silica diagenesis in eocene shallow water platform carbonates southern pyrenees spectacularly developed lower eocene chert in the corones platform carbonates of the spanish pyrenees is concentrated within a restricted brackish water laminated ostracod rich facies which also contains abundant sponge spicules the chert occurs as nodular bedded and mottled varieties and four petrographic types of quartz are developed microquartz length fast lf chalcedony megaquartz and microspheres 18o values of chert range from 29 6 to 30 9 smow which correspond to a broad isotope rank common for biogenic and diagenetic replacement cherts calcian dolomite crystals with high fe and na are disseminated within the microquartz and lf chalcedony but are absent from the megaquartz and host carbonate the chert is closely associated with desiccation cracks and with interstratal dewatering structures load casts are silicified and laminae rich in sponge spicules are convoluted early cracks related to dewatering are filled by microquartz and quartz cements ostracod shells within chert are locally fractured those in the host carbonate are commonly flattened late fractures are filled by lf chalcedony and megaquartz there is much evidence for the dissolution of sponge spicules and their calcitization in the carbonate host rock silica for the corones cherts was derived from sponges during early diagenesis and shallow burial early mechanical compaction and sediment dewatering played a major role in sponge spicule dissolution migration of silica rich fluids and the consequent precipitation of chert quartz cements continued to be precipitated into the burial environment

late miocene pleistocene evolution of india eurasia convergence partitioning between the bhutan himalaya and the shillong plateau new evidences from foreland basin deposits along the dungsam chu section eastern bhutan the shillong plateau is a unique basement cored uplift in the foreland of the eastern himalaya that accommodates part of the india eurasia convergence since the late miocene it was uplifted in the late pliocene to 1600 m potentially inducing regional climatic perturbations by orographically condensing part of the indian summer monsoon ism precipitations along its southern flank as such the eastern himalaya shillong plateau ism is suited to investigate effects of tectonics climate and erosion in a mountain range broken foreland system this study focuses on a 2200 m thick sedimentary section of the siwalik group strategically located in the lee of the shillong plateau along the dungsam chu at the front of the eastern bhutan himalaya we have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating combined with sedimentological and palynological analyses we show that 1 the section was deposited between 7 and 1 ma in a marginal marine deltaic transitioning into continental environment after 5 ma 2 depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the shillong plateau had an unexpected limited impact on the paleoclimate of the bhutanese foothills and 3 the diminution of the flexural subsidence in the basin and or of the detrital input from the range is attributable to a slowdown of the displacement rates along the main boundary thrust in eastern bhutan during the latest miocene pleistocene in response to increasing partitioning of the india eurasia convergence into the active faults bounding the shillong plateau

cenozoic magnetostratigraphy of the xining basin ne tibetan plateau and its constraints on paleontological sedimentological and tectonomorphological evolution abstract the xining basin is well known for its thick and continuous sequence of fine grained cenozoic sediments in addition it contains important xiejia fauna which define the xiejian stage of the early miocene in the standard chinese land mammal zonation and it also provides detailed histories of eolian dust deposition which document the aridification of asia monsoon evolution and the initiation of the yellow river in relation to the uplift of the tibetan plateau however the results of magnetostratigraphic dating of the fossil bearing sequence has yielded conflicting ages for the fauna and stratigraphy hindering the use of the sequence for addressing major paleoenvironmental and paleoclimatic issues here we review the paleontological lithological and magnetostratigraphic record of the xining basin and provide a new long and continuous high resolution magnetostratigraphy from the basin center the results show that the observed magnetic polarity zones from the various sections at different sites in the basin exhibit similar magnetic polarity patterns which can be readily correlated in terms of both magnetic polarity zonation and lithofacies in addition they show that the sedimentary sequence of the basin was sub continuously deposited from 54 ma to 4 8 ma the magnetostratigraphic correlations confirm previous constraint of the xiejia fauna in the late oligocene at 25 ma challenging the current chinese mammal land zonation for the early miocene and we suggest that it provides a superior record of eolian dust deposition and river incision in the late pliocene than previous studies there is a close match of the climatic proxy records of the xiejia section dated by magnetostratigraphy with the global climatic record which corroborates the paleomagnetic correlations the evolution of the sedimentary environment provided by the lithofacies demonstrates a complete cycle of basin formation and termination which records the eastern qilian shan experienced three main phases of uplifts slow episodic uplifts at 54 ma and 22 5 ma and a late rapid episodic uplifts at 8 7 ma and since 4 8 3 6 ma this tectonic uplift in the ne tibetan plateau was mostly nearly synchronous responses to the initial and continuing collision of india and asia since 50 5 ma

management of ambiguities in magnetostratigraphic correlation abstract magnetostratigraphy is a powerful tool to provide absolute dating of sediments enabling robust and detailed chronostratigraphic correlations it is based on the correlation of a magnetic polarity column observed and measured in a given sediment section to a magnetic polarity reference scale where polarity changes are well dated via other independent methods however magnetostratigraphic correlations are loose as they are only constrained by binary magnetic chrons i e normal or reversal and their thickness which are both defined from depth variations of the magnetic remanent directions the thickness of a given magnetic polarity zone is a function of time and sediment accumulation rate which may not be stationary leading to ambiguities when performing the correlations to address these ambiguities a numerical method based on the dynamic time warping algorithm is proposed magnetostratigraphic correlations are computed in order to minimise the local variation of the accumulation rate the main advantage of the proposed method is to automatically provide a set of reasonably likely correlations this set can then be scrutinised for further analysis and interpretation however the likelihood of a correlation should be handled carefully as it depends on the information content of the magnetostratigraphic section itself and remain ultimately valid by ancillary constraint nevertheless the method gives consistent results on difficult synthetic cases that simulate abrupt variations of the sedimentation rate insights on true sections debated by previous authors are also given

the neogene xiyu formation a diachronous prograding gravel wedge at front of the tianshan climatic and tectonic implications the tarim and junggar basins in central asia are capped by a thick pile of conglomerates called the xiyu formation that are commonly linked to a change in climate and or accelerated uplift near the plio pleistocene boundary in order to better understand their origin and significance we carried out a combined structural and magnetostratigraphic study in the quilitage syncline southern tianshan where the base of the xiyu conglomerates is observed at both sides of the syncline a balanced cross section shows that even at a local scale the base of the xiyu conglomerates cannot be regarded as a single continuous stratigraphic layer on the southern flank of the quilitage syncline we collected 172 samples collected for magnetostratigraphic dating identify 17 polarity chrons that date the new section from 5 2 to 1 7 ma and constrain the base of the xiyu conglomerate here at 1 7 ma this is 4 2 ma younger than the age of the xiyu previously found on the northern limb of the same syncline together with other magnetostratigraphic studies carried out around the tianshan our study unambiguously demonstrates that the onset of deposition of the xiyu conglomerates is diachronous and that the conglomerates are systematically younger toward the basin consequently the xiyu formation should not be considered as a chronostratigraphic marker related to any particular tectonic or climatic event but is instead a prograding gravel wedge that has prograded over the underthrusting forelands a synthesis of chronologic and structural results yields progradation rates over the last 10 ma on the order of 2 0 mm yr and 3 9 mm yr south and north of the tianshan mountains respectively these rates are comparable to the shortening rate across the tianshan range suggesting that underthrusting is the main factor governing the progradation rate of the xiyu formation

