A depositional model for the terminal Neoproterozoic–Early Cambrian Ara Group evaporites in south Oman

Abstract Six evaporite–carbonate sequences are recognized in the terminal Neoproterozoic–Early Cambrian Ara Group in the subsurface of Oman. Individual sequences consist of a lower, evaporitic part that formed mainly during a lowstand systems tract. Overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts. Detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment, source of brines and basin evolution. At the beginning of the evaporative phase (prograding succession), a shallow-water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas. Minor amounts of highly soluble salts locally record the last stage of basin desiccation. This gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow-water halite from shallow-water sulphate, and shallow-water sulphate from relatively deeper water carbonate respectively. These surfaces record repeated flooding of the basin, dissolution of evaporites and stepwise reduction in salinity. Final flooding led to submergence of the basin and the establishment of an open-water carbonate ramp. Marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines. The Ara Group sequences represent a time of relatively stable arid climate in a tectonically active basin. Strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres, while tectonic barriers simultaneously provided the required restricted conditions. Subsidence allowed evaporites to blanket basinal and platform areas. The study suggests a deep-basin/shallow-water model for the evaporites.     

Halite saltern in the Canning Basin, Western Australia: a sedimentological analysis of drill core from the Ordovician-Silurian Mallowa Salt

The Late Ordovician-Early Silurian Mallowa Salt of the Carribuddy Group, Canning Basin, north-west Australia, is the largest halite deposit known in Australia, attaining thicknesses of 800 m or more within an area of approximately 200 000 km². Study of 675 m of drill core from BHP-Utah Minerals’ Brooke No. 1 well in the Willara Sub-basin indicates that the Mallowa Salt accumulated within a saltern (dominantly subaqueous evaporite water body) that was subject to recurrent freshening, desiccation and exposure. Textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite-mudstone cycles (Type 2) and the less common dolomite/anhydrite-halite-mudstone cycles (Type 1). The borate mineral priceite occurs in the capping mudstones of some cycles, reinforcing the idea of an increasing continental influence toward the top of mudstone-capped halite cycles. The rock salt in both Type 1 and Type 2 cycles typically comprises a mosaic of large, randomly orientated, interlocking halite crystals that formed during early diagenesis. It only partially preserves a primary sedimentary fabric of vertically elongate crystals, some with remnant aligned chevrons. Intraformational hiati, halite karst tubes and solution pits attest to episodic dissolution. Stacked Type 2 cycles dominate; occasional major recharges of less saline, perhaps marine, waters in the same area produced Type I cycles. The envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the Permian Salado Formation in New Mexico and the Permian San Andres Formation of the Palo Duro Basin area in Texas. However, in the Canning Basin the cycles are characterized by a much lower proportion of anhydrite, implying perhaps a greater degree of continental restriction to the basin. The moderately high level of bromine in the Mallowa Salt (156·5 ± 43·5 ppm Br for primary halite, 146·1 ± 54·7 ppm Br for secondary halite) accords with evolved continental brines, although highly evaporative minerals such as polyhalite and magnesite are absent. The bromine levels suggest little or no dissolution/reprecipitation of primary halite and yet, paradoxically, there is little preservation of the primary depositional fabric. The preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths, probably at burial depths of no more than a few metres, and the resultant preservation of primary bromine values in the secondary halite.     

Compaction in halite-cemented carbonates – the Dawson Bay Formation (Middle Devonian) of Saskatchewan, Canada

Halite-impregnated carbonates in the Dawson Bay Formation of Saskatchewan lie between beds of halite and are buried to a depth of 1 km. They exhibit two different diagenetic styles – some resisted compaction and had high pre-salt porosities; others contain compaction-broken fossils and pressure-solution seams. The uncompacted rocks, together with the difficulty of explaining how halite cement could enter the Dawson Bay after overlying bedded halites were deposited, suggest that halite cementation occurred early with only a few tens of metres of overburden. Early diagenetic compaction is suggested by the presence of unbroken, displacive skeletal halite crystals, which cross-cut compaction structures, and by the difficulty of explaining how (1) later compaction could occur in halite-cemented rocks and (2) how pore-fluids could be expelled after surrounding rocks lost their permeability. The organic-rich nature of many carbonates may explain why compaction was both early and extensive, but this explanation fails to explain how similar compaction developed in horizons with lower organic contents. Chemical compaction may also have been enhanced by aragonite dissolution during seawater evaporation or brine dilution. Early chemical compaction in Dawson Bay carbonates indicates that compaction in other carbonates need not signify deep burial diagenesis; neither can compaction be used indiscriminately to identify other diagenetic events as being of deep burial origin. Early halite cementation, as in the Dawson Bay Formation, preserves carbonates at early diagenetic stages and may thus preserve geochemical information unmodified by later diagenesis.     

Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates

Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ¹³C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ¹⁸O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with ¹⁸O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction.     

Diagenesis of spiculites and carbonates in a Permian temperate ramp succession – Tempelfjorden Group,Spitsbergen, Arctic Norway

This paper explores little investigated diagenesis of spicule‐dominated sediments, based on Permian spiculites and cool‐water carbonates of the Tempelfjorden Group in central Spitsbergen. Field observations, petrography, stable isotope geochemistry, and mineralogical and chemical analyses reveal that the strata have been subjected to multistage diagenesis as the result of silica phase transitions at medium burial depths and deep‐burial overprinting. The growth of silica concretions occurred during the opal‐A/opal‐CT conversion and was controlled by the content and distribution of clay and spicules in the sediment, resulting in a variety of megascopic silica fabrics. Opal‐CT was subsequently dissolved, and all silica is now in a stable quartz stage. Petrographically, the rocks are characterized by a variety of chalcedony and quartz cements which perfectly preserve precursor textures. Most cements precipitated from silica‐oversaturated fluids, and their shapes reflect the silica saturation state and geometry of the pore space. Some microquartz and cryptoquartz also formed by a solid–solid inversion (recrystallization) of chalcedony. The cements have δ ¹⁸O values between +30‰ and +20‰ Standard Mean Ocean Water and display a systematic depletion in ¹⁸O from the first to the last crystallized, interpreted to reflect a gradual increase in temperature during burial. The precipitation of quartz cements started in the Middle Triassic when the strata passed the 19°C isotherm at burial depths of ca 600 m, and was completed in the mid‐Cretaceous, 2·3 km beneath the sea floor at temperatures of 75°C. Late diagenetic overprinting of the chert includes fracturing, brecciation and cementation with carbonate cements having δ ¹⁸O values between +2‰ and −30‰ Pee Dee Belemnite and δ ¹³C values between +4‰ and −14‰ Pee Dee Belemnite; they are linked to hot solutions introduced during Cretaceous volcanism or Palaeogene tectonism. This study illustrates the diagenetic pathway during burial of spicule‐rich sediments in a closed system and thereby provides a baseline for studies of more complexly altered chert deposits.     

Environmental and sequence stratigraphic implications of anhydrite textures: A case from the Lower Triassic of the Central Persian Gulf

The Lower Triassic Kangan Formation in the Persian Gulf (South Pars Gas Field) and its adjacent areas are composed of carbonate–evaporite sequences. These sediments were deposited in a shallow marine homoclinal ramp. Study of the anhydrite-bearing intervals shows various structures and textures. The anhydrite structures are mainly bedded, massive, chicken-wire and nodular type and the main textures are felted, sparse crystal, needle shape, lath shape, equant and fibrous. Pervasive and poikilotopic cement together with replacement and porphyroblastic gypsum are accounted as the most common diagenetic features in anhydrite. Evaluation of anhydrite occurrences and features support both primary and secondary formations. The nodular to chicken-wire anhydrite formed under synsedimentary sabkha conditions, whereas anhydrite cements occurred during the late stages of diagenesis (shallow burial stage). Massive to bedded anhydrite could have been formed under subaqueous conditions or originated by coalescing and continued growth of anhydrite nodules in the sabkha zone. Anhydrite fabrics impose a significant control on the reservoir quality of the Kangan carbonates at the South Pars Gas Field. Thick massive and bedded anhydrite could have been formed as an intraformational seals and anhydrite cements occluded pore spaces and reduced the poroperm values. The sequence stratigraphic analysis revealed two depositional sequences in the studied intervals, which are composed of TST and HST. Investigation of anhydrite throughout depositional sequences indicates a change in the content and style of anhydrite texture. Anhydrite content (volume) decreases upward through transgressive system tract (sea-level rise) whereas, it enhances during highstand system tract (sea-level fall). Pervasive and poikilotopic anhydrite cements together with replacement by anhydrite are prevalent features during transgressive and early highstand system tract. At the late HST, with a progradational stacking pattern, anhydrite value increases and felted, radial, equant, crystalline and mosaic texture are the most common anhydrite fabrics. Sequence boundaries that indicate maximum sea level fall and exposure of successions are marked by the broad anhydrite deposits with massive to bedded and chicken-wire structures and various textures that located in late HST package. There is an unambiguous relationship between the microfacies associations, the evaporite textures, and the sea-level fluctuations. This relationship could lead to a predictable pattern that can be of use as a general guide for the sequence stratigraphic interpretations in the area.     

Characteristics of Hydrocarbon Fluid Inclusions and Their Significance for Evolution of Petroleum Systems in the Dabashan Foreland, Central China

Field investigation combined with detailed petrographic observation indicate that abundant oil,gas,and solid bitumen inclusions were entrapped in veins and cements of sedimentary rocks in the Dabashan foreland,which were used to reconstruct the oil and gas migration history in the context of tectonic evolution.Three stages of veins were recognized and related to the collision between the North China block and the Yangtze block during the Indosinian orogeny from Late Triassic to Early Jurassic(Dl),the southwest thrusting of the Qinling orogenic belt towards the Sichuan basin during the Yanshanian orogeny from Late Jurassic to Early Cretaceous(D2),and extensional tectonics during Late Cretaceous to Paleogene(D3),respectively.The occurrences of hydrocarbon inclusions in these veins and their homogenization temperatures suggest that oil was generated in the early stage of tectonic evolution,and gas was generated later,whereas solid bitumen was the result of pyrolysis of previously accumulated hydrocarbons.Three stages of hydrocarbon fluid inclusions were also identified in cements of carbonates and sandstones of gas beds in the Dabashan foreland belt and the Dabashan foreland depression(northeastern Sichuan basin),which recorded oil/gas formation,migration,accumulation and destruction of paleo-reservoirs during the D2.Isotopic analysis of hydrocarbon fluid inclusions contained in vein minerals shows that δ~(13)C_1 of gas in fluid inclusions ranges from-17.0‰ to-30.4‰(PDB) and δD from-107.7‰ to-156.7‰(SMOW),which indicates that the gas captured in the veins was migrated natural gas which may be correlated with gas from the gas-fields in northern Sichuan basin.Organic geochemical comparison between bitumen and potential source rocks indicates that the Lower Cambrian black shale and the Lower Permian black limestone were the most possible source rocks of the bitumen.Combined with tectonic evolution history of the Dabashan foreland,the results of this study suggest that oil was generated from the Paleozoic source rocks in the Dabashan area under normal burial thermal conditions before Indosinian tectonics and accumulated to form paleo-reservoirs during Indosinian collision between the North China block and the Yangtz block.The paleo-reservoirs were destroyed during the Yanshanian tectonic movement when the Dabashan foreland was formed.At the same time,oil in the paleo-reservoirs in the Dabashan foreland depression was pyrolyzed to transform to dry gas and the residues became solid bitumen.     

Replacement of evaporites within the Permian Park City Formation, Bighorn Basin, Wyoming, USA

The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate grains. Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filled with widespread, very coarse, blocky calcite spar. These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement and pore-filling calcites have δ¹³C values ranging from 0·5 to -25·3%, and δ¹⁸O values of -16·1 to -24·3₀ (PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid-Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with the in situ hydrocarbon-rich pore fluids and evaporite minerals, and precipitated calcite cements. The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non-ferroan), and hydrocarbon emplacement. The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.     

Facies analysis of the Trypali carbonate unit (Upper Triassic) in central-western Crete (Greece): an evaporite formation transformed into solution-collapse breccias

The Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.     

Deformation mechanisms of deeply buried and surface-piercing Late Pre-Cambrian to Early Cambrian Ara Salt from interior Oman

We compared microstructures of Late Pre-Cambrian to Early Cambrian Ara Salt diapirs from the deep subsurface (3.5–5 km) of the South Oman Salt Basin and from surface-piercing salt domes of the Ghaba Salt Basin. Laterally, these basins are approximately 500 km apart but belong to the same tectono-sedimentary system. The excellent data situation from both wells and outcrops allows a unique quantification of formation and deformation mechanisms, spanning from sedimentation to deep burial, and via re-activated diapir rise to surface piercement. Microstructures of gamma-irradiated and etched thin sections indicate dislocation creep and fluid-assisted grain boundary migration as the main deformation mechanisms operating in the deep subsurface. Microstructures from the surface are characterised by large ‘old’ subgrain-rich crystals. These ‘old’ grains are partly replaced by ‘new’ subgrain-free and subgrain-poor crystals, which show gamma irradiation-decorated growth bands and fibrous microstructures, indicative of pressure solution creep and static recrystallisation, most likely due to surface piercement and exposure. Using subgrain size piezometry, the maximum differential stresses for the subsurface salt is 1.7 MPa and those for the surface-piercing salt is 3.4 MPa, the latter value displaying the high stress conditions in the diapir ‘stem’ as the salt rises on its way to the surface.     

柴达木盆地西部尕斯库勒盐湖280 ka以来沉积特征

以柴达木盆地西部尕斯库勒盐湖干盐滩6个钻孔岩芯为研究对象,从岩性特征、成盐期、沉积类型、沉积结构、沉积幅度等方面探讨尕斯库勒盐湖沉积特征。研究表明,自280 ka以来尕斯库勒盐湖经历相对湿润-干旱的气候波动和气候演化,在距今43.6 ka左右进入最干旱时期,可能属于柴达木盆地第二次成盐期;沉积结构层分异现象不明显;各成盐期平均沉积速率变化不大,平均沉积速率比新疆和内蒙古地区高; 该湖沉积中心在盐湖的西北部。     

Zur Diagenese fluviatiler Sandsteine

Are cement minerals that form during the first stages of diagenesis indicative of the environment of deposition? Discussions of fluvial, marine, and evaporitic sandstones of Triassic and Upper Carboniferous age (fig. 1) indicate that kaolinite, sudoite, and potassium feldspar cements can be regarded as characteristic of freshwater sandstones, whereas (early) chlorite, analcime and albite cements are found in marine and evaporitic sandstones mainly, provided that influences of meteoric water during diagenesis can be excluded. Humid climate favors kaolinite, arid favors potassium feldspar. Early diagenetic siderite concretions develop in coal swamps. Reddish biotites are not resistant during synsedimentary weathering in humid alluvial plains. As a consequence of increasing pH, temperature, and salinity of the interstitial fluids with increasing burial depth, common sequences of cementation include (1) silicates + quartz, (2) carbonates, (3) sulphates, (4) halite (fig. 3). The formation of quartz — the most important cement — is mainly governed by modifications of the micro-environment (e.g. pressure solution). In the Lower Triassic Buntsandstein, lithification due to quartz cementation occurred at a burial depth of about 1000–1200 m in alluvial as well as in brackisch to marine sandstones. Cementation by potassium feldspar (in the alluvial sandstones) and by analcime transforming into albite (in the brackish to evaporitic sandstones) occurred earlier. This was shown by investigations of minus-cement-porosity and contact strength (fig. 2).     

Ecological succession, palaeoenvironmental change, and depositional sequences of Barremian–Aptian shallow-water carbonates in northern Oman

Abstract Barremian and Aptian shallow‐water carbonate facies (uppermost Lekhwair, Kharaib and Shuaiba Formations) are described from outcrops in northern Oman. Based on facies analysis and bedding pattern, three orders of depositional sequences are defined (third to fifth order) and correlated between sections. Over the course of three third‐order sequences, covering the Barremian to Lower Aptian, a third‐order depositional pattern is documented that consists of a succession of three distinct faunal assemblages: discoidal orbitolinids and calcareous algae were deposited during early transgression; microbialites and microencrusters dominate the late transgressive to early highstand facies; and a rudist‐ and miliolid‐dominated facies is typical of the highstand. This ecological succession was controlled largely by palaeoenvironmental changes, such as trophic level and clay influx, rather than sedimentological factors controlled by variations in accommodation space. Orbitolinid beds and carbonates formed by microbialites and microencrusters seem to be the shallow‐water carbonate response to global changes affecting Late Barremian to Aptian palaeoclimate and palaeoceanography.     

Controls of facies and sediment composition on the diagenetic pathway of shallow-water Heterozoan carbonates: the Oligocene of the Maltese Islands

Relatively few studies have so far addressed diagenetic processes in Heterozoan carbonates and the role that sediment composition and depositional facies exert over diagenetic pathways. This paper presents a study of Oligocene shallow-water, Heterozoan carbonates from the Maltese Islands. We investigate stratigraphic distribution, abundance and timing of diagenetic features and their relationship to sediment composition and depositional facies. The studied carbonate rocks comprise rud- to packstones of the Heterozoan association predominantly containing coralline red algae, bryozoans, echinoids and benthic foraminifers. XRD analyses show that all high-Mg calcite has been transformed to low-Mg calcite and that no aragonite is preserved. Diagenetic processes include dissolution of aragonitic biota, neomorphism of high-Mg calcitic biota to low-Mg calcite and cementation by fibrous, bladed, epitaxial and blocky cements. Stable isotopes on bulk rock integrated with petrographic data suggest that the study interval was not exposed to significant meteoric diagenesis. We interpret early cementation to have taken place in the marine and marine burial environment. The distribution and abundance of early diagenetic features, determining the diagenetic pathway, can be related to the primary sediment composition and depositional texture. Sorting and micrite content are important controls over the abundance of diagenetic features.     

Very early diagenesis in a calcareous,organic-rich mudrock from Jordan

The diagenesis in the organic-rich Cretaceous to Eocene Al Hisa Phosphorite Formation (AHP), Muwaqqar Chalk Marl Formation (MCM) and Umm Rijam Chert-Limestone Formation (URC) formations of Jordan can be linked directly to the fluctuating sedimentary environment of this shelf depositional system in the Middle to Late Eocene, and its influence on the composition of the deposited sediment and the early burial diagenetic environment. Most cementation was early, mostly within the first 10 m of burial, perhaps entirely within the first 100 m of burial. We propose that the siliceous cements are derived from biogenic silica, probably of diatoms, deposited in a shelf of enhanced productivity. Volumetrically, the most important processes were the redistribution of biogenic opal-A (diatoms) and calcite to form pervasive, layered and nodular cements. The formation of the silica and carbonate cements is closely linked through the effects their dissolution and precipitation have on pore fluid chemistry and pH. The chert beds have a biogenic silica origin, formed through replacement of diatoms and radiolaria by opal-CT, and subsequently by quartz. Calcite cement has carbonate derived from microbial diagenesis of organic matter and calcium derived from seawater. The Mg for early dolomite may have been generated by replacement of opal-CT by quartz, ore dissolution of unstable high Mg calcite bioclasts. The silica and carbonate diagenetic processes are both linked to microbial diagenesis of organic matter, and are intimately linked in both time and space, with pH possibly influencing whether a silica or a carbonate mineral precipitates. The paucity of metal cations capable of precipitating as sulphides is crucial to the creation of acidic pore water favourable to silica precipitation, either as opal-CT, chalcedony or quartz. The lack of clay minerals as a sink for the Mg required for opal-CT polymerisation is the principal factor responsible for the remarkably early silica cementation. All the diagenetic processes, with the probable exception of the opal-CT to quartz transition are early, almost certainly within the first 10 m of burial, possibly much less. A paragenetic sequence is presented here based on these two cores that should be tested against a wider core distribution to see whether this diagenetic history can be generalised throughout the basin. Warm bottom water temperatures probably led to silica diagenesis at much shallower burial depths than occurs in many other sedimentary basins. Silicified layers, in turn, commonly host fractures, suggesting that mechanical properties of the strata began to differentiate at a very early stage in the burial cycle. This has wide implications for processes linking diagenesis to deformation.     

Shallow-marine carbonate cementation in Holocene segments of the calcifying green alga Halimeda

Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively, this study shows that early-diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.     

滨里海盆地的岩相古地理特征及其演化

滨里海盆地内充填了巨厚的古生代、中生代和新生代沉积物。在剖面上可分为三套地层组合，即盐下层系、含盐层系和盐上层系。盐下层系为下古生界-下二叠统，包括巨厚的碎屑岩和碳酸盐岩沉积，在泥盆纪-早二叠世，滨里海盆地周缘广大地区普遍发育碳酸盐岩，在许多古隆起上还发育生物礁体，说明这一时期盆地的滨、浅海地带具有浅、清、暖的沉积环境，盆外陆源碎屑供应较少。含盐层系为下二叠统上部孔谷阶。早二叠世中-后期由于持续构造抬升，盆地气候变得干旱，海水变浅，潮上带蒸发环境发育，以致这一时期广泛发育盐类沉积，形成含盐层系，主要由盐岩和硬石膏层构成，并形成许多大小不等的盐丘构造。盐上层系为上二叠统-第四系。晚二叠世-三叠纪盆地又经历了一次大的海侵，为浅海陆棚环境，局部为海陆过渡三角洲相。侏罗纪-白垩纪在总的浅海陆棚环境下，盆地不同地区也形成了湖泊和瀉湖环境。晚二叠世以后形成的盐上层系沉积，主要为碎屑岩，在局部地区有碳酸盐岩。     

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

Abstract Interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ¹⁸O = 0·9 ± 1·0‰, N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ¹⁸O = 2·1 ± 0·7‰, N = 48) than the hypersaline backreef dolomite (δ¹⁸O = 3·6 ± 0·9‰, N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.