base level buffers and buttresses a model for upstream versus downstream control on fluvial geometry and architecture within sequences abstract the effects of downstream base level control on fluvial architecture and geometry are well explored in several broadly similar sequence stratigraphic models cretaceous dakota group strata u s western interior have characteristics reflecting combined downstream and upstream base level controls that these models cannot address particularly three layers of amalgamated channel belt sandstone within this group thicken and are continuous for distances 300 km along dip that stretch the reasonable lengths for which these models are intended to apply as well architecture in up dip reaches records repeated valley scale cut and fill cycles this contrasts with equivalent strata down dip which record channel scale lateral migration with no such valley scale cycles apparent we here introduce the concept of buffers and buttresses to address these observations we assume that river longitudinal profiles are each anchored down dip to some physical barrier e g the sea strand etc that we refer to as a buttress buttress shift is considered the primary downstream control on base level profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high frequency variability in upstream base level controls e g discharge etc all these potential profiles however are bounded above by the profile of highest possible aggradation and below by the profile of maximum possible incision these upper and lower profiles are buffers and they envelop the available fluvial preservation space thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance dakota valley scale surfaces record repeated cut and fill cycles driven by up dip controls and are confined between thick stable buffers equivalent strata down dip record lateral reworking within a thinner channel scale buffer zone that was positioned by downstream controls regression exposed slopes similar to the buffer zone thus buffers were stable for long distances and durations this prompted dip extensive lateral reworking of strata into upstream valley scale and downstream channel scale sheets buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs downstream base level controls on geometry and architecture the model lends general insights into dip oriented variations in fluvial architecture production of sheet vs lens geometry total preservation volumes for fluvial systems and variations in these factors related to contrasting climatic conditions and basin physiography the model can be amended to existing sequence stratigraphic approaches in order to capture dip oriented variations in sequence architecture

petrophysical and paleomagnetic data of drill cores from the bosumtwi impact structure ghana physical properties from rocks of the bosumtwi impact structure ghana central africa are essential to understand the formation of the relatively young 1 07 ma and small 10 5 km impact crater and to improve its geophysical modeling results of our petrophysical studies of deep drill cores lb 07a and lb 08a reveal distinct lithological patterns but no depth dependence the most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta graywacke and metapelites of target lithologies magnetic susceptibility shows mostly paramagnetic values 200 500 10 6 si throughout the core with an exception of a few metasediment samples and correlates positively with natural remanent magnetization nrm and q values these data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite the paleomagnetic data reveals that the characteristic direction of nrm has shallow normal in a few cases shallow reversed polarity which is in agreement with the lower jaramillo n polarity chron direction and is carried by ferrimagnetic pyrrhotite however our study has not revealed the expected high magnetization body required from previous magnetic modeling furthermore the lb 07a and lb08 a drill cores did not show the predicted high content of melt in the rocks requiring a new interpretation model for magnetic data

facies and facies architecture of paleogene floodplain deposits willwood formation bighorn basin wyoming usa abstract paleogene deposits of the willwood formation were analyzed in two areas of the bighorn basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability despite similar facies and facies organization floodplain deposits in the two areas differ in grain size hydromorphy and maturity of the paleosols and scale and organization of ribbon sandstones in the avulsion deposits these differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas two kinds of cumulative floodplain paleosols red and purple paleosols formed on overbank deposits the red paleosols were better drained and formed on less clay rich parent material intense mottling and iron oxide nodules indicate that the purple paleosols which formed on more clay rich parent material were poorly drained the grain size differences reflect basin position in particular distance from a local sediment source avulsion deposits are volumetrically important comprising about half of the floodplain deposits in both areas the avulsion deposits resemble the stage iii splays described by smith et al 1989 smith n d cross ia dufficy j p clough s r 1989 anatomy of an avulsion sedimentology 36 1 24 in terms of characteristics of their ribbon sandstones and the presence of thin sheet sandstones or siltstones paleosols that formed on the fine grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas this study shows that avulsion belt paleosols provide a key to understanding avulsion rate in the stratigraphic record other differences between the willwood avulsion deposits in the two areas and between these ancient deposits and the modern avulsion deposits described by smith et al may reflect 1 problems in clearly recognizing ancient avulsion deposits in the field and 2 sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature at the same time because exposures are three dimensional the willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones

basin wide magnetostratigraphic framework for the bighorn basin wyoming new paleomagnetic data from six different sections in the bighorn basin are combined with previously published results to construct a basin wide magnetostratigraphic framework geomagnetic polarity reversals between chrons c26r c26n c25r c25n c24r and c24n have been identifi ed among multiple stratigraphic sections in different parts of the basin using the new magnetostratigraphic framework paleontological paleobotanical and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world these new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as biohorizon b which is slightly below the chron c24r c24n boundary close to the position of the elmo isotope excursion in marine records backstripping analysis using these new magnetostratigraphic data helps defi ne the time transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin

depth determination for shallow teleseismic earthquakes methods and results the depths of moderate sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone making it possible to determine the vertical positions of earthquakes within the lithosphere this capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions two basic techniques are used for depth analyses surface wave spectra can be used to determine focal depths as mode excitation reflects the behavior of the eigenfunctions with depth the primary limitation on such studies stems from lateral heterogeneity of velocity structure the waveforms of body waves are also diagnostic of focal depth which controls the time separation between the direct arrival and near source surface reflections these studies can be limited by a partial trade off between depth and source time function duration the reliability of the body wave modeling approach now in general use is shown by excellent agreement between depths determined by different investigators using different algorithms the depth determination is robust to typical uncertainties in focal mechanism and near source structure the trade off with source time function duration generally does not produce major difficulties for small events ms 6 5 for larger events the trade off can be resolved using a approach in which for a given depth the time function yielding the best fit to a suite of seismograms is found by deconvolution deconvolution at a series of depths yields a misfit function whose minimum indicates the focal depth tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near source structure it successfully handles multiple sources moderate sized finite sources and horizontally propagating ruptures good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation a number of important tectonic results have emerged from studies using these depth determination methods the maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750 c isotherm this depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low velocity zone indicated by surface wave dispersion the maximum depth of continental intraplate seismicity is also temperature limited but by a lower temperature these observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies the maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400 c this temperature is lower than would be predicted from the intraplate results and may indicate that transforms are either hotter or weaker than expected the depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow 0 8 km the depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process at subduction zones precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate precisely locate the plate interface and map the vertical extent and position of faulting in large earthquakes

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

detailed structural analysis of the rim of a large complex impact crater bosumtwi crater ghana the 1 ma bosumtwi crater in ghana is an 11 km diameter presumably complex well preserved impact structure that is associated with the ivory coast tektite strewnfield detailed structural geologic studies along a complete traverse through the northwestern rim section indicated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor zone 1 is dominated by thick deposits of lithic impact breccia intercalated in places with products of local mass wasting zone 2 contains inward dipping thrust planes conjugate radial fractures isoclinal folding and overturned stratigraphic sequences zone 3 represents a megabreccia zone in which block size decreases upward and outward toward the rim crest the innermost zone 4 is dominated by intense thrust faulting of multiple orientations resulting in complex duplex and lens shaped bodies these deformation styles generally correspond to those previously reported from the rims of simple bowl shaped meteorite impact craters and appear to be characteristic of impact structures in general

a very broad band inversion method for the recovery of earthquake source parameters abstract waveform studies of the earthquake source have traditionally been performed in narrow bands of the seismic spectrum waveform matching of p and sh waves emphasizes frequencies near and above the corner frequency normal mode and surface wave analyses on the other hand are performed on data which contain information only below the corner frequency a new method is presented which formally includes the result of a long period centroid moment tensor analysis in an inversion of broad band body wave phases a parametrized source model is used which allows for the recovery of a detailed source time function as well as the direction and magnitude of rupture propagation the technical details and the performance of the method are demonstrated in an analysis of the october 28 1983 borah peak idaho earthquake in particular it is shown how the combination of the long period and broad band data sets in the inversion improves the resolution of both the focal mechanism and the scalar moment the very broad band method is applied in an analysis of the september 19 1985 michoacan m w 8 0 earthquake two concentrations of moment release are identified in the source time function separated by 25 30 s in time the spatial distribution of moment is well explained both by a model of unilateral rupture towards the east with a propagation velocity of 2 4 km s 1 and by two point sources separated by 95 km it is shown that the p wave data are largely insensitive to the scalar moment and that source time functions obtained without consideration of the total moment release exaggerate the heterogeneity of slip on the fault plane source time functions which do contain the total moment show that a large fraction of the moment was released smoothly over the whole duration 80 s of the event the results suggest that the fault plane was characterized by two regions of high stress drop but that a large portion of slip occurred on surrounding weak portions of the plane

geochemistry of impactites and basement lithologies from icdp borehole lb 07a bosumtwi impact structure ghana in 2004 a drilling project by the international continental scientific drilling program icdp at the bosumtwi impact crater ghana 1 07 myr old and 10 5 km in diameter obtained drill core lb 07a which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure the lb 07a core consists of three sequences 82 29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54 88 m of so called lower impactite of monomict impact breccia with several suevite intercalations and 74 53 m of meta graywacke and altered shale of the basement also containing a number of suevite intercalations major and trace element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein compositions of polymict impact breccias of the crater fill revealed by core lb 07a are compared with the compositions of the ivory coast tektites and the fallout suevites the impactites of the lb 07a borehole appear well homogenized with respect to the silicate component and little change in the ranges of many major and trace element differences is seen along the length of the borehole except for fe2o3 mgo and cao contents much scatter is observed for a number of elements and in many cases this increases with depth it is proposed that any variability in composition is likely the function of clast population differences i e also of relatively small sample sizes no systematic compositional difference between polymict lithic and suevitic impact breccias is evident an indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies the impactites of the borehole generally show intermediate compositions to previously defined target rocks the fallout suevites have comparable major element abundances except for relatively lower mgo contents the ivory coast tektites are generally similar in composition to the lb 07a suevites but broader ranges in mgo and cao contents are observed for the lb 07a suevites

lower eocene alluvial paleosols willwood formation northwest wyoming u s a and their significance for paleoecology paleoclimatology and basin analysis abstract the lower eocene willwood formation of northwest wyoming is a 700 m thick accumulation of alluvial floodplain and channel mudstones and sandstones nearly all of which show paleopedogenic modifications pedogenesis of willwood sandstones is indicated by taproot and vertebrate and invertebrate bioturbation early local cementation by calcium carbonate and thin illuviation cutans on clastic grains pedogenesis in willwood mudstones is indicated by plant bioturbation insect and other invertebrate burrow casts and lebensspuren free iron aluminum and manganese mobilization including hydromorphic gleying sesquioxide and calcareous glaebule formation in lower parts of the solum presence of clay rich and organic carbon rich zones and well differentiated epipedons and albic and spodic horizons probable a horizons are also locally well developed occurrence of variegated paleosol units in thick floodplain mudstone deposits and their association with thin lenticular and unconnected fluvial sandstones in the willwood formation of the central and southeast bighorn basin suggest that these soils formed during times of rapid sediment accumulation the tabular geometry and lateral persistence of soil units as well as the absence of catenization indicate that willwood floodplains were broad and essentially featureless all willwood paleosols were developed on alluvial parent materials and are complex in that b horizons of younger paleosols were commonly superimposed upon and mask properties of suspected a and b horizons of the next older paleosols the soils appear to be wet varieties of the spodosol and entisol groups aquods and ferrods and aquents respectively though thick superposed and less mottled red purple and yellow paleosols resemble some ultisols most willwood paleosols resemble warm temperate to subtropical alluvial soils that form today under alternating wet and dry conditions and or fluctuating water tables the up section decrease in frequency of gley mottles increase in numerical proportion and thickness of red versus orange coloration and increase in abundance of calcrete glaebules indicate better drained soils and probably drier climate in late willwood time this drying is believed to be related to creation of rain shadows and spacing of rainfall but not necessarily decrease in absolute rainfall due to progressive tectonic structural elevation of the mountainous margins of the bighorn basin

single crystal u pb zircon age and sr nd isotopic composition of impactites from the bosumtwi impact structure ghana comparison with country rocks and ivory coast tektites the 1 07 myr old bosumtwi impact structure ghana excavated in 2 1 2 2 gyr old supracrustal rocks of the birimian supergroup was drilled in 2004 here we present single crystal u pb zircon ages from a suevite and two meta graywacke samples recovered from the central uplift drill core lb 08a which yield an upper concordia intercept age of ca 2145 82 ma in very good agreement with previous geochronological data for the west african craton rocks in ghana whole rock rb sr and sm nd isotope data of six suevites five from inside the crater and one from outside the northern crater rim three meta graywacke and two phyllite samples from core lb 08a are also presented providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the bosumtwi crater rocks our rb sr and sm nd data show also that the suevites are mixtures of meta greywacke and phyllite and possibly a very low amount of granite a comparison of our new isotopic data with literature data for the ivory coast tektites allows to better constrain the parent material of the ivory coast tektites i e distal impactites which is thought to consist of a mixture of metasedimentary rocks and possibly granite but with a higher proportion of phyllite and shale than the suevites i e proximal impactites when plotted in a rb sr isochron diagram the sample data points n 29 including literature data scatter along a regression line whose slope corresponds to an age of 1846 160 ma with an initial sr isotope ratio of 0 703 0 002 however due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material this age which is in close agreement with a possible metamorphic age of 1 8 1 9 ga tentatively derived from our u pb dating of zircons is difficult to consider as a reliable metamorphic age it may perhaps reflect a common ancient source whose rb sr isotope systematics has not basically been reset on the whole rock scale during the bosumtwi impact event or even reflect another unknown geologic event

the influence of stratigraphy and facies distribution on reservoir quality and production performance in the triassic skagerrak formation of the uk and norwegian central north sea acknowledgements funding information the triassic phase 2 consortium members bp total jx nippon cairn energy are gratefully acknowledged for their support of this work all reviewers are thanked for their comments and additions which undoubtedly improved the manuscript

alluvial response to differential subsidence sedimentological analysis aided by remote sensing willwood formation eocene bighorn basin wyoming usa a colour landsat thematic mapper image was used to identify and to map on a regional scale alluvial facies in the lower eocene willwood formation each facies covers a distinct geographical area of several hundred square kilometres in the bighorn basin of wyoming usa comparison of the image with laboratory spectra from willwood formation rocks and field exposures shows that the image provides information regarding the colour of mudstones their relative abundances and the relative abundance of sandstones facies were further characterized from field study and published descriptions on the basis of sandbody geometry the types of nodules present in the mudstones and the abundance and geometry of carbonaceous mudstones this information helps to interpret the fluvial environments represented and types of alluvial palaeosols found in the willwood formation four facies have been distinguished the first facies is dominated by sheet sandbodies associated with immature palaeosols the sheets mark the locations of major channels which tend to occur in topographic lows the second facies is characterized by more mature moderately drained soils that required relatively lengthy periods of non deposition facies three and four dominantly consist of drab mudstones and abundant carbonaceous units both formed in poorly drained areas although minor red mudstones demonstrate that drainage conditions periodically improved in facies three facies four underwent prolonged saturation the geographical facies distribution suggests that east west faults previously mapped in the bighorn mountains extend westward into the bighorn basin and were active during early eocene time the lithological heterogeneity of the willwood formation rocks is attributed to warping associated with movement along these basement controlled faults fault activity created topographic gradients which influenced the position of major channel systems and helped produce variable drainage conditions that affected eocene soil development

abrasion set limits on himalayan gravel flux rivers sourced in the himalayan mountain range carry some of the largest sediment loads on the planet yet coarse gravel in these rivers vanishes within approximately 10 40 kilometres on entering the ganga plain the part of the north indian river plain containing the ganges river understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms rapid increase in gravel flux and subsequent channel bed aggradation that is sediment deposition by a river following the 1999 chi chi and 2008 wenchuan earthquakes reduced channel capacity and increased flood inundation here we present an analysis of fan geometry sediment grain size and lithology in the ganga basin we find that the gravel fluxes from rivers draining the central himalayan mountains with upstream catchment areas ranging from about 350 to 50 000 square kilometres are comparable our results show that abrasion of gravel during fluvial transport can explain this observation most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the ganga plain these findings indicate that earthquake induced sediment pulses sourced from the greater himalayas such as that following the 2015 gorkha earthquake are unlikely to drive increased gravel aggradation at the mountain front instead we suggest that the sediment influx should result in an elevated sand flux leading to distinct patterns of aggradation and flood risk in the densely populated low relief ganga plain

the okavango fan and the classification of subaerial fan systems abstract controversy exists over the classification of fluvially formed subaerial fans authors either restrict alluvial fans to debris flow dominated types or extend the spectrum to fans dominated by braided rivers the okavango fan provides an end member which extends this spectrum to fans dominated by meandering and low sinuosity rivers the fan is a large 150 km radial axis shallowly sloping 0 00036 highly vegetated subaerial fan which can be subdivided into four subenvironments these are 1 the entry corridor or panhandle characterized by single and anastomosing meander belts 2 the upper fan characterized by meander belts diverging from the fan apex comprising peat confined meandering channels with interchannel swamps forming thick peats 3 the middle fan with highly confined single and anastomosing low sinuosity rivers and less common prograding meander belts whose channels are confined by thick peats with however little chance of peat preservation and 4 the lower fan in which annual floods from relatively unconfined channels spread over the fan surface and interact with pre existing aeolian and lacustrine deposits with the recognition of this new fan type the spectrum of subaerial fan types can be expressed in terms of 1 debris flow dominated fans of which the death valley fans are a member 2 braided river dominated fans of which the kosi fan is a member and 3 low sinuosity meandering losimean river dominated fans of which the okavango fan is a member this spectrum can be expressed in terms of variation in slope maximum size and percentage of surface vegetation but crucial to the evolution of the various fan types is variation in the flashy to continuous nature of the discharge and the degree of channel confinement evident on the fan surface comparable ancient examples of the three fan types are recognizable many of which provide intermediates between the ideal end members debris flow dominated and braid dominated fan types are known from throughout earth history however the losimean fan type because of its reliance on confining vegetation may only have developed after the devonian period the spectrum of subaerial fan types can be expressed on a triangular field of variation with the vertices defined by the relative importance of the processes which shaped a particular fan system be they debris flows processes associated with braided rivers or processes associated with meandering and low sinuosity rivers

petrography geochemistry and alteration of country rocks from the bosumtwi impact structure ghana samples of the country rocks that likely constituted the target rocks at the 1 07 myr old bosumtwi impact structure in ghana west africa collected outside of the crater rim in the northern and southern parts of the structure were studied for their petrographic characteristics and analyzed for their major and trace element compositions the country rocks mainly meta graywacke shale and phyllite of the early proterozoic birimian supergroup and some granites of similar age are characterized by two generations of alteration a pre impact hydrothermal alteration often along shear zones is characterized by new growth of secondary minerals such as chlorite sericite sulfides and quartz or replacement of some primary minerals such as plagioclase and biotite by secondary sericite and chlorite a late argillic alteration mostly associated with the suevites is characterized by alteration of the melt glass clasts in the groundmass of suevites to phyllosilicates suevite which occurs in restricted locations to the north and to the south southwest of the crater rim contains melt fragments diaplectic quartz glass ballen quartz and clasts derived from the full variety of target rocks no planar deformation features pdfs in quartz were found in the country rock samples and only a few quartz grains in the suevite samples show pdfs and in rare cases two sets of pdfs based on a total alkali element silica tas plot the bosumtwi granites have tonalitic to quartz dioritic compositions the nb versus y and ta versus yb discrimination plots show that these granites are of volcanic arc tectonic provenance provenance studies of the metasedimentary rocks at the bosumtwi crater have also indicated that the metasediments are volcanic arc related compared to the average siderophile element contents of the upper continental crust both country rocks and impact breccias of the bosumtwi structure show elevated siderophile element contents this however does not indicate the presence of an extraterrestrial component in bosumtwi suevite because the birimian country rocks also have elevated siderophile element contents which is thought to result from regional hydrothermal alteration that is also related to widespread sulfide and gold mineralization

