mineralogy and geochemistry of an organic and v cr mo u rich siliceous rock of late permian age western hubei province china abstract organic rich siliceous rocks of late permian age from western hubei province china are markedly high in trace elements including v 2031 g g cr 856 g g mo 245 g g and u 39 2 g g the mineralogy and geochemistry of these siliceous rocks were investigated using various petrological mineralogical and geochemical methods the results show that the siliceous rocks are high in total organic carbon toc which range from 3 88 to 46 99 with a weighted average of 17 83 toc is dominated by amorphous organic debris resulting from intense degradation and lesser amounts of marine microfossils e g foraminifera algae and poriferan the minerals mainly consist of quartz mixed layer illite smectite i s and small proportions of calcite dolomite k feldspar albite pyrite gypsum apatite marcasite jarosite kaolinite and bassanite trace amounts of xenotime y y ree po 4 rhabdophane nd nd ce la po 4 h 2 o chalcopyrite and sphalerite were observed using sem edx the sem edx results indicate that v and cr are mainly within the organic matter and mixed layer i s mo is closely associated with organic matter and sulfides e g pyrite sphalerite and chalcopyrite u is only detected in p bearing minerals especially in xenotime y and rhabdophane nd the sampled interval was deposited in a transitional marine basin between a continental margin and open ocean pelagic setting where the anoxic environment was attributed to intense organic matter degradation and restricted water mass renewal the enriched elements v cr mo and u were mainly derived from submarine hydrothermal activities

polygonal cracks in the seasonal semi translucent co2 ice layer in martian polar areas 1 in this paper we use morphological and numerical methods to test the hypothesis that seasonally formed fracture patterns in the martian polar regions result from the brittle failure of seasonal co2 slab ice the observations by the high resolution imaging science experiment hirise of polar regions of mars show very narrow dark elongated linear patterns that are observed during some periods of time in spring disappear in summer and re appear again in the following spring they are repeatedly formed in the same areas but they do not repeat the exact pattern from year to year this leads to the conclusion that they are cracks formed in the seasonal ice layer some of models of seasonal surface processes rely on the existence of a transparent form of co2 ice so called slab ice for the creation of the observed cracks the ice is required to be a continuous media not an agglomeration of relatively separate particles like a firn the best explanation for our observations is a slab ice with relatively high transparency in the visible wavelength range this transparency allows a solid state green house effect to act underneath the ice sheet raising the pressure by sublimation from below the trapped gas creates overpressure and the ice sheet breaks at some point creating the observed cracks we show that the times when the cracks appear are in agreement with the model calculation providing one more piece of evidence that co2 slab ice covers polar areas in spring

glaucony and sequence stratigraphy a conceptual framework of distribution in siliciclastic sequences abstract detailed analysis of recent literature on glaucony and selected case studies eocene isle of wight miocene northern apennines shows that the presence of glaucony alone is not diagnostic of a specific systems tract of a depositional sequence a reliable sequence stratigraphic interpretation of glaucony bearing units requires additional information on glaucony including 1 spatial distribution 2 maturity distinction between nascent slightly evolved evolved and highly evolved glaucony and 3 genetic attributes differentiation of autochtonous from allochthonous and intrasequential from extrasequential glaucony autochthonous glaucony is common at various stratigraphic levels in the transgressive systems tract tst and the lower highstand systems tract hst showing an upward increase tst and then decrease hst in abundance and maturity the condensed section can be distinguished from the overlying and underlying deposits by the higher concentration and maturity of glaucony allochthonous intrasequential parautochthonous glaucony can be present in the entire tst hst and lowstand systems tract lst generally showing lower concentration and maturity than its autochthonous counterpart allochthonous extrasequential detrital glaucony is present mainly in the lst its concentration and composition depending on the characteristics of the source horizon the associa ion between autochthonous and allochthonous intrasequential and extrasequential glaucony commonly exists in the lst and in the lower tst

de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

geochemical characterisation of fika formation in the chad bornu basin northeastern nigeria implications for depositional environment and tectonic setting late cretaceous shales of the fika formation in the chad bornu basin northeastern nigeria were analysed to define paleoenvironment and source of the organic matter and their relation to tectonic setting the organic carbon and sulphur contents of fika shale samples are in the range of 0 51 2 13 and 0 31 1 65 wt respectively pointing that these shales were deposited in suboxic anoxic marine conditions the biomarker and chemical compositions provide evidence for a major contribution of aquatic algae and microorganisms with minor terrigenous organic matter input moderate salinity stratification and relatively anoxic suboxic bottom water conditions are also likely in the fika shales therefore stratified water column with moderate salinity and relatively anoxic suboxic bottom water conditions have contributed to organic matter om preservation in the fika shale layer fika shale samples are rich in sio2 54 80 wt followed by al2o3 23 75 wt and fe2o3 10 19 wt compared with average shale the analysed shale samples are obviously enriched in al2o3 23 75 wt tio2 1 34 wt and p2o5 0 30 wt indicating that these sediments are rich in clay minerals and represent a good possibility for enhanced organic matter production and enrichment plots of fika shale on bivariate discriminant function diagram suggest an active continental margin setting for the provenance the inferred tectonic setting for the late cretaceous shales of the fika formation of the chad bornu basin is in agreement with the tectonic evolutionary history of the west and central africa during the cretaceous period

relative location of microseismicity abstractapplying the two point paraxial ray tracing we develop a technique for relative location of microseismic events our technique assumes the availability of a perforation shot or an already located microseismic event termed the master for which the paraxial ray tracing has been performed the ray tracing output for the master makes it possible to compute the relative locations of adjacent microseismic events as many as a data set contains with an efficient algorithm that requires no additional ray tracing and reduces to solving a series of simple low dimensional and well behaved optimization problems the relative event location approach discussed in our paper is especially well suited for surface microseismic monitoring because the high accuracy of the paraxial ray approximation in the directions orthogonal to the reference rays typically spanning the stimulated horizons for surface microseismic geometries ensures the calculation of precise event hypocenters at appreciable distances from th

thin section grain size analysis revisited existing proposals for converting thin section data to their sieve equivalents are all flawed in various ways while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using units and of non spherical ellipsoidal grains using both millimetres and units have not been satisfactorily resolved in the literature it can be shown mathematically that the mean thin section diameters of spherical grains or axis lengths of a series of parallel sections through ellipsoidal grains will underestimate the dimensions of the corresponding central section i e one passing through the grain centre by 0 2023 when measured on a volume frequency basis in order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains the dimensions of a series of sections cut in 49 unique directions symmetrically arranged and evenly spread with respect to the ellipsoidal axes were calculated this calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations the data indicated that the mean true nominal diameters d of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters d and major axes of the central sections derived from the observed values by multiplying the millimetre means by 1 1318 and subtracting 0 2023 from the means using the following equations a rough estimate to within c 5 of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millimetres or units

origin and paleoenvironmental reconstruction of phosphorus bearing sandstones of the cambrian xinji formation southwestern margin of the ordos basin china increasing exploration interest in oil and gas hosted by early cambrian strata has focused research efforts on early cambrian sandstones the origin of phosphorus and the paleoenvironment of phosph

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

oceanic anoxia at the precambrian cambrian boundary the precambrian cambrian pc c boundary separates fossils representing two discrete evolutionary phases the neoproterozoic soft bodied ediacarian biotas and cambrian small shelly faunas the biological discontinuity is suspected to have been a result of mass extinction however recent discoveries of the ediacarian biotas in cambrian sediments have led to an understanding that the faunal change was gradual through the pc c transition th u ratios which are high in oxidizing conditions and low in reducing conditions show a considerable positive correlation with 13c values at all studied sites of the pc c boundary this correlation indicates that reported 13c variation across the pc c boundary from numerous localities corresponds to redox variation in the depositional environment the negative 13c anomaly that occurs worldwide at the pc c boundary therefore corresponds to the widespread development of an oxygen deficient shallow marine environment this finding suggests that widespread oceanic oxygen deficiency which has been interpreted to reflect phanerozoic mass extinction events also occurred immediately before the cambrian explosion

oxygen isotope variations in phosphate of biogenic apatites ii phosphorite rocks abstract phosphorites from sedimentary sequences ranging in age from archaean to recent were analysed for 18 o in both the po 4 18 o p and co 3 18 o c in the apatite lattice the oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts the use of the longinelli and nuti 8 temperature equation yields temperatures for recent phosphorites that are in good agreement with those measured in the field the 18 o p values of ancient phosphorites decrease with increasing age these changes with time are not likely to be due to post depositional exchange changes in 18 o values of seawater and variations of temperature with time can account for the 18 o p time trend but the latter explanation is preferred in ancient phosphorites 18 o c in structurally bound carbonate in apatite is not a reliable geochemical indicator

detached forced regressive shoreface wedges at the southern iberian continental palaeomargin early cretaceous betic cordillera s spain abstract the lower cretaceous barremian to early aptian cerrajon formation on the southern iberian continental palaeomargin is represented in the study area by five sharp based shoreface sandstone wedges 5 15 m thick the sandstone packages abruptly pinch out landwards onto outer ramp limestones and marls los villares formation sandstone tongues are interpreted as prograding wave dominated shorefaces distally evolving to thin micro hummocky cross stratified sandstone beds alternating with marls deposited in a storm wave siliciclastic dominated outer ramp in the context of sequence stratigraphy most of the characteristics in outcrops for detached forced regressive deposits have been tested at the sandstone wedges sandstone bases are characterized by a regressive surface of marine erosion rsme underlying a calcarenite containing reworked inner ramp fossils and glauconite from the preceding highstand carbonate dominated outer ramp los villares formation the shoreface sandstones are in turn cut by incised channels e g 2nd sandstone wedge and filled with lithophaga bored sandstone concretions lithophaga bored sandstone concretions are interpreted as deriving from 1 concretion formation in the forced regressive to lowstand shoreface sandstone during early diagenesis 2 exhumation of sandstones hosting concretions along incised channels and 3 coastal winnowing of poorly cemented sandstone host leaving the sandstone concretions which were bored and colonized by coastal fauna in situ incised channels truncating shoreface sandstones and sandstone concretions colonized by coastal fauna are interpreted as recognition criteria for a second sea level fall that occurred after a lowstand sea level stage the term paired forced regressive is used for a systems tract resulting from a double sea level dip two sea level falls occurring during a single sea level change cycle repeated fourth order fall to rise cycles of relative sea level controlled the architecture and facies distribution of system tracts on the south iberian continental palaeomargin during the barremian to early aptian early cretaceous

composition of modern deep sea sands from arc related basins summary petrographic and chemical examination of modern deep sea sands shows a clear distinction between active and passive plate tectonic settings among active settings sands from forearc basins of island arcs and basins from strike slip continental margins can be distinguished but those from other arc related settings overlap considerably in composition in particular sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable chemically arc related sands are very close in composition to ancient greywacke sandstones for instance sodium almost always exceeds potassium thus it is not necessary for sodium to be added to sands diagenetically to make greywackes as has been suggested the influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology the geosynclinal model of global tectonics stimulated much thought on this subject probably the best known ideas are those of krynine e g 1942 subsequently such research declined but the plate model for global tectonics has produced a new wave of interest e g crook 1974 schwab 1975 potter 1978 dickinson suczek 1979 much of this work has relied on data from ancient sediments where diagenetic changes and difficulty in de ciphering the tectonic setting limit the useful ness of the conclusions much more work is needed on recent sediments to establish a base line against which ancient sediments can be compared we have previously reported valloni maynard 1981 the results of our work on the petrography of a set of recent deep sea sands from piston cores in this paper we shall com pare this petrographic information with bulk chemistry and attempt to relate these to tecto nics in arc related settings

origin of the lower cambrian phosphatic bed at vassbo sweden an extensive phosphatic bed characterized by a black apatite cemented sandstone is developed in the uppermost part of the lower cambrian formation over the entire vassbo area sweden the thickness of the layer is generally 0 1 m increasing in places to 0 2 m the lower boundary of the bed is transitional to a sandstone the upper boundary displays a contact that is both sharp and irregular with a middle cambrian bituminous black shale the phosphatic bed was diagenetically formed by inorganic in situ precipitation of apatite in a sandstone at a depth of about 0 2 m below the sediment water interface where reducing conditions were developed as a result of oxygen consumption during the degradation of organic matter sulphur isotope data from associated pyrite suggest that organic processes such as bacterial sulphate reduction were also involved in the formation of the phosphatic bed the depositional environment of the bed was a marine shelf with a zero to very low sedimentation rate the upper part of the apatite cemented sandstone was partly reworked by bottom currents as demonstrated by a board containing numerous pebbles sedimentological as well as chemical data confirm a southern hemisphere position of the baltic shield in the early cambrian in which an oceanic upwelling and high organic productivity was the probable source of the phosphorus along the palaeo west coast

mars south polar spring and summer behavior observed by tes seasonal cap evolution controlled by frost grain size thermal emission spectrometer tes observations of the recession phase of mars south polar cap are used to quantitatively map this recession in both thermal and visual appearance geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior for most of the cap while temperatures remain near the co2 frost point albedos slowly increase with the seasonal rise of the sun then drop rapidly as frost patches disappear over a period of 20 days a cryptic region remains dark and mottled throughout its cold period tes observations are compared with first order theoretical spectra of solid co2 frost with admixtures of dust and h2o the tes spectra indicate that the cryptic region has much larger grained solid co2 than the rest of the cap and that the solid co2 here may be in the form of a slab the mountains of mitchel remain cold and bright well after other areas at comparable latitude apparently as a result of unusually small size of the co2 frost grains we found little evidence for a significant presence of h2o although co2 grain size may be the major difference between these regions incorporated dust is also required to match the observations a self cleaning process carries away the smaller dust grains comparisons with viking observations indicate little difference in the seasonal cycle 12 martian years later the observed radiation balance indicates co2 sublimation budgets of up to 1250 kg m 2 regional atmospheric dust is common localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm

origin of natural gas from the ordovician paleo weathering crust and gas filling model in jingbian gas field ordos basin china abstract the ordos basin the second largest natural gas producing basin in china is one of the most tectonically stable sedimentary basins favorable to natural gas generation and preservation the origin and potential source of the jingbian gas field one of the five giant gas fields in the basin containing the ordovician paleo weathering crust as reservoir strata remains unclear the question is whether gas is generated from carboniferous permian coal measures or ordovician carbonates this study provides abundant gas geochemical evidence which clarifies its origin and source among 82 natural gas samples collected from the ordovician paleo weathering crust in the jingbian gas field c1 c4 gaseous hydrocarbons are dominant with low amounts of non hydrocarbons co2 h2s and n2 the content of c1 c4 ranges from 79 26 to 99 93 with a gas dryness c1 c1 4 of 0 981 0 999 suggesting that natural gases are dry and possibly originate from a high to over mature source the carbon isotopic compositions of methane and its homologues in natural gases from the ordovician paleo weathering crust vary widely with 13c1 13c2 and 13c3 values of 38 9 to 29 1 37 5 to 22 2 and 30 0 to 20 9 respectively the 13c2 and 13c3 values in the jingbian gas field are much more negative than those of typical coal type gas in sulige wushenqi yulin daniudi and kela2 gas fields the carbon isotopic signature suggests that the natural gas in the ordovician paleo weathering crust in the jingbian gas field most likely originates from ordovician carbonates containing sapropelic organic matter the measured d values of methane range from 177 to 155 and support our hypothesis of the natural gases originating from marine sapropelic organic matter the thermal maturity of the natural gases in the ordovician paleo weathering crust is at a high to over mature stage suggesting that the gases originated from secondary cracking of oil and gas at high temperature and pressure thermochemical sulfate reduction tsr might partially play a role in the alteration of gas dryness and 13c1 in those areas where gypsum is present as an effective cap rock mixing of high to over mature secondary cracking gas from the ordovician source and coal type gas from carboniferous permian coal in the jingbian gas field is minor

sedimentology geochemistry and paleoenvironmental reconstruction of the cretaceous yolde formation from yola sub basin northern benue trough ne nigeria abstract sedimentology organic and inorganic geochemical studies were applied on sediments of the cretaceous yolde formation from yola sub basin northern benue trough northeastern nigeria with an attempt to determine the sedimentary facies paleo redox condition and to re construct the palaeo depositional environments eight 8 facies were identified on the basis of lithology grain size ichnofossils degree of bioturbation and sedimentary structures these facies constitute four facies associations the fa 1 offshore marine fa 2 offshore transition to lower shoreface fa 3 middle shoreface and fa 4 upper shoreface the succession of these facies associations indicates storm and wave influenced offshore shoreface depositional environments for the formation molecular organic geochemical investigation suggests deposition under sub oxic marine environment with major contribution of phytoplankton bacteria and minor amount of terrigenous organic matter xrd mineralogical assessment and major and trace elements geochemistry revealed that the yolde formation sediments were sourced from continental areas of passive continental margin setting and deposited in semiarid climate under suboxic shallow marine conditions related to the first marine transgression that occurred in the benue trough during the late cretaceous period

sedimentary phosphorus record from the oman margin new evidence of high productivity during glacial periods 1 the northern region of the arabian sea is one of the biologically most fertile regions of the world oceans with present productivity rates varying between 150 and 2500 mgc m2 day madhupratap et al 1996 this is related to the influence of the southwesterly summer monsoon which causes vigorous upwelling along the oman margin upwelling ceases during northeasterly winter monsoon activity productivity rates however remain relatively high about 800 mgc m2 day related to deep water mixing madhupratap et al 1996 the goal of this study is to verify if during the last glacial period a period in which winter monsoon conditions prevailed productivity rates were similarly high with an analysis of phosphorus phases stable nitrogen isotopes organic matter content and bulk mineralogy of the upper 10 m of the cores of odp hole 724c corresponding to the last 140 000 years sample resolution is 5 kyr we provide new evidence of high productivity during this last glacial period marine isotopic stages 2 3 and 4 this was probably related to the combined effect of 1 increased eolian input of iron containing dust due to dryness on the adjacent continent and stronger winter monsoon and 2 regeneration and diffusion of dissolved phosphorus from the sediments to the water column due to variations in the position and intensity of the oxygen minimum zone these findings suggest that there is no one to one relationship between summer monsoon activity and productivity which emerges to be a quasi persistent phenomenon across glacial and interglacial stages

importance of borehole deviation surveys for monitoring of hydraulic fracturing treatments during seismic monitoring of hydraulic fracturing treatment it is very common to ignore the deviations of the monitoring or treatment wells from their assumed positions for example a well is assumed to be perfectly vertical but in fact it deviates from verticality this can lead to significant errors in the observed azimuth and other parameters of the monitored fracture system geometry derived from microseismic event locations for common hydraulic fracturing geometries a 2 deviation uncertainty on the positions of the monitoring or treatment well survey can cause a more than 20 uncertainty of the inverted fracture azimuths furthermore if the positions of both the injection point and the receiver array are not known accurately and the velocity model is adjusted to locate perforations on the assumed positions several millisecond discrepancies between measured and modeled sh p traveltime differences may appear along the receiver array these traveltime discrepancies may then be misinterpreted as an effect of anisotropy and the use of such anisotropic model may lead to the mislocation of the detected fracture system the uncertainty of the relative positions between the monitoring and treatment wells can have a cumulative nonlinear effect on inverted fracture parameters we show that incorporation of borehole deviation surveys allows reasonably accurate positioning of the microseismic events in this study we concentrate on the effects of horizontal uncertainties of receiver and perforation positions understanding them is sufficient for treatment of vertical wells and also necessary for horizontal wells

influence of a velocity model and source frequency on microseismic waveforms some implications for microseismic locations in this paper we examine the influence of a velocity model and microseismic source frequency on microseismic waveforms and event locations finite difference waveform synthetics are generated based on the cotton valley hydraulic fracture experiment where we vary the vertical heterogeneity of the velocity models as well as the microseismic source frequencies we find that differences between plausible velocity models lead to changes in arrival times of approximately 0 0035 seconds for p waves and 0 0085 seconds for s waves based on the average p and s wave velocities the difference in the p and s wave traveltimes is equivalent to approximately 20 m in location difference significant increases in the waveform coda develop with increasing model heterogeneity and increasing source frequency the presence of signal noise as well as other sources of error e g uncertainty in geophone location will likely lead to further increase in uncertainty in location error estimates thus we note that location error due to incorrect velocity models cannot be ignored

feasibility of joint 1d velocity model and event location inversion by the neighbourhood algorithm using a set of synthetic p and s wave onsets computed in a 1d medium model from sources that mimic a distribution of microseismic events induced by hydrofrac treatment to a monitoring geophone array s we test the possibility to invert back jointly the model and events location we use the neighbourhood algorithm for data inversion to account for non linear effects of velocity model and grid search for event location the velocity model used is composed of homogeneous layers derived from sonic logging results for the case of one and two monitoring wells are compared these results show that the velocity model can be obtained in the case of two monitoring wells if they have optimal relative position the use of one monitoring well fails due to the trade off between the velocity model and event locations

incised valley fill succession affected by rapid tectonic uplifts an example from the uppermost pleistocene to holocene of the isumi river lowland central boso peninsula japan abstract the burial history of the isumi river lowland was reconstructed based on a facies analysis of borehole and geoslicer samples undisturbed sediment samples collected by 4 0 m long 0 35 m wide and 0 01 to 0 05 m deep wedge shaped steel case together with an analysis of molluscan shell assemblages for environmental assessment and 14c age measurements the borehole successions constitute a part of a depositional sequence in a fluvial and wave dominated incised valley the facies succession and age data suggest that the lower part of the succession before 7 0 calendar kilo years before present cal ky bp has in a broad scale an aggradational to retrogradational stacking pattern although a short period of sediment progradation occurred during 9 0 8 5 cal ky bp the upper part 7 0 cal ky bp present is a prograding succession of beach shoreface and bayhead delta sediments deposited in an enclosed bay environment ca 3 5 cal ky bp the facies changes from an open to an enclosed marine environment resulted from barrier formation age measurements indicate that the choja and izumi terrace surfaces formed some time after 6 5 cal ky bp and around 3 5 cal ky bp respectively the temporary progradation during 9 0 8 5 cal ky bp was probably associated with several meters of tectonic uplift which resulted in an increase in the sediment supply owing to intensified fluvial and marine erosion after a tectonic uplift at about 6 4 ky bp a wide strand plain started to form along the bay shore as a result of sediment transport from both north and south of the bay by longshore currents however a barrier developed about 3 5 cal ky bp after the izumi surface was uplifted subsequently this uplift transported sediments probably bypassed the southern part of the lowland and accumulated near the mouth of the bay to form this barrier as is indicated by the poor strand plain development after this time strand plain progradation might have been prevented by strong wave influences from the pacific ocean and longshore currents resulting from both the recession of the sea cliffs south and north of the lowland and from the uplift which shifted the shoreline seaward and changed the shoreline environment from an enclosed bay into a more open marine environment

syndepositional shallow water precipitation of glauconitic minerals abstract numerous studies have demonstrated that glauconitic minerals predominantly form in water depths of mid shelf to upper slope in modern oceans these areas tend to have slow sedimentation rates another commonly cited requisite for glauconitic mineral precipitation cambro ordovician strata from the southwestern us are rich in glauconitic minerals stratigraphic sedimentological and petrographic constraints indicate that the glauconitic minerals are autochthonous in marked contrast to the modern environments of deposition these cambro ordovician strata formed under very shallow water to tidal flat conditions the trough cross stratified deposits of the most glauconitic mineral rich accumulations glaucarenites indicate a high energy environment and probably a normal to high rate of sedimentation the presence of fibroradiated rims of glauconitic minerals on glauconitic mineral pellets echinoderm fragments and quartz grains demonstrates that the cambro ordovician glauconitic minerals precipitated on or in close proximity to the sea floor and prior to calcite precipitation consequently glauconitic minerals must have formed under markedly different conditions in the lower paleozoic than they do today thus the occurrence of glauconitic minerals in the rock record cannot be used a priori as an environmental indicator of either mid shelf and deeper water and or a slow rate of sedimentation

the behaviour of the rare earth elements during mixing of river and sea waters rare earth element concentrations have been measured in organic rich luce river water and coastal sea water concentrations e g 350 1850 pmol kg nd in the water of luce and 45 350 pmol kg nd in luce bay are related to the presence of particles with 30 60 of the ree associated with 0 4 0 7 m particles and to riverine fe concentrations ree fractionation occurs in the river water the submicrometre river water is heavy ree enriched whereas the coarser fraction has a more shale like ree pattern laboratory experiments show that the ree in organic rich river waters are chiefly associated with feorganic matter colloids which flocculate during estuarine mixing preferential removal of heavy ree 95 relative to light ree 60 occurs but no ce anomaly is developed in contrast no ree removal occurs during estuarine mixing with organic poor river water

cross double difference inversion for simultaneous velocity model update and microseismic event location microseismic monitoring is an approach for mapping hydraulic fracturing detecting the accurate locations of microseismic events relies on an accurate velocity model the 1d layered velocity model is generally obtained by model calibration from inverting perforation data however perforation shots may only illuminate the layers between the perforation shots and the recording receivers with limited raypath coverage in a downhole monitoring problem some of the microseismic events may occur outside of the depth range of these layers to derive an accurate velocity model covering all of the microseismic events and locating events at the same time we apply the cross double difference cdd method for the simultaneous inversion of a velocity model and event locations using both perforation shots and microseismic data the cdd method could provide accurate locations in both the relative and absolute sense utilizing cross traveltime differences between p and s phases over different events at the downhole monitoring scale the number of cross traveltime differences is sufficiently large to constrain events locations and velocity model as well in this study we assume that the layer thickness is known velocities of p and s wave are inverted different simultaneous inversion methods based on the geiger s dd and cdd algorithms have been compared with the same input data synthetic and field data experiments suggest that combining both perforation shots and microseismic data for the simultaneous cdd inversion of the velocity model and event locations is available for overcoming the trade offs in solutions and producing reliable results this article is protected by copyright all rights reserved

hirise observations of gas sublimation driven activity in mars southern polar regions iii models of processes involving translucent ice abstract enigmatic surface features known as spiders found at high southern martian latitudes are probably caused by sublimation driven erosion under the seasonal carbon dioxide ice cap the mars reconnaissance orbiter mro high resolution imaging science experiment hirise has imaged this terrain in unprecedented details throughout southern spring it has been postulated kieffer h h titus t n mullins k f christensen p r 2000 j geophys res 105 9653 9700 that translucent co 2 slab ice traps gas sublimating at the ice surface boundary wherever the pressure is released the escaping gas jet entrains loose surface material and carries it to the top of the ice where it is carried downslope and or downwind and deposited in a fan shape here we model two stages of this scenario first the cleaning of co 2 slab ice from dust and then the breaking of the slab ice plate under the pressure built below it by subliming ice our modeling results and analysis of hirise images support the gas jet hypothesis and show that outbursts happen very early in spring

azimuthal anisotropy in microseismic monitoring a bakken case study abstracthydraulic fracturing routinely applied for enhancing the permeability of unconventional oil and gas reservoirs is one of the possible causes for azimuthal anisotropy of the treated formations accounting for both naturally occurring and completion induced azimuthal anisotropy leads to marked improvements in the results of microseismic data processing as illustrated on a data set acquired in the bakken field north dakota usa those improvements include the possibility of modeling the observed shear wave splitting reduction of misfit between the picked and modeled traveltimes of microseismic events and relocation and tightening of the spatial distribution of the event hypocenters in addition and perhaps most importantly for the development of microseismic technology the feasibility of joint inversion of field microseismic data for the event locations and azimuthally anisotropic velocity model containing triclinic layers is demonstrated

small scale polygonal features on mars seasonal thermal contraction cracks in permafrost a time dependent viscoelastic model of thermal stress in martian ice rich permafrost is developed to test the hypothesis that small scale polygonal features observed from orbit and by the viking lander 2 are the result of thermal contraction cracking as commonly occurs in terrestrial permafrost results indicate that significant tensile stress occurs in martian ice rich permafrost as a result of seasonal cycles in the ground temperature using conservative rheological parameters appropriate for ice at low temperatures tensile stresses poleward of about 20 to 30 latitude easily exceed the tensile strength assumed to be 2 to 3 mpa and fractures should readily form in the equatorial regions special conditions may allow tensile stresses to approach the tensile strength these results support a thermal contraction origin of observed small scale polygonal features and emphasize the utility of these features as valuable morphological indicators of ground ice

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

stratigraphic evolution from shoreface to shelf indenting channel depositional systems during transgression insights from the lower pliocene sua member of the basal upper onzole formation borbon basin northwest ecuador abstract shelf indenting canyons and their tributary systems are fairly common constituents of quaternary shelves of active continental margins but they have been rarely reported from older successions recognition of these prominent geomorphologic features in the ancient record has important implications not only for a proper understanding of shoreface to shelf depositional systems but also from a petroleum exploration standpoint as they represent efficient conduits for moving coarse grained river and nearshore borne sediments to the adjacent slope even during periods of relative rise in sea level coastal exposures of the lower pliocene sua member in the surroundings of sua northwest ecuador preserve the unusual juxtaposition of incising submarine channels onto nearshore deposits this succession accumulated along a narrow active continental margin during tectonically induced transgression and affords a rare opportunity to evaluate the stratigraphic evolution of such systems from an outcrop perspective a comprehensive facies characterization combined with application of sequence stratigraphic concepts has led to definition of the following physical surfaces and stratal units in ascending order i a polygenetic regionally extensive erosional surface resulting from the superposition of the wave ravinement surface onto the previous subaerial sequence boundary sb wrs ii a nearshore sand prone lithofacies succession comprising a condensed basal shellbed deepening upwards through lower shoreface bioturbated silty sandstones into inner shelf sandy mudstones iii two steep u shaped erosional features turbidite shelf entrenchment surfaces interpreted as shelf channels deeply incised into the subjacent nearshore sediments and marking an abrupt deepening of facies iv a thick fining upward sedimentary succession laid down within the confines of the channels by high and low density turbidity currents and including both bed load traction and suspended load deposits the overall fining and thinning upward character exhibited by the infill of these channels is thought to reflect decreasing flow energies and is consistent with the gradual cut off of clastic influx to their upper reaches in response to progressive detachment from an adjacent coastal source during relative rise in sea level based on detailed analysis of facies and a sequence stratigraphic interpretation of outcrop data this study contributes to extend the existing sequence stratigraphic schemes further attesting that shelf sediment bypass and deep water sedimentation can take place at sea levels other than lowstand

cold jets in the martian polar caps 1 mars seasonal polar caps display dark ice local darker spots aligned elongate patches and radially dendritic forms that reverse albedo contrast the unexpected variety and sequence of these features are explained on the basis of processes involving co2 dust sand and h2o these processes are largely related to the atmosphere being near its saturation temperature and they have few terrestrial analogies in the simplest case the 1 m thick seasonal cap initially dusty cleans itself and becomes translucent after sunrise and is impermeable over extensive regions except for local vents the slab ice sublimates at the base and is levitated on high pressure gas causing humidity exchange with deeper layers subslab gas converging toward the vents erodes channels in the soil and ejects this material in high velocity jets recent spectral observations indicate great variety in the details

the late early cambrian microbial reefs immediately after the demise of archaeocyathan reefs hunan province south china abstract the archaeocyath bearing reefs that first appeared in south china in the atdabanian were last seen in the lower toyonian e g in the tianheban and jindingshan formations the upper toyonian lower amgan qingxudong formation of hunan province examined in this study includes large sized reefs formed solely by calcimicrobes of epiphyton kordephyton girvanella hedstroemia and renalcis during the middle and upper cambrian small scale stromatolite reefs were formed the qingxudong reefs are therefore interpreted to be the most well developed microbial reefs of the post archaeocyath bearing reefs in the late early cambrian the transition from archaeocyath bearing to purely microbial reefs was caused by a decline in the diversity of metazoans and was accompanied by a reduction in the abundance of reefs possibly related in part to late early cambrian regression the late early cambrian reefs of south china contain a well preserved record of reef succession and provide important insights into the factors that led to the demise of archaeocyath bearing reefs and the subsequent long lasting hindrances to the development of skeletal dominated reefs during the middle and late cambrian

observational evidence for an active surface reservoir of solid carbon dioxide on mars high resolution images of the south polar residual cap of mars acquired in 1999 and 2001 show changes in the configuration of pits intervening ridges and isolated mounds escarpments have retreated 1 to 3 meters in 1 martian year changes that are an order of magnitude larger than can be explained by the sublimation of water ice but close to what is expected for sublimation of carbon dioxide ice these observations support a 35 year old conjecture that mars has a large surface reservoir of solid carbon dioxide the erosion implies that this reservoir is not in equilibrium with the present environment and that global climate change is occurring on mars

origin of silica paleoenvironment and organic matter enrichment in the lower paleozoic niutitang and longmaxi formations of the northwestern upper yangtze plate significance for hydrocarbon exploration abstract lower paleozoic shale is an important source rock and reservoir for gas in the northwestern upper yangtze plate the paleoenvironment and its impact on organic matter enrichment of the shale plays a vital role in the exploration and development of the resource based on the analysis of the results of total organic carbon toc maceral examination bitumen reflectance rb major elements trace elements rare earth elements rees and x ray diffraction xrd of shale samples from the niutitang and longmaxi formations the origin of silica the paleoenvironment and the mode of organic matter enrichment of the shale were examined the shales of the niutitang and longmaxi formations have high toc abundance with type i organic matter and high maturity quartz and clay minerals dominate the mineral constituents leading to characterization as siliceous shale the average sio2 content of the shale in the niutitang and longmaxi formations is above 70 wt al2o3 and tfe2o3 total iron are the next most abundant major elements with average values of 12 80 wt and 1 95 wt in the niutitang formation and average values of 11 44 wt and 4 09 wt in the longmaxi formation compared with the upper continental crust ucc v mo and u are the most enriched elements the average lree hree of the shale samples is very low and the chondrite normalized ree distribution pattern shows sloping lree trends and flat hree trends towards the right the total rare earth elements of the shale vary within the range of 147 85 219 60 ppm and 159 90 222 02 ppm in both the niutitang and longmaxi formations suggesting that the shale has not been subject to obvious hydrothermal fluid activities the correlation diagrams of sio2 al2o3 sio2 toc and siexcess toc the diagrams of hydrothermal origins and the elemental parameters indicate that the silica of the shales from the niutitang and longmaxi formations are mainly of biogenic origins with a lesser contribution from terrigenous detrital sources the ratios of mno2 tio2 and al2o3 al2o3 tfe2o3 of the shale samples all show limited variability illustrating that the shales are mainly deposited on the continental margin the average chemical alteration index cia values of the niutitang and longmaxi formations are 71 25 and 72 77 with little variation and the average sr cu ratios are 4 38 and 2 22 reflecting a warm and humid paleoclimate for both formations the average ni co th u and u readings of the shale samples of the formations are 10 91 0 97 and 1 51 and 12 69 0 43 and 1 75 indicating that the redox conditions during the sedimentation of the formations were anoxic the positive correlations between toc and paleoenvironmental proxies reveal a warm and humid climate and that anoxia had a positive effect on organic matter enrichment these conditions have laid a geological foundation for the enrichment of shale gas and that the upper yangtze plate is favorable for future shale gas exploration

the petroleum geochemistry of the termit basin eastern niger abstract sixty crude oils from the termit basin eastern niger were analysed using biomarker distributions and bulk stable carbon isotopic compositions comprehensive oil to oil correlation indicates that there are two distinct families in the termit basin the majority of the oils are geochemically similar and characterized by low pr ph pristane to phytane ratios and high gammacerane c 30 hopane ratios small amounts of c 24 tetracyclic terpanes but abundant c 23 tricyclic terpane and lower 13 c values for saturated and aromatic hydrocarbon fractions all of these geochemical characteristics indicate possible marine sources with saline and reducing depositional environments in contrast oils from well dd 1 have different geochemical features they are characterized by relatively higher pr ph and lower gammacerane c 30 hopane ratios higher amounts of c 24 tetracyclic terpane but a low content of c 23 tricyclic terpane and relatively higher 13 c values for saturated and aromatic hydrocarbon fractions these geochemical signatures indicate possible lacustrine sources deposited under freshwater suboxic oxic conditions this oil family also has a unique biomarker signature in that there are large amounts of c 30 4 methylsteranes indicating a freshwater lacustrine depositional environment the maturity of the termit oils is assessed using a number of maturity indicators based on biomarkers alkyl naphthalenes alkyl phenanthrenes and alkyl dibenzothiophenes all parameters indicate that all of the oils are generated by source rocks within the main phase of the oil generation stage with equivalent vitrinite reflectance of 0 58 0 87

petroleum geology of cretaceous tertiary rift basins in niger chad and central african republic this overview of the petroleum geology of rift basins in niger chad and central african republic car is based on exploration work by exxon and partners in the years 1969 1991 the work included 50 000 km of modern reflection seismic 53 exploration wells 1 000 000 km2 of aeromagnetic coverage and about 10 500 km of gravity profiles the results outline ten cretaceous and tertiary rift basins which constitute a major part of the west and central african rift system wcars the rift basins derive from a multiphased geologic history dating from the pan african approximately 750 550 ma to the holocene main rifting was probably caused by the stretching and subsidence of african crustal blocks during the breakup of gondwana in the early cretaceous and by r activated plate movements in the early tertiary wcars in the study area is divided into the west african rift subsystem was and the central african rift subsystem cas was basins in niger and chad are chiefly extensional and are filled by up to 13 000 m of lower cretaceous to holocene continental and marine clastics the basins contain five oil 19 43 api and two oil and gas accumulations in upper cretaceous and eocene sandstone reservoirs the hydrocarbons are sourced and sealed by upper cretaceous and eocene marine and lacustrine shales the most common structural styles and hydrocarbon traps usually are associated with normal fault blocks cas rift basins in chad and car are extensional and transtensional and are filled by up to 7500 m of chiefly lower cretaceous continental clastics the basins contain eight oil 15 39 api and one oil and gas discovery in lower and upper cretaceous sandstone reservoirs the hydrocarbons are sourced by lower cretaceous shales and sealed by interbedded lacustrine and flood plain shales structural styles range from simple fault blocks through complex flower structures the main hydrocarbon traps are in contractional anticlines geological conditions favor the discovery of potentially commercial volumes of oil in wcars basins of niger chad and car

comparison of vsp and sonic log data in nonvertical wells in a heterogeneous structure to compare the results of sonic log measurements and of vertical seismic profiling vsp sonic log velocities are used to estimate the corresponding traveltime in the geologic structure which is then compared with the vsp traveltime we show how to calculate the sonic log traveltime in the geologic structure from the sonic log velocities while taking into account the effects of the nonvertical propagation of seismic waves resulting from the vsp source offset and from heterogeneous velocity in the structure together with the effects of the well trajectory deviating from strictly vertical errors caused by the commonly used assumption of vertical propagation may considerably exceed the difference of the measured vsp traveltimes from the sonic log traveltimes

correlating river terrace remnants using an equotip hardness tester an example from the mino river northwestern iberian peninsula abstract this paper describes a new method to distinguish river terrace levels based on clast hardness and degree of weathering measured with an equotip hardness tester the technique was applied to a series of terraces on the mino river in the northwestern iberian peninsula where the lack of suitable material high degree of weathering and intense iron cementation precluded routine dating detailed mapping demonstrated that terrace sediments occupy a range of altitudes that make assignment to a specific terrace and or correlation between levels difficult statistical analysis of the equotip hardness data from quartzite clasts using k means clustering allowed four probable terrace levels to be identified a series of t tests generally supported these groupings a fifth lowermost terrace level was not included in the analysis because of limited exposure above a reservoir clast hardness and degree of terrace weathering were generally consistent with progressive river downcutting the occurrence of faulted sediments however suggested that terrace elevations were modified locally by post depositional tectonic movements which may explain why probable younger terraces in some sectors of the mino river are at higher elevations than older terraces in adjacent sectors the equotip tester helped to resolve stratigraphic uncertainties and to assign deposits to specific terrace levels and was found to be a useful tool to distinguish and correlate river terraces

geochemical evidence for oxygenated bottom waters during deposition of fossiliferous strata of the burgess shale formation abstract deciphering the oxygenation potential of bottom waters during deposition of the fossiliferous strata of the burgess shale is key to understanding the paleoecology of the organisms preserved as fossils and the processes involved in their preservation methods of paleo redox determination that are based upon trace fossils organic carbon content or size analysis of pyrite framboids are problematic when applied to the burgess shale fortunately patterns in redox sensitive trace elements hold great potential for determining paleo redox conditions for these greenschist facies rocks because they are independent of both age and metamorphic grade four burgess shale type bst fossil bearing sections were studied three lie within the burgess shale formation whereas one lies within the slightly younger duchesnay unit of the middle chancellor formation the geochemical proxies of seawater redox conditions that were used are mo u th v cr v v ni ni co and v sc each of these redox sensitive indices suggests that oxygenated conditions prevailed in the overlying seawater during deposition of beds within which bst preservation developed non fossiliferous beds at the base of the mt stephen trilobite beds yield three geochemical indices v v ni v sc ni co that indicate deposition under dysoxic to anoxic conditions thus the only beds to have been deposited under anoxic conditions are barren of fossils of soft bodied taxa further contradicting the notion that bottom water anoxia is a factor in bst preservation the laminated nature of the burgess shale and evidence for subsurface anoxia coupled with evidence for oxygenated bottom waters presence of bacterial mats and surficial trace fossils suggest that the burgess shale was deposited in the exaerobic zone oxic anoxic boundary at the water sediment interface implications of this reinterpretation of the depositional environment of the burgess shale include 1 taphonomic models for bst preservation must account for the presence of oxygen in bottom waters 2 paleoecological interpretations of the burgess shale need to account for a probable autochthonous benthic component in what is a complex assemblage of fossils that includes elements derived from the platform and 3 the possibility of burgess shale communities based upon chemosynthesis and or bacteria grazing must be considered

phosphogenesis and active phosphorite formation in sediments from the arabian sea oxygen minimum zone in this study porewater chemistry solid phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the arabian sea three parameters namely a decrease of the dissolved phosphate and fluoride concentrations with depth saturation with respect to carbonate fluorapatite and the presence of a solid phase ca phosphate mineral all indicate that phosphogenesis is currently taking place at all three sites authigenic apatite precipitation rates vary between 0 076 and 1 04 molp cm 2 yr 1 and are of the same order of magnitude as reported for other high productivity areas precipitation of an intermediate precursor precedes francolite formation in the continental slope sediments on the karachi margin results of a diagenetic p model indicate that phosphogenesis is induced by high rates of organic matter degradation dissolution of fish debris is likely to provide a substantial additional source of phosphate redox iron cycling does not influence phosphogenesis in these environments model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis the highest rate of francolite formation was observed in a boxcore taken on the oman margin where it contributes to the formation of a holocene phosphorite deposit this observation contrasts with previous reports of only old phosphorites in this area phosphorites are presently forming on the oman margin as a result of a deposition of older reworked material from the continental shelf which has undergone an earlier phase of phosphogenesis b a high input of reactive p fish debris and degradable organic matter c a relatively low sediment accumulation rate and d the absence of winnowing on this location holocene phosphorite deposits may be less common on the oman margin than in other coastal upwelling areas because of the narrowness of the shelf and the steepness of the slope which limit the area where phosphorite formation may occur

imaging coseismic rupture in far field by slip patches summary since the end of the 1970s teleseismic waves are used routinely to infer the first order characteristics of the event location depth duration focal mechanism but finding the second order kinematic parameters spatial distribution of slip rupture velocity and more basically the discrimination between the fault plane and the other nodal plane of distant events remains a difficult task classically these events are studied by two different methods either they are seen as a succession of subevents each of which is considered as a point source or like in near field as extended sources where the kinematic parameters are retrieved on a grid the first approach is not physically satisfactory and can lead to erroneous interpretations of the

phosphorus phosphorites and skeletal evolution at the precambrian cambrian boundary clustering of sedimentary phosphate deposits around the precambrian cambrian boundary is related to a global phosphogenic event this event probably resulted from a complex interplay of factors and had a profound effect on the biota with high concentrations of phosphorus in the photic zone producing an increase in the total shallow marine biomass faunal diversification the precipitation of skeletal structures and explosive evolution

analytic solutions to the joint estimation of microseismic event locations and effective velocity model obtaining hypocenters of microseismic events a primary task in mining geothermal and hydraulic fracturing applications of induced seismicity requires a velocity model for computing those hypocenters in our paper relying on a notion that information provided by microseismic events themselves enables one to construct a velocity model and calculate the event hypocenters in that model we derive exact analytic solutions to the joint velocity estimation event location problem for downhole microseismic data acquired in homogeneous isotropic media we show that traveltimes and polarization vectors of the direct p and s waves excited by a microseismic event and recorded by a string of receivers placed in one or two vertical wells not only uniquely constrain the event location and the medium velocities but also entail a straightforward analysis of the uncertainties of those estimates caused by the presence of noise in the data although the p and s wave velocities calculated analytically under the assumption of the medium homogeneity cannot fully absorb the complexity of heterogeneous subsurface models they become the proper effective velocities for a given microseismic event and the corresponding event location which is no longer exact even for noise free data might serve as a useful initial guess for more sophisticated event location techniques that account for the velocity heterogeneity

simultaneous inversion of multiple microseismic data for event locations and velocity model with bayesian inference abstractwe have applied bayesian inference for simultaneous inversion of multiple microseismic data to obtain event locations along with the subsurface velocity model the traditional method of using a predetermined velocity model for event location may be subject to large uncertainties particularly if the prior velocity model is poor our study indicated that microseismic data can help to construct the velocity model which is usually a major source of uncertainty in microseismic event locations the simultaneous inversion eliminates the requirement for an accurate predetermined velocity model in microseismic event location estimation we estimate the posterior probability density of the velocity model and microseismic event locations with the maximum a posteriori estimation and the posterior covariance approximation under the gaussian assumption this provides an efficient and effective way to quantify the uncertainty of the microseismic location estimation and capture the correlation between the velo

glaucony and carbonate grains as indicators of the condensed section omma formation japan the fifth order depositional sequences of the early pleistocene omma formation exposed along the japan sea coast of central japan were formed by glacial eustasy during oxygen isotope stages 50 to 28 in each depositional sequence two ecostratigraphic datums are always present the appearance and disappearance datums of warm water molluscan species these datums are independent of sequence stratigraphic concepts because the establishment of them is based on immigration events of molluscan species associated with glacio eustatic sea level changes determination of time planes shows that the appearance datum seems to occur near the midpoint of sea level rise on the glacial to interglacial shift in deep sea d 18 o records in order to evaluate the significance of the condensed section in sequence stratigraphy and also to facilitate its recognition this study examines the stratigraphic relationship of the condensed section indicators glaucony and carbonate grains with respect to the position of the ecostratigraphic datums in depositional sequences of the omma formation the results show that the maximum concentration of carbonate grains is a more reliable maximum flooding surface indicator than the concentration of glaucony the combination of indicators of condensed section and ecostratigraphic datums represented by incursion epiboles enables the boundary between transgressive and highstand systems tracts to be recognized in the inner shelf parts of depositional sequences moreover truncation of ecostratigraphic datums during sea level falls demonstrates significant erosion at the sequence boundaries integration of climatic palaeoecology and sequence stratigraphy permits a level of correlational precision of the order of a few thousands of years 1998 elsevier science b v all rights reserved

tectonic environment of the devonian gramscatho basin south cornwall framework mode and geochemical evidence from turbiditic sandstones the portscatho formation within the allochthonous unit of the middle and upper devonian gramscatho group is a thick sequence of deep water sandstones and interbedded slates deposited by southerly derived turbidity currents into the gramscatho basin of south cornwall throughout an approximately 3 5 km thick sequence the portscatho formation is petrographically and chemically coherent except that the upper section shows a higher proportion of metamorphic clasts high but variable cr and low uniform zr abundances complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition similar to average upper continental crust but with an admixture of minor intermediate basic material flysch deposition took place in a fore arc setting the presence of an arc to the south of cornwall during the devonian implies that there was subduction at the margin of the gramscatho basin whose ultimate closure was accommodated by the northward stacking of flysch ophiolite nappes

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

geochemistry of precambrian and paleozoic siliciclastic rocks from the iberian range ne spain implications for source area weathering sorting provenance and tectonic setting abstract the major and trace element chemical composition of precambrian and paleozoic shales and sandstones from the iberian range spain has been investigated to determine the provenance and tectonic setting of these rocks as well as to appraise the influence of the weathering hydraulic sorting and recycling processes upon source rock signature the samples studied belong to a prograde sequence from diagenesis to anchizonal grade and are mainly composed of quartz illite mica phases and or kaolinite with chlorite feldspars and carbonates as minor components the major element distribution chemical index of alteration cia indices reflects that recycling processes have been important in homogenizing the composition of shales and sandstones the higher cia indices observed in the silurian and devonian shales 80 and 85 respectively compared with those of precambrian to ordovician and the carboniferous age about 70 indicate that their source area underwent more intense chemical weathering processes possibly due to climatic and or tectonic variations alternatively the source area of the silurian and devonian shales may have been composed of recycled sedimentary materials mineral fractionation is mainly observed in coarser rocks through zircon apatite and xenotime accumulation although the shales are not free of these phases the zircon content of the fine grained rocks is not high enough to affect rare earth element ree contents but phosphate minerals apatite and xenotime at least partially control the ree distribution the influence of apatite and xenotime are slightly higher in the precambrian shales as the phosphate concentration produces a decrease in the la sm ratios chondrite normalized ree patterns and negative eu anomaly size of the studied rocks are similar to that of post archean australian shales paas indicating that they originally come from a differentiated silicic source the higher ree contents in the studied shales in relation to paas indicate that recycling processes in the iberian range sources were probably more intense than that of paas the slight differences among the ree patterns of the different groups of shales probably do not reflect changes in source area composition but instead suggest variations in mineral sorting chemical weathering and or sediment recycling the th sc co th cr th cr v v ni ratios support a primitive silicic source for these rocks with the higher th sc and lower co th in cambrian and ordovician shales indicating a higher proportion of felsic material in their primitive source area the compositional maturity of analyzed sandstones is typical of cratonic environments and their la sc th and zr contents reflect their passive continental margin setting

the instability of a south polar cap on mars composed of carbon dioxide abstract if the martian south polar cap were composed of solid carbon dioxide would it be sufficiently stable against collapse under its own weight this question is examined in the light of new experiments on the creep of solid carbon dioxide and new high resolution topographic mapping of the south polar cap the conclusion from a simple model is that with the strongest of three flow laws compatible with the measurements co 2 is strong enough to support a cap of this material with thickness about 1800 m for 10 7 years or more which is the age suggested by the cratering record however none of the three possible flow laws will allow a 3000 m cap the thickness suggested by stereogrammetry to survive for 10 7 years indicating that the south polar ice cap is probably not composed of pure co 2 ice

geochemistry of the sedimentary rocks from the nanxiong basin south china and implications for provenance paleoenvironment and paleoclimate at the k t boundary cretaceous and tertiary clastic sedimentary rocks from the nanxiong basin south china have been analyzed to constrain their provenance depositional climate and environment evidence from discrimination diagrams for sedimentary provenance and tectonic setting show that the nanxiong basin sediments were derived from typical continental sources geochemical signatures e g eu eu th ti la ti ta ti y b ti and y ti ratios of the claystone are nearly constant suggesting the provenance of the nanxiong basin remained similar throughout the late cretaceous to early paleocene 83 56 ma in contrast rb ti cs ti ratios and toc and caco3 concentrations require an obvious change in climate across the late cretaceous and early paleocene boundary singularly higher caco3 contents and lower toc values and rb ti cs ti ratios in the late cretaceous indicate that a long period extreme dry climate occurred at that time in south china rb ti cs ti ratios and toc values escalated and caco3 contents decreased in the early paleocene suggesting that the climate became relatively wet which resulted in greater vegetation cover the lasting extreme dry climate in the late cretaceous may provide a clue to the extinction of the dinosaurs in the nanxiong basin c 2006 elsevier b v all rights reserved

geochemistry and origin of elements in some uk coals abstract twenty four uk coals ranging in rank with 4 6 37 6 volatile matter were analysed for 46 major and trace elements the samples were obtained from the uk coal bank and are representative of the major uk coal fields the major element distributions are interpreted in terms of the mineralogical variations quartz and kaolinite are largely responsible for the si and al carbonates for ca and mg and pyrite for fe also exerting an influence in some samples are siderite al phosphate minerals and illite based on statistical relationships with the major elements rb cr th ce zr y ga la ta nb and v are thought to be mainly present in the clay minerals and as mo sb tl se and bi and pb are probably present in pyrite strontium and ba are concentrated in a restricted number of samples related to the phosphate minerals germanium is the only element for which a major organic association can be demonstrated elements with an indirect association with the organic matter are na cl and br in porefluids and possibly te the ash content is controlled mainly by the detrital input and the trace elements related to the ash content are therefore those elements associated with the clay minerals variations with rank would appear to be mainly related to the moisture content porefluids the trace elements associated with the quartz and clay minerals are thought to be dominantly detrital in origin the non detrital elements essentially those contained in pyrite are thought to have been incorporated in the depositional environment from waters with enhanced salinities through seawater ingress hence there are positive relationships between s and trace element concentrations

u pb zircon ages of granites from the southern margin of the yangtze block timing of neoproterozoic jinning orogeny in se china and implications for rodinia assembly abstract neoproterozoic jinningian bendong granodiorite and sanfang and yuanbaoshan granites occurred in northern guangxi southern margin of the yangtze block se china and were generally considered to be of orogenic origin that is the collision between the yangtze and huanan blocks during the jinning orogeny these granites were intruded into the mesoproterozoic sibao metamorphic basement and covered unconformably by the neoproterozoic danzhou metasediments they are dominantly peraluminous asi 1 18 1 29 and have low initial end values of 4 8 to 7 6 indicating derivation from a supracrustal source with strong enrichment in rb k and th and relatively low zr hf and higher rare earth elements they are similar to post collision granites zircon u pb ages of 819 9 826 10 and 824 4 ma were obtained for the bendong granodiorite and the sanfang and yuanbaoshan granites respectively by both shrimp and conventional dating methods the age result precludes the existence of any late mesoproterozoic grenville age granitic magmatism in the southern margin of yangtze block together with previous age determinations of ophiolites along the southern margin of the yangtze block the jinning orogeny is referred to the neoproterozoic orogeny which took place between 1 0 and 0 80 ga and likely ended at 0 82 ga the granite formation age of 0 82 ga also supports the basal boundary age of 0 80 ga for the post orogenic sinian system which is probably stratigraphically correlated with the adeladean system in south australia as the link between australia east antarctica and laurentia in the configuration of the supercontinent rodinia the collision between the yangtze and huanan blocks during the neoproterozoic jinning orogeny probably marks the termination of the rodinia assembly

experimental investigation of insolation driven dust ejection from mars co2 ice caps mars polar caps are depending on hemisphere and season partially or totally covered with co2 ice icy surfaces such as the polar caps of mars behave differently from surfaces covered with rock and soil when they are irradiated by solar light the latter absorb and reflect incoming solar radiation within a thin layer beneath the surface in contrast ices are partially transparent in the visible spectral range and opaque in the infrared due to this fact the solar radiation can penetrate to a certain depth and raise the temperature of the ice or dust below the surface this may play an important role in the energy balance of icy surfaces in the solar system as already noted in previous investigations we investigated the temperature profiles inside co2 ice samples including a dust layer under martian conditions we have been able to trigger dust eruptions but also demonstrated that these require a very narrow range of temperature and ambient pressure we discuss possible implications for the understanding of phenomena such as arachneiform patterns or fan shaped deposits as observed in mars southern polar region

simultaneous inversion for microseismic event location and velocity model in vaca muerta formation velocity models play a key role in locating microseismic events however it is usually challenging to construct them reliably traditional model building strategies depend on availability of well logs or perforation shots we simultaneously invert for microseismic event locations and a velocity model under the bayesian inference framework and apply it in a field dataset acquired in the vaca muerta formation at neuquen argentina this methodology enables uncertainty and posterior covariance analysis by matching the moveouts of the p and s wave arrival times we were able to estimate a 1d velocity model to achieve improved event locations various analyses indicate the superiority of this model over a model built with the traditional strategy with this algorithm we can perform microseismic monitoring to fracturing treatments where no perforation data is available in addition we can also apply it for long term passive seismicity reservoir monitoring where changes of reservoir properties are expected

locating microearthquakes induced by hydraulic fracturing in crystalline rock microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3 5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced the induced microseismicity was monitored by a network of five vorehole seismic stations a total of about 800 induced events were reliably located from arrival times event locations show a tabular distribution that strikes 350 sup 0 and dips 65 sup 0 east subparallel to the injection well the injection was intended to produce a fracture system that would hydraulically connect two subparallel wells a lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore the 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties the injected fluid appears to have stimulated a zone of rock rather than simply a single fracture the distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment

sequence of facies at a holocene storm dominated regressive barrier at praia de leste southern brazil abstract along the coast at praia de leste southern brazil 25 41 s latitude a positive sedimentary budget and a lowering of the sea level following the postglacial maximum has given rise to a progradational strandplain barrier that is 3 5 km wide sand quarries that lie 3 5 km from the present day coastline have provided material from exceptional outcrops of 75 of the sequence of holocene barrier facies five facies associations have been identified which correspond to the inner shelf the lower middle and upper shorefaces and the foreshore the architecture of the facies shows a regressive sequence that overlies an erosional surface and downlapping pleistocene sediments at praia de leste the facies association has a thickness of 14 m and is deposited between 2 m above and 12 m below mean paleo sea level the barrier corresponds to a coastal environment that is characterised by medium to low wave energy under the additional influence of episodic storm events the sequence at the praia de leste barrier differs from the sequence of clastic shoreline facies found elsewhere in two main ways the first difference is the high content of fine sediment and plant debris released at around the same time as the formation of the barrier from large estuarine systems and the second is the predominance on the middle and lower shorefaces of swaley cross stratified sand facies with abundant plant debris we also describe two further characteristics of the barrier at praia de leste first the swaley cross stratified sand represents a component of onshore transport that resulted in the accumulation of sediment transported from the shelf second the beach step has been well preserved thereby allowing the continuous tracing of seaward dipping low angle cross stratification to a sigmoidal cross stratified beach step sandy facies finally we have herein been able to improve the precision of the sequence of formation of the depth of the facies by making reference to paleo sea level during the formation of the barrier rather than to present day mean sea level