paleomagnetic study of some cretaceous and tertiary sedimentary rocks of the klamath mountains province california paleomagnetic investigation of cretaceous outliers and tertiary sedimentary strata of the klamath mountains province and of onlapping cretaceous strata has shown the rocks to be largely remagnetized samples studied are from the upper jurassic to upper cretaceous great valley sequence upper cretaceous hornbrook formation eocene montgomery creek formation and oligocene weaverville formation cretaceous samples that survived the remagnetization have a mean remanence direction that is very close to the expected direction of the cretaceous magnetic field at the locality of the klamath mountains data from both primary and remagnetized samples suggest the possibility of 11 5 15 8 of post cretaceous clockwise rotation of the klamath mountains province none of the data from either the primary or remagnetized samples shows evidence of the large amounts 70 of clockwise rotation that other workers have measured for the lower tertiary of the oregon coast range our data indicate that the oregon coast range and klamath mountains province did not behave as a single rigid block during the early tertiary they also suggest that any post oligocene rotation of the klamath mountains province is less than the approximately 30 post oligocene rotation recently proposed for a combined oregon coast range klamath mountains cascade range block

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

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

provenance of franciscan graywackes in coastal california a systematic comparison of available detrital modes for graywacke sandstones of the franciscan subduction complex and for coeval sandstones of the great valley sequence in the california coast ranges indicates that both were apparently derived from the same general sources the inferred provenance terrane was the ancestral sierran klamath magmatic arc from which mixed volcanic and plutonic detritus readily entered the adjacent great valley forearc basin at intervals along the trend of the arc trench system arc derived detritus also bypassed the forearc region through submarine canyons that fed the franciscan trench longitudinal flow along both the great valley trough and the franciscan trench achieved wide dispersal of the turbidite sediment suites of both franciscan and great valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic mean framework modes of 17 franciscan suites comprising 203 individual samples and of 23 great valley suites comprising 410 individual samples range from 14 to 44 quartz grains 15 to 54 feldspar grains and 7 to 71 total lithic fragments the ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes the compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz and feldspar derived principally from erosion of underlying plutons despite major overlap in the compositions of the two sets of samples some franciscan sandstones are somewhat more feldspathic and less lithic than any known great valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical great valley sources higher proportions of non volcanic to volcanic lithic fragments in some franciscan sandstones probably reflect complex recycling processes on the trench slope diagenetic effects in many franciscan suites include apparent wholesale replacement of k feldspar by albite present age control is inadequate to test fully for time dependent trends in the compositions of franciscan sandstones analogous to the known stratigraphic variations in the composition of great valley sandstones this question ought to be investigated in future studies

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

the sevětin astrobleme southern bohemia czechoslovakia the geological geomorphological petrological and geophysical data for the 46 km circular structure sev tin 49 05 n 14 35 e southern bohemia are consistent with characteristic of moderately eroded structures that have originated by hypervelocity meteorite impact the astrobleme dated as late cretaceous probably coniac santon 85 75 m y is dissected into radial fault segments which exhibit a variable level of erosion the classification of the sev tin astrobleme as a probable impact structure is based on the geological and geophysical evidence of central scar about 8 km in diameter shock metamorphism in the centre of the structure and relics of silicified impact breccia including globular and suevite types with structural indications of molten material shatter coning and shock metamorphism are documented for several formations and lithologies ranging in age from the hercynian granite of the crystalline basement through stephanian permian to the upper cretaceous lower klikov formation the morphological expression of the crater rim is preserved in the less eroded segments the central uplift d 21 km approximately 0 5 of the total diameter still preserved in part as a positive morphological feature is partly burried under the post impact upper klikov formation the impact event provided the major tectonic geomorphological and paleogeographic resetting which was known for some time in the evolution of the continental upper cretaceous of this area and expressed in the splitting of the klikov formation into two divisions

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

large pressure gap between the coastal and central franciscan belts northern and central california abstract the nature of the metamorphism of the central belt of the franciscan complex has long been debated did the whole central belt or its melange matrix undergo high p t metamorphism to solve this problem we have investigated the regional distribution of aragonite in these rocks in northern and central california we have discovered more than 300 new localities of aragonite using the staining method by friedman 1959 aragonite occurs as network veins or pods in both metagraywacke and matrix forming metashale and is invariably partially replaced by calcite on its rim thus we conclude that aragonite is stable in all parts of the franciscan central belt from the lower boundary between the coastal and central belts to the topmost part of the central belt and that both the matrix forming melange shale and exotic blocks of the central belt suffered high p t metamorphism we traced the boundary between the central and coastal belts in northern california using the staining method along the gently eastward dipping thrust plane between the underlying coastal and overlying central belts more than 2 5 kbar pressure break is obtained through the lawsonite laumontite aragonite quartz relations such an observed large pressure gap was formed during the exhumation process of the central belt of the franciscan complex

phanerozoic continental accretion and the metamorphic evolution of northern and central california abstract the late paleozoic and younger tectonic fabric of california north of the transverse ranges consists of accreted exotic and parautochthonous lithic belts which extend north south and northwest southeast separated from one another by subvertical strike slip faults and gently to steeply east dipping thrust faults where recognizable folds in general verge to the west and stratigraphic tops face east a rough but systematic westward decrease in geologic age and metamorphic intensity may also be recognized exotic terranes of uncertain provenance include the central metamorphic belt of the klamath mountains and the salina sur obispo composite block of the central coast ranges pre permian sections of the eastern klamath terrane and portions of the sierran calaveras melones complex may also represent far traveled crustal entities allochthonous oceanic basement seamounts and pelagic sediments are additional transported materials widespread in most of the parautochthonous lithotectonic belts that make up much of the rest of northern and central california ophiolitic assemblages are especially abundant along the fault zones which mark boundaries between these units the parautochthonous lithotectonic belts appear to have been constructed in the vicinity of the continental margin and or an offshore arc judging from the nature and provenance of their constituent igneous and sedimentary rocks intimately associated tholeiitic calc alkaline complexes indicate that at least locally ophiolite generation occurred nearby the salinia sur obispo amalgamated terrane exhibits the effects of fragmentation and removal of crust now represented by the suture juxtaposing the two blocks other areas may have been subjected to similar truncations but relationships are not as clear far traveled terranes are characterized by ages of formation and recrystallization foreign to surrounding units the nature of the protoliths and grades of metamorphism developed within them contrast with those of juxtaposed lithotectonic belts on the other hand essentially in situ growth of many californian lithotectonic belts is suggested by a gradually decreasing age of formation and both monotonically decreasing times and grades of metamorphism proceeding towards the pacific basin some parautochthonous units as well as exotic terranes are locally bounded by discontinuous blueschist belts the latter attesting to at least episodic preservation of the effects of subduction metamorphism and offloading into the accreting continental margin phanerozoic evolution of northern and central california thus apparently consisted of a more or less in situ seaward encroachment of the continental margin relative to the craton involving occasional truncation and loss of section with a concomitant semi continuous or episodic incorporation of allochthonous seamounts oceanic crust and deep sea sediments and the less frequent arrival and juxtaposition of exotic microcontinental fragments of problematic but distant source

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

metamorphic evolution of the central metamorphic belt klamath province california an inverted metamorphic gradient beneath the trinity peridotite metabasalts and metasedimentary rocks of the devonian central metamorphic belt comprise the lower plate of the east dipping trinity thrust system in the klamath province an inverted metamorphic gradient is preserved in the central metamorphic belt metamorphic conditions decrease from amphibolite facies adjacent to the trinity thrust through albite epidote amphibolite facies to upper greenschist facies at the base of the central metamorphic belt mineral chemistry mineral assemblages and limited geothermometry suggest that peak metamorphic conditions decrease structurally downward from 650 50 c at the trinity thrust to 500 50 c at the base of the central metamorphic belt under pressures of 5 3 kbar synmetamorphic ab qtz veins up to 1 m thick increase in abundance towards the trinity thrust infiltration of h2o co2 fluids derived from prograde devolatilization reactions in the central metamorphic belt caused extensive hydration and metasomatism of the trinity peridotite the hanging wall block of the trinity thrust zone geological relationships and the preserved inverted metamorphic gradient suggest that the central metamorphic belt formed in an east dipping devonian subduction zone in an oceanic environment the central metamorphic belt appears to represent a discrete slice of accreted oceanic crust several km thick rather than progressively accreted material metamorphic pressures recorded by the central metamorphic belt are intermediate between the 2 kbar pressures recorded in dynamothermal aureoles beneath obducted ophiolites and the 7 10 kbar preserved in subduction related inverted metamorphic gradients the lack of blueschist facies mineral assemblages in the central metamorphic belt may possibly be explained by an anomalously warm geotherm prior to subduction or early shear heating prior to the arrival of wet rocks at depth

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

post middle miocene accretion of franciscan rocks northwestern california deformed sedimentary rocks assigned to the franciscan assemblage in the king range south of cape mendocino northern california are dominantly deep water argillite and sandstone occurring as thick to thin bedded locally channelized marine turbidites of arkosic to andesitic volcani clastic composition these rocks structurally overlie late cretaceous coniacian to campanian basaltic pillow flows flow breccias diabase sills and arkosic sandstone turbidites that also are assignable to the franciscan assemblage and are well exposed along the coast at point delgada radiolarians and planktic and benthic foraminifers indicate that the oldest rocks in the king range may be early tertiary and that the youngest rocks are middle miocene or younger the youngest rocks yet reported from the franciscan assemblage partly coeval miocene relizian and luisian to quaternary rocks representing shallower sedimentary facies of the inner shelf to outer continental slope occupy the eel river basin and isolated fault bounded slivers adjacent to the king range on the north and east wide zones of penetrative shearing that may delineate a middle miocene or younger suture separate the deep water franciscan rocks of the king range from less severely deformed neogene rocks of the eel river basin the king range appears to be a displaced terrane of oceanic basement overlain by paleogene and neogene sedimentary and igneous rocks of continental and oceanic derivation this terrane may have been displaced north or eastward and obductively accreted to california 15 m y ago before or perhaps during northward passage of the pacific farallon north american plate junction

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

catastrophic emplacement of the heart mountain block slide wyoming and montana usa the mechanism that allowed many tens of km of movement of the enormous block slide floored by the rootless heart mountain detachment fault in nw wyoming has long been a puzzle carbonat rich microbreccia that is widespread along the fault and in dikes in the upper plate contains accreted grains indistinguishable from those observed as fallout from volcanic eruption clouds accretionary lapilli and impact ejecta clouds and in intrusive diatremes in these settings and also in industrial processing accreted grains form when particles in a turbulent gaseous suspension containing limited water adhere to a nucleating grain or to each other elongate grains in thick microbreccia have strong but diverse shap preferred orientations unlike those reported from other fault rocks but instead suggestive of turbulent flow and the microbreccia contains layering and other features of sedimentary character that appear to record deposition from suspension rather than frictional processes along a fault we suggest that frictional heating led to dissociation of carbonate rock along the fault producing supercritical co 2 as the suspending medium high co 2 pressure drastically reduced friction along the fault and allowed continuation of catastrophic movement probably initiated by a volcanic or phreatomagmatic explosion resulting in very large displacement on a low dipping surface earlier slower sliding may have occurred but final emplacement was rapid minutes and spectacular

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

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

the ophiolitic north fork terrane in the salmon river region central klamath mountains california the north fork terrane is an assemblage of ophiolitic and other oceanic volcanic and sedimentary rocks that has been internally imbricated and folded the ophiolitic rocks form a north trending belt through the central part of the region and consist of a disrupted sequence of homogeneous gabbro diabase massive to pillowed basalt and interleaved tectonitic harzburgite u pb zircon age data on a plagiogranite pod from the gabbroic unit indicate that at least this part of the igneous sequence is late paleozoic in age the ophiolitic belt is flanked on either side by mafic volcanic and volcaniclastic rocks limestone bedded chert and argillite most of the chert is triassic including much of late triassic age but chert with uncertain stratigraphic relations at one locality is permian the strata flanking the east side of the ophiolitic belt face eastward and depositional contacts between units are for the most part preserved the strata on the west side of the ophiolitic belt are more highly disrupted than those on the east side contain chert argillite melange and have unproven stratigraphic relation to either the ophiolitic rocks or the eastern strata rocks of the north fork terrane do not show widespread evidence of penetrative deformation at elevated temperatures except an early tectonitic fabric in the harzburgite slip fiber foliation in serpentinite phacoidal foliation in chert and mafic rocks scaly foliation in argillite and mesoscopic folds in bedded chert are consistent with an interpretation of large scale anti formal folding of the terrane about a north south hinge found along the ophiolitic belt but other structural interpretations are tenable the age of folding of north fork rocks is constrained by the involvement of triassic and younger cherts and crosscutting late jurassic plutons deformation in the north fork terrane must have spanned a short period of time because the terrane is bounded structurally above and below by middle or late jurassic thrust faults the north fork terrane appears to contain no arc volcanic rocks or arc derived detritus suggesting that it neither constituted the base for an arc nor was in a basinal setting adjacent to an arc sediment source details of the progressive accretion and evolutionary relationship of the north fork to other terranes of the klamath mountains are not yet clear

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

the azuara impact structure spain new insights from geophysical and geological investigations a progress report on geophysical and geological investigations in the 35 40 km diameter azuara probable impact structure northeast spain is given the target is a 10 km thick sequence of palaeozoic mesozoic and cenozoic sediments gravity measurements establish a negative residual anomaly of about 100 m s2 and a mass deficiency of 1 24 1014 kg they suggest the existence of a buried inner ring with half the size of the outer ring the result of preliminary model calculations is compatible with the assumption of a flat structure only a few kilometers deep measurements of the total intensity of the earth s magnetic field do not indicate causative bodies related to the structure geological mapping reveals a tectonic style which is characterized by folds and faults with radial and tangential elements and a strong horizontal component ejecta abundant monomict and polymict breccias dislocated megablocks inverted stratigraphy and a megabreccia up to 80 m thick in the outer ring are evidence for intense and violent deformation networks of typical breccia dikes well known from many impact structure cut through nearly all stratigraphical units various types and generations of breccia dikes occur in addition to previously described shock deformation in quartz and mica we present further evidence of high pressure and high temperature signatures in rocks the age of the impact is estimated to be upper eocene or oligocene we conclude that the formation of the azuara structure is difficult to explain by mechanisms other than impact and we discuss our observations within the scope of the contact compression excavation and modification stages of impact cratering with special attention directed to the peculiarities of the azuara sedimentary target

accretion of the franciscan complex attending jurassic cretaceous geotectonic development of northern and central california scattered igneous activity marks the triassic margin of california but an andean volcanic plutonic arc began to form in the sierra nevada white inyo range and klamath mountains at ca 170 ma during underflow of the oceanic lithosphere this calc alkaline belt supplied volcanogenic erosional debris to the mariposa galice proximal overlap sequence by ca 165 160 ma transpression also generated high pressure low temperature basaltic eclogites garnet blueschists and amphibolites along the middle and late jurassic convergent plate junction at ca 170 155 ma most of these returned surfaceward as tectonic blocks only during the mid cretaceous the high grade metamafic metamorphosed mafic blocks that formed during construction of the emergent arc and derived mariposa galice strata predate the onset of franciscan trench deposition near the end of jurassic time the klamath salient evidently migrated 200 km westward an earliest cretaceous westward stepout of the convergent junction apparently formed directly offshore from the klamath imbricate orogen but to the south preexisting oceanic crust capped lithosphere was trapped landward as the coast range ophiolite andean arc detritus began to accumulate on this ophiolitic basement within the great valley forearc and outboard franciscan oceanic trench at ca 145 140 ma because earliest cretaceous relatively continuous great valley strata were deposited on the stable north american plate protected from both surface and subcrustal erosion forearc terrigenous sedimentation also signaled coeval deposition in the outboard trench 10 25 m y after middle and late jurassic initiation of the continental margin calc alkaline arc voluminous sedimentation and the accretion of franciscan and san joaquin sections of great valley rocks took place during mid and late cretaceous time after a flare up in igneous arc activity at 125 120 ma the youngest sierran granites have ca 85 ma ages reflecting extinction of the magmagenic zone beneath northern and central california and presumably reflecting subhorizontal oceanic plate underflow attending the laramide orogeny feebly zeolitized upper cretaceous and lower tertiary franciscan coastal belt rocks derived from the now inactive arc underwent only low pressure burial metamorphism reflecting shallow offloading and or large scale transpression they were not appreciably subducted although coastal belt units are members of the franciscan lithotectonic assemblage their origin contrasts with the rest of the subduction complex

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

regional detrital zircon provenance of exotic metasandstone blocks eastern hayfork terrane western paleozoic and triassic belt klamath mountains california abstract the addition of oceanic material through accretion underplating was an important process in construction of the convergent transpressive cordilleran margin of north america according to radiolarian ages the late triassic or younger eastern hayfork terrane represents trench deposits that accreted outboard of the coeval north fork immature island arc it consists chiefly of chert and argillite but contains tectonic and olistostromal blocks of preexisting rock units derived from exotic sources that include inboard klamath terranes fine grained terrigenous debris was shed from these eastern terranes but the eastern hayfork melange also contains blocks of quartzofeldspathic metasandstone of uncertain provenance new detrital zircon u pb ages from five blocks of immature variably metamorphosed arkosic to subarkosic sandstone range from late proterozoic to late archean with modes of 1804 1843 1970 2300 and 2550 2585 ma the adjacent inboard north fork terrane contains zircons with phanero

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

metamorphism of a late jurassic volcano plutonic arc northern california usa the metamorphic history of the middle to upper jurassic volcanic and hypabyssal rocks exposed in the klamath mountains and sierra nevada of california is related in part to the rifting of a volcano plutonic arc the callovian to kimmeridgian rocks exposed in the region consist of from north west to south east a back arc ophiolite a rifted volcanic arc and a volcanic arc complex all of these units have been metamorphosed and contain various combinations of the phases chlorite amphibole epidote prehnite and pumpellyite projection of coexisting phases onto the composition plane mgo mgo feo and al2o3 fe2o3 0 75 cao na2o through quartz water albite and epidote results in consistent mineralogical compatibilities within each region but crossing tie lines between regions this suggests that the volcanic and hypabyssal rocks from each region have equilibrated under different intensive conditions the back arc ophiolite in the north has suffered subseafloor high t p hydrothermal metamorphism with geothermal gradients on the order of 100 c km 1 the rifted volcanic arc has undergone synchronous burial hydrothermal and contact metamorphism metamorphic field gradients in the region pass through the prehnite pumpellyite and greenschist facies suggesting geothermal gradients on the order of 30 c km 1 the southernmost volcanic arc complexes contain metavolcanics of the pumpellyiteactinolite and greenschist facies suggesting moderate to high p t metamorphism and geothermal gradients on the order of 20 c km 1 the apparent increase in rifting and calculated geothermal gradients from south east to north west suggest that the observed very low and low grade metamorphism may be a response to enhanced thermal gradients during extension of the volcanic arc this correlation between the extent of rifting and metamorphism is consistent with a model of diastathermal metamorphism of a propagating rift along the western margin of north america during the late jurassic the plate tectonic setting may be analogous to the present day andaman sea region

terrestrial chondrules glass spherules and accretionary lapilli from the suevite ries crater germany abstract until recently no terrestrial analogues of meteoritic and lunar chondrules were known only rare glass spherules from the lonar crater india and black magnetic spherules from various localities have been recorded the impact breccia suevite of the no rdlinger ries crater germany contains both chondrules and glass spherules and in addition accretionary lapilli all of which are found imbedded within the fine grained matrix of the suevite the chondrules display many of the textural features characteristic of meteoritic and lunar chondrules lithic chondrules and fluid drop chondrules are present the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition on the whole fluid drop chondrules lithic chondrules and glass spherules are rare in the suevite with fluid drop chondrules prevailing detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules accretionary lapilli also represent material formed as a result of impact

a middle jurassic to early tertiary andean sierran arc model for the coast belt of british columbia regional tectonic features of the western canadian cordillera can be interpreted in terms of middle jurassic accretion of a single composite superterrane stikinia wrangellia alexander to ancestral north america closure of the intervening cache creek bridge river ocean moved the continental edge to a new position west of the accreted superterrane the coast belt is primarily a succession of superimposed middle jurassic to early tertiary magmatic arcs related to prolonged subduction of pacific ocean lithosphere beneath the new north american margin discrete magmatic pulses separated by lulls or periods of relative quiescence successively overprinted the new margin the insular and intermontane superterranes previously viewed as widely separate entities prior to mid cretaceous time were already amalgamated before an initial middle jurassic overprint produced an ancestral coast belt late jurassic to early cretaceous rift related marine basins mid to late cretaceous compressional structures and early tertiary extensional features coincident with the coast belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary subduction related andean magmatic arc

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

metamorphic evolution of two different eclogites in the franciscan complex california u s a the eclogites in california u s a have been regarded as tectonic blocks which formed 150 160 ma ago and have been tectonically incorporated into younger rocks of the franciscan complex of 90 130 ma however in ward creek northern california two different types of eclogites were identified type i is a typical tectonic block and type ii is a possible in situ eclogite formed during the franciscan metamorphism most eclogites in the franciscan complex are type i eclogites type i eclogites in ward creek consist of garnet omphacite epidote glaucophane phengite rutile barroisite quartz and have coarse grained gneissic texture they have precursors of the epidote amphibolite facies and are overprinted in various degrees by blueschist facies retrograde metamorphism remnants of metamorphic rinds which consist of actinolite talc chlorite and formed during metasomatic interaction with serpentinizing peridotite occur at the margins of type i eclogite blocks type ii eclogites are medium grained and consist of garnet chloromelaniticclinopyroxene epidote glaucophane paragonite phengite rutile quartz petrochemical characters of type ii eclogites are similar to those of in situ eclogitic schists in the ward creek metabasite sequence epidote amphibolite f facies precursors metamorphic rinds and overprint of retrogressive blueschist facies metamorphism are not found in type ii eclogites garnets in type ii eclogites are medium grained unaltered and idioblastic and have higher spessartine contents sp14 25 whereas garnets in type i eclogites are coarse grained and chloritized and have lower spessartine contents sp2 7 the pt conditions of peak metamorphism for type ii eclogites are 380 10 11 5 kb type i eclogites follow a counter clockwise pt path from the epidote amphibolite through eclogite to blueschist facies whereas the presumable in situ type ii eclogites follow a clockwise pt path from the blueschist to eclogite facies

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

age origin and significance of an unconformity that predates island arc volcanism in the northern sierra nevada the paleogeographic position of rocks of the northern sierra nevada relative to north america has long been the topic of debate a detailed study of the lakes basin region of the northern sierra nevada substantiates that the shoo fly complex of early paleozoic age was folded and unconformably overlain by an island arc sequence in late paleozoic time microfossils recovered during the study represent the first in situ fauna ever collected from the shoo fly complex and provide an ordovician silurian lower limit on the early deformation late jurassic nevadan deformation strongly affected all rocks in the region regional considerations suggest that mid paleozoic deformation may have been widespread in the sierra nevada and could indicate an early phase of an eastward prograding suture that ultimately resulted in emplacement of the roberts mountain allochthon of the great basin

permo triassic accretionary subduction complex southwestern klamath mountains northern california the western paleozoic and triassic subprovince of the southwestern klamath mountains has previously been subdivided into three relatively steeply dipping fault bounded litho tectonic terranes in descending structural order from east to west these terranes are 1 the north fork terrane 2 the hayfork terrane and 3 the rattlesnake creek terrane in a general fashion each of these terranes exhibits its own particular set of lithologic structural and temporal relationships that sets it apart from adjacent terranes this threefold subdivision of the southwestern klamath mountains needs modification only as far as the hayfork terrane is concerned the hayfork terrane is actually a composite terrane consisting of a structurally lower assemblage of predominantly basaltic metavolcaniclastic rocks and a structurally higher assemblage of chaotically disrupted metasedimentary rocks juxtaposed along a previously unrecognized major regional thrust these two lithologically and structurally distinct assemblages have been elevated to the status of a terrane in this report and are herein referred to as the western hayfork terrane structurally lower metavolcaniclastic assemblage and the eastern hayfork terrane structurally overlying metasedimentary assemblage and the fault which juxtaposes these two terranes is herein referred to as the wilson point thrust this paper will focus on the eastern hayfork and north fork terranes and their relationship to other roughly coeval tectonic elements of the klamath mountain province the eastern hayfork terrane is best described as a tectono stratigraphic unit consisting of a complex chert argillite and quartzose sandstone melange and broken formation that contains radiolarian cherts as young as late triassic in age scattered throughout this terrane are various types of blocks some of which are clearly exotic metachert is by far the most common type of metamorphic block and these blocks appear to have been derived via sedimentary processes from the paleozoic central metamorphic belt and or the mesozoic stuart fork formation shallow water permian limestone blocks are also plentiful and are interpreted as reefal accumulations which flanked seamounts that became incorporated into the melange and broken formation as proto pacific ocean floor was subducted beneath the western margin of north america during at least the permo triassic to early jurassic the north fork terrane has at its base basaltic rocks tentatively interpreted as an oceanic crustal remnant generated within an open ocean ridge setting that was removed from any appreciable source of continental or volcanic arc derived detritus overlying the basaltic rocks is a sequence of pelagic hemipelagic terrigenous and tuffaceous metasedimentary rocks that have yielded a late triassic and early or middle jurassic radiolarian fauna the metasedimentary rocks of the north fork terrane are lithologically very similar to those of the eastern hayfork terrane both terranes consist predominantly of chert and argillite contain blocks of metamorphic rock and permian limestone and are at least in part of overlapping age however the metasedimentary rocks of the north fork terrane are structurally coherent unlike the chaotic melange and broken formation of the eastern hayfork terrane taken together the north fork and eastern hayfork terranes bear remarkable similarities to both modern and ancient rock assemblages that occupy sites within zones of lithospheric plate convergence and are interpreted as tectonic elements of an underthrust plate margin developed oceanward of a volcanic arc represented by the permo triassic to middle jurassic volcanic strata of the eastern klamath belt

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

cretaceous sinistral strike slip along nacimiento fault in coastal california the san andreas and nacimiento faults of coastal california both separate granitic and metamorphic basement rocks of the salinian block from partly coeval but contrasting mesozoic terranes underlain by the franciscan subduction complex by analogy with neogene dextral strike slip along the san andreas fault cretaceous sinistral strike slip can be inferred along the nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount following restoration of known san andreas and inferred proto san andreas dextral displacements reversal of about 560 km 350 mi of postulated sinistral slip on the nacimiento fault brings four major mesozoic lithotectonic belts of california and baja california into simple al gnment as subparallel terranes related to mesozoic subduction along the continental margin neogene deformation within the san andreas transform system involved a elongation of the salinian block by dextral slip along subsidiary faults that branch from the san andreas fault and b dextral rotation of crustal panels within the transverse ranges latest cretaceous and or earliest paleogene dextral slip along a proto san andreas fault followed the san andreas course in central california but diverged westward in southern california nacimiento sinistral displacements occurred in mid cretaceous to early or medial late cretaceous time after cretaceous emplacement of plutons now within the salinian block but prior to deposition of uppermost cretaceous sedimentary sequences in central california available data on mesozoic relative and absolute plate motions in the mesoameric n region support the likelihood of cretaceous sinistral strike slip subparallel to the california continental margin paleotectonic reconstruction of crustal blocks in california and baja california to their inferred mid cretaceous relative positions shows the salinian block inserted on a bias between the flanking mojave and peninsular ranges blocks salinian granitic rocks thus formed an interior part of the mesozoic batholith belt and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks the similar mesozoic terranes that now lie east and west of the salinian block were then adjacent to one another west of the sierra nevada block available paleomagnetic data neither support nor preclude the reconstruction but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents a correct interpretation of the nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal california

submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc sawyers bar area central klamath mountains california usa new bulk rock oxygen isotope data indicate a complicated history of fluid rock interactions in the upper few kilometers of a basaltic arc attending its mesozoic accretion to the western margin of north america folded multiply recrystallized weakly metasomatized mafic volcanics and interstratified sediments of the sawyers bar terrane an eastern segment of the western triassic and paleozoic belt were investigated the following scenario can now be reconstructed 1 island arc tholeiites iats ocean island basalts oibs and distal turbidites were deposited in a subsea environment during permian and early mesozoic time 170 245 ma basaltic rocks underwent low temperature alteration by seawater recrystallization occurred at 100 200 c and 1 kbar alkali exchange and variable mg enrichment were accompanied by increases in bulk rock 18o values of the greenstones from 6 to approximately 10 o preceeding initial stages of island arc formation 2 middle jurassic 165 170 ma suturing of the seaward oceanic arc structurally beneath a landward 227 ma blueschist terrane resulted in regional deformation and greenschist facies metamorphism pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300 425 c 3 1 kbar 3 granitoid plutons emplaced during late middle jurassic time 160 165 ma heated adjacent wallrocks to 500 600 c at pressures of approximately 2 3 kbar thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high 18o fluids with intercalated greenstones 18o values in iat and oib metavolcanics increased from 9 to 10 along axial portions of ns trending folds to more than 15 o where metabasalts are intimately interlayered with the metasediments construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid rock isotopic exchanges reflecting the p t time evolution of the complex

a gravity derived model for the steinheim impact crater the gravity survey of the steinheim impact crater comprises about 500 gravity stations resulting in abouguer anomaly map of the crater and its surroundings from this map abouguer residual anomaly of the impact structure was deduced which shows considerable character a central negative anomaly with an amplitude of about 2 mgal and a halfwidth of 3 km is surrounded by ring like relative positive and negative anomalies which extend to a radial distance of about 5 6 km the interpretation is based on nine radial profiles with close station spacing model calculations were performed suggesting the steinheim crater to be much larger and of different formation than has been assumed based on the gravity interpretation on a morphological analysis and on additional geological and geophysical observations a model of the steinheim crater and its development is proposed the main characteristics are a 500 600 m deep primary excavation and a final apparent diameter of roughly 7 km contrasting to an up to now favoured shallow excavation and a 3 5 4 km final diameter the model considerations include mass and energy calculations and references to the ries impact crater

significance of mid mesozoic peridotitic to dioritic intrusive complexes klamath mountains western sierra nevada california a mid mesozoic plutonic suite ranging from ultramafic gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the klamath mountains western sierra nevada california the ultramafic gabbroic rocks are clinopyroxene rich and commonly vary from wehrlite to olivine hornblende clinopyroxenite and melagabbro associated dioritic rocks include biotite two pyroxene diorite monzodiorite and the granitic rocks typically range from tonalite to granodiorite these plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite the volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages clinopyroxene plagioclase olivine or hornblende the plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental sialic crust and that at least locally contain juvenile ophiolitic crust generated adjacent to the locus of magmatism thus their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc

time relations and structural stratigraphic patterns in ophiolite accretion west central klamath mountains california new geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone the josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western jurassic belt of the klamath province within the study area the josephine ophiolite was formed by seafloor spreading at about 157 m y before present it was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence both of which comprise the galice formation the galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages a major nonvolcanic source for the galice flysch appears to have been the western paleozoic and triassic belt of the klamath province exposed to the east proximal volcanic arc activity migrated to the site of the josephine galice section by 151 m y and is represented by numerous dikes and sills which intrude the ophiolite and galice formation the preston peak ophiolite is a polygenetic assemblage consisting of 1 a pre mid jurassic tectonitic peridotite amphibolite substrate which represents disrupted and unroofed basement of the western paleozoic and triassic belt and 2 an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m y the mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase clast breccia and hemipelagic deposits a major arc plutonic complex was emplaced into the preston peak ophiolite in at least two pulses at 153 and 149 m y major phases of this complex consist of wehrlite gabbro pyroxene diorite and hornblende diorite the josephine ophiolite is interpreted as the remnants of interarc basin crust the preston peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the josephine interarc basin the galice formation represents a submarine fan complex that was built on juvenile crust of the josephine basin floor during the time interval of 153 to 149 m y the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge the basin shortly thereafter closed by convergent tectonics during the nevadan orogeny resulting in the imbrication of the josephine and preston peak ophiolites and their superimposed arc assemblages the transition from seafloor spreading generation of josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m y the process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the klamath mountains sierra nevada region and perhaps throughout the western cordillera

imbricate low angle faulting in uppermost franciscan rocks south yolla bolly area northern california the south yolla bolly area of northern california contains the intersection of the franciscan complex the klamath mountains and the great valley sequence four distinct lithic units among them the south fork mountain schist as well as several subunits have been discerned in the franciscan rocks of this area these units are composed mostly of coherent or semi coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange and are arranged in largely upright parallel imbricate fault slices four major low angle faults have been clearly recognized paleontologic data indicate that at least one fault places older material over younger lithic units exhibit various degrees of differences in metamorphic mineral assemblages textural grade and amount of internal deformation across the faults with more intensely deformed and metamorphosed material occupying successively higher structural positions these contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and or after metamorphism original bedding and compositional layering in all of the coherent units generally parallel the thrust faults two fault bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert and further overlain by mudstone and graywacke or their metamorphic equivalents the faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock mafic crust interface perhaps as discrete underthrust slices the distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

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

polygenetic ophiolite belt of the california sierra nevada geochronological and tectonostratigraphic development the assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology however this assumption has only been verified in a few locations by an integrated geochronological and structural stratigraphic approach many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind such reconstructions are prone to error without adequate age control on each of the reconstructed fragments this is a significant problem in structurally complex regions where more than one generation of ophiolite may be present in this paper new pb u zircon ages are presented for key locations along a 375 km segment of the western sierra nevada ophiolite belt these age data are combined with structural stratigraphic observations and published ages and significant tectonic implications for the ophiolite belt emerge three different ophiolitic assemblages are recognized with igneous ages of about 300 200 and 160 m y b p rocks of the 300 m y assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange fragments of upper paleozoic seamounts occur in association with the ophiolitic melange and together these assemblages constitute the basement framework for the western sierra pb u and k ar isotopic systematics are complex within this framework and indicate a polymetamorphic history systematics in the 200 and 160 m y assemblages are less complex and give tighter igneous age constraints rocks of the 200 m y assemblage are in a semi intact state with only local tectonite and melange zones rocks of the 160 m y assemblage are intact but nevertheless deformed both the 200 and 160 m y assemblages have equivalent age basinal volcanic sedimentary sequences that lie unconformably above the ophiolitic melange basement in each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age equivalent ophiolite sections these relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement the sierran ophiolite belt is considered to have formed by a multistage process initiated by the early mesozoic tectonic accretion of upper paleozoic sea floor in general proximity to the ancient continental margin regional metamorphism and ophiolitic melange resulted this accretionary nucleus became the basement of jurassic age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m y ophiolites the rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic sedimentary sequences non volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes contact zones between pre existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles protoclastic deformation zones and cross cutting dikes such edge zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges both the 200 and 160 m y basins were destroyed by compressional orogenic episodes shortly after their formational episodes destruction of young ophiolite floored basins may be a common course of events when small oceanic type plates are generated along continental margin environments such tectonic settings are ideal for the emplacement of young ophiolite sheets

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

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

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

character distribution and tectonic significance of accretionary terranes in the central alaska range the central part of the alaska range near mount mckinley is composed of nine separate tectonostratigraphic terranes that were accreted in southern alaska during late mesozoic time these terranes now form long linear fault bounded belts that are subparallel to the denali fault on the north but oblique to the fault on the south the postaccretion right lateral offset along the denali fault system is about 200 km from north to south the major terranes are 1 yukon tanana terrane metasedimentary and metavolcanic rocks mostly undated but including rocks of known late paleozoic age polymetamorphosed with terminal events in late mesozoic 2 pingston terrane isoclinally folded upper triassic deep water silty limestone quartzite and carbonaceous slate folded with upper paleozoic phyllite chert tuff and minor limestone 3 mckinley terrane upper paleozoic flysch chert and minor limestone intruded by large gabbro sills and dikes and overlain by thick piles of triassic porphyritic pillow lava the top of the section is thick sequence of upper mesozoic conglomerate flysch chert and phyllite 4 dillinger terrane very thick sequence of strongly folded lower paleozoic micaceous sandstone turbidites graptolitic shale and deep water limestone locally overlain unconformably by jurassic fossiliferous sandstone or triassic pillow basalt 5 windy terrane heterogeneous assemblage of serpentinite basalt tuff and chert ophiolite with paleozoic and mesozoic flysch and blocks of mid paleozoic fossiliferous limestone 6 mystic terrane predominantly upper paleozoic flysch and conglomerate but also includes lower paleozoic graptolitic shale pillow basalt and shallow water limestone and upper paleozoic fossiliferous limestone sandstone chert and undated pillow basalt 7 chulitna terrane upper devonian ophiolite overlain by upper paleozoic chert volcanic conglomerate limestone and flysch capped by lower triassic limestone and upper triassic red beds basalt and interbedded basalt and limestone later mesozoic rocks are sandstone chert and argillite 8 westfork terrane jurassic chert sandstone conglomerate and triassic and jurassic crystal tuff 9 broad pass terrane upper paleozoic chert tuff and argillite with blocks of devonian and older limestone locally associated with serpentinite these diverse terranes of mixed oceanic and continental affinities are now juxtaposed to form a complex sequence of folded and faulted rootless nappes major suture zones between them are occupied by intensely deformed upper mesozoic flysch all of the terranes differ markedly from wrangellia the southermost coherent accreted terrane in alaska paleomagnetic measurements from triassic rocks prove that wrangellia originated near the equator probably in the southern heimsphere paleolatitudes for terranes in the central alaska range are not yet determined but other lines of evidence biogeographic and lithologic suggest large scale northern transport for some terranes

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

suevite of the ries crater germany source rocks and implications for cratering mechanics suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement glass inclusions and a small amount of sedimentary clasts data are given of the modal composition of fall out suevites deposited at isolated points around the crater and crater suevite forming a layer below post impact lake sediments in the crater cavity fall out suevites contain aerodynamically shaped bombs which are absent in crater suevite taking into account not only large glass fragments and bombs but also the finer fractions the glass content of fall out and crater suevites amounts to 47 and 29 vol respectively crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks mainly granites and a lower series of gneisses and amphibolite based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined suevites contain on the average 46 igneous and 54 metamorphic clasts in constrast weakly shocked and unshocked crystalline ejecta of the ries structure consist of 82 igneous and 18 metamorphic rocks

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