solid bitumen reflectance and rock eval tmax as maturation indices an example from the nordegg member western canada sedimentary basin abstract marine organic rich rock units commonly contain little for vitrinite reflectance v r 0 measurement the most commoly used method of assessing thermal maturity this is true of the lower jurassic nordegg member a type i ii sulphur rich source rock from the western canada sedimentary basin this study examines the advantages and pitfalls associated with the use of rock eval t max and solid bitumen reflectance b r 0 to determined maturity in the nordegg vitrinite reflectance data from cretaceous coals and known coalification gradients in the study area are used to extrapolate v r 0 values for the nordegg t max increases non linearly with respect to both b r 0 and extrapolated v r 0 values a sharp increase in the reflectaance of both solid bitumen and vitrinite occurs between t max 440 450 c and is coincident with a pronounced decrease in hydrogen index values and the loss of solid bitumen and telalginite fluorescence over the same narrow t max interval this t max range is interpreted as the main zone of hydrocarbon generation in the nordegg and corresponds to extrapolated v r 0 values of 0 55 0 85 the moderate to high sulphur contents in the kerogen played a significant role in determining the boundaries of the nordegg oil window a linear relationship between b r 0 and extrapolated v r 0 as proposed elsewhere is not true for the nordegg b r 0 increases with respect to extrapolated v r 0 according to jacob s 1985 formula v r 0 0 618 b r 0 0 40 up to v r 0 0 72 b r 0 0 52 beyond this point b r 0 increases sharply relative to extrapolated v r 0 according to the relatioship v r 0 0 277 b r 0 0 57 r 2 0 91 the break in the b r 0 v r 0 curve at 0 72 v r 0 is thought to signifiy the peak of hydrocarbon generation and represents a previously unrecognized coalification jump in the solid bitumen analogous to the first coalification jump of liptinites

paleopedology and stable isotope chemistry of late silurian vertic paleosols bloomsburg formation central pennsylvania abstract paleosols with vertic vertisol like features occur in repetitively stacked fining upward sequences bounded by marine erosional surfaces in the bloomsburg formation ludlovian to pridolian of central pennsylvania paleosol development was strongly influenced by repeated changes in relative sea level which resulted in subaerial exposure and pedogenesis of marginal marine sediment accumulating along a low energy mud dominated coastline in a delta plain environment bloomsburg paleosols are characterized by very weak horizonation abundant pedogenic slickensides and a micromorphology dominated by sepic fabrics and peds bounded by stress cutans pedogenic carbonate is sparse occurring principally as micrite nodules stable carbon isotope compositions of stratigraphically low pedogen c carbonate mean 13c 1 92 pdb are attributed to pedogenesis proximal to a marine coastline in stratigraphically high paleosols showing evidence of rhizolith development isotopic compositions of pedogenic carbonate suggest a significant contribution of carbon from c3 flora atmospheric p co2 levels estimated for these calcretes 4500 5500 ppmv are significantly greater than present day values low 18o values for pedogenic carbonate mean 18o 12 12 pdb record isotopic exchange between paleosol carbonate and 18o enriched fluids associated with the conversion of original soil smectites to illite and chlorite during burial diagenesis paleosols bounded by marine erosional surfaces show variable degrees of marine hydromorphism attributable to rising water table during transgression evidence for hydromorphism includes extensive reduction of iron as well as relative increases in cao mno mgo p2o5 na2o and sr at the tops of paleosols due to introduction of marine fluids and sediment into open soil fractures and burrows the abundance of vertic features in all bloomsburg paleosols by analogy with holocene vertisols indicates the recurrence of a seasonally wet dry dry 4 8 months year paleoclimate n the central appalachian region at a paleolatitude of about 20 south

the late neogene 87sr 86sr record of lowland himalayan rivers fossil shells and paleosol carbonate from ancestral himalayan river deposits provide a 87 sr 86 sr record of lowland himalayan river water during the late neogene reconstructed 87 sr 86 sr river values increased sharply in the late miocene probably marking the beginning of exhumation of high 87 sr 86 sr metalimestones more in the central than in the western himalayas these results imply that the marine 87 sr 86 sr record may not be a proxy for silicate weathering or consumption of atmospheric co 2 resulting from that weathering

origin distribution and hydrogeochemical controls on methane occurrences in shallow aquifers southwestern ontario canada abstract natural gas reservoirs in organic rich shales in the appalachian and michigan basins in the united states are currently being produced via hydraulic fracturing stratigraphically equivalent shales occur in the canadian portion of the basins in southwestern ontario with anecdotal evidence of gas shows yet there has been no commercial shale gas production to date to provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production such as leakage of natural gas saline water and or hydraulic fracturing fluids and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern ontario an extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow domestic wells in bedrock and overburden aquifers throughout southwestern ontario was utilized in addition select wells n 36 were resampled for detailed dissolved gas composition 13 c of ch 4 c 2 c 3 and co 2 and d of ch 4 dissolved gases in groundwater from bedrock and overburden wells were composed primarily of ch 4 29 7 98 6 mol of total gas volume n 2 0 8 66 2 mol ar o 2 0 2 3 4 mol and co 2 0 1 2 mol ethane was detected but only in low concentrations 100 in situ saturation were found in bedrock wells completed in the upper devonian kettle point formation middle devonian hamilton group and dundee formation and in surficial aquifers overlying these organic rich shale bearing formations indicating that bedrock geology is the primary control on methane occurrences a few n 40 samples showed na cl br evidence of brine mixing with dilute groundwater however only one of these samples contained high 60 in situ saturation ch 4 the relatively low 13 c values of ch 4 89 9 to 57 3 covariance of d values of ch 4 and h 2 o positive correlation between 13 c values of ch 4 and co 2 and lack of higher chain hydrocarbons c 3 in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin the presence or absence of alternative electron acceptors e g dissolved oxygen fe no 3 so 4 in addition to organic substrates controls the occurrence of microbial ch 4 in shallow aquifers microbial methane has likely been accumulating in the study area since at least the late pleistocene to the present as indicated by the co variance and range of d values of ch 4 314 to 263 and associated groundwater 19 to 6 d h 2 o

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

the application of cathodoluminescence to interpreting the diagenesis of an ancient calcrete profile a laterally extensive calcrete profile has been identified in the late asbian lower carboniferous shallow marine shelf limestones of the llangollen area north wales the upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone the profile contains a unique series of early pore filling vadose cements which only occur down to 1 m below the palaeokarstic surface cathodoluminescence reveals that these cements pre date the late pore filling meteoric phreatic cements which occur throughout local asbian lithologies a spar cement stratigraphy has been established for the calcrete profile subaerial vadose cements comprise two generations of non luminescent cement followed by a brightly luminescent generation which occasionally shows an acicular habit this needle fibre calcite represents the final stage of vadose cementation precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone textures visible only with cathodoluminescence provide evidence of recurrent periods of fabric dissolution the most extensive phase of dissolution occurred immediately after the precipitation of the non luminescent subaerial vadose cements several different textures have been recorded each reflecting the morphology of a partially dissolved substrate dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures these probably acted as fluid pathways in the calcrete during early subaerial diagenesis much of the non marine micrite in the calcrete profile appears as needle fibre calcite under cathodoluminescence this acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near surface evaporation since needle fibre luminescence is strongly variable these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters needle fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite probably during the late stages of subaerial diagenesis two generations of subaerial micrite which define a micrite stratigraphy have been distinguished under cathodoluminescence reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged with many diagenetic processes acting simultaneously during a single phase of emergence

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

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

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

assessing thermal maturity of palaeozoic rocks from reflectance of chitinozoa as constrained by geochemical indicators an example from southern ontario canada the reflectance of chitinozoa chr0 was investigated as an alternative technique of determining the level of thermal maturity of organic rich palaeozoic rocks in southern ontario these sedimentary strata which include the ordovician collingwood member and the blue mountain formation as well as the devonian marcellus formation lack vitrinite precluding the application of a standard vitrinite reflectance vr0 technique chr0 shows a proportional increase at marginal to moderate maturities being on average 20 to 25 higher than expected vitrinite reflectance the reflectance data fall into a very narrow range showing a high degree of consistency for each lithostratigraphic unit the average chr0 are as follows collingwood mbr 0 63 georgian bay area and 0 88 toronto area blue mountain fm 0 92 marcellus fm 0 68 correlation with more conventional optical and geochemical maturity parameters obtained from the same set of samples fluorescence of tasmanites leiosphaeridia and gloeocapsomorpha alginite rock eval tmax extract data distribution of terpanes and steranes in extracts indicates that within the area of study the beginning of the catagenetic stage corresponds to chr0 0 65 equivalent vr 0 50 whereas the threshold of significant oil generation is reached at chr0 0 9 equivalent vr 0 70 therefore the blue mountain formation is thermally mature with respect to hydrocarbon generation throughout the whole area of study the collingwood shales are mature only in the toronto area while those occurring in the georgian bay area as well as the marcellus shales have yet to enter the main stage of hydrocarbon generation this integrated approach of assessing thermal maturity shows that chr0 when constrained with other maturity parameters is a very reliable indicator of thermal maturity in lower to middle palaeozoic sedimentary rocks

