Diagenetic Cements Research Articles

Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China

The Ediacaran Period is characterized by the most profound negative carbon isotope (δ13C) excursion in Earth history, the Shuram Excursion. Various hypotheses – including the massive oxidation of dissolved organic carbon (DOC) in the oceans, the weathering of terrestrial organic carbon, or the release and oxidation of methane hydrates and/or expelled petroleum from the subsurface – have been proposed as sources of the 13C-depleted carbon. More recently, it has been suggested that global-scale precipitation of early authigenic carbonates, driven by anaerobic microbial metabolism in unconsolidated sediments, may have caused the Shuram Excursion, but empirical evidence is lacking. Here we present a comprehensive analysis of a Shuram-associated interval from the uppermost Doushantuo Formation in South China. Our study reveals petrographic evidence of methane-derived authigenic calcite (formed as early diagenetic cements and nodules) that are remarkably depleted in 13C – suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane (AOM) – and systematically depleted in 18O relative to co-occurring dolomite. Early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction, triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels. In light of the finding of methane-derived authigenic carbonates at Zhongling, and based on our basin-scale stratigraphic correlation, we hypothesize that the marked 13C and 18O depletion (including their co-variation noted worldwide) in the Shuram Excursion may reflect an episode of authigenesis occurring within a sulfate–methane transition zone (SMTZ). If true, the Shuram Excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the Ediacaran ocean driven by the Neoproterozoic oxidation of surface environments. This paleo-oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals. Our study highlights the significance of an integrated approach that combines petrography, mineralogy, and texture-specific micro-drilling geochemistry in chemostratigraphic studies.

Geochemistry and diagenetic history of the Ordovician Lower Head Formation sandstones, western Newfoundland, Canada

The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises siltstones with very fine grained to fine-grained sandstones. Petrography confirms that these sandstones are matrix rich, essentially wackes, with detrital minerals including quartz, feldspar, biotite, and numerous accessory minerals. Observed petrographic features suggest that the Lower Head sediments are the distal product of erosion, with sediment sourced from the Dashwoods microcontinent and Lushs Bight oceanic tract and thoroughly mixed in an earlier basin prior to final deposition in a trench slope basin. The Lower Head Formation sandstones have low porosity, with early diagenetic cements (C1) and later calcite in crosscutting calcite veinlets (C2). Petrographic, isotopic, and fluid inclusion analyses indicate that C1 cements formed during the early stages of diagenesis. Both δ13C and δ18O isotopes for the C1 calcite cements are isotopically heavier than the C2 calcite veins. Fluid inclusion homogenization temperatures for the later crosscutting C2 calcite generally range between 78 and 116 °C, with a mean of 100.7 °C (±9.75 °C) and fluid salinities of 5.41–15.98 equiv. wt.% NaCl. Fluid inclusion gas analysis from C2 calcite confirms that CO2/CH4 generally has an inverse correlation with N2/Ar. Petroleum-bearing fluid inclusions were also recorded in C2 calcite cements, indicating that these fractures were conduits for hydrocarbon migration prior and (or) during cementation. However, the early cementation and associated low porosity of the Lower Head Formation sandstones indicate that they offer restricted pathways to migrating fluids and volatiles, and any hydrocarbon migration must have been fracture controlled.

CHARACTERIZATION OF THE LATE APTIAN TOP‐DARIYAN DISCONFORMITY SURFACE OFFSHORE SW IRAN: A MULTI‐PROXY APPROACH

The late Aptian disconformity at the top of the carbonate‐dominated Dariyan Formation is a regional‐scale surface recognised throughout SW Iran and adjacent areas to the south. This study investigates the disconformity using wireline logs, petrographic analyses, SEM observations, and CL and stable isotope data from an oilfield offshore SW Iran. The top‐Dariyan surface in the studied field is characterised by erosional incisions and clastic‐filled fissures which, together with secondary porosity in the underlying carbonates, uranium enrichment and meteoric infill cements, are interpreted as subaerial exposure‐related features. The clastic‐filled fissures were identified from gamma‐ray logs in horizontal wells, and infill material is composed of argillaceous carbonates, clays, quartz and iron‐bearing minerals. The correlation of GR logs in 19 vertical wells resulted in the recognition of erosional incisions (up to about 16 m deep) on the top‐Dariyan surface, together with thickening of siliciclastic sediments in the overlying basal part of the Kazhdumi Formation. Back‐filled sediments in erosional incisions may form potential exploration targets. Secondary porosity in the uppermost 20 m of the Dariyan Formation was created by meteoric processes during subaerial exposure. Cathodoluminescence microscopy of diagenetic cements, carbon and oxygen isotope data and elemental analyses were used to reconstruct the diagenetic history of the carbonates in the Upper Dariyan interval.

