Dense Micrite Research Articles

Digital rock physics: Defining the reservoir properties on drill cuttings

Well drilling with core extraction is one of the most expensive procedures in the oil industry. The potential alternative, which may open opportunities for cost-efficient acquisition of petrophysical data, can be an approach based on digital analysis of drill cuttings. The main purpose of this research was to evaluate the possibility of using digital cuttings analysis and rock physics for reservoir properties evaluation. The carbonate samples of core and cuttings were simultaneously collected from the same intervals during the drilling of deviated wells in the Carboniferous reservoir of the oil deposit in the Republic of Tatarstan, Russia. Based on the core analysis using the optical study of thin sections and XRD, we investigated sedimentological and mineralogical features and classified all lithotypes of well intervals. The standard samples were produced from all defined facies and their reservoir properties were studied in routine tests. The largest cuttings from each well interval were separated and their structures were compared using μCT with core subsamples of the same intervals. We assessed the changes of cuttings structure occurring in the drilling process, which include the colmatation of the pore space with drilling fluid and disintegrated rock particles and the formation of secondary fractures. Detailed analysis of cuttings from one well interval consisting of the border between the facies of high porous grainstones with packstones and dense micrite limestones demonstrated that high porous varieties were eroded in the drilling process. The analysis of chemical composition using XRF-mapping, XRD and SEM with EDS showed that most of the cuttings were constituted by disintegrated rock particles or form elements (e.g. separate fusulinids) cemented by drilling mud. However, more durable cuttings from dense micrite limestones facies were generally retained their dominant calcite composition and original rock structure, with only secondary fractures. Finally, we analyzed pore space properties and performed digital simulations of wave propagation velocities to compare the characteristics of these cuttings and core subsamples from this lithotype, scanned in the same μCT settings. The results demonstrate close characteristics of porosity 0.08–0.6%, dominant pore sizes 1–60 μm and velocities Vp = 6130–6240 m/s, Vs = 3180–3230 m/s. However, the comparison of obtained values with measured on standard samples shows a significant difference, which is caused by deviation from REV. The potential use of cuttings for assessing reservoir and petrophysical properties can be possible in case of provision with the factors of cuttings structure alteration, representativity of samples, as well as optimization for these purposes of the drilling process with cuttings selection.

Sedimentary Fabrics for Thrombolite Bioherm in Cambrian Zhangxia Formation in the Western Part of Shandong Province

Thrombolite is a non-laminated fabric of microbial carbonate, which is in sharp contrast with stromatolite that is characterized by the laminated fabric. Thrombolite bioherm developed in Cambrian Zhangxia Formation in the Western part of Shandong Province mainly consists of clots that are composed of dense micrite and microspar. Both the remain of calcified extracellular polymers (EPS) and calcified cyanobacteria fossils are observed within the clot that makes up thrombolite. Within the complex microscopic fabrics, it can be observed that the remain of calcified EPS are marked by the spherical structure, the sheet structure, the mat structure, and the honeycomb-like structure, which further indicate that the formation of clots within thrombolite should belong to a complex organic mineralization process. Together with both the dense micrite and the calcified cyanobacteria fossil, the calcified EPS provides an important clue for understanding the enigmatic formation mechanism of thrombolites. Therefore, this study can provide an important clue and thinking approach for further research in the future.

Diversified calcimicrobes in dendrolites of the Zhangxia Formation, Miaolingian Series (Middle Cambrian) of the North China craton

As a type of non-laminated microbial carbonates, dendrolites are dominated by isolated dendritic clusters of calcimicrobes and are distinct from stromatolites and thrombolites. The dendrolites in the upper part of the Miaolingian Zhangxia Formation at Anjiazhuang section in Feicheng city of Shandong Province, China, provide an excellent example for further understanding of both growth pattern and forming mechanism of dendrolites. These dendrolites are featured by sedimentary fabrics and composition of calcified microbes as follows. (1) The strata of massive limestones, composed of dendrolites with thickness of more than one hundred meters, intergrade with thick-bedded to massive leiolites, formimg the upper part of a third-order depositional sequence that constitutes a forced regressive systems tract. (2) A centimeter-sized bush-like fabric (shrub) typically produced by calcified microbes is similar to the mesoclot in thrombolites but distinctive from clotted fabrics of thrombolites. This bush-like fabric is actually constituted by diversified calcified microbes like the modern shrub as a result of gliding mobility of filamentous cyanobacteria. Such forms traditionally include: the Epiphyton group (which actually has uncertain biological affinity), the Hedstroemia group which closely resembles modern rivulariacean cyanobacteria, and the possible calcified cyanobacteria of the Lithocodium–Bacinella group. (3) Significantly, dense micrite of leiolite is associated with sponge fossils and burrows, and is covered by microstromatolite. The Lithocodium–Bacinella group is a controversial group of interpreted calcified cyanobacteria in the Cambrian that has also been widely observed and described in the Mesozoic. Therefore, dendrolites with symbiosis of leiolites in the studied section provide an extraordinary example for further understanding of growing style of bush-like fabrics (shrubs) of the dendrolites dominated by cyanobacterial mats. Furthermore, the present research provides some useful thinking approaches for better understanding of the history of the Early Paleozoic skeletal reefs and the microbe–metazoan transitions of the Cambrian.

