new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

experimental alluvial fans advances in understanding of fan dynamics and processes alluvial fans are depositional systems that develop because of a disparity between the upstream and downstream sediment transport capacity of a system usually at the base of mountain fronts as rivers emerge from the constrained mountain area onto the plain they are dynamic landforms that are prone to abrupt changes on a geomorphological decades to centuries time scale while also being long term deposition features that preserve sedimentary strata and are sensitive indictors of environmental change the complexity of interactions between catchment characteristics climate tectonics internal system feedbacks and environmental processes on field alluvial fans means that it is difficult to isolate individual variables in a field setting therefore the controlled conditions afforded by experimental models has provided a novel technique to overcome some of these complexities the use of experimental models of alluvial fans has a long history and these have been implemented over a range of different research areas utilising various experimental designs using this technique important advances have been made in determining the primary factors influencing fan slope understanding of avulsion dynamics identifying autogenic processes driving change on fan systems independent of any change in external conditions and the mechanics of flow and flood risk on alluvial fans to name a few however experiments cannot be carried out in isolation thus combining the findings from experimental alluvial fans with field research and numerical modelling is important and likewise using these techniques to inform experimental design if this can be achieved there is potential for future experimental developments to explore key alluvial fan issues such as stratigraphic preservation potential and simulating extra terrestrial fan systems

large scale dam removal on the elwha river washington usa river channel and floodplain geomorphic change abstract a substantial increase in fluvial sediment supply relative to transport capacity causes complex large magnitude changes in river and floodplain morphology downstream although sedimentary and geomorphic responses to sediment pulses are a fundamental part of landscape evolution few opportunities exist to quantify those processes over field scales we investigated the downstream effects of sediment released during the largest dam removal in history on the elwha river washington usa by measuring changes in riverbed elevation and topography bed sediment grain size and channel planform as two dams were removed in stages over two years as 10 5 million t 7 1 million m3 of sediment was released from two former reservoirs downstream dispersion of a sediment wave caused widespread bed aggradation of 1 m greater where pools filled changed the river from pool riffle to braided morphology and decreased the slope of the lowermost river the newly deposited sediment which was finer than most of the pre dam removal bed formed new bars largely pebble granule and sand material prompting aggradational channel avulsion that increased the channel braiding index by almost 50 as a result of mainstem bed aggradation floodplain channels received flow and accumulated new sediment even during low to moderate flow conditions the river system showed a two to tenfold greater geomorphic response to dam removal in terms of bed elevation change magnitude than it had to a 40 year flood event four years before dam removal two years after dam removal began as the river had started to incise through deposits of the initial sediment wave 1 2 million t of new sediment 10 of the amount released from the two reservoirs was stored along 18 river km of the mainstem channel and 25 km of floodplain channels the elwha river thus was able to transport most of the released sediment to the river mouth the geomorphic alterations and changing bed sediment grain size along the elwha river have important ecological implications affecting aquatic habitat structure benthic fauna salmonid fish spawning and rearing potential and riparian vegetation the response of the river to dam removal represents a unique opportunity to observe and quantify fundamental geomorphic processes associated with a massive sediment influx and also provides important lessons for future river restoration endeavors

impact craters and craterform structures in fennoscandia abstract sixty two nearly circular topographic morphological or geophysical structures in fennoscandia and estonia were studied to discover their origin the structures were divided into five classes fifteen were caused by meteorite impacts class a and a four of them lappajarvi janisjarvi mien and dellen contain large volumes of impact melt the age of the recognized impact craters varies from pre historic 3500 b c to 700 ma and their diameters from 0 04 km to 55 km the majority of the other structures are probable class b or possible class c craters for which there is not yet sufficient proof of impact origin there is increasing evidence that some of the large arcuate morphological or geophysical structures class e represent the deeply eroded scars of very old craters but owing to the lack of identified shock metamorphic features or impact generated rocks e g allogenic breccias they cannot so far be classified as impact craters so far no craterform structure or ejecta layer of archaean or early proterozoic age has been found in fennoscandia although statistically remnants of ancient cratering events should be found in the fennoscandian shield new ways of searching for these structures are proposed the impact cratering rate in fennoscandia is 2 4 10 14 km 2a 1 n 12 the very small craters are omitted which corresponds to about two events per 100 ma for the last 700 ma this is a minimum estimate and is higher than the global cratering rate

geophysical investigations of the lake lappajarvi impact structure western finland abstract a circular negative bouguer anomaly approximately 10 mgal in amplitude and 17 km in diameter is associated with the lappajarvi impact structure the related mass deficit is 440 10 11 kg there are irregularities in the anomaly pattern due to differentially faulted bedrock blocks and lateral density contrasts between bedrock shocked bedrock breccia suevite impact melt rock and overburden the minimum amount of impact melt rock was interpreted from the positive residual bouguer anomaly between the centre and the rim in the northwestern part of the crater as being 1 5 2 0 km 3 there are a few small aeromagnetic anomalies up to 200 nt at an altitude of 40 m within the crater which is mostly characterized by a lack of magnetized material strong magnetic anomalies a few thousand nt which are abundant in the surroundings disappear in the crater area the aero electromagnetic aem data show strong anomalies with inphase quadrature ratios close to 1 indicating moderately conducting material within the crater an electromagnetic wide band sampo em profile was measured across the annular depression between the centre and the rim of the crater three layers were interpreted the thickness of the uppermost layer mostly conductive overburden is about 100 200 m and the resistivity 60 170 m in the annular depression beneath the uppermost layer there is a 50 250 m thick layer of suevites and breccias with resistivity of about 17 50 m the resistive lowest layer follows the base of breccia and suevite some separate soundings were carried out near the centre of the crater according to one of these soundings there is a conductive layer of suevite and breccia with a resistivity of 30 40 m beginning at depth of 150 m beneath the exposed impact melt rock the results of geophysical interpretations such as the annular depression between the centre and the rim of the crater the small thickness diameter ratio of the geophysically anomalous part of the crater and the calculated amount of impact melt rock indicate that the lappajarvi structure belongs to the category of complex craters rhomboidal geophysical anomaly patterns in particular aem anomalies parallel to old bedrock structures suggest that the collapse of the transient crater and or subsequent crater modification were controlled by nw se and ne sw trending pre existing fracture and fault zones