thermal maturity source rock potential and kinetics of hydrocarbon generation in permian shales from the damodar valley basin eastern india abstract this study investigates the source rock characteristics of permian shales from the jharia sub basin of damodar valley in eastern india borehole shales from the raniganj barren measure and barakar formations were subjected to bulk and quantitative pyrolysis carbon isotope measurements mineral identification and organic petrography the results obtained were used to predict the abundance source and maturity of kerogen along with kinetic parameters for its thermal breakdown into simpler hydrocarbons the shales are characterized by a high toc 3 4 mature to post mature heterogeneous type ii iii kerogen raniganj and barren measure shales are in mature late oil generation stage rr raniganj 0 99 1 22 rr barren measure 1 1 1 41 vitrinite is the dominant maceral in these shales barakar shows a post mature kerogen in gas generation stage rr barakar 1 11 2 0 and consist mainly of inertinite and vitrinite the 13corg value of kerogen concentrate from barren measure shale indicates a lacustrine marine origin 24 6 30 84 vs vpdb and that of raniganj and barakar 22 72 25 03 vs vpdb show the organic provenance to be continental the 13c ratio of thermo labile hydrocarbons c1 c3 in barren measure suggests a thermogenic source discrete bulk kinetic parameters indicate that raniganj has lower activation energies e 42 62 kcal mol compared to barren measure and barakar e 44 68 kcal mol temperature for onset 10 middle 50 and end 90 of kerogen transformation is least for raniganj followed by barren measure and barakar mineral content is dominated by quartz 42 63 siderite 9 15 and clay 14 29 permian shales in particular the barren measure as inferred from the results of our study demonstrate excellent properties of a potential shale gas system

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

carbonate oxygen isotope paleoaltimetry evaluating the effect of diagenesis on paleoelevation estimates for the tibetan plateau carbonate oxygen isotope paleoaltimetry is based on analysis of the d 18 o value of carbonate precipitated from surface water deciphering the diagenetic history is important for establishing whether particular carbonates are accurate recorders of paleosurface waters which reflect paleoelevation this study provides examples from southern east central and northeastern tibet of approaches aimed at evaluating the diagenetic history of lacustrine micrites and pedogenic carbonates the most desirable technique for avoiding erroneous interpretations related to diagenetic overprinting is to analyze carbonates that are known to be primary such as aragonitic shell material in rocks that do not contain shell material we have compared lacustrine micrites and pedogenic carbonates to diagenetic carbonate phases to determine the effects of diagenesis on the isotopic composition of primary carbonates where the potential effects of diagenesis are subtle or ambiguous we have evaluated the fidelity of the carbonate record from systematic trends in c and o isotopes that agree with other interpretations of paleoenvironment such as high frequency covariance in c and o that corresponds with changes in the mg concentration of carbonates using these strategies we have determined that diagenesis has not affected the isotopic composition of carbonates in the late miocene pliocene thakkhola graben in southern tibet and the oligocene to pliocene linxia basin in northeastern tibet in the case of paleogene basins in east central tibet however ambiguity in data precludes the determination of diagenetic effects d 2004 elsevier b v all rights reserved

elevation or alteration evaluation of isotopic constraints on paleoaltitudes surrounding the eocene green river basin low oxygen isotope values 16 relative to peedee belemnite standard obtained from microbial carbonates of the green river formation have been interpreted as evidence for snowmelt and high elevations surrounding this early eocene lake basin however low values from microbial precipitates could also represent a diagenetic overprint we investigate these alternate hypotheses by measuring the oxygen isotope composition of altered and unaltered freshwater bivalves from the basin we analyzed subannual samples from 3 individuals and bulk samples from 54 individuals subannual samples exhibit clear seasonal fluctuations in oxygen isotope values these fluctuations are large enough to require seasonal shifts in temperature and or the oxygen isotope value of the lake water however the lowest value of any unaltered sample either bulk or high resolution corresponds to lake water 18 o of 12 relative to standard mean ocean water and is not low enough to require the addition of snowmelt to the lake a few bulk samples exhibit very low oxygen isotope values which would seem to suggest snowmelt however these samples also show clear evidence of calcite in x ray diffraction patterns demonstrating the presence of diagenetic precipitates given that 1 diagenetic alteration is a plausible explanation of very low 18 o values 2 alteration has not been examined in microbial carbonates and 3 unaltered aragonitic bivalves provide no independent evidence for low 18 o values for lake waters we conclude that robust isotopic evidence for high elevations surrounding the green river basin during the eocene is currently lacking

comparison of michigan basin crude oils abstract michigan basin oils from the ordovician trenton silurian niagaran and devonian dundee formations have been geochemically compared by gc gc ms and carbon isotope mass spectrometry one oil from each formation was selected for detailed analysis which included measurement of individual n alkane 13 c values the ordovician and devonian oils are strikingly similar to one another yet clearly different from the silurian oil this pattern is unexpected because ordovician and devonian reservoirs are physically separated by the silurian strata from time temperature considerations the devonian oil probably was formed in older strata and has migrated to its present location our analyses suggest a common source for the devonian and ordovician oils

use of carbon isotopes in paleosols as an indicator of the p co2 of the paleoatmosphere measurements of carbon isotopic composition of coexisting paleosol organic matter and carbonate results in improved estimates of the paleo p co2 content of the paleoatmosphere measurements from tertiary paleosols indicate that the levels of co2 in the early eocene middle and late miocene and pliocene were less than 1000 ppmv early cretaceous levels of p co2 were significantly higher of the order of 1500 to 3000 ppmv

origins of some early carboniferous calcrete fabrics revealed by cathodoluminescence implications for interpreting the sites of calcrete formation abstract calcretes from the lower carboniferous marginal marine limestones of south wales germany and belgium exhibit complex calcite microspar fabrics with rhombic irregular and spherical crystals in variable mosaics while many of these fabrics resemble classic recrystallization microspars having irregular crystal forms and non planar crystal boundaries cathodoluminescence microscopy reveals that they result from multiple phases of precipitation and dissolution no evidence for recrystallization by crystal boundary migration or replacement during the transformation of less stable calcite minerals aragonite or high magnesian calcite is seen identical microspar fabrics have been widely described in quaternary calcareous soils which may also be primary fabrics and not due to recrystallization or mineral transformation cathodoluminescence provides a useful tool for interpreting calcrete fabrics and in the case of these carboniferous forms indicates multiple phases of carbonate saturation and leaching reflecting the prominent wetting and drying phases evidenced by associated vertic soils horizons the complex cycles of precipitation and dissolution are characteristic of pedogenic calcretes and the recognition of such features using cathodoluminescence may provide a means of differentiating pedogenic from groundwater calcretes

petrographic and geochemical characterization of organic rich mississippian black shales in the north of spain vegamian formation cantabrian zone abstract black shales 1 60 m thick with minor cherts radiolarites and phosphate nodules of mississippian age middle upper tournaisian vegamian formation occur in the fold and thrust belt of the cantabrian zone n spain at the base of the carboniferous succession which is infilling the variscan foreland basin the vegamian formation was accumulated in a similar sedimentary and paleogeographic context to the upper devonian carboniferous black shales of the appalachian arkoma fort worth western canada and antler foreland basins north america and to the variscan foreland basin in belgium and germany five stratigraphic sections were investigated in the southern branch of the cantabrian zone corresponding to the shallower sectors of the passive margin of the variscan foreland basin in n spain where this stratigraphic unit reaches a thickness of generally the organic matter is dominated by bitumen found either as diffuse masses or concentrated in spaces and cavities often parallel to the bedding planes or filling voids within calcite veins and fractures generally the bitumen masses are homogeneous without secondary porosity but occasionally they show mosaic optical texture indicating thermal transformation anastomosing structures likely related to bituminite are also common and exhibit a large range of reflectances autochthonous organic matter algae bacteria is seen as scarce tasmanites and liptodetrinite whereas allochthonous components vitrinite inertinite are also scarce except in the olleros section the range of reflectances of vitrinite like particles is similar to that of bitumen but the identification is uncertain and therefore the reflectance of bitumen brr has been used for thermal maturity assessment a gaussian like distribution of bitumen reflectances was found for la brana section brr 0 70 whereas a bimodal distribution was observed in olleros brr 0 70 and vegamian sections brr 0 95 and large scatter of readings in millaro brr 1 55 the vitrinite reflectance equivalent estimated from aromatic compounds indicates that most of the existing correlations between vitrinite and bitumen reflectances underestimate the maturity of the lowest reflecting sections and overestimate it in the highest reflecting one ts toc ratios the geochemistry of organic matter and spectral gamma ray measurements suggest a normal marine to dysoxic environment of deposition with only local anoxia at the base of the vegamian formation

tectonics or climate what drove the miocene global expansion of c4 grasslands investigations into the most plausible causes which triggered the late miocene global expansion of c4 grasslands have reached no consensus and are still being debated the global decline in co2 levels in the late miocene has been thought to be the most common driver although c4 grassland expansion was largely confined to the late miocene their first appearance varied significantly in different parts of the old and new world and therefore emphasizes role of local and or regional factors may or may not be in combination with one or more global factors the dynamic himalayan orogen constitutes a significant global tectonic event that is believed to have affected global climate hence a study of the himalayan foreland basin sediments could help in gleaning out possible causes behind this major paleoecologic event which could perhaps be directly or indirectly related stable carbon isotope analyses of a total of 141 pedogenic carbonates in the mio pleistocene siwalik paleosols of the ramnagar sub basin are coherent providing a better insight into paleovegetational changes across the sub basin on temporal and spatial scales furthermore paleovegetational history during oligocene is reconstructed from dagshai paleosols exposed in the subathu sub basin the isotopic results show dominance of c3 vegetation pre 7 ma and dominance of c4 vegetation post 5 ma percentage abundance of c4 vegetation was less than 20 pre 7 ma but increased to more than 40 post 5 ma reaching up to 100 in the youngest analyzed sediments there is exclusive dominance of c3 vegetation during oligocene these results conform to the pattern of change in vegetation documented in other parts of the himalayan belt the global expansion of c4 grasslands largely during late miocene have long been linked with then climate changes particularly brought by declining atmospheric co2 level large scale fires intensification of monsoon seasonality and aridity as demonstrated by various researchers these major hypotheses explaining expansion of c4 grasslands during late miocene are not very convincing due to significant shortcomings associated with them for example expansion of c4 grasslands in non monsoonal regions put serious constraints on the monsoonal intensification hypothesis likewise aridity as being the primary driver of c4 grassland expansion seems hard to support since there is no substantial evidence that could suggest global onset of arid conditions at about same geologic time and some most arid periods like the triassic show no evidence of c4 plants we suggest that the initial lowering of co2 below 450 ppmv created an environment for the beginning of c4 vegetation but the persistence of this threshold value for a considerable time during late miocene appears to be the probable cause of the late miocene global expansion of c4 grasslands irrespective of their time of first appearance it has been deduced through the correlation of himalayan tectonic events with atmospheric co2 levels and paleovegetational changes since upper miocene times that it was indeed the late miocene continuous intense tectonic instability of the himalayas that significantly decreased atmospheric co2 levels and which perhaps played a key role in changing the nature of photosynthetic pathways

sr c and o isotopes in carbonate nodules from the ganga plain evidence for recent abrupt rise in dissolved 87sr 86sr ratios of the ganga abstract the evolution of 87 sr 86 sr ratio of the ganga water during the past 100 ka has been reconstructed in this study the 87 sr 86 sr 13 c and 18 o values have been measured in carbonate nodules from different depths of two sediment cores raised from the indian institute of technology kanpur iitk campus 50 m long and jagdishpur jp 25 m long respectively the 87 sr 86 sr ratios of the carbonate nodules range from 0 7142 to 0 7189 in the iitk and 0 7142 to 0 7367 in the jp cores the nodules in general display significantly lower 87 sr 86 sr ratio compared to contemporary ganga river water at kanpur however values of 87 sr 86 sr ratio are observed in the nodules of the jp core near the surface are consistent with that of the present day ganga water at kanpur as well as groundwater samples from adjacent areas indicate recent abrupt increase in 87 sr 86 sr ratio of the ganga these findings are also consistent with the concomitant increase in 87 sr 86 sr ratio of silicates in contemporary sediments of the ganga at kanpur compared to that of past 100 ka the sudden rise of 87 sr 86 sr ratio of the ganga is probably due to increase in the relative proportion of sr from the lesser himalaya containing silicates and carbonates with higher 87 sr 86 sr ratio the cause for the recent increase in sr contribution from the lesser himalaya is not clear this could be due to enhanced agricultural activities and deforestation and or climatic variability during recent times resulting in more erosion in the lesser himalaya the 13 c and 18 o measured in the nodules from the iitk core vary from 6 8 to 1 6 and 8 3 to 5 4 respectively the co variation of 13 c and 18 o suggest the impact of paleoclimate paleovegetation petrography and chemical composition of carbonate nodules indicate little or no diagenetic alteration

high times on the tibetan plateau paleoelevation of the thakkhola graben nepal east west extension in the tibetan plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift on the basis of this assumption several studies have dated east west extensional structures to determine when the plateau attained its current high elevation however independent estimates of elevation are needed to determine whether extension occurred before during or after the plateau achieved its current elevation because the isotopic composition of meteoric water decreases with increasing elevation significant change in local elevation throughout the thakkhola graben depositional history should be recorded by change in 18 o values of fluvial and lacustrine carbonates the 18 o values of 16 to 23 of thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern tibetan plateau attained its current elevation prior to eastwest extension initiation of thakkhola graben extension is constrained between 10 and 11 ma based on magnetostratigraphy of the older tetang formation the 13 c values of soil carbonates suggest an age younger than 8 ma for the base of the thakkhola formation

lithofacies associations and stable isotopes of palustrine and calcrete carbonates examples from an indian maastrichtian regolith detailed information on semi arid palustrine carbonate calcrete lithofacies associations in a sheetwash dominated regolith setting is sparse this is addressed by studying the lower limestone of the lameta beds a well exposed maastrichtian regolith in central india the general vertical lithofacies assemblage for this unit comprises a basal calcareous siltstones and marls with charophytes ostracods and gastropods b buff micritic limestones associated in their upper parts with calcretized fissure fill sandstones c sheetwash as fissure fill diamictites and thin pebbly sheets locally developed over a few metres and d sandy nodular brecciated and pisolitic calcretes at the top the sequence is regressive with upsection filling of topographic lows by increased sheetwash lateral lithofacies change is marked but there are no permanent open water lake deposits in topographic lows close to the water table marshy palustrine or groundwater calcretes formed whereas on better drained highs brecciation and calcretization occurred prolonged exposure is implied suggesting that shrinkage was the main cause of brecciation evidence for rhizobrecciation and other biological calcrete fabrics is sparse contrasting with the emphasis on root related brecciation in many studies of palustrine lithofacies stable isotope d18o and d13c values are consistent with the palustrine limestones being fed from meteoric derived groundwater with a strong input of soil zone carbon there is overlap of both d18o and d13c values from the various palustrine and calcrete fabrics co occurring at outcrop this suggests that in groundwater supported wetlands conversion from palustrine carbonate to calcrete need not show isotopic expression as the groundwater source and input of soil zone carbon are essentially unchanged cretaceous tertiary d18o and d13c values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands hydrologically closed lakes and marine influenced water bodies tend to result in low negative palustrine d18o and d13c values during brecciation and calcretization the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland marked biological activity e g rhizobrecciation or root mat development during calcretization may lower d13c values where c3 plants are abundant but in shrinkage dominated systems d13c values will be largely inherited from the palustrine limestones

the stable isotope composition of early ground water cements from sandstone in paleoecological reconstruction abstract the carbon isotope composition of soil carbonates in paleosols records the global expansion of cm4 biomass in the late miocene however soil carbonate is not present in many geological sections or if present is often difficult to distinguish from other carbonate cements here we report on the 13c values of sparry early ground water cements in sandstones from the siwalik group in pakistan that record like the soil carbonates from the same sections the dramatic late miocene expansion of c4 vegetation the early cements are confined to densely cemented nodules in the sandstones but can also be found reworked as nodules into adjacent paleo channels demonstrating their formation shortly after burial the early cements are isotopically distinct from later higher temperature calcite cements our results demonstrate that some early ground water cements can be used in a manner similar to soil carbonates to reconstruct vegetation and perhaps pco2 changes in the past

petrographic maturity parameters of a devonian shale maturation series appalachian basin usa iccp thermal indices working group interlaboratory exercise abstract this paper presents results of an interlaboratory exercise on organic matter optical maturity parameters using a natural maturation series comprised by three devonian shale samples huron member ohio shale from the appalachian basin usa this work was conducted by the thermal indices working group of the international committee for coal and organic petrology iccp commission ii geological applications of organic petrology this study aimed to compare 1 maturation predicted by different types of petrographic parameters vitrinite reflectance and spectral fluorescence of telalginite 2 reproducibility of the results for these maturation parameters obtained by different laboratories and 3 improvements in the spectral fluorescence measurement obtained using modern detection systems in comparison with the results from historical round robin exercises mean random vitrinite reflectance measurements presented the highest level of reproducibility group standard deviation 0 05 for low maturity and reproducibility diminished with increasing maturation group standard deviation 0 12 corrected fluorescence spectra provided by 14 participants showed a fair to good correspondence standard deviation of the mean values for spectral parameters was lowest for the low maturity sample but was also fairly low for higher maturity samples a significant improvement in the reproducibility of corrected spectral fluorescence curves was obtained in the current exercise compared to a previous investigation of toarcian organic matter spectra in a maturation series from the paris basin this improvement is demonstrated by lower values of standard deviation and is interpreted to reflect better performance of newer photo optical measuring systems fluorescence parameters measured here are in good agreement with vitrinite reflectance values for the least mature shale but indicate higher maturity than shown by vitrinite reflectance for the two more mature shales this red shift in max beyond 0 65 vitrinite reflectance was also observed in studies of devonian shale in other basins suggesting that the accepted correlation for these two petrographic thermal maturity parameters needs to be re evaluated a good linear correlation between max and tmax for this maturation series was observed and max 600 nm corresponds to tmax of 440 c nevertheless if a larger set of devonian samples is included the correlation is polynomial with a jump in max ranging from 540 to 570 nm up to 440 c of tmax the max mostly reaches up to 500 nm beyond a tmax of 440 c max is in the range of 580 600 nm this relationship places the red shift when the onset of the oil window is reached at tmax of 440 c moreover the correlation between hi and max r2 0 70 shows a striking inflection and decrease in hi above a max of 600 nm coincident with the approximate onset of hydrocarbon generation in these rocks

environmental influences on the stable carbon isotopic composition of devonian and early carboniferous land plants abstract systematic analysis of the stable carbon isotopic composition of fossil land plants 13cp has the potential to offer new insights regarding paleoclimate variation and plant environment interactions in early terrestrial ecosystems 13cp was measured for 190 fossil plant specimens belonging to 10 genera of early to late devonian age archaeopteris drepanophycus haskinsia leclercqia pertica psilophyton rhacophyton sawdonia tetraxylopteris and wattieza and 2 genera of early carboniferous age genselia and rhodeopteridium collected from sites located mainly in the appalachian basin 22 30 s paleolatitude for the full carbon isotopic dataset n 309 13cp ranges from 20 3 to 30 5 with a mean of 25 5 similar to values for modern c3 land plants in addition to a secular trend 13cp exhibits both intra and intergeneric variation intrageneric variation is expressed as a small mean 0 45 13c enrichment of leaves and spines relative to stems that may reflect differential compound specific compositions intergeneric variation is expressed as a much larger to 5 spread in the mean 13cp values of coeval plant genera that was probably controlled by taxon specific habitat preferences and local environmental humidity among early devonian taxa sawdonia yielded the most 13c depleted values 27 1 1 7 reflecting lower water use efficiency that was probably related to growth in wetter habitats and leclercqia haskinsia and psilophyton yielded the most 13c enriched values 23 0 1 6 22 3 1 3 and 24 8 1 6 respectively reflecting higher water use efficiency probably related to growth in drier habitats

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

dynamic fluvial systems and gravel progradation in the himalayan foreland although the large scale stratigraphy of many terrestrial foreland basins is punctuated by major episodes of gravel progradation the relationships of such facies to hinterland tectonism and climate change are often unclear structural reentrants provide windows into older and more proximal parts of the foreland than are usually exposed and thus provide key insights to earlier phases of foreland evolution our magnetostratigraphic studies show that although the major lithofacies preserved within the himachal pradesh structural reentrant in northwestern india resemble neogene facies in pakistan they have a much greater temporal and spatial variability from 11 5 to 7 ma major facies boundaries in himachal pradesh vary by as much as 2 3 m y across distances of 20 30 km and are controlled by the interference between a major southeastward flowing axial river and a major southwestward flowing transverse river a thick but highly confined middle to late miocene conglomerate facies includes the oldest extensive siwalik conglomerates yet dated 10 ma and implies the development of significant erosional topography along the main boundary thrust prior to 11 ma our studies document extensive syntectonic gravel progradation with conglomerates extending tens of kilometers into the undeformed foreland during a period of increased subsidence rate and within 1 2 m y of major thrust initiation overall gravel progradation is modulated by the interplay among subsidence sediment supply and the proportion of gravels in rivers entering the foreland

evolution of porosity and geochemistry in marcellus formation black shale during weathering abstract soils developed on the oatka creek member of the marcellus formation in huntingdon pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic rich shale making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic rich shale we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings this difference is attributed to differences in composition both shales are dominantly quartz illite and chlorite but the oatka creek member at this location has more organic matter 1 25 wt organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops and accessory pyrite during weathering the extremely low porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures some of these pores are either filled with organic matter or air filled but remain unconnected and thus inaccessible to water based on weathering bedrock soil profiles disintegration is initiated with oxidation of pyrite and organic matter which increases the overall porosity and most importantly allows water penetration water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays as these dissolution reactions proceed the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7 while kaolinite and fe oxyhydroxides precipitate eventually near the land surface mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20 as imaged by computed tomographic analysis weathering causes i greater porosity ii greater average length of connected pores and iii a more branched pore network compared to the unweathered sample this work highlights the impact of shale water o 2 interactions in near surface environments 1 black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized and 2 black shales weather more quickly than less organic and sulfide rich shales leading to high porosity and mineral surface areas exposed for clay weathering the fast rates of shale gas exploitation that are ongoing in pennsylvania texas and other regions in the united states may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low temperature processes in our field study

morphology and isotope heterogeneity of late quaternary pedogenic carbonates implications for paleosol carbonates as paleoenvironmental proxies abstract stable isotope values and radiocarbon ages were determined for pedogenic carbonate microsamples from relict soils and palaeosols from the rio grande rift region of new mexico carbonate nodules and clusters were sampled from fluvial and piedmont soils and palaeosols with widely varying exposure durations 7 900 ky pedogenic carbonates from individual soils exhibited irregular relationships between age and depth ranging in radiocarbon age by 13 ky at individual depths 18 o values and to a lesser extent 13 c values likewise had wide ranges within soils however comparing the stable isotope values of carbonates with similar radiocarbon ages within and between soils revealed consistent trends in stable isotope values over time 18 o values of pedogenic carbonates tended to decrease with decreasing age 25 17 radiocarbon ka and increased with decreasing age 13 c values with either depth or age although the oldest palaeosol piedmont carbonates radiocarbon ages of 24 5 19 2 ka are 1 more negative than the younger piedmont palaeosol carbonates pedogenic carbonates in the relict fluvial soil which has by far the longest exposure period showed the greatest range in 13 c values comparing the 13 c values of samples of similar radiocarbon age from the relict fluvial soil indicates a transition from a mixed c 3 c 4 ecosystem at 11 ka to an effectively pure c 3 ecosystem at 9 ka followed by a return to mixed vegetation by 5 ka thus although c 4 vegetation persisted in southern new mexico through the last glaciation drought resistant c 3 vegetation became dominant at 9 ka prior to significant strengthening of the summer monsoon this study illustrates that the stable isotope values of carbonate nodules and clusters from relict soils and or slowly buried palaeosols can be used for palaeoenvironmental reconstructions in the context of a detailed chronologic framework

chitinozoan reflectance a lower palaeozoic thermal maturity indicator abstract chitinozoan reflectance r ch is an innovative technique of thermal maturation determination suitable for lower palaeozoic sediments where the established reflectance scale vitrinite reflectance r v is not applicable due to the lack of vitrinite reflectance measurements are taken from polished sections of chitinozoa a group of lower palaeozoic organic walled marine microfossils these vase shaped palynomorphs are optically isotropic have thick homogeneous test walls and are easy to identify making then ideal for organic petrology from over 1000 samples collected 54 possessed both chitinozoa and vitrinite enabling direct calibration of the two scales a clear linear relationship exists between the two parameters the equation resulting from this study is r ch 1 152 r v 0 08 with a correlation coefficient of 0 989 there is a suggestion of a maturity discontinuity between 1 and 2 r ch but due to the high sample redundancy rate only 5 of samples possessed both vitrinite and chitinozoa it was not possible to supply more data at this maturity level chitinozoan reflectance is a powerful thermal maturity indicator for lower palaeozoic sediments the technique has significant advantages over its maln rivals particularly the ease of particle identification and the fact that it is independent of facies thus quantitative maturity data with a high spatial and temporal resolution can be gained from pre devonian marine sedimentary sequences

