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 provenance formation and implications of reduced carbon phases in martian meteorites this review is intended to summarize the current observations of reduced carbon in martian meteorites differentiating between terrestrial contamination and carbon that is indigenous to mars indeed the identification of martian organic matter is among the highest priority targets for robotic spacecraft missions in the next decade including the mars science laboratory and mars 2020 organic carbon compounds are essential building blocks of terrestrial life so the occurrence and origin biotic or abiotic of organic compounds on mars is of great significance however not all forms of reduced carbon are conducive to biological systems this paper discusses the significance of reduced organic carbon including methane in martian geological and astrobiological systems specifically it summarizes current thinking on the nature sources and sinks of martian organic carbon a key component to martian habitability based on this compilation reduced organic carbon on mars including detections of methane in the martian atmosphere is best described through a combination of abiotic organic synthesis on mars and infall of extraterrestrial carbonaceous material although conclusive signs of martian life have yet to be revealed we have developed a strategy for life detection on mars that can be utilized in future life detection studies

the bow city structure southern alberta canada the deep roots of a complex impact structure geological and geophysical evidence is presented for a newly discovered probable remnant complex impact structure the structure located near bow city southern alberta has no obvious morphological expression at surface the geometry of the structure in the shallow subsurface mapped using downhole geophysical well logs is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure evidence of deformation along the eastern margin of the central uplift includes thrust faulting folding and steeply dipping bedding normal faults were mapped along the northern margin of the annular region isopach maps reveal that structural thickening and thinning were accommodated primarily within the belly river group evidence from legacy 2 d seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure including a series of listric normal faults in the annular region and complex faulting within the central uplift the absence of any ejecta blanket breccia suevite or melt sheet based on available data is consistent with the bow city structure being the remnant of a deeply eroded complex impact structure accordingly the bow city structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks

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

the lappajarvi meteorite crater finland petrography rb sr major and trace element geochemistry of the impact melt and basement rocks abstract impact melt lithologies of the 77 m y old finnish meteorite crater lappajarvi as well as the precambrian target rocks have been studied in detail to identify and characterize different impact melt types clast poor clast rich suevitic melt and to study their chemical major and trace elements and isotopic rb sr compositions in comparison to the composition of the target rocks the rb sr system of the whole melt body including the suevitic melt is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks chemical interactions exchange of alkali elements 87sr redistribution between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt exchange reactions between clasts and melt are determined by thermal activation but the degree of shock metamorphism in the clasts plays an important role too major and trace element distributions in impact melt and basement rocks indicate that the lappajarvi melt body chemically is extremely homogeneous even volatile elements such as zn and cu were not strongly fractionated comparison of the abundances of siderophile elements in the impact melt e g 118 177 ppm cr 195 340 ppm ni 6 12 ppb ir and calculated target rock mixture 79 mica schist 11 granite pegmatite 10 amphibolite e g 85 6 ppm cr 54 8 ppm ni 0 5 ppb ir revealed the chondritic nature c or h chondritic of the meteoritic projectile less than 2 of the meteorite can be detected in the coherent melt whereas the suevitic melt is uncontaminated by the projectile

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

chemical and structural changes induced by thermal annealing of shocked feldspar inclusions in impact melt rocks from lappajarvi crater finland plagioclase and microcline of clastic mineral and rock inclusions in impact melt rocks karnaite were sampled from the coherent melt sheet located in the central crater area of the lappajarvi meteorite crater and analyzed by optical electron optical x ray and microprobe methods both types of feldspars display strong chemical textural and structural alterations compared to the feldspars of their parental crystalline basement rocks as a result of the thermal energy supplied by the surrounding superheated impact melt increase of shock experienced by the clasts prior to their incorporation into the melt results in increasing thermal alteration by the melt in a lower degree of shock both primary plagioclase and microcline transform to checkerboard textured grains that consist of 5 10 m sized subgrains of parallel optical orientation embedded in a mesostasis of eutectically grown sanidine and quartz the subgrains are strongly zoned disordered labradorite to andesine and rimmed by sanidine in a high degree of shock the subgrains are larger and randomly oriented the subgrains are the result of a nonequilibrium fractional crystallization from a liquid state during which a strong chemical exchange between the melt and the feldspar inclusions took place on the basis of model calculations it is concluded that the temperature of the karnaite melt at the time of the incorporation of the clastic debris was in the range of 1800 to 2100 c the equilibration temperature of the clast melt mixture is estimated accordingly to be at least in the range of 1100 to 1140 c or higher depending on the magnitude of the clast s postshock temperature this temperature is above the solidus and in part above the liquidus temperature of a dry granodioritic melt and is also above the solidus temperatures of the feldspars the clast laden melt was at rest when the equilibrium temperature was reached suggesting that the readjustment of the transient crater cavity by central uplifting of the crater took place within a time scale of hours

the multiple impacts hypothesis for mass extinction a comparison of the late devonian and the late eocene abstract application of the lag time multiple impacts hypothesis poag 1997b palaios 12 582 590 poag et al 2001 columbia univ press to the late devonian leads to the prediction that the frasnian famennian pulsed extinctions were triggered by a rapid drop in global temperature that followed an impact produced anomalous warm interval which interrupted the global cooling trend from the middle devonian greenhouse to the early carboniferous icehouse in actualistic comparison with the late eocene the lag time multiple impacts hypothesis would predict that a frasnian interval of multiple impacts should have occurred between 367 7 and 366 7 ma the fact that three impacts the alamo siljan and flynn creek do occur either within this predicted interval or close to it in time is corroborative evidence that the lag time multiple impacts hypothesis may indeed provide the causal mechanism for the frasnian famennian mass extinction based on the application of the lag time multiple impacts hypothesis to the frasnian famennian mass extinction it is here suggested that future searches for evidence of impact events in the late devonian be concentrated in strata that occur in the frasnian transitans to early hassi zonal interval and not in strata immediately below or above the frasnian famennian boundary

high pressure polymorphs in yamato 790729 l6 chondrite and their significance for collisional conditions shock pressure recorded in yamato y 790729 classified as l6 type ordinary chondrite was evaluated based on high pressure polymorph assemblages and cathodoluminescence cl spectra of maskelynite the host rock of y 790729 consists mainly of olivine low ca pyroxene plagioclase metallic fe ni and iron sulfide with minor amounts of phosphate and chromite a shock melt vein was observed in the hostrock ringwoodite majorite akimotoite lingunite tuite and xieite occurred in and around the shock melt vein the shock pressure in the shock melt vein is about 14 23 gpa based on the phase equilibrium diagrams of high pressure polymorphs some plagioclase portions in the host rock occurred as maskelynite sixteen different cl spectra of maskelynite portions were deconvolved using three assigned emission components centered at 2 95 3 26 and 3 88 ev the intensity of emission component at 2 95 ev was selected as a calibrated barometer to estimate shock pressure and the results indicate pressures of about 11 19 gpa the difference in pressure between the shock melt vein and host rock might suggest heterogeneous shock conditions assuming an average shock pressure of 18 gpa the impact velocity of the parent body of y 790729 is calculated to be 1 90 km s 1 the parent body would be at least 10 km in size based on the incoherent formation mechanism of ringwoodite in y 790729

progressive deformation of feldspar recording low barometry impact processes tenoumer impact structure mauritania the tenoumer impact structure is a small well preserved crater within archean to paleoproterozoic amphibolite gneiss and granite of the reguibat shield north central mauritania the structure is surrounded by a thin ejecta blanket of crystalline blocks granitic gneiss granite and amphibolite and impact melt rocks evidence of shock metamorphism of quartz most notably planar deformation features pdfs occurs exclusively in granitic clasts entrained within small bodies of polymict glass rich breccia impact related deformation features in oligoclase and microcline grains on the other hand occur both within clasts in melt breccia deposits where they co occur with quartz pdfs and also within melt free crystalline ejecta in the absence of co occurring quartz pdfs feldspar deformation features include multiple orientations of pdfs enhanced optical relief of grain components selective disordering of alternate twins inclined lamellae within alternate twins and combinations of these individual textures the distribution of shock features in quartz and feldspar suggests that deformation textures within feldspar can record a wide range of average pressures starting below that required for shock deformation of quartz we suggest that experimental analysis of feldspar behavior combined with detailed mapping of shock metamorphism of feldspar in natural systems may provide critical data to constrain energy dissipation within impact regimes that experienced low average shock pressures

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

shock induced changes in density and porosity in shock metamorphosed crystalline rocks haughton impact structure canada abstract shock metamorphism can occur at transient pressures that reach tens of gpa and well over 1000 c altering the target material on both megascopic and microscopic scales this study explores the effects of shock metamorphism on crystalline quartzofeldspathic basement material from the haughton impact structure on devon island arctic canada shock levels were assigned to samples based on petrographic examination of main mineral phases conventional shock classification schemes proved to incompletely describe the haughton samples so a modified shock classification system is presented fifty two crystalline bedrock samples from the clast rich impact melt rocks in the crater and one reference site outside of the crater were classified using this system the shock levels range from 0 to 7 according to the new shock stage classification proposed here i e stages 0 iv after the stoffler classification indicating shock pressures ranging from 0 to approximately 80 gpa the second aspect of this study involved measuring bulk physical characteristics of the shocked samples the bulk density grain density and porosity were determined using a water displacement method a bead displacement method and a hepycnometer results suggest a nonlinear negative correlation between density and shock level such that densities of crystalline rocks with original densities of approximately 3 g cm 3 are reduced to 1 0 g cm 3 at high shock levels the results also show a positive nonlinear correlation between porosity and shock level these data illustrate the effect of shock on the bulk physical characteristics of crystalline rocks and has implications for assessing the habitability of shocked rocks

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

laboratory thermal emission spectroscopy of shocked basalt from lonar crater india and implications for mars orbital and sample data 1 whereas the thermal infrared tir spectra of specific minerals and or igneous and metamorphic rocks have been compared to data of the martian surface tir data of the impactite products of the shock metamorphism of basalt have not been examined in detail the effects of shock on the thermal infrared spectrum of deccan basalt are described here sample collection at lonar crater india yielded four classes of shocked basalt classes 1 2 4 and 5 spectral features attributed to labradorite in tir spectrum of unshocked deccan basalt are absent in the spectrum of class 2 shocked basalt petrography confirms that labradorite has been replaced by diaplectic glass maskelynite the tir spectrum of class 2 shocked basalt is nearly identical to that of the los angeles shergottite the addition of experimentally shocked plagioclase feldspars as tir end members improves fits and lowers rms errors for the deconvolution of the tir spectrum of the maskelynite bearing impactite and the correct mineralogy is chosen to within 5 class 4 shocked basalt contains vesiculated plagioclase glass due to more heat and highly fractured augites two class 5 spectral types have primary si o stretching vibrations at lower wave numbers than si and si k glass end members commonly used in analyses of tir data agreeing with lower sio2 abundances 50 determined from two techniques at least three differences in spectral features exist between the two class 5 spectral types that we attribute to incomplete melting in the class 5a samples based on comparison to the class 4 spectrum these samples and their tir spectra represent excellent analogs for martian shocked basalt and new lithologic end members for use in spectral libraries used to analyze tir data from mars

thermal infrared spectroscopy of experimentally shocked anorthosite and pyroxenite implications for remote sensing of mars 1 the feldspar and pyroxene mineralogies on mars revealed by the thermal emission spectrometer tes on mars global surveyor likely record a variety of shock effects as suggested by petrologic analyses of the martian meteorites and the abundance of impact craters on the planet s surface to study the effects of shock pressures on thermal infrared spectra of these minerals we performed shock recovery experiments on orthopyroxenite and anorthosite samples from the stillwater complex montana over peak pressures from 17 to 63 gpa we acquired emissivity and hemispherical reflectance spectra 350 1400 cm 1 7 29 mm of both coherent chips and fine grained powders of shocked and unshocked samples these spectra are more directly comparable to remotely sensed data of mars e g tes than previously acquired absorption or transmission spectra of shocked minerals the spectra of experimentally shocked feldspar show systematic changes with increasing pressure due to depolymerization of the silica tetrahedra for the spectra of chips this includes the disappearance of small bands in the 500 650 cm 1 region and a strong band at 1115 cm 1 and changes in positions of a strong band near 940 cm 1 and the christiansen feature near 1250 cm 1 spectra of the shocked powders show the gradual disappearance of a transparency feature near 830 cm 1 fewer changes are observed in the pyroxene spectra at pressures as high as 63 gpa spectra of experimentally shocked minerals will help identify more precisely the mineralogy of rocks and soils not only from tes but also from future mars instruments such as minites and themis index terms 3944 mineral physics shock wave experiments 3934 mineral physics optical infrared and raman spectroscopy 5420 planetology solid surface planets impact phenomena includes cratering 5460 planetology solid surface planets physical properties of materials keywords anorthosite pyroxenite shock spectra thermal infrared remote sensing mars

high resolution solid state sodium 23 aluminum 27 and silicon 29 nuclear magnetic resonance spectroscopic reconnaissance of alkali and plagioclase feldspars

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

an unusual occurrence of coesite at the lonar crater india coesite has been identified within ejected blocks of shocked basalt at lonar crater india this is the first report of coesite from the lonar crater coesite occurs within sio2 glass as distinct 30 m spherical aggregates of granular coesite identifiable both with optical petrography and with micro raman spectroscopy the coesite glass occurs only within former silica amygdules which is also the first report of high pressure polymorphs forming from a shocked secondary mineral detailed petrography and nmr spectroscopy suggest that the coesite crystallized directly from a localized sio2 melt as the result of complex interactions between the shock wave and these vesicle fillings

interaction between the summer monsoons in east asia and the south china sea intraseasonal monsoon modes abstract the summer monsoons in east and southeast asia are characterized respectively by the mei yu in eastern china baiu in japan front mbf and by the monsoon trough stretching from northern indochina to the philippine sea these two major monsoon elements are separated by the north pacific anticyclone as indicated by the 850 mb zonal wind and cumulus convection over some key areas a distinct opposite phase intraseasonal variation exists between the two monsoon elements two approaches are adopted to explore the cause of this opposite phase variation which reflects the coupling between the two monsoon components 1 the correlation coefficient patterns between the 850 mb zonal wind monsoon index and the 850 mb streamfunction field and 2 the composite 850 mb streamline charts and the 120 e zonal wind cross sections it is shown that the opposite phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward migrating 30 60 day monsoon tr

nanoscale chemical characterisation of phase separation solid state transformation and recrystallization in feldspar and maskelynite using atom probe tomography the feldspar minerals occur in a wide variety of lithologies throughout the solar system often containing a variety of chemical and structural features indicative of the crystallization conditions cooling history and deformational state of the crystal such phenomena are often poorly resolved in micrometre scale analyses here atom probe tomography apt is conducted on ca rich bytownite and na rich albite plagioclase reference materials experimentally exsolved k feldspar sanidine shock induced plagioclase glass labradorite composition and shocked and recrystallized plagioclase to directly test the application of apt to feldspar and yield new insights into crystallographic features such as amorphisation and exsolution undeformed plagioclase reference materials amelia albite and stillwater bytownite appear chemically homogenous and yield compositions largely within uncertainty of published data within microstructurally complex materials apt can resolve chemical variations across a 20 nm wide exsolution lamella and define major element na k diffusion profiles across the lamella boundaries which appear gradational over a 10 nm length scale in experimentally exsolved k feldspar nnpp 04b the plagioclase glass within the zagami shergottite shows no heterogeneity in the distribution of major elements although the enrichment of fe mg and sr in the bulk microtip points to at least minor incorporation of surrounding phases pyroxene and with that supports a shock melt origin for the glass maskelynite the recrystallization of feldspar during post shock annealing such as in poikilitic shergottite nwa 6342 appears to induce a range of chemical nanostructures that locally effect the composition of the material these findings demonstrate the ability of apt to yield new insights into nanoscale composition and chemical structures of alumniosilicate phases highlighting an exciting new avenue with which to analyse these key rock forming minerals

martian low temperature alteration materials in shock melt pockets in tissint constraints on their preservation in shergottite meteorites we apply an array of in situ analytical techniques including electron and raman microscopy electron and ion probe microanalysis and laser ablation mass spectrometry to the tissint martian meteorite in order to find and elucidate a geochemical signature characteristic of low temperature alteration at or near the martian surface tissint contains abundant shock produced quench crystallized melt pockets containing water in concentrations ranging from 73 to 1730 ppm water content is positively correlated with cl content the isotopic composition of hydrogen in the shock produced glass ranges from d 2559 to 4422 water is derived from two distinct hydrogen reservoirs the martian near surface 500 and the martian mantle 100 in one shock melt pocket comprising texturally homogeneous vesiculated glass the concentration of h in the shock melt decreases while simultaneously becoming enriched in d attributable to the preferential loss of h over d to the vesicle while the pocket was still molten while igneous sulfides are pyrrhotite in composition fe 0 88 0 90 s the iron to sulfur ratios of spherules in shock melt pockets are elevated up to fe 1 70 s which we attribute to shock oxidation of igneous pyrrhotite and the formation of hematite at high temperature the d and cl enrichment and higher oxidation of the pockets as indicated by hematite support a scenario in which alteration products formed within fractures or void spaces within the rock the signature of these alteration products is preserved within shock melt now glass which formed upon collapse of these fractures and voids during impact shock thermal modeling of tissint shock melt pockets using the heat program demonstrates that the shock melt pockets with the greatest potential to preserve a signature of aqueous alteration are small isolated from other regions of shock melt vesicle free and glassy

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

upper eocene tektite and impact ejecta layer on the continental slope off new jersey during leg 150 of the ocean drilling project odp two sites 903c and 904a were cored that have sediments of the same biostratigraphic age as the upper eocene tektite bearing ejecta layer at deep sea drilling project dsdp site 612 core 45x from odp site 904a 4 km north of site 612 contains a 5 cm thick tektite bearing ejecta layer and core 56 from site 903c 8 km north northwest of site 904 contains a 2 cm thick layer of impact ejecta without any tektite or impact glass shocked quartz and feldspar grains with multiple sets of planar deformation features pdfs and abundant coesite bearing grains are present at both sites the major oxide contents trace element compositions and rare earth element ree patterns of the site 904 tektites are similar to those of the site 612 tektites and to north american tektites especially bediasites the sr and nd values for one composite tektite sample from site 904 fall within the range previously obtained for the site 612 tektites which defines a linear trend that if extrapolated would intersect the values obtained for north american tektites the water contents of eight tektite fragments from site 904 range from 0 017 to 0 098 wt and thus are somewhat higher than is typical for tektites the heavy mineral assemblages of the 63 125 m size fractions from the ejecta layers at sites 612 903 and 904 are all similar therefore we conclude that the ejecta layer at all three sites is from the same impact event and that the tektites at sites 904 and 612 belong to the north american tektite strewn field clinopyroxene bearing cpx spherules occur below or in the lower part of the main ejecta layer at all three sites at all three sites the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures site 612 903 and 904 cpx spherules are similar to those found in the caribbean sea gulf of mexico central equatorial pacific western equatorial pacific and eastern indian ocean the cpx event appears to have preceded the north american tektite event by 10 15 ka or less the fining upward sequence at all three sites and concentration of the denser unmelted impact ejecta at the top of the tektite layer at sites 612 and 904 suggest that the tektite bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass geographic variations in thickness of the tektite bearing ejecta layer the lack of carbonate clasts in the ejecta layer and the low cao content of the tektite glass suggest that the ejecta including the tektite glass were derived from the chesapeake bay structure rather than from the toms canyon structure a sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera radiolaria sponges and fish as well as the planktic foraminifera

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

context matters ar ar results from in and around the manicouagan impact structure canada implications for martian meteorite chronology abstract as an analog for interpretations of the ages of martian shergottite meteorites we have conducted an argon isotopic study of plagioclase feldspars exhibiting varying levels of shock from in and around the manicouagan impact structure canada plagioclase from the impact melt sheet at manicouagan yields an age of 215 40 0 16 ma which indicates the time of impact plagioclase from a clast within melt bearing breccias of the melt sheet and a hornfels adjacent to the melt sheet yield ages of 216 3 ma and 218 7 ma respectively which are interpreted to have been reset by contact metamorphism from the impact melt country rocks that were unaffected by the impact gives 849 ma ages consistent with the known grenvillian target rock history maskelynite amorphous plagioclase which has been transformed in the solid state yields an age of 567 6 ma this age is geologically meaningless because it is not consistent with the target age the impact age or regional metamorphic ages at manicouagan our results show that maskelynite argon ages are not meaningful and that context is critical for proper interpretation of impact affected argon ages

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

maskelynite formation via solid state transformation evidence of infrared and x ray anisotropy we present the results of a combined study of shocked labradorite from the lonar crater india using optical microscopy micro raman spectroscopy nuclear magnetic resonance nmr spectroscopy high energy x ray total scattering experiments and micro fourier transform infrared micro ftir spectroscopy we show that maskelynite of shock class 2 is structurally more similar to fused glass than to crystalline plagioclase however there are slight but significant differences preservation of original preimpact igneous zoning anisotropy at infrared wavelengths x ray anisotropy and preservation of some intermediate range order which are all consistent with a solid state transformation from plagioclase to maskelynite

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

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

new triple oxygen isotope data of bulk and separated fractions from snc meteorites evidence for mantle homogeneity of mars we report precise triple oxygen isotope data of bulk materials and separated fractions of several shergotty nakhla chassigny snc meteorites using enhanced laser assisted fluorination technique this study shows that sncs have remarkably identical 17o and a narrow range in 18o values suggesting that these meteorites have assimilated negligibly small surface materials 5 which is undetectable in the oxygen isotope compositions reported here also fractionation factors in coexisting silicate mineral pairs px ol and mask ol further demonstrate isotopic equilibrium at magmatic temperatures we present a mass dependent fractionation line for bulk materials with a slope of 0 526 0 016 1se comparable to the slope obtained in an earlier study 0 526 0 013 franchi et al 1999 we also present a new martian fractionation line for sncs constructed from separated fractions i e pyroxene olivine and maskelynite with a slope of 0 532 0 009 1se the identical fractionation lines run above and parallel to our terrestrial fractionation line with 17o 0 318 0 016 sd for bulk materials and 0 316 0 009 sd for separated fractions the conformity in slopes and 17o between bulk materials and separated fractions confirm oxygen isotope homogeneity in the martian mantle though recent studies suggest that the martian lithosphere may potentially have multiple oxygen isotope reservoirs

