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

sedimentology paleoflow dynamics and flood history of jokulhlaup deposits paleohydrology of holocene sediment sequences in southern iceland sandur deposits abstract this paper examines the sedimentology of a series of deposits associated with catastrophic floods caused by subglacial volcanic eruptions in southern iceland a simple model is proposed for the interpretation of the sediments in terms of changes in flow characteristics of the fluid sediment mix a range of sediment types characteristic of these flood flows is identified the predominant one being thick massive homogeneous poorly structured black pumice fine gravels with 65 p rcent silt clay type e and single lithology boulder deposits type f these flood deposits alternate with or are channeled into more normal sandur deposits comprising heterogeneous poorly sorted clast supported imbricated rounded cobble and pebble gravels type g the dominant sediment sequences are interpreted as representing hyperconcentrated fluid sediment mixtures with dispersive stresses acting to create massive poorly sorted nongraded or inversely graded sediments the latter characterized by large surface boulders the vertical changes in sediment type reflect significant changes in flow conditions during transport and subsequent deposition with an initial massive flood surge followed by flow deceleration sediment freezing and dewatering producing a more fluid series of flows the more fluid phases were associated with extensive erosion and channelization cutting deep scour channels marked by residual streamlined hummocks boulder lags and fields of megaripples leading downstream to a washed sandur plain the paper examines some of the problems associated with the application of paleoflow models for determining former flow dynamics the models adopted suggest that most of the flows occurred as relatively mobile low viscosity flows with little internal resistance to shear while sediment concentrations are estimated to have reached about 50 percent by weight with maximum yield strengths between 2 9 103 and 4 2 105 dn cm 2 maximum viscosities ranged between only about 2 and 8 102 poises a range of paleovelocity models applied to the flood sediment indicates flow rates of between abo t 4 and 13 ms 1 with peak discharges reaching a maximum of 3 3 105 m3 s 1 dating of the deposits by 14c dating and tephrochronology indicates that at least 8 major sediment producing jokulhlaups occurred over a 4 000 year period between c 4500 yr bp and 1357 ad hence although relatively rare the effects of these floods still dominate the proglacial landscape probably producing 85 percent of the total thickness of proglacial sandur sediments

osmium isotopes and crustal sources for platinum group mineralization in the bushveld complex south africa the bushveld complex in south africa is the largest layered mafic intrusion in the world and a major producer of platinum group elements pges economic mineralization is found in the merensky reef and the ug2 and ug1 chromitites and includes osmium bearing laurite ru ir os s 2 the laurite bearing samples have 187 os 186 os ratios of 1 28 1 60 which are more radiogenic than predicted for mantle derived 2 0 ga os 0 9 and indicate a system wide assimilation of crustal os the ug1 chromitite is stratigraphically the lowest and the least radiogenic 1 28 whereas the merensky reef is the highest and most radiogenic 1 41 1 60 hart and kinloch 1989 radiogenic os from crustal assimilation in mantle derived magmas or from hydrothermal fluids can account for these high isotopic ratios assimilation models require that os and by inference other pges are carried in al rich tholeiitic a magmas rather than high mg ultramafic u magmas pyritic black shales in contact with the intrusion may be high in radiogenic os and less than 1 shale assimilation in an a magma can explain the observed ratios late stage hydrothermal fluids driven by the heat of the intrusion could also have carried os and other pges from adjacent rocks into specific layers of the bushveld the os isotope data add to mounting evidence that pges in layered mafic intrusions may be derived from crustal sources and can no longer be considered as solely a product of mantle derived magmatic processes

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

re os isotope systematics of sulfides from felsic igneous rocks application to base metal porphyry mineralization in chile dating sulfides and determining the source of ore forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits here we analyze sulfides from base metal porphyry deposits from chile to demonstrate the potential of the re os system to determine both the age and the source of metals for hydrothermal mineralization cogenetic chalcopyrite bornite pyrite and sphalerite from el teniente ca 5 ma have 187 re 188 os ratios from 0 3 to 21 8 and initial 187 os 188 os ratios from 0 17 to 0 22 a paragenetically late pyrite that has an initial 187 os 188 os of 0 88 indicates that the ore deposit is not isotopically homogeneous in os throughout the formation of the deposit pyrite samples from andacollo ca 100 ma have 187 re 188 os ratios from 15 to 3600 and different isochrons yield ages between 87 and 103 ma consistent with different sericite k ar ages initial 187 os 188 os ratios of 0 2 to 1 1 are similar to those of el teniente the relatively homogeneous 187 os 188 os ratios 0 19 for cogenetic sulfides from el teniente suggest that most of the os in the ore was from the causal intrusive the latestage pyrite probably incorporated os leached from the country rock by meteoric fluids which mixed with magmatic fluids at the periphery of the hydrothermal system we demonstrate that the re os isotope system can be a powerful geochronological tool in hydrothermal ore deposits the large range in re os ratios of sulfides permits the age of mineralization to be well constrained despite isotopic heterogeneities of the hydrothermal fluids

fluvial lacustrine interaction on skeiðararsandur iceland implications for sandur evolution abstract a complex assemblage of fluvial and lacustrine sediments is identified in a major section through proglacial sediments on skei ararsandur south iceland high magnitude low frequency events are identified in both the fluvial and lacustrine sediments however much of the sedimentary succession is composed of normal low magnitude high frequency sediments retreat of skei ararjokull following a surge event created a topographic basin occupied at various times by both fluvial and lacustrine depositional systems and also subjected to a high magnitude jokulhlaup previous studies have examined the processes and events occurring on skei ararjokull in isolation this paper improves the understanding of the way in which processes with varying magnitude and frequency regimes interact and contribute to the sedimentary record of proglacial environments

routing of heterogeneous sediments over movable bed model development a one dimensional numerical model of sediment routing is derived to simulate erosion transport and deposition of individual size density fractions in the bed material within a relatively straight nonbifurcating alluvial channel the reach of interest is subdivided into a number of longitudinal elements of varying width averaged properties during each time step flow depths and velocities in each element are determined from the gradually varied flow equation using the standard step method for backwater calculations the bedload transport rate of each size fraction is calculated from a modified bagnold equation implementing a novel approach that takes into consideration the effects of turbulent fluctuations in the bed shear stress critical shear stresses for entrainment of particles from the mixed bed are determined using relationships that treat grain protrusion and hiding the transport of particles in suspension is modeled using a convection diffusion sediment continuity equation either explicitly

