Early Marine Cementation Research Articles

Did anoxia terminate Ediacaran benthic communities? Evidence from early diagenesis

The Ediacaran oceanic redox landscape was heterogeneous, where many basins had a shallow and highly dynamic chemocline above anoxic (ferruginous or euxinic) or low oxygen (manganous) waters. Seawater mMg/Ca ratio was also high, promoting early diagenetic dolomitisation. How the benthos responded to these conditions is fundamental to understanding their ecological dynamics. Here we utilise redox sensitive elements in early marine carbonate cements to investigate possible water column redox controls on the distribution and growth of the oldest metazoan communities.Skeletal communities in the Zaris Sub-Basin of the Nama Group, Namibia (∼550–547 Ma), grew in shallow waters where fine-grained carbonate sediment often shows evidence of early dolomitisation. Mid-ramp Cloudina reefs are composed of open, highly porous structures that formed multiple, successive assemblages. Each assemblage is terminated by thin (<1 mm), layers of dolomicrite sediment and dolomite cement. All dolomitic lithologies in the Nama Group analysed via Fe speciation suggest precipitation under anoxic ferruginous water column conditions.Reef cements show a paragenetic sequence from synsedimentary to early marine cement and final burial, which we infer were precipitated under dynamic redox conditions. First, acicular pseudomorphed aragonite cement formed under oxic conditions (low Fe and Mn). Next, the presence of iron-rich dolomicrite sediment, often associated with a recrystallised ferroan dolomite crust, suggests that originally aragonitic or calcitic sediment and a high-Mg precursor cement were preferentially dolomitised. Dolomitisation may have been enhanced either via upwelling of deeper water, anoxic, ferruginous seawater, or by later fluid remobilisation from adjacent shales. A following Mn-rich calcite cement is inferred to be early marine due to its inclusion-rich, fibrous form and well-preserved CL zonation. The final blocky cement precipitated under oxic conditions, probably during shallow burial.The cements likely record a general shallow to deeper water transect, from oxic shallow waters to low oxygen manganous waters, then to oxic, shallow burial conditions. We hypothesize that Cloudina reef communities were terminated by episodic sediment incursions during short-term, transgressive cycles, possibly accompanied by upwelled, anoxic, ferruginous and dolomitising waters, although the timing for this is poorly constrained. More generally, such incursions may have terminated Ediacaran benthic communities that grew close to the chemocline.

A shallow marine clinoform system in limestones (Paleocene/Eocene Jafnayn Formation, Oman): geometry, microfacies, environment and processes

This is the first report of Cenozoic shallow marine limestone clinoforms from Oman in an outcrop of exceptional exposure. The depositional system displays a few hundred well-developed foreset beds/clinobeds, representing a paleoslope of ~ 15° (post-compaction), occurring in a 3–5 m-thick stratigraphic interval. In the outcrop, foreset beds can be followed along for approximately 20 m. The foreset beds are composed of packstones to floatstones. They pass into very short, minimally developed bottomset beds with a base-discordant relation (downlap). Directly below the bottomset beds is a marly limestone bed (wackestone to floatstone). Bedding of the topset beds is largely obliterated by bioturbation, causing nodular bedding in the topset segment and in an overlying red algae-bearing unit, consisting of floatstones to rudstones. The clinobeds represent cross-beds within an overall aggradational sequence. We use the geometry of the clinoform systems as a “platform level”. Although the foreset process had been repeated several hundred times, neither the steepness of the paleoslope nor the length/height of the foreset beds changed, suggesting a very gentle slope as on a shelf and not as on a ramp. Because of local evidence for synsedimentary extensional faulting, we assume that such deformation caused the creation of the initial intraplatform slope. Dasycladacean algae indicate that deposition ensued in the photic zone. Above the red algae unit, alveolinid wackestones to floatstones accumulated. The observed microfacies change from the marly limestone to the alveolinid limestone indicates that the studied clinoform system developed within a shallowing-up succession. In the same direction, water energy and coastal influences increased. The microfacies types are compatible with a lagoon setting on a shelf with open circulation. Erosion of the red algae may have been due to moving and abrading skeletal grains, driven by tidal and rip currents and/or boring organisms. Tidal and rip currents are suggested to have shed the red algae fragments and other carbonate debris onto the depositional slope. In the absence of early marine cements, slope stability is attributed to the moderate paleoslope angle, different grain shapes and possibly to encrustation and binding of sediment by red algae. The presence of Ranikothalia sp. demonstrates that the succession dates from Paleocene to lowermost Eocene. Similarly, the red algae Distychoplax biserialis indicates a Paleocene to Lower Eocene age.