Upper Devonian (Frasnian) stromatactis-bearing mud mounds, western Alberta, Canada: reef framework dominated by peloidal microcrystalline calcite

ABSTRACT Two well-preserved mud mounds, approximately 50 m thick, in the Mount Hawk Formation (Upper Devonian, Frasnian) in western Alberta provide an unparalleled opportunity to study the microstructure of this reef type in detail for this time interval. This reveals that they are composed dominantly of peloidal sediments—more than a quarter of mud mound volume—along with dense micrite, particulate micrite, bioclasts, and stromatactis cavities. Five types of peloids are differentiated: bacterial, cyanobacterial, bioclastic, intraclastic, and pseudo-peloids. Bacterial peloids were generated by bacterial metabolic activities with possibly some contribution from organomineralization in areas within spicular networks. Three subtypes of cyanobacterial peloids are distinguished based on whether they are physically reworked calcified filaments, aggregates of calcified coccoids, or precipitated within stromatolite-forming cyanobacterial mats or biofilms. Bioclastic peloids are fully micritized fragments of skeletons and shells. Intraclastic peloids are eroded fragments of early-lithified matrix. Pseudo-peloids represent artifacts of poorly preserved sponge spicular networks reflecting the interplay between dissolution of spicules and organomineralization. The distribution of the various peloid types shows specificity in different microfacies. They prove to be valuable paleoecological indicators, and taken together suggest that the mud mounds accreted within the photic zone above storm wave base but below the fair-weather wave base, during deposition of a transgressive and perhaps highstand systems tracts. They are the products reflecting a dynamic balance between constructional versus destructive processes. Bacterial peloids combined with dense micrite constitute the framework, which in turn makes up close to 40% of the mud mounds. This indicates that microbial activities are responsible for the early-indurated framework. The onset of bacterial peloid formation in the bedded sediment immediately underlying the mud mounds further demonstrates that it may have been a necessary precursor to mud-mound initiation. Bacterial peloids appear to characterize most Paleozoic and Mesozoic reefal mud mounds as an essential framework element.

Microfacies and depositional environments of Jurassic (Callovian) Tuwaiq Mountain Formation in central Saudi Arabia

Three stratigraphic sections of the Callovian Tuwaiq Mountain Formation at Shaib El-Hisyan, Khashm Al-Giddayah and Dirab, near Riyadh City, central Saudi Arabia have been studied to distinguish their microfacies features and the equivalent probable depositional environments. Lithostratigraphically, the Tuwaiq Mountain Formation is subdivided into two lower and upper Tuwaiq members. The part that informally is known as upper Tuwaiq Member is subdivided from base to top into T1, T2, and T3 members, respectively. The microfacies analysis of the Tuwaiq Mountain Formation led to the recognition of 12 microfacies types (FT1–FT12) that are grouped into three associations: (1) open platform facies association (FT1–FT5); (2) high-energy shoals of ooids and patch reefs facies association (FT6–FT9); and (3) restricted carbonate platform facies association (FT10, FT12). The lower part of the Tuwaiq Mountain Formation is characterized by relative abundance of wackestone and wacke/packstone facies that are capped by thin peloidal molluscan floatstone facies. The former indicates a quiet open platform environment with periodic agitation, while the later indicates deposition in a sand shoal environment. The facies of basal unit of the upper Tuwaiq Mountain Formation, T1, are enriched with dense micrite matrix and shell fragments of epifaunal and deep infaunal organisms (e.g., Pholadomya spp.). The microfacies pattern of Baladiyah Member (T1) indicates a quiet open platform with low and sometimes high water circulation. The Maysiah Member (T2) is composed of hard limestone with abundance of coralline microfacies in framestone, grainstone, and packstone textures suggesting a high-energy shallow reefal environment with periods of quiet open platform conditions. This shallow reefal environment becomes more restricted southward, where shales and algal dolomitic grainstones were formed. The Daddiyah Member (T3) that made up of chalky limestone with chert intercalations is predominated by matrix-support lime mudstone, wackestone, and packstone facies. These microfacies reflect open platform with slightly low circulation below fair-weather wave base with period of restricted marine conditions forming rudstone microfacies. Southward, these environments became shallower and subjected to considerable quartz influx in an agitated sand shoal.

