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

sea level driven changes in ocean chemistry at an upper cambrian extinction horizon a combined stratigraphic and isotopic study across a late cambrian mass extinction horizon pterocephaliid ptychaspid biomere boundary in cratonal and miogeoclinal sections of the western united states reveals evidence for a pattern of sea level rise decreasing 87 sr 86 sr ratios and increasing 13 c the positive shift in 13 c reaching maximum values a few metres above the extinction interval is interpreted to reflect elevated primary productivity and an increase in the rate of burial of organic matter during expansion of the oxygen minimum zone the 87 sr 86 sr decrease is interpreted to reflect some combination of increased sea floor spreading rates and decreased continental weathering rates the 13 c and 87 sr 86 sr trends independently support the field based hypothesis that a sea level rise led to catastrophic oceanic overturn possibly culminating in anoxic conditions that ultimately proved lethal to a large fraction of the existing shelf fauna

sedimentological characteristics and facies of the evaporite bearing kirmir formation neogene beypazari basin central anatolia turkey a thick sedimentary sequence comprising fluvial lacustrine and volcano sedimentary rocks is present in the neogene beypazari basin central anatolia these units display considerable lateral facies variation and interfinger with alkaline volcanic rocks along the north eastern margin of the basin the uppermost miocene kirmir formation contains rlumerous evaporite horizons the evaporite sequence is up to 250 m thick and may be divided into four lithofacies in ascending stratigraphical order these are 1 gypsiferous claystotie facies 2 thenardite glauberite facies 3 laminar gypsum facies and 4 crystalline gypsum facies these facies interfinger with one another laterally along a section from the margins to central parts of the basin the lithological and sedimentological features of the kirmir formation indicate fluvial saline playa mudflat hypersaline ephemeral playa lake and very shallow subaqueous playa lake depositional environments which probably were influenced by alternating semi arid and evaporative conditions

active circulation of saline ground waters in carbonate platforms evidence from the great bahama bank measurements of salinity temperature and ground water discharges within the great bahama bank provide evidence of active circulation of near normal ocean water beneath north andros island elevated salinity waters 38 42 derived by density refulx from the great bahama bank flow eastward beneath the island and mix with normal salinity cold ground waters 19 20 c from deeper than 250 m in the adjacent oceans east to west flow of water from the straits of florida may be driven by head differences generated by the florida current which impinges on the western margins of the platform alternatively density differences between the reflux and tongue of the ocean sea waters may cause more local circulation on the eastern flanks these flows have important diagenetic implications particularly in the explanation of pervasive secondary dolomitization widely reported at shallow depths in the bahamas

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

pervasive dolomitization with subsequent hydrothermal alteration in the clarke lake gas field middle devonian slave point formation british columbia canada the clarke lake gas field in british columbia canada is hosted in pervasively dolomitized middle devonian carbonates of the slave point formation the clarke lake field consists mostly of pervasive matrix dolomite and some saddle dolomite the latter varying in volume from about zero in limestones to normally 20 40 locally up to 80 in dolostones over any given 10 m 33 ft core interval some of the saddle dolomite is replacive some is cement and both varieties are associated with dissolution porosity and recrystallized matrix dolomite the major objective of this study is to identify the causes and timing of matrix and saddle dolomite formation specifically whether these dolomites are hydrothermal a comprehensive petrographic and geochemical examination indicates that pervasive matrix dolomitization was accomplished by long distance migration of halite saturated brines during the late devonian to mississippian fluid inclusion homogenization temperatures suggest about 150 uncorrected to 190c corrected at the time of matrix dolomitization these temperatures differ markedly from most published work on the dolomitized devonian reefs in the alberta basin south of the peace river arch where pervasive matrix dolomitization was accomplished by advection of slightly modified seawater at temperatures of about 60 80c and where no hydrothermal influence was ever present the saddle dolomites at clarke lake are not cogenetic with matrix dolomite and are not the product of hydrothermal dolomitization sensu stricto instead they formed through the hydrothermal alteration of matrix dolomite by way of invasion of a gypsum saturated brine during periods of extremely high heat flow and regional plate margin tectonics in the late devonian to mississippian fluid inclusion homogenization temperatures suggest that hydrothermal alteration occurred between 230 uncorrected and 267c corrected which is significantly higher than the maximum temperature of about 190c attained by the slave point formation during burial the sources of the halite and gypsum saturated brines are middle devonian evaporite depositional environments roughly 200 km 124 mi south and or east of clarke lake near the peace river arch

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

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

preservation of organic carbon in dolomitized cambrian stromatolites and implications for microbial biosignatures in diagenetically replaced carbonate rock abstract stromatolites have been a major focus in the search for ancient microbial life however the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized or correlated with their dolomitizing conditions although dolomitization rarely preserves microbial morphology the presence of organic carbon can provide valuable information for characterization of fossils biogenicity syngenicity and indigeneity to their host rock the cambrian allentown formation in new jersey usa is an excellent example of dolomitized stromatolites and thrombolites containing diagenetically modified microbial biosignatures based on xrd and epma data the dolomite composition is typically stoichiometric with varying degrees of cationic ordering the outcrop underwent early dolomitization in a marginal marine setting and later burial diagenesis resulting in multi generational dolomite formation 1 microspar dolomite formed by early diagenetic replacement at or near the surface 2 zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by infilling primary pore spaces within the microspar matrix the rhombic crystals continued to grow outward in alternating stages of fe enriched and depleted fluids which were preserved in zoned rims and revealed by cathodoluminescence and 3 saddle dolomite formed during late stage deep burial with fe and mn rich fluids and occurs as a void filling high temperature phase organic carbon characterized using confocal raman microscopy has an exclusive distribution within the microspar dolomite and the d and g bands characteristics reveal similar thermal alteration to the host rock indicating that the mapped organic carbon is indigenous and syngenetic with the cambrian carbonates the findings presented in this study reveal organic matter found within microspar of various dolomitized facies deriving from different source pools of organic carbon this study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on earth and mars

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

construction of the cambrian and ordovician seawater 87sr 86sr curve abstract the estimated path of seawater 87 sr 86 sr during the cambrian and ordovician is based on 172 analyses the sample suite is largely from locations in oklahoma new york tennessee and texas where rocks of this age are widely exposed and well studied the oldest reliable samples examined in this study are of middle cambrian age and yield an 87 sr 86 sr near 0 70888 sw 19 the estimated ratio rises to near 0 70915 sw 8 in the lower upper cambrian the highest value during the phanerozoic and probably for all geologic time the estimated path drifts lower to near 0 70903 sw 4 at the cambrian ordovician boundary this further declines to near 0 70875 sw 32 at the end of the ibexian and reaches a value near 0 70821 sw 86 at the whiterockian mohawkian boundary the decline in the seawater 87 sr 86 sr continues into later mohawkian time to flatten abruptly near 0 70782 sw 125 this ratio is essentially constant through the latest ordovician the curve rises near to a ordovician silurian boundary value poorly constrained from our data near 0 70784 sw 123 the early paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of sr continental sr dominated seawater from the beginning of the cambrian into late cambrian from that time as the cambrian transgression proceeded and radiogenic crystalline rocks were covered oceanic sr began to dominate sr from low ratio oceanic sources dominated through early mohawkian time an abrupt balance is struck between oceanic and continental sources to end the decline in the late ordovician and maintained until near the end of the ordovician the general curve shape follows the pattern suggested by proposed sealevel changes the rise and fall of seawater 87 sr 86 sr during the cambrian and ordovician characterized seawater variation during all the paleozoic

cathodoluminescene at low fe and mn concentrations a sims study of zones in natural calcites abstract point analyses of natural calcite cements by sims confirm the earlier suggestions that 20 ppm mn is required to activate luminescence whereas fe concentrations in excess of 1400 ppm quench it partly independently of the mn content below this fe threshold luminescence is bright at mn concentrations in excess of 225 ppm in the range 20 225 mn all types of luminescence dull luminescent and nonluminescent can occur this undifferentiated domain is evident also from a compilation of literature data the type of luminescence is not a simple function of fe and mn concentrations other parameters influence cl sinc most diagenetic calcite cements have fe and mn concentrations that place them within the field of mixed cathodoluminescence these results force reassessment of models that equate the type of luminescence with the redox state of the diagenetic environment

c o and sr isotope stratigraphy across the lower middle cambrian transition of the cantabrian zone spain and the montagne noire france west gondwana c o and sr isotopes have been analysed for three sections of the cantabrian zone northern spain and the southern montagne noire france that cross the lower middle cambrian transition or bilbilian leonian transition in its iberian sense the base of the leonian middle cambrian is regionally defined by the first appearance of the trilobite acadoparadoxides mureroensis which is well constrained in the cantabrian zone but absent in the montagne noire in the bilbilian parts of the three sections nearshore to peritidal strata are capped by erosive unconformities the succeeding leonian relative sea level rise is recorded in similar fashion in both environments leading to the deposition of aggradational bioclastic limestones and or transgressive limestone shale couplets the so called griotte facies overlain by offshore shales which relate to platform drowning and coastal smothering during the leonian caesaraugustan transition bilbilian leonian carbonates show a long term trend to higher 13c values that was punctuated by a succession of positive excursions in cremenes valdore cantabrian zone and ferrals les montagnes montagne noire two bilbilian 13c shifts an older uncertain peak at 0 1 0 01 and a younger certain shift at 0 6 1 7 and two leonian shifts peaking at 1 1 1 2 and 1 3 respectively are apparent at genestosa cantabrian zone a fifth early middle caesaraugustan shift is identified peaking at 2 1 these positive excursions and increasing trend are similar to published trends from the lower middle cambrian transition in siberia and australia although in those areas only two 13c excursions have been reported so far the marked negative 13c excursion reported from the lower middle cambrian boundary of laurentia and china is not recorded in our dataset oxygen isotope and trace element ca mn and sr data indicate that our 13c trends are unlikely to be diagenetic features while the lack of correlation with relative sea level fluctuations facies changes or benthic community dynamics means that they could be of potential for global stratigraphic correlation sr isotope compositions for the cantabrian zone succession exhibit consistent values upsection which suggests that seawater 87sr 86sr remained around 0 7089 0 7090 our more limited dataset for the montagne noire region confirms this value and both are consistent with published constraints from laurentia for seawater 87sr 86sr across the lower middle cambrian boundary the only other 87sr 86sr study to cover the boundary interval reported significantly lower 87sr 86sr values not vert similar0 7087 0 7088 indicating perhaps that the lower middle cambrian boundary in siberia lies stratigraphically below its equivalent levels in europe and laurentia however we consider it more likely that these siberian samples have been diagenetically altered to less radiogenic values

climate controlled early dolomite late triassic cyclic platform carbonates hungary abstract the 2 km thick late triassic hungarian carbonate platform has a completely dolomitized lower part and a limestone dominated upper part the platform succession is made up of meter scale possibly precessional 20 ky carbonate cycles in the lower platform main dolomite cycles are completely dolomitized and capped by caliche laminites and pisolites in the upper platform dachstein limestone dolomite is confined to laminite caps of cycles that are bounded by clayey paleosols most of the dolomitization in the platform interior occurred early in tidal flat settings during each high frequency cycle subtidal dolomites are slightly coarser grained are low in fe2 and mn2 and have the heaviest 18o signature this indicates they formed from evaporative oxidizing brines sourced from supratidal flats intertidal supratidal dolomites are fine grained commonly fe2 and or mn2 rich and slightly enriched in 18o compared to the marine calcite cement they formed from marine water that was weakly to moderately reducing dachstein paleosols have light 18o and 13c signatures reflecting meteoric soil waters repeated emergence stabilized the dolomites to low sr2 and na dolomite similar to cenozoic platform dolomites in contrast to these early cyclic dolomites coarse grained platform margin dolomites with very low mn2 and fe2 and light 18o signatures formed as thermally driven warm oxidizing marine water associated with jurassic rifting of the neo tethys pennini ocean circulated through the margin early dolomitization of cycles may have been controlled by high frequency sea level changes but the overall vertical distribution of early dolomite on the platform does not reflect long term eustasy rather the intense dolomitization of the lower platform reflects a semiarid hot subtropical seasonal setting and megamonsoonal climate global cooling and increased humidity toward the latest triassic and early jurassic inhibited pervasive early dolomitization leaving the upper platform largely as limestone

sedimentological studies of neogene evaporites in the northern western desert egypt abstract neogene calcium sulphate deposits at the northern western desert egypt are interpreted as shallow photic subaqueous evaporites study of the evaporite sequences at gebel el hagif and dir el biraqat areas reveals two principal structures similar to those observed in the messinian gypsum in and around the mediterranean sea the structures arranged from base to top are 1 cryptalgal laminites and stromatolitic structures built up by cyanobacterial blue green algal communities in gypsite and gypsarenite units and 2 selenite specchiolino or mirror like gypsum represented by a disordered selenite and b vertically arranged selenite cyclic increase and decrease in salinity are represented by interlamination of gypsum and cryptalgal carbonate laminae general increase of salinity is represented by the general increase in crystals size of gypsum from gypsite to gypsarenite to twinned gypsum crystals more than 5 cm in length at the top of the sequence a regressive evaporite basin model is suggested for the deposition of the studied evaporite sequence dehydration of gypsum into anhydrite through an intermediate bassanite phase and the subsequent transformation of secondary anhydrite into secondary porphyrotopic gypsum are the main diagenetic processes affecting the studied neogene evaporites

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

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

the application of radiocarbon age estimates to the dating of glacial sequences an example from the miami sublobe ohio u s a abstract despite the importance of dating glacial sequences little evidence has been available to highlight the stratigraphic issues associated with the interpretation of age analysis a compilation of radiocarbon data from the miami sublobe of the laurentide ice sheet provides a large enough data set more than 133 dates to identify at least two sources of error sample and unit variability and transportation problems analysis of different samples from the same stratigraphic level commonly differ by more than the reported one sigma error and 2 000 year differences do occur glacial entrainment and transportation emplace older organic material over younger in situ materials and this effect may also give age errors in the data set of up to 2 000 years at a minimum multiple radiocarbon ages from a sequence of units are necessary to assign a reliable age to events within a glacial sequence however representing multiple ages poses its own problems a graphical representation that combines the probability of each sample appears to be one solution

use of strontium isotopes to constrain the timing and mode of dolomitization of upper cenozoic sediments in a core from san salvador bahamas the sup 87 sup 86 sr ratios and the activity ratios of sup 234 u sup 238 u and sup 230 th sup 238 u have been measured in dolomites from a 168 m deep core taken on the island of san salvador bahamas these data suggest two periods of dolomitization the first episode dolomitized miocene age sediments during the latest miocene and the second dolomitized the pliocene portion of the core and was still active as recently as 150 ka the late timing of the second episode argues against penecontemporaneous models of dolomitization for the pliocene sediments instead dolomitization is favored either as a result of mixing zone development during the large pleistocene sea level changes or by movement of seawater through the platform

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

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

origin of lower ordovician dolomites in eastern laurentia controls on porosity and implications from geochemistry the catoche formation of the st george group in eastern laurentia western newfoundland consists of early ordovician arenigian shallow marine platform carbonates 160 m thick which were extensively dolomitized during the course of their diagenetic history the dolomites occur as both replacement and pore filling cements and are a major control on porosity distribution in the formation the origin and diagenetic history of the catoche dolomites at daniel s harbour western newfoundland were analysed in comparison to equivalent successions at port au choix pac and port au port peninsula pap to assay the reservoir potential of these dolomites in eastern laurentia petrographic examination identified at least three generations of dolomites in the catoche formation which are 1 an early replacement sub to eu hedral micritic dolomite 4 m 30 m d1 2 eu to sub hedral dolomite 70 m 1 mm often with cloudy cores and clear rims d2 and 3 subhedral to anhedral saddle dolomite cement 200 m 3 mm d3 the micritic dolomite d1 exhibits a dull cathodoluminescence cl under cathodoluminoscope whereas dolomite d2 exhibits consistent concentric cl zonation some subhedral crystals of d3 appear zoned both in plane polarized light and cathodoluminoscope otherwise d3 exhibits a dull cl stoichiometric dolomite occurs in all three generations with d2 as the dominant dolomite by abundance the low strontium 47 25 ppm content coupled with depleted 18o value of dolomitizing fluids 10 to 11 2 vsmow and near micritic grain size suggests an early precipitation of dolomite d1 at low temperatures of near surface conditions from solutions likely formed by mixing of early ordovician sea and meteoric waters in contrast microthermometric measurements of primary two phase fluid inclusions in dolomite d2 homogenization temperatures of 102 168 c with a salinity range of 19 8 25 eq wt nacl and dolomite d3 homogenization temperatures of 158 190 c with a salinity range of 20 2 22 2 eq wt nacl suggest that both dolomite generations were generated in mid to deep burial settings from high salinity low temperature 200 c hydrothermal fluids likely under suboxic conditions this is consistent with the low sr concentrations for d2 36 4 8 ppm and d3 38 7 9 ppm 18o values of dolomitizing fluids for d2 2 1 to 8 1 vsmow and d3 6 to 8 1 vsmow coupled with fe contents of d2 1684 1096 ppm and d3 1783 7 618 ppm as well as the respective mn d2 131 2 50 and d3 197 5 55 ppm concentrations ree and shale normalized reesn values of catoche carbonates indicate enrichment in rare earth element ree composition of the earliest calcite c1 relative to those of arenig seawater whereas the reesn profiles of the dolomite generations mimic that of calcite c1 the ce ce ce sn and la la la pr pr sn anomalies of the catoche dolomites are consistent with precipitation in equilibrium with source fluids in slightly oxic to suboxic conditions whereas eu eu eu cn anomalies suggest similar source fluids for d2 and d3 results of fluid inclusion gas analysis are consistent with petrographic features and geochemical compositions and support the exclusion of magmatic fluids during dolomitization visual estimates of porosity from thin sections indicate four porous 4 12 horizons vugs and intercrystalline pores are two types of porosity associated with the dolomites with the latter being the dominant type and associated mainly with dolomite d2

