meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

the flank margin model for dissolution cave development in carbonate platforms the bahama islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level the development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens lens position is controlled by sea level which in stable carbonate platforms like the bahamas is a function of glacioeustatic sea level still stands caves in the bahamas that are currently subaerial must have developed during past higher sea levels during the late quaternary sea levels higher than present have been relatively short lived and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation the bahama islands are low lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level past high sea level events greatly reduced the exposed land area of the bahama islands thus also limiting both the catchment for and size of freshwater lenses caves must be younger than the rock in which they are developed most subaerial bahamian caves are found in limestones that are less than 150000 years old development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism the mixing of discharging freshwater with tide pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands

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

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

aragonite to calcite transformation during fresh water diagenesis of carbonates insights from pore water chemistry dissolved strontium and calcium concentrations in fresh water lenses fwl and associated mixing zones mz on two small holocene ooid sand islands in the schooner cays bahamas were monitored during a 1 yr period to quantitatively analyze the transformation of aragonite to calcite the observed characteristics of this mass transfer are functions of climate and hydrology aragonite to calcite transformation in all hydrologic zones is primarily associated with meteoric recharge the transformation occurs throughout the fwl and in the mz to relative salinities of 19 and 36 sea water on the two islands rates of transformation are rapid in all zones and are greatest in the fwl a limestone composed of 100 calcite should form from an aragonite precursor within 4 700 to 15 600 yr in the fwl and within 8 700 to 60 000 yr in the upper mz efficiencies of transformation can vary between hydrologic zones due to pco2 effects yet the efficiency of the entire system fwl mz is high 87 this indicates that most caco3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh water system or upper mixing zone co2 effects fresh water sea water mixing and the differing solubilities between aragonite and calcite all drive the mass transfer the latter is the most significant accounting for up to nine times more mass transfer than co2 effects and at least ten times more mass transfer than fresh water sea water mixing differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle but it apparently becomes ineffective after the rainy season possibly due to the inhibition of calcite precipitation

water chemistry and caco3 dissolution in the saline part of an open flow mixing zone coastal yucatan peninsula mexico along the caribbean coast of the yucatan peninsula brackish ground water mixed fresh water and sea water is channeled through upper pleistocene limestone via fracture controlled caverns in caves cenotes and caletas at xcaret yalku and tancah this open flow coastal mixing zone comprises three major layers 1 an upper dilute zone of gradually increasing salinity with depth slow mixing 2 a thin intermediate zone of rapidly increasing salinity with depth rapid mixing and 3 a lower saline zone of gradually increasing salinity with depth the intermediate layer occurs at different absolute salinities at different localities and it generally corresponds to the level of a notch apparently related to dissolution in the wall rock of the caverns chemical composition of the mixed ground water is consistent with that predicted from theoretical conservative mixing of fresh water and sea water end members sampled in the yucatan coastal region inorganic c alkalinity mg ca and sr ca and 18o and 13c of the mixed water give no indication of dissolution or precipitation of cac03 as the ground water flows rapidly seaward field and petrographic observations however show that aragonite is preferentially dissolving throughout the modern mixing zone and in particular in the saline portion comparison of saturation indices to calculated mixing curves shows that the coastal mixing zone of the eastern yucatan peninsula is undersaturated with respect to cac03 over a wide range of salinity including mixtures of as much as 95 sea water for aragonite and 90 sea water for calcite supersaturation with respect to stoichiometric dolomite is maintained over the whole range of salinities but super saturation with respect to calcian dolomite ca57mg43 can be confined to high salinity mixtures depending on the thermodynamic data base used in calculations these data suggest that 1 dissolution of coastal limestone can take place in the sea water dominated portion of the mixing zone and 2 the mixing zone model for dolomitization can be extended to much higher salinities than is indicated by the widely accepted dorag model

limitations on lowstand meteoric diagenesis in the pliocene pleistocene of florida and great bahama bank implications for eustatic sea level models data from three bahamian and south floridan cores with deeper water facies show that contrary to current models there is no phreatic meteoric diagenesis that can be attributed to large scale sea level lowstands models of meteoric diagenesis in carbonate sediments commonly assume major phreatic alteration during glacio eustatic sea level lowstands however the diagenesis previously attributed to large scale 80 m lowstands is found in shallow water facies that have been repetitively exposed in order to document diagenesis from large scale lowstands an interval is needed that was deposited in deeper water and not immediately exposed to meteoric fluids three cores with such deeper water facies indicate that the majority of phreatic meteoric diagenesis in florida and the bahamas happened during glacio eustatic highstands not during lowstands the data also suggest that diagenetically active meteoric lenses in florida and the bahamas are restricted to within 60 m and perhaps less of the land surface the most likely reasons deeper lenses are chemically inactive are 1 the greater percolation distance allows the water to reach saturation prior to entering the lens and 2 the large distance exceeds the reach of soil derived organic matter known to drive diagenesis in meteoric lenses models that currently assume a constant rate for alteration in meteoric fluids need to accommodate this variation with thickness of the vadose zone this study also questions the use of paleophreatic lenses as records of eustatic sea level as the large scale falls may not leave any record

geohydrology of the laura fresh water lens majuro atoll a hydrogeochemical approach in small limestone islands the depositional history and subsequent chemical interactions between ground water and the aquifer host rock play critical roles in the occurrence movement and chemical quality of ground water the hydrogeochemistry of the laura fresh water lens majuro atoll marshall islands is an example of these relations laura is underlain by two principal hydrologic units the upper unit is a back reef marginal lagoonal deposit which formed during the holocene interglacial stage it is composed of moderately permeable carbonate sediments the lower hydrologic unit consists of highly permeable limestone that was subaerially exposed most likely during a pleistocene glacial lowstand similar stratification is found at bikini and enewetak atolls the upper hydrologic unit contains a calcium bicarbonate rich fresh water lens in which a potable fresh water nucleus as much as 14 m thick occurs on the lagoon side of the island storage in the fresh water nucleus ranged from 1 70 x 10 6 to 2 08 x 10 6 m 3 during 1984 1985 ground water occurrence and flow are governed by an asymmetric distribution of lithofacies about the longitudinal axis of the island and an abrupt increase in permeability at the contact between the upper and lower hydrologic units the highly permeable lower hydrologic unit contains sea water and truncates the fresh water sea water mixing zone the fresh water lens and associated fresh water sea water mixing zone are the site of continuously occurring diagenetic reactions that significantly affect the porosity and permeability of the aquifer non equilibrium dissolution precipitation reactions coupled with variations in co 2 input control the chemical evolution of laura ground water at the present rate of chemical weathering 465 m 3 of sediment are being dissolved and transported to the sea by ground water each year this dissolution results in an annual increase in porosity of 0 01 the primary factors controlling the occurrence and flow of ground water in the leeward reef islet of laura are 1 the depositional history of the upper hydrologic unit which has resulted in a greater accumulation of low permeability fine grained sediments beneath the lagoon side of the island and a high to low permeability coarse to fine grained sediment gradation between the ocean and lagoon and 2 the diagenetic history of the lower hydrologic unit which has resulted in a highly permeable basement

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

mixing zone diagenesis in the subsurface of florida and the bahamas abstract three cores from great bahama bank and southern florida contain examples of mixing zone diagenesis where typical shallow water aragonite rich sediments were altered to low mg calcite the 40 50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric 18o 3 2 0 7 13c 1 6 1 8 to enriched marine isotopic values 18o 1 0 0 3 13c 1 9 0 7 the age of the mixing zones varies from late pliocene to pleistocene so these cores provide three independent examples of mixing zone diagenesis mixing zone diagenesis produced a limestone composed of low mg calcite with petrographic fabrics including moldic porosity blocky to dogtooth calcite cementation and recrystallization of lime mud to micrite and microspar the mixing zone diagenetic model predicts alteration in only part of the mixing zone because the alteration recorded in these cores shows continuous alteration through the entire mixing zone we consider the mixing of waters itself to be largely irrelevant rather diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter the mixing zone has been proposed as a likely location for dolomitization with the exception of some clearly sea floor dolomite in one core unda the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone the lack of dolomite in these mixing zones combined with the extensive literature questioning the chemical reasoning for mixing zone dolomite supports rejection of the mixing zone dolomite model

differential dissolution of a pleistocene reef in the ground water mixing zone of coastal yucatan mexico a geochemical explanation is provided for the extensive dissolution observed along the carbonate coast of the yucatan peninsula mexico mixing of fresh ground water with subterranean caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite mixing zone dissolution caused by ground water discharge is a major geomorphic process in developing caves coves and crescent shaped beaches along the yucatan coast such dissolution has probably been a significant control on permeability and porosity distribution in carbonate rocks in the geologic record

dupuit ghyben herzberg analysis of strip island lenses dupuit ghyben herzberg analysis follows from combination of the continuity equation and darcy s law with the ghyben herzberg principle and the dupuit assumptions of horizontal flow the analysis is used to derive the position of the water table and salt water interface in island lenses in terms of island geometry distribution of hydraulic conductivity k and distribution of recharge r for small islands and cases for which the salt water head is zero application of dupuit ghyben herzberg analysis gives good results because the low r k ratios of natural lenses assure that height width ratios of natural lenses are extremely low 1 30 to 1 100 in earlier publications the dupuit ghyben herzberg differential equation has been integrated with the boundary condition that the water table and interface meet at sea level at the shoreline that is no outflow face the dimensions of an outflow face however are known from results of potential theory analysis integration of the dupuit ghyben herzberg equation with a boundary condition consistent with the potential theory outflow face a places the interface indistinguishably close to its position given by potential theory and b allows calculation of ghyben herzberg ratios interface depth to water table elevation which in this case depart from their usual value of 40 because equipotentials are curved close to the outflow face for natural sized lenses however such analysis is necessary only within a narrow strip within 1 to 5 of the island width of the shoreline outside this strip the regular analysis that ignores the presence of the outflow face positions the interface indistinguishably close to that of the potential theory solution analytical solutions are developed for a number of infinite strip islands it is shown by analysis of the homogeneous rectangular island case that an island can be considered an infinite strip to 0 1 accuracy if its length width ratio is larger than 4 4 asymmetric lenses occur if the island is composed of strips of different k or different r with the greater asymmetry occurring with differences in k a high k basement compresses the root of the lens and thereby decreases the water table in the island a lens perched on impermeable basement has a higher water table than would otherwise occur in the island but the volume of the lens is less

dolomitization and sulfate reduction in the mixing zone between brine and meteoric water in the newly exposed shores of the dead sea pleistocene dolomites are associated either with evaporative processes such as in sabkhas and playas or with a mixing zone where mixing between two different solutions e g seawater and groundwater takes place we report here on the very recent formation of dolomite at shallow depths 6 12 cm in newly exposed gypsum rich sediments on the western shores of the dead sea sem and eds analysis as well as x ray diffraction analysis indicate that this dolomite is euhedral and close to ideal dolomite in composition these dolomite crystals differ from anhedral detrital ca rich dolomites from the cretaceous section of the watershed found in the whole section taking into account the sedimentation rate at the shore vicinity the maximal age of the euhedral dolomite was estimated as less than 60 years in the three profiles analysed euhedral dolomite was most abundant at a single layer where the gypsum content was locally lowered due to bacterial reduction of sulfate the sediments solution at the dead sea shore localities represent a mixing zone between dead sea residual solution and subsurface freshwater springs under such conditions formation of dolomite occurs at highly saline environment where very high mg ca ratios are attained

the massive dolomitization of platformal and basinal sequences proposed models from the paleocene northeast sirte basin libya abstract the paleocene carbonate succession in the northeast sirte basin is composed of two shallowing upward ramp cycles where each cycle is under and overlain by deeper water pelagic facies a significant proportion of each of these two cycles is dolomitized petrographic study supported by geochemical data stoichiometry stable isotopes trace elements and fluid inclusions and integrated with broader tectono sedimentary information has provided the basis for interpreting these paleocene dolomites the use of this integrated approach in the study of dolomites suggests that despite the much publicized uncertainties in interpreting geochemical analyses of ancient dolomites the results of the paleocene dolomites show that the geochemical characteristics are generally consistent with regional stratigraphic distribution and petrographic observations four distinct types of dolomite have been recognized in this part of the sirte basin based on the stratigraphic position and petrographic criteria two of these types have a platformal setting and the other two are basinal the platform varieties consist of dolomicrites and pervasive stratal dolomites the dolomicrites interpreted to be of syn sedimentary origin were probably a product of reflux of seawater with elevated salinity as suggested by palaeoenvironmental analysis and supported by geochemical evidence the average s 80 value is 0 1 pdb the average sr content is 639 ppm the pervasive dolomites were formed during the progradation of the platform sequences and probably stabilized and augmented during shallow burial a meteoric marine mixing zone is thought to have been the most likely process for the formation of these dolomites this interpretation is supported by geochemical evidence the average 18o is 2 4 pdb the average sr content is 72 ppm combined with a favourable stratigraphic position the most characteristic feature related to both mixing zone and reflux dolomitization is the basinward movement of the dolomitizing fluids which suggests that the formation of these platform dolomites was related to a lowstand system tract the two basinal varieties comprise thick over 300 m basinal dolomudstones and fracture filling sparry dolomites the stratigraphic position of the finely crystalline basinal dolomudstones within very thick shale successions as a result of being very close to the depocentre of the sirte basin combined with geochemical evidence the average 18o is 6 4 pdb suggest that the dolomitizing fluids were basin derived with mg2 released from dewatering through compaction of basinal shales the occurrence of this type of dolomite provides one of the rare examples of large scale dolomitization of thick basinal sequences late diagenetic fracture filling dolomites exhibit a structural control on their distribution geochemical evidence including fluid inclusion analysis and the lightest oxygen isotopic signature of 7 3 pdb suggests that highly saline formation brines were the solutions responsible for their formation

acoustic properties of young carbonate rocks odp leg 166 and boreholes clino and unda western great bahama bank acoustic properties determined from logs and discrete measurements along a borehole transect across the slope of the western great bahama bank margin odp leg 166 show a distinct relationship with primary sediment composition and noncarbonate content dominant sediment types are alternating cemented and uncompacted calciturbidites with shallow water derived grains and poorly to uncemented and compacted periplatform ooze of globigerinid dominated wackestone to packstone comparison of the velocity porosity variation from logs and discrete measurements reveals that both tend to follow the trend of generally accepted velocity transforms primary control on the acoustic velocity is exerted by total porosity whereas the remaining variation in velocity is explained by differential diagenesis the less cemented to uncemented and compacted background sediments have velocities that at a given porosity fall below the time average equation for calcite the cemented turbidites however have velocities that are higher than those predicted by the time average equation although it is unclear whether the original volume of aragonite or presence of organic matter controlled this difference it is suggested that the acoustic properties reflect an input signal the presence of noncarbonate material some of which is clay increases basinward and further reduces the acoustic velocities in the background sediment in contrast earlier work on the more proximal slope and nearly pure carbonate intervals in clino and unda cores documents diagenetic differentiation in compositionally similar sediment resulting in alternating acoustically fast and slow layers marginal and nearly pure limestones have a more complex acoustic behavior and hence are difficult to predict though both velocity logs and discrete velocity measurements show comparable general trends logging data show a considerably larger variation at a given porosity value up to 3 5 km s than discrete data which ranges between 1 0 and 2 5 km s to improve the quality of velocity logs it is suggested to groundtruth these data with other logging data general velocity transforms and ideally discrete measurements on core plugs

regional hydrogeochemistry of a modern coastal mixing zone in west central florida groundwater samples were collected along flow paths in the unconfined upper floridan aquifer that cross the inland freshwater recharge area and the coastal discharge area a groundwater flow and solute transport model was used to evaluate groundwater flow and mixing of fresh and saline groundwater along a cross section of the unconfined upper floridan aquifer results show that between 8 and 15 of the fresh and 30 31 of the saline groundwater penetrates to the depth in the flow system where contact with and dissolution of gypsum is likely the deeply circulating fresh and saline groundwater returns to the near surface environment discharging caso4 rich water to the coastal area where it mixes with fresh cahco3 groundwater resulting in a prediction of calcite precipitation in the modern mixing zone

joint modeling of lithosphere and mantle dynamics evaluation of constraints from global tomography models with the advances in technology seismological theory and data acquisition a number of high resolution seismic tomography models have been published however discrepancies between tomography models often arise from different theoretical treatments of seismic wave propagation different inversion strategies and different data sets using a fixed velocity to density scaling and a fixed radial viscosity profile we compute global mantle flow models associated with the different tomography models and test the impact of these for explaining surface geophysical observations geoid dynamic topography stress and strain rates we use the joint modeling of lithosphere and mantle dynamics approach of ghosh and holt 2012 to compute the full lithosphere stresses except that we use hc for the mantle circulation model which accounts for the primary flow coupling features associated with density driven mantle flow our results show that the seismic tomography models of s40rts and saw642an provide a better match with surface observables on a global scale than other models tested both of these tomography models have important similarities including upwellings located in pacific eastern africa iceland and mid ocean ridges in the atlantic and indian ocean and downwelling flows mainly located beneath the andes the middle east and central and southeast asia

recurrent neomorphic and cement microtextures from different diagenetic environments quaternary to late neogene carbonates great bahama bank abstract the cores clino and unda taken near the western margin of great bahama bank contain a diverse suite of quaternary to late neogene carbonates that have a variety of neomorphic and cement microtextures the microtextures of each of the different neomorphosed aragonitic grain types such as corals mollusks and plates of the calcareous green algae halimeda are very similar wherever the neomorphosed grains are present in the cores limestone samples that contain neomorphosed aragonitic fossils are cemented predominantly with blocky calcite despite the similar neomorphic and cement microtextures light stable isotope data reveal that these calcites precipitated in different diagenetic environments in different parts of the cores the neomorphic calcites and associated calcite cements in the upper parts of clino and unda precipitated in meteoric influenced pore waters as indicated by 18o values ranging from 1 9 to 3 7 whereas in the lower part of clino the calcites precipitated in marine pore waters as indicated by 18o values ranging from 0 1 to 2 4 essentially identical diagenetic microtextures were thus produced in different diagenetic environments revealing a significant limitation in the use of microtextural data to determine the diagenetic histories of carbonate rocks

geochemical investigation of salt water intrusion into a coastal carbonate aquifer mallorca spain geochemical processes occurring within the mixing zone on mallorca spain were investigated in relation to the diagenesis of carbonate minerals and to the development of porosity and permeability within the pleistocene limestone aquifer rock core and ground water samples were obtained from the fresh water zone the mixing zone and the top of the sea water zone at two sites near the coast ground water withdrawals in the vicinity of one study site depressed the water table to below sea level resulting in sea water encroachment and a thin mixing zone laboratory tests conducted on the rock core indicated that porosity and hydraulic conductivity were not enhanced within the transitory location of the present day mixing zone vuggy zones in the core correspond to the approximate location of the water table because the ground water was supersaturated with respect to the carbonate minerals dissolution of the limestone aquifer is no predicted for the present day mixing zone results of mass balance and source rock calculations indicated an excess of calcium and strontium in the mixing zone waters in relation to what was expected from conservative mixing these elevated concentrations are hypothesized to result from limestone dissolution in the vadose zone

carbonate diagenesis in a high transmissivity coastal aquifer biscayne aquifer southeastern florida usa abstract cores collected through the biscayne aquifer plio pleistocene in hollywood southeastern florida as part of the hollywood coastal salinity barrier project provided an opportunity to examine the diagenesis of limestones and sandstones at the meteoric to marine water transition of one of the most transmissive aquifers in the world the saline water front and thus coastal mixing zone has migrated landward approximately 1 km in the hollywood area as the result of wellfield withdrawals no changes in mineralogy such as dolomitization cement types and abundances paragenetic sequence or porosity are evident that can be correlated with the current or likely pre development wellfield withdrawals location of the mixing zone an approximately 2 downhole increase in calcite 18 o values is present in a core hmw 6d that penetrates the pre development mixing zone which may be related to either a down hole increase in salinity or to the interaction of meteoric waters with marine carbonate sediments during calcite cementation the biscayne aquifer in hollywood is currently a relatively quiescent diagenetic environment the limited current diagenesis appears to consist of the dissolution of trace skeletal aragonite remaining in the aquifer as suggested by a meteoric water sr ca ratio similar to that of molluscan aragonite it is proposed that a punctuated equilibria model may be applicable to diagenesis in the biscayne and other aquifers in which limestones and sandstones entered a long period of diagenetic stasis after a period of relatively rapid textural and mineralogical stabilization

late pleistocene mixing zone dolomitization southeastern barbados west indies field geochemical and petrographic data for late pleistocene dolomites from southeastern barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters the dolomite is localized in packstones and wackestones from the algalamphistegina fore reef calcarenite facies stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization carbon isotopes in pure dolomite phases average about 15 0 pdb this light carbon is attributed to the influence of soil gas co2 and precludes substantial mixing with seawater a narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint dolomitization likely occurred with as little as a five per cent admixture of seawater strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy the dolomite is characterized by low sodium values low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization a sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix limpid euhedral dolomite cements precipitated in primary intra and interparticle porosity subsequent to replacement dolomitization as waters became progressively less saline dolomite cements alternated with thin bands of syntaxial calcite cement the final diagenetic phase precipitated was a blocky calcite spar cement representing diagenesis in a fresh water lens this sequence of diagenetic features arose as the result of a single fall in eustatic sea level following deposition a stratigraphic eustatic diagenetic model constrains both the timing and rate of dolomitization in southeastern barbados dolomitization initiated as sea level began to fall immediately following the oxygen isotope stage 7 3 high stand some 216 000 yr bp due to the rapidity of late pleistocene glacio eustasy dolomitization locally complete is constrained to have occurred within about 5000 yr

a review of the factors affecting the development of ghyben hertzberg lenses in the bahamas abstract the groundwater resources of the bahamas occur in ghyben hertzberg lenses and they have been quantified on nearly all of the major islands a comparison of how these lenses vary on each island reveals how lens development is controlled by island size shape climate and geology the geological controls of lens formation were poorly understood in the past but are now becoming apparent in light of recent subsurface geological research

carbonate dissolution in a modern mixing zone the potential for dissolution of calcite and aragonite and precipitation of dolomite by the mixing of fresh and saline ground waters has been demonstrated theoretically1 3 although geologists have used these processes in the explanation of the diagenesis of recent4 7 and ancient8 12 carbonates there has been only limited investigation of the geochemical processes of modern mixing zones13 14 here we report the results of chemical and rock profiling through a modern mixing zone on andros island in the bahamas our results demonstrate undersaturation of the waters in the mixing zone with respect to aragonite and calcite and corresponding pervasive dissolution of the carbonate rocks at this level undersaturation is significantly enhanced by bacterial processes which are also responsible for precipitation of an iron rich crust despite chemical supersaturation precipitation of dolomite was not observed thereby casting some doubt on the mixing zone model of dolomitization