free surface effects on the propagation of dynamic rupture dynamic rupture of reverse and normal fault intersecting the surface are investigated in the case of a normal fault nucleating at depth and propagating upwards coupling of rupture radiated stress and free boundary conditions at the surface may induce a shallow secondary nucleation anticipating up to a few seconds the arrival of the main rupture front indeed the free surface induces normal stress fluctuations modifying the fault frictional strength no significant effect on rupture velocity is observed in the case of reverse faulting these incidences are explained by a stress analysis and illustrated by some numerical simulations in the case of dynamic normal faulting in a homogeneous half space the described effects could explain some observations of high frequency radiation close to the surface in documented shallow earthquakes like in kalamata 1986 bouin 1994

calcretes at sequence boundaries in upper carboniferous cyclothems of the sydney basin atlantic canada abstract nodular and associated groundwater calcretes in coal bearing cyclothems of the sydney basin mark the boundary between underlying marginal marine and overlying alluvial deposits they are inferred to represent lowstand surfaces sequence boundaries the host sediments include both sand siltstones and palustrine limestones microfabrics of calcretes with detrital hosts show evidence of replacement displacement and shrinkage alpha fabrics suggesting that the calcretes formed under relatively arid conditions in contrast microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics with root induced brecciation and subordinate shrinkage fabrics the calcretes and host limestones are inferred to have formed in topographic lows thick alluvial sediments with red calcic vertisols overlie the calcretes and formed within the transgressive systems tract under conditions of abundant sediment supply coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract the presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity

isotope geochemistry of fluid inclusions in permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters abstract d and 18o values have been determined for fluid inclusions in 45 samples of permian halite the inclusions are enriched in 18o relative to the meteoric water line but are depleted in d relative to ocean water inclusions with the more positive values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a d 18o diagram inclusions with more negative values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines the isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that permian sea water was isotopically similar to modern sea water connate evaporite brines can have negative values because of the probable hooked isotope trajectory of evaporating sea water and or synsedimentary mixing of evaporite brines with meteoric waters subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested

model of earthquake recurrence role of elastic wave radiation relaxation of friction and inhomogeneity we have constructed a dynamical model of seismicity wherein a fault is embedded in an infinite continuous elastic medium therefore the influence of the energy dissipated in seismic wave radiation oil the sequential history of model earthquakes is fully taken into account in this model the drop in friction at rupture takes place gradually thus introducing a relaxation dimension as an example we consider a finite homogeneous fault that is terminated by infinitely strong barriers at the ends the features of the seismicity are dominated by the stresses that are reflected from the unbreakable barriers there is a strong dependence of the patterns of seismicity on the ratio of the relaxation dimension to the distance between the barriers at the ends for large values of this parameter we find that periodicity begins after a short transient interval and a dominance of the statistical distributions by large events that break completely through the fault from end to end for small values of this parameter smaller scale seismicity is interspersed with the large events and no periodicity is observed within the time spanned by the computations we conclude that major unquenched heterogeneities such as those found at barriers which we suppose arise in nature due to the nonuniform geometry of faults are vital ingredients for generating complex seismic histories as well as giving rise to the complex features of individual earthquake source time functions in models of seismicity as a dynamical process

sources of basinal and mississippi valley type mineralizing brines mixing of evaporated seawater and halite dissolution brine abstract origins of basinal brines and mississippi valley type mvt mineralizing fluids have been separately attributed to evaporation of seawater or dissolution of halite although brines originating from the two processes are not mutually exclusive in basins this study shows that the na br cl br diagram cannot distinguish between evaporated seawater and halite dissolution fluid using the na deficit ca excess diagram which was previously proposed to characterize fluid rock interactions of basinal brines it is shown that most basinal brines including wt mineralizing fluids of the viburnum trend deposits were probably initially a mixture of halite dissolution fluid and evaporated seawater using the same diagram we suggest that the mineralizing brines of the gays river mvt deposit were derived from an aquifer of clastics underlying a thick succession of evaporites where halite dissolution fluid and evaporated seawater could have mixed

sub basin specific pb and sr sources in zn pb deposits of the lower windsor group nova scotia canada several carbonate hosted lead zinc deposits displaying replacement and or open space filling textures occur in the lower windsor group visean of nova scotia canada many of these deposits are distributed within four sub basins kennetcook musquodoboit shubenacadie and river denys over a distance of about 300 km lead isotope ratios in galena from eight deposits and strontium isotope ratios in ore stage calcite from two deposits and in sphalerite from one deposit collectively indicate derivation of these elements directly from sources specific to each sub basin these sources were identified as immediately underlying basement components or sedimentary rocks derived from them

