Dolomite Nodules Research Articles

Laterally-continuous dolomite layers of the Miocene Pisco Formation (East Pisco Basin, Peru): A window into past cyclical changes of the diagenetic environment

Along the Peruvian coast, the sedimentary succession of the East Pisco Basin is exposed in the Ica Desert. At Cerro Los Quesos, laterally continuous dolomite layers characterise the diatomaceous sediments of the P2 sequence of the Miocene Pisco Formation, where a large number of marine vertebrates are exceptionally preserved, many enclosed in dolomite nodules. In this work, cemented layers from this sequence were described and sampled for petrographic, chemical, microscopic and isotopic analyses. Dolomite occurs in continuous 10–50 cm thick well cemented layers, formed by sediment of different nature: biogenic, terrigenous, volcanoclastic, and phosphatic. The underlying sediments exhibit a yellow layer with sparse dolomite crystals, a black layer with abundant Mn-oxides, and a reddish layer rich in Fe-oxides, indicating redox-related fronts. Two generations of dolomite can be recognised: an early diagenetic microcrystalline one, and a sparry one, filling the large cavities. As observed in both thin sections and on broken surfaces, microcrystalline dolomite also fills the inner spaces of the diatom areolae replicating their finest ultrastructure, such as foramina and cribra and replace calcite shells of foraminifera. δ18O and δ13C values from the microcrystalline dolomite of two layers, selected based on the absence of other carbonate phases (e.g. calcite) and the lack of sparry cement, are in agreement with those reported for the Peru margin and fall in the fields of either sulphate-reduction or methanogenesis.All the data point to dolomite precipitation associated with low-temperature early diagenesis that typically occurs in upwelling settings, where high surface water productivity is responsible for high rates of organic carbon flux to the sea bottom and for the cyclical oxygen depletion at the bottom. Such conditions also promote high abundances of marine vertebrates and the exceptional preservation of their skeletons in the sediments.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part II: Carbonate Replacement Sulfide Mineralization During Burial Diagenesis—Implications for Mineral Exploration

AbstractThe Teena Zn-Pb deposit is located in the Carpentaria Zn Province (Australia), which contains some of the largest clastic dominant (CD-type) massive sulfide Zn-Pb deposits in the world. The timing of the main stage of hydrothermal sulfide mineralization in the Teena subbasin is constrained to the midstage of burial diagenesis, during a period of short-lived regional extension. To distinguish hydrothermal alteration from spatially and temporally overlapping burial diagenetic alteration, and to establish the primary controls on hydrothermal mass transfer, it is necessary to evaluate the various foot- and hanging-wall alteration assemblages that formed between early- (eogenesis) and late- (mesogenesis) stage diagenesis. To achieve this, we have statistically evaluated a large lithogeochemistry dataset (n >2,500) and selected a subset (n = 65) of representative samples for detailed mineralogical (X-ray diffraction, illite crystallinity) and petrographic (scanning electron microscopy) analyses; hyperspectral core imaging data were then used to upscale key paragenetic observations.We show that sulfide mineralization was predated by multiple diagenetic alteration assemblages, including stratiform pyrite, dolomite nodules and cement, disseminated hematite and authigenic K-feldspar. These assemblages formed during eogenesis in multiple subbasins across the broader McArthur Basin and are not part of the synmineralization alteration footprint. Whereas pyrite and dolomite formed primarily from the in situ degradation of organic matter, feldspar authigenesis was the product of K metasomatism that was focused along permeable coarse-grained volcaniclastic sandstone beds within the host-rock sequence. The immature volcaniclastic input is broadly representative of the siliciclastic compositional end member in the subbasin, which formed the protolith for phyllosilicate (illite, phengite, chlorite) formation during burial diagenesis. There is no evidence of extensive phyllosilicate alteration in any of the geochemical, mineralogical (illite crystallinity), or petrographic datasets, despite some evidence of K-feldspar replacement by sphalerite in the Lower and Main mineralized lenses. Rather, the high Zn grades formed via dolomite replacement, which is resolvable from a chemical mass balance analysis and consistent with petrographic observations.There are significant exploration implications associated with carbonate-replacement sulfide mineralization during mesogenesis: (1) the capacity for secondary porosity generation in the host rock is as important as its sulfate-reducing capacity; (2) hydrothermal mineralization has a short-range cryptic lateral and vertical synmineralization alteration footprint due to acid neutralization by a carbonate-rich protolith; and (3) the distribution and chemistry of premineralization phases (e.g., pyrite, dolomite nodules) cannot be directly related to the mineralization footprint, which is localized to the 4th-order subbasin scale. Future exploration for this deposit style should therefore be focused on identifying units that contain a mixture of organic carbon and carbonate in the protolith, at favorable stratigraphic redox boundaries, and proximal to feeder growth faults.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part I: Syndiagenetic Base Metal Sulfide Mineralization Related to Dynamic Subbasin Evolution

Abstract Divergent genetic models have been proposed for clastic dominant (CD-type) massive sulfide Zn-Pb mineralization in the Proterozoic Carpentaria Zn Province. Due to varying degrees of tectonic overprint, it has been difficult to accurately constrain structural and paragenetic timing aspects of the CD-type genetic model, and the most basic timing constraints (syngenetic vs. epigenetic, synextension vs. syninversion) remain debated. The recently discovered Teena Zn-Pb deposit is hosted by an exceptionally well preserved subbasin that permits relative timing relationships to be well defined. Using a combination of geophysical, structural, sedimentological, and petrographic datasets, a new model for subbasin development and syndiagenetic hydrothermal replacement mineralization is developed for the Teena mineral system. At Teena, sulfide mineralization was deposited from lateral fluid flow beneath an impermeable seal several hundred meters below the paleosurface and maximum flood surface, after formation of fine-grained diagenetic pyrite (py1) and dolomite nodules. Sulfide mineralization resulted from syndiagenetic carbonate replacement and pore space cementation where thermochemical sulfate reduction took place. The sulfide mineralization is therefore partly cospatial but not cogenetic with its thick pyritic hanging wall, and its lateral alteration footprint is much smaller than predicted by sedimentary exhalative (SEDEX) models. An additional zone of low-grade Zn-Pb mineralization in the footwall W-Fold Shale Member represents a different style of mineralization not previously reported for Carpentaria CD-type Zn deposits: it is associated with strata-bound lenses of hydrothermal dolomite (HTD) that formed by both replacement and carbonate dissolution and infill, which yielded diverse cavity-infill textures that include coarse-bladed dolomite fans cemented by interstitial sphalerite, dolomite, and quartz. Volumetrically minor Zn mineralization is also present in a fault conduit hydrothermal breccia and in hanging-wall synorogenic vein sets derived by hydrothermal leaching and remobilization of Zn from the underlying mineralized zones. Whereas both the Teena and nearby McArthur River Zn-Pb deposits are located along the northern margin of the 3rd-order Hot Spring-Emu subbasin, they formed in separate 4th-order subbasins in association with local extensional growth faults. Growth fault movement in the Teena subbasin was initiated during deposition of the W-Fold Shale Member and persisted episodically until a weak structural disconformity associated with sedimentary facies regression developed in the Upper HYC unit. Shifting patterns in depocenter location, sedimentary facies development, mineralization, and alteration zonation are attributed to progressive growth and linkage of segments of a regionally anomalous ENE-trending, synsedimentary fault zone. Similar patterns of extensional subbasin development were repeated in other Zn-mineralized subbasins throughout the River supersequence across the northern Carpentaria Zn Province, and formed in response to a short-lived episode of north-northwest–south-southeast regional extension around ca 1640 ± 5 Ma, triggered by far-field subduction events.

Origin of the gypsum veins associated with the diagenetic dolomite nodules from the Neogene Marls of South-East Spain

The late Neogene marls from the Lorca, Murcia and Vera basins in S-E. Spain contain abundant dolomite nodules that were formed due to intense methane-rich fluid migration. The pathways for these fluids are evidenced by dense networks of fractures that are crossing the sedimentary layers and eventually the dolomite nodules. The fractures are generally filled by secondary fibrous gypsum that form veins of a few cm thick.The oxygen and sulfur isotopic compositions of the gypsum veins exhibit wide ranges (−2.2 < δ18O‰ VSMOW <6.7; −22.3 < δ34S‰ VCDT <10.5) that are far away from the average δ values of Tortonian-Messinian gypsum from the basins of Lorca, Fortuna and Sorbas (δ18O = 13.8 ± 2.3‰; δ34S = 21.3 ± 1.0‰) precipitated from the late Neogene seawater sulfate. These low δ values of the gypsum veins clearly indicate their diagenetic origin that would have resulted from sulfide oxidation with 18O-depleted water. The isotopic compositions of the diagenetic dolomite nodules close to the gypsum veins (−0.7 < δ18O‰ VPDB <3.7; −11.9 < δ13C‰ VPDB <7.3) are indicative of their formation in methane-rich fluids. In this context, the formation of sulfide is related to the bacterial sulfate reduction that is associated with the anaerobic oxidation of methane. During this process, pyrite coprecipitated with dolomite, either within the dolomite framework or within conduits where the fluids were seeping. The pyrite oxidation occurred later during diagenesis when more or less oxygenated groundwaters circulated within the fractures during the sealevel drawdown of the Messinian Salinity Crisis and after the regional uplift of the sedimentary deposits in relation with the Betic cordillera tectonics.

U-Th chronology and formation controls of methane-derived authigenic carbonates from the Hola trough seep area, northern Norway

We investigated methane-derived authigenic carbonate (MDAC) crusts and nodules from a cold seep site on the northern Norwegian continental shelf in ca. 220m water depth to determine the timing and mode of their formation. Gas bubbling observed during remotely operated vehicle (ROV)-assisted sampling of MDAC crusts revealed ongoing seep activity. Authigenic carbonates were present as crusts on the seafloor and as centimetre-size carbonate-cemented nodules at several intervals within an adjacent sediment core. Aragonite-dominated mineralogy of the MDAC crusts suggests formation close to the seafloor at higher rates of sulphate-dependent anaerobic oxidation of methane (AOM). In contrast, dolomite-cemented nodules are consistent with the formation at the sulphate-methane-transition zone deeper within the sediment at lower rates of AOM. The δ13C-carbonate values of bulk rock and of micro-drilled aragonite samples vary between −22.2‰ and −34.6‰ (VPDB). We interpret the carbon in aragonite to be mainly derived from the anaerobic oxidation of thermogenic methane, with a minor contribution from seawater dissolved inorganic carbon (DIC). AOM activity is supported by high concentrations of AOM-related biomarkers of archaea (archaeol and 2-sn-hydroxyarchaeol) and sulphate-reducing bacteria (iso and anteiso-C15:0 fatty acids) in the crusts. The dolomite nodules exhibit higher δ13C-carbonate values (−12‰ VPDB) suggesting a smaller amount of methane-derived carbon, presumably due to the contribution of DIC migrating from depth, and lower AOM rates. The latter is supported by orders of magnitude lower concentrations of archaeol and sn-2-hydroxyarchaeol in the sediment interval containing the largest dolomite nodules. δ18O values of pure aragonite samples and dolomite nodules indicate the precipitation of carbonate close to isotopic equilibrium with seawater and no influence of gas hydrate-derived water. U-Th dating of two MDAC crusts shows that they formed between 1.61±0.02 and 4.39±1.63ka BP and between 2.65±0.02 and 4.32±0.08ka BP. We infer both a spatial and temporal change in methane flux and related MDAC formation at this seep site. These changes might be caused by regional seismic events that can affect pore pressure or re-activation of migration pathways thus facilitating fluid flow from deep sources towards the seabed.

A multistratigraphic approach to pinpoint the Permian-Triassic boundary in continental deposits: The Zechstein–Lower Buntsandstein transition in Germany

The Central European Basin is very suitable for high-resolution multistratigraphy of Late Permian to Early Triassic continental deposits. Here the well exposed continuous transition of the lithostratigraphic Zechstein and Buntsandstein Groups of Central Germany was studied for isotope-chemostratigraphy (δ13Corg, δ13Ccarb, δ18Ocarb), major and trace element geochemistry, magnetostratigraphy, palynology, and conchostracan biostratigraphy. The analysed material was obtained from both classical key sections (abandoned Nelben clay pit, Caaschwitz quarries, Thale railway cut, abandoned Heinebach clay pit) and a recent drill core section (Caaschwitz 6/2012) spanning the Permian-Triassic boundary. The Zechstein–Buntsandstein transition of Central Germany consists of a complex sedimentary facies comprising sabkha, playa lake, aeolian, and fluvial deposits of predominantly red-coloured siliciclastics and intercalations of lacustrine oolitic limestones. The new data on δ13Corg range from −28.7 to −21.7 ‰ showing multiple excursions. Most prominent negative shifts correlate with intercalations of oolites and grey-coloured clayey siltstones, while higher δ13Corg values correspond to an onset of palaeosol overprint. The δ13Ccarb values range from −9.7 to −1.3 ‰ with largest variations recorded in dolomitic nodules from the Zechstein Group. In contrast to sedimentary facies shifts across the Zechstein-Buntsandstein boundary, major element values used as a proxy (CIA, CIA*, CIA-K) for weathering conditions indicate climatic stability. Trace element data used for a geochemical characterization of the Late Permian to Early Triassic transition in Central Germany indicate a decrease in Rb contents at the Zechstein-Buntsandstein boundary.New palynological data obtained from the Caaschwitz quarry section reveal occurrences of Late Permian palynomorphs in the Lower Fulda Formation, while Early Triassic elements were recorded in the upper part of the Upper Fulda Formation. The present study confirms an onset of a normal-polarized magnetozone in the Upper Fulda Formation of the Caaschwitz quarry section supporting an interregional correlation of this crucial stratigraphic interval with the normal magnetic polarity of the basal Early Triassic known from marine sections in other regions. Based on a synthesis of the multistratigraphic data, the Permian-Triassic boundary is proposed to be placed in the lower part of the Upper Fulda Formation, which is biostratigraphically confirmed by the first occurrence date of the Early Triassic Euestheria gutta-Palaeolimnadiopsis vilujensis conchostracan fauna. Rare records of conchostracans reported from the siliciclastic deposits of the lower to middle Zechstein Group may point to its potential for further biostratigraphic subdivision of the Late Permian continental deposits.

