climatic controls on late pleistocene alluvial fans cyprus alluvial fans are commonly associated with tectonically active mountain ranges and tectonism is frequently held responsible for abrupt coarsening and cyclical sedimentation of alluvial fan sequences whilst it is accepted that tectonism provides the opportunity for alluvial fan development through the creation of topography increasing gradients of fluvial systems supplying sediments and creating accommodation for the storage of sediment flux the role of climate in fan development is frequently neglected the hypothesis that climatically controlled events can produce recognisable sedimentary signatures in alluvial fan deposits is tested in the active supra subduction zone setting of the late pleistocene of southern cyprus this study demonstrates through architectural analysis and the reconstruction of palaeoflood hydrology a recorded pattern of increasing and decreasing palaeoflow dynamics with switches from a wetter to drier mode clearly exhibited by changes in the sedimentology of the fan at the present day cyprus has a semi arid climate and is influenced by a strongly seasonal rainfall pattern largely restricted to the winter months plus rare occurrences of summer cyclones however at precession minima increased activity of western mediterranean depressions produces wetter summers using inference we propose that longer term increases in rainfall increased river discharge as recorded in the fan palaeoflood hydrology and occurred at minima in the precession these periods correlate with the deposition of conglomeratic channels and open framework gravels drier periods are exhibited by sandier braided fluvial deposits shorter term or seasonal change is recorded in the form of 2nd and 3rd low order bounding surfaces this increased activity of mediterranean summer depressions increased precipitation to the wider levantine area and was coincident with increased intensity of the north african and indian ocean sw monsoons the resultant increase in river discharges at precession minima from both the nile and the wider levant resulted in the formation of sapropels in the eastern mediterranean and is recorded as wet periods in speleothem deposits in the soreq and peqiin caves of israel the predominant control of sedimentation on the late pleistocene alluvial fans of southern cyprus was climate

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

geomorphology internal structure and evolution of alluvial fans at motozintla chiapas mexico abstract alluvial fans and terraces develop in diverse regions responding to different tectonic and climatic conditions the motozintla basin is located in the state of chiapas southern mexico and has an e w orientation following the trace of the left lateral polochic fault the evolution of the motozintla basin and the alluvial plain is related to several factors such as fault movement intense erosion by hydrometeorological events and anthropogenic activity this study presents the geomorphology of the alluvial plain that between the villages of motozintla and mazapa de juarez exposes 31 alluvial fans 5 hanging terraces and 13 ramps fourteen of these alluvial fans have been truncated by the polochic fault exposing maximum uplifts of 12 m the internal structure of truncated fans consists of single massive beds monolithologic fans or stacked beds polygenetic fans the fans stratigraphy is made of debris flow deposits separated by paleosols and minor hyperconcentrated flows fluviatile beds and pyroclastic fall deposits the reconstruction of the stratigraphy assisted by radiocarbon geochronology suggests that these fans have been active since late pleistocene 25 ka to the present this record suggests that at least 10 events have been recorded at the fan interior during the past 1840 years one of these events at 355 65 14 c yrs bp cal yrs ad 1438 to 1652 can be correlated across the fans and is likely associated with an extreme hydrometeorologic event the presence of a 165 60 14 c yrs bp cal yrs ad 1652 1949 debris flow deposit within the fans suggests that movement along the polochic fault formed the fans scarp afterwards in fact a historic earthquake along the fault occurred east of motozintla on july 22 1816 with a m w of 7 5 7 75 recent catastrophic floods have affected motozintla in 1998 and 2005 induced by extreme hydrometeorological events and anthropogenic factors therefore scenarios for motozintla involved several types of mass movement processes that pose a serious hazard and threat to the inhabitants of the region

controls on fan development evidence from fan morphometry and sedimentology sierra nevada se spain abstract we have studied the morphology and the morphometric relations between alluvial fans and drainage basins in a bajada system including more than 20 coalescent fans developed since the late pleistocene as a result of the recent uplift of the sierra nevada betic cordillera spain three allocycles of tectonic origin were recognised across which there is a clear evolution from debris flow to sheet flow dominated fans in connection with a decrease in the volume of fines available in the source areas the larger volume of accommodation space created by higher tectonic subsidence in the northern sector favoured vertical accumulation of sediment with the appearance of less elongated fans in the rest of the system where subsidence is less more elongated fans appear with the development of incised channels and irregular distribution of sediment in depositional lobes this involves the appearance of markedly asymmetric transverse profiles as well as lengthy recurrence between the sedimentation events on particular sectors of the fans where headward eroding gullies develop the lithology of the source area where intensely fractured rocks are found is responsible for an important sediment supply and a significant degree of clast sorting from the source area consequently a weak longitudinal trend in particle size can be recognised which influences the predominance of longitudinal constant slope profiles and anomalous relations between both fan area and fan slope and their drainage areas recent intense tectonic activity has caused the appearance of abnormally low slopes in upper sectors of some catchments where mass flows are trapped and the fans present a subsequently higher proportion of sheet flows recent piracy phenomena can be recognised in some of the drainage basins which indicate rejuvenation of the source area in which case the fan presents a rapid increase in the proportion of mass flows with the development of a segmented longitudinal profile due to the recent nature of the processes the morphometric relations in the fan fed by the basin affected by capture do not coincide with the other fans in the area