estimation of erosion deposition and net volumetric change caused by the 1996 skeiðararsandur jokulhlaup iceland from synthetic aperture radar interferometry using repeat pass satellite synthetic aperture radar interferometry we develop a methodology to measure flood induced erosion and deposition and apply it to a record 1996 glacier outburst flood jokulhlaup on skei ararsandur iceland the procedures include 1 coregistration of backscatter intensity images to observe morphological differences 2 mapping of interferometric phase correlation to identify preserved and modified surfaces and 3 construction correction and differencing of pre jokulhlaup and post jokulhlaup topography procedures 1 and 2 are robust and should be widely applicable to other fluvial environments while procedure 3 is complicated by uncertainties in phase measurement baseline estimate and atmospheric effects after a correction procedure involving interpolation of digital elevation model elevation differences across low correlation areas we find 4 m of elevation change are required to calculate volumes of erosion or deposition this condition was satisfied for the 40 km2 proglacial zone of skei ararsandur where we estimate 38 106 m3 of net sediment deposition along the ice margin 2 106 m3 of net erosion in channels downstream and a total net balance of 13 106 these estimates are supported by field observations and survey data collected in 1997

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

the conformable emplacement of the bushveld mafic rocks along a regional unconformity in the transvaal succession of south africa abstract a previously unrecognized regional unconformity occurs in the proterozoic transvaal succession of the kaapvaal province of southern africa the predominantly marine and siliciclastic rocks of the lower two thirds of the pretoria group are unconformably overlain by a terrestrial and largely volcanic sequence the most voluminous portion of which is the rooiberg felsite if the bushveld complex is mentally subtracted from map patterns the rooiberg felsite also locally overlies pre pretoria sequences and the archean crystalline basement the bushveld complex is not lopolithic as commonly supposed instead regionally condordant emplacement of the complex along the unconformity explains both the seemingly discordant footwall features and the lack of pretoria and older strata other than a few xenoliths in the hanging wall the lack of any geochemical evidence for major crustal contamination of bushveld magmas precludes disposing of any such hanging wall strata by wholesale assimilation the bushveld complex the unconformity and the enclosing strata are synformally folded making the bushveld appear to be lopolithic due to this folding and multiple periods of erosion the original extents of the bushveld complex the unconformity and the enclosing strata are now much reduced other smaller mafic intrusions along or immediately below the same regional unconformity probably are bushveld related

alluvial terraces on the ionian coast of northern calabria southern italy implications for tectonic and sea level controls abstract in this paper we present the results of an integrated geomorphological pedological and stratigraphical study carried out along the ionian coast of northern calabria southern italy this area is characterised by the occurrence of five orders of alluvial terraces that are striking features of the landscape where large and steep catchments debouch from the mountain front to the hilly coastal belt field investigations indicate that the deposits of all five terraces are suggestive of shallow gravel bed braided streams on the basis of the age of the pleistocene substratum and morphostratigraphic correlation with marine terraces cropping out in the nearby areas each order has been associated to specific marine oxygen isotope stages consequently we focused on the interplay of allocyclic factors influencing stream aggradation degradation soil features and other climatic proxies suggest that climate didn t play an important role with respect to tectonic and base level changes in controlling fluvial dynamics in particular we recognised that during the middle pleistocene the study area experienced a period of subaerial landscape modelling as suggested by the thick and complex alluvial sequence of the highest terrace t1 the onset of regional uplift marks a change in the geomorphic scenario with tectonic and eustatically driven changes in base level working together in causing switches in fluvial aggradational erosional phases t2 t5 terraces because of the uplift river dissection occurred during phases of sea level fall whereas aggradation phases occurred during periods of climate amelioration sea level rise just before highstands were attained as a consequence the stepped terraces in the study area reflect the interplay between tectonics uplift and sea level changes in which terraces define episodes of relative sea level fall during the late quaternary

geomorphic impact and rapid subsequent recovery from the 1996 skeiðararsandur jokulhlaup iceland measured with multi year airborne lidar the november 1996 jokulhlaup that burst from the vatnajokull ice cap onto skei ararsandur was the highest magnitude flood ever measured on the largest active glacial outwash plain sandur centimeter scale elevation transects measured from repeat pass airborne laser altimetry missions flown in 1996 pre flood 1997 and 2001 show that sediment deposition exceeded erosion across the central skei ararsandur and established an average net elevation gain of 22 cm for the event net elevation gains of 29 and 24 cm occurred in braided channels of the go gjukvo sl and skei ararivers respectively nearly half of these gains however were removed within 4 years and the two rivers contrast strongly in style of erosional depositional impact and subsequent recovery in the go gjukvo sl the 1996 jokulhlaup caused massive sediment deposition up to 12 m near the ice margin and intense bmega formingq of braided channels and bars downstream post jokulhlaup recovery 1997 2001 was characterized by rapid erosion 0 5 m of ice proximal sediments and their transport to downstream reaches and eradication of the mega forms in contrast the skei ara displays minimal post jokulhlaup sediment erosion in its upstream reaches and little change in braided channel relief this contrast between river systems is attributed to the presence of a previously studied 2 km wide ice marginal trench caused by glacier retreat and lowering which contained flows of the go gjukvo sl but not the skei araprior to dispersal onto the outwash plain rapid removal of jokulhlaup deposits from this trench suggests that in terms of long term evolution of the sandur such features only delay downstream migration of jokulhlaup derived sediment these results therefore suggest that the net geomorphic impact of jokulhlaups on surface relief of skei ararsandur while profound in the short term may be eradicated within years to decades d 2005 elsevier b v all rights reserved

