the permian triassic transition and the onset of mesozoic sedimentation at the northwestern peri tethyan domain scale palaeogeographic maps and geodynamic implications the main aim of this paper is to review middle permian through middle triassic continental successions in european secondly areas of middle late permian sedimentation the permian triassic boundary ptb and the onset of triassic sedimentation at the scale of the westernmost peri tethyan domain are defined in order to construct palaeogeographic maps of the area and to discuss the impact of tectonics climate and sediment supply on the preservation of continental sediment at the scale of the western european peri tethyan basins the upper permian is characterised by a general progradational pattern from playa lake or floodplain to fluvial environments in the northern variscan belt domain areas of sedimentation were either isolated or connected to the large basin which was occupied by the zechstein sea in the southern variscan belt during the late permian either isolated endoreic basins occurred with palaeocurrent directions indicating local sources or basins underwent erosion and or there was no deposition these basins were not connected with the tethys ocean which could be explained by a high border formed by corsica sardinia palaeorelief and even parts of the kabilia microplate the palaeoflora and sedimentary environments suggest warm and semi arid climatic conditions at the scale of the whole study area an unconformity more or less angular is observed almost everywhere between deposits of the upper permian and triassic except in the central part of the germanic basin the sedimentation gap is more developed in the southern area where in some basins upper permian sediment does not occur the large sedimentary supply erosion and or lack of deposition during the late permian as well as the variable palaeocurrent direction pattern between themiddle late permian and the early triassic indicate a period of relief rejuvenation during the late permian during the induan all the intra belt basins were under erosion and sediment was only preserved in the extra belt domains the northern and extreme southern domains in the northern domain the central part of the germanic basin sediment was preserved under the same climatic conditions as during the latest permian whereas in the extreme southern domain it was probably preserved in the tethys ocean implying a large amount of detrital components entering themarinewaters mesozoic sedimentation began in the early olenekian the ephemeral fluvial systems indicate arid climatic conditions during this period three distinct areas of sedimentation occur a northern and southern domain separated by an intra belt domain the latter accumulated sediments during the early middle permian and experienced erosion and or no deposition conditions between the middle late permian and the beginning of mesozoic sedimentation dated as anisian to hettangian at the top of the lower triassic another tectonically induced more or less angular unconformity is observed the hardegsen unconformity which is dated as intra spathian and is especially found in the north european basins this tectonic activity created new source areas and a new fluvial style with marine influences at the distal part of the systems during the anisian and ladinian continental sedimentation was characterised by a retrogradational trend in other words the fluvial system evolved into fluvio marine environments attesting to a direct influence of the tethys ocean in the southern and northern domains both at the end of the olenekian spathian and during the anisian the presence of palaeosols micro andmacrofloras indicate less arid conditions throughout this domain

postapocalyptic greenhouse paleoclimate revealed by earliest triassic paleosols in the sydney basin australia the permian triassic boundary in the sydney basin of australia is coincident with a pronounced decrease in 13 c isotopic values of organic carbon the last coals anywhere in the world for all of the early triassic ca 6 m y time and extirpation of the glossopteris flora at the top of the illawarra and newcastle coal measures coal bearing paleosols of the latest permian represent extensive swamplands of the seasonally deciduous glossopteris flora in a humid cold temperate lowland southwest of an andean style volcanic arc stone rolls large ribs of floor rock up into the coal within some of the uppermost coals of the permian coal measures can be interpreted as string bogs of a kind now found in cold climates at latitudes of 68 70 which is compatible with a paleomagnetically estimated paleolatitude of 65 85 s for the sydney basin paleolatitude was not much different for earliest triassic time but paleosols of that age include inceptisols and entisols showing substantial chemical and textural weathering more like soils now forming at latitudes of 40 58 than those within polar circles this anomalous high latitude warmth set in at the permian triassic boundary sedimentation rates increased at the boundary marked by geochemically unusual acidification of clay and a dramatic carbon isotopic excursion these changes in environments and ecosystems can be explained by soil erosion following deforestation implied by the plant extinctions and abundant fungal remains at the permian triassic boundary evidence from paleosols can now be added to that from paleontological and isotopic studies showing that disruption of the carbon cycle at the permian triassic boundary resulted in a co 2 or ch 4 post apocalyptic greenhouse paleoclimate

palaeoenvironments and faunal recovery after the end permian mass extinction abstract strata of the lower triassic werfen formation dolomites northern italy record deposition in a wide range of palaeoenvironments from peritidal to outer ramp settings this enables accurate assessment of faunal recovery following the end permian mass extinction by eliminating facies related effects extinction of the shelly taxa appears to be rapid and occurs in the latest permian tesero oolite horizon disappearance of the various trace fossil taxa appears to be more gradual following the extinction the lower and middle griesbachian are characterised by laminated sediments containing low diversity bedding plane assemblages ichnological geochemical and sedimentological data show evidence of widespread low oxygen conditions normally oxygenated waters return in the late griesbachian and there is a moderate increase in tiering above and below the surface macrofaunal and ichnofossil diversity decrease markedly during the smithian with the appearance of more terrigenous strata possibly reflecting an increased run off and more brackish conditions complete recovery begins in the basal spathian with an increase in tiering above and below the substrate crinoids and ammonoids appear for the first time in the werfen formation and the ichnogenus rhizocorallium re appears global correlation is hindered by poor biostratigraphy in the dolomites region however on a broad scale it appears as though many of the palaeoenvironmental changes are repeated worldwide current available data suggest that the re appearance of some trace fossil taxa e g diplocraterion rhizocorallium may be globally synchronous

permian continental basins in the southern alps italy and peri mediterranean correlations the late carboniferous to permian continental successions of the southern alps can be subdivided into two main tectono sedimentary cycles separated by a marked unconformity sealing a middle permian time gap generally estimated at over 10 ma the lower cycle 1 between the variscan crystalline basement and the early permian is mainly characterised by fluvio lacustrine and volcanic deposits of calc alkaline acidic to intermediate composition which range up to a maximum thickness of more than 2 000 m the upper cycle 2 which is devoid of volcanics is mostly dominated through the mid late permian by alluvial sedimentation which covered the previous basins and the surrounding highs giving rise to the subaerial verrucano lombardo val gardena groden red beds up to about 800 m thick the palaeontological record from the terrigenous deposits of both the above cycles consists mainly of macro and microfloras and tetrapod footprints the age of the continental deposits is widely discussed because of the poor chronological significance of a large number of fossils which do not allow reliable datings however some sections are also controlled by radiometric calibrations the comparison with some selected continental successions in southern europe allows to determine their evolution and set up correlations a marked stratigraphic gap shows everywhere between the above mentioned cycles 1 and 2 as in the southern alps the gap reaches the greatest extent during the mid permian near the illawarra reversal geomagnetic event 265 ma in western europe however such as in provence and sardinia the discussed gap persists upwardly to late permian and early triassic or slightly younger times i e to the onset of the alpine sedimentary cycle even though in northeastern spain iberian ranges balearic islands this gap results clearly interrupted by late guadalupian lopingian deposits the above two major tectonosedimentary cycles reflect in our view two main geodynamic events that affected the southern europe after the variscan orogenesis the late carboniferous early permian transformation of the gondwana eurasia collisional margin into a diffuse dextral transform margin and the middle late permian opening of the neotethys ocean with the onset of a generalised extensional tectonic regime and the progressive westward marine ingression

ultrasonic logging across unconformities outcrop and core logger sonic patterns of the early triassic middle buntsandstein hardegsen unconformity southern germany abstract the recognition of unconformities in thick and monotonous sandstone successions of consistent grain size is often difficult in locally exposed surface outcrop sections and also poorly constrained by changes of petrophysical properties across the boundary surfaces here a mobile ultrasonic device is tested as a time efficient handheld logging tool to investigate relative changes in sandstone cementation and porosity across the early triassic middle buntsandstein hardegsen unconformity in southern and central germany a persistent offset of 18 20 sonic transit time occurs in all tested sections across the unconformity this offset is not restricted to single peaks but clearly separates the different sonic patterns of the hardegsen and solling formations below and above the unconformity integrated thin section analysis porosity permeability measurements and gamma density logging suggest that these changes in sonic transit time are due to major variations in sandstone cementation and the formation of secondary porosity associated with the unconformity field based ultrasonic measurements thus provide a useful addition to established outcrop logging methods and may help in the non destructive in situ identification of porosity cementation heterogeneities in sandstones in the localization of unconformities and in the correlation of borehole measured sequence boundaries to outcrop analogs

triassic climates state of the art and perspectives abstract the climate of the triassic period was characterized by a non zonal pattern dictated by a strong global monsoon system with effects that are most evident in the tethys realm this strong monsoonal regime is related to the aggregation of the pangaean supercontinent which by triassic time was already completed climate oscillations existed within this framework the harsh hot house climatic conditions that characterized the late permian and perhaps precipitated the end permian mass extinction were probably maintained during the early triassic and may account for the impoverished but distinctive faunal and floral lower triassic associations although metazoan reef builders were probably the most affected group carbonate production remained high at least in the western tethys realm the middle triassic was characterized locally by humid episodes although their geographical distribution has yet to be clarified the carnian pluvial event marks an episode of increased rainfall documented worldwide was the most distinctive climate change within the triassic different hypotheses have been proposed for its causes changes of atmospheric or ocean circulation driven by plate tectonics a peak of the global monsoon due to maximum continent aggregation or triggering by the eruption of a large igneous province subsequently the late carnian and norian seem to have been climatically stable although minor climatic changes have recently been described even from this time period finally the end triassic extinction event is also associated with climate change specifically warming and increased rainfall but this evidence comes mostly from the northern parts of the central atlantic magmatic province and the global pattern of climate change at the triassic jurassic boundary has still to be resolved many facets of triassic climate are intriguing and deserve further research however palaeoclimate studies on the triassic have so far been carried out only locally with different proxies those proxies will require inter calibration in order to depict correctly the temporal and geographical patterns of triassic climate

the unusual sedimentary rock record of the early triassic a case study from the southwestern united states abstract sedimentary systems as well as the earth s biota changed dramatically following the end permian mass extinction this global sedimentological transformation is reflected in the widespread occurrence of flat pebble conglomerates subtidal wrinkle structures microbialites and carbonate seafloor fans in various lower triassic sections as a case study a facies analysis of the lower triassic virgin limestone member of the moenkopi formation and the middle and upper members of the union wash formation southwestern united states was conducted and all unusual facies were documented within a palaeoenvironmental framework the occurrence of the flat pebble conglomerates and wrinkle structures is significant because these reflect a long term reduction in vertical bioturbation in the case of the subtidal microbialites and carbonate seafloor fans their formation is linked to an increase in alkalinity the presence of these facies in the uppermost lower triassic strata of the southwestern united states indicates that reduced levels of infaunal bioturbation as well as and perhaps a consequence of unusual ocean chemistry extended for 5 6 million years after the end permian mass extinction suggesting that depositional environments as well as the biota were significantly affected by the extinction event

significance of interdune deposits and bounding surfaces in aeolian dune sands bounding surfaces and interdune deposits provide keys for detailed interpretations of the development shape type wavelength and angle of climb of aeolian bedforms as well as overall sand sea conditions current alternate interpretations of bounding surfaces require very different but testable models for sand sea deposition two perpendicular traverses of jurassic entrada sandstone utah reveal relations among cross strata first order bounding surfaces and horizontal strata these field relations seem explicable only as the deposits of downwind migrating climbing enclosed interdune basins horizontal strata and dune bodies consisting of superimposed smaller crescentic dunes cross stratified deposits a 1 7 km traverse parallel to the palaeowind direction provides a time transgressive view showing continuous cosets of cross strata first order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree changes occur in the angle of climb cross strata structure and interdune deposits these reflect changes in depositional conditions through time a 1 5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first order bounding surfaces and interdune deposits forming flat laterally discontinuous lenticular bodies the distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins such as those on padre island texas hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg the presence of three orders of bounding surfaces indicates dune bodies consisting of smaller super imposed dunes the geometry of first order bounding surfaces is a reflection of the shape of the inter dune basins second order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type third order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes the presence of only two orders of bounding surfaces indicates simple dunes modern and entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet damp and dry conditions interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions

oblique strain partitioning and transpression on an inverted rift the castilian branch of the iberian chain the iberian chain is a wide intraplate deformation zone formed by the tectonic inversion during the pyrenean orogeny of a permian mesozoic basin developed in the eastern part of the iberian massif the n s convergence between iberia and eurasia from the late cretaceous to the lower miocene times produced significant intraplate deformation the nw se oriented castilian branch of the iberian chain can be considered as a key zone where the proposed models for the cenozoic tectonic evolution of the iberian chain can be tested structural style of basin inversion suggests mainly strike slip displacements along previous nw se normal faults developed mostly during the mesozoic to confirm this hypothesis structural and basin evolution analysis macrostructural bouguer gravity anomaly analysis detailed mapping and paleostress inversions have been used to prove the important role of strike slip deformation in addition we demonstrate that two main folding trends almost perpendicular ne sw to e w and nw se were simultaneously active in a wide transpressive zone the two fold trends were generated by different mechanical behaviour including buckling and bending under constrictive strain conditions we propose that strain partitioning occurred with oblique compression and transpression during the cenozoic 2008 elsevier b v all rights reserved

large scale bed configurations macroforms platte river basin colorado and nebraska primary structures and formative processes large scale current bed forms defined here as bed configurations having dimensions commensurate with the size of the turbulent boundary layer are considered by many workers to be hydrodynamically equivalent to the regime bed forms these bed forms were investigated at formative flows bankfull flows on the south platte and platte rivers in colorado and nebraska to determine structures and formative processes results of this study suggest that large scale bed forms here termed macroforms are not hydrodynamically equivalent to the regime bed forms but constitute a unique hierarchical class of bed configurations produced by turbulent vortices that involve the entire boundary layer macroforms are oriented obliquely to the mean flow direction and are attached to one or both channel banks three members of a continuum of geometries are recognized in the channels of the platte river basin the internal stratification for each of the three macroform types recognized in the basin is similar and in its simplest form consists of the coarsening upward sequence apron laminae foreset laminae topset laminae although it may not be possible to differentiate the three macroform types solely on the basis of internal stratification the presence of a distinctive coarsening upward sequence in coarse grained fluvial systems offers a potentially powerful tool for identifying these environments in the geologic record

the early triassic ammonoid recovery paleoclimatic significance of diversity gradients ammonoids recovered much faster than other marine shelly invertebrates after the end permian mass extinction based on a refined global data set at the basin level we investigate the paleobiogeographical global latitudinal and longitudinal diversity patterns in terms of climatic changes during the early triassic such analysis differs from already published qualitative or quantitative studies in that it estimates faunal patterns and endemicity at an ecologically meaningful level of spatial resolution i e at the basin level during the early triassic the global first order trend in increasing ammonoid diversity was accompanied by a progressive change from cosmopolitan to latitudinally restricted distributions this led to the emergence of a clear latitudinal diversity gradient during most of the smithian and spathian stages which entails increased steepness of the sea surface temperature gradient during the late early triassic however two brief episodes of ammonoid cosmopolitanism combined with low global diversity interrupted the first order increasing trend at the very beginning of the smithian and at its very end the longitudinal analysis of smithian distributions indicates a westward decrease of diversity within the tethys which faded away during the spathian analysis of endemicity indicates a rapid biogeographical maturing and structuring of faunas concomitant with the edification of the latitudinal diversity gradient

near bed sediment concentration in gravel bedded streams in this paper we present an analytical model for the concentration of suspended sediments in the proximity of the bed of gravel rivers the model is applicable to high gradient streams carrying sands and finer sediment particles in suspension over poorly sorted gravel beds the analysis is carried out in terms of equations which realistically describe the velocity profile of the carrying stream and the dispersion of heavy particles in turbulent shear flows over stable gravel beds to test our hypotheses we verified the equations using available flume and river data and evaluated their response for various combinations of bed roughness and suspended load characteristics our results indicate that characterization of the near bed concentration can be formulated in terms of a velocity defect representation of the mean turbulent velocity profile and a sediment eddy diffusivity distribution developed for sand bed rivers the near bed concentration is shown to be rather sensitive to variations in the gravel bed relative roughness the depth at which the concentration is evaluated above the virtual bed surface and the ratio between the settling velocity of the suspended sediment and the bed shear velocity

origin of the permian triassic iberian basin central eastern spain abstract the iberian basin was an intracratonic rift basin in central eastern spain developed since early permian times the basin boundary faults were normal listric faults controlling an asymmetric extension propagating northeast with time hercynian or older lineaments controlled the orientation of the iberian basin and extension was accommodated basically in the hanging wall block by the formation of secondary grabens and a central high the basin was related with the coeval ebro catalan and cuenca mancha basins and their connections are discussed subsidence curves show that the early permian early jurassic period of extension can be subdivided into three rifting episodes and a flexural one extension factor increases from 1 17 in the northwest to 1 29 near the mediterranean coast the increasing extension rate was accommodated by transfer faults trending nne ssw more important in the levante area the rift evolution is intermittent and seems to reflect distinct stress fields the collapse of the late hercynian orogen and related increased heat flux extension and rifting is the most probable origin of the iberian basin and related basins the origin of the catalan and the valencia prebetic basins is related to the southwards migration of the hesse burgundy rift along the eastern margin of the iberian microplate

morphological evolution and dynamics of a large sand braid bar jamuna river bangladesh the initiation and evolution of a kilometre scale sand braid bar was monitored during a 28 month survey period from 1993 to 1996 in one of the world s largest braided rivers the jamuna river bangladesh repeated bathymetric surveys through two monsoon flood seasons combined with bar top surveys during exposure of the bar at low flow provide the most detailed chronology of braid bar growth yet compiled for a large sand bed river during rising and peak flow of the 1994 monsoon flood a 1 5 km long 0 5 km wide 12 m high symmetrical mid channel bar was deposited in the centre of a major channel downstream of a zone of flow convergence and significant bank erosion initial deposition and growth of the bar core were probably caused by amalgamation of dunes that are present in the jamuna channels at all flow stages bar top aggradation continued through downstream migration of an accretionary dune front a 3 m high angle of repose slipface that was composed of amalgamated 0 5 to 1 m high dunes at waning and low flow the mid channel bar widened by up to 1 km through the lateral accretion of dunes onto the margins of the initial bar core a low velocity zone in the sheltered wake region of the bar tail led to the accumulation of substantial volumes of silts and clays during the rising and peak flows of the next monsoon flood the mid channel bar extended its bar tail by up to 1 5 km as one of the anabranches became dominant and flow was deflected across the bar tail accretion at the bar tail generated a lobate transverse bar front with a 10 m high angle of repose avalanche face emergence of several smaller bars along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid channel bars the conversion of a mid channel bar to an alternate bar is contrary to many previous descriptions of the braiding process

shifts in late paleozoic atmospheric circulation over western equatorial pangea insights from pedogenic mineral δ18o compositions the d 18 o values of pedogenic calcites phyllosilicates and iron oxyhydr oxides from fossil soils throughout the southwestern united states show systematic paleolatitudinal and temporal trends that indicate a significant change in soil moisture conditions and atmospheric circulation patterns over southwestern euramerica throughout latest pennsylvanian and early permian time a progressive depletion of as much as 6 in mineral 18 o values with increasing distance northward to 108 of the paleoequator indicates a weakening or disruption of zonal easterly flow over the study area by latest pennsylvanian time furthermore elevated d 18 o values suggest a proximal source of paleoprecipitation over the study area perhaps due to initiation of reversed equatorial flow over tropical southwestern euramerica coupling of the geographic and temporal trends defined by pedogenic mineral d 18 o values with those defined by climatically sensitive paleosol pedotypes in the study area along with overall elevated mineral d 18 o values provides some of the strongest evidence that northern hemisphere monsoonal circulation was well established over pangea by early permian time

the lower triassic sedimentary and carbon isotope records from tulong south tibet and their significance for tethyan palaeoceanography abstract the lower triassic sedimentary and carbonate organic carbon isotope records from the tulong area south tibet are documented in their integrality for the first time new age control is provided by ammonoid and conodont biostratigraphy the basal triassic series consists of griesbachian dolomitic limestones similar to the kathwai member in the salt range pakistan and to the otoceras beds in spiti india the overlying thin bedded limestones of dienerian age strongly resemble the lower ceratite limestone of the salt range they are followed by a thick series of dark green silty shales of dienerian early smithian age without fauna that strikingly resemble the ceratite marls of the salt range this interval is overlain by thin bedded light grey fossil rich limestones of middle to late smithian age resembling the upper ceratite limestone of the salt range these are followed by a shale interval of early spathian age that has no direct counterpart in other tethyan sections carbonate production resumes during the late early and middle spathian with the deposition of red bioclastic nodular limestone ammonitico rosso type facies apart from its colour this facies is similar to the one of the niti limestone in spiti and of the spathian nodular limestone in guangxi south china as in other tethyan localities such as spiti the early middle anisian part of the tulong section is strongly condensed and is characterized by grey thin bedded limestones with phosphatized ammonoids as for many other tethyan localities the carbon isotope record from tulong is characterized by a late griesbachian dienerian positive 13ccarb excursion 2 and a very prominent positive excursion 5 at the smithian spathian boundary thus confirming the well documented perturbations of the global carbon cycle following the permian triassic mass extinction event

δ13c depth profiles from paleosols across the permian triassic boundary evidence for methane release stable carbon isotopic analyses of organic carbon 13 c in individual paleosol profiles from permian triassic sequences of antarctica reveal systematic isotopic variations with profile depth these variations are in many cases analogous to those in modern soils which are functions of redox conditions soil development and degree and type of microbial decay in modern soils these isotopic depth functions develop independently from vegetation changes c 3 versus c 4 vegetation and can be diagnostic of soil orders this study shows that soil intrinsic functions can be preserved in the 13 c values of paleosols as old as 260 ma and constitute valuable data for paleoecological interpretations a large carbon isotopic offset of as much as 10 in whole paleosol profiles across the permian triassic boundary indicates significant changes in the soil biogeochemistry and the soil atmosphere system early triassic paleosols are distinctive in their extremely low 13 c values to 42 and often show an anomalous 13 c depth distribution compared to both permian paleosols and modern soils highly depleted 13 c values as the ones in early triassic paleosols are suggested to be associated with microbial methane oxidation methanotrophy this hypothesis implies increased methane concentrations in the early triassic soil atmosphere system increased atmospheric methane was probably partly responsible for the global carbon isotopic shift documented in marine and terrestrial sediments across the permian triassic boundary

the effect of global warming and global cooling on the distribution of the latest permian climate zones abstract the end permian biotic crisis is commonly associated with rapid and severe climatic changes these climatic changes are commonly suggested to have originated from solid earth carbon degassing leading to global warming but aerosol and ash induced cooling induced by lava degassing has been suggested as well the application of an earth system model of intermediate complexity has enabled a visualisation of the major climatic shifts on the supercontinent pangaea caused by rapid temperature changes due to changed radiative properties from greenhouse gases the reconstructed reference climate was validated by latest permian climate indicative sediments to investigate the possible climatic shifts from a set of 22 reconstructions which varied with temperature a minimum global annual mean temperature of 18 2 c for the late permian climate prior to the climatic perturbation event was determined starting from this pre event setup global warming and global cooling scenarios were simulated the response of the end permian climate system to temperature increase and decrease show marked differences while global cooling is followed by major climatic changes in the high latitudes and replacement of boreal biomes by tundra and polar frost the changes during global warming are less pronounced with only locally increasing aridity compensated by humidisation in other regions the different behaviour of the climatic belts under warm and cold conditions is accompanied by different climate sensitivities caused by different strength of the snow cover albedo feedback thus changes in the energy balance of the latest permian surface troposphere system have a 30 higher perturbation potential during cold climate conditions than during warmhouse conditions therefore substantial global cooling resulting in coldhouse climate conditions and an annual global mean temperature below 18 c is more efficient in perturbing the earth palaeoclimate during the end permian warmhouse the results suggest that global cooling mechanisms as injection of sulphur aerosols and ash particles from the siberian traps large igneous province into the late permian palaeoatmosphere have a higher climate perturbation potential than a warming due to carbon greenhouse gases with a similar magnitude of radiative forcing

anisotropic shear strength of a residual soil of sandstone this paper discusses results of laboratory tests carried out with a residual soil originated from the weathering of eolian sandstone from southern brazil parent rock features like microfabric and particle bonding are remarkably well preserved within this residual soil stiffness and shear strength properties were evaluated with consolidated drained cid and consolidated undrained ciu triaxial compression tests undisturbed specimens were tested with two different orientations between the specimen axis and bedding surfaces i e parallel 0 or perpendicular 90 to investigate the effect of anisotropy when cid triaxial tests were performed with 0 the yield surface associated with the structure was much larger than when tests were performed with 90 coincidently ciu tests with 0 showed peak shear strengths much greater than for 90 at comparable test conditions once the peak shear strength was surpassed ciu tests followed collapse type effective stress paths not sh

an early triassic evolving erg system iberian chain ne spain palaeoclimate implications the triassic buntsandstein facies in the north eastern iberian chain central eastern spain previously considered to be fluvial in origin is shown to contain an evolving erg system the preserved erg accumulation comprises a succession that represents the transition from a wadi belt via inner erg margin to a central erg system this study supports early triassic olenekian palaeoclimate models for western europe which predict extremely arid conditions for iberia the two main easterly and southerly palaeowind directions are in accordance with data from other triassic dune fields in europe and corroborate the influence of monsoonal circulation which affected precipitation in the variscan massif to the south and west

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

lower triassic sequence stratigraphy of the western part of the germanic basin west of black forest fluvial system evolution through time and space abstract the aim of this paper is to analyse the fluvial evolution of the lower triassic in the western part of the germanic basin through time and space as well as the impact of the geodynamic and climatic setting on the preservation of fluvial deposits the lower triassic crops out only in the vosges massif and the black forest so well log studies are required to realise sequence stratigraphy correlations and establish comparisons with others parts of the germanic basin in a first step we use well log data analyses to characterise the electrofacies associations in the triassic and then define the well log signatures of each formation in a second step the characterisation and recognition of genetic sequences and their stacking pattern allow us to define seven minor cycles integrated into two major cycles finally the quantification of the lithologies at different stages of basin evolution leads to the reconstruction of paleoenvironmental maps to illustrate facies evolution through space and time a comparison with cycles defined in the germanic basin allows us to propose correlations of the lower triassic on either side of the rhine graben and leads to a discussion of the evolution of fluvial systems through time and space during the scythian the fluvial style is characterised by braided fluvial systems evolving laterally into lake deposits towards the central part of the germanic basin during this stage the basin was a huge depression with very few marine connections in its extreme eastern part the stratigraphic cycles represent rhythmic fluctuations in relative lake level that could be attributed to sediment supply and or lake level variations in an arid setting four minor stratigraphic cycles are observed that are integrated within a single major stratigraphic cycle during the period of the stratigraphic base level rise of the major cycle a maximum of 233 m of sediment would represent a duration of sedimentation in the paris basin of at least 1 8 m y during the period of the stratigraphic base level fall of the major cycle a maximum of 65 m of sediment would have accumulated over 2 5 m y on the western edge of the germanic basin the top of the scythian is marked by a major sedimentary break characterised by a planation surface with preservation of the first paleosols followed by the hardegsen unconformity this unconformity is tectonically deformed leading to the development of a new sedimentation area in the west of the basin the fluvial sedimentation above this discontinuity shows a trend towards enhanced development of floodplain or lacustrine type environments at its western margin with the formation of paleosols the fluvial systems are linked with sabkhas and then with a shallow sea connected to the tethys ocean in this context the stratigraphic cycles are caused by variations in relative sea level and or sediment supply the fluvial deposits are preserved in an exoreic basin

paleozoic vegetation and the siluro devonian rise of fluvial lateral accretion sets a long standing hypothesis links the increased prominence of meandering rivers in the middle paleozoic to the colonization of terrestrial environments by vegetation this hypothesis is tested using a data set of cambrian to devonian fluvial literature and field examination of key stratigraphic units according to some researchers cambrian to mid silurian river systems were braided in planform with a sharp increase in the abundance of meandering rivers during the silurian devonian although meandering systems were largely identified on the basis of thick mudstones and organized channel deposits the data set record of lateral accretion sets appears to be a robust proxy for the abundance of meandering river point bars lateral accretion is first recorded from pridolian lochkovian strata but is noted in nearly 40 of fluvial case studies by the famennian this trend matches the known record of rooted vegetation suggesting that vegetation progressively stabilized river banks and promoted single thread channels however the presence of precambrian and extraterrestrial meandering systems indicates that vegetation is not essential for meandering and the lack of evidence for cambrian to silurian ludlow point bars is surprising if originally present they may largely have been destroyed by extreme floods chute cut offs in coarse grained meandering systems and eolian activity

onset of biotic and environmental recovery from the end permian mass extinction within 1 2 million years a case study of the lower triassic of the meishan section south china abstract paleontological sedimentary and geochemical data obtained from the lower triassic strata of the meishan section south china suggest that onset of biotic recovery coincided with amelioration of paleoceanographic conditions and an increase in ocean productivity the griesbachian meishanorhynchia association from meishan indicates biotic recovery between the recovery stages 1 and 2 both the initial biotic recovery and palaeoceanic environmental amelioration occurred within 1 2 myr after the end permian mass extinction in the late griesbachian

sedimentary evolution of the continental early middle triassic canizar formation central spain implications for life recovery after the permian triassic crisis the permian triassic transition p t was marked by important geochemical perturbations and the largest known life crisis consequences of this event as oxygen depleted conditions and the unusual behavior of the carbon cycle were prolonged during the early triassic interval delaying the recovery of life in both terrestrial and marine ecosystems studies on lower triassic sediments of continental origin as in the case of western europe are especially problematic due to the scarcity of fossils and absence of precise dating the canizar fm is an early middle triassic unit of continental origin of the se iberian ranges e spain a detailed sedimentary study of this unit allows a shedding of light on some unresolved problems of the continental deposits of this age the top of this unit is dated as early anisian by means of a pollen association while the age of its base is here estimated as late smithian or smithian spathian transition different facies associations and architectural elements have been defined in this unit in the western and central parts of the basin this unit shows sedimentary characteristics of fluvial deposits with locally intercalated aeolian sediments while in the eastern part there is an alternation of both aeolian and fluvial deposits sedimentary structures also indicate changes in the climate conditions mainly from arid to semiarid two marked arid periods when well preserved aeolian sediments developed during early middle spathian and spathian anisian transition they alternated with two semiarid but more humid periods during the late spathian and early anisian these conditions basically correspond with the general arid and very arid conditions described for central western european plate during the same period of time the ateca montalban high in the northern border of the study basin must have represented an important topographic barrier in the western tethys separating aeolian dominated areas to the n and ne from fluvial dominated areas to the south the canizar fm has been subdivided into six members a f separated by seven 1 7 major bounding surfaces mbs these surfaces are well recognized laterally over hundred of km and they represent 104 105 my mbs 5 is considered to be of late spathian age and it is a clear indication of tectonic activity represented by a mild unconformity this event represents a change in the sedimentary characteristics reactivation of the unit and from here to the top of the unit are found the first signals of biotic recovery represented by tetrapod footprints plants roots and bioturbation all of these characteristics and the estimated age represented by the mbs 5 event permit this surface to be related to the coeval hardegsen unconformity of central western europe these first signals of biotic recovery can thus be related to an increased oxygen supply due to the new created paleogeographical corridors in the context of this tectonic activity these biotic signals occurred 5 my after the permian triassic limit crisis a similar delay as occurred in other coeval and neighboring basins

late early triassic climate change insights from carbonate carbon isotopes sedimentary evolution and ammonoid paleobiogeography the late early triassic sedimentary facies evolution and carbonate carbon isotopemarine record 13ccarb of ammonoid rich outer platform settings show striking similarities between the south china block scb and the widely distantnorthern indianmargin nim the studied sections are located within the triassic tethyshimalayan belt losar section himachal pradesh india and the nanpanjiang basin in the south china block jinya section guangxi province respectively carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end permian biotic crisis this study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover extinction followed by a radiation occurring around the smithian spathian boundary based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity we demonstrate that the late early triassic smithian and spathian was a time of a major climate change more precisely the end smithian climate can be characterized by a warm and equable climate underlined by a flat pole to equator sea surface temperature sst gradient while the steep spathian sst gradient suggests latitudinally differentiated climatic conditions moreover sedimentary evidence suggests a transition froma humid and hot climate during the smithian to a dryer climate fromthe spathian onwards by analogywith comparable carbon isotope perturbations in the latedevonian jurassic andcretaceouswe propose that high atmosphericco2 levels could have been responsible for the observed carbon cycle disturbance at the smithian spathian boundary we suggest that the end smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased co2 flux possibly originating froma short eruptive event of the siberian igneous province this increase in atmospheric co2 concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms including ammonoids in late smithian oceans