multiple dolomitization and later hydrothermal alteration on the upper cambrian lower ordovician carbonates in the northern tarim basin china abstract thick upper cambrian lower ordovician carbonates were deposited on a shallow marine platform in the northern tarim basin which were extensively dolomitized particularly for the upper cambrian carbonates the resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites based on petrographic textures matrix dolomites consist of very finely to finely crystalline nonplanar a to planar s dolomite md1 finely to medium crystalline planar e s dolomite md2 and finely to coarsely crystalline nonplanar a dolomite md3 minor cement dolomites include finely to medium crystalline planar s e dolomite cd1 and coarsely crystalline nonplanar saddle dolomite cd2 which partially or completely fill dissolution vugs and fractures these cements postdate matrix dolomites but predate later quartz and calcite infills origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography isotopic geochemistry o c and sr and fluid inclusion microthermometry md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines e g evaporated seawater in near surface to very shallow burial settings whereas md2 dolomite was formed from connate seawater in association with burial dissolution and localized mg concentration or cannibalization in shallow burial conditions md3 dolomite however was likely the result of intense recrystallization or neomorphism upon previously formed dolomites e g md1 or md2 dolomite as the host carbonates were deeply buried and influenced by later hydrothermal fluids subsequent cement dolomite and quartz crystals precipitated from higher temperature hydrothermal fluids which were contributed more or less by the extensive permian large igneous province lip activity in tarim basin as evidenced by less radiogenic sr in the cement and parts of matrix dolomites this extensive abnormal hydrothermal activity could also have resulted in recrystallization or neomorphism on the previous matrix dolomites faults fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths however the basin uplift triggered by the late hercynian orogeny from the late permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids resulting in precipitation of later calcites in which lighter c and more radiogenic sr components demonstrate such a switch of fluid properties this study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the permian lip activity within tarim basin and elsewhere

predicting depths of gypsum dehydration in evaporitic sedimentary basins the conversion of gypsum to anhydrite upon burial and heating is accompanied by about a 39 volume decrease a four fold increase in thermal conductivity and consumption of heat accurate back stripping of an evaporite basin and tracking its thermal evolution requires a knowledge of the depth of the gypsum anhydrite transition this depth depends on temperature fluid pressure lithostatic pressure and activity of water in the pore fluid using a finite difference heat conduction program we have determined the depths of transition for gypsum in evolving basins beneath commonly associated sediments and in several tectonic environments represented in the program by different basal heat flows and sedimentation rates the activity of water is kept at 0 93 precipitation f gypsum from seawater all physical properties are recalculated at each time step as temperature increases and porosity decreases with burial an algorithm for calculating thermal conductivities using parallel series mixtures is presented the modeling results show that overlying lithologies and the tectonic environment are two important factors gypsum converts to anhydrite at shallow depths sim400 m when overlain by poor conductors like shale or gypsum in a rift environment and at great depths hypothetically 4 km when overlain by good conductors like salt in a stable cratonic region sedimentation rate and the transient heat sink effect of the endothermic reaction have little effect on transition depth

microbial nucleation of mg rich dolomite in exopolymeric substances under anoxic modern seawater salinity new insight into an old enigma sulfate reducing bacteria are known to mediate dolomite formation under hypersaline conditions but details of the crystal nucleation process are still poorly constrained our laboratory study demonstrates for the first time that desulfobulbus mediterraneus a marine sulfate reducing bacterium mediates primary precipitation of mg rich dolomite under anoxic conditions in media replicating modern seawater chemistry at low temperature 21 c precipitation of crystals was associated with extracellular polymeric substances in a monospecific biofilm providing templates for nucleation by altering the molar mg ca ratio after initial nucleation of single nanospherulites 50 nm growth was mediated by aggregation resulting in spherulites of 2 3 m in diameter nucleation led to differences in mg ca ratios and 44 40ca values among the organic material i e biofilm including cells and extracellular polymeric substances 0 87 0 01 2 sd and 0 48 0 11 2 se respectively the crystals 1 02 0 11 2 sd and 0 08 0 24 2 se respectively and the liquid bulk medium after mineral precipitation 4 53 0 04 2 sd and 1 10 0 24 2 se respectively these data indicate a two step fractionation process involved in the sequestration of ca from the solution into the crystal lattice of the mineral precipitated our results demonstrate the capability of extracellular polymeric substances to overcome kinetic inhibition fostering the formation of kinetically less favorable mg rich dolomite and they also question the applicability of the ca isotopic system as a proxy for paleogeochemistry of seawater

barred olivine chondrules in ordinary chondrites barred olivine chondrules were studied in h l ll3 6 chondrites no important differences were found between barred olivine chondrules in h3 l3 and ll3 chondrites however comparison of type 3 barred olivine chondrules with all chondrule types in h l ll3 ordinary chondrites show that barred olivine chondrules are enriched in feo al2o3 na2o and k2o and are depleted in sio2 and mgo mineral abundances of barred olivine chondrules show that they are enriched in the feldspathic component compared to most chondrules pyroxene is rarely absent and is sometimes a major component pyroxene rich barred olivine chondrules in type 3 chondrites correlate negatively with feldspathic glass rich barred olivine chondrules indicating that pyroxene formed at the expense of olivine bars barred olivine chondrules in ordinary chondrites decrease systematically in cr2o3 and tio2 with increasing petrologic type feo however shows a systematic decrease with petrologic type in h chondrites no change in l chondrites and a systematic increase in ll chondrites the conclusions of this study are 1 barred olivine chondrules are mainly intermediate in major element composition between the abundant fe mg chondrules and the rare al rich chondrules 2 systematic compositional changes in barred olivine chondrules from type 3 6 in h l ll chondrites may be the result of primary differences 3 precursor components that melted to form barred olivine chondrules were enriched in olivine rich and volatile feldspar and feldspathoid rich components the alkalies may have been condensed where alumina was already available

composition of chondrule silicates in ll3 5 chondrites and implications for their nebular history and parent body metamorphism our petrologic studies of 75 type ia and type ii porphyritic olivine chondrules in nine selected ll group chondrites of type 3 3 to type 5 and comparisons with published studies of chondrules in semarkona ll3 0 show that compositions of silicates and bulk chondrules but not overall chondrule textures vary systematically with the petrologic type of the chondrite these compositional trends are due to diffusive exchange between chondrule silicates and other phases e g matrix such as those now preserved in semarkona during which olivines in both chondrule types gained fe2 and mn2 and lost mg2 cr3 and ca2 in a given ll4 5 chondrite the olivines from the two chondrule types are identical in composition enrichments of fe2 in olivine are particularly noticeable in type ia chondrules from type 3 3 3 6 chondrites especially near grain edges chondrule rims grain boundaries and what appear to be annealed cracks compositional changes in low ca pyroxene lag behind those in coexisting olivine consistent with its lower diffusion rates with increasing petrologic type low ca pyroxenes in type ia chondrules become enriched in fe2 and mn2 and depleted in mg2 cr3 and a13 these compositional changes are entirely consistent with mineral equilibration in chondritic material during metamorphism from these compositional data alone we cannot exclude the possibility that chondritic material was metamorphosed to some degree in the nebula but we see no evidence favoring nebula over asteroidal metamorphism nor evidence that the chondrule reacted with nebular gases after crystallization modelling of the equilibration of chondrule olivines suggests that heterogeneous feo concentrations in olivine could be preserved after cooling from 600 c at rates of 1 10 c ma for at least tens to hundreds of millions of years this is consistent with published estimates for the maximum metamorphic temperatures in type 3 chondrites thermal histories derived from metallographic and fission track cooling rates and 4 4 ga 40ar 39ar ages for ordinary chondrites since the lifetime of the solar nebula was not more than 106 years and there is abundant evidence that meteorite parent bodies were heated some even above their melting point we confidently conclude that the formation of type 3 3 5 ordinary chondrites from type 3 0 material by metamorphism occurred in parental asteroids not the solar nebula

stable isotopic signature of a palaeoaquifer mississippian alamogordo member limestones sacramento mountains new mexico usa nine stratigraphic sections each 5 m thick were sampled from the alamogordo member limestones of the lake valley formation sacramento mountains new mexico usa four stratigraphic sections consist entirely of lime mudstone and wackestone whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in 1 8 o and constant 1 3 c values whereas those in the sections solely composed of lime mudstone and wackestone have in general relatively uniform 1 8 o and 1 3 c values the diagenesis of the alamogordo member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre pennsylvanian exposure surface 100 m above this unit however the uniform 1 3 c and downward decrease in 1 8 o values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the alamogordo member rather than descended from the overlying exposure surface this indicates that the basal packstones were probably a conduit for meteoric water this is further supported indirectly by the relatively uniform 1 8 o and 1 3 c values of the lime mud in the sections without basal packstones the implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

