south african modern beach sand and plate tectonics south america is the ideal continent for baseline studies on how the framework composition of modern sands relates to reconstruction of palaeoplates it has well defined active and passive margins great contrasts in climate and relief and is free of continental glaciation fig 1 i report here petrographical studies of its beach sands which show two active margin associations rich in volcanics and plagioclase one along its pacific coast and one along the argentine coast and two associations rich in quartz a passive margin association from argentina to trinidad and a moderately quartz rich association along the caribbean coast tectonic control is thus overwhelming but complications exist along the argentine coast where a dry narrow continent permits active margin sand to mantle a passive margin another surprising result is that quartz rich sands border the crystalline rocks of much of the sera do mar in brazil

holocene floodplain soils along the rio mamore northern bolivia and their implications for understanding inundation and depositional patterns in seasonal wetland settings abstract the llanos de moxos lm in the bolivian amazon basin host one of the largest seasonally inundated savannah landscapes on earth very little is known of this area with regard to sedimentary dynamics soil formation or their relationship to longer term climatic and hydrological variability in this setting here we present a detailed study of three floodplain depositional sequences building up the inundation savannah along the mamore river in the central lm pleistocene sands and silts unit 1 are overlain by thick overbank deposits of mostly holocene age unit 2 on both sides of the mamore river and underline the importance of extensive flooding processes for the late quaternary sedimentary and geomorphic evolution of the lm the fine grained distal overbank sediments show signs of strong modification by hydromorphic processes and reflect spatial and temporal variations in flood inundation patterns in addition widespread dark gray to blackish soil and paleosol horizons are intercalated with the overbank sediments but lack evidence for significant weathering these horizons are likely the result of cumulative incorporation of organic material under conditions of particularly low sedimentation rates therefore their formation should be linked to processes such as channel migration longer term meanderbelt evolution avulsive drainage reorganization changes in sediment supply or climatically induced variations in flooding type frequency or magnitude the common occurrence of these floodplain soils in the early to mid holocene may thus reflect the combined effects of hydrological geomorphic and sedimentary changes during a drier mid holocene

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

insoluble particles in antarctic ice background aerosol size distribution and diatom concentration we have measured insoluble particle size distributions covering the radius range 0 05 1 31 m for six sections of ice core from dome c antarctica two of the sections are from the holocene two are from the last glacial maximum lgm and another two are from the period that preceded it our measurements lead us to the conclusion that the southern hemisphere insoluble background aerosol size distribution in the range of our measurements has not changed significantly over the 26 000 year period that we studied we also compared the concentration of diatoms in a sample of holocene ice with that in two samples of lgm ice and found that the concentration of diatoms whose largest dimension was equal to or greater than 10 m was 20 times larger during the lgm the same as the ratio we measured for the concentration of insoluble particles we interpret this to mean that the higher dust levels were mainly due to an increase in wind strength rather than to increased continental aridity

discriminating depositional environments of sands from modern source terranes using modal analysis abstract composition of modern beach dune and fluvial sands from the gulf of mexico coast and the kapiti and foxton coasts new zealand were compared in order to establish petrological criteria that discriminate between depositional environments and subenvironments the three areas were chosen because of their different sand compositions sand from the gulf of mexico is quartzofeldspathic to feldspatholithic sand from the kapiti coast sands is feldspatholithic and sand from the foxton is lithofeldspathic using a confidence level of 95 petrographic data were plotted as bar charts for each environment beach dune river and for each subenvironment inshore foreshore backshore of the beaches stoss crest lee face of eolian dunes upstream and mouths of the river the best discrimination of beach dune and river environments was achieved for the gulf of mexico sands using q f l and qm k p plots terranes with multiple source rock types such as the gulf of mexico enhance petrological discrimination between sands from beach dune and river subenvironments and environments in terranes with few rock source types such as the kapiti coast only the river sands can be discriminated from those of other environments the single rock source of the foxton coast allows little petrological discrimination between the beach dune and river sands this study examines compositional differences between beach dune and river subenvironments and environments and the degree to which these differences are dependent on the variation of rock types in the source terrane the study also shows the usefulness of q f l qm k p and lv lm ls bar charts the discrimination among beach dune and river sands is partly due to grain size variation

modern sands of south america composition provenance and global significance standard petrographic methods were used to study 604 modern sands from south america of which 351 came from rivers and 253 from beaches in spite of the wide geomorphic contrasts these sands belong to only three great families 1 an immature andean family of lithic arenites rich in volcanic and metamorphic grains that covers about 30 of south america 2 a cratonic association rich in quartz that covers about 62 of the continent and 3 a transitional molasse association which occurs between these two and covers only about 8 of the continent the grand arithmetic average q f rf ratio for the entire continent is 60 11 29 and the area weighted carbonate free average qa fa rfa is 68 10 22 a superior continental estimate and the first ever for an entire continent

