hydrocarbon potential and depositional environment of the lower cretaceous black mudstones and shales in the coastal guangdong province china abstract the mesozoic especially the cretaceous sequences in the northern south china sea scs are important potential targets of oil and gas exploration however they are currently poorly understood we use organic petrology and geochemistry to evaluate the hydrocarbon potential and study depositional environment of the cretaceous black mudstones and shales from the dayawan section in the guangdong province china these mudstones and shales yield total organic carbon toc contents of 0 63 2 76 and most samples meet the requirement for higher toc source rocks organic matters in the mudstones and shales were sourced mainly from higher plants as well as subordinate benthic macroalgae organic petrology and geochemical data suggested that the organic matters were dominated by type iii and minor type ii and generally have low thermal maturity thus we evaluated the black mudstones and shales have a potential for hydrocarbon generation especially gas prone pr ph trace elements and mixture source of organic matter suggested that these black mudstones and shales formed in a coastal lacustrine environment characterized by weakly oxidizing weakly reducing conditions and low salinity a marine transgression developing in the middle part of the section was recognized generally the northern scs which exhibits the more marine influence likely contains source rocks with higher toc contents and better organic matter types the cretaceous source rocks with higher liquid hydrocarbon generation potential may occur locally in the pearl river mouth basin

the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

identification of some bicyclic alkanes in petroleum abstract bicyclic alkanes containing the drimane skeleton have been identified in crude oils and sediments by synthesis of model compounds two other bicyclic alkanes whose skeletons do not conform to the isoprene rule have also been identified from mass spectral data a correlation between the relative concentration of c 29 steranes and diasteranes and drimane and two c 15 bicyclanes in an extensive sample suite suggests a similar mechanism is involved in the formation of diasteranes and the two c 15 bicyclanes in the subsurface the widespread occurrence of drimanes and drimane related compounds in crude oils and sediments indicates that these compounds are derived from a ubiquitous source the presence of methyldrimanes in a sample rich in methylhopanes suggests that drimanes and hopanes may be derived from similar precursors

assessment of shale oil potential using a new free hydrocarbon index abstract shale oil is currently a highlight and frontier field in petroleum industry however the assessment and quantitative evaluation of oil content in the shale is challenging which is one of the critical issues in understanding the resource potential here we explore a new free hydrocarbon index based on a case study in the currently most successful shale oil play case in china i e the eocene hetaoyuan formation in the biyang depression of the nanxiang basin eastern china this index s 1 represents the difference between hydrocarbon generation quantity and pyrolysis free hydrocarbon value s 1 it is new in that it considers not only the residual oil contents in the rocks like previous parameters but also the hydrocarbon expulsion and migration conditions that have rarely been focused before when s 1 1 is around 0 it is implied that the generated oil nearly has not expulsed and resided in the shale and thus there is also a good shale oil potential in contrast when s 1 0 it is implied that certain amounts of generated oils have migrated to external reservoirs and thus there is a poor shale oil potential this method has potential general implications and the results in the biyang can be used in the practical shale oil exploration

evidence for gloeocapsomorpha prisca in late devonian source rocks from southern alberta canada abstract organic rich intervals within the late devonian camrose member of southern alberta canada show optical and chemical characteristics very similar to those of ordovician samples whose organic matter is dominated by gloeocapsomorpha prisca derived alginite this includes saturate fraction gas chromatograms dominated by n alkanes with a pronounced odd carbon number up to c19 and much lower abundances of c19 n alkanes acyclic isoprenoids and biomarkers microscopy reveals these samples contain g prisca like alginite with a morphology that previously has been suggested for the growth of this organism under higher salinity conditions which is in agreement with the depositional environment of the camrose member one unusual feature of the g prisca rich samples is the very high concentration of c21 and c23n alkylnaphthalenes and n alkylbenzenes in the aromatic fractions of their extracts the origin of these compounds is not known but their occurrence could be related to the high concentrations of c21 and c23n alkyl phenols that have been noted in pyrolysates of estonian kukersites by previous workers other camrose member extracts show very different distributions that are more typical for carbonates deposited under anoxic higher salinity conditions many oils that occur in nisku formation reservoirs in the enchant hays field in southern alberta show saturate fraction gas chromatograms and other evidence that indicate an important contribution to these oils from the g prisca rich intervals these are thought to have a contribution from camrose member source rocks hence caution should be used in only attributing g prisca geochemical characteristics to cambro ordovician oils as on rare occasions they can also occur in younger source rocks and their derived oils

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

the determination of estuarine diffusion coefficients using a fluorimetric dye tracing technique abstract an improved dye injection technique has been used to collect new field data to study the transverse and vertical mixing in a section of the conwy estuary these results define the full three dimensional spatial concentration distribution of the dye plume estimates of the diffusion coefficients are given and were found to be strongly influenced by the transverse shear induced density variations and bed topography the results suggest that an analogy with fickian diffusion appears inappropriate on flood tides because of the importance of secondary circulations during this tidal phase

organic geochemistry petrology and conventional and unconventional hydrocarbon resource potential of paleogene saline source rocks in eastern china the biyang sag of the nanxiang basin abstract basins containing paleogene organic rich shales formed in a saline lacustrine environment are widespread in eastern china and they provide sources of hydrocarbons in numerous petroleum fields however research on these basins has been patchy which limits our understanding of the hydrocarbon source rocks to address this problem we selected the relatively understudied nanxiang basin as the study area and more specifically the third member of the paleogene hetaoyuan formation eh3 which is the main hydrocarbon generating layer in the biyang sag of this basin in this paper we report on the organic petrology and geochemistry of eh3 and further analyze both its conventional and unconventional hydrocarbon potential our results show that the organic matter om within the shales of eh3 had bioprecursors that were mainly phytoplankton bacteria and benthic algae with relatively small contributions from animal and higher plant materials the shales have an average om abundance total organic carbon or toc of 2 96 wt thus meeting the criterion of a good quality source rock their organic matter is mainly type i to type ii and in terms of thermal maturity they range from low maturity to mature thus displaying the characteristics required to generate oil the results of our molecular geochemical analyses show that the sediments of eh3 in the biyang sag belong predominantly to deep or semi deep lake facies and they were deposited in a reducing environment and may have been affected by a marine incursion the evidence for this includes pr ph values of

