paleoaltimetry from stable isotope compositions of fossils stable isotope systematics of phosphatic vertebrate and carbonate invertebrate fossils are reviewed emphasizing external vs biological controls on isotope compositions and their variation external controls include elevation and isotopic lapse rates temperature atmospheric circulation patterns aridity and changes to precipitation seasonality e g development of monsoons biological controls include temperature regulation and temperature dependent biological activity water balance behavior and rates of hard part secretion and maturation some key factors include what sources of water an animal samples how those sources respond isotopically to elevation and the isotopic sensitivity of a biologic tissue to environmental changes general research design criteria are enumerated including numbers of fossils and analyses required as well as uncertainties in the interpretability of isotopic shifts isotope zoning from fossil teeth and shells from the indian foreland demonstrate comparable seasonal monsoon signals at 11 ma vs today implying a high and broad plateau at that time mean isotope compositions of fossil teeth from the lee of the cascade range central oregon show a pronounced decrease in 18 o initiating 7 ma signaling topographic rise associated with impingement of basin and range extension on the arc mean compositions and zoning in fossil shells from the northern rocky mountains indicate high elevations and topographic relief in the late cretaceous and early paleogene paleoaltimetry will principally benefit from better estimates of temperature because temperature lapse rates are less variable than isotopic lapse rates and because the combination of temperature and isotopic studies may help distinguish between local isotopic changes and distal ones caused by catchment or rain shadow effects

paleohydrology of the upper miocene bicorb lake eastern spain as inferred from stable isotopic data from inorganic carbonates abstract the hydrological evolution of the upper miocene bicorb lake eastern spain has been established based on sedimentology carbonate mineralogy and stable isotopic data from inorganic carbonates from the upper lacustrine unit three main subunits have been recognized in subunit a the sedimentological features suggest a hydrologically closed lake although the isotopic data are not conclusive the isotopic compositions of primary carbonates from subunit b display a distribution typical for a hydrologically closed lake and allow the differentiation of four developmental stages the first stage was defined by the deposition of gypsum lmc dolomite and aragonite indicating large variations in the water mg ca ratio during the second and third stages only lmc was deposited suggesting only minor mg ca fluctuations the 13c and 18o pattern of the second stage shows a well defined covariance r 0 8 while in the third stage the isotopic data display a negative covariant relationship r 0 8 changes in dissolved inorganic carbon composition are considered the main factor to explain the variations from positive to negative covariance in the fourth stage the lake waters underwent large mg ca fluctuations as indicated by the deposition of lmc hmc and aragonite the isotopic trend is also covariant finally subunit c is characterized by lmc and minor amounts of hmc suggesting minor changes in mg ca ratio the isotopic features of this subunit are more congruent with a hydrologically open lake the lowest 13c and 18o values are very similar in the three subunits reflecting the most dilute conditions of the lake water and a uniform isotopic composition of the local meteoric water during these periods the changes in water balance and solute inputs recorded in the mineralogy and isotopic signatures of carbonates were controlled by the diapiric evolution and changes in the e p balance

controls on sand ramp formation in southern namibia sand ramps have the potential to provide rich palaeoenvironmental information in dryland regions wherein proxy records are typically scarce however current knowledge of the geomorphic controls and processes of sand ramp formation is limited this study provides a data rich examination of the key factors controlling sand ramp formation the location and morphology of 75 sand ramps in southern namibia are examined the sediments and chronologies of 10 sand ramps are studied in detail using 51 osl dates and 83 grain size and loi samples heavy mineral assemblages are used to determine the provenance of 10 samples and osl sensitivity is used to explore geomorphic processes of 8 samples sand ramp morphology can be grouped into one of four classes of increasing size and complexity and is closely linked to the available accommodation space heavy mineral assemblages indicate local sediment sources and all 75 studied sand ramps are within 4 km of a large ephemeral river channel or within 5 5 km of a dune field therefore accommodation space and sediment supply are identified as the primary controls of sand ramp formation sedimentology and osl sensitivity suggest a complex interplay of aeolian fluvial and colluvial processes contribute to sand ramp formation with large variability observed between ramps three of the ten dated sand ramps have been present in the namibian landscape for 100 ka eight sand ramps show episodic deposition between 75 12 ka and five show evidence of surface reworking over the past 2 ka environmental sensitivity is likely to be linked to the size and availability of the accommodation space therefore individual sand ramps are expected to reflect local environmental conditions recording when an abundant sediment supply coincided with available accommodation space whilst a regional analysis of multiple sand ramps with chronometric data offers the potential to identify larger scale palaeoenvironmental controls of sediment supply

several distinct wet periods since 420 ka in the namib desert inferred from u series dates of speleothems abstract the scarcity of numerical dates of the arid areas in southern africa is a challenge for reconstructing paleoclimate this paper presents a chronological reconstruction in the central part of the namib desert namibia for the last 420 000 yr it is based on 230th u dates tims from a large stalagmite and a thick flowstone layer in a small cave located in the hyper arid central namib desert the results provide for the first time evidence of three or possibly four succeeding wet periods of decreasing intensity since 420 ka through which speleothem deposited at approximately 420 385 ka 230 207 ka and 120 117 ka following the 100 ka milankovitch cycle speleothem growth was not recorded for the holocene these wet periods interrupted the predominantly dry climate of the namib desert and coincided with wet phases in deserts of the northern hemisphere in the murzuq basin sahara the negev israel the nafud desert saudi arabia and the arid northern oman arabian peninsula

oxygen and carbon isotope trends and sedimentological evolution of a meromictic and saline lacustrine system the holocene medicine lake basin north american great plains usa abstract the succession of sedimentary subfacies in the medicine lake basin south dakota usa documents numerous changes in total dissolved solids tds lake level and the nature and stability of meromixis during the holocene the lake formed about 10 6 ka b p when coarse clastics and massive fine grained muds were deposited in a well mixed freshwater system subsequently the lake became saline and meromictic depositing a finely laminated unit composed of variegated and gray muds organic facies gypsum and aragonite laminae phototrophic bacterial plate communities bpc at the chemocline were significant producers of pelagic organic matter relatively high lake levels at least 10 m higher than today but saline and meromictic conditions predominated through the early to mid holocene at the end of this phase lake level dropped and stable meromixis was disrupted sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities bmc spread over the lake replacing bpcs as the predominant microbial producers when lake level rose again massive and laminated units were deposited under less saline conditions another pronounced episode of low lake level and high salinity followed the most recent sediments are sapropels massive or banded muds and littoral sands the 18o record from authigenic aragonite shows more positive values during the arid mid holocene interval with an abrupt increase at the end of this period followed by generally more negative values during the cooler and wetter late holocene the small range of 18o values 3 suggests that the lake was well buffered against high frequency climatic cycles probably as a result of groundwater modulation we postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system either bpcs or bmcs exerted major controls on the carbon cycle and consequently on the development of isotopic trends linking 13c and 18o values more negative 13c values during the early and mid holocene are interpreted as a consequence of the predominance of 12c enriched microbial carbon from bpcs examples of both covariant and inverse covariant isotope trends between 13c and 18o values occur in the medicine lake record inverse covariant trends characterize long periods of stable meromixis when bpcs predominate over bmcs we hypothesize that well developed water stratification caused by greater influx of relatively 18o depleted freshwater induces higher productivity at the chemocline and consequently more positive 13c values in the precipitated carbonates unstable phases of meromixis or holomictic conditions are characterized by a predominance of bmcs or other biogenic producers over bpcs and a number of other factors combine to promote covariant trends the medicine lake record is consistent with the paleoclimatic interpretation for the northern great plains of a more arid early and middle holocene followed by an abrupt transition to a relatively cooler and wetter late holocene the sedimentary succesion in medicine lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord and suggest a late holocene climate punctuated by arid episodes

sand ramps in the golden gate highlands national park south africa evidence of periglacial aeolian activity during the last glacial abstract sand ramps have been identified in a side valley on the southern side of the little caledon river in the golden gate highlands national park free state south africa the site lies in the foothills of the northern drakensberg maluti mountains and this study aims to explore the potential of such deposits to add to the sparse palaeoenvironmental record of the region up to 11 m of aeolian sand banked against north facing slopes is exposed by fluvial incision through the deposits revealing interbedded fluvial lenses and palaeosols the region is currently devoid of significant aeolian sediment transport or deposition and supports well developed grassland vegetation these deposits testify to former conditions different from those of today which would have been characterised by depleted vegetation and a different wind and or climatic regime biogenic proxies are absent from the deposits but optically stimulated luminescence osl dating of the sediments reveals that they accumulated at intervals throughout much of the last glacial event from around 45 to 16 ka these dates are consistent with those of vegetation changes from palaeoecological sites elsewhere in the region and also with dated evidence for niche glaciation further south in the high drakensberg evidence from this study supports significant landscape changes in the drakensberg during the last glacial with the foothills in the region of golden gate highlands national park subject to aeolian processes under a periglacial environment

effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains drummond c n wilkinson b h lohmann k c and smith g r 1993 effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains palaeogeogr palaeoclimatol palaeoecol 101 67 79 oxygen isotopic compositions of upper miocene 9 2 m y lacustrine limestone from northwestern wyoming range from 21 5 to 35 2 0 pdb and are the lightest yet reported for a non marine carbonate sequence these values require that lake water was greatly depleted in 180 relative to modern meteoric precipitation and indicate that most inflow was sourced as meltwater from glaciers in the adjacent gros ventre range assessment of climatic factors influencing the isotopic composition of global meteoric water indicates either that nearly all of miocene precipitation in the southern rocky mountains was derived from large lake systems in the snake river plain and northern great basin to the west or that gros ventre catchment elevations were up to 2300 m higher than at present because erosion rates in modern alpine regions suggest that subaerial denudation could only account for up to 1200 m of post miocene elevation reduction any additional lowering must reflect the influence of post laramide epeirogeny during basin and range extensional tectonism lacustrine isotopic data therefore provide boundary conditions on the timing and magnitude of changes in late cenozoic paleoclimate topography hydrology and tectonism in the western wyoming portion of the rocky mountains

tectonic significance of paleobotanically estimated climate and altitude of the late eocene erosion surface colorado erosion beveled the laramide front range uplift in colorado to a surface of low relief by the end of the eocene this study uses j a wolfe s new multivariate climate analysis techniques to determine the paleoelevation of this regional surface by examining the overlying 35 ma florissant flora a multiple regression model explaining 93 3 of the variance in mean annual temperature was developed using wolfe s dataset of 31 leaf physiognomic character states for 86 modern vegetation sites these character states were scored on 29 species collected from one facies of the florissant lake beds the paleotemperature estimate of mean annual temperature 10 7 plus minus 1 5c derived from these data when combined with sea level temperature and terrestrial lapse rate implies a late eocene paleoelevation of 2 4 2 7 km pliocene uplift is thus not required to explain the present elevation of 2 5 km it is unclear when and why the southern rocky mountains achieved this elevation magmatic crustal thickening can explain the late eocene high elevation of the southern rockies but neither this mechanism nor compressive thickening explains why the great plains which are tied to the florissant elevation by the wall mountain tuff were also high this paleoelevation estimate indicates that regional surfacesmore of planation could be formed at high elevation in the eocene probably because of peculiarities of the eocene climate less

climbing and falling dunes in valles marineris mars 1 multiple occurrences of wall dunes are found several kilometers above the valles marineris canyon floor dune slip face orientation and bed form morphologies indicate transport direction and whether the wall dunes are climbing dunes or falling dunes on earth these types of dunes form in a unidirectional wind regime and are strongly controlled by the local topography newly acquired mars reconnaissance orbiter mro images and topography of the walls of valles marineris show similar sand dune morphologies as wind blown sediment has interacted with local and regional topography primarily found in melas and coprates chasmata these climbing and falling dunes are relevant for understanding aeolian sediment flux sediment sources and wind directions falling dunes show photogeologic and thermophysical evidence of their sand being provided by adjacent outcrops

interpreting geoproxies of late quaternary climate change in african drylands implications for understanding environmental change and early human behaviour abstract climate and environmental change in the late quaternary are widely regarded as key drivers of early human development and dispersal in africa robust records of change are required to assess the nature of potential impacts today s late quaternary palaeoenvironmental palaeoclimate reconstructions from the interior southern african basin kalahari are more spatially detailed have a stronger chronometric underpinning and span a longer time range than those of 10 20 years ago but are no less controversial reconstructions for the last 150 ka are usually interpreted in terms of changes in hydrological balance from conditions more humid or more arid than today and expressed in climatological terms as the changing latitudinal interplay between monsoon easterlies and tropical and mid latitude westerlies with more data than ever before why then is the record not yet delivering a more coherent picture of regional climate change this paper deliberately addresses current challenges and controversies including issues often ignored or neglected with the view that it is essential to do so if more robust interpretations of the past are to be generated in the future sources of palaeo data from the interior are predominantly geoproxies reconstructions not based on highly resolved organic or isotope records but on interpretation of spatially extensive geomorphological proxies of past environments despite being increasingly datable using osl their inclusion in syntheses has proved complex not least when interpretations appear to conflict with records from other sources consequently they are sometimes removed from regional syntheses of climate change these omissions are often replaced by spatially exaggerated interpretations of higher quality local records allowing more spatially coherent timeslice interpretations to be presented yet this is counter intuitive to the spatial complexity of conditions present in the palaeoenvironmental proxies or in climatic and environmental systems today in addition the quest to link regional records to master records from ice and ocean cores can force interpretation within a temporal scale that may be inappropriate for their analysis underplaying the reality of complexity within the environmental system there are not necessarily good or bad records of terrestrial palaeoenvironments in interior southern africa rather there are records that contain different suites of information about past land surface hydrological and climate conditions preserved at temporal scales which can sometimes be incomparable consideration of these factors may resolve apparent conflicts between datasets resulting in a better understanding of past environmental dynamics and revealing the nature of environmental variability which may be a key facet to which human development has responded

stable isotope based paleoaltimetry abstractthe quantitative estimation of paleoaltitude has become an increasing focus of earth scientists because surface elevation provides constraints on the geodynamic mechanisms operating in mountain belts as well as the influence of mountain belt growth on regional and global climate the general observation of decreasing 18o and 2h values in rainfall as elevation increases has been used in both empirical and theoretical approaches to estimate paleoelevation these studies rely on the preservation of ancient surface water compositions in authigenic minerals to reconstruct the elevation at the time the minerals were forming in this review we provide a theory behind the application of stable isotope based approaches to paleoaltimetry we apply this theory to test cases using modern precipitation and surface water isotopic compositions to demonstrate that it generally accords well with observations examples of the application of paleoaltimetry techniques to himalaya tibet and the andes are discussed

fault chronology and uplift history of the southern wind river range wyoming implications for laramide and post laramide deformation in the rocky mountain foreland the stratigraphy provenance and structural involvement of tectogenic sedimentary rocks along the southern and western margins of the wind river range in western wyoming record a complex laramide and later uplift history and identify the timing and position of individual faulting events the chronology of these events is 1 main uplift of the range by displacement on the wind river fault between 90 and 49 to 50 ma 2 the formation of reverse faults in the hanging wall of the wind river fault ca 50 to 51 ma 3 sagging of the toe of the wind river fault into sedimentary fill of the green river basin between 46 and 23 ma 4 uplift of the core of the range along reactivated precambrian shear zones between 30 and 23 ma and 5 collapse of the southern part of the range along normal faults beginning at 13 ma the sedimentary record of uplift and erosion indicates that erosion and basin fill kept pace or caught up with uplift so that by the end of the laramide there was little topographic relief between the crest of the range and the adjacent green river basin the present topography of the range characterized by a high level erosion surface and distinct crestal ridge derives from uplift of the erosion surface by the late laramide reverse faulting post laramide uplift of the crest of the range and neogene exhumation initial uplift of the range as early as 110 ma and prior to displacement on the wind river fault suggests either a pre laramide event of unknown origin or that laramide crustal shortening began nearly 35 m y earlier than commonly assumed

how dry was dry late pleistocene palaeoclimates in the namib desert abstract evidence for late quaternary changes in climate in the namib desert is geographically scattered and often poorly dated the available evidence suggests periods of increased river discharge and groundwater flow centered on 8 12 20 24 and 26 32 ka bp based on modern analogues these palaeohydrologic changes likely involved increased rainfall and runoff in the escarpment zone east of the desert there is little evidence to suggest significantly increased rainfall within the desert during these periods except perhaps during the interval prior to 26 ka bp these data reinforce earlier conclusions that the namib has experienced mostly arid to hyperarid conditions throughout the quaternary

luminescence dating of sand ramps in the eastern mojave desert abstract until recently the timing of the movement of sand and accumulation of sand by aeolian processes in the eastern mojave desert has remained the subject of speculation the results of a luminescence dating program involving 78 samples of material from nine sand ramp complexes have enabled recognition of regional and local patterns of sand accumulation in the eastern mojave the study area extends from lake manix in the west to the colorado river in the east the periods of accumulation have been identified on the basis of quartz tl dating and potassium feldspar tl and irsl dating at a regional scale two major depositional phases can be identified 1 late pleistocene 20 30 ka with sequences at hank s mountain balch dale lake and the big marias and 2 late pleistocene early holocene 15 7 ka with sequences at cat dune dale lake iron mountain big marias above palaeosol and at the western end of the study area at soldier mountain 20 7 ka late holocene sequences appear to be much more localized and are confined to the west cronese old dad mountain and balch sand ramps the earliest phases of sand accumulation on the ramps studied immediately predate the formation of pluvial lake mojave ca 24 5 ka the later main phases of accumulation coincide with the existence of lake mojave and the end of accumulation on the majority of the sand ramps immediately post dates the final end of intermittent lake mojave iii 9 7 ka the continuity of sand supply to potential deflation areas is seen as a critical condition for accumulation of material on the sand ramps during the late quaternary in the eastern mojave

characteristics and control mechanism of nanoscale pores in lacustrine tight carbonates examples from the jurassic da anzhai member in the central sichuan basin china abstract compared with the studies of nanoscale pores in shale coal and tight sandstone studies associated with lacustrine tight carbonate nanoscale pores and their control mechanisms particularly quantitative studies combining reservoir evaluation and geochemistry remain scarce to address this disparity lacustrine tight carbonate samples collected from the jurassic da anzhai member in the central sichuan basin of china were quantitatively investigated similar to other unconventional reservoirs lacustrine tight carbonate reservoirs contain well developed intraparticle pores interparticle pores and micro cracks with nanoscale storage spaces that dominate the full scale reservoirs the relatively small pores 3 5 nm make a dominant contribution to the nanoscale storage space isotherms and hysteresis loops are type ii and type h3 respectively indicating wedge shaped pores however the hysteresis loops indicate more dead end pores larger pores and more complex microstructures than those found in other unconventional reservoirs the mineral in lacustrine carbonate provides a link between the reservoir and the formation environment strong paleo weathering low salinity a humid rainy paleo climate and poor hydrodynamic conditions are favorable to improve the storage spaces and terrestrial mineral contents but harmful to the formation of authigenic minerals terrestrial minerals including primarily quartz clay and dolomite have well developed nanoscale pores and are positively correlated with the nanoscale storage space whereas authigenic minerals primarily calcite have strong cementation and filling effects and are negatively correlated with the nanoscale storage space

characterization of lacustrine mixed fine grained sedimentary rocks using coupled chemostratigraphic petrographic analysis a case study from a tight oil reservoir in the jimusar sag junggar basin abstract mixed deposits are sediments consisting of external clastic epiclastic or terrigenous intrabasinal components and pyroclastic components the mixture comprising variable amounts of the three components is defined as mixed sedimentary rocks the permian lucaogou formation p2l in the jimusar sag of the junggar basin is a promising tight oil target in western china the fine grained mixed sedimentary rocks of which are rich in organic matter om and two sweet spot intervals with relatively high porosity however the sediment composition provenance and deposition environmental settings have not been studied in detail in this study coupled chemostratigraphic petrographic analysis were used to reconstruct their depositional environments the results show that the fine grained sedimentary rocks have three major sediment sources external clastic input terrigenous clastics intrabasinal autochthonous to parautochthonous components carbonates siliceous skeletal debris and om and pyroclastic input main lithofacies include siltstone fine sandstone mudstone dolomite and tuffite the silt sandstones were mainly sourced from rocks with calc alkaline composition while the tuffaceous sedimentary rocks were sourced from high k calc alkaline rocks elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions whereas the fine grained clastic sediments were deposited under relatively humid conditions the muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones variations of lithofacies and om accumulation of different intervals reflect changing deposition environmental settings and the frequently altered high toc content rocks and good reservoirs are benefit for tight oil formation the work may provide some useful insights and serve as a reference for studying other mixed fine grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere

carbonate oxygen isotope paleoaltimetry evaluating the effect of diagenesis on paleoelevation estimates for the tibetan plateau carbonate oxygen isotope paleoaltimetry is based on analysis of the d 18 o value of carbonate precipitated from surface water deciphering the diagenetic history is important for establishing whether particular carbonates are accurate recorders of paleosurface waters which reflect paleoelevation this study provides examples from southern east central and northeastern tibet of approaches aimed at evaluating the diagenetic history of lacustrine micrites and pedogenic carbonates the most desirable technique for avoiding erroneous interpretations related to diagenetic overprinting is to analyze carbonates that are known to be primary such as aragonitic shell material in rocks that do not contain shell material we have compared lacustrine micrites and pedogenic carbonates to diagenetic carbonate phases to determine the effects of diagenesis on the isotopic composition of primary carbonates where the potential effects of diagenesis are subtle or ambiguous we have evaluated the fidelity of the carbonate record from systematic trends in c and o isotopes that agree with other interpretations of paleoenvironment such as high frequency covariance in c and o that corresponds with changes in the mg concentration of carbonates using these strategies we have determined that diagenesis has not affected the isotopic composition of carbonates in the late miocene pliocene thakkhola graben in southern tibet and the oligocene to pliocene linxia basin in northeastern tibet in the case of paleogene basins in east central tibet however ambiguity in data precludes the determination of diagenetic effects d 2004 elsevier b v all rights reserved

evolution of lacustrine diatomite carbonate cycles of miocene age southeastern spain petrology and isotope geochemistry abstract continental formations of miocene age outcropping in southeastern spain include beds of diatomites in economic quantities irregularly alternating with carbonate beds the minerology and isotope geochemistry of the alternating carbonates and diatomites indicate variable depositional conditions associated with an overall deepening of a lake anhedral dolomite associated with sulfate minerals characterizes the first depositional stage and has heavy oxygen and light carbon isotopic values which are consistent with a formation in a restricted environment with highly evaporated waters increasing dilution of the water is reflected in the presence of abundant aragonite near the base of the assemblage which passes to calcite near the top the gradual upward decrease of both 18o and 13c in these minerals indicates a parallel decrease of the evaporation inflow ratio within the depositional basin also seasonal variations of composition and volume of the water affect both the formation and diagenesi of carbonate minerals

palaeoclimatic evolution during eocene and its influence on oil shale mineralisation fushun basin china the fushun basin is a small explored coal and oil shale bearing cenozoic fault basin in the liaoning province northeast china the basin mainly consists of eocene swamp to lacustrine deposits of the guchengzi to xilutian formation and contains the biggest opencast oil shale mine in asia this mine has provided an ideal opportunity to undertake palaeoclimate reconstruction in this basin based on a single geological profile and the analyses of 93 samples using various approaches namely field geological observation clay mineralogical and geochemical sr ba sr cu stable c and o isotope analyses all of which were compared with palaeobotanical data the eocene climate of fushun basin evolved from warm temperate to north subtropical and generally changed from warm humid to subhumid semiarid paleoclimatic and geochemical parameters shows that the very warm and humid climate during jijuntun formation increased the initial productivity of lake water and caused a steady stratification of the lake water then caused oxygen lack in the bottom of water productivity of the lake provides the mean origin of organic matters for oil shale formation and steady anoxic environment is beneficial for the conservation of organic matters

source rock deposition controlled by tectonic subsidence and climate in the western pearl river mouth basin china evidence from organic and inorganic geochemistry abstract interest in factors controlling lacustrine source rock deposition has increased over the last few decades because this type of deposits contain significant petroleum resources generally tectonic subsidence and climate are the two root causes as they control the accommodation potential water column properties and sources of organic matter in this study coupling organic geochemical and elemental geochemical data two potential source rocks i e the eocene wenchang formation e 2 w and oligocene enping formation e 3 e were investigated two models were finally raised to explain deposition of the two set of source rocks according to their paleoclimatic and tectonic properties the source rock potential shows a strong heterogeneity the second member of the eocene wenchang formation e 2 w 2 is characterized by high organic matter content and oil prone kerogen type in contrast the first member of the eocene wenchang formation e 2 w 1 and the oligocene enping formation e 3 e are characterized by low organic matter content and gas prone kerogen type the primary productivity and depositional environment exhibit notable differences between the two potential source rocks horizons and show an obvious variation from the depocenter to the slope and can be best explained by the coevolution of tectonic subsidence and climate during the e 2 w depositional stage low sediment supply led to mudstone deposited in deep lacustrine environment and resulted in underfilled lake basin the low water inflow provided little terrigenous organic matter low bicadinane perylene and floranthene contents and oxygen besides the low area depth ratio impeded the water circulation thus resulted in shallow thermocline and anoxic suboxic bottom environment abundant dibenzothiophene and high c 35 c 31 22s hopane ratios therefore abundant algae which contributed to the high amorphous organic matter aom content can be preserved the warm and wet climate high mn mg ratios gave birth to autochthonous organism such as dinoflagellates and pavlova gyrans abundant 4 methyl sterane during the e 3 e depositional stage the sufficient sedimentary supply resulted in expanding shallow lacustrine and swamp environment the higher area depth ratio and high sediment supply made environment unstable and can be strongly influenced by external environment broader range of mn mg ratios enough terrigenous organic matter tom was transported to the slope but little to the depocenter the slightly hot and dry climate low mn mg ratios led to decreasing autochthonous organism and evaporation environment the shallow water depth and relative dry climate resulted in saline suboxic dysoxic acid bottom environment the co variation of organic and inorganic indexes indicates the combination is a valid method in reconstructing source rock depositional models

development and environmental significance of an eolian sand ramp of last glacial age central iran abstract a 25 m thick section of mostly eolian sediment is exposed in the stream cut flank of a sand ramp accumulated in a mountain saddle near ardakan playa central iran the well sorted eolian sediments of the ramp contain talus beds and incipient paleosols morphology and bedding structures imply that southeasterly winds were primarily responsible for deposition of the eolian sand optical dating shows that the sand was deposited near the last glacial maximum within about 5000 yr surface stabilization lack of surface scree and the development of a stream cut between the mountain and the ramp show that sediment accumulation ceased after this time the siberian high pressure system may have been established over central iran during the last glacial maximum causing cooler temperatures and a prevalence of southeasterly winds these colder windier conditions would have led to frost shattering and eolian transport both of which are not significant processes today but which are evident as past processes from the ramp sediments

pollen and oxygen isotope analyses on late and post glacial sediments of the tourbiere de chirens dauphine france the sequence of vegetation phases in the late glacial was studied in a sediment section from the bog tourbiere de chirens by means of pollen analysis 18o16o ratios of samples of lacustrine marl obtained from the same profile reflect variations of 18o16o in precipitation and thus provide an additional independent paleoclimatic record the observed 18o16o variations agree well with the climatic history as deduced from pollen analysis the climatic transition from the oldest dryas to the bolling period sensu lato as well as the beginning and end of the younger dryas cold phase is marked by abrupt changes in the 18o16o ratio which were observed also in other regions of the alps these drastic climatic changes probably took place simultaneously over large areas of central europe and occurred within short time spans

extensional basin evolution in the cordillera blanca peru stratigraphic and isotopic records of detachment faulting and orogenic collapse in the andean hinterland 1 sedimentologic provenance oxygen isotope and 40ar 39ar results provide insights into late cenozoic evolution of an extensional hinterland basin in the peruvian andes the 5 6 5 km cordillera blanca composes the glaciated footwall of a low angle normal fault parallel to active contractional structures in the andean fold thrust belt the 200 km long wsw dipping 19 36 cordillera blanca detachment fault accommodated 12 15 km of dip slip displacement inducing subsidence of the hanging wall supradetachment basin which is filled by 1300 m of conglomerate sandstone siltstone and limited carbonate of the upper miocene pliocene lloclla formation lithofacies associations are attributed to lacustrine fan delta to proximal stream dominated alluvial fan sedimentation provenance data record footwall unroofing of jurassic through miocene volcanic and sedimentary rocks during nw directed axial dispersal of sediment provenance data record the appearance of footwall derived granite clasts from the upper miocene cordillera blanca batholith and a change to transverse wsw directed transport variations in the character and preservation of basin fill suggest along strike propagation of the fault through time initial extension and subsidence is constrained to the latest miocene by an 40ar 39ar biotite age of 5 4 0 1 ma for a basal tuff in the lloclla formation estimation of paleolake water composition shows very negative 18o vsmow values 13 6 to 18 2 indicating that high elevations comparable to modern were already attained in the cordillera blanca during earliest normal faulting and basin evolution these results lend support to models suggesting that substantial shortening crustal thickening and surface uplift were necessary precursor conditions for the generation of the cordillera blanca detachment fault

mid miocene barstovian environmental and tectonic setting near yellowstone park wyoming and montana middle miocene ca 10 5 16 0 ma deposits in the greater yellowstone region are confined to the newly defined hepburn9s mesa formation in the yellowstone valley park county montana and the colter formation in jackson hole teton county wyoming the hepburn9s mesa formation was deposited in and adjacent to a perennial saline lake as shown by the mineral assemblage clinoptilotite smectite calcite halite and gypsum and sedimentary features fine grain size laminations mud cracks and mud flakes in pebble sandstone lenses biostratigraphic dating indicates that the saline lake formed prior to 16 0 ma and persisted until after 14 8 ma the presence of a saline lake and a fossil mammal fauna dominated by geomyoid rodents indicates an arid or semi arid climate some of the abundant tuffaceous components of the hepburn9s mesa formation may have blown from coeval volcanic vents represented by the colter formation in jackson hole the hepburn9s mesa and colter formations suggest that both the northern and southern ends of the greater yellowstone region were reacting similarly to tectonism during the middle miocene 1 erosion dominated between ca 18 to 17 ma 2 subsidence of the basins accompanied by extensional block faulting took place from at least 16 0 to 14 8 ma 3 after 14 8 ma but before 8 6 ma uplift occurred in northern yellowstone park strata in the yellowstone valley and jackson hole tilted and eroded then subsided faster than they had prior to 14 8 ma inasmuch as the chronology of extensional deformation and volcanism in the greater yellowstone region is so similar to that in the northern rocky mountains the columbia plateau and the central part of the basin and range all of these areas appear to be related by a common tectonic process

late quaternary linear dune accumulation and chronostratigraphy of the southwestern kalahari implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics abstract the linear dunes of the kalahari now largely inactive have long been identified as having potential palaeoenvironmental significance the application of optically stimulated luminescence osl dating to these dunes in the 1990s provided the first chronology of aeolian accumulation in this region though field methodologies and time consuming multiple aliquot laboratory protocols limited both the depth of sampling in dune bodies and the total number of samples dated in order to permit a more thorough investigation of the potential of these dunes to preserve long chronological records this intensive study presents 71 osl ages from the linear dunes of the southwestern kalahari at witpan south africa sampled with coring equipment at regular and frequent intervals down to bedrock the earliest sand accumulation recorded at witpan is at 104 ka and in spatially discrete locations other evidence of dune activity is recorded at 77 76 57 52 and 35 27 ka although an inherently discontinuous archive the linear dunes of the southwestern kalahari have the potential to record multiple phases of dune construction following the last glacial maximum there is near continuous evidence of dune building with a peak of accumulation recorded from 15 to 9 ka at five individual sites this latter period is generally recognised from other proxy evidence as being unusually arid in this region and such periods of dune activity are likely to be related to intensification of the continental anticyclone during the holocene accumulation has continued at most sites sampled albeit at a lesser intensity this may imply that these dunes are presently not far from thresholds of activation

pattern analysis of dune field parameters as a basic form of pattern analysis the parameters of dune spacing defect density crest orientation and crest length are measured from remote images and treated statistically for dunes at white sands in new mexico the algodones in california the agneitir in mauritania and the namib in namibia statistical populations are identified from frequency plots of dune spacing and crest length field scale calculations of defect density and rose diagrams of crest orientation single populations characterize simple dune fields white sands whereas multiple populations characterize compound complex dunes algodones namib and complex dune fields agneitir as time increases dune fields show an increase in dune spacing and crest length a decrease in defect density more tightly clustered crest orientation and a reduction in the variance associated with measurements of these parameters the results are consistent with models of dune fields as self organizing complex systems in which a characteristic pattern emerges as a function of constructional time because pattern evolution is a function of time it may be possible to use pattern analysis to augment current methods of age determination statistically defined populations can be used in geomorphic backstripping to unstack generations of simple patterns that give rise to complex patterns and to reconstruct each generation in terms of construction time and palaeo wind regime copyright 2006 john wiley sons ltd

laurentide ice sheet extent inferred from stable isotopic composition o c of ostracodes at toronto canada abstract early and middle wisconsin glaciolacustrine sediments exposed at scarborough bluffs toronto canada contain a low diversity benthic ostracode fauna dominated by candona subtringulata and c caudata we have studied the oxygen and carbon isotopic compositions of the two ostracode species from the base of the scarborough section to its top immediately below a late wisconsin till we find an upward decrease in 18 o in both species that is more marked in c caudata the shift in 18 o of ostracode calcite from interglacial values of 6 to 8 to values of 17 immediately below the late wisconsin till is the result of 1 lowering of 18 o of the lake as a result of addition of isotopically light meltwater from an advancing and thickening laurentide ice sheet combined with 2 lowering of 18 o of local precipitation entering the lake due to falling temperatures in the last stages of the lake 18 o of the lake water was between 17 and 21 corresponding to a lake body composed of between 35 and 55 glacial meltwater the lake appears to have been isotopically and thermally stratified for part of its history differences in 13 c values between c caudata and c subtriangulata increase upward in the stratigraphic section this may record enhanced partitioning of carbon isotopes between surface and bottom waters as a result of increased water depths photosynthesis by algae and changes in the input of dissolved organic carbon from the oxidation of organic matter if the age assessments of these sediments are correct then these data provide valuable information regarding the isotopic composition of ancestral lake ontario and also confirm that the basin was only fully glaciated during the late wisconsin

pennsylvanian continental cyclothem development no evidence of direct climatic control in the upper freeport formation allegheny group of pennsylvania northern appalachian basin abstract the occurrence of lacustrine carbonates in the allegheny group upper middle pennsylvanian continental cyclothems of the northern appalachian basin has been attributed to climatic alternation of wet coal deposition and dry carbonate deposition periods however we present evidence from facies associations coal and carbonate petrography and carbonate stable isotope geochemistry for a uniformly wet climate throughout deposition of one of the earliest continental cyclothems the upper freeport formation uff we propose a depositional model comprising a large wetland complex drained by an anastomosed fluvial system containing a mosaic of channels freshwater siliciclastic and carbonate lakes and peat bogs swamps on a siliciclastic floodplain carbonate phases represent bio induced precipitates modified by lake dynamics and not evaporative deposits absence of covariant isotopic trends between 18o and 13c and a relatively small range of 18o values 1 9 to 6 4 pdb that are close to or lighter than predicted paleoprecipitation for the pennsylvanian equatorial regions indicate hydrologically open short residence time lake systems in support of our sedimentary model combined sedimentologic and isotopic data do not indicate reduced effective moisture precipitation evaporation during intervals of limestone deposition the uff lakes developed in a relatively wet climate zone without significant episodic closure due to climatically induced water deficit lithologic patterns within the uff cyclothem are primarily attributed to the evolution of anastomosed fluvial systems controlled by subsidence in an active foreland basin rather than to short term climatic change or eustacy

isotopic hydrology and paleohydrology of the madagi kenya natron tanzania basin during the late quaternary abstract three generations of stromatolites mark the recent extension of paleolakes in the magadi natron basin on the border between tanzania and kenya the last two generations are observed some 50 m above the modern lakes level they are respectively attributed to the beginning of the last interglacial and to the pleistocene holocene transition analysis of the morphology and texture of the stromatolites reveals an ecological zonation between 0 and 10 14 m below the shoreline their relatively high 18o and 13c values reflect a long residence time for the paleolake water which would have been favorable to the establishment of an isotopic equilibrium between atmospheric co2 and the dissolved inorganic carbon despite deep carbon inputs in relation to carbonititic volcanism a study of the 18o 2h and 13c contents in the hydrothermal springs perennial rivers and lacustrine brines indicates that during the last period of significant renewal of the deep saline groundwater precipitation was depleted in heavy isotopes when compared to modern precipitation this difference corresponds to a drop of about 600 1000 m in altitudinal hydroclimatic zones finally a comparison between the modern and paleohydrological data leads us to believe that the stabilization of the paleolake shorelines at the same altitude can be attributed to control by the water table level in the aquifer of the basaltic plateaus east of the basin

oxygen isotopes of marine mollusc shells record eocene elevation change in the pyrenees abstract constraining paleoaltimetry of collisional orogens is critical to understand the dynamics of topographic evolution and climate tectonics retroactions here we use oxygen stable isotope record on oyster shells preserved in marine foreland deposits to examine the past elevation of the pyrenees during the eocene our approach is based on the comparison with the paris basin an intracratonic basin not influenced by orogenic growth the finding of a shift of 1 5 between 49 and 41 ma indicating more negative 18oc in the south pyrenean foreland is interpreted to reflect the inflow of river water sourced from higher elevation in the pyrenees to test this and provide paleoelevation estimate we adopt a morphologic hydrological model accounting for the hypsometry of drainage basin our best fitting model shows that the pyrenees rose up to 2000 m this indicates that the pyrenees reached high elevation in the eocene thus providing new critical constraints on their long term orogenic development 18o of marine mollusc shells are proved potentially attractive for paleoelevation studies especially for mountain belts where elevated continental surfaces have not been preserved

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

spatial and temporal variation of cenozoic surface elevation in the great basin and sierra nevada the surface uplift of mountain belts caused by tectonism plays an important role in determining the long term climate evolution of the earth however the general lack of information on the paleotopography of mountain belts limits our ability to identify the links and feedbacks between topography tectonics and climate change on geologic time scales here we present a 1 8 o and d record of authigenic minerals for the northern great basin that captures the timing and magnitude of regional surface uplift and subsidence events in the western united states during the cenozoic authigenic calcite smectite and chert 1 8 o values suggest the northern great basin region experienced 2km of surface uplift between the middle eocene and early oligocene followed by 1 to 2km of surface subsidence in the southern great basin and or sierra nevada since the middle miocene these data when combined with previously published work show that the surface uplift history varied in both space and time surface uplift migrated from north to south with high elevations in southern british columbia and northeastern washington in the middle eocene and development of surface uplift in north and central nevada in the oligocene this pattern of north to south surface uplift is similar to the timing of magmatism in the western cordillera a result that supports tectonic models linking magamtism with removal of mantle lithosphere and or a subducting slab

synorogenic evolution of large scale drainage patterns isotope paleohydrology of sequential laramide basins in the past decade we and others have compiled an extensive dataset of o c and sr isotope stratigraphies from sedimentary basins throughout the paleogene north american cordillera in this study we present new results from the piceance creek basin of northwest colorado which record the evolving hydrology of the eocene green river lake system we then place the new data in the context of the broader cordilleran dataset and summarize implications for understanding the synorogenic evolution of large scale drainage patterns the combined data reflect 1 a period of throughgoing foreland rivers heading in the sevier fold and thrust belt and flowing east 2 ponding of freshwater lakes in the foredeep as laramide uplifts blocked drainage 3 hydrologic closure that led to both intensive evaporation in the terminal sink of the piceance creek basin and integration of catchments over length scales 1000 km 4 infilling of basin accommodation by southward migrating magmatism in distal catchments leading to the freshening and demise of intraforeland lakes that also stepped south over time

winds and sand movements in the namib sand sea wind regimes in the vicinity of the namib sand sea are high energy unimodal near the coast becoming bimodal or complex inland there is an overall decrease in wind energy and sand transport rates from south to north and west to east such that sand moves from coastal and southern source areas to accumulate in the northern and central parts of the sand sea such a pattern can explain much of the observed spatial variability in dune types sizes and sediment characteristics and lends support to a climatic model of sand sea formation in this region seasonal and daily cycles of wind velocity and direction give rise to episodic sand transport most of which is generated by winds of moderate velocity and frequency

the palaeoenvironmental significance of dune size hierarchies the size of a dune is a function of many factors one of which is the length of the period of consistent wind regime under which it grew ephemeral dunes are defined as those that are adjusted to diurnal wind regimes meso dunes to annual wind regimes and mega dunes to wind regimes that persist for longer periods these longer periods could be major climatic cycles but there seem to be no dunes whose size reaches that allowed by the length of some of these periods there may therefore be a maximum size beyond which mega dunes cannot grow dictated by sand supply aerodynamic processes such as speed up or by the scale of the atmospheric boundary layer these concepts are tested on dunes in the wahiba sands in oman analysed from a landsat image it is hypothesised that a large set of megadunes here formed in the lgm that a smaller set of mega dunes may be of younger dryas age and that there is a meso dune series of holocene age

predicting paleoelevation of tibet and the himalaya from δ18o vs altitude gradients in meteoric water across the nepal himalaya the 18o value of meteoric water varies with elevation providing a means to reconstruct paleoelevation if the 18o values of paleowater are known in this study we determined the 18o values of water 18omw from small tributaries along the seti river and kali gandaki in the nepal himalaya we found that 18omw values decrease with increasing altitude for both transects 18omw vs altitude along the kali gandaki in west central nepal fit a second order polynomial curve consistent with increasing depletion of 18o with increasing elevation as predicted by a rayleigh type fractionation process this modern 18omw vs altitude relationship can be used to constrain paleoelevation from the 18o values of miocene pliocene carbonate 18oc deposited in the thakkhola graben in the southern tibetan plateau paleoelevations of 3800 480 m to 5900 350 are predicted for the older tetang formation and 4500 430 m to 6300 330 m for the younger thakkhola formation these paleoelevation estimates suggest that by 11 ma the southern tibetan plateau was at a similar elevation to modern

response of meteoric δ18o to surface uplift implications for cenozoic andean plateau growth abstract the timing and magnitude of surface uplift provide important constraints on geodynamic models of orogen formation oxygen isotope 18 o and mass 47 isotopolog 47 compositions from terrestrial carbonate sediments have been used with modern isotope and temperature lapse rates to infer past surface elevations of the andes however these paleoaltimetry interpretations are contentious because variations in the oxygen isotope composition in meteoric water 18 o p are caused by changes in elevation orographic and regional climate here we use a limited domain isotope tracking general circulation model to simulate changes in 18 o p and isotopic lapse rates in response to andean surface uplift and to re evaluate 18 o and 47 changes in late miocene carbonates previously associated with rapid andean growth results indicate that andean surface uplift leads to changes in low level atmospheric circulation and an increase in precipitation along the eastern andean flank which influences isotopic source and amount effects simulated changes in andean 18 o p are not systematic with an increase in surface elevation but are instead a function of orographic thresholds that abruptly change regional climate a 18 o p decrease of 5 over the central andes and an increase in isotopic lapse rates up to 0 8 km 1 coincide with andean surface uplift from 75 to 100 of modern elevation these changes in the isotopic signature could account for the entire 3 4 18 o depletion in late miocene carbonate nodules and suggest an andean paleoelevation of 3000 m 75 of modern elevations before 10 ma

linear dune accumulation chronologies from the southwest kalahari namibia challenges of reconstructing late quaternary palaeoenvironments from aeolian landforms abstract the linear dunes of the southern kalahari dunefield constitute one of the major palaeoenvironmental proxies in the region the application of optically stimulated luminescence osl dating since the1990s and advancements in the depth of sampling using augering equipment over the past few years have permitted the reconstruction of linear dune accumulation chronostratigraphies for entire dune profiles from base to crest these methods are applied to four dunes in the mariental stampriet region of the southern kalahari dunefield sampled at predominantly 0 5 m intervals individual dunes record multiple phases of dune construction but with only a few phases recorded consistently between two or more of the dunes results from the 48 osl ages produced here extend the aeolian accumulation record for the southern kalahari dunefield through the last three glacial interglacial cycles with two ages from the early part of mis6 a synthesis of all existing luminescence ages for the southern kalahari reveals that the dunefield has been partially active throughout much of the past 120 ka there are no clear clusters of ages within osl age errors this is in contrast to previous syntheses of ages for this region in addition these new data from mariental stampriet dunes show that clusters in grouped dune osl ages can be spuriously produced as a function of reducing the sampling frequency with depth within the dunes from 0 5 to 1 mintervals this has significant implications for previous conclusions regarding discrete phases of aeolian accumulation based on sampling at 1 m intervals and less vertically intensive sampling techniques the total luminescence data set of 136 ages for the southern kalahari implies that this dunefield has been close to the threshold of reactivation throughout much of the late quaternary

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

a record of rapid holocene climate change preserved in hyrax middens from southwestern africa the discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central namib desert africa is providing unprecedented insight into the region s environmental history high resolution stable carbon and nitrogen isotope records spanning 0 11 700 cal calibrated yr b p indicate phases of relatively humid conditions from 8700 7500 6900 6700 5600 4900 and 4200 3500 cal yr b p with a period of marked aridity occurring from 3500 until ca 300 cal yr b p transitions between these phases appear to have occurred very rapidly often within 200 years of particular importance are 1 the observed relationship between regional aridifi cation and the decline in northern hemisphere insolation across the holocene and 2 the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing together these elements call for a fundamental reexamination of the role of orbital forcing on tropical african systems and a reconsideration of what factors drive climate change in the region the quality and resolution of these data far surpass any other evidence available from the region and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern africa

morphology and architecture of a late pleistocene cliff front dune mallorca western mediterranean cliff front dunes are a common coastal feature but this is the first time that the structures indicative of their formation are documented a late pleistocene cliff front dune in southern mallorca has been studied owing to early lithification of the calcarenite its morphology is well preserved the dune occurs in front of a curved near vertical coastal cliff but the bedform cannot be classified as a fixed echo dune the dune grew with time and its brinkline gradually moved closer to the cliff front in the final stage of evolution the top of the dune reached a height of at least 23 m the dune is composed of wind ripple strata these strata are arranged in critical to supercritical climbing dune cross stratification recording its growth stages the angle of climb of the dune increases toward the cliff and reaches 50 the morphological features of cliff front dunes result from both cliff slope and whether the cliff front is curved or straight

peat marl deposition in a holocene paludal lacustrine basin sucker lake michigan exceptionally thick over 15 m deposits of peat which fill the northern end of sucker lake basin in northcentral michigan document the volumetric importance of allochthonous organic material in a modern coal forming environment organic debris that originates in and is derived from a highly vegetated floodplain immediately upstream is deposited as a lakeward prograding lobe that exhibits features typical of most lacustrine gilbert deltas but is composed almost entirely of organic material this system overlies an additional 7 m of nearly pure brecciated lacustrine carbonate deposited as shallow lake margin benches and emplaced into the deep basin centre by gravity sliding prior to deltaic progradation in the southern end of the basin bottomset beds of fine silt size organic phytoclasts onlap distal facies of progradational bench carbonates which originate in shallow water through calcite encrustation about stems of the dominant macrophyte chara with continued sedimentation a stratigraphic succession in which allochthonous organics will overlie pure allochthonous lacustrine carbonates and in part be overlain by autochthonous carbonates will characterize the northern end of this holocene system in the southern end around the basin margins however autochthonous carbonates will entirely underlie allochthonous organics numerous continuous cores up to 22 m through these units document 1 the importance of sources of allochthonous organic debris in modern coal forming complexes 2 the genetic relationship between nearly pure calcareous and nearly pure carbonaceous facies within these paludal lacustrine settings and 3 the complexity of interrelationships between the several component facies within such continental sedimentary systems

geomorphological hierarchies for complex mega dunes and their implications for mega dune evolution in the badain jaran desert abstract the evolution of mega dunes is sometimes attributed to factors other than the wind but evidence for this is lacking it is assumed that the dominance of wind in maintaining the evolution of mega dunes should be characterized by regular height spacing relationships that have been found for simple dunes or wind ripples which are dominantly formed by the wind in this context we studied the height spacing relationship for the complex reversing mega dunes in the badain jaran desert which features the tallest mega dunes in the world the complex mega dunes were divided into three hierarchical orders according to the cumulative probability plots of dune height and spacing measurements and the coefficients of variability of dune heights and spacings were in accordance with values reported for other deserts the relationship between dune spacing and height for all the three orders of dunes could be expressed reasonably well by a uniform linear function that was also applied to the height wavelength relationship for wind ripples in other deserts this relationship was found to be similar to those for several other deserts and subaqueous bedforms this implies that there should be few unique factors in maintaining the evolution of complex mega dunes in the badain jaran desert compared with the superimposed simple dunes and dunes in other deserts dune fields and subaqueous bedforms and that the tallest mega dunes on the earth can be maintained by the wind

mio pliocene seasonality on the snake river plain comparison of faunal and oxygen isotopic evidence smith g r and patterson w p 1994 mio pliocene seasonality on the snake river plain comparison of faunal and oxygen isotopic evidence palaeogeogr pataeoctimatol palaeoecol 107 291 302 analysis of fish faunas and oxygen isotopic composition of a fish otolith from lacustrine deposits of southwestern idaho provide a means of evaluating regional miocene and pliocene climates a disharmonious assemblage consisting of coldwater salmon and trout and warmwater sunfish and catfish from the chalk hills formation of the snake river plain indicates that the climate of the late miocene was warm and moist with cool summers and mild winters colonization of the lake by deepwater sculpins and whitefish in the pliocene indicates that the climate was moist and equable but with summers cooler than either the miocene or quaternary oxygen isotopic variation among seasonal growth rings in an aragonitic otolith of a pliocene littoral sunfish suggests a seasonal range of temperatures locally more equable than at present extremely depleted values of 6180 in carbonates suggest that the lake was maintained by tributaries from high elevation watersheds with locally low evaporation rather than high precipitation

oxygen isotopic composition of carbonate concretions from the lower cretaceous of victoria australia implications for the evolution of meteoric waters on the australian continent in a paleopolar environment oxygen isotopic data from carbonate cements in concretions have been used to infer the isotopic composition of meteoric fluids present at the time of concretion growth in terrestrial sediments that were deposited within the early cretaceous south polar circle at 75 80 s carbon and oxygen isotope compositions have been determined on over 135 samples of carbonate from 45 concretions taken from 24 localities aptian albian in age in the terrestrial sedimentary basins associated with the otway and strzelecki groups southeastern australia the carbonate cements include calcite having 26 4 13c 19 6and3 6 18o 29 6 or siderite having17 6 18o 30 8 calcite cemented concretions are more abundant and are interpreted to represent early near surface cementation events on the basis of textural evidence such as high 30 porosities at the time of cementation and mineralogical evidence such as the preferential preservation within concretions of labile detrital grains including plagioclase pyroxene and amphibole the oxygen isotopic data indicate that meteoric fluids with very low 18o certainly less than 15 and probably on the order of 20 were involved in the precipitation of the early calcites the extremely low 18o values of the fluids involved in the early diagenesis of both the otway and strzelecki groups suggest that the catchment area of the river system that carried sediments to these basins had a cold high latitude climate with mean annual temperatures less than 5 c and quite possibly below freezing by analogy with the relationship between modern18o distribution of meteoric fluids and climate these new data suggest that the early cretaceous polar regions may not have been ice free

evaluating the use of dune sediments as a proxy for palaeo aridity a southern african case study abstract the dominance of dryland environments in the southern hemisphere makes the study of these regions of critical importance for the development of regional hemispheric and global models of environmental change unfortunately the wetting and drying cycles associated with semi hyperarid climates are not conducive to the preservation of traditional organic proxy data sources the last decade however has seen the development of a number of alternative archives including the application of luminescence dating techniques to dunes and other aeolian deposits in continental situations the existence of relict dune fields has long been thought to be evidence of drier conditions during the pleistocene and direct ages from these features have been interpreted almost exclusively as indicating phases of aridity however an increasing number of ages from a broader range of environments are calling into question the assumption that aeolian activity can be simply equated with aridity presented here is a comparison of dune ages that have been obtained from across southern africa with a range of proxies from both terrestrial and marine records taken as a whole three primary phases of activity can be identified at 60 40 35 20 and 17 4 ka the frequent discordance with other terrestrial records indicating coeval increases in humidity and the close correlation of these phases with wind strength proxies suggest that aridity is unlikely to be the sole or even primary forcing mechanism for aeolian activity in the region and the palaeoclimatic significance of these sedimentary archives needs to be reassessed

age and dynamics of linear dunes in the namib desert bristow c s duller g a t lancaster n 2007 age and dynamics of linear dunes in the namib desert geology 35 6 555 558 sponsorship american chemical society

oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in the afar region of ethiopia abstract the oxygen and carbon isotope ratios in the shells of ancient freshwater gastropods coupled with geological geographical and hydrological data from their area and time of origin can yield information concerning general climatic conditions prevailing at the time the gastropod lived in addition sequential oxygen isotope analyses along the growth spiral of the shell provides indications of seasonal variability in the life span of the organisms such data were collected for some freshwater gastropods collected from two lakes and two springs in the afar region of ethiopia spanning late pleistocene to modern age episodes of high rainfall in the ethiopian highlands were clearly evident in the depleted 18o values in some shells whiel arid spells produced enriched levels in others hot springs introduced an added complication to the isotope record this study on a region of particularly complex hydrology and highly variable environmental history indicates the utility of stable isotope analyses as proxies for past climates but also emphasizes the problems involved

stable isotope paleoaltimetry a critical review stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic in situ formed minerals to illustrate the usefulness as well as potential limitations of this method we review 1 processes controlling the isotopic composition of modern precipitation 2 stable isotope data from modern precipitation across regions of high topography and 3 stable isotope data from authigenic minerals that have been used to infer paleotopography from this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography with uncertainties that critically depend on a detailed understanding of local to global scale paleoclimate during the time interval of interest

palaeoenvironmental reconstruction of lacustrine source rocks in the lower 1st member of the shahejie formation in the raoyang sag and the baxian sag bohai bay basin eastern china abstract the lower 1st member es1l of the oligocene shahejie formation is the most important source rock interval in the raoyang sag and the baxian sag to reveal the controlling factors that affected the distribution of organic rich source rocks we analyzed the lower es1 es1l samples collected from the raoyang sag and the baxian sag using sedimentological and geochemical techniques to reconstruct the lacustrine palaeoenvironmental conditions most of the samples had chemical index of alteration cia values between 70 and 75 which reflect a stable and moderately weathered source under a transitional climate the distribution of depocenters and palaeo water depths indicated that the main catchment area was concentrated in the raoyang sag with a maximum palaeo water depth of 14 m and a wave base depth of 7 5 m stable isotope data from carbonates 18ocarb and 13ccarb showed that the lakes with long residence times in the raoyang sag had a positive correlation and that the lakes with short residence times in the baxian sag had no isotopic covariance during the deposition of the lower es1 the p al and p ti ratios ptotal and carbon isotope values of the organic matter 13corg in the two sags illustrated stable palaeoproductivity without much fluctuation the enrichments of mo u and ratios of c p org b ga and sr ba further indicated saline and anoxic bottom water conditions associated with stable water column stratification in well xi63 and brackish and less anoxic bottom water conditions in well nd1 during the deposition of the main source rock of the lower es1 moreover the plots of mo and u enrichment versus toc and hydrogen index values versus 13c i e the difference between the 13c values of carbonates and organic matter corroborated the inference that the preservation of organic matter was the main factor controlling the formation of organic rich source rocks in the raoyang sag and the baxian sag during the deposition of the lower es1

geochemical evaluation of fenghuoshan group lacustrine carbonates north central tibet implications for the paleoaltimetry of the eocene tibetan plateau abstract a sedimentologic petrologic and geochemical evaluation of lacustrine carbonates from the eocene oligocene fenghuoshan group of north central tibet provides insight into the paleoenvironmental and paleolimnological setting of the hoh xil basin during the mid tertiary fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones these carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial alluvial plain deposits individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward fenghuoshan carbonates also contain abundant ostracode bivalve and charaphyte fossils sedimentologic evidence suggests that carbonate deposition occurred in shallow relatively short lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems stable c and o isotopic analysis of fenghuoshan carbonates show 18o values ranging from 11 7 to 10 3 vienna pee de

evidence for progressive holocene aridification in southern africa recorded in namibian hyrax middens implications for african monsoon dynamics and the african humid period abstract presented here are stable nitrogen isotope data from a rock hyrax procavia capensis middens from northwestern namibia that record a series of rapid aridification events beginning at ca 3800 cal yr bp and which mark a progressive decrease in regional humidity across the holocene strong correlations exist between this record and other terrestrial and marine archives from southern africa indicating that the observed pattern of climate change is regionally coherent combined these data indicate hemispheric synchrony in tropical african climate change during the holocene with similar trends characterising the termination of the african humid period ahp in both the northern and southern tropics these findings run counter to the widely accepted model of direct low latitude insolation forcing which requires an anti phase relationship to exist between the hemispheres the combined dataset highlights 1 the importance of forcing mechanisms influencing the high northern latitudes in effecting low latitude climate change in africa and 2 the potential importance of solar forcing and variations in the earth s geomagnetic shield in determining both long term and rapid centennial scale climate changes identifying a possible mechanism for the variations marking the ahp termination in both the southern and northern tropics

basement balancing of rocky mountain foreland uplifts restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts local zones of shortening and extension occur in hinge zones above transitions in fault curvature high curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover 20 references 4 figures

miocene lacustrine algal reefs southwestern snake river plain idaho straccia f g wilkinson b h and smith g r 1990 miocene lacustrine algal reefs southwestern snake river plain idaho sediment geol 67 7 23 the hot spring limestone is a shallow water algal carbonate within a late tertiary transgressive lacustrine sequence exposed in the southwestern snake river plain this 5 m thick lensoid sequence crops out over an 80 km 2 area that closely approximates original areal extent of nearshore carbonate accumulation reefal bodies consist of closely packed algal cylinders several decimeters in height each of which includes a dense laminated carbonate wall surrounding porous digitate carbonate that radiates outward and upward from one or more hollow tubes these coalesce upsection into separate vertical columns several meters in diameter moderately well sorted terrigenous and molluscan debris deposited between columns during growth indicates these structures were resistant to wave erosion and therefore were true reefs thick rings of littoral carbonate surrounding the upper walls of each column record the final stages of reef development structural attributes exhibited by these miocene carbonate bodies are also common to a number of tertiary and quaternary algal buildups reported from other lacustrine settings although features within the hot spring limestone are complex in gross morphology and structural detail both columnar reefs and algal cylinders display little variation in size shape or internal structure between areas of varying water depth and wave energy thus reflecting the importance of biological processes as well as physical processes during reef development

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

post laramide oligocene uplift in the wind river range wyoming in late oligocene time the crest of the wind river range was uplifted from an erosion surface that formed during laramide late cretaceous early eocene deformation this uplift created the highest peaks in the wyoming foreland and is indicated by rejuvenation of the range as a sediment source and by the physiography of the peaks differential movement of basement blocks in the core of the range is identified by apatite fission track data we distinguish this deformation from earlier laramide compression and neogene normal faulting and speculate that it occurred when precambrian shear zones were reactivated in response to local flexural stress in the laramide subsidence uplift couple

isotopic imprint of climate and hydrogeochemistry on terrestrial strata of the triassic jurassic hartford and fundy rift basins abstract late triassic to early jurassic terrestrial sequences in the hartford and fundy rift basins have distinctive carbon and oxygen isotopic compositions of calcite and dolomite the isotopic data mostly reflect paleoclimatic fluctuations and hydrogeochemistry of the lacustrine playa and fluvial environments dolomites from laminae in three sequences of playa red mudstones and lacustrine gray to black mudstones in the hartford basin have variable isotopic compositions 13c 5 8 to 1 8 pdb 18o 7 2 to 0 7 pdb within any single symmetrical cycle of playa red mudstone lacustrine gray black gray mudstone playa red mudstone there is a systematic change to relatively enriched 13c compositions in dolomite in the grayish black and black mudstones in the center of the cycle these carbon isotopic data suggest that the lacustrine sequences formed as the lakes changed from well mixed with anoxic bottom waters to stratified with anoxic bottom waters where 13c depleted carbon was concentrated in organic matter that was then buried calcites from lacustrine micritic and biomicritic limestones of the scots bay formation of the fundy basin have stable isotopic compositions 13c 4 6 to 2 2 pdb 18o 6 1 to 3 0 pdb that become more enriched in 18o and 13c upward in shallowing depositional sequences these isotopic data reflect initial calcite precipitation when a high inflow of fresh water produced high lake levels followed by progressively lower inflow resulting in lower lake stands and higher salinity due to continuing evaporative loss of surface wat r the lake waters were well oxygenated at all times in the hartford basin caliche calcites in fluvial mudstones and sandstones have isotopic compositions 13c 7 3 to 3 8 pdb 18o 8 0 to 5 6 pdb that reflect paleosol processes during climatic conditions that varied from warm and dry in late triassic time to relatively cooler and probably wetter in the early jurassic isotopic compositions of caliche calcites in redbeds in the fundy basin indicate a parallel climate change from late triassic to early jurassic time but also that the climate was relatively hotter and probably drier over the entir interval as compared to the hartford basin

tertiary lacustrine sediments from sentinel butte north dakota and the sedimentary record of ectogenic meromixis abstract about 10 meters of probable oligocene or miocene lacustrine sediments which crop out on top of sentinel butte in western north dakota consist mostly of greenish gray zeolitic mudstone interbedded with relatively thin layers of dolomite limestone and chalcedony the dolomite layers are several centimeters thick and are mostly dolomicrite with laminae or lenses of arkosic sand and pellets they probably formed by deposition of protodolomite in shallow closed saline lakes the limestone layers are up to 30 centimeters thick their basal portions contain varve couplets of micrite and organic matter graded bedding in micrite laminae and concentration of aeolian silt in organic laminae suggest that deposition of carbonate took place during a relatively brief portion of each year pres rvation of varves fossil fish and coprolites in the limestone imply deposition in a meromictic lake in contrast poor preservation of fish and bioturbation in the upper parts of two limestone beds record transition to holomictic conditions some variations in varve thickness appear rhythmic and possibly related to the sunspot cycle several asymmetric cycles of contrasting carbonate facies ideally consisting of massive dolomite overlying clay and underlying laminated limestone imply that the ancient lake in which the sequence was deposited underwent several episodes of shrinkage and closure during which its salinity increased followed by expansion accompanied by abrupt development of ectogenic meromixis changes in authigenic mineralogy and upward increase in thickness of limestone beds relative to dolomite beds record a long term trend toward freshening of basinal hydrology models involving ectogenic meromixis can explain intimate associations of evaporitic facies with varved sediments in other sedimentary sequences and may be applicable to some precambrian iron formations

early oligocene paleovalleys in southern and central wyoming evidence of high local relief on the late eocene unconformity local relief in the mountains of southern and central wyoming at the end of the eocene was as great as if not greater than it is today this is indicated by large valleys containing the lower oligocene white river formation that extend into the precambrian cores of the medicine bow and laramie mountains there has been little deformation of white river rocks in the valleys or on the margins of the mountain ranges precluding post eocene block uplift as a source of modern topographic relief the regionally widespread low relief subsummit surface was developed during the miocene and is not part of the late eocene unconformity other oligocene valley fils in the rocky mountains indicate that high local relief was widespread during the late eocene

diagenesis of shallowly buried miocene lacustrine carbonates from the hoh xil basin northern tibetan plateau implications for stable isotope based elevation estimates the oxygen isotopic composition of lacustrine carbonate deposits has been frequently used to reconstruct paleoaltimetry of orogenic belts however the primary oxygen isotopic composition of lacustrine carbonate can be partially or wholly altered by post depositional diagenetic processes such as deep burial recrystallization and the interaction with geologically younger meteoric waters to better understand how ancient lacustrine carbonates are diagenetically modified by meteoric waters we present detailed mineralogy petrography stable carbon and oxygen isotopic compositions and major and minor element geochemistry of lacustrine carbonates from the miocene wudaoliang group which underwent only shallow burial 300 m in the hoh xil basin northern tibetan plateau our observations indicate that the primary mineralogy in the wudaoliang carbonate muds consists of a mixture of both low mg and high mg calcite during or shortly after deposition the carbonate muds experienced stabilization and associated carbonate cementation in a closed phreatic meteoric diagenetic system interaction with 18 o depleted meteoric waters during diagenesis slightly reduced 18 o c values but overall the wudaoliang carbonates largely retained primary oxygen isotope compositions our data support previous interpretations that 18 o c values of the wudaoliang group lacustrine carbonates are indicative of the primary 18 o composition of miocene meteoric water and can be used for paleoelevation estimate of the miocene hoh xil basin northern tibetan plateau by demonstrating the oxygen isotopic fidelity of the wudaoliang carbonates we are able to rule out open system meteoric diagenesis from younger meteoric waters and subsequent isotopic alteration this study highlights that integration of scanning electron microscope and cathodoluminescence petrography with stable isotopic and mineralogical data enables the determination of the extent of meteoric diagenetic alteration in lacustrine carbonates particularly whether the diagenetic systems were open or closed with respect to the oxygen isotope

mean airflow patterns upwind of topographic obstacles and their implications for the formation of echo dunes a wind tunnel simulation of the effects of windward slope 1 secondary airflow plays an important role in dune formation and development in the case of the echo dunes that form upwind of an obstacle it introduces considerable complexity because both the initiation and development processes are influenced by airflow patterns in this study we measured the variations of airflow in front of obstacles with different windward stoss slope angles by means of particle image velocimetry in a series of wind tunnel tests the windward slope angle was the key factor in determining the secondary airflow patterns the horizontal velocities decreased and the vertical velocities increased as the airflow approached perpendicular to the obstacle and a vortex of reversed flow formed in front of obstacles with a stoss slope of 60 or steeper the positions of airflow separation and of the core of the reversed vortex were a function of the windward slope angle depending on the position of the reverse vortex and its significance for the formation of echo dunes the windward slope could be divided into three groups 65 or less 70 75 and 80 or more the horizontal velocity profiles deviated from a log linear distribution resulting in five airflow regions with different rates of change of horizontal velocity we discuss the significance of these velocity variations for sand accumulation and the effects of each airflow region on echo dune formation

geochemistry of sediments from quaternary sand ramps in the southeastern mojave desert california abstract trace element analyses of sediment from sand ramps in the bristol trough and clark s pass aeolian corridors of the mojave desert california were conducted to determine their depositional history and relation to sources of sediment within sand transport corridors sand ramps are topographically controlled depositional systems consisting of amalgamated accumulations of aeolian fluvial and talus deposits these landforms contain a variety of deposits formed in different environments and are therefore a valuable source of paleoenvironmental information sediments were studied from three sand ramps the iron mountain big maria and dale lake sand ramps and from two sand sheets rice valley and cactus plain cactus plain is located on the eastern side of the colorado river in arizona but previous researchers have suggested that it is related to the corridors of the western california side geochemical data from units within individual sand ramps indicate that sources for each sand ramp changed through time probably as sediment availability from different local fluvial playa systems changed in response to climate fluctuations analyses also indicate that each sand ramp is composed of sediment from discrete local sources sand deposits in the bristol trough are not integrated and thus the corridor does not act as a coherent sand transport pathway comparisons of sand from cactus plain and sediment from the bristol trough and clark s pass corridors indicates that the cactus plain sand on the east side of the colorado river was not derived from sources on the west side of the river

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain

geomorphology and sediments of sand ramps in the mojave desert abstract sand ramps are amalgamated accumulations of aeolian fluvial and talus deposits they have developed as a result of the interactions of wind blown sand and desert piedmont processes adjacent to mountain ranges in the mojave desert and occur next to regional and local sand transport corridors sand ramps are dominated by aeolian sand that was deposited as sand sheets against the mountain front but also include significant fluvial units periods of geomorphic stability are represented by paleosols and talus accumulations the sand ramps provide a record of the response of aeolian processes to climatic change in the mojave desert most are now relict features and accumulated in periods of higher sediment supply from fluctuating and or desiccating paleolakes during the late pleistocene and early holocene

late miocene magnetobiostratigraphy of crete six upper miocene marine clay sections on crete greece have been subjected to a detailed magnetobiostratigraphic analysis six correlatable polarity zones are recognized and these demonstrate the regional synchrony of planktonic foraminiferal biohorizons by way of correlation to the magnetic polarity time scale of lowrie and alvarez 1981 the cretan sequence is assigned to polarity chronozones 5 anomaly 3a and 6 the new chronology provides an age of 5 6 ma for the first occurrence datum fod of thegloborotalia conomiozea group in the mediterranean an age of 6 0 ma for the fod ofg menardii form 5 and an age of 6 6 ma for the last occurrence datum lod ofg menardii form 4 correlating the polarity record of the new zealand blind river section kennett and watkins 1974 with the magnetic polarity time scale provides an age of 6 0 ma for the evolutionary appearance ofgloborotalia conomiozea which is in complete agreement with the age of 6 1 0 1ma given by loutit and kennett 1979 the demonstrated diachrony of 0 4 ma between the new zealand fod ofg conomiozea and its mediterranean counterpart is explicable in view of the different nature of the two events the one in new zealand being evolutionary and the one in the mediterranean migrational the fod of theg conomiozea group in the tortonian messinian boundary stratotype section coincides with the level proposed by colalongo et al 1979 to mark the base of the messinian since the fod of theg conomiozea group in crete and in sicily are most probably time equivalent the age of the tortonian messinian boundary is fixed at 5 6 ma the youngest sediments incorporated in this study extend into the gilbert chronozone and antedate the main evaporitic phase consequently the messinian evaporitic body is younger than the base of the gilbert chronozone the age of which is fixed at 5 3 ma adopting an age of 5 0 ma for the miocene pliocene boundary would imply that evaporites and post evaporitic lago mare sediments were deposited in some 300 000 years and suggests that in the central parts of the mediterranean basins evaporites must have accumulated at rates of some 3 m per 1000 years