evaporitic mixed water dolomitization on st croix u s v i abstract dolomite exists in only a small area of pliocene carbonates on st croix this area consists of reef lagoonal and platform facies that underlie and rim the shoreline of krause lagoon an embayment that existed on the southern coastline of st croix prior to industrial development in the 1960s since dolomite is not found in similar facies elsewhere on st croix this geographic distribution suggests that the formation of the dolomite was related to hydrologic conditions in krause lagoon the dolomite is calcium rich 57 60 mol ca and exists both as euhedral rhombs 2 75 m in diameter and as a replacement mineral in dolomitized bioclasts bioclasts in the dolomitic strata often show good preservation of microstructure particularly in large benthic forams and coralline algae the mean 87sr 86sr isotopic composition of the dolomite is 0 70887 0 00002 2s n 3 which corresponds to the 87sr 86sr ratio of late miocene seawater however because the dolomite resides in pliocene strata it is difficult to invoke unmodified seawater as an agent of dolomitization dolomitization therefore requires a source of nonradiogenic strontium modern st croix groundwater has 87sr 86sr compositions between 0 7076 and 0 7085 n 4 well below the ratio of both modern seawater and the dolomites mixing calculations show that modern st croix groundwater could be a significant source of nonradiogenic strontium in a dolomite formed f om a two component groundwater seawater mix on the basis of strontium concentration modeling the groundwater component responsible for the st croix dolomites may have ranged from 40 to 80 of the dolomitizing fluids stable isotopic values for the dolomite range from d18o of 0 7 to 3 8 and d13c of 0 6 to 2 4 pdb with increasing d18o and d13c values from the margins to the center of the lagoon the maximum d18o values in these dolomites are too high to have formed fro unaltered groundwater or seawater even accounting for ice volume effects therefore the isotopically heaviest dolomite must have precipitated from fluids enriched in 18o probably as a result of evaporation dolomitization from fluids produced from a mixture of evaporated seawater and st croix groundwater are consistent with the geochemistry and geologic distribution of the dolomite calculations show that such a scenario is possible and may be fairly common despite the relative complexity of the model documented block faulting of the krause lagoon area may have provided a stable hydrologic regime for a long enough time for dolomite to form despite island uplift during the late tertiary other models of dolomitization can be shown to be less likely or untenable on the basis of chemical lithologic or hydrologic criteria

unconventional pattern of reservoir facies distribution in epeiric successions lessons from an outcrop analog lower keuper germany this article presents 1 a process oriented description of an outcrop analog for epeiric successions 2 a discussion of fundamental characteristics of epeiric basin fills and 3 a generic depositional model which may be used for improved reservoir prediction in epeiric settings the case study focuses on the upper ladinian mixed siliciclastic carbonate lower keuper formation it is composed of storm generated tide generated and bioturbated facies paleogeographically the succession shows an unusual lateral energy zonation comprising a seaward low energy zone close to wave base an intermediate reservoir prone high energy zone within wave base and a landward low energy zone above wave base stratigraphically the lower keuper formation is subdivided into meter scale transgressive regressive cycles correlatable over distances of more than 500 km cycle boundaries are interpreted to be isochronous within the resulting chronostratigraphic framework facies distributions have been mapped out in three dimensions revealing that reservoir prone facies are most extensive in shoreline detached positions reservoir bodies are thin have sheetlike geometries but extend for several tens of kilometers and are stacked in an aggradational facies architecture additionally the reservoir prone facies is thicker developed in zones of stronger subsidence a few kilometers wide linked to underlying basement blocks resulting unconventional reservoir prediction strategies for similar shallow marine epeiric settings should target shoreline detached high energy facies belts that are preferentially developed within zones of stronger subsidence controlled by basement tectonics

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

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

dolomitization kinetics of hydrothermal bombs and natural settings abstract during high temperature 150 300 c dolomitization experiments the rate of dolomitization increases with temperature reactant surface area reactant solubility ionic strength and mg2 ca2 of the solution one would predict these results from simple kinetic theory the discoveries that so42 slows the rate li increases the rate and the induction period is long for dolomitization could not have been predicted dolomitization is a three step reaction 1 nucleation nucleation of very high mg calcite 35 40 mole mgco3 vhmc or nonstoichiometric dolomite is followed by nucleation of more stoichiometric dolomite on caco3 the composition of the vhmc or nonstoichiometric dolomite is a function of mg2 ca2 of the solution other variables such as dissolved fe can affect the composition of the nuclei 2 induction period nucleation occurs during the induction period but the most of this period is post nucleation growth of vhmc nonstoichiometric and stoichiometric dolomite nuclei changes in the solution and substrate surface area affect the duration of the induction period 3 replacement period a caco3 is replaced by vhmc or nonstoichiometric dolomite vhmc nucleates faster than stoichiometric dolomite and therefore begins to replace the reactant first b caco3 and vhmc and or nonstoichiometric dolomite are rapidly replaced by stoichiometric dolomite this phase of the reaction is sensitive to solution variables such as mg2 ca2 and ionic strength the three phase reaction model is consistent with eight characteristics of natural dolomites 1 very ca rich dolomite is common only in modern dolomites 2 there is a direct relationship between mg2 ca2 00 solution and mg ca ratio in the dolomite 3 the stoichiometry of dolomites in some ancient rocks is directly related to the percentage of dolomite in the rock 4 suppression of stoichiometric dolomite nucleation allows the persistence of metastable ca mg co3 phases 5 dolomite limestone contacts are often sharp 6 dolomite selectively replaces fine grained caco3 7 dolomite crystals often have cloudy centers and clear rims 8 dolomite textures are mainly determined by the crystal size of the reactant

soil containing rock fragments effects on infiltration abstract infiltration is a very complex process rock fragments affect infiltration by their presence in the soil matrix and on the soil surface procedures for quantifying soil and surface rock fragments are reviewed within the soil the physical properties and hydraulic properties that affect infiltration are shown to be influenced by rock fragments physical properties discussed are bulk density porosity and water content hydraulic properties that affect infiltration are water retention saturated conductivity and unsaturated conductivity rock fragments on the soil surface have been reported to both increase or decrease infiltration rates and amounts