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

multiple step recrystallization within massive ancient dolomite units an example from the dinantian of belgium south of the caledonian brabant wales massif a more than 200 m thick tournaisian to lower visean replacive dolomite unit can be followed for several hundred kilometres from the boulonnais france to aachen germany field observations of features such as karst cavities occurring at the top of the lower visean dolomite which are filled by lower visean crinoidal limestone indicate that dolomitization and karstification took place during the early visean this early development of the dolomite is in agreement with the presence of stylolites cutting the dolomite fabric the minor element composition of the majority of the dolomites remains almost uniform throughout the entire studied area values for fe mn na and sr are normally in the range 700 4700 ppm 15 400 ppm 80 300 ppm and 50 200 ppm respectively the 13c values range 0 72 to 5 31 o mainly reflect the carbon isotopic composition of the precursor limestones the 18o values in contrast are highly variable ranging from 19 15 to 0 85 o this rather large range of 18o values is explained by multiple step re equilibration recrystallization during progressive burial and subsequent uplift of the dolomites these processes are also responsible for the high 87sr 86sr values of the dolomites which range from about 0 7088 to 0 7098 they are distinctly more radiogenic than lower visean marine carbonates 0 7076 0 7078 correlation however of 18o values or 87sr 86sr ratios with dolomite and or cathodoluminescenec cl textures has not been very successful this suggests that recrystallization may remain unrecognized if only petrographic techniques are used nevertheless certain cl textures can be related to specific interactions with the ambient recrystallizing fluids

dolomitization and hydrotectonic model of burial dolomitization of the furongian lower ordovician carbonates in the tazhong uplift central tarim basin nw china implications from petrography and geochemistry abstract the nature of the dolomitizing fluids and genesis of the furongian lower ordovician dolomites and dolomite cements are interpreted on the basis of petrography geochemistry trace elements rees o c sr isotopes and fluid inclusion microthermometry these carbonate strata were deposited on a shallow marine platform in the tazhong uplift in central tarim basin and were extensively dolomitized based on field investigations and petrographic examination two types of fabric retentive dolomitic textures dolomicrites dm and microbial dolomudstone dms and three types of fabric obliterated dolomitic textures very fine to fine crystalline planar e s dolomite rd1 fine to medium crystalline planar e s dolomite rd2 and medium to coarse crystalline nonplanar a planar e s dolomite rd3 are identified two types of dolomite cements medium to coarse crystalline dolomite cement dc1 and coarse to very coarse crystalline saddle dolomite cement dc2 are identified the ree patterns of the analyzed dolomites and calcite cements are characterized using seawater normalization standard the dc2 dolomite cement and later stage calcite cement lscal display positive europium anomalies eu and positive cerium anomalies ce whereas the dm dms rd1 rd2 rd3 and early stage calcite cement escal samples display positive ce light ree lree enrichment and negative eu the dm and dms were formed through seawater flux and mesosaline brine derived from evaporative seawater during early burial seepage reflux dolomitization and microbial dolomitization respectively the preferential occurrence of rd1 along low amplitude stylolites suggest that their formation may have linked to pressure dissolution by which mg ions were probably released for replacement dolomitization during late early burial to early shallow burial dolomitization moreover the tightly packed and curved crystal faces of rd2 and rd3 with irregular overgrowth rims and increasing crystal sizes presumably resulted from the recrystallization and replacement upon the rd1 and or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow burial dolomitization the average range of the homogenization temperatures th of cd2 is 132 4 c 198 7 c and mostly higher than the estimated ambient temperature 95 c 150 c for the furongian lower ordovician strata implying that the growth of dc2 in the study area was the result of hydrothermal dolomitization the comparable isotopic 18o 13c and 87sr 86sr values between rd2 rd3 dc1 dc2 dolomites and coeval neoproterozoic to early phanerozoic seawater values in addition to the relatively high th and salinity levels in these dolomites indicates that their dolomitizing fluid were derived from radiogenic 87sr enrich coeval seawater during late shallow burial dolomitization and hydrothermal dolomitization the hydrotectonic driving model of burial dolomitization of the furongian lower ordovician carbonates in the tazhong uplift resulted from compressional tectonics that affected the tarim basin during cambrian to middle ordovician and late ordovician period this model is characterized by fluid flow in response to significant thermal heating pressure loading and enhanced fluid expulsion by squeezing and the resultant higher temperature mg rich connate waters and hydrothermal basinal brines are conveyed by convective advective flows along permeable horizons fractures and overthrust fault planes resulting in extensive replacement dolomitization of carbonate materials and recrystallization of early formed dolomites forming irregular networks of dolomites and calcite cements in the host carbonate rocks

a reappraisal of dolomite abundance and occurrence in the phanerozoic critical assessment of dolomite abundance throughout the phanerozoic suggests that most large scale dolomite has had an origin related to salinity elevated seawaters the distribution of such dolomite is not directly related to periods of major continental flooding but rather to periods when extensive peritidal sequences or large scale evaporite basins lagoons existed the development of such depositional systems is controlled largely by eustasy paleogeographic tectonic setting and paleoclimate extensive peritidal sequences are characteristic of mature passive margin carbonate platforms that formed during long periods of earth history generally devoid of major continental glaciation e g late precambrian through early paleozoic and mesozoic sea level fluctuations of relatively low amplitude during such periods allowed extensive cyclic peritidal dominated platforms to develop such platforms typically lack raised rims had depositional surfaces that stayed near sea level for tens of millions of years and show little evidence of major drops in sea level that would have exposed the shelf for long periods this would have allowed repeated replenishment of pore waters by mg rich marine fluids over very extensive supratidal surfaces also since the cycles are very thin the downward moving brines could penetrate several cycles on the inner platform causing massive dolomitization in contrast during major glacial periods e g middle to late mississippian to early permian and late cenozoic higher amplitude ice driven fluctuations in sea level led to extensive meteoric water flushing of shallow platforms but related dolomitization has been of minor significance under conditions of elevated mg ca ratio in hypersaline brines however large scale dolomitization could occur in response to the formation of extensive evaporite basins lagoons suitable climatic and paleogeographic tectonic conditions have allowed such dolomitization at various times during the phanerozoic regardless of specific styles of eustatic sea level change lack of massive dolomitization in holocene carbonates may be because neither extensive peritidal sequences nor large scale evaporite basins lagoons are prevalent in modern carbonate environments

early dolomitization and recrystallization of the lower middle ordovician carbonates in western tarim basin nw china abstract the lower middle ordovician shallow marine carbonates in western tarim basin were dolomitized to different degrees the resultant dolomites are mainly concentrated in the successions of platform interior to shallow subtidal shoal and display an upward decrease in abundance based on petrographic textures three types of matrix dolomites were identified 1 very finely to finely crystalline nonplanar a to planar s dolomite md1 2 finely to medium crystalline planar e s dolomite md2 and 3 finely to coarsely crystalline nonplanar a dolomite md3 origins of matrix dolomites are interpreted on the basis of field investigation petrography and isotopic o c and sr geochemistry md1 dolomite was formed from slightly evaporated i e mesosaline to penesaline seawater in near surface to very shallow burial settings possibly facilitated by microbial mediation i e bacterial sulphate reduction bsr md2 dolomite was precipitated from connate seawater stored in coeval limestone in shallow burial realms in contrast md3 dolomite was likely formed as a consequence of intense recrystallization of early formed dolomites e g md1 at depths the formation of early matrix dolomites i e md1 was likely determined by depositional facies and sea level variations of variable orders this study provides a useful analogue to understand the dolomitization processes related to depositional facies and variable order sea level fluctuations in the tarim basin and elsewhere

paleomagnetic and isotopic evidence for the diagenesis and alteration of evaporites in the paleozoic elk point basin saskatchewan canada in the paleozoic elk point basin saskatchewan canada hematite in evaporite rocks of the middle devonian prairie formation records two magnetizations that are very different from those expected when the evaporites were originally deposited the first x magnetization corresponds to cretaceous or cenozoic paleofield directions and contains both normal and reversed polarities the second y magnetization appears to be a composite of cretaceous cenozoic and late paleozoic kiaman reversed polarity directions the x magnetization occurs in anomalous ores and is associated with fine grained hematite occurring predominantly along grain boundaries the y magnetization occurs in normal ore and is associated with sylvite that has hematite both along grain boundaries and within sylvite crystals the latter inferred to be of kiaman age k ar ages of the host sylvites are also composite and are consistent with those inferred from paleomagnetic directions stable isotopic compositions of fluid inclusions in hali

dolomitization by penesaline sea water in early jurassic peritidal platform carbonates gibraltar western mediterranean peritidal carbonates of the lower jurassic liassic gibraltar limestone formation which form the main mass of the rock of gibraltar are replaced by fine and medium crystalline dolomites replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an 460 m thick platform succession the fine crystalline dolomite has d 18 o values either similar to or slightly higher than those expected from early jurassic marine dolomite and d 13 c values together with 87 sr 86 sr ratios that overlap with sea water values for that time indicating that the dolomitizing fluid was early jurassic sea water absence of massive evaporitic minerals and or evaporite solution collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation the occurrence of peritidal facies a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions salinity 72 199 the medium crystalline dolomite has some d 18 o and d 13 c values and 87 sr 86 sr ratios similar to those of early jurassic marine dolomites which indicates that ambient sea water was again a likely dolomitizing fluid however the spread of d 18 o d 13 ca nd 87 sr 86 sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow 500 m burial or that dolomitization was multistage these data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation taking earlier models into consideration it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities normal penesaline to hypersaline provided that there is a driving mechanism for fluid migration the gibraltar dolomites confirm other reports of significant early jurassic dolomitization in the western tethys carbonate platforms

aerobic microbial dolomite at the nanometer scale implications for the geologic record microbial experiments are the only proven approach to produce experimental dolomite under earth s surface conditions although microbial metabolisms are known to induce dolomite precipitation by favoring dolomite growth kinetics the involvement of microbes in the dolomite nucleation process is poorly understood in particular the nucleation of microbially mediated dolomite remains a matter for investigation because the metabolic diversity involved in this process has not been fully explored herein we demonstrate that halomonas meridiana and virgibacillus marismortui two moderately halophilic aerobic bacteria mediate primary precipitation of dolomite at low temperatures 25 35 c this report emphasizes the biomineralogical implications for dolomite formation at the nanometer scale we describe nucleation of dolomite on nanoglobules in intimate association with the bacterial cell surface a combination of both laboratory culture experiments and natural samples reveals that these nanoglobule structures may be 1 the initial step for dolomite nucleation 2 preserved in the geologic record and 3 used as microbial tracers through time and or as a proxy for ancient microbial dolomite as well as other carbonate minerals

