structural evolution of the sarandi del yi shear zone uruguay kinematics deformation conditions and tectonic significance the sarandi del yi shear zone is a crustal scale shear zone that separates the piedra alta terrane from the nico perez terrane and the dom feliciano belt in southern uruguay it represents the eastern margin of the rio de la plata craton and consequently one of the main structural features of the precambrian basement of western gondwana this shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions giving rise to the reactivation of pre existing crustal fabrics in the easternmost piedra alta terrane afterwards pure shear dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions the mylonites resulting from this event were then locally reactivated by a cataclastic deformation this evolution points to strain localization under progressively retrograde conditions with time indicating that the sarandi del yi shear zone represents an example of a thinning shear zone related to the collisional to post collisional evolution of the dom feliciano belt that occurred between the meso to neoproterozoic 600 ma and late ediacaran lower cambrian times

first petrographic results on impactites from the yaxcopoil 1 borehole chicxulub structure mexico the icdp yaxcopoil 1 yax 1 borehole located 60 km south southwest of the center of the chicxulub impact structure intercepted an interval of allogenic impactites depth of 795 895 m petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations unit 1 795 922 m comprises an apparently reworked poorly sorted and graded fine grained clast supported melt fragment bearing suevitic breccia the interstitial material similar to units 2 and 3 is permeated by numerous carbonate veinlets units 2 823 846 m and 3 846 861 m are groundmass supported breccias that comprise green to variegated angular and fluidal melt particles the groundmass of units 2 and 3 comprises predominantly fine grained calcite altered alkali element ca and si rich cement as well as occasional lithic fragments unit 4 861 885 m represents a massive variably devitrified and brecciated impact melt rock the lowermost unit unit 5 885 895 m comprises highly variable proportions of melt rock particles mrp and lithic fragments in a fine grained carbonate dominated groundmass this groundmass could represent either a secondary ydrothermal phase or a carbonate melt phase or both units 1 and 5 contain well preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units all units show diagnostic shock deformation features in quartz and feldspar clasts our observations reveal that most felsic and all mafic mrp are altered they register extensive k metasomatism in terms of emplacement we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume whereby unit 1 was subsequently reworked by resurging water unit 4 represents a coherent impact melt body the formation of which involved a significant proportion of crystalline basement unit 5 is believed to represent an initial ejecta ground surge deposit

potential consequences of a mid pleistocene impact event for the middle stone age occupants of dakhleh oasis western desert egypt abstract while it is expected that prehistoric populations experienced the effects of impacts by extraterrestrial objects there is little record of such events in occupied regions we discuss the ramifications of an impact between 100 and 200 ka into dakhleh oasis at that time experiencing humid phase conditions and inhabited by middle stone age people based on modeling of impact processes using estimated impact parameters the impact blast would likely have felled or dismembered most trees over 280 2400 km 2 of the oasis and inhabitants would have been injured over much of the region although severely only near the blast location the impact would not have vaporized much of the dakhleh paleolake s although some water was probably lost through boiling and through waves initiated by the impact blast water quality would have decreased as lake turbidity increased and soot from regional fires fell into the lake a relatively small event would not have immediately ignited much of the oasis vegetation but sedimentary evidence suggests that impact generated fires did spread to significantly affect the landscape the impact and resulting fires may have triggered mass wasting events as well as significant changes in erosion rates and have substantially affected human uses of the area

u pb and th pb whole rock isochrons from metavolcanic rocks of the carolina slate belt felsic volcaniclastic rocks and massive pyrite of a part of the carolina slate belt in south carolina referred to as the haile brewer block seem to have remained closed systems with respect to u th and pb since 465 ma u pb and th pb whole rock isochrons indicate ages of 466 40 2 and 462 53 ma respectively the pb pb secondary isochron age of the volcaniclastic rocks is 455 ma because of the divergent ways in which u th and pb have behaved during the various alteration events that have affected the haile brewer block metamorphism post tectonic intrusion weathering the isochronous relationships most likely represent the emplacement age of the rocks the preservation of u pb and th pb isochrons is thought to be due to the originally glassy nature of the volcaniclastic rocks and to the relatively impermeable character of the rocks which may have been aided by the sealing of pore space by infiltration of penecon temporaneous silica rich hydrothermal fluids the u pb and th pb whole rock ages are considerably younger than the 550 ma age inferred from u pb zircon ages and ediacaran fossil assemblages for the slate belt in north carolina the younger ages agree however with the presence of sponge spicules no older than middle cambrian in the haile brewer block the younger age of the haile brewer block with respect to other parts of the carolina slate belt indicates that the belt is more complex than has been thought

is the central piedmont suture a low angle normal fault in the crystalline southern appalachians the carolina are terrane is in fault contact with the piedmont terrane along a seismically reflective surface dipping toward the hinterland and called the central piedmont suture the central piedmont suture may be interpreted as a thrust but existing data also support a silurian devonian normal slip origin 1 there are lower grade rocks in the hanging wall than in the footwall 2 a normal fault solution allows simultaneous metamorphism of the piedmont terrane and carolina terrane prior to their juxtaposition along the central piedmont suture 3 mineral ages in the piedmont terrane are older in the west than in the east consistent with an eastward progressive unroofing 4 along the western edge of the carolina terrane a linear belt of devonian subalkalic to alkalic granitoids and gabbro norites with low initial 87sr 86sr ratios may represent mantle derived magmas along the axis of rifting that are contemporary with major crustal extension the westernmost piedmont terrane includes the chauga belt the chauga belt comprises metavolcanic and metaplutonic units similar in rock type and age to those of the western carolina terrane chauga belt rocks are interpreted to be the westernmost exposures of the carolina terrane translated west on the lower plate by extension the piedmont and carolina terranes may thus compose a single lithotectonic element the piedmont terrane would represent the basement on which the arc was constructed the terrane was uplifted during extension along a major low angle normal fault recognized today as the central piedmont suture

kinematics u pb geochronology and 40ar 39ar thermochronology of the gold hill shear zone north carolina the cherokee orogeny in carolinia southern appalachians the gold hill shear zone is the most prominent pre carboniferous structure in the southern appalachian peri gondwanan tract of carolinia common perception based on indirect evidence holds that it is an acadian dextral strike slip shear zone however our recent structural and geochronologic studies directed at the shear zone indicate that it is a complex structure in both time and space structural studies in central north carolina kinematically link deformation in the shear zone to regional shortening structures in both the hanging wall and footwall and indicate that there was a sinistral component to the deformation collectively these structures constitute a regional sinistral transpressional system we obtained nine new u pb zircon ages ediacaran devonian and 12 new 40 ar 39 ar muscovite ages late ordovician middle mississippian these data in conjunction with the regional geology indicate that the shear zone has 12 km of stratigraphic throw and that the main motion on the zone was late ordovician collectively geologic relations structures and the distribution of 40 ar 39 ar ages indicate that the shear zone was reactivated in the late devonian and the middle mississippian the regional late ordovician silurian sinistral transpressive system of which the gold hill shear zone is part represents the most widespread tectonism documented in carolinia it overlaps in time with the ordovician silurian cherokee unconformity in laurentian strata and late ordovician silurian suprasubduction zone magmatism and metamorphism in peri laurentian rocks and consequently is considered a manifestation of the southern appalachian cherokee orogeny marking the accretion of carolinia to laurentia

reprocessing and reinterpretation of cocorp southern appalachian profiles abstract reprocessing of cocorp southern appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement the most important evidence is a nearly continuous series of subhorizontal reflections extending from the valley and ridge province into the piedmont province continuity of subhorizontal reflections becomes tenuous in the inner piedmont careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of events termination of sedimentary rocks beneath the piedmont is interpreted from true amplitude seismic data a zone of detachment continues southeast of the sediment termination as far as the master decollement root zone research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities this phenomenon may occur in the charlotte belt of georgia bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density variation in crustal thickness also contributes to the bouguer gravity gradient no continuous large scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the kings mountain belt and charlotte belt

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

paleoenvironmental changes across the cretaceous tertiary boundary at koshak kazakhstan based on planktic foraminifera and clay mineralogy abstract the koshak section of the mangyshlack peninsula kazakhstan is one of the most complete cretaceous tertiary k t transitions known from the boreal paratethys cretaceous species richness is low 11 to 13 species except for a peak of 20 species near the k t boundary in the uppermost maastrichtian top 50 cm that represents the temporary incursion of low latitude taxa this maximum species richness occurred during climatic warming associated with increased humidity as suggested by clay mineral analyses biofacies analysis suggests external platform conditions at this time followed by a more humid climate a sea level transgression and deepening basinal facies in the lower danian subzone p1a shallower platform conditions resumed in danian subzones p1b and p1c accompanied by a cooler and probably more arid climate no abrupt mass extinction occurred at the koshak k t boundary which is marked by an ir anomaly a clay layer and the first appearance of tertiary planktic foraminifera the influx of lower latitude species ends at or before the k t boundary whereas the majority of the indigenous cretaceous assemblage survived into the danian these data suggest that long term climatic changes may have been the principal factors in the progressive demise of the cretaceous planktic foraminifera in the eastern boreal paratethys

rotation of fabric elements in convergent shear zones with examples from the southern appalachians plane strain shear zones between rigid walls which do not rotate but which converge and move laterally relative to each other are here in referred to as convergent shear zones analysis of the deformation in convergent shear zones indicates the existence of two flow apophyses one parallel to the shear zone wall and the other inclined to the wall modeling of the development of fabrics in convergent shear zones indicates the occurrence of stable orientations in which s and c do not rotate and are oppositely inclined to the shear zone boundary the stable c orientation is parallel to the inclined flow apophysis and also is parallel to the approach velocity vector of the opposing walls of the shear zone if it can be demonstrated from field relationships that the walls of a shear zone were rigid and remained parallel then the occurrence of a flow apophysis inclined in the direction of shear is diagnostic of convergent shear s and c fabrics in the ridge road and gundy creek shear zones of the southern appalachian piedmont are interpreted to indicate a convergent shear regime with an approach velocity vector oriented 10 13 clockwise from the strike of the zones

