Micritic Sediments Research Articles

The role of non-skeletal carbonate component in Mediterranean Coralligenous: new insight from the CRESCIBLUREEF project

In the frame of the project FISR_04543 “CRESCIBLUREEF - Grown in the blue: new technologies for knowledge and conservation of Mediterranean reefs”, we present preliminary data on the ecological and depositional implications of micritic sediments in a coralligenous bioconstruction formed along the Mediterranean shelf in front of the Marzamemi village (Sicily, Italy). The framework of the build- up is mainly built by crustose coralline algae, which in turn create the substrate for a high-diversified epi- and infaunal community. Two types of microcrystalline calcite, tentatively interpreted as allochthonous and autochthonous micrite, strictly related to fine skeletal debris, have been detected. The allochthonous micrite derive from abiotic accumulation of fine sediments in the framework cavities. The autochthonous micrite is deposited in situ through organic-mediated processes. The occurrence of this component allows hypothesizing a possible contribution of non-skeletal carbonate in the strengthening of the primary framework due to its syndepositional cementation.

The microbial carbonate factory of Hamelin Pool, Shark Bay, Western Australia

Microbialites and peloids are commonly associated throughout the geologic record. Proterozoic carbonate megafacies are composed predominantly of micritic and peloidal limestones often interbedded with stromatolitic textures. The association is also common throughout carbonate ramps and platforms during the Phanerozoic. Recent investigations reveal that Hamelin Pool, located in Shark Bay, Western Australia, is a microbial carbonate factory that provides a modern analog for the microbialite-micritic sediment facies associations that are so prevalent in the geologic record. Hamelin Pool contains the largest known living marine stromatolite system in the world. Although best known for the constructive microbial processes that lead to formation of these stromatolites, our comprehensive mapping has revealed that erosion and degradation of weakly lithified microbial mats in Hamelin Pool leads to the extensive production and accumulation of sand-sized micritic grains. Over 40 km2 of upper intertidal shoreline in the pool contain unlithified to weakly lithified microbial pustular sheet mats, which erode to release irregular peloidal grains. In addition, over 20 km2 of gelatinous microbial mats, with thin brittle layers of micrite, colonize subtidal pavements. When these gelatinous mats erode, the micritic layers break down to form platey, micritic intraclasts with irregular boundaries. Together, the irregular micritic grains from pustular sheet mats and gelatinous pavement mats make up nearly 26% of the total sediment in the pool, plausibly producing ~ 24,000 metric tons of microbial sediment per year. As such, Hamelin Pool can be seen as a microbial carbonate factory, with construction by lithifying microbial mats forming microbialites, and erosion and degradation of weakly lithified microbial mats resulting in extensive production of sand-sized micritic sediments. Insight from these modern examples may have direct applicability for recognition of sedimentary deposits of microbial origin in the geologic record.

Occurrence and origin of glauconite in the Cambro-Ordovician Bliss Formation of southern New Mexico and West Texas (U.S.A.)

ABSTRACTIn southern New Mexico and West Texas, USA, the Late Cambrian–Early Ordovician Bliss Formation is a relatively thin and dominantly siliciclastic succession that was deposited on the Proterozoic basement during a major global transgression. The Bliss Formation can be divided into two members: 1) a lower, coarser-grained member composed mostly of sandstone (quartz arenite and subarkose) that lacks glauconite and calcite cement, and 2) an upper, finer-grained member that includes glauconitic sandstone, arkosic sandstone, and mixed siliciclastic–carbonate siltstone to fine-grained sandstone, intercalated with thin carbonate beds of grainstone, packstone, and rudstone. Iron oolite and oolitic sandstone are locally exposed at the base of the upper member. Sandstone of the lower member represents upper-shoreface to foreshore deposits, whereas sedimentary structures in the upper member indicate deposition in a middle- to lower-shoreface setting, and locally in a tidal-flat environment. Intercalated carbonate beds are storm layers (tempestites). Glauconite grains are abundant in the upper member as mostly rounded to well-rounded, spheroidal to ovoidal pellets. Two types of glauconite grains are present: homogeneous, dark green grains with high K2O contents (> 8 wt.%; stage 4) and mottled pellets composed of a mixture of glauconite and apatite. Glauconite of the Bliss Formation is not autochthonous as proposed by earlier workers, but of allochthonous (parautochthonous) origin. Thus, the glauconite grains were reworked from deeper shelf environments in northern Mexico to the south and were transported and deposited under regressive–transgressive conditions. Mottled glauconite grains formed by the reworking of phosphatized and glauconitized micritic sediments during regression and were transported and deposited by storm-induced currents, particularly in storm layers during transgressive events. Homogeneous, mature glauconite grains probably were derived from the reworking of glauconitized fecal pellets or completely glauconitized micritic sediments during regressive–transgressive cycles. The world-wide occurrence of glauconite in Cambrian–Early Ordovician sediments indicates that glauconite formation during that period can be considered as a “global event.”