thermal infrared spectroscopy and modeling of experimentally shocked plagioclase feldspars thermal infrared emission and reflectance spectra 250 1400 cm 1 7 40 m of experimentally shocked albite and anorthite rich rocks 17 56 gpa demonstrate that plagioclase feldspars exhibit characteristic degradations in spectral features with increasing pressure new measurements of albite ab98 presented here display major spectral absorptions between 1000 1250 cm 1 8 10 m due to si o antisymmetric stretch motions of the silica tetrahedra and weaker absorptions between 350 700 cm 1 14 29 m due to si o si octahedral bending vibrations many of these features persist to higher pressures compared to similar features in measurements of shocked anorthite consistent with previous thermal infrared absorption studies of shocked feldspars a transparency feature at 855 cm 1 11 7 m observed in powdered albite spectra also degrades with increasing pressure similar to the 830 cm 1 12 0 m transparency feature in spectra of powders of shocked anorthite linear deconvolution models demonstrate that combinations of common mineral and glass spectra can replicate the spectra of shocked anorthite relatively well until shock pressures of 20 25 gpa above which model errors increase substantially coincident with the onset of diaplectic glass formation albite deconvolutions exhibit higher errors overall but do not change significantly with pressure likely because certain clay minerals selected by the model exhibit absorption features similar to those in highly shocked albite the implication for deconvolution of thermal infrared spectra of planetary surfaces or laboratory spectra of samples is that the use of highly shocked anorthite spectra in end member libraries could be helpful in identifying highly shocked calcic plagioclase feldspars

pressure induced amorphization in plagioclase feldspars a time resolved powder diffraction study during rapid compression abstract the pressure induced amorphization of the two endmembers of the plagioclase na1 xcax al1 xsi3 xo8 solid solution anorthite caal2si2o8 and albite naalsi3o8 has been studied as a function of compression rate by means of time resolved powder diffraction anorthite and albite were compressed in a diamond anvil cell to 80 gpa at multiple rates from 0 05 gpa s to 80 gpa s the amorphization pressure decreases with increasing compression rate this negative strain rate sensitivity indicates a change in deformation mechanism in the plagioclase solid solution endmembers from brittle to ductile with increasing compression rate the presented data support the previously proposed shear deformation mechanism for the amorphization of plagioclase furthermore amorphization progresses over a wide pressure range suggesting heterogeneous amorphization similar to observations based on recovered material from shock compression experiments of plagioclase our experiments support the contention that amorphization pressures for plagioclase may occur at lower pressures than usually considered

exploring the variability of argon loss in apollo 17 impact melt rock 77135 using high spatial resolution 40 ar 39 ar geochronology 40ar 39ar incremental heating experiments on whole rock lunar samples commonly provide evidence of varying degrees of radiogenic 40ar 40ar loss however these experiments provide limited information about whether or not 40ar is preferentially lost from specific glasses minerals or polyphase domains ultraviolet laser ablation microprobe uvlamp 40ar 39ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40ar 39ar dates potassium rich glass and k feldspar in the mesostasis are the dominant sources for ar released during low temperature steps of published 40ar 39ar release spectra for this rock while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute ar predominantly to intermediate to high temperature steps additionally uvlamp analysis of a mm scale plagioclase clast demonstrates the potential to use stranded 40ar diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly while some submillimeter clasts of plagioclase are distinctly older than the melt other small clasts yield dates younger than the oldest melt components in 77135 plausibly due to subgrain fast diffusion pathways and or 40ar loss during brief episodes of reheating at high temperatures our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the moon s evolution

shock induced amorphous textures in plagioclase manicouagan quebec canada the textural relationships and structural states of optically isotropic labradorite from the manicouagan quebec impact structure have been examined by light optical and transmission electron tem microscopy two distinct diaplectic glasses have been recognized based on their contrasting morphology timing and the inferred modes of formation the earliest isotropic bands and grain scale isotropism maskelynite optically exhibit a gradational in situ transformation from crystalline plagioclase with preservation of relict textures twins grain boundaries the same transformation from crystalline to amorphous structure is observed in tem to occur heterogeneously at scales on the order of the unit cell the progressive transformation of optical properties reflects an increase in the volume fraction and eventual coalescence of these amorphous units this maskelynite type diaplectic glass is interpreted to form in the solid state directly from crystalline material during the compressional phase of the shock wave the other isotropic material occurs in spatially discrete tensiongashes and planar deformation features pdfs that overprint the maskelynite type glass this second type of diaplectic glass pdf type is developed homogeneously within a given glass band and exhibits sharp crystal glass boundaries in contrast to the gradational boundaries of the maskelynite type glass pdf type glass is interpreted to form by melting in tensional release zones during passage of the rarefaction wave these observations emphasize the ability of naturally shocked rocks to preserve subtle evidence of variations in the shock process from highly transient events

integrated sequence stratigraphy of neogene deposits new jersey continental shelf and slope comparison with the exxon model we have evaluated the neogene stratigraphic succession beneath the continental shelf off new jersey using high quality seismic and well data to 1 construct geologic cross sections that relate seismic geometries to lithofacies in a prograding tectonically quiescent setting 2 apply a sequence stratigraphic approach to subdivide the stratigraphic section and relate the characteristics of the deposition observed sequences to the published exxon model and 3 determine the resolution of the available biostratigraphy in the correlation of the offshore new jersey depositional sequences to the published eustatic cycle chart in the offshore new jersey area the well developed progradational geometries in the neogene section the high quality seismic data and the relatively abundant lithologic information from wells allow us to use the results of this study to predict lithofacies distribution in similar but less geologically constrained settings depositional sequences are characterized by similar overall stacking patterns within component systems tracts however the new jersey sequences are different in several ways these differences include the lack of type 2 unconformities an absence of leveed channel deposits within lowstand wedges very poorly developed transgressive systems tracts and sandstone rich high regressive highstand systems tracts these differences are explained in terms of the limited neogene accommodation on the newmore jersey shelf that resulted from slow subsidence rates and a second order miocene eustatic fall paleontologic data bracket the ages of the individual sequences but are not detailed enough at present to correlate precisely to the time scale and inferred global sea level records based on the available biostratigraphy however neogene depositional sequences from offshore new jersey have a cyclicity consistent with interpreted third order eustatic fluctuations on the global cycle chart 18 refs 6 figs 1 tab less

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

origin of lunar very high potassium vhk basalts a combination of endogenous and exogenous processes abstract very high potassium or vhk basalts have been described from the apollo 14 landing site at fra mauro on the lunar near side many of the known samples are clasts in polymict breccias with a few as new work completed on fourteen vhk basalts include crystal size distributions csds and in situ chemical analyses and element maps collected by electron probe microanalysis epma csds completed on three vhk basalt samples indicate that these basalts are not impact melts and represent endogenous melts of the lunar interior potassium k element maps reveal the spatial relationship between the k rich material and the basalt clasts with attached breccia matrix if present in situ epma analyses have identified two distinct types of k rich material k feldspar and k rich glass with these new data an additional mechanism for an exogenic petrogenesis of vhk basalts is proposed that involves the impact process vhk basalts can be divided into two groups based on density and viscosity vhk 1 basalts formed when a hot impact ejecta covered the granite felsite rich matrix material containing high al basalt clasts heat from the impact ejecta partially melted and possibly evaporated nearby k bearing materials which then infiltrated and contaminated the high al clasts vhk 2 basalts are consistent with the hypothesis of granite assimilation by a high al magma as seen in the higher abundances of network forming elements

microscopic lamellar deformation features in quartz discriminative characteristics of shock generated varieties the appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments lamellar deformation features from known shock and or impact environments are sharp parallel closely spaced and tend to extend in a continuous manner across entire grains they have specific well documented orientations and occur in multiple sets per grain lamellar deformation features from other geologic environments including tectonic and explosive volcanic only superficially resemble shock features they are generally less well defined slightly curved wider spaced and tend to extend in a discontinuous manner across parts of grains they have relatively random orientations only some of which correspond to known shock orientations and generally occur as single sets per grain on the basis of these observations we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock these so called planar features differ from microscopic lamellar deformation features produced by other dynamic processes therefore the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven