genesis of the late eocene bedded halite in the qaidam basin and its implication for paleoclimate in east asia abstract it has long been suggested that the eocene climate in east asia was characterized by three latitudinally distributed zones one middle arid zone and two humid zones to its north and south the middle arid zone was firstly proposed based on the widespread red beds and evaporites in a nww trending zone of china however this long standing model has recently been challenged by increasing doubts about the effectiveness of these arid climate indicators the qaidam basin as one of the eocene evaporites bearing basins in china can be studied to test whether the bedded evaporites can validly indicate arid climate over 300 m halite beds have been found in the late eocene upper xiaganchaigou uxg formation in a local saline center shizigou area of the qaidam basin its coexistence with source rocks and limited distribution together with the absence of red beds make it difficult to explain the halite genesis with the conventional climatic controlled playa model based on stratigraphic correlation lithological and mineralogical observation and oxygen carbon and chlorine isotopic analysis we identify several characteristics about the shizigou saline center 1 undulated lake floor 2 shallow water 3 limited hydrothermal impact 4 evaporation induced precipitation 5 high eolian influence and 6 fluctuated climate background according to these investigations we suggest a new model for the genesis of the bedded halite in the qaidam basin the tectonic climatic model the key point for this model is that the undulated topography caused by tectonic activity and the fluctuated climate together leaded to the hydrological isolation of the shizigou area the shizigou area was isolated from the larger brackish qaidam paleolake for independent evaporation during arid climate stage while connected with it during humid climate stage causing alternatively water influx and evaporation and inducing the occurrence of interbedded halite and source rocks thus the widespread bedded halite in the eocene strata over china may indicate fluctuated rather than arid climate in east asia

vegetation changes and hydrological fluctuations in the central ebro basin ne spain since the late glacial period saline lake records abstract although the central ebro basin northeastern iberian peninsula is both the northernmost semi arid area in europe and one of the regions with the largest biodiversity it has been insufficiently studied in terms of past climate variability due to the scarcity of suitable sites for palaeoenvironmental analyses previous studies from ephemeral saline lakes in the area mainly based on palynological data show abrupt and rapid arid humid transitions throughout the last glacial cycle highlighting a complex palaeohydrological evolution new cores from two saline lakes la playa and la salineta in the los monegros area provide multi proxy records including sedimentology geochemistry and pollen indicators this study together with a detailed review of the main saline records from the central ebro basin enables us to reconstruct the main features of the palaeoclimate evolution during the last glacial cycle one of the main results of this study is the alternation of humid and dry phases as a characteristic of the climate evolution during the late glacial additionally the study suggests an important role of the increased flow from the pyrenean rivers during deglaciation in the hydrological balance of the central ebro basin the early holocene is the wettest period of the studied sequences contrasting with the aridity of the middle holocene interval which is frequently absent as a result of intense aeolian erosive processes the lack of archaeological remains associated with the middle holocene neolithic also supports increased aridity that would have impeded human settlements although anthropogenic activity partially masks the climate signal from the palynological data in the uppermost part of the studied sequences there is some sedimentological evidence for a climate change during the last 2000 yr resulting in a recovery of average saline lake levels in the central ebro basin

early eocene asian climate dominated by desert and steppe with limited monsoons abstract paleoenvironmental reconstructions illustrated that asia was dominated by a zonal climate pattern in the eocene epoch with an almost latitudinal arid semiarid band extending from the eastern coast of china to central asia however the mechanism behind the formation of the broad zonal arid semiarid region and the possible occurrence of a monsoonal system remains unclear for that period here using a reconstructed land sea distribution and topography of 50 ma we employed a global climate model to simulate the early eocene climate in china a set of experiments with consideration to different topography orbital parameters vegetation sea surface temperature and atmospheric co2 concentration demonstrated that a large extent of arid semi arid region with annual precipitation less than 800 mm was distributed latitudinally between 20 n and 40 n paleolatitude in china this arid region was controlled by desert bwh and steppe bsh climates according to the koppen climate classification systems on the north of the zonal desert steppe region a mediterranean like climate occurred in central asia these simulations agree well with the geological reconstructions of a zonal desert steppe region and indicate that this pattern was primarily caused by subtropical high pressures associated with strong descending air however it is likely that the aridity in the zonal desert steppe was reduced occasionally by intensification of monsoon circulations over east asia the change in orbital parameters causing enhanced insolation in summer was the most likely factor that could enlarge the land sea thermal contrast and further intensify the monsoon circulations in the eocene epoch though its effect was reversible and impermanent in addition simulations show that east asian monsoon did not play a leading role in chinese paleogene climate although it may have affected eastern china occasionally

evaporites and siliciclastics of the permian nippewalla group of kansas usa a case for non marine deposition in saline lakes and saline pans the mid permian nippewalla group of kansas consists of bedded evaporites red bed siliciclastics and grey siliciclastics deposited in a non marine environment lithologies and sedimentary features indicate lacustrine and aeolian deposition subaerial exposure and palaeosol formation grey siliciclastic mudstones characterized by planar and convolute laminations ostracods peloids and plant material represent a freshwater brackish perennial lake facies bedded anhydrites containing gypsum crystal pseudomorphs clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes bedded halites consist of chevron and cumulate crystals dissolution surfaces and pipes and mudcracked microcrystalline salt crusts which were deposited in saline pans dominated by flooding evaporative concentration and desiccation chaotic halite composed of red bed mudstone and siltstone with displacive halite crystals formed in saline mudflats red bed mudstone and siltstone with little or no displacive halite but with abundant cracking root and plant features suggest deposition in a dry mudflat red bed sandstone composed of well sorted well rounded quartz grains cemented with halite indicate aeolian and rare shallow water deposition most deposition took place in halite dominated ephemeral saline lakes surrounded by saline and dry mudflats sandflats and sand dunes evaporation desiccation flooding and wind played significant roles in this environment the nippewalla group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non marine acidic saline pan system in the mid continent of north america the problem of distinguishing between ancient marine and non marine evaporites as well as recognizing those evaporites deposited in acid settings with detailed field core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed in addition interpretations of mid permian palaeoclimate data in the form of short term air temperature proxies within longer term wet dry trends have been made these data provide a new palaeogeographic and palaeoclimatic model for the mid permian of western pangaea

