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

rare earth element characteristics of the carboniferous huanglong formation dolomites in eastern sichuan basin southwest china implications for origins of dolomitizing and diagenetic fluids abstract marine dolostones of carboniferous huanglong formation constitute major gas reservoir rocks in eastern sichuan basin however the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored the current study attempts to investigate the ree characteristics of dolomites using seawater normalization standard and therefore discusses the origins of dolomitizing and diagenetic fluids on the basis of continuous 47 33 m long core samples from the second member of huanglong formation c 2 h 2 in eastern sichuan basin low th sc and hf concentrations 0 791 10 6 4 751 10 6 and 0 214 10 6 respectively random correlation between total ree concentration ree and fe or mn abundance and seawater like y ho ratios mean value of 45 612 indicate that the carbonate samples are valid for ree analysis based on petrographic characteristics four dolomite types are identified including micritic sized dolomite type dol 1 fine to medium sized dolomite type dol 2 medium to coarse sized dolomite type dol 3 and coarse to giant sized saddle dolomite type dol 4 dol 1 dolomites characterized by positive ce anomaly mean value of 6 398 light ree lree enrichment and heavy ree hree depletion with mean lree hree ratio of 12 657 show micritic calcite like ree patterns indicating seawater origin of their dolomitizing fluids dol 1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology dol 2 dolomites particularly subtype dol 2a barely developing vuggy porosity also show micritic calcite like ree patterns suggesting their dolomitizing fluids were seawater or seawater derived fluids this inference is confirmed by low fe and mn concentrations which range from 651 g g to 1018 g g mean value of 863 g g and 65 g g to 167 g g mean value of 105 g g respectively whereas homogenization temperatures t h mean value of 103 c indicate that dol 2 dolomites were formed under burial environment dol 3 dolomites in form of cements of dol 2 dolomites show similar ree patterns to their host minerals i e dol 2 dolomites indicating their parent source was possibly derived from dol 2 dolomites dol 3 dolomites have high fe and mn concentrations with mean values of 3346 g g ranging from 2897 g g to 3856 g g and 236 g g ranging from 178 g g to 287 g g respectively indicating the involvement of meteoric water meanwhile it confirms that the dissolution in dol 2 dolomites was caused by meteoric water leaching positive eu anomalies mean value of 1 406 in dol 4 dolomites coupled with high homogenization temperatures mean value of 314 c suggest that dol 4 dolomites precipitated from hydrothermal fluids high fe and mn concentrations mean values of 2521 g g and 193 g g respectively in dol 4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks

archaean oil migration in the witwatersrand basin of south africa abstract the late archaean witwatersrand supergroup of south africa hosts the largest known gold uranium pyrite ore deposits oil preserved in fluid inclusions in quartz grains in siliciclastic sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during the archaean and may have been involved in mineralization processes through reference to phanerozoic analogues oil bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in early syntaxial quartz overgrowths imply that the onset of oil migration coincided with early to intermediate stages of burial while intra granular porosity was still preserved multiple generations of oil migration are indicated by i oil inclusions within early diagenetic cements at different levels in the stratigraphic succession ii more than one type of oil in entrapment sites iii oil entrapment in multiple stages of the quartz paragenetic sequence oil generation and migration are considered to have occurred throughout and for some considerable time after development of the witwatersrand basin consistent with progressive burial and kerogen maturation in more than one tectonic regime oil bearing fluid inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary succession on the kaapvaal craton prior to 2 9 ga oil in the witwatersrand supergroup was most likely derived from multiple source areas with the principal source probably being shales within the lower witwatersrand supergroup the hydrocarbon migration history of the basin has important implications for understanding the textural relationship between gold bituminized oil and uraninite in the giant gold uranium pyrite ore deposits

thermal convection dolomitization induced by the emeishan large igneous province abstract the mechanism of dolomitization has long been debated due to complex sedimentary and diagenetic conditions large scale dolomite bodies were recently discovered in the permian cisuralian qixia formation 284 273 ma in the northwestern sichuan basin providing an opportunity to understand the dolomitization associated with the emeishan large igneous province elip traditionally the dolomites in the sichuan basin have been interpreted as hydrothermal dolomitization because of the hydrothermal impacts from the elip here we report new petrographic and geochemical data on the permian dolomites which we grouped into two categories matrix dolomites or md1 md3 and cement dolomites or cd that support thermal convection dolomitization the petrographic and geochemical features 13c 18o ree mn sr ratios 87sr 86sr ratios suggest that md1 were formed in penecontemporaneous seawater whereas md2 md3 and cd were formed in fluids resembling permian seawater during shallow burial stage of which both dolomitizing fluids are non hydrothermal high formation temperature inferred from fluid inclusion is consistent with the low 18o values of the md2 md3 and cd suggesting that the elip event may have provided the heat for the dolomitization in the qixia carbonate platform the formation of the widespread dolomite bodies was facilitated by the rapid convection exchange between warm pore seawater in the qixia strata of the platform margin and cold water from outside the slope therefore we demonstrate that the dolomites in the northwestern sichuan basin are mainly due to thermal convection dolomitization in the shallow burial stage this study provides important implications for the mechanism of dolomite formation linked to abnormal geothermal activity

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

the oaxaca mexico earthquake of 1931 lithospheric normal faulting in the subducted cocos plate a study of the great earthquake in oaxaca mexico on 15 january 1931 shows that it was a normal fault earthquake striking east west the lack of evidence of surface faulting near the source region1 and the focal depth of 40 km inferred from body wave modelling suggest that the earthquake occurred within the subducted slab the large size of the earthquake ms 8 0 strongly suggests that the rupture broke through the entire lithosphere of the subducted cocos plate other large normal fault earthquakes in the japanese2 aleutian3 peruvian4 and sumba5 arcs show lithospheric faulting seawards of the trench or very close to it the 1931 event however occurred farther down dip in the slab 150 km landward of the trench below the bottom of what appears to be the seismogenic plate interface we speculate here that large although infrequent normal fault earthquakes such as the 1931 oaxaca earthquake may have a role in decoupling at depth the subducted slab from the upper part of the slab and perhaps from the overriding mexican continental plate the epicentral location of the 1858 michoacan earthquake6 ms 7 5 suggests that it may represent another example of lithospheric normal faulting in the middle american arc such earthquakes for which the periods of recurrence are unknown constitute an extremely high seismic risk to population centres of mexico