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

fluvial palaeoenvironments in the intracratonic amazonas basin early miocene early middle miocene colombia abstract based on a detailed study of early miocene to early middle miocene sediments in the caqueta river area amazonas basin colombia a reconstruction of the palaeoenvironmental and palaeogeographical history is made sediment composition sedimentary sequences and transport directions indicate that the succession was deposited by a low sinuosity fluvial system with an anastomosing character that originated from the guyana shield basin subsidence precambrian and palaeozoic palaeorelief and a changing base level controlled deposition in this fluvial system sedimentary environments such as channels abandoned channels crevasse splays backswamps and palaeosoils characterized this system where fluvial flooding was common the early miocene to early middle miocene sequences have been truncated by a more recent fluvial system probably of pliocene to pleistocene age representing the precursor of the present caqueta river palynological analysis suggest that the sediments are part of the retitricolporites and the psiladiporites crototricolpites pollen zones fifteen new species are described which belong to the genera psilatricolpites retitricolpites retitricolporites bombacacidites syncolporites and psilastephanocolporites the palynological assemblages indicate that the area was characterized by palm swamps riparian vegetation and a rather diverse tropical forest intervals rich in marine palynomorphs accompanied by high concentrations of mangrove pollen grains zonocostites suggest periods of marine influences and mangrove development it is thought that the marine intervals are related to the late burdigalian transgression

the impact of quaternary sea level changes on the evolution of the amazonian lowland coring and acoustic surveying 3 5 khz in both rivers and lakes in central amazonia provide additional insights into the late quaternary hydrological and sedimentological development of the amazon river and its tributaries erosion and accumulation phases were found to be linked to quaternary sea level changes the low sea level phase during the last glacial maximum caused deep incision of the amazon river and erosion in major tributaries such as the rio negro 1500 km upstream from the amazon mouth a 3 5 khz profiling suggests a lowering of the water level by at least 30 m at manaus during that phase the slope of the amazon valley must have increased resulting in an increase in bed load transport capacity the subsequent sea level rise caused a backwater effect far upstream with silting up of the amazon valley and the tributary inflows former river systems changed into ria lakes the floodplains of the amazon river the varzea were formed approximately 5000 years ago when the sea level approached its present day level copyright 2009 john wiley sons ltd

relative alteration of microcline and sodic plagioclase in semi arid and humid climates abstract petrographic determination of the degree of alteration of holocene detrital feldspars from first order streams indicates that a broad overlap exists in the amount of alteration on detrital feldspars from contrasting wet and dry climates thus the amount of alteration of detrital feldspars from ancient sandstones is probably an invalid indicator of paleoclimate furthermore regardless of climate detrital plagioclase ab 80 95 generally is 15 20 percent more altered than potassium feldspar in light of this the paleoclimatic significance of the observed relation potassium feldspar more weathered than plagioclase in ancient sandstones todd 1968 needs re evaluation results of this study do suggest a possible preference of limonite plus kaolinite on detrital feldspars from humid climates versus limonite plus smectite on feldspars from semi arid climates however unequivocal substantiation of this difference is still lacking even if the compositional difference can be rigorously demonstrated an inability to distinguish diagenetic from climate controlled surface weathering induced alteration of feldspar will further decrease the utility of feldspar alteration relationships in paleoclimate interpretation

grain size control of river suspended sediment geochemistry clues from amazon river depth profiles residual solid products of erosion display a wide range of size density shape mineralogy and chemical composition and are hydrodynamically sorted in large river channels during their transport we characterize the chemical and isotopic variability of river sediments of the amazon basin collected at different water depths as a function of grain size absolute chemical concentrations and sr and nd isotopic ratios greatly varies along channel depth the al si ratio tightly linked to grain size distribution systematically decreases with depth mostly reflecting dilution by quartz minerals a double normalization diagram is proposed to correct from dilution effects elements define fan shaped patterns and can be classified in three different groups with respect to hydrodynamic sorting during transport in the amazon 1 poorly sorted insoluble elements like al fe th and rees 2 well sorted insoluble elements like zr and ti mainly carried by heavy minerals and 3 alkali na to cs and alkali earth elements mg to ba for which a large variety of patterns is observed related for alkali to their variable affinity for phyllosilicates sr isotopes show that the amazon river at the mouth is stratified the madeira and solimoes derived sediments being preferentially transported near the channel surface and at depth respectively the comparison between the solimoes and madeira rivers shows how the interplay between grain sorting weathering and crustal composition controls the composition of the suspended river sediments

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

iron isotope composition of the bulk waters and sediments from the amazon river basin abstract the present study provides iron concentrations and isotopic compositions determined by multi collector inductively coupled plasma mass spectrometry mc icp ms along with key chemical mineralogical and physical properties of 35 representative bulk unfiltered waters and bulk sediments from the amazon river basin these samples from the amazon river five of its main tributaries the solimoes negro madeira tapajos and trombetas rivers and four sub tributaries the purus jau ucayali and napo rivers were essentially collected during seven field missions conducted for over two years these encompassed the centennial flood of may 2009 and the exceptional low water stage of september october 2010 thereby providing the most extreme hydrological situations that have been recorded over the last hundred years while the data confirmed massive losses of iron up to 19 000 tons day ca 50 of the amazon river bulk water budget in the solimoes and negro rivers mixing zone the fe isotope signatures of these bulk waters behaved conservatively this property allows the use of bulk water fe isotope signature to track iron sources and explain such isotopic signature in terms of simple mixing unfiltered samples from the organic rich black water rivers present light 57 fe relative to the average continental crust composition this contrasts with the composition of the bulk white waters carrying a high mineral suspended load that have 57 fe values undistinguishable from the crustal isotopic signature 0 1 relative to irmm 14 this observation indicates that the fe isotopic composition represents a reliable direct tracer of the iron speciation and therefore of the host phases of iron in its sources specifically the white water 57 fe most likely trace the signatures of igneous and sedimentary sources as well as of their lateritic soil minerals while the bulk black water 57 fe track a preferential release of fe that has gone through a reduction step in the organic rich horizons of tropical podzols as a result of the biological activity this study shows that the total iron transferred by the amazon river represents between 5 and 30 of the world s ocean fe input by rivers and this amazon bulk water iron displays an isotopic composition indistinguishable from that of the average continental crust

regional paleoclimatic and stratigraphic implications of paleosols and fluvial overbank architecture in the morrison formation upper jurassic western interior usa abstract paleosols in the morrison formation upper jurassic from the western interior and colorado plateau regions occur in fluvial overbank and marginal lacustrine depositional facies associated with aggradational settings and at sequence bounding unconformities that mark divisions between major aggradational and degradational successions pedogenic features within these horizons preserve important contextual information about the local and regional paleoclimate and paleoenvironment in which the soils formed floodplain and lake margin paleosols show evidence that most of the morrison basin was characterized by a semi arid to tropical wet dry paleoclimate with fluctuating groundwater conditions a low precipitation to evaporation ratio and weak to moderately seasonal precipitation paleosol ichnofauna show evidence of a diverse and opportunistic flora and fauna that exploited changing conditions and existing nutrient and moisture regimes changes in paleosol type and degree of development over the basin indicate the overall regional paleoclimate was drier in the western and southern portions of the basin vertical trends indicate paleoclimatic conditions over the basin became steadily more humid through time laterally continuous well developed deeply weathered paleosols formed during times of little or no deposition and mark regional unconformities the paleosols at these sequence bounding unconformities serve as useful regional stratigraphic markers to trace genetic packages across the morrison depositional basin and to determine regional accommodation trends

the quaternary of ecuador abstract ecuador is composed of three major regions the coast the cordillera and the oriente or amazon in the eastern amazonic lowlands the quaternary begins with the deposition of the mera mesa formation composed of coarse and medium grained sediments that unit was sedimented during a dry climate forming two large alluvial fans an erosive episode followed with formation of pediments at a later stage the humid climate appeared in the late pleistocene with formation of a landscape of round convex hills and narrow valleys the last important event in the region was the deposition of a thick ash mantle in the early holocene the quaternary of the cordillera is characterized by important volcanic activity glacial processes and the occurrence of cangahua a loess like aeolian silt composed of piroclastic materials lahars debris avalanches and related phenomena are common the coastal region during the quaternary underwent the influence of mud flows debris avalanches and other cordilleran phenomena which aggradated the area san tadeo formation and other units sea level oscillations deposited tidal sediments in several areas during the middle holocene and the occurrence of el nino produced alterations in the oceanic and coastal regime during the late holocene

geochemistry of renuka lake and wetland sediments lesser himalaya india implications for source area weathering provenance and tectonic setting the geochemical investigation of sediments deposited in the renuka lake basin and its adjoining wetland has shown variation in the distribution and concentration of major trace and rees the major elements are depleted in the lake in relation to wetland and that of post archaean australian shale paas except for cao which is strikingly in excess and has a dilution effect on sio2 and other oxides and trace elements the wetland sediments on the other hand are enriched in al2o3 fe2o3 k2o and tio2 and the latter three show a positive correlation with al2o3 in both wetland and lake sediments suggesting their association with phyllosilicates and similar source rocks the enrichment of y zr ni th u and nb in wetland compared to lake and their similarity with paas in the former suggests more clay fractions in the wetland a high zr hf ratio in wetland and lake sediments and a positive correlation of zr with y and hree indicate zr control on hrees however higher zr yb and zr th ratios in wetland compared to lake indicate mineral sorting during the process of lighter particles clays being trapped in wetland soil this is also reflected from negative correlation of gdn ybn with al2o3 and a strong positive correlation with sio2 in wetland sediments the wetland in this context has a control on lake sediment chemistry the chondrite normalized ree patterns are essentially the same for lake as well as wetland sediments but abundance decreases in the former the similarity of pattern with that of paas and negative eu anomaly indicates a cratonic source of sediments in a plot of the individual samples wetland samples cluster while lake samples are separated indicating fractionation of lake sediments a strong positive correlation of lan ybn with al2o3 and a positive correlation of zr lree and zr lan ybn suggest that lrees are controlled by both phyllosilicates and zircon the chemical index of alteration cia indices in lake sediments and in wetland are higher than paas indicating moderate chemical weathering in the source area the petrography lack of felsic magmatic rock fragments and negative correlation between zr gd yb c indicate sedimentary source rocks for the detritus this is in conformity with the lesser himalayan sedimentary sequence belonging to neo proterozoic proterozoic age and constituting lake catchment of renuka the tectonic delineation and discriminant function plots of lake and wetland sediments indicate their cratonic and or quartzose sedimentary orogenic terrain source that has been deposited in a passive margin setting

lowermost amazon river morphology and shallow seismic characteristics abstract the riverbed morphology of the lowermost amazon river was characterized by high resolution acoustic profiling in 21 areas this technique has been used for the first time in this area and allowed the discovery of a variety of reflectors which delineate areas either of sediment transport or of deposition on an older river bottom records obtained with the pinger system made it possible to distinguish three main kinds of reflectors both surface and sub bottom 1 flat 2 wavy and 3 irregular reflection patterns which can be interpreted as strata deposited in a variable mostly high energy setting or reworked structures of initially flat or wavy strata that have been changed mapping of these different reflectors shows that they are not randomly distributed the constant association of a given reflector with a particular morphological setting is indicative of its lithological significance supported by riverbed sampling and erosional depositional processes depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub bottom related to old substrates alter do chao or barreiras formation this may result from inundation and incision caused by quaternary sea level changes previously eroded areas are covered by a wavy layer consisting of very fine to fine sand unit fs which can change locally to medium coarse sand unit ms reflectors are more abundant downstream a minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge moreover the detection of fault scarps interpreted to result from neotectonic activity shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability the dynamic setting of the lowermost amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes the effect of the gurupa arch may result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part

paleoclimatic control on the composition of the paleoproterozoic serpent formation huronian supergroup canada a greenhouse to icehouse transition unlike other feldspathic arenites in the 12 km thick huronian supergroup strata of the 250 350 m thick serpent formation contain abundant plagioclase deposited in a distal alluvial setting the serpent formation consists of arenites with minor shale and siltstone the plagioclase abundance could reflect unroofing of a unique source relative to other huronian units or could be due to less intense chemical weathering major trace and rare earth element ree geochemistry of the serpent formation are used to evaluate these two possibilities serpent formation source rocks form part of the archean superior province which mainly consists of tonalitic granitic and supracrustal assemblages and their metamorphosed equivalents in al2o3cao na2ok2o acnk space shales define a linear array which indicates that the unweathered source contained plagioclase and k feldspar in the ratio of 5 1 this particular array indicates that serpent fluvial systems tapped a variably weathered carapace developed on the superior province rather than purely fresh bedrock a modelled mixture of 80 tonalite and 20 granite group rocks reproduces ree and trace element characteristics of the serpent formation sandstones and shales average eu eu 0 87 lan smn 4 4 gdnybn 1 8 and th sc 1 65 mass balance considerations indicate that less than 5 supracrustal group rocks can be added at the expense of the granite group a unique source for serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition plagioclase k feldspar ratios in serpent sandstones range from 2 1 to 1 2 which achieves mass balance among source mudstone and sandstone compositions when plotted in acnk space achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles these findings indicate that plagioclase preservation in the serpent formation is related to less intense compared with other huronian arenites paleoweathering conditions probably a harbinger of the widespread continental glaciation recorded in the overlying gowganda formation

lower brioverian formations upper proterozoic of the armorican massif france geodynamic evolution of source areas revealed by sandstone petrography and geochemistry abstract formations with interbedded cherts constitute an important part of the lower brioverian succession upper proterozoic age in the armorican massif northwest france these formations are composed of shale sandstone alternations with interbedded siliceous carbonaceous members petrographic and geochemical study of the detrital facies shows that these rocks are compositionally immature the wackes are rich in lithic fragments volcanic fragments 3 20 modal sedimentary and metamorphic fragments 0 7 modal and in feldspar 5 16 from the geochemical point of view they are relatively enriched in fe2 mgo about 5 5 and in alkalis with na 2 o k 2 o ratios greater than 1 the cao contents are low about 0 3 slightly negative eu anomalies are observed eu eu 0 8 their chemical compositions are in agreement with a dominantly acidic source area with deposition in a continental active margin setting compared with other upper proterozoic deposits of the armorican massif the interbedded chert formations appear rather similar to other deposits in north brittany which accumulated in an intra arc or back arc basin environment the formations with interbedded cherts are interpreted as having been deposited during an early stage of magmatic arc activity around 640 630 ma ago in an immature marginal basin the clastic supply to these formations is derived in part from early volcanic products acidic to intermediate which are linked to subduction beneath the north armorican domain another component is inherited from the reworking of 2000 ma old basement relics the opening of the back arc domain with associated basaltic volcanism would bring about a progressive displacement of the interbedded chert depositional basin towards the continental margin

paleogeographic and tectonic implications of quartzose sandstones of barbados the provenance of paleogene sandstones on the island of barbados is relevant to various models of the tectonic evolution of the southern caribbean area modal point counts of 26 sandstones from barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin abundant detrital polycrystalline quartz grains have affinities to a spectrum of low to high rank metamorphic plutonic crystalline source rocks microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances the complex heavy mineral suite of the terrigenous sandstones including glaucophane chloritoid titanite zircon and aluminosilicates supports derivation from orogenic and cratonal terranes due to the complex history of the south american caribbean plate boundary and poor age constraints on timing of deposition widely contrasting models have been proposed concerning the provenance of the sandstones convergence between the aruba orchila arc and continental south america during the paleogene resulted in the formation of an e w trending foreland fold thrust belt and foredeep dominant sediment dispersal was to the east northeast along tectonic strike the present study shows that the terrigenous sandstones on barbados probably were deposited in deep sea fans to the north of present day araya peninsula during and shortly after the late middle eocene probable source terranes include the guayana shield the caribbean mountain system interior foreland fold thrust belt and possibly to a lesser extent the lesser antilles magmatic arc or predecessor arc

amazon fan sands implications for provenance abstract the amazon fan is the third largest modern mud rich submarine fan system in the world located on the equatorial atlantic margin and abyssal plain the fan is believed to have originated as a result of the andean orogeny in the early miocene and has continued to build through to the present day sedimentation rates across the fan vary in time and space and can be as high as 10 25 m k y especially during periods around and during low sea level stands these large scale fan systems offer a unique opportunity to study earth history in particular the comparison between ancient and modern systems in terms of reservoir potential sedimentation processes fan models and provenance studies it is the latter that forms the basis of this contribution in 1994 leg 155 of the ocean drilling program recovered over 4 km of core material from 17 sites on the amazon fan we report on the provenance heavy mineral quartz grain morphology geochemistry and grain size of sands recovered from the fan initial findings indicate that sand compositions vary through a glacial cycle the older units in the upper levee complex u l c have a pyroxene dominated heavy mineral suite and a relatively low proportion of glacially derived quartz grains whilst the younger units in the u l c have an amphibole epidote dominated heavy mineral suite and a larger proportion of glacially derived quartz grains these results suggest either a change in provenance or climate or a combination of both over a glacial cycle core samples studied appear to be unaffected by diagenesis whilst abrasion during transport has had little effect on individual grains

silicate rock weathering and atmospheric soil co2 uptake in the amazon basin estimated from river water geochemistry seasonal and spatial variations abstract using the data of the camrex project 1982 1984 on the water geochemistry of the amazon river and its main tributaries it was possible to assess the silicate rock weathering processes and the associated consumption of atmospheric soil co2 taking into account seasonal and spatial variations this study confirms the important role of the andes in the fluvial transport of dissolved and particulate material by the amazon and it shows for the first time that the silicate weathering rate and atmospheric soil co2 consumption are higher in the andes than in the rest of the amazon basin the seasonal variations exhibit the significant role of runoff as a major factor controlling silicate weathering processes and show that the chemical erosion rates vary greatly from low discharge to high discharge the average weathering rate estimated for the whole amazon basin 15 m my is comparable to other estimations made for other tropical equatorial environments a comparison between physical and chemical weathering rates of silicate rocks for the amazon basin and for each tributary basin show that in the andes and in the amazon trough the soil thicknesses are decreasing whereas in the shield the soil profiles are deepening

multicyclic history of the northern india continental margin northwestern himalaya 1 the geologic evolution of northern india is best recorded in the stratigraphic succession of the zanskar range northwestern himalaya which represents the most complete transect through this ancient continental margin sedimentary history began in the late proterozoic and recorded a late pan african orogenic event around the cambrian ordovician boundary when the gondwana supercontinent was eventually assembled the following long period of epicontinental deposition in shallow seas linked to palaeo tethys lasted until the early permian when a neo tethyan rift began to open between paleo india and the cimmerian microcontinents neo tethyan history can be subdivided into two sedimentary megasequences both recording a major tectonic and magmatic event in the lower part the first one began with breakup in the late permian and lasted until the end of the jurassic the second one started in the early cretaceous with the final detachment of india from gondwana and the opening of the indian ocean and ended with the india eurasia collision in the early eocene the two megasequences can be in turn subdivided into six transgressive regressive supersequences bounded by tectonically enhanced unconformities basal sandstone units of early permian late permian norian callovian early cretaceous and paleocene age are invariably associated with oolitic ironstones or reworked glauco phosphorites and mark the transgre sive part of each supersequence next condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing upward marly units capped by regressive platformal carbonates the six tectono eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of india from the rest of gondwana and document the combination of plate microplate reorganizations and eustatic climatic and oceanographic changes in the tethyan domain after the onset of collision between india and asia close to the paleocene eocene boundary obduction of the remnants of the neo tethys ocean floor onto the indian margin began and the latter underwent multiphase deformation with fold thrust shortening followed by heating and extension after the main metamorphic event ophiolitic nappes were re thrusted and finally emplaced with their sedimentary sole on top of the passive margin succession

provenance of early palaeozoic sandstones southeastern australia part 1 vertical changes through the bengal fan type deposit abstract ordovician strata of the lachlan fold belt in southeastern australia are dominated by a turbidite succession that in its undeformed state was comparable in size to the modern day bengal fan the ancient fan was derived from the northern part of the cambrian ross delamerian orogenic belt that formed at a convergent boundary between plates containing gondwanaland and part of the palaeo pacific ocean respectively substrate to the submarine fan consisted of mafic volcanic rocks with an immature island arc affinity similar to the eocene izu mariana bonin system sandstone compositions determined from several stratigraphically constrained sections demonstrate a uniform framework composition dominated by monocrystalline quartz with less abundant feldspar polycrystalline quartz metamorphic rock fragments sedimentary rock fragments and mica these indicate derivation from a low grade metamorphic terrain up sequence variation is limited to decreasing feldspar and lithic fragments and increasing quartz indicating increased weathering and lowering of topography in the source terrain over time 25 of counted sandstones in the late ordovician interval contain a small component of mafic intermediate volcanic rock fragments that accompanied a change in palaeocurrent direction from eastwards to northwards in the early silurian the ordovician sequence was disrupted by the formation of several major deformation zones between these zones developed several deep marine sedimentary basins that contain a continuous succession from the ordovician into the early silurian early silurian sandstone is similar in composition to underlying early ordovician rocks except that the feldspar component is generally absent early silurian sandstone was derived from uplifted portions of the ordovician sequence and includes texturally inverted types with sandstone containing a bimodal mixture of coarse well rounded quartz and fine angular quartz

the detrital record of cretaceous to pliocene sandstones across the ne south american margin the cretaceous to pliocene erosional history of the ne south american margin depicts the increasing influence of the uplifting caribbean mountain belt petrographic analyses of 59 cretaceous to late pliocene sandstone samples derived from across the islands of trinidad and barbados demonstrate systematic mineralogical changes whereby early cretaceous arkosic arenites gave way to mature quartz arenites by the late cretaceous and caribbean mountain derived lithic rich arenites during the neogene changes in the framework compositions are accompanied by changes in textural attributes with caribbean mountain detritus being notably finer grained than cratonic input silica cements predominate in quartzose sandstones and sericitic and clay cements within litharenites the changing mineralogical compositions record the gradual deformation of the northern south american passive margin and initiation of the andean caribbean mountain foreland basin system with incipient orogenic detritus evident from the latest eocene and unambiguous orogenic input by the late oligocene the preponderance of finer grain sizes within deltaic pliocene sandstones records the later fill of an established foredeep with sediments sourced primarily from erosion of the andean hinterland these interpretations agree with heavy mineral compositional changes and the timing of uplift from geo and thermo chronological data across the margin supplementary material qualitative descriptions sample coordinates and raw point counts are available at https doi org 10 6084 m9 figshare c 3659381

patterns of anabranching channels the ultimate end member adjustment of mega rivers abstract large fluvial systems adjust to a combination of controls to form distinctive channels which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology fluvial geomorphology has commonly classified river channels into meandering straight and braiding patterns which are seen to represent a continuum of channel geometry anabranching patterns rivers with multiple channels however are characteristic of many rivers the identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology the development of this body of knowledge however has been established from medium and small rivers and laboratory flume studies very few of these research ideas developed from analysis of large fluvial systems this paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry discharge w d slope grain size stream power specific stream power and froude number q mean 1000 m 3 s the study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers further with the exception of the lower mississippi alluvial rivers with mean annual discharges greater than 17 000 m 3 s here classified as mega rivers do not generate single thread meandering or typical braided patterns these mega rivers develop anabranching patterns

planform evolution of two anabranching structures in the upper peruvian amazon river we present a study to relate the sinuosity of the main channel and its effect on the dynamics of the secondary channels of anabranching structures for this purpose two locations of the peruvian amazon river were selected 1 a site with a medium to high sinuosity main channel ms site muyuy peru and 2 a site with a low sinuosity main channel ls site at the triple boundary between brazil colombia and peru the main channels for both the ms and ls anabranching structures have freedom to migrate in the lateral direction while at least one of their secondary channels is adjacent to the geological valley for ms and ls sites temporal analysis of planform evolution was carried out using 30 years of satellite imagery from which metrics such as width sinuosity and annual maximum migration rates of main and secondary channels were calculated additionally detailed hydrodynamic and bed morphology field measurements were carried out and a two dimensional shallow water numerical model was developed for a medium to high sinuosity main channel anabranching structure the secondary channels present a dominant mechanism for reworking the floodplain while for the low sinuosity anabranching structure the main channel planform dynamics is dominant flow velocities along the main and secondary channels for low transition and high flow discharges describe that for ms ls site the velocities are much higher along the secondary main channels

petrography and geochemistry of holocene sands in the western gulf of mexico implications for provenance and tectonic setting abstract petrographic and geochemical analysis of recent fluvial beach and dune sediments derived from the same source terrane was carried out in the western gulf of mexico to show the usefulness of the these three environments in determining the tectonic setting of the source petrographic analysis showed that dunes concentrate the less heavy minerals and quartz grains by means of the selectiveness of the wind as a transport agent in contrast the heavier grains such as the rock fragments remain in the beach and fluvial environments because of the high transport energy beach and dune sands do not define a particular tectonic setting because of the selective wind action of the backshore of the beach and the dune environments and the less intense chemical weathering the fluvial sands are the most representative in terms of interpretation of tectonic setting because they reflect the volcanic domain of the source rocks of the trans mexican volcanic belt the geochemical results of all three sedimentary environments reflect the calcalkaline character of the igneous source rocks of the trans mexican volcanic belt the samples fall in a continental island arc margin field with basaltic and andesitic source rocks a correlation and factor analysis indicates an alkali basalt source that contributes to the composition of the beach dune and fluvial sediments the geochemical analysis of the sediments more accurately reflects the tectonic setting regardless of the depositional setting of the sediments

recycling and chemical weathering in tectonically controlled mesozoic cenozoic basins of new zealand sedimentation in molasse basins is controlled by tectonics however recycling and chemical weathering play a critical role in the compositional evolution of a sedimentary succession the cretaceous to pliocene molasse deposits of central otago new zealand are excellent examples of tectonically related deposits that were governed by the effects of chemical weathering and recycling preserved in fault controlled basins floored by flysch deposits of the otago schist the clastic successions contain ubiquitous unconformities and lithofacies consistent with alluvial fluvial and lacustrine depositional settings textural analysis of central otago sandstones establishes a general quartz enrichment and increased mixing of angular and well rounded quartz varieties up section consistent with a history of sediment recycling rare earth element ree patterns which reflect upper crustal compositions are similar for the flysch type otago schist permian early cretaceous a palaeo weathering profile and the overlying molasse deposits the development of quartz arenites is also consistent with high degrees of chemical weathering and erosion of the schist basement which contains numerous quartz veins although recycling has occurred sio2 and tio2 do not consistently show a negative correlation over time this reflects erosion of previously deposited quartz rich sediment and the otago weathering profile which produced an inverse stratigraphy cia values range from 52 for lithic rich coarse grained sandstones and polymictic conglomerate matrices to 93 for coarse grained to pebble rich quartz arenites individual samples were split into finer and coarser grained pairs 2 5 and 2 5 to 1 and were analysed separately the results show that finer grained samples contain higher ree abundances and less sio2 but the coarser grained miocene pliocene samples have higher cia values than their finer grained counterparts these coarse grained deposits are quartz rich and plot erratically on tectonic discrimination diagrams implying that using sio2 poor samples is more reliable for geochemical analysis overall the petrographic and geochemical results indicate that the main factors controlling the composition of the central otago molasse deposits were source composition chemical weathering and recycling studies of this nature can be conducted in archaean tectonically controlled molasse basins that are affected by similar allocyclic factors

geochemistry of mansar lake sediments jammu india implication for source area weathering provenance and tectonic setting abstract the sediment geochemistry including ree of surface and core samples from mansar lake along with mineralogical investigations have been carried out in order to understand the provenance source area weathering hydrolic sorting and tectonic setting of the basin the geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters the major element log values fe 2 o 3 k 2 o vs sio 2 al 2 o 3 and na 2 o k 2 o vs sio 2 al 2 o 3 demarcate a lithology remarkably similar to that exposed in the catchment area the chondrite normalized ree patterns of lake samples are similar to post archaean australian shale paas with lree enrichment a negative eu anomaly and almost flat hree pattern similar to a felsic and or cratonic sedimentary source however the la th sc plot of samples fall in a mixed sedimentary domain close to upper continental crust ucc and paas suggesting sedimentary source rocks for the mansar detritus it also indicates that these elements remained immobile during weathering and transportation the mineralogical characteristic rees and high field strength elements hfse together with the high percentage of metamorphic rock fragments in the siwalik sandstone support a metamorphic source for lower siwalik sediments a very weak positive correlation between zr and sio 2 poor negative correlation with al 2 o 3 negative correlation of la yb n and gd yb n ratios with sio 2 and positive correlation with al 2 o 3 suggest that zr does not dominantly control the ree distribution in mansar sediments the petrographic character and textural immaturity indicate a short distance transport for the detritus the distribution of elements in core samples reflect fractionation the higher zr th and zr yb ratios in coarse sediments and paas compared to finer grained detritus indicate sedimentary sorting plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting it therefore reveals lower siwalik depositional history