Freshwater biodissolution rates of limestone in the temperate climate of the Dinaric karst in Slovenia

Dissolution rates in two freshwater karst systems were determined by using tablets of dense micrite–biopelmicrite Cretaceous limestone. Submerged limestone tablets in riverbeds were subjected to a natural gradient from complete darkness to direct sunlight. Higher light rates significantly (p<0.05) increased the epilithic biomass of phototrophs and the overall dissolution rates, which were highest at the Unica spring (−49.2μma−1), but the exact portion of light-dependent dissolution remains elusive. In the karst river Unica, with its big fluctuations in environmental parameters (e.g., discharge), light rates can be used in estimating the dissolution rates enhanced by phototrophs. Natural biofilms in aquatic systems have important implications for landform evolution, and the impact on limestone dissolution rates is comparable with rates of debris falling from steep slopes.

Paleoclimatic implications of micromorphic features of a polygenetic soil in the Monegros Desert (NE-Spain) .

Pedofeatures can be repositories of information about soil forming factors such as climate. The aim of this work is to provide a model of interpretation of a polygenetic soil in the Monegros desert (Ebro Basin, NE-Spain) and its relationship to environmental changes during the Quaternary. To achieve this goal, the physical, chemical, mineralogical and especially the micromorphic pedofeatures of this profile were studied. Carbonate accumulations extend into all of the horizons of the profile. The paleosol has a thick petrocalcic horizon at the top, with a massive-laminar structure comprising layers of micrite and sparite that sometimes form pendants. Towards its base, the petrocalcic horizon contains a spaced framework of orthic micrite nodules packed between relatively pure micritic laminar bands. Below the petrocalcic horizon, coatings and infillings of microcrystalline calcite occur in old channels, and soft concretions (some of them geodic) indicate an in situ accumulation process (Bkc, calcic horizon). Another calcic horizon with orthic nodules of calcite, impregnative and diffuse (Ckc), is present at the bottom part of the profile. Between the two nodular calcic horizons, two recarbonated argic horizons are found (Btkc and Btk) with coarse orthic nodules of dense micrite superimposed on textural pedofeatures. These textural micromorphic pedofeatures are: (1) interbedded microlaminated clay pockets not associated with current or past pores and (2) microlaminated clay and silt (dusty clay) present as weakly oriented coatings on channel walls. Reduction pedofeatures are associated with textural ones: (1) coatings of manganese oxides around pore channels and cracks, and (2) nodules of manganese and iron oxides within the peds. The presence of calcic horizons alternating with argic horizons, all positioned below the petrocalcic horizon, confirm fluctuations in paleohydrological conditions in the Pleistocene. Its presence indicates that the oldest soil corresponds to a Calcic Luvisol-like pedotype, which is overlain by an Haplic Calcisol-like pedotype and this, in turn, by a Petric Calcisol-like pedotype. This superposition of profiles indicates, within the mentioned climatic changes, a tendency towards increasing dryness during the Pleistocene in the semiarid Ebro Valley.

A panorama of the fossil algae: from cyanobacteria and calcimicrobes to the green calcareous algae

Initiated in the first half of the 19th century, investigationsof calcareous algae flourished after the Second World War,in connection with the intensification of oil exploration incarbonate reservoirs. This field of research immediatelyprovided a new micropaleontological tool for stratigraphicand paleoecological characterization of sedimentary strata.When a representative community working on fossil cal-careous algae had become established by the 1970s and1980s, such investigations reached a peak. This communitygathered at specialized international meetings, where thetop results were shared. The late Professor Erik Flu¨gel wasthe one who organized the first international symposium onfossil algae in Erlangen in 1975. Eric Flu¨gel was wellknown in the international scientific community for hisstudies on carbonate rocks, especially on fossil calcareousalgae. More than that, he was widely appreciated for hisexceptional mentoring qualities, from which numerousgenerations of students in this field benefited. At Erlangen,he founded the carbonate microfacies and calcareous algaeschool; in 1979 he initiated the journal Facies, currently oneof the most prominent publications in the field. As a sign ofour deep appreciation, we dedicate this volume to hismemory.The symposium organized in Erlangen represented thestart of a quadrennial series of international reunions thatsuccessively took place in Paris (1979), Golden, Colorado(1983), Cardiff (1987), Capri (1991), Ankara (1995),Nanjing (1999), Granada (2003), Zagreb (2007), andCluj-Napoca (2011). These scientific meetings encouragedthe development of the study of calcareous algae, as well asinternational collaborations in the field. Accordingly, theInternational Fossil Algae Association (IFAA) decided toinitiate an additional conference series, timed in-betweenthe international meetings. The latter were defined as‘‘regional meetings’’, being organized as fieldwork-basedworkshops. Nevertheless, they always had an internationalcharacter, from the participants and topic point of view.The first regional IFAA symposium took place in Granada(1989), followed by the ones in Munich-Vienna (1993),Krakow (1997), Cluj-Napoca (2001), Ferrara (2005), andMilan (2009). In the last decades, the proceedings of theseinternational and regional symposia summed up the state ofthe art in calcareous algae research. We hope that thecurrent issue of Facies will continue this trend.The current volume gathers some of the presentationsgiven at the 10th International Symposium on Fossil Algae(Cluj-Napoca, September 2011), as well as some otherrecent results in the study of calcareous algae. The con-tributions cover a wide range of topics, from the study ofsome end-products of cyanobacteria and calcimicrobes tostudies of red and dasycladalean green algae.Basso et al. (2013) describe in their contribution thepresent-day formation of microbialites containing cyano-bacteria, bacteria, and microalgae colonies in southern Sinai(Egypt). Based on macroscopic morphology, texture, anddegree of lithification, four categories of microbialites wererecognized. Their presence in the area is considered to resultfrom the combination of local sedimentary dynamics andaccommodation space in a peritidal tropical environmentwith high variations of temperature and salinity.Various fossil microbialites associated with micro-encr-ustersaredescribedbyPles¸ etal.(2013)fromUpperJurassiclimestones in the Southern Carpathians (Romania). Thedepositsmainlyconsistofmassifreeflimestonesincludingavariety of microbialites associated with micro-encrusters.The high abundance of dense micrite, the presence of

Encrusting micro-organisms and microbial structures in Upper Jurassic limestones from the Southern Carpathians (Romania)

Late Jurassic–Early Cretaceous Stramberk-type reef limestones are known from some parts of the Southern Carpathians in Romania. The Upper Jurassic deposits mainly consist of massif reef limestones including a variety of microbialites associated with micro-encrusters. They played an important role in the formation and evolution of the reef frameworks and thus are of significant importance for deciphering the depositional environments. For our study, the most important encrusting organisms are Crescentiella morronensis, Koskinobullina socialis, Lithocodium aggregatum, Bacinella-type structures, Radiomura cautica, Perturbatacrusta leini, Coscinophragma sp., and crust-forming coralline sponges such as Calcistella. Based on microscopic observations, microbial contribution to reef construction is documented by the abundance of dense micrite, laminate structures, clotted, thrombolithic or peloidal microfabrics, constructive micritic cortices, biogenic encrustations and cement crusts, as well as by other types of microbial structures and crusts. Most of the investigated carbonate deposits can be classified as “coral-microbial-microencruster boundstones” which are characteristic for the Intra-Tethyan domain. Their paleogeographical significance is indicated by the presence of many features comparable with carbonate deposits of rimmed platform systems from the Northern Calcareous Alps or Central Apennines. Based on the distribution of the facies and facies associations within the carbonate sequences under study we can distinguish slope and external shelf margin environments. The microbial crusts, the encrusting micro-organisms, and in some cases the syndepositional cements have stabilized and bound the carbonates of the slope facies types. Subsequently, the stable substrate favored the installation of coral-microbial bioconstruction levels.

Belemnite taphonomy (Upper Jurassic, Western Tethys) Part I: Biostratinomy

A detailed taphonomic analysis was performed on 188 belemnites and 56 thin sections from the Pozo Cañada section (External Prebetic), as well as 101 belemnites and 31 thin sections from the Río Segura (Internal Prebetic), both from the Upper Oxfordian–Lower Kimmeridgian. This manuscript is focused on biostratinomic features and a second part is focused on fossildiagenetic aspects. The belemnites correspond to Hibolithes and, less commonly, Belemnopsis. Biostratinomic features, including size, fragmentation, corrasion, microboring and encrustation were analysed in each specimen, using petrographic (conventional, cathodoluminescence, epifluorescence and scanning electron microscopy) techniques.Fragmentation affects the phragmocone and the rostrum cavum, and is usually transversal and rarely longitudinal through the apical line. Isolated fragments of epirostrum are common. Corrasion produces irregular surfaces. Microborings are very common, represented by: a) Fungal hyphae; b) Fascichnus dactilus-like and Scolecia filosa-like borings (cyanobacteria); c) Entobia (sponges), d) large microborings with Y-shaped branching (diameter 100–500μm) subparallel to rostrum surface; e) large cylindrical to ellipsoidal borings related to cirripeds; and f) Palaeconchocelis starmachii (rhodophyte). Microbes and sessile foraminifera (Vinelloidea, Bullopora, Tolypammina, Thurammina, Placopsilina and Subdelloidina) are the main encrusters. Microbial films are composed of dense micrite or clotted-peloidal microfabrics.The biostratinomic features analysed in the belemnite rostra are clearly influenced by the lithofacies and shelf setting. The intensity of corrasion, microborings and encrustation, by indicating time-exposure, help establish which lithofacies shows the highest sedimentation rate. The belemnites from Internal Prebetic lithofacies—that is, the outer shelf deposits (lumpy limestone and lumpy-oncolitic limestone)—present high values of corrasion, microboring and encrustation, thereby indicating long-time exposure. In sharp contrast, belemnites from the lithofacies of the External Prebetic represent deposition in mid-shelf environments (spongiolithic limestone, spongiolithic marl–peloidal limestone, and marl–limestone rhythmite) featuring higher clay content.

An Upper Turonian fine-grained shallow marine stromatolite bed from the Muñecas Formation, Northern Iberian Ranges, Spain

A fine-grained stromatolite bed, laterally continuous on the kilometer scale and with small synoptic relief, crops out in the Muñecas Formation in the Northern Iberian Ranges. The Muñecas Fm. was deposited during the late Turonian on a shallow water platform in the Upper Cretaceous intracratonic Iberian basin.The stromatolite bed has a tabular to domed biostromal macrostructure. Its internal mesostructure consists of planar, wavy to hemispherical stromatoids that display a broad spectrum of microstructures, including dense micrite, bahamite peloids, peloidal to clotted microfabrics, irregular micritic-wall tubes, which are suggestive of algae and filamentous microframeworks, which are suggestive of filamentous cyanobacteria.Various stromatolite growth stages have been linked to the dominance of different accretion processes. The accretion of the entire fine-grained stromatolite involves a complex mosaic of processes: trapping and binding of quartz-silt grains and bahamites, which form the agglutinated parts of some laminae, and microbially induced precipitation, which forms spongiostromic and micritic laminae. Tubiform and filamentous microframeworks resembling porostromatate or skeletal stromatolitic growth were also recognized.Laser ICP-MS measurements of Al, Si, Mg, Mn, Sr, S and Fe were analyzed to detect the influence of siliciclastic inputs and major trends during stromatolite accretion. Carbon and oxygen isotopic compositions from the stromatolite and associated facies were used to identify possible microbial signatures. These data describes a unique and well-preserved example of a shallow marine Upper Turonian fine-grained stromatolite.

Early Ordovician reef construction in Anhui Province, South China: A geobiological transition from microbial- to metazoan-dominant reefs

The Lower Ordovician records a transition from microbial- to metazoan-dominant reefs immediately prior to the Great Ordovician Biodiversification Event. The Hunghuayuan Formation (late Tremadocian-early Floian) of the Jianxin section in Anhui Province, South China, includes well-preserved reefs, which consist of a combination of microbial boundstones and microbial-lithistid sponge-receptaculitid boundstones. The microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of Girvanella-rich layers and thick micrite layers in which the filamentous calcimicrobe Girvanella and bioclasts occur sporadically. Microbial biofilms, including the filamentous microbes, may have trapped and bound lime mud and/or precipitated micrite during their growth and metabolic activities. These activities contemporaneously contributed to substrate stabilisation, encrustation, and the construction of microbialite frameworks (stromatolites). In contrast, the microbial-lithistid sponge-receptaculitid boundstones are characterised by more micritic constituents than skeletal reef-building constituents. Thin and/or domal crusts produced by peloid and dense micrite, together with various proportions of Girvanella, occur upon bioclastic sediments and reef-building skeletal organisms. These micritic crusts with Girvanella therefore played roles as binders and stabilisers, and as consolidators by encrusting reef-building skeletal organisms. In the boundstones, it is remarkable that the degradation of lithistid sponges, due to microbial decay, produced micrite in the sponge bodies. As a result, lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors. These Lower Ordovician reefs, in which microbialites (e.g., stromatolites) dominate but coexist with metazoans (e.g., sponges), were widespread at the time. They provide excellent examples for understanding reefal palaeoecology during the transition from microbe-dominant reefs to reefs enriched in metazoans.

CHAMBERS OF EPIPHYTON THALLI IN MICROBIAL BUILDUPS, ZHANGXIA FORMATION (MIDDLE CAMBRIAN), SHANDONG PROVINCE, CHINA

Abstract Microscopic morphologic variations of Epiphyton thalli in microbial buildups were examined in order to detail controlling factors on morphology and calcification processes, and their implications for identification of calcified microbes. Microbial carbonate of the Zhangxia Formation (Middle Cambrian), Shandong Province, China comprises thrombolites, stromatolites, and Epiphyton buildups, as well as consortia of microbial and metazoan communities. Epiphyton, a rigid framework of the buildups, is subdivided into four types based on characteristics of the branches. Type 1 consists of ∼75-μm-diameter dendritic rods of dense micrite that form bush-shaped and chambered thalli. Type 2 has ∼80-μm-thick branches characterized by transverse segments. Type 3 consists of thin, ∼50-μm diameter micritic branches that form round thalli. Type 4 is characterized by laterally arrayed, branching tubes that form fan-shaped thalli. All morphologic types have bipartite branched filaments as a basic growth pattern. Bus...

Microbial impacts on the genesis of Lower Devonian reefal limestones, eastern Australia

Microbial contributions to reefal limestones are evident in eastern Australian Lower Devonian microbial frame/bindstones, red algal-microbial-stromatoporoid bindstones, and microbial-stromatoporoid bindstones. Varied microbialite textures, such as stromatolites, thrombolites, and leiolites, originated as accumulations and partial aggregations of calcimicrobes, peloids, and micrites, which also derived from microbial activities. In microbial frame/bindstones, calcimicrobes (e.g., Rothpletzella and Wetheredella) and dense micrite layers covered and bound underlying substrates. Stabilized substrates promoted the subsequent construction of layered, domal, and columnar frameworks, which were produced by combined accumulations and intermixed associations of calcimicrobes and micritic microbialites. Microbes flourished in the microbial-stromatoporoid bindstones and red algal-microbial-stromatoporoid bindstones during repeated growth interruptions of the framework-building skeletal organisms. Microbes bored into and eroded the skeletal frameworks to subsequently leave micritic envelopes, on which microbial and skeletal encrustations took place in turn. The importance of microbial colonization on the skeletal frameworks was first as subsidiary encrusters that helped to preserve them from erosion, and second as modifiers of the spaces suitable for succeeding encrusters. Partial aggregations of Renalcis filled in the interstices of the skeletal and microbial frameworks, thereby enhancing their rigidity. The microbial impacts on the genesis of reefal limestones are: (1) origination of components (calcimicrobes, peloids, and micrites); (2) formation of characteristic microbial textures; (3) main and subsidiary reef construction and encrustation; and (4) destruction of these components, textures, and structures, but also the protection of resultant constructions in turn. The Lower Devonian reefal limestones treated herein, surprisingly, preserve excellent records of a variety of microbial impacts. Similar effects may also have been common, although variable in preservation, in other ancient reefal deposits.

Calcimicrobial stromatolites at the Permian–Triassic boundary in a western Tethyan section, Bükk Mountains, Hungary

Basal Triassic carbonates of the Gerennavár Formation (Bükk Mountains, Northern Hungary) were studied. The succession was deposited in an open marine deep ramp environment of the western Tethys, and can be divided into three parts on the basis of the dominant microfacies. The lowermost 0.5 m-thick interval is made up by thin-bedded and bioturbated mudstones which contain ostracods and Earlandia along with relict skeletal fragments. The successive 8 m-thick deposits consist of planar stromatolites with well-preserved microfabrics. The most characteristic microfabric elements are sphere clusters in a dense micrite groundmass, bushy aggregates of micrite clots, bundles of prostrate micrite threads, and peloids. The formation of the spheres and the surrounding dense micrite is attributed to calcification of a coccoid–cyanobacteria dominated mat community. The tufted micrite clot clusters crudely resemble Angusticellularia. A filamentous microbial origin is likely for the thread bundles. The peloids are interpreted either as reworked pieces of calcified mat, or rolled microbial clumps. The reduced preservation of microfabric components in the upper unit refers to less favourable condition for mat development which is probable due to deepening of the depositional environment. The overlying mudstone unit is very poor in fossils. The preservation of microbial microfabrics in the stromatolites indicates that early lithification by carbonate precipitation was widespread and intense even in open marine environments following the end-Permian boundary events. This supports the assumption that significant changes took place in geochemistry of seawater during the Late Permian–Early Triassic time interval.

Marked accumulation patterns characteristic of Lower Devonian stromatoporoid bindstone: Palaeoecological interactions between skeletal organisms and microbes

Stromatoporoid bindstones consisting of coarse alternations of both laminar stromatoporoids (“ Aculatostroma” and Syringodictyon) and microbialites (thrombolites and leiolites) are developed in a biohermal limestone of the Lower Devonian Garra Formation at the Catombal section, south of Wellington, New South Wales, Australia. The stromatoporoid “ Aculatostroma” typically shows a thinly laminar and ragged appearance, producing main frameworks having cryptic spaces within them, and Syringodictyon is also very thinly laminar. Thrombolites are composed of an association of peloids and Girvanella, whereas leiolites are made up of dense micrites with intensive microborings. Similar accumulation patterns are repeatedly shown at each observation scale: 1) the main framework-building “ Aculatostroma” and a combination of secondary framework ( Syringodictyon) and microbialites (leiolites and thrombolites) mesoscopically; 2) Syringodictyon and overlying leiolites and thrombolites at optical microscopic level; and 3) skeletons and intercalating dense micrite layers (leiolites) within Syringodictyon itself at SEM level. Microbialite accumulations on “ Aculatostroma” suggest disruptions of skeletal growth, corresponding to periods of harsh habitat environments, whereas the recurrent appearance of Syringodictyon over microbialites suggests somewhat ameliorated conditions. The resurgence of “ Aculatostroma” as a main frame-builder over a combination of secondary framework and microbialites suggests habitat recovery as well as the competitive superiority of this stromatoporoid. Successions such as “ Aculatostroma” – Syringodictyon – leiolites–thrombolites reflect differences in the tolerances of each responsible organism, and correspond to fluctuating environments. The growth history of stromatoporoid bindstones is as follows: 1) the laminar stromatoporoid “ Aculatostroma” first grew laterally, thereby stabilizing bioclastic sediments; 2) the stromatoporoid growths were partly or totally interrupted in relation to deteriorating habitat conditions, where micritization (leiolites with microborings), and encrustation by thrombolites occurred in succession. However, somewhat ameliorated conditions enabled Syringodictyon to temporarily dominate; 3) surviving parts of “ Aculatostroma” again spread vertically and laterally upon the recovery of optimum conditions and; 4) the ceilings of cryptic spaces produced within “ Aculatostroma” are affected by micritization (leiolites) and then encrustation of thrombolites. Repeated accumulations at various scales indicate antagonistic interrelationships within skeletal organisms and between skeletal organisms and microbes within their habitable ranges. These accumulation patterns thus provide invaluable clues for biotic interactions between skeletal frameworks and microbes, biological successions in accordance with microenvironmental changes, and resultant depositional sequences of bindstones.

Seasonal records of climatic change in annually laminated tufas: short review and future prospects

AbstractMany Recent and fossil freshwater tufa stromatolites contain millimetre‐scale, alternating laminae of dense micrite and more porous or sparry crystalline calcites. These alternating laminae have been interpreted to represent seasonally controlled differences in the biotic activity of microbes, and/or seasonally controlled changes in the rate of calcification. Either way, couplets of these microbially mediated alternating calcified laminae are generally agreed to represent annual seasonality. Combined stable isotope (δ18O and δ13C) and trace element (Mg, Sr, Ba) geochemistry from Recent tufa stromatolites show that seasonal climatic information is available from these calcites. Variability in δ18O (and in one case Mg concentration) has been shown to be controlled primarily by stream temperature change, usually driven by solar insolation. In arid climates, seasonal evaporation can also cause δ18O enrichment by at least 1‰. Variability in δ13C results potentially from: (1) seasonal change in plant uptake of 12C‐enriched CO2; (2) seasonal change in degassing of 12C‐enriched CO2 in the aquifer system; and (3) precipitation of calcite along the aquifer or river flow path, a process that increases δ13C of dissolved inorganic carbon (DIC) in the remaining water. Mechanisms 2 and 3 are linked because calcite precipitates in aquifers where degassing occurs, e.g. air pockets. The latter mechanism for δ13C enrichment has also been shown to cause sympathetic variation between trace element/Ca ratios and δ13C because trace elements with partition coefficients much greater than 1 (e.g. Sr, Ba) remain preferentially in solution. Since degassing in air pockets will be enhanced during decreased recharge when water saturation of the aquifer is lowest, sympathetic variation in trace element/Ca ratios and δ13C is a possible index of recharge and therefore precipitation intensity.High‐resolution geochemical data from well‐dated tufa stromatolites have great potential for Quaternary palaeoclimate reconstructions, possibly allowing recovery of annual seasonal climatic information including water temperature variation and change in rainfall intensity. However, careful consideration of diagenetic effects, particularly aggrading neomorphism, needs to be the next step. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Paleoclimatic implications of pedogenic carbonates in Holocene soils of the Gangetic Plains, India

This study attempts to reconstruct the Holocene climatic history of the Indo-Gangetic Plains based on micromorphological characteristics and stable isotope composition of calcretes in a soil-chronoassociation between the Ramganga and Rapti rivers. The calcrete from the B2 horizon of older soils (QGH4–QGH5: 6500–13 500yearbp) consists of septaric and irregularly shaped nodules of dense micrite and diffused needles associated with the illuvial clay pedofeatures. Fabric of the calcretes of older soils show inclusion of soil constituents and precipitation of carbonates in between weathered layers of micas. Formation of this calcrete took place during the pedogenesis in an arid to semi-arid climate that also influenced vegetation and C4 type biomass such as Chenopodium and Typha angustata spread in the area. A change to warm and humid phase at 6500yearbp led to dissolution and reprecipitation of the calcretes. This led to the formation of blocky crystals and needles of calcite in voids and coarsening of the fabric in the lower horizons. The vegetation pattern also changed to woodland with Anogeissus and Tecomella spp. The younger soils (<2500yearbp) show secondary carbonate accumulation associated mainly along the voids in lower parts of the profiles. These are relatively coarse textured with little or no inclusion of soil constituents. This coarse grained calcrete results from capillary rise from a fluctuating water table in a sub-humid climate similar to present.Carbon and oxygen isotope data suggest that the formation of pedogenic carbonate in old soils (QGH4 and QGH5) by evapotranspiration during arid to semi-arid climate. In the upper horizons, the carbonates are depleted in 13C as their formation was influenced by the sparse vegetation and low soil respiration rate. In the lower horizons, the carbonates show enrichment of 13C due its extensive dissolution–reprecipitation during the humid climate. The younger soils showed the enrichment of 13C in their calcretes and support their formation from fluctuating shallow groundwater table. The oxygen isotope values similar to that of meteoric water of the area indicate calcrete precipitation in equilibrium with the soil water.

Phreatic and Vadose Diagenetic Modification of Pleistocene Limestone: Petrographic Observations from Subsurface of Barbados, West Indies

Data from a shallow cored borehole through 105,000-year-old reef-tract sediments on the south coast of Barbados indicate that the carbonate sediments have been altered more rapidly and more extensively in freshwater phreatic diagenetic environments than in the vadose or marine phreatic diagenetic environments. The upper part of the borehole section has been exposed only to subaerial and shallow vadose processes since emergence from the marine depositional environment about 105,000 years ago. These sediments are recrystallized only partly. Total amounts of cement, mainly needle-fiber cement and dense micrite coatings, are high. Porosity is low. Dissolution, a minor feature in this part of the section, is associated with localized development of needle-fiber fabrics. Those parts of the borehole section which have been occupied by a freshwater phreatic lens have been cemented by calcite microspar. Aragonite grains have been dissolved on a massive scale, resulting in a very well-developed moldic porosity. Neomorphic grain alteration has been volumetrically important. Packstones and grainstones in the lowest part of the borehole are largely unaltered; metastable carbonate mineralogy dominates. This part of the sedimentary column has been subjected alternately to vadose conditions during low stands of sea level (> 70,000 years total) and to marine phreatic conditions during high stands of the sea (< 35,000 years total). Despite prolonged exposure to vadose conditions, diagenetic modification is slight. If the recrystallized lime muds contain greater than 25 percent cement (micrite), then mass-balance calculations suggest large amounts of calcium carbonate have been transported to that part of the sedimentary section exposed to freshwater phreatic diagenetic processes. The source of the additional carbonate may be updip in the aquifer, from mixing of underlying marine pore fluids with the freshwater pore fluids of the coastal water lens, from the overlying shallow vadose zone, or from marine water soon after deposition. Processes which affect early diagenetic modification of carbonate sediments on the south coast of Barbados operate more rapidly and more effectively in the freshwater phreatic environment than in vadose or marine phreatic environments.

Caliche Soil Horizons on Oolitic Shoals and Carbonate Mud Mounds in Carboniferous (Newman Limestone) of Eastern Kentucky: ABSTRACT

A sequence of oolitic calcareous grainstone shoals and calcareous wackestone and packstone-bank and interbank deposits is exposed near Olive Hill, Kentucky. The shoals and banks underwent periods of exposure, as they are capped by beds that represent stages of deposition or caliche weathering in the supratidal zone. The caliche weathering zones are similar to those found on Caribbean Islands and in the southwestern United States. The first type of caliche zone is analogous to the subaerial crusts of the Caribbean area. These crusts are brecciated, often silicified, laminated calcareous mudstones which formed on and within the shoals and mud mounds. They are present as beds or coat fractures which transect bedding planes. Petrographic characteristics include microbrecciation, grains coated with laminated or dense micrite, extensive micritization, pelletoid fabrics, root tubules, and micrite and yellowish-brown spar cements. The second caliche type, similar to those of the southwestern United States, is a nodular, brecciated calcareous mudstone that developed from supratidal and intertidal calcareous muds. The basal part of this caliche unit is a fossiliferous, but intensely brecciated, parent rock; this zone grades upward into a nodular sequence which may be capped by a thin-brecciated, laminated crust. Original depositional fabrics have been obliterated in the nodular section; the nodules are composed of microspar containing breccia clasts and root tubules, and are disrupted by subhorizontal fractures. Both caliche types display End_Page 785------------------------------ desiccation-expansion teepee structures. Caliche clasts are reworked into overlying intertidal-supratidal deposits, indicating that weathering occurred near sea level and contemporaneous with offshore deposition. End_of_Article - Last_Page 786------------