Calcilobes wangshenghaii n. gen., n. sp., microbial constructor of Permian\u2013Triassic boundary microbialites of South China, and its place in microbialite classification

Permian–Triassic boundary microbialites (PTBMs) that formed directly after the end-Permian extinction in the South China Block are dominated by one structure, a lobate-form calcium carbonate construction that created extensive very thin (ca. 2–20 m thick) framework biostromes in shallow marine environments, effectively occupying the ecological position of the prior pre-extinction Permian reefs and/or associated carbonates. In the field, vertical sections show the microbialite is dendrolite (branched) and thrombolite (clotted), but because thrombolite may include branched portions, its structure is overall best classed as thrombolite. In the field and in polished blocks, the microbial material appears as dark carbonate embedded in lighter-coloured micritic sediment, where details cannot be seen at that scale. In thin section, in contrast to the largely unaltered micritic matrix, the microbial constructor is preferentially partly to completely recrystallised, but commonly passes gradationally over distances of a few mm to better-preserved areas comprising 0.1–0.2 mm diameter uneven blobs of fine-grained calcium carbonate (micrite to microsparite). The lobate architecture comprises branches, layers and clusters of blobs ca. 1–20 mm in size, and includes constructed cavities with geopetal sediments, cements and some deposited small shelly fossils. Individual blobs in the matrix may be fortuitous tangential cross sections through margins of accumulated masses, but if separate, may represent building blocks of the masses. The lobate structure is recognised here as a unique microbial taxon and named Calcilobes wangshenghaii n. gen., n. sp. Calcilobes reflects its calcium carbonate composition and lobate form, wangshenghaii for the Chinese geologist (Shenghai Wang) who first detailed this facies in 1994. The structure is interpreted as organically built, and may have begun as separate blobs on the sea floor sediment (that was also composed of micrite but is interpreted as mostly inorganic), by microbial agglutination of micrite. Because of its interpreted original micritic–microsparitic nature, classification as either a calcimicrobe (calcified microbial fossil) or a sedimentary microbial structure is problematic, so C. wangshenghaii has uncertain affinity and nature. Calcilobes superficially resembles Renalcis and Tarthinia, which both form small clusters in shallow marine limestones and have similar problems of classification. Nevertheless, Calcilobes framework architecture contrasts both the open branched geometry of Renalcis, and the small tighter masses of Tarthinia, yet it is more similar to Tarthinia than to Renalcis, and may be a modification of Tarthinia, noting that Tarthinia is known from only the Cambrian. Calcilobes thus joins Renalcis, Tarthinia and also Epiphyton (dendritic form) and others, as problematic microbial structures. Calcilobes has not been recognised elsewhere in the geological record and may be unique to the post-end-Permian extinction facies. C. wangshenghaii occurs almost exclusively in the South China Block, which lay on the eastern margin of Tethys Ocean during Permian–Triassic boundary times; reasons for its absence in western Tethys, except for comparable fabrics in one site in Iran and another in Turkey, are unknown.

Lithistid sponge-microbial reefs, Nevada, USA: Filling the late Cambrian ‘reef gap’

Cambrian–Ordovician sponge-microbial mounds in the Great Basin of the western USA reveal reef structure and composition immediately prior to the Great Ordovician Biodiversification Event (GOBE). Here we describe lithistid sponge-microbial reefs from the upper Cambrian (Furongian, Stage 10) strata of the Arrow Canyon Range, Nevada. The reefs are mound-like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin-walled, bowl-shaped anthaspidellid sponge, columnar microstromatolite fabric, and the calcified microbe Angusticellularia. The reefs formed in low-energy, subtidal environments in which lime mud filled spongocoels and inter-reef spaces around undisturbed, in place, thin-walled sponges. The reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals. The mutually attached lithistid sponges form the main framework of the reefs. These thin-walled and bowl-shaped lithistids most likely were adapted to low-energy environments. Spaces beneath the overhanging sponge walls were filled by microbial carbonates. These include pendent micro-dendritic Angusticellularia attached to dermal sponge surfaces and upward-growing masses of microstromatolites. After death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft-bodied burrowing organisms and keratose-like sponges. These lithistid sponge-microbial reefs, together with an earlier example of late Cambrian (Paibian) dendrolite-lithistid reefs in the same area, characterize skeletal-microbial reefs immediately prior to the GOBE.

A new modular palaeopascichnid fossil Curviacus ediacaranus new genus and species from the Ediacaran Dengying Formation in the Yangtze Gorges area of South China

AbstractNon-biomineralizing Ediacaran macrofossils are rare in carbonate facies, but they offer valuable information about their three-dimensional internal anatomy and can broaden our view about their taphonomy and palaeoecology. In this study, we report a new Ediacaran fossil, Curviacus ediacaranus new genus and species, from bituminous limestone of the Shibantan Member of the Dengying Formation in the Yangtze Gorges area of South China. Curviacus is reconstructed as a benthic modular organism consisting of serially arranged and crescent-shaped chambers. The chambers are confined by chamber walls that are replicated by calcispars, and are filled by micritic sediments. Such modular body construction is broadly similar to the co-occurring Yangtziramulus zhangii and other Ediacaran modular fossils, such as Palaeopascichnus. The preservation style of Curviacus is similar to Yangtziramulus, although the phylogenetic affinities of both genera remain unresolved. The new fossil adds to the diversity of Ediacaran modular organisms.

Origin of Jurassic-Cretaceous neptunian dikes from the Cracow-Częstochowa Upland in southern Poland

Neptunian dikes, representing deposit-filling fractures in older rocks, have long attracted the attention of paleontologists because they may provide unique insights into ancient cryptic submarine cave ecosystems. In the epicratonic Poland, fossil-bearing neptunian dikes are especially well known in Upper Jurassic (mainly Oxfordian) strata. However, the age, origin and faunal composition of these dikes remain the subject of debates. To address these issues, we integrated new paleontological and petrographic data from the Janina Quarry in southern Poland, where neptunian dikes are particularly well exposed. On the basis of sedimentary facies and faunal content occurring as infill of the analyzed dikes, we recognized three distinct stages of Neptunian dike development. Initially, these dikes represented fissures in the sea bottom resulting from the intense Oxfordian and/or Kimmeridgian synsedimentary processes related to Meso- and Neocimmerian tectonic movements. These initial voids were colonized by small crinoids (cyrtocrinids) and brachiopods (lacunosselids), which resulted in the formation of bioclastic limestones. This is the first Mesozoic record of cryptic cyrtocrinids, which implies that migration of this echinoderm group to cryptic habitats has already occurred in the Late Jurassic, consistent with the so-called Mesozoic Marine Revolution. After the initial opening and subsequent rapid sedimentary filling of voids, rejuvenation of these dikes occurred in the Aptian-Cenomanian that was likely induced by the Austrian tectonic phase. Reopened fissures were subsequently filled by rare allochthonous echinoderms and sandstones containing unusually high (5% vol.) content of heavy minerals implying the Bohemian Massif as the most probable source area. The neptunian dikes were finally reopened during the Turonian-Santonian that was probably related to the subhercynnian tectonic phase. They were presumably inhabited by annelids and ultimately filled by a micritic sediment subsequently bioturbated by infaunal echinoids.

Frutexites from microbial/metazoan bioconstructions of recent and Pleistocene marine caves (Sicily, Italy)

Frutexites-like structures were recorded in small bioconstructions from Recent and Pleistocene marine caves. The skeletal/microbialite frameworks hosting Frutexites form in cryptic environments characterized by poor light availability and reduced water circulation leading to confinement, oligotrophic conditions and the development of typical cave communities. Biostalactites and biotic crusts develop in the more confined parts of the both Recent and Pleistocene caves. They are composed of skeletal organisms engulfed in fine micrite sediments deposited in situ via microbial metabolic activity. The Frutexites, composed of Iron and Manganese oxides, are confined to microcavities or microfractures of the bioconstructions. They grow from an original discontinuity surface into the surrounding consolidated micritic sediments. A model based on the increase of the microporosity of micritic sediments due to the dissolution of the original material and precipitation of ferromanganesiferous compounds was proposed. This process occurs during the syndepositional diagenetic stage due to the intrusion in the bioconstruction framework of acidic continental water rich in Iron and Manganese. Mesophilic Fe–Mn autotrophic and chemoheterotrophic bacteria flourish on the microcavities and induce the precipitation of Fe and Mn oxides.

Facies, origin, and palaeontological inventory of an Early Carboniferous neptunian dyke in the Devonian reef limestone near Rösenbeck (Brilon Anticline, Rhenish Mountains)

Abstract. The Devonian reef limestone complex of Rösenbeck near Brilon (Rhenish Mountains) shows numerous neptunian dykes and other hollows which have been filled with Carboniferous siliciclastic as well as fossil-rich carbonate sediments with ammonoids, conodonts, and chondrichthyan fish. While the shales of the infill can be interpreted as autochthonous sediments, the carbonates represent erratic blocks of sediments which were deposited in elevated areas but subsequently eroded and transported as erratic blocks into the karstic cavities. The biota of the carbonates demonstrates that two stratigraphic intervals are represented: (1) the Tournaisian–Viséan boundary interval, which can be identified by the presence of trilobites, and (2) the late Viséan (Asbian–Brigantian), proven by the presence of ammonoids and conodonts embedded in shales that probably have an early Serpukhovian age. Two carbonate facies can be distinguished in the Late Viséan blocks: (1) low-energy micritic sediments with Goniatites crenistria, which are probably equivalent to the crenistria Limestone and (2) high-energy (tempestite) sediments with numerous ammonoid specimens and in a more subordinate role other to fossil groups.

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.

Mississippian (Early Carboniferous) stromatolite mounds in a fore-reef slope setting, Laibin, Guangxi, South China

The Mississippian (Early Carboniferous) is generally a period of scarce carbonate buildups in South China. This study documents outcrops of stromatolite mounds at Mengcun and Helv villages, in Laibin City, Guangxi Province, South China. The stromatolite mounds contain various stromatolite morphologies including laminar, wavy-laminar, domal or hemispheroidal, bulbous, and flabellate-growth columns. Intramound rocks are brachiopod floatstone and dark thin-bedded laminated micrite limestone. Individual stromatolites at Mengcun village are generally 3–6 cm thick and morphologically represent relatively shallow-water laminar (planar and wavy-undulated stromatolites) and deeper-water domal, bulbous and columnar forms. Where mounds were formed, the stromatolites continued growing upward up to 60 cm thick. Thrombolitic fabrics also occur but are not common. Stromatolite microscopic structure shows the bulk of the lamination to consist of wavy microbialite and discrete thin micritic laminae. These mounds are intercalated in deep-water fore-reef talus breccia, packstone formed as a bioclastic debris flow and thin-bedded limestone containing common chert layers of the Tatang Formation (late Visean). Further evidence supporting the deep-water setting of the stromatolite mounds are: (1) a laterally thinning horizon of brachiopod floatstone containing deep-water, small, thin-shelled brachiopods, peloidal micritic sediments and low-diversity, mixed fauna (e.g., thin-shelled brachiopods, tube-like worms and algae) that have been interpreted as storm deposits, (2) common fore-reef talus breccias, (3) lack of sedimentary structures indicating current action, (4) preservation of lamination with sponge spicules, and (5) lack of bioturbation suggesting that the stromatolites grew in a relatively low energy, deep-water setting. The stromatolite mounds are the first described stromatolite mounds in Mississippian strata of South China and contain evidence that supports interpretations of (1) growth history of Mississippian microbial buildups and (2) environmental controls on stromatolite growth and lithification.

Reservoir quality variation on an Eocene carbonate ramp, El Garia Formation, offshore Tunisia: Structural control of burial corrosion and dolomitisation

Study of four closely-spaced El Garia Formation reservoirs (offshore Tunisia) has identified diagenetic controls on variation in reservoir quality. The Ashtart, Didon and Zarat Fields have been affected by significant dissolution porosity, which occurred during burial diagenesis, with vuggy pores preferentially forming in micritic sediment. However, inter-crystalline porosity due to dolomitisation is the principal control on reservoir quality in the Hasdrubal Field. This spatial variation in porosity type reflects, to some extent, the structural and sedimentological isolation of Hasdrubal within the facies trend, surrounded by deep water embayments and, unlike the other studied fields, not directly underlain by salt diapirs. Burial corrosion appears to have formed where mid to outer ramp, mud-rich El Garia Formation facies directly overlie salt domes, a consequence of the circulation of aggressive fluids related to increased heat flow above the salt. Inorganic CO 2, focused into the reservoir prior to hydrocarbon emplacement by faults related to salt movement, is the most likely candidate responsible for the burial dissolution porosity observed in the Ashtart, Didon and Zarat fields. A salt diapir does underlie the most basinward (north-east) margin of Hasdrubal, and minor dissolution porosity is present, but we conclude that the absence of significant dissolution in Hasdrubal is due to its offset position, away from the highly faulted roof zone immediately above the salt. Although we favour this model, we cannot rule out that the absence of dissolution porosity in Hasdrubal may reflect the early emplacement of hydrocarbons from an older (non-Bou Dabbous Formation) source, or differences in the timing of salt movement and consequent fault creation below Hasdrubal compared to the other fields, local temperature and/or acid concentration effects, or a different source of CO 2. Differences in the distribution of dissolution porosity in the Ashtart, Didon and Zarat Fields may be related to the presence of intra-reservoir permeability barriers, due to increased nummulithoclastic (= fragmented Nummulites) content. This may also explain the strata-bound nature of the dolomite in Hasdrubal, which, like the dissolution porosity in Ashtart, is overlain by sediments with an increased nummulithoclastic content. Although dolomite is present in variable quantities in all of the studied fields, the significant volume of dolomite in Hasdrubal may reflect a local ‘plumbing’ effect, with dolomitising fluids entering the reservoir via a fault network that does not affect the other fields.

High-resolution carbon isotope changes in the Permian–Triassic boundary interval, Chongqing, South China; implications for control and growth of earliest Triassic microbialites

High-resolution δ 13C CARB analysis of the Permian–Triassic boundary (PTB) interval at the Laolongdong section, Beibei, near the city of Chongqing, south China, encompasses the latest Permian and earliest Triassic major facies changes in the South China Block (SCB). Microbialites form a distinctive unit in the lowermost 190 cm above the top of the Changhsing Formation (latest Permian) at Laolongdong, comparable to a range of earliest Triassic sites in low latitudes in the Tethyan area. The data show that declining values of δ 13C CARB, well-known globally, began at the base of the microbialite. High positive values (+3 to 4 ppt) of δ 13C CARB in the Late Permian are interpreted to indicate storage of 12C in the deep waters of a stratified ocean, that was released during ocean overturn in the earliest Triassic, contributing to the distinctive fall in isotope values; this interpretation has been stated by other authors and is followed here. The δ 13C CARB curve shows fluctuations within the microbialite unit, which are not reflected in the microbialite structure. Comparisons between microbialite branches and adjacent micritic sediment show little difference in δ 13C CARB, demonstrating that the microbialite grew in equilibrium with surrounding seawater. The Early Triassic microbialites are interpreted to be a response to upwelling of bicarbonate-rich poorly oxygenated water in low latitudes of Tethys Ocean, consistent with current ocean models for the PTB interval. However, the decline of δ 13C CARB may be due to a combination of processes, including productivity collapse resulting from mass extinction, return of deep water to ocean surface, oxidation of methane released from methane hydrate destabilisation, and atmospheric deterioration. Nevertheless, build-up of bicarbonate-rich anoxic deep waters may be expected as a result of the partial isolation of Tethys, due to continental geography; release of bicarbonate-rich deep water, by ocean upwelling, in the earliest Triassic may have been an inevitable consequence of this combination of circumstances.

Micritic fabrics define sharp margins of Wenlock patch reefs (middle Silurian) in Gotland and England

Abstract Silurian reefs are well known to comprise frame-building corals, stromatoporoids and algae, but also a range of calcimicrobial components and micritic sediments of possible microbial origin. The margins of Wenlock patch reefs tend to have diffuse edges that grade into the surrounding bedded facies because of talus shed from the reefs. However, portions of patch reefs show sharp-bounded reef margins, with bedded limestones terminating abruptly against the reef edge. Examples of sharp boundaries where the reef comprises only carbonate mudstone-wackestone with poorly-defined gross fabric, and containing no metazoan framework, have been found in Wenlock patch reefs the UK and Gotland. Although in some cases the micrite may demonstrate a peloidal structure, in others there is no clear structure, broadly fitting the aphanitic (structureless) type of fabric found in leiolites (suspected microbial facies that show no structure). The fabrics are interpreted to have been formed by microbial mediation of micrite precipitation as part of reef construction, and are therefore automicrites. In all cases the sharp reef edges indicate coherence of the micritic fabric, interpreted as a lithified wall against which bedded limestones were deposited. This arrangement supports published interpretations of pronounced topography of Wenlock patch reefs, and shows the presence of steep, vertical and, possibly, overhanging reef margins, formed prior to bedded sediment accumulation. Thus, there is likely to have been a time interval between reef formation and deposition of bedded sediments, possibly caused by reef upward growth in transgressive settings, followed by catch-up of bedded limestone deposition.

Development and compartmentalization of chalky carbonate reservoirs: The Urgonian Jura-Bas Dauphiné platform model (Génissiat, southeastern France)

The chalky limestones of the upper Barremian-middle Aptian Urgonian Limestone Formation of the Jura-Bas Dauphiné platform (southeastern France) are characterised by a total porosity between 17.9 and 24.5%, a threshold radius between 1.04 and 2.13 μm, a trapped porosity between 51.4 and 68.3% and a gas permeability between 161 and 410 mD. They were formed in response to the diagenetic evolution of dominantly micritic sediments deposited during a “calcite sea” interval. Their present-day reservoir qualities and transport properties are controlled by the intercrystalline microporosity of microcrystalline areas (peloids or matrix). This intercrystalline microporosity is the product of a meteoric porewater input (installation of a shallow-burial phreatic environment characterised by a short residence time of porewaters) which promoted an aggrading neomorphism (Ostwald ripening process). The impact of the burial diagenesis on the reservoir qualities and the transport properties of these chalky limestones is limited. The chalky units are presently compartmentalized by horizontal layers of compact limestones characterised by a total porosity between 2.9 and 4.5%, a threshold radius between 0.04 and 0.09 μm, a trapped porosity of 92.4–92.8% and a gas permeability of 2 mD. Like the chalky limestones, the present-day reservoir qualities and transport properties of compact limestones are controlled by the intercrystalline microporosity of microcrystalline areas. The petrophysical differences between the chalky and compact limestones result from differences in the pore network of microcrystalline areas due to an increase in the average size of particles and a modification of crystal–crystal contacts. These petrographical differences reflect an evolution of micritic areas of original sediments during submarine lithifications (precipitation of microcrystalline cements within the pore space and/or an increase in solid–solid contacts by crystal growth) which preceded the aggrading neomorphism induced by the meteoric porewater input.

ABSTRACT: Carbonate Production, Relative Sea Level Fluctuations and Slope Geometry: Case Histories from the Middle-Triassic of the Italian Dolomites

The understanding of carbonate platform reservoirs can be enhanced through the analysis of outcropping counterparts, such as the ones provided by the study platforms (Cernera and Latemar), particularly rich in both marine cements and microbial boundstones. The birth of both coeval platforms was matched with an anoxic event and with the deposition of a potential source horizon, rich in marine organic matter. The evolution of the short lived Cernera Platform was controlled by a very fast subsidence, badly compensated by its aggrading evolution. The lengthening slopes rapidly steeped up, while the cementation intensity increased. The carbonate production was unable to support the fast platform volume increase and the slope deposits therefore became thinner and thinner, forcing the margin to retrograde; the platform eventually drowned and it was covered by condensed pelagic facies and deep water stromatolite-like structures. The Latemar slope experienced a similar lengthening and steeping up evolution; this carbonate system however grew in a comparatively less subsiding area and it was therefore able to keep and eventually catch up the relative sea level increase. While the platform core shallowed from subtidal environments to cyclic emersions, in the adjacent slopes loose bioclastic and micritic sediments gave place to breccia rich in carbonate cements and microbial boundstones. The subsidence then slowed down considerably enabling the platform to laterally prograde, but no margin are presently preserved from this phase. Both platforms were then at least partially sealed by volcanic and terrigenous deposits and incompletely affected by a permeability enhancing dolomitization.

DEPOSITIONAL ENVIRONMENT AND DIAGENESIS OF THE EOCENE JDEIR FORMATION, GABES‐TRIPOLI BASIN, WESTERN OFFSHORE, LIBYA

The late Ypresian (early Eocene) Jdeir Formation was deposited in the Mesozoic‐Cenozoic Gabes‐Tripoli Basin, offshore Libya. The basin developed on the northern passive margin of the African Plate and was relatively unstable being affected by syn‐sedimentary tectonic movements. Deposition was coeval with a relative rise of sea‐level and the subsequent highstand. A lower, thinly‐developed nummulitic bank facies with restricted distribution records the transgressive event and is succeeded by more micritic sediments that record the time of maximum flooding. The succeeding sea‐level highstand is represented by a thick, and widely developed, progradational‐aggradational nummulitic sequence that displays lateral changes across WE‐ESE trending facies belts.Three major lithofacies are recognized in the Jdeir Formation: Nummulites packstone‐grainstone, Alveolina‐Orbitoliteswackestone‐packtone, andFragmental‐Discocyclina‐Assilina wackestone‐packstone, depositedin bank, back‐bank, and fore‐bank environments, respectively. The formation passes to the NNE into the pelagic lithofacies of the Hallab Formation; landward, to the south, it passes into shoreline evaporitic facies of the Taljah Formation. The lithofacies were structurally controlled by contemporaneous and/or syndepositional tectonic movements, with nummulitic facies tending to develop on uplifted areas.Petrographic and petrophysical studies indicate that porosity in the Jdeir Formation is controlled by depositional environment, tectonic setting and diagenesis. The combined effects of salt tectonics, a major unconformity at the top of the formation and meteoric diagenesis have produced excellent‐quality reservoir facies at the Bouri oilfield and in other areas.Porosity is highest in the nummulitic bank facies and lowest in the Alveolina‐Orbitolites micrite facies. Good to excellent reservoir quality occurs in the upper part of the nummulitic packstone‐grainstone facies, especially where these sediments overlie structurally high areas. High rates of dissolution found at the crests of domes and anticlines suggest that early diagenetic processes and features are, in part, structurally controlled. Future exploration success will depend on investigation of similar structures within the Gabes‐Tripoli Basin.Both porosity initiation and preservation are related to early depositional and diagenetic processes. The wide time‐gap between hydrocarbon generation and reservoir formation points to the role of the seal in porosity preservation and rules out the assumption that early emplacement of oil had preserved the porosity.

Possible seismic origin of molar tooth structures in Neoproterozoic carbonate ramp deposits, north China

ABSTRACTThe Xinmincun Formation forms the uppermost unit of a thick Neoproterozoic section which accumulated near the east margin of the North China Block and is overlain by two thin nearshore to continental formations below fossiliferous Lower Cambrian sediments. Although tectonically deformed, sedimentary structures are preserved undeformed on cleavage‐parallel surfaces, and an 80 m section has been reconstructed by correlation across minor folds and faults in the Golden Stone beach area, 50 km NE of the city of Dalian, southern Liaoning province.The measured section shows 65 m of storm‐dominated deposits, consisting of alternations of micrites and sharp‐based graded intraclastic grainstone beds (tempestites), some with rudaceous, commonly erosional or guttered, bases. The top 15 m of section shows three alternations of similar subtidal lithofacies with partly to completely dolomitized peritidal deposits (laminated, sometimes fenestral and desiccated, micrite beds, and intraclastic rudite and grainstone beds). Tempestite beds become thinner and less abundant upwards towards a muddy upper shoreface zone. This environment was characterized in part by the occurrence of micrite with thin or streaky lamination (probable storm‐resuspended sediment), interbedded tempestites, numerous erosion surfaces, and evidence of liquidization and sediment slumping into hollows.Molar tooth structures are widespread in micrite beds of the Xinmincun Formation and are present in lesser abundance in tempestites and liquefied channel‐fills. Originally the structures were sub‐vertical cracks, 1–20 cm long, tapering upwards and downwards. Subsequently they became filled with microspar cement and buckled rigidly during compaction of surrounding sediment during burial. Evidence of repeated episodes of cracking, presence of brecciated cracks and localization of cracking within beds, together with variable degree of development and variation of preferential alignment in plan indicates a mechanical origin. Crack generation may have been by seismic surface waves generated by movement along faults defining either the basin's margins or its internal structure, or possibly by wave action during storms. A seismic origin for molar tooth structure is consistent with other Neoproterozoic occurrences. Their preferential occurrence in Precambrian deposits arises from the relative rigidity of micritic sediment at this time related to lack of bioturbation, incipient cementation and possibly microbial binding.

Hydrothermal Vent Bacterial Community in Ordovician Ophicalcite, Southern Quebec Appalachians

ABSTRACT The oldest known case of chemosynthesis is suggested for Lower Ordovician ophicalcites in the southern Quebec Appalachians (Canada). These ophicalcites are the end product of tectonosedimentary and hydrothermal alteration of ophiolites on the deep sea floor. Peloidal mats (mean d18O -4.6; mean d13C -0.4) coat micritic sediments (mean d18O -4.8; mean d13C +0.1) and ophiolite clasts associated with hydrothermal, low-temperature (» 80°C) vent fracture-filling calcite cements (mean d18O -13.5; mean d13C -0.2). The mats are interpreted to be the fossil analogues of bacterial white slime around modern vents. The mats are locally overlain by botryoidal cements (mean d18O -3.4; mean d13C +0.9) of former high-magnesium calcite mineralogy. Precipitation of this cement was likely mediated by bacteria. Stable-isotope ratios of peloidal mats and botryoidal cements indicate that the exhalatives were free of hydrocarbons and H2S. Modern and ancient hydrothermal vents on ophiolite substrates commonly are typified by high-temperature H2S-rich fluids, sulfide mineralization, and a diverse chemosynthetic fauna. Carbonatized ophiolite successions, however, are significantly different in their vent setting, as indicated by this Ordovician occurrence and a previously described Cretaceous example. The Ordovician ophicalcite is devoid of mineralization and of macro-biological activity. The lack of corrosion of carbonates in the Ordovician ophicalcite suggests minimal rates of H2S venting, and pseudomorphs of sulfates, now calcite, are found in the sediment surrounding the vents. These observations suggest that the venting system of this Ordovician case was somewhat similar to H2S-poor w ite smokers. In the absence of hydrocarbon or sulfide emanations, thermophilic bacterial sulfate reduction likely was active around these Ordovician vents.

Were kinetics of Archean calcium carbonate precipitation related to oxygen concentration?

Archean carbonates commonly contain decimetre- to metre-thick beds consisting entirely of fibrous calcite and neomorphosed fibrous aragonite that precipitated in situ on the sea floor. The fact that such thick accumulations of precipitated carbonate are rare in younger marine carbonates suggests an important change in the modes of calcium carbonate precipitation through time. Kinetics of carbonate precipitation depend on the concentration of inhibitors to precipitation that reduce crystallization rates and crystal nuclei formation, leading to kinetic maintenance of supersaturated solutions. Inhibitors also affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong calcite-precipitation inhibitor, is thought to have been present at relatively high concentrations in Archean seawater because oxygen concentrations were low. The rise in oxygen concentration at 2.2-1.9 Ga led to the removal of Fe2+ from seawater and resulted in a shift from Archean facies, which commonly include precipitated beds, to Proterozoic facies, which contain more micritic sediment and only rare precipitated beds.