implications of flow control by the three gorges dam on sediment and channel dynamics of the middle yangtze changjiang river china the impacts of a dam on the river downstream in terms of hydrology and morphology are determined by a complex mix of variables that includes the patterns of release of water through the dam and the characteristics of the downstream channel scour of the downstream channel is a common response because large dams cause a significant interruption to sediment continuity here we show that in the case of china9s three gorges dam on the yangtze river the outcome is complicated as is commonly the case in large rivers the downstream channel and floodplain system compose an area of long term sediment accumulation and unstable channels with seasonally contrasting erosion and deposition patterns related to the migrating seasonal monsoon rainfall zones in achieving one of the main purposes of this dam that of flood control in the middle and lower basins the pattern of flows released from the dam will closely resemble those seasonal flows that are responsible for channel instability in the middle catchment thus effectively making erosive conditions the most common during a year there is obviously concern about the ultimate impact of sediment storage in the dam on the dynamics of the delta and adjacent coast and we show that this depends on the trajectory and duration of the erosive responses in the middle yangtze basin in this particular case the outcome is of great significance to the well being of the densely populated riparian areas of the river

seasonal variations of sediment discharge from the yangtze river before and after impoundment of the three gorges dam abstract over the past decades 50 000 dams and reforestation on the yangtze river changjiang have had little impact on water discharge but have drastically altered annual and particularly seasonal sediment discharge before impoundment of the three gorges dam tgd in june 2003 annual sediment discharge had decreased by 60 and the hysteresis of seasonal rating curves in the upper reaches at yichang station had shifted from clockwise to counterclockwise in addition the river channel in middle lower reaches had changed from depositional to erosional in 2002 during the four years 2003 2006 after tgd impoundment 60 of sediment entering the three gorges reservoir was trapped primarily during the high discharge months june september although periodic sediment deposition continues downstream of the tgd during most months substantial erosion has occurred supplying 70 million tons per year mt y of channel derived sediment to the lower reaches of the river if sand extraction 40 mt y is taken into consideration the river channel loses a total of 110 mt y during the extreme drought year 2006 sediment discharge in the upper reaches drastically decreased to 9 mt only 2 of its 1950 1960s level because of decreased water discharge and tgd trapping in addition dongting lake in the middle reaches for the first time changed from trapping net sediment from the mainstem to supplying 14 mt net sediment to the mainstem severe channel erosion and drastic sediment decline have put considerable pressure on the yangtze coastal areas and east china sea

lhasa terrane in southern tibet came from australia the u pb age and hf isotope data on detrital zircons from paleozoic metasedimentary rocks in the lhasa terrane tibet defi ne a distinctive age population of ca 1170 ma with e hf t values identical to the coeval detrital zircons from western australia but those from the western qiangtang and tethyan himalaya terranes defi ne an age population of ca 950 ma with a similar e hf t range the ca 1170 ma detrital zircons in the lhasa terrane were most likely derived from the albany fraser belt in southwest australia whereas the ca 950 ma detrital zircons from both the western qiangtang and tethyan himalaya terranes might have been sourced from the high himalaya to the south such detrital zircon connections enable us to propose that the lhasa terrane is exotic to the tibetan plateau system and should no longer be considered as part of the qiangtang greater india tethyan himalaya continental margin system in the paleozoic reconstruction of the indian plate as current models show rather it should be placed at the northwestern margin of australia these results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern tibet and indicate that the lhasa terrane evolved as part of the late precambrian early paleozoic evolution as part of australia in a different paleogeographical setting than that of the qiangtang greater india tethyan himalaya system

new paleomagnetic results from the ediacaran doushantuo formation in south china and their paleogeographic implications abstract a new paleomagnetic pole is obtained from the top of member 3 of the doushantuo formation in the jiulongwan section of the yangtze gorges area in south china a total of 191 samples from 23 sites were collected and subjected to stepwise alternating field af and thermal demagnetization after the removal of a soft component sc of viscous magnetic remanence acquired in present geomagnetic field a high temperature component hc likely carried by magnetite was isolated the hc includes vectors from 147 samples using principal component analysis and arc constraints from 44 samples using remagnetization great circle analysis both the vectors only and the combined vectors and arc constraints of the hc passed a reversal test this is the first ediacaran paleomagnetic remanence from the south china block scb that passes a reversal test and it is interpreted as a primary remanence the hc vectors were thus used to calculate the paleomagnetic pole and in combination with arc constraints were used for polarity interpretation the vectors give a mean direction of d 75 2 i 41 0 95 2 5 n 147 after bedding correction and a corresponding paleomagnetic pole at 23 9 n 187 0 e dm dp 3 0 1 8 this pole differs significantly from previously published results of the doushantuo formation which may have been subjected to paleozoic remagnetization our results provide a 23 5 1 8 n paleolatitude for the yangtze gorges area of the scb and suggest several alternative paleogeographic options for the ediacaran cambrian amalgamation of east gondwana depending on chronostratigraphic correlations