The Late Cryogenian Warm Interval, NE Svalbard: Chemostratigraphy and genesis

The Late Cryogenian Warm Interval (LCWI) refers to a non-glacial interval that separates presumed representatives of the Sturtian and Marinoan panglaciations. Its duration is poorly constrained radiometrically and its deposits are relatively poorly known in most geographic regions. This paper aims to constrain the duration, palaeoenvironments and petrogenesis of such deposits in the classic region of NE Spitsbergen, Svalbard. The succession comprises a 200–205m dolomitic shale (Macdonaldryggen Member, known as E3, of the Elbobreen Formation) overlain by oolitic dolomite Slangen Member (E4), 15–25m thick, with limestone developed at top and base of E3 in the south of the area. The assumed age context of the succession has been confirmed by the presence of a typical Sturtian cap carbonate profile of negative to positive δ13C, and primary Sr isotope compositions of basal E3 limestones <0.7072 and of upper E3 limestones of 0.7076.At the base of E3, interstratification of cap carbonate with ice-rafted and redeposited glacial sediments occurs. Early diagenetic stabilization of carbonate mineralogy from a precursor, possibly ikaite, to calcite or dolomite is inferred. E3 is predominantly dolomitic silt-shale, with sub-millimetre lamination, lacking sand or current-related sedimentary structures. Thin fine laminae are partly pyritized and interpreted as microbial mats. Dolomite content is 25–50%, with δ13C values consistently around +4‰, a value attributed to buffering by dissolution of a precursor metastable carbonate phase. Local calcite cement associates with low δ13C values. The carbonates form silt-sized, chemically zoned rhombic crystals from an environment with dynamically changing Fe and Mn. Three-dimensional reconstructions of cm-scale disturbance structures indicate that they represent horizontally directed sock-like folds, developed by release of overpressure into thin surficial sediment overlying an early-cemented layer.A shoaling upwards unit near the top of E3 displays calcium sulphate pseudomorphs in dolomite in the north, but storm-dominated limestones in the south, both being overlain by peritidal oolitic dolomites, exposed under the succeeding Wilsonbreen glacial deposits. There is no Trezona δ13C anomaly, possibly implying top-truncation of the succession.Regular 0.5m-scale sedimentary rhythms, reflecting subtle variations in sediment texture or composition occur throughout E3 and are interpreted as allocyclic. They are thought to be mainly primary in origin, locally modified slightly during early diagenetic cementation. Rhythms are proposed to represent ca. 18kyr precession cycles, implying 6–8Myr deposition between glaciations.

Temporal and bathymetric resolution of nautiloid death assemblages in stratigraphically condensed oozes (New Caledonia)

AbstractCephalopod shells can be affected by postmortem transport and biostratigraphic condensation, but direct estimates of the temporal and spatial resolutions of cephalopod assemblages are missing. Amino acid racemisation calibrated by 14C demonstrates a centennial‐scale time averaging (&lt;500 years) of Nautilus macromphalus in sediment‐starved, epi‐ and mesobathyal pelagic environments. The few shells that are thousands of years old are highly degraded. The median occurrence of dead shells is at 445 m depth, close to the 300–400 m depth where living N. macromphalus are most abundant. Therefore, dead shells of this species accumulate at a centennial temporal resolution and with excellent bathymetric fidelity. Dead Nautilus shells exist for only a few hundred years on the seafloor, in contrast to the biostratigraphically condensed mixture of extant foraminifers and foraminifers that went extinct during the Pleistocene. Cephalopod shells that do not show any signs of early diagenetic cementation are unlikely to be biostratigraphically condensed.

Multi-scale analysis of porosity in diagenetically altered reservoir sandstone from the Permian Rotliegend (Germany)

We present a multi-scale 3D analysis of the pore network in a Rotliegend reservoir sandstone, both at the scale of the sand grains and the scale of the diagenetic clay cements that grow in the intragranular volume (IGV). For this we combine optical microscopy, micro-CT, Broad Ion Beam polishing, Scanning Electron Microscopy, Woods Metal Porosimetry and Focussed Ion Beam–SEM tomography. In cm-scale layers the samples show differences in IGV and sand fabric. In the layers where IGV is higher, there is less diagenetic cement and the IGV forms a connected network; in the layers where the sand fabric is more compacted, larger pores are connected via the pore network in the diagenetic clay cements with pore throat size down to 20 nm in diameter. We show that the pore network in all types of diagenetic clay is connected and conclude that the total gas volume that can be produced from such a reservoir sandstone includes the pore volume in the diagenetic clay cements. We predict a major change in transport properties when the diagenetic clay fill renders the IGV disconnected.

