new age constraints for the proterozoic aravalli delhi successions of india and their implications abstract proterozoic sedimentary successions of india are important archives of both the tectonic history of the indian subcontinent and the geochemical evolution of earth surface processes however the lack of firm age constraints on many of these stratigraphic units limits their current utility here we present new detrital zircon age data from strata of the southern aravalli delhi orogenic belt adob and the rajasthan vindhyan successions the alwar group of the southern delhi supergroup yielded a large population of 1 2 ga detrital zircon grains which refutes the 1 9 1 7 ga age assertion for this unit detrital zircon age distributions from the southern alwar group differ strongly from the alwar group of the north delhi belt demonstrating miscorrelation between these two regions the jhamarkotra formation of the lower aravalli group contains a large population of 1 9 1 7 ga detrital zircon grains therefore the unit cannot be 2 1 ga as traditionally assumed age distributions of the aravalli and delhi supergroups are similar to those of the lower and upper vindhyan successions and we postulate contiguous sediment sources for both regions with strata of the tectonically deformed adob representing the distal margin equivalents of the vindhyan successions additionally a late paleoproterozoic age for the jhamarkotra formation nullifies the hypothesis that the markedly positive carbonate 13c values in this unit are linked to the 2 3 2 0 ga lomagundi jatuli positive isotope excursion the potential of a large late paleoproterozoic ca 1 7 ga positive 13c excursion contrasts with the long held view of a prolonged period of carbon isotope stasis during the so called boring billion

siliceous sponge expansion and phosphogenesis in a shallow water environment in the malyi karatau range kazakhstan during the precambrian cambrian transition abstract ocean oxygenation during the precambrian cambrian transition is thought to be one of the key factors for the rapid diversification of animals known as the cambrian explosion siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth resulting in an increase in oxygen levels of shallow water to assess the global importance of the impact of ecosystem engineering by sponges we evaluated the palaeo environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the precambrian cambrian transition we investigated three lithosections in the malyi karatau range in southern kazakhstan the lowermost chert samples of these sections are typically characterized by comparably low 30si values and exhibit molds of siliceous sponge spicules upsection the occurrence of cross bedding structures and granular phosphorites document the shallowing of the depositional environment where overall increasing 30si values and increasing al si indicate less settlement of sponges and higher suspended particle loads corresponding carbon isotope records of the carbonate fraction decrease from near zero to negative carbon isotope values averaging 5 9 the 13ccarb values overall coincide with y ho the redox indicator ce ce and mass fractions of carbonate fluorapatite suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment the studied sections document the ecological requirements for sponge settlement and proliferation such as the presence of a hard substrate and sufficiently low suspended particle concentrations and or low wave energy this work suggests that the widespread appearance of karsted surfaces around the precambrian cambrian transition that were subsequently submerged during the cambrian transgression would have created permissive habitats for sponges and large phosphate deposits the capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the precambrian cambrian transition

a positive δ 13 c carb anomaly in paleoproterozoic carbonates of the aravalli craton western india support for a global isotopic excursion abstract carbon isotope measurements carried out on carbonate samples from different localities of the early proterozoic aravalli craton western india yield positive 13 c values up to 11 20 pdb this widespread isotopically anomalous carbonate province reported from this subcontinent coincides with the appearance of stromatolites and the development of red beds these observations support a global paleoenvironment change during the paleoproterozoic as reported from other parts of gondwanaland including the kapvaal craton africa and the sao francisco craton brazil

potential photosynthetic impact on phosphate stromatolite formation after the marinoan glaciation paleoceanographic implications abstract this study investigated the origin and the depositional age of phosphate stromatolites contained in the neoproterozoic salitre formation brazil the stromatolites exhibited columnar shapes and were intercalated with laminated dolostones their depositional environment was interpreted to have been an evaporitic ramp where erosion and reworking by waves prevailed laser ablation inductively coupled plasma mass spectrometry measurements of the stromatolites yielded a u pb age of 616 32 ma suggesting that they were deposited after the marinoan glaciation several types of filamentous structures were recognized in the stromatolites many of them were pseudofossil ambient inclusion trails but some were possibly microfossils to consider the general presence of photosynthetic microorganisms on the surface of shallow water stromatolites the involvement of microbial photosynthesis in phosphate stromatolite formation was expected numerical calculations determined that photosynthesis could have induced phosphate mineral precipitation at the time of deposition if the phosphorus concentration was above ca 5 m to achieve such high concentration at the shallow ocean globally elevated phosphorus concentration was considered to be the ultimate factor in addition to the local process es including upwelling and or evaporation therefore the phosphate stromatolites in the salitre formation are evidence of the development of a phosphorus rich ocean after the marinoan glaciation

regional variations in the carbon isotopic composition of phosphorite from the early cambrian lower tal formation mussoorie hills india abstract early cambrian lower tal phosphorites of the mussoorie syncline have been analyzed for their carbon isotopic composition of apatite bound carbonate and organic carbon two mine excavations at maldeota and durmala located on two limbs of the syncline and 14 km apart were subjected to detailed sampling along selected stratigraphic profiles field and microscopic evidence suggest a shallow marine environment at the site of phosphorite formation characterized by small scale fluctuations in the sedimentation milieu isotope data have been utilized to characterize differing redox conditions prevalent during phosphorite formation at durmala and maldeota in addition a marked negative carbon isotope excursion has been recorded at the dolomite phosphorite contact defined by the krol tal stratigraphic boundary it is thought to reflect the influx of 12 c and p rich waters from a deeper part of the sedimentary basin based on the isotopic composition of carbon in the phosphorite sulfur in the associated pyrite and petrological data it has been inferred that the durmala phosphorite reflects deposition under a suboxic diagenetic environment and the maldeota phosphorite formed within suboxic to anoxic sulfate reducing conditions

timing and petroleum sources for the lower cretaceous mannville group oil sands of northern alberta based on 4 d modeling the lower cretaceous mannville group oil sands of northern alberta have an estimated 270 3 billion m3 bcm 1700 billion bbl of in place heavy oil and tar our study area includes oil sand accumulations and downdip areas that partially extend into the deformation zone in western alberta the oil sands are composed of highly biodegraded oil and tar collectively referred to as bitumen whose source remains controversial this is addressed in our study with a four dimensional 4 d petroleum system model the modeled primary trap for generated and migrated oil is subtle structures a probable seal for the oil sands was a gradual updip removal of the lighter hydrocarbon fractions as migrated oil was progressively biodegraded this is hypothetical because the modeling software did not include seals resulting from the biodegradation of oil although the 4 d model shows that source rocks ranging from the devonian mississippian exshaw formation to the lower cretaceous mannville group coals and ostracode zone contributed oil to mannville group reservoirs source rocks in the jurassic fernie group gordondale member and poker chip a shale were the initial and major contributors kinetics associated with the type iis kerogen in fernie group source rocks resulted in the early generation and expulsion of oil as early as 85 ma and prior to the generation from the type ii kerogen of deeper and older source rocks the modeled 50 peak transformation to oil was reached about 75 ma for the gordondale member and poker chip a shale near the west margin of the study area and prior to onset about 65 ma from other source rocks this early petroleum generation from the fernie group source rocks resulted in large volumes of generated oil and prior to the laramide uplift and onset of erosion 58 ma which curtailed oil generation from all source rocks oil generation from all source rocks ended by 40 ma although the modeled study area did not include possible western contributions of generated oil to the oil sands the amount generated by the jurassic source rocks within the study area was 475 bcm 2990 billion bbl