br cl na systematics in illinois basin fluids constraints on fluid origin and evolution the authors present here bromide chloride and sodium data for fluids from reservoirs of ordovician through pennsylvania age in the illinois basic which suggest that remnant marine fluids contribute significantly to fluid cl budgets cl br and nabr ratios for ordovician through devonian formation fluids are relatively uniform and near those for seawater despite greater than a factor of ten range in cl concentration in contrast fluids from mississippian and pennsylvanian reservoirs separated from older reservoirs by the new albany shale group have more variable fluid cl br and na br ratios most of which are significantly greater then those of seawater the 1 1 stoichiometry of cl and na increases for mississippian and pennsylvanian formation fluids is consistent with halite dissolution nevertheless br systematics and mass balance considerations indicate that he overall cl budget of illinois basin formation fluids appears to be more significantly influenced by the contribution from subaerially evaporated seawater than by halite dissolution

a strontium oxygen and hydrogen isotopic composition of brines michigan and appalachian basins ontario and michigan abstract 87 sr 86 sr ratios of brine from samples from the michigan and appalachian basins in ontario and michigan covering the stratigraphic interval from the cambrian to mississippian vary from 0 708 to 0 711 with the exception of the salt units of the salina formation silurian most values are greater than seawater for the time in question indicating water rock interaction the sources of the radiogenic sr has not been identified all samples plot below the gmwl in 18 o 2 h space with the cambrian and ordovician samples closest to the line mixing of brines meteoric and glacial pleistocene water is indicated in some cases the more concentrated brines from each stratigraphic unit show a very narrow spread in values all the ordovician brines show a narrow range over a 200 km area for sr o and h isotopes indicating extensive lateral migration of the fluids strontium in the brine has not equilibrated isotopically with its host rock in some cases the late stage minerals saddle dolomite calcite and anhydrite have the same 87 sr 86 sr ratios as the brine indicating that they precipitated from the brine in isotopic equilibrium

geochronology of the monteregian hills alkaline igneous province quebec three distinct groups of alkaline rocks can be recognized on the basis of silica content in the monteregian hills quebec strongly undersaturated slightly undersaturated to critically saturated and oversaturated the first two groups were derived from basanite and alkali picrite magmas respectively and yield distinct ages of intrusion clustered about 118 ma and 136 ma the age data are not readily explained by the translation of the north american plate across a fixed mantle hotspot but because the alkali picrite magmas were derived from greater mantle depths than the basanite magmas the data do suggest an upward migration of the zone of melting this change in the depth of melting may be a response to the upward transport of a metasomatic fluid perhaps with a coupled rise in mantle isotherms

δ18o values of mudrocks more evidence for an 18o buffered ocean abstract the whole rock 18 o of mudrocks as well as the 18 o of the clay sized silicate fraction does not change in any systematic manner over paleozoic and late proterozoic time therefore the 18 o of the ocean has not changed except a per mil or so as the result of glaciation deglaciation over the last billion years of earth history

