Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia

Spencer Gulf is a large (ca 22 000 km²), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m^?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m^?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m^?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec^?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.     

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

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

Palaeoclimate of the Late Jurassic of Portugal: comparison with the Western United States

Investigation of the palaeoclimatic conditions associated with Upper Jurassic strata in Portugal and comparison with published palaeoclimate reconstructions of the Upper Jurassic Morrison Formation in western North America provide important insights into the conditions that allowed two of the richest terrestrial faunas of this period to flourish. Geochemical analyses and observations of palaeosol morphology in the informally named Upper Jurassic Lourinhã formation of western Portugal indicate warm and wet palaeoclimatic conditions with strongly seasonal precipitation patterns. Palaeosol profiles are dominated by carbonate accumulations and abundant shrink‐swell (vertic) features that are both indicative of seasonal variation in moisture availability. The δ¹⁸O_SMOW and δD_SMOW values of phyllosilicates sampled from palaeosol profiles range from +22·4‰ to +22·7‰ and ?53·0‰ to ?37·3‰, respectively. These isotope values correspond to temperatures of formation between 32°C and 39°C ± 3°, with an average of 36°C, which suggest surface temperatures between 27°C and 34°C (average 31°C). On average, these surface temperature estimates are 1°C higher than the highest summer temperatures modelled for Late Jurassic Iberia using general circulation models. Elemental analysis of matrix material from palaeosol B‐horizons provides proxy (chemical index of alteration minus potassium) estimates of mean annual precipitation ranging from 766 to 1394 mm/year, with an average of approximately 1100 mm/year. Palaeoclimatic conditions during deposition of the Lourinhã formation are broadly similar to those inferred for the Morrison Formation, except somewhat wetter. Seasonal variation in moisture availability does not seem to have negatively impacted the ability of these environments to support rich and relatively abundant faunas. The similar climate between these two Late Jurassic terrestrial ecosystems is probably one of the factors which explains the similarity of their vertebrate faunas.     

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian–Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole‐rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque–Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole‐rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well‐preserved belemnite shells (non‐luminescent to slightly luminescent) typically show stable isotope values of +4·3‰ to –0·7‰δ¹³C, +0·7‰ to –3·2‰δ¹⁸O, and trace element contents of <32 μg g^–1 Mn, <250 μg g^–1 Fe, >950 μg g^–1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low‐Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non‐luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their δ¹³C and δ¹⁸O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or high‐frequency secular variations in sea‐water chemistry superimposed on the long‐term isotopic trend. A pronounced positive carbon‐isotope excursion (up to +4·3‰) is documented in the early Toarcian serpentinus biozone, which correlates with the Toarcian δ¹³C maximum reported in other European and Tethyan regions.     

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.     

Isotopic constraints on growth conditions of multiphase calcite–pyrite–barite concretions in Carboniferous mudstones

Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ¹³C=?28·7‰ PDB and δ¹⁸O=?1·6‰ PDB through to δ¹³C=?6·9‰ PDB and δ¹⁸O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ¹³C=?12·0 to ?10·1‰ PDB and δ¹⁸O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ³⁴S=+24·8‰ CDT and δ¹⁸O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ³⁴S=+6 to +11‰ CDT and δ¹⁸O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.     

Development of microbial carbonates in the Lower Cretaceous Codó Formation (north‐east Brazil): Implications for interpretation of microbialite facies associations and palaeoenvironmental conditions

The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time. Additionally, microbialites have a demonstrated economic potential to serve as excellent hydrocarbon reservoirs. The Lower Cretaceous Codó Formation, located in the Parnaiba Basin of north‐east Brazil, comprises a unique stratigraphic sequence of up to 20 m thick, well‐preserved carbonate microbialites. Deposited in a continental basin during the initial break up and separation of South America from Africa in the Early Cretaceous, this lacustrine carbonate sequence provides an excellent example to investigate the palaeoenvironmental conditions controlling microbialite facies development. Based on macroscopic and microscopic observations of outcrop and drill core samples, four microbialite facies (stromatolite, lamina, massive and spherulite) were defined and distinguished by textures and microbial fossil content. Changes in facies type are related to alternating palaeo‐water depths, as reflected by ⁸⁷Sr/⁸⁶Sr cycles resulting from fluctuations in the sources of meteoric water. Clumped isotope measurements of stromatolitic fabrics yield precipitation palaeo‐temperatures with an average value of 35°C. The δ¹⁸O values of bulk carbonate (?6·8 to ?1·5‰ Vienna Pee Dee Belemnite) imply precipitation from water with calculated δ¹⁸O values between ?1·6‰ and 1·8‰ Vienna Standard Mean Ocean Water, reflecting precipitation from variably modified meteoric waters. The δ¹³C values of bulk carbonate (?15·5 to ?7·2‰ Vienna Pee Dee Belemnite) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter. Combined, the data indicate that the evolution of the Codó Formation occurred in a closed lacustrine palaeoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations. The results provide new insights into palaeoenvironmental settings, biogenicity and early diagenetic processes involved in the formation of ancient carbonate microbialites and, by extension, improve the knowledge of the reservoir geology of correlative units in deep waters offshore Brazil.     

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

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

Origin of quartz cements in some sandstones from the Jurassic of the Inner Moray Firth (UK)

The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix-supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM-CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM-CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a-axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ¹⁸O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ¹⁸O composition of +20·0‰. Estimates of the δ¹⁸O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ¹⁸O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.     

Physical and chemical growth conditions of Ordovician organogenic deep-water dolomite concretions: implications for the δ18O of Early Palaeozoic sea water

The estimated depth of formation of authigenic dolomite concretions in the Middle Ordovician Cloridorme Formation, Quebec, ranges from < 1 m to 150–200 m below sea floor (mbsf) (mostly between < 1 and 25 mbsf), based on centre‐to‐margin variations in minus‐cement porosity (80–90% to 45–75%). Formation depths are > 350 mbsf (25–17% porosity) in the Lower Ordovician Levis Formation. Outward‐decreasing δ¹³C_VPDB values (10·2–0·8‰) suggest precipitation in the methane generation zone with an increasing contribution of light carbonate derived by advection from thermocatalytic reactions at depth. Anomalously low δ¹⁸O_VPDB values (centre‐to‐margin variations of ?0·4 to ?7·5‰) give reasonable temperatures for the concretion centres only if the δ¹⁸O of Ordovician sea water was negative (?6‰) and the bottom water was warm (> 15 °C). The 3–5‰ lower values for the concretion margins compared with the centres can be explained if, in addition, volcanic‐ash alteration, organic‐matter decomposition and/or advection of ¹⁸O‐depleted water lowered the δ¹⁸O of the pore water further by 2·0–4·0‰ during the first 25–200 m of burial. Reasonable growth temperatures for the margins of 17–20 °C are compatible with a lowering of the isotopic ratios by 1 to < 1·3‰ as a temperature effect. The systematic concentric isotope zonation of the concretions suggests that the well‐ordered near‐stoichiometric dolomite is a primary feature and not the result of recrystallization. Diagenetic dolomite beds of the Cloridorme Formation appear to have formed by coalescence of concretions, as shown by randomly sampled traverses that indicate formation at different subsurface depths. Growth of the Cloridorme dolomites was probably limited by calcium availability, at least 50% of which was derived from connate water, and the remainder by diffusion from sea water. Dolomite precipitation was favoured over calcite by very high sedimentation rates, the abundance of marine organic matter in the host sediment and a correspondingly thin sulphate reduction zone. Deep‐seated concretion growth in the Levis Formation required either internal sources for the participating ions (carbonate dissolution event) or porewater advection along faults.     