Depositional constraints and diagenetic pathways controlling petrophysics of Middle Miocene shallow-water carbonate reservoirs (Leitha limestones), Central Paratethys, Austria-Hungary

Middle Miocene shallow-water limestones, the so-called Leitha limestones, comprise a widespread Middle Miocene lithological unit of the Central Paratethys Sea. It includes mainly limestones composed of corallinacean algae and other fossil debris and were deposited in shoals of various basins such as the Vienna, Eisenstadt-Sopron and Styrian basins of Austria where they form potential hydrocarbon reservoir rocks. Petrophysical parameters including porosity, permeability, density, P-wave velocity and magnetic susceptibility measurements were performed and evaluated. The sampled sites of Mannersdorf and Wöllersdorf quarries and drill-hole Bruckneudorf are located in the Vienna Basin and share the same principal limestones depositional facies that is massive, closely-packed rudstone facies. This facies has low porosities below 10%, low permeability lower than 1.2 mD except one samples at 68.5 mD, high bulk density of dry samples up to 2.64 g/cm3 and high P-wave velocity up to 6761 m/s. In Mannersdorf, a second facies type of soft porous coarse-grained rudstone shows significantly higher porosity (up to 47%), higher permeability up to 124.3 mD, lower bulk density of dry samples less than 1.8 g/cm3 and lower P-wave velocity less than 2798 m/s. This latter facies retains primary porosity and secondary porosity enhanced by dissolution. In contrast, the former facies type suffered intensive cementation, which destroyed the primary porosity, and in some cases, subsequent later dissolution and karstification enhanced secondary porosity partly.Kummer (Austria) and Fertőrákos (Hungary) quarries are located within the Eisenstadt-Sopron Basin. The well-sorted grainstone facies has high porosity up to 50%, extremely high permeability up to 31238 mD, low bulk density of dry samples less than 1.71 g/cm3 and low P-wave velocity less than 3219 m/s. The poorly-sorted massive rudstone facies recorded only in Kummer quarry is less porous (porosities range from 22 to 31%) and less permeable. The role of dissolution predominates in these limestones besides compaction and pressure solution in the massive rudstone facies. Limestones from the Rosenberg quarry of the Styrian Basin show two facies types: (1) wackestone to packstone facies with lower porosity (up to 16.4%) and lower permeability ranges from 0.16 to 0.91 mD, compared to (2) the packstone facies with higher porosity (21%) and higher permeability of 1.83 mD. The influence of diagenesis on both facies is similar, represented by early marine cementation followed by partial dissolution and finally meteoric blocky cementation.The depositional influence on petrophysics of Leitha limestones, for instance, packing, sorting and grain size represents the basis for the differences in facies types in the same locality and among various localities. This depositional influence controls the subsequent stepwise diagenetic evolution the facies. Local depositional environments and their spatial evolution play a significant role on reservoir properties such as, at Mannersdorf the two facies types are varied due to the primary depositional characteristics and hence, the diagenetic evolution. Diagenetic pathways are principally similar in the different localities and basins; however, different stages of diagenesis are recorded due to different burial and exhumation histories.

Quantifying the impact of early calcite cementation on the reservoir quality of carbonate rocks: A 3D process-based model

The reservoir properties of carbonate rocks are controlled by both deposition and diagenesis. The latter includes the early precipitation of calcite cements, which can exert a strong control on the evolution of subsequent diagenetic pathways. We quantify the impact of early marine cement growth in grainstones on evolving pore space by examining trends in the relationship between cementation and permeability using a 3D process-based model (Calcite3D). The model assumes varying proportions of polycrystalline and monocrystalline grain types, upon which we grow isopachous and syntaxial calcite cement types, respectively. We model two syntaxial cement shapes, compact and elongated, that approximate the geometries of typical rhombohedral calcite forms.Results demonstrate the effect of cement competition: an increasing proportion of monocrystalline grains creates stronger competition and a reduction in the impact of individual grains on final calcite cement volume and porosity. Isopachous cement is effective in closing pore throats and limiting permeability. We also show that the impact of syntaxial cement on porosity occlusion and therefore flow is highly dependent on monocrystalline grain location and the orientation of crystal axes. This demonstrates the importance of diagenetic overprint in controlling the evolution of rock properties, but also that this process can be essentially random. We also show that diagenesis alone can create notable heterogeneity in the permeability of carbonates.While Calcite3D is successful in modelling realistic changes in cement volumes and pore space morphology, modelled permeabilities (0.01−30D) are above the range reported in reservoir grainstones due to the very high permeability of the initial synthetic sediment deposit (58.9D). Poroperm data generated by Calcite3D, however, exhibits a linear relationship between the logarithms of porosity and permeability with a high coefficient of determination, as observed in natural media.

