geology and petrology of enormous volumes of impact melt on the moon a case study of the orientale basin impact melt sea abstract lunar basin forming impacts produce enormous volumes 10 5 km 3 of impact melt all known basin forming impacts combined may produce 10 8 km 3 of impact melt 1 20th the volume of the lunar crust despite their volumetric importance the geology and petrology of massive deposits of impact melt on the moon have been little studied in part because most basin impact melt deposits are old and have been obscured or buried by subsequent impact cratering and mare infill we investigate the geology and model the petrology of fresh massive impact melt deposits in the relatively young 930 km diameter orientale basin models of impact melt production combined with geologic analyses based on new lola topographic data suggest that most of the impact melt 2 3 produced by the orientale forming impact occurs in a 15 km thick impact melt sheet better described as an impact melt sea 350 km in diameter with a volume of 10 6 km 3 we anticipate that the orientale melt sea has undergone large scale igneous differentiation since terrestrial impact melt sheets such as manicouagan sudbury and morokweng less than a tenth of the thickness and a hundredth of the volume of the orientale melt sea have differentiated we develop a model for the cumulate stratigraphy of the solidified orientale impact melt sea a modeled cumulate stratigraphy occurring below a quench crust and anorthositic fallback breccia with an 8 km thick layer of norite overlying a 4 km layer of pyroxenite and a basal 2 km thick layer of dunite produced by equilibrium crystallization of a homogenized melt sea consistent with vigorous convection in that melt sea is supported by remotely sensed norite excavated by the central peak of maunder crater from 4 km depth generally we predict that very large basin forming impacts including the south pole aitken spa basin forming impact produce melt seas with a cumulate stratigraphy similar to that of the orientale melt sea impact melt differentiation may explain apparently anomalous lithologies excavated in the spa basin interior we note that impact melt differentiates are slow cooled the orientale melt sea took on the order of 10 5 years to solidify and if meteoritic siderophiles fractionate into metal or sulfide layers may not be siderophile enriched therefore impact melt differentiates may pass for pristine highland plutonic rocks in the lunar sample suite these predictions can be tested with current and future mission data

numerical modeling of asteroid survivability and possible scenarios for the morokweng crater forming impact the fate of the impactor is an important aspect of the impact cratering process defining impactor material as surviving if it remains solid i e does not melt or vaporize during crater formation previous numerical modeling and experiments have shown that survivability decreases with increasing impact velocity impact angle with respect to the horizontal and target density here we show that in addition to these impactor survivability depends on the porosity and shape of the impactor increasing impactor porosity decreases impactor survivability while prolate shaped polar axis equatorial axis impactors survive impact more so than spherical and oblate shaped polar axis equatorial axis impactors these results are used to produce a relatively simple equation which can be used to estimate the impactor fraction shocked to a given pressure as a function of these parameters by applying our findings to the morokweng crater forming impact we suggest impact scenarios that explain the high meteoritic content and presence of unmolten fossil meteorites within the morokweng crater in addition to previous suggestions of a low velocity and or high angled impact this work suggests that an elongated and or low porosity impactor may also help explain the anomalously high survivability of the morokweng impactor

impact melt and projectile bearing ejecta at barringer crater arizona abstract our understanding of the impact cratering process continues to evolve and even at well known and well studied structures there is still much to be learned here we present the results of a study on impact generated melt phases within ejecta at barringer crater arizona one of the first impact craters on earth to be recognized and arguably the most famous we report on previously unknown impact melt bearing breccias that contain dispersed fragments of the projectile as well as impact glasses that contain a high proportion of projectile material higher than any other glasses previously reported from this site these glasses are distinctly different from so called melt beads that are found as a lag deposit on the present day erosion surface and that we also study it is proposed that the melts in these impact breccias were derived from a more constrained sub region of the melt zone that was very shallow and that also had a larger projectile contribution in addition to low and high fe melt beads documented previously we document ca mg rich glasses and calcite globules within silicate glass that provide definitive evidence that carbonates underwent melting during the formation of barringer crater we propose that the melting of dolomite produces ca mg rich melts from which calcite is the dominant liquidus phase this explains the perhaps surprising finding that despite dolomite being the dominant rock type at many impact sites including barringer crater calcite is the dominant melt product when taken together with our estimate for the amount of impact melt products dispersed on and just below the present day erosional surface it is clear that the amount of melt produced at barringer crater is higher than previously estimated and is more consistent with recent numerical modeling studies this work adds to the growing recognition that sedimentary rocks melt during hypervelocity impact and do not just decompose and or devolatilize as was previously thought this has implications for understanding the processes and products of impacts into sedimentary rocks and for estimating the amount of climatically active gases released by impact events

a tungsten isotope approach to search for meteoritic components in terrestrial impact rocks abstract the identification of meteorite impact structures on earth is based on two main criteria the presence of shock metamorphic effects in the crater rock ejecta and or the confirmation of an extraterrestrial meteoritic component in breccias or melt rocks for the latter both high elemental abundances of siderophile elements especially the platinum group elements with corresponding meteoritic inter element ratios and the osmium os and chromium cr isotopic signatures characteristic of meteorites have been used successfully inspired by earlier suggestions of a meteoritic component in archean rocks based on tungsten w isotope anomalies here we explore the possible use of 182 w which has been produced by the decay of now extinct 182 hf t 1 2 8 9 ma as a tracer of meteoritic component in terrestrial material each group of meteorites has w isotopic compositions that are distinct from each other and from the terrestrial crust 182 w has already been used to try to identify the impactor at the k t boundary by analyzing the sediments and ni rich spinel in the present study we broaden the field of investigation by choosing a different approach namely analyzing a variety of known impact rocks we measured the w isotope composition in four samples from different impact structures gardnos norway morokweng south africa vredefort south africa ries germany as well as in two samples from a distal ejecta layer k t boundary samples from gams austria and berwind canyon usa all these samples are unambiguously impact produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies in all these samples the isotopic composition of w is identical with analytical error to that of the earth s continental crust and no 182 w anomalies are present even in the samples containing a significant percent level meteoritic component therefore we conclude that in contrast to the cr or os isotopes w isotopes are not suitable for the identification of meteoritic components in terrestrial rocks

constraints on central uplift structure from the manicouagan impact crater recent drilling operations at the 90 km diameter late triassic manicouagan impact crater of quebec canada have provided new insight into the internal structure of a complex craters central region previous work had indicated that the impact event generated a 55 km diameter sheet of molten rock of relatively consistent originally 400 m thickness floran et al 1978 the drilling data reveals melt sheet thicknesses of up to 1500 m with kilometer scale lateral and substantial vertical variations in the geometry of the crater floor beneath the melt sheet the thickest melt section occurs in a 1500 m deep central trough encircled by a horseshoe shaped uplift of precambrian basement the uplift constitutes a modified central peak structure at least part of which breached the melt sheet mineralogical and compositional segregation differentiation of the thicker melt sheet section coupled with a lack of fractionation in the thinner units shows that the footwall geometry and associated trough structure were in place prior to melt sheet solidification marked lateral changes in sub melt sheet basement relief support the existence of a castellated footwall that was created by high angle impact related offsets of 100s to 1000s of meters this indicates that deformation during the modification stage of the cratering process was primarily facilitated by large displacement fault systems this work suggests that manicouagan is a central peak basin with rings which does not appear to fit with current complex crater classification schemes

sedimentary mercury enrichments as a marker for submarine large igneous province volcanism evidence from the mid cenomanian event and oceanic anoxic event 2 late cretaceous oceanic anoxic event 2 oae 2 during the cenomanian turonian transition 94 ma was the largest perturbation of the global carbon cycle in the mid cretaceous and can be recognized by a positive carbon isotope excursion in sedimentary strata although oae 2 has been linked to large scale volcanism several large igneous provinces lips were active at this time e g caribbean high arctic madagascan ontong java and little clear evidence links oae 2 to a specific lip the mid cenomanian event mce 96 ma identified by a small 1 positive carbon isotope excursion is often referred to as a prelude to oae 2 however no underlying cause has yet been demonstrated and its relationship to oae 2 is poorly constrained here we report sedimentary mercury hg concentration data from four sites three from the southern margin of the western interior seaway and one from demerara rise in the equatorial proto north atlantic ocean we find that in both areas increases in mercury concentrations and hg toc ratios coincide with the mce and the oae 2 however the increases found in these sites are of a lower magnitude than those found in records of many other mesozoic events possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events combined the new mercury data presented here are consistent with an initial magmatic pulse at the time of the mce with a second greater pulse at the onset of oae 2 possibly related to the emplacement of lips in the pacific ocean and or the high arctic

siderophile rich particles in the melt rocks at the e clearwater impact structure quebec their characteristics and relationship to the impacting body the composition of the sampled melt rocks at the 22 km diameter e clearwater impact structure indicates the presence of 8 c 1 material the meteoritic component is fractionated with refractory siderophiles up to 30 times c 1 abundances concentrated in ten to hundred micron sized magnetic particles these particles consist of the ni sulphide millerite and what is assumed to be a mixture of refractory silicates and magnetite with grain sizes of 1 m the larger particles have a core rim structure with millerite and occasionally very minor galena and possibly pentlandite in the core an origin as a combination of altered meteoritic metal and condensed meteoritic silicate is favored for the origin of the siderophile rich particles if 8 meteoritic material is taken as the average meteoritic contamination in the melt then the e clearwater projectile may have impacted with a velocity of 17 km s 1 peak shock pressures would have been of the order of 300 gpa sufficient to vaporize the silicate component but only melt the metal component of the projectile as the meteoritic material was being driven down into vaporized melted target rocks during the initial stages of impact the melted fe ni metal underwent oxidation fe was removed and meteoritic silicate material recondensed on the cooler essentially ni metal as cavity excavation proceeded these ni metal silicate oxide particles were incorporated in the melt their refractory nature prevented thermal digestion and sulphur in the melt reacted with the metal to produce millerite on final equilibration if this hypothesis is correct it suggests that the e clearwater projectile was a c 2 or c 3 chondrite both of which are compatible with the trace element composition of the melt rocks clearwater lake is a twin impact structure formed by an asteroid pair it is still not clear however what type of projectile formed the 32 km diameter western structure where the surface melt rocks contain no identifiable meteoritic signature

the meteoritical bulletin no 95 the meteoritical bulletin no 95 reports 1093 282 non antarctic and 801 antarctic newly approved meteorite names and their recovery histories macroscopic descriptions petrography mineral compositions and geochemistry meteorites reported include lunar meteorites eucrites mesosiderites angrites ureilites an acapulcoite and h l ll r co cm ck and cv chondrites three new falls the bunburra rockhole australia eucrite and the recent nov 2008 buzzard coulee canada h4 chondrite and tamdakht morocco h5 chondrite are reported

channel networks a geomorphological perspective the study of channel networks has been dominated since 1966 by the random model however recent work has shown 1 that although the topological properties of small networks conform to the random model more closely than those of large ones even small networks exhibit systematic deviations from topological randomness and 2 that the topological and length properties of channel networks are controlled to a large degree by the spatial requirements of subbasins and the need for these subbasins to fit together in space by the size sinuosity and migration rate of valley bends and by the length and steepness of valley sides the factors that control the density properties of channel networks vary with the scale of the investigation and the geomorphic processes governing channel initiation although progress has been made toward a satisfactory stream junction angle model further work is needed the evolution of channel networks has been investigated by a variety of methods including the development of conceptual and simulation models the monitoring of small scale badland and experimental drainage basins and the substitution of space for time the morphology of most channel networks is largely inherited from the past or strongly influenced by inherited forms inasmuch as there is no way of ever knowing the origin or complex history of such networks the use of stochastic models in their study seems unavoidable

geochemical signatures and magmatic stability of terrestrial impact produced zircon abstract understanding the role of impacts on early earth has major implications to near surface conditions but the apparent lack of preserved terrestrial craters 2 ga does not allow a direct sampling of such events ion microprobe u pb ages ree abundances and ti in zircon thermometry for impact produced zircon are reported here these results from terrestrial impactites ranging in age from 35 ma to 2 ga are compared with the detrital hadean zircon population from western australia such comparisons may provide the only terrestrial constraints on the role of impacts during the hadean and early archean a time predicted to have a high bolide flux ti in zircon thermometry indicates an average of 773 c for impact produced zircon 100 c higher than the average for hadean zircon crystals the agreement between whole rock based zircon saturation temperatures for impactites and ti in zircon thermometry at a tio2 1 implies that ti in zircon thermometry record actual crystallization temperatures for impact melts zircon saturation modeling of archean crustal rock compositions undergoing thermal excursions associated with the late heavy bombardment predicts equally high zircon crystallization temperatures the lack of such thermal signatures in the hadean zircon record implies that impacts were not a dominant mechanism of producing the preserved hadean detrital zircon record

characterization of the rill geometry over straight hillslopes through spatial scales abstract experimental evidence is presented which shows that there exists a spatial scale over which ergodic assumptions are applicable for rilled hillslope surfaces the property of expected spatial rill density esrd is defined and this property is shown to stabilize after the averaging interval is more than 20 ft over the hillslope the rilled hillslope geometry may be readily quantified in terms of this easily measurable quantity the rill geometry has a significant influence on the surface distribution of overland flow and sediment transport an analysis of the spatial scales suggests that a continuum representation may be utilized in modeling these hydrodynamic flow phenomena for a complete characterization of the rilled hillslope geometry the distributions of the rill widths and rill depths at various locations along the hillslope have been presented these preliminary results show that the mean rill widths the mean rill depths and the esrd increase with increasing slope lengths the distributions of the rill widths and rill depths suggest that these quantities may be modeled by truncated gaussian or gamma distributions at different spatial locations the rill geometry properties may be related to hydrologic and geomorphologic properties of the hillslope it is suggested that the esrd is a better descriptor of the rill geometry on the hillslope than the rill density because of the wide variations exhibited by the distributions of the rill widths

the early cretaceous barents sea sill complex distribution 40ar 39ar geochronology and implications for carbon gas formation abstract mafic igneous rocks of cretaceous age 80 130 ma scattered around the arctic ocean are commonly referred to as the high arctic large igneous province halip we have mapped out the distribution of halip igneous rocks in the barents sea region over the past decade based on integrated seismic gravity magnetic interpretation field work review of publications and analyses of new and vintage borehole and field samples the mapping reveals abundant igneous rocks in the northern and eastern barents sea covering an area of 900 000 km2 with a conservative volume estimate of 100 000 to 200 000 km3 of intrusions the igneous province is dominated by sheet intrusions injected into triassic and permian sedimentary rocks hydrothermal vent complexes are rare and only two potential vent complexes have been identified on seismic data in the eastern barents sea we have further done extensive radiometric dating of the igneous samples in the barents sea region new 40ar 39ar dating of thirteen samples from svalbard reveal ages of crystallization and alteration the large age span 60 140 ma for the raw ages is likely due to partial or complete overprint of the k ar system in plagioclase and the age of the magma emplacement is better represented by u pb tims ages only one of our 40ar 39ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published u pb tims ages of 122 125 ma the new data clearly document that relying on published data from the k ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40ar 39ar degassing data i e k ca we propose that the magmatism on svalbard and franz josef land represents a distinct magmatic event near the barremian aptian boundary 125 ma in the barents sea this early cretaceous barents sea magmatism resulted in the formation of the bssc barents sea sill complex bssc age rocks are also present in arctic canada sverdrup basin and on bennett island new siberia islands the massive injection of hot magma into potentially organic rich sediments in the eastern and northern barents basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles we estimate that up to 20 000 gt of carbon were potentially mobilized corresponding to 175 trillion barrels of oil equivalent the production rates and fate of the carbon gases are uncertain however we speculate that rapid release of aureole greenhouse gases methane may have triggered the oceanic anoxic event 1a oae1a and the associated negative 13c excursion in the early aptian some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east barents basin

morokweng south africa a large impact structure of jurassic cretaceous boundary age geophysical petrographical and geochemical data indicate the presence of a large impact structure in the area around morokweng northwest province south africa possibly up to 340 km in diameter drill cores from the center of the structure show a thick layer of impact melt rocks with high abundances of cr ni co and the platinum group elements consistent with the presence of up to 5 of a chondritic component ion probe dating of zircons extracted from the impact melt yielded a 206 pb 238 u age of 146 2 1 5 ma and a 208 pb 232 th age of 144 7 1 9 ma these are indistinguishable from the age of the jurassic cretaceous boundary following the identification of the chicxulub impact structure of cretaceous tertiary boundary age the discovery of a second large impact structure at a previously established major chronostratigraphic boundary strengthens suggestions that large impact events have been major factors in the evolution of the earth

40ar 39ar thermochronology of the fossil ll6 chondrite from the morokweng crater south africa studies of meteorites are based mostly on samples that fell to earth in the recent past i e a few million years at most the morokweng ll chondrite meteorite is a particularly interesting specimen as its fall is much older ca 145 ma than most other meteorites and because it is the only macro meteorite clast width intersected in drill core 25 cm found in a melt sheet of a large impact structure when applied to the morokweng meteorite 40ar 39ar thermochronology provides an opportunity to study 1 effects associated with pre impact and post impact processes and 2 collision events within a potentially distinct and as yet unsampled asteroid population a single multi grain aliquot yielded an inverse isochron age of 625 163 ma this suggests a major in space collisional event at this time we have modeled the diffusion of 40ar within the meteorite and plagioclase during and after the 145 ma impact on earth to tentatively explain why pre terrestrial impact 40ar has been preserved within the plagioclase grains the 145 ma terrestrial impact age is recorded in the low retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 15 ma and thus confirms that age information about major terrestrial or extraterrestrial impacts can be recorded in the k rich mineral phases of a meteorite and measured by the 40ar 39ar technique more studies on fossil meteorites need to be carried out to understand if the rough 0 6 ga age proposed here corresponds to major ll chondrite asteroid population destructions or rather to an isolated collision event

morokweng impact structure northwest province south africa geophysical imaging and shock petrographic studies abstract the existence of a large near circular structure centred on the area around morokweng 23 32 e 26 31 s in the northwest province of south africa and possibly extending into southern botswana is inferred from aeromagnetic and gravity data results of surface geological studies support earlier suggestions that this feature could represent an impact structure samples from an autochthonous ironstone breccia exposure about 47 km to the west of the centre of this structure and several samples of allochthonous quartzite from south of the central part of the structure contain shock metamorphosed quartz with impact diagnostic planar deformation features pdfs the geophysical evidence is suggestive of an original size of this impact structure of at least 70 km but several annular anomalies outside of the study region could be interpreted as indicating a diameter of 300 340 km the age of this structure is only loosely constrained but is believed to be younger than the 2 25 2 5 ga transvaal supergroup

feldspathic clast populations in polymict ureilites stalking the missing basalts from the ureilite parent body abstract polymict ureilites dag 164 165 dag 319 dag 665 and eet 83309 are regolith breccias composed mainly of monomict ureilite like material but containing 2 vol of feldspathic components we characterized 171 feldspathic clasts in these meteorites in terms of texture mineralogy and mineral compositions based on this characterization we identified three populations of clasts each of which appears to represent a common igneous generally basaltic lithology and whose mafic minerals show a normal igneous fractionation trend of near constant fe mn ratio over a range of fe mg ratios that extend to much higher values than those in monomict ureilites the melts represented by these populations are unlikely to be impact melts because the ubiquitous presence of carbon in polymict ureilites the regolith of the ureilite parent body implies that impact melts would have crystallized under conditions of carbon redox control and therefore have highly magnesian mafic mineral compositions with constant mn mg ratio therefore these melts appear to be indigenous products of igneous differentiation on the ureilite parent body upb complementary to the olivine pigeonite residues represented by the majority of monomict ureilites the most abundant population is characterized by albitic plagioclase in association with pyroxenes phosphates ilmenite silica and incompatible element enriched glass model calculations suggest that it formed by extensive fractional crystallization of the earliest melt s of precursor materials from which the most magnesian shallowest olivine pigeonite ureilites formed a less abundant population characterized by labradoritic plagioclase may have formed from melts complementary to more ferroan olivine pigeonite ureilites and derived from deeper in the upb the third population characterized by the presence of olivine and augite could only have formed from melts produced at greater depths in the upb than the olivine pigeonite ureilites many other feldspathic clasts cannot be positively associated with any of these three populations because their mafic mineral compositions exhibit carbon redox control however they may be products of early crystallization of basaltic melts produced on the upb before carbon was exhausted by reduction partial melting on the ureilite parent body was a fractional or incremental process melts were produced early in upb history and most likely extracted rapidly thus preserving primitive chemical and oxygen isotopic signatures in the residues

geophysical anomalies and quartz deformation of the warburton west structure central australia abstract this paper reports geophysical anomalies and intra crystalline quartz lamellae in drill cores from the warburton west basin overlapping the border of south australia and the northern territory the pre upper carboniferous 450 300 km large warburton basin north eastern south australia is marked by distinct eastern and western magnetic gravity and low velocity seismic tomography anomalies quartz grains from arenite core samples contain intra crystalline lamellae in carbonate quartz veins and in clastic grains similar to those reported earlier from arenites volcanic rocks and granites from the warburton east basin universal stage measurements of quartz lamellae in both sub basins define miller bravais indices of 10 12 and 10 13 in situ quartz lamellae occur only in pre late carboniferous rocks whereas lamellae bearing clastic quartz grains occur in both pre late carboniferous and post late carboniferous rocks the latter likely redeposited from the pre late carboniferous basement quartz lamellae in clastic quartz grains are mostly curved and bent either due to tectonic deformation or to re deformation of impact generated planar features during crustal rebound or and post impact tectonic deformation seismic tomography low velocity anomalies in both warburton west basin and warburton east basin suggest fracturing of the crust to depths of more than 20 km geophysical modelling of the cooper basin which overlies the eastern warburton east basin suggests existence of a body of high density 2 9 3 0 gr cm 3 and high magnetic susceptibility si 0 012 0 037 at a depth of 6 10 km at the centre of the anomalies in the warburton west basin a large magnetic body of si 0 030 is modelled below 10 km with a large positive gravity anomaly offset to the north of the magnetic anomaly in both the warburton east and warburton west the deep crustal fracturing suggested by the low velocity seismic tomography complicates interpretations of the gravity data universal stage measurements of quartz lamellae suggest presence of both planar deformation features of shock metamorphic derivation and deformed planar lamella the latter may be attributed either to re deformation of impact generated lamella impact rebound deformation or and post impact tectonic deformation the magnetic anomalies in the warburton east and west sub basins are interpreted in terms of 1 presence of deep seated central mafic bodies 2 deep crustal fracturing and 3 removal of devonian and carboniferous strata associated with rebound of a central uplift consequent on large asteroid impact further tests of the warburton structures require deep crustal seismic transects

igneous rocks formed by hypervelocity impact abstract igneous rocks are the primary building blocks of planetary crusts most igneous rocks originate via decompression melting and or wet melting of protolith lithologies within planetary interiors and their classification and compositional petrographic and textural characteristics are well studied as our exploration of the solar system continues so too does the inventory of intrusive and extrusive igneous rocks settings and processes the results of planetary exploration have also clearly demonstrated that impact cratering is a ubiquitous geological process that has affected and will continue to affect all planetary objects with a solid surface whether that be rock or ice it is now recognized that the production of igneous rocks is a fundamental outcome of hypervelocity impact the goal of this review is to provide an up to date synthesis of our knowledge and understanding of igneous rocks formed by hypervelocity impact following a brief overview of the basics of the impact process we describe how and why melts are generated during impact events and how impact melting differs from endogenic igneous processes while the process may differ we show that the products of hypervelocity impact can share close similarities with volcanic and shallow intrusive igneous rocks of endogenic origin such impact melt rocks as they are termed can display lobate margins and cooling cracks columnar joints and at the hand specimen and microscopic scale such rocks can display mineral textures that are typical of volcanic rocks such as quench crystallites ophitic porphyritic as well as features such as vesicles flow textures and so on historically these similarities led to the misidentification of some igneous rocks now known to be impact melt rocks as being of endogenic origin this raises the question as to how to distinguish between an impact versus an endogenic origin for igneous like rocks on other planetary bodies where fieldwork and sample analysis may not be possible and all that may be available is remote sensing data while the interpretation of some impact melt rocks may be relatively straightforward e g for clast rich varieties and those with clear projectile contamination we conclude that distinguishing between impact and endogenic igneous rocks is a non trivial task that ultimately may require sample investigation and analysis to be conducted caution is therefore urged in the interpretation of igneous rocks on planetary surfaces

late jurassic age for the morokweng impact structure southern africa abstract a roughly 70 km diameter circular feature buried beneath the kalahari sands in south africa is revealed on regional aeromagnetic maps boreholes drilled into the centre of the structure intercept a 250 m thick sheet of quartz norite interpreted as an impact melt which overlies brecciated and shock metamorphosed basement granite zircons recovered from the quartz norite yield u pb ages of 145 0 8 ma and biotites provide ar ar ages of 144 4 ma these data provide strong evidence for the occurrence of a late jurassic impact crater the morokweng impact structure 100 m beneath the surface

fe rich and k rich mafic spherules from slumped and channelized chicxulub ejecta deposits in the northern la sierrita area ne mexico spherule deposits commonly interpreted as ejecta from the chicxulub impact at yucatan mexico are present in many k t cretaceous tertiary sections geological mapping of the northern la sierrita area ne mexico revealed the presence of 1 multiple spherule deposits embedded in late maastrichtian marls which are folded or disaggregated breccia like they are up to 6 m thick locally present in two outcrop areas and show limited lateral continuity these deposits consist of mm cm sized spherical to drop shaped vesiculated spherules angular to filamentous ejecta fragments and abundant carbonate they are interpreted as primary ejecta fallout deposits that have been affected by subsequent local slumps slides liquefaction and debris flows welded components suggest an initial ground surge like ejecta dispersion mode 2 a spherule deposit 10 60 cm thick that constitutes the base of a channelized sand siltstone deposit at or close to the k t boundary and is characterized by wide lateral continuity it is of similar petrologic composition to deposit 1 though slightly enriched in terrigeneous detritus thus reflecting influx from proximal shelf areas it is interpreted to result from debris flows and turbidite currents though no size sorting and abrasion of ejecta has been observed petrological mineralogical and geochemical criteria suggest that ejecta components from both types of spherule deposits are similar and originated from the chicxulub impact with multiple deposits produced by subsequent remolding reworking and redeposition spherules and fragments have an fe 25 30 wt al mg rich and si poor 25 wt sio2 composition and are altered to chlorite and iron oxides though rare k rich mafic glass 50 wt sio2 5 8 wt k is also present they contain ti fe k rich schlieren fe mg rich globules and rare m sized metallic and sulfidic ni co rich inclusions carbonate as clasts and within spherules and fragments shows textures indicative of quenching and or liquid immiscibility although potential ejecta fractionation and alteration make accurate evaluation difficult this composition suggests an ejecta origin mainly from mafic lithologies and carbonaceous sediments in addition to a contribution from intermediate felsic rocks and the possibility of meteoritic contamination

platinum group elements in the morokweng impact structure south africa evidence for the impact of a large ordinary chondrite projectile at the jurassic cretaceous boundary radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in earth history e g the cretaceous tertiary k t and jurassic cretaceous j k boundaries the morokweng impact crater in south africa is coincident in age with the j k boundary however the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks meteorites are highly enriched in the platinum group elements pge which have been widely used as a tool for identifying the presence of a meteorite signature here we present new pge analyses from the morokweng impact melt sheet our data reveal high pge concentrations and high degree of pge correlation through the melt sheet regression analysis was used to determine the projectile pge signature and constrain input from the terrestrial target rocks the closest match to morokweng is the pge signature of ordinary l or ll chondrite meteorites which is broadly in agreement with the results of an earlier cr isotope study the results of these independent studies provide strong evidence that a large ordinary chondrite projectile struck the area of morokweng in the late jurassic

origin of limestone marlstone cycles astronomic forcing of organic rich sedimentary rocks from the cenomanian to early coniacian of the cretaceous western interior seaway usa abstract we present an integrated multidisciplinary study of limestone marlstone couplets from a continuously cored section including parts of the upper buda limestone the entire eagle ford group boquillas formation and lower austin chalk from the shell iona 1 research borehole texas usa which provides a 8 million year myr distal clastic sediment starved intrashelf basin record of the early cenomanian to the earliest coniacian stages results show that despite variable yet minimal diagenetic overprints several unambiguous primary environmental signals are preserved and support greater water mass ventilation and current activity promoting increased silica carbonate productivity during the deposition of limestone beds compared to deposition of marlstone beds which reflect greater organic matter productivity and preservation furthermore our astronomical analyses demonstrate that the limestone marlstone couplets in the iona 1 core reflect climatic forcing driven by solar insolation resulting from integrated milankovitch periodicities in particular we propose that obliquity and precession forcing on the latitudinal distribution of solar insolation may have been responsible for the observed lithological and environmental variations through the cenomanian turonian and coniacian in this mid latitude epicontinental sea setting our data also suggests that rhythmic lithological alternations deposited in greenhouse periods in general may simply reflect climate driven cycles related to earth sun dynamics without the need to invoke significant sea level variations

drainage network structure and the diameter magnitude relation drainage network structure can be described in terms of elongation or compaction by using the network diameter and magnitude the relation between network diameter and magnitude is the topologic equivalent of the well known relationship between mainstream length and drainage basin area diameter increases with magnitude but at a decreasing rate as networks are sampled over a greater size range downstream variations in river network structure reflect regional geomorphic environment erosional history and space filling by tributaries of different sizes the sequence of tributary magnitudes down a mainstream is not random large tributaries preempt drainage area and tend to be followed downstream by a number of small tributaries in fluvial landscapes free from geological constraints river networks tend to become relatively more elongate in structure with increasing size downstream

discovery of a 25 cm asteroid clast in the giant morokweng impact crater south africa meteorites provide a sample of solar system bodies and so constrain the types of objects that have collided with earth over time meteorites analysed to date however are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time large objects are widely believed to be completely melted or vaporized during high angle impact with the earth consequently identification of large impactors relies on indirect chemical tracers notably the platinum group elements here we report the discovery of a large 25 cm unaltered fossil meteorite and several smaller fragments within the impact melt of the giant 70 km diameter 145 myr old morokweng crater south africa the large fragment clast resembles an ll6 chondrite breccia but contains anomalously iron rich silicates fe ni sulphides and no troilite or metal it has chondritic chromium isotope ratios and identical platinum group element ratios to the bulk impact melt these features allow the unambiguous characterization of an impactor at a large crater furthermore the unusual composition of the meteorite suggests that the morokweng asteroid incorporated part of the ll chondrite parent body not represented by objects at present reaching the earth

which tributaries disrupt downstream fining along gravel bed rivers tributaries and other lateral sediment sources can have a significant impact on river bed sediment texture and in turn on channel form sufficiently voluminous or distinct sediment inputs redefine the mainstem grain size distribution punctuating downstream maturation and isolating a sequence of discrete sedimentary links within these links fining processes usually dominate such that models of sorting and abrasion when applied to individual links provide reasonable predictions of grain size change links represent the fundamental natural unit within which fining models can be tested developed and applied identification of significant lateral sources is therefore important yet beyond vague references to relative tributary size sediment load and sediment calibre no criteria exist for the a priori discrimination of such sources in this paper a procedure for identifying significant lateral tributary sources without the benefit of grain size information is outlined a high resolution characterisation of bed material texture along two canadian gravel bed rivers facilitated classification of all their perennial tributaries as either significant or insignificant three absolute tributary basin parameters and their relative counterparts chosen to reflect the likely controls on tributary significance are then used to develop a discriminant function which isolates a large proportion of significant tributaries while minimising incorrect classifications examination of consistently misclassified anomalous tributaries reveals the importance of lateral source spacing and of inconsistencies in the geomorphic history of the contributing basins in turn a general tributary categorisation procedure is suggested which includes a logistic regression model for attaching probability statements to individual classifications the generality of the discriminant and logistic functions cannot be assessed because of the lack of other suitable data sets

midlatitude shelf seas in the cenomanian turonian greenhouse world temperature evolution and north atlantic circulation 1 an 8 million year record of subtropical and midlatitude shelf sea temperatures derived from oxygen isotopes of well preserved brachiopods from a variety of european sections demonstrates a long term cenomanian temperature rise 16 20 c midlatitudes that reached its maximum early in the late turonian 23 c midlatitudes superimposed on the long term trend shelf sea temperatures vary at shorter timescales in relation to global carbon cycle perturbations in the mid cenomanian and the late turonian two minor shelf sea cooling events 2 3 c coincide with carbon cycle perturbations and times of high amplitude sea level falls although this evidence supports the hypothesis of potential glacioeustatic effects on cretaceous sea level the occurrence of minimum shelf sea temperatures within transgressive beds argues for regional changes in shelf sea circulation as the most plausible mechanism the major carbon cycle event in the latest cenomanian oceanic anoxic event 2 is accompanied by a substantial increase in shelf sea temperatures 4 5 c that occurred 150 kyr after the commencement of the 13c excursion and is related to the spread of oceanic conditions in western european shelf sea basins our oxygen isotope record and published 18o data of pristinely preserved foraminifera allow the consideration of north atlantic surface water properties in the cenomanian along a transect from the tropics to the midlatitudes on the basis of fossil derived 18o estimated w ranges and modeled salinities temperature salinity density ranges were estimated for tropical subtropical and midlatitude surface waters accordingly the cenomanian temperate shelf seas waters have potentially the highest surface water density and could have contributed to north atlantic intermediate to deep waters in the preopening stage of the equatorial atlantic gateway

siderophile rich inclusions from the morokweng impact melt sheet south africa possible fragments of a chondritic meteorite an 870 m thick diameter 30 km melt sheet associated with the morokweng impact structure south africa contains cr rich silicate inclusions up to 30 mm in diameter as well as disseminated ni rich sulphides and oxides that commonly occur in mutual intergrowths although the silicate inclusions are highly altered the mineral chemistry and petrographic evidence along with broadly chondritic re os systematics and platinum group element pge signatures all support the view that the silicate inclusions represent relict fragments of an impactor with compositions similar to ordinary chondrites the ni rich sulphides and oxides are significantly enriched up to 35 chondrites in pge ni and cr although the bulk pge signature of the melt sheet is chondritic the sulphides and oxides display mm and sub mm scale fractionation of pge into different inclusion components a model is presented whereby pieces of the projectile react with the relatively more oxidising impact melt into which they fell resulting in oxidation and fractionation of the metal silicate fragments to form the inclusion assemblages this may be the first reported occurrence of a large melt sheet 300 km3 contaminated by swarms of discrete pebble sized fragments of asteroid material the high proportion of meteorite fragments in the morokweng melt sheet suggests either that the structure was formed by a relatively low velocity 18 km s impact event or that the angle of impact was optimum for retaining maximum amounts of meteoritic material within the crater

watershed morphometry and time to hydrograph peak abstract morphometric complexities often render geomorphic parameters ineffective in maintaining stable relationships over space with various aspects of hydrograph shape drainage density for example may relate either directly or indirectly with time to hydrograph peak depending upon environmental characteristics variations in climate geology and degree of geomorphic development in large measure stimulate morphometric complexities the suggestion is made in this research that the hypsometric curve an expression of overall basin slope be combined with basin area and or relief to promote transferable hydrologic models three aspects of hypsometric curve form are compared to time to peak following intense storms in small agricultural experimental watersheds in west virginia oklahoma and vermont a significant multiple regression correlation relationship is forthcoming between time to peak and hypsometric form with no direct control for basin area relief relief ratio or drainage density hypsometric skewness is related to headward drainage development mid basin slope is assessed through hypsometric density kurtosis and the rate of change in upper and lower basin slope is quantified by hypsometric density skewness while it is not suggested that hypsometry preclude the use of other important watershed parameters like drainage area it is proposed that hypsometric techniques might be employed along with basin area and or relief to increase the predictive power and transferability of rainfall runoff models

clearwater east impact structure a re interpretation of the projectile type using new platinum group element data from meteorites platinum group element pge concentrations and ratios obtained from samples of the clearwater east impact melt have been used along with other siderophile element ratios to classify the impacting projectile as a carbonaceous chondrite this is at odds with recent chromium isotope analyses that suggest ordinary chondrite type material is present the present study reviews and reinterprets the available pge data in the light of new pge data from meteorites and concludes that the pge ratios in the impact melt are most consistent with ordinary possibly type l chondrite source material not carbonaceous chondrites therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites

dating terrestrial impact structures u pb depth profiles and u th he ages of zircon we investigate the presence of epitaxial overgrowth rims and reset zircon complete loss of radiogenic lead pb from terrestrial impactites to constrain the occurrence of such phenomenon in impact environments and their possible use in dating impact events we also explore u th he dating of zircon to evaluate this geochronometer in accurately identifying impact ages particularly when no dateable melt sheet exists our results show that u th he ages of zircon from the brecciated and presumably shocked target can accurately date an impact event and provides another tool to determine impact ages when no melt sheet exists an alternative to problematic interpretations of commonly used apparent 40ar 39ar plateau ages no evidence of epitaxial overgrowth rims and or reset zircon was observed suggesting that zircons within shocked impactites have remarkably slow pb diffusion and possibly explaining the relatively few reset grains reported in terrestrial impactites