Geochemistry of Triassic Carbonates: Exploration Guide to Pb–Zn Mineralization in North Tunisia

The Triassic carbonate rocks in Northern Tunisia (Nappes, Domes, Jurassic Mountains zones), consist of massive carbonates, clays and gypsum with authigenic minerals. These are associated with several Pb–Zn deposits and occurrences. At Jebel Ichkeul, Bechateur and Oum Edeboua, these Triassic carbonates exhibit enrichment in Pb (0.32 to 228 ppm), Zn (17 to 261 ppm), Cd (5 to 6 ppm) and Co (0.3 to 89.5 ppm), with respect to their average contents in crustal carbonates. The enrichment is more pronounced at Oum Edeboua (near the ore zone). Permeability is one of the most effective factors of dispersion of metallic trace elements, causing the development of geochemical halos. The genetic relationship of the Triassic carbonate rocks with the ore deposits was controlled by diapirism and tectonic movements, which favored mineralization along the Triassic‐cover contact as well as the remobilization of metals from the mineralized rocks. Analysis of metallic trace elements in Triassic rocks provides clues to the presence of possible mineral deposits. These could be effectively used for both geochemical interpretation and mineral exploration. Carbon and O‐isotope data (– 9.3‰ < δ¹³C < +3‰; +21.9 < δ¹⁸O < +31‰) suggest that the Triassic carbonates of all study areas have marine carbonates as their origin; some of them show significantly lower δ¹⁸O values indicating some exchange with hydrothermal fluids. Calcites associated with mineralization at Oum Edeboua have δ¹³C of –6.2‰ to –8.22‰ and δ¹⁸O of +24.88‰ to +25‰. The C‐isotope compositions of these calcites are ¹³C depleted, indicating an organic origin.     

Palaeoenvironmental reconstruction of a middle Miocene alluvial fan to cyclic shallow lacustrine depositional system in the Calatayud Basin (NE Spain)

ABSTRACT The middle Miocene sedimentary fill of the Calatayud Basin in north‐eastern Spain consists of proximal to distal alluvial fan‐floodplain and shallow lacustrine deposits. Four main facies groups characteristic of different sedimentary environments are recognized: (1) proximal and medial alluvial fan facies that comprise clast‐supported gravel and subordinate sandstone and mudstone, the latter exhibiting incipient pedogenic features; (2) distal alluvial fan facies, formed mainly of massive mudstone, carbonate‐rich palaeosols and local carbonate pond deposits; (3) lake margin facies, which show two distinct lithofacies associations depending on their distribution relative to the alluvial fan system, i.e. front (lithofacies A), comprising massive siliciclastic mudstone and tabular carbonates, or lateral (lithofacies B) showing laminated and/or massive siliciclastic mudstone alternating with tabular and/or laminated carbonate beds; and (4) mudflat–shallow lake facies showing a remarkable cyclical alternation of green‐grey and/or red siliciclastic mudstone units and white dolomitic carbonate beds. The cyclic mudflat–shallow lake succession, as exposed in the Orera composite section (OCS), is dominantly composed of small‐scale mudstone–carbonate/dolomite cycles. The mudstone intervals of the sedimentary cycles are interpreted as a result of sedimentation from suspension by distal sheet floods, the deposits evolving either under subaerial exposure or water‐saturated conditions, depending on their location on the lacustrine mudflat and on climate. The dolomite intervals accumulated during lake‐level highstands with Mg‐rich waters becoming increasingly concentrated. Lowstand to highstand lake‐level changes indicated by the mudstone/dolomite units of the small‐scale cycles reflect a climate control (from dry to wet conditions) on the sedimentation in the area. The spatial distribution of the different lithofacies implies that deposition of the small‐scale cycles took place in a low‐gradient, shallow lake basin located in an interfan zone. The development of the basin was constrained by gradual alluvial fan aggradation. Additional support for the palaeoenvironmental interpretation is derived from the isotopic compositions of carbonates from the various lithofacies that show a wide range of δ¹⁸O and δ¹³C values varying from ?7·9 to 3·0‰ PDB and from ?9·2 to ?1·7‰ PDB respectively. More negative δ¹⁸O and δ¹³C values are from carbonate‐rich palaeosols and lake‐margin carbonates, which extended in front of the alluvial fan systems, whereas more positive values correspond to dolomite beds deposited in the shallow lacustrine environment. The results show a clear trend of δ¹⁸O enrichment in the carbonates from lake margin to the centre of the shallow lake basin, thereby also demonstrating that the lake evolved under hydrologically closed conditions.     

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.     