Constraining Temperatures of Formation and Composition of Dolomitizing Fluids In the Upper Devonian Nisku Formation (Alberta, Canada) With Clumped Isotopes

Abstract Three dolomite phases in the burial dolomitization system of the Nisku Formation in central Alberta (Canada) were measured for carbonate clumped isotopes. Independent temperature constraints using fluid inclusions and traditional oxygen isotope geothermometry allow us to test the application of this method using a new calibration curve extrapolated to higher temperatures. These three phases of dolomite correspond to three diagenetic stages in the burial history. The oldest phase is replacive gray matrix dolomite formed during relatively shallow burial depths at average clumped-isotope temperatures (Δ47T) of +80 ±7°C. Previous temperature estimates from standard oxygen isotope geothermometry ranged from 50 to 80°C. A calculated δ18O(H2O) of 3 ± 1.4‰ VSMOW points to precipitation from Devonian seawater modified by evaporation and/or water–rock interaction. The first phase of saddle dolomite cement formed by pressure-solution during intermediate burial depths yielded average Δ47T of 96 ±4°C (previously calculated at ~80°C) and δ18O(H2O) of 4 ± 1‰ VSMOW. A second phase of saddle dolomite formed in association with thermochemical sulphate reduction during deep burial records average Δ47T of 151 ±5°C, comparable to fluid inclusion temperatures ranging from 125 to 145°C. The average δ18O(H2O) of the fluid that formed this saddle dolomite phase is calculated as 8 ± 1‰ VSMOW, suggesting much more advanced water–rock interaction. Δ47T are consistent with previous temperature estimates from a combination of geological data conventional O-isotope and fluid-inclusion thermometry. Another important finding is that the Δ47 signatures are preserved even after reaching deeper burial and/or during uplift and cooling, which points to a closure temperature in dolomites in excess of 180°C. Thus, dolomite appears to be a better phase for clumped-isotope analysis than calcite for rocks that have been exposed to higher temperatures upon further burial. This study demonstrates the power of the clumped-isotope thermometer to reconstruct complex diagenetic histories in burial diagenetic environments where multiple generations of diagenetic cements can be analyzed individually.

Multi-scale analysis of porosity in diagenetically altered reservoir sandstone from the Permian Rotliegend (Germany)

Abstract We present a multi-scale 3D analysis of the pore network in a Rotliegend reservoir sandstone, both at the scale of the sand grains and the scale of the diagenetic clay cements that grow in the intragranular volume (IGV). For this we combine optical microscopy, micro-CT, Broad Ion Beam polishing, Scanning Electron Microscopy, Woods Metal Porosimetry and Focussed Ion Beam–SEM tomography. In cm-scale layers the samples show differences in IGV and sand fabric. In the layers where IGV is higher, there is less diagenetic cement and the IGV forms a connected network; in the layers where the sand fabric is more compacted, larger pores are connected via the pore network in the diagenetic clay cements with pore throat size down to 20 nm in diameter. We show that the pore network in all types of diagenetic clay is connected and conclude that the total gas volume that can be produced from such a reservoir sandstone includes the pore volume in the diagenetic clay cements. We predict a major change in transport properties when the diagenetic clay fill renders the IGV disconnected.

Taphonomy of ‘cystoids’ (Echinodermata: Diploporita) from the Napoleon quarry of southeastern Indiana, USA: The Lower Silurian Massie Formation as an atypical Lagerstätte

Diploporites, like most pelmatozoan echinoderms, are characterized by complex multi-element skeletons prone to complete disarticulation if left exposed for extended periods of time. Consequently, preservation of abundant articulated diploporite thecae is rare and generally restricted to environments where rapid burial events could catastrophically entomb individuals. One such environment is represented by the lower portion of the Wenlock-age Massie Formation at the New Point Stone quarry near Napoleon, southeastern Indiana, which we recognize as a Konservat-Lagerstätte. Yet, taphonomic evidence does not support live burial. Diploporites (Holocystites spp.) are dominantly preserved as intact thecae without any portions of brachioles and with damage (plate shifting and plate loss/plate jumbling) on one side. Post-mortem encrustation is common, but epibionts are typically present on the well-preserved sides of thecae. Geopetal sediment-fill of thecae shows that the well-preserved side commonly faced upward following burial. This taphonomic state is paradoxical, as burial of live diploporites would have resulted in preservation of complete individuals with no encrusters and only compaction-induced damage; rapid burial of diploporites following a short interval of exposure would have resulted in preservation similar to observed patterns, but with encrusters and geopetal infills indicating that the poorly preserved side faced up; and burial after extended exposure would have resulted in thorough disarticulation of thecae. Rather, a variable and, in some cases, complex taphonomic history is suggested for diploporites from the Napoleon quarry, with at least some individuals having experienced one or more brief episodes of exhumation prior to final burial. Early diagenetic cementation of initial theca-filling sediment is the most likely mechanism for keeping thecae intact during subsequent exposure.

Late Jurassic biogeochemical microenvironments associated with microbialite-coated unionids (Bivalvia), Asturias (N Spain)

The Vega Formation (Upper Jurassic, Asturias, Spain) contains two Kimmeridgian outcrops – the Abeu and Huerres sections – with bivalves coated with carbonate microbialites (oncoids). The bivalve shells in the Abeu section were replaced early by calcite, while those of the Huerres section were dissolved and the moulds filled by late diagenetic cements. For the Abeu shells, there was a loss of Sr and a gain of Mg, Fe and Mn during the replacement process, but much of the original organic matter was retained. Part of the S present can be linked to this organic matter, and part to the sulphate that replaced the carbonate groups in the calcite. The thin microbialites inside the shells of some of the Huerres bivalves have allowed us to estimate the influence of the growth rate on the incorporation of trace elements in calcite. With an approximately six times lower growth rate, the internal microbialite incorporates less Mg and S and more Fe and Mn than the external one, while maintaining the Sr content. The trace element content (Mg, S, Sr, Fe and Mn) of the Huerres and Abeu microbialites are similar; these coatings may therefore have been formed in the same type of freshwater but in different microenvironments. Given the substantial sulphate content of the microbialites from both sections, this freshwater probably came from hot springs. However, the high Fe/Mn ratio of the Abeu microbialites suggests a reducing environment during their formation. Indeed, δ18O and δ13C data confirm that the Abeu and Huerres microbialites were formed in different microenvironments: the positive covariance for δ18O and δ13C of the Abeu microbialites and replaced shells (r2=0.83) and the wide range of variation for δ18O (2.80‰) together suggest that the microbialite crusts grew in isolated ponds where evaporation was dominant. In the Huerres microbialites the positive covariance for δ18O and δ13C (r2=0.59) and the narrow range of variation for δ18O (0.74‰) indicate that the Huerres section was formed in a more open environmental system where degassing predominated over evaporation.

Experimental calibration of silicon and oxygen isotope fractionations between quartz and water at 250 °C by in situ microanalysis of experimental products and application to zoned low δ30Si quartz overgrowths

The interpretation of silicon isotope data for quartz is hampered by the lack of experimentally determined fractionation factors between quartz and fluid. Further, there is a large spread in published oxygen isotope fractionation factors at low temperatures, primarily due to extrapolation from experimental calibrations at high temperature. We present the first measurements of silicon isotope ratios from experimentally precipitated quartz and estimate the equilibrium fractionation vs. dissolved silica using a novel in situ analysis technique applying secondary ion mass spectrometry to directly analyze experimental products. These experiments also yield a new value for oxygen isotope fractionation. Quartz overgrowths up to 235μm thick were precipitated in silica–H2O–NaOH–NaCl fluids, at pH12–13 and 250°C. At this temperature, 1000lnα30Si(Qtz–fluid)=0.55±0.10‰ and 1000lnα18O(Qtz–fluid)=10.62±0.13‰, yielding the relations 1000lnα30Si(Qtz–fluid)=(0.15±0.03)*106/T2 and 1000lnα18O(Qtz–fluid)=(2.91±0.04)*106/T2 when extended to zero fractionation at infinite temperature. Values of δ30Si(Qtz) from diagenetic cement in sandstones from the basal Cambrian Mt. Simon Formation in central North America range from 0 to −5.4‰. Paired δ18O and δ30Si values from individual overgrowths preserve a record of Precambrian weathering and fluid transport. The application of the experimental quartz growth results to observations from natural sandstone samples suggests that precipitation of quartz at low temperatures in nature is dominated by kinetic, rather than equilibrium, processes.