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

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

high spatial resolution geochemistry and textural characteristics of microtektite glass spherules in proximal cretaceous paleogene sections insights into glass alteration patterns and precursor melt lithologies abstract using laser ablation inductively coupled plasma mass spectrometry la icp ms we have conducted spatially resolved trace element analysis on fresh unaltered microtektite glasses linked to the cretaceous paleogene k pg boundary chicxulub crater and on their surrounding alteration phases this unique approach offers the opportunity to study in situ and at high spatial resolution both the mixing of different target lithologies and the variation of the major and trace element budget during the alteration process in addition two dimensional element distribution maps reveal important geochemical information beyond the capabilities of single spot laser drilling glasses from two localities in opposite quadrants from the source crater were studied at the beloc locality haiti the glass population is dominated by the presence of yellow high ca glass and black andesitic glass formed by admixture of carbonate dolomite anhydrite platform lithologies with crystalline basement these glasses alter according to the well established hydration palagonitization model postulated for mafic volcanic glasses rees become progressively leached from the glass to below the detection limit for the applied spot size while immobile zr hf nb and ta passively accumulate in the process exhibiting both inter element ratios and absolute concentrations similar to those for the original glass in contrast the arroyo el mimbral locality ne mexico is characterized by abundant green glass fragments high in si al and alkalis and low in mg ca fe low si black glass is less abundant though similar in composition to the black glass variety at beloc the alteration pattern of high si al green glass at the mimbral locality is more complex including numerous competing reaction processes ion exchange hydration dissolution and secondary mineral precipitation generally controlled by the ph and composition of the surrounding fluid all green high si al glasses are hydrated and variably enriched in sr ba and cs indicating preferred adsorption from seawater during hydration despite the onset of ion exchange reactions which only seem to have affected the alkalis the trace element composition of the green high si al glass is still largely representative of the original melt composition refining the geochemical signature of altered melt lithologies may advance our current understanding of glass stability in the natural environment and provide insight into the origin and emplacement of ejecta material during crater formation

the carolina terrane in northwestern south carolina u s a late precambrian cambrian deformation and metamorphism in a peri gondwanan oceanic arc the carolina terrane comprises an exotic volcanic island arc in the hinterland of the southern appalachian orogen the western portions of the carolina terrane consist of zoned mafic ultramafic plutonic complexes intruded into a pile of basalts and basaltic andesites this sequence of rocks has been interpreted to represent an episode of intra arc rifting prior to regional metamorphism and deformation new u pb zircon ages from the mean crossroads complex in northwestern south carolina along the central piedmont suture confirm relative ages inferred from detailed mapping two foliated metadiorites yield u pb dates of 579 4 and 571 16 ma interpreted to be crystallization ages a foliated metaquartz diorite yields a u pb date of 538 5 ma interpreted to be a crystallization age an undeformed unmetamorpbosed diorite intruding these metamorphosed zoned complex intrusive rocks yields an age of 535 ma hence we believe that intra arc rifting and regional metamorphism and foliation development both occurred between 580 and 535 ma while petrographic and ar ar studies support subsequent middle to late paleozoic regional metamorphic overprint s or at least static recrystallization and or uplift through hornblende biotite muscovite blocking temperatures for ar the 535 ma undeformed unmetamorphosed intrusion suggests late precambrian early cambrian regional metamorphism and deformation was the event responsible for the regional metamorphic fabric in this area of the piedmont these observations contradict the idea that this metamorphism and fabric development are related to presumed early paleozoic accretion of the carolina arc to laurentia as well as correlations with middle ordovician fabric elements in the eastern piedmont instead this fabric is interpreted to record a changing plate kinematics e g migration of triple junction or change in relative motion vectors during semicontinuous diachronous development of an island arc on the gondwanide margin

microtektites and mass extinctions evidence for a late devonian asteroid impact glass spherules similar to microtektites have been found near the frasnian famennian boundary f f upper devonian at senzeilles belgium contemporaneous with one of the largest marine mass extinctions of the phanerozoic these spherules exhibit a wide range of compositions and display teardrop dumbbell and compound morphologies analogous to microtektites in addition they lack crystallites have few or no vesicles and have a low content of volatile material these characteristics are supportive of an impact origin the siljan ring sweden and charlevoix structure quebec canada are candidate craters of this age the presence of microtektites near the f f boundary supports the hypothesis that an impact caused the upper devonian worldwide benthic mass extinctions

deep in the heart of dixie pre alleghanian eclogite and hp granulite metamorphism in the carolina terrane south carolina usa the central part of the carolina terrane in western south carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the carolina slate belt to the se and amphibolite facies metavolcanic and metaplutonic rocks of the charlotte belt to the nw this region termed the silverstreet domain is characterized by penetratively deformed felsic gneisses granitic gneisses and amphibolites mineral assemblages and textures suggest that these rocks formed under high pressure metamorphic conditions ranging from eclogite facies through high p granulite to upper amphibolite facies mafic rocks occur as amphibolite dykes as metre scale blocks of coarse grained garnet clinopyroxene amphibolite in felsic gneiss and as residual boulders in deeply weathered felsic gneiss inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies con ditions all samples have been partially or wholly retrograded to amphibolite assemblages we infer the following p t t history 1 eclogite facies p t conditions at 1 4 gpa 650 730 c 2 high p granulite facies p t conditions at 1 2 1 5 gpa 700 800 c 3 retrograde amphibolite facies p t conditions at 0 9 1 2 gpa and 720 660 c this metamorphic evolution must predate intrusion of the 415 ma newberry granite and must postdate formation of the charlotte belt and slate belt arcs 620 to 550 ma comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis eclogite and high p granulite facies metamorphism in the silverstreet domain may coincide with a 570 535 ma event documented in the western charlotte belt or to a late ordovician early silurian event the occurrence of these high p assemblages within the carolina terrane implies that prior to this event the western carolina terrane charlotte belt and the eastern carolina terrane carolina slate belt formed separate terranes the collisional event represented by these high pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to laurentia

reinterpretation of adcoh and cocorp seismic reflection data with constraints from detailed forward modeling of potential field data implications for laurentia peri gondwana suture abstract to better constrain the structure of the laurentian peri gondwana suture zone maps and a 2 dimensional regional cross section model constrained by seismic data and surface geology have been developed by forward and inverse modeling the aeromagnetic and gravity fields the central piedmont suture cps the boundary between the laurentian inner piedmont and the peri gondwanan carolina terrane is a low angle thrust fault 30 ramping up from an alleghanian mid crustal detachment at depths of about 12 km adcoh and cocorp seismic data image anticlinal structures in the footwalls of the hayesville thrust and the cps above the alleghanian decollement the footwall rocks have previously been interpreted as paleozoic shelf strata on the basis of sub horizontal seismic reflectors however the high densities required to fit the observed gravity anomaly suggest that the folded footwall reflectors may need to be reinterpreted as horse blocks or duplex structures of grenvillian basement the appalachian paired gravity anomaly can be explained by an increase in crustal thickness and a decrease in upper crustal density moving northwestward from the carolina terrane toward the appalachian core a change in lower crustal density is not required so that grenville basement rocks may extend farther to the southeast than previously thought the 5 to 10 km of alleghanian uplift and exhumation predicted by p t crystallization data compiled in this paper can be easily accommodated by thrusting on four major low angle thrust systems great smoky mountain thrust gsmt hayesville brevard and cps unroofing of metamorphic core complexes by normal faulting may therefore not be required to explain the observed exhumation alleghanian collision along the southeastern appalachian margin was predominately orthogonal to strike consistent with the previous reconstructions that call for the counter clockwise rotation of gondwanan west africa creating head on collision in the southern appalachians and at least 370 km of shortening