Geochemical peculiarities of the formation of the Novoshirokinskii gold-polymetallic deposit,Eastern Transbaikalia,Russia

The fluid inclusions in minerals and isotope composition of sulfur in sulfides and carbon and oxygen in carbonates are studied for the Novoshirokinskii gold-polymetallic deposit. The ore-forming fluids are characterized by the following physico-chemical and isotope-geochemical parameters: temperature of 290–100°C, salinity of 13–2.5 wt % NaCl-equiv., δ¹⁸O from +8 to 0‰, δ¹³C of 2.5 ± 0.5‰, and δ³⁴S of 10.5 ± 1.0‰. It is concluded that the Late Proterozoic-Early Cambrian carbonaceous-terrigenous and carbonate rocks were involved in the Late Jurassic ore-magmatic system.     

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.     

Geology,Fluid Inclusions,and Isotopic Geochemistry of the Jinman Sediment‐Hosted Copper Deposit in the Lanping Basin,China

The Jinman Cu polymetallic deposit is located within Middle Jurassic sandstone and slate units in the Lanping Basin of southwestern China. The Cu mineralization occurs mainly as sulfide‐bearing quartz–carbonate veins in faults and fractures, controlled by a Cenozoic thrust–nappe system. A detailed study of fluid inclusions from the Jinman deposit distinguishes three types of fluid inclusions in syn‐ore quartz and post‐ore calcite: aqueous water (type A), CO₂–H₂O (type B), and CO₂‐dominated (type C) fluid inclusions. The homogenization temperatures of CO₂–H₂O inclusions vary from 208°C to 329°C, with corresponding salinities from 0.6 to 4.6 wt.% NaCl equivalent. The homogenization temperatures of the aqueous fluid inclusions mainly range from 164°C to 249°C, with salinities from 7.2 to 20.2 wt.% NaCl equivalent. These characteristics of fluid inclusions are significantly different from those of basinal mineralization systems, but similar to those of orogenic or magmatic mineralization systems. The H and O isotope compositions suggest that the ore‐forming fluid is predominantly derived from magmatic water, with the participation of basinal brine. The δ³⁴S values are widely variable between ?9.7 ‰ and 9.7 ‰, with a mode distribution around zero, which may be interpreted by the variation in physico‐chemical conditions or by compositional variation of the sources. The mixing of a deeply sourced CO₂‐rich fluid with basinal brine was the key mechanism responsible for the mineralization of the Jinman deposit.     

A Fluid Inclusion Study of Vein-type Copper Mineralization in the East Wulasigou Pb–Zn–Cu Deposit, Altay, Northwestern China

The Wulasigou Cu-Pb-Zn deposit,located 15 km northwest of Altay city in Xinjiang,is one of many Cu-Pb-Zn polymetallic deposits in the Devonian Kelan volcanic-sedimentary basin in southern Altaids.Two mineralizing periods can be distinguished:the marine volcanic sedimentary PbZn mineralization period,and the metamorphic hydrothermal Cu mineralization period,which is further divided into an early bedded foliated quartz vein stage(Q1) and a late sulfide-quartz vein stage(Q2) crosscutting the foliation.Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit:H_2O-CO_2 inclusions,carbonic fluid inclusions,aqueous fluid inclusions,and daughter mineral-bearing fluid inclusions.Microthermometric studies show that solid CO_2 melting temperatures(T_(m,CO2)) of H_2O-CO_2 inclusions in Ql are from-62.3℃ to-58.5C,clathrate melting temperatures(T_(m,clath)l) are from 0.5 C to 7.5 C,partial homogenization temperatures(T_(h,CO2)) vary from 3.3℃ to 25.9℃(to liquid),and the total homogenization temperatures(T_(h,tot)) vary from 285℃ to 378℃,with the salinities being 4.9%-15.1%NaCl eqv.and the CO_2-phase densities being 0.50-0.86 g/cm~3.H_2O-CO_2 inclusions in Q2 have T_(m,CO_2) from-61.9℃ to-56.9℃,T_(m,clath)from 1.3℃ to 9.5℃,T_(h,CO2) from 3.4℃ to 28.7℃(to liquid),and T_(h,tot) from 242℃ to 388℃,with the salinities being 1.0%-15.5%NaCl eqv.and the CO_2-phase densities being 0.48-0.89 g/cm~3.The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and180-370 MPa,respectively.The δ~(34)S values of pyrite from the volcanic sedimentary period vary from2.3‰ to 2.8‰(CDT),and those from the sulfide-quartz veins fall in a narrow range of-1.9‰ to 2.6‰(CDT).The δD values of fluid inclusions in Q2 range from-121.0‰ to-100.8‰(SMOW),and theδ~(18)O_(H2O) values calculated from δ~(18)O of quartz range from-0.2‰ to 8.3‰(SMOW).The δD-δ~(18)O_(H2O)data are close to the magmatic and metamorphic fields.The fluid inclusion and stable isotope data documented in this study indicate that the vein-type copper mineralization in the Wulasigou Pb-Zn-Cu deposit took place in an orogenic-metamorphic enviroment.     