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

age and duration of the emeishan flood volcanism sw china geochemistry and shrimp zircon u pb dating of silicic ignimbrites post volcanic xuanwei formation and clay tuff at the chaotian section abstract the age and duration of the emeishan basalts sw china remain poorly constrained largely due to the severe thermo tectonic overprinting of the ar ar system and failure to obtain zircon separates from erupted basalts in an attempt to solve this problem geochemical analyses and shrimp zircon u pb dating have been carried out on rare felsic ignimbrite in the uppermost of the emeishan lava succession the xuanwei formation which immediately overlies the emeishan basalts and a clay tuff at the middle late permian boundary at the chaotian section clastic rocks of the lowermost xuanwei formation group 1 in eastern emeishan large igneous province lip have a geochemical affinity to the emeishan felsic volcanic rocks whereas the overlying sediments group 2 are compositionally more akin to mafic components of the emeishan traps this is the reverse of volcanic sequence of the central emeishan lip where the felsic extrusives sit above predominant mafic rocks it is likely that the clastic rocks are water transported sediments resulting from erosion of the volcanic rocks in the center of the emeishan lip this interpretation is further supported by the general agreement between the age of the lowermost xuanwei formation 257 4 ma 260 5 ma and that of the silicic ignimbrite 263 4 ma and the clay tuff at the middle late permian boundary at chaotian 260 5 ma these ages interpreted as the termination age of the emeishan volcanism are indistinguishable within error from the middle late permian boundary age 260 4 0 4 ma and the main stage 259 262 3 ma of the emeishan volcanism inferred from dating of mafic and alkaline intrusions in the emeishan lip all these suggest that the emplacement of the emeishan volcanism took place over a very short interval moreover the temporal link and geochemical analyses suggest that the chaotian clay at the middle late permian boundary was genetically related to the emeishan silicic volcanism this together with the fact at that both the emeishan basalts and the chaotian clay rest on the maokou formation leads us to infer that the emeishan basalt was emplaced at the middle late permian boundary in this sense the emeishan volcanism can be regarded as a boundary event and its age is presumed at 260 ma although more precise dating is required both stratigraphic correlation and chronologic data presented in this paper lend supports to the notion that the emeishan volcanism was one of likely causes of the end guadalupian mass extinction

dolomitization of the silurian niur formation tabas block east central iran fluid flow and dolomite evolution abstract the silurian niur carbonates at dahaneh kalut and ozbak kuh sections tabas block eastern central iran have been pervasively dolomitized four types of dolomite consisting of replacive dolomite rd1 rd3 and saddle dolomite cements cd were discriminated by texture cathodolouminscence petrography and geochemical analyses rd1 dolomite occurred as a very fine 10 25 m planar e to planar s dolomite that replaced marine limestones rd2 dolomite is a planar s to planar e medium crystalline dolomite 100 400 m rd3 dolomite is a very coarse 400 m 2 mm non planar crystalline dolomite with sweeping extinction and cd dolomite occurred as voids and fractures filling saddle dolomite 5 10 mm with strong sweeping extinction field and petrographic observations demonstrated that rd3 and cd occur only at the dahaneh kalut section and are related to mafic volcanic rocks and fractures at the base of the niur formation the 18 o 8 45 to 6 06 vpdb and 13 c 0 01 to 0 99 vpdb values suggest that rd1 dolomite was precipitated at the early stage of dolomitization from silurian seawater whereas rd2 dolomite formed from similar fluids at higher temperatures upon burial the 18 o 13 80 to 9 47 and 13 c 1 66 to 0 76 values and fluid inclusion data t h 122 to 175 c salinity 18 68 to 24 wt nacl eq revealed that rd3 and cd were precipitated from hot saline fluids with a magmatic origin the mn fe na and sr concentrations of rd3 and cd suggest that the responsible dolomitizing fluids were more enriched with respect to these elements and had a different origin relative to the rd1 and rd2 dolomitizing fluids these results revealed that the late ordovician to early silurian passive rifting phase generated a local thermal gradient that increased heat flow and thermal convection of diagenetic fluids that led to the precipitation of rd3 and cd as pervasive hydrothermal dolomite the network of fractures and faults that formed during the rifting phase provided open conduits for hydrothermal fluids these results emphasize the importance of both tectonic setting and thermal conditions in the generation and circulation of dolomitizing fluids as well as in massive dolomitization

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

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

fluid rock interaction history during stabilization of early dolomites upper knox group lower ordovician u s appalachians abstract the lower ordovician upper knox group is characterized by shallowing upward peritidal cycles that are extensively dolomitized to form massive dolomite most 85 replacive dolomite is stratigraphically controlled composed of very fine to medium crystalline planar to nonplanar early dolomite composed of zones 1 2 and 3 dolomites remaining dolomite occurs as medium to coarsely crystalline nonplanar late replacive dolomite zones 2 and 3 late dolomites that selectively replaces limestone associated with stylolites and fractures or forms massive dolomite fronts complexly zoned zone 2 to 6 saddle dolomite cements make up less than 5 of all dolomite and primarily fill solution voids and fractures early dolomites are near stoichiometric and relative to early ordovician marin carbonates are similar to or significantly depleted in 18o and sr slightly to significantly enriched in mn and fe and have similar 87sr 86sr values relative to early dolomites late zones 2 and 3 dolomites are depleted in 18o and st enriched in mn and fe and have similar 87sr 86sr values early dolomites are interpreted to have formed from modified seawater during seaward progradation of tidal flats within each high frequency fifth and fourth order cycle duration late dolomites based on their texture and geochemistry formed as a result of recrystallization of early dolomites as direct replacement of limestone at elevated temperatures and as cement in fractures and secondary solution voids covariant trends between textures and geochemical compositions for early dolomite indicate that presentday compositions of knox early dolomite record a history of progressive diagenetic modification stabilization or recrystallization involving varying degrees of changes of textural and geochemical compositions during multiple episodes of dolomitization early dolomites wit the most extensively altered textures are more stoichiometric more depleted in 18o and sr and more enriched in mn and fe relative to texturally less altered early dolomite initial stabilization may have occurred syndepositionally in modified seawater continued stabilization of early dolomite likely occurred in fresh or mixed waters of a middle ordovician meteoric aquifer associated with knox unconformity development final stabilization occurred by deep burial brines at elevated temperatures as indicated by 1 extensive replacement and overgrowth of early dolomite by late dolomites 2 a progressive increase in crystal size and abundance of nonplanar crystal boundaries with increase in replacement by late dolomites and 3 the similarity of the geochemical compositions of texturally most extensively altered early dolomites and late dolomites

