the effects of logging on frequency and distribution of landslides in three watersheds on vancouver island british columbia abstract three hundred and sixty three landslides in three watersheds that totaled 382 km2 were identified from air photographs beginning at a date that preceded logging to the present the three watersheds all lie on vancouver island however they have different precipitation regimes topography and amounts logged landslide areas in the watersheds varied in size from 200 m2 to more than 1 ha nearly 80 of the landslides were debris slides 15 were debris flows and the remainder primarily rock falls following logging the number of landslides increased substantially in all watersheds although the amount of increase was variable approximately 11 3 and 16 times in macktush creek artlish river and nahwitti river respectively other analyses of changes in landslide density also produced highly variable results with the number of landslides increasing between 2 4 and 24 times further 2 12 times more landslides reached streams following logging activities densities for landslides impacting streams increased for the period of record from 1 5 to 10 times following logging activities the densities were substantially greater where only landslides that reached streams since development began in a watershed were considered roads had the greatest spatial impact in the watersheds compared to their total area with frequencies determined to have increased by 27 12 and 94 times for macktush artlish and nahwitti respectively the results highlight the relative impact of roads and their role in slope stability

isotopic dating of an archean bolide impact horizon hamersley basin western australia detailed geologic field work in the hamersley basin of western australia has identified a single horizon that contains predominantly sand sized spherules similar to those found in impact ejecta such as at the cretaceous tertiary boundary evidence suggests that these spherules represent a reworked distal strewn field formed by a large bolide impact in late archean time this so called spherule marker bed occurs throughout the main body of the hamersley basin and in stratigraphically equivalent but shallower water lithologies in the northeastern corner carbonate constitutes both a matrix component of the spherule marker bed and minor interbeds in the local stratigraphic section we have utilized carbonate pb pb dating methods to provide for the first time an age estimate 2541 18 15 ma for this important marker bed and therefore of the bolide impact event samples from both facies define within error an identical isochron age that is also consistent with the few zircon based age estimates for the hamersley succession these findings highlight the great potential of carbonate pb pb geochronology in the dating of archean and proterozoic sedimentary rocks

geochemistry of precambrian carbonates 3 shelf seas and non marine environments of the archean a comprehensive whole rock study of mineralogical chemical and isotopic attributes of archean carbonates suggests that their lithologies and facies have been controlled by tectonic setting in the first two papers of this series we have shown that the dominant lithology of sedimentary carbonates in greenstone belt settings is limestone fe rich dolostone of ankeritic composition in older examples in this paper we suggest that the archean shelf sequences are mostly dolostone and the contemporaneous lacustrine playa lakes are characterized by limestone facies the present study is of the shelf environments of the archean represented by the pongola supergroup of south africa and the hamersley group wittenoom and carawine dolomite of australia the lacustrine playa examples have been sampled from the ventersdorp supergroup of south africa and the fortescue group of australia geological trace element and oxygen isotope considerations of the shelf carbonates suggest that their original mineralogy may have been aragonite and that the pongola dolostones probably represent a direct dolomitization product of this precursor in contrast the stabilization of the hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone prior to dolomitization subsequently both sequences but the pongola supergroup much more so were subjected to pervasive thermal events which overprinted their chemical and isotopic signatures consideration of the best preserved samples yields 0 9 2 1 and 6 l pdb for the 13c and 18o respectively for the initial archean signature if compared to phanerozoic carbonates the archean 13c values are comparable but the oxygen isotope measurements again support the primary nature of 18o depletion in ancient marine sediments regardless of its ultimate cause the 87sr86sr of 2 5 ga old sea water was not more than 0 7033 if this value reflects the contemporaneous sea water it would indicate the commencement of a significant 87sr input from the growing and aging continents it is however entirely possible that the slight enrichment in radiogenic 87sr relative to the contemporaneous mantle represents only an unresolved artefact of post depositional alteration if so the 87sr86sr of the 2 5 ga old sea water may still have been within a mantle range the lacustrine limestones retain a strong facies control in their trace element attributes they are also 4 depleted in 13c compared to marine carbonates and strongly depleted in 18o 19 l pdb these light 18o values are somewhat enigmatic unless the lakes were of the high altitude latitude variety the 87sr86sr of 0 711 for the ventersdorp lakes suggests that their drainage basin incorporated not only volcanic rocks but also aged continental crust archean carbonates of all lithologies and facies are enriched in mn and fe compared to their phanerozoic counterparts

interaction between co2 rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical co2 conditions experiments and modeling abstract a test site for co 2 geological storage is situated in hontomin burgos northern spain with a reservoir rock that is mainly composed of limestone during and after co 2 injection the resulting co 2 rich acid brine gives rise to the dissolution of carbonate minerals calcite and dolomite and gypsum or anhydrite at depth may precipitate since the reservoir brine contains sulfate experiments using columns filled with crushed limestone or dolostone were conducted under different p p co 2 conditions atmospheric 1 10 3 5 bar subcritical 10 10 bar and supercritical 150 34 bar t 25 40 and 60 c and input solution compositions gypsum undersaturated and gypsum equilibrated solutions we evaluated the effect of these parameters on the coupled reactions of calcite dolomite dissolution and gypsum anhydrite precipitation the crunchflow and phreeqc v 3 numerical codes were used to perform reactive transport simulations of the experiments within the range of p p co 2 and t of this study only gypsum precipitation took place no anhydrite was detected and this only occurred when the injected solution was equilibrated with gypsum under the p p co 2 t conditions the volume of precipitated gypsum was smaller than the volume of dissolved carbonate minerals yielding an increase in porosity up to 4 a decrease in t favored limestone dissolution regardless of p co 2 owing to increasing undersaturation with decreasing temperature however gypsum precipitation was favored at high t and under atmospheric p co 2 conditions but not at high t and under 10 bar of p co 2 conditions the increase in limestone dissolution with p co 2 was directly attributed to ph which was more acidic at higher p co 2 limestone dissolution induced late gypsum precipitation long induction time in contrast to dolostone dissolution which promoted rapid gypsum precipitation moreover owing to the slow kinetics of dolomite dissolution with respect to that of calcite both the volume of dissolved mineral and the increase in porosity were larger in the limestone experiments than in the dolostone ones under all p co 2 conditions 10 3 5 and 10 bar by increasing p co 2 carbonate dissolution occurred along the column whereas it was localized in the very inlet under atmospheric conditions this was due to the buffer capacity of the carbonic acid which maintains ph at around 5 and keeps the solution undersaturated with respect to calcite and dolomite along the column 1d reactive transport simulations reproduced the experimental data carbonate dissolution and gypsum precipitation for different p p co 2 t conditions drawing on reaction rate laws in the literature we used the reactive surface area to fit the models to the experimental data the values of the reactive surface area were much smaller than those calculated from the geometric areas

geological evidence for a strewn field of impact spherules in the early precambrian hamersley basin of western australia predominantly sand sized spherules of k feldspar with spherulitic vesicular and other crystalline textures help to define a unique marker bed in the 2 6 ga wittenoom formation which persists for more than 300 km across the hamersley basin of western australia these spherules usually occur as a minor constituent of a single turbidite but locally they are restricted to centimeter thick lenses of pure spherules along a single horizon in the argillite close beneath it in the shallower northeastern part of the basin similar spherules as well as larger more heterogeneous particles likewise occur at only one horizon but here they are a minor constituent of a thick dolomitic debris flow deposit as the wittenoom formation and the carawine dolomite are thought to be stratigraphically equivalent this debris flow deposit is believed to be a proximal equivalent of the spherule bearing turbidite on the basis of their similarity to microtektites and microkrystites in shape size and mode of occurrence the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the hamersley basin by a major bolide impact moreover the mass of the spherules preserved in the hamersley basin is of the same order of magnitude as the estimated masses of microtektite glass in major cenozoic strewn fields despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller the layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial the flow however is not believed to have been a direct result of the proposed impact if the spherule bearing layers constitute a chronostratigraphic marker as suggested here there are important implications for the deposition of carbonates and banded iron formations bif within the hamersley basin similar spherules also occur at a single horizon 250 m higher stratigraphically in the brockman iron formation suggesting that two impacts occurred near the hamersley basin in relatively rapid succession correlative strata may be present in the transvaal basin of south africa

the chemical conditions of the late archean hamersley basin inferred from whole rock and pyrite geochemistry with δ33s and δ34s isotope analyses the well preserved late archean sedimentary rocks of the fortescue and hamersley basins in western australia offer fascinating insights into early earth ocean chemistry prior to the great oxidation event goe in this study we use a combination of whole rock geochemistry la icpms trace element analysis of sedimentary pyrite and pyrrhotite and shrimp si sulfur isotope analyses to elucidate the chemical changes in these sedimentary rocks these proxies are used to examine chemical conditions of the ocean during the late archean two to three periods of oxygen enrichment prior to the deposition of banded iron formations bif can be identified one minor stage of general increase in whole rock enrichment factors and trace element content of pyrite is observed up stratigraphy in the jeerinah formation fortescue basin and a more substantial stage is present in the paraburdoo and bee gorge members of the wittenoom formation hamersley basin some of the trace element enrichments indicate organic matter burial flux ni cr zn co and cu which suggests an increase in biological productivity if the increased biological activity reflects an increase in cyanobacteria activity then an associated increase in oxygen is likely to have occurred during the deposition of the bee gorge member an increase in atmospheric oxygen would result in continental weathering of sulfide and other minerals increasing the trace element content of the water column via erosion and avoiding excessive depletion of trace elements due to drawdown in seawater since some of these trace elements may also be limiting nutrients such as mo and se for the cyanobacteria the degree of biological productivity may have further increased due to the increasing amount of trace elements introduced by oxygenation in a positive feedback loop these periods of increased productivity and oxygen rise stopped prior to the onset of bif deposition in the hamersley basin this may be due to the ocean reaching an oxidation threshold enabling the precipitation of hematite and magnetite bif the bif deposition caused depletion of ocean nutrients such as phosphate severely limiting the growth of cyanobacteria and thus limiting further oxygen production

experimental and modeling study of the interaction between a crushed marl caprock and co 2 rich solutions under different pressure and temperature conditions abstract carbon storage is regarded as a potential technology to reduce co 2 emissions this study is concerned with co 2 storage in a reservoir rock limestone which is a deep saline aquifer containing a nacl and sulfate rich groundwater in equilibrium with calcite and gypsum the reservoir rock is overlain by a very low permeability formation marls which acts as a caprock during and after co 2 injection the resulting co 2 rich acid solution gave rise to mineral dissolution and precipitation processes these reactions led to changes in porosity pore structure and permeability of the caprock and also to the alteration of its co 2 seal capacity flow through experiments using columns filled with three crushed samples marly limestone bituminous black shale and marl which constitute the hontomin caprock unit were conducted under different p total p co 2 atmospheric 1 10 3 5 bar subcritical 10 10 bar and supercritical 150 37 bar conditions and t 25 and 60 c using gypsum undersaturated and gypsum equilibrated solutions to evaluate the hontomin caprock reactivity 1d reactive transport modeling of the results enabled us 1 to better understand the overall processes that bring about the large dissolution of calcite minor dissolution of aluminosilicates and the precipitation of gypsum kaolinite zeolites and some aluminum and iron oxides and oxyhydroxides and sulfates and 2 to evaluate their implication for the hydrodynamic properties of the caprock an increase in porosity over the column length which mostly occurred near the inlet was calculated

