rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments deposition of volcanic ash into aqueous environments leads to dissolution of adsorbed metal salts and aerosols increasing the bioavailability of key nutrients volcanogenic fertilization events could increase marine primary productivity leading to a drawdown of atmospheric co2 here we conduct flow through experiments on unhydrated volcanic ash samples from a variety of locations and sources measuring the concentrations and fluxes of elements into de ionized water and two contrasting ocean surface waters comparisons of element fluxes show that dissolution of adsorbed surface salts and aerosols dominates over glass dissolution even in sustained low ph conditions these surface ash leachates appear unstable decaying in situ even if kept unhydrated volcanic ash from recent eruptions is shown to have a large fertilization potential in both fresh and saline water fluorine concentrations are integral to bulk dissolution rates and samples with high f concentrations display elevated fluxes of some nutrients particularly fe si and p bio limiting micronutrients are released in large quantities suggesting that subsequent biological growth will be limited by macronutrient availability importantly acidification of surface waters and high fluxes of toxic elements highlights the potential of volcanic ash leachates to poison aqueous environments in particular large ph changes can cause undersaturation of caco3 polymorphs damaging populations of calcifying organisms deposition of volcanic ash can both fertilize and or poison aqueous environments causing significant changes to surface water chemistry and biogeochemical cycles

geochemistry and sedimentology of the lower silurian longmaxi mudstone in southwestern china implications for depositional controls on organic matter accumulation abstract spatial and temporal changes of lithofacies and abundance of organic matter in mudstones control distribution of prospective unconventional petroleum reservoirs in a sedimentary basin lithofacies characterization is an essential prerequisite to understanding of organic matter accumulation depositional processes and water column chemistry this research combines geochemical analyses and detailed sedimentologic observations in order to investigate the depositional controls on organic matter abundance in the prolific black mudstone of the long 1 member of the lower silurian longmaxi formation in southwestern china six primary lithofacies deposited at variable marine water depths and under differing water column chemistry were identified from an 83 m long core based on microscopic observations of sediment texture and structure and biota and analyses of total organic carbon toc content mineralogy and trace element abundances our results show that toc content is well correlated to biogenic quartz content and the non detrital components of v u mo ni and cu suggesting the accumulation of organic matter in the studied mudstone was controlled by high paleoproductivity and anoxic water conditions the good correlations also suggest that the abundant organic matter was produced by algal blooms which are typically associated with radiolarian thrive because of the symbiotic relationship between algal and radiolarian the low mo toc ratios of the three lithofacies formed in deep anoxic environments were similar to the ratios of modern sediments deposited in anoxic euxinic environments suggesting moderate basin restriction during deposition the mudstone in the lower longmaxi formation may be part of the globally abundant organic rich mudstones of early silurian age whose depositional mechanism have yet to be fully explored the distribution of lithofacies in the studied interval shows an overall trend from deep water to shallow water depositional environments we developed a detailed depositional model to interpret the evolution of depositional environments of the lower longmaxi formation this study provides an example to how better characterize unconventional hydrocarbon systems through coupling of rigorous geochemical and sedimentological analysis

magnetic field modelling and interpretation of the himalayan tibetan plateau and adjoining north indian plains abstract a prominent magnetic low is seen over the himalayan tibetan plateau and the northern indian shield region in satellite anomaly data here we model the sources for the observed magnetic low in the region the induced magnetic field is modelled based on a crustal magnetisation model computed in a geographical information system with horizontal layers containing susceptibility derived from the geology and a tectonic map of the region and global seismic crustal thickness model the modelling results indicate that the upper crust of the himalayan tibetan plateau is magnetised while much of the middle crust and the lower crust appears non magnetic thermal modelling corroborates the predictions placing the curie isotherm in the southern tibetan plateau no deeper than 10 km the results suggest that the sources of the magnetic low observed in the region are likely to be a thin magnetised crust because of the elevated curie isotherm in this region the indian shield region is believed to extend up to the foothills of the himalayas in the north but most of the shield is obscured by a thick sediment cover of the gangetic plains to the north the magnetic field modelling suggests a weakly magnetic crust in the region indicating either the indian shield may not extend up to the base of the himalayas or the lower crust in the region has been modified

carbon isotopes and event stratigraphy near the ordovician silurian boundary yichang south china abstract the organic carbon isotope data through the unweathered ordovician silurian boundary strata at the wangjiawan riverside section which is c 180 m southeast of the wangjiawan north section the gssp for the base of the hirnantian stage upper ordovician show that a positive 13 c org excursion begins just below the base of the hirnantian stage and peaks in the lower part of the n extraordinarius biozone this is followed by an interval of slightly reduced 13 c values and a second peak of 2 above pre hirnantian values which occurs in the lower part of the n persculptus biozone upper hirnantian the peaks in 13 c org values can be correlated well with episodes of glacial expansion described from africa and peri gondwanan europe evidence from sedimentological faunal and geochemical data from south china as well as the evidence of glacial sediments in north africa all suggests a short lived glaciation of 1 ma in the ordovician south pole region consisting of at least two major pulses the first phase of gradual glacial expansion began just before the hirnantian epoch and the second phase ended with rapid melting in the late hirnantian early n persculptus biozone

productivity or preservation the factors controlling the organic matter accumulation in the late katian through hirnantian wufeng organic rich shale south china abstract the wufeng organic rich shale is widespread in the upper yangtze platform and it is one of the most advantageous shale gas producers in china its deposition commenced with the onset of the boda warming event in the late katian and terminated at the hirnantian glacial maximum the depositional mechanism of the wufeng formation was poorly understood owing to its relatively thin thickness being interpreted as the result of organic matter om accumulation in a shelf setting under anoxic conditions this study integrated detailed geochemical analyses based on the core samples from three boreholes which are located in the outer middle and inner shelf of the upper yangtze platform in order to investigate the depositional environments during the deposition of the wufeng formation geochemical redox proxies mo ef u ef and v cr and productivity proxies p baxs sixs indicate that the deposition of the wufeng formation was associated with four intervals wf1 to wf4 each characterized by its unique geochemical features related to sea level fluctuations black siliceous mudstones deposited during wf1 and wf2 are relatively organic lean due to low productivity less favorable preservation conditions and terrestrial dilution whereas black siliceous mudstone deposited during wf3 and wf4 is organic rich due to high paleoproductivity and conducive preservation conditions the geochemical proxies cd mo co x mn associated with the modern upwelling settings were replicated in the wufeng formation revealing that the outer shelf of the upper yangtze platform was dominated by persistent upwelling the upwelling driven primary productivity is considered as the first order control for the om enrichment at the outer shelf of the upper yangtze platform seasonal upwelling may influence the middle shelf of the upper yangtze platform where both primary productivity and preservation played essential roles in the om accumulation in contrast the om accumulation at the inner shelf was fundamentally driven by good preservation conditions

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

crustal structure of the india asia collision zone southern tibet from indepth mt investigations abstract project indepth was initiated in 1992 to develop a better understanding of the deep structure and mechanics of the himalaya tibet region and magnetotelluric surveying was added during indepth phase ii in 1995 broadband and long period data were acquired in 1995 along a n s transect crossing the india asia collision zone the yarlung zangbo suture at 90 e longitude however these data failed to penetrate to the deep crustal and mantle depths due to low solar activity and the presence of the thick conductive crust deep crustal and upper mantle information is key to test the model of subcretion to southern tibet by the stiff indian mantle lid in order to try to obtain such information and to test along strike variability of the yarlung zangbo suture zone a magnetotelluric experiment was performed in 2001 roughly along longitude 92 e containing an ultra long period component mt data were acquired at 28 locations along the new profile with long period data at 15 of these and ultra long period data at five of these 15 initial comparisons of the models generated from the 1996 and 2001 datasets indicate little variation in electrical conductivity structure along strike