extraterrestrial impact events the record in the rocks and the stratigraphic column abstract the known terrestrial impact record is a biased sample of a much larger population of impact events the biases are due to the modifying effects of terrestrial geologic processes coupled with incomplete searches for impact structures and impact related materials terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but because of post impact modification by terrestrial geologic process are recognised by the occurrence of shock metamorphic effects in some cases siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body similar shock metamorphic effects and a siderophile anomaly in k t boundary materials are indicative of a major impact event which has been correlated with the formation of the chicxulub structure mexico evidence of a small number of other impacts occur in the stratigraphic record most commonly as tektite or microtektite horizons in some cases they are known to be accompanied by geochemical anomalies in other cases a number of ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact the majority of known impact events in the stratigraphic record are from relatively recent geologic time logic dictates however that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts d 20 km have the potential to cause atmospheric blow out and thus global dispersion of some of the impact products geochemical detection however of such events may not be easy in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material in addition non chondritic bodies may result in no appreciable geochemical anomaly in view of this any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials although in the case of impacts into oceanic crust this too will be difficult given the k t experience however and the fact that large scale impact on earth is a natural consequence of the character of the solar system the potential of impacts to provide local and global marker horizons can not be ignored similarly the fact that impacts may have the potential to result in short term biologic or climatic excursions can not be dismissed arbitrarily when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record

a thick tethyan multi bed tsunami deposit preserving a dinosaur megatracksite within a coastal lagoon barremian eastern spain abstract a thick multiple bed tsunami deposit consisting of sandstones and conglomerates has been discovered and investigated in the camarillas formation 130 6 128 4 ma barremian age in eastern spain the tsunami deposit is interbedded within red mudstones deposited in mud flats of a back barrier system it crops out along 7 km in length and at its base a great number of dinosaur tracks assigned to sauropods ornithopods and theropods have been preserved as natural casts then constituting an exceptional regional megatracksite associated with tsunami deposits on the basis of sedimentological features and the lateral and vertical architecture of the involved lithofacies up to five couplets of inflow backflow deposits formed by a tsunami wave train have been recognized overlying the tracks although sedimentation mainly took place during backflow currents inflows led to the removal of sand from a fronting barrier island and the rip up of lagoonal carbonate and clay pebbles depositing them in the protected back barrier lagoon its unusually great thickness is interpreted among others as being the result of the filling of the previous low topography of the back barrier lagoon

the cretaceous palaeogene boundary at gorgonilla island colombia south america the discovery of a new cretaceous palaeogene k pg bathyal marine sequence on gorgonilla island sw colombia extends the presence of chicxulub impact spherule deposits to the pacific region of northern south america and to the eastern pacific ocean the gorgonilla spherule layer is approximately 20 mm thick and consists of extraordinarily well preserved glass spherules up to 1 1 mm in diameter about 70 90 of the spherules are vitrified and their chemical composition is consistent with haiti beloc impact glass spherules normal size grading delicate spherule textures welded melt components and an absence of bioturbation or traction transport suggest that the gorgonilla spherule layer represents an almost undisturbed settling

tsunami induced conglomerates in miocene upper bathyal deposits chita peninsula central japan abstract the tsubutegaura conglomeratic tsunamiites occur in the middle section of the miocene storm related sand silt alternation system in the chita peninsula central japan deposition of the system in the upper bathyal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological palaeogeographical and sedimentological studies two coupled units each built up by a conglomerate layer with an overlying sandstone layer and the alignment of lenticular sedimentary bodies are exceptional in this sequence some typical lenticular bodies confined within sedimentary troughs consist of a boulder bearing conglomerate layer and calcareous sandstone layer of the lower couple and another conglomerate layer of the upper couple the tuffaceous sandstone layer of the upper couple is distributed more widely than the other lithologies the framework gravels in the coupled conglomerates form a clast supported fabric and are quite notably monomictic the clasts are angular and imbricated partly forming peculiar gravel clusters due to traction current transportation conspicuous laminations develop in the coupled sandstone layers antidunes with chute and pool structure in the upper couple sandstone layer indicate deposition from an upper flow regime current on the other hand the calcareous composition of the lower couple sandstone layer reveals shallow water provenance thixotropic deformations and diastasis cracks in the siltstone bed immediately beneath the layers of the couples show that a severe earthquake tremor and a rapid change of water pressure occurred just before the deposition of the tsunamiites submarine debris flows due to collapse of a fault scarp in a shallow bank and the ensuing wash by two pulses of the tsunami induced ebb current comprise the scenario for the formation of the tsubutegaura conglomeratic tsunamiites

biotic effects of the chicxulub impact k t catastrophe and sea level change in texas abstract biotic effects of the chicxulub impact the k t event and sea level change upon planktic foraminifera were evaluated in a new core and outcrops along the brazos river texas about 1000 km from the chicxulub impact crater on yucatan mexico sediment deposition occurred in a middle neritic environment that shallowed to inner neritic depths near the end of the maastrichtian the sea level fall scoured submarine channels which were infilled by a sandstone complex with reworked chicxulub impact spherules and clasts with spherules near the base the original chicxulub impact ejecta layer was discovered 45 60 cm below the sandstone complex and predates the k t mass extinction by about 300 000 years results show that the chicxulub impact caused no species extinctions or any other significant biotic effects the subsequent sea level fall to inner neritic depth resulted in the disappearance of all larger 150 m deeper dwelling species creating a pseudo mass extinction and a survivor assemblage of small surface dwellers and low oxygen tolerant taxa the k t boundary and mass extinction was identified 40 80 cm above the sandstone complex where all but some heterohelicids hedbergellids and the disaster opportunistic guembelitrids went extinct coincident with the evolution of first danian species and the global 13c shift these data reveal that sea level changes profoundly influenced marine assemblages in near shore environments that the chicxulub impact and k t mass extinction are two separate and unrelated events and that the biotic effects of this impact have been vastly overestimated

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

new field evidence for an exposure of the appalachian decollement at the east end of the pine mountain terrane georgia detailed field mapping at the east end of the pine mountain terrane in central georgia indicates that the box ankle fault is an 4 km thick ductile shear zone separating grenvillian basement of laurentian affinity in the footwall from allochthonous accreted terranes in the hanging wall fabric data from the box ankle fault indicate that it is a gently southeast dipping northwest vergent thrust fault recently discovered structural relations together with geochronological data indicate that the box ankle fault is of late paleozoic age we argue that the box ankle fault is a surface exposure of the appalachian decollement the east end of the pine mountain terrane may be the only place in the entire orogen where this fundamentally important structure is exposed for study

the hyco shear zone in north carolina and southern virginia implications for the piedmont zone carolina zone boundary in the southern appalachians the recently recognized hyco shear zone is a first order structure in the southern appalachians that separates sharply contrasting rocks of the carolina and milton terranes in north central north carolina and southern virginia traditionally this boundary has been viewed as a comformable contact between infrastructural milton rocks and suprastructural carolina rocks in contrast our work indicates that shallowly dipping sillimanite gneisses of the milton terrane comprise the footwall of the ductile shear zone whereas moderately to steeply dipping greenschist to amphibolite facies carolina terrane gneisses and metavolcanics form the hangingwall the hyco shear zone has a sharply arcuate surface trace it trends east northeast in the area of hyco lake north carolina and abruptly changes trend to north northeast near the north carolina virginia state line the zone is moderately to steeply south southeast dipping along the hyco lake segment and structures there indicate dominantly dextral strike slip along the virginia segment the zone is moderately to shallowly east dipping with structures indicating thrusting of the carolina terrane over the milton terrane timing constraints on synkinematic plutons indicate that the shear zone is an early alleghanian feature the geometry kinematics and timing of the shear zone are compatible with the interpretation that the hyco lake and virginia segments of the zone represent lateral and frontal ramps respectively in an overall alleghanian thrust regime recognition of the shear zone in conjunction with geological geophysical and isotopic contrasts across the zone negates established views that the milton rocks are a comformable higher grade portion of the carolina terrane these contrasting features instead are remarkably similar to those found along the central piedmont suture the tectonic boundary between the piedmont and carolina zones from south carolina to central north carolina we propose that the hyco shear zone is the northern equivalent of this structure thus our new data on the hyco shear zone have significant regional implications concerning the nature timing and kinematics of this zone boundary

gale crater and impact processes curiosity s first 364 sols on mars abstract impact processes at all scales have been involved in the formation and subsequent evolution of gale crater small impact craters in the vicinity of the curiosity msl landing site and rover traverse during the 364 sols after landing have been studied both from orbit and the surface evidence for the effect of impacts on basement outcrops may include loose blocks of sandstone and conglomerate and disrupted fractured sedimentary layers which are not obviously displaced by erosion impact ejecta blankets are likely to be present but in the absence of distinct glass or impact melt phases are difficult to distinguish from sedimentary volcaniclastic breccia and conglomerate deposits the occurrence of individual blocks with diverse petrological characteristics including igneous textures have been identified across the surface of bradbury rise and some of these blocks may represent distal ejecta from larger craters in the vicinity of gale distal ejecta may also occur in the form of impact spherules identified in the sediments and drift material possible examples of impactites in the form of shatter cones shocked rocks and ropy textured fragments of materials that may have been molten have been observed but cannot be uniquely confirmed modification by aeolian processes of craters smaller than 40 m in diameter observed in this study are indicated by erosion of crater rims and infill of craters with aeolian and airfall dust deposits estimates for resurfacing suggest that craters less than 15 m in diameter may represent steady state between production and destruction the smallest candidate impact crater observed is 0 6 m in diameter the observed crater record and other data are consistent with a resurfacing rate of the order of 10 mm myr considerably greater than the rate from impact cratering alone but remarkably lower than terrestrial erosion rates

chicxulub impact event is cretaceous paleogene boundary in age new micropaleontological evidence abstract high resolution and quantitative planktic foraminiferal biostratigraphy from two se mexico stratigraphic sections bochil guayal shows that the chicxulub related complex clastic unit ccu is synchronous with the ejecta rich airfall layer and the cretaceous paleogene k pg catastrophic mass extinction horizon in the el kef tunisia and caravaca spain sections the lowermost danian h holmdelensis subzone biozone p0 was identified in both sections in a thin dark clay bed just above the ccu proving that such bed is chronostratigraphically equivalent to the k pg boundary clay of the el kef stratotype these new micropaleontogical data confirm that the k pg impact event and the chicxulub impact event are the same one this contradicts the suggestion by others that the chicxulub impact predated the k pg boundary by about 300 ka

magnetic characterization of cretaceous tertiary boundary sediments rock magnetic properties across several k t boundary sections have been investigated to reveal any possible magnetic signature associated with the remains of the impact event at the end of the cretaceous studied sections locations vary in distance to the chicxulub structure from distal agost and caravaca spain through closer odp hole 1049a blake nose north atlantic to proximal el mimbral and la lajilla mexico a clear magnetic signature is associated with the fireball layer in the most distal sections consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization sirm and a decrease in remanence coercivity magnetic properties in these sections point to a distinctive ferrimagnetic phase probably corresponding to the reported mg and ni rich highly oxidized spinels of meteoritic origin at closer and proximal sections magnetic properties are different although there is an increase in susceptibility and sirm associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections and with a similar limonitic layer on top of the spherule bed that defines the boundary at blake nose the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine grained goethite based on previous geochemical studies at blake nose and new geochemical and pge abundance measurements performed in this work at el mimbral this goethite rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of fe there is not enough evidence to assert that this fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material magnetic geochemical and iridium data reject it as a primary meteoritic phase

impact melts and glasses on mars 1 dark mobile materials on mars are widely considered to be volcanically derived windblown particulates such a conclusion is consistent with the large volcanic edifices on mars and the assumption that a volatile rich crust would not favor significant impact melt generation several post viking discoveries however provide new perspectives for reconsidering alternative origins not only for some of the mobile dark veneers but also certain materials at the pathfinder and viking landing sites first certain dark materials on mars do not exhibit the expected mafic absorption features in their reflectance spectra second the pathfinder mission revealed unexpected feldspathic compositions of vesicular blocks that lead to a preliminary conclusion that they represent andesites or basaltic andesites rather than basalts third recent discoveries of holocene to miocene impacts in argentine loessoid deposits provide new insights for the nature of melts generated from eolian sediments fourth global dispersal of impact melt products are now recognized in the terrestrial record throughout the cenozoic consequently the generation and preservation of impact glass on mars should be reconsidered possible occurrences include possible tektite like strewnfields downrange from recent oblique impacts dark avalanches emerging from backwasting cliffs of easily eroded unconformable deposits global dispersal of glassy ejecta from hesperian and amazonian craters of chicxulub scale and dark mobile veneers emerging from noachian craters we propose that the mobile dark materials comprising many of the named features of mars as well as some blocks at the pathfinder viking landing sites may represent impact melt products

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

late maastrichtian age of spherule deposits in northeastern mexico implication for chicxulub scenario in the la sierrita area of nuevo leon mexico three spherule layers are present and separated from the overlying siliciclastic deposits by up to 6 m of pelagic marls the marls are of latest maastrichtian age plummerita hantkeninoides cf1 zone micula prinsii zone and deposited under normal pelagic conditions with no significant evidence of reworking or slumping original deposition of the spherule layers occurred during the last 300 ka of the maastrichtian and well prior to the cretaceous tertiary k t boundary event thus if the spherules in northeastern mexico provide critical evidence of an impact at chicxulub this impact predates the k t boundary

ocmulgee fault the piedmont avalon terrane boundary in central georgia the ocmulgee fault is a fundamental boundary within the piedmont of central georgia separating north american piedmont terrane rocks from exotic rocks of the avalon carolina terrane contrasts in stratigraphy metamorphic grade and aeromagnetic signature coincide with the fault trace the fault comprises a broad zone ca 2 km of foliation dipping steeply southeast and containing localized syn and post thermal peak right lateral strike slip shear zones oblique dextral convergence between the avalon and peidmont terranes most likely around 350 ma produced the steep zone and its attendant dextral shear zones steep foliation within the fault zone resulted in its predisposition to later faulting

when the sky fell on our heads identification and interpretation of impact products in the sedimentary record research on impact products mechanisms and consequences has benefited greatly in the last ten years from the highly publicized cretaceous tertiary kt boundary debate this debate has led to an increasing awareness among earth scientists of the importance and potential effects of impact events in earth history with more and more researchers realizing the role played by impacts and becoming familiar with the characteristics of impact materials it is likely that in the coming years an increasing number of impact events will be identified in agreement with the earth cratering rate estimations grieve 1984 shoemaker et al 1990 the increasing recognition of impact event is demonstrated by the discovery or identification in recent years of several impact craters some of large diameter chicxulub 180 km in yucatan mjolnir 40 km in the barents sea ames 16 km in oklahoma avak 12 km in alaska pretoria saltpan 1 1 km and kalkkop 640 m both in south africa sediments record impact events in the form of impact glass shocked minerals ir anomalies and perhaps also polymic chaotic breccias and or tsunami deposits identification of impact products particularly in ancient rocks is hampered by poor preservation diagenetic alteration metamorphism misinterpretation or even lack of interest or awareness of their significance impact products represent very short term events most likely geographically localized and stratigraphically concentrated over a thin interval of sediments and often diluted by normal background deposition finding these impact signatures requires very detailed petrographic and geochemical investigations with closely spaced sampling a few cm such detail is not commonly used when studying ancient mesozoic and paleozoic sediments

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

generation segregation ascent and emplacement of alleghanian plutons in the southern appalachians the carboniferous permian alleghanian orogeny in the southern appalachians produced a distinctive 45 m y long intrusive event that formed a ne trending band of dispersed composite plutons covering 10 500 km 2 of these plutons 75 are metaluminous biotite or amphibole biotite granitoids with lower crustal sources and 20 are peraluminous biotite muscovite garnet or cordierite biotite granitoids with more evolved crustal sources the remaining 5 are gabbroids intermediate rocks and possible alkaline granitoids the granitoid to gabbroid intermediate rock area ratio is 180 1 the exposed intrusive rocks were produced at a relatively slow rate of 2 km 3 linear km m y along the orogen indicating an inability of the source to produce much melt inefficient melting mechanisms or difficulty of magma segregation and ascent the absence of spatial trends in rock composition and age of emplacement the small amount but relative abundance of the granitoids and coincidence with con

docking carolina mid paleozoic accretion in the southern appalachians accretion of the extensive carolina zone to laurentia is one of the most critical unresolved problems in southern appalachian tectonics synthesis of southern appalachian tectonism reveals three lines of evidence from across the orogen that indicate a late ordovician to silurian time of accretion a tectonic unconformity in native laurentian rocks an extensive magmatic pulse in the piedmont zone of the orogen and active uplift in the carolina zone are all related to this accretionary event structural geometries in the carolina zone suggest that accretion involved sinistral transpression this new model for carolina accretion is consistent with models for the accretion of northern appalachian peri gondwanan terranes collectively their timing and mode of accretion to laurentia place tectonic constraints on paleozoic global reconstructions

confirmation of the carolina slate belt as an exotic terrane an assemblage of middle cambrian atlantic faunal province trilobites has been found in the rocks of the carolina slate belt near batesburg south carolina geologic and paleomagnetic data suggest that the carolina slate belt and the adjacent charlotte belt constitute an exotic terrane that was accreted to north america in early to middle paleozoic time

the association of folds and veins in shear zones abstract paired hook shaped asymmetric folds with shared limbs cut by axial planar veins dikes or fractures are common in glaciers and rocks the sense of offset across the vein or fracture is characteristically opposite to that expected from the sense of fold asymmetry the kinematics of development of the folds and the relationship of the folds to the fractures or veins can be understood by considering the nature of glacial flow and by utilizing simple clay experiments in zones of simple shear dilational fractures may form at about 45 to the shear zone boundaries the perturbation of strain around such a fracture during its development and subsequent rotation and closure under continued deformation causes layers or pre existing foliation cut by the fracture to become folded thus folding is a consequence of the fracturing and not vice versa the precise shape of the fold depends on the nature of the perturbation and particularly whether or not the fracture remains for a time open or fluid filled and thus has walls unable to sustain a shear stress during vein rotation paired hook folds provide evidence for non coaxial strain and are good sense of shear indicators however not all folds with veins parallel to their axial surfaces form in the way described in this paper in some cases vein emplacement occurs late during fold development or post dates folding in these cases offsets along the veins are consistent with fold asymmetry

