channel arrangements and depositional styles in the sao lourenco fluvial megafan brazilian pantanal wetland abstract the brazilian pantanal is an extensive lowland tropical basin characterized by the presence of fluvial megafans and seasonally inundated savanna floodplain wetlands with an area of about 16 000 km 2 the sao lourenco is the second largest megafan in the pantanal three distinct fluvial channel styles that formed at different times during the late quaternary are found here a geomorphological and sedimentary assessment of these depositional patterns provides valuable insight on the environmental context of their evolution new optically stimulated luminescence data indicate that the upper five meters of sediment in the sao lourenco megafan has been accumulating since the late pleistocene ancient fan lobes located in upper and intermediate fan settings consist of medium and coarse grained fluvial sands and exhibit well preserved distributary braided paleochannels on their surfaces as the megafan evolved through time pleistocene lobes were incised by a prominent valley filled with holocene aged meander belt deposits which consist of silts interbedded with very fine sands and clays currently the incised valley is a zone of sediment bypass modern deposition occurs along the distal toe of the megafan system where lobes characterized by distributary channel levee ridges are widespread these features formed by progradation of avulsion belts into a broad swampy floodbasin which caused the lower portion of the meander belt to be abandoned the significant differences observed in intra fan morphology appear to be linked to the variability in effective precipitation fan lobes deposited with braided distributary channels occurred under relatively dry conditions in the late pleistocene by contrast aggradational meander belt deposits and lobes with distributary channel levee ridges formed during fluctuating precipitation conditions of the holocene when the pantanal emerged from deglacial aridity modern lobes form under heavy seasonal flooding and deposition occurs in response to very rapid and common avulsion events these results have implications for interpreting the complexity of megafan facies in similar continental basins

paleogene synorogenic sedimentation in the altiplano plateau and implications for initial mountain building in the central andes sedimentologic data and palynological ages from paleogene clastic deposits of the northern and central altiplano plateau suggest foreland basin development in the central andes by mid paleocene time the nonmarine potoco formation 3000 6500 m thick constitutes the majority of cenozoic basin fill the potoco overlies the santa lucia formation 50 300 m thick previously dated as mid paleocene by mammal fossils and magnetostratigraphy new geochronologic data for the potoco formation include late eocene to oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels these ages and published 40 ar 39 ar and k ar ages from overlying upper oligocene lower miocene volcaniclastic rocks indicate 1 nondeposition or greatly reduced deposition average sediment accumulation rates 10 m m y from mid paleocene to middle eocene time top santa lucia to lowermost potoco formation followed by 2 rapid deposition average sediment accumulation rates up to 500 m m y throughout late eocene and oligocene time majority of potoco formation lithofacies and paleocurrent data confirm both depositional phases a 20 100 mthick interval of superimposed paleosols in the basal potoco supports reduced sediment accumulation and low rates of subsidence during mid paleocene to middle eocene time the overlying main body of the potoco contains facies assemblages and stratigraphic architecture dominantly nonero

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

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

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

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

early to middle tertiary foreland basin development and the history of andean crustal shortening in bolivia a 2 5 km thick succession of tertiary strata in the eastern cordillera of southern central bolivia consists of predominantly fluvial and lacustrine deposits the age of the base of the succession is middle paleocene and its upper part which is erosionally truncated is probably late oligocene early miocene the lower 50 130 m of the succession consists of interbedded fluvial sandstone and mudstone including minor paleosols of the santa lucia formation these strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the santa lucia formation and lower part of the impora formation that may represent much of late paleocene eocene time above the paleosol zone is a thin zone 10 m of lacustrine carbonate and clastic rocks in the uppermost impora formation and this in turn is overlain by a 2000 m thick upward coarsening succession of clastic fluvial deposits in the cayara camargo and suticollo formations paleocurrent data indicate that fluvial channels that deposited the santa lucia impora and cayara formations flowed mainly westward whereas channels responsible for the camargo and suticollo formations flowed generally eastward modal sandstone petrographic data show a long term evolution from subarkosic during santa lucia deposition to quartz arenitic during impora and cayara deposition to sublitharenitic during camargo and suticollo deposition compositions we argue that the lithofacies and sediment accumulation history of this succession are most consistent with deposition in an eastward migrating foreland basin system e mail decelles geo arizona edu critical to this interpretation is the zone of extreme stratigraphic condensation in the lower impora formation the inferred upper paleocene eocene part of the succession the abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous postrift thermal subsidence resulted in a gradual continuous decay of early tertiary sediment accumulation rates stratigraphic condensation is however consistent with passage of the forebulge through the region an interpretation that also can be reconciled with the record of foreland basin development in the altiplano to the west and gross sediment accumulation patterns in the subandean zone and modern foreland basin to the east simple flexural modeling and palinspastic reconstruction of the complete cenozoic migration history of the foreland basin system suggest that 1000 km of foreland lithosphere has been underthrust westward beneath the andean orogenic belt at the latitude of southern bolivia the amount of middle and lower crust that would have been added to the andean infrastructure is sufficient to explain the present crustal thicknesses in bolivia