Evaluation of meteoric calcite cements as a proxy material for mass-47 clumped isotope thermometry

Meteoric diagenetic cements are ubiquitous throughout geologic history, affecting most carbonate exposures worldwide. They can often be difficult to interpret, as it is usually difficult to separate the influences of water δ18O and temperature on isotopic signals contained within the carbonate rock body. Despite this difficulty in interpretation, meteoric phreatic cements can potentially be a useful proxy material, as they form slowly in equilibrium at mean annual temperature and are not affected by any biogenic effects that can bias other proxy materials.We applied the mass-47 clumped isotope paleothermometer to Pleistocene and Holocene carbonates from Bermuda and Barbados in order to investigate the effects of meteoric diagenesis on Δ47 signals, and to determine their suitability as a paleotemperature proxy. Phreatic calcite cements are found to record the same temperatures as unaltered carbonate sediments, while any sample exhibiting vadose characteristics is biased towards unreasonably hot apparent formation temperatures. Burial heating and re-equilibration are not geologically viable explanations for the anomalously hot temperatures recorded in vadose cements, as neither Bermuda or Barbados has any burial history. Instead, it is likely that precipitation in the vadose zone occurs on timescales quicker than isotopic equilibrium can be achieved, driven by a combination of CO2 degassing and evaporation, which have been previously shown to cause problems in speleothems and pedogenic carbonates.We conclude by suggesting that meteoric phreatic calcites may be an ideal phase for paleotemperature reconstruction, as they accurately record mean annual temperatures and form under equilibrium conditions, while also being resistant to further mineral driven diagenesis. Vadose cements, and any sample likely affected by processes similar to vadose diagenesis, should be avoided for climate reconstructions using the mass-47 clumped isotope thermometer.

Diagenesis and Diagenetic Evolution of Deltaic and Neritic Gas‐Bearing Sandstones in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin: Implications for Depositional Environments and Sequence Stratigraphy Controls

AbstractGas‐bearing deposits in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin consist of shoreface sandstones of the highstand systems tract (HST) and transgressive systems tract (TST), and deltaic sandstones of the lowstand systems tract (LST) and falling stage systems tract (FSST). Detailed petrographic observations suggest that the diagenetic features and related evolution of these deposits cannot be simply characterized and demonstrated in the depth domain. However, the occurrence of diagenetic minerals systematically depends on the studied interval within the HST, TST, LST, and FSST; therefore, diagenesis in this region can be better constrained when studied in the context of the depositional environments and sequence stratigraphic framework. The eogenetic processes in such settings include: (1) microcrystalline siderite precipitated as concretions in almost all environments and systems tracts, which inhibited further mechanical compaction; (2) grain dissolution and kaolinitization occurred in shoreface HST sandstones and deltaic LST and FSST sandstones; (3) glaucony was locally observed, which did not clearly reflect the controls of facies or sequence stratigraphy; and (4) cementation by pyrite aggregates occurred in the shoreface HST sandstones and deltaic LST sandstones. The mesogenetic diagenesis includes: (1) partial conversion of kaolinite into dickite in deltaic LST sandstones, and minor chlorite cementation in deltaic FSST sandstones; (2) transformation of kaolinite into illite and quartz cementation in deltaic LST and FSST sandstones; (3) frequent precipitation of ankerite and ferroan calcite in shoreface TST sandstones and early HST sandstones, forming baffles and barriers for fluid flow, with common calcite in shoreface HST sandstones as a late diagenetic cement; and (4) formation of dawsonite in the deltaic LST and FSST sandstones, which is interpreted to be a product of the invasion of a CO2‐rich fluid, and acts as a good indicator of CO2‐bearing reservoirs. This study has thus constructed a reliable conceptual model to describe the spatial and temporal distribution of diagenetic alterations. The results may provide an entirely new conceptual framework and methodology for successful gas exploration in the continental margins of offshore China, thus allowing us to predict and unravel the distribution and quality evolution of clastic reservoirs at a more detailed and reliable scale.

Geochemistry of Phanerozoic diagenesis on Victoria Island, NWT, Canada

Victoria Island, in Arctic Canada, is one of the largest islands in the world, but its geology has remained largely unmapped and unstudied owing to its remoteness. Base-metal and hydrocarbon showings have been reported from the island, but the origin and prospectivity of these showings remain enigmatic. Regionally extensive, void-filling Phanerozoic diagenetic phases (dolomite, calcite, quartz) are conspicuous in two of the most widespread carbonate units, the Neoproterozoic Wynniatt Formation and the Cambro–Ordovician Victoria Island formation, recording repeated post-depositional movement of fluid through the rocks. These phases were studied to (a) determine whether they could have been associated with movement of metalliferous or petroliferous fluids, (b) establish a diagenetic base-line for a very large and economically unexplored area, and (c) determine whether the fluid-flow events could have been related to known episodes of deformation and mineralisation elsewhere in the Canadian Arctic archipelago. Using an innovative protocol combining in situ SIMS analysis of O and S isotopes with LA-ICP-MS trace-element analysis, the geochemical conditions attending the diagenetic evolution of host dolostone and cement phases were determined. The Wynniatt Formation dolostone contains four Phanerozoic cements: saddle dolomite, brown dolomite, replacive calcite and late calcite. Average δ18O (SMOW) values of Wynniatt Formation saddle and brown dolomite, replacive calcite, and late calcite cements are 24.7‰, 7.7‰, and 6.9‰, respectively. The PAAS-normalised rare earth element patterns of the dolostone, dolomite, and replacive calcite are smooth and flat with slightly positive Ce and Y anomalies and MREE-enrichment; late calcite cement, in contrast, has negative Ce and positive Y anomalies. Recrystallisation of the host dolostone by a reduced, saddle-dolomite-precipitating fluid in a fluid-dominated system altered the isotopic and REE pattern of the dolostone to saddle-dolomite values. This fluid had interacted with shale at depth and then mixed with a relatively high-salinity fluid. Brown dolomite precipitated from this fluid after a decrease in salinity, and oxygenated meteoric water later infiltrated the system and precipitated the calcite cements.The Victoria Island formation dolostone contains two cements: quartz and dolomite. Host dolostone, quartz, and dolomite have average δ18O (SMOW) values of 31.7‰, 18.7‰, and 18.6‰, respectively, whereas δ34S (V-CDT) for framboidal pyrite in the quartz cement averages −7.5‰. The host dolostone has flat PAAS-normalised REE patterns with a positive Eu anomaly, no Ce anomaly, and a zig-zag HREE pattern, whereas the dolomite cements have two REE patterns: one similar to the dolostone (flat) and the other with LREE-enrichment, and negative La and Eu anomalies. These data suggest that a reduced, hydrothermal fluid altered and partially silicified the host dolostone. Quartz precipitated from a seawater-sourced fluid that had incorporated metals at depth and mixed with bacterially reduced sulphur at the site of precipitation. The dolomite cement records the change from a rock-dominant system to a fluid-dominant system. This fluid was sourced from seawater and interacted with REE-phosphate minerals.The results suggest that precipitation of the diagenetic cements took place before peak burial associated with the Ellesmerian Orogeny (late Devonian–early Carboniferous). The most probable cause of fluid migration, therefore, is the Ellesmerian Orogeny for the majority of the diagenetic phases. High-latitude meteoric fluid then infiltrated the system at some time after the Ellesmerian Orogeny. Similar ages, temperatures, compositions, and precipitation mechanisms as those associated with the Cornwallis zinc district (Polaris mine), highlight a possible relation to known base-metal mineralising fluids.The results of this study provide much needed baseline knowledge of the post-depositional history of Victoria Island. It also illustrates that a “less-is-more” approach, using detailed micro-analytical methods on carefully selected and well-constrained samples, allows one to decipher a complex diagenetic history involving the mixing of fluids from different reservoirs and frequent changes in redox state, both of which may be overlooked using conventional macro-scale approaches.

Taphonomy of the Ediacaran Podolimirus and associated dipleurozoans from the Vendian of Ukraine

Fossiliferous strata of the late Ediacaran Lomoziv Member of the Mohyliv Formation, once known from now flooded outcrops along the Dniester River, have been recently exposed in a large quarry near the Novodnistrovs’k electric plant dam in Podolia, Ukraine. Finely bedded arkosic sandstone with claystone/siltstone intercalations is locally rich in ‘elephant skin’ surfaces indicating original presence of microbial mats. Imprints of soft-bodied organisms frequently occur on the sole surfaces of the sandstones. There are no signs of early diagenetic cementation with iron sulfides and the fossils are usually strongly compacted, with low relief. Specimens of bilaterally symmetrical latest Precambrian animals newly collected at the quarry offer additional evidence that their bodies had a complex internal anatomy. This relates especially to Podolimirus mirus, previously known from only fragmentary specimens representing only marginal parts of its chambered organ (‘quilt’). It appears that there is a large region of the body in front of the ‘quilt’, with a lobate organ resembling bifurcating anterior structures earlier thought to represent intestinal diverticula in other dickinsoniids. The ‘quilt’ (presumably a dorsal muscular organ) of Podolimirus has a deep medial sinus that may have hosted a cylindrical axial organ analogous to that reported in the dickinsoniids from northern Russia and Australia.

