global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

discrepancies between different organic maturity indicators in a coal series affected by an abnormal thermal event visean niger abstract classical petrographical and geochemical approaches have been used to study visean coal seams sampled in two areas 50 km apart in northern niger t max values given by rock eval pyrolysis and individual molecular ratios determined on hopanes and steranes are in agreement with the burial history of the two sets of samples studied the difference of maturity of the two sample sets revealed by these various parameters is consistent with their current difference in burial depth 400 m only small changes of these parameters with depth were observed owing to the small depth interval sampled at both sites 15 and 40 m these variations also appear to be related to normal burial diagenesis no maturity differences between the two sample sets are revealed by vitrinite reflectance data and atomic h c ratios which are in the same range at both sites but display very large variations with depth r v 0 60 1 40 and h c at 0 80 0 90 to 0 55 these data indicate rapid entrance into the oil window this conclusion also being supported by microspectrofluorimetric observations showing oil production and expulsion and by the presence of pyrobitumens containing large devolatilization vacuoles in the deepest samples the abnormally high maturity gradient could have been caused by a hydrothermal event the present observations on coals confirm and supplement previous conclusions drawn from the study of organic materials disseminated in zn pb sulfide ore bearing sedimentary rocks the difference in sensitivity shown by the various maturity indicators is considered very likely to be due to differences in the kinetics of the transformations on which they each depend

stable isotope s c chemostratigraphy and hydrocarbon biomarkers in the ediacaran upper section of sierras bayas group argentina the neoproterozoic sedimentary successions of the sierras bayas group sbg are situated within the tandilia belt buenos aires province argentina the loma negra formation lnf from the upper part of the sbg has been investigated using a combination of inorganic organic and isotopic geochemical methods in order to determine the stable isotope composition of carbonates delta c 13 car delta o 18 car kerogen delta c 13 ker pyrite and carbonate associated sulfur delta s 34 py delta s 34 cas concentrations of major trace and rare earth elements and hydrocarbon distributions were also investigated this data set was used to further the understanding of the interactions between paleo biodiversity and paleoenvironmental conditions for terminal ediacaran post gaskiers shelf deposits the high delta s 34 py values exceeding the coeval delta s 34 cas values in green micritic limestones of the lower lnf may be explained by a combination of different events such as globally low seawater sulfate concentrations an increased rate of bacterial sulfate reduction and a decrease in sulfate levels in the near bottom deepest water layer inherited from glaciations negative ce anomalies in the dark grey micritic limestones of the upper lnf suggest they were deposited under oxidizing conditions however higher concentrations of fe mo zn and ree and the occurrence of authigenic s 34 enriched pyrite suggest that the sediments were maintained under reducing conditions the distributions of hydrocarbon biomarkers e g hopanes maximizing at c 29 are in line with this hypothesis and indicate a diverse microbial community including primary producers such as cyanobacteria e g terminally branched monomethyl alkanes hopanoid distribution phototrophic bacteria e g acyclic isoprenoids c phytane the unsteady delta c 13 car ker values reflect changes in primary biomass due to relative contributions of bacterial microorganisms using different photosynthetic carbon fixation pathways the lowest delta c 13 car ker values coincided with the latest biomarker signal interpreted as the signature of green non sulfur bacteria using a less c 13 fractionation pathway the combined biogeochemical features of the loma negra formation indicated a well stratified water column with oxygenated surface waters oxygen poor bottom waters and anoxic sediments which helped to refine a correlation with the polanco formation from the arroyo del soldado group in uruguay c 2013 elsevier b v all rights reserved

the neoproterozoic glacial record in the rio de la plata craton a critical reappraisal neoproterozoic glacial successions have been described in south america but the glacial deposits of the rio de la plata craton have been neglected in previous studies addressing the global distribution of glacially influenced successions the rio de la plata craton contains neoproterozoic glacial deposits in the sierra del volcan formation tandilia system argentina glacial influenced deposits in the playa hermosa and zanja del tigre formations dom feliciano belt uruguay and suspected glacially influenced deposits in passo da areia sao gabriel block brazil the tandilia system glacial record includesdiamictites dropstonesand rhythmitesdepos ited in glaciomarine conditions in a tectonically stable depositional setting the dom feliciano belt includes a thin section with ice rafted clasts in carbonates and a thicker section containing diamictites rhythmites outsized clasts and deformed beds in a volcano sedimentary succession the sao gabriel block occurrence deserves more attention to confirm any glacial influence in the fine grained part of the succession glaciation is considered to be contemporaneous with the gaskiers glaciation 580 ma with the exception of the carbonates with dropstones that may represent a previous event correlative with one of the glaciations described in the kalahari craton prior to kalahari rio de la plata assembly in the proto western gondwana margin

yardang geometries in the qaidam basin and their controlling factors abstract the hyperarid qaidam basin features extensive fields of yardangs covering an area of 40 000km2 sculpted in tectonically folded sedimentary rocks we extracted the geometries of 16 749 yardangs such as length to width ratio l w spatial density and spacing from multi source remote sensing data provided by google earth we classified the yardangs into four types based on their l w short axis 1 2 whale back 2 6 hogsback 6 10 and long ridge 10 210 we interpreted the yardang geometries in the context of their geologic setting bedding orientation location along anticline crests or syncline troughs and lithologic heterogeneity our results show that the yardang geometries in the qaidam basin are mainly controlled by the structural geology and rheology of the sedimentary rocks e g strike and dip of bedding the presence or absence of interbedded soft and hard beds and structural position with folds the angle between geomorphically effective wind directions and the strike of bedding and the relative cumulative wind shear force where two geomorphically effective wind directions are present our analysis revealed the following 1 nearly 69 of the yardangs with long ridge and hogsback geometries are distributed in syncline areas whereas 73 of the yardangs with short axis geometries are distributed in anticline areas 2 the l w ratio of yardangs exposed along the windward limbs of anticlines is lower than that of yardangs exposed along the leeward limbs and 3 in the westernmost parts of the basin yardangs are locally sculpted into mounds by two geomorphically effective wind directions

the tandilia system of argentina as a southern extension of the rio de la plata craton an overview the southernmost outcrops of the rio de la plata cratonic region are exposed in the tandilia system in eastern argentina the geological evolution comprises mainly an igneous metamorphic paleoproterozoic basement named buenos aires complex which is covered by neoproterozoic to early paleozoic sedimentary units which display subhorizontal bedding the basement of calc alkaline signature consists mainly of granitic tonalitic gneisses migmatites amphibolites some ultramafic rocks and granitoid plutons subordinate rock types include schists marbles and dykes of acid and mafic composition tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids the tectonic scenario seems related to juvenile accretion event 2 25 2 12 ga along an active continental margin followed by continental collision 2 1 2 08 ga after u pb zircon data the collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks the tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution the basement was preserved from younger orogenies such as those of the brasiliano cycle after a long paleoweathering process the sierras bayas group c 185 m thick represents a record of the first neoproterozoic sedimentary unit siliciclastic dolostones shales limestones superposed by cerro negro formation c 150 400 m thick siliciclastics assigned to upper neoproterozoic age the final sedimentary transgression during early paleozoic was the balcarce formation c 90 450 m thick deposited over all the mentioned precambrian units based on all the geological background a tectonic evolution is offered

sr ca mg ca ratios and sr and stable isotopes of biogenic carbonates from the late miocene velona basin central apennines italy provide evidence of unusual non marine messinian conditions abstract the sr ca and mg ca ratios of fossil ostracod and mollusc shells from a late miocene early messinian lacustrine sequence in the velona basin central apennines italy were used to calculate the sr ca and mg ca of waters in which these taxa lived sr ca of these waters were always higher than the sr ca calculated for the messinian seawater despite the broad salinity ranges fresh water oligo mesohaline inferred from the paleontological analyses the estimated mg ca water from the velona samples are below the mg ca value for coeval seawater even though they represent relatively high values when compared with many non marine waters the salinity variations of the velona lacustrine system recorded by changes in faunal assemblages are linked to variable inputs of na cl from deep groundwaters and are not reflected in significant changes in sr ca and mg ca of the waters calculated from shell chemistry the combination of 18 o signatures and sr isotopic ratios from molluscs from the velona basin suggests a non marine environment during some episodes of the lake history given the saline inputs the waters in the lake would attain a large solute concentration oligo to mesohaline although they retained the meteoric 18 o signature we compare the paleohydrochemical inferences from the studied sequence with other messinian records and we show the importance of combining paleoecological and geochemical proxies to obtain consistent paleoenvironmental reconstructions

elevation modelling and palaeo environmental interpretation in the siwa area egypt application of sar interferometry and radargrammetry to cosmo skymed imagery abstract digital elevation models produced from cosmo skymed imagery were used to delineate palaeo drainage in a wide area surrounding the siwa and al jaghbub oases of the western sahara desert egypt and libya this new generation of synthetic aperture radar imagery is suitable for this purpose because of its high spatial resolution and capacity to penetrate dry surface sediments different techniques such as radar interferometry and radargrammetry were used to produce digital elevation models these were assessed for accuracy and then combined to produce a single elevation model of the area the resulting elevation model was used to support the geological study and palaeo environmental interpretation of the area it revealed buried features of the landscape including inactive palaeo drainage systems drainage features were extracted from the elevation model using geographical information systems results were combined and assessed with respect to geological field data as well as data from the literature previous studies in the area suggest that a wide river probably the old nile river flowed into the libyan palaeo sirte before the late messinian drawdown of the mediterranean sea during the late messinian lowering of the sea the fluvial system changed shape and carved deep canyons throughout north eastern africa the reported findings on the key siwa area were used to precisely delineate the physiography of the modern drainage network and to confirm findings from our previous geological research in the area

first ediacaran fauna found in western africa and evidence for an early cambrian glaciation ediacara type body fossils are reported here for the first time from the west african craton in algeria this assemblage comprises medusoids comparable to medusinites sp and nimbia a trace fossil and a new genus of body fossil with uncertain affinity the fossils are in sandstones of the neoproterozoic cover separated by a major unconformity from the overlying tillite considered until now to be late neoproterozoic most ediacaran fauna radiated after the varanger ice age and tillites associated in ahaggar with molasse of the pan african belt are overlain by carbonates that were deposited during deglaciation and that locally contain early cambrian shelly fossils therefore we interpret the glaciogenic diamictite as early cambrian in age

variation in bed level shear stress on surfaces sheltered by nonerodible roughness elements 1 direct bed level observations of surface shear stress pressure gradient variability turbulence intensity and fluid flow patterns were carried out in the vicinity of cylindrical roughness elements mounted in a boundary layer wind tunnel paired corkscrew vortices shed from each of the elements result in elevated shear stress and increased potential for the initiation of particle transport within the far wake while the size and shape of these trailing vortices change with the element spacing they persist even for large roughness densities wake interference coincides with the impingement of the upwind horseshoe vortices upon one another at a point when their diameter approaches half the distance between the roughness elements while the erosive capability of the horseshoe vortex has been suggested for a variety of settings the present study shows that the fluid stress immediately beneath this coherent structure is actually small in comparison to that caused by compression of the incident flow as it is deflected around the element and attached vortex observations such as these are required for further refinement of models of stress partitioning on rough surfaces

aeolian erosional lineations in the libyan desert dakhla region egypt abstract this paper presents results of a study of aeolian erosion at the landscape scale there have been few such studies in desert geomorphology compared to those focused on individual landforms and on rock surface sculpture the present study area lies in the southern part of the libyan desert in south central egypt between 25 and 27 n 29 and 30 e bedrock comprises paleogene limestone of various lithologies climate is hyperarid the following topics are treated i correspondences between rock outcrop belts and belts of terrain lineated by aeolian erosion and unlineated terrain ii the effect of lithology on aeolian lineation which works through the presence or absence of chert in the limestones lineation is produced by aeolian erosion in chert free poor rocks whereas erosion of chert rich rocks produces a desert pavement that armors the surface suppressing erosion iii the effect of large valleys eroded upwind of lineated terrain which deflect winds and trap sand ending erosion so that downwind lineations are reduced and finally erased by weathering iv sample lineated landscapes in the area show stages of evolution arranged in the time domain into a proposed cycle of aeolian erosion the cycle progresses from initial smooth plain to grooves separating long blade shaped ridges to segmentation of ridges into shorter blades along diagonal joints to streamlining of shorter blades and size reduction to final planation in any one wind parallel swath of lineated terrain stages in the cycle progress downwind in the space domain so that upwind landscapes are more advanced in the cycle v the structure of air flow responsible for lineation is still uncertain but initial grooving may respond to stable longitudinal horseshoe vortices or self organized regularity of erosion by random vortices subsequently the erosion pattern is fixed by evolving relief vi large smooth basins within the lineated terrain were carved by aeolian erosion before oxygen isotope stage ois 5 70 130 ka as were similar basins along the dakhla piedmont below the scarp little geomorphic change has occurred since so in this field of aeolian erosional lineations aels the aeolian erosion cycle may have occupied 10 times as long since continental scale aridity set in at ca 2 4 ma there have been only two cycles in this area a completed cycle earlier than the current incomplete one is indicated by rare small yardangs riding piggyback on larger ones

neoproterozoic to lower palaeozoic successions of the tandilia system in argentina implication for the palaeotectonic framework of southwest gondwana the cryogenian to uppermost ediacaran successions of the tandilia system sierras bayas group and cerro negro formation in central eastern argentina is exceptional because of its unmetamorphosed and nearly undeformed character its sediment provenance trend and the absence of any identified glacial deposit and the deposition of warm water carbonates we decipher a dramatic change in the basin evolution from small scale depositional areas during the neoproterozoic to a larger basin related to an active continental margin throughout the uppermost ediacaran the base of the succession is represented by immature detritus of alkaline composition villa monica formation but towards the top of this formation the material is sorted and reworked nonetheless still reflecting in its detritus the local rocks the clastic deposition is interrupted by diagenetic overprinted dolomites the unconformable overlying quartz arenitic ediacaran cerro largo formation reworked the cryogenian villa monica formation and contains mainly felsic granitic and metamorphic basement material of slightly wider variety while the dominant alkaline geochemical signature in rocks of the villa monica formation disappears based on diagenetic petrographic and sedimentological data we can interpret the unconformity representing a longer time of erosion the cerro largo formation shows a transition to mudstones and the heterolithic facies of the olavarria formation the top of the sierras bayas group is represented by limestones loma negra formation which are discordantly overlain by the uppermost ediacaran cerro negro formation the latter displays detrital material derived from a continental arc mafic and felsic sources several arc related geochemical proxies th sc 20 are recorded in the sediment detritus as are syn depositional pyroclastites the absence of volcanic material in the underlying rocks allows proposing that the cerro negro formation is related to an active continental margin fringing gondwana terra australis orogen as a retro arc or retro arc foreland basin

yardangs on earth and implications to mars a review abstract yardangs as typical aeolian landforms are extensively identified in arid hyper arid areas on earth and other terrestrial bodies in this review based on analyses of morphology distribution age climate and geologic condition of yardangs worldwide we 1 assess the impacts of various controlling factors on yardang development to generalize a model based on morphology 2 illustrate the climate driven mechanism for yardangs 3 propose a geomorphological evolutionary model of yardang fields and 4 discuss potential implications of the above two models of yardang evolution on earth to mars it is demonstrated that both deflation and abrasion work on yardang development and it is the lithology to determine which one will be more effective in addition the wind action dominates the general aerodynamic form of yardangs while non aeolian factors account for the diversity in yardang morphology thus a model including four stages and initiating mechanisms is generalized for yardang processes from initiation to demise the synthetic analyses of meteorological data chronology and paleoclimatic proxies show that yardang development is mostly controlled by westerlies and would be accelerated during glacial periods and interrupted by lake formation during interglacial periods due to dry humid fluctuations and changes in atmospheric circulation driven by variations in solar radiation at orbital scales this leads to the zonal distributions of yardangs near 30 in both hemispheres depressions would be created or enlarged in yardang fields during glacial period along with activities of dust release and transport leading to aeolian deposition e g loess downwind the depressions then could be turned into lakes during wet phase of interglacial period and evolved into new yardangs in the following glacial period due to wind erosion this climate controlled geomorphological process under glacial interglacial cycles suggests that high accumulation rate in sediments from enclosed inland basins e g the qaidam basin doesn t mean high resolution climatic record due to the hiatus by wind erosion indicating the significance of robust chronology establishment before any climatic correlation under the rules of earth based yardang evolution it is potential to identify the specific level of development and explore the geomorphological processes as well as the climatic background for yardangs on mars

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

sedimentological evidence concerning the paleoceanography of the cretaceous western north atlantic ocean abstract black clays developed as a result of progressively intensifying intermittent anoxic conditions from the early through mid cretaceous the preservation of primary sedimentary structures of authigenic minerals such as pyrite and siderite and of significant amounts of organic carbon testify to the stagnant conditions intermittent anoxic conditions prevailed in the deepest portion of the basin resulting from vertical migration of an intrasediment anoxic interface middle aptian variegated clays within the black clay sequence at several low latitude sites suggest a widespread oxygenation event of short duration occurred there greater thicknesses of black clays accumulated in low latitudes implying that surface or intermediate depth currents redistributed the sediments south ward millimeter scale laminae characterizing the early and mid cretaceous sediments represent seasonal sediment input where seasons during the cretaceous had durations measured in years cenomanian variegated clays succeeded the black clay interval and represent deposition under totally oxygenated conditions thus the sluggish and stagnant conditions present in the early and mid cretaceous were followed by increased current activity these currents were derived from more northern latitudes as iberia was rifted from north america cooler temperatures presumably increased the oxygen capacity of the water and its ability to sink into the deep basin to relieve the stagnant conditions present the late cenomanian through late campanian was a time of widespread marine transgression resulting in the deep ocean becoming a sediment starved basin clastics and carbonates accumulated on the shelves as the calcite compensation depth ccd shallowed appreciably the late campanian through maastrichtian interval is characterized by calcium carbonate accumulation throughout much of the deep basin a significant lowering of sea level paralleled by a deepening ccd caused this sedimentation pattern

canons revisited and reviewed lester king s views of landscape evolution considered 50 years later fifty years after its publication lester king9s canons of landscape evolution is reviewed and is considered in light of subsequent trends and actual developments some of his ideas such as the role of scarp recession and the antiquity of some surfaces remain current his broad view of the world and his interest in major relief anticipated trends that are now fashionable some of his interpretations however and in particular his downgrading of the importance of structural factors and his linking of scarp retreat and pedimentation have not stood the test of time other concepts such as the etch or two stage origin of forms which were mooted but not fully appreciated in king9s day have come to the forefront and technological advances in dating and survey particularly of the ocean floors have signaled new perspectives in landscape interpretation nevertheless king9s was a courageous attempt to provide guidelines for landscape study

origin of mid cretaceous black shales in the benue through nigeria abstract black and dark gray laminated shales with large amounts of both terrestrial and marine organic material are common in upper cenomanian to lower santonian rocks of the benue trough these shales were deposited during global mid cretaceous marine transgressions when widespread high organic carbon sedimentation took place the benue trough in those times was the site of a shallow density stratified anoxic epeiric sea with estuarine type circulation from this reservoir nutrient rich oxygen depleted waters were fed into the northern parts of the south atlantic ocean

geomorphology and quaternary geology of the dakhla oasis region egypt abstract dakhla oasis 25 5 n 29 e occupies a structurally localized depression at 90 140 m above sea level 1200 km 2 in area below a 300 m escarpment bordering the libyan plateau western desert of egypt semi arid intervals of the quaternary period generated eight sedimentary formations separated by erosion during hyper arid intervals sediments comprise three generations of colluvial fluvial fanglomerates two generations of tabular spring laid clastic and chemical sediments two generations of mound springs and basinal sediments of fluvio lacustrine evaporitic pluvio aeolian and aeolian origins discussion of these sediments is organized according to geomorphic region from north to south plateau scarp and piedmont lowland and cuesta plain chronological evidence is restricted to many radiocarbon dates of holocene cultural material associated with playa sediments 9 4 5 ka a th u isochron age of 62 0 7 6 ka for basinal evaporites and two th u ages of 176 and 170 ka for a boulder of derived travertine the regional quaternary sequence is reconstructed from stratigraphic and geomorphic relationships of the sediments and erosion surfaces it is broadly similar to sequences earlier reconstructed in the topographically similar kharga oasis region 150 km to the east and kurkur oasis 400 km to the south southeast speculations on chronology and driving mechanism are offered in conclusion

escarpment retreat in sedimentary tablelands and cuesta landscapes landforms mechanisms and patterns abstract although the general concept that escarpments in layered rocks retreat through time was offered more than 150 years ago recognizing the exact mechanisms patterns and rates of retreat remains a challenge in this paper we provide the state of the art of the theme of escarpment retreat reviewing both the classic contributions often forgotten but important case studies and the latest research in separate parts of the paper we discuss 1 terminology applied to the geomorphology of escarpments 2 the main assumptions and pathways of long term evolution of escarpments including conceptual models presented so far 3 processes involved in cliff retreat mainly weathering mass movements and erosion 4 the role of spring erosion and fluvial processes leading to fluvial dissection and development of canyons present in many tablelands 5 the possible role of climate change in escarpment evolution 6 rates of escarpment retreat in conclusions we summarize the major advances in our understanding of escarpment evolution of the last two three decades such as the recognition of the key role of emerging groundwater in non uniform escarpment retreat of an important role of silica dissolution and underground removal of dissolved solids increasing appreciation of rock properties in governing the patterns of escarpment retreat the proposal that cliffs do not necessarily retreat via catastrophic rock slope failures but may disintegrate in situ and the first attempts to date escarpment retreat using cosmogenic isotopes finally the remaining gaps and possible research priorities for the future are identified

book reviewthe geomorphology of egypt landforms and evolution volume 1 the nile valley and the western desert nabil s embabi ed the egyptian geographical society special publication cairo egypt 2004 447 pp hardbound us 60 available from egyptian geographical society 109 qasr el aini street p o box mohamed farid cairo egypt isbn 977 5821 04 5

chemostratigraphy of the lower arroyo del soldado group vendian uruguay and palaeoclimatic implications abstract new c and o isotopic determinations from the vendian lower arroyo del soldado group are reported and combined with sedimentologic and biostratigraphic data on the basis of different geochemical and petrographic criteria the primary nature of the c and sr isotopic signature is shown positive d 13 c values characterize the mainly siliciclastic upper yerbal formation which contains oxide facies bif and a diverse assemblage of skeletal fossils including cloudina riemkeae a series of positive and negative d 13 c excursions occur up section in the overlying polanco formation mainly composed of limestones and limestone dolostone rhythmites the transition to the overlying barriga negra and cerro espuelitas formation which consists of conglomerates and shales cherts bif respectively is marked by a further negative excursion on the basis of sedimentary structures a correlation of d 13 c with palaeobathymetry is established positive d 13 c peaks are associated with high sea level stand high organic carbon burial and relatively higher microfossil diversity while negative d 13 c excursions occur in carbonates with less organic matter less microfossil diversity and are always associated to regressions these observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients p n fe was the key factor controlling neoproterozoic glaciations the mentioned models are discussed in view of the new data the occurrence of at least four cold periods in the upper vendian is envisaged these cold periods led to sea level fall and possibly glaciation at higher latitudes the absence of glacigenic rocks associated to negative d 13 c excursions in the arroyo del soldado group is probably due to the tropical setting of the basin finally the upper vendian age of the lower arroyo del soldado group is confirmed on the basis of c and sr isotopes the onset of carbonate deposition at the base of the polanco formation is estimated at 580 ma by comparison with existing global isotopic curves

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

hairpin erosional marks horseshoe vortices and subglacial erosion abstract crescentic erosional marks wrapped around obstacles and extending into straight grooves are termed hairpin erosional marks they are u shaped to semi circular in cross section are deepest around the upstream shoulders of the obstacles and range over seven orders of magnitude in scale some are asymmetrical with one groove better developed than the other the straight grooves may broaden and shallow rapidly in the downflow direction upstream grooves merge with rather than cross downstream crescentic marks grooves are distinct from glacial abrasion forms and many are not striated hairpin erosional marks are interpreted as being products of scour by horseshoe vortices in turbulent fluids this interpretation is supported by the close similarity of these marks to scours produced experimentally and in nature by horseshoe vortices estimated reynolds numbers show that such vortices are probable around obstacles in wind and flowing water but are extremely unlikely in ice aeolian hairpin erosional marks related to yardangs are illustrated to support this conclusion the surface texture cross sectional shapes and spatial patterns of hairpin erosional marks also discount formation by ice a model for the evolution of isolated erosional drumlins from an initial field of hairpin erosional marks extends earlier research on a meltwater hypothesis for drumlins

geomorphology and origin of yardangs in the kumtagh desert northwest china yardangs are typical wind eroded landforms that are found in most of the world s major deserts this paper describes the morphology of yardangs in the kumtagh desert analyzes their control factors and discusses their development processes age and rates of formation yardangs in the kumtagh desert lack the ideal streamlined morphology and the length to width ratio close to 4 1 that have been previously described as being due to their unique controlling factors which include a combination of stratigraphic structures wind erosion and fluvial erosion fluvial erosion plays a particularly important role in the development of the yardangs though wind continuously sculpts their morphology the development of yardangs can be divided into four stages an embryonic stage adolescence maturation and recession most yardangs in the kumtagh desert are in the mature to receding stages we determined an approximate age for the yardangs based on the erosion rates estimated by several researchers on this basis the yardangs in the kumtagh desert appear to have formed during the holocene

mineral magnetic properties of surface soils and sands across four north african transects and links to climatic gradients 1 in light of their earlier success in characterizing and differentiating soils sediments and atmospheric particulates magnetic measurements were carried out on samples of north african surface soils and sands the main aims were to document spatial variations in magnetic properties explore their possible links to climatic gradients and their likely basis and thereby help to differentiate potential sources for sahara sahel derived dusts samples were collected along four broadly north to south orientated transects located in egypt niger mali and benin and southern togo these transects cover major climatic gradients ranging from hyper arid to tropical across the rainfall range spanned by the niger and mali transects 800 mm yr there are significant north to south gradients in magnetic mineral concentrations this is seen most clearly in the concentrations of the finest ferrimagnetic minerals magnetite maghemite which are strongly represented in the clay fraction concentrations of the imperfect antiferromagnetic minerals predominantly haematite also increase with increasing rainfall by analogy with studies spanning a similar rainfall gradient in areas of more uniform lithology we suggest that the most likely cause for these trends is increased chemical weathering linked to the progressively wetter climate across the sahara sahel transition within the set of samples from egypt rainfall 100 to 5 mm yr there is a marked gradient in the coercivity of remanence in the antiferromagnetic minerals present with coercivity increasing southwards paralleling the trend toward increasing aridity given the strong spatial discrimination demonstrated especially in the niger and mali transects these potentially climate linked gradients hold out the promise of discriminating between dust source regions

redox sensitive trace element distribution in the loma negra formation in argentina the record of an ediacaran oxygenation event abstract the limestones of the loma negra formation provide an excellent opportunity to examine the behaviour of rare earth elements plus yttrium rey due to the near zero diagenetic alteration of the carbonate system the terrigenous supply to this unit was minimal or scarce in relation to the depth of seawater in which the limestones accumulated the interpreted palaeoenvironment suggests that limestones could have originated by suspension fall out in a shallow open marine ramp the diagenetic features composition and original microtextures are considered good criteria for investigating their rey composition burial diagenesis products were limited and limestones largely preserved their original texture and organic matter as well as their isotope and trace element composition the study of rey distribution in the loma negra formation in different settings of the tandilia system revealed systematic chemostratigraphic patterns related to uniform palaeo redox conditions the general light rare earth element depletion marked positive la y and gd anomalies as well as the clear negative ce anomalies are further features typically ascribed to oxidizing seawater conditions from these results it can be concluded that the loma negra formation documents an oxygenation event that stands out in the overall anoxic ediacaran ocean this record may be correlated with the mid ediacaran oxygenation event ca 580 ma this may support the hypothesis that the clymene ocean was well oxygenated and connected during the ediacaran at least in the southwestern border of gondwana

isotope sr c and u pb shrimp zircon geochronology of marble bearing sedimentary series in the eastern sierras pampeanas argentina constraining the sw gondwana margin in ediacaran to early cambrian times abstract the sierras de cordoba metasedimentary series consists of marbles and metasiliciclastic rocks of ediacaran to early cambrian age ca 630 and 540 ma that underwent high grade metamorphism during the collisional pampean orogeny in the early cambrian the ages of the marbles were determined from the sr isotope composition blind dating of screened samples of almost pure calcite marble and were further constrained with c and o isotope data and u pb shrimp detrital zircon ages of an interbedded paragneiss two groups of samples are recognised with sr isotope composition ca 0 7075 and 0 7085 that are considered stratigraphically significant the first is inferred early ediacaran the second late ediacaran to early cambrian the sierras de cordoba metasedimentary series is correlated for the first time with marble bearing metasedimentary series in several sierras to the west and north of sierras de cordoba e g the difunta correa sedimentary sequence and the ancajan series implying that all were probably parts of an originally extensive sedimentary cover these series bear evidence of sedimentary sources in the mesoproterozoic and paleoproterozoic basement of the western sierras pampeanas part of the large mara continental block and farther west laurentia in terms of the age of limestones marbles and detrital zircon patterns the sierras de cordoba metasedimentary series differs strongly from the older section of late ediacaran to early cambrian puncoviscana formation of northwestern argentina which mostly outcrops in northern sierra chica and sierra norte with sedimentary input from western gondwana sources the sierras de cordoba metasedimentary series and the puncoviscana formation were probably juxtaposed during the pampean orogeny along a complex suture zone that was further folded and or imbricated at mid crustal depths the peak of metamorphism was attained at 527 2 ma according to the evidence found here most of the sierras pampeanas to the west of the sierras de cordoba were part of the lower colliding plate during the final amalgamation of sw gondwana

palaeoenvironment and holocene land use of djara western desert of egypt abstract the results of the interdisciplinary project acacia support the assumption of a more humid climate at djara on the egyptian limestone plateau which is a hyper arid desert today during the early and mid holocene the ancient plant and animal inventories give new impetus for the suggestion of an interfingering of two climatic regimes the winter rains from the north and west and the summer monsoonal rains from the south on the latitude of djara a playa sediment sequence the composition of plant and animal taxa as well as the reconstructed settlement patterns indicate a semi arid climate with alternating more humid and drier conditions the concentration of prehistoric sites in the djara depression points to locally favourable conditions in contrast to the surrounding plateau surface the widespread catchment and a distinct system of palaeochannels offered fresh water over a period of time due to the run off from the plateau surface after rain events although the ecological conditions were better during the holocene humid phase than they are today a sedentary way of life was improbable the hydrological constraints require altogether highly mobile subsistence strategies shells of the nile bivalve aspatharia sp spathopsis sp give evidence for contacts between djara and the nile valley which remains beside the egyptian oases an important retreat area with perennially available water the decrease of radiocarbon dates and related archaeological sites around 6300 bp c 5300 cal bc indicate the depopulation of the djara region as a consequence of the drying trend while the drop off of the 14 c dates can also be observed in other desert research areas of the acacia project we date the end of the holocene humid phase about 300 years earlier than previously suggested

correlation of ordovician diamictites from argentina and south africa using detrital zircon dating abstract the results of detrital zircon u pb geochronology suggest a maximum depositional age of 485 7 2 ma for a glacial diamictite from the sierra del volcan in eastern argentina tandilia system earlier interpretations associated the deposit with the neoproterozoic snowball earth9 hypothesis the data allow direct correlation for the first time between early palaeozoic deposits in both south america and south africa connecting the glacial deposits from southern bolivia to central argentina with those in south africa pakhuis formation on the basis of these results a new distribution map of glacial cover corresponding to the hirnantian stage can be developed supplementary material sampling and analytical techniques data a simplified map and bse images are available at http www geolsoc org uk sup18384

geomorphic indicators of holocene winds in egypt s western desert abstract geomorphic mapping of egypt s western desert from landsat mss images reveals oriented aeolian landforms that record in part holocene winds wind directions reconstructed from these landforms indicate the dominance of n s airflow from 30 n to 20 n turning clockwise southward to ne sw conformable with modern circulation a second direction appears over western egypt w between 30 n and 26 n nw between 26 n and 20 n cross cutting aeolian landforms show that w nw winds are older than the n ne winds geomorphic evidence abundant south to 26 n and less abundant to 20 n also indicates that w and nw winds were early holocene palaeowesterlies some evidence also indicates that they extended eastward to at least 30 e perhaps to the red sea these winds steered moist atlantic mediterranean air masses to egypt sustaining early holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20 n upon aridification beginning after 5 kyr bp yardangs oriented west to east were eroded in early holocene basinal sediments in western egypt indicating that these winds continued there for 1 2 kyr until 3 4 kyr bp optically stimulated luminescence osl ages of surface sand sheet in southern egypt indicate that the present north south winds were established ca 3 4 kyr bp at the same time as the northern savanna boundary was stabilized at its present position

rare earth elements in the terminal ediacaran bambui group carbonate rocks brazil evidence for high seawater alkalinity during rise of early animals abstract rare earth elements plus yttrium rey mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater here we investigate rey patterns in two carbonate sections from the ediacaran bambui group sao francisco basin brazil which comprise its two lowermost transgressive regressive second order sedimentary sequences shale normalised distributions vary with the sequence stratigraphy framework in the basal 2nd order sequence carbonate samples from the basal sequence transgressive systems tract display light rey lrey distributions slightly depleted to enriched that reflect input of freshwater possibly in a post glacial episode upwards carbonate rocks from the early highstand systems tract ehst yielded lrey enriched distributions which progressively turns into lrey shale normalized depleted distributions on samples from the late highstand systems tract lhst this portion of the sequence also displays y positive anomaly in some cases carbonate samples from the upper second order sequence do not display coherent patterns ce ce values 1 in most samples throughout the two sections suggest permanent anoxia of seawater the rey change from the ehst to lhst in the basal sequence marks an important paleoenvironmental overturn in the basin with increasing alkalinity in seawater driving rey fractionation and lrey depletion confinement of the basin in the inner areas of west gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent thus delivering a higher ionic influx to a restricted setting increasing alkalinity during lhst cloudina sp fragments were reported in this stage of the bambui group and in carbonate rocks with high sr mass fractions in other west gondwana basins supporting the hypothesis that the high alkalinity of seawater during late ediacaran may have driven the appearance of the first biomineralizing organisms

megaripples at wau an namus libya a new analog for similar features on mars abstract transverse aeolian ridges tars are curious aeolian megaripples on mars but there are few sites identified thus far to study as terrestrial analogs a new terrestrial analog site for tars has been found in and around an isolated volcano caldera in the sahara desert of libya the wau an namus volcano is potentially active and recognizable from space because of its strong albedo contrast wau an namus has large dark aeolian megaripple bedforms with the same horizontal length scales and overall planform patterns as observed for tars on mars some new patterns for these tar like features also have been found at wau an namus such as stripe like and clustered patterns which may shed new light on the formation of these features on both planets other aeolian features such as sand dunes and dust devil tracks are also evident in this region this is the first terrestrial analog site with dark megaripple features similar to some tars on mars the dark halo around wau an namus visible from space represents a unique site to study the formation and evolution of these mysterious features and thereby further our understanding of the probable recent history of the wind regime on mars

discrimination of fluvial and eolian features on large ignimbrite sheets around la pacana caldera chile using landsat and srtm derived dem abstract satellite images and ground based observations were used to investigate the morphology and origin of valleys and ridges across large ignimbrite provinces around la pacana caldera n chile we studied 12 separate sites that included five different ignimbrite units that range in age from 5 6 0 5 ma to 1 35 0 15 ma over 400 features from within these study sites were quantified study sites typically have dimensions of 25 35 km measurements were made using landsat tm etm and srtm derived dem and shaded relief images morphological measurements were made of the ridges and channels including the direction of regional slope length orientation and gradient of each feature a classification of the different ignimbrite erosion influences resulting in varying landscape morphologies identified 4 landscape types fluvial eolian modified and complex we conclude that the most prevalent features are fluvial derived channels but the most distinctive are linear wind derived ridges the orientation of these ridges suggests that strong uni directional winds from the northwest have been constant over a long period of time 1 my

paleoenvironmental implications of two phosphogenic events in neoproterozoic sedimentary successions of the tandilia system argentina fil gomez peral lucia consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet la plata centro de investigaciones geologicas universidad nacional de la plata facultad de ciencias naturales y museo centro de investigaciones geologicas argentina

local δ34s variability in 580ma carbonates of northwestern mexico and the neoproterozoic marine sulfate reservoir abstract many 34s records have been produced from carbonate associated sulfate cas in order to understand the oxidation state of the neoproterozoic oceans but interregional correlation is complicated by the absence of robust chronostratigraphic markers here a globally correlatable stratigraphic interval containing the wonoka shuram w s 13c excursion was analyzed to explore variability in the sulfur isotope record in the excursion containing units the local 34s record from multiple closely spaced sections in sonora mexico was examined to explore potential heterogeneities and then these were compared to more distant sections elsewhere in sonora the inception and isotopic minimum of the w s excursion is located in the clemente formation and coincides with an extensive oolite marker bed five sections containing this marker bed span 25 km of lateral distance exhibit significant variability in 34scas range 18 6 to 27 6 vcdt and high variability in cas concentration range 1200 ppm unlike the 34scas values cas concentrations show strong negative correlation with mn sr ratios and fecarb concentrations consistent with cas removal upon diagenetic recrystallization and an absence of an accompanying sulfur isotopic fractionation indeed samples containing low cas concentrations exhibit petrographic characteristics consistent with diagenetic recrystallization including fabric destructive neomorphism and relatively coarse crystalline textures coeval w s strata in death valley oman and the least altered sonora samples record a decrease in 34scas and an increase in cas concentration however the magnitude of the changes are distinct perhaps reflecting differential response to a transient oxidation event in contrast time equivalent facies of south china do not exhibit a decrease in 34scas nor a relative increase in cas concentrations the variability in geochemical characteristics likely developed from local and or regional marine 34ssulfate heterogeneity although neoproterozoic oceanic heterogeneity in 34ssulfate has been proposed it has not been reported on such close spatial scales in addition the existence of variability among the sonora sections in which strata were all deposited at similar depth indicates that heterogeneity could have occurred laterally finally these findings demonstrate that while cas concentrations may be drastically affected by diagenesis 34scas may retain primary signatures

integrated correlation of the vendian to cambrian arroyo del soldado and corumba groups uruguay and brazil palaeogeographic palaeoclimatic and palaeobiologic implications abstract the corumba group of sw brazil and the arroyo del soldado group asg of uruguay are correlated on the basis of litho bio and chemostratigraphy both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf in the corumba basin sedimentation began in the varangerian represented by the glaciomarine puga formation a series of sea level fluctuations coupled with climatic changes are recorded up section while uppermost deposits of the asg are of lowermost cambrian age sedimentation ceased in the latest vendian in the corumba basin an assemblage of six species of organic walled microfossils dominated by bavlinella faveolata and soldadophycus bossii three species of vendotaenids and two species of skeletal fossils cloudina and titanotheca is described from the corumba group the vendotaenid eoholynia corumbensis sp nov is described from siltstones of the guaicurus formation an important diversity of skeletal fossils in the corumba arroyo del soldado and nama groups points to favourable vendian palaeoclimatic conditions in sw gondwana preliminary carbon isotopic data show a series of alternating positive and negative excursions corroborating the upper vendian age indicated by fossils for both units previously reported strontium isotopic data are also consistent with this age it is postulated that the corumba and asgs were deposited onto the same shelf which opened to the east the rio de la plata superterrane craton extends farther to the north than previously expected or it was already amalgamated with the amazonian craton by vendian times collision of the platform with the parana block caused closure of the basin during the cambrian early ordovician finally models of neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed

the carbon isotopic composition of proterozoic carbonates riphean successions from northwestern siberia anabar massif turukhansk uplift thick carbonate dominated successions in northwestern siberia document secular variations in the c isotopic composition of seawater through mesoproterozoic and early neoproterozoic early to early late riphean time mesoproterozoic dolomites of the billyakh group anabar massif have delta 13c values that fall between 0 and 1 9 permil versus pdb with values in the upper part of the succession yusmastakh formation consistently higher than those of the lower ust il ya and kotuikan formations consistent with available biostratigraphic and radiometric data delta 13c values for billyakh carbonates compare closely with those characterizing early mesoproterozoic carbonates about 1600 1200 ma worldwide in contrast late mesoproterozoic to early neoproterozoic limestones and dolomites in the turukhansk uplift exhibit moderate levels of secular variation only the lowermost carbonates in the turukhansk succession linok formation have delta 13c values that approximate billyakh values higher in the turukhansk succession delta 13c values vary from 2 7 to 4 6 permil with outliers as low as 5 0 permil interpreted as diagentically altered again consistent with paleontological and radiometric data these values compare well with isotopic values from 1200 to 850 ma successions elsewhere five sections measured in different parts of the turukhansk basin show nearly identical patterns of variation confirming that carbonate delta 13c correlates primarily with time and not facies the siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra and interbasinal correlation of mesoproterozoic and early neoproterozoic rocks

tracking connection and restriction of west gondwana sao francisco basin through isotope chemostratigraphy abstract c and sr isotope compositions of carbonate rocks from the intracontinental sao francisco basin can track ocean connections and restriction the lower three formations of the bambui group can be grouped into three chemostratigraphic intervals ci recording different evolution stages of the basin lowermost ci 1 comprises the basal cap carbonates of the sete lagoas formation displaying an initial c negative excursion followed by a coeval c and sr positive excursions 13 c values from 5 to 0 and 87 sr 86 sr ratios from 0 7074 to 0 7082 in 10 m of stratigraphic record it marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression ci 2 comprises carbonates of the middle portion of the sete lagoas formation with 13 c values around 0 and 87 sr 86 sr ratios around 0 7082 that matches those observed worldwide for the late ediacaran it records the onset of a gondwana sea pathway connecting several epicontinental basins allowing migration of index fossil cloudina sp uppermost ci 3 starts after a major positive excursion in the 13 c values reaching 16 and a steepened decrease of 87 sr 86 sr ratios to 0 7075 which are lower than those expected for the ediacaran cambrian boundary this interval comprises the upper sete lagoas serra de santa helena and lagoa do jacare formations and records the end of the connection of the sao francisco basin to the gondwana sea pathway setting a restricted epeiric sea restriction was probably caused by late ediacaran uplifting of orogenic belts surrounding the basin other west gondwana cloudina bearing units also display the same mismatch in the sr isotope ratios suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

chemostratigraphy and diagenetic constraints on neoproterozoic carbonate successions from the sierras bayas group tandilia system argentina carbon and oxygen isotopic data combined with a detailed diagenetic study obtained from undeformed and unmetamorphosed dolomite and limestone from the neoproterozoic sierras bayas group argentina provide a new record of isotopic stratigraphic variations useful for regional and global correlations the sierras bayas group is subdivided into the villa monica sandstones and dolomites cerro largo siliciclastic rocks and loma negra limestones formations unconformably overlain by the cerro negro formation siliciclastic rocks with basal marls and grouped into four depositional sequences bounded by unconformities the

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

early holocene basinal sediments of the dakhleh oasis region south central egypt abstract twenty samples of artifactual ostrich eggshell and hearth charcoal firmly to loosely associated with basinal lacustrine playa and sand sheet sediments in the dakhleh oasis region of south central egypt yield radiocarbon ages between ca 8800 and ca 4700 yr b p the sediments record variable sedimentary responses to an early holocene pluvial interval in this virtually rainless region differences of hydrogeology and morphometry among and within basin types complicate paleoclimatic interpretation

evaluating the life expectancy of a desert pavement abstract this paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the sonoran desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements this synthesis identifies vital factors key factors and site specific factors promoting pavement stability hyperaridity is the vital factor in pavements surviving for 106 years or more aided by minimal bioturbation and clast size reduction three key factors aid in pavements surviving for 104 to 105 years accumulation of allochthonous dust underneath pavement cobbles a flat topography and a lack of headward retreating swales or gullies a unified explanation for pavement longevity however did not emerge from a literature review because a variety of site specific factors can also promote pavement antiquity including resistant bedrock beneath the pavement disk shaped cobbles to promote dust accumulation and microclimatological and ecological reasons for minimal bioturbation both key and site specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the sonoran desert central arizona the buildup of cosmogenic 10be and in situ 14c optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between 30 and 332 ka with conditions for longevity including flat surface topography pavements underlain by consolidated granitic bedrock a lack of headward retreating gullies and swales 87sr 86sr analyses indicating the infiltration of allochthonous dust floating disk shaped pavement cobbles and a quartzite lithology resistant to disintegration however 10be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate these weaker materials allowed the growth of headward retreating swales that in turn promoted exposure of newer gravels by surface erosion

paleo climatic and paleo environmental evolution of the neoproterozoic basal sedimentary cover on the rio de la plata craton argentina insights from the δ13c chemostratigraphy abstract the sierras bayas group of the tandilia system constitutes the neoproterozoic sedimentary cover of the rio de la plata craton in argentina that accumulated amid the breakup of the rodinia supercontinent and subsequent assembly of gondwanaland evidence for glaciation in the villa monica formation vmf at the base of the succession comes in the form of iron rich laminated sediments containing dropstones composed predominantly of basement crystalline rocks and quartzites that are sequentially overlain by a phosphatic mudstone and a 40 m thick stromatolitic dolomite subtidal facies preserve columnar forms similar to post glacial tubestone stromatolites seen in the neoproterozoic records these morphologies suggest rapid growth associated with elevated seawater alkalinity and high rates of carbonate accumulation records the vmf dolomites in our four studied sections near olavarria sierras bayas area reveal a pronounced negative to positive 13 c up section that is similarly to these cap carbonates and others worldwide our sedimentological and geochemical study of the vmf sections reveal consistent carbon and oxygen isotope trends that may be useful for detailed intra basinal correlations samples of the vmf fabric retentive dolomite preserve an unusually narrow range of non radiogenic strontium isotopic compositions 0 7068 to 0 7070 that are consistent with cryogenian limestone facies in the potential namibian and brazilian equivalents exceptional preservation of 87 sr 86 sr compositions suggest the possibility of primary dolomite precipitation in post glacial seawater and furthermore that ree patterns and distributions may yield similar insights to redox conditions in the depositional basin in particular the vmf dolomites reveal depleted lree abundances a negative ce anomaly positive la and gd anomalies and low y ho values as a whole these observations suggest oxidizing post glacial seawater conditions associated with significant freshwater inputs into the basin global warming and increases in primary productivity and organic carbon burial linked to the buildup of oxygen the positive 13 c trend and the increase in stromatolite biodiversity in the vmf

organic facies of middle cretaceous black shales in deep north atlantic the middle cretaceous black shales of the deep north atlantic consist of alternating layers of sediment rich in total organic carbon toc that contains abundant amorphous types of organic matter and sediment poor in toc that contains mostly woody and coaly types of organic matter the amorphous materials are derived mostly from marine organisms in the eastern north atlantic off africa in the caribbean and on the demerara rise off surinam and from higher land plants in the western north atlantic and off europe marine amorphous material is important off africa because this was an area of upwelling and highly productive surface water amorphous marine organic matter was deposited across the entire north atlantic basin in the cenomanian in response to an ocean wide upwell ng event connected with the opening of the equatorial atlantic the difference between the toc rich and background toc layers reflects deposition of the former under reducing conditions which allowed the preservation of labile amorphous materials of marine and terrestrial orgin and of the latter under oxidizing conditions when nearly all that was preserved was refractory organic material

sedimentation de la matiere organique dans le nord est du bassin de paris consequences sur le depot des argilites carbonees du toarcien inferieur organic matter sedimentation in the northeast of the paris basin consequences on the deposition of the lower toarcian black shales abstract a borehole realised by the agence nationale pour la gestion des dichets radioactifs andra near the village of montcomet aisne in the paris basin allowed to core about 1 km of jurassic series the organic matter om associated to the drilled series has been characterised by various well proven methods rock eval pyrolysis palynofacies study and saturated hydrocarbon analysis the series received variable but notable amounts of inputs of om of terrestrial origin during all the jurassic these inputs probably originated from the ardennes massif near the study area the conditions of preservation of these inputs being poor during the early jurassic improved markedly at the beginning of the domerian and then apparently remained rather constant at least up to the callovian during three distinct and rather well limited periods important amounts of algae derived om added to the background of terrestrial om the most important and the best known of these episodes corresponds to the black shales of the base of the lower toarcian lateral equivalent of the well known schistes carton these results suggest that in the paris basin the deposition of om rich sediments occurred during episodes of high planktonic production happening when medium conditions were favourable to om preservation the quantitative and qualitative fluctuations of the om all along the studied series accompany and describe three of the large transgression regression cycles recently evidenced in the paris basin guillocheau 1991 late carnian toarcian aalenian bathonian and callovo oxfordian one episode of accumulation of planktonic om appears to have occurred in each of these cycles in the neighbourhood of transition transgression regression in the present state of knowledge only the confinement behind sills in a structured basin seems capable of explaining the triggering of episodes of planktonic production in the paris basin these conditions of confinement appear to have mainly resulted from a notable structuration of the platforms and of the adjacent troughs due to 1 subsidence during the tectonic cycles and 2 to distensive tectonic activity during tectono eustatic cycles

origin and characteristics of yardangs in the um al rimam depressions n kuwait article i nfo although arabia hosts one of the largest deserts in the world studies regarding aeolian erosional features in general and yardangs in particular are rare the principal aim of this study is to delineate and investigate the various forms of yardangs in the um al rimam depressions in the northern part of kuwait the study area consists of two main depressions connected by neck area made up of continental sediments both depressions are surrounded by a dissected escarpment and their floors host muddy playas on lithological bases three main types of yardang have been identified 7 yardangs are sculptured in sandstones of lower fars formation lower to middle miocene 23 are carved in calcretic rocks of ghar formation oligocene to lower miocene and 11 yardangs are developed in quaternary sediments their mean orientation nw 297 coincides with the prevailing direction of the strong local northwesterly wind called the shamal the average length width ratio is 1 5 1 the wide geographic distribution of yardangs around the margins of the northern depression is due to sand blasting this is explained by the annual sand accumulation measured in the study area where the northern depression shows 143 times higher accumulation than the southern depression the neck area and the platform above the depressions also the field measurements for annual erosional rates show that the sides of the yardangs are more affected by erosion compared to their middle parts remarkably slight mineralogical variations were detected within the surface sediments of um al rimam depressions and the quaternary muddy yardangs which indicate a common origin the age of yardangs is unknown but annual erosional rates on a small quaternary muddy yardang suggest that they range in age between 44 and 1500 years depending on yardang size it is believed as indicated from field observation and interpretation of aerial photos that the majority of yardangs are pre existing bedrock forms these forms were developed by paleo surface hydrological processes which were modified by wind action in latter phase

a meltwater origin for antarctic shelf bedforms with special attention to megalineations the geomorphology of troughs crossing the antarctic shelf is described and interpreted in terms of ice stream hydrology the scale of tunnel channels on the inner shelf and the absence of sediment at their mouths are taken to infer catastrophic drainage drumlins on the inner and outer shelves with pronounced crescentic and hairpin scours are also interpreted as products of catastrophic flow gullies and channels on the continental slope and turbidites on the rise and abyssal plain point to abundant meltwater discharge across the shelf attempts to explain this morphology and sedimentology in terms of release or discharge of meltwater by pressure melting strain heating darcian flow or advection in deforming till are shown to be unrealistic we suggest that meltwater flow across the middle and outer shelves might have been in broad turbulent floods which raises the possibility that megascale glacial lineations msgl on the shelf might originate by erosion in turbulent flow this possibility is explored by use of analogs for msgl from flood and eolian landscapes and marine environments an extended discussion reflects on objections that stand in the way of the flood hypothesis

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

wind erosion under cold climate a pleistocene periglacial mega yardang system in central europe western pannonian basin hungary abstract mega yardangs are known to form under extremely arid warm or in some instances moderate climates here we report on the mega yardang field of the pannonian basin hungary which was uniquely formed under periglacial probably semi arid conditions and discuss its implications on factors controlling yardang formation in general the yardang field is composed of advanced streamlined forms and of less aerodynamic ridges separated by well developed corridors the length of the largest features exceeds 60 km their height varies around 150 m together with ventifacts deflation hollows and wind blown sand areas the yardang field is part of an integrated eolian feature system driven by northerly to northwesterly winds dating of ventifact surfaces with in situ produced cosmogenic nuclides places the onset of significant wind erosion to the early pleistocene 1 5 ma osl dated young loess from which well developed yardangs were sculpted and wind blown sands show that eolian action continued until the early holocene the long temporal range of wind erosion compared to average yardang formation rates and the rarity of advanced yardang forms indicate intermittent and relatively short most probably millennial scale deflation periods since the early holocene the system has been inactive and demonstrates that yardangs can persist in a recognizable form for several millennia even under less arid conditions the main control on yardang formation appears to be the existence of strong unidirectional winds in the pannonian basin facilitated mainly by the channeling effect of topographic lows in the surrounding mountain chains yardangs can form even under semi arid climates however the rarity of these occurrences suggests that this requires a special association of environmental conditions yardang formation is facilitated by homogeneous lithology namely by widespread weakly consolidated sandy sediments the distribution of yardang fields is controlled by vertical neotectonic movements which provided uplifted areas to be sculptured and topographic obstacles that sheltered downwind sediments faulting can be excluded as a controlling factor in mega yardang formation nor is the role of jointing supported

black shales and carbon isotopes in pelagic sediments from the tethyan lower jurassic detailed sampling and analysis of jurassic pelagic limestones and marls from italy hungary and switzerland have enabled construction of an isotope stratigraphy across the pliensbachian toarcian boundary with resolution to the zonal level the oxygen isotope record is unremarkable the carbon isotopes however show two positive excursions one relatively minor during the pliensbachian margaritatus zone subnodosus subzone the other more major during the toarcian early falciferum zone where a maximum 13c value of 4 52 pdb is attained these intervals are known to be favoured periods of organic rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record a comparable phenomenon has been documented from the cenomanian turonian boundary in the cretaceous where it has been referred to the influence of an oceanic anoxic event some italian sections spanning this lower jurassic interval contain organic rich shales in the falciferum zone the isotopic signatures from their included locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve carbon isotopes from the organic carbon itself are extremely negative falling to 33 pdb and in one section examined in detail correlate with the calcium carbonate content of the shales they may reflect a partial change to a non calcified planktonic biota during deposition of this lime poor interval possibly responding to upwelling and increased fertility of near surface waters the onset of upwelling may have been as early as spinatum tenuicostatum zone time that is at the pliensbachian toarcian boundary

recent environmental change and prehistoric human activity in egypt and northern sudan abstract this paper reviews the various late quaternary records that are available from western egypt and northern sudan which includes more than 500 published radiocarbon dates and various sedimentary archives from local landscape components including palaeolakes soils drainages wadis and archaeological sites this palaeoenvironmental compilation frames the spatial and temporal context of local cultural activities when the region was most hospitable 9000 6000 bp at this time monsoonal weather influenced the portion of the african continental interior creating enough convective rainfall for occasional surface water storage in this part of the modern sahara rapid hydroclimatic changes play a key role in geomorphic evolution and resource availability as watering holes formed and dried up in the early to middle holocene neolithic people developed various subsistence strategies including opportunistic hunting of small animals e g gazelle and hare and food related e g wild sorghum millet and legumes activities gathering plant cultivation and livestock rearing during its wettest phases during the monsoonal maximum the area was drought prone sustaining a meager steppe shrub desert flora further desertification and aeolian deflation during the middle and late holocene fostered technological innovation migration and settlement as well as the further development of agrarian communities and complex culture

geochronology of la tinta upper proterozoic sedimentary rocks argentina abstract olavarria sierras bayas barker san manuel and balcarce mar del plata fine grained sedimentary rocks from la tinta formation the pre cenozoic cover of the tandilia region were studied using the rb sr and k ar geochronology the mineralogical study of the fine fraction has shown that only the olavarria sierras bayas area presents suitable material comprising typical sedimentary clays affected only by diagenetic processes two rb sr isochrons were obtained from olavarria sierras bayas rocks they show 1 an age of 769 12 ma with 87 sr 86 sr 0 0 7121 0 0005 for aust quarry rocks and 2 an age of 723 21 ma with 87 sr 86 sr 0 0 7171 0 0012 for cerro negro and losa quarries rocks considering the above mentioned isochron data and the mineralogy of the clays studied the conclusion is drawn that the ages obtained reflect the isotopic setting of a late diagenetic process dated back to nearly 720 ma k ar data also support the rb sr isochrons and the late diagenetic clay origin the lower section of la tinta sequence in the sierras bayas area must then be considered as upper proterozoic in age these new data support the recently reported stratigraphical divisions and ages

discrimination of fluvial eolian and neotectonic features in a low hilly landscape a dem based morphotectonic analysis in the central pannonian basin hungary abstract the godoll hills a low relief terrain within the central pannonian basin in hungary is characterised by moderate tectonic deformation rates although typical tectonic landforms are not clearly recognisable in the study area this paper succeeded in discriminating between tectonically controlled landforms and features shaped by fluvial erosion or deflation with no tectonic control dem based morphometric parameters including elevation slope and surface roughness enabled the delineation of two nw se trending spearhead shaped ridges separated by a wide rectilinear valley of the same strike although directional statistics suggested possible tectonic control of nw se striking landforms precise morphometry completed with an analysis of subsurface structures rejected their tectonic preformation deflation plays a significant role in shaping the area and the presence of two large scale yardangs separated by a wind channel is proposed in temperate continental areas of europe no deflational landforms of such scale have been described so far suggesting that pleistocene wind power in periglacial areas was more significant than it was previously thought characteristic drainage patterns and longitudinal valley profiles enabled the recognition of areas probably affected by neotectonic deformation a good agreement was observed between locations of quaternary warping predicted by the morphometric study and subsurface structures revealed by the tectonic analysis zones of surface uplift and subsidence corresponded to anticlinal and synclinal hinges of fault related folds in low relief and slowly deforming areas where exogenous forces may override tectonic deformation only the integrated application of morphometric and subsurface structural indications could assure correct interpretation of the origin of various landforms while a morphometric study alone could have led to misinterpretation of some morphometric indices apparently suggesting tectonic preformation on the other hand the described morphological expression of subsurface structures could verify quaternary age of the deformation