tectonic inversion in the santa barbara system of the central andean foreland thrust belt northwestern argentina 1 the santa barbara system sbs of northern argentina is a 400 km long segment of the subandean foreland thrust belt it is characterized by predominantly west verging relatively high angle thrust faults many of these faults are reactivated normal faults from one branch of a complex cretaceous to paleogene rift system heteroaxial folding in parts of the sbs is probably due to a slight component of range parallel dextral strike slip motion acting on preexisting faults striking north and ne during inversion regional balanced cross sections along two transects across the northern sbs indicate that the major faults flatten into a detachment in the basement at about 10 km depth neogene e w contraction in the sbs is of the order of 21 26 km rift extension is not very well constrained but was probably less than 10 km the structural style of the sbs differs from the thin skinned subandean thrust belt to the north and from the large wavelength sierras pampeanas basement uplifts to the south the changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style

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

a magma mass beneath casa diablo further evidence from reflected seismic waves a forty station seismographic network operating in long valley caldera in california during the summer of 1997 recorded a number of earthquakes with unusual secondary phases the phases can be seen on several stations throughout the network allowing the use of particle motions and differential travel times to examine their nature and points of origin based on amplitude frequency and travel time considerations it appears that some of the phases are mode converted s to p reflec tions from a zone below the casa diablo area near mammoth lakes ca the locus of reflection points was imaged using time domain signal migration and stacking under the assumptions of first order scattering theory the results indicate that the reflecting zone is a curved 1 5 km by 2 km surface 7 6 km below and 1 4 km west of the casa diablo geothermal plant previous studies show low p wave velocities and high s wave attenuations in the same location as the reflecting zone an amplitude analysis of the suspected reflec tions suggests they are produced by large contrasts in material properties with the possible range of transitions including solid to partially molten or dry to fluid satu rated rock given these observations plus the volcanic history of the caldera and its recent tectonic activity the most straightforward interpretation of the reflections is that they come from either the top of a sill or intrusion or the overlying hydrothermal system of such structures

neogene to quaternary broken foreland formation and sedimentation dynamics in the andes of nw argentina 25 s 1 the northwest argentine andes constitute a premier natural laboratory to assess the complex interactions between isolated uplifts orographic precipitation gradients and related erosion and sedimentation patterns here we present new stratigraphic observations and age information from intermontane basin sediments to elucidate the neogene to quaternary shortening history and associated sediment dynamics of the broken salta foreland this part of the andean orogen which comprises an array of basement cored range uplifts is located at 25 s and lies to the east of the arid intraorogenic altiplano puna plateau in the salta foreland spatially and temporally disparate range uplift along steeply dipping inherited faults has resulted in foreland compartmentalization with steep basin to basin precipitation gradients sediment architecture and facies associations record a three phase 10 5 and 2 ma east directed yet unsystematic evolution of shortening foreland fragmentation and ensuing changes in precipitation and sediment transport the provenance signatures of these deposits reflect the trapping of sediments in the intermontane basins of the andean hinterland as well as the evolution of a severed fluvial network present day moisture supply to the hinterland is determined by range relief and basin elevation the conspiring effects of range uplift and low rainfall help the entrapment and long term storage of sediments ultimately raising basin elevation in the hinterland which may amplify aridification in the orogen interior