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

precise age determination of mafic and felsic intrusive rocks from the permian emeishan large igneous province sw china abstract radiogenic isotopic dating using different mainly micro beam analyses techniques of rocks from the emeishan large igneous province elip has yielded a range of ages from the capitanian to the early triassic in contrast estimates based on paleontological and paleomagnetic data suggest the elip was emplaced rapidly 2 ma and that volcanism ended by 257 ma new zircon ca tims u pb ages that have superior precision compared to ages from micro beam analyses were obtained from strategic intrusive rocks of the panxi region inner zone of the elip the huangcao 258 9 0 7 ma daheishan 259 1 0 5 ma woshui 259 6 0 5 ma and cida 258 4 0 6 ma syenitic and granitic plutons and three mafic dykes 259 2 0 4 ma 259 5 0 8 ma 257 6 0 5 ma yielded a narrow range of ages between 257 ma and 260 ma the new results are consistent with the estimates from magneto biostratigraphic data and a rapid emplacement model the new wuchiapinqian ages of the huangcao and daheishan plutons suggests that magmatism may have lasted significantly less than 10 ma as previously suggested and the precise ages of the mafic dykes refute the hypothesis that emeishan high ti basalts represent the waning stages of the elip magmatism the short duration of magmatism also implies that degassing of the host sediments mostly carbonates and thus releasing of greenhouse gasses occurred within a substantially shorter interval than previously thought which significantly changes the loading of atmosphere per time unit the latter corollary potentially has severe environmental effects that may have contributed to causing the end guadalupian mass extinction

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

hydrothermal dolomite reservoir in the precambrian dengying formation of central sichuan basin southwestern china abstract the precambrian dengying formation is maximum buried carbonate reservoir in the sichuan basin reservoir types are thought to be dominated by sedimentary facies or karst controlled precursory sedimentary fabrics have been intensively superimposed by medium to coarse grained dolomite in most areas dolomitized intervals contain planar and saddle dolomite quartz and few hydrothermal replacive minerals fluid inclusion analyses of dolomite suggests that rocks are formed at temperatures ranging from 132 6 c to 218 7 c in the presence of dense brines while the dolomite phases are demonstrated by negative 18 o and 13 c vpdb values strontium isotopes enriched in 87 sr and the fluid source could support the conclusion the dolomites of the dengying formation in central sichuan basin that formed around basement rooted wrench faults in turn mainly oriented towards the north south and east west strike slip faults are detectable lastly the grabens take the form of negative flower structures the result of an intra cratonic rift that took place during the sinian and early cambrian period through tensional faulting our primary contention is that basement fault which resulted in the magmatic or deep clastic fluids migration was key for the formation of the obvious high temperature coarse dolomite and saddle dolomite replacement subsequently hot fluids that circulated within the matrix dolomite were aided by fractures or vugs and 1 leached into the dolomite producing vugs and pores 2 precipitated saddle dolomite and 3 led to hydrofracturing fractures enlargement and further brecciation the dolomite eventually formed porous hydrocarbon reservoirs through diagenesis this model better illustrates how fluids that originated from deep basin migration along strike slip transfer faults and fractures flowed out to structures in precambrian basement rooted faults inheriting the rift in the cambrian the data involved offers a fresh perspective pertinent to deep hydrocarbon exploration of dolomite reservoirs in southwestern china

dolomitization of the capitan formation forereef facies permian west texas and new mexico seepage reflux revisited interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines this study looked at the partially dolomitized forereef facies of the capitan formation to test the extent of reflux dolomitization and evaluate the possible role of the near backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids the capitan formation forereef facies ranges from 10 to 90 dolomite most of the dolomite is fabric preserving and formed during early burial after marine cementation before and or during evaporite cementation and before stylolitization within the forereef facies dolomite follows depositional units with debris flow and grain flow deposits the most dolomitized and turbidity current deposits the least the amount of dolomite increases with stratigraphic age and decreases downslope within the reef facies dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and in contrast is completely dolomitized this dolomite distribution supports dolomitization by sinking fluids oxygen isotopic values for fabric preserving dolomite 1 8 o 0 9 1 0 n 101 support dolomitization by sea water to isotopically enriched sea water these values are closer to the near backreef dolomite 1 8 o 2 1 0 7 n 48 than the hypersaline backreef dolomite 1 8 o 3 6 0 9 n 11 therefore the fabric preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near backreef carbonate lagoon the occurrence of mesosaline brine reflux in the capitan formation has important implications for dolomitization in forereef facies and elsewhere first any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present secondly such brines may travel significant distances vertically provided permeable pathways such as fractures are present therefore the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration

permian palaeogeographic evolution of the jiangnan basin south china abstract the most striking geographic feature of the permian of south china is the jiangnan basin which intimately reflects the interplay between the yangtze and cathaysia blocks geographic and sedimentological evolution suggests that the jiangnan basin superposed on a precambrian collisonial megasuture and a folded early paleozoic deep basin was an extensional intracratonic basin it opened in the longlinian and became more passive and restricted in the latest maokouan the remarkably similar geographic patterns of the jiangnan basin in the early and late palaeozoic suggest that the yangtze and cathaysia blocks were coherent during the paleozoic

multiple phase fracture and fault controlled burial dolomitization upper devonian wabamun group alberta abstract recent data from the subsurface tangent eaglesham and normandville fields located east of the peace river arch in northern alberta indicate that these wabamun group subtidal carbonates have undergone a complex diagenetic history related to multiple stages of fracturing and faulting burial dolomitization which generated mainly matrix dolomites was responsible for the development of porous reservoir rocks which are spatially highly variable primary porosity was reduced by marine and early burial diagenesis which formed micrite syntaxial overgrowths granular blocky and coarse fracture filling calcite cements minor amounts of microdolomite and floating dolomite rhombs formed during shallow burial further porosity reduction by mechanical compaction and cementation except for minor dissolution and initial fracturing resulted in predominantly tight limestones matrix dolomitization is associated with the second most extensive phase of fracturing which greatly increased both the porosity and permeability of the tightly cemented wabamun limestones dolostones which comprise 85 matrix dolomite are the principal reservoir rock but have highly variable porosity large dissolution cavities developed locally after formation of the matrix dolomites the third phase of fracturing crosscuts the matrix dolomites in the tangent field these fractures locally form irregular cavity systems which are partly filled with finely crystalline and saddle dolomites these fine crystalline dolomites probably represent dolomite silt and sand derived from brecciation of the earlier matrix dolomite during faulting renewed faulting was responsible for rotation of breccia clasts saddle dolomite was precipitated during at least two phases on the sides and undersides of some clasts and in fractures late stage bladed anhydrites and fracture filling coarse calcites precipitated during hydrocarbon generation presumably in the late cretaceous except locally where source rocks may have been heated by ascending hot brines the timing of dolomitization and fracturing is difficult to determine early fractures most likely occurred in the late devonian to early carboniferous during early burial and mechanical compaction the second phase of fracturing probably occurred during the mississippian to pennsylvanian when the normandville and dunvegan faults underwent movement during the foundering of the peace river arch the third phase of fracturing is linked to the more rapid subsidence and burial by the cretaceous and early tertiary clastic wedge which presumably reactivated some of the earlier faults the combined stratigraphic petrophysical and geochemical data support the following interpretations 1 early dolomite rhombs and patches resulted from the movement of seawater through semi lithified sediment near the sediment water interface or during early compaction 2 matrix dolomites formed at temperatures of between 40 and 65 c and depths of about 500 to 1500 m probable fluid sources for matrix dolomites were devonian brines buried with these and underlying sediments and expelled by mechanical compaction possibly aided by seismic pumping along stage ii fracture fault conduits 3 sucrosic saddle and reworked dolomites postdate stage iii fractures and occurred during deep burial most saddle dolomites formed at temperatures around 100 c but a few formed near 200dc these dolomites formed from warm highly saline basin brines and or from hot brines that ascended along fractures and faults and other stratigraphic conduits from deeper parts of the basin and or the crystalline basement

dolomite formation in the shallow seas of the alpine triassic the alpine triassic units of switzerland northern italy and western austria offer an extensive geological archive in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context recent studies clearly demonstrate that large amounts of the alpine triassic dolomites are late diagenetic or hydrothermal nevertheless as part of multiple generations of diagenetic overprint some generations of fine crystalline ca rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments in this review three cases of alpine triassic dolomites are discussed where dolomite rocks may have formed during or soon after sedimentation the sedimentary facies indicate contrasting palaeoenvironmental conditions and hence document three different possible processes of dolomite formation i in the dolomite mountains northern italy dolomite beds of the partly isolated middle triassic anisian ladinian latemar platform are confined to the very top of shallowing upward lagoonal facies cycles ii dolomite beds of the san giorgio basin southern switzerland an intra platform basin that opened during the anisian ladinian transition are associated with organic carbon rich shales which were deposited in a deeper water environment under anoxic conditions iii in the entirely dolomitized platform facies of the dolomia principale hauptdolomit formation a very early generation of fine crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles this platform extended over thousands of square kilometres along the tethys margin during the late triassic carnian and norian and large amounts of carbonate were deposited under hypersaline sabkha like conditions representing three distinct depositional environments these three different triassic systems show features in common with several dolomitization models developed from the study of modern dolomite forming environments for example the sabkha model the evaporative lagoon lake model the organogenic model and the microbial model although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation dolomite forming processes during the triassic were apparently quite different from the modern world in terms of distribution and scale recent developments in stable isotope geochemistry and high resolution geochemical probing offer the possibility to make better reconstructions of triassic palaeoceanographic conditions and suggest a non actualistic approach to better understand dolomite formation during the triassic

coupled δ44 40ca δ88 86sr and 87sr 86sr geochemistry across the end permian mass extinction event abstract we report high resolution high precision 44 40ca 88 86sr and 87sr 86sr records spanning the permian triassic boundary ptb from the meishan and dajiang carbonate successions in south china the goal of the study was to understand the behavior of ca and sr isotopes during a time period in earth history characterized by severe biological and environmental perturbations including a major mass extinction flood basalt volcanism ocean acidification and sea level fluctuations dajiang displays negative 44 40ca and 88 86sr excursions and invariant 87sr 86sr ratios in the

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

δ18o and δ13c of permian brachiopods a record of seawater evolution and continental glaciation abstract two hundred sixty four 18 o and 13 c values of permian articulate brachiopod shells were analyzed and 81 of these values were characterized as well preserved and biostratigraphically well defined these were then utilized for construction of baseline oxygen and carbon isotope curves for the permian interval in addition 21 13 c whole rock values are reported for the wordian and capitanian the early permian asselian to artinskian times are characterized by 2 5 decrease in oxygen isotope values from 0 7 to 3 3 v pdb this is attributed to a 4 7 c increase in temperature in the southern urals concomitant with the retreat of the permo carboniferous ice sheets and return of the 18 o depleted melt water into the oceans the late permian samples from iran jolfa at kuh e ali bashi and china meishan yield 18 o values and presumably temperatures similar to those that followed the termination of the large scale glaciation in the lower permian in between the upper kungurian to capitanian samples from the delaware basin guadalupe mountains are enriched in 18 o at 1 5 to 3 we have no definitive explanation for these data but tentatively suggest that the anomaly can potentially be a result of evaporative enrichment of seawater in 18 o due to intracratonic arid setting of the basin the 18 o enriched nature of the zechstein samples 1 2 to 2 5 on the other hand is in all probability a reflection of the high evaporation rates in the zechstein sea the permian interval is characterized by a relatively constant 13 c at about 4 the exceptions are again the brachiopods from the delaware basin guadalupe mountains which show 1 6 increase in the guadalupian to values of up to 5 9 in the wordian a tentative explanation as in the case of oxygen is based on the proposition that the semi enclosed delaware basin was likely stratified with sequestration of the 13 c depleted carbon to the deeper water layers and a complementary 13 c enrichment in the upper oxygenated layer the coeval open ocean water dic may have been similar to that of the remainder of the permian interval at 4 as indicated by whole rock carbonate samples from oman sicily and iran in the latest permian the trend mimics the well known 13 c drop at the permian triassic boundary

hydrothermal dolomitization of basinal deposits controlled by a synsedimentary fault system in triassic extensional setting hungary dolomitization of relatively thick carbonate successions occurs via an effective fluid circulation mechanism since the replacement process requires a large amount of mg rich fluid interacting with the caco3 precursor in the western end of the neotethys fault controlled extensional basins developed during the late triassic spreading stage in the buda hills and danube east blocks distinct parts of silica and organic matter rich slope and basinal deposits are dolomitized petrographic geochemical and fluid inclusion data distinguished two dolomite types 1 finely to medium crystalline and 2 medium to coarsely crystalline they commonly co occur and show a gradual transition both exhibit breccia fabric under microscope dolomite texture reveals that the breccia fabric is not inherited from the precursor carbonates but was formed during the dolomitization process and under the influence of repeated seismic shocks dolomitization within the slope and basinal succession as well as within the breccia zones of the underlying basement block is interpreted as being related to fluid originated from the detachment zone and channelled along synsedimentary normal faults the proposed conceptual model of dolomitization suggests that pervasive dolomitization occurred not only within and near the fault zones permeable beds have channelled the fluid towards the basin centre where the fluid was capable of partial dolomitization the fluid inclusion data compared with vitrinite reflectance and maturation data of organic matter suggest that the ascending fluid was likely hydrothermal which cooled down via mixing with marine derived pore fluid thermal gradient is considered as a potential driving force for fluid flow

