chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

perspectives on proterozoic surface ocean redox from iodine contents in ancient and recent carbonate abstract the proterozoic eon hosted the emergence and initial recorded diversification of eukaryotes oxygen levels in the shallow marine settings critical to these events were lower than today s although how much lower is debated here we use concentrations of iodate the oxidized iodine species in shallow marine limestones and dolostones to generate the first comprehensive record of proterozoic near surface marine redox conditions the iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters to assess the validity of our approach neogene quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate iodine contents despite the potential for diagenetic loss maximum proterozoic carbonate iodine levels are elevated relative to those of the archean particularly during the lomagundi and shuram carbon isotope excursions of the paleo and neoproterozoic respectively for the shuram anomaly comparisons to neogene quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends the baseline low iodine levels in proterozoic carbonates relative to the phanerozoic are linked to a shallow oxic anoxic interface oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes however the diagnostically low iodine data from mid proterozoic shallow water carbonates relative to those of the bracketing time intervals are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans this redox instability may have challenged early eukaryotic diversification and expansion creating an evolutionary landscape unfavorable for the emergence of animals

wave velocities and attenuation of shaley sandstones as a function of pore pressure and partial saturation we obtain the wave velocities and quality factors of clay bearing sandstones as a function of pore pressure frequency and partial saturation the model is based on a biot type three phase theory that considers the coexistence of two solids sand grains and clay particles and a fluid mixture additional attenuation is described with the constant q model and viscodynamic functions to model the high frequency behaviour we apply a uniform gas fluid mixing law that satisfies the wood and voigt averages at low and high frequencies respectively pressure effects are accounted for by using an effective stress law by fitting a permeability model of the kozeny carman type to core data the model is able to predict wave velocity and attenuation from seismic to ultrasonic frequencies including the effects of partial saturation testing of the model with laboratory data shows good agreement between predictions and measurements

carbon isotopic profile across the bilara group rocks of trans aravalli marwar supergroup in western india implications for neoproterozoic cambrian transition abstract the rocks of marwar supergroup in the trans aravalli sector in western india are presumed to span the time interval between neoproterozoic and early cambrian this predominantly unfossiliferous marine sedimentary sequence is characterized by a lower arenaceous facies jodhpur group middle carbonate facies bilara group and upper argillaceous arenaceous facies nagaur group rocks the sedimentation has been essentially in a shallow basin described either as the fore land slope of the rising aravalli mountains or a sag basin which developed and evolved due to subsidence of the updomed crust during neoproterozoic malani magmatism that failed to open rifts the carbon isotopic profile for the bilara group carbonate rocks in the lower part shows marked oscillations and broadly negative 13 c character with negative anomalies as low as pdb observed near the base of dhanapa formation lower unit and pdb in the overlying gotan formation middle unit the upper part of the profile shows a gradual positive shift the carbon isotopic signatures of the bilara group rocks can be correlated with the end neoproterozoic early cambrian vendian tommotian carbon isotopic evolution curve extremely low 13 c values indicate the glaciation related cold climatic postulates of the end neoproterozoic followed by the warmer climatic conditions as indicated by the positive shift the carbon isotopic data for gotan formation carbonates at variance with the globally observed 13 c trends for early tertiary do not support the recently proposed tertiary age for the bilara group

lithification of peritidal carbonates by continental brines at fisherman bay south australia to form a megapolygon spelean limestone association abstract groundwater springs in the supratidal zone at fisherman bay south australia are the sites of aragonite precipitation and cementation of holocene marine carbonates lithification which commenced less than 3000 years bp and is still active has formed a cavernous limestone containing megapolygons tepees and speleothems including pisoliths floe aragonite and aragonitic pool deposits the emerging waters are from a continental source and have evolved from low alkalinity waters of pleistocene sand and clay coastal plain aquifers which have passed through an underlying tertiary marine carbonate aquifer the resulting waters have high pco2 total carbonate ca and sulphate concentrations they are close to saturation with respect to aragonite and their mmg2 mca2 ratios approach or exceed the critical aragonite precipitation value aragonite is precipitated from these waters when they approach the surface and evolve co2 features which taken together may diagnose ancient examples of this process include 1 primary aragonitic cements with high msr2 mca2 values 2 nonmarine 34s values in associated gypsum 3 two superimposed networks of surface polygons one delineated by extensional boundaries the other by tepees 4 high water vadose zone isopachous grain cements 5 interconnected speleothem lined cavities and 6 the presence of evaporites only in surface sediments possible ancient examples of peritidal carbonate lithification by saline spring waters rising from confined aquifers are recognized in descriptions of carbonates from west texas lombardy and the atlas mountains the areal extent of each of these deposits suggests that the process may be a geologically important feature and its products may be diagnostic of semi arid or arid zone paralic sedimentation

production of subtidal tubular and surficial tempestites by hurricane kate caicos platform british west indies abstract storm infilling of deep open callianassa burrows with molluscan to peloidal packstone was the dominant style of preservable sedimentation produced by the passage of hurricane kate across caicos platform british west indies on 18 november 1985 this moderate hurricane was the first to affect an actively studied portion of the carbonate platform environments of the south florida bahama region in 19 years a time during which many of our concepts of storm sedimentation have evolved prior to hurricane kate much of the shallow platform interior peloidal packstone to grapestone grainstone had a sparse seagrass cover and a highly irregular bottom caused by abundant callianassa mounds hurricane kate flattened the mounded bottom of the platform interior and caused minor smothering and erosion of the seagrass beds a thin surficial tempestite though deposited over much of the platform was difficult to identify because it lacked a coarse base instead much of the reworked surface sediment poured down the large callianassa burrow entrances and filled the vast burrow networks with either a coarse molluscan packstone or peloidal skeletal packstone storm burrow filling tubular tempestites effectively removed the coarse fraction from contributing to a surficial storm layer ten percent of the upper 75 cm of platform interior sediment consists of tubular tempestites from hurricane kate