the modern foreland basin system adjacent to the central andes regional variations in sediment thickness internal structures average elevation and bouguer gravity define a four component foreland basin system adjacent to the central andes in the most proximal part of the foreland basin system the eastern subandean zone and westernmost chaco plain 1 3 km of cenozoic deposits overlies active folds and thrusts of the frontal andean orogenic wedge these wedge top deposits pass cratonward into a foredeep depozone containing a 3 4 km thick sedimentary prism that tapers toward and locally pinches out against a broad wavelength forebulge in the central eastern chaco plain the forebulge is underlain by precambrian mesozoic rocks and is largely covered by a thin veneer of quaternary alluvium east of the forebulge a thin 0 5 km saucer shaped accumulation of sediment beneath the pantanal wetland represents a back bulge depozone ancient counterparts of these four depozones can be identified in the central andes suggesting that modern basin architecture is the result of continuous eastward migration of the coupled orogenic wedge and foreland basin system since the late cretaceous paleocene

erosion rate determination from foreland basin geometry the geometry of foreland basins is controlled by a dynamic balance between thrust belt migration flexural subsidence and fluvial deposition to improve our ability to quantify the relationships between tectonics climate and foreland basin geometry i developed analytic solutions for basin topography and stratigraphy using a classic two dimensional mathematical model of foreland basin evolution model predictions for basin topography are successfully tested against observed profiles along a humid to arid climatic gradient in the central andes using published estimates for the thrust belt migration rate flexural parameter and thrust front basin depth in this region i show that basin topographic profiles can be used to estimate the upstream erosion rate at any point along a foreland basin basin averaged erosion rates estimated in this way vary from 0 025 to 0 045 mm yr in the central andes increasing from semiarid to humid climates

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

detrital zircon u pb ages provide provenance and chronostratigraphic information from eocene synorogenic deposits in northwestern argentina paleogene clastic sedimentary rocks in the puna plateau of northwestern argentina contain valuable information about the timing and location of early mountain building in the central andes because these rocks generally lack tuffaceous facies only paleontological ages have been available we present u pb ages from detrital zircons in the conglomeratic eocene geste formation of the central puna plateau the zircon ages indicate that the geste formation was derived from nearby high relief ranges composed of ordovician metasedimentary rocks a small population of ca 37 35 ma grains also confirms the late eocene stratigraphic age of the geste formation and suggests that u pb detrital zircon ages may provide a new tool for determining depositional ages and provenance of widespread paleogene deposits in the central andes

the signature of river and wind borne materials exported from patagonia to the southern latitudes a view from rees and implications for paleoclimatic interpretations abstract riverine and wind borne materials transferred from patagonia to the sw atlantic exhibit a homogeneous rare earth element ree signature they match well with the ree composition of recent tephra from the hudson volcano and hence this implies a dominance of material supplied by this source and other similar andean volcanoes due to the trapping effect of proglacial and reservoir lakes the larger patagonian rivers deliver to the ocean a suspended load with a slightly modified andean signature that shows a ree composition depleted in heavy rees in this paper we redefine patagonia as a source of sediments which is in contrast with other sources located in southern south america quaternary sediments deposited in the northern and to a lesser extent in the southern scotia sea and most of the dust in ice cores of east antarctica have ree compositions very similar to the loess from buenos aires province and to patagonian eolian dust however we rule out buenos aires province as a holocene major source of sediments similarly to buenos aires loess a proximal facies it is likely that the ree compositions of most sediment cores of the scotia sea and antarctica reflect a distal transport of dust with an admixed composition from two main sources a major contribution from patagonia and a minor proportion from source areas containing sediments with a clear upper crustal signature e g western argentina or from bolivia s altiplano evidence indicates that only during the last glacial maximum patagonian materials were the predominant sediment source to the southern latitudes

cenozoic foreland basin system in the central andes of northwestern argentina implications for andean geodynamics and modes of deformation 1 cenozoic strata in the central andes of northwestern argentina record the development and migration of a regional foreland basin system analogous to the modern chaco parana alluvial plain paleocene lower eocene fluvial and lacustrine deposits are overlain by middle upper eocene hypermature paleosols or an erosional disconformity representing 10 15 myr this supersol disconformity zone is traceable over a 200 000 km2 area in the andean thrust belt and is overlain by 2 6 km of upward coarsening eastward thinning upper eocene through lower miocene fluvial and eolian deposits middle miocene pliocene fluvial lacustrine and alluvial fan deposits occupy local depocenters with contractional growth structures paleocurrent and petrographic data demonstrate westerly provenance of quartzolithic and feldspatholithic sediments detrital zircon ages from cenozoic sandstones cluster at 470 491 522 544 555 994 and 1024 1096 ma proterozoic mesozoic clastic and igneous rocks in the puna and cordillera oriental yield similar age clusters and served as sources of the zircons in the cenozoic deposits arc derived zircons become prominent in oligo miocene deposits and provide new chronostratigraphic constraints sediment accumulation rate increased from 20 m myr during paleocene eocene time to 200 600 m myr during the middle to late miocene the new data suggest that a flexural foreland basin formed during paleocene time and migrated at least 600 km eastward at an unsteady pace dictated by periods of abrupt eastward propagation of the orogenic strain front despite differences in deformation style between bolivia and northwestern argentina lithosphere in these two regions flexed similarly in response to eastward encroachment of a comparable orogenic load beginning during late paleocene time

the effect of inherited paleotopography on exhumation of the central andes of nw argentina differential exhumation in the puna plateau and eastern cordillera of nw argentina is controlled by inherited paleostructures and resulting paleotopography related to the cretaceous salta rift paleomargins the ceno zoic deformation front related to the development of the andean retro arc orogenic system is generally associated with 4 km of exhumation which is recorded by cenozoic apatite fi ssion track aft and u th sm he ages he ages in the eastern cordillera of nw argentina new aft ages from the top of the nevado de cachi document oligocene ca 28 ma cooling which combined with existing data indicates exhumation of this range between ca 28 ma and ca 14 ma however some of the highest ranges in the eastern cordillera preserve cretaceous ages indicative of limited cenozoic exhumation samples collected from an similar to 3 km elevation transect along the northern part of the sierra de quilmes paleorift fl ank laguna brava show aft ages between ca 80 and ca 50 ma and he ages between ca 45 and ca 10 ma another set of samples from an similar to 1 km elevation transect farther to the southwest la quebrada shows cretaceous aft ages between ca 116 ma and ca 76 ma and mainly cretaceous he ages in agreement with aft data analysis of existing aft and he ages from the area once occupied by the salta rift reveals a pattern characterized by cretaceous ages along paleorift highs and cenozoic ages within paleorift hanging wall basins and later foreland basin depocenters this pattern is interrupted by the sierras pampeanas at similar to 28 degrees s which record mid cenozoic ages our data are consistent with a complex inherited pattern of pre andean paleostructures likely associated with paleotopography which was beveled by the cenozoic regional foreland basin and reactivated during the late neogene ca 10 ma strongly controlling the magnitude of cenozoic uplift and exhumation and thus cooling age distribution this combined with variable lithologic erodibility resulted in an irregular distribution of themochronological ages

a late pleistocene holocene wetland megafan in the brazilian amazonia abstract despite the growing interest in megafans definitions provided for this type of environmental setting have not yet been widely agreed upon a record of sedimentary facies distribution in both space and time including a larger number of analogs is particularly needed for improving megafan facies models this work focuses on a large fan like feature from an amazonian wetland in northern brazil morphological data based on remote sensing as well as sedimentary facies and radiocarbon analyses were integrated to propose that this feature is related to a megafan system active during the late pleistocene to holocene the megafan displays a divergent drainage network gently dipping slope and concave up and convex up longitudinal and transverse profiles respectively near surface deposits correspond to fining and coarsening upward sands related to active channels and overbank sand sheets terminal fan lobes sediments are interbedded with abandoned channel floodplain and lake pond muds morphostructural analyses and drainage anomalies revealed a geological setting affected by reactivation of pre existing faults contemporaneous with sediment accumulation establishment of a megafan system in this wetland most likely occurred within a slightly tectonically subsiding basin under favorable climatic conditions during wet seasons high water discharge would have favored sediment transport from highlands into this depositional site high summer temperatures and drought under a monsoonal regime kept the water levels low the described megafan could serve as an analog for contemporary tropical wetland megafans formed under a monsoonal climate regime

anisotropic three dimensional behavior of a normally consolidated clay an experimental study is presented of the influence of the intermediate principal stress on the stress strain pore pressure and strength characteristics of a normally consolidated natural anisotropic clay san francisco bay mud under undrained conditions consolidated undrained triaxial compression tests and triaxial tests with independent control of all three principal stresses on cubical specimens were performed the stress strain behavior and the pore pressure characteristics as well as the effective stress failure surface can be described as being cross anisotropic key words anisotropic soils clays deformation shear strength triaxial tests

an integrated sedimentary systems analysis of the rio bermejo argentina megafan character in the overfilled southern chaco foreland basin abstract the chaco foreland is well known for its spectacular fluvial megafans and this basin frequently serves as a modern analog for studies that use the geological record to investigate tectonic processes in convergent orogenic systems yet little sedimentary data has been recovered from the southern chaco particularly at the latitude of northern argentina which limits both its utility as an analog and our understanding of andean megafan development in the subtropics the southern chaco is particularly amenable for a modern time slice sedimentary systems analysis as environmental gradients controlled by tectonics and climate are relatively well documented deposition in the southern chaco foreland is dominated by the rio bermejo which was studied by integrating geomorphological observations made using geographic information systems with analyses of composition texture and u pb ages of modern detrital sediments remote sensing data reveal changes in rio bermejo channel width sinuosity and bifurcation from west to east across the foreland which are compatible with aspects of geomorphic models for distributive fluvial systems the rio bermejo fully spans the foreland and connects with the south flowing rio paraguay fluvial sediments and reworked loess bury and obscure any surficial expression of a flexural forebulge therefore the rio bermejo megafan rbm stands as an important example of a megafan in an overfilled foreland basin variability in rbm morphology and landforms appears to have only subtle linkages to flexural partitioning of the chaco which is likely a consequence of sediment supply overwhelming available accommodation trend surface analysis highlighted topographic contrasts in rbm architectural elements and indicates the presence of distributary channels with variable surficial expression shallow accommodation marked by ponds is most pervasively developed at the eastern end of the rbm this pattern is consistent with model predicted locations of a back bulge depozone although three environmentally distinct hinterland areas contribute sediment to the rbm sands from the foreland cannot distinguish these zones rather provenance is controlled chiefly by parent lithologies and transport distance sands in the rbm plot within the recycled orogen provenance field mean qtfl 77 6 17 and compositional maturity increases with distance from the thrust front such that quartzose sublitharenites dominate in the distal back bulge distinct intra foreland gradients are captured in sand texture and clay mineral abundances rio bermejo sand sizes become progressively finer towards the forebulge and back bulge mean 98 m whereas mean particle sizes in the wedgetop and foredeep are 280 and 195 m respectively detrital clays suspended in the rio bermejo consist of illite smectite chlorite and kaolinite in rank order of abundance with smectite concentrated in the foredeep due to reworking of the semiarid floodplain by water and wind the results of this study provide novel sedimentary ground truth for distributive fluvial systems in overfilled retroarc forelands that can be used to improve facies models and interpretations of the geological record

tributary distributary and other fluvial patterns what really represents the norm in the continental rock record abstract this paper evaluates the recent claim made by weissmann et al 2010 that deposits of distributive fluvial systems dfs may form the bulk of the continental fluvial record weissmann et al 2010 define dfs as a pattern of channel and floodplain deposits that radiate outward from an apex that is located where the river enters the sedimentary basin as such dfs are fan like systems on which multiple channels coexist and distribute water and sediment across the fan published criteria for the recognition of dfs weissmann et al 2010 are 1 a radial pattern of channels from the dfs apex 2 downslope decreases in channel size 3 down dfs grain size decreases and 4 a lack of lateral channel confinement however in weissmann et al 2010 and subsequent papers only the first of these criteria is applied rigorously thus allowing a variety of types of fluvial system including avulsive incised anabranching and coastal plain distributary systems to potentially be classified uncritically as dfs an ancient succession formed by dfs should preserve mostly small channel bodies with a limited range of dimensions and no outsized channel bodies channel bodies in dfs dominated successions should also decrease in size in a down palaeoslope direction and show palaeocurrent relationships indicative of radial dispersal in order for the term dfs to be applied these characteristics should also be established within stratigraphic bodies that formed coevally however very few examples have yet been published that satisfy these criteria if the claim that dfs dominate the alluvial stratigraphic record is valid then few if any sedimentary bodies formed by large channels should be preserved a review of published research demonstrates however that the alluvial stratigraphic record contains a broad diversity of fluvial network styles including abundant representatives of tributary avulsive anabranching and incised in addition to distributary types it is also clear that the deposits of large main stem rivers that are comparable to the big rivers of the modern world are abundantly preserved in the stratigraphic record an analysis of the planimetric area of modern tributary and distributary fluvial systems is presented demonstrating that tributary systems are also likely to represent a far larger proportion of the ancient sedimentary record than dfs the body of evidence presented herein from modern and ancient alluvial systems thus suggests that the hypothesis of weissmann et al 2010 should be wholly rejected dfs do not dominate the continental stratigraphic record

geomorphic and sedimentary response of rivers to tectonic deformation a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings abstract rivers are extremely sensitive to subtle changes in their grade caused by tectonic tilting as such recognition of tectonic tilting effects on rivers and their resultant sediments can be a useful tool for identifying the often cryptic warping associated with incipient and smaller scale epeirogenic deformation in both modern and ancient settings tectonic warping may result in either longitudinal parallel to floodplain orientation or lateral normal to floodplain orientation tilting of alluvial river profiles alluvial rivers may respond to deformation of longitudinal profile by 1 deflection around zones of uplift and into zones of subsidence 2 aggradation in backtilted and degradation in foretilted reaches 3 compensation of slope alteration by shifts in channel pattern 4 increase in frequency of overbank flooding for foretilted and decrease for backtilted reaches and 5 increased bedload grain size in foretilted reaches and decreased bedload grain size in backtilted reaches lateral tilting causes down tilt avulsion of streams where tilt rates are high and steady down tilt migration combing where tilt rates are lower each of the above effects may have profound impacts on lithofacies geometry and distribution that may potentially be preserved in the rock record fluvial sedimentary evidence for past tilting is traditionally based on the assumption that depositional features reminiscent of modern fluvial tectonic effects are evidence for past tectonic effects where it is closely associated with historically active structures or where non tectonic causes cannot be invoked however caution must be exercised when using these effects as criteria for past or current tectonic warping as these effects may be caused by non tectonic factors these non tectonic causes must be eliminated before tectonic interpretations are made

unsteady evolution of the bolivian subandean thrust belt the role of enhanced erosion and clastic wedge progradation abstract the subandean fold and thrust belt of bolivia constitutes the easternmost part of the andean orogen that reflects thin skinned shortening and eastward propagation of the andean deformation front the exact interplay of tectonics climate and erosion in the deposition of up to 7 5 km of late cenozoic strata exposed in the subandes remains unclear to better constrain these relationships we use four w e industry seismic reflection profiles eight new zircon u pb ages from mio pliocene sedimentary strata and cross section balancing to evaluate the rates of thrust propagation shortening and deposition pinch out migration eastward thrusting arrived in the subandean belt at 12 4 0 5 ma and propagated rapidly toward the foreland unit approximately 6 ma this was followed by out of sequence deformation from ca 4 to 2 1 ma and by renewed eastward propagation thereafter our results show that the thrust front propagation and deposition pinch out migration rates mimic the sediment accumulation rate the rates of deposition pinch out migration and thrust propagation increased three and two fold respectively 8 mm a 3 3 mm a at 8 6 ma the three fold increase in deposition pinch out migration rate at this time is an indication of enhanced erosional efficiency in the hinterland probably coupled with flexural rebound of the basin following the pulse of pinch out migration the subandean belt witnessed rapid 80 km eastward propagation of thrusting to the la vertiente structure at 6 ma as there is no evidence for this event of thrust front migration being linked to an increase in shortening rate the enhanced frontal accretion suggests a shift to supercritical wedge taper conditions we propose that the supercritical state was due to a drop in basal strength caused by sediment loading and pore fluid overpressure this scenario implies that climate controlled variation in erosional efficiency was the driver of late miocene mass redistribution which induced flexural rebound of the subandean thrust belt spreading of a large clastic wedge across the basin and subsequent thrust front propagation

geomorphology and late quaternary of the chaco south america the chaco is a large tropical plain located in the interior of south america consisting of parts of argentina paraguay and bolivia it is 840 000 km2 in area and is characterized by forests savannas and extensive swamps which give it a marked climatic and biogeographic identity it encompasses five huge alluvial fans built by the major rivers which cross the region salado bermejo pilcomayo parapeti and grande the fans are composed of several sedimentary units deposited during different times of the late quaternary under diverse climates two fluvial terraces appear at the apex of each fan the older one is probably late pleistocene in age the second was formed in postglacial times humid climates such as the present one favoured the generation of soils and stable fluvial belts drier climates led to widespread sedimentation along small ephemeral channels and large spill outs during two intervals on in the late quaternary glacial maximum and the late holocene dry climates occurred in the region leading to the formation of dune fields and loess mantles

