clustered impacts experiments and implications impact by clusters of projectiles rather than a single projectile can result from several processes atmospheric breakup tidal breakup and ejecta from a large primary impact experiments have been performed in order to establish the characteristics of such events over a wide range of impact velocities 15 m s to 6 km s at very low impact velocities 15 200 m s clustered impacts were produced by launching a grouped projectiles of aluminum shot steel shot iron filings and sand at moderate to high velocities 0 8 6 km s pyrex spheres were shattered above the target during passage through aluminum foil or paper thereby forming a well defined cluster of fragments of overall radius rc the ratio of the overall radius rc of the cluster to the radius rs of a solid impactor of the same mass provides a measure of the cluster dispersion sand and compacted pumice targets were used in order to compare qualitatively the additional effect of slight differences in target strength the experiments reveal marked contrasts between impacts by clusters of projectiles and impacts by a single solid body tight clusters rc rs 20 displace a factor of 10 less mass this reduction in cratering efficiency is largely expressed as a shallow crater with an aspect ratio diameter depth as large as 30 for open clusters the size and velocity of the cluster as well as the density and strength contrast between the impactor and target can dramatically affect crater morphology open clusters impacting compacted pumice produce a flat hummocky floor with an incipient multiring pattern whereas a tight cluster impacting the same target produces a central floor mound oblique impacts by symmetrical clusters form a characteristic array of v shaped ridges pointing uprange the apex angle of the ridges depends on the cluster dispersion and impact angle inventories of postimpact projectile material reveal that the projectile is largely retained on the surface and spread downrange from the impact direction the amount of projectile material retained on the surfaces increases with decreasing impact angle from the horizontal and with increasing strength of the target relative to the projectile clustered impact craters and lunar secondary craters bear striking similarities over a broad range of morphologic features on this basis and on the basis of reasonable models of ejecta curtain structure we suggest that these experiments provide new clues for understanding ejecta emplacement around large lunar impact craters when the ejecta curtain around large impacts is viewed as a thick wall of debris and clustered impactors are viewed as a unit section of such a curtain then experimental results indicate that the continuous ejecta facies even for lunar craters larger than 100 km could contain as much as 90 primary material such a conclusion contrasts with many existing models that derive mixing ratios implicitly based on single noninteracting impact events beyond the continuous ejecta facies impacting clusters of ejecta provide a physical basis for understanding the wide variety of secondary morphologies and the large range in spectral signatures of primary material in crater rays

the toms canyon structure new jersey outer continental shelf a possible late eocene impact crater abstract the toms canyon structure 20 22 km wide is located on the new jersey outer continental shelf beneath 80 100 m of water and is buried by 1 km of upper eocene to holocene sedimentary strata the structure displays several characteristics typical of terrestrial impact craters flat floor upraised faulted rim brecciated sedimentary fill but several other characteristics are atypical an unusually thin ejecta blanket lack of an inner basin peak ring or central peak being nearly completely filled with breccia seismostratigraphic and biostratigraphic analyses show that the structure formed during planktonic foraminiferal biochron p15 of the early to middle late eocene the fill unit is stratigraphically correlative with impact ejecta cored nearby at deep sea drilling project dsdp site 612 and at ocean drilling program odp sites 903 and 904 22 35 km southeast of the toms canyon structure the toms canyon fill unit also correlates with the exmore breccia which fills the much larger chesapeake bay impact crater 90 km diameter 335 km to the southwest on the basis of our analyses we postulate that the toms canyon structure is an impact crater formed when a cluster of relatively small meteorites approached the target site bearing n 50 e and struck the sea floor obliquely

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

quaternary ichnofacies model for paleoenvironmental and paleosealevel interpretations a study from the banas river basin western india abstract integration of data from ichnological and other interdisciplinary sources from the quaternary banas river basin india has permitted the reconstruction of a composite ichnofacies model for the interpretation of paleoenvironments and paleosealevels a glacioeustatic transgressive regressive system tract of the ancient arabian sea has been recognized from the superposition of ichnofacies the pleistocene shallow marine psilonichnus ichnofacies ichnogenus psilonichnus skolithos arenicolites and thalassinoides of infaunal polychaete crab and bivalve origin developed in the vadpag vend segment of the lower banas sequence corresponds to interglacial high sea stands the overlying late pleistocene holocene continental coprinisphaera scoyenia mixed ichnofacies paleosol association of delicate surficial traces of terrestrial insects and river floodplain association of aquatic traces of worms and crustaceans formed in the radhanpur varahi segment of the upper banas sequence represents the last glacial regressive mode of sealevel levee bound oriented crustacean burrows may be used as new ichnological tools to locate buried paleochannels and aquifers the juxtaposition of levees with alternate worm burrow and barren zones and floodplains with gastropod and barren zones indicates monsoonal lows and highs in the holocene the overprinting of the coprinisphaera elements on the scoyenia forms has resulted from the recent sub recent environmental degradations rising temperature spread of desert shrinking saltmarshes and weakened fluvial forces probably related to global warming and other greenhouse processes the readable ichnological signatures of the quaternary glacioeustatic sealevel fluctuations and consequent climatic changes qualify the banas model for wider application

cometary impacts into ocean their recognition and the threshold constraint for biological extinctions the montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied the impact crater is 45 km in diameter and was formed at 50 8 ma by a fall of probably an old cometary nucleus 3 4 km in diameter into shallow 600 m ocean comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim instead the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent return water flow into the excavating cavity by this process most of the fall out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land at a microscopic scale the shock metamorphism features are about the same as those for land impacts geochemically impacts of comet nucleii may not leave a recognizable signature at the impact horizon except for a minor increase in iridium thus stratigraphic horizons associated with extinctions and or major changes in biota have to be closely examined for other impact indicators like the presence of tectites glass spherules and quartz grains with shock features occurrence of megatsunami wave deposits extensive erosion on continental margins margin failures and faunal mixing above erosional unconformities are other potential impact indicators there is no single indicator that can provide sufficient proof of an impact event such interpretations have to be based on multiparameter studies of global extent since many of the impact indicators are only of regional extent the lack of extinction of any marine plankton genera or of bottom dwellers at the montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus 4 km in diameter the calculated frequency for a cometary impact which could result in a 10 extinction of marine genera is about 6 10 7 yr 1 and for the k t boundary type of extinctions about 2 10 8 yr 1 even allowing for a large degree of uncertainty in these estimates it is unlikely that the biological extinction events for the last 250 ma identified by sepkoski 1990 could have been all caused by meteorite impacts

large impact crater in the barents sea seismic profiles in the central barents sea record a conspicuous structure exhibiting gross morphological and structural characteristics closely resembling those of large impact craters this feature named the mjolnir structure is 39 km in diameter and of jurassic early cretaceous age it probably formed by the impact of an asteroid or comet 0 7 2 5 km in diameter into a shallow sea underlain by 5 km of sedimentary section if the impact interpretation is correct mjolnir is one of the largest well preserved craters in the errestrial record

doomed pioneers gravity flow deposition and bioturbation in marine oxygen deficient environments isolated horizons of thalassinoides and gyrolithes burrows appear in exclusive association with gravity flow deposits within sequences of nonbioturbated hernipelagic sedimentary rocks of the miocene monterey formation of california and the oligocene miocene san gregorio formation of baja california these burrowed levels are not associated with other ichnogenera such as zoophycos and chondrites we infer a causal relation between gravity flow deposition and the presence of thalassinoides and gyrolithes and suggest that these gravity flows entrained thalassinidean crustacea upon deposition in oxygen deficient environments the surviving borrowers reworked substantial quantities of aminated commonly organic rich sediments in an environment from which they were previously excluded the persistence of or the ecologically rapid return to oxygen depleted conditions limited the survival time and ecological complexity of the transported infaunal dwellers and rendered them doomed pioneers ecological and physiological data support this hypothesis thalassinidean crustacea have the capability to endure turbulent transport and survive up to 5 7 days of anoxia without being severely limited in their biological activities the accurate recognition of doomed pioneer trace fossil assemblages as ephemeral ecological phenomena in otherwise laminated successions may contribute to a better understanding and interpretation of paleo oxygen levels and basin history

delayed climate cooling in the late eocene caused by multiple impacts high resolution geochemical studies at massignano italy abstract high resolution studies 13c 18o and elemental abundances were done in rocks at and below the gssp for the eocene oligocene e o boundary at massignano italy in addition to an earlier known ir anomaly at 5 61 m which is possibly linked to the popigai impact event we confirm the presence of two additional ir anomalies in the intervals from 6 00 to 6 40 m and from 10 00 to 10 50 m with maximum values of 259 32 ppt at 6 17 m and 149 24 ppt at 10 28 m respectively the lower ir anomaly might be derived from the chesapeake bay impact event whereas for the other one no impact event is known similar 13c and 18o trends related to the two ir anomalies indicate that the ir anomaly at 10 28 m might be also derived from an impact into a continental shelf similar to the chesapeake bay impact event 18o values decrease in the high ir layers to 1 16 and 1 17 respectively which together with the negative shifts in 13c in the ir rich levels indicate a warm pulse superimposed on a general late eocene cooling trend that is characterized by 18o values ranging between 0 6 and 0 4 the release of methane hydrate after an impact in a continental shelf or seafloor or impacts of 12c rich comets during a 2 2 million year long comet shower respectively could produce these more negative carbon and oxygen excursions compared to the continuously decreasing trend over the whole late eocene massignano section

ocean drilling project hole 689b spherules and upper eocene microtektite and clinopyroxene bearing spherule strewn fields montanari et al 1993 reported a positive ir anomaly in the upper eocene sediments from ocean drilling program hole 689b on the maud rise southern ocean vonhof 1998 described microtektites and clinopyroxene bearing cpx spherules associated with the ir anomaly in hole 689b and suggested that they belong to the north american and equatorial pacific cpx strewn fields respectively we searched a suite of 27 samples taken through the spherule layer from hole 689b and we recovered 386 microtektites and 667 cpx spherules we studied the petrography of the microtektites and cpx spherules and determined the major element compositions of 31 microtektites and 14 cpx spherules using energy dispersive x ray analysis we also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis we found that the peak abundance of cpx spherules is 2 cm below the peak abundance of the microtektites 128 7 m below sea floor which suggests that the cpx spherule layer may be slightly older 3 5 ka the microtektites are mostly spherical and are generally transparent and colorless they are similar to the north american microtektites in composition the biggest differences being their generally lower na2o and generally higher zr ba and ir up to 0 3 ppb contents we agree with vonhof 1998 that the hole 689b microtektites probably belong to the north american tektite strewn field we calculate that the number of microtektites 125 m cm2 at hole 689b is 52 this number is close to the concentration predicted by extrapolation of the trend of concentration vs distance from the chesapeake bay structure based on data from other north american microtektite bearing sites thus the north american strewn field may be at least four times larger than previously mapped the hole 689b cpx spherules range from translucent yellow to opaque black but most are opaque tan to dark brown they are generally spherical in shape and all are 125 m in diameter some contain ni rich spinels in addition to clinopyroxene microlites the cpx spherules are petrographically and compositionally similar to cpx spherules previously found in the northwestern atlantic ocean caribbean sea gulf of mexico equatorial pacific and eastern indian ocean the abundance and widespread geographic occurrence of these spherules suggest that the strewn field may be global in geographic extent assuming a global extent we estimate that there may be at least 25 billion metric tons of cpx spherules in the strewn field based on age size and geographic location we speculate that the 100 km diameter popigai crater in northern siberia may be the source of the cpx spherule layer

structural outer rim of chesapeake bay impact crater seismic and bore hole evidence nine seismic reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the chesapeake bay impact crater the rim is manifested as a 90 km diameter ring of terraced normal fault blocks which forms a 320 m 1200 m high rim escarpment the top of the rim escarpment is covered by a 20 m 30 m thick ejecta blanket the escarpment encircles a flat floored annular trough which is partly filled with an 250 m thick breccia lens exmore breccia the exmore breccia overlies a 200 m 800 m thick interval of slumped sedimentary megablocks which in turn rests on crystalline basement rocks all postimpact strata upper eocene to quaternary sag structurally into the annular trough and most units also thicken as they cross the rim into the crater postimpact compaction and subsidence of the exmore breccia have created extensive normal faulting in overlying strata