geomorphologic relationships and ages of soils on alluvial fans in the rio general valley costa rica abstract soil geomorphic relationships on alluvial fans in the rio general valley costa rica show that the sequence of alluvial fan surfaces and soil development on them is related chiefly to uplift of the bordering mountain ranges and not significantly to climatic change the oldest and most strongly developed soils are found only on the highest surfaces successively younger and less developed soils have lower positions oxisols characterize the oldest surfaces alfisols the intermediate ones and entisols the youngest surfaces minimum ages of these surfaces are estimated from the extrapolation of sedimentation rates solum development rates and the rate of channel incision based on two radiocarbon dates to range from about 45 000 to 65 000 years for the oldest to approximately 100 years for the most recent surface pedons representative of the soils on the principal alluvial surfaces are described in terms of the physical chemical and mineralogic characteristic we propose that the soils studied are a chrono catena that represents soils development through time for this region and can be used to establish a minimum time scale by which to guage stage and rate of soil development in a humid tropical environment

sediment soil sequences in the granada basin as evidence for long and short term climatic changes during the pliocene and quaternary in the western mediterranean abstract since the tortonian the carbonate dominated granada basin has received various terrestrial sediments late miocene pliocene early and mid pleistocene alluvial fans and late pleistocene slope deposits and loesses these deposits include many paleosols two hundred and forty samples of paleosols and sediments in 20 representative pliocene and pleistocene profiles were analysed by standard methods the long sequences with up to 85 paleosols indicate climatic cycles over at least 5 ma erosion and sedimentation occurred under dry conditions without vegetation cover during periods of geomorphodynamic activity soil formation occurred under wet conditions with closed vegetation during periods of geomorphodynamic stability the pedogenetically effective climate has been subtropical mediterranean since the early pliocene decreasing soil formation intensity indicates decreases of humidity in the mid late pliocene and late pleistocene

ice formation in ash influenced clouds after the eruption of the eyjafjallajokull volcano in april 2010 1 the influence of volcanic ash on heterogeneous ice nucleation in tropospheric clouds is investigated on the basis of 90 observed cloud cases the clouds were observed with polarization lidars at the two central european earlinet stations leipzig 51 3 n 12 4 e and maisach 48 2 n 11 3 e 25 km northwest of munich germany in volcanic aerosol layers which originated from the strong eruptions of the icelandic eyjafjallajokull volcano in april 2010 case studies of evolving boundary layer cumuli and long lasting free tropospheric cloud events with unusual behavior mixed phase cloud complex cirrus deck are discussed a clear impact of ash is observed the ice nuclei concentration derived from the lidar observations has been estimated to range from 2 20 per liter in the boundary layer and from 100 300 per liter at cirrus level the statistical analysis based on the 90 evaluated cloud cases revealed that all observed cloud layers with cloud top temperatures of below 15 c contained ice typically under non volcanic aerosol conditions such a high fraction of ice containing clouds is not reached before temperatures decrease below 25 c over central europe

the development of a laterally confined laboratory fan delta under sediment supply reduction abstract in previous fan delta experiments the effect of lateral confinement was generally ignored as these fans were usually unconfined with semiconical geometries however in gorge areas fan development is usually laterally confined by valley walls this study investigates autogenic processes of fan deltas in a laterally confined experimental tank the experiment is divided into three phases the sediment supply is held constant within each phase so the autogenic processes of the fan are separated from the allogenic forcings results indicate that laterally confined fan deltas have higher progradation and aggradation potential more regular channel braiding and more even transverse sedimentation than unconfined fans besides responses of fan deltas to sediment supply reduction are investigated in this research at the initiation of the second and third phases sediment feed rates are instantaneously reduced so that the allogenic forcings are predominant observations show that under sediment supply reduction channelization on fan deltas are more pronounced and durations of the fluvial cycles are longer the adjustment of fan morphology becomes slower as the self regulation capacity of the fan decreases with reduced sediment supply

luminescence ages for alluvial fan deposits in southern death valley implications for climate driven sedimentation along a tectonically active mountain front abstract controversy exists over whether alluvial fan sedimentation along tectonically active mountain fronts is driven by climatic changes or tectonics knowing the age of sedimentation is the key to understanding the relationship between sedimentation and its cause alluvial fan deposits in death valley and throughout the arid southwestern united states have long been the subjects of study but their ages have generally eluded researchers until recently most mapping efforts have recognized at least four major relative age groupings q1 oldest q2 q3 and q4 youngest using observed changes in surface soils and morphology relation to the drainage net and development of desert pavement obtaining numerical age determinations for these morphologic stages has proven challenging we report the first optically stimulated luminescence osl ages for three of these four stages deposited within alluvial fans along the tectonically active black mountains of death valley deposits showing distinct remnant bar and swale topography q3b have osl ages from 7 to 4 ka whereas those with moderate to poorly developed desert pavement and located farther above the active channel q3a have osl ages from 17 to 11 ka geomorphically older deposits with well developed desert pavement q2d have osl ages 25 ka using this osl based chronology we note that alluvial fan deposition along this tectonically active mountain front corresponds to both wet to dry and dry to wet climate changes recorded globally and regionally these findings underscore the influence of climate change on alluvial fan deposition in arid and semi arid regions