Chemical and isotopic architecture of the belemnite rostrum

Macrofossil calcite is of great importance for quantitative reconstructions of palaeoenvironment and palaeoseasonality. The calcite rostra of belemnites, Jurassic to Cretaceous marine invertebrates, are especially suited for such investigations, because they are comparatively large and are structured by growth bands. Despite their use in chemostratigraphic and palaeoenvironmental studies, much of the internal variability of geochemical signatures in rostra is poorly understood.Here, multiple profiles through a belemnite rostrum of Passaloteuthis bisulcata (∼183Myr old) were analyzed for δ13C and δ18O values as well as Mg/Ca, Sr/Ca and Mn/Ca ratios. Geochemical signatures of the central 1–2mm of the profiles indicate diagenetic cementation along the apical zone, for which original porosity of up to 40% can be inferred. The overall δ13C and δ18O values of the other, well preserved parts of the belemnite fluctuate by >1 per mil, but are nearly uniform within single growth bands. In contrast, Sr/Ca and Mg/Ca in the well-preserved parts show growth-rate and crystal-shape related variability. Close to the central apical zone, strongly bent calcite crystals are enriched in Mg (up to 70%) and Sr (up to 50%). Through the remainder of the rostrum, higher calcite precipitation rate can account for Mg depletion of ∼15% and Sr enrichment of ∼15% with respect to co-genetic calcite precipitated at a slower rate. No indication for temperature control on Mg/Ca or Sr/Ca is detected in the investigated specimen. Overall, the new findings indicate that δ13C and δ18O analyses of belemnite rostra produce consistent results regardless of the sampling area within the rostrum, and that growth rate effects on element incorporation are minor with respect to the control exerted by secular changes in seawater composition through time. Additionally, the central part of the rostrum, where strong calcite crystal bending is observed, should be avoided for sampling when studying elemental composition of the calcite for palaeoenvironmental reconstructions.

Hydrothermal activity during tectonic building of the Variscan orogen recorded by U-Pb systematics of xenotime in the Grès Armoricain formation, Massif Armoricain, France

In the Saint-Aubin-des-Châteaux deposit (Massif Armoricain, France), the Ordovician Gres Armoricain sandstones have undergone several fluid-rock interaction events, including diagenetic cementation and orogenic base metal-As-Sb-Au mineralisation. Ironstone layers interbedded in the sandstones contain several generations of spectacular authigenic xenotime overgrowths that formed around detrital zircon grains in response to successive hydrothermal events. Textural and chemical characterisations allow to distinguish three generations of xenotime overgrowths, differing notably in their REE characteristics. In-situ U-Pb data obtained on these xenotime overgrowths show that their U-Pb systematics were largely disturbed by successive hydrothermal events over about 90 Ma between ~ 330 and ~ 420 Ma, a time interval encompassing most phases of the construction of the Variscan orogen in France. The younger dates cluster around ~ 330–340 Ma and likely correspond to the age of the deposition of massive sulphides and base-metals in the Saint-Aubin-des-Châteaux deposits, which is consistent with the structural contexts where they formed. Finally, this study shows that similarly to monazite, another phosphate widely used for U-Pb and Th-Pb dating studies, the U-Pb chronometric system in xenotime appears to be highly sensitive to fluid circulations.

Evidence of bacteriogenic iron and manganese oxyhydroxides in Albian–Cenomanian marine sediments of the Carpathian realm (Poland)

The Albian and Cenomanian marine sediments of the Silesian and Tatric basins in the Carpathian realm of the Western Tethys contain ferric and ferromanganese oxyhydroxides, visible macroscopically as brown stainings. They coat calcareous bioclasts and mineral clasts, fill pore spaces, or locally form continuous, parallel microlayers, tens of micrometers thick. Light-microscope (LM) and scanning-electron-microscope (SEM) obser- vations show that the coatings contain elongated capsules, approximately 3–5 µm across and enriched in iron and manganese, which may be remnants of the original sheaths of iron-related bacteria (IRB). Moreover, the ferric and ferromanganese staining observed under LM is similar to bacterial structures, resembling the sheaths, filaments and rods formed by present-day bacteria of the Sphaerotilus–Leptothrix group. All of the possible bacteria-like structures are well preserved owing to processes of early diagenetic cementation. If the observed structures are fossil IRB, these organisms could have played an important role in iron and manganese accumulation on the sea floor during Albian–Cenomanian time. The most plausible source of metals for bacterial concentration in the Silesian Basin might have been submarine low-temperature hydrothermal vents, as previously was hypothesized for Cenomanian–Turonian deposits on the basis of geochemical indices.