miocene sedimentary environment and climate change in the northwestern qaidam basin northeastern tibetan plateau facies biomarker and stable isotopic evidences abstract facies biomarker and stable isotopic records from the miocene lacustrine sediments in the northwestern qaidam basin were investigated to reconstruct the miocene sedimentary environment and climatic history three distinct facies can be recognized these include the following 1 gray black laminated mudstone and marlstone which represent a semi deep fresh to semi brackish lake environment 2 gray yellowish massive mudstone marlstone and siltstone and 3 yellowish massive sandstone which imply a shallow brackish lake environment the decreasing c 27 c 31 and c 27 c 29 c 31 c 33 values the increasing acl mean chain length values of n alkanes and the vertical evolution of sedimentary environments indicate the overall intensified aridity which is considered to be an integrated result of high elevation of the himalaya tibetan system retreat of the paratethys and global cooling high fluctuations of the 18 o values and primary dolomite contents reveal the hydrologically closed paleolake with intermittently open conditions in the study area during middle late miocene the qaidam basin is suggested to be hydrologically segmented based on the stable isotopic data comparison between the study area and the northeastern area the most negative end of the oscillations of the 18 o values indicating the minimal evaporation which likely represents the isotopic ratio of the meteoric water surprisingly conveys stability in the shang youshashan and shizigou formations and displays a positive 2 5 shift this significant shift was probably due to the climatic aridification and air mass changes around 10 8 ma rather than the global cooling

early middle miocene topographic growth of the northern tibetan plateau stable isotope and sedimentation evidence from the southwestern qaidam basin abstract this study presents facies analysis and carbonate stable isotopic records of a 4435 m thick oligocene middle miocene 30 to 11 ma stratigraphic section from the southwestern qaidam basin which place constraints on the interplay between topographic growth and climatic hydrologic changes in the northern tibetan plateau three types of carbonate including marl limestone pedogenic carbonate and carbonate cement were collected and went through detailed screening for diagenesis carbonate cements are primary early diagenetic carbonates formed in shallow ground water setting the majority of marls limestones and pedogenic carbonates are primary micritic carbonates whereas a few of them showing alterations were sub sampled to avoid diagenetic phases a negative shift in the 18o values of marls limestones and depositional environment change from palustrine to shallow lacustrine associated with paleohydrological change at 20 ma are likely caused by tectonic activity in adjacent ranges around 15 14 ma a decrease of 1 5 in the most negative 18o values of carbonate cements is interpreted to reflect the topographic growth of surrounding mountain ranges i e altun shan east kunlun range this inference is supported by the sedimentary facies change from marginal lacustrine to fluvial that was accompanied by an abrupt increase in sedimentation rate finally a 1 5 positive shift in the most negative 18o values of carbonate cements and pedogenic carbonates enriched 13c values of pedogenic carbonates as well as sedimentary environment change from fluvial to braided river alluvial fan at 13 12 ma most likely reflect a regional aridification event observed over a large area of the central asia this aridification event may have been associated with continued topographic growth of the northern tibetan plateau to a critical elevation to block moisture e g westerlies from penetrating the interior of central asia

the thermo tectonic evolution of the southern congo craton margin as determined from apatite and muscovite thermochronology abstract the southern irumide belt sib of zambia consists of predominantly mesoproterozoic terranes that record a pervasive tectono metamorphic overprint from collision between the congo and kalahari cratons in the final stages of gondwana amalgamation this study applies multi method thermochronology to samples throughout southern zambia to constrain the post collisional phanerozoic thermo tectonic evolution of the region u pb apatite and 40ar 39ar muscovite data are used to constrain the cooling history of the region following congo kalahari collision and reveal ages of c 550 450 ma variations in the recorded cooling ages are interpreted to relate to localised post tectonic magmatism and the proximity of analysed samples to the congo kalahari suture apatite fission track data are used to constrain the low temperature thermo tectonic evolution of the region and identify mean central ages of c 320 300 210 200 and 120 110 ma thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the phanerozoic carboniferous to permian triassic heating is suggested to relate to the development of karoo rift basins found throughout central africa and constrain the timing of sedimentation in the basin permian to jurassic cooling is identified in a number of samples reflecting exhumation as a result of the mauritanian variscan and gondwanide orogenies subsequent cooling of the majority of samples occurs from the cretaceous and persists until present reflecting exhumation in response to larger scale rifting associated with the break up of gondwana each model reveals a later phase of enhanced cooling beginning at c 30 ma that if not an artefact of modelling corresponds to the development of the east african rift system the obtained thermochronological data elucidate the previously unconstrained thermal evolution of the sib and provides a refined regional framework for constraining the tectonic history of central africa throughout the phanerozoic

how to find high quality petroleum source rocks in saline lacustrine basins a case study from the cenozoic qaidam basin nw china abstract despite having made significant contributions to global hydrocarbon occurrences source rocks in saline lacustrine basins show great mineralogical and organic geochemical heterogeneity and the controlling mechanism is weakly studied here we use the cenozoic qaidam basin as an example to compare the organic and inorganic parameters of fine grained sediments in different lacustrine facies and to figure out the characterization and distribution of effective petroleum source rocks in saline lacustrine systems organic inorganic database of six selected boreholes show that in the bench margin high gradient lacustrine system fine grained sediments in sublittoral brackish mesosaline zones containing low chlorides

chlorine isotopic geochemistry of salt lakes in the qaidam basin china abstract the isotopic compositions of chlorine in salt lake brine coexisting with halite oil field water and river water from the qaidam basin qinghai china have been examined using high precision measurement of chlorine isotopes based on thermal ionization mass spectrometry of the cs 2 cl ion 37 cl values vary from 2 05 to 2 94 with average values of 2 94 for hot spring water 1 35 for river water 0 38 for oil field brine 0 40 for saline water and 0 63 for salt lake brine the preliminary results show isotopic fractionation of chlorine in salt lake brine during salt precipitation the variation of the 37 cl value in these salt lakes is mainly associated with brine evaporation and halite precipitation and with the hydrochemistry of the input water the chlorine isotopic composition may reflect the degree of evaporation of the salt lake brine and may be used to distinguish brines that are in equilibrium with precipitating salts and brines that have dissolved earlier evaporates

do red beds indicate paleoclimatic conditions a permian case study abstract terrestrial red beds have long been interpreted as desert deposits by comparison with modern red deserts more recently red beds have been interpreted as evidence of seasonally dry conditions and a permo triassic pangean monsoon red beds of cala viola sardinia are identified as paleosols and used to reconstruct late permian paleoclimatic conditions reconstruction of paleoenvironmental conditions based on the paleosols of the cala viola indicates warm humid conditions with no evidence of dry conditions as in a desert or of extreme seasonality as in a monsoon instead it is suggested that the red color of the paleosols is a result of former good drainage and that red color in general does not indicate specific paleoclimatic conditions

evidence for deep water deposition of abyssal mediterranean evaporites during the messinian salinity crisis scientific drilling of the abyssal evaporites beneath the deepest parts of the mediterranean basin gave rise to the idea that the mediterranean sea completely evaporated at the end of the messinian herein we show using new organic geochemical data that those evaporites were deposited beneath a deep water saline basin not in a subaerial saltpan as originally proposed abundant fossil organic lipids were extracted from evaporites in mediterranean deep sea drilling project cores the archaeal lipid distribution and new analyses using the ace salinity proxy and tex86 temperature proxy indicate that surface waters at the time of evaporite deposition had normal marine salinity ranging from 26 to 34 practical salinity units and temperatures of 25 28 c these conditions require a deep water setting with a mixed layer with normal marine salinity and an underlying brine layer at gypsum and halite saturation after correction for isostatic rebound our results indicate maximum drawdown of 2000 m and 2900 m relative to modern sea level in the western and eastern mediterranean basins respectively our results are consistent with previously proposed scenarios for sea level drawdown based on both subaerial and submarine incision and backfilling of the rhone and nile rivers which require messinian sea level drops of 1300 m and 200 m respectively this study provides new evidence for an old debate and also demonstrates the importance of further scientific drilling and sampling of deeper part of the abyssal messinian units

step wise change of asian interior climate preceding the eocene oligocene transition eot abstract understanding the global climate change from greenhouse to icehouse conditions at the eocene oligocene transition eot 34 million years ago requires climatic records from oceanic as well as continental realms of the key late eocene doubthouse period preceding this switch here we report integrated stratigraphic results from well dated late eocene continental mudflat to saline lake paleoenvironments of the xining basin northeastern tibetan plateau western china recording regional and global change cyclostratigraphic analysis strongly suggests continuous dominance of the 41 kyr obliquity cycle in the whole late eocene interval down to the base of polarity chron c18n 2n at 39 ma with additional input of the 100 kyr eccentricity cycle up to the base of chron c13r at 34 7 ma this might imply that high latitude climates dominated the area long before the eot probably related to incipient ice volume fluctuations furthermore our results reveal two paleoenvironmental deterioration steps preceding the eocene oligocene transition the first step occurs in the top of chron c17n 1n at 36 6 ma this age closely corresponds to 1 the high altitude pollen appearance in chron c16 2r at 36 4 ma in the same section 2 the recently dated final retreat of the tarim sea in western china and 3 a shift from precession to obliquity dominance in the atlantic ocean this near co occurrence suggests global change at this time we hypothesize this change is related to an increase in incipient ice sheet volume leading to passing threshold conditions for the high altitude pollen appearance and tarim sea retreat finally leading to decreased moisture availability in the xining basin at the second step in the base of chron c13r at 34 7 ma a substantial increase in clastic sedimentation rates is observed this might relate to increased climate variability preceding the greenhouse to icehouse transition at the eot that prevented landscapes to attain equilibrium configurations

glauberite halite association of the zaragoza gypsum formation lower miocene ebro basin ne spain glauberite is the most common mineral in the ancient sodium sulphate deposits in the mediterranean region although its origin primary or diagenetic continues to be a matter of debate a number of glauberite deposits of oligocene miocene age in spain display facies characteristics of sedimentologic significance in particular those in which a glauberite halite association is predominant in this context a log study of four boreholes in the zaragoza gypsum formation lower miocene ebro basin ne spain was carried out two glauberite halite lithofacies associations a and b are distinguished association a is composed of bedded cloudy halite and minor amounts of massive and clastic glauberite association b is made up of laminated to thin bedded clear macrocrystalline massive clastic and contorted lithofacies of glauberite and small amounts of bedded cloudy halite transparent glauberite cemented by clear halite as well as normal graded and reverse graded glauberite textures are common this type of transparent glauberite is interpreted as a primary subaqueous precipitate gypsum thenardite or mirabilite are absent in the two associations the depositional environment is interpreted as a shallow perennial saline lake system in which chloride brines association a and sulphate chloride brines association b are developed the geochemical study of halite crystals bromine contents and fluid inclusion compositions demonstrates that conditions for co precipitation of halite and glauberite or for precipitation of na sulphates mirabilite thenardite were never fulfilled in the saline lake system

cenozoic environmental evolution of the qaidam basin and its implications for the uplift of the tibetan plateau and the drying of central asia abstract a complete cenozoic pollen sequence of the qaidam basin has been constructed with absolute age control the cenozoic environmental evolution of the qaidam basin and its implications for the uplift of the tibetan plateau and the drying of central asia during the cenozoic era is discussed based on the interpretation of the pollen sequence the commencement of eolian deposition in central china and the change of climatic distribution pattern of china the results suggest that in the qaidam basin relatively dry climates prevailed in the eocene oligocene and pliocene pleistocene these dry periods resulted from the effects of a subtropical high pressure zone and the uplift of the surrounding mountains and the tibetan plateau respectively about a 600 km northward drift of western china has probably occurred during the late cenozoic resulting in the eocene oligocene arid climate belt twisting about six degrees of latitude the tibetan plateau reached probably 1500 m elevation by the latest miocene 8 5 ma not as high as it was thought previously the cenozoic environmental evolution of northwest china does not support the idea that the retreat of the paratethys sea played an important role in the late cenozoic drying of central asia at least not in northwest china

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

a direct comparison between the historical record of lake level and the δ18o signal in carbonate sediments from lake turkana kenya we compared oxygen isotopic profiles from authigenic calcite in nine freeze cores from lake turkana kenya with the historical record of lake levels for the past 105 years 210pb dating of the cores indicates that sediment in two of the cores spans the complete historical record we observed spatial variability in 6180 and 617c values between cores and an offset from expected isotopic equilibrium values in several of the cores both of these characteristics can be explained by proximity to river input in model calculations of lake level we muted the spatial variability of the s1 o dataset by using a normalized and stacked record there is good agreement between the lake level history modeled using the normalized and stacked record and the historical record of lake level the lake level record derived from the s180 data indicates that lake level fluctuated by 15 m during the last century our model calculations indicate that s o records can be used to generate quantitative lake level curves even using datasets with significant spatial variability the isotopic composition of carbonates has been used in numerous lacustrine studies to determine paleoclimatic information fluctuations in the isotopic record preserved in carbonates are created mainly by variations in the temperature and composition of the water in which the carbonate formed in turn these variations can be caused by climatic changes in the lake basin carbonates are therefore potentially an archive of the paleoenvironment of a lake basin in tropical or closed basin settings shifts in the oxygenstable isotopic composition s180 of carbonates are often attributed to shifts in the evaporation to input e z ratio of a lake previous workers have used the relationship between changes in the s180 value of carbonates and the e z ratio of a lake basin to discuss qualitative changes in paleo lake levels although the relationship between the o composition of lake water and the various inputs and outputs to a basin has been described quantitatively gat 1984 phillips et al 1986 benson and white 1994 only one quantitative comparison between a historical lake level record and lake level determined from the ai80 composition of carbonates has been attempted li 1995

