on the tectonics of the himalaya and the tibet plateau new data for the gravity anomalies and the uplift rate in the himalaya and the tibet plateau have imposed strict constraints on acceptable models for present tectonic processes in this region through two dimensional finite element modeling incorporating realistic rheology for the crust and upper mantle a model is found which satisfactorily predicts both the observed uplift rate and the changes of the gravity anomalies this model shows that 1 the himalaya is dynamically supported 2 the northern margin of the indian plate underthrusts the himalaya and at the same time the entire tibet thickens at a rate of 2 mm yr 3 the partition of the total shortening across the region is about 15 in the ganges basin 30 in the himalaya and 55 in tibet and 4 the extent of deformation reaches beyond the northern boundary of tibet

models of aggradation versus progradation in the himalayan foreland a frequent goal of decompaction analysis is to reconstruct histories of basin subsidence and tectonic loading in marine environments eustatic and paleobathymetric uncertainties limit the resolution of these reconstructions whereas in the terrestrial basins these ambiguities are absent it is still necessary to account for depositional slopes between localities in order to analyze three dimensional patterns of subsidence we define two end members for depositional surfaces aggradation and progradation the relative importance of either end member is a function of the interplay between the rate of net sediment accumulation and the rate of basin subsidence the models predict the patterns of major drainages transverse versus longitudinal and the way in which provenance should be reflected within different portions of a basin consequently paleocurrent and provenance data from the ancient stratigraphic record can be used to distinguish between these endmembers the subhorizontal depositional surfaces that dominate during times of aggradation provide a well defined reference frame for regional analysis of decompacted stratigraphies and related subsidence depositional slopes during progradation can not be as precisely specified and consequently yield greater uncertainties in reconstructions of subsidence these models are applied to the mio pliocene foreland basin of the northwestern himalaya where sequences of isochronous strata have been analyzed throughout the basin these time controlled data delineate a distinctive evolution from largely aggradational to largely progradational depositional geometries as deformation progressively encroaches on the foreland such a reconstruction of past depositional surfaces provides a well constrained reference frame for subsequent integration of subsidence histories from throughout the foreland

fluvial facies model of the ganga river sediments india abstract the river ganga flows from the northwest to the southeast following the lineaments developed in response to the himalayan tectonics ganga sediments show a large number of facies developments throughout the study area from hardwar to kachhla the active channel deposits are marked by the development of large scale facies in gravel and coarse sand small scale facies comprising fine sand silt and clay are developed in inactive channel deposits and overbank deposits the depositional model shows a progressive decrease in particle size thickness of units and scale of cross bedded sets from the lower to the upper parts of sequences as well as from upstream to downstream the sedimentation of the ganga basin is seasonally and tectonically controlled

configuration of late archaean chilimanzi and razi suites of granites south central zimbabwe craton from gravity modelling geotectonic implications the subsurface geometry of five representative late archaean chilimanzi and razi suite plutons in the zimbabwe craton zc has been investigated by gravity modelling constrained in part by surface geology density measurements and seismic information to determine their 3d configuration and infer tectonic context of emplacement the generally k rich massive homogeneous monzogranites are characterised by large bouguer gravity lows up to 30 mgal amplitude whose gradients outline their spatial extent well the southernmost plutons and their anomalies have general trends paralleling the north marginal zone nmz of the limpopo orogenic belt lb predictive gravity models indicate that the density contrast of the chivi batholith cb adjacent to the volcanic arc like belingwe greenstone belt extends to a depth of about 13 km the nearby razi pluton rp which intrudes the zc lb boundary appears to have been emplaced at shallower depths levels the gravity model suggests a thickness of about 5 6 km and a moderate to shallow dip to the southeast under the nmz compatible with syn kinematic intrusion during overthrust of the lb over the zc the smallest nalatale granite ng is on average 2 5 km thick under the fort rixon greenstone belt but includes a root up to 4 5 km thick under the anomaly peak and a steep contact with the tonalite gneiss to the east these granites follow the general power law for pluton dimension and are similar in this respect to the classical wedge shaped plutons extending largely in one direction with large aspect ratios length l thickness t 7 however the overall shape of the rp is typical of a diapir width w 5 l t 15 overall the cb and the zg are interpreted as massive deep rooted batholithic intrusions l t 10 contrary to some geological interpretations of these late post kinematic intrusions as sheet like bodies emplaced at relatively shallow levels in the crust on the other hand the chb appears to be a tabular intrusion probably fed by dykes it exhibits a lateral extent much greater than the vertical one outlining a sheeted geometry w t 7 l t 18 the geophysical evidence together with geological and fabric data support and or confirm the two main granite configurations sheets and batholith and thus also confirm the two main modes of emplacement dyke and diapirism or ballooning plutonism this is consistent with other known batholiths on the zc but considered unusual for plutons of the same age and spatially close when compared to other archaean cratons