expansion of c4 grasses in the late miocene of northern pakistan evidence from stable isotopes in paleosols stable isotopic clay mineralogic and bulk chemical analyses were conducted on paleosols of the neogene siwalik sections in northern pakistan in order to reconstruct floodplain environments over the past 17 ma the stable carbon isotopic composition of soil carbonate mean 13c pdb 10 2 and associated organic matter mean 13c pdb 24 1 in paleosols representing 17 7 3 ma reveal that floodplain vegetation was dominated by c3 plants at 7 3 ma a shift toward more positive carbon isotopic values began signaling the gradual expansion of c4 grasses onto the floodplain from 6 ma to present carbon isotopic values for paleosol carbonate mean 13c pdb 0 6 and organic matter mean 13c pdb 14 4 are uniformly enriched in 13c indicating the presence of nearly pure c4 grassland the scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1 0 1 25 m yr or less for the entire 17 ma of record paleosols in the lower portion of the section lack organic a horizons but have reddish b horizons often containing secondary iron oxide nodules leaching depths of soil carbonate in these older paleosols are typically greater than those in the plio pleistocene part of the section where organic a horizons are common and b horizons are markedly more yellow the combined evidence suggests that the mature paleosols in the pre 7 3 ma part of the record are dominantly calcareous alfisols or mollisols that once underlay nearly pure c3 vegetation perhaps trees and shrubs while calcareous mollisols underlying c4 grassland dominate the upper part of the record the carbon and oxygen isotopic trends in the paleosol record in pakistan are also evident in the diet of fossil mammals and in paleosols from nepal thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent wide local effects such as the development or intensification of the asian monsoon driven by uplift of the tibetan plateau may have led to the expansion of c4 grasses if however the expansion of c4 grasses proves globally synchronous then a larger scale cause such as a marked decrease in co2 may be the driving mechanism

a reactive transport model for marcellus shale weathering abstract shale formations account for 25 of the land surface globally and contribute a large proportion of the natural gas used in the united states one of the most productive shale gas formations is the marcellus a black shale that is rich in organic matter and pyrite as a first step toward understanding how marcellus shale interacts with water in the surface or deep subsurface we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions constrained by soil and water chemistry data the simulation was carried out for 10 000 years since deglaciation assuming bedrock weathering and soil genesis began after the last glacial maximum results indicate weathering was initiated by pyrite dissolution for the first 1000 years leading to low ph and enhanced dissolution of chlorite and precipitation of iron hydroxides after pyrite depletion chlorite dissolved slowly primarily facilitated by the presence of co2 and organic acids forming vermiculite as a secondary mineral a sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases co2 and o2 specific surface area and flow velocity of infiltrating meteoric water the soil chemistry and mineralogy data could not be reproduced without including the reactive gases for example pyrite remained in the soil even after 10 000 years if o2 was not continuously present in the soil column likewise chlorite remained abundant and porosity remained small if co2 was not present in the soil gas the field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1 3 orders of magnitude smaller than surface area values measured for powdered minerals small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings in addition some mineral surface is likely interacting only with equilibrated pore fluids an increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far from equilibrium conditions we conclude from these observations that availability of reactive surface area and transport of h2o and gases are the most important factors affecting rates of marcellus shale weathering of the in the shallow subsurface this weathering study documents the utility of reactive transport modeling for complex subsurface processes such modelling could be extended to understand interactions between injected fluids and marcellus shale gas reservoirs at higher temperature pressure and salinity conditions

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

inverse correlation between the molybdenum and uranium isotope compositions of upper devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations abstract coupled mo u isotope data from the upper devonian kettle point formation ontario canada provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales in the gore of chatham core the kettle point black shales have high mo concentrations 48 473 g g and consistently high mo u ratios 3 times the mo u ratio of modern seawater suggesting a euxinic depositional environment these shales yield an inverse correlation r 0 89 p

stable isotopic results from paleosol carbonate in south asia paleoenvironmental reconstructions and selective alteration article i nfo we measured 18 o and 13 c values of paleosol carbonate nodules from a variety of sedimentary units in southern asia in order to reconstruct paleoenvironmental conditions and to evaluate for possible diagenetic alteration paleosol carbonate nodules were collected from lower to upper cretaceous strata exposed in two locations in southern tibet and from lower to middle miocene strata in northern india and nepal additional samples were collected from stratigraphically adjacent marine carbonate units to test for possible resetting of stable isotope values based on the implausibly low 18 o values 13 of most of the marine carbonate samples we interpret the 18 o values of the associated paleosol carbonate nodules as being reset and therefore unreliable for paleoenvironmental or paleoelevation reconstruction importantly the 18 o values of 13 c values of marine carbonate rocks fall within the expected primary

hydrocarbon biomarkers thermal maturity and depositional setting of tasmanite oil shales from tasmania australia this study represents the first geological and organic geochemical investigation of samples of tasmanite oil shale representing different thermal maturities from three separate locations in tasmania australia the most abundant aliphatic hydrocarbon in the immature oil shale from latrobe is a c19 tricyclic alkane whereas in the more mature samples from oonah and douglas river low molecular weight n alkanes dominate the extractable hydrocarbon distribution the aromatic hydrocarbons are predominantly derivatives of tricyclic compounds with 1 2 8 trimethylphenanthrene increasing in relative abundance with increasing maturity geological and geochemical evidence suggests that the sediments were deposited in a marine environment of high latitude with associated cold waters and seasonal seaice it is proposed that the organism contributing the bulk of the kerogen tasmanites occupied an environmental niche similar to that of modern sea ice diatoms and that bloom conditions coupled with physical isolation from atmospheric co2 led to the distinctive isotopically heavy 13c values 13 5 to 11 7 for the kerogen 13c data from modern sea ice diatoms 7 supports this hypothesis isotopic analysis of n alkanes in the bitumen 13 5 to 31 suggest a multiple source from bacteria and algae on the other hand the n alkanes generated from closed system pyrolysis of the kerogen 15 are mainly derived from the preserved tasmanites biopolymer algaenan the tricyclic compounds mean 8 both in the bitumen and pyrolysate have a common precursor they are consistently enriched in 13c compared with the kerogen and probably have a different source from the n alkanes the identification of a location where the maturity of the tasmanite oil shale approaches the oil window raises the possibility that it may be a viable petroleum source rock

differential preservation of primary isotopic signatures in silurian brachiopods from northern europe abstract isotopic and geochemical compositions of silurian calcitic brachiopod shells carbonate host rocks and calcite cements from two northern european sites with different diagenetic histories are compared gotland sweden oslo graben norway the sequence exposed on gotland is characterized by repeated synsedimentary subaerial exposure and meteoric influence but only insignificant thermal alteration during shallow burial diagenesis in contrast silurian sediments exposed in the oslo area show only few signs of early exposure but were severely affected by thermal alteration during burial of the succession the preservation potential of brachiopod shells in each basin is different most nonluminescent brachiopods from gotland show well preserved microstructures and trace element contents very similar to those of modern brachiopod shells tightly clustered isotope values of individual brachiopod populations do not suggest significant meteoric alteration of primary marine isotope signatures in contrast there is strong evidence that all brachiopod shells from the oslo area contain diagenetically altered 18o values alteration of norwegian brachiopods is not always reflected in their trace element contents and cathodoluminescence characteristics but under sem the shells often display corrosion and recrystallization scattered oxygen isotope values in nonluminescent brachiopods 18o 18 to 5 5o oo together with low 18o values the case study from the oslo graben demonstrates that high fluid rock ratios and thermal overprinting during burial diagenesis may result in pervasive diagenetic alteration of 18o values in nonluminescent brachiopod shells this alteration can be detected by combined petrographic and geochemical investigations on brachiopod shells diagenetic cements and carbonate host rocks brachiopods from deeply buried and hydrothermally altered sites should not be used for deciphering marine 18o signals however the data presented here argue against a general rejection of lower paleozoic brachiopods as indicators of marine isotope signatures

bakken madison petroleum systems in the canadian williston basin part 1 c21 c26 20 n alkylpregnanes and their triaromatic analogs as indicators for upper devonian mississippian epicontinental black shale derived oils abstract a series of 20 n alkylpregnanes and their triaromatic analogs have been tentatively identified in crude oils and source rock extracts from the western canada sedimentary basin when found in oils in this basin these compounds may serve as molecular indicators for source input from the uppermost devonian mississippian epicontinental black shales crown copyright

application of organic petrography in north american shale petroleum systems a review abstract organic petrography via incident light microscopy has broad application to shale petroleum systems including delineation of thermal maturity windows and determination of organo facies incident light microscopy allows practitioners the ability to identify various types of organic components and demonstrates that solid bitumen is the dominant organic matter occurring in shale plays of peak oil and gas window thermal maturity whereas oil prone type i ii kerogens have converted to hydrocarbons and are not present high magnification sem observation of an interconnected organic porosity occurring in the solid bitumen of thermally mature shale reservoirs has enabled major advances in our understanding of hydrocarbon migration and storage in shale but suffers from inability to confirm the type of organic matter present herein we review organic petrography applications in the north american shale plays through discussion of incident light photographic examples in the first part of the manuscript we provide basic practical information on the measurement of organic reflectance and outline fluorescence microscopy and other petrographic approaches to the determination of thermal maturity in the second half of the paper we discuss applications of organic petrography and sem in all of the major shale petroleum systems in north america including tight oil plays such as the bakken eagle ford and niobrara and shale gas and condensate plays including the barnett duvernay haynesville bossier marcellus utica and woodford among others our review suggests systematic research employing correlative high resolution imaging techniques and in situ geochemical probing is needed to better document hydrocarbon storage migration and wettability properties of solid bitumen at the pressure and temperature conditions of shale reservoirs

organic geochemistry and oil source correlations paleozoic of ohio routine analytical methods and new kerogen kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in ohio in the paleozoic section four good to excellent source units were identified the mississippian sunbury shale the devonian ohio shale and olentangy shale and the shales of the ordovician point pleasant formation these marine shales are dominantly oil prone with maturation levels that span the immature to peak oil generation range no source rocks examined exceeded an ro of 1 0 1 1 or equivalent the reservoired oils in ohio are mostly supermature based on normal alkanes pristane n c17 to phytane n c18 ratios low asphaltene and sulfur contents and high paraffin indices three basic oil groups were identified using oil characterization procedures group 1 consists of cambrian ordovician and some silurian oils group 2 consists of some silurian devonian one mississippian and some pennsylvanian oils and group 3 consists of oil from the mississippian berea sandstone oil source rock correlation techniques permitted the assignment of probable source rocks to each oil group group 1 oil was probably derived from the point pleasant shales group 2 oil was derived from the ohio shale and olentangy shale and group 3 oil came from the sunbury shale because the supermature oils in ohio occur in reservoirs with maturities generally under 0 8 ro the oils would have had to migrate from deep within the appalachian basin migration could have followed fracture zones unconformities or extensive and permeable units such as the silurian clinton sands

correlation of the subsurface lower and middle devonian of the lake erie region the completion of 6 detailed cross sections using the gamma ray logs of 94 closely spaced wells has provided new correlations and confirmed some earlier correlations of the lower and middle devonian strata of the lake erie region these sections connect western new york northwestern pennsylvania northern central and northwestern ohio southwestern ontario and southeastern michigan the lower devonian bois blanc limestone of western new york is continuous with the type bois blanc of northern michigan via southwestern ontario it has been traced southwestward across northern ohio and southward into the lower part of the type columbus limestone in central ohio the edgecliff clarence nedrow and moorehouse members of the overlying middle devonian onondaga limestone of western new york correlate with the upper columbus limestone of ohio the seneca member of the onondaga is equivalent to the overlying delaware limestone the amherstburg and lucas formations of the detroit river group of michigan and ontario can be traced southward across lake erie into the lower and middle parts of the upper columbus of ohio the amherstburg correlates with the edgecliff clarence nedrow and lower moorehouse members of the new york onondaga the lucas correlates with the middle part of the moorehouse member the dundee limestone of northwestern ohio michigan and ontario is an equivalent of the upper moorehouse and seneca members of the new york onondaga limestone it is not coterminous with the delaware of ohio as the base of the dundee is significantly older than the base of the delaware these correlations have been strengthened by tracing the stratigraphic positions of four different ash beds through the upper part of the carbonate sequence three of these ash beds are parts of the type tioga ash bed in the subsurface of northern pennsylvania a major regional unconformity separates the lower and middle devonian carbonate sequence from the overlying middle devonian terrigenous one the delaware and dundee limestones cannot be correlated with any part of the marcellus formation of the new york hamilton group in western new york the cherry valley limestone member of the marcellus formation rests directly and unconformably upon the onondaga in northwestern pennsylvania and northeastern ohio the union springs shale reappears between the cherry valley and the onondaga beneath central lake erie the union springs cherry valley and oatka creek members of the marcellus merge into the bell shale rockport quarry limestone and arkona shale of ontario the arkona can be traced across lake erie into the plum brook and lower olentangy shales of ohio the lower part of the terrigenous sequence of the lake erie region consequently is marcellus in age not skaneateles the prout ohio and hungry hollow ontario limestones are homotaxial and correlate with the centerfield limestone of new york the rockport quarry arkona and hungry hollow can be traced into the silica shale of southeastern michigan and northwestern ohio the widder shale and ipperwash limestone of ontario merge into the tenmile creek dolomite of michigan and ohio

paleoelevation reconstruction using pedogenic carbonates paleoelevation reconstruction using stable isotopes although a relatively new science is making a significant contribution to our understanding of the recent growth of the world s major orogens in this review we examine the use of both light stable isotopes of oxygen and the new clumped isotope 47 carbonate thermometer in carbonates from soils globally the oxygen isotopic composition 18 o of rainfall decreases on average by about 2 8 km of elevation gain this effect of elevation will in turn be archived in the 18 o value of soil carbonates and paleoelevation can be reconstructed provided 1 temperature of formation can be estimated 2 the effects of evaporation are small 3 the effects of climate change can be accounted for and 4 the isotopic composition of the carbonate is not diagenetically altered we review data from modern soils to evaluate some of these issues and find that evaporation commonly elevates 18 o values of carbonates in deserts an effect that would lead to underestimates of paleoelevation some assessment of paleoaridity using qualitative indicators or carbon isotopes from soil carbonate is therefore useful in evaluating the oxygen isotope based estimates of paleoelevation sampling from deep 50 cm in paleosols helps reduce the uncertainties arising from seasonal temperature fluctuations and from evaporation the new clumped isotope 47 carbonate thermometer expressed as 47 offers an independent and potentially very powerful approach to paleoelevation reconstruction in contrast to the use of 18 o values nothing need be known about the isotopic composition of water from which carbonate grew in order to estimate of temperature of carbonate formation from 47 values using assumed temperature lapse rates with elevation paleoelevations can thereby be reconstructed case studies from the andes and tibet show how these methods can be used alone or in combination to estimate paleoelevation in both cases the potential for diagenetic alteration of primary carbonate values first has to be assessed clear examples of both preservation and alteration of primary isotopic values are available from deposits of varying ages and burial histories 47 values constitute a relatively straightforward test since any temperature in excess of reasonable surface temperatures points to diagenetic alteration for 18 o values preservation of isotopic heterogeneity between different carbonate phases offers a check on diagenesis results of these case studies show that one area of south central tibet attained elevations comparable to today by the late oligocene whereas 2 7 0 4 km of uplift occurred in the bolivian altiplano during the late miocene

primary and diagenetic isotopic signals in fossils and hemipelagic carbonates the lower jurassic of northern spain stable isotope and trace element analyses of 230 jurassic pliensbachian toarcian samples from northern spain have been performed to test the use of geochemical variations in fossils belemnites and brachiopods and whole rock hemipelagic carbonates as palaeoceanographic indicators although the succession analysed reinosa area westernmost basque cantabrian basin has been subject to severe thermal alteration during burial diagenesis the samples appear to be well preserved the degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic chemical and cathodoluminescence analyses it is demonstrated that brachiopods and whole rock carbonates although widely used for palaeoceanic studies do not retain their primary marine geochemical composition after burial diagenesis in contrast there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock well preserved belemnite shells non luminescent to slightly luminescent typically show stable isotope values of 4ae3 to 0ae7 d 13 c 0ae7 to 3ae2 d 18 o and trace element contents of 950 l gg 1 sr and sr mn ratios 80 this study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low mg calcite fossil shells and hemipelagic bulk carbonates behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated multiple non luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders yet their d 13 c and d 18 o values exhibited moderate scatter such variability is likely to be related to the palaeoecological behaviour of belemnites and or highfrequency secular variations in sea water chemistry superimposed on the longterm isotopic trend a pronounced positive carbon isotope excursion up to 4ae3 is documented in the early toarcian serpentinus biozone which correlates with the toarcian d 13 c maximum reported in other european and tethyan regions

chitinozoan reflectance in the lower palaeozoic of the welsh basin vitrinite woody matter and spores are generally uncommon or absent in pre devonian sediments to determine organic maturity in the lower palaeozoic alternative methods such as chitinozoan reflectance must be employed chitinozoa are marine vase shaped organic walled microfossils of unknown affinity the reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and due to their relative facies independency a high degree of resolution to regional maturity studies calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre devonian sediments the range of chitinozoan reflectance for the welsh basin is from under 1 to 8 this corresponds to estimated palaeotemperatures of under 100 c to over 300 c the regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the welsh basin is attributed to palaeobasin form the increase in reflectance with age in the rocks of the myddfai steep belt and associated areas allows an estimation of a palaeogeothermal gradient in the silurian sequences around llandovery the gradient is calculated to be in the region of 50 c km 1 similar gradients are derived from other studies clay minerals metabasite mineral assemblages conodont colours the results from myddfai steep belt suggest the thermal peak occurred during the acadian event chitinozoan reflectivity demonstrates that with respect to hydrocarbon generation mature conditions occur on the palaeoplatform where the sedimentary sequences are relatively thin overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed

sensitivity of biomarker properties to depositional environment and or source input in the lower toarcian of sw germany abstract a sequence of lower toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil to source correlation lower liassic marls are representative of a well aerated shallow water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions various properties have been found to vary considerably within these two units amongst the most important are carbon isotopic composition of the kerogen the pristane phytane ratios nickel as opposed to vanadyl porphyrins and the c27 dia regular steranes although maturation within the profile does not change some of the maturation dependent biomarker properties such as the monoaromatic steroid side chain cracking and the tm ts ratio exhibit large changes which can be assigned to diagenetic processes another maturation dependent property the 20s 20r epimerization of c29 steranes exhibits smaller changes which could also be due to early diagenetic processes the study suggests that reducing and oxidizing conditions i e eh and ph in the sediment exert an influence on several biomarker precursor product pathways in maturation studies initial variations due to depositional conditions have therefore to be taken into account

carbon and oxygen isotopic variability in neogene paleosol carbonates constraints on the evolution of the c4 grasslands of the great plains usa abstract understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas we focus on the central and southern portions of the great plains which is the largest continuous grassland in north america and has a well documented mammalian fossil record modern biomass in this region is dominated by c4 grasses but paleontological evidence indicates the region was forested prior to the middle miocene implying the region was formerly dominated by c3 plants we examine long and short term variations in the abundance of c4 grasses and climatic conditions in the central and southern great plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region the carbon isotope composition 13c of paleosol carbonate reflects the proportion of c3 and c4 plants that grew in an ancient soil the long term pattern of 13c values indicates that the percentage of c4 grasses in the great plains was moderate ca 20 throughout most of the miocene increased from 6 4 to 4 0 ma and reached modern levels by 2 5 ma based on the range in 13c values within sections and the mean 13c values for different carbonate morphologies that reflect different intervals of carbonate accumulation the abundance of c4 biomass did not vary substantially on short time scales the variability in c4 biomass on short time scales does not correlate with age of section latitude longitude or the mean percentage of c4 biomass the oxygen isotope composition 18o of soil carbonate is controlled by soil temperature and the 18o value of soil water great plains paleosol 18o values vary considerably in relation to time and latitude removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of neogene climate change quantitative interpretation of the 18o record is difficult without independent control on temperature or the oxygen isotope composition of soil water both of which have probably varied with long term climate change comparisons of measured 18o values for each section with predicted values based on modern temperature data and estimated modern meteoric water 18o values suggest that 1 most miocene sections have 18o values consistent with warmer temperatures and more positively shifted soil water 18o values than today 2 three miocene sections in western nebraska probably had soil water compositions that were more negative than similar aged localities in the region and 3 the plio pleistocene sections have 18o values consistent with cooler temperatures and more negative soil water 18o values than today the paleosol 13c values do not correlate strongly with the pattern of climate change recorded by the 18o values suggesting that long term change in mean temperature is not the main control on the abundance of 13c biomass in the great plains we propose two testable hypotheses to account for our data and other published data that bear on the evolution of the great plains ecosystem

evidence for mixing of re os isotopes at 2 7 ga and support of a remobilized placer model in witwatersrand sulfides and native au abstract this study analyzes re and os contents of au rich sulfides from the carbon leader tau lekoa and mponeng the vcr mponeng and tau lekoa and the black reef modder east as well as native au in the carbon leader to further investigate the detrital placer modified detrital placer and hydrothermal models for the source of au in mines from the witwatersrand an isochron plot of the results from the four locations yields an imprecise isochron age of 2381 140 ma mswd 17 and initial 187 os 188 os 0 24 0 21 isochron plots for the individual reefs also yielded ages that are younger than the deposition of the basin witwatersrand sediments samples that possess higher concentrations of os 100 ppb and low 187 re 188 os ratios to unify all of the data providing a large variation of chronologic information os mixing diagrams for both gold and sulfide minerals produce realistic mixing trends when solving os initial ratios for 2050 ma the two most probable end members for the mixing relationships are a high os concentration non radiogenic source and a low os concentration highly radiogenic source the high concentration non radiogenic os source has been measured in multiple studies and has been clearly derived from the subcontinental lithospheric mantle sclm the low concentration highly radiogenic source is debatable the measured concentrations of os in the witwatersrand ores are much greater than published os concentrations in other hydrothermal ores the data could indicate a redistribution of the detrital os signatures at 2050 ma thus a modified detrital model best explains the previously published data and this data

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

oil generation in the michigan basin a biological marker and carbon isotope approach abstract palaeozoic strata in the michigan basin produced crude oils which despite their rather small volume and economic value are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting based on n alkane profiles biological marker distributions and carbon isotope ratios the oils belong to three main families of different genetic origin and a few less important mixed types silurian oils from salina and niagara limestones have broad n alkane distributions and abundant isoprenoid hydrocarbons a strong phytane over pristane predominance and the lack of diasteranes indicate a carbonate source for these oils chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n alkanes oils found in the calcareous ordovician trenton formation contain n alkane cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils many oils from the devonian dundee reservoirs are very similar in overall composition to the trenton oils devonian traverse oils are considered to be mainly from a devonian source with some contribution of ordovician type oil and are of intermediate maturity based on calculations using kinetic parameters of biological marker reactions considerably deeper subsidence of the devonian source rock in the past is implied

soils at the hyperarid margin the isotopic composition of soil carbonate from the atacama desert northern chile we evaluate the impact of exceptionally sparse plant cover 0 20 and rainfall 2 114 mm yr on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet the atacama desert in northern chile d 13 c and d 18 o values of carbonates from the atacama are the highest of any desert in the world d 13 c vpdb values from soil carbonate range from 8 2 at the wettest sites to 7 9 at the driest we measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of cerling 1984 earth planet sci lett 71 229 240 in which we assumed an exponential form of the soil co2 production function and relatively shallow 20 30 cm average production depths overall we find that respiration rates are the main predictor of the d 13 c value of soil carbonate in the atacama whereas the fraction c3 to c4 biomass at individual sites has a subordinate influence the high average d 13 c value 4 1 of carbonate from the driest study sites indicates it formed perhaps abiotically in the presence of pure atmospheric co2 d 18 o vpdb values from soil carbonate range from 5 9 at the wettest sites to 7 3 at the driest and show much less regular variation with elevation change than d 13 c values d 18 o values for soil carbonate predicted from local temperature and d 18 o values of rainfall values suggest that extreme 80 in some cases soil dewatering by evaporation occurs at most sites prior to carbonate formation the effects of evaporation compromise the use of d 18 o values from ancient soil carbonate to

organic petrology and organic geochemistry of devonian black shales in southwestern ontario canada abstract the hydrocarbon source rock potential of the devonian marcellus and kettle point black shales in southern ontario has been assessed using rock eval pyrolysis gas chromatography gas chromatography mass spectrometry and incident light microscopy both formations contain sufficient hydrogen rich organic matter type ii to be considered possible source rocks the kerogen is predominantly marine in origin this organic matter was deposited in a clastic algal dominated environment and mainly consists of unstructured bituminite and unicellular alginite the organic microfacies indicate shelf paleodepositional settings with a transition to an open basinal region the optical reflectance fluorescence and geochemical rock eval tmax extract yields distributions of steranes and terpanes maturity data indicate that the organic matter in the kettle point formation is immature whereas in the marcellus formation it has reached the early stages of hydrocarbon generation and expulsion some devonian oils occurring in this area have characteristics similar to the marcellus extracts suggesting that more mature equivalent units from the appalchian basin could be their source

stable isotopic composition of pedogenic carbonates of the early cretaceous shimonoseki subgroup western honshu japan abstract abundant pedogenic carbonate nodules are present in the shimonoseki subgroup in the kanmon basin of kyushu and honshu japan the oxygen isotope compositions of these pedogenic carbonates range from 20 1 to 22 8 pdb which is 7 lower than those of the septarian crystallaria in the carbonate nodules the meteoric water composition estimated from the oxygen isotope composition of the soil water in equilibrium with the carbonates is much depleted compared with the estimate from other sources this suggests that the oxygen isotope compositions of the shimonoseki pedogenic carbonates were modified during diagenesis the carbon isotopic compositions of the shimonoseki carbonates range from 5 4 to 7 0 pdb with an average of 6 7 pdb suggesting carbonate formation in soils dominated by c 3 type of vegetation an analysis of these values using cerling s model cerling t e wright v p vanstone s d 1992 further comments on using carbon isotopes in paleosols to estimate the co 2 content of the palaeo atmosphere j geol soc london 148 1991 945 947 149 673 676 indicates that the partial pressure of co 2 in the early cretaceous shimonoseki atmosphere was about 1700 3200 ppmv

fabric retentive non luminescent brachiopods as indicators of original delta 13 c and delta 18 o composition a test abstract fabric retentive non luminescent helderbergian brachiopods from open marine deposits analogous to brachiopods used in other studies to determine paleo oceanic stable isotopic composition exhibit depleted 13c values immediately below the erosional contact and more enriched values of 13c at greater stratigraphic distances below this surface in addition 13c values of these brachiopods display a marked shift across the erosional surface and values above it are essentially the same as those 30 m or more beneath the surface complementing these trends whole rock analyses metastable high agnesian calcite constituents echinoderms as well as early equant calcite cement also exhibit an increased depletion trend in 13c values as the helderberg oriskany contact is approached from below fabric retentive luminescent as well as obviously recrystallized brachiopods display stable isotopic values similar to those of fabric retentive non luminescent brachiopods thus best preserved fabric retentive non luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods diagenetically altered metastable constituents echinoderms stable early meteoric cements and whole rock samples helderberg strata of new york state were affected by early meteoric diagenesis during a siegenian exposure event prior to deposition of the overlying marine oriskany sandstone meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the helderberg oriskany contact all of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters consequently the practice of using fabric retentive non luminescent brachiopods as indicators of their original chemistry and by extension paleo oceanic chemistry is questioned

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

biomarkers as environmental indicators in a carbonate complex example from the middle to upper devonian holy cross mountains poland abstract autochthonous biomarker distributions were studied in an array of different subfacies of a devonian carbonate complex represented in platform reef and off platform shelf basinal systems it revealed that biomarkers are sensitive indicators of depositional environments and may serve to refine palaeoenvironmental interpretations of carbonate complexes based on classical sedimentological facies analyses the contrasting biomarker composition of the carbonate platform and reef complex versus shelf basin deposits is here interpreted as the effect of both more diverse biotic sources of a primary organic matter in the former environments highly scattered results of sterane distribution and contrasting styles of decay and bacterial reworking of the primary biomass peritidal dolostones deposited under anoxic to suboxic conditions and with scarce sedimentological evidence of elevated salinity reveal the highest potential of organic matter preservation among other platform reefal facies their biomarker composition includes several compounds indicating hypersaline conditions and anoxia whereas others in particular gammacerane indicate that the water column was stratified during sedimentation this observation constrains environmental interpretation of these deposits pointing to shallow subtidal lagoons with elevated salinity and low oxygen near bottom waters most probably originating from a salinity controlled stratification of the water column biomarkers diagnostic of green sulphur bacteria were found in both peritidal lagoonal dolostones and shelf basinal facies this indicates the existence of photic zone anoxia in the latter environment thus suggesting a shallow position of the redoxcline the frasnian famennian mass extinction event s apparently had no impact on the biomarker distribution in the shelf basinal system continuous across the stage boundary

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

the takena formation of the lhasa terrane southern tibet the record of a late cretaceous retroarc foreland basin our understanding of the processes involved in the indo asian collision and the construction of the tibetan plateau are in part predicated on our understanding of the tectonic setting and crustal conditions of southern tibet in the time period immediately preceding the indo asian collision late cretaceous several hypotheses have been proposed that describe the middle to late cretaceous tectonic and paleogeographic evolution of southern tibet each with different implications for when and how the tibetan plateau was uplifted we examined the mid upper cretaceous takena formation of the lhasa terrane in southern tibet in order to reconstruct the middle to late cretaceous tectono sedimentary history of this area and test the competing tectonic hypotheses the takena formation consists of 2 km of sedimentary strata that include a lower marine limestone member penbo member and an upper member of clastic red beds lhunzhub member the aptian albian penbo member consists of 250 m of orbitolinid limestone beds that were deposited in a shallow marine seaway during a time when there was little regional subsidence the overlying lhunzhub member 1500 m consists primarily of fl strata that were deposited during a period of increased subsidence overall the lhunzhub member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base to multistory multilateral braided stream deposits near the top the fl uvial sandstone units are lithic rich and contain abundant plagioclase and volcanic grains and paleocurrent data record northwest directed transport these data indicate that the sediment was derived from the gangdese volcanic arc that developed along the southern margin of the lhasa terrane following deposition but prior to ca 70 ma the beds of the takena formation were folded and partially eroded the sedimentary and stratigraphic characteristics of the takena formation are most consistent with deposition in a retroarc foreland basin setting the limestone beds of the penbo member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the gangdese volcanic arc the low subsidence rate recorded in the lowermost strata of the takena formation including the penbo member and the paleosol rich interval of the lhunzhub member is interpreted to be associated with the passage of a fl exural forebulge the overlying upward coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin the late cretaceous folding of the takena formation indicates that the foreland basin strata were eventually incorporated into a fold and thrust belt the upper cretaceous sedimentary strata of the lhasa terrane of southern tibet predict that a north verging fold and thrust belt existed along the southern margin of the lhasa terrane during late cretaceous time prior to the indo asian collision collectively the evidence indicates that the southern margin of the lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the indo asian collision

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

magnetic polarity stratigraphy of the neogene siwalik group at khutia khola far western nepal the middle miocene pliocene siwalik group was deposited in the himalayan foreland basin in response to uplift and erosion in the himalayan fold thrust belt results of thermal demagnetization experiments on samples from the siwalik group in central and western nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies sandstones and paleosols of the siwalik group however generally display highly erratic paleomagnetic behavior during thermal demagnetization on the basis of these observations siltstones from a well exposed 2423 m thick section of the siwalik group in khutia khola far western nepal were sampled for magnetic polarity stratigraphy the siwalik group is composed of informal lower middle and upper members correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13 30 to 7 65 ma the lower middle siwalik boundary occurs at 11 05 ma near the beginning of chron c5n the rate of sediment accumulation increases upsection similar to rate changes previously observed in the pakistan siwalik group and probably in response to increasing proximity of the himalayan thrust belt in the khutia khola section a discordant declination indicates that this region has rotated about a vertical axis 16 6 counterclockwise with respect to the indian subcontinent measurements of 13 c in paleosol carbonate indicate the predominance of c 3 plants until 7 65 ma and the clear presence of c 4 plants higher in the undated portion of the section

laser 40ar 39ar dating of single detrital muscovite grains from early foreland basin sedimentary deposits in india implications for early himalayan evolution in india the dagshai and overlying kasauli formations represent the oldest exposed continental foredeep sediments eroded from the himalayan orogen 40ar 39ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of 28 ma for the dagshai formation at one locality and 25 ma at a second locality whereas deposition of the kasauli formation occurred after 28 ma at two localities and after 22 ma at a third locality this timing suggests that in india the start of substantial exhumation and erosion from the rising himalayan orogen was delayed until 28 ma

cu isotopes and concentrations during weathering of black shale of the marcellus formation huntingdon county pennsylvania usa abstract the complex weathering processes which govern the production of soil from bedrock have proven difficult to understand for many lithologies weathering of black shale is of particular interest because it releases organic carbon and heavy metals as solutes and therefore impacts the health of terrestrial and aquatic ecosystems to understand black shale weathering a geochemical survey was initiated for soils developed on shales of the marcellus formation at a zero order catchment at a satellite site of the susquehanna shale hills critical zone observatory located in jackson corner pennsylvania this formation is an organic and metal rich carbonaceous shale that underlies much of new york pennsylvania ohio and west virginia in this paper we focus on the effects of weathering on variations of cu isotopes in the shale cu concentration data for soil were normalized using ti concentrations to document the mobility of cu relative to bedrock at both the ridgetop and valley floor depletion profiles for cu are documented in the soils the cu in the soils is depleted in 65 cu average 65 cu 0 5 0 2 compared to the parent material average 65 cu 0 03 0 15 consistent with loss of cu from soils the pore waters contain 10 ppb cu on average and are enriched in the heavy isotope average value 65 cu 1 14 0 44 rayleigh fractionation models using the concentration and isotope data of the soils are consistent with pyrite weathering and loss of cu from the ridgetop but downslope transport and cu re precipitation at the valley floor

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

high and dry in central tibet during the late oligocene the time at which the tibetan plateau rose to its present high elevation remains controversial with estimates ranging from 40 ma to more recent than 7 ma new stable isotope analyses of modern and accurately dated ancient paleosol carbonate in the nima basin of central tibet point to an arid climate and high paleoelevation 4 5 5 km comparable to today s setting by 26 ma oxygen isotope values of ancient 26 ma soil carbonate are both very negative and indistinguishable after modest corrections for changes in global climate from the lowest least evaporated oxygen isotope values of modern soil carbonates in the area substantial enrichments in oxygen 18 in paleolacustrine carbonates as well as high carbon isotope values from paleosol carbonates indicate considerable lake evaporation and low soil respiration rates respectively and both are consistent with the present arid climate of the nima area blockage of tropical moisture by the himalaya and perhaps the gangdese shan probably has contributed strongly to the aridity and very negative oxygen isotope values of soil carbonate and surface water in the nima area since at least the late oligocene the maintenance of high elevation since at least 26 ma suggests that any flow of lower crust from beneath central tibet must have been balanced by coeval northward insertion of indian crust beneath the plateau