formation and stability of transverse and longitudinal sand dunes the shape of dunes depends on the history of wind regimes and sand availability in deserts exposed to winds from two different directions but with comparable magnitude dunes are found to be linear ridges which are either perpendicular or parallel to the mean wind direction depending on the angle between the two wind directions these dunes respectively observed for small and large angles between winds are called transverse and longitudinal dunes in both cases their large width hundreds of meters and evolution time scale years strongly limit the investigation of their dynamics and thus our understanding of such structures here we show that under water similar structures can be obtained but at much smaller space and time scales performing controlled experiments together with numerical simulations we highlight the physical mechanisms at play in the formation and long term evolution of these structures we show in particular that while longitudinal dunes are stable and extend in time transverse dunes are unstable they evolve into wavy ridges and eventually break into barchans if the sand supply is too low this fundamental difference is understood through the study of single sand piles and bars exposed to two winds in the case of a large angle between winds a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune such an elongation does not occur for a small angle where a sand pile evolves into a barchan these results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field

segmentation and differential post rift uplift at the angola margin as recorded by the transform rifted benguela and oblique to orthogonal rifted kwanza basins abstract we analyse tectonic and sedimentary field and subsurface data for the angola onshore margin together with free air gravity anomaly data for the offshore margin this enables us to characterize the mode of syn rift tectonism inherited from the precambrian and its impact on the segmentation of the angola margin we illustrate that segmentation by the progressive transition from the benguela transform rifted margin segment to the oblique rifted south kwanza and orthogonal rifted north kwanza margin segments the spatial variation in the intensity of post rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post rift succession of the coastal benguela and kwanza basins upper cretaceous to cenozoic uplifted palaeodeltas erosional unconformities palaeovalleys quaternary marine terraces and perched gilbert deltas the onshore benguela transform margin has a distinctive mainly progradational stratigraphic architecture with long term sedimentary gaps and high elevation marine terraces resulting from moderate upper cretaceous cenozoic to major quaternary uplifting i e 775 1775 mm ky or m ma by contrast repeated synchronous episodes of minor cenozoic to quaternary uplift occurred along the orthogonal rifted north kwanza segment with its cenozoic aggradational architecture short term sedimentary gaps and low elevation pleistocene terraces margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep sea fans along the low lying north kwanza margin sedimentation of the broad cenozoic to pleistocene kwanza submarine fan was probably governed by the width of the kwanza interior palaeodrainage basin combined with the wet tropical neogene climate along the high rising benguela margin the small size of the benguela deep sea fan is related to the interplay between moderate continental sediment dispersal from long lived small catchments and a warm very arid neogene climate however the driving forces behind the epeirogenic post rift uplift of the angola coastal bulge remain a matter of speculation

eolian fluvial and pan playa facies of the tertiary tsondab sandstone formation in the central namib desert namibia abstract the pliocene to recent sand sea in the central part of the 2000 km long namib desert is underlain widely by sediments comprising the tsondab sandstone formation tsf of early to middle tertiary age this formation is the earliest record of terrestrial sedimentation in the central namib desert and formerly was thought to consist of only eolian dune and sand sheet deposits of a desert origin however recent field investigations indicate that the tsf also includes deposits of ephemeral watercourses and pans playas these fluvial and playa sediments are associated intimately with the eolian dune and sand sheet deposits as is likewise the case in the present extreme arid namib desert sedimentary structures and trace fossils in the tsf eolianites resemble those found in the modern sand sea furthermore these eolianites were deposited under a dominant southerly and a subordinate easterly quadrant wind regime which is similar to the present day circulation pattern in the namib since the cold benguela current was established only in the late tertiary the tsf represents a major ancestral arid phase in the namib tract that was controlled by an early anticyclonic wind regime and a geographic location on the rainshadow side of southern africa

grain size dependence of sediment composition and environmental bias in provenance studies abstract this article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition which cause systematic distortion in quantitative provenance analysis environmental bias traditional procedures aimed at eliminating textural noise find limited success the gazzi dickinson method cannot remove hydrodynamic related modal variability multiple size window strategies are time consuming narrow or moving size window strategies represent misleading or impractical short cuts being less convenient options than simply analysing each sample in bulk new concepts introduced here unravel the superposed causes of compositional variability in modern sediments intrasample modal variability fundamentally explained by settling equivalence relationships can be accurately modelled mathematically intersample modal variability principally resulting from selective entrainment can be assessed and removed by a simple principle in absence of provenance changes and environmental bias the weighted average density of terrigenous grains srd index should be equal for each sample and each grain size class of each sample to the weighted average density of source rocks by correcting relative abundances of detrital minerals in proportion to their densities we can restore the appropriate srd index for any provenance and subprovenance type in each sample or grain size class modal variability is effectively reduced by this procedure which can be applied confidently to modern sediments deposited by tractive currents in any environment good results are obtained even for placer sands and finest classes where heavy mineral concentration is strongest such srd correction also successfully compensates for biased narrow window modes thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology after compensating for settling equivalence and selective entrainment effects residual size dependent compositional variability may be provenance related minor in ganga brahmaputra sediments provenance related effects are spectacularly displayed in the nile basin where volcaniclastic silt mixes with basement derived quartzofeldspathic sand and wind blown saharan quartz

mineralogy and geochemistry of the clay fraction of sediments from the namibian continental margin and the adjacent hinterland the continental shelf off namibia can be divided into two sectors at latitude 18 40 s based on the distinctive morphologies of the two regions and on differences in composition of their surficial sediments the kunene shelf is narrow shallow and covered mainly by terrigenous sediment whereas the walvis shelf is wide deep and supports predominantly biogenic sediment the walvis shelf also possesses a number of distinguishing features its outer region has a concave profile which is attributed to upbuilding of the inner shelf during periods of lowered sea level its inner region is covered by an extensive belt of diatomaceous mud parallel to the coast the seaward and landward flanks of this deposit differ in lithology due mainly to the advanced state of dissolution of opal in deep water and to the influx of aeolian terrigenous detritus especially illite close to shore long belts of terrigenous mud extend southwards from both the kunene and orange rivers the currents responsible for transporting this sediment are poleward directed tongues of the angola benguela front in the north and mixed poleward moving south atlantic central water in the south the kunene river carries more smectite to the sea than the much larger orange the distribution of clay minerals on the continental shelf follows that of the adjacent hinterland with smectite and to a lesser extent kaolinite being significant in the north and illite predominating in the south along the upper slope the concentrations of smectite and kaolinite increase due to their importation from the kunene river by poleward directed antarctic intermediate water the geochemical character of riverine clays reflects the rock types exposed in each of the drainage basins compared with marine sediments these clays are enriched in eight of the fourteen elements investigated namely al fe mg co mn pb rb and zn in contrast the offshore element distribution patterns show little resemblance to the patterns of individual drainage basins clays in the marine sediments are enriched relative to the river sediments in ca k cd ni and sr five lithofacies recognized in the unconsolidated marine sediments are based on the relative proportions of opal calcium carbonate phosphorite glauconite and terrigenous components average element concentrations of individual lithofacies are enriched relative to the river clays in cd diatomaceous facies ca cd ni and sr calcareous and facies k and ni slightly glauconitic facies cd and sr terrigenous facies regionally the clay fractions of unconsolidated marine sediments contain high concentrations of al fe mg k mn and rb on the kunene margin due to aluminosilicates being imported by the kunene river ca and sr reach high values on the walvis outer margin in response to upwelling along the coast maximum concentrations of cd are associated with diatomaceous mud on the walvis inner shelf cu ni and zn reach high levels at a few spot locations where organic matter morg has suffered little dilution from other components especially opal and pb is concentrated with glauconitic sediment on the kunene shelf semi consolidated sediments are exposed intermittently on the outer margin they provide evidence that the palaeoclimate became much drier in post miocene times but in the north this picture is complicated by the introduction of sediment from the kunene river during the late tertiary palaeo oceanographic evidence suggests that the northern part of the study area experienced a concomitant decrease in productivity while in the southern part productivity increased