revisiting the paleogene climate pattern of east asia a synthetic review abstract east asian paleogene climates have long been regarded as controlled by the planetary wind system which might result in a climate pattern with three latitudinally distributed zones two humid zones located separately in the north and south were lithologically designated by coals and oil shales while an arid zone in the middle was represented by red beds and evaporites because the middle arid zone was located along 30 n paleolatitude its presence had been further linked with a then subtropical high however this long standing model has recently been challenged by growing evidence from petrology sedimentology paleontology paleobiogeography paleoclimatology and climate modeling here we review the primary data from these disciplines and reinterpret their climate significances to revisit the east asian climate pattern during the paleogene petrologically while the occurrence of coals and or oil shales is accepted as an indicator for overall humid climates that of red beds and or evaporites is highly equivocal to exclusively indicate perennial arid climates unless their origins are carefully investigated in reality generic red beds merely represent an oxidizing environment not essentially associated with a single specific climate type meanwhile evaporites although typically precipitated in arid environments may be deposited in either perennial dry or seasonal monsoonal climates there is no solid evidence so far to convincingly support that the landscape of the so called middle arid zone was dominated by desert and or steppe under a then subtropical high during most of the paleogene the plant function type study additionally suggests that the middle arid zone appears to be lack of xerophytic vegetation even though some xerophytic or sclerophyllous plant taxa did sporadically occur interestingly paleozoological data show that the paleogene mammalian faunas were somewhat equably distributed over east asia strongly suggesting the evident absence of a critical biogeographical or climatic barrier stretched across the middle arid zone as the planetary wind model implied in contrast to the planetary wind model monsoonal or monsoon like paleogene climates have been broadly reported from the northern middle and southern east asia as well as adjacent regions of russia and kazakhstan if only the indicators for humid climates are considered simply due to the uncertainty of those for perennial arid climates east asia must have had a relatively dry region in the continental interior during the late eocene to oligocene transition likely caused by the continentality and or the rain shadow effect along with the global cooling the monsoonal interpretation is highly in agreement with the evidence from floras faunas basin analyses and modeling experiments and well explicates the paleogene climate distribution and seasonal dynamics of east asia however further studies will be largely needed to verify whether uniformly according to the modern criteria the paleogene climates of the east asia interior can be accurately attributed to the arid category

cenozoic deformation history of the qaidam basin nw china results from cross section restoration and implications for qinghai tibet plateau tectonics abstract the deformation and uplifting history of the qinghai tibet plateau is a controversial issue and so far there has been insufficient evidence for interpretation investigations of the cenozoic deformation in the northeastern edge of this plateau may provide some relevant information in this paper the cenozoic deformation history of the qaidam basin has been studied based on results from the balanced cross section restoration of six geological sections they indicate that 1 the qaidam basin has been undergoing continuous shortening since the beginning of cenozoic with two relatively fast shortening phases the first during 42 8 40 5 ma and the second during 2 8 ma present 2 the shortening rate of the qaidam basin reached its maximum value since 2 8 ma and approximately 32 of the average total cenozoic shortening took place in this period and 3 the shortening of all sections reached approximately 50 of their respective total shortening by miocene the cenozoic deformation history of the qaidam basin shows good synchroneity with uplifting history of the qinghai tibet plateau it is therefore proposed that the deformation and uplifting in the south and north edges of the qinghai tibet plateau was almost synchronous with the altyn tagh fault formed at the beginning of india asia collision accordingly it is postulated that since the india asia collision at the beginning of cenozoic the indian plate moved northward continuously with two relatively fast phases the first during 42 8 40 5 ma and the second during 2 8 ma present approximately 50 of the total cenozoic northward displacement of the indian plate of the total amount of crustal shortening and uplifting of the qinghai tibet plateau and of the total cenozoic strike slip displacement of the altyn tagh fault were probably reached by the early miocene moreover another 30 40 might have taken place during 2 8 ma present the periods during 42 8 40 5 ma and during 2 8 ma present may also be the two fast phases for crustal shortening and uplifting of the qinghai tibet plateau and strike slip displacement of the altyn tagh fault

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

stable chlorine isotopes in arid non marine basins instances and possible fractionation mechanisms abstract stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite halite and dissolved chloride in groundwater that has interacted with halite in arid non marine basins has a 37 cl range of 0 3 far greater than the range for marine evaporites basins characterized by high positive 1 to 3 near 0 and negative 0 3 to 2 6 are documented halite in weathered crusts of sedimentary rocks has 37 cl values as high as 5 6 salt excluding halophyte plants excrete salt with a 37 cl range of 2 1 to 0 8 differentiated rock chloride sources exist e g in granitoid micas but cannot provide sufficient chloride to account for the observed data single pass application of known fractionating mechanisms equilibrium salt crystal interaction and disequilibrium diffusive transport cannot account for the large ranges of 37 cl cumulative fractionation as a result of multiple wetting drying cycles in vadose playas that produce halite crusts can produce observed positive 37 cl values in hundreds to thousands of cycles diffusive isotope fractionation as a result of multiple wetting drying cycles operating at a spatial scale of 1 10 cm can produce high 37 cl values in residual halite chloride in rainwater is subject to complex fractionation but develops negative 37 cl values in certain situations such may explain halite deposits with bulk negative 37 cl values future field studies will benefit from a better understanding of hydrology and rainwater chemistry and systematic collection of data for both cl and br

sedimentological and mineralogical records from drill core skd1 in the jianghan basin central china and their implications for late cretaceous early eocene climate change abstract the jianghan basin central china contains a thick continuous sequence of early cenozoic evaporite bearing deposits that provide a wealth of information about early cenozoic climate change and evaporites accumulation in east asia however the paleoclimatic history of the basin and its effect on the evaporites are largely unknown we obtained a deep drill core skd1 2346 5 m long from the late cretaceous to early eocene evaporite bearing deposits in the jianghan basin detailed analyses of the lithofacies mineralogy and geochemistry of the core indicate that salinization and deepening of the lake occurred corresponding to the yuyang formation to the lower xingouzui formation followed by desalting and shallowing corresponding to the upper xingouzui formation reconstruction of the spatiotemporal evolution of the evaporites in the jiangling depression suggests a transition from semi arid conditions in the late cretaceous to extremely arid conditions in the initial stage of the eocene the climate then became semi humid during the deposition of the upper xingouzui formation indicating a monsoon or monsoon like climate in east asia during the early eocene the southward migration of the depocenter of the jiangling depression coupled with a warm and extremely arid climate resulted in the precipitation of evaporites during the accumulation of the upper shashi formation and the lower xingouzui formation these results indicate great potential for identifying potash resources within the subsag in the southern jiangling depression

high resolution magnetostratigraphy of the neogene huaitoutala section in the eastern qaidam basin on the ne tibetan plateau qinghai province china and its implication on tectonic uplift of the ne tibetan plateau abstract the closed inland qaidam basin in the ne tibetan plateau contains possibly the world s thickest 12 000 m continuous sequence of cenozoic fluviolacustrine sedimentary rocks this sequence contains considerable information on the history of tibetan uplift and associated climatic change however work within qaidam basin has been held back by a paucity of precise time constraints on this sequence here we report on a detailed paleomagnetic study of the well exposed 4570 m huaitoutala section along the keluke anticline in the northeastern qaidam basin where three distinct faunas were recovered and identified from the middle miocene through pliocene constrained by these faunal ages the observed thirty three pairs of normal and reversed polarity zones can be readily correlated with chrons 2n 5br of the gpts this study assigns the age of the section to the interval between ca 15 7 ma to 1 8 ma in addition the widely used stratigraphic units the xia youshashan shang youshashan the shizigou and qigequan formations were formed at 15 3 ma 15 3 8 1 ma 8 1 2 5 ma and

sodium sulphate deposits of neogene age the kirmir formation beypazari basin turkey the evaporite member of the kirmir formation was deposited in shallow lacustrine environments during the upper miocene the most soluble minerals of this member can be currently observed in the cayirhan mine the evaporite member which is composed of secondary gypsum at outcrop can be subdivided into a bedded lower unit and a massive upper unit in the bedded lower unit most of the gypsum throughout the basin can be identified as having been transformed from glauberite in the glauberite layers of the cayirhan mine some glauberite textures clear glauberite suggest a primary subaqueous free precipitation on a depositional floor more common however are the glauberite textures indicating an interstitial growth within a clayey magnesitic matrix in the thenardite layers accompanying the glauberite in the cayirhan mine some disruption structures can be assigned to synsedimentary dissolution these structures together with the textures of the thenardite suggest that the original sodium sulphate was mirabilite thenardite being a secondary phase which formed during early to moderate burial diagenesis the massive upper unit in which evidence of sodium bearing minerals is absent is characterized by laminated to banded gypsum and nodular gypsum in the marginal areas of the evaporitic basin whereas thick clast supported gypsum breccias prevail in the northern deeper part of the basin the brecciation of these calcium sulphate layers occurred as a result of synsedimentary gravitative slumping under tectonic control although the sulphur isotopic values d 34 s of the sulphates of the kirmir formation suggest a marine derived brine supply the oxygen isotopic values d 18 o and the strontium ratios 87 sr 86 sr do not support such a supply the origin of the mother brines the glauberite genesis the depositional model of the sodium sulphates and the salinity evolution are discussed d 2002 elsevier science b v all rights reserved

boron isotopic composition of marine and nonmarine evaporite borates abstract here we present the results of the boron isotopic analysis of borates from some well known marine and nonmarine evaporite deposits combining new analyses with analyses from the literature yields a mean 11 b value for nine marine evaporite borate analyses of 25 4 permil 11 b 10 b 4 15 0 02 and a mean for twenty five nonmarine evaporite borates of 7 10 permil 11 b 10 b 4 02 0 04 previous studies have shown that the boron isotopic composition of seawater 11 b 40 permil 11 b 10 b 4 20 is about four percent heavier than the boron isotopic composition of most crustal rocks and minerals we suggest that our results for marine and nonmarine evaporite borates reflect this difference thus it should be possible to use boron isotopic composition as a tracer for boron of marine evaporitic origin applications include borate bearing evaporite deposits and perhaps borate and or borosilicate bearing metamorphic rocks

did the mediterranean sea dry out during the miocene a reassessment of the evaporite evidence from dsdp legs 13 and 42a cores abstract the idea that the mediterranean sea dried out completely during the late miocene messinian 5 6 ma is now widely accepted this idea first published in 1972 hsu 1972a hsu 1972b grew out of the interpretation by deep sea drilling project dsdp scientists that the evaporites recovered from beneath the floor of the mediterranean by drilling on dsdp legs 13 1970 and 42a 1975 were of shallow water origin since that time there have been a number of advances in understanding of evaporite depositional processes and with this in mind we have reexamined the evidence provided by the dsdp evaporite cores we show that the case for deposition of the miocene evaporites of the sub mediterranean under shallow water conditions is based on interpretations that are either equivocal or incorrect several features of these evaporites used by the dsdp workers as evidence of shallow water conditions are more compatible with deposition under deep water below wave base conditions and others can only be considered as of uncertain origin in addition coring has sampled only the upper few tens of meters of the evaporites a factor that seriously limits any interpretation of the origin of the deposit as a whole in view of these findings we suggest that the question of the origin of the miocene evaporites beneath the floor of the mediterranean sea be reexamined by the geologic and oceanographic communities

decoupling of foreland basin subsidence from topography linked to faulting and erosion many mountain ranges in regions of plate convergence are flanked by foreland sedimentary basins that are usually thought to result from flexing of the elastic lithosphere under the passive weight of sediments and thrust sheets emplaced in the mountain belt however in some places the depth of the foreland basin bears little relation to the weight of basin sediments and neighboring mountains implying that this model is too simple and or that additional forces must affect subsidence here i investigate this problem using a numerical model that incorporates coupling between an upthrusting mountain range and a subsiding basin i show that only part of the subsidence reflected in the shape of a foreland basin is a flexural isostatic response to crustal thickening and sediment loading the remaining subsidence results from dragging of the basin margin downward by slip on a range front fault and related flexural deformation of the surrounding lithosphere the maximum basin depth is shown to increase with increasing fault displacement and can become decoupled from the elevation of the mountain range especially as erosion becomes increasingly important deeply eroded model mountain belts may have foreland basins several times deeper than flexural models predict on the basis of surface loads these results challenge the prevailing view that fault bounded foreland basins result solely from the passive weight of mountains and sediments acting on the lithosphere and they highlight that slip on major range front reverse faults when repeated over many earthquake cycles may be recorded in their geometry and stratigraphic records

post break up tectonic inversion across the southwestern cape of south africa new insights from apatite and zircon fission track thermochronometry the south west african margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning recent attention focused on this margin has included theoretical modelling studies of rift processes plate kinematic studies of the opening geometry and timing and empirical studies focused on documenting the crustal structure and offshore sedimentary record here we examine the onshore geomorphic and tectonic response to rifting and breakup with a specific focus on the sw cape of south africa we present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of south africa the data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the early cretaceous 150 130 ma and late cretaceous 100 80 ma respectively both periods of enhanced erosion are regional in extent involved km scale erosion and extend well inland of the current escarpment zone albeit with spatially variable intensity and style the late cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km scale differential displacement and erosion the new aft data do not exclude the possibility of modest surface uplift occurring during the cenozoic but they restrict the depth of regional cenozoic erosion on the western margin to less than c 1 km the inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore orange and bredasdorp basins it is suggested that the late cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the sw cape by local tectonic forces arising from dextral displacement of the falkland plateau along the falkland agulhas fracture zone

paleogene temperature gradient seasonal variation and climate evolution of northeast china abstract continental paleogene climates have been well studied in europe and north america but very little is known from asia because paleoclimatic results have only been reported from particular geological intervals here based on 29 plant assemblages from 8 well age controlled fossiliferous sites we quantitatively reconstruct the climates through most of the paleogene of northeast china and discuss related seasonal variations our results demonstrate that the mean annual temperature mat gradient was fairly shallow 0 27 c 1 latitude during the paleocene throughout this region in the eocene seasonality was high in the region indicated by marked differences in both temperature and precipitation between winters and summers of the sites the paleo east asian monsoon must have had intensified at least in the late mid eocene shown by apparent differences in annual precipitation distribution at all the sites regarding the paleogene climatic evolution of northeast china our quantitative results suggest that mat overall declined from warm in the paleocene and eocene to moderate in the oligocene generally consistent with the trend of marine records but with some distinctions two significant cooling events are recognized in the early and mid eocene with mat 3 4 c and 3 8 c lower and winter temperature 5 8 c and 4 7 c lower respectively in similar magnitudes to corresponding variations in europe and north america furthermore the present results show that mat rebounded in the late mid eocene and then decreased until the oligocene a similar pattern demonstrated in europe during the mid eocene to oligocene interval

chlorine isotopes unravel conditions of formation of the neoproterozoic rock salts from the salt range formation pakistan during the late neoproterozoic the salt range in pakistan was one of the regions where the tethys truncated and marine strata developed the numerous transgressions and regressions that occurred d

synoptic reconstruction of a major ancient lake system eocene green river formation western united states numerous 40ar 39ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the greater green river piceance creek and uinta basins of wyoming colorado and utah constrain 8 m y of the eocene epoch multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance 40ar loss and 39ar recoil when considered in conjunction with existing radioisotopic ages and lithostratigraphy biostratigraphy and magnetostratigraphy these new age determinations facilitate temporal correlation of linked eocene lake basins in the laramide rocky mountain region at a significantly increased level of precision to compare our results to the geomagnetic polarity time scale and the regional volcanic record the ages of eocene magnetic anomalies c24 through c20 were recalibrated using seven 40ar 39ar ages overall the ages obtained for this study are consistent with the isochroneity of north american land mammal ages throughout the study area and provide precise radioisotopic constraints on several important biostratigraphic boundaries applying these new ages average sediment accumulation rates in the greater green river basin wyoming were approximately three times faster at the center of the basin versus its ramp like northern margin during deposition of the underfilled wilkins peak member in contrast sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled tipton and laney members sediment accumulation patterns thus reflect basin center focused accumulation rates when the basin was underfilled and supply limited accumulation when the basin was balanced filled to overfilled sediment accumulation in the uinta basin at indian canyon utah was relatively constant at 150 mm k y during deposition of over 5 m y of both evaporative and fluctuating profundal facies which likely reflects the basin margin position of the measured section the most rapid sediment accumulation for the entire system 1 m k y occurred between 49 0 and 47 5 ma when volcaniclastic materials from the absaroka and or challis volcanic fields entered the green river formation lakes from the north our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the green river formation basins coupled with previously published paleocurrent observations our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes the overall character of eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding laramide landscape freshwater overfilled lakes were initially dominant 53 5 52 0 ma possibly related to high erosion rates of remnant cretaceous strata on adjacent uplifts expansion of balanced fill lakes first occurred in all green river formation basins at 52 0 51 3 ma and again between 49 6 and 48 5 ma evaporative underfilled lakes occurred in various basins between 51 3 and 45 1 ma coincident with the end of the early eocene climatic optima and subsequent onset of global cooling defined from marine record however evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate evaporative terminal sinks were initially located in the greater green river and piceance creek basins 51 3 48 9 ma then gradually migrated southward to the uinta basin 47 1 45 2 ma this history is likely related to progressive southward construction of the absaroka volcanic province which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient the greater green river and piceance creek basins were eventually filled from north to south with absarokaderived detritus at sedimentation rates 1 2 orders of magnitude greater than the underlying lake deposits

late eocene clay boron derived paleosalinity in the qaidam basin and its implications for regional tectonics and climate abstract the qaidam basin located on the northeastern tibetan plateau and containing cenozoic sediments with a maximum thickness of 12 000 m is an ideal place to study the phased uplift of the ne tibetan plateau and regional climate change the estimation of the paleosalinity of sedimentary environments not only helps to evaluate the evolution of lakes in this region but offers insights into contemporaneous climate change we present detailed geochemical and mineralogical investigations from the lacustrine interval of the hongliugou section in the northern qaidam basin to reconstruct salinity fluctuations in the paleolake during the late eocene era 42 0 35 5 ma the clay mineral assemblages mainly contain smectite illite chlorite kaolinite and irregular illite smectite mixed layers clay boron derived paleosalinity estimates equivalent boron content couch s paleosalimeter and b ga ratios along with other proxies sensitive to salinity changes e g rb k ratios and ostracod assemblages collectively indicate an overall brackish sedimentary environment with a higher salinity period at approximately 40 0 39 2 ma this higher salinity period indicates a more arid environment and is probably related to global cooling however the global cooling in late eocene cannot explain the overall stable long term salinity pattern implying that other factors exist we propose that the migration of the yiliping depression depocenter in the northern qaidam and increased orographic rainfall induced by late eocene tectonic activity at the northern margin of the basin might have partly offset the increase in salinity driven by global cooling

magnetostratigraphy of paleogene sediments from northern qaidam basin china implications for tectonic uplift and block rotation in northern tibetan plateau abstract we have carried out a detailed magnetostratigraphic study of tertiary sediments in the northern margin of the qaidam basin northern tibetan plateau a total of 400 paleomagnetic samples with an interval of about 2 m were collected from the shanggancaigou and xiagancaigou formations in the hongsanhan section stepwise thermal demagnetization was used to isolate the magnetic components in these samples a higher temperature component htc was separated between 300 and 680 c from most of samples positive fold and reversal tests suggest a primary origin for the htc a magnetic polarity sequence is established and then compared with the geomagnetic polarity time scale ostracoda assemblages and also a fission track age from an adjacent section suggest a tertiary age of the shanggancaigou and xiagancaigou formations in the hongsanhan area the magnetozones in the xiagancaigou formation may correspond to subchrons c16n c18 ranging from 40 to 35 5 ma mid eocene and the shanggancaikou formation to subchrons c8 2n c15 age from 35 5 to 26 5 ma late eocene to late mid oligocene the accumulation of sandy conglomerate and conglomerate layers on top of the shanggancaikou formation and an abrupt increase in sedimentation rates are probably resulted from the main uplift of the north qaidam at about 30 ma when compared with coeval directions expected from eurasia the mean declinations indicate a slight vertical axis rotation of the northern qaidam basin since the mid eocene sinistral shear strain may be concentrated on the altyn tagh fault atf

two stage evolution of the altyn tagh fault during the cenozoic new insight from provenance analysis of a geological section in nw qaidam basin nw china introductionthe 1600 km long active altyn taghfault atf de nes the northern edgeof the tibetan plateau links thewestern kunlun eastern kunlun andqilian orogenic belts fig 1a and gures prominently in constrainingthe timing mechanism and stress dis sipation of the tibetan uplift e g yinand harrison 2000 tapponnier etal 2001 cowgill et al 2003 aspectof its cenozoic evolution has beenattracting hot debate zhou and gra ham 1996 yue and liou 1999 yinet al 2002 liu et al 2007 and akey issue is the initiation of left slipmovement on the fault which wasconsidered to occur i during lateeocene early oligocene evidenced bythermochronology in the altynmountain chen et al 2001 andsedimentology adjacent to the fault meng et al 2001 yue et al 2001 ritts et al 2004 or ii since middlemiocene through structural analysisin the qaidam basin yuan et al 2008 wang et al 2010 apatite s sion track result along the atf wanet al 2001 chen et al 2002 andcomprehensive analysis of the north ern tibetan plateau burch el et al 1989 wang 1997 meyer et al 1998 yin et al 2002 argued a much earlierinitiation age 49 ma of the westernand central segments of the atf butwith questionable proofs ritts et al 2004 two factors probably lead tothis uncertainty i the atf is toolarge to allow us to nd an appropri ate proxy deciphering accurately itsevolution and ii most extant studiesshare a bias that the atf has beenalways a left slip fault during thecenozoic which we regard as a mis understanding in this paper provenance analysis has beenproved e ective on revealing theuplifting history of mountains andevolution of major faults e g lindseyet al 2007 miao et al 2008 linet al 2009 marsaglia et al 2010 the qaidam basin is the largest sed imentary basin inside the tibetan pla teau and accumulated thick terrestrialsediments during the cenozoic rieseret al 2006 making itself an idealplace for deciphering the uplift andpropagation of the plateau yin et al 2007 2008 as it is bounded by theatf to the northwest fig 1b itssedimentation has possibly been tiedto the development of this large scaleintracontinentalfault fewworkshavebeen done on this subject due to thetough geographic conditions in thispaper we investigated a geologicalsection spanning eocene pliocene inthe nw qaidam basin and analysedin detail provenance of the sediments we link the result to cenozoic evolu tion of the atf and try to illustrate together with some other evidences that the atf was used to be a basalshear zone before its initiation of left slip since middle miocene which rec onciles the two contrasting viewpointson the cenozoic initiation of the atfabovementioned geological settingthe study section is seated at thesouthern side of the atf and incisesthe western limb of the nw trendingganchaigou anticline bounded to thenorth by a north directed reversefault a branch fault of the atf fig 1c the cenozoic strata are wellexposed in the section including thexiaganchaigou xg shangganchai gou sg xiayoushashan xy shangyoushashan sy and shizigou szg formations in an ascendingorder the xg formation consists of redmudstone siltstone and sandstoneattaining bed thickness of 0 2 1 m interbedded with conglomerate andpebble sandstone layers 10 20 cmthick figs 2 and 3a the stratumeven comprises more than 100 mthick calcareous mudstones in theupper part in the drillhole chaishen 3 about 6 km east of the sectionabstract

northward growth of the qimen tagh range a new model accounting for the late neogene strike slip deformation of the sw qaidam basin abstract situated along the western termination of the eastern kunlun mountains the qimen tagh range represents a key area to understand the cenozoic basin range interactions between the northeastern tibetan plateau and the qaidam basin within that region several huge bow like fault systems such as the kunbei and qimen tagh fault systems accommodate the transpressive deformation but their kinematic evolution is still highly debated newly acquired seismic profiles and isopach maps of the late eocene sediments strongly suggest that the kunbei fault system consisting of the kunbei arlar and hongliuquan faults in the southwestern qadaim basin was initially a left lateral strike slip fault system rather than a thrusting system growth strata indicate an early miocene onset age for this strike slip deformation however earthquake focal mechanisms show that the present day tectonic pattern of this fault system is dominated by ne sw transpression as for the qimen tagh fault system numerous linear geomorphic features and fault scarps indicate that it was again a strike slip fault system deformed sediments within the adatan valley prove that strike slip motion prevailed during the pleistocene yet the present day deformation is marked by ne sw transpression collectively the kunbei and qimen tagh fault systems were initially left lateral strike slip fault systems that formed during early miocene and pleistocene respectively colligating with these southward younging left lateral strike slip faulting ages and the fact that these convex northward structures converge to the center segment of active kunlun fault in the east we thus considered the kunbei and qimen tagh fault systems as former western segments of the kunlun fault once located further south in the present day location of that fault these faults gradually migrated northward since the early miocene while their kinematics changed from left lateral strike slip motion to ne sw transpression

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

shallow water evaporitic environments and their source rock potential in the major evaporitic environments on the world s surface today most organic matter accumulates in shallow subaqueous to seasonally subaerially exposed algal mat sediments given the present depositional setting and the most likely flushing by oxidizing water it is unlikely this organic matter could be preserved to form source rocks however if we place such evaporite deposition into a geologic context then there are times in the past when source rocks could have formed in shallow water settings such settings were characterized by hydrological conditions which allowed the retention of hypersaline anoxic pore waters to depths where the organic material was buried deeply enough to generate hydrocarbons when deep basin shallow water evaporite successions were laid down in basins such as the mediterranean in the late miocene the michigan basin in the silurian and other large saline giants then conditions were right for the formation of source rocks within shallow water and salt flat evaporitic environments the evaporites in these saline giants were deposited under conditions of relatively shallow water 50 m the basin never appears to have dried out but water levels changed quickly nearly equal 10 000 years from shallow to deep continual water saturation coupled with saline pore fluids prevented the inflow of fresh oxidizing groundwater into the basin center of shallow water organic rich evaporites immature hydrocarbons derived from such rocks today drip from the 5 5 m y evaporites of sicily in active salt and sulfur mines organic rich sediments could also be preserved to generate hydrocarbons in rapidly subsiding rift basins in such basins rapid burial has prevented the entrance of fresher oxygenated waters and the associated degradation and destruction of the organic matter the early continental riff stage generates the source rocks the ephemeral streams wadis and dune fields generate the reservoirs and the subsequent evaporite stage seals the reservoir

source rock potential of evaporitic environment examination of modern saline lakes solar evaporation ponds and lagoons shows that the evaporitic environment can be very productive of organic matter few species survive in the brines but those that do commonly exist in great profusion in a marine evaporitic embayment the flow of surface currents is persistently toward regions of highest salinity so that there is a continual supply and concentration of nutrients prolific growth of phytoplankton may be similar to that in areas of upwelling in modern oceans only carbonates precipitate in the mesosaline part 4 to 12 salinity of such an evaporitic environment and no great dilution of organic matter by clastic or biogenic sediments occurs because stratification of brine may occur and reducing conditions may be a sociated with the bottom waters much of the organic matter produced can be preserved upon maturation the result may be a rich carbonate source rock frequently unrecognized in the geologic column in the middle east mesosaline conditions are known to have occurred many times from the triassic to cretaceous and may be responsible for the vast reserves of petroleum in the area evaporitic conditions may also have played a part in the petroleum productivity of many other areas including the michigan and paradox basins

tibetan plateau aridification linked to global cooling at the eocene oligocene transition the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed precisely dated evidence details that aridification on the tibetan plateau occurred at the time of the eocene oligocene transition in the marine records this suggests a direct link between the palaeontological and palaeoenvironmental changes recorded in continental asia at the time and global climate cooling continental aridification and the intensification of the monsoons in asia are generally attributed to uplift of the tibetan plateau and to the land sea redistributions associated with the continental collision of india and asia1 2 3 whereas some studies suggest that past changes in asian environments are mainly governed by global climate4 5 6 the most dramatic climate event since the onset of the collision of india and asia is the eocene oligocene transition an abrupt cooling step associated with the onset of glaciation in antarctica 34 million years ago7 8 9 however the influence of this global event on asian environments is poorly understood here we use magnetostratigraphy and cyclostratigraphy to show that aridification which is indicated by the disappearance of playa lake deposits in the northeastern tibetan plateau occurred precisely at the time of the eocene oligocene transition our findings suggest that this global transition is linked to significant aridification and cooling in continental asia recorded by palaeontological and palaeoenvironmental changes10 11 12 and thus support the idea that global cooling is associated with the eocene oligocene transition13 14 15 we show that with sufficient age control on the sedimentary records global climate can be distinguished from tectonism and recognized as a major contributor to continental asian environments

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

diachronous seawater retreat from the southwestern margin of the tarim basin in the late eocene in contrast to the present hyper arid inland basin surrounded by the high mountains of central asia the western tarim basin was once connected with the tajik basin at least in the late eocene when an epicontinental sea extended from the western tarim basin to europe western tarim is a key site for studying the retreat of seawater which was likely caused by the northward indentation of the pamir arc and facilitated by the climatic cooling and eustatic sea level change in the cenozoic here we present a new magnetostratigraphic record from the tarim basin that provides evidence of diachronous seawater retreat from its southwestern margin we studied about 1360 m of well exposed eocene oligocene strata at keliyang in the folded foreland of the west kunlun orogen until now the age of the strata has only been minimally constrained by the presence of late mid eocene marine fossils our biostratigraphic and magnetostratigraphic results demonstrate that the age of the sedimentary sequence ranges from 46 ma to 26 ma mid eocene to late oligocene and the seawater retreat at keliyang took place at 40 ma considering the stepwise northward indentation and uplift of the pamir orogen together with the other previous results we propose that seawater retreat from the southwestern margin of the tarim basin was diachronous in the late eocene ranging from 47 ma to 40 ma the regional indentation uplift and erosion of the pamir orogen played the dominant and important role in controlling the seawater retreat from the southwestern margin of the tarim basin

magnetostratigraphy of the dahonggou section northern qaidam basin and its bearing on cenozoic tectonic evolution of the qilian shan and altyn tagh fault article i nfo the timing of uplift of the tibet plateau has a central role in the development of tectonic models for the tibet plateau and cenozoic global climate change a detailed magnetostratigraphic study of the dahonggou section northern qaidam basin reveal that the section spans from 34 to 8 5 ma and the ages of the shang ganchaigou xia youshashan and shang youshashan formations are from 34 to 22 20 ma 22 20 to 13 ma and 13 to 8 5 ma respectively variations in lithofacies sedimentation rate and magnetic susceptibility k suggest that the southern qilian shan was tectonically inactive and didn t respond to the rapid slip on the altyn tagh fault at 30 ma in contrast the similar sedimentary records in the dahonggou section the xishuigou section along the altyn tagh fault and even more localities along much of the qilian range front imply that the qilian shan and the altyn tagh fault were synchronously tectonically active at about 12 ma the lower k between 12 ma and 8 5 ma in the sediments of the dahonggou section is interpreted to be due to long distanced oxidation and sorting which cause not only that magnetite was oxidated to hematite but also that magnetic minerals are enriched in fine grained sediments and coarse grained sediments bear few magnetic mineral

stratigraphic evidence for a deep eocene lake uinta piceance creek basin colorado relief on a prograding clastic complex is used to estimate the water depth of eocene lake uinta in the piceance creek basin of western colorado relief of as much as 412 in is present today on the complex postdepositional compaction can account for only about 100 m of this relief therefore the remaining 312 m appear to represent paleorelief on the floor of lake uinta the lake was apparently more than 300 m deep authigenic saline minerals were precipitated in the unlithified sediments of the deeper areas of the lake suggesting that at least the lower part of the water column was highly saline the evidence presented here indicates that the deep stratified lake model is more appropriate than the more generally accepted shallow or playa lake model for lake uinta

mineralogy and organic geochemistry of the terrestrial lacustrine pre salt sediments in the qaidam basin implications for good source rock development abstract terrestrial pre salt sediments are not only good lithologic reservoirs similar to marine pre salt carbonates but also are supposed to have high hydrocarbon generation potential because the overlying terrestrial thick bedded salts are generally preserved in basin centers however their status as source rock has attracted much less attention than as reservoir in this paper we choose the qaidam basin as an example and study the minerology organic geochemistry and organic petrology of the pre salt sediments in a hydrocarbon rich unit where abundant petroleum resources have been discovered under halite beds lithoscanner logging of six wells and x ray diffraction results of 378 samples show that terrestrial pre salt sediments are characterized by mixed siliciclastic carbonate sulphate rocks and their relative proportions are laterally varied quartz feldspar and clay minerals are of allochthonous origin and classified as argillaceous endmember while dolomite and ankerite are of autochthonous origin as carbonate endmember organic geochemistry analysis of 118 samples show great variability in toc s2 and tmax values and this variability is not related to mineral composition heterogeneity as hydrogen index is not well correlated with detritus carbonate and sulphate minerals contents indicating the interacting controls of source rocks by both exogenetic and endogenetic processes in brackish mesosaline lakes strong bacterial reworking of organic matter results in the dominant macerals of pre salt sediments as bituminite and oil generation at low maturity levels sealed by impermeable halite beds and in close proximity to reservoirs the generating hydrocarbon by pre salt good source rocks is effectively accumulated which entails hydrocarbon exploration under halite beds in other terrestrial salt bearing basins