relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

diagenesis and formation water chemistry of triassic reservoir sandstones from southern tunisia the fluvial triassic reservoir subarkoses and arkoses 2409 5 2519 45 m of the el borma oilfield southern tunisia were subjected to cementation by haematite anatase infiltrated clays kaolinite and k feldspar at shallow burial depths from meteoric waters subsequently basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths magnesian siderite minor ferroan magnesite and anhydrite the enrichment of siderite in 12c isotope 13cpdb 14 5 to 9 results from derivation of carbon from the thermal decarboxylation of organic matter during further burial the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred followed by the formation of microcrystalline k feldspar and quartz chlorite and illite prior to the emplacement of oil present day formation waters are na ca cl brines evolved by the evaporation of seawater and water mineral interaction and are in equilibrium with the deep burial 3 1 km minerals these waters are suggested to be derived from the underlying silurian and devonian dolomitic mudstones

eolian topography as a control on deposition incorporating lessons from modern dune seas permian cedar mesa sandstone se utah u s a abstract the dominant current model for the accumulation of eolian strata in the rock record emphasizes translation and climb of bedforms and the preservation of continuous beds studies of modern dune fields however show that when large dunes form they create a topographically irregular surface later episodes of eolian activity commonly fill the low lying areas between the older dunes resulting in beds that are laterally discontinuous strata in part of the permian cedar mesa sandstone in southeastern utah require this model in this new model inherited eolian topography influences subsequent deposition detailed gps mapping of an 8 km2 area was used to trace eolian strata through a 20 25 m thick unit in the cedar mesa sandstone the unit is bounded above and below by subparallel erosion surfaces that have up to 5 m of relief within the unit large transverse draa or megadune deposits are spaced 1 km apart locally over 0 5 km wide areas the draa deposits fill the entire 25 m thickness of the unit only to be completely replaced by interdraa deposits a few hundred meters away the draa surfaces were eroded and exposed prior to the filling of the intervening lows with alternations of barchanoid dune deposits sand sheets and ponds which formed in the topographic lows the cedar mesa sandstone illustrates aggradation wherein the important factor is the preserved eolian topography the draas formed an undulating stable surface from which subsequent deposition filled the interdraa low large eolian dunes that influenced subsequent deposition are also observed in several pleistocene to recent dune seas this mechanism offers an alternative to the existing models that require continuous migration of large dunes

late archaean deep marine volcaniclastic sedimentation in an arc related basin the kalgoorlie sequence of the eastern goldfields superterrane yilgarn craton western australia abstract the kalgoorlie sequence 2658 ma is a 3000 m thick succession of volcaniclastic sedimentary rocks and subordinate rhyolite dacite andesite and basalt spatially and temporally associated with subvolcanic sills and dykes of rhyolite and dacite gabbro sills and i type granitoids although basalts are tholeiitic the felsic rocks have ttd affinity the sequence comprises mostly deep marine channel levee turbidite complexes and hemipelagites resedimented syneruptive volcaniclastic deposits are associated with coherent lavas but almost all of the sedimentary fill was recycled from prior cycle volcanic and volcaniclastic sources provenance was mainly dacitic rhyolitic with minor andesitic basaltic and polymictic compositions volcano bound and fault bound architectures and two styles of basin filling are identified progradational or aggradational transverse fans and slope aprons and retrogradational longitudinal fans of a linear trough retrogradation in longitudinal systems is consistent with basin lengthening and deepening six depositional sequences are grouped in four unconformity bounded tectonic stages that record uplift subsidence cycles coeval mafic and felsic magmatism indicate that the tectonic stages represent extensional tectonic settings whereas volcanogenic provenance spatially linked volcanism and plutonic magmatism and deep marine environments support an intra arc site cyclic uplift subsidence and basin lengthening are more consistent with strike slip controls on basin formation than with orthogonal extension the tectonic setting is interpreted to have been a pull apart basin associated with strike slip faulting and amalgamating arc related terranes similar to the modern philippines archipelago the strong similarity between archaean volcaniclastic and modern arc related volcaniclastic environments implies comparable tectonic settings and therefore comparable controlling tectonic factors

the beginning of the buntsandstein cycle early middle triassic in the catalan ranges ne spain sedimentary and palaeogeographic implications abstract the early middle triassic siliciclastic deposits of the catalan ranges ne spain are dominated by aeolian sediments indicating a predominance of arid climate during this time span in sharp contrast with the coeval fluvial sediments found in the castilian branch of the iberian ranges 300 km to the sw the ne sw oriented catalan basin evolved during the middle late permian as the result of widespread extension in the iberian plate this rift basin was bounded by the pyrenees ebro and montalban oropesa highs the permian early triassic age sediments of the catalan basin were deposited in three isolated subbasins montseny garraf prades separated by intrabasinal highs but linked by transversal nw se oriented faults the three subbasins show evidence of diachronic evolution with different subsidence rates and differences in their sedimentary records the buntsandstein sedimentary cycle started in the late early triassic smithian spathian in the central and southern domains garraf and prades with conglomerates of alluvial fan origin followed by fluvial and aeolian sandstones source area of the fluvial sediments was nearby paleozoic highs to the north and west in contrast with the far away source areas of the fluvial sediments in the iberian ranges to the sw these fluvial systems were interacting with migrating aeolian dune fields located towards the s which developed in the shadow areas behind the barriers formed by the paleozoic highs these highs were separating the subbasins under arid and semi arid climate conditions the dominating winds came from the east where the westernmost coast of the tethys sea was located and periods of water run off and fields of aeolian dunes development alternated some of the fluvial systems were probably evaporating as they were mixed into the interdune areas never reaching the sea from the end of the smithian to the spathian the catalan basin and neighbour peri tethys basins of the present day southern france sardinia and minorca islands constituted a geographical arch where arid and semi arid conditions represented an extension of the prevailed arid and hyper arid conditions in surrounding areas of the variscan belt harsh climatic conditions in this area prevented the life recovery in the aftermath of the permian triassic extinction event until the early anisian when more humid climate allowed for the colonisation of the area by plants amphibians and reptiles the boundary between desert areas and semi arid and or seasonal climate domains during the smithian spathian in sw europe can be precisely established in ne iberia between the catalan ebro region and the castilian branch of the iberian ranges to the sw

formation and paleoenvironment of rhizoliths of shiyang river basin tengeri desert nw china abstract rhizoliths found in bed of the late quaternary paleolake zhuyezhe minqin basin central tengeri desert nw china were studied vegetation coverage is present at some locations in the paleolake area although much of the bed area is covered with moving dunes rhizoliths are occasionally eroded out and exposed to the air the morphology of the rhizoliths resembles singular or branching carbonated tubules which are hollow in their central part the rhizoliths are black to grey and are broken and scattered randomly on the sand surface of the lake bed the lacustrine deposits are black greenish silt or silty clay with a large quantity of white petite lake snail shells the possible plant types sources and formation conditions of carbon dioxide and calcium the sedimentary and diagenetic environments and the process of rhizolith formation were discussed via examining rhizoliths macromorphology studying the micromorphology and mineralogy by microscopy cathodoluminescence and scanning electron microscope and studying the chemical compositions of cementing minerals and fragments by energy dispersive x ray spectra the original roots of the rhizoliths belong to hydrophytes such as typha latifolia scirpus maritimus and carex stenophylla lake snails and previous pollen data indicate that the rhizoliths formed in a sedimentary environment of shallow fresh water lake like marginal or palustrine areas during the holocene shallow lake water disturbance by desert wind above the loose sandy sediment or soil was favoring the rhizoliths formation a continuous supply of oxygen through water disturbance led to complete oxidation of roots and producing carbon dioxide dissolution of co2 in water and so carbonic acid production resulted in minerals weathering such as feldspar and primary carbonate particles and the release of k and ca2 ions at presence of co2 and ca2 saturated in the pore water around the roots of hydrophytes within the sediments or soil carbonate precipitation occurred around the root channel and led to rhizoliths formation as tubules the petrifaction process therefore has happened in margin of the paleolake with shallow water and weak redox condition which implies the suitability of rhizolith for reconstructing paleoenvironment

paleoenvironmental implications of early diagenetic siderites of the paraiba do sul deltaic complex eastern brazil abstract abundant early diagenetic siderites occur as spherulites and rhombohedral microcrystalline and macrocrystalline crystals in the cores of the 2 mu 1 rj well drilled in the paraiba do sul deltaic complex rio de janeiro brazil the host sediments of the siderites are siliciclastic hybrid and carbonate deposits intense pedogenetic processes affected the siliciclastic sediments immediately after deposition comprising clay illuviation plants bioturbation feldspar dissolution and iron oxide hydroxide precipitation siderite and pyrite are the main diagenetic constituents the other diagenetic products are kaolinite smectite argillaceous and carbonate pseudomatrix quartz overgrowths diagenetic titanium minerals jarosite and iron oxides hydroxides early diagenetic siderites were separated into four groups based on their elemental and stable isotopic composition as well as on their paragenetic relationships with the other constituents and with the host sediments spherulitic to macrocrystalline siderites from group 1 are almost pure average 94 7 mol feco3 1 2 mol mgco3 2 3 mol caco3 1 8 mol mnco3 and precipitated from meteoric porewaters in continental siliciclastic rocks under suboxic conditions 18ovpdb values range in 10 28 to 5 57 and the 13cvpdb values in 12 68 to 4 33 microcrystalline rhombohedral siderites from group 2 have zonation due to substantial ca and mg substitution core average 78 5 mol feco3 4 2 mol mgco3 15 7 mol caco3 1 6 mol mnco3 edge average 74 0 mol feco3 9 2 mol mgco3 15 6 mol caco3 1 1 mol mnco3 and 13cvpdb and 18ovpdb values of 0 17 and 1 96 precipitated from marine porewaters in packstones wackestones under methanogenic conditions the group 3 is represented by irregular spherulitic siderites with moderate ca and mg substitutions average 80 2 mol feco3 7 9 mol mgco3 11 3 mol caco3 0 6 mol mnco3 with 18ovpdb values ranging from 5 96 to 7 61 and 13cvpdb values ranging from 5 15 to 10 41 the group 4 microcrystalline siderites are magnesium rich average 57 3 mol feco3 31 4 mol mgco3 9 6 mol caco3 1 7 mol mnco3 13cvpdb 1 43 and 18ovpdb 14 09 the group 3 and 4 siderites were formed from brackish porewater under suboxic conditions in hybrid and siliciclastic rocks these variations in siderites are probably related to the paraiba do sul river dynamics to sea level changes and to climatic variations that took place during the quaternary

formation of siderite mg calcite iron sulphide concretions in intertidal marsh and sandflat sediments north norfolk england concretions cemented mainly by siderite mg calcite and iron monosulphide are common in late holocene marsh and sandflat sediments on parts of the north norfolk coast field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate reducing bacteria are active 13c values ranging from 3 to 11 8 mean 5 9 0 suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter 18o values ranged from 6 4 to 0 8 mean 1 0 suggesting that carbonate precipitated in porewaters ranging from pure sea water to sea water diluted with meteoric water chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation some concretions have formed around fragments of wood or metal but others contain no apparent nucleus in field experiments siderite fes and mg calcite were precipitated around several different nuclei within a period of six months we suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron

calcrete in quaternary coastal dunes in southwestern australia a capillary rise phenomenon associated with plants abstract non pedogenic calcrete is forming as a thin sheet 10 50 cm thick just above the water table in the subsurface of a barrier dune terrain in south western australia the calcrete is forming in response to the regional evapo transpirative regime the annual rainfall is high but of short duration and it infiltrates rapidly into the sandy substrates relatively high air temperatures and evaporation coupled with wind stress combine to produce a water vapor deficit across the dune terrain the vegetation responds by drawing upon phreatic water and pellicular water of the vadose zone thereby precipitating caco3 where plants are not drawing phreatic water calcrete is developing by evaporation of water at the top of the zone of capillary rise various structure types of calcrete are developed in the study area namely rhizoconcretionary mottled massive and laminar forms these occur in a definite stratigraphic sequence related to maturity of profile vegetation cover and thickness of the vadose zone rhizoconcretionary calcrete forms in the vadose zone mottled and massive calcrete frequently capped by laminar calcrete develop in the zone of capillary rise these latter calcrete types form a sheet that is ubiquitous and parallel to the water table since it is a feature of the water table the calcrete sheet may potentially transect regional stratigraphic boundaries and unconformities at the local level the various structural types of calcrete may he used as paleowater table indicators