sensitivity of supercell simulations to initial condition resolution abstractobservational and model resolution limitations currently preclude analysis of the smallest scales important to numerical prediction of convective storms these missing scales can be recovered if the forecast model is integrated on a sufficiently fine grid but not before errors are introduced that subsequently grow in scale and magnitude this study is the first to systematically evaluate the impact of these initial condition ic resolution errors on high resolution forecasts of organized convection this is done by comparing high resolution supercell simulations generated using identical model settings but successively coarsened ics consistent with the warn on forecast paradigm the simulations are initialized with ongoing storms and integrated for 2 h both idealized and full physics experiments are performed in order to examine how more realistic model settings modulate the error evolution in all experiments scales removed from the ic wavelengths 2 4 8 or 16 km regenerate within 10 20

character of the alleghanian orogeny in the southern appalachians part iii regional tectonic relations geological and geochronological studies in the eastern piedmont of the southern appalachians have documented a polyphase late paleozoic deformational chronology d 2 d 4 that is contemporaneous with the alleghanian orogeny recorded in the western appalachian foreland the scale and vergence of d 3 anticlinoria in the eastern piedmont are similar to those of anticlinoria in the western appalachians which are related to ramping of underlying thrust surfaces we suggest that the appalachian decollement extends southeastward beneath the entire piedmont province and that all of the piedmont rocks were displaced at least 175 km northwestward relative to north america during the alleghanian orogeny late paleozoic deformational effects in the eastern piedmont thus are considered cogenetic with alleghanian foreland deformation we interpret the alleghanian orogeny to be the result of oblique dextral convergence and collision of gondwana and laurentia

half a century of progress in research on terrestrial impact structures a review abstract the author who investigated the wolfe creek australia in 1962 and edited two benchmark sets of readings on meteorite craters and possible astroblemes in 1977 and 1979 reviews the state of knowledge at the present time the text is concerned with terrestrial impact structures geological features without any consideration of extraterrestrial analogues a handful of definitive publications are drawn on to present the story of terrestrial impact in a single article the text covers historical aspects briefly the effect of target variations the paucity of human observation of such large scale events distinction from volcanic endogenous structures modification by geological processes the transience of the crater initially formed on the target and its subsequent modifications the global geographic distribution of the 174 structures now listed of which a number are dubious attributions their distribution in geological time many ages being known only known to wide limits maximum or minimum values their size distribution calculations of impact frequencies shock effects processes on impact the stages of formation impact into shallow marine and deep sea targets impacts on ice about which little is known and finally the input of impact into biotic extinctions in this last lengthy section the summaries of the conclusions of scientists researching impact on earth and palaeontologists researching biotic impact are set side by side it is concluded that if the recent foraminiferal evidence obtained by gerta keller and associates is taken at its face value the case of impact as a sole agent in extinction is non existent biotic extinction is clearly a complex process involving a number of causes in some cases it was staggered in time and different sets of organisms responded quite differently and surprisingly even in the same extinction event extraterrestrial impact may have been one of the causes in some cases but it may have been regional rather than global in its effects we may never know how much input it had into the record of biotic extinction on earth an enormous amount of new knowledge has arisen from detailed studies of this new family of remarkable geological structures

nickel rich magnesiowiistite in cretaceous tertiary boundary spherules crystallized from ultramafic refractory silicate liquids we have found trace inclusions of ni rich magnesiowastite within grains of magnesioferrite spinel recovered from cretaceous tertiary boundary sediments from dsdp site 596 south pacific 23 51 20 s 169 39 27 w and dsdp site 577 north pacific 3 6 51 n 157 43 40 e measured compositions of these inclusions range from mg 85ni 74fe 17 o to mg 74ni 09fe 1 7 o coexisting magnesioferrite and magnesiowastite can only crystallize from ultramafic refractory mg rich liquids with mgsi 2 atom ratio such liquid compositions cannot form as a result of fractional crystallization and are unknown to occur as a result of terrestrial igneous processes or meteoroid ablation we infer that these minerals crystallized from liquid droplets that equilibrated with silicate vapor at high temperatures probably 2300 c resulting in fractionation of volatile sio2 from more refractory mgo the most plausible source of this high temperature vapor is in the fireball of the major impact event that terminated the cretaceous

basinward transport of chicxulub ejecta by tsunami induced backflow la popa basin northeastern mexico and its implications for distribution of impact related deposits flanking the gulf of mexico ejecta bearing strata are present at the top of cretaceous foreland basin deposits throughout the la popa basin in northeastern mexico in the southeast part of the basin locally thick as much as 4 6 m ejecta rich conglomeratic strata occupy valley like features at a bathymetric break that separated maastrichtian upper shoreface from lower shoreface and prodelta depositional settings clast supported textures normally graded planar conglomerate sandstone couplets upcurrent dipping low angle cross laminae sparse paleocurrent data and transported fossils indicate deposition by south to southeast directed turbulent supercritical flow in the northwest part of the basin ejecta grains are present but less common in correlative deposits sediment ejecta and organisms were eroded from shoreward environments and transported basinward by backflow of run up surge s emplaced against the continent by one or several tsunami s high discharge supercritical offshore directed flow provides a mechanism for transport of voluminous ejecta bearing sediment and late maastrichtian marine organisms into deep water gulf of mexico settings

what is the intrinsic predictability of tornadic supercell thunderstorms abstracta 25 member ensemble of relatively high resolution 75 m horizontal grid spacing numerical simulations of tornadic supercell storms is used to obtain insight on their intrinsic predictabil

what is a mega tsunami abstract no unambiguous and widely accepted definition currently exists for the term mega tsunami this is in spite of the rapidly growing popularity of the expression in the scientific literature especially in recent years following the devastation wrought by the 2004 indian ocean and 2011 tohoku oki tsunamis a comprehensive literature search is revealing we find that although there have been several previous attempts at a definition the term mega tsunami has generally been applied in a rather arbitrary fashion to a number of tsunami characteristics such as wave height or amplitude at both source and distant locations run up height geographical extent and impact this haphazard situation is undesirable in response we propose a stricter definition for mega tsunami that is based solely on initial wave height amplitude at source exceeding 100 m 50 m respectively a source related definition conveniently avoids any difficulties associated with the potential influence of coastal physical attributes e g configuration bathymetry geomorphology on tsunami parameters at affected locations using this definition it becomes apparent that mega tsunamis can only include those rare events on geological timescales generated by large bolide impacts violent volcanic activity or oceanic island flank collapse and possibly extreme tsunamigenic submarine earthquakes most seismically triggered events instead fall into the group of souteigai tsunamis i e unexpected tsunamis which are considered exceptional according to historical experience and local perspectives

the manson impact structure 40ar 39ar age and its distal impact ejecta in the pierre shale in southeastern south dakota the 40 ar 39 ar ages of a sanidine clast from a melt matrix breccia of the manson iowa impact structure mis indicate that the mis formed 73 8 0 3 million years ago ma and is not coincident with the cretaceous tertiary boundary 64 43 0 05 ma the mis sanidine is 9 million years older than 40 ar 39 ar age spectra of mis shock metamorphosed microcline and melt matrix breccia interpreted earlier to be 64 to 65 ma grains of shock metamorphosed quartz feldspar and zircon were found in the crow creek member upper campanian at a biostratigraphic level constrained by radiometric ages in the pierre shale of south dakota that are consistent with the 40 ar 39 ar age of 73 8 0 3 ma for mis reported herein

character of the alleghanian orogeny in the southern appalachians part ii geochronological constraints on the tectonothermal evolution of the eastern piedmont in south carolina a nearly concordant u pb zircon age of 550 4 ma is interpreted to closely date crystallization of the epizonal little mountain metatonalite in the southeastern part of the charlotte belt in south carolina this confirms field studies which indicate that the charlotte belt contains a plutonic metaigneous complex that developed as a sub volcanic arc infrastructure contemporaneous with vulcanism manifested in the carolina slate belt both paleontological and geochronological controls indicate that the south carolina slate belt is mostly younger than 570 ma cambrian whereas the slate belt in north carolina and virginia is mostly of late proterozoic age a regionally significant mid paleozoic ca 340 360 ma thermal event is suggested by discordant 40ar 39ar whole rock age spectra of slate phyllite in the northwestern carolina slate belt and from hornblende in the southeastern charlotte belt it is uncertain if this event was associated with deformation in the eastern piedmont however mid paleozoic deformation has been previously documented elsewhere in the western piedmont a slightly disconcordant u pb zircon age of 317 4 ma is interpreted to closely date initial crystallization of the deformed edge field granite and confirms a record of late paleozoic penetrative deformation in the kiokee belt of south carolina u pb isotope data for the lake murray orthogneiss and clouds creek granite are discordant and suggest that the magmas of these plutons were derived by partial melting of a sialic precam brian source and then emplaced in the eastern piedmont at ca 315 ma prior to or during the early stages of the alleghanian orogeny the thermal maximum of alleghanian regional metamorphism amphibolite facies in the kiokee belt greenschist facies in the southeastern part of the carolina slate belt occurred during ca 295 315 ma during the late carboniferous and early permian the eastern piedmont experienced differential uplift erosion and relatively rapid postmeta morphic cooling isothermal surfaces were folded into an antiform synform antiform configuration corresponding to the kiokee carolina slate and charlotte belts respectively the geochronological data provide the following calibration for the late paleozoic deformational chronology recorded in the kiokee and carolina slate belts d2 lake murray deformation ca 295 315 ma d3 clarks hill deformation ca 285 295 ma and d4 irmo deformation ca 268 290 ma

the evolution of the avalonian plate abstract the avalon platform of newfoundland is the type locality of the avalonian plate that is characterized by late precambrian volcanic and clastic rocks marine at the bottom and terrestrial at the top all are intruded by 650 550 ma granites late precambrian rocks in the southern british isles are similar in both areas northeastern parts of the belt are affected by deformation and metamorphism that have orogenic aspects the orogeny is referred to as the avalonian in newfoundland and monian in britain to the southeast of the avalonian rocks in europe lies another late precambrian cadomian orogenic belt of rocks as a part of the pan african orogenic belt the cadomian and avalonian belts extend into north africa morocco since these belts are ophiolite bearing it is suggested that the plate was bordered by oceanic domains it is therefore proposed that a microcontinental volcanic arc that was generated in the late precambrian was orogenically deformed by collision with north america europe and africa and survived as a coherent terrain until the late carboniferous

isotopic evidence for a tectonic boundary between the kings mountain and inner piedmont belts southern appalachians concordant u pb zircon ages of 320 ma for the deformed high shoals granite of the kings mountain belt kmb extend the range of known alleghanian late paleozoic deformation and metamorphism in the southern appalachian piedmont isotope systematics in kmb rocks suggest an affinity with terranes to the southeast rather than with the inner piedmont belt to the northwest lead isotope systematics in meta igneous rocks of the kmb suggest direct or indirect mantle derivation with little or no crustal contamination

coeval 40ar 39ar ages of 65 0 million years ago from chicxulub crater melt rock and cretaceous tertiary boundary tektites 40 ar 39 ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180 kilometer subsurface chicxulub crater in yucatan mexico has yielded well behaved incremental heating spectra with a mean plateau age of 64 98 0 05 million years ago ma the glassy melt rock of andesitic composition was obtained from core 9 1390 to 1393 meters in the chicxulub 1 well the age of the melt rock is virtually indistinguishable from 40 ar 39 ar ages obtained on tektite glass from beloc haiti and arroyo el mimbral northeastern mexico of 65 01 0 08 ma mean plateau age for beloc and 65 07 0 10 ma mean total fusion age for both sites the 40 ar 39 ar ages in conjunction with geochemical and petrological similarities strengthen the recent suggestion that the chicxulub structure is the source for the haitian and mexican tektites and is a viable candidate for the cretaceous tertiary boundary impact site

distinguishing shocked from tectonically deformed quartz by the use of the sem and chemical etching abstract multiple sets of crystallographically oriented planar deformation features pdfs are generated by high strain rate shock waves at pressures of 12 gpa in naturally shocked quartz samples on surfaces pdfs appear as narrow 50 500 nm lamellae filled with amorphosed quartz diaplectic glass which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions in contrast slow strain rate tectonic deformation pressure produces wider semi linear and widely spaced arrays of dislocation loops that are not glass filled etching samples with hf before examination in a scanning electron microscope sem allows for unambiguous visual distinction between glass filled pdfs and glass free tectonic deformation arrays in quartz this etching also reveals the internal pillaring often characteristic of shock induced pdfs this technique is useful for easily distinguishing between shock and tectonic deformation in quartz but does not replace optical techniques for characterizing the shock features

microspherules in devonian sediments origins geological significance and contamination problems we have studied various microspherules 0 5 mm across that were found in devonian sediments from canada and china during our micropaleontological studies e g conodont picking these spherules are similar in appearance but they are quite different in origin a combined scanning electron microscope energy dispersive x ray x ray microdiffraction and electron microprobe analysis enables us to determine their probable origins phosphatic conodont pearls of biological origin silicate glass microtektites of terrestrial meteorite impact origin and black magnetic iron spherules of probable extraterrestrial origin we also observed some contaminant spherules glue spatters and steel slag of artificial origins studies of natural spherules in sediments can provide important geological information that may be otherwise undetectable

interactive property of large thrust sheets with footwall rocks the sub thrust interactive duplex hypothesis a mechanism of dome formation in thrust sheets abstract recently acquired appalachian ultradeep core hole adcoh project site investigation seismic reflection data and geologic data from the appalachians and several other orogenic belts suggest an important mutually interdependent relationship exists between emplacement of large crystalline thrust sheets and the deforming foreland rocks beneath this relationship suggests isolated domes beneath crystalline thrust sheets may be produced by passive folding of the sheet as a result of formation of an antiformal stack duplex in the platform sedimentary sequence beneath suggestions that domes in crystalline thrust sheets formed by interference of late open folds is doubtlessly still valid in places but the platform duplex mechanism is probably also valid to explain the late doming of many crystalline and other large thrust sheets the dome beneath the shooting creek and brasstown bald windows in the adcoh site region is imaged as an antiformal stack duplex at depth the tallulah falls dome grandfather mountain and mountain city windows and smokies foothills duplex in the site region and elsewhere in the southern blue ridge are all late isolated domes and all are probably or demonstrably underlain by antiformal stack duplexes beneath the blue ridge piedmont composite crystalline thrust sheet the assynt window and footwall duplex benath the arnabol and moine thrusts in scotland and the engadine window in the alps may be similar structures

impact dust not the cause of the cretaceous tertiary mass extinction most of the 3 mm thick globally distributed chicxulub ejecta layer found at the cretaceous tertiary k t boundary was deposited as condensation droplets from the impact vapor plume a small fraction of this layer 14 g was submicrometer size dust the global mass and grain size distribution of the clastic debris indicate that stratospheric winds spread the debris from north america over the pacific ocean to europe and little debris reached high southern latitudes these findings indicate that the original k t impact extinction hypothesis the shutdown of photosynthesis by submicrometer size dust is not valid because it requires more than two orders of magnitude more fine dust than is estimated here furthermore estimates of future impact hazards which rely upon inaccurate impact dust loadings are greatly overstated

did the taconic appalachians continue into southern south america the appalachian mountains now terminating abruptly at the gulf of mexico coastal plain may have formerly continued into southern south america rocks forming the basement of the argentine andes can be interpreted as remnants of an early paleozoic orogen the famatinian belt not unlike the taconic appalachians both orogens are bordered to the west present coordinates by lower paleozoic carbonate platforms bearing the olenellid trilobite fauna that is characteristic of laurentia paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain possibly extending into antarctica during ordovician closure of an ocean basin southern iapetus between laurentia and gondwana the taconic and famatinian segments of the chain may have been truncated during late ordovician separation of laurentia and gondwana along the preexisting late neoproterozoic to cambrian rift system that initiated formation of the ouachita embayment and the southern margin of north america

deposition and age of chicxulub impact spherules on gorgonilla island colombia the end cretaceous mass extinction 66 ma has long been associated with the chicxulub impact on the yucatan peninsula however consensus on the age of this impact has remained controversial because of differing interpretations on the stratigraphic position of chicxulub impact spherules relative to the mass extinction horizon one side argues that the impact occurred precisely at the cretaceous paleogene boundary thus coinciding with the mass extinction the other side argues that the impact predated the cretaceous paleogene boundary based on the discovery of primary impact spherules deposits in ne mexico and texas near the base of planktic foraminiferal zone cf1 dated at 170 k y before the cretaceous paleogene boundary a recent study of the most pristine chicxulub impact spherules discovered on gorgonilla island colombia suggested that they represent a primary impact deposit with an absolute age indistinguishable from the cretaceous paleogene boundary here we report on the gorgonilla section with the main objective of evaluating the nature of deposition and age of the spherule rich layer relative to the cretaceous paleogene boundary the gorgonilla section consists of light gray yellow calcareous siliceous mudstones pelagic deposits alternating with dark olive brown litharenites turbidites a 3 cm thick dark olive green spherule rich layer overlies an erosional surface separating maastrichtian and danian sediments this layer consists of a clast supported normally graded litharenite with abundant chicxulub impact glass spherules lithics mostly volcanic and maastrichtian as well as danian microfossils which transitions to a calcareous mudstone as particle size decreases mineralogical analysis shows that this layer is dominated by phyllosilicates similar to the litharenites turbidites that characterize the section based on these results the spherule rich layer is interpreted as a reworked early danian deposit associated with turbiditic currents a major hiatus 250 k y spanning the cretaceous paleogene boundary and the earliest danian is recorded at the base of the spherule rich layer based on planktic foraminiferal and radiolarian biostratigraphy and carbon stable isotopes erosion across the cretaceous paleogene boundary has been recorded worldwide and is generally attributed to rapid climate changes enhanced bottom water circulation during global cooling sea level fluctuations and or intensified tectonic activity chicxulub impact spherules are commonly reworked and redeposited into younger sediments overlying a cretaceous paleogene boundary hiatus of variable extent in the caribbean central america and north atlantic while primary deposits are rare and only known from ne mexico and texas because of their reworked nature gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred

paleomagnetism and 40ar 39ar ages from the carolina slate belt albemarle north carolina implications for terrane amalgamation with north america paleomagnetic and sup 40 ar sup 39 ar dating studies of late proterozoic metamorphic rocks from the carolina slate belt were undertaken to gain insight into the history of this suspect lithostratigraphic terrane geochronological data suggest a peak metamorphic age of ca 455 ma and a later thermal overprint at ca 185 ma thermal demagnetization indicates two magnetization components a low temperature component yields a pole close to 180 190 ma segments of the north american apparent polar wander path a high temperature component postdates folding and is inferred to have been acquired at ca 455 ma corresponding paleomagnetic pole is lat 20 sup 0 n long 80 sup 0 e although paleomagnetic data from the carolina slate belt remain too ambiguous for construction of a unique polar wander path consideration of available paleolatitudes suggests that the terrane may have been proximal to cratonic north america after ca 455 ma this result contrasts markedly with high southerly ordovician paleolatitudes inferred from paleomagnetic studies within central and northern appalachian terranes

ordovician docking of the carolina slate belt paleomagnetic data paleomagnetic results from eight sites in argillaceous metasediments from union county north carolina in the central carolina slate belt yield a consistent high blocking temperature magnetization component the precision of the mean declination and inclination of all site means is maximized at 50 tilt correction and the improvement in grouping relative to 0 and 100 tilt correction is significant at the 95 confidence level this tends to indicate that the high blocking temperature component was acquired during folding the metamorphism and deformation in this area are rather well constrained by recent 40ar 39ar data to the late ordovician 450 ma we therefore interpret the computed pole position 29 6 n 122 1 e 95 5 11 as late ordovician in age and note that it lies close to some of the late ordovician poles for cratonic north america the paleolatitude 22 2 s of the sampling site is certainly unlike those derived from armorica and gondwana for this time interval and is rather similar to that proposed for the iapetus coast of laurentia during the late ordovician indeed the magnetization and deformation may be contemporaneous with the docking of the carolina slate belt onto the laurentian continent

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

a dual layer chicxulub ejecta sequence with shocked carbonates from the cretaceous paleogene k pg boundary demerara rise western atlantic abstract an up to 2 cm thick chicxulub ejecta deposit marking the cretaceous paleogene k pg boundary the k t boundary was recovered in six holes drilled during odp leg 207 demerara rise tropical western atlantic stunning features of this deposit are its uniformity over an area of 30 km2 and the total absence of bioturbation allowing documentation of the original sedimentary sequence high resolution mineralogical petrological elemental isotopic sr nd and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components the deposit is normally graded and composed predominantly of rounded 0 1 to max 1 mm sized spherules spherules are altered to dioctahedral aluminous smectite though occasionally relict si al rich hydrated glass is also present suggesting acidic precursor lithologies spherule textures vary from hollow to vesicle rich to massive some show in situ collapse others include distinct fe mg ca ti rich melt globules and lath shaped al rich quench crystals both altered glass spherules and the clay matrix site 1259b display strongly negative e nd t 65 ma values 17 indicating uptake of nd from contemporaneous ocean water during alteration finally fe mg rich spherules shocked quartz and feldspar grains few lithic clasts as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0 5 0 7 mm of the deposit the carbonate clasts display in part very unusual textures which are interpreted to be of shock metamorphic origin the preservation of delicate spherule textures normal grading with lack of evidence for traction transport and sub millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air fall deposit not affected by significant reworking the odp leg 207 spherule deposit is the first known dual layer k pg boundary in marine settings it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites its stratigraphy strongly resembles the dual layer k pg boundary deposits in the terrestrial western interior of north america although there carbonate phases are not preserved the occurrence of a dual ejecta layer in these quite different sedimentary environments separated by several thousands of kilometers provides additional evidence for an original sedimentary sequence therefore the layered nature of the deposit may document compositional differences between ballistic chicxulub ejecta forming the majority of the spherule deposit and material falling out from the vapor ejecta plume which is concentrated in the uppermost part

sensitivity of high resolution idealized simulations of thunderstorms to horizontal resolution and turbulence parametrization idealized simulations of deep moist convection are performed with the meso nh model at kilometric scales to assess the impact of the horizontal grid spacing 4 km 2 km 1 km and 500 m and turbulence scheme one dimensional t1d versus three dimensional t3d turbulence the simulations generate one cell which splits into two convective systems a leftward moving multicellular system and a rightward moving supercell objective criteria based on statistical properties and bulk quantities are examined for both systems to characterize the convection they show that the accumulated rainfall and corresponding surface area increase with increasing resolution as does the area covered by the updraughts while the 90th quantile of the intensity of updraught cores decreases the 4 km horizontal grid spacing is set apart as it clearly under resolves the convective motions but the difference between 2 and 1 km horizontal resolutions is larger than between 1 km and 500 m suggesting the beginning of convergence at 500 m also 1 km appears to represent the deep convective structures more correctly than 2 km for a practical weather forecast of organized convective systems t3d induces more mixing and enhances the microphysical processes compared with t1d producing larger amounts of cloud cover and precipitation also the magnitude of the pressure anomaly on the southeastern flank of the supercell is stronger accentuating the path curvature the difference between t1d and t3d becomes perceptible at 2 km pointing out the necessity to deal with horizontal turbulent fluxes at kilometric resolutions although a stronger numerical diffusion added to t1d allows the necessary damping to be introduced at the spectral energy tail it removes a part of the physical mixing and still misses some variance at larger scales the ratio between resolved and total turbulent kinetic energy tke decreases with increasing resolution for both t1d and t3d which is unexpected the main explanation is insufficient turbulent mixing inside convective clouds more pronounced at coarser resolution which is also confirmed by the vertical velocity spectra at 500 m horizontal resolution the subgrid tke is mainly due to dynamical processes with maxima located at the upper level of the convective systems in areas of stronger potential temperatures associated with downdraughts however thermal production is mostly negative underlying the lack of entrainment at the cloud edges

bouguer gravity in northeastern georgia a buried suture a surface suture and granites a bouguer gravity map of an area of northeastern georgia encompassing parts of the inner piedmont charlotte belt and carolina slate belt has been constructed from gravity stations spaced 2 km apart anomalies in the charlotte carolina slate belts are due to metamorphosed mafic rocks in the upper crust depth extent

structural analysis of the carolina inner piedmont terrane boundary implications for the age and kinematics of the central piedmont suture a terrane boundary that records paleozoic laurentia gondwana interactions new field mapping along the carolina inner piedmont terrane boundary in south carolina and eastern georgia reveals preaccretionary synaccretionary and postaccretionary faults the dextral strike slip lowndesville shear zone is adjacent to a 50 km long segment of the terrane boundary however the lowndesville shear zone is correlated eastward with the dextral strike slip beaver creek shear zone which is within the carolina terrane and predates 415 ma the lowndesville shear zone is overprinted by a dextral strike slip phyllonite zone named the deal creek shear zone the deal creek shear zone is correlated eastward with the gold hill silver hill shear zone which is also within the carolina terrane and records dextral strike slip motion between 400 and 325 ma the cross anchor and mulberry creek faults both truncate the lowndesville and the deal creek shear zones and form the terrane boundary the mulberry creek fault probably of triassic jurassic age juxtaposes the lowndesville shear zone adjacent to the carolina inner piedmont terrane boundary west of waterloo south carolina the cross anchor fault is the terrane boundary east of waterloo south carolina and forms the southeastern boundary of the whitmire reentrant crosscutting relationships indicate that the cross anchor fault is the oldest fault which juxtaposes the carolina and inner piedmont terranes in the study area these structural interpretations and available geochronological data indicate that the cross anchor fault is a 325 ma thrust fault and may be the central piedmont suture an early alleghanian suture resolves the problem of inserting the carolina terrane into the western iapetus ocean

analogous tectonic evolution of the ordovician foredeeps southern and central appalachians new stratigraphic data suggest that the diachronous evolution of the ordovician foredeeps in the southern and central appalachians was remarkably similar stratigraphic features that characterize the middle ordovician sevier basin in tennessee and the middle and late ordovician martinsbuirg basin in pennsylvania are in identical ascending order 1 disconformity on the knox group beekmantown group 2 shelf carbonates 3 slope deposits 4 submarine fan turbidites and 5 contourites and muddy turbidites we propose that diachronous attempted subduction of the north american craton beneath southeastern microplates and or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins finally tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites the evolutionary model proposed for the sevier and martinsburg basins closely resembles present day tectonics of the timor foredeep and the adjoining sahul shelf north of australia similar comparisons have also been made for ordovician basins in the northern appalachians analogous tectonic mechanisms therefore appear to have operated diachronously along the eastern margin of north america from tennessee to new england and possibly to newfoundland during ordovician time