137cs and 210pb derived sediment accumulation rates and their role in the long term development of the mkuze river floodplain south africa abstract wetlands are well known to act as sinks for sediment and chemicals in the landscape within the mkuze river floodplain in northern kwazulu natal south africa deposition of clastic sediment occurs predominantly in close proximity to the river channel resulting in the formation of levees over time this leads to channel avulsion a process that may be accelerated by the activities of hippopotami whose trails create hydraulically favourable pathways sedimentation rates determined using the isotopes 210pb and 137cs indicate that the mkuze river floodplain is a relatively rapidly aggrading system that should experience frequent avulsion with average short term rates in the order of 0 25 to 0 50 cm y sediments on the floodplain are also an important sink for solutes which concentrate in the groundwater and precipitate out because of evapotranspiration over long timescales chemical sedimentation affects the landscape by influencing salinity vegetation distribution hydrological flows and local topography the mkuze river floodplain is an actively evolving system which continues to aggrade as a result of the combination of clastic and chemical sedimentation in a region characterised by a strong annual water deficit temporal patterns of clastic and chemical sedimentation are likely to exert influence on the long term development of wetland systems elsewhere in southern africa this study the first on sediment accretion rates using 210pb and 137cs dating for a southern african wetland demonstrates that radioisotopic methods are an important tool that can be applied toward fully understanding wetland formation evolution and functioning in the region

structure of the southern falkland islands continental shelf initial results from new seismic data abstract acquisition of a modern marine dataset has revealed the internal structure of sedimentary basins to the south of the falkland islands for the first time the new data reveal a variety of structural styles and indicate the presence of a sedimentary section ranging from 2 to 10 km in thickness developed in three linked sub basins 1 the eastern part of the malvinas basin lies south west of the islands a prominent regional unconformity developed throughout the area marks the top of gently folded rocks of probable palaeozoic age and their cover of late jurassic volcanics to the north west of the island the underlying strata approach the surface in a structural high which forms a prominent gravity low and may record the continuation of the palaeozoic basin exposed on the islands themselves strata of probable mesozoic and tertiary age rest on the unconformity throughout the area and show a regional southward dip these deposits increase in thickness south westwards towards the depocentre of the malvinas basin midway between the islands and argentina 2 the south falkland basin and burdwood bank lie south of the islands thicknesses increase towards an area of southward dipping extensional fault blocks within the south falkland basin downdip these are buried beneath a prograding and north vergent stack of blind and emergent thrusts at the northern margin of the burdwood bank which is a structural highly lying a shallow water south of the islands thrust loading has resulted in the development of a foreland basin north of the thrust front complex antiformal structures are associated with the thrusting whereas a shallow sedimentary basin developed on the crest of the burdwood bank is interpreted as a piggy bank basin riding on and sourced dominantly from within the thrust pile 3 the falkland plateau basin lies to the east and south east of the islands and is characterized by a varied and extensive mesozoic to tertiary succession of submarine fan deposits thickening eastwards into deeper water igneous intrusives deep in the section may correlate with jurassic volcanic rocks present within the malvinas basin to the west or may record younger rifting associated with the opening of the south atlantic these initial results indicate considerable structural heterogeneity in this part of the falkland islands continental shelf in the light of the hydrocarbon discoveries already made further west significant economic potential is likely stratigraphic traps may occur at regional unconformities and within submarine fan deposits whereas potential structural traps include mesozoic extensional fault blocks and thrust and sub thrust structures generated by tertiary and quaternary compression

chicxulub multiring impact basin size and other characteristics derived from gravity analysis the buried chicxulub impact structure in mexico which is linked to the cretaceous tertiary k t boundary layer may be significantly larger than previously suspected reprocessed gravity data over northern yucatan reveal three major rings and parts of a fourth ring spaced similarly to those observed at multiring impact basins on other planets the outer ring probably corresponding to the basin s topographic rim is almost 300 kilometers in diameter indicating that chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago

simplified process modeling of river avulsion and alluvial architecture connecting models and field data abstract modeling is an invaluable tool for studying sedimentary basin filling and for understanding depositional processes with long recurrence intervals including channel avulsion simplified modeling approaches such as cellular models and process analogue experiments are particularly useful for efficiently exploring alternative hypotheses and evaluating first order controls on river avulsion and alluvial architecture here we review the history and current state of the art in simplified avulsion and alluvial architecture models with a particular focus on how results and insights from these models can be incorporated into field and subsurface studies and vice versa simplified avulsion and alluvial architecture models have proliferated in the past decade providing a wide variety of models to serve as a basis for future coupled field modeling studies we compare features of leading models and discuss avenues for effectively pairing model capabilities with hypotheses and field data outstanding questions highlighted by recent modeling efforts include 1 what thresholds control avulsion initiation in different systems 2 how do floodplain processes and topography influence avulsion dynamics and alluvial architecture 3 what factors determine where avulsion channels stabilize answering these questions will require targeted modeling efforts coupled to data from ancient systems hence our model comparison emphasizes features that can be used to choose or design fit for purpose models and we outline how quantitative data useful for model selection and validation can be obtained from modern systems and ancient deposits matching model goals with targeted questions and model parameters and predictions with quantitative field data will help tighten communication between field and model oriented sedimentary geologists facilitating advances in our understanding of river avulsion and alluvial architecture

large impact crater in the barents sea seismic profiles in the central barents sea record a conspicuous structure exhibiting gross morphological and structural characteristics closely resembling those of large impact craters this feature named the mjolnir structure is 39 km in diameter and of jurassic early cretaceous age it probably formed by the impact of an asteroid or comet 0 7 2 5 km in diameter into a shallow sea underlain by 5 km of sedimentary section if the impact interpretation is correct mjolnir is one of the largest well preserved craters in the errestrial record

evidence for a volcanic rifted margin and oceanic crustal structure for the falkland plateau basin marine geophysical data from the western falkland plateau show strongly lineated magnetic anomalies and a coincident linear gravity high that identify the southeastern margin of a falkland islands block as a volcanic rifted continental margin characteristics of onshore and offshore dykes suggest this margin is most probably slightly younger than 190 ma together with these data seismic reflection profiles and sonobuoy seismic refraction data from the basin province of the falkland plateau east of the islands indicate an elevated oceanic origin for the major part of the falkland plateau basin most probably within the karoo hotspot province these conclusions are compatible with but do not directly indicate the independent rotation of a falkland islands block and closely constrain models of southwest gondwana evolution

persistent alluvial fanhead trenching resulting from large infrequent sediment inputs we present laboratory and field evidence that in mountainous catchment fan systems persistent alluvial fanhead aggradation and trenching may result from infrequent large sediment inputs we suggest that the river fan systems along the fault bounded range front of the western southern alps new zealand are likely to be in a dynamic equilibrium on 103 yr timescales superimposed on which their fanheads undergo long term cumulative episodic aggradation these fanheads are active only in rare events do not take part in the usual behaviour of the catchment fan system and require much longer to exhibit dynamic equilibrium than the rest of the fan these findings 1 increase our knowledge of the effects of extreme events on alluvial fan morphodynamics in humid climates 2 question the general applicability of inferring past climatic or tectonic regimes from alluvial fan morphology and stratigraphy and 3 provide a conceptual basis for hazard zonation on alluvial fans copyright 2006 john wiley sons ltd

geophysical evidence for a large impact structure on the falkland malvinas plateau a large roughly circular structural basin is recognised on the falkland malvinas plateau to the nw of west falkland gran malvina island s 51 00 w 62 00 the basin seen in seismic reflection profiles and evident as a large negative gravity anomaly has a diameter of 250 km the age of the basin is estimated to be late palaeozoic it is completely buried by younger sediments and has no topographic expression on the sea floor we propose that the basin and geophysical anomalies especially the combination of a large circular negative gravity anomaly with a rim of positive anomalies and a marked circular series of positive magnetic anomalies in the same area may be best explained by the presence of a large buried impact structure

quantifying fluvial non linearity and finding self organized criticality insights from simulations of river basin evolution abstract a numerical study was undertaken to investigate non linearity and the potential for self organized criticality soc in the evolution of river basins twenty three simulations were carried out using the authors caesar landscape evolution model in which the magnitude of storm events variability of storm events sediment heterogeneity sources of sediment supply and catchment morphology are systematically varied to evaluate their importance as possible drivers for non linear behavior and soc temporal fluctuations in simulated sediment yield show notable non linear behavior storm magnitude and occurrence of landslides appear to have little impact on variability of the sediment yield when compared to the impacts of sediment heterogeneity rainfall variability and catchment morphology particularly it appears that the non linearity of sediment yields results from the manner in which the catchment processes the variable rainfall rather than just the rainfall variability itself the variations in sediment yield show a power law magnitude frequency distribution which is a possible but inconclusive indicator of soc however several other more qualitative arguments can be made to support the case for soc in these simulations specifically we identify the nature of the critical state and suggest two cascade mechanisms by which the system can organize itself around this critical state combined these arguments indicate that simulated evolution of river basins indeed exhibits soc at least with respect to sediment yield the critical state appears to be an indicator of the connectivity of the drainage network thus the simulations indicate that unlike traditional soc systems the critical state of the system can vary in time as sudden changes in drainage network connectivity may result in sudden changes in the soc behavior of the system