regional cementation from unconformity recharged aquifer and burial fluids mississippian newman limestone kentucky abstract the mississippian newman limestone in kentucky was studied to assess the effects of unconformities within and above the sequence on calcite cementation early cements are low in fe and show a nonluminescent to dull to nonluminescent zonation early nonluminescent cement mainly formed in updip areas from oxidizing meteoric waters in a regional paleoaquifer that formed during regional regression in the late mississippian during late mississippian transgression waters became reducing to form early dull cement up to 600 ppm mn up to 2 700 ppm fe but averaging 570 ppm the aquifer underwent later recharge during mississippian pennsylvanian regression to form a second nonluminescent cement this event continued into the pennsylvanian when the sequence underwent increasing burial earl cements have 13o values that become lighter downdip perhaps reflecting a downdip temperature increase due to burial 13c values are lightest in the north postulated recharge area and reflect input of organically derived co2 in waters there most calcite cementation appears to have been associated with two post newman unconformities which generated regional aquifers and reflects a shift to a wetter climate in late mississippian pennsylvanian time early cement abundance decreases away from postulated recharge areas later iron rich burial cements up to 2 200 ppm mn up to 11 000 ppm fe fill remaining pore spaces compaction induced fractures spalled cement rims and tectonic fractures lined with saddle dolomite and pyrite they formed above 60 c at depths of 1 to 3 km 13o values are compatible with derivation by pressure solution of preexisting calcites in isotopically heavy waters highly modified by burial diagenesis light 13c values reflect isotopically light organic carbon in the deltaic waters entering the subsurface or being expelled from shales or buffering of waters by light c early calcite cements the 13c calcite cements become heavier downdip by equilibration of waters with host limestone newman sediments may have been subjected to late postcementation migration of warm fluids based on secondary fluid inclusions these late stage fluids most likely were chemically complex warm brines 7 17 equiv wt naci 50 to 160 c that resided in the formation for sufficiently short times that conodont cai values were not affected

hydrothermal diagenesis of paleozoic seamount carbonate components we present geochemical results of paleozoic carbonate components brachiopods biogenic low mg calcite blmc crinoids biogenic high mg calcite bhmc and whole rock admixture of aragonite and calcite wrac from open ocean permo carboniferous seamounts of japan situated in the tropical mid panthalassic ocean the diagenetic history of these and other carbonate allochems was determined by visual optical inspection of their external morphology and luminescence by evaluation of their microstructures and by assessment of their trace element and stable isotope distributions the degree and trends of the diagenetic alteration tests were then reconciled with their concomitant sr isotope values and compared to those of the established seawater sr isotope curve the altered carbonate material from three of the four time slices support the novel concept of diagenesis in the presence of marine hydrothermal fluids leading to less radiogenic sr isotope values in addition sr isotope values of unaltered blmc from three of the four time slices do not concur with the values for the concurrent time of the established seawater 87sr 86sr curve an adjustment of their stratigraphic position is untenable and unsatisfactory in light of their biostratigraphic assignment which suggests that sr isotope diagenesis is much more complex and complicated than is espoused in the literature consequently the concept of less radiogenic sr isotope values in diagenetic marine carbonates has profound implications for the veracity of the established seawater 87sr 86sr profile and curve and it needs to be re evaluated in light of the findings

physico chemical environment of pedogenic carbonate formation in devonian vertic palaeosols central appalachians usa the morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the devonian catskill formation in central pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation the carbonate is characterized by three distinct petrographic generations pedogenic rhizoliths and nodules are the earliest precipitated generation and typically consist of dull red brown luminescent micrite clear equant calcite spar cement fills voids in the centres of rhizoliths as well as circumgranular cracks and septarian voids in nodules early spar cements are nonluminescent to dull luminescent whereas later spar cements exhibit bright yellow orange luminescence late stage pedogenic fractures are always occluded with very bright yellow orange luminescent spar cements the incorporation of progressively higher concentrations of mn up to 34 000 ppm into successively younger calcite spar cements without concomitant increases in fe suggests carbonate precipitation from an evolving meteoric water in which mn2 became increasingly mobile over time the increased mobility is possibly due to decreasing eh resulting from oxidation of organic matter after rapid soil burial on the floodplain the amount of fe2 available for incorporation into calcite was limited because most iron was immobile having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 3c relative to nodules which formed stratigraphically higher within the zone of active soil shrink and swell processes nodular carbonate precipitated in proximity to deep cracks in the soil is enriched due to increased gas exchange with isotopically heavy atmospheric c02 accordingly rhizolith compositions will most accurately estimate palaeoatmospheric levels of c02 the use of nodule compositions may result in overestimation of pco by as much as 30

fault displacement accumulation and slip rate variability within the taupo rift new zealand based on trench and 3 d ground penetrating radar data 1 in offshore regions studies based on densely spaced reflection seismic data tied to stratigraphic logs demonstrate that active faults can have variable displacement rates over relatively short distances and short time intervals here we demonstrate how high resolution 3 d ground penetrating gpr data tied to trench derived stratigraphic logs provide similar information for active faults in onshore regions to investigate recent 24 4 ka fault activity within the taupo rift of new zealand we analyze 3 d gpr data acquired over 10 fault strands within the maleme fault zone after correlating three prominent gpr reflection horizons with three faulted chronostratigraphic units observed within a trench we extrapolate the geometries of the horizons over a 150 250 m area of the fault zone and determine slip accumulation patterns and rates profiles of cumulative fault displacement measured for horizons older than 12 5 ka exhibit characteristic displacement distributions by calculating average cumulative displacements with time for five practically complete fault strands we obtain robust slip rate estimates slip rates are variable for time intervals 12 5 ka long suggesting that at least four earthquakes are required for these faults to exhibit uniform slip rates characteristic of their long term behavior

carbon oxygen and strontium isotopes in paleozoic carbonate components an evaluation of original seawater chemistry proxies abstract brachiopods trilobites cements and whole rock matrix material were evaluated for their reliability as proxies of original seawater carbon oxygen and strontium isotope chemistry in this process coeval material was evaluated from individual horizons of formations spanning the permian to cambrian unaltered shells of low mg calcite articulated brachiopods retain original seawater oxygen carbon and strontium isotope compositions as old as late ordovician assessments of older specimens are hampered by a lack of suitable material coeval brachiopod whole rock sets a definitive assessment of cement marine chemistry is difficult due to the paucity of material but despite this caveat cements hold some promise for retaining original carbon and possibly strontium isotope seawater values in contrast the potential of whole rock material as a proxy of original seawater chemistry is quite complex in some but not all instances the carbon isotopic composition of whole rock after detailed scrutiny and evaluation appears to represent an original seawater chemistry signal in a few instances their oxygen isotope compositions reflect original seawater values unlike the stable isotope compositions the strontium isotopes of all studied whole rock material permian to upper cambrian appear not to reflect original seawater chemistry values thus the potential for retaining original seawater isotope chemistry and serving as specific proxies in order of decreasing reliability are 1 unaltered low mg calcite brachiopods c o and sr isotopes 2 pristine marine cements c and sr isotopes and 3 whole rock material c isotopes carbon oxygen and strontium isotope compositions of ordovician trilobites mirror those of coeval unaltered brachiopods cambrian trilobites hold significant promise as an important proxy of original seawater isotope chemistry but further studies are needed to ascertain their full potential the survey of isotopes in some ordovician and cambrian intermediate low mg calcite trilobites demonstrates their potential as an important proxy of original seawater chemistry

origin of dolomite in the phosphatic miocene hawthorn group of florida abstract in addition to large amounts of phosphorite the miocene hawthorn group of florida contains abundant dolomite dolomite is present as disseminated silt size rhombs as friable dolosilt beds and as pore filling cement in dolostone beds and clasts the dolomite formed during early burial diagenesis both in the sulfate reduction zone overlapping and extending below sediment depths of phosphorite formation and in adjacent nonphosphatic shallow water lagoonal environments much of the dolomite is closely associated with the fibrous mg rich clay minerals palygorskite and sepiolite the percent carbonate in the hawthorn group increases from north to south the dominant carbonate mineral in north florida is dolomite whereas dolomite and calcite are both abundant in south florida the 13c values of the dolomite from 1 82 to 6 21 pdb suggest that metastable biogenic carbonate aragonite and high mg calcite and seawater were the predominant sources of carbonate however negative 13c values of dolomite from northeast florida suggest that as much as 30 40 of the carbonate was derived from degradation of organic matter degradation of organic matter enhanced dolomitization by removing sulfate ion and increasing the carbonate alkalinity of the pore waters the oxygen and strontium isotopic values along with moderate na contents indicate a marine origin evaporation of seawater or mixing o seawater and meteoric water were apparently not major factors in dolomite formation the 87sr 86sr ratios of the dolomite range from 0 708129 to 0 708820 and correspond to sr derived ages of 26 to 12 ma the similarity of associated dolomite and phosphorite sr derived ages r2 0 79 combined with geochemical and textural evidence suggest that dolomite and phosphorite formed at approximately the same time during rising or maximum sea level and were reworked during marine regressions reworking of the sediment concentrates disseminated dolomite rhombs and phosphorite grains oxidizes organic matter and exhumes buried dolostone beds to boring and encrusting organisms on the seafloor therefore the presence of dolomite along with phosphorite in reworked sequences can indicate deposition of organic rich sediments from which most of th organic matter has since been removed

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

strontium isotopic composition of devonian dolomites western canada sedimentary basin significance of sources of dolomitizing fluids abstract dolomites from the middle and upper devonian of five regions in the western canada sedimentary basin wwere studied middle devonian winnipegosis formation outcrop keg river formation reefs in the rainbow field and presqu ile barrier and the upper devonian miette buildup and the wabamun group of northern alberta data from the upper middle devonian swan hills formation in the rosevear field and nisku formation reefs were also included in these areas of western canada there are at least three dolomitizing events 1 early dolomites in winnipegosis reefs and the subsurface part of the presqu ile barrier have 87sr 86sr values near 0 7080 wabamun sabkha dolomites have 87sr 86sr values near 0 7083 and all fall on their respective parts of the seawater curve thus the sources of dolomitizing fluids were devonian seawaters 2 secondary matrix dolomites from the presqu ile barrier rainbow rosevear miette nisku and wabamun all have slightly higher 87sr 86sr values 0 7081 0 7094 than the correspnding devonian seawater dolomitizing fluids were probably modified from devonian seawater with small amounts of radiogenic 87sr being added during early compaction from adjacent or underlying clastics or older carbonates 3 late stage saddle dolomites from presqu ile rainbow rosevear miette nisku and wabamum are all highly radiogenic 0 7100 up to 0 7151 except for most pine point saddle dolomites indicating significant input of radiogenic 87sr with likely sources from the underlying cambrian and precambrian clastics and or the precambrian basement at pine point some presqu ile saddle dolomites have sr isotopic values similar to matrix dolomites suggesting a different fluid source possibly from subsurface brines that may have evolved from pressure solution of devonian strata alternatively formation waters derived from the deeper part of the basin may have been progressively diluted with low 87sr 86sr shallow formation waters as they moved updip along the presu ile barrier the 87sr 86sr values support and refine earlier conclusions based on textual evidence c and o isotopes geochemistry and fluid inclusions they considerably limit the sources of the dolomitizing fluids in open diagenetic systems however the timing of the widespread secondary matrix dolomites can only be broadly constrained to sometime during the late paleozoic the similarity of the o and sr isotopes in the replacement dolomites from widely separated locations geographically and stratigraphically tentatively suggests that these dolomites may have precipitated from similar formation waters on a basin wide scale the late stage saddle dolomites and probably the coarsely crystalline dolomites could only have formed near maximum burial conditions in the late cretaceous to early tertiary from warm hydrothermal brines that moved updip from more deeply buried portions of the basin later coarse crystalline calcites precipitated from highly saline brines and are generally more radiogenic than saddle dolomites they apparently formed from more radiogenic formation waters than have curretly been reported from the basin

a hybrid laser tracking gps location method allowing gpr acquisition in rugged terrain ground penetrating radar images of geologic structures provide information about the crucial third dimension which is inaccessible to geologic mapping in order to correlate images with geologic mapping photomosaics and behind outcrop borehole logs radar profiles must be accurately mapped moreover accurate profile elevations are required during processing to avoid introducing into the radar image false structures associated with topographic relief in recent years radar station locations and elevations have been obtained very efficiently from a self tracking laser theodolite operated at the same time the radar data are being acquired the required physical link between radar and laser systems however may make such a system unusable in areas of rugged terrain

incorporating and reporting uncertainties in fault slip rates 1 quantitative slip rate estimates are essential to understanding crustal deformation processes and assessing seismic hazard computing slip rates requires two fundamental ingredients estimates of the age of an offset landforrn or deposit and displacement along the fault of interest because both of these measures contain uncertainty slip rates are inherently uncertain methods to compute and report slip rates have not been standardized and therefore slip rate data are presented inconsistently and are frequently ambiguous in particular slip rate uncertainty is often insufficiently characterized we present a rigorous probabilistic approach to computing and reporting intermediate to long term fault slip rates additionally we have developed freely available software that provides standard age and displacement uncertainty models we demonstrate the method using recent observations from the neodani and death valley fish lake valley fault zones and we compare slip rates determined using this approach with previously published results

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

chemostratigraphy of the cerro victoria formation lower cambrian uruguay evidence for progressive climate stabilization across the precambrian cambrian boundary abstract petrographic isotopic and geochemical data are reported for the cerro victoria formation of the arroyo del soldado group uruguay it is divided into four informal lithostratigraphic units a to d which are mainly composed of an alternation of stromatolitic oolitic and clastic dolostones the genesis of dolomite as a primary or penecontemporaneous precipitate is discussed considering excellent preservation of textural details whereas 87 sr 86 sr ratios in the least altered samples are too high 0 7106 0 7109 to represent seawater values 13 c determinations are interpreted as reflecting near seawater composition the curve obtained for the cerro victoria formation is characterized by negative values at its base 3 5 pdb passing into a slightly positive plateau up section peak values of 0 64 pdb are encountered in the upper unit c the upper cerro victoria formation is characterized by a return to moderately negative values the shape of the 13 c curve is consistent with the data reported for the nemakit daldyn stage of the lower cambrian we suggest that the precambrian cambrian boundary is located at the cerros san francisco cerro victoria transition or slightly lower in the succession thalassinoides bearing dolostones of the upper cerro victoria formation might reach the tommotian or atdabanian in the arroyo del soldado group 13 c excursions parallel sea level oscillations transgressive events being characterized by positive 13 c values falling amplitudes of coupled 13 c sea level fluctuations recorded in these carbonates are in accordance with the trend observed in ediacaran lower cambrian successions worldwide covariance of 13 c and sea level might be explained by bioproductivity fluctuations affecting the carbon cycle atmospheric co 2 concentrations climate ice volume and ultimately sea level while these oscillations were of large magnitude in the late neoproterozoic they progressively faded away in the cambrian giving place to a more stable environment the cause of the amplitude decrease in biogeochemical climatic and sea level perturbations might be the decreasing rates of nutrient input at mid ocean ridges as reflected by rising 87 sr 86 sr values between 800 and 500 ma punctuated by at least three events roughly coincident with rifting episodes of rodinia

rare earth element and nd isotopic variations in regionally extensive dolomites from the burlington keokuk formation mississippian implications for ree mobility during carbonate diagenesis abstract marine carbonates of the burlington keokuk fm miss of the midcontinent region have been subject to two major episodes of regionally extensive 100 000 km2 dolomitization previous studies of the cathodoluminescent petrography sr c and o isotopic compositions and sr fe mn and ca concentrations of burlington keokuk dolostones outline a diagenetic history in which lime mud was pervasively dolomitized by a seawater dominated fluid at many localities this early dolomite generation dolomite i was recrystallized by nonmarine diagenetic fluids at shallow burial depths to produce dolomite ii dolomites i and ii have similar rare earth element ree patterns and abundances and a narrow range of initial 143nd 144nd isotope ratios of 0 511906 0 000025 1 this consistency in the nd isotope ratios and ree patterns along with the presence of anomalously low abundances of cerium in the dolomites cherts glauconite pellets fish apatite and crinoids suggests that 1 seawater derived ree dominated the system and 2 the extensive water rock interaction that produced profound textural and compositional changes during the formation of dolomite ii did not significantly alter the ree patterns or nd isotopic signatures of the dolomites the results of quantitative modeling of isotopic and trace lement exchange during water rock interaction in carbonate systems indicate that extremely large water rock ratios are required to alter the ree signatures of diagenetic carbonates such large water rock ratios can reflect either 1 large extents of fluid flow during carbonate recrystallization or 2 minimal interaction between diagenetic fluids and host rocks during carbonate cement crystallization late vug filling calcite and dolomite have low sm nd and high ce nd n ratios relative to the dolostones such distinct ree signatures probably reflect limited interaction between late diagenetic fluids and burlington keokuk host rocks penecontemporaneous clastic sediments have similar initial 143nd 144nd isotope ratios 0 511857 0 511877 and lower sm nd ratios compared to the dolomites suggesting 1 a common crustal source of ree for the clastics and carbonates and 2 ree fractionation during the formation of carbonate apatite glauconite and other authigenic phases in the marine environment

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

surficial offsets on the central and eastern garlock fault associated with prehistoric earthquakes geomorphic features offset along the central and eastern garlock fault record the amount of surface slip associated with prehistoric earthquakes along the easternmost 90 km of the fault the smallest offsets cluster around 2 3 m of left lateral slip apparently associated with the most recent earthquake on this portion of the fault larger offsets along this part of the fault especially in pilot knob valley cluster around values consistent with 2 4 m of slip in each of the past several events farther west south of el paso mountains offset geomorphic features suggest that each of the past two earthquakes on this stretch of the garlock fault was produced by about 7 m of slip whereas the third event back was produced by about 4 m of slip vertical displacements of geomorphic features range from 0 to 30 of the left lateral offsets within pilot knob valley along the southern side of the slate range vertical displacements are consistently up on the northern side whereas within the avawatz mountains both north and south side up vertical displacements are present on the basis of the geomorphic offsets the geometry of the garlock fault and the precedents set by historical strike slip earthquakes elsewhere a number of different rupture patterns are plausible these range from rupture of the entire garlock fault in a single event with a maximum magnitude of about mw 7 8 to separate rupture of the western segment and of the central and eastern segments combined with approximate magnitudes mw 1 1 and mw 7 5 respectively to separate rupture of even shorter segments producing earthquakes of magnitudes mw 6 6 to mw 7 5 in conjunction with available slip rates for the garlock fault the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600 1200 years south of el paso mountains about 200 750 years in searles valley about 200 1300 years in pilot knob valley and about 200 3000 years near leach lake and in the avawatz mountains

geochemistry of cambro ordovician arbuckle limestone oklahoma implications for diagenetic δ18o alteration and secular δ13c and variation abstract isotopic analyses of 227 limestone samples from the cambro ordovician arbuckle group oklahoma document slow secular changes in the chemistry of the limestones from late cambrian to early ordovician the 18o values of the limestones increase from 10 to 7 pdb 13c values decrease from 0 to 2 pdb and 87 sr 86 sr ratios decrease from 0 7091 to 0 7088 the light 18o values suggest that all arbuckle limestones underwent diagenetic alteration probably caused by meteoric water recharged during the development of the overlying pre middle ordovician unconformity the gradual 18o increase from late cambrian to early ordovician reflects reduced 18o depletion with decreasing burial temperature during alteration although the presence of additional primary secular 18o variation cannot be ruled out the 13c and 87 sr 86 sr variations in accord with 13c and 87 sr 86 sr variations in the literature represent primary secular variations the variations indicate that the 13c value and 87 sr 86 sr ratio of early paleozoic surface seawater decreased from late cambrian to early ordovician the 13c variation during this time period seems to correlate with sea level variation specifically during sea level fall an increase in the rate of oxidation of organic matter caused 13c depletion of inorganic bicarbonate in seawater as a result early ordovician carbonates probably deposited during the regression stage of the latest precambrian to latest early ordovician cycle became 13c depleted relative to late cambrian carbonates the decrease of seawater 87 sr 86 sr ratio from late cambrian to early ordovician may have resulted from decreased riverine sr input caused by decreased rate of continental weathering

multiple dolomitization episodes in deep water limestones of the triassic lagonegro basin southern italy from early reflux to tectonically driven fluid flow abstract massive dolomitization does not commonly occur in deep water carbonates however this regional study on outcrops and subsurface samples of triassic lagonegro basinal carbonates from the southern apennines fold and thrust belt italy shows that the evolution of the lagonegro basin was punctuated by two events of massive dolomitization these processes resulted in two distinct dolomite types displaying different petrographic and geochemical features a medium crystalline and fabric preservative dolomite in the northern area molise campagna and san fele mount pierno contrasts with a medium to coarse crystalline and fabric destructive dolomite in the southern area from pignola to the high val d agri geochemical data increase in 18o with respect to typical late triassic seawater values high sr and low mn and fe content sr isotopes consistent with late triassic seawater indicate that the dolomite in the northern area is the result of an early replacement caused by a marine and possibly oxidizing fluid during shallow burial dolomitizing fluids likely were expelled through reflux from the norian apenninic carbonate platform towards the proximal lagonegro basin this reflux was promoted by the norian peculiar paleoceanographic and climatic conditions in the tethyan region and affected only the proximal basinal facies dolomites in the southern area display a decrease in 18o compared to late triassic seawater values and show lower sr higher mn and fe contents and higher sr isotope ratios compared to the early dolomites of the northern area fluid inclusion microthermometry reveals homogenization temperatures between 70 and 120 c and salinities of 1 9 to 6 4 wt nacl eq for saddle dolomites these geochemical signatures indicate that the southern area was affected by late diagenetic burial dolomitization interpreted to be the result of large scale tectonically driven fluid flow during the second compressional phase characterizing the southern apennines fold and thrust belt emplacement the dolomite distribution across the lagonegro basin northern and southern is possibly controlled by the paleogeographic distribution of the facies proximal vs distal as well as by their proximity to thrust faults the lagonegro case study demonstrates that basinal successions may undergo multiple dolomitization episodes in a manner similar to carbonate platform successions from early shallow to deep burial diagenesis one of the relevant implications is that in areas of restricted oceanographic circulation during periods of aridity reflux dolomitization may extend beyond the platform margin into deep water carbonates having the potential to alter their reservoir quality

depositional processes and basin analysis of messinian evaporites in cyprus messinian evaporites in cyprus resulted from the interplay of mediterranean wide and eustatic sea level changes and local tectonics in an inferred above subduction zone setting distinctive tortonian early messinian pre evaporitic facies include diatomaceous marls and microbial carbonates overlain by a variety of gypsum facies and then by lagoonal lacustrine deposits and local palaeosols facies analysis and comparisons allow construction of a simple model in which evaporites formed in semi isolated small basins not far below global eustatic sea level coarsely crystalline gypsum formedin situ along the margins of small basins and within shallow water lagoons 10 m deep this comprised common banded stacked i e layered selenite swallowtail selenite botryoidal selenite and sugary bedded selenite fine grained gypsum precipitated widely and was reworked into basinal areas 70 m deep by weak traction currents and low density turbidity currents shallow water derived selenite was also reworked basinwards by high density turbidity currents and debris flows slumps indicate tectonic instability more detailed basin analysis can be achieved by study of individual sub basins in the polemi sub basin in the west a lower unit up to 60 m thick comprises basinal gypsum interbedded with gypsum turbidites and mass flow deposits with slumps this is overlain by an extensive mega rudite up to 20 m thick including up to metre sized clasts of marginal gypsum facies above the upper unit up to 70 m thick includes shallow water gypsum e g swallowtails marl and minor microbial carbonates the pissouri sub basin in the south west exposes marginal facies of the upper unit including deltaic elastics and palaeosols the maroni sub basin in the south exhibits a basinal lower gypsum unit with laterally equivalent marginal facies up to 50 m thick overlain by an extensive mega rudite up to 20 m thick finally the mesaoria subbasin in the north exposes relatively marginal gypsum facies in an unstable tectonic setting formation of the polemi pissouri and mesaoria gypsum sub basins relates to coeval extensional faulting and graben development evaporites in south cyprus maroni sub basin formed in elongate basins between former compressional lineaments created by localized early miocene thrusting in the sub basins of west south west and south cyprus large scale slumping of marginal gypsum facies took place towards depocentres to form megarudite debris flows triggered by one or several phases of extensional faulting

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

earthquake scaling fault segmentation and structural maturity abstract slip and length measurements on earthquakes suggest large stress drop variability we analyze an extended set of slip length measurements for large earthquakes m 6 to seek for the possible origin s of this apparent variability we propose that such variability arises from earthquakes breaking a variable number of major fault segments that number depends on the strength of the inter segment zones which itself depends on the structural maturity of the faults we propose new d max l parameterizations based on that idea of multiple segment ruptures in such parameterizations each broken segment roughly scales as a crack while the total multi segment rupture does not stress drop on individual segments is roughly constant only varying between 3 5 to 9 mpa the slight variation that is still observed depends on fault structural maturity more mature faults have lower stress drops than immature ones the new d max l functions that we propose reduce uncertainties with respect to available relationships they thus provide a more solid basis to estimate seismic hazard by integrating fault properties revealed by geological studies

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

temperatures and oxygen isotopic composition of phanerozoic oceans abstract the temperature of ancient oceans is an important constraint for understanding the climate history of our planet the classical oxygen isotope paleothermometry on fossil shells while very proficient when applied to the younger cenozoic portion of the geologic record is believed to yield only unreliable results for the phanerozoic deep time either because the empirically well documented secular trend to more negative 18 o values with increasing age was generated by post depositional recrystallization processes or if primary implies ecologically unpalatable hot early oceans here we present a compilation of 18 o measurements for 58 532 low mg calcite marine shells that cover almost the entire phanerozoic eon argue that the secular decline of about 6 is primary propose that it reflects the changing oxygen isotopic composition of sea water and define a new baseline trend for 18 o of paleo sea water the latter providing a new template for calculation of ambient habitat temperatures of fossil specimens the resulting pattern for fossil taxa foraminifera brachiopods belemnites and bivalves mimics their modern counterparts in temperature ranges and modes this conceptual framework enables application of actualistic concepts to ambient habitat temperatures of fossils and provides us with a long overdue tool for interpretation of deep time geologic history

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

dissolution methods for strontium isotope stratigraphy guidelines for the use of bulk carbonate and phosphorite rocks diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy sis this is especially important for precambrian and cambrian studies for which no suitable skeletal material is available one proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution we applied a similar technique to dissolve a range of lithologies limestone dolostone and phosphorite and compared the strontium isotopic composition of various leaches to contemporaneous seawater our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near primary seawater 87sr 86sr composition once contaminant strontium from secondary calcite and other phases has been removed commonly applied trace element e g mn sr mg ca and isotopic c o screening cut offs were also examined in the light of these results and proved unreliable particularly in the case of dolostones and partially dolomitized limestones we recommend that rigid sample selection and sequential leaching procedures be applied to all sis studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future

a major drop in seawater 87sr 86sr during the middle ordovician darriwilian links to volcanism and climate a large drop in seawater 87 sr 86 sr during the middle ordovician was among the most rapid in the entire phanerozoic new 87 sr 86 sr measurements from nevada indicate that the rapid shift began in the pygodus serra conodont zone of the upper darriwilian stage we use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic sr to the oceans a close balance between volcanic outgassing and co 2 consumption from weathering produced steady p co 2 levels and climate through the middle katian consistent with recent ordovician paleotemperature estimates in the late katian outgassing was reduced while volcanic weathering continued and resulted in a cooling episode leading into the well known end ordovician glaciation

construction of the mississippian pennsylvanian and permian seawater 87sr86sr curve abstract the estimated path of seawater 87 sr 86 sr during the mississippian pennsylvanian and permian is based on 228 analyses the samples are largely from the southern interior of the u s a where rocks of this age are widely exposed multiple analyses from single rock units are used to define the seawater ratio for different time periods the seawater 87 sr 86 sr declines from 0 70812 sw 95 mississippian to an estimated low in the early meramecian of near 0 70755 sw 152 the value rises to near 0 70812 sw 95 in latest mississippian the ratio rises to 0 70830 sw 77 in mid pennsylvanian and decreases to near 0 70809 sw 98 during virgilian time the earliest permian value is near 0 70793 sw 114 after rising to 0 70804 sw 103 in the early leonardian the seawater 87 sr 86 sr falls sharply to reach a low of 0 70672 sw 235 in ochoan time the end of the paleozoic is estimated to be near 0 70707 sw 200 based on samples from china analysis of the paleozoic curve shape shows an alternating dominance of oceanic and continental sr contributions the extended and precipitous decline in the 87 sr 86 sr during mid permian is unmatched during the phanerozoic the contrast of the paleozoic seawater sr curve shape to that in the mesozoic and cenozoic indicates a potentially fundamental change in the balance of oceanic and continental sr contributions at the end of the permian

chemical tracing of salinity sources in lake kinneret sea of galilee israel lake kinneret is a freshwater lake in northern israel that receives a major part of its salt input from unmonitored springs that discharge through the lake s bottom we attempt to characterize the nature of these springs by estimating their chemical composition while the springs around lake kinneret are subject to wide spatial and temporal variations in their ionic concentrations specific sodium na potassium magnesium mg strontium bromine and lithium to chlorine cl ion ratios are almost constant within individual springs and spring groups the radium cl ratio and the d 18 o cl relationship confirm the notion that the spring waters result from recent mixing between saline brines and freshwater available compositional data from past years along with new analyses of the lake and its known springs allow identification of the salinity source that causes the observed deficit in the lake s salt budget e g 91 93 chloride the relative contributions from these saline springs are different for different ions this contribution is highest for bromide 95 decreases to 84 for na and is less than 50 for mg two independent approaches have been used for balancing the salts in the lake and they are as follows 1 an annual mass balance between salt removal and supply of the different ions assuming a steady state lake and 2 simulation of the lake s evolution from 1964 the beginning of salt removal from the lake via the salinity diversion channel until the present both methods predict very similar ionic ratios for the yet unknown average saline spring s testifying to the reliability of both approaches the ionic ratios so obtained closely resemble fuliya 6 tabgha type waters excluding the tiberias and eastern shore springs as significant salt sources this inferred composition of the average unmonitored springs depends strongly on present day diversion of saline springs this diversion thus prevents their flow into the lake the different ionic ratios that identify the various spring groups reflect the respective compositions of the brine pockets that feed them our simulation also shows that the layered structure of lake kinneret enhanced the evolution rate of the lake after the implementation of the salt diversion program in 1964 lake kinneret lk lake tiberias or the sea of galilee is a lake of immense historical and religious significance it is also the only freshwater lake in israel and the main water reservoir for the national water carrier nwc the water supply system that transfers freshwater to central and southern israel about 30 of the country s water supply was pumped from lk in 1995 furthermore with the water shortage in the middle east becoming more and more acute lk is also a waterbody of great political significance the lake is monomictic it is stratified between mid march and december the mixing of a fully stratified to a fully mixed lake takes about 4 5 months though it is a flowthrough lake it is significantly saltier than the jordan river which is its major water source table 1 the composition

finding the buried record of past earthquakes with gpr based palaeoseismology a case study on the hope fault new zealand in places where sedimentation and erosion compete at fast rates part of the record of past earthquakes on faults may be buried hence hidden in the first few metres below the surface we developed a novel form of palaeoseismology of geophysical type based on the use of a dense pseudo 3 d ground penetrating radar gpr survey to investigate such possible buried earthquake traces on a long fast slipping strike slip fault hope fault new zealand at a site terako where marked alluvial conditions prevail we first used lidar data to analyse the ground surface morphology of the 2 km2 site at the greatest resolution nineteen morphological markers were observed mainly alluvial terrace risers and small stream channels that are all dextrally offset by the fault by amounts ranging between 3 and 200 m the measurements document about 10 past earthquake slip events with a mean coseismic slip of 3 3 1 m with the most recent earthquake event having a slip of 3 0 5 m we then investigated a detailed area of the site 400 x 600 m2 with pseudo 3 d gpr we measured 56 similar to 400 m long 510 m spaced gpr profiles 250 mhz parallel to the fault and evenly distributed on either side the analysis revealed the existence of a palaeosurface buried at about 3 m depth corresponding to the top of alluvial terraces of different ages that buried surface is incised by a dense network of stream channels that are all dextrally offset by the fault we measured 48 lateral offsets in the buried channel network more than twice than at the surface these offsets range between 6 and 108 m as observed at the surface yet provide a more continuous record of the fault slip the similarity of the successive slip increments suggests a slip per event averaging 4 4 1 m fairly similar to that estimated from surface data from the total surface and buried 67 offset collection we infer that a minimum of 30 large earthquakes have broken the hope fault at the terako site in the last about 67 kyr with an average coseismic slip of 3 2 1 m a minimum average recurrence time of about 200 yr and a magnitude of at least mw 7 07 4 our study therefore confirms that part of the record of past earthquakes may indeed reside in the first few metres below the surface where it may be explored with geophysical gpr based palaeoseismology developing such a new palaeoseismological tool should provide rich information that may complement surface observations and help to document the past earthquakes on faults

salinity reflux and dolomitization of southern australian slope sediments the importance of low carbonate saturation levels anomalously saline waters in ocean drilling program holes 1127 1129 1130 1131 and 1132 which penetrate southern australian slope sediments and isotopic analyses of large benthic foraminifera from southern australian continental shelf sediments indicate that pleistocene holocene meso haline salinity reflux is occurring along the southern australian margin ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50 salinity a well flushed zone at the top of all holes contains pore waters with normal marine trace element contents alkalinities and ph values dolomite precipitation occurs directly below the well flushed zone in two phases phase 1 is a nucleation stage associated with waters of relatively low ph ca 7 caused by oxidation of h2s diffusing upward from below this dolomite precipitates in sediments 80 m below the sea floor phase 2 dolomite grows on dolomite cores precipitated during phase 1 phase 2 dolomite has d 13 c values similar to those of the surrounding bulk carbonate and high sr values relative to phase 1 dolomite consistent with having formed in waters affected by aragonite and calcite dissolution the nucleation stage in this model phase 1 challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water

depositional and diagenetic models of some miocene evaporites on the red sea coast egypt abstract the miocene evaporites on the red sea coast egypt are interpreted as a sabkha chenier complex which includes deposition in shallow water as well as in supratidal environments costal sabkhas and shallow subaqueous evaporite sequences are recognised the subaqueous sequence is represented by nodular mosaic and massive crystalline gypsum covered by a surface layer of nodular white dense powdery anhydrite the sabkha sequences are represented by a small mounds overlying the older rock unit gebel el rusas formation composed of fenestral stromatolitic and nodular dolomites overlain by a dolomitic limestone layer b thin layer overlying the subaqueous evaporite sequence and composed of fenestral stromatolitic dolomite overlain by a nodular anhydrite layer the nodular structure in the subaqueous as well as the sabkha sequences is formed diagenetically by transformation and recrystallization process the dolomites in both sabkha sequences a and b are characterised by low na concentrations 143 to 755 ppm and low values of 18 o 1 85 to 8 7 pdb this indicates that the dolomites were formed from saline water mixed with fresh water these dolomites are also characterised by a low 13 c 12 95 to 27 1 pdb this shows that the light carbon produced by the degradation of organic matter through the sulphate reduction process has played a significant role in the formation of these dolomites both microcrystalline and megacrystalline white dolomites are recognised in sabkha sequences relatively high concentrations of lead zinc and copper are associated with the high percent of megacrystalline white dolomite

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

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

application of airborne lidar to mapping seismogenic faults in forested mountainous terrain southeastern alps slovenia results are presented of the first airborne lidar survey ever flown in europe for the purpose of mapping the surface expression of earthquake prone faults detailed topographic images derived from lidar data of the idrija and ravne strike slip faults in nw slovenia reveal geomorphological and structural features that shed light on the overall architecture and kinematic history of both fault systems the 1998 mw 5 6 and 2004 mw 5 2 ravne fault earthquakes and the historically devastating 1511 m 6 8 idrija earthquake indicate that both systems pose a serious seismic hazard in the region because both fault systems occur within forested terrain a tree removal algorithm was applied to the data the resulting images reveal surface scarps and tectonic landforms in unprecedented detail importantly two sites were discovered to be potentially suitable for fault trenching and palaeo seismological analysis this study highlights the potential contribution of lidar surveying in both low relief valley terrain and high relief mountainous terrain to a regional seismic hazard assessment programme geoscientists working in other tectonically active regions of the world where earthquake prone faults are obscured by forest cover would also benefit from lidar maps that have been processed to remove the canopy return and reveal the forest floor topography

syndepositional deformation of the permian capitan reef carbonate platform guadalupe mountains new mexico usa abstract permian shelf strata equivalent to the capitan reef are cut by at least 13 closely spaced syndepositional dip slip faults in slaughter canyon guadalupe mountains new mexico usa the control of five of these faults on platform development architecture stratigraphy and diagenesis is revealed by an integrated stratigraphic sedimentological and structural framework within a 500 m wide and 200 m high outcrop window here faulting and fault related deformation acted as a primary control on changes in thickness facies and stratal geometry and resulted in the local steepening shallowing and even the reversal of dip in shelf strata the role of primary depositional relief in controlling changes in thickness and stratal geometry was of secondary importance the relationship between geopetal fabrics average dip 12 towards 141 and bedding shows that during deposition of the yates formation fault growth was concurrent with down to the basin tilting and rotation of at least 6 8 this tilting appears to have been the main control on the down dip expansion of shelf strata towards the basin a further 4 6 of basinward tilt occurred after fault growth during deposition of the tansill formation and later the syndepositional faults reported here have a maximum displacement of 24 m most tip out below asymmetric growth folds and have high displacement distance gradients typical of growth faults and faults cutting unlithified strata the average rate of fault displacement 0 021 m ka and the maximum rates of fault propagation 0 088 0 123 m ka were normally less than the platform accumulation rates 0 053 0 336 m ka thus the faults were normally blind and rarely broke the platform top so that slumps and fault scarp degradation breccias are rare the fault zones were substantially modified by diagenesis during platform development they are up to 9 m wide taper both up and downward have irregular margins and complex fills mainly of sedimentary origin their margins and fill were subject to extensive modification by karstic mixing zone dissolution gravitational collapse and dolomitising fluids consequently tectonic fabrics and kinematic indicators are rare preserved tectonic fabrics consistently indicate a normal and reverse dip slip sense of movement previously these faults were mistaken for neptunian dykes and fissures so that the seven rivers and yates 1 2 hfs shelf stratigraphy has been miscorrelated across them it is apparent that the stratal relationships exposed in the guadalupe mountains do not simply preserve the original depositional morphology of the capitan equivalent shelf the subsidence history stratigraphy and development of the platform succession is more complex than previously thought the study has important implications for many aspects of the capitan system including i shelf reef correlations ii the controls on platform architecture and development iii capitan reef palaeobathymetry iv diagenesis and v the amplitude of sea level changes affecting the platform s stratigraphic development

oligocene miocene strontium isotopes stratigraphic revisions and correlations to an inferred glacioeustatic record this study tests and improves on previously published early and middle miocene 87sr 86sr marine correlations presents sr isotopic age correlations for this interval using the new timescale of cande and kent 1992 and evaluates sr isotopic changes against an inferred glacioeustatic proxy we generated a latest oligocene to early late miocene 87sr 86sr isotope record from ocean drilling program odp hole 747a this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates very good foraminiferal preservation and excellent core recovery comparisons of new 87sr 86sr data from hole 747a with previously published data from deep sea drilling project dsdp sites 608 miller et al 1991a and 588 hodell et al 1991 yield the following results 1 confirmation and refinement of the early miocene sr isotope changes 2 improved definition of the timing of the changes in slope of 87sr 86sr near 15 4 ma and 22 8 ma 3 improved sr isotopic age resolution for the middle miocene with resolution as good as 0 7 m y and 4 identification of an inflection in the sr isotope record at 28 0 ma based on the combined records from dsdp site 522 miller et al 1988 and odp hole 747a we have been unable to determine the cause of middle miocene offset between site 588 and hole 747a data although we believe it may be attributed to problems in the age assignments for hole 588a for the interval 14 11 ma and site 747 for the interval 11 8 ma because hole 747a results provide a better chronology than site 588 for most of the miocene and a better middle miocene sr isotope record than site 608 we propose that hole 747a serves as the best reference section for miocene 87sr 86sr variations from ca 23 to 11 ma using 87sr 86sr data from sites 522 608 and 747a we relate late eocene to early miocene inflections in the 87sr 86sr isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events the decreases deglaciations observed in the 18o record apparently lead the 87sr 86sr inflections by 1 to 1 5 m y

strontium isotope 87sr 86sr stratigraphy of ordovician bulk carbonate implications for preservation of primary seawater values the present study on bulk carbonate 87 sr 86 sr stratigraphy represents a companion work to earlier research that presented a conodont apatite based ordovician seawater 87 sr 86 sr curve for the tremadocian katian stages 485 445 ma here we directly compare the curve based on conodont apatite including some new data not published in earlier work with a new curve based on 87 sr 86 sr results from bulk carbonate from the tremadocian sandbian stages we sampled eight lower to upper ordovician carbonate successions in north america to assess the reliability of bulk carbonate to preserve seawater 87 sr 86 sr and its utility for 87 sr 86 sr chemostratigraphy a high resolution 87 sr 86 sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 sr 86 sr 87 sr 86 sr seawater in total 230 bulk carbonate samples that are paired to conodont samples were measured for 87 sr 86 sr in order to determine the conditions under which 87 sr 86 sr seawater is preserved in bulk carbonate results indicate that well preserved bulk carbonate can faithfully record the 87 sr 86 sr seawater trend but that its 87 sr 86 sr values are commonly more variable than those of conodont apatite on average bulk carbonate samples of the same age vary by 10 20 10 5 compared to 5 10 10 5 for conodont apatite the amount of isotopic alteration of bulk carbonate from seawater 87 sr 86 sr 87 sr 86 sr was determined by taking the difference between 87 sr 86 sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 sr 86 sr values cross plots comparing 87 sr 86 sr values to bulk carbonate sr concentration sr and conodont color alteration indices cai an estimate of the thermal history of a rock body indicate that bulk carbonate is most likely to preserve 87 sr 86 sr seawater minimally altered when either 1 bulk carbonate sr is greater than 300 ppm or 2 carbonate rocks experienced minimal thermal alteration with burial temperatures less than 150 c carbonates with intermediate sr e g between 130 and 300 ppm can also yield 87 sr 86 sr seawater values but results are less predictable and local diagenetic conditions may play a greater role modeling results support the argument that seawater 87 sr 86 sr can be preserved in bulk carbonates with low sr if pore water rock ratios are low 87 sr 86 sr is similar to the seawater 87 sr 86 sr value preserved in limestone bulk carbonate samples that meet these criteria can be useful for high resolution measurements of 87 sr 86 sr seawater with a sample variation on par with fossil materials 5 particularly for successions where well preserved fossil material i e conodonts or brachiopods is not available such as precambrian strata sequences recording mass extinction events or otherwise fossil barren facies these criteria and model predictions based on bulk carbonate sr must be considered in the context of whether a limestone accumulated under calcite seas e g ordovician with relatively high seawater sr ca or aragonite seas in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non seawater sr

pervasive subsurface dolomitization of the nisku formation in central alberta abstract the nisku formation in the subsurface of central alberta canada contains pervasive dolomites that occur mainly as a replacement of reefal limestones the degree of pervasive dolomitization increases downdip along a structural homocline from about 25 to 100 vol and associated replacive anhydrites increase from about 5 to 20 vol pervasive dolomites occur as two types one type consists of almost non porous and impermeable mosaics of cloudy crystals that have diameters of 5 300 microns the other type has significant intercrystalline porosity and permeability and consists of cloudy and clear crystals ranging in size from 100 500 microns cathodoluminescence zonation is rare but modulated structures observed with tem are common both types of dolomite have strong xrd ordering refl ctions and are nearly stoichiometric sr concentrations 25 125 ppm increase and mn concentrations 20 400 ppm decrease upwards in several cores the boron contents of both dolomite types 20 50 ppm are enriched relative to other diagenetic phases 18 ppm the sr isotope ratios vary from 0 7089 0 7107 and are enriched relative to devonian and younger seawater oxygen and carbon isotope ratios cluster around 5 8 permil pdb and 2 0 permil pdb respectively and are similar to those of remnant calcites from the combination of stratigraphic petrographic petrophysical and geochemical data it is inferred that both types of matrix dolomite formed contemporaneously from the same fluids in a flow regime with predominantly upward fluid movement at temperatures between about 40 50 c and at burial depths of about 300 1 000 m most anhydrites formed originally as gypsum during the advanced stages and after matrix dolomitization and later recrystallized to the anhydrites now present based on all accumulated data and on mass balance calculations at least two types of subsurface flow could have caused pervasive dolomitization and gypsification anhydritization 1 expulsion of burial compaction water i e chemically modified seawater with or without additional contributions of m bearing formation waters from deeper parts of the basin e g middle devonian brines and 2 thermal convection of formation fluids originally underlying and or overlying the nisku

isotopic evidence for the early meteoric diagenesis of the reef facies permian reef complex of west texas and new mexico abstract multiple isotopic analyses of former aragonitic marine cements from a single sample of permian reef complex reef facies define a linear covariant trend with a positive slope each individual analysis represents a different mixture of two isotopically distinct phases which are too finely intergrown for separate analysis in order to assess the systematic variation of the isotopic composition of these calcite phases over a paleobathymetric range samples were collected from four sites a through d along a single upper capitan paleoslope reflecting the transition from reef massive through reef foreslope facies on a plot of 13c versus 18o the convergence f four marine cement trends from these four sites indicates that the isotopically enriched phase is invariant whereas the depleted phase a luminescent calcite intergrowth along original marine cement intercrystalline boundaries varies systematically with position on the paleoslope the emplacement of the calcite intergrowth in the marine cements is intimately related to the earliest episodes of equant calcite spar precipitation at the shallowest paleodepths sites a through c the compositions of the first generation of calcite spar spar i clusters around a mean 18o of 8 2 and is progressively less depleted in 13c from site a to site c as this variation is also present in the marine cement trends it is concluded that both the intergrowth and spar i represent successive episodes of calcite precipitation during a single diagenetic event at the deepest paleodepth site d sp r i is absent and the calcite intergrowth is interpreted to have the same composition as the second generation of spar spar ii 12 3 18o and 0 0 13c which can be found throughout the reef facies on the basis of its relatively constant oxygen composition the systematic enrichment of its carbon composition with increasing paleo depth and its absence from the deepest site on the paleoslope we conclude that the spar i diagenetic event represents the establishment of a meteoric phreatic system in the reef facies characterized by well defined downdip gradients in rock water interaction furthermore isotopic evidence of fresh water alteration of allochthonous massive facies reef debris deposited in foreslope strata near site d and below the limit of the spar i diagenetic event suggests that this meteoric system was active contemporaneously with the growth of the reef complex interpretation of the spar ii diagenetic event is less clear cut the associated diagenetic fluid probably did not have marine or hypersaline oxygen signatures as this would require the persistence of aragonite mineralogies to temperatures over 70 c and burial depths of at least i kin spar il was probably precipitated from meteoric water with a 18o similar to that of the spar i meteoric water at temperatures between 30 65 c unlike the spar i diagenetic event the spar ii event shows no association with depositional systematics hence we tentatively suggest that spar ii may have been related to a presumed ochoan or younger regional meteoric or shallow burial phreatic system

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

distribution and interpretation of rare earth elements and yttrium in cenozoic dolostones and limestones on cayman brac british west indies abstract the cenozoic carbonate succession on cayman brac a small island 19 km long and 1 5 to 3 km wide located in the caribbean sea has been dolomitized to varying degrees the rare earth element ree and yttrium y concentrations ree y of 134 carbonate samples 127 from cayman brac and 7 supplementary samples from grand cayman vary from 0 2 to 7 5 ppm average 2 8 ppm n 134 the shale normalized ree y patterns of the carbonate samples akin to those of oxygenated seawater are characterized by 1 lree depletion relative to hree average dy n sm n 1 7 n 125 2 positive la anomalies 3 negative ce anomalies average ce ce 0 4 n 133 and 4 superchondritic y ho molar ratios average y ho 85 n 86 the lack of correlation between the dolomite content of the samples and their ree y dy n sm n la n nd n ce ce or y ho indicates that dolomitization did not have a major impact on ree y signatures and that dolomitization was probably mediated by seawater like fluids the shale normalized patterns e g dy n sm n la n nd n ce ce and y ho vary from formation to formation for samples from the cayman formation the pedro castle formation and the ironshore formation there are subtle but gradual changes in dy n sm n la n nd n ce ce with depth in contrast there are marked changes in la n nd n ce ce y ho and sm nd across the brac unconformity which forms the boundary between the lower oligocene brac formation and the overlying miocene cayman formation variations in the ree y patterns reflect diagenetic processes e g dy n sm n and possibly secular changes in the ree y composition of seawater e g la n nd n y ho and sm nd

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

evidence against the dorag mixing zone model for dolomitization along the wisconsin arch a case for hydrothermal diagenesis ordovician carbonates near the wisconsin arch represent the type locality in ancient rocks for the dorag or mixing zone model for dolomitization field petrographic and geochemical evidence suggests a genetic link between the pervasive dolomite trace mississippi valley type mvt minerals and potassium k silicate minerals in these rocks which preserve a regional hydrothermal signature constraints were placed on the conditions of water rock interaction using fluid inclusion methods cathodoluminescence and plane light petrography stable isotopic analyses and organic maturity data homogenization temperatures of two phase aqueous fluid inclusions in dolomite sphalerite and quartz range between 65 and 120c freezing data suggest a na ca mg cl h2o fluid with salinities between 13 and 28 wt nacl equivalent the pervasive dolomitization of paleozoic rocks on and adjacent to the wisconsin arch was the result of water rock interaction with dense brines at elevated temperatures and it was coeval with regional trace mvt mineralization and k silicate diagenesis a reevaluation of the dorag mixing zone model for dolomitization in conjunction with convincing new petrographic and geochemical evidence has ruled out the dorag model as the process responsible for pervasive dolomitization along the wisconsin arch and adds to the abundant body of literature that casts serious doubt about the viability of the dorag model in general

pleistocene dolomite from the namibian shelf high 87sr 86sr and 18o values indicate an evaporative mixed water origin abstract dolomites from upper pleistocene sandstone and diatomaceous mud recovered from a coast parallel 4 km wide trough on the namibian shelf between hottentot bay and spencer bay have 87sr 86sr ratios between 0 70946 and 0 71309 marine mollusc shells associated with the dolomites have a modern seawater sr isotope ratio of 0 70915 the greater than seawater sr ratios indicate unequivocally that groundwaters were involved in dolomite formation groundwaters containing radiogenic sr derived from weathering of precambrian basement rocks were focused into local troughs mixed with seawater and evaporated in the arid climate the 13cpdb values of 4 0 to 2 4o oo indicate uptake of bicarbonate derived in part from microbial degradation of organic matter and 18opdb values of 1 4 to 6 4o oo suggest that the dolomite cement precipitated from a variably evaporated groundwater seawater mixture dolomite cementation of the basal bioclastic sand and gravel occurred during emergence when sea level was below the trough sill depth of 65 m most likely between 500 and 130 ka overlying calcareous and diatomaceous eemian mud and sand are dolomitic only in the deepest central trough area and contain 26 to 45 wt disseminated dolomite the 13cpdb values of 0 3 to 0 5o oo and 18opdb values of 5 7 to 6 4o oo suggest that the disseminated dolomite formed from an evaporated groundwater seawater mixture largely by replacement of biogenic carbonate when the shelf was subaerially exposed between 75 and 11 ka holocene transgressive shelly gravels and muddy sands contain only reworked pleistocene dolomite but holocene dolomite may occur in coastal salt pans onshore

dissolution methods for strontium isotope stratigraphy whole rock analysis the preparation of samples for strontium isotope stratigraphy sis by whole rock dissolution results in contributions of sr from contaminant phases as well as from the target carbonate phase here we highlight and quantify the errors in 87sr 86sr values that might result when calcium carbonate is dissolved using an inappropriate method we report the variation in 87sr 86sr value of sr released from four samples of differing calcite content and degrees of cementation when subject to different methods of dissolution we show that errors of up to 0 000076 in 87sr 86sr which correspond to errors of 3 68 ma in age can be avoided by careful preleaching of the samples

dolomitization of gas reservoirs the upper permian changxing and lower triassic feixianguan formations northeast sichuan basin china abstract the lower triassic feixianguan formation and to a lesser extent the upper permian changxing formation contain more than ten giant recently discovered dolomite gas fields on either side of a paleo embayment kaijiang liangping bay sichuan basin china this study documents the origin of the dolomite using a combination of petrology fluid inclusion trace element geochemistry and stable isotope and radiogenic isotope analysis of samples from outcrop and core samples from producing gas fields three stages of dolomitization are revealed dolomicrite from the lagoon facies in the feixianguan formation and minor fabric retentive dolomite from reef facies in the changxing formation on the northeast side of kaijiang liangping bay represent the initial dolomite phase caused by reflux of feixianguan brine with salinity close to gypsum saturation 18 wt salinity during the first stage of diagenesis at 30 c the second and dominant dolomite phase fabric retentive dolomite in both the feixianguan and changxing formations on the both sides of kaijiang liangping bay is the result of reflux of diagenetic fluids with salinity ranging from normal feixianguan seawater 3 5 salinity up to mesohaline water 6 5 salinity during early burial diagenesis at about 35 40 c as shown by the dominance of single phase aqueous inclusions shallow formed stylolites that cut the second phase of dolomite and the early triassic marine carbon oxygen and strontium isotope and rare earth element and yttrium ree y signals the subordinate third dolomite phase fabric obliterative dolomite and dolomite cement was due to burial diagenetic recrystallization between 80 and 140 c in the feixianguan and changxing formations on both sides of kaijiang liangping bay the third phase of dolomitization involved water expelled from the slightly younger lower triassic jialingjiang formation marine evaporites and carbonates on the northeast side of kaijiang liangping bay revealed by the high salinity of the aqueous inclusions that homogenize at 80 to 140 c and elevated strontium isotope data the carbon isotope and ree y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input future exploration should be focused on dolomitized high energy sediments e g reef and oolitic shoal and bank in similar triassic tethyan paleogeographic settings that are associated with restricted facies but underlying dolomitized high energy permian strata may also be good exploration targets

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