how to find high quality petroleum source rocks in saline lacustrine basins a case study from the cenozoic qaidam basin nw china abstract despite having made significant contributions to global hydrocarbon occurrences source rocks in saline lacustrine basins show great mineralogical and organic geochemical heterogeneity and the controlling mechanism is weakly studied here we use the cenozoic qaidam basin as an example to compare the organic and inorganic parameters of fine grained sediments in different lacustrine facies and to figure out the characterization and distribution of effective petroleum source rocks in saline lacustrine systems organic inorganic database of six selected boreholes show that in the bench margin high gradient lacustrine system fine grained sediments in sublittoral brackish mesosaline zones containing low chlorides

record of emergent continental crust 3 5 billion years ago in the pilbara craton of australia isotopic data for the earth s oldest rocks1 7 imply that a considerable volume of continental crust existed during the early archaean aeon 3 0 gyr ago but it is not known when this crust first began to form emergent landmasses sedimentary geochemistry suggests8 9that the area of exposed continent was negligible until late in the archaean10 a contention supported by the fact that until now all greenstone supracrustal volcanic and sedimentary successions shown to have been deposited on eroded continental basement have yielded ages of 3 0 gyr here we report the discovery of an angular unconformity an ancient erosion surface beneath rocks of the 3 46 gyr warrawoona group in the pilbara craton of australia currently the oldest known well preserved greenstone succession below the unconformity low grade greenstones older than 3 5 gyr were intruded by voluminous granitoids before erosion as the overlying warrawoona rocks are only mildly metamorphosed slightly deformed and were deposited near sea level we infer that they accumulated on crust that was already rigid cool and buoyant thus by 3 46 gyr ago the sub warrawoona rocks formed an emergent block of continental crust the most ancient known

geochemistry and correlation of crude oils from reservoirs and source rocks in southern biyang sag nanxiang basin china abstract the paleogene hetaoyuan formation a sequence of gray to dark calcareous mudstone and dolomitic mudstones is the main source rock and reservoir in the biyang sag nanxiang basin eastern china organic geochemical analysis study performed on 57 source rock samples from the hetaoyuan formation demonstrates poor to abundant organic matter content ranging from 0 24 5 4 wt toc content characterized by oil prone type ii kerogen eleven crude oil samples from the hetaoyuan formation in southern biyang sag were analyzed using bulk properties and molecular methods to determine their maturity and origin the results suggest that the crude oils exhibit different maturity and source organic matter three groups of oils were identified by integrated analysis of gas chromatography gas chromatography mass spectrometry stable carbon isotope physical properties and gross composition of crude oil group a oils defined as low mature oil are characterized by high density viscosity sulfur content and low saturated hydrocarbon content this group of oils has relatively high values of ph n c 18 c 24 c 26 and gammacerane index indicating that the original organic matter had more contribution from terrigenous plants and was deposited in saline and stratified water columns under anoxic conditions group b oils defined as normal maturity oil are characterized by moderate density viscosity sulfur content and moderate saturated hydrocarbon content this group of oils has relatively low values of ph n c 18 c 24 c 26 and gammacerane index and displays a slight predominance of c 29 steranes indicating that the original organic matter had more contribution from marine algae and was deposited under sub oxic to anoxic conditions with low salinity group c oils defined as light oil are characterized by low density viscosity and abundant saturated hydrocarbons representative source rock samples were selected for further correlation the results of oil source rock correlation suggest that crude oils from the eh 2 member of hetaoyuan formation are mixture derived from source rocks in the eh 2 member and upper layer of the eh 3 member while oils from the upper and lower layer of the eh 3 member correlate with source rocks in the upper and lower layers of the eh 3 member respectively

geochemistry of the late archean banting group yellowknife greenstone belt slave province canada simultaneous melting of the upper mantle and juvenile mafic crust this study investigates the geochemistry and tectonic setting of the 2 66 ga banting group the younger sequence of volcanic rocks in the yellowknife greenstone belt and its relationship to older tholeiitic volcanic rocks of the 2 72 2 70 ga kam group the banting group includes a much higher proportion of felsic volcanic and volcaniclastic rocks than the kam group but mafic to intermediate volcanic rocks are common banting basalts are tholeiitic and are melts of archean depleted upper mantle as are basalts of the kam group in contrast banting dacites and rhyolites have much lower heavy rare earth element abundances and generally have higher initial 143nd 144nd than kam felsic rocks the chemistry of the felsic rocks provides a geochemical signature to distinguish rocks of kam versus banting age where complex structures have obscured the stratigraphy whereas kam felsic rocks evolved from mafic parents by assimilation fractional crystallization processes banting felsic rocks have compositions sim

bottom current depositional model and characterization of the paraburdoo member surrounding the paraburdoo spherule layer hamersley basin western australia abstract the stratigraphy within the wittenoom formation s paraburdoo member of the hamersley basin in western australia has lacked a significant and identifiable marker bed until the recent discovery of the paraburdoo spherule layer psl in this paper we correlate this stratigraphy residing at the three known exposed locations of the psl within the hamersley basin and use these correlations in conjunction with detailed observations and analyses to assess the type of sedimentation recorded by these alternating sand and mud beds we also characterize in detail these same several meters of stratigraphy using the psl as a distinctive marker bed and add to the existing depositional model of the hamersley basin we find that 1 the strata surrounding the psl was not deposited by sediment gravity flows but rather long acting deep marine bottom current s 2 the rhythmicity recorded as alternating sand and mud beds is the result of increasing and decreasing current velocities 3 grain size decreases while mud content increases to the east within the basin 4 bed thinning occurs to the west within the basin 5 microbial activity may be recorded in laminations in the finer grained mud beds present within these strata and 6 the diagenetic history changes across the hamersley basin

development of organic porosity in the woodford shale with increasing thermal maturity abstract using a combination of focused ion beam milling and scanning electron microscopy we describe the evolution of secondary organic porosity in eight woodford shale late devonian early mississippian samples with mean random vitrinite reflectance values ranging from 0 51 ro to 6 36 ro organic porosity was observed to be absent in samples with vitrinite reflectance values of up to 0 90 ro with the first appearance of secondary pores starting with the 1 23 ro sample porosity in the organic matter was unexpectedly absent in a sample with a vitrinite reflectance of 2 00 ro however organic pores were again found in samples with higher thermal maturities porosity when present did not appear to be uniformly distributed among the organic matter that was within less than a micron of each other suggesting important differences in composition of the organic matter thin regions of organic matter were observed between grains raising the possibility that small amounts of the deposited organic matter were compacted between grains to form thin layers and or the structures are part of the secondary organic matter interpreted to be post oil bitumen which was left behind as a residue during oil migration through the shale some regions of porous organic matter appeared to be grain protected whereas others did not which indicates that these non protected porous organic regions may be stress supporting with porosity intact under in situ reservoir conditions these observations suggest that thermal maturity alone is insufficient to predict porosity development in organic shales and other factors such as organic matter composition complicate porosity development

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

cretaceous and paleogene saline lacustrine source rocks discovered in the southern junggar basin nw china abstract saline lacustrine source rocks in china provide the resource basis for many giant oil fields here we report the first discovery of such source rocks in the piedmont area of the northern tianshan mountains in the southern junggar basin of nw china these source rocks are cretaceous and paleogene in age organic geochemistry and trace element data suggest that these mudstones were formed in a brackish saline lacustrine environment characterized by deep water and reducing conditions and in a warm and dry paleoclimate evidence for this environment includes mudstone pr ph values of 0 4 and negative ce and eu anomalies the total organic carbon toc contents of these cretaceous and paleogene mudstones are typically 0 5 with maximum values of 1 45 and 2 22 respectively signifying moderate to high organic matter abundance rock eval and kerogen analyses suggest that the organic matter was dominated by type ii kerogen the mudstones are generally thermally mature suggesting favorable conditions for hydrocarbon particularly oil generation oil source correlation confirms the oil generation of these two sets of source rocks whose development are controlled mainly by reducing environments these cretaceous and paleogene mudstones should therefore be a target for regional exploration in the southern junggar basin in addition to the known gas producing jurassic rocks this oil bearing potential is unique amongst the early cretaceous and eocene oligocene central asian petroliferous basins as the eastern portion of this realm is considered to host predominantly jurassic coal based source rocks giving rise to numerous large gas fields rather than oil resources

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

cooling of the earth in the archaean consequences of pressure release melting in a hotter mantle a model is presented to describe the cooling of the earth in the archaean at the higher archaean mantle temperatures pressure release melting starts deeper and generates a thicker basaltic or komatiitic crust and depleted harzburgite layer compared with the present day situation intrinsic compositional stability and lack of mechanical coherency renders the mechanism of plate tectonics ineffective it is proposed that the archaean continents stabilised early on top of a compositionally stratified root in the archaean oceanic lithosphere hydrated upper crust can founder and recycle through its high pressure phase eclogite eclogite remelting and new pressure release melting generates new crustal material migration of magma and latent heat release by solidification at the surface provides an efficient mechanism to cool the mantle by several hundreds of degrees during the archaean this can satisfactorily explain the occurrence of high extrusion temperature komatiites and lower extrusion temperature basalts in greenstone belts as being derived from the same source by different mechanisms

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

seismic constraints on dynamics of the mantle of the kaapvaal craton abstract shear wave splitting in the upper mantle of the kaapvaal craton in southern africa is measured by applying the sks technique to the records of a portable array of digital broad band seismographs the data are interpreted in terms of lattice preferred orientation in olivine fast wave polarization directions at all stations of the array are close to the absolute plate motion apm direction of southern africa since the end of the jurassic this alignment is similar to that previously reported for the north american craton and suggests that the large scale component of mantle anisotropy on both sides of the atlantic can be related to resistive drag at the base of the plates a likely depth range of the corresponding deformation is 150 400 km analysis of the mantle converted phases suggests that the high velocity mantle root of the kaapvaal craton which translates coherently with the plate resides in the same depth range the bottom of the root is found at a depth close to 380 km we conclude that the root is deformed by the recent plate motion but the deformations are not strong enough to be seen in the available tomographic models

vaalbara earth s oldest assembled continent a combined structural geochronological and palaeomagnetic test the only remaining areas of pristine 3 6 2 7 ga crust on earth are parts of the kaapvaal and pilbara cratons general similarities of their rock records especially of the overlying late archean sequences suggest that they were once part of a larger vaalbara supercontinent here we show that the present geochronological structural and palaeomagnetic data support such a vaalbara model at least as far back as 3 1 ga and possibly further back to 3 6 ga vaalbara fragmented prior to 2 1 ga and possibly as early as 2 7 ga suggesting supercontinent stability of at least 400 myr consistent with neoproterozoic and phanerozoic analogues

organic geochemical characteristics and organic matter enrichment of mudstones in an eocene saline lake qianjiang depression hubei province china abstract the qianjiang depression is a typical eocene hypersaline lake basin developing good source rocks in the qianjiang formation which have recently been identified as a potential target for saline lacustrine shale oil exploration previous studies have only focused on estimates of source rock potential and reservoir quality however studies on organic matter enrichment mechanisms in evaporative environments are limited based on organic geochemical and mineralogical data as well as sedimentological analyses we researched the factors controlling differences in organic matter enrichment in the basin the results show that total organic carbon toc contents are closely associated with lithofacies with distinct variations three main lithofacies associations las were identified 1 glauberite rich la1 with medium toc 2 carbonate rich la2 with high toc and 3 anhydrite rich la3 with low toc the distribution of biomarkers from 14 samples indicate both higher land plants and aquatic organisms were involved in the organic matter accumulations combining experimental results on gammacerane c30 hopane extended tricyclic terpane ratio etr sr ba v ni and sedimentary characteristics we analyzed the sedimentary environments of the three las la1 was deposited in a salinity stratified waterbody with strong evaporation la2 was deposited in a still stratified deep waterbody whereas la3 was deposited in a disturbed saline waterbody sedimentary environments not only controlled the formation of the lithofacies but also influenced the organic matter sources and preservation relatively low salinity strong anoxic condition without physical disturbance appropriate nutritional terrigenous material inputs and algal blooms produced the best organic matter enrichment in the saline lake system

salt structures and hydrocarbon accumulations in the tarim basin northwest china the tarim basin is one of the most important hydrocabon bearing evaporite basins in china four salt bearing sequences the middle and lower cambrian the mississippian the paleogene and the neogene have various thickness and areal distribution they are important detachment layers and intensely affect the structural deformation in the basin the kuqa depression is a subordinate structural unit with abundant salt structures in the tarim basin salt overthrusts salt pillows salt anticlines salt diapirs and salt withdrawal basins are predominant in the depression contraction that resulted from orogeny played a key function on the formation of salt structures growth strata reveal that intense salt structural deformation in the kuqa depression occurred during the himalayan movement from oligocene to holocene with early structural deformation in the north and late deformation in the south growth sequences also record at least two phases of salt tectonism in the yingmaili tahe and tazhong areas low amplitude salt pillows are the most common salt structures and these structures are commonly accompanied by thrust faults the faulting and uplifting of basement blocks controlled the location of salt structures the differences in the geometries of salt structures in different regions show that the thickness of the salt sequences has an important influence on the development of salt cored detachment folds and related thrust faults in the tarim basin salt sequences and salt structures in the tarim basin are closely linked to hydrocarbon accumulations oil and gas fields have been discovered in the subsalt intrasalt and suprasalt strata salt deformation has created numerous potential traps and salt sequences have provided a good seal for the preservation of hydrocarbon accumulations large and small scale faults related with salt structures have also given favorable migration pathways for oil and gas when interpreting seismic profiles special attention needs to be paid to the clastic and carbonate interbeds within the salt sequences because they may lead to incorrect structural interpretation in the tarim basin the subsalt anticlinal traps are good targets for hydrocarbon exploration

stratigraphy and sedimentology of an early archean felsic volcanic sequence eastern pilbara block western australia with special reference to the duffer formation and implications for crustal evolution abstract a sequence of early archean felsic volcanic and volcaniclastic rocks and chert is present in the upper half of the warrawoona group throughout greenstone belts in the eastern pilbara block western australia it includes three formations the duffer formation which makes up the bulk of the sequence consists of up to 5 km of metamorphosed dacitic to rhyolitic volcaniclastic breccia and lava and lesser amounts of tuffaceous rocks and mafic lava it is overlain in several sections by the sandy panorama formation and both are capped by silicified orthochemical and biogenic deposits of the towers formation including both the strelley pool and marble bar cherts these units are identifiable in at least six greenstone belts in the eastern pilbara block the duffer formation is divided into four facies a structureless and graded breccia b complexly organized conglomerate c turbiditic tuff breccia and tuff and d lava these facies represent deposition on predominantly subaqueous volcaniclastic aprons flanking felsic volcanic centers a three stage model of the evolution of these aprons includes 1 an early stage of felsic and mafic eruptions and subaqueous deposition 2 a middle stage during which the bulk of the sequence was deposited involving shallow water to subaerial felsic volcanism and subaqueous deposition and 3 a terminal stage of subaerial felsic eruptions and subaerial deposition post duffer felsic volcanism was marked by explosive pyroclastic activity which resulted in widespread deposition of ash in surrounding shallow water and subaerial systems this phase of activity is represented by the panorama formation after the cessation of volcanism erosion of the emergent vents and regional subsidence led to an interval of orthochemical and biogenic sedimentation represented by the towers formation paleocurrent and facies patterns in the duffer and panorama formations support earlier proposals that the intrusion of granitoid rocks in the eastern pilbara block was coeval with felsic volcanism it seems likely that this volcanism was the surficial expression of plutonism that represented early stages of cratonization in the eastern pilbara the calc alkaline volcanism and depositional styles represented by this sequence are similar to those associated with many phanerozoic arcs the abundance of shallow water facies closely spaced and possibly non linear distribution of felsic centers close association of felsic units with thick sequences of basalt erupted at or near sea level sparse syn volcanic deformation and lack of granitic detritus suggest however that the setting of this archean felsic volcanism was in many details unlike that of most modern arcs

a review of crust upper mantle structure in the precambrian areas of australia and implications for precambrian crustal evolution the western two thirds of the australian continent are underlain by precambrian rocks seismically this region is characterised by negative teleseismic travel time residuals indicating a thick cold tectosphere despite extensive platform cover and phanerozoic crustal reworking in a few intracratonic sedimentary basins the precambrian crust can be classified broadly into archaean and proterozoic cratons and proterozoic mobile belts the archaean cratons have a two layered crust 25 35 km thick with observed velocities in the lowermost crust in the range 6 8 7 2 km s 1 the moho is a distinct feature with a velocity contrast of 1 0 km s 1 vertical reflection probes in the southern yilgarn block yielded record sections that have numerous laterally discontinuous but distinct reflections especially in the lower crustal layer that are especially concentrated around the boundary between the two crustal layers and the moho the seismic reflections and velocities are consistent with a predominantly felsic crust with mafic lenses constituting 20 35 of the lowermost crust and with the metamorphic grade ranging from granulite in the upper to middle crust to garnet granulite and even eclogite at the base of the crust the proterozoic mobile belts and cratons a have multi layered crust 45 50 km thick velocities below 25 km depth are almost always 7 0 km s 1 and reach 7 4 7 6 km s 1 in the lowermost crust velocities such as these are characteristic of gabbroic basaltic underplate the velocity contrast is 1 0 km s 1 at the moho which is less distinct than in archaean terranes record sections from vertical reflection probes show a highly reflective crust with no correlation between seismic layers in the refraction models and the distribution of vertical reflections the contrasting character of the crust in archaean and proterozoic provinces favours a tectonic model for the evolution of precambrian crust in which predominantly felsic archaean protocrust was tectonically reworked and underplated to produce proterozoic crust the simple two layered protocrust became more complicated in the process the introduction of vast amounts of underplate especially near the crust mantle boundary thickened the crust and destroyed the character of the moho it overprinted the reflection character of the crust so that the bands of reflections now do not correlate with the boundaries between the crustal layers the velocities in the lower crust are high because of the underplate

generation of earth s early continents from a relatively cool archean mantle this research has been supported by dfg grant spp 1833 building a habitable earth and magma consolidator grant erc project 71143

garnet growth during amphibolite anatexis implications of a garnetiferous restite formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes we have experimentally simulated the dehydration melting of amphibolite and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage the restite with an emphasis on garnet a natural low k calcic amphibolite was powdered and run in sealed gold capsules in a piston cylinder apparatus at 10 kbar 750 1000 c 1 to 9 day runs and fo 2 ni nio garnet first appears in 8 day runs at 825 25 c above 850 c clinopyroxene and poikilitic garnet comprise most of the restite garnet peaks at 950 c at 32 vol of the material within the capsule or 41 vol of the restite assemblage and disappears between 975 1000 c there is no cross over of fe mg partitioning between coexisting liquids and garnets liquids remain mg poorer than garnets throughout the melting interval but do increase in mg with increasing temperature thus the hotter liquids may be less likely to react with mantle wedge peridotite the modes are consistent with ree patterns of archean tonalitic trondhjemitic rocks generated by the partial melting of garnet bearing mafic crust a garnet clinopyroxenite with 41 vol garnet has a density of 3 5g cm 3 which may induce lower crustal delamination in hot nonthickened amphibolitic crust this crust is seismically indistinguishable from lithosphere or mantle because garnet growth traps water bearing hornblende inclusions the restite may contain up to 0 3 wt water thus delivery of poikilitic garnet to the mantle possibly by delamination or by subduction may be an important mechanism for adding water to the mantle

