characterization of dolomitic rocks from the coastal mixing zone of the floridan aquifer florida u s a abstract petrographic xrd sem cathodoluminescence and microprobe analyses were conducted to characterize dolomitic strata from a core penetrating the coastal mixing zone of the floridan aquifer lithologic geochemical and water chemistry data were used to identify diagenetic trends including modern water rock interaction the depositional environment of these middle eocene carbonate sediments of the avon park formation was intertidal to supratidal sabkha conditions caused penecontemporaneous deposition of evaporites and accessory minerals present water chemistry indicates undersaturation for gypsum and its survival is attributed to sluggish groundwater circulation dolomite crystals were commonly found to be compositionally zoned with crystal centers generally calcium rich and less stable than the more stoichiometric crystal rims three stages of dissolution were recognized from sem studies of these crystals 1 initial development of dissolution pits 2 partial dissolution of the unstable crystal center and 3 complete dissolution of the unstable crystal center leaving hollow dolomite present day mixing zone water is saturated with respect to stoichiometric dolomite near stoichiometric dolomite cement may be filling pores in this zone and non stoichiometric dolomite may be dissolving the instability of calcian dolomite in the mixing zone may be related to compositionally modulated crystal lattice defects pervasive dissolved crystal centers in the saline zone are believed to have been highly disordered penecontemporaneous dolomite stoichiometric dolomite does not appear to be precipitating in this zone because of the presence of competing ions stable carbon and oxygen isotopic values for dolomite are comparable to those reported for plio pleistocene dolomites 14 c activity implies some relatively young dolomite exists probably as rims on older crystals

fossil cyclic calcrete in the kuwait group clastic deposits mio pleistocene of kuwait arabian gulf an investigation of the upper kuwait group clastic sequence mio pleistocene exposed along the jal az zor escarpment in north kuwait revealed several horizons of fossil calcrete field occurrences and textural characteristics of the studied sequence indicated that the fossil calcrete was hosted in cyclic fluviatile muddy sandstones and developed through diagenesis during inter fluvial periods depositional and diagenetic features of the calcretized fluviatile sandstone units suggested that each cycle started with the deposition of a fluviatile muddy host sandstone during a humid period this was followed by a period of semi aridity during which calcrete profiles developed the upper part of the calcrete profiles was weathered and terra rosa type soil was formed during wet seasons this was succeeded by a severe arid climate and the deposition of desert aeolian sand sheets fossil nodular calcrete was only developed within the fluviatile muddy sandstones the calcretized sandstones were lithified in two stages of diagenesis an early stage that was responsible for the authigenesis of microcrystalline calcite and the development of calcrete nodules and a later stage during which macrocrystalline calcite cement was precipitated as an intergranular cement displacing and replacing the clay matrix of the host sediments and filling fractures and cavities in the calcrete nodules the findings and conclusions of the present study will help in understanding the lithostratigraphic setting of the kuwait group clastic sequence in the northern arabian gulf region

pedogenic origin of dolomite in a basaltic weathering profile kohala peninsula hawaii we document stoichiometric dolomite occurring in a nonsaline quaternary soil on the kohala peninsula northwestern hawaii geologic constraints and geochemical and isotopic data confirm that this dolomite is not the result of marine influence or wind blown dust the strontium isotopic composition of the dolomite 87sr 87sr 0 7045 0 7048 is indicative of its derivation primarily from the weathering of basaltic parent material rather than from meteoric water or seawater infiltration of soil waters with elevated mg ca 1 derived from alteration of ferromagnesian minerals such as olivine likely led to dolomitization of early precipitated soil calcite and or to direct dolomite precipitation in the profile this demonstrates that well ordered dolomite can form in a nonmarine environment at temperatures 100 c without undergoing burial diagenesis

dolomitized cells within chert of the permian assistencia formation parana basin brazil abstract dolomitic microscopic structures in the form of microspheres horseshoe shaped objects and thin botryoidal crusts found within microfossiliferous chert within stromatolites of the evaporite bed eb of the permian assistencia formation irati subgroup parana basin brazil have been investigated by means of optical microscopy x ray fluorescence scanning electron microscopy raman spectrometry and energy dispersive x ray spectrometry the microspheres were identified as dolomitized coccoidal cyanobacteria based on similarity in size spheroidal and paired hemispheroidal morphologies and colonial habit to co occurring silicified organic walled cyanobacteria embedded within the same microfabric and rock samples the co occurrence of dolomite pyrite framboids and abundant dispersed carbonaceous material and silicified cells is consistent with a hypersaline depositional environment with abundant cyanobacterial mats and elevated mg2 ca2 ratios and reducing conditions with active anoxic microbial processes near the water bio sediment interface the abundance of extracellular polymeric substances facilitated anoxic microbial processes sulfate reduction providing essential conditions for possible primary microbially induced dolomitization in most of the dolomitized cells dolomite occurs only as an external layer in fully dolomitized cells magnesium is richest in the outermost layer presumably the dolomitization process was favored by the presence of anoxic microbial degraders and negatively charged functional groups at the surface of the cyanobacterial cells botryoidal dolomite rims of silica filled fenestrae formed by a similar process and inherited the botryoidal morphology of the cell as originally lining the fenestrae silicification interrupted the dolomitization of the largely organic biosediment mostly by permineralization but locally by substitution thereby preserving not only dolomitic microspheres but also huge numbers of structurally well preserved organic walled cyanobacteria and portions of microbial mat clearly dolomitization began very early in the microbial mats prior to compaction of the sediment or full obliteration of cellular remains followed very closely by silicification thereby impeding continued degradation and providing a window onto very well preserved permian microbial mats

an underground mining detection system based on dinsar underground mining easily causes casualties and illegal mining is a major contributor to this issue it is a challenge to locate and differentiate illegal mines from approved mines over a vast area over the past few years our research in satellite differential radar interferometry dinsar to monitor underground mine induced surface subsidence has demonstrated the reliability of dinsar in this paper a dinsar based illegal mining detection system dimds is proposed to exploit the geometric spatial and temporal characteristics of those subsidence patterns testing results over a coalfield in asia have proven the efficiency reliability and cost effectiveness of dimds in finding out illegal mines

groundwater dolocretes from the upper triassic of the paris basin france a case study of an arid continental diagenetic facies thick dolomite cemented horizons dolocretes occur within a fluvial sandstone mudstone sequence of late triassic age in the western part of the paris basin france two types of dolomites can be distinguished a nodular dolomitic beds less than a few metres thick which formed within mottled overbank siltstones and mudstones and b massive dolomite up to 16 m thick which occurs in coarse grained channel sandstones and conglomerates the majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and less commonly dolomicrite glaebules irregular spar filled cracks spheroidal dolomite silicification and vuggy porosity are locally abundant in the massive dolomite in contrast biologically induced micromorphological features such as rhizocretions and alveolar septal fabrics were observed in the thin nodular dolomite beds the dolomite is near stoichiometric well ordered and non ferroan 18o values range from 7 7 to 0 4 o pdb and 18o values range from 5 1 to 1 8 0 pdb and no obvious difference in the stable isotopic composition between both types of dolomites was observed sr isotope ratios range from 0 7101 to 0 7126 and are invariably higher than the contemporary triassic sea water a vadose pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures several features however argue against a pedogenic origin for the massive carbonates most notably the absence of biologically induced structures the occurrence in coarse grained channel and not overbank deposits and the great thickness these units are thus interpreted as groundwater in origin phreatic calcretes of quaternary age widespread in inland australia are regarded as a modern analogue for the triassic paris basin dolocretes petrographic observations argue in favour of primary proto dolomite precipitation although early diagenetic replacement of calcite by proto dolomite cannot be ruled out strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non marine continental origin for the groundwaters the poorly ordered and non stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near stoichiometric well ordered dolomite that clearly lacks evidence for pervasive recrystallization

terrestrial freshwater lenses unexplored subterranean oases freshwater lenses are lenticular bodies of fresh tds 1000 mg l groundwater that develop above more saline groundwater within the same host aquifer due in part to buoyancy in contrast to the widely studied situation of freshwater lenses in coastal aquifers the formation location and persistence of freshwater lenses in terrestrial settings are poorly understood this is despite inland aquifers commonly containing saline groundwater particularly in arid and semi arid climates and the local occurrences of freshwater being critical for ecosystems and human endeavour we identify and classify known terrestrial freshwater lenses tfls using four formation categories namely topography geology groundwater surface water interaction and recharge mechanisms the resulting typology highlights the importance of buoyancy in the formation of tfls in otherwise unlikely situations implying that tfls may be more prevalent than previously thought tfls represent some of the most vulnerable and precious freshwater resources on earth that require considerably more research into mechanisms of formation and threats to their existence

paleoenvironmental and paleohydrochemical conditions of dolomite formation within a saline wetland in arid northwest australia abstract groundwater dolocrete occurring within the fortescue marsh a large inland wetland in the pilbara region of northwest australia has been investigated to provide paleoenvironmental and paleohydrological records and further the understanding of low temperature dolomite formation in terrestrial settings over the quaternary period two major phases of groundwater dolocrete formation are apparent from the presence of two distinct units of dolocrete based on differences in depth 18o values and mineral composition group 1 g1 occurs at depth 20 65 m b g l below ground level and contains stoichiometric dolomite with 18o values of 4 02 0 71 group 2 g2 is shallower 0 23 m b g l occurring close to the current groundwater level and contains ca rich dolomite secondary calcite with a comparatively lower range of 18o values 7 74 and 6 03 modelled 18o values of paleogroundwater from which older g1 dolomite precipitated indicated highly saline source water which had similar stable oxygen isotope compositions to relatively old brine groundwater within the marsh developed under a different hydroclimatic regime the higher 18o values suggest highly evaporitic conditions occurred at the marsh which may have been a playa lake to saline mud flat environment in contrast g2 dolomite precipitated from comparatively fresher water and modelled 18o values suggested formation from mixing between inflowing fresher groundwater with saline brine groundwater within the marsh the 18o values of the calcite indicates formation from brackish to saline groundwater which suggests this process may be associated with coeval gypsum dissolution in contrast to the modern hydrology of the marsh which is surface water dependent and driven by a flood and drought regime past conditions conducive to dolomite precipitation suggest a groundwater dependent system where shallow groundwaters were influenced by intensive evaporation

ophiolitic basement to the great valley forearc basin california from seismic and gravity data implications for crustal growth at the north american continental margin the nature of the great valley basement whether oceanic or continental has long been a source of controversy a velocity model derived from a 200 km long east west reflection refraction profile collected south of the mendocino triple junction northern california in 1993 further constrained by density and magnetic models reveals an ophiolite underlying the great valley great valley ophiolite which in turn is underlain by a westward extension of lower density continental crust sierran affinity material we used an integrated modeling philosophy first modeling the seismic refraction data to obtain a final velocity model and then modeling the long wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model the crustal section of great valley ophiolite is 7 8 km thick and the great valley ophiolite relict oceanic moho is at 11 16 km depth the great valley ophiolite does not extend west beneath the coast ranges but only as far as the western margin of the great valley where the 5 7 km thick great valley ophiolite mantle section dips west into the present day mantle there are 16 18 km of lower density sierran affinity material beneath the great valley ophiolite mantle section such that a second deeper present day continental moho is at about 34 km depth at mid crustal depths the boundary between the eastern extent of the great valley ophiolite and the western extent of sierran affinity material is a near vertical velocity and density discontinuity about 80 km east of the western margin of the great valley our model has important implications for crustal growth at the north american continental margin we suggest that a thick ophiolite sequence was obducted onto continental material probably during the jurassic nevadan orogeny so that the great valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle

systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern great basin united states stable carbon and oxygen isotope variations in holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern great basin united states a region with a semi arid climate our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate ecological variations differences in parent material and soil depth 13 c of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth indicating a decrease in the ratio of atmospheric to plantderived co 2 downprofile pedogenic carbonate at the soil air interface approaches a 13 c value in equilibrium with atmospheric co 2 in all three soils observed 13 c profiles for pedogenic carbonate can be described using a one dimensional model for 12 co 2 and 13 co 2 assuming isotopic equilibrium between soil co 2 and soil carbonate the modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil respiration rates at each site 13 c in deep pedogenic carbonate 50 cm varies by about 12 per mil over a 2 440 m elevation range being enriched in 13 c at the lowest elevations the slope of 13 c for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials depletion by 4 6 to 4 7 per mil per 1 000 m increase in altitude between 300 to 2 740 m above mean sea level for the localities studied this concordance makes it likely that there has been complete isotopic exchange between hco 3 in solution and soil co 2 prior to carbonate precipitation soil co 2 and soil respiration rates increase systematically with elevation the plantderived component of soil co 2 indicates that c 3 plants dominate the biomass at most measured sites in agreement with plant surveys calculated equilibrium fractionation factors between soil co 2 and soil carbonate are very similar to those observed again indicating complete isotopic exchange between carbon species in all the soil co 2 and soil carbonate data suggest that the 13 c variation with elevation observed in the soil carbonates results from differing soil respiration rates at each site as well as from variations in the proportion of c 3 to c 4 and cam plants in each site9s surface biomass 18 o values in pedogenic carbonates are higher at lower elevations due in part to the more positive 18 o values for meteoric waters at lower elevations the average 18 o value of deep 50 cm pedogenic carbonate at all sites however is enriched 2 4 to 3 7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters this suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place one or both of these processes also may explain the 18 o decrease in soil carbonate with depth observed in two of three soil profiles

groundwater dolocretes in a lake marginal environment an alternative model for dolocrete formation in continental settings danian of the provence basin france in the long continental history of the provence basin which extended from santonian to oligocene times a major period of palaeosol development occurred in the danian dolocretes developed within floodplain silts and partly from palustrine limestones dolocretes are overlain by pedogenic facies calcretes and palustrine limestones gradational lower and upper limits of profiles succession of nodular coalescent and massive horizons the epigenesis of quartz by dolomite the unimodal crystal size and the euhedral dolomite fabric as well as the absence of biogenic structures and vadose cements show that dolocretes formed in the phreatic zone the exclusive occurrence of dolocretes around the palaeolake or playa suggests that dolocrete formed by the mixing of groundwaters and lake brines which infiltrated the phreatic zone during periods of strong evaporation and lake level lowering the term halo dolocrete is proposed to describe this type of dolocrete deposit subsequent alteration of the dolocrete includes leaching of the central core of dolomite crystals and calcitization calcitization was either fabric destructive type i or fabric preserving type ii and took place during very early diagenesis i e concomitant with calcrete formation or palustrine limestone deposition fabric destructive calcitization is attributed to a drop in the lake level when the upper part of the dolocrete was subjected to vadose zone processes whereas the fabric preserving calcitization resulted from reactions with dilute lake and groundwaters during rise in lake level

deep structure of the western south african passive margin results of a combined approach of seismic gravity and isostatic investigations abstract the passive margin of the south atlantic shows typical features of a rifted volcanic continental margin encompassing seaward dipping reflectors continental flood basalts and high velocity density lower crust at the continent ocean transition probably emplaced during initial seafloor spreading in the early cretaceous the springbok profile offshore western south africa is a combined transect of reflection and refraction seismic data this paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives the velocity modelling revealed a segmentation of the margin into three distinct parts of continental transitional and oceanic crust as observed at many volcanic margins the lower crust is characterised by a zone of high velocities with up to 7 4 km s the conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities found to be 3100 kg m 3 both approaches identified the body to have a thickness of about 10 km yet the gravity modelling predicted the transition between the high density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal finally isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced the results imply isostatic equilibrium over large parts of the profile smaller deviations are supposed to be compensated regionally the calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

mineralogical composition and potential sources of dust fallout deposits in kuwait northern arabian gulf abstract detailed mineralogical investigations of dust fallout deposits collected at eleven locations in kuwait during april 1979 march 1980 were carried out the mineralogy of five size fractions medium sand fine sand very fine sand coarse silt and clay was determined occurrence frequency and potential sources of the mineral grains that form the various size fractions are discussed and the gross mineralogical composition of the dust fallout calculated it was found that dust fallout deposited in kuwait is quartzitic calcareous sandy silt and is mostly derived from the dry mesopotamian flood plain in the middle and south of iraq the occurrence of sand size particles is attributed to pick up from local recent surface deposits

muddy and dolomitic rip up clasts in triassic fluvial sandstones origin and impact on potential reservoir properties argana basin morocco abstract the significance of rip up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition however this study highlights the great value of muddy and dolomitic rip up clast occurrence as a information source about low preservation potential from floodplain deposits and b key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality high resolution petrographic analysis on triassic fluvial sandstones from argana basin t6 and t7 t8 units highlights the significance of different types of rip up clasts as intrabasinal framework components of continental sediments from arid climates on the basis of their composition and ductility three main types are distinguished a muddy rip up clasts b dolomitic muddy rip up clasts and c dolomite crystalline rip up clasts spatial distribution of different types is strongly facies related according to grain size origin of rip up clasts is related to erosion of coeval phreatic dolocretes in different development stages and associated muddy floodplain sediments cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth respectively dolomite crystals are almost stoichiometric this composition is very similar to that of early sandstone dolomite cement supporting phreatic dolocretes as dolomite origin in both situations sandstone diagenesis is dominated by mechanical compaction and dolomite cementation a direct correlation exists between 1 muddy rip up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume igv and 2 occurrence of dolomitic rip up clasts and dolomite cement nucleation in host sandstone occluding adjacent pores but preserving igv both processes affect reservoir quality by generation of vertical and 3d fluid flow baffles and barriers that compartmentalize the reservoir these findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon bearing basins such as the bay of fundy nova scotia canada

crustal structure of the sw moroccan margin from wide angle and reflection seismic data the dakhla experiment part a wide angle seismic models abstract a total 1500 km of seismic reflection and wide angle profiles were acquired off the southern moroccan margin during the dakhla cruise a joint project of ifremer the universities of brest el jadida and lisbon and total the shots along two profiles parallel to the margin and two profiles perpendicular to the margin were also recorded by ocean bottom seismometers obs the profiles perpendicular to the margin were additionally extended on land using 14 stations on the northern profile and 11 stations on the southern profile modelling of the reflection and wide angle seismic data reveals a 10 km deep sedimentary basin including two high velocity carbonate layers lateral crustal thinning is observed from a 27 km thick crystalline continental crust to a 7 km thick oceanic crust occurring over less than 100 km the crystalline continental crust can be divided into two distinct layers of 12 and 15 km thickness the oceanic crust east of the magnetic anomaly m25 displays higher velocities in layer 3 than west of the magnetic anomaly the change in seismic velocity suggests a possible link to changes in accretionary processes of the oceanic crust some regions show seismic velocities between 6 8 and 7 4 km s which could be explained by slightly elevated mantle temperatures during accretion of the crust

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

geochemistry of calcic horizons in relation to hillslope processes southern india abstract the study concerns chemical and mineralogical properties of calcic horizons in relation to their physical environment such as position in the soil profile and the terrain the carbonates were calcitic in upslope positions but dolomite was abundant in valley sites the content of fluorine increased downslope the distribution of the composition in the terrain indicates a sequential precipitation of calcite dolomite and fluorite from soil water and groundwater as the water flows downslope to discharge areas carbonates in vertisols formed from the more basic charnockites were of the low mg calcite type while dolomite occurred only in entisols and alfisols formed from biotite hornblende gneisses manganese in calcic horizons increased as the groundwater level was approached black intercalations in laminated petrocalcic horizons contained elevated concentrations of mn k al and si this may indicate periods of higher soil moisture and clay formation the presence of low mg calcitic carbonates overlying dolomitic ones in valley sites is likely to be due to deposition of calcite rich material from upslope the amount of soil eroded from upslope positions was between 0 2 and 2 m in four investigated profiles

climate fluctuations recorded in phreatic and vadose calcretes of the lower carboniferous clyde sandstone formation of machrihanish kintyre peninsula sw scotland the galdrings cliffs of machrihanish in the kintyre peninsula of scotland expose a lower carboniferous clastic succession that hosts a wide variety of calcretes including thick and massive host replacing phreatic calcrete hardpans hrpchs which are geologically rare and which are the products of a thorough replacement of host minerals by calcite in the mixing zone between fresh and evaporitic groundwaters isotopic values of 53 samples from various calcretes distributed in a c 45 m thick succession provided well defined trends of covariance between carbon and oxygen isotopes thus delineating trends of aridification and humidification from the interplay of precipitation and evaporation rates although values from hrpchs are more constrained all other forms of calcrete in the succession pedogenic nodules pillar calcrete laminar calcrete and invasive phreatic calcrete cement in sandstone follow similar stable isotopic trends an aridification trend preceded the formation of each hrpch occurrence corroborating studies on modern equivalents that suggest that they develop in the more arid range of calcrete formation the better constrained isotopic values of hrpchs compared with those of more common forms of calcrete are interpreted to be indicative of more specific environmental requirements which may in part explain their rarity in the geological record

pseudospherulitic fibrous calcite in paleo groundwater unconformity related diagenetic carbonates paleocene of the ager basin and miocene of the madrid basin spain abstract massive decimeter thick intervals formed by mosaics of subspherical calcite crystals displaying an intracrystalline fibrous microfabric have been found in paleocene and miocene strata in spain the fibrous microfabric is marked by inclusion defined fibers that radiate from the central parts of the crystals which can be termed pseudospherulites and not spherulites because they are single crystals and not polycrystalline aggregates the horizons of pseudospherulitic calcite appear in meter scale laterally persistent levels of crystalline limestones that exhibit complex pseudospar fabrics they are interbedded with peritidal and or lacustrine deposits they grade downward into bedded dolomicrites and they can be linked to unconformities several meters higher in the section above intervening unaltered limestones the pseudospar zones are interpreted as early diagenetic paleo groundwater alteration products after dolomicrites the pseudospherulites are composed of low mg calcite with low contents of fe and mn and they have a stable isotope composition indicative of precipitation from meteoric water they form xenotopic mosaics in which goethite palygorskite and sepiolite may be present as inclusions and in the intercrystalline matrix cathodoluminescence microscopy reveals peripheral growth of the pseudospherulites which results from several phases of precipitation and even of dissolution the first phase of crystal growth may show rhombic habits and may contain corroded dolomite inclusions interpreted as an indication of growth at the expense of dolomicrite by a dissolution precipitation process the second phase of crystal growth is fibrous sensu stricto exhibits lobate growth laminae and shows indications of displacive growth the pseudospherulitic fabric is not relict and the pseudospherulites do not represent speleothems recrystallized microcodium or neomorphosed spherulitic carbonate instead they are interpreted as split crystals that grew within dolomicritic host sediments in part by dissolution precipitation and in part by displacement the hypothesis of a dolomicritic host is supported by the geological and diagenetic setting by the presence of corroded dolomite inclusions within the pseudospherulites and by the relationship between their growth and the authigenesis of sepiolite palygorskite and goethite the pseudospherulites grew in the shallow subsurface at or near the water table from predominantly oxidizing meteoric water and under periodically fluctuating geochemical conditions saturation redox caused by fluctuations of the water table during periods of subaerial exposure the presence of fossil bacteria within the pseudospherulites the anastomosing pattern of the inclusion defined fibers and the lobate growth forms of the fibrous calcite suggest some bacterial influence in the genesis of the pseudospherulitic fabric

replacement of aragonite by quasi rhombohedral dolomite in a late pleistocene tufa mound great salt lake utah u s a abstract fabric preserving quasi rhombohedral dolomite forms as cement and replacement of aragonite in a tufa mound 2 m thick on the western shoreline of the great salt lake the rounded crystals range in size from 5 to 20 m in diameter crystal faces are not pitted and have numerous sharp edged steps and kinks the quasi rhombohedral forms are the result of crystal growth phenomena and not of post crystallization dissolution microprobe analyses n 92 of the dolomite yield an average 46 mole mgco3 in agreement with the 44 to 45 mole mgco3 content determined from x ray diffraction 30 8 2 position of the 104 peak the great salt lake dolomite is poorly ordered with relative ordering of 28 5 and a reflection ratio of 0 9503 compared to a well ordered standard dolomite the great salt lake dolomite has the poorest ordering of any natural dolomite recorded to date more than half of the electron microprobe analyses show elevated sr abundance indicating that dolomite formed in a rock buffered diagenetic system where much of the bicarbonate and ca were supplied by dissolution of aragonite and mg from the lake water preservation of the tufa fabric by dolomite was accomplished by the closely related dissolution of aragonite and precipitation of dolomite in a fluid undersaturated with respect to aragonite and supersaturated with respect to ca rich dolomite formation of spheroids might reflect a high growth rate from a high alkalinity fluid that was undersaturated with respect to other carbonate minerals maintenance of this growth rate might have been aided by the ca enrichment and cation disordering of the precipitating dolomite phase and the large surface area of a sphere compared to that of an inscribed rhomb

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

gravity inversion using seismically derived crustal density models and genetic algorithms an application to the caribbean south american plate boundary summary crustal density models derived from seismic velocity models by means of velocity density conversions typically reproduce the main features of the observed gravity anomaly over the area but often show significant misfits given the uncertainty in the relationship between velocity and density seismically derived density models should be regarded as an initial estimate of the true subsurface density structure in this paper we present a method for estimating the adjustments necessary to a seismically derived density model to improve the fit to gravity data the method combines the genetic algorithm paradigm with linear inversion as a way to approach the non linear and linear aspects of the problem the models are divided into three layers representing the sedimentary column the crystalline crust and the lithospheric mantle the depths of these layers are determined from the seismic velocity model each of the layers is divided into a number of provinces and a density adjustment value is found for each province so that the residual gravity difference between the observed gravity anomaly and the anomaly calculated for the seismically derived model is minimized while keeping between predefined bounds the preferred position of the province boundaries is found through the artificial evolution of a population of solutions given the stochastic nature of the algorithm and the non uniqueness of the problem different realizations can yield different solutions by performing multiple realizations we can analyse a set of solutions by taking their mean and standard deviation providing not only an estimate of the distribution in the subsurface but also an estimate of the associated uncertainty synthetic tests prove the ability of the algorithm to accurately recover the location of province boundaries and the values for a known model when using noise free synthetic data when noise is added to the data the algorithm broadly recovers the features that define the known model despite greater standard deviations of the solutions and the occurrence of artefacts in the mean solutions the algorithm was applied to four profiles across the caribbean south america plate boundary some general patterns in the distribution of were observed consistently in the profiles and are correlated with the interpretations of the velocity models positive values in the sedimentary layer negative values for island arc and extended island arc crust with an abrupt change to positive values in south american crust and positive values in the mantle under the continent and island arc with a transition to negative values under the caribbean oceanic crust

atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps interferogram images derived from repeat pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor other tropospheric variations such as pressure and temperature also induce distortions but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term spatial and temporal changes of 20 in relative humidity lead to 10 cm errors in deformation products and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries in wet regions such as hawaii these are by far the dominant errors in the spaceborne imaging radar c and x band synthetic aperature radar sir c x sar interferometric products the unknown time delay from tropospheric distortion is independent of frequency and thus multiwavelength measurements such as those commonly used to correct radar altimeter and global positioning system gps ionospheric biases cannot be used to rectify the error in the topographic case the errors may be mitigated by choosing interferometric pairs with relatively long baselines as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline for the sir c x sar hawaii data we found that the best longest baseline pair produced a map supporting 100 m contouring whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval in the case of deformation map errors the result is either independent of baseline parameters or else very nearly so here the only solution is averaging of independent interferograms so in order to create accurate deformation products in wet regions many multiple passes may be required rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are 1 to use the longest radar wavelengths possible within ionospheric scintillation and faraday rotation limits 2 for topography maximize interferometer baseline within decorrelation limits and 3 for surface deformation use multiple observations and average the derived products following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions such as hawaii

seismic structure of the crust and upper mantle in the peace river arch region canada four intersecting 300 km long reversed refraction lines within northwestern alberta and northeastern british columbia have been interpreted for crustal and upper mantle seismic velocity structure in the peace river arch pra region of the western canada sedimentary basin the data have been modeled using a two dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals in addition an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles the major features of the structural models are 1 weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low velocity zones within the crust 2 an average sub basement rms crustal velocity of 6 6 km s average upper mantle velocity of 8 25 km s and average crustal thickness of 40 km 3 regional variations in structure which appear related to the dominant n s trending cratonic structure including crustal thickness upper crustal and upper mantle velocities and pmp character and 4 subtle variations in structure that may be associated with the e w trending devonian axis of the pra including a shallowing of high lower crustal velocities thickening of the crust and an anisotropic pmp character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the pra region evidence from the refraction and other geophysical data for the presence of a local crustal expression of the pra is weak but suggests a thermal as opposed to flexural origin for its anomalous vertical movements

crustal structure and erosion of the lofoten vesteralen shelf northern norwegian margin abstract the norwegian continental shelf has been through several rift phases since the caledonian orogeny early cretaceous rifting created the largest sedimentary basins and early cenozoic continental breakup between east greenland and europe affected the continental shelf to various degrees the lofoten vesteralen shelf is located off northern norway bordering the epicontinental barents sea to the northeast and the deep water lofoten basin to the west an ocean bottom seismometer hydrophone obs survey was conducted over the shelf and margin areas in 2003 to constrain crustal structure and margin development this study presents profile 8 03 located between the islands of lofoten vesteralen and the shelf edge the wide angle seismic data were modeled using forward inverse raytracing to build a crustal velocity depth transect gravity modeling was used to resolve an ambiguity in seismic moho identification in the southwestern part results show a crustal thickness of 31 km significantly thicker than what a vintage land station based study suggested profile 8 03 and other obs profiles to the southwest show high sedimentary velocities at or near the seafloor increasing rapidly with depth sedimentary velocities were compared to the velocity depth function derived from an obs profile at the barents sea margin tied to a coincident well log where there is little erosion results from this profile and the crossing profile 6 03 breivik et al 2017 indicate three major erosion episodes late triassic early jurassic tentatively mid cretaceous late cretaceous early cenozoic and a minor late glacial erosion episode off vesteralen

dolomite in permian paleosols of the bravo dome co2 field u s a permian reflux followed by late recrystallization at elevated temperature abstract lower permian abo tubb subsurface strata of northeastern new mexico u s a consist of 30 to 120 m of fluvial and eolian deposits containing numerous dolomitic paleosols dolomite in the abo tubb interval occurs in discrete intervals interpreted as paleosols on the basis of recognizable horizonation and the presence of associated features such as ped structures and rhizoliths the dolomite commonly forms veins rhizoliths and discrete and coalesced nodules petrographic analysis indicates four dolomite types present in paleosol horizons microsparitic pseudospherulitic clear cement and cloudy inclusion rich cement microsparitic dolomite constitutes most of 70 the dolomite and is stoichiometric with relatively low sr and na and elevated fe and mn pseudospherulitic dolomite forms 15 of the dolomite and consists of stoichiometric microspar and spar with abundant fluid inclusions low sr na and fe and elevated mn clear dolomite cement constitutes approximately 12 of the dolomite and is finely to coarsely crystalline with a relatively calcian composition low sr and na and elevated fe and mn the cloudy cement forms a minor 2 phase of very coarsely crystalline rhombs with a calcian composition low sr and na values and relatively high fe and mn all dolomite types exhibit a mottled texture of moderate to bright orange and red luminescence fluid inclusion analysis was not possible on the most common microsparitic dolomite type but analyses on the coarser varieties reveal homogenization temperatures typically ranging from 70 c to 110 c and final melting temperatures of ice ranging from 20 c to 34 c suggesting fluids of high 25 by weight salinity bulk rock carbon and oxygen isotopes of carbonate nodules and rhizoconcretions from four paleosol profiles most commonly exhibit mean values typically ranging between 3 to 5 5 vpdb and 0 5 to 2 8 vpdb respectively sulfur isotopes were analyzed for the overlying cimarron anhydrite interval and an anhydrite nodule near the base of the study interval both samples yielded similar results suggesting that brines that precipitated the cimarron anhydrite also influenced the study interval the dolomite in the abo tubb interval is interpreted to have precipitated in multiple stages solid calcite microinclusions in microsparitic and pseudospherulitic dolomite suggest that pedogenensis and groundwater penecontemporaneous with early permian deposition led to precipitation of calcite rather than dolomite oxygen and sulfur isotope data indicate dolomitization from later reflux of brines associated with deposition of the overlying leonardian cimarron anhydrite petrographic relations and elevated temperatures and salinities recorded by the fluid inclusions in the coarse dolomite are consistent with recrystallization of the earlier dolomite and precipitation of dolomite cement from warm saline fluids of a possible tertiary hydrothermal origin just predating migration of mantle derived gas into the bravo dome reservoir these findings offer insight into questions about the climate significance of dolomite in paleosols origin of dolomite in hydrothermally altered hydrocarbon reservoirs and the long term effect of co2 sequestration in saline aquifers the results show that dolomitic nodules in paleosols may form as replacements of paleosol calcite long after the paleosol formed primary pedogenic dolomite would undoubtedly have climatic and paleoenvironmental significance but recognizing it as forming during soil formation as opposed to later dolomitization is key in establishing that significance also this study provides an example of high temperature recrystallization of low temperature dolomite a process hotly debated in discussions of dolomitized hydrocarbon reservoirs with evidence for hydrothermal fluid flow finally as a natural co2 reservoir the bravo dome system shows that the long term result of dissolution of co2 into brines is predominantly carbonate mineral dissolution

dolomitization in a mixing zone of near seawater composition late pleistocene northeastern yucatan peninsula abstract patches of dolomite occur in cores of reefal limestone from the shallow subsurface on the northeastern coast of the yucatan peninsula this limestone accumulated during an interglacial high stand of sea level about 200 000 years ago dolomitization was preceded by freshwater diagenesis including precipitation of sparry calcite cement stabilization of mg calcitic skeletal fragments and partial dissolution of aragonitic components this suggests a predolomitization lowering of sea level with the consequent freshening of pore water the subsequent precipitation of dolomite indicates a return to high sea level with the consequent increase in mg ca ratio of pore water dolomitization took place during a brief high stand of sea level either shortly after deposition about 200 000 yr bp r more likely about 125 000 yr bp dolomite occurs both as microcrystalline replacement dolomite and as cement the cement is part of the following diagenetic sequence 1 limpid euhedral subhedral calcian dolomite crystals 2 zoned dolomite crystals with zones formed by variations of the calcium magnesium ratio in dolomite 3 layers of alternating calcian dolomite and magnesian calcite or calcite and 4 calcite this sequence represents the progressive freshening of ground water during the initial stage of a fall in sea level average cation composition of the limpid dolomite cement is ca57mg43 electron microprobe analysis zoned cement crystals are composed of ca57 59mg43 41 layers and ca62mg38 layers most of the higher calcium dolomite layers are dissolved forming hollow zone crystals in cement with alternating dolomite and calcite zones the calcite is ca99 97mg1 3 low mg calcite and ca96 93mg4 7 mg calcite the dolomite and mg calcite zones are partially to totally leached 18o compositions of yucatecan dolomite and of modern ground water suggest dolomite precipitation from mixed water ranging from about 75 seawater 25 freshwater to nearly all seawater isotope analyses are for the most stable calcian dolomites more soluble calcium rich dolomite presumably is analyzed with calcite and thought to be isotopically lighter than the less soluble dolomite in the cement sequence the most stable dolomite is followed by more soluble dolomite as ground water becomes less saline isotope analyses together with position of dolomite in the cement sequence suggest the most stable calcian dolomite including limpid dolomite precipitated from mixed water with large proportions of seawater and the less stable more calcian dolomite precipitated from fresher mixed water

calcrete and near surface geology of kuwait city and suburbs kuwait arabian gulf abstract near surface accumulation of pedogenic carbonate calcrete is a common and characteristic feature of the landscape of kuwait city and its suburbs the principal host material for these calcretes is a thick sequence of unconsolidated quartzose sand a typical fully developed soil profile can be divided into five zones in descending order these are 1 a top calcareous aeolian soil 2 a zone of friable sand with powdery or tightly cemented calcrete lumps nodules 3 a massive calcrete zone 4 a mottled calcrete zone and 5 unaltered parent material episodic formation of calcrete is indicated by the occurrence of multiple profiles in which zones are repeated calcrete ooids and pseudo ooids of accretionary origin are characteristic of the upper calcrete zone the carbonate fraction in the upper calcrete zone consists dominantly of low mg calcite with minor amounts of dolomite replacement was a major process in formation of the massive calcrete and the dominant cementing material is largely low mg calcite and or microcrystalline dolomite aeolian dust is considered to be the major source of ions for calcrete formation in kuwait city and suburbs an earlier introduced model is applied to explain the mechanisms of formation and precipitation of low mg calcite and dolomite the calcrete thickness in the study area is explained by episodic deposition of fluviatile sand and calcrete formation during the oligocene through lower miocene period

3 d modelling of alpine mohos in southwestern alps summary we present and discuss a 3 d geometrical model of the moho topography in the southwestern alps to achieve this objective we used the potential of 3 d modelling software i e a 3 d geomodeller to combine gravity seismic and seismological constraints in a same and coherent 3 d space a new regional bouguer gravity anomaly map of the southwestern alps was calculated and filtered to isolate the moho signature in the southwestern alps then two alternative 3 d models were computed with data from the literature based on seismic and seismological constraints the first one represents an european lithospheric mantle decoupled from the european orogenic crust which is back thrusted by the ivrea body whereas the second 3 d model illustrates the subduction of a 20 km thick piece of lower continental crust coupled with the european upper mantle beneath the ivrea body according to the geological knowledge of these units we then assigned a density value to each modelled layer and we computed their gravity effects to compare them with the filtered moho map of the southwestern alps first of all the significant discrepancies of the moho gravity signature generated by these two geometrical models reveal that subduction of continental crust is no more active in the present day configuration of the southwestern alps therefore the first 3 d model was refined by a stochastic 3 d gravity inversion based on this processing our investigations confirm the presence of three superposed mohos in the southwestern alps and underline that

an overview of the surface and near surface geology geomorphology and natural resources of kuwait abstract the surface and near surface geology geomorphology and natural resources of kuwait have been reviewed based on the published work the stratigraphy and structure of the near surface sediments of kuwait have been mainly controlled by the tertiary tectonic activity of the arabian shield the shield started separating from the african mainland by the close of the eocene and drifted northeastward towards the asian plate giving rise to the folding and thrusting of the zagros mountains of iran the tectonic activity superimposed a northwest southeast trend over an earlier north south structural trend and gave rise to parallel ridges and valleys trending northwest southeast through differential erosion it has been possible to identify 15 geomorphic zones and 11 morphostructures from analysis of the surface features of kuwait seven of these morphostructures can be attributed to tectonic processes and two to fluvial processes it has not been possible however to explain satisfactorily the origin of the jal az zor escarpment or the ad dalmaniyah cliff the whole of kuwait can be divided into seven drainage zones of these the internal drainage systems of the ar raudhatain umm al aish drainage system and the ad dibdibba drainage system are hydrogeologically most important the clastic deposits of kuwait group sediments often affected by low temperature carbonate calcite and dolomite and sulfate gypsum precipitation form the surface rock of kuwait these are covered by recent aeolian and residual deposits playa deposits in the enclosed basins and near the coast oolitic sediments beach sands and sabkha deposits the surface and near surface deposits of kuwait contain important reserves of construction sand and gravel as well as the only naturally occurring freshwater reserves of the country an understanding of the nature of these surface deposits is required for the management of environmental issues like sand encroachment land use planning and soil and groundwater pollution

using a genetic algorithm for 3 d inversion of gravity data in fuerteventura canary islands the use of genetic algorithms in geophysical inverse problems is a relatively recent development and offers many advantages in dealing with the non linearity inherent in such applications we have implemented a genetic algorithm to efficiently invert a set of gravity data employing several fixed density contrasts this algorithm determines the geometry of the sources of the anomaly gravity field in a 3 d context the genetic algorithms based on darwin s theory of evolution seek the optimum solution from an initial population of models working with a set of parameters by means of modifications in successive iterations or generations this searching method traditionally consists of three operators selection crossover and mutation acting on each generation but we have added a further one which smoothes the obtained models in this way we have designed an efficient inversion gravity method confirmed by both a synthetic example and a real data set from the island of fuerteventura in the latter case we identify crustal structures related to the origin and evolution of the island the results show a clear correlation between the sources of gravity field in the model and the three volcanic complexes recognized in fuerteventura by other geological studies

distinguishing early groundwater alteration effects from pedogenesis in ancient alluvial basins examples from the palaeogene of southern portugal colour mottling and horizons of secondary carbonates are common in ancient alluvial sequences and are normally interpreted as pedogenic features they have been used to assess palaeoclimates soil drainage conditions and deposition rates palaeogene alluvial deposits in the sado and lisbon basins of portugal exhibit prominent colour variations and mottle patterns as well as carbonate accumulations both at the bases of fining upwards cyclothems and as thick units up to 20 m capping alluvial megasequences however these colour and carbonate features are interpreted as the products of shallow saline reducing groundwaters unrelated to pedogenesis such non pedogenic products are easily mistaken for soil formed ones and criteria for differentiating the two are reviewed to assist interpretations in other alluvial deposits key criteria are thickness gradational tops and bases absence of soil horizon features occurrence in coarser alluvium and prevalence of hydromorphic colour and mottling patterns

pedogenic phreatic carbonates on a middle devonian givetian terrigenous alluvial deltaic plain gilwood member watt mountain formation northcentral alberta canada in the muskeg trough of northcentral alberta the gilwood member contains widespread carbonate deposits that formed within terrigenous mudstone and sandstone hosts stratigraphic depositional and petrographic relationships indicate that these carbonates represent calcretes and dolocretes calcretes observed best with cathodoluminescence display microcrystalline alpha fabrics circumgranular cracks root networks displacive growth fabrics elongate channel voids and rare coloform growths with flower spar similarly dolocretes have microcrystalline alpha fabrics brecciation gradational contacts with host mudstones extensive layered nodular horizons and are associated with anhydrite and pyrite 13c values range between 7 to 1 and 6 to 3 for calcretes and dolocretes respectively oxygen isotopes are more variable and differ with host lithologies 18o of calcretes ranges between 11 to 8 for sandstones and 8 to 3 for mudstones whereas 18o of dolocretes ranges between 3 to 1 for marine mudstones and 6 to 2 for pedogenic mudstones regional mapping indicates that calcretes thicken towards the deepest parts of the muskeg trough widespread dolocretes extend beyond the eastern and western limits of muskeg trough and are useful marker intervals for regional correlations dolocretes of restricted lateral extent are found within gleyed palaeosol mudstones next to calcretized channel sandstones calcrete isotopic values are interpreted as indicative of carbonate precipitation from waters with meteoric water input however the higher 18o values in dolocretes are indicative of a contribution from an isotopically heavier source such as seawater stratigraphically calcretes are most common along the western and northern edges of muskeg trough thus calcrete accumulation was further controlled by meteoric water in flow from the highland to the west and sluggish groundwater flow in muskeg trough in contrast regionally widespread dolocrete horizons appear to have formed from mixing of fresh waters derived from the highland to the west and seawaters introduced from the east regionally restricted dolocretes which are found next to channel sandstones formed from groundwater out flow from the permeable channel sandstones which resulted in calcretization in channel proximal mudstones and dolomitization in channel distal mudstones

relationship between climate and vegetation and the stable carbon isotope chemistry of soils in the eastern mojave desert nevada abstract the relationship between the stable c isotope composition of the soil environment and modern climate and vegetation was determined empirically along a present day climatic transect in the eastern mojave desert the 13c of the soil co2 and carbonates decreased with increasing elevation and plant density even though plant assemblages at all elevations were isotopically similar several factors including differences in the ratios of pedogenic of limestone calcite and differences in past vegetation were considered as explanations of this trend however it appears that in the sparsely vegetated mojave desert the 13c of pedogenic carbonate is controlled by differences in plant density and biological activity this relationship may provide a tool for assessing past vegetational densities as long as the vegetation is isotopically homogeneous

occurrence of phreatic dolocrete within tertiary clastic deposits of kuwait arabian gulf abstract the term dolocrete is used in the present study to describe siliciclastic sediments which are partially or completely cemented and or replaced by terrestrial phreatic dolomite several successive dolocrete profiles occur within the subsurface tertiary fluviatile muddy sand and mudstone deposits in kuwait city three zones were recognized within these dolocrete profiles these are in descending order 1 dolocrete band 2 dolocrete nodules and 3 incipient dolocrete dolocrete also occurs within the compact green mudstone layers the studied dolocrete comprises detrital grains inherited from the host sediment and authigenic minerals precipitated during the dolocretization processes the detrital grains include mainly quartz feldspars illite and smectite the abundance of authigenic minerals dolomite calcite and palygorskite relative to detrital grains reflects the degree of dolocrete maturation mature dolocrete is chiefly composed of dolomite and palygorskite several dolomite fabrics and textures were recognized however dolocretic dolomite is generally present as incomplete rhombohedra with spiky terminations variation in the nature of host sediment and diagenetic conditions are responsible for the degree of dolocretization and diversity of fabrics and textures it is suggested that the dolocrete was developed by the precipitation of dolomite from pore solution saturated with respect to dolomite within the phreatic zone three main sources of chemicals required for dolocretization are suggested 1 mg rich groundwater from the dolomitic dammam formation 2 mg and co 2 released from clay minerals and organic matter in the host sediment mud matrix and 3 infiltrated meteoric water

spheroidal dolomites in a visean karst system bacterial induced origin spheroidal dolomite crystals occur in the karstified top of a dinantian dolomite sequence in eastern belgium the spheroidal dolomite crystals are best developed at the base of the karst system the dolomite crystals are characterized by a spherulitic or dumb bell inclusion pattern and are overgrown by dolomite cements with a rhombohedral outline they are considered to be bacterially related precipitates based on 1 textural similarities with documented bacteriogenic precipitates 2 the presence of bacterial microspheres and framboidal pyrite embedded within the dolomite and 3 their general geological setting the geochemical characteristics of the dolomites and associated minerals support a bacterial origin the ubiquity of framboidal pyrite depleted in 34s 34s 22 4 to 25 5 ocdt testifies to a period of bacterial sulphate reduction the isotopic composition of the spheroidal dolomites 13c 2 4 to 3 2 opdb and 18o 3 8 to 3 4 opdb suggest a contribution from oxidized organic carbon produced during bacterial sulphate reduction sulphate reduction may also result in a concomitant 18o depletion if the system is nearly closed it is however evident from the sulphur isotopic composition of associated framboidal pyrite that the system was fairly open the 18o depletion of the spheroidal dolomite crystals 18o 3 8 to 3 4 opdb and their occurrence adjacent to and within karst cavities suggests a mixing zone origin with a significant proportion of freshwater in it the rhombohedral cement overgrowths have calculated 18o values in the range of 0 to 5 3 opdb which reflect precipitation from normal to slightly evaporated contemporaneous seawater

development of magnesian carbonates in quaternary soils on the island of hawaii abstract pedogenic carbonate minerals have progressively developed on basalt in a quaternary weathering chronosequence 5 350 ka on the island of hawaii the sites were isolated from direct contact with seawater or the water table and were minimally influenced by eolian input throughout their history the carbonates consist chiefly of high magnesium calcite hmc and well ordered stoichiometric dolomite pedogenic development of such minerals is rare and the conditions of their formation bear on interpretations of paleosols and the development of sedimentary magnesian carbonates 87sr 86sr ratios of chronosequence carbonates range from 0 7038 to 0 7048 the data indicate that 75 of the strontium was derived from the weathering of volcanic parent material even in the oldest soils where few primary minerals are available to provide ca and mg this is in contrast to continental calcretes which are commonly dominated by atmospheric inputs and are generally composed of low mg calcite lmc stable isotope data are consistent with carbonate precipitation from soil waters in an arid environment the magnesium content of the carbonate increases with substrate age and textural and mineralogical changes suggest direct precipitation of dolomite as well as dolomitization of early formed lmc and hmc opal was detected in young 5 10 ka samples and gibbsite is present in older samples indicating a shift from silica dominated to aluminum dominated soil solution high concentrations of dissolved silica and aluminum during early weathering likely precluded formation of mg rich secondary clay minerals such as smectite or palygorskite weathering of basalt with a molar mg ca ratio of 0 8 1 5 and the absence of secondary magnesian silicates can account for elevated soil water mg ca ratios 0 7 1 0 in soils 70 ka that led to magnesian carbonate formation over time the formation of a clay rich soil profile prevented rapid evaporation of soil water possibly facilitating crystallization of stoichiometric dolomite

petrography geochemistry and primary origin of spheroidal dolomite from the upper cretaceous lower tertiary maghra el bahari formation at gabal ataqa northwest gulf of suez egypt abstract field petrographical and geochemical studies of the maghra el bahari formation at gabal ataqa in egypt indicate an occurrence of spheroidal dolomite as a primary precipitate petrographically this dolomite is microcrystalline 10 40 m in diameter and lacks any direct or indirect evidence of a precursor mineral each dolomite spherule is composed of a cluster of coalesced incomplete rhombohedral sub units exhibiting a globular body with a hollow core this dolomite is commonly associated with authigenic palygorskite and gypsum based on its associations the spheroidal dolomite occurs in two modes i siliciclastic disseminated dolomite and ii evaporitic associated dolomite on the basis of its elemental and stable isotopic signature the spheroidal dolomite is subdivided into two types type a and type b type a is ca rich with average caco3 and mgco3 equal to 58 20 and 39 91 mol respectively and with average sr 108 ppm average na 656 ppm average mn 72 ppm and 18o ranges from 0 4 to 0 9 pdb type b compared with type a is nearly stoichiometric with average caco3 and mgco3 equal to 49 32 and 43 75 mol respectively with higher average sr 508 ppm average na 3008 ppm average mn 171 ppm and 18o 0 98 to 1 8 pdb slightly positive values of 13c 0 18 to 2 10 pdb are recorded for the two types of dolomite type a belongs to the siliciclastic disseminated dolomite whereas type b is of the evaporitic associated dolomite it is interpreted that the spheroidal dolomite directly precipitated from a solution that was supersaturated with respect to dolomite components and did not develop by dolomitization of precursor carbonates two main sources are suggested for dolomite precipitation hypersaline water and mixed saline fresh water

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

brackish water and evaporitic ca mg carbonates in the holocene lacustrine deltaic deposits of southern mesopotamia authigenic dolomite mg calcite and calcite occur in the holocene near surface marshland ahwar sediments and some subsurface strata of southern mesopotamia dolomite has been detected in evaporitic and brackish marine sedimentary units of both the siliciclastic and carbonate sequences whereas high mg calcite and low mg calcite are mainly restricted to brackish marine and lacustrine carbonate units respectively the dolomite which is polygenetic is highly calcian and has the composition ca 1 12 1 22 mg 0 88 0 78 co 3 2 the calcite is of two distinct phases low mg and high mg consisting of 5 mole or less and 10 mole or more mgco 3 respectively the dolomite samples show a spread of 18 o values from 8 pdb for detrital dolomite 3 to 6 5 for inferred dilute brackish samples 1 to l for mixing zone samples and 3 to 5 for evaporitic types the influence of sea level fluctuations during the holocene evolution of the tigris euphrates delta has resulted in the formation of different sedimentary environments of mixing marine and fresh waters favourable for ca mg carbonates formation evaporitic environments were the main source for dolomite formed in the early holocene

the character and genesis of calcrete in late quaternary alluvial deposits gujarat western india and its bearing on the interpretation of ancient climates late quaternary deposits in mainland gujarat contain sediments deposited in subhumid and semi arid climates the 30 35 m succession shows the presence of vertisols at the base and a red bed horizon ferric calcisol when pedogenic in origin that roughly bisects the succession a widespread development of calcretes is observed throughout the succession the various varieties of calcrete include pedogenic calcrete groundwater calcrete calcrete conglomerate transported calcrete and rhizogenic calcrete pedogenic calcrete nodules associated with vertisols and the red soil show marked differences in morphology dimensions and the distribution of microscopic features these differences arise due to contrasting climate controlled physicochemical environs under which they formed pedogenic calcrete nodules associated with vertisols acquire large 5 10 cm dimensions and are characterised by either showing the presence of a nucleus of soil matter or showing a dense micritic groundmass cut by thick sparitic veins in contrast calcrete nodules that formed in the red soil are 3 cm in size and do not exhibit dense networks of sparitic veins calcretes associated with vertisols and the ferric calcisol also exhibit differences in the morphology of rhizoliths these differences also show up in the distribution of microfabrics of calcretes grain coats are present in rhizogenic and pedogenic calcretes but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes clotted micrite is present in all types except groundwater calcretes sparitic veins however are observed in each type but are relatively less developed in groundwater calcretes a similar distribution of displacive and replacive textures is also seen although some grains in groundwater calcretes showed signs of corrosion the vertisol associated calcretes represent a subhumid 500 700 mm climate whereas the red soil calcretes suggest a semi arid 100 500 mm climate calcretes from the vertisol association show a range in 13c composition constrained between 9 and 5 whereas the red soil calcretes exhibit the whole spectrum of values from 9 to 1 based on mineralogical associations calcretes usually taken to reflect semi arid episodes in the earth s history may be classified further calcretes when associated with sepiolite palygorskite suggest an arid climate mean annual rainfall 50 100 mm when associated with smectite haematite and the absence of hydromorphism a semi arid climate mean annual rainfall 100 500 mm and when found in smectitic vertisols subhumid climates mean annual rainfall 500 700 mm

late pleistocene mixing zone dolomitization southeastern barbados west indies field geochemical and petrographic data for late pleistocene dolomites from southeastern barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters the dolomite is localized in packstones and wackestones from the algalamphistegina fore reef calcarenite facies stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization carbon isotopes in pure dolomite phases average about 15 0 pdb this light carbon is attributed to the influence of soil gas co2 and precludes substantial mixing with seawater a narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint dolomitization likely occurred with as little as a five per cent admixture of seawater strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy the dolomite is characterized by low sodium values low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization a sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix limpid euhedral dolomite cements precipitated in primary intra and interparticle porosity subsequent to replacement dolomitization as waters became progressively less saline dolomite cements alternated with thin bands of syntaxial calcite cement the final diagenetic phase precipitated was a blocky calcite spar cement representing diagenesis in a fresh water lens this sequence of diagenetic features arose as the result of a single fall in eustatic sea level following deposition a stratigraphic eustatic diagenetic model constrains both the timing and rate of dolomitization in southeastern barbados dolomitization initiated as sea level began to fall immediately following the oxygen isotope stage 7 3 high stand some 216 000 yr bp due to the rapidity of late pleistocene glacio eustasy dolomitization locally complete is constrained to have occurred within about 5000 yr

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

the caribbean south american plate boundary at 65 w results from wide angle seismic data 1 we present the results of the analysis of new wide angle seismic data across the caribbean south american plate boundary in eastern venezuela at about 65 w the 500 km long profile crosses the boundary in one of the few regions dominated by extensional structures as most of the southeastern caribbean margin is characterized by the presence of fold and thrust belts a combination of first arrival traveltime inversion and simultaneous inversion of pmp and pn arrivals was used to develop a p wave velocity model of the crust and the uppermost mantle at the main strike slip fault system we image the cariaco trough a major pull apart basin along the plate boundary the crust under the southern caribbean deformed belt exhibits a thickness of 15 km suggesting that the caribbean large igneous province extends to this part of the caribbean plate the velocity structures of basement highs and offshore sedimentary basins imaged by the profile are comparable to those of features found in other parts of the margin suggesting similarities in their tectonic history we do not image an abrupt change in moho depth or velocity structure across the main strike slip system as has been observed elsewhere along the margin it is possible that a terrane of caribbean island arc origin was accreted to south america at this site and was subsequently bisected by the strike slip fault system the crust under the continental portion of the profile is thinner than observed elsewhere along the margin possibly as a result of thinning during jurassic rifting

link between long lasting evaporitic basins and the development of thick and massive phreatic calcrete hardpans in the mississippian windsor and perce groups of eastern canada part of the visean upper mississippian succession in the upper paleozoic maritimes basin of eastern canada is overprinted by massive phreatic calcrete hardpans that can exceed 10 m in thickness and that are characterized by the thorough mineral replacement of most of their host sediment by calcite similar to those that are currently forming around salt lakes in quaternary sediments of central australia the precise timing and paleogeography of this carboniferous event was until now poorly known but lateral correlations over a large study area indicate that phreatic calcretization occurred in the vicinity of large evaporitic basins following a marine transgression and regression cycle in chadian to holkerian times this relation confirms that the previously proposed model for modern analogs in central australia which states a genetic link between the salt lakes and the thick and massive phreatic calcrete hardpans can be applied to ancient environments and that such occurrences may be used to infer the former presence of an evaporitic basin in their vicinity finally our relatively large dataset indicates that the stable isotopes of carbon and oxygen are successful in differentiating ancient marine carbonates from ancient phreatic calcretes 2007 elsevier b v all rights reserved

mapping accumulated mine subsidence using small stack of sar differential interferograms in the southern coalfield of new south wales australia the southern coalfield is located in the sydney basin in the state of new south wales nsw australia the coal seams of the southern coalfield contain high quality hard coking coals which are mostly used for steel production this paper describes an approach developed to study the subsidence associated with underground coal mining activity in the west cliff colliery in the southern coalfield of nsw using multiple sar differential interferograms accumulated subsidence maps have been derived using the approach within the study area results obtained by processing ten 10 alos palsar images showed that the land surface had subsided by more than 700 mm in the area of west cliff colliery longwall 32 during the period of image acquisitions june 2007 to october 2008 the results have been compared with deformation predicted by modelling as well as with the available gps field survey data high correlation has been observed between the dinsar derived subsidence results and the predicted mining induced deformation by comparing the dinsar measured subsidence with the gps derived results between september 2007 and march 2008 the magnitude and trend of the deformation has been confirmed the absolute differences range from 0 to 45 mm with a standard deviation of 8 mm and an average absolute difference of 12 mm

the nature and possible origin of mega dunes in liwa ar rub al khali uae abstract the liwa area is located in the south central part of the uae along the northeastern upwind margin of the ar rub al khali sand sea and covers about 3694 km 2 it has one type of dunes a mega barchan that is compound and complex in form characterized by the superimposition of smaller dunes due to the giant size of barchans interdune areas appear as closed depressions that vary in shape from irregular elongate triangular to circular or semi circular these depressions are mostly covered with small barchans or sabkha deposits with patches of gypcrete in their upper parts scattered flat top mounds are also observed in interdune areas the size of dunes in liwa is directly controlled by quaternary climatic changes multiple generations are superimposed on one another separated by calcretic horizons the climate during dune formation alternated between relatively arid to humid the dune sands are unimodal with a modal class of fine sand moderately to very well sorted and near symmetrical and mesokurtic quartz grains are mainly subrounded to subangular based on the heavy mineral suites the sand came mostly from the ultrabasic rocks that form the ophiolitic sequence of the oman mountains the acidic rocks of iran and from the arabian shield

paleoclimatic reconstruction in northern oman based on carbonates from hyperalkaline groundwaters abstract a paleoclimatic reconstruction for the past 35 000 years for northern oman is based on an unusual approach using travertines and fracture calcites associated with hyperalkaline springs high ph groundwaters ph up to 11 9 discharge from the mantle sequence of the oman ophiolite as the product of modern low temperature serpentinization under arid climatic conditions hyperalkaline discharge occurs at the surface uptake of atmospheric co 2 precipitates characteristic laminated travertines accompanied by strong kinetic depletion of 13 c and 18 o pluvial climates supporting a shallow bicarbonate groundwater flow system and vegetation are recorded by fracture calcites with equilibrium stable isotope contents and calcite replaced roots and stems all such carbonates have modern initial 14 c contents allowing radiocarbon dating and paleoclimatic reconstruction for the late pleistocene and holocene our reconstruction shows a dominantly wet late pleistocene up to 19 000 yr b p when a phase of climatic deterioration began leading to a period of hyperaridity which dominated from ca 16 300 to 13 000 yr b p the early holocene pluvial occurred from 12 500 to ca 6500 yr b p and was followed by renewed climatic deterioration and the current phase of hyperaridity comparison of this paleoclimatic reconstruction with that for lacustrine deposits from the a rub al khali of central saudi arabia and the summer insolation driven monsoon record of east africa and the arabian sea is remarkably good

the role of mixing zone dolomitization in sandstone cementation evidence from the triassic buntsandstein the iberian range spain abstract petrographic and geochemical data suggest that dolomite cement in shallow marine sandstones of the triassic buntsandstein in the ateca horst of the iberian range spain has precipitated in the zone of mixing between phreatic meteoric and marine waters dolomite or rhombohedric crystals consisting of concentric bands of alternating syntaxial dolomite and calcite have filled isolated voids 350 12 mm in diameter subsequently limpid dolomite followed by sparry calcite precipitated in void center as the latest cements the carbonates have low 13 c pdb 9 5 to 6 0 and 18 o pdb 8 1 to 3 2 values and are enriched in radiogenic strontium 87 sr 86 sr 0 710112 to 0 720293 indicating a dominantly meteoric component in the porewaters the overall sequence of carbonate cementation suggests precipitation from porewaters that evolved in the mixing zone with successively increased meteoric influence and finally to fully meteoric probably due to a major sea level fall

kuwaiti dolocrete petrology geochemistry and groundwater origin abstract near surface sediments in southern kuwait show extensive development of duricrusts the host materials are siliciclastic sandstones of the post eocene kuwait group inland the duricrusts are dominantly pedogenic calcrete with some silcrete and gypcrete whereas within 10 20 km of the coast dolomite is the dominant duricrust mineral both these dolocretes and the inland calcretes display a similar maturation sequence in which carbonate rich nodules develop and coalesce carbonate progressively replacing and displacing detrital grains the dolomite of the dolocretes forms mosaics of crystals typically 10 70 m in size varying from simple rhombs to spherulites an intermediate morphology named artichoke dolomite from its appearance in sem is particularly abundant authigenic palygorskite is associated with the dolomite dissolution of cores or zones within dolomite crystals has occurred calcite is present as sparry crystals always post dating dolomite and is the expected precipitate from present day soil and groundwaters chemical analyses of dolomite show highly negative values of 13 c to 10 7 pdb and 18 o varying from 0 6 to 3 3 pdb the oxygen isotope values are interpreted as reflecting evaporation of a marine based fluid manganese values of around 1000 ppm show that this fluid was reducing strontium data show variability reflecting mineral fluid reactions the general absence of metastable carbonates and presence of zoning in dolomite crystals suggests that meteoric dilution of seawater also occurred given that the duricrusts lack biogenic features the light carbon isotope values are taken to indicate oxidation of seeping hydrocarbons the dolocretes are interpreted as groundwater precipitates near the water table of a brackish water body formed at a time of higher relative sea level than today

characterizing the caribbean south american plate boundary at 64 w using wide angle seismic data 1 we present wide angle velocity modeling results from profile 64 w of the broadband ocean land investigation of venezuela and the antilles arc region bolivar project line 64w is a 460 km long approximately north south onshore offshore reflection refraction transect located approximately at 64 w longitude the profile extends across the transform plate boundary between the southeastern caribbean car and south american sa plates east of the profile the plate boundary bends to the north and sa subducts beneath car we utilize first arrival tomography to resolve a smooth velocity field for the sediments and upper middle crust and then use a layered approach to resolve a sharp velocity contrast for the moho simultaneously inverting refracted pn and reflected pmp arrivals we image crustal and upper mantle structure across the plate boundary zone we interpret that the strike slip system that accommodates relative motion between car and sa extends near vertically through the entire crust and offsets the moho we see no evidence supporting a major component of convergence and rather than a wide boundary zone of overlapping lithospheric plates we interpret the plate boundary to be confined to the 33 km wide near vertical strike slip system previously interpreted thrust faults flanking the strike slip system appear to be confined to the upper middle crust and may be related to the detachment of subducting south american lithosphere at the southern terminus of the lesser antilles subduction zone east of 64 w

geomorphological analysis of paleo drainage systems and their environmental implications in the desert of kuwait a well developed drainage network is carved in the hard calcretized and gypcretized gravelly sand of the pleistocene dibdibba formation in northern kuwait the present day aridity suggests that these drainages were developed during pluvial episodes that took place in the post pleistocene time and therefore are considered as paleo drainage detailed morphometrical analysis of the endoeric drainage systems was performed and the degree of correlation among their different morphometric parameters was investigated based on these parameters the studied drainage basins are statistically grouped into three groups such grouping was confirmed by discriminant analysis the categorical data analysis demonstrated the dependence of these groupings on the surface geology regional topography and local geomorphological settings infiltration measurements revealed that the drainage bedrocks have a low infiltration rate 20 cm h 1 whereas the drainage fill deposits have a relatively high infiltration rate 67 30 cm h 1 the impact of the drainage system pattern and morphometry on the hydrological conditions is discussed and potential near surface low salinity to freshwater aquifers is delineated the role of the studied drainage systems in the occurrence of mobile sand and sand dunes rainwater harvesting and land capability are discussed

model for how microbial methane generation can preserve early porosity in dolomite and limestone reservoirs in some dolomite and limestone hydrocarbon reservoirs protection from cementation is a primary factor in porosity preservation we present a model in which methanogens methane producing microorganisms produce methane gas ch4 g that outgasses from solution in pore space creating a two phase system that reduces effective hydraulic conductivity k protecting pore space from cementation methanogens have been implicated in dolomite formation and can generate methane to fill pore space of a model near surface carbonate sediment 37 5 primary porosity with ch4 g in 180 to 4650 yr depending on nutrient levels gas generation results in occlusion of the aqueous phase from pore spaces and throats creating a two phase flow regime reducing the effective hydraulic conductivity of model marine carbonate sand by about 50 from 2 8 to 1 2 cm day in as little as 55 yr therefore reducing aqueous fluid flow necessary for cementation despite rapid burial and complete cessation of methanogenesis effective hydraulic conductivity could take more than 100 000 yr to return to its original value if methanogenesis continues with burial the effective hydraulic conductivity is reduced to zero after 250 yr dolomites can preserve more primary porosity with depth than other carbonates we propose that in addition to increased structural resistance a biogenic model exists for porosity preservation in dolomites that is linked to the activity of methanogens this model represents specific end member cases and illustrates the effect of methane buildup in relationship to the extent of reservoir diagenesis

crustal structure at the contact of the dinarides and pannonian basin based on 2 d seismic and gravity interpretation of the alp07 profile in the alp 2002 experiment summary the area of this study includes the contact between the dinarides and the pannonian basin as a relation between the adriatic microplate and pannonian segment our analysis was carried out on profile alp07 which is a part of the alp 2002 experiment alp 2002 was large international seismic experiment that focused on the lithospheric structure of the eastern alps and surrounding areas profile alp07 is one of several refraction and wide angle reflection profiles located in the transition from the adriatic microplate through the dinarides and into the pannonian basin this 300 km long profile extends from istra croatia to the drava river at hungarian croatian border in a wsw ene direction it is oriented approximately perpendicular to the dinarides the main faults in the adriatic region and the contact between the dinarides and pannonian basin 2 d seismic modelling was done using tomographic inversion and ray tracing techniques the moho depth is the greatest in the area of the dinarides reaching about 40 km and is shallowest 30 20 km in the pannonian basin area at the boundary between these provinces the depth changes rather suddenly in order to obtain additional constraints on the crustal structure 2 d gravity modelling was also performed the layer boundaries were retained from the seismic model as there was no need to change them during modelling since varying densities in the model produced a good fit to the data a geological model was constructed based on both geophysical models three types of the crust were found along profile the dinaridic and the pannonian crusts that are separated by a relatively narrow transition zone the dinaridic upper crust is characterized by low seismic velocities and densities but its lower crust has high velocities and densities the pannonian crust can be seen as unique layer characterized by both low seismic velocities and densities large lateral and vertical changes in densities and seismic velocities can be found in the transition zone troughs in the seismic model at the level of the mohorovicic discontinuity are interpreted as major faults in the lithosphere three main lithospheric faults were identified in northeastern part of the dinarides under the sava depression and under the drava depression the first one may be considered as a result of subduction of the adriatic microplate under the pannonian segment similar movements are also defined within the transition zone where the pannonian segment is gradually rising over on the adriatic microplate

spheroidal dolomites origin by hydrocarbon seepage spheroidal dolomite cements are a pervasive and distinctive feature in five exposed lithologies of different geological age in kuwait the spheroids range in diameter from 5 350 m have concentric zones and nuclei of fluid inclusions with or without a radial fabric and compositions varying from near stoichiometric to ca56 mg44 all spheroids appear to be primary in origin with no evidence of replacement some concentric zones show selective calcitization and or leaching most of these features are confined to within 30 m of the surface consequently spheroidal dolomites recognizable at depth in ancient sequences may be indicative of a local unconformity the dolomite distributions are areally correlative with known areas of hydrocarbon seepage the latter being confined to prominent oil field anticlinal trends it is proposed that the dolomitization was caused by groundwaters carrying hydrocarbons to the surface which were then oxidized to carbon dioxide the gas bubbles may have acted as nuclei for spheroid growth with a possible inducement by bacteria which have an affinity for hydrocarbons the implication is that the regional distribution of spheroidal dolomites may be an indicator of a potential reservoir facies at depth

a spherulitic fabric in selectively dolomitized siliciclastic crustacean burrows northern kuwait abstract in a quaternary siliciclastic tidal bar deposit crustacean burrows have been selectively dolomitized giving two distinct zones of cement fabrics the inner central part of the burrow has a microdolomite fabric in contrast the outer wall shows a variety of coarse dolomites ranging from zoned rhombic or quasi rhombic crystal fabrics to rounded spherulitic fabrics comprising concentric layers and radial fibrous crystallites the latter fabric is derived from other much simpler arrangements but evidence of replacive features is lacking the boundary between the two fabric zones is sharp all evidence points to a primary origin for the spherulitic dolomite the composition of the dolomite varies from ca52 mg48 to ca57 mg43 and both fabric zones in a given burrow show variable dolomite compositions along its length these observations suggest that the composition of the dolomitizing fluids and the kinetics of crystal growth were not uniform during the growth of these cements it is surmised that fluid flow conditions and organic matter very likely related to bacterial activity within the burrows were probably significant controls of selective dolomitization and cement fabrics

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

petrography and diagenesis of cavity fill dolocretes kuwait abstract cavity fill dolocretes within the palaeokarst zone of the middle eocene chertified dolomicrite dammam formation are exposed within a quarry located on the top of al ahmadi ridge in southern kuwait they are characterised by the abundance of zoned and spherulitic dolomite these dolocretes are formed by massive precipitation of primary dolomite into cavity fill siliciclastics within phreatic groundwater conditions during arid to semi arid period successive stages of karstification and dolocretisation are responsible for the development of two types of cavity fill dolocretes namely mature and immature dolocretes the association of diagenetic alunites with the cavity fill dolocretes suggests that hydrocarbon seepage may have played a role in the development of these dolocretes it is suggested that the occurrence of similar diagenetic setting within carbonate sequence may potentially affect their reservoir characteristics

dolomitization of atolls by sea water convection flow test of a hypothesis at niue south pacific the core of the former atoll on niue island south pacific is extensively dolomitized a detailed stable isotope study reveals that the dolomites are uniformly enriched in 18o and 13c 18o 2 8 0 5 13c 2 0 0 3 relative to their carbonate precursor the sr isotope ratios are equally constant with depth yielding 87sr 86sr values between 0 70912 and 0 70916 the observed isotopic compositions point to sea water as the dominant dolomitizing fluid the sr age of the dolomites is younger than the biostratigraphic age mid to late miocene placing the dolomitization event at plio pleistocene time a model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water sea water is drawn through the atoll margin and transferred upward by convective flow delivering mg to the sites of dolomite precipitation transported with the sea water are volcano derived metals fe cu zn mn as evidenced by the chemical gradient in

morphology and origin of dolomite in paleosols and lacustrine sequences examples from the miocene of the madrid basin the northern madrid basin contains paleosols and lacustrine sequences composed of ordered dolomite smectite rich mudstones with analcime deposited in distal alluvial fans affected by pedogenic processes mudstones contain dolomite and silica laminae interpreted as mineralized root mats dolostones with prismatic structure constitute stage iii dolocretes homogeneous lacustrine dolomudstones show some features indicative of subaerial exposure dolomite textures include microcrystalline dolomite coarse crystalline mosaics located in root cavities dolomite dumbbells and dolomite spheroids replacing clays dolomudstones or opal mol values of mgco3 of dolomites vary between 47 67 and 53 34 mol 18o values range between 3 65 and 5 51 vpdb 13c values range between 7 19 and 8 38 vpdb dolomicrite formed in alkaline lakes by direct precipitation and replacement of clays in soils the rhizosphere was favorable for dolomite mosaic and dumbbells formation spheroidal dolomite replaced clays and opal under early phreatic diagenesis trioctahedral smectites silica and analcime indicate that these pedogenic and lacustrine environments were alkaline smectites and or carboxyl groups acted as a catalyst favoring the incorporation of mg into dolomite in these non saline settings the kinetic barriers for dolomite precipitation lowered by high ph and mg enrichment ordered dolomite formation occurred under biogenic and abiogenic conditions during sedimentation pedogenesis and early diagenesis

origin of dolomite cement in chesapeake group miocene siliciclastic sediments an alternative model to burial dolomitization abstract ferroan dolomite and calcite cement fossiliferous sandstone unite in the choptank and calvert formations miocene of western maryland and northern virginia the average composition of the dolomite is ca 55mg 43fe 02co3 though nonferroan dolomite is also present the dolomite is generally euhedral and zoned and it may have fluid inclusion rich cores calcite cements are dominantly composed of equant spar though a fine acicular isopachous rim cement may be present calvert formation dolomite is chemically similar to choptank formation dolomite though the former contains both calcite inclusions and hollow centers a relatively simple burial history for ferroan dolomite and calcite cement coupled with petrographic and isotopic evidence indicates that some of the dolomite has a marine origin possibly under the influence of sulfate reducing organisms later recrystallization may have occurred under the influence of alternating marine and meteoric phreatic conditions zoned dolomite rhombs dolomite with calcite cores and clear dolomite rims indicate episodic crystallization possibly due to multiple periods of phreatic conditions most of the calcite cement is demonstrably meteoric in origin dolomite cement is generally confined to permeable sandy intervals bounded on the top and bottom by clays of low permeability when buried this sequence would be similar to dolomite sequences inferred to be of deep burial in origin the timing of dolomite formation in chesapeake bay sediments is significant in that most of the dolomite has derived its magnesium from sources other than the clays as has been suggested in deep burial dolomitization models

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less