north slope alaska source rock distribution richness thermal maturity and petroleum charge four key marine petroleum source rock units were identified characterized and mapped in the subsurface to better understand the origin and distribution of petroleum on the north slope of alaska these marine source rocks from oldest to youngest include four intervals 1 middle upper triassic shublik formation 2 basal condensed section in the jurassic lower cretaceous kingak shale 3 cretaceous pebble shale unit and 4 cretaceous hue shale well logs for more than 60 wells and total organic carbon toc and rock eval pyrolysis analyses for 1183 samples in 125 well penetrations of the source rocks were used to map the present day thickness of each source rock and the quantity toc quality hydrogen index and thermal maturity tmax of the organic matter based on assumptions related to carbon mass balance and regional distributions of toc the present day source rock quantity and quality maps were used to determine the extent of fractional conversion of the kerogen to petroleum and to map the original toc toco and the original hydrogen index hio prior to thermal maturation the quantity and quality of oil prone organic matter in shublik formation source rock generally exceeded that of the other units prior to thermal maturation commonly toco 4 wt and hio 600 mg hydrocarbon g toc although all are likely sources for at least some petroleum on the north slope we used rock eval and hydrous pyrolysis methods to calculate expulsion factors and petroleum charge for each of the four source rocks in the study area without attempting to identify the correct methods we conclude that calculations based on rock eval pyrolysis overestimate expulsion factors and petroleum charge because low pressure and rapid removal of thermally cracked products by the carrier gas retards cross linking and pyrobitumen formation that is otherwise favored by natural burial maturation expulsion factors and petroleum charge based on hydrous pyrolysis may also be high compared to nature for a similar reason

diagenesis of phosphatic hardgrounds in the monterey formation a perspective from bulk and clumped isotope geochemistry understanding the authigenesis of carbon ate fluorapatite through isotopic geochem istry can yield important information on fundamental geologic processes occurring on continental margins around the world this is particularly true for phosphatic hardgrounds which are often found in regions of upwell ing but of which questions remain about the initial formation and subsequent diagen esis here we apply standard isotopes 13 c 18 o alongside the novel clumped isotope 47 used in this study for the first time to reconstruct the temperature of formation of carbonate ions within the lattice of sedimen tary carbonate fluorapatite we investigated phosphatic hardgrounds of miocene age 12 7 10 8 ma sampled at el capitan state beach in the monterey formation the range of isotopic signatures observed is between 1 5 and 8 0 for 13 c relative to the vienna peedee belemnite vpdb standard and 9 5 and 6 0 vpdb for 18 o and val ues range between 0 599 and 0 615 for 47 the enriched 13 c and depleted 18 o signa tures are suggestive of recrystallization within the methanogenic zone clumped isotope geo chemistry further constrains this transforma tion as having taken place at a temperature of 61 66 c 5 c in line with previous esti mates for maximum burial of the monterey formation based on the silica phase transi tion the calculated 18 o for the connate fluid shows an expected range for seawater composition for the miocene suggesting only minor contribution of silica derived oxygen to the 18 o of carbonate fluorapatite the combined conventional and clumped isotope data set also points out that methanogenesis took place deeper within the sediment in the middle miocene than at present day within the santa barbara basin this study fur thers our understanding of phosphogenesis and potential links to burial processes in the monterey formation and it shows for the first time that the clumped isotope paleother mometer could be used to under stand funda mental geochemical processes in authigenic sedimentary phosphates

rare earth elements and stable isotope geochemistry of early cambrian chert phosphorite assemblages from the lower tal formation of the krol belt lesser himalaya india abstract the neoproterozoic cambrian transitional sequence in the mussoorie and garhwal hills of the lesser himalaya is represented by dolomites of the upper krol formation and chert phosphorite assemblages of the lowermost tal formation these rocks provide valuable information regarding the assumed existence of a stratified ocean during the terminal neoproterozoic and beginning of earliest cambrian nemakit daldynian the isotopic compositions of carbonate and organic carbon in the transition profile can be best interpreted as reflecting an oceanic anoxic event oae prior to the phase of early cambrian phosphate formation the chert phosphorite beds record negative ce and positive eu anomalies while ce is an important paleooceanic redox indicator eu is rather immobile under normal diagenetic conditions the observed eu anomaly is linked to the high ba content in these phosphorites and is indicative of an anoxic or sulfate reducing diagenetic environment the suggested stratification of the ocean during the late neoproterozoic and nemakit daldynian times would have been linked to the reduction of oxygen supply to the zone below the redoxcline causing 12 c and p rich organic matter previously produced in the photic zone to be trapped in large quantities within the deeper anoxic zone this would in turn increase the 13 c content of the water in the photic zone moreover in the deeper anoxic waters reduction of ce4 to ce3 would result in an enrichment of dissolved ce as ce3 upwelling of this anoxic bottom stratum would bring the 12 c and ce enriched waters back to the oxic marine realm however on entering the highly oxic productive zone the excess ce is expected to be removed from the water by oxidation to insoluble ce4 followed by precipitation or preferential scavenging due to these oxidation reactions the waters of the shallow oxic zone would tend to become rapidly depleted in ce in other words ocean mixing and upwelling during the early cambrian were ultimately responsible for the transport of 12 c p and ce to the oxic shelves sharp drops in both 13 c carb and 13 c org support the existence of oceanic anoxia followed by oceanic mixing at this juncture of earth history the large spread of the 34 s values of early diagenetic pyrites within the phosphorite bearing sequence in contrast to the limited spread of 13 c carb values suggests that phosphatisation took place under exclusively suboxic diagenetic non sulfate reducing condition and pyritisation began only when phosphate layers entered the zone of sulfate reduction

genesis of upper proterozoic cambrian phosphorite deposits of india isotopic inferences from carbonate fluorapatite carbonate and organic carbon carbon and oxygen isotope analyses performed on sedimentary carbonates organic carbon kerogen and structurally bound co2 3 groups of carbonate fluorapatite francolite from four widely separated indian phosphorite provinces have shown that each of the deposits is characterized by specific basically facies dependent distribution patterns of 13ccarb 13ocarb 13corg and 13capat comparison of the carbon isotope composition of apatite co2 with that of the coexisting sedimentary carbonate has served to further constrain the genetic type of the individual phosphorite deposit investigated augmenting relevant geological evidence by an additional geochemical discriminator as a whole the results confirm the importance notably of carbon isotope work for elucidating the processes involved in sedimentary phosphorite formation

diagenetic formation of interlayer deficient fluorophlogopite as a clay mineral in early cambrian phosphorite lesser himalaya india the trioctahedral analog of illite the occurrence of a trioctahedral analog of illite the dioctahedral interlayer deficient k mica has long been debated due to the inherent difficulties of determining structure and chemical composition of the extremely fine grained material earlier descriptions based on separated material are equivocal here we describe low temperature diagenetic formation of fluorophlogopite which is interlayer deficient and therefore analogous to illite using high resolution in situ methods transmission electron microscopy tem with preparation by focused ion beam milling combined with wavelength dispersive analysis by field emission gun electron microprobe the average composition is k0 5mg2 8v0 01fe0 005 si3 15al0 85o10 oh 0 65f1 35 including minor amounts of nh4 for charge compensation as determined by electron energy loss spectroscopy the k deficient mg mica occurs in layer packages of 10 layers and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with tem x ray powder diffraction patterns of separated material confirm the absence of smectite components the mineral was identified in phosphorites from the lowermost cambrian tal group mussoori syncline lesser himalayas india the rocks are alternating phosphatic mudstones and phosphatic dolostones at times interbedded with phosphate poor carbonate layers which are rich in organic matter sedimentary fluorophlogopite occurs in both rock types and in two textural associations one in vesicles filled with amorphic organic matter the other as reaction rims around illite which contains up to 5 wt v2o3 in its rims textural arguments favor an early diagenetic formation of both v bearing illite and fluorophlogopite closely associated with organic matter and linked to dolomitization the high f content stabilizes phlogopite to low temperatures our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions

reos isotopes and major and trace element geochemistry of carbonaceous shales aravalli supergroup india impact of post depositional processes abstract this study presents re os isotopes and major and trace elemental geochemistry of carbonaceous black shales and a weathering profile developed on them from the paleoproterozoic deposits of the jhamarkotra formation lower aravalli group india these measurements provide information on the impact of post depositional processes particularly hydrothermal fluid interactions on the 187re 187os geochronology and element mobility in shales in contrast to previously reported compositions for black shales the re concentrations of the jhamarkotra shales are in general lower than their corresponding os concentrations the 187os 188os ratios of the black shales vary widely from 3 5 to 10 5 without any discernible systematic trend with 187re 188os range 0 4 to 17 7 excluding an outlier these 187re 188os ratios are lower by a few orders of magnitude than those generally reported for fresh black shales these observations indicate that the jhamarkotra shales though they seem visually well preserved have undergone post depositional modification resulting in open system behavior of re and os with re more mobile than os higher sio2 tio2 and k2o na2o ratios of the jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal possibly si and k rich fluids the sio2 tio2 ratios an index for hydrothermal alteration show negative correlation with re corg and positive correlation with corg n ratios 42 154 indicating loss of re and nitrogen during hydrothermal fluid interaction earlier studies have shown the presence of re porphyrins n containing organic compounds in black shales thus the observed co variations of sio2 tio2 with re corg and corg n ratios suggest mobilization of re from the porphyrins in the shales further positive correlation between sio2 tio2 and 187os 188os ratios indicates that significant amounts of radiogenic os component were re distributed throughout the jhamarkotra shales during the post depositional processes in the weathering profile developed on black shales the al2o3 normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering on the contrary the al2o3 normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile suggesting limited mobility over the depth of the profile the depth profiles of re al2o3 and 188os al2o3 and 187os 188os ratios in the weathering profile by and large reflect compositions of their presumed parent shales which have gone through significant post depositional changes

triassic oils and potential mesozoic source rocks peace river arch area western canada basin abstract triassic oils and their potential mesozoic source rocks in the peace river arch region western canada basin have been examined using various analytical techniques triassic oils are divided into two groups 1 and 2 on the basis of biomarker composition triassic outcrop samples from the rocky mountain foothills are mature to overmature and contain type ii iii organic matter with toc s ranging from 0 1 to 15 0 wt and hi values up to 445 subsurface core samples from the triassic montney formation are mature to overmature with toc s rarely up to 2 8 wt and hi values up to 398 the triassic doig formation has good to excellent source potential and moderate to high maturity with toc values over 5 0 wt and hi values commonly in excess of 300 the jurassic nordegg member ranges from early mature to overmature with toc s as high as 23 5 wt and hi values greater than 600 where maturity is low oil source rock correlations are established based on biomarker signatures between the group 1 oils and the doig formation group 2 oil was sourced by the nordegg member

oil generation kinetics for organic facies with type ii and iis kerogen in the menilite shales of the polish carpathians the menilite shales oligocene of the polish carpathians are the source of low sulfur oils in the thrust belt and some high sulfur oils in the carpathian foredeep these oil occurrences indicate that the high sulfur oils in the foredeep were generated and expelled before major thrusting and the low sulfur oils in the thrust belt were generated and expelled during or after major thrusting two distinct organic facies have been observed in the menilite shales one organic facies has a high clastic sediment input and contains type ii kerogen the other organic facies has a lower clastic sediment input and contains type iis kerogen representative samples of both organic facies were used to determine kinetic parameters for immiscible oil generation by isothermal hydrous pyrolysis and s2 generation by non isothermal open system pyrolysis the derived kinetic parameters showed that timing of s2 generation was not as different between the type iis and ii kerogen based on open system pyrolysis as compared with immiscible oil generation based on hydrous pyrolysis applying these kinetic parameters to a burial history in the skole unit showed that some expelled oil would have been generated from the organic facies with type iis kerogen before major thrusting with the hydrous pyrolysis kinetic parameters but not with the open system pyrolysis kinetic parameters the inability of open system pyrolysis to determine earlier petroleum generation from type iis kerogen is attributed to the large polar rich bitumen component in s2 generation rapid loss of sulfur free radical initiators in the open system and diminished radical selectivity and rate constant differences at higher temperatures hydrous pyrolysis kinetic parameters are determined in the presence of water at lower temperatures in a closed system which allows differentiation of bitumen and oil generation interaction of free radical initiators greater radical selectivity and more distinguishable rate constants as would occur during natural maturation kinetic parameters derived from hydrous pyrolysis show good correlations with one another compensation effect and kerogen organic sulfur contents these correlations allow for indirect determination of hydrous pyrolysis kinetic parameters on the basis of the organic sulfur mole fraction of an immature type ii or iis kerogen

reflectance spectroscopy and aster based mapping of rock phosphate in parts of paleoproterozoic sequences of aravalli group of rocks rajasthan india abstract this paper applies reflectance spectroscopy and advanced space borne thermal emission reflection radiometer aster data processing to delineate dolomite hosted rock phosphate around udaipur city western india using visible near infrared vnir and short wave infrared swir bands of the aster sensor a number of aster image spectra and field spectra resampled to aster bands of pure dolomite and rock phosphate are compared and found to be consistent in terms of the presence of absorption minima at 2 3 m for dolomite and the absence of the same feature in the spectra of the rock phosphate before attempting spectral mapping aster swir band data are clipped using the spatial extent of host rock which has been prepared from the reference geological map and consequently updated using vnir swir derived image enhanced products at selected part of the study areas two sub pixel spectral methods namely adaptive coherence estimator ace and matched filtering mf methods are applied to aster swir bands to delineate rock phosphate rich zones within dolomite the output images of the above algorithms are integrated in a principal component pc image to delineate phosphate pixels a principal component which has similar sign of eigen vector loading for ace and mf abundance image is used to delineate rock phosphate within dolomite the results are validated in the field using a qualitative colorimetric method and also by comparing image spectra of spectrally anomalous zone with the aster resampled laboratory spectra of rock phosphate samples a number of sporadic but important patches of phosphate rich zones in addition to the main surface exposures of the jhamar kotra mine are confirmed in the field the proposed method is a novel application of spectral mapping of phosphate deposits in dolomitic and carbonatic rocks and present approach of aster data processing can be used to delineate phosphate rich zones occurring within similar geological provinces in any parts of the world

b_na rich palaeoproterozoic aravalli metasediments of evaporitic association nw india a new repository of gold mineralization we report field observations and preliminary petrochemical and stable isotopic compositions of the host rock and sulfide minerals for a recently discovered sulfide hosted gold deposit from the palaeoproterozoic aravalli rocks in northwestern india gold occurs associated with feascu co sulfides in lithofacies of diverse mineralogical composition dominated by amphibole bearing dolomitic marble and albite rich quartzo feldspathic rocks qfr the presence of vugs believed to be after dissolution of halite patches coupled with the development of scapolite bearing assemblages in dolomite marble occurrence of tourmaline rich mineralized rocks and very high na content of carbonates and qfr indicate hypersaline depositional environment for the host rock a narrow spread of 34 scdt values from 10 5 to 12 8 for stratiform arsenopyrite and remobilized pyrrhotite corroborates hypersaline to evaporitic depositional environment the 13 c values for gold bearing carbonates show bimodal distribution one population mean 0 4 v pdb is close to the sea water carbonate values while the other 3 l indicates probable mantle affinity the 18 o values for carbonates mean 18 4 v smow are not discriminative as they appear to have been modified by subsequent metamorphism and hydrothermal activity interpretation of mantellic source for carbon is borne out from the absence of biogenic activity and incompatibility of significantly negative 13 c values in the evaporitic setting of deposition of bhukia rocks we suggest that nab rich fluids channeled through rift related faults were responsible for creating hypersaline conditions that eventually evolved into an evaporitic environment mantle carbon through some still deeper faults was probably responsible for the gold transport and its subsequent deposition in an otherwise uncommon evaporitic repository of carbonates and qfr 2002 elsevier science b v all rights reserved

comparison of petroleum generation kinetics by isothermal hydrous and nonisothermal open system pyrolysis this study compares kinetic parameters determined by open system pyrolysis and hydrous pyrolysis using aliquots of source rocks containing different kerogen types kinetic parameters derived from these two pyrolysis methods not only differ in the conditions employed and products generated but also in the derivation of the kinetic parameters i e isothermal linear regression and non isothermal nonlinear regression results of this comparative study show that there is no correlation between kinetic parameters derived from hydrous pyrolysis and open system pyrolysis hydrous pyrolysis kinetic parameters determine narrow oil windows that occur over a wide range of temperatures and depths depending in part on the organic sulfur content of the original kerogen conversely open system kinetic parameters determine broad oil windows that show no significant differences with kerogen types or their organic sulfur contents comparisons of the kinetic parameters in a hypothetical thermal burial history 2 5 c my show open system kinetic parameters significantly underestimate the extent and timing of oil generation for type iis kerogen and significantly overestimate the extent and timing of petroleum formation for type i kerogen compared to hydrous pyrolysis kinetic parameters these hypothetical differences determined by the kinetic parameters are supported by natural thermal burial histories for the naokelekan source rock type iis kerogen in the zagros basin of iraq and for the green river formation type i kerogen in the uinta basin of utah differences in extent and timing of oil generation determined by open system pyrolysis and hydrous pyrolysis can be attributed to the former not adequately simulating natural oil generation conditions products and mechanisms

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

oxygen and carbon isotope composition of structural carbonate in weathering apatites from laterites southern brazil and western senegal abstract the oxygen 18 o c and carbon 13 c c isotope compositions of the structural carbonate group co 3 in apatites from lateritic profiles were investigated the weathering profiles located in southern brazil and in western senegal are developed on three different types of apatite rich parent rock carbonatite metamorphosed marine phosphorite and sedimentary marine phosphorite the parent rock apatites are of magmatic hydrothermal metamorphic and sedimentary origins the in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically the overall range of measured 18 o c and 13 c c values of apatites of weathering origin 22 to 27 smow for 18 o c and 15 to 10 pdb for 13 c c is much smaller than the range of measured and or published isotope compositions of parent rock apatites 4 35 for 18 o c and 11 to 1 for 13 c c in any profile the apatites of weathering origin can exhibit lower similar or higher 18 o c values than parent rock apatites in contrast their 13 c c values are systematically and significantly lower than those of the parent rock apatites apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition assuming that apatite co 3 fractionates o in a way similar to calcite co 3 the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions the very low 13 c c values exhibited by these apatites indicate that the dominant sources of dissolved co 2 in the soil water are organic the isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

petrological and chemical characteristics of liptinite rich coals from alberta canada a suite of liptinite rich coals from the lower cretaceous mannville group in the subsurface of alberta canada has been studied with respect to its organic petrology and organic geochemistry the samples are rich in fluorescing vitrinite perhydrous sporinite cutinite exsudatinite and bituminite rock eval analysis shows that the coals are enriched in hydrogen hydrogen index up to 320 mg hc g toc and depleted in oxygen oxygen index up to 11 mg co2 g toc the value of tmax ranges from 445 to 460 c indicating a late catagenic level of thermal maturity equivalent to ro 1 0 1 1 but the vitrinite reflectance is between 0 68 and 0 81 indicating that it is lowered by as much as 0 4 the pyrolysis chromatograms of the coal samples are dominated by substituted benzenes toluene and xylenes naphthalenes phenanthrenes substituted phenols cresols and small amounts of n alkanes in the range c20 gas chromatography mass spectrometry of the steranes and terpanes indicates little or no contribution of allochthonous bitumen to the coals in addition the mz 217 sterane traces are dominated by c29 compounds with little c27 and c28 this along with the high terpane to sterane ratio mz 191 to mz 217 8 8 indicates that the bitumen was generated from terrestrially derived organic matter the bitumen in the coals has most likely been autogenerated by the coals themselves in response to thermal stress the originally sapropelic coals were probably deposited in an anaerobic environment and were much enriched in hydrogen this study shows that terrigenous organic matter when associated with an anomalous liptinite content may have the potential to generate bitumen

coal quality coal rank variation and its relation to reconstructed overburden upper cretaceous and tertiary plains coals alberta canada near surface coals in the alberta plains occur in upper cretaceous and tertiary sediments the former represented by the belly river group horseshoe canyon formation and wapiti formation and the latter by the scollard member of the paskapoo formation statistical techniques were used to document both the distribution of and the interrelationships among the components of proximate and ultimate analysis and calorific value determinations the coals discussed range in rank from subbituminous c to high volatile bituminous c these coals are variable in ash content and low in sulfur content the relationships among calorific value dry basis in kj kg and ash dry basis in and calorific value moist mineral matter free basis in kj kg and equilibrium moisture meq in were determined to be cvd 29 677 315 17 ashd and cvmmmf 31 816 442 05 meq respectively the maximum depth of coal seam burial dob in m was reconstructed on the basis of previously published graphs relating the equilibrium moisture loss to depth of coal seam burial using the relationship log10meq 1 865 0 000416 dob a least squares regression analysis of calorific value mmmf basis in k kg on the calculated depth of burial dob in m yielded the equation cvmmmf 14 748 6 25 dob indicating a coalification gradient determined on the reconstructed overburden of 6 25 kj kg m 0 82 btu lb ft of depth or overburden near surface coals in the alberta plains increase in rank toward the west southwest toward the foothills and mountains region because of progressively greater amounts of overburden that existed in that direction during the tertiary erosion has since removed between 900 and 1 900 m 3 000 and 6 200 ft of sediment with the greatest amount of removal occurring in a west southwest direction

age and implications of the phosphatic birmania formation rajasthan india abstract the birmania inlier in western rajasthan india contains important phosphate deposits the depositional age of which is poorly constrained here we provide the first direct age constraints for the phosphate bearing birmania formation using a combination of paleontological and detrital zircon u pb data the occurrence of the multicellular algal fossil wengania exquisita in phosphatic chert of the birmania formation suggest that it was deposited during the ediacaran detrital zircon age distributions contain prominent populations of 1 7 1 9 ga grains with subordinate younger grains that range from 650 to 980 ma the distribution broadly resembles those from neoproterozoic strata from both cratonic and himalayan india but like zircon age distributions from the marwar group lacks 1 0 1 2 ga grains which may suggest that both areas shared similar local sources the lack of zircon grains younger than 650 ma is consistent with an ediacaran depositional age because almost all cambrian or younger strata from india have yielded cambrian or latest neoproterozoic age grains these findings raise the possibility of a previously unrecognized late neoproterozoic episode of phosphogenesis on the india craton the presence of wengania exquisita further supports strong palaeogeographic affinity between the india and south china during the neoproterozoic and cambrian

evaluation of paired carbon isotopic signals from the ediacaran doushantuo phosphorites diagenetic or primary abstract this study reports high resolution c isotopic data of carbonate carbon 13ccarb and organic carbon 13corg from the ediacaran doushantuo carbonate and phosphorite as well as from the underlying tillite of the nantuo formation from the datang mine and chuanyuandong sections for the nantuo tillite in the datang section the striking negative 13ccarb values ranging mostly from 10 to 16 with negative 18o values down to 27 2 and clear covariance between 13ccarb and 18o suggest that nearly all of the primary c and o isotopic compositions have been reset by meteoric diagenesis the near complete modification of the primary isotopic signals may be due to frequent exposure to fresh water conditions during diagenesis and the lithification of porous tillite sediment the extremely negative 13ccarb values down to 16 8 suggest the contribution of 12c enriched dissolved inorganic carbonate which originated from the remineralization of organic carbon during burial diagenesis these observations suggest that caution is needed when using tillite samples for paleoenvironmental reconstruction the correlative lower phosphorite deposits from the datang and chuanyuandong sections show clear covariance between 13ccarb and 18o although the chuanyuandong phosphorites have more negative values of 13ccarb and 18o than the datang section although most of the 18o values of the lower phosphorites may have been altered by secondary diagenetic processes the 13ccarb values of 3 5 and the 13corg values of

paleoenvironmental implications of two phosphogenic events in neoproterozoic sedimentary successions of the tandilia system argentina fil gomez peral lucia consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet la plata centro de investigaciones geologicas universidad nacional de la plata facultad de ciencias naturales y museo centro de investigaciones geologicas argentina

comment on new age constraints for the proterozoic aravalli delhi successions of india and their implications by mckenzie et al precambrian res 238 2013 120 128 the newly discovered 1 9 1 7 ga positive anomaly of carbonate carbon remains to be proven

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

13c enrichment in the palaeoproterozoic carbonate rocks of the aravalli supergroup northwest india influence of depositional environment abstract the dolomitic carbonates from the jhamarkotra formation of the palaeoproterozoic aravalli supergroup are characterized by widely variable carbon isotope ratios 13 c ranging from near zero to as high as 12 v pdb the estimated maximum age ca 2150 ma of the aravalli carbonates help bracketing these with the coeval carbonate bodies of the world that show high positive carbon isotope values the intriguing existence of normal marine 13 c values in some pockets suggest influence of local scale depositional conditions prevailing in different sub basins amongst these two sub basins which showed high 13 c values a hypersaline evaporative condition is considered responsible for the necessary enrichment in one while methanogenesis possibly in conjunction with sulphate reduction processes might have caused such enrichment in the other from the empirical association of the sub basins with profuse stromatolitic phosphorite we infer that the depositional setting that favoured cyanobacterial growth leading to formation of stromatolites prevented growth of methanogenetic archaea in such anoxic environments our study therefore highlights the fact that the early palaeoproterozoic 13 c excursion in the aravalli supergroup is not essentially a time specific event but is greatly dependent on the variation in the depositional palaeoenvironment prevailing in different sub basins

geothermal gradients hydrodynamics and hydrocarbon occurrences alberta canada using coal rank data and the demonstrated genetic link between the present hydrodynamic regime and geothermal gradient pattern it is possible to reconstruct the geothermal history of alberta over the past 60 m y palinspastic adjustment for tectonic compression caused by the major laramide thrusting shows that the predeformation isoreflectance lines increased logarithmically with depth in the late paleocene the geothermal gradient was about 23 c km 1 25 f 100 ft in the eastern alberta plains compared to about 30 c km 1 65 f 100 ft in the western alberta plains a regional trend opposite to the post laramide trends reconstruction of the early eocene surface indicates western uplands with geothermal gradients as low as 21 c km 1 15 f 00 ft and eastern lowlands with geothermal gradients of 27 c km 1 5 f 100 ft compared to the present situation this represents an enhanced topographic surface and a subdued geothermal gradient pattern the genetic relations of topography water table elevation hydrodynamic regime and geothermal gradient pattern in both the early eocene and the present conform to a model developed for any compacted sedimentary basin with subaerial relief in this model on a regional scale high topographic areas have high water table elevations with correspondingly high potentiometric surfaces these areas control the regional recharge of cold meteoric water and hence have low geothermal gradients areas of medium elevation exhibit regional lateral flow and intermediate geothermal gradients regional topographic lows correspond with low potentiometric surfaces and the regional discharge of warm formation waters from deep in the basin results in high geothermal gradients local topographic featur s cause perturbations in this picture study of both the regional pattern and fine detail of the early eocene and present topographic surfaces and geothermal gradient patterns shows the validity of the model the result of the second laramide orogenic pulse was therefore to reverse the geothermal gradient pattern resulting mainly from compaction flow and to impose a topographically controlled hydrodynamic regime with flow in the same direction but with a 9 c km 0 5 f 100 ft decreased geothermal gradient in western alberta and a 4 c km 0 2 f 100 ft increased geothermal gradient in eastern alberta as erosion proceeded during the tertiary the entire alberta basin warmed up until the present temperature regime was attained study of some major hydrocarbon occurrences shows a genetic link between the hydrocarbon occurrence and the present fluid regime which itself is controlled by the present topography for at least the significant upper cretaceous shallow gas reserves in southeastern alberta however their position may already have been determined as early as the beginning of the eocene by the topographic surface at that time therefore the extent to which the present topographic surface is responsible for hydrocarbon accumulation through gravity induced cross formational flow remains unknown because this process was operative as long ago as the early eocene what does not ap ear to be in doubt is the fact that gravity induced cross formational flow does control the hydrodynamic regime which in turn influences the geothermal gradient pattern and the accumulation of hydrocarbons

graphite content and isotopic fractionation between calcite graphite pairs in metasediments from the mgama hills southern kenya abstract amphibolite grade metasediments from the mgama hills region kenya contain conspicuous quantities of graphite most probably derived from organic progenitor materials the highest graphite contents 5 1 20 4 are found in schists whereas calcite marbles intercalated in the sequence contain relatively low amounts 0 1 2 0 the graphitic constituents are consistently enriched in 13 c relative to common sedimentary organic material with the highest isotopic ratios in graphite from the marbles 13 c 7 3 5 0 n 10 carbon isotope fractionations between calcite and graphite mostly vary between 3 3 and 7 1 which comes close to both empirically recorded and thermodynamically calculated fractionations in the temperature range of the upper amphibolite facies 550 650 c however larger values occasionally encountered in the marbles suggest that complete isotopic equilibrium is not always attained in amphibolite facies metamorphism

geochemistry and re os geochronology of the organic rich sedimentary rocks in the jingtieshan fe cu deposit north qilian mountains nw china abstract the jingtieshan group in the north qilian mountains nw china is dominantly composed of banded iron formations bifs copper deposits and organic rich sedimentary rocks ors carbonaceous phyllite x ray diffraction analysis of the ors shows the mineral assemblage to be quartz clay minerals the total organic carbon contents show a range of 0 44 1 72 here we present the results of the geochemistry and re os geochronology of the ors from the jingtieshan group the high values of chemical index of alteration cia chemical index of weathering ciw plagioclase index of alteration pia and th u ratio indicate intense weathering the al 2 o 3 tio 2 zr sc th sc la th ratios high rare earth elements abundances light rare earth elements enrichment normalized to chondrite and distinctly negative eu anomalies suggest that the jingtieshan group ors were derived mainly from felsic volcanic units the new re os isochron age of 1308 100 ma 2 n 6 mswd 23 broadly overlap with the previous published ages determined using sm nd and u pb isotope systems the new age represents the depositional age of the jingtieshan group as well that of bif in the jingtieshan area furthermore the initial 187 os 188 os ratios 0 44 0 07 indicate that the os in the seawater was dominantly derived from hydrothermal fluids 75 the ce anomaly ce ce 0 95 1 00 and v v ni ratios 0 71 0 86 as well as the lack of enrichment in redox sensitive trace elements such as u v zn pb cu ni cr co and mn together with the presence of overlying bif suggest that the jingtieshan area represents a ferruginous deep water succession this and intense submarine hydrothermal activities contributed to the deposition of the jingtieshan bif

13c 12c exchange between calcite and graphite a possible thermometer in grenville marbles the fractionation of 13c between calcite and graphite cc gr is consistently small 2 6 4 8 permil in 34 assemblages from upper amphibolite and granulite facies marbles of the grenville province in 25 samples from the adirondack mountains new york it decreases regularly with increasing metamorphic temperature the fractionations are independent of absolute 13c values of calcite 2 9 to 5 0 for t 600 800 c the adirondack data are described by cc gr 0 00748t c 8 68 this good correlation between and t suggests that carbon isotope equilibrium was attained in these high grade marbles and that the theoretical calculations of this fractionation by bottinga are approximately 2 permil too large in this temperature range because of the relatively high temperature sensitivity suggested by these results and by bottinga s calculations and the pressure independence of isotope fractionation cc gr may provide a very good thermometer for high grade marbles comparison of this field calibration for cc gr vs temperature with results from other terranes supports the utility of cc gr for geothermometry and suggests that graphite is much more sluggish to exchange than calcite that exchange between calcite and graphite occurs at temperatures as low as 300 c and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500 600 c because 13c exchange is an unavoidable metamorphic process at temperatures above 300 c high values of 13c gr in moderate to high grade carbonate bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

simple kinetic models of petroleum formation part iii modelling an open system abstract the movement of petroleum from organic rich source beds to potential carrier beds is the net result of two processes expulsion the release of generated petroleum from kerogen within the source bed and primary migration flow of a petroleum phase through the inorganic void networks of the source bed and all other intervening fine grained rocks expulsion is by far the more important process within the source bed orgas a simple zero dimensional model of the source rock as an open system explicitly models expulsion concurrent with oil and gas generation and oi lgas cracking as a storage threshold problem oil and gas are retained adsorbed absorbed within kerogen until their concentrations exceed the respective sorptive capacity of the residual organic carbon 0 1 and 0 02 g g ck 1 the initial hydrogen index hl 0 controls the important balance between potential sorbate petroleum and sorbant residual carbon using simple geochemical measurements the initial organic carbon is divided into initial oil inert kerogen and oil and gas generative reactive kerogen components reactive kerogens are assigned to one of five global kinetic organofacies with pre determined kinetic parameter sets governing separately rates of oil and gas generation kinetic parameters governing the rates of oil cracking within the source rock are predicted from hl 0 the two most important determinants of source rock behaviour are organofacies and hl 0 at a reference constant heating rate of 2 c ma 1 the projected oil expulsion threshold for typical organofacies members increases in the order a de 100 110 120 135 c f typically expels no oil an order of magnitude increase decrease in heating rate increases decreases these thresholds by ca 15 c typical cumulative gas mass fractions in their ultimate expulsion products are a f 0 25 0 3 0 25 0 65 and 1 0 g g 1 a value of 200 mg hc g c 1 is an approximate minimum hl 0 prerequisite for oil expulsion irrespective of absolute petroleum potential p 0 as a family coals vary dramatically in their expulsion behaviour as their hl 0 ranges both above and velow this value primary migration is efficient for expulsive source rocks movement within the source bed is limited only at low p 0 a source rocks total genetic potential p 0 thickness becomes important during migration outside the source bed it potentially restricts the primary migration distance and the excess of petroleum available for secondary migration and entrapment we evaluate primary migration approximately using a small migration loss factor no more than about 10 saturation of porosity or 1 mg g 1 coupled with an understanding of fluid potential gradients which determine migration direction within the stratigraphic architecture ultimately kerogen degradation and oil to gas cracking in the source rock are the rate limiting processes in petroleum accumulation

positive δ13c excursion in carbonate and organic fractions from the paleoproterozoic aravalli supergroup northwestern india abstract carbon and oxygen isotopic investigations have been carried out on the archean and paleoproterozoic carbonate rocks of the udaipur region in the aravalli mountain belt northwestern india the study has led to the interesting finding of 13 c enrichment in the carbonate rocks of the jhamarkotra formation 13 c carb up to 11 1 v pdb belonging to the 2200 1900 ma paleoproterozoic aravalli supergroup further it is observed that the organic carbon from phosphorite bearing stromatolitic dolomites of the same formation are also enriched in 13 c 13 c org up to 11 1 however it must be emphasized that the 13 c enriched carbonates and the organic fractions are from different stratigraphic levels although in the same formation the former being at a lower stratigraphic level a critical analysis of the field geological petrological as well as the isotopic data indicates that the observed 13 c excursion in the carbonate carbon is not due to local causes such as methanogenesis or evaporitic conditions the shallow water environment of deposition of these carbonates as well as the immediately overlying formations rules out the stratified ocean model for the observed excursion the model involving high sedimentation rates and organic carbon burial has been preferred to explain the 13 c excursion in the carbonates of the jhamarkotra formation the 13 c enrichment in organic fraction occurred in localized regions of the aravalli sedimentary basin where high productivity was supported by phosphorous supply indicating that the diffusion limited pathways of organic carbon fixation were operative in the paleoproterozoic itself the 13 c enriched carbonate rocks of the jhamarkotra formation serve as the indian example for the paleoproterozoic global 13 c excursion

on the late neoproterozoic early cambrian transition events in parts of east gondwanaland abstract the east gondwanaland evolved as a result of break up of the rodinia supercontinent the late neoproterozoic early cambrian geochemical events documented in the rocks of the east gondwanaland and siberia suggest variations in the c s and sr isotopic compositions of the contemporary seawater and systematic distribution of phosphorite and evaporite deposits the geochemical records in the peninsular india himalaya south china iran and oman regions have been discussed and used for establishing late neoproterozoic links of these widely separated sedimentary basins

fractionation between inorganic and organic carbon during the lomagundi 2 22 2 1 ga carbon isotope excursion article i nfo the lomagundi 2 22 2 1 ga positive carbon isotope excursion in shallow marine sedimentary carbonates has been associated with the rise in atmospheric oxygen but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen the amount of oxygen released to the exosphere during the lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon which is poorly characterized because dissolved inorganic and organic carbon reservoirs were arguably larger in the paleoproterozoic ocean at a time of lower solar luminosity and lower ocean redox state decoupling between these two variables might be expected we determined carbon isotope values of carbonate and organic matter in carbonates and shales of the silverton formation south africa and in the correlative sengoma argillite formation near the border in botswana these units were deposited between 2 22 and 2 06 ga along the margin of the kaapvaal craton in an open marine deltaic setting and experienced lower greenschist facies metamorphism the prodelta to offshore marine shales are overlain by a subtidal carbonate sequence carbonates exhibit elevated 13 c values ranging from 8 3 to 11 2 vs vpdb consistent with deposition during the lomagundi positive excursion the total organic carbon toc contents range from 0 01 to 0 6 and 13 c values range from 24 8 to 13 9 thus the isotopic fractionation between organic and carbonate carbon was on average 30 3 2 8 n 32 in the shallow marine environment the underlying sengoma shales have highly variable toc contents 0 14 to 21 94 and 13 c values 33 7 to 20 8 with an average of 27 0 3 0 n 50 considering that the shales were also deposited during the lomagundi excursion and taking 13 c values of the overlying carbonates as representative of the 13 c value of dissolved inorganic carbon during shale deposition a carbon isotope fractionation as large as 37 appears to characterize the production of bulk organic matter in the deeper part of the pretoria basin at that time this enhanced fractionation relative to that observed in shallow water environments likely reflects heterotrophic secondary and chemotrophic productivity at and below a pronounced redoxcline consistent with the euxinic conditions inferred from independent evidence for the deeper part of the pretoria basin greater variability in organic carbon vs carbonate carbon isotopic values on the shallow marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the lomagundi excursion our study suggests that in contrast to the late neoproterozoic and phanerozoic when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers in the paleoproterozoic redox stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline this strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the 13 c values of dissolved inorganic carbon in the paleoproterozoic seawater

provenance of proterozoic basal aravalli mafic volcanic rocks from rajasthan northwestern india nd isotopes evidence for enriched mantle reservoirs abstract komatiites komatiitic basalts and tholeiites from nathdwara rajasthan have been investigated for their nd isotopic characteristics 143nd 144nd vary broadly in the komatiites 0 51131 0 51252 and show a restricted range in komatiitic basalts and tholeiites 0 51120 0 51179 and 0 51141 0 51202 respectively the corresponding epsilon nd nd values indicate an enriched source for these rocks the model ages of komatiitic basalts and tholeiites cluster between 2 2 and 3 0 ga the komatiites show older model ages between 3 5 and 3 8 ga if the correlations found between nd and sm isotopic ratios can be interpreted as isochrones then the komatiites point to the age of 2300 ma and the komatiitic basalts and tholeiites constrain the age of 1800 2000 ma the age of 2300 ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples 1800 2000 ma considering the errors involved collectively the data suggest an age of 2300 1800 ma for the basal aravalli volcanism the variation of nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for sm and nd it may be due to significant mantle heterogeneity in the source region isotopically distinct sources for the komatiites and komatiitic basalts are possible but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall rock material is inconsistent with the observed isotopic ratios the inference of an enriched mantle component appears inevitable as the alternative model of mixing would have generated more siliceous high magnesian boninitic rocks with much lower fe and ti than are observed this enriched material may have been part of the sub continental lithospheric mantle sclm under the aravalli craton which could have generated the basal aravalli magmatic rocks we suggest that during early middle proterozoic the sclm under cratonic regions consisted of relatively fertile mantle capable of generating komatiites tholeiites and alkalic magmas

a model of a mesoscale lens in large scale shear part i linear calculations abstract a simple model is used to study the behavior of a lens shaped eddy in a background flow with uniform horizontal and vertical shear the primary motivation for the work is to understand the influence of large scale shear on mediterranean salt lenses in the canary basin the model eddy is represented by an isolated three dimensional patch characterized by anomalous potential vorticity and its evolution is governed by stratified f plane quasigeostrophic dynamics guided by the available observations the potential vorticity field is chosen to be horizontally piecewise constant and is a linear function of depth within the lens this produces a flow field characterized by a depth dependent solid body rotation within the core of the eddy with speeds decreasing monotonically outside the core in good agreement with observations a family of linearized solutions is discussed representing a lens shaped eddy with a large trapped fluid region which is deformed due to interactions with external shear t

high phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the paleoproterozoic atmosphere abstract the deposition of major precambrian phosphorites was restricted to times of global change and atmospheric oxygenation at both ends of the proterozoic phosphorites formed after highly positive carbon isotope excursions in carbonates deposited during the paleoproterozoic lomagundi jatuli event and the neoproterozoic cryogenian and ediacaran periods the correlative step wise rise in atmospheric oxygen over the proterozoic has been linked to changes in the carbon cycle however the postulated relations between carbon isotope events phosphorites and atmospheric oxygenation remain unexplained paleoproterozoic carbonates of the aravalli supergroup india preserve evidence for cyanobacterial blooms in the form of tightly packed stromatolitic columns in the world s oldest significant sedimentary phosphate deposit restricted basins of the lower aravalli group with stromatolitic phosphorites in jhamarkotra udaipur jhabua and sallopat exhibit near zero 13 c carb values and large ranges of 13 c org values between 33 3 and 10 1 indicative of a complex carbon cycle because phosphate accumulates primarily in oxic sediments these eutrophic microbial ecosystems likely developed within the photic zone of the shallow oxygenated marine realm this is consistent with deposition during the time of increasingly more oxidizing conditions after the great oxidation event goe approximately contemporaneous basins without phosphate deposits from ghasiar karouli negadia umra and babarmal exhibit a range of positive 13 c carb excursions some with values up to 11 2 that suggest high rates of organic carbon burial and others with moderately high 13 c carb values around 6 or 3 that suggest smaller carbon cycle perturbations the 15 n values of all these rocks vary between 0 7 and 3 4 and are consistent with the predominance of nitrogen fixation during cyanobacterial blooms in all basin types such low nitrogen isotope values are interpreted to have arisen from the biological response to high phosphate availability we conclude that increased phosphate availability during and after the paleoproterozoic lomagundi jatuli event likely caused cyanobacterial blooms and was a key factor in the oxygenation of earth s atmosphere increasing oxygenation of the shallow ocean seafloor favored the removal of excess phosphate as authigenic apatite thus dampening effects of weathering increases on organic burial and marine 13 c carb after about 2 0 ga

geochemical changes across the proterozoic cambrian transition in the durmala phosphorite mine section mussoorie hills garhwal himalaya india abstract the transition from the neoproterozoic nemakit daldynian 1 krol e dolomite to the tommotian lower tal phosphorite chert black shale sequence denotes a period of drastic change in the depositional environment and the chemical milieu of the sedimentation basin these changes are reflected by a marked negative excursion of carbon isotopes phosphorus enrichment trace metal content high organic carbon burial and notably the first appearance of prolific small shelly fauna and burrowing organisms recent studies have documented these changes along several stratigraphic profiles in the mussoorie hills of the garhwal himalaya for which a single profile from the durmala phosphorite mine provides a key section discussed here

c o isotope and whole rock geochemistry of proterozoic jahazpur carbonates nw indian craton abstract the jahazpur group nw indian craton meta pelite carbonate quartzite sequence of presumed palaeoproterozoic age unconformably overlies the volcano sedimentary rocks of hindoli group and a tonalitic gneiss leucogranite complex jahazpur granite basement leucogranite the younger component of the jahazpur granite complex has been considered to be coeval with the ca 2 5 ga berach granite the lowermost unit of the jahazpur group the basal conglomerate which unconformably overlies the hindoli group is conformably overlain by a phyllite dolomite phyllite sequence this sedimentary package is developed in two belts designated as the eastern and western jahazpur belts preliminary c and o isotopic results and relevant whole rock geochemical data on representative dolomitic limestone samples are presented and discussed in this paper the lower dolomite unit shows significantly negative to near zero d13c values 4 8 to 0 9 v pdb while the upper dolomite developed in the western belt only shows near zero values various geochemical parameters mn sr and ca mg ratios and non correlative d13c and d18o values indicate well preserved isotopic characteristics the isotopic signatures of jahazpur dolomites are significantly different from the highly positive d13c values reported from the palaeoproterozoic aravalli supergroup rocks occurring around udaipur jhamarkotra formation the jahazpur carbonates show some similarity in the c isotopic characteristics with the delwara formation carbonates which underlie the jhamarkotra formation however the observed negative d13c values in the delwara formation carbonates have possibly resulted from effects of metamorphic decarbonation

high primary productivity and nitrogen cycling after the paleoproterozoic phosphogenic event in the aravalli supergroup india the causes of atmospheric oxygenation in the paleoproterozoic are multifaceted yet poorly understood an increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere a likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate which would have resulted in a fundamental change from the archean nitrogen cycle in this study we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the paleoproterozoic aravalli supergroup india the shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2 0 ga shales in the phosphate domain of the jhamarkotra formation from the lower aravalli group have distinct characteristics from coeval shales in the non phosphate domain carbonaceous shales of the phosphate domain contain relatively high v and u th typical of oxygen deficient environments these shales also have a high organic content of up to 14 wt generally homogeneous 13 corg values around 29 c nwr up to 360 and a large range of 15