isotopic and trace element constraints on the origin and evolution of saline groundwaters from central missouri na ca cl groundwaters with salinities of 1 to 30 discharge from natural springs and artesian wells in mississippian carbonates and ordovician sandstones and carbonates in central missouri carbonate saturation and quartz supersaturation are maintained throughout the salinity range major and trace element and isotopic variations in the waters are used to place constraints on models for rock water interaction and regional hydrology the groundwaters have d values that range from 108 to 45 and 18 o values that range from 14 7 to 6 5 smow these data lie approximately along the meteoric water line ranging from values similar to local recharge in missouri to significantly lower values that are similar to 18 o and d for meteoric recharge in high altitude and high latitude regions of north america the more saline samples have the lowest 18 o and d contents a trend that is opposite to that observed in most other studies of saline waters the h o isotopic correlation and the range in salinity and several major and trace element concentrations in central missouri groundwaters are readily explained by a saline dilute water mixing model 87sr 86sr ratios for the groundwaters range from 0 7155 to 0 7161 these values are significantly higher than previously published values for 1 estimates of 87 sr 86 sr in phanerozoic seawater 0 7068 0 7092 and 2 carbonates of the mississippian burlington keokuk fm 0 7075 0 7105 through which some of the waters migrate as they pass to the surface nd 0 values in the groundwaters range from 10 9 to 8 1 and 147 sm 144 nd ratios range from 0 108 to 0 128 these values are similar to or lower than previously published analyses of carbonates and other authigenic phases from the burlington keokuk fm mean nd 0 7 8 0 8 1 mean 147 sm 144 nd 0 141 the h o sr and nd isotopic data and the results of model calculations preclude 1 models involving the modification of ancient seawater for the origin of the saline waters and 2 extensive interaction between the groundwaters and their host carbonates the waters apparently acquired their 18 o and d values as meteoric recharge and their 87sr 86sr and rare earth element signatures from extraformational crustal sources high 87 sr 86 sr low nd 0 low 147sm 144nd and largely preserved these isotopic signatures during subsequent migration through the carbonates chemical exchange via rock water interaction is required for isotopic exchange to occur therefore the present state of chemical equilibrium between the groundwaters and carbonate aquifer minerals may serve to limit rock water interaction and hence preserve the extraformational isotopic signatures in the waters the integration of geochemical isotopic and hydrologic data on a local and regional scale suggests a history for the central missouri groundwaters involving 1 meteoric recharge in the front range of colorado 2 dissolution of permian halite in the subsurface of kansas 3 interaction with predominantly silicate mineral assemblages in paleozoic strata and possibly precambrian basement with aquisition of crustal sr and ree signatures 4 dilution and migration to shallow aquifer levels in central missouri and 5 mixing with local meteoric recharge through mississippian carbonates with no significant change of the isotopic signatures acquired in stage 3

modelling of outbursts at 26 colliery glace bay nova scotia part 1 outburst history and field data abstract the outbursts experienced at 26 colliery glace bay nova scotia originated from within sandstone river channels just above and sometimes penetrating the coal seam these outbursts caused delays in driving the main headings a considerable loss of revenue and threatened the closure of some areas of the mine a major mine fire has since closed this mine however other mines in the sydney coal field are expected to experience similar problems part 1 of this paper summarises the geology of the sydney coal basin and the geology associated with the outbursts a brief description of the 37 recorded outbursts is presented little data is available to classify the conditions at the outbursts sites but that available on the rock properties gas pressures and field stress conditions is summarised here a better understanding of the mechanisms of these outbursts could pave the way to creating a safer and more profitable operation at neighbouring mines part 2 of this paper proposes a mechanism that explains these occurrences and a model is developed to analyse the influencing parameters part 3 compares the results of the model studies with field observations

geochemistry of tectonically expelled fluids from the northern coast ranges rumsey hills california usa tectonic compression has created abnormally high pressure on deep basinal fluids causing their expulsion from areally exposed upper cretaceous rock along the eastern margin of the california coast ranges the fluids emerge as near neutral perennial sodium chloride springs at high elevations with flow rates as high as 10 l per min higher spring discharges are more common around the exposure of a west vergent fault propagation fold axis spring waters range from 1000 to 27 000 mg l tds the least saline water 18o 7 5 closely represents local meteoric water that mixes with saline fluid 18o 5 3 and forms a slope of 3 5 on a d vs 18o plot a na 125 to 8000 mg l vs cl 150 to 17 000 mg l plot shows a linear dilution trend that extends close to but below the values for modern seawater calcium 75 3000 mg l is considerably enriched relative to seawater and forms a nonlinear trend with chloride in detail the na deficit defined by the difference between the measured na content and the na concentration on a hypothetical seawater dilution line is approximately balanced by the ca excess similarly defined by the seawater dilution line this relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths simultaneous b and 18o enrichment of the fluids accompanied by deuterium depletion further suggest that the seawater modification is influenced by clay diagenesis bicarbonate and sio2 concentrations show an inverse correlation with cl with most waters being saturated or slightly oversaturated with calcite and quartz at the discharge temperatures some freshwater springs with near meteoric stable isotope values may represent mixing of young groundwater from perched aquifers but in many cases the freshwater springs emerge along the same structures and have the same perennial nature as the saline fluids and expulsion of an older fresh groundwater component that is under abnormal fluid pressures cannot be ruled out basinal fluids elsewhere commonly show dilution trends with local meteoric water and in the case of the rumsey hills some of the dilute saline waters may indicate deep penetration of meteoric water 1 km in the pleistocene before the latest tectonic uplift geothermometry of the spring waters maximum 90 c suggest an origin from as deep as 4 0 km this depth is consistent with the depth of the core of a fault propagation anticline below the surface of the rumsey hills developed by active internal deformation of an east tapering wedge beneath the southwestern sacramento valley active tectonic compression causes near lithostatic fluid pressures in the shallow subsurface below the rumsey hills and volume strain within the core of the anticline that results in upward expulsion of the saline fluids from the indicated depths

