Early Diagenetic Dolomite Research Articles

MAGic: A Phanerozoic Model for the Geochemical Cycling of Major Rock-Forming Components

A dynamical model (MAGic) is presented that describes the elemental cycling of sedimentary materials involving sodium, potassium, calcium, magnesium, chloride, carbon, oxygen, iron, sulfur and phosphorous through much of the Phanerozoic. The model incorporates the basic reactions controlling atmospheric carbon dioxide and oxygen concentrations, continental and seafloor weathering of silicate and carbonate rocks, net ecosystem productivity, basalt-seawater exchange reactions, precipitation and diagenesis of chemical sediments and authigenic silicates, oxidation-reduction reactions involving carbon, sulfur, and iron, and subduction-decarbonation reactions. Although MAGic contains feedback and forcing functions adapted from the GEOCARB models (Berner, 1991, 1994; Berner and Kothavala, 2001), these functions are incorporated in a reservoir-reaction scheme that is considerably more detailed. Coupled reservoirs include shallow and deep cratonic silicate and carbonate rocks and sediments, seawater, atmosphere, oceanic sediments and basalts, and the shallow mantle. Model results are reasonably consistent with recently published constraints provided by fluid inclusion, isotopic, floral, and mineralogical records. We have used these results to evaluate sensitivity to uncertainties in the history of the earth-ocean-atmosphere system over the past 500 Ma: the advent of pelagic carbonate sedimentation, the importance of burial versus early diagenetic dolomite formation, the importance of reverse weathering, and the relationship of these processes to seafloor spreading rates. Results include a general pattern of dolomite abundance during periods of elevated seafloor spreading and alkalinity production, elevated atmospheric CO₂ concentrations for most of the Phanerozoic similar to those predicted by GEOCARB, and covariance of seawater sulfate to calcium ratios with magnesium to calcium ratios. These trends are broadly consistent with proxies for seawater composition and the mass-age data of the rock record itself.

The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps)

AbstractIn the Dolomites of northernmost Italy the carbonate‐platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow‐water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a well‐preserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time‐equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen‐depleted environment immediately post‐dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low‐diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post‐dating platform demise, average + 2·4 and − 2·4‰, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower δ13C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that near‐normal marine conditions persisted long after the shutdown of shallow water carbonate‐platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow‐water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal‐marine mounds by dark marlstones of the basal Heiligkreuz Formation.

Neoproterozoic cap carbonates: a critical appraisal of existing models and the plumeworld hypothesis

AbstractEvidence for glaciation during the mid‐late Neoproterozoic is widespread on Earth, reflecting three or more ice ages between 730 Ma and 580 Ma. Of these, the late Neoproterozoic Marinoan glaciation of approximately 635 Ma stands out because of its ubiquitous association with a characteristic, microcrystalline cap dolostone that drapes glacially influenced rock units worldwide. The Marinoan glaciation is also peculiar in that evidence for low altitude glaciation at equatorial latitudes is compelling. Three models have been proposed linking abrupt deglaciation with this global carbonate precipitation event: (i) overturn of an anoxic deep ocean; (ii) catastrophically accelerated rates of chemical weathering because of supergreenhouse conditions following global glaciation (Snowball Earth Hypothesis); and (iii) massive release of carbonate alkalinity from destabilized methane clathrates. All three models invoke extreme alkalinity fluxes into seawater during deglaciation but none explains how such alkalinity excess from point sources could be distributed homogeneously around the globe. In addition, none explains the consistent sequence of precipitation events observed within cap carbonate successions, specifically: (i) the global blanketing of carbonate powder in shallow marine environments during deglaciation; (ii) widespread and disruptive precipitation of dolomite cement; followed by (iii) localized barite precipitation and seafloor cementation by aragonite. The conceptual model presented here proposes that low latitude deglaciation was so massive and abrupt that the resultant meltwater plume could extend worldwide, physically separating the surface and deep ocean reservoirs for ≥103 years. It is proposed that cap dolostones formed primarily by microbially mediated precipitation of carbonate whitings during algal blooms within this low salinity plumeworld rather than by abiotic precipitation from normal salinity seawater. Many of the disruption features that are characteristic of cap dolostones can be explained by microbially mediated, early diagenetic dolomitization and cementation. The re‐initiation of whole ocean circulation degassed CO2 into the atmosphere in areas of upwelling, triggering localized, abiotic CaCO3 precipitation in the form of aragonite fans that overlie cap dolostones in NW Canada and Namibia. The highly oxygenated shallow marine environments of the glacial and post‐glacial Neoproterozoic world provided consistently favourable conditions for the evolutionary development of animals and other oxygenophiles.

Diagenesis and reservoir quality of the Sherwood Sandstone (Triassic), Corrib Field, Slyne Basin, west of Ireland

The Triassic Sherwood Sandstone in the Corrib Field, Slyne Basin west of Ireland on the European continental margin, is a dry gas reservoir with a Mercia mudstone top-seal. Petrographic analysis combined with X-ray diffraction, stable isotope, fluid inclusion and core analysis have been used to assess: timing of growth and origin of mineral cements, whether sandstone diagenesis involved mass flux, the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections. Depositional and early diagenetic characteristics of the Sherwood in Corrib are typical of a semi-arid fluvial environment, containing groundwater of meteoric origin. Early diagenesis included the development of copious dolomite cement, in the form of dolocrete, as well as the formation of abundant clay while less volumetrically important, grain-coating haematite and K-feldspar cement also grew. Burial diagenesis witnessed the initial growth of minor chlorite and albite. Quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 °C. Albite commenced growth at about 90 °C, quartz cement at 100 °C and ankerite at 110 °C. These cements reached the zenith of their development at 105–110 °C for albite, 125–130 °C for quartz and 135–145 °C for ankerite. Siderite and anhydrite are relatively minor, late-stage cements. The formation water has been consistently highly saline during burial and, in terms of mineral cement-derived oxygen stable isotope values, is likely to be a diagenetically evolved version of the initial depositional water suggesting long-term stagnation of the pore-fluids. The diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete. Reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with >200 μm grain size have >50 md permeability. Both shallower and deeper sections than Corrib may have similar porosity and permeability since temperature-dependent diagenetic controls on reservoir quality, e.g. quartz and illite cement, are peripheral.

Control factors and porosity evolution of high-quality sandstone reservoirs of Kela-2 gas field in Kuqa Depression

By using the integrated methods including sandbodies modelling of the outcrops, sedimentary facies of the cores and well logs of the drilled wells, and the reservoir correlation of interwells, it is thought that the sandstone reservoirs of Kela-2 gas field are a suit of high-quality natural gas reservoirs with great thickness, extensive and continuous distribution, high porosity and permeability, and a few barrier beds. Sedimentary facies and microfacies are the main factors controlling the reservoir distribution and interior heterogeneity. Based on a great deal of data of rock’s thin sections, porosity, permeability, and the parameters of capillary pressure, the reservoir diagenesis, controls, mechanism and evolution of pores have been studied. It is considered that compaction in the early stage and diagenetic dolomite and calcite cements have effect on the decline of reservoirs property. Now compaction is chiefly middle to weak. The better reservoirs have no or a few calcite cements. In the early of deep buried stage, there are still mainly remainder primary intergranular pores. The authigenic kaolinite of reservoirs is the production of the dissolution of feldspars and lithic fragments. The dissolution results in the partly modified and broadened secondary intergranular pores. In the late of deep buried stage, structure fissures and overpressure were in favor of improving and preservation of these pores.

Microtextural Characteristics and Origin of Dolomites in the Tepearasi Formation, SW of Beysehir‐Konya, Turkey

Abstract The Tepearasi Formation of the autochthonous Geyikdagi Group in the Central Tauride Belt, SE of Beysehir, is Dogger in age and consists dominantly of massive limestones and greyish dolomites occurring within the middle to upper sections. The total thickness of the dolomitic levels ranges from 100–300 m and laterally extends 500–700 m. Three types of dolomite were distinguished through petrographic analyses: homogeneous, mottled (saddle‐crystalline) and joint‐filling dolomite, which were interpreted to have formed in two different stages, early diagenetic and late diagenetic. The homogeneous dolomite of the early diagenetic stage is light‐coloured and monotonous‐textured and shows the form of a dolosparite mosaic. The mottled dolomite formed in the late diagenetic stage is light‐ to dark‐coloured and coarsely granular idiomorphic. The other type of late diagenetic dolomite, described as the joint‐filling type, presents a crystal growth pattern from the joint walls towards the centre of the joint space. It is associated with coarse calcite crystals as well as primary dolomite crystal clasts which were formed in the early diagenetic stage. In addition to these characteristics, cataclastic texture indicating the influence of tectonism is also observed. Microtexture‐oriented scanning electron microscopy (SEM) studies indicate that mottled dolomites show zonal structures and contain secondary dissolution vugs. SEM studies also revealed the existence of some remains (calcite, clay etc.) in joint‐filling dolomites. Analyses by energy dispersive spectrometry (EDS) indicate the existence of clay minerals (likely to be illite) in the pores of dolomite. Isotope studies conducted to shed light onto the origin of the dolomites of the Tepearasi Formation yielded the results of δ18O=‐2.48 to −3.87%‰ and δ13C=0.93 to 1.12%‰ for the early diagenetic homogeneous dolomites. Mottled and joint‐filling type dolomites of the late diagenetic stage, on the other hand, gave the results of δ18O = −5.42 to −7.12%‰, and δ13C=‐2.29 and −5.70 respectively. X‐Ray diffractometry (XRD) and energy dispersive analysis X‐Ray (EDAX), atomic absorption spectrometry (AAS) analyses, results of petrographic data, and δ18O and δ13C values suggest that the early diagenetic dolomitization in the Tepearasi Formation occurred under the control of hypersaline (rich in Mg) and fresh water mixing zone. The late diagenetic dolomites partly developed under the control of tectonic thrusting.

ABSTRACT: Early Diagenetic Dolomite in Early Triassic Sandstones, Offshore West of Ireland

ABSTRACT: Early Diagenetic Dolomite in Early Triassic Sandstones, Offshore West of Ireland

The Origin and Importance of the Dolomite-Limestone Breccia Between the Lower and Upper Cretaceous Deposits of the Adriatic Carbonate Platform: An Example from Ćićarija Mt. (Istria, Croatia)

On the NE slopes of Ćićarija Mt. (N Istria) a 120-150 m thick complex composed of dolomite-limestone breccia crops out between the Lower and Upper Cretaceous deposits. This studied breccia sequence is of post-sedimentary, tectogenic-diagenetic origin. It was formed bypoly phase tectonic fracture of the Upper Albian to Lower Cenomanian early- and late-diagenetic dolomite succession with relics of recrystallized limestone, which enabled very important subsequent diagenetic alteration. This included partial dissolution, dedolomitization,recrystallization and calcitization of the fine-grained, crushed dolomite matrix, and centripetal dissolution of dolomite fragments and their cementation by calcite and ferroan calcite cements, as wellas the partial collapse of fragments from the roofs of dissolution cavities and limited late-diagenetic silicification (the silica surplus originating from layers of diagenetic quartz from underlying Upper Albian deposits). Such a complex pattern of different events resulted in the high variability of breccia characteristics over relatively small distances, especially near more intensively tectonized zones. The contemporaneous stratigraphic level (Lower to Upper Cretaceous transition) in other parts of the Adriatic Carbonate Platform is also characterised by predominantly late-diagenetic dolomites with relics of limestones (including local occurrences of early-diagenetic dolomites) which are, in more tectonized areas, late-diagenetically altered into tectogenic-diagenetic breccias.

Holocene paleohydrology of playa lakes in northern and central Spain: a reconstruction based on the mineral composition of lacustrine sediments

The focus of this study is the reconstruction of Holocene limnic conditions of playa lakes in central Spain using inorganic components of their sediments. It is demonstrated that the mineralogical sediment characters provide valuable paleoenvironmental information: If dolomites originate from authigenic precipitation or early diagenetic dolomitization , the Mg–Ca-ratio of dolomites, derived from the position of dolomite's major X-ray diffraction line, turns out to be an indicator for the reconstruction of paleosalinity . In contrast, the concentrations of carbonates and sulphatic evaporites and their changing proportion to siliciclastic sediments point to changing depositional environments . The comparison of investigations from different playa lakes and drainage basin systems shows that the interpretation of mineralogical sediment features always has to consider the petrographic, hydrologic, and geomorphic features of the catchment. As for the lacustrine sediments investigated there are no data available yet, a preliminary chronological framework is derived by comparison with results from neighbouring areas. Based on this the hypothesis is put forward that during the so-called Little Ice Age subhumid environmental conditions occurred over the central part of the Iberian Peninsula , while today climatic conditions are dry-subhumid to subarid. Also for times older than the Subatlantic distinct changes of environmental conditions are noticed, but various hiatuses in the sediments make correlation difficult.

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones, Central North Sea: an example of cement growth during bioturbation?

Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non‐ferroan, generally unzoned, subhedral to highly anhedral ‘jigsaw piece’ crystals. These are of a similar size to the detrital silicate grains and typically account for ≈5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other significant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has near‐marine δ18O values and low, positive δ13C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near‐surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore fluid of regionally consistent δ18O. Labile bioclastic debris (e.g. aragonite, Mg‐calcite) supplied isotopically positive carbon to the pore fluids during shallow‐burial dissolution. Such dissolution took place in response to the ambient ‘calcite sea’ conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg2+ and Ca2+ with the sea‐water reservoir maintained a sufficient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed fine calcitic shell debris, before ankerite precipitation during deep‐burial diagenesis.

Hydrothermal dolomites in SW Sardinia (Italy): evidence for a widespread late-Variscan fluid flow event

Abstract Epigenetic replacive and saddle dolomite, frequently forming zebra structures (Geodic dolomite), affected Cambrian limestones and early diagenetic dolomites across large areas of the Iglesiente–Sulcis mining district (SW Sardinia, Italy). The dolomite crops out in an area of more than 500 km 2 and reaches a thickness of up to 600 m (or even more). This rock represents one of the most extensive dolomites of epigenetic origin yet documented. Prior to dolomitization, the Cambrian carbonates underwent ductile deformation and greenschist facies metamorphism. The same is true for the overlying tectonic nappes. Fluid inclusion analysis combined with O- and Sr-isotope data indicate that the dolomitizing fluid can be categorized as a “basinal brine”. The spread in homogenization temperatures shows a gradient with values decreasing from east to west. The higher temperatures (mean of around 100°C) have been measured in eastern parts of the Iglesiente area, whereas the lowermost temperatures (mean of about 85°C) have been found along the western coast of Iglesiente. This fluid certainly can neither have come directly from the low-grade metamorphic lithotypes undergoing dolomitization, nor from the low-temperature metamorphic rocks within the overlying nappes. It is assumed that a late-Variscan hydrothermal event, which coincided with extensional tectonics, set brine circulation into motion. Dolomitization may have occurred within circulation cells, which were driven by high heat flow. Fluids originated in the underlying rocks and circulated upward to affect the overlying Cambrian carbonates. An alternative model is that the dolomitization was caused by hydrothermal brines that originated within basinal areas of the Variscan orogenic pile circulating deep within the crust, and ascended in the External Zone (Iglesiente–Sulcis). Whatever the origin, these reducing saline fluids, which carried radiogenic Sr, dolomitized a large volume of Early Paleozoic rocks. The many subhorizontal open cavities within the Geodic dolomite, only partly filled with cement, might indicate fluid overpressure at the time of dolomitization.

Isotope composition of O and C in Mesosoic carbonate rocks of Slovenia - effect of facies and diagenesis

Isotope composition of nearly 300 mostly Mesozoic limestone and dolomite samples were examined and show that δ18O and δ13C changes depend on mineral compositon of carbonate, increased organic matter admixture and diagenetic processes. Limestone of different sedimentary environments (open or restricted shelf, lagoons, deeper water...) has δ18O ranging from +22 to +30 ‰ SMOW, and δ13C around +2 ‰ PDB. Early-diagenetic dolomites are in comparison to limestone, enriched in the heavy oxygen isotope for 2 to 5 ‰, while late-diagenetic dolomites and de-dolomites are depleted in heavy isotopes, as well as for oxygen as for carbon. Limestone with higher content of organic matter, those samples which were deposited in the freshwater environment - i.e. characean limestone and those which underwent vadose diagenesis, are enriched in light carbon isotopes. The different age of examined carbonate samples is not reflected in the changes of O and C isotope composition.

The geological history of Messinian (upper miocene) evaporites in the Central Jordan Valley (Israel) and how strontium and sulfur isotopes relate to their origin

Evaporites, comprising gypsum, anhydrite, halite and dolomite are described from the Messinian Bira Formation outcrops and from two boreholes (Newe Ur-2 and Zemah-1) in the Central Jordan Valley, Israel. Strontium and sulfur isotopic compositions of the evaporite minerals, and their Sr/Ca and Br/Cl ratios are used to interpret their enviromments of deposition and processes of formation and diagenesis. The brines from which the evaporites precipitated originated from seawater. The processes caused by mixing of surface water, seawater and subsurface brines, resulted in dolomitization and also sulfur reduction. The surface water and subsurface brines reacted with the rocks they drained, including Cretaceous and Eocene carbonates and Neogene basalts. The gypsum deposits in the Central Jordan Valley are interpreted to have formed as a result of evaporation of the magnesium-rich Lake Bira water which became oversaturated with respect to calcite and gypsum. The gypsum was deposited in stratified, relatively closed basins, where a partial reduction of the sulfur resulted in high δ34S of the precipitated gypsum. Gypsum and early diagenetic dolomite formed from the same water bodies. The Bira evaporites in Newe Ur-2 borehole, precipitated from mixtures of sea- and fresh waters with basaltic contribution. The samples from the Lower Gabbro and Halite Unit in the Zemah-1 borehole were deposited from evaporated seawater, which leached basaltic rocks, in closed basins; the Middle Halite Unit formed from seawater, whiles the brines that deposited the Upper Halite Unit leached also basalt rocks.

Dachstein Limestone from Krn in Julian Alps (Slovenia)

The Dachstein limestone from Krn is characterized by Lofer development. The section encompasses 30 cyclothems on average 3-4 metres thick. Intraand supratidal environment of deposition is evident by stromatolites, loferite and tem­ pestile breccias. They were all locally affected by early diagenetic dolomitization. Biomicritic limestone with several horizons of megalodont clams was deposited in shallow and restricted shelf. Basal members A of the Lofer cyclothem are devel­ oped only exceptionally. Solution cavities are fairly common, being infilled by sparry calcite and insoluble rich red limestone. The limestone was deposited on the Julian carbonate platform near its passage part to the southerly lying Slovenian basin. In this part reigned specific circum­ stances that resulted into formation of very numerous corrosion vugs; they are filled in their central parts by variegated marly clay. Interesting are frequent temdoi:10.5474/geologija.1996.006

Late Diagenetic Dolomitization of Lower Ordovician, Upper Knox Carbonates: A Record of the Hydrodynamic Evolution of the Southern Appalachian Basin

Late diagenetic dolomitization of the Lower Ordovician, Upper Knox Group in the southern Appalachian basin was closely associated with widespread secondary porosity development, hydrocarbon migration, and local Mississippi Valley-type mineralization. Regionally extensive (^sim70,000 km2), late diagenetic dolomites consist of replacement dolomites and zoned dolomite cements. Late diagenetic replacement dolomites comprise 15 to 50% of all Knox matrix dolomites. The ^dgr18O (-11.9 to -5.3^pmil), ^dgr13C (-3.8 to +0.9^pmil), and 87Sr/86Sr (0.70895 to 0.70918) values of late diagenetic replacement dolomites overlap with those of the first zone of dolomite cements (zone 2), early replacement dolomites, and Lower Ordovician arine calcites, reflecting rock buffering of initial dolomitizing fluids and extensive neomorphism of replacement dolomites by subsequent late diagenetic fluids. Nonporous to sucrosic, late diagenetic dolomites have porosities (1 to 16%) and permeabilities (0 to 1030 md) significantly greater than those of early diagenetic replacement dolomites and host limestones ( 165°C), saline (13 to 22 wt.% NaCl equivalent) basinal brines that underwent extensive fluid-rock interaction with clastics. Precipitation temperatures of late diagenetic dolomites estimated from fluid inclusion homogenization temperatures and systematic trends in ^dgr18O va ues record a regionally developed, prograde-to-retrograde thermal history. Knox late diagenetic dolomites are interpreted to record the spatial and temporal evolution of large-scale fluid flow systems that developed in response to different burial and tectonic stages of the southern Appalachian basin. The occurrence of zoned dolomite cements in tectonic fractures and breccias, and their close association with noncarbonate diagenetic minerals of Pennsylvanian to Early Permian ages, suggest that most Knox late diagenetic dolomites record deep subsurface (2 to >5 km) fluid migration in response to late Paleozoic Alleghenian tectonism (330 to 265 Ma). Late diagenetic matrix dolomites served as long-lived conduits that focused and channeled diagenetic fluids in the deep subsurface. The occurrence of bitumen in secondary porosity within late diagenetic dolomite indicates that they likely were the most viable reservoirs during hydrocarbon migration in the late Paleozoic.

Early diagenetic dolomite concretions in the late Cretaceous Herring Formation, eastern Marlborough, New Zealand

Dolomite concretions occur in the upper part of the late Cretaceous Herring Formation of eastern Marlborough, New Zealand. Herring Formation sediments are centimetre-bedded, poorly indurated, clay mineral-rich, siltstones or very fine sandstones, and were deposited in a NW-SE trending trough. Differential compaction of surrounding sediments around concretions is ubiquitous. Concretionary dolomite consists of intergrown anhedral to subhedral crystals up to 150 μm across. Pyrite is common. Insoluble residue contents indicate sediment porosities of 70% or more at the time of concretion formation, which is consistent with an origin in the upper few metres of the sediment column. Generally, concretions have near stoichiometric dolomite compositions, although a few are composed of calcite. Dolomite Fe compositions average about 4 mol% FeCO 3, with Mn contents in the range 623–9543 ppm. All concretions contain less Mn than Fe. The average Sr content ranges from 175 to 710 ppm. Concretions can be separated into a “light carbon group” ( δ 13 C = < − 6ℵ PDB , δ 18 O = < −5ℵ PDB ) in the basin centre, and a “heavy carbon group” ( δ 13 C = > −9ℵ PDB , δ 18 O = > −4ℵ PDB which are found at basin margins. Ca 2+ and Mg 2+ were derived initially from pore waters and ultimately from diffusion of overlying seawater. Palaeotemperatures calculated from δ 18O analyses indicated concretions formed at less than 60°C. Burial depths derived from palaeotemperatures are higher than expected for concretions formed in the upper sediment column. However, Sr concentrations are interpreted as indicating that the bulk of dolomite formed by the dolomitization of early diagenetic calcite. This process, along with other competing early diagenetic reactions, caused depletion in 18O in formation waters. The combination of Sr and δ 18O data are interpreted as indicating wholly marine formation without the influence of meteoric fluids. The presence of pyrite and δ 13C data indicate concretions formed in association with SO 4 2− reduction in the upper sediment column. The distribution and abundance of organic matter, metabolized by SO 4 2− reducing bacteria, was therefore probably important in the distribution and formation of dolomite concretions. Dolomitization continued as concretions were buried into the zone of bacterial fermentation. In addition, ° 13C analyses indicate that concretions formed in the basin centre contain an organic carbon component derived from upward diffusing methane, whereas basin margin concretions do not. Concretions ceased forming when they were buried to depths below which Ca 2+ and Mg 2+ could diffuse and when supplies of SO 4 2− and/or organic material became insufficient to support levels of SO 4 2− reducing bacteria. Continued burial resulted in compaction of sediments around concretions. Some basin margin concretions were later exhumed and phosphatised.

Petrographic and geochemical evidence for original aragonite mineralogy of upper Jurassic carbonates (Mozduran formation), Sarakhs area, Iran

Diverse skeletal and non-skeletal grains with abundant red and blue-green calcareous algae, evaporites and early diagenetic dolomites in the Mozduran carbonates are similar to those of modern warm-water shallow-marine carbonates. Isopachous and fibrous intragranular sparry calcite cements resemble modern aragonite morphologies. Concentric and radial ooids are like modern aragonitic forms which are formed in turbulent and in low-energy environments, respectively. Deformed ooids, shattered micritic envelopes and spalled ooids indicate aragonite dissolution during meteoric diagenesis. Meteoric cements are equant, mosaic to drusy calcites. Burial cements only occur as vein fillings.Sr and Na covariance with Mn supports evidence of inversion of aragonite to calcite, with subsequent open phreatic diagenesis. Fe concentrations are a magnitude higher than Mn due to a reducing phreatic environment. High SrNa ratios suggest original aragonite mineralogy.The δ18O and δ13C field of Mozduran limestone lies at the edge of the modern shallow-marine field indicating a range from marine to meteoric cements. Differences in δ18O values between marine calcites and meteoric-derived calcites is similar to that in the modern tropics. The surface marine diagenetic trend line that passes through shallow-marine modern whole carbonates cuts the Mozduran limestone isotope field due to their formation in warm, shallow-marine waters, given that atmospheric CO2 levels were similar to those at present.Increasing Mn and Fe values, decreasing Na and uniform Sr values with increasingly lighter values of both δ18O and δ13C are exlained by extensive meteoric diagenesis. The least altered marine calcite δ18O value of −2.6‰ PDB corresponds to 29 or 24°C, depending on whether δω = 0‰ (of present seawater) and δω = − 1.2‰ (of ancient seawater with no ice caps) are taken as a base. An original calcite mineralogy postulated for some Jurassic limestones is related to a non-tropical origin.

Geochemical differences between subtropical (Ordovician), cool temperate (Recent and Pleistocene) and subpolar (Permian) carbonates, tasmania, australia

Subtropical (10°N) Ordovician carbonates are similar to modern warm-water ones and contain aChlorozoan Biota, diverse non-skeletal grains, abundant micrite, isopachous cements and early diagenetic dolomites and rare evaporites. Sr/Na ratios are around ≥ 3 as in modern warm-water carbonates. Covariance between Mn content and that of Sr and Na indicates stabilization of aragonite to calcite in a semi-closed diagenetic system. Covariance of δ18O and δ13C with Mn, Sr and Na of brachiopods and isopachous cements indicate Late Ordovician marine calcite values were around −5‰ ± 1.0 in δ18O and 1‰ ± 1.0 in δ13C. A small δ18O difference (2‰) between marine calcite and meteoric calcite indicates low latitude meteoric diagenesis. Sr/Mn ratios decrease within increasingly lighter δ13C due to dissolution of calcite in meteoric waters.

Petrography and geochemistry of the Belle Roche breccia (lower Visean, Belgium): evidence for brecciation by evaporite dissolution

ABSTRACTThe lower Visean Belle Roche breccia (east Belgium) displays a number of features that indicate brecciation by evaporite dissolution collapse: the sharp lower contact of the breccia, the gradual transition into the overlying strata, the presence of semi‐continuous beds within the breccia giving it a crude ‘stratification’, and the existence of several types of (calcite, dolomite and silica) evaporite pseudomorphs. Furthermore, the majority of the breccia fragments indicates hypersaline to lagoonal sedimentation conditions. Most of these fragments display an interlocking fabric. The interpretation is also supported by the existence of continuous evaporite beds replaced by carbonates (calcite and dolomite) both under and overlying the breccia.The brecciation history is characterized by gradual subsidence. Multiple brecciation episodes are recognized, and are best seen in the lower breccia which underwent at least two major brecciation episodes. Here, brecciated and veined breccia fragments occur within a microsparite (neomorphosed mud) matrix. Brecciation of these strata was due to the dissolution of interlayered evaporites. The second brecciation event relates to infiltration of meteoric water and to the dissolution of the remaining evaporites. This infiltration was probably triggered by the orogenic event at the end of the Visean (Sudetic orogenic phase). The whole breccia was finally cemented by a blocky calcite.The different lithologies and cements were characterized by their trace element (Mg, Sr, Na, Fe, Mn, K), insoluble residue and organic matter content. Carbon/oxygen isotope data of the cements and replaced evaporite layers helped to place the multiple collapse episodes within a general diagenetic model. Solution‐reprecipitation processes within the original aragonite‐dominated mud fragments, as well as in the early diagenetic dolomite fragments, have been recognized. The geochemical data show that these transformation processes occurred in equilibrium with the same fluid. These processes may have occurred within a freshwater lens very early in the diagenetic evolution or under shallow burial conditions. Cementation of blocky calcite occurred in a meteoric realm under burial conditions.

The carbon- and oxygen-isotope record of the Precambrian–Cambrian boundary interval in China and Iran and their correlation

AbstractThe fossiliferous section at Meishucun of Yunnan, China, is a candidate stratotype section for the Precambrian–Cambrian boundary. Early diagenetic dolomites and phosphorites have been sampled across the boundary interval here, and in the correlated section at Maidiping in Sichuanand Valiabad in Iran, for comparison of their carbon and oxygen isotopes. This is the first such study that is calibrated by biostratigraphy in the interval from the earliest (pre-Tommotian) skeletal fossils to trilobites. Although negative oxygen isotopes indicate a diagenetic signal in the Zhongyicun Member and basal Badaowan Member phosphorites, two carbon-isotope cycles are clearly present and can be correlated in dolomitic rocks between the two sections. The first appearance datum (FAD) of the earliest skeletal assemblage (zone I, Marker A), FAD of diverse micromolluscs (zone II, Marker B) and FAD of Chinese trilobites (zones IV, V) and Marker C appear at similar points on the carbon-isotope curve in the two Chinese sections. Integrated carbon-isotope and early skeletal fossil biostratigraphy is shown to have the potential to correlate further afield, with sections in Iran, as well as with India, Siberia, Morocco and Australia. We suggest that a distinctive positive excursion provides a global marker for the interval between Marker B and C in China and just below the Tommotian Stage of Siberia.