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

morokweng impact structure northwest province south africa geophysical imaging and shock petrographic studies abstract the existence of a large near circular structure centred on the area around morokweng 23 32 e 26 31 s in the northwest province of south africa and possibly extending into southern botswana is inferred from aeromagnetic and gravity data results of surface geological studies support earlier suggestions that this feature could represent an impact structure samples from an autochthonous ironstone breccia exposure about 47 km to the west of the centre of this structure and several samples of allochthonous quartzite from south of the central part of the structure contain shock metamorphosed quartz with impact diagnostic planar deformation features pdfs the geophysical evidence is suggestive of an original size of this impact structure of at least 70 km but several annular anomalies outside of the study region could be interpreted as indicating a diameter of 300 340 km the age of this structure is only loosely constrained but is believed to be younger than the 2 25 2 5 ga transvaal supergroup

late pleistocene holocene river dynamics at the trent soar confluence england uk although river confluences have received geomorphic attention in recent years it is difficult to upscale these studies so confluence dominated reaches are commonly presumed to be either 1 braided or 2 meandering and characterized by laterally migrating channels if the geomorphology of a confluence zone is to be considered over longer timescales changes in river style need to be taken into account this paper uses a combination of remote sensing techniques lidar gpr er borehole survey and chronometric dating to test this differentiation in the confluence zone of a medium sized mixed load temperate river system trent uk which on the basis of planform evidence appears to conform to the meandering model however the analysis of confluence sediment body stratigraphy demonstrates that the confluence does not correspond with a simple meander migration model and chronostratigraphic data suggests it has undergone two major transformations firstly from a high energy braid plain confluence in the lateglacial 25 13 k yrs cal bp to a lower energy braided confluence in the early to middle holocene early holocene 2 4 kyr bp which created a compound terrace second incision into this terrace creating a single channel confluence 2 4 0 5 kyr cal bp with a high sinuosity south bank tributary the river soar the confluence sediment body stratigraphy is characterized by a basal suite of late pleistocene gravels bisected by younger channel fills which grade into the intervening levee and overbank sediments the best explanation for the confluence sediment body stratigraphy encountered is that frequent switching soft avulsions sensu edmonds et al 2011 of the tributary are responsible for the downstream movement of the channel confluence at an average rate of approximately 0 5 m per year dissecting and reworking older braid plain sediments the late holocene evolution of the confluence can be seen as a variant of the incisional frequent channel reorganization avulsion model with sequential downstream migration of the reattachment point copyright 2012 john wiley sons ltd

a method to assess fluvial fan channel networks with a preliminary application to fans in coastal british columbia abstract the channel network on a fluvial fan distributes sediment across the fan surface and determines fan development we present a method to characterize fan channel networks fcns so that the effect of controls e g sediment supply on the fcn can be evaluated we then do a preliminary test of the method using three fans in coastal british columbia the method uses a set of three measures 1 the spatial extent of hydrogeomorphic activity measured as the percentage of the contemporary fan surface area occupied by active channels 2 the topologic structure using node counts to measure channel network complexity and 3 a sediment budget to indicate the proportion of sediment that is stored on the fan loosely characterized by comparing the second largest clast size of sediment at the fan apex or intersection point with the second largest clast at the toe of the fan these measures were applied to two fan deltas and one partial fan delta in northern vancouver island british columbia using channel surveys conducted in 2006 and 2007 all three fans are located within 30 km of each other have similar climatic physiographic and vegetation settings and have natural channel networks each fan has perennial channel flow at the fan apex watershed areas range from 19 5 to 35 6 km2 and contemporary fan areas range from 0 2 to 0 7 km2 the melton relative relief ratio ranges from 0 24 to 0 30 and none of the fans show evidence of debris floods or debris flows in addition to testing the efficacy of the fcn measures we use these fans to explore the question of whether fans in similar geomorphic settings and with similar controls develop similar fcns results show the fans have between 4 7 8 5 of the contemporary fan surface occupied by the active channel network topologic node counts indicate that the two fan deltas have a similar level of channel complexity with 42 54 nodes in total the partial fan delta channel network is approximately half as complex with 23 nodes in total all three fans have boulder sized sediment 30 50 cm at the fan apex on the two fan deltas the largest sediment at the toe of the fan is 10 12 cm indicating that much of the sediment delivered to the fan apex is stored on the fan in contrast the partial fan delta has 18 20 cm sized sediment transported into the axial valley river at the toe of the fan and a wider and steeper outlet channel indicating that a greater proportion of sediment is exported from the fan although the axial river does not visibly incise into the fan deposits the lower channel complexity and coarser sediment output indicates that the axial river truncates the toe of the partial fan delta the two fan deltas have similar fcns supporting the idea that fans in similar geomorphic settings develop similar fcns the partial fan delta is in a different geomorphic setting that was not identified prior to analysis of the fcns and has a different fcn this study also examines the effects of a large regional storm event that occurred between the 2006 and 2007 surveys one fan had increased active area two fans had increased total node counts and two fans increased output clast size after the flood event indicating the measures are sensitive and respond to disturbance further investigations of fcns may have utility in hazard assessments addressing how fans respond to changes in sediment supply or discharge regime how vegetation type affects fcns or the effect of other watershed processes or fan attributes

mexican site for k t impact crater research throughout the caribbean suggests that the geophysical anomalies in the yucatan first noted by penfield and camargo 1981 and called the chicxulub crater could be the site of the impact purported to have caused the k t extinctions a semicircular ring of sink holes known locally as cenotes which correlates with the geophysical anomalies has been identified and it is argued that the origin of the cenote ring is related to postimpact subsidence of the chicxulub crater rim if there is indeed a crater the region within the cenote ring corresponds to its floor and the crater rim diameter is probably larger than 200 km if confirmed as a site of impact the chicxulub crater would be the largest terrestrial impact crater known which is consistent with the uniqueness of the k t global catastrophe

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

the origin of the 1883 krakatau tsunamis abstract hypotheses for the origin of the krakatau tsunamis of 27 august 1883 are reviewed the 151 minute sea wave travel time between krakatau and batavia jakarta first proposed by verbeek cannot be substantiated thus hypotheses for the origin of the tsunamis which rest on very accurate timings of the origins of the air and sea waves such as that of latter 1981 also cannot be substantiated there was one exceptionally large sea wave tsunami on 27 august and many smaller ones most of the smaller waves appear to have been air sea coupled waves rather than true tsunamis of the few sea waves that were probably true tsunamis only one may have originated by emplacement of a pyroclastic flow at a point nearer batavia than krakatau the catastrophic event in the eruption of august 27th was preceded some 40 minutes earlier by a subordinate event which also propagated an air sea coupled wave and a tsunami the largest heard explosion produced an air sea coupled wave but was not associated with a tsunami although the precise time relations between the origins of the air and sea waves cannot be definitely established the most likely scenario for the catastrophic phase of the eruption is broadly similar to that originally proposed by verbeek in which collapse of part of the original volcanic edifice propagated a major volcanic explosion

stratigraphy and paleohydrology of delta channel deposits jezero crater mars abstract the jezero crater open basin lake contains two well exposed fluvial sedimentary deposits formed early in martian history here we examine the geometry and architecture of the jezero western delta fluvial stratigraphy using high resolution orbital images and digital elevation models dems the goal of this analysis is to reconstruct the evolution of the delta and associated shoreline position the delta outcrop contains three distinct classes of fluvial stratigraphy that we interpret from oldest to youngest as 1 point bar strata deposited by repeated flood events in meandering channels 2 inverted channel filling deposits formed by avulsive distributary channels and 3 a valley that incises the deposit we use dems to quantify the geometry of the channel deposits and estimate flow depths of 7 m for the meandering channels and 2 m for the avulsive distributary channels using these estimates we employ a novel approach for assessing paleohydrology of the formative channels in relative terms this analysis indicates that the shift from meandering to avulsive distributary channels was associated with an approximately four fold decrease in the water to sediment discharge ratio we use observations of the fluvial stratigraphy and channel paleohydrology to propose a model for the evolution of the jezero western delta the delta stratigraphy records lake level rise and shoreline transgression associated with approximately continuous filling of the basin followed by outlet breaching and eventual erosion of the delta our results imply a martian surface environment during the period of delta formation that supplied sufficient surface runoff to fill the jezero basin without major drops in lake level but also with discrete flooding events at non orbital e g annual to decadal timescales

lake st martin impact structure manitoba canada magnetic anomalies and magnetizations a ground magnetic survey has further delineated the previously known intense 1000 nt negative anomaly at the centre of the lake st martin impact structure manitoba shocked gneiss in the central uplift has a strong reverse remanent magnetization whose direction is similar to that found after alternating field demagnetization of melt rock specimens from within the impact structure shocked gneiss is interpreted as the source of the central negative anomaly the direction of the remanence is consistent with a permo triassic age for the impact event the magnetization in the shocked gneiss is interpreted as a chemical remanent magnetization carried by hematite formed during an extensive alteration of mafic silicates occurring immediately after the impact

the relationship between channel avulsion flow occupancy and aggradation in braided rivers insights from an experimental model experimental modelling of an aggrading braided river has allowed investigation of the relationship between the frequency of channel avulsion a f the duration of time that the braidplain is occupied by flow the spatial pattern of braidplain sedimentation and how these respond to a change in sediment supply s s model results demonstrate a strong positive relationship between s s and a f and that there is no downstream change in a f over the short braidplain distances ca 100 m modelled herein although a f is strongly dependent on s s the degree of channel switching does not influence the rate or spatial pattern of braidplain sedimentation all experiments used a single central input for water and sediment and the channels occupied the centre of the alluvial basin for a longer period of time than the margins for all sediment supply rates and distances downstream despite this spatio temporal pattern in flow occupancy braidplain sedimentation rates were nearly uniform both downstream and across the basin and increased at approximately the same rate as increases in s s as a consequence less frequent and possibly short lived flows at the margins of the braidplain deposited and preserved more sediment per unit time in comparison with the centre of the basin where flow occupancy was higher an approximate order of magnitude change in sediment supply resulted in only a factor of two change in bed slope probably due to both an increase in channelization and adjustment of the channel form that maintained sediment transport through the basin this result suggests that linear diffusion models are unlikely to be applicable in landscape evolution models that possess aggrading multi thread rivers which are capable of self adjustment in channel number and form

gravity survey of the mount toondina impact structure south australia gravity and seismic reflection data together with geologic mapping indicate that the mount toondina feature in south australia is best interpreted as an eroded 4 km diameter impact structure consisting of a ring structural depression surrounding a pronounced central uplift beds at the center of the structure within the central uplift have been raised as much as 200 m from depth and deformed by convergent flow seismic reflection data indicate that deformation extends to depths of only 800 m at greater depths the reflectors are nearly flat lying indicating little or no deformation gravity data show residual anomalies of 1 0 mgal coincident with the central uplift and a 0 5 mgal low associated with the ring structural depression modeling of the gravity data indicates that relatively high density material occurs within the central uplift whereas the ring depression is filled with low density material the deformation at mount toondina is typical of a complex impact crater the 4 km diameter is consistent with the expected threshold size for complex craters formed in weak to moderate strength sedimentary rocks

experimental study on evolution of lacustrine shallow water delta abstract in the process of delta evolution the flow regime and sediment transport change with the development of topography flow is the dynamic factor of shaping topography while sediment scouring and silting is the direct cause of topography formation and they are constrained and influenced by topographic changes simultaneously in order to explore rules of water and sediment movement and topographic sedimentation of delta under microscale conditions we studied the phenomena rules and evolution mechanism of flow movement sediment transport particle sorting topographic sedimentation and flow path generation merger and recombination under microscale conditions from the perspective of physical experiment the influence of different external conditions on lacustrine shallow water delta evolution and the self organization of delta evolution are studied through comparative experiments under the condition of distinct sediment supplied rates or lake levels and repeatability experiments under the same conditions some rules of flow movement sediment transport and topographic sedimentation such as characteristics of each stage during the process of evolution of lacustrine shallow water delta the periodic change of flow regime and the prediction model the characteristics of sedimentary morphology under different conditions and the influence of stochastic factors on delta evolution are summarized the experiments suggest coupled linkage of interaction of water and sediment self organization of flow path evolution and the equilibrium tendency of delta structure under the action of coupling interaction of multiple factors of water sediment and topographic sedimentation

size and structure of the chicxulub crater revealed by horizontal gravity gradients and cenotes it is now widely believed that a large impact occurred on the earth at the end of the cretaceous period and that the buried chicxulub structure in yucatan mexico is the resulting crater24 knowledge of the size and internal structure of the chicxulub crater is necessary for quantifying the effects of the impact on the cretaceous environment although much information bearing on the crater s structure is available diameter estimates range from 170 to 300 km refs 1a 7 corresponding to an order of magnitude variation in impact energy here we show the diameter of the crater to be 180 km by examining the horizontal gradient of the bouguer gravity anomaly over the structure this size is confirmed by the distribution of karst features in the yucatan region mainly water filled sinkholes known as cenotes the coincidence of cenotes and peripheral gravity gradient maxima suggests that cenote formation is closely related to the presence of slump faults near the crater rim

autogenic avulsion channelization and backfilling dynamics of debris flow fans alluvial fans develop their semi conical shape by quasi cyclic avulsions of their geomorphologically active sector from a fixed fan apex on debris flow fans these quasi cyclic avulsions are poorly understood partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas in this study debris flow fans were experimentally created under constant extrinsic forcing and autogenic sequences of backfilling avulsion and channelization were observed backfilling avulsion and channelization were gradual processes that required multiple successive debris flow events debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia in the channelization phase debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed resulting in longer run out and deeper channels backfilling commenced when debris flows reached their maximum possible length and channel depth as defined by channel slope and debris flow volume and composition after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated the terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low gradient zone forcing deposition consequently the next debris flow was shorter which led to more in channel sedimentation causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout this topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas debris flow avulsion cycles are governed by the same large scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans although the detailed processes are unique to debris flow fans this novel result provides a basis for modelling of debris flow fans with applications in hazards and stratigraphy

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

a test of models of fluvial architecture and palaeosol development camp rice formation upper pliocene lower pleistocene southern rio grande rift new mexico usa numerical experimental and theoretical models of fluvial architecture and palaeosol development are tested with outcrops of upper pliocene lower pleistocene sediment in the southern rio grande rift new mexico the sediment was deposited and subsequently exhumed in the jornada del muerto basin a westward tilted half graben whose footwall corresponds to the rincon hills and san diego mountain fault blocks the axial river the ancestral rio grande shared time between the jornada del muerto basin and the adjacent corralitos basin the ancestral rio grande entered the jornada del muerto basin via a gap between the footwall blocks periodically flowing southward towards san diego mountain or making a broad northward sweep into the northern fluvial salient towards the rincon hills fault block and unfaulted northern edge of the basin ten logged sections up to 35 m thick are correlated using the top of the formation la mesa surface a 1 59 ma pumice conglomerate and a ground water carbonate opal bed additionally one of the sections is dated by reversal magnetostratigraphy consistent with the model of bridge leeder 1979 and bridge mackey 1993a differential tilting of the jornada del muerto half graben resulted in sections directly adjacent to the faults that consist almost exclusively of multistorey channel sands sandstones whereas more distal sections contain a greater proportion of crevasse splay fine sand and overbank mudstone and calcic palaeosols along the axis of the northern fluvial salient a northward decrease in channel floodplain ratio a decrease in channel recurrence interval from 171 kyr to 685 kyr and an increase in the maturity of calcic palaeosols are consistent with southward tilt of the unfaulted northern edge of the basin an upsection decrease in sediment accumulation rate in the northern fluvial salient from 0 036 mm yr to 0 017 mm yr corresponds to an increase in the ratio of channel floodplain facies and in the number of multistorey channel sands sandstones and is consistent with the model of bridge leeder 1979 in which avulsion frequency is independent of sediment accumulation rate stage ii and iii calcic palaeosols indicate 103 105 year of landscape stability and soil formation between periods of floodplain deposition in response not only to basin tilting but also because the ancestral rio grande had multiple paths within the jornada del muerto basin and shared time between the corralitos and jornada del muerto basins

middle and late holocene avulsion history of the river rhine rhine meuse delta netherlands at present little evidence is available on avulsion rates avulsion frequencies and interavulsion periods of aggrading fluvial systems over time scales of millenia the large number of high quality 14c ages from the holocene rhine meuse delta central netherlands enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency 14c histogram analysis indicates that instantaneous avulsion coexistence of successive channel belts for less than a few centuries has dominated in this area interavulsion periods were typically 1000 yr although values of 500 or 5000 yr have also been found avulsion frequency was especially large prior to 4300 cal yr b p indicating a strong control by aggradation rate in turn controlled by sea level rise in addition periods with high avulsion frequency appear to correlate with time intervals characterized by large scale transgressive tendencies the time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style

avulsion cycles and their stratigraphic signature on an experimental backwater controlled delta river deltas grow in large part through repeated cycles of lobe construction and channel avulsion understanding avulsion cycles is important for coastal restoration and ecology land management and flood hazard mitigation emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics however our knowledge of backwater controlled avulsion cycles is limited here we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges shallow bed slopes and subcritical flows the experimental avulsion cycles consisted of an initial phase of avulsion setup an avulsion trigger selection of a new flow path and abandonment of the parent channel avulsions were triggered during the largest floods 78 of avulsions after the channel was filled by a fraction 0 3 0 13 of its characteristic flow depth at the avulsion site which occurred in the upstream part of the backwater zone the new flow path following avulsion was consistently one of the shortest paths to the shoreline and channel abandonment occurred through temporal decline in water flow and sediment delivery to the parent channel experimental synthetic stratigraphy indicates that bed thicknesses were maximum at the avulsion sites consistent with our morphologic measurements of avulsion setup and the idea that there is a record of avulsion locations and thresholds in sedimentary rocks finally we discuss the implications of our findings within the context of sustainable management of deltas their stratigraphic record and predicting avulsions on deltas

experimental alluvial fan evolution channel dynamics slope controls and shoreline growth 1 river deltas and alluvial fans have channelization and deposition dynamics that are not entirely understood but which dictate the evolution of landscapes of great social economic and ecologic value our lack of a process based understanding of fan dynamics hampers our ability to construct accurate prediction and hazard models leaving these regions vulnerable here we describe the growth of a series of experimental alluvial fans composed of a noncohesive grain mixture bimodal in size and density we impose conditions that simulate a gravel sand fan prograding into a static basin with constant water and sediment influx and the resulting fans display realistic channelization and avulsion dynamics we find that we can describe the dynamics of our fans in terms of a few processes 1 an avulsion sequence with a timescale dictated by mass conservation between incoming flux and deposit volume 2 a tendency for flow to reoccupy former channel paths and 3 bistable slopes corresponding to separate entrainment and deposition conditions for grains several important observations related to these processes are an avulsion timescale that increases with time and decreases with sediment feed rate fan lobes that grow in a self similar quasi radial pattern and channel geometry that is adjusted to the threshold entrainment stress we propose that the formation of well defined channels in noncohesive fans is a transient phenomenon resulting from incision following avulsion and can be directly described with dual transport thresholds we present a fairly complete process based description of the mechanics of avulsion and its resulting timescale on our fans because the relevant dynamics depend only on threshold transport conditions and conservation of mass we show how results may be directly applied to field scale systems

relative importance of fluvial input and wave energy in controlling the timescale for distributary channel avulsion 1 existing avulsion models are decoupled from nearshore processes here i explore quantitatively how the interplay of wave energy with fluvial input of sediment and water controls the aggradation rate and avulsion timescale of a single distributary channel my approach rigorously couples a diffusive moving boundary theory of fluvial morphodynamics with a diffusive treatment of shoreface morphodynamics i use this deterministic model to quantify the time required for channel belt superelevation normalized with channel depth to attain a threshold value for nodal avulsion at a specified channel location increasing the long term wave energy relative to fluvial input by an order of magnitude increases longshore sediment dispersal thereby reducing the rate of channel belt aggradation and associated seaward extension and increasing the avulsion timescale by a factor of approximately 50 far field processes eventually limit the ability of wave energy to suppress avulsion

channel evolution on the dammed elwha river washington usa abstract like many rivers in the western u s the elwha river washington has changed substantially over the past century in response to natural and human forcing the lower river is affected by two upstream dams that are slated for removal as part of a major river restoration effort in preparation for studying the effects of dam removal we present a comprehensive field and aerial photographic analysis of dam influence on an anabranching gravel bed river over the past century with the dams in place loss of the upstream sediment supply has caused spatial variations in the sedimentary and geomorphic character of the lower elwha river channel bed sediment is armored and better sorted than on the naturally evolving bed upstream of the dams on time scales of flood seasons the channel immediately below the lower dam is fairly stable but progresses toward greater mobility downstream such that the lowermost portion of the river responded to a recent 40 year flood with bank erosion and bed elevation changes on a scale approaching that of the natural channel above the dams in general channel mobility in the lowest 4 km of the elwha river has not decreased substantially with time enough fine sediment remains in the floodplain that given sufficient flood forcing the channel position sinuosity and braiding index change substantially the processes by which this river accesses new fine sediment below the dams rapid migration into noncohesive banks and avulsion of new channels allow it to compensate for loss of upstream sediment supply more readily than would a dammed river with cohesive banks or a more limited supply of alluvium the planned dam removal will provide a valuable opportunity to evaluate channel response to the future restoration of natural upstream sediment supply

unmetamorphosed sedimentary melange with high pressure metamorphic blocks in a nascent forearc basin setting abstract melanges crop out within unmetamorphosed basal great valley group gvg forearc basin strata and between gvg and the underlying coast range ophiolite cro in the san francisco bay region of coastal california these melanges include high pressure hp metamorphic blocks of the franciscan subduction complex that structurally underlies the unmetamorphosed cro as well as blocks of gvg and cro the matrix consists of foliated shale and serpentinite locally interleaved at centimeter scale the melanges strike and dip parallel to bounding gvg sandstones and conglomerates the matrix locally consists of sedimentary breccia and conglomerate made up of clasts of serpentinite and shale gvg sandstones within and bounding the melanges have detrital serpentinite clasts the field relationships indicate a sedimentary origin of the melanges as olistostromal deposits within latest jurassic to earliest cretaceous basal gvg the melanges correlate to units along the eastern margin of the northern coast ranges about 250 km to the north with restoration of post subduction dextral faulting and differ from the latter in the higher proportion of shale stronger matrix foliation and common occurrence of hp blocks similar units may have mistakenly been assigned to the franciscan owing to the foliated nature of the matrix and occurrence of hp blocks this and the broad distribution of localities indicate that these deposits are more widespread than previously believed exhumation rates of coarse grained hp melange blocks may have been 2 to 10 mm yr or higher based on the ages of similar blocks in the coast ranges burial depth of the blocks and depositional age of the enclosing strata exhumation and deposition of franciscan blocks in these gvg melanges predates preserved accretion of similar materials in the adjacent franciscan by at least 30 m y suggesting subduction erosion of previously accreted material or exhumation of the blocks in forearc serpentinite shale diapirs

chronology and controls of avulsion along a mixed bedrock alluvial river tooth stephen rodnight h duller g a t mccarthy t s 2006 chronology and controls of avulsion along a mixed bedrock alluvial river geological society of america bulletin 119 3 pp 452 461 rae2008

variations of lateral bedrock erosion rates control planation of uplifting folds in the foreland of the tian shan nw china fluvial planation surfaces such as straths commonly serve as recorders of climatic and tectonic changes and are formed by the lateral erosion of rivers a process that remains poorly understood here we present a study of kilometer wide fluvially eroded low relief surfaces on rapidly uplifting folds in the foreland of the southwestern tian shan a combination of field work digital elevation model analysis and dating of fluvial deposits reveals that despite an arid climate and rapid average rock uplift rates of 1 3 mm yr rivers cut extensive 1 2 km wide surfaces with typical height variations of 2 6 kyr the extent of this beveling varies in space and time such that different beveling episodes affect individual structures between times of planation beveled surfaces are abandoned incised and deformed across the folds in a challenge to models that link strath cutting and abandonment primarily to changes in river incision rates we demonstrate that lateral erosion rates of antecedent streams crossing the folds have to vary by more than 1 order of magnitude to explain the creation of beveled platforms in the past and their incision at the present day these variations do not appear to covary with climate variability and might be caused by relatively small much less than an order of magnitude changes in sediment or water fluxes it remains uncertain in which settings variations in lateral bedrock erosion rates predominate over changes in vertical erosion rates therefore when studying fluvial planation and strath terraces variability of both lateral and vertical erosion rates should be considered

human induced stream channel abandonment capture and filling of floodplain channels within the atchafalaya river basin louisiana article i nfo the atchafalaya river basin is a distributary system of the mississippi river containing the largest riparian area in the lower mississippi river valley and the largest remaining forested bottomland in north america reductions in the area of open water in the atchafalaya have been occurring over the last 100 years and many historical waterways are increasingly filled by sediment this study examines two cases of swamp channels b85 m 3 s that are filling and becoming unnavigable as a result of high sediment loads and slow water velocities the water velocities in natural bayous are further reduced because of flow capture by chan nels constructed for access bathymetry flow suspended sediment deposited bottom material isotopes and photointerpretation were used to characterize the channel fi ll on average waterflowing through these two channels lost 23 of the suspended sediment load in the studied reaches along one of the studied reaches two constructed access channels diverted significant flow out of the primary channel and into the adjacent swamp immediately downstream of each of the two access channels the cross sectional area of the studied channel was reduced isotopic analyses of bottom material cores indicate that bed filling has been rapid and occurred after detectable levels of cesium 137 were no longer being deposited interpreta tion of aerial photography indicates that water is bypassing the primary channels in favor of the more hy draulically efficient access channels resulting in low or no velocity flow conditions in the primary channel these swamp channel conditions are typical in the atchafalaya river basin where relict large chan nel dimensions result in flow velocities that are normally too low to carry fine grained sediment constructed channels increase the rate of natural channel avulsion and abandonment as a result of flow capture

a three dimensional numerical model of sediment transport erosion and deposition within a network of channel belts floodplain and hill slope extrinsic and intrinsic controls on floodplain dynamics and alluvial architecture a three dimensional numerical model of sediment transport erosion and deposition within a network of channel belts and associated floodplain is described sediment and water supply are defined at the upstream entry point and base level is defined at the downstream edge of the model sediment and water are transported through a network of channels according to the diffusion equation and each channel has a channel belt with a width that increases in time the network of channels evolves as a result of channel bifurcation and abandonment avulsion the timing and location of channel bifurcation is controlled stochastically as a function of the cross valley slope of the floodplain adjacent to the channel belt relative to the down valley slope and of annual flood discharge a bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value the floodplain aggradation rate decreases with distance from the nearest active channel belt channel belt degradation results in floodplain incision extrinsic extrabasinal allogenic and intrinsic intrabasinal autogenic controls on floodplain dynamics and alluvial architecture were modelled and sequence stratigraphy models were assessed input parameters were chosen based on data from the rhine meuse delta to examine how the model responds to extrinsic controls the model was run under conditions of changing base level and increasing sediment supply rises and falls in base level and increases in sediment supply occurred over 10 000 years rising base level caused a wave of aggradation to move up valley until aggradation occurred over the entire valley frequency of bifurcations and avulsions increased with rate of base level rise and aggradation rate channel belt width varied with water discharge and the lifespan of the channel belt wide connected channel belts and high channel deposit proportion occurred around the upstream inflow point because of their high discharge and longevity less connected smaller channel belts occurred further down valley such alluvial behaviour and architecture is also found in the rhine meuse delta during base level fall valley erosion occurred and the incised valley contained a single wide channel belt during subsequent base level rise a wave of aggradation moved up valley filling the incised valley bifurcation and avulsion sites progressively moved upstream relatively thin narrow channel belts bordered and cut into the valley fill these results differ substantially from existing sequence stratigraphy models the increase in sediment supply from upstream resulted in an alluvial fan most bifurcations and avulsions occurred at the fan apex nodal avulsion and channel belts were the widest and the thickest here giving high channel deposit proportion due to their high discharge and longevity the width and thickness of channel belts decreased down valley due to decreased discharge longevity and aggradation rate this behaviour occurs in modern alluvial fans intrinsic controls also affect floodplain dynamics and alluvial architecture variation of aggradation rate bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years compared with 10 000 years for extrinsic controls this variation is partly related to local aggradation and degradation of channel belts around bifurcation points channel belts were preferentially clustered near floodplain margins because of low floodplain aggradation rate and topography there

effects of vegetation on braided stream pattern and dynamics 1 investigations using a 16 m by 2 m recirculating experimental flume model of an ephemeral braided river indicate that the presence of large and erosion resistant plants within the channel e g trees or shrubs can have a significant impact on channel pattern and planform dynamics simulations show that these plants have two effects first they act as obstructions in some cases forcing the flow to divide this flow separation can allow the deposition of a small island immediately in the lee of the plant splitting the channel the net result is a substantial increase in the number of channels and correspondingly the braid index this is in direct contrast to previous studies where increased levels of vegetation in perennial streams have decreased the braid index second the plants stabilize braid bars and can form relatively stable islands in their lee significantly reducing the longitudinal migration of islands typically associated with braided rivers

autocyclic behaviour of fan deltas an analogue experimental study fan deltas are excellent recorders of fan building processes because of their high sedimentation rate particularly in tectonically active settings although previous research focuses mainly on allogenic controls there is clear evidence for autogenically produced storage and release of sediment by flume and numerical modelling that demands further definition of characteristics and significance of autogenically forced facies and stratigraphy analogue experiments were performed on fan deltas with constant extrinsic variables discharge sediment supply sea level and basin relief to demonstrate that fan delta evolution consists of prominent cyclic alternations of channellized flow and sheet flow the channellized flow is initiated by slope induced scouring and subsequent headward erosion to form a channel that connected with the valley while the removed sediment is deposited in a rapidly prograding delta lobe the resulting decrease in channel gradient causes a reduction in flow strength mouth bar formation flow bifurcation and progressive backfilling of the channel in the final stage of channel filling sheet flow coexists for a while with channellized flow semi confined flow although in cycle 1 this phase of semi confined flow was absent subsequent autocyclic incisions are very similar in morphology and gradient however they erode deeper into the delta plain and as a result take more time to backfill the duration of the semi confined flow increases with each subsequent cycle during the period of sheet flow the delta plain aggrades up to the critical gradient required for the initiation of autocyclic incision this critical gradient is dependent on the sediment transport capacity defined by the input conditions these autogenic cycles of erosion and aggradation confirm earlier findings that storage and release of sediment and associated slope variation play an important role in fan delta evolution the erosional surfaces produced by the autocyclic incisions are well preserved by the backfilling process in the deposits of the fan deltas these erosional surfaces can easily be misinterpreted as climate sea level or tectonically produced bounding surfaces

channel dynamics sediment transport and the slope of alluvial fans experimental study a b s t ra c t we present the results of an experimental study of alluvial fan sedimentation under conditions of constant inflow water discharge qw sediment supply qso and median grain size d the study was designed to complement and test a recently formulated model of alluvial fan sedimentation and to emphasize the interactions between and controls on flow channelization and equilibrium fan slope flow channelization and fan sedimentation were studied under conditions of nearly steady uniform aggradation steady conditions were achieved by imposing a steadily rising base level just in balance with the average sediment aggradation rate experimental inflows covered a wide range of conditions allowing examination of the effects of variations in qw q so and d in both bedload and suspension dominated environments experimental results were most consistent with an expanding flow channel model key experimental findings successfully and quantitatively predicted by the expanding flow theory include 1 straight to slightly concave radial profiles of bedload dominated fans 2 distinctly convexo concave profiles of suspension dominated fans 3 a strong inverse relationship between qw and fan slope 4 a strong but secondary relationship between q so and fan slope and 5 near independence of d and fan slope so long as transport stage is high and bedload transport dominant however potential scale effects in the experiments arose from reduced flow reynolds numbers and incorrect geometric scaling of channel widths no confident conclusions regarding the debate over the relative importance of sheetfloods and braided channel flows can be drawn from the experimental data extrapolation to field scale is best accomplished through appropriate application of the theoretical model herein confirmed against experimental data

experimental steep braided flow application to flooding risk on fans flooding processes occurring on alluvial fans are considerably different from those occurring along single thread rivers with well defined floodplains active erosion rapid sedimentation and the uncertainty in flow path make the prediction of flood evolution and extent difficult based on a large scale experiment this study investigates the long term evolution of the flow on a steep noncohesive sediment surface resembling a complex of merged alluvial fans the results are pertinent to the assessment of flooding hazard on alluvial fans at any given time the average flow occupancy was 21 of the surface however the flow was characterized by active channel switching and overflow processes the percentage of the surface remaining dry was found to decay harmonically with time a reworking time was defined as the time at which half of the surface that was initially dry remained dry whereas the other half was inundated at least once an empirical expression was developed in which reworking time is proportional to the average cross sectional area of flow and inversely proportional to the sediment supply

spatial variation of overbank aggradation rate and its influence on avulsion frequency river avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges and the associated increase in cross valley slope relative to either the down channel slope or the down valley slope the latter is termed the slope ratio in the present paper therefore spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion dominated models of alluvial architecture detailed evidence on centennial to millennial scale floodplain deposition has to date been largely unavailable new data on such long term overbank aggradation rates from the rhine meuse and mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto and can be similar to observations for single overbank floods this leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios potentially resulting in increased avulsion frequencies a revised input parameter for overbank aggradation rate was used in a three dimensional model of alluvial architecture to study its effect on avulsion frequency realistic patterns of avulsion and interavulsion periods 1000 years were simulated with input data from the holocene rhine river with avulsions occurring when the slope ratio is in the range 3 5 however caution should be practised with respect to uncritical use of these numbers in different settings evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value irrespective of whether critical slope ratios are used as in the present study or superelevation as has been proposed by other investigators

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

experimental test of tectonic controls on three dimensional alluvial facies architecture abstract in an extended series of papers by m leeder j r l allen j s bridge j alexander and others the depositional stacking patterns of channel belt deposits were related to tectonic and other external controls we refer to these models collectively as lab models these models established a basic connection between subsidence channel avulsion and the mesoscale channel belt architecture of an alluvial deposit two fundamental predictions resulted from this work 1 channel belt stacking density and hence connectedness is inversely correlated to temporal vertical changes in sedimentation rate and 2 channel belt stacking density and hence connectedness is directly correlated to lateral changes in sedimentation rate in this paper we make use of the experimental earthscape xes facility at the st anthony falls laboratory to examine the effects of temporally and laterally variable subsidence rates on mesoscale alluvial architecture we then compare the experimental results with the main predictions of the early lab models regarding the predictions of the lab models under conditions of high sediment supply and a highly active alluvial system lateral and downstream variation in subsidence geometry and rate have little effect on the details of alluvial architecture we found that the principal architectural signature of changes in subsidence geometry i e laterally variable subsidence rate is stratal tilting and that channel belts are not attracted to the subsidence maximum we hypothesize that the effect of variable subsidence geometry is felt by the fluvial system only if subsidence proceeds faster than the river can adjust to the formation of a topographic low via the deposition of overbank material in the form of splays and sheet sands our results lend impetus to the development of more complete three dimensional numerical models of mesoscale alluvial architecture that couple architecture to broader allogenic forcing mechanisms

fluvial response to an historic lowstand of the great salt lake utah understanding fluvial adjustments to base level changes benefits the fields of sequence stratigraphy geomorphology and petroleum geology this investigation is a modern case study of the channel dynamics of lee creek and the goggin drain two streams that are part of the jordan river drainage into the endorheic great salt lake of northern utah a lacustrine system that has experienced multiple decadal scale base level changes since 1965 the lake level has fluctuated in elevation more than 6 m transitioning from an historic lowstand 1284 m a s l and in 2009 2010 approaching an historic lowstand this study uses detailed aerial images fieldwork and lidar data to link the modern geomorphology and channel hydraulics to specific variations in sediment transport channel form and avulsion behavior although lee creek and the goggin drain are situated only a few kilometers apart and share similar shore zone gradients substrates and vegetation patterns and have been subjected to the same changes in lake level their channel forms have evolved very differently differences in discharge patterns are likely the most influential factor causing the meandering form of lee creek and the braiding channel of the goggin drain despite the differences in discharge total sediment eroded from the two streams is comparable and can be attributed to similar stream power unit stream width in the two streams although lee creek has not recently been avulsive three major avulsions of the goggin drain have taken place since 1965 two possible styles of avulsion are interpreted an allogenic response to changing base level and an autogenic response dictated by channel morphology and hydraulics despite a wealth of available information avulsions cannot be unequivocally attributed to one style or another caution should be used when attempting to link the complex process of avulsion to causal mechanisms copyright 2011 john wiley sons ltd

relationship between sediment supply and avulsion frequency in braided rivers the interplay between sediment supply s s sedimentation rate s r and the frequency of channel avulsion a f exerts a primary control on alluvial architecture in order to investigate the effect of sediment supply on avulsion frequency four froude scale model experiments of an aggrading braided river were undertaken in which the magnitude of s s was progressively increased over an eightfold range the value of a f increases at a rate slower than the increase in s s contrary to the trend previously reported by bryant falk and paola in their experimental study on alluvial fan dynamics these results suggest that the relationship between a f and s s is dependent upon bed slope and that the response of a f to an increase in s s in unconfined braided rivers may be different than that on steep alluvial fans

floodplain stages in the braided ngaruroro river new zealand this study investigates floodplain deposits in the braided ngaruroro river new zealand to characterize and quantify the spatial extent of deposits at different stages of development using a sequence of aerial photography depositional surfaces were mapped and then assigned a floodplain development stage based on age the discharge causing inundation and field observations of sediment and vegetation characteristics the ensemble of floodplain deposits makes up 40 of the river valley the remainder being the active river bed stabilizing bars and mature floodplains dominate the floodplain ensemble while the existence of stabilizing bars points to a recent shift or fluctuation in controlling conditions the extent and location of mature units suggest an earlier change in hydrologic and sediment supply conditions that limited braidtrain mobility mosaics of different aged deposits range from simple two stage mosaics to more complex arrangements of three and four stages in the rapid evolution from stabilized bars to mature floodplains a large amount of fine grained sediment is stored in the ngaruroro river valley highlighting the possibility of finding extensive fine grained facies in the ancient sedimentary record of braided systems understanding floodplain ensembles and their controls will contribute to effective restoration of braided rivers

mjolnir structure an impact crater in the barents sea a systematic search for impact indicators was conducted on a core of late jurassic early cretaceous sedimentary strata from the vicinity of the proposed mjolnir impact structure barents sea a 0 8 m thick section of the core was found to contain unequivocal indicators of meteoritic impact shocked quartz grains and a strong enrichment in iridium the ejecta bearing strata were discovered only 30 km north northeast of the structure within a stratigraphic interval corresponding to the seismically defined deformation event at mjolnir further study of this unusually well presented impact crater ejecta layer pair may help constrain poorly understood aspects of large magnitude meteorite impacts into the oceans 14 refs 4 figs

depositional character and preservation potential of coarse grained sediments deposited by flood events in hyper arid braided channels in the rift valley arava israel abstract four coarse grained braided channels undergoing rapid deposition in the hyper arid arava rift valley have been studied to determine the dynamics of strata deposition by monitored flood events and to ascertain the sedimentologic character of these event deposits the systems span granule sand to sandy gravel bed material having longitudinal bars with simultaneously increasing grains size slope and bed relief scour and fill were monitored along cross sections using scour chains and repeat surveys while event strata fill were described with reference to sediment calibre type of support grading thickness and location unlike many previous investigations of gravel sedimentology the source of information for this study involves not only stratigraphic description but also observation of the concomitant dynamics of the channel bed as expressed by the extent of event deposition the texture of the deposited sediment generally coarsens in sympathy with the surface bed texture of the studied channels the thickness of event deposited sediments increases as the magnitude of flow events increases the stratigraphic record is characterized by 1 sub parallel stratification 2 no grading 3 clast supported gravel infilled with sand granule matrices and 4 rapid sedimentation rates this character is hypothesized to mirror the elevated bedload fluxes in this hyper arid fluvial setting lacking densely vegetated banks the proximal braidplain of this arid environment often contains no extensive fine textured floodplain successions the depth of fill as well as the extent of strata obliteration increase with flood magnitude explaining the increased preservation potential of event strata with increase in flood magnitude it also explains why most event strata including large ones are only partially preserved

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

major circular structure beneath southern lake huron defined from potential field data newly compiled and merged canadian and united states potential field data for the area of southern lake huron reveal a large circular structure that has a diameter that may extend to 100 km the aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43 14 21 n long 82 19 88 w surrounded by a principal ring with a diameter of 50 km and indications of conformable arcuate structure extending to a diameter of about 100 km the prevailing northeast striking aeromagnetic signature of the grenville front and adjacent grenville front tectonic zone is interrupted and may be overprinted by the circular feature the bouguer gravity field shows a strong positive anomaly that extends southeast from the grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field the gravity field illustrates the circular structure with an annular low and central high as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature there is no strong evidence of the feature in the bathymetry data of southern lake huron although the general coastline of the southern end of lake huron closely follows the outline of the main circular feature the interruption of the grenville trends the clear regular circular signature of the feature and the interpreted magnetic source depths at the precambrian surface suggest an event possibly as young as early paleozoic in the absence of basement samples the origin of the feature is unconfirmed however the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the canadian shield