a tsunami deposit at the cretaceous tertiary boundary in texas at sites near the brazos river texas an iridium anomaly and the paleontologic cretaceous tertiary boundary directly overlie a sandstone bed in which coarse grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple laminated very fine grained sandstone this bed is the only sandstone bed in a sequence of uppermost cretaceous to lowermost paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high the most likely source for such a tsunami at the cretaceous tertiary boundary is a bolidewater impact

probable benthic macroalgae from the ediacara member south australia abstract the ediacara biota records the rise of morphologically complex macroscopic eukaryotes including animals and other heterotrophs eltonian ecology demands that ediacaran ecosystems must have been supported by primary producers e g cyanobacteria phytoplanktonic eukaryotes benthic macroalgae but the fossil record of ediacaran primary producers has not been fully documented particularly with respect to benthic macroalgae hampering a complete understanding of ediacaran ecosystems here we provide systematic description of probable benthic macroalgal fossils from the ediacara member of the rawnsley quartzite at the national heritage site nilpena in south australia these fossils include flabellophyton stupendum sp nov f typicum liulingitaenia irregularis sp nov and longifuniculum cf dissolutum previously informally referred to as bundles of filaments bof they are characterized by bundled filaments that can be twisted loosened or otherwise deformed although many are fragmented some specimens of flabellophyton stupendum sp nov f typicum and possibly longifuniculum cf dissolutum bear structures that resemble variants of pseudorhizostomites and mop but interpreted here as holdfasts suggesting that in life these were epibenthic organisms attached to the substrate atypical populations of relatively small ediacara type fossils are found on a bedding surface dominated by benthic macroalgae suggesting that together with microbial mats these primary producers were beneficial for juvenile settlement of ediacara type organisms potentially contributing both oxygen and nutrients although debates about animal affinity dominate ediacaran paleontology this study highlights the importance of benthic macroalgae in ediacaran ecosystems

environmental interpretation and a sequence stratigraphic framework for the terminal proterozoic ediacara member within the rawnsley quartzite south australia abstract fossils of the ediacara biota are confined to preservational windows in the ediacara member of the rawnsley quartzite pound subgroup from the flinders ranges south australia the base of the ediacara member is a type 1 sequence boundary incised into the partly lithified chace quartzite member of the rawnsley quartzite the ediacara member and the upper half of the rawnsley quartzite comprise the rawnsley depositional sequence bounded above by the early cambrian uratanna depositional sequence the rawnsley sequence developed over an erosional surface cut into the chace quartzite member with some 250 m of relief southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone at lowstand local channels were filled by massive sand the highstand systems tract consists of several parasequence sets of laminated siltstone and channelized amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well bedded clean sandstone impressions of soft bodied ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward shoaling delta front environments between storm and fairweather wavebase the ediacara member is in places conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member distinctive microbial mat bound sand laminae described as petee structures are characteristic of the rawnsley quartzite above and below the ediacara member during deposition of the ediacara member colonization of the deeper substrates by microbial mat communities enabled preservation of the ediacara biota the erosional surface at the base of the ediacara member is one of at least four incision events recorded in the terminal proterozoic and early cambrian succession of the adelaide geosyncline the similarity of incision filling lithofacies of the rawnsley and uratanna sequences has in condensed sections led to confusion between the depositional sequences characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the proterozoic cambrian boundary

the global stratigraphy of the cretaceous tertiary boundary impact ejecta the chicxulub crater ejecta stratigraphy is reviewed in the context of the stratigraphy of underlying and overlying rock sequences the ejecta sequence is regionally grouped in a thick polymict and monomict breccia sequences inside the crater and within 300 km from the rim of the crater known from drill holes in and close to the breater and exposures near the border of yucatan and belize b gulf of mexico region 2500 m from the crater with up to 9 m thick complex tsunami wave influenced tektite bearing sequences in shallow marine 500 m deep environments and tektite bearing decimeter thick gravity flow deposits in deep water sites c an intermediate region between 2500 and 4000 km from the crater where centimeter thick tektite bearing layers occur and d a global distal region with a millimeter thin ejecta layer the distal ejecta layer is characterized by sub millimeter sized microkrystites often rich in ni rich spinels and altered clinopyroxene wherever present the ejecta layers mark exactly the sudden mass mortality horizon of the k t boundary what exactly caused the mass mortality is still uncertain but it appears the main event leading to the k t mass extinctions

asteroid impact ejecta units overlain by iron rich sediments in 3 5 2 4 ga terrains pilbara and kaapvaal cratons accidental or cause effect relationships abstract the significance of temporal and spatial associations between asteroid comet impact ejecta units and overlying iron rich sediments including banded iron formation bif jaspilite and ferruginous shale observed in the pilbara craton western australia and the barberton greenstone belt bgb kaapvaal craton south africa is considered such associations include 1 3470 1 1 9 ma impact spherule units and tsunami type breccia overlain by jaspilite in the antarctic chert member central pilbara craton 2 3258 3 ma impact spherule unit s2 in the bgb overlain by bif jaspilite and ferruginous shale 3 3243 4 ma impact unit s3 in the bgb overlain by iron rich sediments ulundi formation 4 two formations of bif and ferruginous shale at the base nimmingarra iron formation and lower part paddy market formation of the gorge creek group central pilbara craton which overlie 3235 3 ma felsic volcanics along a boundary correlated with the bgb s3 unit 5 2629 5 ma impact spherule unit and tsunami type deposit overlain by bif and ferruginous shale marra mamba iron formation central pilbara craton 5 2481 4 ma spherule unit intercalated at the lower part of the dales gorge iron member of the brockman iron formation hamersley basin and possible equivalents in the kuruman formation western transvaal no significant thicknesses of iron rich sediments are known to overlie carbonate hosted impact ejecta units including the bee gorge member bgm of the wittenoom formation 2561 8 ma carawine dolomite eastern hamersley basin pilbara craton monteville formation and reivilo formation griqualand west basin transvaal group or the graensco vallen spherule occurrence southwest greenland the juxtaposition between impact ejecta units and overlying bif and jaspilite may be accidental or purely related to preservation of these units in below wave base environments alternatively this association may hint at enrichment of seawater in soluble iron under low oxygen atmosphere hydrosphere conditions due to enhanced denudation of mafic volcanic terrains uplifted exposed through impact induced tectonic movements and or impact triggered mafic volcanic and hydrothermal activity the general absence of iron rich sediments above carbonate hosted impact ejecta units may be attributed to lack of fe enrichment or to ph and eh conditions unsuitable for the precipitation of iron oxides the near absence of bif in high energy shallow water environments overlying impact ejecta units may reflect retardation of colloidal chemical sedimentation in such environments a systematic association between impact ejecta and iron rich sediments if confirmed may yield useful stratigraphic tracers in the search for impact signatures in early precambrian terrains

microtektite like impact glass associated with the frasnian famennian boundary mass extinction abstract glass spherules of probable impact origin occur in the late devonian hony section in belgium they are found at the stratigraphic level of the frasnian famennian boundary which is marked by one of the largest mass extinctions in the phanerozoic the hony glass spherules are chemically identical to those previously described from the stratigraphically equivalent senzeille section located 90 km to the west southwest in contrast to the senzeille layer the age of the hony spherule layer is well constrained by conodont biostratigraphy the spherules have morphological and chemical characteristics similar to tektites and microtektites they have a broad range of chemical composition with generally high k2o na2o and al2o3 na2o k2o their water content is in the range measured for tektites and impact glasses small inclusions of lechatelierite may be present in the spherules these glass spherules are likely to be the product of an impact event that occurred at or very near the frasnian famennian boundary the target rock was probably a clay rich pelitic sediment intermixed with carbonates the 368 1 ma siljan ring crater sweden is the most likely impact site candidate the hony and senzeille glass spherules represent the first substantial evidence that an impact event may be linked to the frasnian famennian mass extinction

smothering of microbial mats by macrobiota implications for the ediacara biota abstract recent advances in understanding the preservation of the soft bodied neoproterozoic ediacara biota have centred upon the role of microbes in casting the morphologies of static or dead macrobiota on the precambrian sea floor this experimental study explores the preservational effects brought about by the passive movement of dead macro organisms upon a pre established marine microbial mat under laboratory conditions this demonstrates the potential for making multiple impressions of body tissues on the surface of microbial mats by means of passive transport alone this throws doubt on the evidence for active feeding in ediacaran fossils such as dickinsonia and yorgia supplementary material methods are available at http www geolsoc org uk sup18375

textured organic surfaces associated with the ediacara biota in south australia abstract the ediacaran period takes its name from the fossils of the ediacara biota which represent the first appearance of large and diverse assemblages of organisms in the fossil record although the global record of these distinctive body fossils is now well known a previously unrecognized megascopic organic record of textured organic surfaces tos occurs in the ediacara biota however tos is also a feature over a wider range of paleoenvironmental settings where body fossils are unknown in ediacaran siliciclastic successions that have been studied in australia namibia and western north america paleoecological analysis of successive bedding planes of strata from the late ediacaran rawnsley quartzite in the flinders ranges of south australia reveals that tos represent the most common organic features in bedding surface assemblages of the ediacara biota the tos consist of preserved patterned assemblages of textured organic mats fibers and simple tubular body fossils complex ediacara body fossils while striking for their distinctive body plans and dominating some of the beds are relatively minor components of combined overall surface area many elements of tos have previously been miss diagnosed as trace fossils which are in practice limited to two or at most three morphotypes that indicate the presence of bilateria although tos represent a simpler grade of organismic construction than discrete and more complex ediacara body fossils they were preserved in a similar manner marked variability in all components of the biota between successive surfaces suggests that ediacara ecologies fluctuated at short intervals despite an apparently consistent sedimentary regime

spinel from archean impact spherules abstract large meteorite impacts are recorded in distal fall deposits preserved in two early archean greenstone belt sequences 3 2 3 5 ga mapping stratigraphie and geochemistry studies have been used to identify at least four impact layers in the barberton greenstone belt of southern africa and one in the pilbara greenstone belt of western australia spherules that represent impact melt droplets distinguish these impact layers from associated tuffs spinel with unusual compositional traits occurs within spherules in two of these impact layers spinel is the only preserved primary phase and is found within up to 10 of the spherules in the best preserved layers some spherules contain up to 40 spinel which is often the only primary phase that crystallized within the spherules spinel morphologies include dendrites and skeletal octahedra typical of those found in high temperature melts spinel compositions are similar to those of cenozoic impacts including very high ni and ferric iron contents but the archean spinel is distinctive in being composed dominantly of the chalcophile elements fe ni cr and v this spinel is not metamorphic or metasomatic in origin closely associated komatiites in the underlying onverwacht group both fresh and highly altered have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel characteristics of spinel preserved in these archean impact spherules suggest a rather complex series of steps following a large bolide impact initial high temperature reduction produced melt droplets some of which were immiscible silicate metal sulfide density differences allowed separation of some of these immiscible melts which subsequently evolved independently and formed spherules with highly variable chalcophile element contents oxidation during atmospheric reentry increased ni fe and fe 3 fe 2 in the chalcophile siderophile melts archean impact spinel has higher ni fe than cenozoic impact spinel though distinctly lower fe 3 fe 2 which probably reflects a substantially less oxidizing atmosphere during the archean the archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric h 2 o h 2 bolide size is estimated to be 24 km in diameter based on the average diameter of 0 85 mm for the spinel bearing spherules this estimate is consistent with the observed ir fluence in spinel bearing impact layers of approximately 5 3 mg cm2 which would be produced by a 30 km diameter chondritic bolide

bioturbation in burgess shale type lagerstatten case study of trace fossil body fossil association from the kaili biota cambrian series 3 guizhou china abstract cruziana gordia planolites rusophycus and trichophycus are common ichnological elements of the kaili biota new discoveries based on the examination of 323 specimens include eldoniids echinoderms trilobites monoplacophorans and non biomineralizing arthropods that are associated with trace fossils based on the observed effects of bioturbation on the preservation of five different animal groups it is clear that infaunal scavengers deposit feeders were periodically active on the kaili sea floor and were able to reach historic layers yielding exceptionally preserved fossils in general the average level of infaunal activity is absent to moderate ichnofabric index i i 1 to 3 in the kaili substrate by contrast the phycodes beds are completely disturbed by infaunal activity i i 5 observed burrow diameter ranges from diminutive 0 2 mm to normal up to 4 2 mm computed tomography allows us to visualize the precise geometry of the trace fossil body fossil association in three dimensions we concluded that although some kaili infaunal animals could bore through the biomineralized echinoderms they did not appear to have scavenged upon these echinoderms based on our three dimensional reconstruction furthermore kaili burrowers can reach the historic layers containing exceptionally preserved fossils without altering soft tissue preservation on the other hand we used conventional techniques to reveal that one burrow is filled with fecal pellets 100 200 m at its terminal end because most burrows are filled with yellow brown coarse sediment and because there is no compositional difference between the fecal pellets and the surrounding coarse sediment within the burrow the origin of the yellow brown coarse sediment is interpreted here as parautochthonous and or autochthonous and as a result of sediment reworking by deposit or suspension feeders the relative scarcity of soft part preservation in the kaili biota compared to the older chengjiang biota and the younger burgess shale biota may be the result of post burial bio disturbance due to the relatively high intensity of local bioturbation it is plausible that oxygenation in deeper sediments facilitated by bioturbation allowed carcasses in the kaili biota to undergo a more extensive period of normal decay prior to final burial than in other cambrian konservat lagerstatten

geological evidence for a strewn field of impact spherules in the early precambrian hamersley basin of western australia predominantly sand sized spherules of k feldspar with spherulitic vesicular and other crystalline textures help to define a unique marker bed in the 2 6 ga wittenoom formation which persists for more than 300 km across the hamersley basin of western australia these spherules usually occur as a minor constituent of a single turbidite but locally they are restricted to centimeter thick lenses of pure spherules along a single horizon in the argillite close beneath it in the shallower northeastern part of the basin similar spherules as well as larger more heterogeneous particles likewise occur at only one horizon but here they are a minor constituent of a thick dolomitic debris flow deposit as the wittenoom formation and the carawine dolomite are thought to be stratigraphically equivalent this debris flow deposit is believed to be a proximal equivalent of the spherule bearing turbidite on the basis of their similarity to microtektites and microkrystites in shape size and mode of occurrence the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the hamersley basin by a major bolide impact moreover the mass of the spherules preserved in the hamersley basin is of the same order of magnitude as the estimated masses of microtektite glass in major cenozoic strewn fields despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller the layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial the flow however is not believed to have been a direct result of the proposed impact if the spherule bearing layers constitute a chronostratigraphic marker as suggested here there are important implications for the deposition of carbonates and banded iron formations bif within the hamersley basin similar spherules also occur at a single horizon 250 m higher stratigraphically in the brockman iron formation suggesting that two impacts occurred near the hamersley basin in relatively rapid succession correlative strata may be present in the transvaal basin of south africa

complex trace fossils from the terminal proterozoic of namibia we document for the first time the occurrence of the complex trace fossil treptichnus from the terminal proterozoic nama group namibia these traces differ in detail from the cambrian index fossil treptichnus pedum but document precursors with the same basic behavior the producer of the trace is not known in its complexity however this trace fossil provides strong evidence for advanced latest proterozoic bilaterians

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

fragmentation and hydration of tektites and microtektites abstract an examination of data collected over the last 30 years indicates that the percent of glass fragments vs whole splash forms in the cenozoic microtektite strewn fields increases towards the source crater or source region we propose that this is due to thermal stress produced when tektites and larger microtektites fall into water near the source crater while still relatively hot 1150 c we also find evidence low major oxide totals frothing when melted for hydration of most of the north american tektite fragments and microtektites found in marine sediments high temperature mass spectrometry indicates that these tektite fragments and microtektites contain up to 3 8 wt h2o the h2o release behavior during the high temperature mass spectrometric analysis plus high ci abundances 0 05 wt indicate that the north american tektite fragments and microtektites were hydrated in the marine environment i e the h2o was not trapped solely on quenching from a melt the younger ivory coast and australasian microtektites do not exhibit much evidence of hydration at least not in excess of 0 5 wt h2o this suggests that the degree of hydration increases with age in addition we find that some glass spherules with 65 wt sio2 can undergo simple hydration in the marine environment while impact glasses with 65 wt sio2 can also undergo palagonitization

evolutionary relationships within the avalonian ediacara biota new insights from laser analysis abstract we report new high resolution laser scanning of the type material for the earliest complex ediacaran genera charnia bradgatia charniodiscus and ivesheadia from charnwood uk and compare these with beothukis mistakensis gen et sp nov and the recently described taxa charnia wardi charnia antecedens and fractofusus spp from broadly coeval strata in newfoundland we use the laser and other techniques to map the similarities and differences in morphology between these ediacaran rangeomorphs key features are suggested to include the number of growth axes the number and placement of growth tips the presence of radiating or subparallel axes for the first and higher order branches the extent of displayed or undisplayed leaf like rangeomorph9 architecture and the extent of furling of the margins of these leaf like elements these features are then used to propose suggested homologies between these taxa leading to a preliminary phylogenetic hypothesis for the evolution of the avalonian ediacara biota

neonereites uniserialis from c 600 ma year old rocks in western scotland and the emergence of animals a chain of pellets neonereites uniserialis usually attributed to a coelomate or pseudocoelomate metazoan is described from the middle dalradian bonahaven formation of western scotland in sandstones not far above the port askaig tillite this trail lies more than 8 km below u pb zircon dated tayvallich lavas 595 4 ma and predates early deformation plus intrusion by granites at ben vuirich dated at 597 11 and 590 2 ma u pb zircon chronology for early metazoan evolution suggests that this is the earliest putative evidence for coelomates or pseudocoelomate discovered so far with a minimum interglacial age of c 600 ma bp it is from about this time that a fall takes place in the maximum 13 c of carbonates suggestive of a decline in the long term rate of carbon burial one explanation for this decline is the increasing impact of metazoan bioturbation through the late neoproterozoic to cambrian interval

have distal impact ejecta changed through geologic time sand size spherules of former silicate melt have been identified in 10 thin early archean to paleoproterozoic layers that appear to be reworked ejecta from large impacts melt spherules are common in phanerozoic layers of distal impact ejecta but the aggregate thickness of spherules in early precambrian layers is generally 10 to 100 times greater and they display textures rarely seen in phanerozoic spherules these contrasts suggest secular variations in the nature of both impactors and target materials such as a progressive decrease in the size of impactors and a progressive increase in the fraction of earth s surface covered by continental crust

paleoecology of the enigmatic tribrachidium new data from the ediacaran of south australia abstract tribrachidium is a monospecific genus of the ediacara biota found globally in the ediacara member of the rawnsley quartzite flinders ranges of south australia the spatial distribution of tribrachidium across the seafloor is best described as patchy although tribrachidium is the dominant fossil on two of the twenty six beds currently excavated and is present in large numbers on another the genus most commonly occurs as no more than a handful of specimens on a bed tribrachidium size frequency distributions of each of the three beds with more than 5 specimens are all statistically distinguishable from one another additionally the size range on any given bed is smaller than the overall size range observed for the genus these patterns suggest that these organisms lived in populations composed of single generations the beds with numerous tribrachidium come from different facies and are characterized by the presence of dissimilar mixes of taxa and textured organic surfaces indicating that tribrachidium was a generalist able to adapt to a variety environments uniquely the base or internal structure of tribrachidium is also found preserved in both positive and negative relief as a sequence of concentric ridges on beds where tribrachidium is the dominant genus the most parsimonious explanation for the presence of these concentric ridge fossils is that they are the fossilized form of a tribrachidium preserved when the organism was buried upside down flipped over or dead and partially decayed prior to burial

