surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

transience and persistence in the depositional record of continental margins 1 continental shelves and coastal plains are large persistent depositional landforms which are stationary nonmigrating at their proximal ends and characterized by relatively steady long term growth in detail however their surface form and stratigraphic record is built of transient freely migrating landscape elements we derive the timescales of crossover from transient to persistent topographic forms using empirical scaling relations for mean sediment accumulation as a function of averaging time based upon tens of thousands of empirical measurements a stochastic noisy diffusion model with drift predicts all the gross features of the empirical data it satisfies first order goals of describing both the surface morphology and stratigraphic completeness of depositional systems the model crossover from noise dominated to drift dominated behavior corresponds to the empirical crossover from transport dominated autogenic transient behavior to accommodation dominated subsidence persistent behavior which begins at timescales of 102 103 years and is complete by scales of 104 105 years because the same long term scaling behavior emerges for off shelf environments it is not entirely explicable by steady subsidence fluctuations in sediment supply and routing probably have significant influence at short term transient scales the exponents of the scaling relations vary with environment particularly the prevalence of channeled sediment transport at very small scales modeling sediment transport as a diffusive process is inappropriate our results indicate that some of the timescales of interest for climate interpretation may fall within the transitional interval where neither accommodation nor transport processes are negligible and deconvolution is most challenging

astronomically controlled cycles in the lower tertiary of gubbio italy abstract a grouping of marl limestone beds in the tertiary of gubbio is interpreted in terms of sedimentary cycles two methods of recording are used subjective mapping of cycle boundaries in the field and power spectral analysis of bed thickness measurements both result in cycle thicknesses of between 30 and 70 cm the cycle record is compared with the known magnetic stratigraphy and this gives an absolute time estimate of 100 kyr for the cycle it therefore seems likely that the earth s eccentricity period is the ultimate cause of the cycle

incipient granular mass flows at the base of sediment laden floods and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands extraordinary floods commonly produce thick deposits as a result of the entrainment and transport often at high concentrations of large volumes of clastic detritus this paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20 km3 breakout flood from intracaldera lake taupo following the 1 8 ka taupo eruption the proximal deposit is characterised by a strongly bimodal grain size distribution with downstream fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix this combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms i accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity and ii progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor the first mechanism is a product of a nonlinear particle concentration gradient in the sediment laden flow which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies the second mechanism is a function of the superjacent turbulent turbid flow that acted as the primary transporting system this pairing of transport and depositional systems is likely to characterise many sediment laden aqueous flows with clay mineral concentrations too low to develop non newtonian rheologies such as jokulhlaups and lahars that entrain unweathered pyroclastic material qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow as compared to simple water make impossible accurate estimates of flow competence based on shields entrainment functions for beds of both uniform and mixed grain size distribution

real and apparent changes in sediment deposition rates through time 1 field measurements show that estimated sediment deposition rate decreases as a power law function of the measurement interval this apparent decrease in sediment deposition has been attributed to completeness of the sedimentary record the effect arises because of incorporation of longer hiatuses in deposition as averaging time is increased we demonstrate that a heavy tailed distribution of periods of nondeposition hiatuses produces this phenomenon and that observed accumulation rate decreases as t 1 over multiple orders of magnitude where 0 1 is the parameter describing the tail of the distribution of quiescent period length by using continuous time random walks and limit theory we can estimate the actual average deposition rate from observations of the surface location over time if geologic and geometric constraints place an upper limit on the length of hiatuses then average accumulation rates approach a constant value at very long times our model suggests an alternative explanation for the apparent increase in global sediment accumulation rates over the last 5 million years

alluvial architecture paleohydrology and provenance tracking from the neoproterozoic banganapalle formation kurnool group india an example of continental sedimentation before land plants abstract an integrated study involving facies association analysis architectural element analysis paleohydrology and detrital zircon geochronology allowed detail insight of sedimentology and paleogeography of neoproterozoic banganapalle formation bf kurnool group detailed process based facies and paleoenvironmental analysis have led to the identification of fourteen facies types grouped under four different facies associations which record paleoenvironmental settings ranging between mid alluvial fan to distal fluvial plain whereas mass flows hyperconcentrated flows streamlets and sheet floods dominate the depositional process on the bf fan middle to distal surface streams of varied geometry viz braided and ephemeral meandering occupied the proximal and distal bf alluvial plain our study supports the emerging view that even without the buffering effect of land plants channels developed sinuous planform in pre vegetation time subject to low gradient and discharge of alluvial plain and adequate supply of fine detritus from occurrence of boulder gravelly mass flows boundary pebbly streamlet thalweg and pebbly granular sheet flood deposits in the alluvial fan thick silty muddy stable flood plain deposits in the distal alluvial plain and unequivocal southward paleocurrent irrespective of facies association it is inferred that the bf alluvial basin received mixed sediment from long term beveled source in the north and northwest from two u pb detrital zircon geochronology age clusters viz paleo to mesoarchean 3410 ma 3075 ma and neoarchean to late paleoproterozoic 2750 ma 2500 ma provenance for the bf is tracked in the north and northwest of the kurnool basin i e the ttg suite of the peninsular gneiss and volcanics hosted greenstone belts in the central and western dharwar craton a mixing of sediments from two different provenances is inferred a consistent low water discharge with marginal downstream increase is suggested for the bf alluvial system from paleohydraulic analysis except for some occasional spurts of high discharge in mid fan at times of intense rainfall in the hinterland

sediment flux dynamics as fingerprints of catchment rehabilitation the case of western rift valley escarpment of northern ethiopia variability in supply and entrainment of stream bedload particles during storm events is an important hydrogeomorphologic response of steep mountain streams to land management in their catchments because of difficulties in its measurement and absence of accurate measuring equipment bedload transport in mountain streams is however poorly understood this study focuses on analyzing bedload supply and entrainment in 11 streams of sloping 27 65 catchments 0 4 25 km2 in northern ethiopia which have experienced severe deforestation and degradation processes until the first half of the 1980s and considerable rehabilitation thereafter field measurements of the median diameter of the 10 coarsest bedload particles max10 moved in each event n 332 and stream bed particle sampling were carried out in three rainy seasons 2012 2014 event peak discharges were calculated from daily measurements by 11 crest stage gauges using manning s equation percentages of land cover classes in the catchments were detected from high resolution 0 6 m google earth imagery 1 february 2014 morphometric characteristics of the catchments were computed from an aster digital elevation model and topographic maps hydraulic competence analysis for entrainment of the average max10 was carried out using peak discharge stream power and critical shear stress approaches the supply of max10 was positively related to scar density on the surrounding slopes r2 0 50 p 0 05 and catchment area r2 0 36 p 0 05 while negatively related with total forest cover r2 0 63 p 0 05 or vegetation cover r2 0 58 p 0 05 a multiple regression analysis showed that 98 of the variability in max10 is explained by scar density and catchment area entrainment of max10 was positively related to peak discharge qp r2 0 60 p 0 01 stream power r2 0 71 p 0 01 strickler s roughness sn r2 0 86 p 0 01 and critical shear stress c with reference to d50 r50 r2 0 96 p 0 01 and with reference to d84 r84 r2 0 93 p 0 01 the relationships between event max10 and event qp in each catchment were also used to extrapolate the peak discharges that would have prevailed in the first half of the 1980s and the results showed huge reductions in peak discharges p 0 01 as a result of the reforestation interventions overall this study demonstrates that i the supply of bedload in steep tropical mountains is determined negatively by forest or vegetation cover and positively by the presence of scar networks in the sloping parts of the catchments and catchment size ii entrainment of the coarsest bedload particles is determined positively by peak discharge stream power stream bed roughness and most strongly by critical shear stress and iii in steep mountain streams where direct measurement of bedload movement is difficult yet simple in situ measurement of the coarsest bedload particles max10 supplied and entrained in each event helps for understanding the dynamics of sediment flux in relation to catchment rehabilitation

preservation of planar laminae due to migration of low relief bed waves over aggrading upper stage plane beds comparison of experimental data with theory experimental studies of the formation of planar laminae by migration of low relief bed waves over aggrading upper stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point in general the preserved laminae represent only a small proportion generally less than 50 of the height of the largest bed waves in the population the theory developed by paola borgman 1991 relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation was adapted for aggrading conditions and shown to agree well with data these results suggest that the theory can be used to estimate from the distribution of thickness of planar laminae either the distribution of bed wave heights mean aggradation rate mean bed wave length or mean bed wave celerity provided the other parameters can be estimated

geomorphological impacts of an extreme flood in se spain abstract long term field studies in semiarid ephemeral streams are rare these geomorphic data are essential for understanding the nature of the processes in order to develop modelling for risk assessments and management an extreme flood event on 28 september 2012 affected the murcia region of se spain including long tem monitoring sites on two fluvial systems in the guadalentin basin the nogalte and torrealvilla detailed morphological data were collected before and immediately after the event and the amount of morphological change erosion and deposition have been related to peak flow conditions at the sites on the nogalte channel peak flow reached 2500 m3 s 1 at the downstream end of the catchment in less than 1 h the event had a recurrence interval of 50 years based on rainfall records and damage to old irrigation structures the major effect in the braided gravel channel of the nogalte was net aggradation with massive deposition in large flat bars the measured changes in bankfull capacity were highly correlated with most hydraulic variables net changes in cut and fill in cross sections on the nogalte were highly related to peak discharge and stream power but much less so to measures of hydraulic force velocity shear stress unit stream power relationships of amount of erosion to hydraulic variables were much weaker than for amount of deposition which was largely scaled to channel size and flow energy changes on the torrealvilla were much less than on the nogalte and net erosion occurred at all sites sites on the nogalte channel in schist exhibited higher deposition than those of the torrealvilla sites on marl for the same hydraulic values overall less morphological change took place in the extreme event on the nogalte than predicted from some published hydraulic relations probably reflecting the high sediment supply and the hydrological characteristics of the event the results demonstrate the high degree of adjustment of these channels to the occasional high magnitude flash flood events and that such events need to be allowed for in management the detailed quantitative evidence produced by these long term monitoring sites provide valuable rare data for modelling morphological response to flood events in ephemeral channels

climatic and eustatic signals in a global compilation of shallow marine carbonate accumulation rates two of the most important factors that control the accumulation rate of material in carbonate platform environments on geological time scales are climate and eustasy accurately assessing the importance of these interrelated factors through the study of both modern and ancient carbonate facies however is problematic these difficulties arise from both the complexities inherent in carbonate depositional systems and the demonstrable incompleteness of the stratigraphic record here a new compilation of more than 19 000 global phanerozoic shallow marine carbonate accumulation rates derived from nearly 300 individual stratigraphic sections is presented these data provide the first global holistic view of changes in shallow marine carbonate production in response to climate and eustasy on geological time scales notably a clear latitudinal dependence on carbonate accumulation rates is recognized in the data moreover it can also be demonstrated that rates calculated across the last glacial maximum and holocene track changes in sea level in detail the data show that globally averaged changes in carbonate accumulation rates lagged changes in sea level by ca 3 kyr reflecting the commonly observed delay in the response of individual carbonate successions to sea level rise differences between the rates of carbonate accumulation and sea level change over the past 25 kyr ostensibly reflect changing accumulation mode with platform drowning give up mode pervasive during peak early holocene sea level rise followed by a switch to catch up mode accumulation from ca 9 ka to the present carbonate accumulation rates older than the quaternary are typically calculated over time spans much greater than 100 kyr and at these time spans rates primarily reflect long term tectonically mediated accommodation space changes rather than shorter term changes in climate eustasy this finding coupled with issues of stratigraphic incompleteness and data abundance tempers the utility of this and other compilations for assessing accurately the role of climate and eustasy in mediating carbonate accumulation rates through geological time

ordered hierarchy versus scale invariance in sequence stratigraphy sequence stratigraphy has been applied in a wide range of scales of time and space from decimeter thick layers formed within hours to kilometer thick basin fills formed during hundreds of millions of years the traditional approach to practice sequence stratigraphy in this wide range of scales is to subdivide the sediment piles into an ordered hierarchy of sequence cycles of different duration and different architecture an alternative are scale invariant models with fractal characteristics published data confirm two predictions of the ordered hierarchy model sequences of very short duration 200 106 years are symmetrical transgressive regressive cycles however the sequence record in the range of 1 104 200 106 years the principal domain of sequence stratigraphy shows a rather irregular succession of sequences with variable symmetry and bounded by flooding surfaces or exposure surfaces for these time scales scale invariant models are a good first approximation particularly because the evidence for scale invariance and randomness in the stratigraphic record is strong frequency spectra of sea level change as well as rates of sedimentation and rates of accommodation change plotted against length of observation span show basic trends indistinguishable from random walk these trends combined with scale invariant sequence models may be the most efficient tools for across the board predictions on sequences and for locating islands of order in the sequence record

a note on cluster bedform behaviour in a gravel bed river abstract the behaviour of cluster bedforms was investigated in the farma r central italy a study reach was surveyed and the three components of the pebble clusters occuring on the surface of a gravel bar were painted using different colours before and after each flood all the clusters were photographed and a comparison was made this simple methodology allowed the evolution of the painted clusters to be followed and also allowed a survey of the dislodgment of the clusters particles and the addition of new ones the experiment shows that the mobility of the wake grains is higher than that of the stoss particles a pebble cluster generally loses the wake grain first followed by the stoss pebbles the obstacel clast is commonly the last to be entrained and acts as a single particle after the wake and stoss particles have been dislodged different explanations of this unexpected behaviour are postulated and the interaction of pebble with bed armouring and bedload size distribution is discussed

cantor set model of eolian dust deposits on desert alluvial fan terraces available data show that eolian dust accumulation rates on desert alluvial fan terraces are often inversely proportional to the square root of the terrace age for a given area this temporal scaling is similar to that observed in fluvial and marine stratigraphic sequences in which a fractal distribution of hiatuses occurs i e a cantor set eolian dust accumulation on alluvial fan terraces is controlled by regional climatic influences on dust deposition e g dessication of pluvial lakes in the early holocene and local surface characteristics e g vegetation and pavement coverage that control dust preservation in order to interpret the observed scaling in terms of these relative influences i consider two end member models 1 a deterministic model in which dust deposition is controlled by cyclical climatic changes i e glacial interglacial cycles and in which no erosion occurs and 2 a stochastic model in which erosion and deposition take place with equal probability and magnitude during each time step i e a bounded random walk the observed temporal scaling is most consistent with the bounded random walk model suggesting that long term eolian dust accumulation is predominantly controlled by episodic changes in vegetation and pavement coverage over time and that eolian dust deposits are strongly punctuated by episodes of erosional reworking at a wide range of temporal scales

rates of sedimentation and stratigraphical completeness the most obvious result of sedimentation processes is the change in the thickness of accumulated sediment with time whenever the net change over a period of time is positive a record is left of a sedimentation system s existence a stratigraphical section for the stratigrapher the problem that such a section poses is to determine the detailed correspondence of thickness to time1 for the sedimentologist the problem is to infer the nature of the parent sedimentation system and to estimate its parameters a simple stochastic model is used here 1 to illustrate some basic stratigraphical results of clastic sedimentation processes 2 to offer an explanation for the commonly cited systematic decrease of mean sedimentation rate with increasing time span and 3 to assess the concept of stratigraphical completeness it will be assumed that all post depositional non erosional processes such as compaction can be identified and their effects on stratigraphical thickness allowed for

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

depo sequence as tool to convert shoreline trajectory from space domain to time domain abstract deposystems are complex and governed by discrete depo events with variable intervals of stasis or erosion in between since shoreface sediments indicate sea level depo events of shoreline facies are discrete samples of sea level only if these samples are sufficiently regular and frequent will the shoreline trajectory in the space domain accurately reflect the sea level curve in the time domain this study presents a method to convert shoreline trajectory in the space domain to sea level curve in the time domain from artificial miniature deltas one obtains the depo sequence as function of time and uses it to convert depo sequence from time to space domain correlate depo sequence to shoreline trajectory in the space domain and convert shoreline trajectory from space to time domain for natural deltas one would extract the depo set in the frequency domain i e the probability density function of stasis intervals between depo events from the experimental depo sequence and use it to convert shoreline trajectory from space to time domain and infer a range of possible sea level curves this method therewith explicitly includes the uncertainty of the inferred sea level curve

fluvial processes and cyclicity in terminal fan deposits an example from the late devonian of southwest ireland abstract the gun point formation is a sandstone dominated sequence approximately 1 5 2 5 km thick which forms part of a major 6 km late devonian alluvial basin fill succession in southwest ireland it is interpreted as the deposits of a southerly flowing terminal fan system which operated under a semi arid climate the proximal zone was characterized by low sinuosity gravelly feeder channels which passed downstream into a medial zone dominated by a sandy braidplain further south in the distal zone phases of high energy flood activity were centred on transient low sinuosity ephemeral channels and were separated by more quiescent periods of shallow sheetflooding subaerial exposure and non deposition sedimentary parameters such as set thickness coset thickness channel sand body density and maximum sand body thickness all exhibit regular cyclic fluctuations with a periodicity of 160 180 m the cycles are interpreted as evidence of regular discharge variations which may have been a response to the 412 ka orbital eccentricity cycle a second periodicity of approximately 700 m has also been detected and is thought to reflect long term variations in basin subsidence

strain partitioning along the himalayan arc and the nanga parbat antiform shortening along the himalayan arc of continental convergence is approximately in the radial direction if the underthrusting foot wall block india is not deformed the hanging wall block tibet needs to stretch along the arc as suggested by radial grabens in southern tibet in contrast the nanga parbat haramosh massif and the western himalayan syntaxis are part of a 250 km long antiform that strikes in the radial direction northeast and verges northwest the nanga parbat antiform is the structural and topographic expression of arc parallel shortening that compensates for arc parallel extension in southern tibet this shortening is predicted to be as high as 12 mm yr

a fractal model for the distribution of stratigraphic hiatuses several recent studies have suggested that there is a negative log linear relationship between sedimentation rates and the time span over which they are measured i e measured sedimentation rates are slow when measured over long time spans and rapid when measured over short time spans this relationship may be due to the inclusion of more and longer hiatuses over long time spans this paper proposes a mathematical model consistent with this relationship that is based upon the fractal cantor bar this model allows the estimation of the relative number and durations of stratigraphic hiatuses within a given section

autogenic sedimentation in clastic stratigraphy internally generated or autogenic terrestrial and marine sediment transport dynamics can produce depositional patterns similar to those associated with climatic tectonic or sea level changes a central challenge in accurately interpreting the sedimentary archive is determining what scales and types of deposits reflect autogenic controls on sedimentation in different environments autogenic sediment transport dynamics commonly result from intermittent sediment storage in transient landforms which produces episodic spatially discontinuous sedimentation across a basin the transition from localized variable sedimentation to even basin wide sedimentation marks the shift from stochastic landscape dynamics to deterministic deposition responding to the long term balance between sediment supply and the creation of space to accommodate sediment this threshold can be measured in a wide variety of stratigraphic successions and has important bearing on whether climatic tectonic or sea level signals can be

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

environmental change geomorphic processes and land degradation in tropical highlands abstract the international symposium highland2006 held in mekelle ethiopia from 21 to 25 september 2006 was the second time that ethiopia could host a major international scientific event on geomorphological processes highland2006 created a forum for those conducting research on the impacts of environmental change on geomorphic processes and land degradation in east african highlands as well as in similar regions around the globe discussions of results and exchange of experiences during indoor sessions as well as excursions have resulted in this distinctive set of scientific papers the two main questions addressed in this special issue through case studies in tropical mountain areas on all continents are 1 how have environmental changes impacted the type and intensity of geomorphic processes in tropical mountains since late pleistocene times the answer to this question is strongly linked to our knowledge on changes in climate vegetation cover human activities hydrology hillslope and fluvial processes for such environments 2 which factors control the intensity of land degradation its on site and off site impacts in tropical mountains this question addresses the role of natural and anthropogenic factors in the degradation of the vegetation cover and in controlling the intensity of hydrological processes soil erosion landsliding reservoir sedimentation and flooding in parallel with this special catena issue a number of selected papers on land degradation and its mitigation in tropical highlands are presented in a special issue of soil tillage research

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

completeness of the geological record the completeness of a sedimentary section of known timespan may be assessed qualitatively by comparing its thickness with the average accumulation for that timespan average accumulations may be estimated from sediment volume and continental area data quantitative completeness estimation methods based on data compiled from the geological literature have been proposed but we argue that the literature data are significantly biased and cannot support such methods interestingly however a comparison of the literature data and accumulation averages computed from sediment volume data suggests that the thickest known sections may be extremely complete

wavelet analysis of well logging data from oil source rock egret member offshore eastern canada wavelet analysis is a sensitive method for automatically detecting and distinguishing abrupt discontinuities i e faults unconformities cyclicity and gradual changes in sedimentation rate by transforming depth related sedimentary signals i e gamma rays into wavelengths at distinct depth intervals we used wavelet analysis for evaluation of the spatio temporal distribution of oil source rocks and for estimating accumulation rates in a sedimentary basin having high resolution the method was applied to 16 gamma ray logs from the jurassic egret member an oil source rock succession 55 m to 227 m in thickness offshore eastern canada dominant gamma ray cycles having wavelengths varying from 2 8 m western margin of the basin to 24 m eastern part of the basin have been detected by wavelet analysis the coincidence of the ratio of predominant gamma ray cycles with the ratio of milankovitch spectra about 400 100 40 20 k y suggests that climatic cycles are an important factor controlling sedimentary cyclicity in the egret member dominant wavelengths likely represent 100 k y eccentricity giving accumulation times of 1 9 m y for stratigraphically complete sections having 19 successive 2 8 m gamma ray cycles and giving accumulation times of 600 k y for incomplete successions having only 6 cycles up to four discontinuities occur in gamma ray log cyclicity and separate the egret member into subunits we interpret the discontinuities as unconformities or faults and as related to sediments having low petroleum potential the stratigraphic completeness of the egret member is correlated to total mass of organic carbon and decreasing thickness of non source rock intervals having correlation coefficients of r 0 8 and r 0 76 respectively

continuous monitoring of bedload discharge in a small steep sandy channel this paper reports on bedload flux and texture monitored in a natural steep sandy ephemeral channel draining a small gullied sandy watershed the barranca de los pinos 1 32 ha spain bedload flux was continuously monitored with two independent reid type slot samplers bedload texture was determined from the sediment collected in the samplers channel morphology was surveyed with a high spatial resolution with a terrestrial laser scanner the monitored instantaneous bedload fluxes are among the highest measured in natural rivers characterized by high temporal and spatial variability related to the presence of bedforms shallow bars and sand sheets and to the reworking of the dry bed between and at the end of individual flow events the grain size distribution of the bedload indicates equal mobility but bedload texture fluctuates depicting the transport of coarser bar surfaces and of finer grained anabranch surfaces as well as of the overall bed subsurface

