cause of dominance by sheetflood vs debris flow processes on two adjoining alluvial fans death valley california two large adjoining alluvial fans of the panamint range piedmont death valley california are composed of different facies assemblages deposited by contrasting sedimentary processes the anvil spring fan was built solely by water flow processes incised channel floods and sheetfloods whereas the neighbouring warm spring fan has been constructed principally by debris flows the boundary between these fans delineates a sharp provincial piedmont contact between sheetflood dominated fans to the south and debris flow dominated fans to the north factors such as climate catchment area fan area catchment relief aspect vegetation types and density and neotectonic setting are essentially identical for these two fans the key difference between them is that their catchments are underlain by dissimilar bedrock types which weather to yield distinctive sediment suites weathering of the granite and andesite of the anvil fan catchment produces significant volumes of medium to very coarse sand granules pebbles cobbles and boulders but minimal silt and clay in contrast the shale quartzite and dolomite that dominate bedrock in the warm spring catchment weather to yield a wide suite of sedimentary particles spanning from clay to boulders the abundance of mud and the unsorted character of the yielded sediment cause precipitation induced slope failures in the warm spring catchment to transform readily into debris flows this propensity is due to the low permeability of the colluvial sediment which causes added water to become trapped quickly and pore pressure to rise rapidly promoting transformations to debris flows in contrast the limited volume of sediment finer than medium sand yielded from the anvil fan catchment causes the colluvium to have high permeability this factor prevents the transformation of wet colluvium to a debris flow during hydrologically triggered slope failures instead maintaining sediment transport as entrained bed load or suspended load in a water flow

controls on alluvial fan long profiles water and debris flows exiting confined valleys have a tendency to deposit sediment on steep fans on alluvial fans where water transport of gravel predominates channel slopes tend to decrease downfan from 0 10 0 04 to 0 01 across wide ranges of climate and tectonism some have argued that this pattern reflects grain size fining downfan such that higher threshold slopes are required just to entrain coarser particles in the waters of the upper fan whereas lower slopes are required to entrain finer grains downfan threshold hypothesis an older hypothesis is that slope is adjusted to transport the supplied sediment load which decreases downfan as deposition occurs transport hypothesis we have begun to test these hypotheses for alluvial fan long profiles using detailed hydraulic and particle size data in sediment transport models on four alluvial fans in the western u s we find that channel hydraulic radiiare largely 0 5 0 9 m at fan heads decreasing to 0 1 0 2 m at distal margins we find that median gravel diameter does not change systematically along the upper 60 80 of active fan channels as slope declines so downstream gravel fining cannot explain most of the observed channel slope reduction however as slope declines channel bed sand cover increases systematically downfan from areal fractions of 20 above fan heads to distal fan values in excess of 70 as a result entrainment thresholds for bed material might decrease systematically downfan leading to lower slopes however current models of this effect alone tend to underpredict downfan slope changes this is likely due to off channel gravel deposition calculations that match observed fan long profiles require an exponential decline in gravel transport rate so that on some fans approximately half of the load must be deposited off channel every 0 20 1 4 km downfan this leads us to hypothesize that some alluvial fan long profiles are statements about the rate of overbank deposition of coarse particles downfan a process for which there is currently no mechanistic theory

quaternary deposits of the buyuk menderes graben in western anatolia turkey implications for river capture and the longest holocene estuary in the aegean sea abstract the buyuk menderes graben is a seismically active depositional basin in the n s extensional tectonic region of western anatolia turkey it extends in an e w direction and is bounded by the aegean sea to the west the infill of this tectonic basin comprises ca 850 m and 245 m thick clastic sequences of neogene and quaternary respectively and the quaternary part of the basin fill is presented here by the help of seismic sections and boreholes results show that the studied succession was made of unconsolidated mostly fine grained clastic sediments of marine and continental sequences interfingered with each other the marine counterpart forms three relatively short approx 30 km and one long approx 100 km from modern coastline sediment wedges representing sea transgressions onto the graben the last one was a rapid incursion that occurred in the middle holocene forming the longest estuary of the aegean sea in western anatolia the filling of this estuary caused the decline of some historical harbours and settlements in the region while marine based events occurred in the west of the graben alluvial and fluvial processes produced marginal and axial deposits in the east of graben respectively in general the continental succession of quaternary is thinner than its marine equivalent probably due to sea level fluctuations in addition the sedimentation rate increased suddenly during deposition of the last marine sequence estuarine deposits in the holocene it is likely that this was the result of enlargement of the drainage area of the river buyuk menderes due to a river capture that occurred in the latest stages of pleistocene

measuring alluvial fan sensitivity to past climate changes using a self similarity approach to grain size fining death valley california the effects of climate change on eroding landscapes and the terrestrial sedimentary record are poorly understood using mountain catchment alluvial fan systems as simple analogues for larger landscapes a wide range of theoretical studies numerical models and physical experiments have hypothesised that a change in precipitation rate could leave a characteristic signal in alluvial fan sediment flux grain size and down system fining rate however this hypothesis remains largely untested in real landscapes this study measures grain size fining rates from apex to toe on two alluvial fan systems in northern death valley california usa which each have well exposed modern and ca 70 ka surfaces and where the long term tectonic boundary conditions can be constrained between them these surfaces capture a well constrained temporal gradient in climate a grain size fining model is adapted based on self similarity and selective deposition for application to these alluvial fans this model is then integrated with cosmogenic nuclide constraints on catchment erosion rates and observed grain size fining data from two catchment fan systems to estimate the change in sediment flux from canyon to alluvial fan that occurred between mid glacial and modern interglacial conditions in a fan system with negligible sediment recycling an approximately 30 decrease in precipitation rate led to a 20 decrease in sediment flux and a clear increase in the down fan rate of fining supporting existing landscape evolution models consequently this study shows that small mountain catchments and their alluvial fan stratigraphy can be highly sensitive to orbital climate changes over 105 year timescales however in the second fan system it is observed that this sensitivity is completely lost when sediment is remobilised and recycled over a time period longer than the duration of the climatic perturbation these analyses offer a new approach to quantitatively reconstructing the effects of past climate changes on sedimentation using simple grain size data measured in the field this article is protected by copyright all rights reserved

dating recent volcanic rocks through 230th 238u disequilibrium in accessory minerals example of the puy de dome french massif central precise dating of very young 10 000 yr silicic volcanic rocks can be achieved by measurements of 230 th 238 u disequilibria in accessory minerals such as apatite and zircon it is illustrated here by the isochron age of 12 000 1000 yr obtained on a trachyte from the puy de dome french massif central the crystallization age is virtually the same as the eruption age determined by thermoluminescence and 14 c dating this indicates a short time interval a few centuries at most between crystallization and eruption in such small volume silicic magmas

bedrock lithology control on contemporaneous alluvial fan facies oligo miocene southern pyrenees spain climate and tectonics play important roles in controlling processes of transport and deposition on alluvial fans but the bedrock lithology in the fan catchment area is also a significant independent factor adjacent oligo miocene alluvial fan deposits on the northern margin of the ebro basin display contrasting depositional characteristics with one dominated by the deposits of debris flows and the other by deposition from flows of water a difference in clast compositions indicates that the two studied fans the nueno and san julian fans had contrasting bedrock lithology in their drainage basins the proximal facies of the nueno fan body contains matrix supported conglomerate beds with up to 80 pebble clasts of gypsum in a matrix of gypsiferous sand interbedded with gypsarenite beds the drainage basin of this fan was dominated by triassic bedrock consisting of beds of gypsum marl and micritic limestone the san julian fan body comprises clast supported polymict conglomerate beds containing pebbles from triassic cretaceous and palaeogene limestone units that are exposed in the adjacent part of the basin margin the interfingering of the deposits of these two fans demonstrates that they were contemporaneous given the consistent climate the differences in fan depositional processes must therefore be attributed to the contrasting bedrock lithology in their drainage basins a drainage basin consisting mainly of marl and gypsum bedrock provided sufficient fine grained material to generate debris flows whereas more dilute water lain processes dominated where the drainage basin was largely limestone strata

seagrass related carbonate ramp development at the front of a fan delta burdigalian new caledonia insights into mixed carbonate siliciclastic environments abstract past shallow water carbonate environments of the main island of new caledonia nc have been subject to high terrigenous influx derived from the erosion of ultramafic obducted nappes and are therefore a relevant case study for characterizing neritic carbonate production in mixed carbonate siliciclastic systems under a tropical climate more particularly we focused on burdigalian carbonate sedimentary records cropping out on the western coast of nc in the nepoui area based on a comprehensive sedimentological study of cores and outcrops we established a new depositional model of an alluvial fan to carbonate ramp transition shallow water euphotic carbonate production was dominated by seagrass related biota and corals derived from small sized bioconstructions extensional tectonics and associated normal faulting driven by post obduction isostatic rebound favored carbonate ramp aggradation and preservation the carbonate ramp was incised by conglomerate filled terminal distributary channels which indicate that terrigenous inputs remained significant during marine transgression and did not inhibit the development of seagrass and scleractinian carbonate factories phytal substrates induced by seagrass and or macro algae seafloor colonization strongly controlled the nature of carbonate production and promoted the accumulation of foraminiferal coralline algal sediments seagrass development at the front of the fan delta is interpreted to have controlled the preservation of a diverse and significant carbonate production by reducing water turbidity and by limiting the risk of suffocation for filter feeding biota in such a high terrigenous influx setting

relative durability of chert and rhyolite detritus in conglomerates and sandstones with reference to the study of provenance geology modal changes in both gravel and sands occur because of unequal durability of some clastic materials to abrasion selective abrasion causes an elimination of undurable materials and a complimentary enrichment of competent components the fluviatile oligocene noda group northeast japan consists of two formations that include both the competent and undurable clastics modal analyses of rhyolite or chert rich sediments in both formations show that the amount of pebble and granule size chert detritus is nearly constant but the sand size chert fragments remarkably decrease with the decreasing grain size the amount of rhyolite pebbles and the sands however is nearly unchangeable despite the variable clast size results of our study reveal that we need careful thoughts for a restricted size of sediments and the mode of fine grained detritus that include undurable chert sands tends to significantly underestimate the outcrop area of chert bedrocks in the provenance

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

short term evolution of an active basin fan system aspromonte south italy abstract alluvial fans are widespread throughout calabria southern italy but only a minority of them show evidence of current activity by means of interpretation of aerial photographs orthophotos and satellite images comparison of topographic maps reconstruction of major rainfall events and geomorphologic and sedimentologic field observations we reconstructed the recent history of the geomorphic and sedimentation events and the consequent morphological changes of an active basin fan system merging with the bed of an ephemeral stream locally named fiumara the case study is located along the ionian side of the aspromonte massif a sector of calabria where mass movement intensive erosion and channel mass transport are the predominant morphodynamic processes evidence is available of large volumes of debris produced by mass movements and erosion processes affecting the upper sector of the feeder basin the aggressive climate with intense winter storms is the principal cause of erosion and mass movements largest rainfall events occurred in clusters of 4 6 events between the 1930s and the early 1970s at roughly 20 year time intervals the development of the fan appeared as incipient in 1955 air photos activity continues today with events of sediment output from the feeder basin large rainfall events are reflected with a delay of ca eight years by aggradation dissection and progradation retrogradation cycles in the feeder channel and the fan respectively mutual interference between fan development and human activity exists as well as does the interaction between the fan progradation and the floods of the receiving fiumara stream based on the aforementioned survey tools two almost 10 year cycles were recognised feeder channel aggradation owing to the debris input of the feeder subbasin and to the presence of check dams built along the feeder channel continues until the channel slope reaches a threshold value this can also be caused by the trimming of the fan toe by the fiumara the stored alluvium is therefore released resulting in progradation at the fan site until the feeder channel slope decreases subsequently a renewed cycle starts again in response to sediment input from the source area the morphological and sedimentological evolution of the active basin fan system indicates how short term trends and cyclic modifications of such systems can be important especially where tributary basins join larger stream beds of a fiumara type torrent

facies and depositional model of alluvial fan dominated by episodic flood events in arid conditions an example from the quaternary poplar fan north western china

source area and tectonic control on alluvial fan development in the miocene fohnsdorf intramontane basin austria middle miocene alluvial fans in the intramontane fohnsdorf basin of the eastern alps originated along normal faults and linked strike slip faults in a continental half graben setting the fans display considerable facies differences debris flows of the rachau fan are characterized by a sandy matrix and large boulders whereas debris flows of the apfelberg fan are characterized by higher silt and clay content and smaller clasts key control of debris flow facies is the lithology contrast in the fan source areas sand pebbles and large outsized boulders originated predominantly from the resistant augengneissand amphibolite dominated hinterland of the rachau fan whereas a significant higher proportion of mud and silt and smaller boulders have been derived from the apfelberg fan catchment which was dominated by mica schists and marble from harvey a m mather a e stokes m eds 2005 alluvial fans geomorphology sedimentology dynamics geological society london special publications 251 207 216 0305 8719 05 15 the geological society of

debris flow hazards on tributary junction fans chitral hindu kush range northern pakistan abstract the chitral district of northern pakistan lies in the eastern hindu kush range the population in this high relief mountainous terrain is restricted to tributary junction fans in the chitral valley proximity to steep valley slopes renders these fans prone to hydrogeomorphic hazards including landslides floods and debris flows this paper focuses on debris flow hazards on tributary junction fans in chitral using field observations satellite image analyses and a preliminary morphometry the tributary junction fans in the chitral valley are classified into 1 discrete and 2 composite the discrete fans are modern day active landforms and include debris cones associated with ephemeral gullies debris fans associated with ephemeral channels and alluvial fans formed by perennial streams the composite fans are a collage of sediment deposits of widely different ages and formed by diverse alluvial fan forming processes these include fans formed predominantly during mis 2 holocene interglacial stages superimposed by modern day alluvial and debris fans composite fans are turned into relict fans when entrenched by modern day perennial streams these deeply incised channels discharge their sediment load directly into the trunk river without significant spread on fan surface in comparison when associated with ephemeral streams active debris fans develop directly at composite fan surfaces major settlements in chitral are located on composite fans as they provide large tracts of leveled land with easy accesses to water from the tributary streams these fan surfaces are relatively more stable especially when they are entrenched by perennial streams e g chitral ayun and reshun when associated with ephemeral streams e g snowghar or a combination of ephemeral and perennial streams e g drosh these fans are subject to frequent debris flow hazards fans associated with ephemeral streams are prone to high frequency 10 years return period debris flow hazards by comparison fans associated with perennial streams are impacted by debris flow hazards during exceptionally large events with return periods of 30 years this study has utility for quick debris flow hazard assessment in high relief mountainous regions especially in arid to semi arid south central asia where hazard zonation maps are generally lacking

gpr derived architecture of a lahar generated fan at cotopaxi volcano ecuador the internal geometry of volcaniclastic fans produced by aggradation during lahar events is difficult to examine in modern settings because of the frequent lack of three dimensional exposures this makes it challenging to i reconstruct the spatial and temporal evolution of such fans and ii interpret observed facies stratigraphy in the context of lahar flow dynamics from proximal to distal fan reaches this research therefore presents the results of a ground penetrating radar gpr survey of the rumipamba fan at the mouth of the burrohuaycu quebrada on the southwestern flank of cotopaxi volcano a survey grid consisting of 50 individual gpr profiles representing a total length of 19 4 km was constructed covering most of the 4 km2 large fan surface all gpr profiles were collected using a pulseekko 100 with a 400 v transmitter fan sediments consist of sandy and gravelly lahar deposits alternating with volcanic fallout including ash and pumice lapilli at times reworked by fluvial processes deposits could be ground truthed to a depth of 3 m whereas gpr penetration depth reaches 15 m data interpretation was based on classification into 15 distinct radar facies characterized by the nature of their bounding surfaces and or internal features cross referenced where possible with shallow exposures three main facies were identified parallel irregular and clinoform erosional contacts were distinguished from aggradational ones vertical channel fill and lateral accretion flow parallel versus flow transverse and proximal distal variations in deposit architecture were featured the results of this study confirm the existence of two major channel systems in the northern and southern extremities of the fan and the more recent formation of a smaller central fan channel system deposit architecture is complex and facies chronologies illustrate that lahars have affected the entire survey area

alluvial eolian and lacustrine sedimentology of a paleoproterozoic half graben baker lake basin nunavut canada the northeast trending baker lake sub basin was a volcanically active half graben during deposition of ca 1 85 1 76 ga baker lake group drainage was oriented along transverse and axial directions with flow to playa lake and deeper perennial lacustrine depocentres basin marginal streamflow dominated alluvial fans were concentrated along the southern margin and provided sediment from archean crystalline basement rocks these fed transverse gravel and sand bed braided streams alluvial dynamics were characterized by channel aggradation and abandonment abandoned channel belts were sites of floodplain and eolian deposition basin axial braided streams fed northeast and southwest to a depocentre near christopher island where eolian playa and lacustrine environments were intimately linked felsic minette flows were initially erupted from localized centres contemporaneous sedimentary deposits typically contain minor volcaniclastic components that increase in abundance basinward voluminous and widespread younger minette flows prograded outward from volcanic centres contributing significant additional basin infill

evidence for very recent melt water and debris flow activity in gullies in a young mid latitude crater on mars abstract terrestrial debris flows and their deposits are mainly studied and monitored because of their hazardous nature on mars they may serve as geomorphologic indicators of transient liquid water we compared the morphology of debris flow like deposits within a young 0 2 ma mid latitude crater on mars with debris flow fans on svalbard as possible terrestrial analogues it was our objective to constrain whether dry granular flow or processes related to water saturation at or close to the surface were responsible for the formation of the deposits within the crater we found that the morphological attributes of the deposits on mars are very similar to debris flows in svalbard and include overlapping terminal lobes debris tongues and snouts debris flow fans scoured channels with medial deposits debris plugs and clearly defined lateral deposits levees furthermore the interior crater walls display a range of landforms indicating aspect dependent degradation ranging from debris flow dominated pole facing slopes to east and west facing single channel gullies and north facing talus cones granular flow our findings suggest that the debris flows are not related to impact induced heating and release of meltwater we further suggest that degradation of a latitude dependent dust ice mantling unit may only have played a minor role in this youthful terrain instead we propose that the debris flows are mainly formed by melting of very recent snow deposits after the termination of the last martian ice age as such they may represent some of the most recent geomorphological indicators of transient liquid water in the martian mid latitudes the distinct north south asymmetry in degradation further demonstrates that insolation controlled slope processes are surprisingly efficient on mars during the last

alluvial fans and fluvial fans at the margins of continental sedimentary basins geomorphic and sedimentological distinction for geo energy exploration and development alluvial fans and fluvial fans are the most common depositional landforms along the margins of continental sedimentary basins their occurrence is determined by the area relief and hydrology of the catchment which ultimately control the relative ratios of sediment v flow discharge and runoff regimes fundamental morphological and process distinctions exist between these two kinds of deposystem which are seen as essentially different facies associations and internal architectures for the corresponding deposits alluvial fans commonly develop over short radial distances hundreds of metres to a few kilometres and are constructed by ephemeral flash flow events that lead to poor organization of the sedimentary facies and overall architecture of the corresponding deposits by contrast fluvial mega fans are fed by proper river systems which aggrade much larger volumes of clastic sediment over distances of up to a few hundred kilometres distinct channel belt and overbank domains are developed with a marked heterogeneity in the distribution of sedimentary facies represented by hierarchically well organized fluvial deposits the general properties of alluvial and fluvial fans reflect the different potentials for the corresponding successions to host economically attractive oil and gas resources and the different approaches required in exploration and prediction

precambrian basin margin fan deposits mesoproterozoic bagalkot group india abstract three dimensional facies variability in coarse clastic sedimentary rocks breccia conglomerates and coarse grained sandstones at the base of the ramdurg formation suggests terrestrial scree and fans giving way downslope to fluvial sediments along the margin of the mesoproterozoic bagalkot basin in india in consistency with the basic tenet of precambrian alluvial sedimentation fluvial architectural elements and field location specific consistency in channel flow direction support an invariably braided pattern for the rivers although the architectural element packaging pattern does show distinct changes downslope with channel belts thinning and becoming more regular in their geometry with possible persistence of a semi arid climate downslope change in flow durability in the channels was controlled primarily by water discharge depending on position of the channels with respect to the mean level of the water table basin margin slope intersection confined between an unconformity below and a granular lag succeeded by thoroughly wave featured sandstone and argillite carbonate above the coarse and poorly sorted clastic sedimentary rocks of the basal ramdurg are interpreted as a base level lowstand product for which the sedimentation rate exceeded the rate of space creation for sediment accumulation consequently the fan succession as a whole is coarsening upward fluvial sections nonetheless fine upward as the depositional slope gradient became progressively reduced with aggradation of channels tectonics related slope variation along and across the basin margin devoid of vegetation dictated the sediment distribution and sequence building pattern primarily eventual termination of this basin margin depositional system was caused by later enhancement in the rate of base profile rise

evidence for episodic alluvial fan formation in far western terra tyrrhena mars a late noachian aged alluvial fan complex within harris crater in far western terra tyrrhena mars is comprised of two well defined source regions and associated discrete depositional lobes three fan units were recognized based on common morphological characteristics thermal properties and spectral signatures although the entire fan complex has been subjected to extensive erosional degradation the preserved morphologies record episodic fan formation and indicate the type of flow processes that occurred the bulk of the fan surface has morphology consistent with fluvial emplacement while one fan unit exhibits a rugged surface texture with boulders consistent with a debris flow this transition from fluvial to late stage debris flow s suggests a decline in available water and or change in sediment supply the thermal inertia values obtained for all three fan surface units mean values ranged from 318 to 344 j m 2 k

attenuation characteristics of body waves for the bilaspur region of himachal lesser himalaya the attenuation characteristics around bilaspur region of the himachal lesser himalaya have been estimated adopting extended coda normalization method and using a data set of 41 local events 0 5 m l 2 9 that occurred in the region from may 2013 to march 2014 the frequency dependent relations governing the quality factors of p waves q and s waves q in the frequency range from 1 5 to 24 hz are q 43 4 f 1 30 0 04 and q 79 6 f 1 25 0 02 the average estimates of q and q are found to vary from 71 and 125 at 1 5 hz to 2901 and 4243 at 24 hz respectively the q and q estimates are compared to the similar estimates obtained for the other seismically active regions of the himalaya it is found that for the various himalayan regions the q estimates at 1 hz vary between 22 for the kumaon himalaya and 97 for the northwest himalaya whereas q estimates range between 63 for the garhwal himalaya and 127 for the northwest himalaya for the bilaspur region the q q ratio is greater than unity and varies between 1 84 and 1 45 in the frequency range from 1 to 24 hz the region specific attenuation relations can be adopted for estimating earthquake source parameters simulating strong ground motion and assessing seismic hazard for the bilaspur region of himachal lesser himalaya

stream capture and piracy recorded by provenance in fluvial fan strata abstract stream capture and piracy in tectonically active regions have been described in geomorphic systems worldwide however few studies show the influence stream capture has on the rock record we present an analysis of fluvial fan stratigraphy that developed as a result of multiple stream capture events building a complex stratigraphic succession beneath the lawrence livermore national laboratory llnl california the llnl site is located in the southeast portion of the tectonically active livermore basin a transpressional basin in the california coast ranges geomorphic evidence for this stream capture include 1 the arroyo seco enters the basin from the south through an uplifted fault block 2 south of this fault block lies an abandoned arroyo seco fluvial fan 3 north of the fault block in the livermore basin arroyo seco built a 7 km2 fluvial fan apparently forcing the arroyo las positas a smaller stream that enters the basin from the east northward around the arroyo seco fan and 4 a knickpoint exists near the point of capture on arroyo seco stratigraphic evidence reflecting this shift in the arroyo seco position into the livermore basin was evaluated through a provenance study of 215 gravel units from 34 boreholes spaced evenly over the 2 6 km2 llnl site the arroyo seco derives its sediment from both the jurassic cretaceous franciscan assemblage and the altamont hills which are comprised of mesozoic great valley group and tertiary continental sediments the arroyo las positas drains only the altamont hills and thus lacks the franciscan assemblage derived clasts the origin of the individual gravel units was determined by the percentage of franciscan assemblage indicator pebbles red chert green chert and blueschist in the samples through this analysis we determined that high percentage franciscan assemblage derived clasts were present below a depth of approximately 35 m below the surface low percentage franciscan assemblage derived clasts were present at depths between 35 m and 18 m and high percentage franciscan assemblage derived clasts were present from depths of approximately 18 m to the surface of the fluvial fan these results indicate that the arroyo seco flowed north and deposited sediments at the llnl site then was later absent from the basin at which time it formed a fan south of the fault block during this absence of the arroyo seco the arroyo las positas a westerly flowing stream dominated the sediment supply at the llnl site the arroyo seco was then captured by a gully headward eroding through the uplifted fault block redirecting the arroyo seco into the basin once again this history of multiple stream captures created three stratigraphic units with alternating overall channel and paleoflow orientations