the orange river southern africa an extreme example of a wave dominated sediment dispersal system in the south atlantic ocean sediments delivered to the south atlantic ocean by the orange river are fractionated and dispersed northwards and westwards by a vigorous longshore drift system and a number of ocean currents gravels are accreted to the coastline for a distance 300 km north from the orange river mouth sands are transported alongshore for 700 km but are in places along this transport path returned onshore by coastal winds to form the main namib sand sea and other smaller dune fields mud is more widely dispersed westwards northwards and southwards probably by slow moving ocean scale currents into basins on the shelf and onto the continental shelf edge this dispersal system operating since at least eocene times is believed to have originated during a time when there was a late cretaceous early cenozoic uplift of southern africa which resulted in 1 intensification of the existing southerly wind system 2 incision of the orange river which coupled with a shift in climate resulted in a coarsening of its sediment load delivered to the coast 3 a broad weakly subsiding or mildly uplifting inner continental shelf with little accommodation space for the sediment load of the incising orange river

crustal scale structural architecture in central chile based on seismicity and surface geology implications for andean mountain building 1 we document a crustal scale structural model for the central chile andes based on seismicity and surface geology which consists in a major east verging ramp detachment structure connecting the subduction zone with the cordillera the ramp rises from the subducting slab at 60 km depth to 15 20 km below the western edge of the cordillera extending eastward as a 10 km depth flat detachment this structure plays a fundamental role in the andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a simple shear deformation mode indeed despite shortening distribution being very asymmetric 16 km versus 70 km in the western and eastern side respectively the western side is higher and thicker than what is expected yield strength envelopes show strong rheological control on this structure vp and vp vs variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab which coincides with the blueschist eclogite transition therefore subduction processes would control the depth where the major east verging structure may merge with the slab such a ramp flat structure is observed in other parts of the chilean margin hence it seems to be a first order feature in the andean subduction zone this structure delimitates upward the rocks transmitting part of the plate convergence stress from the plate interface and controls mountain building tectonics thus playing a key role in the andean orogeny

ocean climate of the south east atlantic observed from satellite data and wind models the near coastal south east atlantic ocean off africa is a unique and highly dynamic environment comprising the cool benguela current warm angola current and warm agulhas current strong coastal upwelling and the congo river strongly influence primary production much of the present knowledge of the south east atlantic has been derived from ship borne measurements and in situ sensors which cannot generally provide extensive spatial and temporal coverage similarly previous satellite studies of the region have often focused on small spatial areas and limited time periods this paper provides an improved understanding of seasonal and interannual variability in ocean dynamics along the south east atlantic coast of africa using time series of satellite and model derived data products eighteen years of satellite sea surface temperature data are complimented by 7 years of sea level data three years of chlorophyll a data illustrate the seasonal biological response but the time series is not of sufficient length for investigating interannual variability in chlorophyll biomass modelled wind fields are used to describe atmospheric forcing of the surface ocean this is the first synoptic scale description of the south east atlantic from a suite of large spatial coverage long time series products previous studies of seasonal and interannual variability in the region are reviewed and used to interpret key oceanographic features and processes identified in the satellite data key findings of this study are 1 descriptions of seasonal and interannual variability from these data show climate forcing of the south east atlantic coast of africa from both the northern and southern boundaries bimodal seasonal signals of equatorial origin propagate poleward along the angolan coast while the trade winds and events in the agulhas region dominate the agulhas bank and southern benguela the northern benguela is a mixed regime under the influence of forcing from both directions 2 the benguela nino years of 1984 and 1995 are clearly observed in sea surface temperature and sea level anomalies and correspond to anomalously weak southerly winds at the equator these conditions were also observed in 1999 suggesting this too may have been a benguela nino year 3 consideration of putative nino type events in the equatorial atlantic from this and other studies suggests that the frequency of these events is much higher than previously estimated and may be similar to the frequency of el nino southern oscillation enso events in the pacific ocean furthermore years of anomalously strong southerly winds at the equator occur during pacific enso years c 2003 elsevier ltd all rights reserved

dunes on the skeleton coast namibia south west africa geomorphology and grain size relationships simple and locally compound transverse and barchanoid dunes dominate the 2000 km2 skeleton coast dunefield in northwestern namibia south west africa dune height and spacing are closely correlated r 0 89 and decrease across the dunefield from southwest to northeast with an accompanying change from transverse to barchanoid ridges and ultimately barchans the dunes are aligned transverse to the dominant strong south and south southwest onshore winds alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune forming winds to swing to the right over the dunes but resume their original direction beyond grain size and sorting vary at three scales the dune the dune landscape and through the dunefield overall the sands derived from three localities by deflation from beaches supplied by vigorous longshore drift become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing a statistically significant relationship r 0 65 was established between dune spacing and the phi grain size of the coarser fraction of the dune sands demonstrating the importance of the protective effects of coarse grains and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand

sand residence times of one million years in the namib sand sea from cosmogenic nuclides the namib sand sea is one of the world s oldest and largest sand deserts1 yet little is known about the source of the sand in this or other large deserts2 in particular it is unclear whether the sand is derived from local sediment or comes from remote sources the relatively uniform appearance of dune sands and low compositional variability within dune fields3 make it difficult to address this question here we combine cosmogenic nuclide measurements and geochronological techniques to assess the provenance and migration history of sand grains in the namib sand sea we use u pb geochronology of detrital zircons to show that the primary source of sand is the orange river at the southern edge of the namib desert our burial ages obtained from measurements of the cosmogenic nuclides 10be 26al and 21ne suggest that the residence time of sand within the sand sea is at least one million years we therefore conclude that despite large climatic changes in the namib region associated with quaternary glacial interglacial cycles4 5 the area currently occupied by the namib sand sea has never been entirely devoid of sand during the past million years

morphometric analysis of aeolian bedforms in the namib sand sea using aster data the aster global digital elevation model gdem has made elevation data at 30 m spatial resolution freely available enabling reinvestigation of morphometric relationships derived from limited field data using much larger sample sizes these data are used to analyse a range of morphometric relationships derived for dunes between dune height spacing and equivalent sand thickness in the namib sand sea which was chosen because there are a number of extant studies that could be used for comparison with the results the relative accuracy of gdem for capturing dune height and shape was tested against multiple individual aster dem scenes and against field surveys highlighting the smoothing of the dune crest and resultant underestimation of dune height and the omission of the smallest dunes because of the 30 m sampling of aster dem products it is demonstrated that morphometric relationships derived from gdem data are broadly comparable with relationships derived by previous methods across a range of different dune types the data confirm patterns of dune height spacing and equivalent sand thickness mapped previously in the namib sand sea but add new detail to these patterns

upper mantle structure of south america from joint inversion of waveforms and fundamental mode group velocities of rayleigh waves 1 a new tomographic s wave velocity model for the upper mantle beneath south america is presented we developed and applied a new method of simultaneously inverting regional s and rayleigh waveforms and fundamental mode rayleigh wave group velocities to better constrain upper mantle s velocity structure and moho depth we used 5700 rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the south american continent the joint inversion of this data set provided a new three dimensional 3 d upper mantle s velocity model and a moho depth model for south america which fits both the group velocity and regional waveform data sets well new features of the final three dimensional 3 d s velocity and moho depth model correlate well with known geotectonic units on a regional scale the moho depth ranges from 30 km in the central chaco basin to 42 km beneath the amazonian craton and 45 70 km beneath the orogenic andean belt the imaged s velocity indicates an average lithosphere thickness of around 160 km for the amazonian craton high velocities are imaged beneath the amazon and part of the parana and parnao ba basins down to about 150 km low to very low velocities are imaged