influence of climate and dolomite composition on dedolomitization insights from a multi proxy study in the central oman mountains abstract dedolomitization is an important diagenetic process that can yield information on the history of paleo fluids in a given aquifer or outcrop and by extension it offers a window into past hydrologic conditions dedolomitization is also relevant economically as it can alter the porosity and permeability of carbonates thus affecting reservoir quality despite considerable research the process of dedolomitization is still not entirely understood here new findings from the central oman mountains highlight the importance of dolomite chemistry and fluctuating climate on dedolomitization petrographic mineralogical and geochemical data from both altered and pristine dolomite hosted in jurassic carbonate rocks from the sahtan group and outcropping at wadi mistal in the jebel akhdar tectonic window reveal two dolomite bodies with different characteristics stratabound and fault related dolostone the ferroan dolomite crystals are larger in the fault related dolostone whereas the non ferroan dolomite crystals are smaller in the stratabound dolostone petrography reveals a complex suite of alteration textures including pristine dolomite recrystallized dolomite and calcitized dolomite dedolomite iron oxides are present pervasively in the altered rocks and different alteration textures co occur in the same sample relative to unaltered dolomite in this outcrop the recrystallized dolomite is characterized by similar positive carbon isotope values but less negative oxygen isotope values and the calcitized dolomite is characterized by even less negative oxygen isotope values and more negative carbon isotope values based on field data and petrography two phases of alteration are evident an earlier dedolomitization event affecting the stratabound dolomite only and a more pervasive pleistocene holocene alteration phase associated with surface weathering that affected both the fault related dolomite and to a lesser extent the stratabound dolomite stable isotope results further suggest that the more recent subaerial weathering phase happened under two alternating climate regimes dedolomitization was triggered by meteoric fluids that interacted with soil related carbon during humid climatic period s whereas recrystallization of ferroan fault related dolomite happened during more arid climatic period s with less developed soils and lower fluid rock interaction these results suggest that weathering of ferroan fault related dolomite can lead to formation of goethite and recrystallized non stoichiometric dolomite with a reset stable oxygen isotope signature and sometimes to dedolomitization

runoff and sediment yield from topsoils with different porosity as affected by rock fragment cover and position abstract interrill runoff and sediment yield from a topsoil having essentially textural pores and from another with structural pores were measured in the laboratory under simulated rain to investigate the effects of cover and positions of rock fragments the results reveal that the relation between the cover of rock fragments and interrill runoff or sediment yield can be negative as well as positive porosity in the soil near rock fragments determines both the type of relation and the degree to which the positions of rock fragments affect this relation rock fragments increase runoff and sediment yield when well embedded in a surface seal i e a top layer with essentially textural pore spaces a negative relation occurs either where rock fragments are partly embedded in a top layer with structural porosity or where the rock fragments rest on the surface of a soil having either textural or structural pore spaces the time at which macropores at the soil surface close is crucial for then the effect of the cover of rock fragments on the generation of runoff switches from reduction to an increase since cover can have an ambivalent effect on the hydrological and erosion response descriptions of topsoils containing rock fragments should include the type of soil porosity as well as the positions of rock fragments

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

the role of the subcutaneous zone in karst hydrology abstract the subcutaneous zone is the upper weathered layer of rock beneath the soil but above the permanently saturated phreatic zone it is of particular hydrological importance in karst because of its high secondary permeability arising from the considerable chemical solution in this zone however corrosional enlargement of fissures diminishes with depth thus permeability decreases in the same direction with the result that percolation is inhibited except down widened master joints and faults storage of water consequently occurs in this zone particularly after storms the upper surface of this suspended saturated layer in the subcutaneous zone is defined by a perched water table which slopes towards points of rapid vertical percolation the potential induces lateral water movement converging on the most permeable areas such as beneath dolines leakage from the subcutaneous store sustains slow percolation in the vadose zone cross correlation of rainfall with percolation rates in caves in new mexico u s a and new zealand reveal response lags of 2 14 weeks with no apparent relationship to depth below the surface other percolation sites show no correlation with rainfall interpreted as being a consequence of considerable friction in tight fissure networks the recognition of storage and rapid as well as very slow percolation from the subcutaneous zone requires re interpretation of the components of hydrographs from karst springs and of some conceptual models of karst aquifers the importance of subcutaneous storage in sustaining baseflow discharge at some sites must be recognised as must the contribution of subcutaneous water to flood hydrographs methods of estimating the volumes of subcutaneous and phreatic components of karst spring flood hydrographs are presented the paper concludes with a discussion of the significance of subcutaneous hydrologic processes for an understanding of karst geomorphology the desirability of explaining karst landform evolution in terms of hydrologic processes is stressed

soil hydraulic properties on the steep karst hillslopes in northwest guangxi china soil hydraulic properties such as soil infiltration rate and hydraulic conductivity are closely linked to runoff generation and infiltration processes but little is known about them on karst hillslopes the objectives of this paper were to investigate the change in soil stable infiltration rate q s and near saturated hydraulic conductivity k ns in different slope positions and to understand their relationship with rock fragment content and soil texture within the topsoil in subtropical karst regions of southwest china tension infiltrometers 20 cm in diameter were used to measure q s and k ns at pressure head of 20 mm on hillslopes 1 a disintegrated landslide failure and 2 an avalanche slope the change of q s and k ns was great and they mostly had a moderate variability with coefficient of variations cv between 0 1 and 1 0 in the different slope positions on average q s ranged from 0 43 to 4 25 mm min and k ns varied from 0 75 to 11 00 mm min these rates exceed those of most natural rainfall events confirming that overland flow is rare on karst hillslopes from bottom to top q s and k ns had a decrease increase decrease trend due to the presence of large rock outcrops 2 m in height on hillslope 1 but had an increasing trend on hillslope 2 with less complex landform they tended to increase with increase in total rock fragment content 5 250 mm within the topsoil as well as slope gradient on both hillslopes pearson correlation analysis suggested that higher coarse pebble 20 75 mm cobble 75 250 mm and sand 2 0 05 mm contents as well as total rock fragment content could significantly facilitate water infiltration into soils but higher clay 0 002 mm content could restrict water movement this result indicated that rock fragment sand and clay contents may remarkably affect water flow in the topsoil layers and should be considered in hydrological modeling on karst hillslopes in subtropical regions

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

the role and impact of freshwater seawater mixing zones in the maturation of regional dolomite bodies within the proto floridan aquifer usa palaeogene dolostones from the sub surface of florida are ideal for the study of dolomite maturation because they record the early stages of a secondary dolomite overprint without destruction by later diagenetic overprints two distinct dolomite textures occur in the dolostones of the upper eocene ocala and lower oligocene suwannee limestones in west central florida a porous and permeable sucrosic dolomite and a less porous and relatively impermeable indurated non sucrosic dolomite in both textures the initial matrix dolomite is dully luminescent whereas the secondary overprint is dominantly luminescent cement in the suwannee and only neomorphic luminescent dolomite in the ocala the abundance of luminescent dolomite ranges from 2 to 38 which translates to 1 6 km3 of material in the suwannee and 13 5 km3 in the ocala extrapolated trace element contents sr and na and 18o values for the matrix and luminescent end members indicate a marine origin for the matrix dolomite in both units and a freshwater seawater mixing zone origin for the secondary luminescent dolomites the 18o values indicate that a saline middle mixing zone environment overprinted the suwannee but a more dilute mixing zone affected the ocala fluid fluid mixing models constrained by modern floridan aquifer hydrochemistry and extrapolated 87sr 86sr values of the luminescent phases indicate that the mixing zones operated during the late miocene to pliocene in the ocala and affected the suwannee in the pliocene the luminescent suwannee mixing zone cement reduced porosity up to threefold and permeability up to 100 fold which converted many sucrosic dolomites to indurated dolomites by contrast the neomorphic luminescent ocala dolomite did not have an appreciable impact on the maturations although freshwater seawater mixing zones were not the sites of the initial dolomitization the mixing zone environment did dramatically overprint and mature the regionally widespread dolomites of the ocala and suwannee limestones this maturation occurred shortly after formation of the proto floridan aquifer the timing suggests the matrix dolomites were ripe for alteration and that the only prerequisite for mixing zone dolomite is pre existing dolomite substrates to reduce kinetic barriers in contrast to recent claims the results of this study demonstrate that mixing zones can be effective in forming regionally significant amounts of secondary dolomite and influencing the petrophysical maturation of dolomite bodies

the strong diachronous muschelkalk keuper facies shift in the central european basin implications from the type section of the erfurt formation lower keuper triassic and basin wide correlations the transition from the shallow marine upper muschelkalk sea to the lower keuper fluvial plain represents the most diachronous facies shift of the entire germanic triassic the type section of the fluvial lower keuper erfurt formation is described in detail for the first time including biostratigraphic dating of the muschelkalk keuper boundary the type section is integrated into a nne ssw cross section through the central european basin and the muschelkalk keuper facies shift is constrained by high resolution conodont and ceratite biostratigraphy thus the fundamental changes in palaeogeography shifts of facies belts and stratal pattern architecture are visualised forced by a rapid transgression from tethyan waters the shallow marine upper muschelkalk sea attained its maximum flooding in the lower conodont zone 2 sequens pulcher to philippi robustus zones subsequent slow continuous regression to the south was accompanied by step by step progradation of coastal to fluvial plain environments of the lower keuper culminating in a fluvial plain extending to south germany based on stratal patterns an improved sequence stratigraphic interpretation for the upper muschelkalk lower keuper interval is suggested in combination with biostratigraphic arguments the new sequence stratigraphy points to a revised correlation of this interval within the tethyan triassic incorporating the positions of the anisian ladinian and fassanian longobardian boundaries

relation between infiltration and stone cover on a semiarid hillslope southern arizona abstract a knowledge of the spatial distribution of infiltration rates is essential to the application of realistic distributed rainfall runoff models in semiarid areas one method of determining this distribution is to develop a predictive equation based on a readily observable surface property such as stone cover previous studies of the relation between infiltration and stone cover on shrub covered semiarid hillslopes have yielded both positive and negative correlations it is suggested that positive correlations result where infiltration measurements are confined to areas between large shrubs negative correlations on the other hand reflect pronounced shrub intershrub differences in infiltration and stone cover and are found where both shrub and intershrub areas are sampled in this study of a semiarid hillslope in southern arizona both shrub and intershrub areas are sampled and a negative correlation is obtained between infiltration and stone cover this correlation arises from the fact that under shrubs fine sediments have accumulated primarily as a result of differential splash whereas between shrubs such sediments have been selectively removed by a combination of rainsplash and overland flow leaving behind a gravel lag consequently infiltration rates are higher under shrubs than between them owing to the higher percentage of sand in the surface soil the larger quantity of organic matter both within the soil and on its surface and the greater digging and burrowing by animals in addition simulated rainfall experiments imply that shrubs with moderately dense canopies are more effective than adjacent surface gravels in dissipating the kinetic energy of raindrops and thereby reducing surface sealing and promoting infiltration inasmuch as the field site is typical of gentle shrub covered piedmont hillslopes over much of the sonoran desert inverse relations between infiltration rate and stone cover are probably characteristic of such hillslopes and may be used to predict the spatial distribution of infiltration rates for inclusion in rainfall runoff models

nature and origin of organic matter in carbonates from speleothems marine cements and coral skeletons organic matter in speleothem calcite marine carbonate cements and aragonitic coral skeletons was studied to determine its location molecular structure functionality and effect on mineral growth sem analyses showed that inorganically precipitated carbonates incorporate during growth adsorbed organic matter between submicroscopic subunits of the crystals whereas biologically secreted carbonates incorporate biogenic tissue between the crystals molecular fluorescence spectroscopy indicated that low molecular weight fulvic acids are the most important constituents of the organic matter the fulvic acids are probably derived from soils speleothem calcite dissolved organic matter marine carbonates and biological decay products aragonitic coral skeleton

chemical openness and potential for misinterpretation of the solute environment of coastal sabkhat abstract sabkha deposits in the geologic record are commonly used to interpret the environmental conditions of deposition implicit in this use is the assumption that the solute system is chemically closed that is the authigenic minerals represent the composition of the fluids in their environment of origin thermodynamic and mass balance calculations based on measurements of water and solute flux of contemporary abu dhabi coastal sabkha system however demonstrate that the system is open for sodium and chloride where nearly half of the input is lost but closed for sulfur where nearly 100 is retained sulfur and chloride isotopes were consistent with this observation if these sabkha deposits were preserved in the geologic record they would suggest a solute environment rich in sulfate and poor in chloride yet the reverse is true in most coastal sabkha environments capillary forces bring solutes and water to the surface where the water evaporates and halite carnallite sylvite and other soluble minerals are precipitated retrograde minerals such as anhydrite calcite dolomite and gypsum however precipitate and accumulate in the capillary zone beneath the surface of the coastal sabkha because they possess relatively low solubility and are below the surface these retrograde minerals are protected from dissolution and physical erosion occurring from infrequent but intense rainfall events thus they are more likely to be preserved in the geological record than highly soluble minerals formed on the surface

electrical imaging and fluid modeling of convective fingering in a shallow water table aquifer free to read unstable density driven flow can lead to enhanced solute transport in groundwater only recently has the complex fingering pattern associated with free convection been documented in field settings electrical resistivity er tomography has been used to capture a snapshot of convective instabilities at a single point in time but a thorough transient analysis is still lacking in the literature we present the results of a 2 year experimental study at a shallow aquifer in the united arab emirates that was designed to specifically explore the transient nature of free convection er tomography data documented the presence of convective fingers following a significant rainfall event we demonstrate that the complex fingering pattern had completely disappeared a year after the rainfall event the observation is supported by an analysis of the aquifer halite budget and hydrodynamic modeling of the transient character of the fingering instabilities modeling results show that the transient dynamics of the gravitational instabilities their initial development infiltration into the underlying lower density groundwater and subsequent decay are in agreement with the timing observed in the time lapse er measurements all experimental observations and modeling results are consistent with the hypothesis that a dense brine that infiltrated into the aquifer from a surficial source was the cause of free convection at this site and that the finite nature of the dense brine source and dispersive mixing led to the decay of instabilities with time this study highlights the importance of the transience of free convection phenomena and suggests that these processes are more rapid than was previously understood

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

convection in groundwater below an evaporating salt lake 1 onset of instability convective groundwater motion can be generated in aquifers beneath and adjacent to saline lakes such motions strongly control the subsurface distribution of salts the formation of saline minerals and the distribution of contaminants that may be disposed of in the saline lake in particular schemes to use saline lakes as evaporation basins for irrigation waste waters must consider the groundwater dynamics associated with convection the convection is driven by the evaporative concentration of salts at the land surface leading to an unstable distribution of density however the evaporative groundwater discharge can dynamically stabilize this saline boundary layer and inhibit convection in this work we investigate the nature and onset of convection for a range of boundary conditions typically found in saline lakes processes involved in the accumulation of salt at the surface of a groundwater discharge zone and in the gravitational instability of the near surface groundwater are considered results of theoretical stability analysis are applied through laboratory and numerical experimentation to the more complex geometries typically found in the field these comparisons indicate that for large saline lakes the stability of the saline boundary layer can be parameterized by traditional rayleigh criteria in which the aquifer permeability and the evaporation rate from the lake bed are principal controlling factors for typical saline lake environments convection will dominate in sediments whose permeability exceeds approximately 10 14 m2 below this threshold permeability the boundary layer should be stabilized by the evaporative flux resulting in the accumulation of salts and evaporites at the land surface these results provide insight into the predictive behavior of groundwater dynamics and solute distributions in many natural systems

δ13c and δ18o values of triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature abstract a dataset of 160 isotope 13c and 18o values from anisian ladinian carnian and rhaetian articulate brachiopod shells complemented by 158 carbon and oxygen isotope values from whole rock carbonates define the first continuous stable isotope baseline trends for the triassic seawater the carbon isotope data suggest the existence of several short term high amplitude excursions in the early triassic followed by a predominance of values around 0 5 1 during the middle triassic a rise to 3 5 during the carnian plateau at this level during the late carnian to early norian and a 1 5 decline in the middle norian to values around 2 during the late norian rhaetian interval the causation scenarios for these rapid oscillations are at present equivocal but may in part reflect a biological instability of the carbon cycle following the recovery from the end permian extinction event and or an input of mantle derived co2 from enhanced volcanic activity the 18o values from well preserved brachiopods from the tethyan realm range from 3 9 to 0 6 v pdb these values require open marine triassic seawater 18o values close to 0 v smow for the calculated temperatures to be within the range of tolerance of the coexisting reef building corals a 2 18o increase within the uppermost cordevolian and early julian suggests either a distinct temperature decline in the southern alps during that time interval an increase in seawater salinity or their combination oxygen isotope values for the muschelkalk brachiopods range between 6 2 and 2 0 and likely reflect a strong influx of meteoric waters into the germanic basin that shifted the oxygen isotopes to more negative values

multiple episodes of dolomitization and dolomite recrystallization during shallow burial in upper jurassic shelf carbonates eastern swabian alb southern germany abstract the upper jurassic of the eastern swabian alb is composed of oolitic platform sands with associated microbe siliceous sponge mounds at the platform margins they are surrounded by argillaceous or calcareous mudstones and marl limestone alternations deposited in adjacent marl basins partial to complete dolomitization is predominantly confined to the mound facies six types of dolomite as well as one type of ankerite document a complex diagenetic history during shallow burial with multiple episodes of dolomite formation and recrystallization the earliest massive matrix dolomitization is ca rich has slightly depleted oxygen isotope values relative to late jurassic seawater and carbon isotopic values in equilibrium with late jurassic seawater this initial massive matrix dolomitization occurred during latest jurassic to earliest cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50 c hydrologic conditions and mass balance calculations indicate that burial compaction provided sufficient fluids for dolomitization mg is derived from negligibly modified seawater that was expelled from the adjacent off reef strata into the mound facies position of the mounds along the platform margins controlled the distribution of the shallow burial dolomite covariant trends between textural modification increasing stoichiometry partial changes in trace element content mn fe sr and depletion in stable isotopes as well as distinctive cl pattern illustrate two recrystallization phases of the precursor matrix dolomite during further burial at elevated temperatures strong sr enrichment of the second phase of recrystallized dolomite is ascribed to sr rich meteoric waters descending from overlying aragonite bearing reef limestones or evaporite bearing peritidal carbonates late stage coarsely crystalline dolomite cements occur as vug and fracture fillings and formed during burial ankerite associated with sulphide and sulphate minerals and saddle dolomite are assumed to have formed from hydrothermal waters that moved to higher stratigraphic levels along fracture conduit systems that developed during late cretaceous to tertiary alpine orogenesis

dolomitization anhydrite cementation and porosity evolution in a reflux system insights from reactive transport models significant volumes of world hydrocarbon reserves occur in dolostones and the majority of these reservoirs are interpreted to be of reflux origin we used a two dimensional numerical reactive transport model to investigate systematically the temporal and spatial distribution of replacement dolomitization dolomite cementation anhydrite cementation and porosity evolution in a reflux system we tested the sensitivity of reflux dolomitization to brine concentration mesohaline to near halite saturated brines near surface temperature flow rate porosity permeability feedback relationships reactive surface area and the effect of initial porosity permeability heterogeneity simulations support the contention that hypersaline reflux is capable of extensive pervasive dolomitization and that mesohaline reflux dolomitization is viable reflux generated a tabular dolomite body that is thickest close to the brine source and thins basinward incorporation of initial porosity and permeability heterogeneity resulted in a dolomite body with pronounced lateral fingers replacement dolomitization increased porosity up to 8 but postreplacement reflux resulted in the precipitation of minor dolomite cements overdolomitization anhydrite cements that were spatially and temporally associated with replacement dolomitization caused a significant porosity reduction of up to 25 simulated rates of replacement dolomitization by reflux are fast up to three orders of magnitude faster than seawater dolomitization the rate of dolomitization and anhydrite mineralization proved to be critically sensitive to the flow rate and the brine chemistry temperature and reactive surface area were important controls on the rate of dolomitization whereas the feedback relationship between porosity and permeability was a relatively moderate control our reactive transport models predict the general spatial and temporal trends in dolomite porosity and anhydrite observed in some major dolomite reservoirs

strontium and its isotopic composition in interstitial waters of marine carbonate sediments abstract the concentrations and isotopic compositions of strontium in interstitial waters from several dsdp sites where sediments consist chiefly of carbonate oozes and chalks are used as indicators of carbonate diagenesis by reference to a recently produced curve showing detailed variations in the87sr 86sr ratio of seawater with time carbonate sediments of the walvis ridge show increases in interstitial sr2 concentrations in the upper carbonate ooze sections with the highest concentrations near the ooze chalk boundary where maximum rates of carbonate recrystallization occur below this in situ production of sr2 diminishes and there is a diffusive flux of sr to an underlying sink presumably volcanogenic sediments or basalts leading to sr isotopic disequilibrium between carbonates and interstitial waters in some other sites however there is no apparent sr sink at depth and isotopic equilibrium is retained overall diffusive smoothing of profiles exerts an important control on the87sr 86sr ratios although lower ratios than contemporaneous seawater values in the carbonate oozes often correlate with zones of mg2 loss and reflect a combination of a flux of sr2 from the zone of maximum recrystallization rates together with a contribution from the in situ alteration of volcanic matter

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

abu dhabi sabkha persian gulf revisited application of strontium isotopes to test an early dolomitization model a study of brines and diagenetically altered minerals from coastal sabkha of abu dhabi persian gulf was undertaken to test the flood recharge evaporative pumping model for early dolomitization using strontium isotopes as a tracer the sup 87 sr sup 86 sr ratio of the sabkha brines defines a pattern of both horizontal and vertical mixing of coastal seawater and continental ground water consistent with a previously proposed model the sup 87 sr sup 86 sr ratio of the diagenetic dolomites indicates early dolomitization by marine recharge waters the sup 87 sr sup 86 sr ratio of gypsum from the dolomitization area 1 4 to 4 4 km from high tide mark and seaward reflects precipitation from marine brines but more landward 6 to 9 km gypsum and anhydrite record a continental signal in the minerals suggests a paragenetic evaporative sequence dolomite yields gypsum yields anhydrite the dolomitizing fluid is evaporated seawater that recharges the sabkha aquifer during supratidal flooding and is afterward pumped upward through the saturated zone by evaporative processes

source of solutes to the coastal sabkha of abu dhabi an ascending brine model is proposed to address the observed isotope geochemistry solute composition and solute and water fluxes in the coastal sabkha of the emirate of abu dhabi mass balance measurements document that 95 of the solutes are derived from ascending continental brines minor amounts are derived from rainfall and from groundwater entering from up gradient areas nearly 100 of the annual water loss is from evaporation and not lateral discharge direct rainfall on the sabkha and subsequent recharge to the underlying aquifer account for 90 of the annual water input to the system the remaining 10 comes from both lateral and ascending groundwater flow thus the water and solutes in this system are from different sources solute concentrations of conservative i e nonreactive elements in the coastal sabkha covered aquifer are consistent with the fluid pore volumes of ascending brine calculated from hydrologic properties calcium to sulfate ratios and sulfur isotopes are consistent with this source of solute from the underlying tertiary formations recharging rainwater dissolves halite and other soluble minerals on the surface causing the solution to become more dense and sink to the bottom of the aquifer where it vertically mixes with less dense ascending brines solutes are returned to the surface by capillary forces and recycled or lost from the system by eolian or fluvial processes thus the system becomes vertically mixed consistent with the presence of tritium throughout the aquifer but there is essentially no horizontal mixing of seawater with groundwater the observed seawater solutes in the supratidal zone come from interstitial seawater trapped by the rapid progradation of the sediments into the arabian gulf and are not refluxed or laterally mixed the ascending brine model contrasts significantly with both the seawater flooding and evaporative pumping models previously proposed as a source of solutes to the coastal sabkha of the emirate of abu dhabi use of these earlier models leads to incorrect conclusions and raises serious questions about their applicability in the evaluation of sabkhat in the geologic record

a sandy tidal coast in the uppermost muschelkalk and the origin of the muschelkalk keuper boundary in the southwestern germanic basin a tidal coast is documented in mixed siliciclastic and carbonate sands of the uppermost muschelkalk middle triassic along the southwestern margin of the germanic basin in luxembourg the coastal sediments are vertically and laterally stacked channel fills interpreted to have formed in a tidal flat environment the channel fills overlie carbonates of a shallow subtidal ramp the strong progradation of the tidal flat indicates deposition during a late stage of sea level highstand but before sea level fall in their upper part the channel fills are overprinted by a thick paleosol which resulted from subaerial exposure around the time of the muschelkalk keuper boundary the exposure and formation of the paleosol in the subtidal coastal sediments and in basinward sections the deposition of dolomicrites above the muschelkalk keuper boundary in the lowermost keuper both indicate a sea level fall

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

multi phase dolomitization and recrystallization of middle triassic shallow marine peritidal carbonates from the mecsek mts sw hungary as inferred from petrography carbon oxygen strontium and clumped isotope data abstract shallow marine to peritidal carbonates of the triassic csukma formation in the mecsek mts of sw hungary are made up of dolomites limestones and dolomitic limestones that show evidence of a complex diagenetic history integration of petrographic conventional stable oxygen and carbon isotope clumped isotope and strontium isotope data with the paleogeography paleoclimate and burial history of the region revealed four major diagenetic stages stage 1 the peritidal carbonates were dolomitized penecontemporaneously during the middle triassic by refluxing evaporatively concentrated brines stage 2 increasing burial during the late triassic jurassic resulted in recrystallization of the kan dolomite member in an intermediate burial setting stage 3 during the early cretaceous seawater was drawn down and circulated through rift related faults causing renewed recrystallization of the kan dolomite member as well as dolomitization of the kozar limestone member and the underlying limestones in a deep burial setting but only in the vicinity of the faults stage 4 during the late cretaceous and cenozoic thrusting resulted in tectonic expulsion of basinal fluids and precipitation of multiple saddle dolomite cement phases near the faults the results of this study imply that the clumped isotope method integrated with other geochemical data can successfully be applied to identify the nature and potential sources of extra formational diagenetic fluids responsible for dolomitization and recrystallization this study provides conclusive evidence for multi phase dolomitization and dolomite recrystallization over several millions of years middle triassic through early cretaceous and several thousands of meters of burial in the csukma formation in sw hungary furthermore this study is the first to identify fault controlled dolomitization by circulating cretaceous seawater within triassic carbonates of central europe further supporting the viability of the interpretation of dolomitization by seawater initially drawn down and then geothermally circulated through faults in extensional basins

effects of progressive burial on matrix porosity and permeability of dolostones in the foreland basin of the alpine orogen switzerland abstract the changes in rock matrix porosity and permeability that carbonate reservoirs undergo with increasing burial depth are poorly understood this lack of understanding raises the risks involved in exploring and engineering deep reservoirs for geo energy applications to provide more insight into compaction processes the present study examines the effects of progressive burial on two dolomitized mudstone units belonging to the middle triassic muschelkalk within the swiss molasse basin situated in the foreland of the alpine orogen based on investigations of wireline logs and drill cores retrieved from up to 5000 m depth we report the burial modification of crystal textures pore sizes pore geometries and their impact on matrix porosity and permeability within the first 1500 m below surface porosity is found to drop from 40 2 to 18 1 vol and permeability drops from 105 15 to 1 md at depths 3000 m porosity and permeability maintain nearly constant values of 6 2 vol and 1900 m mechanical compaction is inactive and pressure solution at crystal contacts and along stylolites both diagenetic and tectonic without any associated cementation accounts for porosity loss at depths 3000 m collapse of pores by pressure solution is compounded by pore clogging by hydrothermal dolomite introduced by external fluids throughout the entire depth range stylolitization incrementally thins the formations however the dissolved material is not locally reprecipitated

sequence stratigraphic framework of the german triassic abstract the sequence stratigraphic methodology can be readily applied to the cratonic basin fill of the classic german triassic consisting of shallow marine to terrestrial mixed carbonate siliclastic rocks the whole triassic succession represents a second order transgression regression cycle built by third order depositional sequences systems tracts and parasequences many bounding surfaces represent widely used marker beds long used in classical lithostratigraphy using a synthesis of outcrop well log and literature data on stratal geometry facies cycle stacking patterns and paleogeography a regional chart of coastal onlap was constructed within the limitations of the presently available biostratigraphic data the observed cycles appear to correlate fairly well with those in other areas but include a number of additional sequences not included in the haq et al 1988 chart comparative analysis of regional onlap curves from different globally spread triassic basins together with an improved biostratigraphy will be necessary to relate the accommodation changes to eustatic versus tectonic and climatic controls and to produce a refined eustatic chart the german basin could provide a favourable reference point for such an analysis

multiple scale facies and reservoir quality variations within a dolomite body outcrop analog study from the middle triassic sw german basin abstract this study focuses on a porous dolomite body in the proximal part of an epeiric carbonate ramp triassic muschelkalk south german basin analogous to hydrocarbon bearing dolomite reservoirs in the middle east e g khuff and arab formations the dolomite body is made up of shoal lagoonal and peritidal facies types and is built by a three fold hierarchy of cycles 3rd order to 5th order the best reservoir quality occurs in finely crystalline lagoonal dolo mudstones mean k 37 8 md 20 2 with micro intercrystalline and vuggy porosity coarsely crystalline shoal related dolo grainstones show variable reservoir potential mean k 30 1 md 4 4 due to late diagenetic modification reservoir quality significantly decreases in the non dolomitized foreshoal section mean k 4 5 md 3 6 stratigraphically the most porous dolomite occurs in the upper regressive portion of the 3rd order cycle reservoir quality progressively decreases towards the limy bottom of the section outcrop mapping showed high lateral continuity for the distribution of facies types and poroperm values on the scale of several 10s of meters within correlated 5th order cycles considerable facies and poroperm heterogeneity is already observed on the 100s of meters scale on a 1000s of meters scale the facies and lithology distribution is controlled by the geometry of a gently inclined carbonate ramp during regression a progradational pattern is recognized marked by a basinward migration of backshoal facies and a down dip decrease of dolomite body thickness lithology facies and poroperm data from outcrops and cores were used for 3d modeling with the software petrel the modeled variations in the poroperm distribution are consistent with the degree of heterogeneity expected for a combination of laterally persistent lithology and highly variable facies results of this study may help to better understand the reservoir heterogeneity of inner ramp dolomite bodies and can be applied to improve characterization and modeling of similar dolomite reservoirs

marine ostracode provincialism in the late cretaceous of the tethyan realm and the austral province abstract a synthetic analysis based on previous and new data allows the recognition of four main palaeobiogeographic units palaeobioprovinces for the late cretaceous of the tethyan realm and austral province of the south temperate realm south european north central american afro arabian and austral bioprovinces another unit the dinaro hellenic subprovince is located between the south european and the afro arabian bioprovinces special emphasis is given to the spatial and temporal distribution of ostracode assemblages at the specific level and selected key genera within several geographic zones from the tethys north central america southern europe northern africa and the middle east to eastern africa india south africa and australia several factors are responsible for these palaeobiogeographic distributions provincialism and their evolution the position and extension of continental margins oceanic surface current patterns paleobathymetry eustatic sea level changes paleolatitudes and their influence on paleoclimates other parameters are the biology physiology and ecology of the ostracodes therefore ostracode provincialism has a high potential for further paleogeographic inferences such as global tectonic reconstructions

fate of reflux brines in carbonate platforms active reflux may occur during periods of platform top brine generation but the role and fate of these brines after reflux events are uncertain we have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform top salinity simulations suggest that reflux brines originally concentrated to gypsum saturation 150 have a relatively long platform residence time on the order of 100 times the duration of the reflux event when brine generating conditions cease brines will continue to sink through the platform entraining seawater a variant of reflux circulation we term latent reflux mesosaline brines intercepted by drilling of carbonate margins by the ocean drilling program may have originated from pleistocene reflux event s on the adjacent platform top and be currently moving by latent reflux latent reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates including mg for dolomitization

dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes upper muschelkalk switzerland the trigonodus dolomit is the dolomitized portion of the homoclinal ramp sediments of the middle triassic upper muschelkalk in the se central european basin various dolomitizing mechanisms followed by recrystallization have been previously invoked to explain the low 18o high 87sr 86sr extensive spatial distribution and early nature of the replacive matrix dolomites this study re evaluates the origin timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the trigonodus dolomit at 11 boreholes in northern switzerland in each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth crystal size increase change from anhedral to euhedral textures uv fluorescence decrease 18ovpdb decrease from 1 at the top to 6 7 at the base and an 87sr 86sr increase from 0 7080 at the top to 0 7117 at the base thus dolomites at the top of the unit record isotopic values similar to middle triassic sea water 18ovsmow 0 87sr 86sr 0 70775 while dolomites at the base record values similar to meteoric groundwaters from the nearby vindelician high 18ovsmow 4 87sr 86sr 0 712 according to water rock interaction modelling a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends instead the combined isotopic geochemical and petrographic data can be explained by dolomitization via seepage reflux of hypersaline brines into dense horizontally advecting groundwaters that already had negative 18o and high 87sr 86sr values evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization this study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre existing groundwaters can be preserved during seepage reflux dolomitization in low angle carbonate ramps

on a test case for density dependent groundwater flow and solute transport models the salt lake problem like any other computer model density dependent mathematical groundwater models must be verified to ascertain that they accurately represent the physics implied by a governing set of equations very few test cases for density dependent groundwater numerical models exist as such there is still a need for new and more robust tests of these modeling codes in this paper a numerical model of an idealized evaporating salt lake is produced using the two dimensional density dependent saturated unsaturated transport sutra model the results of which are compared with an equivalent laboratory hele shaw cell system developed by wooding et al 1997a b evaporation results in dense brine overlying less dense fluid which is hydrodynamically unstable and leads to downward convection of salt fingers or plumes a comparison of experimental and numerical plume growth shows good spatial and temporal agreement the numerically generated plume pattern is sensitive to changes in random noise level applied just below the evaporation surface that serves as a trigger for the growth of instabilities experimental plume patterns were best matched with a noise level corresponding to 1 of the total salinity difference between boundary layer and background fluid concentrations at saturation in a second comparison the stream function based finite difference model described by wooding et al 1997a b which differs significantly in principle from sutra is shown after revision to give good spatial and temporal agreement with experimental results this test for density dependent groundwater models appears to be the most comprehensive and detailed available to date

experimental investigation of dense solute plumes in an unconfined aquifer model unstable dense aqueous phase contaminant plumes may contaminate larger regions of an aquifer than stable plumes to learn more about the behavior of variable density contaminant plumes experiments were conducted in a flow container packed with homogeneous sand salt nai solutions with different densities were introduced at different rates from a source located on top of the sand into the unconfined aquifer model where horizontal ambient groundwater streams were established with different horizontal velocities salt concentrations were measured with an automated nonintrusive gamma radiation system results show that dense plumes were either stable or unstable important factors determining the stability of the dense plumes were the magnitude of the horizontal darcy velocity and the relative density difference between the dense plume and the ambient groundwater longitudinal and transverse dispersivities estimated from breakthrough curves and concentration profiles determined during the development of stable dense plumes were small indicating advection as the main mode of transport concentration profiles of stable dense plumes taken along vertical transects showed a rapid drop in relative concentration at the bottom of the plumes when dense plumes became unstable salt concentrations fluctuated considerably over time and concentration profiles of unstable plumes showed large mixing zones

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

dolomitization by mixed evaporative brines and freshwater upper miocene carbonates nijar spain abstract upper miocene carbonates nijar spain have been studied in order to test a model for massive dolomitization of western mediterranean reefal carbonates invoking evaporite brines formed during the messinian salinity crisis tortonian messinian reef bearing carbonates at nijar comprise a kilometers wide shelf fringing the sierra alhamilla highland and dip southward into the evaporite bearing nijar basin in the nijar platform pervasive dolomites are present in greater than 75 of tortonian messinian carbonates dolomites transect intra miocene sequence boundaries and occur in the youngest messinian unit at nijar the oolitic terminal carbonate complex tcc dolomites do not transect the miocene pliocene unconformity but do occur as clasts in basal pliocene lime grainstones this constrains the dolomitization as syn or post tcc and pre pliocene in age and suggests that dolomitization occurred within 600 000 years and possibly in less than 200 000 years carbon and oxygen isotopes of nijar dolomites show marked positive covariation with most values ranging from 5 4 to 1 2 d18o pdb and 2 5 to 4 3 d13c pdb the highest d18o values are heavier than reasonable estimates of messinian normal salinity seawater dolomites and therefore imply involvement of evaporative brines calculations of covariations of o and c isotopes during water rock interaction and fluid fluid mixing argue against recrystallization of nijar dolomites an interpretation supported by their ca richness 49 57 mol these mixing calculations show that about 80 of nijar ata are consistent with dolomitization in mixtures of freshwater and a few tens of percent of seawater derived evaporative brine and about 20 are consistent with mixtures of freshwater and normal salinity seawater 87sr 86sr of pure nijar dolomites range from messinian seawater values 0 7089 to high values 0 70928 and show distinct negative covariations with d18o d13c and sr calculations of covariations of 87sr 86sr with d18o and d13c during water rock interaction and fluid fluid mixing argue strongly against recrystallization and argue for their formation by mixtures of freshwater with a few tens of percent of evaporative brines with seawater and with seawater brine mixtures modeling ovariations of 87sr 86sr and sr ca ratios during mixing further supports involvement of evaporative brine and suggests that the freshwaters had low sr ca ratios and relatively high sr and ca concentrations na cl and so4 concentrations in nijar dolomites 200 1700 ppm 300 600 ppm and 600 6500 ppm respectively argue for involvement of evaporative brines and of mixtures of freshwater and seawater when compared with other well characterized dolomites the proposed model is that most dolomitization occurred after reef formation and during and possibly after tcc deposition during multiple relative sealevel changes the model invokes tcc deposition on the nijar shelf during high sealevels and evaporite deposition in nijar basin during low sealevels during which brines in the 5x to 6x seawater range were formed during sealevel rises these brines mixed with seawater or remained as dense basin bottom water in either case the brines mixed with freshwater mainly in a groundwater setting the freshwaters having radiogenic sr and depleted d18o and d13c derived from a weathered mantle on the sierra alhamill the brines and sea water brine mixtures were the sources of mg for the dolomites and their circulation through platform rocks was driven primarily by buoyant circulation of the mixing zone beneath freshwater lenses the nijar model may be applicable to dolomitization in other partially land locked basins having adjoining highlands e g sorbas basin that acted as freshwater recharge areas the mallorcan model which invokes brines and seawater with no freshwater may be appropriate for settings open to the deep mediterranean and distant from highlands e g santa pola spain mellilla morocco

hydrogeologic processes in saline systems playas sabkhas and saline lakes abstract pans playas sabkhas salinas saline lakes and salt flats are hydrologically similar varying only in their boundary conditions thus in evaluating geochemical processes in these systems a generic water and solute mass balance approach can be utilized a conceptual model of a coastal sabkha near the arabian gulf is used as an example to illustrate the various water and solute fluxes analysis of this model suggests that upward flux of ground water from underlying formations could be a major source of solutes in the sabkha but contribute only a small volume of the water local rainfall is the main source of water in the modeled sabkha system with a surprisingly large recharge to rainfall ratio of more than 50 the contribution of seawater to the solute budget depends on the ratio of the width of the supratidal zone to the total width and is generally confined to a narrow zone near the shoreline of a typical coastal sabkha because of a short residence time of water steady state flow is expected within a short time 50 000 years the solute composition of the brine in a closed saline system depends largely on the original composition of the input water the high total ion content in the brine limits the efficiency of water rock interaction and absorption because most natural systems are hydrologically open the chemistry of the brines and the associated evaporite deposits may be significantly different than that predicted for hydrologically closed systems seasonal changes in temperature of the unsaturated zone cause precipitation of minerals in saline systems undergoing evaporation thus during the hot dry season months minerals exhibit retrograde solubility so that gypsum anhydrite and calcite precipitate evaporation near the surface is also a major process that causes mineral precipitation in the upper portion of the unsaturated zone e g halite and carnallite provided that the relative humidity of the atmosphere is less than the activity of water the slope of the fresh brine water interface in saline lake systems is shallower than in fresh seawater interface because of the greater density difference between the fresh brine water bodies the interface between sabkha brines and seawater slopes seaward unlike normal marine fresh water systems that slope landward moreover the brine seawater interface does not achieve steady state because it is pushed toward the sea by the sabkha s brine

early diagenesis driven by widespread meteoric infiltration of a central european carbonate ramp a reinterpretation of the upper muschelkalk abstract meteoric diagenesis of carbonate ramps is often difficult to interpret and can commonly be confused with other coinciding diagenetic processes the middle triassic upper muschelkalk of switzerland provides an insightful case in which the effects of several overprinting diagenetic environments including matrix dolomitization can be clearly unravelled previous studies suggested that diagenesis took place in connate marine waters with later meteoric waters being invoked to explain recrystallization of dolomite in this study diagenetic analyses c o stable isotope ratios thin section point counting cathodoluminescence and uv fluorescence microscopy of calcitic bioclastic samples have revealed that early diagenesis pre stylolitization and the accompanying porosity evolution did not occur exclusively in the presence of marine fluids five sequential stages of diagenesis have been identified marine shallow burial mixing zone meteoric and dolomitization marine diagenesis induced precipitation of bladed and inclusion rich syntaxial cements that fluoresce strongly under uv light both cements account for a mean 7 5 vol reduction in the porosity of bioclastic beds shallow burial diagenesis likely induced mouldic porosity and associated fluorescent dog tooth cementation based on light oxygen isotope and elevated strontium isotope ratios matrix aragonite calcite neomorphism is interpreted to have occurred in a mixture of marine and meteoric fluids the combination of shallow burial and mixing zone processes reduced porosity on average by 4 8 vol evidence for subsequent meteoric diagenesis is found in abundant dog tooth and blocky calcite cements that have mean 18 o vpdb of 9 36 and no signs of recrystallization these meteoric cements reduced porosity by a further 13 4 vol percolation of meteoric water through the ramp was driven by hydraulic gradients on an adjacent basement high which was exposed by a cycle of early ladinian regressions following meteoric diagenesis the upper muschelkalk was dolomitized by refluxing brines this complex history of diagenesis resulted in moderate porosities in dolomitized rocks up to 20 and low porosities

strontium isotope evolution of late permian and triassic seawater the 87sr 86sr values based on brachiopods and conodonts define a nearly continuous record for the late permian and triassic intervals minor gaps in measurements exist only for the uppermost brahmanian lower part of the upper olenekian and middle norian and only sparse data are available for the late permian these 219 measurements include 67 brachiopods and 114 conodont samples from the tethyan realm as well as 37 brachiopods and one conodont sample from the mid european middle triassic muschelkalk sea the late permian lower triassic interval is characterized by a steep 1 3 10 3 rise from 0 7070 at the base of the dzhulfian to 0 7082 in the late olenekian a rate of change comparable to that in the cenozoic in the mid triassic anisian and ladinian the isotope values fall to 0 7075 followed again by a rise to 0 7081 in the middle late norian the 87sr 86sr values decline again in the late norian sevatian and rhaetian to 0 7076 the sharp rise in the 87sr 86sr values during the late permian early triassic was coincident with widespread clastic sedimentation because of the paucity of tectonic uplifts the enhanced erosion may have been due to intermittent humid phases during mainly an arid interval coupled with the absence of a dense protective land plant cover following the mass extinction during the latest permian the apex of the 87sr 86sr curve at the olenekian anisian boundary coincides with cessation of the large scale clastic sedimentation and also marks the final recovery of land vegetation as indicated by the renewed onset of coal formation in the middle triassic the rising 87sr 86sr values from the middle carnian to the late norian coincide with the uplift and erosion of the cimmeride indosinian orogens marking the closure of the palaeotethys the subsequent rhaetian decline that continues into jurassic pliensbachian toarcian boundary on the other hand coincides with the opening of the vardar ocean and its eastern continuation in the izmir ankara ophiolitic belt samples from the upper muschelkalk are more radiogenic than the global trend this may reflect separation of the basin from the open ocean due to strong meteoric influx from a large land mass in the north the germanic basin became increasing brackish up section in the north and east but because of the high evaporation rates the salt content was not much reduced in the southern and central basin where a rich but increasingly endemic marine fauna survived

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

influence of rock fragments on the water retention and water percolation in a calcareous soil abstract the water retention properties of a calcareous soil containing rock fragments have been determined in the laboratory thanks to pressure plate measurements done on both the fine earth and the rock fragments from the soil the available water content awc has been calculated from these data we have shown that when the rock fragments are neglected the awc can be overestimated by 39 when we do not neglect their volume but when their hydraulic properties are not considered the awc can be underestimated by 34 by using a reservoir model we have also calculated the effect of rock fragments on water percolation to groundwater depending of the climatic characteristics of the year the underestimation of percolation when we neglect the rock fragments can reach up to 14 9 and the overestimation when we neglect their hydraulic properties can be equal to 15 8 these findings emphasise the role of the rock fragments on the water supply in stony soils