age measurement of the submarine montagnais impact crater seven laser generated 40ar 39ar age spectra of melt rocks from the newly described submarine montagnais crater are presented while the age spectra are not completely straight forward to interpret the evidence is very strong that the impact occurred approximately 51 m y ago thus the montagnais event is associated with neither the k t nor the north american tektite events such an age is also totally out of phase with the 26 32 m y cycles proposed recently for the meteorite impact and extinction records as it falls roughly midway between two of the most precisely dated points in such proposed cycles the k t boundary and the north american tektite event

ancient impact structures on modern continental shelves the chesapeake bay montagnais and toms canyon craters atlantic margin of north america abstract three ancient impact craters chesapeake bay 35 7 ma toms canyon 35 7 ma montagnais 51 ma and one multiring impact basin chicxulub 65 ma are currently known to be buried beneath modern continental shelves all occur on the passive atlantic margin of north america in regions extensively explored by seismic reflection surveys in the search for oil and gas reserves we limit our discussion herein to the three youngest structures these craters were created by submarine impacts which produced many structural and morphological features similar in construction composition and variability to those documented in well preserved subaerial and planetary impact craters the subcircular chesapeake bay diameter 85 km and ovate montagnais diameter 45 50 km structures display outer rim scarps annular troughs peak rings inner basins and central peaks similar to those incorporated in the widely cited conceptual model of complex impact craters these craters differ in several respects from the model however for example the montagnais crater lacks a raised lip on the outer rim the chesapeake bay crater displays only small remnants of a raised lip and both craters contain an unusually thick body of impact breccia the subtriangular toms canyon crater diameter 20 22 km on the other hand contains none of the internal features of a complex crater nor is it typical of a simple crater it displays a prominent raised lip on the outer rim but the lip is present only on the western side of the crater in addition each of these craters contains some distinct features which are not present in one or both of the others for example the central peak at montagnais rises well above the elevation of the outer rim whereas at chesapeake bay the outer rim is higher than the central peak the floor of the toms canyon crater is marked by parallel deep troughs and linear ridges formed of sedimentary rocks whereas at chesapeake bay the crater floor contains concentric faults and compression ridges formed in rocks of the crystalline basement the chesapeake bay crater is distinguished further by its cluster of at least 23 adjacent secondary craters the north american tektite strewn field a widespread deposit of distal ejecta is thought to be derived from the chesapeake bay impact perhaps with a small contribution from the toms canyon impact no ejecta field is known to be associated with the montagnais impact no immediate major extinction event is directly linked to any of these three impacts there is evidence however that the chesapeake bay and toms canyon impacts helped initiate a long term pulse of warm global climate whose eventual dissipation coincided with an early oligocene mass extinction event 2 ma after the impacts

trace fossils from marginal lacustrine deposits of the cretaceous jinju formation southern coast of korea abstract trace fossils from the cretaceous jinju formation of the south coast of korea occur in interbedded medium to fine grained sandstone of a moderate energy lake shoreface and mudstone shales of a periodically exposed low energy shallow lake and interfingerings of both of the above the trace fossils are represented by beaconites antarcticus beaconites coronus cochlichnus anguineus diplichnites octopodichnus cf didactylus diplichnites type a diplichnites isp helminthopsis hieroglyphica palaeophycus sulcatus palaeophycus tubularis planolites annularis planolites beverleyensis protovirgularia dichotoma skolithos magnus taenidium barretti and sauropod tracks the diplichnites specimens are among the first arthropod trackways recorded from the cretaceous two of the four trackway bearing ichnocoenoses present in the study area are considered representative of composite ichnofacies sinusoidal burrows or trails and simple irregular interface burrows or trails probably formed in wet possibly submerged soupground softground conditions in low or medium energy lake margins and represent the mermia ichnofacies trackways and meniscate burrows probably formed on the subsequently subaerially exposed lake margins following lake regression and represent an overprinting by the scoyenia ichnofacies the other two trackway bearing ichnocoenoses are from the scoyenia ichnofacies alone

an earthquake study in the lake baringo basin of the central kenya rift a 100 80 km2 earthquake recording network was operated for three months january march 1990 in the lake baringo region of the kenya rift valley twenty nine seismic sites were occupied by short period stations over a region including the elgeyo escarpment the kerio valley the tugen hills and lake baringo itself eighty local events of ml 2 0 have been located within 50 km of the network these events are situated within the central part of the rift showing some association with the main rift faults but mainly clustering beneath lake baringo at a depth of about 5 km and occurring as swarm activity ninety percent of the events occurred at depths shallower than 12 km the brittle ductile transition zone in this area is determined at a depth of 12 16 km similar to that in the lake bogoria area immediately to the south a preliminary study of focal mechanisms of suitable events indicates wnw ese extension across the lake baringo basin and suggests the presence of sub vertical rupture surfaces beneath the lake these may be caused by the insertion of basic dykes into the upper crust and the rupture process assisted by high temperature and pressure geothermal fluids the inversion of local p wave arrival time data for the upper crustal velocity structure beneath lake baringo identifies a low velocity zone beneath surface geothermal activity a relationship previously demonstrated for the lake bogoria region

when a cosmic impact strikes the sea bed most bolides that collide with the earth hit the sea limited knowledge about marine target impacts hampers predictions about their perilousness this study presents geological features that are particular to craters formed at sea the features are most likely a result of the influence of the target on the cratering process marine target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed when the crater diameter is large compared to the water depth the crater resembles its counterparts that are formed on land craters formed in deeper water are concentric and often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater

new evidence concerning the age and biotic effects of the chicxulub impact in ne mexico abstract in the 1990s the chicxulub impact was linked to the k t boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact generated tsunami deposit since that time a preponderance of evidence has failed to support this interpretation revealing long term deposition of the sandstone complex the k t boundary above it and the primary impact spherule ejecta interbedded in late maastrichtian marls below based on evidence from mexico and texas we suggested that the chicxulub impact predates the k t boundary impact tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in mexico represents slumps and widespread tectonic disturbance although no such evidence has been presented the decades old controversy over the cause of the k t mass extinction will never achieve consensus but careful documentation of results that are reproducible and verifiable will uncover what really happened at the end of the crectaceous this study takes an important step in that direction by showing 1 that the stratigraphically older spherule layer from el penon ne mexico represents the primary chicxulub impact spherule ejecta in tectonically undisturbed sediments and 2 that this impact caused no species extinctions

end cretaceous cooling and mass extinction driven by a dark cloud encounter abstract we have identified iridium in an 5 m thick section of pelagic sediment cored in the deep sea floor at site 886c in addition to a distinct spike in iridium at the k pg boundary related to the chicxulub asteroid impact we distinguish the contribution of the extraterrestrial matter in the sediments from those of the terrestrial matter through a co ir diagram calling it the extraterrestrial index f ex this new index reveals a broad iridium anomaly around the chicxulub spike any mixtures of materials on the surface of the earth cannot explain the broad iridium component on the other hand we find that an encounter of the solar system with a giant molecular cloud can aptly explain the component especially if the molecular cloud has a size of 100 pc and the central density of over 2000 protons cm 3 kataoka et al 2013 2014 pointed that an encounter with a dark cloud would drive an environmental catastrophe leading to mass extinction solid particles from the hypothesized dark cloud would combine with the global environment of earth remaining in the stratosphere for at least several months or years with a sunshield effect estimated to be as large as 9 3 w m 2 the dark cloud would have caused global climate cooling in the last 8 myr of the cretaceous period consistent with the variations of stable isotope ratios in oxygen barrera and huber 1990 li and keller 1998 barrera and savin 1999 li and keller 1999 and strontium barrera and huber 1990 ingram 1995 sugarman et al 1995 the resulting growth of the continental ice sheet also resulted in a regression of the sea level the global cooling which appears to be associated with a decrease in the diversity of fossils eventually led to the mass extinction at the k pg boundary

establishing the link between the chesapeake bay impact structure and the north american tektite strewn field the sr nd isotopic evidence the chesapeake bay impact structure which is about 35 ma old has previously been proposed as the possible source crater of the north american tektites nat here we report major and trace element data as well as the first sr nd isotope data for drill core and outcrop samples of target lithologies crater fill breccias and post impact sediments of the chesapeake bay impact structure the unconsolidated sediments cretaceous to middle eocene in age have srt 35 7 ma of 54 to 272 and ndt 35 7 ma ranging from 6 5 to 10 8 one sample from the granitic basement with a tndchur model age of 1 36 ga yielded an srt 35 7 ma of 188 and an ndt 35 7 ma of 5 7 the exmore breccia crater fill can be explained as a mix of the measured target sediments and the granite plus an as yet undetermined component the post impact sediments of the chickahominy formation have slightly higher tndchur model ages of about 1 55 ga indicating a contribution of some older materials newly analyzed bediasites have the following isotope parameters 104 to 119 srt 35 7 ma 5 7 ndt 35 7 ma 0 47 ga tsrur and 1 15 ga tndchur which is in excellent agreement with previously published data for samples of the nat strewn field target rocks with highly radiogenic sr isotopic composition as required for explaining the isotopic characteristics of deep sea drilling project dsdp site 612 tektites were not among the analyzed sample suite based on the new isotope data we exclude any relation between the na tektites and the popigai impact crater although they have identical ages within 2 errors the chesapeake bay structure however is now clearly constrained as the source crater for the north american tektites although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites

early post impact sedimentation around the central high of the mjolnir impact crater barents sea late jurassic abstract the mjolnir bolide created the 40 km diameter mjolnir crater when it impacted the black mostly anoxic clays of the hekkingen formation in the paleo barents sea about 142 2 6 million years ago the normally calm 300 500 m deep epicontinental depositional environment was suddenly disrupted by the dramatic effect of the impact resulting in a brief period of extreme sediment reworking and redeposition the hypoxic to anoxic depositional conditions characteristic of the hekkingen formation returned to the impact site soon after the collapse when the major modification phases of the mjolnir crater were completed we have studied a shallow core 121 m long retrieved from the flanks of the central high in the mjolnir crater the core shows a complex depositional succession of the ragnarok formation which is related to both the uplift and the subsequent collapse and drowning of the central high the basal part of the core consists of chaotically organised large folded slabs of pre impact substrate which we infer to be related to the rapid steepening of the slope of the central high during its rising shortly after the impact the slump deposits are overlain by a diamict which is interpreted to originate from debris flows that originate by liquefaction and subsequent remoulding and remobilisation of sediment from the collapsing central high the diamict is in turn covered by a brecciated graded mudstone that records the action of impact related tsunami and the subsequent submergence of the impact crater a sequence of mainly debris flow and turbidite deposits separates the impact related deposit from the overlying shelf sediments of the hekkingen formation and forms the last post impact sedimentary recorder of the presence of a central high in the crater

applications of ichnology in lacustrine sequence stratigraphy potential and limitations abstract in comparison with their marine counterparts lacustrine ichnofaunas have not been extensively used in sequence stratigraphy application of trace fossils in continental sequence stratigraphy cannot be based simply on the extrapolation of marine sequence stratigraphy marine substrate controlled ichnofacies in particular the firmground glossifungites ichnofacies develop in stable and cohesive substrates reflecting erosive exhumation of the sediment in contrast in lacustrine sequences substrate controlled ichnofossils only rarely indicate erosional exhumation because they are commonly related to desiccation of water bodies the tripartite division of overfilled balanced fill and underfilled lakes provides a comprehensive framework to evaluate the potential and limitations of lacustrine ichnofaunas in sequence stratigraphy based on a number of case studies spanning the three lake basin types a model of trace fossil distribution in overfilled balanced fill and underfilled basins is proposed overfilled lake basins contain well developed softground trace fossils mermia and skolithos ichnofacies and the softground suite of the scoyenia ichnofacies that are useful to delineate parasequences and parasequence sets fluvial discharge into overfilled lakes usually generates density currents that oxygenate lake bottoms allowing the establishment of epifaunal and infaunal communities additionally these are freshwater lakes where no stress due to hypersalinity occurs leading to the development of a relatively diverse benthos land plant derived organic matter is the prime source of nutrients favoring the development of a deposit feeding benthic fauna in permanently subaqueous low energy zones firmground suites are rare because such large lakes usually do not experience desiccation abundant firmground trace fossil suites of the scoyenia ichnofacies occur in balanced fill lakes but softground assemblages are usually depauperate lowstand deposits contain abundant and widespread ichnofaunas commonly meniscate trace fossils and arthropod trackways of the scoyenia ichnofacies lake hydrology is closed during lowstands and salinity usually increases imposing a stress factor on the lake biota and therefore softground ichnofaunas are depauperate ichnofaunas from turbidite systems in balanced fill lakes are less abundant and diverse than those from overfilled lacustrine turbidites freshwater conditions are common during transgression but dysaerobic conditions may prevail imparting a stress factor on the benthic biota in hydrologically closed lakes the scoyenia ichnofacies is widespread but the mermia ichnofacies is commonly absent or impoverished the scoyenia ichnofacies is associated with lowstand desiccated substrates in underfilled lakes the density of arthropod trackways may be high forming tracked omission surfaces some of these omission surfaces may represent sequence boundaries expressed by co planar surfaces of lowstand and subsequent flooding rapid changes in depositional conditions reflecting desiccation during vertical aggradation can lead to the formation of composite ichnofabrics that reflect successive bioturbation events

geochemical evidence for a comet shower in the late eocene analyses of pelagic limestones indicate that the flux of extraterrestrial helium 3 to earth was increased for a 2 5 million year my period in the late eocene the enhancement began 1 my before and ended 1 5 my after the major impact events that produced the large popigai and chesapeake bay craters 36 million years ago the correlation between increased concentrations of helium 3 a tracer of fine grained interplanetary dust and large impacts indicates that the abundance of earth crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced these observations provide evidence for a comet shower triggered by an impulsive perturbation of the oort cloud

seismic images of magmatic rifting beneath the western branch of the east african rift 1 we have performed a tomographic study using a joint data set that includes local and teleseismic events recorded by a temporary network in the western branch of the east african rift system from the travel time residuals we derive a three dimensional model of seismic p wave velocity anomalies for the crust and upper mantle down to a depth of 80 km particular attention is paid to the verification of the inversion results by various resolution tests the results show that the eastern rift shoulder is characterized by relatively high seismic velocities lower velocities are obtained beneath the entire length of the rift valley and the rwenzori mountains a prominent feature is observed north east of the mountain range here we detected a vertically oriented cylindrical low velocity anomaly with maximum amplitudes in the middle crust and the upper mantle lithosphere we suggest that this anomaly indicates reservoirs of molten material related to the ongoing rifting process within this segment of the rift just above this anomaly at depths between 5 and 16 km earthquake swarms exist the observed reduction in p wave velocity is used to provide constraints on the possible melt content and temperature anomaly in the uppermost mantle the observed 3 5 p velocity decrease can be explained by melt fraction up to 2 3 3 or alternatively by a temperature increase of at least 248 to 376 k and even higher temperature anomalies are possible if lower ambient temperatures in the reference mantle are assumed probably the two effects act in combination

the chesapeake bay bolide impact a convulsive event in atlantic coastal plain evolution abstract until recently cenozoic evolution of the atlantic coastal plain has been viewed as a subcyclical continuum of deposition and erosion marine transgressions alternated with regressions on a slowly subsiding passive continental margin their orderly succession modified mainly by isostatic adjustments occasional appalachian tectonism and paleoclimatic change this passive scenario was dramatically transformed in the late eocene however by a bolide impact on the inner continental shelf the resultant crater is now buried 400 500 m beneath lower chesapeake bay its surrounding peninsulas and the continental shelf east of delmarva peninsula this convulsive event and the giant tsunami it engendered fundamentally changed the regional geological framework and depositional regime of the virginia coastal plain and produced the following principal consequences 1 the impact excavated a roughly circular crater twice the size of rhode island 6400 km2 and nearly as deep as the grand canyon 1 3 km deep 2 the excavation truncated all existing ground water aquifers in the target area by gouging 4300 km3 of rock from the upper lithosphere including proterozoic and paleozoic crystalline basement rocks and middle jurassic to upper eocene sedimentary rocks 3 synimpact depositional processes including ejecta fallback massive crater wall failure water column collapse and tsunami backwash filled the crater with a porous breccia lens 600 1200 m thick at a phenomenal rate of 1200 m hr the breccia lens replaced the truncated ground water aquifers with a single 4300 km3 reservoir characterized by ground water 1 5 times saltier than normal sea water chlorinities as high as 25 700 mg l 4 a structural and topographic low created by differential subsidence of the compacting breccia persisted over the crater at least through the pleistocene in the depression are preserved postimpact marine lithofacies and biofacies upper eocene lower oligocene lower miocene not known elsewhere in the virginia coastal plain 5 long term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the postimpact strata many of these faults appear to reach the bay floor and may be potential hazards for motion sensitive structures in population centers around chesapeake bay near surface fracturing and faulting generated by the impact shock may extend as far as 90 km from the crater rim 6 having never completely filled with postimpact sediments the sea floor depression over the crater appears to have predetermined the location of chesapeake bay 7 as large impact craters are principal sources for some of the world s precious metals it is reasonable to expect that metal enriched sills dikes and melt sheets are present in the inner basin of the crater in addition to these specific consequences the crater and the convulsive event that produced it have widespread implications for traditional interpretations of certain structural and depositional features of the atlantic coastal plain particularly in southeastern virginia

mapping chicxulub crater structure with gravity and seismic reflection data the chicxulub crater buried under the yucat n platform of mjxico affords one of the most favourable opportunities on earth for structural studies of a large complex impact crater due to its preservation and accessibility the crater was buried soon after rim erosion in an environment with only minor subsequent tectonic disturbance it is also of interest as the chicxulub impact apparently ended the cretaceous period and the crater s ejecta blanket is known globally at hundreds of localities the buried crater is half overlain by a coastal plain and half by a shallow sea allowing exploration by both marineand land based techniques as well as hybrid combinations the radial symmetry of the crater allows application of data acquired in either terrain to the entire structure using the common data sets of magneticand gravity field anomalies gravity data sets covering the yucat n platform e g 1 2 reveal an 180 km diameter low with internal concentric structure e g 3 4 that is the expression of the chicxulub crater impact lithologies and crater fill the gravity signature is complicated by regional anomalies and early modifications to the crater rim and the northern half of the crater is not well imaged due to the scarcity and imprecision of marine data also an 20 km wide gap in survey coverage due to shallow water occurs between the land data and the nearest seismic profile of camargo and suarez 5 fig 1 which roughly corresponds to the beginning of offshore coverage during september and october 1996 a marine gravity survey was successively conducted on 32 and 10 5 m vessels to survey this gap and offshore seismic lines of the imperial college birps u of texas seismic project offshore coverage was 2 400 line km followed by 1 500 line km inshore with 1 3 and 1 9 mgal unadjusted crossovers respectively data precision was strongly influenced by sea conditions preliminary results from the marine surveys confirm the provisional ties of gravity gradient features to the seismic line the perimeter fault crater fill margin slump faults and collapsed transient cavity margin may all be linked to gradient features unfortunately the clearest crater expression in the gravity field is on the east side where the seismic lines lose resolution at the basin margin to the west the strong peripheral gradient splits with one component corresponding to the perimeter slump fault and a larger feature corresponding to the basin margin this splitting is interpreted as resulting from prompt backwash of crater ejecta into the northwestern part of the crater regional gradients also obscure the crater signature to a greater extent to the west asymmetry of the central high is indicated although a weak feature may extend to the ne the surrounding sharp low continues until it abuts the regional high to the nw additional land surveys support the correlation of the cenote ring to gravity gradient features and suggest that a weak high may correlate to the rim uplift in some places

montagnais a submarine impact structure on the scotian shelf eastern canada the first impact crater in the ocean has been identified on the outer continental shelf off nova scotia canada the crater is well preserved and buried by 510 m of tertiary and quaternary marine sediments the crater is a circular structure at least 45 km wide and 2 7 km deep with a central structural uplift the uplift is at least 1 250 m and possibly 2 900 m in height its upper surface is 11 5 km across and it has a central pit 3 5 km wide in the center an exploratory oil well located near the center of the structure encountered cambro ordovician metamorphic rocks of the basement which are fractured with planar deformation features in quartz representative of pressures in the 8 to 12 gpa range the uplifted basement is covered by 552 m of breccia shock induced features are common they include planar lamellae in quartz and feldspars partial to total isotropization of silicates and the occurrence of mixed tectosilicate glasses some of which exhibit flow textures with total dissociation of the mafic components to ill defined aggregates these features are diagnostic of a 35 to 50 gpa shock level for parts of the breccia the top of the breccia is covered by a 40 5 m thick suevite zone two crystalline melt rock horizons 72 and 34 m thick are enclosed in the breccia even though the feldspars in the melt are labradorite to bytownite in composition the chemical composition of the rock is rhyolitic with 70 77 of sio 2 similar to the composition of the basement there is no obvious enrichment in siderophile elements except an increase in iridium 0 1 0 3 ppb the occurrence of a thick section of impact melt rocks and breccia capping the central uplift is a feature not recognized in other complex impact craters and may be a function of impact into a marine environment dating by 39 ar 40 ar indicates that the impact occurred at about 51 ma late early eocene and this is confirmed by biostratigraphic data from immediately overlying sediments the compositional and structural imprint left on the montagnais structure by the colliding body points to the impactor being most probably the nucleus of a comet

impact origin of the chesapeake bay structure and the source of the north american tektites seismic profiles drill core samples and gravity data suggest that a complex impact crater 35 5 million years old and 90 kilometers in diameter is buried beneath the lower chesapeake bay the breccia that fills the structure contains evidence of shock metamorphism including impact melt breccias and multiple sets of planar deformation features shock lamellae in quartz and feldspar the age of the crater and the composition of some breccia clasts are consistent with the chesapeake bay impact structure being the source of the north american tektites

deep sea drilling project site 612 bolide event new evidence of a late eocene impact wave deposit and a possible impact site u s east coast a remarkable 60 m thick upward fining polymictic marine boulder bed is distributed over 15 000 km sup 2 beneath chesapeake bay and the surrounding middle atlantic coastal plain and inner continental shelf the wide varieties of clast lithologies and microfossil assemblages were derived from at least seven known cretaceous paleocene and eocene stratigraphic units the supporting pebbly matrix contains variably mixed assemblages of microfossils from the same seven stratigraphic units along with trace quantities of impact ejecta tektite glass and shocked quartz the youngest microfossils in the boulder bed are of early late eocene age on the basis of its unusual characteristics and its stratigraphic equivalence to a layer of impact ejecta at deep sea drilling project dsdp site 612 new jersey continental slope the authors postulate that this boulder bed was formed by a powerful bolide generated wave train that scoured the ancient inner shelf and coastal plain of southeastern virginia the most promising candidate for the bolide impact site identified on seismic reflection profiles is 40 km north northwest of dsdp site 612 on the new jersey outer continental shelf

widespread erosion and mass failure from the 51 ma montagnais marine bolide impact off southwestern nova scotia canada12 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120241 the 51 ma montagnais impact crater on the outer scotian shelf is well known but the potential effects from the impact event on the slope and rise seaward of the crater have until now remained poorly understood through detailed seismic stratigraphic correlation and ties to available wells we define a threefold seismic stratigraphic subdivision for upper cretaceous to eocene strata on the shelf and slope calibrated to the most recent biostratigraphic results using this framework we identify a number of depositional and erosional products that are temporally consistent with a late ypresian impact event within the limits of seismic and biostratigraphic resolution we link a series of prominent failure scarps on the outer shelf and upper slope to a single widespread mass transport deposit mtd on the lower continental slope rise and abyssal plain failed material amassed in a large debris field referred to here as the montagnais mtd it covers an area of 93 000 km2 and travelled up to 580 km f

late eocene impact ejecta geochemical and isotopic connections with the popigai impact structure late eocene microtektites and crystal bearing microkrystites extracted from dsdp and odp cores from the atlantic pacific and indian oceans have been analyzed to address their provenance a new analysis of nd and sr isotopic compositions confirms previous work and the assignment of the uppermost microtektite layer to the north american tektites which are associated with the 35 5 ma 85 km diameter chesapeake impact structure of virginia usa extensive major element and nd and sr isotopic analyses of the microkrystites from the lowermost layer were obtained the melanocratic microkrystites from sites 216 and 462 in the indian and pacific oceans possess major element chemistries sr and nd isotopic signatures and sm nd t chur model ages similar to those of tagamite melt rocks in the popigai impact structure they also possess rb sr t ur model ages that are younger than the tagamite t chur ages by up to 1 ga which require a process as yet undefined of rb sr enrichment these melanocratic microkrystites are consistent with a provenance from the 35 7 ma 100 km diameter popigai impact structure of siberia russia while ruling out other contemporaneous structures as a source melanocratic microkrystites from other sites and leucocratic microkrystites from all sites possess a wide range of isotopic compositions 143 nd values of 16 to 27 7 and 87 sr values of 4 1 354 0 making the association with popigai tagamites less clear these microkrystites may have been derived by the melting of target rocks of mixed composition which were ejected without homogenization dark glass and felsic inclusions extracted from popigai tagamites possess 143 nd and 87 sr values of 26 7 to 27 8 and 374 7 and 432 4 respectively and t chur and t ur model ages of 1640 1870 ma and 240 1830 ma respectively which require the preservation of initially present heterogeneity in the source materials the leucocratic microkrystites possess diverse isotopic compositions that may reflect the melting of supra basement sedimentary rocks from popigai or early basement melts that were ejected prior to homogenization of the popigai tagamites the ejection of melt rocks with chemistries consistent with a basement provenance rather than the surface 1 km of sedimentary cover rocks atypically indicates a non surficial source to some of the ejecta microkrystites from two adjacent biozones possess statistically indistinguishable major element compositions suggesting they have a single source the occurrence of microkrystites derived from a single impact event but in different biozones can be explained by 1 diachronous biozone boundaries 2 post accumulation sedimentary reworking or 3 erroneous biozonation

chicxulub the third dimension of a multi ring impact basin the buried 65 ma chicxulub impact structure in mexico is the largest well preserved impact crater known on earth seismic reflection data have revealed chicxulub to be a multi ring basin it has the morphology of the largest impact craters in the solar system we use these seismic data to relate surface morphology and near surface structure to deeper deformation within the crust to provide the first high resolution look into the third dimension of a multi ring impact basin we observe three distinct topographic ring types crater rims peak rings and outer rings each is associated with a different style of deep deformation crater rims are the head scarp of the terrace zone formed during the collapse of the transient cavity the peak ring at chicxulub appears to have formed when the central uplift collapsed overthrusting and overriding the terrace zone the impact has affected the whole crust the outer rings at chicxulub are linked to whole crustal deformation in which the middle crust and lower crust have moved inward and downward strong reflections that dip craterward at 35 cut the entire crust and connect normal faulting in the sedimentary section with zones of downthrown moho at a crater radius of 35 55 km weakly developed exterior rings appear as thrust faults with small offsets these appear to be the progenitors of the more significant normally faulted outer rings in multi ring basins

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

geological record and reconstruction of the late pliocene impact of the eltanin asteroid in the southern ocean in 1995 an expedition on board the research vessel fs polarstern explored the impact site of the eltanin asteroid in the southern ocean the only known asteroidimpact into a deep ocean basin analyses of the geological record of the impact region place the event in the late pliocene 2 15 myr and constrain thesize of the asteroid to be 1 km the explosive force inferred for this event places it at the threshold of impacts believed to have global consequences and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events which are common on geological timescales

trace fossils and their palaeoecological significance in lower cretaceous lacustrine conservation deposits el montsec spain abstract la pedrera and la cabrua at el montsec spain are two lower cretaceous lacustrine conservation deposits konservat lagerstatten that have provided a rich diverse and well preserved faunal and floral fossil record although the lithographic limestones where the body fossils are found are finely laminated and essentially undisturbed by bioturbation epigenic trace fossils occur on the surface of certain laminae trace fossils at la pedrera include the fish trail undichna britannica and the arthropod trackway hamipes didactylus while the ichnospecies found at la cabrua are steinsfjordichnus brutoni gordia arcuata cochlichnus anguineus and undichna britannica all but the last corresponding to the activity of small invertebrates the trace fossil occurrences record short term events in a lake bottom that did not support any benthic life for most of its history differences in the trace fossil assemblages for the two localities agree with sedimentological data in attributing a more distal position for la pedrera

fish and tetrapod trace fossils from the upper carboniferous of puertollano spain abstract well preserved fish and tetrapod trace fossils occur in carboniferous stephanian c beds at puertollano they represent the first evidence of vertebrate ichnites in the carboniferous of spain the fish swimming traces are assigned to undichna britannica and undichna unisulca the dimensions and morphological features of the traces indicate that orthacanthus xenacanthidae elasmobranchii a shark very common in the basin might have been the trace producer a morpho functional analysis of the traces suggests an axial undulatory anguilliform mode of locomotion of xenacanth sharks this conclusion is consistent with the eel like body found in all known xenacanthids tetrapod trackways referred to puertollanopus microdactylus ichnogen et ichnosp nov and resting traces are described both walking and resting tracks are assigned to a small terrestrial tetrapod dimensions and proportions of the tracks and size and morphology of the impressions of manus and pes suggest a microsaur amphibian or a small reptile as the track maker this represents the first evidence of these vertebrates in the basin the superposition of fish trails and tetrapod trackways strongly suggests tidal processes in a marginal marine depositional setting this interpretation agrees with recent sedimentological geochemical and palynological data from the puertollano basin

integrated geophysical analysis supporting the impact origin of the mjolnir structure barents sea abstract gravity magnetic and seismic traveltime anomalies observed at the 40 km diameter mjolnir impact structure reveal a distinct spatial correspondence with the radially zoned seismic structure the gravity anomaly is dominated by a 2 5 mgal 14 km wide centrally located high superimposed on a 45 km diameter low that attains minimum values of 1 5 mgal the magnetic anomaly field exhibits several local low amplitude anomalies within the 100 nt range located towards the periphery while seismic mapping of a prominent originally planar reflector beneath the structure brings out a central pull up traveltime anomaly on the order of 80 ms in terms of impact origin the integrated geophysical modelling based on the characteristic bowl shaped seismic disturbance beneath the structure supports the differentiation into a central uplift and a peripheral region interaction of several impact cratering processes such as impact induced porosity increase due to brecciation mass transport during collapse and structural uplift explains the modelled physical properties associated with the disturbance the modelling further substantiates the interpretation of the mjolnir structure as an impact crater and demonstrates the incompatibility of alternative geological origins such as salt or clay diapirism and igneous intrusions

crustal tomography of the central kenya rift abstract during the kenya rift international seismic project 1990 krisp90 experiment the shots of the seismic refraction programme were recorded by a two dimensional teleseismic network this was installed in the central part of the kenya rift and on its shoulders from lake baringo in the north to lake naivasha in the south p wave first arrivals were used to determine lateral crustal velocity heterogeneities with a three dimensional inversion programme mainly high velocity zones were found which can be correlated with other geophysical anomalies and geological features this comparison shows that the high velocity bodies found within the crust are likely indicators for mafic cumulates or magma chambers these evolved during the major magmatic episodes which are typical indicators for an active rift

late eocene microtektites and radiolarian extinctions on barbados a late eocene microtektite layer has been found in three exposed sections on barbados west indies these microtektites and associated tektite fragments are compositionally similar to the north american tektites and thus like the caribbean and gulf of mexico microtektites may belong to the strewn field of north american microtektites which have been found in deep sea deposits from the caribbean sea across the pacific and into the northeastern indian ocean1 2 the north american layer has been reported to be approximately synchronous with several radiolarian extinctions3 4 and with an anomaly in iridium concentrations5 7 and it has been hypothesized that the microtektites the extinctions and the iridium anomaly are all related to an asteroid impact we report here that the radiolarian extinctions predate the microtektite layer and that the highest ir concentrations apparently coincide with the extinctions thus although the extinctions may be causally related to the ir anomaly the microtektites appear not to be

tomographic study of local earthquake data from the lake bogoria region of the kenya rift valley summary the 3 d p wave and s wave velocity structure in the upper crust beneath the lake bogoria region of the kenya rift valley is obtained by the simultaneous inversion of local earthquake arrival time data for velocity and hypocentral parameters the data were obtained from seismograms recorded by the krisp85 kenya rift international seismic project 1985 lake bogoria earthquake network and inverted using a modified version of thurber s 3 d iterative simultaneous inversion program the results are consistent with the inversion of teleseismic traveltime residuals from the same network and correlate with the mapped surface geological features there is a high velocity anomaly located at depth beneath the goituimet volcanic region in the southwest of the study area which may be associated with basic igneous intrusives or metamorphism there is a low velocity anomaly in the north of the study area which can be correlated with the geothermal activity around the southern shores of the lake the main feature is the variation in velocity across the legisianana emsos bogoria fault with velocities up to 5 per cent higher than the average to the east and up to 2 5 per cent lower to the west

evidence for ocean water invasion into the chicxulub crater at the cretaceous tertiary boundary the possibility of ocean water invasion into the chicxulub crater following the impact at the cretaceous tertiary boundary was investigated based on examination of an impactite between approximately 794 63 and 894 94 m in the yaxcopoil 1 yax 1 core the presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval abundant occurrence of nannofossils of late campanian to early maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation the maximum grain size of limestone lithics and vesicular melt fragments and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite the upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater most likely by high energy currents association with ocean water invasion

meteoroid mayhem in ole virginny source of the north american tektite strewn field new seismic reflection data from chesapeake bay reveal a buried 85 km wide 1 5 2 0 km deep peak ring impact crater carved through upper eocene to lower cretaceous sedimentary strata and into underlying pre mesozoic crystalline basement rocks a polymictic late eocene impact breccia composed mainly of locally derived sedimentary debris determined from four continuous cores surrounds and partly fills the crater structural and sedimentary characteristics of the chesapeake bay crater closely resemble those of the miocene ries peakring crater in southern germany we speculate that the chesapeake bay crater is the source of the north american tektite strewn field

the maastrichtian danian maimara tracksite yacoraite formation salta group quebrada de humahuaca argentina environments and ichnofacies implications fil console gonella carlos alfredo consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet tucuman instituto superior de correlacion geologica universidad nacional de tucuman facultad de ciencias naturales e instituto miguel lillo departamento de geologia catedra geologia estructural instituto superior de correlacion geologica argentina

the age of the popigai impact event and its relation to events at the eocene oligocene boundary ages ranging from the late cretaceous 65 myr to the oligocene 29 myr have been reported for the 100 km diameter popigai impact structure1 2 on the anabar shield central siberia these ages3 4 5 6 7 8 overlap the timing of several possible impact related features including the cretaceous tertiary and eocene oligocene stratigraphic boundaries the north american tektites9 and the recently reported occurrences of an iridium anomaly and shocked quartz in late eocene deposits in northern italy10 here we report age determinations of several popigai impact melt rocks using the 40ar 39ar step heating technique to constrain the age of the impact event our results are consistent with a late eocene impact age of 35 7 0 2 myr 2 coincident in time with the impact deposits found in italy as this age is also similar to that of the north american tektites which have been associated with the chesapeake bay impact structure in the eastern united states11 12 13 there seem to have been at least two large and essentially contemporaneous impacts during the late eocene