growth of megaspherulites in a rhyolitic vitrophyre megaspherulites occur in a rhyolitic vitrophyre near the base of a thick rhyolitic vitrophyre that occupies a late eocene to early oligocene volcanic tectonic basin near silver cliff colorado diameters of the megaspherulites range from 0 3 to over 3 66 m including an alteration layer the megaspherulites are compound spherulites they are holocrystalline very fine to fine grained composed of sanidine and quartz and surrounded by a thin sanidine quartz rind and an alteration layer containing mordenite and 15 a montmorillonite megaspherulite crystallization began soon after the vitrophyre was emplaced as the result of sparse heterogeneous nucleation under highly nonequilibrium conditions spherulite growth proceeded in a diffusion controlled manner under lowered viscosity as a result of a high water content 5 to 7 wt which also contributed to sparse nucleation when nucleation did occur it was at a large t 245 350 c on near critical sized nuclei once the spherulites began to grow continued nucleation was on existing growth cones and new independent spherulites did not form sanidine crystals grew with a fibrous habit which is favored by a large t restricting fibril lengths and diameters between 10 30 and 3 7 m respectively during crystallization these growth cones impinged upon each other resulting in fibril cone free areas these cone free interconal areas consist of coarser fine grained phases dominated by quartz which crystallized from the melt as it accumulated between the crystallizing sanidine fibrils of the cones the anhydrous nature of the sanidine and quartz suggests that their crystallization concentrated a water rich residual fluid enriched in ca mg and fe and depleted in si k and na at the megaspherulite vitrophyre interface this fluid phase enclosed the megaspherulites resulting in the hydrothermal alteration of the vitrophyre forming a hydrothermal alteration layer as indicated by the presence of the minerals mordenite and montmorillonite

sedimentation and recent history of a freshwater wetland in a semi arid environment loboi swamp kenya east africa loboi swamp is a 1ae 5k m 2 freshwater wetland situated near the equator in the kenya rift valley the climate is semi arid precipitation is 700 mm year 1 and evapotranspiration is 2500 mm year 1 some of the wetland water is currently used for irrigation an interdisciplinary study was conducted on the geology hydrology pedology and biology of the wetland to determine its origin and history and to assess its longevity under present hydrological conditions sedimentary records from two piston cores 1ae8 and 4 m long indicate that the present wetland developed during the late holocene on a lowrelief alluvial plain floodplain deposits sandy silts are capped with wetland sediments organic rich clay and peat which began to form at 700 bp the swamp is dominated by typha domingensis pers 80 and floating cyperus papyrus l 20 it is fed by warm springs t 35 c ph 6ae4 6ae9 emanating from grid faults of the rift floor water compositions suggest that sources are dominated by shallow meteoric water with little contribution from deeper geothermal fluids siderite concretions in the floodplain silts reflect the fe reducing conditions that developed as the surface became submerged beneath the water table the pollen record captured both local and more regional vegetation showing the prevailing dry rift valley climate despite development of the wetter conditions on the valley floor the diatom record also suggests a dramatic change in local hydrology the combined biological records of this semi arid wetland indicate an abrupt change to wetter conditions most probably as a result of a regional change in climate rift tectonics provided accommodation space maintained the wetland at or below the water table and enabled spring recharge the size of the modern wetland has been reduced by about 60 since 1969 which suggests that the system may now be under hydrological stress due to anthropogenic impacts from land use change

formation of alunite jarosite and hydrous iron oxides in a hypersaline system lake tyrrell victoria australia abstract alunite kal 3 so 4 2 oh 6 and jarosite kfe 3 so 4 2 oh 6 are common weathering products of aluminosilicates and pyrite long range transport of the constituents forming these minerals and the subsequent deposition of alunite and jarosite in an evaporite setting have not been previously documented such conditions for the occurrence of alunite and jarosite were investigated in a hypersaline system where acid groundwater enriched in k al fe iii and so 4 enter a saltplaya lake sediment cores were studied by thin section xrd and sem eds groundwater and pore water chemistries were also analyzed the results show that alunite and jarosite occur together or as individual layers and cements within the top 20 cm of the sediments where the groundwater is most concentrated due to evaporation hydrous fe oxides also occur as cements or vein fillings with the alunite and jarosite but are distributed throughout the 70 cm sediment column studied the results are consistent with a model in which alunite and jarosite precipitate as a result of the evaporation of water thermodynamic modeling indicates that the pore water in the playa sediments maybe in equilibrium with alunite jarosite and hydrous fe oxides

microbial control on the precipitation of modern ferrihydrite and carbonate deposits from the cezallier hydrothermal springs massif central france abstract actively precipitating hydrous iron oxides and carbonates are commonly associated with cool co2 rich spring waters in the massif central france chemical isotopic and mineralogical analyses and thermodynamic calculations were undertaken to elucidate the processes involved in the formation of these deposits hydrous iron oxide particles rich in al si ca and as are the first to form in the springs accompanying the iron oxides are layered carbonate deposits composed mainly of well crystallised calcite sem examination of both oxides and carbonates sampled at the outlet of the springs shows that they basically consist of grape shaped agglomerates of small 300 nm to 1 m anhedral crystals light acid etching of these agglomerates reveals colonies of spherical nannobacteria water trace partitioning between oxide and carbonate favours the hydrous oxide with as pb zn th concentration the microscopic features of both types of deposit indicate that microbial mediation probably contributes to their formation

pluvial conditions in the eastern sahara following the penultimate deglaciation implications for changes in atmospheric circulation patterns with global warming during the transition from the penultimate glacial to the last interglacial period isotope stages 6 5e or terminal ii large freshwater lakes abruptly infilled sandy depressions in arid regions of the eastern sahara geochemical and mineralogical studies of samples from lacustrine sequences in the western desert of southern egypt indicate 1 the sediments are chalk composed of authigenic chemical precipitates predominantly low mg calcite with carbonate contents averaging about 80 the low dilution of the chemical precipitate by detrital input implies that physical erosion and transport of detrital grains were minimized by extensive vegetation cover in the catchment area the fauna and flora associated with the lake deposits and middle paleolithic occupation sites on the paleo shorelines attest to a more amenable environment 2 vastly different rainfall or atmospheric circulation patterns than at present must have existed based on consistently negative 18opdb values for the chalk as low as 9 5 which would require precipitation from lake waters significantly more depleted in 18o than the modern local spring water 18osmow 1 8 3 the lakes were stable perennial water bodies with relatively long residence times sufficient to produce isotopic covariant signals in the chalks apparently the lakes disappeared as abruptly as they appeared without going through a terminal phase of evaporative enrichment extremely arid conditions again prevailed during the transition from the last glacial to the present interglacial terminal i a similar change from arid to pluvial conditions occurred thus it is proposed that during transitions from glacial to interglacial climates rapid global warming can lead to temporary alteration to atmospheric circulation with an intersification of the southwest monsoons bringing large quantities of isotopically light moisture to the eastern sahara

response of aeolian systems to holocene climatic and hydrologic changes on the northern margin of the sahara a high resolution record from the chott rharsa basin tunisia four millennial scale latest pleistocene to holocene climatic cycles are recorded by aeolian lacus trine strata along the southern margin of chott rharsa a playa situated within an active tectonic basin along the northern margin of the sahara desert in tunisia cycles are characterized by 1 a humid phase of dunefield stabilization and high lake levels and 2 an arid phase of playa desiccation and deflation with dunefield construction the cycles overall define a progressive drying trend and correlate well with a detailed chronology from morocco and with the sahara region in general for the latest pleistocene and early holocene climatic events appear more regional in nature for the later holocene an isolated outcrop of aeolian deposits dated at 86 ka bp argues for records of older cycles in the chott rharsa basin that were subsequently removed by long term deflation in the area

