final amalgamation of the tianshan and junggar orogenic collage in the southwestern central asian orogenic belt constraints on the closure of the paleo asian ocean abstract the tianshan and junggar orogenic collage occupies the southwestern part of the central asian orogenic belt and was assembled by collision accretion of several continental blocks and island arcs during late paleozoic early mesozoic time following the consumption of the junggar and south tianshan oceanic basins in the western segment of the paleo asian ocean considerable and continuing controversy has surrounded when where and how this orogenic collage was eventually amalgamated which plays a crucial role in understanding the formation of the central asian orogenic belt this review synthesizes the most recent data and geologic evidence that place critical constraints on the closure history of the oceanic domains a comprehensive data compilation indicates that the formation of ophiolites along major sutures in the tianshan and junggar region lasted to the early carboniferous for the kalamaili suture zone to the mid carboniferous for the west junggar region and the north tianshan suture zone and to the late carboniferous for the south tianshan suture zone ca 325 310 ma ultra high pressure metamorphism along the western part of the south tianshan suture occurred nearly synchronously with the collision between the tarim craton and the central tianshan yili block the oceanic closure and collision events were sequentially followed by widespread and intensive a type granitic and bimodal magmatism which commenced in the mid carboniferous along the kalamaili belt and in the latest carboniferous to earliest permian in other regions contemporaneously large scale strike slip shearing took place along major faults subparallel to main sutures with a dominant dextral sense and deformation time since the carboniferous permian transition available data and geological evidence support an eastward propagating scissor like closure of the western segment of the paleo asian ocean along the south tianshan and north tianshan suture zones over a period from the mid to end carboniferous with the south tianshan belt representing the final suturing site in the early permian the tarim mantle plume likely merely affected the southwestern part of the assembled tarim and tianshan region as indicated by regional discrepancy in type and geochemistry of early permian a type granitoids the amalgamation of the tianshan and junggar orogenic collage was associated with the tectonic bending of the kazakhstan orocline leaving a piece of trapped oceanic crust in its middle part beneath the present junggar basin

deep hydrocarbons in the northwestern junggar basin nw china geochemistry origin and implications for the oil vs gas generation potential of post mature saline lacustrine source rocks abstract deep 4500 m sedimentary units in petroliferous basins are a frontier field for petroleum exploration and exploitation worldwide this study focuses on the northwestern junggar basin of nw china and presents the results of a comprehensive geochemical analysis of the source rocks and hydrocarbons hosted by deep reservoirs in this region including data obtained from rock eval pyrolysis carbon isotope biomarker and light hydrocarbon analyses these data provide insights into the exploration potential of deep hydrocarbons in the study area and in other areas with similar geologic settings i e saline lacustrine petroleum systems results indicate that the deep hydrocarbons in the northwestern junggar basin are dominated by oil with both the oil and natural gas being derived mainly from the lower permian fengcheng formation this formation was deposited in a heterogeneous saline lacustrine environment which led to the generation of geochemically diverse oil compositions consequently fengcheng derived oils can be divided into subcategories based on source rock lithology i e hypersaline alkaline muddy dolomites saline dolomitic mudstones and brackish mudstones a small number of deep oils in the study area were derived from the carboniferous lower permian jiamuhe formation the deep hydrocarbons in the study area generally accumulated in near source environments indicating that prospective areas are present in the form of fengcheng derived oils in the depocenter of the mahu sag especially oils derived from hypersaline alkaline muddy dolomites and saline dolomitic mudstones these source rocks are likely to have generated oil even under the highly mature conditions associated with deep burial this might explain why discoveries within saline lacustrine basins are generally dominated by oil implying that these conditions might also exist within the deeper parts of saline lacustrine basins worldwide

petroleum migration and mixing in the northwestern junggar basin nw china constraints from oil bearing fluid inclusion analyses abstract abundant oil bearing fluid inclusions are present in four reservoir sandstone samples from the slope and fault zone areas in the northwestern margin of the junggar basin nw china free oil in intergranular pores adsorbed oil on grain surfaces and petroleum in inclusions hosted by mineral grains of these samples were analyzed by gas chromatography and gas chromatography mass spectrometry analytical results indicate similar biomarker distributions in the three bitumen fractions of the samples collected from the slope area which correlate well with characteristic of the source rocks in the lower permian fengcheng formation this is in contrast to the fault zone where the three bitumen fractions vary significantly in their geochemical signatures suggesting that all of the three permian source sequences rocks including the lower permian jiamuhe and fengcheng formations and the middle permian lower wuerhe formation have been mature and contributed to the oils in the fault zone the presence of an unresolved hydrocarbon hump and a full range of unaltered n alkanes in the gas chromatograms of samples collected from the fault zone indicate an earlier hydrocarbon biodegradation and subsequent injection of fresh oils distinctly different from that in the slope area petrographic and microthermometric investigations of oil bearing fluid inclusions suggest a single oil charge in the slope area to the lower triassic reservoir occurring during the late triassic to early jurassic time in the fault zone oil migration and mixing took place mainly during the late triassic to early jurassic and cretaceous followed by gas migration in the tertiary period the different petroleum migration and mixing histories in the slope and fault zones are attributed to the effect of fault controlled oil and gas migration

remote sensing and petroleum seepage a review and case study when reservoirs leak varying quantities of oil and gas migrate to the surface as macroseeps which are visible and microseeps which are invisible this review article describes the mechanisms of seepage and the resulting surface manifestations in relation to optical high resolution remote sensing data oil pools and tar deposits macroseeps often can be detected directly by remote sensing microseeps are more difficult to study using remote sensing but they give rise to vegetation stress and cause geochemical alterations in soil and rocks which can be studied indirectly using hyperspectral sensors an integrated methodology is presented to combine various geoscience and remote sensing datasets for seepage detection a combination of red edge modelling algorithms and spectral matching tools is identified that provides a validated technique for onshore microseepage detection these remote sensing tools are not only important for petroleum exploration but also have environmental implications because seeps emit greenhouse gases a statistical data integration approach was developed based on bayesian assumptions which can be used to integrate hyperspectral remote sensing data other satellite remote sensing data and ancillary field geological and geochemical datasets for modelling microseepage in a case study from the ventura basin santa barbara in southern california probe 1 data from the 1998 geosat group shoot are integrated with field and subsurface geological and geochemical data to predict possible sites of hydrocarbon microseepage

the origin of natural gas and the hydrocarbon charging history of the yulin gas field in the ordos basin china abstract the genetic type source and charging history of natural gas in the yulin gas field in the ordos basin have been studied by combining the carbon isotopic composition of natural gas and geochemical characteristics of light hydrocarbons with carbon isotope fractionation model results and fluid inclusion analysis the carbon isotopic composition of methane and ethane in the yulin gas field is relatively enriched in 13c with 13c1 values ranging from 35 3 to 29 8 average value 32 4 and 13c2 ranging from 26 3 to 23 5 average value 24 8 the c7 light hydrocarbons are predominated by methylcyclohexane accounting for 65 8 to 80 9 average value 71 6 which is characteristic of coal derived gas furthermore the gas geochemistry indicates that although marine limestone source rocks are present in the region the contribution of oil associated gas from this source to the yulin gas field is quite low based on the empirical relationship between 13c1 and ro of gas source rocks and the kinetic isotope fractionation predictions for gases generated in the yulin gas field the observed 13c of methane is heavier than that of natural gas only originated from in situ coal measures this result shows that gas in the yulin gas field is contributed not only by the natural gas generated from local coal measure source rocks but also from the higher maturity natural gas sourced from the coal measure source rocks to the south or southwest of this gas field fluid inclusion analysis proves that the accumulation of natural gas in the yulin gas field has the characteristics of continuous charge

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

prediction of natural gas composition in ultradeep sandstone reservoirs petroleum generation is kinetically controlled and depends on time and temperature but is essentially independent of rock type ultimately it leads to a thermodynamically stable assemblage with a composition that is controlled by temperature pressure and local mineralogy we used a free energy minimization computer program to calculate the composition of gases in thermodynamic equilibrium with various mineral assemblages in clastic reservoirs from 10 000 to 40 000 ft 3048 12 192 m in clean sand reservoirs methane concentration falls with increasing depth and is eliminated by 25 000 ft 7620 m if a graphitic residue is present additional methane is generated and methane is still present at 40 000 ft 12 192 m it appears that gas composition is influenced by the balance between the reaction of graphite with water to give methane and the oxidation of methane by water to form carbon dioxide and hydrogen in systems with iron minerals hematite decreases and magnetite increases as methane is oxidized high temperatures that accompany increasing depth of burial lead to the thermal decomposition of calcite cements and the carbon dioxide generated dilutes any methane to give uneconomic gas compositions hydrogen sulfide is generated when the reservoir mineral assemblage contains sulfur bearing minerals such as pyrite and anhydrite 12more figs 6 tabs less

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

carbon and hydrogen stable isotope variations in kerogen during laboratory simulated thermal maturation carbon and hydrogen stable isotope measurements on sapropelic and humic kerogen isolated from laboratory heated sediments reveal several isotopic trends an initial depletion of 13c and d 2h or deuterium in kerogen isolated from the heated sapropelic sediment appears to be the result of isotopic heterogeneity interaction with soluble organic matter and or volatilization of a heat labile fraction of the protokerogen during the main isotopic trend the sapropelic kerogen becomes enriched in 13c and d by about 5 and 50 ppt parts per thousand respectively theoretical calculations based on a rayleigh fractionation process suggest that this trend is largely controlled by the generation of isotopically light methane directly from the ker gen this argument also applies to the main hydrogen isotopic trend for the humic kerogen which becomes enriched in d by about 90 ppt the main 13c trend for the humic kerogen however results in a maximum enrichment in 13c of only about 1 6 ppt and appears to be largely controlled by factors other than methane a final depletion trend in 13c and d occurs for the sapropelic kerogen at very high temperatures this may be due to a reduced isotopic fractionation at higher temperatures or to the indirect generation of methane from kerogen through a liquid intermediate which is subsequently coked to pyrobitumen

mixed marine and lacustrine input to an oil cemented sandstone breccia from brora scotland abstract a rounded cobble collected from a beach near brora scotland contains angular brecciated sandstone clasts cemented together by crude oil no oil occurs within the tight dolomite cemented clasts except along fractures provenance of the cobble is uncertain however the lithology and shape of the clasts suggests that they are fault gouge breccia composed of old red sandstone from the nearby helmsdale fault oil emplacement presumably along the fault cemented the breccia prior to transport and weathering of the cobble independent source and biodegradation parameters support mixing of two charges of oil like oil from the nearby beatrice field the oil cement contains 24 n propylcholestanes and carotane that indicate marine and lacustrine source rocks respectively the oil cement also contains pristane phytane residual n alkanes and a complete series of 25 norhopanes which indicate mild and heavy biodegradation respectively the first charge consisted of lacustrine devonian oil that was heavily biodegraded at shallow depth the origin of the second charge is unclear unlike beatrice oil the oil cement contains oleanane and shows a higher 24 24 27 nordiacholestane ratio suggesting input from a cretaceous or younger paralic marine shale source rock although cretaceous and paleocene source rocks are inferred to exist in various depocenters in the northeast atlantic margin subcrops of these rocks offshore brora are thermally immature alternately the second charge could originate from an effective middle jurassic source rock that contains oleanane this hypothesis is supported by mass spectrometric confirmation of oleanane in an extract from the marine equivalent of the middle jurassic brora coal in the nearby ukcs 11 30 2 well oleanane in crude oil is not positive proof of a cretaceous or younger source rock for the oil

evolution of caledonian deformation fabrics under eclogite and amphibolite facies at vardalsneset western gneiss region norway the vardalsneset eclogite situated in the western gneiss region sw norway is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation the eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite actinolite clinozoisite kyanite quartz paragonite phengite and rutile the rocks record a five stage evolution connected to caledonian burial and subsequent exhumation 1 a prograde evolution through amphibolite facies t 490 63 c is inferred from garnet cores with amphibole inclusions and bell shaped mn profile 2 formation of l s tectonite eclogite t 680 20 c p 16 2 kbar related to the subduction of conti nental crust during the caledonian orogeny lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the l s fabric was coaxial 3 formation of sub horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90 disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3 locally occurring mylonitic eclogites t 690 20 c p 15 1 5 kbar with top w kinematics may indicate however that non coaxial deformation was also active at eclogite facies conditions 4 development of a widespread regional amphibolite facies foliation t 564 44 c p 10 3 8 1 kbar quartz veins and development of conju gate shear zones indicate that coaxial vertical shortening and sub horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions 5 amphibolite facies mylonites mainly formed under non coaxial top w movement are related to large scale movement on the extensional detachments active during the late orogenic extension of the caledonides the structural and metamorphic evolution of the vardalsneset eclogite and related areas support the exhumation model including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust

peculiarities of salt lake sediments as potential source rocks in china abstract hypersaline lake sediments are widely distributed in china and can be good sources for petroleum the palaeogene sediments of the jianghan basin in eastern china are a typical example composed of salt lake evaporite clastic facies in fault subsided basins their organic matter is variable but is dominated by type ii kerogen the biological marker characteristics of core extracts and oils from the jianghan basin include i low oep values for n alkanes and occasionally an even odd predominance ii a high concentration of phytane iii a relatively high gammacerane 17 h 21 h hopane ratio and iv abundant mainly nickel porphyrins sample guang 33 k 33 is typical of the immature oils found in the jianghan basin geological and geochemical data both indicate that it was generated from source rocks in the maturity range r 0 0 45 0 55 its characteristics are similar to those of immature source rocks with high concentrations of phytane steranes triterpanes and nickel porphyrins and for an oil extremely low values for molecular parameters of maturity such as c 29 20s 20s 20r 14 h 17 h steranes c 32 22s 22s 22r 17 h 21 h hopanes and dpep dpep etio porphyrins

organic geochemistry of freshwater and alkaline lacustrine sediments in the green river formation of the washakie basin wyoming u s a abstract two members of the green river formation in the washakie basin have been analysed by organic geochemical and organic petrographic techniques and the results placed in a sequence stratigraphic framework one of these members the laney shale was deposited in a hydrologically closed alkaline lake under an arid climate and was characterized by high concentrations of alginite rich organic matter whereas the other the luman tongue was deposited in a hydrologically open freshwater lake under a humid climate and consisted of organic poor profundal lake mudstones and coaly lake margin sediments potential source rocks in both lake types have the potential for generating high wax oil at high subsurface temperatures 150 175 c at a heating rate of 5 k ma but the alkaline system is clearly more prolific the most abundant biological marker hydrocarbons were the 4 methylsteranes in both lake types though dinosteranes were found only in the laney shale oleanane was absent from the luman tongue despite an abundance of angiosperm palynomorphs intraformational heterogeneities were best documented for the laney shale where molecular differences in kerogen type occur at the parasequence level with early transgression maximum transgression and rejuvenation stages of lake history having its own kerogen type and distinctive high wax oil fingerprint thermal lability was shown to be linked to kerogen structure in particular the presence of alicyclic and oxygen containing moieties and these in turn are correlated with total organic carbon content and thence productivity degree of preservation some kerogen components appear to have originated via selective preservation whereas others may have been formed by diagenetic condensation reactions

exhumation and metamorphism of an ultrahigh grade terrane geochronometric investigations of the sudete mountains bohemia poland and czech republic the sudete mountains ne bohemian massif czech republic and poland preserve abundant eclogitic and granulitic centimetre to decimetre scale boudins enveloped in a predominantly migmatitic matrix published geochronometry and thermobarometry from the uhp and uht rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units however the timing of stage 2 metamorphism and associated unroofing is less well constrained new in situ ion microprobe th pb monazite results together with complementary u pb zircon and electron microprobe analyser total pb monazite results on 11 amphibolite facies gneissic to migmatitic samples place important temporal constraints on the second stage of uhp and uht metamorphism exhumation the orlica snieznik dome records uhp metamorphism occurring at 375 ma and subsequent exhumation to midcrustal levels in supra barrovian conditions at c 345 330 ma in contrast the western gory sowie block preserves evidence of hp granulite conditions at c 400 ma and exhumation to mid crustal levels at 380 370 ma revealing a c 30 million years difference in exhumation events between the neighbouring terranes the eastern gory sowie block preserves ages similar to the orlica sneiznik dome suggesting that different preserved metamorphic cooling histories are juxtaposed across the sudetic marginal fault the bounding niemcza shear zone yields preliminary th pb dates that range from 380 8 ma to 283 2 ma preserving a protracted metamorphic record that spans the exhumation history of the region the distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low viscosity crust under supra barrovian conditions near the mid crustal high strength lid during oblique transpressional convergence

electron microprobe dating of monazites from western carpathian basement granitoids plutonic evidence for an important permian rifting event subsequent to variscan crustal anatexis accessory monazites from 35 granitoid samples from the western carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages on the basis of their th u and pb contents the sample set includes representative granite types from the tatric veporic and gemeric tectonic units in most cases lower carboniferous variscan ages have been obtained however a much younger mid permian age has been recorded for the specialised s type granites of the gemeric unit and several small a and s type granite bodies in the veporic unit and the southern tatric unit this distinct permian plutonic activity in the southern part of the western carpathians is an important although previously little considered geological feature it appears to be not related to the variscan orogeny and is interpreted here to reflect the onset of the alpine orogenic cycle with magma generation in response to continental rifting the voluminous carboniferous granitoid bodies in the tatric and veporic units comprise s and i type variants which document crustal anatexis accompanying the collapse of a compressional variscan orogen sector the variscan magmas were most likely produced through the remelting of a subducted precambrian volcanic arc type crust which included both igneous and sedimentary reworked volcanic arc material although the 2 errors of the applied dating method are quite large and typically 10 20 ma for single samples it would appear from the data that the variscan s type granitoids 333 367 ma are systematically older than the variscan i type granitoids 308 345 ma this feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post collisional variscan crust in accordance with recently published zircon ages this study shows that the western carpathian basement must be viewed as a distinct eastern tectonomagmatic province in the variscan collision zone where the post collisional crustal melting processes occurred 20 ma earlier than in the central sector south bohemian batholith hohe tauern batholith

acidic biomarkers from albacora oils campos basin brazil abstract acidic biomarkers of 8 different oil samples from albacora oil field campos basin brazil were analyzed in order to obtain additional information about biodegradation and thermal evolution for this purpose we selected oils lacustrine saline origin showing different biodegradation levels and at different distances from the fault that acted as migration pathway in the area previous analyses using hydrocarbon biomarkers suggested a mix of oil charges characterizing two distinct oil cycles interrupted by meteoric water circulation in these reservoirs analyses of the naphthenic acid biomarkers revealed a parallelism between the hydrocarbon and the acidic biomarkers notwithstanding there is evidence that the carboxylic acid biomarkers more resistent to biodegradation can provide information on past reservoir history when most important neutral biomarkers have been biodegraded the limitations to the use of these biomarkers are their de novo synthesis in the biodegradation process and the complexity of the sample manipulation prior to the analysis

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

the 40ar 39ar ages from the west sudetes ne bohemian massif constraints on the variscan polyphase tectonothermal development

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

significance of coniferous rain forests and related organic matter in generating commercial quantities of oil gippsland basin australia contrary to the conventional belief that humic coal generates primarily gas 3 billion bbl of recoverable oil has been discovered in the humic coaly succession of the fluviodeltaic latrobe group upper cretaceous tertiary that serves as both the reservoir and the source for hydrocarbons in the offshore gippsland basin of southeastern australia evidence for generation of liquid hydrocarbons from the coaly succession includes 1 similarity of n alkane distribution in the oil and in the coal extracts 2 high wax content of oil up to 27 by weight 3 high ratio of pristane phytane in oil 5 6 and 4 dominance of c29 steranes in the oil in the gippsland basin coniferous rain forests dominated by kauri vegetation flourished in a raised bog setting present temperate climate and kauri vegetation of new zealand are considered to be the modern analog to the gippsland basin the coniferous vegetation provided large quantities of hydrogen rich exinite macerals such as cutinite and resinite with potential to generate oil high rainfall raised ground water level low oxygen high acidity and low nutrient conditions of a raised bog setting were suitable for preserving organic matter a comparison of gas chromatograms of oils in the gippsland basin with gas chromatograms of oils generated by hydrous pyrolysis in the laboratory from the immature source rocks suggests that the paraffinic fraction of the oil was derived from coal and the naphthenic fraction was derived chiefly from resin

multistage variscan magmatism in the central tauern window austria unveiled by u pb shrimp zircon data u pb shrimp ages of nine variscan leucocratic orthogneisses from the central tauern window austria reveal three distinct pulses of magmatism in early carboniferous visean late carboniferous stephanian and early permian each involving granitoid intrusions and a contemporaneous opening of volcano sedimentary basins a similar relationship has been reported for the carboniferous parts of the basement of the alps further to the west e g the external massifs in switzerland after the intrusion of subduction related volcanic arc granitoids 374 10 ma zwolferkogel gneiss collisional intrusive granitic anatectic and extrusive rhyolitic dacitic rocks were produced over a short interval at ca 340 ma augengneiss of felbertauern 340 4 ma hochweisenfeld gneiss 342 5 ma falkenbachlappen gneiss 343 6 ma this early carboniferous magmatism which produced relatively small volumes of melt can be attributed to the amalgamation of the gondwana derived tauern window terrane with laurussia avalonia probably due to the oblique nature of the collision transtensional phenomena i e volcano sedimentary troughs and high level intrusives and transpressional regimes i e regional metamorphism and stacked nappes with anatexis next to thrust planes evolved contemporaneously the magmas are mainly of the high k i type and may have been generated during a short phase of decompressional melting of lithospheric mantle and lower crustal sources in the late carboniferous a second pulse of magmatism occurred producing batholiths of calc alkaline i type granitoids e g venediger tonalite 296 4 ma and minor coeval bodies of felsic and intermediate volcanics heuschartenkopf gneiss 299 4 ma peitingalm gneiss 300 5 ma prior to this magmatism several kilometres of upper crust must have been eroded because volcano sedimentary sequences hosting the heu schartenkopf and peitingalm gneisses rest unconformably on 340 ma old granitoids the youngest permian period of magma generation contains the intrusion of the s type granatspitz central gneiss at 271 4 ma and the extrusion of the rhyolitic schonbachwald gneiss protolith at 279 9 ma these magmatic rocks may have been associated with local extension along continental wrench zones through the variscan orogenic crust or with a permian rifting event the permian and the above mentioned late carboniferous volcano sedimentary sequences were probably deposited in intra continental graben structures which survived post variscan uplift and alpine compressional tectonics

new insights on the green river petroleum system in the uinta basin from hydrous pyrolysis experiments the tertiary green river petroleum system in the uinta basin generated about 500 million bbl of recoverable high pour point paraffinic crude oil from lacustrine source rocks a prolific complex of marginal and open lacustrine source rocks dominated by carbonate oil shales containing up to 60 wt type i kerogen occur within distinct stratigraphic units in the basin petroleum generation is interpreted to originate from source pods in the basal green river formation buried to depths greater than 3000 m along the steeply dipping northern margin of the basin producing fields in the altamont bluebell trend have elevated pore fluid pressures approaching 80 of lithostatic pressure and are completed in strata where open fractures provide permeability active hydrocarbon generation is one explanation for the origin of the overpressured reservoirs in this study experiments were undertaken to examine the mechanisms of hydrocarbon generation and accumulation in the uinta basin we combined analyses of representative source rocks from the entire green river stratigraphic section with detailed laboratory simulation experiments using both open and closed system pyrolysis this information provides new insights on lacustrine source rock lithofacies gas oil source rock correlations hydrocarbon generation kinetics and basin modeling the results show that the basal green river formation contains a unique type i source facies responsible for generation of paraffinic crude oils the classic type i oil shales in the upper green river formation correlate well with low maturity aromatic asphaltic samples we determined kinetic parameters for the source rocks and used them to develop basin models for hydrocarbon generation the models show that hydrous pyrolysis kinetic parameters are more consistent with the natural data in terms of predicted timing and extent of oil generation as compared to models using rock eval kinetics begin page 1334

thermal history of sedimentary basins maturation indices and kinetics of oil and gas generation temperature is the most sensitive parameter in hydrocarbon generation thus reconstruction of temperature history is essential when evaluating petroleum prospects no measurable parameter can be directly converted to paleotemperature maturation indices such as vitrinite reflectance tmax from rock eval pyrolysis spore coloration thermal alteration index tai or concentration of biological markers offer an indirect approach all these indices are a function of the thermal history through rather complex kinetics frequently influenced by the type of organic matter their significance and validity are reviewed besides the problems of identification e g vitrinite and interlaboratory calibration it is important to simultaneously interpret kerogen type and maturation and to avoid difficult conversions from one index to another geodynamic models where structural and thermal histories are connected are another approach to temper ture reconstruction which could be calibrated against the present distribution of temperature and the present value of maturation indices kinetics of kerogen decomposition controls the amount and composition of hydrocarbons generated an empirical time temperature index tti originally introduced by lopatin does not allow such a quantitative evaluation due to several limitations no provision for different types of kerogen and different rates of reactions poor calibration on vitrinite reflectance it is of limited interest unless one has no access to a desk top computer kinetic models based on a specific calibration made on actual source rock samples can simulate the evolution of all types of organic matter and can provide a quantitative evaluation of oil and gas generated examples from the jurassic source rocks of the paris basin monterey formation of california green river shales of utah paleozoic source eds of the algerian sahara and miocene rocks of the mahakam delta indonesia illustrate various aspects of the discussion geological geochemical models are the most efficient way to integrate geological seismic and geochemical data and they should greatly help to reduce the risk in exploration

constraints on the duration of high pressure metamorphism in the tauern window from diffusion modelling of discontinuous growth zones in eclogite garnet an eclogite sample from the grossgockner region of the hohe tauern austria contains garnet with a pronounced compositional discontinuity between a mn rich core and an fe rich rim this jump in composition was caused by a garnet consuming reaction followed by growth of the garnet rim omphacite and marks the prograde transition from epidote amphibolite to eclogite facies metamorphism garnet growth ended at peak metamorphic conditions of 570 c 17 kbar but intracrystalline diffusion continued until about 450 c 4 kbar on the retrograde path this garnet overgrowth texture represents a natural diffusion couple and a time span of 1 myr was calculated from the diffusion profile developing out of the original sharp compositional step for typical crustal densities this time corresponds to a minimum average velocity in the range 4 6 7 4 cm yr 1 for vertical movement which is one of the fastest exhumation rates reported the diffusion of all divalent cations of four profiles was modelled both analytically and numerically both approaches gave comparable results but the times computed for each element were always discrepant up to a factor of 2 variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits numerical modelling does not require the simplifications introduced in the analytical approach on the other hand error propagation was computationally unfeasible with this method

geochemistry and origin of natural gas in the petroliferous mahu sag northwestern junggar basin nw china carboniferous marine and permian lacustrine gas systems abstract the mahu sag located in the northwestern junggar basin of nw china is a renowned carboniferous marine permian lacustrine hydrocarbon depocenter that in theory could host both oil and gas however as ongoing hydrocarbon exploration and production are focused solely on oil few gas prospects have been discovered in this paper we discuss the possible occurrence of natural gas deposits based on a comprehensive study of gas geochemistry including gas composition carbon isotopes and compositions of light hydrocarbons c5 c7 and biomarkers in retrograde condensates combined with geological information available for the study area our results indicate that three types of natural gas occur in the region a sapropelic type gas derived from the lower permian fengcheng formation p1f a humic type gas sourced from carboniferous strata c and the overlying lower permian jiamuhe formation p1j and a humic type gas derived from the middle permian lower wuerhe formation p2w the p1f sourced sapropelic type gas accumulated during at least two distinct periods of time yielding low maturity oil associated gases and mature to high maturity kerogen oil cracking gases the carboniferous and p1j sourced humic type gases are mainly high maturity to over mature kerogen oil cracking gases without obvious geochemical evidence for a multi stage accumulation the p2w sourced gas is a mature humic type gas of these three types of gas the former two have the potential to form significant gas deposits and constitute valid exploration targets making the mahu sag a prospective area for gas exploration favorable areas for gas occurrence include deep anticlines hosting both kerogen and oil cracking gases the inferred exploration targets occur at significant depths which is why little drilling has been undertaken in these areas and major gas discoveries have yet to be made

lacustrine organic facies a biomarker study using multivariate statistical analysis abstract four distinct depositional settings in the devonian orcadian basin of scotland have been characterised by their minerology and organic geochemistry with the help of multivariate statistical analysis the data visualise the organic matter source input the depositional environment and the level of thermal maturity principal component analysis proved invaluable for extracting information from a data set composed of a large number of parameters the thermal maturity of the majority of the samples lies within a restricted range in the oil window and biomarker composition reflects predominantly organic input and depositional environment basin margin deposits are clearly differentiated from deeper water central lake deposits salinity and anoxicity variations are seen within the latter type of deposits

origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry muhs d r bettis iii e a aleinikoff j n mcgeehin j p beann j skipp g marshall b d roberts h m johnson w c benton r 2008 origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry gsa bulletin 120 11 12 1378 1407

heterogeneous extrusion and exhumation of deep crustal variscan assembly geochronology of the western tatra mountains northern slovakia abstract the nature and style of mid crustal assembly and exhumation during continental collision has been investigated in the tatra mountains of the western carpathians the pre alpine basement of the western carpathians represents the easternmost exposure of the variscan orogen in europe which marks the collision of laurasia with gondwanian affiliated terranes during the palaeozoic the tatric crystalline unit of the western tatra in northern slovakia displays an inverted metamorphic sequence where a high grade unit comprising migmatites with relicts of eclogite has been thrust over a lower grade mica schist unit new geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the western tatra the upper unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1 6 gpa to 1 0 1 2 gpa at 750 800 c which lead to intensive re equilibration at high pressure granulite facies conditions comparable to the peak metamorphic conditions of the host migmatite both eclogite and migmatite shared a retrograde p t path following the insertion of the eclogite assemblage into the migmatites the metamorphic evolution of the lower unit mica schist is constrained to peak p t conditions of 0 6 0 8 gpa and 640 and 660 c followed by retrogression this suggests that different rock types of the western tatra metamorphic core shared only their exhumation path from mid crustal levels id tims sm nd dating of garnet from eclogite yields a whole rock garnet isochron age of 337 10 ma with an initial e nd isotopic composition of 8 3 in situ u pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c 380 ma whereas monazite from a migmatite away from the eclogite preserves a robust 340 11 ma age which is indistinguishable from sm nd garnet age and u pb age of zircons in the anatectic leucosome of the migmatite 347 7 ma a younger monazite age population from the migmatite of 300 16 ma is consistent with 40 ar 39 ar mica ages of c 310 ma this argues for a contemporaneous and likely shared exhumation path of the assemblage pair in situ monazite total pb analyses from the lower unit mica schists yields xenocrystic and c 370 ma ages but no geochronologic evidence for peak variscan tectonism exhumation of the deep crustal root occurred most probably in a two stage process the timing of the high pressure eclogite facies metamorphism before the onset of exhumation into the mid crust was likely between c 380 ma and 360 ma subsequent exhumation into the middle crust was coeval with migmatite generation at c 340 ma and garnet diffusion modeling suggest 30 c ma cooling rates the exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid crustal levels at a time coeval with anatexis and subsequently extruded with mid crustal material to the upper crust

classification and thermal history of petroleum based on light hydrocarbons abstract classifications of oils and kerogens are described two indices are employed termed the heptane and isoheptane values based on analyses of gasoline range hydrocarbons the indices assess degree of paraffinicity and allow the definition of four types of oil normal mature supermature and biodegraded the values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type aliphatic and aromatic kerogens are definable only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group 41 in the sample set this group was evidently generated at subsurface temperatures of the order of 138 149 c 280 300 f defined under specific conditions of burial history it is suggested that all other petroleums are transformation products of normal oils

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

permian metamorphic event in the alps in addition to the well known variscan and alpine orogenic cycles a permian thermal event occurred in the european alps the geodynamic background of this event is not well understood here we suggest that the event is a reflection of lithospheric thinning accompanied by magmatic underplating causing partial melting of the lower crust and low pressure high temperature metamorphism the event was terminated by early triassic opening of the meliata ocean and was followed by sag stage subsidence during slow lithospheric cooling this last stage of the evolution allowed the onset of the mesozoic marine evolution with the orogen wide formation of triassic carbonate platforms well known from the dolomites and the northern calcareous alps the combined evidence for magmatism metamorphism and late subsidence identifies the permian event of the alps as an excellent example of underplating events suggested as a cause for low pressure high temperature terrains worldwide

the origin of 4 methyl steranes and pregnanes from tertiary strata in the qaidam basin china abstract a geochemical study of oils and source rocks in tertiary saline lake facies from the qaidam basin of china demonstrates that pregnanes are mainly derived from the thermal degradation of organic matter in source rocks synchronous with hydrocarbon genesis whereas 4 methyl steranes may principally be a product of microbial activity during diagenesis

thick skinned versus thin skinned thrusting rheology controlled thrust propagation in the variscan collisional belt the southeastern bohemian massif czech republic austria the variscan nappe assembly within the southeastern bohemian massif includes 1 crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels 2 nappe assembly of footwall units under very low to low grade metamorphic conditions and 3 frontal thin skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate thick skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin skinned tectonic character of foreland units 40ar 39ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during late variscan thrusting between 340 ma and 325 ma the 40ar 39ar mineral ages together with previously published 40ar 39ar and sm nd data record systematically decreasing variscan mineral ages structurally downward into footwall units this is explained by rapid exhumation and cooling synchronously with thrusting along crustal scale ramps during foreland directed nappe propagation the location of decollement zones within specific structural levels was apparently controlled by crustal scale rheological con trasts late variscan cooling ages contrast with cadomian 40ar 39ar hornblende and muscovite ages recorded within the structural basement immediately below the major variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during variscan thrusting these record a sequence of cadomian magmatic and metamorphic events in foreland units including 1 pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions 610 ma 2 greisen formation during a cadomian collisional event 600 ma and 3 intrusion of calcalkaline granitoid bodies with i type affinities 590 ma

geochemical application of sterane and triterpane biomarkers to a description of oils from the taranaki basin in new zealand abstract the distributions of sterane and terpane biomarkers in a suite of 25 oils from onshore and offshore regions of the taranaki basin of new zealand have been determined by gc ms features of these distributions include the presence of relatively high concentrations of a number of c 24 tetracyclic terpanes the virtual absence of tricyclic terpanes a predominance of c 29 normal iso and dia steranes and the presence of a number of non hopanoid triterpanes in one member of the suite urenui 18 h oleanane is the most abundant component m z 191 ion chromatogram of the terpane class of biomarkers based upon the distributions of 18 h oleanane the mckee and moturoa families of oils can be delineated these terpane distributions are qualitatively similar to those encountered in crude oils from a number of far eastern basins and from the handil field of the mahakam delta indonesia the taranaki oils are of approximately the same thermal maturity maturity parameters calculated from the distributions of c 29 steranes and of 17s h 21 h and 17 h 21s h hopanes confirm recent findings of incomplete nuclear isomerisation in oils derived from tertiary sediments an attempt has been made to correlate the mckee 1 and maui 4 crude oils with potential source rocks from depths of 3708 m and 3825 m respectively the rocks studied have attained insufficient maturity to have been the sources of these oils

high abundance of carotanes in the brackish saline lacustrine sediments a possible cyanobacteria source abstract the organic rich oil shales of low maturity and tight oils from the middle permian lucaogou formation were formed in a brackish saline lacustrine environment in the jimusaer sag junggar basin they are characterized by remarkably abundant carotane and carotane the biological sources and favourable sedimentary environments of carotanes formation remain obscure organic geochemical analyses of 17 oil shales and 20 tight oils from the jimusaer sag reveal that reducing evaporitic and carbonate rich shallow water environment is favourable for carotane accumulation geochemical studies show that carotane abundance is controlled by both source of organic matter and sedimentary environment rather than thermal maturity in the study area even though both higher plants and microalgae are reported to be precursors of carotanes cyanobacteria may be the main biological sources of carotanes in the lucaogou formation based on cyanobacteria derived biomarkers such as the c39 2 6 dimethyl benzene substituted isoprenoid 2 methylhopanes 7 and 8 monomethylhepadecanes the airborne volcanic ash containing ample elements e g mg and fe caused by the intermittent volcanic eruptions during the middle permian most likely contributed to the flourish of cyanobacteria in the study area

hydrocarbon generation potential of triassic mudstones in the junggar basin northwest china triassic mudstones in the junggar basin of northwest china especially those of the upper triassic baijiantan formation which formed during a lake flooding event are the most important set of cap rocks in the basin these mudstones presumably have hydrocarbon generation potential although this potential has not been documented here we provide new organic geochemical and geological background data on these mudstones and discuss their hydrocarbon generation potential geochemical analyses were performed on available samples collected from more marginal shallow lake facies in the central basin area these analyses indicated that these rocks contain about 1 0 total organic carbon toc the organic matter type is predominantly type iii kerogen derived mainly from terrestrial higher plants the organic matter underwent maturation in uplifted areas and reached higher maturity levels in depressions within the basin thus the triassic mudstones may have a potential for hydrocarbon generation during the lake flooding period especially for gas and hence deserve attention in future exploration more research needs to be conducted on the generation potential of these units including oil besides gas as few deep lake facies samples from depressions in the basin are currently available for investigation

lacustrine carbonate platforms facies cycles and tectonosedimentary models for the presalt lagoa feia group lower cretaceous campos basin brazil studies of lacustrine carbonate rocks in continental rifts have received huge interest in recent years because of their great economic value in the south atlantic however most existing facies and tectonosedimentary models for carbonate platforms are based on marine carbonate systems whereas models for nonmarine systems are scarce the main aim of this paper is to establish such models and to further our understanding of the hydrocarbon bearing late synrift lower cretaceous carbonate successions of the campos basin brazil this paper is based on a proximal to distal industrial data set of three dimensional 3 d seismic cores and well logs from the coqueiros formation coquina southern campos basin the dominant carbonate facies in the coqueiros formation are mollusk rich grainstones rudstones and floatstones which form the main reservoir facies the 3 d seismic interpretations show an oblique extensional rift system characterized by a series of grabens half grabens accommodation zones and horsts oriented northeast southwest to north northeast south southwest three tectonic domains are recognized based on structural style stretching factors and subsidence rates as well as facies and different types of lacustrine carbonate platforms proximal rift margin areas are characterized by a series of half grabens with footwall and hanging wall dip slopes of shallow lacustrine carbonates and fluviodeltaic mixed carbonate and siliciclastic deposits in marginal hanging wall basins central areas are carbonate rich with platforms established over horst blocks surrounded by deeper water carbonate facies distal areas have the highest amount of stretching and subsidence and accumulate the thickest carbonate successions over a template of buried horsts and grabens the entire carbonate succession underlies a thick layer of aptian salt which forms the seal to this prolific hydrocarbon system

the hydrocarbon generation potential and migration in an alkaline evaporite basin the early permian fengcheng formation in the junggar basin northwestern china abstract the permian fengcheng formation in the mahu sag in the junggar basin represents deposition in a rare alkaline saline lake with hydrologically closed conditions alkaline saline brines and intense volcanic activity the fengcheng formation contains large volumes of high quality source rocks which form not only in the depocenter but also in the wide shallow lake margins the source rocks analyzed in this study are different from those in most other ancient lakes in the following aspects genesis and occurrence of organic matter kerogen type lithological composition and hydrocarbon migration pathway this study showed that the fengcheng formation has high hydrocarbon generation potential and that combinations of multiple paths enhanced the hydrocarbon expulsion and migration benthic microbial mats and algae that bloomed in the alkaline saline lake represent the dominant types of organic matter due to their specific composition the evaporites may have catalyzed hydrocarbon generation via organic inorganic interactions while intergranular seams in bedded laminated evaporites provided paths for hydrocarbon migration stylolites with high contents of brittle minerals and low clay content also allowed favorable paths for hydrocarbon expulsion and migration combining hydrocarbon expulsion fractures in the organic rich laminae fractures and microfractures caused by compressive tectonic stress pathway networks formed that enhanced the efficiencies of both hydrocarbon expulsion and migration

a novel microbial hydrocarbon degradation pathway revealed by hopane demethylation in a petroleum reservoir abstract the origin of 25 norhopanes is controversial despite their abundance in many biodegraded oils in a biodegraded oil reservoir we found concomitant and directionally opposite changes with depth in the concentrations of certain 17 hopanes and their 17 25 norhopane counterparts demonstrating microbially induced demethylation of hopanes these observations suggest a hitherto unknown biodegradation mechanism and provide insight into the poorly understood process of natural petroleum biodegradation

geochemical characteristics and origin of sodium carbonates in a closed alkaline basin the lower permian fengcheng formation in the mahu sag northwestern junggar basin china abstract the lower permian fengcheng formation in the mahu sag of northwestern china is well known for its alkaline lacustrine deposits however previous research is scarce due to this formation s notable burial depth up to 5000 m and limited oil and gas exploration using xrd xrf micro xrf and c o isotope analyses we determined 1 the stratigraphic trends in mineral compositions 2 the cyclic depositional processes and 3 paleoclimate variations the petrologic and geochemical data elucidate depositional and diagenetic processes in the mixed siliciclastic volcaniclastic and chemical strata of the fengcheng formation sodium carbonate minerals in the studied core are dominated by wegscheiderite na2co3 3nahco3 and trona na2co3 nahco3 2h2o nahcolite nahco3 and a mg na carbonate mineral northupite na2co3 mgco3 nacl also occur although their abundance is obviously lower than that of wegscheiderite and trona these substances grew as bottom growth crystals from lakes overlying tuffs the volumetrically important ca na carbonate mineral shortite na2co3 2caco3 and some northupite precipitated diagenetically from shallow groundwater brines in unlithified dark colored siliciclastic sediment and volcanic ash shortite mainly precipitated from the ca na co3 rich shallow groundwater whereas northupite precipitated from the mg na co3 cl rich shallow groundwater sodium carbonates and tuffs fine grained tuffaceous rocks and siliciclastic rocks formed repetitive centimeter and meter scale cycles that are shown as dark colored lithological layers during lake expansion and contraction stages or during volcanic eruptions the brine composition which controlled the precipitation of sodium carbonates was influenced strongly by volcanism and volcanism related hydrothermal activity changes in climate and volcanic eruptions controlled the deposition of alternating beds of sodium carbonates and dark colored lithologies changes in climate caused the expansion and contraction stages in the lakes during relatively humid climate stages diluted water entered the basin and resupplied the lake water with calcium and magnesium during flooding events sandy to silty beds were deposited by turbidity currents during this stage during arid climate stages bedded sodium carbonates grew in the lakes and their co occurrence suggests high atmospheric pco2 and high temperatures the ashes that are provided by volcanic eruptions during arid climate stages can also stop the precipitation of sodium carbonates

a stable carbon isotope and biological marker study of polish bituminous coals and carbonaceous shales abstract biological marker and carbon isotopic compositions of coals and carbonaceous shales from the upper carboniferous strata of the upper silesian uscb lower silesian lscb and lublin lcb coal basins were determined to assess depositional conditions and sources of the organic matter n alkane sterane and isoprenoid distribution and carbon isotope ratios are consistent with an origin from higher plants in some cases pristane phytane pr ph ratios of carbonaceous shales roof and floor shales are 29 stereoisomers typical but not conclusive of higher plant origin carbonaceous shales exhibit a wider range of sterane composition suggesting local significant input of algal organic matter significant amounts of benzohopanes and gammacerane are present in some coals although benzohopanes are present at least in small amounts in samples from many different environments they have been reported to occur most commonly in marine environments the present study seems to provide the first example where benzohopanes have been reported in significant amounts in terrestrial organic matter gammacerane is abundant in rocks or sediments deposited in carbonate or highly saline marine environments the finding of high gammacerane concentrations in the coals expands the depositional settings in which it has been observed and questions its utility as an independent indicator of hypersaline carbonate environments stable carbon isotope composition of coals and type iii kerogen in carbonaceous shales as well as correlation of stable carbon isotope composition of saturated and aromatic hydrocarbons in carbonaceous shales from both the uscb and the lscb indicate terrigenous origin bitumens are always co genetic with associated coals and kerogens isotopic data reveal that sofer s genetic classification of oils is not applicable to organic matter in coals

cretaceous evolution of a metamorphic core complex the veporic unit western carpathians slovakia p t conditions and in situ40ar 39ar uv laser probe dating of metapelites alpine metamorphism related to the development of a metamorphic core complex during cretaceous orogenic events has been recognized in the veporic unit western carpathians slovakia three metamorphic zones have been distinguished in the metapelites 1 chloritoid chlorite garnet 2 garnet staurolite chlorite 3 staurolite biotite kyanite the isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system k2o feo mgo al2o3 sio2 h2o kfmash the isograds are roughly parallel to the north east dipping foliation related to extensional updoming along low angle normal faults thermobarometric data document increasing p t conditions from c 500 c and 7 8 kbar to c 620 c and 9 10 kbar reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies 40ar 39ar data obtained by high spatial resolution in situ ultraviolet uv laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the late cretaceous mean dates are between 77 and 72 ma however individual white mica grains record a range of apparent 40ar 39ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous the reconstructed metamorphic p t t path is clockwise reflecting post burial decompression and cooling during a single alpine orogenic cycle the presented data suggest that the veporic unit evolved as a metamorphic core complex during the cretaceous growth of the western carpathian orogenic wedge metamorphism was related to collisional crustal shortening and stacking following closure of the meliata ocean exhumation was accomplished by synorogenic orogen parallel extension and unroofing in an overall compressive regime

an alkaline lake in the late paleozoic ice age lpia a review and new insights into paleoenvironment and petroleum geology abstract the ca 300 ma fengcheng formation in the junggar basin of nw china is reported to develop a rare alkaline lake soda lake in this study based on an expanded literature survey and new insights from geological and geochemical data the development of this ancient alkaline lake resulted from a convergence of favorable paleotectonic paleogeographic and paleoclimatic conditions a detailed characterization of the alkaline lake includes evidence for a ph of 9 0 indicated by 15n data average 18 4 the presence of alkali minerals such as trona and shortite globular bacteria like fossils low contents of clay minerals organic and inorganic geochemistry indicating high salinity strongly reducing conditions and hydrothermal activity and a complete sedimentary evolution sequence of an archetypal alkaline lake the studied lake deposit is presented as the first such deposit worldwide to generate a large oil reserve thus allowing scientific insights to be gained into the controls on hydrocarbon generation in alkaline lakes the alkaline lacustrine source rocks have high contents of organic matter favorable organic matter type abundant oil and poor natural gas multiple phases of hydrocarbon generation and high hydrocarbon volumes the genesis and development of this alkaline lake were controlled jointly by evaporation and volcanic hydrothermal activity however some aspects of this type of lake remain unclear and future work should explore the integrated evolution of biological climatic petroleum geology systems with respect to alkaline lakes and their associated deposits and their coupling research with worldwide event then namely glacial episode iii of the late paleozoic gondwanan glaciation our data suggest that the fengcheng formation is a well preserved archive of late paleozoic ice age lpia geology and geochemistry around 300 ma

metamorphic evolution and fluid composition of garnet clinopyroxene amphibolites from the tatra mountains western carpathians relics suggesting an early high pressure eclogite facies stage have been identified in the garnet and clinopyroxene bearing amphibolites of the tatra mountains in the variscan basement of the western carpathians slovakia in these rocks primary omphacite cpx i has been wholly converted to symplectites of diopside cpx ii and plagioclase apart from minor inclusions in the garnet cores amphibole and plagioclase are secondary minerals formed in the kelyphitic rims between garnet and clinopyroxene several generations of amphibole pargasite hornblende cummingtonite actinolite are evidence of a transformation down to greenschist facies conditions thermobarometric calculations from mineral inclusions in the garnet cores yield 670 700 c and 10 15 kbar recording the initial path from upper amphibolite to eclogite facies conditions the attainment of the eclogite facies stability field is inferred from the composition of a reconstructed omphacite jd36 implying a minimum peak pressure of 15 16 kbar conditions of around 650 c and 8 10 kbar record the post eclogite breakdown and partial re equilibration in the amphibolite facies region the earliest fluid inclusions contain high density nitrogen dominated water absent fluid younger are polyphase brines and two phase h2o n2 ch4 inclusions with signs of heterogeneous entrapment of coexisting gas rich and water rich immiscible phases pure nitrogen 0 5 mol ch4 is considered to have been the major component during the high pressure metamorphism in contrast the later aqueous inclusions are interpreted to represent retrogression related fluids the brines have originated by leaching and re entrapment of saline melt inclusions observed in tonalitic trondhjemitic layers of the amphibolite enclosing the eclogitic relics a similar mechanism is assumed for the origin of the h2o n2 ch4 inclusions these represent relics of the primary eclogite facies related n2 inclusions re equilibrated and re entrapped during retrogression under increasing water activity a possible source of n2 and cl rich fluids was primary amphibole which decomposed during the prograde metamorphism from the amphibolite to the eclogite facies the metamorphic pt path is generally clockwise reflecting a steep increase in pressure and temperature during burial followed by decompression and cooling the retrograded eclogites occur in boudins within an allochthonous unit of an inverted metamorphic sequence their exhumation was facilitated by tectonic transport along a ductile shear zone during the variscan orogeny

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

first permian early triassic zircon ages for tin bearing granites from the gemeric unit western carpathians slovakia connection to the post collisional extension of the variscan orogen and s type granite magmatism this study presents the first preliminary u pb zircon data on tin bearing s type granites from the gemeric unit of the western carpathians slovakia u pb single zircon dating controlled by cathodoluminescence suggests crystallization of the gemeric granites during permian to early triassic 303 241 ma time post crystallization low temperature metamorphic overprint is reflected by partial pb loss in zircons these gemeric granites are younger than the highly fractionated s type tin and rare element bearing leucogranites in the european variscides they may have resulted from partial melting triggered by increased heat flow from the mantle below the continental crust and most probably intruded during the post collisional extension and initial rifting of the variscan orogenic belt during alpine orogeny the gemeric granites were affected by a low temperature deformation and metamorphism

eocene tectonometamorphism on serifos western cyclades deduced from zircon depth profiling geochronology and mica thermochronology abstract phengite bearing metasedimentary sequences preserving relict glaucophane from serifos western aegean are indicative of high pressure conditions during cycladic blueschist metamorphism depth profiling and conventional sims u pb geochronology and o isotope analysis on zircon from highly strained orthogneisses at the base of the sequence reveal xenocrystic zircon with varying degrees of recrystallization one end member population has needle like or prismatic morphologies is internally oscillatory zoned with little to no rim material created during recrystallization these zircons yield triassic to carboniferous ages elevated th u and 18 o mostly ranging between 4 and 11 in marked contrast a separate zircon population has a spongy structure created by complete recrystallization of the pre existing crystal these zircons possess low th u and flat hree patterns and yield eocene ages with 18 o 7 chondrite normalized ree profiles are extremely depleted within this spectrum other zircons show variable states of recrystallization and mixed ages regression of u pb data from three mylonitic orthogneiss samples that exhibit near complete recrystallization and similar age depth profiles yields c 40 ma ages these new data are the first geochronological constraints that successfully demonstrate an eocene metamorphic event in the western cyclades and new 40 ar 39 ar phengitic mica ages of 38 32 ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter

stable carbon isotopes of amorphous kerogens from phanerozoic sedimentary rocks amorphous kerogens from phanerozoic sedimentary rocks may be divided into two groups on the basis of their 13c values h amorphous kerogens have isotopic values in the range of 24 to 20 per mil and l amorphous kerogens have isotopic values in the range of 35 to 26 per mil the occurrence of amorphous kerogens in the intermediate isotopic range from one rock unit in the data set suggests a weak continuum may exist between these two groups phytoplankton and to a lesser extent zooplankton and bacteria are considered to be the precursors for both kerogen types a plausible explanation for this dichotomy may be found in the source of co2 utilized by phytoplankton which is considered to be controlled by environmental factors h amorphous kerogens are expected to occur in silled basins that are overlain by expansive well circulated deep 500 m water where the prevailing source of carbon in the photic zone is atmospheric derived co2 conversely l amorphous kerogens are expected to occur in restricted basins that are overlain by stratified shallow 200 m water where the prevailing source of carbon in the photic zone may be organic derived co2

application of biological markers in the assessment of paleoenvironments of chinese non marine sediments abstract in this study we employ molecular organic geochemical data for 22 representative sediment samples from chinese non marine sedimentary basins in an attempt to characterize and assess the non marine depositonal paleoenvironments the samples investigated include oilshale shale mudstone and sodium sulphate rich sediments ranging in age from cretaceous to tertiary it is evidenced by geological and geochemical data that those sediments studied originate from lacustrine environments ranging from freshwater brackish fresh water to hypersaline the aliphatic fractions of the sediment extracts were analyzed by gc and gc ms and relative abundance of compounds and compound classes of normal acyclic and cyclic alkanes were measured from mass chromatograms the results reveal significant differences in the chemical features of the various sample groups which enable them to be characterized and identified in terms of source and or environment related parameters the diagnostic parameters that allow this assessment are the relative abundances of n alkanes acyclic isoprenoid alkanes and 4 methyl steranes hopane sterane ratio gammacerane and homohopane indexes etc in this study data interpretation was also assisted by pattern recognition through principal component analysis the results show that the sample groups of different environmental origin could be unambiguously distinguished on scores plot

thermochronology of the west sudetes bohemian massif rapid and repeated eduction in the eastern variscides poland and czech republic the sudete mountains of northeastern bohemian massif were amalgamated during the closure of the rheic ocean culminating with variscan orogenesis and contain occurrences of high pressure granulite and small relict ultrahigh pressure eclogite formed during subduction we performed 40 ar 39 ar thermochronometry on primarily amphibolite facies gneisses and schists from two crustal blocks within the sudetes gory sowie and orlica snieznik massifs hornblende and mica plateau ages from the mountainous portion of the gory sowie reflect relatively rapid cooling between 382 1 ma and 373 0 5 ma following peak conditions at ca 400 ma kyanite sillimanite and cordierite garnet mineral growth textures denote near isothermal decompression during eduction concordant hornblende and biotite cooling ages obtained from the eastern topographically flat region adjacent to the niemcza shear zone indicate markedly younger cooling at 337 0 8 ma 40 ar 39 ar results from the snieznik mountains 50 km to the south yield plateau cooling ages for white mica and biotite between 341 1 to 337 0 6 ma and 342 1 to 334 0 6 ma respectively and are remarkably similar in age to an amphibolite facies overprinting episode mica analyses from the orlica mountains also yield cooling ages between 338 0 9 to 335 0 5 ma thermochronometry illustrates temporally disparate cooling histories but similar metamorphic evolutions between the gory sowie block and the orlica snieznik complex in both terranes it took 35 m y from the time of peak high pressure conditions to residence in the upper crust and possibly as little as 10 m y between high pressure and mid crustal supra barrovian events these results in light of regional geochronology illustrate the rapid localized and repetitious nature of eduction exhumation of the sudetic orogenic root including the eclogite and hp granulite assemblages during protracted subduction

prediction of source rock characteristics based on terpane biomarkers in crude oils a multivariate statistical approach abstract the distributions of eight tricyclic and eight pentacyclic terpanes were determined for 216 crude oils located worldwide with subsequent simultaneous rq mode factor analysis and stepwise discriminate analysis for the purpose of predicting source rock features or depositional environments five categories of source rocks are evident nearshore marine i e paralic deltaic deeper water marine lacustrine phosphaticrich source beds and ordovician age source rocks the first two factors of the rq mode factor analysis describe 45 percent of the variation in the data set the tricyclic terpanes appear to be twice as significant as pentacyclic terpanes in determining the variation among samples lacustrine oils are characterized by greater relative abundances of c 21 diterpane and gammacerane nearshore marine sources by c 19 and c 20 diterpanes and oleanane deeper water marine facies by c 24 and c 25 tricyclic and c 31 plus c 32 extended hopanes and ordovician age oils by c 27 and c 29 pentacyclic terpanes although thermal maturity trends can be observed in factor space the trends do not necessarily obscure the source rock interpretations also since bacterial degradation of crude oils rarely affects tricyclic terpanes biodegraded oils can be used in predicting source rock features the precision to which source rock depositional environments are determined might be increased with the addition of other biomarker e g steranes and stable isotope data using multivariate statistical techniques

carbon isotope geochemistry of paleozoic oils from big horn basin abstract twenty crude oils from paleozoic reservoirs in the big horn basin wyoming were fractionated into light hydrocarbons saturates monoaromatics diaromatics polyaromatics and polars and asphaltenes amounts and isotopic composition of each fraction were found to be internally consistent with the degree of maturation of the oil confirming the established single source origin for these oils a variation of approximately three per mil in the carbon isotopic composition of the whole oil and individual fraction was explained as being caused by maturation the isotope type curves for these oils the variations in isotopic composition as a function of compound group type were not found to be as simple as commonly believed other alteration processes such as migration and deasphalting might have influenced the pattern of the isotope distributions among fractions the conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil showed that the maturation of crude oil consists of consecutive processes leading from larger and more polar components into smaller and less polar components culminating in natural gas calculations were made to project the amount of condensates associated with the crude oils and the amount of natural gas which had been generated from crude oils of a given maturity

the permian hybrid petroleum system in the northwest margin of the junggar basin northwest china in this paper we define the concept of a hybrid petroleum system for the permian petroleum system in the northwest margin of the junggar basin by documenting the complexity in its formation and evolution it is termed hybrid because there are three source rock sequences seven reservoir caprock combinations and three petroleum events and multiple source rocks source the same reservoir and the same source rocks also feed multiple reservoirs the mahu sag centre is the petroleum source area with three sets of source rocks dark shale of the lower permian jiamuhe formation black shale of the lower permian fengcheng formation and black shale of the middle permian lower wuerhe formation the reservoir rocks include volcanic and clastic rocks of upper carboniferous to lower cretaceous age mainly characterized by alluvial conglomerate and sandstone there are three sets of ultimate caprocks among which the mudstone of the upper triassic baijiantan formation is the most important due to its wide distribution throughout almost the whole northwest margin and greatest thickness up to 300 m whereas the lower jurassic and lower cretaceous mudstone acts as an effective seal oil and gas were generated mainly during the middle late permian late triassic and middle jurassic early cretaceous the reconstruction of the oil filling history shows that the three source rocks charged reservoirs mainly in three geologic intervals with common mixing of oils from different sources and or with different maturities these three hydrocarbon charges coincide with the three main stages of petroleum generation related to the hercynian indosinian and yanshannian orogenies during these three petroleum events oil and gas migrated along the migration framework controlled by faults unconformities and good permeability reservoir rocks the faults are mainly well developed and episodically active thrust faults and secondary faults as a result the permian petroleum system in the northwest margin of the junggar basin is hybrid

on the negative carbon isotope excursion across the wuchiapingian changhsingian transition a regional event in the lower yangtze region south china abstract previous studies have reported faunal changeover and carbon isotope excursions through the wuchiapingian changhsingian w c transition a geologically significant period of time that bridged two major extinction events the end permian and guadalupian lopingian boundary mass extinctions however the causes of the carbon isotope excursion across the w c interval remain poorly understood this study presents sedimentological and geochemical data from three well exposed sections in the lower yangtze region south china a strong negative carbon isotope excursion in organic matter is recorded across the w c interval and correlates well with other sections in south china sedimentology and elemental geochemistry indicate that the lower yangtze region experienced a distinct regional deepening during the w c transition inducing bottom water redox changes increased primary productivity possible causes of the negative carbon isotope excursion at the w c boundary include selective preservation of organic matter during early diagenesis under anoxic euxinic conditions and changes in terrigenous organic matter input both of these factors were controlled by regional relative sea level rise triggered by contemporaneous tectonic movements

variable late neogene exhumation of the central european alps low temperature thermochronology from the aar massif switzerland and the lepontine dome italy 1 several recent studies proposed an important increase in exhumation rate in the western european alps since circa 5 4 ma in order to assess potential spatial differences in exhumation histories we present new apatite fission track aft and apatite u th he ahe ages from the central aar massif guttannen area switzerland and the western lepontine dome formazza area italy internal u th zoning in apatites explains alpha ejection corrected ahe ages that are older than the corresponding aft ages in this study a qualitative interpretation of aft and ahe age elevation relationships suggests a two phase 9 7 and 5 3 ma exhumation scenario affecting the central alps with a stronger expression of the pliocene signal in the formazza area however a quantitative evaluation of exhumation scenarios using the 3 d heat equation solver pecube highlights the existence of several other likely scenarios casting doubt on the validity of a qualitative interpretation of the age elevation relationships in formazza scenarios suggested by quantitative modeling include continuous denudation at a rate of 750 m ma and a one step exhumation rate change from 300 to 1000 m ma at 5 ma in guttannen they include continuous denudation at a rate of 400 m ma with valley deepening and two periods of higher exhumation rate increasing from 300 to 700 m ma repeatedly at 9 7 and at 5 3 ma contingent upon further flexural isostatic modeling the magnitude of exhumation recorded in the axial region of the alps since circa 5 ma does not appear sufficient to solely explain the denudation recorded in the north alpine foreland basin

investigating biological nitrogen cycling in lacustrine systems by ft icr ms analysis of nitrogen containing compounds in petroleum abstract biological nitrogen cycling plays an important role in bio environmental and petroleum geochemistry until now most research has focused on marine environments using nitrogen isotopes and in contrast nitrogen cycling in lacustrine environments remains poorly understood to fill this knowledge gap we present a case study of crude oils from permian brackish and saline alkaline lacustrine rocks in the junggar basin northwestern china to complement the large variation in nitrogen isotope values in lacustrine sediments we analyzed the pyrrolic n containing compounds of crude oil using negative ion electrospray ionization fourier transform ion cyclotron resonance mass spectrometry esi ft icr ms our results show that pyrrolic compounds derived from biological sources are more abundant in oils from brackish lacustrine settings i e the jimusar sag oils of the junggar basin than in oils from alkaline lacustrine settings i e the mahu sag oils of the junggar basin which is mainly due to greater higher plant contributions to the latter higher plants provide abundant alkaloids and chlorophyll that are the biological precursors of pyrrolic n containing compounds in addition anoxic sulfate type brackish lacustrine settings provide abundant ferrous iron and sulfide that promote n fixation this study provides the first insights into biological n cycling in brackish and alkaline lacustrine systems which has significance for fossil fuel resources and their environmental impacts our data suggest that the n cycling in lacustrine systems might have an important role in entire earth system n budget

new perspective on aptian carbon isotope stratigraphy data from δ13c records of terrestrial organic matter carbon isotope analyses were performed on detrital woody materials in aptian mid cretaceous marine sediments of central hokkaido northern japan a positive 13cwood excursion from 25 4 to 21 8 following a remarkable negative isotope shift is recognized in the early aptian and a small positive anomaly is also found in the latest aptian this 13cwood profile is exactly in phase with the 13ccarbonate curve from a pacific guyot when the age of the guyot carbonates is revised using published sr isotope stratigraphies the highly conformable 13c profiles of pacific and hokkaido sections suggest that 13c compositions of aptian marine and terrestrial carbon reservoirs changed simultaneously by the same amplitude within the ocean atmosphere biosphere system fluctuation patterns of tethyan 13ccarbonate curves are slightly different from those of pacific and hokkaido sections such differences in 13c profiles may be attributed to the local paleoceanographic setting of the aptian mediterranean tethys

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

clasts of variscan high grade rocks within upper visean conglomerates constraints on exhumation history from petrology and u pb chronology in this paper u pb zircon monazite and rutile data for crystalline rocks deposited as clasts in the upper visean conglomerates at the eastern margin of the bohemian massif are reported u pb data of spherical zircon from three different granulite clasts yielded a mean age of 339 0 0 7 ma 2 while oval and spherical grains of another granulite pebble define a slightly younger date of 337 1 1 1 ma these ages are interpreted as dating granulite facies metamorphism thermochronology and the derived pressure temperature p t path of the granulite pebbles reflect two stage exhumation of the granulites near to isothermal decompression from at least 44 km to mid crustal depths of around 22 km was followed by a near isobaric cooling stage based on reaction textures and geothermobarometry minimum average exhumation rate corresponds to 2 8 4 3 mm year 1 the extensive medium pressure high temperature overprint on granulite assemblages is dated by u pb in monazite at c 333 ma this thermal event probably has a close link to generation and emplacement of voluminous moldanubian granites including the cordierite granite present in clasts this granite was emplaced at mid crustal levels at 331 3 ma u pb monazite whereas the u pb zircon ages record only a previous magmatic event at c 378 ma eclogites and garnet peridotites normally associated with high pressure granulites are absent in the clasts but exotic subvolcanic and volcanic members of the ultrapotassic igneous rock series durbachites of the bohemian massif have been found in the clasts it is therefore assumed that the clasts deposited in the upper visean conglomerates sampled a structurally higher tectonic unit than the one that corresponds to the present denudation level of the moldanubicum of the bohemian massif the strong medium temperature overprint on granulites dated at c 333 ma is attributed to the relatively small size of the entirely eroded bodies compared with the presently exposed granulites

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy

40ar 39ar dating of the inner carpathians variscan basement and alpine mylonitic overprinting abstract the 40ar 39ar dating method has been applied to mylonitic rocks from the carpathian mountains these are variscan metamorphic and magmatic rocks that were overprinted during the alpine orogeny the inner western carpathians are conventionally described as a nappe pile formed by northward movement and then subjected to intense strike slip and normal faulting resulting in the formation of mylonitic rocks precise stratigraphie ages provide clear constraints for the geodynamic interpretation related to the succession of alpine events such as the pre gosauian phase during the turonian 90 ma the 40ar 39ar method allows to define the age of the ancient variscan basement and the ages of the alpine overprinting in the high and low tatras mineral ages of around 340 350 ma are yielded by gneisses micaschists and amphibolites providing a younger limit for the variscan metamorphism the emplacement cooling of the variscan granites is defined between 305 and 335 ma these rocks are overprinted by several alpine tectonothermal events the major events are dated at 65 and 86 88 ma during which the mylonites were generated an older event around 140 ma earlier defined by the rb sr method is now confirmed by the 40ar 39ar method dating of phengites extracted from high pressure metamorphic triassic rocks from the kosice area yields new data for the age of this event at 155 ma