origin of dolomite in the middle devonian guanwushan formation of the western sichuan basin western china abstract the carbonates in the middle devonian guanwushan formation d2g of the western sichuan basin are extensively dolomitized three types of replacement dolomite can be distinguished 1 micritic microcrystalline dolomite 100 m with anhedral subhedral dolomite crystals type 3 the anhedral to subhedral fine crystalline crystals in type 3 dolomite are truncated by a stylolite which indicates that the formation of type 3 dolomite most likely occurred simultaneously or predated the stylolite formation stratigraphic petrographic and geochemical data indicate that the 87sr 86sr ratios 0 70839 to 0 71117 of the d2g dolomites which are higher than those of the contemporaneous seawater 0 70780 0 70830 were affected by the underlying siliciclastic rocks of the jinbaoshi formation d2j pingyipu formation d1p and silurian strata the dolomitizing fluids incorporated elemental sr which tends to be abundant in crustal fluids the rare earth element ree compositions of the d2g dolomites are characterized by enrichment in light rees and depletion in heavy rees ce and eu indicating that the dolomites of the d2g inherited the characteristics of the contemporaneous seawater and formed in a low temperature relatively oxic environment the 18o values of type 1 and type 2 dolomites 6 3 to 0 43 and 7 2 to 1 69 respectively are slightly higher than those of seawater derived dolomite suggesting that these dolomites may be related to the evaporation of seawater during the penecontemporaneous stage the 18o values of type 3 dolomite 8 32 to 0 83 are slightly lower than those of the seawater derived dolomites indicating that this dolomite may have formed at a higher temperature during shallow moderate burial type 1 dolomite has higher 18o values sr2 concentrations and total ree ree concentrations and slightly lower mn2 concentrations than the other two types of dolomite suggesting that type 1 dolomite formed via capillary concentration dolomitization from evaporated seawater type 2 dolomite has lower 18o values and ree contents and slightly higher mn2 and sr2 concentrations than type 1 dolomite suggesting that type 2 dolomite formed via the seepage reflux process from denser seawater type 3 dolomite has a stylolite lower 18o values sr2 concentrations and ree concentrations and slightly higher mn2 concentrations and mg ca values suggesting that type 3 dolomite formed during shallow moderate burial

mass transfer between mudstone sandstone interbeds during diagenesis as revealed from the type and distribution of carbonate cements in the eocene beach bar sandstones bohai bay basin abstract the characteristics heterogeneous distribution and origins of carbonate cements within beach bar sandstones in the boxing sag and implications on the mass transfer from mudstone to sandstone during diagenetic process was investigated systematically by an integrated analysis invoking petrography carbon and oxygen stable isotopes and porosity permeability with specific references to the contrasts among the sharp s change lithology s mudstone s sandstone interbeds the gradual g change lithology g mudstone g sandstone interbeds and the center of sandstones there are distinct differences in the occurrence content distribution carbon and oxygen isotopic compositions and precipitation temperatures of the carbonate cements among the s sandstone g sandstone and center of sandstone material sources of carbonate cements in the s sandstones are mainly derived from the diagenetic alterations of the nearby s mudstones through mass transfer with the participation of organic co2 while in the g sandstones the material sources are mainly internal and originated from the calcareous matrix dissolution of feldspars and conversion of clay minerals provided the main materials for the precipitation of carbonate cements in the center of sandstones mass transfer of carbonate cements mainly occurred between the s mudstones and the s sandstones with limited transmission distance which controlled the distribution of carbonate cements in the s mudstones there was almost no mass transfer between the g mudstones and g sandstones because of the small concentration gradient instead the depositional components and recrystallization controlled the carbonate distribution of the g sandstones

chemical and physical studies of type 3 chondrites v the enstatite chondrites abstract we report instrumental neutron activation analysis determinations of 19 major minor and trace elements in three enstatite chondrites based on these and literature data on the bulk and mineral composition of enstatite chondrites we discuss the history of the type 3 or unequilibrated enstatite chondrites and their relationship with the other enstatite chondrites the type 3 enstatite chondrites have e chondrite lithophile element abundances and their siderophile element abundances place them with the eh chondrites well resolved from the el chondrites moderately volatile chalcophile elements are at the low end of the eh range and cr appears to be intermediate between eh and el we suggest that the type 3 enstatite chondrites are eh chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock produced sulfurous liquids the oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas 30 higher than the others or with the loss of a plagioclase rich liquid the mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration or in some instances with shock effects

compilation and time series analysis of a marine carbonate δ18o δ13c 87sr 86sr and δ34s database through earth history abstract the sr s o and c isotope database of marine carbonates contains over 55 000 published isotope values of low mg calcite from diagenetically little altered phanerozoic fossil shells as well as samples of whole rocks and calcite cements of ordovician to archean age carbon and oxygen isotope data for the shell material are divided into habitat subsets high mid low latitude and deep sea and whole rock data are separated by mineralogy into calcite dolomite subsets trend correlation wavelet and spectral analyses on gaussian filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results 1 oxygen isotope trends from the high latitude and deep sea habitats are almost indistinguishable through the last 115 ma consistent with the existence of the oceanic conveyor belt throughout this interval 2 all oxygen isotope habitat records show a strong coherent 30 45 ma 38 ma cyclicity throughout the cretaceous and the cenozoic 3 up to 70 of the multi million year variability in the 18o record of the last 115 ma can be simulated by the following equation 18 o 0 64 sin 2 t 120 ma 0 9 x sin 2 t 38 3 ma 1 1 with x ranging from 0 4 0 6 for the low high latitude and deep sea habitats to 0 8 for the mid latitude realm 1 a 120 20 ma cycle occurs in the paleozoic and neoproterozoic 18o record consistent with paleoclimate variability as interpreted from sedimentological and faunal records 2 the offset of 13c values between deep water and high latitude vs surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the cretaceous 3 sr and s isotope records exhibit a 60 70 ma cyclicity throughout the phanerozoic

modern marine dolomite cement in a north jamaican fringing reef minor quantities of ordered dolomite delta sup 18 o 2 0 per thousand pdb composition ca sub 1 22 mg sub 0 78 co sub 3 sub 2 have formed from near normal seawater in a subtidal hardground as part of a modern fringing coral reef crystals 5 mu m in diameter precipitated within the past 1 8 ka in the form of syntaxial fringes on mg calcite marine cements and skeletal allochems the crystals have a fine modulated microstructure and c reflections both apparently formed during crystal growth

a two dimensional regional basin model of williston basin hydrocarbon systems institut francais du petrole s two dimensional model temispack is used to discuss the functioning of petroleum systems in the williston basin along a 330 km long section focusing on four regional source intervals ordovician yeoman formation lower devonian winnipegosis formation upper devonianulower mississippian bakken formation and mississippian lodgepole formation thermal history calibration against present temperature and source rock maturity profiles suggests that the williston basin can be divided into a region of constant heat flow of about 55 mw m2 away from the nesson anticline and a region of higher heat flow and enhanced thermal maturity in the vicinity of the nesson anticline original kinetic parameters used in the calibration were derived or each of the four source rocks from rock eval yield curves bakken overpressures are entirely due to oil generation not compaction disequilibrium very low bakken vertical permeabilities range from 0 01 to 0 001 nd are matched against observed overpressures whereas bakken porosities based on the model and confirmed by measurements are inferred to be also unusually low around 3 mature bakken shales do not seem to have reached hydraulic fractionation thresholds except perhaps locally in regions of extensional tectonic stress hydraulic fracturing cannot be viewed as a pervasive mechanism driving bakken oil expulsion our expulsion model confirms the high residual bakken oil saturations and explains the low residual s1 toc by the low bakken shale porosities approximately 85 of the oil generated in the bakken shales is predicted to have been expelled out of bakken strata which agrees with observed s1 toc trends madison group accumulations at the subcrop edge are found to be fed by lodgepole sourced oil only in particular by the rich middle lodgepole interval these accumulations probably result from a three dimensional migration pattern madison accumulations in the nesson anticline are found to be fed mostly by lodgepole sourced oil mixed with minor amounts of bakken sourced oil the vast majority of the expelled bakken oils are lost in the madison group at very low saturations this explains the low conventional oil resource associated with bakken sources by recent geochemical studies expulsion and migration occurred no earlier than the atest cretaceous paleocene in the yeoman and no earlier than the eocene in other source rocks consistent with temporal controls on oil migration flow models show no restriction to expulsion and migration of yeoman oil this suggests a potential undiscovered oil resource in ordovician and silurian strata of williston basin northeast of the nesson anticline

oxygen isotope composition of holocene dolomite formed in a humid hypersaline setting holocene dolomite forming in supratidal carbonate sediments of bonaire netherlands antilles is precipitated from hypersaline fluids in which the mg ca ratio is elevated by evaporation and gypsum precipitation kinetic effects and water vapor exchange between the humid atmosphere and hypersaline brines limit the 18o values of the brines to between 0 7 and 2 1 standard mean ocean water the mean 18o value of dolomite is 1 0 peedee belemnite n 8 which is lower than values of dolomite precipitated from hypersaline brines in less humid environments bonaire dolomite is also isotopically lighter than dolomites interpreted to have formed in normal marine waters and mixed marine and or meteoric waters calculated fractionation between dolomite and calcite formed from bonaire brines is between 1 5 and 3 5 the high end of this range is consistent with previously proposed mineralogic fractionations of 3 1 and 3 2 between dolomite and calcite

stabilization of early formed dolomite a tale of divergence from two mississippian dolomites abstract several large hydrocarbon accumulations in alberta canada are hosted in dolomitized successions of stacked thin sabkha capped cycles of visean age porosity is micro intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved two such fields are here considered that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures the dolomite in the upper debolt formation of the dunvegan field nw alberta forms planar e or microsucrosic fabrics with crystals in the 1 20 m range the dolomite is non ferroan ca rich average of 58 mol caco 3 and poorly ordered its stable isotopic signatures range from 0 12 to 3 4 vpdb for 18 o mean 1 3 and 0 9 to 4 3 vpdb for 13 c mean 2 6 the average radiogenic 87 sr 86 sr ratio for this dolomite is 0 7077 both sets of values are consistent with dolomite precipitation from mississippian marine or modified marine evaporated seawater these parameters are strongly reminiscent of holocene protodolomites and hence suggestive of a sabkha dolomitization process shallow seepage reflux or evaporitive pumping this dolomite with its high associated porosity average of 15 and up to 38 relatively unaltered mineralogical and chemical signatures both preserved despite 4 km of burial depth suggests a very unique set of relatively non reactive physico chemical conditions during burial likely a closed system in contrast the dolomite from the mount head formation of the shell waterton field sw alberta has undergone measurable neomorphic alteration in several stages in deeper burial environments open system such alteration has affected its crystal size range 18 o ranges between 1 5 in the least altered dolomite and 13 2 vpdb 13 c ranges from 3 9 to 1 5 vpdb and 87 sr 86 sr ranges from 0 7078 to 0 7090 the dolomite has retained a degree of non stoichiometry with an average ca content of 55 mol caco 3 the mount head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial and are in this respect representative of the norm for dolostones in the western canada sedimentary basin

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

87sr86sr isotopic evolution of lower carboniferous seawater dinantian of western europe abstract the chemical and isotopic evolution of ocean water mirrors the dynamics of the earth system the original seawater signature can be measured only indirectly for the palaeozoic samples with the greatest utility for such an approach are the secondary layers of articulate brachiopods for a correct interpretation of isotope data samples that have suffered alteration by diagenetic processes must be avoided trace element analysis is frequently utilized as an analytical tool the present study deals with the delineation of a detailed 87 sr 86 sr curve for the dinantian this in turn enables us to search for causative factors of the observed isotopic oscillations a total of 175 lower carboniferous dinantian brachiopod shells from a variety of western european locations have been analysed for their 87 sr 86 sr ratios for 152 of these shells quantitative geochemical tests on diagenetic alteration based on trace element repartitioning during recrystallisation have been performed the concentrations of ca mg sr mn and fe were routinely measured by inductively coupled plasma atomic emission spectroscopy icp aes analysing the diluted phosphoric acid that remained after carbonate dissolution for stable isotope gas preparation in addition 71 samples have been analysed by proton induced x ray emission spectroscopy micro pixe despite large differences in the analysed masses 3 6 mg for icp aes as low as 10 ng for pixe both methods yield comparable data for the sr and mn contents any discrepancies can be related to inhomogeneities of the shells e g punctae of these shells 73 have sr and mn contents of 600 ppm and the sr isotope record of the dinantian seawater is characterised by a decline in 87 sr 86 sr ratio from 0 7082 at the devonian carboniferous transition to 0 7076 in the mid visean superimposed on this trend are higher order fluctuations with a periodicity in the ma range the dinantian seawater curve may potentially serve as a geochronological and correlation tool particularly for the hastarian to lower chadian interval where the attainable resolution is 1 ma this is better than the resolution available by biostratigraphy the higher order wiggles for the most part can be generated by changing the riverine flux of sr or its isotopic ratio within reasonable geological scenarios some spikes however are too large and too short to be explained by the above phenomena these apparent spikes may reflect instead the fact that their duration has been underestimated due to unreliable geochronology and or the presence of unrecognised hiatuses

the interaction of tectonics climate and eustasy in controlling dolomitization a case study of cenomanian turonian shallow marine carbonates of the iberian basin

dolomitization on an evaporitic paleoproterozoic ramp widespread synsedimentary dolomite in the denault formation labrador trough canada abstract the denault formation 2 1 1 9 ga crops out in the labrador trough northeastern quebec and western labrador rocks surrounding the town of schefferville quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm influenced shallow ramp mid ramp facies consist of intraclastic grainstones with hummocky cross stratification hcs swaley cross stratification scs current ripples and graded event beds further outboard grainstones grade into deeper water laminites that are composed of even mm scale couplets of flat lying organic and dolomudstone laminae scours within the laminites suggest periodic storm activity laminites gradually grade into outer ramp deep water shales an isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km present day this succession can be interpreted as the remnant of a near continuous margin or may simply represent an isolated accumulation that developed on a pre existing topographic high the presence of gypsum pseudomorphs in all lithofacies indicates that the denault margin was restricted and evaporitic four paragenetic stages are recognized in the diagenetic evolution of the denault formation 1 carbonate deposition contemporaneous marine cementation authigenic gypsum growth and precipitation of authigenic chert 2 synsedimentary mimetic dolomite precipitation 3 pore rimming and pore occluding shallow burial dolomite cement and 4 fabric destructive sutured anhedral burial dolomite gypsum crystals occur in all lithofacies form the nuclei of interstitial dolomite rhombs average 10 m in length and often display swallowtail twinning paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water mg ca ratios and lowering its dissolved sulfate concentrations such an interpretation may explain why there is an abundance of synsedimentary dolostone in the precambrian and the relative paucity of phanerozoic analogs

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

host rock dolomitization and secondary porosity development in the upper devonian cairn formation of the fairholme carbonate complex south west alberta canadian rockies diagenesis and geochemical modelling the upper devonian carbonate reefs in west central alberta are important petroleum reservoirs that are well known for their extensive secondary porosity an outcrop analogue study indicates that an early matrix selective dolomitization event occurred which is characterized by a major late devonian sea water component with increased salinity because of evaporation it is interpreted that the matrix replacive dolomite formed during the famennian as the result of a combination of both seepage and latent reflux dolomitization although an additional type or overprinting of later immediate burial dolomitization cannot be excluded formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores most typically stromatoporoids geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case geochemical models consistent with this analysis and interpretations can reproduce the secondary porosite and suggest a viable dolomitization process for the localities studied

distribution of porosity and permeability in platform dolomites insight from the permian of west texas saller and henderson 1998 presented an article concerning porosity and permeability variations in two west texas oil fields north riley and south cowden fields these fields occur within dolomitized permian platform carbonates near the basinward shelf edge of the central basin platform a paleogeographic element that influenced permian deposition in west texas their carefully documented study outlines several anomalous aspects concerning porosity distribution they noted in particular that porosity distribution is not strictly facies dependent but that porosity tends to increase in a basinward direction they suggested that porosity developed largely during dolomitization by the action of hypersaline evaporative fluids that flowed downdip in a basinward direction during the permian these brines were generated in restricted shelf interior lagoons and moved downdip parallel with bedding in a shallow reflux type of hydrologic system driven by subsurface fluid density differences in the permian clear fork and grayburg formations as shown here in figure 1 see figure 18 in saller and henderson 1998 figure 1 part a shows the scenario proposed by saller and henderson 1998 for porosity creation dolomitization and dolomite cementation of the permian clear fork and grayburg formations of west texas they suggested that macrodissolution of calcium carbonate occurred contemporaneously with dolomitization and dolomite cementation this process of brine backreaction however would cause a net increase of porosity in the shelf interior the opposite of what is observed part b shows a two stage scenario proposed here to circumvent the problem of limestone macrodissolution by shelf interior brines that must have been initially in approximate bulk equilibrium with calcium carbonate the first stage records dolomitization by replacement of limestone with little macrodissolution in stage two evaporative shelf interior brines may have evolved in equilibrium with dolomite only and became undersaturated with calcite and aragonite reflux migration of these brines downdip may

insights into the dolomitization process and porosity modification in sucrosic dolostones avon park formation middle eocene east central florida u s a abstract the avon park formation middle eocene in central florida u s a contains shallow water carbonates that have been replaced by dolomite to varying degrees ranging from partially replaced limestones to highly porous sucrosic dolostones to less commonly low porosity dense dolostones the relationships between dolomitization and porosity and permeability were studied focusing on three 305 m long cores taken in the city of daytona beach stable isotope data from pure dolostones mean 18o 3 91 v pdb indicate dolomite precipitation in eocene penesaline pore waters which would be expected to have been at or above saturation with respect to calcite nuclear magnetic log derived porosity and permeability data indicate that dolomitization did not materially change total porosity values at the bed and formation scale but did result in a general increase in pore size and an associated substantial increase in permeability compared to limestone precursors dolomitization differentially affects the porosity and permeability of carbonate strata on the scale of individual crystals beds and formations at the crystal scale dolomitization occurs in a volume for volume manner in which the space occupied by the former porous calcium carbonate is replaced by a solid dolomite crystal with an associated reduction in porosity dolomite crystal precipitation was principally responsible for calcite dissolution both at the actual site of dolomite crystal growth and in the adjoining rock mass carbonate is passively scavenged from the formation which results in no significant porosity change at the formation scale moldic pores after allochems formed mainly in beds that experienced high degrees of dolomitization which demonstrates the intimate association of the dolomitization process with carbonate dissolution the model of force of crystallization controlled replacement provides a plausible explanation for key observations concerning the dolomitization process in the avon park formation and elsewhere 1 volume for volume replacement at a crystal scale 2 coupled growth of dolomite crystals and dissolution of host calcium carbonate matrix and 3 automorphic replacement by euhedral dolomite crystals the force of crystallization model also does not require an influx of externally derived water that is undersaturated with respect to calcite to dissolve calcite a fact that could simplify diagenetic models of porosity generation in dolostones the later addition of external carbonate can result in a substantial reduction in porosity by the precipitation of dolomite cement which could convert a high porosity sucrosic dolostone into a dense paleozoic type dolostone

melt inclusion geochemistry and computer modeling of trachyte petrogenesis at ponza italy abstract the punta della guardia trachytic unit on the island of ponza italy is thought to represent the latest stage in the evolution of a parent basaltic melt in a shallow magma chamber undergoing fractional crystallization to test this hypothesis melt inclusions mi have been analyzed to constrain the crystallization history of the ponza trachyte the trachyte is a weakly porphyritic lava that contains phenocrysts of plagioclase k feldspar clinopyroxene and biotite melt inclusions were observed and measured in plagioclase clinopyroxene and k feldspar the crystallized devitrified melt inclusions were heated to dissolve melt the crystals quenched to a homogeneous glass and then analyzed for major elements by electron microprobe two different heating protocols one involving rapid heating in a one atmosphere furnace and the other involving slower heating in a microscope heating stage were used to homogenize the inclusions in order to investigate possible effects of heating rates on inclusion composition no detectable difference was found melt inclusion compositions suggest that the magma evolved from an early phase dominated by clinopyroxene plagioclase crystallization to a late stage in which melt composition was controlled by precipitation of two feldspars the potential basaltic parent magma does not crop out on ponza although more primitive and coeval basalts and trachybasalts are found on nearby ventotene island which is considered to be co magmatic with ponza using the program melts we have compared results obtained from melt inclusions in the ponza trachyte with melt compositions predicted for fractional crystallization of a magma having the composition of the ventotene trachybasalt crystallization trends predicted by melts are consistent with data from melt and host phases although differences in measured and predicted compositions are significant in some cases in spite of these differences the model results and measured compositions are consistent with the hypothesis of a genetic link between the magmatism of ponza and ventotene

isotope stratigraphy of the european carboniferous proxy signals for ocean chemistry climate and tectonics abstract carboniferous brachiopods from western europe and the former ussr have been utilized as carriers of isotopic proxy signals for paleoceanography and paleoclimatology during the assemblage of pangea this is of particular interest because this time interval coincides with the transition from greenhouse to icehouse conditions the 18 o values 3 ma running means show an overall 18 o increase from about 7 1 to 3 1 during the carboniferous the rise in the 18 o is stepwise with major shifts in the mid tournaisian and at the visean serpukhovian transition the comparable 13 c record shows a tournaisian peak at 5 a visean oscillation at about 2 5 1 a serpukhovian rise of more than 3 a plateau around 5 during the bashkirian and moscovian a 4 drop during the kasimovian and another rise of about 3 during the gzhelian the 87 sr 86 sr record shows an early carboniferous decline from 0 7083 to a mid visean minimum of 0 7077 followed by a rapid serpukhovian rise to a bashkirian and moscovian plateau at 0 7087 and a slight decline during the kasimovian and gzhelian the covariance of isotopic trends and the coincidence with geological phenomena indicates that the long term variations in ocean chemistry and climate may be coupled to the tectonic evolution of the earth system the namurian witnesses the onset of hercynian orogeny initiation of the main phase of carboniferous glaciation formation of the extensive coal deposits and the rise in 87 sr 86 sr 13 c and 18 o all this suggests that a tectonically controlled climate change resulting in cooling and increased carbon storage in the organic reservoir was the major triggering factor for all the above phenomena the late carboniferous glacial episode was interrupted by a warm interval in the stephanian the carbon and oxygen isotope data indicate that this episode may have been restricted to the early kasimovian a proposition at odds with the poorly dated record of glacial deposits that are interpreted as indicating a warm climate for the entire stephanian in addition the pattern of short lived peaks in 13 c and 18 o in the tournaisian and possibly visean may be interpreted as a reflection of short lived glacial episodes a proposition consistent with geologic records

late diagenetic calcitization of anhydrite from the mississippian of saskatchewan western canada mississippian nodular anhydrites beneath an unconformity in the subsurface of southern saskatchewan are locally replaced by calcite pyrite and celestite triassic clastics above the unconformity are green rather than red and a usually developed subunconformity alteration zone where carbonates are dolomitized and porosity is filled with anhydrite is absent the unconformity lacks karstic features unlike in the usa and probably formed in a hyperarid climate mississippian anhydrites near the unconformity are not preferentially dissolved nor were they extensively hydrated anhydrite calcitization occurred only after the unconformity was shallowly buried by redbeds and it probably involved sulphate reducing bacteria hydrogen sulphide generated by bacteria reduced redbed pigments the replacement calcite contains pseudomorphs and relicts of anhydrite and pseudomorphs of secondary gypsum these indicate calcitization occurred only after original mississippian gypsum was altered to anhydrite and this in turn was partially converted back to secondary gypsum beneath the unconformity replacement occurred concurrently with the formation elsewhere of the dolomitized zone beneath the unconformity sulphur isotopic ratios of replacement pyrite are depleted relative to mississippian sulphate values consistent with the activities of sulphate reducing bacteria carbon isotopic ratios of replacive calcites however do not support this interpretation and are identical to those of mississippian limestones simple replacement of sulphate by pore water bicarbonate in equilibrium with host limestones is unlikely because protons generated during the reaction should have created acidic conditions in which calcite would have dissolved a full explanation of the calcitization remains elusive but may involve replacement occurring in an active groundwater system and or bacterial sulphate reduction occurring upstream of the site of calcitization

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

direct physical evidence of dolomite recrystallization the possibility of recrystallization is a long standing barrier to deciphering the genetic origin of dolomites there is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event results from 65 new high temperature dolomite synthesis experiments 1 m 1 0 mg ca ratio solutions at 218 c demonstrate dolomite recrystallization affecting stoichiometry cation ordering and nanometre scale surface texture the data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization which occur by various dissolution precipitation reactions during the first phase induction period no dolomite forms despite favourable conditions the second phase replacement period occurs when ca rich dolomite products with a low degree of cation ordering rapidly replace calcite reactants during the replacement period dolomite stoichiometry and the degree of cation ordering remain constant and all dolomite crystal surfaces are covered by nanometre scale growth mounds the third phase primary recrystallization period which occurs in the experiments between 97 and 100 dolomite is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering the end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat laterally extensive layers the fourth phase of the reaction secondary recrystallization period occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time inferences of recrystallization in natural dolomite based on cation order or stoichiometry of dolomite usually depend on assumptions about the precursor dolomite subjected to recrystallization if it is assumed that the experimental evidence presented here is applicable to natural low temperature dolomites then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization

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

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

unusually large subsidence and sea level events during middle paleozoic time new evidence supporting mantle convection models for supercontinent assembly new evidence from basin modeling documents a nearly synchronous episode of rapid subsidence between middle devonian and earliest mississippian time in almost all preexisting margins and interior basins of north america the subsidence events appear to have been accompanied by a large sea level rise relative to the stable cratonic platform herein we suggest that the timing and magnitude of basin subsidence and sea level change are compatible with predictions by recent mantle convection models for the assembly of supercontinents if so the sea level and subsidence events were consequences of the onset of accretion of pangea above a major zone of mantle downwelling

low and high frequency sea level changes control peritidal carbonate cycles facies and dolomitization in the rock of gibraltar early jurassic iberian peninsula the rock of gibraltar comprises two tectonically separated limbs of an isolated klippe of liassic gibraltar limestone formation both limbs have similar c 400 m thick sequences of inner carbonate platform facies arranged in high frequency metre scale shallowing upward peritidal cycles with emergent caliche caps four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures lithologies facies and biota when compared with other liassic cycles from fault bound platforms of the western mediterranean region all are found to be of similar scale facies and cycle type likely common origins are through milankovitch band allocyclicity or autocyclic tidal flat progradation superimposed on regional subsidence within the gibraltar limestone high frequency cycles are superimposed on a low frequency third order cyclicity that is revealed through the use of fischer plots to control the occurrrence of facies biota high frequency cycle types and dolomitization falling sea level and lowstand phases with reduced accommodation space are typified by restricted inner platform facies and cycles and by early reflux dolomitization transgressive and highstand phases with more accommodation space are characterized by the absence of early dolomites the incoming of inner platform microfossils i e foraminifera and calcareous algae and by less restricted marine facies i e oncoids shelly rudstones packstones and grainstones fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas

early mississippian climate based on oxygen isotope compositions of brachiopods alamogordo member of the lake valley formation south central new mexico stable isotope compositions of brachiopod shells from the alamogordo member of the lake valley formation south central new mexico provide information about early mississippian paleoclimate the alamogordo member was deposited on a ramp at the southwestern edge of the north american craton at lat 20 s it consists of cherty lime mudstone to lime packstone predominantly mudstone and wackestone strata and scattered mud cement waulsortian mounds the brachiopods analyzed lived at water depths estimated to have been between 100 and 300 m the 1 8 o values of well preserved shells of a single species of brachiopod average 2 3 relative to pdb the peedee belemnite isotope standard this average represents a temperature of 21 26 c if the isotopic composition of the seawater 1 8 o s w was between 0 and 1 as inferred from pleistocene and holocene 1 8 o s w and other considerations sea surface temperature would have been 26 31 c after we applied a 5 c correction for water depth this tropical climate for an early mississippian low latitude site agrees with previous isotopic and climate modeling results but contrasts with a temperate climate inferred from comparison of the lithology and biota of the alamogordo member with those of modern temperate carbonates

the origin of dolomites in tertiary sediments from the margin of great bahama bank abstract based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on great bahama bank we have determined three different environments of formation for the dolomites that are common throughout the pliocene and miocene parts of these cores the first environment of dolomitization occurs in association with development of nondepositional surfaces dolomite typically forms below each of these surfaces the concentration and extent of which is governed by the length of the period of nondeposition these dolomites are recognized by their association with the nondepositional surfaces characteristic positive 18o values indicative of formation from cold bottom waters and 18o and sr profiles with depth that suggest formation in the presence of diffusive temperature and sr gradients the second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes the dolomite forming here is termed background dolomite this is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space dolomitization by this mechanism uses a local source of mg2 and consequently the dolomite never constitutes more than between 5 and 10 of the sediment this type of dolomite is characterized by extremely high sr concentrations which reflect concentrations of sr2 in the pore fluids from which it formed the high concentrations of sr2 in the pore fluids arise through the continued recrystallization of metastable aragonite and high mg calcite to dolomite and low magnesium calcite driven by oxidation of organic material by sulfate not only does sulfate reduction provide an additional thermodynamic drive for recrystallization but because the absolute concentration of sr2 in the pore fluids is governed by the solubility of celestite srso4 removal of sulfate allows the sr2 ca2 ratio of the interstitial fluid to become much higher than normally encountered the final type of environment of dolomitization is associated with coarse grained reefal sediments the pervasive nature of the dolomitization and the relatively normal sr concentrations suggest the circulation of normal marine water in a relatively open system

chemical and physical studies of type 3 chondrites vi siderophile elements in ordinary chondrites abstract the abundances of fe ni co au ir ga as and mg have been determined by instrumental neutron activation analysis in 38 type 3 ordinary chondrites 10 of which may be paired and 15 equilibrated chondrites classification of type 3 ordinary chondrites into the h l and ll classes using oxygen isotopes and parameters which reflect oxidation state fa and fs in the olivine and pyroxene and co in kamacite is difficult or impossible bulk compositional parameters based on the equilibrated chondrites have therefore been used to classify the type 3 chondrites the distribution of the type 3 ordinary chondrites over the classes is very different from that of the equilibrated chondrites the ll chondrites being more heavily represented the type 3 ordinary chondrites contain 5 to 15 percent lower abundances of siderophile elements and a compilation of the present data and literature data indicates a small systematic decrease in siderophile element concentration with decreasing petrologic type the type 3 ordinary chondrites have like the equilibrated ordinary chondrites suffered a fractionation of their siderophile elements but the loss of ni in comparison with au and ir is greater for the type 3 chondrites these siderophile element trends were established at the nebula phase of chondritic history and the co variation with petrologic type implies onion shell structures for the ordinary chondrite parent bodies it is also clear that the relationship between the type 3 and the equilibrated ordinary chondrites involves more than simple closed system metamorphism

the oxygen and carbon isotope composition of carboniferous fossil components sea water effects the aragonitic molluscs and lime mud of the pennsylvanian buckhorn asphalt deese group of southern oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea water in addition 18o values indicate that the pelecypods precipitated their shells during the warmer months of the year the coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year for the orthocone nautiloids the 18o values suggest that they precipitated their shells in deeper cooler water the low mg calcite brachiopods of the mississippian lake valley formation of new mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea water the 18o and 13c values of the buckhorn and lake valley faunas in conjunction with other published results suggest that carboniferous sea water was on a average depleted in 18o by 1 5 2 pdb relative to recent sea water however the 13c value of 2 6 2 pdb for average carboniferous sea water is similar to that of recent ocean water early diagenetic alteration of metastable carbonates probably occurs in a meteoric sea water mixing zone in this zone the oxygen and carbon isotopic compositions of these components are increased by about 2 4 pdb over their marine composition

isotopically zoned carbonate cements in early paleozoic sandstones of the illinois basin δ 18 o and δ 13 c records of burial and fluid flow abstract sem sims imaging and analysis of 18 o and 13 c in sandstones from a transect through the illinois basin usa show systematic m scale isotopic zonation of up to 10 in both carbonate and quartz cements of the middle ordovician st peter and cambrian mt simon formations quartz 18 o values are broadly consistent with the model of hyodo et al 2014 wherein burial and heating in the illinois basin is recorded in systematically zoned quartz overgrowths observations of zoned dolomite ankerite cements indicate that they preserve a more extended record of temperature and fluid compositions than quartz including early diagenesis before or during shallow burial and late carbonates formed after quartz overgrowths many carbonate cements show innermost dolomite with 18 o values 21 25 vsmow that are too low to have formed by deposition at low temperatures from ancient seawater 18 o 3 and most likely reflect mixing with meteoric water a sharp increase in fe content is commonly observed in zoned carbonate cements to be associated with a drop in 18 o and an abrupt shift in 13 c to higher or lower values these changes are interpreted to record the passage of hot metal rich brines through sandstone aquifers that was associated with mississippi valley type mvt pb zn deposits ca 270 ma of the upper mississippi valley local variability and individual trends in 13 c are likely controlled by the sources of carbon and the degree to which carbon is sourced from adjacent carbonate units or thermal maturation of organic matter quartz overgrowths in sandstones provide an excellent record of conditions during burial heating and pressure solution whereas carbonate cements in sandstones preserve a more extended record including initial pre burial conditions and punctuated fluid flow events

genesis of upper cambrian lower ordovician dolomites in the tahe oilfield tarim basin nw china several limitations from petrology geochemistry and fluid inclusions abstract thick upper cambrian lower ordovician carbonate units in the tahe oilfield of northern tarim basin were extensively dolomitized during their diagenetic history petrographic examination identified three types of matrix dolomites one type of cement dolomite and one type of saddle dolomite in these units micritic to very finely crystalline nonplanar matrix dolomite d1 finely crystalline planar s e matrix dolomite d2 medium to coarsely crystalline nonplanar a matrix dolomite d3 finely to medium crystalline planar e cement dolomite cd and coarsely crystalline planar s to nonplanar saddle dolomite sd isotopic studies c o and sr elements measurements and fluid inclusion analysis were combined to constrain the origins of dolomitizing fluids the occurrence of near micritic dolomite crystals as well as isotopic compositions which are consistent with the values and ratios of upper cambrian lower ordovician marine limestones coupled with the rare earth elements ree patterns with negative ce anomalies collectively suggest precipitation of d1 dolomite at low temperatures of near surface settings from mesosaline to penesaline seawater d1 dolomite characterized by high mg ca ratios and mgo values moderate sr concentrations and low mn and fe values was formed in the early diagenetic stage and scarcely experienced recrystallization the dolomitizing fluid of d2 dolomite was derived from connate seawaters resulting from mechanical compaction and burial dissolution under shallow burial conditions which is indicated by textures isotopic compositions and ree patterns the relatively low mgo contents and mg ca ratios constrain the formation of the d2 dolomite to a closed diagenetic system with a limited local supply of mg2 ions the large overlaps of isotopic compositions between d3 dolomite and earlier dolomite as well as the similar ree patterns and depleted sr contents indicate that d3 dolomite was likely the result of intense recrystallization upon previous matrix dolomites in a deeply buried formation fluid system the petrographic features and geochemical attributes of cd dolomite strongly indicate progressive growth or recrystallization in the burial environment inclusions with high temperatures developed only in the outer edges of cd crystals demonstrate diffusionally mediated recrystallization during the later hydrothermal activity sd dolomite is interpreted to directly precipitate from hydrothermal fluids as evidenced by high homogenization temperature measured from fluid inclusions and high sr contents high salinity levels suggesting the existence of a very saline fluid deep basinal brine the enriched radiogenic strontium isotope coupled with positive eu anomalies indicate that hydrothermal fluids are likely to be crustal magmatic since early hercynian orogeny long period tectonic activities were responsible for the hydrothermal alteration high angle faulting which appeared as strike slip faulting initiated along basement rooted faults likely acted as the important pathway that channeled the hydrothermal fluids from the depths initially hydrothermal fluids with elevated pressure high temperature and low fluid rock conditions were likely responsible for hydrofracturing and brecciation when distal fluids of cooler temperatures flowed into permeable formations the open fluid dominated system with higher fluid rock conditions facilitated the occurrence of dissolution vugs and precipitation of sd dolomite this study provides a basis for understanding the deep burial carbonate reservoirs in tarim basin by clarifying the process of dolomitization

chondrules and other components in c o and e chondrites similarities in their properties and origins we describe three types of chondrules that are common to h3 ll3 cm2 co3 and cv3 chondrites low and high feo porphyritic olivine chondrules contain olivine with fa0 3 8 and fa5 50 respectively and can easily be distinguished petrographically poikilitic pyroxene chondrules have 1 20 vol olivine fa0 2 8 enclosed by low ca pyroxene fs0 5 7 and also occur in e chondrites these three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups metallic fe ni grains containing 0 1 1 cr si and p are present in low feo olivine chondrules from all type 2 and least metamorphosed type 3 chondrites metal compositions reflect reduction during chondrule formation in the nebula not nebular condensation opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy and probably have related origins we conclude that chondrules in all known chondrite groups share similar nebular origins

calcium isotope record of phanerozoic oceans implications for chemical evolution of seawater and its causative mechanisms a total of 280 brachiopods of ordovician to cretaceous age complemented by published data from belemnites and planktonic foraminifera are used to reconstruct the evolution of calcium isotope composition of seawater 44 40casw over the phanerozoic the compiled 44 40casw record shows a general increase from 1 3 nist srm 915a at the beginning of the ordovician to 2 at present superimposed on this trend is a major long term positive excursion from the early carboniferous to early permian as well as several short term mostly negative oscillations a numerical model of the global cycles of calcium carbon magnesium and strontium was used to estimate whether the recorded 44 40casw variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans the model uses the record of marine 87sr 86sr ratios as proxy for seafloor spreading rates a record of oceanic mg ca ratios to estimate rates of dolomite formation and reconstructed atmospheric co2 discharge and erosion rates to estimate continental weathering fluxes the model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed 44 40casw trends suggesting that the isotope signatures of these fluxes must also have changed as a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits i e the oscillating calcite aragonite seas the ultimate control of the calcium isotope budget of the phanerozoic oceans appears to have been tectonic processes specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic mg ca ratio which in turn controlled the dominant mineralogy of marine carbonates hence the 44 40casw as to the causes of the short term oscillations recorded in the secular 44 40casw trend we tentatively propose that these are related to variable rates of dolomite formation and or to changing chemical composition of the riverine flux in particular and ratios induced by variable proportions of silicate vs carbonate weathering rates on the continents

recrystallization versus neomorphism and the concept of significant recrystallization in dolomite research abstract the term neomorphism originally introduced in 1965 as a substitute for the term recrystallization in limestones is now in use also for dolostones however only very few dolomite researches use the term neomorphism even though it is well and strictly defined many do not know what this term means and using the term commonly leads to confusion among carbonate researchers as well as between them and other geologists partially or largely for these reasons most dolomite researchers use the term recrystallization rather than neomorphism in addition the term neomorphism does not encompass all the important characteristics of recrystallization known today it is recommended therefore to abandon the term neomorphism in dolomite research and apply the term recrystallization with an extended definition that includes those characteristics that commonly are involved in recrystallization and that can be measured in dolomites and dolostones textural changes size shape structural changes ordering strain compositional changes stoichiometry isotopes trace elements including zoning fluid inclusions and changes in the paleomagnetic properties for practical applications the absence presence and or degree of recrystallization is often important for a genetic interpretation of dolostones if changes via recrystallization in texture structure composition and or paleomagnetic properties results in data ranges that are larger than the original ones which must be known for reference a dolomite dolostone is said to be significantly recrystallized and the reset properties are no longer representative of the fluid and environment of dolomitization but characterize the last event of recrystallization however not all measurable properties must be reset during recrystallization in this case inherited properties represent the event of dolomitization whereas reset properties represent the fluids and process of recrystallization in recrystallization resets only one of 10 properties e g 87 sr 86 sr ratios beyond the range of the pristine samples within the analytical errors this dolomite is significantly recrystallized with respect to sr isotopes yet insignificantly recrystallized with respect to the other nine properties in either case values of those properties that are identical to the pristine reference values can be used for genetic interpretations of dolomitization the concept of significant recrystallization is of great use in genetic interpretations of dolomites dolostones in particular a reliable interpretation of the chemistry of the dolomitizing fluids no longer depends on an absolute recognition of recrystallization rather it is sufficient to recognize that a dolomite is insignificantly recrystallized application of this principle also shows that many ancient dolomites and dolostones are insignificantly recrystallized finally the concept of significant recrystallization is independent of mineralogy hence it can and should be applied also to limestones

secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems insights from reactive transport modelling dolomitization often plays a critical role in the pore network development of platform carbonates with implications for reservoir quality distribution understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies here the role of secular variations in seawater mg ca as a control on dolomitization and early porosity modification was evaluated using one dimensional reactive transport models and fluids based on modern aragonite sea mississippian and aptian calcite sea seawaters the sensitivity of dolomitization to a range of extrinsic controls brine salinity temperature fluid flow rate and pco2 and to intrinsic reactivity of the sediments effective reactive surface area was also explored simulations suggest faster calcite replacement by dolomite for seawaters with higher mg ca indicating that dolomitization potential is determined more by mg ca rather than saturation index increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density driven fluid flux in addition to brine composition effective surface area of precursor sediments and temperature exert a critical control on replacement rate while secular variations of ph and carbonate alkalinity associated with changes in pco2 are only secondary controls above flow rates of 0 01 m yr 1 replacive dolomitization is reaction limited rather than flux limited favouring alteration of fine grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low post replacement dolomite cementation is flux dependent and thus favoured in areas of high head gradient and high permeability sediments and contrary to replacement supersaturation is a more important driver than mg ca while uncertainties remain regarding low temperature dolomitization kinetics the capability of numerical simulations to decouple individual controls provides new insights which can be used in conjunction with traditional comparative sedimentology to generate more rigorous conceptual models for individual reservoir settings

stable isotopic composition of meteoric calcites evidence for early mississippian climate change in the mission canyon formation montana abstract the lower mississippian mission canyon formation of central to southwestern montana was deposited under dominantly semiarid to arid climatic conditions during osagean to early meramecian times following deposition a pronounced climatic shift to more humid conditions occurred during middle meramecian times this climatic change is indicated by extensive post depositional karst fabrics and in the stable isotopic composition of early meteoric calcite cements and diagenetically altered sediments early meteoric calcite cement in mission canyon limestones is generally nonluminescent and fills intergranular and fenestral porosity petrographic data indicate that this cement formed during intermittent subaerial exposure of the mission canyon platform during osagean times this initial generation of meteoric calcite cement has 18o values from 8 1 to 2 6 pdb these data and the oxygen isotopic values from nonluminescent skeletal grains and micrite in host limestone indicate that osagean meteoric water may have had 18o values as low as 6 0 smow a second generation of petrographicalty similar but isotopically distinct calcite cement fills biomolds and porosity within solution collapse breccias in the mission canyon formation this cement generation postdates earlier nonluminescent osagean calcite cement and is volumetrically most abundant near the top of the mission canyon formation 18o values from these cements and from nonluminescent lime mudstone clasts and matrix in solution collapse breccias range from 13 8 to 8 2 pdb these data indicate that meramecian meteoric water may have had 18o values as low as 12 0 however a higher temperature burial overprint on the 18o values of the calcite cement cannot he ruled out the more positive 18o values of the osagean calcite components probably indicate warm and arid conditions during short term 104 yr subaerial exposure along intraformational sequence and parasequence boundaries the more negative 18o values from meramecian calcite components and the extensive karst associated with the post mission canyon unconformity may have developed because of cooler and more humid climatic conditions and possible rain out effects during middle meramecian times a dramatic shift towards cooler and more humid climatic conditions may be coincident with the onset of major continental glaciation in the early carboniferous the post mission canyon unconformity has been attributed to a major fall in sea level that may have glacio eustatic origins growth of continental glaciers during a time of global cooling would have caused migration of polar fronts further toward the paleoequator these polar fronts in turn would have pushed moist mid latitude weather systems toward the paleoequator resulting in cooler more humid conditions in low latitude settings during icehouse times

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

numerical modeling of reflux dolomitization in the grosmont platform complex upper devonian western canada sedimentary basin the upper devonian grosmont platform in the western canada sedimentary basin is a pervasively dolomitized giant heavy oil reservoir with reserves of 317 billion bbl of bitumen the principal type of grosmont platform dolomite formed early and on the basis of stratigraphic and geochemical evidence is interpreted as early diagenetic reflux dolomite we use a numerical ground water flow model to investigate the viability of reflux to dolomitize the grosmont platform we simulate reflux at four key stages of platform evolution incorporating the transient effects of changes in platform architecture rock properties and the salinity of platform top waters the pattern and magnitude of reflux is critically controlled by permeability and the distribution of platform top brines which are concentrated up to gypsum saturation reflux flow is focused in the relatively permeable carbonates of the grosmont formation and is from the platform interior toward the platform margin the 120 m thick shales of the ireton formation that separate the grosmont and cooking lake formations restrict cross formational flow and brine transport during a 100 k y period of relative sea level rise and platform top drowning brines of reflux origin continue to sink and entrain platform top waters latent reflux where the intervening aquitards are thin or absent reefs of the leduc formation capture reflux brines from the overlying grosmont platform and focus cross formational brine transport lateral contrasts in salinity are sufficient to drive a series of free convection cells in the relatively permeable reefs of the leduc formation computed distributions of fluid flux in conjunction with magnesium mass balance calculations that incorporate the range of uncertainty particularly in permeability support the suggestion that the reflux of gypsum saturated brines could have formed much if not most of the dolomite in the grosmont formation in the 1 6 m y available

dolomitization and neomorphism of mississippian visean upper debolt formation blueberry field northeastern british columbia geologic petrologic and chemical evidence petrographic chemical and isotopic studies of the mississippian visean upper debolt formation in the blueberry field british columbia canada reveal that dolomitization was the result of several diagenetic events and that neomorphic alteration of these dolomites significantly modified their original chemical signatures these studies also demonstrate how tectonics play an important role in controlling and modifying reservoir dolomites in the area petrographic investigations have documented two early dolomite phases 1 early matrix dolomite and 2 pervasive dolomite and two later generations 3 coarse cement and 4 pseudomorphic replacement of crinoids early matrix dolomite occurs as small average 25 m subhedral to euhedral crystals that replace the matrix of carbonate mudstones wackestones and packstones petrographic evidence suggests that early matrix dolomite had a relatively early precompaction origin possibly from marine fluids however geochemical evidence indicates that later fluids have altered their original geochemical signatures copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 1 1996 revised manuscript received august 16 1996 final acceptance january 19 1997 2department of earth sciences university of windsor windsor ontario canada n9b 3p4 present address chauvco resources ltd 2900 255 5th avenue s w calgary alberta t2p 3g6 canada 3department of earth sciences university of windsor windsor ontario canada n9b 3p4 this research was supported by individual operating grants from canada s natural sciences and engineering research council nserc awarded to i s al aasm and additional funding from pancanadian petroleum resources limited and suncor inc s durocher acknowledges support from an ontario graduate scholarship s brame talisman energy l williams phillips petroleum resources ltd g burrowes and i mcllreath pancanadian petroleum ltd and j packard rhomb carbonate consulting ltd are thanked for the valuable information insights and ideas for tackling the complexities of the blueberry field thanks are also due to j veizer university of bochum and university of ottawa for strontium isotope analysis an earlier version of this paper benefited from critical reviews by m malone texas a m university c viau shell canada ltd s schutter j dravis dravis interest inc and an anonymous reviewer

diagenetic evolution of the upper cretaceous limestone and sandstone exhumed reservoirs in the western basque cantabrian basin north spain abstract the basque cantabrian basin ne spain has been considered one of the most interesting areas for hydrocarbon exploration in the iberian peninsula since the 60th to 70th of last century this basin is characterized by the presence of numerous outcrops of tar sands closely associated with fractures and triassic diapirs the aims of this work is to establish the diagenetic evolution of the upper cretaceous reservoir rocks with special emphasis in the emplacement of oil and their impact on reservoir quality the studied rocks are constituted of carbonates and sandstones that contain massive quantities of bitumen filling vugs and fractures petrographic results indicate that the carbonate rocks from maestu outcrops are bioclastic grainstones and wackestones whereas the tar sandstones from atauri and loza outcrops are dominated by quartzarenites and subordinated subarkoses the paragenetic sequence of the main diagenetic phases and processes include pyrite bladed and drusy calcite cement calcite overgrowths silicification of bioclasts and microcrystalline rhombic dolomite cement and first stage of oil emplacement blocky calcite cement coarse crystalline calcite cement calcitized dolomite calcite veins saddle dolomite and stylolites filled by the second phase of oil entrance together with the above mentioned diagenetic alterations the arenites are affected by early kaolinitization of feldspars and the scarce formation of clay rim and epimatrix of illite all sandstones and dolomitized carbonate rocks show high intercrystalline and intergranular porosity which is full by biodegraded hydrocarbons solid bitumen the biodegradation affects alkanes isoprenoids and partially hopanes and steranes saturated hydrocarbons aromatics hydrocarbons like naphthalenes phenanthrenes dibenzothiophenes and triaromatics are also affected by biodegradation results indicate that the first hc emplacement corresponds to early stage of calcite and dolomite cementation and the second and more important emplacement is related to fracturation processes resulting in the formation of excellent reservoirs

recrystallization of dolomite evidence from the monterey formation miocene california dolomites from the upper calcareous siliceous member of the miocene monterey formation exposed west of santa barbara california were analysed for geochemical isotopic and crystallographic variation the data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids recrystallization is recognized by the following compositional and crystallographic variations dolomites have decreasing 18o and 13c compositions decreasing sr contents and increasing mg contents with increasing burial depths and temperatures from east to west in the study area 18o values vary from 5 3 in the east to 5 5 pdb in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west 13c values correlate with 18o and decrease from 13 6 in the east to 8 7 pdb in the west sr concentrations correlate positively with 18o values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west mol mgco3 values inversely correlate with 18o values and increase from a minimum of 41 0 in the east to a maximum of 51 4 in the west rietveld refinements of powder x ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering the fraction of the ca sites in the dolomites that are occupied by ca atoms increases slightly with the approach to stoichiometry the fraction of the mg sites occupied by mg atoms strongly correlates with mol mgco3 even in early diagenetic non stoichiometric dolomites there is little substitution of mg in ca sites during recrystallization the amount of mg substituting for ca in ca sites decreases even further most of the disorder in the least recrystallized non stoichiometric dolomites is related to substitution of excess ca on mg sites

korra korrabes a new large h3 chondrite breccia from namibia a new large ordinary chondrite has been recovered from near the strewn field of gibeon iron meteorites in namibia and is designated korra korrabes after the farm property on which the specimens were found in 1996 2000 a total of 140 kg of related specimens were recovered including a large stone of 22 kg and hundreds of smaller objects between 2 g and several kilograms cut surfaces indicate that korra korrabes is a breccia containing 10 20 of light grey brown clasts up to 3 cm across in a uniform darker grey brown host that contains abundant round chondrules and irregular grains of fe ni metal and troilite up to 1 cm across the vast majority of the stone is unshocked although some clasts show mild shock features stage s2 and one chondrule fragment is moderately shocked stage s3 weathering grade varies between w1 and w2 microprobe analyses indicate variable compositions of olivine fa13 8 27 2 n 152 percent mean deviation 7 82 and low ca pyroxene multiply twinned clinobronzite fs8 4 27 8 n 68 there is excellent preservation of magmatic textures and mineralogy within many chondrules including normally zoned olivine fa13 8 18 9 and low ca pyroxene fs0 2 20 9 phenocrysts and abundant glass some of whose compositions are unusually alkaline na2o k2o 13 6 16 3 wt and ca deficient cao 0 0 75 wt seemingly out of magmatic equilibrium with associated clinoenstatite or high al calcic clinopyroxene crystals textural and mineralogical features indicate that korra korrabes is an h3 chondrite breccia which represents the largest and least equilibrated stony meteorite yet recovered from namibia it is now one of the four largest unequilibrated ordinary chondrites worldwide

dolomitization and dolomite neomorphism in the back reef facies of the bonneterre and davis formations cambrian southeastern missouri the back reef facies of the bonneterre and davis formations consists of dolomitized cryptalgalaminates and partially dolomitized peloid mudstones these subfacies were deposited on tidal flats and in shallow lagoons that existed in the st francois mountains of southeast missouri during late cambrian time the back reef facies is bounded on the seaward side by dolomitized algal bioherms and grainstone banks which host the southeast missouri mississippi valley type mvt lead zinc copper ore deposits cryptalgalaminates are replaced by both fine crystalline planar dolomite and coarse crystalline nonplanar dolomite peloid mudstones are partially replaced but mainly by coarse crystalline nonplanar dolomite planar dolomite replacing cryptalgalaminates shows gradational transitions into coarse crystalline nonplanar dolomite the transitions are accompanied by progressive increase in average crystal size and an increased skewness toward the coarse size fraction planar dolomite is enriched in 18 o relative to stratigraphically equivalent marine limestones coarse crystalline nonplanar dolomite which replaces both cryptalgalaminates and peloid mudstones has delta 13 c and delta 18 o values transitional between those of planar dolomite and late dolomite cements associated with mvt mineralization fluid inclusion analysis of nonplanar dolomite and associated dolomite cements indicates precipitation at elevated temperatures 60 to 114 degrees c by saline basinal fluids these data indicate that the back reef facies was subjected to a complex diagenetic history involving both early and late diagenetic dolomitization and dolomite neomorphism fine crystalline planar dolomite was formed during early diagenetic dolomitization of the cryptalgalaminate subfacies by a fluid approaching seawater composition coarse crystalline nonplanar dolomite was formed as a result of neomorphism of pro existing planar dolomite and dolomitization of peloid mudstones after burial accompanying mvt mineralization in southeast missouri

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

origin of dolomite in the arab d reservoir from the ghawar field saudi arabia evidence from petrographic and geochemical constraints a significant proportion of oil production from the kim meridgian arab d strata in the ghawar field saudi arabia originates from dolomitized rocks stratigraphic petrographic and geochemical data suggest that at least four episodes of dolomitization affected these sediments the lower portion of the arab d zone 3 is only partially dolomitized with the dolomite frequently being associated with firm grounds we propose that these dolomites were formed on an outer ramp setting with a maximum water depth of 50 m during a period of nondeposition with the dolomitization process being promoted by the oxidation of organic material and the diffusion of mg 21 from the overlying seawater the dolomites in zone 2b are geochemically dis tinct compared to those in zone 3 in that they have relatively positive oxygen isotope compositions 2 1t o22 compared to 26 5 the relatively positive oxygen isotope composition and the geochemical sim ilarity of zone 2b to the dolomites in zone 1 which are intimately associated with the overlying evaporites has led us to conclude that the zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments these hypersaline fluids bypassed zone 2a by moving through the grain dominated sediments early cemen tation and dolomite formation made these units more susceptible to later fracturing that affected the entire arab d formation this frac turing allowed higher temperature fluids to leach the dolostones there by removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites as a result of this later dolomitization event rocks that were only partially dolomitized were leached creating units with extremely high permeability and porosity super k intervals dolomites in the lower zone 3 were recrystallized during burial by the normal geothermal gradient leading to the pre sent negative oxygen isotope values zone 1 dolomites are petrograph ically distinct from zone 2 dolomite in that they are mimetic and fabric preserving although they are geochemically similar this mimetic style of dolomitization occurs immediately adjacent to the overlying anhy drite and is interpreted to have formed very shortly after deposition from hypersaline brines recovery and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log as a result of the importance of dolomite in the formation of the super k intervals within the arab d an understanding of the paragenesis of these zones is important not only in predicting super k formation within the arab d but also as an analogue for understanding other dolomitized res ervoirs with high permeability and porosity previous characterization work of dolostones from the arab d in the ghawar field cantrell et al 2004 has identified three types of dolomite with distinctive petrographic stratigraphic and geochemical characteristics fabric preserving non fabric preserving and baroque dolomite fabric pre serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved fabric preserving dolomite contains low concentrations of fe average 247 ppm fe and relatively positive oxygen isotope values average

the subsidence of the williston basin a detailed comparison between the pre cretaceous stratigraphic record in the williston basin and the available information about phanerozoic sea level changes indicates that most stratigraphic breaks in the basin reflect eustatic control the basin appears to have been subsiding continuously and steadily about the same centre for most of the phanerozoic above a thick 55 km continental crust deposition kept up with subsidence and most of the sedimentary record was laid down close to sea level erosional breaks in the stratigraphic record may represent periods of removal of strata during episodes of low sea level a possible explanation of this essentially steady nonexponential subsidence record is that a mafic subcrustal body is undergoing transformation to eclogite

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

bedding parallel lenticular soft sediment deformation structures a type of seismite in extensional settings abstract soft sediment deformation structures ssdss with bedding parallel lenticular morphology have been occasionally reported and called different names such as loop bedding or pillow bed however there is no understanding of the formative conditions that give rise to them versus other ssdss similar bedding parallel lenticular dolomite bodies tens of centimeters thick with quite variable lengths 30 cm 30 m occur across a large area 3000 km2 exclusively in the st louis member late vasean mississippian of the slade formation in east central kentucky usa field investigations indicate that the lenticular structures along with other associated ssdss represent a persistent deformed horizon that is correlative with and occurs symmetrically around the more intensely deformed big sinking bed which was previously interpreted as earthquake induced ssdss based on multiple lines of evidence the lenticular structures are interpreted to represent distal expressions of the same seismic event that generated the proximal big sinking bed the lenticular structures were subsequently dolomitized and their selective dolomitization is attributable to their brittle deformation during the earthquake which resulted in preferentially developed fracture porosity that allowed more efficient circulation of mg2 fluids throughout the lenticular structures petrographic and stable isotopic evidence also suggests that the selective dolomitization postdates the deformation probably related to a brine reflux system developed during the first marine transgression following subaerial exposure of the st louis carbonates based on comparative analysis with other global cases the bedding parallel lenticular structures are interpreted to represent a distinct type of ssds comparable to boudinage in tectonic deformation their formation reflects a situation in which seismically elevated pore fluid pressure is lower than vertical confining stress but higher than horizontal confining stress a prominent occurrence of lenticular type ssds is a potential indicator of syndepositional extensional stress regime which is testified by tectonic settings of all known cases including the kentucky dolomite bodies

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

petrography and geochemistry of early stage fine and medium crystalline dolomites in the middle devonian presqu ile barrier at pine point canada the petrography and geochemistry of fine and medium crystalline dolomites of the middle devonian presqu ile barrier at pine point western canada sedimentary basin are different from those of previously published coarse crystalline and saddle dolomites that are associated with late stage hydrothermal fluids fine crystalline dolomite consists of subhedral to euhedral crystals ranging from 5 to 25 m mean 8 m the dolomite interbedded with evaporitic anhydrites that occur in the back barrier facies in the elk point basin fine crystalline dolomite has 18 values between 1 6 to 3 8 pdb and 87sr 86sr ratios from 0 7079 0 7081 consistent with derivation from middle devonian seawater its sr concentrations 55 225 p p m mean 105 p p m follow a similar trend to modern little bahama seawater dolomites its rare earth element ree patterns are similar to those of the limestone precursors these data suggest that this fine crystalline dolomite formed from middle devonian seawater at or just below the sea floor medium crystalline dolomite in the presqu ile barrier is composed of anhedral to subhedral crystals 150 250 m mean 200 m some of which have clear rims toward the pore centres this dolomite occurs mostly in the southern lower part of the barrier medium crystalline dolomite has 18o values between 3 7 to 9 4 pdb mean 5 9 pdb and 87sr 86sr ratios from 0 7081 0 7087 mean 0 7084 sr concentrations from 30 to 79 p p m mean 50 p p m and mn content from 50 to 253 p p m mean 161 p p m and negative ce anomalies compared with those of marine limestones the medium crystalline dolomite may have formed either 1 during shallow burial at slightly elevated temperatures 35 40 c from fluids derived from burial compaction or more likely 2 soon after deposition of the precursor sediments by middle devonian seawater derived from the elk point basin these results indicate that dolomitization in the middle devonian presqu ile barrier occurred in at least two stages during evolution of the western canada sedimentary basin the geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water rock ratios were large during neomorphism