Geochemical modelling of formation damage risk during CO2 injection in saline aquifers

This study provides an understanding of the impact of geochemical reactions during and after CO2 injection into a potential storage site. The results of calculations of geochemical reactivity of reservoir rock and of cap rock during and after CO2 injection were performed using a geochemical simulator, with the calculations showing that for these conditions up to 0.5 mol of CO2 can be dissolved per kg of water. The risk of dissolution of primary cements was considered and identified. In addition, the potential of carbonation reactions to permanently sequester CO2 was considered, although these reactions were shown to be very slow relative to other processes. The implications for security of storage are that while dolomite nodules exist in the sandstone formation, these do not contribute significantly to the overall rock strength, and hence the risk of dissolution of the formation or caprock causing significant leakages pathways is very low. Further calculations were performed using a commercial reservoir simulation code to account for brine evaporation, halite precipitation and capillary pressure re-imbibition. The impact on injectivity was found not to be significant during continuous and sustained injection of CO2 at a constant rate. Capillary pressure effects did cause re-imbibition of saline brine, and hence greater deposition, reducing the absolute porosity by up to 13%. The impact of the halite deposition was to channel the CO2, but for the configuration used there was not a significant change in injection pressure.

The characteristics of large-sized dolomite nodules occuring in the Bessho and Aoki formations

The characteristics of large-sized dolomite nodules occuring in the Bessho and Aoki formations

The dolomite nodules enclosing fossil marine vertebrates in the East Pisco Basin, Peru: Field and petrographic insights into the Lagerstätte formation

Abstract The Mio-Pliocene Pisco Formation (Peru) is a world-famous marine vertebrate Lagerstatte. Several fossil specimens are wrapped up in dolomitic nodules. Some others lie in the sediment displaying dolomite only in bone cavities (e.g., mesorostral canal and endocranium). With the aim to understand whether the precipitation of the dolomitic nodules influenced the formation of the Lagerstatte, we collected field data on a high number of fossil vertebrates and conducted petrographic and mineralogical analyses on samples representative of the variable development of concretions. Our results revealed positive relationships between size, completeness and articulation of skeletons and the presence of an external nodule. Clear evidence of chemoautotrophic communities that thrived on the carcasses is scarce. Microborings are often found in the cortical bone tissues together with iron oxides; the former are left by microorganisms feeding on the carcass, the latter are traces of former Fe sulphides, a product of organic matter degradation. We suggest that an early burial of the skeletons was a determinant factor in the development of dolomite concretions, since it allowed methanogenesis and anaerobic sulphate reduction exploiting the lipids in the bones and the organic matter dispersed in the sediments. Dolomite precipitation was driven by the same bacteria operating during the suphophilic stage of whale-fall communities. Textural observations imply that dolomite precipitated shortly after the burial of carcasses. The increase of alkalinity generated by sulphate reduction and methanogenesis caused a rapid precipitation of the dolomite within skeletal cavities and prevented the degradation of the bones and diagenetic compression of skeletons; the nodules themselves prevented erosion of fossils after exhumation. Therefore, nodule formation had a crucial role in the development of the Pisco Lagerstatte.

Methanogenesis and clay minerals diagenesis during the formation of dolomite nodules from the Tortonian marls of southern Spain

Dolomite nodules are widespread within the Tortonian marls of Fortuna and Lorca basins in southern Spain. They occur as large bodies of various forms (round, ovoid, tabular) that are parallel or secant relative to the stratification. They are massive and present sometimes internal conduits that are considered as drains used for the migration of fluids.This study brings new results on the mineralogy and elemental and isotopic geochemistry of these dolomite nodules to better describe the processes that were occurring during their formation. The diagenetic reactions that have driven authigenic dolomite precipitation within the sediments involved on the one hand methanogenesis that produced 13C-poor CH4 and 13C-rich CO2, on the other hand carbonate and silicate weathering by the CO2-rich solutions, that released respectively alkalinity and cations in pore solutions. Moreover, the distribution of the major elements (Si, Al, K, Na, Ca, Mg, Fe) in the marls and dolomite nodules indicates that these elements were redistributed within the sedimentary formation during the diagenetic reactions without external inputs by the circulating fluids. These observations thus confirm the link between authigenic dolomite formation and clay minerals diagenesis in sediments where methanogenesis was active.

Reconstituição paleoambiental das formações Motuca e Sambaíba, Permo-Triássico da Bacia do Parnaíba no sudoeste do Estado do Maranhão, Brasil

The interval between the Late Paleozoic and Early Mesozoic was marked by paleogeographic and paleoclimatic global changes, partly attributed to catastrophic events. The intense continentalization of the supercontinent Pangaea of End-Permian propitiated the development of extensive deserts that succeeded the coastal and platform environments of Early Permian. The records of these events in northern Brazil are found in intracratonic basins, particularly in the Permo-triassic succession of the Parnaiba Basin. The facies and stratigraphic outcrops analysis of this succession allowed the individualization of 14 sedimentary facies grouped into four facies associations (FA): FA1 and FA2 related to deposits of Motuca Formation and, FA3 and FA4, representative of the base of Sambaiba Formation. The FA1 - Shallow lake/Mudflat consists of red laminated mudstone with lenses of gypsum, calcite and marl, besides lobes of sigmoidal sandstones. The FA2 - Saline pan consists of lenticular bodies of laminated gypsum, nodular gypsum and gypsarenite, overlapped by greenish mudstones with dolomite nodules and palygorskite. The FA3 - sand sheet and FA4 - dunes field are formed, respectively, for orange cream sandstones with even parallel stratification and medium- to large-scale cross-bedding. In the contact between Motuca and Sambaiba formations occurs a deformed interval, laterally continuous for hundreds of kilometers. Brecciated and contorted bedded siltstones and mudstone (Motuca Formation) and sandstone with sinsedimentary faults/microfaults, convolute lamination and mud-filled injection dykes (Sambaiba Formation) are interpreted as seismites triggered by high magnitude earthquakes (>; 8 according Richter scale).

Reconstituição paleoambiental das formações Motuca e Sambaíba, Permo-Triássico da Bacia do Parnaíba no sudoeste do Estado do Maranhão, Brasil

The interval between the Late Paleozoic and Early Mesozoic was marked by paleogeographic and paleoclimatic global changes, partly attributed to catastrophic events. The intense continentalization of the supercontinent Pangaea of End-Permian propitiated the development of extensive deserts that succeeded the coastal and platform environments of Early Permian. The records of these events in northern Brazil are found in intracratonic basins, particularly in the Permo-triassic succession of the Parnaíba Basin. The facies and stratigraphic outcrops analysis of this succession allowed the individualization of 14 sedimentary facies grouped into four facies associations (FA): FA1 and FA2 related to deposits of Motuca Formation and, FA3 and FA4, representative of the base of Sambaíba Formation. The FA1 - Shallow lake/Mudflat consists of red laminated mudstone with lenses of gypsum, calcite and marl, besides lobes of sigmoidal sandstones. The FA2 - Saline pan consists of lenticular bodies of laminated gypsum, nodular gypsum and gypsarenite, overlapped by greenish mudstones with dolomite nodules and palygorskite. The FA3 - sand sheet and FA4 - dunes field are formed, respectively, for orange cream sandstones with even parallel stratification and medium- to large-scale cross-bedding. In the contact between Motuca and Sambaíba formations occurs a deformed interval, laterally continuous for hundreds of kilometers. Brecciated and contorted bedded siltstones and mudstone (Motuca Formation) and sandstone with sinsedimentary faults/microfaults, convolute lamination and mud-filled injection dykes (Sambaíba Formation) are interpreted as seismites triggered by high magnitude earthquakes (>; 8 according Richter scale).

Dolomite in Permian Paleosols of the Bravo Dome CO2 Field, U.S.A.: Permian Reflux Followed by Late Recrystallization at Elevated Temperature

Abstract Lower Permian Abo–Tubb subsurface strata of northeastern New Mexico, U.S.A. consist of 30 to 120 m of fluvial and eolian deposits, containing numerous dolomitic paleosols. Dolomite in the Abo–Tubb interval occurs in discrete intervals, interpreted as paleosols on the basis of recognizable horizonation and the presence of associated features such as ped structures and rhizoliths. The dolomite commonly forms veins, rhizoliths, and discrete and coalesced nodules. Petrographic analysis indicates four dolomite types present in paleosol horizons: microsparitic, pseudospherulitic, clear cement, and cloudy (inclusion-rich) cement. Microsparitic dolomite constitutes most of (70%) the dolomite, and is stoichiometric, with relatively low Sr and Na and elevated Fe and Mn. Pseudospherulitic dolomite forms 15% of the dolomite, and consists of stoichiometric microspar and spar, with abundant fluid inclusions, low Sr, Na, and Fe, and elevated Mn. Clear dolomite cement constitutes approximately 12% of the dolomite and is finely to coarsely crystalline with a relatively calcian composition, low Sr and Na, and elevated Fe and Mn. The cloudy cement forms a minor (2%) phase of very coarsely crystalline rhombs with a calcian composition, low Sr and Na values, and relatively high Fe and Mn. All dolomite types exhibit a mottled texture of moderate to bright orange and red luminescence. Fluid-inclusion analysis was not possible on the most common (microsparitic) dolomite type, but analyses on the coarser varieties reveal homogenization temperatures typically ranging from 70°C to 110°C and final melting temperatures of ice ranging from −20°C to −34°C, suggesting fluids of high (25% by weight) salinity. Bulk-rock carbon and oxygen isotopes of carbonate nodules and rhizoconcretions from four paleosol profiles most commonly exhibit mean values typically ranging between~ −3 to −5.5‰ (VPDB) and −0.5 to 2.8‰ (VPDB), respectively. Sulfur isotopes were analyzed for the overlying Cimarron Anhydrite interval and an anhydrite nodule near the base of the study interval. Both samples yielded similar results, suggesting that brines that precipitated the Cimarron Anhydrite also influenced the study interval. The dolomite in the Abo–Tubb interval is interpreted to have precipitated in multiple stages. Solid calcite microinclusions in microsparitic and pseudospherulitic dolomite suggest that pedogenensis and groundwater penecontemporaneous with Early Permian deposition led to precipitation of calcite rather than dolomite. Oxygen and sulfur isotope data indicate dolomitization, from later reflux of brines associated with deposition of the overlying (Leonardian) Cimarron Anhydrite. Petrographic relations and elevated temperatures and salinities recorded by the fluid inclusions in the coarse dolomite are consistent with recrystallization of the earlier dolomite and precipitation of dolomite cement from warm saline fluids of a possible Tertiary hydrothermal origin, just predating migration of mantle-derived gas into the Bravo Dome reservoir. These findings offer insight into questions about the climate significance of dolomite in paleosols, origin of dolomite in hydrothermally altered hydrocarbon reservoirs, and the long-term effect of CO2 sequestration in saline aquifers. The results show that dolomitic nodules in paleosols may form as replacements of paleosol calcite long after the paleosol formed. Primary pedogenic dolomite would undoubtedly have climatic and paleoenvironmental significance, but recognizing it as forming during soil formation, as opposed to later dolomitization, is key in establishing that significance. Also, this study provides an example of high-temperature recrystallization of low-temperature dolomite, a process hotly debated in discussions of dolomitized hydrocarbon reservoirs with evidence for hydrothermal fluid flow. Finally, as a natural CO2 reservoir, the Bravo Dome system shows that the long-term result of dissolution of CO2 into brines is predominantly carbonate mineral dissolution.

Environmental factors influencing streamwater composition on sandstone (Vosges Mountains)

The influence of sandstone composition, precipitation, vegetation and relief on streamwater composition at base flow was studied in 95 forested catchments located in the sandstone part of the Vosges Mountains (N.E. France). Catchments lie on four main types of sandstone. Information was acquired for each catchment using a geographic information system (GIS) and spatially distributed databases. Sulphate concentration is mainly negatively correlated to the annual rainfall and positively to the relative area covered by conifers in the catchment. Nitrate is positively correlated to altitude. Sodium, Cl and Si concentrations are strongly inter-correlated and negatively related to the annual precipitation and the relative are ao f the conglomerate layer in the catchment. Calcium, Mg, ANC (Acid Neutralising Capacity) and pH are negatively correlated to precipitation, and positively correlated to the relative area of the sandstone layer containing dolomitic nodules. This study emphasizes the role of annual rainfall amounts in controlling stream chemistry at base flow.

Isotopic compositions of diagenetic dolomites in the Tortonian marls of the western Mediterranean margins: evidence of past gas hydrate formation and dissociation

Previous studies in the Lorca basin, S.E. Spain (Sediment. Geol., 2002) have shown that the diagenetic dolomitic concretions from the Tortonian marls are related to the past occurrence of gas hydrates which were rapidly dissociated when the sea level dropped during the Messinian salinity crisis. These conclusions were based on the oxygen and carbon isotopic compositions of these dolomites which indicated that the carbonate diagenesis occurred either in 13C-rich/ 18O-poor solutions as it is typical in gas hydrate systems or in 13C-poor/ 18O-rich solutions generated during gas hydrate decomposition. We have recently discovered similar dolomitic nodules in other Tortonian marly successions from S.E. Spain and Northern Morocco. The oxygen and carbon isotopic compositions of the diagenetic dolomites from these areas in Spain and Morocco are very similar to those from the Lorca basin. This demonstrates that the dolomitic nodules from the Tortonian marls of the Betic and Rifian basins were all related to the same diagenetic environments where pore fluids were modified by gas hydrate formation–dissociation processes. These overall observations in S.E. Spain and Northern Morocco suggest that the dissociation of gas hydrates during the Messinian salinity crisis was not a local phenomenon but corresponded to a major event which affected both sides of the Western Mediterranean margins; it might have been initiated during the first major restriction step that started around 6.7 Ma in the Mediterranean Sea, and probably climaxed at 5.9 Ma when the Mediterranean sea level dropped by more than 1000 m during the salt deposition stage.

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

AbstractThe 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.

Methane-related authigenic carbonates of eastern Mediterranean Sea mud volcanoes and their possible relation to gas hydrate destabilisation

Nautile submersible investigations of mud volcanoes and brine seep areas of the eastern Mediterranean Sea during the MEDINAUT cruise in November 1998 discovered extensive areas of authigenic carbonate crusts associated with methane emissions. Carbonate crusts form pavements, round slabs and circular mounds on the central, most active parts of mud volcanoes and in a fault-related valley where brines have accumulated to form a submarine brine lake. Authigenic carbonate nodules have been recovered from the same areas during the MEDINETH cruise in July 1999. Large 13C depletions of authigenic calcite, aragonite and dolomite indicate methane as a major carbon source for the carbonate. Crust pavements are formed when methane from a freshly emplaced, methane-charged mud flow is oxidised at the seafloor. In this environment, where bottom waters provide the sulphate and magnesium, aragonite is favoured versus calcite and accounts for the majority of the methane-related authigenic carbonates. Calcite, when present, contains significant amounts of Mg2+ (high-Mg calcite), and possibly other divalent ions in its crystal lattice. In areas of brine seep and accumulation, dolomitic nodules are present at shallow depth in the sediment. The 18O enrichment of the authigenic carbonates (up to 4‰ greater than calculated values for carbonates precipitating from modern eastern Mediterranean bottom waters) is interpreted as due to precipitation from 18O-rich fluids rather than as a temperature effect. The source of the 18O-rich fluids may be multiple and possibly includes the destabilisation of gas hydrates present at shallow subbottom depth, and the seepage of relic Messinian brines.

A Namurian A (Silesian) permineralized flora from the Carrière du Lion at Engihoul (Belgium)

A permineralized flora was discovered in the Carrière du Lion, at Engihoul (southern margin of the Namur Synclinorium, Belgium). This flora is preserved in dolomitic nodules within ampelitic alum shale. The goniatites Isohomoceras subglobosum and Homoceras beyrichianum allow this shale to be correlated with the Chokierian (Namurian A — Silesian — Carboniferous). The plant association includes Dubiocarpon elegans (Endogonales?), Lepidophloios sp., Lepidocarpon lomaxii (Lepidodendrales), Arthropitys sp. (Equisetales), Psaronius sp. (Marattiales), a group of sporangia incertae sedis, whose affinities are discussed, Myeloxylon sp. (Medullosales), and fragments of pycnoxylic secondary xylem. This flora is interesting because of its Namurian A age: the Arthropitys and Dubiocarpon specimens are the oldest known to date, and the Psaronius specimen is the oldest occurrence of a petrifaction of this genus.

Sulphur isotope geochemistry of the ores and country rocks at the Almadén mercury deposit, Ciudad Real, Spain

Seventy-four new S isotope analyses of ore minerals and country rocks are given for the Hg deposit of Almadén. The spread of the cinnabar δ 34 S is narrow within each of the three orebodies, but the δ 34 S average values differ sufficiently between them (mean δ 34 S: San Nicolas = 0.2 ± 1.1 %., San Francisco = 8.1 ± 0.7%., San Pedro = 5.9 ± 1.0%.) to indicate three different mineralization episodes and possibly processes. The unweighted mean for all cinnabar samples is 5.6%. and the S source is considered to be the host-rocks, either the Footwall Shales ( δ 34 S = 5.5%.) or the spilites ( δ 34 S = 5.1 ± 1.3%.). For geometric and chronologic reasons, the former seem the best potential source. However, the high δ 34 S values of the San Francisco cinnabar cannot be explained without addition of heavy S from reduction of seawater sulphate. Orderly distributions of the δ 34 S values are observed in all three orebodies: 1. (1) their increase from the stratigraphic bottom to the top in the San Pedro orebody is explained by a Rayleigh process, and 2. (2) the maxima in the centres of the San Francisco and San Nicolas orebodies are explained by mixing of the S transporting hydrothermal fluids with seawater within the sediments. Associated pyrite and cinnabar were deposited under isotopic disequilibrium, probably because the low solubility of cinnabar caused rapid precipitation of cinnabar. The different morphological pyrite types have their own isotopic δ 34 S signatures. The spilites are notably enriched in S ( n = 3; average S content = 0.56%) compared to normal basalts (1000 ppm) and have an average δ 34 S = 5.1 ± 1.3%.. The linear relationship between the δ 34 S and the S content of the spilites is interpreted as a mixing line between mantle S and a constant S source, probably an infinite open reservoir. An incomplete basalt-seawater reaction at nearly constant temperature is the best explanation for this relation. The S (predominantly pyrite) of the black shales ( n = 3; δ 34 S from −6.7%. to 14.4%.) is interpreted as resulting from bacterial seawater sulphate reduction under variable euxinic conditions. The stratabound pyrite ( n = 16) found in the sediments near, but independent of the Hg ores, also has a wide range of δ 34 S between 15.0%. and 16.4%., with a mode of about 8%.; the pyrite rims of the diagenetic dolomite nodules ( n = 2; δ 34 S = −14.7 ± 0.35%.) in the Hanging Wall Shales display the most negative values found at Almadén.

Mineralogia y sedimentologia del yacimiento de saponita de Yuncos (Toledo)

Mineralogy and textures of saponite-bearing bentonites and related facies have been studied in quarries located southwest of Yuncos village (Toledo) in the Madrid Neogene Basin. Results allow us to establish three stages related with the evolution of sedimentary environments, where bentonite deposits (middle stage) are at the boundary between lacustrine (lower stage) and distal alluvial facies (upper stage). Lower stage : Regressive mud flat deposits with paleopedogenic features, made up of green clays facies with variable detrital content and hydromorphic mottling at topo Clay mineralogy association: Smectite (tri-dioctahedral)-illite-(kaolinite). Middle stage : Transitional stage of shallow flooding with variable development of paleopedogenic features. The facies association is made up of saponitic brown clays with several textures (massive, fissile-Iaminated, brecciated) and occasional carbonates as calcite ooids or dolomite nodules. Clay mineralogy association: Smectite (trioctahedral)-illite-sepiolite-( kaolinite) . Upper stage : Progradation of distal alluvial fan deposits. The facies association is made up by reddish to brownish mudstones often with bioturbation features and hydromorphic mottling, with sporadic sandy inserts. Clay mineralogy association: Smectite (tri-dioctahedral)- illite-sepiolite-(kaolinite-chlorite). Illite, dioctahedral smectite, kaolinite and chlorite are interpreted as inherited clay minerals, whilst saponite and sepiolite are of authigenic origino Saponite origin is early diagenetic both by dioctahedral smectite transformation and neoformation, being an inhomogeneous genetic process so that the purest bentonite is located far away from distal alluvial progradation where the flooding is longer in time. From a genetic point of view, sepiolite is later than trioctahedral smectite sometimes being in short fibers on saponite sheets, at others in longer fibers as a cement, both cases related to salinity-alkalinity changes in the environment.