Strength characteristics of Japan Trench borehole samples in the high-slip region of the 2011 Tohoku-Oki earthquake

The 2011 Tohoku-Oki earthquake demonstrated that the shallowest reaches of plate boundary subduction megathrusts can host substantial coseismic slip that generates large and destructive tsunamis, contrary to the common assumption that the frictional properties of unconsolidated clay-rich sediments at depths less than ∼5km should inhibit rupture. We report on laboratory shearing experiments at low sliding velocities (<1mm/s) using borehole samples recovered during IODP Expedition 343 (JFAST), spanning the plate-boundary décollement within the region of large coseismic slip during the Tohoku earthquake. We show that at sub-seismic slip rates the fault is weak (sliding friction μs=0.2–0.26), in contrast to the much stronger wall rocks (μs>∼0.5). The fault is weak due to elevated smectite clay content and is frictionally similar to a pelagic clay layer of similar composition. The higher cohesion of intact wall rock samples coupled with their higher amorphous silica content suggests that the wall rock is stronger due to diagenetic cementation and low clay content. Our measurements also show that the strongly developed in-situ fabric in the fault zone does not contribute to its frictional weakness, but does lead to a near-cohesionless fault zone, which may facilitate rupture propagation by reducing shear strength and surface energy at the tip of the rupture front. We suggest that the shallow rupture and large coseismic slip during the 2011 Tohoku earthquake was facilitated by a weak and cohesionless fault combined with strong wall rocks that drive localized deformation within a narrow zone.

Neoproterozoic microbialites in outcrops of the Qarn Alam salt dome, central Oman

ABSTRACT Limestones and mixed limestone and dolomite facies from the Neoproterozoic to early Cambrian Ara Group are exposed as blocks and rafts by the surface-piercing Qarn Alam salt dome in central Oman. These limestones and dolomites compose laminite-stromatolite-thrombolite-evaporite shallowing-up successions, and are remarkable in that they contain well-preserved microbial textures and fossils (both calcite and dolomite with very small but significant silicates and other mineral species) as well as pristine syn-depositional to very early diagenetic cements from the first stages of sediment lithification. The facies are described at scales ranging from outcrop (1–100 m) to the SEM (μm-scale). Outcrop-scale sedimentology and high-resolution stratigraphy are described in detail, and petrographic and geochemical analyses are recorded. The depositional environment is interpreted to have been shallow marine subtidal to intertidal and hypersaline supratidal, with low-energy tidal flats and channels, lagoons or salinas, and continental sabkhas. Both calcitic and dolomitic phases show microbial fossils and structures with fabrics of mineralised extra-cellular polymeric substances (EPS). The occurrence of syn-sedimentary primary dolomitic matrix in thrombolites is interpreted to result from the degradation of a thicker microbial mat, during or after growth, which provided the right micro-environmental conditions for the precipitation of dolomite. A caliche crust and sabkha evaporites (the white band) cap the laminite-thrombolite succession and together with karst breccias, fracture fills and neptunian dykes, record an emersion at the top of each of the depositional units. Stable isotopes of carbon and oxygen of the microbialite to evaporite facies show close values for δ13C (+2‰ to +4‰) but a broader range of δ18O (+0.5‰ to -5‰). These values, and their spread recorded within sets of laminae, indicate little to no diagenetic resetting and therefore should be close to original equilibrium values for seawater and early diagenetic fluids. Later diagenetic cements in fractures show entirely different values with δ13C in the range of -2‰ to -6‰, and δ18O from -7.5‰ to -11‰. Whereas dolomite shows no post-depositional diagenetic modification and records preservation of finely detailed EPS mineralisation, the calcite of clumps of clots and mesoclots shows neomorphism with reorganisation into crudely fascicular-optic crystals that cut across primary sediment and early diagenetic cement fabrics. Preservation of both sedimentary facies and the fossil record is remarkable for these ca. 540 million year old rocks and indicates that diagenesis had little effect on the microbialites at Qarn Alam.