field relations and strontium isotope systematics of the marginal rocks of the eastern bushveld complex the well exposed basal contact of the eastern bushveld complex was mapped at scales of 1 10 000 and 1 20 000 along 130 km of strike and through a 9 km vertical section field relations petrography and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups a broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone a gabbroic group occurs above this level each group consists of a number of closely related rock types most of which have sill analogs rocks of the first group b1 include pyroxenites norites and peridotites all of which may have quench textures all contain orthopyroxene en 78 plagioclase an 65 and chromite with subordinate clinopyroxene and biotite rare earth element patterns are mostly strongly fractionated ce n yb n approximately 10 and have 40x chondritic abundances at ce their initial strontium isotope ratios at 2 05 b y fall in the range 0 7032 to 0 7057 and rb sr ratios approximate 0 2 the gabbroic group consists of microcrystalline two pyroxene gabbros b2n and b3n and their coarser grained equivalents b2 and b3 that border the upper critical and main zones respectively mineral assemblages are dominated by plagioclase approximately an 55 clinopyroxene and orthopyroxene approximately en 63 with abundant magnetite and traces of ilmenite and biotite their rare earth element patterns are less fractionated than b1 rocks the b2n and b2 family have ce n yb n approximately 6 and 30x chondritic abundances at ce the b3n and b3 rocks have ce n yb n approximately 3 6x chondritic abundances at ce and pronounced positive eu anomalies strontium isotope initial ratios at 2 05 b y fall in the range 0 706 to 0 708 and the rb sr ratios are approximately 0 02 whole rock and sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial sr isotope ratio r 0 similar to that of the transition zone 0 7056 a fraction of such liquid enriched in cumulus olivine crystals when mixed with up to 30 percent of a pyroxenitic liquid with low r 0 0 7034 yields a product having the isotopic and compositional characteristics of the peridotite sills a continuum exists in the initial sr ratios through the pyroxenitic and gabbroic marginal rocks suggesting that these represent mixtures of at least two sources with distinct initial sr isotope ratio characteristics it is argued that neither magma aging nor crustal contamination adequately accounts for the sr isotope characteristics of the marginal rocks or their derived cumulates the unusual chemistry of the pyroxenitic early liquids sio 2 approximately 55 mgo approximately 13 coupled with high concentrations of incompatible elements should be considered primary

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

modelling of erosion processes associated with rainfall triggered lahars following the 2011 cordon caulle eruption chile abstract in this work we investigate the sheet and rill erosion process as a possible lahar initiation mechanism in two catchments located in villa la angostura argentina southern andes in association with the 2011 eruption of cordon caulle chile a few days after the climactic phase of the eruption several lahar events occurred in villa la angostura where 15 cm of tephra mantled the slopes the physically based model lisem openlisem 2013a b was applied to two small catchments where sheet and rill erosion of tephra deposits was observed model input parameters related to vegetation soil surface and infiltration were obtained from field measurements geotechnical tests and the literature a total of 20 scenarios were simulated using a range of values of rainfall intensity and duration and hydraulic conductivity only 16 of these scenarios resulted in erosion results show that hydraulic conductivity ks and rainfall intensity significantly affect the runoff volume with the smallest ks i e the tephra deposits associated with the largest fraction of fine ash being associated with the largest eroded volumes and the largest discharges of water and sediment the scenarios with the highest hydraulic conductivity did not show erosion for any modelled rainfall intensity nonetheless we conclude that sheet and rill erosion were not the principal mechanism for rain triggered lahars associated with the cordon caulle eruption but mostly shallow landslide and similar erosion mechanisms contributed to lahars in villa la angostura in fact the eroded volumes calculated with lisem are significantly smaller than the volume calculated with shallow landslide models for the same catchments trigrs

physically based modelling of soil erosion induced by rainfall in small mountain basins abstract in coarse grained unsaturated soils heavy rainstorms may cause either shallow landslides or superficial soil erosion the triggering of both of these phenomena is related to infiltration runoff and overland flow which are key processes requiring investigation in particular the quantitative estimates of the sediment yield at the outlet of mountain basins requires suitable physically based modelling in this paper the available approaches to soil erosion analysis are first reviewed and the capabilities and limitations of a physically based model are then evaluated through a case study of two small mountain basins the results obtained are in good agreement with those in the literature and with specific field data from a test area specifically for distinct realistic rainfall scenarios soil suction is found to be a key factor in the spatial temporal evolution of infiltration and runoff inside a mountain basin and soil suction and rainfall intensity greatly influence the total peak discharge of water and sediments the maximum volumetric concentration of sediments transported by water to the outlet of a mountain basin is found to be primarily related to the basin s specific morphometry

the upper critical zone of the bushveld complex and the origin of merensky type ores the mg number mg mg fe atomic ratio of bronzite in each unit at the top of the critical zone of the bushveld complex decreases upward in a manner consistent with fractional crystallization the base of each unit is marked by an increase in the mg number suggesting that crystallization of fresh magma is responsible for the initiation of the unit bulk trace and major element concentrations in pegmatoids of the merensky reef which itself forms the basal part of one of a series of these cyclic units are similar to those in medium grained feldspathic bronzitite the base of each unit is also marked by pockets or a layer of sulfide with a higher ni cu and much higher pt cu ratio than the sulfides of the remainder of the unit thus the pt tenor alternates from high to low three times upward through the three units these data indicate strongly that the pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum group elements there are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous merensky reef a model based on that of campbell et al 1983 in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma accounts well for all of our observations and many other features of the upper part of the critical zone

micromechanical modelling of rainsplash erosion in unsaturated soils by discrete element method abstract the rainsplash erosion is one important mechanism in natural and artificial slopes and the raindrops impact contributes to the amount of solids conveyed at the outlet of a mountain basin rainsplash erosion involves individual or small clusters of soil particles it is a dynamic temporally and spatially variable process and mainly occurs in unsaturated soils where capillarity forces allow steep slopes to be stable the discrete element method dem was firstly applied in this paper for the analysis of rainsplash erosion assuming realistic rainfall intensities and a range of both slope steepness and capillary forces in a parametric analysis the dem numerical results highlight the specific role of the slope steepness capillarity force and rainfall intensity towards the final volume of the solid eroded from the ground surface at the computational domain it is also valuable that the numerical results are in good agreement with literature formulations and increasing power law functions were found to relate well the eroded volume to rainfall intensity

the distribution and main geologic features of discordant bodies of iron rich ultramafic pegmatite in the bushveld complex transgressive bodies of iron rich ultramafic pegmatite composed predominantly of varying proportions of hortonolite clinopyroxene ilmenite and ti magnetite are abundantly distributed throughout the layered mafic sequence of the bushveld complex new information from field and underground studies together with interpretations of detailed magnetic surveys is presented along with an overview of the available literature some whole rock and mineralogical chemical data are also included within the bushveld complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized these include besides the main group of iron rich ultramafic pegmatite bodies of magnesian dunite and the well known platiniferous ultramafic or dunitic pipes transgressive often pipelike bodies composed almost entirely of fe ti oxides occur in the main and upper zones of the complex which although part of the iron rich ultramafic pegmatite suite are described separately composite bodies in which massive fe ti oxides occur as a core within a peripheral shell of silicate rich pegmatite are also recognized the major occurrences of iron rich ultramafic pegmatite are documented and depicted in a series of maps their distribution is subject to two major stratigraphic controls iron rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones it is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus ti magnetite layer in the upper zone it is also evident that these rocks become increasingly iron rich toward the top of the layered sequence so that in the upper zone fe ti oxide pegmatite is the dominant variety many bodies particularly the large ones are structurally controlled and occur in disturbed areas characterized by faulting and post bushveld dikes stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron rich ferromagnesian silicates and fe ti oxides this iron rich material is considered to represent the accumulated and trapped late stage residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large pipelike bodies of iron rich ultramafic pegmatite these bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids the authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron rich ultramafic pegmatite is an integral feature of the layered sequence

the petrogenesis of a platiniferous dunite pipe in the bushveld complex infiltration metasomatism by a chloride solution the driekop dunite pipe crosscuts part of the noritic layered sequence of the upper critical zone in the eastern bushveld complex the central orebody of the pipe contains platinum group minerals base metal sulfides magnetite and iron rich olivine the country rock forms a collapse structure around the pipe with the layered sequence becoming almost vertical near the contact with the pipe it is proposed that a high temperature chloride solution flowed through irregular channel ways along a structural weakness near the axis of the pipe and reacted with the noritic host rocks leading ultimately to the formation of dunite reaction fronts advanced at different rates outward from the channelways and produced a sequence of metasomatic zones characteristics of infiltration metasomatism that are evident in the pipe include the movement of material over large distances in predominantly one direction the formation of a monomineralic rock the occurrence of a marginal zone that is enriched by displaced components and plateaus in the compositions of mineral solid solutions the reactions responsible for the major changes in the bulk composition of the host rocks were desilication of orthopyroxene to produce olivine and dissolution of plagioclase the 67 percent volume loss accompanying these reactions can account for the structural collapse of the critical zone around the pipe some olivine in the pipe is too fayalitic to have been produced by simple desilication of host rock orthopyroxene the enrichment of olivine in iron can take place bymg 2 sio 4 2fecl sub 2 sub aq fe 2 sio 4 2mgcl sub 2 sub aq forsterite fayalite aqueous chloride complexes are also believed to have played an important role in the transport and precipitation of the platinum group elements the base metals and other cations interdependent chemical reactions involving a chloride solution olivine magnetite and the platinum group elements can account for the preferential association of the ore with iron rich olivine infiltration metasomatism by a high temperature chloride solution may have an important bearing on the interpretation of other features of the bushveld complex

shesed a physically based distributed erosion and sediment yield component for the she hydrological modelling system abstract shesed is introduced as a physically based spatially distributed erosion and sediment yield component for the existing she hydrological modelling system for use at the catchment scale for hillslopes represented spatially by the she grid square network shesed simulates soil erosion by raindrop impact leaf drip and sheet overland flow without rilling and the transport of the eroded material by overland flow for channels the component simulates the erosion of bed material and the downstream transport of this material together with that supplied by overland flow in the channel sediment routing procedure it is assumed that the flow can carry any available load of fine sediments less than 0 062 mm in diameter but for coarser sediments the load is limited by the calculated capacity transport rate of the flow the channel component also allows for bed armouring application of shesed to rainfall induced sediment yield events at two agricultural catchments areas 5 1 and 6 4 ha in iowa shows generally good reproduction of the observed temporal variations in sediment yield application to a 3 3 km reach of the east fork river wyoming for a 37 day period shows good reproduction of observed sediment discharge magnitudes but some discrepancy in the timing of the simulated sediment discharge peak

platinum group element mineralization in the critical zone of the western bushveld complex i sulfide poor chromitites below the ug 2 primary stratiform platinum group element pge mineralizations associated with chromitites in the critical zone of the bushveld complex south africa are confined to cumulate layers that are regionally persistent over tens of kilometers four categories ofchromitite are recognized layers at the bases of cycles in the lower critical zone type i layers at the bases of cycles in the upper critical zone type ii thin layers in the intermediate parts of cycles type iii and stringers associated with orthopyroxene pegmatoids type iv these latter may not be of primary magmatic origin nevertheless probably each type of chromitite contains anomalous concentrations of pge chromitites at the bases of cycles comprising the lower group lg middle group mg and upper group ug are further categorized into types ia lg 1 lg 4 ib lg 5 mg 1 iia mg 2 ug 1 and iib ug 2 and above on the basis of lithostratigraphy chromite and pge chemistry and sulfide content type iib chromitites contain significant quantities of sulfides and are not investigated here new whole rock and electron microprobe chemical analyses of the sulfide poor type ia ib and iia chromitites cu contents 1 8 low pge contents 2 type ib chromitites occur in pyroxenite cycles and form an intermediate group these regular variations with increased height are interpreted as a differentiation trend pge are concentrated in sulfide poor chromitites as a result of chromitite control a process not fully understood but thought to involve a combination of direct nucleation of platinum group minerals and localized s saturation it is the crystallization of copious quantities of chromite from hybridized melts close to the crystal liquid interface that triggers this process but magma mixing essential to drive the melt into the chromite field is the overall cause replenishment of the magma chamber was from a single magma batch because the chromitites below the ug 2 are not associated with major isotopic inflections huge volumes of dense hot magma were fed as basal flows along the crystal liquid interface the magma chamber was compartmentalized at an early stage possibly in response to basement tectonics and lateral mixing over distances of 150 km is an alternative mechanism for achieving sufficiently high r values repeated replenishment of the magma chamber resulted in mixing with resident liquid that became increasingly differentiated with height type ia chromitites are ascribed to replenishment and mixing with the relatively primitive lower zone residue type ib crystallized as a result of mixing with lower critical zone resident liquid and type iia due to mixing with more evolved upper critical zone resident liquid differentiation of pge with height is a critical part of the process which culminates in the occurrence of higher grade pge mineralization in sulfide bearing chromitites ug 2 merensky reef in the uppermost part of the upper critical zone