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

comment on extraterrestrial influences on mantle plume activity by d h abbott and a e isley earth planet sci lett 205 2002 53 62

piecing together the puzzle of the ediacara biota excavation and reconstruction at the ediacara national heritage site nilpena south australia abstract patterns of origination and evolution of early complex life on this planet are interpreted largely from the fossils of the precambrian soft bodied ediacara biota excavation and reconstruction of beds of the ediacara member of the rawnsley quartzite at the national heritage ediacara fossil site nilpena in the flinders ranges of south australia has exposed nearly 300 square meters of fossiliferous bedding planes as a result the taphonomy and sedimentology of the succession are well constrained rendering it possible to disentangle ecological from environmental and taphonomic signals the fossil record that is preserved on these beds is a function of a number of factors including 1 sedimentary environment 2 initial colonization 3 nature of textured organic surfaces 4 duration of time between episodes of disturbance and 5 biostratinomic processes the excavation and reconstruction of beds at nilpena yield an exceptional and unique opportunity to examine not only the taxonomic composition of ediacara communities but also their ecological character at various stages of development preserved ecological snapshots of fossil assemblages range from immature communities of small bodied individuals associated with poorly developed organic mats to communities characterized by a high diversity of macrofaunal taxa wide range of body sizes and the presence of dense textured organic surfaces

noble metal abundances in an early archean impact deposit we report detailed analyses on the concentrations of the noble metals pd os ir pt and au in an early archean spherule bed s4 of probable impact origin from the lower fig tree group barberton greenstone belt south africa compared to other sedimentary deposits of known or suspected impact origin some noble metals are present in exceptionally high concentrations noble metal abundances are fractionated relative to abundances in chondrites with ratios of osir ptir pdir and auir at only 80 80 41 and 2 of these values in ci chondrites although an extraterrestrial source is favored for the noble metal enrichment the most plausible cause of the fractionation is by regional hydrothermal metasomatic alteration

geological and geochemical record of 3400 million year old terrestrial meteorite impacts beds of sand sized spherules in the 3400 million year old fig tree group barberton greenstone belt south africa formed by the fall of quenched liquid silicate droplets into a range of shallow to deep water depositional environments the regional extent of the layers their compositional complexity and lack of included volcanic debris suggest that they are not products of volcanic activity the layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt komatiite and ci carbonaceous chondrite the spherules are thought to be the products of large meteorite impacts on the archean earth

clavate borings in a miocene cetacean skeleton from tarragona ne spain and the fossil record of marine bone bioerosion abstract clavate borings found in the tympanic bulla of a miocene cetacean from el camp de tarragona basin constitute the first evidence of the ichnogenus gastrochaenolites in bones of an autochthonous cetacean carcass previous records of similar trace fossils on marine bones were described from transported or reworked remains based on their morphology and ichnotaxonomy the borings are assigned to the activity of pholadid bivalves which would have colonized the skeletal carcass on the sea floor after removal of covering soft tissues sedimentological and paleontological data indicate a low energy depositional setting with low sedimentation rate which would have provided the temporal window for bivalve colonization this new finding contributes to widen our knowledge of bioerosion in marine vertebrate skeletons

vendobionta and psammocorallia lost constructions of precambrian evolution the non availability of biomineralized skeletons and low levels of predation led vendian evolution along strange avenues the ediacara type vendobionta appear to represent a kingdom in which foliate shapes large sizes and the necessary compartmentalization were achieved by quilting of the skin rather than by multicellularity psammocorallia in contrast are interpreted as coelenterates that constructed an internal sand skeleton both were immobile soft bottom dwellers that had high population densities and both became preserved by obrutional accidents thus they render fossil snap shots in which the original distributional patterns age structures and standing biomasses of populations are accurately recorded

miocene clypeaster from valencia e spain insights into the taphonomy and ichnology of bioeroded echinoids using x ray micro tomography abstract taphonomic analysis and x ray micro tomography ct were applied to clypeaster specimens from the miocene of fuente del jarro valencia spain this enabled the identification of various preservational features including encrustation bioerosion abrasion and post depositional cracking approximately 15 of the studied specimens were affected by encrusters and or borers of particular interest was a single intensely bored specimen preserving numerous cross cutting bioerosion structures these structures exhibit a distinctive morphology consisting of clavate or flask shaped chambers with a circular to oval cross section a narrow neck region near the figure of eight shaped aperture and a chimney extending outside the clypeaster test the excellent preservation and characteristic morphology as revealed by the ct scan coupled with the observation that in some cases articulated bivalve shells are still present within the chambers allows for the identification of the tracemaker as the boring bivalve rocellaria reaffirming these tube dwelling animals as borers burrowers and crypt builders the trace fossils described herein are semi endoskeletal dwellings representing a combination of bioerosion through the plates of the echinoids bioturbation in the form of burrowing into the sediment infill of the clypeaster specimen and carbonate secretion resulting in a crypt the morphologies of the different dwellings are influenced by the limited available space within the echinoid tests this study confirms the importance of the relatively stable and thick clypeaster shells as benthic islands in cenozoic sandy littoral settings the high degree of multiple colonizations as well as the uniform orientation of the burrows suggests a long term stable position of this secondary substrate at or near the sediment surface

iridium anomaly but no shocked quartz from late archean microkrystite layer oceanic impact ejecta samples from a ca 2 54 ga layer rich in microkrystites sand size spherules of former silicate melt in the hamersley group western australia are enriched in ir up to 50 fold over associated carbonates and shales but few of the other siderophile elements display significant anomalies or chondritic interelement ratios however similar concentrations and interelement ratios are observed in ejecta from the ca 590 ma acraman impact structure and have been attributed to diagenetic redistribution the microkrystite layer also contains sand size quartz grains with internal textures typical of regionally metamorphosed rocks rather than evidence of shock metamorphism we suggest that the microkrystites were created by an impact that took place in a deep ocean basin rather than on a continent and that the associated quartz is epiclastic detritus brought in by unusually high energy waves and or currents not as ballistic ejecta

comment on dickinsonia liftoff evidence of current derived morphologies by s d evans m l droser and j g gehling palaeogeography palaeoclimatology palaeoecology 434 28 33 2015 abstract dickinsonia is an enigmatic fossil of ediacaran age variously interpreted as a lichen foraminiferan placozoan cnidarian or worm and now evans et al 2015 have provided documentation of new and informative specimens from nilpena in the flinders ranges of south australia they consider missing marginal slivers evidence for soft bodied free living dickinsonia lifted by current but such marginal dismemberment of dickinsonia by currents requires attachment of the rest of the body to the substrate more firmly than body cohesion unidirectional orientation of missing pieces of dickinsonia was considered evidence of shallow marine storms but such directed currents are found in floods or tsunamis finally curved conchoidal fracture of the missing slivers reveals that the unskeletonized carapace of dickinsonia was not always soft and flexible but sometimes hardened perhaps by desiccation or freezing

new evidence for a large palaeoproterozoic impact spherules in a dolomite layer in the ketilidian orogen south greenland an unconformable sedimentary succession deposited between c 2130 1848 ma on archaean gneisses of the foreland of the palaeoproterozoic ketilidian orogen includes a layer with coarse sand sized silicate spherules the layer is c 1 m thick and consists mainly of coarse diagenetic dolomite in addition to c 18 spherules the layer also contains 3 well sorted very fine quartzose sand and 6 larger intraclasts of chert and carbonate the spherules were previously interpreted as microfossils vallenia sp because of their spheroidal shapes and inclusions of carbonaceous matter the spherules are reinterpreted as replaced impact ejecta because they have shapes typical of splash form microtektites some contain possible examples of replaced skeletal spinel crystals perlitic cracks and devitrification spherulites and non spherical particles with shapes and textures of typical glassy and scoriaceous volcanic ash are absent the carbonaceous matter is attributed to hydrocarbons that migrated into the spherule layer from elsewhere in the sedimentary succession the spherules were reworked after deposition probably as a result of turbidity currents or storm or impact induced waves analysis of one spherule bearing sample revealed only 0 02 ppb iridium a value comparable with low iridium abundances in distal layers of other terrestrial impact ejecta the spherules in south greenland are the first distal impact ejecta recognized in mid precambrian strata they represent a major impact because their aggregate thickness exceeds the thickest spherule accumulations reported from the cretaceous tertiary boundary layer given their loosely constrained age and the implied scale of the impact the ketilidian spherules could be distal ejecta from either the vredefort south africa c 2025 ma or sudbury canada c 1850 ma impacts

nature and origin of carbon in the archaean witwatersrand basin south africa the nature and origin of the carbonaceous matter carbon in the gold bearing reefs of the archaean witwatersrand basin of south africa has long been a subject of controversy and debate the importance of the carbon results from its direct association with the gold grade the studies reported here were directed at resolving the nature and origin of carbon techniques used include stope scale macroscopic mapping mesoscopic hand specimen examination and microscopic studies incorporating the use of optical reflectance measurement macroscopic mapping has demonstrated the braided habit of carbon bodies over tens of square metres with a close parallel to sedimentology mesoscopic observations however show the vein like carbon seams are often cross cutting sedimentary features with the carbon occupying an anastomosing set of bed parallel fractures microscopic observations show a globular form for the carbon and inclusions of uraninite particles in seam carbon the globules are generally modified into elongated spindles oriented perpendicular to bedding optical properties are dominated by generally high reflectance and anisotropy with a swirling liquid crystal structure classifying seam carbon as carbonaceous mesophase it is this fine mesophase texture that has previously been wrongly attributed to plant algal morphology highly significantly the optical anisotropy is bed perpendicular rather than bed parallel as in normal sedimentary burial indicating mesophase formation during fracture opening both reflectance and bireflectance of the seam mesophase show a range of values at all scales no stratigraphic or depth trends were apparent though geographic variation points to localization of the maximum temperature event s given high anisotropies maximum reflectance values in the 4 8 r o range and minima in the 1 2 r o range give arithmetic mean reflectance values of about 4 0 r o kinetic modelling equates these values with short term 50 000 years temperature events of c 400 c or burial temperatures of c 280 c lower reflectance values of c 2 0 r o obtained from the shales suggests thermal detachment of permeable and impermeable beds a characteristic of pulsed hydrothermal flux the included uraninite grains influence the orientation of the surrounding anisotropic mesophase demonstrating that uraninite predated the mesophase formation event regional palaeo stress frozen into the mesophase anisotropy indicates a nw se deviatoric stress the combination of hydrocarbon radiation and hydrothermal fluid flow marks witwatersrand basin pyrobitumens out as being different from conventional oil putative source rocks for the oil are identified in the intra basinal witwatersrand shale units lithological control in carbon seams is explained by the poro perm and mechanical properties of the unconformity sequence of footwall reef conglomerates and hanging wall quartzites favouring fracture along the unconformity surfaces an overpressured oxidizing fluid created the fracture network that hosted the oil and also introduced uranium as uranyl ion on meeting a reducing agent oil uraninite precipitated stabilizing the oil by polymerization further hotter hydrothermal flux then coked the bitumen to often fibrous spindle shaped mesophase continued flow deposited gold in the available pore spaces most notably those formed by lateral contraction between mesophase spindles the speculative role of methane in the reductive precipitation of gold is indicated by the presence of gold in any co eval porosity largely independent of the mineral matrix

ejecta layer at the cretaceous tertiary boundary bass river new jersey ocean drilling program leg 174ax a continuously cored borehole drilled at bass river new jersey recovered a cretaceous tertiary k t succession with a 6 cm thick spherule layer immediately above the boundary below the spherule layer the cretaceous glauconitic clay is extensively burrowed and contains the uppermost maastrichtian micula prinsii calcareous nannofossil zone spherical impressions of spherules at the top of the cretaceous indicate nearly instantaneous deposition of ejecta from the chicxulub impact the thickest ejecta layer shows clearly that a single impact occurred precisely at k t boundary time above the spherule layer the glauconitic clay contains the planktonic foraminiferal p0 and p alpha zones indicating 1 a complete k t succession and 2 continuous deposition interrupted only by fallout of the ejecta layer clay clasts within a 6 cm interval above the spherule layer contain cretaceous microfossils and may be rip up clasts from a tsunami or possibly a megastorm event extinction of the cretaceous planktonic foraminifers and burrowing organisms occurs abruptly at the k t boundary thus the bass river k t succession unequivocally links the chicxulub bolide impact to the mass extinctions at the end of the mesozoic

first non destructive internal imaging of rangea an icon of complex ediacaran life the origins of multicellular life have remained enigmatic due to the paucity of high quality three dimensionally preserved fossils rangea was a centimetre to decimetre scale frond characterised by a repetitive pattern of self similar branches and a sessile benthic lifestyle fossils are typically preserved as moulds and casts exposing only a leafy petalodium and the rarity and incompleteness of specimens has made it difficult to reconstruct the three dimensional 3d morphology of the entire organism this in turn has led to many differing interpretations of its morphology and phylogenetic affinities here we use high resolution x ray micro computed tomography microct to investigate the 3d internal morphology of rare exceptionally preserved ironstone fossils of rangea from the nama group in southern namibia our investigation reveals a series of structures that represent boundaries between individual fronds or structural elements that divide into smaller secondary and tertiary elements leading to a repetitive pattern of branches these elements surround an internal core of a distinctly different texture and internal appearance there is no distortion of the walls of the primary elements thus we conclude that rangea likely had a rigid or semi rigid skeleton like structure that prevented buckling or compression and maintained integrity during life we compare these findings with previous interpretations of rangea morphology and present new insights on the architecture of internal structures such as the central core and the overall appearance of this complex ediacaran life form our insights based on microct scans of these rare uniquely preserved specimens provide a more accurate interpretation of the 3d morphology essential for determining the true affinities and modes of life of the ediacaran biota during this early stage in the evolution of complex macroscopic life

extraterrestrial ni rich spinel in upper eocene sediments from massignano italy evidence of a major cosmic event at the end of the eocene is given by the finding of at least one possibly two or more impact horizons containing microtektites microkrystites shocked quartz and unusually high iridium concentrations we report here the discovery in the global stratotype of the eocene oligocene boundary at massignano in italy of ni rich spinel a mineral that forms by the interaction of meteoritic bodies with the earth s atmosphere a maximum concentration of 800 crystals mg 1 corresponding to a flux of 2 107 crystals cm 2 is observed in a well dated layer 35 7 0 4 ma that also contains shocked quartz and iridium the crystals are found clustered in flattened spheroids the probable relic of their host bodies the chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects as proposed for the spinel bearing spheroids found at the cretaceous tertiary boundary the most likely explanation is that the dust particles were generated by a large cometary impact whether this impact is the one that produced microtektites and or microkrystites or another one that may have occurred shortly after or before is still questionable additional searching for ni rich spinel at other upper eocene sections may help to answer this question

changing biodiversity of mediterranean irregular echinoids from the messinian to the present day morphological and palaeoecological changes in irregular echinoid diversity during the late miocene messinian stage have been analysed at the scale of the sorbas basin of southeast spain biometrical and statistical analyses established the validity of species differentiation an example is given for messinian pliocene and recent schizaster species s saheliensis and s canaliferus palaeoecological analyses show that irregular echinoid faunas record environmental changes especially sea level variations with contrasting ecomorphological gradients for deepening and shallowing trends deepening is indicated by the stratigraphic succession of clypeaster altus associated clypeaster marginatus spatangus schizaster and then brissopsis species whereas shallowing is indicated by the opposite succession the survival of brissopsis typically a stenohaline and relatively deep marine organism in the marls interbedded within the uppermost messinian gypsum of the sorbas basin testifies to the existence of marine connexions between this basin and other mediterranean marine areas during the end of the evaporitic episode tabulation of irregular echinoid species diversity at eight time intervals between the tortonian messinian boundary and the present day inclusive for the mediterranean basin as a whole indicates a progressive increase in diversity from the latest tortonian to the upper part of the pre evaporitic messinian before a fall in diversity during the salinity crisis however survivals after the crisis are very numerous representing 78 of the maximum pre evaporitic value

regional variations in spinel compositions an important key to the cretaceous tertiary event spinel found in spherules from cretaceous tertiary boundary sediments exhibits a wide range in composition and is distinguished from typical igneous spinel by high mg al and ni relatively low ti and cr and high fe2o3 feo solid solution compositions range from nearly pure magnetite to magnesioferrite to relatively pure mgal2o4 spinel the ni spinel trevorite is also a common component a regional variation in composition is also observed spinel from two north pacific sites has higher mg and al and lower ni and fe than spinel from two european sites and one south atlantic site the most probable source of the spinel is crystallization from molten silicate droplets produced from a major impact event the regional variations may provide a key to locating the impact site s

chromium isotope evidence in ejecta deposits for the nature of paleoproterozoic impactors abstract non mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra terrestrial materials in terrestrial impact rocks paleoproterozoic spherule layers from greenland graenseso and russia zaonega as well as some distal ejecta deposits lake superior region from the sudbury impact 1849 0 3 ma event have been analyzed for their cr isotope compositions our results suggest that 1 these distal ejecta deposits are all of impact origin 2 the graenseso and zaonega spherule layers contain a distinct carbonaceous chondrite component and are possibly related to the same impact event which could be vredefort 2023 4 ma or another not yet identified large impact event from that of similar age and 3 the sudbury ejecta record a complex meteoritic signature which is different from the graenseso and zaonega spherule layers and could indicate the impact of a heterogeneous chondritic body

early archean silicate spherules of probable impact origin south africa and western australia layers of sand sized silicate spherules in 3 2 to 3 5 ga archean greenstone belts in south africa and western australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences the layers and particles appear to be unrelated to volcanic activity and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts if so these spherules are the oldest known terrestrial impact products if these objects are representative of a larger population of early archean impact features it may be possible to directly estimate the meteorite flux on the early archean earth

sedimentology and carbon sulphur geochemistry of the velkerri formation a mid proterozoic potential oil source in northern australia abstract the velkerri formation is a flat lying 400 m thick shale unit of mid proterozoic age that occurs in the mcarthur basin of northern australia it underlies an area approximately equivalent to that of the netherlands and contains intervals tens of metres thick highly rich in organic matter toc values are commonly 1 to 3 but exceed 5 at three levels the maximum value measured was 8 7 toc the region has not been buried deeper than about 2500 m and the organic matter is mature two of these intervals represent impressive oil source beds jackson et al reported the discovery of live oil from the lower interval thereby raising considerable interest in the petroleum potential of the sequence initial reconnaissance geochemical studies of material from one drillhole indicated deposition in a large lake or barred bay donnelly and crick detailed facies analysis however indicates deposition in a variety of sub environments on an epicratonic marine shelf in most of these sub environments the depositional interface was beyond the effects of normal wave and tidal current activity the stratigraphic relationships between the velkerri formation and associated wave and tide deposited shoreface sandstones of the bessie creek and moroak sandstones and the widespread presence of glauconite throughout the velkerri formation are incompatible with the lacustrine barred bay setting suggested by donnelly and crick the results of a subsequent systematic geochemical sampling program of a second drillhole presented here provide some support for a marine origin although c s plots berner and raiswell have been successful in differentiating between marine and freshwater environments in phanerozoic rocks their use in the proterozoic should be approached with caution and preferably only within the constraints of a full consideration of depositional and diagenetic influences on c s fe systematics degree of pyritiation of iron dop raiswell et al and c fe s plots aller et al as used in this study are additional tools suitable for classifying the depositional environment due to the limited regional information it is difficult to classify the velkerri formation into one of the published models for anoxic basin sediments it shows some similarities to several european mesozoic oil prone beds which developed during major marine transgressions e g hallam

echinoid assemblages as a tool for palaeoenvironmental reconstruction an example from the early miocene of egypt abstract a rich lower miocene echinoid fauna has been investigated from gebel gharra nw of suez egypt the ca 140 m long section consists of a siliciclastic lower part and a carbonate dominated upper part this corresponds to a general transgression regression cycle in all 27 different echinoid taxa were recognised the level of taxonomic identification varies depending on test completeness and preservation of specific morphological characters the palaeoecology of the echinoids was inferred using a functional morphological approach and actualistic comparisons a wide variety of ecological habitats are represented with the presence of regular as well as irregular sea urchins epibenthic as well as endobenthic forms as well as a wide range of interpreted burrowing depths for different irregular echinoids seven different echinoid assemblages were distinguished which differ with respect to the species diversity skeletal taphonomy and sedimentary environment 1 the parascutella assemblage displays spectacular mass accumulations of sand dollars accumulated by proximal storm deposits and winnowing 2 the cidaroid echinacea assemblage represents a slightly deeper moderate energy environment with a highly structured habitat and corresponding variety of regular and irregular sea urchins 3 a spatangoid assemblage with a diverse fauna of burrowing echinoids 4 the transported assemblage represents an allochthonous collection of echinoids from shallow water coarse sandy substrates 5 the mixed assemblage representing a slightly shallower low to moderate energy environment with reduced sedimentation rates 6 a clypeaster martini assemblage characterising a shallow higher energy environment 7 finally the poorly diverse phyllacanthus assemblage from shallow water carbonates diversity variations within the assemblages are correlated primarily to substrate variation burrowing depths as well as taphonomic factors the transgression regression cycle is well reflected by the echinoid assemblages which show a general deepening of depositional environment followed by shallowing upward tendencies

a new ecological model for the 565 ma ediacaran biota of mistaken point newfoundland abstract ediacaran macrofossils immediately precede the first animal ecosystems of the cambrian period and have excited much attention for potentially containing ancestors of the modern animal phyla ediacaran fossil surfaces have been variously interpreted as vertically tiered census populations comparable to animal ecosystems or as osmotrophic ecosystems exploiting turbulent flow regimes these hypotheses are highly dependent upon the taphonomic interpretation of mistaken point newfoundland as a consensus population representing the associations of the organisms in life reanalyses of these surfaces however indicate that this taphonomic model is inconsistent with the range of preservation states seen in each taxon we present a new taphonomic model for mistaken point surfaces in newfoundland canada and describe it using an index of decay this reveals a complex history of life death and decay at these sites data gathered from large scale casting projects in newfoundland show that only 15 40 of individuals were killed by ash flows rather than the whole population as previously interpreted this shows that ediacaran organisms at mistaken point newfoundland had a diverse range of ecological relationships with the microbial mat system frondose organisms are engaged in chemical exchange with the sediment and the water column with the stem penetrating through the microbial mat reclining forms such as fractofusus lived exclusively above the microbial mat and drew nutrients from the dead organisms on which they typically grew the ecology is based on organism interaction with the sediment water interface quite unlike that of modern vertically tiered metazoan communities that have developed since the start of the cambrian period

spindle shaped ediacara fossils from the mistaken point assemblage avalon zone newfoundland the mistaken point assemblage of the ediacara fossils is dominated by rangeomorphs with homologous fractal branching elements the most distinctive are the fusiform fossils herein named fractofusus misrai n gen and n sp and fractofusus andersoni n gen and n sp although endemic to the newfoundland portion of the avalonia terrane they dominated deep sea floor communities below the photic zone in the mid ediacaran period ca 575 560 ma their biological affinities remain uncertain but their architecture suggests a phylogenetic position near the base of the metazoa

impact spherules from karelia russia possible ejecta from the 2 02 ga vredefort impact event spherule beds of possible impact origin have been discovered in two drill cores in the paleoproterozoic zaonega formation karelia northwest russia spherules are found within the dolomite matrix of in situ brecciated sedimentary dolostones spherules are millimeter size and generally round although teardrop and dumbbell morphologies are present spherules contain up to 0 75 ppb ir with a ru ir of 2 indicating a mixing of target rocks with a minor chondritic component the age of the zaonega formation is constrained between limits of 1 975 0 024 ga sm nd and 1 980 0 057 ga pb pb and 2 050 ga re os which brackets the age of the 2020 ma vredefort impact structure in south africa and suggests that the spherule beds could represent ejecta from that event if the link is confirmed the size of the spherules and thickness of the beds suggest that the distance from the impact site was 2500 km thereby constraining the paleogeographic distance between the fennoscandian shield and kaapvaal craton during the late paleoproterozoic

age and origin of early proterozoic dolerite dykes in south west greenland samples of three dolerite dykes from south west greenland give a pooled weighted mean rb sr whole rock isochron age of 2 130 65 ma initial 87sr 86sr ratios are 0 70155 0 00018 for one of the dykes naujat and 0 70277 0 00012 for the other two torssut and akinaq a fourth dyke qaqarssuaq did not yield an isochron torssut samples have significantly lower initial 207pb 204pb ratios than samples from the naujat dyke indicating that the torssut dyke has been affected by crustal contamination

evidence of organic structures in ediacara type fossils and associated microbial mats ediacara type fossils represent a group of soft bodied organisms mainly known from imprints in proterozoic coarse grained siliciclastic sedimentary rocks circular compressions of beltanelliformis brunsae and remains related to ediacara type fossils such as cucullus fraudulentus and mucuplagum primitivum are reported here in an organic mode of preservation from the neoproterozoic doushantuo and liulaobei formations of china they can be interpreted as prokaryotic colonies a charnid fossil with circular attachment disc and stalk but torn off frond is documented in a three dimensional and partly organic mode of preservation from the neoproterozoic ust pinega formation white sea coast russia according to their morphology and structure the charniidae are not regarded by us as pennatulaceans modern myxobacteria illustrate that macroscopic size complexity and even compartmentalization can also be developed by prokaryotic colonies part of the ediacara type fossils may therefore represent prokaryotic colonies or symbiotic organisms involving prokaryotes finally direct evidence indicates that biofilms with associated prokaryotic sheaths preserved in both organic and pyritic fashion form the wrinkled surfaces elephant skin that were preferentially colonized by ediacara type fossils this finding supports previous interpretations based on comparative morphological and sedimentological approaches that ancient wrinkle structures were microbial mats

moderate velocity oblique impact sliding production of shocked meteorite textures and palaeomagnetically important metallic spherules in planetary regoliths we detail the production of metallic spherules in laboratory oblique shock impact experiments and their applicability 1 to textures in a partly shock melted chondritic meteorite and 2 to the occurrence of palaeomagnetically important fine iron or iron alloy particles in the lunar regolith samples recovered from 29 44 gpa 800 ns experiments revealed melting and textures reminiscent of metallic spherules in the yanzhuang h chondrite including dumbbell forms and other more complex morphologies our experiments demonstrate that metallic spherules can be produced via oblique impact sliding at lower velocities 1 85 km s 1 than are generally assumed in previous work associated with bulk shock melting and that oblique impact sliding is a viable mechanism for producing spherules in shock induced veins in moderately shocked meteorites significantly our experiments also produced fine metallic iron alloy spherules within the theoretical narrow size range a few tens of nanometers for slightly ellipsoidal particles for stable single domain ssd particles which are the most important palaeomagnetically since they can record lunar and planetary magnetic fields over geological time periods the experiments also produced spherules consistent with superparamagnetic sp and multidomain md particle sizes the fine ssd and sp particles on the lunar surface are currently thought to have been formed predominantly by space weathering processes our experiments suggest that oblique shock impact sliding may be a further means of producing the ssd and sp iron or iron alloy particles observed in the lunar regolith and which are likely to occur in the regoliths of mercury and other planetary bodies

three dimensional microct analysis of the ediacara fossil pteridinium simplex sheds new light on its ecology and phylogenetic affinity abstract ediacara fossils often exhibit enigmatic taphonomy that complicates morphological characterization and ecological and phylogenetic interpretation such is the case with pteridinium simplex from the late ediacaran aar member in southern namibia p simplex is often preserved as three dimensional 3d casts and molds in coarse grained quartzites making detailed morphological characterization difficult in addition p simplex is often transported distorted and embedded in gutter fills or channel deposits further obscuring its morphologies by utilizing microfocus x ray computed tomography microct techniques we are able to trace individual specimens and their vanes in order to digitally restore the 3d morphology of this enigmatic fossil our analysis shows that p simplex has a very flexible integument that can be bent folded twisted stretched and torn indicating a certain degree of elasticity in the analyzed specimens we find no evidence for vane identity change or penetrative growth that were previously used as evidence to support a fully endobenthic lifestyle of p simplex instead evidence is consistent with the traditional interpretation of a semi endobenthic or epibenthic lifestyle this interpretation needs to be further tested through microct analysis of p simplex specimens preserved in situ rather than transported ones the elastic integument of p simplex is inconsistent with a phylogenetic affinity with xenophyophore protists instead its physical property is consistent with the presence of collagen chitin and cellulose an inference that would provide constraints on the phylogenetic affinity of p simplex

impact spherules as a record of an ancient heavy bombardment of earth the fossilized remnants of vaporized asteroids called spherules can be used to infer that the flux of asteroid impacts on earth 3 5 billion years ago was much greater than it is now the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets impact craters are the most obvious indication of asteroid impacts but craters on earth are quickly obscured or destroyed by surface weathering and tectonic processes1 earth s impact history is inferred therefore either from estimates of the present day impactor flux as determined by observations of near earth asteroids or from the moon s incomplete impact chronology2 3 4 asteroids hitting earth typically vaporize a mass of target rock comparable to the projectile s mass as this vapour expands in a large plume or fireball it cools and condenses into molten droplets called spherules5 for asteroids larger than about ten kilometres in diameter these spherules are deposited in a global layer spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found1 here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers the impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3 5 billion years ago than it is now this conclusion is consistent with a gradual decline of the impactor flux after the late heavy bombardment

the early paleozoic development of bioturbation evolutionary and geobiological consequences abstract bioturbation the physical and chemical mixing of sediments by burrowing animals is a critical engineering process in modern seafloor environments and exerts an important control on not only benthic ecology and sediment properties but also ocean wide biogeochemical cycling well mixed sediments have long been assumed to appear at the precambrian cambrian boundary with the first occurrence of the index fossil and three dimensional burrow treptichnus pedum however field based analyses synthesizing ichnological stratigraphic sedimentological and taphonomic data collected from lower paleozoic successions worldwide indicate that sediment mixing in marine shelf environments remained limited until at least the late silurian 120 million years after the precambrian cambrian transition in spite of early advances in complex modes of infaunalization mixed layer development had a much more gradual trajectory and did not occur in step with the first appearance of three dimensional burrows the delayed appearance of modern style intensities of sediment reworking postdating both the cambrian explosion and the great ordovician biodiversification event may be due to the late stage radiation of mobile infaunal deposit feeders biological bulldozers the organisms which are in modern marine settings the most efficient bioturbators the protracted development of the sediment mixed layer holds important implications for contemporaneous biogeochemical cycling and oxygenation of the ocean atmosphere system the delayed development of intensive sediment mixing may have also mediated exceptional preservation of both fossilized soft tissues and shallow tier trace fossils and may therefore have contributed to the anomalous preponderance of konservat lagerstatten and exceptionally preserved shallow tier trace fossils and bioglyphs characteristic of the lower paleozoic stratigraphic record

physical and chemical evidence of the 1850 ma sudbury impact event in the baraga group michigan an ejecta layer produced by the sudbury impact event ca 1850 ma occurs within the baraga group of northern michigan and provides an excellent record of impact related depositional processes this newly discovered 2 4 m thick horizon accumulated in a peritidal environment during a minor sea level lowstand that punctuated a period of marine transgression common ejecta clasts include shock metamorphosed quartz grains splash form melt spherules and tektites accretionary lapilli and glassy shards suggesting sedimentation near the terminus of the continuous ejecta blanket sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact generated tsunami wave train observed platinum group element anomalies ir rh and ru within the sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite documenting and interpreting the detailed characteristics of the sudbury ejecta horizon in michigan have yielded a fingerprint to identify this chronostratigraphic marker in other paleoproterozoic basins for the first time a foundation exists to assess the consequences of the sudbury impact on precambrian ocean chemistry and early life

taphonomic comparison between recent and fossil sand dollars abstract the taphonomy of a recent and a fossil sand dollar are compared the recent echinodiscus auritus originates from a shallow water carbonate environment in the red sea the fossil parascutella hobarthi is found in micaceous sands of the lower miocene austrian molasse zone both species show strong similarities in constructional morphology including the flattened overall shape details of the surface morphology as well as the presence of an internal support system three taphonomic features are remarkable different 1 lethal and non lethal predation especially high in the recent e auritus 2 encrustation of the test especially by coralline algae in the fossil parascutella and 3 radial cracking on implosion of the body cavity caused by sediment loading on the fossil tests differences in taphonomic signatures can be correlated with depositional environment predation and encrustation as well as factors related to burial of the fossil specimens sediment loading and diagenesis

depositional and preservational environments of the ediacara member rawnsley quartzite south australia assessment of paleoenvironmental proxies and the timing of ferruginization abstract the paleoenvironmental setting in which the ediacara biota lived died and was preserved in the eponymous ediacara member of the rawnsley quartzite of south australia is an issue of longstanding interest and recent debate over the past few decades interpretations have ranged from deep marine to shelfal to intertidal to terrestrial here we examine the evidence in support of and against various paleoenvironmental interpretations of the fossiliferous ediacara member as well as exploring in depth a range of paleoenvironmental proxies that have historically been employed in such studies we emphasize the importance of reconciling paleoenvironmental analysis with an understanding of sedimentological processes and outline which proxies are consistent with this approach a careful assessment of paleoenvironmental parameters is essential to the reconstruction of the depositional and early diagenetic history of the ediacara biota and thus the physical chemical and biological factors that shaped the development and the fossilization of these earliest examples of complex life we find no compelling evidence for a terrestrial setting for the ediacara member and strong support for a shallow marine depositional environment

changes in the trace fossil biota across the proterozoic phanerozoic boundary trace fossils became relatively diverse in shallow water clastic seas in the late proterozoic vendian with a further significant increase in abundance diversity and complexity in the tommotian and atdabanian little change followed in the remainder of the lower palaeozoic traces typical of deeper water facies evolved in shallow water during the vendian and early cambrian and may have slowly migrated into the deep ocean during the remainder of the lower palaeozoic there are a limited number of short ranging ichnogenera which may be useful for correlation but the first appearance of ichnogenera may be a more satisfactory method three trace fossil zones covering the ranges redkino and kotlin rovno and tommotian lower atdabanian are recognized and may be useful for world wide correlation

boron paleosalinity proxy for deeply buried paleozoic and ediacaran fossils abstract boron content is an indicator of paleosalinity for mesozoic and cenozoic clayey sediments but is compromised by clay diagenesis in deeply buried sediments of paleozoic and ediacaran age this study of north american russian and australian paleozoic and ediacaran fossils showed tight covariance of b ppm and k wt confirming common wisdom that boron is carried largely by illlite smectite clays ratios of b k 40 g g distinguish modern marine and freshwater sediments but this discrimination threshold declines to 21 g g for sequences with bituminous coal just below the anchimetamorphic zone and to 5 g g for sequences within the anchimetamorphic zone toward greenschist facies marine and non marine threshold b k values can be predicted from weaver and kubler indices of illite crystallinity as a measure of b depletion during deep burial and metamorphism deviations from that expectation b k weaver and b k kubler are positive in marine rocks and negative in non marine rocks trilobites stromatolites and other marine fossils were correctly identified as marine by this proxy and fossil land plants identified as non marine though in some cases by small margins because of declining b content with increased clay crystallinity cryogenian to devonian problematic quilted fossils dickinsonia arumberia rangea pteridinium ernietta aspidella rutgersella protonympha are indistinguishable from devonian to triassic plants using this proxy and significantly different from trilobites brachiopods ammonites and other securely marine fossils

geochemical evidence for an impact origin for a late archean spherule layer transvaal supergroup south africa a late archean layer rich in sand sized spherules of former silicate melt in the monteville formation transvaal supergroup south africa has ir concentrations as high as 6 4 ppb and is clearly enriched in ir relative to associated tuffs carbonates and shales the monteville spherule layer is also enriched in other siderophile elements including the platinum group elements pges the pges in the spherule layer produce a flat meteorite like pattern when they are normalized to chondritic abundances the abundances of ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the hamersley basin of western australia that the mineral compositions textures and sedimentary structures of the spherule layers in the transvaal supergroup and hamersley basin are also very similar suggests that they were all formed by the same processes we think that the best way to explain the high ir concentrations and other characteristics of the monteville spherule layer is that it represents distal impact ejecta there are however significant differences between the monteville spherule layer and early archean spherule layers in the barberton greenstone belt including much higher average and maximum concentrations of ir in the latter the data presented here clearly show that each precambrian spherule layer is unique and needs to be characterized individually as is true for the impact spherule layers of the phanerozoic

multiple routes to mass accumulations of clypeasteroid echinoids a comparative analysis of miocene echinoid beds of sardinia abstract although clypeasteroid echinoids are very common in the shallow water sediments of the miocene of sardinia they have received little attention as far as their origin is concerned in this paper three sand dollar dominated mass accumulations from the lower miocene of sardinia are studied and compared in order to understand their genesis and palaeoecology two clypeasteroid assemblages were found in the sarcidano region central sardinia within the nurallao formation late oligocene burdigalian the third sand dollar mass occurrence crops out in the logudoro region northern sardinia within the calcari di mores formation late burdigalian these fossil echinoid deposits show a low taxonomic diversity two assemblages are dominated by the genus amphiope while the other is dominated by parascutella with subordinate clypeaster an integrated stratigraphic taphonomic and sedimentological approach allows different genetic mechanisms for these shallow water sandy shoreface deposits to be inferred the origin of these deposits is related to ecological morphological and sedimentological factors the sand dollars of cuccuru tuvullao nuragus show high degrees of abrasion and fragmentation and represent a multiple in situ reworked accumulation while the assemblage of duidduru genoni with low abrasion and variously oriented complete and fragmented specimens constitutes an example of a proximal storm deposits the sand dollar bed of monte sa loca chiaramonti consists of a dense accumulation of well preserved mainly concordant oriented juvenile and adult specimens and is considered to represent an autochthonous assemblage this paper explores the role of detailed sedimentological and taphonomic analysis in the palaeoecological and genetic interpretations of the sand dollar beds and points out the widespread distribution and diversification of these echinoids in the miocene deposits of the mediterranean area

discovery of distal ejecta from the 1850 ma sudbury impact event a 25 70 cm thick laterally correlative layer near the contact between the paleoproterozoic sedimentary gunflint iron formation and overlying rove formation and between the biwabik iron formation and overlying virginia formation western lake superior region contains shocked quartz and feldspar grains found within accretionary lapilli accreted grain clusters and spherule masses demonstrating that the layer contains hypervelocity impact ejecta zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca 1878 ma and 1836 ma the sudbury impact event which occurred 650 875 km to the east at 1850 1 ma is therefore the likely ejecta source making these the oldest ejecta linked to a specific impact shock features particularly planar deformation features are remarkably well preserved in localized zones within the ejecta whereas in other zones mineral replacement primarily carbonate has significantly altered or destroyed ejecta features

evidence of sensory driven behavior in the ediacaran organism parvancorina implications and autecological interpretations abstract the ancient in situ fossil seafloor communities of the ediacara biota present an unparalleled window into the assembly of the earliest complex macroscopic organisms including early animals on earth ca 555 million years ago mya the unique preservation style of ediacara fossil seafloors preserves whole communities virtually frozen in time including both living and dead organisms at the time of burial this phenomenon where the fossilized organisms are arranged as they were in life offers an unparalleled opportunity to examine ecological patterns in some of the earliest examples of animal communities in deep time the small anchor shaped fossil genus parvancorina is common among the ediacara biota however its morphology and ecology have received little attention here we describe a population of juvenile parvancorina preserved on a section of fossil seafloor recently excavated from the characteristic ediacara member from ediacara conservation park in the flinders ranges south australia we applied spatial methods to the sample population of parvancorina n 202 and found that they demonstrated two size clusters distinguishing juveniles from adults and further analyses showed that the smaller specimens tended to be spatially aggregated for the first time among any ediacara taxon we found that this sample population of parvancorina demonstrated a strong bimodal orientation suggesting that orientation played an important behavioral role in its autecology the aggregated spatial distribution and bimodal orientation of parvancorina likely resulted from behavioral responses to the influence of benthic currents suggesting that parvancorina had a complex sensory network and was capable of motility

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

ventogyrus a possible siphonophore like trilobozoan coelenterate from the vendian sequence late neoproterozoic northern russia complex internal anatomy is documented for the first time in a neoproterozoic soft bodied invertebrate new data suggests that ventogyrus originally described as a sessile boat shaped form belonging to the enigmatic neoproterozoic group petalonamae is actually composed of three identical modules arranged around an axial rod each module has one longitudinal and numerous transverse septa which delimit chambers of decreasing size from one end to the other three distinct longitudinal channels with alternating lateral branches make up an internal network perhaps a circulatory system ventogyrus is a most unusual form particularly in the quality of its three dimensional preservation of a complex internal structure and a three fold symmetry a symmetry shared by many ediacarans ediacaran metazoans of the late neoproterozoic are attracting growing attention as key evidence of an early radiation of the metazoa different specialists consider many if not all of these fossilized remains are representatives of a totally extinct group while others suggest that at least some ediacarans gave rise to living groups glaessner 1984 seilacher 1989 1992 gehling 1991 jenkins 1992 runnegar fedonkin 1992 fedonkin 1992 2003 fedonkin waggoner 1997 dzik 2003 the perplexing issue remains what are the exact relationships between these enigmatic ediacaran organisms and to which known groups might they belong current studies are underway in a variety of fields comparative anatomy molecular and developmental biology lipps et al 1998 ayala et al 1998 valentine 2004 valentine et al 1999 peterson butterfield 2005 fig 1 location of ventogyrus fossil locality starred and geological setting of the outcrop on the onega river sandstone lenses are shown in cross section from vickers rich p komarower p eds the rise and fall of the ediacaran biota geological society london special publications 286 187 194 doi 10 1144 1262sp286 14 0305 8719 07 15 00 the geological society of london 2007 of the more than 200 species of ediacaran metazoan fossils described to date only a few show any details of their internal anatomy ventogyrus chistyakovi ivantsov grazhdankin 1997 is exceptional because abundant fossils are preserved in three dimensions complex casts record imprints of this form s internal structure in a fine grained sandstone the fossils occur in the lower part of the ust pinega formation a part of the vendian sequence which crops out along the winter coast of the white sea in russia s north to the west of arkhangel sk chistyakov et al 1984 ivantsov grazhdankin 1997 fedonkin et al 1999 originally ventogyrus was interpreted as a solitary sessile immobile organism reaching up to 60 mm wide 120 mm long and 40 mm high thought to have been buried in life position the supposed boat like shape of this organism was structurally supported by a single longitudinal septum with additional septa branching off it with several orders of branching these septa surrounded regular chambers that decreased in volume with increasing distance from the central axis one unique unpaired chamber occurs at one end of the structure the impression of a thin layer of enclosing tissue is preserved as an external cast and the branching channels of an internal tubular system are preserved in exquisite detail ivantsov grazhdankin 1997 the construction in ventogyrus one longitudinal septum with numerous transverse septa surrounding chambers was originally supposed to fig 2 ventogyrus chistyakovi paleontological institute russian academy of science moscow pin no 4564 1006 1014 light from the top internal cast of several modules yarnema onega peninsula white sea region northern russia f coffa m a fedonkin a y ivantsov 188

post fossilization processes and their implications for understanding ediacaran macrofossil assemblages fossil assemblages from newfoundland s avalon peninsula preserve diverse examples of the enigmatic ediacaran macrobiota offering some of the earliest evidence for large and complex multicellular life these fossils are exposed on extensive coastal bedding planes in extraordinary abundances permitting palaeoecological studies based on census data from spatially extensive palaeocommunities such studies have been used to constrain the reproductive strategy and phylogenetic placement of ediacaran organisms geological mapping and stratigraphic correlation in the mistaken point ecological reserve reveal that some fossil bearing surfaces can be tracked over distances of several kilometres these laterally extensive surfaces reveal that the modern processes by which the sediment overlying a fossil surface is removed may impose important controls on the observed composition of fossil assemblages weathering and erosion along with factors associated with tectonics metamorphism and discovery are here grouped as post fossilization processes and introduce biases that are often not explicitly accounted for in palaeoecological studies specifically post fossilization processes may differentially influence the preservational fidelity of individual specimens on a given surface and generate features that could be mistaken for original morphological characters we therefore recommend that post fossilization processes must be considered when undertaking palaeoecological studies in ediacaran successions in newfoundland and potentially elsewhere

textural constraints on the formation of impact spherules a case study from the dales gorge bif paleoproterozoic hamersley group of western australia impact ejecta about 2 5 gyr old in the ds4 layer of the dales gorge bif hamersley group western australia are so well preserved that many original textures such as vesicles and microlites are faithfully preserved about 65 of the particles in the layer originated as impact ejecta of which 81 are splash forms the remaining 19 are angular but the splash forms and angular particles have the same composition mainly diagenetic stilpnomelane and k feldspar and share a common suite of internal textures some particles contain randomly oriented microlites texturally identical to plagioclase in basalts most splash forms have rims of inward growing crystals that may have formed from the melt perhaps nucleated by impinging dust or via thermal devitrification the rims clearly formed in flight because in broken particles which make up about 13 of the splash forms they are generally not present on broken surfaces the origin of the angular particles is uncertain but they may represent solid ejecta given the large sizes and variable shapes of the splash forms they are probably droplets of impact melt emplaced ballistically this is largely by analogy to the k t boundary layer but ds4 splash forms differ from k t spherules in important ways suggesting the k t model is not universal the occurrence of basaltic ejecta from a large impact highlights its scarcity in the stratigraphic record despite the areal abundance of oceanic crust the diverse textures formed via in flight crystallization suggest particle paths in the plume are more complex than is generally appreciated

dickinsonia liftoff evidence of current derived morphologies abstract dickinsonia an iconic member of the ediacara biota is abundant in the ediacaran deposits found at the nilpena field site south australia despite exquisite fossil preservation at this site many specimens of dickinsonia appear to be incomplete with an apparently missing piece on the periphery orientation measurements from specimens on three fossil beds suggest that these so called missing pieces are aligned irrespective of the axial orientation of dickinsonia the nonrandom orientation of incomplete specimens matches that of other aligned structures found on two of these beds the preferred directionality of this feature suggests the molding of incomplete specimens under the influence of current activity prior to or during burial we propose that this feature originates where part of a dickinsonia was lifted off of the substrate during a storm event and that sand was deposited beneath this lifted portion this model suggests that dickinsonia was easily separated from the sea floor and was not attached to the substrate on which it lived this is consistent with the data from dickinsonia footprints suggesting that dickinsonia was mobile

oil preserved in fluid inclusions in archaean sandstones oil is generally thought to be geologically young as it is thermodynamically unstable when subjected to elevated temperatures over long periods in open systems1 2 indeed almost all petroleum production comes from rocks younger than 400 million years ref 3 although the oldest known oil occurs in rocks 1 650 myr old4 suitable source rocks were abundant in older geological successions5 and circumstantial evidence suggests that some of these generated hydrocarbons early in their history6 here we report the discovery of oil preserved in fluid inclusions in sandstones dating back 3 000 myr most inclusions lie within healed microfractures confined to individual detrital quartz grains indicating that their oil was emplaced before archaean or palaeoproterozoic metamorphism sealed all voids and thus came from older in some cases archaean sources the fluid inclusions apparently acted as inert pressure vessels that protected the oil from subsequent degradation by circulating fluids and mineral catalysts because of its great age this oil can potentially yield valuable information about the size and diversity of the early biosphere particularly if it contains molecular fossils biomarkers of the primordial organisms from which it was derived

the sedimentary record of extraterrestrial impacts in deep shelf environments evidence from the early precambrian abstract impact ejecta layers in four formations in the hamersley basin western australia and in one formation from the transvaal supergroup south africa show striking evidence for impact related reworking each layer contains sand sized spherules of a former silicate melt that resemble those found in well documented impact layers given available isotopic age dates of associated strata and uncertainties in stratigraphic correlation these layers represent a minimum of three and a maximum of five impacts between ca 2 49 and 2 63 ga all of these layers were deposited below wave base in deep shelf environments yet they show a common suite of sedimentary features indicating deposition and reworking under high energy conditions these features occur in a consistent order 1 extensive erosion including the transport of meter scale rip up clasts 2 reworking by waves 3 synwave to postwave offshore directed bottom return flow and 4 later reworking by sediment gravity flows we interpret the con

tektites and microtektites key facts and inferences abstract tektites are generally small black rounded silicate glass bodies found scattered over several widely separated areas of the earth s surface called strewn fields four major tektite strewn fields are known 1 australasian 2 ivory coast 3 czechoslovakian and 4 north american tektites from these four strewn fields have been dated at 0 7 1 15 and 34 ma respectively most tektites are splash forms i e spheres teardrops dumbbells etc but some have undergone a second period of melting to produce ablated forms tektites with a blocky layered appearance are called muong nong types microtektites microscopic tektites belonging to the australasian ivory coast and north american strewn fields have been found in deep sea sediments the geographic occurrence and abundance of microtektites at each site have been used to determine the size and mass of each strewn field the australasian ivory coast and north american strewn fields contain 100 million 20 million and 1 billion tonnes of glass respectively although tektites resemble obsidian they can be distinguished from terrestrial volcanic glasses by their composition and petrography tektites for example are drier than volcanic glasses and have higher feo fe 2 o 3 ratios in addition tektites unlike volcanic glasses contain lechatelierite particles sio 2 glass and are devoid of primary crystallites although some authors favor a lunar volcanic origin most of the compositional and petrographic data indicate a terrestrial impact origin the major trace and isotopic compositions of tektites indicate that the parent material was a terrestrial sedimentary deposit and the presence of lechatelierite coesite and shocked relict inclusions favors an impact origin furthermore two tektite strewn fields have been associated with known impact craters and tektites from one strewn field have been found associated with impact ejecta unlike the cretaceous tertiary boundary layer microtektite layers are not associated with mass extinctions or iridium anomalies

oceanic mega impacts and crustal evolution lunar mare crater counts the terrestrial impact flux and astronomical observations of asteroids and comets define a consistent impact rate of 4 6 10 15 km 2 yr 1 within the inner solar system since the end of the late heavy bombardment 3 8 ga coupled with the observed crater size vs cumulative crater size frequency relationship of n d c 1 8 n cumulative number of craters of diameter d c these rates imply formation on earth of more than 450 d c 100 km diameter craters more than 50 d c 300 km diameter craters and more than 20 d c 500 km diameter craters geochemical and isotopic constraints require that more than 80 of the projectiles impacted on time integrated oceanic crust since the late heavy bombardment the injection of shock energies calculated at 10 8 mt tnt equivalent by a d p 10 km diameter projectile may result in propagating fractures and rift networks thermal perturbations and ensuing magmatic activity examinations of the geologic record for correlated impact and magmatic fingerprints of such events remain inconclusive in view of isotopic age uncertainties potential but unproven connections may be represented by the 1 cretaceous tertiary boundary ca 65 ma impact s onset of the carlsberg ridge spreading deccan volcanism and onset of the mantle plume of the emperor hawaii chain 2 jurassic cretaceous boundary ca 145 ma impacts onset of gondwana breakup including precursors of the east african rift structures 3 permian triassic boundary ca 251 ma impact s siberian norilsk traps and early triassic rifting and 4 the 3 26 ga basal fig tree group east transvaal ir rich and ni rich quench spinel bearing impact spherules and contemporaneous igneous tectonic activity tests of the theory require further identification and isotopic dating of distal ejecta impact spherule condensates and meteoritic geochemical anomalies

synchroneity of the k t oceanic mass extinction and meteorite impact blake nose western north atlantic a 10 cm thick layer of green spherules occurs precisely at the biostratigraphic boundary between the cretaceous and paleogene k t boundary at ocean drilling program site 1049 lat 30 08 n long 76 06 w the spherulitic layer contains abundant rock fragments chalk limestone dolomite chert mica books and schist as well as shocked quartz abundant large cretaceous planktic foraminifera and rounded clasts of clay as long as 4 mm interpreted as altered tektite glass probably derived from the chicxulub impact structure most of the cretaceous foraminifera present above the spherule layer are not survivors since small specimens are conspicuously rare compared to large individuals instead the cretaceous taxa in paleocene sediments are thought to be reworked the first paleocene planktic foraminifera and calcareous nannofossil species are recorded immediately above the spherule bed the upper part of which contains an iridium anomaly hence deposition of the impact ejecta exactly coincided with the biostratigraphic k t boundary and demonstrates that the impact event was synchronous with the evolutionary turnover in the oceans these results are consistent with a reanalysis of the biostratigraphy of the k t boundary stratotype which argues that shallow marine k t boundary sections are not biostratigraphically more complete than deep sea k t boundary sites

how well do fossil assemblages of the ediacara biota tell time patterns of origination evolution and extinction of early animal life on this planet are largely interpreted from the fossils of the precambrian soft bodied ediacara biota spanning nearly 40 m y of the terminal ediacaran period localities containing these fossils are loosely considered as part of either the avalon white sea or nama associations these associations have been interpreted to have temporal paleobiogeographic preservational and or paleoenvironmental significance surprisingly elements of all three associations occur within the ediacara member of the rawnsley quartzite of south australia an analysis of over 5000 specimens demonstrates that fossil distribution is strongly controlled by facies and taphonomy rather than time or biogeography and that individual taxa vary considerably in their environmental tolerance and taphonomic integrity the recognition that these taxa represent organisms living in various distinct environments both juxtaposed and shared holds strong implications for our interpretation of the record of early animal life on this planet and questions the biostratigraphic utility of the three associations furthermore although in situ soft bodied preservation provides a unique perspective on composition of benthic fossil assemblages the record should not be interpreted as a simple snapshot fossil beds represent a range of preservational modifications varying from current winnowed census samples of benthic communities at different depths and ecological maturity to entirely transported assemblages unless the appropriate environments and taphonomic conditions are present for certain taxa the absence of a particular taxon may or may not indicate its extinction in space or time