palustrine carbonates and the florida everglades towards an exposure index for the fresh water environment abstract palustrine carbonates are shallow fresh water deposits showing evidence of subaqueous deposition and subaerial exposure these facies are common in the geological record the intensity of modification is highly variable depending on the climate and the length of emergence palustrine limestones have previously been interpreted as marginal lacustrine deposits from fluctuating low salinity carbonate lakes but several problems remain with existing facies models 1 palustrine carbonates possess a lacustrine biota but commonly display fabrics similar to those of calcretes and peritidal carbonates 2 the co occurrence of calcrete horizons and karst like cavities is somewhat unusual and appears to indicate contemporaneous carbonate precipitation and dissolution in the vadose zone 3 the dominance of gray colors indicates water saturation apparently inconsistent with the evidence for strong desiccation overprint 4 profundal lake deposits are generally absent from palustrine sequences and sublittoral facies commonly make up only a small proportion of total thicknesses 5 no good modern analogue has been identified for the palustrine environment analogy with the florida everglades suggests a re interpretation of palustrine limestones not as pedogenically modified lake margin facies but as the deposits of extensive very shallow carbonate marshes the distribution of environments in the everglades is determined by the local hydrology reflecting the control of seasonal water level fluctuations and topography climate and topography were the main controls on deposition of ancient palustrine carbonates as in peritidal sequences aggradational cycles are capped by a range of lithologies evaporites desiccation and microkarst breccias calcretes lignite or coal horizons etc permitting interpretation of the climate careful analysis of lateral facies variations may permit reconstruction of subtle topography consideration of the florida everglades as a modern analogue for the palustrine environment has suggested the development of an exposure index for fresh water carbonates

palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information much of the work performed in this area has focused on either of two types of terrestrial carbonate palustrine carbonates or calcretes pedogenic and groundwater yet their simultaneous study shows there may be a gradual transition from one form to the other revealing the interplay between pedogenic sedimentary and diagenetic processes three main factors control the formation of these carbonates the position of the water table the host rock and the period of sub aerial exposure in pedogenic calcretes precipitation of carbonate takes places mostly in the vadose zone above the water table and within a previous host rock or sediment in groundwater calcretes the precipitation of carbonate also occurs within a previous host rock and around the groundwater table in palustrine carbonates however the precipitation of lime mud occurs in a lacustrine water body palustrine carbonates necessarily form on previous lacustrine mud whereas both types of calcretes may form on any type of sediment or soil the sub aerial exposure time needed to form palustrine carbonates may by relatively short even a season whereas pedogenic calcretes need more time several years to millions of years groundwater calcretes do not form on the topographic surfaces so there is no need of sub aerial exposure however stable surfaces favour the development of thick groundwater calcretes small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features causing difficulties in the interpretation of these carbonates the formation of these carbonates is controlled by palaeoenvironmental factors both commonly form in semi arid climates arid climates are also suitable for calcretes but sub humid conditions are more suitable for palustrine carbonates more indications of climatic conditions may be obtained through the analysis of the d18o content of both calcretes and palustrine carbonates and from the depth of the horizon containing carbonate nodules in pedogenic calcretes vegetation is also important in the formation of these types of carbonates data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13c signal of the primary carbonates which in pedogenic carbonates has also been used to estimate atmospheric pco2 during the phanerozoic these terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins their presence and characteristics can be used as indicators of aggradation subsidence or accommodation rates and therefore as indicators of different tectonic regimes even though the study of these carbonates has notably increased in recent years much less is known about them than about marine carbonates presently there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins with a need to include the carbonates analysed in this paper

was himalayan normal faulting triggered by initiation of the ramgarh munsiari thrust and development of the lesser himalayan duplex the ramgarh munsiari thrust is a major orogen scale fault that extends for more than 1 500 km along strike in the himalayan fold thrust belt the fault can be traced along the himalayan arc from himachal pradesh india in the west to eastern bhutan the fault is located within the lesser himalayan tectonostratigraphic zone and it translated paleoproterozoic lesser himalayan rocks more than 100 km toward the foreland the ramgarh munsiari thrust is always located in the proximal footwall of the main central thrust northern exposures toward the hinterland of the thrust sheet occur in the footwall of the main central thrust at the base of the high himalaya and southern exposures toward the foreland occur between the main boundary thrust and greater himalayan klippen although the metamorphic grade of rocks within the ramgarh munsiari thrust sheet is not significantly different from that of greater himalayan rock in the hanging wall of the overlying main central thrust sheet the tectonostratigraphic origin of the two different thrust sheets is markedly different the ramgarh munsiari thrust became active in early miocene time and acted as the roof thrust for a duplex system within lesser himalayan rocks the process of slip transfer from the main central thrust to the ramgarh munsiari thrust in early miocene time and subsequent development of the lesser himalayan duplex may have played a role in triggering normal faulting along the south tibetan detachment system

biogenic caliches in texas the role of organisms and effect of climate abstract biogenic constituents are ubiquitous and abundant in the caliches of texas investigation of 51 caliche profiles on various host strata alluvium limestone igneous rocks etc across approximately 900 km of texas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenic constituents these profiles exhibit significant differences in thickness varying from centimeters to meters and maturity varying from i to vi all of the different caliche facies are composed of low mg calcite biogenic features generally occur in the upper part of the profiles including the uppermost portion of massive caliche horizons platy horizons laminar crusts and pisoids the main biogenic caliche facies include rhizoliths calcified root structures stromatolite like laminar crusts and coated grains compared to the abiogenic massive micritic to microsparitic calcite groundmass biogenic constituents are morphologically distinct these biogenic constituents are composed of several microscopic mineral components including calcified filaments needle fiber calcite e g single crystalline needles and needle pairs triangular crystals and polycrystalline chains of rhombohedrons spherulites micro rods and nano spheres a large number of calcified root cellular structures and micro organisms e g fungal filaments actinomycetes and rod like bacteria are also present plant roots as well as soil biota produce distinctive structures and also enhance lithification by inducing calcite precipitation in the caliches i e biologically controlled or influenced processes host strata did not significantly influence the abundance nor type of biogenic features in the caliches in contrast climate had an evident effect on the development of biogenic constituents in these caliches in terms of the amount as well as type the thickness of laminar crusts and grain coatings and the abundance of biotic constituents within those facies decrease as the climate shifts from subhumid and subarid in southeast and central areas to subarid and arid in west and northwest texas in addition root structures and micro rods diminish significantly from the subhumid east to the arid west

palustrine lacustrine and alluvial facies of the norian owl rock formation chinle group four corners region southwestern u s a implications for late triassic paleoclimate abstract the upper triassic norian owl rock formation was deposited in a low gradient floodbasin at a subtropical paleolatitude the lower part of the formation consists predominantly of fine grained siliciclastic lithofacies deposited by sheetflood and sinuous streams on a muddy floodplain during a period of continuous basin aggradation nodular calcretes are increasingly mature higher in the formation suggesting increasingly episodic depositional conditions the upper part of the formation consists mostly of interbedded fine grained siliciclastic facies and laterally continuous ledges of limestone and sandstone the predominant limestone facies has brecciated to peloidal fabrics spar filled circumgranular cracks and root channeling the subordinate limestone facies displays wavy to irregular argillaceous lamination desiccation cracks and oscillation ripples and is vertically and laterally gradational with the brecciated facies the upper owl rock formation records deposition of aggrading sequences of alluvial sediments deposited during base level rise capped by highstand carbonates deposited in small perennial and ephemeral carbonate lakes and ponds base level lowstand in an overall semiarid climate resulted in extensive pedogenesis of the limestone and laterally equivalent alluvial facies basin wide variations in base level are interpreted as resulting from climatic fluctuations this depositional model is consistent with an interpreted trend towards aridification on the colorado plateau during the late triassic as pangea drifted northward from one climate zone to another

comparison of lacustrine successions and their palaeohydrological implications in two sub basins of the triassic cuyana rift argentina fil benavente cecilia andrea consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet mendoza instituto argentino de nivologia glaciologia y ciencias ambientales provincia de mendoza instituto argentino de nivologia glaciologia y ciencias ambientales universidad nacional de cuyo instituto argentino de nivologia glaciologia y ciencias ambientales argentina

sedimentology diagenesis and ichnology of cretaceous and palaeogene calcretes and palustrine carbonates from uruguay abstract the cretaceous mercedes formation and paleogene queguay formation deposits cropping out in w and s uruguay comprise two terrestrial limestone units that are very rich in trace fossils the study of these units permits to propose a sedimentological model for palustrine limestones and calcretes in which the distribution of different types of trace fossils is considered the study units include three main types of deposit lacustrine limestones palustrine limestones and calcretes the lacustrine limestones are relatively homogeneous and contain gastropods charophytes and ostracods but no trace fossils they were deposited in a relatively more perennial lacustrine environment the palustrine limestones include four different facies desiccated mudstones nodular limestones granular limestones and gravel sheets the desiccated mudstones indicate a lesser degree of pedogenic modification and the granular limestones a higher degree the gravel sheets are an indication of the reworking of previous limestones deposits during low lakewater levels most of the palustrine limestones except the gravel sheets contain the same bioclasts as the lacustrine limestones plus a variety of trace fossils such as rebuffoichnus sciuttoi fictovichnus gobiensis and different ichnospecies of celliforma the calcretes are either massive groundwater or laminar the massive calcretes are sandy limestones made up of a carbonate matrix and cements the laminar calcretes root mats which contain alveolar septal structures occur as centimetre thick layers and can be seen in all types of deposit they contain the same trace fossil association as the palustrine limestones the massive calcretes are poorer in such fossils the distribution of trace fossils in these environments is under strong facies control and provides good evidence of subaerial exposure and semi arid climates all the limestones are partially replaced and cemented by opal and quartz but in all cases the primary structure is preserved silicification occurred under groundwater meteoric conditions overall the limestones facies calcretes palustrine and lacustrine and their lateral distribution likely reflect the existence of wetland environments in semiarid to sub humid climates these climatic conditions were interrupted by a period of increased precipitations probably the early eocene climatic optimum

lateral variation in geometry of thrust planes and its significance as studied in the shumar allochthon lesser himalayas eastern bhutan abstract the features mapped indicate a sse striking displacement vector for the shumar thrust of eastern bhutan sections parallel to this direction show that the frontal flat segment of the thrust plane along with the subthrust and the hanging wall rocks has been folded into a synform a longitudinal section however shows a cuspate lobate shape broad concave upward curves meeting along sharp crests of the thrust plane which laterally cuts up through the subthrust rocks the broad salients and sharp re entrants defining the multiple lobate pattern of the thrust trace is due to the cuspate lobate lateral curvature which is a primary feature of the thrust plane each cusp combined with the forward or backward inclination of the thrust plane imparts a scoop like shape several of which are joined by transverse ridges parallel to the displacement vector the s surfaces transposed parallel to the limbs and axial surfaces of the isoclinal early folds in the allochthon have been progressively reoriented parallel to the curviplanar thrust surface later post thrusting folds superposed on these initially curved surfaces have curved hinges which trend at high angles to the displacement direction as a result the bent hinges are congruent to the laterally curved thrust surface below the evolution of the shumar thrust can be explained by lateral propagation along concave upward trajectories thrust fronts propagating from multiple nuclei subsequently coalesced to form a single thrust surface the ridges along which the thrusts coalesce have the form of lateral ramps

assessing the record and causes of late triassic extinctions accelerated biotic turnover during the late triassic has led to the perception of an end triassic mass extinction event now regarded as one of the big five extinctions close examination of the fossil record reveals that many groups thought to be affected severely by this event such as ammonoids bivalves and conodonts instead were in decline throughout the late triassic and that other groups were relatively unaffected or subject to only regional effects explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms regression during the rhaetian with consequent habitat loss is compatible with the disappearance of some marine faunal groups but may be regional not global in scale and cannot explain apparent synchronous decline in the terrestrial realm gradual widespread aridification of the pangaean supercontinent could explain a decline in terrestrial diversity during the late triassic although evidence for an impact precisely at the boundary is lacking the presence of impact structures with late triassic ages suggests the possibility of bolide impact induced environmental degradation prior to the end triassic widespread eruptions of flood basalts of the central atlantic magmatic province camp were synchronous with or slightly postdate the system boundary emissions of co2 and so2 during these eruptions were substantial but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved a substantial excursion in the marine carbon isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion although the triggering mechanism and climatic effects of such a release remain uncertain d 2003 elsevier b v all rights reserved

stratigraphic record of the early mesozoic breakup of pangea in the laurasia gondwana rift system rift basins of the central atlantic margins cam of north america and morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early mesozoic the best known is the newark basin of new jersey new york and pennsylvania where an astronomically calibrated magnetic polarity time scale is developed lacustrine cycles of milankovitch origin are commonly present in cam basins with the period changing from 10 ky paleoequator with coals to 20 ky 4 10n to perhaps 40 ky northward with evaporites cycles of100 ky 413 ky and2 my are also important four mostly unconformity bounded tectonostratigraphic sequences are present the anisian ts i is fluvial and eolian ts ii ts iv late triassic to early jurassic consist of tripartite lacustrine sequences caused by extension pulses the newark basin accumulation rate history allows comparison with quantitative rift basin models the north american plate s slow northward drift resulted in a relative shift of climate although the rapid humidification during the latest triassic and early jurassic is associated with a sea level rise the triassic jurassic mass extinction is of independent origin plausibly impact related

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

the triassic timescale based on nonmarine tetrapod biostratigraphy and biochronology abstract the triassic timescale based on nonmarine tetrapod biostratigraphy and biochronology divides triassic time into eight land vertebrate faunachrons lvfs with boundaries defined by the first appearance datums fads of tetrapod genera or in two cases the fads of a tetrapod species definition and characterization of these lvfs is updated here as follows the beginning of the lootsbergian lvf fad of lystrosaurus the beginning of the nonesian fad cynognathus the beginning of the perovkan lvf fad eocyclotosaurus the beginning of the berdyankian lvf fad mastodonsaurus giganteus the beginning of the otischalkian lvf fad parasuchus the beginning of the adamanian lvf fad rutiodon the beginning of the revueltian lvf fad typothorax coccinarum and the beginning of the apachean lvf fad redondasaurus the end of the apachean beginning of the wasonian lvf near the beginning of the jurassic is the fad of the crocodylomorph protosuchus the early triassic tetrapod lvfs lootsbergian and nonesian have characteristic tetrapod assemblages in the karoo basin of south africa the lystrosaurus assemblage zone and the lower two thirds of the cynognathus assemblage zone respectively the middle triassic lvfs perovkan and berdyankian have characteristic assemblages from the russian ural foreland basin the tetrapod assemblages of the donguz and the bukobay svitas respectively the late triassic lvfs otischalkian adamanian revueltian and apachean have characteristic assemblages in the chinle basin of the western usa the tetrapod assemblages of the colorado city formation of texas blue mesa member of the petrified forest formation in arizona and bull canyon and redonda formations in new mexico since the triassic lvfs were introduced several subdivisions have been proposed lootsbergian can be divided into three sub lvfs nonesian into two adamanian into two and revueltian into three however successful inter regional correlation of most of these sub lvfs remains to be demonstrated occasional records of nonmarine triassic tetrapods in marine strata palynostratigraphy conchostracan biostratigraphy magnetostratigraphy and radioisotopic ages provide some basis for correlation of the lvfs to the standard global chronostratigraphic scale these data indicate that lootsbergian uppermost changshingian induan and possibly earliest olenekian nonesian much of the olenekian perovkan most of the anisian berdyankian latest anisian and ladinian otischalkian early to late carnian adamanian most of the late carnian revueltian early middle norian and apachean late norian rhaetian the triassic timescale based on tetrapod biostratigraphy and biochronology remains a robust tool for the correlation of nonmarine triassic tetrapod assemblages independent of the marine timescale

significance of forelimb folds in the shumar allochthon lesser himalaya eastern bhutan abstract the shumar allochthon is the most widespread of all the thrust sheets constituting the lesser himalaya of eastern bhutan the pre to syn thrusting folds of the allochthon have been affected by an unusual type of superposed folding called here forelimb folding in this type of superposed folding the alternate limbs of pre existing folds are buckled and thickened while the other set of limbs is stretched and thinned the hinges of these pre to syn thrusting folds are also bent producing plane non cylindrical folds analysis of these fold structures within the allochthon indicate simple shear strain generated in the course of forward movement of the thrust sheet within the shear zones some of the larger folds were oriented such that one set of limbs the forelimbs were inclined towards the shear direction and the other set the backlimbs were inclined in the opposite direction in the course of progressive shearing the forelimbs were shortened and buckled producing the forelimb folds and the backlimbs were stretched and thinned the forelimb folds also indicate the shear sense in the course of progressive shearing the forelimb folds have been boudinaged and rotated producing rootless folds with axes parallel to the thrust direction

footwall geometry and the rheology of thrust sheets abstract the inter relationships between the exact footwall geometry and the rheology of thrust sheets are investigated deviations in the thrust fault surface from an ideal plane will induce a local heterogeneous deformation the resulting deformation processes depend upon the rate of thrust sheet displacement the geometry of the feature causing heterogeneous flow the deformation conditions and the lithologies involved two classes of features are particularly important in causing heterogeneous deformation in thrust sheets the first features are small perturbations on bedding planes which may be inherited sedimentary structures or produced during layer parallel shortening the second class of features are ramps where the thrust sheet climbs up the stratigraphic section displacement over these features causes repeated cyclic straining in the hanging wall during movement the strain rates associated with deformation at perturbations ramps of different geometries and different displacement rates are estimated and used to discuss the influence of footwall geometry on the structural evolution of a thrust sheet particular attention is given to the range of fault rocks and deformation microstructures preserved after movement over a footwall with a complex geometry perturbations are suggested to be important in the localization of ramps either because they create sticking points near the fault tip during propagation or because they induce eventual failure in the hanging wall after the movement over a number of these features raises the accumulated damage to a critical level analysis of the influence of the exact geometry of ramps on deformation processes during displacement leads to two important conclusions firstly the exact geometry of ramps i e the maximum dip angle and the straining distance from a flat to this maximum angle may be used to estimate a maximum displacement rate of the thrust sheet secondly the listric geometry of ramps may be an equilibrium shape adjusted to the displacement rate and the rheology of the hanging wall adjustments towards the final geometry may involve the generation of shortcuts on either hanging or footwall which reduce the imposed deformation rate in the hanging wall during displacement

late triassic paleogeography of the southern cordillera the problem of a source for voluminous volcanic detritus in the chinle formation of the colorado plateau region the upper triassic chinle formation of the colorado plateau contains voluminous volcanic detritus evidently derived from a source to the south volcanic rocks exposed in southern arizona and northern sonora have been assumed to represent this source terrane but u pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the chinle and thus not a source for the volcanic detritus in the chinle igneous rocks of known or possible late triassic age in nevada california or northeastern mexico are possible sources but a clearly defined source terrane for the volcanic detritus in the chinle has not been identified tectonic removal of the source terrane by rifting or strike slip offset though not proven is a possibility

the late triassic timescale age and correlation of the carnian norian boundary the late triassic timescale is poorly constrained due largely to the dearth of reliable radio isotopic ages that can be related precisely to biostratigraphy combined with evident contradictions between bio stratigraphic and magnetostratigraphic correlations these problems are most apparent with regard to the age and correlation of the carnian norian boundary base of the norian stage we review the available age data pertaining to the carnian norian boundary and conclude that the long norian in current use by many workers which places the carnian norian boundary at 228 ma is incorrect the evidence supports a norian stage that is much shorter than proposed by these workers so the carnian norian boundary is considerably younger than this close to 220 ma in age critical to this conclusion is the correlation of the carnian norian boundary in nonmarine strata of europe and north america and its integration with existing radioisotopic ages and magnet ostratigraphy three bio stratigraphic datasets palynomorphs conchostracans and tetra pods reliably identify the same position for the carnian norian boundary within normal limits of bio stratigraphic resolution in nonmarine strata of the chinle group american southwest newark supergroup eastern usa canada and the german keuper these biostratigraphic datasets place the carnian norian boundary at the base of the warford member of the lower passaic formation in the newark basin and as was widely accepted prior to 2002 this correlates the base of the norian to a horizon within newark magnet ozone e13n in recent years a correlation based solely on magnetostratigraphy has been proposed between the pizzo mondello section in sicily and the newark section this correlation which ignores robust biostrati graphic data places the norian base much too low in the newark basin section at the base of the lockatong formation correlative to a horizon near the base of newark magnet ozone e8 despite the fact that this correlation is falsifiable on the basis of the bio stratigraphic data it still became the primary justification for placing the carnian norian boundary at 228 ma based on newark cyclo stratigraphy the long norian created thereby is unsupported by either bio stratigraphic or reliable radioisotopic data and therefore must be abandoned while few data can be presented to support a carnian norian boundary as old as 228 ma existing radio isotopic age data are consistent with a norian base at 220 ma although this date is approximately correct more reliable and precise radio isotopic ages still are needed to firmly assign a precise age to the carnian norian boundary

dissected paleotopography and base level changes in a triassic fluvial sequence at one stratigraphic level in petrified forest national park in arizona fluvial mudrocks of the chinle formation are truncated by areally extensive deep scours that are filled dominantly by pedogenically modified mudrocks color differences between paleosols developed on scoured and scour fill deposits facilitate recognition of the dissected interval the mudstone filled scour system records a period of channel incision and widespread gullying caused by lowered base level although it is difficult to document subsidence rates in the chinle basin the tectonic setting suggests that the enormous scour system could be the result of fluctuations in regional thermal activity during late triassic time similar scour and fill should occur in other fluvial sequences however because they both truncate and are filled by mudrocks identification can be difficult recognition of them is important because they are indicators of base level fluctuations and thus are useful in unraveling the depositional history of a basin

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

cyanobacterial tufa calcification in two freshwater streams ambient environment chemical thresholds and biological processes abstract calcareous freshwater streams near bad urach southwest germany were studied to determine the environmental limits to cyanobacterial calcification daily variations in temperature ph calcium concentration and alkalinity were measured at seasonal intervals from september 1993 to january 1995 in two small woodland streams with lengths of 1 0 and 1 5 km the principal cause of supersaturation in these fast flowing streams is inorganic carbon dioxide outgassing from resurging groundwater locally enhanced by turbulence at waterfalls and cascades photosynthetic uptake of carbon dioxide and temperature effects are negligible nonetheless organic substrates particularly cyanobacteria dominated microbial mats and biofilm significantly localize precipitation by providing suitable nucleation sites precipitation rates on artificial substrates up to 2 2 mm yr on limestone correlate with high saturation index values copper substrates inhibited microbial colonization and received negligible encrustation tufa formation and external calcium carbonate encrustation of cyanobacteria are conspicuous where the annual wateq4f saturation index exceeds 0 8 and are slightly below 0 7 calcium carbonate impregnation of cyanobacterial sheaths has not been observed we infer that in these fast flowing streams cyanobacteria utilize co2 in photosynthesis whereas elsewhere in sluggish freshwater cyanobacteria utilize hco3 leading to sheath impregnation by calcium carbonate even where saturation index is only 0 2 0 3 thus photosynthetic influence on cyanobacterial calcification appears to be negligible in fast flowing co2 rich streams and cyanobacterial calcification takes the form of external encrustation in slow flowing co2 poor streams and lakes cyanobacteria preferentially utilize bicarbonate and sheath impregnation can result modern tropical marine carbonate shelves have saturation indices commonly in the range 0 62 0 82 and cyanobacterial calcification is negligible extrapolation suggests that cyanobacterial calcification could occur in present day seas if saturation index exceeded 0 8 this level corresponding with heavy calcification in the freshwater streams studied may have been widespread in marine environments in the geological past at times when calcified cyanobacteria and environmentally influenced non skeletal calcium carbonate precipitates were abundant

patterns of folding within nappes and thrust sheets examples from the variscan of southwest england folding is a common response to deformation within nappes and thrust sheets and a wide variety of attitudes of folds can result two examples of patterns of variably oriented folds from southwest england are analysed in terms of thrust tectonics in northern cornwall the millook nappe contains a downward progression from upright to recumbent folding with a zone of oblique or sheath folds at its base these changes are related to an increasing simple shear component towards the base of the nappe in southwestern cornwall d1 and d2 deformations are related to the thrust emplacement of the lizard complex f1 folds in rocks below and to the north of the lizard complex have variable axial orientations alternating zones of folds trending normal and oblique to the transport direction have been mapped the oblique folds are interpreted as initiating in zones of differential movement within thrust sheets later d2 shear zones cut up section ahead of the advancing lizard complex in which case they result in refolding of f1 folds beneath the lizard complex these d2 shear zones cut down section extending but rarely refolding the d1 structures

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals

alternative model of thrust fault propagation a widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory we propose an alternative model that applies gretener s beam failure mechanism to a multilayered sequence invoking compatibility conditions which demand that a thrust propagate both upsection and downsection we suggest that ramps form first at shallow levels and are subsequently connected by flat faults this hypothesis also explains the formation of many minor structures associated with thrusts such as backthrusts wedge structures pop ups and duplexes and provides a unified conceptual framework in which to evaluate field observations

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

lacustrine carbonates and pedogenesis sedimentology and origin of palustrine deposits from the early cretaceous rupelo formation w cameros basin n spain the berriasian rupelo formation of the w cameros basin consists of a 2 200 m thickness of marginal and open lacustrine carbonate and associated deposits open lacustrine facies contain a non marine biota with abundant charophytes both stems and gyrogonites ostracods gastropods and rare vertebrates carbonate production was mainly biogenic the associated marginal lacustrine palustrine facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history the succession was laid down in a low gradient shallow lake complex characterized by wide fluctuations of the shoreline carbon and oxygen stable isotope analyses from the carbonates show non marine values with ranges of 13 from 7 to 11 and 18 from 3 to 7 5 differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake analyses from the entire sample suite plot on a linear trend isotopic compositions become lighter with increasing evidence of pedogenic modification this suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil derived co2 the stable isotope data thus support evidence from petrography and facies relations that palustrine limestones form through pedogenic modification of lake carbonates

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

the middle carnian wet intermezzo of the stuttgart formation schilfsandstein germanic basin abstract the middle carnian wet intermezzo mcwi of the stuttgart formation schilfsandstein and age equivalent strata of the northwestern tethys occurred entirely within equivalents of the upper subzone of the austrotrachyceras austriacum ammonoid zone of the late julian its duration is estimated to be only about 0 7 0 8 myr in both the germanic basin and the northwestern tethys the warm climate during the mcwi was characterized by a rate of precipitation that exceeded somewhat the evaporation but was not so great as to be pluvial the mcwi was related to the atmospheric circulation of a megamonsoonal system that was characterized by strong moisture laden northwesterly flowing trade winds that rose as they reached the estimated 2000 3000 m high eastern shoulder uplift of a huge rift causing them to drop an extraordinary amount of rain this eastern shoulder uplift lay within the caledonides of modern day western scandinavia this region only between 30 and 50 n palaeolatitude had a truly pluvial climate and the huge amounts of fresh water dropped there transported large amounts of siliciclastics from this rift shoulder uplift southward into the germanic basin before deposition of the siliciclastics an early julian eustatic sea level fall caused widespread erosion in the germanic basin in the later part of late julian a transgression from the eastern gates with the concurrent strong fresh water influx from the north flooded the centre of the northern germanic basin with a shallow brackish sea in which the osterhagen horizon basisschichten of the lower stuttgart formation was deposited in the upper osterhagen horizon the salinity rapidly decreased from mesohaline through mio and oligohaline to fresh water levels in southern germany the basisschichten formed entirely within fresh water or very low salinity brackish environments later a slight subsidence of the southwestern germanic basin shifted the main outflow of fresh water toward the southwestern end of the basin creating local brackish conditions in the northern marginal part of the northwestern tethys e g in the lunz beds during this time interval tidal influence also can be found in the stuttgart formation deposited in palaeoestuaries of the southwestern germanic basin adjacent to the tethys the very strong fresh water influx from the north however prevented these estuaries from developing a strongly elevated salt content so that only fresh water to oligohaline brackish faunas are found there such as the eberstadt bivalve conchostracan fauna at the base of the tuvalian the megamonsoonal system either disintegrated or else the trade winds shifted their principal flow direction this caused the climate within the germanic basin and in the nearby northwestern tethys sea to become arid again as it had been before the deposition of the stuttgart formation these changes in the atmospheric circulation patterns also terminated the pluvial climatic regime to the north along the scandinavian eastern rift shoulder uplift this in turn ended the transport of huge amounts of siliciclastics from this region southward into the germanic basin and ended the deposition of the schilfsandstein after this the hypersaline sabkha and playa sedimentation of the weser formation began which was accompanied by some minor marine ingressions in the southwestern germanic basin

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

palynological correlation of carnian humid pulses throughout western tethys abstract the carnian pluvial event is an upper triassic swing toward increased rainfall best documented in the tethys tropical belt and mainly recognized on its effects on sedimentary systems as the demise of carbonate platforms and the enhanced siliciclastic supply in some portions of western tethys and particularly in the raibler schichten of the northern calcareous alps and in the dolomites the carnian pluvial event is recorded by four discrete siliciclastic pulses in the raibler schichten three siliciclastic intervals are documented and take the name of first second and third shale in order to correlate these pulses at the basinal scale the study of the palynological assemblages in a section of the raibler schichten near rubland austria and of the classical succession of the lunz nappe have been carried out more than eighty palynomorph species are recorded most of them correspond to typical carnian morphospecies of the carnian pluvial event in europe on the base of their stratigraphic distribution three different palynomorph assemblages are described aulisporites astigmosus assemblage lagenella martinii assemblage and granuloperculatipollis rudis assemblage a fourth older assemblage is also informally defined a palynostratigraphic correlation between the nca the germanic basin and the dolomites has been obtained the four siliciclastic pulses recognized in the raibler schichten and lunz are also found in the dolomites and yield the same palynological associations the base of the schilfsandstein that record the carnian pluvial event in the germanic basin is correlated to the second or third siliciclastic pulse first or second shale of the raibler schichten siliciclastic pulses are thus correlated throughout western tethys and can thus be interpreted as short lived humid climate wiggles within the carnian pluvial event a quantitative analysis of the carnian successions of the nca highlights a shift toward hygrophytic elements in the uppermost julian and an abrupt return to xerophytic associations in the lower tuvalian the palynological hygrophytic associations that characterize the aulisporites astigmosus and lagenella martinii assemblages are present in many other parts of europe and middle east suggesting that the carnian pluvial event determined a humid climate perturbation in a wide area of the northern hemisphere spanning from tropical to high latitudes

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

scaling properties of thrust fault traces in the himalayas and inferences on thrust fault growth abstract map traces of thrust faults in the himalaya show a conspicuous cuspate lobate pattern i e broad lobes joined by sharp angular cusps for each lobe we can draw a chord by joining the apices of two adjacent cusps and define amplitude a by perpendicular distance of apex of the lobe from its chord the lobes show a linear scaling defined by the relationship a cl where l is the chord length and c is a constant the l a ratio remains nearly constant over about 1 71 orders of magnitude spread of the l values of the sampled lobes suggesting that a is proportional to l the trace pattern indicates that the thrust surfaces are laterally curved due to the growth of fault segments from multiple nuclei along concave upward laterally curved trajectories the propagating surfaces coalesce along sharp angular ridges this lateral curvature gives a furrow like shape to each segment and a fluted appearance to the thrust surfaces the invariant nature of the l a ratio supports self similarity and is manifest as a linear scaling of a against l of the lobes the self similarity further suggests the possibility of thrust fault growth along a characteristic curved surface

differential transport and continuity of thrust sheets abstract structural geologists long have strived to establish basic tenets describing the interaction between thrust displacement and lateral extent of thrust sheets individual non metamorphic thrust sheets can be partitioned into segments that exhibit linear variations in displacement along strike the differential transport between endpoints of each segment define an along strike bulk shear angle which describes the spatial rate of change in displacement along the length of the fault empirical measurements of differential transport angles for continuous thrust sheets from different north american fold thrust belts yield bulk shear angles that do not exceed 35 40 without formation of tear faults these values might thus be considered a limit for coherent behavior of continuous non metamorphic thrust sheets thereby providing constraints on three dimensional interpretations of fold thrust belts

orogen transverse structures in the eastern himalaya dextral riedel shear along the main boundary thrust in the garu gensi area arunachal pradesh india implication in hydrocarbon geoscience abstract structural mapping and analyses reveal deformation in the rocks of the lesser himalayan miri and gondwana and lower siwalik dafla successions across the miri thrust mt and the main boundary thrust mbt in the garu gensi area arunachal himalaya india the north dipping gondwana sequence is sandwiched between the miri group of rocks in the north and the lower siwalik dafla sandstones in the south the gondwana rocks folded four times d1 d4 detail outcrop mapping and structural analyses substantiated by satellite images elucidate that the miri the gondwana and the lower siwalik rock sequences in the area are traversed by two sets of nw and ne trending cross fault zones with 70 100 m thickness mappable for 70 km the cross faults trend nw around the locality daring while in the western part around gensi they trend ne the westernmost fault slipped dextrally and the easternmost one at daring sinistrally these transverse faults offset the mbt 0 52 0 59 km for the western and 0 58 1 41 km for the eastern faults the deformation in the gondwana sequence may be constrained as syn himalayan i e india asia collisional event this was followed by a compression from e ese due to the post eocene indo burmese collision the culminating phase of combined india asia and indo burmese compression is manifested in field with a strong ne directed compression with dextral reidal shear developing in the transverse conjugate fault zones in the area with maximum displacement towards se cluster of the earthquake data indicates record of the events m 6 within 30 80 km depth both n and s of the mbt in the study area further the southerly continuation of the transverse faults below the alluvium can provide vital clue in studying the potential hydrocarbon trap in the northern fringe of the brahmaputra river

a late triassic lake system in east greenland facies depositional cycles and palaeoclimate abstract the upper triassic fleming fjord formation of the jameson land basin in east greenland contains a well exposed succession 200 300 m thick of lake deposits the malmros klint member 100 130 m thick is composed of cyclically bedded intraformational conglomerates red siltstones and fine grained sandstones and disrupted dolomitic sediments paleosols the cyclicity is composite with cycles having mean thicknesses of 25 5 9 and 1 6 m the overlying carlsberg fjord beds of the orsted dal member 80 115 m thick are composed of structureless red mudstones rhythmically broken by thin greyish siltstones this unit also has a composite cyclicity with cycles having mean thicknesses of 5 0 and 1 0 m the uppermost tait bjerg beds of the orsted dal member 50 65 m thick can be divided into two units a lower unit is composed of cyclically bedded intraformational conglomerates or thin sandstones red mudstones greenish mudstones and yellowish marlstones an upper unit is composed of relatively simple cycles of grey mudstones and yellowish marlstones recognized cycles have mean thicknesses of 5 6 and 1 6 m the lake deposits contain evidence of seasonal orbital and long term climatic change seasonal change is documented by numerous desiccation surfaces especially in the malmros klint member and carlsberg fjord beds orbital change is suggested by the composite cyclicity and long term climatic change is indicated by the systematic upwards change in sedimentary characteristics of the lake deposits the sedimentary features of the malmros klint member suggest lacustrine deposition in a dry climate that fluctuated between desert and steppe conditions the carlsberg fjord beds probably record lacustrine lake deposition in a rather constant dry steppe climate while the tait bjerg beds record lake sedimentation in a climate that fluctuated between dry steppe and warm moist temperate in the tait bjerg beds the upward change in cycle characteristics indicates a shift towards more humid conditions climatic deductions from sedimentary facies are in good agreement with climate maps of laurasia as simulated by numerical climate models palaeomagnetic data indicate a northward drift of east greenland of about 10 from ca 25 n to ca 35 n in the middle to late triassic the fleming fjord formation which represents ca 5 m y of the late triassic interval was deposited during latitudinal drift of 1 2 it is possible that the observed long term upward shift in climatic indicators within the formation can be ascribed to plate drift but southward shift of climatic belts could also have been of importance

strains and folds within thrust sheets an analysis of the heilam sheet northwest scotland abstract the changes of shape and orientation of fossil worm burrows have been used to carry out a detailed strain analysis of the heilam thrust sheet in the moine thrust zone northwest scotland the strains have been factorised into components of layer parallel shear 1 layer normal shear 2 and layer parallel shortening or elongation variations in the 1 shear are largely lithologically controlled some strain is due to flexural processes as the thrusts and overlying beds change dip over thrust ramps or are tightened by later deformation while there may also be a component of drag which increases towards the overlying thrust this predating the shear due to flexure a contoured map of the values suggests that major folds within the thrust sheet developed as a result of stick or drag on the floor thrust prior to ramp climb this led to differential movement in the transport direction causing layer parallel shortening accompanied or followed by buckling of the layers a map showing layer normal shear strains 2 suggests that the heilam sheet forms a major surge zone the central part having moved further wnw than the northern and southern parts this has led to components of lateral shear along parts of the sheet especially in the north producing curvature of the imbricate faults

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

stability of atmospheric co2 levels across the triassic jurassic boundary the triassic jurassic boundary 208 million years ago is associated with widespread extinctions in both the marine and terrestrial biota the cause of these extinctions has been widely attributed to the eruption of flood basalts of the central atlantic magmatic province this volcanic event is thought to have released significant amounts of co2 into the atmosphere which could have led to catastrophic greenhouse warming but the evidence for co2 induced extinction remains equivocal here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations spanning a 20 myr period across the triassic jurassic boundary using a standard diffusion model we interpret these isotopic data to represent a rise in atmospheric co2 concentrations of about 250 p p m across the boundary as compared with previous estimates of a 2 000 4 000 p p m increase the relative stability of atmospheric co2 across this boundary suggests that environmental degradation and extinctions during the early jurassic were not caused by volcanic outgassing of co2 other volcanic effects such as the release of atmospheric aerosols or tectonically driven sea level change may have been responsible for this event

three dimensional geometry of thrust surfaces and the origin of sinuous thrust traces in orogenic belts insights from scaled sandbox experiments abstract sinuous traces of emerging thrust tips comprising multiple salients and recesses are commonly observed in orogenic belts e g lesser himalayas of india nepal and bhutan and in accretionary prisms e g nankai trough off the coast of japan lateral along the strike of the deformation zone variation in the depths of foreland basins i e variable sediment thickness or in the strength of the basal detachment or presence of a curved indenter has been traditionally cited to explain the formation of salients in fold and thrust belts although they are not applicable in all cases in the present work we have carried out four series of scaled analog model experiments using dry quartz sand changing the dip of the basal decollement 0 or 5 and the basal friction b 0 5 or 0 3 to investigate the 3d shape of thrust surfaces under varying overall boundary conditions but without any lateral variation of these parameters within the models the experimental results show that under all boundary conditions thrust surfaces are curved both in their dip and strike directions i e spoon shaped in 3d multiple concave upward and convex upward segments constitute a thrust surface which produces a sinuous trace when the tip line intersects the earth s surface it is also shown that thrust surface curvatures occur at different scales and the overall thrust surface roughness corrugations has a self affine fractal geometry

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

true polar wander an analysis of cenozoic and mesozoic paleomagnetic poles a true polar wander path is presented for the past 180 ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data a relative displacement of 22 10 is observed the motion between the two reference frames has not been smooth episodes of rapid motion on the order of 80 100 mm yr include the present 65 50 ma 115 85 ma and 180 160 ma periods of slow or little polar motion less than 20 mm yr are observed at 50 5 ma and 160 115 ma the effect of plate motions is removed using morgan s plate motion model a19 this model is tied to a hotspot reference frame assumed to be fixed in the mantle the rotated paleopoles are grouped by ages and then averaged they are found to cluster about points distinct from the north pole as defined by the hotspots these points trace out a path of motion between the rotation axis and the hotspots or mantle this is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle paleomagnetic poles of the past 7 m y are examined independently this eliminates a dependence on the fixed hotspot hypothesis since plate motions are essentially negligible for this time span there is a deviation of 5 2 between the present geographic north pole and the average of the paleopoles at 5 ma this is too great to attribute to plate motion interpreted as true polar wander the rate of this polar motion is about 1 m y astronomical measurements for the past 80 years have recorded actual motion of the earth s rotational axis up to 1 m y in the direction of eastern canada this agrees in rate with paleomagnetic polar motion of the past 7 m y and in both rate and direction for the past 2 m y

magnetic polarity stratigraphy and paleolatitude of the triassic jurassic blomidon formation in the fundy basin canada implications for early mesozoic tropical climate gradients a magnetic polarity stratigraphy is established for a continuously cored 360 m thick section of the entire cyclical lacustrine sequence and part of the sandy fluvial and eolian basal sequence of the late triassic blomidon formation in the fundy basin in nova scotia canada in conjunction with published results for the newark and dan river basins the fundy basin record allows us to precisely map the spatio temporal distribution of climate sensitive continental lithofacies along a paleolatitudinal transect in eastern north america corresponding to the tropics of pangea in the late triassic indicators of high humidity such as coals and black shales tend to occur within 5 of the paleoequator and eolian dunes and other evidence of aridity occurs within 10 of the paleoequator although steep the latitudinal climate gradient in the interior of the pangea supercontinent is not that different from the average modern zonal variation in the balance of evaporation to precipitation even though the distribution of continents was radically different and there is no evidence of continental ice sheets in the late triassic we suggest that the common perception of a dry paleoequator in the triassic is largely an artifact of observational bias because compilations of paleoclimate proxies usually have spatio temporal resolutions that are inadequate to delineate narrow zonal climate belts giving an aliased impression of ancient climate fl 2000 elsevier science b v all rights reserved

lacustrine and palustrine facies in the bembridge limestone late eocene hampshire basin of the isle of wight southern england abstract the carbonate lake system represented by the late eocene bembridge limestone formation isle of wight southern england comprises two main facies lacustrine and palustrine these facies are arranged vertically in transgressive regressive cycles corresponding to a model of carbonate lakes of low gradient and fluctuating margins with extensive palustrine fringes these lakes developed in a coastal plain of limited relief where two paleogeographical domains predominate a lacustrine domain is represented in northern and eastern sections in the isle of wight by cycles representing upward passage from central lacustrine sub facies marl marly limestone through marginal lacustrine sub facies biomicrite to sub facies indicative of exposure brecciated nodular limestone a palustrine domain predominant in west wight is represented by a predominantly pedogenic palustrine facies with clotted peloidal to ooidal textures and some intercalated laminated laminar calcrete horizons within the pedogenic facies textures have evolved from those of unaltered biomicrites through clotted peloidal to dense ooidal to open ooidal textures a third domain developed in close geographical proximity to more saline waters is represented by gypsiferous lake margin facies found in north wight in which microlenticular gypsum developed post depositionally as a result of evaporation following subaerial sediment exposure in the lacustrine domain there appears to be an association between cyclicity and changes in base sea level reflecting a likelihood that the lakes were part of a paralic coastal plain complex with distal marine connections alternatively the cyclicity might have developed as a result of the alternation of wetter and drier climatic periods

paleosols trace fossils and precipitation estimates of the uppermost triassic strata in northern new mexico abstract this study provides a detailed pedogenic evaluation of two upper triassic late norian through rhaetian stratigraphic intervals in new mexico in order to assess the climate and ecology of the latest triassic which ended in a mass extinction the two study areas are located in north central and east central new mexico and are separated by 200 km each section contains abundant paleosols of varying maturity with features that reflect an arid to semiarid climate there is little pedogenic variation throughout the strata at each location and a typical paleosol profile is about 1 m thick and has an ab bw bk bc horizon succession bkm bss bssk or bssg horizons are present in some paleosols micromorphological features suggest dominantly well drained sola e g abundant carbonate nodules illuviated clay with minor periods of moist or saturated conditions e g femn concretions femn coatings and hypocoatings sepic plasmic fabrics trace fossils are abundant in these strata and are dominated by taenidium serpentinum and root traces depth to carbonate functions estimate that mean annual precipitation was between 200 and 450 95 mm relative to location 1 eastern new mexico location 2 north central new mexico produced higher paleo precipitation estimates and has stronger and more abundant sepic plasmic fabrics in thin sections the presence of a gleyed paleosol camborygma eumnkenomos and slickensides at location 2 also suggests conditions wetter than at location 1 taxonomically all of the paleosols in this study appear to be entisols or aridisols and can be grouped into seven representative pedotypes of varying maturity by comparing these paleosols to modern soils this study demonstrates that the late triassic western interior during the late norian to rhaetian was arid to semiarid and supported a desert shrub environment that had localized and periodic moist or saturated soil conditions

hangingwall strain a function of duplex shape and footwall topography abstract the concept of piggy back thrust tectonics implies that foreland is progressively accreted onto a developing thrust sheet during duplex formation lateral shape changes in duplexes in the hangingwall of a thrust and corrugations in the footwall will fold higher thrust sheets to give culminations and depressions balancing of parts of high level thrust sheets with lower sheets and foreland requires a sequence of extensional and compressional strains orientated normal to the thrust transport direction culmination walls will be sites of strike parallel extension subsequent adjacent culminations will compress early culmination walls which will result in a sequence of irrotational strains examples of this geometry are given from the moine thrust zone of northwest scotland the model allows a re examination of strains and hangingwall evolution in some thrust sheets in the helvetic and external zones of the alps

fresh water carbonates from the lower freshwater molasse oligocene western switzerland sedimentology and stable isotopes abstract the lower freshwater molasse of western switzerland comprises a thick series of continental deposits of late oligocene early miocene age fresh water carbonates occur at two main horizons within this succession lower chattian palustrine carbonates calcaires inferieurs show a shallow fresh water fauna flora of ostracods and charophytes but display clear evidence of subaerial exposure in the form of glaebule pisoid formation circumgranular cracking rhizoliths and microkarstic cavities repeated desiccation led to locally intense modification by pedogenesis these horizons record the development of shallow ephemeral lakes at the northern margin of the swiss molasse basin stable isotope analyses were carried out on 46 calcite samples data from matrix and intraclasts reveal typical fresh water compositions 13c 5 to 7 5 18o 5 5 to 9 a weak co variant trend suggests deposition in hydrologically closed lakes although the range of 13c values recorded is also consistent with pedogenetic overprinting associated with subaerial exposure spar cements and vadose crystal silts display 13c compositions from 6 to 7 but show light 18o values from 10 to 11 recording formation from meteoric waters upper chattian carbonates calcaires d eau douce et dolomie include algal ostracodal laminites interpreted as profundal lake deposits as well as bioturbated shallow lake charophyte ostracod mudstones and wackestones sandy wackestones and white dolomite stable isotope analyses 20 calcites 2 dolomites show 13c values from 2 to 6 and 18o from 6 to 11 data for laminites and biomicrites plot in different fields variations in carbon isotopic composition may be attributable to higher organic productivity during high lake stands the laminites show heavier 13c values or to areal differences in the proportions of biogenic authigenic and detrital carbonate 13c values are generally higher in areas of greater detrital carbonate supply the heavier 18o values shown by the dolomites 13 c 3 18 o 0 to 1 suggest formation from evolved evaporitic lake waters these carbonates record deposition in an extensive possibly deeper lake complex subject to clastic input from the alps in the south

the blake geomagnetic event transition geometry dynamical characteristics and geomagnetic significance abstract two records of the blake geomagnetic event have been obtained from marine cores in the mediterranean the upper bound for duration of the event calculated from accurate oxygen isotope stratigraphy and or tephrochronology is 4000 years one record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved the directional changes do not occur at constant speed rather a stop and go behaviour is observed as in detailed records of reversals the virtual geomagnetic pole vgp path confined to two bands of longitude over the americas and antipodal to them is virtually identical to the only other path of this event obtained from lacustrine sediments in the western usa suggesting that a dipolar component may be present in the transitional field during the blake event the two bands of longitude coincide with those of many recent reversals suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time

carbonate deposition in an ephemeral siliciclastic alluvial system jurassic shuttle meadow formation newark supergroup hartford basin usa abstract nonmarine carbonate accumulation from a sedimentologic perspective has multiple possible origins clastic load springs groundwater discharge soil development biochemical seasonal changes cave speleothems and eolian influx many of these carbonates contain diagnostic textures from which origin is easy to ascertain however differentiating between clastic and spring groundwater carbonates especially in depositional systems dominated by siliciclastics is not always simple here criteria for recognition are developed subsurface hydrology and tectonic origin of the depositional basin source rocks surrounding the basin sedimentologic packaging and extent of the carbonates in question as well as their sedimentary structures and fossil content and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution the present study focuses on a nonmarine carbonate deposit found within the jurassic shuttle meadow formation of the hartford basin one of many rift basins which formed as a result of the opening of the north atlantic ocean during the late triassic to early jurassic the upper 50 m of the formation at plainville connecticut is interpreted as continental in origin and is composed of five facies red ripple cross laminated muddy siltstones to sandstones fr sr massive red siliciclastic mudstones fm buff fine to medium grained horizontally laminated sandstones sh red to white disrupted calcareous shales fl3 and rare micritic carbonates pm facies fr sr sh and fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism similar to the channel country drainage system of central australia in addition preservation of lateral accretion sandstone sheets accretionary benches indicate that the shuttle meadow alluvial system was rapidly accreting the anomalous existence of carbonate beds in facies fl3 and pm in the smf bedload dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin no direct evidence for a spring groundwater input is apparent the shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment the intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration desiccation cracks and burrowing

pedogenic carbonate isotopes as evidence for extreme climatic events preceding the triassic jurassic boundary implications for the biotic crisis the triassic jurassic boundary is associated with widespread marine and terrestrial extinctions but there is disagreement regarding the existence and extent of climatic changes that may have driven the biotic crisis here we apply quantitative isotopic climate proxies in order to construct two age equivalent relatively continuous temperature and p co 2 records that span the eight million years preceding the triassic jurassic boundary and that supersede previous terrestrial records in temporal resolution the 18 o data suggest that mean annual temperatures mat increased by 7 9 c from the late norian to the rhaetian in association with the peak increases in p co 2 levels the 13 c data suggest relatively low late norian p co 2 levels 1500 ppmv and at least two periods of extreme p co 2 levels 3000 ppmv preceding the triassic jurassic boundary these estimates are consistent with a recent late triassic climate model that suggests the effects of increased p co 2 levels on pangea would cause severe climatic consequences including a global mat increase of 6 c 10 c in some regions while it is possible that periods of increased aridity could have resulted in erroneously high estimates of both temperature and p co 2 levels it is likely that climate was still fluctuating during the end of the triassic although our data precede the triassic jurassic boundary many studies conclude that the mass extinction took place over a more prolonged period beginning in the late triassic thus climate may have been a significant driving mechanism of the late triassic extinctions

syngenetic formation of grainstones and pisolites from fenestral carbonates in peritidal settings abstract experiments with unconsolidated muds and petrofabric studies of some ancient peritidal carbonate rocks suggest that intense fenestral alteration of micritic sediments can lead to the in situ formation of grainstones of diagenetic origin intermediate stages in this process have been recognized in many ancient rocks the contemporaneous diagenetic modification of such grainstones commonly results in the formation of pisoliths and associated features that mimic those found in marine pisolites as well as in pisolitic caliche deposits of either marine or meteoric origin

calcrete conglomerate case hardened conglomerate and cornstone a comparative account of pedogenic and non pedogenic carbonates from the continental siwalik group punjab india the occurrence of authigenic carbonates formed in three different environmental situations within the continental siwalik group has been used to compare the lithological and petrographic characters of the contrasted lithofacies the three lithofacies are 1 calcrete conglomerate 2 case hardened conglomerate 3 cornstone pedogenic nodular calcrete the calcrete conglomerate facies laterally intertongues with the channel conglomerates it consists of pisolites which are interpreted to have formed from carbonate rich spring waters emerging on to the gravelly substrate of dry abandoned channels the laminae characteristics of these pisolites are distinctly different from those of marine origin and also from comparable biogenic materials case hardened conglomerate occurs in the youngest part of the siwalik stratigraphic column in boulder conglomerates having limestone as the principal clast component this lithofacies has resulted from cementation of the conglomerate through continued dissolution and re precipitation of calcite by meteoric water downwards from the surface it displays a coarsely crystalline sparry calcite cement with no evidence for displacive growth or replacement by calcite cornstones nodular calcrete occur in several sedimentary cycles of the middle siwalik sub group these are immature and commonly associated with thinly bedded sandstones levee and red shales overbank this lithofacies is a result of concentration of carbonate through capillary action associated with pedogenic activity ooids developed in cornstone are essentially micritic in nature and usually composed of less than five indistinct laminae

geometry and kinematics of the darjeeling sikkim himalaya india implications for the evolution of the himalayan fold thrust belt abstract the darjeeling sikkim himalaya lies in a zone where the convergence vector between the indian and eurasian plates is almost perpendicular to the himalayan deformation front current outcrop pattern preserves high grade hanging wall rocks of the main central thrust mct and the pelling thrust pt within 5 km of the mountain front u pb shrimp analysis on zircon grains from mct sheet record two 206pb 238u age groups of ca 2575 and ca 943 ma and from the pt 1835 13 ma the lesser himalayan duplex in the footwall of the mct is composed of two duplex systems a structurally higher hinterland dipping duplex and a lower duplex system that varies from hinterland dipping in the north to an antiformal stack in the middle and foreland dipping duplex in the south the foreland dipping horses and reactivation of the roof thrust resulted in large translation of the overlying mct and pelling thrust sheets the duplex also resulted in a plunge culmination forming a broad n s trending anticline a transport parallel restorable balanced cross section reveals a minimum shortening of 450 km 81 south of the south tibetan detachment system and an average long term shortening rate 20 mm yr although the study area lies in a zone of high convergence angle with the deformation front the surface structures do not record the greatest shortening of the orogen the shillong plateau and a tranverse zone lying southeast of the study area have accommodated a part of the convergence related shortening thereby under representing the shortening in the sikkim himalayan fold thrust belt lateral variations in the original width of the lesser himalayan basin may have played a role in partitioning the shortening in the fold thrust belt

tectonics of a part of the shumar allochthon in eastern bhutan abstract the lesser himalaya of eastern bhutan is made up of a pile of thin thrust sheets stacked on one another and the shumar allochthon lies at the top of this pile the shumar allochthon is a composite tectonic unit divisible into two structurally discordant sub units the upper sub unit system b is made up of a layered sequence of quartzite phyllite and marble that repeats several times through the sequence several subsidiary thrusts in the lower sub unit system a form a imbricate system arranged as a hinterland dipping duplex this imbricate system is spatially related to a transverse footwall ramp where the shumar thrust has cut up the underlying buxa stratigraphic sequence at the crest of this ramp the shumar thrust and the roof thrust of the duplex join forming a splay a detailed map of a part of the shumar allochthon reveals that alternate blocks parcels or horses of metasediments and sheared granitic gneisses constitute the stack of imbricates in this duplex analysis of the imbricate system indicates accretion of footwall elements in the course of forward propagation of the shumar allochthon and thus provides an idea about the nature of the basement on which the allochthon moved these accreted footwall elements suggest that the thrust sheets of the lesser himalaya of eastern bhutan moved and now rest on a basement of precambrian gneisses the top part of this gneissic basement was mobilized and involved in the thrusting process as is shown by the horses of the basement repeated at several levels within the shumar allochthon isolated patches of younger sediments on the precambrian gneisses were also involved in the thrusting

recent freshwater ooids and oncoids from western lake geneva switzerland indications of a common organically mediated origin abstract the shallow water sediments of western lake geneva are composed mainly of sand size coated grains ranging from oncoids to ooids in surficial sediments the coatings consist of anhedral microcrystals of low mg calcite and often show a well developed intercrystalline porous network containing bacteria and residues of degraded mucus nucleation occurs in close association with organic films formed by mucus produced by filamentous cyanobacteria and diatoms similarities between oncoid and ooid cortex crystal size shape and arrangement indicate that the processes controlling the building of these types of particle are identical in subsurface sediments the microstructure of the coatings changes considerably the intercrystalline porosity is reduced by aggrading neomorphism and the micritic porous and organic rich coatings are transformed into a dense xenotopic mosaic with scattered molds of filamentous cyanobacteria this occurrence of ooidal oncoidal sands in lake geneva may help in interpreting ancient lacustrine formations and brings additional information on the processes of genesis and diagenesis of coated grains

record of the carnian wet episode in strata of the chinle group western usa the late triassic carnian wet episode was an interval of humid climate evident in the lowermost strata of the upper triassic chinle group in the western usa chinle deposition began with the development of major river systems of the shinarump formation and equivalents laterally equivalent to and or overlain by floodplain deposits containing kaolinitic gleyed to spodic palaeosols and local coal beds the chinle strata immediately above these wet episode strata are deposits of smaller rivers that lack coal or carbonaceous strata and contain non kaolinitic palaeosols that are locally calcareous and vertic indicative of drier but strongly seasonal climates the lowermost chinle strata contain fossil tetrapods palynomorphs and conchostracans that are consistent with other data that indicate that the basal chinle strata are of julian age and immediately overlying lower chinle strata are early tuvalian so these strata are the age of the carnian wet episode identification of a long norian stage places the norian base in the chinle group near the top of the strata of the wet episode which means that norian strata sit directly on early carnian strata but there is no unconformity at this level to support recognition of such a long norian in the chinle section

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

pennsylvanian to jurassic eolian transportation systems in the western united states peterson f 1988 pennsylvanian to jurassic eolian transportation systems in the western united states in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 207 260 the direction of sediment transport in eolian sandstones of permsylvanian to jurassic age was interpreted from crossbedding resultants vector means obtained from studies of eolian rocks in the western u s supplemented by data from the few eolian units of eastern north america these were compiled from the published or unpublished theses hterature from unpublished fidd data contributed by colleagues or from measurements made for this study in addition new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies regionally the crossbedding indicates northeasterly northerly or northwesterly winds present coordinates from pennsylvanian through most of middle jurassic time a rather abrupt change in wind directions occurred in late middle jurassic time late part of the cauovian age when westerly wind patterns developed by the late jurassic the winds shifted to southwesterly calculations of the consistency factor vector mean strength made from region wide analyses of the resultants indicate fairly unidirectional winds from the permsylvanian through the early jurassic middle jurassic circulation was more varied judging from crossbedding studies in the lower part of the entrada sandstone crossbedding in upper jurassic eolian rocks of wyoming and south dakota yielded a random pattern but upper jurassic rocks farther south on the colorado plateau and adjoining areas show a return to a fairly unidirectional pattern comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns however the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns although they do not cause noticeable horizontal deflections in wind patterns small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field

models of miocene marginal lacustrine sedimentation in response to varied depositional regimes and source areas in the madrid basin central spain abstract a succession of lake complexes formed through miocene times in the madrid basin central spain the complex formed during the aragonian stage provides an outstanding opportunity to evaluate the role of source areas and depositional regimes in the construction of coeval marginal lacustrine deposits sedimentary facies of the aragonian lake indicate a shallow lake that was fringed to the nw and s by arkosic alluvium to the n by litharenitic alluvial fans and to the ne and e by alluvial fans distally dominated by large floodplains marginal lacustrine deposits in the arkosic realm mostly consist of mg smectites and dolomite sequences that laterally interfinger with sepiolite beds which were deposited in small discontinuous ponds in the distal alluvial fan areas occasionally micaceous rich sandy streams entered the lake forming sheets at their entry points in contrast the northern margin of the lake was formed by muddy flats which were influenced by fluctuation in the water table in this setting illitic mudstones are commonly interbedded with biotite rich flood sands showing a sheet like geometry meandering channels occasionally cut into the mud flats facies assemblages on this side of the lake were mainly derived from low grade metamorphic areas to the northeast the marginal lacustrine facies consist of silty clays carbonate paleosols and paludal to lacustrine limestones these facies reflect an interfingering between the lake and a wide stream floodplain system derived from a carbonate source area the mosaic of marginal lacustrine facies in the madrid basin during the aragonian is discussed in the light of lithological and geochemical factors related to variations in source areas

carbonate facies of the upper triassic ojo huelos member san pedro arroyo formation chinle group southern new mexico paleoclimatic implications abstract the upper triassic adamanian lvf ojo huelos member of the san pedro arroyo formation chinle group is a distinctive carbonate rich unit that occurs in the lower chinle section of central new mexico the member consists mainly of micritic lime mudstones ostracodal wackestones to grainstones peloidal grainstones and distinctive pisolitic rudstones interbedded with fine grained siliciclastic mudstones most limestones exhibit some evidence of pedogenic brecciation and root penetration and porous fabrics similar to those of modern limestone tufas occur locally the interbedded mudstones are typically lenticular and commonly display a blocky ped fabric in which subequant peds are separated by sparry calcite veins fossils from the ojo huelos member are freshwater darwinulid ostracodes various freshwater fishes and aquatic amphibious tetrapods metoposaurs and phytosaurs we interpret the carbonate facies as the deposits of carbonate lakes ponds and wetlands that were partly spring fed whereas the interbedded and surrounding mudstones were alluvial in origin the groundwater and overland hydrology of the region was likely controlled by the relative proximity to an upland recharge area in the mogollon highlands to the south but sedimentary fabrics record strong overprinting by desiccation and pedogenic reworking consequently we interpret the ojo huelos member as recording a climate that varied from subhumid to semi arid which caused episodic falls in the hydrologic base level this resulted in landscape degradation exemplified by significant pedogenic and erosional reworking of the carbonate sediments and fluvial incision

pangea in permian to jurassic time the apparent polar wander paths apwps for the northern and southern continents have been redetermined using revised time scales and data when plotted on the conventional pangea pangea a the paths are distinct but essentially linear and parallel from late carboniferous to late permian time both paths then undergo a sharp change in direction and converge so as to overlap in early to middle jurassic time if the permo triassic magnetic field was an axial dipole then paleomagnetic data show that pangea was mobile during most and possibly all of the mid triassic to mid jurassic interval because of the uncertainties in pole ages and positions and errors in measurements the new apwps are consistent with a range of pangeas in particular the data are compatible with two paleomagnetic pangeas a2 and a new class of pangea here named the d pangeas these resemble morel and irving s 1981 pangea b and the c pangeas of smith et al 1981 unlike the a2 pangeas the b c and d pangeas all require large 2000 km relative motion between the northern and southern continents geological data require the transition from an earlier pangea to pangea a to take place along a transform zone here named the pangea transform zone or ptz the ptz is considered to be technically analogous to the great glen fault of scotland four types of possible ptz are examined passing through either the western or eastern end of the brunswick magnetic anomaly of eastern north america or north and south of iberia the geological data suggest that those pangeas that require large displacements on ptzs that pass east of the brunswick magnetic anomaly are improbable b and c pangeas are snapshots giving precise paleomagnetic reconstructions d pangeas fit long stretches of the apwps as well as possible a2 pangeas fit the apwps moderately well but also require the geological displacement to be smaller the differences in displacement between the d and a2 pangeas reflect geometrical relationships between euler poles ptz curvature and the apwps the general geological evidence suggests that d pangeas are less probable than a2 pangeas but that they cannot be rejected transition from a2 to a is most likely to have occurred via a sequence of rotations about different poles rather than a single rotation in detail the conventional pangea requires modification africa must be broken into at least two fragments along the benue trough and its northern continuation northwest africa then fits more tightly into northern south america which modifies the apwps and reduces the gap between them by including the effects of continental extension and using detailed geological data the gap can be reduced to one in which the a63 circles touch or overlap non dipole fields or an expanding earth need not be invoked to explain the mismatch of the apwps on the conventional pangea the finite rotation required for the older pangea to pangea a is very close to van der voo and french s 1974 single rotation for a2 to a but is actually the sum of three smaller rotations that reflect the modification to pangea a continental extension and the postulated ptz displacement of the possible ptzs a ptz parallel to the west african continental magnetic anomaly with a displacement of about 170 km gives a better fit of this anomaly to the east coast magnetic anomaly of north america further refinements to pangea a2 will depend on a better knowledge of stretching factors on the passive continental margins of the modified pangea a and incorporation of rotations due to major faults such as the south atlas fault in morocco

triassic environments climates and reptile evolution abstract a consideration of all the available data on triassic vertebrate faunas and their stratigraphic location reveals a relatively sudden extinction event among the last of the mammal like reptiles and the herbivorous rhynchosaurs in the norian of the upper triassic this event was apparently quickly followed by the radiation of the dinosaurs also in the norian this conclusion suggests that competition was not the main factor in the initial success of the dinosaurs but opportunistic radiation following the extinction of major reptile groups a global review of triassic sedimentary facies shows that there were climatic and floral changes towards the end of the triassic it is envisaged that increasing aridity in the later triassic resulting from plate motions and particularly affecting gondwanaland and southwestern laurasia brought about floral changes and then the reptile extinctions with the rapid evolution of new floras of conifers and bennettitaleans the dinosaurs came to dominate all terrestrial faunas within the space of only a few million years

jurassic hot spring deposits of the deseado massif patagonia argentina characteristics and controls on regional distribution abstract the deseado massif santa cruz province argentinean patagonia hosts numerous middle to late jurassic age geothermal and epithermal features represented by siliceous and calcareous chemical precipitates from hot springs sinters and travertines respectively hydrothermal breccias quartz veins and widespread hydrothermal silicification they indicate pauses in explosive volcanic activity marking the final stages in the evolution of an extensive jurassic ca 178 151 ma volcanic complex set in a diffuse extensional back arc setting heralding the opening of the atlantic ocean published paleo hot spring sites for the deseado massif plus additional sites identified during our recent field studies reveal a total of 23 locations five of which were studied in detail to determine their geologic and facies associations they show structural lithologic textural and biotic similarities with miocene to recent hot spring systems from the taupo and coromandel volcanic zones new zealand as well as with modern examples from yellowstone national park u s a these comparisons aid in the definition of facies assemblages for deseado massif deposits proximal middle apron and distal siliceous sinter and travertine terraces and mounds with preservation of many types of stromatolitic fabrics that likely were controlled by formation temperature ph hydrodynamics and fluid compositions locally the mapped hot spring deposits largely occur in association with reworked volcaniclastic lacustrine and or fluvial sediments silicic to intermediate lava domes and hydrothermal mineralization all of which are related to local and regional structural lineaments moreover the numerous geothermal and significant epithermal those with published minable resources deposits of the deseado massif geological province mostly occur in four regional nnw and wnw hydrothermal structural belts northwestern northern central and southern defined here by alignment of five or more hot spring deposits and confirmed as structurally controlled by aeromagnetic data the northern and northwestern belts in particular concentrate most of the geothermal and epithermal occurrences hence jurassic hydrothermal fluid flow was strongly influenced by the most dominant and long active geological boundaries in the region the outer limits of the deseado massif horst itself

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

regional paleohydrologic and paleoclimatic settings of wetland lacustrine depositional systems in the morrison formation upper jurassic western interior usa abstract during deposition of the upper jurassic morrison formation water that originated as precipitation in uplands to the west of the western interior depositional basin infiltrated regional aquifers that underlay the basin this regional groundwater system delivered water into the otherwise dry continental interior basin where it discharged to form two major wetland lacustrine successions a freshwater carbonate wetland lacustrine succession formed in the distal reaches of the basin where regional groundwater discharged into the denver julesburg basin which was a smaller structural basin within the more extensive western interior depositional basin an alkaline saline wetland lacustrine complex lake t oo dichi formed farther upstream where shallower aquifers discharged into the san juan paradox basin which was another small structural basin in the western interior depositional basin these were both wetlands in the sense that groundwater was the major source of water input from surface and meteoric water was limited in both basins lacustrine conditions developed during episodes of increased input of surface water inclusion of wetlands in our interpretation of what had previously been considered largely lacustrine systems has important implications for paleohydrology and paleoclimatology the distal carbonate wetland lacustrine deposits are well developed in the morrison formation of east central colorado occupying a stratigraphic interval that is equivalent to the lower morrison but extends into the upper morrison formation sedimentologic paleontologic and isotopic evidence indicate that regional groundwater discharge maintained shallow hydrologically open well oxygenated perennial carbonate wetlands and lakes despite the semi arid climate wetland deposits include charophyte rich wackestone and green mudstone lacustrine episodes in which surface water input was significant were times of carbonate and siliciclastic deposition in scarce deltaic and shoreline deposits marginal lacustrine deposits include ooid and skeletal packstone grainstone siltstone and sandstone distal lacustrine units are skeletal mudstone wackestone microbialites and laminated siliciclastic mudstone differentiation between wetlands and distal lacustrine units is not always possible palustrine features magadi type chert mtc and evaporites record episodes of increased aridity and exposure farther upstream during deposition of the upper part of the brushy basin member the ancestral uncompahgre uplift imposed a barrier to shallow eastward flowing groundwater that discharged into the san juan paradox basin on the upstream side of the uplift this created the closed hydrologic setting necessary for development of an alkaline saline wetland lacustrine complex lake t oo dichi silicic volcanic ash delivered by prevailing winds from calderas west and southwest of the basin contributed to the pore water evolution in the sediments a distinctive lateral hydrogeochemical gradient reflecting increasing salinity and alkalinity in the pore waters altered the ash to a variety of authigenic minerals that define concentric zones within the basin the basinward progression of diagenetic mineral zones is smectite clinoptilolite analcime potassium feldspar albite the groundwater fed wetlands were shallow and frequently evaporated to dryness scarce laminated gray mudstone beds record distinct episodes of freshwater lacustrine deposition that resulted from intermittent streams that carried detritus well out into the basin

late palaeozoic mesozoic geomagnetic reversal paths and core mantle boundary summary an analysis of transitional virtual geomagnetic poles vgps from the late permain early triassic through to the late jurassic early cretaceous shows them to be located in preferred areas which coincide with present day zones of fast seismic velocity at the core mantle boundary cmb as has been observed for late cainozoic transitional vgp paths it is therefore suggested that the similar geometries of late palaeozoic mesozoic and late cainozoic polarity transitions have been impressed by sources of the cmb which have persisted through the mesozoic and tertiary to the present

paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks climate is a primary control on sediment flux from continental sources into sedimentary systems in warm climates siliciclastic input is greatest under highly seasonal rainfall nonseasonal conditions favor formation of end member chemical rocks perennially wet climates are conducive to coal formation whereas dry climates produce carbonates and or evaporites stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive regressive events and tectonics

nature and origin of through going discontinuities in nonmarine foreland basin strata upper cretaceous montana implications for sequence analysis two laterally traceable discontinuities montana united states that vary in character and origin provide a means of subdividing fully nonmarine strata into genetic packages that are likely related to changes in relative sea level both discontinuities are situated 100 km inland of contemporaneous shorelines and both are characterized by features that render them distinguishable from other surfaces in the alluvial record e g pervasive oxidation persistent and unusually thick lags extraformational clasts the lower discontinuity probably formed due to a fall in eustatic sea level and appears analogous to a marine sequence boundary the upper discontinuity probably formed due to a tectonically induced increase in the rate of base level rise and lacks characteristic

plant taphonomy in incised valleys implications for interpreting paleoclimate from fossil plants paleoclimatic interpretations of the upper triassic chinle formation colorado plateau based on plants conflict with those based on the sedimentary rocks the plants are suggestive of a humid equable climate whereas the rocks are more consistent with deposition under highly seasonal precipitation and ground water conditions fossil plant assemblages are limited to the lower members of the chinle formation which were deposited within incised valleys that were cut into underlying lower to middle triassic and older rocks in contrast the upper members of the formation which were deposited across the fluvial plain after the incised valleys were filled have few preserved fossil plants the taphonomic characteristics of the plant fossil assemblages within the stratigraphic and hydrologic context of the incised valley fill sequence explain the vertical and lateral distribution of these assemblages the depositional hydrological and near surface geochemical conditions were more conducive to preservation of the plants fossil plant assemblages in fully terrestrial incised valley fills should be taphonomically biased toward riparian wetland environments if those assemblages are used to interpret paleoclimate the paleoclimatic interpretations will also be biased the bias may be particularly strong in climates such as those during deposition of the chinle formation when the riparian wetlands may reflect local hydrologic conditions rather than regional climate and should be taken into account when using these types of plant assemblages in paleoclimatic interpretations

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

climatic and biotic change in the late triassic generally arid conditions that pervaded much of europe and north america during the late triassic were interrupted by a wet monsoonal climatic phase during middle and late carnian times extensive fluviatile sandstones deposited at this time throughout the region occur within a thick sequence of playa lake mudstones the sandstones occasionally contain kaolinite suggesting a humid climate an extreme 13 c depletion in a shallow marine sequence of this age in israel has been interpreted as evidence for an influx of freshwater a widespread change from carbonates to clastics in marine sequences at this time may also be climate related water course cave systems in limestone areas exposed during the late triassic indicate high levels of runoff during the middle and late carnian the marine invertebrate fauna shows a significant turnover at the end of the early carnian the terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the carnian norian boundary these periods of biotic change appear to be synchronous with the onset and cessation of a carnian humid phase the change to a monsoonal climate during this interval has been documented over more than 90 of longitude between 5 and 50 north of the triassic equator it may have been caused by rising atmospheric co 2 levels due to volcanism associated with the incipient dispersal of pangaea

palustrine sedimentation and associated features grainification and pseudo microkarst in the middle miocene intermediate unit of the madrid basin spain abstract a rather thick sequence 50 75 m of lacustrine limestones forms the uppermost part of the intermediate unit middle miocene in the ne part of the madrid basin central spain a gradual change from distal alluvial to open lake facies is recorded in this area red mudstones passing vertically to strongly mottled limestones are typical of the transition from distal alluvial to marginal lake deposits a wide variety of facies occur in the marginal lacustrine facies association all of them showing pedogenic features e g root traces alveolar septal structures pseudo microkarst micrites and biomicrites are interpreted as open lacustrine deposits although not as strongly pedogenically modified as the marginal lacustrine facies they still show evidence of subaerial exposure the carbonate sequence was deposited in a low gradient shallow lake system in which slight fluctuations in the water level caused extensive emergence pedogenic processes affected most of the lake deposits but modification was more intense in the marginal facies biogenic activity and mechanical reworking at lake margins led to the formation of pedogenic diagenetic pseudo microkarst and peloidal and or intraclastic limestones

abiotic versus biotic controls on the development of the fairmont hot springs carbonate deposit british columbia canada the relict fairmont hot springs deposit formed largely of carbonates covers an area of 0ae 5k m 2 and is up to 16 m thick the triangle shaped discharge apron which broadens down valley is divided into a proximal part with beds dipping at 10 and a distal part with beds dipping at 10 to 15 the deposit is formed of the 1 basal macrophyte 2 lower carbonate 3 middle clastic 4 upper carbonate and 5 upper clastic sequences two charcoal samples embedded in the lower carbonate sequence yielded dates of 8690 90 and 8270 70 cal yr bp indicating that much of the deposit formed post glacially during the early to mid holocene deposit aggradation ceased in the mid to late holocene when the fairmont creek valley was incised the lower and upper carbonate sequences which are the thickest sequences are composed of nearly equal parts of travertine abiotic and tufa biotic with feather dendrite travertine radiating dendrite travertine and stromatolite tufa dominating competition between calcite precipitation rates and biotic growth rates controlled the distribution of tufa and travertine across the discharge apron calcite and biotic growth rates were controlled largely by flow velocity across the apron which in turn was controlled by topography and regular fluctuations in spring water discharge volume during times of high spring discharge slow sheet flow over the proximal part of the apron promoted stromatolite growth whereas fast turbulent flow on the distal part of the apron induced rapid feather dendrite formation during times of low spring discharge quiescent shallow evaporative pools conducive to radiating dendrite formation formed on the proximal part of the apron whereas slow flow on the distal part promoted stromatolite growth facies with high calcite supersaturation experienced rapid abiotic dendrite growth that precluded most biotic growth