strontium and carbon isotope constraints on carbonate solution interactions and inter aquifer mixing in groundwaters of the semi arid murray basin australia abstract strontium and carbon isotopes are used to study the effect of carbonate mineral dissolution and inter aquifer mixing on the chemical evolution of groundwater from the murray group limestone aquifer in semi arid se australia the 87 sr 86 sr ratio of groundwater evolves from 0 7097 at the basin margin to a less radiogenic value of 0 7084 about 250 km down gradient which is similar to the 87 sr 86 sr ratio of the aquifer carbonate matrix the concomitant increase in 13 c dic sr ca and mg ca ratios in the groundwater along a 250 km transect suggests that incongruent dissolution of high mg calcite controls the carbonate geochemistry in this aquifer further down gradient the groundwater is characterized by a relatively more radiogenic 87 sr 86 sr ratio caused by upward leakage and mixing with more radiogenic groundwater from the renmark group sand aquifer a mixing model using 87 sr 86 sr ratio suggests that the fraction of water that contribute to the murray group aquifer through upward leakage from underlying renmark group aquifer ranges from 15 to 85

strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

microstructural changes accompanying the opal a to opal ct transition new evidence from the siliceous sinters of geysir haukadalur iceland opaline silica opal a has formed in marine lacustrine and geothermal environments throughout geological time during diagenesis opal a normally changes to opal ct then opal c and finally to quartz such changes commonly destroy the original fabrics and any fossils that opal a contained the physical changes that accompany the opal a to opal ct transition however are known poorly x ray diffraction analyses electron microprobe analyses and high resolution high magnification scanning electron microscope imagery of siliceous sinters from the geysir geothermal area in iceland show that opal a is formed of heterometric arrays of randomly packed microspheres up to 5 m diameter with neighbouring spheres commonly being joined by small connection pads in contrast enlarged spheres lepispheres inverse opal two types and spindle frameworks with hexagonal motifs characterize opal ct the textures in opal ct which vary on a microscale reflect the complex interplay between dissolution e g inverse opal and precipitation e g enlarged spheres spindle frameworks that probably was mediated by groundwater in a near surface environment the processes deciphered from these young rocks should however be applicable to sedimentary opal a and opal ct of all ages irrespective of their origin

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

physico chemical and environmental controls on siliceous sinter formation at the high altitude el tatio geothermal field chile abstract el tatio geothermal field is located 4270 m above sea level in the altiplano northern chile siliceous sinter deposits from el tatio were studied to understand the influence of water chemistry and the extreme climatic conditions on their textures and mineralogy the results of this study show that the mineralogy of el tatio sinters include of opal a and accessory minerals such as halite gypsum and cahnite ca 4 b 2 as 2 o 12 4h 2 o which precipitate by full evaporation of high arsenic boron and calcium thermal waters el tatio sinters show a high degree of structural disorder probably linked to cation incorporation into the silica structure and or the occurrence of micro to nano scale accessory minerals the high content of cations in the thermal waters is strongly tied to relatively high silica precipitation rates considering silica concentration in water 147 285 mg l sio 2 precipitation rate reach 2 5 kg m 2 per year based on in situ precipitation experiments the particular environmental conditions of this high altitude geothermal area that produce high water cooling rate and high evaporation rate may also be responsible for the fast silica precipitation low environmental temperatures create freezing related sinter textures i e silica platelets and micro columns ridges silicified microbial filaments are also characteristic of el tatio sinters and they are indicative of water temperature and hydrodynamic conditions at the moment of sinter formation however sinter textural interpretation in a high altitude andean context must be done carefully as specific relationships between microbial and hydrodynamic textures are found

application of fourier transform infrared spectroscopy to silica diagenesis the opal a to opal ct transformation abstract an important goal in silica diagenesis research is to understand the kinetics of opal transformation from noncrystalline opal a to the disordered silica polymorph opal ct because the conventional technique for monitoring the transformation powder x ray diffraction xrd is applicable only to phases with long range order we used fourier transform infrared spectroscopy ftir to monitor the transformation we applied this technique combined with xrd and tem to experimental run products and natural opals from the monterey formation and from siliceous deposits in the western pacific ocean using a ratio of two infrared absorption intensities i472 cm 1 i500 cm 1 the relative prop rtions of opal a and opal ct can be determined the progress of the transformation is marked by changes in slope of vs depth or time when a sufficient stratigraphic profile is available there are three stages in the opal a to opal ct reaction 1 opal a dissolution 2 opal ct precipitation whose end point is marked by completion of opal a dissolution and 3 opal ct ordering during which tridymite stacking is eliminated in favor of cristobalite stacking

piggyback basin in the sevier orogenic belt utah implications for development of the thrust wedge timing data on a frontal thrust system indicate that the sevier thrust wedge of central utah advanced rapidly to its ultimate structural front in 30 m y between early albian and campanian time it then persisted by out of sequence thrusting for an additional 25 m y until the early eocene the key to the history of the frontal thrust is a piggyback basin that formed on the hanging wall unconformity bounded sedimentary units within the basin thin both westward and eastward onto structures generated by thrust faulting up to 860 m of upper campanian to lower eocene strata accumulated between these structures which consist of a ramp anticline in the west and a thrust duplex in the east the newly recognized interaction of coeval deposition and thrust deformation permits revised limits on the inception and longevity of the frontal thrust system

geochemical evolution of groundwater in the carbonate aquifers of sierra de segura betic cordillera southern spain summary sierra de segura betic cordillera with a total area of over 3000 km 2 is the source of the two principal rivers in southern spain the guadalquivir and the segura due to the orographic effect of these mountains precipitations are considerably more abundant than in nearby lowland areas where the climate is semi arid sierra de segura is constituted of mesozoic and cenozoic sedimentary rocks among which there are thick limestone dolomitic formations which have given rise to extensive outcrops of permeable materials in geomorphological terms there is a large plateau intensively karstified that constitutes the main recharge area discharge takes place via a large number of springs of which the 50 most important add up to a mean spring flow of about 13 500 l s the active geochemical processes in aquifers of sierra de segura with their corresponding time sequence are dissolution of co 2 dissolution of calcite incongruent dissolution of dolomite dedolomitization exsolution of co 2 and precipitation of calcite more evolved water has higher temperature magnesium content and mg ca ratio therefore these parameters can be utilised as indicators of the degree of hydrochemical evolution in addition a good correlation between water temperature and magnesium concentration or mg ca ratio indicates that an increase in temperature accelerates the kinetics of the dissolution of dolomite finally the distribution of the temperatures in the vadose zone determined by atmospheric thermal gradient implies an apparent stratification of the predominant hydrochemical processes and of the groundwater physical and chemical characteristics

