sulfide formation in reservoir carbonates of the devonian nisku formation alberta canada an ion microprobe study the processes affecting sulfur during diagenesis in carbonates have been investigated by ion microprobe analysis of 34s values of pyrite marcasite and anhydrite from the devonian nisku formation in the western canada sedimentary basin pyrite and marcasite from three nisku wells have 34s ranging between 35 and 20 cdt most lie between 35 and 5 cdt the 34s values of sulfides are heterogeneous within individual thin section sized samples varying by as much as 25 the 34s values increase by up to 50 with increasing well depth in two different nisku wells and 34s also increases by up to 15 as grain size increases from 40 to over 100 m in individual samples the appearance of native sulfur is accompanied by an increase of up to 30 in pyrite 34s values bulk and ion probe analyses of anhydrite are relatively uniform 34s 22 to 30 cdt the predominately low 34s values of nisku sulfides indicate bacterial sulfate reduction textural relations indicate that fe sulfide formation in nisku carbonates may have occurred by two different mechanisms much of the sulfide has 34s values that suggest that it was associated with bacterial sulfate reduction although most fe sulfides did not form until after pervasive matrix dolomitization depths of 300 1000 m other sulfide may have formed later during deep 4 km burial via thermochemical sulfate reduction the range in 34s values in a single thin section and correlations between pyrite morphology and isotopic values suggest that sulfate reduction was a very localized process and that the sulfate reduction environment varied considerably on a small scale

how many oceans meliata vardar and pindos oceans in mesozoic alpine paleogeography the timing of rifting and subduction in the meliata vardar and pindos zones of the hellenic dinaric carpathian region indicates that they cannot be considered remnants of a single ocean basin in contrast to several recently published mesozoic paleogeographies sea floor spreading in the meliata zone culminated in middle triassic ladinian time and ceased in the late triassic and subduction occurred from this time until late jurassic closure in contrast sea floor spreading in the vardar zone began in early jurassic time and culminated in the middle jurassic and subduction occurred from latest jurassic time in the pindos zone sea floor spreading was initiated in the early triassic with late jurassic and early tertiary subduction

authigenic potassium feldspar a tracer for the timing of palaeofluid flow in carbonate rocks northern calcareous alps austria

late jurassic blueschist facies pebbles from the western carpathian orogenic wedge and paleostructural implications for western tethys evolution in spite of the absence of ophiolitic slices at the surface some traces of the lost tethys ocean are recorded along the pieniny klippen belt pkb a narrow decollement thrust system sutured at the transpressive boundary between the outer and inner carpathians the enigmatic precollisional evolution of western carpathians can be deciphered from some late albian to campanian flysch conglomerates which display chrome spinel grains ophiolitic detritus and pebbles of blueschist facies tholeiitic metabasalts yielding a 40ar 39ar plateau age of 155 4 0 6 ma other detrital components are represented by extrabasinal pebbles of limestones arc volcanics and igneous to metamorphic basement rocks from southern sources our results suggest a markedly northward extension of the sublongitudinal triassic vardar meliata ocean and its subduction since the late middle jurassic supposedly balanced westward by coeval spreading in the ligurian piedmont basin of the apennine western alpine tethys a lateral kinematic connection between these diachronous and roughly parallel tethys branches was provided on the north by a left lateral east west trending shear zone running from the swiss austrian penninic domain to the northern carpathians this reconstruction replaces the classic model of two paired north penninic and south penninic oceanic basins and eastern homologues with the brianconnais hochstegen and czorstin microcontinents in between the late jurassic early cretaceous evolution of the carpathian active margin was characterized by subduction metamorphism and accretion of a wide orogenic wedge in this time the shallowing to deeply subsiding basins inferred from facies analyses on the sedimentary units of the pkb were likely floored by individual sections of the growing wedge later some exhuming blueschist ophiolitic units of the wedge were uplifted to the surface and functioned in the albian campanian as an exotic ridge supplying clasts to the forearc basin finally the colliding wedge became a cryptic paleostructure when since the latest cretaceous it disappeared beneath the inner carpathian orogenic lid and was incorporated within the eastward moving infrastructure of the carpathian orocline

conodont colour alteration indices palaeotemperatures and metamorphism in the northern calcareous alps a general view the middle and eastern parts of the northern calcareous alps nca can be subdivided into two distinct units with a lateral boundary marked by abrupt changes in the conodont colour alteration index cai values the first of these is a northern unit tirolikum with a relatively homogeneous distribution of no or low grade conodont alteration cai 1 0 2 0 the thermal overprint is thought to be relatively young and related to a heat flow from the tauern crystallization the second unit consists of the juvavic nappe system juvavikum which is distributed along the southern rim of the nca but also covers some of the northern parts of the tirolikum with respect to its cai distribution the juvavikum is more heterogeneous on a regional and local scale with some local cai inversions the juvavikum additionally shows distinctly different sets of cai values one with weak cai 1 0 1 5 and another with strong alteration cai 5 5 7 0 at present the highest known thermal overprint measured in the nca the metamorphism is relatively old as it predates the late jurassic early cretaceous gravity tectonic emplacement of the juvavikum onto the tirolikum the high cai values of parts of the juvavic nappe system are though to be related to tectonic burial in an accretionary wedge formed parallel to the closure of the vardar ocean the low cai values of the tirolikum apparently exclude a direct juxtaposition of the two units at the time of metamorphism

new evidence for block and thrust sheet rotations in the central northern calcareous alps deduced from two pervasive remagnetization events 1 we present 81 paleomagnetic sites early triassic to early cretaceous from the central sector of the northern calcareous alps nca eastern alps austria and germany stepwise thermal demagnetization defines three magnetic directions mostly carried by low unblocking temperature and low coercivity minerals j3 350 j2 500 j1 575 and 680 j3 and j2 show positive inclinations whereas j1 very seldom is of dual polarity the fold tests show that a j3 can be interpreted as a postfolding and posttilting remagnetization and j2 as a postfolding and pre syn tilting j1 can be considered as primary because of the occurrence of two polarities and evidence presented by other authors in the area all three components show a systematic and significant clockwise rotation after comparing with the expected european references j2 or j1 are marked by higher rotation values than j3 j2 shows different inclinations depending on the structural position north or southward dips considering the structural evolution and the observed inclinations the first postfolding and pretilting remagnetization event j2 could have taken place between late cretaceous and eocene times but certainly before the thrusting of the nca over the rhenodanubian flysch and northward tilting caused by the stacking of the lower austroalpine nappes the second postfolding and posttilting remagnetization j3 would have been acquired after the final thrusting of the nca over penninic units from then the nca behaved as a set of rigid blocks recording the main stage of vertical axis clockwise rotation 65 in average associated with the continental collision the variable degree of rotation in the different positions from 40 to 134 can be explained by individual vertical axis rotation in a block system trying to adjust to space problems the constant declination differences between j2 and j3 25 in average would reflect the rotation related with the lateral differences of shortening during the oblique thrust of the austroalpine units over the penninic units

authigenic feldspar as an indicator of paleo rock water interactions in permian carbonates of the northern calcareous alps austria abstract dolostones interbedded with upper permian evaporites at the base of the northern calcareous alps contain abundant authigenic k feldspar two petrographically structurally and isotopically distinct generations of k feldspar can be distinguished crystals composed of an inclusion rich core and a clear rim and optically unzoned transparent crystals both feldspar types have essentially identical k feldspar end member compositions with 99 5 mole or component low oxygen isotope ratios 16 1 to 18 1 smow suggest precipitation from 18o enriched saline fluids at temperatures in excess of 140 c 40 r 39ar plateau age spectra of five samples range from 145 1 to 144 1 ma early berriasian and suggest that both types of feldspar were formed within an interval that did not exceed 2 m y rb sr model ages range from 152 to 140 ma assuming that the burial diagenetic regime was buffered with respect to strontium by the associated marine permian evaporites authigenic k feldspar records two distinct events of hot brine flow most likely triggered by tectonic movements detachment and by an increase in the subsurface temperature in response to thrust loading

stochastic wave radiation by the gulf stream abstract a linear small amplitude model of rossby waves forced by idealized meanders is presented in order to ascertain whether or not the observed low frequency motions near the gulf stream are capable of being so generated two crucial ingredients are shown to be necessary the meandering activity must vary in the downstream direction and the meanders must have a transient behavior cast as a stochastic average over a large number of meanders the predicted amplitudes of the kinetic energy and reynolds stresses are similar to those observed implications for the forcing of a mean flow are discussed

origin of vein hydrothermal carbonates in triassic limestones of the espadan ranges iberian chain e spain dolomitization and hg bearing veins in the lower triassic limestones of the espadan ranges iberian chain e of spain

the alpine haselgebirge formation northern calcareous alps austria permo scythian evaporites in an alpine thrust system abstract multiphase deformed and weakly metamorphosed permo scythian evaporites of the northern calcareous alps eastern alps of austria were deposited in an aborted rift arm of the northwest tethys they represent marine precipitates laid down in a basin surrounded by alluvial fans and mudflats mean bromide values of halite 13 53 ppm as well as s isotope ratios of sulphate minerals 14 0 9 cdt indicate that the contribution of non marine waters to the marine brine composition was insignificant the most striking feature is the haselgebirge structure a chaotic melange of shales silt and sandstones anhydrites carbonates and scarce magmatites kirchner 1980 embedded in a clayey halite matrix this melange resulted from severe tectonization caused by a variety of deformational processes halokinesis diapirism gravitational sliding and alpine thrust tectonics

pyrite framboids and their development a new conceptual mechanism it has been suggested that the two morphologies of sedimentary pyrite framboids and euhedra may reflect two distinct pathways of pyrite formation framboids form indirectly via iron monosulphides whereas euhedra form from direct precipitation from solution a third pathway which is bridging these two forms is proposed here namely the continuous growth from a monosulphide globule through framboids to a euhedral single crystal it is also suggested that framboids probably occur over a range of three orders of magnitude from the least complex microframboids through framboids to polyframboids

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

fluid rock reactions in an evaporitic melange permian haselgebirge austrian alps tectonically isolated blocks of carbonate rocks present within the anhydritic haselgebirge melange of the northern calcareous alps record a complex history of deformation and associated deep burial diagenetic to very low grade metamorphic reactions fluids were hot up to 250 c and reducing brines charged with carbon dioxide individual carbonate outcrops within the melange record different regimes of brine rock reactions ranging from pervasive dolomite recrystallization to dedolomitization early diagenetic features in these carbonates were almost entirely obliterated matrix dolomite alteration was related to thermochemical sulphate reduction tsr recognized by the replacement of anhydrite by calcite pyrite native sulphur pyrite associated with tsr is coarsely crystalline and characterized by a small sulphur isotope fractionation relative to the precursor permian anhydrite carbonates associated with tsr show low fe mn ratios reflecting rapid reaction of ferrous iron during sulphide precipitation as a result tsr related dolomite and calcite typically show bright mn ii activated cathodoluminescence in contrast to the dull cathodoluminescence of many ferroan carbonate cements in other deep burial settings in addition to carbonates and sulphides silicates formed closely related to tsr including quartz k feldspar albite and k mica 40ar 39ar analysis of authigenic k feldspar yielded mostly disturbed step heating spectra which suggest variable cooling through the argon retention interval for microcline during the late jurassic this timing coincides with the recently recognized subduction and closure of the meliata hallstatt ocean to the south of the northern calcareous alps and strongly suggests that the observed deep burial fluid rock reactions were related to jurassic deformation and melange formation of these permian evaporites

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

petrological and kar and40ar39ar age constraints for the tectonothermal evolution of the high pressure meliata unit western carpathians slovakia abstract the meliata high pressure unit exposed in the southern part of the western carpathians consists of four groups of rocks which are characterized by different lithologies and metamorphic histories 1 the most common are triassic marbles intercalated with metabasalts and glaucophane bearing phyllites phengite from metabasalts and phyllites record k ar and40ar 39ar plateau ages of 152 155 ma which are interpreted to date the high pressure metamorphism 2 blueschist facies mafic rocks and subordinate glaucophane bearing micaschists devoid of marbles which contain relic low si muscovite and newly formed high si phengite the40ar 39ar data obtained for the micas indicate palaeozoic ages of 375 380 ma for the early amphibolite facies muscovite and a 250 ma age for the secondary blueschist facies phengite 3 glaucophane free phyllites and phengite quartzite with low si and high si micas respectively textural features indicate a polyphase evolution characterized by a greenschist facies metamorphism followed by a blueschist facies overprint the high si micas provide evidence for an early middle jurassic age 172 ma of the high pressure metamorphism 4 amphibolite facies slices and blocks overprinted by blueschist facies metamorphism represent old basement rocks regarding the differences in lithologies metamorphic conditions and radiometric ages of high pressure micas older age for continental vs younger age for oceanic rocks subduction of a continental wedge prior to that of oceanic crust can be assumed this interpretation is based also on palaeogeographical reconstruction of the mesozoic western tethys between the apulian promontory and the tisza bihor continental block

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

the evolution of the meliata hallstatt ocean and its significance for the early evolution of the eastern alps and western carpathians abstract the evolution of the meliata hallstatt ocean and of its southern and northern margin in the inner western carpathians are described the main rifting began during the pelsonian and the sea floor spreading ended at the beginning of the middle carnian contemporaneously with the raibl event the subduction began in the latest triassic and the final closing of the ocean accompanied by uplift of the adjacent marginal areas was dated as basal oxfordian remnants of the oceanic suboceanic sequence occur in two tectonic positions 1 obducted nappes that contain mostly as tectonic melanges the whole middle triassic to middle jurassic sequence including large bodies of dismembered ophiolites of ladinian to early carnian age 2 evaporite melanges at the base of nappes that are derived from the northern margin of the ocean these evaporite melanges consist of late permian evaporitic matrix and blocks of a dismembered ladinian to lower carnian ophiolitic sequence in the inner western carpathians both the northern and southern marginal sequences of the meliata hallstatt ocean and the transitions into the carbonate platforms are present in different nappes the southern marginal development is characterized by middle carnian distal clastic raibl beds and by middle jurassic rhyolithic volcanism both missing in the northern marginal development because the subduction related middle jurassic volcanism contemporaneous with the turbidites in the southern meliaticum is restricted to the marginal area south of the meliata hallstatt ocean southward directed subduction is indicated to the south on the adjacent carbonate platform a jurassic basin with aalenian to bajocian ophiolites opened this may be a back arc basin in the eastern alps only the northern marginal zone and the adjacent carbonate platform are preserved but parts of the dismembered ophiolites in the haselgebirge may be of ladinian to late carnian age like in evaporitic melanges in the same tectonic position in the inner western carpathians the final closing of the meliata hallstatt ocean is indicated by the abrupt end of the turbiditic sedimentntation in the oceanic suboceanic domain and by uplift in the marginal areas where lower oxfordian radiolarites are overlain by shallow water upper oxfordian to tithonian or neocomian limestones silica nappe hallstatt nappes with the evidence of the final closing of the meliata hallstatt ocean near the middle late jurassic boundary the cimmerian orogenesis in the genetic sense of engor 1984 1985 is now also proven in the inner western carpathians and in the eastern alps a continuation of the meliata hallstatt ocean into the pontide paleotethys cimmerian ocean sensu kozur 1990 through the transylbanian oceanic domain and the strandzha unit is assumed a connecction with the vardar ocean is impossible because in the vardar zone no triassic ophiolites or basic volcanics are present and the ophiolites have jurassic age

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

40ar 39ar dating of the inner carpathians variscan basement and alpine mylonitic overprinting abstract the 40ar 39ar dating method has been applied to mylonitic rocks from the carpathian mountains these are variscan metamorphic and magmatic rocks that were overprinted during the alpine orogeny the inner western carpathians are conventionally described as a nappe pile formed by northward movement and then subjected to intense strike slip and normal faulting resulting in the formation of mylonitic rocks precise stratigraphie ages provide clear constraints for the geodynamic interpretation related to the succession of alpine events such as the pre gosauian phase during the turonian 90 ma the 40ar 39ar method allows to define the age of the ancient variscan basement and the ages of the alpine overprinting in the high and low tatras mineral ages of around 340 350 ma are yielded by gneisses micaschists and amphibolites providing a younger limit for the variscan metamorphism the emplacement cooling of the variscan granites is defined between 305 and 335 ma these rocks are overprinted by several alpine tectonothermal events the major events are dated at 65 and 86 88 ma during which the mylonites were generated an older event around 140 ma earlier defined by the rb sr method is now confirmed by the 40ar 39ar method dating of phengites extracted from high pressure metamorphic triassic rocks from the kosice area yields new data for the age of this event at 155 ma

sr and nd isotopes at the prmian triassic boundary a record of climate change abstract we present a detailed curve of seawater 87sr 86sr for the middle permian to triassic based on analyses of conodonts from overlapping sections in the u s a pakistan correlated using conodont biostratigraphy the isotope ratio decreased in the middle permian at an average rate of 0 000062 ma 1 reached a minimum in the capitanian 257 258 ma and increased in the late permian at an average rate of 0 000097 ma 1 the late permian rate of increase was roughly two and a half times greater than the average increase over the past 40 ma and approximately equal to the highest cenozoic rates which occurred over much shorter time intervals modeling results suggest that decreasing middle permian 87sr 86sr ratios were driven by changes in the riverine sr flux to the oceans while increasing ratios in the late permian triassic are attributed to both increased riverine 87sr 86sr and flux the reduced middle permian riverine flux coincides with extreme continental aridity associated with the formation of pangea and recorded by massive evaporite deposits in addition mountains in the equatorial region of pangea may have created a rain shadow thereby minimizing precipitation in regions that currently contribute the bulk of chemical weathering products to the ocean increasing riverine 87sr 86sr in the late permian is suggested by the observation that 143nd 144nd values decrease at the same time however the source of radiogenic sr is not known frequently cited mechanisms for increasing 87sr 86sr in runoff such as glaciations and continent to continent collisions coincide instead with decreasing seawater 87sr 86sr in the middle permian one possible source may have been deep erosion into older orogens associated with a dramatic increase in chemical weathering in the late permian the cause of enhanced weathering appears to have been increased levels of atmospheric co2 with associated global warming and increased humidity proposed sources of co2 include dissociation of gas hydrates and oxidation of organic matter during extreme sea level regression as well as volcanic emissions from siberian traps eruptions continental floral and faunal distributions are consistent with this interpretation as are oceanic 13c patterns and variations in shallow water sediment lithologies

geochemical modeling of the madison aquifer in parts of montana wyoming and south dakota stable isotope data for dissolved carbonate sulfate and sulfide are combined with water compo sition data to construct geochemical reaction models along eight flow paths in the madison aquifer in parts of wyoming montana and south dakota the sulfur isotope data are treated as an isotope dilution problem whereas the carbon isotope data are treated as rayleigh distillations all reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the madison limestone the geochemical reaction models indicate that the dominant groundwater reaction in the madison aquifer is dedolomitization calcite precipita tion and dolomite dissolution driven by anhydrite dissolution sulfate reduction ca mg2 ina cation exchange and halite dissolution are locally important particularly in central montana the groundwater system is treated as closed to c02 gas from external sources such as the soil zone or cross formational leakage but open to c02 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer the computed mineral mass transfers and modeled sulfur isotopic composition of madison anhydrites are mapped throughout the study area carbon 14 groundwater ages adjusted for the modeled carbon mass transfer range from modem to about 23 000 years b p and indicate flow velocities of 7 7 ftlyr 2 1 26 5 rnlyr most horizontal hydraulic conductivities calculated from darcy s law using the average 14 c flow velocities are within a factor of 5 of those based on digital simulation the calculated mineral mass transfer and adjusted 14 c groundwater ages permit determination of apparent rates of reaction in the aquifer the apparent rate of organic matter oxidation is typically 0 12 jlidol uyr sulfate and to a lesser extent ferric iron are the predominant electron acceptors the kinetic biochemical fractionation of 34 s between sulfate and hydrogen sulfide is approximately 44 o at 25 c with a temperature variation of

nacacl saline formation waters frio formation oligocene south texas usa products of diagenesis abstract saline formation waters from reservoirs in oligocene deltaic clastic sedimentary rocks frio formation south texas are derived from three sources mixtures characterize most samples accounting for the wide range in chemistry low salinity water is derived from cenozoic shales in contrast saline water having variable br cl ratios is allochthonous and is derived from at least two sources in mesozoic strata which underly the cenozoic units saline water in this area regardless of br cl ratio contains variable high concentrations of ca calcium is derived primarily from dissolution of plagioclase as demonstrated by petrography 87 sr 86 sr analyses and chemical modelling the mccl 2 or carpenter function of saline formation waters in this area thus can be derived from water rock interaction and cannot be used to identify the ultimate source of solutes reverse osmosis is an unimportant processes in this area and 18 o and d values converge on rock buffered values of approximately 8 o and 30 o respectively

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

dating fault gouges geological faults are fractures in the earth s crust along which there has been some displacement and are thus zones of weakness which are of importance in planning civil engineering projects constructing underground caverns and conducting earthquake research to assess the potential for future displacements and fluid circulation along faults it is helpful to establish the age of the latest episode of mineral growth in fault zones however there have been few attempts to date recrystallized and newly grown minerals developed in fault zones at low temperature1 4 here we report k ar and rb sr ages for clay fractions from two fault zones in the eastern alps and compare them with ages of the same fine fractions from the undeformed host rock the results suggest that the bulk of the clays along these faults were formed during early cretaceous and tertiary times respectively no major displacements seem to have taken place in recent times

quaternary rupture behavior of the karakoram fault and its relation to the dynamics of the continental lithosphere nw himalaya western tibet abstract the karakoram fault is a major strike slip system that partitions strain between the compressional regime of the nw himalaya and the translational and extensional fault systems of western tibet although seismic and geodetic observations suggest that the fault is currently locked a high millennial slip rate infers a dominant role in the deformation of western tibet we examine all quaternary data available for the karakoram fault and attempt to explain the temporal and spatial variability of fault slip rate we show that the karakoram fault stress level could be modulated by coulomb stress changes associated with varying seismicities on the subducting indian plate beneath western tibet our model has implications for the understanding of stress transfer between large fault systems and their interaction with lithosphere deformation we suggest that while the long term deformation of tibet may be modeled as a partially viscous medium the slip rates of major faults may vary by one order of magnitude during the quaternary our analysis may resolve the frequently observed discrepancy between millennial and decadal records of fault slip rates

origin of deformed halite hopper crystals pseudomorphic anhydrite cubes and polyhalite in alpine evaporites austria germany the alpine haselgebirge formation represents an upper permian to lower triassic evaporitic rift succession of the northern calcareous alps eastern alps although the rocksalt body deposits are highly tectonised consisting mainly of protocataclasites and mylonites of halite and mudrock the early diagenetic history can be established from non tectonised mudrock bodies cm sized euhedral halite hopper crystals formed as displacive cubes within mud just during shallow burial the crystals were deformed by subsequent compaction later migrating fluids led to the replacement of halite by anhydrite retaining the shapes of deformed halite cubes polyhalite formed from subsequent enhanced fluid migration mudrock provided water by dewatering while potassium and magnesium were dissolved from primary salt minerals when these fluids interacted with sulphates polyhalite precipitated 40ar 39ar analyses date the polyhalite from within the retaining shapes of deformed halite hopper shaped cubes from two localities to ca 235 232 ma middle triassic at this time ca 20 25 ma after sedimentation polyhalite crystallised at shallow levels

a strontium and sulfur isotopic study of permo triassic evaporites in the northern calcareous alps austria pervasively deformed permian and to a lesser extent triassic evaporites of the northern calcareous alps form a tectonic melange collectively known as alpine haselgebirge isotopic compositions of s and sr and trace element concentrations of sr and ba in ca sulfate minerals were investigated in terms of their stratigraphic and paleoenvironmental significance the majority of the samples yielded 34s values ranging from 9 8 to 13 8 indicating precipitation from late permian seawater with local modifications due to bacterial sulfate reduction bsr 87sr86sr ratios of these samples range widely from 0 7070 to 0 7098 but most samples show sr isotope values between 0 7070 and 0 7074 overlapping with the accepted late permian seawater values comparison of both s and sr isotopic compositions of individual samples reveals distinct trends of isotopic enrichment and depletion that can be linked to early diagenetic bsr mixing with continental surface or groundwaters and burial diagenesis concentrations of sr tend to be lower and ba values show a tendency toward values higher than those reported from marine evaporites implying variable diagenetic alteration triassic evaporites are volumetrically less important in the haselgebirge than their permian counterparts and are characterized by marine upper scythian to lower anisian s and sr isotope values in three localities hitherto unknown inclusions of these triassic evaporites were identified within the haselgebirge melange

karstification without carbonic acid bedrock dissolution by gypsum driven dedolomitization aggressive karstification can take place where dolomite and gypsum are in contact with the same aquifer gypsum dissolution drives the precipitation of calcite thus consuming carbonate ions released by dolomite lake banyoles in northeastern spain is a karst lake supplied by sublacustrine springs and karstic collapse is occurring in the immediate vicinity of the lake lake water is dominated by mg ca and so 4 hco 3 and is supersaturated with calcite that is actively accumulating in lake sediments water chemistry sulfur isotope composition local stratigraphy and mass balance modelling suggest that the primary karst forming process at at lake banyoles is dedolomitization of basement rocks driven by gypsum dissolution karstification takes place along the subsurface contact between the gypsiferous beuda formation and the dolomitic perafita formation this process is here recognized for the first time to cause karstification on a large scale this is significant because it proceeds without the addition of soil generated carbonic acid gypsum driven dedolomitization may be responsible for other karstic systems heretofore attributed to soil generated carbonic acid