Sedimentological and marine eogenetic control on porosity distribution in Upper Cretaceous carbonate turbidites (central Albania)

Results of a detailed petrographic and stable isotope study illustrate that sedimentological differences and eogenetic dissolution/precipitation processes controlled porosity distribution within carbonate turbidites of the Ionian basin (central Albania). Based on lithology characteristics and porosity distribution observed in outcrop, individual turbidite beds can be subdivided into four distinct intervals, i.e. from base to top: (A) a non‐porous wackestone/floatstone or packstone followed by (B) porous packstone–grainstone that grades into (C) wackestone and (D) non‐porous mudstone with pelagic foraminifera. Wackestone interval C is characterized by an alternation of porous and non‐porous laminae. Changes in turbidity current flow regime controlled the initial presence of matrix micrite giving rise to both matrix‐ and grain‐supported lithologies within turbidite sequences. These are non‐porous and porous, respectively. Four eogenetic diagenetic processes (dissolution, cementation, neomorphism and compaction) acted shortly after deposition and modified primary porosity characteristics and distribution. Alteration by meteoric water is excluded based on the continuous burial until the Oligocene of the studied deep marine carbonates. Moreover, the stable isotope data with values between −2·1‰ and +0·7‰ for δ¹⁸O_V‐PDB and between +1‰ and +3‰ for δ¹³C_V‐PDB, favour alteration by marine‐derived pore‐waters. Compaction and aggrading neomorphism occurred dominantly in intervals characterized by higher matrix micrite content, i.e. the floatstone base and the wackestone–mudstone upper turbidite part. Framework‐stabilizing cementation occurred dominantly in the packstone–grainstone middle part of the turbidite beds. In the latter porous lithologies, matrix micrite was not compacted because of the grain fabric and the framework‐stabilizing cements. Here, neomorphism of micrite into microrhombic euhedral calcite occurred and microporosity was preserved.     

Characterization of C‐isotope variability in the Delhi Supergroup,northwestern India and the Mesoproterozoic ocean

Marine carbonate rocks of the Delhi Supergroup of northwestern India show little deviation in whole‐rock δ ¹³Ccarb and δ ¹⁸Ocarb values, which generally are around 0 and –10‰ respectively. These narrow ranges and almost constant δ ¹³Ccarb values persist despite close sampling through long sections. The data suggest that the global rate of organic carbon burial was probably constant during deposition of the Delhi Supergroup. The nearly invariant C isotopic profile of the Delhi Supergroup is similar to C isotopic profiles of Mesoproterozoic carbonates older than 1.3 Ga, as reported from different parts of world. Carbonate units on the western margin of the Delhi Supergroup however, have on average moderately positive δ ¹³C values (from 2 to +4.96‰). These high δ ¹³C carbonates may represent the Mesoproterozoic–Neoproterozoic transition (from ～1.25 to ～0.85 Ga), a period characterized by high positive δ ¹³C values globally.     

Aragonite stromatolitic buildups from Santorini (Aegean Sea,Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp ) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ¹³C_PDB (0·16 to 12·97‰) with a narrower variation for δ¹⁸O_PDB (?0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ¹³C = 12·40 ± 0·43‰ and δ¹⁸O = 2·42 ± 0·77‰, n = 21). The δ¹³C and δ¹⁸O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, ⁸⁷Sr/⁸⁶Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO₂ gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.