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

organic geochemical characteristics and oil generating potential of the upper jurassic safer shale sediments in the marib shabowah basin western yemen the organic rich safer shales exposed in the north central part of onshore marib shabowah basin are evaluated and their depositional environments are interpreted total organic carbon contents toc of the shales range from 1 02 16 8 wt and yield hydrogen index hi values ranging from 130 to 820 mg hc g toc consistent with mainly type ii with minor contributions from type i and mixed types ii iii kerogens the safer shale samples have vitrinite reflectance values in the range of 0 5 1 0 ro indicating early mature to peak mature stage for oil generation tmax values range from 429 438 c which are in reasonably good agreement with vitrinite reflectance data kerogen microscopy shows that the safer shales are characterized by high amounts of organic matter consisting predominantly of yellow fluorescing amorphous organic matter and alginite of marine origin this is supported by their high content of hydrogen rich type ii and i oil prone kerogen the biomarker distributions of the upper jurassic safer extracts are characterized by dominant low to medium molecular weight compounds n c14 to n c20 low pr ph ratio 1 0 high phytane n c18 ratios 0 82 2 68 and predominant regular sterane c27 all biomarker parameters clearly indicate that the organic matter was derived from marine algal inputs and deposited under anoxic reducing conditions hypersaline conditions also prevailed during deposition of these sediments as indicated by the presence of gammacerane

organic geochemistry and petrology of lower cretaceous black shales in the qiangtang basin tibet implications for hydrocarbon potential abstract lower cretaceous black shales have recently been discovered in the qiangtang basin especially in its northern basin tibet implying a new interval with hydrocarbon resource potential in the region this potential however has not been investigated to date and is addressed in this paper based on organic petrology and geochemistry data from the representative shengli river outcrop organic petrology including optical and scanning electron microscope observations indicates that the organic matter om within these shale sequences has precursors from abundant benthic algae some bacteria and amorphous om and a few land plants suggesting that the black shales were deposited in a generally reducing platform lagoonal environment favorable for om preservation the shales have high total organic carbon toc contents 1 74 7 71 wt type ii kerogen tending to type iii and high thermal maturity vitrinite reflectance equivalent of ca 1 3 ro biomarkers in the shales suggest deposition under reducing brackish or saline water with aquatic benthic organisms bacteria and amorphous om as the dominant input thus the results of organic petrology organic geochemistry and biomarker geochemistry are generally consistent and imply that the shengli river black shale samples have significant hydrocarbon resource potential and most likely produced gas due to relatively high maturity and gas prone kerogen the neighboring black shales within the same stratigraphic interval as the shengli river shales if having moderate om maturity and oil prone kerogen can be expected to generate oil the presence of well developed micropores and fractures and abundant brittle minerals within the black shales suggests that they may have unconventional hydrocarbon resource potential especially in intervals with high toc 4 0 wt these results provide new data and understanding for regional hydrocarbon exploration

late cretaceous late turonian coniacian and santonian petroleum source rocks as part of an oae tarfaya basin morocco abstract late turonian coniacian and santonian source rock samples from a recently drilled well tafaya sondage no 2 2010 in the tarfaya basin were analyzed for quantity quality maturity and depositional environment of the organic matter om to our knowledge such a thick sequence of organic matter rich turonian to santonian source rocks was investigated in that great detail for the first time organic geochemical and organic petrological investigations were carried out on a large sample set from the 200 m thick sequence in total 195 core samples were analyzed for total organic carbon corg total inorganic carbon contents and total sulfur ts contents rock eval pyrolysis and vitrinite reflectance measurements were performed on 28 samples chosen on the basis of their corg content non aromatic hydrocarbons were analyzed on selected samples by way of gas chromatography flame ionization detection gc fid and gc mass spectrometry gc ms the organic matter rich carbonates revealed a high source rock potential representing type i kerogen and a good preservation of the organic matter which is mainly of marine phytoplankton origin hi values are high 400 900 mg g corg and in a similar range as those described for more recent upwelling sediments along the continental slope of north africa ts corg ratios as well as pristane over phytane ratios indicate variable oxygen content during sediment deposition all samples are clearly immature with respect to petroleum generation which is supported by maturity parameters such as vitrinite reflectance 0 3 0 4 tmax values 401 423 c production indices s1 s1 s2 0 1 as well as maturity parameters based on ratios of specific steranes and hopanes

the origin type and preservation of organic matter of the barremian aptian organic rich shales in the muglad basin southern sudan and their relation to paleoenvironmental and paleoclimate conditions barremianeaptian organic rich shales from abu gabra formation in the muglad basin were analysed using geochemical and petrographic analyses these analyses were used to define the origin type of organic matters and the influencing factors of diagenesis including organic matter input and preservation and their relation to paleoenvironmental and paleoclimate conditions the bulk geochemical characteristics indicated that the organic rich shales were deposited in a lacustrine environment with seawater influence under suboxic conditions their pyrolysis hydrogen index hi data provide evidence for a major contribution by type i ii kerogen with hi values of 400 mg hc g toc and a minor type ii iii contribution with hi values 400 mg hc g toc this is confirmed by kerogen microscopy whereby the kerogen is characterized by large amounts of structured algae botryococcus and structureless amorphous with a minor terrigenous organic matter input an enhanced biological productivity within the photic zone of the water columns is also detected the increased biological productivity in the organicrich shales may be related to enhanced semi arid humid to humid warm climate conditions therefore a high bio productivity in combination with good organic matter preservation favoured by enhanced algae

volcanic sedimentary and tectonostratigraphic environments of the 3 46 ga warrawoona megasequence a review abstract the warrawoona megasequence comprises a 3 46 ga assemblage of tholeiitic and calc alkaline volcanic rocks interlayered with cherty sedimentary rocks deposition occurred in a range of shallow to deeper water environments ranging from the shoreline to distal deposits in sediment starved basins volcanism provided the dominant or only source of clastic sediment sedimentary rocks also contain evaporites probable stromatolites other evidence for the existence of microbial life and possible evidence of meteorite impact the tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic arc or near arc settings the compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc alkaline suites developed above subduction zones the calc alkaline suite has a complex petrogenesis which probably involves mantle derived basaltic melts intermediate to silicic melts derived from subducted mafic crust magma mixing and fractional crystallization intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the warrawoona megasequence contains examples of early archaean volcanic arc and near arc assemblages

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

evolutionary characteristics of the sags to the east of tan lu fault zone bohai bay basin china implications for hydrocarbon exploration and regional tectonic evolution abstract some cenozoic half grabens sags have been found to the east of the tan lu fault zone in the bohai bay basin east china the evolutionary history of these sags is consistent with the entire bohai bay basin besides some remarkable characteristics which had not been mentioned in previous studies of the bohai bay basin are presented and they are the keys to understand the regional tectonic evolution among them two characteristics are emphasized 1 the structural and evolutionary features of the sags were markedly changed at about 38 ma 2 the paleogene basin underwent several times of uplifts and denudations during paleogene the first characteristic shows that the normal faulting was the dominant structural deformation before 38 ma and after then the strike slip movement might initiate to influence the basin the second characteristic indicates remarkable reducing of the present day residual basin relative to the original basin and implies the episodic uplifting of the jiaoliao massif besides our estimation on the hydrocarbon exploration potential of these sags shows that part of them can sustain the formation of medium large oilfield independently while part can contribute to the formation of large oilfield

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

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

zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the ivrea verbano and strona ceneri zones nw italy abstract in order to improve the understanding of thermal tectonic evolution of high grade terranes we conducted a systematic study of textures ree content and u pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the ivrea verbano and strona ceneri zones italy this study documents the behaviour of these accessory minerals in the presence of melt the absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting the retrograde path is marked by i growth of new zircon rims r1 and r2 around inherited cores in the mesosome ii crystallisation of stubby zircon grains in the leucosome especially at granulite facies and iii crystallisation of new monazite in the mesosome stubby zircon grains have a distinctive fir tree zoning and a constant th u ratio of 0 20 together these features reflect growth in the melt conversely the new zircon grains with r1 rims have dark prismatic habits and th u ratios in the ivrea verbano zone the interquartile range iqr of u pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt accordingly they provide an indication on the minimum duration for high temperature metamorphism and partial melting of the lower crust 20 ma and 30 ma in amphibolite and granulite facies respectively in amphibolite facies zircon crystallisation between 310 and 294 ma iqr is interpreted to reflect metamorphic peak condition and earlier retrograde history conversely monazite crystallisation between 297 and 271 ma iqr reflects cooling under 750 c to a temperature close to the solidus in granulite facies zircon crystallisation between 295 and 265 ma iqr is interpreted to reflect high temperature conditions which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850 950 c the observed decrease of u pb ages in metamorphic zircon and monazite from amphibolite to granulite facies i e from the middle to the lower crust is interpreted to record slow cooling and crystallisation of the variscan orogenic root at the transition from orogenic collapse to opening of the tethys ocean

detrital zircon u pb lu hf and o isotopes of the wufoshan group implications for episodic crustal growth and reworking of the southern north china craton abstract the paleoproterozoic orogens of the north china craton ncc provide important windows to evaluate the crustal evolution history during early precambrian here we report results from sims zircon geochronology and oxygen isotopes as well as la icpms zircon hf isotopic analyses on detrital zircons from mesoproterozoic neoproterozoic sandstones of the wufoshan group from the southern ncc to probe the paleoarchean paleoproterozoic crustal evolution our data show episodic magmatism during the paleoarchean neoarchean and paleoproterozoic as inferred from the 207pb 206pb age range of 3514 2751 ma and the well defined 207pb 206pb weighted mean ages of 2669 15 ma 2510 8 ma 2402 39 ma 2298 36 ma 2179 10 ma 2058 16 ma 1940 6 ma 1874 7 ma and 1819 5 ma the similarity in u pb geochronology and hf isotopes of the archean detrital zircons from the wufoshan group to those from the paleoproterozoic songshan group reveal that the 2669 ma zircons record a major phase of neoarchean crustal growth and that 2510 ma zircons correspond to the latest neoarchean crustal reworking the 18o depleted isotopic features displayed by some detrital zircons are absent in those from songshan group and together with the rarity of core rim textures which were widespread in archean zircons from the songshan group we suggest that the source materials for the mesoproterozoic neoproterozoic wufoshan group are different from the archean cratonic source for the paleoproterozoic songshan group we propose that the zircons were probably sourced from the newly formed trans north china orogen tnco our paleoproterozoic zircon data provide robust evidence for continuous and complex reworking of the mesoarchean to early neoarchean crust in the southern ncc during early paleoproterozoic at ca 2402 ma 2298 ma and 2179 ma the middle paleoproterozoic ages 2058 ma 1940 ma and 1874 ma mark a partial cycle of oceanic crust subduction to continental collision during the formation of the tnco the extreme oxygen isotopic variation with the fairly restricted range in hf isotopes demonstrates that the components from both high temperature and the low temperature rock water interaction induced by the release of hydrothermal fluids from the subducted altered oceanic crust must have played a significant role during the growth of the zircons the paleoproterozoic thermal event at 1819 ma records post collisional uplifting as evidenced by an apparent negative correlation in the hf t vs 18o plot possibly inherited from the mixing of the ancient crust and the juvenile input during the oceanic subduction and continental collision integrated with the information from granulite facies metamorphic rocks in the orogenic root of the tnco our data suggest that the episodic paleoproterozoic magmatism was mainly induced by the tectonics of plate subduction and subsequent continental collision leading to the final cratonization of the ncc following the mesoproterozoic neoproterozoic sedimentation of the paleoproterozoic zircons except for the pb loss during the subsequent thermal events there was no major alteration in the hf and oxygen isotopic compositions thus the remarkably large variation of oxygen isotopes from the lower than normal mantle to the supracrustal value 18o 2 28 10 09 in the paleoproterozoic detrital zircons further demonstrates that these zircons came from the uplifted and exposed upper part of the tnco our detrital zircon data provide convincing evidence for the repeated crustal reworking in early paleoproterozoic and the onset of modern style plate tectonics in the ncc in middle paleoproterozoic

archaean crustal growth through successive partial melting events in an oceanic plateau like setting in the tanzania craton the detrital zircon population in quartzitic conglomerates from the northern tanzania craton yield ages between 2640 ma and 2790 ma which includes most of the igneous history from this part of the craton the igneous evolution is characterised by mafic volcanism with an oceanic plateau like geochemical signature at 2800 ma followed by diorite and tonalite trondhjemite granodiorite dominated magmatism between 2790 and 2700 ma which transitioned into more evolved high k magmatism between 2700 and 2620 ma the ehf values of the detrital zircons range from 2 4 to 1 4 and change with time from radiogenic hf pre 2700 ma 98 positive ehf to unradiogenic hf post 2700 ma 41 positive ehf the petrological progression from mafic to felsic crust is reflected in the detrital age distribution and ehf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic plateau like environment

characterization of gas shale pore systems by porosimetry pycnometry surface area and field emission scanning electron microscopy transmission electron microscopy image analyses examples from the barnett woodford haynesville marcellus and doig units the nanometer scaled pore systems of gas shale reservoirs were investigated from the barnett marcellus woodford and haynesville gas shales in the united states and the doig formation of northeastern british columbia canada the purpose of this article is to provide awareness of the nature and variability in pore structures within gas shales and not to provide a representative evaluation on the previously mentioned north american reservoirs to understand the pore system of these rocks the total porosity pore size distribution surface area organic geochemistry mineralogy and image analyses by electron microscopy were performed total porosity from helium pycnometry ranges between 2 5 and 6 6 total organic carbon content ranges between 0 7 and 6 8 wt and vitrinite reflectance measured between 1 45 and 2 37 the gas shales in the united states are clay and quartz rich with the doig formation samples being quartz and carbonate rich and clay poor higher porosity samples have higher values because of a greater abundance of mesopores compared with lower porosity samples with decreasing total porosity micropore volumes relatively increase whereas the sum of mesopores and macropore volumes decrease focused ion beam milling field emission scanning electron microscopy and transmission electron microscopy provide high resolution 5 nm images of pore distribution and geometries image analysis provides a visual appreciation of pore systems in gas shale reservoirs but is not a statistically valid method to evaluate gas shale reservoirs macropores and mesopores are observed as either intergranular porosity or are confined to kerogen rich aggregates and show no preferred orientation or align parallel with the laminae of the shale networks of mesopores are observed to connect with the larger macropores within the kerogen rich aggregates

pore types in the barnett and woodford gas shales contribution to understanding gas storage and migration pathways in fine grained rocks the identification of organoporosity microscale and nanoscale pores within organic matter in shales its importance to storage and perhaps transfer of gas molecules through shales and methods for gathering three dimensional images of the pores such as by argon ion milling and or field emission scanning electron microscopy have all been well documented and discussed for unconventional gas shales however other types of pores exist within shales that can be important to storage and migration of gas and oil and other technologies are available for their identification and imaging the different pore types found in the barnett and woodford shales are described and classified in this article scanning electron microscopy revealed the presence of porous floccules in both the barnett and woodford shales which appear similar to laboratory produced floccules and to those in other ancient shales published experimental and observational studies indicate that floccules are hydraulically equivalent to coarser grains and are transported by traction processes current induced microsedimentary structures and textures within the barnett and woodford shales as well as the preserved floccules suggest such processes were active during transport and deposition pore spaces between the floccules are open and can provide storage space as well as permeability pathways for gas molecules through the shales pores are also found within organic matter that occurs both as discrete particles and as adsorbed coatings around clay grains in the shale they are referred to here as organopores porous fecal pellets are also common in the barnett shale preserved fossil fragments such as organic walled spores and inorganic sponge spicules have hollow central chambers which may remain partially or completely open even after burial intraparticle pores occur between grains of various minerals e g pyrite framboids microchannels within shale matrix which may be the bounding surfaces of scours or microsedimentary structures may also provide permeability pathways for hydrocarbon migration mircrofractures are also common and their initiation might be related to mineral crystal structure when present in sufficient quantity these pore types offer potential gas and oil molecule storage spaces and permeability pathways through the shales

on the formation of continental silicic melts in thermochemical mantle convection models implications for early earth abstract important constituents of archean cratons formed in the early and hot history of the earth are tonalite trondhjemite granodiorite ttg plutons and greenstone belts the formation of these granite greenstone terrains is often ascribed to plate tectonic processes buoyancy considerations however do not allow plate tectonics to take place in a significantly hotter earth we therefore propose an alternative mechanism for the coeval and proximate production of ttg plutons and greenstone like crustal successions that is when a locally anomalously thick basaltic crust has been produced by continued addition of extrusive or intrusive basalts due to partial melting of the underlying convecting mantle the transition of a sufficient amount of basalt in the lower crust to eclogite may trigger a resurfacing event in which a complete crustal section of over 1000 km long sinks into the mantle in less than 2 million years pressure release partial melting in the complementary upwelling mantle produces large volumes of basaltic material replacing the original crust partial melting at the base of this newly produced crust may generate felsic melts which are added as intrusives and or extrusives to the generally mafic crustal succession adding to what resembles a greenstone belt partial melting of metabasalt in the sinking crustal section produces a significant volume of ttg melt which is added to the crust directly above the location of subduction presumably in the form of a pluton this scenario is self consistently produced by numerical thermochemical mantle convection models presented in this paper including partial melting of mantle peridotite and crustal meta basalt the metamorphic p t conditions under which partial melting of metabasalt takes place in this scenario are consistent with geochemical trace element data for ttgs which indicate melting under amphibolite rather than eclogite facies other geodynamical settings which we have also investigated including partial melting in small scale delaminations of the lower crust at the base of a anomalously thick crust and due to the influx of a lower mantle diapir fail to reproduce this behavior unequivocally and mostly show melting of metabasalt in the eclogite stability field instead

carbonate precipitation in the alkaline lake specchio di venere pantelleria island italy and the possible role of microbial mats abstract alkaline lakes like the hydrothermally affected lake specchio di venere pantelleria island central mediterranean are typical geological settings harbouring calcified microbial mats the present work is focused on the discrimination between biotic and abiotic processes driving carbonate precipitation in this lake using hydrochemical mineralogical and isotopic data hydrochemical analyses demonstrate that the lake is nearly 10 fold supersaturated with regard to aragonite and seasonally reaches hydromagnesite supersaturation microscopic observations depict organosedimentary laminated structures consisting of microbial communities and aragonitic precipitates which are rather disseminated in pores than directly linked to microorganisms oxygen isotopic data indicate that authigenic carbonate crystallisation from evaporating water is the dominant precipitation process further suggested by the absence of textural evidence of diagenetic processes conversely the observed 13c values reflect an influence of extracellular polymeric substances eps on carbon fractionation during the precipitation process due to the selective sequestration of 12c in the biomass the above considerations suggest that at lake specchio di venere the carbonate precipitation is mainly of inorganic nature but a minor role played by biologically influenced processes in microbial mats is not excluded

origin of the 3500 3300 ma calc alkaline rocks in the pilbara archaean isotopic and geochemical constraints from the shaw batholith abstract calc alkaline plutonic rocks intruded at 3450ma comprise a major component of the shaw batholith in the archaean east pilbara block western australia new whole rock pb isotopic geochronology confirms the extent of these rocks but a minor plutonic phase is dated at 3338 52 ma and represents a second plutonic event of the same age as much of the nearby mt edgar batholith the sm nd isotopic systematics of the 3450ma rocks imply their derivation from a heterogeneous source which probably included a slightly older crustal component as well as a depleted mantle component the 3338 52 ma pluton includes components derived from crustal sources older than 3600 ma the geochemistry and sm nd isotopic systematics of these rocks are consistent with crustal growth in the early archaean from upper mantle sources as depleted as the modern upper mantle the shaw batholith calc alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation a suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals principally hornblende and possibly sphene magnetite and epidote and into a leucocratic component containing quartz plagioclase and k feld par these archaean calc alkaline plutonic rocks in common with rocks from many other archaean calc alkaline provinces exhibit very fractionated ree patterns with depleted hree contents a feature considered to result from equilibrium with garnet at depth in lower crustal regions the geochemistry of the pilbara archaean calc alkaline rocks is identical to the subset of modern continental margin calc alkaline plutonic rocks with fractionated ree patterns such as those from the central and eastern peninsular ranges batholith western usa the tectonic setting in which the archaean calc alkaline rocks formed is still not known this reflects both uncertainty associated with the petrogenesis and environments of modern calc alkaline rocks as well as the limited knowledge of the precise timing and relationships of plutonic depositional and tectonic events in the pilbara archaean

using distributions and stable isotopes of n alkanes to disentangle organic matter contributions to sediments of laguna potrok aike argentina when using biomarkers such as n alkanes as tools for paleo environmental reconstructions it is imperative to determine their specific sources for each setting toward that goal we analyzed a set of various potential organic matter om sources such as aquatic and terrestrial plants dust and soils from laguna potrok aike lpa and surrounding areas in southern patagonia we determined chain length distributions and hydrogen d and carbon 13c isotopic compositions of n alkanes of different om sources in order to quantify their relative contributions to lake sediments our results reveal that the mid chain n alkane n c23 is predominantly produced by submerged aquatic plants whereas long chain n alkanes n c29 n c31 are derived from various terrestrial sources we estimated their relative contributions to the sediment using two approaches i e based on the n alkane distributions and their d and 13c values both approaches result in similar estimates of aquatic and terrestrial contributions for mid and long chain n alkanes to the sediment 62 73 of the mid chain n c23 alkanes originate from aquatic sources while 66 77 of the long chain n alkanes originate from dust and 14 30 from terrestrial plants our study shows that mid chain n alkanes such as the n c23 alkane in lpa are derived mainly from aquatic macrophytes and thus have the potential to record changes in lake water isotopic composition in contrast the n c29 alkane reflects the isotopic signal of various terrestrial sources from southern patagonia

structural evolution of the middle archean coastal pilbara terrane western australia 1 the middle archean coastal pilbara terrane is composed of two lithotectonic units the karratha and cleaverville roebourne supercomplexes the karratha supercomplex lies tectonically beneath the cleaverville roebourne supercomplex and consists of 3270 3250 ma granitic batholiths and metamorphosed terrigenous sediments it is identified as a juvenile continental crust with tonalite trondhjemite granodiorite type granites the cleaverville roebourne supercomplex is interpreted as a dismembered immature island arc which has a restored crustal thickness of 20 km this supercomplex consists of 3195 ma bimodal volcanic chemical sedimentary sequences metabasite low k granitic gneiss and peridotite regionally distributed granite porphyries circa 3020 3000 ma have intruded all of the coastal pilbara terrane detailed structural analyses and zircon u pb dating suggest that the coastal pilbara terrane records two phases of deformation the first phase is an immature island arc and micro continental crust collision and is represented by top to the northwest thrusting d1 circa 3100 3020 ma this phase of deformation an arc continent collision appears to represent an important transition in the formation of continental crust the 3020 ma granite porphyry event may be related to syn post collision igneous activity and may reflect a second crustal thickening associated with magmatic underplating the second phase of deformation is characterized by regional scale left lateral strike slip faulting d2 circa 3020 2925 ma and by more localized right lateral strike slip faulting d3 circa 2925 2770 ma this phase of craton scale strike slip faulting may represent stabilization of the newly formed continental crust

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

metamorphism and melting of picritic crust in the early earth abstract partial melting experiments with models of archean oceanic crust maoc with 11 13 and 15 wt mgo are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of ttg like melts and continental crust on the early earth the approach investigates the possibility that the average early crust composition was comparatively mgo rich and evolved to lower magnesium content during the secular cooling of the earth high pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in maoc 15 to predominantly tonalitic melts in maoc 11 and higher melting temperatures with increasing magnesium in the bulk composition tonalitic melts were generated in maoc 11 and 13 at pressures 12 5 kbar along with the residual phases garnet clinopyroxene plagioclase quartz orthopyroxene in the presence of quartz and at lower pressures in the absence of amphibole basaltic melts were generated at pressures 15 kbar predominantly in the presence of granulite facies residues such as amphibole garnet plagioclase orthopyroxene that lack quartz in all maoc compositions the tonalitic melts generated in maoc 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern morb is a viable source for the generation of early archean high si low mgo melts and therefore ttg like continental crust in the archean the favoured settings for the generation of these melts at pressures up to 15 kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones both without interaction of the melts with the mantle during passage to the surface tonalitic melts may have formed in deeper subduction zones at 20 kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction this process may have become more important during the progressive cooling of the earth

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events