new constraint on the division of cambrian time a u pb zircon age of 530 7 0 9 ma for a volcanic ash in the lower placentian series of the avalonian terrane in southern new brunswick provides a precise point for calibration of early cambrian biologic evolution the ash is from the middle rusophycus avalonensis zone in saint john new brunswick this date is on subtrilobitic lower cambrian strata that may be approximately correlative with the sub tommotian manykaian stage of siberia and the lowest meishucunian stage of the south china platform this age when combined with published geochronologic data from the cambrian suggests that the manykaian stage may have lasted approximately 13 m y and that the tommotian may have been considerably shorter

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

reconstructing the avalon continent marginal to inner platform transition in the cambrian of southern new brunswick a west to east marginal to inner avalonian platform transition comparable to that in southeast newfoundland and southern britain is present in the cambrian of southern new brunswick the saint john caton s island hanford brook area lay on the marginal platform and its thick uppermost precambrian lower lower cambrian is unconformably overlain by trilobite bearing upper lower cambrian an inner platform remnant is preserved in the cradle brook outlier 60 km northeast of saint john in contrast to the marginal platform sequences the cradle brook outlier has a very thin lower lower cambrian and has middle lower cambrian strata bonavista group not present on the marginal platform the cradle brook lower cambrian closely resembles inner platform successions in eastern massachusetts and trinity and placentia bays southeast newfoundland a limestone with camenella baltica zone fossils on cradle brook seems to be the peritidal limestone cap of the subtrilobitic lower cambrian known in avalonian north ame

chemical sedimentology and paleoenvironmental history of lake olduvai a pliocene lake in northern tanzania lake olduvai was a saline highly alkaline closed basin lake in which lacustrine sediments of bed i and lower bed ii of olduvai gorge were deposited between ca 1 92 and 1 70 ma the lake consisted of a lacustrine area the central basin cb surrounded by lake margin terrain that was alternately flooded and dried the lacustrine deposits consist largely of authigenic clay and carbonate minerals zeolites k feldspar chert and pyrite claystones of the cb and most of the eastern lake margin elm terrain consist chiefly of mg rich authigenic illite smectite and interlayered illite smectite i s these clays were formed penecontemporaneously with detrital clay deposition by interaction of the detrital clays with the saline highly alkaline water the lake olduvai clays average 65 70 mg 2 sio 2 and alkali ions from lake water and the remaining 30 35 is detrital clay lacustrine clays of the elm indicate that this area was flooded by saline water for substantial periods of time hominids occupied the elm at times of lower lake levels when the clays were dried calcite is the most abundant carbonate mineral and occurs chiefly as sand size euhedral crystals dispersed in claystone and less commonly as calcite crystal limestones and nodules the sand size calcite crystals were formed diagenetically at shallow depths possibly from a gaylussite and or micritic calcium carbonate precursor dolomite occurs chiefly in the form of dolostones some of which are probably of replacement origin the oxygen isotope composition of lake water and pore fluid of the cb varied over 11 as determined from calculated delta 18 o h 2 o values of authigenic minerals clay minerals were formed chiefly in the more saline waters sand size calcite crystals were generated mostly from less saline pore fluids and limestone nodules were formed chiefly from dilute pore fluid dolostones were formed from fluids of varying salinity hydrogeology of the central basin was complex and postdepositional fluid movements are documented by differing delta 18 o h 2 o values for different authigenic minerals of the same samples pore fluids moved within the lake deposits for a considerable period of time after deposition as shown by 18 o values of quartz and k feldspar formed at different times downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits lake olduvai became overall shallower during the deposition of bed i and the subaerial exposure increased leading to occasional playa conditions these changes very likely resulted from increasing aridity and tectonic activity the lake that deposited lower bed ii was larger than that responsible for upper bed i and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the bed i lake

duration of the early cambrian u pb ages of volcanic ashes from avalon and gondwana volcanic zircons from three ashes give a u pb date of 511 1 ma on trilobite bearing upper lower cambrian upper branchian series strata of southern new brunswick that correlate into the siberian middle botomian toyonian stage interval this very young age on the late but not latest early cambrian of avalon is consistent both with a 519 1 ma age on the older caerfai bay shales of south wales that are tentatively correlated into strata with the oldest avalonian trilobites lower branchian and with a 517 1 5 ma age on the antatlasia gutta pluviae zone trilobites of morocco determination of a 522 2 ma zircon age on the moroccan subtrilobitic lower cambrian lie de vin formation is consistent with an earlier reported 521 7 ma age from the lie de vin but suggests that a 526 4 ma age on australian trilobite bearing lower cambrian rocks may be too old a 33 ma duration of the avalonian early cambrian and an 8 ma length of the avalonian trilobite bearing lower cambrian support proposals that

reconstructing the avalonia palaeocontinent in the cambrian a 519 ma caliche in south wales and transcontinental middle terreneuvian sandstones an early cambrian caliche on the st non9s formation emended is the base of the caerfai bay formation unit term changed at caerfai bay south wales subaerial exposure and the caliche mean the two formations were not genetically related units the st non9s is an older sand sheet likely tidalitic not delta related referred to avalonian depositional sequence ads 2 and the caerfai bay is a shallow mud basin unit refered to ads 4a the similar random formation upper ads 2 in north american avalonia has a basal age of c 528 ma and is unconformably overlain by red mudstones or sandstones in fault bounded basins on the avalonian inner platform coeval british sandstones lower hartshill wrekin st non9s brand hills are unconformably overlain by latest terreneuvian ads 3 or epoch 2 ads 4a units dates of 519 ma on caerfai bay ashes give an upper bracket on the late appearance of avalonian trilobites and suggest an ads 2 4a hiatus of several million years post st non9s and post random basin reorganization led to abundant caerfai bay formation volcanic ashes and sparse brigus formation ashes in newfoundland the broad extent of erosional sequence boundaries that bracket lithologically similar to identical units emphasize that east and west avalonia formed one palaeocontinent the inner platform in southern britain was larger than the midlands craton a tectonically defined later palaeozoic area unrelated to terminal ediacaran early palaeozoic depositional belts the cool water successions of early palaeozoic avalonia were distant from coeval west gondwanan carbonate platforms

late ediacaran paleogeography of avalonia and the cambrian assembly of west gondwana abstract the latest proterozoic tectonic evolution of avalonia one of the largest peri gondwanan terranes bears on the issue of gondwana amalgamation during the transition from rodinia to pangea new paleomagnetic results from mid late ediacaran strata in the newfoundland sector of avalonia confirm substantial apparent motion of that terrane between ca 590 and 560 ma consistent with a true polar wander interpretation for that interval when the avalonian poles are mapped onto complementary paleomagnetic records from west africa and laurentia the avalonia terrane may occupy any of four possible positions all of which are far removed from those cratons choosing the most kinematically parsimonious location leading toward gondwana assembly we find that avalonia was most likely proximal to amazonia during the ediacaran cambrian transition with a substantial 3000 km separation between amazonia and west africa at that time our results thus favor the existence of a wide clymene ocean and subsequent cambrian final assembly of west gondwana providing a key snapshot onto the evolving rodinia gondwana supercontinental transition

the lower middle cambrian stages and stratotypes from the iberian peninsula a revision lower middle cambrian regional stages of the iberian peninsula cordubian ovetian marianian bilbilian acadoparadoxides and solenopleuropsidae stages are now reconsidered in the light of new stratigraphic and palaeontological trace fossils trilobites and archaeocyaths studies these stages are re described with selected stratotypes and chronostratigraphic boundaries the names leonian and caesaraugustian are formally proposed to replace the provisional stages of acadoparadoxides and solenopleuropsidae respectively a correlation chart of the lower middle cambrian lithostratigraphic units from the iberian peninsula is also presented two regessive transgressive cycles are proposed to explain the lithological and palaeontological peculiarities through the lower middle cambrian sequence in the iberian peninsula

pseudocryptic species of the middle cambrian trilobite eodiscus hartt in walcott 1884 from avalonian and laurentian newfoundland two species of the middle cambrian trilobite eodiscus hartt in walcott 1884 e punctatus salter 1864 and e scanicus linnarsson 1883 have been reported from several paleocontinents however in their respective type areas of avalonian britain and baltica sweden both species are poorly documented from moulds preserved in siliciclastic mudstone that are variably compacted and distorted moreover variation in such characters as surface sculpture between putative occurrences suggests that widespread use of these names may mask species differentiation within and between paleocontinents detailed examination of eodiscus sclerites that are exquisitely preserved in full relief in limestone from the manuels river formation of avalonian newfoundland and the shallow bay formation of laurentian newfoundland demonstrates the presence of multiple species that are distinct from both e punctatus and e scanicus we interpret them as a group of pseudocryptic species that are comparable to groups that are no

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

effects of bioturbation on carbon and sulfur cycling across the ediacaran cambrian transition at the gssp in newfoundland canada the initiation of widespread penetrative bioturbation in the earliest phanerozoic is regarded as such a significant geobiological event that the boundary between ediacaran and cambrian strata is de

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

the succession of small shelly fossils especially conoidal microfossils from english precambrian cambrian boundary beds small shelly fossils especially conoidal microfossils have potential for biostratigraphic correlation of precambrian cambrian boundary rocks where evolutionary successions of taxa can be recognized a succession of hyoliths brachiopods and species of sunnaginia torellella eccentrotheca and rhombocorniculum can be recognized in the home farm member of nuneaton and or the comley limestone of shropshire england revised ranges of taxa are given and sunnaginia neoimbricata n sp s parva n sp s angulata n sp eccentrotheca grandis n sp and several new records are described from nuneaton and comley the similarity in the sequence of small shelly fossils in england and siberia indicates that a correlation can be achieved between the two areas it is suggested that the lower tommotian to lower atdabanian correlates with the hartshill formation of nuneaton of which the condensed home farm member spans the mid tommotian to lowest atdabanian while the upper atdabanian to lower botomian correlates with the condensed comley limestone of shropshire

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

mixed siliciclastic carbonate deposition in an early cambrian oxygen stratified basin chapel island formation southeastern newfoundland abstract the upper portion of member 3 and member 4 of the uppermost precambrian vendian lower cambrian chapel island formation represents a large scale shoaling up deposit dominated by shelf siliciclastic mudstone medium scale higher order shoaling up cycles at the top of member 3 and in member 4 are capped by peritidal limestones that contain a wide variety of stromatolitic buildups and algal mud mounds sedimentation took place in an oxygen stratified basin with accumulation of gray dysaerobic mudstone offshore and green and red mudstone in shallower more oxygenated waters biofacies models for dysaerobic basins can be applied to this sequence and help in understanding the distribution of trace fossils skeletal fossils shells and pyritic steinkerns and diagenetic features these da a are important for the understanding of the stratigraphic distribution of shelly fossils within this lowest cambrian unit a mixed carbonate siliciclastic facies model is presented in which carbonate sediments formed in intertidal areas along the shoreline of a siliciclastic muddy shelf the carbonate shoreline prograded during periods of increasing rate of relative sea level fall or decreasing rate of relative sea level rise restriction of carbonate sediments to the peritidal environment may have been due to a combination of higher rates of carbonate sediment production and accumulation in the peritidal zone seaward dilution by siliciclastic mud and a paleogeographic position near the latitudinal climatic extreme for carbonate production

a refined chronology for the cambrian succession of southern britain abstract three dated u pb zircon ash beds from biostratigraphically constrained avalonian successions of shropshire england and pembrokeshire wales delimit the traditional lower9 middle9 cambrian boundary and resolve a problematic regional correlation in shropshire a date of 514 45 0 36 0 81 including tracer calibration and 238 u decay constant errors ma from near the top of the lower comley sandstone formation provides a maximum age for the boundary between cambrian stages 3 and 4 and a date of 509 10 0 22 0 77 including tracer calibration and 238 u decay constant errors ma from the basal quarry ridge grits upper comley sandstone formation provides a minimum age for the boundary between cambrian stages 4 and 5 and thus series 2 and 3 these dates offer a calibration of early metazoan evolution by directly constraining the age of the intervening comley limestones which contain diverse small shelly fossils in addition to trilobites and also a key early occurrence of exceptional three dimensionally preserved arthropods in pembrokeshire an ash bed from the caerfai bay shales formation dates to 519 30 0 23 0 77 including tracer calibration and 238 u decay constant errors ma equivalent to a horizon low in the lower comley sandstone formation of shropshire possibly around the level at which trilobites make their first local appearance supplementary material appendix 1 a table of isotope data is available at http www geolsoc org uk sup18444

tidal deposits of the lower cambrian random formation eastern newfoundland facies and paleoenvironments the avalon tectonostratigraphic zone occupies about 25 of the area of insular newfoundland late precambrian development of the avalon terrane is distinctly different than that of other parts of the appalachian orogen and cratonic north america late precambrian volcanism and tectonic instability gave way to deposition of about 2 km of cambrian and lower ordovician dominantly epiclastic sediments on a slowly subsiding relatively stable shelf platform the lower cambrian random formation is part of the platformal sequence and is bounded by regional disconformities over much of the newfoundland avalon zone except around fortune bay where it is underlain by older terrestrial and shallow marine sediments the random formation was deposited during a time of global sea level rise and consists of up to 250 m of shoreline nearshore and open shelf deposits that record macrotidal conditions and periodic storm activity cross bedded quartzarenites occur in units to about 50 m thick one exceptional unit is 110

neoproterozoic to early cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the yangtze platform china abstract small shelly fossils ssfs occur widely on the yangtze platform and have great potential for biostratigraphic subdivision of pre trilobitic early cambrian strata based on the ssf record of the shallow water realm five biozones can be recognized for the meishucunian stage in ascending order the biozones are anabarites trisulcatus protohertzina anabarica assemblage zone paragloborilus subglobosus purella squamulosa assemblage zone watsonella crosbyi assemblage zone formerly heraultipegma yunnanensis zone poorly fossiliferous interzone sinosachites flabelliformis tannuolina zhangwentangi assemblage zone in addition one ssf biozone is recognized for the overlying qiongzhusian stage pelagiella subangulata taxon range zone the formerly used siphogonuchites triangularis paragloborilus subglobosus zone and heraultipegma yunnanensis zone are discussed and redefined approximately 80 species of ssfs were screened for their spatial and temporal distribution on the yangtze platform variations in lithofacies and biofacies can be recognized throughout the yangtze platform extending over an area of 2000 900 km in a deeper water shelf setting the first zone is represented by the protohertzina anabarica kaiyangites novilis assemblage zone while younger zones are not recognized in the southern region at the northern platform margin the qiongzhusian is represented by the ninella tarimensis cambroclavus fangxianensis assemblage zone and the rhombocorniculum cancellatum taxon range zone the southeastern shaanxi western hubei region followed a slightly different lithological and faunal development than the rest of yangtze platform indicating a stronger similarity with parts of east gondwana some taxa such as w crosbyi p subangulata r cancellatum microdictyon effusum a trisulcatus protohertzina unguliformis and p anabarica occur nearly worldwide and support an international correlation of early cambrian sequences between the yangtze platform and smaller west gondwanan blocks siberia newfoundland and australia the six investigated zones of the yangtze platform comprise an interval spanning the early nemakit daldynian to the late atdabanian early botoman stage on the siberian platform palaeobiogeographic analysis revealed a strong taxic similarity between the yangtze platform and the tarim platform a smaller number of species are shared with other west gondwanan platform fragments such as india and iran palaeobiogeographic results do not support the previously reported position of the south china block between australia and siberia during the early cambrian

early cambrian teichichnus dominated ichnofabrics and palaeoenvironmental analysis of the caerfai group southwest wales uk abstract the early cambrian caerfai group of pembrokeshire in southwest wales yields an intensely bioturbated teichichnus dominated ichnofabric and provides evidence for the early evolution of infaunal organisms three units of the caerfai group comprise the focus of this study in sections exposed at caerfai bay in the st non s sandstone few primary sedimentary structures are observed bedding being predominantly massive with subordinate parallel and current ripple cross lamination the topmost st non s sandstone contains common soft sediment deformation phenomena and collectively deposition is believed to have taken place in the proximal subaqueous reaches of a steep fronted delta system teichichnus is the dominant trace with subordinate planolites palaeophycus and possible rhizocorallium the overlying caerfai bay shales were deposited from mass flows with evidence of downslope creep suggesting deposition on a steep delta front sandstone debrites are common as are tuffaceous beds with evidence of reworking by bottom currents sandstone and tuff event beds contain common skolithos and arenicolites with less frequent planolites interbedded mudstones contain infrequent teichichnus the caerfai bay shales coarsen and thicken upwards into the caerbwdy sandstone debrite sandstone beds dominate and bed thickening upsection indicates progradation of the delta system the dominant trace fossil within the unit is teichichnus which is constrained to the top few centimetres of individual sandstone beds less common are planolites and palaeophycus the size and abundance of teichichnus in the caerfai group are similar to or exceed those in contemporaneous sections in avalonia and baltica local environmental factors were probably the primary control on trace fossil distribution substrate and salinity are proposed as the main influences on ichnodiversity and size in the caerfai group teichichnus is most abundant in the st non s sandstone where the proximal reaches of the delta experienced reduced salinity and possibly higher oxygen levels burrow diameter progressively decreases upsection and bioturbation becomes less pervasive in subsequent units of the caerfai group this indicates that by the cambrian stage 2 significant advances into reduced salinity environments had occurred along with the dispersal of the teichichnus producer throughout the avalonia baltica seaway correlation within avalonia indicates that a transgression in the cambrian stage 2 initiated the deposition of the caerfai group marking the onset of the welsh basin

properties and development of channel calcretes in a mountain catchment tabernas basin southeast spain nonpedogenic channel calcretes of up to 3 5 m thick occur in a number of locations across the tabernas basin in almeria province southeast spain they represent the last major phase of late quaternary calcium carbonate cementation to affect this semiarid drainage basin channel calcretes are situated within the basal parts of sequences of 12 m thick poorly sorted coarse grained and schist dominated fluvial gravels that were deposited within confined bedrock channels channel calcretes occupy the full basal width of the bedrock channels within which they occur and mostly crop out near the mouths of tributary valleys to the main rambla de tabernas drainage system six profiles from five separate channel calcrete outcrops were logged and sampled all samples were analysed in thin section and under scanning electron microscope sem from these analyses the macro and micromorphological properties were identified and the mode of origin of channel calcrete profiles was assessed results indicate that all channel calcretes are massively cemented by calcite and appear uniform at the field profile scale microscale analyses reveal that detrital grains within the calcretes are initially coated by micrite which is overlain by grain coating and pore filling sparite a series of broad trends in cement type and micromorphology are recognised within individual field profiles all profiles exhibit an increasing degree of calcite crystal size in a down profile direction most field profiles predominantly contain micrite in upper parts with an increasing percentage of sparite towards the base some profiles are however dominated by sparite and show increasing crystal size and occurrence of euhedral crystals towards the profile base many profiles also contain evidence of postcalcite cementation diagenesis in the form of dissolution of calcite crystal faces or replacement of the calcite cement by amorphous silica with alteration mostly occurring in the lower parts of profiles the trends within the calcrete fabric appear to have developed in conjunction with a fluctuating water table with the increased crystal size and occurrence of euhedral crystals towards the base of profiles arising from greater duration of wetting in basal zones cementation in some field profiles occurred at depths of up to 12 m within the host sediment well below the zone of capillary rise and pedogenesis and in the absence of significant organic activity as such channel calcretes in the tabernas basin may represent an ideal opportunity to observe the influence of groundwater upon cementation of coarse grained sediments by carbonate

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

microcodium an extensive review and a proposed non rhizogenic biologically induced origin for its formation abstract microcodium has been previously described as a mainly cenozoic calcification pattern ascribed to various organisms a review of the available literature and our data reveal two peaks in microcodium abundance the moscovian early permian and the latest cretaceous paleogene a detailed analysis of late paleozoic and cenozoic examples leads to the following new conclusions typical microcodium forming unilayered corn cob aggregates of elongated grains and thick multilayered palisade replacing structures cannot be linked to smaller grained intracellular root calcifications as became widely accepted after the work of klappa klappa c f 1979 calcified filaments in quaternary calcretes organo mineral interactions in the subaerial vadose environment j sediment petrol 49 955 968 typical microcodium is recognized from the early carboniferous with doubtful devonian reports to quaternary as a biologically induced mineralization formed via dissolution precipitation processes in various aerobic ca rich soil and subsoil terrestrial environments morphology and 13c signatures of microcodium suggest that neither plants algae or roots and root associated mycorrhiza regulate the formation of these fossil structures non recrystallized microcodium grains basically consist of slender 1 5 4 m curved radiating monocrystalline prisms with occasionally preserved hyphae like morphology thin 0 5 3 m hypha like canals can also be observed these supposed hyphae may belong to actinobacteria however thin fungal mycelia cannot be excluded we propose a model of microcodium formation involving a mycelial saprotrophic organism responsible for substrate corrosion and associated bacteria capable of consuming acidic metabolites and caco3 reprecipitation into the microcodium structures

carbonate deposition and hydrocarbon reservoir development at the precambrian cambrian boundary the ara group in south oman abstract the ediacaran early cambrian ara group in the south oman salt basin consists of six evaporite carbonate cycles a0 a1 to a6 that record tectono eustatic sea level changes the a4 cycle developed at the precambrian cambrian boundary 542 ma it forms a single depositional sequence with evaporites deposited in a lowstand systems tract lst overlying carbonates represent an open marine shallowing upward ramp succession that developed in transgressive tst and highstand systems tracts hst the low energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high the tst facies include sulfates evaporite carbonate laminites and organic rich carbonate laminites that record initial flooding deepening of the basin and establishment of an outer ramp depositional environment carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity subsequent hst facies comprise mostly fine grained clastic carbonates and stromatolites that formed in middle and inner ramp settings these facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp sediment was redistributed in suspension by muddy turbidity currents muddy debris flows storm and shallow water currents during the late hst carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying lst evaporite unit a combination of strong tectonic subsidence and transient flooding caused significant shallow water evaporite deposition to occur not only down dip but also on top of the former carbonate platform where several hundreds of meters of evaporites accumulated the transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit the presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties distribution of organic material and carbonate mud can be linked to specific environmental conditions low physical and biogenic disturbance of sediment and stratified water mass and the sequence stratigraphic position low flux of fine grained carbonate during tst in contrast diagenetic evaporite formation has largely degraded reservoir quality in porous shallow water facies near the top of the a4c

chemostratigraphy of the cerro victoria formation lower cambrian uruguay evidence for progressive climate stabilization across the precambrian cambrian boundary abstract petrographic isotopic and geochemical data are reported for the cerro victoria formation of the arroyo del soldado group uruguay it is divided into four informal lithostratigraphic units a to d which are mainly composed of an alternation of stromatolitic oolitic and clastic dolostones the genesis of dolomite as a primary or penecontemporaneous precipitate is discussed considering excellent preservation of textural details whereas 87 sr 86 sr ratios in the least altered samples are too high 0 7106 0 7109 to represent seawater values 13 c determinations are interpreted as reflecting near seawater composition the curve obtained for the cerro victoria formation is characterized by negative values at its base 3 5 pdb passing into a slightly positive plateau up section peak values of 0 64 pdb are encountered in the upper unit c the upper cerro victoria formation is characterized by a return to moderately negative values the shape of the 13 c curve is consistent with the data reported for the nemakit daldyn stage of the lower cambrian we suggest that the precambrian cambrian boundary is located at the cerros san francisco cerro victoria transition or slightly lower in the succession thalassinoides bearing dolostones of the upper cerro victoria formation might reach the tommotian or atdabanian in the arroyo del soldado group 13 c excursions parallel sea level oscillations transgressive events being characterized by positive 13 c values falling amplitudes of coupled 13 c sea level fluctuations recorded in these carbonates are in accordance with the trend observed in ediacaran lower cambrian successions worldwide covariance of 13 c and sea level might be explained by bioproductivity fluctuations affecting the carbon cycle atmospheric co 2 concentrations climate ice volume and ultimately sea level while these oscillations were of large magnitude in the late neoproterozoic they progressively faded away in the cambrian giving place to a more stable environment the cause of the amplitude decrease in biogeochemical climatic and sea level perturbations might be the decreasing rates of nutrient input at mid ocean ridges as reflected by rising 87 sr 86 sr values between 800 and 500 ma punctuated by at least three events roughly coincident with rifting episodes of rodinia

island arcs and their role in the evolution of the western arabian shield abstract two northeast trending late precambrian ensimatic island arc systems in western saudi arabia are described each displays lithotec tonic provinces that conform to those found in modern examples of plate convergence the younger hijaz arc system 800 700 ma contains a thick fore arc sequence of al ays group turbidites overlying an accretionary prism of metamorphosed and fragmented volcanosedimentary rocks and associated ophiolites of the farri group and the al hinu formation the older asir arc system 900 700 ma is exposed at a much deeper erosional level in which only remnant arc and fore arc deposits are present it is identi fied in part by a frontal arc assemblage of abundant dioritic intrusions and high grade metamorphic rocks that are interpreted as a high t low p belt a mafic volcanic assem blage in th back are c area appears to deline ate an interarc basin situated between the at ta if frontal arc and a lithologically similar remnant arc west of bishah about 700 ma ago the hijaz island arc collided with the asir arc system along a southeast dipping subduction zone now rec ognized as the bi r umq port sudan suture epeirogeny during this period led to the crea tion of an intramassif basin in the hijaz arc system and an accretionary basin in the asir arc system these were filled by volcanic and shallow water to continental molasse depos its of the hadiyah and fatimah groups re spectively continued convergence between 700 and 675 ma ago resulted in development of the northeast trending samran fold belt the collision event marks the cessation of an episode of southeast convergence that is in terpreted as the initial phase in the progres sive eastward evolution of the arabian nubian shield introduction

middle eocene through early oligocene climate history and paleoceanography in the southern ocean stable oxygen and carbon isotopes from odp sites on maud rise and kerguelen plateau mackensen a and ehrman w u 1992 middle eocene through early oligocene climate history and paleoceanography in the southern ocean stable oxygen and carbon isotopes from odp sites on maud rise and kerguelen plateau mar geol 108 l 27 paleogene stable oxygen and carbon isotopes were measured in foraminifera from odp sites 689 and 690 at maud rise in the atlantic ocean sector of the southern ocean and from sites 738 744 748 and 749 at the southern kerguelen plateau in the indian ocean sector these data were compared with sedimentological data from the same sample set both benthic and planktic 6180 values document a cooling trend beginning around 49 5 ma at all sites during the late middle eocene planktic lso values indicate a steepening latitudinal temperature gradient from 14 c at the northern sites towards 10 c at the southernmost sites terrigeneous sand grains of probably ice rafted origin and clay mineral assemblages point to the existence of a limited east antarctic ice cap with some glaciers reaching sea level as early as middle eocene time around 45 5 ma between 45 and 40 ma average paleotemperatures were between 5 and 7 c in deep and intermediate water masses while near surface water masses ranged between 6 and io c during the late eocene between 40 and 36 ma average temperatures further decreased to 4 5 c in the deep and intermediate water masses and to 5 8 c near the sea surface abruptly increasing 61so values at approximately 35 9 ma exactly correlate with a sharp pulse in the deposition of icerafted material on the kerguelen plateau a dramatic change in clay mineral composition and an altered southern ocean circulation indicated by a differentiation of benthic 61 c values between sites increasing opal concentrations and decreasing carbonate contents for planktic and benthic foraminifera this 80 increase ranges between 1 0 and 1 3 o and between 0 9 and 1 4 o respectively we favour a hypothesis that explains most of the 61so shift at 35 9 ma with a buildup of a continental east antarctic ice sheet consequently relatively warm oligocene antarctic surface water temperatures probably are explained by a temperate wet based nature of the ice sheet this would also aid in the fast build up of an ice sheet by enhancing the moisture transport on to the continent

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

precambrian cambrian boundary global stratotype ratified and a new perspective of cambrian time the precambrian cambrian boundary global stratotype is the base of the phycodes pedum zone at fortune head southeastern newfoundland the initial two stage radiation of the phanerozoic fauna the appearance of phanerozoic aspect trace producers and subsequent origin of diverse skeletalized metazoans was earliest cambrian fossil assemblages with deep complex burrows or originally mineralized skeletal fossils with exception of cloudina are early cambrian or younger correlation into avalon indicates that strata once referred to the proterozoic e g manykaian or nemakit daldynian and rovno stages are lowest cambrian and that the siberian tommotian stage is middle lower cambrian the thick sequence and complex epeirogenic history of the placentian series as compared with the trilobite bearing branchian series suggest that most early cambrian time is represented by the subtrilobitic lower cambrian

the tectono metallogenic evolution of the arabian shield abstract the arabian shield comprises a group of diverse terranes accreted during the late proterozoic their boundaries are suture zones characterised by tectonised ultramafic bodies aligned within distinct orogenic belts the asir hijaz and midyan terranes consist of a series of late proterozoic immature island arcs and together form the western half of the shield in the east however the afif and ar rayn terranes have a continental affinity with the afif terrane being much older at least early proterozoic and possibly archaean metallogeny within the arabian shield is also diverse and includes cu ni and cr mineralisation associated with proterozoic oceanic crust or ophiolite arc type volcanogenic base and precious metal deposits associated with formerly active plate margins shear zone hosted mesothermal gold mineralisation related to accretionary and strike slip tectonics as well as sn w ta mo ree mineralisation associated with post cratonisation alkali granite magmatism of late proterozic to early phanerozoic age it is possible to trace a systematic evolution in metallogeny both within each terrane and throughout the craton as a whole as a function of crustal development and tectonics although the age range of rocks exposed in the shield is from early proterozoic to tertiary all known styles of primary metallogeny are associated with its cratonisation in the late proterozoic

spectroscopy of the mineralized tonalite diorite intrusions bulghah gold mine area saudi arabia effects of opaques and alteration products on fieldspec data abstract this paper aims to reveal the spectral characteristics of the mineralized tonalite diorite intrusions exposed at bulghah gold mine area saudi arabia using fieldspec spectroradiometer and landsat etm data gold mineralization at bulghah gold mine is hosted mainly by syn to late orogenic tonalite diorite intrusions aligned in n s direction and is associated mainly with cataclastic zones and quartz veins based on field petrography and fieldspec data two main tonalite diorite groups namely a and b are recognized group a recorded at the mine area is characterized by low flat spectral profiles with overall low reflectance values 10 these low values are attributed to the presence of high content of trans opaque phases hematite and goethite as indicated by petrographic study spectral profiles of group b recorded at bulghah north area show high reflectance values 40 with three main absorption features around the 1 45 m 2 20 m and 2 35 m wavelength regions these absorption features are attributed mainly to the presence of clay minerals sericite chlorite and carbonate alteration products to discriminate the above mentioned tonalite diorite groups band ratios and minimum noise fraction mnf techniques are conducted landsat false color composite band ratio image 7 4 r 4 1 g and 4 5 b discriminates easily the bluish brick red tonalite diorite intrusions from the surrounding rock units but failed to distinguish the above mentioned two tonalite diorite groups on mnf3 image the two tonalite diorite groups can be distinguished easily in which group a has white color whereas group b has gray color the present study proved the usefulness of fieldspec spectral profiles and the processed landsat etm data for discriminating and delineating the mineralized tonalite diorite intrusions exposed at the study area

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

roter kamm impact crater namibia geochemistry of basement rocks and breccias abstract the roter kamm crater in the southern namib desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts to better define the variety of target rocks and breccias we studied the petrography and chemical composition of a new suite of twenty eight basement and breccia samples from the roter kamm crater based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two layer target region an upper layer of gariep metasediments schist marble quartzite and sandstone overlying the crystalline basement of the namaqualand metamorphic complex the basement was also heavily intruded by coarse grained quartz veins and quartz and quartz feldspar pegmatites the clast population in the melt breccias indicates that impact induced melting involved mainly metasedimentary target rocks with rarely detected contributions from pegmatite and granite granodiorite three varieties of melt breccias can be defined 1 schistose 2 quartzitic melt breccias 3 true impact melt breccias these melt breccia types are chemically heterogeneous and even the impact melt breccias may have been produced in situ and not from a coherent melt body the shapes of the schistose melt breccias previously thought to be ejected impact breccias are most likely caused by erosion and these breccias are now interpreted to be locally derived the crater basement as exposed at the rim was structurally severely affected and at least locally considerable thermal energy was generated during formation of large volumes of cataclastic mylonitic and pseudotachylitic breccias analyses of mylonite and pseudotachylites from the crater rim as well as their respective host rocks show that these breccias were mainly formed from local material analyses of pseudotachylite like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar similar to their tectonically produced counterparts although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated several breccias compared to their host rocks do show evidence of severe chemical alteration chloritisation and sericitisation the presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact induced hydrothermal event in the crater area

early palaeozoic peri gondwana terranes new insights from tectonics and biogeography following the late neoproterozoic early cambrian breakup of the supercontinent rodinia gondwana evolved as one of the principal continental masses on earth embracing most of south america africa australasia antarctica much of western europe and parts of asia around its margins were various other terranes that had varying tectonic and biogeographical affinities with the main continental block this book incorporates a series of reviews and multidisciplinary research papers that together explore the tectonic palaeogeographical and palaeobiogeographical evolution of the elements that made up the peri gondwanan collage the stratigraphical scope of the coverage embraces the late precambrian through early devonian providing a comprehensive overview of structural stratigraphical and biological evolution through this significant interval of earth history integration of these various processes throughout the volume will be of broad based interest to a wide range of geoscientists

stepwise mass extinctions and impact events late eocene to early oligocene abstract species ranges and relative abundances of dominant planktonic foraminifers of eight late eocene to early oligocene deep sea sections are discussed to determine the nature and magnitude of extinctions and to investigate a possible cause effect relationship between impact events and mass extinctions late eocene extinctions are neither catastrophic nor mass extinctions but occur stepwise over a period of about 1 2 million years four stepwise extinctions are identified at the middle late eocene boundary the upperglobigerapsis semiinvoluta zone theg semiinvoluta globorotalia cerroazulensis zone boundary and at the eocene oligocene boundary each stepwise extinction event represents a time of accelerated faunal turnover characterized by generally less than 15 species extinct and in itself is not a significant extinction event relative species abundance changes at each stepwise extinction event however indicate a turnover involving 60 of the population implying major environmental changes there microtektite horizons are present in late eocene sediments one in the upperg semiinvoluta zone 38 2 ma and two closely spaced layers only a few thousand years apart in the lower part of thegloborotalia cerroazulensis zone 37 2 ma each of the three impact events appears to have had some effect on microplankton communities however the overriding factor that led to the stepwise mass extinctions may have been the result of multiple causes as there is no evidence of impacts associated with the step preceding or the step following the deposition of the presently known microtektite horizons

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

integrated correlation of the vendian to cambrian arroyo del soldado and corumba groups uruguay and brazil palaeogeographic palaeoclimatic and palaeobiologic implications abstract the corumba group of sw brazil and the arroyo del soldado group asg of uruguay are correlated on the basis of litho bio and chemostratigraphy both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf in the corumba basin sedimentation began in the varangerian represented by the glaciomarine puga formation a series of sea level fluctuations coupled with climatic changes are recorded up section while uppermost deposits of the asg are of lowermost cambrian age sedimentation ceased in the latest vendian in the corumba basin an assemblage of six species of organic walled microfossils dominated by bavlinella faveolata and soldadophycus bossii three species of vendotaenids and two species of skeletal fossils cloudina and titanotheca is described from the corumba group the vendotaenid eoholynia corumbensis sp nov is described from siltstones of the guaicurus formation an important diversity of skeletal fossils in the corumba arroyo del soldado and nama groups points to favourable vendian palaeoclimatic conditions in sw gondwana preliminary carbon isotopic data show a series of alternating positive and negative excursions corroborating the upper vendian age indicated by fossils for both units previously reported strontium isotopic data are also consistent with this age it is postulated that the corumba and asgs were deposited onto the same shelf which opened to the east the rio de la plata superterrane craton extends farther to the north than previously expected or it was already amalgamated with the amazonian craton by vendian times collision of the platform with the parana block caused closure of the basin during the cambrian early ordovician finally models of neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed

shocked quartz from the late eocene impact evidence from massignano italy this paper reports the discovery of shocked quartz grains within a well dated upper eocene marlstone at the massignano outcrop ancona italy the shocked quartz reaches a maximum concentration of 1 6 0 4 grains per cubic centimetre of bulk rock a modest ir anomaly is also coincident with the shocked quartz and together they strongly support an impact origin moreover this impact layer has a precise interpolated radiometric age of 35 7 0 4 ma although impact evidence found throughout the world indicates that at least two large impacts did occur during this time the actual number timing and environmental effects of these impact events remain unclear the late eocene was a critical time in earth history because of significant climatic biotic and environmental crises

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

the proterozoic cambrian transition within the charnian supergroup of central england and the antiquity of the ediacara fauna a revised interpretation for the charnian supergroup is suggested from ichnological and lithological comparisons with the cambrian of nuneaton and other parts of the avalonian superterrane late neoproterozoic volcaniclastic sediments contain an ediacara fauna in the maplewell group these are succeeded by one or more inferred erosional breaks and overlain by passive margin sediments quartzarenites glauconitic greywackes pelites of the brand group which yield cambrian trace fossils including abundant large teichichnus this transformation is compared with that between the neoproterozoic caldecote volcanic formation and the cambrian hartshill formation at nuneaton the hanging rocks formation of the brand group lies above a major erosional break but shares a volcanogenic island arc petrology with the underlying maplewell group conglomeratic sandstones at the base of the brand hills formation contain purple pelite clasts comparable with underlying lithologies plus clasts of weathered diorite showing granophyric textures identical to those seen in the enigmatic south charnwood diorites nd sm isotope studies confirm the geochemical similarity between these diorites and those that intruded the caldecote volcanics at nuneaton previously u pb zircon dated at 603 2 ma these strong similarities with the nuneaton succession suggest that deposition of the brand hills formation in the cambrian was preceded by the local unroofing of diorite and erosion of underlying mudrocks much as seen in other parts of avalonia a date of 603 2 ma for the south charnwood diorites provides a minimum age for the local ediacara fauna but this is in need of substantiation

late eocene north american microtektites and clinopyroxene bearing spherules approximately 35 m y ago two events separated by only 10 000 20 000 years each spread roughly 1 billion metric tons of silicate spherules over large areas of the earth s surface the older event produced mostly clinopyroxene bearing spherules cpx spherules the cpx spherules have been found in the caribbean sea gulf of mexico equatorial pacific and eastern equatorial indian ocean the cpx spherule layer is associated with an iridium anomaly and appears to coincide with the extinction of several species of radiolaria the composition and widespread distribution of the cpx spherules indicate that they were formed by an impact event the younger layer of spherules consists of microtektites belonging to the north american tektite strewn field based on age and elemental and isotopic composition the north american microtektites have been found in the western equatorial atlantic on barbados in the caribbean sea and in the gulf of mexico the north american microtektite layer is not synchronous with an iridium anomaly nor does it appear to be associated with any major changes in the microfossils neither the cpx spherule event nor the north american tektite event appears to be associated with a climatic change but no detailed studies have been made across either layer excluding the cpx layer which contains mostly crystal bearing spherules the evidence at the present time indicates that there is only one late eocene microtektite layer and it is part of the north american strewn field

comet showers as a cause of mass extinctions if at least some mass extinctions are caused by impacts why do they extend over intervals of one to three million years and have a partly stepwise character the solution may be provided by multiple cometary impacts astronomical geological and palaeontological evidence is consistent with a causal connection between comet showers clusters of impact events and stepwise mass extinctions but it is too early to tell how pervasive this relationship may be

further shrimp geochronology on the early cambrian of south china zircon 206pb 238u ages are reported for bentonites from two cambrian sections in south china the meishucun and the fandian the analyzed zircons vary widely in age because of the presence of detrital and altered grains spot ages were corrected for calibration drift during analysis and the age of reference zircon sl13 was calibrated as 580 ma against other standards using sims use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages two age groups occur within the meishucun bed 5 bentonite of which the larger at 518 7 2 5 ma is interpreted as a post depositional zircon growth event and the other at 539 4 2 9 ma as the volcanic age bentonites from the meishucun bed 9 and its fandian equivalent have two age groups 526 5 1 1 ma and 540 2 1 0 ma we interpret the younger of these as the tuff magmatic age and the older as detrital small shelly fossils indicate meischucunian bed 5 is an equivalent of the late nemakit daldynian and its 539 4 2 9 ma age both closely times the base of the cambrian and is a maximum for the basal tommotian the 526 5 1 1 ma for bed 9 and its equivalents approximates the close of the tommotian

chemical composition of north american microtektites and tektite fragments from barbados and dsdp site 612 on the continental slope off new jersey abstract we have determined the major element and trace element content of tektite fragments and microtektites found in deep sea sediments from barbados and dsdp site 612 on the continental slope off new jersey the major element and trace element contents were determined using energy dispersive x ray analysis and instrumental neutron activation analysis respectively the compositions are consistent with the conclusion that the samples from both occurrences belong to the north american tektite strewn field the chemistry of the samples from both occurrences is however not identical with the bediasite or georgianite chemistry there are differences especially in the lower sodium and higher k and cr and possibly higher volatile element contents of the tektite fragments from dsdp site 612 the differences in chemistry between bediasites georgianites and the tektites from the sites analyzed here show that there are geographical variations in composition in the north american strewn field like in the czechoslovakian or australasian strewn field the relatively high volatile element contents e g sb and au of the site 612 tektite fragments together with the occurrence of mineral inclusions and a layered structure of some specimens indicates that these specimens are or close to muong nong type tektites if the dsdp site 612 tektites are muong nong type then the source crater has to be close to that site and is therefore probably on the continental shelf as had previously been suggested tektites on land usually occur in deposits which are much younger than the tektites the so called age paradox the association of tektites and microtektites together in one layer with a stratigraphic age that is consistent with the radiometric age would seem to resolve this paradox

the granulitic impactite suite impact melts and metamorphic breccias of the early lunar crust abstract an important and poorly understood group of rocks found in the ancient lunar highlands is called feldspathic granulitic impactites rocks of the granulite suite occur at most of the apollo highlands sites as hand samples rake samples clasts in breccias and soil fragments most lunar granulites contain 70 80 modal plagioclase but they can range from anorthosite to troctolite and norite previous studies have led to different interpretations for the thermal history of these rocks including formation as igneous plutons long duration metamorphism at high temperatures and short duration metamorphism at low temperatures this paper reports on a study of 24 polished thin sections of lunar granulites from the apollo 15 16 and 17 missions we identify three different textural types of granulitic breccias poikilitic granoblastic and poikilitic granoblastic breccias these breccias have similar equilibration temperatures 1100 50 c as well as common compositions crystal size distributions in two granoblastic breccias reveal that ostwald ripening took place during metamorphism solid state grain growth and diffusion calculations indicate relatively rapid cooling during metamorphism 0 5 to 50 c year and thermal modeling shows that they cooled at relatively shallow depths 200 m in contrast we conclude that the poikilitic rocks formed by impact melting whereas the poikilitic granoblastic rocks were metamorphosed and may have partially melted these results indicate formation of lunar granulites in relatively small craters 30 90 km in diameter physically associated with the impact melt breccia pile and possibly from fine grained fragmental precursor lithologies

early paleozoic avalon gondwana unity an obituary response to palaeontological evidence bearing on global ordovician silurian continental reconstructions by r a fortey and l r m cocks

marine biotic signals across a late eocene impact layer at massignano italy evidence for long term environmental perturbations the eocene oligocene transition marks the passage from greenhouse conditions to an icehouse state with progressive global cooling starting in the early middle eocene the late eocene presents substantial evidence for extraterrestrial impacts whose effects on living organisms and climatic changes are still not completely clear a high resolution microfloral and faunal investigation has been carried out in a 4 m thick segment of the massignano global stratotype section and point for the eocene oligocene boundary the studied interval includes a late eocene 35 7 0 4 myr old impactoclastic layer containing several cosmic signatures the impact event recorded at massignano had no abrupt dramatic effects on marine biota in terms of extinction however significant quantitative changes in the calcareous plankton and dinoflagellate cyst assemblages occurred 60 kyr after the impact event the observed pattern is intepreted as reflecting a long term re organization of water structure

late cambrian middle furongian shallow marine dysoxic mudstone with calcrete and brachiopod olenid lotagnostus faunas in avalonian cape breton island nova scotia the common belief that organic rich mudstones formed in quiescent distal settings is further weakened by study of an upper cambrian leptoplastus lower peltura superzones succession in the chesley drive group in avalonian cape breton island that is comparable to alum shale successions in baltica dramatic sea level likely eustatic changes are now recognized by punctuation of deposition of shallow wave influenced black mudstone with brachiopod orusia lenticularis and olenid trilobite bearing limestones by offlap and formation of a subaerially cemented calcrete clast conglomerate subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with leptoplastus superzone l ovatus zone trilobite sclerites dynamic processes are shown by wave ripples in the mudstone and limestone sorting and winnowing of fossil rudstones and pre compactional fracture of the conglomerate and rudstones orusia rudstones in the succession below the conglomerate are regarded as analogues of eoorthis and billingsella rudstones in the biomere extinction intervals of the laurentian basal sunwaptan the lowest orusia rich beds are no older than the p spinulosa zone but as elsewhere in avalonia they range into the higher leptoplastus cape breton and even the peltura britain new brunswick superzones rare agnostoid sclerites in lower peltura superzone ctenopyge tumida zone olenid rudstone resemble those traditionally assigned to lotagnostus trisectus in avalonian britain and sweden and are distinct from laurentian l americanus an l americanus zone cannot be identified in avalonia or baltica and the first appearance datum fad of purported l americanus is not suitable as a standard for the base of the highest cambrian stage

first evidence for cambrian glaciation provided by sections in avalonian new brunswick and ireland additional data for avalon gondwana separation by the earliest palaeozoic abstract the first evidence for cambrian glaciation is provided by two successions on the avalon microcontinent the middle lowest cambrian middle terreneuvian series and fortunian stage stage 2 boundary interval has an incised sequence boundary overlain by a fluvial lowstand facies and higher olive green marine mudstone on hanford brook southern new brunswick this succession in the lower mystery lake member of the chapel island formation may be related to melting of an ice sheet in avalon the evidence for this interpretation is a muddy diamictite with outsized up to 10 cm in diameter proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone that eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with watsonella crosbyi zone fossils 650 km away in avalonian eastern newfoundland a sea level rise within the watsonella crosbyi chron at ca 535 ma may correspond to a unnamed negative 13 c excursion younger than the basal cambrian excursion bace and the zhuce excursion in stage 2 of the upper terreneuvian series cambrian dropstones are now also recognized on the northern gander margin of avalon in continental slope rise sedimentary rocks in southeast ireland although their age early middle cambrian is poorly constrained dropstones in the booley bay formation provide additional evidence for cambrian glaciation on the avalon microcontinent besides providing the first evidence of cambrian glaciation these dropstone deposits emphasize that avalon was not part of or even latitudinally close to the terminal ediacaran cambrian tropical carbonate platform successions of west gondwana

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

suspect terranes and accretionary history of the appalachian orogen the north american connection or the grenvillian continental crust that separated from eastern north america during the initiation of the appalachian orogenic cycle is poorly defined within the present orogen a variety of terranes occur outboard of the north american miogeocline all structurally uncoupled and therefore suspect stratigraphic analysis indicates that appalachian accretion progressed from the miogeocline outward the boundaries of the earliest accreted western terranes are marked by melange and ophiolite complexes later boundaries between eastern terranes are steep mylonitic zones and brittle faults accretionary events defined by stratigraphic analysis correspond to times of major deformation plutonism and metamorphism in the history of the orogen

eocene oligocene transition in the southern ocean history of water mass circulation and biological productivity high resolution records of carbon and oxygen isotopes and benthic foraminiferal accumulation rates for the eocene oligocene section at ocean drilling program site 689 maud rise weddell sea paleodepth about 1500 m were used to infer variations in paleoproductivity in relation to changes in climate and ventilation of the deeper water column the benthic foraminiferal abundance and isotope records show short term fluctuations at periodicities of 100 and 400 ka implying orbitally driven climatic variations both records suggest that intermediate depth water chemistry and primary productivity changed in response to climate during the eocene productivity increased during cold periods and during cold to warm transitions possibly as a result of increased upwelling of nutrient rich waters in the oligocene in contrast productivity maxima occurred during intervals of low d18o values presumably warmer periods when a proto polar front moved to the south of the location of site 689 this profound transition in climate productivity patterns occurred around 37 ma coeval with rapid changes toward increasing variability of the oxygen and carbon isotope and benthic abundance records and toward larger amplitude d18o fluctuations therefore we infer that at this time temperature fluctuations increased and a proto polar front formed in conjunction with the first distinct pulsations in size of the antarctic ice sheet we speculate that this major change might have resulted from an initial opening of the drake passage at 37 ma at least for surface and intermediate water circulation

oxygen and carbon isotope stratigraphy of early cambrian carbonates in southeastern newfoundland and england carbonate rocks have been sampled through predominantly siliciclastic sediments above the precambrian cambrian global stratotype level in southeastern newfoundland to assess their potential for oxygen and carbon isotope stratigraphy comparable successions were sampled at nuneaton and comley in england greatly depleted 18 o signals are attributed to widespread thermal alteration during deep burial and granitic intrusion including within the stratotype region carbon isotope ratios appear to have been less affected and these are described from nine sections a provisional composite 13 c curve is based on non ferroan pink nodular and bedded micrites several 13 c excursions occur in the fossiliferous bonavista group and allow the position of the tommotian atdabanian boundary to be identified chemostratigraphic correlation of the new precambrian cambrian boundary stratotype may however prove difficult because of the lack of suitable well preserved carbonates the search must begin for a comparable reference section allowing global correlation of the boundary level using chemostratigraphy as well as biostratigraphy

geochronology of the cambrian a precise middle cambrian u pb zircon date from the german margin of west gondwana a volcanic tuff 1 0 m above the base of the triebenreuth formation in the franconian forest provides the first precise and biostratigraphically bracketed date within the traditional middle cambrian the first illustration of fossils from the triebenreuth formation in this report and their discussion allow a more highly refined correlation within the middle cambrian a weighted mean 206 pb 238 u date of 503 14 0 13 0 25 0 59 ma on zircons from this subaerial pyroclastic tuff was determined by u pb chemical abrasion isotope dilution mass spectrometry ca tims techniques at c 6 0 7 0 ma younger than the base of the traditional middle cambrian in avalonia the new west gondwanan date from east central germany suggests that estimates of 500 ma for the base of the traditional upper cambrian and 497 ma on the base of the furongian series may prove to be too old biostratigraphically well bracketed dates through most of the middle cambrian series 3 and below the upper upper cambrian upper furongian series do not exist an earlier determined 494 4 3 8 ma date from the southwell group of tasmania may actually prove to be a reasonable estimate for the age of the base of the traditional upper cambrian until high precision dates are determined on the base of the traditional upper cambrian and base of the furongian series the rates of biotic replacements and geological developments and the durations of biotic zones in the middle series 3 and upper cambrian furongian series remain as best guesses

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