facies model of a semiarid freshwater wetland olduvai gorge tanzania the sedimentology and stratigraphy of a freshwater wet land in early pleistocene 1 75 ma volcaniclastic deposits olduvai gorge tanzania are characterized using texture mineralogy micro fossils and stratigraphic description to create a depositional facies model the freshwater wetland was located on the margin of a semi arid closed basin containing a playa lake the lake margin deposits are dominated by two distinct lithologies waxy claystones and earthy sediments waxy sediments are olive green dense claystones that con tain authigenic minerals such as trona and represent a fluctuating saline alkaline lake hay 1976 earthy sediments are beige friable siltstones that contain siliceous microfossils bone fragments and minor pebbles they represent the freshwater wetland microfossils from the earthy sediments and diatomites indicate freshwater marsh biota with some rare salt tolerant species associated lithofacies are diatomite carbonate and sandstone interpretation of vertical and lateral lithofacies variations in the 2 km 2 wetland based on 50 excavations identified seven distinct sub environments spring deposits perennial marsh pool ephemeral wet land fringing wetland wetland channel and ephemeral stream the stratigraphic sequence estimated to represent an 40 50 kyr long interval indicates that persistent 10 3 yr wet periods prevailed dur ing the deposition of thick waxy clays when the lake expanded and flooded the wetlands and equally long intervening dry periods caused the lake to contract allowing the flourishing of wetlands and accretion of earthy sediment the lithofacies associations in the olduvai paleo wetland provides an important first step in the development of a de positional environmental model for freshwater wetlands that can be tested elsewhere particularly in arid and semiarid settings

one million year old groundwater in the sahara revealed by krypton 81 and chlorine 36 measurements of 81 kr kr in deep groundwater from the nubian aquifer egypt were performed by a new laser based atom counting method 81 kr ages range from 2 10 5 to 1 x 10 6 yr correlate with 36 cl cl ratios and are consistent with lateral flow of groundwater from a recharge area near the uweinat uplift in sw egypt low 2 h values of the 81 kr dated groundwater reveal a recurrent atlantic moisture source during pleistocene pluvial periods these results indicate that the 81 kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrolugic problems

ages of quaternary pluvial episodes determined by uranium series and radiocarbon dating of lacustrine deposits of eastern sahara abstract as documented by radiocarbon dating and geoarchaeological investigations the now hyperarid northwestern sudan and southwestern egypt experienced a period of greater effective moisture during early and middle holocene time about 10 5 ka we have used the uranium series technique to date lacustrine carbonates from bir tarfawi bir sahara east wadi hussein oyo depression and the great selima sand sheet localities results indicate five paleolakeforming episodes occurred at about 320 250 240 190 155 120 90 65 and 10 5 ka four of these five pluvial episodes may be correlated with major interglacial stages 9 7 5e and 1 the 90 65 ka episode may be correlated with substage 5c or 5a our results support the contention that past pluvial episodes in north africa corresponded to the interglacial periods farther north ages of lacustrine carbonates from existing oases and from the sand sheet fail to indicate pluvial conditions between about 60 and 30 ka age results and field relationships suggest that the oldest lake and ground water deposited carbonates were much more extensive than those of the younger period and that carbonate of the latest wet periods were geographically localized within depressions and buried channels

mantle plumes and flood basalt stratigraphy in the parana south america geochemical studies of the parana continental flood basalts in brazil have led to the recognition of distinct magma types which have been used to infer the internal stratigraphy of the lava pile the overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying parana basin sedimentary rocks imply that the site of magnatism migrated 750 km toward the north during this volcanic event this has important implications for recent plume related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood basalt magmatism it is difficult to ascribe the magnitude and direction of the shift in the locus of the parana magmatism to movement of the brazilian lithosphere relative to the underlying tristan mantle plume instead it is suggested that the observed lava distribution and internal structure of the parana etendeka flood basalt province were imposed by the intracacies of the rifting process during the initial opening of the south atlantic ocean

groundwater in the wetlands of the okavango delta botswana and its contribution to the structure and function of the ecosystem abstract the okavango delta of northern botswana is a large 40 000 km2 alluvial fan located at the terminus of the okavango river the river discharges about 10 km3 of water onto the fan each year augmented by about 6 km3 of rainfall which sustains about 2500 km2 of permanent wetland and up to 8000 km2 of seasonal wetland interaction between this surface water and the groundwater strongly influences the structure and function of the wetland ecosystem the climate is semi arid and only 2 of the water leaves as surface flow and probably very little as groundwater flow the bulk of the water is lost to the atmosphere the okavango river also delivers about 170 000 tonnes of bedload sediment and about 360 000 tonnes of solutes to the delta each year most of which are deposited on the fan bedload is deposited in the proximal permanent wetland whilst much of the solute load is deposited in the seasonal wetland notwithstanding the high evapotranspirational loss saline surface water is rare between 80 and 90 of the seasonal flood water infiltrates the ground recharging the groundwater beneath the flood plains and the many islands on the flood plains the remainder is lost by evaporation this groundwater reservoir is transpired into the atmosphere by both aquatic vegetation on the flood plains and terrestrial vegetation on the islands and the water table is steadily lowered following passage of the seasonal flood trees which are almost exclusively confined to islands are particularly important as they lower the water table beneath islands relative to the surrounding wetlands there is therefore a net flow of groundwater towards islands accumulation of dissolved salts in this groundwater leads to precipitation of solutes mainly of silica and calcite in the soils beneath island fringes and the islands grow by vertical expansion islands are thus an expression of the chemical sedimentation taking place on the fan sodium bicarbonate accumulates in the groundwater beneath island centres and this impacts on the vegetation leading ultimately to barren island interiors dense saline brine thus produced subsides under density driven flow this cycling of seasonal flood water through the groundwater reservoir thus plays a key role in creating and maintaining the biological and habitat diversity of the wetland and inhibits the formation of saline surface water

introduction to this special section low frequency seismic geophysicists in the seismic exploration industry have long recognized the potential benefits of extending the bandwidth of seismic measurements to well below 10 hz low frequencies generated by a seismic shot travel much farther through the subsurface than do higher frequencies just as the low frequencies in thunder travel farther in the atmosphere thus the amount of power available in the low end of the seismic spectrum is especially important for deep exploration targets such as subsalt and sub basalt plays and for deep crustal studies in many areas of the world higher frequencies are often too attenuated scattered and dispersed to provide useful signal for probing such objectives also low frequencies play a critical role in the inversion process by which seismic data are converted into acoustic impedance sections and ultimately into reservoir properties surface seismic data usually lack low frequencies necessary for accurate inversion so they are commonly augmented by data from well log me

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

faunal remains from a middle pleistocene lacustrine marl in dakhleh oasis egypt palaeoenvironmental reconstructions abstract vertebrates and invertebrates associated with lithic artifacts are reported from a later middle pleistocene horizon in dakhleh oasis probably dating to isotope stage 7 this represents the first middle pleistocene fauna of this stage from a site in the egyptian western desert and demonstrates the presence of extensive permanent lakes along the margin of the libyan escarpment the fauna includes two freshwater snails limnaea stagnalis planorbis planorbis a catfish clarias sp two reptiles varanus cf griseus trionyx sp four water birds grus grus anas platyrhynchos anas sp small arenaria interpres and one land bird struthio camelus and probably eleven mammals phacochoerus aethiopicus hippopotamus amphibius camelus thomasi pelorovis antiquus damaliscus large antelope gazella sp large gazella cf dorcas small antelope equus capensis and possibly hyaena hyaena most of the fossils derive from a near shore marl deposit in a freshwater lake that occupied a depression whose northern boundary coincides with the present southern margin of the oasis in its eastern end casts of the stems of reeds are also present the animals represent a proximal freshwater tied fauna of obligatory aquatics and those who require freshwater during much of their activity a distal savanna fauna of grazers browsers or rooters who migrate to water on a diurnal cycle and probable transitory avian migrants attracted to the palaeolake in passage human presence is attested by flakes and bifaces found in situ

constraints on pleistocene pluvial climates through stable isotope analysis of fossil spring tufas and associated gastropods kharga oasis egypt abstract stable isotope analyses of fossil spring carbonates tufas and freshwater gastropods melanoides tuberculata from the currently hyperarid western desert of egypt indicate that this region received enough precipitation to support a small perennial lake during the height of the oxygen isotope stage 6 5e pluvial event and a substantial volume of spring discharge during prior pluvial phases tufa and gastropod oxygen isotope ratios are generally low relative to modern precipitation in the region confirming that the majority of pleistocene pluvial precipitation came from an atlantic source as determined by previous workers mean tufa 13c 2 pdb suggests a significant proportion of c4 vegetation in the recharge area of the tufa depositing springs while the paucity of tufas with 13c greater than 0 suggests that equilibrium between spring water and atmospheric co2 represented an endpoint for the evolution of spring water carbon though spring carbonate deposition requires a substantial increase in humidity in central egypt relative to modern conditions high variation in tufa 18o up to 4 within individual stratigraphic units as well as positive covariance between tufa carbon and oxygen isotope ratios reflect significant effects of evaporation on spring water during tufa formation similarity in 18o and 13c values of tufas from different stratigraphic units indicates that climatic conditions did not vary significantly among pluvial phases recorded by spring carbonate deposition the lack of substantial carbonate formation at tufa localities during the holocene pluvial phase however suggests that tufa deposition represents substantially wetter conditions than have existed during holocene time

evidence for a 200 100 ka meteorite impact in the western desert of egypt abstract in this study we present evidence in the form of unusual silicate glasses for a meteorite impact event 200 100 ka in the dakhleh oasis western desert egypt these glasses known locally as dakhleh glass were derived from the shock melting of a series of unconsolidated sediments underlain by interbedded carbonates sandstones and phosphate rich lithologies hypervelocity impact in to a volatile rich target resulted in the production of impact glasses with cao and al 2 o 3 contents of up to 25 and 18 wt respectively other notable properties include the presence of globules of immiscible calcite and pyrrhotite melt phases shattered quartz grains and fragments of silicified plant matter dating of geoarchaeological artefacts associated with the dakhleh glass support preliminary 40 ar 39 ar data indicating a 200 100 ka age for the impact event geoarchaeological evidence indicates that archaic homo sapiens and early modern humans continually inhabited this region of the western desert during this period while it is unclear at present whether the dakhleh glass was formed during a cratering event or a large aerial burst the effect on the environment and inhabitants of dakhleh would have been catastrophic

complexity of saharan paleoclimate reconstruction and implications for modern human migration abstract chronological and geochemical data from scarp and scarp foot depression sediments ssds have been used for deciphering saharan paleoclimates and patterns of migration of anatomically modern humans amh u th dating of thick accumulations of ssds indicates prevalent deposition during long lasting cool glacial periods marine isotope stage mis 6 8 10 and 12 and limited deposition during warm interglacial periods mis 5 7 and 13 in contrast quaternary sediments associated with short residence hydrologic systems in sinai yielded mis 2 osl ages of 27 7 to 10 2 ka the lack of ssds of mis 2 ages and the wide range of warm and cool stages in eastern sahara is attributed to 1 lengthy travel times of groundwater prior to discharge and deposition of ssds from deep groundwater originating from distant sources and 2 sampling and dating of ssds from depressions where natural discharge has been apparently continuous through wet and dry periods given that the previously dated depression ssds could have been deposited during dry periods and that the ages of ssds reflect the timing of groundwater discharge rather than the ages of the wet periods during which recharge occurred we suggest that earlier interpretations of saharan paleoclimate and amh migrations that heavily relied on dating of ssds should be revisited

effusive silicic volcanism in the parana magmatic province south brazil evidence for locally fed lava flows and domes from detailed field work abstract lava flows and dome complexes of silicic composition were identified in the lower cretaceous parana magmatic province pmp at rio grande do sul state southern brazil detailed mapping and image analysis reveals significant volumes of effusive deposits aligned according to main lineaments likely representing the fissural systems that fed the three palmas type silicic units different structures indicative of effusive emplacement lava domes lobated flows sheet flows and autobreccias are very common in the study area and are probably also more abundant than previously thought in whole pmp silicic magmatism in fact effusive deposits seem predominant in the three distinct silicic units identified in the area since no remnants of pyroclastic components have been identified the vitreous dacites that make up the upper flows of the basaltic andesite to dacite barros cassal sequence are clearly effusive as indicated by their occurrence as thin sheet flows the much thicker early caxias do sul dacites occur mostly as lava flow lobes and pancake like of low to moderate viscosity and lava domes the younger high sio2 santa maria rhyolite unit shows unequivocal examples of effusive deposits at its lower portion as lobated flows formed by vesicle rich obsidian in spite of higher viscosities relative to the previous units 106 pa s it is probable that the very low h2o contents 1 wt of these rhyolite melts associated with high discharge rates resulted in an effusive nature in most to this unit

assessing long term habitability at an eastern sahara oasis esr dating of molluscs and herbivore teeth at dakhleh oasis egypt abstract in the northeastern sahara electron spin resonance esr dating of when animals lived documents their habitability in dakhleh oasis egypt a middle pleistocene paleolake s covered 1700 km 2 at eastern locality dak348 10 m thick remnant lacustrine marls yielded pleistocene fauna rare artefacts and plant casts no obvious unconformity exists within these deposits from upper horizons a hartebeest tooth esr dated at 195 11 ka correlates with marine isotope stage mis 7 while molluscs from a stratigraphically higher horizon averaged 89 10 ka correlating with mis 5a b at western locality dak006 upslope deflation has left a temporally mixed surficial lag numerous lagged tooth fragments independently dated by esr correlate with mis 5 through 17 fragments from a slope sand unit correlate with mis stages 3 through 6 one bovid tooth associated with younger middle stone age artefacts in the base of the sand dated at 84 7 ka mis 5a b molluscs from romano byzantine backdirt at a breached artesian vent dated to 8 15 1 ka suggesting that ponds formed during mis 1 and 2 even without well defined sedimentary contexts esr frequency data indicate that the oasis was habitable for herbivores during at least twelve stages in the mid late quaternary and therefore likely also for humans