the ongeluk basaltic andesite formation in griqualand west south africa submarine alteration in a 2222 ma proterozoic sea abstract the ongeluk lavas form part of the palaeoproterozoic transvaal griqualand west supracrustal sequence of the archaean kaapvaal craton of south africa they form a thick shallow marine volcanic sequence of pillow lava massive flows and hyaloclastite which together with their comagmatic subaerial hekpoort correlate once covered most of the craton in this study the magmatic composition and alteration features of ongeluk samples were distinguished using geochemistry and radiogenic isotopes the ongeluk hekpoort magma was a basaltic andesite derived from anomalous kaapvaal craton lithosphere with a long history of high u pb high rb sr and chur like sm nd ratios little evidence for magmatic fractionation was found although contamination of the lower lavas with cr rich material is evident in both hekpoort and ongeluk sequences the alteration of ongeluk lavas is characterised using diagrams which compare pillow core rim pairs hyaloclastite and massive lava samples with the estimated original magma composition alteration is ascribed to a shallow 87 sr 86 sr ratios decreased from magmatic 0 7071 towards seawater 0 7025 during submarine alteration the rb sr system also reveals a much later 1200 ma phase of ca and rb mobility in a few samples in contrast the submarine alteration affected pb isotope ratios so little that a 2222 13 ma isochron age is regarded as a refinement of earlier dates for the ongeluk hekpoort extrusion the ongeluk hyaloclastites acted as a sink for k and rb in the overlying sea while the mn and ca entering the sea could have contributed to the overlying kalahari manganese deposits

surficial patterns of debris flow deposition on alluvial fans in death valley ca using airborne laser swath mapping data debris flows are a common event in mountainous environments they often possess the greatest potential for destruction of property and loss of lives in these regions delimiting the spatial extent of potential damage from debris flows relies on detailed studies of the location of depositional zones current research indicates debris flow fans have two distinct depositional zones however the two zones were derived from studies containing detailed analyses of only a few fans high resolution airborne laser swath mapping alsm data is used to calculate profile curvature and surface gradient on 19 debris flow fans on the eastern side of death valley the relationship between these parameters is assessed to 1 identify if debris flow fans are accurately represented by two depositional zones and 2 to assess how these terrain parameters relate to one another at the individual fan scale the results show at least three zones of deposition exist within the sampled fans these zones do not hold consistent when individual fan morphometry is analyzed in conjunction with localized fan surface gradients fans with consistently shallower gradients exhibit numerous depositional zones with more subtle changes in profile curvature steeper gradient fans exhibit significantly fewer zones with more pronounced local changes in profile curvature the surface complexity of debris flow fans is evident from these analyses and must be accounted for in any type of hazard studies related to these features

the importance of earthquake induced landslides to long term slope erosion and slope failure hazards in seismically active regions abstract this paper describes a general method for determining the amount of earthquake induced landsliding that occurs in a seismically active region over time this determination can be used as a quantitative measure of the long term hazard from seismically triggered landslides as well as a measure of the importance of this process to regional slope erosion rates and landscape evolution the method uses data from historical earthquakes to relate total volume of landslide material dislodged by an earthquake to the magnitude m and seismic moment m0 of the earthquake from worldwide data a linear regression relation between landslide volume v and m0 is determined as v m0 1018 9 0 13 where v is measured in m3 and m0 is in dyn cm to determine the amount of earthquake generated landsliding over time this relation is combined with data on seismic moment release for a particular region which may be derived from either earthquake history or fault slip data the form of the m0 v relation allows the rate of production of earthquake induced landslides over time to be determined from total rate of seismic moment release without regard to the distribution of individual events thus simplifying and generalizing the determination application of the method to twelve seismically active regions with areas ranging from 13 275 to 2 308 000 km2 shows that erosion rates from earthquake induced landslides vary significantly from region to region of the regions studied the highest rates were determined for the island of hawaii new zealand western new guinea and the san francisco bay region of california significantly lower rates were determined for iran tibet the sierra nevada great basin region of california and central japan for the time period from 715 ad to the present intermediate rates were determined for peru southern california onshore california turkey and central japan for the time period from 1586 ad to the present to determine the relative long term importance of seismically triggered landslides these erosion rates are compared to erosion rates calculated for other slope processes and to rates calculated from fluvial sediment discharge comparisons with other slope processes indicate that earthquake induced landslides are the predominant agents of slope erosion on the island of hawaii in the san francisco bay region and in western new guinea for hawaii the san francisco bay region and sierra nevada great basin region of california the erosion rates calculated for earthquake induced landslides also exceed the regional erosion rates calculated from fluvial sediment discharge

compilation and time series analysis of a marine carbonate δ18o δ13c 87sr 86sr and δ34s database through earth history abstract the sr s o and c isotope database of marine carbonates contains over 55 000 published isotope values of low mg calcite from diagenetically little altered phanerozoic fossil shells as well as samples of whole rocks and calcite cements of ordovician to archean age carbon and oxygen isotope data for the shell material are divided into habitat subsets high mid low latitude and deep sea and whole rock data are separated by mineralogy into calcite dolomite subsets trend correlation wavelet and spectral analyses on gaussian filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results 1 oxygen isotope trends from the high latitude and deep sea habitats are almost indistinguishable through the last 115 ma consistent with the existence of the oceanic conveyor belt throughout this interval 2 all oxygen isotope habitat records show a strong coherent 30 45 ma 38 ma cyclicity throughout the cretaceous and the cenozoic 3 up to 70 of the multi million year variability in the 18o record of the last 115 ma can be simulated by the following equation 18 o 0 64 sin 2 t 120 ma 0 9 x sin 2 t 38 3 ma 1 1 with x ranging from 0 4 0 6 for the low high latitude and deep sea habitats to 0 8 for the mid latitude realm 1 a 120 20 ma cycle occurs in the paleozoic and neoproterozoic 18o record consistent with paleoclimate variability as interpreted from sedimentological and faunal records 2 the offset of 13c values between deep water and high latitude vs surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the cretaceous 3 sr and s isotope records exhibit a 60 70 ma cyclicity throughout the phanerozoic

the guatemala earthquake of 1816 on the chixoy polochic fault abstract forty historical documents have been found which describe a previously unknown very large earthquake on 22 july 1816 this event occurred along the strike slip boundary between the caribbean and north american plates in guatemala and southern mexico modified mercalli intensities are estimated from these accounts and a rough isoseismal map is constructed the damage pattern indicates that the causative fault was the left lateral chixoy polochic fault for which no damaging earthquake has previously been reported damage of modified mercalli intensity vii or greater covered an area of at least 13 000 km 2 extending over 340 km from alta verapaz province in guatemala westward to san cristobal ias casas in chiapas mexico the area of intensity vii indicates a moment of 1 10 28 dyne cm while the estimated length of the intensity vii isoseismal indicates a more likely figure of 3 5 10 27 dyne cm or an equivalent magnitude m w of 7 1 2 to 7 3 4 the reported aftershock sequence is compatible with an earthquake in this magnitude range these data demonstrate that the chixoy polochic fault is currently an active part of the caribbean north american plate boundary and is capable of producing very large earthquakes

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

evolution of fluvial terraces in response to climate change and tectonic uplift during the pleistocene evidence from kamienica and ochotnica river valleys polish outer carpathians abstract fluvial terraces of kamienica and ochotnica river valleys recorded vertical movements of the earth s crust and oscillatory changes in the pleistocene climate seven terrace levels are present in the valleys usually built of a bedrock surface overlain by a fluvial sediment cover of various thickness these flights of terraces formed a staircase like relief during the cyclical evolution of the valley bottoms during the interglacial periods valley bottoms widened and bedrock platforms were formed that later became the base on which fluvial deposits accumulated the main phases of aggradation took place during the glacial periods and partly also in the interglacials incision and deepening of the valleys were mainly confined to the phases of glacial interglacial transition additionally also during interglacials and the interglacial glacial transitions while the area was uplifted constantly throughout the pleistocene results of the research were supported by published studies on simulation and modelling

testing models of pre goe environmental oxidation a paleoproterozoic marine signal in platform dolomites of the tongwane formation south africa abstract carbonate facies of the upper tongwane formation preserve a largely overlooked record of pre goe paleoproterozoic seawater this inventory has survived despite a complex paragenetic history involving both diagenetic and contact metamorphic processes bif mineral assemblages are dominated by a medium grade grunerite overprint and the formation of prograde and retrograde riebeckite the massive dolomite member a platform top carbonate which caps the succession is characterized by marine rey sn patterns lacking significant negative ce sn anomalies 13 c carb values of between 1 83 and 2 95 vpdb that are slightly elevated above paleoproterozoic normal marine values systematically decreasing 13 c carb values measured in monotonously recrystallized talc bearing slope carbonates reflect interaction with devolatilization fluids during contact metamorphism caused by the intrusion of the bushveld igneous complex slope carbonates offer no insight into marine paleoredox conditions thus despite contact metamorphism paleoproterozoic marine signals are retained in platform top dolomites however they do not support claims for widespread oxygen accumulation in shallow depositional environments of the transvaal basin on the eve of the goe

tectonic and geochronological constraints on late archaean and palaeoproterozoic stratigraphic correlation within and between the kaapvaal and pilbara cratons recent radiometric dating of carbonates and banded iron formations on the kaapvaal craton southern africa has suggested that late archaean and palaeoproterozoic carbonate sedimentation was diachronous across the craton we present new shrimp u pb zircon ages of 2583 5 ma and 2588 7 ma for two samples of a tuff bed 320 km apart in the upper oak tree formation transvaal supergroup these ages in conjunction with published data support previously established sequence stratigraphic correlations that show the temporal equivalence of late archaean carbonates in the region a review of the sequence stratigraphy and geochronology of the mount bruce megasequence set on the pilbara craton western australia and the newly named highveld megasequence set on the kaapvaal craton indicates a remarkably similar geohistory both cratons are characterized by a divergent megacycle hamersley range and vaal megasequences followed by a convergent megacycle chichester range and drakensberg megasequences our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time c 2590 ma which permits more meaningful geotectonic correlations between them

