distribution and properties of soft weathered bedrock at 1 m depth in the contiguous united states the weathered bedrock zone is increasingly recognized as an important part of ecological and hydrologic systems but its distribution is poorly known in the contiguous united states we used spatial and laboratory characterization data from the us department of agriculture usda natural resources conservation service to assess the distribution and soil like properties of soft weathered bedrock saprock and saprolite within the 48 contiguous united states because usda soil inventories generally do not extend below 2 m and because the upper 1 m is clearly involved in ecosystem function and vadose zone hydrology we restricted our inquiry to soft weathered bedrock within 1 m of the land surface soft weathered bedrock within 1 m of the land surface is widespread throughout the contiguous united states underlying at least 6 of the land area in depth analysis of three states showed that soft weathered bedrock at the 1 m depth underlies 22 of the total land area in california 33 in wyoming and 18 in north carolina soft weathered bedrock hosts pedogenic activity as indicated by morphological features such as roots clay films and iron fe manganese mn oxide concretions recorded in pedon descriptions in the database the physical and chemical properties of soft weathered bedrock are often similar to those of the overlying soil suggesting that in many respects soft weathered bedrock behaves like soil it supplies water and nutrients to plants whose roots penetrate into it and it modulates throughflow runoff to streams for a more complete understanding of soft weathered bedrock systematic data are needed on its thickness across landscapes and a consistent terminology for its various forms needs to be adopted and widely used copyright 2012 john wiley sons ltd

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

spring related carbonate rocks mg clays and associated minerals in pliocene deposits of the amargosa desert nevada and california the amargosa desert of nevada california is a spring fed structural basin containing large amounts of pliocene carbonate rocks and mg clays deposited in playas marshland ponds and flood plains pliocene basin fill deposits dated at 2 4 to 3 2 ma are subdivided into three lithofacies designated tpa tld and tpl tpa is chiefly limestone and was deposited in marshland and ponds tld consists chiefly of limestone and montmorillonite claystone that were deposited in varied environments including ponds and flood plains tpl is almost entirely mg clays limestone and dolomite which were deposited in playas and associated marshland the largest area of playa sediments termed east playa is in the east central part of the basin limestone and dolomite seepage mounds and caliche breccia musses were formed along zones of ground water leakage within east playa chemically precipitated mg smectite and sepiolite form large deposits in tpl sepiolite was formed in water of low salinity whereas mg smectite was formed in water of higher salinity authigenic celadonitic illite and k feldspar are commonly associated with mg smectite and are very likely reaction products of montmorillonite in saline alkaline water silicic vitric ash layers were locally altered to phillipsite and k feldspar in saline alkaline water and to montmorillonite as a result of leaching by water of low salinity carbonate rocks are locally silicified along seepage zones carbonate minerals of the pliocene deposits have stable isotopic compositions that vary with their lithofacies tpa limestones deposited in or near areas of spring discharge have low 18o values averaging 16 6 tld limestones recrystallized in the vadose zone have higher 18o values averaging 18 1 carbonate minerals of tpl are the most 18o rich with calcite averaging 20 3 and dolomite 24 3 reflecting evaporative concentration in playas much of the dolomite is of replacement origin yet is in approximate isotopic equilibrium with the calcite it replaces relatively pure sepiolite has lower 18o values than do mg smectite clays which fits with the lower salinity inferred for sepiolite precipitation sepiolite and mg smectite disseminated in limestone fall outside the oxygen isotopic limits of the relatively pure clays possibly because of diagenetic recrystallization the pliocene climate was wetter than that of the present and springs were more widespread in the amargosa basin this dolomite may have formed in water isotopically and chemically about the same as that of the calcite and dolomitization may have been chiefly a result of the differing crystallization kinetics of calcite and dolomite and of fluctuating water chemistry in a playa environment the saline alkaline water of east playa resulted from evaporation of a mixture of alkali bicarbonate runoff from the volcanic terrane to the north with ca mg bicarbonate water discharged in springs from the ash meadows ground water system whose recharge area is principally in paleozoic carbonate rocks to the east and northeast seepage along fault zones in east playa was initially of ash meadows type low in silica and changed to siliceous water derived from volcanic rocks resulting in silicification of carbonate deposits caliche breccias along the western margin of the basin record a similar change in the composition of ground water seepage which may be attributed to a period of decreased precipitation about 2 5 ma which reduced recharge from nearby carbonate rocks relative to that from more distant volcanic rocks to the north and northwest

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

possible influences upon lake development in the east african rift valleys the rift valley lakes of east africa exhibit considerable variety in size hydrology hydrochemistry and sedimentary regime although tectonics and climate are ultimately responsible for lake formation vol canism may exert a significant influence upon the lake characteristics in the eastern rift kenya ethiopia extensive volcanic activity has disrupted drainage and helped create small shallow lake basins weathering of volcanic rocks produces runoff waters rich in na and hco 3 and smectite is a common clay mineral in contrast the western rift uganda mozambique has localized volcanism the lakes are large and deep with significant fluvial influents and effluents dissolved mg 2 k and so 4 are proportionately greater and detrital mineralogy is more variable carbonates and evaporites also reflect the hy drochemical differences in addition the topographic elevation of the lake within the rift may determine the composition and rate of sedimentation

influence of late quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont eastern mojave desert california radiocarbon dating of late quaternary deposits and shorelines of lake mojave and cation ratio numerical age dating of stone pavements dorn 1984 on the adjacent soda mountains piedmont provide age constraints for alluvial and eolian deposits these deposits are associated with climatically controlled stands of lake mojave during the past 15 000 yr six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces as well as on soil geomorphic data all but one of these stratigraphic units were deposited in response to time transgressive climatic changes beginning approximately 10 000 yr ago increased eolian flux rates occurred in response to the lowering of lake mojave and a consequent increase in fine sediment availability increased rates of deposition of eolian fines and associated salts influenced pedogenesis stone pavement development and runoff infiltration relations by 1 enhancing mechanical weathering of fan surfaces and hillslopes and 2 forming clay and silt rich surface horizons which decrease infiltration changes in alluvial fan source areas from hillslopes to piedmonts during the holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less permeable soils inferred increased in early to middle holocene monsoonal activity resulted in high magnitude paleo sheetflood events on older fan pavements this runoff triggered piedmont dissection which in turn caused increased sediment availability along channel walls thus runoff infiltration changes during the late quaternary have occurred in response to eolian deposition of fines pedogenesis increased sheetflood activity in the holocene and vegetational changes which are related to many complicated linkages among climatic change lake fluctuations and eolian hillslope and alluvial fan processes

the geopolitics of nuclear waste more debate has begun over questions related to the safety of high level waste disposal at the yucca mountain site in the nevada desert an engineering geologists jerry szymanski one of the department of energy s doe own staffers in las vegas has proposed that the 15 billion repository would sit on top of an intensely active structure that if altered by an earthquake would send a slug of ground water up from deep within the mountain into the waste storage area this theory has already been slammed in two formal reviews and has virtually no support among geologists however enough doubt has been raised that much more geological testing will be necessary to prove or disprove szymanski s theory nevada state officials are also using all methods to thwart or block the project the question of the origin of a series of calcium carbonate and opal veins exposed in an exploratory pit trench 14 near the top of the mountain is also far from answered the doe and us geological survey may have to collect much more information on the quantity size and location of carbonate sites in the area at a high financial outlay to the us government before a complete casemore on the origin of the material in trench 14 can be made less

systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern great basin united states stable carbon and oxygen isotope variations in holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern great basin united states a region with a semi arid climate our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate ecological variations differences in parent material and soil depth 13 c of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth indicating a decrease in the ratio of atmospheric to plantderived co 2 downprofile pedogenic carbonate at the soil air interface approaches a 13 c value in equilibrium with atmospheric co 2 in all three soils observed 13 c profiles for pedogenic carbonate can be described using a one dimensional model for 12 co 2 and 13 co 2 assuming isotopic equilibrium between soil co 2 and soil carbonate the modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil respiration rates at each site 13 c in deep pedogenic carbonate 50 cm varies by about 12 per mil over a 2 440 m elevation range being enriched in 13 c at the lowest elevations the slope of 13 c for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials depletion by 4 6 to 4 7 per mil per 1 000 m increase in altitude between 300 to 2 740 m above mean sea level for the localities studied this concordance makes it likely that there has been complete isotopic exchange between hco 3 in solution and soil co 2 prior to carbonate precipitation soil co 2 and soil respiration rates increase systematically with elevation the plantderived component of soil co 2 indicates that c 3 plants dominate the biomass at most measured sites in agreement with plant surveys calculated equilibrium fractionation factors between soil co 2 and soil carbonate are very similar to those observed again indicating complete isotopic exchange between carbon species in all the soil co 2 and soil carbonate data suggest that the 13 c variation with elevation observed in the soil carbonates results from differing soil respiration rates at each site as well as from variations in the proportion of c 3 to c 4 and cam plants in each site9s surface biomass 18 o values in pedogenic carbonates are higher at lower elevations due in part to the more positive 18 o values for meteoric waters at lower elevations the average 18 o value of deep 50 cm pedogenic carbonate at all sites however is enriched 2 4 to 3 7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters this suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place one or both of these processes also may explain the 18 o decrease in soil carbonate with depth observed in two of three soil profiles

ancient lacustrine hyperpycnites a depositional model from a case study in the rayoso formation cretaceous of west central argentina abstract hyperpycnal flows originate when sediment laden fluvial discharges enter standing lower density water because of their excess density the flows plunge near the river mouth and continue to travel basinward as a quasi steady and fully turbulent underflow the related deposits are hyperpycnites and constitute a particular type of turbidite with poorly known facies and facies tracts although hyperpycnal flows seem to be quite common in present times their occurrence in fossil strata is poorly documented this paper addresses the characteristics and depositional processes of shallow lacustrine sandy hyperpycnites on the basis of the field analysis of well exposed lower cretaceous strata rayoso formation in the neuquen basin of west central argentina the rayoso formation is composed of clastic and minor evaporitic red beds up to 1200 m thick deposited in a shallow perennial lake of variable salinity affected by long lived hyperpycnal flows main clastic facies are composed of fine grained sandstones with climbing ripples and plane beds other common sandstone facies include massive beds and low angle cross stratification most sandstone facies are related to traction plus fallout processes and often show a vertical fluctuation between sedimentary facies originated under different traction plus fallout conditions within single beds these fluctuations are interpreted to be evidence of deposition from flow fluctuations in sustained hyperpycnal flows most beds internally show the existence of three depositional phases acceleration ap erosion plus bypass ep and deceleration dp which record the complete evolution of a single long lived hyperpycnal flow at a fixed point additionally the depositional evolution of a single long lived hyperpycnal flow with distance records initially the progressive basinward migration of the ap and ep phases and finally an overall deposition under the dp phase both in proximal and distal areas this evolution provides an adequate explanation for the basinward extension of channelized features and for the occurrence of fine grained sandstones with climbing ripples both in proximal and distal positions within the same hyperpycnal system consequently facies analysis derived from application of the bouma sequence is not valid for deposits of quasi steady hyperpycnal flows

lacustrine sedimentation processes and patterns during effusive and explosive volcanism challis volcanic field idaho abstract lacustrine deposits in the eocene challis volcanic field accumulated in a deep lake bordered by steep ranges volcanic and nonvolcanic sediment was transported into the lake via a major dispersal system from the north high density turbidity currents transported this sediment throughout the lake system paleozoic rocks in the adjacent ranges were secondary sediment sources this sediment was trapped along the shoreline and only a small fraction was transported offshore by turbidity currents the frequency of turbidity currents varied with changes in eruption style thus turbidite accumulation patterns are a faithful record of changes in volcanic activity and associated landscape processes effusive volcanism that characterized the early activity in the challis volcanic field produced little sand size sediment and had little impact on lacustrine sedimentation patterns relatively slow settling from suspension characterized lake sedimentation during this time turbidity currents were infrequent the resulting accumulation pattern is a single sheet of turbidite sandstone encased in thick bedded mudrock initial eruptions at the van horn peak cauldron complex altered sedimentation patterns dramatically by increasing the frequency of turbidity currents a turbidite ramp prograded into the study area marking a major increase in sedimentation rate sediment composition and turbidite accumulation patterns record changes in landscape processes syneruption deposits are stacked turbidite sheets early turbidites contain mostly sediment derived from the fresh pyroclastic material the proportion of sediment from pre eruption sources increases upward through the stacked sheets recording a change in sediment sources at the end of the syneruption period ramp growth continued through this part of the period although at a much slower rate

distal fluvio lacustrine volcaniclastic resedimentation in response to an explosive silicic eruption the pliocene mushono tephra bed central japan the pliocene mushono tephra bed central japan is characterized by fluvio lacustrine volcaniclastic resedimentation at distal locations from the volcanic source sedimentary responses to an explosive eruption are well recorded in the resedimented mushono tephra bed immediately after the eruption large volumes of primary volcaniclastic material were reworked from proximal areas and discharged into the distal basin overloading of fluvial systems by pyroclastic debris resulted in the development of low sinuosity active fluvial channel systems that carried sediment laden hyperconcentrated flows the fluvial system flowed into a lake more than 150 km from the inferred source location resulting in delta formation and progradation primary pyroclastic fall deposits are only a few centimeters thick but are overlain by reworked volcaniclastic deposits more than 5 m thick which show coarsening upward successions representing fluvio lacustrine deltaic progradation the reworked succession consists of six sedimentary facies which can be interpreted as prodelta mouth bar distributary channel hyperconcentrated flow inter distributary lowland or floodplain and slump deposits rounded cobble sized pumice clasts in prodelta and distributary channel deposits and thick reworked deposits imply that the majority of resedimented volcaniclastic material was derived from voluminous proximal ignimbrite rather than thin fine grained distal fallout ash material vertical facies changes are concordant with a decrease in sediment yields with time lateral facies distributions correspond to episodic influxes of volcaniclastic sediments that controlled the morphology of discrete fluvio lacustrine deltas the distal sedimentary response to an explosive silicic eruption is thus clearly recorded in vertical and lateral facies distribution of reworked volcaniclastic sediments

stable isotopic evidence for a pedogenic origin of carbonates in trench 14 near yucca mountain nevada layered carbonate and silica encrust fault fractures exposed in trench 14 near yucca mountain site of the proposed high level nuclear waste repository in southern nevada comparison of the stable carbon and oxygen isotopic compositions of the fracture carbonates with those of modern soil carbonates in the area shows that the fracture carbonates are pedogenic in origin and that they likely formed in the presence of vegetation and rainfall typical of a glacial climate their isotopic composition differs markedly from that of carbonate associated with nearby springs the regional water table therefore remained below the level of trench 14 during the time that the carbonates and silica precipitated a period probably covering parts of at least the last 300 000 years

ordovician marine volcanic and sedimentary record of rifting and volcanotectonism snowdon wales united kingdom wales was part of a continental convergent plate margin for 50 m y during ordovician time the crustal stress regime was one of sinistral transtension due largely to oblique subduction early uplift and subsequent growth of subaerial arc volcanoes were followed by development of a broad fault bound marine basin 150 km wide in an infra arc or back arc setting within the basin relatively narrow grabens became sites of pronounced subsidence and bimodal basalt rhyolite volcanism the grabens developed in fault splays above steep crust penetrating discontinuities that focused extension and plumbing of magmas at these sites at snowdon the bedded pyroclastic formation records submarine and island basaltic volcanism with associated sedimentation at the site of an active rhyolitic caldera volcano the volcanoes formed on the axis of the snowdon graben and the basaltic succession which contains several unconformities due to marine abrasion at sea level shows a complex history of alternate uplift and subsidence the uplift is attributable to resurgence of rhyolitic magma and was 336 m in total subsidence due mainly to crustal extension was 500 m at a rate of 500 m m y the bedded pyroclastic formation shows clear evidence of tectonic influence in the timing location and style of basaltic magmatism repeated reactivation of magmatic plumbing systems demonstrates fault or fault intersection controlled channeling of magmas in the upper crust episodes of relatively vigorous subsidence preceded copious magmatism and both intrusive and extrusive basalt magmas exploited contemporaneously active normal faults pronounced topographic ponding of extrusive products and highly variable trends of fault block subsidence and rotation evident from the volcanogenic sedimentary deposits testify to chaotic piecemeal displacements on faults spaced commonly at

average infiltration at yucca mountain over the next million years 1 an uncertainty distribution is developed to describe areal average infiltration at yucca mountain averaged over the next million years the distribution uses infiltration tabulator for yucca mountain itym model results to estimate uncertainty in net infiltration given decadal average climate driven with two independent estimates for potential future climatic sequences both future climatic sequences use orbital mechanics linked to paleoclimatic proxies to estimate future climate states but one uses primarily biotic indicators and the other uses primarily abiotic indicators to link paleoclimate to orbital stage abiotic indicators suggest that present day climate variation by latitude in the great basin is reasonably analogous to glacial scale climate variation at a single location once orographic lake effect and insolation influences are accounted for in the southern sierra nevada abiotic indicators suggest that mean annual precipitation was 1 7 1 9 times larger than present and mean annual temperature was 5 6 7 c cooler than present at the last glacial maximum the influence of decadal to millennial scale variability is estimated using the 8000 year methuselah walk tree ring record which suggests that even large inferred fluctuations over the holocene only modestly increase 8000 year average infiltration combining the future climate projections with the infiltration model uncertainty distribution yields approximately lognormal uncertainty distributions for projected million year areal average net infiltration with a mean of 41 mm yr and coefficient of variation of 0 81 even if anthropogenic effects significantly alter climatic patterns relative to past climate cycles the projected anthropogenic period is sufficiently brief that million year averages are not likely to be substantially influenced

use of carbon isotopes in paleosols as an indicator of the p co2 of the paleoatmosphere measurements of carbon isotopic composition of coexisting paleosol organic matter and carbonate results in improved estimates of the paleo p co2 content of the paleoatmosphere measurements from tertiary paleosols indicate that the levels of co2 in the early eocene middle and late miocene and pliocene were less than 1000 ppmv early cretaceous levels of p co2 were significantly higher of the order of 1500 to 3000 ppmv

evidence for high frequency cyclic fault activity from high resolution seismic reflection survey rukwa rift tanzania a high resolution seismic survey of the rukwa rift has imaged the upper 300 m of the pliocene holocene section the sediments are characterized by aggradational deposition in shallow lacustrine conditions that episodically dried out overall the reflection package expands northeastwards towards the lupa fault half graben bounding fault isopach maps for nine intervals 6 m to 65 m thick show four periods of marked expansion of the section towards the lupa fault interpreted to represent times of fault activity or infilling of fault created topography during the early stage of fault quiescence there are three periods of little or much reduced expansion interpreted to represent infilling of the accommodation space between the lake floor and the lake surface during periods of fault inactivity cyclicity of such high frequency has not been widely reported for faults which penetrate the crust the periods of activity show remarkably similar displacement patterns with no lateral migration of the main depocentre implying that seismic and aseismic slip has persistently clustered in the same parts of the fault identification of high frequency fault cyclicity has implications for sequence stratigraphy in tectonically active areas since there is a tendency to assume faults do not have cyclic effects on the scale of tens of thousands of years

late quaternary environmental changes in the upper las vegas valley nevada abstract five stratigraphic units and five soils of late pleistocene to holocene age crop out in dissected badlands on corn creek flat 30 km northwest of las vegas nevada and at tule springs nearer to las vegas the record is dominantly fluvial but contains evidence of several moister marsh forming periods the oldest unit b dates perhaps to the middle wisconsin and the more widespread unit d falls between 30 000 and 15 000 yr b p unit d therefore correlates with pluvial maximum lacustrine deposits elsewhere in the great basin standing water was not of sufficient depth or extent during either period to form lake strandlines between 14 000 and 7200 yr b p unit e standing surface water gradually decreased a trend also apparent in great basin pluvial lake chronologies during the same period groundwater carbonate cementation and burrowing by cicadas cicadae accompany the moist phase units after 7200 yr b p increased wind action decreased biotic activity and at least 25 m of water table lowering accompanied widespread erosion of older fine grained deposits based on pack rat midden and pollen evidence this coincides with major vegetation changes in the valley from sagebrush dominated steppe to lower mohave desertscrub

overview of calcite opal deposits at or near the proposed high level nuclear waste site yucca mountain nevada usa pedogenic hypogene or both calcite opal deposits cod at yucca mountain were studied with respect to their regional and field geology petrology and petrography chemistry and isotopic geochemistry and fluid inclusions they were also compared with true pedogenic deposits tpd groundwater spring deposits gsd and calcite vein deposits cvd in the subsurface some of the data are equivocal and can support either a hypogene or pedogenic origin for these deposits however sr c and o isotope fluid inclusion and other data favor a hypogene interpretation a hypothesis that may account for all currently available data is that the cod precipitated from warm co2 rich water that episodically upwelled along faults during the pleistocene and which upon reaching the surface flowed downslope within existing alluvial colluvial eluvial or soil deposits being formed near or on the topographic surface the cod acquired characteristics of pedogenic deposits this subject relates to the suitability of yucca mountain as a high level nuclear waste site

downstream dilution of a lahar transition from debris flow to hyperconcentrated streamflow nearly instantaneous melting of snow and ice by the march 19 1982 eruption of mount st helens released a 4 106 m3 flood of water from the crater that was converted to a lahar volcanic debris flow through erosion and incorporation of sediment by the time it reached the base of the volcano over the next 81 km that it traveled down the toutle river the flood wave was progressively diluted through several mechanisms a transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater when the total sediment concentration had decreased to about 78 by weight 57 by volume the hyperconcentrated lahar runout flood wave transporting immense quantities of sand in suspension continued to experience progressive downstream dilution although turbulence was significantly dampened by the extremely high suspended load very large standing waves and antidune waves were observed the hyperconcentrated lahar runout flow deposited an unusual faintly stratified coarse sand which locally contained small isolated gravel lenses very similar deposits in the quaternary stratigraphy of mount st helens and other cascades volcanoes suggest that lahars may be more frequent than previously recognized

holocene explosive eruptions in the rungwe volcanic province tanzania abstract the holocene explosive eruptive record of rungwe and ngozi volcanoes of the rungwe volcanic province in tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole rock major and trace element analyses of tephra samples this reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the holocene tephro chronostratigraphy we show evidence of two catastrophic ngozi eruptions and five rungwe pumice fallout deposits and also identify several more intercalated poorly preserved pumice and ash deposits the ngozi eruptions probably played a role in shaping the present day caldera the rungwe record includes a ca 2 2 km3 deposit of a plinian style eruption dated at ca 4 ka a sub plinian one at ca 2 ka and at least three additional smaller scale fallout deposits the rungwe explosive eruptive record shows that the volcano has been frequently active in its late holocene past this study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment

a 250 000 year climatic record from great basin vein calcite implications for milankovitch theory a continuous record of oxygen 18 18o variations in the continental hydrosphere during the middle to late pleistocene has been obtained from a uranium series dated calcitic vein in the southern great basin the vein was deposited from ground water that moved through devils hole an open fault zone at ash meadows nevada between 50 and 310 ka thousand years ago the configuration of the 18o versus time curve closely resembles the marine and antarctic ice core vostok 18o curves however the u th dates indicate that the last interglacial stage marine oxygen isotope stage 5 began before 147 3 ka at least 17 000 years earlier than indicated by the marine 18o record and 7 000 years earlier than indicated by the less well dated antarctic 18o record this discrepancy and other differences in the timing of key climatic events suggest that the indirectly dated marine 18o chronology may need revision and that orbital forcing may not be the principal cause of the pleistocene ice ages

reworked pyroclastic beds in the early miocene of patagonia reaction in response to high sediment supply during explosive volcanic events abstract two meter scale pyroclastic levels are interbedded within the early miocene succession of the estancia 25 de mayo patagoniense transgression and santa cruz formations in the foreland austral or magallanes basin argentina the lower pyroclastic level lpl is a tabular body interbedded within offshore marine deposits laterally continuous for 30 km and varying in thickness from few centimeters to around 4 m grain size grades from coarse to extremely fine ash with upward fining along with a northeastern fining trends structureless fine to very fine tuffs dominate and rare parallel laminations are the only tractive sedimentary structures the upper pyroclastic level upl lies within low energy fluvial deposits and is laterally discontinuous and it is composed by lenticular bodies reaching a maximum of 15 m thick and 100 m wide with a concave up base and a plane top grain size range is similar to the lpl but it coarsens upward the lower portion of the upl shows parallel lamination current ripple lamination and mud drapes with large pumice lapilli and plant debris whereas the upper portion shows parallel lamination and trough cross stratification both pyroclastic levels are composed mainly of pumice grains and glass shards with minor proportions of quartz and plagioclase crystals and lithic fragments the lpl shows no mixing with epiclastic material whereas the upl shows an upward increase in epiclastic material and an upward increment in the scale of cross bedding the large thickness in relation to the possible emission center and the content of plant debris of the lpl does not suggest a direct submarine ash fallout origin the lpl is interpreted as a deposit of hyperpycnal flows generated at the coastal zone when tephra laden rivers plunged into the ocean large amounts of well preserved plant debris support the hypothesis of a terrestrial source of the sediments the upl is entirely composed of tractive deposits so an ash fallout origin is disregarded this together with the lenticular shape and the alluvial plain origin of the encasing sediments suggests accumulation within fluvial channels cycles of upper flow regime parallel lamination current ripple lamination and mud drapes at the lower portion suggest short lived turbulent flows that initially filled semi abandoned channels they were followed by sheet floods and channel reactivation expressed by large scale cross bedding the low degree of particle mixing observed in both levels is explained by the inability of streams to erode the substrate as they are suddenly over saturated with pyroclastic sediments during and after the eruption the grain size distribution of the lpl and geochemical data indicate a contemporaneous volcanic source located to the west southwest in the andean ranges where the south patagonian batholith is presently located

geochemical and c o sr and u series isotopic evidence for the meteoric origin of calcrete at solitario wash crater flat nevada usa calcite rich soils calcrete in alluvium and colluvium at solitario wash crater flat nevada usa contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi arid southwestern united states nevertheless a ground water discharge origin for the solitario wash soil deposits was proposed in a series of publications proposing elevation dependent variations of carbon and oxygen isotopes in calcrete samples discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at yucca mountain nevada site of a proposed high level nuclear waste repository new geochemical and carbon oxygen strontium and uranium series isotopic data disprove the presence of systematic elevation isotopic composition relations which are the main justification given for a proposed ground water discharge origin of the calcrete deposits at solitario wash values of 13c 4 1 to 7 8 per mil 18o 23 8 17 2 87sr 86sr 0 71270 0 71146 and initial 234u 238u activity ratios of about 1 6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at yucca mountain and are incompatible with a ground water origin for the calcrete variations in carbon and oxygen isotopes in solitario wash calcrete likely are caused by pedogenic deposition from meteoric water under varying quaternary climatic conditions over hundreds of thousands of years

sequence stratigraphy of a condensed low accommodation succession lower upper cretaceous dakota sandstone henry mountains southeastern utah cretaceous low accommodation deposits have been extensively studied in the subsurface of the western interior of north america because of their prolific hydrocarbon production and remaining potential understanding the stratigraphic complexities of these deposits in the subsurface relies strongly on detailed outcrop analogs in this study the dakota sandstone was examined along 100 km 62 mi of semicontinuous outcrop between the towns of hanksville and ticaboo in the henry mountains of southeastern utah this region represented a low accommodation setting located over the forebulge of the cretaceous western interior basin during accumulation of the unit the dakota sandstone is 0 to 38 m 125 ft thick of cenomanian age and records multiple cycles of sediment accumulation the dakota sandstone is subdivided into two condensed top truncated stratigraphic sequences the upper of which contains two parasequences the basal parts of both sequences are composed of braided fluvial conglomerates and sandstone overlain by tidally influenced fluvial sandstone inclined heterolithically stratified estuarine mudstone carbonaceous shale and coal the overlying parasequences consist of coarsening upward lower to upper shoreface mudstone sandstone tidal inlet deposits and oyster shell concentrations these facies define tripartite subdivisions of depositional environments typical of wave dominated estuaries the fluvial deposits may represent lowstand deposits but overall sediments accumulated during transgressive systems tracts tst the parasequences recorded in the henry mountains are similar to the dakota sandstone of northwestern new mexico and to high frequency sequences identified in the kaiparowits plateau approximately 80 km 50 mi to the southwest which suggests eustatic driving mechanisms the best potential for hydrocarbon reservoirs occurs in fluvial sandstones and conglomerates

evidence for an earthquake triggered basin collapse in saguenay fjord canada abstract on february 5 1663 perhaps the largest earthquake ever witnessed in eastern north america struck the saguenay fjord basin with such force that landslides and submarine slides were triggered over a wide region over 3 km 3 of clayey holocene sediments collapsed from the margins of the fjord resulting in amalgamated debris flow and slide deposits that covered much of the basin floor these deposits reached thicknesses of up to 100 m and extended over an area of 100 km 2 the ambient seafloor was compressed under the impact and load of these debris masses and or liquefied under the cyclic stress of the seismic wave a 0 2 km 3 landslide also occurred at this time blocking the saguenay river and possibly facilitating an extra large spring freshet discharge biotracers planktonic and benthic foraminifera arcellaceans pollen and wood in conjuction with mass physical properties of sediment cores and high resolution seismic and sidescan profiler data have been used to identify local and distal sediment sources including their original water depth of deposition and their subsequent mode of mass gravitational transport the landslide material was eroded by the river and super elevated sediment concentrations are considered responsible for a long lasting 28 days turbidity current that entered the saguenay fjord depositing a single 0 3 km 3 turbidite the 2 to 10 m thick turbidite is considered to have been self igniting having eroded into the surface of the collapsed basin debris paleo hydraulic calculations suggest that this turbid flow averaged 0 45 m s and reached a flow thickness of 30 m

calcretes from a palaeosinkhole in jurassic palaeokarst subbetic southern spain abstract calcretes have been recognized in the bottom of a palaeokarstic sinkhole etched into early jurassic carbonates from the subbetic southern spain these are 3 to 10 cm thick and they show an internal structure in which three units can be distinguished the lower unit is composed of alternate irregular bands of unfossiliferous micrite and sparry calcite in the micrite several features point out the activity of endolithic microorganisms as fungi and bacteria the middle unit which has a laminated structure with palisade calcite spar and some vadose silt is interpreted as a speleothem and finally the upper unit shows calcrete pisoids and rhizocretions the isotope ratio analysis gives results in which 13 c ranges from 10 1 to 10 9 and 18 o ranges from 5 7 to 7 0 these values are very similar to those from palaeosols of other ages and localities which is an important argument favouring emersion stages with continental deposition in an island with an arid or semi arid climate nevertheless these calcretes outcrop in association with a palaeokarstic relief reflecting wet climate before the calcrete formation

the distribution and discrimination of shallow authigenic carbonate in the pliocene pleistocene palomas basin southern rio grande rift nine types of authigenic carbonate are present in the pliocene pleistocene alluvial fan and axial fluvial sediment of the palomas half graben in the southern rio grande rift pedogenic and other vadose carbonate includes 1 pedogenic carbonate of stage ii and stage iii morphology underlying bw and bt horizons 2 mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone 3 pedogenic calcic nodules and tubules in eolian sand and 4 gully bed cement of proximal fan conglomerates formed by infiltration and evaporation of surface runoff shallow groundwater carbonate exists as 5 thin 30 50 cm massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe 6 thick 1 5 3 m massive beds deposited by lateral flow of groundwater or at springs and 7 thin 30 cm calcified root mats associated with near surface water saturated sediment or springs phreatic spar cements 8 occupy the interstices of conglomerates and sandstones and locally exist as 9 oriented concretions groundwater carbonates are best developed near the toes of the large hanging wall derived alluvial fans whereas phreatic cement preferentially exists in footwall derived alluvial fan conglomerates pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons a predominantly vertical arrangement of root traces peds desiccation cracks and calcic tubules and by gradational contacts the 13 c and 18 o values are similar among pedogenic and shallow groundwater carbonate although locally nodules in eolian sand and gully bed cement have higher 18 o values perhaps due to the effects of evaporation some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of 13 c and 18 o authigenic carbonate in footwall derived alluvial fan sediment has consistently higher values of 13 c than that in hanging wall derived sediment which may reflect differences in vegetative type and or density on either side of the basin

u and sr isotopes in ground water and calcite yucca mountain nevada evidence against upwelling water hydrogenic calcite and opaline silica deposits in fault zones at yucca mountain nevada have created considerable public and scientific controversy because of the possible development of a high level nuclear waste repository at this location strontium and uranium isotopic compositions of hydrogenic materials were used to test whether the veins could have formed by upwelling of deep seated waters the vein deposits are isotopically distinct from ground water in the two aquifers that underlie yucca mountain indicating that the calcite could not have precipitated from ground water the data are consistent with a surficial origin for the hydrogenic deposits

a 22 kyr bp sedimentological record of lake rukwa 8 s sw tanzania environmental chronostratigraphic and climatic implications abstract the sedimentological study of a 12 8 m long core r96 i from lake rukwa tanzania 8 s provides a new record of past lake level fluctuations that took place in response to changes of the regional climate since the last glacial period from 21 to 15 cal kyr bp nearshore and swamp marsh environments are evidenced from clastic deposition macrophytes debris and ca mg carbonate enrichments a transgressive sequence starting around 15 cal kyr bp lead to the humid lacustrine optimum between 15 and 7 cal kyr bp from 12 to 10 cal kyr bp anoxic lake bottom environments were favored by the concomitant effects of high primary productivity increased subsidence and inputs of volcanic ash in the lake the middle holocene 7 3 cal kyr bp is characterized by high concentrations of silt carbonates and low organic content which indicate the occurrence of relatively oxic shallow and saline depositional environments especially around 7 and 3 4 cal kyr bp although grainsize and toc profiles suggest that shallow environments likely persisted in the upper holocene low mg concentration values for the last 3 kyr may indicate a trend toward relatively more dilute environments the two main lake rukwa low stand periods at 21 15 and 7 3 cal kyr bp correspond remarkably well with the insolation maximum at the equator which occurred at 17 kyr bp for the spring equinox and at 6 kyr bp for the autumn equinox respectively we suggest that during these periods of minimum inter hemispheric insolation gradients monsoon circulation weakened in south equatorial regions and the itcz was located north of 8 s in east africa

transient calcite fracture fillings in a welded tuff snowshoe mountain colorado abstract the core from two boreholes 13 1 and 19 2 m depth drilled 500 m apart in the fractured welded tuff near the summit of the snowshoe mountain colorado 47 30 n 106 55 w had unique petrographic and hydrodynamic properties borehole sm 4 had highly variable annual water levels in contrast to sm 1a whose water level remained near the land surface core samples from both boreholes n 10 and 11 were examined petrographically in thin sections impregnated with epoxy containing rhodamine to mark the pore system features and were analyzed for matrix porosity and permeability core from the borehole sampling the vadose zone was characterized by open fractures with enhanced porosity around phenocrysts due to chemical weathering fractures within the borehole sampling the phreatic zone were mineralized with calcite and had porosity characteristics similar to unweathered and unfractured rock at the top of the phreatic zone petrography indicates that calcite is dissolving thereby changing the hydrogeochemical character of the rock i e permeability porosity reactive surface area and mineralogy radiocarbon ages and c and o stable isotopes indicate that calcite mineralization occurred about 30 to 40 ka ago and that there was more than one mineralization event results of this study also provide some relationships between primary porosity development from 3 types of fracture in a welded tuff

geology and geophysics of the rukwa rift east africa exploration in the rukwa rift using gravity and seismic reflection surveys fieldwork and drilling has defined the structure and stratigraphy of the basin in greater detail than any other part of the western rift the stratigraphy comprises precambrian basement karroo sandstones shales and coals upper miocene red beds and miocene recent lacustrine and fluvial sediments during miocene recent rifting the greatest sediment input apparently came from axial fluvial systems flowing from the northwest and southeast the southwestern area experienced alternating shallow lacustrine and fluviodeltaic conditions during the miocene recent cenozoic age rift structures have a dominant nw se and a subordinate n s trend the nw se trend tends to follow a precambrian basement and later karroo structural trend ne sw seismic lines indicate up to 10 km extension of the tertiary section in a direction oblique to the probable e w regional extension direction in the southeastern portion of the basin both karroo and tertiary quaternary sediments expand into the lupa fault zone reaching thicknesses of up to 3 and 7 km respectively tertiary quaternary sediment thicknesses decrease northwestward accompanied by a decrease in the amount of extension and a broadening of the basin as extension is transferred to the lake tanganyika rift

pedogenic carbonates in texas stable isotope distributions and their implications for reconstructing region wide paleoenvironments abstract pedogenic carbonates are ubiquitous in paleosols through geologic history and have been widely used to speculate on paleoclimatic conditions during their formation they are widespread and well developed in large areas of texas a majority are of late quaternary age and have formed across a range of climatic conditions on various types of substrates to evaluate the usefulness of pedogenic carbonates as environmental indicators this study investigates regional scale trends in stable isotope compositions of pedogenic carbonates across approximately 900 km of texas the 13c values of 49 pedogenic carbonate profiles 182 analyses on various host strata e g limestone igneous rocks range from 10 6 to 0 7 v pdb most 13c values are evenly distributed between 9 and 1 and the 18o values range from 9 1 to 0 2 v pdb mainly between 6 and 2 there is no correlation between carbon and oxygen isotopic values r2 0 0002 pedogenic carbonates in the texas gulf coastal plains southern high plains and west texas exhibit higher 13c values than those in south and central texas which corresponds to the spatial change in vegetation between c4 dominated and c3 dominated plants the marked increase in 13c values of the pedogenic carbonates from the tertiary strata into the overlying quaternary strata confirms the invasion of a c4 biomass into the southern high plains since the early pliocene from east to west the 18o values of late quaternary pedogenic carbonates decrease gradually a trend that mimics the spatial variation of 18o values of modern meteoric water primarily due to the increased distance from the gulf of mexico source of most of the meteoric water in contrast to the regional trends the stable isotope values of some pedogenic carbonates developed on marine limestone and calcareous alluvium in central texas and south texas have isotopic signatures inherited from the host strata in addition some of the 13c values of pedogenic carbonate nodules in the gulf coastal plains have been affected by saline groundwater soil carbonates formed in the shallow parts of the profile such as petrocalcic horizons may have also been influenced by the atmospheric co2 nevertheless climatic trends are readily apparent in the isotopic data thus demonstrating their potential value in region wide paleoclimatic analyses

climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania the interactions between climatic and volcanic forcing on diatom communities contained in a 50 000 year sedimentary sequence from lake massoko tanzania were examined at the century scale 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels whereas at the millennial scale diatom inferred shifts in precipitation evaporation based on conductivity optima and diatom life form ratios were broadly similar to lake level reconstructions from lake rukwa lake malawi and others in the region some fluctuations of lake massoko are consistent with the precession driven changes in insolation but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone sea surface temperatures are strongly correlated to multimillennial scale climate patterns over this region of africa

stable isotope distribution in maastrichtian vertebrates and paleosols from the haţeg basin south carpathians abstract located in the south carpathians in romania the ha eg basin contains a thick sequence of maastrichtian continental deposits yielding a rich dinosaur and mammalian fauna stable isotope analyses of both calcretes and dinosaur crocodilian and turtle remains from two localities tustea and sibisel were integrated in order to reconstruct environmental conditions during maastrichtian time and to gain further insights into the metabolism and behaviour of the vertebrates the large difference observed between the 18o and 13c of the eggshells and the surrounding mudstones as well as the preservation of the 9 difference between the oxygen isotope composition of the telmatosaurus eggshell and tooth enamel indicate that diagenesis has not significantly altered the primary isotopic signal the presence of smectite in the fraction less than 2 m and the preservation of dinosaur eggshell structure also indicate that diagenesis was not significant in the studied outcrops stable isotope compositions of both calcretes and phosphatic remains suggest warmer conditions during the deposition of the tustea sequence mean annual temperature of around 14 c than during the deposition of the sibisel sequence mean annual temperature of 10 c the intra tooth 18o patterns for zalmoxes and allodaposuchus show different magnitudes of isotopic variation with lower values for tustea and higher for sibisel the calculated 18obody water enrichment for kallokibotion bajazidi is similar to that found in living turtle taxa by contrast in the case of allodaposuchus the isotopic enrichment is higher than for recent taxa this suggests that for allodaposuchus the body water was less buffered by a watery environment which probably indicates more time spent outside water i e more terrestrial habit the 18o values for the teeth of telmatosaurus and zalmoxes are similar to those of allodaposuchus suggesting that at the investigated sites the body temperature of both dinosaurs was similar to that of the crocodile the isotopic composition of calcretes teeth and eggshells indicates a c3 vegetation and diet with 13c values between 27 and 29 pdb and the absence of large scale habitat partitioning between the dinosaurs

quaternary ostracodes and molluscs from the rukwa basin tanzania and their evolutionary and paleobiogeographic implications abstract much of the spectacular biodiversity of the african great lakes is endemic to single lake basins so that the margins of these basins or their lakes coincide with biogeographic boundaries longstanding debate surrounds the evolution of these endemic species the stability of bioprovinces and the exchange of faunas between them over geologic time as the rift developed because these debates are currently unsettled we are uncertain of how much existing distribution patterns are determined by modern hydrological barriers versus reflecting past history this study reports on late quaternary fossils from the rukwa basin and integrates geological and paleoecological data to explore faunal exchange between freshwater bioprovinces in particular with lake tanganyika lake rukwa s water level showed large fluctuations over the last 25 ky and for most of this period the lake contained large habitat diversity with different species assemblages and taphonomic controls along its northern and southern shores comparison of fossil and modern invertebrate assemblages suggests faunal persistence through the last glacial maximum but with an extirpation event that occurred in the last 5 ky some of the molluscs and ostracodes studied here are closely related to taxa or part of clades that are currently endemic to lake tanganyika but others testify to wider and perhaps older faunal exchanges between the rukwa bioprovince and those of lake malawi and the upper congo in particular lake mweru the rukwa basin has a long history of rifting and lacustrine conditions and at least temporarily its ecosystems appear to have functioned as satellites to lake tanganyika in which intralacustrine speciation occurred paleontological studies of the rukwa faunas are particularly relevant because of the basin s important role in the late cenozoic biogeography of tropical africa and because many of the molecular traces potentially revealing this history would have been erased in the late holocene extirpation

textural and isotopic evidence for ca mg carbonate pedogenesis abstract models for evaluating the terrestrial carbon cycle must take into account not only soil organic carbon represented by a mixture of plant and animal remains but also soil inorganic carbon contained in minerals mainly in calcite and dolomite thick soil caliches derived from weathering of mafic and ultramafic rocks must be considered as sinks for carbon storage in soils the formation of calcite and dolomite from pedogenic alteration of volcanic tephras under an aridic moisture regime is studied in an unusually thick 3 m soil profile on gran canaria island canary islands spain the biological activity of the pedogenic environment soil respiration releases co2 incorporated as dissolved inorganic carbon dic in waters it drives the formation of low magnesian calcite and calcian dolomite over basaltic substrates with a 13c negative signature 8 to 6 vs v pdb precipitation of authigenic carbonates in the soil is accompanied by the formation of mg rich clay minerals and quartz after the weathering of basalts mineralogical textural compositional and isotopic variations throughout the soil profile studied indicate that dolomite formed at greater depths and earlier than the calcite the isotopic signatures of the surficial calcite and deeper dolomite crusts are primary and resulted from the dissolution precipitation cycles that led to the formation of both types of caliches under different physicochemical conditions dolomite formed within a clay rich matrix through diffusive transport of reactants it is precipitated from water with more negative 18o values 1 5 to 3 5 vs v smow in the subsoil compared to those of water in equilibrium with surficial calcite thus calcite precipitated after dolomite and directly from percolating solutions in equilibrium with vadose water enriched in 18o 0 5 to 1 5 due to the evaporation processes the accumulation of inorganic carbon reaches 586 1 kg m 2 in the soil studied which means that the carbon sequestration capacity of mafic rocks must be taken into account for certain terrestrial settings dolomite together with calcite should be assessed when quantifying carbon stored in arid semiarid soils as a result of the natural weathering processes

thermal history of the unsaturated zone at yucca mountain nevada usa abstract secondary calcite silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry temperatures and timing of past fluid movement in the unsaturated zone at yucca mountain nevada the proposed site of a high level radioactive waste repository the distribution and geochemistry of these deposits are consistent with low temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone however the discovery of fluid inclusions in calcite with homogenization temperatures th up to 80 c was construed by some scientists as strong evidence for hydrothermal deposition this paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition mineral precipitation temperatures in the unsaturated zone are estimated from calcite and fluorite hosted fluid inclusions and calcite 18o values and depositional timing is constrained by the 207pb 235u ages of chalcedony or opal in the deposits fluid inclusion th from 50 samples of calcite and four samples of fluorite range from 35 to 90 c calcite 18o values range from 0 to 22 smow but most fall between 12 and 20 the highest th and the lowest 18o values are found in the older calcite calcite th and 18o values indicate that most calcite precipitated from water with 18o values between 13 and 7 similar to modern meteoric waters twenty two 207pb 235u ages of chalcedony or opal that generally postdate elevated depositional temperatures range from 9 5 to 1 9 ma new and published 207pb 235u and 230th uages coupled with the th values and estimates of temperature from calcite 18o values indicate that maximum unsaturated zone temperatures probably predate 10 ma and that the unsaturated zone had cooled to near present day temperatures 24 26 c at a depth of 250 m by 2 4 ma the evidence of elevated temperatures persisting in ash flow tuffs adjacent to parent calderas for as much as 8 ma is a new finding but consistent with thermal modeling simulations using the heat code demonstrate that prolonged cooling of the unsaturated zone is consistent with magmatic heat inputs and deep seated sub water table hydrothermal activity generated by the large magma body 8 km to the north that produced the 15 11 ma ash flows and ash falls that make up yucca mountain the evidence discussed in this and preceding papers strongly supports unsaturated zone deposition of the secondary minerals from descending meteoric waters although depositional temperatures reflect conductive and possibly vapor phase convective heating of the unsaturated zone related to regional magmatic sources until perhaps 6 ma depositional conditions similar to the present day unsaturated zone have prevailed for at least the past 2 4 ma

interplay of structural climatic and volcanic controls on late quaternary lacustrine deltaic sedimentation patterns in the western branch of the east african rift system rukwa rift basin tanzania abstract this paper presents the first detailed outcrop based sedimentologic investigation of the pleistocene holocene upper lake beds succession in the rukwa rift basin located in the western branch of the east african rift system southwestern tanzania the goal of this investigation is to examine the sedimentary facies and reconstruct the depositional environments of this important archive of quaternary climate and environmental change eleven diagnostic facies associations comprising 24 facies were identified and provide the basis for recognition of three key deposystems 1 alluvial to fluvial channel system 2 lake delta system and 3 profundal lacustrine system analysis of paleocurrent indicators and sandstone provenance indicate widely dispersed source regions and drainage patterns that were strongly influenced by major border fault systems and episodic volcanism six stratigraphic sequences a f ranging from 2 to 17 m thick were identified based on stratal stacking patterns and the development of sequence bounding unconformities and lacustrine flooding surfaces sedimentation processes facies architecture and stratigraphic packaging record a complex interplay between quaternary climate fluctuations and intense episodic volcanism in the nearby rungwe volcanic province set against large scale tectonic controls associated with synchronous development of the east african rift system sequence stratigraphic analysis reveals that the rukwa rift basin episodically shifted between a balanced fill lake basin and an overfilled lake basin deep water basin wide lake expansion occurred at different times during the late quaternary the final depositional sequence preserved in the basin a fluvial underfilled lake basin initiated 7 9 ka and has persisted to the present day high frequency climate change played the key role in sequence development in the upper lake beds however voluminous rift related volcanism and erosion of abundant labile volcanic materials from the rungwe volcanic province as well as syntectonic evolution of the rift led to high sedimentation rates and transformation of flash floods and debris flows in the hinterlands rift margin to hyperpycnal flows towards the basin depocenter

stable carbon and oxygen isotopes of pedogenic carbonates ajo mountains southern arizona implications for paleoenvironmental change abstract in this study we compare the stable isotope composition of late holocene pedogenic carbonate from a late holocene soil developed on the pediment below the ajo mountains in southern arizona usa with measurements of the 18 o of the soil water throughout the seasonal cycle and with 18 o and 13 c measurements on older dated soils the comparison with the soil water composition shows that the pedogenic carbonate is forming in equilibrium with highly evaporated soil water at the highest summer temperatures the 13 c of the carbonate apparently reflects equilibrium with co 2 of an isotopic composition consistent with the present mixture of c3 desert shrubs and cam cactus comparison with glacial period soil carbonates shows that there has been little change in the 18 o but that the 13 c was about 4 5 heavier independent evidence indicates that glacial period summer temperatures were probably much cooler and that the 18 o of precipitation was also lighter the increase in the carbonate water fractionation factor with decreasing temperature probably counteracted the decrease in the 18 o of precipitation producing little net change in the 18 o of the soil carbonate we attribute the decrease in the 13 c to replacement of a glacial period c4 grassland by holocene c3 cam desert shrubs and succulents this replacement was probably in response to an increase in temperature and reduction in summer precipitation at the end of the last glacial period the 13 c of older soil carbonates indicates that landscape was dominated by grassland for most of the past 700 ka and that the present vegetation likely represents a relatively brief anomaly

reply to the comment on origin timing and temperature of secondary calcite silica mineral formation at yucca mountain nevada by y v dublyansky s z smirnov and g p palyanova

early pleistocene lake formation and hominin origins in the turkana omo rift abstract prior research has correlated the formation of plio pleistocene lakes in east africa to global regional climate changes and interpreted the lacustrine basins as significant settings of hominin evolution paleo lake lorenyang from the turkana omo rift is important to these issues as its marginal deposits contain some of if not the earliest currently known specimens of acheulian stone tools and african homo erectus magnetostratigraphic and sedimentological evidence indicates that the oldest preserved paleo lake lorenyang deposits are dated at 2 148 2 128 ma and derive from the nw turkana basin predating those from the omo basin by 100 kyr and the ne turkana basin by 190 kyr apparently the lake expanded asynchronously in the rift potentially due to a volcano tectonic influence on the location of drainage networks depositional slopes or topographic elevation differences within and between the basins at the time of flooding the onset of the lake temporally coincides with the eruption of basalt lava flows dated to 2 2 2 0 ma that blocked the southeast outlet of the turkana basin this provides a plausible mechanism for hydrologic closure and lacustrine basin formation through volcano tectonic impounding it also points to a non climatic cause for the initial formation of paleo lake lorenyang at 2 14 ma first appearances for african h erectus 1 87 ma and acheulian tools 1 76 ma in the turkana omo rift postdate the lake s initial formation by about 270 kyr and 380 kyr respectively such timing differences contrast with studies that correlate all three to the 400 kyr eccentricity maximum at 1 8 ma although the turkana omo rift is just one example it does provide alternative insights to views that link climate hominin evolution and lake formation in east africa

east african climate change and orbital forcing during the last 175 kyr bp abstract variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long term fluctuations in african lake levels the understanding of such relationships is essential for designing climate prediction models for the tropics our assessment of the nature and timing of east african climate change is based on lake level fluctuations of lake naivasha in the central kenya rift 0 55 s 36 20 e inferred from sediment characteristics diatoms authigenic mineral assemblages and 17 single crystal 40ar 39ar age determinations assuming that these fluctuations reflect climate changes the lake naivasha record demonstrates that periods of increased humidity in east africa mainly followed maximum equatorial solar radiation in march or september interestingly the most dramatic change in the naivasha basin occurred as early as 146 kyr bp and the highest lake level was recorded at about 139 133 kyr bp this is consistent with other well dated low latitude climate records but does not correspond to peaks in northern hemisphere summer insolation as the trigger for the ice age cycles the naivasha record therefore provides evidence for low latitude forcing of the ice age climate cycles

structure and stratigraphy of the rukwa rift abstract combining recently acquired multifold seismic data with well and gravity information and field mapping a comprehensive picture of the structure and stratigraphy of the rukwa rift has emerged the rukwa rift lies between the tanganyika and nyasa malawi rifts in the western branch of the east african rift system in southwest tanzania the rukwa rift is a nw trending half graben basin that is 350 km long and 40 km wide unlike the neighboring tanganyika and nyasa rifts there is no evidence of half graben polarity reversals in the rukwa rift the nw trending boundary fault system lies on the northeastern side of the basin and comprises a series of listric faults most internal faults also show listric forms and trend n s oblique to the boundary fault the basal sedimentary section is the permo triassic karroo sequence this is overlain by the red bed sandstone sequence in which both mesozoic and tertiary fauna have been found the cenozoic lake bed sequence is the highest unit and covers nearly all of the present basin sediment thicknesses commonly reach 7 km and attain a maximum of 12 km at the southeastern end of the basin the lake bed sequence is the thickest unit in the main depocentre but the karroo sequence is often the thickest unit on the shoaling side of the half graben the rukwa rift is here interpreted to have evolved as a strike to oblique slip pull apart basin based on numerous indications of nw trending strike slip faulting

new biostratigraphic age dates from the lake rukwa rift basin in western tanzania geologic note 1 the rukwa rift basin in western tanzania is a segment of the western branch of the east african rift system three major stratigraphic units have been recognized in the basin from oldest to youngest karroo supergroup the red sandstone and the lake beds these units have traditionally been assigned the following ages respectively based upon lithologic correlations permian triassic jurassic cretaceous and tertiary recently microfloral assemblages collected from sidewall cores and cuttings from two deep exploration wells drilled in the basin have provided new data on the age of these formations the biostratigraphic data suggest the formations should be assigned the following ages karroo supergroup late permian red sandstone neogene miocene late pliocene and the lake beds late pliocene holocene the new age dates indicate that the post karroo to pre miocene history of the basin is one of relative quiescence and the modern rift is a relatively young feature this interpretation is more consistent with the tectonic histories of the other basins of the western rift

sims analyses of minor and trace element distributions in fracture calcite from yucca mountain nevada usa abstract fracture lining calcite samples from yucca mountain nevada obtained as part of the extensive vertical sampling in studies of this site as a potential high level waste repository have been characterized according to microbeam scale 25 30 m trace and minor element chemistry and cathodoluminescent zonation patterns as bulk chemical analyses are limited in spatial resolution and are subject to contamination by intergrown phases a technique for analysis by secondary ion mass spectrometry sims of minor mn fe sr and trace ree elements in calcite was developed and applied to eighteen calcite samples from four boreholes and one trench sims analyses of ree in calcite and dolomite have been shown to be quantitative to abundances bulk chemical signatures noted by vaniman 1994 allowed correlation of minor and trace element signatures in yucca mountain calcite with location of calcite precipitation saturated vs unsaturated zone for example upper unsaturated zone calcite exhibits pronounced negative ce and eu anomalies not observed in calcite collected below in the deep unsaturated zone these chemical distinctions served as fingerprints which were applied to growth zones in order to examine temporal changes in calcite crystallization histories analyses of such fine scale zonal variations are unattainable using bulk analytical techniques in addition lree particularly ce scavenging of calcite precipitating solutions by manganese oxide phases is discussed as the mechanism for ce depletion in unsaturated zone calcite

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

a paleomagnetic record from tangle lakes alaska large amplitude secular variation at high latitudes a paleomagnetic study of a core from the tangle lakes area of alaska has provided information about the nature of secular variation between 2 700 and 4 600 years b p the data indicate that the amplitudes of fluctuations in inclination and declination were substantially larger than those which would be expected from the present configuration of the geomagnetic field from this single site it is not possible to determine whether the observed secular variation arises mainly from the dipole or the non dipole component of the field if the dipole component is responsible then the data indicate that about 3 350 years b p the geomagnetic pole shifted by about 20 in a period of less than 150 years this shift is comparable to one proposed to have occurred about 500 years ago

the effect of vegetation on infiltration in shallow soils underlain by fissured bedrock abstract mean annual infiltration above the high level waste repository proposed to be sited at yucca mountain nevada has a large impact on assessments of repository performance ongoing investigations of infiltration processes have identified the relatively horizontal caprock environment above portions of the repository as a potentially large source of infiltrating waters due to shallow permeable soils above a moderately welded tuff with large soil filled fissures the combination of shallow soils and fissured bedrock allows rapid penetration of wetting pulses to below the rooting zone plant uptake can strongly reduce net infiltration in arid environments with high water storage capacity and despite the low water storage capacity there is a relatively high vegetation density in this environment the apparent discrepancy between high vegetation density and low water storage motivates the study of plant hydrologic interactions in this semiarid environment field observations were coupled with plant and landscape scale models to provide insight into plant hydrologic interactions several lines of evidence including i linear plant growth features observed on aerial photographs ii comparisons of plant cover within the fissured environment and comparable environments lacking fissures and iii direct excavations all suggest that the widely spaced soil filled fissures are conducive to plant growth even when fissures are buried at soil depths exceeding 30 cm results from a mechanistic simulation model for root growth into fissures suggest that the additional sheltered plant available soil water within fissures provides a competitive advantage for plant establishment therefore plants that germinate above a fissure are more likely to survive in turn developing linear features above fissures having established that plants preferentially root within soil filled fissures in the caprock environment a set of simulations were performed to examine the hydrologic consequence of plant roots within fissures at the landscape scale the response to three rainfall amounts was simulated for the largest storm fluxes at the fissure bottom peaked at 1 4 weeks after the storm when plant uptake was not active but were eliminated when fissures had active vegetation when plants were active within a fissure uptake eliminated net infiltration in the fissure regardless of the size of the storm two plant related mechanisms reduced total flux through the plant filled fissures i transpiration during fissure flow and ii wetting pulse retardation due to drier fissures prior to rain the first mechanism appears to be dominant in these simulations results suggest that transpiration may strongly limit net infiltration i e total deep percolation flux escaping the plant root zone significant infiltration can occur however when plants are dormant so that most infiltration would be expected to occur during winter

tectonic controls on rift basin morphology evolution of the northern malawi nyasa rift radiometric k ar and ar 40 ar 39 age determinations of volcanic and volcaniclastic rocks combined with structural gravity and seismic reflection data are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern malawi nyasa rift and to characterize changes in basin and flank morphologies with time faulting and volcanism within the tukuyu karonga basin began at approximately 8 6 ma when sediments were deposited in abroad initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief extensions primarily by a slip along the border fault and subsequent regional isostatic compensation led to the development of a 5 km deep basin bounded by broad uplifted flanks along the low relief basin margin opposite border fault younger stratigraphic sequences commonly onlap older wedge shaped sequences although their internal geometry is often progradational intrabasinal faulting flankuplift and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2 5 ma when cross rift transfer faults developed to link the tukuyu karonga basin to the rukwa basin local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin s depocenter sequence boundaries are commonly erosional along this low relief hanging wall margin and conformable in the deep lake basin the geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake irrespective of paleoclimatic fluctuations