a unique lacustrine mixed dolomitic clastic sequence for tight oil reservoir within the middle permian lucaogou formation of the junggar basin nw china reservoir characteristics and origin abstract the middle permian lucaogou formation in the jimusaer sag of the southeastern junggar basin nw china was the site of a recent discovery of a giant tight oil reservoir this reservoir is unusual as it is hosted by lacustrine mixed dolomitic clastic rocks significantly differing from other tight reservoirs that are generally hosted by marine lacustrine siliciclastic calcitic sequences here we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field petrological geophysical including seismic and logging and geochemical data field and well core observations indicate that the lucaogou formation is a sequence of mixed carbonate mainly dolomites and terrigenous clastic mainly feldspars sediments that were deposited in a highly saline environment the formation is divided into upper and lower cycles based on lithological variations between coarse and fine grained rocks in particular dolomites and siltstones are interbedded with organic rich mudstones in the lower part of each cycle whereas the upper part of each cycle contains few dolomites and siltstones tight oil accumulations are generally present in the lower part of each cycle and dolomites and dolomite bearing rocks are the main reservoir rocks in these cycles including sandy dolomite dolarenite dolomicrite and a few dolomitic siltstones optical microscope back scattered electron and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon related fluids

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

diagenetic history of the proterozoic carbonates and its role in the oil field development in the baikit anteclise southwestern siberia abstract the meso to neoproterozoic kamo group and ediacaran carbonate rocks of the baikit anteclise southwestern siberian platform russia were studied to reconstruct the history of development of the reservoir rocks and oil field the kamo group is composed of 3 5 to 4 km thick dolostone and rarely limestone succession that was deformed before the ediacaran time post depositional processes played the key role in forming the reservoir properties of the kamo group during the stage of deposition and subsidence 1500 720 ma silicification cementation and recrystallization caused development of a dense and low permeability matrix pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments including increase in mn up to 0 093 and fe up to 3 11 contents and 87sr 86sr ratios up to 0 7239 decrease in sr concentrations down to 11 ppm and the development of flat ree normalized patterns folding and uplift in the baikit basin began about 720 ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot 70 130 c and highly saline up to 28 7 wt fluids geochemical e g paas normalized patterns of ree and isotopic 87sr 86sr 18o and 13c signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks uplift caused subaerial erosion intense dissolution and karstification which penetrated down to 200 300 m below the surface folding uplift and peneplenization were important for porosity and permeability development in the dolostones of the kamo group the cap rocks were subsequently formed during the ediacaran and paleozoic sedimentation cycle the oil field developed in the paleozoic oil produced from the source rocks of the madra vedreshev and iremeken formations of the kamo group was dissipated during the uplift and erosion before the ediacaran sedimentation after deposition of the ediacaran and cambrian strata source rocks of the iremeken formation might have released oil when they passed the oil window but most of oil in the kuyumba field was derived from younger ediacaran source rocks and migrated through fault network from the flanks the kureika and cis sayan yenisey syneclises into the reservoir of the baikit anteclise that was a topographic high at that time

origin and modification of early dolomites in cyclic shallow platform carbonates yeongheung formation middle ordovician korea abstract the middle ordovician yeongheung formation korea consists of numerous metre scale shallowing upward cycles which were deposited on a shallow marine carbonate platform depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology whereas those developed on the outer platform were partially dolomitized the amount of replacement dolomite within a metre scale shallowing upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences the cross platform distribution of dolomites and their stratigraphic position in the sequences suggest that the yeongheung dolomite formed syndepositionally the common existence of pseudomorphs of evaporite minerals and nodules in the yeongheung formation indicates that dolomitization of the yeongheung formation occurred in a sabkha type tidal flat environment it also occurred during each cycle of deposition influenced by high frequency sea level oscillations the present day yeongheung replacement dolomites are characterized by near stoichiometric to stoichiometric composition 49 9 to 52 1 caco3 mole and low strontium concentration 52 to 133 ppm the 18o value of the replacement dolomites ranges from 7 7 to 5 2 pdb this is much depleted relative to that of contemporary sabkha type dolomites these geochemical characteristics of yeongheung dolomite indicate significant post depositional modification the common existence of oomolds leached skeletal grains and dissolved evaporites in the yeongheung formation illustrates invasion of meteoric water into the sediments recrystallization of yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter 18o values and lower strontium concentration

diagenetic stabilization of the upper permian dalan formation persian gulf basin abstract the upper permian dalan formation is a supergiant gas reservoir in the persian gulf basin reservoir quality is controlled by the original facies type dolomitization and anhydrite cementation this study focuses on the texture petrographic analysis bulk stable isotope values and trace element composition in order to identify the original mineralogy of carbonate grains and cements and to assess the synsedimentary and diagenetic processes including the timing and setting of dolomitization the limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone and neomorphism and low mg calcite cementation blocky and drusy crystals syntaxial overgrowths in the active freshwater phreatic zone the relatively pristine 18o and 13c values low mn and relatively high sr and na contents indicate that this meteoric diagenesis was relatively limited the remaining porous space was cemented by blocky calcite later during burial four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites dolomicrite dolomicrosparite dolosparite and saddle dolomite dolomicrite formed syndepositionally from the evaporated permian seawater or by reflux of dense evaporated brines during shallow burial the second major episode of dolomitization dolomicrosparite and dolosparite was driven by reflux of warmer and more saline fluids at greater depth which affected both previously non dolomitized and dolomitized sediments the final diagenetic stabilization during deep burial is associated with extensive pressure solution fracturing vein filling of calcite and dolomite cements and crystallization of saddle dolomite

significant sink of ocean eddy energy near western boundaries ocean eddies generated through instability of the mean flow are a vital component of the energy budget of the global ocean modelling combined with satellite altimetry data suggests that the energy from westward propagating eddies is scattered and eventually dispersed when they reach the western boundary of an ocean basin

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

withdrawn petrographic and origin of authigenic carbonate minerals and the associated fluid evolution in the lacustrine carbonate siliciclastic mudrocks a case from the low cretaceous of the erennaoer sag erlian basin ne china

fluid compartmentalization of devonian and mississippian dolostones western canada sedimentary basin petrologic and geochemical evidence from fracture mineralization1 integrated petrographic geochemical and fluid inclusion study of fracture mineralization and associated host rock in selected mississippian and devonian carbonates extending from southeastern alb

strontium isotopic compositions of cambrian upper miaolingian furongian series dolomites from south eastern sichuan basin china significance of sources of dolomitizing fluids and timing of dolomitization abstract tracing the nature of dolomitizing fluids and constraining the dolomitization time are of great significance for understanding dolomite genesis and reservoir formation especially for ancient dolomites strontium isotopes can be used as very useful tools to study this aspect thick dolomite bodies developed in the loushanguan group of the upper miaolingian furongian series cambrian in the south eastern margin of the sichuan basin china these dolomites are probably good hydrocarbon reservoirs little research has been carried out on the sr isotopes in these ancient dolomites three different textures of dolomites have been identified in the study area dolomicrite fabric retentive oolitic dolomite and fabric obliterative dolomite the dolomicrite displays a higher sr isotopic value avg 0 71052 compared with the contemporaneous seawater as well as a high sr and low mn content these chemical characteristics indicate that the dolomitizing fluids of the dolomicrite were mainly derived from modified seawater mixed with a little continental input the 87 sr 86 sr ratios avg 0 70936 of the fabric retentive oolitic dolomites are close to those of normal seawater indicating that the dolomitizing fluids were derived from the contemporaneous seawater and that they possibly formed soon after sedimentation as a result of an early replacement during shallow burial the low sr isotopic values avg 0 70854 of the fabric obliterative dolomites indicate that these dolomitizing fluids are related to late cambrian seawater and that the dolomites are the result of recrystallization of the fabric retentive dolomites in a relatively closed diagenetic system during burial with combined characteristics of relatively high sr and low mn contents the recrystallization began approximately 490 ma and was completed not earlier than 485 ma this study is not only beneficial to understand the characteristics and multistage process of the dolomitization of ancient cambrian dolomites but provides a reference for sourcing dolomitizing fluids and constraining the dolomitization time using sr isotopes

