neoproterozoic iron formation an evaluation of its temporal environmental and tectonic significance article neoproterozoic iron formation nif provides evidence for the widespread return of anoxic and ferruginous ba sins during a time period associated with major changes in climate tectonics and biogeochemistry of the oceans here we summarize the stratigraphic contextof neoproterozoic iron formation and its geographic and temporal dis tribution it is evident that most nif is associated with the earlier cryogenian sturtian glacial epoch although it is possible that some nif may be ediacaran there is no incontrovertible evidence to support this age assignment the paleogeographic distribution of nif is consistent with anoxic and ferruginous conditions occurring in basins within rodinia or in rift basins developed on its margins consequently nif does not require whole ocean anoxia simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and or detrital sourcesofiron are capable ofsupplying sufficient ironto account forthe mass of the largernif occurrences this con clusion is reinforced by the available geochemical data that imply nif record is a mixture of hydrothermal and de trital components a common thread that appears to link most if not all nif is an association with mafi cv olcanics

deterministic mapping of reservoir heterogeneity in athabasca oil sands using surface seismic data bitumen reserves in oil sands in alberta canada represent one of the biggest such deposits in the world the athabasca region contains the bulk of this resource and the lower cretaceous mcmurray formation contains the most significant target interval inclined heterolithic strata and associated sand accumulations comprise most of the formation however the distribution of bitumen in the formation varies due to the high degree of facies heterogeneity throughout the deposit this lithological heterogeneity causes difficulties in interpreting geology and estimating the bitumen distribution

estimation of effective porosity using geostatistics and multiattribute transforms a case study the middle eocene kalol formation in the north cambay basin of india is producing hydrocarbons in commercial quantity from a series of thin clastic reservoirs these reservoirs are sandwiched between coal and shale layers and are discrete in nature the kalol formation has been divided into eleven units k i to k xi from top to bottom multipay sands of the k ix unit 2 8 m thick are the main hydrocarbon producers in the study area apart from their discrete nature these sands exhibit lithological variation which affects the porosity distribution low porosity zones are found devoid of hydrocarbons in the available 3d seismic data these sands are not resolved and generate a composite detectable seismic response making reservoir characterization through seismic attributes impossible after proper well to seismic tie the major stratigraphic markers were tracked in the 3d seismic data volume for structural mapping and carrying out attribute analysis the 3d seismic volume was inverted to obtain an aco

archean banded iron formations yilgarn block western australia banded iron formation is a widely distributed although relatively minor lithology in the archean yilgarn block western australia occurring in supracrustal sequences composed mainly of metamorphosed volcanogenic rocks in the northern and eastern parts of the yilgarn block and also in highly deformed and metamorphosed predominantly metasedimentary sequences in the gneissic terrain in the southwest in the central part of the yilgarn block structural mapping and stratigraphic reconstruction indicates that individual banded iron formation units extend over an area of at least 40 x 60 km and could extend over a much larger area most banded iron formation units appear to be 5 to 50 m thick and units of 100 to 150 m are uncommon three scales of layering occur in iron formation units a interlayering of banded iron formation and fe shale fe shales range in thickness from several millimeters to 5 to 6 m b mesobanding is the most prominent structure in the banded iron formations c millimeter scale laminations including microbanding occur in both banded iron formation and fe shales other structures in the banded iron formation include pods macules and small spheroidal nodules despite modification by metamorphic and deformation processes which among other effects have caused thinning of mesobands all the structures can readily be equated with those in the hamersley basin iron formations there is a wide range in metamorphic grade at the lowest grades banded iron formation contains quartz magnetite greenalite stilpnomelane minnesotaite siderite and ankerite fe shales contain chamosite stilpnomelane siderite and pyrite the highest grades are characterized by quartz magnetite clinopyroxene orthopyroxene in iron formation and hornblende almandine or biotite assemblages in fe shale mineral assemblages are essentially identical to those in many metamorphosed proterozoic iron formations the bulk chemical compositions of the yilgarn block iron formations are very similar to other precambrian iron formations from very different stratigraphic settings fe shales have highly variable compositions characterized by a marked positive al 2 o 3 tio 2 correlation and high cr contents the similarity in the bulk chemical composition of iron formations in the yilgarn block and other precambrian sequences implies that deposition of these iron formations occurred in very similar chemical environments it is suggested that although fe shales may be related in some way to volcanic activity the main constituents of the banded iron formations are not directly derived from volcanic sources banded iron formations appear to have been precipitated from ocean waters which relative to younger oceans contained higher concentrations of fe and si in solution

