river damming late quaternary rockslides in the otz valley region tyrol austria abstract the otz valley and adjacent regions in tyrol austria have been repeatedly affected by large rockslope failures following deglaciation six rockslides each over 107 m3 in volume were emplaced into the otz and inn valleys five of which formed persistent rockslide dams even though catastrophic rockslope failures are short lived events commonly minutes they can have long lasting impacts on the landscape for example large fans have built in the otz valley and knickpoints persist at the former dam sites even though the otz river has eroded through the deposits during the past thousands of years exact age constraints of rockslide dam failure however are still scarce empirical geomorphic stability indices from the literature successfully identified the least and the most stable dams of this group whereas the rest remain inconclusive with some indices variably placing the dams in the stable unstable and uncertain categories this shows a that further index calibrations and b better age constraints on dam formation and failure are needed and c that the exact processes of dam failure are not always trivial to pinpoint for ancient partially breached dams this study is a contribution towards better constraining the nature and landscape impact of dam formation following large rockslope failures

long runout rockslide on glacier at tsar mountain canadian rocky mountains potential triggers seismic and glaciological implications a rhomboidal shaped rockslide with a length of 2 2 km and an area of 1 35 km2 covers 30 of a small clemenceau icefield group cirque glacier by means of remote sensing airborne photography climate and seismic data analysis the extent timing possible trigger mechanisms and rockslide effects on the glacier are discussed an ml 2 0 seismic event on 14 september 2000 represents the rockslide and has a spindle shaped waveform and high energy in the low frequencies based on a strong signal duration of 56 108 seconds the rockslide had an average runout velocity of 22 45 m s 1 stress fatigue and a combination of rain and snow followed by rapid melt and night time freezing could have co triggered the slide additional seismic events with epicentres 40 km from the cirque were recorded 26 times since 1985 and range between ml 0 9 3 3 but none were caused by or triggered large mass movement with a rock source volume of 1 6 106 m3 and a deposited volume of 2 4 106 m3 the sum of bulking and entrainment is between 25 and 150 five melt seasons post event levees 20 30 m wide tower 30 m over the surrounding glacier rockslide characteristics and its fahrboschung of 15 height length h l 0 28 indicate that this rockslide is quite typical for long runout rockslides on glaciers however the basal friction coefficient was higher than found on other glaciers and could relate to the presence of medial moraines transverse to the flowline and to pre slide debris cover the difference between the coefficient of friction in the main flowline 0 11 0 14 and that in a lobe confined between two medial moraines 0 065 suggests that confinement increases mobility by 70 115 apart from reduced ablation under the rock debris effects of the rockslide on the glacier were minimal copyright 2010 john wiley sons ltd

contemporary changes in sediment yield in an alpine mountain basin due to afforestation the upper drome in france abstract the baurieres plain within the upper drome river basin was used to reconstruct recent changes in sediment supply in relation to changes in land use within an alpine catchment a considerable body of archival information is available furthermore the plain acts as a natural sediment trap and the reach basin interaction has not been disrupted by human activity based on archival data channel geometry measurements dendrogeomorphological and radionuclide analysis cs137 and unsupported pb210 the trends in channel change and sediment supply over the past two centuries are assessed and their causes are interpreted dendrogeomorphology and radionuclide profiles show that the floodplain is characterised by a decrease in sedimentation rate in the 1960s the ex pb210 profiles also suggest a spatial modification of the relative contribution in sediment supply of the catchment bedload yields are estimated to be at least 26 m3 km 2 year 1 between 1928 and 1996 based on an estimate of storage over a channel length of 11 5 km archival data concerning bedload removal from traps yields an estimate of 25 6 m3 km 2 year 1 between 1993 and 2001 these two values are very comparable suggesting no major modification in bedload transport within the reach during the 20th century if the bedload supply has not strongly decreased in the studied reach during this period the bedload sources have changed the volume of sediment stored in the beaurieres area between 1928 and 1996 corresponds to at least 40 of the sediment delivered by channel degradation from an upstream alluvial reach both changes in floodplain sedimentation as well as changes in bedload and suspended contributions from catchment sources are interpreted as responses to land use but also flow regime hillslope sediment production strongly decreased due to planned hillslope afforestation and torrent regulation at the end of the 19th century and spontaneous hillslope afforestation resulting from grazing decline mainly in the two decades following world war ii the observed change in suspended sediment supply which occurred around 1960 1965 has a clear synchronicity with spontaneous catchment afforestation following world war ii after this period a decrease in sediment supply a change in source but also a decrease in peak flow were observed changes in run off are complex and cannot be caused with only land use change flood hydrographs underwent peak decreases and duration increases through the 20th century because of increase in water retention capacity of the forested catchment in addition flood seasonality has changed with september and october flood events being much less frequent in the last part of the 20th century

rock fall hazard assessment in asar hill ancient mabolla city mugla sw turkey this paper discusses the potential rock fall hazard of asar hill this flat topped hill containing upper miocene conglomerate and finer grained sedimentary rocks hosts to the ancient mabolla city the seasonal temperature differences are high in this rainy region block sizes bed and joints attitudes were measured along three profiles during field study schmidt hammer was applied to grains and matrix of conglomerates for determining strength in situ one joint set in profile a 3 in profile b and 4 in profile c were determined via dips analysis an intersection of joints and bedding directed 25 35 nw caused the block formation control slope angle and mass movement type at least 10 blocks 0 6 1241 085 tons were fixed on top of profiles two blocks on top of the eastern profile a contain archaeological ruins after back analysis for determining restitution coefficients rocfall version 4 9 software program was used to determine possible end points of those blocks most of the blocks can reach the valley and pass the other side which includes settlements and gardens consequently the possible rock fall hazard in asar hill is threatening visitors and settlers the cleaning of detached blocks on top of slope is one of the easiest ways however in situ preservation of blocks with archaeological ruins via tensioned rock anchors and or patterned rock bolts has vital importance relocation of settlers and gardens opening trench around hill and establishing green belt forest would be considered as other measures

silicon isotope fractionation during microbial reduction of fe iii si gels under archean seawater conditions and implications for iron formation genesis abstract microbial dissimilatory iron reduction dir is a deeply rooted metabolism in the bacteria and archaea in the archean and proterozoic the most likely electron acceptor for dir in marine environments was fe iii si gels it has been recently suggested that the fe and si cycles were coupled through sorption of aqueous si to iron oxides hydroxides and through release of si during dir evidence for the close association of the fe and si cycles comes from banded iron formations bifs which consist of alternating bands of fe bearing minerals and quartz chert although there has been extensive study of the stable fe isotope fractionations produced by dir of fe iii si gels as well as studies of stable fe isotope fractionations in analogous abiologic systems no studies to date have investigated stable si isotope fractionations produced by dir in this study the stable si isotope fractionations produced by microbial reduction of fe iii si gels were investigated in simulated artificial archean seawater aas using the marine iron reducing bacterium desulfuromonas acetoxidans microbial reduction produced very large 30 si 28 si isotope fractionations between the solid and aqueous phase at 23 c where 30 si solid aqueous isotope fractionations of 3 35 0 16 and 3 46 0 09 were produced in two replicate experiments at 32 fe iii reduction solid phase fe ii fe total 0 32 this isotopic fractionation was substantially greater than that observed in two abiologic controls that had solid phase fe ii fe total 0 02 0 03 which produced 30 si solid aqueous isotope fractionations of 2 83 0 24 and 2 65 0 28 in a companion study the equilibrium 30 si solid aqueous isotope fractionation was determined to be 2 3 for solid phase fe ii fe total 0 collectively these results highlight the importance of fe ii in fe si gels in producing large changes in si isotope fractionations these results suggest that dir should produce highly negative 30 si values in quartz that is the product of diagenetic reactions associated with fe si gels such si isotope compositions would be expected to be associated with fe bearing minerals that contain fe ii indicative of reduction such as magnetite support for this model comes from recent in situ si isotope studies of oxide facies bifs where quartz in magnetite rich samples have significantly more negative 30 si values than quartz in hematite rich samples

monitoring numerical modelling and hazard mitigation of the moscardo landslide eastern italian alps abstract the moscardo torrent basin eastern italian alps is a high risk site since a large roto translational landslide might dam the torrent with the consequence of increasing the possibility of large debris flow events creating a threat for the infrastructures and the socio economic activities of the villages that dot the valley below the landslide whose volume is estimated 2 million m3 has been monitored since 2006 with inclinometers electric piezometers and a gps network the velocity along the entire body of the landslide averages 1 0 1 5 cm per month the shear surface develops at depths varying from 9 to 10 m to 55 62 m while the groundwater table is almost constant throughout the year despite a cumulative rainfall of the area that usually reaches 2000 mm year the movements were simulated in a numerical model in order to estimate the stabilization effect obtained by different types of possible countermeasures the simulation was carried out using flac 2d with creep modelling visco elasto plastic model of the medium in the sliding zone was assumed allowing to determine the relation between time and displacement a 10 year displacement trend starting from the initial situation of 2006 was simulated moreover seismic conditions were taken into consideration with a quasi static approach by applying a horizontal acceleration the numerical model was built and validated on the basis of the data retrieved from geological investigations as well as from inclinometric and gps measurements the results show that an accurate and well planned multidisciplinary approach can help the decision makers in the choice of the most effective engineering solution for the mitigation of landslide hazard and risk

numerical analysis of initiation and progressive failure in natural rock slopes the 1991 randa rockslide the 1991 randa rockslide in the swiss alps involved several complex mechanisms relating to geological mechanical and hydrological processes for which no clear trigger can be asserted this paper investigates the concept of progressive failure and the numerical modelling of rock mass strength degradation in natural rock slopes using the randa rockslide as a working example results from continuum i e finite element modelling are presented to illustrate a hypothesis suggesting that initiation of a progressive rock mass degradation process ultimately leading to failure began following deglaciation of the valley below discontinuum distinct element modelling is then applied to investigate the underlying mechanisms contributing to the episodic nature of the rockslide finally the use of a hybrid method that combines both continuum and discontinuum techniques to model fracture propagation are discussed in the context of modelling progressive slide surface development linking initiation and degradation to eventual catastrophic failure

analyses of past and present rock slope instabilities in a fjord valley implications for hazard estimations abstract for quantitative hazard assessment it is necessary to define the magnitude frequency distribution and a temporal model of the landslide frequency this is often complicated for large rock slope failures due to the lack of significant numbers of large rock slope failures in inventories of a given homogeneous region or sparse information about the timing an inventory of 108 rock slope failure deposits within the fjords of the storfjord region including a relative rockslide chronostratigraphy was available for this study the temporal distribution of rock slope failures is characterized by a rapid response with a high frequency of failures directly after deglaciation followed by a lower constant frequency since 9 000 years bp for the entire storfjord region the largest recorded volumes failed directly after deglaciation volumes larger than 5 10 6 m 3 have not been recorded within the last 9 000 years in addition 17 rockslide scars and on the other hand 17 potential rock slope instabilities have been mapped at the northeastern flank of tafjord which is a branch of the storfjord volume frequency relations have been developed for the three inventories from tafjord comprising i fjord deposits ii rockslide scars and iii potential instabilities they result in very similar distributions over the entire volume range all three inventories are underlying the same processes that lead to destabilisation and finally failure additionally the compilation of the three inventories is independent and based on different methodologies the similarity of the volume frequency distributions is thus verifying the different inventories hence the obtained volume frequency relation can be used to assess the rockslide hazard for the study area present day annual expectable frequencies for different volume classes have been assessed based on the obtained volume frequency relation of the fjord deposits for example the average annual frequency for rock slope failures of v 1 10 6 m 3 originating from the northeastern flank of tafjord is 1 1670 based on a lognormal distribution this regional hazard can be used to improve previously existing qualitative susceptibility rankings for potential unstable rock slopes within the tafjord region the resulting probabilities are very low however the uncertainties of all hazard estimates are very high

fe isotope fractionation during fe ii oxidation by the marine photoferrotroph rhodovulum iodosum in the presence of si implications for precambrian iron formation deposition abstract the iron fe isotopic composition of precambrian iron formations ifs besides providing geological context through its mineralogical properties was suggested to function as a biosignature that can be used to infer a potential microbial role in the formation of the deposited fe minerals anoxygenic phototrophic fe ii oxidizing bacteria photoferrotrophs capable of oxidizing fe ii anoxically using light energy were potentially involved in fe ii oxidation in anoxic or suboxic precambrian oceans the effect of si on fe isotopic fractionation between aqueous fe ii and fe si co precipitates has been investigated before however it is currently unknown how stable fe isotopes are fractionated during enzymatic fe ii oxidation under marine hydrogeochemical conditions and particularly how the presence of si affects the fe isotope composition and the isotopic exchange among different fe phases we therefore studied fe isotope fractionation during fe ii oxidation by the marine photoferrotroph rhodovulum iodosum in simulated precambrian seawater amended with 1 mm dissolved si our results show that the change in the fe isotope compositions over time for both the initial aqueous fe ii fe aq and the fe iii precipitates fe ppt follow a rayleigh distillation model moreover the fractionation e 56 fe ppt aq determined independently from either 56 fe aq or 56 fe ppt data resulted in a value of 2 3 0 3 2sd n 6 this value differs from the fractionation factor determined previously for fe ii oxidation by r iodosum in the absence of si where the fractionation calculated from 56 fe aq i e 0 96 1 18 was different from that calculated from 56 fe ppt 1 96 1 98 this difference was attributed to isotopic exchange processes with soluble and sorbed fe species the present study suggests that si present in precambrian oceans retards fe isotopic exchange likely through combined effects of complexation of dissolved fe species by si and sorption of si to fe iii minerals thus lowering sorption of fe ii to the fe iii minerals which is necessary for isotopic exchange in summary our data suggests that si in ancient oceans played a key role for the fe isotope composition of fe iii minerals that were deposited by photoferrotrophic iron oxidation in precambrian oceans by minimizing subsequent isotope exchange and recrystallization processes with aqueous fe ii

deformation induced silica redistribution in banded iron formation hamersley province australia abstract the formation of banded iron formations bif remains controversial despite their potential to provide key information on precambrian atmospheres and hydrospheres it is widely agreed that bif are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases many models address the chemical transformation of such precursor iron oxide phases into bif during compaction and diagenesis however the formation of chert and the influence of physical forces in this process have received less attention microstructural analysis of bif from the hamersley province western australia reveals that significant amounts of silica were redistributed by dissolution precipitation creep during both diagenesis and regional scale deformation this physicochemical process led to silica remobilisation and volume loss by stress induced dissolution of microcrystalline quartz in an aqueous fluid the dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low pressure zones leading to local volume increase these processes were further enhanced by rheological contrasts between different minerals resulting in significant variations of chert band thickness microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations cpo in quartz grains within chert layers the cpo fabrics record strain regimes e g pure and simple shear extension and shortening that modified quartz aggregates by dissolution precipitation creep providing new insights into the metamorphic and deformation history of bif we document microstructures that indicate that non coaxial deformation was active during diagenesis and subsequent deformation of the hamersley province bif further relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution precipitation creep providing a possible explanation for the destruction of primary features we show that different generations of chert alternate on a micro scale suggesting that the role of deformation has been underestimated in many aspects of bif research the discovery of dissolution precipitation creep in low grade metamorphic bif provides a potential link between the physical and chemical processes that occur within these rocks after deposition in the hamersley province dissolution precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events we also suggest that chert layers without cpo record the least overprint by diagenesis and as the most pristine form of bif should be targeted for future paleo environmental research

planar lamellar substructures in quartz both single and multiple sets of lamellar features in quartz grains can be produced in shock experiments and are observed at known impact sites they can also be produced in static constant strain rate experiments and are observed at known tectonic and explosive volcanic sites the former are strictly planar and parallel and generally conform to rational crystallographic orientations in the quartz crystal structure the latter can also be planar and parallel but sometimes show slight to moderate curvature with lamellar bifurcations and variable lamellar widths their orientations show a variety of forms with in general a broader angular distribution than that of the impact generated lamellae deductions of physical conditions of deformation from optical morphology spacing multiplicity and orientation information alone remain somewhat conjectural full characterization by transmission electron microscopy tem appears to be necessary

surface exposure dating of the flims landslide graubunden switzerland abstract sixteen boulder and bedrock surfaces related to the flims landslide volume 8 12 km3 were dated with 36cl and 10be exposure ages range from 4900 250 yr to 15 440 1480 yr including corrections made due to snow cover and karst erosion ages of 11 410 590 yr and 13 340 1090 yr were obtained on bedrock surfaces outside of the landslide zone these are minima for deglaciation of segnes valley and provide constraints on possible amounts of inherited nuclides based on seven boulder ages which range from 8200 260 to 9520 990 yr a mean of 8900 700 years is calculated for the flims landslide we exclude three outliers one significantly older and two significantly younger than the others and the ages from the cassons bedrock site from the mean calculation the latter is excluded as shielding due to snow and vegetation cover is difficult to constrain there the oldest boulder on the landslide yielded an age 15440 1480 yr more than 5000 years older than any other boulder age suggesting that it included the surface of the pre slide bedrock the exposure ages are consistent with a single failure event the erratics and patches of till lying on the landslide debris must have been carried piggy back on top of the landslide rather than having been deposited by the late pleistocene vorderrhein glacier the flims rockslide is about the same age as the kofels 9800 yr and kandertal 9600 yr rockslides all three occurred during the marked transition to warmer wetter conditions during the early holocene

replacement origin for hematite in 2 5 ga banded iron formation evidence for postdepositional oxidation of iron bearing minerals banded iron formations bifs are central to interpretations about the composition of the precambrian ocean atmosphere and biosphere hematite is an important component of many bifs and its presence has been used as evidence for the former presence of hydrous ferric oxyhydroxides that formed from the oxidation of dissolved ferrous iron in seawater however textural evidence for the origin of hematite is equivocal new petrographic results show that hematite in unmineralized bif from the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia including morphologies previously interpreted to represent ferric oxyhydroxide precipitates formed via fluid mediated replacement of iron silicates and iron carbonates along sedimentary layering the lateral transition from stilpnomelane and siderite rich laminae to hematite dominated laminae is interpreted to reflect progressive stages of in situ alteration of reduced mineral assemblages by oxygen bearing fluids rather than changes in the chemistry of the water column during deposition although morphologies previously ascribed to primary hematite are present they are related to mineral replacement reactions raising doubts about the petrographic criteria used to identify original hematite hematite replacement in unmineralized bif postdated deposition and possibly metamorphism and predated modern weathering from a regional perspective it appears to be a distal signature of the processes that were responsible for iron ore mineralization which involved the deep infiltration of oxygen bearing meteoric fluids the mineral replacement reactions recorded in the dales gorge member are unlikely to be unique and probably occurred in bifs elsewhere at some point in their history the observation that at least some of the hematite in unmineralized bif did not form directly from ferric oxyhydroxides implies that hematite is not a reliable proxy for the composition of the precursor sediment or the redox chemistry of the ocean the oxidation of ferrous rich phases after deposition suggests that the precursor sediments of bif originally had a more reduced bulk composition this raises the possibility that in an ocean with negligible molecular oxygen and elevated si and fe the growth of iron rich clay minerals was favored over hematite

diagnostic characteristics and paleodynamics of supraglacial rock avalanches innerdalen western norway abstract misinterpretation of rock avalanche deposits in mountain environments is not uncommon because of the complex interactions between rock avalanches and glaciers this paper for the first time presents a detailed description of suspicious rock boulder deposits which are remnants of a rock avalanche onto a late pleistocene glacier the boulder deposits cover around 1 1 km 2 and are distributed over an area of around 7 5 km 2 and a variety of landforms three closely nested distinct terminal moraines 0 45 km 2 and dislocated terminal moraines 0 34 km 2 entirely built up of large boulders lateral moraines that decrease in elevation over a short distance and connect to the terminal moraines small patches 0 02 km 2 of boulders that occur in islands on a peat dominated valley floor and large patches of boulder fields 0 25 km 2 in size hanging 350 m above the valley floor the latter are disconnected from any potential source area 10 be cosmogenic nuclide ages of the three terminal moraines give an indistinguishable late pleistocene age of 13 6 1 4 ka whereas the largest boulder field that is located on the opposite side of the valley from the failed mountain yields a 10 be age of 14 1 0 4 ka which is slightly older but indistinguishable from the moraine deposits within uncertainty margins all boulder deposits add up to a volume of 31 10 6 m 3 the interpretation of a single rock avalanche source for those landforms is further confirmed by numerical runout modeling using dan3d a failure of the entire volume from skarfjellet mountain would result in a runout pattern that is identical to that of the distributed boulder fields when considering that the valley was filled with a glacier around 350 m thick the rock avalanche would not reach the position of the terminal moraines however a glacial re advance could have transported the boulders to that position the late pleistocene rock avalanche was followed by a holocene rock avalanche with a volume of 23 10 6 m 3 and a typical lobate deposit damming the innerdalen valley and creating a lake 10 be ages indicate that this event occurred 7 97 0 94 ka ago structural measurements performed on high resolution lidar scans show a strongly jointed source rock with three joint sets with dip dip direction 81 034 86 331 and 77 354 and a foliation with 22 003 the kinematic of the failures can thus be a combination of small scale wedge failures and planar sliding on shallow dipping foliation our detailed description can enable a better identification and interpretation of similar deposits in other mountain areas

archean banded iron formations of india abstract archean banded iron formations bifs are important lithologies for understanding the early earth processes greenstone belts in the four archean cratonic nucleii of peninsular india namely singhbhum bastar dharwar and bundelkhand host bif units that can be important candidates for case studies the bifs in these greenstone successions represent both algoma and superior type associations the oldest record comes from the paleoarchean southern eastern and western iron group of the singhbhum craton and the sargur supracrustals of the dharwar craton mesoarchean and neoarchean greenstone belts in the dharwar and bundelkhand cratons record a number of cycles of bif deposition in a spectrum of depositional settings from manganiferous arenite carbonate shelf associations to deep water volcanics and graywacke associations the bifs from the neoarchean bailadila group in the bastar craton were deposited in siliciclastic shelf association compositional variations in major trace and ree patterns have been evaluated from published data high grade iron ores are mainly mined from the bifs and locally mn ores occur in association as well microbial carbonates associated with some of the paleoarchean as well as neoarchean iron manganese formations suggest early life activities the lithological association of the archean iron formations have been explained in terms of both active margin and passive margin depositional settings bifs in association with manganiferous formations as old as paleoarchean time provide an opportunity to probe into the redox stratification of oceanic hydrosphere and in turn the oxygenation of early atmosphere

dust to dust evidence for the formation of primary hematite dust in banded iron formations via oxidation of iron silicate nanoparticles abstract conventional models for the deposition of banded iron formations bifs envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide hydroxide particles in surface waters that settled to form laterally extensive layers of iron rich sediment a fundamental tenet of this model is that fine grained hematite so called dusty hematite in least altered bifs represents the dehydration product of original oxide hydroxide precipitates however this premise has never been proven we have investigated the origin of the earliest formed iron oxides in chert in well preserved bifs of the 2 63 2 45 billion year old hamersley group australia we find that laminated chert in bifs show progressive stages of in situ alteration from gray green chert containing iron silicate nanoparticles to red chert with abundant hematite dust analysis of textures by transmission electron microscopy of samples from the transition zone between gray green and red chert reveals that dusty hematite formed after partial dissolution of iron silicate nanoparticles by the precipitation of iron oxides in resulting cavities these observations suggest that hematite dust is not a relict of an original seawater precipitate but the end product of post depositional oxidation our observations are consistent with paleomagnetic results from the hamersley group which record two major phases of magnetic remanence carried by hematite that post date deposition by more than 200 million years our results may provide an alternative explanation for the origin of jasper in bifs deposited before the start of the great oxidation event about 2 4 billion years ago if correct it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early precambrian seawater furthermore our results suggest that the primary iron precipitate in bifs was iron silicate mud that was silicified at or just below the sediment water interface a hypothesis that requires neither dissolved oxygen nor photosynthetic life but was an inorganic chemical process reflecting anoxic oceans enriched in iron and silica

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

evidence from laminated chert in banded iron formations for deposition by gravitational settling of iron silicate muds the deposition of clay sized particles by settling from suspension within a water column typically forms highly porous 80 90 water muds dominated by uniformly distributed and randomly oriented particles the depositional microfabric of muds is rapidly destroyed during burial resulting in mudstones with a strong preferred particle orientation here we show that laminated chert in banded iron formations from the hamersley group australia and ghaap group south africa preserve laminae of nanometer sized particles of greenalite with textures that resemble the fabric of freshly deposited muds the oldest such fabrics in the rock record the preservation of the textures implies that the nanoparticles were frozen by silica cement on or just below the seafloor an interpretation supported by their presence in intraformational chert clasts and nodules with strong differential compaction the co occurrence of stacks of plates forming thicker aggregates with multiple face to edge contacts indicates that the greenalite may have been deposited as loosely formed flocs the common occurrence in certain horizons of iron silicate microgranules dispersed in the greenalite mud resembles aggregates of clay particles documented in modern muds these textures support models for active seawater precipitation of the iron rich phases and point to collision and flocculation of particles during sediment transport upon deposition amorphous silica rapidly occluded the available porosity and entombed the precipitates in a chemically inert cement that limited burial related compaction and hindered diagenetic reactions providing a taphonomic view of the earliest mineralogy and texture of iron formations

10be and 26al production rates deduced from an instantaneous event within the dendro calibration curve the landslide of kofels otz valley austria surface exposure dating requires the knowledge of cosmogenic nuclide production rates when determining time integrated production rates the exposure ages of the calibration samples need to be accurately known the landslide of kofels austria is very well suited for this purpose it is the largest landslide in the crystalline alps of austria dating back to 7800 100 years bc ams 14c dating of buried wood which is well within the 14c dendro calibration curve exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10be and 26al to calculate sea level high geomagnetic latitude 60 open sky radionuclide production rates corrections were applied for altitude and latitude for shielding by surrounding mountains for sample geometry vegetation and snow cover and for sample thickness the production rates for an exposure age of 10 000 years are 5 75 0 24 10be atoms yr g sio2 and 37 4 1 9 26al atoms yr g sio2 a 26al 10be ratio of 6 52 0 43 can be calculated the influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13 000 years bp

recent channel adjustments in alluvial rivers of tuscany central italy drastic channel adjustments have affected the main alluvial rivers of tuscany central italy during the 20th century bed level adjustments were identified both by comparing available topographic longitudinal profiles of different years and through field observations changes in channel width were investigated by comparing available aerial photographs 1954 and 1993 98 bed incision represents the dominant type of vertical adjustment and is generalized along all the fluvial systems investigated the arno river system is the most affected by bed level lowering up to 9 m whereas lower incision generally less than 2 m is observed along the rivers of the southern part of the region human disturbances appear to be the dominant factors of adjustments the main phase of vertical change occurred during the period 1945 80 in concomitance with the phase of maximum sediment mining activity at the regional scale the second dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel based on measurements of channel width conducted on aerial photographs 38 of the reaches analysed experienced a narrowing greater than 50 of the initial channel width the largest values of channel narrowing were observed along initially braided or sinuous with alternate bars morphologies in the southern portion of the region a regional scheme of channel adjustments is derived based on initial channel morphology and on the amounts of incision and narrowing different styles of channel adjustments are described rivers that were originally sinuous with alternate bars to braided generally became adjusted by a moderate incision and a moderate to intense narrowing in contrast sinuous meandering channels mainly adjusted vertically with a minor amount of narrowing copyright 2003 john wiley sons ltd

static and dynamic simulation of a 700 m high rock slope in western norway abstract static and dynamic rock slope stability analyses were performed using a numerical discontinuum modelling technique for a 700 m high rock slope in western norway the rock slope has been investigated by the geological survey of norway ngu which has been carrying out rock slide studies for the county more and romsdal in western norway the purpose of numerical modelling was to estimate the volume of the rock mass that could potentially slide under static and dynamic forces this estimation was required to assess the run up heights tsunami in a fjord that could potentially be caused by the rockslide three cases have been simulated for predicting the behaviour of the rock slope first an initial static loading is applied in the numerical model to simulate the prevailing rock mass conditions at the site second saturated and weathered joint conditions are modelled by reducing the residual friction angle along the discontinuities of the rock mass in doing so the model simulates the effect of degradation of discontinuities in the rock slope third a dynamic loading based on peak ground accelerations expected in the area is applied to simulate dynamic earthquake conditions these numerical studies have provided some useful insights into the deformation mechanisms in the rock slope both sliding and rotation of blocks start to occur once the residual friction angle along the discontinuities is reduced and when the region is shaken by a strong earthquake the results indicate that due to variations in the inclination of discontinuities the entire slope does not become unstable and that down slope sliding and rotation of blocks occur mainly on the top layers of the slope within the range of parameter values considered for this study it is unlikely that the whole rock slope can be destabilised the study provides an illustration of how the geo mechanical properties of a rock mass can be integrated in a discontinuum rock slope model which is used for predicting the behaviour of the slope under existing environmental and earthquake conditions this model has helped not only to better understand the dynamics of the rockslide but also to estimate the potential rock volume that can become unstable when subjected to static and dynamic loads

numerical modelling of the gravity induced destabilization of a slope the example of the la clapiere landslide southern france abstract a finite difference two dimensional model with hooke mohr coulomb properties and topography derived from the dem are used to reproduce the la clapiere landslide the principal factor defining the gravity driven destabilization of the model is a gradual reduction in the cohesion this reduction simulates a degradation of the material properties with time because of weathering alteration processes the inelastic deformation fracturing and faulting first occur at mountain scale and results in normal fault formation causing crest sagging later the failure process is concentrated in the lower part of the slope and leads to the formation of a localized fault subparallel to the slope surface at a depth of ca 100 m this corresponds to the initiation of the la clapiere landslide and its propagation upslope a slow crest sagging continues during the whole model evolution

examples of multiple rock slope collapses from kofels otz valley austria and western norway abstract this paper describes the effects of large massive rock slope failures on subsequent slope stability three examples of large rock slope failures from the austrian alps and norway demonstrate that failure increases the probability of further collapses at kofels austria a holocene rock slope failure several km3 in size filled the otz valley the morphology of the deposits indicates that at least one subsequent failure occurred along the head scarp of the first failure most likely a slide of similar size debris of the second landslide slid over the older deposits forming the famous kofels frictionite at least three rock slope failures all of them in excess of 106 m3 occurred from the same mountainside within the last 6000 8000 years at tafjord in western norway the most recent of these failures in 1934 triggered a destructive tsunami five large failures with volumes 50 000m3 occurred at ramnefjell norway within 50 years two of them caused considerable damage and a large number of death due to the formation of destructive tsunamis two dimensional finite element models of rock slope stability before and after the kofels and tafjord landslides show that massive rock slope failures produce a irregular slopes parts of which are as steep or steeper than the slope before failure and which represent new zones of instability and b zones of weakness related to slow slope deformation and related cracking

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

kinematic evolution of fault ramps and its role in development of landslides and lakes in the northwestern argentine andes the quebrada del toro and quebrada de las conchas in the cordillera oriental of northwestern argentina are notable for repeated landsliding and damming of major drainages both areas correspond to displacement transfer zones along major thrust systems that developed during the neogene uplift of the central andes during this period displacement transfer zones were areas of subdued relief that localized where rivers cross the fault systems however neotectonic deformation converted the transfer zones into restraining bends along strike slip systems resulting in enhanced uplift at river crossings the antecedent streams responded by rapidly downcutting into the ascending hanging wall blocks producing precipitous local relief the extreme local relief that was controlled by the kinematic evolution of faulting in both areas provides an effective precondition for landsliding which may have been ultimately triggered by seismicity and gravitational failure aided by enhanced undercutting during past humid conditions

structural and lithological controls on large quaternary rock avalanches sturzstroms in arid northwestern argentina landsat thematic mapper tm analysis aerial photograph interpretation and field studies of the semiarid puna plateau adjacent eastern cordillera and sierras pampeanas of argentina lat 24 28 s have revealed the existence of at least 55 rock avalanche deposits with volumes larger than 10 6 m 3 that formed by the collapse of entire mountain fronts the spatial distribution of landslide deposits is not random but it clusters along mountain fronts bounded by active faults inspection in the field reveals five principal controls on the distribution of these events the source area of the rock avalanches has two topographic constraints 1 vertical relief contrasts between the breakaway zone and the mountain front must exceed a threshold of 400 m and 2 the slope inclinations must be steeper than 20 rock avalanches are restricted to three types of lithology granites low grade metamorphic rocks and coarse clastic sediments structural controls are very important rock avalanches are controlled by planar structures such as bedding planes exfoliation joints minor faults and cleavage that all dip toward the valley in addition major slide clusters occur along mountain fronts that underwent quaternary reverse fault reactivation of former transfer faults with strike slip kinematics the trigger mechanism for the majority of these landslides is interpreted to be seismic although the ages of some major slides are about 30 ka and they may correspond to a more humid interval in southern south america

the youngest big impact on earth deduced from geological and historical evidence the comparison of events that followed the great impact at the cretaceous tertiary boundary with those contained within the flood myths suggests an analogous cosmic cause this paper proposes that there is geological evidence to support this thesis which is also indicated by a careful analysis of the natural phenomena described in flood traditions the time the sites the cause the detailed course of the events and the consequences of this catastrophe can hope to be reconstructed

tiny particles building huge ore deposits particle based crystallisation in banded iron formation hosted iron ore deposits hamersley province australia abstract the world s major source of iron ore is hosted in precambrian banded iron formations these chemical meta sedimentary rocks are composed of alternating laminae of iron oxide minerals and chert despite the economic significance of high grade iron ore deposits controversy persists after decades of research on how banded iron formations became upgraded to form iron ore the fundamental requirement for iron ore formation is the removal of vast amounts of chert coupled with an increased concentration of iron oxide minerals here we assess the fate of colloidal hematite inclusions encapsulated in chert after quartz dissolution and examine their role in the formation of hematite ore we have analysed hematite ores from the hamersley province australia using a combination of petrography high resolution electron microscopy and x ray diffraction these techniques reveal the presence of abundant nano and microscale hematite particles that we suggest form the building blocks of larger hematite crystals within the iron ore our textural observations indicate that hematite colloids that were previously encapsulated inside the microcrystalline quartz grains in chert layers are released during quartz dissolution and subsequently reassemble in a self similar fashion to from new microplaty hematite crystals via non classical crystallisation pathways progressive growth and fusion leads to the transformation of hematite microplates into hematite bands which resemble pre existing iron oxide laminae of banded iron formations in contrast to previous models we observe the direct transformation of banded iron formations to microplaty hematite and have not found evidence that significant amounts of hematite formed through metamorphism of goethite or that intermediate carbonate minerals were involved during the upgrading of banded iron formations to pure hematite ore given the strong evidence for hypogene fluids found in many deposits we have also assessed the role that such warm highly saline fluids may have played during the evolution of iron ore using insights from crystal chemistry we conclude that fluid infiltration impacts many aspects of iron ore formation by controlling hematite colloid liberation and aggregation and finally controlling the transformation of the colloids into macroscopic crystals of hematite via non classical crystallisation mechanisms our study underlines the significance of hypogene fluids in the upgrading of banded iron formation to iron ore however we suggest that their influence was mainly passive as hematite was not precipitated directly from these fluids

electron microscopy study of hyalomylonites evidence for frictional melting in landslides abstract hyalomylonites from langtang nepal and kofels otz valley austria were investigated by transmission electron microscopy tem the matrix is glassy at both localities rarely it is crystallized and devitrified to dendrites and skeletal microlites of plagioclase alkali feldspar and biotite the matrix is chemically heterogeneous with schlieren and formed by partial to almost complete melting of host rocks of granitic to granodioritic composition locally glasses with pure quartz plagioclase and alkalifeldspar composition were found from sio 2 glass minimum temperatures of 1520 c are estimated at both hyalomylonite localities melting appears to be the result of frictional heating on the gliding plane of a large landslide the host rocks were deformed in a brittle mode microstructures typical of plastic deformation at high temperatures are displayed by inclusions and were probably inherited from previous tectonic events

geological model of the aknes rockslide western norway abstract aknes is known as the most hazardous rockslide area in norway at present and is among the most investigated rockslides in the world representing an exceptional natural laboratory this study focuses on structural geology and the usage of geophysical methods to interpret and understand the structural geometry of the rockslide area the interpretations are further used to build a geological model of the site this is a large rockslide with an estimated volume of 35 40 million m 3 derron et al 2005 defined by a back scarp a basal shear zone at about 50 m depth and an interpreted toe zone where the sliding surface daylights the surface the rockslide is divided into four sub domains experiencing extension in the upper part and compression in the lower part structural mapping of the area indicates that the foliation of the gneiss plays an important role in the development of this rockslide the upper boundary zone of the rockslide is seen as a back scarp that is controlled by and parallel to the pre existing steep foliation planes where the foliation is not favourably orientated in regard to the extensional trend the back scarp follows a pre existing fracture set or forms a relay structure the foliation in the lower part dipping 30 to 35 to s sse seems to control the development of the basal sliding surface with its subordinate low angle trust surfaces which daylights at different levels the sliding surfaces are sub parallel to the topographic slope and are located along mica rich layers in the foliation geophysical surveys using ground penetrating radar gpr refraction seismic and 2d resistivity profiling give a coherent understanding of undulating basal sliding surface in the subsurface the geophysical surveys map the subsurface in great detail to a depth ranging from 30 40 m for gpr to approximately 125 m for refraction seismic and 2d resistivity profiling this gives a good control on the depth and lateral extent of the basal sliding surface and its subordinate low angle thrusts drill cores and borehole logging add important information with regard to geological understanding of the subsurface fracture frequency fault rock occurrences geophysical properties and groundwater conditions both in outcrops and or drill cores constrain the geometrical and kinematic model of aknes rockslide

the origin of clay minerals in active and relict hydrothermal deposits samples of fe oxide rich hydrothermal sediments were collected from active and inactive portions of the transatlantic geotraverse tag hydrothermal field on the mid atlantic ridge clays separated from tag metalliferous sediments in this study all consist of al poor nontronite oxygen isotope thermometry of the clays yields formation temperatures of 54 67 c for samples from the inactive alvin mound compared with 81 96 c for samples from the active tag site the latter are the highest recorded temperatures for authigenic hydrothermal clays sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater dominated fluids containing less than 15 hydrothermal end member fluid in contrast nontronite from the inactive alvin mound has 87sr 86sr values that closely resemble that of detrital north atlantic clays suggesting a dominantly continental source for the sr rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid crystallographic fractionation of the trivalent ree is apparent in the heavy ree enrichments for all nontronite samples the inferred formation mechanism for nontronite rich fe oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids at the inactive alvin site in contrast the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and fe oxyhydroxide these two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposit

causes of 20th century channel narrowing in mountain and piedmont rivers of southeastern france extensive channel narrowing in southeastern france provides an illustration of geomorphic response to land use changes the study region comprises a range of environments from large piedmont and intramountain gravel bed rivers to small mountain streams field measurements and analysis of historical data demonstrate two distinct periods of channel change from 1850 to 1950 channel narrowing is interpreted to be the result of a recovery process in response to widespread channel destabilization induced by major floods during the second half of the 19th century at the time the largely deforested basins were highly responsive to flooding whereas the recovery process was accelerated by floodplain and basin scale land use changes afforestation and torrential control works which in turn reduced sediment delivery and enabled vegetation development in channels from 1950 to 1970 channel narrowing accelerated in most of the studied rivers this recent phenomenon is considered as a human induced fluvial adjustment directly related to forest development on river margins and human abandonment of intensive floodplain land uses at the same time long profile degradation occurred as a result of long term bedload supply decrease in small mountain streams channel narrowing is mainly explained by channel incision which seems to progress from upstream where sediment sources are progressively stabilized by afforestation copyright 2002 john wiley sons ltd

downstream variation in grain size along an alpine river analysis of controls and processes downstream variation in bed material size was studied along a 115 km reach of the piave river eastern alps italy this river drains a region producing high sediment yields and has a braided pattern along half of its length during the last century the river system has been affected by several human impacts hydroelectric dams diversions gravel mining which have strongly altered flow regime and sediment dynamics the aim of the study is to analyse the changes of bed material size in the downstream direction and to explain these changes in the light of both natural and anthropogenic factors surface material was sampled at 35 sites using the grid by number method the collected data give a detailed picture of the downstream variation in sediment size which is characterised by a complex pattern rather than by a simple decreasing trend an explanation of this variation was found through identification of discontinuities significant steps in the downstream trend and factors which can determine changes in sediment texture both natural lateral sediment sources and anthropogenic factors e g barrages are significant in this river system and they provide an explanation for most of the observed discontinuities seven out of eight discontinuities few tributaries disrupt downstream fining while as expected the barrages produce important changes in sediment texture fining processes were investigated in the lower part of the study reach where the lateral sediment sources and the barrages have minor effects on the bed material exponential models provide acceptable explanations of the downstream variations and give fining coefficients equal to 0 027 and 0 028 km 1 respectively for d50 and d84 evidence suggests that both abrasion and sorting are important in determining the downstream fining observed but it is likely that sorting is the dominant process

the biogeochemistry of ferruginous lakes and past ferruginous oceans abstract anoxic and iron rich ferruginous conditions prevailed in the ocean under the low oxygen atmosphere that occurred through most of the archean eon while euxinic conditions i e anoxic and hydrogen sulfide rich waters became more common in the proterozoic ferruginous conditions persisted in deep waters ferruginous ocean regions would have been a major biosphere and earth surface reservoir through which elements passed through as part of their global biogeochemical cycles understanding key biological events such as the rise of oxygen in the atmosphere or even the transitions from ferruginous to euxinic or oxic conditions requires understanding the biogeochemical processes occurring within ferruginous oceans and their indicators in the rock record important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes their sediments commonly host siderite and ca carbonates which are important precambrian records of the carbon cycling lakes that were ferruginous in the past or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid depth oceanic waters during the proterozoic modern ferruginous meromictic lakes which host diverse anaerobic microbial communities are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans such lakes are believed to be rare but regional and geological factors indicate they may be more common than previously thought while physical mixing processes in lakes and oceans are notably different many chemical and biological processes are similar the diversity of sizes stratifications and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems near shore off shore silled basins or those dominated by terrestrial or hydrothermal element sources ferruginous systems both extant and extinct lacustrine and marine host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of earth s surface environment

numerical modeling of the june 17 2017 landslide and tsunami events in karrat fjord west greenland on june 17 2017 the western coast of greenland was the site of a tsunami which flooded several villages killing 4 people and destroying 11 houses in the village of nuugaatsiaq this tsunami was triggered by a subaerial landslide which occurred in a fjord 32 km ene of nuugaatsiaq this paper presents the numerical modeling of this landslide of sim 50 million hbox m 3 and of the tsunami propagation from its source to nuugaatsiaq the landslide is considered as a granular flow under gravity forces and the water waves generated are related to the displacement of the sea bottom the results obtained are similar in amplitude to our inferences from videos i e three water waves between 1 and 1 5 m arriving at nuugaatsiaq with a period of roughly 3 min and are also in general agreement with the amplitude 1 m resulting from deconvolution of oscillations recorded on a horizontal seismogram operating at nuugaatsiaq nuug according to the field survey performed by fritz et al egu general assembly conference abstracts vol 20 of egu general assembly conference abstracts p 18345 2018a on july 2017 a second mass next to the landslide is threatening karrat fjord a sensitivity study is realized on its volume with 2 7 14 and 38 million hbox m 3 reaching the sea the shape of the water waves is found to be independent of volume and linearity is observed between the volume and the water wave heights finally the orientation of the slide does not seem to influence either the period or the shape of the generated water waves

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

seismic signatures of landslides the 1990 brenda mine collapse and the 1965 hope rockslides abstract we present the well documented case of a 2 10 6 m 3 rockslide that was recorded by seismographs to distances of several hundred kilometers and might have entered the canadian catalog as a bona fide earthquake except for independent knowledge to the contrary motivated by the observation of this event we reexamined the seismograms associated with the 1965 hope slide in british columbia the largest known historical landslide in the canadian cordillera and conclude that these seismograms were the signatures of two rockslides and not of hypothetical tectonic earthquake triggers as previously suggested mathews and mctaggart 1969 wetmiller and evans 1989 these events raise the general question of differentiation between seismic signals from landslides and true earthquakes we suggest an lp sp discriminant as the most obvious although a resolvable mechanism solution may be even more convincing efficiency of energy conversion from potential to seismic is also a diagnostic aid and correlates well with the slope of the slide detachment surface

incorporating seepage losses into the unsteady streamflow equations for simulating intermittent flow along mountain front streams 1 seepage losses along numerous mountain front streams that discharge intermittently onto alluvial fans and piedmont alluvial plains are an important source of groundwater in the basin and range province of the western united states determining the distribution of seepage loss along mountain front streams is important when assessing groundwater resources of the region seepage loss along a mountain front stream in northern nevada was evaluated using a one dimensional unsteady streamflow model seepage loss was incorporated into the spatial derivatives of the streamflow equations because seepage loss from streams is dependent on stream depth wetted perimeter and streambed properties a two dimensional variably saturated flow model was used to develop a series of relations between seepage loss and stream depth for each reach this method works when streams are separated from groundwater by variably saturated sediment two periods of intermittent flow were simulated to evaluate the modeling approach the model reproduced measured flow and seepage losses along the channel seepage loss in the spring of 2000 was limited to the upper reaches on the alluvial plain and totaled 196 000 m3 whereas 64 of the seepage loss in the spring of 2004 occurred at the base of the alluvial plain and totaled 273 000 m3 a greater seepage loss at the base of the piedmont alluvial plain is attributed to increased streambed hydraulic conductivity caused by less armoring of the channel the modeling approach provides a method for quantifying and distributing seepage loss along mountain front streams that cross alluvial fans or piedmont alluvial plains

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

iron silicate microgranules as precursor sediments to 2 5 billion year old banded iron formations banded iron formations bifs are chemical sedimentary rocks comprising alternating layers of iron rich and silica rich minerals that have been used to infer the composition of the early precambrian ocean and ancient microbial processes however the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues petrographic examination of the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia reveals the presence of abundant silt sized microgranules composed of stilpnomelane the microgranules are most common in the least altered bif where they define sedimentary laminations implying a depositional origin we suggest that the precursor mineral was an iron rich silicate that formed either in the water column or on the seafloor the microgranular texture may have developed due to clumping of amorphous mud forming silt sized floccules the microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular rich lamina overlain by amorphous mud with dispersed microgranules the lamina sets collectively define plane lamination structure probably of the lower flow regime the microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in bif we propose that for most of the early precambrian the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation

the formation and impact of landslide dams state of the art abstract the blocking of river courses by mass movements is very common in mountainous areas with deep and narrow valleys landslide dams may pose serious threats to people and their livelihoods downstream in the case of abrupt dam failure since the publication of benchmark reviews of costa and schuster 1988 and korup 2002 there is a growing number of studies focusing on the formation stability and short term impacts of landslide dams this review combines the insights of all these studies builds on current concepts of landslide dams and suggests ways to unify terminologies and classifications we furthermore present a new worldwide database compiled from literature data it contains 410 landslide dams 1 million m3 in volume that were formed since 1900 since these have the most complete data entries these data show that dam longevity is among other factors correlated with the type of landslide forming the dam those formed by rock debris avalanches and rockslides have longest lifespans however the influence of landslide type or material on dam longevity decreases with time after dam formation to ensure consistency in the next database generation we suggest guidelines for data collection to provide a solid basis for evaluating dam stability and governing factors a preliminary classification matrix for landslide dam stability that combines topographic setting and the internal structure of the dam body is another outcome of our review furthermore an evaluation of the various geomorphic stability indices proposed in the literature regarding their suitability and limitations in assessing dam formation and stability shows that they predict the probability of dam formation reasonably well but that their application to longevity estimates requires further assessment the geomorphic impacts of landslide dams in the short medium and long term are summarized and illustrated with key examples finally for a better understanding of the factors controlling dam stability we recommend to 1 include dam composition and sedimentary structures in future case studies 2 maintain and update the worldwide database for sound statistical analyses 3 refine landslide dam stability indices and test them for different landslide types and 4 study hazard cascades related to multiple dams in one watershed for long term landscape evolution studies we suggest to 5 quantify terrestrial sediment flux related to landslide dams 6 detect ancient landslide dams in river profiles and 7 further exploit the sediment archives in former impoundment areas

making magnetite late again evidence for widespread magnetite growth by thermal decomposition of siderite in hamersley banded iron formations abstract banded iron formations bifs are marine chemical sedimentary rocks that were widely deposited before the start of the great oxidation event 2 45 2 32 ga they represent an important archive into the chemistry of the precambrian oceans and atmosphere and the evolution of the marine biosphere it is believed that bifs were derived from ferric oxides hydroxides that were deposited by iron oxidizing bacteria or by reaction with oxygen released by cyanobacteria hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide hydroxide precipitate whereas magnetite is thought to have formed from ferric oxides hydroxides via bacterial iron reduction and oxidation of microbial biomass however studies are equivocal about when and how magnetite formed in bifs we present the results of a detailed petrographic study of magnetite in bifs of the hamersley group western australia we find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction instead most of the magnetite formed by replacement of siderite and other ferrous rich minerals after burial our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern pilbara craton 200 350 c the thermal decomposition of siderite could account for much of the magnetite in the hamersley bifs 3feco3 h2o fe3o4 3co2 h2 the relationship between magnetite and the primary precipitates which are preserved as greenalite nanoparticles and microgranules in chert bands suggests that much of the magnetite formed in siderite rich iron silicate bands that developed by compaction of non silicified iron silicate muds these bands acted as reactive pathways for fluid flow and the formation of magnetite if most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron our results are consistent with models for the deposition of bifs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial

abiologic silicon isotope fractionation between aqueous si and fe iii si gel in simulated archean seawater implications for si isotope records in precambrian sedimentary rocks abstract precambrian si rich sedimentary rocks including cherts and banded iron formations bifs record a 7 spread in 30si 28si ratios 30si values yet interpretation of this large variability has been hindered by the paucity of data on si isotope exchange kinetics and equilibrium fractionation factors in systems that are pertinent to precambrian marine conditions using the three isotope method and an enriched 29si tracer a series of experiments were conducted to constrain si isotope exchange kinetics and fractionation factors between amorphous fe iii si gel a likely precursor to precambrian jaspers and bifs and aqueous si in artificial archean seawater under anoxic conditions experiments were conducted at room temperature and in the presence and absence of aqueous fe ii fe ii aq results of this study demonstrate that si solubility is significantly lower for fe si gel than that of amorphous si indicating that seawater si concentrations in the precambrian may have been lower than previous estimates the experiments reached 70 90 si isotope exchange after a period of 53 126 days and the highest extents of exchange were obtained where fe ii aq was present suggesting that fe ii fe iii electron transfer and atom exchange reactions catalyze si isotope exchange through breakage of fe si bonds all experiments except one showed little change in the instantaneous solid aqueous si isotope fractionation factor with time allowing extraction of equilibrium si isotope fractionation factors through extrapolation to 100 isotope exchange the equilibrium 30si 28si fractionation between fe iii si gel and aqueous si 30sigel aqueous is 2 30 0 25 2 in the absence of fe ii aq in the case where fe ii aq was present which resulted in addition of 10 fe ii in the final solid creating a mixed fe ii fe iii si gel the equilibrium fractionation between fe ii fe iii si gel and aqueous si 30sigel aqueous is 3 23 0 37 2 equilibrium si isotope fractionation for fe si gel systems is significantly larger in magnitude than estimates of a near zero solid aqueous fractionation factor between pure si gel and aqueous si indicating a major influence of fe atoms on si o bonds and hence the isotopic properties of fe si gel larger si isotope fractionation in the fe ii bearing systems may be caused by incorporation of fe ii into the solid structure which may further weaken fe si bonds and thus change the si isotope fractionation factor the relatively large si isotope fractionation for fe si gel relative to pure si gel provides a new explanation for the observed contrast in 30si values in the precambrian bifs and cherts as well as an explanation for the relatively negative 30si values in bifs in contrast to previous proposals that the more negative 30si values in bifs reflect hydrothermal sources of si or sorption to fe oxides hydroxides

kinematics of an alpine retrogressive rockslide in the japanese alps the aresawa rockslide in the japanese alps experienced a partial collapse 5 10 105 m3 in 2004 followed by an accelerated downward movement of the head area rather than stabilization this paper discusses the kinematics of a retrogressive rockslide area and factors promoting slope instability subsequent to the 2004 collapse on the basis of geodetic surveys and meteorological observations from 2006 to 2010 the unstable area covers at least 2 3 104 m2 and consists of five active blocks the main blocks are separated from the stable rock slope by pre existing sackung features that follow the strike of the bedding plane shear zones below these sackung features have developed into slip planes potentially causing the next collapse in particular the propagation of a slip plane causing movement at 70 cm yr 1 in the head area is promoted by bidirectional movement it consists of slow dip slip movement in snow accumulating periods and rapid side slip movement in snow melting and snow free periods both of which occur on the same slip plane this bidirectional movement appears to result from the combination of a loss of lateral support by the 2004 event and the immature state of the slip plane the primary trigger of the acceleration in the unstable area is the stress release by the 2004 collapse in addition the presence of a potential slip plane below the sackung features also promotes slope instability suggesting that sackung features behind a rockslide area may predefine the new head scarp of a forthcoming collapse copyright 2012 john wiley sons ltd

petrology and geochemistry of the boolgeeda iron formation hamersley basin western australia abstract the boolgeeda iron formation and overlying turee creek group hamersley basin western australia represent a conformable succession of sediment deposited between 2 45 and 2 22 ga this interval of geologic history is of significant interest because it spans the great oxidation event goe when oxygen first accumulated in earth s atmosphere here we present geochemical and petrographic data from the uppermost 45 m of the boolgeeda iron formation and an additional 30 m of the overlying kungarra formation mudstones as sampled from the turee creek drilling project 1 drill core tcdp1 this core captures the termination of bif deposition in the hamersley basin and coincides with a global decline in bif deposition in the paleoproterozoic we provide a continuous high resolution chemostratigraphic dataset of major and trace element concentrations as well as fe speciation data to assess the relationship between the rise of atmospheric oxygen and the subsequent decline in bif deposition we also highlight the interplay between local and global controls on the preservation of redox signatures including the rise and fall of local base level input of weathered continental material influx of reduced sulfur species associated with the continental weathering and global increases in atmospheric oxygen we interpret boolgeeda deposition to have taken place under shallow water oxic conditions overlying anoxic ferruginous deep water intermittent periods of oxidative weathering led to influxes of redox sensitive trace elements from land as continental sulfide minerals were weathered this also led to the temporary disappearance of mif s but o2 remained below the threshold capable of completely eliminating s isotope signatures associated with the mif s recycling

precipitation of iron silicate nanoparticles in early precambrian oceans marks earth s first iron age the early ocean was characterized by anoxic iron rich ferruginous conditions before the rise of atmospheric oxygen 2 45 b y ago a proxy for ferruginous conditions in the ancient ocean is the deposition of banded iron formations bifs which are iron and silica rich chemical sediments whose constituents were largely derived from seawater although experiments simulating ancient ocean chemistry support the rapid growth of iron silicate phases the main iron precipitates are hypothesized to have been ferric oxyhydroxides the paradox between the prevailing reducing conditions and the deposition of oxidized iron phases is explained by biologically mediated oxidation in the water column new high resolution microscopy of bifs and shales throughout the 2 63 2 45 b y old hamersley group australia reveals the presence of vast quantities of nanometer sized iron silicate particles in laminated chert the nanoparticles are finely disseminated in early diagenetic chert and locally define sedimentary lamination indicating that they represent relicts of the original sediments by inference from experimental studies simulating the composition of the early precambrian ocean we suggest that the nanoparticles precipitated from anoxic seawater enriched in silica and dissolved iron and were silicified upon deposition the prevalence of iron silicate nanoparticles implies that they were pervasive background precipitates in ferruginous silica enriched oceans forming the primary sediments of bifs during periods of enhanced submarine mafic volcanism our results imply that silicate precipitation was a major sink of seawater iron and silica before the great oxidation event and because of the reactivity of nanoparticle surfaces may also have influenced the transport and geochemical cycling of trace metals and nutrients our hypothesis that the basic building blocks of bifs were predominantly iron silicate muds rather than iron oxides and or hydroxides may lead to new insights into seawater chemistry on the early earth and the role of biology in the deposition of bifs

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

sedimentology of rock avalanche deposits case study and review abstract detailed sedimentological studies concerning different facies types and the factors influencing their development such as lithology topography substrates etc were carried out at the carbonate tschirgant rockslide rock avalanche deposit in the eastern alps of austria several depositional facies and sub facies have been identified at this and other sites by many researchers these facies are found throughout the debris regardless of depth or travel distance and result from highly heterogeneous stress distributions in time and space during emplacement one of the most important recent observations in terms of emplacement dynamics is that shear is not confined to the base but is distributed throughout the debris except the coarse carapace due to the preservation of source stratigraphy in these non turbulent mass movements any horizontal facies or grain size variations need to be regarded in context since they strongly depend on source stratigraphy and hence the spatial distribution of lithologies i e geological units of different mechanical properties for example some lithologies do not form a boulder carapace due to absence of large in situ blocks in the source rock mass however at tschirgant slight facies maturation with distance within the same lithology is observed i e clast fragmentation progresses to create the distinct facies characteristic grain size distribution only after some distance travelled these details are revealed through facies based sampling and analysis studies based on bulk sampling in some cases observed progressively reduced grain sizes with distance and depth which might be a function of added variations within different facies sampled together in this paper the wealth of observations on rockslide and rock avalanche sedimentology of the past decades is combined with a detailed case study of a lithologically relatively simple deposit to outline global and site specific depositional structures and facies with the aim to summarize review and refine the current state of knowledge on rock avalanche and rockslide emplacement processes

neotectonics and catastrophic failure of mountain fronts in the southern intra andean puna plateau argentina new cosmogenic nuclide exposure ages of rock avalanche deposits in front of sierra laguna blanca in the arid argentine puna plateau demonstrate the effects of westward propagation of reverse faulting and the influence of active faulting on slope oversteepening and subsequent gravitational collapse the 21 ne exposure ages of 8 superposed landslides with volumes 0 3 km 3 are between ca 150 and 430 ka and indicate an average recurrence interval of large gravitational mountain front failures of 27 5 k y for the ultimate 7 landslides landsliding did not occur after 150 ka along this sector of the sierra laguna blanca the termination of landsliding activity correlates with reverse fault propagation into the piedmont area of the sierra laguna blanca the exposure age of ca 130 ka of a tectonically uplifted terrace postdates initiation of fault activity in the piedmont region and the fault scarp height of 120 m indicates important continued uplift in the piedmont no further offsets occurred at the mountain front due to this lack of tectonic activity at the mountain front no further collapse has occurred indicating that in this environment catastrophic mountain front failure is mainly controlled by tectonically driven slope oversteepening

subaqueous rock avalanche deposits exposed by post glacial isostatic rebound innfjorddalen western norway abstract this paper presents a detailed description of deposits and landforms of multiple rock avalanches in western norway one of which fell onto water saturated sediments in innfjorddalen below the former water level deposits of the latter are now exposed on the valley floor due to post glacial isostatic rebound at least three rock avalanches from the same source at grafonnfjellet mountain have occurred during late glacial and post glacial time and their deposits are distributed over an area of 1 44 km 2 in the valley these rock avalanches have volumes of 15 1 10 6 m 3 5 4 10 6 m 3 and 0 3 10 6 m 3 and yielded cosmogenic radionuclide 10 be ages of 14 3 1 4 ka 8 79 0 94 ka and 1 028 0 380 ka respectively the youngest event dates within uncertainty limits to a historic rock avalanche in the year 1611 12 ce the rock avalanches formed a stratified succession of deposits the rock avalanche deposits 1 38 m 2 have lobate forms have frontal rims and parallel ridges extend across the valley floor and up the opposite slope and form dams on the valley floor isolated hills comprised of rock boulders 0 61 km 2 interpreted to be toma hills are disconnected from the main rock avalanche deposits by a 520 m wide zone of deformed valley fill sediments trenches and a ground penetrating radar survey of these deposits indicate large scale deformation or liquefaction numerical runout modeling of the rock avalanches with the code dan3d supports the interpretation of their landforms and sources and highlights their runout behavior

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

sedimentology of the 3 3 ga upper mendon formation barberton greenstone belt south africa the mendon formation is the uppermost unit of the 3 5 3 26 ga onverwacht group in the barberton greenstone belt south africa it consists of a cyclic stack of komatiitic volcanic units separated by thin cherty sedimentary layers in most areas the uppermost mendon formation is a sedimentary interval characterized by black chert banded black and white chert and banded ferruginous chert although the detailed patterns of lithofacies in different sections are more complex previously reported zircon u pb ages suggest that mendon deposition could represent more than 70 myr of time between 3 334 ma and 3 260 ma this study presents sedimentological and petrographic observations of the upper mendon formation from across the central part of the barberton greenstone belt in order to investigate sediment sources depositional processes and environments of sedimentation the dominant mode of sedimentation was quiet settling of carbonaceous grains and in the deepest sections below storm wave base fine ferruginous material resulting in finely laminated black and grey chert in situ carbonaceous laminations are rare suggesting that benthic microbial mat growth had little direct influence on deposition the hemipelagic background deposition was punctuated by occasional inputs of fine pyroclastic debris formation and deposition of silica granules and reworking by infrequent storm events storm deposits are represented by coarse grained poorly sorted intraclast breccias some of which include distinctive intraclasts sampling lithofacies that are not observed in situ despite considerable lateral variability correlative temporal trends are resolvable in many mendon sections there is an upward deepening of the overall depositional setting recorded in the oldest upper mendon sections consistent with the previous interpretation that mendon time was characterized by rifting lowe 1994a lowe 1999a younger mendon cycles include thick relatively ferruginous basal sections interpreted to reflect the deepest water deposition these sections are capped by black chert and silicified ashes with more evidence of disturbance and reworking by storms reflecting gradual shoaling this sedimentological analysis is broadly consistent with previous geochemical and tectonic analyses and provides a better picture of depositional patterns during uppermost onverwacht time before the distinct change in tectonic regime marked by impact spherule layer s2 and the onset of fig tree group orogenesis and related siliciclastic deposition

channel adjustments and vegetation cover dynamics in a large gravel bed river over the last 200 years article i nfo the timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bed river the piave river eastern italian alps that was heavily impacted by human activities training structures hydropower schemes and gravel mining have been analyzed by historical maps aerial photos repeated topographic measurements and geomorphological surveys results show that the channel underwent a strong narrowing during the twentieth century but with a faster pace during the 1970s 1990s and with an associated shift from a dominant braided pattern to a wandering morphology bed incision up to 2 m mostly from gravel mining has been documented for this period large areas of the former active channel were colonized by riparian forests both as islands and as marginal woodlands the ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend with evidence of vegetation erosion channel widening even though a significant aggradation phase is not present we conclude that alteration of sediment regime has played a major role on the long term channel evolution however only relevant flood events rin10 15 years appear to determine substantial island erosion and therefore the proportion of island vs channel area fluctuates depending on flood history

why did the 1756 tjellefonna rockslide occur a back analysis of the largest historic rockslide in norway abstract on 22 february 1756 the largest historically recorded rockslide in norway took place at tjelle in the langfjord western norway the rockslide created three displacement waves of up to 50 m in height that caused 32 casualties and destroyed most houses and boats along the shores of the langfjord the trigger and contributing factors leading to the tjellefonna rockslide are largely unknown and even seismic triggering has previously been suggested this study provides a thorough back analysis of the tjellefonna rockslide using detailed geomorphological engineering geological and tectonic field mapping in combination with topographic reconstructions bathymetry analysis volume estimations and numerical slope stability analysis the back scarp and eastern flank of the tjellefonna scar form several tens of meter high rock walls while the basal failure surface and other parts of the scar are covered by rock avalanche debris that extend from the back scarp down to the bottom of the langfjord the rockslide occurred in granodioritic gneisses with variably developed metamorphic foliation that is folded and strike parallel to the fjord two prominent fault zones are present in close proximity to the tjellefonna scar one is steeply se dipping tjelle fault while the other one is sub horizontal to shallow se dipping ritlehamran fault both fault zones are linked to the more trondelag fault complex with one of its branches forming the langfjord lineament and probably also the faults at tjellefonna additionally there are four persistent joint sets that together with the metamorphic foliation and the tjelle fault define the back scarp of the rockslide and give a fracturing of the rock mass corresponding to a geological strength index gsi of 45 55 the gsi decreases significantly to 10 20 in the fault zones which form distinct weakness zones in the rock slope volume estimates based on a reconstruction of the ante rockslide topography range from 9 3 to 10 4 million m 3 which is lower than previous volume estimates 12 15 million m 3 large portions of the failed rock mass remained on land and only approximately 3 9 million m 3 entered the fjord the observed discontinuities in the rock mass at tjellefonna do not allow for a simple kinematic failure mechanism due to the lack of moderately se dipping structures the basal failure surface was most likely not composed of a single structure but of a complex interplay of fault zones metamorphic foliation joints and broken rock bridges numerical slope stability modelling highlights that weak fault zones are essential for the development of the failure surface over a long time this progressive failure was likely aided by low to medium magnitude earthquakes that are frequent in the region numerical slope stability modelling and historical accounts suggest however that heavy long lasting rainfall was the triggering factor for the 1756 tjellefonna rockslide rather than an earthquake

fe isotopes of a 2 4 ga hematite rich if constrain marine redox conditions around the goe abstract the hematite and manganese rich hotazel iron formation griqualand west basin south africa was deposited at a key moment in time close to the goe between 2 4 and 2 3 ga it stratigraphically overlies the ongeluk formation comprising thick flood basalts which in turn interfinger with and cover the makganyene formation diamictites the inferred remnants of the first paleoproterozoic snowball earth interval no extensive research has been conducted to date on the basal part of the hotazel formation due to poor exposure though it constitutes an important link between a period of large scale ice cover extensive volcanism and the onset of atmospheric oxygenation here we present a detailed petrographic geochemical and fe isotope study of a roughly 3 metre long drill core exposing the ongeluk to hotazel contact our results show that after the cessation of ongeluk volcanism primary precipitation of fe iii oxyhydroxides from the photic surface zone of the original basin became the dominant sedimentation mechanism negative 56fe values between 0 26 and 0 50 in micro drilled hematite rich chert indicate that surface water 56fe compositions at the time of deposition were depleted yet 56fe and bulk rock fe mn values are still substantially higher 1 2 than those reported higher up in the mn rich layers of the hotazel sequence suggesting that redox potentials were still comparatively limited during the earliest stages of the hotazel depositional environment the base of the hotazel formation thus forms a transitional interval between precipitation from essentially ferruginous seawater as recorded in the ghaap group bifs and from isotopically and chemically highly evolved surface waters as demonstrated by the hotazel mn rich layers in the absence of fe ii as a strong reducing agent large volumes of photosynthetic oxygen may have eventually escaped into the atmosphere leading to the onset of atmospheric oxygenation our results thus contradict previous models that place the onset of the goe before the hotazel formation concurrent with and mechanistically linked to the makganyene and ongeluk events instead we show that the hotazel basal sediments are still pre goe consistent with their circa 2 4 ga age and the continuation of mif s higher in the stratigraphic record

giant rockslides from the inside the growing body of research on large scale mass wasting events so far has only scarcely investigated the sedimentology of chaotic deposits from non volcanic terrestrial landslides such that any overarching and systematic terminological framework remains elusive yet recent work has emphasized the need for better understanding the internal structure and composition of rockslide deposits as a means to characterise the mechanics during the final stages of runout and emplacement we offer a comprehensive overview on the occurrence of rock fragmentation and frictional melt both at different geographic locations and different sections within large 10 6 m 3 rockslide masses we argue that exposures of pervasively fragmented and interlocked jigsaw cracked rock masses basal melange containing rip up clasts and phantom blocks micro breccia and thin bands of basal frictionite are indispensable clues for identifying deposits from giant rockslides that may remain morphologically inconspicuous otherwise these sedimentary assemblages are diagnostic tools for distinguishing large rockslide debris from macro and microscopically similar glacial deposits tectonic fault zone breccias and impact breccias and thus help avoid palaeoclimatic and tectonic misinterpretations let alone misestimates of the hazard from giant rockslides moreover experimental results from mossbauer spectroscopy of frictionite samples support visual interpretations of thin sections and demonstrate that short lived 1500 degrees c in the absence of water occurred at the base of several giant moving rockslides this finding supports previous theories of dry excess runout accompanied by comminution of rock masses down to gm scale and indicates that catastrophic motion of large fragmenting rock masses does not require water as a potential lubricant

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

on the dynamics of the kofels sturzstrom geomorphologic data from the kofels landslide in the otz valley are compared with the results of computer modelling the study discusses the dynamic information that can be extracted from the data and argues that a standard coulomb friction model cannot adequately explain the observed features

the formation of magnetite in the early archean oceans abstract banded iron formations bifs are iron and silica rich chemical sedimentary rocks that were deposited throughout much of the precambrian the biological oxidation of dissolved fe ii led to the precipitation of a ferric oxyhydroxide phase such as ferrihydrite in the marine photic zone upon burial ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological fe iii reduction coupled to the oxidation of buried microbial biomass however it has always been intriguing as to why the oldest bifs are characteristically magnetite rich while bifs formed after the neoarchean are dominated by hematite here we propose that some magnetite in early archean bif could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot fe ii rich hydrothermal fluids that existed in the deeper waters we conducted experiments that showed the reaction of fe ii with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite our model further posits that with the progressive cooling and oxidation of the earth s oceans the above reaction shuts off and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism

widespread deposition of greenalite to form banded iron formations before the great oxidation event abstract banded iron formations bifs are precambrian sedimentary rocks interpreted to have been precipitated from anoxic seawater prior to the first permanent rise in atmospheric oxygen in the great oxidation event at ca 2 45 2 32 ga bifs hold the key to understanding the chemistry of the oceans and atmosphere and how these interacted with microbial life prior to and during the evolution of oxygenic photosynthesis to unlock this information it is essential to know how bifs formed that is what minerals were precipitated by what mechanisms and in which environments bifs are found in almost all depositional settings at times when input of clastic detritus was lacking for example submarine proximal volcanic environments basin floor slope deep marine shelf and shallow shelf settings high resolution electron microscopy of finely laminated bifs and ferruginous cherts that range in age from 3 45 to ca 2 4 ga and that preserve depositional features indicates that the original sediment was a very fine mud composed of nanoparticles of the iron silicate greenalite in places the greenalite mud experienced very early silicification on the sea floor enabling preservation of the mineral and its depositional textures very fine grained siderite and hematite in laminated bifs post date dehydration of the early silica cement and are not primary minerals greenalite nanoparticles are found in bifs from all depositional settings indicating a common origin likely precipitation resulting from mixing of plume water from hydrothermal vents with ambient seawater the nanoparticles were carried throughout the oceans and were deposited on the seafloor and on continental margins to form the primary sediments of bifs

multi stage structural and kinematic analysis of a retrogressive rock slope instability complex preonzo switzerland abstract large catastrophic rock slope failures are difficult to predict because the underlying mechanisms causing slope accelerations are difficult to study under in situ conditions for slope failures with compound basal rupture planes not only fracture propagation and slip in the basal rupture surface but also within the landslide body control the displacement evolution and time to failure the preonzo instability complex in southern switzerland failed several times since 2002 and offers a unique opportunity to study these mechanisms under in situ conditions the largest failure at preonzo occurred in may 2012 and was conditioned by large slope parallel nne ssw fracture zones which are intersected by ne and sw and e w striking faults creating diverging lateral boundaries whereas in the head scarp region oblique and flexural toppling along pre existing fracture sets is the prevailing mechanism sliding of either planar or wedge type is the preferred kinematic mode facilitated by the new discontinuities developed within the basal rupture surfaces these dominating mechanisms leading to a classical rock slope collapse could only be revealed in retrospect we compare fracture patterns in stable ground tectonic and unloading fractures with new fractures related to rock slope failure observed on basal rupture surfaces and in the head scarp simplified mechanical and kinematic analyses show that old pre existing tectonic fractures cannot accommodate substantial deformations under the in situ stress conditions and propagate connect through curved wing cracks forming at high angles this situation favors the development of new nearly slope parallel fractures which are interpreted as synthetic p shears along localized shear zones fractographic markings on these fractures indicate a continuous and relatively fast formation which presumably has taken place during a period of about 60 years prior to catastrophic failure this detailed description of the multi stage evolution of slope failure at preonzo represents a unique data set for future numerical studies of progressive failure in crystalline rocks

seafloor silicification and hardground development during deposition of 2 5 ga banded iron formations banded iron formations bifs are important archives of the ancient oceans atmosphere and biosphere but fundamental questions remain about their origin it is widely assumed that bifs were derived from layers of ferric oxyhydroxides and silica that precipitated directly from a water column that was enriched in dissolved iron and silica the reported lack of current generated structures and clastic particles beyond mud grade and the perceived basin scale extent of laminae is regarded as evidence for uninterrupted pelagic settling with no sedimentary reworking new sedimentological and petrographic results show that laminated cherts in the 2 5 ga dales gorge member of the brockman iron formation western australia preserve textures indicative of in situ brecciation immediately below the seafloor and the deposition of intraformational sandstones composed of chert clasts in a chert matrix chert intraclasts have two sedimentary components silt sized microgranules and submicron sized particles indicating that the original sediment comprised iron rich silicate muds that were cemented on or just below the seafloor by pore filling silica silicified muds were episodically eroded by density currents and the resulting detritus was transported as sand sized clasts and locally deposited in a matrix of microgranules and mud our results support the hypothesis that high concentrations of silica in early precambrian seawater favored episodic silica cementation of sediments on the seafloor we suggest that competition between sediment accumulation and seafloor silica cementation with subsequent differential compaction explains primary layering in bifs between beds of relatively thickly laminated chert and beds of thinly laminated iron rich minerals the thickest laminated chert beds are interpreted to represent intervals when seafloor silicification outpaced deposition of hydrothermal muds forming the equivalent of phanerozoic hardgrounds at sequence boundaries

changes in the magnitude and transformation of flood waves subsequent to the channelization of the raba river polish carpathians alterations in flood flows of the raba river are examined to determine the influence exerted on flood waves by changing morphological conditions with stable vertical channel position the river increased its sinuosity during the 1920s to 1940s and the change was accompanied by a growing tendency to flood wave attenuation the temporal change in flood wave transformation is typical of a developing low flow system subsequently streambed degradation has been induced due to channnelization works which straightened and narrowed the river flood waves became progressively more flashy as channel incision progressed the increase in magnitude of flood waves passing the deepened reach was greatest for bankfull flows and diminished for lower in bank flows and higher overbank flows the tendency to magnification of peak discharges has been also found in other carpathian rivers which were considerably degraded in the 20th century in response to channelization introducing an empirically found correcting factor into the analysis of the ratio of outflow to inflow peak discharges shows how the conditions of peak flow transformation in a reach have changed since the beginning of the study period a marked coincidence between changes in vertical channel location and variations in the corrected peak discharge ratio proves channel changes to be a very important reason for the growing flood hazard in southern poland gradient oversteepening and channel narrowing caused by channelization lead to formation of a river system having a steep straight narrow and deep channel such a morphology distinguishes the system from natural low flow and high flow systems reduced floodplain water storage and self acceleration of flow concentrated in a channel zone make flood waves progressively more flashy on their way down the channelization formed system

hematite replacement of iron bearing precursor sediments in the 3 46 b y old marble bar chert pilbara craton australia the history of atmospheric oxygen prior to the great oxidation event 2 45 2 2 ga is not well understood hematite in the marble bar chert from a nasa funded drill hole abdp1 in the pilbara craton australia has been cited as evidence for an oxygenated ocean 3 46 b y ago however isotopic data from the same drill hole have been used to argue for an anoxic ocean it is generally agreed that the hematite is primary representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis here we present new petrographic evidence from the marble bar chert in drill hole abdp1 that shows that hematite in jasper bands formed via mineral replacement reactions the hematite mostly occurs as sub micron sized inclusions within chert so called dusty hematite that are typically arranged into polygonal clusters surrounded by a rim of clear quartz resembling shrinkage structures the lateral transition from laminated chert enclosing minute inclusions of greenalite siderite and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron bearing minerals the presence of hematite rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite this interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries the occurrence of dusty hematite in fractures sedimentary laminae and the outer margins of polygonal clusters containing greenalite is consistent with fluid mediated oxidation of iron rich precursor minerals the presence of syn sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition the abundance of dusty greenalite inclusions which are texturally the earliest components of the laminated chert suggests that the precursor sediment contained an iron rich clay mineral our results show that hematite has replaced ferrous rich minerals after deposition and provide a mechanism to explain the origin of hematite in the marble bar chert which is consistent with the origin of hematite in adjacent basalts a secondary origin for hematite invalidates arguments for an oxygen bearing ocean 3 46 b y ago and provides a viable explanation for the formation of archean jasper bands our findings show that misinterpretations about the origin of hematite in early precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere

rock avalanches clusters along the northern chile coastal scarp abstract rock avalanche clusters can be relevant indicators of the evolution of specific regions they can be used to define the type and intensity of triggering events their recurrence and potential probability of occurrence the progressive damage of the rock mass the mechanisms of transport and deposition as well as the environmental conditions at the time of occurrence this paper tackles these subjects by analyzing two main clusters of rock avalanches each event between 0 6 and 30 mm 3 separated by few kilometers and located along the coastal scarp of northern chile south of iquique it lies hence within a seismic area characterized by a long seismic gap that ended on april 1st 2014 with a m w 8 2 earthquake the scar position high along the coastal cliff supports seismic triggering for these clusters the deposits relative positions are used to obtain the sequence of rock avalanching events for each cluster the progressive decrease of volume in the sequence of rock avalanches forming each cluster fits well the theoretical models for successive slope failures these sequences seem to agree with those derived by dating the deposits with ages spanning between 4 kyr and 60 kyr an average uplift rate of 0 2 mm yr in the last 40 kyr is estimated for the coastal plain giving a further constraint to the rock avalanche deposition considering the absence of reworking of the deposits volume estimates and datings allow the estimation of an erosion rate contribution of about 0 098 0 112 mm km 2 yr 1 which is well comparable to values presented in the literature for earthquake induced landslides we have carried out numerical modeling in order to analyze the mobility of the rock avalanches and examine the environmental conditions that controlled the runout in doing so we have considered the sequence of individual rock avalanches within the specific clusters thus including in the models the confining effect caused by the presence of previous deposits bingham rheology was the most successful at explaining both the distance and the geometry of the observed events

reverse engineering mother nature shale sedimentology from an experimental perspective abstract experimental study of the sedimentology of shales can take a variety of forms at its simplest one can experiment with suspensions in a glass jar and try to understand their settling behavior or one can manipulate mud in a tank or bucket to gain insights into its rheology this approach was championed over a century ago by sorby and the insights gained can be quite profound more recently tank and settling tube experiments of animal sediment interactions compaction behavior and sediment unmixing via re suspension have proven to be highly informative in spite of their simplicity flumes can be used to obtain quantitative information about depositional and erosional parameters and to generate fundamental bedforms in flume experiments however it is of critical importance that the flume be designed in a way that flocculated materials move under shear stress conditions that would be reasonable in natural environments although much flume work on muds has been conducted by hydraulic engineers the transfer of that knowledge to sedimentology is hampered by the fact that engineers and sedimentologists are interested in different though not mutually exclusive products from such experiments engineers and hydrologists are commonly concerned with quantifying fluid flow properties whereas sedimentogists are particularly interested in the sedimentary products that result from a variety of flow conditions recent sedimentologically oriented flume studies have shown that muds can form deposits at flow velocities and shear stresses that would suffice to transport and deposit medium grained sand mud suspensions are prone to flocculation and the resulting floccules travel in bedload and form ripples that accrete into beds the latter finding suggests that many laminated shales were deposited from currents rather than by settling from slow moving or still water there are many other sedimentary features in shales that can potentially be reproduced in flume studies and in the future serve to provide a quantitative basis for shale sedimentology

greenalite precipitation linked to the deposition of banded iron formations downslope from a late archean carbonate platform banded iron formations bifs were deposited as deep water facies distal to the late archean campbellrand carbonate platform transvaal supergroup south africa they are traditionally interpreted to have formed from iron oxides hydroxides and silica that precipitated when upwelling hydrothermal water enriched in fe2 and silica mixed with cooler shallow seawater enriched in dissolved oxygen the ferric oxides hydroxides were then converted to hematite or in the presence of organic matter reduced and incorporated into siderite new high resolution microscopy of bifs from distal facies of the campbellrand platform questions that interpretation revealing the presence of abundant greenalite nanoparticles interpreted to represent primary precipitates from ferruginous seawater consistent with recent results from equivalent aged bifs of the hamersley group western australia the particles locally define primary sedimentary lamination and are engulfed in diagenetic chert that preserves shrinkage structures considered to have formed during dehydration and recrystallization of amorphous silica cement the replacement of greenalite nanoparticles by coarser grained oxide silicate and carbonate minerals with destruction of primary textures and structures indicates that iron silicate muds were originally much more widespread based on our findings we propose a new model for the deposition of the bifs involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor we suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids upon mixing with cooler more alkaline seawater the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron silicate nanoparticles over vast areas of the seafloor the close association between iron and silica during the deposition of bifs may be explained by the precipitation of iron silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles furthermore on the seafloor iron silicate nanoparticles presumably acted as nucleation sites for dissolved silica promoting early diagenetic silica precipitation and cementation of the iron rich muds our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of bifs