87sr 86sr record of permian seawater abstract a population of 169 permian articulate brachiopod shells was analysed for their 87 sr 86 sr ratios 51 of these characterised as well preserved and stratigraphically well defined are utilized for delineation of the permian seawater strontium isotope trend the 87 sr 86 sr curve shows values of about 0 7080 in the lowermost permian asselian followed by a gradual decline to 0 70685 in the capitanian and a minor rise in the dzhulfian and lower dorashamian to values of 0 70715 just below the permian triassic boundary the early permian decrease in the strontium isotope curve that commences in the early sakmarian is coincident with the advancing deglaciation of the gondwana and with the increased aridity in large parts of the pangaea these factors may have led to a reduced continental weathering of rb rich silicate rocks and thus to the decline in seawater 87 sr 86 sr starting with the artinskian the opening of the neotethys and the associated widespread basaltic volcanism supplied low radiogenic strontium to seawater from an enhanced hydrothermal flux in the capitanian basaltic volcanism in the entire palaeotethys ceased and this may have been the reason for a slightly more radiogenic seawater isotopic composition during the lopingian higher input of riverine sr due to expansion of humid areas may have been a contributory factor

petrography and geochemistry of fault controlled hydrothermal dolomites in the riopar area prebetic zone se spain abstract the present paper reports the first detailed petrographical and geochemical studies of hydrothermal dolomites related to mvt zn fe pb deposits in the riopar area mesozoic prebetic basin se spain constraining the nature origin and evolution of dolomitizing and ore forming fluids mapping and stratigraphic studies revealed two stratabound dolostone geobodies connected by other patchy bodies which replace carbonate units of upper jurassic to lower cretaceous ages these dolostones are associated to the w e trending san jorge fault indicating a main tectonic control for fluid flow seven different dolomite types were identified i matrix replacive planar s red i ii matrix replacive planar e red ii iii planar e sucrosic cement sud iv non planar grey saddle dolomite cement sad i pre dating zn fe pb sulfides v non planar milky to pinkish saddle dolomite cement sad ii post dating zn fe pb ores vi ore replacive planar e porphyrotopic pod and vii planar s cloudy cement ced meteoric calcite types were also recognized the different dolomite types are isotopically characterized by i depleted 18 o from 25 1 to 27 6 v smow and 13 c from 2 3 to 0 9 v pdb values compared to upper jurassic to lower cretaceous limestone signature 18 o 27 6 to 30 9 v smow 13 c 0 5 to 3 2 v pdb and ii 87 sr 86 sr ratios for the main dolomitization phases red and sud 0 70736 0 70773 close to the jurassic and cretaceous carbonate values 0 70723 0 70731 whereas more radiogenic values 0 70741 0 70830 for saddle dolomites sad related to the zn fe pb sulfide mineralization prevailed after fluid interaction with rb bearing minerals microthermometrical studies on two phase liquid and vapor fluid inclusion populations in planar and non planar dolomites and sphalerite show homogenization temperatures between 150 and 250 c these data indicate that both planar and non planar dolomite textures formed at high temperatures under hydrothermal conditions in deep burial diagenetic environments the main dolomitizing phase red i red ii and sad i shows low to moderate fluid inclusions salinity 5 14 wt eq nacl whereas the dolomitization related to ore precipitation sphalerite and sad ii spreads to higher salinity values 5 25 wt eq nacl these data may respond to a mixing between a low salinity fluid fluid a less than 5 wt eq nacl and a more saline brine fluid b more than 25 wt eq nacl at different fluid proportions

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

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

formation of coarsely crystalline hydrothermal dolomite reservoirs in the presqu ile barrier western canada sedimentary basin late stage coarsely crystalline replacement dolomite and associated saddle dolomite cement form a widespread diagenetic facies in the middle devonian presqu ile barrier that extends southwestward from pine point to the subsurface of the foothills in northeastern british columbia these dolomites create hydrocarbon reservoirs in otherwise tight limestones in the subsurface of the northwest territories and northeastern british columbia and host mississippi valley type deposits at pine point the coarsely crystalline replacement dolomite and saddle dolomite cement are interpreted to have formed during burial because they replace blocky sparry calcite cements occur continuously across the sub watt mountain unconformity postdate stylolites and earlier replacement dolomites and overlap sulfide mineralization the dgr18o values of both replacement and saddle dolomites increase eastward updip along the presqu ile barrier from 16 pmil pdb in the deeper subsurface of northeastern british columbia to 7 pmil pdb at pine point whereas the corresponding homogenization temperatures of saddle dolomite fluid inclusions decrease from 178 to 92 c the 87sr 86sr ratios of coarsely crystalline and saddle dolomites decrease eastward along the pres u ile barrier from about 0 7106 in the subsurface of northeastern british columbia to 0 7081 at pine point these geochemical trends suggest a possible basin scale migration of hot radiogenic dolomitizing fluids updip eastward along the presqu ile barrier such large scale fluid movements probably were related to western canada sedimentary basin tectonic compression and sedimentary loading which occurred at least twice during early burial between the late devonian and early carboniferous and during deep burial between the late jurassic and early tertiary

silicic ash beds bracket emeishan large igneous province to 1 m y at 260 ma this research was supported by the open fund plc201404 of state key laboratory of oil and gas reservoir geology and exploitation chengdu university of technology the national natural science foundation of china 41502109 project funded by china postdoctoral science foundation 2015 m582528 and the fundamental research funds for the central universities chengdu university of technology

permian 87sr 86sr chemostratigraphy from carbonate sequences in south china abstract a continuous strontium isotope profile with high resolution sampling is desirable to calibrate multiple global events realize temporal correlation among different continents and understand tectonic history of the permian period in this paper we analyzed 90 micritic limestone samples from south china and 7 samples from iran with the aim to gain new 87sr 86sr data and obtain a curve of sr isotopes for the permian the concentrations of mg fe mn and sr are used to discriminate possible diagenesis of the samples a comparison of those 87sr 86sr values between south china and southern urals confirms that sr isotope proxy has the potential to be used for time calibration and correlation in regions where biostratigraphic or radiometric markers are not available the 87sr 86sr curve obtained in this study shows a trend comparable with the locally weighted scatterplot smoothing lowess regression curve from the previous studies a continuous decline trend from the latest carboniferous to late guadalupian was shown and the lowest 87sr 86sr values in the late capitanian was constrained within the interval from the conodont jinogondolella shannoni j altudaensis to j xuanhanensis zones it is followed by an increasing trend to more radiogenic values in the lopingian with a minor fluctuation in the early wuchiapingian we consider that mid ocean ridge spreading and increasing continental weathering during the transition from convergence to the breakup of pangea as the dominant factors driving the changes of the seawater sr isotope ratios

pressure solution features in a shallow buried limestone abstract three distinct types of pressure solution features are found in the alpena limestone devonian michigan stylolites solution seams and fitted fabric texture the style of pressure solution is different in grainstones packstones and wackestones the difference is interpreted to be the result of preferential cementation of the grainstones well cemented grainstones typically have stylolites whereas solution seams and fitted fabric texture are more common in poorly cemented grainstones packstones and wackestones pressure solution occurs at e80 percent of the lithologic transitions this is probably due to competency contrasts between adjacent units that material dissolved at pressure solution surfaces is not locally reprecipitated is indicated by porous allochems which abut against stylolites and units of abundant intergranular pressure solution fitted fabric texture which lack cement the style of pressure solution in the alpena limestone and its relationship to cementation is also observed in the tuscarora quartz arenite silurian appalachian basin

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

spatial patterns of diagenesis during geothermal circulation in carbonate platforms geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations we used reactive transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick in the simulations porosity and permeability decrease with depth to account for sediment compaction dolomitization of a platform consisting of medium grained 0 0 5 mm sediments occurred in a broad band ranging from 2 5 km depth near the margin to 1 5 km depth near the platform center the area of dolomitization is deep enough that temperatures exceed 50 c but not so deep that low permeabilities restrict mass transport complete dolomitization in the center of this zone is estimated to require at least 60 my incorporation of permeability contrasts permeable beds and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent dolomitization created magnesium depleted calcium rich fluids in less than 10 ky and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available

formerly aragonitic limestones associated with tillites in the late proterozoic of death valley california abstract the upper proterozoic virgin spring and sour dough limestones occur within the glaciogenic kingston peak formation of the death valley region eastern california the virgin spring limestone is black thinly laminated and consists of microscopic spheroids and scattered ooids contained within coarse neomorphic spar an original aragonite mineralogy is inferred and this is supported by high contents of strontium up to 3 650 ppm thin sandstone beds occur within the limestone and their features suggest a storm origin the sour dough limestone has been metamorphosed and deformed but shows similar sedimentary structures and a high sr content both limestones are interpreted as deeper water outer shelf deposits and their sedimentation is related to a sea level rise through deglaciati n the virgin spring limestone has a typical marine 13c signature average 2 0 pdb but the 18o is very negative 15 8 pdb the latter is attributed more to the burial diagenetic calcitization of an originally aragonitic sediment than to precipitation from 18o depleted seawater

a source and wave propagation study of the copalillo mexico earthquake of 21 july 2000 mw 5 9 implications for seismic hazard in mexico city from inslab earthquakes the copalillo earthquake of 21 july 2000 m w 5 9 is the closest well located inslab event to mexico city ever to be recorded in this study we analyze local and regional broadband and accelerometric recordings to determine the source parameters of the earthquake and the attenuation of ground motion with distance and to obtain a preliminary estimate of the seismic hazard posed to the city by such events our results show that the earthquake occurred at a depth of about 50 km most probably in the subducted oceanic crust the waveform inversion discriminates between the two nodal planes the fault plane defined by the following strike 305 dip 32 and rake 80 the rupture propagated nearly unilaterally along the strike toward northwest with a small downdip component the observed source spectrum can be well explained by an 2 source model with m 0 6 0 10 25 dyne cm and a stress drop of 360 bar we find that high frequency ground motion f 3 hz which is related to a max during inslab earthquakes is not amplified at ciudad universitaria cu a hill zone site in the valley of mexico that is known to suffer amplification at low frequencies 0 1 f m w 7 0 event could give rise to an a max value of 30 40 gal the cu recordings indicate that the a max value of 30 gal could have a return period of about 40 yr about the same as from shallow dipping thrust earthquakes along the mexican subduction zone which have been regarded as posing the highest hazard for the city an inslab earthquake with an a max value of about 40 gal could cause heavy damage to small buildings at certain locations of the city we conclude that seismic hazard from inslab earthquakes to mexico city has so far been underestimated

dolomite mountains and the origin of the dolomite rock of which they mainly consist historical developments and new perspectives beginning in the late 18th century the dolomite mountains in northern italy have been the location for major sedimentological developments from the discovery of the mineral dolomite to the formulation of the coral reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region further the dolomite mountains have inspired voluminous research into the origin of dolomite questioning whether dolomite is a primary precipitate or a secondary replacement product recently with the recognition that microbes can mediate dolomite precipitation a new geomicrobiological approach combining the study of modern natural environments with bacterial culture experiments is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record this three pronged methodology applied to the study of dolomite formation holds great promise for future research into the dolomite problem and provides a new impetus to revisit the dolomite mountains in the 21st century

magnetostratigraphic constraints on two stage eruptions of the emeishan continental flood basalts temporal and spatial variations of high and low contents of ti basalts in the permian emeishan large igneous province elip of southwest china and vietnam are crucial for modeling of continental basalt formation and the nature of mantle lithosphere interaction a combined geochemical and magnetostratigraphic study was carried out at yanyuan in the inner zone of the elip geochemical results indicate that the basalts are low ti lt basalts ti y 484 2 511 4 in the luomapu section and high ti ht basalts ti y 558 2 658 9 in the gongmushan section systematic paleomagnetic sampling and stepwise thermal demagnetization demonstrate that the higher temperature component shows two distinct characteristic remanent magnetizations with northeasterly upward and sww downward directions these results indicate a simple magnetostratigraphic normal and reversed polarity pattern from the lower and upper parts of the section respectively by comparing with available magnetostratigraphic and paleomagnetic data carried out in the elip these results imply that the eruption of the emeishan basalts can be divided into early normal polarity subchron and late reversal subchron stages the early stage included lt basalts in the inner zone and most of the ht basalts in the intermediate and outer zones and the late stage contained a part of lt and ht basalts in the upper parts of the sections in the inner zone and a few in the intermediate zone of the elip correlating with available middle late permian magnetostratigraphy and the permian magnetic time scale the new magnetic polarity sequence result suggests that the elip formed over a very short time interval 1 myr

middle upper permian carbon isotope stratigraphy at chaotian south china pre extinction multiple upwelling of oxygen depleted water onto continental shelf abstract in order to examine the causal relationships between the carbon cycle in a shallow euphotic zone and the environmental changes in a relatively deep disphotic zone at the end guadalupian middle permian isotopic compositions of carbonate carbon 13ccarb of the guadalupian lopingian upper permian rocks were analyzed in the chaotian section in northern sichuan south china by analyzing exceptionally fresh drill core samples a continuous chemostratigraphic record was newly obtained the ca 65 m thick analyzed carbonate rocks at chaotian comprise three stratigraphic units i e the limestone unit of the guadalupian maokou formation the mudstone unit of the maokou formation and the lower part of the wuchiapingian lower lopingian wujiaping formation in ascending order the limestone unit of the maokou formation is characterized by almost constant 13ccarb values of ca 4 followed by an abrupt drop for 7 to 3 in the topmost part of the unit in the mudstone unit of the maokou formation the 13ccarb values are rather constant around 2 although distinct three isotopic negative excursions for 3 from ca 2 to 1 occurred in the upper part of the unit in the lower part of the wujiaping formation the 13ccarb values monotonously increase for 5 from ca 0 to 5 the present data newly demonstrated four isotopic negative excursions in the topmost part of the maokou formation in the capitanian late guadalupian at chaotian it is noteworthy that these negative excursions are in accordance with the emergence of an oxygen depleted condition on the relatively deep disphotic slope basin on the basis of litho and bio facies characteristics they suggest multiple upwelling of oxygen depleted waters with dissolved inorganic carbon of relatively low carbon isotope values along the continental margin from the deeper disphotic slope basin to the shallower euphotic shelf slightly before the end guadalupian extinction although the negative excursions at chaotian are apparently correlated with the previously proposed large negative excursion in the middle capitanian in south china the age difference according to the biostratigraphic constraints clearly exclude this interpretation the isotopic negative excursions at chaotian are unique and no similar isotopic signal in the same period has been reported elsewhere the multiple upwelling of oxygen depleted waters onto the euphotic shelf may have represented local phenomena that occurred solely around northwestern south china

formation of saddle dolomites in upper cambrian carbonates western tarim basin northwest china implications for fault related fluid flow abstract the saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in upper cambrian carbonate of western tarim basin suggesting that formation of the saddle dolomites is likely related to fault controlled fluid flow they partially fill in fractures and vugs of replacement dolomite the saddle dolomites exhibit complex internal textures commonly consisting of core and cortex in comparison with the matrix dolomites the saddle dolomites show lower sr content and 87 sr 86 sr ratios higher fe and mn content and more negative 18 o values combined with high th 100 130 c of primary fluid inclusions it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite bearing middle cambrian strata and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites the cores precipitated from the formation water mixed by deep brines at the early stage of fault activity and the cortexes precipitated from the deep fluid with higher temperatures through the middle cambrian later in summary the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity which also resulted in high porosity in upper cambrian carbonate in western tarim basin

genesis of evaporite associated platform dolomites case study of the main dolomite zechstein upper permian leba elevation northern poland dolomitization of the zechstein late permian main dolomite carbonates of northern poland was penecontemporaneous and or very early diagenetic well ordered stoichiometric dolomites are associated with the basinal facies the platform dolomitesare relatively poorly ordered and usually non stoichiometric most samples are highly enriched in 13c as in other zechstein carbonates 6 o values show large variations from 5 l to 7 4 there is an isotope zonation of the examined dolomites the isotope signature indicates that dolomites formed from variable solutions of meteoric water seawater and evaporitic brines of possible marine or continental origin once initiated dolomitization proceeded despite the evolution of dolomitizing brines this evolution explains the occurrence of lagoonal dolomites with common evidence for dissolution in the lower part of sections compared with well developed rhombohedra in the upper part crystal zoning suggests the initiation of dolomite growth in hypersaline water and progressive dilution by fresh water there is isotopic evidence for migration of continental waters into the basin presumably following sea level fall at the end of the deposition of the main dolomite influence of fresh water on syndepositional dolomitization well established in the main dolomite strongly suggests that similar relationships may be characteristic for other evaporite associated dolomites as well

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

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

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

carboniferous permian carbon isotope stratigraphy of successions from china yangtze platform usa kansas and russia moscow basin and urals abstract the pennsylvanian and early permian were times of extreme sea level changes of 100 to perhaps up to 200 m for example glacio eustatic sea level changes are well documented from the us midcontinent moscow basin and yangtze platform where more than 100 stacked subtidal to supratidal sequences have been described in contrast carbonates developed in the ural mountains and south guizhou of the yangtze platform were deposited in ramp and slope settings 13c values of whole rock samples from the us midcontinent and moscow basin were intensively affected by diagenesis only well preserved brachiopod shells maintained their original carbon isotope ratios in contrast limestones deposited in the deeper water slope environments were not affected by meteoric diagenesis and are interpreted to have mainly retained their carbon isotope ratios mean 13c values of slope carbonates of the yangtze platform increase from about 3 during the visean and serpukhovian to 5 at the carboniferous permian boundary and decrease to values around 2 in the early kungurian this positive 13c excursion coincides with low 13c values in the yangtze platform successions which were reset by meteoric diagenesis the highest 13c values recorded in the slope succession coincide with the inferred maximum glaciation that caused pronounced sea level lowstands short lived but significant negative 13c excursions in the chinese slope succession are interpreted to reflect changes in ocean circulation due to sea level rises which caused enhanced production and or preservation of organic matter which influenced the subsequent early diagenetic cementation

rupture history of september 30 1999 intraplate earthquake of oaxaca mexico mw 7 5 from inversion of strong motion data near source strong motions are inverted to estimate the rupture history of intraslab normal faulting september 30 1999 oaxaca mexico earthquake two focal mechanisms harvard and neic cmt solutions are tested for the source geometry the inversion with the ne dipping fault plane of the harvard solution best matches the data strike 295 dip 50 rake 82 we estimated the slip distribution on the fault and the associated rupture front propagation as well as the rise time the inversion results show that the rupture mainly propagated from ese to wnw and slightly downdip with an average rupture velocity of about 3 km s the rise time ranges between 1 and 2 s the slip distribution on the fault is mainly concentrated in two interconnected patches with a maximum slip of 2 5 m located about 20 km and 40 km wnw of the hypocenter most of the slip is released at an average depth of 45 km a smaller area with a maximum slip of 1 5 m is also observed close to the hypocenter the total co seismic moment released is equal to 1 8 1020 nm

fault controlled dolomitization in a rift basin there are numerous examples of fault controlled so called hydrothermal dolomite htd many of which host economic mineral deposits or hydrocarbons but there remains a lack of consensus as to how they form in particular multiple phases of diagenetic overprinting can obscure geochemical fingerprints study of a cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds the hammam faraun fault hff block within the suez rift egypt hosts both massive and stratabound dolostone bodies non fabric selective massive dolostone is limited to the damage zone of the fault while fabric selective stratabound dolostone bodies penetrate nearly 2 km into the footwall oligo miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the hff escape of fluids from the incipient hff into the lower thebes formation led to differential stratabound dolomitization once the hff breached the surface fluid circulation focused along the fault plane to form younger massive dolostone bodies this study provides a snapshot of dolomitization during the earliest phases of extension unobscured by subsequent recrystallization and geochemical modification contrary to many models stratabound dolomitization preceded non stratabound dolomitization fluids were hydrothermal but with little evidence for rapid cooling and brecciation common to many htd bodies these results suggest that many of the features used to interpret and predict the geometry of htd in the subsurface form during later phases of structural deformation perhaps overprinting less structurally complex dolomite bodies

relationship between karstification and burial dolomitization in permian platform carbonates lower khuff oman large breccia fabrics associated with karst constitute an important structure in massive limestone successions the dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids but subsequently breccia fabrics can also govern the migration of later fluids therefore breccias are highly relevant features to capture for reservoir characterisation outcrop analogues for lower khuff units in the middle east present in the central oman mountains reveal brecciated fabrics up to 10 s of meters in diameter these brecciated units are closely associated with dolomite bodies of late diagenetic origin based on an integrated set of data the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids the exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids based on the composition of the clasts and matrix in the breccias two dolomitization events are interpreted to have affected the succession one prior to early diagenetic ed dolomite and one after brecciation late diagenetic dt2 dolomite dolomite of shallow burial origin ed dolomite was only observed as clasts within breccia and is much more frequent than late diagenetic medium to deep burial dolomite clasts thus the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization late diagenetic replacive dolomite dt2 dolomite forms 90 of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization but is rarely present as clasts stable isotope measurements 18o 2 5 to 6 vpdb and 13c 2 9 to 4 8 vpdb suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids the dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities thus early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the oman mountains and potentially in other similar settings elsewhere

primary dolomite in the late triassic travenanzes formation dolomites northern italy facies control and possible bacterial influence in the late carnian late triassic a carbonate clastic depositional system including a distal alluvial plain flood basin and sabkha tidal flat and shallow carbonate lagoon was established in the dolomites northern italy the flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered a dolomite phase made of sub micrometre euhedral crystals with a mosaic microstructure of nanometre scale domains was identified in stromatolitic laminae of the flood basin embedded in clay this dolomite is interpreted here as primary and has a nearly stoichiometric composition as opposed to younger early diagenetic not primary dolomite phases which are commonly calcian this primary dolomite was shielded from later diagenetic transformation by the clay the stable isotopic composition of dolomite was analyzed along a depositional transect the 13c values range between ca 6 and 4 with the most 13c depleted values in dolomites of the distal alluvial plain and flood basin and the most 13c enriched in dolomites of the tidal flat and lagoon uniform 18o values ranging between 0 and 3 were found in all sedimentary facies it is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria a multi step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite but replacement and overgrowth were facies dependent dolomites of the landward clay rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps and partly retain an isotopic signature consistent with bacterial sulphate reduction with 13c as low as 6 in contrast dolomites of the marine clay free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties and exhibit 13c values of ca 3 major shifts of 13c values strictly follow the lateral migration of facies and thus mark transgressions and regressions

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

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

permian gondwanaland paleoenvironment inferred from carbon and oxygen isotope records of brachiopod fossils from sydney basin southeast australia abstract the 13 c and 18 o records of permian fossil brachiopods with biostratigraphical age control from southeastern australia have been constructed to investigate the high latitude paleoenvironment of gondwanaland all samples were thin sectioned and examined using petrographic and cathodoluminescence microscopy for evaluating shell preservation elemental compositions of select brachiopod shells were measured using electron probe microanalysis to further evaluate shell preservation within these intervals the average 13 c values of the best preserved non luminescent nl portion of the brachiopod shells are all greater than 5 1 with the highest value 7 0 at lower wandrawandian siltstone late kungurian these carbon isotope values are similar to those of contemporaneous tropical eastern pangea russian platform but heavier than those of western pangea north america the oxygen isotope difference between brachiopods of high southern latitudes and low latitudes fluctuated during early to middle permian the mean 18 o values of the high latitude nl brachiopod shells were more positive than those of the low latitudes in early sakmarian 0 3 early late artinskian 1 1 late kungurian 1 0 roadian 0 2 and wordian 0 7 comparable to those of tropical western pangea in early artinskian 2 1 and early kungurian 2 5 and more negative than those of tropical western pangea in late late artinskian 3 9 assuming the oxygen isotope value for permian seawater in southeast australia was 1 the calculated oxygen isotope temperatures were mostly between 12 c and 16 c reflecting southeast australia s cool high latitude temperature conditions higher apparent oxygen isotope temperatures for the upper pebbly beach formation 29 c late late artinskian berriedale limestone 21 c early artinskian and snapper point formation 22 c early kungurian may be attributed to a combination of warmer climate and lower seawater 18 o in this region

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

structurally controlled hydrothermal dolomite reservoir facies an overview structurally controlled hydrothermal dolomite htd reservoir facies and associated productive leached limestones are major hydrocarbon producers in north america and are receiving increased exploration attention globally they include multiple trends in the ordovician locally silurian and devonian of the michigan appalachian and other basins of eastern canada and the united states and in the devonian and mississippian of the western canada sedimentary basin they also occur in jurassic hosts along rifted atlantic margins in the jurassic cretaceous of the arabian gulf region and elsewhere hydrothermal dolomitization is defined as dolomitization occurring under burial conditions commonly at shallow depths by fluids typically very saline with temperature and pressure t and p higher than the ambient t and p of the host formation the latter commonly is limestone proof of a hydrothermal origin for htd reservoir facies requires integration of burial thermal history plots fluid inclusion temperature data and constraints on timing of emplacement hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary exhalative lead zinc ore bodies and htd hosted mississippi valley type sulfide deposits all three hydrothermal deposits show a strong structural control by extensional and or strike slip wrench faults with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall transtensional sags above negative flower structures on wrench faults are favored drilling sites for htd reservoir facies saddle dolomite in both replacive and void filling modes is characteristic of htd facies for many reservoirs matrix replacive dolomite and saddle dolomite appear to have formed near contemporaneously and from the same fluid and temperature conditions the original host facies exerts a major influence on the lateral extent of dolomitization resultant textures pore type and pore volume breccias zebra fabrics shear microfractures and other rock characteristics record short term shear stress and pore fluid pressure transients particularly proximal to active faults high temperature hydrothermal pulses may alter kerogen in host limestones a process designated forced maturation basement highs underlying sandstone and or carbonate aquifers probably overpressured and overlying and internal shale seals and aquitards also may constrain or influence htd emplacement although many questions and uncertainties remain particularly in terms of mg and brine source and mass balance recognition and active exploration of the htd play continues to expand increasing use of three dimensional seismic imagery and seismic anomaly mapping combined with horizontal drilling oblique to linear trends defined by structural sags helps to reduce risk