neoproterozoic sedimentation and tectonics of the laurentian midcontinent detrital zircon provenance of the jacobsville sandstone lake superior basin usa and canada

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

half graben filling processes in the early phase of continental rifting the miocene namurungule formation of the kenya rift abstract the miocene namurungule formation crops out on the eastern flank of the kenya rift representing basin fill that developed in association with miocene rifting the formation is characterized by large volumes of volcaniclastics supplied passively by pyroclastic fall facies analysis reveals that this formation consists mostly of lacustrine delta deposits with minor alluvial fan deposits at its base the conspicuous occurrence of a flood plain facies with infrequent channel fill deposits in the lower part of this formation suggest that the drainage area was limited in the early stage of deposition pyroclastic fall would therefore have been an important source of sediment during the early stages of rift development the delta deposits are divided into two distinct successions based on lithological characteristics separated by a thick pyroclastic layer in the middle part of the formation the stacking pattern of the lower succession is retrogradational whereas the upper succession is characterized by a pile of prograding bodies both delta successions are interpreted to have accumulated in an underfilled basin the change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake level rise assuming constant sediment supply the apparent difference in flooding scale between the lower and upper successions is attributed to the topographical widening of the basin flat similar successions also occur in another basin in the east african rift ngorora formation central kenya in which pyroclastic sediments are dominant and upward decrease in sedimentation rate is recognized the differing in the stacking patterns between the namurungule formation and the ngorora formation is probably induced by basin width the narrow namurungule basin seems to have been sensitive to increased sediment supply owing to the expansion of the catchment area

palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information much of the work performed in this area has focused on either of two types of terrestrial carbonate palustrine carbonates or calcretes pedogenic and groundwater yet their simultaneous study shows there may be a gradual transition from one form to the other revealing the interplay between pedogenic sedimentary and diagenetic processes three main factors control the formation of these carbonates the position of the water table the host rock and the period of sub aerial exposure in pedogenic calcretes precipitation of carbonate takes places mostly in the vadose zone above the water table and within a previous host rock or sediment in groundwater calcretes the precipitation of carbonate also occurs within a previous host rock and around the groundwater table in palustrine carbonates however the precipitation of lime mud occurs in a lacustrine water body palustrine carbonates necessarily form on previous lacustrine mud whereas both types of calcretes may form on any type of sediment or soil the sub aerial exposure time needed to form palustrine carbonates may by relatively short even a season whereas pedogenic calcretes need more time several years to millions of years groundwater calcretes do not form on the topographic surfaces so there is no need of sub aerial exposure however stable surfaces favour the development of thick groundwater calcretes small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features causing difficulties in the interpretation of these carbonates the formation of these carbonates is controlled by palaeoenvironmental factors both commonly form in semi arid climates arid climates are also suitable for calcretes but sub humid conditions are more suitable for palustrine carbonates more indications of climatic conditions may be obtained through the analysis of the d18o content of both calcretes and palustrine carbonates and from the depth of the horizon containing carbonate nodules in pedogenic calcretes vegetation is also important in the formation of these types of carbonates data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13c signal of the primary carbonates which in pedogenic carbonates has also been used to estimate atmospheric pco2 during the phanerozoic these terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins their presence and characteristics can be used as indicators of aggradation subsidence or accommodation rates and therefore as indicators of different tectonic regimes even though the study of these carbonates has notably increased in recent years much less is known about them than about marine carbonates presently there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins with a need to include the carbonates analysed in this paper

variations in mid continent rift magma volumes consistent with microplate evolution 1 modeling of gravity data along the 1 1 ga failed mid continent rift system shows systematic patterns in magma volume between and along the rift s two arms the volume of magma increases towards the lake superior region consistent with magma flowing away from a hotspot source there the west arm experienced significantly more magmatism these patterns are consistent with a model in which the two rift arms acted as boundaries of a microplate the volume of magma along the west arm increases with distance from the euler pole indicating that it acted essentially as a spreading ridge whereas the much smaller magma volumes along the east arm are consistent with its acting as a leaky transform this view of the rift system s evolution is compatible with the rift being part of an evolving plate boundary system rather than an isolated episode of midplate volcanism

a marine origin for the late mesoproterozoic copper harbor and nonesuch formations of the midcontinent rift of laurentia abstract the c 1 1 ga copper harbor and nonesuch formations of the keweenawan supergroup exposed along the canadian united states shorelines of lake superior are part of the surface exposures of the laurentian midcontinent rift these units have long been considered non marine in origin and have figured prominently in ideas regarding the evolution of microbial life and the redox conditions of earth s ocean atmosphere system at the close of mesoproterozoic time however these rocks also host hydrothermal metal deposits the emplacement of which may have compromised primary geochemical signals that are used to underpin those ideas here we highlight new sedimentological observations to provide an independent framework for assessing the depositional setting and geochemistry of those strata we show that the totality of sedimentological features leads to the conclusion that parts of the upper copper harbor formation and the entirety of the nonesuch formation were deposited along tide and wave influenced shorelines and in shallow marine settings under evaporitic conditions evidence for this interpretation includes the abundance of flaser wavy linsen and pinstripe bedding ubiquity of reactivation surfaces and mud drapes associated with all ripple forms superposed sets of ripple cross lamination showing bimodal herring bone sediment transport directions desiccation cracks and metre scale hummocky cross stratification further evaporite fabrics and pseudomorphs after gypsum in the upper 200 m of the copper harbor formation and in numerous stratigraphic positions within the nonesuch formation indicate that the water body was saline not fresh the emerging palaeogeographic image is one of a large shallow marine embayment with fringing sabkha like shorelines ideas about late mesoproterozoic biospheric evolution and earth s surface redox and oxygenation that rely on the nonesuch formation and copper harbor stromatolites having been deposited within a lacustrine setting require reassessment

the flin flon paleosol and the composition of the atmosphere 1 8 bybp within the 1800 to 1900 my old flin flon snow lake greenstone belt amisk group volcanics are overlain by missi group fluvial sediments several localities along the missi amisk contact the volcanics show evidence of subaerial weathering field relationships mineralogical evidence and chemical analyses confirm that this alteration zone is a paleosol pedogenic fabrics and mineralogy were somewhat obscured by greenschist grade metamorphism associated with the hudsonian orogeny 1750 my this is especially true in the upper meter of the paleosol where metamorphic paragonite and sericitic micas developed in a crenulated fabric this metamorphism did not however obliterate the imprint of weathering on the amisk volcanics features characteristic of well drained modern soils are evident in the paleosol corestones of spheroidally weathered pillow lavas occur at depth within the paleosol cr horizon the corestones decrease in size upward and eventually disappear into a hematite rich horizon at the top of the paleosol these macroscopic changes are accompanied by a decrease in cao and mgo and by an increase in al2o3 tio2 and total iron toward the paleosol missi contact ferrous iron decreases upward toward the contact feo was apparently oxidized to ferric iron and retained within the paleosol during weathering the oxidation and retention of iron within the flin flon paleosol indicates that po2 was probably or 10 2 p a l at the time of weathering the behavior of iron in the flin flon paleosol contrasts sharply with its behavior in the 2200 my hekpoort paleosol which is strongly depleted in iron this difference suggests that a significant increase in the ratio of po2 pco2 in the atmosphere took place between 2200 and 1800 mybp

tutorial on seismic interferometry part 1 basic principles and applications seismic interferometry involves the crosscorrelation of responses at different receivers to obtain the green s function between these receivers for the simple situation of an impulsive plane wave propagating along the x axis the crosscorrelation of the responses at two receivers along the x axis gives the green s function of the direct wave between these receivers when the source function of the plane wave is a transientas in exploration seismology or a noise signalas in passive seismology then the crosscorrelation gives the green s function convolved with the autocorrelation of the source function direct wave interferometry also holds for 2d and 3d situations assuming the receivers are surrounded by a uniform distribution of sources in this case the main contributions to the retrieved direct wave between the receivers come from sources in fresnel zones around stationary points the main application of direct wave interferometry is the retrieval of seismic surface wave responses from ambient noise and the subsequent tomographic determination of the surfacewave velocity distribution of the subsurface seismic interferometry is not restricted to retrieving direct waves between receivers in a classic paper claerbout shows that the autocorrelation of the transmission response of a layered medium gives the plane wave reflection response of that medium this is essentially 1d reflected wave interferometry similarly the crosscorrelation of the transmission responses observed at two receivers of an arbitrary inhomogeneous medium gives the 3d reflection response of that medium one of the main applications of reflected wave interferometry is retrieving the seismic reflection response from ambient noise and imaging of the reflectors in the subsurface a common aspect of direct and reflected wave interferometry is that virtual sources are created at positions where there are only receivers without requiring knowledge of the subsurface medium parameters or of the positions of the actual sources

insects and crustaceans from the latest early early middle triassic of poland two stratigraphical horizons in the palegi clay pit a new triassic paleontological site within buntsandstein deposits latest olenekian early anisian in age in the holy cross mountains poland

the 1100ma sturgeon falls paleosol revisited implications for mesoproterozoic weathering environments and atmospheric co2 levels abstract during the active rifting stage of the 1100 ma midcontinental rift in north america alluvial sediments were deposited intermittently between basalt flows on the north and south shores of present day lake superior at times of depositional quiescence paleosols developed in both areas on the alluvial sediments and on the antecedent basalt new results from the sturgeon falls paleosol in michigan characterizing the weathering processes at the time of its formation indicate moderate maturity high degrees of hydrolysis and leaching and a low degree of salinization geochemical provenance indices indicate a homogeneous source for the paleosols and in contrast to earlier work there is little evidence for k metasomatism as a result atmospheric co 2 levels of 4 6 pre industrial atmospheric levels were calculated using a mass balance model this result is consistent with previous calculations from nearly contemporaneous paleosols from the other side of the keweenawan rift and from the 100 ma younger sheigra paleosol in scotland the calculated co 2 values are also consistent with the calculated weathering environment proxies that indicate weak to moderate weathering at this time frame and suggest that the higher greenhouse gas loads indicated by paleoproterozoic paleosols had dissipated by the mid late mesoproterozoic

chemistry of modern soils as a guide to interpreting precambrian paleosols precambrian paleosols are potentially one of our best sources of information about the composition of the atmosphere early in earth s history to be used this way however an alteration profile must be shown to be an in situ product of vadose processes data from modern soils show that the ratios of less mobile elements such as zr ti and al can be used as a preliminary screen to test for the in situ nature of a profile if the ti zr ratio departs from that of the host rock by more than 40 or the ti al ratio departs by more than 50 for moderate degrees of weathering then a significant allochthonous contribution to the profile is indicated this technique correctly separates in place from allochthonous horizons when applied to independent data from two modern profiles a review of published precambrian profiles identifies two often cited examples denison and pronto as suspect based on these criteria in the interpretation of paleoatmospheres it is also necessary to show that fe is controlled not b

sulfur and carbon isotopic evidence for metabolic pathway evolution and a four stepped earth system progression across the archean and paleoproterozoic abstract the earth s mantle has provided a ready redox gradient of sulfur compounds so2 h2s since the stabilization of the crust and formation of the ocean over 4 billion years ago and life has evolved a multitude of metabolic pathways to take advantage of this gradient these transitions are recorded in the sulfur and carbon isotope signals preserved in the rock record in the genomic records of extant microorganisms and in the changing mantle and crust structure composition and cycling here we have assembled approximately 20 000 sulfur 34s 33s 36s and carbon 13c isotope data points from scientific publications spanning over five decades of geochemical analyses on rocks deposited from 4 0 to 1 5 ga we place these data in the context of molecular clock and tectonic and surface redox indicators to identify overarching trends and integrate them into a holistic narrative on the transition of the earth s surface towards more oxidizing conditions the greatest extreme in 34s values of sulfide minerals 45 5 to 54 9 and sulfate minerals 13 6 to 46 6 as well as 13c values in carbonate minerals 16 8 to 29 6 occurred in the period following the great oxidation event goe while the greatest extremes in organic carbon 13c values 60 9 to 2 4 and sulfide and sulfate mineral 33s and 36s values 4 0 to 14 3 and 12 3 to 3 2 respectively occurred prior to the goe from our observations we divide transitions in earth s history into four periods period 1 4 00 to 2 80 ga during which geochemical cycles were initialized period 2 2 80 to 2 45 ga during which s and c isotope systems exhibit changes as conditions build up to the goe period 3 2 45 to 2 00 ga encompassing the goe and period 4 after 2 00 ga after which s and c isotopic systems remained relatively constant marking a time of earth system geochemical quiescence using these periods we link changes in s and c isotopes to molecular clock work to aid in interpreting emerging metabolic functions throughout earth s history while underscoring the need for better proxies for robust evolutionary analyses specifically results indicate 1 an early development of sulfide oxidation and dissimilatory sulfite reduction followed by disproportionation and then sulfate reduction to sulfite resulting in a fully biologically mediated sulfur cycle by 3 25 ga 2 support for the acetyl coenzyme a pathway as the most likely earliest form of biologically mediated carbon fixation following methanogenesis 3 an increasingly redox stratified ocean in the neoarchean with largely oxic surface water and euxinic bottom water during the first half of the paleoproterozoic and 4 that secular changes in earth system crustal cycling dynamics and continent formation likely played a key role in driving the timing of the goe finally based on geochemical data we suggest that the paleoproterozoic be divided into a new era of the eoproterozoic from 2 45 to 2 00 ga and the paleoproterozoic from 2 00 to 1 60 ga

life associated with a 2 76 ga ephemeral pond evidence from mount roe 2 paleosol dark sericitic material at and near the top of the 2 765 0 01 ga mount roe 2 paleosol in western australia contains 0 05 0 10 wt organic carbon with delta 13c values between 33 and 51 pdb peedee belemnite such negative isotopic values strongly indicate that methanotrophs once inhabited this material the textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds that these ponds became desiccated and that heavy rains transported the material to its present sites the discovery of methanotrophs associated with the mount roe 2 paleosol may extend their geologic record on land by at least 1 5 b y methanotrophy in this setting is consistent with the notion that atmospheric methane levels were or 20 mu atm during the late archean the radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the late archean

mineral weathering and soil development in the earliest land plant ecosystems land colonization by plants and their fungal and bacterial symbionts during the paleozoic was fundamental to the evolution of terrestrial ecosystems but how these early communities influenced mineral weathering and soil development remains largely unknown we investigated cryptogamic ground covers cgcs in iceland to identify modern analogous communities and to characterize soil structure and biologically mediated weathering features using a novel application of x ray microcomputed tomography we show that moss dominated cgcs and their soils are not adequate analogues of early communities comparisons with the 407 ma rhynie chert scotland biota indicate that modern cgcs dominated by lichens liverworts and their associated symbionts fungi cyanobacteria are more representative of early soil forming communities liverwort and lichen soils are thin and their depth and complexity are constrained by the size and growth form of the dominant plants or lichens they are aggregated and stabilized by cyanobacteria mycorrhizal and lichenized fungi rhizoids and associated exudates smectite was associated with liverwort but not with moss cgc soils soil grain dissolution features are diverse and attributable to different organisms e g bacteria fungi and types of interaction e g symbiosis we postulate that such features provide a novel indirect means of inferring biotic interactions in paleosols

sedimentary provenance and weathering processes in the 1 1 ga midcontinental rift of the keweenaw peninsula michigan usa abstract fluvial alluvial lacustrine and debris flow sedimentary rocks from the keweenaw peninsula michigan usa are geochemically characterized to examine weathering processes and provenance these sedimentary rocks result from the erosion weathering and redistribution of effusive and plutonic igneous lithologies during periods of volcanic quiescence associated with the failed 1 1 ga north american midcontinental rift three units comprising the sedimentary igneous mixed portage lake volcanics coarseness and source varied copper harbor conglomerate and the quartz rich post rift fill unit of the jacobsville sandstone were studied over a geographically wide and lithologically varied range of localities across the keweenaw peninsula michigan provenance indicators such as ti zr and ti al ratios and la th sc ternary diagrams reveal that the portage lake volcanic conglomerates are near source and that the copper harbor conglomerate is primarily derived from felsic uplifted basement and recycled orogen lithologies to the east the jacobsville sandstone is a combination of pulses of sources from the top to the bottom of the stratigraphic section 1 felsic arkosic arenites sourced from the erosion and uplift of basement orogenic volcanic and plutonic rocks top 2 felsic arkosic subarkosic arenites sourced from the erosion of uplifted basement recycled orogeny and reworking of primary sedimentary and metasedimentary rocks middle and 3 quartz arenites eroded from the cratonic interior highly mature this work can consequently aid in structuring a stratigraphic nomenclature for the jacobsville sandstone both petrographic data and weathering indices such as the chemical index of alteration cia and the paleosol hydrolysis metric al ca na mg k indicate low degrees of weathering during syn rift deposition at 1 1 ga but increases with evolution of the rift in post rift units toward the late mesoproterozoic early neoproterozoic values for the cia throughout the mesoproterozoic from this and other localities have the same trend to that of proposed atmospheric co2 concentrations derived from paleosol data demonstrating that characterizing the weathering trends of floodplain deposits can be a useful addition to deciphering past paleoclimatic conditions in the precambrian geologic record

taphonomic analysis in lacustrine environments two different contexts for triassic lake paleofloras from western gondwana argentina abstract during the earliest triassic several rift basins developed along the western margin of gondwana associated with the pre breakup of pangea they were filled by exclusively non marine sediments including alluvial fluvial and lacustrine deposits in the ischigualasto villa union basin the lacustrine deltaic succession is placed in the los rastros formation and consists of several coarsening upward cycles of black shale siltstone and sandstone the paleontologic content of the succession includes abundant floral remains related to the dicroidium type flora invertebrates conchostracans insects and vertebrates fishes a temnospondyl amphibian ichnites at the cerro puntudo area in the cuyana basin the lacustrine succession forms the upper part of the cerro puntudo formation and consists of limestone stromatolitic limestone mudstone sandstone and tuff the paleontologic content includes scarce floral remains and rhizoliths invertebrates are represented exclusively by traces associated with ichnofacies of skolithos and scoyenia and vertebrates by a fragment of the pelvic girdle of a basal arcosaur the taphonomic analysis performed in the two triassic lacustrine successions allows recognition of two different taphonomic histories for the plant remains the los rastros lake preserved both autochthonous originated in the littoral zone and allochthonous originated in the upstream fluvial system elements the offshore lacustrine area was dominated by autochthonous well preserved elements and allochthonous plant debris and wood which formed time averaged accumulations the delta deposits are characterized by allochthonous elements with varied preservational conditions usually showing evidence of mechanical degradation and accumulation within a short time autochthonous and allochthonous material were preserved in the los rastros lake by means of anoxic conditions in the offshore lacustrine area and high sedimentation rates in the delta in contrast the cerro puntudo lake preserved only autochthonous elements originated in the littoral zone including rhizoliths and foliar material which formed autochthonous and parautochthonous accumulations at the littoral zone in spite of aerobic conditions this was the result of tuffaceous material that enhanced preservation thus these very different lacustrine environmental contexts are showing different preservational modes the fossil assemblages allowed the reconstruction of the original communities from this part of gondwana thus the los rastros lake margins were characterized by shrubs and small trees of ginkgoales and corystospermales and herbaceous members of the sphenophyta the sphenophytes were also the dominant floral component along the river margins whereas the corystospermales cycadales pteridophyta and conifers formed the woodland upstream probably related to the floodplains of a trunk fluvial system the littoral zone of the cerro puntudo lake was dominated mainly by herbaceous sphenophytes and lycopsids these fossil assemblages characterize the paleoflora associated with lacustrine systems a fuller understanding of the processes that generate these assemblages is essential for comparisons with other continental paleobotanical records in the middle triassic of gondwana e g australia south africa