detrital sediment fluxes from continents to oceans abstract detrital sediment is carried from land to the sea by three agents rivers glaciers and winds the shoreline is an arbitrary boundary within the detrital sediment transport system which extends from a site of origin across areas of temporary storage to a site of long term deposition the most important of the agents moving sediment across the land is river transport estimated to be in the order of 20 10 12 kg of sediment annually at present analysis of drainage basins indicates that relief and runoff are the most important factors in determining the sediment load of rivers the competence of rivers to transport sediment is governed by the volume flow gradient and the sediment load itself today most large rivers are fed by snowmelt in highland areas runoff from rainfall in the drainage basin and groundwater inflow along the river course water is lost to evaporation and groundwater infiltration river courses can often be divided into two segments a degradational section in which the gradient is relatively steep and little temporary storage of sediment takes place and an aggradational section where the gradient is sharply reduced through meandering and where large scale temporary sediment storage forms a flood plain lakes trap sediment inland and prevent its transport to the sea today many high and mid latitude rivers are interrupted by lakes of glacial origin there are also some large areas of internal drainage that deliver no sediment to the sea the load carried by rivers has been markedly altered by human activity and may have doubled over the past few thousand years only to be reduced in the past century by the widespread construction of dams the ancient use of fire in hunting and its subsequent use in clearing land has increased erosion extensive deforestation and cultivation processes have also increased the sediment supply dam construction is a relatively new factor and affects the sediment transport system by trapping sediment before it can reach the sea the resulting lower sediment supply from rivers is at least in part compensated by increased coastal erosion glacial erosion is difficult to estimate there is an ongoing controversy whether ice sheets are effective erosive agents or not estimates of the present global flux of glacial detritus range from 0 8 50 10 12 kg annually with the lower value most probable the dust flux is in the order of 0 5 to 0 9 10 12 kg annually but may vary greatly with time

u pb ages sr nd isotope geochemistry and petrogenesis of kimberlites kamafugites and phlogopite picrites of the alto paranaiba igneous province brazil abstract the kimberlites kamafugites and phlogopite picrites of alto paranaiba igneous province apip southern brazil span a range between 91 and 78 ma with new in situ more tightly constrained u pb ages the kimberlites show porphyritic texture with olivine xeno and phenocrysts plus phlogopite fe ti cr oxides and perovskite microcrysts set in a carbonate rich matrix the kamafugites are feldspar free rocks represented by ugandites and mafurites ugandites and mafurites are porphyritic and contain olivine and clinopyroxene phenocrysts in a fine grained groundmass composed by clinopyroxene perovskite apatite magnetite phlogopite leucite and or analcime in ugandites and by olivine clinopyroxene amphibole phlogopite perovskite magnetite kalsilite and ba zeolites in mafurites phlogopite picrites have a pseudo fluidal porphyritic texture with olivine phenocrysts and abundant phlogopite microcrysts in a groundmass composed by olivine spinel apatite perovskite calcite and rare garnet new in situ sr and nd isotopic data on perovskites 87 sr 86 sr i 0 70467 0 70565 and 143 nd 144 nd i 0 51222 0 51233 fall within the known ranges of apip rocks 87 sr 86 sr i 0 70431 0 70686 143 nd 144 nd i 0 51205 0 51280 the apip magmas derived from a source assemblage made up of an old metasomatized mica carbonate garnet lherzolite that did not suffer interaction with convective mantle nor with any hypothetical melts derived from a trindade mantle plume geochemical modelling show that low degree melting f 0 5 2 of such a source can produce compositions resembling the apip rocks the geochemical and isotopic composition of the magmas the calculated degrees of partial melting the composition of the calculated source and the absence of a hot spot track from goias to alto paranaiba igneous provinces can be explained with the presence of chemically and mineralogically heterogeneous mantle sources that melted at different pressures

ree and selected trace elements in kimberlites from the kimberley area south africa abstract new data by ina methods on sc cs ree ta th and u in 11 kimberlites 1 phlogopite garnet harzburgite xenolith and 1 eclogite xenolith from the larger kimberley area are used to constrain models for kimberlite genesis kimberlite magmas are produced by melting of 61 clinopyroxene 60 olivine 22 phlogopite and 3 garnet of mantle rocks linked with precipitation of 46 orthopyroxene the accumulation of the incompatible elements la ce sr rb in kimberlites relative to garnet harzburgite has been modelled with the distribution coefficients of major mantle minerals to melts there is not much conformity between computed and observed abundances the degree of partial melting is estimated as 2 3 based upon kimberlite clinopyroxene balances complete melting of accessory mantle phases phlogopite phosphates carbonates etc and fractional melting of clinopyroxene eventually plus phlogopite of metasomatically altered garnet harzburgite can have contributed the major proportion of incompatible elements in kimberlites and melilite bearing olivine nephelinites the major fraction of hree in the latter rocks must have been supplied by mantle clinopyroxene and not by garnet this precludes the general discrimination between garnet peridotite and spinel peridotite equilibrated melts by means of the hree concentration of primary mantle magmas

geochemistry and geodynamic implications on the source of parana etendeka large igneous province evidenced by the 128 ma rosario 6 kimberlite southern brazil abstract the rosario 6 is a non diamondiferous hypabyssal kimberlite located above the rio de la plata craton and near the south eastern edge of the parana basin in southern brazil it is petrographically an inequigranular texture macrocrystal kimberlite fresh and the groundmass exhibits a microporphyritic texture and round megacrysts of olivine which are derived from disaggregated mantle xenoliths olivine is also present as macrocrysts microphenocrysts and in the groundmass together with phlogopite and apatite these microphenocrysts are immersed in a groundmass of olivine monticellite phlogopite catio3 perovskite apatite mg chromite and mg ulvospinel and melilite a mesostasis assemblage of phlogopite melilite soda melilite akermanite and calcium carbonate is segregated from the groundmass its geochemical signature is similar to those of transitional kimberlites of kaapvaal craton south africa and the u pb ages of 128 ma on perovskite reveal that rosario 6 kimberlite post dates the main pulse of volcanism in the parana etendeka large igneous province lip the high ti content of some minerals such as mg chromite mg ulvospinel phlogopite and melilite and the presence of perovskite suggest a ti rich source the petrographic geochemical and isotopic data indicate that the rosario 6 kimberlite source is a depleted mantle metasomatized by h2o rich fluids co2 rich and silicate melts derived from the recycling of an ancient subducted oceanic plate eclogite before the south atlantic opening although several authors indicate the influence of tristan da cunha plume for the generation of alkaline magmatism associated to the parana etendeka flood basalts our data demonstrates that tristan da cunha plume has no chemical contribution to the generation of rosario 6 kimberlite except by its thermal influence

sediment exchange between amazon mudbanks and shore fringing mangroves in french guiana time series remote sensing data sediment cores and water column profiles of fluid properties and suspended sediment concentration were utilized to examine the mechanisms and volumes of sediment exchange between the mangrove fringe and the coastal zone in french guiana remote sensing estimates of sediment yield from mangrove shoreline erosion by wave attack during the 30 year periods when offshore amazon mudbanks are not adjacent to a 30 40 km long section of the shoreline are 6 23 million tons year 1 7be 137cs and 210pb inventories in sediment cores from inner 5 m water depth mudbank areas suggest the mangrove erosion occurs primarily during the january april period of high wave energy and is responsible for the bulk of sediment deposition on the leading edge of the mudbank remote sensing estimates of shoreline accretion associated with onshore flux of sediment from a mudbank are also large 8 26 million tons year 1 radiotracer generated strata age relationships indicate that the lower intertidal zone of the leading depositional edge of the mudbank is a relict surface suggesting the offshore mudbank is decoupled from the shoreline water column profiles and radiochemical signatures point to fluid mud suspensions 10 400 g l 1 as the primary mechanism for delivering sediment across the relict intertidal zone to create the accretion at the shoreline if the estimates of sediment volume involved in shoreline accretion and erosion in the studied area are extrapolated along the entire 1400 km long guianas coastline possessing offshore mudbanks the annual exchange of sediment to and from the mangrove fringe is at least equivalent to the average input of new sediment from the amazon