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

near k t age of clastic deposits from texas to brazil impact volcanism and or sea level lowstand near k t boundary clastic deposits from texas mexico haiti guatemala and brazil often described as impact generated tsunami deposits are stratigraphically below well defined k t boundary horizons and appear not to be causally related to the k t boundary event stratigraphic evidence indicates that their deposition began during the last 170 200 kyr of the maastrichtian coincident with a major eustatic sea level lowstand that lowered sea level by as much as 70 100 m clastic deposition ended a few tens of thousands of years before the k t boundary during a rapidly rising sea level the presence of glass in clastic deposits in haiti northeastern mexico and yucatan suggests that the sea level lowstand coincided with a time of major volcanism or pre k t boundary bolide impact

tem study of meteorite impact glass at new zealand cretaceous tertiary sites evidence for multiple impacts or differentiation during global circulation abstract study by transmission electron microscopy of samples from the cretaceous tertiary k t boundary clay at flaxbourne river and woodside creek new zealand has revealed the occurrence of nanometer sized meteorite impact derived glass the average glass composition is exceptionally ca rich and is distinct from other glass found on earth apart from glass inferred to be of impact origin at mexican and haitian k t sites the glass shards are partially altered to montmorillonite like smectite with the dominant interlayer cation ca reflecting the composition of the parent glass the data imply a heterogeneous global distribution in composition of k t boundary impact glass si rich and ca rich in mexico and haiti si rich in denmark and ca rich in new zealand this heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the chicxulub impact site however recent discovery of an impact crater of k t boundary age in ukraine raises the possibility of impact clusters which produce material of heterogeneous composition

shock induced devolatilization of calcium sulfate and implications for k t extinctions the devolatilization of calcium sulfate which is present in the target rock of the chicxulub mexico impact structure and dispersal in the stratosphere of the resultant sulfuric acid aerosol have been suggested as a possible mechanism for the cretaceous tertiary extinctions we measured the amount of so 2 produced from two shock induced devolatilization reactions of calcium sulfate up to 42 gpa in the laboratory caso 4 sio 2 casio 3 so 3 caso 4 cao so 2 1 2o 2 we found both to proceed to a much lower extent than calculated by equilibrium thermodynamic calculations reaction products are found to be 10 2 times those calculated for equilibrium upon modeling the quantity of sulfur oxides degassed into the atmosphere from shock devolatilization of caso 4 in the chicxulub lithographic section the resulting 9 10 16 to 6 10 17 g in sulfur mass is lower by a factor of 10 100 than previous upper limit estimates the related environmental stress arising from the resultant global cooling and fallout of acid rain is insufficient to explain the widespread k t extinctions

tektite bearing deep water clastic unit at the cretaceous tertiary boundary in northeastern mexico the hypothesis of cretaceous tertiary k t boundary impact on yucatan mexico predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves an outcrop along the arroyo el mimbral in northeastern mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic marl sequence deposited at more than 400 m water depth the marls were found to be unsuitable for determining magnetostratigraphy but foraminiferal biostratigraphy places the clastic unit precisely at the k t boundary we interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact the elastic unit comprises three main subunits 1 the basal spherule bed contains glass in the form of tektites and microtektites glass spherules replaced by chlorite smectite and calcite and quartz grains showing probable shock features this bed is interpreted as a channelized deposit of proximal ejects 2 a set of lenticular massive graded laminated beds contains intraclasts and abundant plant debris and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water 3 at the top several thin ripple beds composed of fine sand are separated by clay drapes they are interpreted as deposits of oscillating currents perhaps a seiche an iridium anomaly 921 23 pg g is observed at the top of the ripple beds our observations at the mimbral locality support the hypothesis of a k t impact on nearby yucatan

chicxulub impact spherules in the north atlantic and caribbean age constraints and cretaceous tertiary boundary hiatus the chicxulub impact is commonly believed to have caused the cretaceous tertiary boundary mass extinction and a thin impact spherule layer in the north atlantic and caribbean is frequently cited as proof we evaluated this claim in the seven best north atlantic and caribbean cretaceous tertiary boundary sequences based on high resolution biostratigraphy quantitative faunal analyses and stable isotopes results reveal a major cretaceous tertiary boundary unconformity spanning most of danian subzone p1a 1 and maastrichtian zones cf1 cf2 400 ka in the nw atlantic bass river core odp sites 1049a 1049c and 1050c in the caribbean odp sites 999b and 1001b the unconformity spans from the early danian zone p1a 1 through to zones cf1 cf4 3 ma only in the demerara rise odp site 1259b is erosion relatively minor and restricted to the earliest danian zone p0 and most of subzone p1a 1 150 ka in all sites examined chicxulub impact spherules are reworked into the early danian subzone p1a 1 about 150 200 ka after the mass extinction a similar pattern of erosion and redeposition of impact spherules in danian sediments has previously been documented from cuba haiti belize guatemala south and central mexico this pattern can be explained by intensified gulf stream circulation at times of climate cooling and sea level changes the age of the chicxulub impact cannot be determined from these reworked impact spherule layers but can be evaluated based on the stratigraphically oldest spherule layer in ne mexico and texas which indicates that this impact predates the cretaceous tertiary boundary by about 130 150 ka

pdf orientations in shocked quartz grains around the chicxulub crater we measured 852 sets of planar deformation features pdfs in shocked quartz grains in impactite samples of the yaxcopoil yax 1 core and from 4 cretaceous tertiary k t boundary deposits the monaca the cacarajcara and the pealver formations in cuba and dsdp site 536 within 800 km of the chicxulub crater in order to investigate variations of pdf orientations in the proximity of the crater orientations of pdfs show a broad distribution with peaks at 1013 1012 and 1112 plus r z 1011 orientations with minor c 0001 s 1121 t 2241 plus x 5161 and m 1010 plus a 11 20 orientations planar deformation features with c 0001 orientation are relatively more abundant in the proximity of the chicxulub crater than in distal sitessuch as north america the pacific ocean and europe this feature indicates that in the proximity ofthe crater part of the shocked quartz grains in the k t boundary deposits were derived from the lowshock pressure zones moreover the orientations of pdfs with 1122 plus r z 1011 are high in our studied sites and frequencies of these orientations decrease with increasing distance from the crater on the other hand absence of c 0001 and the rare occurrence of pdfs with 1122 plus r z 1011 orientations in the sample from the yax 1 core that was taken at the top of the impactite layer of the chicxulub crater suggests that the sampling horizon that reflects a certain cratering stage is also an important factor for variations in shocked quartz

high resolution planktonic foraminiferal analysis from the cretaceous tertiary boundary at ain settara tunisia evidence of an extended mass extinction the ain settara section located in the kalaat senan area of north central tunisia spans the cretaceous tertiary k t boundary which is characterized by a red layer and a thin non bioturbated boundary clay sediment accumulation across the k t boundary at ain settara was probably interrupted by three short hiatuses and or condensed sedimentation the first hiatus occurs at the top of the cf1 zone the second hiatus condensation occurred just below the boundary clay and the third hiatus at the p0 p1a boundary in the earliest danian these hiatuses are marked by weak unconformities bioturbation and sudden disappearances appearances of species which are known to disappear evolve sequentially in continuous sections quantitative high resolution planktonic foraminiferal analysis across zones cf1 upper p0 and p1a 1 reveals an extended and selective mass extinction all 41 of the species which disappeared at or below the k t boundary are rare to very rare and primarily ecologically specialized keeled deeper dwelling tropical subtropical forms globotruncana globotruncanita gublerina planoglobulina rosita contusotruncana racemiguembelina their combined relative abundance varies between 10 and 15 of the total population at the end of the maastrichtian the k t crisis thus appears more catastrophic when viewed in tropical subtropical assemblages and based on analysis of larger species 200 m which preferentially includes the more specialized forms though in fact the k t mass extinction actually involved a relatively small part of the foraminiferal population in terms of relative abundance the pattern of extinction and changes in dominant population at ain settara appear to be very similar to the planktonic foraminiferal turnover of the other north central tunisian sections el kef elles the selective mass extinction pattern suggests that the catastrophic effects of the bolide impact superimposed those related to long term environmental changes such as variations in temperature sea level and associated water mass changes

biogeographical significance of cambrian trilobites from the carolina slate belt abundant middle middle cambrian trilobites of the ptychagnostus atavus interval zone are described from the upper asbill pond formation informal name near batesburg south carolina the fauna containing at least nine species has strong affinities with cool water faunas of both armorica and baltica but little affinity with warm water shelf faunas of laurentia this is compatible with previous conclusions that the carolina slate belt is an exotic terrane that was accreted to north america during the early or middle paleozoic further evaluation however introduces an alternative possibility that the fauna could have lived in deep cool water environments along the periphery of laurentia and subsequent displacement need not necessarily have been great combined faunal and stratigraphic components of the carolina slate belt do not precisely match those of any known region but may be closest to those of bohemia although a southpolar location in proximity to armorica baltica and gondwana seems likely a definitive placement in a reconstruction of the cambrian world is not yet possible included in the trilobite fauna are the agnostoids hypagnostus mammillatus h parvifrons peronopsis fallax ptychagnostus sp and tomagnostus fissus the polymeroids include agraulos sp paradoxides cf polonicus paradoxides sp and skreiaspis sp

the chicxulub impact event and its environmental consequences at the cretaceous tertiary boundary abstract an impact mass extinction hypothesis for the cretaceous tertiary k t boundary transition has been confirmed with multiple lines of evidence beginning with the discovery of impact derived ir in k t boundary sediments and culminating in the discovery of the chicxulub impact crater likewise a link between the chicxulub impact crater and k t boundary sediments has been confirmed with multiple lines of evidence including stratigraphic petrological geochemical and isotopic data the environmental effects of the chicxulub impact event were global in their extent largely because of the interaction of ejected impact debris with the atmosphere the environmental consequences of the chicxulub impact event and their association with the k t boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet