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

palaeoenvironmental reconstruction and biostratinomic analysis of the jurassic yanliao lagerstatte in northeastern china abstract the middle upper jurassic yanliao lagerstatte contains numerous exceptionally preserved fossils of aquatic and land organisms including insects salamanders dinosaurs pterosaurs and mammaliaforms despite extensive study of the diversity and evolutionary implications of the biota the palaeoenvironmental setting and taphonomy of the fossils remain poorly understood we reconstruct both the palaeoenvironment of the daohugou area one of the most famous yanliao fossil areas and the biostratinomy of the fossils we use high resolution stratigraphic data from field investigation and excavations to document in detail the stratigraphic succession lithofacies facies associations and biostratinomic features of the lagerstatte our results show that frequent volcanic eruptions generated an extensive volcaniclastic apron and lake s in the studied area the frequent alternation of thin lacustrine deposits and thick volcaniclastic apron deposits indicates either that the studied area was located in the marginal regions of a single lake where the frequent influx of volcaniclastic apron material caused substantial fluctuations in lake area and thus the frequent lateral alternation of the two facies or that many short lived lakes developed on the volcaniclastic apron most terrestrial insects were preserved in the laminated normally graded siltstone claystone and tuff facies that form many thin intervals with deposits of graded sandstone siltstone and tuff in between within each interval the terrestrial insects occur in many laminae associated with abundant aquatic organisms but are particularly abundant in some laminae that directly underlie tuff of fallout origin most of these terrestrial insects are interpreted to have been killed in the area adjacent to the studied palaeolake s during volcanic eruptions their carcasses were transported by influxes of fresh volcaniclastic material primarily meteoric runoff and possibly minor distal pyroclastic flow into the palaeolake s and were buried in palaeolake deposits prior to extended decay probably due to a combination of rapid vertical settling ash fall and water turbulence

geomorphological and sedimentological analysis of a catastrophic flash flood in the aras drainage basin central pyrenees spain abstract on august 7th 1996 an intense and short duration convective storm occurred over the 18 6 km 2 aras drainage basin central pyrenees spain this high relief basin is composed of three subbasins aso betes and la selva and feeds the aras alluvial fan in the gallego river valley this alluvial fan had been drained by an artificial channel about 125 m 3 s at bank full capacity more than 30 check dams in its feeder channel the aras barranco had been previously filled by earlier sediments the heaviest rain was over the betes subbasin total rainfall 178 4 mm maximum rainfall intensity of 153 mm h for a 10 min time interval was estimated most of the rainfall fell in a 70 min period this storm resulted in high runoff causing catastrophic damage and significant geomorphic changes in the drainage basin especially in the betes subbasin the high discharge concentrated in the aras barranco destroyed most of the check dams flushing out a great amount of debris major channel trenching and widening occurred in this barranco when the confined sediment laden flash flood reached the basin mouth it sheet flooded the southern sector of the aras fan depositing a massive amount of debris on this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe a first stage peak discharge of sheet flooding deposited a coarse boulder lobe burying the artificial channel at the fan head and causing a darnming effect on the water flood during the second stage discharge decline the flood made its way through the fan head incising the previous debris accumulation and splitting into two main flow paths

deep water formation in the north pacific and deglacial co2 rise deep water formation in the north atlantic and southern ocean is widely thought to influence deglacial co 2 rise and climate change here we suggest that deep water formation in the north pacific may also play an important role we present paired radiocarbon and boron isotope data from foraminifera from sediment core md02 2489 at 3640 m in the north east pacific these show a pronounced excursion during heinrich stadial 1 with benthic planktic radiocarbon offsets dropping to 350 years accompanied by a decrease in benthic 11 b we suggest that this is driven by the onset of deep convection in the north pacific which mixes young shallow waters to depth old deep waters to the surface and low ph water from intermediate depths into the deep ocean this deep water formation event was likely driven by an increase in surface salinity due to subdued atmospheric monsoonal freshwater flux during heinrich stadial 1 the ability of north pacific deep water npdw formation to explain the excursions seen in our data is demonstrated in a series of experiments with an intermediate complexity earth system model these experiments also show that breakdown of stratification in the north pacific leads to a rapid 30 ppm increase in atmospheric co 2 along with decreases in atmospheric 13 c and 14 c consistent with observations of the early deglaciation our inference of deep water formation is based mainly on results from a single sediment core and our boron isotope data are unavoidably sparse in the key hs1 interval so this hypothesis merits further testing however we note that there is independent support for breakdown of stratification in shallower waters during this period including a minimum in 15 n younging in intermediate water 14 c and regional warming we also re evaluate deglacial changes in north pacific productivity and carbonate preservation in light of our new data and suggest that the regional pulse of export production observed during the bolling allerod is promoted by relatively stratified conditions with increased light availability and a shallow potent nutricline overall our work highlights the potential of npdw formation to play a significant and hitherto unrealized role in deglacial climate change and co 2 rise

distribution and provenance of the middle miocene eagle mountain formation and implications for regional kinematic analysis of the basin and range province conglomeratic strata from middle miocene sections in the central resting spring range and nearby eagle mountain california contain a clast assemblage including marble orthoquartzite fusulinid grainstone and coarse 1 m monzogabbro interstratified with tephras yielding laser fusion 40 ar 39 ar ages of 11 6 13 4 and 15 0 ma petrographic and geochronologic evidence ties the clast assemblage to a source area in the southern cottonwood mountains california 100 km west northwest of their present location in the upper 100 m of the resting spring range section conglomerates are derived almost exclusively from the southern cottonwoods source and sandstone modes are as much as 50 angular plagioclase derived from the monzogabbro the lack of dilution of this detritus by other sources and sedimentary features in both sections indicate 1 that deposition occurred on an alluvial fan with a north northeast paleoslope and 2 that transport of the gravels by sedimentary processes was probably 20 km north northeast in a direction normal to the present azimuth to their source therefore we interpret most or all of the net east southeast transport as a result of extensional and strike slip faulting between the cottonwood mountains and the resting spring range since 11 12 ma restoration of these deposits to a position 10 20 km north northeast of the eastern margin of the monzogabbro source east margin of the hunter mountain batholith yields a net tectonic displacement of the cottonwood mountains relative to the resting spring range of 104 km n67 w this result confirms previous reconstructions based on the restoration of isopachs in the cordilleran miogeocline pre cenozoic structural features and other proximal tertiary deposits in the region

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

alluvial fan to storm dominated shelf transition in the mesoproterozoic singhora group chattisgarh supergroup central india abstract mesoproterozoic siliciclastic strata at the basal part of the singhora group central india provide an example of a pre vegetation alluvial fan and braid delta depositional system detailed process based facies and paleoenvironmental analysis have led to the identification of 15 facies types grouped under four different facies associations which record paleoenvironmental settings ranging between continental alluvial fan and distal marine shelf two formations investigated are the rehtikhol formation formed in a conglomeratic or coarse arenaceous alluvial fan braid plain and delta setting and the overlying saraipalli formation which preserved the sedimentation record of a storm infested inner to distal shelf setting the preponderance of granular and sandy sediment gravity flow products with subordinate rock fall deposits within the alluvial fan system together with a pervasive west northwesterly paleocurrent is suggestive of a gently sloping beveled source area in the east southeast distal to the inferred fan a braid plain system developed with the decrease in the depositional slope streams within the braid plain had widths up to several hundred meters the braid plain system at the margin of the marine basin developed a delta with the deposition of wave reworked fluvial dominated sediments the crest trends of wave generated bedforms within the inferred delta sediments depict the northeast southwest shoreline interpreted for the singhora sea the prodeltaic basinal part of the singhora braid delta was storm infested the inner shelf acted as a zone of throughput with high velocity storm flows eroding deep narrow scours gutter casts and is interpreted as a zone of bypass operation of geostrophic currents on the singhora shelf has been inferred from the directional relationship of gutters with the inferred orientation of the shoreline a proximal distal relationship has been established within the shelf products based on the relative dominance of erosional and depositional features the singhora alluvial fan and braid delta facies are arranged in a fining upward sequence denudation of the source area increasing bathymetry with relative rise in sea level or a combination of both is inferred both extrabasinal and intrabasinal forcings possibly contributed to the deepening upward trend

sedimentation of the late miocene bahe formation and its implications for stable environments adjacent to qinling mountains in shaanxi china abstract an important key site along the bahe river in the lantian area shaanxi province northern china exposes a conformable sedimentary sequence that records the transition from early late miocene fluvial sediments into late miocene and pliocene aeolian red clay and finally into pleistocene loess although the red clay and pleistocene loess in northern china have been intensively studied over the years the bahe formation beneath the red clay in the lantian area has been less well studied here we present the preliminary results of a detailed lithostratigraphical investigation of the bahe formation on the northern slopes of the bailuyuan plateau and discuss the style of sedimentation during its deposition the bahe formation in the lantian area is composed of fluvial and lacustrine deposits laid down in the weihe graben during the late miocene six main facies assemblages have been distinguished 1 massive or crudely bedded conglomerates 2 cross stratified conglomerate and sandstone deposits 3 minor sandstone deposits 4 fine grained deposits 5 gritty mudstone and sandstone deposits and 6 marl deposits these assemblages represent the deposits of active channels crevasse splays sheet floods and floodplains with paleosols and lakes thick and laterally pervasive units of fine grained sediments formed as suspension fall out on the floodplain indicating low energy conditions and a relatively gentle surface gradient in the area are by far the most common sedimentary component channel related sandstones and conglomerates indicate that the rivers had a low sinuosity and were braided to anastomosing types the bahe fluvial system operated for several million years in a tectonically inactive setting in a vegetated area under a relatively arid climate this environmental interpretation is consistent with that derived from the interpretation of the vertebrate fossil fauna recovered from the sediments

response of plio pleistocene alluvial systems to tectonically induced base level changes vera basin se spain tectonics are perceived to be a major control on the positioning and long term evolution of alluvial systems by increasing slope gradients through uplift and tilting or by changing local base level incision can be stimulated leading to a switch in the active area of sedimentation an example of such a sedimentary response to tectonic activity is provided by well exposed late pliocene early pleistocene alluvial sediments of the salmeron formation from the western margins of the vera basin se spain early stage palaeogeographic reconstructions demonstrate the occurrence of two alluvial fan bodies with distinct palaeocurrent and provenance signatures that suggest sediment source areas from the north sierra lisbona and south sierra de bedar of the study area late stage reconstructions suggest fan abandonment and indicate the occurrence of a braided river system sourced from the sierra de bedar in the south proximal parts of this braided river are incised by up to 100 m into underlying fan sediments sourced from the sierra de bedar in distal areas incision is negligible and the braided system forms a conformable sedimentary succession with underlying fan sediments sourced from the sierra lisbona the switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the vera basin during the early pleistocene extensional faulting resulted in uplift and subsidence leading to localized tilting of depositional surfaces in distal areas of the southern fan increased stream power resulted in headward incision through mid and proximal fan areas once the system became fully trenched distal base level controls became effective in generating an enlarged catchment area and continuing incision the resultant changes dramatically increased both sediment and water discharge to the alluvial system and a switch to braided river sedimentation

early ordovician alluvial fan deposits of the marmul oil field south oman fine grained shaly sands of the mahwis formation haima group form one of the reservoir intervals of the marmul field six lithofacies can be distinguished from cores shale clast conglomerates trough cross stratified sands horizontally stratified sands massive sands rippled sands and interbedded shales and sands horizontally stratified sands make up 45 of the cored reservoir two scales of fining upward sequence occur small sequences are 0 3 1 5 m thick and are interpreted as sheetflood deposits on the distal portions of semi arid alluvial fans the larger sequences are 25 50 m thick can be correlated on well logs fieldwide and probably represent periodic tectonic rejuvenation and erosional lowering of the sediment source area it is interpreted from regional correlation sediment provenance and a knowledge of possible source areas that the alluvial fans were predominantly sandy had radii of several tens of kilometres and formed an extensive apron bordering a highland area which lay to the southeast the thinly bedded and shaly character of these sheetflood sands causes problems of petrophysical log evaluation and hence oil in place estimation reservoir quality is related to lithofacies occurrence and variation in grain size the large scale fan sequences correspond to an upward decrease in porosity and permeability and form useful subdivisions for field scale correlation and reservoir prediction

impact of the magnitude and frequency of debris flow events on the evolution of an alpine alluvial fan during the last two centuries responses to natural and anthropogenic controls the dynamics and the surface evolution of a post lgm debris flow dominated alluvial fan tartano alluvial fan which lies on the floor of an alpine valley valtellina northern italy have been investigated by means of an integrated study comprising geomorphological field work a sedimentological study photointerpretation quantitative geomorphology analysis of ancient to modern cartography and consultation of historical documents and records the fan catchment meteoclimatic geological and geomorphological characteristics result in fast rates of geomorphic reorganization of the fan surface 2 km2 the dynamics of the fan are determined by the alternation of low return period catastrophic alluvial events dominated by non cohesive debris flows triggered by extreme rainstorms which caused aggradation and steepening of the fan and avulsion of its main channel with periods of low to moderate streamflow discharge punctuated by low to intermediate magnitude flood events causing slower but steady topographic reworking the most ancient parts of the fan surface date back at least to the first half of the 19th century but most of the fan surface has been restructured after 1911 mainly during the debris flow dominated events of 1911 and 1987 phases of rapid fan toe incision and fan degradation have been recognized since the 1930s or 1940s the tartano fan has been subjected to a state of deep entrenchment and narrowing of the main trunk channel and distributary area post little ice age climate change and present day surface uplift rates have been considered as possible explanations for the observed geomorphic evolution but tectonic or climatic controls cannot account for the order of magnitude of the erosional pace anthropogenic controls plausibly override the natural ones in particular the building of a dam in the late 1920s about 2 km upstream of the fan seems to have triggered fan dissection having altered the sediment discharge through sediment retention copyright 2011 john wiley sons ltd

tectonic and climatic control on geomorphological and sedimentary evolution of the mercure basin southern apennines italy abstract the morpho tectonic and sedimentary evolution of the mercure intramontane basin calabria lucania boundary southern apennines has been assessed through facies analysis morphostratigraphy and geomorphological correlation with adjacent areas the mercure basin one of the most active seismogenic zones of the southern apennines is a favorable area for reconstructing the main stages of landscape evolution of the axial zone because of its capability to record changes in base level during the quaternary in addition the presence of both erosional and depositional palaeosurfaces is a useful marker for reconstructing tectonic and morphogenetic events and hence to detect the role played by tectonics and climate in its genesis evolution and extinction the present study identifies the key role of tectonics and denudation combined with high frequency floods as mechanisms controlling alluvial sedimentation in the study area during endorheic conditions denudational processes driven by pulses of extensional deformation of the basin margin caused strong alluvial inputs that resulted in the development of alluvial fans alluvial facies are mainly characterized by turbulent subaerial hyperconcentrated flood flows deposited during the glacial semi arid conditions of mis 14 the retrogradational stacking pattern of the alluvial system indicates decreasing rates of tectonic activity along with declining river gradients the mercure coalescing alluvial fans were inundated by lake transgression during mis 13 in response to i abrupt tectonic subsidence at the basin margins and ii large decrease of coarse sediment supply due to the interplay among climate tectonics and catchment size changes in this regard it is suggested that tectonic control on the drainage network along with climate and long term slope evolution may have caused marked pulses in sediment supply thus influencing the arrangement of facies associations in the sedimentary succession in addition the study points out that the main tectonic landforms developed during each period of the landscape evolution well correspond with some active fault segments

sedimentology of a sandur formed by multiple jokulhlaups kverkfjoll iceland abstract the kverkfjoll sandur in north iceland is the furthest upstream of a suite of fluvial landforms extending for 200 km along the jokulsa a fjollum river incision of the sandur exposes over 3 km of sedimentary sections up to 15 m in height a sandur wide well bedded succession of matrix rich cobble gravel and pebble granule gravel with individual beds 0 2 to 0 5 m thick indicates that the sandur is primarily the product of sandur wide sheet floods with sediment rich hyperconcentrated flows and also some debris flows and channelised turbulent flows this interpretation is evidenced by bedded hyperconcentrated flow deposits occurring as laterally extensive tabular depositional units that dominate the entire sandur reflecting the unconfined nature of the flow clast supported boulder gravel units interpreted as the product of macroturbulent flow occur in relatively narrow but deep channels the sedimentary succession is interpreted as the product of at least six volcanically generated catastrophic outburst floods jokulhlaups during the little ice age the sedimentology of these little ice age flood deposits on a small high gradient sandur contrasts strongly with the deposits of volcanically generated jokulhlaups on large low gradient coastal sandar and sandar associated with retreating glaciers which have been the basis for most previous models of jokulhlaup sedimentation

controls and evolution of fan delta systems in the miocene pohang basin se korea abstract the miocene pohang basin southeastern korea occurs in the eastern part of the ne sw trending yangsan fault which has experienced right lateral strike slip deformation since the eocene during the miocene however the strike slip movement was negligible and instead was under extensional regime large scale drainage systems were developed at the junction between the strike slip zone and the oblique transfer faults acting as major sources for coarse grained sediments high rate of sediment supply and rapid subsidence of the hanging wall resulted in the progradation of large scale gilbert type fan deltas doumsan duksung and gohyun systems which all show a radial distributional pattern along the steeply inclined footwall scarps created by ne sw trending faults the drainage systems were relatively small here low rate of sediment supply formed scree apron type malgol and yugye systems and steep faced slope type maesan system fan deltas based on variations in sedimentary facies and distribution architecture and overlapping patterns the fan delta systems in the pohang basin can be divided into four stages of evolution during the early stage early miocene alluvial fans prograded into shallow marine setting in the doumsan and duksung systems high rate of sediment supply and rapid rise of relative sea level resulted in the transformation of shoal water type to gilbert type foresets in the malgol and maesan systems rapid sea level rise accompanied with low rate of sediment supply caused transformation of alluvial fans to scree apron type and steep faced slope type fan deltas respectively in the second stage subsidence of the hanging wall formed large scale truncation surfaces in the doumsan and duksung fan deltas coarse grained sediments were continuously supplied in the doumsan system whereas only fine grained sediments were deposited in the duksung system in the malgol system sediment supply was almost terminated in this stage in the third stage middle miocene sediment supply rate of the doumsan fan delta decreased abruptly and the system was overlain by fine grained gilbert type deposits the duksung fan delta was overlain by the maesan and gohyun systems the maesan system prograded further basinward during the last stage late miocene sediment supply of the doumsan and maesan systems was also terminated and the entire basin was draped by fine grained biogenic materials mainly diatoms

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

facies characteristics of talchir formation jharia basin india implications for initiation of gondwana sedimentation abstract a long phase of nondeposition that prevailed in peninsular india since the late proterozoic came into an end with the onset of gondwana sedimentation the talchir formation the lowest unit of gondwana supergroup deserves special attention for understanding the late paleozoic geological history in this part of the globe signatures of three major events have been identified in the talchir formation in different gondwana basins of india 1 intracratonic rifting 2 permo carboniferous glaciation and 3 marine incursion but a unique model to explain the specific relation between these events is yet to be formulated the present study reveals that the talchir succession of jharia basin of eastern india bears certain evidences which may lead to a possible solution to this problem the lower part of the succession is represented by a glacigenic conglomerate which underwent brecciation possibly due to hydraulic fracturing during deglaciation facies analysis leads to the conclusion that a thick fan deposit presumably produced through resedimentation of preexisting unconsolidated glacial sediments represents the rest of the talchir succession the presence of fragments of brecciated conglomerate within this fan deposit confirms a significant break in the sedimentation history three fining upward cycles can be identified within the proximal part of the fan succession the lower and the middle cycles bear the signature of emplacement of subaqueous high density flows and variations in facies pattern developed through progressive flow transformation the upper cycle however is represented by sheet flood and stream flow deposits products of dominantly suspension fall out over a vast area with periodic influx of coarser material constitute the distal fan deposit the fan succession is unconformably overlain by a fluvial succession which in turn interfingers with the fluviolacustrine deposits of the younger barakar formation derived from the opposite side of the basin this indicates asymmetric pattern of basin filling as expected in a continental extensional setting the overall character of this succession indicates that 1 emplacement of the intracratonic rift system was a post glaciation event 2 initial phase of deposition was dominated by resedimentation of the preexisting glacial debris and 3 marine incursion probably followed deglaciation and persisted for a short time until isostatic equilibrium was attained before the rift system came into existence

sedimentary processes vertical stratification sequences and geomorphology of the roaring river alluvial fan rocky mountain national park colorado abstract the roaring river alluvial fan formed on 15 july 1982 in rocky mountain national park colorado by a catastrophic flood that was generated by a dam failure the fan covers an area of 0 25 square km has a radial length of 0 7 km and is up to 14 m thick sedimentation occurred in three phases each producing a distinct fan lobe initial sedimentation was by a noncohesive sediment gravity flow which deposited two levees on the proximal boundaries of lobe i the levees consist of a poorly sorted mixture of logs sand pebbles cobbles and boulders the first two lobes were built primarily by sheetflooding which deposited imbricated boulders in trains behind obstacles that formed as jams between boulders or logs and upright trees horizontally laminated granule and sand sedimentation took place down fan from the boulders due to deceleration of the expanding sheetflood thin to medium interbedded sand and cobble pebble gravel couplets were deposited by sheetflooding on the third lobe gravel was transported as bedload by supercritical flow and deposited locally where antidunes broke sand was transported as suspended load and deposited where flood velocity locally decreased due to destruction of antidunes increased roughness or flow separation around the low amplitude gravel bed forms the flood rechannelized at the distal end of lobe iii due to constriction between the fan and the valley margin deposits in the upper rechannelized reaches consist of crudely bedded cobble and pebble gravel and interstratified pebble gravel and backset bedded sand deposition was by supercritical flow in the lower reaches planar cross bedded sandy pebble gravel and climbing ripple horizontal trough cross bedded and backset bedded sand were deposited by supercritical and subcritical flow the flood deposit was modified during waning flood stage and during the three years following the flood by noncatastrophic discharge events these events formed braided distributary channels by erosion into the top of the sheet flood deposits fan building took place mostly by catastrophic unconfined discharge whereas much of the present fan surface consists of braided channels that formed by erosion into the sheetflood deposits by noncatastrophic discharge

diamondiferous neoarchean fan delta deposits western superior province canada sedimentology and provenance abstract most fluvial diamond placers occur in non aggrading braided systems where the diamonds are preferentially trapped by very rough substrates the recognition that some of the neoarchean diamond bearing breccias near wawa ontario canada are actually fluvial conglomerates deposited by a mixture of traction current and mass flow processes provided the opportunity to add information on aggradational diamond placer systems the 454 m thick leadbetter conglomerate was studied to ascertain the depositional processes that led to the development and provenance of the placer the conglomerate is composed of a series of weakly sheared and highly viscous debris flows interbedded with boulder to pebble conglomerates representing longitudinal bars and channels pebble conglomerate and sandstone channels become more dominant upwards with a decrease in mass flow processes an abrupt marine transgression flooded this fan delta assemblage ending deposition of the coarse grained lithofacies the lowest 100 m of the conglomerate is composed of clasts derived solely from the underlying mafic volcanic assemblage above this zone the clast types become more diverse and felsic volcanic clasts become progressively more important upwards whole rock geochemistry performed on coarse grained sandstone samples indicates that major sediment contributions came from mafic rocks and nearby diamondiferous lamprophyres and related breccias though the possible existence of a localized kimberlitic source cannot be excluded geochemistry also indicates that minor enrichments of heavy minerals formed on the alluvial fan delta surface but the primary cause of the conglomerate s diamondiferous nature was a nearby diamond bearing source rock

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

stratigraphic record of pleistocene faulting and basin evolution in the borrego badlands san jacinto fault zone southern california sedimentary rocks in the borrego badlands southern california contain a record of pleistocene crustal deformation during initiation and evolution of the san jacinto fault zone we used detailed geologic stratigraphic and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault controlled sedimentation in this area the base of the 300 to 500 m thick ocotillo formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the borrego formation at 1 05 0 03 ma this coincides with regional scale progradation of ocotillo formation sand and gravel and appears to record initiation of strike slip faults in the southwestern salton trough at ca 1 1 ma thickness trends clast compositions paleocurrents and distribution of paleosols provide evidence for initiation of the east coyote mountain fault at ca 1 05 ma followed by onset of nne ward basin tilting obliquely toward the santa rosa segment of the clark fault at ca 1 0 ma stratigraphic omission of the ocotillo formation and progressively older units southwest of the coyote creek fault beneath the fonts point sandstone provides evidence that tilting to the northnortheast was related in part to growth of the san felipe anticline during deposition of the ocotillo formation map and stratigraphic data suggest that the coyote creek fault in the western borrego badlands postdates ocotillo deposition and thus appears to have propagated southeast into the study area at ca 0 6 ma the fonts point sandstone is a thin sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the borrego badlands the base of the fonts point sandstone changes from a conformable contact in a narrow belt southeast of the inspiration point fault where it is dated at 0 6 0 02 ma to an angular unconformity on the folded ocotillo formation northwest of the fault the pattern of stratal truncation records initiation of the inspiration point fault at ca 0 6 ma this coincides with a major structural reorganization in the san jacinto fault zone that initiated the modern phase of north south shortening and erosion in the southwestern salton trough

catalogue of large alluvial fans in martian impact craters abstract we conducted a systematic global survey using thermal emission imaging system infrared themis ir coverage 100 m pixel to search for large alluvial fans in impact craters on mars our survey has focused on large fans apron areas greater than 40 km 2 usually located in craters greater than 20 km in diameter due to the resolution of the themis images and mars orbiter laser altimeter mola coverage we find that the host craters are found to have a distinctive diameter range from 30 150 km the fans generally cluster in three geographic areas southern margaritifer terra southwestern terra sabaea and southwestern tyrrhena terra however several outliers do exist the alluvial fans do not form in a particular orientation along the crater rim nor are they associated with the location of current high rim topography fan area magnitude and variability increase with crater diameter while fan concavity magnitude and variability increase with decreasing crater diameter smaller fan aprons in general have higher more variable concavity the source of the water forming these fans is uncertain given the challenges of accommodating the global distribution pattern and formation patterns within the craters

sedimentological response of an alluvial system to neogene thrust tectonics atacama desert northern chile the llano de la paciencia is a thrust sheet top basin in which the sedimentological and topographic evolution can be linked to thrust tip propagation it is an elongate gravel plain which borders the salar de atacama a major intermontane basin in the andean forearc of northern chile the llano is bounded to the west by the cerros de purilactis a cretaceous paleocene sequence uplifted by the frontal domeyko thrust the eastern margin of the llano is formed by the cordillera de la sal which was uplifted by a linked back thrust frontal thrust system the salar de atacama is thus divided into a number of discrete sub basins the llano de la paciencia the pampa visachita and the western sub llano which from east west are bounded by the cordillera de la sal the northern imbricates and the ignimbrite back thrust two phases of sedimentological evolution can be distinguished within the llano on the basis of quaternary to recent sediment dispersal patterns initially phase 1 alluvial fan lobes prograded eastwards into the main salar de atacama basin subsequently uplift of the cordillera de la sal deflected drainage systems southwards parallel to the structural strike these phase 2 alluvial deposits drain into the salar de atacama at the lateral termination of the cordillera de la sal frontal thrust in places where thrust tip ramps are emergent within the llano gullies have been incised into the drainage pathways this has resulted in the reworking of the early phase 1 gravels and progradation of the phase 2 fan lobes the evolution of the llano de la paciencia illustrates the close link between topography and syntectonic alluvial drainage patterns in an arid intermontane basin