kimberlitic sources of super deep diamonds in the juina area mato grosso state brazil abstract the juina diamond field in the 1970 80s was producing up to 5 6 million carats per year from rich placer deposits but no economic primary deposits had been found in the area in 2006 2007 diagem inc discovered a group of diamondiferous kimberlitic pipes within the chapadao plateau chapadao or pandrea cluster at the head of a drainage system which has produced most of the alluvial diamonds mined in the juina area diamonds from placer deposits and newly discovered kimberlites are identical they have super deep origins from the upper mantle and transition zone field observations and petrographic studies have identified crater facies kimberlitic material at seven separate localities kimberlitic material is represented by tuffs tuffisites and various epiclastic sediments containing chrome spinel picroilmenite manganoan ilmenite zircon and diamond the diamond grade varies from 0 2 1 8 ct m3 chrome spinel has 30 61 wt cr2o3 picroilmenite contains 6 14 wt mgo and 0 2 4 wt cr2o3 manganoan ilmenite has less than 3 wt mgo and 0 38 1 41 wt mno the 176hf 177hf ratio in kimberlitic zircons is 0 028288 0 28295 with ehf 5 9 8 3 and lies on the average kimberlite trend between depleted mantle and chur the previously known barren and weakly diamondiferous kimberlites in the juina area have ages of 79 80 ma in contrast zircons from the newly discovered chapadao kimberlites have a mean 206pb 238u age of 93 6 0 4 ma corresponding to a time of magmatic activity related to the opening of the southern part of the atlantic ocean the most likely mechanism of the origin of kimberlitic magma is super deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto kimberlitic magma

from static to dynamic provenance analysis sedimentary petrology upgraded abstract the classical approach to sandstone petrology established in the golden years of plate tectonics and based on the axiom that detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes has represented a benchmark for decades the composition of sand and sandstone however simply provides us with a distorted image of the lithological structure of source terranes and gives us little clue whether they are allochthonous or autochthonous orogenic or anorogenic young or old what we may able to see reflected in detrital modes is the nature of source terranes continental arc oceanic and the tectonostratigraphic level reached by erosion in space and time the proposed new approach to the petrology of sand and sandstone 1 starts with a simple classification scheme circulated since the 1960s which is purely descriptive objective and free of ill defined ambiguous terms and 2 focuses on the nature and tectonostratigraphic level of source terranes further steps are essential to upgrade provenance analysis acquiring knowledge from modern settings is needed to properly identify and wherever possible correct for physical and chemical processes introducing environmental and diagenetic bias and thus address nature s complexities with adequate conceptual tools equally important is the integration of multiple techniques ideally including bulk sediment multi mineral and single mineral methods bulk sediment petrography remains the fundamental approach that allows us to capture the most precious source of direct provenance information represented by the mineralogy and texture of rock fragments bulk sediment geochemistry applicable also to silt and clay carried in suspension is a superior method to check for hydraulic sorting chemical weathering and fertility of detrital minerals in different sediment sources detrital geochronology thermochronology and isotope geochemistry reveal the diverse time structures of source rocks and have become necessary complementary techniques in modern provenance analysis inferences on geodynamic processes need independent geological information and come last but if tackled properly they can lead us much farther than the standard label obtained by using triangular diagrams uncritically as if they were infallible oracles

climate control on early cenozoic denudation of the namibian margin as deduced from new thermochronological constraints abstract the processes that control long term landscape evolution in continental interiors and in particular along passive margins such as in southern africa are still the subject of much debate e g braun 2018 although today the namibian margin is characterized by an arid climate it has experienced climatic fluctuations during the cenozoic and yet to date no study has documented the potential role of climate on its erosion history in western namibia the brandberg massif an erosional remnant or inselberg provides a good opportunity to document the cenozoic denudation history of the margin using the relationship between rock cooling or exhumation ages and their elevation here we provide new apatite u th sm he dates on the brandberg inselberg that range from 151 12 to 30 2 ma combined with existing apatite fission track data they yield new constraints on the denudation history of the margin these data document two main cooling phases since continental break up 130 myr ago a rapid one 10 c myr following break up and a slower one 1 2 c myr between 65 and 35 ma we interpret them respectively to be related to escarpment erosion following rifting and continental break up and as a phase of enhanced denudation during the early eocene climatic optimum we propose that during the early eocene climatic optimum chemical weathering was important and contributed significantly to the denudation of the namibian margin and the formation of a pediplain around the brandberg and enhanced valley incision within the massif additionally aridification of the region since 35 ma has resulted in negligible denudation rates since that time

geochemistry and mineralogy of kimberlites from the arkhangelsk region nw russia evidence for transitional kimberlite magma types abstract the arkhangelsk kimberlite province akp is situated in the north of the baltic shield within the buried southeastern portion of the kola kuloi craton it forms part of the extensive devonian magmatic event of the northern baltic shield and kola peninsula two main groups of kimberlites can be distinguished within the province 1 kimberlites from the diamondiferous zolotitsa field that have geochemical and isotopic affinities with group 2 kimberlites and lamproites 2 diamond poor ti fe rich kimberlites from other arkhangelsk fields that have geochemical and isotopic affinities with group 1 kimberlites however the zolotitsa and ti fe rich kimberlites have mineralogical characteristics that are not typical for their respective assigned kimberlite group classifications both groups of arkhangelsk kimberlites are apparently transitional to group 1 kimberlites group 2 kimberlites and lamproites as they are defined elsewhere in the world an associated kimberlite from the mela sill complex has strong affinities with carbonatites the low al2o3 high ni and cr contents and high mg in both groups of kimberlites indicate strongly depleted lherzolitic harzburgitic mantle sources trace element patterns show a variable enrichment of incompatible elements and strong lree enrichment however kimberlites from the zolotitsa field have overall lower trace element abundances and less steep ree patterns suggesting a higher degree of partial melt and or a less enriched source compared to that of the ti fe rich kimberlites a calciocarbonatite of the mela sill complex has trace element and ree patterns typical of other carbonatites closely associated with kimberlites 87 sr 86 sr i and 143 nd 144 nd i isotope compositions of the arkhangelsk kimberlites and carbonatite reveal that at least two mantle sources are required to explain the isotopic variation 1 most of the zolotitsa and mela kimberlites and the mela carbonatite are derived from an ancient enriched lithospheric source emi 2 the ti fe rich kimberlites are derived from a plume related asthenospheric mantle source with an isotopic composition close to bulk earth present day pb isotope compositions reveal that the zolotitsa kimberlites have values close to group 1 kimberlites however the ti fe rich kimberlites generally have slightly more radiogenic pb isotope values

ultra long distance littoral transport of orange sand and provenance of the skeleton coast erg namibia abstract quantitative provenance analysis based on high resolution bulk petrography and heavy mineral data on beach and dune sands integrated with detrital zircon geochronology and chemical analyses of pyroxene garnet and staurolite demonstrates that sand derived from the orange river is carried by powerful and persistent longshore currents as far as northern namibia and southern angola 1750 km north of its mouth this is the longest cell of littoral sand transport documented so far compositional forward modeling indicates that 80 of dune sand in the skeleton coast is orange derived the remaining 20 being supplied by slow erosion of the damara orogen chiefly via the swakop river a decrease in basaltic rock fragments and pyroxene with relative enrichment in garnet staurolite tourmaline and other metamorphic minerals north of walvis bay indicates that only one third of beach sand in the 350 km dune free gap between the namib and skeleton coast ergs is orange derived the remaining two thirds being supplied largely by the swakop river draining the damara orogen although volcanic gravel becomes dominant in beaches of the skeleton coast north of the uniab mouth detritus from cretaceous etendeka lavas accounts for only 4 of beach sand reflecting limited sand generation in the arid catchment contributions from the kunene river to either dune or beach sands is ruled out indicating that gem diamonds found in placer deposits along the coast of namibia are all derived from the orange river

the hydrothermal system of the domuyo volcanic complex argentina a conceptual model based on new geochemical and isotopic evidences abstract the domuyo volcanic complex neuquen province argentina hosts one of the most promising geothermal systems of patagonia giving rise to thermal manifestations discharging hot and cl rich fluids this study reports a complete geochemical dataset of gas and water samples collected in three years 2013 2014 and 2015 from the main fluid discharges of this area the chemical and isotopic composition d h 2 o and 18 o h 2 o of waters indicates that rainwater and snow melting are the primary recharge of a hydrothermal reservoir located at relative shallow depth 400 600 m possibly connected to a second deeper 2 3 km reservoir reactive magmatic gases are completely scrubbed by the hydrothermal aquifer s whereas interaction of meteoric waters at the surface causes a significant air contamination and dilution of the fluid discharges located along the creeks at the foothill of the cerro domuyo edifice thermal discharges located at relatively high altitude 3150 m a s l namely bramadora are less affected by this process as also shown by their relatively high r ra values up to 6 91 pointing to the occurrence of an actively degassing magma batch located at an unknown depth gas and solute geothermometry suggests equilibrium temperatures up to 220 240 c likely referred to the shallower hydrothermal reservoir these results confirming the promising indications of the preliminary surveys carried out in the 1980 s provide useful information for a reliable estimation of the geothermal potential of this extinct volcanic system although a detailed geophysical measurements is required for the correct estimation of depth and dimensions of the fluid reservoir s

heavy mineral beach placers in southeastern australia their nature and genesis southeastern australia has been one of the world s most productive heavy mineral sand provinces concentrations of detrital rutile zircon ilmenite and minor monazite occur as beach placers and in dunes in high stand barriers of quaternary age along the present coast although individual mineral grains are of terrigeneous origin the heavy mineral deposits themselves are not linked to specific source rocks in the hinterland rather they arise under the action of waves during the building of coastal sand barriers a number of fractionating mechanisms that concentrate heavy minerals operate during marine transgressions and under highstand conditions to produce three different types of beach placers type a deposits occur along erosional discontinuities in leaky embayments or between barriers of different age here repeated storm reworking has produced beach face concentrations on drift aligned sectors of coast by a process termed littoral bypassing fractionation type b heavy mineral deposits are found in back barrier washover facies and sometimes as condensed sections at the rear of swash aligned barriers in coastal embayments these were produced by a process of transgressive barrier fractionation that reworked heavy minerals from shelves undergoing erosion during marine transgressions type c heavy mineral deposits occur in prograded barriers and represent episodic influxes of remobilized heavy mineral rich sand that originated elsewhere through the two fractionation mechanisms mentioned above necessary environmental conditions for these fractionating mechanisms to operate include low rates of clastic sediment supply and long periods of weathering and abrasion to create a mature heavy mineral suite an energetic swell wave climate driving large sand fluxes onshore and alongshore and changing sea levels especially marine transgressions that have the effect of moving heavy minerals from the shelf onto the present coast because of the last mentioned effect the mineral prospectivity for the southeastern australian continental shelf is not high

scales of transient convective support beneath africa beneath sub equatorial africa there is convincing evidence for a gigantic slow velocity anomaly within the lower mantle that is inferred to be a thermochemical superplume here we define the spatial and temporal pattern of uplift across the northern edge of this putative superplume by analyzing stratal geometries and stacking velocities of neogene sedimentary rocks these marine deposits are imaged on a dense grid of seismic reflection profiles that straddle the southwest african coastal shelf inverse modeling of stacking velocity profiles demonstrates that there have been at least two significant phases of uplift a post pliocene uplift event increases from 0 to 500 m elevation over a distance of 1000 km in close agreement with geophysical estimates of the present day variation of dynamic topography an earlier phase of uplift occurred in middle oligocene time ca 30 35 ma ago we propose a two stage development of epeirogeny in which a middle oligocene phase of uplift may be related to initiation and growth of the african superplume post pliocene denudation records rapid 100 250 m ma vertical motions generated by upper mantle convection which vary on length scales of 500 1000 km our results support the existence of a convective circulation system that has operated on different temporal and spatial scales beneath the african plate

climatic zonation and weathering control on sediment composition angola abstract complementary mineralogical and geochemical datasets on fluvial beach and dune samples collected along the atlantic margin of subequatorial southwestern africa are used to investigate the relationships between provenance and climatic controls on sediment composition and to test the reliability of different geochemical and mineralogical weathering proxies as climatic indicators the studied n s trending coastal region is characterized by strong latitudinal and inland climatic gradients and thus represents an excellent natural laboratory in which to study the effects of climatic induced weathering on sediment composition although the mineralogy and geochemistry of suspended load muds closely reflects the different weathering intensities over both latitudinal and inland climatic gradients the composition of mud and sand samples are strongly affected by sediment provenance consequently weathering parameters such as the al e values estimating the degree of depletion in element e relative to the ucc standard display complex patterns of variation especially for sand samples by assuming a typical order of bulk sediment mobility na ca sr mg k ba rb anomalously high or low al values placing a specific element off the expected mobility order are considered as an indicator of source rock control on sediment composition the composition of detritus recycled from meso cenozoic strata reflects the cumulative effect of successive sediment cycles with recycling processes affecting to a different extent the diverse weathering proxies in particular al na appears to be more strongly affected by recycling in muds than in sands among all mineralogical and chemical parameters those that correlate best with rainfall in the drainage areas are al na for sands al mg for muds and smectite content only in areas of low rainfall in the geological and geomorphological setting of sw africa these proxies turn out to be better climate estimators than the classical weathering indices cia or wip this case study reminds us to carefully consider source rock control and mixing with recycled detritus when drawing inferences on climatic conditions based on weathering indices

major and trace element compositional variation of phlogopite from kimberlites and carbonatites as a petrogenetic indicator abstract ferromagnesian trioctahedral micas are a common macrocryst phenocryst phase in kimberlites and carbonatites we explored the possibility of using the chemistry of ferromagnesian micas to discriminate between these rocks in order to assess their economic potential we determined the extent of major and trace element variations of micas from 14 selected kimberlite and carbonatite localities worldwide representing several different kimberlite fields and alkaline carbonatite provinces phlogopite macrocrysts from kimberlites follow two principal evolutionary trends primary macrocrystic involving an increase in ba al ti cr zr nb and sr contents at decreasing mg towards the rim and transitional macrocryst groundmass trend involving an increase in kinoshitalite component at decreasing fe ti and cr contents micas from dolomite calcite carbonatites cover a much greater compositional range than those from kimberlites defining evolutionary trends towards annite tetra ferriphlogopite and kinoshitalite our data confirm that macrocrystic micas from kimberlites cannot be reliably distinguished from carbonatitic phlogopite on the basis of their major element compositions exceptions are fe al rich fe fe mg 0 2 and na rich 0 3 wt na 2 o micas that seem to be virtually restricted to carbonatites the generally higher levels of na in carbonatitic micas relative to those from kimberlites can be explained by the lower na abundances in the latter relative to carbonatites our data indicate that the trace element composition of mica determined by la icp ms can be used as a reliable petrogenetic indicator kimberlitic phlogopite is characteristically enriched in cr up to 12030 ppm ni up to 1600 ppm and co 40 70 ppm and depleted in mn 280 ppm nb 28 ppm sr 55 ppm sc 8 ppm zr 12 ppm and to some extent cs 7 ppm and ta 3 ppm with respect to carbonatitic micas 315 ppm cr 470 ppm ni 34 ppm co up to 17200 ppm mn 1080 ppm nb 830 ppm sr 80 ppm sc 105 ppm zr 27 ppm cs and 19 ppm ta with the exception of zr the observed differences in trace element composition of mica probably reflect the contrasting trace element geochemistry of kimberlitic and carbonatitic magmas the depletion of kimberlitic phlogopite in zr and possibly nb ta can be explained by sequestration of these elements in the earlier crystallized ilmenite variations in ba rb and v contents are similar in mica from both rock types and cannot be used as a petrogenetic indicator

turbidite megabeds in an oceanic rift valley recording jokulhlaups of late pleistocene glacial lakes of the western united states abstract escanaba trough is the southernmost segment of the gorda ridge and is filled by sandy turbidites locally exceeding 500 m in thickness new results from ocean drilling program odp sites 1037 and 1038 that include accelerator mass spectrometry ams 14c dates and revised petrographic evaluation of the sediment provenance combined with high resolution seismic reflection profiles provide a lithostratigraphic framework for the turbidite deposits three fining upward units of sandy turbidites from the upper 365 m at odp site 1037 can be correlated with sediment recovered at odp site 1038 and deep sea drilling program dsdp site 35 six ams 14c ages in the upper 317 m of the sequence at site 1037 indicate that average deposition rates exceeded 10 m k yr between 32 and 11 ka with nearly instantaneous deposition of one 60 m interval of sand petrography of the sand beds is consistent with a columbia river source for the entire sedimentary sequence in escanaba trough high resolution acoustic strat