diapirism initiated by the bushveld complex south africa the bushveld complex induced the development of a unique type of diapir during its intrusion these diapirs are preserved as large domal structures exposed on the margin and within the northeastern mafic layered sequence they were produced by gravitational loading and heating of argillaceous and arenaceous floor rocks gravity induced deformation is restricted to rocks above a detachment level that corresponds approximately with the 550 c peak metamorphic isotherm and with lithological contacts of contrasting competency strongly foliated and boudinaged contact metamorphosed sedimentary rocks between domes were deformed by lateral extension along flow lines directed toward dome culminations at the highest structural levels domes have bulbous geometries and the enclosing layered igneous rocks are deformed into series of outward verging folds that define a broad rim syncline deformation within the domal cores is represented by constriction that produced radial curtain type folds that have steeply plunging lineations and concentrically oriented folds in the outer shell diapirism is closely linked to magma emplacement mechanisms floor folds in the country rocks were initiated in the interfinger areas of a fingered intrusion with the coalescence of intrusion fingers into a single sheet interfinger folds matured into large diapiric domes that rose to the upper levels of the magma chamber strain rates estimated from strain analyses dome geometry and model cooling calculations are on the order of 10 14 s 1 corresponding to diapiric uplift rates of 0 6 cm yr

micro characteristics of soil aggregate breakdown under raindrop action abstract raindrop splash is considered to be the first step in soil erosion and it is also a contribution to such erosion aggregate breakdown due to raindrop splash causes crusting and soil erosion in this study the micro characteristics of soil aggregate breakdown under the action of three sizes of raindrops were examined using synchrotron based x ray micro computed tomography sr ct the results showed that soil aggregate breakdown was related to the size of raindrops was mainly caused by large raindrops during a rainfall event and formed an enrichment zone transition zone and dense airtight zone of aggregates in the surface soil the shape index and the amount of aggregate in the splashed soil was higher than in the un splashed soil especially for micro aggregate fragments

strontium isotope evidence for preserved density stratification in the main zone of the bushveld complex south africa the bushveld complex formed from two isotopically distinct magmas magma addition and mixing can be tracked by measuring initial strontium isotope ratios of cumulates rocks with ratios of 0 7065 lie below the lower 2 2 km of the main zone ratio 0 7085 whereas those with ratios of 0 7073 lie above it this unit represents a liquid layer intruded into the density stratified magma pile near the base of the chamber the merensky reef was produced by mixing of the pile with underlying trapped liquid enriched in precious metals

correlation between chromite composition and pge mineralization in the critical zone of the western bushveld complex detailed mineralogical investigations of chromite in the lower and critical zones in the northwestern sector of the bushveld complex have revealed significant compositional variations with regard to modal proportions host rock lithology and stratigraphic height superimposed on these variations are long range systematic trends in the composition of chromite in the massive layers these long range trends are closely linked with the evolution of the silicate cumulates the massive chromitite layers are divided into two types type 1 comprises the chromitites hosted entirely within ultramafic cumulates while type 2 chromitites are within cyclic units in which plagioclase cumulates occur the types are also distinguishable by their respective contents of platinum group elements pges and distribution patterns thereof viz the ratios between ru os ir and pt pd rh or relative element proportions both of which display a systematic change with height in accordance with chromite composition the relation between silicate geochemistry chromite composition and pge tenor leads to the development of a model explaining the formation of pge mineralized sulphide poor chromitite layers in the critical zone of the bushveld complex

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

icelandic jokulhlaup impacts implications for ice sheet hydrology sediment transfer and geomorphology abstract glaciers and ice sheets erode entrain and deposit massive quantities of debris fluxes of subglacial meltwater exert a fundamental control on ice dynamics and sediment transport budgets within many glacial systems outburst floods jokulhlaups constitute high magnitude high frequency fluxes of meltwater relative to normal ablation controlled discharge this paper presents a synthesis of research on recent icelandic jokulhlaups and their geomorphological and sedimentary impact we identify jokulhlaup impacts within subglacial englacial and proglacial settings and discuss their wider significance for ice sheet hydrology sediment transfer and geomorphology because jokulhlaups erode deposit and re work sediment simultaneously they usually cause significant glaciological and sedimentological impacts jokulhlaups that propagate as subglacial flood waves often produce widespread hydromechanical disruption at the glacier base recent icelandic jokulhlaups have been recognised as highly efficient agents of reworking subglacial sediment and glacial sediment entrainment models of jokulhlaup impact therefore need to encompass the sub and englacial environment in addition to the proglacial zone where research has traditionally been focussed most jokulhlaups transport sediment to proglacial sandar and often directly to oceans where preservation potential of the impact is greater proglacial jokulhlaup deposits form distinctive sedimentary assemblages coupled with suites of high energy erosional landforms this study of modern jokulhlaup processes and sedimentary products may be useful for the interpretation of meltwater processes associated with quaternary ice sheets

regional trends in the platinum group mineralogy of the critical zone of the bushveld complex south africa a wealth of quantitative data on the types volume distribution and mode of occurrence of the platinum group minerals residing in the merensky reef the ug 2 chromitite layer and platreef has been obtained over a twelve year period with the result that general platinum group mineral types for unexplored areas can perhaps now be predicted the platinum group minerals of the ug 2 and overlying merensky layers although locally different in composition mode of occurrence and size show close ties when viewed on a regional scale especially with regard to their pt pd sulfide and pt fe alloy contents thus if in one particular area the merensky reef contains pt pd sulfides as the predominant platinum group minerals then it can be safely inferred that pt pd sulfides will be a significant component of the ug 2 layer in that area the occurrence of pt fe alloy also follows this principle the distribution of pt fe alloy is in fact a major factor in differentiating the ores in the vicinity north and south of the pilansberg alkaline complex from those of the same layers farther along the arcuate strike northeastern and southeastern horns of the western bushveld and has led to an overall concept relating platinum group mineral types to postulated centers of increased volatile activity or perhaps feeder sites of the complex the mineralogy of reef disturbances such as potholes is discussed the mode of occurrence of the platinum group minerals varies between base metal sulfide silicate and chromite associations and the size of platinum group minerals in the various reefs is given solid solution or lattice substitutions of platinum group elements in the base metal sulfides account for the very important latent fraction of the total platinum group elements as distinct from platinum group minerals of the bushveld complex rhodium in the merensky reef occurs almost wholly in solid solution within pentlandite whereas rh minerals occur in the ug 2 layer chromite tends to host microscopic to submicroscopic inclusions of laurite and does not appear to take ru into the crystal lattice the probability of solid solution of platinum group elements in silicate and other oxide phases is discussed

on erosion characteristics of compacted loess during wetting procedure under laboratory conditions the erosion of engineered slope surfaces has become an urgent problem in loess regions in china which has a serious impact on the safety and stability of the slope projects in this paper to study the effect of the confining pressure dry density of the slope surface and water flow velocity during wetting procedure on the erosion characteristics of compacted loess we developed a laterally confined uniaxial compression erosion apparatus which was characterized by the fact that it can change the confining pressure of the being eroded soil sample to analyze the loess erosion rates and particle size distributions of the eroded sediments the fractal dimensions of the eroded sediments were calculated by fractal theory and their variations were analyzed results show that the erosion rate is approximately linearly related to the confining pressure the higher pressure has a tendency to move towards the free top of soil sample the erosion rate increases compared with the water flow velocity the higher confining pressure plays a greater role in the erosion rate the influence of water flow velocity on the erosion rate decreases with the increase of dry density the erosion rate decreases the higher dry density enhances the ability of soil sample to resist water erosion the confining pressure changes the particle size distribution of the eroded sediments of loess samples the median diameter d50 increases however the fractal dimension decreases with the increase in confining pressure the confining pressure of slope surface has a negative effect on the loess anti erosion characteristics the results in this paper would provide detailed information on the mechanisms of loess erosion which could contribute toward prevention of surface erosion of high fill slopes in loess regions

small scale cyclicity in alpine proglacial fluvial sedimentation abstract this paper compares contemporary sedimentation in an alpine proglacial braided river with deposits from inactive areas of the braidplain this approach allows confidence in determining the key characteristics of alpine proglacial braided river deposits the diagnostic features are 1 repeated coarse fine couplets in fine grained sediment

splash erosion under natural rainfall on three soil types in ne spain article i nfo article history splash detachment and transport of soil particles by raindrops impacting a soil surface are the initiating mechanisms of water erosion the amount of splash depends on rainfall characteristics mainly kinetic ener gy and intensity and soil properties experimental results of rainfall and splash monitoring in three soil types under natural rainfall in ne spain are presented some 27 rainfall events were evaluated during which high rates of soil splash were measured 6 06 g per splash cup stressing its importance as an erosion process on bare soils sources of variation of soil splash were analyzed by a linear mixed effects lme model significant relationships were found between the splash erosion rates and the rainfall erosivity index ei30 no significant differences in erosion were found among the soil types analyzed the lme model explained 55 of variance in the erosion rates and most of the residual variability 74 was due to differences between splash cups within a single soil type and event i e random effects

routing of heterogeneous sediments over movable bed model verification a one dimensional numerical model of heterogeneous size density sediment transport has been developed to simulate the movement of graded sediments in natural and laboratory flow reaches predicted temporal and spatial variations in bed and armor layer grain size distributions eroded grain size distribution eroded thicknesses and total bedload transport rates compare quite favorably to observed variations in flumes the san luis canal colorado and the east fork river wyoming for a large variety of flow scales and flow conditions the main advantages of this model over others is the high degree of accuracy of model results obtained using only bed and flow variables as input the treatment of turbulent fluctuations of bed shear stress the minimization of calibration factors and the explicit treatment of multiple grain densities in addition only the active layer thickness must be calibrated

measurements of rain splash on bench terraces in a humid tropical steepland environment soil loss continues to threaten java s predominantly bench terraced volcanic uplands sediment transport processes on back sloping terraces with well aggregated clay rich oxisols in west java were studied using two different techniques splash on bare cropped or mulched sub horizontal 2 3 terrace beds was studied using splash cups of different sizes whereas transport of sediment on the predominantly bare and steep 30 40 deg terrace risers was measured using a novel device combining a gerlach type trough with a splash box to enable the separate measurement of transport by wash and splash processes measurements were made during two consecutive rainy seasons the results were interpreted using a recently developed splash distribution theory and related to effective rainfall erosive energy splash transportability i e transport per unit contour length and unit erosive energy on the terrace risers was more than an order of magnitude greater than on bare terrace beds 0 39 0 57 versus 0 013 0 016 g m j 1 this was caused primarily by a greater average splash distance on the short steep risers 11 cm versus c 1 cm on the beds splashed amounts were reduced by the gradual formation of a protective pavement of coarser aggregates in particular on the terrace beds soil aggregate size exhibited an inverse relationship with detachability i e detachment per unit area and unit erosive energy and average splash length and therefore also with transportability as did the degree of canopy and mulch cover on the terrace risers splash creep and gravitational processes transported an additional 6 50 of measured rain splash whereas transport by wash played a marginal role copyright 2002 john wiley sons ltd

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

sph run out modelling of channelised landslides of the flow type abstract rainfall induced channelised landslides of the flow type may reach high velocities and travel large run out distances the proper modelling of the propagation stage is fundamental to risk analysis and management the correct selection of mass rheology is fundamental to this procedure and the variation of pore water pressures and bed entrainment along the landslide path must also be properly taken into account all of these processes can be consistently simulated through a quasi 3d depth integrated coupled sph model in this study such a model is applied to a relevant case history from southern italy for which an advanced dataset is available the numerical results provide a satisfactory back analysis of the case history and clearly indicate that the bed entrainment rate and the extent of the erodible areas affect the propagation paths the velocities and the deposition heights on the basis of the results general insights into this type of landslide are discussed with reference to different hypotheses for propagation modelling

dispersal of glacially derived sediment over part of the continental shelf of south iceland and the geometry of the resultant sediment bodies abstract the southern continental shelf of iceland is crossed by a series of troughs 200 300 m below sea level with intervening banks 100 120 m below sea level the troughs were eroded by the pleistocene ancestors of the large valley glaciers which descend from the vatnajokull ice cap a study of sedimentation and sediment history in one such trough its flanking banks and the adjacent coastal belt is presented coarse sediments discharged from the glaciers have built out as a massive coastal sediment wedge which has filled up the mouth of an earlier fjord of which the shelf trough is an extension strong bottom currents across the flanking banks transport sand to depths of 140 m the proximal part of the trough is infilled by muddy sediments derived from gravity flows down the face of the coastal sediment wedge and by sediment transported over the bank to the west the trough infill is strongly asymmetric being banked up against the western flank of the trough deep turbidite channels occur in the middle part of the trough and on its flanks strong bottom currents over the shelf edge have denuded the outer part of the trough of sediments a pleistocene phase of sedimentation on the shelf is represented by very thick till accumulations on the banks a major moraine which extends across the inner part of the trough reflects a phase of glacier stillstand during the last retreat of ice across the shelf and forms a dam behind which debris flow sediments derived from the coastal sediment wedge have accumulated

lithofacies variations within sandur deposits the role of runoff regime flow dynamics and sediment supply characteristics abstract the icelandic sandur plains have for long acted as classic sites for models of braided river behaviour and of associated lithofacies assemblages however many of the icelandic sandurs are subject to repeated catastrophic flood events or jokulhlaups which are likely to have played a significant role in sandur construction established lithofacies models based on the icelandic sandurs have failed to take account of the varying runoff regimes which characterise the different sandur environments in addition sediment inputs to sandur channel systems are highly variable fluctuating in response both to varying runoff patterns and to advance and retreat of the source glacier that affect sediment availability within the proglacial zone this paper describes the main lithofacies types and vertical profiles that characterize the sandurs of south iceland and demonstrates that they can largely be attributed to extreme runoff events with few deposits typical of braided river facies occurring within the sedimentary record a preliminary qualitative model is developed that allows interpretation and prediction of vertical lithofacies profiles derived from glacial outwash in terms of i the drainage mechanism which in turn controls the dominant runoff population ii sediment availability and iii the spatial variability in flow dynamics

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

rhenium osmium evidence for regional mineralization in southwestern north america more than 40 base metal porphyry ore deposits in southwestern north america are associated with the laramide orogeny about 90 million to 50 million years ago rhenium osmium dates on molybdenite a rhenium enriched sulfide common in many of the deposits reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception magnitude or duration of magmatism deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization rhenium osmium ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago most deposits that formed in the oldest interval are within the older precambrian basement of northwestern arizona whereas the younger deposits are restricted to the younger precambrian basement in southern arizona and northern mexico synchronous widespread mineralization indicates that similar crust mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes

field experiments constraining the probability distribution of particle travel distances during natural rainstorms on different slope gradients rain splash erosion is an important soil transport mechanism on steep hillslopes the rain splash process is highly stochastic here we seek to constrain the probability distribution of splash transport distances on natural hillslopes as a function of hillslope gradient and total precipitation depth field experiments were conducted under natural precipitation events to observe splash travel on varying slope gradients the downslope fraction of splash transport on 15 degrees 25 degrees and 33 degrees gradients were 85 96 and 96 respectively maximum splash transport lmax was related to the rain splash detachment of soil particles and slope gradient an empirical relationship of lmax to the precipitation depth and gradient was obtained it is linearly proportional to hillslope gradient and logarithmically related to precipitation depth measured splash distances were calibrated to the fully two dimensional 2d model of splash transport of furbish et al journal of geophysical research 112 f01001 2007 that is based on the assumption that radial splash distances are exponentially distributed calibrated values of mean splash transport distances are an order of magnitude greater than those previously determined in a controlled laboratory setting we also compared measured data with several one dimensional 1d probability distributions to asses if splash transport distances could be better explained by a heavy tailed probability distribution rather than an exponential probability distribution we find that for hillslopes of 15 degrees and 25 degrees although a lognormal probability distribution best describes the data we find its likelihood is nearly indistinguishable from an exponential distribution based on computing maximum likelihood estimators for all 1d distributions exponential log normal and weibull at 33 however we find stronger evidence that measured travel distances are heavy tailed

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

magnitude and frequency in proglacial rivers a geomorphological and sedimentological perspective abstract proglacial fluvial sedimentary systems receive water from a variety of sources and have variable discharges with a range of magnitudes and frequencies little attention has been paid to how these various magnitude and frequency regimes interact to produce a distinctive sedimentary record in modern and ancient proglacial environments this paper reviews the concept of magnitude and frequency in relation to proglacial fluvial systems from a geomorphic and sedimentary perspective rather than a hydrological or statistical perspective the nature of the meltwater inputs can be characterised as low magnitude high frequency primarily controlled by ablation inputs from the source glacier or high magnitude low frequency primarily controlled by exceptional inputs the most important high magnitude low frequency inputs are catastrophic outburst floods often referred to by the term jokulhlaup icelandic for glacier burst glacier surges are an additional form of cyclical variation impacting the proglacial environment which briefly alter the volumes and patterns of meltwater input the sedimentary consequences of low magnitude high frequency discharges are related to frequent variations in stage the greater directional variability that sediment will record and the increased significance of channel confluence sedimentation in contrast the most significant characteristics of high magnitude low frequency flooding include the presence of large flood bars and mid channel jokulhlaup bars hyperconcentrated flows large gravel dunes and the formation of ice block kettle hole structures and rip up clasts glacier surges result in a redistribution of low magnitude high frequency processes and products across the glacier margin and small floods may occur at the surge termination criteria for distinguishing magnitude and frequency regimes in the proglacial environment are identified based on these major characteristics studies of quaternary proglacial fluvial sediments are used to determine how the interaction of the various magnitude and frequency regimes might produce a distinctive sedimentary record consideration of sandur architecture and stratigraphy shows that the main controls on the sedimentary record of proglacial regions are the discharge magnitude and frequency regime sediment supply the pattern of glacier advance or retreat and proglacial topography a model of sandur development is suggested which shows how discharge magnitude and frequency in combination with sandur incision and aggradation controlled by glacier advance and retreat can control sandur stratigraphy

application of the 187re 187os system to black shale geochronometry the decay of 187re to 187os provides a tool for determining depositional ages of black shales re and os concentrations and os isotopic compositions of whole rock samples of the bakken shale a mississippian devonian boundary black shale yield a whole rock isochron with an age of 354 49 ma this age is in agreement with the accepted age of the bakken shale 360 ma the initial 187os186os ratio of the isochron is 6 2 3 3 this value is indistinguishable from the 187os186os ratios observed in modern black sea sediments the concentration of common os in bakken shale samples is strongly correlated with total nitrogen indicating that a large fraction of the os in these samples is associated with a hydrogenous component which overwhelms any os supplied to the sediment in association with detrital material or cosmic dust the dominance of the hydrogenous component imparts a relatively homogeneous initial 187os186os ratio to the sediment at the time of deposition the degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity the pattern of scatter is consistent with postdepositional mobilization of re and or os on a small spatial scale we suggest that this mobilization may be a consequence of petroleum formation and migration in the bakken shale

sulfide and platinum mineralization in the merensky reef evidence from hydrous silicates and fluid inclusions the base metal sulfides of the merensky reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates biotite and phlogopite crystallized in the vicinity of sulfides from a volatile enriched highly fractionated intercumulus melt amphibole chlorite and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase biotite is often accompanied by zircon rutile and quartz accessory quartz hosts a complex suite of h2o nacl cacl2 co2 ch4 fluid inclusions which have thus far not been described from the merensky reef the earliest fluid inclusion compositions are nacl h2o with less than 10 vol water co2 coexisting with a halite daughter crystal and brine and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations the maximum trapping temperature is around 730 c at 4 to 5 kb pressure later inclusion generations are h2o nacl co2 h2o and pure co2 and ch4 the presence of cl rich fluids during the intercumulus stage of the crystallizing merensky reef is directly related to the mode of sulfide precipitation prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as h2s sulfur saturation causes reaction of h2s with oxides of the silicate melt to form a sulfide melt plus water during reaction the magma is enriched in water until a separate fluid unmixes it carries all compounds with high fluid melt partition coefficients as well as metals capable of forming oh and cl complexes precious metals are assumed to have fractionated into the cl rich fluid as cl complexes rather than being dissolved in the sulfide melt during the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets the model proposed explains the distribution pattern of platinum group minerals in the merensky reef better than any orthomagmatic mineralization concept offered so far

the platinum group element and gold contents of the marginal rocks and sills of the bushveld complex previous work has defined three suites of sills and marginal rocks adjacent to and below the rustenburg layered suite of the bushveld complex namely an ultramafic mgo approximately 30 a magnesian basaltic mgo approximately 12 and a tholeiitic suite mgo approximately 7 these three suites are thought to represent magmas which have intruded and been incorporated into the rustenburg layered suite at various times during its formation the elements ir ru rh pd pt and au have been analyzed in 17 samples representative of the three suites the ultramafic sills have the highest total noble metal content 46 ppb followed by the magnesian basaltic suite 37 ppb and the tholeiitic suite 21 ppb the ultramafic sills are characterized by a relatively fiat chondrite normalized platinum group element pattern with an average pt ir ratio of 3 52 and a pronounced rh anomaly the magnesian basaltic and tholeiitic suites both have steep positive chondrite normalized patterns with average pt ir ratios of 16 14 and 40 5 respectively and no rh anomaly modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes of cumulate olivine and chromite that were intruded into the floor during the crystallization of the lower portion of the rustenburg layered suite the enrichment of ir ru and rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills the chondrite normalized patterns found for the merensky reef and the platreef are similar to those of the magnesian basaltic magma the pattern found for the ug 2 is much flatter and in some respects very similar to that found for the ultramafic sills on a worldwide basis the platinum group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites the magnesian basaltic magma does have one of the highest pt contents measured for a magma

hydrogeological implications of glacial landscape evolution at skeiðararsandur se iceland abstract patterns of groundwater recharge and flow in present day and palaeo glacial environments are governed by both geomorphology and subsurface sedimentology this paper assesses the hydrogeological implications of glacial landscape evolution and develops a conceptual model of glacial landscape evolution and hydrogeological interaction the ice marginal zone of skei ararsandur se iceland was selected as a case study of a temperate actively receding glacier landsystem that is affected by both surging behaviour and periodic glacier outburst floods jokulhlaups the groundwater table elevation was measured approximately every 2 5 days throughout two six week summer field seasons in 2000 and 2001 and once in march 2001 using a network of shallow piezometers and hydraulic conductivity was calculated based on grain size distribution analysis of sampled aquifer materials the hydraulic conductivity of the near surface aquifer at the study site spans three orders of magnitude and is highest and most varied in areas inundated by the november 1996 jokulhlaup the shallow water table in the study area responds dynamically to precipitation events with the greatest fluctuation in an enclosed moraine lake basin with a perched aquifer underlain by ice and in a zone of outwash fans proximal to the ice margin the seasonal decline in water table is greatest at the ice margin reflecting the reduced winter recharge from the stagnant ice zone landsystems with morainic relief are associated with localised moraine precipitation fed groundwater systems these localised groundwater systems are superimposed on the intermediate regional scale groundwater systems that discharge further down sandur and offshore and are dominated by the gently sloping geomorphologically uniform outwash plain groundwater fed lakes formed in depressions from ice blocks deposited during jokulhlaups may provide important ecological niches the investigations at this site demonstrate that glacial outburst floods and glacier margin fluctuations produce a suite of sediments landforms and environments that affect the hydrogeological system

a conceptual model for alpine proglacial river channel evolution under changing climatic conditions this paper integrates concepts derived from the literature to focus upon interactions between riparian vegetation and river channel dynamics in alpine glacier basins discussion of the nature and variability of discharge and sediment regimes of alpine glacier fed rivers downstream variations in the physical character of the river channel and corridor consequent downstream variations in lateral processes and regional variations in alpine glacier dynamics lead to the proposal of a conceptual model of proglacial river channel riparian vegetation interactions under changing climatic conditions

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small