environmental isotopes meet 3d geological modelling conceptualising recharge and structurally controlled aquifer connectivity in the basalt plains of south western victoria australia summary to develop a conceptual model of recharge and inter aquifer connectivity for a complex aquifer system in southwestern victoria australia a three dimensional 3d geological model was constructed and used to examine the influence of the regional geology on groundwater composition and age as well as recharge mechanisms the major aquifers are three basalts differentiated on age and geomorphological features and an underlying palaeovalley deep lead system the upper fine grained sediments of the deep lead form an aquitard that separates the basalt from the permeable sands and gravels beneath recharge calculations chloride mass balance and water table fluctuation methods salinity stable isotope tritium and radiocarbon data show that volcanoes and the youngest basalts although volumetrically minor form preferential recharge areas for all the basalts up to 30 mm year and also recharge the deep lead sediments through volcanic necks that penetrate the aquitard infiltration through the thick clay rich soils on the older basalts is small typically 1 2 mm year and saline resulting in a progressive increase of groundwater salinity along the flow path in these basalts the comparison of fault location with groundwater age in the deep lead aquifer suggests that in some areas faults exert significant structural control on groundwater flow this study demonstrates the value of integrating geological hydrogeological and hydrochemical data to identify preferential recharge areas inter aquifer connectivity and the influence of faults on groundwater flow in a complex aquifer system

segmentation and differential post rift uplift at the angola margin as recorded by the transform rifted benguela and oblique to orthogonal rifted kwanza basins abstract we analyse tectonic and sedimentary field and subsurface data for the angola onshore margin together with free air gravity anomaly data for the offshore margin this enables us to characterize the mode of syn rift tectonism inherited from the precambrian and its impact on the segmentation of the angola margin we illustrate that segmentation by the progressive transition from the benguela transform rifted margin segment to the oblique rifted south kwanza and orthogonal rifted north kwanza margin segments the spatial variation in the intensity of post rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post rift succession of the coastal benguela and kwanza basins upper cretaceous to cenozoic uplifted palaeodeltas erosional unconformities palaeovalleys quaternary marine terraces and perched gilbert deltas the onshore benguela transform margin has a distinctive mainly progradational stratigraphic architecture with long term sedimentary gaps and high elevation marine terraces resulting from moderate upper cretaceous cenozoic to major quaternary uplifting i e 775 1775 mm ky or m ma by contrast repeated synchronous episodes of minor cenozoic to quaternary uplift occurred along the orthogonal rifted north kwanza segment with its cenozoic aggradational architecture short term sedimentary gaps and low elevation pleistocene terraces margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep sea fans along the low lying north kwanza margin sedimentation of the broad cenozoic to pleistocene kwanza submarine fan was probably governed by the width of the kwanza interior palaeodrainage basin combined with the wet tropical neogene climate along the high rising benguela margin the small size of the benguela deep sea fan is related to the interplay between moderate continental sediment dispersal from long lived small catchments and a warm very arid neogene climate however the driving forces behind the epeirogenic post rift uplift of the angola coastal bulge remain a matter of speculation

sedimentary processes in the carnot formation central african republic related to the palaeogeographic framework of central africa abstract the depositional environment provenance and processes of emplacement of the detrital material of the mesozoic carnot formation are defined by bedding and sedimentological analysis of its main facies and are reconstructed within the palaeogeographic framework of central africa the clastic material was laid down between probably the albian and the end of the cretaceous in a nnw oriented braided stream fluvial system that drained into the doba trough chad and probably also into the touboro basin cameroon the material was derived from weathering of the underlying devonian carboniferous mambere glacial formation and of the precambrian schist quartzite complex located to the south of the carnot formation these results provide useful indications as to the provenance of diamonds mined in the southwest central african republic

brittle tectonic evolution along the western margin of south africa more than 500 myr of continued reactivation abstract the brittle structural history of western south africa has been investigated by remote sensing and field studies to build a conceptual scheme for its 500 ma long evolution paleostress tensors were computed from a significant fault slip dataset and a relative geochronological succession of brittle deformation events was established this was aided by separating in time faulting events through the usage of cretaceous weathering horizons silicified fluvial deposits paleosols and 77 54 ma olivine melilitite plugs as time markers the oldest features recognized formed during four compressional episodes assigned to the neoproterozoic pan african evolution this history is expressed by sub vertical conjugate fracture sets and fits well the inferences derived from remote sensing the greatest compressive direction rotated from nw se to nne ssw and finally to almost e w a subsequent ene wsw oriented extensional episode is associated with the local effects of the opening of the atlantic ocean and was followed by a second ca e w extensional episode linked to the well acknowledged mid cretaceous 115 90 ma event of margin uplift a late santonian 85 83 ma nw se compressive paleostress deformed the late cretaceous sequences and was in turn followed firstly by a renewed episode of ne sw extension and later by ca nne ssw late maastrichtian 69 65 ma shortening the latter is broadly coeval with the emplacement of the gamoep magmatic suite a phase of wnw ese cenozoic extension is assigned to the extensional phase recorded in the okawango delta interpreted as reflecting propagation of the east african rift system into southern africa no stress tensor was computed for the present day wegener anomaly stress field oriented nw se however in situ stress measurements were used to perform slip tendency analysis which indicates that under the currently existing stress conditions wnw ese and nnw sse striking faults are critically stressed and are the most likely reactivated in agreement with the present seismicity

pre existing intra basement shear zones influence growth and geometry of non colinear normal faults western utsira high heimdal terrace north sea abstract pre existing intra basement shear zones can induce mechanical and rheological heterogeneities that may influence rifting and the overall geometry of rift related normal faults however the extent to which physical and kinematic interaction between pre existing shear zones and younger rift faults control the growth of normal faults is less well understood using 3d reflection seismic data from the northern north sea and quantitative fault analysis we constrain the 3d relationship between pre existing basement shear zones and the geometry evolution and synrift depositional architecture of subsequent rift related normal faults we identify ne sw and n s striking rift faults that define a coeval middle jurassic early cretaceous non colinear fault network ne sw striking faults are parallel to underlying intra basement shear zone the faults either tip out above or physically merge with the underlying shear zone for faults that merges with the basement shear zone a change from tabular to wedge shaped geometry of the hangingwall synrift strata records a transition from non rotational to rotational extension faulting which we attribute to the time of rift fault s linkage with the shear zone following downward propagation of its lower tip n s striking faults are oblique to and offset rather than link with intra basement shear zones these observations highlight the selective influence pre existing intra basement shear zones may or may not have on evolving rift related normal faults

the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

tectonic history along the south gabon basin anomalous early post rift subsidence an integrated study of the south gabon margin south atlantic based on reflection seismic and well data has been performed to quantify tectonic activity a regional profile crossing the entire basin together with subsidence analysis highlights important aspects of the post rift history the most striking event in the margin evolution appears to be the anomalous extra subsidence during the early post rift period characterized by high sedimentation rates equivalent to one third of the syn rift subsidence although the presence of evaporite layers restricts knowledge of the underlying structures essentially composed of pre rift and syn rift sequences the outcome of this post rift tectonic study has strong implications for the rifting history the early post rift subsidence patterns can be related to a high thermal anomaly during the early rifting thermal state of the lithosphere these findings are highly relevant for petroleum system studies and have implications for hydrocarbon generation

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

coagulation and transport of sediments in the gironde estuary the distribution of suspended particle size and concentration were measured along the gironde estuary france from the river seaward to the ocean the suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage sediments discharged by the rivers coagulate upon encountering the very low salinities 0 2 of the upper estuary confirmed with laboratory experiments and are then transported and deposited by currents in the remainder of the estuary this coagulation coupled with estuarine circulation produces a turbidity maximum which is offset between the surface and bottom waters the floc size maximum is oceanward of the turbidity maximum and is likewise offset along the estuary by about 30 km the estuary can be subdivided into the following zones 1 coagulation 2 hydrodynamic landward of the null point and 3 hydrodynamic seaward of the null point initial coagulation appears to be completed in coagulation zone 1 and particles are transported and settled with very little floc breakage and recoagulation in zones 2 and 3 only the floc settling velocities coupled with estuarine circulation control the concentration and size distributions of flocs in the water column and eventually control the deposition of sediments

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

evaporation induced 18o and 13c enrichment in lake systems a global perspective on hydrologic balance effects growing pressure on sustainable water resource allocation in the context of global development and rapid environmental change demands rigorous knowledge of how regional water cycles change through time one of the most attractive and widely utilized approaches for gaining this knowledge is the analysis of lake carbonate stable isotopic compositions however endogenic carbonate archives are sensitive to a variety of natural processes and conditions leaving isotopic datasets largely underdetermined as a consequence isotopic researchers are often required to assume values for multiple parameters including temperature of carbonate formation or lake water delta o 18 in order to interpret changes in hydrologic conditions here we review and analyze a global compilation of 57 lacustrine dual carbon and oxygen stable isotope records with a topical focus on the effects of shifting hydrologic balance on endogenic carbonate isotopic compositions through integration of multiple large datasets we show that lake carbonate delta o 18 values and the lake waters from which they are derived are often shifted by 10 parts per thousand relative to source waters discharging into the lake the global pattern of delta o 18 and delta c 13 covariation observed in 70 of the records studied and in several evaporation experiments demonstrates that isotopic fractionations associated with lake water evaporation cause the heavy carbon and oxygen isotope enrichments observed in most lakes and lake carbonate records modeled endogenic calcite compositions in isotopic equilibrium with lake source waters further demonstrate that evaporation effects can be extreme even in lake records where delta o 18 and delta c 13 covariation is absent aridisol pedogenic carbonates show similar isotopic responses to evaporation and the relevance of evaporative modification to paleoclimatic and paleotopographic research using endogenic carbonate proxies are discussed recent advances in stable isotope research techniques present unprecedented opportunities to overcome the underdetermined nature of stable isotopic data through integration of multiple isotopic proxies including dual element c 13 excess values and clumped isotope temperature estimates we demonstrate the utility of applying these multi proxy approaches to the interpretation of paleohy droclimatic conditions in ancient lake systems understanding past present and future hydroclimatic systems is a global imperative significant progress should be expected as these modern research techniques become more widely applied and integrated with traditional stable isotopic proxies c 2015 elsevier ltd all rights reserved

tectonic evolution of the sergipano fold belt ne brazil during the brasiliano orogeny abstract the sergipano fold belt sfb represents the southern segment of a brasiliano age 600 ma continental collision zone which extends over ne brazil and continues into central africa as the north equatorial fold belt major sinistral transcurrent fault shear zones and thrusts separate domains with individual lithological tectonometamorphic and magmatic characteristics the principal steeply dipping strike slip zones trend east west to wnw ese lithological correlations across the shears have not proved possible which suggests that displacements were probably large but as yet are unquantified major folding and thrusting affected all the domains folds have a southerly vergence with nw se to wnw ese trends consistent with a ne sw to nne ssw shortening direction although small differences in shortening trends exist between individual domains the southernmost allochthonous domain of carbonate to clastic shelf slope sediments was subject to a single phase of folding and thrusting onto the sao francisco craton accompanied by greenschist facies metamorphism north of this domain lies the macurure domain which is separated from the southernmost allochthonous domain by the major sao miguel do aleixo transpressive shear zone polyphase deformation amphibolite facies metamorphism and massive granitic intrusion characterize this domain north of the macurure domain lie the migmatites and granites of the poco redondo domain which represent a deeper crustal level they are separated from the macurure domain by the belo monte shear a late tectonic leucogranite in the poco redondo domain yields a 600 ma rb sr isochron finally the poco redondo domain was overthrust from the north by the caninde gabbroic complex associated with carbonates pelites and amphibolites metamorphosed to greenschist and upper amphibolite facies this has previously been interpreted as an ophiolite sequence although the relative movements or absolute ages of the domains are unknown their distinct geological differences suggest that they are separate crustal blocks which underwent large lateral movement during the brasiliano collision

chloride imbalance in a catchment undergoing hydrological change upper barwon river southeast australia documenting whether surface water catchments are in net chemical mass balance is important to understanding hydrological systems catchments that export significantly greater volumes of solutes than are delivered via rainfall are not in hydrologic equilibrium and indicate a changing hydrological system here an assessment is made of whether a saline catchment in southeast australia is in chemical mass balance based on cl the upper reaches of the barwon river southeast australia has total dissolved solids tds concentrations of up to 5860mg l and cl concentrations of up to 3370mg l the high river tds concentrations are due to the influxes of groundwater with tds concentrations of up to 68 000mg l between 1989 and 2011 the median annual cl flux from the upper barwon catchment was 17 8 106kg 140kg a ha this represents 340 2230 of the annual cl input by rainfall to the catchment major ion and stable isotope geochemistry indicate that the dominant source of solutes in the catchment is evapotranspiration of rainfall precluding mineral dissolution as a source of excess cl the upper barwon catchment is not in chemical mass balance and is a net exporter of solutes the chemical imbalance may reflect the transition within the last 100ka from an endorheic lake system where solutes were recycled producing shallow groundwater with high tds concentrations to a better drained catchment alternatively a rise in the regional water table following land clearing may have increased the input of groundwater with high tds concentrations to the river system

strontium and carbon isotope tracers and the origins of soil carbonate in south australia and victoria abstract calcium carbonate occurs in soils in association with a variety of rock types in coastal and inland south australia and victoria the 87 sr 86 sr ratios of the coastal soil carbonates fall between 0 7094 and 0 7098 showing that the ocean is the principal source of calcium to nearly all soils near the coast and that contributions from bedrock are generally small an exception is the soil formed on the young volcano at mt gambier in which the average ratio for carbonate is about 0 7070 compared to 0 7041 for the volcanic ash the basaltic ash of the volcano is readily weathered and rich in calcium roughly 40 of strontium and 10 to 20 of the calcium in the soil carbonate derives from the ash and the balance from marine sources the 87 sr 86 sr ratios from soil carbonate gradually increase inland reaching values of 0 7152 these higher ratios reflect at least in part the increasing 87 sr 86 sr ratio of inland dust in contrast the ocean does not contribute significantly to the carbon pool of the soils we studied the 13 c pdb values of soil carbonates on kangaroo island and the higher rainfall areas of eastern victoria fall between 11 0 and 8 7 indicating that the carbonate formed in equilibrium with the c3 dominated vegetation typical of the region in the warmer inland areas to the north such as the eyre peninsula the 13 c value of soil carbonate averages 5 3 reflecting the higher proportion of c4 plants in the local biomass mixing with marine sources of carbonate cannot account for the observed carbon isotopic patterns the flux of carbon into soils from the ocean therefore must be small compared to that derived from plant respiration and decay

on the reactivation of extensional fault systems in terranes that have undergone substantial extension three sets of faults dominate a shallow to steep dipping commonly rotational normal faults b a really extensive shallow dipping normal detachment faults and c steep dipping transfer faults that strike at high angles to the normal faults these fault systems may extend through a large fraction of the crust reactivation of these fault systems will depend primarily on the relative strengths of the faults shear zones and their host rock and their orientation in the prevailing stress field it is concluded that reactivation is generally mechanically favoured but that it will probably only take place when the fault shear zones are in near ideal orientations consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse thrust reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles examples of these styles are described from the bass strait basins of southeastern australia because extended terranes commonly underlie sedimentary basins for example on passive continental margins reactivation of extensional faults may be a key control on the tectonic evolution of such basins i e basin inversion

tectonic inheritance crustal architecture and contrasting structural styles in the venezuela andes surface and subsurface data image contrasting structural styles for the two flanks of the venezuela andes in the north a flexural basin developed in neogene times between the andes and the lake maracaibo north verging thrusts are mainly detached in the pre cretaceous substratum and form a deeply buried antiformal stack secondary decollement levels occur both in the upper cretaceous and tertiary strata accounting for the passive roof thrust of a conventional frontal triangle zone in the south the barinas basin hardly compares with a flexural basin it is largely dominated by both north and south verging basement involving structures paleogene normal faults are locally inverted and caribbean nappes are frequently reactivated or refolded by younger oblique neogene andean structures seismic profiles in this area also attest to the strong neogene structural inversion of upper jurassic lower cretaceous grabens hercynian or more likely early paleozoic structures account for local preexisting crustal heterogeneities reactivated during both the tethyan rifting and the andean deformations two trans andean regional profiles have been balanced and constrained by an inversion of the gravimetric data they imply a progressive deepening of the northern moho and a south dipping subduction of the infracontinental lithospheric mantle of the maracaibo block the shortening for both sections averages 60 km palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the andes and minimize the possible effects of a progressive right lateral escape of the maracaibo block with respect to stable south america along the bocono fault strain partitioning during neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary strike slip motion in the allochthon along the bocono fault and an asymmetric subduction wedging at depth

clay illuviation and mechanical clay infiltration is there a difference abstract soil scientists attribute the presence of coatings of oriented clay in soils and sediments to the process of clay illuviation which is a process that occurs at the earth s surface and may penetrate to a depth of several metres geologists attribute coatings of oriented clay in coarse sediments to a process of mechanical infiltration of muddy river water or muddy overland flow clay illuviation is a climate dependent process seasonal climate which is restricted to the vadose zone the source of the translocated material is internal i e material from upper soil layers is transported to deeper layers the resulting accumulation horizons are up to 2 m thick and continuous in clayey deposits but in sandy deposits they are discontinuous and may reach a depth of several metres the process of mechanical infiltration is not climate related and its effect is not restricted to the vadose zone but may reach far below the ground water table the source of the transported material is external it is provided by muddy water entering the soil the accumulations are supposed to be deep and continuous the environment of muddy waters with higher electrolyte concentrations than water that percolates the soil upon first rain and a gradient of particles coarser than clay should however preclude mechanical infiltration because 1 clays will tend to flocculate 2 both coarser suspended material and flocculated clay will tend to clog the pore system and 3 below ground water transport of suspended material will be virtually impossible because of low percolation velocity in addition flocculated clay will not produce oriented coatings upon desiccation the model of mechanical infiltration is not supported by theory or observations we suggest that the process of mechanical infiltration of clay needs further study to prove whether it is different from clay illuviation in soils the distinction between the two is of utmost importance for the interpretation of paleosols

petroleum geology of tunisia recent discoveries and important oil shows have proven the existence of hydrocarbons in newly identified depocenters and reservoirs in general except for some areas around the producing fields tunisia is largely underdrilled the national company etap has decided to release data and to publish a synthesis on the petroleum geology of tunisia the geology of tunisia provides a fine example of the contrast between alpine folding which typifies northern tunisia and the african craton area of the saharan part eastern tunisia corresponds to an unstable platform forming plains or low hills and extending eastwards to the shallow pelagian sea there are a wide variety of basins central and northern tunisia represents a front basin the saharan ghadames basin or the chott trough are sag basins the gulf of gabes was formed as a distension margin the gulf of hammamet is a composite basin and several transversal grabens cut across the country including offshore and are rift type basins all these features are known to be oil prolific throughout the world two large fields and many modest size pools are known in tunisia oil and gas fields in the surrounding countries namely the saharan fields of algeria and libya the large bouri fieldmore offshore tripolitania and discoveries in the italian part of the straits of sicily suggest a corresponding potential in tunisia exposed paleogeographic and structural maps balanced sections and examples of fields and traps will support an optimistic evaluation of the future oil exploration in tunisia less