barrier evolution and placer formation at bujuru southern brazil abstract this paper demonstrates the link between the evolution of a coastal barrier in southern brazil during the late holocene and the formation of a large volume of eolian disseminated heavy mineral deposits our data set is based on an earlier heavy mineral prospecting campaign 1991 and on 10 new shallow vibrocores 2 5 m long the model presented has three main steps of barrier evolution the first step is the recycling of coastal plain deposits during the postglacial marine transgression which ended at 5 6 ka when heavy minerals were incorporated into beach and washover facies of a transgressive barrier the second step is the shoreward retreat of the barrier under a slow and small sea level fall during the last 5 6 ka this second step eroded and recycled sediments from the pleistocene substrate which acted as an extra source of heavy minerals heavy minerals were concentrated in backshore deposits by wave action during barrier recession the third step is the erosion and transport of backshore sands by onshore winds into an inter barrier depression in the form of transgressive dune deposits these deposits contain an average of 4 66 1 02 disseminated heavy minerals 1494 samples this eolian placer deposit has started to form 1 ka ago and is still under formation

stable and ultrastable heavy minerals of alluvial to nearshore marine sediments from central portugal facies related trends abstract the pliocene to pleistocene sedimentary record from the western iberia coastal margin is a thin succession maximum thickness around 70 m derived from the metamorphic and igneous variscan iberian massif and from older sedimentary deposits it comprises a wide variety of facies representative of deposition in environments ranging from inner shelf to alluvial fan the facies are stacked in an overall regressive sequence this sequence may be divided into lower order sequences with thin transgressive portions not always present and thicker prograding portions given the limited burial diagenetic transformation the differences in heavy mineral hm assemblage can be explained by provenance hydraulic segregation and chemical and mechanical selection transparent heavy minerals susceptible to acid treatment were not considered in this heavy mineral analysis after the acid treatment the hm signal is relatively monotonous except for the mica rich assemblages in alluvial facies fed from the variscan iberian massif by short streams the hm assemblages are dominated by tourmaline andalusite zircon and staurolite however it is still possible to recognize two opposite trends in hm composition linked to facies evolution the first trend is widespread along the coastal margin although typical of the earlier deposits it is characterized by a vertical increase in staurolite and andalusite proportions coincident with prograding and regressive facies evolution relative sediment starvation during retrogradation when compared to progradation favoured the recycling of previous deposits leading to higher proportions of ultrastable minerals tourmaline zircon and rutile hydraulic segregation in the nearshore zone also contributes to increased zircon content in the very fine sand fraction the second trend is observed in stratigraphically higher sediments deposited when environments were displaced basinward this trend is common in relatively uplifted western sectors of the coastal margin where sand and gravel dominate the record it is characterized by a vertical increase in the content of ultrastable minerals mostly tourmaline and zircon throughout the regressive sequence it may be explained by alluvial storage and the recycling of previous nearshore and coastal plain facies recycling favoured by relative sediment starvation or low accommodation conditions plays a major role in these hm assemblages

weathering indices as climate proxies a step forward based on congo and sw african river muds abstract despite the influence of other geological and geomorphological factors chemical weathering at the earth s surface is strongly controlled by climate thus a measure of weathering intensity determined from soils or sediments should provide information about the climatic conditions associated with their formation available geochemical and mineralogical data on modern fluvial and marine muds from different regions of southern africa and its atlantic continental margin are used to review the links between sediment composition and climatic properties together with the possible causes of variance although river muds may not be generated exclusively in a single sedimentary cycle and erosion and weathering processes do not necessarily take place in a spatially homogeneous way significant relationships between mineralogical and geochemical signatures of river mud and rainfall in the corresponding catchment area were recognised our study shows that the composition of clay is strongly influenced by climatically driven weathering whilst coarser mud fractions tend to be more affected by provenance grain size hydraulic sorting and recycling in the marine environment the climatic signal may be lost even in clay because of hydraulic fractionation authigenic mineral growth and mixing with foreign particles given the ubiquitous character of fluvial muds and the easy and non expensive methods available for separating and analysing clay fractions their geochemical fingerprints represent a most precious source of information concerning climate any geochemical parameter used as a regional proxy of climate however still requires that the diversity of geological geomorphological and biological factors that affect its value are cautiously considered

a classification of metamorphic grains in sands based on their composition and grade abstract an operational classification of metamorphic grains in sands and sandstones is proposed with the aim of enhancing data reproducibility among operators and the potential of high resolution bulk petrography in provenance studies for each of four protolith compositions metapelite metapsammite metafelsite metacarbonate metabasite six archetype grains displaying increasing degree of recrystallization and foliation development are illustrated such a classification grid is specifically devised as a subsidiary tool for point counting with the gazzi dickinson method traditional qfr parameters can also be easily recalculated from the data set obtained thus meeting all possible needs decker and helmold 1985 suttner and basu 1985 an experiment shows that usage of visual comparison standards effectively minimizes operator variation and allows retrieval of crucial information during point counting in a reproducible way a petrogenetic grid is presented as a subsidiary tool for classifying grains that include index minerals and to help correlation with dense mineral data the metamorphic index mi is introduced as an estimator of average metamorphic grade of source rocks our classification an extension of concepts used first in the study of arc continent collision in taiwan dorsey 1988 and successfully expanded to interpret the evolution of continent continent collision in the himalayas najman and garzanti 2000 white et al 2002 proves to be fruitful in provenance analysis of foreland basin sediments shed from alpine type thick skinned collision orogens particularly when integrated with dense mineral geochemical and geochronological data

mineralogy of the core yrd 1101 of the yellow river delta implications for sediment origin and environmental evolution during the last 1 9myr abstract in this study we report the high resolution heavy mineral records from a 200 30 m long borehole yrd 1101 on the northern coast of modern yellow river delta yrd covering the last 1 90myr the results show that the heavy mineral assemblages are dominated by epidote followed by hornblende garnet and hematite limonite in the yellow river yr sediments the content of hornblende is higher than the epidote and the epidote to hornblende ratios e h are lower than the core yrd 1101 the e h show two distinct lower values at depositional unit du 4 2 117 9 125 6 m and du 4 1 to du 2 13 5 99 3 m indicating the obvious influence of yr sediments the heavy mineral composition of du 5 is distinct from yr sediments and interpreted as the result from heterogeneous provenance of the adjacent ancient rivers while du 4 to du 1 resemble the upper middle reaches of the yr sediments the different sediment types mud sandy silt and silt of core yrd 1101 do not contain markedly different transparent heavy mineral assemblages in terms of concentration and mineral ratios no matter how sedimentary environment changes the ratios of epidote and hornblende show very stable values in du 4 1 and du 3 due to the influence of yr sediments the transparent heavy minerals such as hornblende epidote and other stable minerals which were relatively stable are controlled primarily by provenance while the sedimentary environment mostly influences the flaky minerals and the authigenic minerals this study provides evidence regarding the history of the lower reaches of the yr and yrd since the early pleistocene highlighting river s important role for the evolution of the western coast of the bohai sea bs

using provenance analysis in an anthropocene natural laboratory abstract globally anthropogenic perturbations to sediment dispersal systems produce detrital signals that are evident in the recent sedimentary record we review the study of anthropogenic perturbations to sediment dispersal systems from the perspective of detrital signals and discuss the use of provenance analysis in anthropocene sedimentology anthropocene provenance analysis is illustrated by a case study from the monterey bay sediment dispersal system mbds of coastal and offshore central california u s a the mbds records two phases of alterations to sediment flux an early increase in fine flux recognized in increased sedimentation rates and a later decrease in coarse flux recognized in detrital provenance signatures results from the mbds suggest that a multi methodological approaches employing provenance analysis and detailed short timescale 102 104 yr depositional chronology markers combined with the study of other anthropocene indicators like the introduction of non natural detritus are valuable tools for describing natural sedimentary processes in addition to human alterations to natural systems the recognition and description of such perturbations to sediment supply is increasingly recognized as an important variable in understanding the effects of human driven landscape alteration particularly in the face of a changing climate the broad concepts and approach outlined in this study provide tools that can extend environmental observations past the period of historical record and can be used to understand past and predict future impacts of altered sediment flux in response to drivers like climate change and sea level rise

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

physical controls on sand composition and relative durability of detrital minerals during ultra long distance littoral and aeolian transport namibia and southern angola 2014 international association of sedimentologists sediment in coastal namibia to southern angola is supplied dominantly from the orange river with minor additional fluvial input and negligible modifications by chemical processes which makes this a great test case for investigating physical controls on sand texture and composition this study monitored textural mineralogical and geochemical variability in beach and aeolian dune sands along a ca 1750 km stretch of the atlantic coast of southern africa by using an integrated set of techniques including image analysis laser granulometry optical microscopy raman spectroscopy and bulk sediment geochemistry these results contrast with previous reports that feldspars and volcanic detritus break down during transport that sand grains are rounded rapidly in shallow marine environments and that quartzose sands may be produced by physical processes mechanical wear is unable to modify the relative abundance of detrital components including pyroxene and mafic volcanic rock fragments traditionally believed to be destroyed rapidly the sole exceptions are poorly lithified or cleaved sedimentary metasedimentary rock fragments readily lost at the transition to the marine environment and slow settling flaky micas winnowed and deposited offshore coastal sediments tend to be depleted in relatively mobile amphibole preferentially entrained offshore or re deposited in sheltered beaches while less mobile garnet is retained onshore no detrital mineral displays a significant increase in grain roundness after 300 to 350 km of longshore transport in high energy littoral environments from the orange mouth to south of the namib erg but all minerals get rapidly rounded after passing into the dunefield pyroxene and opaques get rounded faster than harder quartz and garnet but sand mineralogy remains unchanged excepting strong transient selective entrainment effects physical processes are unable to modify sand composition significantly selective mechanical breakdown can be largely neglected in quantitative provenance analysis of sand and sandstone even in the case of ultra long distance transport in high energy environments dominated by strong persistent winds and waves

historical evolution of the columbia river littoral cell abstract this paper details the historical coastal evolution of the columbia river littoral cell in the pacific northwest of the united states geological data from a d 1700 and records leading up to the late 1800s provide insights to the natural system dynamics prior to significant human intervention most notably jetty construction between 1885 and 1917 all reliable surveys charts and aerial photos are used to quantify decadal scale changes at the three estuary entrances and four sub cells of the littoral cell shoreline bathymetric and topographic change over three historical intervals 1870s 1920s 1920s 1950s and 1950s 1990s are integrated to provide an understanding of sediment sharing relationships among the littoral cell components regional morphological change data are developed for alongshore segments of approximately 5 km enabling comparisons of shoreline change to upper shoreface and barrier volume change within common compartments the construction of entrance jetties at the columbia river 1885 1917 and grays harbor 1898 1916 has profoundly affected the evolution of the littoral cell and has accentuated the morphological coupling between the inlets ebb tidal deltas shorefaces and barriers the jetties induced erosion of the inlets and offshore migration of ebb tidal deltas the change in boundary conditions at the entrances enabled waves to rework the flanks of ebb tidal deltas and supply enormous quantities of sand to the adjacent coasts over several decades the initial sand pulses have been dispersed alongshore up to tens of kilometers from the estuary entrances winter waves and coastal currents produce net northward sediment transport across the shoreface while summer conditions tend to induce onshore sediment transport and accumulation of the upper shoreface and barriers at relatively high rates historical shoreline progradation rates since jetty construction are approximately double the late prehistoric rates between 1700 and the 1870s erosion rates of the mid to lower shoreface to the south of the jettied estuary entrances have typically been greater than the accumulation rates of the upper shoreface and barrier suggesting that the lower shoreface has been an important source of littoral sediments over decadal and longer time scales until recent decades sediment supply from the ebb tidal delta flanks and lower shoreface has largely masked the decline in columbia river sediment supply resulting from flow regulation and dredging disposal practices with the contemporary onset and expansion of coastal erosion adjacent to the jettied estuary entrances proper management of dredged sediment is imperative to mitigate the effects of a declining sediment budget

sedimentary processes controlling ultralong cells of littoral transport placer formation and termination of the orange sand highway in southern angola this study focuses on the causes modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on earth so far sand derived from the orange river is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal namibia to angola all four dunefields are terminated by river valleys where aeolian sand is flushed back to the ocean and yet sediment transport continues at sea tracing an 1800 km long submarine sand highway sand drift would extend northward to beyond the congo if the shelf did not become progressively narrower in southern angola where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths garnet magnetite placers are widespread along this coastal stretch indicating systematic loss of the low density feldspatho quartzose fraction to the deep ocean more than half of mocamedes desert sand is derived from the orange river and the rest in similar proportions from the cunene river and from the swakop and other rivers draining the damara orogen in namibia the orange fingerprint characterized by basaltic rock fragments clinopyroxene grains and bimodal zircon age spectra with peaks at ca 0 5 ga and ca 1 0 ga is lost abruptly at namibe and beach sands further north have abundant feldspar amphibole epidote suites and unimodal zircon age spectra with a peak at ca 2 0 ga documenting local provenance from palaeoproterozoic basement along with this oblique rifted continental margin beach placers are dominated by fe ti cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the orange littoral cell high resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres providing essential information for the correct reconstruction of source to sink relationships in hydrocarbon exploration and to predict the long term impact of man made infrastructures on coastal sediment budgets

origin and diagenetic evolution of gypsum and microbialitic carbonates in the late sag of the namibe basin sw angola abstract ephemeral evaporitic conditions developed within the uppermost part of the transgressive late sag sequence in the namibe basin sw angola leading to the formation of extensive centimetre to metre thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals the onshore pre salt beds examined in this study are located up to 25 m underneath the major mid aptian evaporitic succession which is typified at the outcrop by gypsiferous bambata formation and in the subsurface by the halite dominated loeme formation carbonate evaporite cycles mostly occur at the top of metre thick regressive parasequences which progressively onlap and overstep landward the former faulted rift topography or fill major pre salt palaeo valleys the sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules dissolution related gypsum breccia and are cross cut by thin satin spar gypsum veins nodular and fine grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates whereas gypsum veins and breccia result from telogenetic processes the carbonates display a broader diversity of facies characterised by rapid lateral variations along strike thin dolomitic and calcitic bacterial mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds dissolution related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra to intertidal sabkha and constitute high frequency hypersaline precursor events prior to the accumulation of the giant saline mid aptian bambata and loeme formations petrographic and geochemical analyses reveal successive dissolution recrystallisation and cementation phases that occurred during burial uplift and exhumation implying a complex diagenetic evolution of both gypsum and carbonates influenced by pore fluids of diverse composition which distinctly varied from meso to telogenetic domains

phlogopite and tetra ferriphlogopite from brazilian carbonatite complexes petrogenetic constraints and implications for mineral chemistry systematics abstract the results of a mineral chemistry study of phlogopite and tetra ferriphlogopite in carbonatites and associated alkaline silicate rocks from the brazilian carbonatite complexes of jacupiranga tapira and catalao are presented a wide textural variety of mica is found ranging from primary magmatic crystals to late stage metasomatic phases primary micas from the carbonatites and from the associated plutonic silicate rocks follow distinct evolution paths with magma differentiation in all three complexes micas from the silicate rocks evolve from phlogopite towards annite although in jacupiranga they are more al rich among the micas crystallising from carbonatite liquids those from jacupiranga are typically al and mg rich while in tapira and catalao they are extremely al poor metasomatic micas bridge the gap between primary micas from silicate rocks and from carbonatites the complete phlogopite tetra ferriphlogopite series is reported from the catalao and tapira complexes this study shows that micas from carbonatite complexes may span a wide compositional range largely overlapping the fields of micas from several types of alkaline ultrapotassic rocks especially regarding ti and al contents the use of phlogopite composition and evolution to discriminate between different types of alkaline rocks should be undertaken with caution

two modes for dune orientation earth s sand seas dune fields experience winds that blow with different strengths and from different directions in line with the seasons in response dune fields show a rich variety of shapes from crescentic barchans to star and linear dunes these dunes commonly exhibit complex and compound patterns with a range of length scales and various orientations which up to now have remained difficult to relate to wind cycles here we develop a model for dune orientation that explains the coexistence of bedforms with different alignments in multidirectional wind regimes this model derives from subaqueous experiments which show that a single bidirectional flow regime can lead to two different dune orientations depending on sediment availability i e the erodibility of the bed sediment availability selects the overriding mechanism for the formation of dunes increasing in height from the destabilization of a sand bed with no restriction in sediment availability or elongating in a finger on a non erodible surface from a localized sand source these mechanisms drive the dune orientation therefore dune alignment maximizes dune orthogonality to sand fluxes in the bed instability mode while dunes are aligned with the mean sand transport direction in the fingering mode applied to earth s deserts the model quantitatively predicts the orientation of rectilinear dunes and their superimposed patterns this field study suggests that many linear dunes on earth elongate from sources and are simply aligned with the mean sand transport direction

characteristics of diamondiferous plio pleistocene littoral deposits within the palaeo orange river mouth namibia abstract the orange river the principal conduit transporting diamonds from hinterland sources to the namibian coast in post cretaceous times is characterised by an extreme wave dominated delta that has given rise to a progression of coarse rudaceous littoral deposits preserved onshore for 150 km north of the mouth under the long lived prevailing vigorous wave wind and northward longshore drift regimes the orange river outfall has been reworked into amongst others a series of economically viable diamondiferous plio pleistocene onshore gravel beach deposits these placers comprise spits and barrier beaches in the proximal reach within the palaeo orange river mouth that after ca 5 km northwards merge into extensive but narrow linear beaches that in turn after ca 70 km give way to pocket beaches gravel and diamond size decreases northwards away from the ancestral orange river mouth the linear and pocket beach types have considerably higher diamond content but lower average diamond stone size than the two proximal units that are characterised by low diamond grade but comparatively large average diamond size given the risk of delineating low grade alluvial diamond deposits accurately we present here sedimentological reconstructions of the subtidal intertidal and supra tidal facies that constitute the spit and barrier beach sequences based largely on face mapping of exploration trenches and open cast mine cuts as well as the results of large tonnage sampling campaigns diamond distribution is also linked convincingly to basic littoral processes that were operational within the palaeo orange river mouth during the complex transgression that gave rise to the 30 m package in plio pleistocene times in both the spit and barrier beach settings the intertidal deposits prove to be the most promising targets whereas the subtidal sediments are the least economic the constant raking associated with coarse cobble boulder sized gravel foreshore deposits in an energetic micro tidal wave regime increased the average diamond stone size in the intertidal deposits to 1 to 2 carats per stone cts stn but the lack of fixed trapsites no competent footwall within the palaeo orange river mouth at that level prohibited the accumulation of substantially enriched diamondiferous gravels consequently grades of only 1 5 to 6 carats per 100 tons cpht are realised the highest grades 2 to 6 cpht are found in the landward facing intertidal beach deposits on the spits where gentle reworking in that sheltered environment had somewhat enriched and preserved the diamond content significantly the low average stone size of ca 0 5 cts stn in this lower energy setting probably reflects that of the general diamond population available at that time in contrast the sand rich subtidal deposits in the spit sequence return the lowest grades 0 1 to 0 5 cpht similar to those in the slightly younger subtidal transgressive boulder lags of the barrier beaches however the stone size in the spit subtidal sediments is also low 0 1 to 0 5 cts stn due to the highly mobile fine grained character of those deposits whereas that in the subtidal transgressive lag is large 2 to 3 cts stn as a result of the local semi permanent turbulence associated with the boulder sized clasts in these gravel sheets diamond distribution is therefore also influenced by littoral facies and associated beach types in addition to the spatial and temporal parameters that have already been documented for the onshore marine placers of the southern namibian coast

terrigenous sediment export from the western margin of south africa on glacial to interglacial cycles abstract similar to many shelves globally pleistocene terrigenous sediment deposition on the western shelf of south africa is strongly influenced by high amplitude sea level fluctuations over glacial to interglacial climate cycles here variations in the glauconitic quartzose sand and terrigenous mud content of gravity cores from the western slope of south africa are used to evaluate the dynamics of shelf sediment export over climate cycles of the last 450 kyr calcareous ooze on the slope contains a mean of 0 2 to 8 wt glauconitic quartzose sand derived from erosion of neogene outer shelf sediments and 13 to 40 wt terrigenous mud sourced from the orange river quartz sand content on the slope increases during glacial periods and is sharply reduced across glacial terminations and throughout interglacial highstands relative sea level changes on the margin estimated from the quartz sand content on the slope are consistent with the timing but not amplitude of proposed eustatic sea level records a possible mechanism of sand erosion on the outer shelf is the vertical mixing forced by internal tides which may intensify over the shelf break as sea level is lowered during glacials turbidites are rare and bottom currents are generally capable of suspending mud but not sand except locally along the margin estimated variations in the bulk sedimentation rate and terrigenous mud mass accumulation rate on the slope suggest that export of terrigenous mud off the shelf during glacial lowstands is transported beyond the slope at intermediate and deeper water depths to the southern ocean

raman spectroscopy in heavy mineral studies abstract raman spectroscopy is an innovative tool with tremendous potential serving as a fun damental complement to a variety of provenance methods including heavy mineral analysis anddetrital geochronology because of its accuracy ef ciency and versatility the results of theraman technique are indispensable for fully reliable identi cation of heavy minerals in grainmounts or thin sections thorny long standing problems that cannot be solved con dently witha polarizing microscope alone such as the determination of opaque and altered heavy minerals of detrital grains as small as a few microns or of colourless crystals with uncertain orientationand rounded morphology can nally be addressed although the method can be highly automa tized the full ability and experience of the operator is required to combine raman data with theoptical information obtained under the microscope on the same grains which is essential for theef cient application of the method in provenance studies this article provides exemplaryramanspectrausefulforthecomparisonand determinationof over70 differentopaqueandtrans parent heavy mineral species commonly found in sediments conveying speci c information onthe genesis of their source rocks and thus is particularly useful in provenance diagnoses andpalaeotectonic reconstructions supplementary material detailed information on the lasers used and the origin of the analysedminerals is available at http www geolsoc org uk sup18615it is the scattering of light by atoms and moleculesthat gives us the light of the sky the blue colour ofthe deep sea and the delicate opalescence of largemasses of clear ice raman 1928 pp 368 369

development and steady states of transverse dunes a numerical analysis of dune pattern coarsening and giant dunes we investigate the development and steady states of transverse dunes for ranges of flow depths and velocities using a cellular automaton dune model subsequent to the initial bed instability dune pattern coarsening is driven by bed form interactions collisions lead to two types of coalescence associated with upstream or downstream dominant dunes in addition a single collision ejection mechanism enhances the exchange of mass between two adjacent bed forms throughpassing dunes the power law increases in wavelength and amplitude exhibit the same exponents which are independent of flow properties contrary to the wavelength dune height is limited not only by flow depth but also by the strength of the flow superimposed bed forms may propagate and continuously destabilize the largest dunes we identify three classes of steady state transverse dune fields according to the periodicity in crest to crest spacing and the mechanism of size limitation in all cases the steady state is reached and maintained through the dynamic equilibrium between flow strength and dune aspect ratio in the limit of low flow strength where it becomes the primary factor of size limitation the bed shear stress in the dune trough regions is close to its critical value for motion inception comparisons with natural dune fields suggest that many of them may have reached a steady state finally we infer that the sedimentary patterns in the model may be used to bring new constraints on the development of modern and ancient dune fields