soil mineralogy trends in california landscapes california s diverse environmental gradients serve as natural experiments for examining controls on soil mineral distribution in landscapes in this paper we use example soilscapes from throughout california to examine how lithology climate topography and duration of pedogenesis interact to produce distinctive weathering environments and characteristic suites of soil minerals seven soil geomorphic sequences were assembled from the literature to illustrate major soil mineralogical trends 1 granitic terrain of the peninsular ranges 2 granitic terrain of the central sierra nevada 3 andesitic terrain of the northern sierra nevada 4 fluvial terraces on the east side of the great valley 5 marine terraces of the central coast 6 ultramafic terrain of the klamath mountains and 7 an alluvial fan in the mojave desert results of this analysis show that kaolin is present in virtually all pedons irrespective of climate parent material age or topographic position kaolin does not form in ultramafic soils due to insufficient aluminum many secondary clay minerals reflect the state s strong climatic influence with palygorskite smectite and vermiculite in the dry hot environments hydroxy interlayered minerals gibbsite and short range ordered minerals in the cool moist environments in arid and semi arid regions the distribution of calcite gypsum and soluble salts is strongly related to patterns of eolian dust deposition and water infiltration and leaching

cracks in desert pavement rocks further insights into mechanical weathering by directional insolation the formation of cracks is a fundamental first step in the physical weathering of rocks in desert environments in this study we combine new field data from the mojave u s gobi mongolia and strzelecki australia deserts that collectively support the hypothesis that meridional cracks cracks with orientations not readily attributable to rock anisotropies or shape in boulders or cobbles form due to tensile stresses caused by directional heating and cooling during the sun s daily transit the new studies indicate that rock size surface age and latitude play important roles with respect to their influence on rock fracture rock size and pavement surface age exert an influence on the development of rock cracks as the average clast size of mature desert pavements may be at or below the threshold clast size for thermal cracking of rocks latitude controlled seasonal temperature variations play a key role as demonstrated by 1 tightly clustered mean resultant orientations that differ by latitude as predicted in mcfadden et al 2005 and 2 very cold wintertime temperatures and strong diurnal gradients that may favor crack development in wintertime given the likelihood for strong clast heating during early morning hours the consistent evidence for meridional cracks in surfaces of diverse age and desert environments climate vegetation and distance of clast transport indicate that directional insolation may play the key role in initially generating and propagating rock fractures rather than a secondary role as implied in recent field and modeling studies of physical weathering in deserts

surface control of desert pavement pedologic process and landscape function cima volcanic field mojave desert california desert pavement is a distinctive feature widespread across arid lands of the world it plays a dynamic role in geomorphic hydrologic and ecologic processes where desert pavement predominates infiltration is limited and rainfall is delivered as runoff to nearby bare ground areas where shrubs cluster desert pavement surfaces may appear monotonously flat and barren but we have found instead that they are a complex association of landscape and hydrologic elements governed by their surface characteristics previously we identified six unique surface mosaic types that accurately capture the subtle but distinct variations in surface clast arrangements for a desert pavement landscape formed on a single aged basalt flow in the mojave desert we now report that these surface mosaics predict the spatial distribution of fundamental desert vegetation and soil characteristics characteristics of soil morphology and texture the leaching depth of soluble salts percent plant cover and shrub species diversity are remarkably consistent for each mosaic type across a 580 000 year old basalt flow even when measured 1 km apart hydrologic character is distinctly different between desert pavement and bare ground regions and vegetation distributions reflect the spatially heterogeneous soil moisture where desert shrubs cluster on the three bare ground surface mosaics leaching is deep removing most soluble salts to below the 50 cm depth where shrubs are absent or few on the three desert pavement mosaics leaching depths are shallow with soluble salt depth

soil horizon development and the tempo and modes of vegetation change during the holocene in a sonoran desert basin usa silt rich and clay rich vesicular horizons v horizons that develop beneath desert pavements limit infiltration and plant available soil moisture a radiocarbon date from buried charcoal constrains the timing of emplacement of a sonoran desert alluvial fan deposit to the very end of the pleistocene approximately 12 7 ka the surface of the fan is dotted with 1 5 3 m light colored plant scars that mark places once occupied by larger perennial plants a few very large ring like clones of larrea tridentata have persisted on the surface and represent plants that may have initially established as long ago as the early holocene the gravelly fan deposit initially would have possessed high infiltration capacities and fostered the establishment of vegetation two principal sets of factors have driven vegetation change over the course of the holocene during the first part of the holocene differences in life history attributes of various species dispersal and colonization vs competitive abilities would have contributed to a succession of perennial plant occupants culminating with the predominance of the slow growing long lived but competitively dominant l tridentata during the latter part of the holocene and continuing to the present continued development of the pavement and associated v horizon has increasingly diminished the capacity of the surface to absorb precipitation and is responsible for the more recent phase involving progressive elimination of vegetation development of soil features that markedly influence soil hydrology is a major component of vegetation change during the late quaternary in the more arid portions of the sonoran and mojave deserts

microtopographic evolution of lava flows at cima volcanic field mojave desert california microtopographic profiles were measured and power spectra calculated for dated lava flow surfaces at cima volcanic field in the eastern mojave desert of california in order to quantify changes in centimeter to meter scale roughness as a function of age for lava flows younger than about 0 8 m y roughness over all spatial scales decreases with age with meter scale roughness decreasing slightly more than centimeter scales flows older than about 0 8 m y show a reversal of this trend becoming as rough as young flows at these scales modeling indicates that eolian deposition can explain most of the change observed in the offset or roughness amplitude of power spectra of flow surface profiles up to 0 8 m y other processes such as rubbling and stone pavement development appear to have a minor effect in this age range changes in power spectra of surfaces older than about 0 8 m y are consistent with roughening due to fluvial dissection these results agree qualitatively with a process response model that attributes systematic changes in flow surface morphology to cyclic changes in the rates of eolian soil formation and fluvial processes identification of active surficial processes and estimation of the extent of their effects or stage of surficial evolution through measurement of surface roughness will help put the correlation of surficial units on a quantitative basis this may form the basis for the use of radar remote sensing data to help in regional correlations of surficial units

relation between infiltration and stone cover on a semiarid hillslope southern arizona abstract a knowledge of the spatial distribution of infiltration rates is essential to the application of realistic distributed rainfall runoff models in semiarid areas one method of determining this distribution is to develop a predictive equation based on a readily observable surface property such as stone cover previous studies of the relation between infiltration and stone cover on shrub covered semiarid hillslopes have yielded both positive and negative correlations it is suggested that positive correlations result where infiltration measurements are confined to areas between large shrubs negative correlations on the other hand reflect pronounced shrub intershrub differences in infiltration and stone cover and are found where both shrub and intershrub areas are sampled in this study of a semiarid hillslope in southern arizona both shrub and intershrub areas are sampled and a negative correlation is obtained between infiltration and stone cover this correlation arises from the fact that under shrubs fine sediments have accumulated primarily as a result of differential splash whereas between shrubs such sediments have been selectively removed by a combination of rainsplash and overland flow leaving behind a gravel lag consequently infiltration rates are higher under shrubs than between them owing to the higher percentage of sand in the surface soil the larger quantity of organic matter both within the soil and on its surface and the greater digging and burrowing by animals in addition simulated rainfall experiments imply that shrubs with moderately dense canopies are more effective than adjacent surface gravels in dissipating the kinetic energy of raindrops and thereby reducing surface sealing and promoting infiltration inasmuch as the field site is typical of gentle shrub covered piedmont hillslopes over much of the sonoran desert inverse relations between infiltration rate and stone cover are probably characteristic of such hillslopes and may be used to predict the spatial distribution of infiltration rates for inclusion in rainfall runoff models

gravity field of the orientale basin from the gravity recovery and interior laboratory mission the orientale basin is the youngest and best preserved major impact structure on the moon we used the gravity recovery and interior laboratory grail spacecraft to investigate the gravitational field of orientale at 3 to 5 kilometer km horizontal resolution a volume of at least 3 4 0 2 106 km3 of crustal material was removed and redistributed during basin formation there is no preserved evidence of the transient crater that would reveal the basin s maximum volume but its diameter may now be inferred to be between 320 and 460 km the gravity field resolves distinctive structures of orientale s three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle the crustal structure of orientale provides constraints on the formation of multiring basins

stages and rate of the gravel shattering process by salts in desert reg soils abstract gravel shattering by salts is a common process in gravelly young to mature desert reg soils the shattered gravel is found on the surface and in the b and c horizons down to a maximum depth of 50 cm field studies on the process of salt weathering in deserts are scarce therefore the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the negev desert the studied reg soils were developed on two alluvial fan chronosequences in the negev desert the fan of nahal ze elim and the fan of nahal shehoret the development of the shattering process can be divided into five stages a e based on chemical physical and micromorphological features of the reg soils stages a d very young yo young reg soils represent the early holocene to the present some 14 000 years salinity values are between 0 27 16 mmho cm and the percentage of the shattered gravel is between 20 and 70 the shattered gravel however still maintains its original shape stage e mature reg soil represents the middle pleistocene to early holocene or 500 000 years salinity values at this stage reach 62 mmho cm although part of the gravel does not keep its original shape and starts to disperse in the profile the percentage of shattered gravel is only 75 80 it was found that the process of salt shattering in reg soils does not depend on concentrations or mineralogy of the salts but on the micro environmental conditions of the soil profile the process requires extreme and rapid variations of temperatures and moisture conditions such as found close to the soil surface to cause salt crystallization in a confined system with time the sealing of the soil surface through the development of desert pavement and a gravel free b horizon results in a change in the soil micro environment this change impedes the process irrespective of the amounts concentrations and types of salt minerals which exist in the profile the fact that in extreme arid regions the gravel shattering process develops primarily close to the reg soil surface suggests that it can be used to identify buried reg soils and to reconstruct climatic and micro environmental conditions of paleosols

the vesicular layer and carbonate collars of desert soils and pavements formation age and relation to climate change abstract the vesicular fine grained a horizon av is the widespread ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages now known to form mostly through accumulation of eolian dust recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation changes in the magnitude of eolian activity and effective leaching related to quaternary climatic changes are also hypothesized to have influenced the evolution of the av horizon numerical modeling geochronologic and field laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of av how the changing av horizon increasingly influences soil infiltration and carbonate translocation and accumulation and the control that clasts of the evolving pavement exert on pedogenic processes results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived eolian material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by av horizon formation the early development of a weakly to moderately developed av horizon directly over gravelly parent material in late and middle holocene soils moderately influences soil infiltration but significant leaching of very soluble materials and some carbonate in dust are permitted in older pleistocene soils however the texturally more mature av and underlying cumulic nongravelly horizons more strongly limit the rate and depth of leaching and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material other aspects of av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate referred to as carbonate collars development of a numerical model that integrates soil hydrology a co2 production diffusion model calcite kinetics and thermodynamic considerations composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation model results combined with results of 13c and 18o analyses of collar carbonate demonstrate how precipitation of calcite on pavement clasts and within the av is favored at a depth much shallower than that indicated by the classic carbonate depth climate relationship of jenny and leonard jenny h j leonard c d 1935 functional relationships between soil properties and rainfall soil science 38 363 381 and arkley arkley r j 1963 calculations of carbonate and water movement in soil from climatic data soil science 96 239 248 or simulated by numerical models of carbonate accumulation simultaneous development of thick carbonate collars and the av horizon requires the sustained pavement clast av horizon coupling for at least centuries to possibly millennia new thermoluminescence ages also indicate that much of the av horizon formed in the holocene and that it is certainly much younger than the older pleistocene pavements this supports the previously proposed hypothesis that increased dust flux during the pleistocene to holocene transition triggered and or greatly accelerated av horizon development an understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments but it also provides important clues into processes of pavement evolution and preservation of av horizons during long glacial periods the av horizon is not merely an insignificant surficial zone of recent dust accretion instead its development profoundly influences the genesis of desert soils and pavements

geomorphology on geologic timescales evolution of the late cenozoic pacific paleosurface in northern chile and southern peru abstract the atacama desert on the western margin of the central andes is one of the driest and oldest deserts in the world it is defined by a distinct and ancient surface known as the pacific paleosurface pps or atacama paleosurface the age of this surface is determined as the time at which sediment deposition ceased and the surface was effectively abandoned early studies suggested that this abandonment took place between 14 and 10 ma and was related to both the uplift of the andes and the onset of hyperaridity in the region here we provide a regional re examination of the pps compiling existing work on the underlying geology sedimentology surface exposure dating and seismic profiling we also present new multispectral satellite maps of the pps and 45 new cosmogenic 3he and 21ne surface exposure ages in order to constrain the formation age and the preservation and incision history of the paleosurface we conclude that the pps is not a single paleosurface but instead is a mosaic of smaller surfaces that were formed by aggradational and degradational processes over 19 million years or more and should be termed collectively as the pacific paleosurfaces the time at which individual paleosurfaces formed is related to regional climate where the location of each is controlled by regional tectonic activity cosmogenic surface exposure ages suggest that the surfaces are a record of regional scale climate events

clast size controls and longevity of pleistocene desert pavements at lathrop wells and red cone volcanoes southern nevada formation of desert pavement and accretionary soils are intimately linked in arid environments well sorted fallout scoria lapilli at lathrop wells 75 80 ka and red cone ca 1 ma volcanoes southern nevada formed an excellent parent material for pavement allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular a av horizons variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation pavement becomes ineffective when clasts can saltate under the strongest winds while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation necessary for av development contrary to predictions that all pavements above altitudes of 400 m would have been reset in their development after late pleistocene vegetation advances the soils and pavements show clear differences in maturity between the two volcanoes this indicates that either the pavements and or soils develop slowly over many tens of thousands of years and then are very stable or if they are disrupted by vegetation advances subsequent pavements are re established with successively more mature characteristics

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

spatial and temporal variability of water percolation and movement in a system of longitudinal dunes western negev israel subsurface lateral flow has previously been identified in dune areas where the average annual rainfall exceeds 200 mm very little is known about subsurface flow in sandy deserts with less than 100 mm of rainfall per year the present study deals with the water regime in a system of longitudinal dunes in the negev desert sixteen boreholes were dug down to a depth of 6 m across a sandy ridge and the adjacent corridor soil moisture and water movement were monitored with a neutron probe during two consecutive years the first year had been relatively wet and in the following year the rainfall was slightly lower than the long term average the data obtained show that in an average year water percolation is limited to 60 cm with no lateral water movement deeper percolation in the range 180 400 cm occurs in response to rainy spells with about 100 mm of cumulative rain within two months the process involves subsurface lateral flow and water movement by the piston effect the lateral flow on the flanks of the dune is explained by differences in hydraulic conductivities within the sloping layers 1997 by john wiley sons ltd

rock varnish microlamination dating of late quaternary geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts in this study we establish a generalized late quaternary i e 0 300 ka varnish layering sequence for the drylands of western usa and tentatively correlate it with the specmap oxygen isotope record we then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late quaternary geomorphic features in the study region vml dating of alluvial fan deposits in death valley of eastern california indicates that during the mid to late pleistocene 5 15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation during the holocene interglacial time however 0 5 1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate vml dating of alluvial desert pavements in death valley and the mojave desert reveals that pavements can be developed rapidly

dynamical processes on desert pavements and the healing of surficial disturbances abstract mature desert pavements are traditionally regarded as hallmarks of stability but their stability is dynamic not static in a study aimed at documenting this dynamic stability and its role in healing surface disturbances experiments were performed over a 5 yr period on small cleared patches or plats on pavement surfaces in panamint valley california these experiments show that stones from plat edges begin to resurface the clearing at rates of about 1 per year on 40 cm square plats and 10 per year on 10 cm square plats stones contributing to the regenerated pavement have smaller average diameters than stones on the surrounding pavement cavities 5 10 cm deep formed in mature pavement by removal of embedded boulders fill by ravel and slope failure after five years cavity depth has been reduced by as much as 60 forty year old boulder cavities are nearly completely refilled and have been repaved by smaller than average pavement stones gaps caused by removal of small stones 2 3 cm have completely healed in 5 yr displacement of surface stones by small animals is a major component of the healing process

cosmogenic 36cl ages of quaternary basalt flows in the mojave desert california usa abstract basalt flows provide excellent opportunities for calibration and intercomparison of quaternary dating methods remote sensing methods and rates of geomorphic processes the immediate motivation for this study was to provide chronology for a blind test of the utility of rock varnish microstratigraphy as an indicator of the age of flow emplacement five basaltic eruptive centers in the mojave desert of california were sampled for cosmogenic 36cl analysis multiple samples were taken from most centers and with one exception produced good agreement assuming a surficial erosion rate of 1 mm kyr 1 the flows yielded the following ages amboy crater 79 5 ka pisgah crater 22 5 1 3 ka cima field i cone 27 1 3 ka cima field a cone 21 1 6 ka and 11 5 1 5 ka cima field flow of unidentified origin 46 2 ka the ages from the cima i and a cones are in good agreement with previous cosmogenic 3he dating ages from the three previously undated flows are significantly older than previous estimates based on flow appearance tanzhou liu performed varnish microstratigraphic analysis on samples collected from the same sites his results were submitted for publication without knowledge of the 36cl ages his age estimates agree well with the 36cl ages for the three previously undated flows strongly supporting the validity of varnish microstratigraphy as a chronological correlation tool

relation between sediment yield and gradient on debris covered hillslopes walnut gulch arizona the relation between overland flow erosion and gradient on debris covered semiarid hillslopes in walnut gulch experimental watershed arizona is investigated by conducting three sets of field experiments under simulated rainfall on two different substrates each set of experiments yields a convex upward sediment yield gradient relation with a vertex at about 12 the key to understanding this relation is the relation between runoff and gradient on slopes less than 12 runoff appears to increase very slowly with gradient and so sediment yield increases with gradient mainly in response to the increase in the downslope component of gravity on slopes steeper than 12 runoff decreases rapidly as gradient increases this decrease in runoff outweighs the increase in the down slope component of gravity as gradient increases and causes sediment yield to decrease in all three sets of experiments runoff is controlled by either stone cover or stone size and stone size increases with gradient inasmuch as stone cover and stone size are indicators of mantle stoniness these findings suggest that curvilinear relations between runoff and gradient and hence convex upward relations between sediment yield and gradient occur only on hillslopes with coarse weathering mantles in which stoniness increases with gradient different combinations of substrate and climate may produce such hillslopes they are especially likely to develop in arid and semiarid climates but they may also form in alpine periglacial and humid ones where substrates are favorable hillslopes on which mass movement processes are the dominant denudational agents will not have convex upward relations

characterization of lava flow degradation in the pisgah and cima volcanic fields california using landsat thematic mapper and airsar data digital landsat thematic mapper tm observations multi frequency polarimetric airsar radar data microtopographic profiles derived from helicopter borne photography and field measurements were analyzed over the pisgah and cima volcanic fields and lavic lake playa mojave desert california surface radiance factors were extracted from tm data using radiative transfer procedures to model the atmosphere and surface simultaneously airsar data were calibrated to backscatter specific cross sections using corner reflectors deployed at a number of locations analyses focused on the use of key field sites to understand compositional and textural controls on tm band 5 1 55 to 1 75 m radiance factors and c 5 6 cm l 24 cm and p 68 cm band specific cross sections for the pisgah and lavic lake sites the tm data are explained using a linear mixing model of basalt and fine grained sediment clay to sand sized grains of clay minerals quartz feldspar carbonate and lithic fragments radiance factors with the addition of shadows for the rough a9a flow on the other hand airsar data are shown to be primarily controlled by surface roughness as there is a strong correlation between specific cross section and surface height standard deviation for the elongate flows at the cima volcanic field which exhibit a mix of pahoehoe and a9a properties we find a positive linear relationship between tm band 5 radiance factors and flow ages for the 3 m y age range represented specific cross sections remain constant until about 0 14 ma and then decrease with age up to about 0 56 ma on the other hand data for the two older flows 0 70 and 0 85 ma show increasing specific cross sections with increasing age we interpret the increase in tm band 5 radiance factors from 0 05 to 0 25 with increasing flow age and the decrease in specific cross sections 8 db change at 35 incidence angle for l band for flows up to 0 56 ma in age as due to smoothing by mass wasting and infill by fine grained aeolian deposits the increased specific cross sections for older flows are interpreted to be a consequence of surface roughness generated by fluvial dissection

s saturation history of nain plutonic suite mafic intrusions origin of the voisey s bay ni cu co sulfide deposit labrador canada the voisey s bay deposit is hosted in a 1 34 ga intrusion composed of troctolite olivine gabbro and ferrogabbro the sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites and also occurs as veins along structures in adjacent paragneiss a dyke is connected to the base of the north wall of the eastern deeps intrusion and the entry point of this dyke into the chamber is the locus of the eastern deeps nickel sulfide deposit a detailed exploration in the area between the eastern deeps and the ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke the eastern deeps deposit is surrounded by a halo of moderately to weakly mineralized variable textured troctolite vtt that reaches a maximum thickness above the axis of the eastern deeps deposit along the northern wall of the eastern deeps intrusion the massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the vtt and there is a marked break in ni tenor of sulfide between the two sulfides hosted in the dyke tend to have low metal tenors ni 100 2 5 3 5 sulfides in eastern deeps massive and breccia ores have intermediate ni tenors ni 100 3 5 4 and disseminated sulfides in overlying rocks have high ni tenors ni 100 4 8 four principal processes control the compositions of the voisey s bay sulfides coarse grained loop textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the pd ir ratio which is due to mineralogical variations where pentlandite is enriched in pd and ir is dispersed throughout the mineral assemblage a decrease in ir and rh from the margin of the ovoid toward cubanite rich parts at the central part of the ovoid is consistent with fractionation of the sulfide from the margins toward the center of the ovoid the ovoid ores have higher ni and pd tenor than the eastern deeps massive sulfides this is consistent with both a higher r factor and greater degree of silicate parental magma evolution in the ovoid than the eastern deeps the disseminated sulfides surrounding the eastern deeps deposit have some of the highest ni and pd tenors at voisey s bay which are indicative of not only more primitive magmas but also higher r factors than the ovoid or the eastern deeps vtt and normal textured troctolite of the eastern deeps that contain trace sulfide have 0 1 3 ppb pt and 0 1 3 ppb pd whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of pt and pd troctolites and olivine gabbros from other parts of the voisey s bay intrusion and other nain plutonic suite intrusions including the kiglapait newark bay barth island mushua and nain bay south intrusion also have low platinum group element abundances although it is possible that this is a signature of a widespread sulfide saturation event that pre dated ore formation at voisey s bay it is more likely that platinum group element pge depletion is a product of the source melting process where low degrees of melting resulted in the retention of pge in the mantle source if so this indicates that pge depletion should be used with caution as an exploration tool in the nain plutonic suite

cosmogenic 3he surface exposure dating of stone pavements implications for landscape evolution in deserts the formation of stone pavements a ubiquitous gravel armor mantling landforms in arid regions of the world has been previously attributed to erosion by wind and water or alternating shrinking and swelling of soil horizons implying that gravel is concentrated at the land surface in a time transgressive manner a newly proposed model for pavement evolution differs from these models in that pavement clasts are continuously maintained atthelandsurfaceinresponsetodepositionandpedogenicmodificationofwindblowndust in situ cosmogenic 3 he surface exposure ages on volcanic and alluvial landforms in the mojave desert of california are used to understand pavement evolution over geologic time scales and to test this new model these exposure ages are stratigraphically consistent show internal consistency at each site and for stone pavements adjacent to pristine continuously exposed volcanic bedrock are indistinguishable at the 1 level we conclude that stone pavements are born at the surface and that pavements may provide one of the longest term records of geologic hydrologic and climatic processes operating on desert surfaces

blind testing of rock varnish microstratigraphy as a chronometric indicator results on late quaternary lava flows in the mojave desert california rock varnish is a manganiferous dark coating ubiquitous in desert landscapes to test the validity of varnish microstratigraphy as a chronometric indicator varnish samples were collected from radiometrically dated and undated late quaternary lava flows in amboy cima and pisgah volcanic fields avf cvf pvf in the mojave desert of california western united states varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the younger dryas and heinrich events in the north atlantic region microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western us sites with available radiometric age constraints based on this regional chronology varnishes from the a flow h flow and a stone pavement surface in the cima volcanic field were estimated to be 16 5 24 74 85 and 74 85 ka respectively these ages are consistent with previously published cosmogenic 3 he ages of 18 20 72 74 and 80 85 ka for these geomorphic surfaces varnishes from the i flow at cima yielded a puzzling age estimate of 39 ka which is consistent with an older 3 he age of 37f6 ka reported for the i flow but inconsistent with a younger 3 he age of 31f7 ka and a cosmogenic 36 cl age of 27f1 3 ka for the same flow reinterpretation of the original varnish age data with knowledge of then available field mapping results of the i flow suggests that the i cone is polycyclic and different flow units were probably unintentionally sampled in the field the revised varnish ages of 30 and 39 ka for the i flow thus may be in good agreement with their corresponding 3 he and 36 cl ages in a blind test of the method varnishes from the phase 1 flow at pisgah an unnamed flow called here the ivflow at cima and the amboy flow were estimated to be 24 30 46 60 and 74 85 ka respectively these ages agree well with 36 cl ages of 22 5f1 3 46f2 and 79f5 ka reported for the same flows by phillips geomorphology 2002 these test results provide convincing evidence that varnish microstratigraphy once radiometrically calibrated can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western us drylands d 2002 elsevier science b v all rights reserved

k ar dating of the cima volcanic field eastern mojave desert california late cenozoic volcanic history and landscape evolution thirty four k ar ages and supporting paleomagnetic measurements from flows of the cima volcanic field provide a detailed volcanic history and a temporal basis for analysis of evolving landforms the cima field has undergone three periods of volcanic activity spanning late miocene through latest pleistocene time 1 7 6 to 6 5 2 4 5 to 3 6 and 3 1 0 to at least 0 015 m y ago these precisely dated sequences afford a unique opportunity to quantify the long term effects of pedogenesis and erosion on volcanic landforms in an arid environment morphologic and pedologtc data indicate that the most stable geomorphic surfaces in the field occur on flows between 0 25 and 0 75 m y old younger flows are dominated by eolian aggradation and outcrop rubbling and older flows are dominated by surface runoff and fluvial dissection a process response model involving progressive pedogenesis and a subsequent shift from infiltration to surface runoff is proposed to explain this temporal variation in land surface stability