), high temperature (T) fluids with the surrounding host-rock. A striking feature of hydrothermal dolomite bodies is the pattern development and periodicity of zebra textures, whereby alternating units of replacement dolomite (RD) and saddle dolomite (SD) form symmetrical RD-SD/SD-RD patterns. Zebra textures are often considered to be diagnostic of these elevated P/T conditions, but the roles of mechanical deformation and the localization of strain during dolomitization have received limited attention. Here we evaluate the effect of P/T perturbations on the genesis of zebra textures, alongside how strain-hardening mechanisms promote their characteristic pattern development. Published fluid inclusion homogenization and carbonate clumped isotope temperatures were compiled from the literature and the offset between these data were used as a geobarometer. Based on these porefluid pressures, a series of rock deformation experiments were conducted to reproduce zebra textures in the laboratory. Cylindrical rock samples were held in an annealed copper jacket and deformed in axisymmetric extension. As the rock underwent tensile failure, the copper jacket locally deformed by intracrystalline plasticity, strain-hardened, and stabilized each opening-mode fracture. As a result, a succession of closely spaced fractures formed along the length of the sample. In natural geological settings, an analogous process is inferred, whereby dilatancy hardening, precipitation hardening, and the stress shadow effect promote the rhythmicity that is a defining feature of zebra textures. Lastly, the effects of P/T perturbations on the solubility of dolomite, for a range of different fluid compositions, were evaluated using the Pitzer aqueous model in PHREEQC. This interdisciplinary study presents novel insights into the geomechanical and hydrochemical interaction between metasomatic fluids and carbonate rocks, which are of critical importance to our understanding of carbonatehosted ore deposits in sedimentary basins worldwide.

This contribution identifies biogenic structures created by modern birds foraging in marginal aquatic settings and provides descriptions to facilitate their identification in the rock record. Biogenic structures related to foraging can be separated into those created by bills, such as peck marks, probe marks, gape marks, dabble marks, sweep marks, and bill-stir marks. Biogenic structures created by feet include stir tracks and paddle pits. Peck marks are created during visual foraging and result in shallow, solitary or paired, random or clustered, circular to subcircular pits and grooves. Probe marks are created during tactile foraging but are similar to peck marks, differing solely in their greater depth of penetration. Gape marks are formed when birds open their bill in the sediment resulting in elongated grooves. Dabble marks are larger ovoid divots emplaced by broad-billed waterbirds in subaqueous settings. Bill stirring occurs when a bird swishes its bill in a narrow trend on the sediment surface. Sweep marks are arcuate grooves emplaced in the sediment when long-billed birds forage by sweeping their bill side-to-side across the sediment-water interface.Birds shuffling their feet in soft sediment is termed 'foot-stirring' and results in overprinted, side-by-side trackways. Foot-paddling dewaters the sediment and produces various pit morphologies with massive fill. Trackways emplaced during foraging are commonly characterized by variable stride length, stutter steps, and sudden changes in direction. 'Trample grounds' are produced by gregarious foraging flocks of birds. It is anticipated that illustrating and describing the structures produced by these behaviors will facilitate recognition of these commonly overlooked traces.

The Abu Dhabi coast, particularly the supratidal zone known as the " sabkha " , has been a subject of geological interest in recent decades due to its relevance as an analogue for subsurface petroleum reservoirs. The intertidal zone (Al Qantur lagoon) has received less attention, especially regarding neoichnological investigations. This study addresses this research gap by combining modern sedimentological and neoichnological observations. Three recurring depositional textures are identified in the sediments examined: the bioturbated peloidal grain- stone, microbially laminated bindstone, and bioclastic rudstone. Observed burrows are generally of two types; those attributed to worms ( Nereis polychaetes) and those to decapod crustaceans. Crustaceans homogenize sediments in distal areas by excavating dwelling structures, while worms act as mobile deposit feeders. The relatively low complexity and diversity of burrowing morphologies in Abu Dhabi are attributed to heightened environmental stressors, including salinity, heat, and subaerial exposure, as well as upward seepage of hypersaline continental brines. Porewater analyses indicate a stabilization of salinity within burrows, suggesting the infauna irrigate their burrows to cope with elevated salinity. Crustacean burrows enhance solute advection in the lower intertidal zone by increasing substrate permeability, facilitating the penetration of less-saline surface waters to greater depths. This study provides a unique neoichnological examination of Abu Dhabi's intertidal zones and assesses the impact of bioturbation on solute advection, an aspect often overlooked in contemporary carbonate sedimentological studies.