A new Middle Ordovician bivalve–siliceous sponge–microbe reef-building consortium from North China

A new reef-building consortium from a Middle Ordovician succession of the western North China Platform is described, consisting mainly of bivalves, siliceous sponges, and microbial carbonates (Epiphyton-like, Renalcis-like and Girvanella calcified microbes, and microcrystalline microstromatolites), in addition to minor brachiopods, Amsassia and Rhabdotetradium. The bivalves are thin-walled and mostly articulated, indicating in situ preservation. The siliceous sponges are characterized by regularly spaced spicule networks embedded within micrite, which partly grade into peloidal textures. Three main types of bivalve–sponge associations are found: (1) larger bivalves (2–13mm) encrusted by sponges, (2) sponges occupying internal spaces of larger bivalves, and (3) smaller (0.2–0.4mm) bivalves embedded within sponge spicule networks. Microbial carbonates either cover the upper surfaces of siliceous sponges and bivalves, or occur independently as centimeter-scale patches. The reefal boundstones were constructed mainly by bivalves, siliceous sponges and microbes, which were subsequently encrusted and stabilized by additional sponges and microbes. Extensive early marine cementation forming fibrous cement helped stabilize the reef framework. The co-occurrence of bivalves and siliceous sponges is possibly analogous to modern-day counterparts in which sponges encrust bivalves or bivalves are living within sponges, suggesting a symbiotic relationship. The bivalve–siliceous sponge–microbial reefs of this study, together with other Ordovician reefs, represent the changeover from microbial- to skeletal-dominated reefs during the Middle Ordovician. The current example may represent an ancestral association of bivalve–siliceous sponge–microbe reefs, similar to those in the Mesozoic and Cenozoic, thus shedding light on the roots of such associations.

Carbonate sedimentary characteristics of the beach rocks around Qilian Islets and Cays, Xisha Islands: Implication for coral reef development and decline

Beach rocks, developed on the coast of Qilian Islets and Cays, Xisha Islands, China, represent carbonate sedimentary formation on a typical biogenic coast adjacent to coral reef areas. The subaerial and submarine distribution and natural outcrop sections of beach rocks around three islets were surveyed, traced, measured and mapped to determine their development and depositional successions. Carbonate sedimentary characteristics of the beach rocks show that more than 95% of sediments are sourced from adjacent reef flat and reef crest, and are dominated by reef-building coral skeletal fragments and detritus, calcareous algae, and other skeletal grains of reef dwellers. Bioclastic content, algal and microbial content, sorting and roundness of skeletal grains and porosity in depositional successions change with sediment source, sediment accumulation rate, grain characteristics and textures of sediments, hydrodynamic intensity and cementation features. Cementation of the beach rocks is characterized by the binding action of algae and microbes, the micritization caused by boring of endolithic algae and microbes, the early marine cementation, and the early meteoric cementation. Algal and microbial activity played a major role in micritization of skeletal grains of beach rocks. The beach rock formation is mainly affected by the algal and microbial content in beach sediments, the sediment supply from reef flat, the grain characteristics of beach sediments, cementation and diagenesis process, the hydrodynamic intensity driven by wind and monsoons, and sea-level changes. The developmental dimensions and spatial extent of coral reef architecture, the flourishing and extinction of reef-building organisms in reef ecosystem, the abundance of algae and microbes and the diversity of reef dwellers may influence wave energy dissipation and sediment transport patterns in reef flat, thus alter the carbonate sediment budget of the coastal environment.

The earliest Phanerozoic carbonate hardground (Cambrian Stage 5, Series 3): Implications to the paleoseawater chemistry and early adaptation of hardground fauna

Carbonate hardgrounds are lithified seafloors formed by synsedimentary cementation of carbonate sediments, which dominantly occur during the period of calcite seas. The earliest typical Phanerozoic hardground known until now was reported from the Furongian of USA, which was suggested to indicate onset of the early Paleozoic calcite sea period. In this study, we report hardgrounds from the early and middle parts of the Cambrian Series 3 (Stage 5 and Drumian) of the North China Platform, which predate previously reported hardgrounds. The hardground surfaces developed on oolitic grainstone, oncolitic wackestone, and microbialite (thrombolite and dendrolite), which sharply truncate the underlying deposits. The radial fibrous calcite cements between the carbonate grains below the hardground surfaces indicate that the cements formed by early marine cementation. EPMA analysis reveals that the fibrous cements typically consist of low-Mg calcite. The hardgrounds are sometimes encrusted by microbialites and coated by hematite, suggesting long exposure to the open seawater after formation of the surface. In addition, detailed review on the sedimentological studies of Cambrian Series 3 to Furongian deposits throughout the world reveals that there may be several other hardgrounds during these times, which could have been overlooked. The abundant occurrence of hardgrounds in Cambrian Series 3 deposits suggests that the general paleoseawater chemistry was suitable to induce synsedimentary cementation of low-Mg calcite, implying that seawater chemistry would have changed from the aragonite to calcite seas during the Cambrian Series 3 or even earlier period. Metazoan encrustors and macroborers possibly could not have adapted to the newly appeared substrate condition yet, until the latest Cambrian Series 3.