petrography and major element geochemistry of the permo triassic sandstones central india implications for provenance in an intracratonic pull apart basin abstract detrital mode composition of feldspars and heavy minerals and major element chemistry of sandstones from the permo triassic succession in the intracratonic satpura gondwana basin central india have been used to investigate provenance the talchir formation the lowermost unit of the succession comprises glacio marine and glacio fluvial deposits the rest of the succession base to top comprising the barakar motur bijori pachmarhi and denwa formations largely represent variety of fluvial depositional systems with minor fluvio deltaic and fluvio lacustrine sedimentation under a variety of climatic conditions including cold warm arid sub humid and semi arid qfl compositions of the sandstones indicate a predominantly continental block provenance and stable cratonic to fault bounded basement uplift tectonic setting compositional maturity of sandstones gradually increases upwards from the early permian talchir to the middle triassic denwa but is punctuated by a sharp peak of increased maturity in the barakar sandstones this temporal change in maturity was primarily controlled by temporal variation in fault induced basement uplift in the craton and was also influenced by climatic factors plots of different quartz types suggest plutonic source rocks for the talchir sandstones and medium to high rank metamorphic plus plutonic source rocks for the younger sandstones composition of alkali feldspars in the permo triassic sandstones and in different precambrian rocks suggests sediment derivation from felsic igneous and metasedimentary rocks compositions of plagioclase in the talchir and bijori sandstones are comparable with those of granite acid volcanic and metasedimentary rocks of the precambrian basement suggesting the latter as possible source rare presence of high k plagioclase in the talchir sandstones however indicates minor contribution from volcanic source rock exclusively plagioclase bearing metasedimentary rock tonalite gneiss and mafic rocks are the probable sources of plagioclase in the upper denwa sandstones quartz rich nature of the sandstones predominance of k feldspar over plagioclase and albite rich character of plagioclase in the sandstones is consistent with deposition in an intracratonic pull apart basin like the satpura gondwana basin composition of garnet and its comparison with that from the precambrian basement rocks suggests mica schist and amphibolite as possible sources predominance of dravite variety of tourmaline in the permian sandstones suggests sediment supply from metasedimentary rocks presence of both dravite and schorl variety of tourmaline in subequal amount in the triassic sandstones indicates sediment derivation from granitic and metasedimentary rocks however schorl bearing rocks are absent in the basement complex of the study area a cn k plot suggests granites acid volcanic rock and meta sediments of the basement as possible sources of the talchir sandstones and metasedimentary rocks for the barakar to pachmarhi sandstones the denwa sandstones were possibly derived from k feldspar free plagioclase bearing metasediments mafic rocks and tonalite gneiss chemical index of alteration cia values suggest low intensity source rock weathering for the talchir sandstones and higher intensity source rock weathering for the others various bivariate plots of major oxides composition of the sandstones suggest passive to active continental margin setting and even arc tectonic setting for a few samples

quartz arenites review and interpretation abstract quartz arenites orthoquartzites are exceptional sands pettijohn et al 1972 that in the past have formed extensive and commonly thick formations they are somewhat enigmatic in that they do not appear to be forming at the present time this paper examines the origin of quartz rich sands documents the range of environments in which they are found and suggests a model for their genesis quartz sand is most commonly formed by tropical weathering of coarse grained quartz bearing rocks such as granite it may be deposited as texturally immature fluvial sandstones generally under a humid climate with drying of the climate quartz rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts quartz sand when transported into shallow marine environments becomes very well rounded through reworking by tidal currents such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings quartz rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones precambrian orthoquartzites are commonly thick and widespread when compared to younger examples the thinness of post silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles there is an abundance of shallow marine and fluvial quartz arenites in the early paleozoic and the cretaceous this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents the shallow marine examples are commonly associated with bituminous sometimes phosphatic shale glauconite and possibly chamositic ironstone there is an abundance of eolian quartz arenites in the permo triassic this is related to a widespread dry continental climate during assembly of the pangean supercontinent

deep freeze petrology of lena river sand siberia modern sand from the vast 2 430 000 km 2 high latitude lena river watershed occupies a low lying mostly unglaciated craton covered by thick widespread permafrost totally different from the watersheds of large temperate and tropical rivers whose sands have been previously studied to establish modern baselines for the interpretation of ancient sandstones fifty nine lena sands are rich in rock fragments and feldspar and reflect well the three major tectonic provinces that underlie it platform shield and collisional suture although the platform and shield sands are still very immature long exposure to cool middle tertiary climates at first and dry sub arctic to arctic climates later is the underlying cause of the mineralogically immature river sands of the lena river