jurassic aeolian oolite on a palaeohigh in the sundance sea bighorn basin wyoming aeolian limestones are widespread in the quaternary record and have been identified in outcrops and cores of late palaeozoic strata these rocks have been interpreted as a low latitude signal of glacio eustatic sea level fluctuations and have not been previously reported from the mesozoic or from other episodes of earth history generally believed to have been non glacial numerous lenticular bodies of cross stratified oolite lie near the contact between the lower and upper members of the mudstone dominated lower sundance formation middle and upper jurassic in the bighorn basin of north central wyoming usa the lenses up to 12 m thick contain sedimentary structures diagnostic of aeolian deposition inversely graded laminae within thick sets of cross strata were deposited by climbing wind ripples adhesion structures and evenly dispersed lag granules are present in flat bedded strata at the bases of several of the oolite bodies thin sections reveal abundant intergranular micrite of vadose origin the lenses appear to represent virtually intact isolated aeolian bedforms that migrated across a nearly sand free deflation surface when the sundance sea transgressed the dunes a thin 1 m thick wave rippled oolite veneer formed on the upper surface of the aeolianite previous workers primarily on the basis of sedimentary structures in the veneer interpreted the oolite lenses as tidal sand bodies the dunes provide clear evidence of widespread subaerial exposure on the crest and north flank of the sheridan arch this structural high was delineated by previous workers who demonstrated thinning of pre upper sundance formation strata and localized development of ooid shoals ooids that formed in shoals on the windward southern side of the palaeohigh were exposed and deflated during lowstand thin scour filling ooid grainstone lenses that crop out in the southern part of the study area represent remnants of the marine beds that sourced the aeolianites farther north down wind oolitic dunes prograded over thinly laminated lagoonal silts when relative sea level began to rise the uncemented dunes were buried under fine grained marine sediment as the lee side of a low relief island was inundated

the rift to break up evolution of the gulf of aden insights from 3d numerical lithospheric scale modelling abstract the gulf of aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore recent surveys showed that the spatio temporal evolution of the gulf of aden rift system is dominated by three fault orientations displacement orthogonal wsw rift parallel wnw and an intermediate e w trend the oldest parts of the rift that are exposed onshore feature displacement orthogonal and intermediate directions whereas the subsequently active necking zone involves mainly rift parallel faults the final rift phase recorded at the distal margin is characterised by displacement orthogonal and intermediate fault orientations we investigate the evolution of the gulf of aden from rift initiation to break up by means of 3d numerical experiments on lithospheric scale we apply the finite element model slim3d which includes realistic elasto visco plastic rheology and a free surface despite recent advances 3d numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved we address this issue by proposing a simple post processing method that uses the surface stress tensor to evaluate stress regime extensional strike slip compressional and preferred fault azimuth the described method is applicable to any geodynamic model and easy to introduce our model reproduces the observed fault pattern of the gulf of aden and illustrates how multiple fault directions arise from the interaction of local and far field tectonic stresses in an evolving rift system the numerical simulations robustly feature intermediate faults during the initial rift phase followed by rift parallel normal faulting at the rift flanks and strike slip faults in the central part of the rift system upon break up displacement orthogonal as well as intermediate faults occur this study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new insights in the timing of fault successions of the gulf of aden and continental rifts in general

sideritic ironstones as indicators of depositional environments in the weald basin early cretaceous se england the lower cretaceous wealden sideritic ironstones have a wide occurrence and great potential to aid the reconstruction of the depositional environments of the weald basin in se england however mineralogical and geochemical datasets on the ironstones are scarce in the literature geochemical and mineralogical data on the sideritic ironstones are presented from the wadhurst clay formation within the weald basin the mineralogy of the ironstones was examined using a panalytical x pert pro x ray diffractometer and panalytical s highscore plus software elemental composition of the ironstones was measured using a panalytical minipal2 ed xrf benchtop x ray spectrometer the examination of the mineralogy of the wealden ironstones confirms the presence of early diagenetic siderites the trace element assemblage shows that the sideritic ironstones are chemically pure pointing to a freshwater origin the sideritic ironstones reveal anoxic conditions and palaeo salinity in the basin more generally it is suggested that the composition of the host rocks has significant controls on the composition of sideritic ironstones in sedimentary basins this work reinforces the importance of the composition of sideritic ironstones as useful non traditional data for understanding the depositional settings of sedimentary basins especially when traditional datasets are not readily available or insufficient

high resolution ultrasonic measurements as proxies to resolve clastic reservoir heterogeneity in a salt cemented gas reservoir petrophysical properties and their heterogeneity within sandstone bodies are key parameters in the evaluation of hydrocarbon and geothermal reservoirs however common tools applied to constrain the porosity distribution pattern in borehole cores are commonly time consuming and destructive or suffer from a resolution limited to the meter to decimeter scale we examine the applicability of nondestructive high resolution sonic hrs logging of well core sections in the ultrasonic frequency range as a method providing porosity proxy data at the centimeter scale in a clastic sedimentary sequence the middle solling sandstone member a gas bearing reservoir offshore from the netherlands is used as a test sample it consists almost entirely of clean cross bedded to massive eolian dune and dry sand flat deposits that are salt plugged to variable degrees plots of hrs logging data versus core plug porosity values show a positive linear relationship that develops the highest coefficient of determination r2 0 86 in structureless eolian dune sands most probably caused by the lack of bedding related anisotropies there once calibrated for a particular facies type this correlation enables the calculation of porosity proxy data from sonic transit time values acquired at centimetric steps high resolution sonic logging thus contributes a reliable and time efficient highly spatially resolving quantification of reservoir heterogeneities at centimeter scale and turns out to be a suitable tool for the nondestructive in situ detection of high porosity zones in otherwise uniform sandstone successions moreover plots of closely spaced hrs logging derived porosity proxy data significantly improve interpolation between single wider spaced core plug porosity data points

siderite concretions indicators of early diagenesis in the gammon shale cretaceous the upper cretaceous gammon shale in the northern great plains contains abundant siderite concretions that formed during the early compactional history of the fine grained marine sediments the relative depth of siderite precipitation can be estimated on the basis of textural mineralogical and isotopic evidence siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate 75 85 preserve uncompacted sediment textures and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater delta 18 o nearly equal e 0 to 2 per thousand pdb in contrast lower volume percent carbonate values 55 75 and relative decreases in 18 o 16 o values characterize concretions that formed longer after deposition and or at greater depths in the accumulating sediments both volume percent carbonate and delta 18 o values are strongly correlated with independently determined sediment accumulation rates concretions in rapidly deposited sediments formed at shallow depths 200 m later in the burial history the gammon served as both source rock and reservoir for accumulations of biogenic methane and like the siderite concretions the methane was generated during early diagenesis both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen because of chemical requirements siderite did not precipitate until after the onset of methane generation therefore if the depth of siderite formation is known the minimum depth for the onset of methane accumulation can be inferred the presence of abundant pyrite in slowly accumulated gammon sediments suggests by analogy to data from recent sediments that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate in slowly accumulating sediments most organic matter was probably consumed by aerobic organisms and sulfate reducing anaerobic bacteria whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation

extension of berner s new geochemical classification of sedimentary environments to ancient sediments using stable isotopes of carbon and oxygen in siderite nodules and lithology of host shales one can recognize geochemical environments proposed by berner in devonian carboniferous sediments of the appalachian basin his oxic post oxic and sulfidic diagenetic environments are well represented sediments of the methanic environment may be present but the evidence is equivocal the sulfidic sediments can be further subdivided into well laminated and bioturbated corresponding to the presence of h 2 s in the overlying water in the first case but confined to the sediment pore waters in the second journal abstract

giant desiccation fissures filled with calcareous eolian sand hermosa formation pennsylvanian southeastern utah abstract at two stratigraphic intervals within the upper member of the upper pennsylvanian hermosa formation calcareous eolian sand fills downward tapering fissures that are as much as 18 cm wide and 5 7 m deep fissure fillings define orthogonal polygons 10 m or more in diameter one of the host beds is primarily composed of subtidally deposited limestone the other is a thinly laminated nonmarine red siltstone both systems of fissure fillings are directly overlain and underlain by large scale cross stratified calcareous eolianites the limestone host bed contains chert pseudomorphs after gypsum compaction of host rocks contorted fissure fillings and caused doming of eolian strata over each fissure platy mineral grains in fissure fillings are aligned subparallel to bedding in the host rocks supporting the view that the fissures were passively filled rather than forcefully injected these ancient fissure systems are similar in scale and pattern to those that define giant desiccation polygons in numerous great basin playas the pennsylvanian fissures like their holocene counterparts probably formed when groundwater tables dropped from shallow levels within fine grained impermeable deposits into underlying aquifers greatly decreasing the extent of the capillary fringe our study of the fissures and host rocks supports the hypothesis that carbonate grains within the eolianites were deflated from uncemented marine sediments that were broadly exposed during regressive intervals

age constraints on recycled crustal and supracrustal sources of archaean metasedimentary sequences eastern goldfields province western australia evidence from shrimp zircon dating abstract shrimp u pb zircon dating of sedimentary volcanic and subvolcanic intrusive rocks from unconformity bounded sequences in the eastern goldfields province defines maximum depositional ages for those sequences the kambalda sequence which records a backarc basin was deposited prior to 2700 ma and from at least 2715 ma the spargoville sequence which records an arc adjacent volcano bound basin was deposited between 2700 and 2683 ma the kalgoorlie sequence which records an intra arc rift basin comprises two tectonic stages with depositional ages of 2681 2670 and 2661 2655 ma deposition of the spargoville and kalgoorlie sequences overlapped with the intrusion of granitoids and subvolcanic porphyries thereby indicating that those granitoids were emplaced during crustal extension younger submarine fan kurrawang sequence and braided fluvial merougil and jones creek sequences deposits of a remnant ocean basin were deposited at when coupled to palaeocurrent data provenance dating of detrital zircon suites fingerprints southeast oriented amalgamation of felsic detritus from multiple sources within longitudinal depositional systems and longitudinal reworking of felsic detritus from single sourced transverse systems most felsic detritus was recycled from coeval volcanoplutonic arcs and the orogen to the remnant ocean basin sequences with zircon ages ranging from 2730 to 2660 ma older felsic sources with age ranges of 3570 3130 3030 2900 2870 2770 and 2760 ma could have been structural enclaves within the volcanoplutonic arcs or orogen but were more likely within a distant cratonic basement when coupled with stratigraphic data it is concluded that detrital zircon age distributions preserve evidence for the sedimentary recycling of coeval volcanoplutonic arc or orogenic sources uplifted basin floor and dissected arc successions and cratonic basement virtually all source terrains have apparently vanished presumably through the combination of tectonic and sedimentary recycling and tectonic severance increased use of detrital zircon suites from archaean sedimentary sequences is advocated in order to quantify the balance between sedimentary recycling of felsic rocks from contemporaneous volcanoplutonic sources which themselves document crustal growth in arc environments and from relict volcanoplutonic and crustal basement sources which document the degree of crustal recycling

megarhizoliths in pleistocene aeolian deposits from gran canaria spain ichnological and palaeoenvironmental significance the pleistocene dune field of tufia located on the east of gran canaria spain contains different stratigraphic levels of indurated pillar like structures that are interpreted as megarhizoliths the megarhizoliths occur at the top of different aeolian sets and reach 31 5 cm in diameter and over 1 m in height these scattered free standing vertical cylindrical to slightly conical columns usually appear as hollow cylinders displaying elliptical cross sections aligned with the prevailing wind on the leeward side of some specimens the external wall shows a tail of rock matrix resembling a sort of wind shadow these tails and other remains of the associated rock matrix show a texture composed of long horizontal parallel cylinders orientated with the wind internally the most complete structures show five concentrically arranged zones zone a is a central pore corresponding to the cavity originally occupied by the root zones b and c which include alveolar and laminated carbonate textures indicating that carbonate precipitation was mostly induced by the roots and their associated microorganisms and zones d and e consisting mostly of aeolian sands in d the sand grains show thin micritic coatings whereas in e vadose aragonite cements can be seen on the grain surface suggesting a less biogenic influence in their formation the degree of cementation and the time of the precipitation of carbonate around the roots controlled the preservation of these zones thus in some cases zones b c and or d are not preserved cylinders are up to 30 the diameter of the root that nucleated them the presence of the megarhizoliths at the top of the aeolianite beds indicates that aeolian sedimentation halted several times allowing soil formation and plant colonisation during slightly more humid periods the occurrence of megarhizoliths is further proof of the alternation of arid and slightly more humid climates in the north atlantic during the last glacial period it is also noted that they may be misinterpreted as animal trace fossils or tree trunk casts resulting in incorrect ichnological or palaeoenvironmental interpretations

