detrital modes of upper paleozoic sandstones derived from antler orogen in nevada implications for nature of antler orogeny

the tectonic significance of u th pb ages of monazite inclusions in garnet from the himalaya of central nepal abstract textural observations chemical zoning and 196 in situ la icp ms 232 th 208 pb and 238 u 206 pb dates of monazite inclusions in greater himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite cenozoic prograde and retrograde monazite are commonly intergrown and both of these components are frequently intergrown with lower paleozoic monazite because zones in most inclusions are too small to date and because some inclusions show little zoning of y and th chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of th pb dates comparison of 238 u 206 pb and 232 th 208 pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions most cenozoic monazite crystallized between 42 29 and 22 12 ma and these age ranges may record prograde and retrograde metamorphism of greater himalayan rocks respectively dates younger than 12 ma are discordant without exception the

provenance of lower cretaceous wolong volcaniclastics in the tibetan tethyan himalaya implications for the final breakup of eastern gondwana abstract lower cretaceous volcanic lithic arenites widely distributed in the tethyan himalaya provide insights into the continental breakup of eastern gondwana in southern tibet they are represented by the wolong volcaniclastics the volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts whereas rhyolitic dacitic volcanism becomes the predominant source of the upper strata geochemical analyses of basaltic grains and of detrital cr spinels from the wolong volcaniclastics demonstrate the alkaline character of the volcanism and suggest within plate tectonic setting for the volcanism zircon u pb ages confirm that this volcanism continued from 140 ma to 119 ma hf isotope data on these early cretaceous zircons indicate that their parental magmas were mantle derived but in the later stage of magmatic activity mantle derived magmas were mixed with partial melts derived from the continental crust the lower cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of greater india the onset of volcaniclastic deposition in the himalayas appears to become progressively younger toward the west but it ended synchronously during the late albian 102 ma the low volume of volcanic rocks and their intra plate tectonic setting suggest that they are the result of decompressional melting along extensional deep seated fractures cross cutting the continental crust and reflect changes in regional intra plate tectonic stresses when greater india began to separate from the australia antarctica supercontinent

tethyan and indian subduction viewed from the himalayan high to ultrahigh pressure metamorphic rocks abstract the himalayan range is one of the best documented continent collisional belts and provides a natural laboratory for studying subduction processes high pressure and ultrahigh pressure rocks with origins in a variety of protoliths occur in various settings accretionary wedge oceanic subduction zone subducted continental margin and continental collisional zone ages and locations of these high pressure and ultrahigh pressure rocks along the himalayan belt allow us to evaluate the evolution of this major convergent zone 1 cretaceous 80 100 ma blueschists and possibly amphibolites in the indus tsangpo suture zone represent an accretionary wedge developed during the northward subduction of the tethys ocean beneath the asian margin their exhumation occurred during the subduction of the tethys prior to the collision between the indian and asian continents 2 eclogitic rocks with unknown age are reported at one location in the indus tsangpo suture zone east of the nanga parbat syntaxis they may represent subducted tethyan oceanic lithosphere 3 ultrahigh pressure rocks on both sides of the western syntaxis kaghan and tso morari massifs formed during the early stage of subduction exhumation of the indian northern margin at the time of the paleocene eocene boundary 4 granulitized eclogites in the lesser himalaya sequence in southern tibet formed during the paleogene underthrusting of the indian margin beneath southern tibet and were exhumed in the miocene these metamorphic rocks provide important constraints on the geometry and evolution of the india asia convergent zone during the closure of the tethys ocean the timing of the ultrahigh pressure metamorphism in the tso morari massif indicates that the initial contact between the indian and asian continents likely occurred in the western syntaxis at 57 1 ma west of the western syntaxis the higher himalayan crystallines were thinned rocks equivalent to the lesser himalayan sequence are present north of the main central thrust moreover the pressure metamorphism in the kaghan massif in the western part of the syntaxis took place later 7 m y after the metamorphism in the eastern part suggesting that the geometry of the initial contact between the indian and asian continents was not linear the northern edge of the indian continent in the western part was 300 to 350 km farther south than the area east of the nanga parbat syntaxis such en baionnette geometry is probably produced by north trending transform faults that initially formed during the late paleozoic to cretaceous gondwana rifting farther east in the southern tibet the collision occurred before 50 6 0 2 ma finally high pressure to ultrahigh pressure rocks in the western himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data

cambrian depositional history of the zanskar valley region of the indian himalaya tectonic implications a well preserved cambrian section in the zanskar valley of northern india has previously been interpreted to record the transition from a passive to an active tectonic margin related to cambrian ordovician orogenesis this interpretation has been used to support the tectonostratigraphic interpretation of other successions across the tethyan himalaya our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these cambrian deposits no definitive record of impending cambrian ordovician orogenesis is recorded in these late middle cambrian rocks a critical transition from an 125 m thick stromatolite bearing carbonate deposit the karsha formation into shale and sandstone of the kurgiakh formation was interpreted to represent tectonically induced drowning of a carbonate platform siliciclastic strata of the kurgiakh formation were thought to record deep water flysch deposition in a tectonically active foreland basin next to an arc trench system this interpretation was based on sandstone beds with classic bouma sequences we show that these event beds in the kurgiakh instead contain hummocky cross stratification quasi planar lamination and combined flow ripple stratification all of which reflect deposition in shallow marine storm influenced environments thus although the karsha carbonate platform may have been drowned it did not culminate in deep sea flysch deposition and this in turn eliminates a major line of evidence linking kurgiakh deposition to the onset of cambrian ordovician orogenesis other aspects of cambrian ordovician deposits of northern india also shed doubt on the proposed link between kurgiakh sedimentation and the cambrian ordovician orogenic event first our improved biostratigraphic database suggests that the transition from the karsha carbonate to the kurgiakh formation may have predated the main phase of cambrian ordovician orogenesis as recorded by overlying ordovician molasse by as much as 20 30 my second published data from the ordovician molasse indicate northward paleocurrents which are parallel to those recorded by siliciclastic deposits of the parahio formation below the karsha and thus are at odds with standard models of foreland basin development for the cambrian ordovician event our sedimentological analysis of depositional cycles of the parahio formation indicates that these strata record storm influenced environments from offshore marine to shoreface to fluvial settings this is at odds with previous paleoenvironmental interpretations that ranged from deep sea flysch to intertidal deposits paleocurrent data for marine and fluvial facies of the parahio formation in both zanskar and the spiti valley to the south indicate northeast sediment transport this supports the view that the parahio and overlying carbonate of the karsha formation record the ancient northern passive margin of india during the cambrian and that these strata may be distal equivalents of the younger cambrian deposits of the lesser himalaya

the crystalline units of the high himalayas in the lahul zanskar region northwest india metamorphic tectonic history and geochronology of the collided and imbricated indian plate in the high himalayan belt of northwest india crustal thickening linked to palaeogene collision between india and eurasia has led to the formation of two main crystalline tectonic units separated by the syn metamorphic miyar thrust the high himalayan crystallines sensu stricto hhc at the bottom and the kade unit at the top these units are structurally interposed between the underlying lesser himalaya and the very low grade sediments of the tibetan nappes they consist of paragneisses orthogneisses minor metabasics and chiefly in the hhc leucogranites the hhc registers a polyphase metamorphism with two main stages designated as m1 and m2 a metamorphic zonation with high temperature recrystallization and migmatization at middle structural levels and medium temperature assemblages at upper and lower levels in contrast the kade unit underwent a low temperature metamorphism rb sr and u th pb isotope data point to derivation of the orthogneisses from early palaeozoic granitoids while the leucogranites formed by anatexis of the hhc rocks and were probably emplaced during miocene time most of the complicated metamorphic setting is related to polyphase tectonic stacking of the hhc with the cooler kade unit and lesser himalaya during the himalayan history however a few inconsistencies exist for a purely himalayan age of some ml assemblages of the hhc as regards the crustal derived leucogranites the formation of a first generation mixed with quartzo feldspathic leucosomes was possibly linked to melt lubricated shear zones which favoured rapid crustal displacements at upper levels they intruded during stage m2 and the latest movements along the syn metamorphic miyar thrust but before juxtaposition of the tibetan nappes along the late metamorphic zanskar fault

nappe tectonics extension and metamorphic evolution in the indian tethys himalaya higher himalaya se zanskar and nw lahul in the southern part of the indian tethys himalaya between se zanskar and nw lahul nappe tectonics with transport toward the wsw during the main deformation phase has been ascertained if the stratigraphy of the different units considered as nappes is combined it results in a composite section ranging from lower cambrian to middle cretaceous a metamorphism of amphibolite facies in the lowest unit is associated with the nappe stacking the lower miocene cooling ages of metamorphic biotite and muscovite suggest that the nappe emplacement starts at the oligocene or at the oligocene eocene boundary after the compressional phases an extension provokes the reactivation of older main thrusts as normal fault and the formation of a steeply dipping normal fault this interpretation is based on detailed mapping

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

subsidence history of the tethys himalaya abstract this article reconstructs the evolution of the passive northern margin of peninsular india facing the neotethys from the earliest rifting stages in the carboniferous to final collision with the active southern margin of asia in the early eocene classical techniques of basin analysis were applied to an extensive and coherent stratigraphic data base facies analysis biostratigraphic dating and palaeoenvironmental interpretation of an over 10 km thick sedimentary succession from eight main composite sections allowed us to draw accurate sedimentation and backstripped subsidence curves and to place quantitative constraints on the palaeotectonic scenario inferred from the sedimentary record in each of the investigated composite sections three major stratigraphic gaps are consistently recognised in the rift sequence the rift unconformity mississippian the carboniferous permian hiatus largely kasimovian to asselian and the break up unconformity mid sakmarian two or even all three gaps may merge into a single hiatus spanning up to 70 ma overall gaps are associated with rift shoulder uplift which after calculation of backstripped subsidence sums up to 600 m at least in three of the investigated sections thermal subsidence and low sedimentation rates are documented by the mid permian to middle triassic drift sequence deposited while the neotethys was expanding between northern gondwana and the detached peri gondwanan blocks a marked increase in accumulation rates is recorded in the carnian norian but calculations of uniform lithospheric stretching suggest that extension was minor and associated with tectonic processes affecting distant parts of gondwana the tethys himalayan margin was uplifted around the jurassic cretaceous boundary at the onset of a major volcanic episode finally uplift exceeding in magnitude all previous events is documented by the uppermost units of the stratigraphic column deposited during the earliest stages of collision between india and asia in the latest paleocene to early eocene

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

compositional suites of terrigenous deep sea sands of the present continental margins the compositional variability shown by sands of present continental margins was determined through conventional microscopic techniques this allowed the establishment of a descriptive classification of deep sea terrigenous sands based on a restricted number of commonly used parameters the fundamental sand types include 1 qf suite sands in which quartzose and or feldspathic grains predominate carbonate l bearing sands on average richer in lithics occur within this suite 2 ql suite in which quartzose and or lithic grains predominate little compositional diversification is apparent within this suite 3 fl suite in which lithic and or feldspathic grains predominate two compositional members can be identified based on the k feldspar plagioclase ratio 4 lv suite consists of sands in which volcanic grains largely dominate the lithic types vitric and serpentine sands occur within this suite the suites are related to distinct source area types defined as follows 1 cratonic block 2 sedimentary orogenic belt 3 volcanic orogenic belt 4 volcanic arc some sands cannot be easily assigned to a single compositional suite because petrologic parameters overlap with an adjacent suite such critical compositions are commonly arkosic using roundness of the framework components it is possible to define a category identified as matured sands which are greatly enriched in resistates

middle ordovician darriwilian conodonts from southern tibet the indian passive margin implications for the age and correlation of the roof of the world

patterns in the aggregate stability of mancos shale derived soils abstract salt and sediment contributions to the colorado river and its tributaries pose economic and environmental concerns for the united states and mexico land use decisions promoting the aggregation of mancos shale derived soils are one way to reduce the transportation of salts and sediments to water resources we used a simple field test of soil aggregate stability to determine the site characteristics influencing the soil stability of sedimentary marine shale in the gunnison gorge national conservation area in southwestern colorado ninety six 1 m 2 plots were intensively sampled to explore relationships between soil stability and the biological chemical and physical site characteristics analysis of variance showed decreased soil stability when erosion features were present at a sample location suggesting the applicability of stability test results to the erosion potential of mancos shale derived soils p 0 02 analysis using generalized linear models with a negative binomial distribution and non parametric regression tree models determined that the best predictor variable for soil stability was the percent cover of visible biological soil crust identifying the importance of biological soil crust in the stabilization of mancos shale soils is important for land use planning because biological soil crusts respond to changes in land use the results of this study lead to improved interpretation of the field soil stability test and may assist in the definition of important ecological or functional thresholds in many arid and semiarid environments

hydrogen uptake and diffusion in callovo oxfordian clay rock for nuclear waste disposal technology abstract the callovo oxfordian clay rich rock formation is currently considered as host rock barrier in the french geological repository facility for radioactive waste meuse haute marne after the closure of the facility hydrogen gas is expected to develop mainly from anaerobic corrosion of steel containers and other iron containing structures gas pressure build up could impact the safety of the repository it is therefore important to acquire in depth knowledge on the interaction between hydrogen gas and surrounding clay rock in terms of uptake ability and diffusion hydrogen uptake capacity was evaluated on dried clay rock samples i at 20 k to allow for hydrogen liquefaction and determine the maximal h 2 uptake of the clay and ii at typical pressure and temperature conditions expected to develop in the repository up to 363 k and a hydrogen pressure of 40 60 bar h 2 absorption on the dried raw callovo oxfordian start to saturate at about 30 40 bar and the average adsorption above 40 bar is about 0 1 in weight quasi elastic neutron scattering spectroscopy was used to study the diffusion mechanism of hydrogen gas in the clay rock at the microscopic scale and to determine hydrogen self diffusion coefficients in the dry samples in the temperature range 25 300 k neutron data suggested that hydrogen diffuses in the dry clay rock according to fick s law the findings reported in this work can help to better understand the behavior of h 2 in clay rock samples

palaeozoic strike slip rift pulses and palaeogeography in the circum mediterranean tethyan realm abstract five main rifting cycles supported by different types of field evidence including magmatism sedimentation and tectonics have been recognized in the circum mediterranean area western tethys realm before the opening of the jurassic tethys proper they occurred roughly in the late cambrian early ordovician early silurian late devonian dinantian early mid permian and mid triassic times each of these rifting cycles began with intracontinental mainly shallow marine conditions evolving usually to basinal environments of quite different bathymetry the palaeozoic ccd has been proved to be much less deep than the post liassic one minor suboceanic seaways cut across and fragmented the gondwana south european plate following the early palaeozoic rifting cycles the long mid palaeozoic devono dinantian rifting cycle migh possibly have resulted in a wider system of suboceanic sea ways connecting a palaeo tethys ocean to the east of gondwana with the rheic ocean to the west of it the birth of palaeo tethys or of its westward extension would have been directly linked with the gradual death of the devono mississippian rheic ocean a complex large scale dextral transform system is inferred to have connected the two respectively expanding and contracting oceans fragmentation and mutual displacement of microblocks in the connection area the circum mediterranea area and southern europe resulted from strike slip oblique rift pulses with coeval local contractional and extensional processes a major change of the gondwana rotational vector is believed to have produced the mid late carboniferous transpressional strike slip hercynian orogen this is a good explanation of the peculiar thermal regime of the hercynian diastrophism all over europe there is no support for the hypothesis of a very wide more than 3000 km palaeozoic ocean separating northern europe from southern europe or gondwanaland this idea was mainly based on poor palaeomagnetic data and is heavily contradicted by palaeobiogeographical and geological data furthermore it ignores the dramatic shortening of the hercynian orogen all over europe the early palaeozoic to early mesozoic history of the circum mediterranean area involves the cyclic recurrence of oblique rifts culminating in the early jurassic strike slip rift and subsequent limited opening of the tethys proper to avoid confusion in terminology one should use prefixes referring to age bathymetry physiography and geotectonic setting to qualifying the general term tethys e g shallow cretaceous tethys basinal intracontinental devonian tethys oceanic jurassic tethys etc

cambrian ordovician orogenesis in himalayan equatorial gondwana an early paleozoic tectonic event the kurgiakh orogeny has long been known from the western tethyan himalaya and it is conspicuously recorded by an angular unconformity between cambrian marine shelf deposits and coarse ordovician conglomerate as well as widespread granitic plutons although the lateral extent of this event is poorly known two regions in the central and eastern himalaya the kumaon of india and klippen of bhutan contain conglomerate units that may be correlative with this event in the kumaon the ralam formation which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca 512 ma is overlain by sandstone containing the arthropod walking trace diplichnites gouldi an ichnospecies known from ordovician and younger strata the late early cambrian trilobite redlichia sp recovered from the underlying martoli group indicates that the conglomerate is post early cambrian and based on stratigraphic analysis also likely records the kurgiakh orogeny in central bhutan the lower paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix these units have also been assigned to a variety of ages but rocks that apparently underlie these strata contain the furongian late cambrian zonal trilobite kaolishania granulosa here we show that detrital zircon ages of a conglomerate clast no grains younger than ca 781 ma are distinctly older than those of the matrix which contains a large peak at 490 500 ma thus these conglomerate units are also potential lateral correlatives of the ordovician conglomerate deposits of the western himalaya our paleontological and geochronologic data suggest that the kumaon and bhutan conglomerate units and possible equivalents in tibet correlate with those of the western himalaya this increases their known along strike extent by over 1000 km bringing the total extent to 1200 km thus the kurgiakh orogeny spanned much of the himalayan orogen along the northern gondwanan margin and reflected a widespread tectonic reorganization of the peri gondwanan realm to andean type margins recent work indicates that it also likely extended into the lhasa block and both the north and south china blocks this event was associated with widespread ca 500 ma arc related plutonism recorded in the himalaya and with abundant detrital zircon grains of that age in younger himalaya strata based on current fossil data the duration of the hiatus associated with the cambrian ordovician boundary which separates pre and postdeformational strata ranges from 22 to 36 m y available geochronological data do not provide greater constraint if the event was close in age to the depositional age of the ordovician molasse then the main phase of tectonic uplift erosion and deposition may have been much later than the presumed cambrian ordovician boundary interval age of ca 485 ma

latest cretaceous himalayan tectonics obduction collision or deccan related uplift abstract diverging interpretations and incompatible scenarios have been proposed for the early stages of himalayan history numerous researchers have postulated that northern india was involved in ophiolite obduction arc continent or continent continent collision during the late cretaceous or early paleocene but firm geological evidence was never produced in this article we argue against orogenic events predating the late paleocene when the neotethys ocean was still open the tethys himalayan sedimentary record testifies to anorogenic evolution primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of deccan lavas and eventually followed by thermal subsidence major stratigraphic gaps in pelagic sediments suggest that such tectono magmatic episode started to affect the base of the indian plate in the campanian or possibly even in the santonian 10 to 20 ma before the climax of deccan flood basalt eruptions the abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the indian subcontinent during the maastrichtian was followed by progradation of coastal quartzarenites along the northern indian margin in the early paleocene sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose felsitic volcanic rock fragments and cr spinels many of which with the same geochemical fingerprint as deccan spinels and newly found throughout the maastrichtian to danian succession resisted the combined effect of subequatorial weathering and subsequent diagenesis and testify that detritus from deccan basalts reached the indian passive margin as far as south tibet at the close of the early paleocene india drifted away from the seychelles block and thermal subsidence led to widespread carbonate deposition along the tethys himalaya this article illustrates how mega events of magmatic upwelling followed by lithospheric cooling may control passive margin sedimentation and stratigraphic patterns as occurred in northern india first in the early cretaceous and next in the latest cretaceous paleocene

the reactivity of carbonates in selected soils of southern spain the properties of many soils of arid and semiarid areas are influenced by the presence of carbonates this study was conducted to characterize the reactivity of carbonates in the surface horizons of 25 soils entisols inceptisols vertisols and alfisols of southern spain the samples were analyzed for calcium carbonate equivalent cce active calcium carbonate equivalent acce predominant carbonate mineralogy mg content particle size distribution psd of carbonates by sedimentation and by a ph stat acid dissolution method and surface area sa of carbonates by n2 adsorption and by 45ca exchange from the differences between intact and decalcified soil samples as well as by estimates from the psd described above the cces ranged from 70 to 778 g kg 1 of soil and the acces ranged from 48 to 285 g kg 1 dolomite was present in only eight samples and in the dolomite free samples calcite generally had mg ca mg ratio 0 03 the carbonate psd as determined by sedimentation differed from the psd calculated from the ph stat data with the latter method giving a much higher proportion of claysize carbonates the average sas of carbonates as estimated from the psd data were 0 49 m2 g 1 soil for the sedimentation method and 3 2 m2 g 1 for the ph state method the average sas determined by the n2 and the 45ca methods were 6 0 and 2 3 m2 g 1 respectively the average specific surface areas ssas of soil carbonate m2 g 1 cce were 1 5 7 5 8 8 and 22 for the sedimentation 45ca exchange ph stat and n2 methods respectively the low sa and ssa values from the sedimentation method were a consequence of the occurrence of porous aggregates which have considerably higher equivalent spherical diameters than the particles of which these aggregates are composed the ph stat and 45ca methods gave sa values of similar magnitude to each other probably because both methods are dependent on actual reactive surface area instead of aggregate size of the carbonate mass the higher values obtained by the n2 method might indicate that n2 is physisorbed on carbonate surfaces that are slowly reactive to acid attack or 45ca exchange the ph stat sa was highly correlated with acce which measures the reactivity of carbonates to oxalate to obtain a rapid estimate of the reactivity of soil carbonate the acce method because of its simplicity may be preferred

cambrian biostratigraphy of the tal group lesser himalaya india and early tsanglangpuan late early cambrian trilobites from the nigali dhar syncline precise biostratigraphic constraints on the age of the tal group are restricted to 1 a basal level correlative with the anabarites trisulcatus protohertzina anabarica assemblage zone of southwest china 2 a level near the boundary of the lower and upper parts of the tal group correlative with the early tsanglangpuan stage drepanuroides zone and 3 an interval low in the upper part of the tal group correlative with later in the tsanglangpuan stage palaeolenus zone these correlations are based on small shelly fossil and trilobite taxa other chronostratigraphic constraints include the marked negative 13 c isotopic excursion coincident with the transition from the krol group to the tal group this excursion is used as a proxy for the precambrian cambrian boundary in several sections worldwide and if applied to the lesser himalaya indicates that the boundary is at or just above the base of the tal group the upper parts of the tal group may be of middle or late cambrian age and might form proximal equivalents of sections in the zanskar spiti region of the tethyan himalaya both faunal content and lithological succession are comparable to southwest china furthering recent arguments for close geographic proximity between the himalaya and the yangtze block during late neoproterozoic and early cambrian time trilobites from the uppermost parts of the sankholi formation from the nigali dhar syncline are described and referred to three taxa one of which drepanopyge gopeni is a new species they are the oldest trilobites yet described from the himalaya

late cambrian deformation in the lesser himalaya abstract the tons valley situated in the central easternmost part of the himachal lesser himalaya adjoining the garhwal himalaya shows geological features suggestive of a strong pre tertiary deformational episode the paleoproterozoic dharagad group overlain by the mesoproterozoic deoban and neoproterozoic simla groups rest as a thrust sheet over the middle cambrian chilar formation which occurs as windows and also as tectonic slivers within the thrust sheet designated as the dharagad thrust sheet dts the mineral lineation inclination of tectonic slivers and overturned beds suggest that the dts was translated from the ne the westernmost and southwesternmost leading edges of the dts are exposed at subathu and morni wnw and wsw respectively of the tons valley the position of the leading edges of the dts vis a vis the windows in the tons valley suggest a minimum translation of about 50 km for the dts the simla group at subathu and the deoban at morni forming parts of the dts constitute basement for the thanetian lutetian subathu formation of the himalayan foreland basin hfb this stratigraphic relationship unambiguously demonstrates that the simla and the deoban groups forming leading edges of the allochthonous dts were already translated and emplaced at subathu and morni before the creation of the hfb in which the deposition commenced with the subathu formation in thanetian it implies that the dts was translated from the ne to the present position at subathu and morni in pre thanetian time there is no direct evidence to constrain the age of the thrusting in view of regional regression in late cambrian a distinct angular unconformity between the cambrian and the overlying ordovician early paleozoic metamorphism and extensive development of early paleozoic granites and their rapid exhumation a late cambrian age is suggested for the dts thrusting not only the direction of movement of the dts is same as that of the tertiary thrust sheets but also cambrian folds are co axial with the tertiary folds this strange coincidence shows that similar kinematic field existed during two tectonic events a ridge like the present central crystalline axis was elevated between the tethyan and lesser himalayan basins which contributed zircons of the early cambrian age to both basins

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

pan african magmatism and sedimentation in the nw himalaya correlation of early palaeozoic pan african 500 50 ma granites that intruded the chail salkhala haimanta formations in the lesser himalaya zanskar crystallines and lower taglang la of tso morari crystallines in the northwestern himalaya is based on the field relationship tectonic setting mineralogical and geochemical characteristics and isotope dating of the granites these granite plutons exhibit identical petrographical and geochemical character the mineralogical composition of the granites is quite similar consisting of quartz k feldspar plagioclase feldspar biotite muscovite garnet tourmaline cordierite andalusite and sillimanite fibrolite the granite which are massive and inequigranular in the core of the plutons show strongly foliated character indicating development of ductile shear zone at the margins these are peraluminous s type granites having high a cnk value 1 presence of normative corundum rounded shape of zircon and high initial sr ratio suggest crustal source of the granites mantle normalized spider diagram exhibits similar characters for all these granitoids the intrusion of the pan african granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from palaeoproterozoic the sudden break in sedimentation towards the terminal phases of the lower cambrian has been observed in almost all parts in lesser as well as the tethys himalaya occurrences of large number of plutons along different tectonic belts of northwestern himalaya are indicative of widespread tectono thermal event during early palaeozoic 500 50 ma the bracketing of the two features like the break in sedimentation during post late cambrian and the intrusion of granites around 500 50 ma is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the pan african orogeny in africa

rb sr geochronology of granites and gneisses from the mount everest region nepal himalaya nineteen samples of granites orthogneisses and paragneisses from the high himalaya basement nappe s of the mount everest region have been dated by the rb sr method the post metamorphic tourmaline leucogranites of the upper imja drangka nuptse lhotse glacier have high initial sr87 sr86 characteristic of an anatectic origin from crustal material a whole rock isochron age of 52 m y early eocene has been obtained for the samples from the granite body of lhotse glacier apparently sr isotopic homogenization was not reached throughout the much larger nuptse granite

multicyclic history of the northern india continental margin northwestern himalaya 1 the geologic evolution of northern india is best recorded in the stratigraphic succession of the zanskar range northwestern himalaya which represents the most complete transect through this ancient continental margin sedimentary history began in the late proterozoic and recorded a late pan african orogenic event around the cambrian ordovician boundary when the gondwana supercontinent was eventually assembled the following long period of epicontinental deposition in shallow seas linked to palaeo tethys lasted until the early permian when a neo tethyan rift began to open between paleo india and the cimmerian microcontinents neo tethyan history can be subdivided into two sedimentary megasequences both recording a major tectonic and magmatic event in the lower part the first one began with breakup in the late permian and lasted until the end of the jurassic the second one started in the early cretaceous with the final detachment of india from gondwana and the opening of the indian ocean and ended with the india eurasia collision in the early eocene the two megasequences can be in turn subdivided into six transgressive regressive supersequences bounded by tectonically enhanced unconformities basal sandstone units of early permian late permian norian callovian early cretaceous and paleocene age are invariably associated with oolitic ironstones or reworked glauco phosphorites and mark the transgre sive part of each supersequence next condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing upward marly units capped by regressive platformal carbonates the six tectono eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of india from the rest of gondwana and document the combination of plate microplate reorganizations and eustatic climatic and oceanographic changes in the tethyan domain after the onset of collision between india and asia close to the paleocene eocene boundary obduction of the remnants of the neo tethys ocean floor onto the indian margin began and the latter underwent multiphase deformation with fold thrust shortening followed by heating and extension after the main metamorphic event ophiolitic nappes were re thrusted and finally emplaced with their sedimentary sole on top of the passive margin succession

proterozoic sedimentation and pan african geodynamic development in the himalaya abstract the purana or proterozoic lower cambrian continental margin sedimentary succession of the lesser himalaya comprises three lithological divisions a lower argillo arenaceous group a middle carbonate succession and an upper argillo calcareous assemblage the poorly sorted basal sediments consisting of wacke slate alternations of the lower group were emplaced by currents including turbidity currents which in the early stage flowed northwards from the aravalli and satpura mountains of peninsular india lenticular deposits of diamictite within the flysch succession represent submarine slides penecontemporaneous basaltic lavas and associated tuffs interbedded with shallow water arenites making up the bulk of the upper part of the lower group were related to rifting of the basins the overlying dolomites and dolomitic limestones were laid down on a very shallow stable shelf and are characterised by columnar branching stromatolites and lenticular units of crystalline magnesite and associated talc lenticular diamictites towards the top of the upper group of slates and limestone are harbingers of tectonic movements culminating towards the end of the early cambrian in an upheaval which abruptly terminated sedimentation throughout the lesser himalaya and peninsular india and caused a pronounced interruption of sedimentation in the tethyan domain in the northern himalaya the cambro ordovician 500 25 ma granites intruding the lesser himalayan thrust sheets of lower proterozoic metamorphic rocks and the upwarped northern front of the himalaya signify an important tectonic event that terminated the purana proterozoic lower cambrian sedimentation in the indian subcontinent this seems to be related to the tectonic movements that occurred towards the close of the late proterozoic or early cambrian

modern shallow water graded sand layers from storm surges bering shelf a mimic of bouma sequences and turbidite systems abstract a sequence of graded sand layers interbedded with mud extends offshore over 100 km from the yukon delta shoreline across the flat shallow 100 kin such shallow water graded layers off lobate deltas may be distinguished from similar deep water turbidites by 1 the predominance of trough cross lamination perhaps resulting from wave oscillation effects in the proximal part of the system and 2 gradation to common shallow marine fossils bioturbation and storm lag layers n distal areas

differentiation of pedogenic and lithogenic carbonate forms in texas abstract sixteen pedons representing soils developed from limestone and fluvial deltaic sediments were sampled in central and western texas 300 800 mm annual precipitation to differentiate pedogenic secondary and lithogenic inherited carbonate forms by field and laboratory techniques lithogenic carbonate forms identified in the field included indurated limestone bedrock and coarse limestone fragments lithorelicts field identified pedogenic forms included petrocalcic horizons thin laminar cappings on indurated limestone carbonate joint seams pendants on pebbles and carbonate films and threads on ped faces small

structures and textures in triassic braided stream conglomerates bunter pebble beds in the sherwood sandstone group north staffordshire england in the northern parts of the needwood and stafford eccleshall basins england the pebble beds of the sherwood sandstone group contain thick successions of texturally mature fluvial pebble cobble conglomerates which are organized into either horizontal or cross stratified sets the horizontally lying sets generally coarser grained and more poorly sorted than the cross bedded sets are usually disorganized and either matrix or clast supported although thin lenses of well sorted occasionally openwork units interpreted as falling stage phenomena are often present the cross stratified conglomerates have foresets exhibiting remarkable textural organization with a coarse bimodal sometimes matrix supported part grading upwards or being abruptly overlain by a finer well sorted occasionally openwork part and finally capped by sandstone these rhythmic textural changes are attributed partly to an avalanche process at high stage and partly to falling stage conditions the most common types of vertical association are thick successions of horizontally bedded conglomerates up to 20 m and sequences of an upwards coarsening nature 2 12 m in which cross stratified sets are overlain by flat lying sets the environment of deposition of the gravels is interpreted as one in which water depths at high stage were greater than depths in most modern braided stream plains proglacial or alluvial fan but shallower than depths associated with the pleistocene catastrophic floods from which texturally mature giant gravel bars have been recorded recent braided streams with relatively confined channels and considerable bar channel relief are better analogues in particular medial or mid channel bars with a two tier structure subaqueous and partly emergent portions may explain the upward coarsening sequences in which horizontally lying conglomerates overlie cross stratified conglomerates the thicker sequences of horizontally stratified conglomerates represent proximal longitudinal bar deposits sheets of pebbly sandstone and argillaceous sandstone lying between the conglomerates and commonly occurring towards the top of the succession largely represent deposition from sandwaves and dunes finer interbedded argillaceous sandstones siltstones and mudstones are interpreted as overbank and waning flood deposits basin forming tectonism of increasing intensity probably caused the initial coarsening upwards of the lower part of the succession whilst more stable tectonic conditions and decreasing relief on the margins of the basins and in the areas of provenance in the midlands and the hercynides account for the upwards fining of the upper part of the succession

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

ordovician successions in southern central xizang tibet china refining the stratigraphy of the himalayan and lhasa terranes abstract the ordovician stratigraphy of southern central xizang tibet has been revised based on new conodont data recovered from 43 samples in four stratigraphic units and their integration with existing nautiloid and graptolite data the histiodella holodentata and pygodus serra biozones have been identified respectively in the alai and jiaqu formations of the chiatsun group exposed near alai village in nyalam county within the himalayan terrane and the yangtzeplacognathus foliaceus subbiozone lower part of the pygodus serra biozone in the sangqu formation exposed at the guyu section within zayu county in the lhasa terrane recognition of these biozones has increased the precision of correlation of the middle upper darriwilian strata in the region regional reassessment of the ordovician stratigraphy permitted by new biostratigraphic data has allowed revised definitions for the chiatsun and keerduo groups and the sangqu and xainza formations the chiatsun group is defined herein to include three lithologically distinctive formations in descending order the jiaqu the alai and the adang formations the stratigraphic age for the jiaqu and alai formations in the type area ranges from the middle darriwilian histiodella holodentata biozone to middle katian hamarodus brevirameus biozone but the age of the adang formation remains less certain

stratigraphic correlation of cambrian ordovician deposits along the himalaya implications for the age and nature of rocks in the mount everest region the depositional age and stratigraphic correlations of metamorphosed and variably deformed rocks of mount everest are poorly known because of limited recovery of diagnostic fossils detailed study of cambrian and ordovician strata from along the length of the himalaya has produced a coherent stratigraphy that stretches from northern india to tibet our work also demonstrates that the north col formation rocks everest series between the qomolangma and lhotse detachments of the south tibetan detachment system still locally preserve sedimentary textures and primary stratigraphy that match those within cambrian strata similar to 1100 km to the west in northern india this demonstrates a coherency of depositional systems and stratigraphic architecture for cambrian deposits along much of the himalaya tethyan margin it also allows for the first time identification of precise depositional ages of several units in the everest region in particular the yellow band carbonate and directly underlying siliciclastic strata which are both shown to be late middle cambrian in age detrital zircon data presented herein for a sample from these siliciclastic strata contain a similar age spectrum to those from middle cambrian strata in northern india as well as grains as young as ca 526 ma both of which support the depositional age and continuity of depositional systems along the length of the himalaya highly fractured rocks of the ordovician lower chiatsun group in the hanging wall of the south tibetan detachment system in nyalam 75 km to the west of everest correlate with ordovician strata of the mount qomolangma formation on mount everest our correlations indicate that the base of the summit pyramid of everest the foot of the third step is composed of a 60 m thick white weathering thrombolite bed the top of this ancient microbial deposit crops out only 70 m below the summit of mount everest

problems in applying preservation facies and sequence models to sinian neoproterozoic glacial sequences in australia and asia abstract the sinian 800 530 ma is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate eustasy and possibly tectonics understanding sinian stratigraphy may help interpret the co evolution of life and environment during the period though sinian successions are poorly dated and difficult to correlate preservation facies and sequence models based on mesozoic recent successions may be used to interpret them preservation models based on basin subsidence permit estimates of how thick and complete successions can be in different tectonic settings and how long it took for them to be deposited facies models for rift craton shelf slope and basin environments help in identification of these different tectonic settings sequence models facilitate interpretation of the interplay of tectonic subsidence eustasy rates of sedimentation and climate such models applied to the sinian successions of australia and asia indicate the following great shallowing upwards megacycles occur in sinian successions and each ends with regional glaciations deep rift basins formed around 850 800 ma preserve the oldest megacycles but these basins soon were filled with rapidly deposited glaciogenic sediments relatively complete sequences are found only in deep basins subject to repeated rifting cratons and shallow shelves preserve only the youngest megacycles older deposits have been removed by younger glacial advances deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types but these sequences are not developed to the same extent in all blocks problems in correlating the four great megacycles recognized elsewhere with those of asia will persist until the requisite facies analysis and u pb dating of interbedded volcanics are done

phosphate sorption by calcium carbonate in some soils of the mediterranean part of spain abstract we studied the properties of caco3 and phosphate sorption of 36 calcareous soil samples collected across the mediterranean part of spain owing to its low surface area and to its relatively low phosphate sorption capacity caco3 did not influence sorption markedly other soil components of calcareous soils such as fe oxides or silicate clays appear to be quantitatively more important at least for sorption at low phosphate equilibrium concentrations

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

the himalayan passive margin from precambrian to cretaceous times abstract the himalaya passive margin should be separated into the indian himalaya and the pakistan himalaya divided by the nanga parbat haramosh basement uplift in the northwestern syntaxes the indian passive margin shows a complex history involving splitting of microcontinents off the northern gondwana margin from early paleozoic times until the jurassic in contrast the pakistan himalaya formed part of the stable indian shelf until separation from africa started in the jurassic each margin has a distinct stratigraphic history until the mid mesozoic after which they show similar histories until the tertiary the himalaya mountains in india consist of three distinct tectonic units juxtaposed during the neogene by southward thrusting of the northern indian precambrian to mesozoic passive margin the lesser himalaya on the south forms a late precambrian passive margin usually directly overlain by late cretaceous to tertiary clastics its stratigraphy is closest to that of the indian shield in contrast the high himalaya has a thin ordovician to carboniferous shelf sequence deposited after early ordovician deformation and granite intrusion above this thick permian and mesozoic shelf sequences mark the separation of continental blocks off the northern indian margin and the opening of the neotethys ocean the north himalaya formed the slope and basin of this ocean and consists of reactivated paleozoic gneiss domes overlain by thick mesozoic sediments which pass in the northeastern himalaya into an enormously thick sedimentary sequence resting on oceanic crust paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the main central thrust between the lesser and high himalaya this thrusting together with a further 250 km of erosion at the front of the nappes has removed the entire inner shelf of the himalaya passive margin and can explain the startling contrast between the high and lesser himalaya stratigraphic sequences but there are still discrepancies particularly in the eastern himalaya where precambrian basement is juxtaposed with the north himalaya mesozoic slope these discrepancies can be partly resolved by strike slip movements roughly parallel to the himalaya which have removed parts of the northern indian passive margin such faults of the requisite orientation displacement and age occur in southeast asia where their cumulative displacements can add up to several thousand kilometres similar faults of undoubted pre tertiary age occur in the pakistan himalaya here the passive margin shows a much simpler history of jurassic to miocene subsidence

the precambrian cambrian boundary problem and its prospects northwest himalaya india the precambrian cambrian boundary problem is studied in an areno argillaceous sequence in the kashmir and spiti valley northwest himalaya india in kashmir a rich and diversified microbiota cryptarchs and algae of the late precambrian and low lower cambrian trace fossils are recorded in the spiti valley the yield of microbiota is poor and the trace fossils are late lower cambrian modified journal abstract

regressive transgressive sedimentation in the ordovician sequence of the spiti tethys basin himachal pradesh india abstract the spiti basin comprises about 10 000 m of more or less fossiliferous cambrian to cretaceous sedimentary succession in the tethyan tibetan himalaya the ordovician strata in this basin consist of dominant sandstone with minor mudstone siltstone facies and locally developed basal conglomerate and persistent topmost minor carbonates this sequence has been regarded variously as continental and or marine to stretch out the environment of deposition a detailed study on field characteristics palaeocurrents petrography and reported marine fauna and trace fossils was carried out and suggests three major depositional environments viz fluvial transitional and marine the fluvial facies assemblage is characterised by fine to coarse grained multistoried sheets of sublithic arenite with abundant planar cross stratification and low mudstone content and is interpreted as having formed in a braided river environment the generalised palaeoslope is towards ne and nw the transitional facies comprises fine to medium grained sublithic to subarkosic arenites having sheet and lenticular sandstones with small to medium scale trough and planar cross stratification and subordinate herringbone cross stratification sigmoidal reactivation surfaces flaser and tidal bedding etc these features indicate the influence of both non marine fluvial and marine tidal flat environments the marine facies consists of fine to medium grained subarkose sublithic arenite and quartz wacke as well as sandy dolomicrite in the upper part the sandstone facies are generally multistoried having a sheet geometry and are interstratified with mudstone the important sedimentary structures include trough planar and hummocky cross stratifications and parallel lamination the palaeoflow direction is generally towards ne and nw with some minor fluctuations toward the south and west landward side these characteristic features reveal deposition in the nearshore shore zone three different stages of regression and transgression are recognised stage i resulted in the deposition of fluvial facies on the earlier marine domain cambrian representing a regressive phase caused by differential uplift and subsidence probably triggered by magmatic intrusion and or regional tectonics stage ii represents channelising of the fluvial system on tidal flats marking an initiation of transgression finally stage iii denoting the development of shore zone facies caused by basin subsidence and a change in sea level is interpreted as a transgressive phase

micromorphic record and interpretations of carbonate forms in the rolling plains of texas abstract the rolling plains of texas comprises about 10 million hectares of soils developed in permianage deposits most soils occur on erosional landscape positions soils have ustic moisture regimes and thermic temperature regimes these soils have developed in a wide variety of sediments ranging from red bed shales to sandstones most of the soils have carbonate enriched bk horizons while parent materials are slightly to non calcareous pedogenic calcite usually occurs as nodules calcans neocalcans and crystallaria calcite nodules are the dominant form in upper bk horizons while the other forms occur primarily in lower bk cr horizons additional forms of calcite include pseudomorphs after gypsum and equant anhedral calcite which fill fracture or joint seams and sand calcite crystals poikilotopic these are considered pedogenic although formed under an earlier climatic regime dolomite a post depositional lithogenic mineral occurs as nodules intercalary crystals and clustered to dispersed microsparite crystals micromorphic forms of calcite indicate that the soils have been through several major and minor fluctuations in the soil chemical environment on stable landscape positions carbonate minerals preserve the long term pedogenic record of soil development

water dispersible clay in calcareous soils of southwestern spain abstract water dispersible clay wdc is a good indicator of the risk of soil erosion by water and the consequent losses of nutrients and contaminants in overland flow we measured the content and studied the properties of wdc in 26 samples of calcareous xeralfs xerepts and xererts of southwestern spain collected from fields under different crop and tillage management the soils ranged widely in total clay content 60 455 g kg 1 calcium carbonate equivalent cce

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

cryptalgal metazoan bioherms of early ordovician age in the st george group western newfoundland bioherms are common in the st george group a sequence of shallow water carbonate rocks deposited on the western continental shelf of iapetus ocean they range from small heads and metre sized mounds to extensive banks and complexes many metres thick and hundreds of metres in lateral extent the cores of these bioherms are principally composed of thrombolites unlaminated branching columnar stromatolites structures quite distinct from laminated stromatolites which are common in intertidal beds associated with thrombolites is a diverse fauna of burrowing invertebrates trilobites nautiloids pelmatozoans brachiopods gastropods rostroconchs and archaeoscyphiid sponges on the basis of framework building components three main bioherm types are distinguished 1 thrombolite mounds 2 thrombolite lichenaria or sponge mounds and 3 thrombolite lichenaria renalcis reef complexes the framework of the last is the most complex with abundant cavities and a demonstrably uneven growth surface of thrombolites corals and free standing renalcis heads walls and roofs some cavities were active sediment conduits while others were protected their roofs draped with renalcis and their walls coated by cryptalgal laminites these bioherms possess the attributes of shallow water ecologic reefs they span a critical time gap in the development of reefs the transition period from algal dominated bioherms of the precambrian and cambrian to the metazoan dominated bioherms of the middle ordovician and remaining phanerozoic

sedimentary evidence of a cambro ordovician orogenic event in the northwestern himalaya abstract the late proterozoic early paleozoic sequence of the tethys himalaya provides evidence of an orogenic episode close to the cambrian ordovician boundary late pan african event pre middle cambrian sediments were deposited on a terrigenous tidal flat gradually passing upward to a peritidal carbonate platform the succession shows an overall transgressive trend which is reproduced at smaller scale by second and third order coarsening upward cycles terminated by the landward shift of carbonate facies during highstands of sea level sandstone petrography suggests a passive continental margin setting for this part of the sequence in middle cambrian times the carbonate platform underwent gradual drowning and outer shelf slope shales with intercalated tuffaceous material erupted from a nearby immature volcanic arc were deposited the shales are overlain by basinal turbidites attesting to the growth of a sandstone lobe enclosed in lobe fringe siltstones in the middle ordovician unconformable alluvial fan conglomerates and braid plain sandstones derived from uplifted sedimentary sequences indicate erosion of a newly formed orogenic belt comparison with early paleozoic and recent counterparts allows to interpret the cambro ordovician sedimentary evolution in terms of foreland basin development the thermal subsidence of a passive margin temporarily reduced by the passage of a peripheral bulge was enhanced strongly by flexural downwarping towards a subduction zone collision of a buoyant continental margin at the trench hampered subduction rapid uplift ensued with formation of a foreland fold thrust belt shedding sedimentary detritus into the adjacent molasse basins

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

controls on cenozoic exhumation of the tethyan himalaya from fission track thermochronology and detrital zircon u pb geochronology in the gyirong basin area southern tibet the gyirong basin southern tibet contains the record of miocene pliocene exhumation drainage development and sedimentation along the northern flank of the himalaya the tectonic controls on basin formation and their potential link to the south tibetan detachment system stds are not well understood we use detrital zircon zft and apatite aft fission track analysis together with detrital zircon u pb dating to decipher the provenance of gyirong basin sediments and the exhumation history of the source areas results are presented for nine detrital samples of gyirong basin sediments aft zft and u pb two modern river sediment samples zft and aft and six bedrock samples zft from transect across the gyirong fault bounding the basin to the east the combination of detrital zircon u pb and fission track data demonstrates that the gyirong basin sediments were sourced locally from the tethyan sedimentary sequence this provenance pattern indicates that deposition was controlled by the gyirong fault active since 10 ma whose vertical throw was probably 5000 m rather than being controlled by normal faults associated with the stds the detrital thermochronology data contain two prominent age groups at 37 41 and 15 18 ma suggesting rapid exhumation at these times a 15 18 ma phase of rapid exhumation has been recorded widely in both southern tibet and the himalaya a possible interpretation for such a major regional exhumation event might be detachment of the subducting indian plate slab during the middle miocene inducing dynamic uplift of the indian plate overriding its own slab

tectonostratigraphic subdivisions of the himalaya a view from the west 1 indian plate rocks in the central himalaya have traditionally been divided into orogen parallel fault bound tectonostratigraphic zones a straightforward westward extrapolation of these zones has proved problematic in part because of a lack of consensus on the existence or significance of major faults within the metamorphic zone of the indian plate in pakistan where more than 10 locations for the main central thrust mct have been proposed we address this ambiguity by systematically tracing established central himalayan tectonostratigraphy around the western himalayan syntaxis and across pakistan this exercise reveals the following stratigraphic and structural relationships 1 there is a westward decrease in neogene shortening across the himalayan fold and thrust belt such that there is no age equivalent thrust in pakistan with displacement and metamorphic juxtaposition equivalent to the central himalayan mct 2 shortening across the fold and thrust belt in western pakistan is concentrated in the unmetamorphosed foreland as opposed to the metamorphic zone in the central himalaya 3 lesser himalayan higher himalayan and tethyan rocks are in stratigraphic order within the metamorphic zone of pakistan which appears to be the metamorphic equivalent of kashmir tethyan stratigraphy 4 the combination of early paleozoic and late paleozoic tectonism in pakistan has locally eliminated upper proterozoic higher himalayan rock and lower to middle paleozoic tethyan rock from the metamorphic zone of pakistan 5 late cretaceous and or early paleocene proto himalayan deformation in the pakistan foreland telescoped and eroded stratigraphy prior to the main phase of himalayan orogeny 6 tectonostratigraphic zones are offset in eastern pakistan by the transverse jhelum balakot fault 7 there is no evidence within the indian plate of pakistan for a large scale normal fault system comparable to the south tibetan detachment system 8 stratigraphy as well as the age and tectonic setting of deformation and metamorphism must be taken into account when drawing tectonostratigraphic zones

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

geologic and u pb geochronologic evidence for early paleozoic tectonism in the dadeldhura thrust sheet far west nepal himalaya abstract the dadeldhura thrust sheet inm western nepal consists of proterozoic lower paleozoic sedimentary and plutonic rocks and their metamorphic equivalents that rest structurally on proterozoic strata of the lesser himalayan sequence although regional metamorphism and ductile deformation were widespread during tertiary thrust emplacement relicts of early paleozoic tectonism are preserved locally new field and geochronologic studies together with the findings of previous workers indicate that this early paleozoic tectonism included 1 regional metamorphism to at least garnet grade 2 regional folding of a thick metamorphic sequence into a broad east west trending syncline 3 outcrop scale folding of metasedimentary rocks 4 emplacement of cambro ordovician granitic bodies during and after the metamorphism and deformation 5 uplift and erosion of the metamorphic sequence with garnet grade rocks locally exposed at the surface and 6 derivation of ordovician conglomeratic sandstones from the early paleozoic orogen similar records of metamorphism deformation and uplift erosion have been found in other regions of the himalaya indicating that rocks of the dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system future tectonic models of himalayan orogenesis must accommodate this early paleozoic event