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

numerical simulation of seismicity due to fluid injection in a brittle poroelastic medium summary it has recently been shown that rather small perturbations in eiective stress due to uid injection or withdrawal may trigger microseismic events such events typically in the magnitude range below magnitude 4 ml have similar characteristics to normal tectonic earthquakes with double couple focal mechanisms implying a dominant shear motion at the source in this paper we examine the nature of this mechanical uid rock interaction for the case of uid injection in a hydrocarbon reservoir our model is based on the combination of a model of seismicity in dry rocks and a model of pore uid pressure diiusion the former involves a nite diierence approximation of the equation of motion and the latter follows a lattice boltzmann approach they are coupled via the concept of eiective stress applied both to the mohr coulomb rupture criterion and to the volumetric elastic deformations which in turn perturb the pore pressure the simplifying assumptions are that the uid and solid phases have the same bulk moduli and that strain compatibility conditions are not included 2 d plane strain simulations of uid injections in an anisotropically pre stressed elastic brittle medium illustrate the capabilities of the model spontaneous inception and growth of shear fractures in both the near and far elds and explicit calculation of their seismic radiation which feeds back into pore pressure perturbations they also show that as expected the amount and rate of stress drop during the brittle rupture control the size and spatial density of the resulting fractures our model should prove to be relevant at least in two cases as a simulation tool in the investigation of correlations between injection and producer wells which are thought to be of geomechanical origin in certain oil elds and as a forward model in the inversion of focal mechanisms of pore pressure induced shear events

sulphur in pennsylvania coals of atlantic canada geologic and isotopic evidence for a bedrock evaporite source abstract higher sulphur contents in coals are commonly explained by the proximity of the original peat to marine waters during deposition as the sulphate ions in seawater provide an abundant source of sulphur however pennsylvanian coals of the maritimes basin which commonly contain 5 8 sulphur are associated with a predominantly freshwater biota and alluvial sedimentary features and evidence for marine beds is scare several workers have suggested that the sulphur was derived from dissolution of thick sulphate evaporites in the underlying mississippian windsor group rather than from pennsylvanian seawater we set out to test this hypothesis using a biogeochemical model derived from studies of sulphur isotopes in modern peat swamps during the bacterial decomposition of plant material the h 2 s generated from groundwater sulphate is fixed as metal sulphides which are enriched in the lighter 32 s isotope by 15 or more sulphides formed chemically during burial will be much less fractionated isotope ratios of sulphur bearing products can thus be used to trace the original source of the sulphur by comparison with a source of known isotopic composition eight evaporite samples from the upper windsor group in the sydney basin have a mean 34 s value of 15 sixteen pyrite samples from five coal seams in the overlying morien group range from 14 8 to 5 34 s the pyrite values follow the pattern predicted for derivation from windsor sulphates disseminated pyrite generally contains more highly fractionated sulphur than nodular pyrite suggesting that much of the former was generated by bacterial action whereas the latter was produced during later burial isotopic ratios for soluble sulphate in the coal correspond closely to those for the associated pyrite because mississippian and pennsylvanian seawaters showed a similar range of 34 s values the test does not preclude a pennsylvanian marine source of sulphate both geological and isotopic data however are consistent with a windsor evaporite source

numerical model of seismic rupture simple numerical models of the cellular automaton type have been proposed recently as an analogy for seismic faults those showed interesting features of spontaneous rupture evolution lomnitz adler and lemus diaz 1989 or even seismic recurrence bak and tang 1989 it is possible to incorporate realistic rheology and tensorial physics into this kind of model to extend it to a portion of crust instead of a single fault plane and to simulate its time evolution a numerical model based on physics of continuum media instead of a cellular automaton can simulate dynamic rupture in two dimensions the medium can be submitted to increasing load which is introduced by an imposed displacement at the boundaries space and time being discretized by a finite difference scheme the equation of dynamics is used at each time step a rupture criterion allows each node in the grid of the medium to fail if a threshold is reached the brittle threshold is allowed to fluctuate from point to point in the medium according to a chosen distribution failure is represented by a loss of shear stiffness in the rock element concerned hence we distinguish two states in our medium the unbroken viscoelastic rheology allowing for a slight attenuation and the broken state similar to a viscous fluid the viscosity allows introduction of a kind of dynamic friction at the broken point when failure takes place at a point elastic stress is then locally released and radiates sometimes causing rupture of neighbor points and localization of fracture along fissures or faults this gives realistic synthetic seismograms and source slip history

brines in the carboniferous sydney coalfield atlantic canada abstract formation waters within upper carboniferous sandstones in the sub sea prince and phalen coal mines nova scotia originated as residual evaporative fluids probably during the precipitation of windsor group lower carboniferous salts which underlie the coal measures salinity varies from 7800 to 176 000 mg l and the waters are na ca cl brines enriched in ca sr and br and depleted in na k mg and so4 relative to the seawater evaporation curve br cl and na cl ratios suggest that the brine composition corresponds to an evaporation ratio of as much as 30 the brines lie close to the meteoric line on h o isotopic plots but with a compositional range of 18o from 4 18 to 6 99 and of d from 42 4 to 23 5 distant from modern meteoric or ocean water mine water composition contrasts with that of nearby salt spring brines which are inferred to have originated through dissolution of windsor group evaporites by modern meteoric waters however a contribution to the mine waters from halite dissolution and from br in organic matter cannot be ruled out present concentrations of several elements in the brines can be explained by water rock interaction the original windsor brines probably moved up into the overlying coal measure sandstones along faults prior to the late triassic the high salinity and irregular salinity distribution in the phalen sandstones suggests that the brines have undergone only modest dilution and are virtually immobile in contrast prince waters show a progressive increase in salinity with depth and are inferred to have mixed with surface waters basinal brines from which these modern formation fluids were derived may have been important agents in base metal and ba mineralisation from the mid carboniferous onwards as saline fluid inclusions are common in zn pb sulphide deposits in the region

hydrological processes in the tyrrell basin southeastern australia abstract the tyrrell basin in semi arid southeastern australia provides an ideal setting where saline acid groundwaters evaporites redbeds syngenetic sulphides and thick groundwater brine bodies are all present and in many instances are actively forming today the relatively flat landscape and the associated regional groundwater systems with their concomitantly low hydraulic gradients create a setting in which is developed a clearer than normal aerial separation of a wide range of complex interwoven physico chemical processes detailed work in the tyrrell basin over a number of years has enabled the establishment of a hydrogeological framework which forms the basis for unravelling otherwise complicated hydrochemical interactions between groundwater systems and salt lakes indeed the interactions between groundwater flow systems and the regional groundwater discharge landscape in general and the spring zones of salt lakes in particular have important implications bearing upon the solution migration deposition and concentration of salts and metals by regional groundwater systems in arid and semi arid areas

seawater sr isotope variations through time a procedure for constructing a reference curve to date and correlate marine sedimentary rocks a new curve of seawater 87 sr 86 sr variation through the past 450 m y enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties we discuss the method by which this curve was constructed strictly standardized 87 sr 86 sr ratios and biostrat igraphic data for 1300 87 sr 86 sr analyses of marine minerals were input into a data base objective criteria were used to assign each data point a reliability score these scores were used as initial weights in a robust nonparametric regression lowess to obtain a curve of seawater 87 sr 86 sr variation with rigorous 95 confidence intervals for prediction of age uncertainty precise dating and stratigraphic correlation are feasible for much of the phanerozoic the main problem with future sr isotope stratigraphy is likely to be sample quality we discuss guidelines for sample selection

the brine underlying the oak ridge reservation tennessee usa characterization genesis and environmental implications abstract the deep hydrogeologic system underlying the oak ridge atomic reservation contains brine its origin was assessed using existing and newly acquired chemical and isotopic data the proposed model which best fits the data is modification of residual brine from which halite has been precipitated potential sources for this brine are in the appalachian mountains in the east where bedded halite is documented and where the hydraulic gradient needed to move the residual brine exists other models such as ultrafiltration and halite dissolution were also evaluated in places contaminants such as radionuclides heavy metals nitrates and organic compounds have reached the deep system the chemical and isotopic features of the brine were examined with respect to its potential discharge contaminated in places into shallow freshwater systems the observations suggest that whereas the origin of the salts in the brine may be very old influx of recent water takes place consequently the brine is not isolated in terms of recharge and discharge from the overlying active and fresh water bearing units

geochemistry and origin of formation brines from the paris basin france 2 saline solutions associated with oil fields abstract oil fields from the paris basin are located in three geological levels dogger middle jurassic rhaetian and keuper upper triassic the origin of water and dissolved salts in saline solutions associated with oil is investigated using the hypothesis that cl and br are conservative in solution in a diagram of cl br ratios vs cl the points of the dogger and the keuper aquifers lie in the zone of primary brines whereas those of the rhaetian are located in the zone of secondary brines the na cl ratio is very high in all of the saline solutions suggesting either a relatively poorly evolved brine at the beginning of halite deposition or some halite dissolution all of the saline solutions especially those from the dogger aquifer are clearly diluted dilution by meteoric waters would not alter the cl br ratio high concentrations of lithium rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures of secondary brines formed by dissolution of halite and small amounts of sylvite and ca sulphate mass balance calculations of the excess of cl with respect to na k allows the fraction x of water derived from the primary brine to be calculated assumptions are that all na and k is derived from the secondary brine and that the cl content of the primary brine is constrained by bischofite saturation at 11 000 mmol kg 1 the typical proportion of water from the primary brine waters is 0 7 in the dogger formation and 2 4 in the keuper values of the concentration factor cf expressed as the ratio of the molal concentration of a conservative ion in the evolved brine to its concentration in seawater have been calculated for lithium solutions from the dogger and keuper aquifers have cfli values of 1 8 103 and of 7 7 103 respectively in the rhaetian aquifer the linear relationship between heavy isotope contents of the brine 2h and 18o and the cl content indicates a mixture of two water sources the first one is meteoric water as defined by the intersection of the formation water line with the global meteoric water line 2 h 65 18 o 9 3 the other end member is enriched in heavy isotopes and in cl and is attributed to seawater that has dissolved evaporites mainly halite however the presence of small amounts of an extremely evolved brine is required to account for the bromide lithium rubidium and boron contents of the formation brine from the rhaetian aquifer as this contribution is very limited in amount it cannot affect the heavy isotope content of the formation water mass balance equations are solved for ternary mixtures of meteoric water seawater and brine the same constraints as above no na and k supplied by the primary brine and clbrine 11 000 mmol kg 1 are applied it is also assumed that the seawater component has the same ionic contents as modern seawater and that the salt content of the meteoric component is negligible the fraction of primary brines is x 1 4 according to the cl balance this estimate agrees with that obtained from the lithium balance assuming that the primary brine component was the same as that of the keuper formation water mass balance calculations also indicate that the so42 content of the rhaetian formation water is exclusively due to the secondary brine component in the three saline solutions the very high ca2 contents and high ca 2 mg 2 ratios may be attributed to reaction of the very evolved brine free of so2 4 with gypsum or anhydrite the result is an exchange of mg2 for ca2 which causes a release of ca2 to the solution and a precipitation of secondary mg sulphate this process can also account for the high strontium contents of the analysed samples values of the ratio 87 sr 86 sr are very high in the keuper formation water due to close contact of the solutions with the granitic basement or with granite derived detrital deposits values in the rhaetian and dogger aquifers equal those of upper triassic marine deposits the stable isotope 34s and 18o content of the dissolved aqueous so2 4 indicates a triassic origin for the bulk of sulphate involved in the evolution of formation waters from the rhaetian and from the dogger all geochemical indicators thus show a vertical migration of triassic brines however aqueous so2 4 from the keuper has a permian isotopic signature suggesting a reworking in solution of salts from this period

a stochastic model for fracture growth on a heterogeneous seismic fault summary we present a model for the propagation of seismic fracture on a heterogeneous fault on which the strength and applied stress vary locally along the fault plane in a random fashion the propagation of fracture is controlled by three macroscopic parameters the average of the difference between the applied stress and fault strength a measure of fault heterogeneity and the average coupling between elementary faults evolution of fracture can then be represented by an epidemics model whose domains of slip are obtained from percolation theory the geometry of these domains contains the principal features of both barrier and asperity models

a study of fluid inclusions in sulfide and nonsulfide mineral phases from a carbonate hosted zn pb deposit gays river nova scotia canada this paper reports on a detailed fluid inclusion study of synore sphalerite calcite and postore calcite fluorite quartz barite mineral phases at the gays river zn pb deposit 2 4 mt 8 6 zn 6 3 pb nova scotia canada the deposit is hosted by dolomitized visean carbonate rocks in the basal part of the windsor group that formed during a marine transgression on terrestrial clastics of the tournaisian horton group mineralization is dominantly of replacement style with lesser porosity infilling and mainly occurs in the fore reef of a carbonate bank the shallow water equivalent of the laterally extensive laminated algal and bituminous macumber formation paragenetically constant volume dolomitization of the host rock is followed by euhedral manganiferous dolomite cement then sphalerite and galena syn to postore calcite with trace amounts of barite fluorite and quartz occlude the remaining porosity fluid inclusions types include 1 aqueous l rich or solids including halite 2 methane 3 h 2 o co 2 4 monophase l and v types and 5 liquid petroleum or aqueous phase type 1 inclusions are most abundant types 2 and 3 are rare and type 5 are postore type 4 inclusions are mostly related to postentrapment changes e g necking thermometric data indicate that the mineralizing fluids were high salinity nacl cacl 2 h 2 o brines with 20 to 28 wt percent nacl equiv hydrohalite ice melting relationships indicate considerable variation in the nacl nacl cacl 2 ratio of the fluids 0 2 1 lower salinity fluid inclusions 0 16 wt nacl equiv are restricted to postore calcite and barite combined sem eds analyses of decrepitate mounds identify na ca and mg as the major solute components homogenization temperatures t h for all mineral phases range from 70 degrees to 250 degrees c but ranges of or 10 degrees to 20 degrees c occur within isolated groups of inclusions homogenization data on liquid petroleum inclusions and associated aqueous inclusions in fluorite are 120 degrees to 150 degrees c whereas three petroleum inclusions in syn or postore calcite have t h values of 53 degrees 53 degrees and 167 degrees c bulk crush gas chromatography indicates that inclusion fluids contain up to 1 4 mole percent combined co 2 and ch 4 with the most abundant condensable gases in sphalerite and galena since the high pressures or 2 000 bars required to retain this gas as dissolved species in the fluid are incompatible with the local stratigraphy it is inferred that the gas was either picked up along the fluid path and transported as an immiscible phase or was produced locally in the carbonate bank by thermal degradation of organic material the data are interpreted to indicate that a high temperature or 250 degrees c saline ca 25 wt nacl metalliferous fluid migrated into the carbonate bank where it reacted with reduced sulfur generated by thermochemical sulfate reduction and mixed with an equally saline but lower temperature fluid the variability of the nacl nacl cacl 2 ratio of the fluids suggests contamination by dissolution of the host dolostone this dissolution may also have provided some of the co 2 component of the gas the pressure of the mineralizing environment is constrained at ca 400 bars by the presence of liquid petroleum aqueous and methane inclusions and their respective isochoric projections the high homogenization temperatures for the mineralizing fluids may reflect the regional setting of the deposit that is it is located on the margins of a large stable craton in an area that was structurally active during the carboniferous and within which widespread thermal disturbances are known the high temperatures also suggest affinities with the irish carbonate hosted base metal deposits

simulation of the frictional stick slip instability a lattice solid model capable of simulating rock friction fracture and the associated seismic wave radiation is developed in order to study the origin of the stick slip instability that is responsible for earthquakes the model consists of a lattice of interacting particles in order to study the effect of surface roughness on the frictional behavior of elastic blocks being rubbed past one another the simplest possible particle interactions were specified corresponding to radially dependent elastic brittle bonds the model material can therefore be considered as round elastic grains with negligible friction between their surfaces although breaking of the bonds can occur fracturing energy is not considered stick slip behavior is observed in a numerical experiment involving 2d blocks with rough surfaces being rubbed past one another at a constant rate slip is initiated when two interlocking asperities push past one another exciting a slip pulse the pulse fronts propagate with speeds ranging from the rayleigh wave speed up to a value between the shear and compressional wave speeds in agreement with field observations and theoretical analyses of mode ii rupture slip rates are comparable to seismic rates in the initial part of one slip pulse whose front propagates at the rayleigh wave speed however the slip rate is an order of magnitude higher in the main part of pulses possibly because of the simplified model description that neglected intrinsic friction and the high rates at which the blocks were driven or alternatively uncertainty in slip rates obtained through the inversion of seismograms particle trajectories during slip have motions normal to the fault indicating that the fault surfaces jump apart during the passage of the slip pulse normal motion is expected as the asperities on the two surfaces ride over one another the form of the particle trajectories is similar to those observed in stick slip experiments involving foam rubber blocks bruneet al 1993 additional work is required to determine whether the slip pulses relate to the interface waves proposed by brune and co workers to explain the heat flow paradox and whether they are capable of inducing a significant local reduction in the normal stress it is hoped that the progressive development of the lattice solid model will lead to realistic simulations of earthquake dynamics and ultimately provide clues as to whether or not earthquakes are predictable

paleomagnetism of the gays river zinc lead deposit nova scotia pennsylvanian ore genesis the gays river zinc lead deposit is the largest known mississippi valley type deposit in the mississippian windsor group of atlantic canada thermal and af demagnetization show that both ore and host rock have very similar magnetic characteristics stability tests constrain the magnetization to be mid mississippian to late pennsylvanian the pole position from the ore and host rock sites falls next to the pennsylvanian apw path suggesting an 8 clockwise rotation of the meguma platform relative to the craton regional thermal history and geochemical evidence suggest that the zinc lead ore magnetization and host rock remagnetization are coeval

diagenetic history of eocene wilcox sandstones south central texas abstract sandstones and shales of the lower eocene wilcox group extend from outcrop to depths greater than 5 km where temperatures exceed 200 c eight authigenic minerals have been identified quartz kaolinite calcite ankerite and albitized plagioclase are volumetrically significant whereas chlorite illite albite overgrowths and dolomite are present only in trace amounts quartz kaolinite and ankerite cements are present over the entire depth range sampled 1528 to 4446 m calcite cement occurs only in sandstones shallower than 3300 m albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m no systematic variations in the chemical composition of calcite with depth no zoning within calcite or ankerite and no compositional differences between pore filling and grain replacing calcite or ankerite were found calcite cements have 87 sr 86 sr ratios generally less than 0 708 whereas ankerite cements are significantly enriched in 87sr the oxygen isotopic composition of carbonate cement ranges between 9 and 14 pdb for calcite and between 8 and 12 for ankerite with no systematic variation with depth calculated 18o values for authigenic quartz and kaolinite range from 22 to 31 smow and from 14 to 17 respectively volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation secondary porosity generation or smectite to illite transformation in encasing shales the isotopic composition of strontium in calcite cement shows most of it to be of eocene or older marine carbonate origin ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone pore water system

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

chemical osmosis reverse chemical osmosis and the origin of subsurface brines abstract calculations using recently tabulated values of density and osmotic coefficient for nacl h2o indicate that overpressuring is more than adequate to overcome chemical osmosis and drive reverse chemical osmosis in sedimentary sequences the best demonstrated overpressuring mechanism is the rapid deposition of fine grained sediments the dehydration of gypsum contributes to overpressuring for brief time intervals at shallow depths whereas water evolved during the protracted conversion of smectite to illite is probably a subordinate but continuing contributor to overpressuring at greater depth occurrences of overpressuring in sedimentary sections older than cretaceous indicate that post depositional mechanisms such as tectonic compression and aquathermal pressuring must also operate the latter may be of major importance in geothermal areas with adequate low permeability seals and a nontrivial contributor in areas of normal geothermal gradient because of shales that sharply decrease normal fluid flow the strongest arguments for the importance to present day brine compositions of membrane concentration of sea water solutes are 1 1 the correlation of d values of water molecules of pore fluid with those of local meteoric water 2 2 the need for major sources of mg2 and cl in apparently evaporite free basins even where dissolution of halite is a major contributor of solute reverse chemical osmosis still operates to leak relatively dilute water of the associated diagenetic chemical reactions that of mg2 with limestone to form dolomite is particularly effective in generating concentrated cl brines rich in ca2 it decreases the concentration of mg2 increases that of ca2 and decreases those of both so42 and co32 by precipitating caco3 and caso4 because of the ca2 common ion effect

origin and evolution of formation waters from oil fields on the norwegian shelf abstract stable isotopes of h and o and chemical analyses of formation waters from upper triassic jurassic clastic reservoirs and cretaceous carbonate reservoirs offshore norway are employed to interpret the origin and evolution of these waters in both types of reservoir the formation waters become progressively enriched in total dissolved solids and the heavy isotope of o with increasing depth whereas deuterium varies erratically between 20 and 40 smow bromine analyses and stable isotope data indicate that the formation waters are mixtures of a brine end member formed by evaporation of sea water beyond the point of halite saturation and meteoric water the chemical and isotopic composition of the formation waters and general geological considerations suggest that the brines originated as connate waters in permian evaporites and were later expelled into overlying strata there is a disequilibrium stable isotope distribution between the bulk chalk formations and the formation waters and only minor diagenetic modification of the formation waters appears to have taken place in these reservoirs the formation waters from the clastic reservoirs on the other hand are modified by diagenetic reactions