transient surface ocean oxygenation recorded in the 2 66 ga jeerinah formation australia many paleoredox proxies indicate low level and dynamic incipient oxygenation of earth s surface environments during the neoarchean 2 8 2 5 ga before the great oxidation event goe at 2 4 ga the mode tempo and scale of these redox changes are poorly understood because data from various locations and ages suggest both protracted and transient oxygenation here we present bulk rock and kerogen bound nitrogen isotope ratios as well as bulk rock selenium abundances and isotope ratios from drill cores sampled at high stratigraphic resolution through the jeerinah formation 2 66 ga fortescue group western australia to test for changes in the redox state of the surface environment we find that both shallow and deep depositional facies in the jeerinah formation display episodes of positive primary 15n values ranging from 4 to 6 recording aerobic nitrogen cycling that requires free o2 in the upper water column moderate selenium enrichments up to 5 4 ppm in the near shore core may indicate coincident oxidative weathering of sulfide minerals on land although not to the extent seen in the younger mt mcrae shale that records a well documented whiff of atmospheric oxygen at 2 5 ga unlike the mt mcrae shale jeerinah selenium isotopes do not show a significant excursion concurrent with the positive 15n values our data are thus most parsimoniously interpreted as evidence for transient surface ocean oxygenation lasting less than 50 my extending over hundreds of kilometers and occurring well before the goe the nitrogen isotope data clearly record nitrification and denitrification providing the oldest firm evidence for these microbial metabolisms

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

correlating multiple neoarchean paleoproterozoic impact spherule layers between south africa and western australia abstract well studied successions in the griqualand west basin south africa and the hamersley basin western australia that both straddle the archean proterozoic boundary have been correlated on the basis of numerous lithostratigraphic similarities each succession hosts a minimum of three impact spherule layers implying a minimum of three large impacts by extraterrestrial objects over a time span of about 140 million years the occurrences of the youngest spherule layers in both basins in comparable stratigraphic positions in banded iron formations that have already been correlated suggest the youngest pair of layers were formed by a single impact close similarities in stratigraphic setting age and petrographic characteristics of the spherules as well as the restriction of distinctive irregular melt particles to only these layers suggest the oldest layers in the two basins were likewise formed by a single impact in contrast the middle layers on the two continents differ significantly in both their stratigraphic positions and the textures of the spherules suggesting they are products of different impact events these results suggest that using impact spherule layers to establish a global network of high resolution stratigraphic markers for early precambrian successions may be an achievable goal but careful comparisons between potentially correlative layers will be necessary to achieve it

why some alluvial rivers develop an anabranching pattern anabranching rivers have been identified globally but a widely accepted and convincing theoretical explanation for their occurrence has remained elusive using basic flow and sediment transport relations this study analyzes the mechanisms whereby self adjusting alluvial channels can anabranch to alter their flow efficiency sediment transport capacity per unit of stream power it shows that without adjusting channel slope an increase in the number of channels can produce a proportional decrease in flow efficiency a finding particularly relevant to understanding energy consumption in some braided rivers however anabranching efficiency can be significantly increased by a reduction in channel width as occurs when vegetated alluvial islands or between channel ridges form the counteracting effects of width reduction and an increasing number of channels can cause with no adjustment to slope an otherwise unstable system underloaded or overloaded to achieve stability as with other river patterns anabranching can be characterized by stable equilibrium or accreting disequilibrium examples copyright 2007 by the american geophysical union

multiple generations of carbonaceous material deposited in apex chert by basin scale pervasive hydrothermal fluid flow abstract the pilbara craton in western australia contains the best preserved and most complete record of archean rocks in the world as such they are some of the most studied rocks in the world paleontologists isotopic geochemists geologists and geobiologists have all investigated these rocks for clues about the early biosphere and atmosphere here we show using high resolution transmission electron microscopy that the carbonaceous material found in the apex chert and potentially in other associated units was formed by multiple processes such as abiotic catalytic synthesis and or biological synthesis we use these data as well as the geological history of the craton to demonstrate that when the rocks of the pilbara craton experienced a high degree of post depositional hydrothermal alteration carbonaceous material could have been remobilized and redeposited as the carbonaceous material within the apex chert samples was formed over nearly a billion years bulk chemistry even at the micron level will be unable to unambiguously delineate the presence of life in these ancient rocks although nanoscale observations may provide a way forward in the search for ancient life

functional relationships between vegetation channel morphology and flow efficiency in an alluvial anabranching river 1 water and sediment flux interactions are examined in magela creek an alluvial anabranching sand bed river in the northern australian tropics dense riparian vegetation stabilizes the channels and floodplains thereby preventing erosional instability at flow depths up to 6 2 times bankfull and discharges up to 15 times bankfull narrow anabranching channels characterize 92 of the alluvial reach and transport bed load more efficiently than short reaches of wide single channels yet overall 29 12 of the bed load is sequestered and the average vertical accretion rate is 0 41 0 17 mm yr 1 along the 12 km study reach the most effective discharge for transporting sediment 40 45 m3 s 1 is consistent at all 5 stations 10 channels examined and is equivalent to the channel forming discharge it has an average recurrence interval of 1 01 years occurs for an exceptionally long portion 13 15 of the annual flow duration and averages a remarkable 2 1 times bankfull the high flow efficiency i e bed load transport rate to stream power ratio of the anabranches is facilitated by low width depth channels with banks reinforced by vegetation colonnades of bank top trees confine high velocity flows overbed i e over the channel bed at stages well above bankfull at even larger overbank flows momentum exchange between the channels and forested floodplains restrains overbed velocities in some cases causing them to decline thereby limiting erosion magela creek exhibits a complicated set of planform cross sectional and vegetative adjustments that boost overbed velocities and enhance bed load yield in multiple channels while restraining velocities and erosion at the largest discharges

hydraulic and morphological characteristics of middle and upper reaches of the parana river argentina and brazil the parana river basin is a 2 600 000 km2 south american fluvial system most of which is in brazil and argentina the channel is about 4000 km long the mean annual discharge is 17 000 m3 s 1 and the annual sediment load is more than 150 000 000 tons the basin of the paraguay river its main tributary covers about 38 of the parana basin this paper compares the upper and middle channels of the parana hydraulic morphological and sedimentological features of the parana river upstream and downstream of the confluence with the paraguay were analyzed in order to evaluate the changes in these variables the effect of the paraguay on water and suspended sediment discharges was examined the paraguay river contribution explains most of the variation in channel conditions upstream and downstream of the confluence the channel pattern however remains braided the high bed load of the parana river in proportion to the total sediment discharge 21 in the upper course 25 in the middle course determines the similarity in drainage

supercritical carbon dioxide brine rock reactions in the madison limestone of southwest wyoming an experimental investigation of a sulfur rich natural carbon dioxide reservoir abstract the madison limestone on the moxa arch southwest wyoming usa is a sulfur rich natural co 2 reservoir a series of hydrothermal experiments was performed to evaluate multi phase co 2 h 2 o brine rock reactions and processes in this reservoir and to test the hypothesis that this reservoir is a natural analog for geologic carbon sulfur co sequestration idealized madison limestone dolomite calcite anhydrite pyrite and na cl so 4 2 brine i 0 5 molal reacted at 110 c and 25 mpa for approximately 81 days 1940 h supercritical co 2 was then injected and the experiment continued for approximately 46 days 1100 h a parallel experiment was performed without supercritical co 2 to provide a basis of understanding for the interaction of supercritical co 2 with the brine rock system two additional experiments were conducted in the same manner but without anhydrite in the starting mineral assemblage to examine supercritical co 2 sulfur reactivity injection of supercritical co 2 decreases ph by 2 5 to 3 3 units increases eh by 0 19 to 0 23 v and drives reaction pathways along the pyrite anhydrite saturation boundary of an eh ph diagram the dolomite calcite anhydrite mineral assemblage and reaction textures that are produced are consistent with those observed in the natural co 2 reservoir the mineral assemblage does not change following emplacement of supercritical co 2 instead minerals dissolve mobilize and re precipitate anhydrite precipitates in the dolomite calcite pyrite experiment following injection of supercritical co 2 and provides a mineral trap for sulfur anhydrite precipitation decreases so 4 2 activity ultimately leading to mineralization of co 2 experimental results support the hypothesis that the madison limestone on the moxa arch is a natural analog for geologic carbon sulfur co sequestration

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

channel pattern and river floodplain dynamics in forested mountain river systems channel pattern effectively stratifies the dynamics of rivers and floodplains in forested mountain river systems of the pacific northwest usa straight channels are least dynamic with relatively slow floodplain turnover and floodplains dominated by old surfaces braided channels are most dynamic with floodplain turnover as low as 25 years and predominantly young floodplain surfaces island braided and meandering channels have intermediate dynamics with moderately frequent disturbances erosion of floodplain patches maintaining a mix of old and young surfaces return intervals for the erosion of floodplains increase in the order of braided island braided meandering and straight 8 33 60 and 89 years respectively a threshold for the lateral migration of a channel occurs at a bankfull width of 15 20 m the most likely mechanism underlying this threshold is that larger channels are deep enough to erode below the rooting zone of bank vegetation above this threshold channels not confined between valley walls exhibit channel patterns distinguishable by slope and discharge and slope discharge domains can be used to predict channel patterns meandering and braided patterns are most consistently identified by the model and prediction errors are largely associated with indistinct transitions among channel patterns that are adjacent in plots of slope against discharge locations of straight channels are difficult to identify accurately with the current model the predicted spatial distribution of channel patterns reflects a downstream decline in channel slope which is likely correlated with a declining ratio of bed load to suspended load ecological theory suggests that biological diversity should be highest where the intermediate disturbance regime of island braided channels sustains high diversity of habitat and successional states through time

structural controls and timing of fault hosted manganese at woodie woodie east pilbara western australia abstract high grade fault hosted manganese deposits at the woodie woodie mine east pilbara are predominantly hydrothermal in origin with a late supergene overprint the dominant manganese minerals are pyrolusite braunite and cryptomelane the ore bodies are located on or near the unconformities between the neoarchean carawine dolomite and the paleoproterozoic pinjian chert breccia weathering product of carawine dolomite and sedimentary units of the overlying ca 1300 1100 ma manganese group stratabound manganese is typically located above or adjacent to steep fault hosted manganese the ore bodies range in size from 0 2 to 5 5 mt with an average of 0 5 mt historically over 35 mt of manganese has been mined at woodie woodie and current ore resources are 29 94 mt at 39 94 mn 6 96 fe resource and reserves statement june 2011 consolidated minerals pty ltd manganese mineralization at woodie woodie is related to northwest southeast directed extension and basin formation during the mesoproterozoic basin architecture is generally well preserved and major manganese occurrences are localised along growth faults which down throw the pinjian chert breccia into local extensional basins manganese ore bodies are typically located on steep 2nd and 3rd order structures that extend off the major growth faults mineralized structures display a dominant northeast trend reflecting the direction of maximum dilation during northwest southeast extension a paragenetic sequence is identified for the manganese ore at woodie woodie with early hydrothermal braunite pyrolusite cryptomelane todorokite hausmannite overprinted by late supergene oxides preliminary fluid inclusion studies in quartz crystals intergrown with pyrolusite and cryptomelane indicate that primary and pseudosecondary inclusions display a range of salinities from 1 to 18 eq wt nacl and trapping temperatures estimated to be from 220o to 290o at 1 kbar pressure a lead manganese oxide coronadite is common in manganese ores at woodie woodie and pb isotope studies of 40 lead rich ore samples from 16 pits indicate mineralization occurred within an age range of 955 1100 ma a mixed source is suggested for the lead but was predominantly basalts and or volcanogenic sedimentary units e g jeerinah formation of the ca 2700 ma fortescue group the typically high mn fe ratios and enrichment in elements such as pb as cu mo zn are consistent with a dominantly hydrothermal origin for the manganese at woodie woodie supergene manganese is distinguished from hypogene manganese by a marked enrichment in ree in the supergene manganese an early structural framework established during neoarchean rifting provides a major structural control on manganese ore distribution the woodie woodie mine corridor is located in a zone of oblique strike slip extension on major northwest trending transform faults and north trending oblique normal faults a major transform structure at the southern end of the woodie woodie mine corridor jewel southwest fault zone likely acted as a major fluid conduit for manganese bearing hydrothermal fluids and this would account for the concentration of significant manganese ore occurrences to the north and south of this structure

the holocene alluvial plain of the middle amazon river brazil abstract the fluvial belt of the amazon river is a complex of quaternary sedimentary units of different ages and formation conditions present position of the channel and the morphology and size of the alluvial plain are related to neo tectonic lineaments in the study area the main channel of the solimoes is nearly straight the river may be considered anastomosed albeit with few islands and anabranches the main holocene units are the impeded floodplain and the channel dominated floodplain the channel dominated floodplain is a complex mosaic of fluvial forms including younger channels active sandbars islands levees scroll dominated plains and abandoned belts the impeded floodplain is a widespread unit characterised by a very flat surface and round or irregularly shaped lakes the impeded drainage flood plain is only partially affected by the present floods and even those parts affected by floods receive little or no sediment from the main channel more important sediment contribution is by alluvial plain channels from the solimoes branches that generate deltas in the impeded floodplain lakes the impeded floodplain is interpreted to be an earlier stage in the evolution of the holocene alluvial belt the formation was marked by widespread vertical accretion and a channel morphogenetically less active than at present the later stage is associated with an active main channel and the paranas and generation of a scroll dominated plain this very large river system had a complex and delayed response to variations in the hydrological conditions induced by changes in the regional climate during the late holocene

patterns of anabranching channels the ultimate end member adjustment of mega rivers abstract large fluvial systems adjust to a combination of controls to form distinctive channels which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology fluvial geomorphology has commonly classified river channels into meandering straight and braiding patterns which are seen to represent a continuum of channel geometry anabranching patterns rivers with multiple channels however are characteristic of many rivers the identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology the development of this body of knowledge however has been established from medium and small rivers and laboratory flume studies very few of these research ideas developed from analysis of large fluvial systems this paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry discharge w d slope grain size stream power specific stream power and froude number q mean 1000 m 3 s the study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers further with the exception of the lower mississippi alluvial rivers with mean annual discharges greater than 17 000 m 3 s here classified as mega rivers do not generate single thread meandering or typical braided patterns these mega rivers develop anabranching patterns

riparian vegetation and island formation along the gravel bed fiume tagliamento italy after more than 300 years of river management scientific knowledge of european river systems has evolved with limited empirical knowledge of truly natural systems in particular little is known of the mechanisms supporting the evolution and maintenance of islands and secondary channels the dynamic gravel bed fiume tagliamento italy provides an opportunity to acquire baseline data from a river where the level of direct engineering intervention along the main stem is remarkably small against a background of a strong alpine to mediterranean climatic and hydrological gradient this paper explores relationships between topography sediment and vegetation at eight sites along the active zone of the tagliamento a conceptual model of island development is proposed which integrates the interactions between large woody debris and vegetation geomorphic features sediment calibre and hydrological regime islands may develop on bare gravel sites or be dissected from the floodplain by channel avulsion depositional and erosional processes result in different island types and developmental stages differences in the apparent trajectories of island development are identified for each of the eight study sites along the river the management implications of the model and associated observations of the role of riparian vegetation in island development are considered in particular the potential impacts of woody debris removal riparian tree management regulation of river flow and sediment regimes and changes in riparian tree species distribution are discussed

climatic variations in the argentine plains during the last 18 000 years the last deglacial hemicycle was characterized by a general increase in temperature and precipitation in the region with a few significant departures from this general trend the present ne sw climatic gradient was maintained throughout the entire period except in the upper holocene the following sequence of events is apparent if the present climate is taken as a reference base 1 a 18 000 8500 yr b p arid and cool with aeolian sand ad loess deposition patagonian fauna climatic isolines temperature precipitation etc were located some 750 km northeast of their present positions 2 b 8500 3500 yr b p humid subtropical with brazilian fauna pedogenesis and fluvial dynamics climatic limits migrated about 800 900 km southwest of their former positions 3 c 3500 1000 yr b p dry subtropical aeolian dynamics the normal latitudinal climatic gradient was interrupted by the occurrence of an anticyclonic centre which stabilized the climate over an area of some 1 600 000 km2 4 d 1000 yr b p little ice age climate was similar to the present one over much of the plains but the northeastern extremity was warmer 5 e little ice age climatic deterioration in the southern belt was characterized by generalized aeolian activity and migration of isolines more than 150 km to the northeast in that area 6 f present climate 19th and 20th centuries subtropical humid in the east and dry in the west

influence of the flow rate on dissolution and precipitation features during percolation of co2 rich sulfate solutions through fractured limestone samples abstract calcite dissolution and gypsum precipitation are expected to occur when injecting co2 in a limestone reservoir with sulfate rich resident brine if the reservoir is fractured these reactions will take place mainly in the fractures which serve as preferential paths for fluid flow as a consequence the geometry of the fractures will vary leading to changes in their hydraulic and transport properties in this study a set of percolation experiments which consisted of injecting co2 rich solutions through fractured limestone cores was performed under p 150 bar and t 60 c flow rates ranging from 0 2 to 60 ml h and sulfate rich and sulfate free solutions were used variation in fracture volume induced by calcite dissolution and gypsum precipitation was measured by x ray computed microtomography xcmt and aqueous chemistry an increase in flow rate led to an increase in volume of dissolved limestone per unit of time which indicated that the calcite dissolution rate in the fracture was transport controlled moreover the dissolution pattern varied from face dissolution to wormhole formation and uniform dissolution by increasing the flow rate i e pe from 1 to 346 fracture permeability always increased and depended on the type of dissolution pattern

origin evolution and sedimentary records of islands in large anabranching tropical rivers the case of the upper parana river brazil abstract anabranching patterns are characteristic of the largest rivers in the world we have chosen to study the parana river because of its worldwide importance among the great rivers and the large database available from over four decades of research in this paper we present results on the classification origin evolutionary processes and sedimentary records of islands that form anabranching patterns in the upper parana river in brazil these islands are formed by in channel processes that drive their evolution from central bar islands and by off channel processes that form floodplain excised island both types of islands can increase in area by the annexation of lateral bars forming composite islands the islands formed by floodplain excision are normally larger than the other types with ages of up to 14 6 ka the sedimentary records of these islands are relatively simple central bar islands present a lower lithosome comprising sandy channel deposits sp st sm and sm o facies that correspond to central and lateral bar formation followed by a mud sequence fm fl and fm o facies deposited by vertical accretion events during flood stages floodplain excised islands have the same sedimentary sequence as the floodplain from which they were detached the in channel island forming processes described in this paper may have been operating since the beginning of the holocene the understanding of island origin and evolution processes presented in this paper is fundamental to ecological and geomorphological studies as in the sustainable management of large anabranching rivers

co2 rock brine interactions in lower tuscaloosa formation at cranfield co2 sequestration site mississippi u s a a highly integrated geochemical program was conducted at the cranfield co2 enhanced oil recovery eor and sequestration site mississippi u s a the program included extensive field geochemical monitoring a detailed petrographic study and an autoclave experiment under in situ reservoir conditions results show that mineral reactions in the lower tuscaloosa reservoir were minor during co2 injection brine chemistry remained largely unchanged which contrasts with significant changes observed in other field tests field fluid sampling and laboratory experiments show consistently slow reactions carbon isotopic composition and co2 content in the gas phase reveal simple two end member mixing between injected and original formation gas we conclude that the reservoir rock which is composed mainly of minerals with low reactivity average quartz 79 4 chlorite 11 8 kaolinite 3 1 illite 1 3 concretionary calcite and dolomite 1 5 and feldspar 0 2 is relatively unreactive to co2 the significance of low reactivity is both positive in that the reservoir is not impacted and negative in that mineral trapping is insignificant

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

field evidence of eros scale asteroids and impact forcing of precambrian geodynamic episodes kaapvaal south africa and pilbara western australia cratons abstract the role of asteroid and comet impacts as triggers of mantle crust processes poses one of the fundamental questions in earth science i present direct field evidence for close associations between impact ejecta fallout units major unconformities and lithostratigraphic boundaries in archaean and early proterozoic terrains including abrupt changes in the composition of volcanic and sedimentary assemblages across stratigraphic impact boundaries with implications for the nature and composition of their provenance terrains as originally observed by d r lowe and g r byerly in the barberton greenstone belt eastern kaapvaal craton south africa 3 26 3 24 ga asteroid mega impact units are closely associated with the abrupt break between an underlying simatic mafic ultramafic volcanic crust and an overlying association of turbidites banded iron formations felsic tuff and conglomerates of continental affinities contemporaneous stratigraphic relationships are identified in the pilbara craton western australia evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains likely dominated by mafic compositions attributed to impact triggered oceanic volcanic activity relationships between impact and volcanic activity are supported by the onset of major mafic dyke systems associated with 2 48 ga and possibly the 2 56 ga mega impact events

decimeter scale mapping of carbonate controlled trace element distribution in neoarchean cuspate stromatolites abstract ancient stromatolites can provide key insights into the early evolution of life on earth neoarchean fenestrate stromatolites from the 2520 ma upper nauga formation transvaal supergroup south africa preserve cuspate morphologies they possess clearly delineated support and drape structures interpreted as dolomitized microbial mat material petrographic observations show that the biogenic structures are composed of planar s to non planar ferroan dolomite encased in ferroan calcite including herringbone calcite textures the cuspate stromatolites were analyzed using synchrotron rapid scanning x ray fluorescence srs xrf and more conventional techniques to determine i whether element distributions could be distinguished in ancient stromatolites at both cm to dm scales ii whether element distributions show variation between biogenic and abiogenic textures and iii the sample s paragenesis the distributions of ca fe mn pb cu as br al si p and s directly correspond to dolomitized stromatolitic structures and show trace element distributions are principally controlled by calcite and dolomite occurrence dolomite formation was mainly driven by seawater derived fluids given the high concentrations of fe and retention of marine shale normalized rare earth element and yttrium reysn patterns however the spatial association of dolomite to stromatolite structures may reflect microbially influenced mineral nucleation given the complexity of this sample s paragenetic evolution trace metal distributions cannot be conclusively tied to specific metabolic pathways bioaccumulation or passive binding however the results show srs xrf can be used for quantifiable spatial in situ investigation of ancient microbialites

iridium anomalies and shocked quartz in a late archean spherule layer from the pilbara craton new evidence for a major asteroid impact at 2 63 ga a thin 1 5 mm spherule layer in ca 2 63 ga shale from the jeerinah formation pilbara craton northwestern australia has been identified at two new localities the layers have ir concentrations as high as 15 5 ppb significantly higher than the surrounding carbonaceous shale 1 5 ppb the sand sized spherules display quench and devitrification textures and are interpreted as former silicate melt droplets that were replaced by k feldspar carbonate and sulfide during diagenesis in one spherule layer sample an angular quartz grain 100 mm in size with planar deformation features was found which represents the oldest known shocked grain in distal ejecta by 2000 m y the survival of shocked quartz in ca 2 63 ga rocks which have undergone multiple metamorphic events suggests that their absence in other impact ejecta layers may not only be a question of preservation the presence of shocked quartz and anomalously high ir contents in a layer containing melt spherules provides compelling evidence for an extraterrestrial impact with a target area that was at least partly silicic favoring a continental impact site estimates based on geochemical data suggest that the spherule layer comprises as much as 2 3 wt of a chondritic meteorite component if proposed correlations with the carawine eastern pilbara craton and monteville south africa layers are correct then the combined ejecta blanket represents fallout from a single major impact with an areal distribution of 32 000 km 2 among the largest yet documented in the precambrian rock record

iridium anomaly but no shocked quartz from late archean microkrystite layer oceanic impact ejecta samples from a ca 2 54 ga layer rich in microkrystites sand size spherules of former silicate melt in the hamersley group western australia are enriched in ir up to 50 fold over associated carbonates and shales but few of the other siderophile elements display significant anomalies or chondritic interelement ratios however similar concentrations and interelement ratios are observed in ejecta from the ca 590 ma acraman impact structure and have been attributed to diagenetic redistribution the microkrystite layer also contains sand size quartz grains with internal textures typical of regionally metamorphosed rocks rather than evidence of shock metamorphism we suggest that the microkrystites were created by an impact that took place in a deep ocean basin rather than on a continent and that the associated quartz is epiclastic detritus brought in by unusually high energy waves and or currents not as ballistic ejecta

carbonate pbpb ages of the wittenoom formation and carawine dolomite hamersley basin western australia with implications for their correlation with the transvaal dolomite of south africa abstract pb isotopic analyses of carbonates from the hamersley group western australia yield the following results 1 a late archean minimum age of 2 5 ga is definitely established for the deposition of both the deeper water basinal wittenoom formation and the shallow water carawine dolomite stratigraphic correlation of the transvaal dolomite south africa with these two formations is supported by the present pb isotope age data 2 a pb pb isochron age of 2505 37 ma for the paraburdoo member lower part of the wittenoom formation is best interpreted as a late diagenetic event which took place 100 ma after the initial deposition 3 a pb pb isochron age of 2346 38 ma for the bee gorge member most likely represents a still later local event of calcite replacement in fractures and veins 4 a pb pb isochron age of 2541 32 ma for carawine dolomite could represent a regional diagenesis slightly earlier than the paraburdoo carbonates if the carawine carbonates were deposited contemporaneously with the wittenoom formation alternatively it may represent the time of early diagenesis which closely followed the initial sedimentation a study of trace element abundances and stable isotope compositions has revealed a major difference in fe and mn contents and 18 o values between the wittenoom and carawine dolomite samples this suggests that the two suites of dolomites have undergone separate diagenetic histories with different chemical and oxygen isotopic modifications

iridium anomalies and shocked quartz in a late archean spherule layer from the pilbara craton new evidence for a major asteroid impact at 2 63 ga comment and reply comment simonson 1992 1 simonson and hassler 1997 2 and simonson et al 2000 3 discovered extraterrestrial impact ejecta in the hamersley and fortescue groups pilbara craton western australia and noted the absence of shocked quartz grains simonson et al 1998 4 rasmussen and

anabranching and maximum flow efficiency in magela creek northern australia 1 anabranching is the prevailing river pattern found along alluvial tracts of the world s largest rivers hydraulic geometry and bed material discharge are compared between single channel and anabranching reaches up to 4 times bank full discharge in magela creek northern australia the anabranching channels exhibit greater sediment transporting capacity per unit available stream power i e maximum flow efficiency mfe simple flume experiments corroborate our field results showing the flow efficiency gains associated with anabranching and highlight the prospect of a dominant anabranch which is found in many anabranching rivers these results demonstrate that anabranching can constitute a stable river pattern in dynamic equilibrium under circumstances in which a continuous single channel would be unable to maintain sediment conveyance we propose the existence of a flow efficiency continuum that embraces dynamic equilibrium and disequilibrium vertically accreting anabranching rivers

direct dating of paleomagnetic results from precambrian sediments in the amazon craton evidence for grenvillian emplacement of exotic crust in se appalachians of north america abstract we apply a new diagenetic dating technique to determine the age of magnetization for precambrian sedimentary rocks in the sw amazon craton two new paleomagnetic poles are reported from the rocks of the aguapei gp red beds of the fortuna fm p lat 59 8 n p lon 155 9 e a 95 9 5 k 14 18 sites n n 128 115 q 5 and the reverse polarity mudstones of the overlying vale da promissao formation p lat 49 5 n p lon 89 3 e a 95 12 5 k 30 6 sites n n 94 80 q 4 the fortuna fm magnetization is hosted by massive interstitial hematite cement and constitutes a post depositional remanence the age of diagenesis of the red beds is well constrained by the 1149 7 ma u pb age of authigenic xenotime rims on detrital zircons determined by shrimp analysis the magnetite hosted remanence of the vale da promissao fm may be detrital in origin but the age of deposition is poorly constrained the reliable and precisely dated fortuna fm paleomagnetic pole fixes the paleogeographic position of the amazon craton near the se appalachians portion of north america at 1 15 ga these data demonstrate a mobile grenvillian link between these two cratons and support the recent identification of amazon crust in the blue ridge province region of north america

direct dating of stromatolitic carbonates from the schmidtsdrif formation transvaal dolomite south africa with implications on the age of the ventersdorp supergroup pb isotopic analyses of stromatolitic limestones from the schmidtsdrif formation transvaal dolomite of south africa give a late archean isochron age of 2 557 plus minus 49 2 sigma ma the new date significantly changes the position of the archean proterozoic boundary in the kaapvaal province and suggests that a possible division of the transvaal supergroup is necessary it also resolves the long standing controversy about the age of the ventersdorp supergroup whose zircon u pb data of ca 2 7 ga must be more acceptable than the rb sr dates of about 2 3 ga the study provides a new example that demonstrates the great potential of employing the u pb systems of carbonate rocks for direct dating of sedimentary sequences

neoarchean crustal evolution in western shandong province of the north china craton the role of 2 7 2 6 ga magmatism abstract the neoarchean era is an important period for the formation of continental crust in the early earth the north china craton ncc records magmatism during 2 8 2 7 ga and 2 6 2 5 ga with the latter more extensive however the 2 7 ga rock assemblages are significantly different from those of the 2 5 ga rocks based on detailed field investigation we present results of zircon geochronology by using shrimp and la icp ms techniques on the shanggang pluton in the central belt of the western shandong province wsp the shanggang pluton is composed of 2 71 2 69 ga strongly deformed tonalite on the eastern and northern margins with the central part occupied by younger 2 67 2 65 ga and 2 63 2 61 ga trondhjemite most zircon grains with apparent 207pb 206pb ages of 2 67 2 66 ga in our samples have low th u ratios 0 05 0 20 resulted from low to moderate th contents of 10 130 ppm and relatively higher u contents of 100 3000 ppm consistent with those of anatectic zircons some of grains carry 2 7 ga inherited cores our study reveals that the crustal re working event at 2 67 2 66 ga might have been more widespread than currently known although the 2 67 2 66 ga zircons have positive ehf t values close to that of coeval depleted mantle wan et al 2014 together with data from previous studies it is notable that more trondhjemite and even some granite were generated during 2 67 2 60 ga relative to the older ttg suites 2 72 2 69 ga the younger ones 2 67 2 60 ga are dominated by trondhjemite the compositional variations of ttg suites through the early to middle neoarchean suggest that the 2 67 2 60 ga magmatic and anatectic events should be related to the compositional differentiation of the archean continental crust of the wsp

investigation of porosity and permeability effects from microstructure changes during limestone dissolution we studied experimentally the dissolution of a porous limestone core during co2 enriched water injection we measured the changes in porosity and permeability arising from modifications of the pore network geometry and the fluid rock interface a methodology based on periodic x ray microtomography imaging was implemented to record the evolution of the time and scale dependent microstructures with a spatial resolution of 4 91 m two processes were successively involved in the rapid permeability increase of the sample as documented from microscale to core scale measurements

carbonate petrography kerogen distribution and carbon and oxygen isotope variations in an early proterozoic transition from limestone to iron formation deposition transvaal supergroup south africa the transition zone comprises campbellrand microbialaminated replacing cryptalgalaminate limestone and shale with minor dolomite conformably overlain by the kuruman iron formation of which the basal part is characterized by siderite rich microbanded iron formation with minor magnetite and some hematite containing units the iron formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones the sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees c and pressures of 2 kbars carbonate minerals in the different rock types are represented by primary micritic precipitates now recrystallized to microsparite early precompactional sparry cements and concretions deep burial limpid euhedral sparites and spar cements precipitated from metamorphic fluids in close contact with diabase sills paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them kerogen occurs as pigmentation in carbonate crystals as reworked organic detritus in clastic textured carbonate units and as segregations of kerogen pigment around late diagenetic carbonate crystals locally kerogen may also be replaced by carbonate spar carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies integration of sedimentary petrographic geochemical and isotopic results makes it possible to distinguish between depositional early diagenetic deep burial and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence major conclusions are that deep burial thermal decarboxylation led to 13c depletion in euhedral ferroan sparites and 13c enrichment in kerogen organic carbon metamorphic sparites are most depleted in 13c carbonates in oxide rich iron formations are more depleted in 13c than those in siderite rich iron formation whereas the kerogens in oxide banded iron formations bif are more enriched this implies that the siderite rich iron formations were not derived from oxide rich iron formation through reduction of ferric iron by organic matter organic matter oxidation by ferric iron did however decrease the abundance of kerogen in oxide rich iron formation and led to the formation of isotopically very light sparry carbonates siderite and calcmicrosparite both represent recrystallized primary micritic precipitates but differ in their 13c composition with the siderites depleted in 13c by 4 6 per mil on average relative to calcmicrosparite this means that the siderites were precipitated from water with dissolved inorganic carbon depleted in 13c by about 9 per mil relative to that from which the limestones precipitated this implies an ocean system stratified with regard to total carbonate with the deeper water from which siderite rich iron formation formed depleted in 13c iron formations were deposited in areas of very low organic matter supply depletion of 13c may therefore derive not from degradation of organic matter but from hydrothermal activity a conclusion which is supported by 18o composition of the carbonate minerals and trace element and rare earth element ree compositions of the iron formations

anabranching in mixed bedrock alluvial rivers the example of the orange river above augrabies falls northern cape province south africa abstract anabranching is characteristic of a number of rivers in diverse environmental settings worldwide but has only infrequently been described from bedrock influenced rivers a prime example of a mixed bedrock alluvial anabranching river is provided by a 150 km long reach of the orange river above augrabies falls northern cape province south africa here the perennial orange flows through arid terrain consisting mainly of precambrian granites and gneisses and the river has preferentially eroded bedrock joints fractures and foliations to form multiple channels which divide around numerous large up to 15 km long and 2 km wide stable islands formed of alluvium and or bedrock significant local variations in channel bed gradient occur along the river which strongly control anabranching style through an influence on local sediment budgets in relatively long 10 km lower gradient reaches 0 0013 within the anabranching reach local transport capacity exceeds sediment supply bedrock crops out extensively and channels flow over an irregular bedrock pavement or divide around rocky islands channel incision into bedrock probably occurs mainly by abrasion with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting mixed bedrock alluvial anabranching also occurs in a number of other rivers worldwide and appears to be a stable and often long lived river pattern adjusted to a number of factors commonly acting in combination 1 jointed fractured granitoid rock outcrop 2 erosion resistant banks and islands 3 locally variable channel bed gradients 4 variable flow regimes

prolonged history of episodic fluid flow in giant hematite ore bodies evidence from in situ u pb geochronology of hydrothermal xenotime abstract absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the earth s crust yet many ore deposits remain undated because suitable mineral chronometers have not been found the origin of giant hematite ore deposits which are hosted in precambrian banded iron formations bifs remains contentious several models have been formulated based on different sources and timing for the mineralizing fluids supergene metamorphic syn orogenic late orogenic extensional collapse and syn extensional precise geochronology of the ore offers a means of discriminating between these models in this study two u pb chronometers xenotime and monazite have been identified in high grade hematite ore bodies from the mount tom price mine in the hamersley province northwestern australia both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite hematite pseudomorph after magnetite and microplaty hematite indicating that the xenotime and monazite precipitated during mineralization in situ u pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events our results suggest that ore genesis may have commenced as early as 2 15 ga with subsequent hydrothermal remobilization and or mineralization at 2 05 ga 1 84 ga 1 67 ga 1 59 ga 1 54 ga 1 48 ga and 0 85 ga the location of the ore bodies along ancient fault systems and the coincidence of at least some of the u pb phosphate dates with episodes of tectonothermal activity in the adjacent proterozoic capricorn orogen implies that fluids were channelled through major structures in the southern pilbara craton during discrete phases of tectonic compression and extension our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow in contrast to the aforementioned models our results imply that iron ore formation was probably a long lived multi stage process spanning more than one billion years

recognition of the sedimentary architecture of dryland anabranching anastomosing rivers abstract the application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions from stratigraphy are to be reliable from modern observations it appears that anabranching anastomosing rivers can be divided into separate humid and dryland classes each class has dissimilar characteristics to the other necessitating different criteria for their recognition in the rock record the criteria being used by sedimentologists to recognize deposits of anabranching rivers are derived solely from modern examples in humid regions heavily biased to a few specific cases in canada these criteria are dominated by the concept of ribbon sandstone bodies encased in fine grained floodplain sediment with an overall low sand to mud ratio in the succession there has been a general failure to appreciate that dryland anabranching rivers have characteristics significantly different from these and almost certainly leave a sedimentary record quite dissimilar to that previously expected avulsion or obtrusion the gradual formation of an alternative channel are the characteristic dynamic of anabranching rivers forming and maintaining coexisting multiple anabranch channels avulsion or obtrusion require stream banks to have a high resistance to erosion usually achieved by cohesive bank sediments or the reinforcement of banks by vegetative root mats in some drylands riparian vegetation can be sparse and bank resistance may depend in large part on a high clay content here channel fills are mud dominated and floodplains have a uniform fine grained character which is usually structureless because of intense pedoturbation and some bioturbation the evidence from such present day dryland anabranching rivers suggests that their sedimentary record is likely to be one of laterally extensive mud rich beds with weak or enigmatic fluvial indicators elsewhere dryland anabranching systems are sand dominated their banks reinforced with riparian gallery forests sustained by infrequent flow and a shallow water table though the stratigraphy of these has barely been investigated they are clearly different from humid region anabranching systems for their channels and islands are predominantly sandy anabranching rivers with prominent channel sandbodies separated by muddy islands have never been described from contemporary drylands the interpretation of a dryland origin for ancient stratigraphic successions with this form is made therefore without evidence that they can actually exist in the light of this such interpretations should be reevaluated

late holocene channel changes of the middle trent channel response to a thousand year flood record this paper presents recent work on the floodplain sedimentology of the middle trent using data from gravel pits archaeological sites and documentary sources the middle trent has been unusually active during the holocene in comparison with other large lowland rivers in the british isles the holocene floodplain fill is dominated by sands and gravels with abundant structural evidence of changes in channel pattern and channel type a thousand year record of channel change has been reconstructed from palaeochannels and gravel units with radiocarbon dating of brushwood palaeomagnetic dating of fine channel fills dendrochronological dating of timber structures and dating via archaeological typologies the trent also has a reasonably well recorded flood history at least since the 11th century ad a comparison of the flood record and channel change indicates that the same degree of morphological and sedimentary response is not necessarily associated with floods of similar magnitudes i e there is no constant relationship between event magnitude and landform change instead the response seems dependent on the existing state of the channel and medium term trajectory of channel change there is evidence at both the hemington and colwick reaches of a cycle of channel change involving a change in channel typology and dating evidence that this may have migrated downstream the results of this work provide a medium term perspective on channel change which may be more appropriate for large british rivers than short term monitoring for both model validation and planning purposes

iridium anomalies and shocked quartz in a late archean spherule layer from the pilbara craton new evidence for a major asteroid impact at 2 63 ga comment and reply reply we welcome a discussion of the jeerinah impact layer and its likely stratigraphic correlation glikson accepts the interpretation of the jeerinah impact layer as a distal ejecta layer as first proposed by simonson et al 2000 1 but questions our correlation of the jeerinah impact layer with

the dissolution kinetics of major sedimentary carbonate minerals abstract among the most important set of chemical reactions occurring under near earth surface conditions are those involved in the dissolution of sedimentary carbonate minerals these minerals comprise about 20 of phanerozoic sedimentary rocks calcite and to a significantly lesser extent dolomite are the major carbonate minerals in sedimentary rocks in modern sediments aragonite and high magnesian calcites dominate in shallow water environments however calcite is by far the most abundant carbonate mineral in deep sea sediments an understanding of the factors that control their dissolution rates is important for modeling of geochemical cycles and the impact of fossil fuel co 2 on climate diagenesis of sediments and sedimentary rocks it also has practical application for areas such as the behavior of carbonates in petroleum and natural gas reservoirs and the preservation of buildings and monuments constructed from limestone and marble in this paper we summarize important findings from the hundreds of papers constituting the large literature on this topic that has steadily evolved over the last half century our primary focus is the chemical kinetics controlling the rates of reaction between sedimentary carbonate minerals and solutions we will not attempt to address the many applications of these results to such topics as mass transport of carbonate components in the subsurface or the accumulation of calcium carbonate in deep sea sediments such complex topics are clearly worthy of review papers on their own merits calcite has been by far the most studied mineral over a wide range of conditions and solution compositions in recent years there has been a substantial shift in emphasis from measuring changes in solution composition to determine batch reaction rates to the direct observation of processes occurring on mineral surfaces using techniques such as atomic force microscopy afm however there remain major challenges in integrating these two very different approaches a general theory of surface dissolution mechanisms currently lacking although see lasaga and luttge science 291 2001 2400 is required to satisfactorily relate observations of mineral surfaces and the concentration of dissolved components studies of aragonite high magnesian calcites magnesite and dolomite dissolution kinetics are much more limited in number and scope than those for calcite and provide at best a rather rudimentary understanding of how these minerals are likely to behave in natural systems although the influences of a limited number of reaction inhibitors have been studied probably the greatest weakness in application of experimental results to natural systems is understanding the often profound influences of foreign ions and organic matter on the near equilibrium dissolution kinetics of carbonate minerals

carbon dioxide reaction processes in a model brine aquifer at 200 c and 200 bars implications for geologic sequestration of carbon the reactive behavior of supercritical co2 under conditions relevant to geologic storage and sequestration of c is largely unknown experiments were conducted in a flexible cell hydrothermal apparatus to determine the extent of fluid rock reactions in addition to carbonate mineral precipitation that may occur in a brine aquifer aquitard system that simulates a saline aquifer storage scenario the system was held at 200 c and 200 bars for 59 days 1413 h to approach steady state then injected with co2 and allowed to react for another 80 days 1924 h in addition to magnesite precipitation silicate minerals quartz plagioclase microcline and biotite in the aquifer and the aquitard display textures etch pits mineralization indicating significant reaction changes in elemental abundances in the brine following addition of co2 include ph decrease and enrichment in cl partly due to supercritical co2 desiccation of the brine geologic sequestration systems have potential for geochemical reactions that extend beyond simple aqueous dissolution of co2 and precipitation of carbonate these reactions may produce geochemical and geotechnical consequences for sequestration and provide important characteristics for monitoring and evaluation of stored co2 a n understanding of multi phase equilibrium relationships between supercritical co2 and aquifer brine systems also raises new questions for a variety of geologic systems multi phase fluid equilibria may for example account for the large amounts and heterogeneous distributions of calcite cement in a wide variety of geologic systems particularly in sedimentary basin sandstones published by elsevier science ltd

changes in a large regulated tropical river the parana river downstream from the porto primavera dam brazil abstract dams disturb in the fluvial dynamics by changing the natural cycle of the rivers interfering with the transport deposition processes and affecting river ecology the brazilian part of the parana basin has the largest hydrometrical power potential of the country with more than 150 hydroelectric power dams that transformed the parana river and its tributaries in a succession of lakes this research aims to analyze the changes in certain fluvial variables in the last natural downstream reach of the upper parana river after closure of the porto primavera dam data acquired before and after dam construction showed changes in water discharge bank erosion flood pulse bed load grain size discharge concentration of suspended load and bedform morphology and size those alterations generated changes in river ecology especially on fish reproduction benthic community and rotifers variety and density a review of the obscure regional literature generally in portuguese is also presented

nitrogen isotope evidence for stepwise oxygenation of the ocean during the great oxidation event abstract the earth s oxygenation represents one of the most important environmental drivers of life s evolution with the first rise known as the great oxidation event goe corresponding to unpreceded accumulation of atmospheric o2 changes in the flux of marine nutrients and possibly global glaciations however the detailed evolution of the goe is still debated as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation here we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from two drill cores t2 and t3 in the early paleoproterozoic turee creek group western australia to track the redox evolution of marine conditions during the goe t2 core samples of the kungarra formation which consists of clastic sedimentary rocks overlaid by the glaciogenic meteorite bore member were deposited 2 31 ga ago t3 core intercepts from bottom to top quartzite of the koolbye formation and shales and stromatolitic carbonates of the kazput formation which were deposited around 2 25 ga samples from t2 show minor variations of 13corg avg 34 5 1 7 n 30 with no significant difference between siliciclastic and glaciogenic sedimentary rocks in contrast t3 samples display an increase in 13corg from 32 0 to 24 8 n 54 from shales to carbonates in both t2 and t3 cores 13corg values are inversely correlated with al2o3 suggesting a strong petrological control on 13corg values inferred here as resulting from variable contributions of detrital organic matter bulk n contents are low from 13 5 to 56 7 ppm and 15 7 to 53 4 ppm in t2 and t3 samples respectively the 15n values show a bimodal distribution with one mode at 2 6 in t2 and another at 8 8 in t3 independent from lithological variations this 15n values shift between t2 and t3 is interpreted as reflecting a change from dominating n2 fixers to no3 assimilating organisms this implies an increase of no3 availability and thus of o2 concentration during the time interval separating the deposition of t2 and t3 sediments dissolved no3 and o2 concentrations of the turee creek marine basin are estimated from two models using n isotope data the dissolved no3 concentration has an upper limit ranging from 1 91 to 3 04 m about one order of magnitude below the average value of modern oceans the lower limit for dissolved oxygen concentration ranges from 1 8 to 4 4 m which is two orders of magnitude lower than modern oceans together with previous studies the present data place quantitative constraints on the redox changes associated with the great oxidation event and illustrate a stepwise increase of no3 bioavailability between 2 31 to 2 25 ga in relation with increasing o2 level

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

experimental evaluation of mixed fluid reactions between supercritical carbon dioxide and nacl brine relevance to the integrity of a geologic carbon repository abstract the reactive behavior of a mixed fluid supercritical co2 and brine under physical chemical conditions relevant to geologic storage and sequestration in a carbon repository is largely unknown experiments were conducted in a flexible cell hydrothermal apparatus to evaluate fluid rock aquifer plus aquitard reactions that may adversely impact the integrity of the repository a 5 5 molal nacl brine rock system was held at 200 c and 200 bars 20 mpa for 32 days 772 h to approach steady state then injected with co2 and allowed to react for an additional 45 days 1079 h in a separate experiment at 200 c and 200 bars the system was allowed to react for 77 days 1845 h without injection of co2 corroded magnesite and euhedral siderite crystallized in a paragenetic sequence after co2 injection nucleation and growth of siderite on shale suggests the aquitard is a reactive component in the system changes in elemental abundances in the brine following addition of co2 include ph decrease and depletion of sodium due to accelerated growth of analcime a ph increase follows pressure and temperature decrease and loss of saturated co2 from acidic brine silica concentrations and dissolution rates are enhanced and silica precipitation inhibited in the acidic brine geochemical reactions in a carbon repository extend beyond ph decrease and carbonate mineral precipitation rock dominated reaction systems yield to acid dominated and related reactions controlled by mixed fluid equilibria i e a fluid dominated system escape of co2 or migration of brine from the repository into overlying aquifers may cause silica super saturation and increased alkalinity due to mixed fluid phase equilibria these geochemical changes could be monitored in aquifers as indicators of repository integrity return of silica super saturated brine to a rock dominated reaction system buffered to neutral ph conditions may enhance precipitation of quartz chalcedony or amorphous silica in addition to the potential effects beneficial or deleterious that silica super saturation and precipitation may hold for repository performance an understanding of the effects of multi phase equilibrium relationships between supercritical co2 and dissolved silica in aquifer brine systems also raises new questions for a variety of geologic systems multi phase fluid equilibria may for example account for quartz cements in some sedimentary basin sandstones and quartz vein mineralization in some ore districts

u pb zircon age constraints on the hamersley spherule beds evidence for a single 2 63 ga jeerinah carawine impact ejecta layer seven spherule beds interpreted to be distal impact ejecta were recently found in late neoarchean and early paleoproterozoic successions of northwestern australia and south africa inadequate age constraints have led to uncertainty about the number of impacts they represent and their correlation here we report two new isotopic ages from the pilbara region of western australia zircons from a tuff in the carawine dolomite 30 m below the carawine spherule layer give a 2 0 7 pb 2 0 6 pb age of 2630 6 ma suggesting that the carawine and ca 2 63 ga jeerinah spherule layers formed during a single impact significantly increasing the known area of the strewn field to 17 000 km 2 this date also requires that the carawine dolomite was probably time equivalent to the uppermost jeerinah formation and lowermost marra mamba iron formation negating previous lithostratigraphic correlations and suggesting that the deposition of banded iron formation was synchronous with carbonate precipitation in a shallow water platform a tuff in the mount mcrae shale yielded a zircon 2 0 7 pb 2 0 6 pb age of 2504 5 ma providing new age constraints on the overlying wittenoom and dales gorge spherule layers the new dates show that the hamersley spherule layers represent three impacts that occurred ca 2 63 ga 2 56 2 50 ga and 2 50 2 48 ga the three hamersley spherule beds broadly correlate with three beds in south africa highlighting the possible global extent of the impact ejecta fallout the layers potentially provide three geologically instantaneous markers spanning 150 m y across the archean proterozoic boundary possibly underpinning a chronostratigraphic framework across two continents and constraining late neoarchean paleogeography

the role of vegetation in the formation of anabranching channels in an ephemeral river northern plains arid central australia as the distribution and abundance of vegetation in drylands is often controlled by the greater availability of water along river channels riparian vegetation has the potential to influence significantly dryland river form process and behaviour this paper demonstrates how a small indigenous shrub the inland teatree melaleuca glomerata influences the formation and maintenance of anabranching channels in a reach of the ephemeral marshall river northern plains arid central australia here the marshall is characterized by ridge form anabranching where water and sediment are routed through subparallel multiple channels of variable size which occur within a typically straight channel train channels are separated by channel train ridges narrow flow aligned vegetated features or by wider islands by providing a substantial element of boundary roughness dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velocities flow depths and sediment transport resulting in flow diversion bank and floodplain erosion and especially sediment deposition ridges and islands represent a continuum of forms and their formation and development can be divided into a three stage sequence involving teatree growth and alluvial sedimentation 1 teatrees colonize a flat sandy channel bed initiating the formation of ridges by lee side accretion individual ridges grow laterally vertically and longitudinally and maintain a geometrically similar streamlined lemniscate form that presents minimum drag 2 individual ridges grow in size and interact with neighbouring ridges causing the lemniscate forms to become distorted ridges in the lee of other ridges tend to be protected from the erosive effects of floods and survive whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges tend to be removed 3 ridges lengthen and coalesce with downstream ridges eventually subdividing the channel train into well defined anabranches this sequence turns a channel initially obstructed with dense and chaotic stands of teatrees into a well organized system of ridge form anabranches in the moderate to low gradient marshall river which is colonized by an abundance of within channel vegetation and subject to declining downstream discharges this helps to minimize flow resistance thereby maintaining an efficient water and sediment flux copyright 2000 john wiley sons ltd

regional controls on geomorphology hydrology and ecosystem integrity in the orinoco delta venezuela interacting river discharge tidal oscillation and tropical rainfall across the 22 000 km2 orinoco delta plain support diverse fresh and brackish water ecosystems to develop environmental baseline information for this largely unpopulated region we evaluate major coastal plain shallow marine and river systems of northeastern south america which serves to identify principal sources and controls of water and sediment flow into through and out of the orinoco delta the regional analysis includes a summary of the geology hydrodynamics sediment dynamics and geomorphic characteristics of the orinoco drainage basin river and delta system because the amazon river is a major source of sediment deposited along the orinoco coast we summarize amazon water and sediment input to the northeastern south american littoral zone we investigate sediment dynamics and geomorphology of the guiana coast where marine processes and holocene history are similar to the orinoco coast major factors controlling orinoco delta water and sediment dynamics include the pronounced annual flood discharge the uneven distribution of water and sediment discharge across the delta plain discharge of large volumes of water with low sediment concentrations through the rio grande and araguao distributaries water and sediment dynamics associated with the guayana littoral current along the northeastern south american coast inflow of large volumes of amazon sediment to the orinoco coast development of a fresh water plume seaward of boca grande disruption of the guayana current by trinidad boca de serpientes and gulf of paria and the constriction at boca de serpientes

2 7 ga crustal growth in the north china craton evidence from zircon u pb ages and hf isotopes of the sushui complex in the zhongtiao terrane abstractthe major precambrian crustal growth in the north china craton ncc has been thought to have occurred at 2 5 ga here we present secondary ion mass spectroscopy sims and laser ablation inductively coupled mass spectrometry la icp ms zircon u pb ages and hf isotope compositions from dioritic and tonalite trondhjemite granodiorite ttg gneisses of the neoarchean paleoproterozoic metamorphic terrane in the central ncc zircons from the trondhjemitic samples yield emplacement ages at and ma those from the dioritic gneiss yield an age of ma the ttg gneisses are characterized by a na rich and k poor nature strongly fractionated rare earth element patterns high sr 418 318 and sr y 78 49 low y 5 4 6 4 and ybn 3 0 3 6 and low mgo 0 89 1 10 mg 47 42 cr 39 20 and ni 11 8 nb ta 16 12 which suggests that magmas were derived from the partial melting of a flatly subducted oceanic slab the dioritic gneiss shows higher mgo 3 17 mg 50 cr 44

variations in neoarchean microbialite morphologies clues to controls on microbialite morphologies through time the neoarchean carawine formation hamersley group western australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments these distinctive structures are distributed in five shallow subtidal and two deeper water facies in the oakover area of the carawine formation the shallow subtidal facies are composed of biohermal and bedded stromatolites centimetre scale ridge shaped microbialites and wavy laminated microbialites the deeper water facies are composed of fenestrate microbialites planar laminated dolostone and dolostone with rolled up microbial laminae microbialites in the carawine formation lie within a continuum of archean to proterozoic microbial facies some shallow water microbial facies in the carawine formation are similar to proterozoic facies such as large bioherms internally composed of a variety of stromatolite morphologies in contrast fenestrate microbialites grew in quiet subtidal environments and are common in archean rocks but have not been documented in similar proterozoic environments the similarity of shallow water facies across the archean proterozoic transition before and after the oxidation of the atmosphere and surface oceans indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes rather stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment in contrast the absence of fenestrate microbialites from proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans

anastomosing river deposits sedimentation rates and basin subsidence magdalena river northwestern colombia south america situated in a tectonically active foreland basin the magdalena river consists of vertically accreting levee confined channels and adjacent extensive wetlands which are interpreted as an anastomosing river sedimentary system equivalent rates of basin filling and subsidence average 3 8 mm yr 1 based on 18 14c dates from five bore holes drilled to depths of 55 m and sediment transport budgets from 35 years of measurement located in a savanna tropical climate anastomosing river deposits of the magdalena are remarkably similar to the anastomosing deposits of the upper columbia river in a temperate cold climate in western canada suggesting that climate is not a controlling factor of anastomosis the geometry of anastomosing channel fills in the magdalena consists of stratigraphically non uniform low sinuous narrow stringers of sand up to 30 m thick by 600 m wide a width depth ratio of 20 thin 1 2 m off channel crevasse splay sand sheets extend laterally up to 10 km distance when buried both sand deposits become encased by lacustrine or marsh mud to form stratigraphic traps while there are few modern anastomosing river systems as compared to braiding and meandering there may be a disproportionately large number of ancient anastomosed fluvial rock sequences due to the rapid rate of vertical accretion such a different depositional style and geometry of sand bodies have considerable significance in the interpretation of some ancient fluvial rock sequences because it provides an alternative to the meandered and braided river deposition models

geochemical evidence for an impact origin for a late archean spherule layer transvaal supergroup south africa a late archean layer rich in sand sized spherules of former silicate melt in the monteville formation transvaal supergroup south africa has ir concentrations as high as 6 4 ppb and is clearly enriched in ir relative to associated tuffs carbonates and shales the monteville spherule layer is also enriched in other siderophile elements including the platinum group elements pges the pges in the spherule layer produce a flat meteorite like pattern when they are normalized to chondritic abundances the abundances of ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the hamersley basin of western australia that the mineral compositions textures and sedimentary structures of the spherule layers in the transvaal supergroup and hamersley basin are also very similar suggests that they were all formed by the same processes we think that the best way to explain the high ir concentrations and other characteristics of the monteville spherule layer is that it represents distal impact ejecta there are however significant differences between the monteville spherule layer and early archean spherule layers in the barberton greenstone belt including much higher average and maximum concentrations of ir in the latter the data presented here clearly show that each precambrian spherule layer is unique and needs to be characterized individually as is true for the impact spherule layers of the phanerozoic

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

paraburdoo spherule layer hamersley basin western australia distal ejecta from a fourth large impact near the archean proterozoic boundary we report the discovery of the paraburdoo spherule layer which consists entirely of replaced impact produced melt spherules it is distal ejecta from a large impact close to the archean proterozoic boundary in the hamersley basin of western australia the spherule layer occurs in the paraburdoo member of the wittenoom formation and was deposited ca 2 57 ga date via u pb age interpolation in a deep shelf environment the layer consists of microkrystites rich in plagioclase and ferromagnesian crystals replaced by k feldspar and a phlogopite like sheet silicate respectively the skeletal textures indicate rapid cooling of a melt with mafic to ultramafic composition and suggest oceanic target rocks the paraburdoo spherule layer is 2 cm thick normally graded and was probably deposited by suspension settling as there is no evidence of reworking it is strikingly similar to the reivilo spherule layer in the griqualand west basin south africa appears to provide a third impact related high resolution stratigraphic correlation between these two basins and points to a high frequency of large impacts around the archean proterozoic boundary

genesis and sedimentary record of blind channel and islands of the anabranching river an evolution model abstract blind channel bc is a fluvial feature formed by attachment of a lateral sand bar to an island or riverbank it consists of a 10 to 20 m wide and hundreds to thousands meters long channel parallel to the island or bank closed at its upstream end by accretion to the island it is an important feature in anabranching rivers that plays an important role in both the island formation and river ecology this paper discusses the formation processes functioning evolution and the sedimentary record of a blind channel related landforms and its context on island development in the upper parana river the evolution of this morphologic feature involves 1 formation of a lateral or attachment bar beside an island with the development of a channel in between 2 vertical accretion of mud deposits during the flood and vegetal development on the bar 3 the upstream channel closure that generates the blind channel and 4 annexation of the blind channel to the island a blind channel is semilotic to lentic that is not totally integrated to the dynamics of the main active channel and that acts as a nursery for fingerlings and macrophytes the sedimentary facies succession of bcs are relatively simple and characterized by cross stratified sand covered by organic muddy sediments based on facies analysis of 12 cores we identified a succession of environments that contribute to the formation of islands channel bar blind channel pond and swamp blind channel formation and its related bar island attachment are relevant processes associated with the growing of large island evolution in some anabranching rivers

mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

morphology hydrology and evolution of the anastomosing ovens and king rivers victoria australia anastomosing channels form the characteristic stream pattern of the riverine plain of southeastern australia investigations of the ovens and king rivers in northeastern victoria show that the individual channels and reaches of channels vary markedly in channel morphology and age young channels have large bankfull flows low sinuosity large meander wavelength steep gradients and are unstable they are incised channels that are rapidly eroding their bed and banks old channels however have small bankfull flows high sinuosity small meander wavelength gentler gradients and are relatively stable the multiple channels develop by avulsion as individual channels become older and more sinuous they also become more hydraulically inefficient thus increasing proportions of flood discharge are displaced overbank this overbank flow concentrates in relatively straight flood plain depressions and it erodes a new channel which develops by both up and down valley incision down valley incision is initiated at outflow points and up valley incision is initiated where the overbank flow rejoins the main channel with time both channel segments join to form a new anabranch of the anastomosing channel system large anastomosing channel networks have been identified in ancient rocks by stratigraphers and sedimentologists on the basis of studies of the anastomosing ovens and king rivers it is very likely that these channel networks are composed of channels of different ages hence even at one stratigraphic level not all of the anabranches functioned simultaneously nevertheless as a result of anabranch incision which intersects older channels at depth a three dimensional network of paleochannels can form that permits fluid flow throughout the network

upb geochronologic constraints on deposition of the campbellrand subgroup transvaal supergroup south africa abstract a volcanic ash bed that crops out 40 50 m below the contact between the campbellrand subgroup and the kuruman iron formation transvaal supergroup south africa yields an u pb zircon age of 2521 3 ma this new age demonstrates that the campbellrand malmani carbonate platform was deposited during late archean time and is the oldest extensively preserved carbonate platform known in addition the age constrains deposition of the kuruman griquatown and penge iron formations to latest archean to earliest paleoproterozoic time although a previously reported pb pb age of 2557 49 ma for dolomites at the base of the campbellrand subgroup is broadly consistent with this u pb zircon age new pb isotopic data for carbonate cements and massive sulfides lie along the same pb pb array as the dolomites this relationship implies that the isochron is the result of post depositional resetting of the u pb system in dolomite by fluids responsible for sulfide mineralization and that the age which corresponds to its slope has no geological significance

evaluation of the impact of co2 co contaminant gas aqueous fluid and reservoir rock interactions on the geologic sequestration of co2 lowering the costs of front end processes e g separation in the geologic sequestration of co2 can dramatically lower the overall costs one possible approach is to sequester less pure co2 waste streams that are less expensive or require less energy to separate from flue gas or a coal gasification process etc the objective of this research is to evaluate the impacts of co2 itself as well as an impure co2 waste stream on geologic sequestration using reaction progress models reactive transport simulators and analogous reactive transport experiments run in a plug flow reactor specifically we are investigating the potential for co injecting the sox nox and h2s present in coal fired waste streams along with the co2 we present here the results of reactive transport simulations made to investigate the long term impact of dissolved co2 h2s and so2 on carbon sequestration in the frio fm tx the results suggest that addition of relatively large amounts of h2s to a co2 injection should not adversely impact injectivity or sequestration compared to the injection of co2 alone the co injection of so2 however could produce significantly different results if conditions are such that so2 can oxidize to sulfate in the long term the simulations suggest that carbonate mineral formation can sequester a significant fraction of the injected carbon through the formation of calcite magnesite and dawsonite

screening and ranking of sedimentary basins for sequestration of co2 in geological media in response to climate change sedimentary basins are suitable to different degrees for co2 geological sequestration as a result of various intrinsic and extrinsic characteristics of which the geothermal regime is one of the most important warm basins are less favorable for co2 sequestration than cold basins because of reduced capacity in terms of co2 mass and because of higher co2 buoyancy which drives the upward co2 migration a set of 15 criteria with several classes each has been developed for the assessment and ranking of sedimentary basins in terms of their suitability for co2 sequestration using a parametric normalization procedure a basin s individual scores are summed to a total score using weights that express the relative importance of different criteria the total score is ranked to determine the most suitable basin or region thereof for the geological sequestration of co2 the method is extremely flexible in that it allows changes in the functions that express the importance of various classes for any given criterion and in the weights that express the relative importance of various criteria examples of application are given for canada s case and for the alberta basin in canada

neotectonics and channel evolution of the lower ivinhema river a right bank tributary of the upper parana river brazil abstract the ivinhema river an important right bank tributary of the upper parana river cuts a series of terraces geomorphic surfaces floodplains and associate features through terrace mapping sediment composition absolute 14 c tl dating and morphostructural analysis this paper presents the composed longitudinal profile and the structural analysis of the lower course of the ivinhema river the tectonic model of the study area constitutes a sequence of nw tilting fault blocks formed from antithetic faults that control channel sinuosity longitudinal profile floodplain distribution and valley asymmetry the area began to be structured during the paleocene with formation of the oligomict conglomerate probably under an arid climate another tectonic reactivation in the mio pliocene boundary defined the divides of the upper parana and paraguay river basins with formation of the polymict conglomerate an active depositional phase taken place at the tertiary quaternary limit produced the massive reddish clayey sand sediments very common in the area during the late pelistocene 42 5 1 7 and 31 1 1 7 ka bp the parana and ivinhema rivers constructed a sedimentary sequence of gravelly sand deposits in a braided channel environment at the beginning of the holocene a new tectonic pulse lifted up a group of blocks parallel to the parana river valley developed a series of antithetic faults tectonic intensity associated with the pluvial stage hypsithermal phase between 8 and 6 ka changed the channel pattern of the river from braided to meandering channel evolution during the late pleistocene and holocene was also influenced by climatic changes

geochemistry of 2 63 2 49 ga impact spherule layers and implications for stratigraphic correlations and impact processes thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long distance stratigraphic correlation of precambrian successions seven formations in western australia and south africa clustered around the archean proterozoic boundary each contain a single spherule layer all of which were deposited between ca 2 49 and 2 63 ga analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component etc in all six layers based on pge ratios and or cr isotopic composition the amount of etc varies from marginally detectable to clear and obvious it generally amounts to ca a few percent both pge ratios and cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition in contrast all spherule layers older than ca 3 0 ga that have been analyzed were produced by carbonaceous chondritic impactors normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in pge ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation the geochemical data also provide a means to test proposed intra and intercontinental stratigraphic correlations cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2 63 ga in contrast clear geochemical differences between the middle layers both deposited ca 2 54 ga suggest they were not produced by the same impact event the youngest banded iron formation hosted layers on both continents have also been correlated but no geochemical data are available on the african layer to test this at present

depositional age of the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa and implications for low level re os geochronology and paleoproterozoic global correlations abstract we present new re os isotope data from the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa to further evaluate low level re os geochronology of organic rich sedimentary rocks and improve global correlations of paleoproterozoic sedimentary rocks the klipputs member contains re 0 3 0 8 ppb total os 34 177 ppt and common os 192 os 10 66 ppt abundances that overlap upper crustal abundances of 0 2 2 ppb 30 50 ppt and 12 21 ppt respectively five samples yield a model 1 re os date of 2479 20 ma 22 ma if the 187 re decay constant uncertainty is included 2 mean square of weighted deviates mswd 0 01 using the cr vi h 2 so 4 digestion protocol dissolution of the same sample powders using inverse aqua regia yields a model 3 re os date of 2862 190 ma 2 mswd 148 comparison of re os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital os whereas the cr vi h 2 so 4 medium did not the re os date of 2479 22 ma for the klipputs member is consistent with published geochronological ages from underlying and overlying stratigraphic units suggesting the cr vi h 2 so 4 protocol yields accurate re os depositional ages from ors containing very low levels of hydrogenous re and hydrogenous radiogenic os hence there is potential for widespread application of low level re os ors geochronology including sedimentary successions deposited before the great oxidation event the initial 187 os 188 os 0 12 0 01 from the klipputs member cr vi h 2 so 4 regression indicates that dissolved os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and or cosmic dust rather than oxidative weathering of the radiogenic upper crust in light of the re os depositional age for the klipputs member we propose that the stratigraphic units in the upper hamersley group above the brockman iron formation western australia are younger than the ghaap group diamictite from the doradale formation in the ghaap group is constrained to be older than 2479 22 ma and thus may not be correlative with glacial deposits assigned to the oldest paleoproterozoic glaciation

effect of dissolved h2so4 on the interaction between co2 rich brine solutions and limestone sandstone and marl abstract the effect of pure and impure carbon dioxide co 2 on geological storage is uncertain oxidation of impurities such as sulfur dioxide so 2 and hydrogen sulfide h 2 s which may be catalyzed by co injected oxygen or nitrogen oxides leads to the formation of sulfuric acid h 2 so 4 and a decrease in ph of the formation water we investigated the effect of 0 005 mol l 1 h 2 so 4 corresponding to a worst case of 0 4 so 2 in the flue gas anticipating total conversion of so 2 into h 2 so 4 on the reactivity of the reservoir limestone and sandstone and cap marl rocks of hontomin spain at p pco 2 10 bar and 60 c during 24 days using flow through column experiments aqueous element concentrations were measured from fluid extracts obtained periodically throughout the experiments to infer fluid rock reactions over time results were modeled with the crunchflow reactive transport code the added h 2 so 4 lowered the ph of the injected brine by 1 5 ph units with respect to the ph of 3 6 of the h 2 so 4 free brine in both h 2 so 4 free and h 2 so 4 rich brine calcite dissolution fostered gypsum precipitation a comparison between the reactivity of the rocks reacted in h 2 so 4 free and h 2 so 4 rich brine showed that calcite dissolution and gypsum precipitation rates were increased by 27 48 and 25 75 respectively in h 2 so 4 rich brine overall rock porosity increments in h 2 so 4 rich brine were 2 9 3 6 3 7 4 8 and 2 1 2 7 for sandstone limestone and marl respectively porosities in h 2 so 4 rich brine were 6 23 and 250 higher respectively than under pure co 2 modeled porosity increments in the acid inlet zone in h 2 so 4 rich brine for sandstone limestone and marl were 28 29 44 45 and 24 28 respectively corresponding to an increase of 1 250 and 25 respectively relative to h 2 so 4 free brine gypsum precipitation was consistently higher in marl than in limestone and sandstone indicating kinetically favorable conditions for gypsum precipitation within the cap rock our results provide relevant data for long term storage simulations of impure co 2 injection

crustal growth and reworking of the eastern north china craton constraints from the age and geochemistry of the neoarchean taishan ttg gneisses abstract the taishan complex is located in the central part of the eastern north china craton ncc and preserves extensive neoarchean ttg gneisses and potassic granitoids their formation age geochemical features and petrogenesis can provide important insights into understanding the crustal growth and reworking events involving in the evolution of the craton previous studies were mainly focused on geochronology of these neoarchean rock assemblages but lacked study on their petrogenesis and related tectonic regime in this study we carried out lithological geochronological and geochemical analyses for the representative ttg gneisses of the taishan complex u pb zircon age dating results reveal that the taishan ttg gneisses were mainly emplaced at 2718 2548 ma lu hf isotopic data show uniform positive zircon ehf t values 2 2 to 6 6 with hf model ages of 2949 2665 ma indicating that the ttg gneisses were mostly derived from juvenile crustal materials extracted from the depleted mantle during the mesoarchean to neoarchean the 18o values of zircons vary from 4 6 to 6 7 indicating that the source region was not significantly affected by the supracrustal materials geochemical data show that these ttg gneisses have high na2o 3 81 4 83 wt and ratio of zr sm 24 80 but show low mg 40 56 content of sr 292 508 ppm and ratios of sr y 35 6 66 6 and nb ta 10 1 15 9 these ttg gneisses were probably derived from partial melting of juvenile crustal materials under a relatively low pressure environment which were equilibrated with a certain amount of amphibole and plagioclase and minor garnet in the residue it indicates that a large scale magmatic underplating could be a most possible tectonic regime operating in this region during the neoarchean our new results combined with the available data from the adjacent areas reveal a significant crustal growth event in the eastern ncc at ca 2 94 2 70 ga which was followed by the pervasive crustal reworking event at ca 2 70 2 55 ga and regional metamorphism at ca 2 55 2 45 ga

facies and architectural analysis of channel sandy macroforms in the upper parana river abstract the upper parana river in the area under analysis has a multichanneled pattern with many islands and channel bars channel bars are sandy macroforms classified as central and lateral bars this paper presents a sedimentological analysis of these bars according to their textures facies and facies architecture central bars are unstable deposits situated in the highest flow energy sites of the main channel with intensive processes of deposition and erosion they are composed of cross stratified medium to coarse sand sp and st facies lateral bars are very stable macroforms situated in the lower flow energy areas with no or very few erosional processes where flaser and climbing rippled fine sand to mud sr so fl and fm facies are the most common sediments

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

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

the role of mineral heterogeneity on the hydrogeochemical response of two fractured reservoir rocks in contact with dissolved co2 abstract in this study we compare the hydrogeochemical response of two fractured reservoir rocks limestone composed of 100 wt calcite and clast poor sandstone composed of 66 wt calcite 28 wt quartz and 6 wt microcline in contact with co 2 rich sulfate solutions flow through percolation experiments were performed using artificially fractured limestone and sandstone cores and injecting a co 2 rich sulfate solution under a constant volumetric flow rate from 0 2 to 60 ml h at p 150 bar and t 60 c measurements of the pressure difference between the inlet and the outlet of the samples and of the aqueous chemistry enabled the determination of fracture permeability changes and net reaction rates additionally x ray computed microtomography xcmt was used to characterize and localize changes in the fracture volume induced by dissolution and precipitation reactions in all reacted cores an increase in fracture permeability and in fracture volume was always produced even when gypsum precipitation happened the presence of inert silicate grains in sandstone samples favored the occurrence of largely distributed dissolution structures in contrast to localized dissolution in limestone samples this phenomenon promoted greater dissolution and smaller precipitation in sandstone than in limestone experiments as a result in clast poor sandstone reservoirs the larger increase in fracture volume as well as the more extended distribution of the created volume would favor the co 2 storage capacity the different distribution of created volume between the limestone and sandstone experiments led to a different variation in fracture permeability the progressive stepped permeability increase for sandstone would be preferred to the sharp permeability increase for limestone to minimize risks related to co 2 injection favor capillary trapping and reduce energetic storage costs 2d reactive transport simulations that reproduce the variation in aqueous chemistry and the fracture geometry dissolution pattern were performed using crunchflow given that calcite dissolution reaction at ph

avulsion mechanisms on the okavango fan botswana the control of a fluvial system by vegetation a study of the avulsion of a major distributory channel on the alluvial fan 22 000 km2 in area of the okavango river in northern botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp the channels are vegetatively confined a primary channel defined here as a channel which receives water and sediment directly from the fan apex aggrades vertically as a result of bedload deposition the rate of aggradation increases downchannel and may exceed 5 cm yr 1 in the distal reaches rapid aggradation is associated with a decline in flow velocity this initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris further reducing flow rate and causing the channel water surface to become elevated thereby increasing rate of water loss from the channel accelerating blockage and aggradation the channel ultimately fails enhanced water loss from the channel promotes the growth of flanking swamp vegetation which confines the failing channel increased flow through the swamp erodes pre existing hippopotamus trails producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel the region of failure of the primary channel migrates upstream accompanied by headward propagation of the secondary channel system the swamp distal to the failed primary channel dessicates and is destroyed by peat fires secondary channels are stable and not prone to blockage comparison with avulsions described in other river systems indicates that the influence of plants in the okavango river system is exceptionally strong

rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2 5 billion years ago the history of the growth of continental crust is uncertain and several different models that involve a gradual decelerating or stepwise process have been proposed1 4 even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea subaerial landmasses with estimates ranging from about one billion to three billion years ago5 7 the area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans 8 and therefore connects earth s crustal evolution to surface environmental conditions9 11 here we use the triple oxygen isotope composition of shales from all continents spanning 3 7 billion years to provide constraints on the emergence of continents over time our measurements show a stepwise total decrease of 0 08 per mille in the average triple oxygen isotope value of shales across the archaean proterozoic boundary we suggest that our data are best explained by a shift in the nature of water rock interactions from near coastal in the archaean era to predominantly continental in the proterozoic accompanied by a decrease in average surface temperatures we propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near modern average elevation and aerial extent roughly 2 5 billion years ago the use of triple oxygen isotope data from continental shales spanning the past 3 7 billion years suggests that continental crust with near modern average elevation and extent emerged about 2 5 billion years ago

the upper parana river brazil geomorphology sedimentology and paleoclimatology this study involves the geomorphological sedimentological and paleoclimatological aspects of the upper parana river at porto rico state of parana brazil the alluvial plain is composed of a braided system main channel with island and sandy bars and an anastomosing system involving secondary channels tributaries and a complex of swamps pools and natural levees river discharge in its upper course varies from 8400 to 13 000 m3 s minimum of 2550 m3 s and maximum of 33 740 m3 s solid discharge reaches 27 106 tons year for suspended sediments and 3 106 tons year for load sediments through echobathymetric survey bed forms were grouped in ripples megaripples dunes and sand waves a detailed study of the sedimentology heavy minerals and facies architecture was applied to the two major geomorphic provinces and their subenvironments channel province with sand bar and bed form deposits and overbank province with natural levee flood basin crevasse and island deposits palynological preliminary studies show that during the late pleistocene the area was dominated by grassland and savannas under drier climatic conditions since the beginning of the holocene there has been a generalized transition to a humid phase and the present broadleaf forest occupies the area some hypotheses can be formulated for the evolution of the area 1 the first order architectural element channel ch was generated during the pleistocene associated with climatic changes and tectonism the alluvial valley was filled up by alluvial colluvial deposits at the end of the pleistocene channel deposits present 40 ka bp 2 tectonism and climatic changes atlantic climatic optimum generated a new valley bottom 5 8 m below the former level and a wide meandering plain was built up 4000 4500 bp 3 a recent fluvial system created a new terrace 3 m above the normal water level

extending the paleogeographic range and our understanding of the neoarchean monteville impact spherule layer transvaal supergroup south africa