relationship between the origin of organic rich shale and geological events of the upper ordovician lower silurian in the upper yangtze area abstract based on detailed sedimentological observations and geochemical analyses four geological events have been identified and a genetic depositional model has been established within a sequence stratigraphic framework the organic rich siliceous shale interbedded with dense bentonite layers in the transgressive systems tract 1 tst 1 is attributed to the first stage of high frequency volcanism which started at 447 62 ma associated with the collision of the cathaysia and yangtze blocks the presence of the organic fair lime mudstone shelly limestone indicates that hirnantian glaciation in the highstand systems tract hst the occurrence of organic rich siliceous shale interbedded with scarce bentonite layers in the transgressive systems tract 2 tst 2 is attributable to the second stage of low frequency volcanism which started at 443 83 ma and to postglacial effects the formation of organic fair mixed shale with silty laminae is attributed to bottom current activity in the early highstand systems tract ehst the occurrence of organic poor clay rich shale is due to terrigenous input in the late highstand systems tract lhst our research also shows that the high marine productivity caused by volcanic ash and a high burial rate in an anoxic depositional setting led to organic matter and siliceous mineral enrichment in the tst 1 and tst 2 the favorable organic matter preservation conditions and the postglacial biotic recovery played significant roles in the formation of the organic rich shale in the tst 2

controls on organic matter accumulation in the triassic chang 7 lacustrine shale of the ordos basin central china abstract the triassic chang 7 formation is a rich lacustrine source rock in the ordos basin central china petrographic and geochemical analyses were performed in order to research the mechanism of organic matter om enrichment in the chang 7 shales bulk geochemical data indicate that the om rich shales were formed in a fresh and brackish water which is not usually the case for high toc total organic carbon content sediments therefore there must be some special conditions favorable for om enrichment in chang 7 shales petrography and element geochemistry show that the chang 7 om rich shales are abundant in om enriched laminae framboidal pyrites collophanes and some elements including iron fe diphosphorus pentoxide p2o5 copper cu vanadium v molybdenum mo and uranium u high bio productivity algae blooming and anoxic depositional environments are critical for om enrichment as the organic carbon contents increase the fe p2o5 cu v mo and u element contents increase as well and the ratios of u th and v v ni are growing many geochemical data demonstrate that the water was oxic while the environment under the sediment water interface was anoxic as a result of a high organic supply that sufficiently consumed oxygen taking into account the geological settings and our previous studies we propose that the frequent volcanic events and hydrothermal activity caused by the collision of the north china landmass with the yangtze landmass and consequential formation of the qingling mountains had a significant role in the om enrichment in the fresh and brackish lacustrine shales deposition of volcanic ash into aqueous environments might lead to the increase of some key nutrients such as fe and p2o5 which might enhance primary bio productivity new evidence for hydrothermal activity the reddingite was demonstrated in this study not only can the hydrothermal activity provide elements necessary for life but is also favorable for formation of anoxic settings the deposition of the chang 7 lacustrine om rich shales can be explained using the high bio productivity driven model which is related to volcanic events and hydrothermal activity as a consequence of regional tectonic movements an anoxic depositional setting is beneficial for om preservation and slow sedimentation rates in the deep lacustrine environments are favorable for om enrichment

a comprehensive model of the quiet time near earth magnetic field phase 3 summary the near earth magnetic field is caused by sources in the earth s core ionosphere magnetosphere lithosphere and from coupling currents between the ionosphere and the magnetosphere and between hemispheres traditionally the main field low degree internal field and magnetospheric field have been modelled simultaneously with fields from other sources being modelled separately such a scheme however can introduce spurious features especially when the spatial and temporal scales of the fields overlap a new model designated cm3 comprehensive model phase 3 is the third in a series of efforts to coestimate fields from all of these sources this model has been derived from quiet time magsat and pogo satellite and observatory hourly means measurements for the period 1960 1985 it represents a significant advance in the treatment of the aforementioned field sources over previous attempts and includes an accounting for main field influences on the magnetosphere main field and solar activity influences on the ionosphere seasonal influences on the coupling currents a priori characterization of the influence of the ionosphere and the magnetosphere on earth induced fields and an explicit parametrization and estimation of the lithospheric field the result is a model that describes well the 591 432 data with 16 594 parameters implying a data to parameter ratio of 36 which is larger than several popular field models

lithofacies and depositional mechanisms of the ordovician silurian wufeng longmaxi organic rich shales in the upper yangtze area southern china the ordovician silurian organic rich shales in the wufeng and longmaxi formations are widespread in southern china and are the most important shale gas producers in china the shales display varying total organic carbon content ranging from 0 2 to 6 and are composed of black siliceous mudstone shelly limestone siliceous clay rich mudstone gray silty shaly interlaminated mudstone gray argillaceous mudstone and calcareous or dolomitic mudstone detailed stratigraphic correlation of these formations was achieved and a high resolution sequence stratigraphy was constructed based on the integration of well log data mineralogical geochemical and petrophysical analyses the wufeng formation and the guanyinqiao bed were interpreted to be a condensed section with four cycles of sea level fluctuation the longmaxi formation consists of three members the long 1 member was interpreted to be a third order depositional sequence that can be subdivided into a transgressive systems tract an early highstand systems tract and a late highstand systems tract within the constructed isochronal stratigraphic framework the characteristics and genetic mechanism of different shale lithofacies were studied the deposition of black siliceous mudstone of high total organic carbon in the wufeng formation and the long 1 transgressive systems tract was likely influenced by volcanic activity the guanyinqiao shelly limestone or calcareous mudstone was associated with the ordovician silurian transition when glaciation and mass extinction took place the gray silty shaly interlaminated mudstone of moderate total organic carbon in the long 1 early highstand systems tract was the product of bottom flow intrusion the deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the long 1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts the distribution pattern of different lithofacies varies greatly in the upper yangtze area and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry a better understanding of the distribution pattern and depositional history of the wufeng and longmaxi formations might provide guidance for future profitable shale gas exploration

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

comparison of radiolarian and sedimentologic paleoproductivity proxies in the latest miocene recent benguela upwelling system abstract estimating past ocean productivity from ocean sediments often gives different results depending on the measurement used we have examined a suite of paleoproductivity proxies in latest miocene recent sediments from dsdp site 532 and odp site 1084 two deep sea sections underlying the benguela upwelling system off the atlantic coast of southern africa the productivity history of this system has been previously established via organic carbon concentration diatom floras and alkenone based estimates of surface water temperature and shows a change from low productivity in the early pliocene to sustain high productivity in the late pliocene recent each of our samples was split and simultaneously analysed for several proxies of ocean productivity including organic carbon toc carbonate abundance of opaline radiolarians accumulation rate of benthic foraminifera bfar the radiolarian faunal composition indices upwelling radiolarian index uri and the water depth ecology index wade other proxies for opal and carbonate dissolution plus stable isotopes of benthic foraminifera comparisons between proxies in the same measured samples between sites in downcore plots and to the published productivity record for this region suggest that toc and radiolarian faunal composition particularly the wade index are good indicators of past productivity albeit with different sensitivities log linear correlation wade toc r 0 78 n 65 p

a modern vs permian black shale the hydrography primary productivity and water column chemistry of deposition the sediment currently accumulating in the cariaco basin on the continental shelf of venezuela has an elevated organic carbon content of approximately 5 is accumulating under o2 depleted bottom water conditions so42 reduction is composed dominantly of foraminiferal calcite diatomaceous silica clay and silt and is dark greenish gray in color upon lithification it will become a black shale recent studies have established the hydrography of the basin and the level of primary productivity and bottom water redox conditions these properties are used to model accumulation rates of cd cr cu mo ni v and zn on the seafloor the model rates agree closely with measured rates for the uppermost surface sediment the model is applied to the meade peak phosphatic shale member of the phosphoria formation a phosphate deposit of permian age in the northwest united states it too has all of the requisite properties of a black shale although the deposit is a world class phosphorite it is composed mostly of phosphatic mudstone and siltstone chert limestone and dolomite it has organic carbon concentrations of up to 15 is strongly enriched in several trace elements above a terrigenous contribution and is black the trace element accumulation defines a mean primary productivity in the photic zone of the phosphoria basin as moderate at 500 g m 2 year 1 organic carbon comparable to primary productivity in the cariaco basin the source of nutrient enriched water that was imported into the phosphoria basin upwelled into the photic zone and supported primary productivity was an o2 minimum zone of the open ocean the depth range over which the water was imported would have been between approximately 100 and 600 m the mean residence time of bottom water in the basin was approximately 4 years vs 100 years in the cariaco basin the bottom water was o2 depleted but it was denitrifying or no3 reducing rather than so42 reducing

paleoceanographic applications of trace metal concentration data abstract recent studies have identified a range of new applications of trace metal concentration data in the analysis of paleoceanographic systems in restricted anoxic marine systems trace metal toc ratios can provide insight into the degree of watermass restriction and estimates of deepwater renewal times in such systems secular changes in sediment trace metal ratios may provide evidence of the chemical evolution of basinal deepwaters in response to differential rates of trace metal removal to the sediment the degree of deepwater restriction in silled basins is generally controlled by eustatic elevations with higher lower sea levels resulting in lesser greater watermass restriction short term ca million year drawdown of the trace metal inventory of seawater has occurred repeatedly in conjunction with oceanic anoxic events which result in elevated rates of trace metal removal to widespread anoxic facies long term eon scale changes of the trace metal composition of seawater have occurred in response to secular changes in atmospheric oceanic redox conditions and their effects on trace metal cycling caution must be exercised in evaluating trace metal patterns in paleomarine systems however 1 hydrographic analyses based on trace metal toc relationships can be undertaken only on systems that had anoxic deepwaters and 2 the influence of redox variation on trace metal accumulation patterns outweighs that of hydrographic factors in some paleomarine systems

the downward continuation of magsat crustal anomaly field over southeast asia a general method for the local downward continuation at ground level of geopotentials measured by low orbiting satellites at different altitudes is proposed such a continuation is the inverse solution of the external dirichlet problem the stability of the solution with respect to the altitude and the distribution of data points is discussed principal component analysis is shown to perform well and leads to a stable solution the method proposed has the advantage of being directly applicable to measured data points which are not evenly spaced and at different altitude as an illustration it is applied to magsat data over southeast asia two independent maps of the vertical component of the anomaly field at a constant altitude near ground level are derived for dawn descending orbits and dusk ascending orbits data separately the two maps reveal similar anomalies and confirm the correlation of the magsat derived anomaly field with the mosaic structure of the continental crust in southeast asia stable continental blocks are associated with positive anomalies and intervening collision belts correspond to well defined negative anomalies a very prominent low is observed above the himalaya and extends over the tibet plateau confirming an anomalous crustal and thermal structure of the plateau

differential enrichment of shale gas in upper ordovician and lower silurian controlled by the plate tectonics of the middle upper yangtze south china abstract the upper ordovician wufeng formation and lower silurian longmaxi formation marine shale are the major shale gas exploration and development target in the middle upper yangtze region although three national shale gas demonstration areas have been built and several high yield shale gas wells have been discovered in the wufeng formation and longmaxi formation shale gas in these areas has not been fully commercialized because the enrichment degree of shale gas varies greatly in different tectonic settings by analyzing the formation processes and distribution characteristics of the organic rich shale reconstructing the burial history and hydrocarbon generation history and comparing the shale gas enrichment and preservation conditions in different tectonic settings in south china this paper points out that 1 the craton depression and foreland basin on the cratonic margin are favorable zones for the development of organic rich shale because the yangtze plate is small the foreland basin is greatly affected by the input of terrigenous clastic and the thickness and area of organic rich shale in the craton basin is larger than that in the foreland basin the organic rich shale is mainly distributed in south sichuan deep water shelf and west hubei east chongqing deep water shelf and the enrichment interval is the dillcellograptus complexus cystograptus vesicolus graptolite zone from the wufeng formation to the bottom of longmaxi formation are favorable intervals for shale gas enrichment 2 since the late jurassic significant differential intracontinental tectonic deformation uplift and denudation occurred in the middle upper yangtze region moderate compression and uplift were beneficial to the development of micro fractures and bedding fractures in shale resulting in good preservation conditions of shale gas reservoirs with a high pressure coefficient in contrast strongly deformed areas where faults and high angle large fractures were developed are not conducive to the preservation of shale gas reservoirs 3 the craton depression of the middle upper yangtze region where organic rich shale was deposited is favorable to shale gas enrichment because it provides a good material basis and also a relatively stable and compression resistant tectonic environment for later tectonic deformation and uplift derived destruction based on the analysis and evaluation of tectonic construction transformation conditions under the macro control of plate tectonics it can well explain the existing commercial shale gas discoveries and failure cases and can guide the optimization of future exploration targets

constraints on the accumulation of organic matter in upper ordovician lower silurian black shales from the lower yangtze region south china abstract the late ordovician to early silurian o s was an important period in which many significant geological events occurred during the o s transition organic rich black shales were widely deposited and became important unconventional energy sources paleoproductivity detrital input and redox conditions are the three major factors controlling organic matter om accumulation although the effect of each factor is relatively clear the complex influences of these factors remain poorly understood specifically the evolution of these factors and the corresponding influence on the accumulation of om during the o s transition has not yet been systematically investigated this paper investigates the constraints on om accumulation in o s transition black shales from the lower yangtze region based on graptolite fossil identification zircon u pb dating and geochemical analyses of a well sequenced drilling profile the results show that i the sy 1 core sequence was well constrained from graptolite zones wf2 447 62 ma to lm5 441 57 ma across the o s boundary with decreasing sedimentation rates srs in the upper katian wf2 to wf3 and increasing srs in the lower rhuddanian ii complementary volcanic inputs katian and rhuddanian and upwelling events hirnantian exerted strong control over paleoproductivity iii orogenic processes in south china controlled detrital inputs and redox conditions iv om accumulation in the lower yangtze area across the o s transition occurred in three stages in stage 1 wf2 wf3 strong volcanism boosted productivity whereas om preservation was not favored in the oxic environment in stage 2 wf4 lm3 upwelling and volcanism resulted in high productivity and provided the optimal configurations for om enrichment in stage 3 lm4 and later rare om was preserved due to terrestrial dilution and unfavorable preservation conditions eventually wf4 lm3 became the prominent om rich biozones due to the effective coordination of the controlling factors namely upwelling volcanism and redox conditions

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

crust and upper mantle shear velocity structure beneath the tibetan plateau and surrounding regions from interevent surface wave phase velocity inversion average a priori shear velocity models are constructed for the tien shan tarim basin pamir hindu kush himalaya and ne india these models are shown to account for most of the lateral velocity heterogeneity at wavelengths 1000 km interevent fundamental mode rayleigh and love wave phase velocities were measured in the period range 32 200 s and were inverted for path averaged shear velocity structures these in turn were regionalized to estimate average isotropic structures beneath the six regions listed above low upper mantle velocities are observed beneath the western and eastern tien shan the tarim basin structure is poorly constrained but exhibits very low upper crustal velocities probably due to deep sedimentary accumulation and either very high upper mantle velocities exist 4 8 km s in a layer 70 km thick or the lithosphere is very thick 180 km low upper mantle velocities are observed beneath the chang thang region the central and the northeast plateau but not beneath the southern or southeastern plateau the crustal velocity gradient with depth is zero in the east central plateau consistent with lower crustal basaltic intrusions there but is positive to the west and southeast of this zone upper mantle structures show that the indian lithosphere could have subducted beneath the entire plateau only if either 1 mantle in the depth range 115 185 km has been altered or removed to reduce its average velocity or 2 the indian lithosphere is only 85 km thick

resolution of direction of oceanic magnetic lineations by the sixth generation lithospheric magnetic field model from champ satellite magnetic measurements the champ satellite continues to provide highly accurate magnetic field measurements from decreasing orbital altitudes 350 km at solar minimum conditions using the latest 4 years 2004 2007 of readings from the champ fluxgate magnetometer including an improved scalar data product we have estimated the lithospheric magnetic field to spherical harmonic degree 120 corresponding to 333 km wavelength resolution the data were found to be sensitive to crustal field variations up to degree 150 down to 266 km wavelength but a clean separation of the lithospheric signal from ionospheric and magnetospheric noise sources was achieved only to degree 120 this new mf6 model is the first satellite based magnetic model to resolve the direction of oceanic magnetic lineations revealing the age structure of oceanic crust

sea level changes control organic matter accumulation in the longmaxi shales of southeastern chongqing china abstract fine grained sediments in the ordovcian silurian o s longmaxi formation on the yangtze platform represent a significant shale gas source and reservoir in south china the formation of the organic rich black rock has been attributed to high primary productivity suboxic anoxic environments and or low detrital dilution however these claims remain to be debated here we investigated total organic carbon toc contents iron speciation major and trace elements of the o s longmaxi shale in the inner shelf qianqian 1 drillcore southeastern chongqing south china our results show that the longmaxi formation is composed of black shale with high toc values 4 7 0 6 in the lower member lm 0 6 m and silty shale with low toc values 1 3 0 5 in the upper member um 6 77 m the data from the lm suggests high primary productivity ferruginous conditions punctuated with episodes of intermittent euxinia and low terrigenous fluxes in contrast data for the um indicate reduced primary productivity ferruginous conditions and higher terrigenous fluxes leading to dilution of the organic matter a good coupling of local redox conditions with primary productivity and p al ratios rather than water mass restriction suggests that water column anoxia was mainly driven by high primary productivity and had a minor effect on primary productivity through marine p cycling changes in primary productivity might result from variable upwelling intensities associated with the fluctuations of sea levels variations in local redox conditions and terrigenous fluxes are mainly controlled by sea level fluctuations this study thus suggests that sea level fluctuations and related environmental effects play key roles in organic matter accumulation through primary productivity redox state and detrital dilution these findings highlight that multiple factors are responsible for organic matter accumulation at this location during the o s transition in south china

the end ordovician glaciation and the hirnantian stage a global review and questions about late ordovician event stratigraphy abstract this paper proposes a global review of hirnantian event stratigraphy the hirnantian gssp in south china is tentatively correlated with latest ordovician strata from the peri gondwanan glacial regions problems of biostratigraphical correlation are highlighted at a worldwide scale the major biostratigraphically useful fossil groups graptolites chitinozoans brachiopods conodonts acritarchs are analysed and their limits for global correlation of the uppermost ordovician are discussed palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective late ordovician global biostratigraphical scheme as an alternative correlative tool the hice hirnantian isotopic curve excursion event is often put forward in the literature however carbon isotope chemostratigraphy shows like biostratigraphy some limits to the present state of knowledge no good independent biostratigraphical control of the hice exists in both shallow carbonate deposits and deeper shaly ones recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic 13corg and inorganic 13ccarb carbon species further complicating the use of the hice as an isochronous benchmark all of these difficulties for hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future precise late ordovician and early silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the big five extinctions

sedimentary environmental controls on petrology and organic matter accumulation in the upper fourth member of the shahejie formation paleogene dongying depression bohai bay basin china abstract the upper fourth member of the eocene shahejie formation es4s is a typical example of a bohai bay basin lacustrine shale these sedimentary units are characterized by markedly different mineral compositions compared to marine siliciclastic shales thus understanding the characteristics and sedimentary environment of lacustrine shales enables analysis of their origin enrichment of organic matter om distribution of organic rich shales and exploration targets for shale oil and gas based on thin sections and field emission scanning electron microscope observations from well core samples combined with x ray diffraction and geochemistry the mineralogy lithofacies organic and geochemical characteristics as well as the sedimentary environment of the es4s shale are analyzed in this paper the results show that across the study area the es4s shale is mainly composed of calcite clay minerals and quartz but also contains subordinate dolomite feldspar pyrite and anhydrite on the basis of mineral composition total organic carbon toc content and sedimentary structures eight lithofacies are identified the toc content of samples ranges between 0 15 and 11 4 with an average of 2 27 while om primarily consists of type i kerogen based on paleoclimate and paleosalinity data as well as redox conditions and primary productivity the sedimentary environment of the es4s shale can be further divided into four intervals intervals i ii iii and iv and these four intervals have large differences rather than persistently warm damp climate and reducing condition the evolution of these intervals is also analyzed in detail in this study and the controls on lithofacies and om accumulation are also discussed the results of this study suggest that the development of mineral compositions and frequent vertical lithofacies associations are strongly controlled by the cyclic evolution of the sedimentary environment thus the accumulation of om occurs as the result of a number of inter connected factors including primary productivity terrestrial inputs and variation in oxidizing reducing conditions across a given interval these factors can either work together or one can play a leading role

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

lithofacies and sequence stratigraphy of the barnett shale in east central fort worth basin texas ten barnett shale lithofacies have been recognized in a 223 ft 68 m long core from johnson county texas eight of these lithofacies match those previously identified in the main producing area of the newark east barnett shale field in the northern part of the fort worth basin but two new lithofacies have been identified in this core resedimented spiculitic mudstone lithofacies and lag deposits both of which are indicative of a relatively higher energy environment and downslope resedimentation of shallower water deposits the recognition of cyclical stacking patterns of the lithofacies condensed sections css and transgressive surfaces of erosion were the keys to establishing the sequence stratigraphic framework in these fine grained rocks which consists of seven stratigraphic intervals in the lower barnett shale and nine stratigraphic intervals in the upper barnett shale spectral gamma ray uranium and thorium logs aided in this objective and are recommended for future sequence stratigraphic studies of these and other shales the sequence stratigraphic framework reveals that the lower barnett shale in this area was deposited mainly in a low energy relatively deep water environment somewhat far from a terrigenous source area which probably lies to the northwest by contrast the upper barnett shale was deposited in an oxygenated shallower water environment which had a source area from the west and southwest sides of the basin the higher frequency of sea level fluctuation during development of the upper barnett shale most probably indicates periodic tectonic activity perhaps associated with a structural high that was susceptible to sea level fluctuations alternatively it could have resulted from the onset of glaciations in gondwanaland during this time this higher frequency may indicate that the upper barnett is chesterian in age because cyclicity was higher than during the osagean and meramecian stages if so there may be more high frequency cycles than recognized in this core siliceous sponge spicules are more common in this core than in more northerly cores so more brittle facies might prevail in the southern part of the fort worth basin high gamma ray log responses which are caused by a high total organic carbon and or in situ phosphate minerals are commonly found in css and can be used for regional correlations however high gamma ray phosphatic deposits that have been resedimented to downslope positions by sediment gravity flows are an exception to the previous statement correlation of the barnett stratigraphic intervals now provides a north to southeast stratigraphic framework along the fort worth basin relative hydrocarbon potential rhp is an organic geochemical parameter applied to this core and found to provide an indicator of marine transgressions and regressions we recommend continued testing and use of the rhp parameter for high frequency sequence stratigraphic analysis of unconventional gas shales

fracture development in paleozoic shale of chongqing area south china part one fracture characteristics and comparative analysis of main controlling factors abstract natural macro and micro fractures in the longmaxi shale lower silurian from yuye 1 well and the niutitang shale lower cambrian from yuke 1 well are characterized regional geological structural evolution and shale geochemistry including toc and mineral composition etc are analyzed the controlling factors of natural fractures in palaeozoic shale in the southeast of chongqing have been confirmed which have been analyzed qualitatively or semi quantitatively the results show that fractures are abundant in both the longmaxi shale and niutitang shale due to the multiphase strong tectonic movement in the southeast of chongqing therefore tectonic factors are the principal factors that control fracture development in the paleozoic shale of this region followed by the mineral composition of the shale vertically the mineral composition of shale in yuke 1 well has a high dispersion degree and a strong heterogeneity the dispersion coefficients of quartz and dolomite were 0 48 and 1 respectively therefore the development and distribution of fractures are controlled by quartz and dolomite content in the niutitang shale moreover the quartz content shows positive correlation with the fractures quantity and fractures are most abundant where dolomite content up to 64 7 while the mineral composition of shale in yuye 1 well has a low dispersion degree and a strong homogeneity in a vertical position the dispersion coefficients of quartz and plagioclase were 0 15 and 0 26 respectively which shows unconspicuous control effect for fractures therefore the development and distribution of fractures are mainly controlled by the toc content moreover the toc content shows positive correlation with the fractures quantity fractures are sealed by calcite quartz and pyrite although sealed fractures do not contribute to the permeability of the reservoir they are important planes of weakness that tend to be reactivated by hydraulically induced fractures sem images show that intergranular pore is connected by microfractures thereby the validity of porosity has been improved the mixed network of microfractures and micropore provide enhanced permeability or storage capacity for the reservoir

influence of volcanic and hydrothermal activity on organic matter enrichment in the upper triassic yanchang formation southern ordos basin central china abstract based on the field investigation of oil shales in the southern ordos basin detailed observations and systematic sample analyses of core samples from a well that penetrated an oil shale the influence of volcanic and hydrothermal activity on the chang 8 chang 7 members of the yanchang formation on organic matter om enrichment was geochemically and petrologically analyzed according to maceral analyses and rock pyrolysis the kerogen types of the samples in the study area are widely distributed those of the oil shale with the highest om abundance are mostly type ii i and are immature or in a stage of low maturity six layers of tuff and tuffaceous mudstone were identified by core observation and element characteristics they are characterized by higher total rare earth element ree and ce values and th contents and lower la yb n and eu values mineral assemblages of anhydrite pyrite marcasite and relevant diagrams of the elemental geochemistry confirmed the existence of hydrothermal deposits the high yn hon and u th ratios and total sulfur ts contents indicate that the chang 7 member experienced four episodes of intense hydrothermal activity early in the chang 73 member volcanic ash and hydrothermal fluids brought in a large amount of nutrients which not only increased primary productivity but also increased the salinity of the water column enhanced anoxia in the sedimentary environment of the lake bottom promoted the preservation of om and finally formed the oil shale layer with the richest om however the strong volcanic eruption in the late chang 73 member led to changes in the regional climate causing it to dry and become colder and resulting in a decrease of primary productivity earthquakes triggered by volcanism resulted in turbidite deposits which destroyed the preservation and diluted the enrichment of om this study reveals in detail the influence of volcanic and hydrothermal activity on om enrichment during the evolution of the basin and thereby deepens our theoretical understanding of om enrichment

subduction zone volcanic ash can fertilize the surface ocean and stimulate phytoplankton growth evidence from biogeochemical experiments and satellite data volcanoes confront earth scientists with new fundamental questions can airborne volcanic ash release nutrients on contact with seawater thereby excite the marine primary productivity mpp and most notably can volcanoes through oceanic fertilization affect the global climate in a way that is so far poorly understood here we present results from biogeochemical experiments showing that 1 volcanic ash from subduction zone volcanoes rapidly release an array of nutrients co limiting algal growth in vast oceanic areas 2 at a speed much faster minute scale than hitherto known and that marine phytoplankton from low iron oceanic areas can swiftly within days utilize iron from volcanic sources we further present satellite data possibly indicating an increase of the mpp due to the seaward deposition of volcanic particulate matter our study supports the hypothesis that oceanic iron fertilization with volcanic ash may play a vital role for the development of the global climate

changes in marine productivity and redox conditions during the late ordovician hirnantian glaciation abstract changes in marine productivity and redox conditions during the end ordovician hirnantian glaciation and ordovician silurian transition were investigated through mo isotope and major and trace element analyses of the wangjiawan hubei province and nanbazi guizhou province sections from the yangtze platform of south china katian shales of the wufeng formation which yield the graptolites dicellograptus complanatus dicellograptus complexus and paraorthograptus pacificus were deposited under euxinic conditions at both localities as shown by high mo ef u ef and 98 mo values a major sea level regression during the hirnantian glaciation resulted in shallowing and a shift toward better oxygenated conditions within the yangtze sea as well as deposition of thin bedded siliceous sediments calcirudite debris flows and limestone turbidites of the kuanyinchiao formation the base of which correlates with the first phase of the end ordovician mass extinction the termination of the hirnantian glaciation at the top of the kuanyinchiao formation was associated with a major sea level transgression a rapid expansion of euxinia in the yangtze sea as documented by a return of high mo ef u ef and 98 mo values and the second phase of the mass extinction during which the cool adapted hirnantian fauna went extinct the long term cooling trend of the middle and late ordovician which culminated in the hirnantian glaciation was driven by enhanced burial of organic carbon as documented by 13 c carb 13 c org and 34 s py records increased organic carbon burial was linked to high rates of marine productivity as shown by high toc and biogenic ba concentrations especially at the deeper less restricted wangjiawan locale producing the hice hirnantian isotopic curve excursion 13 c carb excursion the locus of organic carbon burial during the hirnantian crisis shifted to deeper water environments that were located outside the study region the relatively rapid onset and termination of the hirnantian glaciation were probably due to crossing of tipping points in the late ordovician climatic oceanic system

model for k bentonite formation evidence from zoned k bentonites in the disturbed belt montana thirty four k bentonites from the marias river formation upper cretaceous in the montana disturbed belt have been studied in detail a 2 5 m thick k bentonite is zoned mineralogically and chemically the upper and lower contacts of the bed being more illitic and k rich than the middle of the bentonite k ar dates of illite smectite separated from the bed are also zoned yielding older ages at the contacts than at the center a model for the formation of k bentonites enclosed by k rich shale is presented in which k bentonites form from smectite bentonites during a thermal event t 100 200 c potassium is derived from mineralogic breakdown probably mica and k feldspar in the shale and migrates by diffusion into the bentonite

laminae characteristics and influence on shale gas reservoir quality of lower silurian longmaxi formation in the jiaoshiba area of the sichuan basin china abstract widely developed in shale strata laminae are the most typical shale sedimentary structure to investigate their influence on shale gas reservoir quality we conducted complementary studies by integrating microscopic observations geochemical analyses field emission scanning electron microscopy fe sem imaging and mercury intrusion capillary pressure micp techniques the microscopic observation of thin sections enabled us to detect the laminae characteristics including the density and maximum thickness of individual lamina and three types and stages of lamina development were identified the total organic carbon toc content mineral composition natural fracture pore type pore size distribution porosity permeability and tortuosity were investigated in these different laminae types using fe sem micp and geochemical analysis combining the laminae characteristics and pore structure we concluded that the organic pores and total pore area are mainly affected by the toc content whereas the development of the laminae controls the inorganic macropore scale and affects the pore size distribution in addition we found that the excessive development of shale laminae is not conducive to organic matter accumulation because of the change in redox condition and because it has an adverse impact on the formation of complex artificial fractures during hydraulic fracturing

radiolarian and sedimentologic paleoproductivity proxies in late pleistocene sediments of the benguela upwelling system odp site 1084 the changing composition of radiolarian faunas from late neogene deep sea sediments has been used in recent years as a proxy for changes in marine paleoproductivity we examine radiolarian faunas organic carbon content toc opal and coarse fraction components over the last 270 000 years in sediments from odp hole 1084a drilled in a high productivity upwelling region within the benguela upwelling system off the west coast of africa age control is provided by stable oxygen isotope measurements of benthic foraminifera prior research has established that late pleistocene glacial intervals in this upwelling system generally had higher productivity than interglacials the radiolarian wade water depth ecology paleoproductivity index correlates well with toc and opal in these samples and all three parameters change in synchrony with the benthic isotope curve over all but the mis 5e 6 time interval wade inferred productivity is significantly higher in glacials than interglacials we conclude that the wade index is a useful proxy for paleoproductivity at this location as are also opal and organic carbon accumulation rates carbonate and carbonate based indices such as the accumulation rate of benthic foraminifera bfar by contrast do not correlate well either to productivity indices or to the glacial interglacial cycle and are interpreted to primarily reflect carbonate dissolution

underestimation of the authigenic fraction of cu and ni in organic rich sediments abstract there are two possible techniques to estimate the degree of authigenic or non lithogenic metal enrichment in sediments first the authigenic metal fraction can be calculated according to m auth m total m al background al total while m is the metal of interest and al is aluminum second a metal enrichment factor ef is calculated according to ef m al total m al background both techniques rely on the appropriate use of the m al ratio of the lithogenic background for the latter the m al ratio of average shale or crust is widely applied as lithogenic background in marine geochemistry however as we show here the lithogenic background based on average shale or crust is too high for cu and ni as a consequence the authigenic fraction of cu and ni in sediments and suspended particulate matter spm is significantly underestimated as shown for examples from various environments upwelling areas of peru chile and the gulf of california the norwegian kyllaren fjord and the german wadden sea by correlating cu al and ni al with toc in the samples the apparent m al ratio of the background can be estimated a prerequisite for this technique is high linearity and high quality of correlation r 2 0 8 otherwise the lithogenic background cannot be exactly determined the resulting higher authigenic level of cu and ni may promote their use as productivity indicators while the status of cu and ni may shift from moderately to highly enriched when the enrichment is assessed via efs

analysis of marine environmental conditions based onmolybdenum uranium covariation applications to mesozoic paleoceanography article i nfo article history accepted 13 september 2011 available online 17 september 2011 patterns of uranium molybdenum covariation in marine sediments have the potential to provide insights re garding depositional conditions and processes in paleoceanographic systems specifically such patterns can be used to assess bottom water redox conditions the operation of metal oxyhydroxide particulate shuttles in the water column and the degree of water mass restriction the utility of this paleoenvironmental proxy is due to the differential geochemical behavior of u and mo 1 uptake of authigenic u by marine sediments begins at the fe ii fe iii redox boundary i e suboxic conditions whereas authigenic mo enrichment re quires the presence of h2s i e euxinic conditions and 2 transfer of aqueous mo to the sediment may be enhanced through particulate shuttles whereas aqueous u is unaffected by this process in the present study we examine u mo covariation in organic rich sediments deposited mostly in the western tethyan region during oceanic anoxic events oaes of early jurassic to late cretaceous age our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard ing water mass restriction and the operation of particulate shuttles in depositional systems these insights will help to address contentious issues pertaining to the character and origin of mesozoic oaes such as the degree to which regional paleoceanographic factors controlled the development of the oaes

effects of nanoporosity and surface imperfections on solid bitumen reflectance bro measurements in unconventional reservoirs abstract bitumen reflectance bro is an alternative maturity indicator to vitrinite reflectance vro in lower paleozoic and marine rocks where few higher plant macerals can be found in unconventional petroleum rocks locally exuded and or migrated oil are retained in the tight rocks and thermally and diagenetically consolidated into solid bitumen and pyrobitumen finding suitable bitumen macerals for bro measurements pose a challenge in indicating the proper thermal maturity this study reports the effects of surface imperfections caused by nanoporosity surface granularities and impurities on measuring bro in core samples from the jurassic nordegg shale shale oil and the triassic montney siltstone tight gas in the western canada sedimentary basin the degree that bitumen s surface imperfections influence any given reflectance measurement was controlled by changing the probe size area the larger the measuring probe the higher the degree of surface imperfections is included in bro readings the effect is quantified in this paper showing up to 27 diminishment of mean bro values when the probe size increased from 0 31 to 5 85 m2 the mode in which bitumen accumulates within the rock has a strong influence on the degree of surface imperfections bitumen which is disseminated within clay matrix introduces a high degree of nanoporosity and mineral interference and hence will result in lowest bro in contrast inclusions of high reflecting micrinite or diagenetic minerals such as pyrite result in higher bro these effects can vary at the microscopic scale leading to multiple populations of reflectance readings and difficult interpretations of thermal maturity

magnetic anomaly modeling at the indo eurasian collision zone abstract one of the consistent features of satellite crustal magnetic anomaly maps is the large negative anomaly in the indo eurasian collision zone this region is modeled by the introduction of magnetic blocks representing the tibetan asian and himalayan mountain regions the crustal thickness of these blocks is based on the principle of isostasy it has been shown that the removal of the core field from the magsat data removes most of the long wavelength anomalies associated with the ocean continent susceptibility contrast and that some of the observed anomalies after processing of the satellite data are a manifestation of this contrast to account for this the final model is a superposition of the above block model on a global model of typical continent and ocean susceptibility called a standard earth magnetization model semm a combination of the three blocks superposed on the semm reproduces the major features of the observed magsat anomalies block model susceptibilities of the asian and himalayan regions are the same as for the continental portion of the semm albeit with a greater crustal thickness thus the crust in these regions is typical by contrast the satellite magnetic data require an integrated susceptibility in the tibetan region which is considerably lower than typical two possible explanations are suggested one that the origin and tectonic history of the tibetan region differs from the neighboring regions in such a way as to result in lower susceptibility this is consistent with current thinking that the tibetan plateau is made up of several fragments or micro continents which were attached to asia prior to the collision of india with asia the other possible explanation is that the curie isotherm in the tibetan region is elevated compared to the asian and himalayan regions

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

facies patterns and geography of the yangtze region south china through the ordovician and silurian transition abstract lithofacies and biofacies changes through the ordovician and silurian transition on the yangtze platform are coincident with stepwise mass extinctions recovery events global sea level changes and regional palaeogeographic configurations facies patterns through three intervals the late mid ashgill hirnantian and early rhuddanian indicate that black shale occupied most of the yangtze platform region during the late mid ashgill and early rhuddanian while the hirnantian possesses more diverse facies types the black shale was replaced by carbonate facies during the hirnantian coincident with a global sea level low stand biofacies changes from the late mid ashgill graptolitic to the hirnantian shelly facies were apparently signalled by gradual and diachronous existence of a mixed facies the manosia facies in the yangtze region however the biofacies change was abrupt from the hirnantian to the early rhuddanian when graptolitic facies occupied most areas on the yangtze platform leaving only a very narrow limited belt of shelly facies along the southern margin of the platform in northern guizhou the early rhuddanian unified biofacies pattern is a result of a global sea level rise which may be synchronized with a recovery bioevent of graptolites after hirnantian mass extinction

contourite drift types and their distribution in the north and south atlantic ocean basins abstract patterns of sediment accumulation under the control of deep bottom currents depend on interacting factors intensity of the regional deep circulation morphology of the sea floor coriolis effects abundance of terrigenous and biogenic supplies subsequent turbidity of deep nepheloid layers and depth of the ccd based on many examples of bottom current deposits from the atlantic and from other oceans we recognise three major types of contourite accumulation 1 giant elongate drifts 2 contourite sheets and 3 channel related drifts they generally occur in distinct topographic settings and show different geometries facies and rates of deposition in the north atlantic strong bottom circulation an open basin system and high sediment supply have led to the construction of large elongate contourite drifts which have striking relief 200 1000 m above the surrounding sea floor and are composed of muddy contourites rich in calcareous pelagic material in the south atlantic strong bottom currents are often trapped in gyre like circulation patterns around topographically closed basins or are accelerated through deep narrow connecting passages the resulting deposits are mainly extensive contourite sheets covered by fields of large sediment waves and channel related drifts including contourite fans constructed at the downstream exit of deep passages

paleo environmental cyclicity in the early silurian yangtze sea south china tectonic or glacio eustatic control abstract the yangtze sea of the south china craton experienced strong environmental perturbations during the first 5 6 myr of the silurian the lower silurian lungmachi formation in the jiaoye 1 drillcore chongqing china records three sea level cycles stratigraphic sequences characterized by rapid deepening at the base of each cycle followed by slow shallowing each deepening event was associated with shifts toward higher marine productivity more intensely reducing bottomwater conditions reduced watermass restriction lower detrital input and enhanced siliciclastic weathering intensity we infer control by glacio eustasy in which deglacial sea level rises and concurrent climatic warming triggered the observed environmental changes shallowing was associated with renewed glaciation and characterized by the opposite set of conditions the results of this study thus provide evidence in support of continued continental glaciation during the early silurian i e following the termination of the major end ordovician hirnantian glaciation the study core also exhibits a long term shallowing trend through the entire lower silurian the origin of which may have been tectono epeirogenic uplift of the south china craton during the kwangsian orogeny

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

k bentonites in ordovician silurian transition from south china implications for tectonic evolution in the northern margin of gondwana numerous k bentonites from the ordovician silurian o s transition in south china record important information about the geodynamics of volcanic activity in the northern margin of gondwana a series of k bentonite beds have been identified in the zhoujiaxi group early silurian in foreland basins from central hunan province china they are dominantly composed of illite with minor kaolinite volcanogenic minerals include quartz feldspar biotite and lesser apatite and zircon u pb zircon ages from three beds 442 8 1 8 442 2 1 9 and 441 6 2 0 ma are compatible with the age of the o s global boundary geochemical results indicate calc alkaline felsic magmas derived from continental crust and erupted in a subduction related collisional environment isopach schemes and grain size reveal that the volcanic ashes were sourced from a volcanic event likely with a volcanic explosivity index of 8 and transported 300 1000 km away from the palaeo sse the volcanism was associated with northwestern subduction of the zhenghe dapu ocean beneath the southeastern south china block scb on the northern margin of gondwana the zhenghe dapu fault might be a suture zone implying the consumption of the zhenghe dapu ocean during collision between the nanhai terrane and the scb this study supports that the wuyi yunkai orogenic belt is a collision type orogen rather than an intraplate belt supplementary material plots and geochemical data are available at https doi org 10 6084 m9 figshare c 5027096

fifth generation lithospheric magnetic field model from champ satellite measurements 1 six years of low orbit champ satellite magnetic measurements have provided an exceptionally high quality data resource for lithospheric magnetic field modeling and interpretation here we describe the fifth generation satellite only magnetic field model mf5 the model extends to spherical harmonic degree 100 as a result of careful data selection extensive corrections filtering and line leveling the model has low noise levels even if evaluated at the earth s surface the model is particularly suited for inferring large scale structure and composition of the lithosphere it is also meant to serve as the long wavelength part of continental and global scale marine and aeromagnetic anomaly maps

weakly magnetic crust in the canadian cordillera abstract current models of continental crust favor an increase in magnetization with depth here we report a counter example from the canadian cordillera where almost a full thickness of non magnetic continental crust is suggested by joint interpretation of magnetic and seismic data the magnetic field over the cordillera is characterized by complex short wavelength 100 km portion of the cordilleran field is subdued and mainly featureless and suggests a lack of magnetic sources at greater depths seismic reflection and refraction data from three major transects in the yukon and british columbia canada support this interpretation and indicate that sedimentary like formations make up the majority of the crust the dominance of shallow upper crustal magnetization in the canadian cordillera contrasts with the generally held view that the lower continental crust is the primary source for long wavelength magnetic anomalies sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology the canadian cordillera appears to be an example of a non magnetic lower crust overlain by a more magnetic upper crust that is however not magnetized strongly enough to produce significant long wavelength magnetic anomalies

bottom current controlled sedimentation a synthesis of the contourite problem abstract an overview of the main items concerning deep bottom current controlled deposits is presented these include the definition of contourites their processes of deposition and facies characteristics contourite drift types and seismic patterns the interplay of turbidity and bottom current processes and the correlation between bottom current processes and global changes in climate sea level from this discussion several of the more significant problems are highlighted which we believe should be priority targets for future research 1 identification of contourites especially in ancient series where they are interbedded with turbidites and other facies 2 refinement of the numerous parameters used as tracers of palaeocirculation patterns 3 understanding the interaction between the different types of bottom currents and also between bottom current and other deep water processes 4 distinction in the sedimentary record between the effects of short time scale bottom current variations due to regional causes like benthic storms and geological scale current fluctuations related to climatic or astronomic control 5 understanding the control of glacial interglacial climatic cycles on the nature and rate of contourite deposition

organic matter accumulation of the wufeng longmaxi shales in southern sichuan basin evidence and insight from volcanism abstract the wufeng longmaxi formations wf lm fms in the sichuan basin are sets of graptolite bearing shale strata containing high frequency bentonite layers in order to investigate the influence of volcanism on the organic matter om accumulation in shales detailed observations of several typical profiles and wells were performed also analyses including total organic carbon toc contents and mineral compositions were conducted on 126 shale and 5 bentonite core samples major and trace element analyses were conducted on the selected 25 shale and 5 bentonite samples in the wf lm fms of well lu a results show that wf and lm1 4 members are the bentonites concentrated sections while lm5 6 are the bentonites relatively concentrated sections and no bentonite is found in lm7 9 members the high potassium k content of bentonites have the typical characteristics of intermediate acid calcium alkaline volcanic rocks such as andesite and dacite which probably originated from intraplate collision or volcanic arc structural backgrounds related to the collision belt of the cathaysia block to the yangtze block productivity indicators p content excess si p al cu al and ni al in the wf and lm1 3 members are all significantly higher than those in the lingxiang lx and lm4 5 members the higher biological productivity during the ordovician silurian o s transition was attributed to the massive and intensive volcanism the fertilization by volcanic ash brought abundant biologically relevant nutrient elements n p si fe and al into the ocean thus triggering the phytoplankton bloom and enhancing primary biological productivity all the redox condition indexes moef uef nief vef mo toc corg p and u th positively correlate with toc content during the o s transition the intensive and massive volcanic eruptions triggered global cooling sea level changes late ordovician mass extinction and primary productivity increases thus promoting anoxic and possibly intermittently euxinic bottom conditions in addition the deposited bentonite layers had a strong capacity for oxygen isolation volcanism should be considered when modeling the om accumulation of the wf lm shales

a sulfidic driver for the end ordovician mass extinction the end ordovician extinction consisted of two discrete pulses both linked in various ways to glaciation at the south pole the first phase starting just below the normalograptus extraordinarius zone particularly affected nektonic and planktonic species while the second pulse associated with the normalograptus persculptus zone was less selective glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end ordovician extinction but a general consensus is lacking we have used geochemical redox indicators such as iron speciation molybdenum concentrations pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key hirnantian sections these indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end ordovician extinction and that these conditions expanded towards the second pulse even though the n extraordinarius zone appears to have been a time of oxygenated deposition pyrite is significantly enriched in 34s in our sections as well as in sections reported from south china this suggests a widespread reduction in marine sulfate concentrations which we attribute to an increase in pyrite burial during the early hirnantian the s isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well we argue that euxinic conditions prevailed and intensified in the early hirnantian oceans and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites in the n persculptus zone an interval associated with a major sea level rise our redox indicators suggests that euxinic conditions and ferruginous in some places encroached onto the continental shelves in our model the expansion of euxinic conditions during the n extraordinarius zone was generated by a reorganization of nutrient cycling during sea level fall and we argue overall that these dynamics in ocean chemistry played an important role for the end ordovician mass extinction during the first pulse of the extinction euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep water benthic and nektonic organisms during the second pulse the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats

sequence stratigraphy and depositional environments of the shamokin union springs member marcellus formation and associated strata in the middle appalachian basin organic carbon rich shales of the lower marcellus formation were deposited at the toe and basinward of a prograding clinothem associated with a mahantango formation delta complex centered near harrisburg pennsylvania distribution of these organic carbon rich shales was influenced by shifts in the delta complex driven by changes in rates of accommodation creation and by a topographically high carbonate bank that formed along the findlay algonquin arch during deposition of the onondaga formation specifically we interpret the union springs member shamokin member of the marcellus formation and the onondaga formation as comprising a single third order depositional sequence the onondaga formation was deposited in the lowstand to transgressive systems tract and the union springs member was deposited in the transgressive highstand and falling stage systems tract the regional extent of parasequences systems tracts and the interpreted depositional sequence suggest that base level fluctuations were primarily caused by allogenic forcing eustasy climate or regional thermal uplift or subsidence instead of basement fault reactivation as argued by previous workers paleowater depths in the region of marcellus formation black mudrock accumulation were at least 330 ft 100 m as estimated by differences in strata thickness between the northwestern carbonate bank and basinal facies to the southeast geochemical analysis indicates anoxic to euxinic bottom water conditions these conditions were supported by a deep stratified basin with a lack of circulation

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

deep electrical conductivity structure beneath the southern part of the indo gangetic plains analysis of magnetotelluric data near the southern boundary of the indo gangetic basin reveals a clear two dimensional structure two dimensional numerical modelling of the data has been carried out the e polarization response is found to be very similar to the local one dimensional response a study has been carried out to remove the screening effect on the magnetotelluric data caused by the top layer sediments after the removal of the top layer sediments a best fit model of the averaged magnetotelluric field data has been found which has a very thick resistive upper crust and a conducting lower crust the conductance of the lower crust is found to be much less than the values of lower crustal conductance obtained in eastern siberia in contrast the upper mantle below the indo gangetic plains has a resistivity significantly greater than that of the upper mantle below eastern siberia

quantitative stratigraphy of the wufeng and lungmachi black shales and graptolite evolution during and after the late ordovician mass extinction article i nfo graptolite bearing black shales are widely distributed on the yangtze platform in south china based on 19 sec tions systematically studied recently we conducted a quantitative stratigraphic analysis of the graptolite biodi versity change during and after the late ordovician mass extinction the combination of graphic correlation and rarefaction methods make it possible to evaluate the effect of sampling intensity on biodiversity estimates graptolite diversity reached its peak in the dicellograptus complexus chron and then a gradual decline to the p pacificus chron it was followed by a two step mass extinction the first and major phase occurred in the metabolograptus extraordinarius chron and the second and minor phase in the latest m persculptus chron the second phase of the extinction was mostly represented by the elimination of the diplograptina survivors from the major extinction event but did not show significant decline in standardized diversity curve thereafter a moderate increase in richness appeared in the early and mid parakidograptus acuminatus chron which was followd by a considerable decrease of graptolite richness in the late p acuminatus chron and early c vesiculosus chron the present study also indicates slow and uneven sedimentation rates in the studied sections from mid katian to early rhuddanian including a relatively slower sedimentation rate during the hirnantian epoch the latter andthehiatusatthe honghuayuanandguanwusections probablyreflecttheeffectsofthe hirnantian glaciation and regional tectonic movements

factors controlling organic matter accumulation in the upper ordovician lower silurian organic rich shale on the northeast margin of the upper yangtze platform evidence from petrographic and geochemical proxies abstract organic rich shales are widespread in the upper ordovician lower silurian wufeng longmaxi formations on the northeastern margin of the upper yangtze platform and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in china however organic matter accumulation in these shales needs further research especially in the areas which experienced an extensive upwelling and volcanism here this work integrates detailed petrographic and geochemical proxies namely redox sensitive trace elements rstes and ratios of si al ti al from two ordovician silurian sections on the northeast margin of upper yangtze platform in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the ordovician silurian transition data of si al and ti al ratios indicate that the deposition of black gray siliceous mudstones in the lower wufeng formation commenced with a transgression triggered by the boda warming event which led to a reduced detrital input based on rstes the black carbonaceous shales of the upper wufeng formation are organic rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation abundant nutrients supplied by the post glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column which lead to high concentrations of organic matter in lower longmaxi formation although the water column conditions were not conducive to the accumulation of organic matter it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle longmaxi formation the rstes especially cd mo co mn reveal that the organic matter accumulation of the wufeng longmaxi formation during ordovician silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input

high resolution geochemistry and sequence stratigraphy of the hushpuckney shale swope formation eastern kansas implications for climato environmental dynamics of the late pennsylvanian midcontinent seaway abstract the hushpuckney shale member of the swope formation missourian stage eastern kansas is the core shale of a kansas type cyclothem formed during the late transgressive to early regressive phases of a late pennsylvanian glacio eustatic cycle coeval high frequency climato environmental dynamics of the midcontinent seaway are preserved in the kgs orville edmonds no 1a study core as centimeter scale variation in major components organic carbon authigenic sulfides and phosphate detrital siliciclastics organic macerals trace element redox proxies mo u v zn and ichnofabric features benthic o 2 levels declined sharply from the base of the black shale 0 cm went sulfidic at 4 cm and reached a redox minimum at 21 cm above this level redox potential gradually rose fluctuating between sulfidic and nonsulfidic conditions from 35 cm to the black shale gray shale contact at 52 cm onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft bodied organisms comprised of deep tiered tracemakers tolerant of low o 2 conditions trichichnus and shallow tiered tracemakers favoring soupground helminthopsis or firmground substrates the zoophycos phycosiphon schaubcylindrichnus planolites association most geochemical records exhibit a low order cycle spanning the full 52 cm thickness of the black shale submember reflecting a dominant glacio eustatic control the base and top of the black shale submember record lateral migration of the pycnocline across the midcontinent shelf during the transgressive and regressive phases respectively the maximum flooding surface mfs is at 17 23 cm an interval containing the euxinic peak and characterized by high concentrations of illite representing a cratonic siliciclastic flux and terrestrial organic macerals the product of transient increases in humidity weathering rates and the export of coal swamp vegetation associated with the interglacial highstand of the swope cyclothem although a transgressive surface of maximum starvation sms cannot be confidently identified a phosphate rich regressive condensation surface at 28 34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth a correlative shift in dominance from terrestrial to marine derived organic macerals reflects increased upwelling of nutrient rich deepwaters and enhanced primary productivity penecontemporaneous paleogeographic and climatic factors e g semirestricted circulation monsoonal precipitation predisposed the midcontinent seaway toward sensitivity to high frequency climato environmental fluctuations which are preserved as 2 to 7 cm thick cycles in the study core the 52 cm thick black shale submember contains 12 such high order cycles which have an estimated average duration of 2 to 9 kyr implying a sub milankovitch band i e millennial scale climatic control the results of this study are relevant to the sequence stratigraphic interpretation of kansas type cyclothems the dynamics of gondwanan ice age glacio eustatic cycles and the development of boundary conditions for late pennsylvanian paleoclimate models

factors influencing shale gas accumulation in the lower silurian longmaxi formation between the north and south jiaoshiba area southeast sichuan basin china abstract owing to successful shale gas exploration and development wells were drilled into the longmaxi shale formation in the jiaoshiba area southeast sichuan basin china however shale gas production in longmaxi varies greatly between the north and south jiaoshiba areas to determine the factors controlling the shale gas accumulation mechanism the total organic carbon toc content mineralogy porosity permeability total gas content gas preservation conditions and tectonic activities of longmaxi shale in both the north and south jiaoshiba areas were studied in detail the results show that the concentration of organic matter and minerals in the longmaxi shale differed slightly between the south and north jiaoshiba areas and the lower longmaxi shale long 1 member which is used in commercial shale gas production is characterized by a high quartz content and low clay content in addition the longmaxi shale in the north jiaoshiba area is characterized by high porosity and low permeability while that in the south jiaoshiba area has low porosity and high permeability two different reservoir types were identified in the jiaoshiba area the major storage spaces are pores in the longmaxi shale reservoirs located in the north jiaoshiba area while the storage capacity of shale reservoirs in the south jiaoshiba area is predominantly provided by fractures shale gas mainly exists as free gas in the northern jiaoshiba area due to the good gas preservation conditions however owing to the development of abundant faults in the south jiaoshiba area a large amount of free gas was lost causing absorbed gas to dominate the shale reservoir in this area

stability of bubbles in a linear elastic medium implications for bubble growth in marine sediments 1 methane bubbles in muddy fine grained sediments grow initially through a process of elastic expansion punctuated by discrete fracture events lefm growth the ability of the surrounding sediments to support a stress and actively resist expansion can under conditions of low gas production or high sediment toughness result in the cessation of growth and the presence of stable bubbles thus it is possible for a bubble to stop growing despite the presence of a source in the sediments that continues to produce gas this contrasts with growth of bubbles in a fluid medium which cannot support a stress and so will continue to grow as long as a surrounding source provides gas this no growth condition is the result of the coupling between gas supply methane production or supersaturation and the sediment mechanics here we quantify this condition and present a criterion for the switch between no growth and the lefm growth regimes we apply this theory to the sediments of eckernforde bay in the kiel bight germany and despite the absence of measurements for the key sediment mechanical properties we can provide a qualitative explanation for the sizes and shapes of the observed bubble population with depth in the sediment we also show how the release of hydrostatic pressure can stimulate growth by pushing otherwise stable bubbles into the lefm growth regime this could provide a mechanism for the release of bubbles during periods of low water such as during low tide or wave events

tibetan satellite magnetic low evidence for widespread melt in the tibetan crust the tibetan plateau is associated with a pronounced satellite magnetic low forward modeling of this anomaly demonstrates that the crust underlying the tibetan plateau is weakly magnetic compared to the crust on either side and that the boundaries of the weakly magnetic region coincide closely with the topographic edges of the plateau himalaya kun lun because there are no obvious changes in the bulk magnetization of the crust associated with the principal terrane boundaries within the plateau banggong and yarlung zangbo sutures the low likely manifests hot crust beneath tibet forward modeling of the magnetic anomaly with simple assumptions suggests that the curie isotherm is likely to reside in the upper crust across the tibetan plateau 15 km depth implying that granite minimum melt temperature 600 650 c is also achieved in the upper crust across the plateau 16 18 km depth this inference is consistent with the shallow depth extent of earthquakes in tibet and with the recent suggestion that melt may be areally widespread within the tibetan crust