physical hydrogeology and environmental isotopes to constrain the age origins and stability of a low salinity groundwater lens formed by periodic river recharge murray basin australia summary a low salinity total dissolved solids tds 25 000 mg l is marked by a hectometer to kilometer scale transition in tds concentrations that is not stratigraphically controlled rather this boundary represents a mixing zone with the regional groundwater the position of which is controlled by the rate of recharge from the river the lens is part of an active and dynamic hydrogeological system that responds over years to decades to changes in river levels the lens has shrunk during the drought of the late 1990s to the mid 2000s and it will continue to shrink unless regular high flows in the murray river are re established over longer timescales the rise of the regional water table due to land clearing will increase the hydraulic gradient between the regional groundwater and the groundwater in the lens which will also cause it to degrade replacement of low salinity groundwater in the lens with saline groundwater will ultimately increase the salinity of the murray river reducing its utility for water supply and impacting riverine ecosystems

neocomian to early aptian syn rift evolution of the normal to oblique rifted north gabon margin interior and n komi basins abstract the north gabon coastal rift basins consist of a set of 130 150 long segment asymmetrically tilted half grabens interior basin and 000 020 short segment en echelon half grabens n komi basin separated by 040 060 major transverse faults tectono sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation during berriasian to early barremian times uniform uniaxial 040 060 extension was responsible for the stretching of the brittle upper crust over a 100 km wide domain during late barremian early aptian times the main locus of extension stepped westward resulting in severe end rift uplift and erosion of the failed interior and n komi rift basins early cretaceous coastal rifts in north gabon display a wide range of styles from oblique rifting n komi basin normal rifting interior basin to transform rifting the pre existing precambrian tectonic fabric exerts a strong control over the mode and over the 100 300 km scale segmentation of the rifting

classification of large scale subaqueous bedforms a new look at an old problem sepm bedforms and bedding structures abstract a symposium entitled classification of large scale flow transverse bedforms was convened at the 1987 mid year meeting of sepm in austin texas with the purpose of examining the problems involved in classifying large subaqueous flow transverse bedforms developed in fluvial intertidal and marine environments and recommending changes in nomenclature the consensus of the participants is that despite the wide spectrum of morphologies of large scale flow transverse bedforms excluding antidunes they all occupy a similar position in the lower flow regime sequence between ripples and upper plane bed the wide variety of forms is a reflection of secondary effects such as channelization fluctuating water levels and unsteady and reversing flows the bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1 000 m the symposium panel proposes therefore that they should have only one name dune dune is preferred as it has historical precedence over other terms in use such as megaripple and sand wave the term dune should be modified by rimary descriptors of shape i e 2 d or 3 d and size based on spacing small 0 6 5 m medium 5 10 m large 10 100 m or very large 100 m and the adjective subaqueous when it is important to distinguish them from eolian dunes the panel recommends a morphologically based classification that is descriptive with an underlying genetic rationale second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature

the northeastern brazil and gabon basins a double rifting system associated with multiple crustal detachment surfaces analysis of structural stratigraphic and gravimetric data from the reconcavo tucano and sergipe alagoas basins in northeastern brazil and the gabon basin in africa shows that these basins originated in a double rifting system associated with multiple crustal detachment surfaces and that the direction of dip of the detachment surfaces was reversed at the vaza barris fault system this geometry is in agreement with models and data from modern rifts but it requires the existence of a predominantly extensional stress regime in the northern part of the south atlantic during the early stages of oceanic evolution evidence for the existence of such a stress regime is given by application of kinematic constraints to early cretaceous reconstructions of the south atlantic ocean interpretation of the regional geologic setting suggests that the double rifting system and the point of final continental rupture were controlled by the preexisting structural orientation of the tectonic provinces in the precambrian basement

tidal and fluvial controls on the internal architecture and sedimentary facies of a lobate estuarine tidal bar the plassac tidal bar in the gironde estuary france abstract estuarine tidal bars emplaced in estuaries are complex sediment bodies composed of sand and mud provided by rivers this study focuses on the plassac tidal bar which is the unique lobate shape tidal bar of the gironde estuary located at the very upstream extremity of the bay head delta because no detailed sedimentological studies have been made on such geomorphological category of tidal bar its stratigraphy was investigated using a dense grid of 65 very high resolution seismic profiles ground truthed by 6 to 9 meter long cores and time controlled by historical bathymetric surveys the base of the tidal bar consists of a tidal erosional surface this surface has eroded muddy tidalites corresponding to the maximum flooding surface of the gironde estuary the bar is composed of two vertical sequences of facies composed of thickening up cross stratified fine to medium grained sand beds at the top and decimeter thick mud layers at the base those mud layers constitute the master bedding of the bar and are dipping in a channelward direction perpendicularly to the tidal flows and to the sediment transport directions one phase of lateral accretion of the plassac tidal bar is correlated with variations in fluvial discharge of the last decades and suggests a climate control on the internal architecture of this sandbar low water periods lead to the upstream migration of the turbidity maximum and to the deposition and consolidation of the muddy master bedding planes during the subsequent flood periods the muddy master bedding planes are partially eroded and then buried by the fluvially sourced sandy mini flood lobes that merge with the tidal bar the internal architecture of the plassac tidal bar is controlled by a lateral accretion process its master bedding surfaces which record this process are dipping perpendicularly to the axis of the tidal flows all the other tidal bars of the bay head delta around a dozen exhibit an elongated morphology and prograde longitudinally by seaward frontal accretion occurring in the axis of the tidal flows the master bedding surfaces which record this process are seaward dipping along the axis of the tidal flows those variations in internal accretion surfaces orientation of tidal bars are of considerable importance and must be taken into account for studies of ancient tidal deposits paleoenvironment reconstructions and for heterogeneities characterization and quantification of reservoir properties in these types of sandstones

facies and architecture of an estuarine tidal bar the trompeloup bar gironde estuary sw france abstract this study describes the sedimentary facies and internal architecture of a modern estuarine tidal bar present in the bay head delta of a wave and tide dominated estuary the gironde estuary in southwest france based on 51 cores 2 to 5 m long this work demonstrates that such bars are composite sedimentary bodies made up of individual sigmoidal sand units isolated from one another by thick muddy layers the bar appeared in the study area around 1874 since then its evolution has been characterized by phases of rapid downstream bar migration alternating with periods of bar abandonment during the periods of active bar progradation which commonly last between 10 and 20 years the bar progrades up to 7 km seaward and forms a narrow 200 800 m wide sand ridge up to 6 m thick during abandonment phases which also last around 10 20 years the bar is starved of sand and a mud layer 1 to 3 m thick is deposited the vertical succession formed by the seaward progradation of the tidal bar records these alternating phases of growth and abandonment a typical vertical succession in the bar system is 8 m thick and comprises several superimposed upward coarsening units of cross stratified sand deposited during phases of seaward progradation separated by a thick layer of thinner bedded mud and sand laminae deposited during phases of bar abandonment when a tidal bar builds up to the supratidal zone marshes accumulate and the bar becomes a permanent island in the estuary the large scale architecture of the bar is thus made of an alternation of potential sandy reservoir units and muddy internal seals the reservoir units are characterized by a sigmoidal shape the muddy layer which is deposited between the two sigmoidal units is a thick continuous potential internal seal this complex internal architecture is due to the fact that there is a considerable amount of mud present in the bay head delta of the gironde estuary it is thought to be representative of the architecture of tidal bars in the inner part of wave and tide dominated estuaries

the character and origin of lacustrine source rocks in the lower cretaceous synrift section congo basin west africa lower cretaceous synrift lacustrine shales from the congo basin west africa have been analyzed with sedimentological and geochemical techniques to characterize source rock quality and identify triggers for deposition of intervals richest in organic carbon the sequence includes a lower active rift section deposited during active faulting and subsidence overlain by an upper late rift section deposited during reduced faulting and subsidence total organic carbon toc averages 2 3 wt throughout the active rift siliciclastic shale section 6 in marls in the lower part of the late rift section and 1 2 in deltaic shales in the upper part of the late rift section organic matter consists of mixed types i and iii kerogen in the active rift shales pure type i kerogen in the late rift marls and a type i and iii mixture in the late rift deltaic shales redox proxies indicate that the deep lake was relatively reducing throughout deposition of the active rift and lower late rift sections therefore enhanced anoxia did not trigger deposition of the richest source rocks decreased sedimentation rates in the late rift do not account for the full increase in toc nor the shift on organic matter type the richest source rocks are associated with high rates of organic productivity and chemical sedimentation indicating that flux of dissolved components to the rift lake including nutrients for algae growth was critical we propose that reduced topography associated with the late rift was necessary for efficient cycling of plant derived carbon into soil carbonate and ultimately the rift lake and for enhancing chemical weathering and nutrient flux

basement control in the development of the early cretaceous west and central african rift system abstract the structural framework of the precambrian basement of the west and central african rift system wcars is described in order to examine the role of ancient structures in the development of this early cretaceous rift system basement structures are represented in the region by large pan african mobile belts built at ca 600 ma surrounding the 2 ga west african congo and sao francisco cratons except for the small gao trough eastern mali located near the contact nappe of the pan african iforas suture zone along the edge of the west african craton the entire wcars is located within the internal domains of the pan african mobile belts within these domains two main structural features occur as the main basement control of the wcars 1 an extensive network of near vertical shear zones which trend north south through the congo brazil nigeria niger and algeria and roughly east west through northeastern brazil and central africa the shear zones correspond to intra continental strike slip faults which accompanied the oblique collision between the west african congo and sao francisco cratons during the late proterozoic 2 a steep metamorphic nw se trending belt which corresponds to a pre pan african ca 730 ma ophiolitic suture zone along the eastern edge of the trans saharian mobile belt the post pan african magmatic and tectonic evolution of the basement is also described in order to examine the state of the lithosphere prior to the break up which occurred in the earliest cretaceous after the pan african thermo tectonic event the basement of the wcars experienced a long period of intra plate magmatic activity this widespread magmatism in part relates to the activity of intra plate hotspots which have controlled relative uplift subsidence and occasionally block faulting during the paleozoic and the early mesozoic this tectonic activity was restricted to west of the hoggar west of air and northern cameroon during the late jurassic early cretaceous the st helena mantle plume may have had an important effect in weakening the lithosphere across a broad zone from brazil to sudan however causes of extension and stretching are likely to be found in a plate tectonic framework at that time the opening of both the indian and atlantic oceans were the dominant forces which favoured the development of the wcars and its propagation by strike slip movements and extensional displacements along pre existing pan african lithospheric zones of weakness

infiltrated clays in fluvial jurassic sandstones of reconcavo basin northeastern brazil abstract fluvial sandstones and conglomerates of the sergi formation a jurassic unit of reconcavo basin northeastern brazil contain significant amounts of interstitial detrital clays which are interpreted as the product of early mechanical infiltration developed in an arid semi arid environment within such an environment the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium concentrating the clays in the upper phreatic zone near the sources of influent seepage petrographic textures considered to be typical of mechanically infiltrated mi clays are herein presented using examples from the sergi formation these textures include 1 ridges and bridges 2 geopetal fabrics 3 loose aggregates 4 anisopachous coatings of tangentially accreted lamellae cutans 5 massive aggregates and 6 shrinkage patterns developed during diagenesis in addition their detrital aspect as revealed by sem observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays in most hydrocarbon reservoirs in the sergi formation the clays described in this paper are among the main controls on porosity and permeability distribution commonly zones with maximum clay concentration are the main internal permeability barriers within the sergi formation reservoirs

fractured reservoir modeling from well data to dynamic flow methodology and application to a real case study in illizi basin algeria abstract fault arrays and natural fractures distribution strongly influence subsurface fluids migration trapping and production it is critical to develop methodologies that can be used to accurately characterize reservoir volumes as present day exploration and appraisal campaigns become increasingly focused on tight or low porosity reservoirs a common method used to model the distribution and intensity of subsurface fracture sets is the discrete fracture network dfn technique shortcomings of the dfn technique include the evaluation of fracture attributes computational aspects in the case of large fields and most importantly issues related to upscaling thus the aim of this work is to present a simplified methodology for fractured reservoir characterization based on the distribution of fracture intensity as a continuous property fracture intensity was calculated from image well logs data and then distributed in the reservoir according to specific fracture drivers the case study is related to a large appraisal gas field located in the illizi basin southern algeria where late ordovician glacial deposits are the primary reservoir levels in which the presence of faults and fractures strongly enhance well performances the final fracture intensity model was obtained by implementing a workflow in a commonly used commercial geomodeling software and calibrated by means of well test data analysis the implemented methodology is a useful tool for large fractured reservoir characterization when dfn technique is hardly applicable for computational reasons or the level of uncertainty does not support a performing discrete analysis

understanding the salinisation process for groundwater in an area of south eastern australia using hydrochemical and isotopic evidence abstract increases in groundwater salinity down gradient are a common occurrence in catchments throughout australia exacerbating the effects of dryland salinisation in discharge areas groundwater hydrochemical and isotopic data from the willaura catchment in south eastern australia which is affected by both primary natural and secondary human induced salinity allowed the factors that cause the down gradient trend in salinisation within this catchment to be determined the overall salinity of the groundwater 0 8 70 ms cm is controlled by evapotranspiration which concentrates cyclic salts in the unsaturated zone greatly increasing the salinity of the soil water to 3700 6400 mg l cl the progressive addition of this saline soil water to fresher groundwaters recharged on the catchment margins causes a gradual increase in salinity down gradient the groundwater stable isotope compositions show that most recharge occurs in winter and spring groundwater salinities increase around the saline lakes of the discharge zone due to direct evaporation from shallow watertables and minor dissolution of halite as indicated by cl br ratios mineral water interactions within the aquifers cause a slight overall reduction in salinity conversion of kaolinite to smectite and illite and cation exchange of na for ca2 on smectites cause na cl mg2 cl k cl si cl and hco3 cl ratios to progressively decline and ph to gradually increase down gradient oxidation of organic matter causes a strong decrease in the redox potential of the groundwaters down gradient the groundwater ages increase from modern containing tritium on the catchment margins to 8000 years based on 14c dating at the end of the 25 km flow path the older groundwaters are isotopically heavier than the younger waters probably due to recharge during a wetter climate in the early holocene the groundwater ages show clearly that the progressive addition of saline soil water causing groundwaters to become more saline down gradient has occurred throughout the holocene and may explain the occurrence of primary salinisation within many australian catchments

detrital clay grain coats in estuarine clastic deposits origin and spatial distribution within a modern sedimentary system the gironde estuary south west france

in situ neutron flux 36cl production and groundwater evolution in crystalline rocks at stripa sweden the meteoric input of36cl due to cosmogenic or nuclear weapon produced36cl cannot contribute significantly to the36cl present in the saline groundwaters up to 700 mg l 1 cl from the stripa granite the extent of in situ production of36cl has been estimated on the basis of the neutron fluxes within the granite and its surrounding leptite the36cl present in the groundwaters is attributed to either admixture of labelled cl from the leptite with cl from the granite or to the total derivation of groundwater chlorinity within the leptite followed by radiochemical ingrowth of36cl during subsequent groundwater residence within the granite the chloride derived from the leptite may be either matrix chloride or chloride from an external source which has had a long residence time within the leptite the implications of36cl in situ production for the estimation of groundwater residence times and for the geochemical evolution of groundwater chlorinity are discussed

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

sources of chloride and implications for 36cl dating of old groundwater southwestern great artesian basin australia we investigate the relative importance of a number of processes that affect cl and 36cl distribution in the southwestern flow systems of the great artesian basin australia to provide more precise estimates of age and flow and recharge rates in situ production of 36cl secular variations in the rate of production of 36cl and its subsequent fallout as well as rates of diffusion of stable cl from adjacent aquitards mask the interpretation of rates of decay of 36cl cl along hydraulic gradients and can preclude estimating absolute groundwater ages however mean flow velocities calculated from absolute cl concentrations are 0 24 0 03 m yr and are a more reliable parameter than calculated ages because fewer assumptions and approximations are required recharge rates to the main cadna owie formation and algebuckina sandstone aquifer of the great artesian basin estimated from cl mass balance in the unconfined aquifers are 0 16 0 08 mm yr and were probably up to a factor of 3 higher 30 kyr ago as indicated by decreasing cl and 18o concentrations along the head gradient

differences in groundwater and chloride residence times in saline groundwater the barwon river catchment of southeast australia abstract the residence times of groundwater and chloride and the processes contributing to the development of saline total dissolved solids tds up to 45 379 mg l groundwater within the barwon river catchment of southeast australia were investigated using major ion stable isotope 18 o 2 h and 13 c and radioactive isotope 3 h 14 c 36 cl geochemistry the elevated groundwater salinity in the region is primarily due to evapotranspiration and recycling of solutes in saline lakes with minor contributions from weathering of halite silicate and calcite minerals groundwater residence times estimated from 14 c vary from modern to 20 ka for groundwater with lower 14 c activities the estimated residence times vary significantly depending on the assumed flow model and the 14 c activity of recharge chloride residence times downgradient of lake murdeduke a saline through flow lake in the centre of the catchment are greater than the corresponding groundwater residence times due to the recycling of cl within the lake precise estimates of chloride residence time could not be determined using 36 cl due to r 36 cl in precipitation being lower than that of groundwater this is most likely due to r 36 cl values in rainfall having been higher in the past than they are at present due to climate variability 18 o 2 h and 13 c values also suggest that the region has experienced increasingly more evaporative conditions with time the results of this study demonstrate that while cl is a useful tracer of hydrological processes it must be applied carefully in arid and semi arid regions of the world in particular recharge rates calculated using chloride mass balance may be underestimated where recycling of cl has occurred

tectonostratigraphic development of the interior sudan rifts central africa abstract in the muglad melut and blue nile rift basins of interior sudan three major episodes of rifting concomitant subsidence and nonmarine nonvolcanic sedimentation are recognized these three rifting cycles which spanned 140 to 95 ma fl 95 to 65 ma f2 and 65 to 30 ma f3 resulted in the accumulation of up to 5400 4200 and 5400 m of sediments respectively in the muglad basin the best known and largest of the sudan rift basins each rifting cycle consists of 1 a basal sandstone unit at least near rift margins that is followed by 2 an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone and 3 a capping blanket of fluvial and alluvial sandstone the shale dominated portions of these cycles were deposited in a closed drainage basin during active faulting the fluvial and alluvial blanket sands were deposited in an open drainage basin during the thermal sag phase following each tectonic cycle the early cretaceous f1 intracontinental rifts of interior sudan were linked to major rifts spreading centres in the proto south atlantic by the dextral wsw trending central african shear zone and to the indian ocean via the nw trending anza rift in kenya in the muglad basin f1 deformation involved high strain rates rapid syn rift crustal stretching and subsidence and the formation of deep fault bounded tensional and transtensional pull apart basins during the f2 and f3 deformations the rates of subsidence and stretching were much lower and were focused within smaller geographic areas structural elements include asymmetric half grabens and less common full grabens with central highs the three superimposed tectonic episodes resulted in the subsidence of nnw to nw trending rift sub basins this gave rise to a wide variety of normal fault geometries displacements and growth histories planar domino style and listric normal f1 fault arrays are modeled the rotated f1 basement blocks typically are asymmetric and low standing and favour ne directed growth and material transport the f2 and f3 normal faults which have both ne and sw directed polarities often are listric and decouple younger syn rift strata from older f1 rotated and locked planar basement faults palinspastic restorations and forward modeling of three regional cross sections suggest that f1 faults account for 65 80 of total crustal extension stretching factors across the interior sudan rifts vary between 1 25 and 1 40 this range is considered to be a minimum since restored sections do not cross f3 depocenters total crustal extension across the muglad plus the melut rift basins exceeds 75 km

a new precipitation weighted method for determining the meteoric water line for hydrological applications demonstrated using australian and global gnip data summary the relationship between 2h and 18o in precipitation at a site known as the local meteoric water line lmwl is normally defined using an ordinary least squares regression olsr which gives equal weighting to all data points regardless of the precipitation amount they represent however smaller precipitation amounts are more likely to have a lower d excess due to re evaporation of raindrops below the cloud base or biases in the sampling method in this paper we present an equation for a precipitation amount weighted least squares regression pwlsr that will correct these biases for use in groundwater and surface hydrology applications new lmwl equations are presented for australian sites in the global network of isotopes in precipitation gnip where the pwlsr consistently produces a lmwl with a larger gradient than the olsr perth and alice springs exhibit the largest change in slope this is consistent with the higher frequency of small monthly precipitation amounts with low d excess values occurring at these sites in summer for perth and throughout the year for alice springs the pwlsr method was also applied to 288 stations in the gnip data base n 36 and the difference between the slopes of the lmwls a slopepwlsr slopeolsr calculated for these stations the mean change in slope a was 0 12 with 56 of sites showing an increase in slope or positive a value and 44 having a decrease in slope or negative a sites with mediterranean climates showed the greatest increase in slope the magnitude of the change in slope followed some general trends showing a positive correlation with average 2h and 18o composition and rainfall variability and negative correlation with period of record n

handbook of environmental isotope geochemistry vol 1 the terrestrial environment a p fritz and j ch fontes editors elsevier amsterdam 1980 xii 532 pp u s 90 25 dfl 185 00

dawsonite cement in the triassic lam formation shabwa basin yemen a natural analogue for a potential mineral product of subsurface co2 storage for greenhouse gas reduction abstract the origin and permeability effects of carbonate cement blamed for poor reservoir quality in the upper triassic turbidite sandstones of the lam formation in yemen have been investigated using petrography x ray diffraction stable isotopes fluid inclusions and thermodynamic modelling in order of growth in the sandstone the carbonate cements include ferroan calcite dawsonite naalco3 oh 2 ferroan dolomite and siderite ferroan dolomite is the most abundant cement there is an inverse correlation between permeability and total carbonate cement volume supporting the earlier assertion that carbonate cements are the main control on reservoir quality in these sandstones ferroan calcite replaced and was sourced by marine limestone rock fragments that are abundant in the turbidite sandstone the later cements were probably also at least partly sourced from the marine limestone rock fragments stable isotope data from ferroan calcite ferroan dolomite and siderite show that diagenesis occurred in formation water with oxygen isotopes typical of closed system diagenesis and that only minor organic derived carbon dioxide was involved in diagenesis dawsonite while not common as a diagenetic mineral in sandstones has recently been suspected of being a routine consequence of co2 storage in the subsurface for greenhouse gas reduction in the lam formation dawsonite found at concentrations of up to 8 vol of the rock grew at the expense of detrital feldspar minerals under conditions of elevated partial pressure of carbon dioxide isotopic data show that dawsonite grew in the presence of carbon dioxide with a 13c value of about 4 to 2 this is distinctly different from the carbon isotope ratio of the gas phase carbon dioxide now present in the sandstone 27 which had an organic source rock origin the dawsonite 13c data probably represent a mixture between carbonate from detrital marine limestone rock fragments and carbon dioxide from a moderately negative source possibly of magmatic or basement origins dawsonite may thus be the result of a mass influx of deep e g magmatic carbon dioxide that pushed the lam formation into the dawsonite stability field at the expense of albite

the deep structure of the south atlantic kwanza basin insights from 3d structural and gravimetric modelling abstract we present results from 3d gravimetric modelling of the kwanza basin offshore angola accomplished to investigate the deep crustal structure beneath the basin and discuss our findings with respect to their implications for the opening of the central south atlantic the kwanza basin is located in the southern part of the central south atlantic although the post rift evolution of the kwanza basin is well studied little is known about the basin s early history this is mainly due to the missing knowledge of its crustal structure owing to the masking effect of an up to 4 km thick salt layer which seismically obscures the underlying basement to get an insight into the deeper structure of this part of the angolan margin we combined 3d structural isostatic and gravimetric modelling 2d seismic reflection data was used to determine the structural setting and the configuration of the stratigraphic units in the sedimentary part of the basin whereas its crustal structure was constrained by isostatic and gravity modelling our modelling results indicate that high density lower crustal bodies similar to ones found in the northern part of the central segment are present in the southern part and thus seem to be a general feature of the crust of the central south atlantic thinning of the crust occurs gradually rather than in an abrupt manner and block faulting seems to have played a significant role during rifting as is indicated by a strongly structured upper crust in contrast we find little evidence for ductile thinning or exhumation of the deeper lithosphere as previously suggested for the northern segment of the central south atlantic in its southern segment it appears more likely that magmatic intrusions prevented mantle exhumation by sealing crustal detachment faults and giving the margin a distinct magmatic signature

reactivation of intrabasement structures during rifting a case study from offshore southern norway abstract pre existing structures within crystalline basement may exert a significant influence over the evolution of rifts however the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood using borehole constrained 2d and 3d seismic reflection data from offshore southern norway we identify and constrain the three dimensional geometry of a series of enigmatic intrabasement reflections through 1d waveform modelling and 3d mapping of these reflection packages we correlate them to the onshore caledonian thrust belt and devonian shear zones based on the seismic stratigraphic architecture of the post basement succession we identify several phases of reactivation of the intrabasement structures associated with multiple tectonic events reactivation preferentially occurs along relatively thick c 1 km relatively steeply dipping c 30 structures with three main styles of interactions observed between them and overlying faults i faults exploiting intrabasement weaknesses represented by intra shear zone mylonites ii faults that initiate within the hangingwall of the shear zones inheriting their orientation and merging with said structure at depth or iii faults that initiate independently from and cross cut intrabasement structures we demonstrate that large scale discrete shear zones act as a long lived structural template for fault initiation during multiple phases of rifting

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

mesozoic and early tertiary rift tectonics in east africa abstract a complex history of crustal extension occurred in east and central africa during the mesozoic and early tertiary beginning in the late jurassic this resulted in a large system of rifts the central african rift system that spanned from central sudan to southern kenya late jurassic rifting is best documented in the white and blue nile rifts of the sudan and records east west extension in half graben that were connected by large scale shear zones and pull apart basins early cretaceous rifting re activated jurassic basins and spread to the large south sudan rifts and anza rift in kenya by the late cretaceous the extension direction shifted to the ne sw and the presently observed large scale rift geometry was established in the early tertiary some mesozoic basins were again reactivated while other regions underwent wrench faulting and basin inversion the large number of basins preserved in the central african rift system can be used to construct an evolutionary model of continental rift tectonics early phases of extension at low strains produced alternating half graben accommodation zone geometries similar to those observed in most young and active continental rifts at higher strains some border faults were abandoned so that through going simpler active fault systems could evolve this is interpreted as representing a switch from complex oppositely dipping detachment structures with strike dimensions of 50 150 km to regional detachment structures that continue for hundreds of kilometers parallel to the rift this change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate non rotational high angle normal faulting dominates in the development of these late basin geometries deciphering similar rift basin histories from passive continental margins may in many cases exceed the limits of available reflection seismic data east west striking shear zones including the umm hani and south sudan shears served as basin terminating structures in the late jurassic rift system these shear zones were later reactivated during the cretaceous and early tertiary controlling the positions of some accommodation zones and jogs apparent offsets in rift trends the south sudan shear may have continued to play a role in neogene extensional tectonics as it connects jogs in both the abu gabra anza rift jurassic to early tertiary and turkana chew bahir rift neogene to recent

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

chlorine 36 dating of very old groundwater 1 the great artesian basin australia chlorine 36 has many advantages as a dating tool for very old groundwater these advantages include a suitable half life 3 01 x 10 sup 5 years simple geochemistry conservative behavior in groundwater and a general absence of subsurface sources at levels comparable to the atmospheric input recent advances in tandem accelerator mass spectrometry have permitted the analysis of sup 36 cl at low abundance expected following residence in the subsurface for 10 sup 6 years or more in order to test the suitability of sup 36 cl for dating very old groundwater the sup 36 cl cl ratios of 26 groundwater samples from the great artesian basin of australia have been measured groundwater ages calculated from the sup 36 cl data compare favorably with ages computed independently from hydrodynamic simulations

le hoggar oriental bloc cratonise a 730 ma dans la chaine pan africaine du nord du continent africain abstract the djanet tafassasset unit in eastern hoggar is a segment of a pre pan african fold belt trending nw se and is cut by syn to late tectonic calc alkaline batholiths u pb dating of zircons from a granite of this suite gives a crystallisation age of 729 8 ma the eastern hoggar was hence stabilized at about 730 ma i e at the time of growth of both the arabo nubian shield and north mali systems of magmatic arcs during the pan african continental collision 600 ma the eastern hoggar behaved as a rigid block in between the saharan nigerian belt and the mozambique belt

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

deformation of oceanic crust in the eastern gulf of guinea role in the evolution of the cameroon volcanic line and influence on the petroleum endowment of the douala rio muni basin

fault rocks of the kaltungo lineament ne nigeria and their relationship with benue trough tectonics in the upper part of the benue trough of nigeria several major ne sw lineaments are essential components in the structure of both the pan african basement and the cretaceous sediments in order to estimate their influence on the geodynamics of the trough detailed structural analyses have been carried out on the fault rocks of one of them the kaltungo lineament it cross cuts a basement inlier emerging from lower to middle cretaceous sediments this inlier of granitic rocks exhibits a wide fault zone trending n50e which displays an anastomosing network of both ductile and brittle shear zones the former are expressed by mylonitic bands generated by a pan african dextral wrenching on a regional scale the latter are represented by fault rocks of the cataclasite series related to a sinistral wrenching which occurred during the mesozoic the fault zone extends into the cover where it is seen as a thin crush zone characterized by conjugate strike slip and reverse faults from a computer dynamic analysis of microfaults an earlier compression n s shortening axis was found only within the basement whereas a second one n140e shortening axis appears in both sediments and basement the first palaeostress field corresponds to cataclasite genesis which preceded or coincided with the formation of pull apart basins along the sinistral kaltungo wrench fault which initiated the upper benue trough during the lower cretaceous sedimentary basins and uplifted zones kaltungo inlier were formed on releasing and restraining oversteps of such a fault the above statement may be extended to other similar faults at the scale of the entire trough the second palaeostress field corresponds to the late cretaceous compression responsible for the large scale folding in the sediments and the formation of fault breccia along the sinistral kaltungo lineament

chlorine 36 dating of very old groundwater 2 milk river aquifer alberta canada the milk river aquifer in southern alberta canada consists of sandstone interbedded between thick shale units the groundwater is confined and discharges by both upward and downward leakage through the shales the concentration of cl increases by about 2 orders of magnitude downgradient through the aquifer this cl increase complicates 36cl dating of the system climatic changes flushing of connate water introduction of cl from older water below the aquifer and ion filtration have been proposed as possible explanations for the increase the 36cl data show a consistent decrease of the 36cl cl ratio downgradient but an increase in the 36cl concentration downgradient in certain parts of the aquifer dates calculated from the 36cl cl ratio show a reasonable distribution in light of the hydraulic controls on the system whereas dates calculated from the 36cl concentration give negative ages this suggests that ion filtration is responsible for the cl increase and that the 36cl cl dates are to be preferred we estimate water ages to be more than 2 m y near the distal end of the aquifer

mise en evidence de complexes ophiolitiques dans la chaine du mayombe gabon angola et nouvelle interpretation geotectonique abstract the precambrian mayombian belt was formed by the closure of an oceanic basin by subduction processes during late kibarian time 1000 ma ago this closure led to a collision between a western eburnean craton 2000 ma and two older eastern cratons the chaillu in the north 2500 ma and the kasai in the south 2700 ma after this collision marked by the tectonic emplacement of ophiolitic complexes and the generation of palingenetic granites the nyanga and niari eastern miogeosyncline was formed between 1000 and 600 ma ago the deformations of the miogeosynclinal sequence resulted from the late orogenic phases and the uplift of the axis of the belt this new model of the evolution is similar to those recently proposed for the pan african damara ribeira orogens south of the equator and the pharusian dahomeyan west africa

hydrogeochemical and isotopic constraints on the origins of dryland salinity murray basin victoria australia abstract combined hydrogeological and hydrogeochemical data allow flow systems and the origins of solutes in the honeysuckle creek area of the southeastern murray basin which is an area affected by dryland salinity to be constrained recharge occurs both on the uplands that are composed of fractured violet town volcanic rocks and the riverine plain that comprises sediments of the shepparton and coonambidgal formations groundwater from the violet town volcanics has low salinity 20 mmol l groundwater has intermediate cl br ratios 600 1000 which indicate that the high salinities do not simply result from halite dissolution rather mixing of groundwater homogenises cl br ratios and evaporation as a consequence of a shallow water table is the dominant process that increases salinity oxygen and h isotopes also indicate that mixing and evaporation have occurred these results indicate that land use over the whole region not just the uplands needs to be considered in any salinity management plans additionally future development of salinity is controlled by depth to the water table on the plains and the efficiency of recharge rather than by salt stores halite or brines in the unsaturated zone

geochemistry s vital contribution to solving water resource problems abstract as part of the events celebrating 40 a of iagc it is fitting to trace the modern evolution and development of hydrogeochemistry however fascination with water quality can be traced back more than 2 ka in the post war years hydrogeochemistry was influenced heavily by the advances in other disciplines including physical chemistry metallurgy and oceanography hydrological applications of isotope science also developed rapidly at this time and important advances in analytical chemistry allowed multi element and trace element applications to be made experimental studies on equilibrium processes and reaction kinetics allowed bench scale insight into water rock interaction consolidation of knowledge on processes in groundwaters and the current awareness of hydrogeochemistry by water professionals owe much to the work of robert garrels john hem and co workers in the early 1960s studies of down gradient evolution enabled a field scale understanding of groundwater quality and geochemical processes as a function of residence time dissolution and precipitation processes in carbonate and non carbonate aquifers redox processes cation exchange and salinity origins emerging water resource and water quality issues in the 1960s and 70s permitted the application of hydrogeochemistry to contaminant and related problems and this trend continues the impacts of diffuse pollution from intensive agriculture waste disposal and point source pollution from urban and industrial sources relied on geochemistry to solve questions of origin and attenuation in semi arid regions facing water scarcity geochemical approaches have been vital in the assessment of renewability and characterising palaeowaters the protection and new incoming regulation of water resources will rely increasingly on a sound geochemical basis for management

subsurface seepage of seawater across a barrier a source of water and salt to peripheral salt basins many of the great phanerozoic salt basins share a common paleogeography in which a deep lake or sea is separated from the ocean by a narrow barrier examples include the aptian evaporites of the south atlantic rift basins and the messinian salt deposits of the mediterranean sea and the north caspian depression marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins we test an alternate hypothesis that subsurface seepage of seawater through the barrier was a significant source of water influencing both water level and water composition our model permits flux of water and salt into and out of the basin via 1 seepage through the barrier 2 evaporation from the water surface 3 rainfall and or rivers and 4 incorporation into basin sediments our investigation focuses on three settings a modern analog in the gregory rift of northern kenya the messinian mediterranean basin and the early cretaceous aptian salt basins of the south atlantic we test our model through a numerical simulation applying reasonable values for the dimensions of the barrier differences in water level and hydraulic conductivities to calculate flow rates through the barrier modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant depending on the climate size of the peripheral basin and hydraulic conductivity of the barrier the flux of salt is significant over the full range of modeled hydraulic conductivities producing brines in the peripheral lake and or sea within 0 5 m y in most cases this demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions and that marine transgressions across the barrier are not necessary to explain the apparent high salinities

constraining groundwater flow residence times inter aquifer mixing and aquifer properties using environmental isotopes in the southeast murray basin australia environmental isotopes particularly d 18 o d 2 h and d 13 c values 87 sr 86 sr ratios and a 14 c constrain geochemical processes recharge distribution and rates and inter aquifer mixing in the riverine province of the southern murray basin due to methanogenesis and the variable d 13 c values of matrix calcite d 13 c

decoupling of solutes and water in regional groundwater systems the murray basin australia documenting the origins residence times and movement of groundwater and the solutes that it contains is critical to understanding hydrogeological systems this study uses cl mass balance to determine the cl accession time i e the time required for cl to accumulate and cl to estimate the residence times of cl in the victorian portion of the murray basin southeast australia much of the murray basin contains saline groundwater with total dissolved solids tds concentrations commonly 14 000 mg l and locally up to 300 000 mg l the total mass of cl stored in the victorian portion of the basin is estimated as between 12 400 and 47 100 mt using present day rainfall totals and cl concentrations in rainfall the cl accession time is 170 to 650 ka aquifer thicknesses and groundwater salinity both increase westwards in this part of the murray basin consequently the cl accession times increase westward from 0 1 0 6 ka to 286 1080 ka by contrast c activities of the majority of the groundwater are 2 pmc and commonly much higher notwithstanding the difficulty in correcting c residence times the widespread occurrence of groundwater with above background c activities implies that groundwater residence times are generally 30 ka which is substantially shorter than the cl accession times rcl the ratio of cl to total cl 10 values of the groundwater are between 20 and 230 and are uncorrelated with cl concentrations while it is difficult to determine precise cl residence times the observation that the rcl values are significantly higher than those that represent secular equilibrium with the aquifer matrix rcl of 5 to 10 indicates that they are up to a few hundred thousand years and similar to the cl accession times the rcl values together with the geology of the aquifers stable isotope ratios and major ion geochemistry precludes halite dissolution incorporation of connate water or the long term diffusion of cl from clays as mechanisms of producing the elevated cl concentrations rather it is most likely that the high cl concentrations result from recycling of solutes in saline lakes and playas or repeated cycles of evapotranspiration in the unsaturated zone

postrift stress field inversion in the potiguar basin brazil implications for petroleum systems and evolution of the equatorial margin of south america rifting and related normal stress regime in the equatorial continental margin of brazil ceased during the late cretaceous when the stress regimes in eastern south america and west africa changed to induce strike slip or reverse motion in this study we explore the postrift tectonic geomorphic magmatic and sedimentary responses to stress changes using the potiguar basin the easternmost basin in the equatorial margin of brazil as a case study we use field and topographic data 2d seismic reflection lines vertical electric soundings and geochronological and borehole data to constrain the stress evolution of the potiguar basin from the late cretaceous to the quaternary discussing the role of basin inversion on sedimentation and landforms our results indicate the presence of two strike slip stress regimes after rifting the first stress field sf1 occurred from late cretaceous to the middle miocene and consisted of a n s oriented maximum subhorizontal compression and an e w oriented extension the second stress field sf2 took place from the middle miocene to the present day and included subhorizontal e w to nw se compression combined with n s and ne sw subhorizontal extension emplacement of volcanic rocks occurred along transtensional faults with a principal peak during sf1 at 20 30 ma and a subordinate peak during sf2 at 5 10 ma in response to shortening during sf2 a 70 km long and 50 km wide dome formed where marine miocene strata were uplifted to 250 m asl this uplift induced the displacement of alluvial channels away from the dome anticlines formed by transpression along the main ne sw striking faults during both sf1 and st2 acted as traps in the petroleum system similar shifts and stress field inversions documented in other areas of the brazilian continental margin are consistent with the neogene rise of the andes and may have implications for reconstructing the tectonic history of the equatorial atlantic margin of south america

helium 4 characteristics of groundwaters from central australia comparative chronology with chlorine 36 and carbon 14 dating techniques summary helium isotope and concentration characteristics were determined for a suite of groundwater samples from the amadeus basin in central australia two study areas include a wellfield south of alice springs and the dune plains and mututjulu aquifers near uluru measurements of 36cl cl and 14c on the same sample suite enable us to assess the relative applicability of the three groundwater chronometers over a range of anticipated groundwater residence times ages and to investigate possible causes of discordant ages derived from the different groundwater dating techniques results from the analyses of 39 groundwater samples reveal helium 4 4he concentrations that range from 0 80 to 98 8 10 7 cm3 stp g 1 h2o in the alice springs samples and from 0 47 to 65 6 10 7 cm3 stp g 1 h2o in the uluru samples 4he concentrations yield uncorrected groundwater residence times i e time since recharge of between modern to 2500 ka near alice springs and modern to 1600 ka near uluru assuming an effective porosity of 20 and uranium and thorium contents of 1 7 and 6 1 ppm respectively 36cl cl ratios on the same samples range from 93 to 158 10 15 near alice springs and from 80 to 335 10 15 near uluru representing groundwater residence times near alice springs from modern to 200 ka and from modern to 300 ka near uluru percent modern carbon pmc on the same samples ranged from 64 9 to 12 5 pmc near alice springs and from 93 5 to 30 ka for the amadeus basin groundwater samples the 4he method uncorrected over estimates groundwater residence time compared to 36cl and 14c techniques this implies the presence of an extraneous he component or basal flux of he j0 to reconcile groundwater 4he and 14c residence times it is necessary to adopt j0 values between 0 and 30 10 8 cm3 stp he cm 2 a 1 which supplements in situ produced he within the aquifer adoption of j0 values over this range lowers 4he residence times relative to 36cl and produces closer agreement between the 4he and 14c chronometers the extraneous he component basal flux in amadeus basin samples is dominated by radiogenic crustal 4he without a detectable amount of mantle derived 3he we conclude that the stable tectonic regime albeit with the presence of major faults within the basin inhibits the input of mantle volatiles to the groundwater system so that in this region diffusion is the dominant transport mechanism for he in the crust

the permo jurassic alkaline province of tadhak mali geology geochronology and tectonic significance the tadhak alkaline complexes located along the eastern edge of the west african craton in northeastern mali intrude the 2 ga old basement or the overlying pan african nappes and are covered by cretaceous sediments this magmatic activity was accompanied by doming and rifting northnortheast trending normal faults define two crustal segments exhibiting variously sized 1 km to 17 km intrusive complexes these comprise eroded undersaturated ring complexes with occasional carbonatites in the west and high level phonolitic plugs with shallow intrusive nepheline syenites in the east new rb sr isotopic data indicate 100 ma of within plate magmatic activity in the area without any obvious age progression in addition to the adrar tadhak complex 262 7 ma the following intrusions have been dated tirkine 215 11 ma anezrouf 184 14 ma and tidjerazraze in imanal 161 5 ma a clear break thus exists with the cambrian alkaline granite province of the iforas located on the opposite side of the pan african suture 87sr 86sr initial ratios are similar throughout between 0 7044 and 0 7049 which indicates a bulk earth isotopic composition the conclusion of exclusively mantle origin dupal composition previously demonstrated for the adrar tadhak complex can now be extended to the other complexes this implies repeated tapping of a single mantle source throughout the life of the province the described magmatic and tectonic permo jurassic activity has been triggered by intraplate stresses focussed along the pre existing pan african suture zone together with similar features in europe and africa the permo jurassic tadhak alkaline province heralds the progressive break up of pangaea and the opening of the atlantic ocean

groundwater residence time and palaeohydrology in the otway basin south australia 2h 18o and 14c data stable isotopes 2h 18o and 13c and radiocarbon 14c have been used in conjunction with chemical data to evaluate recharge mechanisms groundwater residence time and palaeohydrology within the confined dilwyn sand aquifer in the gambier embayment of the otway basin this aquifer does not receive recharge down gradient of the hydraulic hinge line and data have been interpreted along two discrete flow lines the mean residence time of groundwater determined by 14c in the confined aquifer from the hydraulic hinge position to the sea a distance of about 50 km along an inferred flow path is approximately 12 800 years the corresponding hydraulic travel time calculated from darcy s law is approximately 49 000 years the apparent discrepancy may be a result of eustatic sea level lowering during the last glacial because the groundwater system is hydraulically connected to the sea lower sea level would result in increased gradients and a decrease in groundwater residence time variations in stable isotopic composition along flow lines suggest a number of recharge mechanisms stable isotope data indicate progressive depletion of 2h and 18o in the groundwaters over the past 30 000 years groundwaters older than about 10 000 years b p were recharged either during a cooler climate climatic regime lower precipitation evapotranspiration and temperature and or the vapour source s had travelled over greater continental mass than those recharged over the past 10 000 years lower chloride concentrations in some of the older groundwaters indicate lower evapotranspiration rates in the recharge areas before 10 000 years ago

coagulation rates of clay minerals and natural sediments abstract coagulation experiments conducted using illite kaolinite montmorillonite and four natural sediment samples in blade and couette reactors with solutions of various salinities yielded collision efficiency factors significantly higher than those in the previous literature indicating that coagulation begins at lower salinities 1 2 1 the natural sediments do not generally behave as might be predicted for a mixture of the standards of the minerals of which the natural sediment samples were comprised the half times for coagulation of the particles indicate that in nature the coagulation process would probably be completed in the first few parts per thousand salinity

integrated depositional model for the cenomanian turonian organic rich strata in north africa abstract during the late cenomanian early turonian c t oceanic anoxic event oae2 organic rich strata was deposited in rift shelf basins and slopes across north africa and in deep sea basins of the adjacent oceans based on a review of published and unpublished information this paper documents the distribution and organic richness of c t strata across the whole region within a palaeogeographic framework and systematically analyses the conditions and processes which controlled their deposition previously the c t in north africa has been most intensively studied in southern morocco tarfaya and tunisia only little data is availabe for other parts of north africa namely algeria libya and egypt because distribution of c t corg strata there becomes more patchy a general decrease in peak organic richness and black shale thickness occurs from west to east partly as a result of upwelling along the moroccan atlantic coast and the absence of upwelling in the eastern mediterranean area furthermore in the confined central atlantic the oxygen minimum zone omz in many places reached down to the deep sea floor 3 4 km while the lower limit of the omz along the north african tethys was much shallower and underlain by oxic water masses as documented by high resolution biostratigraphic and chemostratigraphic data c t black shale deposition in most areas outside the upwelling zone are restricted to a strong eustatic latest cenomanian transgressive phase triggered by this sea level rise the omz impinged onto the north african continental shelf and the margins of intrashelf basins which mostly formed during the early cretaceous as halfgrabens important units containing c t organic rich strata in the region are the atlantic tarfaya black shales morocco western sahara black shales and phtanites in the moroccan and algerian atlas the bahloul fm in the se constantine basin and in northern and central tunisia the etel fm in the sirte basin the al hilal fm in cyrenaica libya the abu roash fm in the abu gharadig and fayium basins western desert egypt the daliyya fm along the ne sinai palestine isreal coast and the shueib fm in west central jordan

experimental nucleation and growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces abstract laboratory experiments were performed to investigate high temperature 100 150 c authigenic formation of clay coatings smectites chlorites on clean feldspar and quartz surfaces artificial formation waters were used with mg concentrations in the range of north sea brines experiments were run for 21 50 days in brines composed of magnesium chloride and sodium carbonate adjusted to circum neutral ph the experiments suggest that the silica activity is the main factor determining if grain coating smectites or chlorite form the clay minerals form easily on both clean quartz and feldspar surfaces but chlorite coatings were formed only on feldspar surfaces while smectite coatings were formed on both feldspar and quartz the chlorite morphology varies between honeycomb edge to face and rosette patterns while all smectite formed with honeycomb like textures the clay coatings produced in this study are morphologically similar to naturally occurring diagenetic clay minerals in natural sediments and sedimentary rocks the formation of clay coatings is promoted by pre existing clay drapings on mineral grains the growth substrates used in this study were not coated with such a natural precursor materials this suggests that the nucleation of clay coatings in nature may be possible on clean quartz and feldspar surfaces but pre existing clay minerals may impact the allowed levels of supersaturations and thereby the formation of the grain coating phases

development of texture contrast soils by a combination of bioturbation and translocation abstract soils and weathering profiles in a wide variety of parent materials and environmental settings exhibit coarse over fine vertical textural contrasts where these cannot be attributed to inherited texture contrasts or erosion deposition the most common explanations are based on translocation eluviation illuviation which removes clays from surface layers and deposits them in the subsoil or bioturbation where preferentially fine material is delivered to the surface by organisms from whence erosional winnowing creates a coarse surface layer in some soils of the lower coastal plain of north carolina u s a neither explanation is sufficient to explain the observed texture contrasts a heuristic model based on a combination of translocation of fine material from surface to subsoil and bioturbation driven delivery and recycling of material to the surface can explain the observed vertical textural contrasts the key elements in the model are coastal plain sediments which include some fine material eluviation illuviation by percolating water delivery of additional fine and mixed grain size material to surface by bioturbation making it available for translocation concentration of fine material originally scattered throughout the parent material in a b horizon and maintenance of vertical moisture fluxes by bioturbation the model is supported by morphological evidence of the key mechanisms argillic horizons that are finer than both the surface layers and underlying parent material evidence that argillic horizon formation is not limited by the rate of clay synthesis and the absence of texture contrasts in nearby soils formed from dune sands which lack fines

re evaluation of the petroleum potential of the kufra basin se libya ne chad does the source rock barrier fall abstract the kufra basin is a large underexplored palaeozoic intracratonic sag basin in se libya and ne chad with extensions into nw sudan and sw egypt the basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to cretaceous geologically the basin is very similar to the murzuq basin in sw libya which recently presented libya with its largest oil discovery for over a decade most of the hydrocarbon play elements known from the murzuq basin also occur in the kufra basin thick porous cambro ordovician sandstones are present and would form good reservoirs lower silurian shales may act as effective seals and there are potential structural traps in seismically defined fault blocks however the source rock availability in the kufra basin is currently unclear one of the two main source rock candidates in the basin is a lower silurian shale unit tanezzuft formation the tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by agip between 1978 and 1981 hot shales developed at the base of this widespread silurian shale unit form important source rocks in many areas of north africa and arabia these hot shales are interpreted to have been deposited in palaeodepressions such as incised valleys of the preceding lowstand or intrashelf basins during the initial transgression after the melting of the late ordovician ice cap the areal distribution of the organic rich unit is therefore discontinuous fieldwork in the kufra basin has shown that the basal tanezzuft horizon is not exposed on the northern and eastern margins of the basin deep infracambrian rift grabens have been interpreted on seismic lines from the kufra basin and in analogy to oman and algeria could contain organic rich infracambrian deposits the infracambrian succession in the kufra basin may contain a second major potential source rock and warrants further investigation

structural segmentation inversion and salt tectonics on a passive margin evolution of the inner kwanza basin angola the kwanza basin angola is divided into the inner and outer kwanza salt basins separated by a chain of synrift basement highs on which aptian 112 122 ma salt is thin or absent north to northwest trending basement structures in the inner kwanza basin have repeatedly been reactivated since neocomian 144 127 ma rifting reactivation formed three northwest striking fold and thrust belts near basement uplifts the thrust belts are bounded by northeast striking rift related transfer fault zones that were apparently reactivated during subsequent shortening three episodes of postrift basement involved shortening are documented in the inner kwanza basin 1 albian early cenomanian 112 96 ma 2 senonian 89 65 ma and 3 oligocene holocene 34 0 ma we relate the albian early cenomanian event to ridge push the senonian event to global plate reorganization and oligocene holocene shortening to uplift of the african superswell structural segmentation of the inner kwanza basin controlled the evolution of salt structures adjacent to basement uplifts diapirs were initiated as buckle folds some anticlines were unroofed by erosion and evolved into passive salt walls elsewhere broad salt walls were triggered by either detached extension or basement block uplift these walls grew until they exhausted their supply of salt thereafter dissolution rates exceeded rates of salt inflow so the walls began to subside withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous trough fill ranges in age from cenomanian to pliocene and this age varies greatly from trough to trough and along strike within troughs

chronology and geodynamic setting of cretaceous cenozoic rifting in west and central africa abstract the development of the early cretaceous to palaeogene west and central african rift system which extends from nigeria benue trough to kenya anza trough can be related to the build up of intraplate tensional stresses during the break up of gondwana which caused reactivation of pre existing zones of lithospheric weakness repeated changes in the intraplate stress regime of africa are reflected by phases of crustal extension alternating with episodes of compression many of these events can be correlated with changes in rates of seafloor spreading in the central and south atlantic oceans as reflected in flowline patterns the west and central african rifts can be considered as typical passive rifts which evolved in response to the build up of intraplate stresses however the st helena hot spot appears to have been located beneath the equatorial plate boundary at approximately 120 ma and may have played an important role in weakening the lithosphere during extension

recent and old groundwater in the niebla posadas regional aquifer southern spain implications for its management summary the niebla posadas np aquifer in southern spain is one of the main groundwater sources for the lower guadalquivir valley a semiarid region supporting an important population agriculture and industry to contribute to the understanding of this aquifer the assessment of sustainable use of groundwater the residence time of groundwater in the np aquifer has been estimated using 3 h 14 c and 36 cl along the flow paths recharged groundwater mixes with nacl type waters and undergoes calcite dissolution and is further modified by cation exchange ca na consequently the water loses most of its calcium and the residual 13 c dic in the groundwater is isotopically enriched further modifications take place along the flow path in deeper zones where depleted 13 c dic values are overprinted due to so 4 2 and iron oxide reduction triggered by the presence of organic matter dating with 3 h 14 c and 36 cl has allowed the differentiation of several zones recharge zone 30 ky an apparent link between the tectonic structure and the groundwater residence time zonation can be established regional faults clearly separates deep zone 1 from the distinctly older age 30 ky deep zone 2 from the estimated residence times two groundwater areas of different behavior can be differentiated within the aquifer

rb sr isochron from gboko and ikyuen rhyolites and its implications for the age and evolution of the benue trough nigeria summary rhyolites overlying the basement granitic gneisses and granites adjacent to the selimentfilled benue trough were studied rb sr isochron age of 113 3 ma and initial 87 sr 86 sr rato of 0 7067 were obtained from two extrusive bodies which form gboko and ikyuen hills it is shown that their eruption has some relation to crustal doming at the onset of the benue rifting the published age of the oldest exposed sedimentary infill of the trough is albian as a result of the investigation of these volcanic rocks from the basement margin however an earlier and absolute age for the initiation of the rift is implied and a late barremian to early aptian age is proposed for the formation of this continental rift

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

using 3h and 14c to constrain the degree of closed system dissolution of calcite in groundwater abstract this study uses 3h concentrations 14c activities a14c 87sr 86sr ratios and 13c values to constrain calcite dissolution in groundwater from the ovens catchment se australia taken in isolation the 13c values of dissolved organic c dic and 87sr 86sr ratios in the ovens groundwater imply that there has been significant calcite dissolution however the covariance of 3h and 14c and the calculated initial 14c activities a014c imply that most groundwater cannot have dissolved more than 20 of 14c free calcite under closed system conditions rather calcite dissolution must have been partially an open system process allowing 13c and 14c to re equilibrate with co2 in the unsaturated zone recognising that open system calcite dissolution has occurred is important for dating deeper groundwater that is removed from its recharge area in this and other basins the study is one of the first to use 14c and 3h to constrain the degree of calcite dissolution and illustrates that it is a valuable tool for assessing geochemical processes in recharge areas

active concomitant counterclockwise rotation and northwards translation of africa during the albian campanian time a paleomagnetic study on the wadi natash alkaline volcanic province 104 78ma south eastern desert egypt abstract africa was a key tectonic unit in the geologic evolution of the tethyan realm in the middle late cretaceous during the rifting of the south equatorial atlantic ocean this research sheds light on the tectonic evolution of africa during the albian campanian time the two end members of the wadi natash alkaline volcanic province wnavp 104 78 ma in the south eastern desert of egypt were studied paleomagnetically the wnavp 24 5 n 34 25 e is made up of a thick succession of differentiated lava flows grading from alkali olivine basalt aob followed by hawaiite mugearite and benmoreite flows with abundant sandstone interbeds all intruded by a large number of trachyte phonolite tr ph circular lava domes and ring dykes forming conspicuous landmarks in the province the oldest flows of the aob 104 7 ma and the youngest tr ph plugs and ring dykes 86 78 ma as well as the interbedded turonian taref sandstone formation previously known as nubian sandstone were sampled all over the province which covers more than 500 km 2 the isothermal remanent magnetization irm study pointed to magnetite family as the major remanence in wadi natash volcanics with little contribution from goethite hematite minerals on the other hand goethite and or hematite are the dominant remanence carriers in the interbedded taref sandstone after the progressive stepwise thermal demagnetization of all samples the characteristic remanence chrm direction for each rock type was visually isolated and its best fit line was calculated followed by the site means rock unit means and the paleomagnetic poles this paleomagnetic study revealed that 1 in the tilt corrected coordinates the mean chrm of the oldest aob flows n 12 sites was dec inc 340 10 k 54 95 6o yielding a north pole at 55 n 250 e the chrm of the youngest tr ph ring dykes and plugs n 10 sites was dec inc 354 3 5 k 60 95 6 3o yielding a north pole at 66 5 n 229 e 2 on the other hand the site mean chrms of the taref sandstone interbeds n 8 sites in the present day in situ coordinates are clustered around the present day geomagnetic field with a mean dec inc 3 33 k 37 95 9 3 yielding a north pole at 83 n 191 e the two paleomagnetic poles of the aob and the tr ph are in accordance with albian campanian poles from africa including madagascar and those rotated from the main cratons such as the north american craton europe including greenland and iberia south america and india using the published plate kinematics and paleomagnetic euler pole pep rotation parameters paleogeographically the two obtained poles of wadi natash volcanics place the african plate at two different azimuths and paleolatitudes during albian cenomaniam and santonian middle campanian tectonically these two paleomagnetic poles point to an active synchronous northwards translation associated with a counterclockwise rotational convergence of africa towards eurasia during the albian campanian time between the time of the aob eruption 104 7 ma and the intrusion of the tr ph 86 78 ma africa concomitantly rotated counterclockwise about 14 and translated northwards towards eurasia about 6 of latitude