facies architecture of an archean komatiite hosted ni sulphide ore deposit victor kambalda western australia implications for komatiite lava emplacement nickel sulphide mineralisation is located in structural embayments or troughs at the base of thick basal komatiite lava flows at the victor ore body kambalda western australia primary contact relations and textural and vesicle distribution of the host komatiite lavas particularly the development of coherent or quench fragmented margins is consistent with emplacement of komatiites under laminar flow conditions however the absence of sedimentary units beneath komatiites with coherent flow tops in the ore environment seems contradictory and suggests both turbulent and passive laminar emplacement respectively the presence of erosive basal contacts in the ore environment suggests the komatiite lava was initially turbulent and probably open channel fed it is during this initial stage that both erosion and deposition of the nis deposits occurred as the flow evolved widened and thickened laminar flow conditions prevailed the komatiites are inferred to have flowed in a laminar state through the development of interior magma tubes beneath surface crusts and to have grown endogenously

canons revisited and reviewed lester king s views of landscape evolution considered 50 years later fifty years after its publication lester king9s canons of landscape evolution is reviewed and is considered in light of subsequent trends and actual developments some of his ideas such as the role of scarp recession and the antiquity of some surfaces remain current his broad view of the world and his interest in major relief anticipated trends that are now fashionable some of his interpretations however and in particular his downgrading of the importance of structural factors and his linking of scarp retreat and pedimentation have not stood the test of time other concepts such as the etch or two stage origin of forms which were mooted but not fully appreciated in king9s day have come to the forefront and technological advances in dating and survey particularly of the ocean floors have signaled new perspectives in landscape interpretation nevertheless king9s was a courageous attempt to provide guidelines for landscape study

diagenetic overprinting of the sphaerosiderite palaeoclimate proxy are records of pedogenic groundwater delta18o values preserved meteoric sphaerosiderite lines msls defined by invariant d 18 o and variable d 13 c values are obtained from ancient wetland palaeosol sphaerosiderites millimetre scale feco3 nodules and are a stable isotope proxy record of terrestrial meteoric isotopic compositions the palaeoclimatic utility of sphaerosiderite has been well tested however diagenetically altered horizons that do not yield simple msls have been encountered well preserved sphaerosiderites typically exhibit smooth exteriors spherulitic crystalline microstructures and relatively pure 95 mol feco3 compositions diagenetically altered sphaerosiderites typically exhibit corroded margins replacement textures and increased crystal lattice substitution of ca 2 m g 2 and mn 2 for fe 2 examples of diagenetically altered cretaceous sphaerosideritebearing palaeosols from the dakota formation kansas the swan river formation saskatchewan and the success s2 formation saskatchewan were examined in this study to determine the extent to which original early diagenetic d 18 o and d 13 c values are preserved all three units contain poikilotopic calcite cements with significantly different d 18 o and d 13 c values from the co occurring sphaerosiderites the complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses the dakota and swan river samples ultimately yield distinct msls for the sphaerosiderites and mcls meteoric calcite lines for the calcite cements the success s2 sample yields a covariant d 18 o vs d 13 c trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters the calcite cements in the success s2 formation yield meteoric d 18 o and d 13 c values a stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end member compositions modelled hyperbolic fluid mixing curves for the success s2 formation suggest precipitation from fluids that were 25 sea water

observations of the lineament ore relation geological and geophysical features of the australian continental crust follow systematic distribution patterns characterized by major linear discontinuities or lineaments these lineaments form the floor plan of the regional tectonic framework and appear to represent fundamental crustal thresholds and corridors of disturbance along which maximum crustal energies have been channelled the effects are variously expressed as lineament associated intensifications of intrusion deposition dislocation deformation metamorphism and mineralization in australia over two decades of systematic study led to the recognition that major australian ore deposits are related to major lineaments this was a retrospective observation that established the relation for known deposits the subsequent prospective use of the relation in mineral exploration is exemplified in the discovery of the giant lineament related olympic dam cu u au deposit at roxby downs south australia major australian examples of the lineament ore relation are described in comparison with apparently similar relations in north america and these are briefly considered in regional and global contexts

major bounding surfaces in aeolian sandstones a climatic model large areas of fixed vegetated dunes occur in the sahel and southern margin of the sahara these dunes were active during late pleistocene times and have been stabilized as a result of a change to more humid less windy conditions in the holocene the stabilized surfaces which are in part of erosional origin are of regional extent within aeolian sand sequences such surfaces form the highest order bounding surfaces since regional bounding surfaces of this type occur in all the world s major deserts it is suggested that they should also be present in some ancient aeolian sandstone sequences

carbonate pseudomatrix in siliciclastic carbonate turbidites from the oquirrh wood river basin southern idaho abstract upper pennsylvanian to lower permian mixed siliciclastic carbonate sandy turbidites from the oquirrh wood river basin in southern idaho contain 20 to 60 modal percent microspar and pseudospar previous interpretations suggested that neomorphism of detrital lime mud produced the observed carbonate textures the original detrital lime mud content based on these interpretations indicates matrix rich poorly sorted turbidite deposits however observed turbidite hydrodynamics and grain size data from experimental and naturally occurring sandy turbidite deposits indicate that ta tc intervals of sandy turbidites are generally moderately well sorted with low matrix content fluorescence microscopy reveals that the carbonate fraction of these mixed siliciclastic carbonate turbidites contains micritized skeletal grains and fusulinids and algal peloids these micritized grains and peloids were physically compacted and neomorphosed to form a carbonate pseudomatrix formation of carbonate pseudomatrix is analogous to formation of pseudomatrix in siliciclastic lithic sands which includes crushing and recrystallization of lithic grains grain size analysis of siliciclastic and slightly compacted carbonate grains indicates that these are moderately well sorted turbidite deposits with similar grain size populations in both fractions the presence of rare noncompacted bioclasts which have experienced only minor micritization suggests that micritization control ed grain integrity and therefore later pseudomatrix generation lack of recognition of carbonate pseudomatrix could lead to erroneous interpretations of carbonate petrology identification of carbonate pseudomatrix is important to the study of mixed siliciclastic carbonate gravity flow deposits this study demonstrates the value of fluorescence microscopy in the recognition of carbonate pseudomatrix

introducing an improved multi proxy approach for paleoenvironmental reconstruction of loess paleosol archives applied on the late pleistocene nussloch sequence sw germany to study paleoenvironmental and paleoclimatic changes in loess paleosol sequences lps most proxies except for screening parameters have often been applied at low sampling density typically 1 2 per stratigraphic unit this might lead to results that overlook paleovegetation effects that were not only restricted to paleosols but also affected underlying loess material here a new profile was investigated in the loess paleosol sequence lps at nussloch sw germany using a high resolution multi proxy approach to improve our understanding of the late pleistocene record which serves as a standard sequence in central europe this approach included several geophysical and geochemical parameters as well as frequencies of root related features grain size distribution and osl dating enabled correlation of the new section with the nearby standard profile p4 revealing considerable variations of the thickness of stratigraphic units due to local small scale variations in depositional conditions and paleogeomorphology of longitudinal greda structures magnetic susceptibility revealed weathering of the entire lps under more humid conditions than those described for se european and chinese lps it further enabled a rough differentiation between an interval that was influenced by pedogenic processes lower pleniglacial to lower part of the upper pleniglacial and loess that was not markedly affected by soil formation color measurements confirmed these results and enabled the identification of several paleosols in the upper pleniglacial for the uppermost part of the lps low magnetic susceptibility and inorganic geochemical parameters such as low fe ti ratios indicated only weak or no pedogenic influence in contrast considerable variations e g in carbonatic carbon contents and organic carbon nitrogen c n ratio might be related to the high frequency of calcified root remains up to 200 rhizoliths m 2 and up to 12 500 microrhizoliths m 2 in certain depth intervals of the lps entailing incorporation and degradation of organic matter frequencies of biopores which are suggested to derive largely from former root growth and to a lesser extent from insect and earthworm bioturbation were related to paleosols and thus confirmed the pedostratigraphy an improved assessment of terrestrial sedimentary archives can be achieved by high resolution multi proxy analysis and three dimensional investigation of terrestrial sediments that are prone to bioturbation c 2014 elsevier b v all rights reserved

sem analysis of siderite cements in intertidal marsh sediments norfolk england abstract siderite concretions and more continuous cemented layers are fairly common in intertidal marsh sediments on the coast of east anglia concretions from the norfolk coast at warham have been examined by scanning electron microscopy in the secondary electron and backscattered electron modes and by microprobe analysis siderite occurs primarily as passive void filling cement and as coatings on quartz grains no evidence of quartz replacement or displacement was found in the sections examined the siderite is somewhat variable in composition but is predominantly relatively pure with less than 6 ionic magnesium substitution x ray powder diffraction and sem analyses have shown that small amounts of calcite greigite and iron monosulphides are also present siderite is currently forming at very shallow depths in rapidly accreting marsh sediments which have large amounts of decaying organic matter high bacterial activity and low interstitial concentrations of dissolved sulphate and h 2 s

the anatomy of shallow crustal transpressional structures insights from the archaean carajas fault zone amazon brazil abstract the carajas fault zone cfz is the most prominent structure in the e w trending carajas strike slip system casss an ancient upper crustal fault network of probable late archaean age that cuts across the precambrian amazonian craton in brazil the subvertical faults reactivated pre existing high grade basement shear zone fabrics itacaiunas shear zone and display a complex long lived history of movement 1 ga dominated by oblique and strike slip displacements a postulated early regional phase of dextral movement downfaulted units of low grade to unmetamorphosed basement rocks into dilational bends and offsets in the casss subsequent sinistral transpression ca 2 6 ga led to faulting and folding during partial inversion of the cover rocks in these dilational sites with much of the associated deformation focused in the immediate vicinity

characteristics origin and significance of mesoproterozoic bedded clastic facies at the olympic dam cu u au ag deposit south australia mesoproterozoic bedded clastic facies occur in the olympic dam breccia complex which hosts the olympic dam cu u au ag deposit south australia contacts of the bedded clastic facies with breccia of the olympic dam breccia complex are faulted and or brecciated fragments of the bedded clastic facies occur in the breccia complex the bedded clastic facies comprise five main facies associations four of which are organised into mappable units these associations have distinct textures and components and have not been mixed major sources of detritus were felsic and mafic volcanic units and granitoids of the 1590 ma gawler silicic large igneous province archean and paleoproterozoic zircons in well sorted quartz rich sandstone indicate that older gawler craton basement successions also contributed sediment to the bedded clastic facies accumulation of the bedded clastic facies was at least partly contemporaneous with rhyolitic explosive eruptions from vent s probably tens of km away because bubble wall shards are present in tuffaceous mudstone facies deformation of the bedded clastic facies prior to final lithification produced folds that have diverse shapes and sizes and that lack cleavage the folds have a reasonably consistent gentle plunge to the northeast these soft sediment folds could be related to slumping on a northeast striking paleoslope that dipped either to the northwest or southeast the depocenter in which these facies accumulated may have been bounded by a combination of northeast striking and northwest striking faults because the bedded clastic facies occur at the top of the olympic dam breccia complex and contributed clasts to it the fault controlled sedimentary basin was in place when the breccia complex and the olympic dam ore deposit formed

microbial processes controls on the shape and composition of carbonate concretions abstract shape size and mineralogical chemical compositions of carbonate concretions vary considerably the questions to be answered are why do they form at all what processes are involved why do they so often form as regular spheroids why do they sometimes form laterally extensive bands what controls whether the mineral composition is calcite dolomite rhodochrosite or siderite or mixtures of them the questions are addressed and the answers illustrated by selected examples carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents organic matter microbes take advantage of the energy source and are believed to be involved in most of the various processes pore waters in steadily accumulated sediments show a depth related succession of chemical and carbon stable isotope compositions this results from degradation of organic matter proceeding via an orderly succession of processes during burial each mediated by a specific microbial group aerobic oxidation using dissolved oxygen occurs first followed by mn iv reduced to mn ii as the oxidant fe iii reduction and sulphate reduction are probably more significant methanogenesis is the final deepest microbial process the common product of all these reactions is co 2 or dissolved hco 3 which can result in carbonate supersaturation the specific process controls the chemical and isotopic compositions of pore water and leads to a characteristic mineralogy of any carbonate precipitated however the processes may not proceed successively and localised micro environments occur recent work on siderite concretions in very young salt marsh sediments norfolk u k has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth concretions in ancient rocks can be described in terms of the same processes observed in modern sediments laterally extensive siderite bands in bullhouse quarry westphalian u k resulted from interaction of two sets of complementary processes shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation the model is supported by a mass and charge balance calculation similar complementary processes formed spheroidal concretions gammon shale usa but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction the spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model kimmeridge bay uk dolomite ledges are marine analogues to the bullhouse examples for the pyrite rimmed calcite jet rock concretions early jurassic yorkshire u k the rate of production of sulphide from sulphate reduction locally exceeded availability of fe ii to precipitate pyrite outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore water evolution however all represent an expression in ancient rocks of microbiological processes similar to those operating today

synapsid burrows and associated trace fossils in the lower jurassic navajo sandstone southeastern utah u s a indicates a diverse community living in a wet desert ecosystem abstract enigmatic structures in the lower jurassic navajo sandstone near moab utah occur at three localities below carbonate beds that record interdune lake deposits these structures are interpreted as mammal and therapsid burrows based on their architectural and surficial burrow morphologies type i large diameter and type ii mega diameter burrows type i burrows include sinuous tunnels y and t branched tunnels sinuous ramps and chambers and weather into mounds averaging 33 m 22 m and extend 1m above the surface type i burrows are dorsoventrally flattened in cross section averaging 9 3 cm wide and 4 2 cm high and are sand filled and structureless type i burrows mostly have smooth walls though some have scalloped walls type ii burrows are simple inclined tunnels 35 cm wide and 20 cm high and exhibit well preserved bilobate morphology along the underside of the tunnel the walls preserve a series of 3 or 4 thin 4 8 mm inclined scratch marks from the upper part of the wall and along the floor the great complexity and high density of type i burrow systems is best explained by multiple individuals living together in social groups these burrows are more complex than early triassic therapsid burrows from south africa and are most similar to burrows of extant social e g voles and eusocial mammals e g naked mole rats type ii burrows were likely constructed by therapsids based on burrow size and comparison to permian and triassic therapsid burrows from south africa and antarctica

alluvial sandstone composition and paleoclimate ii authigenic mineralogy abstract various empirical and theoretical arguments suggest that early formed cement in sandstone is a product of meteoric water whose chemistry is controlled by climate the nature and distribution of early silicate cement in gondwana fountain and cutler sandstone support this supposition sandstones from all three units have experienced a broadly similar diagenetic history characterized by two stages of authigenesis stage i was typified by neomorphic development of kaolinite chlorite smectite quartz and additionally in the cutler formation laumontite stage ii was dominated by replacement reactions involving the production mostly of illite iron oxide carbonate minerals and mostly in the gondwana sandstone a second generation of quartz thermodynamic relations hydrologic constraints and radiometric dating of the late stage illite indicate that early authigenesis took place within a few million years of deposition at burial depths measured in hundreds of meters consequently the nature of the silicate cement was a function of the groundwater chemistry as controlled by climate during times of relative aridity ionic concentration of the groundwater was high and high proportions of smectite chlorite and more rarely laumontite formed such was the case in petrofacies i iv v of the gondwana supergroup for a definition of gondwana petrofacies see part i of this study in the cutler fm and in the upper 150 m of the fountain fm during times of high precipitation pore water was dilute which promoted authigen c formation of kaolinite and quartz this is represented by sandstones in petrofacies ii and vi of the gondwana and by sandstones in the lower 200 m of the fountain fm illite is especially abundant as a pseudomorphic replacement of kaolinite in the older gondwana sandstones illitization apparently began at burial depths of about 1 600 m at a temperature of about 75 c illite is not common in the cutler formation and is present only in subordinate amounts in the fountain formation from the study area the 18o values of early clay cement in the gondwana sandstones range systematically from base to top from 5 00 to 13 20 the systematic variation in 18o value of gondwana clay reflects the gradual migratory drift of the gondwana harm toward lower latitudes through time average 18o values for cutler cement 13 23 and fountain cement 19 82 are greater than the analogous values for gondwana cements this is consistent with the lower latitude setting of deposition for the cutler and fountain relative to the latitudinal location of the gondwana basins however the values are anomalously lower than what have been observed in modern day neoformed clays in weathering profiles from low latitudes

hydrologic framework of a sabkha along arabian gulf during the last 4 000 to 5 000 years sedimentary offlap and a relative fall in sea level have resulted in the development of broad gently seaward sloping about 1 3 000 planar areas or sabkhas along much of the south shore of the arabian persian gulf the depth of the water table below the sabkha surface increases with distance from the sea until it reaches 1 0 to 1 5 m after which it remains constant when the water table depth is in the range of 1 0 to 1 5 m the sabkha surface has reached a state of deflational equilibrium the water table beneath the sabkha always slopes seaward indicating that the direction of ground water flow is toward the sea the subsurface flow system beneath the sabkha is part of a regional seaward flowing groundwater regime in the seaward part of the sabkha a downward flow component is superimposed on the general seaward ground water flow pattern as a result of the infiltration and percolation of water from shallow bodies of seawater that are propelled inland over the sabkha surface by strong onshore winds farther inland where marine flooding does not occur the vertical flow component is directed upward in response to evaporative losses at the sabkha surface the net rate of evaporation from the central parts of the sabkha is on the order of 6 cm factor of 2 of ground water per year a regional ground water flow system which causes the water table level to rise as the sabkha progrades appears to be a requirement for the development and the preservation of broad sabkhas fluctuations in sea level during sabkha development influence the rate of progradation and the thickness of sedimentary facies the refluxing of sabkha brines beneath the lagoons in the abu dhabi area indicates a mechanism for preserving anhydrite during marine transgressions thus permitting the development of stacked sabkha successions which are common in the rock record

integrated ichnofacies models for deserts recurrent patterns and megatrends abstract although it is commonly assumed that the trace fossil record of eolian dunes and associated environments is invariably poor a systematic review of the available information indicates that this is not necessarily the case a model involving five main phases of colonization of desert environments through the phanerozoic is proposed in this paper the first phase cambrian silurian involved animal incursions into coastal dune fields directly from the sea although it is unlikely that these animals would have remained for long periods of time in coastal deserts the second phase devonian reflects the activities of dune pioneers that left their fluvial habitat to enter temporary or permanently into inland deserts the third phase carboniferous permian involved the colonization of deserts by tetrapods the fourth phase triassic cretaceous involved a major exploitation of the infaunal ecospace as reflected by the appearance of more varied behavioral patterns in sub superficial structures the fifth phase paleogene recent reflects the appearance of the ecological dynamic that characterizes modern desert communities the invertebrate ichnofacies for eolian dunes is re named herein as the octopodichnus entradichnus ichnofacies honoring the seminal work of previous workers who simultaneously tackled the issue of eolian dune ichnofacies the chelichnus ichnofacies is retained for vertebrate trace fossil assemblages in eolian settings both ichnofacies occur in mobile and temporary stabilized sandy substrates subject to frequent erosion and deposition and to strong seasonality desert settings consist of complex mosaics of habitats or physical units associated with organism activity trace fossil distribution can be understood as reflecting the partitioning of desert settings in a mosaic of landscape units characterized by water content and its temporal fluctuations nutrient availability nature of the substrate and the dominant organisms present in turn desert systems are dynamic entities that change as a response to regional climate landscape units such as eolian sand seas salt flat and playa lake systems ephemeral rivers and alluvial fans interact in response to regional scale climate variations in hyper arid arid and semiarid climatic settings ancient deserts completely developed under hyper arid climatic conditions rarely preserve trace fossils due to the absence of moisture near the surface however the alternation of wet periods may represent windows for life development and thus preservation of biogenic structures arid deserts display complex patterns of dunes combined with dry wet and flooded interdunes dry desert elements e g dunes interdunes sand sheets typically record the entradichnus octopodichnus and chelichnus ichnofacies slight rises in regional precipitation produce elevation of the water table and increase of fluvial discharges that provide water and sediment to the system these processes may result in the local concentration of trace fossils in wet interdunes and ephemeral fluvial systems illustrating the scoyenia and chelichnus ichnofacies in semiarid systems playa lakes expand by the addition of freshwater evolving into freshwater lakes and fluvial systems may become more common lake margins and fluvial overbanks typically contain trace fossil assemblages that may be ascribed to the scoyenia ichnofacies

imaging links between lithospheric architecture and surface geology in the proterozoic curnamona province australia 1 potential field magnetic and gravity analysis is an effective method to investigate crustal scale architecture and provides a means of linking continental scale geophysical data sets e g seismic tomography with detailed geological and geophysical studies here we image deep crustal structure of the proterozoic curnamona province australia in aeromagnetic and gravity data sets and establish spatial and temporal links between deep structures and near surface geological features by superimposing first vertical derivative responses on upward continued images upward continued gravity and aeromagnetic data show nw se ne sw e w and n s oriented long wavelength discontinuities and boundaries to anomalies which we interpret as deep seated structures these structures predate the oldest known tectonic events recorded in outcropping rocks and appear to have been long lived thereby influencing the tectonic history of the province for example outcropping cambrian shear zones evident in vertical derivative data appear to have nucleated off displaced segments of a once continuous deep seated structure of proterozoic origin that has been imaged in upward continued data long wavelength gradients defining e w and nw se structures in upward continued aeromagnetic data intersect beneath the dominantly felsic benagerie igneous province in the central province and are coincident with a large negative gravity anomaly interpreted as felsic igneous rocks that are coeval to the volcanic pile tectonic implications of this study include 1 deep seated structures in the curnamona province have protracted histories 2 these structures appear to have controlled the location and distribution of younger shear zones igneous activity and possibly basin subsidence in the upper crust 3 geophysical lineaments are likely to have evolved and propagated during multiple tectonic cycles and hence record the protracted evolution of a geological province

periodic accumulation and destruction of aeolian erg sequences in the permian cedar mesa sandstone white canyon southern utah usa the permian cedar mesa sandstone represents the product of at least 12 separate aeolian erg sequences each bounded by regionally extensive deflationary supersurfaces facies analysis of strata in the white canyon area of southern utah indicates that the preserved sequences represent erg centre accumulations of mostly dry though occasionally water table influenced aeolian systems each sequence records a systematic sedimentary evolution enabling phases of aeolian sand sea construction accumulation deflation and destruction to be discerned and related to a series of underlying controls sand sea construction is signalled by a transition from damp sandsheet ephemeral lake and palaeosol deposition through a phase of dry sandsheet deposition to the development of thin chaotically arranged aeolian dune sets the onset of the main phase of sand sea accumulation is reflected by an upward transition to larger scale ordered sets which represent the preserved product of climbing trains of sinuous crested transverse dunes with original downwind wavelengths of 300 400 m regularly spaced reactivation surfaces indicate periodic shifts in wind direction which probably occurred seasonally compound co sets of cross strata record the oblique migration of superimposed slipfaced dunes over larger slipfaceless draa each aeolian sequence is capped by a regionally extensive supersurface characterized by abundant calcified rhizoliths and bioturbation and which represents the end product of a widespread deflation episode whereby the accumulation surface was lowered close to the level of the water table as the sand sea was progressively cannibalized by winds that were undersaturated with respect to their potential carrying capacity aeolian sequence generation is considered to be directly attributable to cyclical changes in climate and related changes in sea level of probable glacio eustatic origin that characterize many permo carboniferous age successions sand sea construction and accumulation occurred during phases of increased aridity and lowered sea level the main sand supply being former shallow marine shelf sediments that lay to the north west sand sea deflation and destruction would have commenced at or shortly after the time of maximum aridity as the available sand supply became exhausted restricted episodes of non aeolian accumulation would have occurred during humid interglacial phases accumulation and preservation being enabled by slow rises in the relative water table subsidence analysis within the paradox basin together with comparisons to other similar age successions suggests that the climatic cycles responsible for generating the cedar mesa erg sequences could be the product of 413 000 years so called long eccentricity cycles by contrast annual advance cycles within the aeolian dune sets indicate that the sequences themselves could have accumulated in just a few hundred years and therefore imply that the vast majority of time represented by the cedar mesa succession was reserved for supersurface development

early diagenetic siderite as an indicator of depositional environment in the triassic rewan group southern bowen basin eastern australia early concretionary and non concretionary siderites are common in subsurface triassic sandstones and mudrocks of the rewan group southern bowen basin a detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks the siderites are extremely pure containing 85 97 mol feco3 and are commonly enriched in manganese 13c pdb values are highly variable ranging from 18 4 to 2 9 whereas 18o pdb values are very consistent ranging from 14 0 to 10 2 mean 11 9 1 0 the elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation implying that host rocks accumulated in totally non marine environments the carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate carbon dioxide generated by methane oxidation and methanogenesis very low 13c values demonstrate that contrary to current views highly 13c depleted siderite can be produced at shallow burial depths in anoxic non marine sediments

sequence stratigraphic evolution and cyclicity of an ancient coastal desert system the pennsylvanian permian lower cutler beds paradox basin utah u s a abstract the lower cutler beds comprise a 200 m thick succession of at least 12 repeating cyclic packages of strata each 10 18 m thick that are of mixed eolian fluvial and shallow marine origin these accumulated during the latest pennsylvanian to early permian in the paradox foreland basin of southeast utah each depositional cycle comprises a lower unit composed of a bioclastic wackestone or a bioclastic sandy calcarenite facies both of shallow marine origin the upper units in each cycle are of nonmarine eolian dune and interdune and fluvial channel and floodplain origin the mixed nonmarine and shallow marine nature of the inferred paleoenvironments of the lower cutler beds is significant because the preserved expression of the nonmarine eolian and fluvial units in each cycle demonstrates an external climatic control on system development whereas the marine units in each cycle together with associated transgressive and regressive surfaces demonstrate repeated episodes of relative sea level change key stratigraphic surfaces have been traced from the continentally dominated northeastern portion of the basin into apparently contemporaneous shallow marine dominated sections in the southwest of the basin interdependent changes in both climate and relative sea level change are shown to have generated the preserved cycles which are interpreted to represent high frequency sequences that are bounded by erosional unconformities sequence boundaries and paraconformities diastems relative sea level lowstand was coincident with climatic aridity at which time exposure of a broad low relief coastal plain enabled the generation of a substantial eolian sediment supply that was available for transport by a paleowind aligned parallel to the trend of the paleocoastline resulting in dune field construction and accumulation along a coastal belt episodes of relative sea level rise were coincident with a transition to a more humid paleoclimate during which the eolian sediment supply was greatly reduced resulting in dune field deflation and associated expansion of the fluvial system across the deflated former dune field surface marine transgression occurred first in a series of estuarine embayments via the back flooding of broad but shallow fluvially incised valley systems later transgression culminated in marine incursion over the former low relief floodplain episodes of relative sea level highstand were coincident with maximum climatic humidity the high frequency sequences make up a composite sequence set which itself records a longer term relative sea level cycle at this larger scale a lowstand systems tract is recorded in the lower part of the studied lower cutler beds succession whereas a substantial transgressive systems tract is developed in the upper part a regionally extensive maximum flooding surface defines the top of the lower cutler beds the preserved thickness and number of high frequency cycles together with their relation to the long term subsidence trend of the paradox basin records a periodic switching of both climate and relative sea level the temporally linked nature of which demonstrates that glacio eustasy exerted a dominant control on the origin of the preserved succession the lower cutler beds preserve evidence for the linkage and feedback between combined climatic and eustatic allogenic controls on sedimentation and their role in generating high frequency milankovitch style cycles in mixed clastic carbonate successions

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

episodic deposition and preservation of eolian sands a late paleozoic example from southeastern utah the upper pennsylvanian and lower permian stratigraphic sequence in canyonlands national park contains about 40 flat topped eolian sand bodies rhizoliths burrows and traces of former evaporites abound at the tops of these tabular genetic units upper surfaces of units were created by eolian deflation to the level of the groundwater table analysis of cross stratification reveals that sands accumulated via migration and climb of relatively small dunes brief episodes of sand accumulation were triggered by regression and were terminated when upwind sand supplies were depleted upper surfaces of genetic units are diastems and represent much more geologic time than the rocks themselves

model for sandstone carbonate cyclothems based on upper member of morgan formation middle pennsylvanian of northern utah and colorado the upper member of the 200 m 660 ft thick morgan formation middle pennsylvanian consists of 5 25 m 16 82 ft thick very fine grained quartz sandstone units that are interbedded repetitively with 0 5 11 m 1 6 36 ft thick oolitic bioclastic peloidal and micritic carbonate units a modified markov chain analysis verified the presence of 2 kinds of repetitive regressive dominant shoaling upward vertical sequences the carbonate sequence consists of from bottom to top a a knifesharp contact between sandstone and the overlying carbonate unit b planar bedded bioclastic grainstones or cross bedded oolitic grainstones c fragmented fossil and intact fossil wackestones d peloidal and evaporitic wackestones and mudstones e peloidal and ostracod gastropod grainstones and packstones f planar bedded to low angle cross bedded fossiliferous calcareous sandstones commonly comprising the transitional contact between carbonate and overlying sandstone the sandstone sequence consists of from bottom to top f planar bedded to low angle cross bedded fossiliferous calcareous sandstone g medium scale trough and wedge cross bedded sandstones punctuated by bioturbated zones dominated by ophiomorpha nodosa h large to very large scale wedge and planar tabular cross bedded sandstones i deformed cross bedded sandstone erosionally truncated by the next overlying carbonate unit both sequences define a new conceptual depositional model that may explain the origin of many other widespread interbedded quartz sandstone carbonate successions deposited on the western edge of the north american craton during pennsylvanian and permian time one depositional phase employs virtually instantaneous transgression followed by dominantly progradational subtidal carbonate shelf to shoal conditions the other involves progradationof sandstones across intertidal and subtidal carbonate environments eustatic changes related to gondwana glaciation were probably the primary control on sedimentation the quartz sandstone carbonate cyclothems defined by this study have potential as targets for hydrocarbon exploration both eolian dune sandstones and dolomitized shelf carbonate strata are locally important reservoir rocks in the subsurface in parts of the western overthrust belt in utah and wyoming modified journal abstract

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

a broad scale structural characteristic of major nickel sulfide deposits of western australia during the past ten years western mining corporation has carried out specific tectonic studies of ore occurrences in the western australian nickel belt at various scales ranging from those appropriate to an individual ore shoot to broader scales relating to regional and continental tectonic patterns these studies indicate that nickel mineralization on a regional scale is repeatedly localized at a particular structural conjunction where the host rock is transected discordantly by a relatively narrow belt or corridor of disturbance rendered visible by processing patterns of geologic and geophysical data for reasons of exploratory advantage many of the results of the studies have remained confidential for various periods of time western mining corporation has recently authorized the release of data which show that the same structural signature at a regional scale is repeated in the four principal ni sulfide deposits of the western australian nickel belt namely kambalda perseverance mt keith and mt windarra in each of these instances the associated linear structural corridor is parallel to the giant kalgoorlie shark bay magnetic lineament a major double edged belt of crustal disturbance which cuts west northwesterly for 800 km through the nickel belt the regional lineament through the kambalda deposits is contained within the kalgoorlie shark bay magnetic lineament zone while the perseverance mt windarra and mt keith deposits lie respectively on separate parallel west northwest corridors several hundred kilometers to the north the regional west northwesterly structural discontinuities related to individual nickel deposits are typically upward of 80 km in length and variably from one to several kilometers in width

applying spatial prospectivity mapping to exploration targeting fundamental practical issues and suggested solutions for the future abstract despite many decades of development spatial prospectivity modelling is not yet widely used or accepted throughout the global mineral exploration industry a common criticism of the method is that it is not practically useful because it has a bias to mature well known areas and generates excessively large areas of high prospectivity it is suggested that the reason for this is not primarily related to limitations in the prospectivity mapping algorithms but rather to issues relating to the use of input data sets specifically it is common that the input data such as geological interpretations do not uniformly and objectively represent the search space of interest omit critical targeting relevant geoscientific elements such as major deep seated ore controlling structures and have a large degree of unrecognised dependence it is considered that these problems are not in principle barriers to the eventual successful deployment of this technology however future approaches to spatial prospectivity modelling need to explicitly address these concerns it is suggested that the most effective method may be a hybrid of subjective human geological interpretation and objective machine based analysis that captures the best aspects of these alternative approaches i e an intelligence amplification ia rather than an artificial intelligence ai approach a roadmap is proposed for improving the effectiveness of spatial prospectivity modelling that has implications for the broader community interested in mineral exploration targeting

paleosols trace fossils and precipitation estimates of the uppermost triassic strata in northern new mexico abstract this study provides a detailed pedogenic evaluation of two upper triassic late norian through rhaetian stratigraphic intervals in new mexico in order to assess the climate and ecology of the latest triassic which ended in a mass extinction the two study areas are located in north central and east central new mexico and are separated by 200 km each section contains abundant paleosols of varying maturity with features that reflect an arid to semiarid climate there is little pedogenic variation throughout the strata at each location and a typical paleosol profile is about 1 m thick and has an ab bw bk bc horizon succession bkm bss bssk or bssg horizons are present in some paleosols micromorphological features suggest dominantly well drained sola e g abundant carbonate nodules illuviated clay with minor periods of moist or saturated conditions e g femn concretions femn coatings and hypocoatings sepic plasmic fabrics trace fossils are abundant in these strata and are dominated by taenidium serpentinum and root traces depth to carbonate functions estimate that mean annual precipitation was between 200 and 450 95 mm relative to location 1 eastern new mexico location 2 north central new mexico produced higher paleo precipitation estimates and has stronger and more abundant sepic plasmic fabrics in thin sections the presence of a gleyed paleosol camborygma eumnkenomos and slickensides at location 2 also suggests conditions wetter than at location 1 taxonomically all of the paleosols in this study appear to be entisols or aridisols and can be grouped into seven representative pedotypes of varying maturity by comparing these paleosols to modern soils this study demonstrates that the late triassic western interior during the late norian to rhaetian was arid to semiarid and supported a desert shrub environment that had localized and periodic moist or saturated soil conditions

effect of bacteria on the elemental composition of early diagenetic siderite implications for palaeoenvironmental interpretations many ancient early diagenetic siderite concretions are thought to have been precipitated by microbially mediated reactions and their trace element chemistry has been used to infer palaeoenvironments of formation e g marine versus freshwater in this study pure cultures of the microorganism geobacter metallireducens were used to precipitate siderite in the laboratory at a range of temperatures 18 40 c magnesium and calcium in three different ratios 3 1 as in seawater 1 1 and 1 3 as in freshwater or manganese were added to some cultures at each incubation temperature to study trace element incorporation the siderite produced exhibited the rhombohedral crystal form typical of concretionary siderite however this microbial siderite did not simply retain the trace element chemistry of the water from which it precipitated as is assumed in palaeoenvironmental interpretations instead manganese and calcium incorporation were found to be inversely proportional to the rate of microbial activity which itself is dependent upon the nutritional status of the microorganisms magnesium incorporation was found to be rate independent and at high mg ca ratios it caused inhibition of calcium incorporation in short the influence of microorganisms on the trace element composition of early diagenetic siderite must be taken into account in order to produce valid palaeoenvironmental interpretations

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

dolines of the pleistocene dune calcarenite terrain of western eyre peninsula south australia a reflection of underprinting abstract a field of middle and late pleistocene coastal foredunes occupies much of western eyre peninsula south australia the rolling surface reflects the morphology of the stacks of calcarenite dunes that underlie the area hardpan calcrete is well developed in relation to the present as well as to earlier dune surfaces the region is a typical karst in that surface drainage is lacking there are a few shallow and short caves but solution pipes and dolines are abundantly developed some dolines including several of the larger forms occur high in the local topography and are also aligned in groups they are attributed to underprinting to the diversion of groundwaters into fractures in the pre pleistocene basement and the concentration of solution in the limestone above such zones low permeability calcrete horizons within the dune sequence have probably disturbed groundwater circulation and also form a stable framework preventing major collapse and preserving both dolines and caves

ancient subaerial exposure and freshwater leaching in sandstones the authors present an example of leaching by meteoric water in the jurassic magnus sandstone member of the northern north sea the interpretation is based on information from several independent techniques including downhole natural gamma ray spectrometry detailed petrographic analysis and cross sectional time slice reconstructions from seismic data the initial lines of evidence that prompted further investigation into freshwater involvement in the magnus sandstone member were brackish formation water compositions and a potassium depletion shown by the gamma ray spectrometry log in sandstone beneath the unconformity at the base of the cretaceous section subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution resulting in precipitation of kaolinite and creation of secondary porosity the latter also revealed by conventional core analysis porosity and permeability measurements finally cross sectional time slice reconstructions from a regional seismic line across the magnus oilfield strongly suggested subaerial exposure of the magnus sandstone member in early cretaceous time and consequent freshwater leaching of the magnus sandstone member and generation of the unconformity at the base of the cretaceous the combination of several lines of evidence from the magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones

isotopic composition of siderite as an indicator of depositional environment compilation of available oxygen and carbon isotopic data for siderites from different depositional environments indicates that marine and continental siderites are characterized by distinctive compositional fields the most notable difference between the fields is the generally higher 13 c values of continental vs marine siderites this difference appears to reflect the fact that marine sediments generally undergo a more extensive period of sulfate reduction than do the continental sediments in which siderite forms analysis of siderite geochemistry may assist in determination of depositional environment in cases where the available sedimentological evidence is ambiguous

komatiites and nickel sulphide orebodies of the black swan area yilgarn craton western australia 1 petrology and volcanology of host rocks the black swan succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km nne of kalgoorlie within the 2 7 ga eastern goldfields greenstone province of the yilgarn craton the komatiite stratigraphy comprises a steep dipping east facing package about 700 m in maximum thickness and about 2 5 km in strike length fig 1 which hosts a number of economically exploitable ni sulphide orebodies including the silver swan massive ore shoot approximately half a million tonnes at about 10 5 ni the sequence can be subdivided into a lower felsic unit comprising coherent and autobrecciated facies of multiple dacite lava flows an upper eastern and lower western ultramafic unit each showing marked lateral facies variation and an upper felsic unit coeval with the eastern ultramafic unit the komatiite sequence has been metamorphosed at sub greenschist facies in the presence of high proportions of co2 rich fluid giving rise to pervasive talc carbonate and talc carbonate quartz assemblages with extensive preservation of pseudomorphed igneous textures cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession the degree of penetrative deformation is generally very low and original stratigraphic relationships are largely intact in much of the sequence the eastern ultramafic unit and western ultramafic unit are interpreted as components of a single large komatiite flow field representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows the western ultramafic unit which hosts the bulk of the high grade massive and disseminated ores is a sequence dominated by coarse grained olivine cumulates 2 km wide and up to 500 m thick with major magma pathways represented by thick homogenous olivine mesocumulate piles at its northern and southern ends respectively 400 and 200 m thick the sequence between the two major pathways consists of olivine orthocumulates ooc with minor spinifex textured intervals the unit is capped by a persistent spinifex textured crust less than 1 m thick and is locally vesicular the eastern ultramafic unit contains the black swan cumulate zone a 500 m thick sequence of very coarse grained hopper textured locally vesicular ooc containing disseminated sulphides in its lower 200 m the zone is flanked to the north and south by complexly interdigitated sequence of highly irregular spinifex capped olivine cumulate rich flow lobes between 1 and 100 m thick and dacitic lavas and tuffs the complexity of the 3 d spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite combined with thermal or thermomechanical erosion the eastern and western units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava which developed localised thermo mechanical erosion channels in the dacitic substrate komatiite and dacite eruption was synchronous giving rise to complex interdigitation and extensive contamination and hybridisation

the benagerie shear zone 1100 myr of reactivation history and control over continental lithospheric deformation abstract a newly identified northwest southeast oriented deeply rooted steep to vertical large scale structural system within the proterozoic curnamona province australia which we term the benagerie shear zone is imaged in regional magnetic and gravity datasets in this study we use a combination of field analysis and quantitative geophysical methods to establish a 1100 myr history of activity along the benagerie shear zone during which the location of younger geological structures are influenced by the pre existing shear zone this deformational system is interpreted to have 1 aided ascent and emplacement of the ca 1600 ma ninnerie magmatic supersuite 2 controlled the loci of nucleation of normal faults during rifting and continental breakup at ca 800 ma and 3 influenced the development of fold structures as well as acting as a plane co linear to the rotation axis of pre existing normal faults such that they were steepened and reactivated as strike slip structures during the ca 500 ma delamerian orogeny we interpret that the benagerie shear zone has not undergone uni directional propagation during its evolution but rather through reactivation was a primary influence on controlling the nucleation of neoproterozoic rift faults thereby playing a major role in accommodating strain over a significant period of the evolution of the curnamona province this study demonstrates that crustal scale shear zones can evolve over hundreds of millions of years have strike lengths of hundreds to thousands of kilometers and have vastly different surface expressions along strike

the depositional environments and tectonic development of a mesozoic intra arc basin atacama region chile a thick succession of continental redbeds was deposited in a 50 km wide intra arc basin on the andean active continental margin in the atacama region of northern chile during early cretaceous times upper jurassic to early cretaceous marine limestones were buried by the seaward progradation of a succession of coastal dunes saline lakes and sandflats aeolian dune fields migrating towards the east across these coastal plains became stabilized by the growth of vegetation interdune alluvial areas between the sand dunes and dune fields developed into extensive alluvial braid plains which were in turn superimposed by alluvial fans these fans were inundated by a regionally extensive saline lake produced by tectonic or volcanic damming of the sedimentary basin this lake dried up leaving a large area of playa lake mudflats the climate was warm and semi arid with a low and seasonal rainfall parts of the area supported a substantial vegetation of woody plants together with a vertebrate fauna of dinosaurs pterosaurs and crocodiles the continental redbeds were derived from a volcanic source and were deposited on continental crust in a deep but narrow north south elongated fault bounded graben this extensional basin formed in an intra arc setting within an active andesitic volcanic chain upwards coarsening sedimentary successions were the product of uplift of the fault bounded margins of the basin

volcanological and structural controls on mineralization at the mount keith and cliffs komatiite associated nickel sulfide deposits agnew wiluna belt western australia implications for ore genesis and targeting the mount keith region of the agnew wiluna belt in western australia is perhaps best known for hosting the giant komatiite associated disseminated ni sulfide deposit known as mkd5 it also hosts the sizeable cliffs massive ni sulfide deposit which is the eighth largest in the archean yilgarn block a concerted program of regional scale three dimensional model construction has integrated a plethora of geological geochemical and geophysical data and led to a much better understanding of the tectonic and volcanic architecture of this ancient greenstone belt the disseminated mkd5 orebody occupies a curvilinear thickening in the olivine adcumulate core of the host mount keith ultramafic unit the orebody plunges shallowly to the south from surface over a distance of 3 5 km before bending sharply to plunge steeply to the north northwest at depth the thickest part of the orebody and the best ni grades are associated with the bend in the overall ore trend this also marks the approximate intersection point of two growth faults that appear to control the thickness variations in the adcumulate core and the plunge variations in the ore trend the olivine adcumulate core is mantled by olivine orthocumulate rocks which are best developed above and below the flanks of the orebody and a fractionated sequence of pyroxenitic and gabbroic rocks occurs at the top of the mount keith ultramafic unit above the inside of the bend in the ore trend these observations suggest a strong volcanological control on the position of the mkd5 orebody and the local curvilinear thickening in the adcumulate core is interpreted as a fluid pathway the locus of which was in turn controlled by the intersection of two broadly orthogonal growth faults the cliffs ultramafic unit consists of a lower olivine ortho to mesocumulate unit which varies in thickness along strike overlain by a sequence of thin spinifex textured komatiite flow units intercalated with variably sulfidic tuffaceous shales the cliffs massive ni sulfide deposit occurs at the base of a relative thickening to 100 m in the lower olivine ortho to mesocumulate unit the orebody is typically less than 6 m thick 500 m wide and plunges gently south from surface over a distance of 1 5 km terminating in a structurally complex zone across which there are marked differences in the internal stratigraphy and thickness of the cliffs ultramafic unit the orebody has relatively low tenor maximum 8 ni typically 5 ni and the tenor is strongly zoned with extremely low tenors 1 3 ni developed on the flanks a barren exhalative massive fe sulfide unit is typically present at the base of the cliffs ultramafic unit on the flanks of the orebody and probably acted as a source of sulfur the tenor zonation pattern suggests that the melting and entrainment of the underlying exhalative sulfide horizon occurred more vigorously in the center of the lava pathway the structurally complex zone at the southern end of the cliffs orebody is interpreted as a possible vent location this would explain the relatively low tenor of the orebody and the tenor zonation through limited mixing and entrainment of melted exhalative iron sulfide melt with the komatiite lava the poorly incised nature of the lava pathway and the abrupt change in internal komatiite stratigraphy across this zone there is good evidence that the southern terminations of both the mkd5 and the cliffs orebodies are marked by growth faults at a belt scale these points correspond roughly with the intersection of nne and nw trending lineaments these lineaments are on the order of 5 to 10 km in strike length and are visible in regional geophysical datasets both gravity and magnetics they are likely to mark the position of accretionary structures that were active at the time of volcanism and that probably controlled the volcanic architecture of the komatiite flow field and the location of ni sulfide accumulations within komatiite lava pathways repeated reactivation of these early crustal scale structures during subsequent orogenesis resulted in their upward propagation as sets of late brittle structures of broader significance to the understanding of the generation of komatiite associated nickel sulfide deposits is the conclusion that many such deposits may be developed proximal to vent rather than in a distal setting as has been proposed for the kambalda camp

styles of interaction between aeolian fluvial and shallow marine environments in the pennsylvanian to permian lower cutler beds south east utah usa the pennsylvanian to permian lower cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the paradox foreland basin fill exposed in and around the canyonlands region of south east utah aeolian facies comprise i sets and compound cosets of trough cross bedded dune sandstone dominated by grain flow and translatent wind ripple strata ii interdune strata characterized by sandstone siltstone and mudstone interbeds with wind ripple wavy and horizontal planar laminated strata resulting from accumulation on a range of dry damp or wet substrate types in the flats and hollows between migrating dunes and iii extensive near flat lying wind rippled sandsheet strata fluvial facies comprise channel fill sandstones lag conglomerates and finer grained overbank sheet flood deposits shallow marine facies comprise carbonate ramp limestones tidal sand ridges and bioturbated marine mudstones during episodes of sand sea construction and accumulation compound transverse dunes migrated primarily to the south and south east whereas south westerly flowing fluvial systems periodically punctuated the dune fields from the north east several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds these surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo water table during episodes of aeolian quiescence fluvial systems became more widespread forming unconfined braid plains that fed sediment to a coastline that lay to the south west and which ran approximately north west to south east for at least 200 km shallow marine systems repeatedly transgressed across the broad low relief coastal plain on at least 10 separate occasions resulting in the systematic preservation of units of marine limestone and calcarenite between units of non marine aeolian and fluvial strata to form a series of depositional cycles the top of the lower cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north eastward directed marine transgression into the basin prior to the onset of exclusively non marine sedimentation of the overlying cedar mesa sandstone styles of interaction between aeolian fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small scale autocyclic behaviour of competing coeval depositional systems and larger scale allocyclic changes that record system response to longer term interdependent variations in climatic and eustatic controlling mechanisms the architectural relationships and system interactions observed in the lower cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea level and that these two external controls probably underwent cyclical change in harmony with each other in the paradox basin during late pennsylvanian and permian times this observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio eustatic driving mechanism

high resolution stratigraphic architecture and lithological heterogeneity within marginal aeolian reservoir analogues marginal aeolian successions contain different lithological units with variable geometries dimensions and spatial distributions such variations may result in considerable heterogeneity within hydrocarbon reservoirs developed in successions of this type which poses a high risk to their efficient development here such heterogeneity is described and characterized at inter well 1 km scales using two well exposed outcrop analogues of end member marginal aeolian deposits from the permian cedar mesa sandstone and jurassic page sandstone of south central utah usa the sedimentology and stratigraphic architecture of the cedar mesa sandstone was studied in a 1 2 km2 area in the indian creek region of southern utah where the interval consists of interbedded fluvial and aeolian deposits representative of a fluvial dominated erg margin the page sandstone was studied in a 4 3 km2 area near escalante close to the utah arizona border where it consists of interbedded sabkha and aeolian deposits representative of a transitional marine erg margin the three dimensional stratigraphic architectures of both reservoir analogues have been characterized in order to establish the dimensions geometries and connectivity of high permeability aeolian sandstones facies architecture of the aeolian sabkha deposits is characterized by laterally continuous aeolian sandstone layers of relatively uniform thickness that alternate with layers of heterolithic sabkha deposits aeolian sandstones are thus likely to form vertically unconnected but laterally widespread flow units in analogous reservoirs facies architecture in the aeolian fluvial deposits is more complex because it contains alternating intervals of aeolian sandstone and fluvial heterolithic strata both of which may be laterally discontinuous at the studied length scales aeolian sandstones encased by fluvial heterolithic strata may form small isolated flow units in analogous reservoirs although the limited continuity of fluvial heterolithic strata results in vertical connectivity between successive aeolian sandstones in other locations these architectural templates may be used to condition zonation schemes in models of marginal aeolian reservoirs

climate induced sediment palaeosol cycles in a late cretaceous dry aeolian sand sheet marilia formation north west bauru basin brazil aeolian sand sheets which are characterized by low relief surfaces that lack dunes are common in arid and semi arid climatic settings the surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects or unstable such that it is affected by deflation or sedimentation the maro lia formation late cretaceous may be interpreted as an ancient aeolian sand sheet area where alternating phases of stability and instability of the accumulation surface have been recorded detailed field studies were carried out in several sections of the maro lia formation where cyclic alternations of palaeosols and aeolian deposits were evident using palaeopedological and facies analysis methods supported in the laboratory by the analysis of rock samples cut and polished in slabs thin sections scanning electron microscope images and x ray diffraction data from the clay minerals the deposits comprise three lithofacies that in order of abundance are characterized by i translatent wind ripple strata ii flood deposits and iii ephemeral river channel deposits palaeosols constitute on average 65 of the vertical succession three types of palaeosols pedotypes are recognized i aridisols ii entisols and iii vertisols erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits the palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the maro lia formation to be defined as a flat area dominated by aeolian sedimentation with subordinate ephemeral river sedimentation and characterized by a dry climatic setting with occasional rainfall the climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development a climate controlled model is proposed in which i the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases and ii the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation thereby increasing the availability of sediment during the drier climatic phases the ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment

climate changes during the early middle triassic transition in the e iberian plate and their palaeogeographic significance in the western tethys continental domain abstract until recently the climate of the early middle triassic at low latitudes was broadly considered as generally temperate warm with no major climate oscillations this work examines the climate of this period through a detailed study of the sedimentary plant soil and mineral records of continental rocks buntsandstein facies in eastern iberian basins our findings indicate temporal climate variations for these near equator 10 14 n regions and unveil the significance of such variations in the southern laurasian domain the climate of iberia s early triassic was mainly dominated by alternating brief although the beginning of the anisian aegean was represented by alternating arid and semi arid to subhumid intervals during the bithynian and pelsonian clearly wetter climates are recorded by the succession consisting of alternating semi arid to semi humid intervals this general tendency was interrupted by three short but marked intervals two humid intervals in the late bithynian and one arid period near the bithynian pelsonian boundary iberia was crossed by prominent irregular highs separating marked corridors or isolated areas this palaeogeography prevailing since variscan tectonics clearly conditioned dominant climates and their geographical distribution no clear climate belts developed in these conditions however isolated internal climate zones separated by elevated areas are identified this palaeogeographic configuration and the low latitudinal position of iberia determined central iberia highs in the southernmost border of laurasia beyond which more humid conditions clearly extended towards the equator reaching the present day moroccan meseta and argana basin

eolian origin of upper paleozoic sandstones southeastern utah abstract throughout its outcrop in southeastern utah the permian cedar mesa sandstone contains large scale crossbedding and is devoid of marine fossils except for sand size skeletal fragments although this rock unit has been interpreted by several recent workers as shallow marine stratification identical to that documented in modern eolian dunes is ubiquitous in the cedar mesa thin inversegraded laminations within crossbeds represent the unique product of climbing wind ripples this type of stratification is the best evidence for an eolian origin of the cedar mesa such an interpretation is strongly supported by other evidence vertebrate trackways and simple small cylindrical burrows constitute a distinctively nonmarine ichnofauna rhizoliths root casts are abundant at many stratig aphic levels in the underlying rico and hermosa formations sandstones with physical and biogenic sedimentary structures identical to those in the cedar mesa are interbedded with crossbedded limestones with abundant megafossils and marine traces contacts between sandstones and limestones are sharp gradational relationships would be expected if both sandstones and limestones accumulated subaqueously siliciclastic pebbles are present in some of these carbonate rocks suggesting that the well sorted nature of the sandstones cannot be attributed to the unavailability of coarse clasts palcocurrent data from the sandstones and limestones show significantly different vector means and variance southward pinchout of marine carbonates indicates that winds were onshore winnowing of silicicla tics from coarser carbonate detritus created a strong dichotomy in bulk mineralogy between marine and eolian sediments reinterpretation of the sandstones necessitates reappraisal of ideas concerning the mineralogical maturity of these sediments the relative importance of eolian and shallow marine transport processes to the destruction of labile siliciclastics is unknown

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

thin eolianites interbedded within a fluvial and marine succession early proterozoic whitworth formation mount isa inlier australia abstract the early proterozoic whitworth formation in the mount isa inlier australia contains the oldest known thinly bedded eolian dune and amalgamated interdune deposits and records the complex interplay of eolian fluvial and marginal marine depositional processes the whitworth formation accumulated during the waning phases of rifting in a north south trending trough that began between 1808 and 1790 ma and ended before 1740 ma three eolian facies are recognized 1 cosets of cross strata 2 sets of horizontal to low angle strata and 3 isolated sets of cross strata facies el records preservation of dunes with low angles of climb without intervening interdune deposits sets of facies e2 are interpreted as either sand sheet preserved dune plinths or amalgamated interdune deposits facies e3 is encased in facies e2 and consists of lenses of cross strata with irregular tops this association of facies is comparable to recent gypsum dune and interdune deposits at white sands new mexico facies e3 reflects limited dune preservation during migration and accumulation at or near the groundwater table vertical change from amalgamated interdune deposits to dune without interdune deposits records dunefield initiation or lateral migration from a peripheral to a more centralized position within the dunefield increased sedimentation rates promoted preservation of dune deposits represented by facies e1 decreased sedimentation rate and possible limited sediment bypassing resulted in amalgamation of interdune deposits on the dunefield periphery eolian facies e1 and e2 are interbedded with ephemeral river deposits three ephemeral river facies are recognized 1 cosets of horizontal strata 2 cosets of horizontal strata and overlying trough cross beds and 3 cosets of medium scale trough cross beds ephemeral river facies f3 often are preserved in channels incised into eolian facies e3 associated marginal marine facies consist of 1 thin bedded arenites of intertidal to supratidal origin and 2 trough and tabular planar cross bed sets and cosets that resulted from migration of subaqueous dunes within a tide dominated subtidal setting interbedded fluvial and eolian deposits reflect either autogenic processes such as reworking of fluvial sands by eolian processes or incursion of a fluvial system into a dunefield related to allogenic processes such as climatic change dune deposits in the whitworth formation probably developed under arid to semi arid conditions as indicated by extensive ephemeral river deposits presence of evaporite pseudomorphs and absence of wet interdune deposits interbedded eolian fluvial and marginal marine deposits resulted from relative sea level fluctuations related to a combination of eustasy and intrabasinal tectonics

yakabindie revisited volcanological and structural controls on the komatiite hosted six mile well and goliath north deposits and implications for the architecture of the 2 7 ga rift event in the agnew wiluna belt yilgarn craton western australia the yakabindie area hosts significant deposits of disseminated fe ni cu sulfide in the six mile well 2 0 mt contained ni and goliath north 0 6 mt contained ni dunite lenses these dunite lenses form part of the same komatiitic ultramafic unit that hosts the giant mount keith mkd5 deposit 2 6 mt contained ni to the north but the morphology and internal stratigraphy of the host unit differs significantly three dimensional modeling of the yakabindie region reveals that each dunite lens is located at the intersection between steeply dipping nnw and nne trending synvolcanic faults which appear to have acted as conduits from which komatiitic lava vented onto the seafloor the igneous contacts dip more shallowly to the west and describe a w facing wedge of olivine cumulate rock probably less than 1 km in plunge extent and cored by adcumulate rock the mineralization at both six mile well and goliath north consists of multiple stacked lenses of disseminated fe ni cu sulfide associated with the olivine adcumulate core this core is interpreted as the pathway position that focused the flow of komatiite lava away from the vent rather than being linked by sheeted bodies of orthocumulate rocks as is typically the case along strike to the north the dunite lenses at yakabindie appear to be capped by a strike continuous sequence of thin komatiite flow units there is no evidence of the fractionated sequences of pyroxenite and dolerite which occur in pathway positions within the mount keith ultramafic unit to the north and which reflect late stage ponding and in situ fractionation of komatiite lava three sets of synvolcanic structures are recognized in the broader agnew wiluna belt and a model of oblique extension at ca 2 7 ga is proposed a nnw trending set of structures marks the overall rift orientation at ca 2 7 ga but is probably inherited from an earlier ca 2 81 ga phase of rifting a nw trending set is thought to represent ca 2 7 ga transfer faults but may also have much older roots these structures produce marked sinistral offsets in both the greenstone stratigraphy and the gravity response and can be used to divide the agnew wiluna belt into rift segments the third set of early structures trends nne subparallel with the volcanic grain of the greenstones as inferred from the orientation of komatiite lava pathways these are thought to have originated as extension orthogonal normal faults at ca 2 7 ga at the regional scale there is a strong spatial association between the larger fe ni cu sulfide deposits of the agnew wiluna belt and the locus of postulated nw trending transfer zones whereas at the deposit scale intersecting nw and nne trending synvolcanic faults exert a control on the location and best development in terms of thickness and or grade of fe ni cu sulfide mineralization in some deposits these spatial associations have major implications for exploration targeting at both regional and deposit scale

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

the relationships between sedimentary structures transport directions and dune types in mesozoic aeolian sandstones atacama region chile extensive occurrences of upper jurassic to lower cretaceous aeolian sandstones form part of a thick sequence of terrigenous red beds in northern chile these sediments accumulated in a n s elongated intermontane back arc basin on the landward side of an active volcanic chain this volcanic chain which was produced by subduction beneath the continental margin provided the source of almost all the clastic sediments the aeolian sands formed isolated dune fields covering only part of the depositional basin at any one time cosets of cross bedded sandstone up to 40 m thick represent the deposits of linear draas cross bedded sets which average between 0 5 and 1 m in thickness were produced by the migration of small dunes down and along the gentle lee slopes of the draas the majority of foreset laminations in the cross bedded sets are thin parallel and persistent they are the product of the lateral migration of dunes which developed oblique to the prevailing winds foreset laminations dip at low angles averaging 19 well below the angle of repose of dry sand most of the laminations originated by tractional processes rather than by grainflow avalanching or grainfall this conclusion indicates that the dunes were gently undulating with no slip faces the orientation of the direction of maximum dip of foreset laminations does not provide a reliable indicator of the palaeowind direction although individual sets or cosets commonly show a unidirectional pattern significant variations have been recorded between sets of the same age at nearby locations and vertically between one coset and the next variations in vertical sections were produced by the superimposition of draas with different orientations

early diagenetic spherulitic siderites from pennsylvanian palaeosols in the boss point formation maritime canada the boss point formation of maritime canada comprises alternating successions of braidplain sandstone lacustrine mudstone and lacustrine fluvio deltaic sandstone and mudstone these rocks were deposited within an active strike slip basin during the early pennsylvanian westphalian a at a palaeolatitude of 8 s palaesols comprise a small but distinct proportion of the formation and typically occur at the tops of fining upward sandstone siltstone cycles spherulitic siderites from the boss point formation occur within sandy and silty palaeosols they occur as large 200 m and small 200 m diameter spherulitic nodules within calcrete and show either a radially arranged ferroan calcite and siderite spar morphology or contain a core of ferroan calcite surrounded by a later formed sheath of siderite analytical data indicate that with respect to ca mg mn wide compositional variation occurs within the siderites the boss point formation siderites are similar to but contain less ca for a given mg mn ratio than previously published siderite compositional data published data together with the results from the boss point formation indicate that a continuum exists in freshwater siderites between the relatively mn depleted and the mg enriched analytical fields that were previously reported the relatively low ca values determined in the boss point formation samples reflect the general lack of calcium in the sedimentary basin during sedimentation both chlorite and smectite clays may have been important sources of iron it is concluded that the spherulitic siderites are eogenetic and formed within small anoxic ponds rich in organic matter under reducing and low dissolved sulphate conditions with time these pools dried up ferroan calcite precipitating as calcrete nodules around the earlier formed siderite spherulites

rhizoliths in ancient eolianites abstract rhizoliths are abundant within permo pennsylvanian eolian rocks of southeastern utah they are also present at the tops of several eolian sand bodies within the pennsylvanian fountain formation of southeastern wyoming and at a single stratigraphic level within the jurassic navajo sandstone of southeastern utah ranging in diameter from less than 1 mm to 15 cm these cylindrical structures are composed of siliciclastic free micrite and calcite spar cylinders ramify throughout the uppermost 2 3 m of thin flat topped sand bodies large cylinders are compound rhizoliths and display alveolar fabric a feature resulting from rootlet calcification that is characteristic of modern and ancient calcretes development of rhizoliths required flat geomorphically stable substrates for plant growth highly permeable surficial sediments and sources of calcium ions the restriction of rhizoliths to planar deflation surfaces suggests that plants were unable to colonize or stabilize eolian bedforms paleozoic rhizoliths appear to be restricted to coastal areas with onshore winds associated evaporites suggest an arid to semi arid paleoclimate rhizoliths formed when carbonate allochems derived from an upwind epeiric sea were dissolved and calcite reprecipitated around plant roots quaternary rhizoliths are best developed in similar sedimentologic and geographic settings the abundance of rhizoliths and carbonate allochems in quaternary and late paleozoic low latitude coastal dunes suggests that they are genetically related to glacio eustatic sea level fluctuations

stratigraphic evolution of an aeolian erg margin system the permian cedar mesa sandstone se utah usa the permian cedar mesa sandstone of south east utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which in terms of palaeogeographic setting reflects a transition from a dry erg centre through a water table controlled aeolian dominated erg margin to an outer erg margin subject to periodic fluvial incursion the erg margin succession represents a wet aeolian system accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes and a range of dry damp and wet flooded interdune elements is recognized variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre locally interconnected damp and or wet interdune ponds in the aeolian dominated erg margin and fully interconnected fluvially flooded interdune corridors in the outer erg margin relationships between aeolian dune and interdune units indicate that dry damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration temporal variation in the rates of water table rise and bedform migration determined the angle of climb of the erg margin succession such that accumulation rates increased during periods of rapidly rising water table whereas sediment bypassing zero angle of climb occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table during these periods in the outer erg margin the expansion of fluvially flooded interdunes in front of non climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass flood supersurfaces a spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the cedar mesa erg margin succession

stable isotopic characterisation of francolite formation stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits values of 13c and 34s in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions the exceptions are concretionary francolite from namibia which has an isotopic composition in keeping with its formation within organic rich sediments and that from offshore morocco which has an isotopic signature of the anoxic suboxic interface onshore deposits from jordan mexico south africa and possibly the permian phosphoria formation in the western u s a are substantially depleted in18o they appear to be too altered for deductions to be made about their environments of formation in other onshore deposits which are unaltered or minimally altered the isotopic composition suggests that some formed within sulphate reducing sediments sedhura morocco whilst francolite from the georgina basin of australia formed at the oxic anoxic boundary where oxidation of biogenic h2s decreases the 34s of pore water in general pelletal samples show non oxic isotopic signatures whilst non pelletal samples show oxic isotopic signatures but samples from namibia peru ica plateau and the californian and moroccan margins are exceptions to this rule morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature

crustal signature of late archaean tectonic episodes in the yilgarn craton western australia evidence from deep seismic sounding abstract deformation in the greenstone supracrustal rocks of the eastern goldfields province of the archaean yilgarn craton in western australia is delaminated from the underlying basement along a regional detachment surface presently at 3 7 km depth this might suggest that the history of crustal deformation cannot be inferred with any certainty from that of the greenstones however seismic images of the crust below the greenstones show structures that can be interpreted in terms of a series of tectonic events similar to those within the greenstones the upper crust below and to the west of the greenstones is largely unreflective with interpreted west dipping reverse faults the middle crust is reflective with reflector geometry implying thickening by west directed thrust stacking and the lower crust has a fabric indicative of ductile deformation these reflection fabrics imply crustal shortening probably during the late archaean regional d2 ene wsw shortening event they were subsequently overprinted and disrupted by structures consistent with regional nnw sse strike slip d3 faulting and probably younger more localised d4 faulting the seismic images of the crust therefore show that the crust suffered tectonic events in which both the order and direction of deformation are similar to those of the greenstones this is evidence that the whole crust deformed when the greenstones deformed however the scale and style of deformation vary with depth through the crust and include thrusting and probably folding in the upper crust thrust stacking in the middle crust and ductile deformation in the lower crust the length scale wavelength of structures in the middle and lower crust is greater than that in the greenstones

distribution of calcrete in holocene coastal sands in relationship to climate southwestern australia seven study areas located over 320 km of coastal plain in southwestern australia show the distribution of groundwater and vadose calcrete within holocene coastal sands age 7 100 yr bp over a regional climate gradient the calcrete exhibits a range of structures that include rhizoconcretionary mottled massive and laminar types groundwater calcrete composed of mottled massive and laminar types occurs as a thin sheet in the zone of capillary rise just above the water table vadose calcrete is predominantly rhizoconcretionary at all sites where it is well developed the groundwater and vadose calcretes are closely associated with vegetation calcrete is well developed in southern areas that are humid further north as the rainfall decreases and evaporation increases calcrete is less common until it is absent or rare at the semiarid northern limit of the study region its limit of occurrence is correlated with a region whose rainfall is ca 800 mm annum evaporation is ca 1 900 mm annum and mean summer temperature is 23 degrees c this study shows therefore that a trend toward a drier climate may not be conducive to the development of calcrete although such a view prevails in the literature

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

paleohydrologic controls on soft sediment deformation in the navajo sandstone abstract many workers have noted the presence of contorted cross strata in the navajo sandstone and other ancient eolianites and have recognized their significance as indicators of sediment saturation during the accumulation history horowitz 1982 proposed a general model for the production of such features in ancient ergs by episodic seismically induced liquefaction of accumulated sand a key feature of that popular model is the prevalence of a flat water table characteristic of a hyper arid climatic regime during deformation under arid climatic conditions the water table is established by regional flow and liquefaction is limited to the saturated regions below the level of interdune troughs however various paleohydrological indicators from navajo sandstone outcrops point toward a broader range of water table configurations during the deformation history of that eolianite some outcrops reveal extensive deformation complexes that do not appear to have extended to the contemporary depositional surface these km scale zones of deformation affecting multiple sets of cross strata and grading upward into undeformed crossbeds may represent deep water table conditions coupled with high intensity triggers which produced exclusively intrastratal deformation such occurrences contrast with smaller scale complexes formed within the zone of interaction between the products of soft sediment deformation and surface processes of deposition and erosion the horowitz model targets the smaller scale deformation morphologies produced in this near surface environment this study examines the implications of a wet climatic regime for the horowitz deformation model it demonstrates how a contoured water table characteristic of humid climates may have facilitated deformation within active bedforms as well as in the accumulation intra dune deformation would enable deflation of deformation features during the normal course of dune migration more parsimoniously accounting for the frequent occurrence of erosionally truncated deformation structures in the navajo sandstone the production of such erosional truncations during bedform climb and aggradation of the accumulation and the dramatic fluctuations in the water table required to deposit dry eolian sand deform those deposits under saturated conditions and then dry the deformed sand to enable deflation

the mineral chemistry near infrared and mid infrared reflectance spectroscopy of phengite from the olympic dam iocg deposit south australia article i nfo phengite is the main potassic dioctahedral mica identified at the olympic dam iron oxide copper gold iocg deposit south australia where its mineral chemistry is quite variable these differences can be explained by con trasting degrees of hydrothermal alteration in the heavily sericitized ore bearing rocks the phengites display a lower si content a higher al content and a lower mg number than the phengites from the weakly sericitized alter ation halo that surrounds the deposit variations are also observed in the near and mid infrared reflectance spectra collected from phengite bearing rocks in the near infrared high al phengite produces a spectral absorption feature at 2 206 m andthisfeature isdisplacedto2 213 m forlow al phengite inthe mid infrared high al phengite pro duces a strong reflectance peak at 9 59 m whereas this peak is observed at 9 57 mi n the spectra from low al phengite additional peaks were also identified at 10 98 12 22 and 13 33 m these were most intense in the spectra from high al phengite a drill core profile was produced using the results of the spectral analysis that shows the change in phengite mineral chemistry and phengite abundance as a function of depth in general near and mid infrared reflectance spectroscopy can be used to characterize the aluminum content of potassic dioctahedral micas like phengite and this information can be used to infer the degree of sericitic alteration that has occurred as a result of hydrothermal fluid flow

sedimentary evolution of the continental early middle triassic canizar formation central spain implications for life recovery after the permian triassic crisis the permian triassic transition p t was marked by important geochemical perturbations and the largest known life crisis consequences of this event as oxygen depleted conditions and the unusual behavior of the carbon cycle were prolonged during the early triassic interval delaying the recovery of life in both terrestrial and marine ecosystems studies on lower triassic sediments of continental origin as in the case of western europe are especially problematic due to the scarcity of fossils and absence of precise dating the canizar fm is an early middle triassic unit of continental origin of the se iberian ranges e spain a detailed sedimentary study of this unit allows a shedding of light on some unresolved problems of the continental deposits of this age the top of this unit is dated as early anisian by means of a pollen association while the age of its base is here estimated as late smithian or smithian spathian transition different facies associations and architectural elements have been defined in this unit in the western and central parts of the basin this unit shows sedimentary characteristics of fluvial deposits with locally intercalated aeolian sediments while in the eastern part there is an alternation of both aeolian and fluvial deposits sedimentary structures also indicate changes in the climate conditions mainly from arid to semiarid two marked arid periods when well preserved aeolian sediments developed during early middle spathian and spathian anisian transition they alternated with two semiarid but more humid periods during the late spathian and early anisian these conditions basically correspond with the general arid and very arid conditions described for central western european plate during the same period of time the ateca montalban high in the northern border of the study basin must have represented an important topographic barrier in the western tethys separating aeolian dominated areas to the n and ne from fluvial dominated areas to the south the canizar fm has been subdivided into six members a f separated by seven 1 7 major bounding surfaces mbs these surfaces are well recognized laterally over hundred of km and they represent 104 105 my mbs 5 is considered to be of late spathian age and it is a clear indication of tectonic activity represented by a mild unconformity this event represents a change in the sedimentary characteristics reactivation of the unit and from here to the top of the unit are found the first signals of biotic recovery represented by tetrapod footprints plants roots and bioturbation all of these characteristics and the estimated age represented by the mbs 5 event permit this surface to be related to the coeval hardegsen unconformity of central western europe these first signals of biotic recovery can thus be related to an increased oxygen supply due to the new created paleogeographical corridors in the context of this tectonic activity these biotic signals occurred 5 my after the permian triassic limit crisis a similar delay as occurred in other coeval and neighboring basins

the langmuir volcanic peridotite associated nickel deposits canadian equivalents of the western australian occurrences the 2 7 b y old abitibi greenstone belt is a crudely s shaped zone of metamorphosed volcanic subvolcanic and sedimentary rocks and associated intrusions through it are scattered a dozen small nickel deposits in volcanic peridotites their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous mgo with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows at least one deposit dumont is hosted by a dunitic intrusion others e g dundonald langmuir 2 are partially hosted in sulfidic iron formations mining of the langmuir 2 deposit permitted extensive underground mapping core logging and sample collecting the other deposits are known mainly from poor surface exposures and drill core the langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in western australia a magmatic origin is supported by the localization of the main ore zone in a paleotrough by the presence of a possible volcanic feeder beneath the ore and by the regular sequence through the basal ultramafic flow of massive ore overlain by net textured ore then by barren peridotite and lastly by a spinilex textured cap to the flow an unusual spinilex textured and sulfide bearing rock and skeletal chromite also support a magmatic origin as do the base and precious metal contents of the ores the high s se ratios averge 23 000 and slightly lower sulfur isotopic ratios average 0 5 per mil delta 34 s compared to most magmatic nickel sulfide deposits are similar to the ratios for windarra western australia and may be explained by assimilation of sulfide rich cherty iron formation other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore and the high sulfur content of some ores these features result from deformation and greenschist facies metamorphism but surviving features of an earlier magmatic history argue against the ores being formed metamorphically evidence of contemporaneous fumarolic activity was found at the south end of the mine but its importance as an ore forming process was not supported by detailed petrological and geochemical studies limited data from the other deposits in the abitibi belt suggest a common origin for the langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced the volume of contained nickel sulfides the presence of sulfidic iron formations and the severity of alteration and metamorphism the deposits in western australia probably were formed by similar processes in particular the langmuir deposits display strikingly similar sulfide chemistries to the windarra deposits which are probably dae to assimilation of sulfide rich cherty iron formation