the paleoproterozoic snowball earth a climate disaster triggered by the evolution of oxygenic photosynthesis although biomarker trace element and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2 8 giga annum before present ga and perhaps as early as 3 7 ga a skeptical examination raises considerable doubt about the presence of oxygen producers at these times geological features suggestive of oxygen such as red beds lateritic paleosols and the return of sedimentary sulfate deposits after a 900 million year hiatus occur shortly before the 2 3 2 2 ga makganyene snowball earth global glaciation the massive deposition of mn which has a high redox potential practically requires the presence of environmental oxygen after the snowball new age constraints from the transvaal supergroup of south africa suggest that all three glaciations in the huronian supergroup of canada predate the snowball event a simple cyanobacterial growth model incorporating the range of c fe and p fluxes expected during a partial glaciation in an anoxic world with high fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on timescales as short as 1 million years as the geological evidence requiring oxygen does not appear during the pongola glaciation at 2 9 ga or during the huronian glaciations we argue that oxygenic cyanobacteria evolved and radiated shortly before the makganyene snowball

origin of 3 45 ga coniform stromatolites in warrawoona group western australia a new occurrence of conical and branched pseudocolumnar stromatolites in archean dolostones in the pilbara region australia contributes significant new morphologic information on such structures these remains are interpreted as probably representing in part microbially mediated accretionary growth surfaces in an archean hypersaline depositional basin the structures comprise laterally linked pseudocolumns of centimeter width and decimeter height with first order conical laminae of as much as 15 cm of synoptic relief and apical angles of 30 80 the conical laminae are modified by a second order centimeter scale low amplitude primary corrugate lamination with crests and troughs occasionally stacked to form satellitic obliquely directed pseudocolumns bedding surfaces exhibit a preferred direction of elongation of the cones an orientation that is orthogonal and unrelated to the trend of younger folding the microstructure is secondary the stromatolites are better preserved than those previously known from chert in the warrawoona succession the remains exhibit certain distinct morphologic attributes corresponding to those in younger stromatolites such as displayed by thyssagetes and jacutophyton whose biogenicity is generally accepted although difficult to demonstrate conclusively the conical warrawoona forms may represent the oldest known precursor of these taxa

giant ooids in a neoproterozoic carbonate shelf simla group lesser himalaya india an analogue related to neoproterozoic glacial deposits abstract this study is the first comprehensive documentation of giant ooids and related facies associations in the kunihar formation proterozoic simla group lesser himalaya himachal pradesh analysing the understanding of processes and controls on development of giant ooids based on field observations supplemented by outcrop based facies analysis petrography and delineation of environmental variations four facies associations have been delineated i peritidal siliciclastic carbonate fa1 ii shelf lagoon fa2 iii reef complex fa3 iv fore reef slope fa4 deposition of giant ooids and associated facies associations of kunihar formation occurred in a carbonate rimmed shelf with high tidal influence size of giant ooids from kunihar formation is the largest as compared to giant ooids from other geological formations kunihar giant ooids developed when normal ooids were washed from ooid shoals intertidal into slightly deeper regions shallow subtidal resulting in the increased dimension of ooids in suspension due to higher hydrodynamics scanning electron microscopy sem studies support microbial origin of ooids giant ooids and stromatolites of kunihar formation increased microbial activity in kunihar formation is attributed to increase in nutrients by virtue of weathering of underlying darla volcanics abundant carbonate and microbially induced sedimentary structures miss deposition in lower part of simla group points to increased microbial activity which likely increased the volume of oxygen in neoproterozoic atmosphere ushering in ice house conditions during the subsequent deposition of blaini group giant ooids associated with neoproterozoic glacial deposits throughout the world occupy stratigraphic positions below above or between glaciations simla group is another example where giant ooids lie stratigraphically below marinoan blaini tillites increased magmatic activity and weathering before and during the neoproterozoic glaciations increased nutrients in marine waters which increased algal growth thus giant ooids were deposited due to such phases of increased microbial activity before after glaciation and during interglacial periods

morphological record of oxygenic photosynthesis in conical stromatolites conical stromatolites are thought to be robust indicators of the presence of photosynthetic and phototactic microbes in aquatic environments as early as 3 5 billion years ago however phototaxis alone cannot explain the ubiquity of disrupted curled and contorted laminae in the crests of many mesoproterozoic paleoproterozoic and some archean conical stromatolites here we demonstrate that cyanobacterial production of oxygen in the tips of modern conical aggregates creates contorted laminae and submillimeter to millimeter scale enmeshed bubbles similarly sized fossil bubbles and contorted laminae may be present only in the crestal zones of some conical stromatolites 2 7 billion years old or younger this implies not only that cyanobacteria built proterozoic conical stromatolites but also that fossil bubbles may constrain the timing of the evolution of oxygenic photosynthesis

a reconstruction of archean biological diversity based on molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup hamersley basin western australia abstract bitumens extracted from 2 7 to 2 5 billion year old ga shales of the fortescue and hamersley groups in the pilbara craton western australia contain traces of molecular fossils based on a combination of molecular characteristics typical of many precambrian bitumens their consistently and unusually high thermal maturities and their widespread distribution throughout the hamersley basin the bitumens can be characterized as probably of archean age accepting this interpretation the biomarkers open a new window on archean biodiversity the presence of hopanes in the archean rocks confirms the antiquity of the domain bacteria and high relative concentrations of 2 methylhopanes indicate that cyanobacteria were important primary producers oxygenic photosynthesis therefore evolved 2 7 ga ago and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere moreover the abundance of cyanobacterial biomarkers in shales interbedded with oxide facies banded iron formations bif indicates that although some archean bif might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria those in the hamersley group formed as a direct consequence of biological oxygen production biomarkers of the 3 methylhopane series suggest that microaerophilic heterotrophic bacteria probably methanotrophs or methylotrophs were active in late archean environments the presence of steranes in a wide range of structures with relative abundances like those from late paleoproterozoic to phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late archean 900 ma before visible fossil evidence indicates that the lineage arose sterol biosynthesis in extant eukaryotes requires molecular oxygen the presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least 1 of the present atmospheric level pal and may have been sufficient to support aerobic respiration

the unusual 3d interplay of basement fault reactivation and fault propagation fold development a case study of the laramide age stillwell anticline west texas usa abstract subsurface fault geometries have a systematic influence on folds formed above those faults we use the extraordinarily well exposed fold geometries of the laramide age stillwell anticline in west texas usa to develop a strain predictive model of fault propagation fold formation the anticline is a 10 km long nw trending ne vergent asymmetric fold system with an axis that displays a map view left stepping en echelon pattern we integrated field observations geologic and structural data cross sections and 2d kinematic modeling to establish an unusual 3d two stage model of contractional fold formation including 1 reverse reactivation of a pre existing nw striking sw dipping left stepping en echelon normal fault system in paleozoic basement rocks to generate monoclinal flexures in overlying layered cretaceous carbonate rocks and 2 the formation of a subsequent flat ramp fault system that propagated horizontally along a mechanically weak clay rich cretaceous unit before ramping up at the hinge of the pre existing monocline system strain is focused within the forelimb of the system in front of the propagating fault tip and is accommodated by a combination of interlayer slip flat ramp faulting and fracturing proximal to planes of slip this strain predictive model can be applied to similar less well exposed contractional systems worldwide and provides a new unusual example of laramide age contractional deformation

the role of strain in area constant detachment folding abstract the classical constant area relationship for balancing cross sections is reformulated to accommodate layer parallel strain leading to a general equation for the layer parallel strain in a fold formed above a stratigraphically fixed detachment horizon the strain required for area balance can be calculated from a knowledge of the fold shape and the depth to detachment the strains predicted from a simple kinematic model are realistic in the order of 10 30 layer parallel shortening the strain may be penetrative or occur as small scale disharmonic folds and faults the tip top anticline a fold at the leading edge of the wyoming thrust belt is used to illustrate the applicability of this reformulation the computed layer parallel strains in the well controlled upper portion of the structure are less than 5 because most of the shortening is accommodated by second order folds and faults

slip partitioning along major convergent plate boundaries along plate boundaries characterized by oblique convergence earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors consequently relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back arc regions the deformation behind the trench may take the form of strike slip motion back arc extension or some combination of both we observe from our analysis of the harvard moment tensor catalog that convergent arcs characterized by back arc spreading specifically the marianas and new hebrides are characterized by a large degree of slip partitioning however the observed rates directions and location of back arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate in the case of the sumatran arc where partitioning is accommodated by strike slip faulting in the overriding plate the degree of partitioning is similar to that observed along the marianas but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors in the case of the alaskan aleutian arc where back arc spreading is also absent the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence if partitioning in the alaskan aleutian arc is accommodated by strike slip faulting within the upper plate the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either 1 the ratio of the depth extent of strike slip faults behind the trench z s to the subduction thrust z t increases westward along the arc 2 the dip of the subduction thrust increases westward along the arc or 3 the strength of the subduction thrust decreases westward along the arc

characteristics of a tonian reef rimmed shelf before the onset of cryogenian insights from neoproterozoic kunihar formation simla group lesser himalaya abstract the kunihar formation records evolution of a unique rimmed shelf during the tonian before the cryogenian marinoan glaciation of krol formation lesser himalaya the mixed siliciclastic carbonate system of neoproterozoic kunihar formation provokes interest for outcrop based facies analysis and sequence stratigraphy in order to frame a better understanding on the mechanism and control of its deposition the formation records a 450 m thick mixed siliciclastic carbonate rimmed shelf five facies associations have been identified peritidal fa1 lagoonal fa2 barrier island ooidal shoal fa3 reef complex fa4 and fore reef fa5 facies associations two systems tracts were delineated transgressive tst and highstand hst systems tracts tst is marked by dominant aggradation of stromatolitic carbonates while siliciclastic incursion surpassed carbonate deposition during highstand it conforms to the lower carbonate upper clastic model of mixed siliciclastic carbonates during the precambrian reef rimmed shelves were considered more abundant in the archaean and palaeo mesoproterozoic this study determines that development of rimmed shelves was more common during the neoproterozoic than previously speculated neoproterozoic reef rimmed shelves are concentrated more within the cryogenian and ediacaran period stromatolitic reef rimmed shelf of kunihar formation is the only rimmed shelf recorded in the tonian period till date increased microbial stromatolitic accretion was propelled by surge in nutrients due to chemical weathering of underlying tholeiitic basalt darla volcanics associated with the rifting of supercontinent rodinia with increasing interest of tonian cryogenian transition as a key period in both planetary and organic evolution kunihar formation represents a terminal reef rimmed shelf system that evolved due to microbial stromatolites in the absence of complex organisms subsequent to this cryogenian and ediacaran biota took over as the dominant reef builders

depositional setting of the 2 1 ga francevillian macrobiota gabon rapid mud settling in a shallow basin swept by high density sand flows the depositional setting of the 2 1 ga fill to the franceville basin of gabon is important for understanding the habitat energy and availability of light and oxygen and taphonomy of recently discovered early macro organisms buried in black shales in unit fb the available data bearing on the stratigraphy and sedimentology of unit fb provide new insight into processes acting on the palaeo seafloor the shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands the latter poubara sandstones were emplaced during a forced regression during the terminal infill of fault bounded sub basins following a stage characterised by a ferruginous to anoxic water column the structureless sandstones were deposited from high density gravity currents along with a locally strong bottom oscillation of the water column tuft structures preserved in cyanobacterial mats together with the position of the macro organisms at the top of the sandstone beds within associated black shales point to a water depth of less than 80 m the relative sea level fall that drove deposition of the poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows this article is protected by copyright all rights reserved

nature and distribution of diagenetic phases and petrophysical properties of carbonates the mississippian madison formation bighorn basin wyoming usa abstract the purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs to do so a pluridisciplinary approach was carried out integrating sedimentologic and diagenetic studies on the madison formation lower carboniferous 357 340 my which is a carbonate reservoir in subsurface the madison formation outcropping in the bighorn basin wyoming usa is a 340 m thick carbonate series composed of four to seven 3rd order depositional sequences s1 to s7 depending of the palaeogeographic location the first three sequences s1 to s3 were deposited under arid and warm conditions during tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp s1 to s2 to a wide platform s3 it also probably favoured the early calcite cementation isopachous and syntaxial calcite cements of the subtidal deposits and the early dolomitization d1 of the supratidal to intertidal sabkha ones in addition the very flat profile occurring during s3 was also responsible for the postponed dolomitization d2 of s1 s2 due to reflux of brines at various degrees depending on the palaeogeographic location the deposition of s4 to s7 under humid conditions during visean times were associated with 1 a decrease of the carbonate accumulation and of the dolomitization 2 dissolution processes at micro pores network and macro scales karst collapse breccia and calcite cementation c1 the spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases thus the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution karsts collapse breccias or dolomitization the burial diagenesis of the madison formation was characterized by 1 a dolomitization phase d3 in proximal parts of the platform leading to an increase of porosity and permeability and 2 calcite cementations c2 c3 in the distal parts of the platform leading to a decrease of these properties the madison formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations climates burial history and structures

stromatolite reef from the early archaean era of australia the 3 430 million year old strelley pool chert spc pilbara craton australia is a sedimentary rock formation containing laminated structures of probable biological origin stromatolites determining the biogenicity of such ancient fossils is the subject of ongoing debate however many obstacles to interpretation of the fossils are overcome in the spc because of the broad extent excellent preservation and morphological variety of its stromatolitic outcrops which provide comprehensive palaeontological information on a scale exceeding other rocks of such age here we present a multi kilometre scale palaeontological and palaeoenvironmental study of the spc in which we identify seven stromatolite morphotypes many previously undiscovered in different parts of a peritidal carbonate platform we undertake the first morphotype specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation finally we argue that the diversity complexity and environmental associations of the stromatolites describe patterns that in similar settings throughout earth s history reflect the presence of organisms

archaean stromatolites from the ngesi group belingwe greenstone belt zimbabwe preservation and stable isotopes preliminary results abstract the petrology and stable isotope chemistry of cyanobacterial stromatolites of archaean age 2 7 ga from the cheshire and manjeri formations of the belingwe greenstone belt in zimbabwe have been examined palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes the cheshire formation shows little evidence of either anion or cation exchange during metamorphism and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80 c the manjeri formation only slightly older but overlain by a thick volcanic sequence shows a low grade of metamorphism and isotope ratios that are consistent with a metamorphic temperature of around 200 c

tungsten in archean banded iron formation of the former sherman and adams mines ontario possible mineralization processes anomalous tungsten occurs in archean banded iron formation from the previous sherman and adams mines ontario in sherman samples from three open pits and one drill core site with one exception anomalous tungsten occurred only in the sherman north pit adams mine samples from three pits carried anomalous tungsten in samples of chlorite rich silicate facies statistical evaluation showed that 10 ppm tungsten was anomalous with respect to the entire data set the implications of restricted tungsten distribution as seen in sherman occurrences were evaluated with respect to tungsten mineralization processes scenarios considered involve syngenetic metamorphic and magmatic hydrothermal processes thermal constraints provided by iron bearing silicate assemblages and oxygen isotopes attest to contact metamorphism as a potential mechanism for tungsten redistribution within iron formation however tungsten of magmatic hydrothermal origin emplaced as an epigenetic component is considered the essential miner

microbially mediated carbonate precipitation in a hypersaline lake big pond eleuthera bahamas microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability this can result in the precipitation of carbonate minerals and thus the formation of microbialites here the microbial metabolic activity the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake big pond bahamas were determined to identify the driving forces in microbialite formation a conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented geochemical modelling indicated an oversaturation with respect to carbonates including calcite aragonite and dolomite but these minerals were never observed to precipitate at the mat water interface this failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium a layer of high mg calcite was present 4 to 6 mm below the surface of the mat just beneath the horizons of maximum photosynthesis and aerobic respiration this carbonate layer was associated with the zone of maximum sulphate reduction it is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface i e the cyanobacterial layer of the mat bind calcium both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances each process accounting for approximately half of the total consumption and low molecular weight organic carbon liberating calcium and producing inorganic carbon the combination of these geochemical changes can increase the carbonate saturation index which may result in carbonate precipitation in conclusion the formation and degradation of extracellular polymeric substances as well as sulphate reduction may play a pivotal role in the formation of microbialites both in marine and hypersaline environments

archean stromatolites in michipicoten group siderite ore at wawa ontario small bioherms with complex conical stromatolites attributable to thyssagetes occur at the top of the siderite orebody in the late archean 2 75 2 70 ga helen iron formation at the macleod mine 4 km northeast of wawa horizontal sections show round to polygonal outlines 2 to 20 cm across longitudinal sections show stacked convex to inflexed angulate laminae the lamina shapes are primary and are relatively undeformed the microstructure is secondary with dark laminae outlined by coarse secondary pyrite pyrrhotite and magnetite and light laminae by very fine grained siderite the stromatolites are the first to be reported from siderite they are relatively well preserved primary biosedimentary structures that provide conclusive morphologic evidence of biologic activity in the shallow water photic zone in the late archean basin in which the carbonate of the michipicoten group accumulated they also indicate that the ore zone was most likely a carbonate originally rather than a tuff as postulated in some early accounts these structures now constitute the southeasternmost documented occurrence of archean stromatolites in north america they also are the oldest known representatives of the thyssagetaceae

what caused the rise of atmospheric o2 abstract oxygenic photosynthesis appears to have evolved well before o 2 levels increased in the atmosphere at around 2 4 ga this has led to numerous suggestions as to what may have kept o 2 suppressed and then eventually allowed it to rise these suggestions include changes in the recycling of carbon and sulfur relative to water or hydrogen a switch from dominantly submarine to dominantly subaerial volcanism gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases a decline in deposition of banded iron formations a decline in nickel availability and various proposals to increase the efficiency of photosynthesis several of these different mechanisms could have contributed to the rise of o 2 although not all of them are equally effective to be considered successful any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates which shows relatively little change with time apart from transient but occasionally spectacular excursions the reasons for this constancy are explored here but are not fully resolved in the process of making these comparisons a self consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on earth like exoplanets

archaean stromatolites from the steep rock group northwestern ontario canada reinvestigation of the late archaean stromatolites of the steep rock group has shown that a wide variety of forms is present including domed and tabular bioherms and biostromes both columnar and noncolumnar structures are present branching is common in some columnar forms facing directions in the stromatolites are consistent with other field evidence showing that the base of the steep rock group is an unconformity

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

sulfur and carbon isotopes in archean banded iron formations implications for sulfur sources abstract sulfur isotope ratios of pyrite from two late archean banded iron formations were used to constrain possible sources of sulfur the iron formations studied were the temagami iron formation which hosts the sherman mine near temagami and the boston iron formation including the adams mine south of kirkland lake both in ontario in both cases algomantype oxide facies magnetite chert ores predominate the sulfur studies were supplemented by limited carbon isotopic data for the sherman mine iron formation in the sherman mine occurs on both limbs of a regional synformal structure each iron formation unit caps a thick calc alkaline andesite dominated volcanic sequence with one band hosted in sedimentary rocks of turbidite origin the adams mine includes four major iron formation bands interbedded with mafic and ultramafic volcanic rocks and greywacke of turbidite origin the sulfur isotope statistics for averages of pyrites from these three units are sherman mine volcanic hosted suite 34 s 1 7 4 6 n 14 sherman mine sediment hosted suite 34 s 1 4 1 6 n 43 and adams mine 34 s 1 8 2 9 n 17 these averages differ by from juvenile sulfur and together with their limited dispersions suggest a predominantly unfractionated source of sulfur it is argued that this sulfur has been derived from the associated volcanic piles most of the pyrite occurs as disseminated grains in oxide facies iron formation and interbedded clastic sediments a distinctive nodular pyrite in the sherman mine sediment hosted suite occurs in cherty graphitic sulfide facies iron formation while the sulfur isotopic properties of this subset 34 s 0 0 0 8 are not suggestive of biogenic sulfur the geological setting and carbon isotopic compositions of associated graphite 13 c 34 imply that anaerobic bacterial processes were active in this reducing environment the apparent lack of biogenic sulfide is rationalized by models suggesting that archean marine basins differed from phanerozoic analogues in various parameters which influence sulfur isotopic characteristics it is argued that biogenic sulfide is probably a significant component of the sulfur in sherman mine nodular pyrite

depositional facies and platform architecture of microbialite dominated carbonate reservoirs ediacaran cambrian ara group sultanate of oman intrasalt carbonates of the ediacaran cambrian ara group constitute a significant reservoir element of the intrasalt stringer play in oman in which dolomitic carbonates are encased in salt at depths of 3 to 7 km 1 9 to 4 3 mi these reservoir carbonates have significant microbial influences although ara group reservoirs are mostly latest precambrian the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded in lacustrine settings where calcified invertebrates are not a significant source of carbonate or after periods of mass extinction before faunal recovery a broad range of carbonate facies provides the context in which to understand the origin of the microbialite dominated reservoirs developed across both ramp and rimmed shelf profiles major facies associations include carbonate evaporite transition zone deep ramp and slope subtidal microbialites clastic textured carbonates and restricted peritidal carbonates microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties and that help in predicting the location of reservoir fairways microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities 50 shallow subtidal pustular laminites with cm scale variability of lamina morphology deeper subtidal crinkly laminites that show mm scale variability of lamina morphology and intertidal tufted laminates that show mm to cm scale tufted textures other reservoir facies are more conventional grainy carbonates including ripple cross stratified grainstone packstone hummocky cross stratified grainstone packstone flat pebble conglomerate ooid and intraclast grainstone packstone and cloudina grainstone packstone these facies are almost invariably dolomitized and all have moderate to excellent reservoir quality these facies comprise carbonate platforms broken up during salt tectonics that range up to 160 m 525 ft in thickness and extended laterally prior to halokinesis for tens to over 50 km 31 mi the distribution of reservoir facies follows sequence stratigraphic predictions with microbialites occurring in every accommodation profile late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build ups whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate

repetitive cyclical volcanism in the late archean larder lake group near kirkland lake ontario implications of geochemistry on magma genesis abstract the larder lake group is the basal stratigraphic unit of the upper supergroup the younger magmatic cycle of the abitibi greenstone belt of the superior province three major suites of extrusive and hypabyssal rocks are recognized a a high magnesian suite of komatiite to mg basalt b a tholeiitic series with mg rich and fe rich basaltic varieties and c a low magnesian series of basalt to rhyodacite fourteen such subcycles were recognized in the boston iron range south of kirkland lake and are grouped in four major stratigraphic intervals the entire larder lake group magma series evolved from a peridotitic mantle source through various stages of partial melting and fractional crystallization lree enriched tholeiites are interpreted as the initial products of low degrees of melting of a chondritic mantle both magnesian suites show lree depleted patterns and identical zr y and zr ti ratios suggesting that their parental magmas were generated by partial melting of the residual mantle from which the tholeiitic melts had been previously derived the low magnesian suite which is genetically related to a komatiitic magma series includes compositions as silicic as rhyodacite

facies architecture and sequence stratigraphic features of the tumbiana formation in the pilbara craton northwestern australia implications for depositional environments of oxygenic stromatolites during the late archean abstract the tumbiana formation ca 2 7 ga is a succession developed in response to late archean crustal extension in the pilbara craton northwestern australia the formation is characterized by intercalations of stromatolitic carbonates and provides a perspective of the evolution of photosynthetic organisms which are interpreted to have been responsible for the oxygenation of earth surface environments in early earth here we investigated the depositional setting of the stromatolitic carbonates for the better understand of the environments which may have controlled the massive development of oxygenic photosynthesis we studied lithofacies and sequence stratigraphic features of the tumbiana formation in the redmont area a southern margin of the northern pilbara craton on the basis of three dimensional analyses of onshore outcrops the tumbiana formation as much as 170 m thick unconformably overlies the early to middle archean basement rocks and is unconformably overlain by the maddina formation that consists of mafic tuffaceous sediments and basalt lavas the lower part of the tumbiana formation is characterized by planar and trough cross stratified conglomerates which are interpreted to be an alluvial fan deposit shed from the northern hinterlands and developed mainly in local depressions of the basement rocks during late lowstand and early transgressive stages the base of the middle part is defined by a transgressive surface of erosion overlain by conglomerates and or breccias that pass upward to mafic tuffaceous mudstones and sandstones intercalated with accretionary lapillis breccias and basalt pillow lavas mudstones are commonly laminated and sandstones contain current and wave ripple lamination parallel lamination hummocky cross stratification and trough cross stratification in general the middle part exhibits an overall fining and coarsening upward pattern and is interpreted to indicate transgressive and regressive shelf to coastal lithofacies successions in particular the uppermost horizon of the middle part is characterized by sandstone beds with herringbone structures and desiccation cracks and is interpreted to indicate an intertidal deposit developed during late highstand and or following falling stages the middle part of the formation in general thickens to the northern proximal area and eventually flattened out the topographic irregularity of the basement rocks the upper part is characterized by stromatolitic carbonates associated with minor mudstones and wave and current rippled sandstones and does not exhibit distinct lateral variation in thickness and vertical lithofacies organization the base of the upper part is characterized by a transgressive lag deposit the ensuing transgression over the intertidal deposits of the uppermost middle part is interpreted to have developed a new accommodation space for the vertical stacking of stromatolites with different external shapes from elongated small domal and columnar types associated with desiccation cracks to stratiform and large domal types these changes may have responded to the increase in paleowater depth possibly from intertidal to subtidal environments in conclusion stromatolites in the upper tumbiana formation in the redmont area are interpreted to have developed in coastal and shallow marine environments in response to the rise in relative sea level rather than in ephemeral saline lakes as proposed by previous studies however the outcome of the present research does not necessarily deny the previously proposed interpretation but suggests spatial variations in depositional environments of the tumbiana formation from the north to south

u pb dating of calcite veins reveals complex stress evolution and thrust sequence in the bighorn basin wyoming usa reply we report u pb absolute ages of calcite cements from a diffuse vein network documented in the bighorn basin wyoming usa where distinct systematic vein sets developed at the front of the thin skinned sevier orogen during laramide layer parallel shortening and during thick skinned laramide thrusting and folding the u pb age distribution illustrates 1 an outward eastward transmission of sevier orogenic stress from 89 7 2 9 ma 2 in the west and from 75 3 2 8 ma in the east and 2 an inward westward development of laramide related fracturing and folding 72 3 0 ma and 45 4 1 8 ma respectively in the east 60 5 4 6 ma and 27 9 1 1 ma respectively in the west which is in accordance with the known sequence of exhumation of the major laramide basement arches our results also show that the stress related to laramide compression first overprinted the stress related to sevier compression in the sedimentary cover around major basement uplifts this study highlights the utility of u pb calcite geochronology as a powerful tool for constraining complex sequences of deformation in orogenic forelands

age and timing of igneous activity in the temagami greenstone belt ontario a preliminary report the southernmost remnants of archean supracrustal and intrusive rocks in eastern ontario are exposed through a window in the early proterozoic huronian supergroup near the town of temagami u pb zircon ages from this area indicate the presence of some of the oldest felsic magmatism so far discovered in this portion of the superior province the iceland lake pluton 2736 2 ma and a nearby rhyolite flow are contemporaneous which establishes that at least some of the intrusive rocks in the region are synvolcanic and coeval with the oldest volcanic cycle the youngest plutonic activity is the emplacement of a late rhyolite porphyry dike at 2687 2 ma an age that is bracketed by the 2675 2700 ma emplacement ages of late internal plutons found throughout the abitibi subprovince the 2736 ma dates however are older than the nearest portion of the exposed abitibi some 120 km to the north near kirkland lake

depositional evolution of an extinct sinter mound from source to outflow el tatio chile siliceous sinter deposits from el tatio chile preserve a wide variety of depositional environments and biosignatures from high temperature 85 c vent proximal facies to distal deposits dominated by silicified microbial mats four cores were drilled into an el tatio sinter mound and associated distal apron to investigate changes in hydrothermal environments over geologic timescales sedimentary and geochemical analysis of multiple sinter cores records the initiation and accretion of diverse depositional features still observed today in el tatio facies adjacent to hydrothermal vents are dominated by laminated sinter crusts on the steep margins of a high temperature pool with sparse microbial preservation outer margins of the same pool contain extensive sinter columns up to ten centimeters in length precipitated during repeated cycles of pool overflow and subsequent evaporation low relief hydrothermal pools also form minor deposits within distal debris aprons and analogous pools are still active close to sampling locations debris aprons are dominated by palisade tufted and arborescent microbial fabrics with distinct mat textures revealing well preserved microfossils surficial deposits in all cores feature detrital rich and microbially influenced sinters overlying higher temperature facies indicating a relative decrease in hydrothermal activity over time geochemical proxies for hydrothermal fluids and de trital input match depositional interpretations based on sedimentary structures 14 c ages from core deposits extend the mound s history by 11 000 years recording at least three thousand years of sinter deposition on top of glacial sandstones 13 337 10 232 y cal bp importantly this work provides a detailed depositional model unavailable through surficial sedimentology alone

antarctic microbial mats a modern analog for archean lacustrine oxygen oases the evolution of oxygenic photosynthesis was the most important geochemical event in earth history causing the great oxidation event goe 2 4 b y ago however evidence is mixed as to whether o2 production occurred locally as much as 2 8 b y ago creating o2 oases or initiated just prior to the goe the biogeochemical dynamics of possible o2 oases have been poorly constrained due to the absence of modern analogs however cyanobacteria in microbial mats in a perennially anoxic region of lake fryxell antarctica create a 1 2 mm o2 containing layer in the upper mat during summer providing the first known modern analog for formation of benthic o2 oases in lake fryxell benthic cyanobacteria are present below the oxycline in the lake mat photosynthesis rates were slow due to low photon flux rate 1 2 mol m 2 s 1 under thick ice cover but photosynthetic o2 production was sufficient to sustain up to 50 mol o2 l 1 sandwiched between anoxic overlying water and anoxic sediments we hypothesize that archean cyanobacteria could have similarly created o2 oases in benthic mats prior to the goe analogous mats may have been at least partly responsible for geological evidence of oxidative weathering prior to the goe and habitats such as lake fryxell provide natural laboratories where the impact of benthic o2 oases on biogeochemical signatures can be investigated

using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts abstract we investigate changes in shortening displacement and fold geometry to understand the detailed along strike structural variation within fold thrust belts and infer thrust growth and linkage mechanisms field observations from the vercors in se france are used to characterise deformation style in the region parallel cross sections are constructed analysed and used to create shortening and thrust displacement profiles from the northern to southern vercors sections show changes in structural style and shortening accommodation from thrust dominated in the north to fold dominated in the south the total shortening distance in the vercors does not change significantly along strike 3400 4650 m however displacements along individual thrust zones do vary significantly and displacement profiles show a range in displacement gradients 16 107 m km despite relatively simple shortening patterns in the vercors sections show a more complex 3d internal structure of the fold thrust belt thrust displacements and geometries suggest both large scale thrust zones and small scale thrusts are soft linked transferring displacement along strike through transfer zones short soft linked thrust segments indicate an intermediate stage of thrust growth and linkage well documented for normal fault systems which form prior to the formation of thrust branches and hard linked displacement transfer

morphological adaptations of 3 22 ga old tufted microbial mats to archean coastal habitats moodies group barberton greenstone belt south africa microbial life was well established and widespread by the paleoarchean however the degree of evolutionary advancement such as microbial motility intra and inter species interactions phototropism or oxygenic photosynthesis by that time remains highly debated the 3 22 ga moodies group in the barberton greenstone belt bgb south africa are earth s oldest well preserved siliciclastic tidal deposits they exhibit a unique assemblage of microbial mats providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings the fossil mats are preserved as kerogenous laminations 0 5 1 mm thick that can be traced laterally for 15 km in a 1000 m thick succession of fine to coarse grained tidal sandstones and conglomerates we here present a detailed stratigraphic and depositional facies analysis documenting the association of the three principal mat morphotypes with specific environmental settings 1 planar type in coastal floodplain 2 wavy type in intertidal and 3 tufted type in upper inter to supratidal facies all mat types indicate a flourishing phototrophic biota moreover the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments

early non marine life evaluating the biogenicity of mesoproterozoic fluvial lacustrine stromatolites abstract the record of life on land or in non marine environments during the precambrian is sparse limiting our ability to understand life outside of marine settings before the advent of animals stromatolites from such environments are known but demonstrating stromatolite biogenicity remains difficult as stromatolite growth can be controlled by a spectrum of biologic chemical and biologically mediated processes stromatolites from the mesoproterozoic 1 09 ga copper harbor conglomerate an alluvial fan fluvial and lacustrine deposit offer an interesting test for the presence and nature of microbial life in shallow mesoproterozoic non marine settings stromatolites from a siltstone facies are interpreted as biogenic as they contain detrital rich laminae likely indicative of trapping and binding by microbes and fenestral fabrics suggestive of desiccation or lift off structures in mats via the presence of gas perhaps o2 from photosynthesis or other gases from mat decay the stromatolites formed as microbial mats grew over a mudflat or sandflat with carbonate filled desiccation cracks on an erosive topography and thus provide evidence for life in a very shallow predominantly desiccated environment stromatolites from a conglomerate facies are microdigitate and record both isopachous laminae with radial fibrous calcite fans and botryoids typically considered abiogenic in origin as well as wavy conical laminations likely indicative of the former presence of microbial mats the conglomerate facies stromatolites are interpreted to have formed in a flooded braidplain setting with restricted circulation this study supports the suggestion that microbial communities were abundant in non marine environments in the midcontinent rift during the mesoproterozoic it also highlights how variable environmental factors can influence stromatolite growth even in similar depositional settings and with a consistent microbial presence

methylhopane biomarker hydrocarbons in hamersley province sediments provide evidence for neoarchean aerobiosis abstract bacteriohopanepolyols with a ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa when preserved in ancient rocks the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies considering both their occurrences in modern organisms and their proterozoic and phanerozoic sedimentary distributions hopanes carrying 2 methyl or 3 methyl substituents are proposed to be derived from cyanobacteria and oxygen respiring methanotrophs respectively here we report the distribution of methylhopanes in 2 72 2 56 billion year old neoarchean rocks from the hamersley province on the pilbara craton the relative abundance of c 31 3 methylhopane but not that of c 31 2 methylhopane shows a strong correlation to the carbon isotopic composition of co occurring kerogen insoluble particulate organic matter the unanticipated nature of this relationship provides evidence for a shallow water locus of carbon cycling through aerobic oxidation of ch 4 and coincidentally a means to demonstrate biomarker syngenicity 2 methylhopanes are most abundant in both shale and carbonate from shallow water sediments and a positive correlation to carbonate abundance contrasts to variable but generally lower 2 methylhopane abundances of deeper water facies these observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow water ecosystems providing molecular oxygen fixed carbon and possibly fixed nitrogen

environmental control on diverse stromatolite morphologies in the 3000 myr pongola supergroup south africa a unique outcrop of partly silicified dolomite in the white umfolozi section of the pongola supergroup south africa indicates that stromatolites were diverse and adapted to a range of shallow tidal depositional settings 3000 myr ago composite columnar stromatolitic bioherms 0 7 1 6m high and 0 4 1 0m in diameter formed along the margins of a tidal channel they were flanked away from the channel by flat stratiform and small domical stromatolites growing in low energy tidal flat environments conical stromatolites 0 05 0 30m high and 0 03 0 10m in diameter accreted in high energy coarse grained carbonate sand along the bottom of the tidal channel the stromatolites probably formed through the activities of filamentous oxygen producing photoautotrophic cyanobacteria

discovery of major basement cored uplifts in the northern galiuro mountains southeastern arizona implications for regional laramide deformation style and structural evolution lowell institute for mineral resources at the university of arizona arizona geological society j harold courtright scholarship

morphological signatures of microbial activity across sediment and light microenvironments of lake vanda antarctica abstract cyanobacteria dominated microbial mats in lake vanda grow with pinnacles and ridges separated by prostrate mat rocks protrude over microbial mats on the lake bottom to create localized dm scale gradients in sedimentation and irradiance the effects of sedimentation on pinnacle and ridge growth were isolated from photosynthetic activity by contrasting growth across microenvironmental gradients sedimentation rate was measured as the mass of sand and mud sized sediment in mat that accumulated over 11 years and the incident light was modeled near and under rocks by reconstructing topography using structure from motion techniques morphologically diverse pinnacles and ridges were documented in both exposed and sheltered mat microenvironments in addition to growing downward from the underside of overhanging rocks mat that grew with 40 irradiance under overhangs did not have consistent differences in pinnacle density or ridge abundance as a function of sedimentation rates or irradiance when compared to exposed mat however their morphology did change significantly with changes in the direction of incident irradiance where irradiance was

archaean quartz arenites in the canadian shield examples from the superior and churchill provinces abstract units of remarkably pure archaean quartz arenite occur in the northwestern part of the superior province and in the northern terrane of the western churchill province rae province of the canadian shield in the superior province silica cemented quartz arenites of archaean age are well preserved in several greenstone belts the example from the keeyask lake sedimentary assemblage displays tabular planar and trough cross beds ripple marks reactivation surfaces and pebble lag deposits in spite of penetrative deformation and greenschist grade metamorphism primary textures are extremely well preserved showing framework grains to be very well rounded and sorted the succession of keeyask lake quartz arenite beds is overlain by siltstones containing small scale stratiform domal and columnar stromatolites a shallow marine environment of deposition is inferred detrital heavy minerals include pyrite magnetite zircon tourmaline apatite sphene and topaz in the northern part of the western churchill province rae province archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures this has led to speculation that some of these units are metachert or recrystallized vein quartz but local preservation of primary textures and structures provides clear evidence of epiclastic origin in the example described herein quartz arenites of the woodburn lake group display sparse occurrences of trough and tabular planar cross beds channels ripple marks and pebble lag deposits probable environments of deposition for these quartz arenites include fluvial systems and shallow marine shelf settings the occurrence of unequivocal quartz arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix now mainly muscovite and rare detrital grains of accessory heavy minerals predominantly zircon and opaque iron oxides pyrite and other sulphides have been introduced along fractures but some intergranular sulphide grains may be of detrital origin the principal source for the quartz arenites in both areas must have been quartz rich granitoid rocks conditions of intense chemical weathering are indicated the widespread occurrence of extremely mature quartz arenites throughout archaean terranes of the canadian shield and in other shields of the world are suggestive of crustal stability during early earth history the association of quartz arenites and ultramafic rocks uncharacteristic of younger terranes is now recognized in many archaean greenstone belts of the canadian shield

in situ iron isotope analyses of pyrite and organic carbon isotope ratios in the fortescue group metabolic variations of a late archean ecosystem abstract the biogeochemical cycle of the late archean ocean is important for understanding the relationships between biological activity and oxygenation of the atmosphere and ocean based on the detailed geological survey of the fortescue group in the redmont area in pilbara craton western australia we carefully selected 44 samples for iron isotope analyses which consist of sandstones stromatolitic carbonate rocks alternating mudstone sandstone rocks mudstones and cherts our in situ analyses of 56fe values of 210 pyrite grains in these samples show a large variation from 4 2 to 3 0 we also analyzed 128 and 40 carbon isotope compositions of organic 13corg 51 8 to 10 3 and inorganic 13ccarb 6 1 to 0 6 carbons respectively microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio most pyrite grains with positive 56fe values show hexagonal rectangular and parallelogram shapes which may replace former iron oxide crystal systems hematite magnetite and goethite respectively in contrast more than half the pyrite grains with negative 56fe values show irregular forms the correlation allows the possibility to solve the origin and the formation process of each grain of pyrite the positive 56fe values suggest the partial oxidation of iron in an oxygen limited environment some pyrites show very lower 56fe values below 2 2 suggesting a biological origin probably due to microbial iron reduction on the other hand the pyrite is accompanied by isotopically very light organic carbon 13corg 51 8 to 40 which indicates aerobic or anaerobic methanotrophy the coexistence of the low 56fe values and low 13c values in the some rocks suggests anoxic oxidation of methane by iron reduction aom ir the iron and carbon isotopes demonstrate the metabolic variations of microorganisms in a late archean shallow marine environment

archean stromatolites from abitibi greenstone belt quebec canada the oldest fossils in eastern north america are neoarchean stromatolites discovered near joutel in the northern part of the abitibi subprovince of the superior craton they are found in allochthonous limestone blocks as much as 2 m across in 2730 2720 ma volcanic breccia and turbiditic volcaniclastic rocks of the joutel volcanic complex the stromatolites which include thyssagetacean and pseudocolumnar forms represent an assemblage transported to greater depth from biohermal or biostromal complexes formed in shallow water settings on a submerged volcanic edifice

late archean molecular fossils from the transvaal supergroup record the antiquity of microbial diversity and aerobiosis abstract cores recovered during the agouron griqualand drilling project contain over 2500 m of well preserved late archean transvaal supergroup sediments dating from ca 2 67 to 2 46 ga bitumen extracts of these strata were obtained using clean drilling sampling and analysis protocols that avoided overprinting syngenetic molecular fossil signatures with contaminant hydrocarbons comparisons of biomarker contents in stratigraphically correlated intervals from diverse lithofacies in two boreholes separated by 24 km as well as across a 2 gyr unconformity provide compelling support for their syngenetic nature the suite of molecular fossils identified in the late archean bitumens includes hopanes attributable to bacteria potentially including cyanobacteria and methanotrophs and steranes of eukaryotic origin this molecular fossil record supports an origin in the archean eon of the three domains of cellular life as well as of oxygenic photosynthesis and the anabolic use of o2

chambered structures from the ediacaran dengying formation yunnan china comparison with the cryogenian analogues and their microbial interpretation enigmatic chambered structures have been reported forming reef frames in cryogenian interglacial carbonates prior to the commonly acknowledged microbial metazoan reefs at the terminal ediacaran and interpreted as fossils of possible sponge grade organisms a better constraint on the affinity of these structures is partly hindered by few analogues in other time periods this study describes similar structures from peritidal dolostones of the ediacaran denying formation from yunnan china samples were investigated using optical microscopy and three dimensional 3 d reconstruction based on grinding tomography the dengying chambered structures are comparable with cryogenian structures in basic construction but are not frame building and show variations in overall shape and inhabiting facies two dimensional 2 d cross sections show that thin homogeneous micritic laminae are the basic building blocks of the chamber walls thick walls represent parallel accretion of these laminae and thin walls developed from the angular growth of a single lamina or merging of multiple laminae in 3 d space the laminae primarily correspond to continuous surfaces which sometimes contain sub circular holes while a few represent filamentous elements connected to the surfaces the morphological features and growth pattern of the dengying chambered structures indicate that they are likely to be calcified microbial constructions rather than skeletal remains of basic metazoans however aside from the cryogenian and dengying examples comparable chambered constructions with laminae based architecture are yet unknown from other fossil or extant microbialites further work investigating related structures is needed to determine the microbial consortia and controlling environmental factors that produced these chambered structures

fracture controlled paleohydrogeology in a basement cored fault related fold sheep mountain anticline wyoming united states new geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of sheep mountain anticline wyoming united states oxygen carbon and strontium isotope studies were performed on prefolding and fold related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results we show that most of the cements precipitated from paleogene meteoric fluid stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth t 110 c this implies that fluids migrated along the basement thrust underlying the fold and or at the base of the cover above the fault tip the fluids circulated rapidly in the diffuse synfolding and early folding fracture network the zone of preferential migration of the warm fluids is currently located in the backlimb of the fold which supports some of the previously published structural interpretation of the subsurface this study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement involved shortening in laramide style forelands

the interplay of fold mechanisms and basement weaknesses at the transition between laramide basement involved arches north central wyoming usa abstract horizontally shortened basement involved foreland orogens commonly exhibit anastomosing networks of bifurcating basement highs here called arches whose structural culminations are linked by complex transition zones of diversely oriented faults and folds the 3d geometry and kinematics of the southern beartooth arch transition zone of north central wyoming were studied to understand the fold mechanisms and control on basement involved arches data from 1581 slickensided minor faults are consistent with a single regional shortening direction of 065 evidence for oblique slip vertical axis rotations and stress refraction at anomalously oriented folds suggests formation over reactivated pre existing weaknesses restorable cross sections and 3d surfaces constrained by surface well and seismic data document blind ene directed basement thrusting and associated thin skinned backthrusting and folding along the beartooth and oregon basin fault systems between these systems the basement cored rattlesnake mountain backthrust followed basement weaknesses and rotated a basement chip toward the basin before the ene directed line creek fault system broke through and connected the beartooth and oregon basin fault systems slip was transferred at the terminations of the rattlesnake mountain fault block by pivoting to the north and tear faulting to the south in summary unidirectional laramide compression and pre existing basement weaknesses combined with fault propagation and rotational fault bend folding to create an irregular yet continuous basement arch transition

a field guide to finding fossils on mars the martian surface is cold dry exposed to biologically harmful radiation and apparently barren today nevertheless there is clear geological evidence for warmer wetter intervals in the past that could have supported life at or near the surface this evidence has motivated national aeronautics and space administration and european space agency to prioritize the search for any remains or traces of organisms from early mars in forthcoming missions informed by 1 stratigraphic mineralogical and geochemical data collected by previous and current missions 2 earth s fossil record and 3 experimental studies of organic decay and preservation we here consider whether how and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early mars we conclude that noachian hesperian fe bearing clay rich fluvio lacustrine siliciclastic deposits especially where enriched in silica currently represent the most promising and best understood astropaleontological targets siliceous sinters would also be an excellent target but their presence on mars awaits confirmation more work is needed to improve our understanding of fossil preservation in the context of other environments specific to mars particularly within evaporative salts and pore fracture filling subsurface minerals

laramide structures in basement and cover of the beartooth uplift near red lodge montana ramp mechanisms associated with decoupling of a 6 7 km thick basement slab may have been responsible for progressively more and more horizontal components of thrusting of the northeast corner of the beartooth uplift near red lodge montana as part of a nearly right angle corner of the uplift two apparent tear faults bound a 7 km long block of laramide mountain front structures new roadcuts and a deep well through basement refine geometry of range overthrusting and show that these apparent tear faults are really pivoting normal faults that cut frontal thrust structures on either side of an uplifted corner flap a ship s prow analogy of late stage horizontal thrust motion is proposed with the bow wave causing uplift and rotation of the corner flap volumetric adjustments associated with late stage stuffing of basin material beneath frontal thrusts plus deeper duplexing of basement beneath the uplift helped define final details of range geometry a mechanism probably applicable elsewhere in the middle rocky mountains

hybrid carbonates in situ abiotic microbial and skeletal co precipitates abstract abiotic bioinduced and biocontrolled carbonates are process based sediment categories they successively reflect increasing levels of biotic control over carbonate precipitation from aqueous solutions are often closely linked to depositional environment and change with time hybrid carbonates are intimate in situ combinations of two or more of these categories hybrid carbonates are widespread and diverse in the marine geological record and reflect large scale changes in carbonate precipitation through time they also occur in fluvial and lacustrine carbonates marine and non marine reef systems and methane seep deposits plots of hybrid carbonate composition in time and space reveal complex backtracking and looping patterns that reflect changes in environmental conditions and biological processes of carbonate production recognition of hybridity emphasizes the importance of distinguishing abiotic and bioinduced precipitates until they were diversified by skeletal carbonates in the late proterozoic precambrian hybrid carbonates were abiotic bioinduced combinations during the phanerozoic hybrid carbonates were conspicuous during periods of overlap or transition between intervals of microbial and skeletal carbonate abundance microbial skeletal dual hybrids are common during the cambrian mid ordovician late devonian mississippian and late jurassic early cretaceous abiotic microbial skeletal triple hybrids were common from late pennsylvanian to mid triassic shallow marine hybrid carbonates declined in abundance after the mid cretaceous although late cenozoic reefs contain some striking examples of microbial skeletal hybrids recognition of hybrid carbonates draws attention to fundamental processes underlying carbonate sedimentation and their patterns and drivers of change in time and space

sedimentology chemostratigraphy and stromatolites of lower paleoproterozoic carbonates turee creek group western australia the ca 2 45 2 22 ga turee creek group western australia contains carbonate rich horizons that postdate earliest proterozoic iron formations bracket both paleoproterozoic glaciogenic beds and the onset of the great oxidation event goe and predate ca 2 2 2 05 ga lomagundi jatuli c isotopic excursion s as such turee creek carbonate strata provide an opportunity to characterize early paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change here we report on the stratigraphy sedimentology petrology carbon isotope chemostratigraphy and stromatolite development for carbonate rich successions within the pre glacial part of the kungarra formation and the postglacial kazput formation kungarra carbonate units largely occur as laterally discontinuous beds within a thick predominantly siliciclastic shelf deposit while this succession contains thin microbialite horizons most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix c isotopic values show marked variation along a single horizon and even within hand samples reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn and post depositional remineralization of organic matter in contrast the kazput carbonates consist of subtidal stromatolites grainstones and micrites deposited on a mixed carbonate siliciclastic shelf these carbonates exhibit moderate 13c values of 2 to 1 5 and likely preserve a c isotopic signature of seawater kazput carbonates thus provide some of the best available evidence that an interval of unexceptional c isotopic values separates the lomagundi jatuli c isotopic excursion s from the initiation of the goe as inferred from multiple sulfur isotopes loss of mass independent fractionation the kazput formation also contains unusual m scale stromatolitic buildups which are composed of sub mm laminae and discontinuous convex upward lenticular precipitates up to a few mm in maximum thickness laminae interpreted as microbial mat layers contain quartz and clay minerals as well as calcite whereas precipitate lenses consist of interlocking calcite anhedra sometimes showing faint mm scale banding these cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats or as local seafloor precipitates that formed on or within surface mats it is possible that both mechanisms interacted to form the unique kazput stromatolites these microbialites speak to a distinctive interaction between life and environment early in the paleoproterozoic era

the antiquity of oxygenic photosynthesis evidence from stromatolites in sulphate deficient archaean lakes the tumbiana formation about 2700 million years old was largely deposited in ephemeral saline lakes as judged by the unusual evaporite paragenesis of carbonate and halite with no sulfate stromatolites of diverse morphology occur in the lacustrine sediments some with palimpsest fabrics after erect filaments these stromatolites were probably accreted by phototropic microbes that from their habitat in shallow isolated basins with negligible sulfate concentrations almost certainly metabolized by ozygenic photosynthesis

3 43 billion year old stromatolite reef from the pilbara craton of western australia ecosystem scale insights to early life on earth abstract the 3 43 billion year old strelley pool chert pilbara craton western australia contains compelling evidence of early archaean life in the form of kilometre sized remnants of an ancient stromatolitic carbonate platform reviewing and building on earlier studies we examine the fossilized remains of the platform to seek ecosystem scale insights to earth s early biosphere examining the evidence for biosedimentation and the importance and effect of different environmental processes on biological activity both vertical and lateral trends show that stromatolite abundance and diversity are greatest in the area interpreted as an isolated partially restricted peritidal marine carbonate platform or reef where there is virtually no trace of hydrothermal or terrigenous clastic input in contrast stromatolites are poorly developed or absent among hydrothermal volcaniclastic or terrigenous clastic sedimentary facies and are absent in deeper marine settings that are laterally equivalent to shallow marine stromatolitic facies hydrothermal veins some of which were previously interpreted as vents that exhaled fluids from which the stromatolitic structures precipitated are shown to postdate the stromatolites on the platform stromatolite facies associations varied between different palaeoenvironments but some stromatolite types occurred across different palaeoenvironments highlighting the combined influence of biological and environmental processes on stromatolite formation the regional distribution of stromatolites in the palaeoenvironment suggests a biological response to variations in water depth sediment influx and hydrothermal activity with stromatolite formation favoured by relatively normal shallow marine environments with low clastic chemical sedimentation rates and no direct input from high temperature hydrothermal systems the lithology structure and fabrics of the stromatolites and their close association with abundant evaporite crystal pseudomorphs indicate that evaporitic precipitation was probably the dominant non biological process that contributed to stromatolite formation the study supports a biological interpretation for the origin of the stromatolites and reveals compelling evidence for the conditions that favoured biological activity on the early earth and formation of macroscopic biosignatures that could be preserved for most of earth s history

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

increased mud deposition reduces stromatolite complexity microbial mat growth can produce diagnostic delicate structures in low energy environments but mat morphology and thus preservation of delicate structures as morphological biosignatures is affected by mud accumulation in lake joyce antarctica varying rates of siliciclastic mud deposition affected the morphology of partially calcified microbial mats these mats were characterized along a shore parallel transect from higher to lower sedimentation regions of the lake mats with high mud accumulation rates were predominantly flat with sparse pinnacles at lower sedimentation rates mats developed more pinnacles and more delicate features including vertical webs of biofilm suspended between pinnacles morphological transitions suggest that mud deposition disrupts growth of delicate microbial structures like biofilm webs and inhibits pinnacle growth at higher accumulation rates the simplification of mat morphology with increasing mud accumulation has important implications for the distribution of stromatolite types in the rock record stromatolites with intricate biofilm structures are rare possibly due to the paucity of appropriate environments rather than the intrinsic properties of microbial communities communities capable of forming delicate microbialites likely produced morphologically simpler pinnacles and flat mat in environments with significant siliciclastic mud or micrite influx thus delicate mats have narrow environmental ranges for growth and temporal or spatial variations in siliciclastic mud or micrite sedimentation rates bias the record of microbial activity preserved in stromatolites

kinematic model of detachment folding including limb rotation fixed hinges and layer parallel strain abstract detachment anticlines formed above a stratigraphically fixed detachment horizon and without thinning in the syncline will in general require layer parallel shortening a kinematic model for detachment folds is presented that has planar limbs and is area balanced but requires bed length changes that can be expressed as layer parallel strain the fold grows primarily by limb rotation not by limb lengthening at constant dip the kinematic evolution of the geometry and strain is examined fold geometry is well predicted and the strain estimated in applications of the model to two map scale detachment folds a triangle zone from the front of the wyoming thrust belt and a duplex cored anticline from the appalachian plateau

neoarchean 2 7 ga lacustrine stromatolite deposits in the hartbeesfontein basin ventersdorp supergroup south africa implications for oxygen oases abstract the hartbeesfontein basin contains the most extensive deposits of archean lacustrine stromatolites on the kaapvaal craton with stromatolitic facies occurring over 100 km 2 in beds up to 7 m thick stromatolitic dolostones and cherts both preserve evidence of microbial processes dolomitic stromatolites have grumelous microspar textures between organic rich laminae suggestive of carbonate precipitation within microbial mats stromatolitic laminae within chert preserve detrital material beyond the angle of repose indicating the trapping and binding of grains by microbial mats stromatolitic cherts also preserve fenestral textures and filamentous microfossils many fenestrae have rounded shapes surrounded by filamentous laminae and appear to have formed in situ within stromatolite fabrics before lithification fenestrae within stromatolitic chert resemble hourglass associated fenestrae noted from recent silica stromatolites from yellowstone national park and are interpreted to originate from gas bubbles forming within stromatolite building mats the preservation of delicate structures in hartbeesfontein stromatolitic chert e g filamentous microfossils and gas related fenestrae implies rapid lithification of microbial mats while the mm to cm scale of fenestrae indicate equally rapid rates of microbial gas production textural and mineralogical evidence associated with gas related fenestrae support the presence of oxygenic photosynthesis which in turn strengthens previous hypotheses on archean lakes as potential oxygen oases before the great oxidation event

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

new study of microbial mats from the mesoproterozoic jixian group north china evidence for photosynthesis and oxygen release abstract microbial mats were well established and widespread during the proterozoic eon playing a pivotal role in atmospheric oxygen content in this study microbial mats were systematically investigated from the 1600 ma to 1400 ma jixian group north china three types of microbial mats were recognized including microbial mat dominated by filaments mmf microbial mat dominated by coccoids mmc microbial mat dominated by filaments and coccoids mmfc these microbial mats were mainly built by siphonophycus kestron siphonophycus solidum myxococcoides sp and coniunctiophycus majorinum due to in situ preservation five sedimentary structures related to photosynthesis and oxygen release were discovered in these microbial mats the alternating light dark laminae and bottleneck structure may be caused by immediate mineralization and phototropism of filaments in a hypersaline environment bubble structure fenestrae structure and hourglass structure probably were related to oxygen release and their emergence fill the gap between oxygenic photosynthesis of microbial mat and atmospheric oxygen content in proterozoic eon

transition of microbiological and sedimentological features associated with the geochemical gradient in a travertine mound in northern sumatra indonesia abstract modern travertines carbonate deposits in ca rich hydrothermal water with high pco2 often display a changing environment along the water path with corresponding variability in the microbial communities we investigated a travertine bearing hot spring at the blue pool in northern sumatra indonesia the thermal water of 62 c with high h2s 200 m and pco2 1 atm developed a travertine mound 70 m wide the concentrations of the gas components h2s and co2 decrease immediately after the water is discharged while the dissolved oxygen ph and aragonite saturation increase in the downstream direction responding to the geochemical gradient in the water the surface biofilms change color from white to pink light green dark green and brown as the water flows from the vent this corresponds to microbial communities characterized by chemolithoautotrophs halothiobacillaceae purple sulfur bacteria chromatiaceae anaerolineaceae and co occurrence of green non sulfur bacteria chloroflexales cyanobacteria and green sulfur bacteria chlorobiales respectively in an environment with a certain level of h2s 1 m sulfur digestion and anoxygenic photosynthesis can be more profitable than oxygenic photosynthesis by cyanobacteria the precipitated carbonate mineral consists of aragonite and calcite with the proportion of aragonite increasing downstream due to the larger mg2 ca2 ratio in the water or the development of thicker biofilm where the biofilm is well developed the aragonite travertines often exhibit laminated structures that were likely associated with the daily metabolism of these bacteria the microbiological and sedimentological features at the blue pool may be the modern analogs of geomicrobiological products in the early earth biofilm of anoxygenic photosynthetic bacteria had the potential to form ancient stromatolites that existed before the appearance of cyanobacteria

basement balancing of rocky mountain foreland uplifts restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts local zones of shortening and extension occur in hinge zones above transitions in fault curvature high curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover 20 references 4 figures

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

rocky mountain foreland uplifts products of a rotating stress field or strain partitioning stress tensor analysis of minor faults near the structural range fronts of diversely oriented laramide foreland uplifts in the central rocky mountains wyoming indicates that paleo 1 was horizontal and oriented nearly perpendicular to uplift trends trend directions for 1 vary from east northeast in the north trending laramie range to northeast in the northwest trending bighorn mountains to north at east trending casper mountain and in the east trending central owl creek mountains although these compression directions are compatible with models invoking temporally changing stress directions during the laramide orogeny the apparent similarity in timing of uplift in these ranges suggests that the computed stress tensors may not reflect regional stress conditions recent stress determinations in transpressional regimes suggest that strain partitioning may be an important factor in considering local stress patterns in the vicinity of mountain belt structures oblique to regional trends in light of these studies caution should be used in interpreting regional paleostress from structures measured close to the tectonic fronts of laramide uplifts

microfossil assemblage from the 3400ma strelley pool formation in the pilbara craton western australia results form a new locality abstract a new microfossil locality for the ca 3400 ma strelley pool formation spf in the pilbara craton western australia was discovered in the panorama greenstone belt large and morphologically diverse microfossils 20 m up to 100 m along the major dimension are abundant in a carbonaceous laminated black chert of shallow water origin that was originally collected in 2010 and relocated in 2011 just south of the trendall reserve state geoheritage reserve r50149 over 300 small spherical and lenticular flanged and non flanged lenses carbonaceous cellular microfossils are present in each replicated thin section measuring 2 5 cm 3 4 cm in addition film like structures and tubular filaments were discovered narrow carbonaceous filaments of several types are also present in and around the secondary cavity fill cherts although their biogenicity is uncertain colonies of spherical and lenticular structures are often observed colonies composed of small spheroids can be classified into at least two groups based on the sizes of the individuals their mode of clustering is variable ranging from tight to loose and from spherical to random colonies of lenticular structures are often composed of a mixture of flanged and non flanged lenses in addition to those solely composed of either flanged or non flanged lenses the size ranges of these three types are not statistically distinguishable therefore it is currently unknown whether flanged and non flanged lenses represent different species or different life stages of the same species although more research will be needed to establish the taxonomic framework for these ancient microfossils they provide evidence that the coastal region of the 3 4 ga strelley pool sea was inhabited by various microbial communities reflecting the early stage diversification of life on earth

stromatolites of the late archean back river stratovolcano slave structural province northwest territories canada nine stromatolite localities in the back river volcanic complex occur at the boundary between 2692 ma felsic dome flow complexes marking the latest eruptions of this stratovolcano and overlying turbiditic sedimentary rocks of the beechy lake group yellowknife supergroup stromatolites form lenses isolated within coarse volcanic breccia at margins of felsic dome flow complexes and 2 m thick bioherms that extend laterally for hundreds of metres thin units contain wavy laminae and open spaced linked mounds which form thin encrustations on breccia blocks or clusters of mounds with low synoptic relief thick successions comprise undulatory flat laminated dolomite that contains wrinkled wavy laminae pseudocolumnar forms and locally elongate low relief mounds these units typically contain millimetre scale layers of fine volcanic ash at regular intervals testifying periodic explosive eruptions during deposition of microbial mats the stromatolites which are identified by gross morphology and distin

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

the association of tourmalinite with stratiform scheelite deposits three types of stratiform scheelite deposits are recognised they all commonly have a close spatial association with tourmalinite regional prograde calc silicate rocks in precambrian rift settings commonly contain 100 ppm wo3 with enrichment to 0 5 1 wo3 in retrograde metamorphic assemblages at fold hinges joints faults shears and granite and pegmatite dyke contacts the calc silicate progenitor was probably a impure dolomitic carbonate sediment amphibolite hosted scheelite deposits occur in metamorphosed altered mafic volcanics formed in a narrow rift graben scheelite occurs both as stratiform and stratabound footwall stringer ore which has undergone minor remobilization during tectonism tourmalinites are rarely the host for stratabound quartz scheelite wolframite veins remobilized from the host tourmalinite and associated metavolcanics during tectonism

the meaning of stromatolites stromatolites document microbial interactions with sediments and flowing water throughout recorded earth history and have the potential to illuminate the long term history of life and environments modern stromatolites however provide analogs to only a small subset of the structures preserved in archean and proterozoic carbonates thus interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian scale dependent models of microbial responses to nutrient availability seawater chemistry influx of sediment grains shear and burial models that integrate stromatolite scales macroscopic organization and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity an improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms tha

structure timing and kinematics of the early eocene south fork slide northwest wyoming usa abstractthe south fork slide sfs is exposed over an area of 800 km2 in northwest wyoming and its evolution is connected in space and time to the adjacent and overlying 48 87 ma heart mountain slide hms the youngest rocks deformed as part of the sfs are the eocene willwood formation sands and mudstones which have a u pb detrital zircon youngest depositional age tuffzirc calculation of 51 80 1 04 1 08 ma as well as a spectrum of older zircons that were eroded from the sevier highlands to the west and the archean beartooth uplift to the north n 379 u pb zircon ages the sfs is overlain by allochthonous paleozoic carbonate and eocene volcanic rocks of the hms the sfs deformation involves shallowly plunging thin skinned folds that trend northeast southwest where the slip surface could have been the top of the jurassic gypsum springs formation detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin skinned belt as well as the fac

microbiological processes in banded iron formation deposition banded iron formations have been studied for decades particularly regarding their potential as archives of the precambrian environment in spite of this effort the mechanism of their deposition and specifically the role that microbes played in the precipitation of banded iron formation minerals remains unresolved evidence of an anoxic earth with only localized oxic areas until the great oxidation event ca 2 45 to 2 32 ga makes the investigation of o2 independent mechanisms for banded iron formation deposition relevant recent studies have explored the long standing proposition that archean banded iron formations may have been formed and diagenetically modified by anaerobic microbial metabolisms these efforts encompass a wide array of approaches including isotope ecophysiological and phylogeny studies molecular and mineral marker analysis and sedimentological reconstructions herein the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post depositional alteration are described the main findings of recent years are summarized and compared here and suggestions are made regarding cross disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition

multistage multidirectional tertiary shortening and compression in north central new mexico the nature of laramide deformation in the southern rocky mountains remains highly debated advocates of north south strike slip faulting favor transpression and northward displacement of the colorado plateau whereas advocates of east west shortening by thrust faulting contest evidence for large right lateral displacements minor faults n 2552 were measured to determine the laramide to holocene structural evolution of north central new mexico multimodal slickenline and ideal 1 orientations as well as consistent crosscutting relationships indicate multiphase multidirectional faulting the oldest set of thrust and strike slip faults indicates east northeast to east trending shortening and compression the faults cut rocks as young as the upper paleocene lower eocene diamond tail formation strike slip faults from a second phase of northeast to north northeast trending shortening and compression cut the eocene galisteo formation but do not cut the 27 ma galisteo dike elsewhere later north striking strike slip faults cut 24 ma igneous units indicating north northeast to north trending shortening and compression of mid tertiary age subsequent north striking normal faults related to rio grande rifting commonly reactivate the mid tertiary strike slip fault planes these results show the validity of both strike slip and thrust hypotheses in north central new mexico although neither is adequate in exclusion of the other early laramide east west thrusting probably formed the north trending laramide arches of the region later counterclockwise rotation of regional shortening and compression directions may have caused transpression opening some late laramide axial basins and causing limited northward displacement of the colorado plateau subsequent mid tertiary strike slip deformation may be a missing link between paleogene laramide shortening and neogene rio grande rifting