The major-ion composition of Carboniferous seawater

The major-ion chemistry (Na+, Mg2+, Ca2+, K+, SO42−, and Cl−) of Carboniferous seawater was determined from chemical analyses of fluid inclusions in marine halites, using the cryo scanning electron microscopy (Cryo-SEM) X-ray energy-dispersive spectrometry (EDS) technique. Fluid inclusions in halite from the Mississippian Windsor and Mabou Groups, Shubenacadie Basin, Nova Scotia, Canada (Asbian and Pendleian Substages, 335.5–330Ma), and from the Pennsylvanian Paradox Formation, Utah, USA, (Desmoinesian Stage 309–305Ma) contain Na+–Mg2+–K+–Ca2+–Cl− brines, with no measurable SO42−, which shows that the Carboniferous ocean was a “CaCl2 sea”, relatively enriched in Ca2+ and low in SO42− with equivalents Ca2+ >SO42−+HCO3−.δ34S values from anhydrite in the Mississippian Shubenacadie Basin (13.2–14.0 ‰) and the Pennsylvanian Paradox Formation (11.2–12.6 ‰) support seawater sources. Br in halite from the Shubenacadie Basin (53–111ppm) and the Paradox Basin (68–147ppm) also indicate seawater parentages.Carboniferous seawater, modeled from fluid inclusions, contained ∼22mmol Ca2+/kg H2O (Mississippian) and ∼24mmol Ca2+/kg H2O (Pennsylvanian). Estimated sulfate concentrations are ∼14mmol SO42−/kg H2O (Mississippian), and ∼12mmol SO42−/kg H2O (Pennsylvanian). Calculated Mg2+/Ca2+ ratios are 2.5 (Mississippian) and 2.3 (Pennsylvanian), with an estimated range of 2.0–3.2.The fluid inclusion record of seawater chemistry shows a long period of CaCl2 seas in the Paleozoic, from the Early Cambrian through the Carboniferous, when seawater was enriched in Ca2+ and relatively depleted in SO42−. During this ∼200Myr interval, Ca2+ decreased and SO42− increased, but did not cross the Ca2+–SO42− chemical divide to become a MgSO4 sea (when SO42− in seawater became greater than Ca2+) until the latest Pennsylvanian or earliest Permian (∼309–295Ma). Seawater remained a MgSO4 sea during the Permian and Triassic, for ∼100Myr.Fluid inclusions also record a long interval, from the Early Cambrian to the Middle Devonian, when seawater had low Mg2+/Ca2+ ratios (<2) that coincide with calcite seas. The Mg2+/Ca2+ ratio of seawater rose from 0.9 in the Middle Devonian, to 2.5 in the Middle/Late Mississippian, 2.3 in the Middle Pennsylvanian, and 3.5 in the Early Permian. The transition from calcite seas to aragonite seas, established from the mineralogy of oölites and early marine cements, occurred in the Late Mississippian. Fluid inclusions show that seawater Mg2+/Ca2+ ratios rose above 2 by the Middle to Late Mississippian coinciding exactly with the shift to aragonite seas. Aragonite seas existed for ∼100Myr, from the Late Mississippian until the Late Triassic/Early Jurassic.

Soft-sediment deformation structures in Cambrian siliciclastic and carbonate storm deposits (Shandong Province, China): Differential liquefaction and fluidization triggered by storm-wave loading

This paper focuses on soft-sediment deformation structures (SSDS) in both siliciclastic and carbonate storm deposits of the Cambrian Mantou and Chaomidian formations (Shandong Province, China) in order to understand their deformation mechanisms and possible triggers. Siliciclastic SSDS (e.g., sand volcanoes and pillows) and carbonate SSDS (e.g., sedimentary dykes, grainstone-matrix breccias, deformed cross-beddings, and marlstone-matrix breccias) occur exclusively in hummocky cross-stratified fine sandstone and peloidal grainstone, respectively. The siliciclastic SSDS formed in porous and permeable surface sediment, whereas the carbonate SSDS formed under shallow burial conditions when early marine cementation prevailed. The various deformation structures resulted mainly from differences between siliciclastic and carbonate sediment conditions and resulting deformation mechanisms. Sand volcanoes formed by upward extrusion of over-pressured fluidized sandy sediment, whereas sand pillows resulted from upward injection of lower density muddy sediment into overlying liquefied sandy sediment. Sedimentary dykes and grainstone-matrix limestone breccias formed as a result of differential liquefaction and fluidization of heterogeneously cemented carbonate sediment, whereas deformed cross-beddings and marlstone-matrix limestone breccias developed by the thixotropic liquidization and injection of clayey sediment, and further disruption of thin peloidal layers. Sedimentary facies analysis and analysis of the physical processes indicate that the SSDS were most likely caused by cyclic loading of storm waves.

Diagenesis of a SE Asian Cenozoic carbonate platform margin and its adjacent basinal deposits

This study provides insights into the diagenesis of a SE Asian Tertiary carbonate platform and the variations in alteration that occur across platform top, margin, slope and adjacent basinal deposits. A petrographic, cathodoluminescent and stable isotopic study allowed evaluation of the diagenesis of the Kedango Limestone of Borneo. The Kedango Carbonate Platform developed as a predominantly low to moderate energy system in the semi-enclosed marine embayment of the Kutai Basin during the Eocene to Miocene. Early alteration through pervasive micritisation and minor isopachous fringing, syntaxial overgrowth and bladed to radiaxial cements was due to marine diagenesis on the basis of limited, relatively enriched stable-isotope compositions and non-luminescent cathodoluminescent (CL) characters. Minor early marine cementation formed just in the shallowest-platform top and margin deposits, and was likely promoted in high energy portions of the platform due to wave and/or current activity. Dissolution cavities, banded calcite cements with variable CL characteristics and depleted stable-isotope compositions, micritic sediment infiltration and alveolar textures are minor features, but together are indicative of localised subaerial emergence and meteoric diagenesis. The most prevalent and pervasive diagenetic feature of the deposits is the neomorphic alteration and replacement of metastable bioclasts and micritic matrix, together with contiguous calcitisation of pore spaces. Burial fluids with a marine character are inferred as the parent diagenetic fluid, since stable-isotope compositions for the neomorphic spar is largely consistent with precipitation from SE Asian Oligocene–Miocene seawater in a burial environment. Tectonic subsidence, the protected semi-enclosed marine embayment setting, and probably terrestrial runoff, together with component variability reflecting local environmental variations, are inferred to have been major controlling influences on the diagenesis of the Kedango Platform. Variations in diagenesis across bathyal, slope and shallow-platform deposits of the Kedango Limestone are not as marked as in other platforms from the region.

Facies of Devonian fore reef limestones: a quantitative study (Iberg Reef, Harz Mts., Germany)

Facies in a 238-m-long core drilled through the fore reef of the Devonian (Givetian–Frasnian) Iberg Reef is largely characterised by detrital deposits. Fibrous, early marine cements are common. Stromatoporoids, corals, and crinoids are the dominant reef-building taxa. Brachiopods, cephalopods, and conodontophorids belong to the reef dwellers. Reef destroyers include gastropods and rare echinoids. Quantitative analysis of abundance data allows for dilineation of five facies including detritus-rich grainstones–rudstones, cement- and detritus-rich grainstones–rudstones, cement-rich rudstones, stromatoporoid rudstones, and crinoid grainstones. Time-series analysis indicates cyclic sedimentation that might have been related to mid–late Devonian third-order sea-level variations. However, the long-term (mid-Devonian to Early Carboniferous) development of the Iberg Reef including the post-reef seamount stage was presumably controlled by subsidence.

Diagenetic Pattern in the Citarate Carbonate Rocks, Cilograng Area, Lebak Regency, Banten Province

DOI: 10.17014/ijog.v7i3.142The carbonate sequence overlies conformably the tuffaceous sandstone unit, and in turn is conformably underlain by the tuff-sandstone unit, both of which are members of the Citarate Formation. The Citarate carbonate rocks were deposited in an open platform back reef environment, which was temporarily drowned by local sea level rise. Regional Middle Miocene deformation formed NNE-WSW trend faults and E-W trend folds in the researched area. This paper discusses the nature of diagenetic alteration of the Citarate carbonate rocks based on petrographic analyses of twenty surface samples. Carbonate rocks from bottom to top comprise algae packstone, packstone-grainstone, coral-algae packstone, and foraminifer wackestone-packstone. Fragments of coral, coralline red algae, and large foraminifera are the dominant bioclasts in most of the observed samples, whereas echinoids and bivalves are less abundant; they are set in a recrystallized micrite matrix. Planktonic foraminifera are abundant only in few samples. Fragments of plagioclase, igneous volcanic rocks, pyroclastic rocks (tuff), and much less abundant quartz are commonly present in all the studied samples. A generalized diagenesis includes early marine cementation by fibrous aragonite, compaction, aragonite dissolution and/or neomorphism, precipitation of equant-grained calcite cement in a phreatic environment, dissolution to form moldic porosities, dolomitization, the formation of stylolites and fractures, and precipitation of late ferroan calcite during burial. Multiple carbonate cements occur as pore-filling phases, with ferroan calcite cementation taking place during later-stage burial. Secondary porosities were formed during different stages in diagenetic processes, such as dissolution, dolomitization, and stylolite and fracture formations. Although precipitation of nonferroan and ferroan calcite cement occluded porosities, porosity enhancement during early selective dolomitization might still be significant. Current observations also revealed the presence of intraparticle, micro-vuggy, and fracture porosities in different samples.