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

the eucarro rhyolite gawler range volcanics south australia a 675 km3 compositionally zoned lava of mesoproterozoic age the eucarro rhyolite is remarkable in being far more voluminous 675 km3 and extensive than most felsic lavas 1 km3 its dimensions are comparable to the largest known volcanic units felsic ignimbrites and flood basalts it is one of several voluminous felsic units within the mesoprot erozoic ca 1592 ma gawler range volcanics an intracontinental intraplate volcanic province in south australia and is typical of the voluminous felsic magmatism associated with laurentia cf belt supergroup midcontinent united states the lava is 300 m thick with groundmass textures and lithofacies characteristics consistent with cooling and degassing as a single unit there are gradational compositional and textural variations throughout its highly elongate distribution 225 km east west 74 wt sio2 pyroxene is the dominant ferromagnesian phase but at the base in the west primary amphibole and olivine are present granitic and mafic clasts are ubiquitous but most abundant 5 vol in western and eastern parts of the unit the main eucarro magma was compositionally zoned and several smaller batches of compositionally distinct magma were tapped during the eruption concentrations of granitic clasts coincide with changes in bulk composition and may record the rupture of wall rock between nearby but separate magmas the distribution subtle lateral variations in composition and texture and local presence of compositional flow banding along the northern margins suggest that the eucarro rhyolite was derived from several source vents located along an east trending fissure system although very widespread outflow of the eucarro lava was probably no more than a few tens of kilometers from source vents

stratigraphy and volcanic facies architecture of the torres syncline southern brazil and its role in understanding the parana etendeka continental flood basalt province abstract the torres syncline is a large structure that constitutes the eastmost outcrop of the parana etendeka cfb in south american side and this work focuses the stratigraphy and facies architecture of the volcanic pile in the syncline the volcanic sequence along the study area permits the division of three regions main valley intermediate zone and south hinge each of them with distinct stratigraphy which probably reflects the structural evolution of the syncline the stratigraphy of the torres syncline is composed by 1 botucatu palaeoerg 2 basic volcanic episode i 3 basic volcanic episode ii 4 acidic volcanic i 5 basic volcanic episode iii and 6 acidic volcanic episode ii the five volcanic episodes recognized in study area can be related to five volcanic facies architecture compound braided tabular classic tabular lobate escoriaceous dome field acidic lavas and tabular flows acidic lavas the basic episode i is composed by pahoehoe flows with a compound braided facies architecture that covered the botucatu palaeoerg the basic episode ii is a tabular classic facies architecture predominantly composed by simple flows 10 20 m thick reaching the total thickness of 500 m in main valley the acidic episode i is exposed in main valley and south hinge and is composed by acidic lavas forming lava dome field facies architecture with a thickness of 150 m the basic episode iii is predominantly constituted by a flows with tabular lobate escoriaceous facies architecture the acidic episode ii is constituted by tabular flow volcanic facies acidic flows and outcrops all along the study area the torres syncline constitute the eastmost on shore exposures of the parana etendeka cfb in south american side and detailed stratigraphic volcanological and structural studies in these area coupled with correlation with huab basin nw namibia africa will aim the understanding of the gondwana breakup process and the early stages of the south atlantic margin opening

pan floor barchan mounds ntwetwe pan makgadikgadi botswana their origin and palaeoclimatic implications abstract the variety of forms assumed to be dunes and palaeolakes of northern botswana have long been perceived as geoproxies for the elucidation of quaternary palaeoclimates mounds on the floor of ntwetwe pan have been regarded as modified barchans pertaining to a time in the past when conditions were drier than at present the morphology hydrogeology and biology of these placed in the wider context of the palaeolacustrine hinterland indicate that they are not dunes but subaqueous features spring mounds the morphology is inconsistent with barchans conspicuous features are the elevated eyes within the enclosing arms of the crescentic forms that occur furthest from the pan edge seepages discharge from their flanks commonly about 7 m higher than the dry saline pan floor on the top of forms nearer the pan margins permanent pools of potable water occur similarly elevated above the floor hydraulic pressure to sustain the upward discharge of groundwater is provided by the excellent recharge through silicate karst depressions in the hinterland particularly those of the 936 m palaeolake floor groundwater discharge still operating today was formerly much stronger and associated with lacustrine conditions a chronological framework is provided by archaeological remains on the palaeolake floors of the hinterland these indicate that the oldest palaeolake floor pertaining to the quaternary is the 920 m level periods of spring mound formation are associated with both this and the 912 m palaeolake the implications for future luminescence dating targeting palaeoenvironmental reconstruction are discussed in the light of this new model of mound formation

wetland diagenesis and traces of early hominids olduvai gorge tanzania lateral variations in whole rock and clay geochemistry of basal bed ii claystones in olduvai gorge tanzania reflect water quality differences across the eastern lacustrine plain 1 75 myr ago bulk ba sr and na2o k2o mgo al2o3 range from 1 4 to 4 2 and from 0 7 to 1 4 respectively and indicate leaching of lacustrine claystones beneath freshwater wetlands at times following lake retreat bulk mgo al2o3 0 3 1 0 and molar mg al 0 5 3 9 ratios of 0 2 m clays reflect alteration of mg rich lacustrine clays these indicators point to freshest conditions near locality 43 of hay 1976 hwk east leakey 1971 moderate conditions to the east locality 40 mck and high salinity and alkalinity to the west localities 85 vek 45 flk clay geochemistry and artifact abundances are well correlated r 0 67 p 0 005 suggesting a relationship between paleo water quality and hominid paleoecology this pattern is consistent with predictions of greatest artifact discard loss around freshwater sources where scavanging opportunities were greatest for hominids this quantifies a relationship between artifact density distribution and a paleoecological proxy over landscape scales for the first time in early stone age archaeology in contrast fossil bone abundance is uncorrelated r 0 14 p 0 6 reflecting more complex taphonomic processes quantitative tests of landscape scale land use models are important for understanding early hominid behavior and its evolution

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

analysis of coleopterans from late holocene natural spring deposits in south central sweden abstract the usage history of a natural spring deposit in associated glacial till was investigated via analysis of coleopteran remains found in peat this material was recovered during archaeological excavations in a spring outflow in south central sweden the geographical region of southern sweden has been heavily affected by uplift following the terminal glaciation weichselian in scandinavia with the study area for this project having risen above sea level about 1200 bc the spring was found in connection to the northern part of a large settlement and religious area dating from the younger bronze age and pre roman iron age 1100 300 bc however the area has been used for agricultural land up until the present day sampling was conducted for interpretation of the usage history of the site with the insect fragments showing exceptional preservation as a result of unusual conditions in the sediments 14 c dating from the lower to upper part of the spring yielded a stratigraphic range of 670 870 to 660 810 ad respectively a half circle boulder construction provides clear evidence that the spring was used early in prehistory probably during the bronze age and potentially excavated at irregular periods the coleopteran assemblage was dominated by beetles indicative of open landscapes grazing land and forested environments this closely resembles the vegetation setting of today in which the spring is situated on the border between arable land and mixed deciduous and coniferous forest a high frequency of carabid beetles suggests that the spring may have functioned as a pitfall trap and correlated well with the relative inorganic to organic compound composition of the ground conditions the spring was probably abandoned in the late iron age 600 800 ad identification of two red listed threatened species that are not present in the current fauna implies that radical change in the agricultural landscape over the last 1200 years could be negatively affecting diversity dynamics within the local fauna

uranium isotopic study of artesian and pluvial contributions to the nubian aquifer western desert egypt the groundwater systems of the nubian aquifer beneath six major oases baris kharga dakhla farafra bahariya and siwa in the western desert of egypt have been studied to establish their sources and mixing volumes using uranium isotopes one hundred six groundwater samples from different depths of the nubian aquifer have been analyzed for uranium content and 234u 238u activity ratio ar the aquifer under the western desert is known to have more than one source of water at bahariya and farafra oases the nubian artesian water migrating from the south has been augmented by local recharge during pluvial times at siwa oasis in the northwestern desert a shallower aquifer component migrating from the north or west is also present at dakhla kharga and baris oases in the southwestern desert the main source is the nubian artesian water migrating from southeast uweinat upland and northwest sudan these water masses have distinctive uranium isotopic signatures the nubian aquifer water is characterized by very low u concentrations 1 5 the shallow northwest aquifer water also has a high ar but a much higher u concentration the locally recharged pluvial waters have high u concentrations 0 1 ppb but low ars near unity a diagnostic derivative parameter is excess 234u content the deep nubian aquifer water is characterized by a relatively low excess 234u 0 02 ppb equivalent while the shallow siwa water has a very high excess 234u ar 1 u conc the bahariya and farafara waters are also high in this component probably because of pluvial water percolation through phosphate rich strata at all oases u isotopic mixing diagrams show that the deep aquifer sources predominate however the pluvial contributions are significant ranging from about 5 at remote baris oasis to about 26 at the more northerly farafra oasis the observed lowering of potentiometric surfaces in the western desert is caused not only by pumping at a rate greater than inflow from the aquifer systems but also by the withdrawal of pluvial water which in modern times is not being replaced at all

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

dating lacustrine episodes in the eastern sahara by the epimerization of isoleucine in ostrich eggshells the eggshell of the african ostrich struthio camelus closely approximates a closed system for the retention of indigenous proteinaceous residues epimerization of the protein amino acid isoleucine follows linear first order kinetics in laboratory simulations nearly to racemic equilibrium and the variation in d l ratio within a single fragment or between fragments of the same age is significantly less than in other carbonate systems these observations suggest that the extent of isoleucine epimerization aile ile ratio in ostrich eggshell offers the potential for high resolution geochronology of quaternary deposits from the simulation experiments and dated early holocene samples for which we have in situ mean annual sediment temperature measurements arrhenius parameters have been calculated the activation energy is 30 33 kcal mol 1 similar to that of other carbonate systems we have measured the aile ile ratio in ostrich eggshell associated with lacustrine episodes at bir tarfawi and bir sahara east two depressions in what is currently the hyperarid eastern sahara the ratios can be used directly to indicate qualitatively the time represented by each series of lake sediment and to correlate disjunct lacustrine deposits within and between the basins uranium series disequilibrium dating of algal mats contained within some of the lake beds indicate that a major wet interval occurred about 130 ka ago using the u series date for calibration the amino acid ratios are used to date the most recent lacustrine interval to about 100 ka b p and two older intervals one about 200 25 ka b p and an older interval that occurred prior to 250 ka ago

sedimentology of acid saline lakes in southern western australia newly described processes and products of an extreme environment naturally acid saline systems with ph values between 1 7 and 4 are common on the yilgarn craton of southern western australia a combination of physical and chemical processes here yield a previously undescribed type of modern sedimentary environment flooding evapoconcentration desiccation and eolian transport at the surface as well as influx of acid saline groundwaters strongly influence these lakes halite gypsum kaolinite and iron oxides precipitate from acid hypersaline lake waters shallow acid saline groundwaters affect the sediments of the lakes and associated mudflats sandflats channels and dunes by precipitating early diagenetic halite gypsum iron oxides clays jarosite and alunite these modern environments would likely yield a rock record composed mostly of bedded red siliciclastic and reworked gypsum sand alternating with less common beds of bottom growth gypsum and halite with alteration by early diagenetic features diagnostic of acid saline waters this documentation of sedimentary processes and products of modern acid saline environments is an addition to the comparative sedimentology knowledge base and an expansion of the traditional models for classifying brines implications include better interpretations of terrestrial redbeds and lithified martian strata improved acid remediation methods new models for the formation and occlusion of pores and the new setting for finding previously undescribed extremophiles

esr in the 21st century from buried valleys and deserts to the deep ocean and tectonic uplift abstract electron spin resonance esr dating can date many materials including hydroxyapatite in enamel and some fish scales aragonite and calcite in corals molluscs some travertine and calcrete and quartz from ash fluvial deposits and some flint dating studies using these materials have numerous potential applications in many varied quaternary settings esr dating uses signals resulting from trapped charges created by radiation in crystalline solids ages are calculated by comparing the accumulated radiation dose in the dating sample with the internal and external radiation dose rates produced by natural radiation in and around the sample and produced by cosmic radiation when compared to other dating techniques age agreement has been excellent for teeth corals molluscs and quartz recent improvements have included using a more complex modelling technique to calculate the cosmic dose rates and more detailed modelling techniques for dealing with variable external dose rates methods in development include using quartz from buried fluvial valleys to date geomorphic surfaces and using the signals in barnacles and benthic foraminifera for dating fossils or their associated sediment new chronometer applications recently developed include using coral and mollusc dates to build sealevel curves and to monitor volcanic activity and tectonic uplift using tooth and mollusc dates to assess water availability in deserts and using isochron data to assess u uptake processes into teeth when coupled with other geochemical and geomorphological techniques esr can provide the chronometric control to build paleoclimatic and other paleoenvironmental records many other applications are possible from heating studies for artefacts to dating sulphates and other minerals on distant planets

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

cosmogenic radiogenic and stable isotopic constraints on groundwater residence time in the nubian aquifer western desert of egypt measurements of radiochlorine sup 36 cl radiogenic noble gases sup 4 he and sup 40 ar and stable chlorine isotope ratios were obtained to assess the residence time of groundwater in the nubian aquifer of the western desert of egypt measured sup 36 cl cl ratios yield apparent residence times from approx 0 2 to 1 2 x 10 sup 6 years in the deep 600 1200 m groundwater assuming constant cl and le 0 16 x 10 sup 6 years in the shallow 600 m groundwater values of delta sup 37 cl in the groundwater strengthen the application of the sup 36 cl dating method by constraining cl sources and identifying groundwater mixing dissolved gases were measured in some of the deep groundwater samples measured sup 4 he concentrations indicate accumulation of radiogenic sup 4 he that is qualitatively consistent with the age progression indicated by the sup 36 cl cl ratios but the flux of external sup 4 he from the underlying crust has not been quantified and is not constant throughout the aquifer concentrations of sup 40 ar range from 3 3 to 6 7 x 10 sup 4 ccstp g and indicate excess air incorporation at recharge measured sup 40 ar sup 36 ar ratios do not exceed the atmospheric ratio a two dimensional numerical hydrodynamic transect of the aquifer was modeled from the areamore of the uweinat uplift to the northern bahariya oasis predicted groundwater velocities in the deep portion of the aquifer are 0 5 3 5 m yr with groundwater residence times up to 9 x 10 sup 5 years residence times up to 1 3 x 10 sup 6 years are predicted in the confining shale aquifer properties are estimated by using the model to fit the measured sup 36 cl cl ratios under these conditions hydrodynamic residence times are within about 30 percent of those calculated from sup 36 cl when mixing of cl sup is accounted for in the highest cl sup deep groundwaters by mutually calibrating multiple methods hydrodynamic sup 36 cl and sup 4 he a consistent picture of the nubian aquifer has emerged in which lateral flow from a southern recharge area dominates the deep horizons while shallow horizons contain younger autochthonous recharge less

paleoenvironmental reconstruction and water balance of a mid pleistocene pluvial lake dakhleh oasis egypt mid pleistocene lacustrine sediments exposed as erosional remnants in dakhleh oasis in the western desert of egypt provide evidence for a more humid climate than that which persists today the sediments range in facies from palustrine to fully lacustrine and generally reflect an increase in water depth upward through the strata ironstone spring deposits associated with the lacustrine sediments and modern artesian discharge in the oasis indicate that nubian aquifer water was probably an important component of the water balance but the high carbonate content of the lake sediments requires the input of surface drainage derived from the nearby limestone capped libyan plateau although definitive shorelines have not been observed a maximum lake size of 1735 km2 is derived from extrapolation of the elevations of the deposits onto the modern deflated topography theoretical water balance modeling suggests that in addition to the discharge derived from the nubian aquifer 410 860 mm yr of rainfall would be required over the dakhleh oasis drainage basin in order to maintain the lake at hydrologic steady state the persistence of climatic and hydrologic conditions amenable to maintaining a shallow freshwater lake would have allowed the region to be inhabited by a savanna fauna as well as early human groups

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

age and isotopic constraints on pleistocene pluvial episodes in the western desert egypt abstract north africa has undergone drastic climatic changes over the past several hundred thousand years the timing of humid intervals called pluvials was investigated by uranium series disequilibrium dating of travertines from the kurkur oasis western desert egypt the youngest and best dated travertines 70 160 ka are found in wadi kurkur and include spring and lacustrine units exposed as 2 to 3 m high terraces travertines having an age of approximately 191 220 ka are exposed by differential erosion as linear mounds produced by spring systems over fracture zones in ancient wadis the oldest travertines having ages 260 ka are extensive cap limestone units above the oasis and were deposited in paludal and lacustrine environments oxygen isotope ratios were measured for the wadi travertines 18o values ranging from 16 7 to 19 1 smow and for spring mound travertines 18 5 20 5 equilibrium oxygen isotope fractionation calculations indicate that the kurkur travertines were deposited from waters having 18o values similar to ancient western desert groundwaters 11 sonntag et al 1978a b thorweithe 1990 sultan et al 1996 the ages of the travertines correspond to times of monsoonal maxima eustatic sea level high stands and interglacial maxima rainfall producing these groundwaters and travertines was significantly fractionated during atmospheric transport in contrast to modern meteoric waters 2 09 joseph et al 1992 implying a distant source for the pluvial waters increased precipitation recharge of western desert groundwaters and resultant travertine deposition are interpreted to be consequences of milankovitch insolation cycle forcing through enhanced atlantic and indian ocean monsoons during interglacial time periods

comparison of the composition of iron pans in danish burial mounds with bog iron and spodic material abstract excavations of danish burial mounds dated to the south scandinavian early bronze age 1700 1000 bc occasionally show thin iron pans encapsulating the interior of the mounds when strongly cemented these pans can be classified as placic horizons the encapsulated core was often wet and anaerobic and in jutland well preserved oaken log coffins with remnants of organic artefacts and human beings were discovered various theories for the development of these pans have been proposed the chemical composition of the cement of two iron pans in burial mounds is compared with that from a bog iron deposit and a spodic horizon this shows that the cement from the iron pans is similar to that in bog iron and not to the spodic material

late quaternary paleohydrology of the central atacama desert lat 22 24 s chile in northern chile precipitation in the high andes 3500 m recharges groundwater systems that flow down the pacific slope and feed large aquifers in the hyperarid atacama desert wetlands which are often found along the base of the andes mark locations where the water table intersects the land surface at these locations paleo wetland deposits which are present as terraces between 3 and 20 m above modern wetlands record past water table heights along the andean front and are used to reconstruct changes in groundwater discharge paleo wetland deposits in the central atacama desert lat 22 24 s record an episode 15 4 9 ka of high water tables followed by an episode 8 3 ka of moderately high water tables elevated water tables result from increased groundwater discharge and ultimately from enhanced recharge in the andes the concordance of results from three separate hydrologic systems suggests that changes in groundwater discharge and recharge are regional and reflect climatic fluctuations this interpretation is supported by close agreement with other paleoclimatic records in the region the periods of greater groundwater discharge were separated by episodes 9 8 and 3 0 ka of significant groundwater lowering and stream incision implying greatly diminished discharge the central atacama and andes lat 22 24 s receive precipitation mainly from moist air masses transported from the amazon basin by the south american summer monsoon sasm increases in groundwater recharge are therefore thought to reflect an increase in the frequency and or moisture content of sasm air masses crossing the andes fluctuations in sasm precipitation have previously been linked to summer insolation in the southern hemisphere the wettest period in the central atacama 15 4 9 ka however coincides with a minimum in austral summer insolation at 10 ka suggesting that regional summer insolation is not a dominant influence on the sasm instead intensification of the sasm may be linked to extraregional forcings such as the walker circulation

fossil spring deposits in the southern great basin and their implications for changes in water table levels near yucca mountain nevada during quaternary time fossil spring deposits are common in the southern great basin and their distribution provides important constraints on the hydrologic response of the regional water table to climate change this information is crucial because the proposed high level nuclear waste repository at yucca mountain will be located 200 400 m above the modern water table water tables will rise in response to a future return to glacial climates but the magnitude of the change and the consequences for radionuclide travel times and overall repository integrity are key uncertainties increased recharge during past pluvial periods in the spring mountains and sheep range caused water tables to rise and ground water to discharge over broad expanses of the las vegas valley system and in nearby pahrump sandy and coyote springs valleys in contrast other valleys in the region contain only small areas of pleistocene discharge resulting from local damming of ground water by faults that cut valley alluvium in these instances which include the valley wells area and piute and northern coyote springs valleys smaller ranges such as the clark and new york mountains supplied the moisture the change in water table levels since the last full glacial period varies between and within valleys from as little as 10 m in several areas to 95 m in the coyote springs valley at yucca mountain the water table has probably changed by 115 m in response to climate change the spring deposits and the mollusk faunas found with them often misinterpreted as lacustrine in origin share many essential features with active spring systems in northeast nevada deposits associated with discharge mainly consist of pale brown silt and sand that is entrapped by dense stands of phreatophytes covering valley bottoms when water tables are high pale green mud containing a mix of aquatic semiaquatic and moist terrestrial mollusks accumulates in wet meadows and marshes associated directly with spring discharge the record in subbasins of the las vegas valley system is dominated by late wisconsin age sediments although pond sediments and alluvium belonging to at least one older pre wisconsin pluvial period are also locally exposed deposits from two even earlier episodes of spring discharge both of which also occurred during rancholabrean time 10 to 450 ka are exposed in the pahrump chicago piute and coyote springs valleys and in the valley wells the records from the pahrump and coyote springs valleys are especially well exposed and likely extend back to at least early pleistocene time

paleolandscape and paleoenvironmental interpretation of spring deposited sediments in dakhleh oasis western desert of egypt abstract iron rich sediments in dakhleh oasis western desert of egypt have been recognized as spring mounds and as archaeological sites where paleolithic materials have been recovered detailed sedimentologic investigation and mapping of these features reveal that spring mound sediments were deposited in a shallow vegetated wetland formed by the discharge of iron rich nubian aquifer waters along the southern margin of the oasis controlled largely by localized faulting and the variable paleotopography of the basin floor the spring sediments represent peri lacustrine or lake independent features and can be differentiated from fully lacustrine deposits on the basis of their sedimentary characteristics as well as the presence of goethite and jarosite in a region where authigenic deposition during pleistocene pluvial activity principally resulted in tufas and lacustrine marls spring mound formation incorporated sediment through the ponding of surface water aeolian entrapment by local vegetation and the formation of iron precipitates in a low energy oxidative and acidic environment that was not dependent upon surface water inputs into the dakhleh basin the potentially interpluvial nature of this water resource makes it an important sedimentary archive for archaeological investigations in the basin the dakhleh spring mounds record a unique groundwater controlled paleoenvironment providing the first evidence of a bog iron in egypt and one of the few occurrences of iron rich wetland remnants in the modern sahara

holocene paleoenvironmental paleoclimatic and geoarchaeological significance of the sheikh el obeiyid area farafra oasis egypt abstract the present paper focuses on the combination of archaeological and geological data sets in order to obtain a synthesis of holocene occupational and environmental history in the sheikh el obeiyid region of the farafra depression egypt in this region the epipalaeolithic and saharan pastoral neolithic sites are distributed along different landscape elements of the sheikh el obeiyid plateau escarpment and bir el obeiyid depression respectively three phases of playa formation were recorded at the bir el obeiyid basin and at the top of the sheikh obeiyid plateau corresponding to humid conditions during the early mid holocene the 14c dates obtained from the excavated hearths at sheikh el obeiyid indicate the presence of three occupation phases confirmed by the different characteristics of the lithic assemblages and playa generations pi pii and piii early holocene occupation is associated with lithic artefacts of high blade and microlithic index and backed retouched tools the middle holocene is represented by two occupations the early and late middle holocene the former is linked with bifacial knives gouges discoidal side scrapers tools with foliated retouch and lens shaped arrowheads the last horizon points to a final middle holocene exploitation phase of the area probably by small groups of herders who periodically visited the region when the climate had already started to deteriorate the sheikh el obeiyid village lies on the second erosion surface of the northern plateau it is characterised by 30 circular or elliptical structures of various sizes grouped together and located near the edge of the terrace commanding the bir el obeiyid playa below radiocarbon dating of charcoal recovered from the structures indicates that the village was used between 7930 123 and 7930 144 cal bp

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

environmental change and archaeology lake evolution and human occupation in the eastern sahara during the holocene abstract the west nubian palaeolake is the most large scale hydrographic evidence in the eastern sahara of the early to mid holocene wet phase that affected northern africa it is the result of a significant increase in local rainfall due to the northward shift of the tropical rainfall belt a series of fieldwork based differential gps dgps measurements along several profiles across the west nubian palaeolake basin provides the first precise topographic data from this up to a 5330 km2 large palaeolake feature in combination with sedimentological geochemical and archaeological results an almost complete picture of significant palaeoclimatic changes and human occupation during the early to mid holocene for this region is presented different stages of palaeolake evolution ranging from non existence of the lake through stable freshwater conditions to its extinction were identified in the period from 9400 to 3800 14c yr bp these lake stages coincide with phases of intensive human inhabitation between ca 6300 and 3500 14c yr bp and include at least four settlement phases distinguishable by style of pottery these are known from adjacent areas of the palaeolake region emphasizing strong prehistoric cultural connections in the eastern sahara during the highstands of the palaeolake in the early to mid holocene the dotted wavy line pottery relates to the early khartoum type culture with its supra regional distribution from the nile valley to the chad and possibly with slightly different forms even to the atlantic coast later in the holocene western nubia with its large palaeolakes and migration paths along palaeowadis such as wadi howar acted as an important natural and cultural link between the nile valley and the chad basin until the region was deserted during the fourth millennium bp

microbial structures in oolitic iron formations ferriferous ooids and microoncoids occur in many sedimentary iron formations these ferriferous coated grains are found in fine grained carbonate rich groundmasses iron mineral encrusted microbiota are observed in both the coated grains and the groundmass the branching nature and other morphological features of the microorganisms suggest fungal origins for the oolitic iron ores particularly of the lower jurassic lorraine minette the similarity of the microbial structures in coated grains and their groundmasses suggests that both had developed within microbial mats growing under calm environmental conditions the contribution of stromatolitic marine fungal mats to the fast extraction and immobilization of iron and thus to the genesis of iron ores is demonstrated observations on well preserved freshwater derived fungal stromatolites of the tertiary and laboratory experiments with organotrophic fungal mats confirm the findings on the jurassic and imply a general role of microorganisms in the formation of such deposits

uranium isotopic evidence for the origin of the bahariya iron deposits egypt abstract the economic iron ore deposits of egypt are located at bahariya oasis in the lower middle eocene limestone the main iron minerals are goethite hematite siderite pyrite and jarosite manganese minerals are pyrolusite and manganite gangue minerals are barite glauconite gibbsite alunite quartz halite kaolinite illite smectite palygorskite and halloysite geochemical comparison between the ore and the nubia sandstone showed that the ore is depleted in the residual elements al ti v and ni and enriched in the mobile elements fe mn zn ba and u which indicates that the bahariya iron ore is not a lateritic deposit despite the deep weathering in this area on the other hand the nubia sandstone showed depletion in the mobile elements which demonstrates the leaching process in the nubia aquifer the presence of such indicator minerals as jarosite alunite glauconite gibbsite palygorskite and halloysite indicate that the ore was deposited under strong acidic conditions in fresh water isotopic analyses of the uranium in the amorphous and crystalline phases of the ore in the country rocks and dissolved in the nubia aquifer water all support the conclusion that u and fe were precipitated together from warm ascending groundwater u and fe display strong co variation in the ore and the 234u 238u activity ratio of the newly precipitated u in the country rock and the leached component of u in the groundwater are identical there is only slightly more uranium in the amorphous phase than in the crystalline and only a slightly lower 234u 238u activity ratio suggesting that the iron in the two phases have a similar origin comparison of the excess 234u in the water and in the total ore leads to the conclusion that the precipitation of the u and by inference the iron occurred within the last million years however that both precipitation and leaching of u have occurred over the last 300 000 years is evidenced by the extreme 230th 234u disequilibria observed in some of the samples some of the amorphous depositional events have been very recent perhaps within the last 10 000 years

late wisconsin groundwater discharge environments of the southwestern indian springs valley southern nevada abstract badland exposures in the indian springs valley southern nevada contain evidence of formerly widespread spring and seep discharge the stratigraphic position and appearance of most of these deposits suggests correlation with late wisconsin 30 000 to ca 10 000 yr b p marsh sediments in nearby las vegas valley previously all these deposits have been loosely described as lacustrine because of the presence of extensive green mudstones associated with aquatic mollusks however this association also typifies modern groundwater discharge environments in many basins of northeast nevada such as the steptoe valley basins often without hydrographic closure such analogs best explain the origin of late wisconsin fine grained deposits in the unclosed southwestern arm of the indian springs valley key features of these depositional systems are the lack of shoreline deposits the presence of a broad belt of subaerially deposited plae brown silts surrounding spring wet meadow and marsh deposits and the intermixture of terrestrial and aquatic mollusks in most horizons where mollusks occur

carbonates as evidence for groundwater discharge to the nile river during the late pleistocene and holocene abstract the african humid period ahp 15 5 ka is the latest in a series of saharan pluvials studied using carbonates evidence from desert paleolakes indicate that water tables rose across the region caused by increased precipitation during these wet periods and that this higher groundwater discharged at the surface creating a wetter environment beyond what increased seasonal precipitation could support alone carbonate cemented material and sediments have been noted at key archaeological sites such as khartoum hospital and al khiday and at site distributions along wadi howar in the deserts west of the nile extensive surface discharge of groundwater is the accepted generator of paleolakes and contributed to wadi flow in contrast studies in the nile valley have focused on the river itself as the source for any moisture to explain documented effects of a higher water table carbonate morphology and oxygen isotope composition of carbonate samples are used to develop a conceptual model demonstrating their accumulation in areas near the nile river as a result of a regional rise in groundwater driven by greater precipitation morphological evidence shows a variety of formative environments at key topographic locations indicating water availability above modeled contemporary nile river flood levels new results presented here from oxygen isotopes show locations dominated by evaporative enrichment at higher elevations a transition to stable isotopic conditions at lower elevations and ultimately a return to more enriched values at the lowest elevations that would have undergone intermixing of discharging groundwater and floodwater from the nile for areas beyond the river s flood extent or an associated river fed alluvial aquifer the only perennial moisture source capable of maintaining an isotopic equilibrium in a monsoonal climate would be groundwater results coincide with isotopic patterns and values observed in carbonates from studied paleolakes in the desert this model provides an explanation for the carbonate accumulation at archaeological sites while offering greater understanding of contemporary water availability for vegetation animals and human populations and whether the nile river functioned as the dominant water source for cultural groups in the valley during the early to mid holocene as it did for the later agricultural populations

an integrated approach for identification of potential aquifer zones in structurally controlled terrain wadi qena basin egypt abstract wadi qena basin represents one of the most promising regions for future development in egypt fresh water supplies are crucial for such plans we provide an integrated remote sensing landsat aster dem geoeye 1 geophysical aeromagnetic isotopic 18 o 2 h field stratigraphic and structural interpretation and geochemical major dissolved ions approach to delineate zones of potential groundwater resources in wadi qena basin four water bearing horizons were sampled fractured crystalline aquifer nubian aquifer system nas post nubian aquifer system pnas and the quaternary aquifer findings include 1 spatial analysis of remote sensing data in a gis environment indicates extensive structural deformation by dextral faults trending ne sw i e qena safaga shear zone qs and sinistral faults trending nw se i e najd shear zone and sufficient surface water supply from the east through wadi fattera sub basin 2 analysis of geophysical data indicates that these faults control the water bearing horizons in the subsurface 3 isotopic analysis reveals four isotopic groups including two end members one mixed group and one mixed and evaporated group group i highly depleted fossil nubian waters range 18 o from 6 39 to 6 74 and 2 h from 48 21 to 52 46 group ii modern waters in fractured basement range 18 o from 1 41 to 1 51 and 2 h from 5 46 to 6 04 group iii a mixed cluster between nas and modern waters range 18 o from 4 82 to 5 05 and 2 h from 33 28 to 38 54 and group iv samples which have both mixing between the nubian and meteoric waters and also have a considerable deviation from the global meteoric water line gmwl range 18 o from 0 58 to 4 69 and 2 h from 19 59 to 38 68 4 samples with a mixed isotopic signature in group iii which tap the nas and are located along the main channel of wadi fattera area 3600 km 2 provide evidence for modern recharge along surface exposures of the nas and enhanced infiltration along deep seated faults 5 samples with a mixed isotopic signature in group iv which tap the quaternary and pnas aquifers and are located along deep seated faults provide evidence of artesian upward leakage from the deep nas into the shallower quaternary and pnas aquifers the present study improves our understanding of the role of structural control and modern recharge in exploration for aquifer potential in arid environments

evolution and architecture of a large felsic igneous province in western laurentia the 1 6 ga gawler range volcanics south australia the gawler range volcanics grv of south australia are an intracontinental subaerial large igneous province lip formed during supercontinent assembly in the mesoproterozoic like many lips the grv are intraplate were erupted over a geologically short time 2 million years and are dominated by voluminous lavas however the grv are overwhelmingly dominated by felsic units the grv are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved furthermore they are well exposed in very gently northward and eastward dipping sections up to 4 km thick allowing assessment of the facies architecture and evolution of this felsic volcanic lip the evolution of the grv can be clearly separated into two main stages initial eruptions at numerous volcanic centres produced small to moderate volume geochemically distinct felsic lavas and lava domes together with ignimbrites and minor mafic and intermediate lavas forming a sequence 0 5 to 3 km thick volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from phanerozoic felsic volcanic provinces however the second stage produced at least three voluminous felsic units each of which represents about 1000 3000 km3 of magma moreover the dominance of evenly porphyritic textures and lack of pyroclastic textures pumice shards broken crystal lithic clasts in these units suggests that they were erupted effusively and flowed as lavas each of these felsic lavas are generally dominated by a single uniform composition and commonly mingled with a subordinate and compositionally distinct lava

grain size distribution function of polymodal sediments in hydraulic and aeolian environments and numerical partitioning of the sedimentary components most continental sediments are polymodal composed of overlapping components of which the grain size generally obeys some type of natural distribution the grain size components and their function types can be determined from frequency and cumulative curve plots in order to define the function formula of the grain size distribution the function parameters can be estimated by fitting a defined function formula to the measured grain size data of the sample which simultaneously achieves numerical partitioning of the sedimentary components genetic analysis of grain size components of hydraulic and aeolian sediments demonstrates the following environmental implications fluvial sediment is composed of isolated saltation and suspension components the sediments in closed lake basins are dominated by a suspension silt clay component with a small proportion of saltation sand the fine sand component makes up the majority of desert sand overlapping with a small proportion of fine dust aeolian loess is composed of two overlapping components a short suspension time silt component and a long suspension time fine component aeolian material in the north pacific deep sea sediments is dominated by long suspension time fine dust the fine component in aeolian sediments shows a consistent grain size distribution and genetic connection from the desert sand loess of northern china to the north pacific ocean which is mainly transported by westerly winds and is dispersed in the atmosphere forming a background dust c 2002 elsevier science b v all rights reserved

were springline carbonates in the kurkur dungul area southern egypt deposited during glacial periods the tufa deposits in the kurkur dungul area southern egypt date from marine isotope stage mis 11 to mis 1 springs across the region were active during glacial periods with sea level below 50 m reflecting changed atmospheric circulation over the indian ocean as well as peak interglacial periods during times of low sea level reduced indonesian throughflow promoted formation of an indian ocean warm pool and anomalous rainfall on its western margin we suggest that egypt lies at the intersection of westerly maghrebian and easterly mashriqian rainfall provinces which show different timing with relation to orbital forcing and different source water regions tufa growth periods are therefore not mechanistically linked to humid periods or sapropel events identified elsewhere stable isotope and t 47 data are also inconsistent with these spring systems being part of a larger system spanning northern africa and lack a clear interaction between northern hemisphere heating and mid latitude rainfall we also follow previous researchers in concluding that formation of springline deposit formation was probably delayed compared with rainfall owing to aquifer flow distances this delay is unlikely to be sufficient to explain why rainfall is out of phase with movements of the monsoon belts but may complicate interpretation of these records supplementary material a lithofacies description and supplementary figures and tables are available at https doi org 10 6084 m9 figshare c 5246661

oxygen isotope geochemistry of the mesozoic volcanics of the etendeka formation namibia the etendeka formation volcanics consist of a bimodal association of basalts and quartz latites forty three new whole rock oxygen isotope analyses are reported for all the major magma types all the rocks except a minor suite of dolerites have higher 18o values than normal mantle the basic rocks average of 29 8 8 have significantly different 18o to the acid rocks average of 10 14 4 these data are apparently consistent with previously published petrogenetic models which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived however mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock the latter being significantly higher in most cases one of the basic magma types the tafelberg basalts shows mutual positive correlations between 18o sio2 and sr if these correlations are due to crustal contamination then as much as 45 contamination is required by material having a 18o value of 15 which is the maximum observed value in the damaran basement rocks in the absence of pyroxene phenocryst 18o data for the high sr tafelberg basalts they are aphyric it is not possible to confirm that contamination has taken place an alternative explanation is that the correlation between sr and sio2 resulted from assimilation coupled with fractional crystallization afc before emplacement post eruption alteration resulted in a correlation between sio2 18o because the material with the most si o bonds was able to concentrate 18o more effectively the limited mineral data for the quartz latites suggests that there is some source heterogeneity a pyroxene 18o value of 10 for a southern etendeka quartz latite is consistent with a crustal source