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

tectonic and geothermal controls on dolomitization and dolomitizing fluid flows in the cambrian lower ordovician carbonate successions in the western and central tarim basin nw china abstract the deeply buried cambrian lower ordovician carbonates in the western and central tarim basin were extensively dolomitized in the course of their diagenetic history petrographic geochemical o c sr isotopes and trace elements and fluid inclusion studies have been used in this study to elucidate the origins of the ancient massive dolomites based on detailed examination dolomicrite and microbial or algal laminated dolomites dml and three types of crystalline dolomites fine crystalline nonplanar a s dolomite rd1 fine to medium crystalline planar e s dolomite rd2 and medium to coarse crystalline nonplanar a dolomite rd3 are identified nonplanar a dolomite saddle cement cd and early and later stage calcite cement as vug fracture infills are identified most samples of dml still possess relics of algal stromatolitic and anhydritc sediment fabrics common in sabkha dolomites formed during penecontemporaneous supratidal dolomitization the occurrence of rd1 and rd2 along low amplitude stylolites in most samples indicates that their formation was somewhat linked with pressure dissolution by which minor mg ions were probably released for replacive dolomitization during early shallow burial dolomitization moreover the curved crystal faces and the tightly packed crystals of rd2 and rd3 with irregular overgrowth rims and increasing crystal sizes may have 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 cd dolomites is 87 1 c to 161 5 c mostly lower than the estimated ambient temperature 60 c to 240 c for the cambrian lower ordovician strata implying that the growth of the cd dolomites in the study area was of geothermal origin during late shallow to deep burial dolomitization the large overlap of 18o 13c and 87sr 86sr values of rd2 rd3 and cd dolomite suggest that the dolomitizing fluids that precipitated the dolomite cement apparently inherited the composition of the remnant saline connate seawater preserved in host limestones dolomites and siliciclastics in the terreneuvian to cambrian series 3 strata through fluid rock interaction the conceptual model to explain tectonic influence on dolomitizing fluid flows show that faults fractures and permeable horizons which evolved from a strong tensional compressional tectonic during the late hercynian orogeny in this basin effectively provided conduits through which extensive cross formational and ascending higher temperature saline rich dolomitizing fluids were driven by thermal convective advective circulation due to geothermal variation forming irregular networks of dolomites cd dolomite and calcite cements in the host carbonate rocks

observations of overturning in the thermocline the context of ocean mixing abstract the time sequence of events that lead to internal wave breaking and ocean turbulence is investigated data are obtained from depths 100 400 m with a repeat profiling ctd and a coded pulse doppler sonar the instruments were deployed from r p flip during february march 1995 while stationed 30 km west of point arguello california as an aspect of the marine boundary layer experiment although the water depth at the site is 1500 m both rms shear and diapycnal diffusivity as inferred from the average rate and size of overturning events increase with depth below 250 m a deep source of wave energy is implied depth time series of 6 4 m shear s 2 m strain n2 n2 where n is the buoyancy frequency 6 4 m gradient richardson number ri n2 s2 and 2 m effective strain rate the depth derivative of ctd inferred vertical velocity are obtained at 4 minute intervals over a 9 day 100 400 m domain the occurrence of overturns static instabilities of vertical scale 2 m in the observed density

convectively driven mixing in the bottom boundary layer closely spaced vertical profiles through the bottom boundary layer over a sloping continental shelf during relaxation from coastal upwelling reveal structure that is consistent with convectively driven mixing parcels of fluid were observed adjacent to the bottom that were warm by several millikelvin relative to fluid immediately above on average the vertical gradient of potential temperature in the superadiabatic statically unstable bottom layer was found to be 21 7 3 1024 km 21 or 6 0 3 1025 kg m24 in potential density turbulent dissipation rates increased toward the bottom but were relatively constant over the dimensionless depth range 0 4 1 0z d where d is the mixed layer height the rayleigh number ra associated with buoyancy anomalies in the bottom mixed layer is estimated to be approximately 1011 much larger than the value of approximately 10 3 required to initiate convection in simple laboratory or numerical experiments an evaluation of the data in which the bottom boundary layer was unstably stratified indicates that the greater the buoyancy anomaly is the greater the turbulent dissipation rate in the neutral layer away from the bottom will be the vertical structures of averaged profiles of potential density potential temperature and turbulent dissipation rate versus nondimensional depth are similar to their distinctive structure in the upper ocean during convection nearby moored observations indicate that periods of static instability near the bottom follow events of northward flow and local fluid warming by lateral advection the rate of local fluid warming is consistent with several estimates of offshore buoyancy transport near the bottom it is suggested that the concentration of offshore ekman transport near the bottom of the ekman layer when the flow atop the layer is northward can provide the differential transport of buoyant bottom fluid when the density in the bottom boundary layer decreases up the slope

structurally controlled hydrothermal fluid flow in an extensional tectonic regime a case study of cretaceous qamchuqa formation zagros basin kurdistan iraq abstract shallow water carbonate rocks of the lower cretaceous qamchuqa formation host a significant reserve of oil in the nne arabian plate southwestern zagros sutured belt in the kurdistan region iraq such carbonates consist of several dolostone bodies with unique and complex arrays of saddle and zebra dolomite textures especially in fractured intervals integration of field core description microstructural analysis petrographic stable c o and sr isotopes and fluid inclusion analyses reveals two main phases of multiple fluxes of hydrothermal fluid circulation during the zagros orogeny at the end of cretaceous and cenozoic times an early regional non focused hydrothermal dolomitization and a subsequent fault and fracture controlled hydrothermal fluid flow which led to replacement and cementation by a range of saddle dolomite and zebra textures several generations of matrix dolomite saddle dolomite and calcite cement were identified the host dolostone has been affected by shortening folding fracturing and thrust faulting saddle dolomite pipes in particular were found to be associated with en echelon folding the morphology and areal extent of the zebra dolomite are controlled by the pore geometry of the host dolostone in relationship to fracturing and faulting both matrix and saddle dolomite cements have comparable overlapping stable isotopic values ranging from 0 39 to 3 57 and 0 12 to 3 11 vpdb respectively for 13c and 18o values range from 10 53 to 4 47 and 12 87 to 7 79 vpdb respectively 87sr 86sr ratios of pervasive matrix dolomite range from 0 707702 to 0 707839 fall close to late cretaceous seawater values slightly more radiogenic values of saddle dolomites ranging from 0 707759 to 0 708024 resulted from mixing of basinal brines with connate waters isotopic signatures of later calcite cements show more radiogenic sr ratios from 0 707755 to 0 708198 and negative 13c values 1 76 to 7 45 possibly related to late diagenetic processes due to incursion of meteoric waters during subaerial exposure fluid inclusion data of pervasive matrix dolomite and saddle dolomites show homogenization temperatures th with a wide range from 71 3 to 169 7 c and 82 3 to 188 2 c and salinity between 14 4 to 25 4 and 15 6 to 27 9 wt nacl eq respectively these values demonstrate the effect of hydrothermal fluid flow of the alteration of host dolomite and formation of saddle dolomite in fractures and faults

estimates of the temperature flux temperature gradient relation above a sea floor the relation between the ux of temperature or buoyancy the verti cal temperature gradient and the height above the bottom is investigatedin an oceanographic context using high resolution temperature measure ments the model for the evolution of a strati ed layer by balmforthet al 1998 is reviewed and adapted to the case of a turbulent owabove a wall model predictions are compared to the average observa tional estimates of the ux exploiting a ux estimation method proposedby winters d asaro 1996 this estimation method enables the dis entanglement of the dependence of the average ux on the height abovethe bottom and on the background temperature gradient the classicaln shaped ux gradient relation is found in the observations model andobservations show similar qualitative behaviour despite the strong sim pli cations used in the model the results shed light on the modulationof the temperature ux by the presence of the boundary and support theidea of a turbulent ux following a mixing length argument in a strati edow furthermore the results support the use of thorpe scales close to aboundary if su cient averaging is performed suggesting that the thorpescales are a ected by the boundary in a similar way as the mixing length

dolomitization of the leduc formation upper devonian southern rimbey meadowbrook reef trend alberta abstract upper devonian leduc carbonate buildups along the southern part of the rimbey meadowbrook reef trend in alberta buried at depths of 2300 3500 m have been extensively dolomitized dolomitization occurred first by pervasive replacement and later by minor cementation in general replacement dolomites are composed of fabric destructive fine to medium 60 250 m planar subhedral to anhedral crystals replacement dolomitization postdates submarine cementation and deposition of shales and carbonates from the overlying ireton formation and overlaps stylolitization it predates dolomite cementation that is interpreted to have originated from hydrothermal fluids in a shallow burial environment prior to basin tilting this combined with the d18o mean 4 5 0 4 pdb and 87sr 86sr mean 0 70853 values in the replacement dolomites suggests that replacement dolomitization occurred also prior to basin tilting probably during the antler orogeny in the subsurface at a temperature of about 45 c and a depth of about 500 m from slightly modified late devonian seawater two generations of dolomite cements postdate replacement dolomites a coarse and planar e s cement dolomite c1 and a later coarser nonplanar cement dolomite c2 dolomite c1 has oxygen mean d18o 4 73 pdb and strontium mean 87sr 86sr 0 70855 isotopic compositions similar to replacement dolomites these similarities combined with the local association of dolomite c1 with stylolites suggest that some dolomite c1 precipitated from fluids whose composition reflects pressure solution of the preexisting replacement dolomites the composition of fluid inclusions in dolomite c1 mean th 117 c mean salinity 19 wt acl equivalent indicates that most dolomite c1 precipitated from warm hydrothermal brines little variation in fluid inclusion homogenization temperatures occur with burial depth suggesting that dolomite c1 precipitated prior to basin tilting and may be related to antler orogenic events dolomite c2 postdates dolomite c1 is relatively more depleted in 18o mean d18o 9 06 pdb and has a more radiogenic 87sr 86sr ratio 0 71087 preliminary fluid inclusion data from the crimson field suggest precipitation temperatures of about 150 c thus dolomite c2 originated from fluids of higher temperatures and different composition than do omite c1 the petrographic paragenetic and geochemical characteristics of replacement dolomites in this study are similar to those of other leduc buildups along the reef trend and of other devonian dolostones of western canada supporting a model of large scale basinwide fluid flow the pervasive distribution of replacement dolomites along the rimbey meadowbrook reef trend and the similar petrographic and geochemical character of dolomites from both the leduc and cooking lake formations is consistent with the hypothesis that the underlying cooking lake platform acted as a subsurface conduit system for the dolomitizing fluids a potential regional fluid source may have been formation fluids including residual evaporite brines expelled tectonically during the antler orogeny

comparison of differential diagenesis of two oolites on the lower triassic platform margin ne sichuan basin implications for the co evolution of rock structure and porosity abstract two oolites of the lower triassic feixianguan formation on the margins of the kaijiang liangping k l trough in the ne sichuan basin were deposited in the upper parts of similar shallowing upward sequences and are capped by a subaerial exposure surface however they have contrasting diagenetic histories and reservoir qualities the sour oolitic dolostone reservoirs on the eastern side of the k l trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side a petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co evolution of oolite structure and pore network on both sides of the k l trough eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface the layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement during the subsequent dolomitization the early formed oolite structure were mostly inherited by the dolostones on the eastern side with fabric retentive textures in the layers close to the exposure surface and fabric destructive textures in the layers far from the exposure surface during deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss whereas the dolostones generally retained more porosity and have better connectivity of the micropore network the higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation rather than the creation of new pores porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution the heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution reprecipitation process induced by thermochemical sulfate reduction during which small amounts of net porosity are calculated to have been created the oolitic limestones are the best reservoirs close to the exposure surface whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface these results provide some insights into the diagenetic controls on oolite structure and porosity and have implications for predicting the porosity of deeply buried oolitic reservoirs so have global theoretical and practical significance

dating penecontemporaneous dolomitization in carbonate reservoirs paleomagnetic petrographic and geochemical constraints the predominant dolomitization in the mississippian debolt formation western canada sedimentary basin was hypothesized to be early in three gas fields based on stable oxygen and carbon isotopic values and on strontium isotope ratios that are similar to postulated mississippian dolomite values and seawater ratios respectively as the absolute age could not be determined by this method paleomagnetism was used to place constraints on the age of the dolomitization the magnetic analyses on debolt specimens from the same wells revealed three magnetization components that could be tied to geologic diagenetic events 1 a low temperature 350 degreesc and coercivity 80 mt c component of primary or early diagenetic origin that is found mostly in the fine grained limestone and dolomitic muds the preservation of a primary or early diagenetic magnetization combined with the preservation of primary isotopic values indicates that little or no extrabasinal fluid flow is likely to have occurred thus in this particular area orogenically induced fluid flow cannot explain the presence of the cretaceous b magnetization consideration of the potential methods for forming this b component suggests that it is likely an in situ chemical remanence resulting from either hydrocarbon migration or dissolution and reprecipitation of fe rich minerals in the original pore fluids

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

diagenesis of fossil ostracods implications for stable isotope based palaeoenvironmental reconstruction a protocol for assessing the diagenetic alteration of ostracod carapaces for use in palaeoenvironmental reconstruction is proposed based on comparison between fossil carboniferous and recent specimens recent ostracods have a carapace ultrastructure of fine 0 5 m diameter crystals of primary low magnesium calcite the degree of recrystallisation in fossil ostracods is assessed based on the carapace ultrastructure and geochemistry six diagenetic stages are identified 1 replacement of the original finely crystalline calcite carapace with neomorphic calcite 2 framboidal and euhedral pyrite growth 3 ferroan calcite carapace replacement and cavity void filling cement 4 ferroan dolomite carapace replacement and void fills 5 siderite growth and 6 sphalerite and barite mineralisation stage 1 occurs during early shallow burial stages 2 3 soon after and stages 4 6 during much later burial possibly hydrothermal alteration ostracods and macrofossils exhibiting diagenetic stages 1 5 were analysed for oxygen and carbon isotope ratios the oxygen isotope compositions clearly reflect the diagenetic stages the most negative oxygen isotopes 18o 12 9 represent the precipitation of ferroan dolomite stage 4 and the most positive values of the neomorphic calcite 18o 0 9 represent stage 1 and are within the predicted range of carboniferous seawater 18o 4 to 0 the diagenetic protocol established here is specific to palaeozoic fossil ostracods from carbonate and clastic sediments that were subject to shallow burial diagenesis and later hydrothermal alteration nevertheless the approach could be used to assess the utility of fossil ostracods for palaeoenvironmental interpretation from a wide range of geological settings

prefrontal bore mixing rainbow ridge a 1 950 m deep upthrusted ultramafic block along the axis of the mid atlantic ridge has an active hydrothermal vent system at 2 400 m on its western slope however within one kilometer from the vent excessive temperatures are barely measurable probably due to strong turbulent mixing this mixing is studied here using a 400 m long high resolution temperature sensor array moored with a 600 m ranging 75 khz acoustic doppler current profiler rich internal wave turbulence was recorded characterized by 100 200 m upshoots and 200 m large overturning in particular near the end of the warming phase of the up and down moving tide these highly nonlinear internal waves of tides interacting with buoyancy frequency waves extend up to 400 m above the sloping bottom of the ridge whilst a turbulent bottom boundary layer could barely be defined the more intense turbulence higher up in the water column is suggested to lead to the strong dispersion of the hydrothermal plume

multistage dolomitization in late jurassic early cretaceous platform carbonates berdiga formation basoba yayla trabzon ne turkey implications of the generation of magmatic arc on dolomitization abstract late jurassic early cretaceous berdiga formation which is widely distributed in an e w trending belt in the eastern pontides ne turkey is composed of platform carbonates the formation shows varying lithofacies changing from supratidal to platform margin reef both laterally and vertically the studied basoba yayla trabzon ne turkey area comprises one of the typical exposures of formation in the northern zone of eastern pontides in this area the lower parts of the formation are pervasively dolomitized by fabric destructive and fabric preserving replacive dolomites that are ca rich and nonstoichiometric ca56 60mg40 44 replacement dolomites rd are represented by 18o values of 15 3 to 4 2 vpdb 13c values of 3 7 1 5 vpdb and 87sr 86sr ratios of 0 70675 0 70731 petrographic and geochemical evidence implies that rd dolomites are formed before compaction at shallow moderate burial depths from late jurassic early cretaceous seawater and or partly modified seawater as a consequence of rock water interaction and they are recrystallized at elevated temperatures during subsequent burial during subsequent late cretaceous diagenetic process when the region became a magmatic arc rd1a and rd3 dolomite were recrystallized by hydrothermal fluids of marine origin the dolomite cements cd overlying the low amplitude stylolites have similar geochemical charasteristics than rd1a and rd3 dolomite and are characterized by high homogenization temperatures th this might indicate that cd dolomite was precipitated from hydrothermal fluids of marine origin that recrystallized the rd1a and rd3 dolomite the fact that cd dolomite comprising the latest phase of dolomitization is overlain by coarsely crystalline mosaic calcite and silica cements may indicate that diagenetic fluids were undersaturated with respect to dolomite and oversaturated with respect to calcite and silica