a new method for interpolating ages between calibrated control points based on foraminiferal concentrations abstract the new method relies on the assumption that foraminiferal accumulation rates are uniform or are at least steadier than bulk sediment accumulation rates over the time scale of intervals between calibrated control points where this condition is fulfilled interpolated ages can be assigned to sampled horizons n in each age bracketed interval by apportioning the difference in ages between control points at the bottom and top of each age bracketed interval t to samples in each interval this is done on the basis of the proportional subdivision of the sum total of weight standardized counts of foraminifera cn in each interval which can be simply expressed as t cn cn the new method is described here in relation to well cuttings samples but it can be adapted for use with whole rock samples from cores and outcrops a case study of the pukearuhe 1 well demonstrates the method its application and outputs in relation to deep water miocene age strata in taranaki basin new zealand the study of this well shows that foraminiferal accumulation rates may not be a precise guide for elapsed time but in tectonically active basins and in settings on the margins of basins where foraminiferal concentrations can be shown to be regulated by influxes of siliciclastic sediment it is better to interpolate ages between calibrated control points on the basis of a uniform foraminiferal accumulation scale than a uniform bulk sediment accumulation scale

estimating stratigraphic completeness the completeness of a stratigraphic section is difficult to estimate in practice because stratigraphic completeness is a measure that is based entirely in time the suggestion that the stratigraphic completeness of a section of known timespan might be estimated by comparing that section s thickness against the global average thickness of sediment accumulated in that timespan would thus be of immense practical value if only that relationship between completeness and thickness could be demonstrated regrettably there appears to be insufficient data to warrant this assertion

fault sourced alluvial fans and their interaction with axial fluvial drainage an example from the plio pleistocene upper valdarno basin tuscany italy abstract the present study deals with the fault sourced alluvial fan deposits of the plio pleistocene upper valdarno basin northern apennines italy different phases of alluvial fan aggradation progradation and backstep are discussed as possible effects of the interaction among fault generated accommodation space sediment supply and discharge variations affecting the axial fluvial drainage the upper valdarno basin located about 35 km se of florence is filled with 550 m palustrine lacustrine and alluvial deposits forming four main unconformity bounded units i e synthems the study alluvial fan deposits belong to the two uppermost synthems montevarchi and torrente ciuffenna synthems and are early to middle pleistocene in age these deposits are sourced from the fault bounded ne margin of the basin and interfinger with axial fluvial deposits alluvial fan deposits of the montevarchi synthem consist of three main intervals i a lower interval which lacks any evidence of a depositional trend and testify balance between the subsidence rate i e fault activity and the amount of sediment provided from the margin ii a coarsening upward middle interval pointing to a decrease in subsidence rate associated with an augment in sediment supply iii a fining upward upper interval locally preserved documenting a phase of tectonic quiescence associated with a progressive re equilibration of the tectonically induced morphological profile the basin scale unconformity which separates the montevarchi and torrente ciuffenna synthems was due to the entrance of the arno river into the basin as consequence of a piracy this event caused a dramatic increase in water discharge of the axial fluvial system and its consequent embanking such an erosional surface started to develop in the axial areas and propagated along the main tributaries triggering erosion of the alluvial fan deposits alluvial fan deposits of the torrente ciuffenna synthem accumulated above the unconformity during a phase of tectonic quiescence and show a fining upward depositional trend this trend was generated by a progressive decrease in sediment supply stemming out from upstream migration of the knickpoints developed during the embanking of the axial system

on traction carpet sedimentation traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows they have a convex up velocity profile and comprise a lower frictional and an upper collisional region the frictional region having a particle concentration more than 80 of the packed bed is characterized by nearly continuous grain contacts low strain rate and hampered grain segregation on the other hand the collisional region has a particle concentration between 15 and 80 of the packed bed and is characterized by active grain collisions and higher strain rate deposition from the traction carpets occurs via progressive aggradation of the bed rather than via en masse freezing while the downward grain flux from the overlying flow maintains them the thickness of a traction carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure which is related to nonuniform distribution of particles when a thick frictional region develops however the inverse size distribution in the collisional region is poorly recorded in the deposits depositional features of traction carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions the thickness ratio is further controlled by the applied shear stress sediment fallout rate and grain size generally a collisional region is better developed beneath a highly competent and coarse grained gravelly flow whereas a thick frictional region is developed beneath a heavily sediment laden fine grained sandy flow this explains the more common occurrence of inverse grading in coarse grained deposits

traction carpet stratification in turbidites fact or fiction abstract the spaced stratification that characterizes some thick deep water sandstones consists of a stack of layers 5 10 cm thick that are inversely graded at their base and structureless at their top these layers have been attributed to deposition from a succession of collapsed inertia layers traction carpets prior to collapse each layer was believed to have been maintained by shear from the overriding turbidity current this depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope instead shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer and deposition sho ld be a gradual process leaving no record of sequentially active inertia layers inertia layers that do form under large turbidity currents are probably never as thick as 5 10 cm based on available data from shear cell experiments instead inertia layers are probably about 1 cm thick similar to the thickness of the inversely graded basal part of each layer in field examples of spaced stratification the rest of each inversely graded stratum probably forms due to rapid fallout from suspension the basal inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies hence spaced stratification may not be a structure produced beneath steady flows but may record strong y fluctuating hydrodynamic conditions and vigorous burst sweep cycles in large turbidity currents

concepts of sediment deposition and accumulation applied to the continental shelf near the mouth of the yangtze river a fundamental understanding of how sedimentary strata form requires distinction between deposition and accumulation of particles deposition refers to the temporary emplacement of particles on the seabed accumulation is the net sum of many episodes of sediment deposition and removal the difference between rates of deposition and accumulation affects the ability of an environment to record sedimentary events radionuclide profiles of 234 th and 210 pb within the seabed can be used to determine 100 day deposition rates and 100 yr accumulation rates respectively on the continental shelf near the mouth of the yangtze river short term deposition rates are about 4 4 cm month whereas accumulation rates integrated over a century are an order of magnitude less about 5 4 cm yr

a new look at sedimentation rates and the completeness of the stratigraphic record several recent papers have suggested that completeness of stratigraphic sections can be estimated by using the ratio of long term sedimentation rates to short term rates we show that the median short term sedimentation rates used in these papers are erroneous and are mostly an artifact arising from improper data manipulation to demonstrate the artificial character of short term median rates we have generated a series of plots assuming a constant sedimentation rate with fixed variance and defined limits of measurement precision these plots demonstrate that increasing variance in the time required to deposit sediments will result in spuriously high short term median sedimentation rates a new method for calculating median sedimentation rates which eliminates the problem of spurious median rates is then applied to make an improved estimate of expected completeness recalculated median values show that sedimentation rates for pelagic sediments are close to identical for both short term and long term tim

a geomagnetic polarity timescale for the permian calibrated to stage boundaries the reverse polarity kiaman superchron has strong evidence for at least three or prob ably four normal magnetochrons during the early permian normal magnetochrons are during the early asselian base ci1r 1n at 297 94 0 33 ma late artinskian ci2n at 281 24 2 3 ma mid kungurian ci3n at 275 86 2 0 ma and roadian ci3r an at 269 54 1 6 ma the mixed polarity illawarra superchron begins in the early wordian at 266 66 0 76 ma the wordian capitanian interval is biased to normal polarity but the basal wuchiapingian begins the beginning of a significant reverse polarity magnetochron lp0r with an overlying mixed polarity interval through the later lopingian no significant magnetostratigraphic data gaps exist in the permian geomagnetic polarity record the early cisuralian magnetochrons are calibrated to a succession of fusulinid zones the later cisuralian and guadalupian to a conodont and fusulinid biostratigraphy and lopingian magnetochrons to conodont zonations age calibration of the magnetochrons is obtained through a bayesian approach using 35 radiometric dates and 95 confidence intervals on the ages and chron durations are obtained the dating control points are most numerous in the gzhelian asselian wordian and changhsingian intervals this significant advance should provide a framework for better correlation and dating of the marine and non marine permian

self organized complexity in geomorphology observations and models the focus of this paper is to relate fundamental statistical properties of landforms and drainage networks to models that have been developed in statistical physics relevant properties and models are reviewed and a general overview is presented landforms and drainage networks are clearly complex but well defined scaling laws are found coastlines topography contours and lakes are classic self similar fractals the height of topography along a linear track is well approximated as a brownian walk a self affine fractal this type of behavior has also been found in surface physics for example the surface roughness of a fracture an applicable model is the langevin equation the heat equation with a stochastic white noise driver this model also reproduces the statistics of sediment deposition drainage networks were one of the original examples of self similar fractal trees an important advance in quantifying the structure of drainage networks is the application of the tokunaga fractal side branching statistics a classic problem in statistical physics is the diffusion limited aggregation the resulting tree like structures have been shown to also satisfy the tokunaga statistics a modified version of the diffusion limited aggregation model reproduces the statistics of drainage networks it is concluded that the models developed in statistical physics have direct applicability to the fundamental problems in geomorphology

estimation of completeness of stratigraphical sections using empirical data and theoretical models the completeness of a stratigraphical section is the fraction of time intervals of some specified length t that have left a record a record is left when some sediment is deposited during the interval and is not subsequently eroded a complete section contains no hiatuses longer than t the completeness of a section varies with t and its accumulation rate varies with the length of the time span over which it is measured plots of measured accumulation rate against time span are an empirical means of estimating completeness simple theoretical models help extrapolate meagre data and identify bias completeness at time scale t can depend upon three general properties of the accumulation history the age of the section the long term net accumulation rate drift and the unsteadiness of the sedimentation rate the way unsteadiness is measured depends upon the kinds of fluctuations that can be recognized in the accumulation rate to describe random fluctuations a standard deviation of rates is sufficient one dimensional brownian motion is a model of random fluctuations that explains many aspects of stratigraphical completeness regular periodic fluctuations in accumulation rate may be described in terms of wavelength and amplitude completeness is an increasing function of t drift and in the periodic case the wavelength of fluctuations it decreases with increasing standard deviation of accumulation rate and the amplitude of periodic fluctuations the completeness and thickness of stratigraphical sections are weakly positively associated

the transport of gravel in an ephemeral sandbed river the channel dynamics of an ephemeral sandbed river the metsemotlhaba in southeast botswana were studied over a five year period using tagged magnetic particles of pebble and cobble size scour chains and topographic cross sections with particular emphasis on the three dimensional dispersal of gravels patterns of scour and fill and depth of active layer two major flow events of equivalent magnitude occurred moving the tagged particles in december 1987 a mean distance of 837 m at a mean burial depth of 0 40 m and in march 1991 a mean distance of 263 m at a mean burial depth of 0 39 m the volume of mobile sediment based on scour depth and distance of travel was 2 7 times greater in the december 1987 event in which the mean scour depth was almost twice the mean burial depth of the tagged particles the distribution of distance of movement was asymmetrical in this first flood when the tracer started from a surface location but was monotonic thereafter intervening small to medium events yielded limited tracer movement with a mean burial depth equivalent to depth of scour the tracer moved in the low and transitional flow regimes burial depth distribution followed the gamma model field data confirm that longitudinal transport is independent of particle size and shape and strongly skewed with respect to distance whilst depths of scour in excess of 1 m for high magnitude events suggest that scour values predicted from the empirical equation of leopold et al underestimate by an order of magnitude copyright 1999 john wiley sons ltd

bedforms and sediment transport processes in the ephemeral streams of kobo basin northern ethiopia abstract bedforms and channel deposits architecture were studied in the ephemeral streams of the structural basin of kobo in northern ethiopia the froude numbers calculated at bankfull flow for a few of the study reaches are in agreement with the extensive occurrence of horizontal planar lamination small bedforms not previously reported in the literature are found and described as produced mainly by scouring rather than accretion processes equations to predict particle entrainment proved that large individual boulders standing scattered on the stream bed can be transported at bankfull flow for long distances direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of divisions with specific characteristics this model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models in contrast with recent theories of flat asymmetrical bed waves progradation at froude numbers close to unity

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

morphology and sediment dynamics of ephemeral stream terminal distributary systems in the kobo basin northern welo ethiopia channel morphology bedforms and sedimentary structures of the distributary systems of ephemeral streams in the structural basin of kobo northern ethiopia are analysed the basin is characterised by a semi arid monsoon affected climate most of the rivers originate in the western margin range and dry up in the middle of the basin where they form large distributary systems they show characteristics different from those of the terminal fans and floodouts previously reported in the literature no flow data are available but bankfull discharge for several study reaches has been obtained by field survey of a number of cross sections hydraulic geometry streambed gradient and bed material grain size distribution in both the main stems and the distributary channels two different types of terminal distributary systems have been identified bird foot and lobate the first type by far predominates in the kobo basin bird foot distributary systems were recognized to consist of five distinctive reach units while the lobate splays have a simpler morphology and are thought to evolve into the former more complex type a description of channel morphology bedforms and sedimentary structure distribution is given and these characteristics are compared to those of other similar depositional systems reported in the literature other features such as the lack of scour pockets around boulder particles scattered on the streambed surface and the interaction of individual large particles with massive or horizontal laminated sand on top of typical upward fining channel sequences are also highlighted

a sem cathodoluminescence study on weathering of quartz grains in faults central japan summary cathodoluminescence cl of quartz grains or particles was observed using a scanning electron microscope the quartz grains were collected from fault gouge and fault breccia within the atotsugawa fault central japan and the quartz particles were obtained by crushing parent rocks of the fault such as granite and gneiss in an iron bowl in comparison with particles from the parent rocks those from the fault showed the following differences cl inherited from the parent rock cl caused by faulting and cl modified by weathering or alterations of the fault these differences are attributed to the faulting and subsequent weathering the cl observations were in general agreement with secondary electron observations and it is recommended that both methods should be used in parallel

stochastic and deterministic controls on stratigraphic completeness and fidelity compilations of sedimentation rates determined across a large range of time spans and different sedimentary environments attest to the discontinuous nature of sedimentation and support the reasoning that hiatuses pervade sedimentary successions at multiple spatial and temporal scales the relationship between sedimentation rate and time span of observation also emphasises how stratigraphic completeness is a time span dependent concept the scaling of stratigraphic completeness with time span alludes to the likely stochastic and fractal nature of the stratigraphic record it also implies that the relationship between stratal thickness and time is inherently non linear and this has obvious implications for the way we should expect temporal processes to be recorded stratigraphically order within the stratigraphic record may arise from deterministic processes in stratigraphy most notably through astronomically influenced sedimentation intuitively a strongly ordered stratigraphic record due to the presence of cyclicity in sedimentation increases stratigraphic completeness at time spans equal to and longer than the time span of the cyclicity and minimises the non linearity between stratal thickness and time here geological data and simple modelling are used to support this supposition but these results also highlight how stochastic variability inherent in depositional processes may sensitise certain successions to missed cycles and incompleteness even in the presence of strong cyclicity this stochastic variability has the potential to mask evidence of astronomical forcing of sedimentation it is also shown how hiatuses affect the ability of a succession to faithfully record temporal processes and this is true regardless of whether hiatuses are deterministically controlled or random or indeed whether a succession is considered complete or not consequently stratigraphic completeness and stratigraphic fidelity are not synonymous terms

inferring autogenically induced depositional discontinuities from observations on experimental deltaic shoreline trajectories palaeo shoreline is a commonly used proxy for palaeo sea level but only if deposition is continuous and constant will shoreline trajectory t l completely capture sea level time series e t artificial deltas were generated in the eurotank flume facility under stepwise tectonic subsidence periodic sea level fluctuation and two periodic water discharge scenarios one in phase and the other out of phase with sea level independent input variables tectonic subsidence y sea level e and water discharge q controlling sediment supply s were varied and dependent output variable shoreline trajectory twas monitored these experiments confirm that deposition is discontinuous even for continuous sediment supply and this hinders the inference of sea level curve from shoreline trajectory these results justify the here developed methodology for converting shoreline trajectory from the space domain to the time domain thereby improving the accuracy of the inferred sea level curve 2015 john wiley sons ltd

quantifying the stratigraphic completeness of delta shoreline trajectories understanding the incomplete nature of the stratigraphic record is fundamental for interpreting stratigraphic sequences methods for quantifying stratigraphic completeness for one dimensional stratigraphic columns defined as the proportion of time intervals of some length that contain stratigraphy are commonplace however quantitative assessments of completeness in higher dimensions are lacking here we present a metric for defining stratigraphic completeness of two dimensional shoreline trajectories using topset foreset rollover positions in dip parallel sections and describe the preservation potential of a shoreline trajectory derived from the geometry of the delta surface profile and the kinematics of the geomorphic shoreline trajectory two end member forward models are required to fully constrain the preservation potential of the shoreline dependent on whether or not a topset is eroded during base level fall a laboratory fan delta was constructed under nonsteady boundary conditions and one dimensional stratigraphic column and two dimensional shoreline completeness curves were calculated results are consistent with the hypothesis derived from conservation of sediment mass that completeness over all timescales should increase given increasing dimensions of analysis stratigraphic trajectories and completeness curves determined from forward models using experimental geomorphic trajectories compare well to values from transects when subsampled to the equivalent stratigraphic resolution as observed in the actual preserved sequence the concept of stratigraphic completeness applied to two dimensional trajectory analysis and the end member forward models presented here provide novel tools for a conceptual understanding of the nature of stratigraphic preservation at basin scales

the detection and illustration of regular sedimentary cycles using walsh power spectra and filtering with examples from the lias of switzerland power spectral analysis provides an objective method for the detection of regular cyclicity in data recording some oscillating parameter or time series digitization of measured stratigraphic sections is a convenient method for generating long time series during the examination of pelagic sequences for cyclicity related to orbital variations or milankovitch cycles the application of walsh power spectral analysis to stratigraphic sections is outlined and band pass filtering using the walsh transform introduced it is shown that variations in sedimentation rate hiatuses bioturbation and variations in compaction have the effect of damping or destroying spectral peaks the wavelength ratios of spectral peaks are often used during studies of milankovitch cycles to decide which orbital cycles are involved however in some circumstances hiatuses are able to alter wavelength ratios averaged walsh power spectra were generated for three liassic limestone shale sequences from breggia gorge switzerland the 21 000 year 21 ka precession cycle was detected in all three cases the 100 ka eccentricity cycle in two cases and the 41 ka obliquity cycle in one filtering revealed that the supposed 21 ka sedimentary cycles are grouped into packets of four or five small and large amplitude cycles the packets reflect the 100 ka modulation of the 21 ka precession cycle local disruption of the packaging can be used to locate hiatuses in pelagic strata

effect of sand supply on transport rates in a gravel bed channel in a series of flume experiments using constant discharge flow depth and gravel feed rate sand feed rates were varied from 0 16 to 6 1 times that of gravel the bed slope decreased with increasing sand supply indicating that the gravel could be transported at the same rate along with increasing amounts of sand at smaller shear stresses prediction of river response to an increase in sediment supply requires prediction of mutual changes in bed composition and transport and therefore a transport model defined in terms of the grain size of the bed surface a recent model provides satisfactory prediction of the experimental observations and indicates the general response of gravel beds to increased sand supply an increase in sand supply may increase the sand content of the river bed and the mobility of gravel fractions which can lead to bed degradation and preferential evacuation of these sediments from the river

the pulse of calm fan deltas abstract at the heart of interpreting the history of earth surface evolution preserved in the rock record is distinguishing environmental allogenic forcing from internally generated autogenic noise allogenic deposits classically have been recognized by their cyclic nature which apparently results from periodic changes in base level sediment supply or tectonics autogenic deposits which are quite variable in their origin and scale are caused by the nonlinearity of sediment transport and might be expected to have a random or scale free fractal signature here we describe a robust mechanism that generates cyclic deposits by an autogenic process in experimental fan deltas sheet flow over the fan surface induces deposition and an increase in fluvial slope and curvature to a point where the surface geometry is susceptible to a channelization instability similar to channel initiation on hillslopes channelized flow results in incision and degrading of the fan surface to a lower slope releasing a

box cores from the equatorial pacific 14c sedimentation rates and benthic mixing abstract carbon 14 determinations on box cores of calcareous ooze from the western and eastern equatorial pacific suggest that patterns of mixed layer ages sedimentation rates and mixed layer thicknesses are controlled by gradients of carbonate dissolution and fertility and by small scale redeposition processes mixed layer ages range from 3000 to 7000 years with a mode between 4000 and 5000 years sedimentation rates range from 0 8 to 2 4 cm 1000 years mixed layer depths calculated according to the box model of mixing range from 7 cm to 16 cm observed thicknesses are about one fourth smaller than calculated ones

stratigraphic completeness tiered trace fossils provide a measuring stick trace fossil associations commonly display a characteristic vertical zonation tiering within the substrate when burrows of a particular tier become truncated by turbidites tempestites or other results of catastrophic events they provide a simple but quantitative caliper for estimating stratigraphic incompleteness the minimum amount of penecontemporaneous sea floor erosion can be determined by types and sequences of trace fossils preserved below event deposits

dynamic interactions between lahars and stream flow a case study from ruapehu volcano new zealand three lahars were sampled in the whan gaehu river on the eastern flank of ruapehu volcano new zealand at 23 5 and 42 km from the source as they were flowing on september 27 and 29 and october 6 1995 the lahars were generated by water explosively ejected from the highly mineralized crater lake we used the chemical contrast between the lahars and resident stream water in their paths to describe a four phase model of a noncohesive lahar wave in a river channel 1 ambient stream water pushed ahead of the lahar in a process of miscible displacement due to hydrodynamic dispersion 2 a zone of mixing between the stream water and the lahar that increases in length with distance from source 3 a remnant of the original lahar least diluted by stream water that decreases in length and dilutes downstream 4 the tail of the lahar surge peak discharge occurs at the end of phase 1 as water is pushed ahead of the lahar peak sediment concentration occurs at the end of phase 2 where debris entrainment by the flow is at its greatest i e in front of the lahar proper deposits record only phases 2 and 3 of the lahar wave phase 1 flow left only a tide line of organic debris and phase 4 deposits were rapidly eroded by later streamflow downstream dilution by stream water eventually caused transformation of phases 2 and 3 of the lahars from debris flow to hyperconcentrated streamflow and then to normal streamflow as the flows became progressively finer grained and more turbulent

downstream changes in dryland river channels the northern plains of arid central australia abstract many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods flow transmission losses and typically few tributary inflows beyond the headwaters along the sandover bundey sandover bundey and woodforde rivers on the northern plains of arid central australia downstream channel changes are broadly similar in upland zones small rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones where channels and narrow floodplains are confined by bedrock alluvial terraces or aeolian dunes in lower gradient lowland zones channels and floodplains remain confined and in the absence of tributary inflows channel cross sectional areas and discharges decrease downstream confining landforms are not present in floodout zones which results in splay formation increased floodplain widths and marked overall downstream decreases in cross sectional areas eventually channelised flow and bedload transport terminate although occasional large floods continue across extensive unchanneled alluvial surfaces termed floodouts these broad similarities apart downstream changes along the three rivers differ in detail the sandover is largely a single thread channel whereas many reaches of the sandover bundey and woodforde are anabranched on the small woodforde river downstream decreases in parameters such as cross sectional area and width are roughly linear on the larger sandover and sandover bundey downstream changes are more irregular particularly through the floodout zones where there are marked fluctuations in widths depths and bed slopes the irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers on the northern plains as in other drylands complex interactions between discharge sediment transport slope patterns of tributary drainage bank sediment type and vegetation result in variable patterns of downstream channel change