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

seasonal variations in the geochemistry of shallow groundwater hosted in granitic till the groundwater chemistry in a small catchment in northern sweden has been studied for a period of 15 months including two snowmelt events the groundwater was sampled from two wells in a slope

early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis most ancient organic microfossils delicately preserved in 3d have been found in cherts although entombment within silica has been shown to promote morphological preservation the impact of early silicification on the molecular evolution of fossilized microorganisms during burial remains poorly constrained here we report results of advanced fossilization experiments performed under pressure 250 bars and temperature 250 c conditions typical of sub greenschist facies metamorphism for different durations up to 100 days on microorganisms experimentally entombed or not within a silica gel the experimental residues have been characterized using xrd and xanes spectroscopy the present study demonstrates that entombment within silica limits the degradation of microorganism molecular signatures likely through specific chemical interactions despite the progressive conversion of silica into quartz during the experiments extrapolation of the present results suggests that such protection may persist during geological timescales the present experimental study provides molecular evidence that in addition to morphologies cherts may support the chemical preservation of remains of ancient life the present results thus constitute a step forward towards the reconstruction of the original chemistry of pu tative fossilized microorganisms

amorphous silica maturation in chemically weathered clastic sediments abstract a detailed understanding of silica postdepositional transformation mechanisms is fundamental for its use as a palaeobiologic and palaeoenvironmental archive amorphous silica opal a is an important biomineral an alteration product of silicate rocks on the surface of earth and mars and a precursor material for stable silica phases during diagenesis amorphous silica gradually and gradationally transforms to opal ct opal c and eventually quartz here we demonstrate the early stage maturation of several million year old opal a from deeply weathered early cretaceous and ordovician sedimentary rocks of the great artesian basin central australia x ray diffraction scanning electron microscopy and electron probe microanalyses show that the mineralogical maturation of the nanosphere material is decoupled from its chemical properties and begins significantly earlier than micromorphology suggests non destructive and locally highly resolved x ray microdiffraction xrd2 reveals an almost linear positive correlation between the main peak position 3 97 to 4 06 a and a new asymmetry parameter ap heating experiments and calculated diffractograms indicate that nucleation and growth of tridymite rich nanodomains induce systematic peak shifts and symmetry variations in diffraction patterns of morphologically juvenile opal a our results show that the asymmetry parameter traces the early stage maturation of amorphous silica and that the mineralogical opal a ct stage extends to smaller d spacings and larger fwhm values than previously suggested

transport of radionuclides in an unconfined chalk aquifer inferred from u series disequilibria abstract u series disequilibria measured in waters and rocks from a chalk aquifer in france have been used as an analog for long term radionuclide migration drill core samples from a range of depths in the vadose zone and in the saturated zone as well as groundwater samples were analyzed for 238 u 234 u 232 th and 230 th to determine transport mechanisms at the water rock interface and to quantify parameters controlling the migration of radionuclides isotope measurements in rocks were done by tims whereas 234 u 238 u and 230 th 232 th activity ratios in water samples were measured by multi collector icp ms both depletion and enrichment in 234 u relative to 238 u were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water table oscillation respectively the correlation between 230 th 232 th activity ratios and 87 sr 86 sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples water samples are all characterized by 234 u 238 u 1 resulting from recoil effect of 234 th groundwaters are characterized by a more radiogenic signature in 87 sr 86 sr than the rocks moreover 230 th 232 th activity ratios in the waters are lower than in the rocks and increase with distance from the water divide which suggests that th transport is controlled by colloids formed during water infiltration in the soil a 1 d transport model has been developed in order to constrain the u series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time dependent composition for the solid phase this model permits a prediction of the time scale of equilibration of the system and an estimation of parameters such as weathering rate distribution coefficients and recoil fractions retardation factors of 10 35 and from 1 10 4 to 2 10 5 were predicted for u and th respectively and can be used to predict the migration of radionuclides released as contaminants in the environment at the scale of our watershed 32 km 2 a characteristic migration time from recharge to riverine discharge of 200 600 yr for u and 0 2 3 7 myr for th was obtained

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

structure and cenozoic kinematics of the eastern cordillera southern bolivia 21 s 1 the eastern cordillera is a main morphotectonic unit of the central andes in southern bolivia it is predominantly composed of ordovician sedimentary rocks as thick as 10 km revision of the ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections improved estimates of crustal shortening and a new kinematic model for the eastern cordillera new estimates of total shortening in the eastern cordillera are up to 95 km 41 44 higher than previous estimates and sufficient to thicken the andean crust to its present state between the eastern foreland and the magmatic arc after initial tertiary uplift at 58 ma first thrusts developed in the middle eocene to early oligocene 40 30 ma exhuming the central part of the eastern cordillera tectonic shortening culminated in late oligocene and early miocene time 25 20 ma when west facing thrust systems developed in the central and western part of the eastern cordillera thrusting terminated in the western part by the end of the early miocene and middle miocene tectonic activity was restricted to minor strike slip faulting in the central part thrusting remained active until the late miocene by 10 ma shortening in the eastern cordillera essentially ceased kinematic fault analyses indicate clockwise rotation of the shortening direction from nne ssw or ne sw in eocene early oligocene time over e w to wnw ese or even nw se in late oligocene to middle miocene time nevertheless wnw ese directed shortening and subvertical extension predominate sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the bolivian orocline it is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along ne trending transfer zones

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

tracing the continental diagenetic loop of the opal a to opal ct transformation with x ray diffraction abstract opaline silica forms in a variety of continental deposits on the earth s surface the maturation from opaline silica to quartz is traditionally traced with x ray diffraction that suggests a gradual phase transformation and structural ordering however the identification and categorization of the gradual opal a to opal ct transformation remains challenging due to potential mineral impurities and the transient structure of opaline silica here we use x ray powder diffraction to identify peak parameters that characterize the maturation from opal a to opal ct in continental settings we analyzed inclusion free opals from non sinter continental deposits and the products of opal mixing and heating experiments twenty five samples were analyzed from australia ethiopia slovakia usa mexico and madagascar and represent opal from a range of host rocks and formation conditions xrd analysis reveals that opal peak parameters constitute a diagenetic loop and that the structural order is decoupled from the low tridymite cristobalite ratio at large d values systematic changes of the width and position of a subsidiary peak at ca 2 5 a accurately reflect the low tridymite cristobalite ratio and degree of ordering our results suggest that the identification of individual stages in the opal a to opal ct transformation sequence requires the application of multiple peak parameters that complement or replace existing maturation indicators this information is essential for a precise silica identification and genetic reconstruction especially of open continental systems with internal re distribution and external input of aqueous silica

isotopic and noble gas study of chalk groundwater in the london basin england abstract the chemical and isotopic composition of groundwater from 52 sites in the london u k area was determined as part of a project aimed at assessing the spatial variation in the age of chalk groundwater and in determining the relationship between fracture and matrix groundwater in this dual porosity system systematic changes in groundwater chemistry take place in the downgradient direction in response to several chemical processes these processes include early concentration by evaporation and congruent dissolution of calcite followed by widespread incongruent dissolution and ion exchange in addition to local oxidation reduction reactions gypsum dissolution and saline intrusion as a result of the above processes chalk groundwater follows an evolutionary path from ca bicarbonate type to na bicarbonate type the age of chalk groundwater was modelled using14c 13c 3h 2h and 180 there is a general increase in the groundwater age in a downgradient direction with the oldest water found in n central areas of the basin groundwater in the unconfined zones and in areas s of the greenwich fault is almost entirely of unevolved modem composition carbon 14 modelling suggests that chalk groundwater in the s basin is generally less than 10000 a old while that in the north is generally between 10000 and 25000 a old the presence of3h in concentrations of up to 7 tu in groundwater which yields ages of several 1000 a however indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer results of palaeorecharge temperature determinations using 2h 180 and noble gas analytical results suggest that significant devensian recharge did indeed occur in the aquifer a model of the development of the chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow occurs predominantly in the fracture system the upper 50 m of the aquifer was flushed with fresh water during the 2 3 106 a of the quaternary and therefore meteoric water largely replaced the tertiary and cretaceous marine water that previously saturated the system most processes which control the chemistry of the groundwater occur in the matrix where the surface area is exceptionally high although fracture flow dominates the flow regime diffusion from the matrix into the fracture porosity controls the chemistry of chalk groundwater