Biofilms are mucilaginous-organic layers produced by microbial activity including viruses. Growing biofilms form microbial mats which enhance sediment stability by binding particles with extracellular polymeric substances and promoting growth through nutrient cycling and organic matter accumulation. They preferentially develop at the sediment-water interface of both marine and non-marine environments, and upon the growing surfaces of modern tufa and travertine. In this context, however, little is known about the factors, environmental or anthropogenic, which affect viral communities in freshwater spring settings. To explore this issue, geochemical and metagenomic data were subjected to multidimensional analyses (Principal Component Analysis, Classical Multidimensional Scaling, Partial Least Squares analysis and cluster analysis based on beta-diversity), and these show that viral composition is specific and dependent on environment. Indeed, waters precipitating tufa and travertine do vary in their geochemistry with their viruses showing distinct variability between sites. These differences between virus groups allow the formulation of a viral proxy, based on the Caudoviricetes/Megaviricetes ratio established on the most abundant groups of viruses. This ratio may be potentially used in analysing ancient DNA preserved in carbonate formations as an additional source of information on the microbiological community during sedimentation.

Mineralogy and texture of diagenetic phases in the Aptian Pre-Salt Barra Velha Formation are described, quantified and compared by facies and structural setting to understand their spatial and temporal distribution, and to develop predictive concepts for their genesis. This study examined data from eight wells from one oil-field in the Santos Basin. Calcite is the most abundant mineral and occurs with fibro-radial texture as spherulites and shrubs and sparse microcrystalline mudstone. The delta 18O values from calcite spherulites and shrubs suggest water of similar composition and temperature, but they have different delta 13C values. Mudstones show lower delta 18O, suggesting warmer lake water and/or lower evaporation, whereas delta 13C values indicate a variable, but commonly strong influence of biogenic CO2. Dolomite with rhombohedral habit was the first to precipitate, followed by lamellar, saddle and anhedral varieties. Rhombohedral dolomites show a positive delta 13C-delta 18O correlation and a similar range in values to spherulites and shrubs, suggesting similar lake water. The lamellar dolomite is related to biofilms, whereas anhedral dolomite is attributed to mixing of pore and meteoric waters. Lamellar and anhedral dolomites have similar isotopic values, precipitating after rhombohedral dolomite in slightly warmer and/or less evaporatively concentrated pore water. Saddle dolomite is related to hydrothermal fluids that percolated the formation during early diagenesis. Silica occurs as replacement of primary calcite and Mg-clay, but also as a cement and more rarely as a depositional chert. Both dolomite and silica display a complex range of petrographic textures, many of which are facies dependent. This study focusses on the most important phases of the paragenetic sequence that took place pre-burial and are (1) formation of Mg-clay, calcite spherulites and shrubs, (2) partial dolomitisation of shrubs and spherulites and precipitation of rhombohedral and lamellar dolomites, (3) precipitation of saddle dolomite, matrix and carbonate dissolution and (4) anhedral dolomite and all textures of precipitated or substituted silica. Mineralogy and texture of diagenetic phases in the Aptian Pre-Salt Barra Velha Formation are described, quantified and compared by facies and structural setting to understand their spatial and temporal distribution, and to develop predictive concepts for their genesis. This study focusses on the most important phases of the paragenetic sequence that took place pre-burial and are (1) formation of Mg-clay, calcite spherulites and shrubs, (2) partial dolomitisation of shrubs and spherulites and precipitation of rhombohedral and lamellar dolomites, (3) precipitation of saddle dolomite, matrix and carbonate dissolution and (4) anhedral dolomite and all textures of precipitated or substituted silica.image

Key Messages: (1) Early diagenetic horizons (hardgrounds) are found locally in the middle-upper intertidal zone. Variability in the concentrations of elements and ions are more prominent in the middle-upper intertidal zone than in the lower intertidal zone. (2) Bioturbation by crustaceans in distal portions of the intertidal zone is capable of completely homogenizing substrates, consequently increasing permeability and allowing deeper solute diffusion. Mixing of the sediment potentially inhibits the formation of early diagenetic horizons. (3) Pore water chemistry and sediment geochemistry reveals that supratidal continental brines diffuse down ramp and add additional solutes to the intertidal zone which mixes with seawater.

Reactive transport modelling is increasingly deployed to quantitatively evaluate conceptual models of diagenetic processes. However, construction of models of complex systems involves trade‐offs between accuracy and simplification. This tension is explored for models of fault‐associated dolomitisation by sea water convection in a syn‐rift carbonate platform, evaluating the contribution of incorporating stratigraphic growth and fault propagation. Simulations of the high heat flux southern margin of the Derbyshire Platform (Northern England), with heterogeneous matrix permeability that reflects the evolving stratal architecture and burial compaction focusses dolomitisation in more permeable units at all depths. A permeable platform margin fault zone enhances dolomitisation in a broad area on the upper slope and margin, and to a lesser but significant extent, across the interior as platform top waters are entrained and discharge via the fault. Stepwise simulation of flow and reactions during stratigraphic growth suggests that static models over‐predict dolomite abundance in younger sediments and show how regions optimally supplied with reactants and heat to drive dolomite formation migrate vertically and laterally during platform growth. Dolomitisation intensity increases with depth due to greater time for reactions and kinetically favourable temperatures. Adding the fault zone to this model focusses and accelerates flow, giving a more spatially restricted dolostone body and reducing dolomitisation temperature. Changes in fault connectivity with the surface of the evolving platform shift fluid flow pathways and change the rate and temperature of dolomite formation. Results concur with petrographic, isotopic and geochemical observations of the early dolomite on the Derbyshire Platform. This work demonstrates the importance of understanding diagenesis as the product of an evolving set of processes that respond to geological and palaeoenvironmental changes rather than as a sequence of individual diagenetic events. This is particularly critical for reactions, such as dolomitisation by geothermal convection of sea water, which occur over timescales synchronous with platform development.

Pedorelicts occur sporadically in coastal exposures of Middle to Late Triassic continental sediments, at Portishead, SW England. The clasts include aeolian and bedrock constituents, calcrete textures, vesicles and other pedogenic features reflecting a hot semi-arid palaeoenvironment. Interpreted as being derived from an upland soil, they are classified in palaeosol terms as Calcic Protosol. In the most significant exposure, cryptic tubular trace fossils in branching networks occur within these pedorelict clasts in a localised fluvial deposit. This extremely rare palaeoecological archive offers a glimpse into continental upland life in a Pangaean desert landscape. It is documented for its own merit, to support continuing studies of its palaeobiology and to prompt investigation for comparable deposits. The pedorelicts occur within the Mercia Mudstone Marginal Facies (MMMF), close to its basal unconformity with underlying Paleozoic strata, at the margin of the Somerset Basin. The soil from which they are derived is interpreted to have developed in an upland regolith over Tournaisian bedrock of interbedded limestone and siltstone. These weathered and, with aeolian dust containing calcite and iron minerals and siliciclastic sand, formed a structured soil. This was aided by intermittent light and moderate rainfall that promoted mainly vadose pedogenic calcretization with displacive calcite crystallisation and siliciclastic grain breakage. In places, the soil was vesicular, as an 'Av' horizon, and in others, it was bioturbated with the development of calcite-lined tubules and unlined tunnels. At intervals of perhaps 104 to 105 years, catastrophic deluges eroded the regolith and transported clasts downslope towards the basin to form onlapping coarse clastic beds typical of the MMMF. The soil structure disintegrated but fragments that became pedorelicts were segregated in a fault-controlled palaeo-valley, possibly partly as a debris-flow, to be deposited as localised conglomeratic lenses with fluvial sand. After burial by further sedimentation, this deposit underwent diagenetic calcite cementation and baryte mineralisation before its present exposure by coastal erosion. (c) 2022 The Geologists' Association. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

The calculation of a reliable temperature dependent dolomite solubility product constant (Ksp°−dol) has been the subject of much research over the last 70 years. This study evaluates log10(aCa2+/aMg2+) using PHREEQC (Pitzer approach) for a screened subset (n=11,480) of formation waters in the U.S. Geological Survey National Produced Waters Geochemical Database V2 (PWGD), an extensive inventory of 165,960 formational waters from a range of sedimentary lithologies in North America up to 6.6 km depth (Blondes and others, 2016). Through extensive ground-truthing against datasets sourced from Texas Gulf Coast basin and the Mississippi Salt Dome basin we establish that both the geochemical data from the PWGD and a new geothermal model of the US that is used to determine temperatures at-formation-depth to be reliable data sources. The vast majority (at least 90%) of PWGD samples have log10(aCa2+/aMg2+)-temperature values that are interpreted to be indicative of calcite-dolomite equilibrium and controlled by bulk mineral equilibria rather than Mg-calcite surface phases. Using statistical models with different parameterizations (different Maier-Kelly formulas, mixed-effects models with various random effects and linear models) log10(aCa2+/aMg2+) values (outcome variable) are regressed against temperature (fixed effect) calculating Ksp°−dol between 0-200°C using the well constrained calcite solubility product (Ksp°−cal). Local effects that modify log10(aCa2+/aMg2+) values are evaluated through the addition of random effects to the mixed model which improves the statistical reliability of the Ksp°−dol estimate and enables the determination of Ksp°−dol for local dolomite phases. The nature of these local effects is open to interpretation, but we suggest the primary influence on log10(aCa2+/aMg2+) values is the stoichiometry of the equilibrium dolomite phase within individual fields that systematically modifies log10(aCa2+/aMg2+) values. We discount the influence on log10(aCa2+/aMg2+) values from the ionic strength of the solution, the equilibration with anhydrite and chlorite group minerals, the illitization of smectite and albitization of feldspar. For the dolomite solubility equation; CaMg(CO3)2 (s) ↔ Ca2+(aq)+ Mg2+(aq) + 2CO3(2-)(aq) (1)the mixed-effects model chosen as most representative yields a pKsp°−dol (log10Ksp°−dol); pK(sp-dol)= 1.47545×10^1 - 6.24959×10^-2∙T(K) - 3.99350×10^3∙1/T(K) (2)At 25°C pKsp°−dol = -17.27±0.35, which is close to prior estimates including the most recent experimental value reported by B´en´ezeth and others, 2018 (pKsp°−dol = -17.19±0.3). This validates this study’s approach and enables conclusions to be drawn via a meta-analysis of a contaminated, though expansive dataset.

Calcite microcrystals and associated microporosity are ubiquitous and extensively developed in Jurassic and Cretaceous carbonate sequences in the Middle East. Clumped isotope analyses of calcite microcrystals in the Lower Cretaceous Thamama-B strata in UAE reservoirs indicate temperatures of 60-90 degrees C and burial of 1.5-2.5 km suggesting formation synchronous with and updip of Late Cretaceous ophiolite obduction at the Eastern Arabian continental margin. Assuming that recrystallization of precursor calcite to calcite microcrystals requires initially undersaturation to drive dissolution/re-precipitation a basin-scale 2D reactive transport model (RTM) was constructed. The model is constrained by hydro-mechanical simulations and used to quantitatively evaluate the hypothesis that the formation of calcite microcrystals and associated microporosity is driven by expulsion of compaction fluids during rapid burial. The combined influence of fluid flux and cooling results in trace net calcite dissolution (porosity increase <0.1 vol %) focused at depths of 2.7-5.1 km. The presence of even minor amounts of minerals with common ions (dolomite and anhydrite) induces additional dissolution but does not change its' spatial distribution. Whilst RTMs only yield a minimum estimate of the degree of recrystallization that likely occurs driven by calcite disequilibrium, simulations suggest these reactions occur at temperatures of 95-170 C, markedly higher than those estimated from clumped isotopes of the calcite microcrystals. Mixing of fluids leaked from underlying strata up faults into the lateral flow system could play an important role in burial diagenesis, with the slow leakage dissolving up to ten times greater mass of calcite than a shorter pulse of equivalent fluid volume. A previously unrecognized effect of the introduction of H2S(aq)-rich fluids, derived from accelerated thermal maturation or thermochemical sulphate reduction (TSR), into formation water with high CO2(aq) concentration is CO2 degassing that drives net calcite precipitation (<3 vol %). This suite of numerical simulations suggest that calcite disequilibrium will have occurred in fluids expelled during ophiolite obduction, but that any associated recrystallization will have occurred at depths greater than those inferred from temperatures measured in the calcite microcrystals. Recrystallization at shallower depths may have been associated by fluid-mixing around the injection points, but reaction kinetics suggests that laterally pervasive alteration would require very rapid flow rates. These results suggest that alternative mechanism(s) are needed to be considered to explain the extensively developed calcite microcrystals and associated microporosity in Mesozoic carbonates of the Middle East.

There has been increasing recognition of fine-grained dolomitic sediments in continental rift basins, but research has tended to focus on their petrography, geochemistry and genesis, rather than the pore characteristics that determine tight oil and gas reservoir quality. This paper presents a classification of pore types for dolomitic sediments influenced by hydrothermal processes from the early Cretaceous Erlian Basin, and examines relations between pore types to infer underlying genetic processes and controls as a precursor to predicting their distribution. We recognize three pore types: (1) primary porosity, which includes inter-crystalline pores between hydrothermal minerals within granules and nano-scale pores within fine matrix; (2) secondary dissolutional porosity, including inter-crystalline dissolved pores in granules, mouldic pores in laminae and bandings and selectively dissolved pores in argillaceous matrix, as well as large vugs in breccias; and (3) fracture porosity, including low-angle dissolved fractures, bedding-parallel microfractures and high-angle tectonic fractures. Both hydrothermal and hydrocarbon-related fluids caused significant dissolution, generating secondary pores that form the largest volume of reservoir storage. Channeling of aggressive fluids via low-angle fractures localized dissolution in the matrix and significantly enhanced permeability. Vertical fractures have a lesser impact on porosity but contribute to improved pore network connectivity. These relationships reflect dolomite accumulation within a tectonically-active extensional setting, with associated fault and fracture networks providing conduits for hydrothermal fluids. These fluids are responsible for formation of the dolomitic reservoir rocks and their diagenesis and may result in a characteristic suite of pore types. The high heat flux ensured rapid maturation of source rocks and may have played an important role in generating additional dissolutional porosity. Given the tight nature of reservoirs in such lacustrine systems, a systematic understanding of the nature and evolution of the pore network is key to identifying productive "sweet spots" in the vicinity of faults for de-risking future hydrocarbon exploration and also in developing such settings for geothermal energy.

Almost 230 years after it was first described, formation of the mineral dolomite remains enigmatic. Dolomitic rocks are abundant throughout the geological record, and most are thought to form by replacement of limestone. Recent work has identified substantial volumes of high‐temperature primary dolomite deposited in Cretaceous lacustrine rift basins in Northern China. Here we provide the missing conceptual understanding of the fundamental processes that may have formed such dolomitic sediments by combining an analysis of the physics and chemistry of sub‐lacustrine hydrothermal systems with new data on the mineralogy and geochemistry of the dolomites. This novel mechanism accounts for systematic changes in the characteristics of the dolomite sequence that intimately link with the evolution of convection within the fault damage zone and venting of brine‐rich mixtures to the lake basin, and provides a new model to be tested against dolomitic sediments in high‐enthalpy settings such as rifting basins.

Neogene dolomites are common in island settings, and they have been used to understand the complex processes of dolomitization. The island of Bonaire was one of the first locations where the concept of reflux dolomitization was applied. Given this historical significance, here we re-evaluate the importance and nature of reflux using a previously unstudied set of outcrops in Bonaire. Mio-Pliocene units in Seru Grandi in northwestern Bonaire show well-defined bodies of dolomite that extend along clinoform surfaces beneath a subhorizontal erosional unconformity. The dolomite distribution suggests early dolomitization as a result of fluids moving downwards through a succession of subtidal facies, as expected from the reflux model. However, non-stoichiometric compositions, weak 18O enrichment, low trace element concentrations and the sedimentological context of the dolomites suggest that refluxing fluid was more likely to be mesohaline in composition rather than hypersaline as previously proposed. In addition, our findings suggest that the fluids did not dolomitize the succession uniformly, but rather elongate bodies of dolomite were developed within selected clinoforms through different events. This variability may be due to temporal changes in the flux, chemistry of reactive fluids and textural changes of original sediment, whereas downdip trends may reflect evolution of magnesium exchange efficiency with distance from the brine source and reactivity of the rock.

Dolomites occur extensively in the lower Cretaceous along syn‐sedimentary fault zones of the Baiyinchagan Sag, westernmost Erlian Basin, within a predominantly fluvial–lacustrine sedimentary sequence. Four types of dolomite are identified, associated with hydrothermal minerals such as natrolite, analcime and Fe‐bearing magnesite. The finely‐crystalline dolomites consist of anhedral to subhedral crystals (2 to 10 μ m), evenly commixed with terrigenous sediments that occur either as matrix‐supporting grains (Fd1) or as massive argillaceous dolostone (Fd2). Medium‐crystalline (Md) dolomites are composed of subhedral to euhedral crystals aggregates (50 to 250 μ m) and occur in syn‐sedimentary deformation laminae/bands. Coarse‐crystalline (Cd) dolomites consist of non‐planar crystals (mean size >1 mm), and occur as fracture infills cross‐cutting the other dolomite types. The Fd1, Md and Cd dolomites have similar values of δ 18 O (−20·5 to −11·0‰ Vienna PeeDee Belemnite) and δ 13 C (+1·4 to +4·5‰ Vienna PeeDee Belemnite), but Fd2 dolomites are isotopically distinct ( δ 18 O −8·5 to −2·3‰ Vienna PeeDee Belemnite; δ 13 C +1·4 to +8·6‰ Vienna PeeDee Belemnite). Samples define three groups which differ in light rare‐earth elements versus high rare‐earth elements enrichment/depletion and significance of Tb, Yb and Dy anomalies. Medium‐crystalline dolomites have signatures that indicate formation from brines at very high temperature, with salinities of 11·8 to 23·2 eq. wt. % NaCl and T h values of 167 to 283°C. The calculated temperatures of Fd1 and Cd dolomites extend to slightly lower values (141 to 282°C), while Fd2 dolomites are distinctly cooler (81 to 124°C). These results suggest that the dolomites formed from hydrothermal fluid during and/or penecontemporaneous with sediment deposition. Faults and fractures bounding the basin were important conduits through which high‐temperature Mg‐rich fluids discharged, driven by an abnormally high heat flux associated with local volcanism. It is thought that differing amounts of cooling and degassing of these hydrothermal fluids, and of mixing with lake waters, facilitated the precipitation of dolomite and associated minerals, and resulted in the petrographic and geochemical differences between the dolomites.

The dominant paradigm for petrogenesis of high-temperature fault-controlled dolomite, widely known as “hydrothermal dolomite” (HTD), invokes upwelling of hot fluid along faulted and fractured conduits from a deep over-pressured aquifer. However, this model has several inherent ambiguities with respect to fluid sources and their dolomitisation potential, as well as mechanisms for delivering enough of these reactive fluids to form substantial volumes of dolomite. Here, we use generic 2D and 3D reactive transport simulations of a single transmissive fault system to evaluate an alternative conceptual model whereby dolomitisation is driven by seawater being drawn down into the subsurface and heated. We examine the evolution of fluid chemistry and the distribution of diagenetic alteration, including predictions of the rate, distribution, and temperature of HTD formation, and consider the possible contribution of this process to the Mg budget of the world's oceans. The simulations suggest that it is possible for convection of seawater along the fault damage zone to form massive dolomite bodies that extend hundreds of metres vertically and along the fault within a timescale of a few tens of thousands of years, with no significant alteration of the country rock. Dolomitisation occurs as a gradient reaction by replacement of host limestones and minor dolomite cementation, and it results in the discharge of Mg2+-poor, Ca2+-rich fluids to the sea floor. Fluids sourced from the basement contribute to the transport of heat that is key for overcoming kinetic limitations to dolomitisation, but the entrained seawater provides the Mg2+ to drive the reaction. Dolomite fronts are sharper on the “up-flow” margin where Mg2+-rich fluids first reach the threshold temperature for dolomitisation, and the “down-flow” dolomite front tends to be broader as the fluid is depleted in Mg2+ by prior dolomitisation. The model demonstrates spatial contrasts in the temperature of dolomitisation and the relative contribution of seawater and basement-derived fluids which are also commonly observed in natural fault-controlled dolomites. In the past, such variations have been interpreted in terms of major shifts in the system driving dolomitisation. Our simulations demonstrate that such changes may also be a product of emergent behaviour within a relatively stable system, with areas that are dolomitised more slowly recording the effect of changes in fluid flow, heat, and solute transport that occur in response to diagenetic permeability modification. Overall, our models robustly demonstrate that high-temperature fault-controlled dolomite bodies can form from mixed convection and act as a sink for Mg in the circulating seawaters. In addition, comparison of our 3D simulations with simplifications to 2D indicate that 2D models misrepresent critical aspects of the system. This has important implications for modelling of systems ranging from geothermal resources and mineralisation to carbonate diagenesis, including hydrothermal karstification and ore genesis as well as dolomitisation.