a profound meteoric water influence on genesis in the permian waterberg coalfield south africa evidence from stable isotopes abstract the coal bearing sediments of the grootegeluk formation of the waterberg coalfield contain siderite ankerite and calcite of various forms that precipitated during deposition and diagenesis and have highly variable d18o and d13c values spherulitic siderite which is present predominantly as early diagenetic nodules 1 5 mm in diameter in coal and carbonaceous mudrock in the lower part of the grootegeluk formation has relatively high d13c values up to 8 pdb and d18o value between 9 and 15 v smow granular siderite d13c values l9 to 14 and d18o values between 12 and 22 ankerite is present only in minor amounts d18o values between 16 and 20 and d13c values 12 the oxygen and carbon isotopic values of cleat filling calcite which is present only in coal seams has d18o values between 12 and 22 and d13c values between 15 and 7 fine grained calcite disseminated in carbonaceous mudrock and as inclusions in kaolinite is present only at the base of the grootegeluk formation and has d13c values between 7 and 3 and d18o values 15 the calcite lenses are interpreted to be synsedimentary and to have precipitated from depositional waters with d18o value of 13 spherulitic siderite and disseminated calcite precipitated during early diagenesis dominated by anaerobic microbial decomposition of organic matter granular siderite formed at higher temperatures d13c values 12 of the cleat filling calcite indicate a predominantly organic source for the carbon but the erratic and wide range of the d18o values do not allow distinction between precipitation daring burial or later during uplift of the sediments stable isotope data of the carbonates are consistent with precipitation from meteoric water but estimation of temperatures of formation are not reliable because burial processes result in highly d18o depleted pore water

changing fluvial environments across the permian triassic boundary in the karoo basin south africa and possible causes of tetrapod extinctions abstract the karoo supergroup is a 12 km thick succession of sedimentary rocks that accumulated in a large intracratonic retroarc foreland basin in southwestern gondwana the strata record 100 million years of almost continuous sediment accumulation from the permo carboniferous 300 ma through to the early jurassic 190 ma under a range of climatic regimes and within several tectonically controlled sub basins alluvial sediments dominate the succession from the late permian onwards fossils of synapsid reptiles and early dinosaurs are sufficiently common to be used in a ten fold biostratigraphic subdivision of these strata the permian triassic boundary in the karoo succession is marked by a major extinction of the herbivorous dicynodonts which co incides with a rapid change in fluvial facies this study uses field observations of the sedimentary facies palaeosols and in situ fossils of well exposed p tr boundary sequences in the southern karoo basin to provide evidence of environmental changes that may have caused the tetrapod extinctions in the main karoo basin below the p tr boundary sequence the sediments comprise drab greenish grey mudrocks with a few thick 3 12 m multistoried laterally accreted fine grained sandstone bodies the mudrocks host hydromorphic palaeosols with palustrine carbonate horizons and mostly disarticulated skeletal remains of dicynodon assemblage zone fauna above a transition zone of approximately 20 m thickness these strata change into reddish brown mudrocks with numerous thin 0 5 5 m ribbon sandstone bodies and more continuous sheet sandstones calcic palaeosols with rhizocretions nodular horizons and pervasive calcareous encrustation of articulated skeletons of lystrosaurus and associated fauna of the lystrosaurus assemblage zone within a 20 m thick interval of the transitional strata fossils of dicynodon and lystrosaurus occur together the facies transition that co incides with the latest permian dicynodont extinction is interpreted as a change in fluvial style from meandering to low sinuosity channels with general drying of the floodplain habitats and redistribution of vegetation belts the horsetails in particular became significantly reduced in areal distribution the changes in fluvial style in the karoo basin at the end of the permian were triggered by a pulse of thrusting in the southerly source area dated at 247 ma 2 ma which brought about rapid progradation of a large sandy braided fan system the katberg sandstone fm into the central parts of the basin a concomitant basinward migration of depositional environments allowed the lystrosaurus communities that were adapted to dry floodplains to colonise the entire basin to the detriment of most of the wet floodplain dicynodon fauna that lived there

timing of mammal like reptile extinctions across the permian triassic boundary in south africa the rate timing and pattern of change in different regions and paleoenvironments are critical for distinguishing among potential causes for the permian triassic p t extinction carbon isotopic stratigraphy can provide global chronostratigraphic control we report a large 13 c excursion at the p t boundary and no long term permian 13 c trends for samples from the interior of pangea stratigraphic gaps between available samples limit the resolution of our 13 c curve but the excursion is within a 15 m thick zone of overlap between permian and triassic taxa sedimentological and taphonomic observations demonstrate that this 15 m interval does not represent geologically instantaneous deposition together these data support a rapid and globally synchronous p t event but suggest that it occurred over a geologically resolvable interval of time

interpretation of new aeromagnetic anomaly data from the central antarctic peninsula new high resolution aeromagnetic anomaly data from the central antarctic peninsula reveal a magnetic signature typical of a magmatic arc system analysis of the new aeromagnetic map together with susceptibility data and 2 5 dimensional modelling support the interpretation of the pacific margin anomaly as reflecting a mid crustal batholithic province consisting of magnetite rich plutons examination and analysis of the magnetic anomaly map shows variations in magnetic character along the antarctic peninsula these variations in conjunction with geological and other geophysical data sets are interpreted as indicating distinct segments of continental crust of particular importance is the identification of three distinct areas of basement on the eastern side of the antarctic peninsula of which the central area 66 s to 68 appears to be gneissic basement at a relatively high crustal level to the north of 66 s the basement is formed of late palaeozoic triassic accretionary material and to the south of 68 s the basement is unknown segmentation is a common feature of arcs worldwide and previous studies of the antarctic peninsula have linked the continental segmentation to the arrival of a series of ridge crests at the subducting pacific margin this study indicates that the major segmentation of the continental crust is linked to the pre cretaceous crustal structure of the peninsula which was subsequently modified by extension during the cenozoic it is suggested that this cenozoic extension was caused by a reduction in plate convergence rates at two distinct times rather than the arrival of ridge segments at the trench as previously proposed

joint geophysical and petrological models for the lithosphere structure of the antarctic peninsula continental margin summary the antarctic peninsula ap is a composite magmatic arc terrane formed at the pacific margin of gondwana through the late mesozoic and cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions subduction may be active today in the northern part of the ap adjacent to the south shetland islands the subduction system is confined by the shackleton and hero fracture zones the magmatic arc of the ap continental margin is marked by high amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the south shetland islands and trench the sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition which were intruded in the cretaceous due to partial melting of upper mantle and lower crustal rocks 2 d gravity and magnetic models provide new insights into crustal and upper mantle structure of the active and passive margin segments of the northern ap our models incorporate seismic refraction constraints and physical property data this enables us to better constrain both moho geometry and petrological interpretations in the crust and upper mantle model along the dss 12 profile crosses the ap margin near the anvers island and shows typical features of a passive continental margin the second model along the dss 17 profile extends from the drake passage through the south shetland trench islands system and bransfield strait to the ap and indicates an active continental margin linked to slow subduction and on going continental rifting in the backarc region continental rifting beneath the bransfield strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

do red beds indicate paleoclimatic conditions a permian case study abstract terrestrial red beds have long been interpreted as desert deposits by comparison with modern red deserts more recently red beds have been interpreted as evidence of seasonally dry conditions and a permo triassic pangean monsoon red beds of cala viola sardinia are identified as paleosols and used to reconstruct late permian paleoclimatic conditions reconstruction of paleoenvironmental conditions based on the paleosols of the cala viola indicates warm humid conditions with no evidence of dry conditions as in a desert or of extreme seasonality as in a monsoon instead it is suggested that the red color of the paleosols is a result of former good drainage and that red color in general does not indicate specific paleoclimatic conditions

do stable isotope data from calcrete record late pleistocene monsoonal climate variation in the thar desert of india late pleistocene terrestrial climate records in india may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete petrography shows that calcrete nodules in quaternary sediments of the thar desert in rajasthan are pedogenic with little evidence for postpedogenic alteration the calcrete occurs in four laterally persistent and one nonpersistent eolian units separated by colluvial gravel thermoluminescence and infrared and green light stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units i iv of ca 70 000 60 000 ca 60 000 55 000 ca 55 000 43 000 and ca 43 000 25 000 yr respectively the youngest eolian unit v is 10 000 yr old and contains no calcrete stable oxygen isotope compositions of calcretes in most of eolian unit i in the upper part of eolian unit iv and in the nonpersistent eolian unit range between 4 6 and 2 1 pdb these values up to 4 4 greater than values from eolian units ii and iii are interpreted as representing nonmonsoonal18o enriched normal continental waters during climatic phases when the monsoon weakened or failed conversely 25 000 60 000 yr old calcretes eolian units ii and iii probably formed under monsoonal conditions the two periods of weakened monsoon are consistent with other paleoclimatic data from india and may represent widespread aridity on the indian subcontinent during isotope stages 2 and 4 the total variation in 13c is 1 7 0 0 1 7 and 13c covaries positively and linearly with 18o 13c values are highest when 18o values indicate the most arid climatic conditions this is best explained by expansion of c4grasses at the expense of c3plants at low latitudes during glacial periods when atmosphericpco2was lowered c4dominance was overridingly influenced by global change in atmosphericpco2despite the lowered summer rainfall

origin and palaeoenvironmental significance of lystrosaurus bonebeds in the earliest triassic karoo basin south africa abstract earth experienced its most devastating extinction event at the end of the permian period 251 million years ago ma despite an estimated 75 to 90 loss of species globally in both marine and terrestrial realms across the permian triassic boundary ptb around 20 of the terrestrial tetrapod genera in southwestern gondwana survived and were immediately joined by a number of immigrant taxa to occupy most of the vacant niches of the earliest triassic preserved in the karoo basin of south africa is an almost continuous stratigraphic record of terrestrial sedimentation through the ptb that hosts a fossil record of ecosystem collapse survivorship and recovery the adaptation of the mammal like reptiles therapsids of the lystrosaurus assemblage zone to a highly seasonal potentially drought prone semi arid earliest triassic karoo basin is associated with changes in modes of fossilisation isolated dicynodont skulls and postcranial elements are commonly found in the latest permian however in the earliest triassic the dicynodonts occur as articulated curled up skeletons and multi individual monotaxic bonebeds lack of epiphyses and relatively small skull length confirm that the bonebeds comprise several subadult lystrosaurus declivis l declivis carcasses no significant evidence for hydraulic bone concentration along with clusters of ribs in life position points to complete carcasses being present at the site of death and suggests that animals behaviourally congregated before perishing together the bonebeds are hosted by an 8 m thick horizon of floodplain mudrocks in the lower katberg formation named the lystrosaurus abundant zone the bonebed horizon is overlain by sand filled mud cracks capped by coarse sediments indicative of rapid deposition during waning floods stable isotope analyses of pedogenic and early diagenetic calcite nodules in association with the bonebeds yield average 13 c values ranging from 9 5 to 5 and 18 o values of 13 5 to 16 respectively these isotope values support a seasonally cold semi arid climate at high latitude 55 s the presence of vertebrate burrow casts on bonebed horizons and evidence of shelter sharing suggests that tetrapods were attempting to escape extreme climatic conditions aggregation behaviour of subadult lystrosaurus during unusual cold snaps may best explain the origin of bonebed assemblages

taphonomy and preservation of a monospecific titanothere assemblage from the washakie formation late eocene southern wyoming an ecological accident in the fossil record abstract more than twenty partial skeletons all cf mesatirhinus sp were discovered in overbank deposits associated with sandstone channels within the adobe town member of the washakie formation washakie basin southwest wyoming taphonomic and sedimentologic data indicate a mass mortality event with reduced post mortem transport of the carcasses such a mass death assemblage of a large mammal taxon is a rare occurrence for the paleogene record accordingly position and orientation of all the bones were recorded to make possible an accurate detailed map of the deposit the remains principally include disarticulated skeletons of both sexes from juveniles to old adult some axial portions and limbs remained articulated others suggest partial articulation the collected remains were scattered in loose associations within an area of about 7 5 9 m recent erosion sheetwash has destroyed an unknown extent of the original mass burial to the east and south of the excavation but the 68 oriented bonebearing blocks collected represent about 50 of the exposed site the skeletal remains occurred within a single unit of poorly sorted but generally upward fining quartz grit with a basal mudflake conglomerate the bones are not abraded but have suffered from severe compaction damage and recent freeze thaw action the sediment represents a mass flow accumulation deposited during the waning stages of a flash flood the large number of individuals of a single taxon suggests that part of a herd of titanotheres was killed in a single mass mortality event the cause of death is not known but it is likely that the accumulation represents a small fraction of a large herd that was crossing a flooded river the carcasses drifted downstream to be left as a carcass jam on the flood plain as the floodwaters withdrew it follows that the assemblage represents a close approach to a single taxon slice from a terrestrial biocoenosis and allows interpretations to be made about the autecology social behavior age composition and community structure of titanotheres decomposition of the integument and other tissues of partially buried carcasses may have been a factor contributing to the pockety nature of the sediment

new geochemical and palaeontological data from the permian triassic boundary in the south african karoo basin test the synchronicity of terrestrial and marine extinctions abstract the end permian mass extinction epme is widely recognised as the largest mass extinction in phanerozoic history in marine strata the main extinction event is well constrained and has been radiometrically dated to an interval of some 60 kyr approximately 251 9 million years ago however the age and duration of the epme in the terrestrial realm as well as its possible synchronicity with that of the marine realm is debated here we shed light on issues pertaining to the identification and position of the terrestrial epme in southern africa using recently collected sedimentological facies sequences palaeontological biostratigraphic ranges geochemical stable isotope analyses and detrital zircon id tims data from a new site in the xhariep district of the south african karoo basin we demonstrate that the permian triassic boundary sequence containing evidence for phased tetrapod extinctions is time equivalent with the marine extinction we conclude that the terrestrial epme recorded in the karoo may be regarded as essentially synchronous with the epme currently defined in the marine realm and was likely the result of the same volcanically induced atmospheric disturbances this study describes the first single vertical succession of vertebrate and plant fossils that span the terrestrial permian triassic boundary that are also well constrained both by relative stable isotopes and absolute detrital zircon geochronology dating methods

revised tectonic implications for the magnetic anomalies of the western weddell sea abstract ten years after the usac u s a rgentina c hile project which was the most comprehensive aeromagnetic effort in the antarctic peninsula and surrounding ocean basins questions remain regarding the kinematics of the early opening history of the weddell sea key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the weddell sea and merging the usac data set with the other magnetic data sets in the region for this purpose we reprocessed the usac data set using a continuation between arbitrary surfaces and equivalent magnetic sources the equivalent sources are located at a smooth crustal surface derived from the existing bathymetry topography and depths estimated by magnetic inversions the most critical area processed was the transition between the high altitude survey over the antarctic peninsula and the low altitude survey over the weddell sea that required downward continuation to equalize the distance to the magnetic source this procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion the enhancement of the mesozoic sequence permits refining the interpretation of the seafloor spreading anomalies in particular the change in shape and wavelength of an elongated positive in the central weddell sea suggests that it was formed during the cretaceous normal polarity interval the older lineations in the southwestern weddell sea are tentatively attributed to susceptibility contrasts modeled as fracture zones numerical experimentation to adjust synthetic isochrons to seafloor spreading lineations and flow lines to fracture zones yields stage poles for the opening of the weddell sea since 160 ma to anomaly 34 time the corresponding reconstructions look reasonable within the known constraints for the motions of the antarctic and south america plates however closure is not attained between 160 and 118 ma if independent published east antarctica africa south america rotations are considered the lack of closure may be overcome by considering relative motion between the antarctic peninsula and east antarctica until 118 ma time an important component of convergence

pattern of vertebrate extinctions across an event bed at the permian triassic boundary in the karoo basin of south africa the extinction of vertebrates around the time of the permian triassic boundary has long been regarded as a gradual event occurring over millions of years our new field investigations of fluvial strata in the central and southern karoo basin of south africa have revealed the presence of an event bed coinciding with a mass extinction of terrestrial fauna and flora the bed is in a sedimentary sequence that is marked by a reddening of flood plain mud rocks and a change from high to low sinuosity river channel systems here we show that the pattern of vertebrate taxa disappearing below this boundary and the subsequent appearance of new taxa above the boundary are consistent with a relatively sudden possibly catastrophic event perhaps of 50 000 yr duration or less

depositional environments of the lower permian dwyka diamictite and prince albert shale inferred from the geochemistry of early diagenetic concretions southwest karoo basin south africa abstract the upper dwyka and lower ecca groups in the karoo basin of south africa document the climatic and palaeoenvironmental changes associated with the final permo carboniferous deglaciation of the gondwana supercontinent the depositional environments of these groups have until recently been interpreted on the basis of sedimentological and palaeontological evidence here we use the geochemistry of early diagenetic concretions septarian calcite concretions from the upper dwyka group and phosphatic chert concretions and beds from the lower ecca group to infer the depositional environment of these rocks in the southwestern karoo basin 18o values 7 8 to 8 9 smow suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than permian seawater 13c values 15 to 3 pdb indicate that the carbon was derived from a mixture of craton derived calcareous material and organic matter bacterially degraded in the lower sulphate reduction to upper methanogenesis zones during early burial diagenesis the rare earth element ree patterns sr concentrations and 87sr 86sr ratios 0 716 0 737 significantly greater than permian seawater 0 708 together also support the interpretation that calcite and phosphatic concretions formed in glacial fresh water sediments

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

palaeoecological implications of an upper cretaceous tetrapod burrow bauru basin peiropolis minas gerais brazil abstract we describe a globally rare example of a tetrapod burrow from the upper cretaceous bauru group bauru basin from peiropolis minas gerais state brazil the sedimentary succession containing the burrow includes a rich vertebrate assemblage comprising fish podocnemid turtles mesoeucrocodylians saurischian dinosaurs among others the burrow is composed of an oblique tunnel 30 oval in cross section with a horizontal and sub oval terminal chamber it is 1 3 m long from the midpoint of its inferred entrance to the midpoint of the bottom of the chamber it occurs in the upper portion of a sandstone succession interpreted as a braided channel deposit and the burrow fill comprises medium grained sandstone with mudstone intraclasts derived from fluvial floodplain facies it is overlain by other fluvial channel deposits analyses suggest that the burrow was dug after the filling of the braided channel and during the pedogenesis of its exposed upper surface based on burrow morphology and size the most plausible producer of this burrow is a notosuchian mesoeucrocodylian such as small to mid sized notosuchians e g sphagesaurids the bauru group has an extensive fossil record of notosuchians with disparate morphologies and it is noteworthy that the small sized notosuchian labidiosuchus amicum comes from the same unit as the burrow moreover arid to semi arid conditions have been inferred for fossil bearing rocks of this unit and as such the data here presented add to our palaeoecological knowledge of cretaceous mesoeucrocodylians in gondwana moreover it constitutes a new cretaceous record of a tetrapod burrow during a period when such ichnofossils are globally rare

tectonic segmentation of the northern antarctic peninsula discrete segments of the southeast pacific ocean crust were subducted beneath the antarctic peninsula in late cretaceous and cenozoic time the segment boundaries of the subducted ocean crust are reflected in the overlying continental crust by transverse megafractures these are usually obscured by contemporaneous magmatism but may be revealed by later fault movements the transverse fracture zones divide graham land and its offlying islands into segments that have different histories of magmatism uplift and erosion and postorogenic sedimentation metalliferous mineralization is controlled in part by the transverse fracture zones

mass extinction of peat forming plants and the effect on fluvial styles across the permian triassic boundary northern bowen basin australia abstract the most spectacular extinction event in earth s history occurred across the permian triassic boundary in the northern bowen basin a major coal bearing sedimentary basin in eastern australia a long lived c 9 myr cold climate peat mire ecosystem collapsed at the permian triassic boundary when the vast majority c 95 of peat producing plants became extinct the environmental change marked by the permian triassic boundary is expressed as an abrupt and sharp change in sedimentary regime at the contact between the rangal coal measures and the sagittarius sandstone the stratigraphic record shows no diminution in the thickness lateral extent or spatial distribution of coal seams prior to the boundary event the abrupt ecological shift at the permian triassic boundary was coincident with and interrelated to a change in landscape attributes and fluvial style the boundary shift is considered to reflect a short period radical atmospheric change accompanied by an abrupt change in plant ecosystems however palynological data indicate that it was preceded by a more gradual gross taxonomic progression in the floral succession the boundary shift is unlikely to reflect change in the tectonic setting of the northern bowen basin because the detrital character of clastic sediment supply shows no provenance change within the boundary sequence the late permian fluvial style is characterised by large scale up to 1 km wide sandstone dominated low sinuosity trunk river channel deposits the trunk river channels were flanked by extensive levee composite crevasse splay systems channel tracts were relatively stationary in position over enduring periods and developed stacked sediment accumulations up to 30 m thick the constrained character of the late permian trunk river systems was most likely due to progressive compaction of thick tracts of peat substrate and the stabilising effect of vegetation adjacent to the channel complex the well developed crevasse splays coupled with the low sinuosity style of the fluvial channels might suggest a perennial fluvial system characterised by short discharge periods as common in high latitude settings the fluvial architecture of the sagittarius sandstone the basal formation of the lower triassic rewan group is characterised by sheet like elements suggestive of broad shallow channels in a deforested braid plain setting the channel deposits are considered to represent highly mobile sandy systems dominated by a flashy runoff regime the mass extinction of plants in the northern bowen basin at the permian triassic boundary thus had a significant impact on the early triassic landscape and fluvial architecture

short paper the significance of mid palaeozoic basement in graham land antarctic peninsula a mid palaeozoic basement to the antarctic peninsula is confirmed by a geochronological study of the gneissose country rock of the mesozoic magmatic arc of eastern graham land rb sr whole rock ages of 410 15 and 426 12 ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the antarctic peninsula during the silurian sm nd data on garnet whole rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late carboniferous times probably as a result of plutonism

revision of platte river alluvial facies model through observations of extant channels and barforms and subsurface alluvial valley fills abstract the platte river of nebraska central usa has been used by sedimentologists as a type example of a sandy braided river system for some forty years despite this designation however the details of the river s subsurface stratigraphy have never been evaluated and so the extant facies model for the platte river remains unvalidated this paper analyzes the holocene stratigraphy of the central and lower platte river it utilizes newly acquired data from wireline logs sediment cores aerial photography and ground penetrating radar gpr traverses to characterize subsurface geology in addition to surface sediment and geomorphic mapping sampling and analysis of serial aerial imagery cores and gpr profiles show that the lithosome of the active platte is 6 8 m in thickness and consists predominantly of medium sand although macroform features are common on the surface of the modern active central and lower platte river in nebraska the shallow subsurface stratigraphy is dominated by mesoform cross bedding and planar stratification overlying channel scour fills this differs markedly from the vertical succession predicted by the platte river facies model of miall 1977 data from inactive portions of the central and lower platte river however show macroform cross bedding indicative of the downstream and cross stream migration of linguoid and transverse bars which would be predicted by the published model the typical architecture of the holocene channel belt sand body in the areas studied thus comprises a lower unit dominated by macroform scale cross bedding overlain by an upper unit dominated by mesoform scale cross bedding and scour fills furthermore time series analysis of aerial photography of the river in the vicinity of grand island nebraska shows that 1 the river has become dominated by aggressively vegetated subaerially exposed channel bed domains termed vegetated channel islands that superficially resemble bars and 2 that this stretch is no longer dominated by downstream accreting macroform bars which were abundant in the 1960s and 1970s contraction of the active channel in this region has accompanied these changes somewhat independently of discharge variation new vertical sequence facies models are presented to account for these differences our data show the futility of seeking to characterize a fluvial system by means of a single vertical sequence model and furthermore indicate that the holocene facies architecture of the sandy braided platte river varies both in space and in time the dominance of mesoform scale structure in the shallow subsurface of the central and lower platte river likely reflects the ongoing process of gradual abandonment of the system it further highlights the need for caution in interpreting subsurface structure from surface geomorphology in fluvial systems

structure and evolution of cenozoic arc magmatism on the antarctic peninsula a high resolution aeromagnetic perspective structure and evolution of cenozoic arc magmatism on the antarctic peninsula a high resolution aeromagnetic perspective

global climate perturbations during the permo triassic mass extinctions recorded by continental tetrapods from south africa several studies of the marine sedimentary record have documented the evolution of global climate during the permo triassic mass extinction by contrast the continental records have been less exploited due to the scarcity of continuous sections from the latest permian into the early triassic the south african karoo basin exposes one of the most continuous geological successions of this time interval thus offering the possibility to reconstruct climate variations in southern laurasia from the middle permian to middle triassic interval both air temperature and humidity variations were estimated using stable oxygen 18op and carbon 13cc isotope compositions of vertebrate apatite significant fluctuations in both 18op and 13cc values mimic those of marine records and suggest that stable isotope compositions recorded in vertebrate apatite reflect global climate evolution in terms of air temperature oxygen isotopes show an abrupt increase of about 8 c toward the end of the wuchiapingian this occurred during a slight cooling trend from the capitanian to the permo triassic boundary ptb at the end of the permian an intense and fast warming of 16 c occurred and kept increasing during the olenekian these thermal fluctuations may be related to the emeishan south china and siberian volcanic paroxysms that took place at the end of the capitanian and at the end of the permian respectively vertebrate apatite 13cc partly reflects the important fluctuations of the atmospheric 13c values the differences with marine curves being likely due to the evolution of local humidity both the oxygen and carbon isotope compositions indicate that the ptb was followed by a warm and arid phase that lasted 6 ma before temperatures decreased during the late anisian toward that of the end permian environmental fluctuations occurring around the ptb that affected both continental and marine realms with similar magnitude likely originated from volcanism and methane release

geochemical and c o sr and u series isotopic evidence for the meteoric origin of calcrete at solitario wash crater flat nevada usa calcite rich soils calcrete in alluvium and colluvium at solitario wash crater flat nevada usa contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi arid southwestern united states nevertheless a ground water discharge origin for the solitario wash soil deposits was proposed in a series of publications proposing elevation dependent variations of carbon and oxygen isotopes in calcrete samples discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at yucca mountain nevada site of a proposed high level nuclear waste repository new geochemical and carbon oxygen strontium and uranium series isotopic data disprove the presence of systematic elevation isotopic composition relations which are the main justification given for a proposed ground water discharge origin of the calcrete deposits at solitario wash values of 13c 4 1 to 7 8 per mil 18o 23 8 17 2 87sr 86sr 0 71270 0 71146 and initial 234u 238u activity ratios of about 1 6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at yucca mountain and are incompatible with a ground water origin for the calcrete variations in carbon and oxygen isotopes in solitario wash calcrete likely are caused by pedogenic deposition from meteoric water under varying quaternary climatic conditions over hundreds of thousands of years

anatomy of a mass extinction sedimentological and taphonomic evidence for drought induced die offs at the permo triassic boundary in the main karoo basin south africa abstract the southern part of the karoo basin of south africa contains a near complete stratigraphic record of the permo triassic boundary ptb isotope and magneto stratigraphy confirm that these predominantly fluvial strata are approximately the same age as zircon dated marine ptb sections 252 mya by august 2013 our team had found 579 in situ vertebrate fossils mostly of the clade therapsida in ptb exposures at three widely separated locations in the southern karoo basin biostratigraphic ranges of the various taxa found in each of the sections reveal three separate phases of die off within the same roughly 75 metre thick stratigraphic interval displaying the same sequence of sedimentary facies interpreted as indicative of climatic drying increased seasonality and the onset of an unpredictable monsoon type rainfall regime the three phases of an inferred ecologically stepped mass extinction are phase 1 45 m 30 m below ptb datum brought on by lowered watertables which led to loss of shallow rooting groundcover in the more elevated proximal floodplain areas and the disappearance of the smaller groundcover grazing herbivorous dicynodonts and their attendant small carnivores phase 2 20 0 m below ptb datum is the main extinction that occurs in massive maroon grey mudrock culminating in an event bed of laminated reddish brown siltstone mudstone couplets this facies reflects progressively unreliable rainfall leading to vegetation loss in proximal and distal floodplain areas the larger tree browsing herbivores and their attendant carnivores are confined to watercourses before finally disappearing phase 3 25 30 m above ptb datum occurs in massive maroon siltstone facies with evidence of climatic aridity including the accumulation of mummified carcasses buried by windblown dust all of the surviving permian taxa disappear within 30 m of the ptb temporal resolution based on accretion rates and pedogenic maturity of each stratigraphic section reveals that phase 1 and phase 2 die offs lasted 21 000 and 33 000 years separated by a short period of 7000 years where no disappearances are recorded and this was followed by 50 000 years of stasis for the final extinction phase lasting only 8000 years that removed all the permian survivor taxa we propose that the recorded disappearances are real rather than preferential preservation failure and that they represent drought induced die offs moving progressively up the food chain as the terrestrial ecosystem collapsed the latter mostly likely caused by volcanogenic greenhouse gas emissions and rapid global warming

the relationship between channel avulsion flow occupancy and aggradation in braided rivers insights from an experimental model experimental modelling of an aggrading braided river has allowed investigation of the relationship between the frequency of channel avulsion a f the duration of time that the braidplain is occupied by flow the spatial pattern of braidplain sedimentation and how these respond to a change in sediment supply s s model results demonstrate a strong positive relationship between s s and a f and that there is no downstream change in a f over the short braidplain distances ca 100 m modelled herein although a f is strongly dependent on s s the degree of channel switching does not influence the rate or spatial pattern of braidplain sedimentation all experiments used a single central input for water and sediment and the channels occupied the centre of the alluvial basin for a longer period of time than the margins for all sediment supply rates and distances downstream despite this spatio temporal pattern in flow occupancy braidplain sedimentation rates were nearly uniform both downstream and across the basin and increased at approximately the same rate as increases in s s as a consequence less frequent and possibly short lived flows at the margins of the braidplain deposited and preserved more sediment per unit time in comparison with the centre of the basin where flow occupancy was higher an approximate order of magnitude change in sediment supply resulted in only a factor of two change in bed slope probably due to both an increase in channelization and adjustment of the channel form that maintained sediment transport through the basin this result suggests that linear diffusion models are unlikely to be applicable in landscape evolution models that possess aggrading multi thread rivers which are capable of self adjustment in channel number and form

jet topography interactions affect energy pathways to the deep southern ocean abstractin the southern ocean strong eastward ocean jets interact with large topographic features generating eddies that feed back onto the mean flow deep reaching eddies interact with topography where turbulent dissipation and generation of internal lee waves play an important role in the ocean s energy budget however eddy effects in the deep ocean are difficult to observe and poorly characterized this study investigates the energy contained in eddies at depth when an ocean jet encounters topography this study uses a two layer ocean model in which an imposed unstable jet encounters a topographic obstacle either a seamount or a meridional ridge in a configuration relevant to an antarctic circumpolar current frontal jet the authors find that the presence of topography increases the eddy kinetic energy eke at depth but that the dominant processes generating this deep eke depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography i

calcretes in semi arid alluvial systems formative pathways and sinks late quaternary deposits in gujarat western india show an abundant development of calcretes three major sinks of carbonate in the alluvial deposits are recognized 1 groundwater calcretes 2 pedogenic calcretes and 3 calcrete conglomerates groundwater calcretes originate from carbonate saturated waters travelling preferentially along stratification planes pedogenic calcretes form through soil forming processes typically in extra channel areas calcrete conglomerates occur as ribbons sheets and lenses due to the reworking of both pedogenic as well as groundwater calcretes as a result a pathway of calcretization develops that has the route groundwater calcrete to pedogenic calcrete to calcrete conglomerate the formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that 1 apart from aeolian dust river waters are also a major source of carbonate and 2 pedogenic carbonates may attain large sizes at accelerated rates due to the presence of pre existing groundwater calcretes consequently the maturity of a soil may be overestimated if determined by following established morphogenetic sequences

rb sr geochronology of graham land antarctica thirty five new rb sr whole rock and mineral isochrons are reported for igneous rocks from graham land magmatism was essentially continuous but peaks of activity occurred in early jurassic early cretaceous late cretaceous and eocene times the oldest dated granite is triassic 209 3 ma volcanic rocks were erupted throughout this history and some previously supposed upper jurassic volcanic rocks must be mid cretaceous or younger a change in magma type from uniformly acid to predominantly basic or intermediate combined with westward migration of the arc may be partly responsible for previously reported transverse geochemical variations there is a marked trend of decreasing initial 87 sr 86 sr ratio of the plutonic rocks with time from c 0 707 to 0 704 although volcanic rocks tend to have higher values this trend is ascribed to the waning influence of an as yet unidentified pre magmatic crustal basement due to increasing depth of magma generation close parallels to the observed age pattern imply continuity of the andean belt through southern south america into graham land back to early mesozoic times and are not compatible with reconstructions of gondwana in which the antarctic peninsula is located w of south america

the sedimentology and alluvial architecture of a large braid bar rio parana argentina abstract this paper presents 30 km of ground penetrating radar gpr data from a mid channel bar in the sixth largest river in the world the rio parana argentina gpr profiles with depth of penetration up to 12 m below the bar surface were collected from a sandy braid bar 3 km long by 1 km wide on a grid with a 200 to 400 m spacing two facies were found to dominate the sedimentary architecture the principal facies 83 of total facies comprises trough and planar cross strata related to the migration of dunes with the thickness of the cross strata decreasing towards the bar surface the second significant facies 15 is high angle generally 10 20 strata that typically form by accretion at the bar margins or bartail clay drapes the rio parana gpr surveys are compared with other gpr studies of sandy braid bars of different sizes from the south saskatchewan wisconsin and jamuna rivers the dominance of dune deposits is ubiquitous to all rivers with each also possessing a significant proportion of large scale high angle strata however two differences were found to exist between the deposits of these rivers 1 the compound bar deposits of smaller rivers contained greater proportions of the fills of cross bar channels which suggests a potential role for discharge as a factor in shaping the alluvial architecture through its impact on the frequency of sediment reworking over the bar tops and 2 the thickness of large scale high angle sets decreases with the age of the bar which suggests that the deposits of older bars may provide more useful geometrical analogues for interpreting ancient successions than smaller transient or recent bar forms that have undergone only limited modification

further evidence of fracture zone induced tectonic segmentation of the antarctic peninsula from detailed aeromagnetic anomalies aeromagnetic anomaly data collected between 67 s and 70 s crossing the antarctic peninsula and adjacent offshore areas show a prominent nw se trend in the magnetic fabric apparent lateral offsets previously recognized in the pacific margin anomaly have been mapped in detail and are shown to be much smaller than previously suggested a 35 km wide zone of subdued magnetic anomalies at the western edge of the pacific margin anomaly bounded by these apparent offsets is interpreted as a downfaulted block of the mafic intermediate batholith thought to be responsible for the pacific margin anomaly the trends of both fracture zones and magnetic lineaments strongly support the link between faulting in the antarctic peninsula magmatic arc and offshore tectonics

early triassic aggradational and degradational landscapes of the karoo basin and evidence for climate oscillation following the p tr event the lower triassic katberg formation beaufort group karoo supergroup in south africa is a fine grained arenaceous unit deposited under ephemeral bedload dominated fluvial conditions and previously interpreted as representing a continuous stratigraphic record its genesis was linked directly to the end paleozoic extinction event and was considered to exhibit the results of a trend towards aridification the present study documents the lowermost katberg formation exposed at carlton heights northern cape province to test the aridification hypothesis four lithofacies vary in both vertical and lateral relationships with feldspathic sandstone dominating the stratigraphy pisolith size nodular conglomerate consists of pedogenically derived carbonate nodules originally precipitated within aridisols none of which are either exposed or reported from the formation rather the only evidence of extreme aridification is found within channel lag barform and overbank deposits marking landscape degradation landscape aggradation is characterized by two feldspathic sandstone geometries where 1 medium bedded planar bedded or ripple laminated sheet sandstone is interpreted as ephemeral sheetflood deposits and 2 thick bedded cross laminated multi lateral lenticular sandstone is interpreted as associated with deeper sand bed braided anabranching systems overbank fines consist of polyphase siltstone paleosols in which bioturbated inceptisols were overprinted by gleysols wherein large pedogenic carbonate nodules precipitated under a high regional water table evidence exists for periodic drying in these wet regimes but no evidence exists for extreme aridity hence the lowermost katberg formation superficially exhibits only an aggradational sedimentological record but significant diastems exist as evidenced by the concentration of aridisol precipitates at the base of each degradational cycle a model for the early triassic in this part of the karoo basin is presented wherein this record is interpreted as a function of climate oscillations rather than either episodic uplift in the cape fold belt or any ecosystem response to the end permian extinction

anoxia as a cause of the permian triassic mass extinction facies evidence from northern italy and the western united states facies faunal and geochemical evidence from the permian triassic boundary sediments of the dolomites and idaho indicates a major anoxic event in the earliest triassic in both regions the basal beds consist of finely laminated micrites with common syngenetic pyrite the only fauna consists of occasional bedding plane assemblages of lingula or clacaria a typical lower dysaerobic assemblage this is a level where previous studies have shown a major negative carbon isotope excursion and a cerium anomaly in the dolomites the pyritic micrite directly overlies strata containing a diverse and typically permian marine fauna of algae foraminifers including fusulinids and articulate brachiopods implying an abrupt extinction in contradiction to many previous views sequence stratigraphic analysis of the dolomite boundary sediments reveals a minor sequence boundary in the late permian followed by extremely rapid transgression leading to the development of the relatively deep water pyritic micrite a maximum flooding surface at the permo triassic boundary a further pulsed deepening in the lower griesbachian recorded in both the dolomites and idaho lead to the widespread establishment of dysaerobic facies it is clear that most of the extinctions occurred at the erathem boundary although the subsequent failure of the marine benthos to fill the empty ecospace in the ensuing griesbachian may have been due to the widespread development of dysaerobic conditions

interpretation of reconnaissance gravity and aeromagnetic surveys of the antarctic peninsula two dimensional models are presented for the crustal structure of the antarctic peninsula a mesozoic cenozoic magmatic arc on the southeast pacific margin the models are constrained by measured rock properties the west coast magnetic anomaly wcma is caused by a zone of mafic plutons which is 70 150 km wide and over 1500 km long in modelling of bouguer anomalies isostatic compensation is maintained by crustal thickness variations the model moho is deepest 35 km beneath the mid cretaceous axis of the arc and shallowest 20 km beneath a late cenozoic marginal basin positive isostatic residual gravity anomalies are associated with the wcma and a mafic terrane in the mesozoic accretionary complex three broad areas of consistent crustal structure are identified the junctions of these areas are sinistral fault zones which displace the curvilinear magnetic physiographic and gravity patterns by several tens of kilometers

ridge trench collision induced switching of arc tectonics and magma sources clues from antarctic peninsula mafic dykes compositions and distributions of mafic dykes in the antarctic peninsula continental arc show that tapping of several mantle sources was tectonically controlled in the cretaceous to tertiary between 135 ma and 55 ma calc alkaline dykes intruded the arc in the late cretaceous however between 95 ma and 65 ma there was a pulse of compositionally diverse magmatism this change resulted from collision of an ocean spreading centre with the trench as a consequence non partitioned dextral transtensional shear in the overriding plate became partitioned into strike slip and extensional domains calc alkaline magmatism was therefore replaced by strike slip related shoshonitic magmatism towards the rear arc and extensionrelated tholeiitic magmatism towards the fore arc oib like dykes were emplaced because of the break in otherwise continuous subduction during the early tertiary subduction continued but ceased after a late tertiary ridge trench collision

uplift rates from a new high density gps network in palmer land indicate significant late holocene ice loss in the southwestern weddell sea the measurement of ongoing ice mass loss and associated melt water contribution to sea level change from regions such as west antarctica is dependent on a combination of remote sensing methods a key method the measurement of changes in earth s gravity via the grace satellite mission requires a potentially large correction to account for the isostatic response of the solid earth to ice load changes since the last glacial maximum in this study we combine glacial isostatic adjustment modelling with a new gps dataset of solid earth deformation for the southern antarctic peninsula to test the current understanding of ice history in this region a sufficiently complete history of past ice load change is required for glacial isostatic adjustment models to accurately predict the spatial variation of ongoing solid earth deformation once the independently constrained effects of present day ice mass loss have been accounted for comparisons between the gps data and glacial isostatic adjustment model predictions reveal a substantial misfit the misfit is localized on the southwestern weddell sea where current ice models under predict uplift rates by approximately 2 mm yr 1 this under prediction suggests that either the retreat of the ice sheet grounding line in this region occurred significantly later in the holocene than currently assumed or that the region previously hosted more ice than currently assumed this finding demonstrates the need for further fieldwork to obtain direct constraints on the timing of holocene grounding line retreat in the southwestern weddell sea and that grace estimates of ice sheet mass balance will be unreliable in this region until this is resolved

arc evolution a magnetic perspective from the antarctic peninsula the antarctic peninsula is a mesozoic cenozoic magmatic arc built on palaeozoic and younger basement it was formed by processes related to the subduction of pacific ocean floor at its western margin although subduction has now ceased along most of its length the peninsula features all the tectonic components commonly associated with a developing arc system basement accretionary complex magmatic arc arc related basins intra arc extension and post subduction volcanism seventeen thousand kilometres of high resolution aeromagnetic data have recently been collected in a transect across part of the arc covering an area 290 by 230 km and incorporating examples of most of the above tectonic components the new map reveals distinct magnetic signatures which can now be related to each of these components in a way that was not possible with reconnaissance data sets a characteristic magnetic anomaly pattern for each component is described and comparisons drawn with magnetic studies of other arc regions

can we distinguish flood frequency and magnitude in the sedimentological record of rivers consideration of the origin of alluvial deposits and their paleoenvironmental interpretation has traditionally involved two schools of thought that they are either the result of processes that on average have acted uniformly through time or that they are related to exceptional events that occur infrequently despite the long running debate of gradualism versus catastrophism within the earth sciences there are surprisingly few quantitative data to assess the magnitude of events that produce alluvial sedimentary successions this paper reports on a unique natural experiment where surface digital elevation model dem and subsurface ground penetrating radar gpr data were taken immediately prior to and after a large 1 in 40 yr flood event on the sandy braided south saskatchewan river canada results show that although this high magnitude flood reworked the entire braidplain the scale of scour and style of deposition was similar to that associated with lower magnitude annual floods the absence of a distinct imprint of this large flood within the deposits is related to the fact that as river discharge rises and begins to flow overbank channel width increases at a much faster rate than flow depth and thus the rate of increase in channel bed shear stress declines hence rather than being a product of either frequent or rare events alluvial deposits will likely be created by a range of different magnitude floods but discriminating between these different scale events in the rock record may be extremely difficult

disruption of playa lacustrine depositional systems at the permo triassic boundary evidence from vyazniki and gorokhovets on the russian platform abstract permo triassic sections at vyazniki and gorokhovets provide evidence on terrestrial events at or close to the permo triassic boundary the time of the largest ever mass extinction the sedimentary succession records the overrun of a muddy playa lacustrine depositional system by major channel belts transporting sand grade sediments biostratigraphy of sections at vyazniki and gorokhovets zhukov ravine shows that this event occurred either at the very end of the permian or 8 m above in the sections the timing and nature of this event which records increased sediment flux from the ural mountains is closely comparable with that from the southern uralian foreland basin the vyazniki and gorokhovets sections are 800 km from the mountain front and in a separate depositional basin which strengthens the case that increased sediment flux from the urals at the permo triassic boundary is related to devegetation of upland catchments increasing sediment yield and a switch toward low frequency but high magnitude discharge events increasing sediment delivery the interbedding of fluvial and aeolian deposits provides further evidence for climatic instability and extremes in the early triassic supplementary material detailed reports on the ostracodes and fossil fish remains from the zhukov ravine sections are available at http www geolsoc org uk sup18402

a multitaxic bonebed near the carboniferous permian boundary halgaito formation cutler group in valley of the gods utah usa vertebrate paleontology and taphonomy abstract the carboniferous permian c p transition records a shift in the composition and environmental setting of tropical flora and vertebrate assemblages across western and central pangea here we report the discovery of a rare multitaxic bonebed in the lower halgaito formation cutler group in valley of the gods and its vicinity san juan county southeast utah usa the assemblage which comprises carcasses and disarticulated bones and teeth preserves aquatic and semi terrestrial elements including xenacanth chondrichthyans actinopterygians sagenodontid lungfish and the temnospondyl amphibian eryops together with terrestrial taxa including the synapsids ophiacodon navajovicus edaphosaurus and sphenacodon and a hitherto undescribed araeoscelidan reptile sedimentological paleontological and strontium isotopic evidence indicates the bonebed formed as a slackwater deposit at the confluence of a major freshwater stream channel and its tributary the deposit probably formed through late stage flooding of a likely bottlenecked fluvial system consistent with increasingly episodic or seasonal precipitation concentrating carcasses of immature and mature shore dwelling animals comparisons with vertebrate assemblages in the cutler group of new mexico usa suggest that the assemblage is correlative with the lower or middle assemblage of the el cobre canyon formation cobrean and coyotean land vertebrate faunachrons lvfs and as such is of latest carboniferous age our findings strengthen the hypothesis that dryland adapted vertebrate communities proliferated earlier in westernmost pangea well before earliest permian time compared to other tropical regions

vertical structure and transport of the antarctic circumpolar current in drake passage from direct velocity observations 1 the structure of the antarctic circumpolar current acc in drake passage is examined using 4 5 years of shipboard acoustic doppler current profiler adcp velocity data the extended 1000 m depth range available from the 38 khz adcp allows us to investigate the vertical structure of the current the mean observed current varies slowly with depth while eddy kinetic energy and shear variance exhibit strong depth dependence objectively mapped streamlines are self similar with depth consistent with an equivalent barotropic structure vertical wavenumber spectra of observed currents and current shear reveal intermediate wavenumber anisotropy and rotation indicative of downward energy propagation above 500 m and upward propagation below 500 m the mean observed transport of the acc in the upper 1000 m is estimated at 95 2 sv or 71 of the canonical total transport of 134 sv mean current speeds in the acc jets remain quite strong at 1000 m 10 20 cm s 1 vertical structure functions to describe the current and extrapolate below 1000 m are explored with the aid of full depth profiles from lowered adcp and a 3 year mean from the southern ocean state estimate sose a number of functions including an exponential are nearly equally good fits to the observations explaining 75 of the variance fits to an exponentially decaying function can be extrapolated to give an estimate of 154 38 sv for the full depth transport

early triassic terrestrial tetrapod fauna a review abstract the permian triassic mass extinction ptme ca 252 mya was one of the most severe biotic crises of the phanerozoic eliminating 90 of marine and terrestrial species this was followed by a long period of recovery in the early and middle triassic which revolutionised the structure of both marine and terrestrial ecosystems triggering the new ecosystem structure of the mesozoic and cenozoic entire new clades emerged after the mass extinction including decapods and marine reptiles in the oceans and new tetrapods on land in both marine and terrestrial ecosystems the recovery is interpreted as stepwise and slow from a combination of continuing environmental perturbations and complex multilevel interaction between species in the new environments as ecosystems reconstructed themselves here we present a review of early triassic terrestrial tetrapod faunas geological formations and outcrops around the world and provide a semi quantitative analysis of a data set of early triassic terrestrial tetrapods we identify a marked regionalisation of early triassic terrestrial tetrapods with faunas varying in both taxonomic composition and relative abundance according to palaeolatitudinal belt we reject the alleged uniformity of faunas around pangaea suggested in the literature as a result of the hot house climate in addition we can restrict the tetrapod gap of terrestrial life in the early triassic to palaeolatitudes between 15 n and about 31 s in contrast to the earlier suggestion of total absence of tetrapod taxa between 30 n and 40 s there was fairly strong provincialism following the ptme according to cluster analysis of a taxon presence matrix entirely consistent with early triassic palaeobiogeography unexpectedly the overall pattern for early triassic terrestrial tetrapod faunas largely reflects that of the late permian suggesting that the recovery faunas in the early triassic retained some kind of imprint of tetrapod distributions according to palaeogeography and palaeoclimate despite the near total extinction of life through the ptme

i a e a advisory group meeting on stable isotope reference samples for geochemical and hydrological investigations vienna austria september 19 21 1983

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

cold stage formation of calcrete nodules in the chinese loess plateau evidence from u series dating and stable isotope analysis this pilot study shows that some calcrete nodules from the base of palaeosols in the chinese loess sequences can be dated by u series isochron methods the ages obtained suggest that the nodules did not form during interglacial climate stages rather they formed either during interglacial glacial transitions or glacial climate stages thus the nodules appear not to have formed contemporaneously with the soils beneath which they immediately lie but significantly post date them post pedogenic injection of carbonate into stratigraphically lower horizons is therefore identified this finding contradicts previous assumptions regarding the timing of nodule formation carbon and oxygen isotope analyses of the nodules identify isotopic compositions which independently suggest that climatic conditions cooler and possibly drier than at present prevailed at the time of nodule formation

crustal structure of the antarctic peninsula sector of the gondwana margin around anvers island from geophysical data abstract we investigated the crustal structure of the northern antarctic peninsula ap a composite magmatic arc terrane at the pacific margin of gondwana it was formed in mesozoic cenozoic by collision accretion of allochthonous terranes to the gondwana margin during subduction and progressive ridge collision of former phoenix plate under the ap the ap is distinguished by belts of high amplitude gravity and magnetic anomalies inferred to indicate the presence of plutons of mafic composition comprising the huge ap batholith for a better understanding of the lithosphere structure and tectonic processes of the ap continental margin we performed 2d gravity and magnetic modeling along line iii iii which crosses the margin south of anvers island our model incorporates data on seismic refraction dss 13 and reflection line 8545 03 profiles and physical property data our modeling shows that the crust of the ap margin near anvers island is of continental type and comprises two crustal domains which were welded together during mid cretaceous collision on the east there is a thick 40 km high velocity density continental crust of the gondwana margin which occupies the ap inner shelf and mainland and larsen ice shelf the space between the gondwanian block and oceanic crust of the bellingshausen sea is occupied by thinner 26 km crust of lower densities of supposed accreted terrane characterized by gravity anomalies caused probably by accretional tectonics as a result of cretaceous subduction two belts of magnetic anomalies forming the pma pacific margin anomaly probably belong to different crustal domains the western belt of the pma relates to accretional block while the eastern belt of the pma corresponds to the gondwana margin block this may represent an evidence that the pma relates to a tectono magmatic event occurred after collision of gondwana margin and accreted terrane in mid cretaceous and had been imprinted on already formed continental margin

the eastern palmer land shear zone a new terrane accretion model for the mesozoic development of the antarctic peninsula a major ductile fault zone the eastern palmer land shear zone has been identified east of the spine of the southern antarctic peninsula this shear zone separates newly identified geological domains and indicates that during late jurassic terrane accretion and collision two and possibly three separate terranes collided resulting in the palmer land orogeny the orogeny is best developed in eastern palmer land and eastern ellsworth land there shallow marine sedimentary rocks of the latady formation and a metamorphic and igneous basement complex of possible lower palaeozoic to pre early jurassic age are thrust and folded this forms an arcuate east directed foreland fold and thrust belt up to 100 km wide and 750 km long parallel to the axis of the antarctic peninsula the newly identified antarctic peninsula domains include 1 a parautochthonous eastern domain that represents part of the margin of the gondwana continent comparable to the western province of new zealand the ross province superterrane of marie byrd land the eastern series of south central chile the pampa de agnia and tepuel rocks of north patagonia and the cordillera darwin rocks of tierra del fuego 2 a suspect central domain that represents an allochthonous microcontinental magmatic arc terrane comparable to the median tectonic zone of new zealand the amundsen province superterrane of marie byrd land and coastal cordillera of north chile and 3 a suspect western domain with strong similarities to the eastern province of new zealand western series of south central chile and chonos metamorphic complex of north patagonia that represents either a subduction accretion complex to the central domain or another separate crustal fragment although an allochthonous terrane hypothesis for the antarctic peninsula remains to be fully tested this has much in common with models for the new zealand and south american parts of the pacific margin of gondwana the identification of a potential allochthonous terrane continent collision zone allows us to define the edge of the gondwana continent in the antarctic peninsula sector of the supercontinent margin which has implications for mesozoic reconstructions of gondwana

helical burrow casts of therapsid origin from the beaufort group permian of south africa abstract large helical burrow casts of vertebrate origin have been discovered at several localities in the lowermost strata of the teekloof formation of the beaufort group permian in the southwestern karoo basin south africa they are not only the oldest but also the first vertebrate burrows of helical geometry to be found in rocks of pre mammalian age the burrow casts are vertically disposed spiralling tubes filled with fine grained sandstone or siltstone often with a calcareous cement tube diameters gradually increase from 60 mm in the upper entrance decline through two non contacting dextral coils to about 160 mm at the base of the spiral whereupon the tube straightens and widens into a horizontal terminal chamber burrow depths range from 0 5 0 75 m and the ramp angle ranges from 10 to 32 although within any single spiral it is constant articulated skeletons of the small dicynodont diictodon have been found within the terminal chambers and spirals of three sand filled burrows linear ridges preserved on the outer walls and floor of some of the terminal chambers resemble scratch marks possibly made by the claws and beak of diictodon during burrow excavation the sedimentary sequences at two separate localities are interpreted as having been deposited by episodic sheet floods and distributary splay fans in the proximal floodbasin areas flanking an aggrading meanderbelt rise comparison is drawn between helical burrow casts in the lower beaufort and other recorded occurrences in particular daimonelix a beaver burrow from the miocene of nebraska fundamental similarities far outweigh differences to the extent that the beaufort helical burrows may be grouped with the daimonelices it is concluded that the excavation of large helical burrows an activity usually attributed to rodents was pre empted by mammal like reptiles some 50 million years before the first anatomical mammals evolved

deev jahi model of the permian triassic boundary mass extinction a case for gas hydrates as the main cause of biological crisis on earth the smoking gun revealing the secrets of the end permian mass mortality is a unique 1 2 m thick layer consisting of 5 20 cm long crystals of calcite that occurs precisely at the permian triassic boundary ptb in iran armenia turkey and china this layer is interpreted as synsedimentary abiotic seafloor cement indicative of precipitation from a highly carbonate supersaturated seawater its 13 c composition 13 c 0 pdb is 4 to 5 pdb lower than the typical upper permian values 4 to 5 pdb suggesting the involvement of massive amounts of gas hydrate ch 4 13 c 60 pdb the temporal coincidence of the cement layer with the ptb suggests that the process that promoted seafloor cementation was also responsible for the biological crisis a cementation model is developed based on accumulation dissociation cycle of gas hydrates which also explains the mass extinction at the ptb the upper permian accumulation period of gas hydrates ended abruptly adjacent to the ptb and the dissociation event began releasing 3 2 to 4 7 10 18 g ch 4 into the ocean oxidation of ch 4 in the water column created a seawater that was charged with co 2 an oceanic acid bath and had lower than normal o 2 content but not anoxic this oceanic acid bath first dissolved suspended fine grained carbonate particles and small calcareous organisms followed by extensive dissolution of platform carbonates raising ca 2 and hco 3 concentrations of seawater when the release of ch 4 declined the acid bath ocean became a soda ocean precipitating massive amount of seafloor cements observed globally at the ptb the study suggests that prior to cement precipitation the ptb ocean was charged with co 2 warm had low oxygen high ca 2 and high hco 3 concentrations these conditions collectively created stressful conditions causing the marine mass mortality the leakage of ch 4 to the atmosphere produced a super hot climate resulting in the biological devastation on land the proposed kill mechanism is developed on the basis of the physical clue the cement layer left behind by the killing process the change in ocean chemistry the accumulation dissociation cycles of gas hydrates also explain the 13 c pattern of marine carbonates and the periodicity of mass extinction events during the phanerozoic accumulation periods were long 5 to 20 my providing favorable conditions for ecosystem development pardeess phase the dissociation events were short and catastrophic 10 to 500 ky causing low oxygenation super hot climate and biological devastation doozakh phase it appears that most mass extinctions of the phanerozoic have been related to the internal working of the earth system during the phanerozoic methane has played the role of the deev jahi a female demon in persian mythology whose task is to attack the earth every so often to kill life on land and in sea

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

paleoclimatic implications of pedogenic carbonates in holocene soils of the gangetic plains india abstract this study attempts to reconstruct the holocene climatic history of the indo gangetic plains based on micromorphological characteristics and stable isotope composition of calcretes in a soil chronoassociation between the ramganga and rapti rivers the calcrete from the b2 horizon of older soils qgh4 qgh5 6500 13 500 year bp consists of septaric and irregularly shaped nodules of dense micrite and diffused needles associated with the illuvial clay pedofeatures fabric of the calcretes of older soils show inclusion of soil constituents and precipitation of carbonates in between weathered layers of micas formation of this calcrete took place during the pedogenesis in an arid to semi arid climate that also influenced vegetation and c4 type biomass such as chenopodium and typha angustata spread in the area a change to warm and humid phase at 6500 year bp led to dissolution and reprecipitation of the calcretes this led to the formation of blocky crystals and needles of calcite in voids and coarsening of the fabric in the lower horizons the vegetation pattern also changed to woodland with anogeissus and tecomella spp the younger soils bp show secondary carbonate accumulation associated mainly along the voids in lower parts of the profiles these are relatively coarse textured with little or no inclusion of soil constituents this coarse grained calcrete results from capillary rise from a fluctuating water table in a sub humid climate similar to present carbon and oxygen isotope data suggest that the formation of pedogenic carbonate in old soils qgh4 and qgh5 by evapotranspiration during arid to semi arid climate in the upper horizons the carbonates are depleted in 13c as their formation was influenced by the sparse vegetation and low soil respiration rate in the lower horizons the carbonates show enrichment of 13c due its extensive dissolution reprecipitation during the humid climate the younger soils showed the enrichment of 13c in their calcretes and support their formation from fluctuating shallow groundwater table the oxygen isotope values similar to that of meteoric water of the area indicate calcrete precipitation in equilibrium with the soil water

tetrapod burrows from the middle upper triassic chanares formation la rioja argentina and its palaeoecological implications abstract we describe tetrapod burrows from the uppermost middle lower upper triassic latest ladinian early carnian chanares formation ischigualasto villa union basin la rioja province northwestern argentina the burrows were found in different localities of the unit but restricted to the lowermost 17 m of the formation they occur at the same interval as a tetrapod assemblage composed of erpetosuchid and basal paracrocodylomorph pseudosuchians basal traversodontid and chiniquodontid eucynodontians and stenaulorhynchine rhynchosaurids this assemblage differs from the typical chanares fauna that occurs in stratigraphically higher levels the burrows were found within the deposits of moderately developed palaeosols formed in volcanically influenced braided fluvial facies under semi arid climate conditions the burrow systems are cylindrical to sub cylindrical in cross section up to 25 cm in diameter and characterized by long multiple branching tunnels several metres long that meander horizontally or are slightly inclined

analysis of marrow cavity fillings as a tool to recognise diverse taphonomic histories of fossil reptile bones implications for the genesis of the lower muschelkalk marine bone bearing bed middle triassic żyglin s poland abstract the bone bearing bed from yglin s poland which likely represents the oldest lower muschelkalk accumulation of reptile remains in the se part of the germanic basin has been investigated in order to determine its genesis the basic methods were supported by petrographic analysis of the marrow cavity and large inter trabecular pore space fillings of fossil bones which was used to check its usefulness to identify the environments where the bones were initially deposited and to decipher taphonomic histories of the remains the bone bearing bed is a composite deposit which consists of three distinct layers from bottom to top micritic limestone mudstone bioturbated autochthonous mud crinoidal limestone grainstone to packstone calcirudite and shell rich limestone packstone to wackstone calcirudite the crinoidal limestone layer the main bone bearing bed is recognised as the proximal tempestite deposited in the mid ramp zone the petrographic analysis of the fillings reveals the prevalence of minute ostracod carapaces accompanied by other grains embedded together with micrite to microspar such compositions suggest that these sediments may have been inserted into the bone pore spaces in lagoons and tidal flat ponds features of some bones record their early diagenetic burial and lithification before the final redeposition the isopachous spar blocky spar and weathered pyrite document changes in the chemical composition of fluids that flowed into the bone interiors the burrow found in the marrow cavity of specimen iguam zov 7 provides evidence that some remains were inhabited by minute or juvenile invertebrates all recognised features indicate that both invertebrate and vertebrate bioclasts included in the tempestite were initially deposited in various settings of the carbonate ramp and in the end redeposited as a result of a heavy storm or a hurricane the examined bone bearing bed represents time averaged assemblage which originated due to hydraulic concentration of the vertebrate bioclasts the petrographic analysis of the fillings is a valuable tool to identify complex taphonomic pathways of vertebrate bioclasts

permo triassic fluvial systems in the southeastern karoo basin south africa abstract a prominent clastic wedge of latest permian to early triassic age katberg sandstone prograded northwestwards into the main karoo basin from a southerly source area as a sedimentary response to renewed tectonism associated with the cape fold belt proximal to distal relationships within this clastic wedge and its relationships with underlying and overlying formations reveal a variety of stream types which reflect changing tectonic and climatic conditions towards the close of permian times the depositional area of the southeastern karoo basin was crossed by meandering river channels cutting through earlier formed floodplain deposits balfour formation tectonic rejuvenation of the source area in early triassic times led to steeper gradients and a sharp increase in the supply of coarser grained detritus as a result alluvial fans developed in areas adjacent to the source terrane and the river channels became braided depositing only sands katberg sandstone with muds and silts being carried down into the most distal parts of the floodplain burgersdorp formation subsequent denudation of the source area gradually reduced slopes and allowed the source ward shift of the distal facies until it eventually overstepped the earlier formed braided stream deposits accompanying the changes in tectonic conditions was a change in climatic which also influenced stream type the late permian to early triassic period records a general change to warmer climatic conditions following the widespread glaciation of the early to middle permian thus the lowest beaufort group sediments were probably deposited in warm temperate to humid conditions with later deposits being laid down under an increasingly arid regime

a new magnetic map of the weddell sea and the antarctic peninsula abstract as part of the antarctic digital magnetic mapping project admap workers from vniiokeangeologia russia the british antarctic survey uk and the naval research laboratory usa have brought together almost all of the available magnetic data in the area 0 120 w 60 90 s the final map covers the whole weddell sea and adjacent land areas the antarctic peninsula and the seas to the west an area comparable in size with that of the usa this paper describes the methods used during the compilation of the map and reviews briefly some of the main features shown on it distinct magnetic provinces are associated with precambrian rocks of the east antarctic craton highly extended continental crust in the weddell sea embayment the arc batholith of the antarctic peninsula and oceanic crust of the northern weddell sea which was created as a direct consequence of south america antarctica plate motion and oceanic crust generated at the pacific antarctic ridge the magnetic anomaly map thus provides an overview of the fragmentation of south western gondwana and the tectonic development of the weddell sea sector of antarctica

δ13c values of carbonate nodules across the permian triassic boundary in the karoo supergroup south africa reflect a stinking sulfurous swamp not atmospheric co2 abstract paleosol macromorphology petrography and stable isotope geochemistry of carbonate nodules from permo triassic p t boundary strata of the central karoo basin south africa are presented in order to evaluate their utility as geochemical archives of permian triassic paleoatmospheric chemistry and their potential to provide a terrestrial carbon isotope chemostratigraphy for correlation with contemporaneous global marine 13 c records paleosol morphologies across p t boundary strata of the central karoo basin indicate the region was a poorly drained seasonally to continuously flooded bottomland radiaxial calcite microspar and micrite microtextures are observed in the calcite nodules associated with p t paleosol profiles micritic calcite is typical of soil carbonate that forms in well drained soils characterized by open system gaseous diffusion between the soil and global atmosphere whereas more coarsely crystalline textures and radiaxial calcite are more typical of crystallization in phreatic environments with limited gaseous diffusion between the soil and global atmosphere in this regard only those nodules composed of micritic calcite can be considered as a potentially reliable source of proxy information for paleoatmospheric 13 c values across the p t boundary the 13 c values of all carbonate nodules from two overlapping measured sections near carlton heights south africa range from 24 4 to 1 8 whereas micritic calcite nodule 13 c values range from 7 0 to 21 3 comparison of calcite 13 c values with penecontemporaneous organic matter 13 c values indicate that calcite 13 c values 13 c values rather these more negative carbonate nodule 13 c values likely reflect calcite crystallization under poorly drained swampy conditions characterized by a semi closed chemical system that evolved independently of earth s troposphere therefore previously reported negative carbon isotope shift s measured from carbonate nodules in the karoo basin p t strata do not provide a record of atmospheric 13 c and probably do not provide a viable carbon isotope stratigraphy that can be realistically correlated with global p t marine carbon isotope stratigraphies