the siwaliks of pakistan time and faunas in a miocene terrestrial setting magnetostratigraphy provides unparalleled chronologic precision for long terrestrial sedimentary records the siwaliks of the potwar plateau northern pakistan present perhaps the best example of a sequence of well dated faunal horizons spanning most of the neogene in a single terrestrial biogeographic province temporal control on fossil localities tied to multiple paleomagnetic sections is ca 0 1 m y less given special conditions of short magnetozone duration or superpositional relationships new correlations are presented for two long sections in the lower siwaliks showing the power and the limitations of mag netostratigraphic correlation within the siwalik depositional setting correlation of potwar fossil localities to those of an east african section also dated paleomagnetically are established with fossil localities from those two biogeographic provinces shown to be coeval on a scale of 10 5 yr the composite biostratigraphy rendered by the thick well exposed well dated potwar siwalik

climatic versus tectonic controls of fan sequences lessons from the dead sea israel tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated the pleistocene fans of the dead sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin exposures are excellent the rivers are currently incising in response to the low lake levels of the holocene producing cliffs 50m high these reveal an interdigitation of lake deposits dominantly fine grained carbonates and the coarse river gravels which characterize the fan deposits the bounds of each facies are sharp river sediments are superimposed on lake sediments with little evidence of either reworking or loading in turn aragonite crusts are draped over alluvial gravels indicating the sudden return of still water conditions neither can be explained by gradual adjustment of the hinterland river system after tectonic shock instead it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment in interpreting basin development in ancient settings it is always tempting to assume that coarse deposits are indicative of tectonism however the dead sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence

stratigraphic sequences and their chronostratigraphic correlation abstract the exxon sequence models of posamentier and coworkers incorporate two concepts which i question they suggest that rivers grade to a bayline but evidence from modern major rivers indicates that they do not they also proposed that fluvial sedimentary accomodation space is generated by the seaward shift in equilibrium profiles during sea level fall but this is too simplistic a model based on the use of schematic continental margin configurations which do not incorporate real slope and relief data in most cases a fall in base level leads to deltaic progradation and upstream fluvial incision most coastal plain fluvial wedges result from two processes sea level rise with the drowning and filling of estuaries and active source area tectonism major tectonic events adjacent to foreland basins such as terrane collisions lead to the development of sequences of third order type duration approximately 1 15 m y changes in intraplate stress regimes resulting from changes in regional plate kinematics have a similar episodicity and can be shown to be responsible for some third order cycles on extensional continental margins examination of the chronostratigraphic basis of the mesozoic cenozoic cycle chart of haq and coworkers reveals internal inconsistencies and an unrealistic downplaying of the built in potential for error this chart is not an approved global standard and it should be used with extreme caution

neogene mammalian faunal change in southern asia correlations with climatic tectonic and eustatic events the fluvial neogene siwalik formations of northern pakistan span long time intervals with only minor hiatuses and being highly fossiliferous are uniquely suited for studies of change in mammalian faunas magnetostratigraphic correlations of a critical stratigraphic section give dates for 45 middle and late miocene biostratigraphic events these mark either first appearances or extinctions in the mammal fauna and show that in the siwaliks there were major fauna turnovers at between 20 and 16 ma and at 9 5 and 7 4 ma two minor faunal events are dated at 13 2 and about 12 ma many species making their first appearance were immigrants from europe or africa and indicate when connections to those regions existed immigration and extinction were the dominant modes of faunal change in situ evolution was much less important the siwalik biostratigraphic record correlates closely to climatic oceanographic and tectonic events which probably controlled immigration into southern asia abiotic events were therefore important factors affecting evolution of the mammal communities

evolution of the lithosphere in pakistan abstract the geological setting of pakistan in the framework of the modern concept of plate tectonics is unique in the sense that within an area of about 800 000 km 2 critical tectonic junctions of different interacting plates and microplates are present in an environment where field exposures are excellent here we discuss the dynamics of these various plate boundaries two types of active plate boundaries are conspicuous 1 convergent boundaries characterized by continent continent collision obduction and thrusting in the northern region of the himalaya and by oceanic crust subduction with a volcanic arc and a wide accretionary wedge in the southern region of chagai and makran 2 a transform boundary the chaman transform zone characterized by very large strike slip and lesser thrusting the chaman transform zone connects the makran convergence zone where oceanic lithosphere is being subducted beneath the lut and afghan microplates with the himalayan convergence zone where the indo pakistan lithosphere is underthrusting eurasia the chaman zone is at present an intracontinental plate boundary with oblique motion characterized by north south strike slip faults and eastward thrusting and folding in the kirthar sulaiman mountain belt this mountain belt the northwestern margin of the indo pak subcontinent was an atlantic type margin from the late paleozoic until the cretaceous in the cretaceous the continental margin became a plate boundary a thrust belt was formed in the paleocene and fragments of the oceanic crust were obducted either as thrust sheets muslimbagh or as an ophiolitic melange bela and waziristan

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

evolution of catskill upper devonian river systems intra and extrabasinal controls abstract the frasnian catskill magnafacies of the catskill region new york state represents alluvium deposited in a foreland basin adjacent to the acadian fold thrust belt facies characteristics vary in space and time in association with a series of transgressive regressive sequences on the order of 102m thick away from the paleoshoreline and upsection the channel belt sandstone bodies within a given regressive sequence show 1 an increase in mean grain size from fine to medium and coarse sand 2 an increase in amount of large scale cross stratification relative to small scale cross stratification and planar stratification 3 a decrease in the inclination of lateral accretion surfaces and more common truncation of their upper parts by chute channel fills 4 an increase in th proportion of sandstone filled channels relative to those filled with sandstone plus mudstone the overbank sandstone mudstone beds concomitantly increase in mean grain size the proportion of channel belt deposits relative to overbank deposits increases slightly with distance from the paleoshoreline and the degree of overlap connectedness of channel belt sandstone bodies increases markedly paleohydraulic analysis shows that channels had low sinuosity less than 1 25 and were predominantly unbraided channels ranged from tens to hundreds of meters wide and were meters in depth channel slopes ranged from approximately 0 0001 to 0 0003 increasing with mean grain size of channel deposits but decreasing as bankfull discharge increased bankfull mean velocity varied little from 0 5 to 0 7 m s with distance from the paleoshoreline channels decreased in sinuosity and increased in sediment size slope and braiding index increase in proportion and connectedness of channel belt deposits with distance from the paleoshoreline probably reflects upvalley increase in channel belt width floodplain width ratio rather than variation in deposition rate avulsion frequency ratio du ing regression and transgression these spatial facies changes are reflected in respectively coarsening upward and fining upward vertical sequences eustasy probably had a minor influence on frasnian regressive transgressive sequences compared to the effects of varying rates of tectonic uplift sediment supply and subsidence there is no evidence of long term climatic change either in the distribution of river channel geometries or in the distribution of paleosols and plant and animal activity on the extensive floodplains

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

isochronous fluvial systems in miocene deposits of northern pakistan a palacomagnetic isochron dated at about 8 1 myr bp and detailed lithostratigraph of a 40 m interval exposed along strike for 40 km establish the depositional patterns of two contemporaneous interfingering fluvial systems in the upper part of the meddle siwalik sequence the two systems referred to as the buff and blue grey differ in unit shape lithofacies bedding sequence palaeocurrent direction and sand composition interfingering occurs along the south west north east strike of the outcrops with the palaeodrainage directions of the two systems generally perpendicular to this line the axis of the blue grey system which deposited widespread sheet sands and silts lay toward the south west end of the study area the more complex axis of the buff system which deposited shoe string sand bodies and lage volumes of silt and clay lay toward the north east the source area for both systems was the rising himalyan belt to the north and noth east of the study area at maximum extent the blue grey system occupied a channel belt at least 25 km wide channel belt widths and depths for the buff system are 1 3 km and 3 7 m respectively current directions averge 94 for blue grey sands and 136 for buff sands blur grey sands contain 20 more rock fragments and are otherwise less mature than buff sands the buff system shows a verticla pattern of avulsion palaeosol formation and floodplain aggradation which we attribute to autocyclic processes of parallel rivers the blue grey system shows phases of erosion accompaniced laterally by plaeosol formation folowed by valley fill and overfowing of interfluve surfaces theis pattern may be caused by allocyclic presses affecting the source area we interpret the blue grey system as a major drainage from the interior himalayas perhaps the ancestral indus and the buff system as a complex of smaller drinages along the mountain front which were probably ributaries to the bluegrey syste vertebrate fossils including hominoid primates from the area are almost exclusively associated with lithofacies of the buff system and this probably refects both taphonomic and palaeoecological differences between the two systems

record of sea level fall in shallow water hemipelagic strata case study and numerical modelling terra nova 00 1 7 2010 abstract hemipelagic rhythmites of the bohemian cretaceous basin and two dimensional numerical modelling of coexisting hemipelagic and siliciclastic depositional systems reveal important details on hemipelagic sequence stratigraphy lowstand systems tracts of c 400 ka sequences are marked by distinct up to 100 ka phase lags between maxima in bottom shear stress omission surfaces and maxima in siliciclastic deposition mudstone intercalations in the shallowest zone of this hemipelagic setting importantly sediment redistribution caused by local differences in bathymetry induces up to 100 ka shifts in lithological markers thus changing the hemipelagic expression of sea level falls both results underscore the frequently neglected fact that lithological and geochemical data on siliciclastic fluxes e g quartz contents ti al provide nonlinear proxies of eustatic and relative sea level changes hemipelagic records of climatic and orbital forcings can be markedly distorted in phases and magnitudes two dimensional modelling can be instrumental in extracting unbiased signals

paleomagnetism of the middle siwalik formations of northern pakistan and rotation of the salt range decollement abstract sampling for paleomagnetic study has been carried out at seven localities in the potwar plateau region of pakistan in the vicinity of the salt range in all 253 sites consisting of three samples per site were studied the sites were taken from the chinji nagri dhok pathan and upper siwalik formations which span the last 14 m y of earth history the formations have been deformed in the ongoing himalayan orogeny and are exposed today in sections which dip at angles of from 10 to 90 in different directions a characteristic direction of magnetization was isolated at all sections by thermal demagnetization at temperatures of from 550 to 650 c the sediments have been folded within the last 2 m y and because of a positive fold test it can be shown that the characteristic magnetization of these sediments was acquired before folding four of the sections have directions of magnetization which have been rotated in a counter clockwise fashion up to 40 however sections taken along the indus river the north flank of the soan syncline and the south flank of the salt range show small counter clockwise rotations of under 10 these observations are in agreement with previous suggestions that strata in the salt range have been moved to the south and rotated in a counter clockwise fashion the observations reported here indicate that the paleomagnetic poles derived from cambrian and permian formations within the salt range have probably also suffered rotation and should be removed from consideration in gondwanaland reconstruction of the paleozoic

shifting of the kosi river northern india the kosi river shifts laterally over the himalaya foreland plain by continual minor cutoffs and bank cutting and by episodic major shifts across watersheds by moving into and then out of preexisting adjacent less actively aggrading streams migration is unidirectional because after a channel is filled to instability floodwater will drain preferentially into a new adjacent low rather than across it to the next watershed or back to the last abandoned channel major shifts seem stochastic and autocyclic they do not correlate with the many severe quakes and floods that undoubtedly helped prime the system for shifts

changes in paleocurrents during the development of an obliquely convergent plate boundary sulaiman fold belt southwestern himalayas west central pakistan abstract pre orogenic cretaceous shelf sandstones at the northern and southern ends of the sulaiman range in west central pakistan show basically westward dispersal which is off the craton however paleocene marine sandstones in the south spread east southeast and early eocene deltaic sandstones in the north spread southeast and north these are mostly onto the craton and indicate slope reversal of the cretaceous shelf post oligocene fluvial molasse shows mostly southward drainage a 3852 m section of molasse in the northern sulaiman range comprises a lower 2378 m of claystones and upwardly fining sandstones that represent meandering rivers and an upper 1474 m that are dominated by conglomerates that represent gravelly braided rivers paleocurrents in the lower meandering part of the section were generally southward but very variable whereas those in the upper braided part were more uniform and show eastward flow a 3146 m section of molasse in the southern sulaiman range comprises 1 a basal 1160 m sequence of claystones and sandstones meandering streams with very diverse flow directions overall to the southwest 2 a middle 1603 m sequence dominated by pebbly sandstone with large bedforms big sandy braided rivers with less diverse flow to the southwest and 3 an upper 383 m conglomeratic sequence gravelly braided streams that still flow uniformly southwest rivers in the early molasse paralleled the modern sulaiman range in the north and flowed obliquely into it in the south which suggests no uplift of the modern front of the range until the onset of modern transverse eastward drainage and the associated influx of conglomerates in present and admittedly uncertain lithostratigraphic terms uplift of the modern front of the range apparently began about 700 m lower in the north than in the south the northern and southern molasse sections otherwise show broadly similar histories of river development

the chronology of intermontane basin development in the northwestern himalaya and the evolution of the northwest syntaxis abstract the northwest syntaxis delineates a complex zone where the northwesterly trending himalayan ranges meet the northeasterly trending hindu kush and indus kohistan ranges the southern margin of the hindu kush himalayan collisional belt is delineated by a series of imbricate thrusts that transect the northern edge of the modern indo gangetic foredeep the kashmir and peshawar basins are embedded in this still developing thrust belt and are symmetrically oriented about the northwest syntaxis consideration of chronologic stratigraphic structural and geophysical data from the syntaxial zone permits the construction of a model for intermontane basin development and the evolution of the northwest syntaxis during the late cenozoic the formation of the kashmir and peshawar basins results from the transfer from the north of the locus of thrusting and uplift to the southern margins of the basins during the pliocene the morphotectonic emergence of the ancestral pir panjal and attock ranges along the southern margins of the two basins coincides with changes in the patterns of sedimentation and deformation both within the basins and in the bounding foredeep to the south contrasting styles of tectonic deformation on opposite sides of the syntaxis are interpreted as a response to differences in the strength of sediment basement coupling across the syntaxis

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

stratigraphic modeling of foreland basins interpreting thrust deformation and lithosphere rheology the authors incorporate the processes of erosion and deposition in a numerical model to predict the stratal geometries and facies patterns produced during episodic thrusting in a nonmarine foreland basin the resultant stratigraphic record is characterized by a stairstepped facies package in which each retrogradation of facies toward the thrust marks the onset of a thrusting event the retrogradation of facies coincides with the migration of the forebulge toward the thrust and the generation of an erosional unconformity in the past changes in basin wavelength during basin evolution have been interpreted to record viscoelastic relaxation of the lithosphere this model suggests that changes in basin wavelength are a natural consequence of the interplay between thrust and sediment loading on an elastic lithosphere

fining upward cycles in the sandy braided river deposits of the westwater canyon member upper jurassic morrison formation new mexico abstract vertical and lateral profiles demonstrate that fining upward cycles are present in the sandy braided fluvial units of the westwater canyon member an upper jurassic morrison formation sheet sandstone in northwestern new mexico the technique of bounding surface classification successfully delineates these cycles on three scales of sedimentary influence namely the macroform scale cycle the channel scale cycle and the sheet scale cycle autocyclic macroform scale cycles are generally 1 6 m in thickness and bounded above and below by 3rd or higher order surfaces one of which must be of 3rd or 4th order channel scale cycles deposits infilling large scale scours and possibly individual channels are 1 10 m in thickness bounded above and below by laterally inextensive 5th or 6th order surfaces sheet scale cycles in contrast are 4 16 m in thickness the products of autocyclic channel avulsion processes bounded both above and below by laterally extensive planar 5th or 6th order surfaces a proposal by previous workers that lithofacies cyclicity also exists in the westwater canyon member is examined and rejected following field work and markov chain analysis further theoretical discussion of markov chain analysis points out several problems with the identification of cycles including the need for proper definition of the bounding surfaces that mark lithofacies transitions

astronomical forcing of meandering river behaviour milankovitch cycles in devonian of east greenland abstract a more than 1100 m thick succession dominated by single and multistorey sandstone bodies from the upper devonian kap graah group in east greenland is interpreted as a meandering river and associated overbank deposits the sandstone bodies display well developed lateral accretion bedding and are interpreted to be deposited as point bars in a well exposed section 215 m thick individual sandstone bodies have been analized quantitatively in order to compare the individual palaeochannel characteristics the results indicate cyclic variations in discharge channel slope and channel size the cycles are of the order of c 20 m thick with superimposed c 100 m mega cycles both are interpreted as reflecting cyclic variations in precipitation rate the cycles are interpreted as being governed by astronomical forcing of the climate reflecting milankovitch type precession cycles in the order of 20 000 years with modulation by c 110 000 years eccentricity mega cycles this study confirms that the precession and eccentricity cycles exhibit the main forcing on low latitude monsoonal climate

a biostratigraphic zonation of the middle and upper siwaliks of the potwar plateau of northern pakistan abstract two biostratigraphic reference sections are designated for the middle and upper siwalik formations of northern pakistan the stratigraphic ranges of 41 mammalian taxa are established in the reference sections and the two sections are correlated to each other and to the standard magnetic polarity time scale by the magnetic polarity stratigraphy the resulting stratigraphic and magnetostratigraphic framework is used to define four contiguous biostratigraphic interval zones the proposed interval zones are bounded by five important faunal events which are identified in the reference sections the four proposed interval zones and the approximate ages in the reference sections of their lower and upper boundaries are 1 the hipparion s l interval zone 7 4 9 5 m y b p 2 the selenoportax lydekkeri interval zone 5 3 7 4 m y b p 3 the hexaprotodon sivalensis interval zone 5 3 2 9 m y b p and 4 the elephas planifrons interval zone 2 9 1 5 m y b p the paleomagnetic and biostratigraphic evidence indicates that the tatrot beds from which pilgrim developed his tatrot faunal zone are between 2 5 and 3 4 m y b p with a preferred maximum age of 2 9 m y b p the biostratigraphic evidence indicates the sethi nagri locality y311 the primary source of pilgrim s nagri faunal zone is in the hipparion s l interval zone the paleomagnetic evidence suggests an approximate age of 9 m y b p a major faunal event at 9 5 m y b p introduced equids suids and large giraffes from eurasia into the indain subcontinent a second more diffuse faunal event between 5 3 and 2 9 m y b p introduced several african taxa as well as equus and cervids from eurasia

a synthetic stratigraphic model of foreland basin development the processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy we present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry topography and facies patterns in the model steady crustal shortening occurs according to a wedgethickening model erosion and deposition follow a diffusive process and the lithosphere is compensated elastically erosion and deposition are controlled by the transport coefficients r of the diffusion equation for a range of thrust velocities and lithospheric rigidities transport coefficients are of order 102 103 m yr in the mountain belt these values are much higher than those derived from the study of scarp degradation in the sedimentary basin transport coefficients of order 104 m2 yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition rapid thrusting results in a narrow underfilled basin while slow thrusting results in a wide overfilled basin in addition by varying the erosional and depositional transport coefficients while holding other parameters constant we generate both overfilled and underfilled basins these results suggest that changes in the rate of thrust loading the climate or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere clearly to understand either the long term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata the manner in which a basin was filled must be considered we apply the model to the evolution of the modern sub andean foreland and find that an erosional transport coefficient of 3000 m2 yr and a depositional transport coefficient of 20 000 m2 yr successfully predict the observed basin geometry

controls on synorogenic alluvial fan architecture beartooth conglomerate palaeocene wyoming and montana late palaeocene uplift of the beartooth range in northwestern wyoming and southwestern montana generated the beartooth conglomerate along the eastern and northeastern flanks of the range systematic unroofing sequences and intraformational unconformities folds and faults in the conglomerate attest to deposition during uplift along the eastern flank at least three ancient alluvial fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies the two southern fans consist of 700 m of sedimentary clast conglomerate and subordinate sandstone dominated by hyperconcentratedflow and stream flow facies the next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud matrix supported debris flow facies as well as stream flow facies the northernmost depositional system consists of arkosic channellized fluvial conglomerate and sandstone overbank mudstone and crevasse splay sandstone units palaeocurrent data indicate eastward dispersal away from the beartooth range outstanding exposure of the beartooth conglomerate allows facies to be mapped on lateral photographic mosaics a seven fold hierarchy of bounding surfaces and enclosed lithosomes exists in the beartooth conglomerate first through fourth order surfaces are analogous to first through fourth order surfaces that recently have been documented in sandy fluvial facies with one exception sediment gravity flows are bounded by first order surfaces fifth order surfaces are either erosional e g lateral migration of fanhead trench or accretionary e g aggradation of fan surface during backfilling of trench and construction of lobes on lower fan during entrenchment on upper fan some fifth order surfaces coincide with intraformational angular unconformities and are thus the result of long term fanhead entrenchment following uplift of the upper part of the fan sixth order surfaces bound individual fan packages that are several hundred metres thick and 10 km2 in area the enclosed sixth order lithosomes are distinguishable in terms of petrofacies and lithofacies a single seventh order surface bounds the entire beartooth conglomerate lower order lithosomes are produced by intrinsic processes of fan construction fifth order lithosomes can be attributed to both extrinsic and intrinsic controls sixth and seventh order lithosomes are generated by extrinsic controls

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

laramide thrust generated alluvial fan sedimentation sphinx conglomerate southwestern montana the uppermost cretaceous lower tertiary sphinx conglomerate crops out over an area of approximately 20 km2 8 mi2 in the madison range of southwestern montana the sphinx consists of more than 1 000 m 3 300 ft of synorogenic boulder and cobble conglomerate derived from a late cretaceous laramide uplift that was located in the area presently occupied by the madison river valley palynological and radiometric age data indicate that the sphinx was deposited 75 58 ma and that thrusting and folding of the deposit had largely ceased by 56 ma compositions of sphinx clasts and paleocurrent data indicate that the sphinx was produced by uplift and unroofing of mesozoic and paleozoic rocks located on two thrust sheets to the west and southwest the lower s hinx was deposited on the distal portions of an eastward prograding alluvial fan system clast assemblages and lithofacies indicate that deposition of the middle sphinx was controlled by a combination of progradation in response to ongoing thrusting and an influx of resistant clasts derived from middle paleozoic carbonates in the source area deposition of the upper sphinx was probably controlled by source lithology as the influx of very coarse resistant clasts from middle and lower paleozoic carbonates overwhelmed the fan system s ability to organize its load of sediment by normal fan processes a preliminary facies model for thrust generated alluvial fan deposits predicts intraformational deformation cannibalization of proximal synorogenic fan facies and abrupt compositional breaks in response to episodes of thrusting

evolution of miocene fluvial systems in the himalayan foredeep through a two kilometer thick succession in northern pakistan abstract sedimentological variations through a kilometer thick coarsening upward succession in the miocene himalayan foredeep basin fluvial chinji and nagri formations of the siwalik group are documented in the chinji village area on the potwar plateau of northern pakistan from the chinji formation upwards into the nagri formation 1 the proportion of major sandstone bodies increase relative to mudstone dominated beds 2 average thickness and mean grainsize of channel deposits increase 3 sediment aggradation rates increase 4 there is a reported change in sediment providence 5 paleocurrents are generally to the southeast and vary little upsection and 6 paleosols are more distinct lower in the section where there is also more evidence of ponding of water on the floodplain smaller scale 100 m thick cyclic variations in the proportion of major channel sandstone bodies also occur in both formations in the modern himalayan basin sediment dispersal away from the mountain belt is associated with 1 major rivers that flow along the basin axis 2 large rivers that drain substantial areas of the mountain belt and deposit low relief sediment fans extending hundreds of kilometers into the basin and 3 smaller rivers in fan and interfan areas that drain local areas and carry finer sediment loads sediment variations through the chinji nagri formation boundary can be related to a shifting of depositional environments within such fan and interfan areas major basin axial rivers are not exposed in this area it appears that kilometer thick upsection variations in these deposits record the progradation of the sediment fan formed by a large river system river discharge on the order of 104 m3 s over the floodplain of a smaller river system river discharge on the order of 103 m3 s large scale vertical sediment variations over hundreds of meters to kilometers appear to reflect changes in the basin subsidence and the rate of sediment input to the basin over time scales on the order of 105 106 years upsection variations can not easily be related to climatic change or eustatic variations in sea level the most likely explanation for upsection changes is tectonism deposition rates and shifting positions of river systems within the basin responded more quickly to changes in basin subsidence rate whereas increases in the proportion of sandstone bodies and increases in floodplain slopes lagged behind deposit variations in the siwalik group clearly record variations between different scale river systems delivering sediment to the basin as well as the overall basin evolution

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

paleomagnetic chronology fluvial processes and tectonic implications of the siwalik deposits near chinji village pakistan a 2800 m section of siwalik strata containing the stratotypes for both the chinji and nagri formations has been dated by magnetic polarity stratigraphy and the observed polarity zonation has securely been correlated with the chron 17 7 segment of the time scale the base of the section is the base of the kamlial formation which occurs near the top of chron 17 18 3 m y the kamlial chinji formation boundary occurs in the middle of chron 15 14 3 m y the chinji nagri boundary near the bottom of chron 10 10 8 m y and the nagri dhok pathan boundary at the chron 8 9 boundary 8 5 m y the siwalik deposits near chinji village consist of four distinct classes of fluvial cycles each with a characteristic periodicity a first order of 10 7 years a second order of 10 6 years a third order of 10 4 10 5 years and a fourth order of 10 0 10 4 years the river system responsible for siwalik sedimentation flowed from west to east and had properties much like the modern ganges river syste

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

estimating the error of age interpolation in sedimentary rocks magnetostratigraphic data can provide information on rates of sediment accumulation within a single sedimentation system over time spans from 104 to 106 yr the short term rate of sediment deposition varies with time the apparent average rate over any longer interval also depends on the relative durations of periods of deposition stasis non deposition and erosion while the average rate can be used to infer the time of occurrence of an event from its stratigraphical position the inferred age has an uncertainty deriving from the variability in rate of sediment accumulation over all shorter timescales we analyse here variability in sediment accumulation rates provided by the magnetostratigraphy of miocene siwalik sediments from pakistan for long periods 106 yr sediment accumulation is approximately linear through time over short intervals 104 105yr however there is considerable variability to provide an error term for an absolute age interpolated between boundaries of polarity units we use a resampling technique similar to the statistician s bootstrapping we illustrate this approach by estimating a standard error for the interpolated age of a biostratigraphical datum the first appearance of hipparionine equids in the siwalik sequence near the town of khaur the first appearance of hipparion in the khaur sequence is 9 22 0 09 myr

distal alluvial fan sediments from the upper jurassic of portugal controls on their cyclicity and channel formation the lusitanian basin of west central portugai is an iberian atlantic margin basin during a late jurassic rifting event a series of alluvial fans distributed coarse basement derived clastics southeastward from the western margin of the basin the distal part of this sequence which is about 140 m thick consists of heterolithic massive sandstone and massive mudrock facies arranged in crude fining upwards cycles 1 15 m thick these form discrete packets of sediment in a strongly aggrada tional sequence and are interpreted as the products of sheetflooding as the active area of sedimenta tion on the fan surface switched channelized flow was not necessarily an early feature of cycle development considerable fluctuations in discharge occurred and were probably related to climatic factors large quantities of tractional and suspended sediment were transported at high stages and were rapidly deposited as flows waned caliche soil profiles indicate a seasonal semi and climate however other features such as widespread soft sediment deformation the lack of desiccation structures and the absence of evaporites suggests that areas of active sedimentation often remained wet three fining upward megacycles are distinguished these indicate rapid advance of fan toes triggered by tectonic activity followed by gradual retreat

paleomagnetic isochrons unsteadiness and non uniformity of sedimentation in miocene fluvial strata of the siwalik group northern pakistan in complement with stratigraphic and sedimentologic studies the time resolution provided by magnetic polarity stratigraphy is a powerful tool that can be used to identify assess and quantify both temporal variability unsteadiness in sediment accumulation over a particular stratigraphic sequence and spatial variability non uniformity in sedimentation across a depositional system chronostratigraphic variability is documented and evaluated in the miocene channel and floodplain sediments of the chinji formation part of the extensive siwalik foreland sequence located on the potwar plateau of northern pakistan magnetic polarity stratigraphies have been developed in multiple vertical sections correlated along strike allowing for paleomagnetic isochrons to be traced throughout the sequence and providing a chronologic framework in which to identify unsteadiness and non uniformity in patterns of sediment accumulation analysis of variability over different time scales is accomplished by constructing simu

depositional response to foreland deformation in the carboniferous of eastern kentucky the appalachian basin is a typical foreland basin that has subsided episodically under the loads of successive thrust sheets the dynamics of basin subsidence and upwarping of basin margin arch systems reflects the flexural properties of the lithosphere quantitative models of the lithosphere depend on assumed rheological properties this study attempts to test these geodynamic models in a part of the appalachian basin mississippian deposition is correlated with waning acadian tectonism orogenic quiescence coincided with basin overdeepening and uplift of the cincinnati arch or forebulge although the foreland basin was mudstone dominated shoaling along the crest of the basin margin arch system resulted in reworking and wedges of unconformities in contrast the onset of alleghenian orogenesis during the pennsylvanian is reflected in a thick wedge of terrigenous clastic sediments the pennsylvanian basin fluctuated between underfilled restricted marine and overfilled nonmarine conditions rapid response of the basin to thrust sheet loading is inferred these characteristics of mississippian and pennsylvanian deposition support a viscoelastic model of the lithosphere