the belize margin revisited 1 holocene marine facies belize is best known for its 260 km long barrier reef and associated lagoon reef shoals but also exhibits a complete transition from near shore siliciclastics to pure carbonate deposits across a narrow shelf lagoon and in addition has three of the very few occurrences of caribbean atolls published holocene facies relationships on the belize shelf have been semi quantitative in that they are based on quantitative point count thin section data in the northern shelf lagoon and on qualitative estimates of thin section composition in the southern shelf lagoon this contrast has been rectified by point counting southern shelf lagoon thin sections and incorporating the result with published point count data from the northern shelf supplemented by additional information to produce a modified factor analysis facies distribution for the entire shelf the resulting nine facies are extended seaward to include previously published analysis of sediment on the three belize off shelf atolls resulting in a total of 11 facies in addition the carbonate mud distribution has been mapped over both the shelf and atolls with the not unexpected result that in places the distribution pattern clearly indicates a barrier platform source of lagoon deposited carbonate mud dunham s limestone classification terminology has been used as a basis for description to make these relationships more useful to those reconstructing ancient limestone depositional environments

early dolomitization and recrystallization of carbonate in an evaporite basin the middle devonian ratner laminite in southern saskatchewan canada this study provides an example of early dolomitization of basinal limestone associated with a giant evaporite deposit the middle devonian ratner carbonate is extensively to completely replaced by microcrystalline to finely crystalline dolomite in the southern and central part of southern saskatchewan in the elk point basin in the northern part however individual cored intervals of the ratner laminite are either completely dolomitized or contain no microcrystalline to finely crystalline dolomite stratigraphic petrographical and sr isotopic data suggest that the ratner dolomite has formed penecontemporaneously from dense evaporitic seawater that percolated downward the distribution of the ratner dolomite is highly localized in the northern part of the study area suggesting that the downward flow of mg 2 rich fluids was restricted by interbedded anhydrite but where the fluids penetrated the underlying carbonate deposits were dolomitized the depletion in 18 o isotope varying from 4 5 to 6 6 pdb and the coarser and inhomogeneous textures of the ratner dolomite are considered to be the result of recrystallization which is interpreted to occur at elevated temperatures related to a thermal anomaly during the late devonian and carboniferous burial

microbial deposits in the aftermath of the end permian mass extinction a diverging case from the mineral mountains utah usa the lower triassic mineral mountains area utah usa preserves diversified smithian and spathian reefs and bioaccumulations that contain fenestral microbialites and various benthic and pelagic organisms ecological and environmental changes during the early triassic are commonly assumed to be associated with numerous perturbations productivity changes acidifica tion redox changes hypercapnia eustatism and temperature changes post dating the permian triassic mass extinction new data acquired in the mineral mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures these data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the smithian early spathian the studied section records a large scale depositional sequence during the dienerian spathian interval during the transgression depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral microbial limestones shallow subtidal marine sponge microbial reefs to deep subtidal mud dominated limestones storm induced deposits microbialite sponge reefs and shallow subtidal deposits indicate the regression three microbialite associations occur in ascending order i a red beds microbialite association deposited in low energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure ii a smithian microbialite association formed in moderate to high energy tidal conditions where microbialites include stromatolites and associated carbonate grains oncoids ooids and peloids and iii a spathian microbialite association developed in low energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation hydrodynamics bathymetry and grain supply this study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions due to the presence of numerous metazoans associated with microbialites throughout the smithian increase in accommodation and spathian decrease in accommodation the commonly assumed anachronistic character of the early triassic microbialites and the traditional view of prolonged deleterious conditions during the early triassic time interval is questioned

