evaluation of kinetic uncertainty in numerical models of petroleum generation oil prone marine petroleum source rocks contain type i or type ii kerogen having rock eval pyrolysis hydrogen indices greater than 600 or 300 600 mg hydrocarbon g total organic carbon hi mg hc g toc respectively samples from 29 marine source rocks worldwide that contain mainly type ii kerogen hi 230 786 mg hc g toc were subjected to open system programmed pyrolysis to determine the activation energy distributions for petroleum generation assuming a burial heating rate of 1c m y for each measured activation energy distribution the calculated average temperature for 50 fractional conversion of the kerogen in the samples to petroleum is approximately 136 7c but the range spans about 30c 121 151c fifty two outcrop samples of thermally immature jurassic oxford clay formation were collected from five locations in the united kingdom to determine the variations of kinetic response for one source rock unit the samples contain mainly type i or type ii kerogens hi 230 774 mg hc g toc at a heating rate of 1c m y the calculated temperatures for 50 fractional conversion of the oxford clay kerogens to petroleum differ by as much as 23c 127 150c the data indicate that kerogen type as defined by hydrogen index is not systematically linked to kinetic response and that default kinetics for the thermal decomposition of type i or type ii kerogen can introduce unacceptable errors into numerical simulations furthermore custom kinetics based on one or a few samples may be inadequate to account for variations in organofacies within a source rock we propose three methods to evaluate the uncertainty contributed by kerogen kinetics to numerical simulations 1 use the average kinetic distribution for multiple samples of source rock and the standard deviation for each activation energy in that distribution 2 use source rock kinetics determined at several locations to describe different parts of the study area and 3 use a weighted average method that combines kinetics for samples from different locations in the source rock unit by giving the activation energy distribution for each sample a weight proportional to its rock eval pyrolysis s2 yield hydrocarbons generated by pyrolytic degradation of organic matter

th u disequilibrium dating of middle and late pleistocene wood and shells from banks and victoria islands arctic canada an attempt has been made at dating middle and late pleistocene deposits from banks and victoria islands using the th u disequilibrium method geological correlations are difficult to establish because of the discontinuous nature of the terrestrial units and the relative absence of datable fossil wood and shells the th u geochemical system likely remained closed for extended periods because of the presence of permafrost which implies very limited or absent water circulation the th u age determinations obtained confirmed the previously established stratigraphic framework and provide chronological information in that samples of sangamonian age are differentiated from those that are younger wisconsinan and holocene or older middle pleistocene the th u disequilibrium method carefully used in areas where water circulation is limited can thus provide chronological control for sediments older than those that can be dated by the radiocarbon method

paleoclimate and location of the border between mediterranean climate region and the saharo arabian desert as revealed by speleothems from the northern negev desert israel speleothem bearing karstic caves of the northern negev desert southern israel provides an ideal site for reconstructing the paleoclimate and paleo location of the border between mediterranean climate region and the saharo arabian desert major periods of speleothem deposition representing humid periods were determined by high resolution 230 th u dating and corresponding studies of stableisotopecompositionwereusedtoidentifythesourceofrainfallduringhumidperiodsandthevegetationtype majorhumidintervals

the relationship between current and paleo groundwater base levels abstract base levels are basically divided into two categories oceanic and marine base levels and continental terminal endorheic ones the marine domain serves as the ultimate base level for both surface and groundwater drainage systems continental inland depressions very often serve as terminal endorheic base levels to both surface drainage and groundwater systems many of the current both marine and inland base levels served also in the past as paleo base levels fluctuations of base level elevations affect the adjoined groundwater system in several ways such as changes in groundwater levels location of divides depths of fresh saline water interfaces and the amount of flow to the base levels this paper provides examples from many different parts of the world that describe the relationship between current and paleo base levels the examples include marine base levels as well as inland endorheic ones both currently above and below sea level close to and distant from the sea

palaeoclimate in the saharan and arabian deserts during the middle palaeolithic and the potential for hominin dispersals to disperse out of sub saharan africa it was necessary for hominins to cross the deserts of either the sahara and or arabia thus understanding the palaeoclimate of the saharo arabian region is central to determining the role these deserts played in the peopling of the planet when did they act as barriers and when were they more humid opening dispersal routes across them to address these questions we have conducted a temporal and spatial evaluation of dated sites from 20 to 350 ka using combined probability density function pdf and geographical information system gis analyses of all sites dated using uranium thorium u th or optically stimulated luminescence osl methods radiocarbon dates were not considered because of contamination problems in this time range the results show that during mis 2 there is little evidence for humidity in arabia as would be expected during the height of the last glacial maximum however the sahara shows a sharp rise in probability at the beginning of mis 2 peaking near the boundary with mis 3 at 29 ka there appear to be brief periods of humidity in mis 3 and 6 though at different times in the sahara ca 37 44 138 154 and 180 ka and arabia ca 40 54 and 163 ka during mis 5 both regions show much evidence for humidity with pdf peaks corresponding to insolation maxima though not all maxima are represented in either the saharan or arabian record this situation can be explained by eccentricity modulated precession when eccentricity is strong insolation is enhanced but also more variable and the desert climate is generally more humid particularly at times of high insolation the opposite happens when eccentricity is low and deserts tend to be more arid but local factors exert more of an influence on climate affecting the timing and strength of the brief humid periods experienced so that they no longer coincide with insolation maxima the spatial distribution of humid sites is compatible with a number of different modern human dispersal theories southern arabia experienced humid periods centred on 54 ka and 125 ka and this could have facilitated dispersal from east africa to southern arabia and beyond via the bab el mandab the sahara shows considerable evidence for humidity during mis 5 and may have had dispersal across its expanse at this time

geophysical and petrological evidence for the presence of an anomalous upper mantle beneath the sahara basins algeria abstract north of 20 n the northwest part of the stable african shield is characterized by an anomalous high heat flow zone 80 110 mw m 2 this east west elongated zone is oblique to major proterozoic structural units and cannot be related to differences in crustal heat production maximum heat flow occurs in the southern part of the sahara basins 100 120 mw m 2 only a mantle heat flow contribution of 60 70 mw m 2 can account for this very high surface heat flow the estimated geotherms suggest that the temperature of upper mantle is anomalously high and partial melting may be present at shallow depths this hypothesis is supported by the existence of a long wavelength negative component of the gravity field that corresponds with the high heat flow axis and with the very low lithospheric s wave velocities 4 2 4 4 km s down to 160 km obtained from surface wave studies in addition the chemistry of the scarce cenozoic melilitite lavas illizi volcanic district geographically associated with the thermal anomaly as well as the mineralogy of their inclusions phlogopitized garnet spinel peridotite and alkali clinopyroxenite show that the upper mantle beneath this region is locally highly metasomatised and partially melted from these preliminary results it appears that the northwest african upper mantle has been modified by recent thermal rejuvenation processes including gas transfer that are likely to be more extensive in the southern part of the sahara basins

two high levels of continental waters in the southern tunisian chotts at about 90 and 150 ka uranium and thorium content and activity ratios were measured on 20 samples 19 molluscs and 1 calcareous concretion from the great chotts in southern tunisia results were studied by the isochron method and by age frequency histograms they suggest that two major flood episodes took place about 150 and 90 ka uranium activity ratios indicate a ground water supply of continental origin in agreement with biological indicators which show large variations in salinity from fresh to marine like conditions the existence of a lacustrine phase radiocarbon dated to 17 40 ka as previously suggested for the northern sahara is highly questionable

kirchhoff image propagation seismic imaging processes are in general formulated under the assumption of a correct macrovelocity model however seismic subsurface images are very sensitive to the accuracy of the macrovelocity model this paper investigates how the output of kirchhoff inversion migration changes for perturbations of a given 3 d laterally inhomogeneous macrovelocity model the displacement of a reflector image point from a perturbation of the given velocity model is determined in a first order approximation by the corresponding traveltime and slowness perturbations as well as the matrix of the beylkin determinant the required traveltime derivatives can be calculated with ray perturbation theory using this result a new computationally efficient kirchhoff inversion migration technique is developed to predict in parallel a series of subsurface images for perturbations of a given macrovelocity model during a single inversion migration process applied to the unmigrated seismic data these images are constructed by su

north slope alaska source rock distribution richness thermal maturity and petroleum charge four key marine petroleum source rock units were identified characterized and mapped in the subsurface to better understand the origin and distribution of petroleum on the north slope of alaska these marine source rocks from oldest to youngest include four intervals 1 middle upper triassic shublik formation 2 basal condensed section in the jurassic lower cretaceous kingak shale 3 cretaceous pebble shale unit and 4 cretaceous hue shale well logs for more than 60 wells and total organic carbon toc and rock eval pyrolysis analyses for 1183 samples in 125 well penetrations of the source rocks were used to map the present day thickness of each source rock and the quantity toc quality hydrogen index and thermal maturity tmax of the organic matter based on assumptions related to carbon mass balance and regional distributions of toc the present day source rock quantity and quality maps were used to determine the extent of fractional conversion of the kerogen to petroleum and to map the original toc toco and the original hydrogen index hio prior to thermal maturation the quantity and quality of oil prone organic matter in shublik formation source rock generally exceeded that of the other units prior to thermal maturation commonly toco 4 wt and hio 600 mg hydrocarbon g toc although all are likely sources for at least some petroleum on the north slope we used rock eval and hydrous pyrolysis methods to calculate expulsion factors and petroleum charge for each of the four source rocks in the study area without attempting to identify the correct methods we conclude that calculations based on rock eval pyrolysis overestimate expulsion factors and petroleum charge because low pressure and rapid removal of thermally cracked products by the carrier gas retards cross linking and pyrobitumen formation that is otherwise favored by natural burial maturation expulsion factors and petroleum charge based on hydrous pyrolysis may also be high compared to nature for a similar reason

a middle late pleistocene palustrine lacustrine travertine system borj edouane unit nw tunisia sedimentology stable isotopes and palaeohydrological implications the carbonate borj edouane unit represents the northernmost documented middle late pleistocene continental carbonate deposits in tunisia the unit consists of two main facies associations asymmetrically distributed 1 calcrete palustrine laminar crust 2 microbialites the first accumulated carbonate formed in the distal zone of alluvial fans calcretes and in shallow lakes subjected to periodic exposure palustrine facies that occupied the westernmost strip of the basin the second association formed in permanent lakes stromatolites oncoids and planar stromatolites located in the central areas of the basin fed by springs associated with a concentration of vertical faults in the eastern area of the basin travertine facies at time of deposition these facies communicated with the lake via a transitional lacustrine travertine environment transitional travertine stromatolite facies all facies show a restricted range of 18o values around 5 8 0 5 suggesting formation from paleoprecipitation largely of atlantic derivation brought about by westerly winds as for the 13c values these show a wide range 6 4 to 11 7 overall relatively high values for all facies with the lower values corresponding to the calcrete palustrine association and the higher ones linked to the microbialite facies the trend in travertines probably mimics an increasing influence of carbon derived from dissolution of carbonate bedrock of the cretaceous substrate while that of palustrine samples records increasing respired c contribution the sedimentological and isotopic characteristics of the borj edouane unit reflect wetter conditions at the time of deposition and point to a humid period at the middle late pleistocene transition

epeirogenic motions of africa as inferred from cretaceous shoreline deposits epeirogenic motions can be quantified if paleodatum planes can be recognized these motions have been determined for the time interval mid cretaceous cenomanian to recent in continental africa using the vertical displacement of ancient shoreline sediments and other nearshore or low elevation deposits that are inferred to represent a cenomanian sea level datum plane the cretaceous trans saharan seaway brought shoreline sediments deep into the african continental interior and additional data suggest that most of the remaining terrestrial regions were also low lying during the cenomanian the present elevations of the sea level sediments indicate that large portions of africa have undergone epeirogenic uplift since the cenomanian the character of the motions in north africa can be described as small scale high amplitude 3 km doming separated by relatively flat lying areas of deposition the largest single epeirogenic feature is the uplift associated with red sea and east african rift development the timing of post cenomanian epeirogenic motions can be further constrained using paleontologic k ar faulting paleodrainage and sedimentary data the most likely causal mechanism for african epeirogeny may be hot spot activity reflective of the mantle convection regime underlying the african lithosphere

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

petroleum generation and migration in the ghadames basin north africa a two dimensional basin modeling study the ghadames basin contains important oil and gas producing reservoirs distributed across algeria tunisia and libya regional two dimensional 2 d modeling using data from more than 30 wells has been undertaken to assess the timing and distribution of hydrocarbon generation in the basin four potential petroleum systems have been identified 1 a middle upper devonian frasnian and triassic triassic argilo greseux inferieur tag i system in the central western basin 2 a lower silurian tannezuft and triassic tag i system to the far west 3 a lower silurian tannezuft and upper silurian acacus system in the eastern and northeastern margins and 4 a lower silurian tanezzuft and middle upper devonian frasnian system to the east southeast the lower silurian tanezzuft source rock underwent two main phases of hydrocarbon generation the first phase occurred during the carboniferous and the second started during the cretaceous generating most hydrocarbons in the eastern libyan basin the frasnian shales underwent an initial minor generative phase in the central depression during the carboniferous however the main generation occurred during the late jurassic cenozoic in the western and central depression the frasnian shales are currently only marginally mature in the eastern part of the basin modeling indicates that the alpine eocene exhumation of the eastern libyan basin margin had a significant control on the timing of hydrocarbon generation from the lower silurian source rock the preferred burial history model calibrates source rock maturity data by incorporating late exhumation and reduced subsidence prior to the hercynian carboniferous orogeny as a result the tannezuft shales preserve their generative potential into the mesozoic cenozoic with renewed hydrocarbon generation during subsequent reburial which can migrate to post hercynian carboniferous traps hence favoring the preservation of hydrocarbon accumulations

prestack time migration velocity analysis using remigration trajectories abstractwe have developed a prestack time migration tool for local improvement of the seismic migration velocity model the method is based on remigration trajectories that describe the position of an image point in the image domain for different source receiver offsets as a function of the migration velocity it determines kinematic migration parameters using local slope information of migrated seismic reflection events these parameters in turn are used to locally correct the velocity model the main advantage of this technique is that it allows us to carry out a moveout correction not just at a fixed point in a zero offset poststack time migrated gather but varying through all offsets of a common image gather taking into account the reflection point displacement in the midpoint direction in other words it provides for time migration velocity analysis from prestack data we have tested the feasibility of the method on synthetic data from four simple models and the sigsbee2b data our tests deter

u series measurements in tyrrhenian deposits from mallorca further evidence for two last interglacial high sea levels in the balearic islands the campo de tiro type section for the tyrrhenian ecostratigraphic beds of the balearic islands shows four indurated littoral conglomerates and beach rocks unconformably superimposed the lower two units 1 and 2 are separated by a thin layer of reddish continental silts both contain a typical eutyrrhenian fauna whereas a neotyrrhenian fauna characterizes the overlying units 3 and 4 thu measurements by thermal ionisation mass spectrometry on mollusk shells from these deposits yielded ages of 135 ka unit 1 117 ka units 2 and 3 and a scatter of ages around 100 ka unit 4 the stratigraphic relationships and thu data indicate i that uranium was uptaken by mollusks shells but arca sp each time during a relatively short early diagenetic interval before cementation ensured a fair closure of the radioactive system ii two high sea stands characterized the last interglacial isotopic substage 5e of the balearic islands area iii the duration of this episode was 17 ka and iv the change in faunal assemblages and disconformity observed between units 2 and 3 are due to fluctuations in sea level and surface water conditions which occurred during the second high sea level episode of the last interglacial

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

poststack velocity analysis by separation and imaging of seismic diffractions small geologic features manifest themselves in seismic data in the form of diffracted waves which are fundamentally different from seismic reflections using two field data examples and one synthetic example we demonstrate the possibility of separating seismic diffractions in the data and imaging them with optimally chosen migration velocities our criteria for separating reflection and diffraction events are the smoothness and continuity of local event slopes that correspond to reflection events for optimal focusing we develop the local varimax measure the objectives of this work are velocity analysis implemented in the poststack domain and high resolution imaging of small scale heterogeneities our examples demonstrate the effectiveness of the proposed method for high resolution imaging of such geologic features as faults channels and salt boundaries

u series isochron dating a generalized method employing total sample dissolution the age of a sedimentary deposit can be determined by measuring 238u 234u 232th and 230th in two or more samples of the deposit with the requirement that these samples have different u th ratios but an identical 230th232th ratio when deposited this paper presents the theoretical and experimental aspects of the dating method the latter involves techniques for securing samples of varied u th ratios from the same deposit plotting isochrons of the radiochemical data obtained on totally dissolved samples for age analysis and estimating the errors involved although applications to impure carbonates and lacustrine evaporite sediments are given as examples in this paper the method potentially can be generalized for dating a variety of authigenic deposits such as rock varnish peat opal sulfate minerals and marine phosphorites

uranium series dated authigenic carbonates and acheulian sites in southern egypt field investigations in southern egypt have yielded acheulian artifacts in situ in authigenic carbonate deposits caco 3 cemented alluvium along the edges of nowaggraded paleovalleys wadi arid and wadi safsaf uranium series dating of 25 carbonate samples from various localities as far apart as 70 kilometers indicates that widespread carbonate deposition occurred about 45 141 and 212 ka thousand years ago most of the carbonate appears to have been precipitated from groundwater which suggests that these three episodes of deposition may be related to late pleistocene humid climates that facilitated human settlement in this now hyperarid region carbonate cements from sediments containing acheulian artifacts provide a minimum age of 212 ka for early occupation of the paleovalleys

palhydaf palaeohydrology in africa program objectives methods major results abstract the palhydaf program aims to reconstruct the late quaternary 130 ka b p to present environmental and hydrological changes of northwest africa and to interpret them in terms of palaeoclimate for testing coupled atmospheric ocean circulation models lacustrine sequences were obtained from closed basins which lie along two meridional transects southern tunisia southern niger and algeria mali particular attention is given to groundwater storage and periods of recharge of the aquifers the chronological framework is based on 14c dates conventional and ams methods and or the th u disequilibrium method in favourable cases e g mali correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge various environmental markers mineralogy elemental chemistry stable isotopes biological remains are used for palaeoenvironmental purposes quantitative estimates of palaeoclimatic variables e g air humidity may be attempted through the combination of geochemical and palaeobiological data the major results already obtained are summarized for the following individual sectors 1 southern tunisia 1 northern algeria 2 southern algeria 3 northern niger 3 mali 4 southern niger at the present stage of investigation the following new inferences can be drown one or several humid episode s occurred in the northern sahara between 150 and 75 ka b p several factors suggest that these humid conditions also affected lower latitudes although their southern extent cannot be given precisely at present the previously postulated existence of large lakes at 20 35 ka b p is not confirmed this time interval seems to coincide with raised water tables but not to a major humid phase in all cases the 14c chronologies obtained for the late pleistocene have to be checked by comparison with other dating methods e g th u disequilibrium after the arid period of the last glacial maximum the return of humid conditions occurred in two major steps at 12 and 9 5 9 ka b p for both the southern and the northern sahara

humidity changes in southern tunisia during the late pleistocene inferred from u th dating of mollusc shells abstract calcareous deposits mainly consisting of mollusc shell accumulations which have been dated by the u th disequilibrium method mark the shorelines of paleolake highstands in the great chotts area of southern tunisia the 5 sites studied consist of discontinuous accumulations of fossils of marine like organisms e g cerastoderma glaucum melania tuberculata melanopsis praemorsa cerithium rupestre u th isochron plots and age frequency histograms for 39 shell samples are reported here limited variations for u content and 234u 238u activity ratios ar of shells support the hypothesis of closure of the geochemical system with respect to this element it is remarkable that 234u 238u ar of shells collected in chott fejej or chott jerid are clustered around different values reflecting probably different groundwater recharge from the continental intercalaire ci or complexe terminal ct aquifers furthermore waters collected near wadi el akarit show 234u 238u ar values comparable to those observed for shells 14c determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka bp the u th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30 95 100 130 150 and 180 200 ka for the episode centred around 30 ka the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event moreover the comparison of 13c and 18o with those of older humid pleistocene phases when very large palaeolakes have been recorded suggests that these young carbonate shells are not related to a true highstand lake it is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place it should be noted that this period was less arid than the present

megalakes in the sahara a review the sahara was wetter and greener during multiple interglacial periods of the quaternary when some have suggested it featured very large mega lakes ranging in surface area from 30 000 to 350 000 km2 in this paper we review the physical and biological evidence for these large lakes especially during the african humid period ahp 11 5 ka megalake systems from around the world provide a checklist of diagnostic features such as multiple well defined shoreline benches wave rounded beach gravels where coarse material is present landscape smoothing by lacustrine sediment large scale deltaic deposits and in places tufas encrusting shorelines our survey reveals no clear evidence of these features in the sahara except in the chad basin hydrologic modeling of the proposed megalakes requires mean annual rainfall 1 2 m yr and a northward displacement of tropical rainfall belts by 1000 km such a profound displacement is not supported by other paleo climate proxies and comprehensive climate models challenging the existence of megalakes in the sahara rather than megalakes isolated wetlands and small lakes are more consistent with the sahelo sudanian paleoenvironment that prevailed in the sahara during the ahp a pale green and discontinuously wet sahara is the likelier context for human migrations out of africa during the late quaternary

recharge and paleo recharge of groundwater in different basins in tunisia abstract assessment of the recharge is of strategic importance for a sustainable development of the aquifer resource especially in arid and semi arid areas this study synthesizes and compares the results of isotope hydrology investigations obtained for different aquifer systems in tunisia the 2h 18o values and 14c and 3h contents are distinct in unconfined and confined groundwater and vary from northern to southern parts of tunisia reflecting different recharge conditions low 2h and 18o values of deep confined groundwater in the southern and central parts agrees with groundwater residence times that suggest an important phase of recharge during the holocene and pleistocene under paleo climatic conditions that are quite humid from the current the study of paleo groundwater signals of these aquifers indicates that the transition from the end of the pleistocene to the holocene is associated with a much humid climate to give a widespread age signal in southern and central deep groundwater in tunisia furthermore the current climatic conditions are the determining factors in the recharge process of tunisia aquifers so they allow only very low recharge rates of most aquifers as a result paleo water is dominating in many groundwater basins particularly where the extraction of water exceeds the present recharge

freshwater to marine like environments from holocene lakes in northern sahara continuous swamp and lacustrine sequences from the northern edge of the great western erg are dated at 9 300 3 000 yr bp diatoms ostracods molluscs and foraminifera show great changes in salinity ranging from freshwater to marine like environments the stable isotope composition of carbonates indicates that the evaporation of groundwater supplies is responsible for these fluctuations in salt contents until now evidence for the occurrence of a wet climatic phase of early and middle holocene age in the north north west sahara was based on a restricted number of 14c dates from scattered lacustrine deposits and paleosols1 8 the present study complements this knowledge and highlights the importance of local hydrological factors in interpreting palaeolimnological changes

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

palaeohydrological corridors for hominin dispersals in the middle east 250 70 000 years ago abstract the timing and extent of palaeoenvironmental connections between northeast africa the levant and the arabian peninsula during the middle and late pleistocene are critical to debates surrounding dispersals of hominins including movements of homo sapiens out of africa although there is evidence that synchronous episodes of climatic amelioration during the late middle and late pleistocene may have allowed connections to form between northern africa and western asia a number of palaeoclimate models indicate the continued existence of an arid barrier between northern arabia and the levant here we evaluate the palaeoenvironmental setting for hominin dispersals between and within northeast africa and southwest asia during marine isotope stages mis 7 5 using reconstructions of surface freshwater availability as an environmental proxy we use remotely sensed data to map palaeohydrological features lakes wetlands and rivers across the presently hyper arid areas of northern arabia and surrounding regions integrating these results with palaeoclimate models palaeoenvironmental proxy data and absolute dating to determine when these features were active our analyses suggest limited potential for dispersals during mis 7 and 6 but indicate the formation of a palaeohydrological corridor the tabuk corridor between the levant and the arabian interior during the mis 6 5e glacial interglacial transition and during mis 5e a recurrence of this corridor following a slightly different route also occurred during mis 5a these palaeohydrological and terrestrial data can be used to establish when proposed routes for hominin dispersals became viable furthermore the distribution of arabian archaeological sites with affinities to levantine assemblages some of which are associated with homo sapiens fossils and the relative density of middle palaeolithic assemblages within the tabuk corridor are consistent with it being utilised for dispersals at various times

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

chronologie th u des concretions calcaires des varves du lac glaciaire de deschaillons wisconsinien inferieur late pleistocene lake deschaillons varves from the central st lawrence lowlands represent a lithostratigraphic unit whose upper limit is defined by the overlying gentilly till which was laid down

shorelines in the sahara geomorphological evidence for an enhanced monsoon from palaeolake megachad the sahara desert is the most extensive desert on earth but during the holocene it was home to some of the largest freshwater lakes on earth of these palaeolake megachad was the biggest landsat tm images and shuttle radar topography mission srtm digital topographic data reveal numerous shorelines around palaeolake megachad at its peak sometime before 7000 years ago the lake was over 173 m deep with an area of at least 400 000 km2 bigger than the caspian sea the biggest lake on earth today the morphology of the shorelines indicates two dominant winds one northeasterly that is consistent with the present day winds in the region the other originated from the southwest we attribute it to an enhanced monsoon caused by a precessionally driven increase in northern hemisphere insolation subsequent desiccation of the palaeolake is recorded by numerous regressive shorelines in the sahara desert

a dynamic explanation for the origin of the western mediterranean organic rich layers the eastern mediterranean sapropels are among the most intensively investigated phenomena in the paleoceanographic record but relatively little has been written regarding the origin of the equivalent of the sapropels in the western mediterranean the organic rich layers orls orls are recognized as sediment layers containing enhanced total organic carbon that extend throughout the deep basins of the western mediterranean and are associated with enhanced total barium concentration and a reduced diversity dysoxic but not anoxic benthic foraminiferal assemblage consequently it has been suggested that orls represent periods of enhanced productivity coupled with reduced deep ventilation presumably related to increased continental runoff in close analogy to the sapropels we demonstrate that despite their superficial similarity the timing of the deposition of the most recent orl in the alboran sea is different than that of the approximately coincident sapropel indicating that there are important differences between their modes of formation we go on to demonstrate through physical arguments that a likely explanation for the origin of the alboran orls lies in the response of the western mediterranean basin to a strong reduction in surface water density and a shoaling of the interface between intermediate and deep water during the deglacial period furthermore we provide evidence that deep convection had already slowed by the time of heinrich event 1 and explore this event as a potential agent for preconditioning deep convection collapse important differences between heinrich like and deglacial like influences are highlighted giving new insights into the response of the western mediterranean system to external forcing

petroleum generation and accumulation in the berkine basin eastern algeria the berkine ghadames basin is regarded as an important hydrocarbon play in eastern algeria an evaluation of organic source facies the application of petroleum geochemistry and an extensive basin modeling study were carried out to improve the understanding of the petroleum charge of the basin geochemical data indicate that the llandoverian wenlockian and the frasnian hot shales are the most important oil prone source rocks for the known hydrocarbon accumulations in the basin maturity data reflect the late cretaceous tertiary burial and are consistent with a 60 63 mw m2 heat flow in the vicinity of the anticlines and 50 mw m2 heat flow farther away from the folded area a two dimensional basin modeling study revealed the importance of poorly constrained parameters such as the amount and rate of hercynian erosion on hydrocarbon generation and trapping efficiencies the model shows that a cretaceous tertiary petroleum generation and emplacement for both source rocks can better explain the different gas and oil fields than can an earlier hercynian event the faults related to the triassic jurassic rifting are most certainly permeable and can be considered as major migration pathways for petroleum in contrast the overpressure that developed along the el biod hassi messaoud axis in the ordovician reservoirs was probably caused by the low permeability of the faults bounding the anticlines that therefore prohibited any lateral or vertical fluid movements the model also indicates two petroleum systems that have triassic reservoirs 1 a frasnian sourced system that is confined to the central part of the basin and 2 a llandoverian wenlockian sourced system in the western part of the basin migration into paleozoic reservoirs was followed by dismigration into overlying triassic traps after the cretaceous compressive movements

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

terrestrial heat flow in the sirt basin libya and the pattern of heat flow across northern africa we report 66 new heat flow and 24 new heat production measurements from the sirt basin a late jurassic miocene sedimentary depression in north central libya underlain by late proterozoic basement heat flow determinations were made using bottom hole temperatures from oil wells and thermal conductivity measurements from drill core and cuttings heat production measurements come from core samples of basement rock heat flow is fairly uniform throughout the basin with a mean of 72 9 s d mw m 2 it is not clear if heat flow from the sirt basin is elevated as a consequence of its origin as a late mesozoic rift the difference between the mean basin heat flow and the global mean heat flow from tectonically undisturbed late proterozoic terrains 55 17 mw m 2 is 17 mw m 2 but this difference lies within the uncertainties associated with these mean heat flow estimates if heat flow from the sirt basin is elevated it could be caused by enhanced crustal heat production and need not be attributed to thermal alteration of the lithosphere related to basin formation mean crustal heat production is 3 9 2 1 w m 3 1 2 to 3 times greater than surface heat production in other proterozoic terrains in africa from west to east the pattern of heat flow across northern africa is characterized by high 80 110 mw m 2 heat flow throughout most of northwestern africa normal to perhaps slightly elevated heat flow in the sirt basin low to normal 35 55 mw m 2 heat flow in egypt inboard of the red sea and high heat 75 100 mw m 2 flow along the red sea high heat flow near the red sea and in northwestern africa along the mediterranean coast can be readily attributed to cenozoic tectonic activity but high heat flow in the paleozoic sahara basins of southern algeria is harder to understand within the tectonic framework of northern africa a possible explanation advanced previously is that elevated heat flow in the sahara basins arises from a regional thermal anomaly within the north african lithosphere if that explanation is correct then the heat flow distribution in the sirt basin and in egypt away from the red sea suggests that the postulated lithospheric thermal anomaly does not extend beyond the sahara basins to the east

time migration velocity analysis by image wave propagation of common image gathers image wave propagation or velocity continuation describes the variation of the migrated position of a seismic event as a function of migration velocity image wave propagation in the common image gather cig domain can be combined with residual moveout analysis for iterative migration velocity analysis mva velocity continuation of cigs leads to a detection of those velocities in which events flatten although image wave continuation is based on the assumption of a constant migration velocity the procedure can be applied in inhomogeneous media for this purpose cigs obtained by migration with an inhomogeneous macrovelocity model are continued starting from a constant reference velocity the interpretation of continued cigs as if they were obtained from residual migrations leads to a correction formula that translates residual flattening velocities into absolute time migration velocities in this way the migration velocity model can be improved iteratively until a satisfactory result is reached wi

uranium series age estimates and paleoclimatic significance of pleistocene tufas from the lahontan basin california and nevada abstract an extended chronology of lahontan basin lake levels based on uranium series age estimates correlates with the global ice volume record lake highstands occur at or shortly after times of maximum ice sheet size moderate size lakes occur when the global ice volume is about 80 of its maximum the data indicate that lake levels rise and fall relative to the proximity of the mean position of the jetstream when the continental ice sheet is above some threshold size or shape it appears that the large scale circulation and climatic conditions are right for producing lakes of moderate to large size within the lahontan basin

ghadames basin southern tunisia a reappraisal of triassic reservoirs and future prospectivity recent discoveries of oil and gas in triassic reservoirs of the ghadames basin eastern algeria suggest that similar potential exists to the east in the southern tunisian part of the province for decades southern tunisia has remained an underexplored part of the basin in large part owing to older geological interpretations that focus on paleozoic reservoirs and to limitations in seismic data quality resulting from statics problems that have rendered trap identification inaccurate or tenuous new analyses of previously and recently acquired geological and geochemical data as well as improvements in seismic data acquisition and processing have begun to reverse older notions regarding limited hydrocarbon presence in southern tunisia log correlations indicate that productive sand rich intervals of the major triassic reservoir in the region the trias argilo greseux inferieur tagi can be correlated over large distances suggesting significant continuity into lightly drilled and undrilled areas analyses of geochemical data reveal a more complex multistage history of hydrocarbon maturation and migration than previously believed much greater volumes of petroleum have been generated and a larger variety of traps have been created than can be accounted for by older interpretations where added to the use of new field and processing parameters for seismic surveys these factors have together led to important upgraded evaluations of basin potential in southern tunisia

uranium isotopes as tracers of groundwater evolution in the complexe terminal aquifer of southern tunisia abstract the complexe terminal ct multi layer aquifer is formed by neogene paleogene sand deposits upper senonian campanian maastrichtian limestones and turonian carbonates the chemical composition and isotopes of carbon and uranium were investigated in groundwater sampled from the main hydrogeological units of the ct aquifer in southern tunisia we paid special attention to the variability of uranium contents and isotopes ratio 234u 238u to provide a better understanding of the evolution of the groundwater system uranium concentrations range from 1 5 to 19 5 ppb typical of oxic or mildly reducing conditions in groundwaters the lowest concentrations are found southeast of the study area where active recharge is supposed to take place when looking at the isotope composition it appears that all the samples including those from carbonate levels are in radioactive disequilibrium with significant 234u excess a clear cut distinction is observed between turonian and senonian carbonate aquifers on the one hand with 234u 238u activity ratios between 1 1 and 1 8 and the sandy aquifer on the other hand showing higher ratios from 1 8 to 3 2 the distribution of uranium in this complex aquifer system seems to be in agreement with the lithological variability and are ultimately a function of a number of physical and chemical factors including the uranium content of the hosting geological formation water rock interaction and mixing between waters having different isotopic signatures significant relationships also appear when comparing the uranium distribution with the major ions composition it is noticeable that uranium is better correlated with sulfate calcium and magnesium than with other major ions as chloride or bicarbonate the 14c activities and 13c values of dic cover a wide range of values from 1 1 pmc to 30 2 pmc and from 3 6 to 10 7 respectively 14c model ages estimated by the fontes and garnier model are all younger than 22 ka and indicate that the recharge of ct groundwater occurred mainly during the end of the last glacial and throughout the holocene

burial history and thermal evolution of the northern and eastern saharan basins the numerical one dimensional reconstruction of the burial history and thermal evolution of the sedimentary section and basement was conducted for 32 wells in the east algerian basins with the help of the galo basin modeling system these reconstructions allowed the performance of a quasi two dimensional analysis of the burial history of the basins changes in rock temperatures the lithosphere thickness and maturation conditions of organic matter during basin development along the four profiles crossing the study region the modeling assumed that the highest thermal activation in the history of the saharan basins with the heat flow more than 100 mw m2 and the thinnest lithosphere on the order of 25 35 km took place during the late carboniferous to permian in the dahar and oued el mya basins the uplift responsible for this activation caused erosion of 2000 3000 m of ordovician to lower carboniferous strata the ghadames and illizi basins were subjected at the same time to more moderate erosion and thermal activation with heat flow less than 75 mw m2 in the triassic cretaceous interval maximum subsidence occurred in the northern areas which were characterized by maximal thermal activation and the thinnest lithosphere in the permian at present the situation is opposite to that in the permian the highest thermal regime occurs in the southern areas of the study region and especially in the illizi basin where heat flow reaches and even exceeds 100 mw m2 and the thickness of the lithosphere decreases as much as 30 km the modeling also assumes stretching of the lithosphere in the northeastern and central parts of the illizi basin with maximal amplitude of about 1 16 during the cenozoic the analysis shows that the hercynian erosion could account for only a few of the abrupt changes in vitrinite profiles in the saharan basins and that triassic and later intrusive activity and associated hydrothermal heat transfer accurately explain the steplike character of maturation profiles

recharge and paleorecharge of the deep groundwater aquifer system in the zeroud basin kairouan plain central tunisia abstract the multilayered aquifer of zeroud basin is characterized by the hydrodynamic complexity due to natural geology and anthropogenic management features stable and radioactive isotope data have been used to investigate the origin of the groundwater and its recharge area and to understand the deep groundwater system within the southern kairouan sedimentary basin most of the stable isotopic data indicate that most deep groundwater samples derived either from meteoric water or from the zeroud river and were not affected by any significant degree of evaporation during recharge current tritium concentration in the groundwater is very low however it proved useful in the qualitative identification of modern recharge and mixing of recent and old groundwater radiocarbon deduced ages range from more than 30 000 years to modern a modern recharge characterizes the deep aquifer upstream of the basin the groundwater with ages range from more than 30 000 years and depleted of heavy isotopes are found in the deeper aquifer aquifer b downstream of the basin and could be inherited from paleorecharge

marine deposits 35 000 25 000 years old in the chott el djerid southern tunisia the chott el djerid is a large salt covered depression in southern tunisia fig 1 the late nineteenth century idea that the basin had been invaded by the sea1 2 has been superseded by the view that any quaternary flooding of the chott must have resulted from climatic changes as the fossils found in its sediments are not exclusively marine and there is no indication of the fluctuations in sea level or of the crustal movements that would be required to fill and then empty the basin2 3 we report here evidence for a marine connection 35 000 25 000 years ago and show that it implies subsequent uplift by some 80 metres in an area long regarded as either stable2 or prone to subsidence throughout the quaternary4

a pleistocene lacustrine episode in southeastern libya fossiliferous lacustrine deposits of middle pleistocene age have been found in central fezzan libya 1 on each side of the east west depressions of the wadi shati south of the precambrian qarqaf anticline1 8 they represent the shoreline of a former brackish water lake fig 1 which some 130 000 yr bp covered an area about 100km long and 10 20 km wide at present the area may be classified as hyperarid since 1978 a collaborative study of the fossil assemblages has been undertaken it included macro and micropalaeontological determination th u dating stable isotope and chemical analyses in parallel with sedimentological and archaeological studies of the sites here we consider the th u dating and the isotopic palaeohydrology of the lacustrine episode

paleohydrologie isotopique de l oued el akarit sud tunisie au pleistocene superieur et a l holocene previous data from gobert and howe coque and page on the upper quaternary deposits from wadi el akarit are expanded through radiocarbon and stable isotope measurements 18c and 13c from carbonates 18o and 34s from sulphates these new results report on detailed stratigraphic profiles and indicate fluviatile and palustrine environments during the late pleistocene vegetal dams linked to spring activity caused water stagnation fine grained sediments were deposited with appreciable amounts of organic matter which locally developed into some peaty levels in the upper part of the typical cross section located on the left bank about 100 m upstream of the railway bridge pleistocene deposits show a palustrine carbonate crust this formation is attributed to the drainage induced by the major marine regression at glacial maximum during the holocene stagnation periods still occurred interrupted by flood episodes marked by bottom transport and coarse deposits in the typical cross section deposits are well drained and oxidized however downstream a stagnation episode takes place with a rich malacofauna dominated by cerastoderma cardium the uneven distribution of rainfall is documented by the alternation of fine and coarse deposits and it is accentuated by the climatic evolution toward aridity which results in the formation of a gypsum crust after 4000 yr bp later exceptional floods removed sediments from the wadi channel cutting the upper quaternary deposits for about 13 m along the central course of the wadi radiocarbon ages on carbonates are discussed according to various possible origins of total dissolved carbon atmospheric co2 organic activity in the basins and aquifer supply a simple exchange mixing model between aquifer bicarbonate and atmospheric co2 is used for calculations of initial carbonate activities chemical isotopic and hydraulic steady states are assumed in this treatment present day 14c and 13c contents of aquifer bicarbonate are thus adopted for past values the model leads to acceptable corrected ages for pleistocene carbonate samples corrected ages are younger than apparent ages by a few thousands of years and become consistent with ages given by organic matter from nearby levels however holocene samples appear overcorrected their crystallization appears to be controlled by modern organogenic co2 holocene 14c ages are probably correct 18o contents of carbonate indicate a continental origin for environmental water nodules and carbonate concretions crystallized in bottom sediments through upward leakage of confined aquifers they show 18o contents compatible with present day stable isotope compositions of artesian springs this supports the concept of a good time constancy in stable isotope contents shell carbonate values indicate a variable evaporation rate from the same initial waters 18o and 34s contents of gypsums are also compatible with aqueous sulphates from springs the data set falls within the range of permo triassic sulphate and suggests deep hydraulic connections within aquifer systems the entire formation appears to be of continental origin the occurence of cardium rich basins can only be attributed to the initial salinity of springs waters 7 9 g 1 1 sightly increased by evaporation this signifies that one can not deduce any tectonic uplift from their location with respect to sea level

on the ages of humid holocene and late pleistocene phases in north africa remarks on late quaternary climatic reconstruction for the maghreb north africa by p rognon discussion sur l article de rognon p paru en 1987 dans palaeogeogr palaeoclimatol palaeoecol vol 58 p 11 34

smiles and frowns in migration velocity analysis reliable seismic depth migrations require an accurate input velocity model inaccurate velocity estimates will distort point diffractors into smiles or frowns on a depth section for both poststack and prestack migrated sections high velocities cause deep smiles while low velocities cause shallow frowns on migrated gathers however for prestack images in the offset domain high velocities cause deep frowns while low velocities cause shallow smiles if the velocity is correct there will be no variation in the depth migration as a function of offset and no smiles or frowns in the offset domain we explain migration responses both mathematically and graphically and thereby provide the basis for depth migration velocity analysis

quantifying paleorecharge in the continental intercalaire ci aquifer by a monte carlo inversion approach of 36cl cl data abstract past variations of the recharge is estimated in the atlas mountains the main recharge area of the continental intercalaire ci aquifer one of the major saharan aquifers over the last 775 kyr in this mediterranean climatic context continental archives generally record single humid events grouped in hypothetical time interval and do not offer continuous chronicles of precipitation we propose to use spatially distributed 36 cl cl data as a temporal constraint to infer the past recharge in the atlas area based on a simplified but robust climatic scenario assuming a piston model we apply a markov chain monte carlo mcmc inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods the interglacial recharge values vary from a few mm yr 1 to more than 60 mm yr 1 glacial recharge is less than 1 mm yr 1 these values are then analyzed in terms of intensity and allow questioning some initial hypothesis especially the generally accepted value of the initial 36 cl cl value around 133 10 15 at at 1 our analysis suggests a higher value around 175 10 15 at at 1 this approach allows us to bring out paleoclimatic information retained in these continental archives especially computed recharges provide reliable evidence that marine isotope stage mis 5 was more humid than the holocene period in agreement with marine archives that document wet condition in the north sahara during that period