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

variations in neoarchean microbialite morphologies clues to controls on microbialite morphologies through time the neoarchean carawine formation hamersley group western australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments these distinctive structures are distributed in five shallow subtidal and two deeper water facies in the oakover area of the carawine formation the shallow subtidal facies are composed of biohermal and bedded stromatolites centimetre scale ridge shaped microbialites and wavy laminated microbialites the deeper water facies are composed of fenestrate microbialites planar laminated dolostone and dolostone with rolled up microbial laminae microbialites in the carawine formation lie within a continuum of archean to proterozoic microbial facies some shallow water microbial facies in the carawine formation are similar to proterozoic facies such as large bioherms internally composed of a variety of stromatolite morphologies in contrast fenestrate microbialites grew in quiet subtidal environments and are common in archean rocks but have not been documented in similar proterozoic environments the similarity of shallow water facies across the archean proterozoic transition before and after the oxidation of the atmosphere and surface oceans indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes rather stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment in contrast the absence of fenestrate microbialites from proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans

hydrological properties of topopah spring tuff laboratory measurements the purpose of this work is to study the transport of water both the liquid and vapor phases in tuff from the topopah spring welded unit under conditions expected in the near field environment of a high level nuclear waste container a naturally fractured sample of topopah spring tuff approximately 8 cm in diameter and 10 cm long from yucca mountain at the nevada test site nevada was studied using as a pore fluid natural groundwater recovered from a well in which the principal producing horizon is the topopah spring member confining pressure sample temperature and pore pressure were held at values that simulate expected in situ near field conditions shortly after emplacement of the container results of this work are comparable with results from previous similar experiments on 2 54 cm diameter samples of topopah spring tuff during the more than 6 month experiment duration water permeability decreased about 3 orders of magnitude the most rapid measured permeability change between about 600 and 100 darcy occurred when the sample temperature was increased from room temperature to 89 c subsequently water permeability decreased in a fairly monotonie manner to a value of about 1 darcy this behavior probably resulted from a decrease in the aperture of the natural fracture possibly reflecting transport and redeposition of silica which was already present in the fracture this process also resulted in a weak bonding of the sample halves previous experiments on two smaller naturally fractured topopah tuff samples resulted in fracture healing presumably by the same mechanism the degree of healing indicated by bonded tensile strength and change in fluid permeability seems to be sample dependent the distribution of electrical resistivity indicates that the sample dehydrates and rehydrates nonuniformly this is consistent with our findings on smaller samples computed impedance tomographs indicate that during each of the five subsequent dehydrations of the sample water first left the matrix adjacent to the fracture surface then escaped the sample through the fracture aperture drying then progressed into the matrix around the fracture rehydration of the dry sample was not quite the reverse of the dehydration process water initially entered the fracture along the fracture edge and the matrix was then wetted progressively from these points however the matrix was also wetted along paths apparently unrelated to fracture flow

magnesian calcite cements and their diagenesis dissolution and dolomitization mururoa atoll the most ubiquitous syn sedimentary cements affecting mururoa atoll are composed of magnesian calcite three main types are distinguished fibrous bladed and sparitic on the basis of petrography morphology and mgco3 concentration of the constituting crystals while peloid infills a particular form of hmc chemical precipitation also exist petrographic evidence and isotopic signatures are compatible with marine precipitation mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of mg calcite cements these alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology the first stage of the partial dissolution of mg calcite induces the development of chalky white friable zones within the initially crystalline hard cement layers at ultrascale this is due to the creation of micro voids along the elongate cement fibres advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and or between superposed cement layers mg calcite dissolution is related to meteoric diagenesis during periods of quaternary exposure the creation of voids within mg calcite layers is due to the mechanical removal of previously altered calcium carbonate a process suggesting marine or non marine water flow probably in the vadose environment selective dolomitization of mururoa cements involves alternations of calcite and dolomite which form successive cement like rinds within primary cavities at mururoa these alternations are the result of selective dolomitization of the pre existing mg calcite cements rather than successive precipitation of calcite and dolomite selective dolomitization of mg calcite cements at mururoa indicates that a given cement succession is not necessarily a simple chronological sequence oxygen isotope values of dolomites are enriched in 186 by about 3 pdb within calcite dolomite pseudo alternations the dolomitizing fluid at mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization

acoustic measures of partial gas saturation in tight sandstones the acoustic properties of tight sandstones are very sensitive to partial gas water saturation measurements at 5 khz show that compressional velocity drops sharply with undersaturation extensional and shear losses at 5 khz are large in both fully and partially saturated sands the shear loss qs 1 declines linearly with increasing gas content the extensional loss qe 1 peaks at 0 80 water saturation q 1 is frequency dependent the attenuation is well explained by hydrodynamic relaxation in the grain contacts the measurements indicate velocity dispersions consistent with this mechanism ultrasonic velocities at 500 khz are roughly 10 25 higher than acoustic velocities at 5 khz the ultrasonic velocities exhibit a weak dependence on saturation frame moduli are unrelaxed at these frequencies

water saturation effects on elastic wave attenuation in porous rocks with aligned fractures summary elastic wave attenuation anisotropy in porous rocks with aligned fractures is of interest to seismic remote sensing of the earth s structure and to hydrocarbon reservoir characterization in particular we investigated the effect of partial water saturation on attenuation in fractured rocks in the laboratory by conducting ultrasonic pulse echo measurements on synthetic silicacemented sandstones with aligned penny shaped voids fracture density of 0 0298 0 0077 chosen to simulate the effect of natural fractures in the earth according to theoretical models our results show for the first time contrasting variations in the attenuation q 1 o fp and s

dolomitization in a mixing zone of near seawater composition late pleistocene northeastern yucatan peninsula abstract patches of dolomite occur in cores of reefal limestone from the shallow subsurface on the northeastern coast of the yucatan peninsula this limestone accumulated during an interglacial high stand of sea level about 200 000 years ago dolomitization was preceded by freshwater diagenesis including precipitation of sparry calcite cement stabilization of mg calcitic skeletal fragments and partial dissolution of aragonitic components this suggests a predolomitization lowering of sea level with the consequent freshening of pore water the subsequent precipitation of dolomite indicates a return to high sea level with the consequent increase in mg ca ratio of pore water dolomitization took place during a brief high stand of sea level either shortly after deposition about 200 000 yr bp r more likely about 125 000 yr bp dolomite occurs both as microcrystalline replacement dolomite and as cement the cement is part of the following diagenetic sequence 1 limpid euhedral subhedral calcian dolomite crystals 2 zoned dolomite crystals with zones formed by variations of the calcium magnesium ratio in dolomite 3 layers of alternating calcian dolomite and magnesian calcite or calcite and 4 calcite this sequence represents the progressive freshening of ground water during the initial stage of a fall in sea level average cation composition of the limpid dolomite cement is ca57mg43 electron microprobe analysis zoned cement crystals are composed of ca57 59mg43 41 layers and ca62mg38 layers most of the higher calcium dolomite layers are dissolved forming hollow zone crystals in cement with alternating dolomite and calcite zones the calcite is ca99 97mg1 3 low mg calcite and ca96 93mg4 7 mg calcite the dolomite and mg calcite zones are partially to totally leached 18o compositions of yucatecan dolomite and of modern ground water suggest dolomite precipitation from mixed water ranging from about 75 seawater 25 freshwater to nearly all seawater isotope analyses are for the most stable calcian dolomites more soluble calcium rich dolomite presumably is analyzed with calcite and thought to be isotopically lighter than the less soluble dolomite in the cement sequence the most stable dolomite is followed by more soluble dolomite as ground water becomes less saline isotope analyses together with position of dolomite in the cement sequence suggest the most stable calcian dolomite including limpid dolomite precipitated from mixed water with large proportions of seawater and the less stable more calcian dolomite precipitated from fresher mixed water

microfossils from silicified stromatolitic carbonates of the upper proterozoic limestone dolomite series central east greenland silicified flake conglomerates and in situ stratiform stromatolites of the upper proterozoic c 700 800 ma limestone dolomite series central east greenland contain well preserved microfossils five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages providing a broad palaeontological representation of late proterozoic peritidal mat communities comparison of assemblages demonstrates that the taxonomy and diversity of mat builder dweller and allochthonous populations all vary considerably within and among horizons the primary mat builder in most assemblages is siphonophycus inornatum a sheath forming prokaryote of probable but not unequivocally established cyanobacterial affinities an unusual low diversity unit in bed 17 is dominated by a different builder tenuofilum septatum while a thin cryptalgal horizon in bed 18 is built almost exclusively by siphonophycus kestron although variable taphonomic histories contribute to observed assemblage variation most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments twenty two taxa are recognized of which two are described as new myxococcoides stragulescens n sp and scissilisphaera gradata n sp

effects of partial liquid gas saturation on extensional wave attenuation in berea sandstone extensional wave attenuation measurements on berea sandstone were made during increasing imbibition and decreasing drainage brine saturations measurements on samples with both open pore and closed pore surfaces were made using the resonant bar technique the frequency dependence was examined using the forced deformation method the attenuation was found to be dependent on saturation history as well as degree of saturation and boundary flow conditions the sample with open pore surface had a larger attenuation which peaked at greater brine saturations than the sample with closed pore surface during drainage the attenuation reached a maximum at about 90 brine saturation as opposed to about 97 brine saturation during imbibition the variation of the size and number of air pockets within the rock can account for this discrepancy the magnitude of the attenuation peak value decreases substantially with decreasing frequency to the extent that no attenuation peak with saturation was apparent at seismic frequencies say below 100 hz

experimental evidence for the biot gardner theory seismic wave attenuation has been a subject of interest during the last 40 years because it may be of use in interpreting seismic data from this attenuation parameter more detailed information about the lithology of the subsurface may be deduced if we understand the absorption mechanisms by which dissipation of seismic energy is governed we are therefore studying in the laboratory the effects of different parameters such as porosity permeability pore fluid and saturation state on the absorption of seismic waves in porous rocks over a wide spectrum ranging from seismic to ultrasonic frequencies burkhardt et al 1986

the vendian succession of northeastern spitsbergen petrogenesis of a dolomite tillite association abstract the sedimentary record of late precambrian time is magnificently displayed in the highland snowfields of northeastern spitsbergen svalbard vendian strata are represented essentially by the polarisbreen group which consists mostly of dolostone and includes two dolomitic glacial units the oldest sediments in the polarisbreen group compose the elbobreen formation c 400 m which is divided into four laterally persistent members the lower carbonate member e1 125 m contains a distinctive basal dark grey limestone with microspar filled synaeresis cracks suggested to be of lagoonal origin and associated with minor dolostone shale and chert higher parts of the member are dominantly dolostone partly stromatolitic with some shale and sandstone shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite penecontemporaneous dolomite is partially overprinted by microsparry dolomite thought to be of groundwater origin the redefined petrovbreen member e2 consists of diamictite and other detrital dolostone pronounced thickness variations 2 40 m are thought to be original depositional features the member represents the deposits of a short glacial period in which the following depositional processes are inferred lodgement massive diamictite subaqueous meltout massive and bedded diamictite ice rafting lithologies bearing dropstones and possibly also diamictite redeposition by sediment gravity flows some diamictite and conglomerate rhythmite and shale current winnowing thin tabular conglomerate subaerial or subaqueous meltwater action channelled conglomerate and sandstone periglacial shrinkage diamictite wedge fillings the macdonaldryggen member e3 230 m is a monotonous succession of shaly dolostone of lagoonal origin it grades up into the slangen member e4 25 m which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment the wilsonbreen formation 160 m represents a return to glacial deposition but this time longer lasting and with substantial extra basinal material represented the gropbreen member w1 28 107 m and the ormen member w3 44 139 m consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a middle carbonate member w2 3 30 m which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone the same depositional processes can be recognised as in the petrovbreen member but the wilsonbreen formation is overall of somewhat more continental aspect lower proportion of rhythmite and dropstone structures in addition there are basal breccia and crack fillings formed by frost shattering of the underlying cemented dolostone tabular sandstone thought to be formed by wave reworking of outwash a striated terrestrial cobble pavement supraglacially derived breccia horizons periglacial wedges filled by sand and the w2 assemblage of possible lacustrine origin the dracoisen formation 525 m represents an abrupt return to non glacial conditions an upward deepening wave dominated succession of pure dolostone d1 20 m and impure dolostone d2 105 m is succeeded by offshore black shale d3 150 m and then by a very shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics d5 10 m separating dolomitic sandstone and shale d4 80 m and d6 150 m the contact with the transgressive cambrian sandstones above is poorly exposed deposition of the succession dominantly under marine conditions is inferred but it is difficult to rule out a lacustrine environment at any particular horizon this dolomite tillite association can be explained by penecontemporaneous and minor secondary dolomite formation in marginal environments with warm climatic indicators at some levels being sharply interrupted because of rapid climatic changes by glacial sediments containing abundant detrital dolomite since the latter sediments make up only 17 of the 1080m thick succession glacial conditions only occupied a small proportion of vendian time

secular changes in the amount and texture of dolomite the amount of dolomite in cenozoic carbonates is lower than throughout most of the paleozoic a difference that could be due to changes in either chemical conditions or physical conditions that lead to extensive dolomitization the abundance of mimetic texturally retentive dolomites in the cenozoic suggests that chemical conditions were unusually favorable for dolomite formation and therefore it must be physical conditions that explain the paucity of cenozoic dolomites one reasonable explanation for the paucity of cenozoic dolomites is that glacio eustatic sea level fluctuations reduced the amount of time carbonate sediments were in contact with dolomitizing solutions

sedimentology mineralogy and isotopic analysis of pellet lake coorong region south australia gravity cores of holocene sediments from a shallow ephemeral lake in the coorong region pellet lake southeastern coastal australia show a mineral assemblage and sequence particular to its hydrology the mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core i e organic rich aragonite to magnesite hydromagnesite aragonite followed by a relative decrease in salinity i e magnesite aragonite hydrornagnesite to aragonite hydromagnesite in the upper portions of the core this sequence is capped by 0 4 m of micritic dolomite and minor amounts of hydromagnesite with the relative abundance of dolomite increasing upwards three stratigraphically and spatially distinct dolomite units upper lower and margin are recognized using stable carbon and oxygen isotope data unit cell calculations and mgc03 mole per cent data of the dolomite detailed x ray diffraction xrd analyses of samples with more than 80 dolomite shows that the dolomite is ordered average unit cell parameters calculated from the xrd patterns indicate that the upper dolomite unit has crystal lattices expanded in the c direction c 16 09 a relative to ideal dolomite c 16 02 a and contracted in the a direction a 4 796 a relative to ideal dolomite a 4 812 a the mol fraction of mgc03 in the upper dolomite shows up to 4 0 f 2 0 mole per cent excess mg in the dolomite crystal lattice calculated from xrd this unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated mg ca ratios transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution the modulated microstructure found in calcium rich dolomite is completely lacking dolomite ordering reflections are present in electron diffraction patterns but are weak stable oxygen and carbon isotope values of the upper dolomite are tightly grouped ave 6l80 x 7 55 6i3c 4 10 yet show three upward lightening oxygen cycles the oxygen cycles correlate with three upward decreases in the calculated mg content of the dolomite zone these cycles may indicate the increased importance of rain water dilution of the brine at times when the water in the lake was at its shallowest levels analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit the lower dolomite is close to stoichiometric has less evaporitic stable isotope values than the upper dolomite and has only a slightly expanded c axis the margin dolomite is ca rich has a more homogeneous microstructure and has expanded a and c axes the abundance of relatively soluble mg bearing phases such as hydromagnesite and magnesite may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment this process may leave a finely laminated dolomicrite deposit which retains little if any evidence of evaporite minerals

coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification draken formation upper riphean svalbard the draken formation 120 250 m of northeast spitsbergen svalbard forms part of a thick upper proterozoic carbonate platform succession it consists predominantly of intraformational dolomitic conglomerates with excellent textural preservation six main lithofacies were recognized in the field quartz sandstones stromatolitic mats conglomerates with silicified intraclasts dolostone conglomerates with desiccated mudrocks oolitic pisolitic grainstones and fenestral dolostones a series of five main gradational biofacies were recognized from silicified and rare calcified microfossils biofacies 1 represents low energy subtidal benthos erect filaments and plankton acritarchs and vase shaped microfossils whereas biofacies 2 to 5 are microbial mat assemblages with filamentous mat builders and associated dwellers and washed in plankton ranging from basal intertidal to high intertidal supratidal colour values a measure of the lightness of the colour shade of sawn rock samples were quantified using a munsell chart and exhibit a pronounced variation means of major groups varying from 4 0 to 5 95 across the spectrum of subtidal to supratidal sediments as inferred from other criteria the lightening in progressively more exposed sediments is related to lowering of organic carbon contents probably mainly by oxidation six types of early cement have been recognized calcite microspar type 1 is common as a subtidal cement in many proterozoic formations whereas types 2 subtidal isopachous fringes 3 subtidal hardground dolomicrite and 4 intertidal meniscus dolomicrite are very similar to phanerozoic examples except for their dolomitic mineralogy types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena they characterize the fenestral lithofacies and compare with modern supratidal cements consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching dolomitization silicification were all significant syndepositional processes altering the original metastable carbonates the data set provides evidence for a spectrum of peritidal environments including ooid shoals protected subtidal tidal sandflats and protected carbonate mudflats different sections show a preponderance of particular facies the coastal lithofacies continuum was completely dolomitized unlike offshore to ooid shoal facies of adjacent formations dolomitization thus bears a relationship to depositional bathymetry although hydrodynamics clearly have a role the potential importance of whiting precipitation in raising mg ca in marginal marine environments is also stressed

eocene spheroidal dolomite from the western sirte basin libya scanning electron micrographs of crypto crystalline 4 m dolomites found in cores taken from the lower eocene facha member reservoir of the zella and aswad fields in libya show that they consist of tiny globular bodies mixed with anhedral crystals producing a chalky texture the sedimentary sequence indicates deposition in upper intertidal supratidal arid environments and in saline lagoons the crypto crystalline dolomite is facies controlled and probably formed as a primary precipitate similar to those reported from present day subaqueous environments in the coorong of south australia and holocene supratidal crusts off florida such chalky textured dolomites could form local fluid migration barriers within hydrocarbon reservoirs

the compositional evolution of meso and neoproterozoic carbonate rocks abstract petrochemical data on meso and neoproterozoic riphean and vendian carbonates from the urals and siberia suggest that upper riphean carbonate rocks differ from both older and younger equivalents in their lower dolomite limestone and higher strontium abundance it is suggested that the directional increase in the ca content of the earth s sedimentary shell results from the hydrothermal interaction of seawater with oceanic basalts manifest in a progressive reduction of dolomite formation throughout geological history superimposed cyclical variation in dolomite limestone may reflect secular changes in the rate of co 2 introduction into the atmosphere in particular the relative low dolomite limestone ratio seen in upper riphean carbonates may reflect low p co 2

a review of the origin and setting of tepees and their associated fabrics carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal lacustrine and subaerial carbonate shelf sediments these are commonly disrupted and brecciated when the surface area of these crusts increases in the subtidal environment megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand similar megapolygons form in the supratidal lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and or cement also causes the area of the hardgrounds to expand the arched up antiform margins of expansion megapolygons are known as tepees the types of tepees found in the geological record include 1 submarine tepees which form in shallow carbonate saturated waters where fractured and bedded marine grainstones are bound by isopachous marine phreatic acicular and micritic cements the surfaces of these brecciated crusts have undergone diagenesis and are bored unlike tepees listed below they contain no vadose pisolites or gravity cements 2 peritidal and lacustrine tepees are formed of crusts characterized by fenestral pisolitic and laminar algal fabrics this similarity in fabric makes these tepees of different origins difficult to separate peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical they are associated with fractured and bedded tidal flat carbonates their fracture fills contain geopetal asymmetric travertines of marine vadose origin and or marine phreatic travertines and or terra rossa sediments the senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common they include groundwater tepees which form over evaporitic boxwork carbonates and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas but the dominant sediment type is carbonate mud these latter tepee crusts are coated and crosscut by laminated micrite the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions in time intervals of days to years cement morphologies reflect this and the crusts often contain gravitational meniscus vadose cements as well as phreatic isopachous cement rinds 3 caliche tepees which are developed within soil profiles in a continental setting they are formed by laminar crusts which contain pisolites and fractures filled by micritic laminae microspar spar and terra rossa most of the cements are gravitational and or meniscoid in ancient carbonates when their cementation and diagenetic fabric can be interpreted tepee structures can be used as environmental indicators they can also be used to trace the evolution of the depositional and hydrological setting

laboratory measurements of seismic attenuation in sandstone strain versus fluid saturation effects abstractin the past few decades great attention has been focused on uncovering the physics of seismic wave attenuation in fluid saturated rocks however the relationship among many variables affecting attenuation is still not completely clear for instance although the role of strain in enhancing friction dissipation is relatively well known for dry rocks it remains unclear how and how much it affects attenuation in fluid saturated rocks we experimentally measured attenuation in the extensional mode in berea sandstone at strains between 7 8 10 7 and 1 9 10 5 and at frequencies in the seismic bandwidth 1 100 hz these strains were similar to those typically observed in seismic exploration 10 6 we also measured the transient fluid pressure caused when a stepwise stress was applied resulting in such strains for the studied strain range our results indicated that 1 the overall attenuation in dry berea sandstone increased linearly with strain 2 the frequency dependent component of attenuati

cement origin of supratidal dolomite andros island bahamas abstract dolomite in supratidal carbonates of northwest andros island bahamas can be seen by means of scanning electron microscope sem to be cement in primary porosity and not replacement of the carbonate sediments as one widely held view suggests these microcrystalline calcian dolomite cements commonly occur in lithified lime mud crusts but they also occur in unconsolidated sediments mainly as intragranular precipitates the voids in which the dolomite occurs vary in size from minute wall pores in rotaliid forams and endolithic borings to major intra and interparticle pores in the larger pores dolomite commonly forms an isopachous fringe and may be followed by either aragonite or high magnesium calcite hmc cement silt sized bioclasts and pellets within crusts are often lithif ed by dolomite cement which may totally fill much of the interparticle porosity within unpelleted lime mud the interparticle porosity may also be filled by dolomite which engulfs the needle mud particles the needle mud inclusions or molds visible on polished etched sections of this dolomite filled mud are equivalent in abundance size and shape to the mud particles in non dolomitic unlithified muds embedded in plastic that equivalence indicates that dolomite replacement of mud has not occurred framboidal pyrite present in a few samples appears to have formed prior to or contemporaneously with dolomite cement the presence of pyrite and the depleted 13c values 3 2 to 3 9 per mil of dolomite cemented crusts sug est that sulfate reduction may have had a role in the formation of andros island dolomite cement other holocene and older dolomites from similar settings should be reexamined by means of sem to determine if they formed partly or wholly as a cement rather than by replacement

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

salina margin tepees pisoliths and aragonite cements lake macleod western australia their significance in interpreting ancient analogs tepee structures banded aragonite cements and pisoliths are currently forming in lake macleod a carbonate evaporite salina in western australia although lake macleod is separated from the indian ocean by a barrier it lies 3 4 m below sea level which promotes the seepage of seawater through the barrier and its discharge from vents and seepage mounds around the margin of the salina discharging waters have precipitated and diagenetically altered carbonate sediments within these seepage mounds to form tepee structures of lithified protodolomite overlying cavities that are lined with banded aragonite cement and floored by both cement and pisoliths significant variations in 18o 5 1 pdb and 13c 5 5 pdb of the aragonite cements were documented and are thought to record shifts in the isotopic composition of the water brought about by the effects of evaporation influx of meteoric water and oxidation of organic water carbon 14 dating of cements indicates that cementation began about 3400 b p and has proceeded at a rate of about 0 2 to 0 4 mm 100 yr the highest rate occurring during evaporative episodes by analogy with lake macleod and other australian salinas peritidal tepee structures and associated diagenetic carbonates in the permian capitan reef complex may owe their origin to speleanlike diagenesis operative in a marine groundwater discharge zone

facies specific calcitic and bimineralic ooids from middle and upper cambrian platform carbonates western newfoundland canada abstract oolites in the port au port group consist of intercalated gray oolite deposited on the shallow subtidal mobile fringe of ooid sand shoals and brown oolite which accumulated on intertidal sand flats along the shoal crests gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe the present fabric of the ooids is the result of protracted burial diagenesis cortices of finely and coarsely preserved radial ooids excluding radial concentric ones and superficial radial ooids were mg calcite originally they changed to calcite still retaining fine fabric and subsequently to a variety of cortical fabrics during shallow burial and altered to blocky fabrics during deep burial concentric ooids and radial concentric ooids were bimineralic originally composed of aragonite now micrite and blocky calcite and mg calcite now radial fibrous calcite primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial resulting voids were filled with blocky calcite during deep burial correlation of original ooid mineralogy fabric and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in cambrian time this correlation is absent in modern deposits suggesting that more widespread chemical and organic controls now prevail

velocity and attenuation in partially saturated rocks poroelastic numerical experiments we use a poroelastic modelling algorithm to compute numerical experiments on wave propagation in a rock sample with partial saturation using realistic fluid distribution patterns from tomography scans frequencies are in the range 10 to 500 khz the rock is a homogeneous isotropic sandstone partially filled with gas and water which are defined by their characteristic values of viscosity compressibility and density we assume no mixing and that the two different pore fills occupy different macroscopic regions the von karman self similar correlation function is used employing different fractal parameters to model uniform and patchy fluid distributions respectively where effective saturation is varied in steps from full gas to full water saturation without resorting to additional matrix fluid interaction mechanisms we are able to reproduce the main features of the variation in wave velocity and attenuation with effective saturation and frequency as those of published laboratory experiments furthermore the behaviour of the attenuation peaks versus water saturation and frequency is similar to that of white s model the conversion of primary p wave energy into dissipating slow waves at the heterogeneities is shown to be the main mechanism for attenuating the primary wavefield fluid gas patches are shown to affect attenuation more than equivalent patches in the permeability or solid grain properties

holocene dolomitization of supratidal sediments by active tidal pumping sugarloaf key florida abstract sugarloaf key is an area where holocene dolomitization is occurring calcium rich dolomite is found in a 0 25 to 10 cm thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted pleistocene miami oolite radiocarbon ages of the crust range from 160 years b p to 1420 years b p with a corresponding increase in dolomite content from 0 to 80 the relatively high permeability of the underlying pleistocene oolite and low permeability of the holocene carbonate mud results in a tidal lag between surface water and the partly confined aquifer consequently seawater is pumped upward and downward through the holocene sediment during spring tides the highest concentration of dolomite is found in areas where the sediment layer is thinnest and tidal pumping is most effective limited analyses of surface and subsurface water taken at intervals throughout the pumping cycle suggest that the dolomitizing fluid is essentially florida bay water the earliest diagenesis is caused by precipitation of dolomite cement which occurs as 0 1 to 0 3 micron subrounded crystallites that show no distinct crystal form during further cementation and somewhat later during replacement of preexisting crystallites poorly ordered dolomite forms as 1 to 5 micron euhedral rhombs x ray diffraction data indicate that the crystallized dolomite is better ordered and less calcium rich than the dolomite composed of crystallites

pedogenetic and diagenetic fabrics in the upper proterozoic sarnyere formation gourma mali abstract several diagenetic fabrics are described in the uppermost member of the sarnyere formation an upper proterozoic dolomite 700 600 ma micronodular micrite micro sparite showing nodulisation processes orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal porostromata boundstone further evolution in a vadose zone creates a network of cavities which are later filled by cave stromatolites micropopcorn micrite and multilayered cements a rhythmical repetition of facies evidences a polyphased pedogenetic history leading to a 30 m thick highly complex section such numerous and diversified structures are rarely preserved especially in proterozoic rocks but each of them have equivalents in recent examples of caliche soil profiles

sedimentary textures of recent belizean peritidal dolomite abstract recent surface and buried dolomite crusts and massive dolomite beds on ambergris cay belize have formed on topographically low relief peritidal flats whereon the hydrologic system is characterized by a dominant phreatic diagenetic overprint as a consequence of their low subaerial exposure index such dolomites and associated undolomitized surface sediments lack many of the textural and microfabric features that are considered diagnostic of the peritidal environment the progressive dolomitization of precursor sandy fossiliferous sediments wackestones to packstones is accompanied by a textural change to dolowackestones with allochem ghosts and a porosity system that resembles but is distinct from syndepositional fenestral fabrics formed in unconsolidated or partly consolidated peritidal sediments the microcrystalline dolomite cements in these rocks are predominantly of phreatic aspect and only rare examples of possible vadose diagenetic fabrics are observed these texturally anomalous peritidal dolomites characterized by a markedly subtidal appearance may have fossil counterparts in many ancient carbonate tidal flat sequences

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

biot gardner theory of extensional waves in porous rods many measurements have been made on fluid saturated porous rods executing extensional flexural and torsional motion measurements for extensional and flexural motion yield a loss parameter for young s modulus waves qy and the measurement for torsional motion yields qs for shear waves qp has then been calculated for compressional waves in bulk rock on the assumption that the fluid saturated rock is an isotropic solid i point out the fallacy of computing qp from these measurements and also urge workers to recognize the losses due to simple fluid viscosity in interpreting their data on extensional waves in rods by application of published theory i show that peaks in attenuation of extensional waves are to be expected at frequencies of several hertz to several kilohertz depending upon rod radius computed curves are compared with published measurements on navajo sandstone saturated with water ethanol and n decane in each case computed peak frequency agrees with published measurements shift of th

sedimentary talc in neoproterozoic carbonate successions abstract mineralogical petrographic and sedimentological observations document early diagenetic talc in carbonate dominated successions deposited on two early neoproterozoic 800 700 million years old platform margins in the akademikerbreen group svalbard talc occurs as nodules that pre date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates in the upper fifteenmile group of the ogilvie mountains nw canada the talc is present as nodules coated grains rip up clasts and massive beds that are several meters thick to gain insight into the chemistry required to form early diagenetic talc we conducted precipitation experiments at 25 c with low so4 synthetic seawater solutions at varying ph mg2 and sio2 aq our experiments reveal a sharp and reproducible ph boundary at 8 7 only above which does poorly crystalline mg silicate precipitate increasing mg2 and or sio2 aq alone is insufficient to produce the material the strong ph control can be explained by mg silica complexing activated by the deprotonation of silicic acid above 8 6 8 7 ft ir tem and xrd of the synthetic precipitates reveal a talc like 2 1 trioctahedral structure with short range stacking order hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite hydrated talc and eventually to talc this formation pathway imparts extensive layer stacking disorder to the synthetic talc end product that is identical to neoproterozoic occurrences early diagenetic talc in neoproterozoic carbonate platform successions appears to reflect a unique combination of low al concentrations and by inference low siliciclastic input near modern marine salinity and mg2 elevated sio2 aq and ph 8 7 because the talc occurs in close association with microbially influenced sediments we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry specifically ph and alkalinity increases driven by anaerobic fe respiration

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

platform to basin facies evolution the carbonates of late proterozoic vendian gourma west africa abstract during the vendian uppermost proterozoic the gourma basin was an east west intracratonic feature that opened eastward into an ocean the southern edge of the gourma basin preserves facies belts representing land to deep basin environments shoreward facies consist of tidal belts of microbially laminated dolomites or porostromata dolomites interstratified with clastic carbonates of local origin landward supratidal facies consist of finely laminated sedimentary rocks with sizeable desiccation features but no evaporites the upper part of this unit is affected by a periodic emersion resulting in a complex succession of paleosol profiles offshore of the tidal facies are lagoon facies rocks that were protected from the open sea by a carbonate buildup composed of a framework of stromat lite bioherms associated with detrital dolomites indicating a high energy environment slope facies consist of a thick succession of ribbon limestones with some graded beds synsedimentary slumping and faulting of the ribbon limestones indicate a significant depositional slope allochthonous deposits are sporadically interstratified with the ribbon limestones and consist of muds with small grains derived from the platform deposits polygenic breccias conglomerates composed only of ribbon limestone intraclasts and oolitic calcarenites shelf facies rocks are pervasively dolomitized but original textures are well preserved including fine grained structures the ribbon limestones on the other hand are not dolomitized except for dolomitic layers caused by pressure solution the distribution of facies in gourma basin carbonates corresponds well with ideal models of wilson 1975 comparisons with present day facies distributions are less satisfactory especially considering the dominance of cyanobacteria over all environments for which there is no modern day counterpart comparisons with the modern environment however suggest a subtropical semi arid climate during deposition of the gourma strata

diagenesis of late proterozoic carbonates the beck spring dolomite of eastern california abstract petrographic and compositional attributes of limestone and dolostone in the beck spring dolomite of eastern california indicate that late proterozoic mineralogies isotopic compositions and diagenetic processes were similar to those inferred for younger phanerozoic marine carbonate sequences allochems in limestones include strontium rich ooids with well preserved concentric banding and cortices of equant neomorphic calcite that commonly exhibit dissolution and compaction features cements include microdolomite rich columnar marine calcite and equant spar calcites are depleted relative to dolomites and become enriched with stratigraphic depth acicular and columnar marine cement and allochems in dolostones display fabrics identical to those in limestones but are isotopically distinct dolomite ooids columnar and acicular cement crystal silt and early luminescent spar cement from four stratigraphic sections define overlapping positive oxygen carbon covariant fields that trend toward depleted calcite compositions early luminescent dolomite spar cement is unconformably overlain by depleted ferroan dolospar fabric data suggest that marine cement and ooids were precipitated as magnesian calcite and aragonite isotopic data suggest that metastable components locally stabilized to calcite during subaerial exposure while laterally dolomitization took place in mixed marine meteoric waters early dolospar precipitation began during replacive dolomitization and late ferroan dolospar precipitated during burial lack of evidence for hypersaline conditions suggests that late proterozoic normal marine carbonate isotopic composition was 0 5 2 018o and 5 3 0 5913c