near wellbore vsp imaging without overburden in seismic exploration a complex overburden can cause defocused and distorted seismic images of deeper target areas while in surface seismic imaging it is difficult to overcome this problem vertical seismic profile vsp borehole seismic imaging with its well oriented recording geometry has proven able to circumvent the overburden s detrimental impact recently vspnear wellboresaltflankimagingwithout knowledge of the overburden has shown encouraging results in this paper we introduce another method for a more common imaging purpose imaging near wellbore horizons without knowledge of the overburden a key component of ourmethodisareverseray tracingtechniquethatusesactual vspdirect wave traveltime picks and extrapolates through a local velocity model to obtain the direct wave traveltimes in the vicinity of the well the extrapolated traveltimes then serve as references for migration of the vsp reflected data from near wellbore horizons the merits of this method are that all source side effects such as shot position and triggeringtimestaticsareavoided anditcanefficientlyandrobustly imagethenear wellborehorizonsbecauseitdoesnotneedan estimated overburden velocity model prone to errors however the image is limited to a diamond shaped area around the well whose size is proportional to the vsp array a synthetic vsp experiment on the seg eage 2d overthrust modelshowsthatthismethod withalocalevenanestimated 1d velocity model effectively images structural features suchasdipdirectionanddiscontinuitiesofthenear wellbore horizons

unconventional distribution of facies in a continental rift basin the pliocene pleistocene mangas basin south western new mexico usa the pliocene early pleistocene mangas basin in sw new mexico usa was a n nw trending full graben that changed southward to an eastward tilted half graben unlike the facies distribution predicted in existing models the half graben part of the mangas basin was characterized by broad alluvial fans derived from the footwall scarp smaller hangingwall derived alluvial fans and a shallow closed lake lake buckhorn that locally lapped onto the hangingwall hills the distribution of facies within the full graben part of the mangas basin was also unlike that predicted in current models primarily because of a broad belt of alluvial fan sediment derived from the eastern footwall scarp and a narrow belt of axial fluvial sediment adjacent to the western footwall scarp the distribution of facies in the mangas basin does not appear to have been controlled by the eastward tilt of the floor of the half graben or see saw motion of the floor of the full graben as predicted by existing models but rather by the large size of the alluvial fans on the eastern side of the basin these fans were derived from large high relief catchments on the footwall scarp of the mogollon mountains the uplift of which began during early miocene this example illustrates how earlier uplift and drainage development in a mountain range may influence facies distribution in a younger extensional basin

imaging of offset vsp data acquired in complex areas with modified reverse time migration a modified reverse time migration algorithm for offset vertical seismic profiling data is proposed this algorithm performs depth imaging of target areas in the borehole vicinity without taking into account the overburden originally recorded seismograms are used reliable results can be obtained using only the velocity profile obtained along the well the downgoing wavefield emitted from a surface source is approximated in the target area using the transmitted p wave recorded by the receivers deployed in the well this is achieved through a reverse time extrapolation of the direct transmitted p wave into the target area after its separation in offset vertical seismic profiling seismograms generated using a finite difference scheme for the solution of the scalar wave equation the proposed approach produces kinematically reliable images from reflected pp and ps waves and furthermore can be applied as a salt proximity tool for salt body flank imaging based on the transmitted ps waves our experiments on synthetic data demonstrate that the modified reverse time migration provides reliable depth images based on offset vertical seismic profiling data even if only the velocity profile obtained along the borehole is used

pedogenesis and geochemical alteration of the picture gorge subgroup columbia river basalt oregon reddened clay rich horizons between basalt flows have historically been identified as baked zones or as zones of lateral groundwater movement the reddening has been attributed to thermal or chemical alteration of the permeable portion of the basalt flow many of these red horizons are instead paleosols that record significant hiatuses in basalt flow emplacement paleosols were identified in the field on the basis of features such as horizonation ped morphology and root traces terrestrial gastropods fossil plants and peats also occur locally plots of weathering indices such as base loss are consistent with a pedogenic origin for these horizons ca na mg fe 21 and p have been extensively lost from the a and bt horizons of the paleosols consistent with modern weathering patterns k and rb have been added to the upper portions of the profiles and lost in the lower portions of the profiles relative to the parent material with rb added disproportionately historically this pattern of enrichment has been cited as evidence of k metasomatism the apparent metasomatism can be explained using a new model in which there was extensive felsic volcanism contemporaneous with pedogenesis and k and rb were likely added by windborne ash the apparent discordance between k and rb contents of the paleosols can be explained by the presence of land plants because k is an important electrolyte and rb does not serve any major biological role this model could account for apparent k metasomatism in other paleosols following the advent of land plants and may be applicable to older paleosols as well

triassic climates state of the art and perspectives abstract the climate of the triassic period was characterized by a non zonal pattern dictated by a strong global monsoon system with effects that are most evident in the tethys realm this strong monsoonal regime is related to the aggregation of the pangaean supercontinent which by triassic time was already completed climate oscillations existed within this framework the harsh hot house climatic conditions that characterized the late permian and perhaps precipitated the end permian mass extinction were probably maintained during the early triassic and may account for the impoverished but distinctive faunal and floral lower triassic associations although metazoan reef builders were probably the most affected group carbonate production remained high at least in the western tethys realm the middle triassic was characterized locally by humid episodes although their geographical distribution has yet to be clarified the carnian pluvial event marks an episode of increased rainfall documented worldwide was the most distinctive climate change within the triassic different hypotheses have been proposed for its causes changes of atmospheric or ocean circulation driven by plate tectonics a peak of the global monsoon due to maximum continent aggregation or triggering by the eruption of a large igneous province subsequently the late carnian and norian seem to have been climatically stable although minor climatic changes have recently been described even from this time period finally the end triassic extinction event is also associated with climate change specifically warming and increased rainfall but this evidence comes mostly from the northern parts of the central atlantic magmatic province and the global pattern of climate change at the triassic jurassic boundary has still to be resolved many facets of triassic climate are intriguing and deserve further research however palaeoclimate studies on the triassic have so far been carried out only locally with different proxies those proxies will require inter calibration in order to depict correctly the temporal and geographical patterns of triassic climate

2 d wavepath migration prestack kirchhoff migration km is computationally intensive for iterative velocity analysis this is partly because each time sample in a trace must be smeared along a quasi ellipsoid in the model as a less costly alternative we use the stationary phase approximation to the km integral so that the time sample is smeared along a small fresnel zone portion of the quasi ellipsoid this is equivalent to smearing the time samples in a trace over a 1 5 d fat ray i e wavepath so we call this wavepath migration wm this compares to standard km which smears the energy in a trace along a 3 d volume of quasi concentric ellipsoids in principle single trace migration with wm has a computational count of o n1 5 compared to km which has a computational count of o n3 where n is the number of grid points along one side of a cubic velocity model our results with poststack data show that wm produces an image that in some places contains fewer migration artifacts and is about as well resolved as the km

local vertical seismic profiling vsp elastic reverse time migration and migration resolution salt flank imaging with transmitted p to s waves to avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model we introduce a vertical seismic profiling vsp local elastic reverse time migration rtm method for salt flank imaging by transmitted p to s waves this method back projects the transmitted ps waves using a local velocity model around the well until they are in phase with the back projected pp waves at the salt boundaries the merits of this method are that it does not require the complex overburden and salt body velocities and it automatically accounts for source side statics in addition the method accounts for kinematic and dynamic effects including anisotropy absorption and all other unknown rock effects outside of this lo cal subsalt velocity model numerical tests on an elastic salt model and offset 2d vsp data in the gulf of mexico using a finite difference time domain staggered grid rtm scheme partly demonstrate the effectiveness of this method over interferometry

co2 drawdown and cooling at the onset of the great oxidation event recorded in 2 45 ga paleoweathering crust abstract the archean atmosphere is thought to have been devoid of oxygen but instead containing high concentrations of greenhouse gases such as co2 and possibly ch4 that were required to keep the earth surface environments warm enough for maintenance of liquid water under faint young sun conditions earlier studies have suggested that the co2 concentrations must have been as high as over 1000 times present atmospheric levels pal to overcome the effects of weaker solar luminosity however more recent studies of precambrian paleosols imply existence of at least tenfold lower but still high pco2 levels the appearance of minute amounts of atmospheric o2 at the transition of archean and proterozoic is well documented however the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated different paleoclimate proxies of the 2 45 ga kuksha paleoweathering crust suggest its formation in a cool temperate climate paleoatmospheric pco2 estimates by geochemical and isotopic mass balance models suggest remarkably low paleoatmospheric co2 levels with the best guess estimates of atmospheric co2 between 1 and 10 pal we speculate that the kuksha paleoweathering crust either predates or partly overlaps with the onset of huronian glaciation and corresponds to a time interval of low co2 level during the first recorded large glaciation in earth history

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

imaging above an extended reach horizontal well using converted shear waves and a rig source abstractwe evaluated a method for using 3c vertical seismic profile data to image acoustic interfaces located between the surface source and a downhole receiver array the approach was based on simple concepts adapted from whole earth seismology in which observed compressional and shear wavefields are traced back to a common origin however unlike whole earth and passive seismology in which physical sources are imaged we used the observed compressional and shear wavefields to image secondary sources scatterers situated between the surface source and the downhole receiver array the algorithm consisted of the following steps first estimating the receiver compressional wavefield second using polarization to estimating the shear wavefield third deconvolving the shear wavefield using estimates of the source wavelet obtained from the direct compressional wave fourth the compressional and shear wavefields were back projected into the volume between the source and receivers where finally an imag

revisiting hcn formation in earth s early atmosphere abstract using a new photochemical model the hcn chemistry in earth s early atmosphere is revisited we find that hcn production in a ch 4 rich early atmosphere could have been efficient similar to the results of a previous study zahnle 1986 for an assumed ch 4 mixing ratio of 1000 ppmv hcn surface deposition increases from 2 10 9 cm 2 s 1 at fco 2 3 to more than 1 10 10 cm 2 s 1 30 tg yr at fco 2 0 3 and 1 these conditions may well have applied throughout much of the archean eon 3 8 2 5 ga prior to the origin of life and the advent of methanogens hcn production rates would likely have been at 1 10 7 cm 2 s 1 or lower thereby providing a modest source of hcn for prebiotic synthesis

taphonomy of insects in carbonates and amber abstract the major taphonomic processes that control insect preservation in carbonate rocks limestones travertines and nodules are biological insect size and wingspan degree of decomposition presence of microbial mats predation and scavenging environmental water surface tension water temperature density and salinity current activity and diagenetic authigenic mineralisation flattening deformation carbonisation the major taphonomic processes that control the preservation of insects in fossil resins amber and copal are different but can be considered under the same headings biological presence of resin producers size and behaviour of insects environmental latitude climate seasonality resin viscosity effects of storms and fires soil composition and diagenetic resin composition insect dehydration pressure carbonisation thermal maturation reworking oxidation these taphonomic processes are geographically and temporally restricted and generate biases in the fossil record nevertheless where insects occur they may be abundant and very diverse taphonomic processes may impact on phylogenetic and palaeobiogeographic studies in determining the timing of the origin and extinction of insect groups and in identifying radiations and major extinctions taphonomic studies are an essential prerequisite to the reconstruction of fossil insect assemblages to interpreting the sedimentary and environmental conditions where insects lived and died and to the investigation of interactions between insects and other organisms

the sturgeon falls paleosol and the composition of the atmosphere 1 1 ga bp a paleosol is exposed along the north bank of the sturgeon river some 25 km sw of baraga michigan the paleosol was developed on hydrothermally altered keweenawan basalt and is overlain by the jacobsville sandstone textures mineralogy and chemical composition change gradually upwards from unweathered metabasalt through the paleosol to the contact of the paleosol with the jacobsville sandstone many of these changes are similar to those in modern soils developed on basaltic rocks however k has clearly been added to the paleosol probably by solutions which had equilibrated with k feldspar in the jacobsville sandstone the keweenawan basalt was oxidized quite extensively during its conversion to greenstone during weathering the remaining fe2 was oxidized to fe3 and was retained in the paleosol the composition of the parent greenstone and its change during weathering can be used to define an approximate lower limit to the ratio of the o2 pressure to the co2 pressure in the atmosphere during the formation of the paleosol formula see text free o2 must have been present in the atmosphere 1 1 ga ago but its partial pressure could have been 10 3 times lower than in the atmosphere today

lacustrine carbonates and pedogenesis sedimentology and origin of palustrine deposits from the early cretaceous rupelo formation w cameros basin n spain the berriasian rupelo formation of the w cameros basin consists of a 2 200 m thickness of marginal and open lacustrine carbonate and associated deposits open lacustrine facies contain a non marine biota with abundant charophytes both stems and gyrogonites ostracods gastropods and rare vertebrates carbonate production was mainly biogenic the associated marginal lacustrine palustrine facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history the succession was laid down in a low gradient shallow lake complex characterized by wide fluctuations of the shoreline carbon and oxygen stable isotope analyses from the carbonates show non marine values with ranges of 13 from 7 to 11 and 18 from 3 to 7 5 differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake analyses from the entire sample suite plot on a linear trend isotopic compositions become lighter with increasing evidence of pedogenic modification this suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil derived co2 the stable isotope data thus support evidence from petrography and facies relations that palustrine limestones form through pedogenic modification of lake carbonates

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

climate controls on soil respired co2 in the united states implications for 21st century chemical weathering rates in temperate and arid ecosystems abstract the most recent ipcc 2007 report predicts that regional scale precipitation patterns will change significantly in the 21st century as a result of increasing atmospheric co 2 the amount of respired co 2 stored in the soil atmosphere is heavily influenced by climate and the concentration of co 2 in soils controls the concentration of dissolved co 2 co 2aq in pore water involved in chemical weathering therefore we can expect changes to chemical weathering rates as a result of climate change which can influence the global carbon cycle through the consumption of co 2 during continental silicate weathering these changes may even be important on human timescales to predict changes to the co 2aq of soil water we have produced an extensive literature review of soil respired co 2 measurements to study the spatial variability of soil respired co 2 we show that respired co 2 concentrations and co 2aq vary with precipitation this study focuses on the western united states where we find a strong relationship between summer average soil respired co 2 and mean annual precipitation for soils forming in or below 900 mm yr 1 precipitation r 2 0 91 the correlation breaks down when higher mean annual precipitation rates are considered restricting the use of this relationship to arid to subhumid precipitation regimes we estimate the response of co 2aq in soil pore water in the western united states to projected anthropogenic co 2 emissions using this new relationship and projected changes in precipitation simulated by the north american regional climate change assessment program for the decade 2051 2060 according to our model co 2aq in soil pore water is expected to decrease by up to 50 in the southwestern united states and to increase by up to 50 in areas of the northern and central great plains these results have important implications for co 2 consumption and changes to terrestrial carbon cycling in the next century

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

a conceptual tectonostratigraphic model for seismic facies migrations in a fluvio lacustrine extensional basin one of the most fundamental differences between lacustrine basins and many marine basins is the stratigraphic response to differential tectonic subsidence because the water volume of a lake is finite tilting of the valley floor redistributes the lake water if the lake water level is below a hinge point fulcrum that experiences no net subsidence the lake shoreline is translated toward the site of subsidence and a deeper lake forms this tectonostratigraphic model for rift lake sedimentation is tested in a seismic and sequence stratigraphic study of deltaic and lacustrine deposits that accumulated in a cenozoic extensional basin beneath goshute valley nevada seismic sequences exhibit an internal cyclic stacking hierarchy of seismic reflections interpreted as the stratigraphic response of a lowstand paleolake to tectonism forced regression and subsequent lake level restoration transgressive and highstand systems tracts

seismic interferometry using multidimensional deconvolution and crosscorrelation for crosswell seismic reflection data without borehole sources crosswell reflection method is a high resolution seismic imaging method that uses recordings between boreholes the need for downhole sources is a restrictive factor in its application for example to time lapse surveys an alternative is to use surface sources in combination with seismic interferometry seismic interferometry si could retrieve the reflection response at one of the boreholes as if from a source inside the other borehole we investigate the applicability of si for the retrieval of the reflection response between two boreholes using numerically modeled field data we compare two si approaches crosscorrelation cc and multidimensional deconvolution mdd si by mdd is less sensitive to underillumination from the source distribution but requires inversion of the recordings at one of the receiver arrays from all the available sources we find that the inversion problem is ill posed and propose to stabilize it using singular value decomposition the results show that the reflections from deep boundaries are retrieved very well using both the cc and mdd methods furthermore the mdd results exhibit more realistic amplitudes than those from the cc method for downgoing reflections from shallow boundaries we find that the results retrieved from the application of both methods to field data agree well with crosswell seismic reflection data using borehole sources and with the logged p wave velocity

the precambrian nonesuch formation of the north american mid continent rift sedimentology and organic geochemical aspects of lacustrine deposition abstract the precambrian keweenawan trough in the lake superior region of north america is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks one unit in this sequence the none such formation is composed of black to green siltstones and shales and is conformable with the underlying alluvial copper harbor conglomerate and the overlying fluvial freda sandstone the formation is exposed as a lens in northern michigan and wisconsin and attains a maximum thickness of 200 m based on the integration of outcrop and core data nine facies have been recognized in the formation each has been assigned to one of three genetic facies assemblages a marginal lacustrine assemblage characterized by interbedded lithic sandstones siltstones mudstones and sandstone shale couplets is interpreted as a record of deposition on a sandflat mudflat complex a lacustrine assemblage is characterized by massive to well laminated dark shaly siltstones carbonate laminites shales siltstones and mudstones these sediments were probably deposited in a progressively shallowing perennial lake bottom conditions were anoxic and became increasingly more oxic as lake regression progressed a gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown massive horizontally laminated and rippled fine sandstones siltstones and mudstones of the fluvial lacustrine assemblage interactions among rates of subsidence rates of sedimentation fluctuations in lake level possible changes in climate and differences in tectonic setting controlled sedimentation in the nonesuch lake and resulted in variable vertical facies sequences total organic carbon toc analyses show a strong correlation between organic richness and the shale facies lacustrine assemblage and mudstone facies marginal lacustrine assemblage detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms and to a limited extent by stable carbon isotope mass spectrometry the geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation e g microbial degradation accounts for the difference rather than maturity or variable source input

changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half graben an example from the triassic ischigualasto villa union basin argentina well exposed triassic rift strata from the ischigualasto villa union basin nw argentina include a 80 to ca 515 m thick lacustrine dominated package that can be correlated across a half graben using key stratigraphic surfaces sequence boundaries lacustrine flooding surfaces and forced regressive surfaces the characteristics of the synrift lacustrine fill in different parts of the half graben have been examined and the mechanisms controlling sedimentation inferred a variety of sedimentary environments are recognized including volcaniclastic floodplain mildly saline lake and playa lake offshore lacustrine delta front to fluvial dominated and wave dominated deltas distributary and fluvial channel and interdistributary bay the succession can be divided into four stratigraphic sequences ss1 to ss4 the oldest of which ss1 contains volcaniclastic fluvial and saline lake deposits it is thickest close to the western border fault zone reflecting more rapid subsidence here accommodation exceeded sediment and water input during ss1 the second and third sequences ss2 and ss3 mark the onset of widespread lacustrine sedimentation reflecting a balance between accommodation creation and water and sediment fluxes sequences ss2 and ss3 are represented by offshore meromictic lacustrine and deltaic deposits the latter mostly sourced from the flexural and southern axial margins of the half graben the presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone the youngest sequence ss4 is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies palaeocurrents and sandstone composition suggesting a switch in sediment supply to the footwall margin to the nw the change in the sediment source is related to reduced footwall uplift the possible presence of a relay ramp and or supply from a captured antecedent drainage network during ss4 the rate of creation of accommodation was exceeded by the sediment and water discharge the stratigraphic evolution of lacustrine strata in the half graben was mainly controlled by tectonic processes including subsidence rate and the growth and evolution of the border fault zone but changing climate inducing changes in water balance and lake level and autocyclic processes delta lobe switching were also important

geochemistry and mineralogy of the 2 45 ga pronto paleosols ontario canada abstract the ontario canada pronto paleosols which formed around 2 45 ga on archean granite were studied through field observation mineralogical study by microscope and microprobe and chemical analyses of major and 45 trace elements including rare earth elements rees since the first full description of its mineralogy and chemistry of major elements by gay and grandstaff gay a l grandstaff d e 1980 chemistry and mineralogy of precambrian paleosols at elliot lake ontario canada precambrian res 12 349 373 the pronto paleosols are among the most cited paleosols of the archean early proterozoic era though several questions have been raised as to whether they are true in place ancient soils results of this study show that the studied section meets the criteria for being in place paleosols furthermore the profile has no explicit traces of being strongly affected by hydrothermal alteration the ree containing accessory minerals were destroyed through weathering and secondary phosphate minerals with rees formed all of these observations support the weathering origin of the pronto paleosols however the profile should be identified as a saprolith rather than paleosol as even the uppermost samples exhibit the granitic texture of the parent rock the samples in this study were collected mainly from two sites site i and site ii saprolith samples from site i show the retention of fe and increases of the fe iii fe ii ratios and the chemical index of alteration cia values toward the unconformity as for the site ii samples the fe ti ratios show slight decreases with increasing cia values in the upper part of section whereas these samples also show the same trend of increasing fe iii fe ii ratios as the samples from site i microprobe analyses show that fe from the parent rock distributes mainly in mafic minerals as hornblende biotite or chlorite which were completely destroyed during the early stages of weathering in the saprolith section fe is present in sericite aggregates in the form of obscure patches of iron oxide tens of micrometers in size all these characteristics indicate that i the fe was not lost during soil formation at site i and slight amount of fe was lost at site ii and ii ferrous fe was quantitatively transformed to ferric fe i e oxidation during soil formation in the samples from both sites the ratios of al ti and si ti decrease toward the top of the profile together with the destruction of minerals toward the unconformity such as apatite biotite and chlorite the depletion of al and si suggests that the parental granite was weathered by an acidic weathering solution during the archean early proterozoic era the high atmospheric partial pressure of co 2 p co 2 is the most probable candidate for producing the acidic weathering solution

physical volcanology of mid proterozoic plateau lavas the keweenawan north shore volcanic group minnesota the north shore volcanic group a thick sequence of plateau lavas of keweenawan proterozoic 1 100 m y age was erupted during the development of the midcontinent rift of north america it contains many well exposed physical features that imply a range of eruptive styles and flow mechanisms these include a high volume and high effusion rate floods of basalt and basaltic andesite which ponded and crystallized largely after flow had ceased b moderate to small volume and effusion rate flows and small floods some probably building low shields and c small flows to very large flows and ignimbrites of icelandite and rhyolite lacking visible vent areas viscosities calculated from 183 chemical analyses correlate well with the physical features the style of volcanism combined characteristics of the columbia plateau and the snake river plain plains volcanism and was very similar to that of the tertiary of eastern iceland

precambrian paleosols and atmospheric co2 levels abstract precambrian atmospheric pco2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols in addition to theoretical flaws with that model recalculation of pco2 levels using more recent thermodynamic data gives significantly lower values a new model based on the mass balance of weathering in paleosols gives consistent results from three separate 2 2 ga old paleosols the calculated pco2 value of 23 3 3 times present atmospheric levels is insufficient to overcome the faint young sun paradox suggesting that another greenhouse gas such as ch4 was present at elevated levels relative to the present atmosphere to account for the discrepancy applying this new approach to other precambrian paleosols a proterozoic pco2 curve is generated which indicates consistently high pco2 from 2 5 to 1 8 ga ago and a substantial drop in atmospheric pco2 at some time between 1 8 and 1 1 ga ago

constraining the thermal history of the north american midcontinent rift system using carbonate clumped isotopes and organic thermal maturity indices abstract the midcontinent rift system mrs is a late mesoproterozoic 1 1 ga sequence of volcanic and sedimentary rocks exposed in the lake superior region of north america the mrs continues to be the focus of much research due to its economic mineral deposits as well as its archive of precambrian life and tectonic processes in order to constrain the post depositional thermal history of the mrs samples were analyzed for carbonate clumped isotope composition and organic thermal maturity clumped isotope values from sedimentary early diagenetic samples were partially reset during burial to temperatures between 68 and 75 c solid state reordering models indicate that maximum burial temperatures of 125 155 c would reset the clumped isotope values to the observed temperature range prior to the onset of regional cooling and uplift clumped isotope results from late stage veins in the white pine mine encompass a greater temperature range 49 116 c indicative of spatially variable hydrothermal activity and vein emplacement after burial temperatures fell below 100 c during regional cooling and uplift clumped isotope and organic thermal maturity data do not indicate significant spatial differences in thermal history along the mrs observed variability in bulk organic matter composition and biomarker indices are therefore more likely a result of shifts in primary productivity or early degradation processes these results demonstrate that the mrs experienced a spatially consistent relatively mild thermal history 125 155 c and is therefore a valuable archive for understanding the late mesoproterozoic environment

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria filamentous and shrub like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca 1200 ma society cliffs formation baffin and bylot islands arctic canada are 400 m y older than previously reported examples in vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms ccms and is a direct ecophysiological link to atmospheric co 2 concentration ccms are induced in present day cyanobacteria under experimental conditions when pco 2 is below 0 36 10 times present atmospheric level pal society cliffs calcified cyanobacteria consequently imply pco 2 levels of 2 of 7 10 pal in the late mesoproterozoic combined petro graphic experimental and modeling results therefore suggest that mesoproterozoic pco 2 concentrations were not substantially different from phanerozoic values and were significantly less than previous estimates of up to 200 pal assuming 10 lower solar luminosity in the late mesoproterozoic pco 2 levels of 10 pal or less require the presence of additional greenhouse gases for maintenance of an ice free earth at 10 pal pco 2 methane concentrations of 100 200 ppm would have been sufficient to sustain warm earth surface conditions the low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of earth9s proterozoic biosphere

weathering and paleosol formation in the 1 1 ga keweenawan rift abstract at 1 1 ga the north american craton began to rift resulting in flood style basaltic volcanism during the phase of active tectonism some of that basalt was weathered and redistributed by fluvial processes new observations near the northwestern shore of lake superior have also revealed the presence of seven paleosols formed on weathered immature sediments derived from the basalt that are described here for the first time only three other paleosols have previously been described from this part of the geologic record so these new paleosols represent a significant new discovery the paleosols are weakly developed and exhibit physical and chemical characteristics similar to phanerozoic entisols in contrast to many precambrian paleosols the keweenawen paleosols have been subject to minimal post burial alteration among the paleosols three distinct pedofacies are recognized 1 waterlogged gleyed fluvial proximal paleosols 2 dry fluvial distal paleosols and 3 cumulative fluvial proximal paleosols despite these facies differences using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt with one of the paleosols showing an additional extra basinal rhyolitic component by considering the mass balance behaviour of a variety of alkali and alkaline earth elements it is possible to rule out significant potassium metasomatism suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions and that they form an important new source of data about mesoproterozoic weathering

comparative sequence stratigraphy of low latitude versus high latitude lacustrine rift basins seismic data examples from the east african and baikal rifts abstract lakes baikal malawi and tanganyika are the world s three largest rift valley lakes and are the classic modern examples of lacustrine rift basins all the rift lakes are segmented into half graben basins and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time in the early stages of rifting basins are fed primarily by flexural margin and axial margin drainage systems at the climax of syn rift sedimentation however when the basins are deeply subsided almost all the margins are walled off by rift shoulder uplifts and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas flexural margin unconformities are commonplace in the tropical lakes but less so in high latitude lake baikal lake levels are extremely dynamic in the tropical lakes and in low latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems evaporation is minimized in relation to inflow in the high latitude lake baikal and in most high latitude systems and consequently major sequence boundaries tend to be tectonically controlled in that type of system the acoustic stratigraphies of the tropical lakes are dominated by high frequency and high amplitude lake level shifts whereas in high latitude lake baikal stratigraphic cycles are dominated by tectonism and sediment supply variations

reduced time migration of transmitted ps waves we develop the novel theory of transmitted ps migration and show that ps transmitted arrivals in a gulf of mexico vertical seismic profile vsp data set can be migrated to accurately image a salt sheet even though the receiver array is below the transmitting boundary we also show that migrating transmitted arrivals is effective in illuminating the base of an orebody invisible to pp reflections in general interfaces that bisect wavepath propagation i e the source and receiver are on opposite sides of the interface and therefore invisible to pp reflections can be imaged by migration of ps transmitted waves these results suggest that migration of ps transmitted waves opens new opportunities in imaging nearly vertical impedance boundaries that are typically invisible to conventional reflection imaging of crosswell and vsp data we also present a new interferometric method denoted as reduced time migration which uses the arrival time difference between the direct p wave and subsequent events to incre

inversion for salt flank geometry using transmitted p and s wave travel times accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion we describe the application of deformable layer tomography dlt to invert for the geometry of salt flank using travel times of transmitted p and s waves from surface sources to downhole receivers the dlt allows us to take advantage of a common situation that the salt velocity is known and generally invariant but we need to determine the variable geometry of salt flank we demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical seismic profiling vsp survey we first use picked p wave arrivals to invert for the salt flank geometry the sinusoid shape of the salt flank is estimated fairly by the dlt due to the uneven p wave raypath coverage as an improvement we further incorporate the s wave arrivals in the dlt the picking of s wave arrivals is assisted with modeled s wave arrivals based on the p wave dlt model the dlt solution using both p and s wave arrivals delineates the salt flank geometry more accurately than that using p wave arrivals alone the salt flank delineation using dlt can complement with the conventional salt proximity and migration methods it could also serve as constraints or the initial model for the full waveform inversion of salt geometry

lacustrine deltas in a mesozoic alluvial sequence from camp hill antarctica sedimentary rocks of late mesozoic age exposed at camp hill northern antarctic peninsula are associated with calc alkaline volcanic rocks they represent deposition on a fault controlled floodplain with marginal alluvial fans on a volcanic arc finely laminated mudstone and occasional graded laminae were deposited from suspension and by density underflow currents respectively in small shallow lakes thickening and coarsening upward sandstone bodies overlying the lake deposits are interpreted as lacustrine deltas of which two types are preserved 1 gilbert type with steep foresets and 2 mouth bar type which lack steep foresets sections through the latter type reveal the presence of sub environments characteristic of fluvial dominated marine deltas i e prodelta distributary mouth bar and distributary channel abandoned mouth bars resulting from avulsion are recognized it is suggested that the processes which operated during formation of the mouth bar deltas resulted from hyperpycnal flow by contrast the gilbert type delta is thought to be the consequence of a reduced inflow of suspended sediment causing homopycnal flow and thorough mixing of the river and lake waters

constructing new seismograms from old earthquakes retrospective seismology at multiple length scales if energy emitted by a seismic source such as an earthquake is recorded on a suitable backbone array of seismometers source receiver interferometry sri is a method that allows those recordings to be projected to the location of another target seismometer providing an estimate of the seismogram that would have been recorded at that location since the other seismometer may not have been deployed at the time at which the source occurred this renders possible the concept of retrospective seismology whereby the installation of a sensor at one period of time allows the construction of virtual seismograms as though that sensor had been active before or after its period of installation here we construct such virtual seismograms on target sensors in both industrial seismic and earthquake seismology settings using both active seismic sources and ambient seismic noise to construct sri propagators and on length scales ranging over 5 orders of magnitude from 40 m to 2500 km in each case we compare seismograms constructed at target sensors by sri to those actually recorded on the same sensors we show that spatial integrations required by interferometric theory can be calculated over irregular receiver arrays by embedding these arrays within 2 d spatial voronoi cells thus improving spatial interpolation and interferometric results the results of sri are significantly improved by restricting the backbone receiver array to include approximately those receivers that provide a stationary phase contribution to the interferometric integrals finally we apply both correlation correlation and correlation convolution sri and show that the latter constructs fewer nonphysical arrivals

the biostratigraphic importance of conchostracans in the continental triassic of the northern hemisphere conchostracans or clam shrimp order conchostraca sars are arthropods with a carapace consisting of two chitinous lateral valves triassic conchostracans range in size from 2 to 12 5 mm long and are common in deposits that formed in fresh water lakes isolated ponds and brackish areas their dessication and freeze resistant eggs can be dispersed by wind over long distances therefore many conchostracan species are distributed throughout the entire north ern hemisphere in the late permian to middle triassic interval several of these forms are also found in gondwana many wide ranging conchostracan species have short stratigraphic ranges making them excellent guide forms for subdivision of triassic time and for long range correlations the stratigraphic resolution that can be achieved with conchostracan zones is often as high as for ammonoid and conodont zones found in pelagic marine deposits this makes con chostracans the most useful group available for biostratigraphic subdivision and correlation in continental lake deposits upper triassic gondwanan conchostracan faunas are different from conchostracan faunas of the northern hemisphere in the norian some slight provincialism can be observed even within the northern hemisphere for example the sevatian redondestheria seems to be restricted to north america and acadiestheriella n gen so far has been found only in the sevatian deposits from the fundy basin of southeastern canada here we establish a con chostracan zonation for the changhsingian late permian to hettangian early jurassic of the northernhemispherethat forthemostpart isverywellcorrelatedwiththemarinescale thiszona tion is especially robust for the changhsingian to early anisian late ladinian to cordevolian and rhaetian to hettangian intervals for most of the middle and upper triassic this zonation is still preliminary five new genera six new species and a new subspecies of conchostracans are described that are stratigraphically important half of the eight stage boundaries of the triassic have been defined by a bio event within a marine global stratotype and point gssp locality and these definitions have been accepted by both the international subcommission on triassic stratigra phy and the international commission on stratigra phy the remaining four stage boundaries are nearing final definition in the lower triassic both the base of the induan priority brahmanian stage base of triassic and the base of the next younger olenekian stage have been firmly defined in the middle triassic there is wide agree ment that the defining species for the base of the anisian stage should be chiosella timorensis in the gssp candidate site at desli caira romania but there has not yet been a formal vote on this the base of the overlying ladinian stage however has been firmly defined in the upper triassic the base of the carnian has been firmly likewise defined but there is not yet a final defi nition for the boundaries of the overlying norian and rhaetian stages a consensus has not been reached on a defining species for the base of the norian or its gssp locality but all of the different proposals under consideration do at least fall within a rather narrow stratigraphic interval for the base of the rhaetian misikella posthernsteini kozur mock has been chosen as the defining species by the international working group on the rhaetian stage and the gssp candidate locality at steinbergkogel austria has been studied in detail by a group under the leadership of l krystyn vienna and presented to the participants of the international conference on upper triassic sub divisions zonations and events in bad goisern in the autumn of 2008 the base of the overlying hettangian stage base of the jurassic has been defined so far only by a working group as the fad first appearance datum of psiloceras spelae guex taylor rakus bucher the final definition of the triassic stages within marine gssp sections will be completed in the near future but more than 50 of known triassic rocks are of continental origin therefore the main task of triassic stratigraphers in thefuture will besubdivid ing and correlating terrestrial strata both between

salt flank delineation by interferometric imaging of transmitted p to s waves we describe how vertical seismic profile vsp interferometric imaging of transmitted p to s ps waves can be used to delineate the flanks of salt bodies unlike traditional migration methods interferometric ps imaging does not require the migration velocity model of the salt and or upper sediments in order to image the salt flank synthetic elastic examples show that ps interferometric imaging can clearly delineate the upper and lower boundaries of a realistic salt body model results also show that ps interferometric imaging is noticeably more accurate than conventional migration methods in the presence of static shifts and or migration velocity errors however the illumination area of the ps transmitted waves is limited by the width of the shot and geophone aperture which means wide shot offsets and deeper receiver wells are needed for comprehensive salt flank imaging interferometric imaging results for vsp data from the gulf of mexico demonstrate its superiority over the traditional migration method we also discuss other arrivals that can be used for interferometric imaging of salt flanks for comparison reduced time migration results are presented which are similar in quality to those obtained for interferometric imaging we conclude that ps interferometric imaging of vsp data provides the geophysicist with a new tool by which salt flanks can be viewed from both above and below vsp geophone locations

the chemical signature of source rocks in size fractions of holocene stream sediment derived from metamorphic rocks in the wet mountains region colorado u s a eight stream sediments draining primarily mixed silica rich and silica poor metamorphic rocks in the wet mountains colorado were analyzed at their source for their mineralogic and selected elemental composition the low silica source rocks include amphibolite hornblende schist and muscovite biotite schist the high silica source rocks include granodioritic to leucocratic granitic gneisses the high silica source rocks tend to contain higher concentrations of ree th hf and ba lower concentrations of fe sc and cr higher la sc and thsc ratios and more negative eu anomalies than the low silica source rocks especially those low silica source rocks derived from basic sources rather than weathered silicic sources the stream sediments draining this mixed metamorphic rock terrain are dominated by sands and gravels and they are composed of feldspar quartz and 25 heavy minerals the silts contain more total biotite hornblende pyroxene and opaque minerals and less quartz and feldspar than the sands and gravels sediment especially the silts derived from mixed low and high silica sources tends to be intermediate in elemental composition between those of the end member sources so there is some hope that source rock signatures may be preserved in the sediment elemental ratios of la sc th sc la co th co and ba co and th la sc plots most clearly distinguish sediment derived from the most silicic and most basic source rocks in addition finer sediment near the source can potentially distinguish silicic sources higher ree and more negative eu anomalies from those of basic sources the clay and silt fractions contain negative eu anomalies normalized to the mcps north american mid continent platform sediment and the sands and gravels have positive eu anomalies normalized to the mcps most size fractions are similar to more enriched in la sc th sc la co and th co ratios than the mcps this suggests local sources in the wet mountains are on the average more silica rich than those that formed the mcps

relations between deformation and sediment hosted copper mineralization evidence from the white pine part of the midcontinent rift system detailed studies over the past decade have significantly extended and revised our knowledge of the geologic history of the well known white pine mining district of northern michigan and indicate that the location of faults exerted a strong control on copper mineralization in this part of the midcontinent rift system at white pine there is evidence for three episodes of faulting 1 synsedimentary faulting 2 subsequent high angle dominantly normal faulting and 3 thrusting two stages of copper mineralization are present at white pine and in the nearby presque isle syncline the first main stage mineralization formed a classic sediment hosted stratiform copper deposit during early diagenesis synsedimentary faults may have provided important conduits for cupriferous brines flowing from underlying red beds of the copper harbor conglomerate into the reduced silts and shales of the nonesuch formation where main stage copper sulfides and native copper were precipitated the second stage of copper mineralization was synchronous with thrusting and introduced additional copper to the white pine ore body and the presque isle syndine thrust faults and cogenetic tear faults provided conduits for second stage mineralizing fluids collectively these observations indicate strong control by regional deformation on fluid migration and mineralization in the rocks of the midcontinent rift similar to proposed relations between deformation and mineralization in other tectonic settings

causes and consequences of low atmospheric pco2 in the late mesoproterozoic abstract based upon various proxy theoretical and model constraints paleoproterozoic atmospheric p co 2 was much higher than phanerozoic levels however relatively little is known about the transition between the two climate states here geochemical mass balance from 1 1 ga old midcontinent rift system usa paleosols is used to reconstruct atmospheric p co 2 during the mesoproterozoic the calculations robustly indicate low atmospheric p co 2 p co 2 minimum is best explained as the culmination of a long term c burial event by the biosphere that is also indicated by marine carbonate 13 c changes during the mesoproterozoic and which is consistent with changes in the biosphere including increased stromatolite abundance and diversity evolution of sulfur utilizing bacteria and the spread of microbial mats into continental environments

local migration with extrapolated vsp green s functions we have developed a novel vertical seismic profile vsp imaging method that requires only a local velocity model around the well this method is denoted as a local migration where the model based forward propagation and backward propagation operators are computed using a local velocity model between the well and the target body the velocity model for the complex overburden and salt body is not needed and the source side statics are automatically accounted for in addition kinematic and dynamic effects including anisotropy absorption and other unknown undefined rock effects outside of this local velocity model are mostly accounted for numerical tests on an acoustic 2d sigsbee vsp data set an elastic salt model and offset 2d vsp data in the gulf of mexico partly validate the effectiveness of this method by accurately imaging the sediments next to the vertical well

paleolimnology evolution in rift basins the ischigualasto villa union basin central western argentina during the triassic abstract extensional basins offer a high preservation potential for sedimentary and fossil records nonmarine rift basins contain diverse environments including fluvial and lacustrine lake deposits have long been recognized as significant archives of the paleoenvironmental record the ischigualasto villa union basin represents the only nearly continuous continental triassic record with up to 6000 m of alluvial fluvial and lacustrine deposits the deep lacustrine interval is known as the los rastros formation and consists of several lacustrine deltaic cycles a detailed facies analysis allowed the characterization of five facies associations and two types of cycles 1 a shoreline dominated progradational type i cycle containing facies association a offshore lacustrine deposits and facies association b marginal palustrine deposits along the shoreline of the lake or facies association c lacustrine siliciclastic coastal deposits and 2 a delta dominated progradational type ii cycle characterized by facies association a offshore lacustrine and prodelta deposits facies association d delta front deposits and facies association e delta plain deposits the sedimentary evolution of the chanares los rastros sequence is defined within this extensional setting as one depositional sequence with three system tracts lowstand transgressive and highstand system tract biotic abiotic and sedimentologic data allowed the interpretation of the limnologic dynamics of the los rastros paleolake as a deep freshwater lake within a half graben exhibiting meromixis and showing a change from moderate eutrophic conditions with low oxygen content to relatively oxygenated oligotrophic conditions the successive fluctuations recorded in lake level are considered in the sequence stratigraphic framework of bohacs et al to reflect the relative balance of sediment and water input versus accommodation in an overfilled lacustrine basin

processes controlling the distribution of ti and al in weathering profiles siliciclastic sediments and sedimentary rocks abstract the ratio of ti to al varies greatly in primary igneous source rocks these elements are considered to be relatively immobile in most weathering regimes so that tio2 al2o3 ratios have been used to investigate the provenance of sediments and sedimentary rocks in modern weathering profiles that have undergone moderate weathering ti al ratios remain fairly constant so that siliciclastic sediments derived from them should have ti al ratios like that of the source materials under extreme weathering conditions however the upper most weathered parts of profiles commonly show a marked increase in ti al ratio that is thought to be due to preferential translocation of al rich phases under steady state weathering conditions one particular level of the profile may act as the main source of detrital material and may thus control the ti al ratio of sediments produced from it in a glacio fluvial setting minimum weathering moderate sorting there is considerable variation in ti content but al is nearly constant this is due to concentration of ti bearing mafic minerals such as biotite in the fine fraction in modern sediments derived from moderately weathered and sorted material the ti al distribution reflects mixing of al and ti enriched fine grained materials with sands that are depleted in these elements when a plot of tio2 vs al2o3 is constructed for a suite of such sands and muds the resultant trend differs significantly from that shown by a similar suite of unweathered glacio fluvial sediments comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of tio2 vs al2o3 may provide insight into weathering and depositional history in addition to their use as provenance indicators

extracting earth s elastic wave response from noise measurements recent research has shown that noise can be turned from a nuisance into a useful seismic source in seismology and other fields in science and engineering the estimation of the system response from noise measurements has proven to be a powerful technique to convey the essence of the method we first treat the simplest case of a homogeneous medium to show how noise measurements can be used to estimate waves that propagate between sensors we provide an overview of physics research dating back more than 100 years showing that random field fluctuations contain information about the system response this principle has found extensive use in surface wave seismology but can also be applied to the estimation of body waves because noise provides continuous illumination of the subsurface the extracted response is ideally suited for time lapse monitoring we present examples of time lapse monitoring as applied to the softening of soil after the 2011 tohoku oki earthquake the detection of a precursor to a landslide and temporal changes in the lunar soil

imaging internal multiples from subsalt vsp data examples of target oriented interferometry seismic interferometry has become a technology of growing interest for imaging borehole seismic data we demonstrate that interferometry of internal multiples can be used to image targets above a borehole receiver array by internal multiples we refer to all types of waves that scatter multiple times inside the model these include for instance interbed intrasalt and water bottom multiples as well as conversions among them we use an interferometry technique that is based on representation theorems for perturbed media and targets the reconstruction of specific primary reflections from multiply reflected waves in this interferometry approach we rely on shot domain wavenumber separation to select the directions of waves arriving at a given receiver using a numerical walkaway waw vsp experiment recorded by a subsalt borehole receiver array in the sigsbee salt model we use the interference of internal multiples to image the salt structure from below in this numerical example the interferometric i

speculations on the origin of the north american midcontinent rift cannon w f and hinze w j 1992 speculations on the origin of the north american midcontinent rift in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 49 55 the midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began volcanism continued along with extension and deep rift subsidence for the ensuing 15 m y much of the basaltic magma including some of the earliest flows was formed by partial melting of isotopically primitive asthenosphere contained in the plume head the intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head as the plume head spread beneath the lithosphere it stretched the overlying lithosphere radially away from the lake superior region the triple junction of the rift system and partially melted to form the great volume of basalt and related intrusive rocks of the region the plume arrived beneath a continent that was under compression as a result of the ongoing grenville orogeny that affected a large region east of the rift that compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned

sedimentary facies in an extensional and deep lacustrine depositional system the pliocene tiberino basin central italy abstract the tiberino basin was formed during plio pleistocene extensional tectonic activity in central italy the pliocene sediments that fill this structure consist predominately of fine grained clastic lithofacies deposited in a lacustrine system four facies associations have been distinguished facies association a was deposited in a deep offshore lacustrine environment and consists of massive laminated bluish gray marly clays that lack benthonic fauna but have preserved organic matter leaves wood this facies association is the main lacustrine deposit whereas the other three represent marginal facies facies association b is interpreted as a delta system deposit it has gilbert type sandy gravel bodies with foresets up to 50 m high and prodelta bodies consisting of laminated to massive marly clays that alternate with sandy and gravelly mud strata association c corresponds to a coastal environment where two sub environments are recognized 1 a wave dominated coastline characterized by interbedded muddy and sandy strata with sedimentary structures attributed to hummocky cross stratification and 2 clayey silts and lignites interpreted as a coastal wetland a spectacular fossil forest is preserved in situ within this unit association d formed on the distal part of a muddy alluvial fan consists of sheet flood clayey sandy silts that alternate with incipient paleosols rare ribbon channel fills are also present paleoenvironmental reconstruction indicates that the tiberino basin was occupied by a narrow deep meromictic lake during the pliocene the size shape depth and sub environment distribution of this lake were controlled by tectonic phenomena such as the high subsidence rate which caused the lake to be large and deep only in the few coastal areas that had high sedimentary input were wetland or alluvial fan deposits formed

geometrical considerations in the acquisition of borehole interferometric data for imaging near vertical features design of field experiments abstractseismic interferometry applied to walkaway vertical seismic profile data has significant potential for imaging the steeply dipping structures often encountered in hard rock minerals exploration using the interferometry process surface shots can be redatumed to the borehole receivers resulting in virtual shot gathers the virtual shot gathers can then be processed using a standard common midpoint cmp processing flow carrying out this procedure for a subvertical borehole results in a geometry that is optimal for imaging structures that are near vertical field acquisition parameters play a critical role in recovering reliable virtual source images we evaluated the major factors that play a role in designing a field acquisition program with the objective of providing guidance to field practitioners the major issue to be considered is insuring that correlation gathers created in the interferometry process have a stationary phase component that when summed produces events with correct timing and

calcium carbonate cement caliche in keweenawan sedimentary rocks 1 1 ga upper peninsula of michigan abstract a complex caliche profile and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the lake superior basin during and after continental basaltic volcanism as interflow conglomerates of the portage lake volcanics and as the oronto group in conglomerates the carbonate cement forms discontinuous zones up to 60 cm thick and several meters in length consisting of up to 25 vol carbonate whereas the caliche horizon at the centennial mine shows typical upward grading features characteristic of pedogenic caliche the widespread sparry carbonate cement is less diagnostic it is interpreted also as caliche for the following reasons 1 it predates regional epidote metamorphism 2 a caliche profile has been described from the same strata 3 textures characteristic of caliche such as carbonate rimmed pebbles pendulose calcite on pebbles sand grains floating in a carbonate matrix and exfoliated chlorite 4 possible vadoids 5 the carbonate rich zones are widespread stratigraphically and geographically in basinward inclined alluvial fans and 6 the distribution of these zones and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate several lines of evidence suggest that the prevailing climate may have been arid with a fluctuating perhaps seasonal rainfall pattern required for caliche 1 a paleolatitude about 30 north 2 carbonate filled shrinkage cracks denoting high evaporation 3 intermittent flashy or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud chip conglomerate enormous previously cemented mudstone rip ups and desiccation cracked mud drapes interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi arid intermontane basin flanked by alluvial fans and with a central saline lake

was the midcontinent rift part of a successful seafloor spreading episode the 1 1 ga midcontinent rift mcr the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the north american craton is traditionally considered a failed rift formed by isolated midplate volcanism and extension we propose instead that the mcr formed as part of the rifting of amazonia precambrian northeast south america from laurentia precambrian north america and became inactive once seafloor spreading was established a cusp in laurentia s apparent polar wander path near the onset of mcr volcanism recorded by the mcr s volcanic rocks likely reflects the rifting this scenario is suggested by analogy with younger rifts elsewhere and consistent with the mcr s extension to northwest alabama along the east continent gravity high southern appalachian rocks having amazonian affinities and recent identification of contemporaneous large igneous provinces in amazonia

neoarchean paleoweathering of tonalite and metabasalt implications for reconstructions of 2 69ga early terrestrial ecosystems and paleoatmospheric chemistry abstract field and laboratory investigations of a 2690 83 ma 207pb 206pb age of saganaga tonalite unconformity exposed in outcrop in northeastern minnesota usa reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions weathering profiles are characterized by a 5 50 m thick regolith that consists of saprolitized saganaga tonalite and paulson lake succession basaltic metavolcanic rocks retaining rock structure which is cross cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits the regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal saprock as well as extrabasinal detritus thin section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars exfoliation and chloritization of biotite and weathering of fe mg silicates and cu fe sulfides weathering of fe ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of fe oxides geochemical analyses of the tonalite assuming immobile tio2 during weathering ti j show depletion of sio2 al2o3 na2o cao mgo and mno and to a lesser degree of k2o relative to least weathered parent materials significant fe was lost from the tonalite a paleoatmospheric pco2 of 10 50 times pal is estimated based on geochemical mass balance of the tonalite profile and assuming a formation time of 50 500 kyr interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate dolomite overprinting patterns of release of p and fe and retention of y and cu in tonalite are consistent with recent laboratory experiments of granite weathering and with the presence of acidic conditions in the presence of organic ligands produced for example by a primitive microbial community during weathering cu metal in the profile may document lower po2 than present day at the surface comparison with previous studies of weathered tonalite and basalt denison 2 45 2 22 ga in ontario canada reveal general similarities in paleoweathering with our study as well as important differences related to lower paleoatmospheric po2 and terrestrial biosignature for the older minnesota profile a falling water table in the alpine lake locality is presumed to have promoted formation of this gossan like deep weathering system that extends to 50 m depth

the copper harbor conglomerate a late precambrian fining upward alluvial fan sequence in northern michigan the keweenawan copper harbor conglomerate exposed in northern michigan is part of a thick succession of volcanics and terrigenous clastics deposited in the keweenawan trough a failed intracontinental rift it is a fining upward wedge of volcanogenic conglomerate and sandstone deposited as proximal to distal braided stream and sheet flood facies on coalesced alluvial fans and sand flats this unit along with the lower portion of the overlying nonesuch shale consists of an alluvial fan shallow lake sequence that filled the rift during and following volcanic activity the fining upward nature of the sequence reflects a waning sediment supply probably due to lowering of the source area following faulting the copper harbor conglomerate exhibits attributes of both arid fan and humid fan alluvial models coarse grained facies are lithologically similar to alluvial fan units deposited in modern humid regions conversely finer distal copper harbor facies exhibit numerous features indicating flashy discharges and desiccation processes typical of deposition in modern arid regions ambiguity of sedimentary features useful for climatic inference probably reflects the limited applicability of holocene models to more ancient alluvial sequences because precambrian hydraulic regimes developed under various climatic settings may have differed from their holocene counterparts

reconstruction of ancient lake environments using both autochthonous and allochthonous fossils abstract eocene lacustrine paleoenvironments in western north america have been reconstructed using evidence from assemblages of autochthonous and allochthonous fossils seasonal variation is recorded in varved sediments that contain fossil assemblages indicative of deep water off shore deposition stratigraphic variation at one locality records a transgression from a coal swamp through near shore lacustrine environments to more off shore lacustrine environments fish assemblages preserved in this section change in the taxa represented and in the taphonomic characteristics of their preservation terrestrial fossils insects and plants change in abundance concomitantly a large proportion of the taxonomic and taphonomic variation between fish assemblages from other formations and localities is directly correlated with the near shore to off shore variation insect and plant assemblages also differ greatly between near shore and off shore depositional environments if one s goal is to reconstruct ancient lacustrine communities sampling programs must be designed to compensate for these biases

palaeoenvironmental interpretation of an upper triassic deposit in southwestern gondwana argentina based on an insect fauna plant assemblage and their interactions abstract we present a multidisciplinary study of the potrerillos formation at the quebrada del durazno locality cuyana basin central western argentina providing novel information on the upper triassic biota and its palaeoenvironmental context the sedimentological and palaeoecological analysis suggests that the potrerillos formation section at the quebrada del durazno locality represents deltaic plain deposits with significant development of black carbonaceous shales and coal facies formed in semi permanent swamps and or ponds within interdistributary bays with intermittent episodes of flooding allowing the establishment of an abundant hydro hygrophytic vegetation and creating environments suitable for the development of a rich fauna of insects spinicaudatans conchostracans and fishes the entomofauna is represented by hemipterans odonatans mecopterans orthopterans grylloblattids coleopterans dipterans and miomopterans the plant fossils are typical of the dicroidium flora represented by corystospermales peltaspermales cycadales gnetales ginkgoales and sphenophytes plant insect interactions hole marginal and surface feeding piercing and sucking and oviposition are described for the first time from the triassic of the cuyana basin pointing to the importance of the vegetation as a food and shelter and also providing a suitable reproduction site for several groups of herbivorous insects

early triassic early olenekian life in the interior of east gondwana mixed marine terrestrial biota from the kockatea shale western australia abstract a new terrestrial marine assemblage from the lower beds of a thin outcrop section of the kockatea shale in the northern perth basin western australia contains a range of fossil groups most of which are rare or poorly known from the lower triassic of the region to date the collection includes spinose acritarchs organic cemented agglutinated foraminifera lingulids minute bivalves and gastropods ammonoids spinicaudatans insects austriocaridid crustaceans actinopterygians a temnospondyl like mandible plant remains and spores and pollen of these groups the insects crustaceans and macroplant remains are recorded for the first time from this unit palynomorphs permit correlation to nearby sections where conodonts indicate an early olenekian smithian age the locality likely represents the margin of an early triassic shallow interior sea with variable estuarine like water conditions at the southwestern end of an elongate embayment within the east gondwana interior rift sag system preserved along the western australian margin monospecific spinose acritarch assemblages intertwined with amorphous organic matter may represent phytoplankton blooms that accumulated as mats and suggest potentially eutrophic surface waters the assemblage represents a mixure of marine and terrestrial taxa suggesting variations in water conditions or that fresh brackish water and terrestrial organisms were transported from adjacent biotopes some of the lower dark shaly beds are dominated by spinicaudatans likely indicating periods when the depositional water body was ephemeral isolated or subjected to other difficult environmental conditions the biota of the kockatea shale is insufficiently known to estimate biotic diversity and relationships of individual taxa to their permian progenitors and triassic successors but provides a glimpse into a coastal zone from the interior of eastern gondwana specialist collecting is needed to clarify the taxonomy of many groups and comparisons to other lower triassic sites are required to provide insights into the pattern of biotic decline and recovery at the end permian crisis

sedimentology of a lacustrine fan delta system miocene horse camp formation nevada usa a 1600 m thick succession of the miocene horse camp formation member 2 exposed in east central nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan delta environments and nearshore and offshore lacustrine environments these four depositional environments are distinguished by particular associations of individual facies 14 defined facies subaerial and subaqueous fan delta facies associations include ungraded matrix and clast supported conglomerate normally graded matrix and clast supported conglomerate ungraded and normally graded sandstone and massive to poorly laminated mudstone subaqueous fan delta deposits typically have dewatering structures distorted bedding and interbedded mudstone the subaerial fan delta environment was characterized by debris flows hyperconcentrated flows and minor sheetfloods the subaqueous fan delta environment by debris flows high and low density turbidity currents and suspension fallout the nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments high energy oscillatory wave currents probably related to a large fetch reworked grains as large as 2 cm into horizontally stratified sand and gravel offshore directed currents produced uncommonly large typically 1 2 m thick trough cross stratified sandstone in addition stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high energy conditions alternating with periods of carbonate precipitation under reduced energy conditions massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout a depositional model constructed for the horse camp formation member 2 precludes the existence of all four depositional environments at any particular time rather phases characterized by deposition in subaerial fan nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan delta subaqueous fan delta and offshore lacustrine deposition this model suggests that high energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge thus preventing development of a subaqueous fan delta environment and promoting development of a well defined nearshore lacustrine environment low energy nearshore currents induced by shallow water along the

keweenawan age caliche paleosol in the lower part of the calumet and hecla conglomerate centennial mine calumet michigan the calumet and hecla conglomerate a keweenawan interflow sedimentary unit that is 1 1 b y in age consists at the centennial mine of a felsitic conglomerate upper part and of 1 2 m of siltstone and shale along the base in these fine grained sediments there is a complex submature caliche soil profile represented by an upward increase in calcite and corresponding changes in calcite structures there is also caliche in conglomerate all of this calcite predates the period of regional epidote alteration the caliche is interpreted to have developed in slowly aggrading sediments that were deposited on a rock floored fluvial plain and took place during the initial stages of slope reversal of the lava plain from southerly to northerly this kind of carbonate accumulation in sediments requires a highly seasonal temperate or tropical climate and represents 15 000 yr of weathering the corresponding keweenawan paleolatitude has been reported to be 30 n

no asymmetry in geomagnetic reversals recorded by 1 1 billion year old keweenawan basalts 1 1 billion year old volcanic rocks in north america are thought to record asymmetric geomagnetic reversals indicating non axial dipolar behaviour of the magnetic field high resolution data from ontario suggest that the reversals were instead symmetric and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of north america

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting