geomorphological evidence for jokulhlaups from kverkfjoll volcano iceland abstract jokulhlaups glacial outburst floods are known to have drained along the jokulsa a fjollum river in iceland during the holocene however little is known about their number age source and flow characteristics this paper provides detailed geomorphological evidence for jokulhlaups that have routed from the kverkfjoll ice margin and hence into the jokulsa a fjollum erosional evidence of jokulhlaups from kverkfjoll includes gorges cataracts spillways subaerial lava steps and valley wide scoured surfaces depositional evidence includes wash limits boulder bars cataract fill mounds terraces slackwater deposits and outwash fans some of these landforms have been documented previously in association with jokulhlaups however subaerial lava surfaces that have been scoured of the upper clinker gorges within pillow hyaloclastite ridges gorges between pillow hyaloclastite ridges and subaerial lava flows subaerial lava lobe steps cataract fill mounds and boulder run ups are previously undocumented in the literature these landforms may therefore be diagnostic of jokulhlaups within an active volcanic rifting landscape the nature and spatial distribution of these landforms and their stratigraphic association with other landforms suggest that there have been at least two jokulhlaups through kverkfjallarani the biskupsfell eruption occurred between these two jokulhlaups kverkfjallarani jokulhlaups were very strongly influenced by topography geology and interevent processes that together determined the quantity and nature of sediment availability such controls have resulted in jokulhlaups that were probably fluidal turbulent and supercritical over large areas of the anastomosing channel bed kverkfjallarani jokulhlaups would have had highly variable hydraulic properties both spatially and temporally the knowledge of flow characteristics that can be gained from jokulhlaup impacts has implications for recognising jokulhlaups in the rock record and for hazard analysis and mitigation within similar landscapes and upon other glaciated volcanoes

a redox stratified ocean 3 2 billion years ago abstract before the great oxidation event goe 2 4 2 2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen producing microorganisms recently however it has been proposed that transient oxygenation of shallow seawater occurred between 2 8 and 3 0 billion years ago here we present a novel combination of stable fe and radiogenic u th pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3 2 ga based on analysis of the manzimnyama banded iron formation bif fig tree group south africa this unit is exceptional in that proximal shallow water and distal deep water facies are preserved when compared to the distal deep water facies the proximal samples show elevated u concentrations and moderately positive 56 fe values indicating vertical stratification in dissolved oxygen contents confirmation of oxidizing conditions using u abundances is robustly constrained using samples that have been closed to u and pb mobility using u th pb geochronology although redox sensitive elements have been commonly used in ancient rocks to infer redox conditions post depositional element mobility has been rarely tested and u th pb geochronology can constrain open or closed system behavior the u abundances and 56 fe values of the manzimnyama bif suggest the proximal shallow water samples record precipitation under stronger oxidizing conditions compared to the distal deeper water facies which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time this work therefore documents the oldest known preserved marine redox gradient in the rock record the relative enrichment of o 2 in the upper water column is likely due to the existence of oxygen producing microorganisms such as cyanobacteria these results provide a new approach for identifying free oxygen in earth s ancient oceans including confirming the age of redox proxies and indicate that cyanobacteria evolved prior to 3 2 ga

reappraisal of hydrocarbon biomarkers in archean rocks hopanes and steranes found in archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes but the syngeneity of these hydrocarbon biomarkers is controversial to resolve this debate we performed a multilaboratory study of new cores from the pilbara craton australia that were drilled and sampled using unprecedented hydrocarbon clean protocols hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit but when they were detectable they had total hopane 37 9 pg per gram of rock and total sterane 32 9 pg per gram of rock concentrations comparable to those measured in blanks and negative control samples in contrast hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389 5 pg per gram of rock and 1 039 pg per gram of rock respectively polycyclic aromatic hydrocarbons and diamondoids which exceed blank concentrations exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1 000 ng per gram of rock in hydropyrolysates from the ultraclean cores these results demonstrate that previously studied archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons therefore existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by 2 7 billion years ago although suitable proterozoic rocks exist no currently known archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation so future exploration for archean biomarkers should screen for rocks with milder thermal histories

palaeoflood estimates of pleistocene coarse grained river terrace landforms rio almanzora se spain abstract a series of palaeoflood estimation techniques are applied to an inset sequence of middle to late pleistocene river terrace landforms associated with the rio almanzora se spain the study area is a 7 km long transverse reach where 4 terrace levels level 1 highest and oldest level 4 lowest and youngest document some 200 m of incision across an uplifted basement block during the pleistocene for the broader region river terrace aggradation is attributed to increased sediment supply during glacial to interglacial transitions level 1 marine oxygen isotope stages mois 12 11 l2 10 9 l3 8 7 and l4 6 5 within the transverse reach terrace levels 2 3 and 4 are characterised by coarse boulder rich gravels d max 2 5 m organised into 3 5 m high cross beds level 4 is characterised by a series of km scale abandoned meander loops with levels 2 and 3 showing evidence for similar degraded high sinuosity forms the fluvial setting is interpreted as a braided river environment with lateral and longitudinal gravel barforms using field derived sedimentary boulder size etc and morphological width slope etc features flow competence and regime based methods were applied palaeoflood estimates varied from 40 to 2859 m 3 s with mean maximum discharges showing an increase through time l2 278 l3 413 l4 1361 m 3 s palaeoflood estimates were compared to flood events associated with the ephemeral modern rio almanzora catchment 2500 km 2 to establish whether palaeoflood values were reasonable estimations a 46 year 1963 2009 modern flood gauge record shows infrequent flood events with discharges typically 3 s a rare flood event in 1973 5680 m 3 s provides an upper value for comparison with the palaeoflood estimates although estimates appear reasonable they should be considered as minimum values due to 1 some disparities between flow depths derived from the palaeoflood equations 0 7 4 m and field based evidence 3 5 m high cross beds 2 issues related to field sampling of boulders from sections and 3 uncertainties as to the relationship between flood hydrology and transportation of boulders formation of 3 5 m high cross beds and the position of the terrace remnants in the river valley the palaeoflood estimates suggest that relatively large discharges occurred during valley floor aggradation in the glacial interglacial transitions periods characterised by low vegetation cover and high sediment yield discharge increases through time can be explained by the long term drainage evolution of the rio almanzora characterised by an expanding catchment area linked to river capture driven by regional uplift patterns the 3 5 times larger discharge estimates recorded in terrace level 4 are coeval with large and rapid changes in precipitation or elevated seasonal discharge linked to the mois 6 5 glacial interglacial climate transition overall within tectonically active regions affected by river capture fluvial systems could be responding to loss or expansion of catchment areas long term palaeoflood records from inset flights of river terraces could store such signals and may mask complicate any climate palaeoflood relationships

jokulhlaup deposits in proglacial areas abstract this paper discusses the main causes and characteristics of jokulhlaup glacier burst floods and explores the extent to which they generate depositional landform and sediment assemblages that are distinct from those of normal braided river outwash type i outwash two main jokulhlaup outwash environments are identified type ii outwash produced by sudden drainage of ice dammed lakes and type iii associated with drainage during subglacial geothermal activity and distinguished by deposits resulting from high sediment concentrations and hyperconcentrated flows in fluid flows especially ones yielding type ii outwash the most common deposits are large scale expansion bars and locally eddy and pendant bars and mega ripples or dunes both forms normally composed of large scale gravel cobble cross bedding often capped by an imbricated boulder lag a type b2 lithofacies sequence the armour is absent only where runoff decreased too rapidly to allow surface winnowing other jokulhlaup facies include extensive boulder beds type c inverse normally graded cobble beds type ds ice proximal debris flow deposits and deformed bedding containing diamicton clasts types g and h and slack water sediments type a type iii outwash is dominated by massive homogeneous flood surge granules underlain by pre surge gravels and capped by post surge fluid bedforms reflecting deposition during both the rising and falling limbs of the flood hydrograph type e4 the paper demonstrates that jokulhlaups do generate distinctive assemblages of depositional landforms and sediments and concludes with a model of the dominant lithofacies sequences and associated landforms in proglacial environments subject to jokulhlaup drainage

the lemniscate loop comparisons with the shapes of streamlined landforms the lemniscate loop is a streamlined geometric form which has a simple algebraic equation containing a parameter whose value governs its overall length to width dimensions the lemniscate has been employed in geomorphology including comparisons with glacial drumlins and erosional streamlined islands in the channeled scabland of eastern washington and within the channels on mars these past comparisons have mainly been visual the present study involves more detailed quantitative comparisons between the lemniscate loop symmetrical airfoils drumlins and various islands formed by water flows the analyses focus primarily on the length to width ratios of these landscape features and their consistency with the analytical lemniscate shape many drumlins are found to be similar to the streamlined lemniscate but an equal number are more closely approximated by an elliptical geometric form the streamlined islands in rivers in the channeled scabland and in the martian channels all have nearly the same ave

gradient irregularity in the herbert gorge of northeastern australia the seventy kilometre long herbert gorge of northeastern australia preserves a record of past floods in slackwater deposits and palaeostage indicators step backwater modelling of water surface profiles indicates that discharges ranging from 11000 to 17000 m3s 1 have occurred six times in the gorge during the last 900 years these flood reconstructions provide insight into the role of extreme flows in shaping bedrock channel morphology in particular the hydraulics of extreme flows can be related to boulder transport and to the location of large boulder bars large boulder bars occur throughout the herbert gorge being best developed at loci of stream power minima along the inside of bends at tributary junctions and at obstructions in the channel caused by bedrock highs only the flows exceeding approximately 8000 m3 s 1 are competent to transport the boulders which constitute the bars in the straight channel reaches the boulder accumulations and bedrock highs have a fairly regular spacing which appears to be independent of lithologic or structural controls the bars provide an efficient means of energy dissipation and they are interpreted as a result of the inherent high turbulence of flow in a steep channel the regular spacing of the bars and their correspondence with the hydraulics of large flows suggest that the bars and associated bedrock highs may represent a self regulating mechanism akin to the pool riffle sequence of alluvial channels it may therefore be appropriate to view bedrock channels as deformable on the timescale of extreme discharges

middle late pleistocene landscape evolution of the dover strait inferred from buried and submerged erosional landforms abstract prominent landforms either buried or preserved at the seafloor provide important constraints on the processes that led to the opening and present day configuration of the dover strait here we extend previous investigations on two distinct landform features the fosse dangeard and lobourg channel to better understand the poly phase history of their formation and inferences for the opening and pleistocene evolution of the dover strait the fosse dangeard consist of several interconnected palaeo depressions their morphology and spatial distribution are interpreted to be the result of plunge pool erosion generated at the base of north eastward retreating waterfalls their infills comprise internal erosional surfaces that provide evidence for the occurrence of several erosional episodes following their initial incision the lobourg channel comprises various sets of erosional features attesting to the occurrence of several phases of intense fluvial and or flood erosion the last one of these carved a prominent inner channel which truncates the uppermost infill of the fosse dangeard the morphology of the lobourg inner channel and the erosional features associated with its incision strongly resemble landforms found in megaflood eroded terrains indicating that this valley was likely eroded by one or several megafloods our study therefore corroborates the existence of waterfalls in the dover strait at least once during the pleistocene epoch it also provides evidence of the occurrence of multiple episodes of fluvial and flood erosion including megafloods finally this study allows us to establish a relative chronology of the erosional depositional episodes that resulted in the present day morphology of this region

cellularly preserved microbial fossils from 3 4 ga deposits of south africa a testimony of early appearance of oxygenic life abstract fossil record of earliest earth s life is scant and restricted to simple kerogenous filaments spheres and lenticular microfossils which origin and taxonomic affiliation are still ambiguous here we report clusters of cell like bodies found in massive and weakly laminated black cherts of the 3 4 ga kromberg formation onverwacht group barberton greenstone belt south africa known earlier for benthic microbial mats and microfossils morphological traits and mineralization of the kromberg microfossils match those known from modern and fossil cyanobacteria micro raman and sem eds analyses showed that the cell like bodies fossilized mostly due to early mineralization with al silicates enclosing dispersed carbonaceous kerogenous matter derived from their thermally altered cell remains although in widespread opinion the early archean life is predominantly represented by benthic microbial mats the random non laminated distribution of the studied cyanobacteria like microfossils in the kromberg cherts is suggestive for probably benthic planktonic life cycle of these microbiota the paper also discusses how the morphological similarity of the kromberg fm cell like microfossils to geologically much younger and extant coccoidal cyanobacteria may influence the debate concerning the time of origin of earth s oxic atmosphere

the tertiary lavas and sediments of northwest rhum inner hebrides several small outliers of tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from torridonian sediments and tertiary granophyre erosion continued during the period of sediment and lava accumulation four formations are recognized these are in order of increasing age the orval formation hawaiite and basaltic hawaiite lavas the guirdil formation icelandite lavas interbedded conglomerates the upper fionchra formation tholeiitic basaltic andesite lavas hyaloclastite deposits basal conglomerate and the lower fionchra formation alkali and transitional basalt basaltic hawaiite and hawaiite lava flows basal conglomerate each is separated by an erosional interval clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses torridonian sediments igneous rocks derived from the rhum tertiary central complex including allivalites and tertiary lavas of local origin but also including in the oldest conglomerates tholeiitic basalts not now preserved on or near rhum prior to and during lava and sediment accumulation erosion on rhum had cut down to a level similar to that of the present day although not to the extent that high grade thermally altered rocks which are a marked feature of the central complex were being eroded in any quantity a sequence of east west trending valleys possibly initiated on the line of the earlier main ring fault drained the area of the central complex which then as now must have been high ground small lakes occasionally formed in the valleys allowing the accumulation of fine grained sediment with plant remains and promoting the formation of hyaloclastite deposits when buried by later flows no source for any of the lava formations is preserved on rhum they are thought to have come from feeders north of rhum possibly near canna and to have ponded against the hills and valleys near and in the central complex the oldest tholeiitic lavas not now found in situ were followed by alkali and transitional flows compositionally similar to the skye main lava series but characteristically feldsparphyric the most mafic also contain phenocrysts of magnesian olivine with included cr al rich spinels and aluminous spinel both the early alkalic transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures apart from a few n s to nw se trending basalt dykes the lava formations represent the youngest tertiary igneous event on rhum

paleohydraulic reconstruction of the largest glacial lake missoula draining s glacial lake missoula glm was the source of earth s largest known peak flood discharges onto the channeled scabland northwestern united states the original studies of glm were based on geomorphological mapping of lake sediments and shorelines more recently studies have emphasized the computational fluid dynamics of downstream megaflood routing and characteristics in the channeled scabland and adjacent areas however actual details of the lake draining processes glm have received little scientific attention in this paper we employ two dimensional hydraulic modelling to understand the paleoflow conditions for different scenarios of draining glm at its highest stands values are calculated for three glm drainage hydrographs 2 6 13 17 10 6 m 3 s 1 that previous studies had estimated for outlet flows consistent with regional mapping and with the original interpretation of fluvial geomorphology by other studies the modelling shows that lake silt sequences in the gorges of glm must postdate the most highly energetic outburst events from the lake this is because the flows generated in the lake by megaflood outflows are sufficiently energetic to erode any accumulated silt deposits in contrast the gravels underlying the silts in these zones include boulder sized clasts largescale cross stratification and 70e100 m high bars forms all of which indicate very high energy flood flow conditions the modelling also shows that the paleohydraulic conditions which developed in the lake basin during the largest megaflood outflows would be capable of accounting for the observed bedrock scour at rainbow lake pass various high eddy deposits eddy bars of flood gravel in gorges and the giant ripple marks subaqueous gravel dunes that were classically described in the 1940s previous studies interpreted the glacial lake silt rhythmites and related deposits such as the ninemile section as evidence for numerous fillings and drainings of glm our modelling suggests that these inferred events must have been relatively low in energy and magnitude relative to the earlier outburst s that emplaced the flood gravels subaqueous gravel dunes and scabland like erosion surfaces

geomorphic effectiveness sandur development and the pattern of landscape response during jokulhlaups skeiðararsandur southeastern iceland by contrast with other historical outburst floods on skeitararsandur the 1996 jokulhlaup was unprecedented in its magnitude and duration attaining a peak discharge of f53 000 m 3 s in 17 h using a combination of field sampling and remote sensing techniques landsat tm sar interferometry airphotos and laser altimetry we document the sandur wide geomorphic impacts of this event these impacts varied widely across the skeitararsandur and cannot be singularly attributed to jokulhlaup magnitude because pre jokulhlaup glacial dynamics and the extant setting largely conditioned the spatial pattern type and magnitude of these impacts topographic lowering and asymmetric retreat of the ice front during the late twentieth century has decoupled the ice sheet from the moraine sandur complex along the central and western sandur this glacial control in combination with the convex topography of the proximal sandur promoted a shift from a primarily diffuse source braided outwash system to a more point sourced channelized discharge of water and sediment deposition dominated within the proglacial depression with approximately 3 8 10 7 m 3 of sediment and along channel systems that remained connected to subglacial sediment supplies this shift to a laterally dissimilar channelized routing system creates a more varied depositional pattern that is not explicitly controlled by the concave longitudinal profile down sandur laterally contiguous units therefore may vary greatly in age and sediment character suggesting that current facies models inadequately characterize sediment transfers when the ice front is decoupled from its sandur water was routed onto the sandur in a highly organized fashion and this jokulhlaup generated major geomorphic changes including sandur incision in normally aggradational distal settings and eradication of proximal glacial landforms dating to fa d 1892 d 2002 elsevier science b v all rights reserved

the problem of deep carbon an archean paradox black carbon rich chert dikes are common features of early archean terranes and are especially prominent on the pilbara craton of western australia the dikes cut across the local stratigraphy before truncating abruptly at specific stratigraphic horizons where they can be seen to interfinger with the stratigraphic succession these uniquely archean dike complexes are enigmatic and raise significant questions about the source of the carbon they contain is it biogenic and therefore indicative of the early biosphere or abiotic and perhaps indicative of prebiotic conditions that led to the appearance of the first life on earth the strelley pool chert which is exposed on the pilbara craton of western australia forms part of the warrawoona group and rests unconformably on older ultramafic volcanics the formation which is generally less than 30 m thick consists of a relatively consistent lithologic succession found over a wide area locally a basal transgressive sandstone is overlain by a sequence of carbonate barite black and white laminated chert and grey green chert which is locally stained blue green by weathered fuchsite the formation is intersected by normal faults about which the depositional successions thickens and thins the mineralogy and geochemistry including stable isotope data of the underlying dike complex closely parallels that of the lithologies preserved in the strelley pool chert similarly the depositional sequence of minerals in voids within the dike complex parallels that of the depositional sequence above this new data from the strelley pool chert and its associated dike complex show that the black chert dikes are an integral part of a hydrothermal mineralogical complex the complex accumulated around vents fed by growth fault controlled dikes that extended to a depth of more than 2 km below the archean paleo seafloor the mineralogy of the sedimentary succession reflects deposition during the declining temperature regime of an evolving hydrothermal system driven by an intruding granitoid complex below the fault system and associated hydrothermal complex evolved in the tensional environment developed during doming above the intermittently intruding granitoid complex that forms the framework of the pilbara protocontinent carbonaceous compounds contained within the black chert dikes could have been generated abiotically by a process similar to fischer tropsch synthesis at elevated temperatures 200 300 c and pressures 500 bar at a paleo depth of 2 km within the hydrothermal system the primordial medium in which life first evolved may thus have been a silica rich slurry with a low ph and a high co 2 content which contained basic abiotic organic building blocks fundamental to the origins of life the abiotic organic output from these hydrothermal systems may overwhelm any early biospheric geochemical signal and may therefore be the most difficult environment in which to recognise the early biospheric record 2005 elsevier b v all rights reserved

holocene uplift and rapid fluvial erosion of iceland a record of post glacial landscape evolution abstract in actively deforming regions fluvial systems are strongly regulated by uplift river geometries record histories of vertical motions that can be used to examine the driving forces generating topographic relief iceland s rapidly evolving landscapes provide an opportunity to disentangle histories of uplift generated by postglacial rebound volcanism dynamic support and plate spreading broad knickzones observed along iceland s large rivers and its powerful waterfalls and deep canyons hint that regional processes have generated significant relief we combine high resolution drone photogrammetry and cosmogenic 3he dating of fluvial terraces to measure the erosional history of one of iceland s largest knickzones jokulsarglufur in the northeast part of the island progressive younging of terraces indicates knickpoint propagation rates of up to 70 cm a 1 during the last 8 ka knickpoint velocities appear to be controlled partly by toppling of basalt columns these rates were used to calibrate a model that inverts iceland s drainage networks for uplift rate histories calculated uplift and isostatic calculations indicate that rifting sub plate support and isostatic adjustment resulted in tens to hundreds of meters of regional holocene uplift our results suggest regional uplift and fluvial erosion can rapidly generate hundreds of meters of relief in post glacial landscapes

biogenic origin for earth s oldest putative microfossils carbonaceous microbe like features preserved within a local chert unit of the 3 5 ga old apex basalt in western australia may represent some of the oldest evidence of life on earth however the biogenicity of these putative microfossils has been called into question primarily because the sample collection locality is a black carbon rich brecciated chert dike representing an archean submarine hydrothermal spring suggesting a formation via an abiotic organic synthesis mechanism here we describe the macromolecular hydrocarbon structure carbon bonding functional group chemistry and biotic element abundance of carbonaceous matter associated with these filamentous features these characteristics are similar to those of biogenic kerogen from the ca 1 9 ga old gunflint formation although an abiotic origin cannot be entirely ruled out it is unlikely that known abiotic synthesis mechanisms could recreate both the structural and compositional complexity of this ancient carbonaceous matter thus we find that a biogenic origin for this material is more likely implying that the apex microbe like features represent authentic biogenic organic matter

fill and spill of giant lakes in the eastern valles marineris region of mars the existence of hesperian age 3 7 3 4 ga surface water bodies on mars is a contentious issue often conflicting with favored climate models extensive lakes are proposed to have filled parts of valles marineris during this period yet evidence for their presence and temporal continuity is poorly constrained here we report geomorphic and chronologic evidence for the initiation and demise of a voluminous lake system within the basins of eastern valles marineris we find that independent kilometer deep lakes were present here well after the wetter global climate optimum that characterized the previous noachian epoch 4 1 3 7 ga relative and impact crater chronologies of flood channels emerging from lake basins indicate relatively late lake spillover in the early amazonian ca 3 0 ga drawdown of the lake and cessation of interbasin sedimentation may be recorded by a similar early amazonian ca 3 1 ga crater retention age on the surface of capri mensa a 4 km tall sulfate bearing interior layered deposit the topography data demonstrate that incision of the bedrock barriers between the basins during spillover was driven by a dramatic local base level difference between the lake surface and downstream basin floors we postulate that the lake spillover process created an integrated drainage routing system between a voluminous equatorial water supply and the northern plains basin

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

architectural analysis of a volcaniclastic jokulhlaup deposit southern iceland sedimentary evidence for supercritical flow the 1918 eruption of the glacially capped katla volcano southern iceland generated a violent jokulhlaup or glacial outburst flood inundating a large area of m rdalssandur the proglacial outwash plain where it deposited ca 1 km3 of volcaniclastic sediment the character of the 1918 jokulhlaup is contentious having been variously categorized as a turbulent water flow a hyperconcentrated flow or as a debris flow based on localized outcrop analysis in this study outcrop based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi stationary antidunes and down current migrating antidunes and from regular based bedsets associated with transient chute and pool bedforms all of which are associated with turbulent transcritical to supercritical water flow conditions antidune wavelengths range from 24 to 96 m corresponding to flow velocities of 6 to 12 m sec 1 and average flow depths of 5 to 19 m this range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts architectural analysis of the 1918 jokulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jokulhlaup that could not have been achieved through localized outcrop analysis the observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions the physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations regarding modes of deposition and formative flow hydraulics are to be made

the rise of atmospheric oxygen when did atmospheric oxygen rise to near present concentrations and what led to this rise as kasting explains in his perspective recent results have provided stronger than ever evidence that the switch from an anoxic to an oxic atmosphere occurred around 2300 millions years ago but researchers remain divided over the mechanism catling et al resurrect a previously proposed mechanism involving hydrogen escape to space with a couple of new twists whether their mechanism is viable depends on the oxidation state of the continental and oceanic crust at this crucial time of earth9s evolution

palaeontology communication arising thermal alteration of the earth s oldest fossils microscopic carbonaceous structures found in ancient rocks could provide clues to early life on earth if they turn out to be genuine fossil microorganisms here we show that thermal alteration of microbial remains embedded in a mineral matrix may significantly change their original morphology and produce structures that resemble those of what are claimed to be the earth s oldest fossils1 these observations may shed light on the controversy2 3 that surrounds these microfossils from the 3 465 myr old apex chert of the early archaean warrawoona group in northwestern australia

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

discussion of field evidence and hydraulic modeling of a large holocene jokulhlaup at jokulsa a fjollum channel iceland by douglas howard sheryl luzzadder beach and timothy beach 2012 this paper discusses howard et al 2012 who reconstruct the peak discharge of a glacial outburst flood or jokulhlaup for part of the jokulsa a fjollum in north central iceland they propose that this flood was the largest on earth weconsider that the magnitude of the jokulhlaup proposed by howard et al 2012 warrants much more robust field evidence and demands more carefully parameterised hydraulic modelling for these reasons we firstly i present their study in the context of previous research ii highlight issues with attributing landforms and sediments to jokulhlaups and iii consider uncertainty regarding the timing and magnitude of jokulhlaups along the jokulsa a fjollum we argue herein that whilst a range of landforms and sediments that are attributable to jokulhlaups can be observed along the jokulsa a fjollum these are not necessarily diagnostic of jokulhlaups secondly we critically discuss iv the major underlying assumptions of their study and v their calculations and subsequent interpretations these assessments lead us to consider that the proposal by howard et al 2012 of the largest flood on earth is highly unrealistic especially when due consideration is given to a possible source area and a trigger mechanism

convergent evolution of abrading flow obstacles insights from analogue modelling of fluvial bedrock abrasion by coarse bedload upstream facing convex surfaces ufcs are formed by bedload abrasion in bedrock rivers and indicate the recent significant action of bedload abrasion in causing channel incision beyond this little is known of the dynamics of ufcs and the effect of substrate and bedload properties on rates and distribution of bedload abrasion for these bed roughness elements grain size populations from 1 to 8 cm b axis in 1 or 2 cm bin widths were used to bombard preshaped marble and limestone targets bolted to the base of an annular flume the control of initial shape and lithology of the target and the erodent grain size and lithology were investigated by monitoring the evolution of the target form using laser scanning at predefined time intervals eleven experiment suites were carried out containing three initial target shapes constructed from two lithologies four bedload erodent grain sizes of either granodiorites or limestone or clear water flow all 10 targets abraded by bedload evolved from their initial form into a steady state time invariant form producing ufcss steady state forms were closely similar for all targets despite different initial conditions bedload grain size has a strong control on this equilibrium form related to the transit path of the grains when moving over the target whilst initial target form has only a weak control steady state morphology is achieved more rapidly with harder erodent bedload particles and or softer targets upstream facing convex surface stoss sides were characterised by a brighter sugary granular appearance on the rock forming grain scale increasing erodent grain size for a fixed bedload mass increased the bulk abrasion rate at fixed flow speed and discharge no detectable erosion occurred for a limestone block in clear water flows under the same flow conditions indicating solution and cavitation were insignificant mechanisms of erosion in this study during the experiment suites suspended load abrasion was also found to be an insignificant mechanism in eroding lee or lateral sides in natural settings the initial formation of ufcss can occur for homogenous and or jointed substrates in close association with plucking or alternatively for heterogeneous substrates by variation in substrate erodibility

glacial lake vitim a 3000 km3 outburst flood from siberia to the arctic ocean abstract a prominent lake formed when glaciers descending from the kodar range blocked the river vitim in central transbaikalia siberia glacial lake vitim evidenced by palaeoshorelines and deltas covered 23 500 km2 and held a volume of 3000 km3 we infer that a large canyon in the area of the postulated ice dam served as a spillway during an outburst flood that drained through the rivers vitim and lena into the arctic ocean the inferred outburst flood of a magnitude comparable to the largest known floods on earth possibly explains a freshwater spike at 13 cal ka bp inferred from arctic ocean sediments

evidence for holocene megafloods down the tsangpo river gorge southeastern tibet lacustrineandalluvialterracesandsedimentsrecordtheextentofatleasttwoholoceneglaciallydammedlakesimmediatelyupstreamofthe tsangpo rivergorge at the eastern syntaxis of the himalaya the larger lake covered 2835 km 2 with a maximum depth of 680 m and contained an estimated 832 km 3 of water the smaller lake contained an estimated 80 km 3 of water radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 f 40 and 9870 f 50 14 c yr b p for the higher set of lake terraces and 1220 f 40 and 1660 f 40 14 cy r b p forsedimentsfromthelowerterraces catastrophicfailureoftheglacialdamsthatimpoundedthelakeswouldhavereleasedoutburstfloods down the gorge of the tsangpo river with estimated peak discharges of up to 1 to 5 10 6 m 3 s 1 the erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post glacial megafloods down the tsangpo river were likely among the most erosive events in recent earth history d 2004 university of washington all rights reserved

the tuareg shield terranes revisited and extended towards the northern gondwana margin magnetic and gravimetric constraints abstract the trans saharan belt is one of the most important orogenic systems constitutive of the pan african cycle which at the end of the neoproterozoic led to the formation of the gondwana supercontinent it is marked by the opening and closing of oceanic domains collision of continental blocks and the deformation of thick synorogenic sedimentary basins it extends from north to south over a distance of 3000 km in africa including the nigerian shield and the tuareg shield as well as their counterparts beneath the phanerozoic oil rich north and south saharan sedimentary basins in this study we take advantage of potential field methods magnetism and gravity to analyze the crustal scale structures of the tuareg shield terranes and to track these pan african structures below the sedimentary basins offering a new 1000 km extent the map interpretations are based on the classical potential field transforms and two dimensional forward modeling we have identified geophysical units and first order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures in particular we are able to highlight curved terminations which in the trans saharan context have been still poorly documented we provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the tuareg shield up to the atlas belt these units highlight the contrasted rheological behavior of the tuareg tectonostratigraphic terranes during i the northerly pan african tectonic escape characteristic of the trans saharan belt and ii the north sahara basin development especially during intraplate reworking tied to the variscan event the discovery of a relatively rigid e w oriented unit to the south of the atlas system and on which the escaping pan african terranes were blocked offers a new perspective on the structural framework of the north gondwana margin it will help to understand how occurred the rendezvous of the n s oriented pan african terranes and the e w oriented cadomian peri gondwanan terranes

mg silica precipitation in fossilization mechanisms of sand tufa endolithic microbial community mono lake california abstract the detection of mg silica associated with endolithic biofilms of cyanobacteria and bacterial communities living in lake shore sand tufa formations in mono lake california has been observed using scanning electron microscopy with backscattered electron imaging sem bse and energy dispersive x ray spectroscopy eds mg accumulated in cell walls of living cyanobacteria is interpreted to promote mg silicification with structural preservation while non cyanobacterial prokaryotes are not fossilized however extracellular polymeric substances around the living communities were prominent as nucleation sites for authigenic mineral precipitation a mg silicate served to stabilize the biofilm remains further diagenesis of these textures resulted in occlusion of the cytoplasm pore spaces by carbonate precipitates this precipitate damaged the microfossil structures but the mg silica fabric was still present the paleontological significance of mg silica precipitates as a possible biosignature of the former presence of endolithic biofilms is addressed

a review of glacier outburst floods in iceland and greenland with a megafloods perspective the very largest glacier outburst floods have been termed megafloods given their volume and peak discharge that definition might be revised because those floods have become understood due to their distinctive and pervasive landscape impacts at least three floods in iceland can be categorized as megafloods since they produced impressive bedrock canyons and giant fluvially transported boulders glacier lake outburst floods glofs in greenland might also have megaflood type attributes given the enormous lake volumes drained we therefore here present the first review of glacier outburst floods in greenland sites isvand russell glacier kuannersuit glacier lake tininnilik two unnamed lakes near amitsulooq ice cap and iluliallup tasersua base camp lake lake hullett qorlortorssup tasia imaersartoq tordenso north midternaes and an outlet glacier of the a p olsen ice cap overall megaflood type landscape impacts in iceland tend to be best preserved and most easily identified inland although there has also been extensive offshore sedimentation there are very few reported impacts of glacier outburst floods in greenland in greenland ice dam failure causes frequent flooding compared to the volcanically triggered floods in iceland and this combined with the proximity of the greenland glacier lakes to the coast means that most proglacial channels in greenland are flood hardened and most landscape impact is likely to be offshore in estuaries and fjords future floods with megaflood type attributes will occur in iceland induced by volcanic activity in greenland they will be induced by extreme weather and rapid ice melt any potential landscape impact of these future floods remains open to question

application of 2d hydrodynamic modelling to high magnitude outburst floods an example from kverkfjoll iceland abstract high magnitude outburst floods cause rapid landscape change and are a hazard to life property and infrastructure however high magnitude fluvial processes and mechanisms of erosion transport and deposition are very poorly understood and remain largely unquantified this poor understanding is partly because of the inherent difficulty of directly measuring high magnitude outburst floods but also because of the limitations and assumptions of 1d models and other palaeohydrological methods which reconstructions of high magnitude floods have to date relied upon this study therefore applies a 2d hydrodynamic model sobek to reconstructing a high magnitude outburst flood this method offers the first calculations of high magnitude fluvial flow characteristics within an anastomosing network of simultaneously inundated channels including sheet or unconfined flow simultaneous channel and sheet flow flow around islands hydraulic jumps multi directional flow including backwater areas hydraulic ponding and multiple points of flood initiation 2d modelling of outburst floods clearly has the potential to revolutionise understanding of high magnitude spatial and temporal hydraulics and high magnitude flow phenomena geomorphological and sedimentological processes and hence rapid fluvial landscape change this potential for new understanding is because of the now wide availability of high resolution dem data for large and often inaccessible areas and the availability of remotely sensed data that can parameterise outburst flood sources such as glacial lakes for example additionally hydraulic models and computing power are now sufficient to cope with large 5 000 000 grid cells areas of inundation and large 100 000 m3 s 1 peak discharges

the role of multiple glacier outburst floods in proglacial landscape evolution the 2010 eyjafjallajokull eruption iceland the 2010 eyjafjallajokull eruption in iceland provided a unique opportunity to quantify the evolution of proglacial geomorphology during a series of volcanogenic jokulhlaups glacial outburst floods 140 events time lapse imagery and repeat terrestrial laser scans before and directly after the eruption show that the jokulhlaup of 14 april 2010 composed 61 of the 57 106 m3 total discharge of the combined events and had the highest peak discharge for the two main flood events but only deposited 18 of the total volume of sediment in front of gigjokull glacier the majority of sediments 67 of a total volume of 17 12 106 m3 were deposited by the 15 april 2010 jokulhlaup and this was followed by extensive reworking by a series of smaller jokulhlaups over the following 29 days that deposited 15 of the total sediment the geomorphological and sedimentary signatures of the two largest jokulhlaups associated with the onset of the eruption have either been reworked by later floods or are buried by later flood deposits consequently the ice proximal posteruption landscape cannot be used to reconstruct the characteristics or magnitudes of either of the two largest jokulhlaups the findings support a complex response model in which peak discharge and the bulk of the sediment transported is decoupled by changing routing mechanisms and water sediment ratios during the eruption

origin and large scale erosional features of glacial lake spillways in the northern great plains deglaciation of the northern great plains produced two basic types of fluvial systems 1 depositional regimes produced by the action of glacial melt water that formed outwash plains and 2 highly erosive systems produced by the sudden rapid drainage of glacial lakes that formed spillways spillways are huge deeply incised trenches that display all of the characteristics of fluvial channels glacial lakes drained in weeks or months and discharges of 10 5 m 3 s were attained at bankfull stage seven spillway systems studied were formed as a consequence of the sudden rapid draining of glacial lakes many other large valleys in the mid continent area and other glaciated regions were probably formed by the same process spillways consist of deeply incised inner channels commonly flanked by broad scoured outer zones initial stages of flow produced shallow anastomosing channels continued erosion produced longitudinal grooves streamlined erosional residuals and surficial boulder lag deposits inner channels developed by the enlargement of one or more centrally located longitudinal grooves the development of erosional residuals was initiated by the incision of shallow anastomosing channels the interchannel areas were progressively modified from irregular quadrilateral or elliptical shapes to an equilibrium minimum drag lemniscate shape as glacial lake drainage proceeded correlations between length and width length and area width and area and length width ratio and k a shape parameter of the lemniscate loop are very high for all erosional residuals studied although only 20 of the residuals display an equilibrium lemniscate shape stable relationships between length width and area of erosional residuals are therefore established very early and are maintained throughout the erosional modification to the lemniscate shape the only shape factor tested which can be used to quantitatively differentiate lemniscate from nonlemniscate erosional residuals is the position of maximum width as measured by the ratio of the length from the lee to the point of maximum width to the total length xm l the mean xm l ratio for erosional residuals in a spillway segment is an indication of the relative duration of flood flows that produced the spillway

an example of the relationship between rift and dome recent geodynamic evolution of the hoggar swell and of its nearby regions central sahara southern algeria and eastern niger abstract from essential gravimetrical and petrological data we propose a new model of geodynamic evolution for the volcanic hoggar swell and its nearby regions the eastern hoggar the tenere troughs and the north hoggar troughs correspond to a single distensive structure which developed during the late mesozoic by reactivation of n s and nw se trending pan african faults in response to the stress field probably associated with the central atlantic opening contemporaneously as suggested by the xenoliths entrained by the cenozoic alkali basaltic eruptions of the eastern hoggar an extensive lower crustal basic magmatism of tholeiitic affinity and probably associated with crustal stretching may have developed beneath the hoggarian part of this distensive structure on the other hand at the hoggar level a transverse lineament oriented ne sw oued amded cross cuts the distensive structure this lineament subsequently controlled the localization of the mio plio quaternary volcanic districts and possibly upper mantle modifications through reheating partial melting metasomatism and magmatic veining produced by astenospheric material transfer gas fluid kimberlitic to carbonatitic magmas resulting in a density reduction on the other hand the high deformation of the peridotitic inclusions of the cenozoic alkali basalts emplaced along the oued amded lineament may indicate that this lineament has also controlled the emplacement of small sized asthenospheric diapirs into the upper mantle thus in the hoggar it appears that doming was probably preceded by a rifting phase and that doming and cenozoic volcanism were possibly controlled by the intersection of this rift by a transverse wrench fault

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

reconstruction of the largest holocene jokulhlaup within jokulsa a fjollum ne iceland abstract glacial outburst floods jokulhlaups have a significant role for landscape evolution in ne iceland a number of jokulhlaups have routed from the northern margin of vatnajokull during the holocene in this study reconstruction of the largest holocene jokulhlaup along jokulsa a fjollum ne iceland was undertaken using the hec ras hydraulic modelling and hec georas flood mapping techniques with a digital elevation model dem derived from ers insar data and field based wash limit evidence the largest jokulhlaup produced extensive erosional and depositional landforms across an inundated area of 1390 km2 and is calculated to have had a peak discharge of 0 9 106 m3 s 1 power per unit area within this jokulhlaup varied from 6 to 46 000 w m 2 jokulhlaup hydraulics are related to geomorphogical evidence at three key sites in va alda upptyppingar and mo rudalur sub areas in order to explain the abrupt spatial variation of the flood characteristics on a regional scale and to relate erosional and depositional features to spatial variations in jokulhlaup hydraulics these process form relationships of the largest jokulhlaup along the jokulsa a fjollum are compared with large outburst floods elsewhere the largest jokulsa a fjollum jokulhlaup had a factor of 20 times smaller discharge and a factor of 20 times lower power per unit area than altai palaeoflood the largest known flood on the earth

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

bedrock detection beneath river terrace deposits using three dimensional electrical resistivity tomography abstract we describe the use of a fully volumetric geophysical imaging approach three dimensional electrical resistivity 3d ert for bedrock detection below mixed sand and gravel deposits typical of fluvial valley fill terraces we illustrate the method through an analysis of terrace deposits of the great ouse river uk where up to 4 m of sand and gravel have filled the valley bottom during the latest pleistocene we use an edge detector to identify the steepest gradient in first derivative resistivity profiles which yields an estimate of bedrock depth verified by drilling to a precision better than 0 2 m average and 0 4 m standard deviation comparison of a range of drilling techniques at the site has revealed that borehole derived interface depths suffered from levels of uncertainty similar to those associated with the 3d ert indicating that the reliability of bedrock interface depths determined using these two approaches is comparable in this case the 3d ert method provides a high spatial resolution that enabled a previously unknown erosional bedrock structure associated with the change from deeper first terrace to second terrace deposits to be identified in the great ouse valley the method provides a relatively quick method to quantify terrace fill volume over large sites to a greater degree of precision than currently available

catastrophic flood of the mediterranean after the messinian salinity crisis around 5 6 million years ago the mediterranean sea became disconnected from the world s oceans and during the period that followed known as the messinian salinity crisis sea levels fell dramatically as the waters evaporated the water returned about 5 3 million years ago but the details about this the largest known flood in earth s history remain sketchy a study making use of boreholes seismic data and numerical modelling suggests that it was a catastrophic event though it started as a trickle that may have lasted several thousand years as much as 90 of the incoming water was transferred in less than two years such an abrupt flood may have involved peak rates of sea level rise of more than 10 metres a day 5 33 million years ago in an event known as the zanclean flood atlantic waters refilled a mostly desiccated mediterranean sea which had become disconnected from the world s oceans however the nature abruptness and evolution of this flood remain poorly constrained using borehole and seismic data and a model study it is now suggested that 90 per cent of the water was transferred in a short period of a few months to two years with peak rates of sea level rise in the mediterranean possibly reaching more than 10 metres per day the mediterranean sea became disconnected from the world s oceans and mostly desiccated by evaporation about 5 6 million years ago during the messinian salinity crisis1 2 3 the atlantic waters found a way through the present gibraltar strait and rapidly refilled the mediterranean 5 33 million years ago in an event known as the zanclean flood4 the nature abruptness and evolution of this flood remain poorly constrained4 5 6 borehole and seismic data show incisions over 250 m deep on both sides of the gibraltar strait that have previously been attributed to fluvial erosion during the desiccation4 7 here we show the continuity of this 200 km long channel across the strait and explain its morphology as the result of erosion by the flooding waters adopting an incision model validated in mountain rivers this model in turn allows us to estimate the duration of the flood although the available data are limited our findings suggest that the feedback between water flow and incision in the early stages of flooding imply discharges of about 108 m3 s 1 three orders of magnitude larger than the present amazon river and incision rates above 0 4 m per day although the flood started at low water discharges that may have lasted for up to several thousand years our results suggest that 90 per cent of the water was transferred in a short period ranging from a few months to two years this extremely abrupt flood may have involved peak rates of sea level rise in the mediterranean of more than ten metres per day

glacier dammed lakes and geological work of glacial superfloods in the late pleistocene southern siberia altai mountains quaternary glacier dammed lakes ofsouthern siberia produced cataclysmic superfloods floodstreams at the initial and final glacial stages when the ice dams were unstable consequently the initial surface was greatly and geologically instantly transformed morphologic associations ofmountainous scablands developed similar to the diluvial complexes ofthe channeled scabland in north america giant diluvial ramparts and terraces bars diluvial berms and giant current ripple relief diluvial erosional and evorsional forms including spillways outburst and oversplash gorges and dry waterfalls are also present the discharges of the glacial superfloods which have been calculated by independent methods were hundreds ofcubic meters per second at the locations where the giant current ripple marks were formed up to millions of cubic meters per second within the gorges maximum discharges of18 10 6 m 3 s 1 were characteristic ofthe diluvial floods in the runoff channel f rom the chuya kuray system ofthe glacierdammed lakes below the mouth ofthe kuray depression as f as the settlement ofchibit the power ofthose floods reached 10 6 wm 2 these values exceed the hydraulic parameters ofthe jok ulhlaups from late quaternary lake missoula in north america the floods of which previously were considered the most powerful freshwater streams in earth history the last phenomenal glacial superflood with discharges in excess of1 10 6 m 3 s 1 occurred in the chuya and katun valleys no later than 13 000 years ago afterwards the glacier dammed lakes degraded simultaneously with the degradation of the glaciers the glacier dammed lakes ofthe intermontane basins in southern siberia disappeared completely at some time af ter 5000 years ago calculations ofthe hydraulics and the estimates ofthe geological activity ofthe quaternary jok ulhlaups mean that the diluvial processes of relief formation can now be viewed as the most powerful exogenous process r 2001 published by elsevier science ltd

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

late quaternary megafloods from glacial lake atna southcentral alaska u s a abstract geomorphic stratigraphic geotechnical and biogeographic evidence indicate that failure of a pleistocene ice dam between 15 5 and 26 ka generated a megaflood from glacial lake atna down the matanuska valley while it has long been recognized that lake atna occupied 9000 km2 of south central alaska s copper river basin little attention has focused on the lake s discharge locations and behaviors digital elevation model and geomorphic analyses suggest that progressive lowering of the lake level by decanting over spillways exposed during glacial retreat led to sequential discharges down the matanuska susitna tok and copper river valleys lake atna s size 50 ka duration and sequential connection to four major drainages likely made it a regionally important late pleistocene freshwater refugium we estimate a catastrophic matanuska megaflood would have released 500 1400 km3 at a maximum rate of 3 106 m3 s 1 volumes for the other outlets ranged from 200 to 2600 km3 and estimated maximum discharges ranged from 0 8 to 11 3 106 m3 s 1 making lake atna a serial generator of some of the largest known freshwater megafloods

canyon formation constraints on the discharge of catastrophic outburst floods of earth and mars catastrophic outburst floods carved amphitheater headed canyons on earth and mars and the steep headwalls of these canyons suggest that some formed by upstream headwall propagation through waterfall erosion processes because topography evolves in concert with water flow during canyon erosion we suggest that bedrock canyon morphology preserves hydraulic information about canyon forming floods in particular we propose that for a canyon to form with a roughly uniform width by upstream headwall retreat erosion must occur around the canyon head but not along the sidewalls such that canyon width is related to flood discharge we develop a new theory for bedrock canyon formation by megafloods based on flow convergence of large outburst floods toward a horseshoe shaped waterfall the model is developed for waterfall erosion by rock toppling a candidate erosion mechanism in well fractured rock like columnar basalt we apply the model to 14 terrestrial channeled scablands washington snake river plain idaho and asbyrgi canyon iceland and nine martian near ares vallis and echus chasma bedrock canyons and show that predicted flood discharges are nearly 3 orders of magnitude less than previously estimated and predicted flood durations are longer than previously estimated from less than a day to a few months results also show a positive correlation between flood discharge per unit width and canyon width which supports our hypothesis that canyon width is set in part by flood discharge despite lower discharges than previously estimated the flood volumes remain large enough for individual outburst floods to have perturbed the global hydrology of mars

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

paleohydrology of late pleistocene superflooding altay mountains siberia cataclysmic flooding is a geomorphological process of planetary significance landforms of flood origin resulted from late pleistocene ice dammed lake failures in the altay mountains of south central siberia peak paleoflows which exceeded 18 x 10 6 cubic meters per second are comparable to the largest known terrestrial discharges of freshwater and show a hydrological scaling relation to floods generated by catastrophic dam failures these seem to have been earth9s greatest floods based on a variety of reconstructed paleohydraulic parameters

radiocarbon dating of mid holocene megaflood deposits in the jokulsa a fjollum north iceland two megafloods in the canyon of the jokulsi i fjollum the major northern routeway for glaciovolcanic floods from vatnajokull have been closely dated by 14c ams dates from betula macrofossils within peat immediately below beds of flood deposited sand ages of c 4415 and c 4065 yr bp 5020 and 4610 cal yr bp are consistent with the presence of the hekla 4 tephra c 3830 yr bp resting on the upper surface of the younger flood sand these sediments are correlated across the jokulsa a fjollum canyon with the upper flood sands in a stack recording around 16 flood events deposits on both sides of the canyon were trimmed by the last megaflood after the hekla 3 tephra fall at c 2900 yr bp and the highest holocene flood stages were at the culmination of a series peaking at c 3500 yr bp these floods have wider palaeoclimatic significance because they require the formation of large subglacial reservoirs below vatnajokull therefore the dated floods indicate that a large composite ice cap covered volcano

the cenozoic volcanic province of tibesti sahara of chad major units chronology and structural features using both field relationships and some absolute ages the sequence of volcanic units in the cenozoic tibesti volcanic province tvp chad is established as follows 1 plateau volcanism between at least 17 and 8 ma consisting of flood basalts and silicic lava plugs with intercalated ignimbritic sheets in the upper basalt succession increasing in amount upwards ages decrease from ne to sw following the migration of the small nw se flexures concentrating the feeding dike swarms 2 late miocene large central composite volcanoes exhibiting diverse and original structures some of them tarso toon ehi oye and tarso yega are located along a major nne fault representing the main tectonic direction in tibesti since precambrian times 3 construction of three large ignimbritic volcanoes associated with significant updoming of the basement ending with the collapse of large calderas voon about 5 7 ma emi koussi 2 4 1 33 ma and yirrigue 0 43 ma 4 basaltic activity starting at about 5 7 ma and essentially consisting of cinder cones and associated lava flows tarso toh tarso ahon and tarso emi chi and 5 final volcanic activity represented by post yirrigue caldera activity in the tarso tousside volcanic complex and especially ehi tousside the only active volcano in tibesti plus ehi timi and ehi mousgou volcanoes similar to ehi tousside the two tectonic directions controlling some volcanic features of the province correspond to the major old lithospheric structures delimiting the volcanic province namely the great nw se tassilian flexure to the sw and a major ne nne fault zone to the e unusual conditions of uplift and erosion in the tvp enable exceptional exposure of the internal structure of its volcanoes

catastrophic impact of extreme flood events on the morphology and evolution of the lower jokulsa a fjollum northeast iceland during the holocene abstract the impact of extreme flood events is rarely considered in studies of long term landscape evolution despite the potential for catastrophic landscape change in a short period of time here we use an integrated approach of geomorphological mapping topographic analysis and geophysical surveys to identify and quantify the impact of extreme flood events jokulhlaups along the jokulsa a fjollum iceland where evidence for the action of such floods is widespread on microspatial to macrospatial scales the apex of the 28 km long jokulsargljufur canyon is characterised by a complex network of palaeo flood channels and large vertical knickpoints such as dettifoss 54 m high and hafragilsfoss 20 m high downstream the forvo valley contains large terraces of boulder rich deposits 50 m thick 3 km long near the outlet of the canyon is asbyrgi a dry canyon 3 km long 1 km wide up to 90 m deep with eroded cataracts and scabland morphology immediately upstream and 90 m above the current river channel topographic analysis and electrical resistivity tomography surveys show that 0 144 km3 of rock was eroded from asbyrgi during its formation 10 000 years ago and just 4 of this eroded volume is currently filled with sediment deposits up to 5 m thick deposited boulders across the canyon floor of asbyrgi demonstrate that the discharge of the jokulhlaup that formed the canyon was at least 39 000 m3 s 1 we present a model for the evolution of the lower jokulsa a fjollum and the jokulsargljufur canyon during various stages of an extreme flood event reconstruction of the early holocene flood event includes the initiation and development of different canyons before the capture of all floodwater within one canyon at the end we tie the evolution of the lower jokulsargljufur canyon to established chronology of flood events during the holocene farther upstream and highlight the dominant impact of extreme flood events over background processes in this landscape

lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya the stratigraphic record of the eastern murzuq basin has been importantly influenced by deformation resulting in angular and or deeply erosional unconformities though the overall context is intracratonic major transgressive events and the ordovician glaciation are nevertheless documented allowing the delineation of tectonic eustasy or climate driven unconformities lower palaeozoic key events and related unconformities that characterize the north gondwana platform have therefore a signature in the eastern murzuq basin the basement cover unconformity also known as the infra tassilian surface truncates all the deformed and metamorphosed lower cambrian and older rocks above is a middle cambrian to lower ordovician megasequence murizidie and hasawnah fms which is in turn truncated by an intra ordovician angular unconformity this megasequence is unconformably overlain by a middle ordovician hawaz fm to silurian tanzzuft and akakus fms megasequence which includes the upper ordovician glaciogenic unit mamuniyat fm bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations tillites and glaciotectonic structures the middle ordovician to silurian megasequence is finally truncated by a base devonian angular unconformity overlain by fluvial sandstones regarding the possibility that those fluvial deposits may be as younger as late devonian in the eastern murzuq basin based on palaeoflora the so called caledonian unconformity might be here a much younger mid eifelian surface and the occurrence of the lower devonian tadrart fm is questioned the upper ordovician glacial erosion surface which is sometimes referred to as the taconic unconformity usually truncates middle ordovician strata in the murzuq basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra ordovician deformation event in the murizidie southeastern murzuq basin the infra tassilian surface the intra ordovician unconformity and the upper ordovician glacial erosion surface amalgamate together here an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision the main part of which relate to the intra ordovician tectonic event the upper ordovician climate related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the lower palaeozoic although it presents most of the physical attributes of tectonic driven unconformities

bedrock detection using 2d electrical resistivity imaging along the peikang river central taiwan article i nfo we use the two dimensional 2d electrical resistivity imaging eri method to detect the contact between sediment and bedrock in a modern fluvial system we performed eight 2d electrical resistivity surveys along the peikang river in kuohsing area central taiwan with a pole pole array of 1 m electrode spacing to produce a spatial resolution of about 1 m the resistivity data were inverted into subsurface electrical structures using the least squares inversion techniques then the contact between the bedrock and sediment was found by using an image processing technique called laplacian edge detection led which represents an objective approach in interpreting various geophysical images numerical modeling demonstrates that isoresistivity lines are not in good agreement with the hypothetical interfaces between the bedrock and sediment but the zero lines of the laplacian operation are strikingly accurate our results indicate a well defined boundary in the resistivity structure that can be used to estimate the quantity of sediments covering bedrock thus highlighting the utility of this technique in studies of landscape evolution sediment transport and sediment budgets

retreat of a giant cataract in a long lived 3 7 2 6 ga martian outflow channel we describe the evolution of an similar to 600 m deep tributary outflow channel to ares vallis mars high resolution topography image analysis and crater statistics indicate that this tributary canyon developed by the upstream migration of a large similar to 300 m tall cataract during multiple flood events that span similar to 1 b y of mars history 3 7 2 6 ga issuing from hydapsis chaos these floods were initiated at a similar time and occurred over a similar time range to flooding in ares vallis suggesting a potential regional control on flood initiation and chaos formation in addition we provide evidence that cataract retreat and significant incision within the tributary canyon occurred only after a series of downcutting events within ares vallis topography data and crater statistics taken from the floor of ares vallis indicate an similar to 300 m base level drop that coincides temporally with an early amazonian ca 2 6 ga flood event and cataract formation within the tributary canyon the results both confirm the hypothesis of long term multiple flood events within martian outflow channels and demonstrate the influence of base level change on their incision

hydrodynamics and geomorphic work of jokulhlaups glacial outburst floods from kverkfjoll volcano iceland jokulhlaups glacial outburst floods occur frequently within most glaciated regions of the world and cause rapid landscape change infrastructure damage and human disturbance the largest jokulhlaups known to have occurred during the holocene within iceland drained from the northern margin of vatnajokull and along the jokulsa a fjollum some of these jokulhlaups originated from kverkfjoll volcano and were routed through anastomosing high gradient and hydraulically rough channels landforms and sediments preserved within these channels permit palaeoflow reconstructions kverkfjoll jokulhlaups were reconstructed using palaeocompetence point measurements slope area one dimensional and depth averaged two dimensional 2d hydrodynamic modelling techniques the increasing complexity of 2d modelling required a range of assumptions but produced information on both spatial and temporal variations in jokulhlaup characteristics the jokulhlaups were volcanically triggered had a linear rise hydrograph and a peak discharge of 50 000 100 000 m3 s 1 which attenuated by 50 75 within 25 km frontal flow velocities were 2 m s 1 but as stage increased velocities reached 5 15m s 1 peak instantaneous shear stress and stream power reached 1 104 n m 2 and 1 105 w m 2 respectively hydraulic parameters can be related to landform groups a hierarchy of landforms is proposed ranging from the highest energy zones erosional gorges scoured bedrock cataracts and spillways to the lowest energy zones of valley fills bars and slackwater deposits fluvial erosion of bedrock occurred in kverkfjallarani above 3 m flow depth 7m s 1 flow velocity 1 102 n m 2 shear stress and 3 102 w m 2 stream power fluvial deposition occurred in kverkfjallarani below 8 m flow depth 11 m s 1 flow velocity 5 102 n m 2 shear stress and 3 103 w m 2 stream power hence erosional and depositional envelopes have considerable overlap probably due to transitional flow phenomena and the influence of upstream effects such as hydraulic ponding and topographic constrictions for example holocene kverkfjoll jokulhlaups achieved geomorphic work comparable to that of other late pleistocene megafloods this work was a result of steep channel gradients topographic channel constrictions and high hydraulic roughness rather than to extreme peak discharges the kverkfjoll jokulhlaups have implications for landscape evolution in north central iceland for water sediment inputs into the north atlantic and for recognizing jokulhlaups in the rock record 2d hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes because it only requires an input hydrograph and a digital elevation model to run a model rather than suites of geomorphological evidence and field surveyed valley cross sections for example copyright 2006 john wiley sons ltd

catastrophic flooding origin of shelf valley systems in the english channel britain became geographically isolated from continental europe when high interglacial seas flooded the shallow english channel and north sea shelf areas but a tenuous link remained the weald artois chalk ridge between southeast england and northwest france just how this isthmus was breached making britain an island has been a matter of conjecture a new bathymetric map of the sea floor may solve the mystery it reveals a large bedrock floored valley containing landforms including grooves and streamlined islands consistent with a megaflood event caused when a rock dam at the dover strait breached draining a large pro glacial lake in the north sea basin a hypothesis for the permanent isolation of britain from mainland europe during interglacial high sea levels has been confirmed gupta et al analysed a new regional bathymetric map of part of the area finding a valley with landforms indicating large scale subaerial erosion by high magnitude water discharges they suggest breaching of a rock dam at the dover strait instigated drainage of a large lake in the north sea basin megaflood events involving sudden discharges of exceptionally large volumes of water are rare but can significantly affect landscape evolution continental scale drainage patterns and climate change1 it has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the english channel the narrow seaway between england and france2 3 4 this hypothesis has remained untested through lack of direct evidence and alternative non catastrophist ideas have been entertained for valley formation5 6 here we analyse a new regional bathymetric map of part of the english channel derived from high resolution sonar data which shows the morphology of the valley in unprecedented detail we observe a large bedrock floored valley that contains a distinct assemblage of landforms including streamlined islands and longitudinal erosional grooves which are indicative of large scale subaerial erosion by high magnitude water discharges our observations support the megaflood model in which breaching of a rock dam at the dover strait instigated catastrophic drainage of a large pro glacial lake in the southern north sea basin2 we suggest that megaflooding provides an explanation for the permanent isolation of britain from mainland europe during interglacial high sea level stands7 and consequently for patterns of early human colonisation of britain together with the large scale reorganization of palaeodrainage in northwest europe4

early archean fossil bacteria and biofilms in hydrothermally influenced sediments from the barberton greenstone belt south africa abstract sem imaging of hf etched 3 3 3 5 ga cherts from the onverwacht group south africa reveals small spherical 1 m diameter and rod shaped structures 2 3 8 m in length which are interpreted as probable fossil coccoid and bacillar bacteria prokaryotes respectively preserved by mineral replacement other possibly biogenic structures include smaller rod shaped bacteriomorphs 900 m textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature 220 c hydrothermal solutions

arctic rockwall retreat rates estimated using laboratory calibrated ert measurements of talus cones in longyeardalen svalbard holocene rockwall retreat rates quantify integral values of rock slope erosion and talus cone evolution here we investigate holocene rockwall retreat of exposed arctic sandstone cliffs in longyear

biogeochemistry earth s redox evolution the timing and nature of changes in the chemistry of the early oceans are intensely debated geochemical analyses show that a prominent transition to sulphidic marine conditions 1 8 billion years ago may have been restricted to near shore environments

gravity study of the central african rift system a model of continental disruption 1 the ngaoundere and abu gabra rifts abstract a regional compilation of published and unpublished gravity data for central africa is presented and reveals the presence of a major rift system called here the central african rift system it is proposed that the junction area between the ngaoundere and abu gabra rift arms in western sudan forms an incipient intraplate triple junction with the as yet unfractured but domally uplifted and volcanically active darfur swell it is only the darfur swell that shows any similarities to the uplift and rift history of east africa the other two rifts arms are considered to be structurally similar to the early stages of passive margin development and thus reflect more closely the initial processes of continental fragmentation than the structures associated with rifting in east africa

morphology sedimentology and palaeohydraulic significance of large gravel dunes altai mountains siberia coarse gravel bedforms which resulted from pleistocene glacial outburst floods are identified as subaqueous dunes comparison of the morphology of these fossil structures with modern dunes shows that the form of two dimensional 2 d transverse dunes and 3 d cuspate and lunate dunes developed in coarse gravels is comparable with sand dune morphology within lesser scale geophysical flows the similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform internal structure indicates that flow over 2 d dunes was relatively uniform but over 3 d bedforms flow was locally variable flow separation and complex streaming of flow occurred over the steepest 3 d dunes cross beds are thin and few approach the angle of repose consequently most dunes did not migrate primarily by avalanching but by stoss entrained gravel transported over the crests rolling down and depositing on the lee slopes the lee formed when flood power was waning some dunes were siightly planed down during falling stage because lee side cross beds tend to be steeper than the angle of the preserved lee slope however silt rich caps indicate that any height reduction was contemporary with the final deposition of foresets post flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs lee side sediments are often finer than the stoss slope sediments which indicates

filamentous microfossils from the 3 500 myr old onverwacht group barberton mountain land south africa the swaziland supergroup barberton mountain land south africa has long been regarded as a promising location for the earth s oldest fossils because it includes some of the most ancient well preserved sedimentary rocks many of which contain carbonaceous matter although there have been numerous reports of microfossils from swaziland group rocks1 7 the biogenicity of most of the structures has been questioned8 10 although some of the organic spheroids are probably biogenic10 11 the best early archaean simple spheroids are generally regarded as possible microfossils 10 because organic spheroids may form abiotically in several ways12 the discovery of less simple biological morphologies is therefore important in establishing the existence of early life forms in the early archaean uniformly sized curving filaments especially tubular ones are difficult to explain as anything other than the fossil remains of filamentous organisms here we report the discovery of numerous filaments from two different stratigraphical positions in the 3 500 myr old onverwacht group of the swaziland supergroup their morphologies and abundance provide convincing evidence for the existence of bacteria or cyanobacteria like organisms on the earth during the early archaean this supports recent reports of similar filamentous microfossils from 3 500 myr old rocks from western australia13