meteoric water participation in the genesis of jurassic cherts in the subbetic of southern spain a significant indicator of penecontemporaneous emergence abstract in the eastern sector of the betic cordillera outcrops of the internal subbetic display chert lenses in the upper parts of oolitic limestones camarena formation mid jurassic the stratigraphy indicates a sedimentary record related to synsedimentary tectonics tilting the sediments were deposited in different tectonic blocks at varying depths the camarena formation is mainly made up of shallow platform oolitic limestones and was deposited on a pelagic swell a shallow isolated platform that developed far from continental areas the chert lenses appear in those stratigraphic units deposited in the lowermost sunken fault blocks which display biosiliceous facies overlying the camarena formation the chert mainly consists of megaquartz formed by replacement of the oolitic host rocks without an opaline precursor the part of the host that has been most affected by the silicification is the carbonate cement the petrological features indicate a slow replacement from solutions which were poor in silica and cations comparing the textures and fabric of the cherts and their host rocks it is deduced that the silicification took place after compaction cementation fracturing and local dissolution of the oolitic limestones the stable isotopic data 18o 13c obtained from the chert oolitic host rock and overlain biosiliceous rocks display significant differences the host rocks camarena formation and overlying biosiliceous rocks have isotopic signatures that are typical of marine environments or shallow marine diagenesis however the chert isotopic values are interpreted to suggest meteoric water participation in the silicification process the timing of the chertification is suggested to have been between the bathonian beginning of the deposition of the biosiliceous source facies and the aptian first appearance of this chert as clasts in breccia beds in palaeogeographic terms the optimum timing would have been during emergence i e between the bathonian and tithonian the exposed areas could have acted as recharge areas of meteoric water into the regional aquifer system the involvement of meteoric water in the silicification processes gives new support to the hypothesis that pelagic swells were subjected to episodic exposure

depositional remanent magnetization toward an improved theoretical and experimental foundation the first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field yet redeposition experiments showed a strong and quasi linear dependence on the external field this empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades we present redeposition experiments which suggest instead that the relationship between depositional remanence drm and applied field may frequently be curved for magnetic fields in the range of the earth s numerical simulations using a flocculation model can explain the redeposition data and suggest that drm will be significantly non linear when the flocs are small several microns there is a strong dependence of floc size on salinity particularly in low salinity environments floc size has a profound influence on the efficiency of drm hence low salinity environment may give results with poor reproducibility the size of the floc in which magnetic particles are embedded is not accounted for in current methods of normalization yet is the most important parameter on the bright side however it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floc size distribution and settling times can be constrained

comment on magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china baochun huang john d a piper shoutao peng tao liu zhong li qingchen wang rixiang zhu earth planet sci lett 2006 doi 10 1016 j epsl 2006 09 020 abstract the recent publication of magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china by b c huang j d a piper s t peng t liu z li q c wang r x zhu earth planet sci lett 2006 doi 10 1016 j epsl 2006 09 020 discusses the cenozoic uplift history of the tianshan mountains by studying the magnetostratigraphy of paleogene to neogene continental sediments from two sections located in the kuche basin at the northern edge of the tarim basin to support their conclusion they reinterpreted a magnetostratigraphic study of the yaha section which lies 10 km south of their sections we previously published j charreau s gilder y chen s dominguez j p avouac s sen m jolivet y li and w wang magnetostratigraphy of the yaha section tarim basin china 11 ma acceleration in erosion and uplift of the tianshan mountains geology 34 3 2006 181 184 here 1 we argue that the interpretations of the sedimentation rate changes they proposed for the kuche sections are partially invalid 2 we disagree with their reinterpretation of the age of the yaha section and 3 we think that the way they interpret their ams data is incorrect

cosmogenic burial ages reveal sediment reservoir dynamics along the yellow river china burial dating using cosmogenic al 26 and be 10 is an emerging technique for establishing chronologies of fluvial deposits ranging in age from ca 0 5 ma to 5 ma the determination of burial age however requires an assumption that sediment is buried with a known initial al 26 be 10 ratio using a sequence of well dated fluvial terraces along the yellow river in lanzhou china we demonstrate that this assumption may be violated when upstream sediment sources include neogene sedimentary basins samples of fluvial gravel from six terraces yield burial ages ranging from ca 0 83 ma to 2 81 ma comparison of these results to independent ages determined from magneto stratigraphic and loess paleosol studies demonstrates a high degree of correspondence between ages in the lower three terraces in the higher terraces however burial ages are systematically too old implying a previous burial history these results can be exploited to place quantitative constraints on the provenance of sediment in the terrace deposits our results reveal that rapid incision and excavation of late neogene sedimentary basins ca 1 7 ma led to injection of fluvial sediment with low initial al 26 be 10 ratios dilution of this source through time occurred as incision progressed into upstream bedrock ranges our results suggest that cosmogenic nuclides can illuminate the timing of long term storage and remobilization of sediment in continental scale river systems

application of dynamic programming to the correlation of paleoclimate records 1 signal matching is a powerful tool frequently used in paleoclimate research but it is enormously time consuming when performed by hand previously proposed automatic correlation techniques require very good initial fits to find the correct alignment of two records a new technique presented in this paper utilizes dynamic programming to find the globally optimal alignment of two records geological realism is instilled in the solution through the definition of penalty functions for undesirable behavior such as unlikely changes in accumulation rate examples with synthetic and real data demonstrate that the dynamic programming technique produces accurate high resolution results with much less effort than hand tuning or preexisting automated correlation techniques

chert appearance in the cueva bedon carbonate platform upper cretaceous northern spain nodular chert from the cueva bedon carbonate platform late turonian earliest coniacian occurs above the type 1 sequence boundary and in association with a transgressive systems tract deposit it formed by selective replacement of grainstones packstones and thalassinoides burrow fillings other minor types of selective silicification include the replacement of anhydrite nodules forming quartz geodes and the partial or total replacement of oysters and inoceramids the chert nodules consist predominantly of microquartz with volumetrically minor fibrous quartz length fast chalcedony and megaquartz the silica has a mainly biogenic source siliceous sponge spicules and radiolarians lepisphere textures that are indicative of an intermediate opal ct precursor are evident in chert nodules which indicates the chert formed by a maturation process from opala to opal ct to quartz the 18 o values of microquartz suggest a precipitation from marine fluids the fabric of quartz geodes consists of irregular bands of heterometric megaquartz quartzine and lutecite spherulites and a well developed band of petaloidal megaquartz that contains small anhydrite inclusions bivalve shells are replaced mainly by fibrousquartz quartzine lutecite which forms extensive beekite rings the silica source of the replaced fossils is also biogenic but silicification couldhave occurred by direct quartz replacement without an opal ct precursor field and petrographic evidence indicate that silicification was an early diagenetic process that affected both uncompacted and compacted sediments andwas inhibited when the dolomitization began there seems to exist a syncrony in age late turonian earliest coniacian between the nodular chert in the cueva bedon carbonate platform and the bedded and nodular chert ofthe plentzia carbonate turbidites in the deeper basque arc

overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the madrid miocene basin spain the miocene distal alluvial fan and palustrine deposits of the madrid basin paracuellos de jarama area were examined to establish the sequence of its pedogenic diagenetic processes and the main factors controlling them in this area the diagenetic processes operated not only on carbonates as commonly studied but also in high magnesium clays and opaline cherts this paper provides a dynamic model for saline alkaline lake margins that complements the existing freshwater palustrine model three sections ber pel and upc containing limestones sepiolite dolostones and opaline cherts were examined these represent the entire transition from alluvial fan deposit to lake environment the alluvial fan deposits pel section show the most complex sequence of processes after a weak edaphic imprint the primary mudstone and siltstones are affected by silicification opaline levels formed mainly by selective replacement of sepiolites and later dolomitization several types of dolomite are present rounded crystals spherules and globular bodies being the most characteristic in the ponds situated at the foot of the alluvial fans or in interfan areas sepiolite precipitated within free water bodies ber section and an intense pedogenesis led to the formation of palustrine sepiolite deposits later the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging in the shallow lakes upc section palustrine limestones and sepiolite were deposited the carbonates are partially affected by silicification the sepiolite becoming completely silicified the opaline levels clearly preserve the structure of the limestones and sepiolite host rocks all the opaline levels show local aging processes the silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies in the distal alluvial fan sediments of the pel section the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures the silicification environment was therefore not easy to determine in the ber and upc section the main silicification took place in groundwater influenced environments but evidence was seen of cements and silicifications in vadose environments in all study settings the overlapping of pedogenic vadose and groundwater processes was controlled by changes in the position of the water table four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements the majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin in all the studied environments the sepiolite deposits were preferentially silicified their fibrous structure and high absorption capacity helping to retain interstitial fluids moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments the dissolution of the sepiolite caused an increase in silica in the interstitial fluids favouring silicification and ruling out the need of a biogenic source the replacement of limestones by silica is usually driven by variations in ph and salinity which are very common in surface environments such as shallow lakes and soils

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

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

tibetan uplift prior to the eocene oligocene climate transition evidence from pollen analysis of the xining basin uplift of the tibetan plateau and the himalayas since the onset of the indo asia collision is held responsible for asian aridifi cation and monsoon intensifi cation but may also have gradually cooled global climate leading to the 34 ma eocene oligocene transition to unravel the interplay between tibetan uplift and global climate proxy records of asian paleoenvironments constrained by accurate age models are needed for the paleogene period here we report the 38 ma appearance of high altitude vegetation recovered from palynological assemblages in precisely dated lacustrine sediments from the xining basin of the northeastern tibetan plateau region this result confi rms previous evidence for important regional uplift in the central and northern tibetan plateau regions during the early stage of the indo asia collision this is consistent with the idea that the associated increase in rock weathering and erosion contributed to lowering of atmospheric co 2 leading to the eocene oligocene transition

a carbonate submarine fan in a fault controlled basin of the upper jurassic betic cordillera southern spain the middle member of the loma del toril formation kimmeridgian lower tithonian intermediate units betic cordillera consists of up to 250 m of resedimented carbonate material three units have been distinguished the lower unit a is composed of conglomerates that are interpreted as deposited in a major valley on the lower slope of a basin margin unit b calcarenites with some conglomerate intercalation is interpreted as distributary channel deposits and unit c calcarenites as the result of poorly developed depositional lobes of a submarine fan the three units form a recessional sequence they cannot be related to a transgression because the kimmeridgian lower tithonian in the prebetic zone where epicontinental sediments exist is clearly regressive the upper member of the loma del toril formation made up of pelagic limestones with sporadic calcarenites or even thin conglomerate intercalations is best interpreted as a basin plain facies lateral facies relationships suggest that down faulting of the basin floor controlled the development of the fan the scarce occurrence of turbidite beds in the basin plain facies the prevailing channelized facies and the obvious lack of overbank deposits suggest a transport system of low efficiency with fan deposition at the base of slope the underlying jurassic strata cropping out along fault scarps coeval carbonate shelf material and upper slope deposits were the main sources of turbiditic resediments with respect to basin morphology sedimentary processes and fan geometry this jurassic turbidite basin can be compared with the modern california continental borderland ancient analogues have been described by reinhart 1977 and price 1977

miocene humid intervals and establishment of drainage networks by 23 ma in the central sahara southern libya terrestrial and lacustrine neogene and quaternary sediments in the libyan fezzan provide key evidence for paleoclimate changes in the central sahara associated with lake megafezzan understanding of holocene and late pleistocene deposits is resolved but the age of older sediments is not we provide the first high resolution chronology and stratigraphy of the neogene deposits in the fezzan basin and so also the central sahara the sediments are divided into three unconformity bounded units the oldest unit comprising the shabirinah and brak formations is dated using magnetostratigraphy the shabirinah formation is a succession of lacustrine and fluvial units locally with humid and arid paleosols which progressively show evidence of increasing aridity up through the succession the overlying brak formation is a pedogenically modified palustrine limestone at basin margin locations all these units are dated to the early aquitanian to late serravallian in the early to mid miocene having formed prior to major volcanic fields to the east during the mid late aquitanian widespread stromatolitic lake sediments developed in se fezzan in the late burdigalian palustrine carbonate units developed that typically pass laterally into mixed clastic paleosol carbonate units that characterise basin margin situations the serravallian aged brak formation is a highstand deposit developed during maximum lake extent which formed due to restriction of basin drainage to the north and east due to growth of the jabal as sawda volcanic centre and uplift of the sw shoulder of the sirte basin gradual aridification of the central sahara occurred from the early miocene but this trend was periodically interrupted by humid phases during which lake megafezzan developed the hyperaridity of the central sahara must have developed after 11 ma and the main drainage networks from the fezzan basin were established before 23 ma in the oligocene indicating the great antiquity of major central saharan river basins