the role of base level change in the dissection of alluvial fans case studies from southeast spain and nevada abstract tectonics climate and base level are the underlying controls of alluvial fan development on many fans base level is stable but where base level change occurs it may itself be a response to tectonics or climatic change tectonically induced base level change will be temporally independent of climatic change and will show spatial variability depending on the rate of propagation of tectonically induced regional dissection this is illustrated by the tabernas fans southeast spain the timing of climatically induced base level change follows the timing of climatic change itself and will show more consistency of spatial relationships within groups of fans affected alluvial fans which toe out in coastal zones or at lake shorelines may be influenced by climatically controlled base level changes conventionally a fall in base level would be expected to cause dissection of the distal fan zone however the reverse may occur with fan progradation taking place in response to a base level fall and dissection occurring in response to a base level rise two contrasted case studies are considered based in part on previously published work from dry region alluvial fan zones the coastal cabo de gata fans in southeast spain and the stillwater and cold springs fans in nevada usa in these areas base level changes occurred in response to late quaternary eustatic changes of sea level and changes in the levels of pluvial lakes respectively within both areas there are fans not affected by base level change cabo de gata west coast and inland fans cold springs fans and others where base level change has been important in fan evolution cabo de gata east coast fans stillwater fans the differences in geomorphic regime between those fans influenced by base level changes and those not are expressed in morphometric contrasts and different fan profile and plan characteristics within those fans influenced by base level changes the different mechanisms have influenced the location and elevation of the zone of incision the differences can be attributed in part to the temporal interactions of base level change with climatically induced changes in sediment supply and in part to differences in gradients of the foreshore exposed by the falling sea or lake base levels

subaerial debris flow deposition in the upper paleozoic cutler formation western colorado abstract outcrops of the lower part of the cutler formation near gateway colorado contain proximal alluvial fan deposits of two principal lithofacies the diamictite lithofacies d consists of a variety of sharply bounded massive muddy beds which are interpreted as deposits of subaerial debris flows the gravelly sandstone lithofacies s consists of amalgamated beds of laminated and cross bedded conglomeratic sandstones which are interpreted as water deposited sediments clasts in s are rarely larger than 0 5 m in maximum diameter but clasts in d are commonly much larger up to 8 m both normal and inverse grading are visible in d but grading is generally not well developed in either lithofacies imbrication of clasts although more pronounced and more consistent in s is fairly commo in both lithofacies four subfacies of diamictite are distinguishable on the basis of clast packing and grading massive matrix supported dram graded matrix supported dmg inversely graded clast supported dci and massive clast supported dcm given the hydrodynamic and rheologic properties of debris flows as reported by previous workers four debris flow regimes are postulated on the basis of varying water content and clast interaction subfacies dram and drag typify deposits of flows having viscous clast interactions subfacies dci represents deposits of flows having inertial or colliding clast interactions subfacies dram comprises deposits of flows with demonstrably high yield strengths subfacies dcm represents deposits of flows in which yield strength was insufficient to support most clasts this classification specifically recognizes turbulence selective deposition and bedload transport of clasts as essential characteristics of certain dilute debris flows it also accommodates as debris flow deposits some beds which have features usually associated with aqueous deposition including clast frameworks channelization imbrication and normal grading

alluvial fan and catchment initiation by rock avalanching owens valley california abstract the north long john alluvial fan of the inyo mountains piedmont owens valley ca was catastrophically initiated by a prehistoric early holocene rock avalanche this avalanche resulted from the collapse and disintegration of the central part of a 1 1 2 0 km range front bedrock facet comprising the divide between the catchments of two large adjoining alluvial fans failure rapidly produced and transferred 25 million m 3 of new sediment to the piedmont where it was deposited in the trough between two coalesced fans these deposits are of unstratified angular muddy bouldery and cobbly pebble gravel present in a u shaped form this form consists of lateral levees 10 to 60 m tall that lead 1 6 km from the range front to a 108 m high distal snout the avalanche sediment comprise the initial platform for and still dominate the nascent north long john fan rock avalanche failure also created a 100 m deep concave scar in the range front that now serves as the fan catchment the slopes of this catchment are mantled by colluvial sediment generated from intense shear along the avalanche detachment surface thus the avalanche served as an alluvial fan starter kit by building the fan platform and by creating a range front catchment lined with sediment because of this unique and recent origin the north long john fan and its catchment are in an early stage of development lacking features such as old surfaces and a well developed drainage net typical of the older adjoining alluvial fans and catchments post avalanche fan activity has been dominated by the deposition of 4 million m 3 of sediment by debris flows these flows including one in 1984 were instigated by the concentration of flashy thunderstorm precipitation across the colluvium of the nascent catchment debris flows have built a conical tract that extends 1 km from the fan apex and partially buries the older avalanche deposits the debris flow tract consists of muddy pebbly cobbly boulder gravel in levees 50 200 cm tall in the steep 12 16 upper segment and boulder free lobes in the lower sloping 5 distal segment this tract terminates in a pond behind the avalanche snout where recessional debris flow pebbly mud has been trapped this pond eventually filled and was breached by a deep channel that now transects the snout of the rock avalanche a second debris flow tract 300 m in radius has prograded outward from the end of the avalanche bypass channel the facies of this tract are identical to those of the proximal fan an abundance of colluvial sediment mantling the steep slopes of the catchment indicates that the north long john fan will continue to enlarge by thunderstorm induced debris flows

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

human impact on geomorphic processes and hazards in mountain areas in northern spain abstract the temporal occurrence of slope movements their contribution to relief evolution and human influence on those processes in two study areas of northern spain are presented the work is based on temporal analyses covering 100 ka in one study area and 43 years in the other temporal analysis has been the basis for quantitatively assessing the magnitude of human influence and developing landslide susceptibility and hazard models with known independently tested prediction capability the results obtained in one study area show a relationship between landsliding periods and increasing precipitation during upper pleistocene and holocene significant increases of landslide frequency and mobilisation rate were also found around 5500 and 200 years ago those moments coincide with two periods of intensified human presence and activities neolithic and industrial revolutions the increase observed represents about one order of magnitude from pre neolithic to present a similar increase has been found between 1954 and 1997 in the other study area the latter increase shows no relationship with changes in climate parameters or seismic activity a fairly good correlation has been found between landslide frequency and socioeconomic indicators of human activity sedimentation rates in two neighbouring estuaries were determined and significant increases particularly in the second part of last century were also found evidence obtained suggest that the increases observed in the frequency of slope instability events and therefore hazard denudation and sedimentation rates are due to a greater extent to indirect geomorphologic changes caused by human action rather than climate change detailed analysis of landslide frequency during that 43 year period has also been used to produce and validate landslide susceptibility models and obtain landslide hazard maps with known prediction accuracy validation tests were carried out comparing susceptibility maps based on landslides that have occurred in the past with the ones occurring in several later periods prediction capability of models could thus be determined validation of susceptibility maps using data derived from the temporal analysis of landslides was used to obtain hazard maps expressed as probability of new ruptures in a given time period on the basis of past trends in terrain behaviour possible impacts of a growing influence of human activities on the terrain have been incorporated into the assessment of future hazards by considering different scenarios

tectonic controls on the morphometry of alluvial fans around danehkhoshk anticline zagros iran article i nfo alluvial fans are important landforms where their morphology and morphometry reflect changes in tectonic climate base level and drainage basin characteristics along the margins of tectonically active mountain ranges like the zagros mountains alluvial fans are generally assumed to act as useful landforms for identify ing the level of tectonic activity the purpose of this paper is to evaluate the relationship between active tec tonics and morphometric characteristics of alluvial fans around danehkhoshk anticline in the simply folded belt of zagros morphometric characteristics of alluvial fans such as area fa slope sf length of base bf width length ratio w l radius r sweep angle sa and entrenchment e as well as valley floor width to height ratio vf and strata dips of anticline limbs dal were measured the study area was sub divided into eight tectonic zones and then the mean values of the above mentioned parameters were calculated in each zone result reveals that values of sa bf and e are directly proportional to dal the poor relationships between catchment characteristics slope and area and fan parameters are probably due to extensive karstic landforms of catchments having complex hydrologic systems and hence result in complex catchment fan relations the highly entrenched fans with high sweep angles and long bases are characteristic of tectonically active fronts of danehkhoshk anticline having v shaped valleys higher vf values steep triangular facets and more rotated limbs higher dal values apart from the tectonic control on fan development the fan head entrenchment and negative accumulation spaces on most alluvial fans can be attributed to decreased sediment load and discharge the drier the present day climate regime

the tabernas alluvial fan and lake system southeast spain applications of mineral magnetic and pedogenic iron oxide analyses towards clarifying the quaternary sediment sequences abstract mineral magnetic and soil iron oxide data are applied to questions of relative age correlation of alluvial fans and lake sediments in the tabernas basin southeast spain within a context of interaction between tectonics and climatic change within the tabernas basin the sediment sequences and morphological evolution of late quaternary alluvial fans suggest climatic change as the primary control the fans toe out at the upper margins of a former lake created in response to tectonic uplift magnetic and iron oxide data from soils particularly dithionite extractable iron fe d and frequency dependent magnetic susceptibility fd accord with the relative age relationships suggested by more conventional field based geomorphic observations magnetic data from the lake sediments suggest the main provenance characteristics of the sediments but also reveal a shift in sediment sources towards the end of the lake period probably during the late pleistocene to sediment supplied from a more active fluvial system from soil erosion within the sierra de los filabres part of the catchment hence although the locations of the fans and the existence of the lake relate primarily to tectonics the fan sequences themselves appear to be primarily climatically controlled and there is evidence of a climatic influence over the source of sediment input into the lake during the late pleistocene

delineation of alluvial fans from digital elevation models with a gis algorithm for the geomorphological mapping of the earth and mars alluvial fans are prominent depositional geomorphic features present in nearly all global climates on earth and also found on mars in this study we present a geographic information system gis algorithm designed for the semi automated detection of alluvial fans that are connected to their contributing upstream drainage network from the analysis of digital elevation models dems through a combination of spatial analysis procedures the gis algorithm generates maps of alluvial fans and their upstream source drainage and watersheds tests of the algorithm in areas with well known alluvial fans indicate that this new gis procedure is capable of high accuracy mapping of the fan apexes and correct delineation of fan deposits in both arid and humid climates possible future applications of the gis algorithm presented in this study include the systematic survey of alluvial fans at the local regional and planetary scales important for geologic hazard assessment studies on the evolution of climate analysis of continental sedimentary environments understanding of the interplay between the endogenous dynamics and exogenous processes and the evaluation of natural resources

slope of alluvial fans in humid regions of japan taiwan and the philippines abstract this study constructed a database for 690 alluvial fans in humid regions of japan taiwan and the philippines and analyzed their slopes and other morphometric parameters such as the area and relief ratio of the source area this type of comprehensive geomorphological research on alluvial fans in humid regions has been limited although numerous studies have dealt with fans in arid and semi arid regions semi conical depositional landforms larger than 2 km2 and steeper than 0 11 0 002 m m were selected as alluvial fans about 60 of the fans formed during the holocene and the rest 40 formed during the pleistocene mean fan slopes for both all the quaternary fans and holocene fans follow a lognormal frequency distribution the distribution also fits slopes of fans belonging to a certain areal range the area and relief ratio of the source area which correlate well with the fan slope also follow a lognormal distribution indicating the dimensions of fan basin systems vary gradually rather than abruptly these findings contrast with a previous notion that depositional landforms rarely have slopes of 0 5 0 009 to 1 5 0 026 and hence only semi conical landforms steeper than 1 5 should be called alluvial fans

debris flow and hyperconcentrated flood flow deposits in an alluvial fan northwestern part of the cretaceous yongdong basin central korea abstract the alluvial fan deposits in the cretaceous yongdong basin korea consist of conglomerates with a muddy or sandy matrix and sandstones with thick or thin laminations the conglomerates and sandstones occur commonly in couplets constituting apparently single sedimentation units facies transitions in the deposits can be summed up in a tripartite facies sequence 1 a clast supported conglomerate with a muddy or sandy matrix and parallel clast alignment facies a and b 2 a matrix supported coarse tail normally graded conglomerate with random clast orientation facies c and thickly stratified pebbly sandstone facies d and 3 laminated sandstone facies e the clast supported conglomerate is interpreted as deposits of debris flows dominated by frictional grain interactions development of pervasive parallel clast alignment lacking large floating clasts and inverse grading suggests deposition via incremental aggradation rather than en masse freezing the matrix supported conglomerate an

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

eustatic and tectonic control on late quaternary alluvial fans along the tyrrhenian sea coast of calabria south italy abstract the late quaternary evolution of the western coastal range northern calabria has been reconstructed by means of a multidisciplinary approach we have focused on the role of allocyclic mechanisms tectonics and base level change considered as factors influencing alluvial fan development in particular we identify no clear evidence for a major influence of climate age constraints were provided by a tephra layer and a paleosol coupled with geomorphological correlations middle pleistocene perched terraces and fan relicts hanging above fault scarps indicate that tectonics caused switches in fan dynamics between aggradational and erosional phases as tectonic activity declined during late middle pleistocene eustatically driven changes in base level possibly plus low rates of tectonic subsidence represent the main factor which controlled late pleistocene fan development the two major fan aggradation phases occurred during periods of climate amelioration prior to highstands whereas high sea levels ois 5 and 1 promoted channel incision into the fan surfaces this continued during periods of climate deterioration with a seaward progradation of alluvial deposition sea level rise is seen as creating accommodation space producing very open fans locally within mountain embayments the fans are characterised by a mushroom shape in plan view and by a retrograde stacking pattern of alluvial fan facies

impacts of holocene climatic variations on alluvial fan activity below snowpatches in subarctic quebec abstract in the lake guillaume delisle area of subarctic quebec storm generated alluvial fans have been active sporadically throughout the holocene in this study we propose that the persistence of late lying snowpatches in fan catchments during holocene cold episodes promoted alluvial fan activity by lowering the precipitation threshold required to trigger a torrential event this hypothesis was tested by characterizing the depositional processes responsible for alluvial fan formation below snowpatches and by reconstructing the holocene alluvial fan activity stratigraphic and sedimentary analyses conducted on seven alluvial fans revealed that they were deposited by torrential activity leading to waterlaid transient or hyperconcentrated deposition the chronology of the storm generated alluvial fans based on 22 radiocarbon dates indicates that torrential activity was enhanced during the cooler late holocene i e after ca 3500 cal yr bp snowier winters and cooler summers were beneficial to nival activity allowing the persistence of larger snowpatches throughout the summer and fall seasons rainfall induced thaw of such snowpatches during rainstorm events is inferred to have contributed to alluvial fan activity by increasing water availability three peaks of alluvial activity occurred during the late holocene 2950 2750 1900 1400 and 800 300 cal yr bp and are indicative of increased storminess resulting in higher fan activity increased fan activity during cooler episodes was concurrent with increased runoff activity in the immediate pronival area this stresses the importance of nivation below snowpatches and pinpoints the role of nivation in enhancing geomorphological activity during period of cooler and more humid climate in subarctic environments

effective timescales of coupling within fluvial systems this paper presents a review of the coupling concept in fluvial geomorphology based mainly on previously published work coupling mechanisms link the components of the fluvial system controlling sediment transport down the system and the propagation of the effects of base level change up the system they can be viewed at several scales at the local scale involving within hillslope coupling hillslope to channel coupling and within channels tributary junction and reach to reach coupling at larger scales coupling can be considered as zonal coupling between major zones of the system or as regional coupling relating to complete drainage basins these trends are illustrated particularly by the examples of hillslope to channel coupling in the howgill fells northwest england badland systems in southeast spain alluvial fans in spain usa and uae and base level induced dissection of neogene sedimentary basins in southeast spain as the spatial scales increase so do the timescales involved effective temporal scales relate to magnitude and frequency characteristics recovery time and propagation time the relative importance changing with the spatial scale for downsystem coupling at the local scale the first two are important with propagation time increasing in importance in larger systems especially in those involving upsystem coupling related to base level change the effective timescales range from the individual event with a return period of decades through decadal to century timescales for downsystem coupling to tens to hundreds of thousands of years for the basinwide response to base level change the effective timescales influence the relative importance of factors controlling landform development

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

desert pavement characteristics on wadi terrace and alluvial fan surfaces wadi al bih u a e and oman abstract in arid mountain areas the dating and correlation of alluvial depositional surfaces is often uncertain especially in regions where the geomorphology is not well known surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages pleistocene wadi terraces and associated alluvial fans occur in wadi al bih u a e and oman for which correlations and age relationships are not known three age related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after miocene uplift of the region the younger two groups of terraces and fans are inset within the older group to identify the gross effects of pavement development comparisons have been made between terrace surface and subsurface particle size distributions the older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces this reflects the degree of pavement development particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces criteria for the classification of pavements were developed based on clast fracturing and angularity size sorting packing and surface texture from which a simple index of pavement development has been derived other properties rock varnish and weathering characteristics were also recorded but these proved to be less discriminatory than pavement characteristics the pavement data have been augmented by observations on soils detailed studies of pavements on terraces 8 sites 12 samples covering the three age groups and fans 5 sites 10 samples covering the three age groups allow differentiation between age groups the three terraces show three different age related pavement types expressed by differences in the pavement development index weakly developed pavements little fracturing sub rounded clasts some modification of the depositional fabric incipient soil development stage i caco3 accumulation occur on the youngest terrace and fan surfaces moderately developed pavements clast fracturing sub angular clasts moderate sorting and packing deeper soil development stage ii caco3 accumulation occur on the middle terrace and fan surfaces well developed pavements complete clast fracturing into small angular fragments mature sorting and packing of the pavement surface deep soil development with strong horizonation stage iii caco3 accumulation occur on the highest terrace and oldest fan surfaces there are minor differences between the youngest pavements on terraces and fans which reflect initial sedimentological differences these differences become less as the pavements develop on the basis of comparative studies the oldest terrace is estimated to date from sometime prior to ca 100 ka bp the second terrace and the most extensive fan surface from the late pleistocene and the youngest terrace and fan phase from the latest pleistocene or early holocene

sedimentology of the narmada alluvial fan western india abstract the narmada alluvial fan is one of the world s largest with an axial length of 23 km the architecture is dominated by debris flow deposits gms facies matrix support a clay content of 3 and clast contact indicate that the clast support mechanism resulted from a combination of buoyancy and dispersive pressure the other faci facies gsh planar cross stratified gravel facies gp1 and gp2 sand sheet facies sm and trough cross stratified sand facies st gms gsh and sm facies are debris flow and sheet flow deposits that aggraded the fan whereas gp1 and st are channel bars and channel fills that dominated the fan between major flood events the fan is characterised by subrounded to rounded clasts the rounding is due to the elongated catchment area upstream of the fan apex as clasts are rounded during prolonged bed load transport and are temporarily arrested upstream of the fan apex as channel bars these clasts are remobilized and entrained in debris flows on the fan during events of anomalous discharge storm events the basalt clasts show a progressive fall in maximum clast size from 150 cm to 10 cm away from the fan apex the narmada river exhibits discharges of up to 60 000 m3 s but due to reconfinement of the feeder channel resulting from tectonic reactivation of pre existing lineaments during the late pleistocene this does not aggrade the fan tectonism has influenced the location of the depositional site has provided the necessary physiographic contrast and has played an important role in the erosion of the fan whereas climate controlled primary and secondary processes have determined the nature of alluvial architecture

the october 1902 plinian eruption of santa maria volcano guatemala abstract the october 1902 eruption of santa maria volcano guatemala was one of the largest this century it was preceded by a great earthquake on april 19 centered at the volcano as well as numerous other major earthquakes the 18 20 hour long plinian eruption on october 25 produced a column at least 28 km high reaching well into the stratosphere the airfall pumice deposit covered more than 1 2 million km2 with a trace of ash and was only two meters thick at the vent white dacitic pumice dark gray scoriaceous basalt with physically and chemically mixed intermediate pumice and loose crystals of plagioclase hornblende hypersthene biotite and magnetite make up the juvenile components of the deposit lithic fragments are of volcanic plutonic and metamorphic origin the plinian deposit is a fine grained crystal rich single pumice fall unit and shows inverse grading mapping of the deposit gives a volume of 8 3 km3 within the one mm isopach crystal concentration studies show that the true volume erupted was at least 20 km3 equivalent to 8 5 km3 of dense dacite and that 90 of the ejecta was less than 2 mm in diameter the plinian volume eruption rate averaged 1 2 105 m3s 1 and the average gas muzzle velocity of the column exceeded 270 ms 1 a total of 8 3 1018 j of energy were released by the eruption a knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms this eruption was the major stratospheric aerosol injection in the 1902 1903 period however mid to low latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease this may be explained by the sulfur poor nature of dacite magmas suggesting that volatile composition rather than mass of volatiles is the controlling parameter in climatic response to explosive eruptions