isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s a the wrong figure 5 was printed with this article the correct figure follows fig 5 thin section photomicrographs of the characteristic lithologies in yucca mound a and b leopard

effects of grand cycle cessation on the diagenesis of upper cambrian carbonate deposits in the southern appalachians u s a abstract the vertical transition from the mainly subtidal alternating shale and carbonate units or grand cycles of the conasauga group middle to upper cambrian to the peritidal dolostone of the knox group upper cambrian to lower ordovician marks a major change in the early paleozoic passive margin sedimentation of the southern appalachians the grand cycles represent an interplay between intrashelf shale basin and carbonate platform deposition the end of grand cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost conasauga group strata the maynardville formation the maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone this change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the maynardville and in conjunction with the changes in the regional facies distribution at the end of grand cycle deposition extended its influence upon the burial diagenesis of these deposits the subtidal deposits of the maynardville contain a variety of calcite cements dolomite is not abundant in these deposits where it occurs mainly as a fine crystalline ferroan phase associated with argillaceous layers and pressure dissolution features the local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits diagenesis of clay minerals and pressure dissolution of carbonates ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite the subtidal deposits also contain saddle dolomite cement associated with mississippi valley type mvt minerals which indicate the involvement of externally derived fluids during late burial diagenesis of these strata the peritidal deposits on the other hand are extensively dolomitized fine crystalline dolomite most likely represents early diagenetic replacement of tidal flat carbonate sediment coarse crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines fenestrae desiccation cracks and evaporite dissolution voids are occluded with dolomite cement which is commonly complexly zoned zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement saddle dolomite in pore centers dissolutional voids and tectonic fractures formed during late burial from warm fluids associated with the migration of mvt brines the effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal maynardville and the underlying conasauga group carbonate deposits and between the peritidal deposits of the upper maynardville and the rest of the overlying knox group the depositional environments and early diagenesis also affected the burial diagenesis the changes in regional facies distribution caused by carbonate platform progradation at the end of grand cycle deposition resulted in a decrease in the proportion of siliciclastic components this decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits the maynardville is conformably overlain by carbonate deposits of the knox group this allowed the maynardville knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the ordovician foreland sevier basin to the southeast during the conversion from a passive into a convergent continental margin the nolichucky shale separates the maynardville knox succession from the carbonate deposits of the underlying conasauga group which were in contrast affected by the migration of burial fluids from the cambrian intrashelf shale basin to the west

a short term post lomagundi positive c isotope excursion at c 2 03 ga recorded by the wooly dolomite western australia it is generally accepted that carbon isotope variations in seawater were muted between c 2 06 ga after the end of the lomagundi carbon isotope excursion lcie and c 1 3 ga evidence is presented here that c 30 myr after the end of the lcie the biogeochemical cycle of carbon experienced a short term c 2 myr high amplitude up to 8 4 v pdb perturbation recorded in the horseshoe rift basin western australia the basin was initiated at c 2 03 ga with deposition of fluvial and shallow marine sandstones followed by the eruption of flood basalt and culminated with the deposition of platform carbonates and accompanying volcaniclastic and siliciclastic sediments wooly dolomite the horseshoe rift basin during deposition of the wooly dolomite was fault compartmentalized but connected to an ocean six depositional sequences make up the wooly dolomite sequence 1 records establishment of a carbonate platform conformably on basalt and coevally with volcaniclastic sedimentation all other sequences have dominant carbonate platform deposits and are unconformity bounded sequence 3 contains a c 57 m thick section with 13 c enriched carbonates bracketed between carbonates with close to 0 carbon isotope values further high resolution chemostratigraphic studies may reveal a more complex pattern of carbon isotope variations during the boring billion years but without precise geochronology similar short term carbon isotope excursions in carbonate successions could be incorrectly correlated to the lcie supplementary material table 2 including chemical and isotopic data sample locations and their position in figure 8 is available at https doi org 10 6084 m9 figshare c 2868055

paleoaquifer and deep burial related cements defined by regional cathodoluminescent patterns middle ordovician carbonates virginia middle ordovician ramp carbonates virginia were deposited in a subsiding foreland basin bordered on the southeast by tectonic highlands ramp carbonates were lithified in part by turbid marine cements but major cementation was by nonferroan clear rim and equant cements zoned defined by cathodoluminescence clear cements consist of nonluminescent oldest bright and dull youngest cements the zonation relates to increasingly reducing conditions of pore waters zoned cements in peritidal beds best developed in southeastern belts have complex zonations pendant to pore rimming fabrics and are associated with crystal silt which abuts all cement zones solutional cavities and erosional surfaces which locally truncate dull cement these cements are meteori vadose to shallow phreatic cements in northwestern exposures of peritidal beds are dominated by non zoned dull cement which lacks abundant evidence of early near surface precipitation major cementation of subtidal facies occurred under burial conditions burial cements best developed in southeastern belts have a simple zonation reflecting progressive burial up to 7 5 km 4 5 mi of the carbonate ramp shallow burial nonluminescent cement formed from oxidizing meteoric waters which expelled anoxic connate marine waters these meteoric waters were carried by aquifers from tectonic upland recharge areas on the southeastern basin margin bright cement formed under more reducing conditions following stagnation of the paleoaquifer with burial and possibly was precipitated at depths of 2 to 3 km 1 to 2 mi burial cements in northwestern exposures of subtidal beds are dominated by dull cement initial generations of which precipitated from downdip portions of aquif rs deeper burial dull calcite and ferroan dolomite cements largely formed at burial depths of 2 to 3 km 1 to 2 mi and temperatures of 75 to 135 c 165 to 275 f or more associated with hydrocarbon formation emplacement during the late devonian to mississippian latest clear dull cement fills tectonic fractures and was emplaced during late paleozoic alleghenian deformation probably at temperatures of 200 to 300 c 390 to 575 f and depths of 5 to 7 km 3 to 4 5 mi deeper burial diagenesis appears to be genetically linked to late paleozoic mississippi valley type mineralization in the southern appalachians zoned peritidal and burial cements are confined mainly to southeastern portions of the ramp where cementation was influenced by meteoric waters shed from tectonic uplands on the southeast and carried northwest by paleoaquifers northwestern portions of the ramp were influenced very little by upland sourced meteoric waters and nonzoned dull cements precipitated from relatively reducing waters the distribution of nonluminescent cement in middle ordovician subtidal facies defines the regional distribution of oxidizing portions of the paleoaquifer system such incursion of upland sourced oxidizing meteoric waters into ramp carbonates should be a common feature of foreland basin carbonates deposited adjacent to tectonic uplands furthermore foreland basin carbonates that undergo progressive burial should show a simple nonluminescent to bright to dull sequence of cement zones the close association of zoned cements and regional uplands in the middle ordovician sequence indicates the importance of assessing regional geologic relationships environmental parameters geologic history and tectonics in understanding regional cementation patterns and cementation processes of anci nt carbonate platforms

stable isotopic signature of a palaeoaquifer mississippian alamogordo member limestones sacramento mountains new mexico usa nine stratigraphic sections each 5 m thick were sampled from the alamogordo member limestones of the lake valley formation sacramento mountains new mexico usa four stratigraphic sections consist entirely of lime mudstone and wackestone whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in 1 8 o and constant 1 3 c values whereas those in the sections solely composed of lime mudstone and wackestone have in general relatively uniform 1 8 o and 1 3 c values the diagenesis of the alamogordo member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre pennsylvanian exposure surface 100 m above this unit however the uniform 1 3 c and downward decrease in 1 8 o values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the alamogordo member rather than descended from the overlying exposure surface this indicates that the basal packstones were probably a conduit for meteoric water this is further supported indirectly by the relatively uniform 1 8 o and 1 3 c values of the lime mud in the sections without basal packstones the implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer

mentawai fault zone off sumatra a new key to the geodynamics of western indonesia the geodynamic evolution of the western part of the sunda arc is controlled by the change from frontal subduction of the indo australian plate along java to oblique subduction along sumatra this obliquity gives rise to the sumatra fault zone that links the accretionary zone of the andaman sea to the sunda strait previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south consequently it has been proposed that the area between the subduction zone and the fault zone i e the sumatra sliver platelet was deformed an oceanographic cruise on the indonesian ship r v baruna jaya iii was designed to study this area seismic reflection data reveal the existence of a major strike slip fault parallel to the sumatra fault zone that we called the mentawai fault zone located in the fore arc area just east of the mentawai islands it is at least 600 km long thus the sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction

new understandings of ni mo mineralization in early cambrian black shales of south china constraints from variations in organic matter in metallic and non metallic intervals abstract the well known ni mo ores hosted in early cambrian black shales of south china are one of the research highlights in economic geology for the past few decades however their origin is complex and still debated here based on a case study in the huangjiawan ore of zunyi city guizhou province we generate several new understandings regarding ni mo mineralization through a comparative investigation of organic matter in metallic and non metallic stratigraphic intervals including abundance type maturity and relationship to mineralization we find new direct evidence for biotic impacts on mineralization organic matter and rhodophyte cystocarps red algae in particular may be significantly correlated to mineralization as it accumulates mineralized ni and mo however this organic material as well as disseminated and amorphous organic matters is not the sole and predominant factor controlling mineralization as implied from the nonlinear correlation between organic matter abundance maturation and mineralization other fluid sources e g hydrothermal and or seawater also contribute to mineralization which may be influenced by hydrothermal activity ni and mo may have mineralized independently as suggested by their differential accumulation in different structures of the cystocarps different relationships between organic matter abundance and thermal maturation and mineralized element concentration as well as the large variation in element accumulation coefficients the history of mineralization is complex as ni and mo may be or not be deposited together during the same stage of mineralization these results might also have broader implications for understanding the origin of sediment hosted ores worldwide

strontium isotopic compositions of cambrian upper miaolingian furongian series dolomites from south eastern sichuan basin china significance of sources of dolomitizing fluids and timing of dolomitization abstract tracing the nature of dolomitizing fluids and constraining the dolomitization time are of great significance for understanding dolomite genesis and reservoir formation especially for ancient dolomites strontium isotopes can be used as very useful tools to study this aspect thick dolomite bodies developed in the loushanguan group of the upper miaolingian furongian series cambrian in the south eastern margin of the sichuan basin china these dolomites are probably good hydrocarbon reservoirs little research has been carried out on the sr isotopes in these ancient dolomites three different textures of dolomites have been identified in the study area dolomicrite fabric retentive oolitic dolomite and fabric obliterative dolomite the dolomicrite displays a higher sr isotopic value avg 0 71052 compared with the contemporaneous seawater as well as a high sr and low mn content these chemical characteristics indicate that the dolomitizing fluids of the dolomicrite were mainly derived from modified seawater mixed with a little continental input the 87 sr 86 sr ratios avg 0 70936 of the fabric retentive oolitic dolomites are close to those of normal seawater indicating that the dolomitizing fluids were derived from the contemporaneous seawater and that they possibly formed soon after sedimentation as a result of an early replacement during shallow burial the low sr isotopic values avg 0 70854 of the fabric obliterative dolomites indicate that these dolomitizing fluids are related to late cambrian seawater and that the dolomites are the result of recrystallization of the fabric retentive dolomites in a relatively closed diagenetic system during burial with combined characteristics of relatively high sr and low mn contents the recrystallization began approximately 490 ma and was completed not earlier than 485 ma this study is not only beneficial to understand the characteristics and multistage process of the dolomitization of ancient cambrian dolomites but provides a reference for sourcing dolomitizing fluids and constraining the dolomitization time using sr isotopes

three dimensional numerical simulation of the subduction dynamics in the sunda arc region southeast asia abstract the sunda arc has been modelled as a test subduction zone with geometrical complexities and varying dynamics we simulated spatially the structural heterogeneities of the subduction zone and carried out our computations by a three dimensional finite element approach the aim is to estimate the lateral variation in the effective horizontal compressive stress in the interplate region which can produce the present day observed seismogenic stress field in our simplified approach the stress field has been modelled by varying the traction ridge push while keeping the age dependent gravitational body force slab pull constant our result suggests that there is a considerable amount of stress dissipation in the decoupled segment of java at sumatra the required traction is much greater than that expected from the age dependency relations in our modelling the transition from the dominance of the horizontal compressive stress to that of the gravitational pull in the regional stress field occurs around a depth range of 150 200 km lateral variation in the subducted slab volume emerges crucial to the seismogenic stress field and the derived pattern of displacement subduction of the lighter oceanic rise material should enhance the stress level the effect of abrupt lateral variation in structure e g sudden changes in the slab dip or slab depth may also be significant on the regional stress field viscoelastic modelling attempted for estimating the temporal variation in subduction dynamics suggests that the thrust level will increase southward of the present day thrust zone in java while strike slip fault type stress field will be developed south of sumatra

early cambrian ocean anoxia in south china the cause of the most marked changes in the evolution of life which define the first order stratigraphic boundary between the precambrian and the phanerozoic eon remains enigmatic and a highly topical subject of debate a global ocean anoxic event triggered by large scale hydrogen sulphide h 2 s release to surface waters has been suggested by wille et al on the basis of two data sets from south china and oman to explain the fundamental biological changes across the precambrian cambrian pc c boundary here we report a new precise shrimp u pb zircon age of 532 3 0 7 million years myr ago fig 1 for a volcanic ash bed in the critical unit that reflects the ocean anoxic event the lowermost black shale sequence of the niutitang formation in the guizhou province south china this age is significantly younger than the precise pc c boundary age of 542 0 0 3 myr ago approximately 10 myr younger than the extinction of the ediacaran fauna and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the pc c boundary

nickel isotopic variation in black shales from bohemia china canada and finland a reconnaissance study we present 60ni values for black shales determined by double spike mc icp ms the samples comprise paleoproterozoic talvivaara ni zn co cu black shales from finland neoproterozoic black shales from the tepla barrandian unit czech republic early cambrian ni mo rich black shales from the yangtze craton and devonian ni zn pge black shales from yukon canada in addition the sample set includes a black smoker sample from the logatchev hydrothermal field mid atlantic ridge the 60ni values vary from 0 84 0 09 to 0 62 0 04 2sd with a median of 0 10 n 28 ni isotopic compositions were predominantly lighter than those of abiotic terrestrial and extraterrestrial samples 0 15 and 0 27 mantle 0 23 gall et al 2017 present day seawater 1 44 cameron and vance 2014 dissolved ni from riverine input 0 84 cameron and vance 2014 ferromanganese crusts 0 9 2 5 gall et al 2013 devonian mississippian organic rich marine sediments lower jurassic organic rich marine sediments 0 2 2 5 average 0 92 n 18 porter et al 2014 and euxinic sediments of the black sea 0 14 0 51 vance et al 2016 however the range of 60ni values in our black shale samples was close to that of the weathering products of mafic ultramafic rocks ore and soil samples ranging from 0 60 to 0 30 ratie et al 2015 spivak birndorf et al 2018 ni sulfide ores hosted by archean komatiites from australia and canada 0 10 to 1 03 average 0 70 n 8 gueguen et al 2013 and archean ni rich magmatic sulfides from zimbabwe 0 28 to 0 47 n 6 hoffman et al 2014 based on our observations and considering the extremely low contribution of direct biological uptake of ni and a dominant ni residence in early fe ni sulfides we suggest that our mostly light ni isotopic compositions in metal rich black shales result from sulfidization of organic matter and ni removal into sulfides

multi carbonate component reconstruction of mid carboniferous chesterian seawater δ13c abstract this study presents the first stratigraphically continuous 13ccarb record for the chesterian interval developed from antler foreland basin platform successions the bulk carbonate derived 13c curve was constructed using 700 biostratigraphically constrained samples providing a 105 yr resolution a multi component study of 13c and 18o variability in petrographically and geochemically screened abiotic and biogenic calcite components delineates the degree of preservation and 13c heterogeneity in these ancient epicontinental carbonates furthermore it constrains the influence of kinetic and vital effects and local to regional scale c cycling on their 13ccarb values the 13c and 18o values of micrites and bulk carbonates overlap regardless of lithofacies and show minimal intrasample variability overall biogenic calcite 13c and 18o values are higher than the average isotope compositions of bulk carbonates and micrites although 13c values in contemporaneous samples are only minimally 0 3 2 std err of 0 3 different relative to much larger differences in 18o values although brachiopod 13c and 18o values are higher than average bulk carbonate and micrite values their 13c values exhibit considerable isotopic variability by taxa and ultrastructure notably the 13c values of brachiopod secondary shell layers overlap with bulk carbonate micrite and marine cement 13c values conversely 13c values of brachiopod tertiary prismatic layers including composita are on average 2 to 5 higher than the 13c values of contemporaneous bulk carbonates micrites and brachiopod secondary shell layers an observed average 2 decrease in the 13c values of secondary cross bladed laminar layers in productids relative to contemporaneous bulk carbonate micrite and marine cement values suggests a previously unrecognized kinetic and or vital effect on carboniferous productid 13c values given that they have the highest 18o values of all analyzed components the observed higher 13c values in these tertiary prismatic layers are comparable to greater than the 13c enrichment 1 previously documented in pennsylvanian composita the overlap in 13c values of bulk carbonates with diagenetically screened microdrilled matrix marine cements and brachiopod secondary shell layers indicates that the chesterian bulk carbonate derived 13c values may reliably record to within 1 the evolution of ambient seawater 13c in the outer carbonate ramp of the antler basin comparison of the chesterian 13c record to the bulk carbonate c isotope curve derived from the arrow canyon nv carbonate succession further suggests that these bulk carbonate derived 13c records are reliable proxies of the composition and trends in seawater 13c regionally along the equatorial eastern panthalassan margin the newly resolved chesterian stratigraphy documents multiple 13c excursions including previously unrecognized significant c isotope shifts a shift to higher frequency and amplitude shorter term 13c fluctuations in the upper third of the chesterian stage occurred simultaneously with a change in stratigraphic cyclicity from dominantly 3rd order to 4th order carbonate cycles suggesting a mechanistic linkage between ambient seawater 13c and sea level fluctuations in the antler basin this change in the character of the shorter term 13c fluctuations coupled with a subsequent significant 4 positive c isotope excursion across the mid carboniferous boundary previously recognized in all carboniferous curves is interpreted to record the onset of late paleozoic glaciation and glacio eustasy

sector zoning in calcite cement crystals implications for trace element distributions in carbonates abstract compositional sector zoning in calcite cement crystals from a sedimentary limestone is represented by differences of mg content of over 1300 ppm or 95 and of sr and mn contents of up to 400 ppm or roughly 50 and 80 respectively between crystallographically nonequivalent time synchronous growth sectors 10 1 1 sectors are enriched with both mg and mn relative to 10 1 4 sectors and other sectors show both greater and lesser enrichment this systematic zoning pattern requires effective distribution coefficients for these trace elements to have been different for growth on different faces differential growth rate effects for nonequivalent faces are shown to be unimportant relative to differential growth face element preferences surface crystal structure and differing coordination environments are regarded as the principal factors influencing sector related elemental partitioning differences in calcite and dolomite

paleopedology and geochemistry of late mississippian chesterian pennington formation paleosols at pound gap kentucky usa implications for high frequency climate variations abstract climate during the late mississippian late chesterian in the southern appalachian basin was characterized by alternating periods of aridity and humidity pennington formation paleosols at pound gap record climate and ecological changes for latest chesterian time ending at the mississippian pennsylvanian systemic boundary forty paleosols were identified described and assigned to seven pedotypes inferred soil orders considered as analogs include histosols entisols inceptisols alfisols and oxisols but are dominated by vertisols classification of an oxisol was determined by field and geochemical evidence of intense leaching and kaolinite dominated clay mineralogy field and micromorphologicial evidence suggests a polygenetic character of the vertisols resulting from changing soil drainage through time variations in soil drainage are quantified using proxy estimates of inferred soil processes such as base loss leaching lessivage and oxidation using the cia k proxy mean annual precipitation map estimates range from 519 to 1361 mm yr changes in map correspond with variation in inferred soil processes the flora of this time period in response to variations in precipitation and soil drainage also changes through time as evidenced by changes in abundance and depth of root traces reconstructed ecosystems range from sparse vegetative cover with shallow tabular root systems in early pennington soil development to dense deeply penetrating root systems suggestive of arborescent floral associations at the top of the succession approaching the mississippian pennsylvanian boundary this study provides greater resolution of changing climate and pedogenic processes than what is provided in previous studies of late mississippian climate and suggests that the documented variability in paleosol types and soil drainage might represent the record of high frequency climate changes likely associated with expansion and contraction of the paleo intertropical convergence zone itcz

an sem study of microfossils in the black shale of the lower cambrian niutitang formation southwest china implications for the polymetallic sulfide mineralization abstract the black shale in the lower cambrian niutitang formation on the southeastern margin of the yangtze platform hosts a layer of polymetallic sulfide ores containing extremely high concentrations of ni up to 3 8 wt and mo up to 7 5 wt abundant micrometer sized microfossils were observed in the ni mo sulfide enriched layer under scanning electron microscopy the microfossils include vesicles with clear organic wall structures and permineralized internal contents these structures resemble unicellular green algae both biomass adsorbed transition metals and biological structural protein metals could be accumulated and mineralized from the blooming algae followed by anoxic microbial reduction and immobilization the biotic impact may be a substantial mechanism for the massive ni and mo sulfide deposition in a typical restricted ocean basin environment

formation of fe mn si oxide and nontronite deposits in hydrothermal fields on the valu fa ridge lau basin abstract hydrothermal fe mn si oxides and nontronite are pervasive in the hine hina vai lili and mariner hydrothermal fields along the central valu fa ridge lau basin morphometric and mineralogical analyses reveal that the iron rich filaments are the most important constituents of these fe mn si oxide deposits both the morphologies and chemical composition of the filaments indicate that neutrophilic fe oxidizing bacteria have played a key role in the formation of these deposits a key process of the formation of these deposits is the creation of a complicated filamentous network in which a series of metabolic activities and passive sorption and nucleation processes occur the precipitation of dissolved si in unsaturated and saturated states leads to a two generation growth model in the hydrothermal vents the precipitation of amorphous opal occurs in a relatively narrow temperature range 41 1 42 9 c based on oxygen isotope analyses indicating a fast precipitation rate of opal a when conductive cooling of the hydrothermal fluid occurrs patchy nontronite in the mariner fields is a product of the direct precipitation from hydrothermal fluids at a temperature of 87 9 c whereas the scattered nontronite at the hine hina field is the product of the replacement of hydrothermal fe si oxides at a temperature of 46 2 c

uranium mineralization and its radioactive decay induced carbonization in a black shale hosted polymetallic sulfide ore layer southwest china the black shale in the lower cambrian niutitang formation southwest china which has been dated to 532 3 0 7 ma by u pb radiometric measurements hosts a sedimentary layer with abnormally high ni mo pge au contents this layer is enriched in ni up to 3 8 wt mo up to 7 7 wt and u up to 595 ppm but its genesis is still controversial here we report the first direct observation of uranium bearing minerals and their radioactive effects on the surrounding matter in the polymetallic sulfide ore x ray absorption fine structure analyses confirmed the reduced valence state of uranium in combination with high resolution electron microscopy and electron probe microanalysis the mineral was identified as coffinite usio4 a strong positive correlation between the sizes of the coffinite crystals and their surrounding carbonized rings reveals that the coffinite is authigenic and its crystallization produced radiation resulted in the radiolysis of surrounding organic matter the association of various biogenic metal sulfides phosphates and abundant organic substances within the ni mo sulfide enriched ore suggests that biological adsorption may have participated in the enrichment of soluble u vi and that microbial sulfate reduction might have facilitated the uranium mineralization

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

evaluating trace elements as paleoclimate indicators multivariate statistical analysis of late mississippian pennington formation paleosols kentucky u s a abstract the temporal and spatial distributions of trace elements in paleosols in relation to soil forming processes and climate have received little attention primarily due to their generally low concentrations 100 ppm and a fundamental lack of knowledge of their behavior in soil systems trace element concentrations of pennington formation paleosols spanning an 8 ma interval in the late mississippian chesterian were analyzed using linear and multivariate statistics of whole rock elemental data linear statistics of the elemental data set show that ti zr nb cs la hf ta w ce and th have the highest correlation through time with r values 0 75 nb served as the proxy trace element for comparison temporal trends in nb closely match trends in lessivage clay formation and accumulation by feldspar weathering mean annual precipitation map and chemical weathering map effectively controls soil hydrology and the accumulation of organic matter and clay map in conjunction with chemical we

regional cementation from unconformity recharged aquifer and burial fluids mississippian newman limestone kentucky abstract the mississippian newman limestone in kentucky was studied to assess the effects of unconformities within and above the sequence on calcite cementation early cements are low in fe and show a nonluminescent to dull to nonluminescent zonation early nonluminescent cement mainly formed in updip areas from oxidizing meteoric waters in a regional paleoaquifer that formed during regional regression in the late mississippian during late mississippian transgression waters became reducing to form early dull cement up to 600 ppm mn up to 2 700 ppm fe but averaging 570 ppm the aquifer underwent later recharge during mississippian pennsylvanian regression to form a second nonluminescent cement this event continued into the pennsylvanian when the sequence underwent increasing burial earl cements have 13o values that become lighter downdip perhaps reflecting a downdip temperature increase due to burial 13c values are lightest in the north postulated recharge area and reflect input of organically derived co2 in waters there most calcite cementation appears to have been associated with two post newman unconformities which generated regional aquifers and reflects a shift to a wetter climate in late mississippian pennsylvanian time early cement abundance decreases away from postulated recharge areas later iron rich burial cements up to 2 200 ppm mn up to 11 000 ppm fe fill remaining pore spaces compaction induced fractures spalled cement rims and tectonic fractures lined with saddle dolomite and pyrite they formed above 60 c at depths of 1 to 3 km 13o values are compatible with derivation by pressure solution of preexisting calcites in isotopically heavy waters highly modified by burial diagenesis light 13c values reflect isotopically light organic carbon in the deltaic waters entering the subsurface or being expelled from shales or buffering of waters by light c early calcite cements the 13c calcite cements become heavier downdip by equilibration of waters with host limestone newman sediments may have been subjected to late postcementation migration of warm fluids based on secondary fluid inclusions these late stage fluids most likely were chemically complex warm brines 7 17 equiv wt naci 50 to 160 c that resided in the formation for sufficiently short times that conodont cai values were not affected

metal sources for the polymetallic ni mo pge mineralization in the black shales of the lower cambrian niutitang formation south china abstract total organic carbon content toc trace element and platinum group element pge concentrations were determined in the black shales of the lower cambrian niutitang formation in the nayong area guizhou province south china in order to study the polymetallic ni mo pge mineralization the results demonstrate that numerous elements are enriched in the polymetallic ores compared to those of the nearby black shale particularly ni mo zn toc and total pge which can reach up to 7 03 wt 8 49 wt 11 7 wt 11 5 wt and 943 ppb respectively the elemental enrichment distribution patterns are similar to those in the zunyi and zhangjiajie areas except that the nayong location is exceptionally enriched in zn whereas positive correlations are observed between the ore elements of the polymetallic ores no such correlations are observed in the black shale these positively correlated metallic elements are classified into three groups co ni cu pge zn cd pb and mo tl toc the geological and geochemical features of these elements suggest that proterozoic and early palaeozoic mafic and ultramafic rocks dolomites and or pb zn deposits of the neoproterozoic dengying formation and seawater could be the principal sources for co ni cu pge zn cd pb and mo tl toc respectively furthermore the chondrite normalized patterns of pges with pd pt pd ir and pt ir indicate that pge enrichment of the polymetallic ores is most likely related to hydrothermal processes associated with the mafic rocks in contrast pge enrichment in the black shale resembles that of the marine oil shale with terrigenous and seawater contributions our investigations of toc trace elements and pge geochemistry suggest that multiple sources along with submarine hydrothermal and biological contributions might be responsible for the formation of the polymetallic ni mo pge mineralization in the black shales of the lower cambrian niutitang formation across southern china

biogenic origin of intergrown mo sulphide and carbonaceous matter in lower cambrian black shales zunyi formation southern china abstract the lower cambrian zunyi formation in southern china hosts complex intergrown phases of mo sulphides and anthracite like carbonaceous matter and graphite mosc pyrite and ni sulphides these sulphides occur in a few centimeters thick polymetallic stratiform layer in black shales above phosphorites cherts dolomites and barite the ore horizon contains 7 wt mo 2 wt ni 19 wt s and 13 wt c org and c tot co 2 58 wt 14 wt cao 1 2 wt as 2000 ppm se 5400 ppm zn 450 ppm u and v and up to 1 ppm platinum group elements three types of mosc phases always including framboidal pyrite are distinguished 1 rounded structures few hundred microns large 2 relict inclusions in diagenetic pyrite 3 grains interstitial to sulphurised and phosphatised organic matter at the nanoscale mosc is represented by nanometric porous rounded structures of mos 2 finely intergrown with carbonaceous matter of typical nano onion structures mosc also contains nanocrysts of quartz clays ca sulphate carbonate and ni sulphides micro raman spectroscopy shows that the carbon phase has a maturation grade of anthracite indicating temperatures below 300 c mosc is mainly composed of 25 wt mo 23 wt s 5 6 wt fe 2 5 wt ni 33 wt c 1 5 wt o 0 6 wt n and several hundreds of ppm of cu as se sb pd zn co and sn electron energy loss spectra show that n is related to nanosized carbonaceous matter the mean c n ratio 57 of mosc is similar to those observed in marine organic matter and those reported from zooplankton at hydrothermal vents the mineralized shales are crosscut by hydrothermal veins composed of carbonate or barite always associated with quartz apatite and fe and zn sulphides a paragenesis also observed as nanocrysts in mosc although mo originated from seawater its extreme concentration is due to an enhanced growth of bacterial consortia in a diffuse hydrothermal vent system these consortia may be dominated by nitrogen fixing and mo dependant hyperthermophilic or mesophilic species containing nitrogenase composed also of mo s fe organic matter decay led thus to the formation of mosc and the included framboidal pyrite

unconformities and porosity development in carbonate strata ideas from a hedberg conference the purpose of this paper is to highlight ideas from a hedberg conference concerning detection of unconformities and associated porosity in carbonate strata many types of information including cores and outcrops seismic data eustatic sea level curves wireline logs biostratigraphic data stable isotope trends cycle stacking patterns and tectonic and basin evolution models can be used to predict and or detect subaerial unconformities all methods of detecting subaerial exposure with these types of information have limits and pitfalls detailed analysis of the stratigraphy sedimentology and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure though an integrated approach using all a ailable information is recommended the relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted subsurface porosity is the product of many factors operating during deposition subaerial exposure and burial several concepts should be recognized when evaluating porosity associated with subaerial unconformities 1 carbonate diagenesis during subaerial exposure rearranges pore networks but usually does not increase the total porosity near surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone in many cases total porosity in carbonates under unconformities is actually reduced during subaerial exposure 2 permeability is likely to change more than porosity during subaerial exposure increasing in some c ses but decreasing in others 3 pore systems evolve with time during subaerial exposure short periods of subaerial exposure 10 000 400 000 yr are often associated with greater porosity than long intervals of subaerial exposure 1 20 million yr prolonged subaerial exposure may change permeability less than porosity because high permeability karst related conduits can form quickly and persist for millions of years 4 many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction cementation and stratal collapse reduce porosity during burial 5 unconformity related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial 6 vugs caverns and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure 7 lithologic changes at unconformities like shales overlying carbonates can influence the flow of subsurface fluids during end page 857 deeper burial resulting in deep burial dissolution localized along unconformities if subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic paleogeographic and climatic framework systematic and hopefully predictable patterns will emerge however we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins and on a smaller scale during reservoir analysis

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

petrology and hydrocarbon potential of microalgal and macroalgal dominated oil shales from the eocene huadian formation ne china abstract a series of oil shales of middle to late eocene age occur in the huadian area of jinlin province northeast china analytical results from 22 rock samples collected from an underground mine in the guanglangtou district in the huadian basin show that the total organic carbon toc contents of the oil shale layers are very high 10 6 wt to 39 6 wt with vitrinite reflectance ro values of 0 41 0 02 and rock eval hydrogen index hi values up to 887 biomarker analyses show a high proportion of hopenes and hopanes with stereochemistry indicative of low maturity detailed organic petrologic analysis demonstrates that the organic matter is dominated by lamalginite and telalginite the presence of botryococcus in the telalginite indicates that the depositional environment of the oil shale is lacustrine this is consistent with low si and s contents and high c s ratios fragments from a newly identified benthic macroalgal rhodophyte were found in some thin layers raising the possibility that the depositional environment of some oil shale layers may have been influenced by marine flooding since rhodophyta are uncommon in freshwater systems two oil shale samples with different algal compositions were selected for assessment of hydrocarbon generation potential sample hd 21 is dominated by phytoplanktic lamalginite with a high hi value of 869 mg hc g toc while sample hd 20 has a lower hi value 490 mg hc g toc and contains high contents of the benthic rhodophyte as well as some phytoplanktic lamalginite hi values and thermal simulation experiments showed that the phytoplankton dominated shale hd 21 has a much higher hydrocarbon generation potential than the rhodophyte dominated oil shale hd 20 even though the toc content of hd 20 was much higher indicating that the benthic macroalgal macerals have a much lower capacity to generate oil when thermally mature

slip vectors and stretching of the sumatran fore arc slip vectors from thrust earthquakes at the java trench southwest of sumatra demonstrate that the sumatran fore arc is not a single rigid plate that is translated to the northwest by oblique plate convergence instead they indicate arc parallel stretching of the fore arc at a uniform strain rate of 3 4 x 10 8 yr the northwestward motion of the fore arc relative to the upper plate southeast asia increases from near zero at the sunda strait to 45 60 mm yr in northwest sumatra and should result in variable slip rates on the sumatran fault

chronostratigraphic significance of cathodoluminescence zoning in syntaxial cement mississippian lake valley formation new mexico abstract echinoderm syntax ial cement crystals have been collected at several stratigraphic levels within the lake valley formation which is overlain by a major pre pennsylvanian subaerial exposure surface the crystals were microsampled along growth bands and yielded high resolution elemental and isotopic information that record fluid evolution during their growth although cement crystals show little variation in cathodoluminescence character and bulk chemistry throughout the regional extent of the formation intracrystalline patterns in minor element and stable isotope chemistry allow for the identification of several settings of cementation including the marine phreatic marine meteoric mixing and meteoric phreatic zones when placed in a regional stratigraphic context crystal growth records enable reconstruction of the temporal and spatial extent of these diagenetic environments isotopic petrographic and stratigraphic constraints indicate that cementation was related to two temporally distinct meteoric systems most lake valley formation syntaxial cement formed in the marine phreatic and marine meteoric mixing zones during the earliest phase of cementation of this cement in excess of 60 formed in the marine phreatic zone and lowermost marine meteoric mixing zone smaller volumes precipitated from progressively fresher mixtures of marine and meteoric fluid during the gradual expansion of a freshwater lens that developed in response to pre pennsylvanian lowering of sea level meteoric phreatic fluids were relatively unimportant during this episode of cementation in contrast cement of entirely meteoric phreatic origin is associated with a later meteoric system but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre pennsylvanian unconforrnity our results indicate that cements in the upper and lower parts of the lake valley formation are genetically and temporally unrelated thus the cement cathodoluminescence stratigraphy which has been previously applied to the lake valley can not be valid over the full extent of the formation this has wider implications our demonstration that diverse and temporally distinct diagenetic settings can produce identical cl characteristics shows that caution should be employed when using cement cl stratigraphy alone to extend diagenetic interpretations over large geographic regions or through substantial stratigraphic successions

coal metamorphism in the upper portion of the pennsylvanian sturgis formation in western kentucky coals from the pennsylvanian upper sturgis formation missourian and virgilian were sampled from a borehole in union county western kentucky the coals exhibited two discrete levels of metamorphism the lower rank coals with vitrinite reflectances indicative of high volatile c bituminous were assumed to represent the normal level of metamorphism a second set of coals with vitrinite reflectances indicative of high volatile a bituminous was found to be associated with sphalerite chlorite and twinned calcite the latter mineral assemblages indicate that hydrothermal metamorphism was responsible for the anomalous high rank consideration of the sphalerite fluid inclusion temperatures from nearby ores and coals and the time temperature aspects of the coal metamorphism suggests that the hydrothermal metamorphic event was in the 150 to 200 c range for a brief time 10 5 10 6 yr as opposed to the longer term 25 50 m y 60 to 75 c ambient metamorphism

ichnofossils and their significance in the cambrian successions of the parahio valley in the spiti basin tethys himalaya india abstract the spiti basin exposes well preserved cambrian successions in the tethys himalaya the present ichnofossil assemblage is reported from the debsakhad member of the kunzum la formation the ichnofossils includes the ichnogenera bergaueria chondrites cruziana didymaulichnus dimorphichnus diplichnites helminthorhaphe merostomichnites monocraterion monomorphichnus nereites palaeopascichnus palaeophycus phycodes planolites rusophycus skolithos scolicia treptichnus etc along with annelid worm burrow and scratch marks these ichnogenera can be assigned to cubichnial repichnial pascichnial to fodinichnial behaviors the ichnofossils reported from this section provide evidence regarding the developmental patterns during the early phase of life in absence of trilobites the present assemblage of ichnofossils is very significant in assigning the age of the debsakhad member the abundance of ichnofossils in sandstone siltstone and in shale beds indicate that the ichnocenosis is dominated by a high behavioral diversity ranging from the suspension to deposit feeders three lithofacies were observed in this section they show a vertical disposition which further reflects general upward coarsening trend ichnofossils are mostly produced by arthropods along with crustacean polychaetes and polyphyletic vermiforms due to the paucity of body fossil as well as microbiota in the lowermost beds of the debsakhad member the precambrian cambrian boundary could not be demarcated however the presence of treptichnus and phycodes can be considered as a horizon marker for the beginning of lower cambrian in this section

correlation of the stratigraphic cover of the pilbara and kaapvaal cratons recording the lead up to paleoproterozoic icehouse and the goe abstract the turee creek basin of the pilbara craton is critical for timing onset of the early paleoproterozoic icehouse and the goe we establish correlation to the koegas postmasburg supersequence of the kaapvaal craton based on a common compressional basin with the pilbara succession deposited near the orogen and the kaapvaal succession near the flexural arch whereas turee creek cyclicity was controlled by tectonic subsidence glacioeustatic influence is discernible in the koegas postmasburg supersequence key features of the turee creek basin are the mid kungarra formation glaciogenic meteorite bore member unconformity bound sequences above the kungarra formation and the basaltic bubbawalyee formation conformably above quartzite 3 on the kaapvaal craton the equivalent glaciogenic unit makganyene formation overlying the sedimentary koegas subgroup and underlying the basaltic ongeluk formation is unconformity bound we correlate the sub ongeluk unconformity with the sub quartzite 3 unconformity and propose tectonostratigraphic equivalence of the ongeluk and bubbawalyee formations there are no equivalents of the post ongeluk hotazel and mooidraai formations above the bubbawalyee formation on the pilbara craton which is significant because the hotazel formation provides the earliest evidence for the goe consequently rather than spanning the goe the turee creek basin provides a chemostratigraphy during its eve all carbonate sediments deposited before and after the first global glaciation meteorite bore makganyene yield carbon isotope values close to 0 establishing that the first carbon isotope excursion occurred long after closure of the turee creek koegas postmasburg basin and onset of the goe it therefore appears that the carbon cycle responded to rather than triggered the goe

slip partitioning and fore arc deformation at the sunda trench indonesia oblique subduction at the sunda trench has produced transpressive deformation of the plate leading edge a major feature is the right lateral great sumatran fault gsf which probably absorbs a significant fraction of the trench parallel shear the kinematics of sunda relative to australia are discussed on the basis of available gps data and geologically determined slip rates on the csf in spite of the uncertainty on the plate motion several robust conclusions can be drawn the predicted obliquity of the convergence increases northward along the sumatra trench up to about 30 slip partitioning is nearly complete along the northern segment of the sumatra trench where the csf probably accommodates most of the trench parallel shear along the southern segment where obliquity is less than about 20 slip partitioning is not complete as indicated by oblique thrusting at the subduction there only a fraction of the trench parallel motion of australia relative to se asia is accommodated along the csf these observations suggest that the leading edge behaves like a plastic wedge except that slip partitioning although incomplete is observed even at low obliquities

is the slip rate variation on the great sumatran fault accommodated by fore arc stretching in oblique convergent subduction areas an obliquity increase should produce a slip rate increase on trench parallel strike slip faults and thus the fore arc platelet should be stretched along the great sumatran fault gsf that is associated with the oblique convergent sunda arc a northward increase of the gsf slip rate occurs parallel with the convergence obliquity transpressional back arc deformation accommodates part of the gsf slip rate variation while no significant fore arc stretching is observed the sumatran case shows that oblique convergence may be accommodated by deformation of a 500 km wide zone from the fore arc to the back arc domains

isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s ameteoric diagenesis of a pennsylvanian phylloid algal mound new mexico

microstratigraphy of calcite cements in pliocene cool water limestones new zealand relationship to sea level burial and exhumation events

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

transtensional basins in the western sunda strait in 1999 a marine geophysical survey collected bathymetric imagery seismic reflection and refraction data to investigate the crustal structure and evolution of the sunda strait which is linked to the obliquely convergent geodynamic setting of the sumatran trench the transtensional character of the study area is manifested in faulted blocks of arc basement and active normal faults detected on both sides of a large graben at the western entrance to the sunda strait the combined interpretation of reflection and wide angle data reveals over 6 km of sediment graben fill associated with substantial crustal thinning the southern part of the study region is only 50 km from the trench and the moho of the downgoing plate is found at a depth of 28 km

sims u pb zircon ages and ni mo pge geochemistry of the lower cambrian niutitang formation in south china constraints on ni mo pge mineralization and stratigraphic correlations abstract in comparison with previous studies on the ni mo pge ore deposits within the deep water lower cambrian black shales of the niutitang formation around hunan and guizhou provinces this study focused on the shallow water ni mo pge enriched layer of the niutitang formation around hubei province by means of an integrated geochronological pge ree and trace element geochemical study the results suggest the ni mo pge enriched layer has a dominant seawater origin whereas the ni mo pge ore deposits have mixed seawater hydrothermal and terrigenous origins trace metals precipitated from ambient seawater under anoxic suboxic settings at exceedingly slow sedimentation rates with episodic injections of fresh oxidized seawater organic matter played an important role in sourcing transporting and precipitating trace metals however decay of organic material by means of sulfate reducing reactions and or fluid reworking overprinted primary relationships with trace metal contents however the positive linear relationships are clearly shown between organic carbon and trace metal contents new sims u pb ages constrained the depositional age of the ni mo pge enriched layer to be 532 527 ma which argues against previous assumption that the anoxic event recorded in the ni mo pge took place at the ediacaran cambrian boundary furthermore new radiometric ages suggest the shuijingtuo niutitang formation could be correlated with shiyantou formation dahai and upper zhongyicun members whereas yanjiahe formation correlated with daibu and lower zhongyicun members in yunnan province respectively

structure and evolution of lithospheric slab beneath the sunda arc indonesia tomographic imaging reveals seismic anomalies beneath the sunda island arc indonesia that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers the sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the mesozoic tethys ocean in accord with previous studies the lithospheric slab was imaged as a continuous feature from the surface to the lower mantle below java with a local deflection where the slab continues into the lower mantle the deep slab seems to be detached from the upper mantle slab beneath sumatra this complex slab structure is related to the tertiary evolution of southeastern asia and the indian ocean region

diagenetic history of pipe creek jr reef silurian north central indiana u s a abstract calcite cements in the silurian ludlovian pipe creek jr reef north central indiana are compositionally zoned with characteristic minor element concentrations and stable isotope signatures and were precipitated in different diagenetic environments superposition and crosscutting relationships allow us to group cement zones and to relate them to the sequence stratigraphic evolution of the reef pipe creek jr reef grew in normal marine waters with the reef top high greater than 50 m above the platform floor flank facies are volumetrically important and are preserved largely as limestone in contrast to most dolomitized silurian reefs in the midcontinent syndepositional marine cements fill primary porosity and synsedimentary fractures and are interlayered with marine internal sediment now low magnesium calcite their isotopic compositions are similar to those of depositional grains and cements estimated to have precipitated from ludlovian sea waters depositional porosity was reduced by 75 by the precipitation of these syndepositional cements which stabilized the steeply dipping flank slope postdepositional clear calcite cements are interpreted as shallow phreatic and burial cements on the basis of their relationship to periods of karstification and fracturing shallow phreatic cements with concentric cathodoluminescent cl zonation precipitated in primary pores and are postdated by fractures and caves filled with middle devonian sandstone cl zonal boundaries are sharp and some near a major stratigraphic unconformity show evidence of dissolution the volumetric abundance of the individual cl zones varies in the reef indicating a complex superposition of waters of varying chemistry and rock water interaction that are probably related to relative sea level changes this important aspect of the reef stratigraphy is recorded only by the diagenetic succession because evidence of earlier sea level changes is removed by a major later regional unconformity burial cements are the youngest diagenetic feature recognized and they rest conformably or unconformably over older cements they exhibit both concentric cl zonation and sectoral zoning they are ferroan to nonferroan and they contain thin sulfide zones along growth band boundaries their isotopic compositions do not overlap with shallow phreatic or marine cement values degraded oil postdates burial cements and is composed of the same sterane class as the devonian age antrim shale the probable source rock this source contrasts with that of reef reservoirs in the michigan basin where silurian strata are commonly the hydrocarbon source

tectonic features of the southern sumatra western java forearc of indonesia 1 multichannel reflection seismic profiles along the active sunda arc where the indo australian plate subducts under the overriding eurasian margin revealed two accretionary wedges the inner wedge i is of assumed paleogene age and the outer wedge ii is of neogene to recent age the inner wedge i is composed of tectonic flakes stretching from southeast sumatra across the sunda strait to northwest java implying a similar plate tectonic regime in these areas at the time of flake development during upper oligocene today wedge i forms the outer arc high and the backstop for the younger outer wedge ii the missing outer arc high of the southern sunda strait is explained by a combination of neogene transtension due to a clockwise rotation of sumatra with respect to java and by arc parallel strike slip movements the rotation created transtensional pull apart basins along the western sunda strait semangka graben as opposed to transpression and inversion on the eastern sunda strait within the new detected krakatau basin the arc parallel transpressional mentawai strike slip fault zone mfz was correlated from the sumatra forearc basin to the northwest java forearc basin off the sunda strait northward bending branches of the mfz are connected with the right lateral sumatra fault zone sfz along the volcanic arc segment on sumatra it is speculated that the sfz was attached to the cimandiri pelabuhan ratu strike slip fault of java prior to the presumed rotation of sumatra and that since the late lower miocene the main slip movement shifted from the volcanic arc position to the forearc basin area due to increasingly oblique plate convergence

gold and platinum in shales with evidence against extraterrestrial sources of metals abstract few black shales contain concentrations of precious metals higher than average continental crust i e 5 ppb au yet au and pt alloys have been reported from the kupferschiefer in poland moreover thin sulfide beds in certain chinese and canadian shales contain several hundred ppb au pd and pt and average 4 mo and 2 5 ni in an association that is difficult to explain volcanic and non volcanic exhalations hydrothermal epigenesis involving either igneous or sedex fluids biogenic processes and low temperature secondary enrichment are among the possible factors involved in deriving ni pge and au for black shales and sulfide beds in black shales extraterrestrial sources have been invoked in some cases e g the cambrian of china however available data on abundances of pge indicate relatively low values for ir

sequence stratigraphic context of syndepositional diagenesis in cool water shelf carbonates pliocene limestones new zealand abstract pliocene limestones in eastern north island new zealand are cool water deposits composed of barnacle epifaunal bivalve bryozoan dominated calcitic biota formed in shoal areas about the margins of a tectonically active forearc basin seaway vertical changes in the thickness and depositional facies of the limestones and fundamentally the position and nature of internal bounding discontinuities within them have enabled identification of high order likely 41 ka transgressive deepening upwards and transgressive regressive deepening then shallowing upwards sequences within the formations syndepositional or sea floor or pre compaction diagenesis in the limestones involves a spectrum of destructive to constructive features including skeletal abrasion bioerosion encrustation dissolution and cementation upward changes in the type and extent of these pre compaction diagenetic features within the discontinuity bounded sequences relate closely to the stacking patterns of depositional facies and their associated skeletal and mineralogical composition consequently it has been possible to develop a series of predictive diagenetic models for cool water shelf carbonates deposited upon and about upthrust antiforms in the forearc that integrate their patterns of syndepositional diagenesis into a sequence stratigraphy based framework driven ultimately by relative sea level changes marine constructive diagenesis characterized by bioencrustation turbid cements and interstratified internal sediments occurs most commonly in those facies close to the sequence bounding discontinuities within the lower levels of transgressive systems tracts tsts and the uppermost levels of regressive systems tracts rsts these sediments equate to the highest energy facies upon antiform tops close to the wave abrasion base where sedimentation rates were low in contrast destructive diagenetic processes typically dominate over constructive ones often exclusively so in the upper part of tsts in highstand systems tracts hsts and in the lower portion of rst carbonates where the sediments were not so influenced by active fluid circulation or strong reworking for prolonged periods but were more rapidly buried within accommodation traps below wave abrasion base about the antiforms the link between the facies motifs and synsedimentary diagenesis in these cool water pliocene limestones helps unlock their otherwise often cryptic high order sedimentary architecture such an interpretive opportunity is not afforded by the subsequent post compaction diagenetic features in the limestones because these are no longer closely related to the sedimentation realm

meteoric calcite cementation diagenetic response to relative fall in sea level and effect on porosity and permeability las negras area southeastern spain abstract a dolomitized upper miocene carbonate system in southeast spain contains extensive upper and lower zones of calcite cementation that cut across the stratigraphy cement textures including isopachous and circumgranular which are consistent with phreatic zone cementation cements in the upper cemented zone are non luminescent whereas those in the lower cemented zone exhibit multiple bands of luminescent and non luminescent cements in the upper cemented zone isotopic data show two meteoric calcite lines mcl with mean 18 o at 5 1 and 5 8 vpdb whereas no clear mcl is defined in the lower cemented zone where mean 18 o for calcite cement is at 6 7 vpdb 13 c values in both cement zones are predominantly negative ranging from 10 to 2 vpdb suggestive of carbon from soil gas or decayed organics measurements of tm ice in primary fluid inclusions yield a mode of 0 0 c in both zones indicating calcite cementation from fresh water these two zones define the positions of two different paleo water tables that formed during a relative sea level fall and erosional downcutting during the plio pleistocene the upper cemented zone pre dated the lower cemented zone on the basis of known relative sea level history meteoric calcite cementation reduced porosity and permeability but measured values are inconsistent with simple filling of open pore space each texture boundstone grainstone packstone wackestone produces a different relationship between percent calcite cement and porosity permeability distribution of cements may be predictable on the basis of known sea level history and the effect of the cementation can be incorporated into subsurface geomodels by defining surfaces of rock boundaries that separate cemented zones from uncemented zones and applying texture specific relationships among cementation porosity and permeability

submarine hydrothermal contribution for the extreme element accumulation during the early cambrian south china throughout the geological history of the earth submarine hydrothermal activity has played an important role in seawater chemistry biological evolution and enrichment of metals in the earth crust however the prospect of hydrothermal activity for extreme element accumulation during the early cambrian a key geological period in south china has not been well constrained this study reports geochemical e g ree and sr isotope investigations of a coarse grained limestone layer and associated calcite veins in zunyi and nayong areas guizhou province to constrain the hydrothermal activity and evaluate the significance of hydrothermal contribution to extreme element accumulation during the early cambrian south china our results reveal positive eu anomalies and higher initial 87sr 86sr ratios 0 7083 0 7150 for carbonate samples than those of early cambrian seawater indicating the presence of hydrothermal processes combined with constraints from the spatial relationships and coincidence with adjacent mineralization these hydrothermal processes provide the most probable contribution for polymetallic ni mo pge mineralization furthermore there are abundant hydrothermal dolomite and barite calcite veins in the dolostone of the dengying formation indicating the occurrence of a variety of hydrothermal fluids overall multi stage hydrothermal pulses with different fluid compositions spanned the ediacaran cambrian transition in south china in particular these hydrothermal fluids with positive eu anomalies and enriched radiogenic sr originating from proterozoic mafic ultramafic rocks may have flowed through the underlying precambrian silicate clastic rocks e g xiajiang banxi and lengjiaxi groups and may have been crucial for the marine environment biological diversity and extreme element accumulation during the early cambrian south china

origin of shallow burial cements in the late visean of the campine basin belgium abstract the diagenesis of late visean buildup and shelf carbonates from three boreholes in the campine basin northern belgium has been studied to determine the early diagenetic pore fluid evolution in the basin the earliest cementation occurred by isopachous fibrous and radiaxial fibrous calcites which precipitated in a marine environment under oxidizing conditions neomorphism during subsequent diagenesis altered these cements extensively precipitation of these first stage cements was followed by an episode of dissolution of carbonate components gastropods pelecypods and echinoderms after this episode non bright and dull luminescent stages successively developed in the shelf carbonates in response to the pore waters becoming progressively more anaerobic fluid inclusions and oxygen stable isotopes in the dull luminescent stage indicate that this cement formed in a meteoric fluid at relatively low temperatures 30 c and thus under burial conditions of a few hundred metres buildup carbonates from the shelf margin show a similar diagenetic evolution but dull luminescent cements appear to have precipitated at higher temperatures 45 c than in the shelf carbonates the meteoric waters responsible for the cementation are interpreted to have been sourced from near the brabant massif where dissolution of visean limestones could have taken place when the limestones to the north were already buried underneath namurian strata lateral gravity driven groundwater flow may be a very important type of fluid flow causing the cementation of shelf carbonates fracturing of the late visean strata started before the precipitation of the meteoric cements but was most intense just before and during the precipitation of the dull luminescent calcites this fracturing probably took place during the sudetic phase at the end of the visean and early namurian times

nature and origin of authigenic rhodochrosite and siderite from the paleozoic aquifer northeast mississippi u s a abstract botryoidal rhodochrosite siderite and pyrite are the principal authigenic phases in fractured chert comprising the paleozoic aquifer of northeast mississippi locally alternating bands of near end member composition rhodochrosite and siderite occur in veinlets up to 2 mm wide in the fractured chert carbon isotope data for the carbonates 13c values ranging from 9 6 to 11 3 pdb indicate that both inorganic and organic c were incorporated into the authigenic phases the paleozoic aquifer is hydraulically connected with the overlying upper cretaceous coastal plain strata groundwater in the coastal plain units is undersaturated with respect to calcite and has relatively high fe and mn contents the latter apparently caused by the dissolution of fe mn oxyhydroxides the natural downward recharge of these waters into the paleozoic aquifer causes calcite dissolution in limestone at the base of the chert which in turn leads to the precipitation of rhodochrosite and siderite additionally pyrite is precipitated as so4 reduction occurs in the same zone rhodochrosite has not been previously documented as an authigenic phase in a fresh water aquifer system suggesting that the geochemical environment necessary for its precipitation is not common the alternating bands of rhodochrosite and siderite observed in this study coupled with available thermodynamic data indicate that the groundwater a mn2 afe2 ratio must have fluctuated over time variations in the ratio are best explained by variable eh conditions at the site where fe mn oxyhydroxide dissolution occurs

geochemical investigation of the lower cambrian mineralised black shales of south china and the late devonian nick deposit canada abstract lower cambrian tommotian black shales of the niutitang formation south china host a thin accumulation 5 20 cm of ni mo platinum group elements pge au ni as zn cu v and rare earth elements ree of all known deposits the late devonian ni mo pge sulphide horizon in black shales of the nick deposit selwyn basin in canadian yukon presents the strongest similarity with the niutitang formation polymetallic layer in the present study samples of the mineralised layer and host shales in south china from two mine sites zunyi and sancha were investigated in order to further characterise the prevailing redox conditions and mechanisms involved in the metal concentration process at both sites additional comparison with the nick deposit mineralised layer was also undertaken bulk geochemical analysis including pge analysis were conducted on all samples along with m scale investigation of metal distributions and associations by x ray fluorescence xrf mapping and synchrotron xrf mapping the comparison of the two sites from south china highlights strong similarities in metal enrichment factors pge and ree distribution patterns are also highly similar and suggest a source from seawater strong variations in elemental distributions at m scale were observed at both sites most likely related to intense variations in redox conditions in the sediment the comparison with the nick deposit highlights a stronger enrichment in ni and zn in the nick deposit and a greater variety of minerals in the niutitang shales the pge and ree distributions of both the nick deposit and the niutitang shales however present highly similar patterns and support an origin from seawater the present study provides further insight into mineralisation style and processes in these mineralised black shales highlighting the importance of redox conditions and re emphasizing the role of organic matter in the formation of these mineralised layers

interpretation of complex non concentric zoning fabrics in sparry calcite cements middle jurassic limestones central england abstract sparry calcite cements in middle jurassic limestones of central england contain complex non concentric compositional zonation detailed petrographic study allows identification and characterisation of the most common zonal patterns although crystallographic indexing of the zones is hampered by the lack of associated concentric zoning and the irregularity of zonal boundaries crystallographic interpretation of the zoning can still be made using the full range of regionally consistent petrographic and geometrical relationships the fabrics can be modelled in terms of a ubiquitous primary sector zonation and a localised and episodic sector specific frequently lamellar zonation compositional variation indicated by staining and cathodoluminescence show that neither of the crystal faces forming the sectors have compositions which necessarily represent equilibrium precipitation from the bulk porefluid analogous zoning fabrics have been documented from many other ancient sparry calcite cements but have not previously been interpreted in this manner their widespread occurrence suggests that important and fundamental kinetic effects associated with growth processes on the propagating crystal surfaces may have a major influence on their trace element contents

a c mos2 mixed layer phase mosc occurring in metalliferous black shales from southern china and new data on jordisite a new phase composed mainly of mo s and c and referred to herein as mosc occurs widely in organic rich metalliferous cambrian black shales in south china mosc which has previously been referred to as jordisite has been studied by scanning electron microscopy sem electron microprobe analysis empa transmission electron microscopy tem powder x ray diffraction xrd extended x ray absorption fine structure exafs and catalytic activity tem data show mosc to have a layered structure with packets resembling molybdenite and graphite like carbon that average five layers in thickness analytical data are consistent with an idealized formula of mo3s6c10 but it commonly contains 1 3 wt each of fe ni and as so that its composition may be better approximated by the formula mo fe ni 3 s as 6c10 selected area electron diffraction saed patterns show a small number of broad inhomogeneous rings corresponding to randomly oriented layers arranged in subspherical cells a single broad weak peak corresponds to a 10 11 a layer spacing in powder xrd diffraction patterns pseudomorphism after fossil bacteria implies an origin by replacement of sedimentary organic material in its chemical properties and structure mosc resembles synthetic compounds used as catalysts for hydrodesulfurization hds in the petrochemical industry the large surface to volume ratio for mosc may be an important factor in its relatively strong hds catalytic activity cotype samples of jordisite from germany previously thought to be amorphous mos2 were also studied by sem and tem jordisite occurs as sequences of a few curved layers that form sub spherical units with an appearance remarkably like that of mosc however the layer spacing is 6 a like that of molybdenite the ratio of mo s is 1 2 and no carbon was detected although it coexists with kerogen jordisite is thus confirmed to be a form of mos2 but because powder diffraction like saed patterns were obtained it is not amorphous

sedimentary characteristics and origin of dolomitic ooids of the terminal ediacaran dengying formation at yulin chongqing south china abstract the ediacaran was a crucial transition period in the evolutionary history of the earth ediacaran ooids in the dengying formation from the yulin section in southwestern china are important archives of paleoenvironmental information although these ooids have been totally dolomitized the cortical fabric characteristics are locally preserved well indicating that these ooids were mimetically replaced the common occurrence of concentric radial fabrics and low level of sr concentrations 60 3 8 8 ppm in the studied ooid cortex is more likely suggestive of a primary low mg calcite mineralogy and that a calcite sea might exist during deposition of the second member of the dengying formation in situ geochemical data for the ooid cortices obtained by laser ablation inductively coupled plasma mass spectrometry show that these ooids have middle rare earth element enrichment prn smn 0 7 0 1 and smn ybn 1 5 0 4 and high fe contents 8200 1800 ppm this suggests ferruginous pore waters which were related to the selective or preferential absorption and release of rees by fe mn oxyhydroxides

characterization of trace elements and carbon isotopes across the ediacaran cambrian boundary in anhui province south china implications for stratigraphy and paleoenvironment reconstruction abstract characteristics of redox sensitive elements and organic carbon isotopes of a drilling section covering the ediacaran cambrian e c boundary at anhui province south china were studied total organic carbon toc organic carbon isotope values of organic compounds abundances of mo v u and ni and ratios of v v ni and th u co vary across the section suggesting that the redox condition is the main factor controlling carbon isotope values of organic matter and sequestration of trace elements the e c boundary is characterized by the lowest organic carbon isotope and the highest ratios of mo toc v toc and u toc in the section which are correlated with reported carbon isotope excursions in other regions this is the first time that signatures of mo v and u in the hetang formation at the lower yangtze platform are used for stratigraphic correlations the results highlight abnormally high ratios of mo toc v toc and u toc near the e c boundary petrological and geochemical characteristics in this section suggests that intense upwelling activities during the e c transition may be the main contributor to 13 c depleted co 2 and enrichment of trace metals the co existing pervasive transgression led to ocean anoxia and facilitated the preservation of 13 c depleted organic matter and trace metal sequestration furthermore oxidation of the bottom ocean water may have completed during the early cambrian a combination of these factors may be among the reasons for significant evolutionary changes during the e c transition

the depositional and diagenetic evolution of the coastal ridges of northwestern egypt since the lower pliocene successive generations of coastal ridges made up of mixed oolitic and biogenic grainstones formed along the mediterranean coastline of egypt west of alexandria and have undergone progressive stages of diagenesis the ridges are complex structures resulting from several phases of deposition in which shallow marine components are partially and progressively reworked as subaerial and aeolian deposits diagenetic alteration resulted from increasing freshening of the pore waters in the ridges diagenesis began with marine phreatic cementation followed by freshwater phreatic cements aggrading recrystallization of microcrystalline calcite to microspar and minor dolomitization non marine hypersaline diagenesis soil profiles developed on the upper surfaces of the oldest dunes

influence of a major exposure surface on the development of microporous micritic limestones example of the upper mishrif formation cenomanian of the middle east microporous platform top limestones of the cenomanian mishrif formation offshore qatar were studied in order to investigate the diagenetic processes associated with the top mishrif subaerial unconformity and its influence on the development of microporosity in underlying carbonates petrographical and stable isotope results indicate that complex diagenetic changes occurred during subaerial exposure of the mishrif formation including pervasive dissolution and meteoric cementation as well as neomorphism of the micritic matrix micrites at the top of the mishrif formation are coarse i e 2 m sub rounded and very dull luminescent under cathodoluminescence in this uppermost part of the studied interval the limestone matrix first underwent dissolution of unstable grains in the vadose zone with subsequent precipitation of low magnesium calcite lmc overgrowths within an oxidising phreatic setting this process explains the poor luminescence of the micrite crystals and their relatively coarse crystallometry which results in the present day in relatively good reservoir properties 13c ratios within the microporous limestones are negative up to 4 v pdb due to the incorporation of isotopically light carbon derived from palaeosols which developed during exposure by contrast fine i e 2 m polyhedral and bright luminescent micritic matrix with higher 13c values up to 4 v pdb and lower permeability are dominant within the underlying carbonate interval these fine micrites are likely to be the product of early neomorphic recrystallisation in the lowermost part of the palaeo aquifer associated with the top mishrif unconformity and were precipitated from meteoric or mixed dysoxic waters which were slightly supersaturated with respect to calcite

carbon oxygen isotope signal of mississippian slope carbonates appalachians usa a complex response to climate driven fourth order glacio eustasy abstract mississippian chesterian ramp slope muds in virginia deposited on the margin of a rapidly subsiding foreland basin were not significantly reset isotopically by regional meteoric aquifer systems that affected units on the adjacent platform carbon and oxygen isotopic ratios of these muds tend to covary at the scale of the twelve 4th order sequences that dominate the platform succession the 13 c values of the slope muds range from 0 5 to 4 pdb and lie in the range of least altered contemporaneous brachiopods from north america indicating little resetting of carbon isotopes by diagenesis there is an approximately 1 to 2 decrease in 13 c in the maximum flooding zone relative to the lowstand followed by a positive shift upward in the highstand before becoming more negative toward 4th order correlative conformities meteoric diagenesis might have reset some sediments just beneath a sequence boundary to lighter values it cannot account for the observed shifts evident throughout the sequences the 13 c enriched values in 13 c profiles could be due to increased productivity during glacial stages the increased productivity during glacial stages was probably due to increased ocean circulation coupled with shallowing which brought the ramp top into waters with more 13 c enriched dissolved inorganic carbon with the opposite effects occurring during deglaciation the 18 o values of muds range from about 2 2 to about 4 6 pdb falling within and beyond the lighter part of the range of contemporaneous north american brachiopods mii h s grossman e l yancey t e 1999 carboniferous isotope stratigraphies of north america implications for carboniferous paleoceanography and mississippian glaciation geol soc am bull 111 960 973 these values likely have been reset by burial diagenesis and perhaps by some meteoric diagenesis just beneath correlative conformities the sequences show negative excursions of 1 to 2 in 18 o in the maximum flooding zone relative to the lowstand and then become more positive upward in the highstand then decrease toward the correlative conformities some of the preserved signal could reflect ice volume changes although this would account for only 0 5 to 1 change the ice volume effect during lowstands plus cooling of ramp top waters and small increases in salinity due to increased aridity in the appalachian basin could account for the remaining 0 5 to 1 5 of oxygen isotope excursions with the opposite effect occurring during high sea levels the 18 o and 13 c of the appalachian ramp slope facies lack the strong 3rd order trends of the mid continental united states and europe the remnant 4th order carbon and oxygen isotope signal is similar in structure to cenozoic deep sea isotope profiles that are due in large part to 4th order glacio eustasy this would be compatible with onset of late paleozoic glaciation of gondwana

new u pb age from the basal niutitang formation in south china implications for diachronous development and condensation of stratigraphic units across the yangtze platform at the ediacaran cambrian transition abstract the black shale dominated niutitang formation covers a large portion of the early cambrian yangtze platform in south china the base of this unit has been traditionally taken as the ediacaran cambrian boundary ca 542 ma recent radiometric ages from the basal niutitang formation and its correlative units however are significantly younger than 542 ma and raise questions about the diachrony of stratigraphic units across the yangtze platform at the ediacaran cambrian transition here we report a new u pb sensitive high resolution ion microprobe shrimp age of 522 7 4 9 ma from a tuffaceous bed of the basal niutitang formation in a deep water section in taoying guizhou province of south china this age in combination with existing ages and biostratigraphic data suggests that the base of the niutitang formation is of tommotian or upper meishucunian age and the ediacaran cambrian boundary is most likely within the liuchapo formation the depositional age of the unusual metal enriched ni mo pge sulfide deposits which are approximately 1 m above the ca 523 ma tuffaceous bed should be early tommotian upper meishucunian rather than early nemakit daldynian lower meishucunian regional correlation confirms the diachronous development and condensation of stratigraphic units across the early cambrian yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the ediacaran cambrian transition in south china

carbonate cement stratigraphy and timing of diagenesis in a miocene mixed carbonate clastic sequence offshore sabah malaysia constraints from cathodoluminescence geochemistry and isotope studies a mixed carbonate clastic sequence is one of several miocene hydrocarbon bearing reservoirs in the northwestern sabah basin this sequence consists of bioclast rich sandstones interpreted as prograding storm shoal deposits the sequence contains a complex variety of carbonate nodules the sequence has undergone nine stages of cementation that completely occluded pores each stage represents a distinctive cement texture precipitating at specific temperatures and burial conditions the diagenetic evolution began with a syndepositional iron free marine calcite ca1 followed by an early methane derived dolomite do1 bladed calcite ca2 blocky vein filling fe calcites ca3ca4 clay mineral associated dolomite do2 fe dolomite and ankerite do3do4 and late iron rich calcite ca5 each stage of cement has distinctive sr2 mn2 mg2 fe2 ca2 and na contents reflecting changes in the palaeo porefluid systems with time the cements are correlatable over the whole study area precipitation occurred from very early to late stages of diagenesis at near surface to 2 0 km depths with progressive burial and temperature increase oxygen isotope values become strongly negative stage ca4 13c 0 5 pdb 18o 5 7 pdb the cements were precipitated at 20 75 c the source of carbon varies from marine dissolved carbonates to fermentation of organic matter methane meteoric waters and possibly hydrothermal fluids strontium isotope dating of do1 with 13c 32 5 pdb 18o 2 60 pdb and do2 13c 3 0 pdb 18o 2 5 pdb dolomites indicates that these dolomitization events took place at 10 5 ma and 8 9 ma respectively assuming precipitation within a completely open seawater system the do1 dolomite was precipitated from a methane derived brine at the boundary between the serravallian and tortonian stages after major uplift and induced faulting at a sea level lowstand the do2 dolomite was precipitated during intermediate to deep burial diagenesis during smectite illite transformation before hydrocarbon emplacement

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

rhenium and osmium isotopes in black shales and ni mo pge rich sulfide layers yukon territory canada and hunan and guizhou provinces china rhenium and osmium abundances and osmium isotopic compositions were determined by negative thermal ionization mass spectrometry for samples of devonian black shale and an associated ni enriched sulfide layer from the yukon territory canada the same composition information was also obtained for samples of early cambrian ni mo rich sulfide layers hosted in black shale in guizhou and hunan provinces china this study was undertaken to constrain the origin of the pge enrichment in the sulfide layers samples of the ni sulfide layer from the yukon territory are highly enriched in re os and other pge with distinctly higher re 192os but similar pt re compared to the black shale host re os isotopic data of the black shale and the sulfide layer are approximately isochronous and the data plot close to reference isochrons which bracket the depositional age of the enclosing shales samples of the chinese sulfide layers are also highly enriched in re os and the other pge re 192os are lower than in the yukon sulfide layer re os isotopic data for the sulfide layers lie near a reference isochron with an age of 560 ma similar to the depositional age of the black shale host the osmium isotopic data suggest that re and pge enrichment of the brecciated sulfide layers in both the yukon territory and in southern china may have occurred near the time of sediment deposition or during early diagenesis during the middle to late devonian and early cambrian respectively

how marine incursion influences the quality of lacustrine source rocks the paleogene nanxiang basin eastern china the impact of marine incursions during transgression i e sea level rises and the shoreline moves landward on the formation and quality of lacustrine source rocks is an important and contentious issue in this study we present a case study of the paleogene hetaoyuan formation in the biyang sag nanxiang basin eastern china paleontological trace element and biomarker data indicate that the hetaoyuan source rocks in this region were influenced by a marine incursion the paleontological evidence indicates that the marine incursion resulted in the introduction of red and brown algae which commonly inhabit marine environments trace element analyses yielded representative evidence of marine incursion e g equivalent boron content 300 ppm and b ga ratio 4 2 biomarker evidence for marine incursion includes c26 c25 tricyclic terpanes ratios of 1 3 which is the threshold for distinguishing marine organic matter from lacustrine using the b ga ratio as a typical paleosalinity indicator it was determined that the influence of marine incursion decreased from the biye 1 to cheng 2 to an 3006 wells with the b ga ratio average decreasing from 7 51 to 6 81 to 3 73 respectively with an increasing extent of marine incursion e g distance landward overall water depth and marine freshwater mixing the primary productivity of organic matter increased and the preservational environment became more reducing these changes resulted in higher contents of organic matter total organic carbon e 2 8 wt and a more favorable type of organic matter for oil generation kerogen type i ii indicating that the marine incursion had a positive effect on the formation of source rocks therefore the formation mechanism of high quality source rocks in coastal lacustrine basins during high sea level periods and associated resource potential might need to be reevaluated e g the campanian lower neslen formation along the margins of the western interior seaway of north america and the terminal oligocene early miocene in the fluvial saldanha bay at the southwestern tip of africa the results also provide useful data for regional oil and gas exploration

micron scale distribution of metals in cambrian metalliferous shales south china insights into local biologically driven redox disequilibrium abstract early cambrian black shales of the niutitang formation are found across the yangtze platform in a 1600 km belt extending across south china a thin organic matter rich layer in the lowermost part of this formation contains exceptional concentrations in mo ni se re os as hg sb ag au pt pd and ag due to their extreme metal enrichment these black shales provide a rare opportunity to study the interface between semi metals metals and biogenic material we report the first detailed m scale investigation of metal distributions and associations in samples from two sites zunyi and sancha located hundreds of km apart using a combination of analytical techniques including x ray fluorescence xrf mapping scanning electron microscopy sem synchrotron based xrf mapping electron probe micro analyser epma particle induced x ray emission pixe probe and x ray absorption near edge structures xanes imaging strong m scale variations in metal and semi metal distributions were highlighted by synchrotron based xrf mapping of samples from both sites at both locations u is present within phosphorite nodules arsenic mo and se are particularly abundant in the organic rich matrix showing strong associations with organic matter nickel however shows different distributions between the two sites it is mainly present in association with the organic matter at the sancha site while it is found in abundance in millerite at the zunyi site suggesting slightly different local conditions at zunyi favouring ni s associations over ni organic matter at both sites biogenic structures were re mineralised with laminations dominated by different metals indicating a likely control of organic matter over metal distributions in addition the xanes imaging highlighted different redox states of as over m scale areas while as 1 appears to be mainly present in pyrite as 3 was mainly detected in association within the carbon and mos2 mixed layer phase mosc overall the present results emphasize the complexity of metal associations in this mineralised layer the significant role of organic matter in the accumulation and precipitation of metals and semi metals in these metalliferous shales and highlight how biogenic activity can induce m scale variations in redox conditions in sediments

sequential vein growth with fault displacement an example from the austin chalk formation texas to determine the opening and precipitation history and characteristics of vein forming fluids analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack seal calcite veins in the austin chalk formation near san antonio texas the veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development they are composed of alternating millimeter to submillimeter thick calcite veinlets and host lithons occasionally crosscut by coarse equant grained secondary calcite veins regular changes in 18o e g 2 6 to 5 6 pee dee belemnite pdb of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed a systematic 18o decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the austin chalk relatively constant 13c approximately 1 4 0 4 pdb for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk there is no spatial variation in trace element composition of the calcite veinlets along the length of veins low sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of sr in calcite during vein formation normal faults in the cretaceous austin chalk were conduits to upwardly mobile vein forming fluids

cathodoluminescence in calcite cements new insights on pb and zn sensitizing mn activation and fe quenching at low trace element concentrations abstract the role of pb and zn as sensitizers of cathodolumines cl in sedimentary carbonates is still speculative as is the relationship between mn fe and cl intensity at low concentrations of mn and fe we address these questions using calcite cements from the lower oligocene suwannee formation of florida which have mean mn pb zn and fe concentrations less than 45 ppm only a small fraction of these cements exhibit visible cl but when present the luminescence is orange to reddish orange trace element contents were measured using a synchrotron x ray fluorescence microprobe and cl intensity was approximated by spot metered photomicrograph exposure times based on mn and fe contents four cement populations can be defined group i is luminescent calcite with mn 25 ppm and molar mn fe 1 8 group ii contains both luminescent and nonluminescent cements all with mn 25 ppm molar mn fe 25 ppm molar mn fe 100 ppm group iv is nonluminescent calcite with mn 25 ppm and variable fe these groupings and statistical analyses of the data mann whitney tests and factor analyses confirm that 25 ppm mn is necessary to initiate visible cl and that mn is the sole control on cl intensity when mn and fe contents are very low 100 ppm the data also establish 100 ppm fe as a new minimum for the quenching of cl when mn is below 200 ppm there is no evidence in our data that pb or zn concentrations in tens of parts per million either activate or sensitize cl the mann whitney test shows that there is no statistical evidence at the 95 confidence level to indicate that luminescent and nonluminescent cements reflect different populations of pb and zn except for group ii cements that have more pb in nonluminescent cements than in otherwise chemically equivalent luminescent cements factor analysis shows that cl intensity in all luminescent cements is unrelated to pb or zn concentrations given that pb or zn sensitizing is lacking in low mn calcites like the suwannee cements it is hard to imagine how these elements could play a significant role in calcites containing even more mn as is typical of most sedimentary carbonates

seismic stratigraphy and subsidence evolution of the northwest sumatra fore arc basin abstract new seismic reflection data collected with the indonesian rv baruna jaya iii under a french indonesian cooperative program allowed to study the geodynamic evolution and the seismic stratigraphy off northern sumatra within the sumatra fore arc domain the direction of subduction of the indian oceanic crust below the sunda arc is normal in front of java and oblique along sumatra as a result parts of the sumatra margin are moving northwards along two large strike slip faults parallel to the subduction trench the sumatra fault in sumatra and the mentawai fault at sea which separates the accretionary prism from the fore arc basin we describe the relationships between the seismic stratigraphy and the sedimentary history and we propose a tectonic evolution of the fore arc domain after widespread uplift and erosion during the paleogene the fore arc basin becomes subsident during the miocene as evidenced by shelf deposits organized in two transgressive regressive sequences 2a 2b produced by tectonics and eustatism this is interpreted as the result of a limited e w extension linked to the incipient play of the sumatra and mentawai faults in pliocene quaternary times the fore arc basin is segmented into several sub basins aceh simeulue and nias basins by compressional zones or by strike slip faults the subsidence rate increased strongly producing sequence 3 with two or four subunits for the pliocene and sequence 4 for the quaternary during this period local variations of the sediments thickness and of the number of sequences indicate that tectonics prevails over eustatism in the fore arc basin the sumatra margin is one of the best examples in the world to study the relationships between sedimentary and tectonic processes in an oblique subduction zone

organic clots and their differential accumulation of ni and mo within early cambrian black shale hosted polymetallic ni mo deposits zunyi south china abstract the celebrated early cambrian ni mo deposits that occur near zunyi city of guizhou province south china are hosted in shales the ore formation has been widely accepted to be related to biotic impact but direct evidence is still lacking in the present study of the huangjiawan ore deposit organic clots are found to occur widely in the ni mo ore beds but not in the barren layers the clots are generally elliptic with sizes ranging from 200 to 600 m the clot morphology is similar to that of modern rhodophyta as reported in the literature we tentatively interpret the clots as rhodophyte cystocarps electron microprobe analysis revealed that the clots have a differential accumulation of ni and mo with ni in the inner part and mo in the outer part of the cystocarp our study seems to have provided direct evidence for the biotic impact on the ore mineralization

dolomitization of gas reservoirs the upper permian changxing and lower triassic feixianguan formations northeast sichuan basin china abstract the lower triassic feixianguan formation and to a lesser extent the upper permian changxing formation contain more than ten giant recently discovered dolomite gas fields on either side of a paleo embayment kaijiang liangping bay sichuan basin china this study documents the origin of the dolomite using a combination of petrology fluid inclusion trace element geochemistry and stable isotope and radiogenic isotope analysis of samples from outcrop and core samples from producing gas fields three stages of dolomitization are revealed dolomicrite from the lagoon facies in the feixianguan formation and minor fabric retentive dolomite from reef facies in the changxing formation on the northeast side of kaijiang liangping bay represent the initial dolomite phase caused by reflux of feixianguan brine with salinity close to gypsum saturation 18 wt salinity during the first stage of diagenesis at 30 c the second and dominant dolomite phase fabric retentive dolomite in both the feixianguan and changxing formations on the both sides of kaijiang liangping bay is the result of reflux of diagenetic fluids with salinity ranging from normal feixianguan seawater 3 5 salinity up to mesohaline water 6 5 salinity during early burial diagenesis at about 35 40 c as shown by the dominance of single phase aqueous inclusions shallow formed stylolites that cut the second phase of dolomite and the early triassic marine carbon oxygen and strontium isotope and rare earth element and yttrium ree y signals the subordinate third dolomite phase fabric obliterative dolomite and dolomite cement was due to burial diagenetic recrystallization between 80 and 140 c in the feixianguan and changxing formations on both sides of kaijiang liangping bay the third phase of dolomitization involved water expelled from the slightly younger lower triassic jialingjiang formation marine evaporites and carbonates on the northeast side of kaijiang liangping bay revealed by the high salinity of the aqueous inclusions that homogenize at 80 to 140 c and elevated strontium isotope data the carbon isotope and ree y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input future exploration should be focused on dolomitized high energy sediments e g reef and oolitic shoal and bank in similar triassic tethyan paleogeographic settings that are associated with restricted facies but underlying dolomitized high energy permian strata may also be good exploration targets

climatic depositional and burial controls on diagenesis of appalachian carboniferous sandstones qualitative and quantitative methods abstract carboniferous quartz lithic and feldspar rich sandstones from different stratigraphic intervals within the central appalachian basin contain evidence for eogenetic and mesogenetic diagenesis important eogenetic products include fe oxide oxyhydroxide siderite kaolinite chlorite and calcite whereas quartz albite illite sericite fe carbonate and secondary porosity are the most important mesogenetic components correspondence and detrended correspondence analysis of the framework grain and diagenetic mineral data set has guided diagenetic interpretations and leads to two major conclusions first minerals that formed early appear to have been influenced by stratigraphic position the distribution of siderite and iron oxide oxyhydroxide is considered to reflect a second order paleoclimatic signature recognized throughout the carboniferous where siderite formed during everwet periods and iron oxide oxyhydroxide during semi arid conditions reflecting differences in redox second framework grain composition controlled the distribution of diagenetic products in the burial environment lithic arenites are deficient in authigenic quartz yet have undergone various degrees of illitization quartz deficiency is attributed to compaction related loss of primary porosity relatively early which inhibited sufficient fluid flow to account for much higher concentrations of quartz cement in the more competent quartz rich sandstones

hydrocarbon generation capability of sinian lower cambrian shale mudstone and carbonate rocks in the sichuan basin southwestern china implications for contributions to the giant sinian dengying natural gas accumulation the giant natural gas accumulation recently discovered within the neoproterozoic dengying formation in the sichuan basin of southwestern china represents a new and important supplement to the ancient neoproterozoic cambrian petroleum systems worldwide however the source of this gas is controversial because there are five possibilities based on geology from bottom to top including the lower sinian doushantuo mudstone the upper sinian dengying algal dolomite the mudstone of the third member of the dengying formation the lower cambrian maidiping argillaceous dolomite and the lower cambrian qiongzhusi shale to improve the understanding of this issue 33 samples from the five possible source rock units in the study area were investigated for their character and maturity via petrology total organic carbon toc pyrobitumen reflectance kerogen carbon isotope and biomarker analyses based on the results we address their hydrocarbon generation capabilities and contributions to the giant gas accumulation results show that present day toc values of the samples range from 0 03 to 2 36 and vary both regionally and stratigraphically with the qiongzhusi shales in the central basin and the weiyuan ziyang area ranking the highest in contrast the organic matter source and maturity within these rocks are relatively homogenous with slight variation throughout the study area the organic matter is predominantly derived from algae and bacteria with different compositions deposited in marine reducing environments the rocks range in thermal maturity from postmature to overmature these results imply that the generation capability of source rock had been high in the past and the gas in the study area is most likely originated from oil cracking and the late generation directly from the kerogen of source rocks with the qiongzhusi shales being the dominant source rock thus the hydrocarbon migration pattern in the study area is a lateral migration from deep seated and younger cambrian source rocks in sags to shallow seated and older sinian dolomite reservoirs in uplifts the migration process occurred in the oil phase with the oil being later cracked the differences in natural gas geochemistry may be controlled by complex factors e g thermal maturity the migration accumulation process and differing contributions from source rocks besides the dominant qiongzhusi regionally the central basin and the weiyuan ziyang area have had high hydrocarbon generation capability and thus should be considered as favorable targets for natural gas exploration in the future this study of china provides an additional example of neoproterozoic cambrian petroleum systems worldwide and thus can help to synthesize the systematics of such systems and understand the sedimentary transition across the precambrian cambrian boundary