origin of massive dolostone the upper knox model abstract previous studies of cambro ordovician dolostones from various sites in the u s midcontinent revealed similar textures and geochemistry prompting similar explanations of their origin before these interpretations can be incorporated in a model to explain other massive dolostones they need verification in a contrasting setting the dupont geohydrological survey dghs well continuously cored through the knox group in west central tennessee provides such a contrast this core provides a complete sample of the knox in the setting of a shallowly buried flat lying structurally and stratigraphically simple platform sequence which contrasts with the relatively deeply buried and structurally complex environments that dominate most prior studies light and luminescence petrography xrd sem eds xrf esr and mass spectrometry were used to document the petrology and geochemistry of the upper knox group lower ordovician from 456 m to 1126 m subsea 2018 ft to 4215 ft below the kelly bushing of the dghs well of the 87 samples 81 are dolostone early e 1 dolomite forms a fine to medium crystalline replacive dolostone that lacks luminescence banding its geochemical signature is 13c 2 83 0 76o oo 18o 5 85 2 01o oo stoichiometry 51 6 1 2 mole ca mnpr 79 27 fe 681 237 ppm mn 79 32 ppm and sr 110 63 ppm e 2 dolomite forms a medium to coarse crystalline replacive dolostone that luminescences with a relatively dull core and brighter red rim its geochemical signature is 13c 2 07 0 32o oo 18o 6 96 1 28o oo stoichiometry 51 0 1 3 mole ca mnpr 74 20 fe 880 843 ppm mn 84 52 ppm and sr 79 29 ppm late l 1 dolomite lines vugs and fractures three limestone beds are unaltered remnants of the precursor to e 2 dolomite calcite is present both as relics in dolomite rhombs and as a vug and fracture fill postdating l 1 dolomite three quartz arenite beds and the presence of 1 to 5 well rounded quartz sand grains in many dolostones denote an intermittent clastic source e 1 dolomite began as syndepositional dolomicrite deposited in an upper intertidal to supratidal environment and it was later modified during shallow burial e 2 dolomite formed when subtidal grainstones and packstones were dolomitized during shallow burial l 1 is a late void fill texture and geochemistry of dolostone in the dghs well is similar to that from coeval appalachian ozark and texas oklahoma units numerous vugs fractures and zones of rubble occur throughout the upper knox core suggesting that multiple exposures occurred during its deposition there is no pattern of change with depth for these features or for any other measured or observed parameter we suggest that massive dolostone formation involving dolomite grain growth calcite replacement and dolomite cement took place in association with a succession of exposure related events the large volume of dolomite thus created required a large amount of magnesium so large in fact that a volume of seawater equal to that of the entire modern ocean must have pumped through collective upper knox exposure surfaces

origin of high h2s concentrations in the upper permian changxing reservoirs of the northeast sichuan basin china abstract intense thermochemical sulfate reduction tsr and up to 18 h 2 s are found in the upper permian changxing formation p 3 ch in the northeast ne sichuan basin china despite that rare gypsum or anhydrite was found in this formation here we present new concentration data of carbonate associated sulfate cas from carbonate host rocks c o and sr isotope data for tsr related calcites and s isotope data for sulfur compounds obtained during this study these data along with spatial temporal changes in palaeogeopressure conditions hydraulic conductivity and the physical capacity indicate that the h 2 s was generated locally from tsr within the p 3 ch reservoirs we propose that the reactive sulfates were derived from cas released during dolomitization and recrystallization of earlier dolomite within the p 3 ch fm and from the cross formational migration of evaporative brines from the lower triassic feixianguan formation t 1 f to p 3 ch fm our calculation shows that the two sources could provide enough so 4 2 for the generation of h 2 s within the p 3 ch reservoirs early downward migration of sulfate rich evaporative brines from the t 1 f formation occurred in near surface and shallow burial diagenetic settings mainly 3 ch fm subsequently tsr calcites precipitated from the pore water have high sr concentrations up to 7767 ppm close to the t 1 f tsr calcites and 87 sr 86 sr ratios mainly from 0 7074 to 0 7078 which are significantly higher than those of late permian seawater but within the range of early triassic seawater

in situ stable isotopic evidence for protracted and complex carbonate cementation in a petroleum reservoir north coles levee san joaquin basin california u s a abstract knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation the stevens sands is a sequence of marine shales and deep sea fan sands that was deposited within the miocene monterey formation in the south central part of the san joaquin basin california during the upper miocene 10 6 ma rapid high precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi collection mode in conjunction with electron microprobe analyses indicate that carbonate cement zones within the stevens sands at north coles levee ncl have had a complex and protracted fluid history three main generations of carbonate cement were identified the relative timing of carbonate cement precipitation within the stevens sands at ncl was estimated using the thermal and burial history of the san joaquin basin in situ oxygen isotope data and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite water and dolomite water precipitation of these cement zones began soon after sediment deposition 7 ma and is ongoing early dolomite was precipitated at a temperature of 10 c near the sediment water interface and soon after sediment deposition calcite cements which are the most abundant variety precipitated semicontinuously between 4 ma and 5 ma at temperatures between 50 c and 65 c and depths of 800 m to 1300 m fe dolomite which is paragenetically late appears to have precipitated at temperatures near 100 c in response to pore pressure reduction which accompanied exploitation of the gas cap within the last 35 years carbon in these cements was likely derived from several sources including marine maturing hydrocarbons and a zone of methanogenesis

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

a stable isotope study of microbial dolomite formation in the coorong region south australia abstract the coorong region of south australia is a classic location for the study of modern dolomite formation a number of ephemeral lakes here contain dolomite whilst other lakes in close proximity contain aragonite with minor magnesite our previous field and experimental work has revealed that sulphate reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area we here use a combination of oxygen carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments we report isotopic data from four dolomite and three aragonite containing lakes 34 s data for dissolved lake and porewater sulphates reveal 34 s enrichment of residual sulphate in all seven lakes consistent with bulk closed system microbial fractionation by sulphate reducing bacteria calculations using temperature dependent oxygen isotope fractionation factors between dolomite and the solution from which it precipitated are consistent with a strong microbial component to carbonate precipitation in these lakes a comparison of 18 o data from dolomite and aragonite sediment and from their associated lakewaters indicates seasonal mineral precipitation occurs late in the evaporative cycle of the lakes within a yoghurt like saturated mud horizon coincident with the zone of sulphate reduction surprisingly 13 c data from the carbonates do not show the characteristic wide range of values often associated with organogenic carbonates but instead show that these bacterially mediated precipitates incorporate carbon primarily from the inorganic lakewater reservoir partially diluted by an organic component a coorong type microbial model of dolomite formation may be particularly applicable to for example precambrian sedimentary dolomites where benthic microbial communities which are today restricted to extreme environments dominated the shallow marine ecosystem prior to the emergence of metazoa

subaqueous shrinkage cracks in the devonian of scotland reinterpreted abundant lenticular straight to curving cracks on bedding planes are commonly interpreted as subaqueously formed shrinkage cracks syneresis cracks the two most influential examples being those in the green river formation picard 1966 and the devonian orcadian basin of scotland donovan and foster 1972 cracked surfaces in the orcadian basin deposits show all gradations between lenticular cracks and well developed polygons of subaerial origin some lenticular cracks are inferred to have formed during subaerial desiccation many of the simplest lenticular cracks also have the shapes of gypsum crystals and originated as pseudomorphs the gypsum crystals were often important in providing nuclei for later desiccation cracks more complex crack patterns of incipient to fully formed desiccation polygons have arisen from desiccation of surfaces containing gypsum crystals or pseudomorphs resulting from gypsum crystal dissolution cracks of all types are preserved by sand infills introduced by wind transport across dried up lake floors reassessment of the literature reporting subaqueous cracks in rocks shows that many of these cracks are likely to have originated in a similar way partly as gypsum crystals and partly by limited subaerial desiccation the remaining examples have either already been reinterpreted in other ways or are insufficiently documented to rule out a subaerial origin there is no clear evidence for subaqueously formed cracks being preserved in rocks

dolomitization of the waulsortian limestone lower carboniferous in the irish midlands the waulsortian limestone lower carboniferous of the southern irish midlands is dolomitized pervasively over a much larger region than previous studies have documented this study indicates a complex multistage multiple fluid history for regional dolomitization partially and completely dolomitized sections of waulsortian limestones are characterized by finely crystalline 0 01 0 3 mm planar dolomite planar replacive dolomite is commonly followed by coarse 0 5 mm nonplanar replacive dolomite and pervasive void filling saddle dolomite cement is frequently associated with zn pb mineralization planar dolomite has average 18o and 13c values pdb of 4 8 and 3 9 respectively these are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the waulsortian 18o 2 2 13c 3 7 mean c and o isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement 7 0 and 3 3 7 4 and 2 4 respectively fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation the petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin possibly in equilibrium with modified carboniferous sea water where the waulsortian was exposed to hydrothermal fluids 70 280 c planar dolomite underwent a neomorphic recrystallization to a coarser crystalline planar and nonplanar dolomite characterized by lower 18o values void filling dolomite cement is isotopically similar to nonplanar replacive dolomite and reflects a similar origin from hydrothermal fluids this history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the irish midlands and provides a framework upon which to test competing models of regional vs localized fluid flow

early diagenetic dolomitization and dedolomitization of late jurassic and earliest cretaceous platform carbonates a case study from the jura mountains nw switzerland e france early diagenetic dolomitization is a common feature in cyclic shallow water carbonates throughout the geologic record after their generation dolomites may be subject to dedolomitization re calcification of dolomites e g by contact with meteoric water during emersion these patterns of dolomitization and subsequent dedolomitization frequently play a key role in unravelling the development and history of a carbonate platform on the basis of excellent outcrops detailed logging and sampling and integrating sedimentological work high resolution sequence stratigraphic interpretations and isotope analyses o c conceptual models on early diagenetic dolomitization and dedolomitization and their underlying mechanisms were developed for the upper jurassic lower cretaceous jura platform in north western switzerland and eastern france three different types of early diagenetic dolomites and two types of dedolomites were observed each is defined by a distinct petrographic isotopic signature and a distinct spatial distribution pattern different types of dolomites are interpreted to have been formed by different mechanisms such as shallow seepage reflux evaporation on tidal flats and microbially mediated selective dolomitization of burrows depending on the type of dolomite sea water with normal marine to slightly enhanced salinities is proposed as dolomitizing fluid based on the data obtained the main volume of dolomite was precipitated by a reflux mechanism that was switched on and off by high frequency sea level changes it appears however that more than one dolomitization mechanism was active pene contemporaneously or several processes alternated in time during early diagenesis percolating meteoric waters obviously played an important role in the dedolomitization of carbonate rocks that underlie exposure surfaces cyclostratigraphic interpretation of the sedimentary succession allows for estimates on the timing of early diagenetic de dolomitization these results are an important step towards a better understanding of the link between high frequency probably orbitally forced sea level oscillations and early dolomitization under mesozoic greenhouse conditions

whiting related sediment export along the middle miocene carbonate ramp of great bahama bank modern aragonite needles are present all along the modern leeward margin of great bahama bank odp leg 166 while middle miocene sediments contain needles only in more distal areas sites 1006 and 1007 in contrast to the rimmed flat topped platform topography during the plio pleistocene the miocene great bahama bank morphology is a carbonate ramp profile this might imply a different location and precipitation type for miocene aragonite needles in this study aragonite needles in miocene sediments were isolated using a granulometric separation method furthermore the isolation of the various carbonate components enables the identification of primary versus diagenetic components the miocene aragonite needles are concentrated in the finest granulometric sediment fractions 12 m the fraction specific geochemical analyses 13c 18o and sr elemental abundance represent useful tools to assess the possible sources of the aragonite mud the geochemical variation of the fractions rich in pristine aragonite needles and the characteristics of the needle morphology point to whiting phenomena as the main sediment source and algal fragmentation as a minor component both components indicate shallow water environments as the main sediment source area ramp top related fine grained particles now present at distal sites were likely exported as suspended material similar to present day transport mechanisms the scarcity of needles at proximal sites is probably linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded the granulometric separation approach applied here enables a better characterization of the finest carbonate particles representing an important step towards the discrimination between primary and diagenetic fine grained components