origins of carbonate in neoproterozoic stromatolites and the identification of modern analogues abstract the viewpoint in this paper is that the distinction of detrital from in situ precipitated carbonate in stromatolites is vital so that the appropriate modern analogues can be sought reviewing literature on modern stromatolites it is clear that marine stromatolites are dominated by detrital sediment although diagenetic processes can obscure the detrital nature of carbonate in mats in low salinity conditions carbonate is dominantly of precipitate origin being triggered by photosynthesis in mats in saline conditions petrographically similar precipitates occur apparently triggered variously by photosynthesis evaporation and organic matter degradation in different cases light carbon isotopes in the carbonate are a good pointer to organic degradation as a carbon source a review of criteria proposed for identifying the origin of carbonate in precambrian stromatolites shows that in different cases reliable evidence both for sediment incorporation and carbonate precipitation has been obtained although many arguments are equivocal the utility of luminescence petrography for positive identification of detritus and carbon isotope data for identification of carbon sources is outlined new petrographic and carbon isotope data are presented from four units canyon formation of east greenland draken and wilsonbreen formations of spitsbergen and atar group mauritania although environmentally diverse each displays evidence for both sediment incorporation and localized carbonate precipitation although in widely varying proportions it is recommended that luminescence studies be carried out for stromatolite microstructural work carbon isotope data indicate no distinction between early inorganic cements stromatolitic and non stromatolitic dolo micrites and indicate a common origin related to water column and microbial mat photosynthesis variably accompanied by evaporation

stromatolitic bioherms and microphytolites from the late proterozoic draken conglomerate formation spitsbergen abstract the late proterozoic draken conglomerate formation akademikerbreen group in eastern spitsbergen displays several prominent stromatolitic bioherms in its lowermost beds the dolomitic bioherms are comprised principally of columnar stromatolites cf gymnosolenoides but include cross bedded microphytolitic grainstones the excellent exposures of the bioherms and microphytolitic grainstones along the margin of petrovbreen glacier and the exquisite preservation of structural and microscopic details permit the documentation and interpretation of these sedimentary and biological associations locally favorable ecologic circumstance or chance colonization initiated stromatolitic mat development which then became self perpetuating to form bioherms until rapid siliciclastic influx overwhelmed the mat building populations intermediate energy conditions in concert with microbial activity and inorganic processes generated the microphytolitic grainstones the interpretation of the biological and sedimentological interactions within these biohermal carbonates microphytolitic grainstones and siliciclastic rocks affords new insights into proterozoic paleoenvironments

calcitic aragonitic and mixed calcitic aragonitic ooids from the mid proterozoic belt supergroup montana a great variety of ooid types occurs within the siyeh and snowslip formations of the mid proterozoic belt supergroup montana cortical layers are inferred to have been composed either of calcite in a radial concentric or radial with dark rays fabric or aragonite in a radial or concentric fabric the calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure others consist of calcite spar without inclusions or columnar calcite which grew radially from the nucleus commonly a calcitic ooid some ooids were wholly composed of calcite others were of aragonite but two phase ooids were common mostly consisting of an inner calcitic part and an outer aragonitic part probable microdolomite inclusions suggest a high mg content of the calcitic cortical layers the depositional environment of these oolites was probably analogous to baffin bay texas where a similar range of ooid types is forming today

neomorphism and cementation in ancient deep water limestones cow head group cambro ordovician western newfoundland canada abstract lithification of thinly bedded lime mudstones and grainstones in the cow head group resulted mainly from precipitation of calcite cement and neomorphic spar at shallow burial depths below the sea floor these crystals are predominantly non ferroan although the iron content and the size of neomorphic spar commonly increase near the margins of nodular and continuous beds of lime mudstone displacive fringes of ferroan fibrous calcite represent the last stage of carbonate precipitation in some limestones most calcite 13c values range between 0 and 3 pdb but 13c values as negative as 15 indicate that bacterial degradation of orgnaic matter variably influenced pore water composition serial analyses from the centres to the margins of nodular and continuously bedded lime mudstones suggest that many of the limestones in this sequence were lithified prior to the precipitation of calcite having the prominent negative 13c values typical of concretionary carbonate calcite 18o values range from 5 to 8 pdb indicating precipitation from 18o depleted pore waters of uncertian origin cathodoluminescence shows that aggrading neomorphism in the lime mudstones did not take place by porphyroid or coalescive consumption of a more finely crystalline mosaic to yield a more coarsely crystalline mosaic instead scattered crystals enlarged by precipitation of progressively younger and eventually ferroan calcite especially at the margins of nodular and continuous limestone beds the cathodoluminescence zoning in calcite crystals is similar to that observed in meteorically altered limestones where pore waters were initially oxidizing but became progressively more reducing with time the preservation of trace element zoning crystal growth microfabrics and 13c values in authigenic calcite suggest collectively that recrystallization did not occur during subsequent burial

rock anelasticity measurements at high pressure low strain amplitude and seismic frequency a new apparatus has been constructed which will ultimately facilitate the study of rock anelasticity under conditions which closely approach those of teleseismic wave propagation simultaneous high pressure and temperature low frequency and strain amplitude and controlled pore pressure of volatiles its performance has been tested in a series of preliminary experiments conducted at room temperature measurements on a steel standard demonstrate the sensitivity of the apparatus to very small departures from ideal elasticity q 1000 experimental data for a fine grained granitic rock show that both the shear modulus g and quality factor q increase sharply with increasing pressure below 100 mpa beyond which pressure both parameters become markedly less pressure sensitive these observations are in accord with those of previous studies at higher frequencies and larger strains and are consistent with the view that the anelasticity of rocks at ambient pressure is dominated by mechanisms operative at open cracks and grain boundaries