chertification histories of some late mesozoic and middle palaeozoic platform carbonates a consistent pattern for the silica sources depositional environments and timing of chertification was observed in a diverse suite of five late mesozoic and middle palaeozoic carbonate sequences the 1 upper greensand cretaceous and 2 portland limestone jurassic of southern england 3 the ramp creek formation mississippian of southern indiana and the 4 lower helderberg group devonian and 5 onondaga limestone devonian of new york state nodular chert formation in all five limestone sequences occurred in sediments that were largely uncemented ghosts of pre chertification carbonate cements are present in some chert nodules but are volumetrically minor in every limestone sequence except the upper greensand chertification occurred after burial to a depth sufficient for intergranular pressure solution and mechanical grain deformation of carbonate sand nodular chert is most abundant in subtidal normal marine wackestones and mudstones that were deposited at or below fair weather wave base and is absent or rare in supratidal intertidal and high energy subtidal limestones and dolomites an intraformational sponge spicule silica source for chert nodules is suggested by direct evidence such as calcitized sponge spicules in the host limestone and circumstantial evidence such as ghosts of sponge spicules in chert nodules and a correlation of chert abundance with depositional environment most molds of siliceous sponge spicules were apparently obliterated by post chertification intergranular compaction we propose that these general trends for the depositional environments silica sources and timing of chertification are representative of most mesozoic to middle palaeozoic platform limestones

the maximum likelihood dating of magnetostratigraphic sections summary in general stratigraphic sections are dated by biostratigraphy and magnetic polarity stratigraphy mps is subsequently used to improve the dating of specific section horizons or to correlate these horizons in different sections of similar age this paper shows however that the identification of a record of a sufficient number of geomagnetic polarity reversals against a reference scale often does not require any complementary information the deposition and possible subsequent erosion of the section is herein regarded as a stochastic process whose discrete time increments are independent and normally distributed this model enables the expression of the time dependence of the magnetic record of section increments in terms of probability to date samples bracketing the geomagnetic polarity reversal horizons their levels are combined with various sequences of successive polarity reversals drawn from the reference scale each particular combination gives rise to specific constraints on the unknown ages of the primary remanent magnetization of samples the problem is solved by the constrained maximization of the likelihood function with respect to these ages and parameters of the model and by subsequent maximization of this function over the set of possible combinations a statistical test of the significance of this solution is given the application of this algorithm to various published magnetostratigraphic sections that included nine or more polarity reversals gave satisfactory results this possible self sufficiency makes mps less dependent on other dating techniques

paleomagnetic and cooling constraints on the duration of the pahute mesa trail ridge eruptive event and associated magmatic evolution black mountain volcanic center southwestern nevada two successive ash flow cooling units of the black mountain volcanic center the pahute mesa and trail ridge members of the late miocene thirsty canyon tuff possess nearly indistinguishable reverse polarity remanent magnetization directions of about 178 declination and 10 inclination these are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition or possibly from a field oriented near the limit of secular variation regardless of the type of geomagnetic field behavior the pahute mesa member was apparently overlain by the trail ridge member before appreciable shift of the anomalous geomagnetic field this strongly suggests that the time between eruption of the two ash flow sheets was very short perhaps less than 100 years a conclusion that is independently supported by the local absence of a complete cooling break between the two units the interval between eruption of the pahute mesa and trail ridge members was too brief for reestablishment of chemical stratification in the black mountain magma system nevertheless a small volume of highly evolved comendite magma developed and was erupted as lava within the black mountain caldera and as a component of the trail ridge member the short repose interval and rapid development of the comendite together with evidence for mixing of magmas of silicic to near mafic composition vogel et al this issue and the presence of air fall tuff at the base of the trail ridge lead us to conclude that eruption of the trail ridge member was probably triggered by injection of hot relatively unevolved magma into the black mountain magma body

chalcedonic quartz and occurrence of quartzine length slow chalcedony in pelagic sediments abstract chalcedony is the most abundant form of quartz in silicified pelagic sediments from the north pacific varieties of chalcedonic quartz present in chert of deep sea origin include chalcedony length fast and zebraic quartzine length slow and lutecite these occurrences of quartzine in known pelagic sediments emphasize the dangers of using quartzine as an indicator of former evaporitic environments quartzine is a diagenetic mineral and does indicate pore fluids rich in sulphate and magnesium in pelagic sediments it is always associated with authigenic barite and in many cases with authigenic dolomite quartzine should not be used by itself as an indicator of any particular environment of deposition

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

reply to the comment by q q qiao et al on cenozoic tectonic deformation and uplift of the south tian shan implications from magnetostratigraphy and balanced cross section restoration of the kuqa depression by tao zhang xiaomin fang chunhui song erwin appel and yadong wang tectonophysics 2014 doi 10 1016 j tecto 2014 04 044 qiao et al 2016 commented on our work zhang et al 2014 and rejected our reinterpretation of the magnetostratigraphic results of huang et al 2006 and li et al 2006 with the results gained using the dynamic time warping algorithm technique dtwat as their main basis however qiao et al 2016 did not provide details of their modeling inputs and in particular the parameters they chose for their calculations where such parameters can have a serious impact upon any results we therefore performed calculations using the same software i e qupydon as qiao et al 2016 using reliable parameter settings the results showed that the interesting correlation of a 6000 minimum cost output completely correlate with the magnetostratigraphic chrons c18r to c3an 1n similar to 40 6 ma which is consistent with our reinterpreted magnetostratigraphic results furthermore we summarized previous biostratigraphic studies of nearby areas the data resulting from this process also supported our reinterpreted magnetostratigraphic correlation we were therefore able to confirm the revised magnetostratigraphic correlation of zhang et al 2014 c 2017 elsevier b v all rights reserved

warming or cooling in the pragian sedimentary record and petrophysical logs across the lochkovian pragian boundary in the spanish central pyrenees abstract high resolution petrophysical correlation methods were applied for the first time to mid paleozoic rocks of the pyrenees the methods included magnetic susceptibility measurements ms gamma ray spectrometry grs and alignment of ms logs using the dynamic time warping dtw algorithm conodont biostratigraphy provided the basic framework necessary for work with the grs and ms logs despite differences in the sediment patterns and accumulation erosion rates the logs from two selected sections in the spanish central pyrenees show a striking symmetry that correlates well with the previously published logs from the barrandian area in the czech republic the high similarity between the petrophysical records from paleogeographically related but distant areas has the potential to contribute to the current discussions about the eustatic and climatic changes that took place in the overall greenhouse settings where evidence for any polar ice sheets is still absent in addition to the extant evidence of a major lochkovian pragian offlap combined with sea level lowering known from the north america evidence about a major sea level fall and drastic reconstruction of the climatic system in the pragian is now expanding over significant parts of the peri gondwanan belt the data from spain provide many new details about this change including the phenomenon of a stratigraphically condensed upper lochkovian a dramatically different type of sedimentation in the pragian and a grs th and ms anomalous stratigraphic interval in the middle pragian the multifaceted evidence allows us to determine the essential parts of the pragian as a still hot and humid period even with the strong differences from the possibly extremely hot lochkovian