a new chemical index of weathering for metamorphic silicate rocks in tropical regions a study from sri lanka abstract weathered and fresh samples of metamorphic rocks from sri lanka were collected from various localities and analysed for major elements by xrf method rigaku kg x system japan the content of water was determined by the ignition method the xrf results obtained from these samples form the basis of a new index of chemical weathering particularly for sri lanka which is named the silica titania index and is calculated as follows silica titania index sio 2 tio 2 sio 2 tio 2 al 2 o 3 tio 2 sio 2 al 2 o 3 100 molecular proportions the index can be used to determine the degree of weathering in chemically weathered silicate rocks of other countries in tropical regions a triangular diagram plots the position of this index the point load strengths of fresh rocksand weathered rocks with different degrees of weathering were correlated with the values of this new chemical index the relative variation in strengths of fresh rocks and weathered rocks clearly indicates its suitability and usefulness for engineering geologists

sequence stratigraphy and cyclicity in the middle eocene of the fayoum ranges western desert egypt implications for regional sea level changes abstract the middle eocene deposits in the fayoum ranges are composed of complex alternative clastic claystone and sandstone and carbonate limestone and dolostone facies and dominated carbonate limestone facies facies are arranged mainly in regression and shallowing upward emergence cycles and sequences field stratigraphic and microfacies analysis of the study area permits recognition of four major sequences reflecting 3rd order cycles depositional sequences and cycles are bounded by subaerial erosive surfaces or caliche deposits ferruginous crust and by their correlative conformities sequence 1 consists of two shallowing upward cycles dominate carbonate facies each of which starts with nummulitic wackestone capped with nummulitic packstone including globigerinatheka subconglobata subconglobata biozone these cycles were deposited under tropical to subtropical conditions as evidenced by the carbonate nature of the rocks and the abundance of nummulites which need warm conditions for their flourishing sequence 2 consists of four emergence cycles based with claystone and capped with wackestone including morozovella lehneri biozone the duration ma of sequence 2 morozovella lehneri zone is 3 05 ma 44 25 ma for the upper of ta3 3 3 to 41 2 ma for the lower of ta3 3 5 sequence 3 includes three rock units includes trucorotaloides rohri biozone the lower unit involves lowstand systems tract the middle unit contains transgressive systems tract and the upper rock unit includes highstand systems tract the lowstand systems tract consists of emergence cycles of mixed clastic carbonate facies clays at the base and capped with wackestone and packstone facies the transgressive systems tract consists of dominant carbonate facies wackestone at the base and capped with packstone facies sequence 4 involves transgressive systems tract the duration of both sequence 3 and sequence 4 has been estimated as 1 8 ma

exploring the geological features and processes that control the shape and internal fabrics of late diagenetic dolomite bodies lower khuff equivalent central oman mountains abstract the lower permian carbonate rock succession of the oman mountains represents a stratigraphical surface analogue for middle eastern lower khuff hydrocarbon reservoirs the characteristics of the carbonate rocks are highly variable both laterally and vertically due to the superposition of several episodes of dolomitization a stratabound early dolomitization event affected the well bedded limestones during triassic times subsequently a later burial dolomitization led to the emplacement of several hundreds of meter wide and tens of meter thick dolomite bodies the goal of our study is to understand the distribution and genetic controls on the formation of late diagenetic dolomite bodies field mapping in wordian to capitanian beds was conducted in the jebal akhdar tectonic window in the central oman mountains to determine the shape internal fabrics and associated structures of late diagenetic dolomite bodies the longest extension of late diagenetic dolomite bodies was found to follow predominantly the strike direction of fracture planes bed to bed contacts the precambrian permian unconformity and the bottom rim of early diagenetic dolomite two groups of diagenetic fabrics were observed in association to fractures and bedding planes first vug related fabrics comprise vuggy pores and zebra dolomite second breccia related fabrics consist of either small scale crackle mosaic breccia fabrics and centimeter sized vugs followed by bedding parallel vugs lined up petrographic analysis revealed three types of diagenetic phases referred to as d1 d2 and d3 the matrix replacive phase d1 represents the main diagenetic phase formed during burial coarse crystalline dolomite rhombs phase d2 and saddle dolomite phase d3 were found along stylolites in vugs and in fractures based on the presence of saddle dolomite and zebra dolomite textures late diagenetic dolomite bodies are interpreted to have formed by the circulation of warm dolomitizing fluids of at least 60 c

relating the spatial variability of chemical weathering and erosion to geological and topographical zones abstract the relationship between the spatial variability of soil elements weathering and erosion rates can be very complex topography abruptly changing elevation gradients and slopes of varying geological composition exert an important influence on weathering trajectories and erosion this complex interconnectivity is only rarely addressed in weathering studies the main objectives of this investigation are therefore to relate weathering and erosion to various and geomorphic units in a dry alpine and threshold landscape having steep slopes kan catchment tehran province iran a number of common weathering indices were tested using a genetic algorithm the best indices are the k na ti ratio the wip weathering index according to parker and the pi product index according to ruxton however the recently suggested 4si m r2 and m f wmin systems discriminate weathering trends more accurately our results show that these soils have a low to moderate weathering stage weathering trajectories point to the active formation of kaolinite and oxyhydroxides river sediments are slightly more weathered and contain the most weathered topsoils and suggest erosion in the catchment is predominantly due to topsoil removal chemical weathering is influenced by the dominant geomorphic units unconsolidated deposits of the quaternary and solid bedrock and landform features combination of altitude and slope aspect does not appear to have a significant impact on weathering denudation erosion rates were determined using meteoric 10be since the fluvial sediments did not contain enough quartz for in situ 10be analyses this procedure has higher uncertainties due to difficulties in estimating the depositional flux of meteoric 10be due to tectonic uplift the erosion rates are very high to maintain a soil layer soil production rates must be also high although annual precipitation is relatively low slope and soil dynamics seem particularly high which leads to a fast turnover of the soil material and maintains the weathering intensity at a rather low to intermediate level

diagenetic evolution of the eocene ramp carbonates a paradigm from the nile valley egypt petrographical and geochemical attributes abstract carbonate rocks of east nile valley in egypt have been investigated as a paradigm for revealing the diagenetic evolution of the eocene ramp carbonates the integration of petrographical elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes i e eogenesis mesogenesis and telogenesis and are related to four diagenetic environments i e marine phreatic meteoric phreatic burial and uplifting environments the paragenetic sequence of the studied carbonates commences with the eodiagenetic episode which includes the marine and meteoric phreatic diagenetic environments the marine phreatic diagenetic environment comprises micritization glauconitization pyritization marine cementation fibrous calcite and syntaxial overgrowth rim cements early dolomitization formation of fine crystalline matrix replacive dolomites and early stages of mechanical compaction before significant burial the meteoric phreatic diagenetic environment includes the neomorphism dissolution meteoric cementation earlier phases of sparry calcite cements and precipitation of megaquartz in the void spaces of the peritidal carbonates during the mesogenesis episode the studied carbonates were exposed to burial diagenesis the shallow burial diagenesis resulted in the creation of point tangenetial contacts between the allochemical constituents and the precipitation of a medium coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores the deep burial diagenetic processes embody the chemical compaction condensed fitted interpeneteration and pressure dissolution fabrics fracturing and burial cementation the telodiagenesis represents the last episode in the diagenetic history of the studied carbonates it witnessed the fracturing meteoric vadose cementation and hematitization processes

geochemical characteristics of upper jurassic lower cretaceous platform carbonates in hazine magara gumushane northeast turkey implications for dolomitization and recrystallization1 the upper jurassic lower cretaceous berdiga formation of the eastern pontides turkey represents a carbonate platform succession composed of pervasively dolomitized intra shelf to deep shelf fac

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

late laramide dolomite recrystallization of the husky rainbow a hydrocarbon devonian reservoir northwestern alberta canada paleomagnetic and geochemical evidence the rainbow field is in reefal carbonates of the middle devonian keg river formation in the western canada sedimentary basin alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical an inclined and a horizontal well core in the dolomite reservoir although they had no viscous remanent magnetization component to use for orientation most specimens had a well defined characteristic remanent magnetization chrm that resides in single to pseudosingle domain magnetite by rotating the mean chrm direction around its core axis a small circle can be generated for each core and the small circles intersect in the true chrm direction of d 168 i 73 5o 95 5 8 k 32 2 its paleopole of 164 e 83 n a95 10 defines a tertiary age with one sigma limits of middle eocene to middle miocene age petrographic examination defines four generations of dolomite matrix dolomite has 60 100 m diameter crystals that were later recrystallized to 200 400 m

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

beryllium isotope geochemistry in tropical river basins abstract the distributions of beryllium 9 and beryllium 10 in rivers within the orinoco and amazon basins have been examined to extend our understanding of their geochemical cycles and to develop their use both in geochronometry and in studying erosional processes beryllium 9 the stable isotope is weathered from mineral lattices before entering the riverine dissolved or exchangeable phase the cosmogenic radioisotope 10be t 1 2 1 5 myr is produced primarily in the atmosphere and is brought into riverine systems via rainfall in dissolved or exchangeable form it may be used to examine the processes which affect beryllium partitioning between the dissolved and particulate phases ancillary data such as major ion distributions provide a basis for selecting regions in which riverine chemistry is dominated by a single geochemical process allowing examination of its effects on be distributions in isolation analyses of 9be in dissolved and suspended material from rivers with a wide range of chemical compositions indicate that its geochemistry is primarily controlled by two major factors 1 1 its abundance in the rocks of the watershed and 2 2 the extent of its adsorption onto particle surfaces the relative importance of these parameters in individual rivers is determined by the extent of interaction with flood plain sediments and the riverine ph this understanding of 9be geochemistry forms a basis for examination of the geochemical cycling of 10be in rivers which are dominated by interaction with sediments the riverine concentration of dissolved 10be is far lower than that in the incoming rainwater indicating that a substantial proportion of it is retained within the soils of the basin or is adsorbed onto riverine particles however in acidic rivers in which the stable dissolved be concentration is determined by the be level in the rocks of the drainage basin dissolved 10be has essentially the same concentration as in precipitation these observations imply that the soil column in such regions must be saturated with respect to 10be and that the ratio of the inventory to the flux does not represent an age as may be the case in temperate latitudes but rather a residence time

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

pore fluid and diagenetic evolution of carbonate cements in lacustrine carbonate siliciclastic rocks a case from the lower cretaceous of the erennaoer sag erlian basin ne china lower cretaceous lacustrine carbonate siliciclastic rocks including siltstones and mudstones form a unique tight oil reservoir in the erennaoer sag in the erlian basin of northeastern china that is characterized by various carbonate cements and volcanic material based on integrated analysis of the petrology mineralogy and carbon and oxygen isotope geochemistry the origin compositions and evolution of carbonate cements and pore fluids are determined five types of carbonate cements are identified from oldest to youngest penecontemporaneous micritic crystalline ferroan dolomite 1 eogenetic micro crystalline ferroan dolomite 2 sparry micro crystalline ferroan calcite 1 mesogenetic micro crystalline non ferroan dolomite 3 and calcite 2 the evolution of the pore fluids also controlled the compositions and distribution of the carbonate cements of the lacustrine mudstone during different periods this may be advantageous to predict the distribution of high quality reservoirs the geochemistry of the paleo pore fluids in these mixed lithological reservoirs are more complicated thus the origin and distribution of pore fluids and their influences on carbonate cementation fluid rock interaction and reservoir quality are complicated and certainly worth studying for hydrocarbon exploration in addition the evolution of the pore fluids of the arshan formation have experienced different salinity fluid environments under alkaline to weak acid conditions

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

10be accumulation in a soil chronosequence we have measured the concentration of the cosmogenic isotope10be in soil samples from various horizons at six sites including three independently dated rappahannock river terraces and a previously undated piedmont soil to which we have assigned an age all of the incident10be can be accounted for in one of these soils and a second is within a factor of two in three soils whose concentrations vary widely with depth a significant fraction of the incident10be cannot be accounted for incomplete sampling and enhanced be mobility caused by organic components are the probable reasons for the low inventory of be from these three soils overall the data from these six sites indicate that10be accumulation could be used to assign ages to soils if be is not mobilized and lost from the soil profile

a deep ocean kelvin helmholtz billow train 1 detailed overturning is observed between 0 5 and 50 m above the sloping side of great meteor seamount canary basin using 100 moored temperature sensors 1 mk accurate sampling at 1 hz while previously reported frontal bores of 40 m amplitude can form with vigorous near bottom motions and sediment resuspension at the beginning of the upslope phase of large e g tidal carrier waves the downslope phase presented here is more permanently turbulent away from the bottom this turbulence is inferred from high resolution temperature space time series which reveal ubiquitous finger like structures it occurs during the clear water tidal phase with low amounts of acoustic scatterers the high frequency finger like motions n n the buoyancy frequency are observed simultaneously with local mode 2 near n inertio gravity waves and overall shear s n they show large temperature variations 5 10 m vertical amplitudes and occasionally develop kelvin helmholtz billows the typical eulerian period of these firstly observed deep ocean billows amounts 50 10 s

cosmogenic nuclide derived rates of diffusive and episodic erosion in the glacially sculpted upper rhone valley swiss alps denudation rates of small tributary valleys in the upper rhone valley of the swiss central alps vary by more than an order of magnitude within a very small distance tens of kilometers morphometric data indicate two distinct erosion processes operate in these steep mountain valleys we determined the rates of these processes using cosmogenic beryllium 10 10be in pooled soil and stream sediment samples denudation in deep glacially scoured valleys is characterized by rapid non uniform processes such as debris flows and rock falls in these steep valleys denudation rates are 760 2100 mm kyr 1 in those basins which show minimal previous glacial modification denudation rates are low with 60 560 mm kyr 1 the denudation rate in each basin represents a binary mixture between the rapid non uniform processes and soil creep the soil production rate measured with cosmogenic 10be in soil samples averages at 60 mm kyr 1 mixing calculations suggest that the debris flows and rock falls are occurring at rates up to 3000 7000 mm kyr 1 these very high rates occur in the absence of baselevel lowering since the tributaries drain into the rhone trunk stream up stream of a knickzone the flux weighted spatial average of denudation rates for the upper rhone valley is 1400 mm kyr 1 which is similar to rock uplift rates determined in this area from leveling the pace and location of erosion processes are determined by the oscillation between a glacial and a non glacial state preventing the landscape from reaching equilibrium copyright 2010 john wiley sons ltd

diagenetic effects on the well yield of dolomite aquifers in slovenia diagenetic effects upon the hydraulic parameters of dolomite aquifers were examined results indicate that dolomites affected only by late burial diagenesis are economically more interesting to exploit than those formed first by early and then also affected by late diagenesis as the values of most studied hydraulic parameters transmissivity specific capacity linearized specific capacity and maximum yields are higher in the former group data were collected from the hydrogeological reports of 298 wells in dolomites from permian to cretaceous age in slovenia classified in a relational hydrogeological database and analyzed by statistical methods the studied parameters are hydraulic conductivity k transmissivity t specific capacity q q s linearized specific capacity ql q d and maximum yields qmax values of the studied parameters are distributed log normally according to most normality tests and span several orders of magnitude statistical tests show significant differences between the late and early late groups for parameters q ql and t and non significant differences for k and qmax

magnesium isotope fractionation during dolostone weathering abstract the element mg and sr isotope ratios of a weathering profile from hubei china have been measured in order to document the behavior of mg isotopes during dolostone weathering the profile is developed in the joint system and the weathering intensity increases from the least weathered dolostone towards the joint plane according to the element and isotope ratios the weathering profile can be divided into the weakly and intensely weathered zones in the weakly weathered zone mg al atomic ratios decrease from 4 99 to 1 75 with increasing weathering intensity while ca al ratios slightly decrease from 13 70 to 9 02 these suggest massive loss of mg and simultaneous conservation of ca 26mg values in the weakly weathered zone slightly decrease from 1 90 to 2 22 lighter than the corresponding protolith 1 90 these element and isotope variations are likely caused by the cooccurrence of dolomite dissolution and calcite re precipitation suggesting that the fluids co existing with the dolostone are saturated with respect to calcite while not saturated with respect to dolomite by contrast both mg al and ca al ratios of intensely weathered samples display a decreasing trend towards the joint plane ranging from 1 62 to 0 16 and 10 24 to 2 05 respectively the significant loss of mg and ca indicates the considerable dissolution of both dolomite and calcite 26mg and 87sr 86sr values increase from 2 22 to 0 41 and from 0 71128 to 0 71368 respectively these observations demonstrate that the isotope compositions of the intensely weathered residues are dominantly controlled by the increasing silicate fractions after carbonate dissolution given that silicates have considerably high 87sr 86sr and 26mg values relative to the carbonates overall our study reveals large mg isotope fractionation during dolostone weathering the direction and extent of mg isotope fractionation are mainly governed by the dissolution and re precipitation of carbonate minerals which are further controlled by the fluid s saturation with respect to carbonate minerals calcite and dolomite therefore cautions should be taken when using whole rock mg isotope data of dolostones to trace paleoclimate change and dolostone genesis primary dolomite could be partially reserved in weakly weathered dolostones and step leaching experiments may help to reveal its bearing mg isotope signals

lithofacies diagenesis and cyclicity of the lower member of the khuff formation late permian al qasim province saudi arabia abstract the lower member of the khuff formation exposed along the western margin of the arabian basin al qasim province saudi arabia comprises seven carbonate and two siliciclastic lithofacies the carbonate lithofacies include laminated lime mudstone ostracode wackestone foraminiferal wackestone intraclastic wackestone burrowed dolostone lithoclastic grainstone and caliche the clastic lithofacies are represented by claystone and quartzarenite these lithofacies are arranged vertically into shallowing upward or emergence cycles the boundaries between which were delineated by field observations peritidal as well as subtidal cycles built up the lower member of the khuff formation the peritidal cycles consist of laminated lime mudstone or subtidal claystone overlain by intertidal burrowed dolostone which is capped in most cases by supratidal lithoclastic grainstone and locally by subaerial caliche deposits these cycles were probably formed by allocyclic mechanism due to the effect of sea level changes and intrabasinal tectonics the subtidal cycles comprise laminated lime mudstone claystone foraminiferal wackestone and ostracode wackestone capped by intertidal burrowed dolostone or intraclastic wackestone these cycles may be formed due to internal factors e g carbonate production changes in water circulation salinity depth chemistry energy temperature and nutrient content diagenetic studies on the depositional cycles show that the basal part of cycle subtidal zone includes micrite cementation micritization and silicification the middle part intertidal zone was affected by early dolomitization while the upper part supratidal zone was subjected to dedolomitization aggrading neomorphism sparry calcite cementation and evaporite carbonate replacement

10be imaging of sediment accretion and subduction along the northeast japan and costa rica convergent margins sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity the seismogenic zone and the subduction recycling of volatiles and continentally derived materials to the earth s mantle identifying and quantifying sediment accretion essential for a complete mass balance across the margin can be difficult seismic images do not define the processes by which a prism was built and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy this contribution reports the first use of cosmogenic 10be depth profiles to define the origin and structural evolution of forearc sedimentary prisms biostratigraphy and 10be model ages generally are in good agreement for sediments drilled at deep sea drilling project site 434 in the japan forearc and support an origin by imbricate thrusting for the upper section forearc sediments from ocean drilling program site 1040 in costa rica lack good fossil or paleomagnetic age control above the decollement low and homogeneous 10be concentrations show that the prism sediments are older than 3 4 ma and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments low 10be in costa rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion

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

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

comparison of ellison and thorpe scales from eulerian ocean temperature observations ocean turbulence dissipation rate is estimated either by means of microstructure shear measurements or by adiabatically reordering vertical profiles of density the latter technique leads to the estimate of the thorpe scale which in turn can be used to obtain average turbulence dissipation rate by comparing the thorpe scale to the ozmidov scale in both cases the turbulence dissipation rate can be estimated using single vertical profiles from shipborne instrumentation we present here an alternative method to estimate the length scale of overturns by using the ellison length scale the ellison scale is estimated from temperature variance just beyond the internal wave band measured by moored instruments we apply the method to high resolution temperature data from two moorings deployed at different locations around the josephine seamount north eastern atlantic ocean in a region of bottom intensified turbulence the variance of the temperature time series just above the internal wave frequency band is well correlated with the thorpe scale the method is based on the time frequency decomposition of variance called maximum overlap discrete wavelet transform the results show that the ellison length scale can be a viable alternative to the thorpe scale for indirectly estimating turbulence dissipation rate from moored instruments in the ocean if time resolution is sufficiently high we suggest that fine structure contaminated temperature measurements can provide reliable information on turbulence intensity

dolomite fluorescence red green quotient a potential new thermal maturity indicator abstract geochemical and petrographic effects of burial temperature on sedimentary organic matter have long been recognized and used as various thermal maturity indicators i e pyrolysis tmax conodont and spore coloration vitrinite bitumen zooclast reflectance and fluorescent spectrometry i e max and red green quotient this study investigates variations in the fluorescence color spectra in the visible light spectrum of authigenic dolomite over a wide range of thermal maturities two sample sets of calcareous shales siltstones from eastern and western canada with sparse dolomite crystals in the rock matrix were selected to examine the relationship between thermal maturity and fluorescent properties of dolomite results from a range of thermal maturities from the oil window to the dry gas window reveal a strong positive correlation between the red green quotient q of dolomite crystals and equivalent vitrinite reflectance vroeqv values this can be optically described as the color shift in fluorescent properties of dolomite with increasing maturity this strong positive correlation between q and thermal maturity can potentially be used as an alternative thermal maturity indicator in calcareous shales with low organic content compilation of worldwide data is required to understand the relationship trend and conversion equations among dolomite q and other established thermal maturity indicators

fluid rock interactions associated with regional tectonics and basin evolution an integrated approach consisting of fracture analysis petrography carbon oxygen and strontium isotope analyses as well as fluid inclusion micro thermometry led to a better understanding of the evolution of fluid rock interactions and diagenesis of the upper permian to upper triassic carbonates of the united arab emirates the deposited carbonates were first marked by extensive early dolomitization during progressive burial the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture filling dolomite quartz and calcite cements after considerable burial during the middle cretaceous sub vertical north south oriented fractures f1 were cemented by dolomite derived from mesosaline to hypersaline fluids upon the late cretaceous maximum burial and ophiolite obduction sub vertical east west fractures f2 were cemented by dolomite dc2 and saddle dolomite ds derived from hot highly saline fluids then minor quartz cement has precipitated in fractures from hydrothermal brines fluid inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot 200 degrees c saline brines 20 to 23 nacl eq through one dimensional burial history numerical modelling the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200 degrees c tectonically driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the late cretaceous maximum burial and lasted until the oligocene miocene compressional tectonics and related uplift the circulation of such hydrothermal brines led to partial dissolution of dolomites dc2 and ds and to precipitation of hydrothermal calcite c1 in new mainly oriented north south f3 and pre existing reactivated fractures the integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history hence this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes which may subsequently affect reservoir properties

the skewness of temperature derivatives in oceanic boundary layers abstract the skewness of the derivative of temperature measured by sensors in the upper ocean boundary layer in convective conditions has been measured and is observed to have a sign opposite to that found in conditions of stable heating this is consistent with observations in the atmospheric boundary layer high resolution measurements in the benthic boundary layer on a continental rise show a variation of skewness during the m2 tidal cycle which dominates the motion with negative skewness consistent with conditions favoring convection occurring during the upslope phase of the motion the observations are compared with laboratory studies of the reflection of internal waves from a slope we suggest that information about the gross features of a turbulent oceanic boundary layer flow may be derived from relatively simple measurements

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

alteration of early formed dolomite during shallow to deep burial mississippian mission canyon formation central to southwestern montana dolomite in the mississippian mission canyon formation osagean to middle meramecian of central and southwestern montana precipitated during multiple episodes of dolomitization and or recrystallization the earliest generation of dolomite most likely precipitated from evaporated sea water in near surface diagenetic environments when the mission canyon platform intermittently aggraded to sea level this dolomite has relatively positive delta sup 18 o values minus 1 5 to 7 5 and typically is nonstoichiometric and trace element enriched a second generation of dolomite has very negative delta sup 18 o values minus 11 2 to minus 1 5 and always is stoichiometric and trace element depleted dolomite with delta sup 18 o values minus 6 8 is in the upper 100 180 m of the mission canyon formation beneath a regional post mission canyon unconformity and karst surface that extends across the entire study area dolomite depleted in sup 18 o may have 1 precipitated initially in a regional mixing zone during post mission canyon subaerial exposure and karstification 2 precipitated at elevated temperatures during later diagenesis or 3 recrystallized from an earlier generation of dolomite during later shallow to deep burial diagenesis very negative delta sup 18 o values and low trace element contents for the second generation of dolomite suggest that diagenetic solutions were dominated by meteoric water whereas delta sup 13 c values indicate that diageneticmore fluids were buffered by osagean carbonate rocks 54 refs 10 figs less

model for how microbial methane generation can preserve early porosity in dolomite and limestone reservoirs in some dolomite and limestone hydrocarbon reservoirs protection from cementation is a primary factor in porosity preservation we present a model in which methanogens methane producing microorganisms produce methane gas ch4 g that outgasses from solution in pore space creating a two phase system that reduces effective hydraulic conductivity k protecting pore space from cementation methanogens have been implicated in dolomite formation and can generate methane to fill pore space of a model near surface carbonate sediment 37 5 primary porosity with ch4 g in 180 to 4650 yr depending on nutrient levels gas generation results in occlusion of the aqueous phase from pore spaces and throats creating a two phase flow regime reducing the effective hydraulic conductivity of model marine carbonate sand by about 50 from 2 8 to 1 2 cm day in as little as 55 yr therefore reducing aqueous fluid flow necessary for cementation despite rapid burial and complete cessation of methanogenesis effective hydraulic conductivity could take more than 100 000 yr to return to its original value if methanogenesis continues with burial the effective hydraulic conductivity is reduced to zero after 250 yr dolomites can preserve more primary porosity with depth than other carbonates we propose that in addition to increased structural resistance a biogenic model exists for porosity preservation in dolomites that is linked to the activity of methanogens this model represents specific end member cases and illustrates the effect of methane buildup in relationship to the extent of reservoir diagenesis

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

paleo erosion rates and climate shifts recorded by quaternary cut and fill sequences in the pisco valley central peru fluvial cut and fill sequences have frequently been reported from various sites on earth nevertheless the information about the past erosional regime and hydrological conditions have not yet been adequately deciphered from these archives the quaternary terrace sequences in the pisco valley located at ca 13 s offer a manifestation of an orbitally driven cyclicity in terrace construction where phases of sediment accumulation have been related to the minchin 48 36 ka and tauca 26 15 ka lake level highstands on the altiplano here we present a 10be based sediment budget for the cut and fill terrace sequences in this valley to quantify the orbitally forced changes in precipitation and erosion we find that the minchin period was characterized by an erosional pulse along the pacific coast where denudation rates reached values as high as 600 80 mm ka600 80 mm ka for a relatively short time span lasting a few thousands of years this contrasts to the younger pluvial periods and the modern situation when 10be based sediment budgets register nearly zero erosion at the pacific coast we relate these contrasts to different erosional conditions between the modern and the minchin time first the sediment budget infers a precipitation pattern that matches with the modern climate ca 1000 km farther north where highly erratic and extreme el nino related precipitation results in fast erosion and flooding along the coast second the formation of a thick terrace sequence requires sufficient material on catchment hillslopes to be stripped off by erosion this was most likely the case immediately before the start of the minchin period because this erosional epoch was preceded by a 50 ka long time span with poorly erosive climate conditions allowing for sufficient regolith to build up on the hillslopes finally this study suggests a strong control of orbitally and ice sheet forced latitudinal shifts of the itcz on the erosional gradients and sediment production on the western escarpment of the peruvian andes at 13 during the minchin period

dolomitization of the late cretaceous paleocene platform carbonates golkoy ordu eastern pontides ne turkey abstract late cretaceous paleocene agillar formation in the eastern pontide zone ne turkey is composed of shallow marine platform carbonates that were buried until uplift in late paleocene time the formation which is represented by 100 m thick reefal limestones around golkoy is completely dolomitized on the basis of petrographic and geochemical features four different replacive and cement dolomite phases are present the replacive dolomites which are cut by low amplitude stylolites include 1 fine crystalline planar s dolomite rd1 and 2 coarsely crystalline planar s e dolomite rd2 two types of dolomite cements were developed in minor amounts 1 a coarse dolomite cement cd1 that mostly fills molds vugs and fractures in rd1 dolomites and 2 rim dolomite cement cd2 that commonly develops on the space facing surfaces of rd2 dolomite the replacive dolomites are nearly stoichiometric ca 49 51 mg 48 50 and have 18 o values of 3 5 to 1 6 vpdb 13 c values of 1 1 to 0 8 vpdb and 87 sr 86 sr ratios of 0 707104 to 707458 sr na mn and fe concentrations of replacive dolomites are 102 171 ppm bdl 371 ppm 244 9941 ppm and 2643 8951 ppm respectively petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from late cretaceous paleocene seawater and or seawater slightly modified by water rock interaction process and then recrystallized by meteoric waters at increasing burial depths and temperatures compared with replacement dolomites cd1 dolomites yield lower 18 o values 4 9 to 5 3 vpdb similar 13 c values 1 2 to 0 5 vpdb and higher 87 sr 86 sr ratios 0 707660 0 707609 the cd1 dolomites overlie the low amplitude stylolites and have homogenization temperatures between 94 and 112 c indicating that they were deposited from basinal brines although replacive dolomites and dolomite cements have similar na bdl 413 ppm mn 259 6990 ppm and fe 2184 6690 ppm concentrations the cements have lower sr contents 63 114 ppm like replacive dolomites cd1 dolomite is nearly stoichiometric ca 49 50 mg 47 49 but cd2 dolomite is slightly ca rich ca 51 52 mg 47 48 the cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite

strong tectonic and weak climatic control of long term chemical weathering rates the relationships among climate physical erosion and chemical weathering have remained uncertain because long term chemical weathering rates have been difficult to measure here we show that long term chemical weathering rates can be measured by combining physical erosion rates inferred from cosmogenic nuclides with dissolution losses inferred from the rock to soil enrichment of insoluble elements we used this method to measure chemical weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates chemical weathering rates correlate strongly with physical erosion rates but only weakly with climate implying that by regulating erosion rates tectonic uplift may significantly accelerate chemical weathering rates in granitic landscapes

origin of dolomite in the late cretaceous paleocene limestone turbidites eastern pontides turkey abstract the late cretaceous paleocene tonya formation of the hacimehmet region in trabzon eastern pontides turkey is composed of limestone turbidites these limestone turbidites were deposited at water depths of around 500 to 600 m and remained buried until uplifted in late paleocene time the lower part of this formation which is made up of clast supported calcirudites and pebbly calcarenites is partially to completely dolomitized two morphological types of dolomite are identified 1 fine to medium crystalline dolomite rd1 2 coarse to very coarse crystalline dolomite rd2 each one of the dolomite types is non stoichiometric ca rich ca 57 61 mg 43 39 and has similar geochemical features and uniform dull red brown luminescence these dolomites have 18 o pdb values from 2 6 to 3 3 average 3 0 13 c pdb values from 2 3 to 3 1 average 2 7 and 87 sr 86 sr ratios from 0 707719 to 0 707772 fluid inclusion data of rd2 dolomites yield homogenization temperatures t h of 48 58 c and salinities of 20 4 to 21 7 wt nacl equivalent the sr concentration in dolomites varying from below a detection limit of 20 to 360 ppm is not homogeneous within each dolomite crystal the na concentrations of dolomites are similar varying from 600 to 760 ppm average 680 ppm and are higher than those for dolomites formed in normal marine environments the fe and mn concentrations range from 2500 to 7000 ppm average 3800 ppm and from 50 to 850 ppm average 330 ppm respectively considering the petrography paragenesis trace element and stable isotope data the dolomites in the tonya formation are interpreted to have formed in the subsurface during mechanical compaction at shallow to intermediate burial depths dolomitization in the tonya formation was most probably driven by seawater that was slightly modified through rock water interaction and in part at elevated temperatures

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

a comparative study of two mississippian dolostone reservoirs in the volga ural basin russia abstract a significant proportion of the remaining oil reserves worldwide is concentrated in dolostone rock bodies the present paper focuses on tournaisian and visean limestones and geologically younger diagenetic dolomite units in two mature oil fields of the volga ural basin russia in contrast to many dolostone hosted hydrocarbon reservoirs worldwide the tournaisian and visean dolostones in the demkinskoye and the onbiyskoye oil fields plot near the low end of the porosity permeability range replacive and porosity destructive burial dolomitization has deteriorated the reservoir properties of these units dolomitization is related to the ascent of hot 170 215 c and saline 22 24 wt fluids as based on fluid inclusion data this temperature range points to an abnormally high temperature background thermal regime of the volga ural basin the onset of burial dolomitization predates oil charge as documented by the fact that oil bearing inclusions are only present in the outermost zones of porosity occluding dolomite cements isotope geochemical data dolomite petrography and fluid inclusion data reveal important similarities between hydrothermal dolostone units in the demkinskoye and the onbiyskoye oil fields based on these data and the detailed cement stratigraphy presented here it is argued that dolomite bodies in both oil fields formed as the result of one main alpine tectonic event pushing hot and saline dolomitizing brines ahead of ascending hydrocarbons

multistage hydrothermal dolomites in the middle devonian givetian carbonates from the guilin area south china pervasive dolomites occur preferentially in the stromatoporoid biostromal or reefal facies in the basal devonian givetian carbonate rocks in the guilin area south china the amount of dolomites however decreases sharply in the overlying frasnian carbonate rocks dolostones are dominated by replacement dolomites with minor dolomite cements replacement dolomites include 1 fine to medium planar e floating dolomite rhombs rd1 2 medium to coarse planar s patchy mosaic dolomites rd2 and 3 medium to very coarse nonplanar anhedral mosaic dolomites rd3 they post date early submarine cements and overlap with stylolites two types of dolomite cements were identified planar coarse euhedral dolomite cements cd1 and non planar saddle dolomite cements cd2 they post date replacement dolomites and predate late stage calcite cements that line mouldic vugs and fractures the replacementdolomiteshaved 18 ovalues from 13ae7to 9ae7 vpdb d 13 cvalues from 2ae 7t o 1 ae5 vpdb and 87 sr 86 sr ratios from 0ae7082 to 0ae7114 fluid inclusiondataofrd3dolomitesyieldhomogenizationtemperatures th of136 149 c and salinities of 7ae2 11ae2 wt nacl equivalent these data suggest that the replacive dolomitization could have occurred from slightly modified sea water and or saline basinal fluids at relatively high temperatures probably relatedtohydrothermalactivitiesduringthelatestgivetian middlefammenian and early carboniferous times compared with replacement dolomites cd2 cements yield lowerd 18 o values 14ae2to 9ae3 vpdb lowerd 13 c values 3ae0 to 0ae7 vpdb higher 87 sr 86 sr ratios 0ae7100 and higher th values 171 209 c whichcorrespondtotrappingtemperatures tr between260and300 c after pressure corrections these data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids derived from underlying siliciclastic deposits and were associated with more intense hydrothermal events during permian early triassic time when the host dolostones were deeply buried the petrographic similarities between some replacement dolomites and cd2 dolomite cements and the partial overlap in 87 sr 86 sr and d 18 o values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids however the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the indosinian orogeny from the early middle triassic as indicated by the decrease in salinities in the dolomite cements in veins 5ae1 0ae4 wt nacl equivalent calcite cements generally yield the lowest d 18 ovalues 18ae5to 14ae3 vpdb variabled 13 cvalues 11ae3to 1ae2 vpdb andhighthvalues 145 170 c andlowsalinities 0 0ae2wt naclequivalent indicating an origin of high temperature dilute fluids recharged by meteoric water in the course of basin uplift during the indosinian orogeny faults were

last glacial to post glacial 10be fluctuations in a sediment core from the academician ridge lake baikal 10be concentrations sediment composition and sediment accumulation rates were determined in a sediment core from the academician ridge of lake baikal the 10be concentrations vary from 5 07 108 atoms g in the last glacial section to 11 25 108 atoms g in the section representing the post glacial climate stage 10be fluxes on the other hand increase toward the last glacial stage coinciding with an increase of the sediment accumulation rate these facts imply that climate related sedimentation is related to both flux and concentration of the nuclide in the sediment core during the last glacial stage arid conditions in the lake s watershed caused a marked increase in flux of clay particles with low 10be content to the lake resulting in an increased flux but lowered concentration of 10be in the sediments it is suggested that the 10be record of the bottom sediments of lake baikal can be used as a proxy indicator of climate change in central asia

temperature statistics above a deep ocean sloping boundary we present a detailed analysis of temperature statistics in an oceanographic observational dataset the data are collected using a moored array of thermistors 100 text m 100 m tall and starting 5 text m 5 m above the bottom deployed during four months above the slopes of a seamount in the north eastern atlantic ocean turbulence at this location is strongly affected by the semidiurnal tidal wave mean stratification is stable in the entire dataset we compute structure functions of order up to 10 of the distributions of temperature increments strong intermittency is observed in particular during the downslope phase of the tide and farther from the solid bottom in the lower half of the mooring during the upslope phase the temperature statistics are consistent with those of a passive scalar in the upper half of the mooring the temperature statistics deviate from those of a passive scalar and evidence of turbulent convective activity is found the downslope phase is generally thought to be more shear dominated but our results suggest on the other hand that convective activity is present high order moments also show that the turbulence scaling behaviour breaks at a well defined scale of the order of the buoyancy length scale which is however dependent on the flow state tidal phase height above the bottom at larger scales wave motions are dominant we suggest that our results could provide an important reference for laboratory and numerical studies of mixing in geophysical flows

micro and nano pores in intrasalt microbialite dominated carbonate reservoirs ara group south oman salt basin abstract dolomitic microbialites form important exploration targets for the oil and gas industry in many parts of the world it is known that a significant portion of their total porosity and potential storage capacity is formed by micropores despite their economic importance few studies attempted to characterize the micro to nanoporosity in dolomitic microbialites recent advances in the field of microscopy now allow to image and analyze their pore systems on a hitherto unprecedented scale this study evaluates pore systems in neoproterozoic ara group carbonates forming part of the deeply buried intra salt a2c hydrocarbon play of the south oman salt basin sosb micro and nanoporosity there is hosted by aphanocrystalline and to a minor extent microcrystalline dolomite image analysis derived from broad ion beam milling and scanning electron microscopy bib sem of representative samples display a contribution between 50 and 100 from micro and nanopores to total porosities which are up to 17 the main diagenetic controls on microporosity in the a2c are the formation and preservation of primary or early diagenetic aphanocrystalline dolomite and the presence of evaporites calcite and solid bitumen petrographic and petrophysical observations show good preservation of the original textural and geochemical attributes in most dolomite samples even after burial to 5 km depth for the past 500 million years this suggests a structurally and hydrodynamically isolated rockbuffered system with a limited extent of cementation confocal laser scanning microscopy clsm reveals significant amounts of hydrocarbons which manifest as degenerated organic material in the carbonates they originated from precursor bacterial organic matter in microbial laminite thrombolite and peloidal source rocks and might have prevented extensive recrystallization of the aphanocrystalline dolomite matrix and further cementation of the porespace primary dolomite might still be preserved in the a2c microbialites as petrographic observations reveal similarities in crystal sizes and the texture of euhedral aphanocrystalline dolomite as observed in comparable sedimentary systems