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

velocity analysis using weighted semblance increasing the resolution of semblance based velocity spectra or semblance spectra is useful for estimating normal moveout velocities as increased resolution can help to distinguish peaks in the spectra the resolution of semblance spectra depends on the sensitivity of semblance to changes in velocity by weighting terms in the semblance calculation that are more sensitive to changes in velocity we can increase resolution our implementation of weighted semblance is a straightforward extension of conventional semblance somewhat surprisingly we increase resolution by choosing an offset dependent weighting function that minimizes semblance we test our method on synthetic and field data and our tests confirm that weighted semblance provides higher resolution than conventional semblance

holocene development of the yellow river s subaqueous delta north yellow sea high resolution seismic profiles from the north yellow sea reveal a 20 40 m thick subaqueous clinoform delta that wraps around the eastern end of the shandong peninsula extending into the south yellow sea this complex sigmoidal oblique clinoform containing an estimated 400 km 3 of sediment overlies prominent relict transgressive surfaces the nearshore topset of the clinoform 50 m contain less than 1 m of holocene sediment with low sedimentation rates 1 mm year in contrast to other clinoforms the shandong clinoform appears to be a compound subaqueous deltaic system with what we interpret to be proximal and distal phases of clinoform development the underlying proximal sequence formed proximally between f11 and 9 2 ka in response to a temporary pause in the rapid postglacial sea level rise after the meltwater pulse 1b mwp 1b and increased discharge from the yellow river to the north yellow sea due to intensification of the summer monsoon a flooding surface appears to separate the proximal and distal phases corresponding to the next rapid sea level rise 9 5 9 2 ka bp mwp 1c since 9 2 ka bp an overlying distal sedimentary sequence has accumulated reflecting the backstepping and shifting river mouth westward to the gulf of bohai some inputs from coastal erosion and nearby small streams may be locally important along shore transport cross shelf advection and upwelling in the north yellow sea have reworked post lgm sediment and have helped maintain the morphology of the clinoform in the shandong mud wedge d 2004 elsevier b v all rights reserved

age provenance and tectonic setting of neoproterozoic to early paleozoic sequences in southeastern south china block constraints on its linkage to western australia east antarctica abstract zircon u pb hf isotopes and whole rock geochemical data are applied to investigate the age provenance and tectonic setting of the late neoproterozoic and overlying sequences from the cathaysia block of south china in order to understand the tectonothermal evolution of this block and its linkage to supercontinents new zircon u pb ages in combination with previous fossil data indicate the lower and upper louziba groups were deposited between 655 and 635 ma cryogenian and 635 and 542 ma ediacaran respectively low cia 65 57 and high icv 1 64 1 05 values for the louziba sandstones are suggestive of immature sediments that experienced mild to weak chemical weathering in their source areas in contrast early paleozoic sandstones have higher cia 80 69 and lower icv 0 95 0 84 values indicative of mature sediments with intensive chemical weathering despite the different degrees of chemical weathering all sedimentary samples show positive correlation between tio2 and al2o3 and negative correlation in plot of la yb n vs zr but lack any correlation between la yb n and p2o5 these data demonstrate sorting of resistant minerals such as rutile monazite and zircon did not affect the chemical signatures of sampled detritus consequently geochemical features in combination with detrital age spectra indicate that the louziba group and overlying sequences record sedimentation in collisional and passive settings with the detritus being derived dominantly from intermediate to felsic igneous rocks late mesoproterozoic ca 1300 1000 ma detritus in the lower louziba group were probably derived primarily from the windmill islands bunger hills 1300 1050 ma and albany fraser belt musgrave 1300 1050 ma regions in east antarctica western australia increasing input of early neoproterozoic 990 950 ma detritus in the uppermost louziba group indicates a greater contribution from the eastern ghats ca 990 950 ma in eastern india and north prince charles mountains ca 990 950 ma in east antarctica this source change was synchronous with the formation of the kuunga 560 530 ma and pinjarra 560 520 ma orogens during assembly of gondwana the sediments of the huanglian formation of the uppermost louziba group likely recorded this event overlying paleozoic sequences then were deposited on passive margin with detritus being derived dominantly from an early neoproterozoic source e g eastern india and east antarctica

stratigraphic sequence and episodes of the ancient huanghe delta along the southwestern bohai bay since the lgm abstract the sedimentary evolution of the ancient huanghe yellow river delta in china is not well documented due to frequent river avulsions and limited deep cores in this paper 22 cores with lengths of 20 m at the coastal plain along the southwestern bohai bay were used the sedimentary facies were determined by grain size and microfossil analysis and carbon 14 dating the strata in the study area since the last glacial maximum lgm are divided into lowstand system tract lst transgressive system tract tst highstand system tract hst and retrograding system tract rst the lst which formed during the lgm to 10 8 ka bp consists of hard mud layer and overlying interfluvial lowland and flood plain deposits the tst that formed during 10 8 8 5 ka bp mainly include coastal marsh onshore sand bars and filled incised valleys the hst that formed during 8 5 2 6 ka bp mainly includes flood plain river channel delta front mouth bar and prodelta the rst that formed after 2 6 ka bp mainly includes crevasse splay interfluvial lowlands and distributary channel about eight episodes of the ancient huanghe delta are observed during the holocene and the names of the deltas are given by corresponding location names of cheniers 1 during 10 8 8 5 ka bp around ys3 and np1 cores without formation of cheniers during the transgression 2 during 8 5 7 4 ka bp around ys1 ys4 and np3 cores the dongsun shuiyicheng delta 3 during 7 4 6 1 ka bp around ck1 core the mashanzi delta 4 during 6 9 5 7 ka bp around gz3 core the miaozhuang zhaizhuang delta 5 during 4 6 3 5 ka bp around ys4 core the wuditai delta 6 during 3 0 2 6 ka bp around hh4 and hh1 cores the qikou delta 7 during 2 6 1 9 ka bp around ys7 and np1 cores the nanpaihe delta and 8 during 1034 bc 1099 bc around wangzidao and dakouhe the wangzidao delta

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

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

introduction from snowball earth to the cambrian explosion evidence from china the neoproterozoic palaeozoic transition npt around 600 ma ago was a critical time interval when the earth experienced fundamental change manifested as climatic extremes snowball earth followed by the emergence and rapid diversification of animals cambrian explosion how animals and environments co evolved and what caused these fundamental changes to the earth system during the npt is a great scientific puzzle which has been a rapidly developing frontier of interdisciplinary research between bio and geosciences south china preserves a complete stratigraphic succession of the npt developed in various facies ranging from shallow to deep marine realms with extraordinarily well preserved successive fossil biotas in various taphonomic settings zhu 2010 fig 1 making it a key area and global focus of studies in the field over recent decades indeed the current narrative of early animal evolution has largely been based on the fossil biotas from south china these include 1 the world9s oldest microscopic animal fossils with cellular details from the early ediacaran weng9an biota doushantuo formation 2 putative macroscopic animal fossils preserved as carbonaceous imprints from the early ediacaran lantian wenghui and miaohe biotas also doushantuo formation 3 typical late ediacaran faunas preserved in dark limestone shibantan biota and as large and poorly mineralized tubular animal fossils gaojiashan biota both from the dengying formation 4 phosphatized small shelly and soft bodied animal fossils from the early cambrian meishucun and kuanchuanpu faunas and 5 cambrian fossil lagerstatten chengjiang guanshan and kaili faunas with typical burgess shale type soft bodied preservation

lowermost amazon river morphology and shallow seismic characteristics abstract the riverbed morphology of the lowermost amazon river was characterized by high resolution acoustic profiling in 21 areas this technique has been used for the first time in this area and allowed the discovery of a variety of reflectors which delineate areas either of sediment transport or of deposition on an older river bottom records obtained with the pinger system made it possible to distinguish three main kinds of reflectors both surface and sub bottom 1 flat 2 wavy and 3 irregular reflection patterns which can be interpreted as strata deposited in a variable mostly high energy setting or reworked structures of initially flat or wavy strata that have been changed mapping of these different reflectors shows that they are not randomly distributed the constant association of a given reflector with a particular morphological setting is indicative of its lithological significance supported by riverbed sampling and erosional depositional processes depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub bottom related to old substrates alter do chao or barreiras formation this may result from inundation and incision caused by quaternary sea level changes previously eroded areas are covered by a wavy layer consisting of very fine to fine sand unit fs which can change locally to medium coarse sand unit ms reflectors are more abundant downstream a minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge moreover the detection of fault scarps interpreted to result from neotectonic activity shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability the dynamic setting of the lowermost amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes the effect of the gurupa arch may result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part

nonstretching nmo correction of prestack time migrated gathers using a matching pursuit algorithm abstractwide azimuth long offset surveys are becoming increasingly common in unconventional exploration plays where one of the key routine processes is maintaining data fidelity at far offsets the conventional nmo correction that processes the data sample by sample results in the well known decrease of frequency content and amplitude distortion through stretch which lowers the seismic resolution and hinders and amplitude variation with offset and azimuth avaz analysis of the long offset signal to mitigate the stretch typically associated with large offsets we use a matching pursuit based normal moveout correction mpnmo to reduce nmo stretch effects in long offset data mpnmo corrects the data wavelet by wavelet rather than sample by sample thereby avoiding stretch we apply our technique 1 to a set of synthetic gathers and 2 as part of a residual velocity analysis workflow to a prestack time migrated data volume acquired over the northern chicontepec basin mexico test results show th

early crustal evolution in the western yangtze block evidence from u pb and lu hf isotopes on detrital zircons from sedimentary rocks abstract in situ u pb and lu hf data on detrital zircons from paleo to neoproterozoic sediments have been used to gain a clearer picture of the regional tectonic background and crustal evolution in the western part of the yangtze block the youngest concordant zircon ages for sedimentary units suggest maximum depositional ages of 1014 ma for the kunyang group 750 649 ma for the sinian sequences and 525 ma for the cambrian sediments the yinmin formation previously assigned to the middle part of the kunyang group was actually deposited after 1667 ma and contains zircons whose ages are dominantly from late archean to paleoproterozoic 2 8 2 7 ga 2 5 2 3 ga and 1 85 ga the heishantou formation in the lower part of the kunyang group has two major age populations of 1 0 ga and 1 8 1 6 ga the sinian and cambrian sedimentary rocks are dominated by neoproterozoic zircons with age peaks at 760 ma and 825 ma consistent with the ages of widespread igneous rocks around the yangtze block the lu hf isotope data suggest that a significant juvenile input took place during archean and neoproterozoic times respectively while crustal reworking was dominant during the paleoproterozoic time the archean paleoproterozoic detritus is isotopically distinct from the archean paleoproterozoic basement exposed in the northern part of the yangtze block suggesting that a subarea of old crust lies beneath the young sediments covering the craton the presence of abundant 1 85 ga zircons suggests that the yangtze block was probably part of the columbia supercontinent during paleoproterozoic time comparisons to the other parts of columbia suggest that the yangtze block could have been adjacent to the north china craton and or australia

earth science mainly in the plain the finding that global mass loss from landscapes is dominated by physical erosion and chemical weathering from flat terrain rather than from mountains challenges our understanding of how earth s surface evolves

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

geochronological constraint on the cambrian chengjiang biota south china the cambrian chengjiang biota of south china provides compelling fossil evidence for the rapid appearance of metazoan phyla in earth history the cambrian explosion however the timing of the chengjiang biota is poorly constrained owing to lack of datable rock materials within the maotianshan shale that yields the fossils here we integrate secondary ion mass spectrometry sims and chemical ablation isotope dilution thermal ionization mass spectrometry ca id tims u pb analyses of detrital zircons from the maotianshan shale to provide high precision geochronological constraint on the chengjiang biota the youngest group of sims u pb detrital zircon dates yields an age peak at 520 ma six zircons from this group are further dated by the ca id tims u pb technique but suggesting that they were not formed from a single zircon growth event thereby neither the age peak nor the weighted mean age defined by the youngest sims u pb dates could represent the maximum depositional age of the maotianshan shale instead the youngest ca id tims u pb date 518 03 0 69 0 71 ma 2 analytical uncertainty incorporates u pb tracer calibration uncertainty provides the first robust maximum age of the chengjiang biota this new geochronological constraint on the chengjiang biota indicates that the cambrian explosion reached its major phase around 518 03 0 69 0 71 ma demonstrating a protracted process 22 myr for the cambrian explosion supplementary material sims and ca id tims zircon u pb data are available at https doi org 10 6084 m9 figshare c 4024849

seismic anisotropy in trinidad a new tool for lithology prediction a great challenge facing explorationists is identification of potential intervals of reservoir quality sand in areas with little well control this is especially acute in deep water where reservoir presence can be elusive and well costs are considerable substantial evidence suggests the occurrence of transversely isotropic ti anisotropy in the subsurface associated primarily with shales while intervals of clastic material show greatly reduced and or near zero ti anisotropy although this has been demonstrated conclusively on core samples the need exists to translate these concepts into a geologic framework on a basinwide and prospect scale this article presents a method for estimating interval anisotropy from nonhyperbolic moveout on common midpoint gathers and relating these values to subsurface geology alkhalifah and tsvankin 1995 demonstrated that for ti media with a vertical symmetry axis vti media only two parameters are sufficient for performing all time related processing such as nmo correction including nonhyperbolic moveout correction if necessary dmo correction and prestack and poststack time migration one of these parameters is dimensionless its departure from zero implies anisotropy the other parameter vnmo is the short spread nmo velocity for horizontal reflectors these two parameters can be obtained solely from surface p wave data using estimates of stacking velocity for reflections from interfaces having two distinct dips alkhalifah and tsvankin 1995 vti media induce nonhyperbolic moveout of reflections that depends on vnmo and at large offsets alkhalifah 1997 showed that these parameters can be obtained by analyzing the moveout at far offsets the direct output from such an inversion or velocity analysis is the effective values of vnmo and that include the combined average influence of the overburden these effective values are converted to interval values using layer stripping techniques using data from offshore africa alkhalifah et al 1996 showed that anisotropic

sedimentation of overbank floods in the confined complex channel floodplain system of the lower yellow river china the channel boundary conditions along the lower yellow river lyr have been altered significantly since the 1950s with the continual reinforcement and construction of both main and secondary dykes and river training works to evaluate how the confined complex channel floodplain system of the lyr responds to floods this study presents a detailed investigation of the relationship between the tempo spatial distribution of sedimentation erosion and overbank floods occurred in the lyr for large overbank floods we found that when the sediment transport coefficient ratio of sediment concentration of flow to flow discharge is less than 0 034 the bankfull channel is subject to significant erosion while the main and secondary floodplains both accumulate sediment the amount of sediment deposited on the main and secondary floodplains is closely related to the ratio of peak discharge to bankfull discharge volume of water flowing over the floodplains and sediment concentration of overbank flow while the degree of erosion in the bankfull channel is related to the amount of sediment deposited on the main and secondary floodplains water volume and sediment load in flood season the significant increase in erosion in the bankfull channel is due to the construction of the main and secondary dykes and river training works which are largely in a wide and narrow alternated pattern along the lyr such that the water flowing over wider floodplains returns to the channel downstream after it drops sediment for small overbank floods the bankfull channel is subject to erosion when the sediment transport coefficient is less than 0 028 while the amount of sediment deposited on the secondary floodplain is associated closely with the sediment concentration of flow over the entire length of the lyr the situation of erosion in the bankfull channel and sediment deposition on the main and secondary floodplains occurred mainly in the upper reach of the lyr in which a channel wandering in planform has been well developed

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

horizon based semiautomated nonhyperbolic velocity analysis abstractwith higher capacity recording systems long offset surveys are becoming common in seismic exploration plays long offsets provide leverage against multiples have greater sensitivity to anisotropy and are key to accurate inversion for shear impedance and density there are two main issues associated with preserving the data fidelity contained in the large offsets 1 nonhyperbolic velocity analysis and 2 mitigating the migration nmo stretch current nonhyperbolic velocity analysis workflows first estimate moveout velocity vnmo based on the offset limited gathers then pick an effective anellipticity eff using the full offset gathers unfortunately estimating vnmo at small aperture may be inaccurate with picking errors in vnmo introducing errors in the subsequent analysis of effective anellipticity we have developed an automated algorithm to simultaneously estimate the nonhyperbolic parameters instead of directly seeking an effective stacking model the algorithm finds an interval model t

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential the mississippian barnett shale serves as source seal and reservoir to a world class unconventional natural gas accumulation in the fort worth basin of north central texas the formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce at present production is mainly confined to a limited portion of the northern basin where the barnett shale is relatively thick 300 ft 92 m organic rich presentday total organic carbon 3 0 thermally mature vitrinite reflectance 1 1 and enclosed by dense limestone units able to contain induced fractures the most actively drilled area is newark east field currently the largest gas field in texas newark east is 400 mi 2 1036 km 2 in extent with more than 2340 producing wells and about 2 7 tcf of booked gas reserves cumulative gas production from barnett shale wells through 2003 was about 0 8 tcf wells in newark east field typically produce from depths of 7500 ft 2285 m at rates ranging from 0 5 to more than 4 mmcf day estimated ultimate recoveries per well range from 0 75 to as high as 7 0 bcf efforts to extend the current barnett play beyond the field limits have encountered several challenges including westward and northward increases in oil saturation and the absence of lithologic barriers to induced fracture growth patterns of oil and gas occurrence in the barnett in conjunction with maturation and burial history data indicate a complex multiphased thermal evolution with episodic expulsion of hydrocarbons and secondary cracking of primary oils to gas

channel adjustments in response to the operation of large dams the upper reach of the lower yellow river the yellow river in china carries an extremely large sediment load river channel form and lateral shifting in a dynamic partly meandering and partly braided reach of the lower yellow river have been significantly influenced by construction of sanmenxia dam in 1960 liujiaxia dam in 1968 longyangxia dam in 1985 and xiaolangdi dam in 1997 using observations from huayuankou station 128 km downstream of xiaolangdi dam this study examines changes in the river before and after construction of the dams the temporal changes in the mean annual flow discharge and mean annual suspended sediment concentration have been strongly influenced by operation of theses dams observations of sediment transport coefficient ratio of sediment concentration to flow discharge at a station hydraulic geometry and bankfull channel form observed from 1951 to 2006 have shown that although variations in flow and sediment load correspond to different periods of dam operation changes in channel form are not entirely synchronous with these the channel has been subject to substantial deposition due to the flushing of sediment from sanmenxia dam resulting in a marked reduction in bankfull cross sectional area flows below bankfull had a greater impact on channel form than higher flows because of very high sediment load at a station hydraulic geometry shows that the variation of channel cross sectional area below bankfull in this wide and relatively shallow system largely depends on changes in width such at a station changes are significantly influenced by 1 events below bankfull and 2 overbank floods bankfull depth is the main component of channel adjustment in that depth adjusts synchronously with channel area the channel adjusts its size by relatively uniform changes in depth and width since 1981 channel morphology is not the product of single channel forming flow frequency it is determined by the combination of relatively low flows that play an important role in fine sediment transport and bed configuration as with relatively high flows that are effective at modifying the channel s morphology the sediment transport coefficient is a useful index for efficiently guiding the operation of the dams in a way that would minimize channel changes downstream sedimentation over the nearly 60 years of study period caused the lower yellow river to aggrade progressively the only significant exception being the two years following completion of sanmenxia dam

causes for continuous siltation of the lower yellow river previous studies indicate that aggradation of a river channel is caused by upriver and or downriver controls but the evaluation of their relative importance is often difficult a method is proposed to isolate the effect of the downriver control based on the slopes of the existing river profile those of the graded profile estimated from the discharge sediment relationship and slope reduction due to local base level rise the method was applied to the rapidly aggrading lower yellow river the downriver control in this case refers to the local base level rise associated with deltaic extension during the period under discussion the result shows that the main portion of siltation along the river is not caused by downriver control but by the slope difference between the existing and the graded profiles over a period within the last 700 years assuming conditions of discharge and sediment load during the period from 1962 to 1985 are reflective of the long term the marked slope difference between the existing and the graded profiles of the river is a result of changes in the river environment in the past several thousand years including the increase of sediment load local base level rise associated with river lengthening by deltaic growth and relative sea level rise due to tectonic subsidence of the coastal alluvial plain

changing pattern of accretion erosion of the modern yellow river huanghe subaerial delta china based on remote sensing images abstract the yellow river shifted from the diaokou course to the qingshuigou course in 1976 and shifted again at the north bank of the qingshuigou course in 1996 as a result the entire modern yellow river delta has experienced complicated patterns of erosion and accretion multitemporal remote sensing data of landsat mss and tm from 1976 to 2000 totaling twenty scenes were used to examine the changing pattern of accretion and erosion of the modern yellow river subaerial delta with gis tools during the period 1976 to 2000 the maximum erosion occurred at the head of the last two abandoned promontories diaokou and shenxiangou promontories where the coastline retreated landward over 7 and 4 5 km respectively with mean net erosion rates of 0 29 and 0 19 km a respectively in contrast the maximum accretion occurred at the head of the qingshuigou promontory where the coastline advanced seaward over 20 km with a mean net accretion rate of 0 83 km a the coastline at the head of the q 8 promontory also advanced seaward 7 km with a mean net accretion rate of 0 29 km a during the period 1996 to 2000 total area of the diaokou and shenxiangou promontories was reduced by 141 3 km 2 with a mean net erosion rate of 6 03 km 2 a total area of the qingshuigou and q 8 promontories increased by 384 16 and 11 88 km 2 respectively with mean net accretion rates of 16 4 and 3 03 km 2 a respectively relative to the diaokou shenxiangou and qingshuigou promontories the west area of the diaokou promontory and south area of the qingshuigou promontory were relatively stable although minor erosion occurred the general pattern of accretion and erosion of the modern yellow river subaerial delta was divided into three stages rapid accretion stage 1976 1984 accretion erosion adjustment stage 1985 1995 and slow erosion stage 1996 present the factors affecting the pattern of erosion and accretion over the yellow river delta such as structural geology the yellow river sediment discharge coastal processes meteorological conditions human activity sea level rise and land subsidence were summarized the cumulative area of increase over the entire subaerial delta is closely correlated with the cumulative yellow river sediment discharge wave induced longshore current is the major driving force to transport sediment from eroded areas along the last two abandoned promontories diaokou and shenxiangou promontories mostly westerly and partially easterly together with these coastal processes is the geographical shift of the yellow river course associated with its discharge regime after 1976 leading to the accretion along the present qingshuigou promontory and then the q 8 promontory

further shrimp geochronology on the early cambrian of south china zircon 206pb 238u ages are reported for bentonites from two cambrian sections in south china the meishucun and the fandian the analyzed zircons vary widely in age because of the presence of detrital and altered grains spot ages were corrected for calibration drift during analysis and the age of reference zircon sl13 was calibrated as 580 ma against other standards using sims use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages two age groups occur within the meishucun bed 5 bentonite of which the larger at 518 7 2 5 ma is interpreted as a post depositional zircon growth event and the other at 539 4 2 9 ma as the volcanic age bentonites from the meishucun bed 9 and its fandian equivalent have two age groups 526 5 1 1 ma and 540 2 1 0 ma we interpret the younger of these as the tuff magmatic age and the older as detrital small shelly fossils indicate meischucunian bed 5 is an equivalent of the late nemakit daldynian and its 539 4 2 9 ma age both closely times the base of the cambrian and is a maximum for the basal tommotian the 526 5 1 1 ma for bed 9 and its equivalents approximates the close of the tommotian

progradation of the yellow huanghe river delta in response to the implementation of a basin scale water regulation program the water and sediment supply to the lower reach and estuary of the yellow huanghe river have been altered to a very significant degree by the construction of numerous small to large reservoirs and the progressive implementation of water and soil conservation measures across the entire drainage basin since the late 1950s followed by a basin scale water regulation program since 2002 there is an urgent need to know how progradation of the yellow river delta yrd an area on which people live and farm has continued and will continue to respond to these controls this study examines the changing patterns of water and sediment supply to the yrd over the period of 1950 2013 and using remote sensing it evaluates the morphological changes of the delta during 1976 2013 although both water and sediment input to the delta have been described as declining significantly since the mid 1960s we show that since 2005 the supply of water has remained at around 20 billion m 3 yr 1 and that for sediment at about 0 13 billion t yr 1 water input has been even slightly increasing the dynamics of extension shrinking and avulsion in river mouth channels and accretion erosion of shorelines is an integrated response to the complex variations in both sediment and water supply this study develops a quantitative relationship between water and sediment supply and the area of land accretion and it predicts the critical condition for land accretion in the yrd to ensure that delta land will not be lost care is needed in the future implementation of the basin scale water regulation program c 2015 elsevier b v all rights reserved

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

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

seismic data processing in vertically inhomogeneous ti media the first and most important step in processing data in transversely isotropic ti media for which velocities vary with depth is parameter estimation the multilayer normal moveout nmo equation for a dipping reflector provides the basis for extending the ti velocity analysis of alkhalifah and tsvankin to vertically inhomogeneous media this nmo equation is based on a root mean square rms average of interval nmo velocities that correspond to a single ray parameter that of the dipping event therefore interval nmo velocities including the normal moveout velocity for horizontal events vnmo 0 can be extracted from the stacking velocities using a dix type differentiation procedure on the other hand which is a key combination of thomsen s parameters that time related processing relies on is extracted from the interval nmo velocities using a homogeneous inversion within each layer time migration like dip moveout depends on the same two parameters in vertically inhomogeneous media namely vnmo

necessary conditions for a meandering river avulsion there is as yet no rational basis for predicting the conditions leading to avulsion of a meandering river here we present a conceptual model and quantify it under simplifying assumptions as a first step toward the construction of a stability diagram for avulsion initiation it is assumed initially that a rectangular crevasse channel of arbitrary depth is cut into the levee of a meandering river because the water entering the crevasse channel is derived from relatively high in the main flow it contains low concentrations of suspended solids consequently the crevasse flow is under capacity and the entrance is eroded deepening of the entrance increases the crevasse channel discharge and the concentration of suspended solids supplied because more sediment laden waters are tapped from the deeper flow in the main channel the crevasse entrance is predicted to deepen until its sediment carrying capacity is satisfied by the suspended solids entering from the main channel whether an avulsion will occur for a particular combination of initial conditions depends upon whether there exists steady state hydraulic and sediment transport conditions for crevasse channel depths that are equal to or less than the main channel depth this conceptual model is quantified by writing the unsteady gradually varied one dimensional conservation of mass and momentum equations for water and sediment transport through a main and crevasse channel sediment transport is computed as the integral of a cross sectional mean velocity and a rouse concentration profile solutions show that stable crevasse channels exist only for particular combinations of the initial height of the crevasse bed relative to the water depth in the main channel and the ratio of initial crevasse bed slope to main channel slope

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

the dramatic changes and anthropogenic causes of erosion and deposition in the lower yellow huanghe river since 1952 the yellow river has been considerably altered over the past half century from human activities these perturbations provide a rare opportunity to document how this natural system has responded to anthropogenic impacts on channel hydrodynamics using a six decade record data set i analyze and summarize changing patterns in erosion and sedimentation of a downstream channel with three units km 3 y gt y and cm y and the response to human impacts upstream the changing pattern of channel bed erosion and deposition in the lower reach during 1952 2007 is divided into five phases in terms of erosion and deposition rates and human activities 1 a rapid and quasi natural deposition phase 1952 1959 0 25 km 3 y or 10 cm y with few main human effects 2 a rapid erosion phase 1960 1964 0 36 km 3 y or 14 cm y in response to heavy sedimentation in the sanmenxia reservoir 3 a rapid deposition phase 1965 1973 0 32 km 3 y or 13 cm y in response to sediment release from the sanmenxia reservoir 4 a moderate erosion and deposition phase with a net deposition 1974 1999 0 10 km 3 y or 4 cm y in response to the sanmenxia reservoir practice of storing clear water and discharging turbid water since 1974 and the intensified soil and water conservation since the 1970s and 5 a slow erosion phase 2000 2007 0 15 km 3 y or 6 cm y in response to the water and sediment regulation and the initial operation of xiaolangdi reservoir the total reduction in sediment delivery by this river is 41 0 gt giga tons during 1959 2007 the sediment delivery reduction are dams and reservoirs 51 soil and water conservation 25 increased water consumption 19 and channel sedimentation 13