neural network seismic reservoir characterization in a heavy oil reservoir the athabasca oil sands in western canada are one of the largest hydrocarbon accumulations in the world due to the low viscosity special production techniques have to be applied to extract the oil one extraction technique is steam assisted gravity drainage sagd which has a simple concept an upper horizontal well injects steam into the producing zone creating a high temperature steam chamber in the formation the heat melts the thick crude and allows it to flow more freely with gravity s asistance to the horizontal production well which is parallel to and 4 6 m below the injection well

evolution of seismic interpretation during the last three decades the ingredients of seismic interpretation include the right mix of geological and geophysical knowledge together with a liberal dose of imagination tempered with a considerable amount of patience seismic interpretation is a skill that one acquires with experience but is constantly reinvigorated with new ideas and tools provided by younger university graduates the prime ingredients for seismic interpretation are seismic reflection data coupled with a geologic depositional and tectonic model which together provide the framework for integrating borehole microseismic and production data resulting in a good reservoir model although commonly used in both engineering and environmental applications most seismic reflection surveys are acquired for oil and gas exploration in both land and offshore areas around the world hydrocarbon accumulations are found at varying depths of a few thousand meters below the earth s surface which are ultimately confirmed by drilling because the cost of drilling closely sp

genetic diversity of manganese deposition in the terrestrial geological record terrestrial manganese deposits formed by hydrothermal sedimentary and supergene processes ancient analogues of modern oceanic hydrothermal deposits formed in spreading centre and subduction related settings and those deposited from terrestrial hot springs are discussed sedimentary mn oxide deposits formed in shallow water at the margins of stratified oceans above the redoxcline during sea level changes mn carbonate deposits probably formed by diagenesis through mn oxyhydroxide reduction coupled with organic matter oxidation climatic variation and basin water stratification responsible for mn concentration were manifestations of atmospheric coz content prompted by tectonism supergene manganese enrichment in continental weathering profiles was mainly dictated by climate topography and drainage systems manganese deposits of diverse genetic types occur in the terrestrial geological record roy 1981 these were produced by direct hydro thermal activity sedimentary processes and continental weathering although the processes may be interrelated each involves distinct mechanisms that place the deposits into specific genetic types some of these processes are best understood in preset day depositional sites the ancient environments were determined by the intensity and style of tectonism volcanism and hydrothermal activity the composition of the atmosphere and the hydrosphere and the development of the biosphere which all varied with time it is my intention to unfold the total panorama of the environmental evolution during different time envelopes that produced a variety of manganese deposits in the past only major deposits and those providing distinctive evidence of their environment of deposition will be considered here

ferride geochemistry of swedish precambrian iron ores chemical analysis for major and trace elements have been performed on 30 swedish precambrian iron ores and on some from iran and chile the swedish ores consist of apatite iron ores quartz banded iron ores skarn and limestone iron ores from the two main ore districts of sweden the bergslagen and the norrbotten province some swedish titaniferous iron ores were also included in the investigation the trace element data show that the swedish ores can be subdivided into two major groups 1 orthomagmatic and exhalative 2 sedimentary within group 1 the titaniferous iron ores are distinguished by their high ti contents from the ferride contents of the kiruna apatite iron ores the ores are considered to be mobilization products of skarn iron ores from the norbotten province

iron formations related to mafic volcanism and ensialic rifting in the southern margin zone of the damara orogen namibia abstract iron formations occur within the metasedimentary metavolcanic sequence of the chuos formation in the southern margin zone of the upper proterozoic damara orogen of south west africa namibia the iron rich lithosome comprises magnetite quartz rock haematite quartz rock and magnetite quartz silicate carbonate rock on the basis of their host rock associations iron formation in diamictite sequences and in amphibolite are distinguished iron formation in amphibolite is developed both as a chemical sediment and as a composite rock consisting of chemical iron rich sediment with admixture of varying amounts of volcanic detritus iron formation in diamictite represents a pure chemical sediment both of the iron rich rocks contain abundant apatite and their manganese contents are low an exhalative origin for the iron formations of the southern margin zone related to mafic volcanism and ensialic rifting is suggested

geochemical patterns of metabasites in the southern part of the damara orogen swa namibia applicability to the recognition of tectonic environment

late neoproterozoic banded iron formation bif in the central eastern desert of egypt mineralogical and geochemical implications for the origin of the gebel el hadid iron ore deposit abstract the banded iron formation bif in the gebel el hadid area is part of the late neoproterozoic island arc ophiolitic assemblage of the central eastern desert of egypt where it alternates concordantly with low grade metavolcaniclastics the bif units are mainly of oxide facies consisting of iron oxide mesobands with intercalations of quartz carbonate chlorite magnetite mesobands in the form of layers and pods geochemically the gebel el hadid bif is characterized by heavy ree enriched nasc normalized patterns rarely with a weak positive eu anomaly the neoproterozoic age of the egyptian bif suggests that they are of rapitan type but their size petrological and geochemical characteristics suggest that they are of algoma type having formed before the final collision between major fragments of east and west gondwana the nature and composition of the bif and associated metavolcaniclastics indicate their formation as a result of submarine hydrothermal activity concomitant with arc volcanism the gebel el hadid bif units were deposited in pulses during periods of quiescent arc activity at ca 717 8 ma u pb age on volcaniclastic zircon submarine diagenesis under suboxic conditions produced an early generation of magnetite hematite and quartz whereas regional metamorphism led to the formation of coarse grained and porphyroblastic magnetite and quartz post metamorphic oxidation led to martitization of magnetite and formation of hematite and specularite

impacts tillites and the breakup of gondwanaland a discussion oberbeck et al 1993 have proposed that some ancient sedimentary rocks such as diamictites and associated deposits widely interpreted as glacigenic should be reassessed in the light of recent advances in the understanding of impact related phenomena such radical departures from accepted interpretations cf deitz 1964 can stimulate review of long accepted more traditional ideas and thus provide a catalyst for new avenues of research the proposed reinterpretations have potentially far reaching consequences for example the possible connections between long term climatic change and tectonic history of the earth fischer 1984 worsley et al 1984 young 1991 would be brought into question some aspects of atmospheric evolution hart 1978 interpretation of stable isotope distribution curves and ideas on the evolution of life knoll 1991 have been linked to inferences about cold climatic episodes glaciations based primarily on study of the rock record in the last two decades following publication of the benchmark paper by schermerhorn 1974 glacial specialists have been particularly circumspect in their interpretations and have developed what they consider to be a reliable set of criteria for the recognition of glacial deposits the following remarks are offered in an effort to reaffirm the validity of some of these criteria and to suggest that the proposed impact theory for the origin of many diamictite bearing sequences is correspondingly

geochemical evidence for magma source heterogeneity and activity of a mantle plume during advanced rifting in the southern damara orogen namibia abstract activity of a mantle plume is indicated by magma source heterogeneity documented in the chemical compositions of metabasaltic rocks in the upper part of the swakop group in the southern damara orogen in contrast metabasaltic rocks in the lower part of the swakop group have homogeneous compositions and activity of a mantle plume during earlier crustal extension is chemically not recorded the basaltic rocks are considered to have been erupted between 750 and 650 ma during continental rifting lower part of the swakop group tholeiitic basaltic magmatism related to normal faulting occurred perpendicular to the main direction of extension the basaltic rocks have been derived from a homogeneous magmatic source and variations in their chemical compositions are related mainly to fractionation processes continuous rifting created subsidence and the formation of a major depositional basin underlain by thinned and stretched continental crust as well as newly formed oceanic crust upper part of the swakop group tholeiitic basaltic magmatism occurred along the axis of the basin over a strike length of at least 400 km the basaltic rocks have been derived by melting of a heterogeneous magmatic source the magma source heterogeneity is considered to indicate the activity of a mantle plume at this late rifting stage the gradual decrease of the degree of magmatic heterogeneity from west to east along strike of the intrabasinal basaltic rocks indicates that the mantle plume was active to the west of the investigated area

mixtite deposits of the damara sequence namibia problems of interpretation abstract mixtite deposits form extensive outcrops in the pan african damara orogenic belt of namibia they occur at two different stratigraphic levels in the lower part of the upper proterozoic damara sequence south african committee for stratigraphy 1980 kroner 1981 the older varianto mixtite is confined to the northern margin of the fold belt no evidence indicating a glacigenic origin has been found but on the southern foreland of the fold belt two possibly coeval mixtite deposits court and blaubeker formation show fair evidence of such an origin the younger chuos mixtitesoccur in three separate zones of the belt all these mixtites have been interpreted as glacigenic sediments and the chuos formation has been used as a stratigraphic marker for the correlation between different parts of the fold belt the present investigation shows that 1 the mixtite deposits show no features indicating a glacial origin 2 the sedimentary features indicate deposition by gravity flow processes 3 turbidites are in several areas closely associated with and even interbedded in the mixtite deposits 4 mixtite deposition is in some areas not confined to the chuos formation proper but began at lower stratigraphic levels or persisted to a higher level 5 in some areas calcareous or dolomitic sediments are interbedded in mixtite or mixtites are interbedded in such formations the mixtites of the chuos formation are interpreted as various kinds of submarine gravity flows slump breccias mass flow slurry grain flows turbidites which were probably triggered by tectonic activity during a stage of differential subsidence that affected the whole geosyncline but was concentrated in three rift zones under this assumption the majority of the mixtites may have been deposited during a limited time spin a different model assuming synorogenic deposition of the pebbly schist mixtites of the southern margin zone is briefly discussed it is concluded that the mixtite deposits are not of glacial origin and can therefore not be regarded as reliable chronostratigraphic markers for correlations between the different facies domains of the damara sequence nor for correlation with other upper proterozoic sequences e g gariep belt katanga belt such a correlation might nevertheless exist if the extensive gravity flows should have been caused by eustatic changes of the sealevel during a time of widespread glaciation on other parts of the globe in this case local mountain glaciers might have contributed material to some of the chuos mixtites

sedimentary and tectonic history of the holowilena ironstone a neoproterozoic iron formation in south australia abstract the holowilena ironstone is a neoproterozoic iron formation in south australia associated with glacial deposits of the sturtian glaciation through a comprehensive field study coupled with optical and scanning electron microscopy x ray fluorescence and x ray diffraction a detailed description of the stratigraphy sedimentology mineralogy and structure of the holowilena ironstone was obtained the holowilena ironstone comprises ferruginous shales siltstones diamictites and is largely made up of hematite and jasper early diagenetic replacement minerals of precursor iron oxyhydroxides and silica these chemical precipitates are variably influenced by turbidites and debris flows contributing clastic detritus to the depositional system structural and stratigraphic evidence suggests deposition within a synsedimentary half graben a model for the holowilena ironstone is proposed in which dense oxic fluids expelled during sea ice formation in the cryogenian pool in the depression of the half graben allowing for long lived mixing with the ferruginous seawater and the deposition of iron oxides this combination of glacial dynamics tectonism and ocean chemistry may explain the return of iron formations in the neoproterozoic

the marine environments encompassing the neoproterozoic glaciations evidence from c sr and fe isotope ratios in the hecla hoek supergroup in svalbard abstract the neoproterozoic era records several important events in earth history the associations between bif deposition snowball earth events and the redox state of seawater are the key to explain the re appearance of banded iron formations bif during the neoproterozoic unraveling ancient iron cycles provides important information about the linkage between the precipitation of bif and the redox condition in the ocean but current iron isotopic data are limited to sediments deposited only during the sturtian glaciations and ediacaran we conducted a detailed geological survey of the tonian to ediacaran sedimentary succession in nordaustlandet island svalbard our chemostratigraphies of 13 c carb from pre sturtian to post marinoan sedimentary successions in the polarisbreen group are consistent with those in coeval sediments we first analyzed iron isotope ratios 56 54 fe of individual pyrite grains with diverse shapes in carbonates black shales quartz arenites and diamictites in order to constrain the redox condition in seawater we found large variations in iron isotope ratios from ca 2 to ca 4 positive 56 54 fe values over 1 0 are only preserved in euhedral and aggregated pyrite grains in black shales of the backlundtoppen formation and the macdonaldryggen member of the elbobreen formation a partial oxidation of ferrous iron in seawater is necessary to explain such high 56 54 fe values which means that deep seawater was ferruginous ferrous iron rich before and after the sturtian glaciation this stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in neoproterozoic seawater this study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the sturtian glaciation if the deep ocean was intrinsically iron rich it is likely that neoproterozoic bifs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation

depositional environment and tectonic implications of the paleoproterozoic bif in changyi area eastern north china craton evidence from geochronology and geochemistry of the metamorphic wallrocks abstract the changyi banded iron formation bif in the eastern north china craton ncc occurs within the paleoproterozoic fenzishan group three types of metamorphic wallrocks interbedded with the bif bands are identified including plagioclase gneisses and leptynites garnet bearing gneisses and amphibolites protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials the garnet bearing gneisses are fe rich pelites contaminated by clastics and the amphibolites are tholeiitic rocks trace elements of la th sc and zr of the plagioclase gneisses and leptynites and the garnet bearing gneisses support that these meta sedimentary rocks were probably derived from recycling of archean rocks with felsic and mafic materials differentiated into different rock types 207 pb 206 pb ages of detrital zircons from the meta sedimentary rocks concentrate at 2 7 3 0 ga confirming their derivation from the archean rocks the presence of several paleoproterozoic detrital zircons 2240 to 2246 ma however also suggests minor involvement of paleoproterozoic materials the archean detrital zircons have e hf t values varying from 0 7 to 7 6 which mainly fall between the 3 0 ga and 3 3 ga average crustal evolution lines on the age vs e hf t diagram further illustrating that the rocks providing materials for the meta sedimentary rocks mainly originated from partial melting of a mesoarchean crust this is strongly supported by their crust like trace element distribution patterns such as nb ta p and ti depletion and ancient nd depleted mantle model ages t dm 2 9 3 4 ga in addition the remarkably high e hf t values 7 5 to 9 3 of the paleoproterozoic detrital zircons constrain the paleoproterozoic materials to originate from a depleted mantle the amphibolites show low sio 2 46 5 to 52 8 wt and high mgo 5 68 to 10 9 wt contents crust like trace element features and low e nd t values 4 5 to 0 3 suggesting that these ortho metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by archean crustal materials since an intra continental environment was required for the formation of the above metamorphic rocks these rocks not only confine the depositional environment of the changyi bif to be an intra continental rift but also support the rifting processes of the eastern ncc during paleoproterozoic

superheavy s isotopes from glacier associated sediments of the neoproterozoic of south china oceanic anoxia or sulfate limitation black shales and mn carbonates interbedded with glacial deposits from the neo proterozoic of southern china exhibit extremely heavy values of pyrite s isotopes that may reflect the peculiar environment of earth at this time 34 s averages 30 at tanganshan and 44 at xiangtan compared with typical values of 0 to 5 found in younger deposits furthermore there is no distinction between the shales and the mn carbonate ores in the neoproterozoic unlike the younger deposits which show much lighter 34 s in the shales than in the mn ores the spread is 25 most other chemical parameters are very similar to both the younger mn deposits and those from the paleoproterozoic the exception is the rare earth elements ree all neoproterozoic fe ores and most neoproterozoic mn ores lack the positive eu anomaly that characterizes archean and paleoproterozoic fe mn accumulations on the other hand neoproterozoic mn deposits have positive ce anomalies on north american shale composite nasc normalized plots in contrast to other mnco 3 ores the ree is also higher than in other mn deposits but lower than in modern deep sea crusts sulfide s values in all neoproterozoic shales tend to be exceptionally variable and to often show much heavier values than can be found in marine strata from the phanerozoic therefore the anomalous 34 s values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects times of enrichment of seawater sulfate in 34 s do not correspond to periods of glaciation so the likely cause of the s isotope patterns is not worldwide glaciation but a generally low level of dissolved sulfate s in the neoproterozoic oceans that allowed modest increases in the amounts of s removed as pyrite to drive down the oceanic s reservoir enough to produce strong rayleigh reservoir effects the abundance worldwide of sturtian age mn and fe deposits indicates an increase in fe flux to the oceans that would have been sufficient to depress so 4 2 levels severely and to result in residu al dissolved s extremely enriched in 34 s ree evidence indicates that most of this enhanced fe and mn flux came from diagenetic remobilization of detrital oxides

geochemical investigation of a neoproterozoic glacial unit the mineral fork formation in the wasatch range utah the mineral fork formation is an isolated unit of neoproterozoic glaciogenic rocks in the wasatch range in northwestern utah geochemical analyses of diamictite and mudstone samples were carried out to document its major and trace element geochemistry comparison of the lower and upper parts of the formation which have been ascribed to different tectonic and sedimentological settings revealed no striking differences but the analyzed samples show a gradual upward increase in cao mgo fe 2 o 3 mno and p 2 o 5 and a decrease in sio 2 tio 2 al 2 o 3 ratios that suggest a provenance throughout the formation with an average composition slightly more mafic than average upper crust the mudstones are more k rich than the diamictites calculation of a chemical index of alteration cia is complicated by the presence of carbonate minerals after making corrections for carbonate content cia values range from 65 to 70 these values are higher than those from paleoproterozoic glaciogenic deposits some mudstones in the upper part of the formation are fe rich 15 fe 2 o 3 approaching the composition of iron formations the ferruginous nature supports previous correlation with fe rich glaciogenic rocks of the rapitan group in the northern canadian cordillera trace elements including rare earth elements suggest that the materials in the mineral fork formation were largely derived from a post archean source most of the widespread glaciogenic rocks of the neoproterozoic were derived from older sedimentary materials whereas the deposits of the earlier great glacial episode in the paleoproterozoic contain a much higher proportion of crystalline materials

chemical physical and micromorphological properties of termite mounds and adjacent soils along a toposequence in zona da mata minas gerais state brazil article i nfo little is known about the effects of neotropical mound building termites in soil chemical and physical properties the influence of soil termite activity on soil characteristics was studied by assessing chemical physical and micromorphological properties of a toposequence of latosols oxisols soil samples were collected from the walls and inner parts of termite mounds and also from adjacent soil a high diversity of termite genera was found in the mounds along the toposequence together with the inquiline termites and other soil dwelling arthropods chemical analyses showed that ph and the contents of organic c and n p ca and mg were significantly higher in termite mounds compared with adjacent areas with an inverse trend for al content significant differences in ph and exchangeable al were observed between soil and mound across the slopes the mound density across the landscape was higher at the upper slope segment followed by the hill top middle slope and lower slope segments considering a lifespan of 30 years and dimensions of termite mounds found in the toposequence we conclude that the textural and chemical uniformity of latosols may be increased following the pedobiological turnover during mound building with local rates varying from 2 1 to 7 5 m 3 ha 1

heterogeneous seawater phosphorus concentrations during the sturtian glaciation evidence from p fe ratios of fulu formation ironstone in south china abstract phosphorus p is one of the key bio limiting elements in the modern ocean and controls marine primary productivity over the geological time scales it is proposed that the ediacaran atmosphere oxygenation and animal evolution might have been triggered by high seawater p concentration in cryogenian oceans seawater p concentration can be reconstructed by the ratio of phosphorus to iron expressed as p fe 100 in ironstones here we measured the p fe ratios of the banded iron formation bif from the cryogenian fulu formation in south china the p fe 100 ratios of the fulu bif vary between 0 08 and 1 81 mean 0 45 n 22 considerably lower than the values of coeval bif deposited in other continents mean 1 24 n 74 variation of p fe ratio of ironstones may suggest a large seawater p concentration gradient during the sturtian glaciation which is comparable to or even higher than that in the modern ocean 1 2 m in north atlantic and 2 6 m in north pacific though being explained in different ways the heterogeneous seawater p concentration requires active organic matter production in the surface ocean and or decomposition in the ocean interior as well as possible onset of thermohaline circulation during the sturtian glaciation finally our study suggests that precipitation of bif in the completely ice covered ocean was unlikely