isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s a the wrong figure 5 was printed with this article the correct figure follows fig 5 thin section photomicrographs of the characteristic lithologies in yucca mound a and b leopard

effects of grand cycle cessation on the diagenesis of upper cambrian carbonate deposits in the southern appalachians u s a abstract the vertical transition from the mainly subtidal alternating shale and carbonate units or grand cycles of the conasauga group middle to upper cambrian to the peritidal dolostone of the knox group upper cambrian to lower ordovician marks a major change in the early paleozoic passive margin sedimentation of the southern appalachians the grand cycles represent an interplay between intrashelf shale basin and carbonate platform deposition the end of grand cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost conasauga group strata the maynardville formation the maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone this change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the maynardville and in conjunction with the changes in the regional facies distribution at the end of grand cycle deposition extended its influence upon the burial diagenesis of these deposits the subtidal deposits of the maynardville contain a variety of calcite cements dolomite is not abundant in these deposits where it occurs mainly as a fine crystalline ferroan phase associated with argillaceous layers and pressure dissolution features the local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits diagenesis of clay minerals and pressure dissolution of carbonates ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite the subtidal deposits also contain saddle dolomite cement associated with mississippi valley type mvt minerals which indicate the involvement of externally derived fluids during late burial diagenesis of these strata the peritidal deposits on the other hand are extensively dolomitized fine crystalline dolomite most likely represents early diagenetic replacement of tidal flat carbonate sediment coarse crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines fenestrae desiccation cracks and evaporite dissolution voids are occluded with dolomite cement which is commonly complexly zoned zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement saddle dolomite in pore centers dissolutional voids and tectonic fractures formed during late burial from warm fluids associated with the migration of mvt brines the effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal maynardville and the underlying conasauga group carbonate deposits and between the peritidal deposits of the upper maynardville and the rest of the overlying knox group the depositional environments and early diagenesis also affected the burial diagenesis the changes in regional facies distribution caused by carbonate platform progradation at the end of grand cycle deposition resulted in a decrease in the proportion of siliciclastic components this decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits the maynardville is conformably overlain by carbonate deposits of the knox group this allowed the maynardville knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the ordovician foreland sevier basin to the southeast during the conversion from a passive into a convergent continental margin the nolichucky shale separates the maynardville knox succession from the carbonate deposits of the underlying conasauga group which were in contrast affected by the migration of burial fluids from the cambrian intrashelf shale basin to the west

diagenesis of a se asian cenozoic carbonate platform margin and its adjacent basinal deposits abstract this study provides insights into the diagenesis of a se asian tertiary carbonate platform and the variations in alteration that occur across platform top margin slope and adjacent basinal deposits a petrographic cathodoluminescent and stable isotopic study allowed evaluation of the diagenesis of the kedango limestone of borneo the kedango carbonate platform developed as a predominantly low to moderate energy system in the semi enclosed marine embayment of the kutai basin during the eocene to miocene early alteration through pervasive micritisation and minor isopachous fringing syntaxial overgrowth and bladed to radiaxial cements was due to marine diagenesis on the basis of limited relatively enriched stable isotope compositions and non luminescent cathodoluminescent cl characters minor early marine cementation formed just in the shallowest platform top and margin deposits and was likely promoted in high energy portions of the platform due to wave and or current activity dissolution cavities banded calcite cements with variable cl characteristics and depleted stable isotope compositions micritic sediment infiltration and alveolar textures are minor features but together are indicative of localised subaerial emergence and meteoric diagenesis the most prevalent and pervasive diagenetic feature of the deposits is the neomorphic alteration and replacement of metastable bioclasts and micritic matrix together with contiguous calcitisation of pore spaces burial fluids with a marine character are inferred as the parent diagenetic fluid since stable isotope compositions for the neomorphic spar is largely consistent with precipitation from se asian oligocene miocene seawater in a burial environment tectonic subsidence the protected semi enclosed marine embayment setting and probably terrestrial runoff together with component variability reflecting local environmental variations are inferred to have been major controlling influences on the diagenesis of the kedango platform variations in diagenesis across bathyal slope and shallow platform deposits of the kedango limestone are not as marked as in other platforms from the region

aspects of alluvial river bank recession some examples from the niger delta the recession of alluvial channel banks occurs as a sequence of mass movements or by gradual dissolution of bank materials criteria for recognizing and interpreting bank recessions due to mass movement and or erosion are presented using back analysis technique slope development processes are reconstructed as a sequence of bank failure episodes and erosional events various mechanisms of mass bank failure and recession as well as causative agents have been identified and analyzed a deterministic sensitivity technique based on partial differentiation can be used for evaluating the contribution of factors to bank recession and as a basis for selecting bank erosion remedial measures

stability of composite river banks the stability of a river bank depends on the balance of forces motive and resistive associated with the most critical mechanism of failure many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size geometry and structure of the bank the engineering properties of the bank material the hydraulics of flow in the adjacent channel and climatic conditions rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt clay bank erosion occurs by fluvial entrainment of material from the lower cohesionless bank at a much higher rate than material from the upper cohesive bank this leads to undermining that produces cantilevers of cohesive material upper bank retreat takes place predominantly by the failure of these cantilevers three mechanisms of failure have been identified shear beam and tensile failure the stability of a cantilever may be analysed using static equilibrium and beam theory and dimensionless charts for cantilever stability constructed application of the charts requires only a few simple measurements of cantilever geometry and soil properties in this analysis the effects of cracks and fissures in the soil must be taken into account these cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface following mechanical failure blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue hence the rate of bank retreat is fluvially controlled even though the mechanism of failure of the upper bank is not directly fluvial in nature this cycle of bank erosion undermining cantilever failure and fluvial scour of the toe operates over several flood events and has important implications for river engineering channel changes and the movement of sediment through fluvial systems

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

triassic latemar cycle tops subaerial exposure of platform carbonates under tropical arid climate the triassic latemar platform in the dolomites italy is the site of several ongoing controversies perhaps the most interesting debate focuses on apparent cyclic deposition within the latemar platform whose nature and duration are still open to debate further disagreement concerns the lack of meteoric diagenesis related isotope shifts at cycle tops that bear circumstantial petrographic evidence for subaerial emergence here an evaluation of the nature of latemar cycle tops is presented combining evidence from previous work and new field petrographic and geochemical data cycle tops are ranked according to increasing exposure duration and spatial extent type i surfaces lacking unequivocal evidence of prolonged supratidal conditions type ii dolomite caps formed in warm evaporitic intertidal lagoonal waters followed by exposure of perhaps intermediate duration type iii clastic rich red calcareous horizons with some showing platform wide extent representing prolonged supratidal conditions and type iv discontinuities in tepee belts genetically related to type ii and iii surfaces but likely representing shorter lived exposure stages petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking this is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that where present are here interpreted as marine vadose in origin the scarcity of carbon and oxygen isotope signatures commonly assigned to subaerial exposure stages is best explained in the context of mid triassic climate the low latitude tropical but arid setting of the latemar situated in the western extension of the tethys ocean its isolation from nearby continental areas and overall short term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops high amounts of aeolian clastic material beneath some cycle tops along with high fe and mn elemental abundances argue for intermittent subaerial conditions this study proposes an enhancement of the classical allan and matthews 1982 isotope model for subaerial exposure under strongly arid climates as the subaerial exposure nature of latemar cycle tops and therefore eustasy as the cause for cyclicity have been previously challenged due to the lack of meteoric induced isotopic signatures the outcome of this study is of significance for the ongoing latemar stratigraphic controversy

syn rift geofluids in fractures related to the early middle miocene evolution of the valles penedes half graben ne spain abstract the valles penedes half graben located in the central part of the catalan coastal ranges is ne sw oriented 10 14 km wide and up to 100 km long and is filled mainly with continental to marine siliciclastic deposits ranging from late oligocene early miocene to late miocene the syn rift burdigalian continental deposits are well stratified s 0 approx 090 15n red beds affected by nnw sse trending nearly vertical extensive fractures middle burdigalian early langhian in age these extensive fractures which cut quasi perpendicularly the stratification are filled with calcite veins presenting two filling stages the first stage is formed by millimetric to centrimetric thick bands of crystalline calcite consisting of palisade calcite cement anhedral spar texture and rhombic calcite cement non to dull orange luminescent with 18 o values ranging from 9 0 to 5 2 pdb 13 c values varying from 9 3 to 6 7 pdb 87 sr 86 sr ratios ranging from 0 71088 to 0 71361 relatively high values of mg and variable values of mn sr and fe this stage is interpreted as subaerial speleothems rimstones which developed in meteoric vadose conditions the second stage is formed by euhedral spar megaspar cement and coconut meat cement with zoned bright yellow to orange luminescence with 18 o values ranging from 16 2 to 9 9 pdb 13 c values varying from 7 4 to 1 9 pdb 87 sr 86 sr ratios ranging from 0 71608 to 0 71800 and relatively low values of mg mn sr and fe this stage is interpreted as subaquatic cements developed in meteoric phreatic conditions the two stages of calcite fillings reflect successive circulation of two meteoric fluids with very different geochemical compositions and represent the change from vadose to phreatic environment during the evolution of the syn rift stage middle late burdigalian to late langhian the meteoric fluid producing the first filling stage was a superficial fluid proceeding directly from the surface influenced by soil co 2 the meteoric fluid generating the second stage was a more evolved fluid which had previously undergone interaction with sediments and reservoir effect the beginning of the thermal subsidence coupled with a change from a more arid to a hotter and perhaps wetter climate could had been the ultimate reasons for the observed evolution

diagenesis of quaternary bahamian beachrock petrographic and isotopic evidence abstract carbonate beachrock is a common feature on many beaches of san salvador island bahamas these well indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward constituent sediment grains are identical to those of adjacent unlithified beach sands beachrock exposed along the northern western and southern coastline of the island is commonly characterized by aragonite needle cement micritized grains and micritic envelopes which are features of early marine phreatic diagenesis these beachrocks have whole rock 13c values of 4 75 pdb and 18o values of 0 75 pdb which are identical to carbon and oxygen isotopic compositions of isolated aragonite needle cement and unlithified beach sand inclusion of recent artifacts in the rock indicates that these deposits are holocene beachrock from one locality on the western side of the island is covered with a calcrete crust indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic rich soils petrographic features include low mg calcite cement evidence of neomorphism of aragonitic cement and micritic rims to low mg calcite random needle fiber calcite calcified filaments and micritic networks whole rock 13c values of these beachrocks range from 3 66 to 6 85 and 18o from 2 16 to 3 68 the wide range of 13c values in these samples is probably a function of the relative proportions of meteoric vadose to meteoric and or marine phreatic cements in individual samples isolated meteoric phreatic cements are only slightly enriched in the light isotopes relative to marine cements whereas meteoric vadose cements are significantly lighter the diagenetic features suggest the beachrock passed through marine phreatic to meteoric phreatic and meteoric vadose diagenetic environments this diagenetic sequence commonly forms in response to sea level lowering evidence of deposition in a shoreline environment and prolonged residence in meteoric phreatic and meteoric vadose environments suggests that these beachrocks may be late pleistocene

characterization and origin of permeability porosity heterogeneity in shallow marine carbonates from core scale to 3d reservoir dimension middle jurassic paris basin france abstract nuclear magnetic resonance nmr stable isotope geochemistry of micro sampled cores nmr well logs and 3d modeling are used to investigate the carbonate permeability porosity heterogeneity along 230 m thick limestones of the paris basin despite the global low porosity and permeability of the limestones two aquifers units with porosity greater than 15 were identified these two aquifers are very different in terms of pore through radii and nmr signal the first one a1 aquifer 1 is a 7 m thick mudstone unit dominated by extended microporosity with pore throat radii of 0 25 m to 0 3 m the second one a2 aquifer 2 is a 15 m thick oolitic grainstone units showing macropores reaching 100 m and pore throat radii of 32 m from core descriptions and wireline logs on 26 wells a 3d static geological model is build the fine tuning of permeability calculations from nmr logs realized along 12 of the wells allows porosity and permeability heterogeneity to be distributed within a 3d model at the reservoir scale area of about 2000 km2 which match the flow behavior illustrated by well tests associated with early meteoric calcite cements and poorly developed burial blocky calcite cements the porous and permeable intervals may be predicted in two stratigraphic and diagenetic considerations firstly the syn sedimentary meteoric dissolution or neomorphism of the initial high magnesium calcite and aragonite particles or clasts into low magnesium calcite particles or cements prevented most mechanical and chemical compaction during the first steps of burial secondly the regional stratigraphic architecture reveals the presence of local permeability barriers which prevented early cretaceous lateral meteoric water circulation and the associated burial calcite cementation

carbon and oxygen isotopic composition of holocene reefal carbonates carbon and oxygen isotopic compositions have been determined for holocene reefal carbonate grains and cements in samples from enewetak bikini bermuda and belize to identify components that exhibit systematic isotopic fractionation relationships these findings are compared with fractionation relationships determined from experimentally precipitated low magnesium calcite lmc aragonite and high magnesium calcite hmc to determine if these relationships are observed in naturally precipitated carbonates compositional variation within individual samples is extreme up to 6 in 13 c pdb and 3 in 18 o pdb experimental fractionation relationships fail to predict the range of observed compositions aragonite cements are enriched 1 0 to 1 4 in 13 c and 1 5 to 2 0 in 18 o relative to calcite equilibrium compositions hmc cements are enriched by 0 3 to 0 4 in 13 c and by 1 8 to 1 9 to 18 o for carbon these results are in agreement with the experimental work of rubinson and clayton for oxygen our results suggest that fractionation factors of both aragonite and hmc may have been underestimated skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co occurring skeletal carbonates

on river cross sectional change in the niger delta a network of dominantly distributary river systems dissects the superficial deposits of the niger delta comprising alluvial sediments changes in river cross sections are instigated mainly by bank failures fluctuations in discharge and bed degradation by fluvial processes the relative importance of factors causing river cross sectional change was ranked based on a deterministic sensitivity technique involving partial differentiation of soil properties flow characteristics and geometrical parameters of the river channels analysis suggests that steep bank inclination and high flow velocity discharge are the major causes of cross sectional change while interlocking of soil grains is the major erosion restraining factor sensitivity coefficients were used further to generate susceptibility indices indicating the vulnerability of channel cross sections to change based on this the risks of channel cross sectional change were compared at different sites

diagenetic history of pipe creek jr reef silurian north central indiana u s a abstract calcite cements in the silurian ludlovian pipe creek jr reef north central indiana are compositionally zoned with characteristic minor element concentrations and stable isotope signatures and were precipitated in different diagenetic environments superposition and crosscutting relationships allow us to group cement zones and to relate them to the sequence stratigraphic evolution of the reef pipe creek jr reef grew in normal marine waters with the reef top high greater than 50 m above the platform floor flank facies are volumetrically important and are preserved largely as limestone in contrast to most dolomitized silurian reefs in the midcontinent syndepositional marine cements fill primary porosity and synsedimentary fractures and are interlayered with marine internal sediment now low magnesium calcite their isotopic compositions are similar to those of depositional grains and cements estimated to have precipitated from ludlovian sea waters depositional porosity was reduced by 75 by the precipitation of these syndepositional cements which stabilized the steeply dipping flank slope postdepositional clear calcite cements are interpreted as shallow phreatic and burial cements on the basis of their relationship to periods of karstification and fracturing shallow phreatic cements with concentric cathodoluminescent cl zonation precipitated in primary pores and are postdated by fractures and caves filled with middle devonian sandstone cl zonal boundaries are sharp and some near a major stratigraphic unconformity show evidence of dissolution the volumetric abundance of the individual cl zones varies in the reef indicating a complex superposition of waters of varying chemistry and rock water interaction that are probably related to relative sea level changes this important aspect of the reef stratigraphy is recorded only by the diagenetic succession because evidence of earlier sea level changes is removed by a major later regional unconformity burial cements are the youngest diagenetic feature recognized and they rest conformably or unconformably over older cements they exhibit both concentric cl zonation and sectoral zoning they are ferroan to nonferroan and they contain thin sulfide zones along growth band boundaries their isotopic compositions do not overlap with shallow phreatic or marine cement values degraded oil postdates burial cements and is composed of the same sterane class as the devonian age antrim shale the probable source rock this source contrasts with that of reef reservoirs in the michigan basin where silurian strata are commonly the hydrocarbon source

two hundred million years of karst history dachstein limestone hungary platform carbonates of the upper triassic dachstein limestone in naszaly hill have been karstified extensively over the past 200 million years they provide an excellent example of polyphase karstic diagenesis that is probably typical of many subaerially exposed carbonate sequences seven karstic phases are recognized in the area each of which include polyphase karstic events the first karst phase was associated with the lofer cycles meteoric waters caused dissolution enlarged fractures and cavities were filled by marine and or vadose silts and cement the second karst phase was caused by local tectonic movements bedding plane controlled phreatic caves were formed and filled by silts the third karst phase lasted from the end of the triassic to the eocene this was a regional multiphase karstic event related to younger composite unconformities bauxitic fill is the most characteristic product of this phase the karst terrain reached its mature or senile stage with very little porosity narrow veins and floating rafts of white calcite marks karst phase 4 which resulted from hydrothermal activity associated with palaeogene magmatism the early rupelian phase of alpine uplift caused large scale rejuvenation of the former karst terrain karst phase 5 subsequently naszaly hill was buried as an area of juvenile karst with significant porosity a second period of hydrothermal activity in the area karst phase 6 was induced by miocene volcanism which resulted in wide pale green calcite veins finally karst phase 7 was of tectonic origin following the most recent miocene uplift of the naszaly hill the carbonates have again become the site of vadose karst development

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

evolution and distribution of porosity associated with subaerial exposure in upper paleozoic platform limestones west texas middle pennsylvanian lower permian limestones in the subsurface of west texas were studied to determine how subaerial exposure and freshwater diagenesis karstification affected porosity distribution in meter scale cycles approximately 87 depositional cycles are present in the gross reservoir interval depths of 2600 3000 m and each cycle is interpreted to represent a glacio eustatic sea level fluctuation using recent radiometric age dating average cycle duration is estimated at 160 000 yr per cycle reservoir grade porosity 4 occurs in 5 25 of the gross reservoir section porosity is stratified occurring in 1 6 m thick intervals in the upper part of cycles that were subaerially exposed however many cycles that were subaerially exposed now lack porosity diagenesis and porosity development have distinct patterns related to duration of subaerial exposure four stages of porosity development are identified 1 very brief or no subaerial exposure estimated at less than 5000 yr caused little or no diagenetic change 2 brief to moderate subaerial exposure estimated at 5000 50 000 yr resulted in most primary pores being filled with calcite cement and dissolution creating fine matrix pores molds and intercrystalline pores 3 moderately long subaerial exposure estimated at 50 000 130 000 yr resulted in cements filling primary pores and some fine secondary pores and dissolution creating small conduit pores vugs fractures fissures 4 prolonged subaerial exposure estimated at greater than 130 000 yr resulted in most primary and secondary matrix pores being filled with calcite cement but dissolution enlarged conduit pores vugs fractures fissures present subsurface porosity in this field preferentially occurs in thick grainstones phylloid algal boundstones and a few wackestone packstones in cycles subjected to brief subaerial exposure stages 2 and 3 matrix porosity molds intercrystalline pores is dominant because most conduit pores formed during prolonged subaerial exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales the distribution of porosity in the southwest andrews area indicates that duration of subaerial exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to subaerial exposure and karstification

meteoric diagenesis marine diagenesis and microporosity in pleistocene and oligocene limestones enewetak atoll marshall islands abstract microporosity occurs in pleistocene and oligocene limestones cored on enewetak atoll in cores of pleistocene limestone 10 80 m deep mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution chalkification leaving a porous meshwork of aragonite needles microporous aragonite is most abundant in 5 10 m thick intervals below zones of intense diagenesis associated with paleo freshwater lenses extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite most calcitized aragonite neomorphic calcite formed by precipitation of sparry calcite in over microporous aragonite aragonitic fossils were not converted to microporous microcrystalline calcite therefore data from the pleistocene of enewetak suggest that widespread microporous microcrystalline calcite does not form directly from pure aragonite at least not in open meteoric systems or mixing zones microporous micrite occurs in oligocene wackestones 930 m deep deposited in a slope environment the microporous micrite is characterized by equant and prismatic crystals of low magnesium calcite lmc equant crystals are anhedral to euhedral and 2 10 m in diameter prismatic calcite crystals are subhedral 5 10 m wide and 20 50 m long stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high magnesium calcite hmc micrite in deep seawater undersaturated with respect to hmc but supersaturated with respect to lmc probably near the sediment water interface mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the thamama group cretaceous persian gulf see moshier 1989 equant microcrystals of calcite in the oligocene micrites are similar to those in microporous parts of the thamama group widespread microporosity in lmc limestones could form by alteration of hmc micrite in mixing zones or in seawater undersaturated with respect to hmc but supersaturated with respect to lmc

diagenetic trends of the pleistocene calcareous ridges mersa matruh area egypt abstract ooids are a major constituent of the pleistocene carbonates along the northwestern coastal zone of egypt the mersa matruh area was selected for a detailed diagenetic study of these rocks 18 o 13 c and sr values were determined on whole rock samples on manually separated ooids and calcite cements plerographic examination shows that these rocks are partly consolidated ooid grainstones with low mg calcite cement in granular voids oxygen isotopic and sr values for the whole rock 0 5 vs pdb and 9300 ppm respectively are consistent with data on limestones of the same age in contrast oxygen isotopic values for ooids ave 0 5 vs pdb are heavier relative to values for low mg calcite cements ave 1 4 vs pdb and sr contents are higher for ooids 10 000 ppm in addition 13 c values are identical for the whole rock and ooids ave 4 5 vs pdb and heavier relative to 13 c values of the calcite cements ranging from 1 4 to 3 8 vs pdb these data may suggest that calcite cements are formed in non diluted pore water under relatively high rock water ratios

coupled simulations of fluvial erosion and mass wasting for cohesive river banks the erosion of sediment from riverbanks affects a range of physical and ecological issues bank retreat often involves combinations of fluvial erosion and mass wasting and in recent years bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models in related work finite element seepage analyses have also been used to account for the influence of pore water pressure in controlling the onset of masswasting this paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion finite element seepage and limit equilibrium stability models are for the first time fully coupled application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where i there is no fluvial erosion and the bank geometry profile remains constant throughout ii there is no fluvial erosion but the bank profile is deformed by simulated mass wasting and iii the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting the results are limited in scope to the specific conditions encountered at the study site but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile or creating overhangs thereby triggering mass wasting however feedbacks between the various processes also lead to unexpected outcomes specifically fluvial erosion also affects bank stability indirectly as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank thereby modulating the evolution of the pore water pressure field consequently the frequency magnitude and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included

late jurassic paleogeography and paleoclimate in the northern iberian basin of spain constraints from diagenetic records in reefal and continental carbonates abstract sedimentation in the northern part of the iberian basin during the early kimmeridgian was characterized by the development of coral reef complexes of the torrecilla en cameros formation which were situated along a seaway that connected the boreal and tethys domains along the northern portion of that seaway close to the boreal sea early diagenesis of the torrecilla fringing reef complex was controlled by rising sea level and by local tectonism leading to alternations of submergence and reefal exposure while exposed reef corals were neomorphosed and dissolved when the corals were submerged secondary porosity was filled by calcite cements precipitated from marine derived waters during the latest stages of reef development rate of sea level rise could no longer keep pace with tectonic uplift and while younger accretionary units continued to be deposited older units were exposed subaerially leading to the precipitation of meteoric calcite cement and to the development of a paleosol this stage of diagenesis also affected the reef complex during late kimmeridgian times as the boreal and tethys coastlines progressively retreated to the north and southeast respectively progressive retreat of the boreal sea and subsequent late jurassic to early cretaceous rifting that formed the cameros basin resulted in the deposition of tithonian continental carbonates directly on these reefal units oxygen isotope compositions of these continental carbonates are 3 more negative than kimmeridgian meteoric cement suggesting an evolution in the isotopic composition of meteoric water in response to an increasing continental effect on meteoric precipitation in contrast early kimmeridgian reef complexes of the torrecilla en cameros formation in the southern sectors of the seaway closer to the tethys sea were exposed during the late kimmeridgian as the tethys coastline retreated leading to dissolution of corals and subsequent precipitation of meteoric calcite cement oxygen isotope compositions of these meteoric cements are 2 more negative than time equivalent meteoric calcites precipitated in the northern torrecilla reef complex given the regional paleogeography this difference suggests that sources of meteoric water must have been different for the northern and southern sectors and probably reflects southward transport of air masses and water vapor from the boreal sea to the northern torrecilla sector and northwest transport of water vapor from the tethys sea to the southern sectors tithonian berriasian continental carbonates which unconformably overlie reef complexes in the southern and northern sectors exhibit an opposite trend with the 18o of continental carbonates in southern sectors being 1 6 more positive than time equivalent carbonates that overlie the torrecilla reef complex to the north this shift in the trend of the isotopes between northern and southern sectors is coeval with paleogeographic changes that occurred in response to late jurassic early cretaceous rifting of the iberian plate and the progressive retreat of boreal and tethys coastlines during this time continental carbonates in both the southern and northern sectors of the seaway precipitated from meteoric waters that were sourced from the tethys sea to the southeast more negative 18o values in the northern torrecilla sector reflect enhanced continental fractionation effects as the influence of the boreal sea derived meteoric waters diminished

early submarine cementation in fore reef carbonate sediments barbados west indies bottom sediments from the sea floor west of barbados between depths of 110 and 324 m are composed of nodular or crusted carbonate deposits individual biogenic sediment grains and the cemented aggregates nodules and crusts are usually more or less altered by bioerosion and may support one or more generations of encrusting organisms on the basis of component analysis of the topmost part of the bottom sediments it is possible to recognize three facies 1 a proximal slope facies down to a depth of about 140 m rich in mollusc fragments benthic foraminifera and bryozoans 2 an upper distal slope facies between about 140 m and about 215 m rich in benthic foraminifera molluses and crustaceans and 3 a lower distal slope facies from about 215 m to at least 300 m dominated by molluscs especially pelecypods with subordinate scleractinians and tubes of the polychaete lygdamus asteriformis the appearance and quantitative importance of the cemented aggregates is also related to these facles in the proximal slope facies only relatively few irregular and very porous nodules are found whereas in the lower distal slope facies aggregates are very common most aggregates are crust like with a smooth upper surface and a more or less irregular knobby lower surface the crusts are massive compared with the nodules of the proximal slope facies and unlike the latter the lower surfaces and walls of larger cavities are usually coated with fe and or mn oxides in the upper distal slope facies a gradual transition between the two types of aggregates is found petrographical and morphological evidence together with carbon and oxygen isotopic data indicates that the nodules and crusts were formed in situ by submarine lithification processes radiocarbon dating of two bulk samples suggests that the cementation took place during late pleistocene and or early holocene

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

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 and porosity development in the middle and upper wabamun group southeast peace river arch alberta canada dolomitization and fracturing are critical to porosity development in most wabamun group reservoirs in the southern part of the peace river arch cores from peoria normandville and eaglesham fields and adjacent areas were examined to determine the relationship between depositional facies dolomitization fracturing and porosity three dimensional 3 d seismic data indicate that most upper wabamun dolomites are coincident with mound like structural highs at the top of the wabamun with structural relief due largely to differential compaction between dolomite and adjacent limestone the dolomite bodies generally are small only 100 300 m across which makes 3 d seismic data very helpful in wabamun exploration when this depositional diagenetic model for dolomitization w s integrated into 3 d seismic interpretations canadian occidentaps discovery rate in the wabamun increased from approximately 25 to 80 dolomitization and porosity are laterally quite variable in the wabamun with the 200 250 m wabamun section changing laterally from porous dolomite to nonporous limestone in less than 1 km sometimes as little as 150 m two generations of replacive dolomite were identified early near surface and late hydrothermal the early dolomite is facies fabric and mineralogy selective with some burrow fills and aragonitic mollusks being preferentially dolomitized in dolomitic limestones complete dolomitization is common in unfossiliferous carbonates mudstones and peloid wackestones and packstones which are interpreted as being deposited on paleotopographic highs and dolomitized by seawater or slightly evaporated seawater during shallow burial most fracturing occurred after early repla ive dolomitization late hydrothermal dolomitization was not strictly facies or fabric selective features associated with hydrothermal dolomites include vugs fractures collapse breccias white dolomite cement anhydrite and coarse calcite cement although anhydrite and coarse calcite cement precipitated distinctly after hydrothermal dolomitization late dolomitization karst like dissolution and collapse apparently were localized near the margins of the early dolomites where adjacent limestones were dolomitized or dissolved dissolution of calcite in partially dolomitized limestones resulted in geopetal accumulations of dolomite rhombs porous sucrosic dolomite in some fractures and burrow fills hydrothermal fluids apparently moved updip through the underlying winterburn and or leduc formations an then upward through porous early dolomites in the wabamun causing dolomitization of adjacent limestones and karst like dissolution stable oxygen isotope ratios of the dolomites are quite variable dgr18o ranges from 2 0 to 11 4 pmil pdb supporting multiple stages of dolomitization during progressive burial

survival patterns of macrobenthic marine assemblages during the end permian mass extinction in the western tethys dolomites italy abstract the ecological competition between brachiopods and bivalves is analysed by means of a quantitative palaeoecologic method applied on four assemblages located within a short stratigraphic interval approximately 2 m thick in the lower tesero member of the werfen formation in the southern alps the assemblages originate from the tesero bulla and sass de putia sections the analysed stratigraphic interval uppermost changhsingian in age is located between the early and heaviest phase of the end permian mass extinction which occurred across the bellerophon werfen formational boundary event boundary and the permian triassic boundary chronological boundary when nearly all the permian stenotopic holdovers disappeared these assemblages are characterised by small sized skeletons lilliput effect which represent an adaptive survival strategy in stressed and harsh habitats resulting from the climatic and palaeoceanographic changes connected with the mass extinction the tesero assemblages are dominated by rhynchonelliform brachiopod orbicoelia bed cnt10 or streptorhynchus bed cnt11a which were mostly attached at the top of shallow microbialitic mounds these assemblages are again dominated by permian stenotopic taxa and show a palaeozoic structure the tesero habitat which again permitted the survival of brachiopods represented one of the last refuges in the western tethys on the contrary the bulla bu9 10 and sass de putia wpk13a assemblages are bivalve dominated and thus show an ecologic structure typical of early triassic post extinction marine benthic communities or palaeozoic stressed marine communities the bivalve dominated assemblages proliferated in prevailing muddy siliciclastic substrates with brief episodes of microbial algal growth the most important environmental limiting factors and leading causes of end permian mass extinction are discussed in terms of palaeoautecologic and palaeosynecologic analysis the different taxonomic composition and ecologic structure of the assemblages is related to palaeogeography including water depth and connections with the open sea the brachiopod dominated assemblage exclusive of the tesero section proliferated in microbial carbonate habitats in near shore environments the bivalve dominated assemblages which were more widespread than the brachiopod assemblages in the dolomites and also occurred in other western tethys localities occur in more open and deeper marine environments in the western tethys margins the local distribution of mixed faunas suggests that the extinction of permian stenotopic taxa was caused by the onset of poisonous water on the shelves originating from deep marine environments this extinction pattern appears to be a regional phenomenon and does not seem be applicable on a global scale the extinction events were controlled by a complex network of interactive factors and the survival of faunal elements was probably stochastic

geochemistry of meteoric calcite cements in some pleistocene limestones in this study the stable isotope and trace element geochemistries of meteoric cements in pleistocene limestones from enewetak atoll western pacific ocean cat island bahamas and yucatan were characterized to help interpret similar cements in ancient rocks meteoric calcite cements have a narrow range of 18o values and a broad range of 13c values in each geographical province these pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location enewetak calcite cements have a mean 18o composition of 6 5 0 pdb and 13c values ranging from 9 6 to 0 4 0 pdb sparry calcite cements from cat island have a mean 18o composition of 4 1 0 and 13c values ranging from 6 3 to 1 1 0 sparry cements from yucatan have a mean 18o composition of 5 7 0 and 13c values of 8 0 to 2 7 0 the mean 18o values of these pleistocene meteoric calcite cements vary by 2 4 0 due to climatic variations not related directly to latitude the 13c compositions of meteoric cements are distinctly lower than those of the depositional sediments variations in 13c are not simply a function of distance below an exposure surface meteoric phreatic cements often have 13c compositions of less than 4 0 0 which suggests that soil derived co2 and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements concentrations of strontium and magnesium are quite variable within and between the three geographical provinces mean strontium concentrations for sparry calcite cements are for enewetak atoll 620 ppm 510 ppm for cat island 1200 ppm 980 ppm and for yucatan 700 ppm 390 ppm equant cements intraskeletal cements and bahamian cements have higher mean strontium concentrations than other cements equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements magnesium concentrations in pleistocene meteoric cements are similar in samples from enewetak atoll mean 1 00 mol mgco3 0 60 mol mgco3 and cat island mean 0 84 mol mgco3 0 52mol mgco3 but yucatan samples have higher magnesium concentrations mean 2 20 mol mgco3 0 84mol mgco3 higher magnesium concentrations in some yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

controls on early diagenetic overprinting in icehouse carbonates insights from modeling hydrological zone residence times using carb3d abstract icehouse carbonate diagenesis is complex with carbonate sequences being exposed to multiple meteoric hydrozones vadose zone freshwater lens mixing zone on multiple occasions a forward modeling approach using carb3d has enabled quantitative exploration of the nature of this diagenetic overprinting by explicit simulation of meteoric hydrozone residence times during evolution of an isolated carbonate platform a sensitivity analysis was performed encompassing the fundamental controls on hydrozone residence time frequency and magnitude of sea level variation subsidence rate and climate the latter via its effect on freshwater lens thickness and rate of dissolutional lowering of the land surface residence time within all meteoric hydrozones is characterized by a high degree of lateral continuity vertical continuity is much more limited with significant variations on a fourth order sequence scale across the platform below sequence boundaries prolonged exposure to the vadose zone results from multiple missed beats when the platform remains exposed at relative sea level maxima overprinting gives total meteoric residence times significantly in excess of the duration of exposure of the overlying sequence boundary there is a hierarchy of eustatically driven overprint cycles with higher frequency cyclicity modifying the primary signal generated by lower frequency cycles fifth order cycles thus have only a minor impact on hydrozone residence times whilst third order seismic scale cycles are a critical control and could be used as a first order approximation of the amount of diagenetic overprinting subsidence has a strong control on residence time with greater overprinting at lower subsidence rates but in high subsidence regimes sediment packages move below the zone of meteoric water influence sufficiently rapidly that residence time can be minimal high land surface dissolution rates which result from a wetter climate remove a large proportion of the subaerially exposed sediment and reduce vadose zone residence time higher effective recharge can increase the thickness of freshwater lens and mixing zones which increases residence times significantly residence times in the vadose zone can be directly linked to a sequence stratigraphic framework but for the freshwater lens and the mixing zone this is possible only with a well constrained relative sea level history