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

effects of carbon dioxide variations in the unsaturated zone on water chemistry in a glacial outwash aquifer abstract the research site at otis air base cape cod massachusetts has been developed for hydrogeological and geochemical studies of sewage effluent contaminated ground water since 1982 research of hydrologic properties transport and chemical and biological processes is ongoing but the origin of background water chemistry has not been determined the principal geochemical process giving rise to the observed background water chemistry is co 2 controlled hydrolysis of na feldspar geochemical modeling demonstrated that co 2 sources could vary over the project area analyses of unsaturated zone gases showed variations in co 2 which were dependent on land use and vegetative cover in the area of groundwater recharge measurements of co 2 in unsaturated zone gases showed that concentrations of total inorganic c in recharge water should range from about 0 035 to 1 0 mmoles l in the vicinity of otis air base flux of co 2 from the unsaturated zone varied for 4 principal land uses ranging from 86 gc m 2 yr for low vegetated areas to 1630 gc m 2 yr for a golf course carbon dioxide flux from woodlands was 220 gc m 2 yr lower than reported fluxes of 500 to 600 gc m 2 yr for woodlands in a similar climate carbon dioxide flux from grassy areas was 540 gc m 2 yr higher than reported fluxes of 230 to 490 gc m 2 yr for grasslands in a similar climate

morphologic and mineralogic transitions from opal a to opal ct in low temperature siliceous sinter diagenesis taupo volcanic zone new zealand abstract the opal a to opal ct silica phase transformation in new zealand s siliceous hot spring deposits sinter is accompanied by recurring textural changes at the micron scale recognition of these changes is essential to extract paleohydrological paleoenvironmental and paleobiological signatures from ancient hydrothermal systems and to recognize diagenetic overprints upon primary depositional signals we examined 39 samples of siliceous sinter from the low temperature 35 c palisade microfacies at orakei korako and te kopia geothermal fields taupo volcanic zone new zealand in three different postdepositional environments least disturbed heat affected weathered friable to indurated samples were collected from modern presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs to sinter 3 500 years old the progressive mineralogical change from noncrystalline opal a to paracrystalline opal ct was traced by x ray powder diffraction and scanning electron microscopy one diagenetic sequence with two morphological pathways spherical and smooth polymeric and monomeric deposition respectively was identified at the micron scale both pathways ultimately lead to formation of typical opal ct bladed lepispheres along the spherical pathway initial morphological restructuring includes formation of circular holes 0 1 to 1 0 m in diameter in opal a spheres 3 0 m in diameter these changes correlate with a shift in the maximum intensity of the opal a x ray scattering broadband from 4 0 a to 4 09 a the characteristic position for opal ct an early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal a ct scattering broadband that is sharp peaked in comparison and centered at 4 09 a a still sharper peaked composite opal ct a x ray diffraction band follows which has developed an incipient tridymite shoulder it is found in samples that display both hexagonal platelets and incipient fuzzy opal ct lepispheres at this late transitional stage patchy replacement textures are visible at the macroscale only with the appearance of numerous well developed opal ct bladed lepispheres is a typical sharp peaked 4 09 a opal ct x ray trace evident with a well defined tridymite shoulder this diagenetic process can produce either porous or vitreous fabrics depending on the degree to which infill of silica has reduced porosity and increased density of the deposit this study provides a textural mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications these modifications continue long after geothermal activity has ceased with little or no burial and occur in horizons or localized patches of a given deposit moreover postdepositional conditions such as heat overprinting or weathering influence the diagenesis of siliceous sinter by increasing its maturation rate

on the strength of interplate coupling and the rate of back arc convergence in the central andes an analysis of the interseismic velocity field 1 we interpret the interseismic crustal velocity field of the central andes using a simple three plate model in which the andean mountain belt is treated as a rigid microplate located between the nazca and south american soam plates we assume that the euler vectors associated with these plates are strictly coaxial and that the surface velocity field can be decomposed into an elastic loading field driven by locking of the main plate boundary and a smaller contribution associated with back arc convergence we obtain our best fit to the geodetic velocities if the main plate boundary is fully 100 locked between depths of 10 50 km and 8 5 of nazca soam plate convergence is achieved in the back arc by underthrusting of the brazilian shield beneath the subandean zone

residence time indicators in groundwater the east midlands triassic sandstone aquifer the east midlands triassic sherwood sandstone aquifer which has been the subject of detailed radiometric age studies is used to investigate both inert and reactive constituents of groundwater as indicators of residence time detailed resampling of the aquifer in 1992 has provided a considerable body of new inorganic geochemistry data though without radiocarbon several inert indicators are defined including the isotopic ratios 18o 2h 36cl noble gas ratios as well as the halogen elements cl br f i and their element ratios these form a group of essentially unreactive tracers primarily reflecting changing rainfall inputs and palaeoclimatic conditions except at outcrop where human impacts are also seen clearly the concentrations of cl mainly from atmospheric sources remain below 25 mg l 1 cl over a distance of some 30 km from outcrop reactive indicators the result of time dependent water rock interactions include 13c mg ca sr ca na cl and show diagnostic trends along the flow lines however the concentrations of certain trace elements li rb cs mn and mo which are not limited by solubility constraints show linear trends along the present day flow gradient this water rock interaction is taking place in groundwaters with low total mineralisation and it can be demonstrated that reactions involving these elements and isotopes are occurring entirely within the aquifer since high salinity groundwaters are found below the sherwood sandstone there is good correlation between some of the elements and 14c activities in the 1977 data set and this has been used to derive a concentration age relationship for the 1992 set of data a chemical timescale for the aquifer is then established using a li and b a combination of five trace elements the age of the fresh groundwater is thus shown to be up to 100 ka bp indicating the likelihood of semi continuous recharge during the devensian glacial period preceding the glacial maximum when no recharge occurred 10 20 ka bp this approach may be of value in extending groundwater dating beyond the radiocarbon timescale as well as interpreting sites where no radiocarbon data are available the scatter from the mean age line indicates those groundwaters which are derived from either rapid or slower than average flow zones within the aquifer as well as age stratification this stratification is borne out by a depth profile of groundwater from a new borehole near to outcrop which is shown by a combination of chemical and isotopic tracers to contain holocene overlying late pleistocene water

delimitation of a cryptic eocene tectono thermal domain in the eastern cordillera of the bolivian andes through k ar dating and 40ar 39ar step heating new k ar ages and 40 ar 39 ar age spectra for samples from granitoid rocks and meta sediments of the cordillera real bolivia reveal a complex mid tertiary tectono thermal history for this segment of the central andean eastern cordillera previous radiometric dating of the cordillera real intrusions defined two areally distinct plutonic domains in the n upper triassic lower jurassic in the s upper oligocene lower miocene our new k ar ages many from deformed intrusions fall between these two episodes for the variably foliated zongo massif there is a marked discordance between biotite and muscovite k ar dates each decreasing consistently from sw to ne across this 8 km wide plutonic centre the age gradients are developed within the foliated and unfoliated facies of the zongo intrusions but do not continue ne into the palaeozoic meta sedimentary rocks for which k ar dates are erratic 40 ar 39 ar step heating analyses of muscovites and most biotites from granitoid rocks reveal that the oldest and youngest micas yield plateaux 210 and 39 ma whereas those with intermediate ages exhibit disturbed spectra a plateau date for a biotite from the ne margin of the zongo massif shows that final cooling through c 300 c occurred in the late eocene the new data reveal that a marked thermal discontinuity existed 39 ma ago at the ne contact of the zongo intrusion additional discordant dates for the yani and illampu intrusions to the nw indicate that the discontinuity was of regional scale there is no direct relationship between this tectono thermal event which affected at least 2000 km 2 and the upper palaeozoic foliation of several of the plutons involved and thus it is cryptic on outcrop and microscopic scales we propose that the thermal discontinuity resulted from late eocene crustal shortening sw verging thrust faulting and attendant cordilleran uplift at the inner eastern boundary of the andean orogen coincident with the incaic deformation defined in peru

heterogeneous behaviour of unconfined chalk aquifers infer from combination of groundwater residence time hydrochemistry and hydrodynamic tools chalk groundwater is an important aquifer resource in france and also in much of northwestern europe large areas between the paris basin the london basin denmark and the northwest of germany as it is intensively exploited for human use with a large proportion reserved for drinking water the improvement of the knowledge on chalk aquifer hydrodynamical functioning and groundwater geochemistry is essential for the management of this resource to achieve this aim we developed a methodology based on a combination of hydrodynamic hydrochemistry and groundwater dating tools a study site of about 500 km2 was selected in ne france where chalk outcrops at 15 sampling points including boreholes and rivers mainly from chalk aquifer drainage groundwater geochemistry were monitored monthly for 2 years from june 2017 to june 2019 groundwater level in boreholes were monitored hourly with in situ water loggers in addition two sampling campaigns for the analysis of cfcs and sf6 have been conducted in boreholes in high and low water level periods to define the groundwater residence time the aquifer at the majority of sampling sites has an inertial behavior with respect to recharge as no impact of individual rainfall event was observed rapid piezometric response following rainfalls was observed at one site suggesting the presence of locally highly developed fracture network contrariwise high spatial and temporal heterogeneities of geochemistry was observed on the study area according to the mixing process piston flow exponential or binary mixing defined at each sampling site groundwater dating indicated rather heterogeneous ages ranging from less than 20 years to more than 50 years a combination of hydrogeological setting residence time and land use information was used to explain the hydrochemical heterogeneities highlighting the main factors governing the chalk groundwater geochemistry including water level fluctuation thickness of the unsaturated zone superficial formations distribution of fracture network aquifer river relations and human activities a conceptual model was proposed to clarify the hydrogeosystem functioning of unconfined chalk aquifers the proposed methodology can be used for the study of other worldwide aquifers chalk or not with the aim to provide appropriate recommendations in terms of water management

opaline silica in young deposits on mars high spatial and spectral resolution reflectance data acquired by the mars reconnaissance orbiter compact reconnaissance imaging spectrometer for mars crism instrument reveal the presence of h 2o and sioh bearing phases on the martian surface the spectra are most consistent with opaline silica and glass altered to various degrees confirming predictions based on geochernicall experiments and models that amorphous silica should be a common weathering product of the basaltic martian crust these materials are associated with hydrated fe sulfates including h 3o bearing jarosite and are found in finely stratified deposits exposed on the floor of and on the plains surrounding the valles marineris canyon system stratigraphic relationships place the formation age of these deposits in the late hesperian or possibly the amazonian implying that aqueous alteration continued to be an important and regionally extensive process on mars during that time

the kinematic history of the central andean fold thrust belt bolivia implications for building a high plateau this paper presents a model for the kinematic evolution of the central andean plateau based on balanced cross sections across the bolivian andes the proposed model links the formation of the andean plateau to the development of the andean fold thrust belt through the creation and propagation of two large basement mega thrusts support for large basement involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the andean fold thrust belt the structurally highest basement thrust raised folds and faults in predominantly lower paleozoic rocks of the eastern cordillera with respect to tertiary rocks in the broad internally drained basin of the altiplano to the west and east verging folds and faults in upper paleozoic rocks of the interandean zone to the east the interandean zone was in turn raised both structurally and topographically with respect to the frontal folds and faults of the fold thrust belt the subandean zone by a second structurally lower basement thrust sheet thus these two megathrusts divide the andean fold thrust belt into four areas of markedly different structural elevations the eastern cordillera can be further subdivided into two zones of west and east vergent folds and thrusts shortening accommodated by the fold thrust belt can be divided among these tectono structural zones and linked to shortening accommodated by the inferred basement megathrusts the proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed 105 km of slip into the eastern cordillera along east vergent and west vergent faults this structure also fed 90 km of eastward slip into the interandean zone the initiation and eastward propagation of a lower basement thrust structurally elevated the interandean zone with respect to the foreland while feeding 65 km of slip into the subandean zone out of sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated 40 km of shortening within the altiplano total cumulative shortening within the cover rocks of the andean fold thrust belt 300 330 km can be balanced by an equivalent amount of shortening along two basement megathrusts to the first order the eastern margin of the central andean plateau defined by the 3 km topographic contour is contiguous with the leading edge of the upper basement megathrust this relationship between the basement highs and the physiographic boundaries of the andean plateau suggests that extensive megathrust sheets involving strong rocks such as crystalline basement or quartzite play an important role in the formation of the central andean plateau and a similar link between megathrust sheets and plateaus may be found in other orogens

opal as a u pb geochronometer search for a standard we report structural geochemical and tims u th pb and u series isotopic data for several samples of precious and common opal which we tested as potential standards for ion microprobe and la icpms isotopic studies the precious opals are low in u contain high concentration of common pb and are not suitable for u pb age determinations in contrast all studied samples of common opal are enriched in uranium and contain sufficiently radiogenic pb for precise calculations of 206pb 238u and 207pb 235u dates complications in the u pb systematics are due to the presence of ubiquitous unsupported 206pb probably derived from initial excess 234u and possibly other intermediate products of 238u decay elevated concentrations and possibly anomalous isotopic composition of initial pb in some of the opals and the presence of two opal generations of different ages in one of the samples despite these imperfections two out of five studied common opals are acceptable as u pb and u series standards at least in the interim the data obtained in this study can be used to guide further search for a better opal standard

consistency of geologic and geodetic displacements during andean orogenesis displacement and convergence rates inferred from gps and marine magnetic data suggest that this trend may be continuing at present hence an increasing fraction of convergence is being absorbed by mountain building this change may reflect increased plate coupling due to decreased sediment supply younger subducting lithosphere or increased normal stress at the interface from the effects of uplift index terms 8102 tectonophysics continental contract orogenic belts 8157 evolution of the earth plate motions past 3040 8158 evolution of the earth plate motions present and recent 3040 8099 structural geology general or miscellaneous

an experimental study of incongruent dissolution of caco3 under analogue glacial conditions this paper addresses the nature of incongruent dissolution of calcite in glacial settings using an experimental approach various caco 3 samples were comminuted using two contrasting techniques dry machine milling and hand grinding of an ice water mixture and dissolved to calcite saturation in both high pco 2 10 2 atm and low pco 2 10 3 5 atmospheric conditions ion yields of ca mg and sr were determined at the end of the experiments leachates displayed enhancement of mg ca and sr ca by a factor of 1 3 8 3 compared with bulk solid carbonate lower mg ca and sr ca ratios under higher pco 2 conditions reflect the percentage dissolution of the carbonate not the pco 2 per se the experimental results imply that reported natural incongruent dissolution is readily reproducible in the laboratory and is primarily dependent on the water rock ratio quantitative analysis of new and previous results suggests that the effect relates to the most reactive 0 15 0 25 of the sample equating volumetrically to the outer few lattice layers and it is interpreted as an anomalous leaching behaviour of calcite surfaces freshly exposed by crushing this phenomenon could serve as an index of the relative efficiencies of fragmenting and dissolutional processes in glacial environments

evidence for major shortening on the eastern edge of the bolivian altiplano the calazaya nappe abstract for the first time evidence is presented of major shortening on the eastern edge of the altiplano in bolivia a tectonic nappe showing at least 40 km of horizontal displacement towards the east is revealed at the altiplano cordillera oriental transition 20 s analysis of the differences in stratigraphy sedimentology and structure between allochthonous and autochthonous units confirms the 40 km of overthrust a geometric analysis of the thrust structures has been made to interpret the three dimensional geometry of the nappe and a deformation sequence is suggested the nappe emplacement is related to the eastward transport of a large allochthonous domain during the late oligocene early miocene the sole thrust of this allocthtonous domain is located in a stratigraphie unit whose three dimensional basin geometry controlled the deformation the existence of the calazaya nappe allows us to explain the exceptional thickness of the altiplano 55 70 km as mainly resulting from the neogene shortening that occurred between the altiplano and the cordillera oriental

sediment accumulation on top of the andean orogenic wedge oligocene to late miocene basins of the eastern cordillera southern bolivia a large volume of tertiary synorogenic sediment accumulated on top of the eastern cordillera of southern bolivia as the andean orogenic wedge was shortened thickened and uplifted oligocene to upper miocene strata were deposited in five basins that were separated by active north trending fold thrust structures of the then frontal part of the orogenic wedge these coarse grained deposits recorded accumulation in the most proximal sector of the andean foreland basin system the wedge top depozone analyses of depositional systems sediment dispersal patterns and clast provenance of 0 6 2 3 km thick oligocene to upper miocene wedge top successions demonstrate that faultpropagation and fault bend folds commonly isolated individual basins while serving as primary sediment sources growth strata which formed by progressive fold limb rotation indicate thrust fault displacement and related folding concurrent with deposition alluvial fans defined most basin margins whereas braided streams or small lakes occupied basin axes diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins growth strata and crosscutting and onlapping relationships between contractional structures and wedge top strata delineate the chronology of fold thrust deformation in the eastern cordillera five new 40 ar 39 ar dates and previously published k ar dates ranging from 30 to 8 ma define an oligocene phase of west vergent backthrusting followed by primarily east vergent thrusting during miocene time timing of displacement on two east vergent thrusts is determined by 40 ar 39 ar ages of tuffs within adjacent growth strata sequences of the tupiza formation conglomerate 16 14 0 06 ma and oploca formation 13 33 0 15 ma 15 7 2 4 ma these ages combined with basin depositional histories demonstrate synchronous and out of sequence thrust displacement during miocene shortening upper crustal contractional deformation and wedge top deposition terminated in the eastern cordillera during late miocene time as the thrust front propagated eastward into the subandean zone continued thrust front migration produced the present day configuration in which eastern cordillera wedge top basins originally developed above the toe of the orogenic wedge at relatively low elevations are now 250 km west of the active thrust front and at 3 km elevation the modern wedge top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrustbounded subandean zone basins and sediment overlying blind structures beneath the westernmost chaco plain in general wedge top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted nevertheless eastern cordillera wedge top deposits have been preserved for 10 30 m y in southern bolivia such long term preservation may reflect the inability of drainage systems to remove sediment mass from this low precipitation region of the central andes retention of sediment mass within the orogenic belt may promote critical thrust wedge conditions in which propagation of the thrust front is favored

modelling the baseline geochemistry of groundwater in a chalk aquifer considering solid solutions for carbonate phases the chalk aquifer of champagne france baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution the water rock interactions are modelled by the speciation code chess from field data and ca mg and sr aqueous concentrations in groundwater the stoichiometries of solid solutions are defined in each stratigraphic unit of the chalk aquifer from bulk geochemistry and chalk mineralogy of samples taken from boreholes the initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach

three dimensional flow and trace metal mobility in shallow chalk groundwater dorset united kingdom abstract the three dimensional groundwater flow and the hydrogeochemical regime have been determined in the bere stream valley north dorset downs southern england the dual porosity characteristics of the portsdown chalk have been established using geophysical and hydrochemical borehole logging chemical properties have been established using major and trace element analyses of depth samples and groundwaters the study site is located at the unconfined confined boundary of the chalk aquifer where it is overflowing in the observation boreholes the chalk dips locally at about 5 m km to the south east under palaeogene confining beds and three distinctive flow horizons may be recognised the chalk groundwater is of ca hco 3 type and three separate geochemical groundwater zones were also determined with depth having different oxygen levels and trace element characteristics 1 a shallow o 2 rich zone with around 80 dissolved o 2 and low trace element concentrations 2 a mixing and transition zone with significant concentrations of trace elements and high trace metal concentrations at its base manganese 29 g l nickel 55 g l cadmium 146 g l and zinc 214 g l 3 a deeper zone with depleted oxygen 5 20 dissolved o 2 and with longer water residence times shown by higher mg ca and k na ratios as well as higher sr and f the groundwater geochemistry in the chalk aquifer is dominated by incongruent reactions with the fine grained carbonate sediments which release trace element impurities to the water some of the metals are co precipitated with mn and fe oxide phases on fissure surfaces whilst producing a purer calcite during subsequent recrystallisation to purer iron and manganese oxides on fissure surfaces under specific geochemical and hydrodynamic conditions trace metals are released into the fissure water the results demonstrate the need to monitor quality stratification and the changes in the groundwater baseline chemistry in areas close to the redox boundary which in the dual porosity chalk is likely to be a diffuse zone with exchange between oxygen poor matrix waters and more oxic water flowing through the fissures

weathering rates from top to bottom in a carbonate environment abstract this study investigates u series sr isotopes major and trace elements in a chalk aquifer system located in eastern france soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil the unsaturated zone and the saturated zone of the aquifer interstitial water was extracted from soils and rocks by a centrifugation technique u series offer a powerful tool to calculate weathering rates because the relative mobility of the u and th isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches as expected the data show very large mobile element depletion in the soil with large 230 th excess relative to 238 u while the rocks show more limited but not insignificant mobile element depletion the u series data have been used to constrain weathering rates based on a 1 d reactive transport model weathering rates in the near surface are about 10 100 times faster than at depth however when integrated over the depth of the cores including the unsaturated and the saturated zones this underground weathering represents more than 30 of the total weathering flux assuming congruent dissolution of carbonates the 234 u 238 u ratios in interstitial water are consistent with solid samples showing 234 u depletion near the surface and an excess 234 u at depth a leaching experiment performed on chalk shows that the excess 234 u in natural waters percolating through carbonate rocks results both from preferential 234 u leaching and direct recoil in the interstitial water a new approach was used to derive the recoil ejection factor based on bet measurements and the fractal dimension of chalk surface consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates

residence time of chalk groundwaters in the paris basin and the north german basin a geochemical approach the comparative geochemical and isotopic study of confined and unconfined chalk groundwaters of the paris basin and the n german basin proves a significant chemical evolution during groundwater flow from the recharge zones to the deep confined aquifer different time dependent geochemical parameters have been tested as dating tools cation ratios sr2 ca2 mg2 ca2 n no 3 noble gas contents as paleotemperature indicators ne ar kr xe radiogenic he 13c 14c 18o 2h 3h cation ratios and 13c show the importance of incongruent dissolution processes in the chalk aquifer water rock interactions were taken into account in a multi step dissolution model to determine radiocarbon groundwater ages the oldest waters in the confined part of the paris basin chalk with maximum 14c ages of 14 000 a b p contain pleistocene recharge components as can be shown by a stable isotope depletion and noble gas temperatures significantly lower than in recent groundwaters chalk waters at the lagerdorf site in northern germany show a distinct stratification with respect to residence times and hydrochemistry

uranium bearing opals products of u mobilization diffusion and transformation processes understanding the retention mechanism of u by amorphous silica i e opal in the environment is of great importance to nuclear waste disposal because opals can retain u for millions of years uraniferous opals from spor mountain and the thomas range utah u s a are examined in terms of their mineralogical chemical and isotopic compositions uranium rich zones composed of most likely vorlanite cauo 4 occur in fibrous opal ct termed lussatite along the interface of the phase with microcrystalline quartz red to black colored precipitates of vorlanite also occur in the interstices between fibers and grains in lussatite and massy opals respectively the high abundance of vorlanite in certain growth zones can be explained by the diffusion of ca and u along boundaries of layers grains and fibers and by the release of ca and u through the transformation of opal a into opal ct and from opal ct into microcrystalline quartz similar o isotope compositions of opal ct and associated microcrystalline quartz indicate that crystallization processes and deposition of subsequent layers of opal occurred from fluids of similar origin and t differences in the isotope and chemical composition between uraniferous opals microcrystalline quartz the sio 2 polymorph moganite and pyrolusite indicate the past occurrence of various alteration processes involving fluids of different composition and t the results of this study indicate that assemblages of opal and microcrystalline quartz can retain u for millions of years as long as confined pore spaces between different generations of opal and quartz or between growth features of opal provide space for the nucleation and adsorption of u bearing phases and species

estimating transmissivity fields and their influence on flow and transport the case of champagne mounts 1 this paper presents a flexible and general methodology that combines hydrogeological and geostatistical modeling techniques to estimate a set of transmissivity fields and their influence on flow and transport the methodology may be applied to any case with only hydraulic head observations even if most of them are concentrated inside a small part of the entire domain of interest it is applied here to the case of the champagne chalk aquifer france where it is shown to be very efficient the methodology is decomposed in three independent parts first a reference head distribution is constructed by kriging in order to constrain the inverse problem as hydraulic heads and elevations are correlated a smoothed digital elevation model is used as external drift the inverse problem is then solved by using a simplified pilot point method with an efficient and easy to use minimization algorithm finally geostatistical simulations combined with flow simulations lead to a set of acceptable transmissivity fields the induced uncertainty is evaluated by calculating tracer concentrations pointing out areas where flow behavior is uncertain and where new borings would be advantageously drilled

geochemistry s vital contribution to solving water resource problems abstract as part of the events celebrating 40 a of iagc it is fitting to trace the modern evolution and development of hydrogeochemistry however fascination with water quality can be traced back more than 2 ka in the post war years hydrogeochemistry was influenced heavily by the advances in other disciplines including physical chemistry metallurgy and oceanography hydrological applications of isotope science also developed rapidly at this time and important advances in analytical chemistry allowed multi element and trace element applications to be made experimental studies on equilibrium processes and reaction kinetics allowed bench scale insight into water rock interaction consolidation of knowledge on processes in groundwaters and the current awareness of hydrogeochemistry by water professionals owe much to the work of robert garrels john hem and co workers in the early 1960s studies of down gradient evolution enabled a field scale understanding of groundwater quality and geochemical processes as a function of residence time dissolution and precipitation processes in carbonate and non carbonate aquifers redox processes cation exchange and salinity origins emerging water resource and water quality issues in the 1960s and 70s permitted the application of hydrogeochemistry to contaminant and related problems and this trend continues the impacts of diffuse pollution from intensive agriculture waste disposal and point source pollution from urban and industrial sources relied on geochemistry to solve questions of origin and attenuation in semi arid regions facing water scarcity geochemical approaches have been vital in the assessment of renewability and characterising palaeowaters the protection and new incoming regulation of water resources will rely increasingly on a sound geochemical basis for management

anhydrite and the sr isotope evolution of groundwater in a carbonate aquifer major element concentrations and 87 sr 86 sr ratios were measured in groundwaters and bedrock from the madison aquifer in western south dakota in this region the madison aquifer is primarily comprised of dolomite belonging to the madison limestone group the purpose of the study was to investigate controls on the downgradient evolution of dissolved sr 2 in a carbonate groundwater system that is recharged by waters with high 87 sr 86 sr ratios draining precambrian basement rocks and to establish the sources of sr 2 added to the waters by reaction with the aquifer lithology a mass balance model following previous workers was used to calculate amounts and effective rates of mineral dissolution and precipitation during groundwater transport along a 240 km flow path both the calculated reaction rates and data for the sr isotope geochemistry of the reacting phases were then used to develop a self consistent and quantitative description of the concentration and isotopic composition of dissolved sr 2 in the aquifer waters the major ion chemistry of madison aquifer groundwater is known to evolve according to dolomite dissolution anhydrite dissolution calcite precipitation and ion exchange with clay minerals dissolved 87 sr 86 sr ratios in the madison aquifer decrease downgradient input waters draining the igneous black hills have 87 sr 86 sr ratios of 0 723 while highly evolved waters in the aquifer have 87 sr 86 sr ratios of 0 708 dissolved sr 2 concentrations undergo a concurrent increase from 1200 to 66 000 nmol l model results indicate that dolomite dissolution exerts a critical control on the major ion chemistry but is not the primary source of nonradiogenic sr 2 as both the dissolution rate and sr concentration of dolomite are very low anhydrite is readily soluble in water has a low 87 sr 86 sr ratio 0 708 and a very high sr concentration 50 000 nmol g anhydrite is also greatly undersaturated in the groundwaters and the downgradient evolution of sr 2 accompanies an 80 fold increase in dissolved so 4 2 while anhydrite has a very low abundance in the aquifer rocks the flow model indicates that anhydrite dissolution provides 300 times more sr 2 per liter of water relative to dolomite dissolution these findings suggest that anhydrite dissolution governs the sr 2 geochemistry of madison aquifer groundwaters whereas dolomite dissolution and calcite precipitation control the bulk chemistry lastly it is shown that the calculation of net mineral masses transferred to solution requires very high relative proportions of anhydrite and clay to dolomite mineral masses obtained by this approach actually represent bulk contributions without consideration of the intrinsic reaction rates of the phases or their modal abundances in the aquifer rocks however consideration of these parameters in a transport equation for flow through porous media shows that the high apparent abundances are not a real requirement this latter approach is consistent with laboratory experiments and indicates that as little as 0 04 wt of very soluble anhydrite and 0 3 wt of exchangeable clay in the aquifer rock are sufficient to produce the observed groundwater chemistry in most of the flow path

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

evidence for rapid groundwater flow and karst type behaviour in the chalk of southern england with the growing importance of groundwater protection there is increasing concern about the possibility of rapid groundwater flow in the chalk of southern england and therefore in the frequency and distribution of karstic features pumping test data although useful in quantifying groundwater resources and regional flow give little information on groundwater flow at a local scale evidence for rapid groundwater flow is gathered from other less quantifiable methods nine different strands of evidence are drawn together tracer tests observations from chalk caves chalk boreholes that pump sand descriptions of adits the nature of water level fluctuations the chichester flood the nature of the surface drainage geomorphological features and the presence of indicator bacteria in chalk boreholes although the evidence does not prove the widespread existence of karstic features it does suggest that rapid groundwater flow should be considered seriously throughout the chalk rapid groundwater flow is generally more frequent close to palaeogene cover and may also be associated with other forms of cover and valley bottoms

petrifaction of wood by silica minerals abstract seventy five woods of australian origin have been examined by means of x ray diffraction optical and scanning electron microscopy and differential thermal analysis the silica is in one or other of the following forms opal a opal ct chalcedony quartz quartz chalcedony opal chalcedony more than half of them consist of opal ct or chalcedony the results indicate that petrifaction by silica involves penetration of the wood via splits or checks permeation of cell walls via a reticulated system of micropores enlargement of the micropore system as cell wall components break down and continuing deposition of silica ata rate and in a manner consistent with maintaining the dimensional stability of the wood deposition of silica in cell lumina and intercellular voids may be sometimes but not always a separate and later event the final stages of lithification involve loss of water and perhaps transformation of one form of silica into another

tracking crystallinity in siliceous hot spring deposits siliceous hot spring deposits sinters entrap paleoenvironmentally significant components and are used as extreme environment analogs in the search for early earth and extraterrestrial life however sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals for ancient hydrothermal settings including those close to the dawn of life these transformations have long since occurred so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change three young sinters preserve the entire diagenetic sequence of silica phases from opal a to quartz the 6000 to 11 500 years bp 70 years sinter at steamboat springs nevada the 1600 1900 160 years bp opal mound sinter at roosevelt hot springs utah and the 456 35 years bp deposit at sinter island taupo volcanic zone new zealand provide an opportunity to track crystallographic mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches worldwide sinter forms from cooling alkali chloride waters as noncrystalline opal a transforming first into noncrystalline opal a ct then paracrystalline opal ct moganite paracrystalline opal c and eventually to microcrystalline quartz in this study these changes were identified by the novel and combined application of electron backscatter diffraction x ray powder diffraction and scanning electron and optical microscopy techniques we show that mineralogical changes precede morphological and accompanied crystallographic transformations during this modification silica particles grow and shrink several times from the micron to nano meter scales via dissolution reprecipitation and recrystallization and diagenesis follows the ostwald step rule all deposits followed nearly identical diagenetic pathways with time as the only variable in the march toward physicochemically stable quartz crystals diagenesis alters original environmental signatures trapped within sinters after five silica phase changes filamentous microfossils are modified but still remain recognizable within sinter from the opal mound and steamboat springs deposits and during the opal a to opal ct silica phase transformations at sinter island therefore delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot spring deposits

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

interpretation of palaeoweathering features and successive silicifications in the tertiary regolith of inland australia detailed studies of morphological micromorphological and geochemical characteristics of silcretes in the deep bleached and weathered regolith across a large area of inland australia have provided a new interpretation of the history of the regolith and its climatic and morphological evolution during the tertiary pedogenic silcretes have distinctive morphological and mineralogical features caused by a succession of phases of silica dissolution and recrystallization resulting from multiple episodes of water infiltration and percolation under alternately wet and dry climates these are the oldest of the regolith features deep bleached profiles formed over a wide area in a variety of substrates ranging from precambrian granites to palaeozoic sandstones cretaceous sediments and tertiary deposits and represent the second major stage in regolith development these profiles in which kaolinite coexists with gypsum alunite and opal formed by reaction of the substrates with saline groundwaters the water table levels of which progressively fell over the region extensive networks of termite burrows constructed to great depth in the bleached regolith followed the water tables down the climate was warm and dry with a high water deficit groundwater silcretes formed near horizontal lenses and pods of porcellanite and jasper in the bleached regolith they preserve the primary fabric of the host rock groundwater silcretes post date the construction of termite burrows and were formed during a rise in groundwater tables across the landscape in places to near surface environments in broad landscape depressions the climate was more humid but the presence of gypsum during silicification demonstrates that the groundwaters were still saline red brown hardpans are the youngest silicification features and represent periods of successive infiltration and percolation and waterlogging during high rainfall or flood events they are confined to low regions in the landscape mineralogical and geochemical analysis of the bleached profiles together with geochemical modelling suggests that ferrolysis is the most likely cause of acidity in groundwater leading to the development of the bleach profiles and or alunite present day groundwater tables are both at low levels and sulphate rich it is possible that acidic alteration leading to bleaching is still active around the extensive playa landscapes in the region

mineralogical characterization of silica sinters from the el tatio geothermal field chile silica sinters deposited from alkali chloride waters in the el tatio geothermal field in northern chile 22 20 s 68 01 w have been characterized by xrd sem tg dta and ftir the dominant silica phase is opal a minor contents of opal a ct and opal ct are also present together with halite nacl sylvite kcl and realgar ass accessory phases include teruggite ca4mgas2b12o22 oh 12 12 h2o sassolite h3bo3 and detrital quartz sio2 fwhm values reflect the immature nature of the studied opal a dta heating experiments of opal a show cristobalite crystallization at 1000 c whereas dta cooling experiments show the cristobalite transformation at 200 c the total weight loss is related to changes in the sinter microtextures mineral phases and organic matter contents ftir spectra show the effects of silica maturation as consequence of the loss of trapped and absorbed water attached to silanols

neogene deformation of the flat subduction segment of the argentine chilean andes magnetostratigraphic constraints from las juntas la rioja province argentina substantial neogene deformation occurred in the part of the andes mountains and the sierras pampeanas that overlie a subhorizontal segment of the subducting nazca plate in northern argentina the history of the volcanic arc suggests that the nazca plate attained the present low angle inclination between about 17 and 10 ma with the region underlain by the flat nazca plate widening eastward thereafter we compare the history of deformation provided by magnetostratigraphy of miocene strata in foreland and intermontane basins to the history of the subducting plate at las juntas in the precordillera thin skinned thrust belt 4 780 m of neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin magnetostratigraphic data complemented by fission track dating of an interbedded tuff indicate that the strata span from 18 to 9 ma the earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the frontal cordillera subsequently the deformation front migrated eastward generating the precordillera thrust belt and finally the sierras pampeanas basement uplifts the rate of strata accumulation varied systematically as new uplifts arose with a maximum rate exceeding 1 0 mm a between 15 and 12 ma a correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed although large scale factors affecting the plate margin beyond the flat slab segment may have partly induced the compressional deformation this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

hydrochemical trends palaeorecharge and groundwater ages in the fissured chalk aquifer of the london and berkshire basins uk the hydrochemical radiochemical stable isotope 14c and dissolved noble gas composition of groundwaters has been determined along two profiles across the confined fissured chalk aquifer of the london basin of southern england and for selected sites in the adjacent berkshire basin during downgradient flow in the london basin aquifer the groundwater chemistry is modified by water rock interactions congruent and incongruent reaction of the carbonate lithology resulting in enhanced mg ca and sr ca ratios and 13c contents with increased residence times redox and ion exchange reactions and towards the centre of the basin mixing with a residual saline connate water stored in the chalk matrix there is evidence from anomalous water chemistries for a component of vertical leakage from overlying tertiary beds into the confined aquifer as a result of historical dewatering of the aquifer dissolved noble gas contents indicate the climate was up to 4 5 c cooler than at present during recharge of the waters now found in the centres of both basins stable isotope 2h and 18o depletions correspond to this recharge temperature change for evolved waters having 13c 8 pdb a negative linear correlation is demonstrated between derived recharge temperatures and 13c values which is interpreted as mixing between relatively warm light isotopic fracture borne waters and cooler stored waters of the matrix having a 13c signature more or less equilibrated with the chalk from geochemical 14c 4he age estimates the abstracted water is interpreted as being either of wholly holocene post devensian glacial origin or an admixture of holocene and late pleistocene pre glacial cold stage interstadial recharge devensian pleniglacial stage waters of the last glacial maximum are not represented

hydrochemical and stable isotope evidence for the extent and nature of the effective chalk aquifer of north norfolk uk in eastern england the chalk aquifer is covered by extensive pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer in this study the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions 18o and 2h are interpreted to reveal the extent and nature of the effective chalk aquifer of north norfolk it is found that the chalk aquifer can be divided into an upper region of fresh groundwater with a cl concentration of typically less than 100 mg l 1 and a lower region of increasingly saline water the transition between the two regions is approximately 50 m below sea level and results in an effective aquifer thickness of 50 60 m in the west of the area but less than 25 m where the eocene london clay boundary is met in the east of the area hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the chalk where the chalk is confined by low permeability chalky boulder clay isotopically depleted groundwater 18o less than 7 5 is present in contrast to those areas of unconfined chalk where glacial deposits are thin or absent 18o about 7 0 the isotopically depleted groundwater is evidence for groundwater recharge during the late pleistocene under conditions when mean surface air temperatures are estimated to have been 4 5 c cooler than at the present day and suggests long groundwater residence times in the confined aquifer elevated molar mg ca ratios of more than 0 2 resulting from progressive rock water interaction in the confined aquifer also indicate long residence times a conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater occupying areas where confined groundwater dates from the late pleistocene is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body

kinetics of incongruent dissolution of carbonates in a chalk aquifer using reverse flow modelling s u m m a r y water rock interactions are major processes that control the solute transfer from natural or anthropo genic sources in aquifers in carbonate aquifers these water rock interactions are mainly characterised by the incongruent dissolutions of carbonates the in situ quantification of these reactions is few studied in the present paper we provide a new quantification of the kinetic rates of simultaneous incongruent dissolution of composite carbonate and dolomite in a chalk aquifer the approach is based on a mass balance model used to calculate the rates of mineral dissolution and precipitation along a 500 m to 4 km flow path starting at the groundwater divide line the groundwater transfer times between the groundwater divide line and the wells are calculated thanks to flow and transport modelling used in a backward particle tracking mode the uncertainties on porosity dispersivity and transmissivity are taken into account the rates are found ranging between 1 5 10 a5 and 1 5 10 a4 mol c5 chalk l a1 w yr a1 for the composite carbonate dissolution between 1 0 10 a7 and 3 0 10 a6 mol dolomite l a1 w yr a1 for the dolomite dissolution and between 1 0 10 a5 and 1 5 10 a4 mol calcite l a1 w yr a1 for the calcite precipitation the results highlight the importance of physical and geological properties of the aquifer

a spatial analysis of structural controls on karst groundwater geochemistry at a regional scale summary the coupled spatial investigation of the geometrical and geochemical properties of a chalk karstic aquifer provides information on the degree to which geologic structure controls aquifer functioning and groundwater quality major ion concentrations in the chalk aquifer of the haute normandie region france were measured at a high spatial resolution more than 100 sampling sites over a 6000 km2 area and mapped the first observation is a continuity of the geochemical properties in spite of the karstic properties of the aquifer principal components analysis of geochemical maps revealed two types of spatial distributions ions with an autochthonous origin ca2 hco3 and ions with a principally allochthonous origin cl na no 3 so 4 2 mg2 was categorised as both autochthonous chalk dissolution and allochthonous brought in by infiltration of tertiary deposits to better understand the spatial distribution of the geochemistry the aquifer geochemistry was compared to the physical properties of the aquifer in particular aquifer thickness representing aquifer geometry and piezometric level representing aquifer flow use of spatial correlation between the geochemical and the geometrical properties provided insight regarding the directional structure of the data and give evidence of directional relations between geochemical and geometrical properties the degree of mineralisation principally composed of ca2 and hco 3 ions increased along the direction of flow corresponding to an increase in chalk dissolution rate along the flowpath the steepest mineralisation gradients were related to an increase in the mg ca ratio evidence of longer residence times and corresponding to zones where aquifer flow capacity is limited because of a decrease of the thickness of the flow section anticlines or faults these results highlight the dominant role played by the geometry and the structural context in controlling aquifer geochemistry

dolomite dissolution rates and possible holocene dedolomitization of water bearing units in the edwards aquifer south central texas abstract rates of dolomite dissolution can be used to test the concept based on geomorphologic evidence that a major part of the edwards aquifer could have formed within the holocene a timeframe of approximately 10 000 years during formation of the aquifer in the edwards limestone cretaceous albian of the balcones fault zone dolomite dissolution and porosity development were synchronous and the result of mixing zone dedolomitization initiation of the mixing zone in the early holocene 11 000 years before present is suggested by the maximum age of formation of major discharge sites that allowed the influx of meteoric water into brine filled dolomitic preaquifer units dedolomitization the dissolution of dolomite and net precipitation of calcite has left aquifer units that are calcitic and 40 vol interconnected pore space the mass of dolomite missing is obtained by comparison of stratigraphically equivalent altered and unaltered units one dissolution rate 1 76 10 4 mmol dolomite kg h 2 o 1 yr 1 is determined from this mass 10 4 yr reaction time and a log linear function describing the increase in mass discharge three orders of magnitude during aquifer formation the second estimated dissolution rate is obtained from the mass transfer of dolomite to solution calculated from the increase in magnesium in pore fluids selected from the modern aquifer to represent a typical flowpath during aquifer formation a reaction time of 10 4 yr for this mass transfer yields a rate of 0 56 10 4 mmol dolomite kg h 2 o 1 yr 1 both of these rates are comparable to modern rates of dolomite dissolution 0 3 to 4 5 10 4 mmol dolomite kg h 2 o 1 yr 1 calculated from measured reaction times in the tertiary floridan aquifer system in florida and the madison aquifer in the mississippian madison limestone of the northern great plains similarity of these rates to the estimated paleo rates of dolomite dissolution supports a 10 4 yr reaction timeframe the holocene reaction time also can be compared to a series of reaction times calculated by assuming that the mass of dolomite missing from the edwards was removed at rates observed in the floridan and madison aquifers these reaction times for complete removal of dolomite range from 2700 to 58 500 yr and span the pleistocene holocene boundary finally an estimated dolomite reaction rate during dedolomitization of the edwards aquifer based on surface area of exposed dolomite mmol cm 2 s 1 millimoles per square centimeter per second may be approximated from reaction times this rate is directly a function of the mass of dolomite removed and the surface area exposed per pore volume passing through the rock the surface area is available from the observed dolomite rhomb size in unaltered rock the rate of pore fluid movement is obtained from the averaged annual discharge rates during formation of the edwards aquifer calculated from all reaction times range from 10 13 to 10 14 mmol dolomite cm 2 s 1 these rates are faster than rates 10 18 mmol cm 2 s 1 measured in the pure laboratory system camg co 3 2 co 2 h 2 o but slower than rates determined in an alpine stream study 10 10 to 10 11 mmol cm 2 s 1 where cold glacial melt water flows over dolostone dolomite dissolution rates from both the edwards and other aquifers support the concept that a major part of the edwards aquifer could have formed within the holocene

a proposed mechanism for the growth of chalcedony the structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica although the paragenesis of chalcedony has provoked marked disagreement among researchers a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures 100 c these conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short chain linear polymers via bridging silica monomers this assembly occurs through a spiral growth mechanism activated by a screw dislocation withb n 2 110 wheren is an integer the proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale such as 1 the direction of fiber elongation along 110 rather than 001 2 the periodic twisting of chalcedony fibers about 110 3 the high density of brazil twin composition planes 4 the common intergrowth of moganite within chalcedony

baseline geochemical conditions in the chalk aquifer berkshire u k a basis for groundwater quality management the chalk aquifer is the most important british aquifer and is also important over much of northern europe aquifer protection requires a sound knowledge of the baseline conditions and how these might vary or have varied with time this detailed geochemical study of a representative area of chalk in berkshire u k includes a consideration of several components 1 the inputs from the atmosphere 2 the interstitial water of the soil and the unsaturated zone 3 the interstitial water in the confined and unconfined sections of the aquifer and 4 the saturated mainly fissure flow along the hydraulic gradient which forms an important water supply of the thames valley region atmospheric inputs form an important source of some elements but the dominant chemical characteristics of the chalk groundwater are acquired during percolation through the soil and the upper unsaturated zone during saturated flow downgradient the chemistry is modified mainly by incongruent reactions of the carbonate matrix and by redox reactions and only to a minor extent by exchange reactions and mixing with residual saline connate water the incongruent reaction of carbonate results in a marked increase in the mg ca ratio and the sr and 13c contents of the groundwater with increased residence time oxygen concentrations are reduced mainly by oxidation of fe2 and the onset of reducing conditions allows dissolved fe2 to increase and rapid denitrification to occur the salinity profile through the confined chalk confirms that residual connate water up to one fifth sea water concentration still remains at depth and this accounts for some salinity increase in the confined groundwater resulting from fissure water pore water diffusional exchange timescales for groundwater movement have been established using tritium radiocarbon and indirectly using inert gas ratios and stable isotope ratios on balance it is concluded that all abstracted water is of holocene age although inert gas temperatures indicate cooler climatic conditions for recharge for some of the confined groundwater the implications for development and aquifer protection are discussed especially the prospect of natural in situ denitrification problems of fe solubility and the recognition of groundwater of different maturities