The Latemar: A flat-topped, steep fronted platform dominated by microbialites and synsedimentary cements

Facies analyses on the Latemar isolated carbonate platform suggest a higher proportion of microbial carbonates compared to literature. This is evident especially in the margin-upper slope zone, where microbial boundstone associated with Tubiphytes, calcareous sponges, botryoidal aragonite and radiaxial fibrous cements extended down to 250 m water depth. The Latemar flat-topped high-relief platform has been subdivided into five different facies belts: inner and outermost platform, margin, upper and lower slope. Each facies belt represents subtidal environments except for the inner platform, which is characterized by a peritidal facies association. Point counting analysis was carried out on ca. 90 thin sections to quantify the different platform components. Microbialite and cements are most abundant: they represent, respectively, ca. 40% and ca. 34% of the whole platform volume. These results suggest that carbonate production at Latemar occurred in a M-Factory. Microbial carbonates, as well as early marine cements are most abundant on the upper slope, margin and outermost platform, although microbialites are found in all facies belts. The presence of supratidal sedimentary facies restricted only to the inner platform and of a completely subtidal outermost platform suggests a new depositional profile for the Latemar. The central portion is occupied by the flat platform interior, cyclically subaerially exposed. Facies of the subtidal outermost platform surround the inner platform and display a progressive increase in depth toward the margin. The margin is submerged and essentially constituted by microbial and cement boundstone. The slope is characterized by steep clinoforms (~ 35°) dominated by in situ microbialites in the upper-middle portion (0–250 m depth), evidently precipitated by non light-dependent microbially influenced processes. The lower slope is mostly detrital, poor in platform top-derived sediments and dominated by margin and upper-slope derived rudstone and grainstone. All these observations are in agreement with the slope-shedding model observed in the Pennsylvanian microbial margin in Asturias (N Spain) and allow the interpretation of the Latemar as a flat-topped, steep fronted platform dominated by microbialites and cement boundstones.

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well-studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time-parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re-deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near-sea floor pore system is a first-order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed.

Vermetid reefs and their use as palaeobathymetric markers: New insights from the Late Miocene of the Mediterranean (Southern Italy, Crete)

Vermetids are a poorly known family of reef-building gastropods characterized by an extremely irregular growth of the shell. Vermetid reefs are known from the Badenian (Middle Miocene) to present-day and their main importance is related to their use as sea-level indicators, strictly associated to the intertidal or immediate subtidal zone. During their evolution, two different vermetid genera have been recognized as reef-builders: Petaloconchus was the major component of vermetid reefs until the Holocene when, for still unknown reasons, it was almost completely replaced by Dendropoma. In the Mediterranean Basin, Petaloconchus reefs have been frequently found in close association to Late Miocene coral bioconstructions. New finding of Petaloconchus reefs within the slope facies of the Salento Messinian reef complex (southern Italy), together with new data from late Tortonian–early Messinian reefs of Crete (Greece) provide further information on the palaeoecology of these peculiar bioconstructions. Stratigraphical and sedimentological contexts of Salento and Crete vermetid reefs have been described, together with an account of their size, shape and inner structure. Biotic (vermetids, serpulids, bryozoans and coralline algae) and abiotic components (early marine cements and intra-reef sediment) have been identified by microfacies analysis, and their percentage-abundance has been reported. These data allow the description of vermetid reefs from two different depositional settings: along the shelf edge and, for the first time, within the coral reef slope. Shelf-edge and slope Petaloconchus reefs show the same inner structure and composition, but they differ by their shape, by the density of the framework, and by the relative proportion of the associated reef-building organisms (serpulids, bryozoans and coralline algae). Additional information on their palaeobathymetric setting have been acquired from the presence of other reef-building biota ( Porites corals and Halimeda green algae) which occur in close proximity to slope vermetid reefs, together with observations on some anatomical feature of Petaloconchus shells. Results clearly indicate for Petaloconchus reefs a palaeobathymetric range spanning from the upper subtidal zone down to 30–50 m of depth. The present study confirms vermetid reefs as important tools for the reconstructions of ancient sea-level, but great attention has to be paid to the identification of the main reef-building vermetid. Actually, while a great number of present-day Dendropoma reefs can be associated to the mean sea-level, Late Miocene vermetid reefs from Salento and Crete indicate for Petaloconchus bioconstructions a wider palaeobathymetric range.

Brachiopods and other fossils from the Permo–Triassic boundary beds of the Antalya Nappes (SW Taurus, Turkey)

An uppermost Permian–Lower Triassic biota of brachiopods, conodonts, algae and foraminifers from the Pamucak and Kokarkuyu formations at Çürük Dağ (Antalya, Turkey) is here described. The brachiopods belong to two different assemblages: a lower assemblage, early Wuchiapingian in age, with Spinomarginifera cf. S. helica, Spinomarginifera cf. S. iranica, Alatorthotetina sp. ind., Orthothetina sp. ind., Ombonia antalyensis nov. sp. and few specimens of Pennospiriferinoidea; an upper assemblage, Changhsingian in age, comprising S. cf. S. iranica, Spinomarginifera cf. S. spinosocostata, Spinomarginifera sp. ind. and Orthothetina sp. ind., characteristic taxa of the low diversity survival brachiopod faunas of latest Permian age (Survival Fauna 1). The occurrence of the conodont Hindeodus cf. praeparvus above the brachiopod fauna confirms its Changhsingian age. The oolitic grainstones at the top of the Pamucak Formation contain Permocalculus sp., Macroporella cf. apachena, species of Hemigordius and Palaeozoic Lagenida. Coarse calcite fibrous cements pervade the oo-bioclastic grainstones, suggesting early marine cementation. The base of the Kokarkuyu Formation is characterized by the disaster forms Earlandia amplimuralis and “Cornuspira” mahajeri, gastropods and ostracods. The conodont Isarcicella lobata has been recovered 31 m above the base of the Kokarkuyu Formation, indicating the occurrence of the second Triassic conodont zone above the parvus biozone and below the staeschei biozone. The faunal content at the transition of the Pamucak and Kokarkuyu formations records the biotic survival in the aftermath of the end-Permian extinction. Facies evolution from lower energy inner platform wackestones and packstones to higher energy open platform oolitic grainstones indicates a transgression at the top of the Pamucak Formation, which continues into the Lower Triassic Kokarkuyu Formation.

Transition metal abundances in microbial carbonate: a pilot study based onin situLA‐ICP‐MS analysis

ABSTRACTThe ability to recognize the former existence of microbes as well as the biological origin of marine precipitates, such as putative microbialites, is crucial for understanding the development and history of early life on Earth. Increasingly, such rocks hold keys to understanding the geochemical evolution of the oceans and linked Earth systems. Vital trace elements previously have received relatively little attention as clues to the origin of carbonate rocks, and low abundance transition elements in particular, have been difficult to analyse in carbonate matrices for technical reasons. We have used laser ablation–inductively coupled plasma–mass spectroscopy for thein situmeasurement of a broad suite of vital transition metals in Late Devonian reefal limestones that contain coeval microbialite (the calcimicrobeRenalcis), stromatoporoid sponge skeleton, early marine cement, and later diagenetic cement. Comparative experiments conducted in two different ion extraction modes determined theoretical detection limits for transition elements on NIST reference material SRM 612. Analyses of NIST glasses SRM 614 and 616 demonstrate accuracy relative to previously published data. On that basis we have identified significant enrichment of the vital elements V, Sn, Cu and Zn within theRenalcis. The stromatoporoid skeleton by contrast is enriched only in V. Earliest cements, which also may have been mediated to some degree by microbial biofilms on the basis of their morphology, show a much smaller degree of enrichment, and later cements show no enrichment, with the exception of Zn, which is concentrated in the latest cement. Fine particulate carbonate sediments (micrite) show variable metal enrichments that are attributable to varying contributions from detrital siliciclastic contamination.Renalciswas also enriched above the sponge and cements in regards to Mn, Cd, Co, and possibly Cr, but at less robust levels. Molybdenum and Sb were found not to be enriched in theRenalcis, and Ni, although clearly very low in concentration, could not be evaluated owing to its high detection limit. We additionally were able to identify specific zones of contamination inRenalcisencountered as the laser drilled deeper into the carbonate. Time resolved analysis allows exclusion of such contaminants from integration into the results. Successful application of the new technique will now allow us to assess metal uptake in ancient carbonates with implications for interpreting the biogenicity of putative microbialites.

Stratigraphic and geochemical framework of the Agouron drill cores, Transvaal Supergroup (Neoarchean-Paleoproterozoic, South Africa)

Two long drill cores through the Transvaal Supergroup (Neoarchean-Paleoproterozoic) record the vertical transition from a mixed siliciclastic-carbonate ramp (Schmidtsdrif Subgroup) through the Campbellrand-Malmani carbonate platform (Campbellrand Subgroup) to deeper-water banded iron formations (Asbestos Hills Subgroup). The cores were drilled in proximal (core GKF01) and distal (core GKP01) positions relative to the margin of the Campbellrand-Malmani carbonate platform. Correlation between cores is based on three types of marker horizons: sequence stratigraphic, volcanic/impact and lithologic markers. Several surfaces can be correlated to the platform succession. Gamma-ray data and bulk-rock XRF geochemistry of siliciclastic mudstones and tuff beds support the correlation. Following flooding of a shallow-water carbonate ramp (Boomplaas Formation of Schmidtsdrif Subgroup), deposition of slope carbonates with minor siliciclastic mudstones, chert and banded iron formation (BIF) occurred below storm wave base. Cycles of in situ microbialites and reworked slope carbonates are prominent in both cores. A final transgressive pulse near the top of the succession led to deposition of thick cherts and iron formations across the former platform, slope and basinal areas. Deposition on the slope was controlled by cementation potential in microbial facies and by sediment input, which in turn was linked to variations in platform margin geometry and changes in accommodation space. Results indicate preferred sediment accumulation in the proximal slope position, while chemical sediments were deposited preferentially beyond the reach of carbonate and siliciclastic detrital sediment. Similarly, sediment input and possibly early marine cementation influenced cycles of microbialites and reworked carbonates. Sequence stratigraphic analysis and siliciclastic mudstone geochemistry suggest a possible sea level fall and a change in provenance of the siliciclastic detritus at the transition from the Monteville Formation to the Nauga Formation (basal Campbellrand Subgroup). These changes were probably related to a major depositional hiatus on the platform.

Accommodation/sedimentation development and massive early marine cementation: Latemar vs. Concarena (Middle/Upper Triassic, Southern Alps)

Massive early marine cementation (MEC) is a major diagenetic feature of some carbonate platforms. The comparison of two Triassic buildups of similar size and preservation but contrasting degree of MEC (Latemar, Dolomites, and Concarena, Lombardic Alps) allows for the investigation of differential cementation on carbonate platforms. The assessment of the response of platform cementation to changes in accommodation/sedimentation led to the identification of fundamental boundary conditions for MEC: (1) the time interval available, i.e. a low rate of creation in accommodation space and (2) low carbonate production. Other important factors are: (3) margin topography (e.g. walled reefs) and (4) effective fluid flow (e.g. wave energy, open and connected cavities provided by rigid frameworks). The Latemar–mainly aggrading and locally retrograding–lacks MEC due to a high A′/ S′ ratio (i.e. creation/destruction of accommodation in time, A′=d A/d t, vs. changes in sediment supply in time, S′=d S/d t) and a low relief of the reefal margin. In contrast, the Concarena–in the beginning slowly and later rapidly prograding–indicates MEC owing to a low A′/ S′ ratio and a distinct topography of the reef. Particular features of the Concarena are cement arrangements of lenticular shape and considerable size in the back-reef domain (up to 3 m in diameter and 2 m in height). These arrangements consist of botryoids and isopachous crusts of radiaxial fibrous calcite cements. They represent one of the main components of the platform margin at Concarena. Cathodoluminescence analyses of cements from both platforms quantifies the influence of shallow and deep burial diagenesis and shows that the majority (60–90%) of all cements are of early marine origin.

Late Early Triassic microbial reefs of the western United States: a description and model for their deposition in the aftermath of the end-Permian mass extinction

Research in the western United States to reinvestigate and characterize the previously reported stromatolites of the latest Early Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation has shown that these microbial build-ups formed patch reef mounds. Detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1–2 m of topographic relief above the seafloor. A petrographic study of these microbial mounds illustrates that they consisted of microbial fabrics, as well as voids with early marine cements and formed in the absence of in situ metazoans. These reef mounds formed during the aftermath of the end-Permian mass extinction that spanned the Early Triassic. Due to the devastation of marine metazoans at this extinction, the Early Triassic has often been deemed a reef gap because no metazoan reefs are found at this time. Whereas colonial metazoan reefs are globally absent from Lower Triassic strata, microbial build-ups similar to these described from the western United States have been found in south China, Armenia, southern Turkey and Greenland. A regional Early Permian to late Early Triassic unconformity exists in the study region. However, analysis of these microbial reefs within a global context suggests that conditions favouring microbial reef development must have existed as long as 4–8 m.y. after the end-Permian mass extinction. A depositional model of these microbial mounds indicates that flooding of the shelf with anoxic and/or CO 2-rich deep waters may have fostered their occurrence.