a relationship between alluvial backswamps and avulsion cycles an example from the willwood formation of the bighorn basin wyoming abstract stratigraphically limited intervals from the lower eocene willwood formation contain laterally extensive carbonaceous shales and ribbon sandstone networks associated with channel avulsion we present data from one such interval that documents the avulsion sequence vertical sections measured along the outcrop of this interval are similar and comprise a basal carbonaceous shale overlain by fine grained deposits on which weakly developed hydromorphic paleosols formed the paleosols enclose and are locally incised by ribbon sandstones some of which cut down to and partly through the carbonaceous shale the ribbons have width thickness ratios between 3 and 13 some ribbons cluster at a particular stratigraphic level which together with paleocurrent trends suggests that they formed channel networks sections are capped by yellow brown paleosols showing moderate pedogenic development we suggest that the carbonaceous shales developed in low lying topogeneous swamps in distal portions of the floodplain far from the trunk channel such a location set limits on the sediment that they received the mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse splay complexes resulting from avulsion of the trunk river the ribbon sandstones represent ancient feeder channels of the avulsion complex the rapid influx of avulsion deposits appears to have been crucial to preserving the organic material and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulation of organic rich deposits in distal alluvial swamps similar deposits are found in other stratigraphic units in the rocky mountain region and the development of these and other organic rich deposits should be reassessed in terms of channel avulsion

evolution of the miocene sedimentary fill of the gediz graben sw turkey abstract the gediz graben is a continental extensional basin filled with miocene to recent sediments the evolution of the graben can be expressed in terms of two phases i miocene half graben phase and ii post miocene graben phase the miocene half graben phase was governed by the southern margin structure evolution of which significantly controlled the stratigraphic variability in the basin alluvial fluvial and lacustrine systems developed in the graben as a function of distance to the southern margin thus alluvial deposits with coarse grained facies dominate the southern margin and grade into finer fractions of fluvial and or lacustrine deposits to the north this depositional architecture repeated three times during the miocene and deposited alasehir caltilik and gediz formations these formations depict distinct thickness and grain size decrease from south to north to emphasize the half graben configuration for the favour of the southern margin the post miocene graben phase started with the faulting of the northern margin this resulted in relatively balanced partitioning of the subsidence between the two margins although the preceding asymmetry was inherited by the graben sediment influx became bipolar and alluvial fan systems developed along the both margins

preliminary determination of focal mechanisms from the inversion of spectral amplitudes of mantle waves abstract we present a method for the preliminary determination of focal mechanisms pdfm based on the inversion of deviatoric moment tensor components from depleted datasets of mantle wave spectral amplitudes the latter are found to be largely insensitive to errors in epicentral estimates which can hamper the inversion of the full complex spectrum under real time operational conditions robust solutions can be obtained from rayleigh wave spectra at as few as three stations and from a combination of rayleigh and love waves at only two stations provided they sample 60 in azimuth the systematic application of this method to a dataset of 43 large earthquakes during 1996 1999 yielded mechanisms rotated an average of only 17 from the published harvard cmt a figure comparing favorably with the case of other catalogues such as the usgs moment tensor solutions

late cretaceous fluvial megafan and axial river systems in the southern cordilleran foreland basin drip tank member of straight cliffs formation and adjacent strata southern utah u s a abstract the upper cretaceous lower campanian drip tank member of the straight cliffs formation in the grand staircase region of southern utah is an upward coarsening sandstone rich low sinuosity fluvial deposit in the cordilleran foreland basin the drip tank member thins from 75 m to 30 m eastward across the basin foredeep approximately parallel to its dominant eastward sediment dispersal direction and thins southward along the foredeep to as little as 17 m near its southern outcrop extent along modern cliff lines it overlies and underlies mudstone rich fluvial strata of the john henry member of the straight cliffs formation and lower member of the wahweap formation respectively both of which were deposited by northeast flowing rivers the john henry drip tank contact lies in an interfingering progradational stratigraphic transition from medium grained lenticular sandstone bodies to tabular coarse grained amalgamated sandstone bodies the contact with the overlying wahweap formation is locally a prominent surface of erosion or sediment reworking sandstone petrography and detrital zircon age populations of the strata indicate detrital sources in basement south of the foreland basin thrust sheets to the west and magmatic arc rocks to the southwest the uppermost part of the john henry member contains as much as 10 feldspar and abundant 1 84 1 60 ga detrital zircons derived from yavapai mazatzal basement rocks in the mogollon highlands the drip tank member ranges from quartzarenite to chert bearing sublitharenite with grenville 1 2 1 1 ga neoproterozoic and early paleozoic detrital zircons derived from proterozoic through jurassic strata exposed in the sevier orogenic belt lower member wahweap litharenites contain abundant thrust belt derived detrital carbonate grains absent from compositionally mature drip tank sandstones as well as a broad range of zircon ages the drip tank member constitutes deposits of a sand rich fluvial megafan or distributive fluvial system that displaced a subjacent but partly time equivalent axial fluvial system eastward as the megafan driven in part by seasonally high discharge fed by monsoonal precipitation prograded across the basin foredeep a thick amalgamated fan axis succession present in northern exposures contrasts with a thinner fan flank succession in southern localities tabular sixth order lithosomes each containing channels on the order of 7 m deep define three individual fluvial fans or fan lobes in the megafan system younger wahweap fluvial channels eroded uppermost drip tank deposits of the fan axis and fan flank transverse fan progradation across the foredeep recorded falling accommodation sediment supply in an aggradational to progradational systems tract erosion and reworking of the upper fan surface record decreased accommodation sediment bypass and erosion prior to reoccupation of the foredeep by the wahweap axial fluvial system the upper contact of the drip tank member is thus the primary sequence boundary in this multi component fluvial system and the lower wahweap represents transitional systems tract deposits accumulated as accommodation sediment supply increased

quantification of a distributive fluvial system the salt wash dfs of the morrison formation sw u s a abstract recent analysis of modern aggradational continental sedimentary basins reveals that sedimentation patterns are dominated by distributive fluvial systems dfss the salt wash member of the late jurassic morrison formation has previously been described as a fan shaped fluvial system this study characterizes facies variations across the salt wash dfs to quantitatively test predicted trends in conceptual dfs models notable proximal to distal trends include a change in total thickness of the fluvial succession from 174 m to 40 m and an average grain size from coarse sand to silt while the percentage of sand decreased from 70 in the proximal region to 8 in the distal region the proportion of amalgamated channel belt deposits decreased from 67 to 0 while floodplain facies and lacustrine deposits increase 38 to 94 and 0 1 to 7 respectively a downstream decrease in average channel belt thickness 15 m to 3 8 m from thickest to thinnest and average story thickness 7 7 m to 2 3 m from thickest to thinnest is also recorded significant downstream changes in deposit architecture were also noted with proximal regions dominated by stacked channel belt deposits with a high degree of amalgamation distal deposits are dominated by floodplain muds and sheet sandstones and sparse ribbon channels with little to no amalgamation of channel deposits this study provides quantified information for an ancient dfss with the aim of providing a dataset that can be used for objective comparison between different dfss as well as providing numerical data to aid resource exploration and modelling efforts

avulsion deposits in lower eocene alluvial rocks bighorn basin wyoming abstract eocene alluvial rocks in the northern bighorn basin can be subdivided into three major kinds fine grained deposits on which cumulative paleosols developed lithologically heterogeneous packages associated with immature paleosols and major sheet sandstone bodies the heterogeneous packages which alternate vertically with the cumulative paleosols are dominated by fine grained deposits and also contain ribbon sandstone bodies and thin 1 m sheet sandstone bodies most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages cut and fill deposits with fine grained fills are locally important because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine grain d facies they are referred to as simple paleosol packages the major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits contrary to classical models of meandering streams only a portion of the fine grained rocks in the study area were deposited by overbank flooding of a trunk channel on the basis of their stratigraphic position and few ribbon sandstones the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel farther from the trunk channel overbank floods deposited alluvium on which the cumulative paleosols formed the greater parts of the simple paleosol packages including their fine grained deposits were deposited rapidly during channel avulsion ribbon sandstone bodies represent ancient splay channels on the avulsion belt simple paleosol packages in the willwood formation support the contention of smith et al 1989 that n many meandering river systems a significant portion of the fine grained deposits were deposited as the trunk channel avulsed deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences

analysis of fluvial architecture in the blackhawk formation wasatch plateau utah u s a using large 3d photorealistic models abstract there are numerous allocyclic and autocyclic factors which influence fluvial sandbody geometry unraveling the relative importance of such factors is challenging because fluvial systems can migrate and avulse across significant distances on the floodplain so that collecting datasets that cover a large enough area with sufficient detail is difficult this problem has been addressed by acquiring an oblique helicopter mounted lidar survey over 16 km of exposed sections from cretaceous coastal plain deposits cropping out in the wasatch plateau of east central utah the exposures of the blackhawk formation occur in steep cliffs which are continuous for several tens of kilometers and have been previously inaccessible for study the recorded lidar data were used to build detailed photorealistic models virtual outcrops from which quantitative dimensional data on fluvial architectural elements mostly sandstone channel bodies were collected those channelized bodies show significant variation in width and thickness 26 of the sandbodies were identified as being multistory channel bodies with a thickness of 9 1 m 4 67 m 1 and a width of 142 6 m 180 7 m which are significantly larger than the measured single story sandbodies 5 9 m and 96 9 m respectively 50 of all sandbodies have aspect ratios smaller 13 1 and the highest values reach 78 1 the spatial constraint of the 397 fluvial sandbodies enabled the recognition of a series of vertical trends within the dataset these trends include a subtle upward increase in width and thickness of both the mean and maximum values this is coupled with a general upward decrease in the number of sandbodies and results in a broadly constant channel to overbank ratio a clear correlation between wider channels with increased distance to the shoreline could also be documented these observations can be explained by considering the entire blackhawk formation as the deposits of a large distributive fluvial system in this model the key control on channel architecture is the distance to the shoreline contrasting with previous interpretations of the blackhawk depositional system in which the rate of accommodation creation in the coastal plain was thought to be the key control on channel geometry and stacking patterns

results of pre drilling potential field measurements at the bosumtwi crater gravity and magnetic measurements were carried out at the bosumtwi crater to determine the geophysical signature of the crater land gravity data was acquired at 163 locations around the structure and on the shore of the lake the separation between the gravity stations was 500 m for radial profiles but 700 1000 m along roads and footpaths that ran parallel to the lakes shore additionally a marine gravity survey was carried out along 14 north south and 15 east west profiles on the lake magnetic data was also acquired along 14 north south profiles on the lake in all marine surveys the line spacing was 800 m and navigation was provided by a garmin 235 echo sounder gps the gravity signature of the crater is characterized by a negative bouguer anomaly with an amplitude of about 18 mgal using the seismic results as constraints the gravity model obtained indicates the central uplift at a depth of 250 m the negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor the impact breccias within the crater and the sedimentary and water infilling of the lake magnetic modeling yielded a model for the causative body which is located north of the central uplift the model has a magnetic susceptibility of 0 03 s i and extends from a depth of 250 to 610 m the causative bodies have been interpreted as impactites

combined remote sensing analyses and landform evolution modeling reveal the terrestrial bosumtwi impact structure as a mars like rampart crater abstract we combine remote sensing and geomorphological analyses with landform evolution modeling to investigate whether or not terrestrial impact crater morphologies are comparable with martian impact craters formed in volatile rich environments we conduct a detailed remote sensing analysis of the terrestrial bosumtwi crater in ghana 10 5 km 1 07 ma investigating its geology and geomorphology this includes a morphometric analysis of the drainage pattern from digital elevation models and the interpretation of multi spectral image data the results are compared with a fresh martian impact crater of similar size for which a hypothetical drainage network and catchment areas are derived we investigate whether the current morphology of bosumtwi crater results from erosion of a typical martian type or lunar type impact crater morphology using landform evolution modeling our results reveal that the morphology of bosumtwi crater cannot be explained by erosion of a lunar like impact crater in contrast the morphological and spectral characteristics of bosumtwi crater exhibit striking similarities to those of martian rampart craters especially double layered ejecta craters we therefore suggest that bosumtwi crater was originally built as a mars like rampart crater and that the current morphology is the result of weakly eroded ejecta ramparts

shock metamorphic petrography and microraman spectroscopy of quartz in upper impactite interval icdp drill core lb 07a bosumtwi impact crater ghana standard and universal stage optical microscope and microraman spectroscopic examination of quartz from the upper impactite interval of the international continental scientific drilling program icdp lake bosumtwi crater drill core lb 07a demonstrates widespread but heterogeneous evidence of shock metamorphism in the upper impactite which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333 4 415 7 m quartz occurs as a major component within metasedimentary lithic clasts and as abundant isolated single crystal grains within matrix the noted quartz shock metamorphic features include phenomena related to a deformation such as abundant planar microstructures grain mosaicism and reduced birefringence b phase transformations such as rare diaplectic quartz glass and very rare coesite c melting such as isolated colorless to dark glassy and devitrified vesicular melt grains and d secondary post shock features such as abundant variable decoration of planar microstructures and patchy grain toasting common to abundant planar deformation features pdfs in quartz are dominated by 1013 equivalent crystallographic planes although significant percentages of 1012 and other higher index orientations also occur notably c 0001 planes are rare significantly the quartz pdf orientations match most closely those reported elsewhere from strongly shocked crystalline target impactites barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 gpa were widely reached pressures exceeding 40 45 gpa were more rare the relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz together with the nature and distribution of melt within suevite suggest a water or volatile rich target for the bosumtwi impact event

bosumtwi impact structure ghana geochemistry of impactites and target rocks and search for a meteoritic component major and trace element data including platinum group element abundances of representative impactites and target rocks from the crater rim and environs of the bosumtwi impact structure ghana have been investigated for the possible presence of a meteoritic component in impact related rocks a comparison of chemical data for bosumtwi target rocks and impactites with those for ivory coast tektites and microtektites supports the interpretation that the bosumtwi structure and ivory coast tektites formed during the same impact event high siderophile element contents compared to average upper crustal abundances were determined for target rocks as well as for impactites chondrite normalized and iron meteorite normalized abundances for target rocks and impactites are similar they do not however allow the unambiguous detection of the presence or identification of the type of a meteoritic component in the impactites the indigenous siderophile element contents are high and possibly related to regional gold mineralization although mineralized samples from the general region show somewhat different platinum group element abundance patterns compared to the rocks at bosumtwi the present data underline the necessity of extensive target rock analyses at bosumtwi and at impact structures in general before making any conclusions regarding the presence of a meteoritic component in impactites

in situ seismic measurements in borehole lb 08a in the bosumtwi impact structure ghana preliminary interpretation in order to assist in the interpretation of previous seismic refraction and reflection surveys a vertical seismic profile was acquired in the lake bosumtwi ghana hard rock core hole lb 08a no seismic reflections are seen in the up going wave field obtained and this observation is consistent with the lack of reflectivity observed in the corresponding 2 d surface seismic profile obtained in earlier studies direct down going p waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open hole hard rock section from a depth of 239 m to 451 m of lb 08a analysis of the observed travel times indicates a nearly constant p wave velocity of 1520 m s through the soft lacustrine sediments in the hard rock however the p wave velocity rapidly increases by nearly 30 from 2600 m s to 3340 m s these values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data these values are low relative to those expected for the metasedimentary protoliths an observation that has been made at other young impact structures of similar size the low velocities together with the fact that they increase so rapidly is suggestive of a decreasing density of fractures and microcracks with depth consequently the seismic velocity trend may provide a proxy measure of damage and hence the decay of the shock pressure from the impact point validation of this requires additional detailed studies of the porosity structure in the core

multi scale classification of fluvial architecture an example from the palaeocene eocene bighorn basin wyoming this work was supported by phase 2 of the fluvial systems research group bp bg chevron conocophillips and total mgms thanks the sao paulo research foundation fapesp 2014 13937 3 the authors also wish to thank numerous residents of the bighorn basin for their kind hospitality and access to land we also thank john howell for discussions on fluvial geometries and nomenclature and isobel buchanan and alistair swan for assistance in the field comments from reviewers andrew miall luca colombera and an anonymous reviewer greatly improved this manuscript

tectonic control on facies distribution of the camp rice and palomas formations pliocene pleistocene in the southern rio grande rift the pliocene pleistocene camp rice and palomas formations in the rio grande rift of southern new mexico provide an excellent test of the role of basin symmetry in the distribution of piedmont and axial fluvial facies in the asymmetrical palomas and northern mesilla basins the axial fluvial facies is characterized by multistory channel sands and sandstones and is concentrated near the locus of maximum subsidence within a few kilometers of the footwall scarp fanglomerate derived from the footwall uplift extends only a few kilometers or less from the footwall scarp whereas alluvial fan and alluvial flat conglomerate sand and mudstone deposited on the hanging wall dip slope occupy a much wider outcrop belt in the symmetrical hatch rincon basin the axial fluvial facies extends to within a few kilometers of both the northern and southern basin margins and has a much higher percentage of fine grained overbank deposits some of which contain calcareous paleosols in all three basins a tongue of fanglomerate as much as 30 m thick prograded over the axial fluvial facies near the end of camp rice and palomas deposition the distribution of facies of the camp rice and palomas formations supports previously published models that predict a two stage history of asymmetrically subsiding basins during tectonically active periods axial river channels preferentially avulse into the topographically lowest area of the alluvial plain which is directly above the axis of maximum subsidence during the postorogenic stage when erosion rate exceeds subsidence rate coarse transverse dispersed sediment progrades toward the center of the basin

alluvial response to the paleocene eocene thermal maximum climatic event polecat bench wyoming u s a abstract the stratigraphic interval spanning the paleocene eocene thermal maximum in the northern bighorn basin wyoming shows changes in the alluvial record that can be tied to a high resolution climate record the complexity and stratigraphic spacing of paleosols change through the study section comparison to the climate record reconstructed from paleosols indicates the changes correspond to paleocene eocene thermal maximum climatic fluctuations in particular the middle of the section contains thick welded paleosols and thin avulsion deposits that link to times of well drained floodplains and lower mean annual precipitation stratigraphic intervals below and above the main part of the paleocene eocene thermal maximum interval correspond to times of less well drained floodplains and higher mean annual precipitation these strata contain thinner paleosols and thick avulsion deposits differences in paleosol complexity and spacing suggest that sediment flux to the depositional site varied in response to precipitation fluctuations associated with the paleocene eocene thermal maximum welded paleosols and thin avulsion deposits indicate reduced floodplain accretion during deposition of the middle of the paleocene eocene thermal maximum intervals with widely spaced paleosols indicate more rapid accretion we hypothesize that drier episodes associated with warming caused reduced vegetation in source areas and promoted erosion and increased sediment yield because precipitation was reduced much of that sediment was stored in upstream reaches of the fluvial system rather than moving to the depositional basin welded paleosols formed because of diminished sediment supply to the basin with a return to wetter conditions during the recovery phase of the paleocene eocene thermal maximum upstream water flux increased stored sediment moved to the basin and vertically spaced thinner paleosols developed the results demonstrate how vertical sections of alluvial paleosols can provide information on how climate fluctuated through time and how the fluvial system responded to climate change

river systems and their sediment flux indo gangetic plains northern bihar india river systems of part of the himalayan foreland northern bihar plains india are described in terms of their channel morphology hydrology and suspended sediment characteristics a simple classification of the river systems based mainly on the source area characteristics is proposed i mountain fed ii foothills fed iii plains fed and iv mixed fed rivers distinct differences are noted between these classes of river systems most rivers show evidence of channel movement mainly by avulsion but cut offs also occur locally the mountain fed rivers are characterized by very high discharge and low suspended sediment concentration and the plains fed rivers have relatively low discharge and high suspended sediment concentration the foothill fed rivers have moderate values of discharge and suspended sediment concentration the mountain fed rivers have built megafans of large extent whereas the foothills fed and plains fed rivers have formed muddy interfan areas semi quantitative estimates of water and sediment flux suggest that about 99 9 of the mass transfer into the plains is water with the remaining 0 1 being sediment and that 10 s of the latter 0 01 of the total is retained in the basin the remainder being transferred to the bengal fan

10be content in clasts from fallout suevitic breccia in drill cores from the bosumtwi impact crater ghana clues to preimpact target distribution rocks from drill cores lb 07a crater fill and lb 08a central uplift into the bosumtwi impact crater ghana were analyzed for the presence of the cosmogenic radionuclide 10be the aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater filling breccia and also to detect meteoric water infiltration within the impactite layer 10be abundances above background were found in two out of 24 samples from the lb 07a core and in none of five samples from the lb 08a core after excluding other possible explanations for an elevated 10be signal we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface our results suggest that in crater breccias were well mixed during the impact cratering process in addition the lack of a 10be signal within the rocks located very close to the lake sediment impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited this may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore space but rather through a localized system of fractures

spillover models for axial rivers in regions of continental extension the rio mimbres and rio grande in the southern rio grande rift usa the pliocene early pleistocene history of the ancestral rio grande and quaternary history of the rio mimbres in the southern rio grande rift new mexico illustrate how axial rivers may alternately spill into and subsequently abandon extensional basins three types of spillover basins are recognized based on the angle at which the axial river enters the basin and whether it descends the hanging wall dip slope or footwall scarp to reach the basin floor in the mimbres basin type the axial river enters and flows through the spillover basin nearly parallel to the footwall scarp resulting in a narrow belt of basin axis parallel channel sand bodies located near the footwall scarp in contrast an axial river may enter a spillover basin at a high angle to its axis either descending the hanging wall dip slope columbus basin type or footwall scarp tularosa basin type and construct a fluvial fan consisting of radiating distributary channels orientated nearly perpendicular to the basin axis faulting exerts significant control on river spillover by creating the topographic gaps through which the axial river moves and by terminating spillover by subsequently uplifting or tilting the gap spillover may also be autocyclic in origin as a result of aggradation to the level of a pre existing gap headward erosion creating and or intersecting a gap or simple river avulsion upstream of a gap predicting facies architecture in the three types of spillover basins is critical to successful subsurface exploration for hydrocarbon reservoirs groundwater aquifers or placer mineral deposits

drill core lb 08a bosumtwi impact structure ghana geochemistry of fallback breccia and basement samples from the central uplift the 1 07 myr old bosumtwi impact structure in ghana west africa which measures 10 5 km in diameter and is largely filled by lake bosumtwi is associated with one of four currently known tektite strewn fields two boreholes were drilled to acquire hard rock samples of the deep crater moat and from the flank of the central uplift lb 07a and lb 08a respectively during a recent icdp sponsored drilling project here we present results of major and trace element analysis of 112 samples from drill core lb 08a this core which was recovered between 235 6 and 451 33 m depth below lake level contains polymict lithic breccia intercalated with suevite which overlies fractured brecciated metasediment the basement is dominated by meta graywacke from fine grained to gritty but also includes some phyllite and slate as well as suevite dikelets and a few units of a distinct light greenish gray medium grained meta graywacke most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types as well as from aqueous alteration processes which have undeniably affected the different rocks suevite from core lb 08a fallback suevite and fallout suevite samples from outside the northern crater rim display some differences in major mainly in mgo cao and na2o contents and minor mainly cr and ni element abundances that could be related to the higher degree of alteration of fallback suevites but also result from differences in the clast populations of the two suevite populations for example granite clasts are present in fallout suevite but not in fallback breccia and calcite clasts are present in fallback breccia and not in fallout suevite chondrite normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other siderophile element contents in the impact breccias are not significantly different from those of the metasediments or compared to target rocks from outside the crater rim so far no evidence for a meteoritic component has been detected in polymict impact breccias during this study in agreement with previous work

on the limits of linear moment tensor inversion of teleseismic body wave spectra the limits of linear moment tensor inversions from long period teleseismic body waves are analysed in detail using inverse methods we focus our attention on single station and few stations methods information on the feasibility of full or deviatoric moment tensor determinations prior to inversion are deduced from the system conditioning the resolution and correlation of the momenttensor components are analysed using the resolution matrix conclusions on the importance and independence of the data are drawn from the information matrix the single station case and its implications are discussed in greater detail as well as possible constraints on the inversion

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

stratigraphic architecture of an experimental basin with interacting drainages abstract the stratigraphic architecture of intracontinental rift basins is defined by a dynamic relationship between depositional processes associated with the basin floor and flanking tributary streams the resulting depositional belts are sensitive to a variety of factors including basin geometry subsidence rate and sediment discharge the experimental earthscape run in 2006 xes06 examined the development of fluvial morphology and alluvial architecture as a function of subsidence and sediment flux in an experimental basin based on the form of a simple half graben sediments from tributary drainages were introduced into the axial stream through toe cutting and realignment of transverse drainage courses to parallel the prevailing axial flow direction transverse sediment contributions to the axial stream were almost equally apportioned over a wide range of sediment discharges tested in the experiments sediment tracers showed a larger contribution of footwall derived sediment into the axial belt probably due to more frequent and aggressive toe cutting by axial streams changes in the axial transverse deposit boundary to external forcing by subsidence and sediment discharge and relatively rapid intrastage stabilization of the depositional belts resembles the large scale self organization observed in moving boundaries that define the morphology of fluviodeltaic systems basin sedimentation was matched to subsidence in order to maintain a constant base level which made the location and width of the axial belt sensitive to the relative sediment fluxes from the transverse systems rather than the axis of maximum subsidence the asymmetrical subsidence pattern and the transverse fan morphology influenced the preservation of sedimentary sequences stage bounding stratigraphic lacunae were well preserved in the hanging wall succession providing a reliable record of basin development

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

fluvial morphology and sediment flux steering of axial transverse boundaries in an experimental basin abstract the distinction between depositional belts associated with the basin axis and flanking piedmont streams is a fundamental attribute of the stratigraphic architecture of intracontinental rift basins spatiotemporal distributions of these lithofacies associations are sensitive to a combination of factors including basin geometry subsidence rate and sediment discharge however most studies have focused on one or two controls and one depositional component axial or tributary of the basin fill succession a new perspective on how these depositional belts develop under simple but precisely controlled boundary conditions of steady subsidence sediment flux and water discharge is achieved through the creation of an experimental stratigraphic succession the experimental earthscape run in 2006 xes06 focused on the geomorphic evolution of sedimentary successions within an asymmetrically subsiding basin analogous to a simple half graben containing four interacting supply points of sediment and water under the imposed conditions the experimental system self organized into an axial stream flanked by transverse fans imposition of various combinations of longitudinal and lateral sediment flux showed that the locations and widths of the axial and transverse systems were strongly controlled by relative sediment fluxes flux steering and less influenced by the location of the subsidence maximum and subsidence rate the axial drainage was dominated by transversely sourced sediment through toe cutting of the transverse fans except during the highest axial sediment discharges footwall fans persisted even under conditions of very large axial sediment discharge aided by topographic inheritance of the steeper transverse depositional slopes

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

the icdp lake bosumtwi impact crater scientific drilling project ghana core lb 08a litho log related ejecta and shock recovery experiments the 1 07 myr old lake bosumtwi impact crater in ghana was drilled within the framework of the international continental scientific drilling project icdp hole lb 08a drilled into the outer flank of the central uplift and with a total depth of 451 m yielded 215 71 m of impact related rocks this paper summarizes observations of the lithological logging on core lb 08a between a depth of 235 6 and 260 m the section consists of a melt bearing allochthonous polymict and mostly clast supported impact breccia down to 418 m the section comprises a rather uniform unit of metagraywacke alternating with phyllite to slate lower greenschist facies few par autochthonous impact breccia bodies and rare impact dike breccias are present the lowermost part of the section contains several centimeter to decimeter thick melt bearing breccia dikes in country rocks identical to those occurring above omnipresent fracturing was mapped in a qualitative manner most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals the maximum shock pressure as evidenced by quartz is below 26 gpa the allochthonous breccias occasionally contain a few vol of melt particles suevites occur outside the crater rim carrying diaplectic crystals coesite and ballen quartz as well as true melt glasses and a variety of lithic clasts among those spectacular staurolite rich mica schists the recorded shock level in the uplifted target rocks is lower than expected and modeled shock recovery experiments with analogue carbonaceous graywackes at 34 and 39 5 gpa yielded nearly complete transformation of quartz into diaplectic glass we therefore exclude a specific shock behavior of the soft fluid rich target material carbonaceous graywackes shales slates in core lb 08a as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the lake bosumtwi impact crater

insights into the morphology geometry and post impact erosion of the araguainha peak ring structure central brazil the 40 km wide araguainha structure is the largest and best exposed complex impact crater in south america it was excavated in flat lying sediments of the intracratonic parana basin where target rocks are well exposed and have remained undeformed since the impact event ca 245 ma despite the excellent state of preservation and exposure information available on the target rock stratigraphy post impact erosion and morphology of the structure is limited our combined field observations and remote sensing analysis demonstrate that araguainha preserves all features of a shallowly eroded peak ring crater the interior of the structure exposes a central peak surrounded by a 5 km wide annular basin and two main ring features 10 12 and 14 18 km from the center analysis of the pre impact stratigraphy present morphology and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin but minimal to nonexistent beyond the inner ring feature the formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls regional stratigraphic data combined with field observations indicate two periods of post impact erosion associated with exhumation of the parana basin we estimate that 250 350 m of fallback deposits and target rocks have been removed by erosion erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure in contrast to previous suggestions that the impact took place in a shallow marine environment our observations are more consistent with an extremely shallow brackish to freshwater lacustrine environment with little effect on the developing impact crater

contribution of seismic tomography in moment tensor inversions using teleseismic surface wave spectra the knowledge of lateral heterogeneities is crucial for path corrections in moment tensor inversions using surface waves after some attempts to use regionalized earth models for very long period surface wave moment tensor inversions recent tomographic earth models offer the possibility to make short period path corrections and therefore retrieve more reliable moment tensors for teleseismic earthquakes first we try to evaluate the precision required for path corrections in comparison with source effects some selected earth models are tested to evaluate how their results compare to those using multiple frequency filtering techniques some real cases illustrate the sensitivity of moment tensor solutions to the different path corrections and it appears clearly that regionalized earth models and tomographic models deduced from long period data alone greater than 150 sec cannot lead to trustworthy broadband moment tensor inversions recent tomographic models using phase velocities at much shorter periods 40 to 200 sec offer a precision comparable to that of the multiple frequency filtering technique both methods lead to acceptable source mechanisms using a small number of stations in more than two cases out of three the use of recent global tomographic models based upon shorter period surface waves might thus be a useful alternative to heavy multiple frequency filtering techniques to automate source studies especially for rapid determinations using a small number of stations

lithological and structural characteristics of the lake bosumtwi impact crater ghana interpretation of acoustic televiewer images an impact event always creates a cloud of ejecta generated through excavation of the target subsequent in filling of the void by crater fill deposits provides a record of post impact processes full core digital photographic scans of core segments from borehole lb 07 in the bosumtwi impact crater provide a complete record of the in fill process the shape orientation and size of clasts within the impact breccia were measured using a best fit ellipsoid approach clast size and variance together with clast orientation data suggest the impact breccias at bosumtwi can be divided into a simple two fold subdivision that loosely agrees with the lithological zonation of a lower monomict breccia overlain by a polymict breccia the lower unit is characterized by a uniform and finer grained clast size together with a uniform flat lying clast orientation the boundary between the two zones is defined by a sharp increase in clast size the upper zone shows an average increase in clast size with decreasing depth but full grain size spectrum together with increased grain size deviation suggest that this is a result of mixing between two populations with different grain size distribution the main population of clasts shows an incremental decrease of clast size with decreasing depth the upper zone also contains weakly defined shallowly dipping clast fabrics which may be suggestive of horizontal transport or deposition onto an inclined surface

paleohydrologic response to continental warming during the paleocene eocene thermal maximum bighorn basin wyoming abstract geologically rapid global warming occurred during the paleocene eocene thermal maximum petm 56 ma several studies have argued that important changes occurred in the hydrological cycle during the petm but results have been inconsistent ranging from global increases in humidity to drier conditions changes in paleosols during the petm in the southeastern bighorn basin document major drying during the body of the event paleosol changes also suggest transitional episodes of climate change that both preceded and followed the petm qualitative semi quantitative and fully quantitative analyses of a 70 m thick interval of paleosols provide a high resolution record of changes in soil moisture and precipitation those changes are compared to changes in temperature determined from 18o values of tooth enamel from the mammal coryphodon a distinct shift to drier soils occurred just prior to the petm a conclusion that is consistent with previous observations that warming began before the onset of the negative carbon isotope excursion associated with the petm paleosols show a progressive drying trend into the lower part of the petm and become even drier in the upper part of the body of the petm purple red paleosols that appear during the recovery phase of the petm indicate wetter soils although they are better drained than paleosols below the onset the purple red paleosols continue for 15 m above the recovery and indicate that wetter soil conditions persisted after the recovery it is not clear whether changes in the paleosols that preceded and followed the petm reflect global forcing factors like orbital cycles or release of carbon that lacks an isotopic label however such mechanisms would provide a unifying explanation for shifts seen in continental and marine environments

mudrock dominated fills formed in avulsion splay channels examples from the willwood formation wyoming mudrock dominated channel fills are common features of the fluvial palaeogene willwood formation of wyoming these fills are small on average 30 m wide and 3 m deep and they vary from simple plugs to more lithologically complex fills with internal scour surfaces some fills preserve plant material and others show pedogenic modification all fills are located within metre thick intervals of weakly pedogenically modified mudstones that surround ribbon sandstones the intervals have previously been attributed to channel avulsion the fine grained channel fills are interpreted as crevassesplay feeder channels within these avulsion deposits variation in fill type particularly the presence of pedogenic features appears to be related to floodplain drainage possible factors that influenced drainage were basin position of the fills and palaeogene climate fluctuations although the role of either is not unequivocal in general channel fills with sombre colours and preserved plant detritus indicating poorly drained conditions dominate locations that were proximal to the palaeogene basin axes fills that contain root traces red and yellow brown matrix colours and mottling which indicate pedogenic modification and episodic subaerial exposure of the sediment dominate locations that were more distal to the palaeogene basin axes eocene climate fluctuations particularly episodes of global warming may have been a secondary control on the kind of fill that developed mudrock dominated channel fills have not been described previously from ancient avulsion deposits their common presence in the willwood formation suggests that they are a typical component of avulsion belts and that similar channel fills in other stratigraphic units should be considered with regard to channel avulsion the variety of fills observed in the willwood formation indicates that avulsion belt channels can experience very different fill histories the descriptive details of fills provided here should help in their recognition elsewhere in addition the nature of the channel fill provides important information on ancient drainage conditions in the avulsion belt pedogenically modified channel fills have been recognized only rarely from the stratigraphic record their presence throughout the willwood formation suggests that they should exist in other alluvial successions and may have been overlooked previously they indicate that periods of sediment exposure alternated with episodic influxes of water and sediment in an active avulsion

integrated 3 d model from gravity and petrophysical data at the bosumtwi impact structure ghana the bosumtwi impact structure of central ghana was drilled in 2004 as part of the international continental scientific drilling program icdp a vast amount of geoscience data is available from the pre site surveys and the actual drilling phase a 3 d gravity model was constructed and calibrated with the available data from the two icdp boreholes lb 07a and lb 08a the 3 d gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data and with the petrophysical data from the lb 08a and lb 07a core materials and the two borehole logs furthermore the model exhibits lateral density variations across the structure and refines the results from previous 2 5 d modeling an important new element of the 3 d model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling

omeonga a possible large impact structure on the eastern kasai province d r congo abstract the omeonga ring structure d r congo shows a remarkable drainage pattern encircling an area up to 45 km wide and encompassing a central smoothed relief 20 km wide this inner circular ridge is elevated about 70 m above the ring depression corresponding to the bed of the unia river which flows between the inner ridge and an outer irregular ridge landsat 7 etm and aster dem show that the structural characteristics resemble those of several wide impact structures known on earth other geological modes of origin that could produce ring structures such as magmatic activity salt diapirism and karst dissolution have been considered however after evaluating the regional stratigraphy the distribution of volcanism and morphometry these processes seem to be rather unlikely if of impact origin the age of the omeonga structure can be constrained to the late cretaceous cenozoic according to the youngest units in which the ring structure was formed

stability of fault plane solutions for the major n italy seismic events in 2012 abstract we propose a critical analysis of the moment tensor solutions of the major seismic events that affected northern italy in 2012 inverting full waveforms at regional distance using the non linear method named inpar we investigate period dependent resolution that affects in particular the solutions of shallow events this is mainly due to the poor resolution of mzx and mzy components of the seismic tensor when inverting signals whose wavelengths significantly exceed the source depth as a consequence instability affects both source depth and fault plane solution retrieval and spurious large compensated linear vector dipole components arise the inversion performed at cutoff periods shorter than 20 s reveals in many cases different details of the rupture process which are supported by independent geodynamical arguments thus we conclude that inversion of full waveforms at cutoff period as short as possible should be preferred

controls on synorogenic alluvial fan architecture beartooth conglomerate palaeocene wyoming and montana late palaeocene uplift of the beartooth range in northwestern wyoming and southwestern montana generated the beartooth conglomerate along the eastern and northeastern flanks of the range systematic unroofing sequences and intraformational unconformities folds and faults in the conglomerate attest to deposition during uplift along the eastern flank at least three ancient alluvial fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies the two southern fans consist of 700 m of sedimentary clast conglomerate and subordinate sandstone dominated by hyperconcentratedflow and stream flow facies the next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud matrix supported debris flow facies as well as stream flow facies the northernmost depositional system consists of arkosic channellized fluvial conglomerate and sandstone overbank mudstone and crevasse splay sandstone units palaeocurrent data indicate eastward dispersal away from the beartooth range outstanding exposure of the beartooth conglomerate allows facies to be mapped on lateral photographic mosaics a seven fold hierarchy of bounding surfaces and enclosed lithosomes exists in the beartooth conglomerate first through fourth order surfaces are analogous to first through fourth order surfaces that recently have been documented in sandy fluvial facies with one exception sediment gravity flows are bounded by first order surfaces fifth order surfaces are either erosional e g lateral migration of fanhead trench or accretionary e g aggradation of fan surface during backfilling of trench and construction of lobes on lower fan during entrenchment on upper fan some fifth order surfaces coincide with intraformational angular unconformities and are thus the result of long term fanhead entrenchment following uplift of the upper part of the fan sixth order surfaces bound individual fan packages that are several hundred metres thick and 10 km2 in area the enclosed sixth order lithosomes are distinguishable in terms of petrofacies and lithofacies a single seventh order surface bounds the entire beartooth conglomerate lower order lithosomes are produced by intrinsic processes of fan construction fifth order lithosomes can be attributed to both extrinsic and intrinsic controls sixth and seventh order lithosomes are generated by extrinsic controls

basin scale predictive models of alluvial architecture constraints from the palaeocene eocene bighorn basin wyoming usa this work was supported by phase 2 of the fluvial systems research group bp bg chevron conocophillips and total ae thanks the university of aberdeen for additional funding and mgms thanks the sao paulo research foundation fapesp 2014 13937 3 the authors also wish to thank numerous residents of the bighorn basin for their kind hospitality and access to land and isobel buchanan and alistair swan for assistance in the field the authors also thank reviewers luca colombera and sian davies vollum and ae christopher fielding for helpful comments on this manuscript

petrographic studies of fallout suevite from outside the bosumtwi impact structure ghana field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the bosumtwi crater rim suevite occurrence there is known from an 1 5 km 2 area its thickness is 15 m the present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous bosumtwi ejecta deposits here we discuss the petrographic characteristics of drill core samples of melt rich suevite macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock granite graywacke phyllite shale schist and possibly slate clasts up to about 40 cm in size shock metamorphic effects in the clasts include multiple sets of planar deformation features pdfs diaplectic quartz and feldspar glasses lechatelierite and ballen quartz besides biotite with kink bands basement rock clasts in the suevite represent all stages of shock metamorphism ranging from samples without shock effects to completely shock melted material that is indicative of shock pressures up to 60 gpa

uppermost impact fallback layer in the bosumtwi crater ghana mineralogy geochemistry and comparison with ivory coast tektites in 2004 an international continental scientific drilling program icdp drilling project at the bosumtwi impact crater ghana 10 5 km in diameter 1 07 myr old was performed to study the sediments that fill the lake as well as the underlying impactites in one lb 05 of 16 cores drilled into the lake sediments the zone between the impact breccias and the post impact sediments was penetrated preserving the final fine grained impact fallback layer this 30 cm thick layer contains in the top 10 cm accretionary lapilli microtektite like glass spherules and shocked quartz grains glass particlesmostly of splash form less than 1 mm size make up the bulk of the grains 70 78 by number in the coarser size fraction 125 m of the top of the fallback layer about one third of all quartz grains in the uppermost part of the layer are shocked with planar deformation features pdfs almost half of these grains are highly shocked with 3 or more sets of pdfs k feldspar grains also occur and some show shock deformation the abundance of shocked quartz grains and the average shock level as indicated by the number of sets of pdfs for both quartz and k feldspar decrease with depth into the layer the well preserved glass spherules and fragments are chemically rather homogeneous within each particle and also show relatively small variations between the various particles on average the composition of the fallback spherules from core lb 5b is very similar to the composition of ivory coast tektites and microtektites with the exception of cao contents which are about 1 5 to 2 times higher in the fallback spherules this is a rare case in which the uppermost fallback layer and the transition to the post impact sediments has been preserved in an impact structure its presence indicates that the impactite sequence at bosumtwi is complete and that bosumtwi is a very well preserved impact crater

fluvial systems in the siwalik miocene and wyoming paleogene abstract the 3 km thick miocene siwalik group himalayan foredeep in northern pakistan and the 2 km thick paleogene fort union willwood formations bighorn basin in wyoming both preserve long records of fluvial deposition adjacent to rising mountain belts depositional environments and associated habitats change across large basins along with changing physiography and with the location of different river systems that may have varied greatly in patterns of channel deposition and the drainage of adjacent floodplain areas deposits exposed in these two basins provide very different records of shifting paleoenvironments and patterns of basin filling these differences reflect distinct patterns and scales of depositional environments the nature of the exposures and the types of sedimentologic studies that have been carried out in each basin the siwalik group fills a basin that extended at least 1000 km along its axis and 150 250 km away from the mountain front comparison of siwalik deposits and modern drainages in the himalayan foredeep suggests the ancient siwalik basin was filled by large rivers that deposited low gradient sediment fans covering areas on the order of 1000 km 2 and by smaller intrafan rivers with more poorly drained floodplains despite the scale of these river systems relative to siwalik exposures in pakistan transitions between different systems have been recognized deposits of coeval river systems in the siwalik basin show pronounced differences in alluvial architecture the character of overbank deposits and the abundance and taphonomy of organic remains in contrast the bighorn basin in wyoming is a relatively small intermontane foreland basin extending 200 km along its axis and about 80 km across bighorn basin strata were deposited by a river that flowed south to north along the basin axis and by smaller rivers that flowed transverse to the basin axis much of this basin is exposed and thus it is possible to reconstruct changing patterns of deposition and environments through time in more detail than in the siwalik basin these patterns indicate changes in basin wide drainage conditions and environments through time but there are also important differences among coeval strata upsection shifts in environments and vertebrate faunas within both the siwalik and bighorn basins may reflect tectonic or climatic forcing but this comparison emphasizes the importance of recognizing deposits from different contemporaneous river systems before inferring such large scale controls on paleoecological change through time

433 eros lineaments global mapping and analysis using images and laser ranging data from the near shoemaker mission we map lineaments on the surface of eros in order to investigate the relationship between surface morphology and interior structure several sets of lineations are clearly related to visible impact craters while others suggest that different parts of the asteroid may have undergone different stress histories some of these sets infer internal structure at least on a local level this structure may derive from eros parent body and suggest although largely coherent eros interior may have portions that have not undergone a common history

polygonal impact craters on dione evidence for tectonic structures outside the wispy terrain abstract plan view impact crater geometries can be indicative of pre impact structures within the target material impact events that occur on a pre fractured surface generate craters exhibiting one or more straight rim segments termed polygonal impact craters pics impact craters that appear to be pics are abundant on the surface of saturn s icy satellite dione both within the wispy terrain a region with large visible fractures and also outside the wispy terrain the non wispy terrain where less evidence for fracturing has been observed in the non wispy terrain subtle lineaments are hypothesized to be ne sw nw se and e w trending fractures suggesting that tectonism may have been an important process in this terrain results of previous studies have shown that pic straight rim segment azimuths pic azimuths commonly parallel pre impact fracture azimuths although disagreements about this relationship exist in the literature we investigated the hypothesis that fractures either subtle or nonvisible with available spacecraft images are present within dione s non wispy terrain our first step was to assess the relationship between pics and pre existing fracture azimuths in the wispy terrain our results from this initial assessment show a parallel relationship between pic azimuths and fracture azimuths based on this correlation in the wispy terrain we find it likely that this relationship would hold true in the non wispy terrain if pics are present we tested for pics using crater rim azimuth data collected from randomly distributed study locations throughout the non wispy terrain from these data we identified widespread pics in this terrain which supports the hypothesis that subtle fractures are also present analysis of the pic azimuth data yield a pattern for these inferred fractures across dione s surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion our results provide evidence for these previously hypothesized events in dione s history and demonstrate that mapping pics and their azimuths is a useful tool for investigating subtle fractures on solar system bodies

chemostratigraphic implications of spatial variation in the paleocene eocene thermal maximum carbon isotope excursion se bighorn basin wyoming 1 the paleocene eocene thermal maximum petm is marked by a prominent negative carbon isotope excursion cie of 3 5 that has a characteristic rapid onset stable body and recovery to near pre cie isotopic composition although the cie is the major criterion for global correlation of the paleocene eocene boundary spatial variations in the position and shape of the cie have not been systematically evaluated we measured carbon isotope ratios of bulk organic matter 13corg and pedogenic carbonate 13ccarb at six petm sections across a 16 km transect in the se bighorn basin wyoming bed tracing and high resolution floral and faunal biostratigraphy allowed correlation of the sections independent of chemostratigraphy the onset of the cie in bulk organic matter at all six sections occurs within a single laterally extensive geosol the magnitude of the cie varies from 2 1 to 3 8 the absolute and relative stratigraphic thickness of the body of the cie in bulk organic matter varies significantly across the field area and underrepresents the thickness of the petm body by 30 80 the variations cannot be explained by basinal position and instead suggest that 13corg values were influenced by local factors such as reworking of older carbon the stratigraphic thickness and shape of the cie have been used to correlate sections estimate timing of biotic and climatic changes relative to the presumed carbon isotope composition of the atmosphere and calculate rates of environmental and biotic change localized controls on 13corg values place these inferences in question by influencing the apparent shape and duration of the cie

is the present the key to the past a global characterization of modern sedimentary basins the stratigraphic record is heavily biased because it is uniquely composed of sediments that were laid down in basins whereas the majority of the present and historic land surface of the planet is composed of areas that are in net long term erosion with no preservation potential global mapping and quantification of the distribution of currently active sedimentary basins suggest that only 16 of earth s terrestrial land surface is within sedimentary basins the remainder of the land is in upland areas that will not be represented in the future rock record furthermore 60 of the modern basin area has an arid climate as opposed to 27 of the land surface tectonic classification of modern basins indicates that intracratonic and foreland basins cover the greatest area whereas forearc extensional and strike slip basins are the least represented by area while this modern snapshot does not account for differences in subsidence rate or basin longevity the mapping and quantification of modern basins highlight the incompleteness of the stratigraphic record and the importance of caution when assuming the present is the key to the past

alluvial fans and fluvial distributary systems oligo miocene northern spain contrasting processes and products sediments of alluvial fans and distributive fluvial sediments were deposited by contrasting processes during the oligo miocene in the north central ebro basin northern spain the coeval depositional systems can be separated on the basis of lithofacies criteria a number of localized deposits each covering only a few square kilometers and lying adjacent to the south pyrenean thrust front are made up of beds of poorly sorted conglomerate sandstone and mudstone that show a radial decrease in bed thickness and grain size these deposits are interpreted as the products of sheet flood events on alluvial fans that formed at the basin margin covering much larger areas of the basin several thousand square kilometers are conglomerate sandstone and mudstone as lensoid and irregular sheet bodies incised into thin bedded sheets of sandstone and mudstone these are interpreted as the deposits of fluvial systems that are considered to be distributary in form because the channel size and grain size of channel fill deposits decrease down paleoflow and because there is a radial pattern of paleocurrents at the distal margins the fluvial facies interdigitate with ephemeral lacustrine and alluvial plain deposits the deposits of the alluvial fans and the fluvial distributary systems are petrographically distinct reflecting separate source areas important components of the alluvial fan deposits are limestone clasts derived from the adjacent thrust front the deposits of the fluvial systems are mainly sand and subordinate gravel derived from rocks of the axial zone of the pyrenean mountain belt the deposits of these systems interdigitate and there are no transitional forms between the two within this stratigraphic package

focal depths and moment tensor representations of shallow earthquakes associated with the great sumba earthquake focal depths in the range 8 to 24 km below the seafloor were determined for seven of the larger shocks associated with the great sumba earthquake of august 19 1977 depth control was achieved by modeling digitized long period p waves recorded by the world wide standard seismic network if the base of the rupture at the time of the mainshock were no greater than the depth of the deepest aftershock the stress drop would be nearly 500 bars amplitude and first motion data from the p waves were inverted for point source moment tensors the inversion of rayleigh and love wave spectra computed from digital seismograms recorded by the seismic research observatories sro and the abbreviated seismic research observatories asro broadened the source study to include earthquakes with moments as small as 1024 dyn cm too small for either traditional first motion solutions or p wave modeling the inversion procedure was applied to data in the passband from 30 to 120 s the propagation phase for the love and the rayleigh waves was calibrated with signals from two earthquakes for which the source phase was inferred from the p wave solutions agreement with the corresponding p wave solutions is nearly the same whether or not the smaller eigenvalues of the inner product matrix are retained in the inversions the double couple components generally account for more than two thirds of the total moment normal faulting along an east to northeasterly strike characterizes the mechanisms in the principal zone of aftershock activity strike slip mechanisms consistent with horizontal compression normal to the arc prevail in the delayed aftershock zone located 200 km northwest of the mainshock of the earthquakes subjected to this surface wave analysis the foreshock stood out as having felt a significantly higher apparent stress perhaps as much as one half an order of magnitude higher than the others these results suggest that accurate source parameters for moderate to small magnitude earthquakes can be determined from surface waves recorded by the sro and asro in particular the mechanisms of small magnitude foreshocks could be obtained in this way

geochronology and mammalian biostratigraphy of middle and upper paleocene continental strata bighorn basin wyoming the bighorn basin of northwestern wyoming preserves one of the most complete records of middle paleocene to lower eocene continental biota the geochronology of this important interval depends partly on numerical calibration of the geomagnetic polarity time scale gpts but the middle and late paleocene parts of the gpts have been poorly constrained radioisotopically a new volcanic ash from the northern bighorn basin provides the first radioisotopic age for upper middle paleocene strata upper selandian and is an important calibration point for the base of polarity chron c26n and the selandian thanetian stage boundary in the gpts sanidine from the ash has a weighted mean age of 59 00 0 30 2 ma based on twenty three 40 ar 39 ar bulk laser fusion analyses the ash corroborates the revised age estimate for the base of chron c26n in the most recent geomagnetic polarity time scale gpts 04 and confirms that c26n is older than depicted in the 1995 gpts we place paleocene mammalian biozones in the bighorn basin into a refined geochronologic framework these biozones are the primary basis for biostratigraphic correlation of middle and upper paleocene continental deposits in north america new paleomagnetic data provide synchronous tie points for correlation among three stratigraphic sections in the northern bighorn basin and to the gpts paleomagnetic correlation study of new fossil material review of known faunas and refined stratigra phy enable us to reevaluate several biozones we define three new zones based on first occurrences of taxa the phenacolemur ti 4b probathyopsis ti 5a revised and copecion cf 3 revised zones the phenacolemur zone includes some faunas previously placed in the plesiadapis churchilli and plesiadapis fodinatus zones ti 4 and ti 5a respectively the probathyopsis zone replaces the p fodinatus zone and the copecion zone replaces the phenacodus ectocion acme zone cf 3 seven of the biozones considered here are subdivisions of the tiffanian land mammalage thetiffanianwasoriginallytypifiedbythemasonpocketfaunainsouthwest ern colorado but it is now much better known from faunas in the northern bighorn basin biostratigraphic evidence and new paleomagnetic data from the bighorn basin indicate that mason pocket is considerably older than was previously recognized and occurs in chron c26r rather than c25r the geochronologic framework presented here helps to constrain the temporal ranges of species that occur in the middle and late tiffanian and it allows the bighorn basin record to be better compared to other faunal floral and paleoclimate records both regionally and globally

bosumtwi impact structure ghana evidence for fluidized emplacement of the ejecta the about 10 5 km diameter bosumtwi impact crater is one of the youngest large impact structures on earth the crater rim is readily noticed on topographic maps or in satellite imagery it defines a circular basin filled by water lake bosumtwi and lacustrine sediments the morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9 10 km from the center of the crater about 2 crater radii this feature comprises a shallow ring depression also described as an annular moat and a subdued circular ridge at its outer edge the origin of this outermost feature could so far not be elucidated based on remote sensing data only our approach combines detailed topographic analysis including roughness mapping with airborne radiometric surveys mapping near surface k th u concentrations and field observations this provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the bosumtwi impact structure the characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow the setting of bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters which are common on mars and venus and also found on icy bodies of the outer solar system e g ganymede europa dione tethys and charon future studies at bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

characterizing avulsion stratigraphy in ancient alluvial deposits guidelines for identifying ancient avulsion deposits were set forth by kraus and wells kraus m j wells t m 1999 recognizing avulsion deposits in the ancient stratigraphical record in smith n d rogers j eds fluvial sedimentology vi special publication of the international association of sedimentologists vol 28 pp 251 268 building on the study by smith et al smith n d cross t a dufficy j p clough s r 1989 anatomy of an avulsion sedimentology 36 1 23 of the modern saskatchewan river system cumberland marshes central canada and serve to characterize avulsion depositional sequences in the ancient willwood and fort union formations paleogene bighorn basin nw wyoming usa we recognize however that the model is not universally applicable to avulsion dominated successions specifically systems which lack defining heterolithic avulsion deposits set forth by kraus and wells kraus m j wells t m 1999 recognizing avulsion deposits in the ancient stratigraphical record in smith n d rogers j eds fluvial sedimentology vi special publication of the international association of sedimentologists vol 28 pp 251 268 observations in several fluvial intervals suggest that the avulsion stratigraphy outlined by kraus and wells kraus m j wells t m 1999 recognizing avulsion deposits in the ancient stratigraphical record in smith n d rogers j eds fluvial sedimentology vi special publication of the international association of sedimentologists vol 28 pp 251 268 represents one category of avulsion stratigraphy found in the rock record but does not capture the nature of avulsion deposits everywhere based on observations using measured sections outcrop photo panels and aerial photographs in the willwood formation eocene wyoming and ferris formation cretaceous paleogene wyoming we present two end member categories of avulsion stratigraphy in ancient deposits stratigraphically abrupt when a main paleochannel is stratigraphically juxtaposed directly atop floodplain overbank deposits and stratigraphically transitional where crevasse splays and other non floodplain overbank deposits stratigraphically precede a main paleochannel this characterization provides a broader more inclusive way to recognize and describe avulsion stratigraphy in ancient deposits and may be an important factor to consider when modeling connectivity in fluvial reservoirs furthermore our observations show that one type of avulsion channel stratigraphy may prevail over another within an ancient basin suggesting that system wide factors such as splay proneness or avulsion style i e aggradational incisional etc slingerland r smith n d 2004 river avulsions and their deposits annual review of earth and planetary sciences 32 257 285 may be primary controls on the type of avulsion stratigraphy deposited and preserved in ancient basin fills

empirical measurements of tectonic moment release in nuclear explosions from teleseismic surface waves and body waves summary a method of source parameter estimation based principally on observations of teleseismic long period surface waves is developed and applied to 71 underground nuclear explosions at the balapan shagan river test site in kazakhstan first we quantify the observed radiation patterns of love and rayleigh waves noting that such data place only three linear constraints on the five parameters of the standard point source model for each explosion the three constraints are the strengths of the isotropic sin2 and cos2q components of the observed pattern where is azimuth the five source parameters are the isotropic moment m the doublecouple moment adt and the strike dip and rake angles qs 6 a these last four parameters characterize the inferred faulting tectonic release surface waves which propagate from different seismic events in a geographically tight cluster to a distant station are similar in shape and for very shallow sources the propagation operator can be described by a single wave shape a differential propagation delay and a single amplitude factor containing the effects of attenuation scattering focusing and geometrical spreading the inverse problem is formulated which iteratively solves simultaneously for the path amplitude factors and the three radiation pattern parameters for all events using observed seismograms as the empirical wave shapes we fit 1110 relative measurements of rayleigh and love wave amplitudes to determine path amplitude factors to 29 globally distributed stations and radiationpattern parameters for the 71 explosions second we present a new method for interpreting such radiation patterns in previous studies two of the five source parameters have typically been fixed by making the very restricted assumption of pure dip slip faulting at 45 dip the assumption leads to estimates of strike that are mostly in the north west south east direction at balapan and to estimates of m that correlate very poorly with estimates based upon short period signals p lg however now that excellent yield estimates have become available we can avoid the assumption of pure dip slip faulting instead we can estimate mi from p and lg wave data thus adding a fourth constraint to help in interpreting the teleseismic surface wave amplitudes the outcome is a method for estimating the strike as of faulting associated with tectonic release for each explosion and for estimating the two combinations m sin 6 cos a and mo sin 26 sin a which respectively are the strike slip and dip slip components of the tectonic release the strike directions of inferred faulting at balapan are found to lie in the range 90 120 which conforms better with limited geological information on faulting in the region than does the trend that has long been inferred from assuming pure dip slip faulting we find that without additional information about individual explosions we are not able to provide a high precision isotropic moment or yield estimate from teleseismic surface waves

recognition of interfluve sequence boundaries integrating paleopedology and sequence stratigraphy in shallow marine and coastal plain strata sequence boundaries are easily recognized at the base of incised valley fills between valleys however it is more difficult to recognize sequence boundaries where they are expressed as interfluve paleosols we suggest that interfluve paleosols can be most reliably identified using a combination of 1 stratigraphic position 2 field observations such as thickness structure color and degree of rooting and 3 micromorphological features such as bioturbation fabric clay coatings ferruginous features and siderite and barite only micromorphology permits recognition of temporal changes in drainage surface stability and protracted pauses in sedimentation that typify these surfaces this three part approach has been successfully employed to identify interfluve sequence bounding paleosols in the cenomanian dunvegan formation in alberta and british columbia canada

ichnofossils of the alluvial willwood formation lower eocene bighorn basin northwest wyoming u s a abstract the ichnofossil assemblage of the lower eocene willwood formation consists of at least nine distinct endichnia that are preserved in full relief four forms three ichnogenera and four ichnospecies are new and represent fodinichnia and domichnia of oligochaete worms an insect or spider an unknown vertebrate probably a mammal and domichnia of an unidentified organism other potential trace makers of the ichnofauna include insects mollusks and decapods in contrast to an egyptian oligocene fluvial ichnofauna produced largely by animals that burrowed in stream channel deposits the willwood assemblage is principally of flood plain origin though the ichnofauna occurs in a variety of paleosol types most of the fossils are restricted in distribution to specific sediment and soil types and within paleosols to specific identifiable horizons this attribute will make them valuable indiced of paleoenvironment once they are better known in other ancient alluvial sequences the environment suggested by the willwood trace fossils damp but not wet soils with fluctuating water tables is consistent with the warm temperate to subtropical possibly monsoonal conditions that are interpreted for the willwood formation by independent evidence of body fossils and paleopedology

recognition and importance of amalgamated sandy meander belts in the continental rock record meandering fluvial channels and their meander belts are common in modern continental sedimentary basins yet compose a minor constituent of the reported fluvial rock record here we document exhumed amalgamated meander belt deposits from the upper jurassic morrison formation utah united states the size of the amalgamated meander belt 9000 km2 is significantly larger than any documented previously and comparable in size to those from modern sedimentary basins we describe a representative outcrop of sandy point bar deposits that shows features considered characteristic of both braided and meandering fluvial systems lateral accretion sets compose 5 of the outcrop area yet point bar morphology is clearly visible in plan view we suggest that difficulties in the identification of sandy amalgamated meander belt deposits indicate that they have gone largely unrecognized in the rock record their recognition has important implications for basin scale reconstructions of fluvial systems and interpretation of tectonic setting