Early Diagenetic Dolomite Research Articles

Characterization of environmental conditions during microbial Mg-carbonate precipitation and early diagenetic dolomite crust formation: Brejo do Espinho, Rio de Janeiro, Brazil

Abstract For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. The mud undergoes an early diagenetic transformation into a 100% dolomite crust on the margins of the lagoon. The biomineralization process, characterized by the variations of the physico-chemical conditions in this environment during the annual hydrological cycle, is integrated with isotopic analysis to define the early diagenetic processes responsible for the formation of both dolomitic mud and crust. The carbon isotope values indicate a contribution of respired organic carbon, which is greater for the crust (δ 13 C=−9.5‰ Vienna Pee Dee Belemnite (VPDB)) than mud (δ 13 C=−1.2‰ VPDB). The oxygen isotope values reflect a moderate degree of evaporation during mud formation (δ 18 O=1.1‰ VPDB), whereas it is greatly enhanced during early diagenetic crust formation (δ 18 O=4.2‰ VPDB). The clumped isotope formation temperature derived for the Brejo do Espinho mud is 34 °C, whereas it is 32 °C for the crust. These temperatures are consistent with the upper range of measured values during the dry season when the lagoon experiences the most hypersaline conditions.

Origin, characteristics and distribution of fault‐related and fracture‐related dolomitization: Insights from Mississippian carbonates, Isle of Man

AbstractViséan limestones on the Isle of Man host numerous examples of fault‐controlled and fracture‐controlled dolomitization, which have been investigated to determine their macro‐scale to micro‐scale characteristics, geofluid origin, timing and relation to basin evolution. Geobodies composed of fabric destructive, ferroan, non‐planar dolomite range from several centimetres to >300 m wide and tens to hundreds of metres long parallel to faults and/or fractures; they have sharply defined margins, cross‐cut stratigraphy and locally finger out along beds or bed boundaries for tens of metres. Larger geobodies accompany NNE–SSW extensional faults with substantial breccia zones. One of these bodies hosts a sphalerite‐rich breccia deposit cemented by dolomite. Saddle dolomite lines or fills vugs and fractures within dolomite geobodies, and is a minor late diagenetic phase in undolomitized limestones. Replacive dolomite has low matrix porosity owing to non‐planar texture and associated cementation, and there is no evidence for subsequent leaching. Three dolomite stages are discriminated by texture, cathodoluminescence petrography and electron microscopy. Disseminated ‘Dolomite 1’ is substantially replaced and may be residual early diagenetic dolomite. Pervasive ‘Dolomite 2’ and ‘Dolomite 3’ have overlapping carbon–oxygen–strontium isotopic and fluid‐inclusion characteristics that indicate precipitation from allochthonous, high‐temperature (98 to 223°C) and high‐salinity (15 to 24 wt% NaCl eq.) brines. These variably equilibrated with host limestones and mixed with resident pore fluids. Overlying mudrocks formed a seal for ascending fluids. Integration of data from the mineral deposit suggests that fault‐fracture systems tapped different deep‐seated fluid reservoirs at different temperatures, and implies fluid interactions with both metamorphic basement and sedimentary cover in large‐scale circulation systems. This phenomenon probably took place during Mesozoic rifting, although an earlier event at the end of the Early Carboniferous cannot be discounted. In either case, a transient heat flow anomaly, previously unrecognized in the Irish Sea region, may be required to account for the hottest fluids.

Diagenetic Geobodies: Fracture-Controlled Burial Dolomite in Outcrops From Northern Oman

SummaryDiagenetic heterogeneities are difficult to predict in the subsurface. Nevertheless, such heterogeneities can be crucial in hydrocarbon exploration. Diagenetic processes can significantly alter petrophysical properties of reservoir rocks, especially in carbonate rocks because of the reactive nature of the carbonate minerals. Dolomitization [i.e., the transformation of calcite (limestone: Lmst) into dolomite] is a common diagenetic process in carbonate rocks. Resulting dolomite bodies have a different pore network than the original Lmst and respond also differently to tectonic stress causing different fracture networks than in the original Lmst. The paper presents an overview of the learning outcomes gained by studying fracture-related dolomite in outcrops of Oman and subsequent laboratory analysis conducted over the last 4 years. A combined structural, petrographic, and geochemical approach was taken to study three dolomite systems occurring in different stratigraphic host-rock (HR) intervals. Structurally controlled dolomitization (i.e., dolomitization along faults and fractures) typically occurs in burial conditions, and the resulting strong permeability anisotropies caused by the dolomite textures can cause major challenges for hydrocarbon exploration.Dolomite bodies in the Precambrian Khufai formation are related to N/S to NNE/SSW fractures, whereas dolomite bodies that mainly occur in the Jurassic HRs occur along reactivated WNW/ESE normal faults. These fracture-related dolomite bodies are generally less than 15 m wide, but can be up to a few hundred meters long. Late-diagenetic dolomite bodies were also recognized in Permian HRs, where they occur at or close to the contact between Permian Lmst and early-diagenetic dolomite. This late-diagenetic dolomite system can be traced laterally for at least hundreds of meters and occurs in wadis that are approximately 40 km apart. Our data indicate that there were several dolomitization events in the geological history of the succession, generating dolomite bodies with different characteristics. This paper highlights the need to understand timing and structural setting of dolomite bodies in the subsurface to improve reservoir management.

Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran

ABSTRACT Here, a case example of a dolomitized Eocene ramp setting from the southeastern Zagros Basin is documented and discussed in the context of published work. This is of significance as well-documented case examples of Eocene dolomitized inner platforms are comparably rare. The same is true for detailed diagenetic studies from the Zagros Basin in general. Three measured field sections were combined with detailed petrographic and geochemical analyses and four main dolomite types were defined. The most significant dolomite type is present in the form of a volumetrically significant occurrence of meter-thick beds of strata-bound dolostones. These dolomites are characterized by near-stoichiometric composition, fabric-retentive and fabric-destructive textures, subhedral to anhedral in shape and most being in the tens-of-microns range. Dolomite δ18O (averaging -2.6‰) values are depleted relative to that expected for precipitation from Eocene seawater (averaging 0‰), while δ13C (averaging -0.1‰) values are within the range of Eocene seawater values (averaging 0.5‰). Dolomite Type II and III 87Sr/86S values from 0.7079 to 0.7086 are somewhat elevated with respect to Eocene seawater (0.7077 and 0.7078). Based on these data, it is suggested that moderately evaporated seawater, via shallow seepage reflux, acted as agent for the initial dolomitization process. Subsequently, early diagenetic dolomites were recrystallized during shallow burial to variable degrees. The absence of volumetrically significant evaporitic deposits indicates that the salinity of porewater during dolomitization was beneath the threshold limit for gypsum precipitation. In addition, ascending saline fluids from deep-seated salt diapirs might have affected dolomitizing fluids.

Anorganski ogljikov cikel v sistemu tla-kamnina-podzemna voda v kraško-razpoklinskih vodonosnikih

The paper presents a systematic analysis of the isotopic composition of carbon (δ13CCaCO3) in carbonate rocks in central Slovenia, representing karst and fissured aquifers, and share of carbon contributions from carbonate dissolution and degradation of organic matter in aquifers, calculated from the mass balance equation. 59 samples of rocks (mainly dolomites) from Upper Permian to Upper Triassic age were analyzed. Samples of carbonate rocks were pulverized and ground to fraction of 45 µm and for determination of δ13CCaCO3 analyzed with mass spectrometer for analyses of stable isotopes of light elements-IRMS. The same method was used for determination of isotopic composition of dissolved inorganic carbon (δ13CDIC) in groundwater for 54 of 59 locations. Values of δ13CCaCO3 are in the range from -2.0 ‰ to +4.1 ‰, with an average δ13CCaCO3 value of +2.2 ‰. These values are typical for marine carbonates with δ13CCaCO3 around 0 ‰, although δ13CCaCO3 values differ between groups depending on the origin and age. Early diagenetic dolomites have relatively higher values of δ13CCaCO3 compared to other analyzed samples. The lowest values of δ13CCaCO3 were observed in Cordevolian and Bača dolomite, probably due to late diagenesis, during which meteoric water with lower isotopic carbon composition circulated in the process of Sedimentation. Values of δ13CDIC range from -14.6 ‰ to -8.2 ‰. Higher δ13CDIC values (-8.2 ‰) indicate a low Proportion of soil CO2 in the aquifer and rapid infiltration, while lower values (-14.6 ‰) indicate higher proportion of soil CO2 in the aquifer and slower infiltration. Calculated contributions of carbon from organic matter / dissolution of carbonates in the karstic and fissured aquifers show a similar proportion (50 % : 50 %).

Evolution, paleoecology and sequence architecture of an Eocene carbonate ramp, southeast Zagros Basin, Iran

ABSTRACTWe document and discuss the results of detailed fieldwork, facies analysis and the subsequent integration of paleoecological evidence from the Middle Eocene carbonate ramp succession in the southeast Zagros Basin (Jahrum Formation). A combination of a sea-level fall and tectonic and diapiric basement uplift favored the initiation of the Jahrum carbonate platform. The lower portions are affected by pervasive, probably early diagenetic dolomitization, whilst the upper Jahrum consists mainly of limestone. Here, the focus is on the limestone portions of the Jahrum Formation. Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the limestone intervals of the Jahrum Formation are interpreted for the upper Middle Eocene (Bartonian). The Jahrum Formation is capped by a major unconformity and overlain by the Lower Oligocene mixed clastic/carbonate Razak Formation.Based on data from field sections, eight facies associations and a series of sub-types have been established, which correspond to inner-, middle-and outer-ramp depositional environments. In their overall context, these data show a southward-dipping inner-ramp-to-basin transect. Towards the Coastal Fars (e.g. Hulur-01 Well) the Jahrum grades laterally into deep-marine Pabdeh foredeep shale units. Based on facies analysis and paleoecological evidence from larger benthic foraminifera, a major transgressive-regressive pattern is recognized in all outcrop sections of the Jahrum. The lowermost stratigraphic units of the formation are here interpreted as a distally steepened ramp. Evidence comes from abundant allochthonous shallow-water facies in the distal, deeper-ramp setting. Shallow-water carbonate clasts were exported towards the basin, a feature that is probably linked to relative sea-level fall control. Furthermore, local to regional basement instabilities by salt diapir-related basement reorganization was arguably of significance. Upsection, evidence is found that the ramp system evolved from a distally steepened to a homoclinal geometry with an overall very gentle slope geometry during the Late Bartonian.The data shown here are significant for those concerned with the Paleogene evolution of the southeast Zagros Basin and provide a well-exposed case example of a Middle Eocene carbonate ramp factory.

Hydrothermal and unexpected diagenetic alteration in Permian shales of the Lodève epigenetic U-deposit of southern France, traced by K–Ar illite and K-feldspar dating

Mineralogical and chemical analyses combined with K–Ar dating of illite and K-feldspars were carried out on bulk samples and different fractions from Permian shales around the Lodève U-deposit to identify diagenetic and hydrothermal alteration episodes and their timing. Authigenic 2M1 and 1M illite with K–Ar ages between 270 and 245Ma are the predominant mineral types in the shales. They are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi-arid conditions. Albite and a first generation of K-feldspar developed by a replacement of analcime, the K-feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite.The hydrothermally altered shales next to the U-deposit contain authigenic quartz, Fe-chlorite, neoformed illite, and a second-generation K-feldspar with K–Ar ages beginning at about 220Ma (Triassic), although Jurassic and Cretaceous ages were also obtained. This younger age spectrum (Jurassic/Cretaceous) is close to previous U–Pb isotope data from the U-deposit, probably reflecting several episodes of U-mobilization. Therefore, the hydrothermal activity extended episodically from Triassic to Cretaceous time within the U-deposit. The hydrothermal temperatures attained locally >220°C as the K–Ar system of illite from early Permian diagenesis is partially reset.Triassic hot fluids ascended along faults into the northern area of the Lodève basin during an initial rifting of the nearby western Tethys Ocean. Further fluid pulses were recorded at Lodève during the opening of the North Atlantic and separation of Europe from Africa (Jurassic/Cretaceous), which also caused the peak hydrothermal activities in Central and Western Europe. As the Lodève area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time.

Microbially mediated dolomite in Cambrian stromatolites from the Tarim Basin, north‐west China: implications for the role of organic substrate on dolomite precipitation

AbstractDolomite [CaMg(CO3)2] is abundant in sedimentary rocks throughout the geological record, but it is rarely found in modern sediments. Also, it cannot be precipitated under low‐temperature conditions in the laboratory without microbial mediation and, as a result, its origin remains a long‐standing enigma. This study reports biologically mediated dolomite precipitation in ancient microbial mats and biofilms from the Cambrian Tarim Basin. The ambient temperature at the time of dolomite precipitation was estimated from δ18O values from early diagenetic dolomite, and the presence of structures associated with extracellular polymeric substances (EPS), is composed of fibres arranged in a reticular pattern, would favour epitaxial crystallization of dolomite on an organic substrate. In addition, poorly crystallized dolomite formed nanocrystal aggregates that strongly resemble the morphology and size distribution observed in microbial culture experiments. These lines of evidence confirm that microbial structures can be preserved in ancient dolomite and validate their use as biosignatures.

Diagenesis of Paleozoic playa-lake and ephemeral-stream deposits from the Pimenta Bueno Formation, Siluro–Devonian (?) of the Parecis Basin, central Brazil

The Parecis Basin is a large intracratonic rift located in central Brazil and filled with Paleozoic carbonate, evaporite and siliciclastic sediments. The occurrence of gas seeps has recently attracted significant exploration interest by the Brazilian petroleum agency and by Petrobras. The continuously cored PB-01-RO well provided the first opportunity to study the depositional environments, diagenetic evolution and hydrocarbon potential of the largely unknown sedimentary successions of the Parecis Basin. The cored lithologies, belonging to the Siluro–Devonian (?) Pimenta Bueno Formation, are interpreted as deposited in playa-lake and ephemeral-stream environments. The deposits display a strong facies control on the diagenetic mineral assemblages and evolution. Diagenetic minerals in the ephemeral-stream deposits include eogenetic hematite and smectitic clay coats and quartz cement, and the mesogenetic process includes precipitation of sulfates (anhydrite and barite) and carbonates (calcite, dolomite and kutnahorite–ankerite–huntite), followed by partial dissolution of these carbonates and sulfates, and of feldspar grains. Telogenetic processes include the precipitation of hematite and kaolinite within secondary pores, and the replacement of anhydrite by gypsum. A second burial phase and mesodiagenesis is indicated by the precipitation of discrete K-feldspar crystals within moldic pores after dissolved feldspars, and by the illitization of etched, telogenetic kaolinite. The playa-lake deposits show early diagenetic dolomitization of lime mud, precipitation of anhydrite nodules and extensive silicification. The anhydrite nodules were replaced by gypsum and chalcedony during telodiagenesis. Potential source rocks are locally represented by organic shales. The fluvial sandstones show fair reservoir quality and limited compaction, as indicated by their intergranular volume, suggesting that the succession has undergone moderate burial. Potential seals for hydrocarbon accumulations are provided by the evaporites and playa-lake mudstones. The presence of an active petroleum system in the basin, as indicated by the gas seeps, suggests a good hydrocarbon exploration potential for the Parecis Basin, gas and stresses the importance of studying the depositional and diagenetic evolution of its sedimentary successions.

Dolomite formation within the methanogenic zone induced by tectonically driven fluids in the Peru accretionary prism

Early diagenetic dolomite formation in methanogenic marine sediments is enigmatic because acidifi cation by CO2, a by-product of methanogenesis, should lead to carbonate dissolution and not precipitation. However, petrographic relationships indicate that dolomite breccia layers with δ13C values of ~+15‰, recovered from the lower slope of the Peru continental margin (Ocean Drilling Program Site 1230), formed deep in the methanogenic zone during tectonic activity of a decollement. Based on radiogenic Sr isotope ratios (87Sr/86Sr > 0.711) and positive δ18O values (+6‰), we present evidence that the dolomite breccias mainly formed from fl uids originating from deep sedimentary units within the accretionary prism, where they interacted with continental crust and/or siliciclastic rocks of continental affi nity. Due to silicate alteration and dehydration, such fl uids are likely alkaline and thus have the potential to neutralize the acidifi cation imposed by the high dissolved CO2 concentrations. This scenario provides a potential mechanism by which dolomite formation can be induced deep in a highly active methanogenic zone.

Authigenesis of native sulphur and dolomite in a lacustrine evaporitic setting (Hellín basin, Late Miocene, SE Spain)

AbstractAbundant sulphur is present in the Late Miocene evaporitic sequence of the lacustrine Hellín basin in SE Spain. Weathering of Triassic evaporites controlled the chemical composition of the Miocene lake. The lacustrine deposits comprise gypsum, marlstones, diatomites and carbonate beds. Sulphur-bearing carbonate deposits predominantly consist of early diagenetic dolomite. Abundant dolomite crystals with a spheroidal habit are in accordance with an early formation and point to a microbial origin. The carbon isotopic composition of the dolomite (δ13C values between −10 and −4‰) indicates mixing of lake water carbonate and carbonate derived from the remineralization of organic matter by heterotrophic bacteria. Dolomite precipitated syngenetically under evaporitic conditions as indicated by high oxygen isotope values (δ18O between +6 and +11‰). Nodules of native sulphur are found in gypsum, carbonate beds and marlstone layers. Sulphur formed in the course of microbial sulphate reduction, as reflected by its strong depletion in34S (δ34S values as low as −17‰). Near to the surface many of the sulphur nodules were in part or completely substituted by secondary gypsum, which still reflects the sulphur isotopic composition of native sulphur (−18 to −10‰). This study exemplifies the role of bacterial sulphate reduction in the formation of dolomite and native sulphur in a semi-enclosed lacustrine basin during Late Miocene time.

From tabular to rhombohedral dolomite crystals in Zechstein 2 dolostones from Scharzfeld (SW Harz/Germany): A case study with combined CL and EBSD investigations

Tabular dolomite crystals found within dolomite rhombs have been investigated by cathodoluminescence (CL) microscopy and spectroscopy combined with electron backscatter diffraction (EBSD) for the first time. The dolomites formed in the Upper Permian Stassfurt Carbonate Ca2 at the southern margin of the German/Polish Zechstein Basin. Cathodoluminescence petrography of the dolostone succession revealed that the dolomites developed in four phases. Electron backscatter diffraction analysis reveals tabular crystal growth during the two first generations, while the last two generations are characterized by rhombohedral crystal shapes. The tabular dolomite cement crystals and their microcrystalline equivalents in matrix and components have a stoichiometric composition with good to very good lattice ordering. Manganese and iron contents of the tabular crystals are low and their carbon and oxygen isotope composition confirms an early diagenetic dolomite formation under marine-evaporitic conditions from precursor carbonates of Upper Permian age. CL spectroscopy reveals that the tabular dolomite generation 1 has a very high percentage of Mn 2+ on the Ca lattice position which results in a visually yellowish-green CL emission. Although relatively increased Mn 2+ contents at the Ca lattice position appear to be rather common in evaporitic dolomites the combination of a tabular crystal shape and a preferred input of Mn 2+ at the Ca lattice position is a remarkable phenomenon. As tabular dolomite crystals so far are exclusively reported from evaporitic diagenetic settings they could be the result of a high Mg/Ca ratio which blocks c-axis orientated growth of dolomite crystal. The occurrence of well ordered dolomite of which the geochemical zoning can be studied in such detail is rare for the earliest, synsedimentary stages of dolomite formation in marine environments, because these early stages commonly consist of not or badly ordered Ca-dolomites. A primary geochemical zoning of such dolomite usually gets lost during stabilisation and transformation to better ordering and stoichiometry.

Paleomagnetic and geochemical evidence for a pre-Triassic age for the Lower Amaranth Member of the Williston Basin of Manitoba (Canada)

Paleomagnetic and geochemical data obtained from six wells in southwestern Manitoba indicate that the Lower Amaranth redbeds were deposited earlier than Jurassic or Triassic, the most commonly cited depositional ages for this formation in the Williston Basin. The magnetization is carried primarily by detrital specular and pigmentary hematite and occasionally magnetite. Inclination-only analysis of paleomagnetic data (83 specimens from 60 plugs) indicates two possible depositional magnetization ages: Devonian–Pennsylvanian (D, found in very few samples) or Carboniferous to Permian, as suggested by the inclination and the polarity of the most predominant magnetization (B). An isolated magnetization (C) could be a mid-Jurassic to Neogene localized fluid flow remagnetization event. The oxygen and carbon isotope values of dolomite (–6.45‰ to 0.30‰ Vienna PeeDee Belemnite (VPDB) δ18O, –1.57‰ to 5.44‰ VPDB δ13C; n = 18) indicate that dolomitization could have occurred anytime between Carboniferous and Jurassic. However, the distribution of these values is a function of the three types of dolomite present in the Lower Amaranth strata: detrital, cement, and matrix replacive and reflects both the primary values and diagenetic overprints. Detrital and cement dolomite show depleted values in both oxygen and carbon isotopes, most likely inherited from the original values of the detrital dolomite, with superimposed effects of recrystallization. The matrix replacive dolomite has no detrital content, and its oxygen isotope values are similar to the expected values for primary or early diagenetic dolomite from Carboniferous to Jurassic times. Mixtures of detrital and replacive matrix dolomite give intermediate oxygen and carbon isotope values.

Comparison of the Late Triassic carbonate platform evolution and Lofer cyclicity in the Transdanubian Range, Hungary and Pelagonian Zone, Greece

For comparative studies of Upper Triassic cyclic platform carbonates, the Transdanubian Range (Hungary) and the Pelagonian Zone (Greece) were chosen. Paleogeographically they represent two distant segments of the passive margin of the Neotethys Ocean. During the Late Triassic, on this wide margin a very extensive tropical carbonate platform domain was developed, referred to as the Dachstein-type carbonate platform system. The Transdanubian Range (TR) represents a segment of a continent-encroaching platform system, whereas the Pelagonian-Subpelagonian Zone (PG) may have been a large isolated platform, surrounded by deep-water basins. The discussed Upper Triassic thick platform carbonates (Fődolomit/Hauptdolomit Formation and Dachstein Limestone in the TR, and Pantokrator Formation in the PG) are made up of cyclically arranged facies deposited under similar environmental conditions in the interior zones of carbonate platforms. Three characteristic major facies types can be distinguished: shallow subtidal-lagoonal, intertidal and supratidal-pedogenic, which correspond to the three typical lithofacies (members C, B and A) of Fischer's (1964) Lofer-cycle. The cycles are usually bounded by discontinuity surfaces related to subaerial exposure and pedogenic alteration. The meter-scale (Lofer) cyclicity is predominant throughout the successions. However, various stacking patterns including symmetric complete, truncated, incomplete, and condensed cycles or even alternating peritidal and subtidal facies without any disconformity are recognized in both areas studied. Pervasive or partial early diagenetic dolomitization affected some parts of the cyclic successions in both areas. However, age-dependence of the early dolomitization was clearly demonstrated only in the TR, where the older part of the platform carbonate succession was subject to pervasive dolomitization, whereas the younger part is non-dolomitized and there is a transitional unit between them. This trend is attributed to the climate changing from semiarid to more humid. The Upper Triassic platform carbonates of the TR and PG show strikingly similar features concerning the litho- and biofacies, the stacking pattern and the thickness of the elementary cycles, despite their distant and different paleogeographic setting within the western Neotethys realm. This suggests a eustatic signal, i.e. the cyclic deposition was essentially controlled by orbitally-forced eustatic sea-level changes, although the contribution of autocyclic mechanisms cannot be excluded either. Definite signatures of subaerial exposure (karstic features and vadose meteoric diagenesis) at and below the cycle boundaries also support allocyclic control. In the northeastern part of the TR the carbonate platform was drowned at the Triassic-Jurassic boundary, whereas platform conditions persisted until the end of the Hettangian in the southwestern part. However, the Hettangian part of the succession is characterized by non-cyclic subtidal limestone, implying an upward-deepening trend. In contrast, in the PG the platform conditions continued until early to middle Liassic, and the Liassic succession is typified by well-developed pedogenic features, suggesting long-lasting subaerial exposure intervals, i.e. an upward-shallowing trend.

Early diagenetic dolomitization and dedolomitization of Late Jurassic and earliest Cretaceous platform carbonates: A case study from the Jura Mountains (NW Switzerland, E France)

Early diagenetic dolomitization is a common feature in cyclic shallow-water carbonates throughout the geologic record. After their generation, dolomites may be subject to dedolomitization (re-calcification of dolomites), e.g. by contact with meteoric water during emersion. These patterns of dolomitization and subsequent dedolomitization frequently play a key role in unravelling the development and history of a carbonate platform. On the basis of excellent outcrops, detailed logging and sampling and integrating sedimentological work, high-resolution sequence stratigraphic interpretations, and isotope analyses (O, C), conceptual models on early diagenetic dolomitization and dedolomitization and their underlying mechanisms were developed for the Upper Jurassic / Lower Cretaceous Jura platform in north-western Switzerland and eastern France. Three different types of early diagenetic dolomites and two types of dedolomites were observed. Each is defined by a distinct petrographic/isotopic signature and a distinct spatial distribution pattern. Different types of dolomites are interpreted to have been formed by different mechanisms, such as shallow seepage reflux, evaporation on tidal flats, and microbially mediated selective dolomitization of burrows. Depending on the type of dolomite, sea water with normal marine to slightly enhanced salinities is proposed as dolomitizing fluid. Based on the data obtained, the main volume of dolomite was precipitated by a reflux mechanism that was switched on and off by high-frequency sea-level changes. It appears, however, that more than one dolomitization mechanism was active (pene)contemporaneously or several processes alternated in time. During early diagenesis, percolating meteoric waters obviously played an important role in the dedolomitization of carbonate rocks that underlie exposure surfaces. Cyclostratigraphic interpretation of the sedimentary succession allows for estimates on the timing of early diagenetic (de)dolomitization. These results are an important step towards a better understanding of the link between high-frequency, probably orbitally forced, sea-level oscillations and early dolomitization under Mesozoic greenhouse conditions.

Hiatal surfaces from the Miocene Globigerina Limestone Formation of Malta: Biostratigraphy, sedimentology, trace fossils and early diagenesis

The Miocene Globigerina Limestone of the Maltese islands contains widespread omission surfaces with very different characteristics and origins. The terminal Lower Globigerina Limestone hardground (TLGLHg) formed during a period of falling sea level. Coccolith assemblages suggest shallowness. Sedimentary structures and trace fossil assemblages, indicate increasing frequency of storm events and erosional episodes, towards the surface. Calcite cementation which took place around Thalassinoides burrows and formed irregular nodules was followed by dissolution of aragonite. It is suggested that lithification was linked to microbial reactions involving organic matter. In contrast two later surfaces, the terminal Middle Globigerina Limestone omissionground (TMGLOg), which marks the Lower to Middle Miocene boundary, and the Fomm-ir-Rih local hardground (FiRLHg) both contain early diagenetic dolomite. Lithification took place in two phases. The dolomite is interpreted to have formed beneath the sea floor; it was subsequently exhumed and partially corroded as the precipitation of calcitic and phosphatic cements took place around burrows open to the circulation of sea water.

Sealevel changes control diagenetic dolomite formation in hemipelagic sediments of the Peru Margin

The first experimentally determined temperature dependent oxygen-18 fractionation factor between dolomite and water at low temperatures [Vasconcelos, C., McKenzie, J.A., Warthmann, R., Bernasconi, S., 2005. Calibration of the δ 18O paleo-thermometer with dolomite formed in microbial cultures and natural environments. Geology 33, 317–320] allows now the precise calculation of temperatures during early diagenetic dolomite precipitation. We use δ 18O values of early diagenetic dolomite beds sampled during ODP Legs 112 and 201 on the Peru continental margin (Sites 1227, 1228 and 1229) [Meister, P., McKenzie, J.A., Vasconcelos, C., Bernasconi, S., Frank, M., Gutjahr, M., Schrag, D.P., 2007. Dolomite formation in the dynamic deep biosphere, results from the Peru Margin, OPD Leg 201. Sedimentology 54, 1007–1032] to calculate paleo-porewater temperatures at the time of dolomite precipitation. We assumed unaltered seawater δ 18O values in the porewater, which is supported by δ 18O values of the modern porewater presented in this study. The dolomite layers in the Pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 °C lower than today. Since Sites 1228 and 1229 are located at 150 and 250 m below sealevel, respectively, their paleo-porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands. Thus, Pleistocene dolomite layers in the Peru Continental margin probably formed during glacial times. This finding is consistent with a model for dolomite precipitation in the Peru Margin recently discussed by Meister et al. [Meister, P., McKenzie, J.A., Vasconcelos, C., Bernasconi, S., Frank, M., Gutjahr, M., Schrag, D.P., 2007. Dolomite formation in the dynamic deep biosphere, results from the Peru Margin, OPD Leg 201. Sedimentology 54, 1007–1032], where dolomite forms episodically at the sulphate methane interface. It was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column, but dolomite layers may only form when the sulphate–methane interface stays at a fixed depth for a sufficient amount of time. We hypothesize that the sulphate–methane interface persists within TOC-rich interglacial sediments, while this zone is buried by TOC-poor sedimentation during glacial times. Thus, the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations. A similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a Milankovitch type pattern observed in the geological record, such as in the Miocene Monterey Formation.

Barite-forming environments along a rifted continental margin, Southern California Borderland

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. δ34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have δ34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time of deposition of the host rocks for the PV samples. This indicates that the mineralizing fluids were not unaltered seawater. We develop a model in which the barites precipitated both below the sediment–water interface and at the seafloor from low-temperature fluids that circulated along faults. The isotopic, chemical, and textural data indicate that the barites formed by several processes. Mesozoic and Cenozoic basement rocks (crystalline and overlying sedimentary rocks), Quaternary basin fill, turbidite fans, and seawater provided various elements for the barites in different environments. The fluids had a deep-seated source and were hydrothermal in the deeper parts of the system for all the barite types, including the seafloor cold-seep deposit, based on Sr isotopes and regional geothermal gradients. These deep-seated fluids mixed with other fluids as they ascended, including seawater at and near the seafloor. The high δ34S values may have resulted from extreme Rayleigh fractionation during bacterially mediated (semi)closed-system sulfate reduction, probably driven by the flux of methane- and hydrocarbon-bearing fluids through basement rocks and basin sediments. Early diagenetic dolomite and pyrite in the host Monterey Formation in the PV Headland also formed in a zone of sulfate reduction, but within sediment buried only centimeters to a few meters and with a predominantly seawater source for the sulfur. Dolomite occurring with vein barite in the PV Headland formed at temperatures in the range of 40–90 °C. The cold-seep barites have δ34S values near that of modern seawater, although still somewhat fractionated. The barites that precipitated below the sediment–water interface have higher δ34S values, suggesting that the fluids were relatively reduced with molar dissolved barium in excess of dissolved sulfate. Those samples were exposed at the seafloor by uplift along faults and are composed predominantly of massive, brecciated, and vein barite.

Tectonic evolution and paleogeography of the Mesozoic Pucará Basin, central Peru

The Pucará Basin of Peru is an elongate trough that subsided landward of a NNW-trending structural high during the Late Triassic–Early Jurassic. It formed as a postrift regional sag as the earlier Triassic fault-controlled Mitu rifts yoked together. The rift and transitional postrift basins were associated with a NW-striking sinistral shear zone that controlled isopachs and facies distributions and resulted in magmatism and mineralization along its trend. A distinct association of later dolomitization and MVT lead–zinc mineralization also occurs with these basin-forming shear zones. Although basaltic and andesitic extrusives are common, there is no evidence that the Pacific margin was a magmatic arc until the upper Pucará, and then only weakly developed in northern Peru. Except in the upper Pucará of northwest Peru, geochemical studies, including whole rock and trace element analyses, indicate that intercalations of volcanic material have intraplate rift affinities. The basin fill has a three-part stratigraphic subdivision, comprising lower and upper carbonate platforms with an intermediate phase of basin overdeepening and sediment starvation that resulted in a regional, organic-rich argillaceous drape. Stratigraphic accumulation was dominated by axial patterns of onlap and progradation, though facies characteristics show it was augmented by periodic flooding of the western basin margin high. Marine invertebrate fossils indicate normal marine salinities. The sedimentological interpretation is based on a SW–NE transect in the southern part of the Pucará Basin. The Chambará (Norian–Rhaetian) and Condorsinga (Toarcian) formations were constructed principally by shallow-water carbonate sedimentation in lagoon-like subtidal, intertidal, and supratidal paleoenvironments. The subtidal carbonate platform is dominated by oolitic grainstones with subordinate bioclastic packstones. Subordinate open-basin facies in the Chambará Formation consist principally of crinoidal packstones and bioclastic wackestones. In the intertidal and supratidal facies, evaporite pseudomorphs are common and generally associated with algal mats and widespread early diagenetic dolomitization. During the Chambará and Condorsinga, subsidence typically was balanced by carbonate production and shallow-water environments prevailed; the basin had the characteristics of an overfilled basin. Conversely, the intermediate late Rhaetian–Sinemurian stage of basin subsidence was marked by underfilled deep water conditions. This widespread transgressive inundation of the Pucará Basin, recorded in the argillaceous Aramachay stratigraphy, correlates with similar events in other Andean basins.

Trends in oolite dolomitization across the Neoproterozoic–Cambrian boundary: A case study from Death Valley, California

Changes in dolomite fabric within the Neoproterozoic–Cambrian stratigraphic succession in Death Valley, California preserve evidence for a first-order biogeochemical change in early diagenetic environments through the Neoproterozoic–Cambrian transition. Oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time. Two key generalizations can be made: (1) late Neoproterozoic dolomitization in Death Valley occurred very early in the paragenetic sequence, was volumetrically significant, and preserved fine detail of the original aragonite or calcite precursor; and (2) Early Cambrian dolomitization in Death Valley occurred late in the paragenetic sequence with burial, was commonly incomplete, and obliterated details of the calcite or aragonite precursor; early diagenetic dolomitization of any kind was absent. The dolomitization fabrics in Death Valley parallel the presumed pattern of seawater Mg/Ca with respect to mimetic dolomitization in the Paleozoic (low Mg/Ca, little/no mimetic dolomitization) and Cenozoic (higher Mg/Ca, more common mimetic dolomitization). However, the volume of Neoproterozoic mimetic dolomitization is immense compared to the Cenozoic examples. We speculate that a fundamental shift in the early diagenetic environment occurred in late Neoproterozoic time, perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment-capping microbial mats. An oscillatory trend in the fabric and volume of dolomite through the Phanerozoic has been suggested, but the termination of volumetrically significant mimetic dolomitization across the Neoproterozoic–Cambrian transition, a critical time in Earth history for many reasons, dwarfs the oscillatory trends noted in the Phanerozoic.