controls on formation and alteration of early diagenetic dolomite a multi proxy δ44 40ca δ26mg δ18o and δ13c approach abstract the full potential of the dolomite ca isotope proxy only unfolds when combined with data of the other main elements c o mg in the crystal lattice of mg carbonates data presented here reveal the level of complexity inherent to dolomite precipitation and alteration environments and add new constraints to the understanding of early diagenetic dolomite formation well constrained precambrian to pleistocene dolomites were investigated representing three characteristic formation and alteration environments i sabkha ii altered marine and iii lacustrine palustrine dolomites primary sabkha dolomites with typically low cation ordering degree cod high 13c and low 44 40ca values contrast with recrystallized sabkha dolomites with relatively high cod low 13c and high 44 40ca values both 13c and 44 40ca values of sabkha dolomite bear witness to the relative effects of kinetic and equilibrium ca isotope fractionation conditions primary sabkha dolomites display 44 40cadolomite fluid ranging from 0 4 to 1 3 whereas recrystallized dolomite is approaching isotopic equilibrium 44 40cadolomite fluid 0 using the fractionation factor deduced from the sabkha dolomite data set recrystallized precambrian dolomite points to a 44 40caseawater pore fluid of about 1 2 srm 915a a value that is distinctively lower compared to previously suggested ones and modern seawater altered marine dolomites display evidence for meteoric overprint indicated by 18o values as low as 4 62 both mg and ca isotope signatures correlate with 18o values of altered marine dolomites whereas 13c values lack correlation with the other isotope systems we propose that freshwater circulated through silicate aquifers prior to reaching the dolostone units and ca mg and o isotopes of altered marine dolomites reflect variable degrees of this meteoric overprint lacustrine palustrine dolomites display a correlation between the isotope values of c mg and ca these dolomites are formed during pulses of marine ingression in swamp playa and lake environments and are thus characterized by water logged conditions anaerobic and saline sulfate rich fluids bacterial sulfate reduction induces dolomite formation and leads to lower 13c and 44 40ca as well as to higher 26mg values the ca isotope proxy acts as a benchmark against which other proxy data can be calibrated or processes tested taking cod and dolomite stoichiometry into consideration the here documented multi proxy isotope approach is promising and provides benchmarks against which proxy signals can be calibrated and tested interestingly even the limited data sets shown here point to patterns that can be interpreted in a meaningful manner that is of relevance for dolomite research

holocene shallow subtidal dolomitization by near normal seawater northern belize calcic dolomite cements compose an average of 5 of the upper 4 3 m of subtidal deposits 18 o 2 compositions of the high sr dolomites mean october 1900 ppm together with near normal salinity and inherently normal mg ca ratio of pore fluids suggest precipitation from near normal seawater tidal and wind driven circulation of seawater through the sediments supplies most of the mg for dolomitization which appears to be promoted by elevated pore water alkalinity resulting from bacterially mediated oxidation of organic matter and locally early stages of methanogenesis rapid dolomitization here supports the idea that significant quantities of dolomite can form syndepositionally from normal seawater in shallow subtidal deposits

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

disseminated jigsaw piece dolomite in upper jurassic shelf sandstones central north sea an example of cement growth during bioturbation unusual textural and chemical characteristics of disseminated dolomite in upper jurassic shelf sediments of the north sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones the dolomite is present throughout the franklin sandstone formation of the franklin and elgin fields as discrete non ferroan generally unzoned subhedral to highly anhedral jigsaw piece crystals these are of a similar size to the detrital silicate grains and typically account for 5 of the rock volume the dolomite crystals are never seen to form polycrystalline aggregates or concretions or ever to envelop the adjacent silicate grains they are uniformly dispersed throughout the sandstones irrespective of detrital grain size or clay content dolomite authigenesis predated all the other significant diagenetic events visible in thin section the dolomite is overgrown by late diagenetic ankerite and bulk samples display stable isotope compositions that lie on a mixing trend between these components extrapolation of this trend suggests that the dolomite has near marine delta18o values and low positive delta13c values the unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near surface zone of active bioturbation sea water provided a plentiful reservoir of mg and a pore fluid of regionally consistent delta18o labile bioclastic debris e g aragonite mg calcite supplied isotopically positive carbon to the pore fluids during shallow burial dissolution such dissolution took place in response to the ambient calcite sea conditions but may have been catalysed by organic matter oxidation reactions bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework continued exchange of mg2 and ca2 with the sea water reservoir maintained a sufficient mg ca ratio for dolomite rather than calcite to form irregular crystal shapes resulted from dissolution of both the dolomite and the enclosed fine calcitic shell debris before ankerite precipitation during deep burial diagenesis

marine derived dolomite in the shallowly buried temperate port campbell limestone miocene otway basin australia the miocene port campbell limestone in the otway basin port campbell embayment south eastern australia is a shallowly buried 350 m temperate carbonate grainstone which consists primarily of benthonic foraminifera bryozoans brachiopods echinoids and planktonic foraminifera volumetrically insignificant calcite cements include scalenohedral blocky and syntaxial overgrowths dolomite is present in variable amounts 1 25 scattered throughout the unit as euhedral rhombs usually comprising 2 of the whole rock volume the dolomite post dates the calcite cements and is mainly an interparticle cement with crystal size varying between 10 and 150 m mean 50 m under cathodoluminescence the dolomite rhombs have a dull core and a nonluminescent outer rim the dolomite is nonstoichiometric ca rich ca54 62mg38 46 with high trace element concentrations the mn concentrations range from 0 to 310 p p m in the crystal cores mean 140 p p m and 80 240 p p m in the crystal rims mean 140 p p m the fe concentrations increase from the crystal cores range 640 5690 p p m mean 2030 p p m to the crystal rims range 2840 9440 p p m mean 6040 p p m whereas the sr concentrations decrease from the crystal cores range 690 1510 p p m mean 1280 p p m to the crystal rims 620 1240 p p m mean 930 p p m the 13cpdb values of the dolomite range between 2 5 and 2 6 whereas the 18opdb values range from 0 3 to 0 6 this dolomite occurrence supports the idea that marine or near marine dolomite can form not only syndepositionally but also in the shallow subsurface of temperate units soon after sediment deposition under reducing conditions the fine grained low permeability nature of the port campbell limestone contributed to the reducing conditions at shallow depth the high trace element concentrations of the dolomite especially in fe and the near marine composition of the dolomitizing fluids as large volumes of meteoric water were inhibited

early dolomitization and recrystallization in shallow marine carbonates mississippian alida beds williston basin canada evidence from petrography and isotope geochemistry abstract the mississippian lower carboniferous alida beds of the northern williston basin canada are carbonate rocks that contain a number of significant hydrocarbon reservoirs these shallow marine carbonate platform deposits consist of abundant very finely to finely crystalline dolomite that occurs mainly in mud dominated sediments dolomite crystals are 4 to 30 m in length subhedral to anhedral in shape and typically replace the matrix but also occur as thin lamellae within ooid cortices textural relations indicate that the dolomite predates all other diagenetic phases including coarse calcite cement anhydrite and bitumen dolomite 18o values 9 3 to 4 5 pdb are depleted relative to that expected for precipitation from osagean late tournaisian seawater 0 5 to 1 5 pdb while 13c values 2 3 to 3 4 pdb are within the range of osagean seawater values 1 8 to 4 8 pdb dolomite 87sr 86sr values 0 70816 to 0 70921 are somewhat elevated with respect to osagean seawater 0 70768 to 0 70817 a trend of increasing average dolomite crystal size with decreasing 18o is observed in matrix replacive dolomite based on petrographic relations 13c compositions and the pervasive occurrence of finely crystalline dolomite initial dolomitization is interpreted to have taken place in the presence of slightly evaporated seawater the absence of depositional evaporites in dolomitized intervals suggests that dolomitization was driven by reflux of slightly evaporated seawater that had not reached gypsum saturation depleted 18o values elevated 87sr 86sr ratios and correlation of dolomite crystal sizes and 18o values suggest that the early formed dolomite was variably recrystallized during burial

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

sediments facies tracts and variations in sedimentation rates of holocene platform carbonate sediments and associated deposits northern belize implications for representative sedimentation rates full text is not available on soar publisher s website http jsedres geoscienceworld org cgi content abstract 74 4 498 doi 10 1306 012004740498

dolomitization of holocene shallow marine deposits mediated by sulfate reduction and methanogenesis in normal salinity seawater northern belize abstract dolomite constitutes an average of 12 of the holocene organic rich sediments over a 15 km2 area of the cangrejo shoals mudbank in northern belize although it defines a laterally persistent stratiform body that averages 3 m thick it is present throughout the 7 6 m thick sediment section these transgressive sediments are less than 6400 years old and were deposited in shallow marine environments of normal salinity the dolomite is dominantly cement and average crystal size is 7 m there are no significant correlations among amount of dolomite vs sediment texture mineralogy porosity or mole mgco3 in associated particulate high mg calcite depth or location on the shoals the dolomites are poorly ordered and calcic 39 5 44 5 mole mgco3 with low mean mn 210 ppm and relatively high mean sr 1034 ppm concentrations there is no evidence of recrystallization or geochemical alteration of the dolomite 18o values of the dolomites range from 0 5 to 2 8o oopdb and the mean value 2 1o oo suggests that the dolomite precipitated from normal salinity pore water dolomite 13c values range from 5 2o oo to 11 6o oopdb mean seawater 13c 0 5o oo which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water dolomitization attending these organodiagenetic reactions apparently was reversible over time and episodic rather than continual precipitation is indicated requisite mg and ca were provided by seawater and by some dissolution of host sediments the most rapid period of dolomitization may have been during early transgression when relatively high sedimentation rates sustained high levels of organodiagenesis and pore water alkalinities

chemosynthetic microbialites in the devonian carbonate mounds of hamar laghdad anti atlas morocco abstract veins cutting across the devonian conical mounds of the hamar laghdad ridge eastern anti atlas morocco were the carriers of geofluids although not completely determined the composition of these fluids seems related to hydrothermal and seepage fluxes that were the primary contributors to the establishment of sub seafloor cryptic environments linking deeper fluid sources to the seafloor the optical and scanning electron microscope investigation of the laminated carbonate infill of these veins has revealed that they consist of stromatolite and oncolite like fabrics in the early and middle devonian mounds the laminated carbonates contain morphologies that are attributable to microbially mediated processes that in turn suggest a direct biological contribution to the deposition of the veins infill microbial evidence include rod shaped and cocci clusters embedded in amorphous membranes iron rich filaments organized as mat like and biodictyon filamentous aggregates or isolated filaments microbial clotted textures and peloids moreover the oncoids from the veins of the middle devonian hollard mound have been interpreted as the mineral replacement of crypto microorganisms as frutexites other microbial evidence are biominerals and include carbonate with spheroids and pyrite framboids in which typifying hydrocarbon seep environments the hollard mound which is the only one with a well documented origin by hydrocarbon seepage shares a number of geomicrobiological evidence with other early devonian mounds these evidence which are supplemented by 13 c depleted data suggest that the infill of the veins system in the hamar laghdad mounds would be the product of chemotrophic chemosynthetic microbial communities that are adapted to crypto habitats sustained by hydrocarbon and other components fluid fluxes

study of an ordovician carbonate with alternating dolomite calcite laminations and its implication for catalytic effects of microbes on the formation of sedimentary dolomite

rock creep and the development of the niagara cuesta this work addresses the post glacial development of the niagara escarpment speci cally the east facing scarp slopes between hamilton and collingwood the escarpment is one of a series of scarps that shape the great lakes basin one interpretation suggests that the escarpment has evolved through homoclinal recession to the west it is therefore viewed as an erosional feature brought about due to uvial action on the weaker shale formations that underlie the cap rock however the deposition of large amounts of drift during the pleistocene glaciation has resulted in the disruption of the preglacial drainage pattern within the great lakes basin therefore the current drainage is typically orthogonal to the scarp face with an absence of uvial action along the base consequently the cuesta is experiencing an extremely slow rate of retreat conventional thinking on the evolution of the present morphology suggests that it developed rapidly following deglaciation due to periglacial processes the concept of the escarpment as a relict feature during the holocene therefore pervades the modern literature however the cambering of the cap rock towards the scarp face and the apparent motion of individual joint bounded blocks indicates that in the absence of exogenetic processes a slow development due to endogenetic processes has dominated the modern development of the escarpment it has been proposed that the present morphology of the cliffed sections may be due to deformation within the shale layers in order to test this hypothesis the rheology of the rocks that form the escarpment was tested and compared to the gravitational stresses that would be expected within the rock mass the results indicate that the compressive strength of the shale layers of the cabot head formation is lower than the expected effective stresses it is therefore concluded that in the absence of high con ning stresses as would be expected near the cliff face slow plastic deformation within this formation is occurring the morphological expression of this deformation is seen in the cambering of the escarpment the dilation of pre existing joints and the rotation of joint bounded blocks

13c enriched carbonate in mississippian mud mounds alamogordo member lake valley formation sacramento mountains new mexico u s a abstract the origin of the carbonate mud constituting paleozoic mid shelf and deeper water mud mounds has long been debated mississippian mud mounds and their laterally adjacent coeval inter mound strata in the lake valley formation are well exposed along cliff faces in south central new mexico the mounds originated on erosional surfaces on a gently sloping ramp in 110 to 250 m water depth carbonate muds that constitute the inter mound strata mud mounds and syndepositional neptunian dikes within the mounds are low mg calcite microspars with similar petrographic cathodoluminescent and fluorescent characteristics carbon and oxygen stable isotope analyses demonstrate that the microspars constituting the inter mound mound and neptunian dikes have average 13c values of 2 6 0 2 3 4 0 6 and 4 8 0 3 pdb respectively additionally well preserved brachiopods from the inter mound strata and from muleshoe mud mound have 13c values markedly different from each other but similar to their surrounding microspars i e brachiopod s 2 7 and 3 9 pdb respectively the stable isotope distributions within each environment similarity of values between microspar and brachiopod data from the same depositional environments as well as agreement of 13c values with other investigations of coeval strata support the conclusion that the measured 13c values are essentially primary values it is envisioned that 13c enriched fluids sourced by bacterial fermentation in the subjacent strata rose into the mounds some through neptunian dikes and influenced the precipitation of the microspars in the mounds and dikes near the sediment water interface these fluids mixed with normal marine waters thus precipitating the microspars or their precursors which have high 13c values that characterize the mud mounds the inter mound microspars and inter mound brachiopods have the lowest 13c values and are in agreement with worldwide coeval marine calcites that is the inter mound microspars formed under normal marine conditions the high 13c values measured in all of the mound related samples indicate that all mounds received a contribution of 13c enriched waters this relationship indicates that microspar precipitation in the mounds was associated with seepage of methanogenic fluids from the underlying strata

dissolved sulfide catalyzed precipitation of disordered dolomite implications for the formation mechanism of sedimentary dolomite abstract dolomite is a common mineral in the rock record however the rarity of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under normal earth surface conditions result in the long standing dolomite problem in sedimentary geology some modern dolomites are associated with sediments where microbial sulfate reduction is active however the role of sulfate reducing bacteria in dolomite formation is still under debate in this study we tested the effect of dissolved sulfide on the precipitation of ca mg carbonates which has been never explored before although dissolved sulfide is one of the major products of microbial sulfate reduction our results demonstrated that dissolved sulfide with a concentration of as low as several millimoles can enhance the mg2 incorporation into the calcitic structure and promote the crystallization of high magnesian calcite and disordered dolomite we also conducted seeded precipitation in experimental solutions containing dissolved sulfide which showed that calcite seeds can inhibit the precipitation of aragonite and monohydrocalcite caco3 h2o and induce more mg2 incorporation we propose that accumulated dissolved sulfide in pore waters in organic rich sediments may trigger the precipitation of disordered dolomite which can be considered as a precursor of some sedimentary dolomite our adsorption experiments revealed a strong adsorption of dissolved sulfide onto calcite faces we suggest that adsorbed dissolved sulfide can lower the energy barrier to the dehydration of mg2 water complexes on the growing carbonate surfaces this study sheds new light on understanding the role of sulfate reducing bacteria in dolomite formation and the formation mechanism of sedimentary dolomite