petrology of fluvial sands from the amazonian foreland basin peru and bolivia modal sand compositions in nine rivers draining the proximal part of the amazonian retroarc foreland basin in peru and bolivia provide support for the recycled orogen provenance model of dickinson and suczek average compositions of 52 sand samples from the rio madre de dios and its tributaries are qfl 74 5 21 qmflt 60 5 35 qplvmlsm 44 10 46 lmlvls 69 18 13 and qmpk 91 4 5 these compositions accurately reflect the predominance of sedimentary and metasedimentary rocks in the cordillera oriental of the andean orogen the ultimate source terrane for the sands of the modern foreland basin rivers of the madre de dios basin can be classified in one of three ways 1 rivers that head in the subandean fold thrust belt 2 rivers that are actively incising a subtle intraforeland uplift overlain by quaternary and upper tertiary foreland basin fill and 3 trunk rivers that collect both type 1 and type 2 rivers the rivers that drain the fold thrust belt carry sands that are slightly more lithic than those carried by rivers that drain the intraforeland uplift the intraforeland derived sands are relatively enriched in volcanic rock fragments lmlvls 70 22 8 whereas sands derived from the fold thrust belt are relatively enriched in sedimentary rock fragments lmlvls 66 9 24 the overriding control on sand composition is the relative area of bedrock type exposed in the fold thrust belt sands reworked from tertiary and quaternary foreland basin fill are relatively enriched in quartz possibly owing to intense weathering of sediments in the soil profile the preservation of predictable and systematic mixing patterns in sand compositions from different rivers suggests that sandstone petrologic data may be augmented by paleocurrent isopach and textural data to facilitate detailed reconstruction of drainage patterns in ancient nonmarine foreland basins

episodic sediment accumulation on amazonian flood plains influenced by el nino southern oscillation continental scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains the spatial extent and depths of such deposits have been described and flood plain accumulation has been determined at decadal timescales but it has not been possible to identify discrete events or to resolve deposition on near annual timescales here we analyse 210 pb activity profiles from sediment cores taken in the pristine beni and mamore river basins which together comprise 720 000 km2 of the amazon basin to investigate sediment accumulation patterns in the andean amazonian foreland we find that in most locations sediment stratigraphy is dominated by discrete packages of sediments of uniform age which are typically 20 80 cm thick with system wide recurrence intervals of about 8 yr indicating relatively rare episodic deposition events ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with la nina events which debouch extraordinary volumes of sediments from the andes we conclude that transient processes driven by the el nino southern oscillation cycle control the formation of the bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon nutrients and pollutants to the amazon main stem

growth and fragmentation of the andean foreland basin during eastward advance of fold thrust deformation puna plateau and eastern cordillera northern argentina 1 the fault bounded cianzo basin represents a cenozoic depocenter between the puna plateau and eastern cordillera of northern argentina analysis of fold thrust relationships nonmarine sedimentation and detrital provenance at 23 24 s helps constrain the origin interconnectedness and subsequent uplift and exhumation of the cianzo basin a potential analog for intermontane hinterland basins in the andes structural mapping reveals a plunging syncline within the 6000 m thick upward coarsening cenozoic clastic succession in the shared footwall of the n striking e directed cianzo thrust fault and transverse ne striking hornocal fault growth strata within upper miocene levels indicate syncontractional accumulation adjacent to the hornocal fault measured stratigraphic sections show upsection changes from 1 paleosol rich distal fluvial sandstones 400 m paleocene eocene santa barbara subgroup to 2 braided fluvial sandstones and mudstones 1400 m upper eocene oligocene casa grande formation to 3 distributary fluvial megafan sandstones and conglomerates 3300 m upper oligocene miocene rio grande formation to 4 alluvial fan conglomerates 1600 m upper miocene pisungo formation the 40ar 39ar geochronological results for interbedded tuffs indicate continuous rio grande deposition from at least 16 34 0 71 to 9 69 0 05 ma sandstone petrographic results define distinct upsection trends in lithic and feldspar content potentially distinguishing the western cordillera magmatic arc from puna eastern cordillera thrust belt sources in addition to growth stratal relationships and 40ar 39ar ages conglomerate clast compositions reflect distinct lithologic differences constraining activation of the cianzo thrust and coeval reverse slip on the reactivated inverted hornocal fault finally detrital zircon u pb ages paleocurrents and facies patterns distinguish local from distal sources revealing a systematic forelandward eastward advance of eocene through upper miocene fold thrust deformation