early middle triassic trace fossils from the luoping biota southwestern china evidence of recovery from mass extinction trace fossils have proven useful for studying the timing and process of biotic recovery after the permian triassic mass extinction ptme recovery stages are defined by comparing successive ichnoassemblages from the latest permian to the early middle triassic lower triassic trace fossils have been explored in some detail but those of the lower middle triassic are less well known here well preserved fossil materials from the luoping biota from yunnan province south china suggest that a fully recovered shallow marine ecosystem was re established by the early middle triassic trace fossil assemblages of the luoping biota are characterized by high ichnodiversity with 14 ichnogenera in the shallow marine environment of an intra carbonate platform basin and nine ichnogenera in the subtidal environment such moderate to high ichnodiversity together with a marked increase in burrow sizes and the common occurrence of key ichnotaxa e g rhizocorallium and thalassinoides suggests that the ichnofauna had reached recovery stage four in contrast non turbiditic strata of the offshore setting record only three ichnogenera with bioturbation indices never exceeding one periodic anoxia in bottom waters was presumably the main control for such a protracted trace fossil recovery in an offshore setting which otherwise aided the fine preservation of body fossils of the luoping biota furthermore event sedimentation turbidite deposits in the offshore setting incorporated moderate ichnodiversity and moderate to high bioturbation indices both interpreted as a result of short term colonization by transported infaunal animals from proximal settings the occurrence of variable crustacean traces e g sinusichnus spongeliomorpha and thalassinoides at luoping and the locomotion traces of marine reptiles together with abundant fishes and fossil decapods highlights the value of trace fossils in ecosystem reconstruction after the ptme

a high resolution geochemical record from the kuche depression constraints on early miocene uplift of south tian shan abstract the building of tian shan is often simplified to a cenozoic reactivation of a paleozoic fold belt during the past decades extensive case studies have arrived at the conclusion that tian shan experienced diachronous uplift during the cenozoic however understanding of the specific location and duration of the mountain building is still hindered by complex tectonic history of the orogenic belt considering the geochemical differences of various tectonic units within the mountains we studied the chemical composition variability for the carbonate free fine grained samples collected from the longest terrestrial outcrop in the kuche depression southern tian shan at a high resolution in order to better constrain the orogenic process our data show significant increases in mobile elements during the interval of 23 17 ma departing substantially from the stable long term weathering trends and limited sedimentary sorting effects we interpret the trend as provenance changes which provide crucial evidence to support that the south tian shan experienced significant surface uplift during the early miocene this argument is supported by other studies conducted on both local and regional scales mineralogical analyses for the study section have shown that since the early neogene a marked increase of detrital limeclasts derived from the south tian shan is accompanied by the decreases of the abundance of garnets derived from an ultrahigh pressure metamorphic fold belt in the central tian shan moreover low temperature thermochronological results revealed that the central tian shan merely experienced pre neogene cooling contrasting to the southern piedmont of the mountain where neogene cooling dominated taken all together we conclude that the uplift of the south tian shan initiated at 23 ma and the paleoelevation may exceed the height of the central tian shan at 17 ma

paleo erosion rates in central asia since 9 ma a transient increase at the onset of quaternary glaciations erosion is a fundamental player of the interactions existing between internal geodynamics and climate in particular through its influence on the carbon dioxide budget however long term 1 ma erosion rates estimated indirectly from sediment budget remain poorly constrained while some studies suggest that worldwide erosion rates increased at the plio pleistocene climatic transition 4 2 ma the validity of this observation and its significance is a matter of debate due to potential biases of the sedimentary record and to the influence of sea level fall on the global sedimentary flux to marginal basins in the present study we estimate erosion rates over the last 9 ma using in situ produced cosmogenic 10be concentrations measured in magnetostratigraphically dated continental sediments we focus on an intracontinental endorheic watershed draining the northern tianshan in central asia a key region regarding the ongoing debate while erosion rates between 0 1 and 1 mm yr 1 are derived from most of our record they reach values as high as 2 5 mm yr 1 from 2 5 to 1 7 ma then after 1 7 ma recent and modern erosion rates fell below 1 mm yr 1 this temporary increase is correlated with the onset of quaternary ice ages and suggests that global climate had a significant and transient impact on erosion

late cenozoic evolution in the pamir tian shan convergence new chronological constraints from the magnetostratigraphic record of the southwestern tianshan foreland basin ulugqat area abstract the northeastern pamir tian shan convergence zone is a key region for understanding ongoing intracontinental mountain building a detailed magnetostratigraphic study combined with color reflectance variations of continental sediments from the 1120 m thick sankeshu section in the south west sector of the tianshan foreland basin of western china yields important insights into the tectonic evolution of this zone correlation with the geomagnetic polarity time scale identifies deposition lasting from 16 7 to 2 6 ma with a marked increase in sedimentation rate at 7 ma a further rapid increase occurred after 2 6 ma with influx of the conglomeratic xiyu formation observed height dependent changes of rock magnetic parameters shape parameter t and ams ellipsoid parameters show that these sediments were influenced by weak deformation with the sediments accumulated before 11 ma recording a signature of compressive deformation from northward indentation by the pamir the succession of sedimentary events in the foreland basin is comparable to previous investigations of magnetostratigraphic and sedimentological analyses and with thermochronology collectively showing that deformation in the tian shan region has been concentrated in miocene and later times the regional correlations resulting from these analyses show that sedimentary events correlate with the episodic nature of regional uplift with the latter inducing climatic changes that are in turn recorded in the sediment record

epigenetic siliceous rocks from the southern part of the krakow czestochowa upland southern poland and their relation to upper jurassic early diagenetic chert concretions abstract the epigenetic siliceous rocks esr are common in the southern part of the krakow czestochowa upland where they are hosted in upper jurassic microbial sponge megafacies as i coatings on the surfaces of fractures and faults ii irregular bodies enclosed within both the upper jurassic limestones and the overlying cenomanian abrasion platform sediments and iii concentric or irregular rims of silicified limestones on early diagenetic chert concretions the distribution of esr is limited to the topmost part of the preserved upper jurassic succession the results of chemical analyses and fluid inclusion microthermometric data clearly demonstrate the hydrothermal origin of the esr the epigenetic silicification was triggered by low temperatures and low saline fluids the age of the esr is indirectly suggested by i the early diagenetic cherts forming the crystallization nuclei ii the silicification of pressure dissolution structures in limestones and iii the silicified carbonate infillings of fractures in chert concretions formed in the local extensional regime enhancing the migration of thermal fluids during the main cenozoic faulting stage of upper jurassic sediments

oligocene miocene magnetostratigraphy and magnetic anisotropy of the baxbulak section from the pamir tian shan convergence zone as the northernmost part of the indo eurasian collision belt the pamir tian shan convergence zone ptcz is a strategic location for understanding intracontinental deformation here we present a magnetostratigraphic investigation of a continuous section from the baxbulak region to better constrain regional tectonic history rock magnetic analyses indicate that hematite and magnetite are the main carriers of characteristic remanent magnetization the resulting polarity sequence allows a distinct correlation to the geomagnetic polarity time scale showing that the section spans the interval of 29 1 20 7 ma rock magnetic results further suggest that paramagnetic and antiferromagnetic minerals dominantly contribute to anisotropy of magnetic susceptibility ams of the sequence thus the ams would indicate the preferred orientations of the mineral grains that are sensitive to tectonic strain at around 26 ma the grouped principal minimum perpendicular to the bedding diverts to a girdle distribution in a n s direction demonstrating the overprint of tectonic fabric to previous weakly deformed sedimentary fabric this would be interpreted as a marked increase in tectonic strain consistent with various evidence from the pamir and the neighboring basin that show the pamir began to migrate northward moreover the coincident changes in distribution of ams principal axes in both direction and magnitude are comparable to the regional counterclockwise rotations observed from paleomagnetic data likely related to orogenesis

diagenesis of pleistocene reef associated sediments from the red sea coastal plain egypt abstract pleistocene reef associated sediments from the red sea coastal plain of egypt consist of carbonate to mixed carbonate siliciclastic sediments the recognized facies are calcareous sandstone skeletal rudstone grainstone wackestone and micritic crusts evidence from cement fabrics aragonite high mg calcite and gypsum suggests that cementation mainly occurred in the intertidal zone under marine vadose conditions however the interaction of these sediments with meteoric waters resulted in the modification and stabilization of the metastable carbonates the replacement of detrital feldspars by carbonates and the silicification of carbonates

silica distribution in interstitial waters and sediments from the southeastern pacific abstract analyses for silica in the interstitial water of five cores from the southeast pacific are presented silica is enriched in these interstitial waters resulting in a vertical flux of silica of between 10 and 50 mol cm 2 yr 1 from the sediment into the overlaying seawater this flux is generated by the dissolution of biogenic silica the dissolution of which is increased in areas of bottom water turbulence the si al and calculated opal leinen 1977 contents of the bulk sediment of these cores are also presented small scale variations over depth intervals of tens of centimetres are present as a result of chaning conditions of sedimentation

a jackknife for magnetostratigraphy we investigate the problem of evaluating the reliability of a magnetostratigraphic section using a jackknife resampling scheme we define a parameter j as the slope relating the percentage of polarity zones of a sampled section remaining after random deletion of sites increasing in proportion to a total of 20 of the initial collection so defined j quantifies the dependence of a magnetostratigraphic record on the distribution of sampling sites based on simulated magnetostratigraphic sections j was found to be an indicator of the approximate percentage of polarity zones present in the original reference sections that were recovered in the sampled section j will perhaps be most helpful as a guide to investigators as to whether further sampling would provide useful information as well as to the non paleomagnetist user of black and white interpretive polarity logs as an estimate of the reliability of a given section

magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china abstract magnetostratigraphic and rock magnetic results from 1006 horizons in paleogene and neogene sediments between the upper kumugeliemu formation and the base of the kuche formation within the kuche depression of the tarim basin nw china are used to evaluate postcollisional uplift and deformation in the tian shan range characteristic remanences with both normal and reversed polarity were isolated by thermal demagnetization generally between 300 and 690 c and a composite magnetostratigraphy is constructed from data at 969 accepted levels correlating with polarity chrons c12n to c3r dated between 31 0 and 5 5 ma on the ck95 geomagnetic polarity time scale observed height dependent changes of rock magnetic parameters km and ams ellipsoid parameters indicate that these sediments were influenced by weak deformation with the succession accumulated before 15 ma recording the effects of compressive deformation this finding together with two substantial increases in accumulation rate at 16 17 and 7 ma define a framework for cenozoic uplift and deformation of the tian shan range we argue that thrusting in the south of the range was probably initiated at 20 ma and followed by rapid uplift at 16 17 ma on the assumption that sediment accumulation in western china was still predominately tectonically controlled at 7 ma another later pulse of rapid uplift with a much larger regional influence is also recognized in the tian shan the kuche depression was subjected to two postcollisional phases of rotational deformation with clockwise rotation prior to 12 ma 1 36 ma succeeded by counterclockwise rotation at 1 64 ma after 12 ma and possibly continuing to the present day this rotational deformation is not temporally related to the history of basin formation resolved from the magnetostratigraphy and may be linked to block interactions with adjoining terranes

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

magnetostratigraphy of the yaha section tarim basin china 11 ma acceleration in erosion and uplift of the tian shan mountains we report a magnetostratigraphic and rock magnetic study of the yaha section located on the southern flank of the central tian shan mountains asia our results show a two fold increase in sedimentation rate as well as marked changes in rock magnetic characteristics ca 11 ma after 11 ma sedimentation rate remained remarkably constant until at least 5 2 ma these findings are consistent with sedimentary records from other sections surrounding the tian shan we conclude that uplift and erosion of the tian shan accelerated ca 11 ma long after the onset of the collision between india and asia and that the range rapidly evolved toward a steady state geometry via a balance between tectonic and erosion processes

ordovician aleman ribbon cherts an example of silicification prior to carbonate lithification abstract the ordovician aleman formation southwestern new mexico is primarily composed of dolomite with ribbons of chert the presence of stromatolites relict evaporite nodules and other features indicates an intertidal to shallow subtidal depositional environment the stromatolitic layers are preferentially silicified with dissolution of opaline sponge spicules providing the main if not sole source of silica silicification occurred in both the vadose and phreatic realms essentially penecontemporaneous with accumulation of the sediment and concomitant with dolomitization this is indicated by the preservation in silicified form of original anhydrite lath morphology central cavities in chert nodules partially filled with geopetal matrix dolomite and matrix dolomite immediately adjacent to conchoidal fractures of silicified carbonate grainstones the latter relationship indicates that the chert was rigid and fractured before the carbonate host sediments were lithified

magnetostratigraphy and biostratigraphy of the neogene deposits of kastellios hill central crete greece abstract the kastellios hill section is situated in central crete and composed mainly of mudstones and silts deposited in lacustrine brackish or estuarine environments several levels of this section have yielded the remains of mammals foraminifera ostracoda pollen and spores the palynological analysis shows that the region was forested in the upper miocene with a vegetation confined to climates warmer than the present the age indicated by foraminifera is early tortonian while the mammals were attributed to late vallesian in continental biochronology of this section 88 m have been sampled for a magnetostratigraphic study several experiments have been made on the magnetic properties of these sediments showing that they are suitable for a paleomagnetic study moreover x ray diffraction and microprobe analysis of 19 samples regularly distributed over the section show that no major changes occurred in the chemical composition from 77 separate sites 241 samples have been demagnetized thermally 215 or by alternating field 26 the main result of this study is the definition of a long reversed polarity period interrupted by two short normal intervals this magnetostratigraphic sequence can be correlated using biostratigraphic data with a part of chron 5 r of the geomagnetic time scale in other words this section is probably younger than 11 47 my and older than 10 30 my

high resolution magnetostratigraphic study of the paleogene neogene strata in the northern qaidam basin implications for the growth of the northeastern tibetan plateau abstract the cenozoic terrestrial intermontane qaidam basin on the northeastern edge of the tibetan plateau contains 12 km of sedimentary rocks that potentially document the accommodation of india asia convergence and the growth of the plateau the chronology remains incomplete hindering cross basin correlation between lithostratigraphic units and their further interpretation here we present a high resolution magnetostratigraphy spanning 5 km of paleogene neogene sequence at dahonggou in the northern qaidam basin based on correlation with the geomagnetic polarity time scale gpts we have dated the section to being between 52 and 7 ma the bottom conglomeratic unit ranging from 52 ma to 44 ma was deposited in high energy environments e g alluvial fan or braided river reflecting the earliest deformation and uplift of the basin bounding qilian shan fold thrust belt in response to india asia collision in addition we identified two major increases in sedimentation rate at 25 16 ma and after 9 5 ma and three phases of lesser increases at 52 44 ma 38 33 ma and 14 6 12 0 ma these increases in sedimentation rate are consistent with regional thermochronology and basin analysis studies which revealed enhanced motion on basin bounding thrust faults we argue that these accelerated sedimentation rates indicate pulsed tectonism in the northeastern tibetan plateau the pulse at 25 16 ma may further relate to phases of strong rainfall linked to an intense monsoon at that time

diagenetic evolution of the upper cretaceous limestone and sandstone exhumed reservoirs in the western basque cantabrian basin north spain abstract the basque cantabrian basin ne spain has been considered one of the most interesting areas for hydrocarbon exploration in the iberian peninsula since the 60th to 70th of last century this basin is characterized by the presence of numerous outcrops of tar sands closely associated with fractures and triassic diapirs the aims of this work is to establish the diagenetic evolution of the upper cretaceous reservoir rocks with special emphasis in the emplacement of oil and their impact on reservoir quality the studied rocks are constituted of carbonates and sandstones that contain massive quantities of bitumen filling vugs and fractures petrographic results indicate that the carbonate rocks from maestu outcrops are bioclastic grainstones and wackestones whereas the tar sandstones from atauri and loza outcrops are dominated by quartzarenites and subordinated subarkoses the paragenetic sequence of the main diagenetic phases and processes include pyrite bladed and drusy calcite cement calcite overgrowths silicification of bioclasts and microcrystalline rhombic dolomite cement and first stage of oil emplacement blocky calcite cement coarse crystalline calcite cement calcitized dolomite calcite veins saddle dolomite and stylolites filled by the second phase of oil entrance together with the above mentioned diagenetic alterations the arenites are affected by early kaolinitization of feldspars and the scarce formation of clay rim and epimatrix of illite all sandstones and dolomitized carbonate rocks show high intercrystalline and intergranular porosity which is full by biodegraded hydrocarbons solid bitumen the biodegradation affects alkanes isoprenoids and partially hopanes and steranes saturated hydrocarbons aromatics hydrocarbons like naphthalenes phenanthrenes dibenzothiophenes and triaromatics are also affected by biodegradation results indicate that the first hc emplacement corresponds to early stage of calcite and dolomite cementation and the second and more important emplacement is related to fracturation processes resulting in the formation of excellent reservoirs

geometry and petrophysical parameters of a calcarenitic turbidite lobe barremian aptian pas de la cluse france abstract several bioclastic resedimentation systems late barremian to early aptian in age outcrop in southeastern france the upslope portion of a calcarenitic turbidite lobe outcrops as a cliff 2 3 km wide and up to 45 m high oriented roughly perpendicular to the direction of palaeoflow it has been studied through 31 sections the studied body is point sourced and corresponds to a channel lobe transition located at the toe of slope of a urgonian carbonate platform on the western edge of the vocontian trough french subalpine chains the internal geometry of the lobe is the result of repeated erosion and filling of channels creating well defined stacked units filling phases result in lobe shaped morphology units evolve from base to top in an ordered way first the sediments exported from the platform and deposited in the basin are represented by decimetre thick sheet like and fine grained bouma a e and ab e sequences in this first unit erosion is limited and represented by shallow and symmetrical channels this initial phase of deposition corresponds to the formation of a lobe with a positive morphology the next phase of system development is more erosional and this is dominated by migrating sinuous channels several metres deep filled with coarse grained poorly graded turbidites these channels are first filled through lateral accretion and then sediment spills over to create a sub lobe the mixture of channel and lobe features could be considered as a carbonate equivalent of the channel levee system of siliciclastic deposits finally the system ends with the deposition of a flat based unit this unit extends for more than 2 km and is composed of the southwards lateral accretion of hundreds of metres wide sequences each composed of a debris flow deposit capped by a coarse grained calcarenite bed these sigmoidal bodies could be the ultimate forms of a spreading out phenomenon of the lobe overall the pas de la cluse system is a coarsening upward body as the grain size is related to the nature of the material this coarsening upward implies either an evolution in the nature of the carbonate material produced on the platform from grainy to bioconstructed a displacement of the source sediment from inner platform to margin or a displacement of the channel head through time from distal to proximal the upward evolution from classic turbidites to debris flow deposits can be explained either by the way the platform facies evolved with time in a depositional sequence or through retrogressive erosion by successive slope failures that cut deeper and deeper into the platform slope although its present day porosity is low the size of the system and the fact that it is encased in marls make it an interesting outcrop analogue for hydrocarbon exploration

silicification of sulphate evaporites and their carbonate replacements in eocene marine sediments tunisia two diagenetic trends samples of chert nodules diagenetic carbonates and evaporites gypsum anhydrite collected from the gypsiferous limestones of the kef eddour member ypressian priabonian near metlaoui and sehib tunisia show selective silicification with great variety in the silicified by products based on d 13 c values which support an organic origin for the carbon carbonates replaced evaporites microbially through bacterial sulphate reduction observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites anhydrite gypsum in the absence of early diagenetic carbonate phases silica with d 18 o values from 25 to 28ae6 standard mean ocean water smow replaced the outer parts of anhydrite nodules at ph 9 in the innermost parts of the anhydrite nodules prevented silica precipitation the record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus the microbial diagenetic replacement of evaporites bacterial sulphate reduction by carbonates calcite aragonite and dolomite favoured silica replacement of carbonates rather than evaporites silica with d 18 o signature of 21 to 26 smow replaced carbonates on a volume for volume basis yielding a more siliceous groundmass and accounting for 90 95 of the nodules the relatively higher d 18 o values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed meteoric marine to marine water the lower d 18 o values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage the silica supply for chert formation could be derived from the reworked bio siliceous deposits diatomites to the west of the basin vestiges of an opal ct precursor undetectable by x ray diffraction xrd were revealed by d 29 si magic angle spinning nuclear magnetic resonance investigations diagenesis of the extraformational and overlying clay rich beds the host limestones are clay poor as shown by xrd measurements and minor volcanogenic and hydrothermal contributions during early diagenetic stages

neogene uplift of the tian shan mountains observed in the magnetic record of the jingou river section northwest china the tian shan mountains constitute central asia s longest and highest mountain range understanding their cenozoic uplift history thus bears on mountain building processes in general and on how deformation has occurred under the influence of the india asia collision in particular in order to help decipher the uplift history of the tian shan we collected 970 samples for magnetostratigraphic analysis along a 4571 m thick section at the jingou river xinjiang province china stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary from this component a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between 1 ma and 23 6 ma with some uncertainty below 21 ma this correlation places precise temporal control on the neogene stratigraphy of the southern junggar basin and provides evidence for two significant stepwise increases in sediment accumulation rate at 16 15 ma and 11 10 ma rock magnetic parameters also undergo important changes at 16 15 ma and 11 10 ma that correlate with changes in sedimentary depositional environments together with previous work we conclude that growth history of the modern tian shan mountains includes two pulses of uplift and erosion at 16 15 ma and 11 10 ma middle to upper tertiary rocks around the tian shan record very young 5 ma counterclockwise paleomagnetic rotations on the order of 15 to 20 which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont

computer aided well log correlation 1 we have devised a computer aided well log correlation scheme using sequence comparisons originally developed for molecular biology and dynamic programming technique employed in operations research the combination of these two methods is known as dynamic waveform matching dwm problems encountered in machine correlation of well logs e g bed thickness changes stratigraphic gaps pinch outs etc may be resolved by dwm we have coded a program xcorr to implement our scheme manual or machine zonation of the logs is performed first then log attributes depth amplitude shape etc of two logs are compared and matched in a pass each matching cycle comprises three passes thus ensuring optimal global not local correlation we found good correspondence between g ologic structures and path diagrams which are the resulting matrix from the dynamic programming algorithm example logs both synthetic and real demonstrate the applicability and efficacy of our approach

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

qaidam basin paleosols reflect climate and weathering intensity on the northeastern tibetan plateau during the early eocene climatic optimum abstract as an important geological archive paleosols have been widely used in reconstruction of paleoenvironmental and paleoclimatic conditions in this study we undertook detailed analyses of the clay mineralogy and carbon and oxygen isotopic compositions of pedogenic carbonates in a series of 13 paleosol horizons within a fluvial section of the eocene 52 44 ma lulehe formation in the qaidam basin pedogenic smectite and illite smectite i s mixed layer minerals are the predominant clay minerals in the paleosols suggesting that the early eocene paleoclimate of the qaidam basin was strongly seasonal with warm wet summers the high levels of pedogenic smectite production and high atmospheric pco2 reconstructed from soil carbonate nodule 13c mean 7 1 0 8 are consistent with elevated temperatures during the early eocene climatic optimum eeco coeval paleosol carbonate nodules yield estimated paleo surface water compositions 18opsw of 7 9 to 6 0 vsmow which are significantly higher than the 18o value of modern summer precipitation in the qaidam basin the estimated temperature difference 5 0 c cooling implies uplift of the study area by nearly 1000 m to its current average elevation of 3000 m above sea level this paleoelevation estimate agrees with modern meteoric water isotopes of global network of isotopes in precipitation gnip sites in asia in the same elevation range

comment on cenozoic tectonic deformation and uplift of the south tian shan implications from magnetostratigraphy and balanced cross section restoration of the kuqa depression by tao zhang xiaomin fang chunhui song erwin appel and yadong wang tectonophysics 2014 doi 10 1016 j tecto 2014 04 044 abstract the recent paper entitled cenozoic tectonic deformation and uplift of the south tian shan implications from magnetostratigraphy and balanced cross section restoration of the kuqa depression by zhang et al tectonophysics 2014 doi 10 1016 j tecto 2014 04 044 discusses the cenozoic tectonic deformation and uplift of the south tianshan mountains by integrating tectonic investigations seismostratigraphic analysis and paleomagnetic dating of the thrust fold belt in the kuqa depression at the southern border of the tianshan to support their conclusions the authors have reinterpreted a high resolution magnetostratigraphic study of the kezilenuer section in a paper entitled magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china by huang et al earth and planetary science letters volume 251 pages 346 364 2006 we note here i that apparent conflicts in definition of the xiyu formation in the kuche depression as proposed by zhang et al 2014 require amplification and ii argue that their age assignment for the kezilenuer section is incorrect and yields an anomalously low sedimentation rate for the infilling of this foreland basin

danghe area western gansu china biostratigraphy and implications for depositional history and tectonics of northern tibetan plateau the danghe area in western gansu province is at the focal point of interaction of the northeastern end of the left lateral altyn tagh fault and growing ramp thrusts of the danghe nanshan mountains along the northern rim of the qinghai tibetan plateau with a thick sequence of tertiary sediments and associated fossil records the danghe area is one of few places on the tibetan plateau that can offer integrated studies of its tectonic history depositional environment and biological records including vertebrate and plant fossils past studies however have not been able to capitalize on the paleontological data or were often misguided by an outdated notion of regional chronology incorrect age estimates in these studies often have profound effects on tectonic interpretations we present new stratigraphic and paleontologic evidence from the danghe area and demonstrate a new chronologic scheme that indicates a much younger age for the majority of the sediments we recognize three packages of sediments in the danghe area 1 oligocene paoniuquan new name formation basal predominantly fine grained dark purple mudstones and siltstones that include the classic yindirte fauna which forms the basis of the tabenbulukian mammal age 2 early miocene to early late miocene tiejianggou new name formation that contains a new platybelodont proboscidean in a coarsening upward sequence terminating in a massive conglomerate and 3 an unnamed late neogene formation that contains another coarsening upward sequence in light of our new stratigraphic framework we reinterpret a previously published magnetic column for the middle sequence in xishuigou as representing chrons c6n through c4ar corresponding to a span of about 20 9 3 ma much younger than has been realized so far this new chronological framework suggests a depositional history of the danghe nanshan that spans at least the early oligocene through late miocene to possibly pliocene early depositions during the oligocene through middle miocene are dominated by fine grained sediments indicating a distant source of sedimentation paleontological data suggest a relatively dry environment as is typical of northern china today by late miocene around 9 12 ma the danghe area began to receive coarse sediments and was much closer to the mountain front the new stratigraphic framework indicates an earlier onset of sedimentation than has generally been assumed but also suggests the presence of sediments much younger than many have realized c 2003 elsevier science b v all rights reserved

chert occurrences in carbonate turbidites examples from the upper jurassic of the betic mountains southern spain in upper jurassic carbonate turbidites of the betic mountains southern spain chert occurs in three morphologies bedded chert nodular chert and mottled chert the last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock the three types of chert are formed by replacement of limestones and are associated with different calcareous facies turbidite packstones of saccocoma and peloids and turbidite lime mudstones of pelagic material contain bedded and nodular cherts the silicification textures are mainly micro and cryptocrystalline quartz with local chalcedonic quartz both length fast and length slow which is more common in the packstones only mottled chert is produced where calcareous breccia beds are silicified mottled chert consists of micro and cryptocrystalline quartz length slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz beds and nodules are the result of early diagenetic silicification with silica derived from the calcitization and dissolution of radiolarians and subordinately sponge spicules whereas mottled chert is the consequence of later silicification in a probably mg rich environment early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water a silica rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert