tectonic subsidence and crustal flexure in the neogene chaco basin of bolivia abstract we examined the evolution of the geometry of the chaco foreland basin of bolivia rio grande parapetiarea latitude 18 50 20 s using seismic reflection gravity and well log data subsidence curves enabled us to identify three tectonic stages with a subsidence rate which ranged from 0 1 to 0 4 km myr during the late miocene we used the bouguer gravity anomaly and the power spectrum of the gravity anomalies to estimate the mean depth of the compensating interfaces in order to understand the evolution of the crustal flexure we used an elastic model with a flexural rigidity of 10 23 n m and crustal thickness of 30 31 km we observed a migration of the forebulge during the neogene of about 90 km analysis of multiple geophysical data in the chaco foreland basin of bolivia permits us to sketch out its evolution and to tentatively describe its successive geometries and topographies since 10 ma

strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

stream driven high density gravelly traction carpets possible deposits in the trabeg conglomerate formation sw ireland and some theoretical considerations of their origin within high density flood flows a prominent mechanism of gravel transport and deposition is by stream driven high density traction carpet with a rheology similar to grain flow these gravel carpets are envisaged to form the basal portion of a bipartite high density flood flow decoupled from an overlying sand and silt laden turbulent flow several examples already documented in the literature are reviewed and an additional case from the lower old red sandstone of southwest ireland is presented two mechanisms of traction carpet initiation are discussed by rapid entrainment of gravel into suspension on rising stage followed by settling into the gravel traction carpet at peak and falling stage and by overconcentration of a normal low density bedload gravel entrainment suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high density sand bearing turbulent flood flow above this tangential shear stress is converted to dispersive pressure which aids buoyancy and quasi static grain to grain contacts in the support of the clasts within the gravel carpet the carpet is thought to have a quasi plastic rheology but behave much like a viscous fluid at high shear rates stream driven gravelly traction carpets are expected to produce sheet like units of clast to matrix supported conglomerate characterized by a parallel or an a p a i clast fabric these units may be ungraded normally or inversely graded depending on the rate of shear the viscosity of the flow and the celerity of deposition

late cenozoic bending of the bolivian andes new paleomagnetic and kinematic constraints 1 new paleomagnetic measurements of essentially undeformed late cenozoic volcanic rocks in the bolivian andes south america constrain rigid body rotations about vertical axes during the last 13 myr on both limbs of the bolivian orocline in the eastern cordillera thermal and alternating field demagnetization was carried out on samples from 52 sites in three major volcanic complexes 1 13 2 ma los frailes ignimbritic volcanics outcropping at 19 5 s 2 9 5 ma morococala ignimbritic volcanics outcropping at 18 s and 3 13 5 ma shoshonitic to acidic lavas and intrusives outcropping at 17 5 s well defined magnetic components were isolated which are interpreted to represent the earth s magnetic field at the time of volcanic activity the mean magnetic vector for site groupings suggests regional tectonic rotations about vertical axes with respect to stable south america of 10 8 clockwise for the 13 2 ma los frailes volcanic complex 8 9 clockwise for only the miocene 13 5 ma los frailes volcanics and 1 18 anticlockwise for the combined 13 5 ma morococala and eucalyptus volcanic complexes these data are consistent with observed shortening gradients on the eastern margin of the bolivian andes in the sub andean zone suggesting rotation and shortening are synchronous a joint inversion for both tectonic rotation and the amount of shortening assuming a linear variation in the amount of rotation along the length of the southern limb of the bolivian orocline shows that rotation of the eastern cordillera in the last 10 15 myr relative to stable south america varies from zero at the oroclinal hinge at 18 s to a maximum of 13 5 clockwise at 22 s decreasing to zero south of 23 s with an average rotation in the range 5 to 10 clockwise concomitant with this shortening in the sub andean zone decreases from a maximum of 86 km at the oroclinal hinge to 47 km at 22 s and then 33 km at 23 s the lack of evidence for large scale internal faulting in the eastern cordillera since 9 ma suggests either bending of the eastern cordillera was accommodated by more distributed small scale faulting or that relative rotation here mainly occurred prior to 9 ma the small average rotation of the eastern cordillera on the southern limb of the orocline is consistent with only very small rotation 10 of the forearc region in northern chile during the last 20 myr rotation of the northern limb is less well constrained but it is also likely to be small 5 and anticlockwise

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

the vast proto tibetan plateau new constraints from paleogene hoh xil basin abstract the surface uplift of the tibetan plateau is the key boundary condition in many cenozoic geological events ranging from global cooling to changes of asian environments during cenozoic however poorly constrained timing for the uplift of tibetan plateau makes these interpretations highly debatable here we report results from sedimentology detrital zircon u pb and lu hf isotopic compositions and paleomagnetic signatures from both the eastern and western hoh xil basins of north central tibetan plateau sedimentary lithofacies and facies associations analyzed in the western hoh xil basin indicate they were deposited in a braided fluvial system and alluvial fan similar with the fenghuoshan group eastern hoh xil basin provenance analyses from conglomerate clast compositions paleocurrent orientations and detrital zircon u pb and lu hf isotopic compositions document sediments in both western and eastern basins were derived from the qiangtang and lhasa blocks these observations in combination with comparative paleomagnetic results imply that hoh xil basin was a single wide basin during paleogene the period of hoh xil basin deposition was coeval with significant period of the early cenozoic uplift and erosion of the qiangtang and lhasa blocks these observations not only reinforce the suggestion that the qiangtang and lhasa blocks were uplifted during eocene to form a proto tibetan plateau but also imply that the proto tibet plateau is vast in areal extent the large dimension and high elevation of the proto tibetan plateau probably contributed to the global cooling during the early eocene

isotopic paleontologic and ichnologic evidence for late miocene pulses of marine incursions in the central andes recognition of an inferred miocene marine incursion affecting areas from colombia through peru and bolivia and into argentina is essential to delineate the south american seaway in bolivia corresponding strata of inferred marine origin have been assigned to the late miocene yecua formation we carried out high resolution 13 c and 18 o isotopic studies on 135 in situ carbonates from 3 outcrops combined with detailed sedimentologic paleontologic and ichnologic analysis four less negative 13 c excursion levels were recorded that coincide well with beds containing marine body barnacle and trace ophiomorpha fossils these strata are interbedded with red green beds containing mudcracks plant roots gypsum and trace fossils of the continental scoyenia ichnofacies our data are significant in that they show for the first time four possible short lived marine incursions in the bolivian central andes during the late miocene the result is constrained by a new u pb date of 7 17 0 34 ma at the top of yecua strata

inferences on the lithospheric structure of campi flegrei district southern italy from seismic noise cross correlation abstract lithospheric v s models are defined in the campi flegrei district southern italy through the non linear inversion of the group velocity dispersion curves of fundamental mode rayleigh waves extracted from ambient noise cross correlations between two receivers and the regional group and phase velocities of the italian cellular lithospheric model 1 1 cells four paths are investigated of which one ischia mis across two adjoining cells the distribution of v s shows a pyroclastic covering with v s increasing from 0 3 0 7 km s to 2 1 km s it rests on a lava or carbonate basement about 5 6 km thick with v s increasing from 2 1 km s to 3 1 km s at about 2 km of depth and rising to 0 6 km towards the island of procida a metamorphic layer is detected at an average depth of 7 7 km with v s of 3 8 3 9 km s about 5 km thick overlying a low velocity layer v s of 3 5 km s at about 11 12 km of depth the v s model along the ischia mis path as average of the models obtained by combining local and regional dispersion data of the two adjoining cells is well consistent with the other paths the moho discontinuity is retrieved at about 23 km of depth with v s of 4 2 km s

forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction tooth s nanson g c 2004 forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction geological society of america bulletin 116 7 8 802 816 rae2008

stream dominated alluvial fan and lacustrine depositional systems in cenozoic strike slip basins denali fault system yukon territory canada ancient stream dominated wet alluvial fan deposits have received far less attention in the literature than their arid semi arid counterparts the cenozoic basin fills along the denali fault system of the northwestern canadian cordillera provide excellent examples of stream dominated alluvial fan deposits because they developed during the eocene oligocene temperate climatic regime in an active strike slip orogen the amphitheatre formation filled several strike slip basins in yukon territory and consists of up to 1200 m of coarse siliciclastic rocks and coal detailed facies analysis conglomerate sandstone percentages c s maximum particle size mps distribution and palaeocurrent analysis of the amphitheatre formation in two of these strike slip basins document the transition from proximal to middle to distal and fringing environments within ancient stream dominated alluvial fan systems proximal fan deposits in the bates lake basin are characterized by disorganized clast supported boulder conglomerate and minor matrix mud supported conglomerate proximal facies are located along the faulted basin margins in areas where c s 80 to 100 and where the average mps ranges from 30 to 60 cm proximal fan deposits grade into middle fan channelized well organized cobble conglomerates that form upward fining sequences with an average thickness of 7 m middle fan deposits grade basinward into well sorted laterally continuous beds of normally graded sandstone interbedded with trough cross stratified sandstone these distal fan deposits are characteristic of areas where c s 20 to 40 and where the average mps ranges from 5 to 15 cm fan fringe deposits consist of lacustrine and axial fluvial facies palaeogeographic reconstruction of the bates lake basin indicates that alluvial fan sedimentation was concentrated in three parts of the basin the largest alluvial fan system abutted the strike slip duke river fault and prograded westward across the axis of the basin two smaller coarser grained fans prograded syntaxially northward from the normal faulted southern basin margin facies analysis of the burwash basin indicates a similar transition from proximal to distal stream dominated alluvial fan environments but with several key differences middle fan deposits in the burwash basin define upward coarsening sequences 50 to 60 m thick composed of fine grained lithofacies and coal in the lower part trough cross stratified sandstone in the middle and conglomerate in the upper part of the sequence upward coarsening sequences 90 140 m thick also are common in the fan fringe lacustrine deposits these sequences coarsen upward from mudstone through fine grained ripple laminated sandstone to coarse grained trough cross stratified sandstone the upward coarsening sequences are basinwide facies independent and probably represent progradation of stream dominated alluvial fan depositional systems coal distribution in the amphitheatre formation is closely coupled with predominant depositional processes on stream dominated alluvial fans the thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility

integrating structural geology and petroleum systems modeling a pilot project from bolivia s fold and thrust belt abstract for the first time a new approach to petroleum systems analysis is presented which allows full integration of tectonic and palinspastic restoration with three dimensional 3d pvt controlled multi component three phase petroleum migration analysis through time a systematic modeling study has been applied to a study area dominated by fold and thrust belts located in the sub andean orogeny near tarija bolivia the project has been performed with a special focus on the simulation technique and on the correct distribution of temperature source rock maturity and pressure development through time with reference to its input data this is the first pilot project presenting a 3d numerical model in a compressional structural regime to which the basin modeling approach has been applied to explain the observed distribution of temperature pressure maturity and petroleum accumulations in general

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

high level surfaces plateau uplift and foreland development bolivian central andes newly dated tertiary strata in the bolivian central andean plateau and synthesis of the tertiary record in the adjacent subandean fold thrust belt constrain the age of deformation in both regions age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high level surface known as the san juan del oro surface new 40 ar 39 ar dates on undeformed strata above the high level surface preclude significant upper crustal shortening within the eastern cordillera after 10 ma tertiary strata within the adjacent subandean region demonstrate that formation of the fold thrust belt occurred after 10 ma on the basis of these data we propose a two stage model of late cenozoic andean growth that links plateau uplift to the development of the fold thrust belt in the first stage early plateau uplift occurred in response to widespread compressional deformation of the plateau eastern cordillera and altiplano during the second stage beginning after 10 ma upper crustal deformation within the plateau terminated and the subandean fold thrust belt developed crustal scale eastward thrusting along the eastern margin of the eastern cordillera drove subandean folding and thrusting the eastern cordillera served as the bulldozer for the deforming subandean wedge

increased sediment accumulation rates and climatic forcing in the central andes during the late miocene new zircon u pb ages from six volcanic ashes in the subandean foothills provide for the first time well constrained depositional ages of the late cenozoic strata in this region the radiometric ages indicate 12 4 0 5 ma 7 93 0 26 ma and 5 94 0 20 ma depositional ages for the bases of yecua tariquia and guandacay formations respectively we used these zircon ages to determine the accumulation rates of the late cenozoic foreland sediments in the subandes our results show a fourfold increase from 130 to 628 m m y in sediment accumulation rates between ca 7 9 and 6 ma during the deposition of andean derived coarsening and thickening upward sandstone dominated tariquia strata this increased accumulation rate correlates well with monsoon intensification and climate variability in south america which was accompanied by the development of fluvial megafan paleodrainage networks in the central andes this correlation suggests that climate may have been an important factor between the late miocene and pliocene

sedimentation in an arid zone anastomosing fluvial system cooper s creek central australia abstract cooper s creek is a large ephemeral multi channel fluvial system in the lake eyre basin of arid central australia the large extent and aggradational regime of the system make it a suitable model for interpreting ancient successions formed under similar environmental conditions the dominant channel pattern in the cooper is a combination of relict braids and active anastomosing channels the braids are the partly buried surface expression of a sand sheet attributed to a pluvial episode about 5 000 years ago whereas the anastomosing pattern is interpreted as an adjustment to the present arid climate tectonism and eolian processes are minor localized influences the anastomosing channels accumulate sand but they occupy a small part of the floodplain on which overbank mud is blanketing the relict braids a preliminary depositional model for arid zone anastomosing fluvial systems comprises a mud dominated succession with minor isolated channel sands other features are deep desiccation cracks minor carbonaceous horizons duricrusts and evaporite horizons

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

arcuate pamir in the paleogene insights from a review of stratigraphy and sedimentology of the basin fills in the foreland of ne chinese pamir western tarim basin abstract the tectonic evolution of the pamir bears essential information for understanding intracontinental tectonic processes within the context of india asia convergence however it is unclear whether the arcuate shape of the pamir is entirely the result of the india asia collision or may have been in part an inherited pre cenozoic feature in this study we address the evolution of the ne chinese pamir during the paleogene by reviewing the stratigraphy of twelve sections in the western tarim basin and the establishment of sedimentary facies the results indicate three episodes of deposition of alluvial coarse sediments in the foreland regions of ne chinese pamir during the 1 early paleocene 2 late paleocene to early eocene and 3 middle eocene to oligocene respectively these are correspondingly attributed to 1 retroarc deformation in an andean type margin of the southern asia during subduction of the neo tethys ocean 2 the immediate response in the pamir to the india asia collision and 3 continued india asia convergence the alluvial conglomerates with grain size up to 5 10 cm imply a transport distance

sedimentation and preservation of the miocene atacama gravels in the pedernales chanaral area northern chile climatic or tectonic control in recent years longitudinal changes on the thin thick skinned tectonic styles of the central andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the peru chile trench the search for paleo climate records is therefore crucial for our understanding of the history of the central andes in this paper we focus on the atacama gravels an extensive blanket of miocene continental deposits filling a neogene paleo valley system along the southern atacama desert in northern chile an east west transect between pedernales and chanaral 26o30 s enabled us to carry out a sedimentological and tectonic study of the atacama gravels based on logging and field observations along the rio salado canyon new 39ar 40ar ages obtained on intercalated and overlying ignimbrites date the beginning of the atacama gravels sedimentation at around the oligocene miocene boundary and cessation of sedimentation in the late miocene thirteen lithofacies included within five facies associations a1 to a5 were identified depositional environments vary from proximal alluvial fan a1 a2 in the precordillera through ephemeral fluvial a3 a4 to distal playa lake a5 in the coastal cordillera no evidences of syn sedimentary deformation have been found showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes and climate change appears to be the dominant controlling factor of sediment preservation a progressive change from semi arid towards hyper arid climatic conditions during the miocene led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the oligocene

basin scale changes in floodplain paleosols implications for interpreting alluvial architecture abstract floodplain paleosols in time equivalent strata of the willwood for mation differ in terms of paleodrainage and degree of pedogenic development ancient soils were more poorly drained and more strongly developed in one area compared to a second area about 45 km away this difference in drainage is attributed to parent material the more poorly drained paleosols formed in an area with less permeable clay rich floodplain deposits in the stratigraphic interval that accumulated more slowly paleosols are more strongly developed less numerous and thicker than in the more rapidly accumulating stratigraphic interval these differences are attributed not only to different sediment accumulation rates which are commonly considered to control the degree of ped ogenic development but also to differences in avulsion frequency a simple model is offered to show how paleosols change with varying basin subsidence rates and the relationship between avulsion frequency and accumulation rate when avulsion frequency increases more rapidly than sediment accumulation rate an increase in subsidence rate causes strongly developed paleosols to be overlain by a stratigraphic interval with paleosols that are less well developed thinner and more densely spaced when avulsion frequency is not related to accumulation rate as early models of alluvial architecture assumed an increase in subsidence rate produces a stratigraphic section in which strongly developed paleosols are overlain by paleosols that are less well developed thicker and more widely spaced study of the thickness and degree of development of floodplain paleosols can be used together with more traditional studies of alluvial architecture to provide a clearer understanding of the factors that influenced the stratigraphic architecture in a particular alluvial basin

archean to recent aeolian sand systems and their sedimentary record current understanding and future prospects the sedimentary record of aeolian sand systems extends from the archean to the quaternary yet current understanding of aeolian sedimentary processes and product remains limited most preserved aeolian successions represent inland sand sea or dunefield erg deposits whereas coastal systems are primarily known from the cenozoic the complexity of aeolian sedimentary processes and facies variability are under represented and excessively simplified in current facies models which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes archean and neoproterozoic aeolian successions remain poorly constrained palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts during the triassic widespread desert conditions prevailed across much of europe during the jurassic extensive ergs developed in north america and gave rise to anomalously thick aeolian successions cretaceous aeolian successions are widespread in south america africa asia and locally in europe spain and the usa several eocene to pliocene successions represent the direct precursors to the present day systems quaternary systems include major sand seas ergs in low lattitude and mid latitude arid regions pleistocene carbonate and holocene modern siliciclastic coastal systems the sedimentary record of most modern aeolian systems remains largely unknown the majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes whereas successions representing linear and star dunes remain under recognized research questions that remain to be answered include i what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology ii what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation iii can sophisticated four dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation and iv is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of earth history when large scale aeolian systems were active or has the evidence to support the existence of other major desert basins been lost for many periods throughout earth history

kinematic flexural and thermal modelling in the central andes unravelling age and signal of deformation exhumation and uplift abstract quantifying age rate and lateral variation of deformation and exhumation in convergent systems relies on integration of geologic map patterns ag and locations of reset thermochronometer systems and synorogenic sediment distribution the central bolivian andes provide an ideal location to examine the influence of variations in shortening and stratigraphic architecture on the structural evolution of the mountain range due to differential age and rates of shortening and distinct sedimentary basin geometries along strike to quantify age and rate of shortening we link thermokinematic modelling of sequentially deformed forward modelled balanced cross sections to synorogenic and thermochronologic histories the preserved basin history in the altiplano and eastern cordillera argues for an early fold and thrust belt located in the now western cordillera with subsequent propagation of shortening eastward around 40 50 ma flexural modelling incorporating isostasy and erosion requires multiple basement thrust sheets with 35 97 km of displacement a temporally evolving effective elastic thickness as well as imposed subsidence in the foreland and uplift in the hinterland is required to reproduce the surface geology increase subandean foreland basin depth limit altiplano sedimentation and facilitate altiplano uplift to modern elevation thermokinematic modelling is compatible with initiation of deformation at 50 40 ma in a marked increase in subandean velocities from 5 5 to 8 10 mm yr from 12 10 ma to present out of sequence thrusting at the westernmost limit of the subandes is required to match measured young and partially reset zircon helium ages out of sequence faulting is supported by high ksn values indicative of active uplift and was likely promoted by the abrupt eastern edge of the paleozoic basin rocks which limited forward propagation of structures and or increased erosion due to focused precipitation our results highlight the importance of incorporating detailed structural modelling in differentiating the geometry kinematics and timing of deformation to reproduce thermochronologic ages and basin histories

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

laramide tectonics and humid alluvial fan sedimentation ne uinta uplift utah and wyoming abstract laramide uplift and erosion of the uinta mountains are recorded in a 10 km long outcrop of the lower eocene wasatch formation on the utah wyoming border this 750 m thick package of interbedded sandstones and conglomerates is dominated by a coarsening then fining upward megasequence 650 m thick that records the growth and abandonment of a humid alluvial fun system during a major cycle of uplift and unroofing of the thrust bounded northern flank of the uinta mountains grain size thickness and lateral extent of channel complex deposits increase upward in the lower 400 m of the sequence reflecting construction and northward progradation of the fan grain size and channel complex thickness decrease upward in the upper 250 m of the sequence reflecting gradual reduction of both sedime t yield and sediment caliber during postorogenic lowering of source area relief within the megasequence coarsening up sequences 10 100 m thick built mainly of channel complex deposits reflect progradation of fan lobes punctuated by periodic fan head avulsions these medium scale sequences in turn comprise small scale fining up cycles 1 10 m thick that reflect such fluvial processes on the fan as bar building discrete flood events and the filling and lateral migration of braided stream channels both the medium scale and small scale sequences are commonly underlain by paleosols evidence of debris flows or other mass movement processes is conspicuously lacking above the main megasequence a second thinner megasequence containing lacustrine mudstones and wave reworked conglomerates indicates that the fan persisted as a locus of coarse clastic deposition during the first of a series of lacustrine transgressions that began as ratios of sediment flux to subsidence rate decreased toward the end of the early eocene partial inundation of the fan during this transgression transformed it into a fan delta

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

temperature variations in lake ice in central alaska usa abstract in winter 2002 03 and 2003 04 thermistors were installed in the ice on two shallow pondsin central alaska in order to obtain data on ice temperatures and their response to increasing anddecreasing air temperatures and flooding and snow ice formation snow depth and density and icethickness were also measured in order to understand how they affected and were affected by icetemperature variability the lowest ice temperature 15 5 8 c and steepest temperature gradient 39 8 8 cm 1 occurred during a 9week period in autumn when there was no snow on the ice withsnow on the ice temperature gradients were more typically in the range 20 to 5 8 cm 1 average icetemperatures were lower during the warmer first winter and higher during the cooler second winterbecause of differences in the depth and duration of the snow cover isothermal ice near the freezingpoint resulted from flooding and snow ice formation and brief episodes of warm weather with freezingrain under these circumstances congelation ice growth at the bottom of the ice cover was interrupted even reversed it is suggested that the patterns in temperatures brought about by the snow ice formationand rain events may become more prevalent due to the increase in frequency of these events in centralalaska if temperature and precipitation change as predicted by arctic climate models

subandean thrust and fold belt of northwestern argentina geometry and timing of the andean evolution the subandean ranges of northwestern argentina are an active thin skinned fold and thrust belt the main detachment level in silurian shales dips 2 3w and all the major east verging faults detach from it important intermediate detachment levels in the devonian shales generate lift off structures and the decoupling of the lower and upper structural levels the subandean thrust belt has a minimum shortening of about 60 km 36 at about 2240 latitude the deformation started at about 8 5 9 ma with the uplift of the el pescado range and the formation of an important back thrust at the cinco picachos range fault generation gets young to the east the pintascayo range uplift started at 7 6 ma while the baja orn range uplift began at about 6 9 ma these two ranges continued growing simultaneously until at least 4 7 ma at san antonio range fault movement began at approximately 4 4 ma and the aguarage uplift started at about 2 7 ma an important stage dominated by out of sequence movements spans from about 4 5 ma to present for both proposed models of shortening the quaternary rates of shortening between 8 and 11 mm yr coincide well with global positioning system results from the area the hydrocarbon generation and migration is contemporaneous with the deformation enhancing the possibilities of hydrocarbon entrapment in the area

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes

miocene forebulge development previous to broken foreland partitioning in the southern central andes west central argentina 1 in the distal region of the modern flat slab segment in the southern central andes an unusual stack of middle miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west these features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system sedimentary thickness farther east and far from the cordilleran tectonic loads suggests accommodation and preservation driven by nonisostatic dynamic subsidence regional overlapping relationships and basin modeling suggest that the modern broken foreland present sierras pampeanas can be interpreted as a reactivation of a formerly partitioned broad forebulge

geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing example from the foreland thrust system of the southern bolivian subandean zone the subandean zone of bolivia is a foreland fold and thrust belt which forms the eastern edge of the central andes mountains between 19 s and 22 s latitude the construction of five balanced cross sections shows that the n s trending subandean zone is characterized by the existence of passive roof duplexes these complex structures can be distinguished by the lithotectonic unit within which duplexing occurs the five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes the sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes apparently these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units while a passive roof duplex was developing the sole thrust stuck and the major horizontal displacements were then transferred either to out of sequence thrusts or to a new sole thrust in a deeper detachment horizon therefore each passive roof duplex would correspond to the orogenic front of the andean range at one very particular time in the history of the subandean zone of southern bolivia from south to north the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front available data do not permit us to explain completely this phenomenom

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

controls on the accumulation of coal and on the development of anastomosed fluvial systems in the cretaceous dakota formation of southern utah alluvial strata of the cretaceous dakota formation of southern utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the sevier orogenic belt these strata contain valley fill deposits anastomosed channel systems and widespread coals the coals constitute a relatively minor part of the dakota formation in terms of sediment volume but may represent a substantial amount of the time represented by the formation the coals are separated by clastic units up to 20 m thick the stratigraphically lowest clastic unit of the dakota formation lies above an unconformity cut into jurassic rocks incised valleys associated with the unconformity are up to 12 m deep two discrete episodes of valley fill sedimentation are recognized including a lower sandstone unit with conglomerate layers and an upper discontinuous coal bearing unit after the valleys filled the area became one of low relief where extensive mires formed peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west the clastic units consist of sandstone mudstone or heterolithic ribbon bodies stacked tabular sandstones and laminated mudstones and contain minor coal beds less than 0 35 m thick ribbon bodies are 1 9 m thick and 15 160 m wide have pronounced basal scours and are filled with both lateral and vertical accretion deposits an anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions the variation in the structure and grain size of channel fills and the presence of abundant tabular sandstones interpreted as crevasse splays although some sandstone bodies have well developed lateral accretion surfaces the overall ribbon geometry indicates that mature meandering streams were not well developed this is in contrast to modern anastomosed systems which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain rather than being a transitional channel pattern related to river avulsion the anastomosed channels of the dakota formation may have formed part of a large inland delta that episodically invaded widespread mires the mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

river systems and their sediment flux indo gangetic plains northern bihar india river systems of part of the himalayan foreland northern bihar plains india are described in terms of their channel morphology hydrology and suspended sediment characteristics a simple classification of the river systems based mainly on the source area characteristics is proposed i mountain fed ii foothills fed iii plains fed and iv mixed fed rivers distinct differences are noted between these classes of river systems most rivers show evidence of channel movement mainly by avulsion but cut offs also occur locally the mountain fed rivers are characterized by very high discharge and low suspended sediment concentration and the plains fed rivers have relatively low discharge and high suspended sediment concentration the foothill fed rivers have moderate values of discharge and suspended sediment concentration the mountain fed rivers have built megafans of large extent whereas the foothills fed and plains fed rivers have formed muddy interfan areas semi quantitative estimates of water and sediment flux suggest that about 99 9 of the mass transfer into the plains is water with the remaining 0 1 being sediment and that 10 s of the latter 0 01 of the total is retained in the basin the remainder being transferred to the bengal fan

climate change across continental sequence boundaries paleopedology and lithofacies of iglesia basin northwestern argentina subdivision of sedimentary rocks into genetically related sequences reflects the interaction between the extrinsic and intrinsic processes that affect a basin s accommodation potential as well as its environment in nonmarine settings sequence bounding surfaces are often difficult to locate and several causal mechanisms for sequences e g tectonism climate change and autogenic variability combine in complex and still poorly understood ways here we draw upon paleosols as sources of evidence for causes of sequence boundaries in the upper miocene lower pliocene continental iglesia wedgetop basin san juan province argentina stable isotope compositions 13c and 18o clay mineralogy inorganic carbon weight percentages and micromorphologic features of paleosols that span 9 0 4 6 ma reveal environmental conditions concurrent with the development of ten continental stratigraphic sequences charaetcristics of compound pedofacies and regionally recognizable abrupt vertical lithofacies changes suggest fluctuations in both accumulation rate and climate across sequence boundaries and between structural subbasins we interpret that the iglesia basin strata accumulated under conditions that were generally semiarid with seasonal precipitation isotopic ratios and soil inorganic carbon weight percentages indicate increased aridity at 6 9 and 5 2 ma each a time of sedimentary hiatus notable enrichment of the 13c signal at 6 9 ma also suggests the onset of dominance of c4 plant biomass in northwestern argentina in combination with seismic data we interpret that four of the ten sequence boundaries formed during intervals of marginal surface uplift concurrent with greater aridity circumstances that led to erosion of proximal strata and starved the central basin of sediment input resulting in hiatuses in sediment accumulation

along strike segmentation of the andean foreland causes and consequences abstract thrust belts of tertiary age characterize the east flank of the andes and part of the adjacent foreland from venezuela to the southern tip of south america this foreland deformation comprises three basically different structural styles 1 thin skinned thrust belts detached within the sedimentary cover 2 thick skinned thrust belts with an inferred basal detachment in the basement at 10 20 km depth and 3 foreland basement thrusts which possibly affect the entire crust alternating foreland structural styles along the orogen often with sharp boundaries produce a segmentation of the andean foreland the changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the south american plate for instance thin skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic crystalline basement unaffected by mesozoic extension in areas of mesozoic rifting normal faults have been reactivated during tertiary contraction resulting in the formation of thick skinned thrust belts the basement becomes involved even if potential decollement levels are present deep seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover the three basic structural styles are unequally efficient in terms of shortening thin skinned belts are typically shortened by 40 70 thick skinned belts by 20 35 and areas of foreland thrusts by less than 10 the structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening available estimates of shortening along the central andes actually suggest that shortening does not vary steadily but has several maxima and minima strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments flat slabs but closely with lithospheric structure the most strongly shortened segment coincides with thick mantle lithosphere suggesting that shortening of the edge of the south american plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the andes this underthrusting of thick lithosphere may have caused steepening of a formerly flat slab in the arica bend region to the normal 25 30 dip presently observed

anatomy and evolution of a lower cretaceous alluvial plain sedimentology and palaeosols in the upper blairmore group south western alberta canada the lower cretaceous albian upper blairmore group is part of a thick clastic wedge that formed adjacent to the rising cordillera in south western alberta regional transgressive intervals are superimposed on the overall regressive succession alluvial conglomerates sandstones and mudstones were deposited in east north eastward draining fluvial systems orientated transverse to the basin axis five facies associations have been identified igneous pebble conglomerate thick sandstone interbedded lenticular sandstone and mudstone thick mudstone with thin sandstone interlayers and fossiliferous sandstone and mudstone the facies associations are interpreted as gravelly fluvial channels sandy fluvial channels sand dominated floodplains mud dominated floodplains and marine shoreline deposits respectively five types of palaeosols are recognized in the upper blairmore group based on lithology the presence of pedogenic features clay coatings root traces ferruginous nodules slickensides carbonate nodules and degree of horizonization the regional distribution of the various types of palaeosols enables a refinement of the palaeoenvironmental reconstruction permitting an assessment of the controls on floodplain evolution in source proximal areas palaeosol development was inhibited by high rates of sedimentation in source distal locations poor drainage resulting from high watertables low topography and lower rates of sedimentation also inhibited palaeosol development the best developed palaeosols containing bt horizons occur in intermediate alluvial plain positions tectonic hinge zone where the floodplains were most stable due to a balance between sedimentation erosion and subsidence rates extrapolating from the upper blairmore group suggests that the tectonic hinge zone of continental foreland basins can be established by palaeosol analysis at the hinge zone soil development is controlled primarily by climate and tectonics and their effect on sediment supply whereas closer to the palaeoshoreline relative sea level fluctuations resulting in poor drainage may have a more significant influence

paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks climate is a primary control on sediment flux from continental sources into sedimentary systems in warm climates siliciclastic input is greatest under highly seasonal rainfall nonseasonal conditions favor formation of end member chemical rocks perennially wet climates are conducive to coal formation whereas dry climates produce carbonates and or evaporites stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive regressive events and tectonics

the character and genesis of calcrete in late quaternary alluvial deposits gujarat western india and its bearing on the interpretation of ancient climates late quaternary deposits in mainland gujarat contain sediments deposited in subhumid and semi arid climates the 30 35 m succession shows the presence of vertisols at the base and a red bed horizon ferric calcisol when pedogenic in origin that roughly bisects the succession a widespread development of calcretes is observed throughout the succession the various varieties of calcrete include pedogenic calcrete groundwater calcrete calcrete conglomerate transported calcrete and rhizogenic calcrete pedogenic calcrete nodules associated with vertisols and the red soil show marked differences in morphology dimensions and the distribution of microscopic features these differences arise due to contrasting climate controlled physicochemical environs under which they formed pedogenic calcrete nodules associated with vertisols acquire large 5 10 cm dimensions and are characterised by either showing the presence of a nucleus of soil matter or showing a dense micritic groundmass cut by thick sparitic veins in contrast calcrete nodules that formed in the red soil are 3 cm in size and do not exhibit dense networks of sparitic veins calcretes associated with vertisols and the ferric calcisol also exhibit differences in the morphology of rhizoliths these differences also show up in the distribution of microfabrics of calcretes grain coats are present in rhizogenic and pedogenic calcretes but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes clotted micrite is present in all types except groundwater calcretes sparitic veins however are observed in each type but are relatively less developed in groundwater calcretes a similar distribution of displacive and replacive textures is also seen although some grains in groundwater calcretes showed signs of corrosion the vertisol associated calcretes represent a subhumid 500 700 mm climate whereas the red soil calcretes suggest a semi arid 100 500 mm climate calcretes from the vertisol association show a range in 13c composition constrained between 9 and 5 whereas the red soil calcretes exhibit the whole spectrum of values from 9 to 1 based on mineralogical associations calcretes usually taken to reflect semi arid episodes in the earth s history may be classified further calcretes when associated with sepiolite palygorskite suggest an arid climate mean annual rainfall 50 100 mm when associated with smectite haematite and the absence of hydromorphism a semi arid climate mean annual rainfall 100 500 mm and when found in smectitic vertisols subhumid climates mean annual rainfall 500 700 mm

tectonic control on fluvial styles the balfour formation of the karoo basin south africa abstract the balfour formation represents a fully fluvial succession of late late permian earliest triassic age which accumulated in the foredeep of the karoo basin during the overfilled phase of the foreland system the lack of a coeval marine environment within the limits of the preserved karoo basin provides an opportunity to study the stratigraphic cyclicity developed during a time when accommodation was solely controlled by tectonics the balfour stratigraphy is composed of a succession of six third order fluvial depositional sequences separated by subaerial unconformities they formed in isolation from eustatic influences with a timing controlled by orogenic cycles of loading and unloading sediment accumulation took place during stages of flexural subsidence whereas the bounding surfaces are related to stages of isostatic uplift the vertical profile of all sequences displays an overall fining upward trend related to the gradual decrease in topographic slope during orogenic loading at the same time an upward change in fluvial styles can be observed within each sequence from initial higher to final lower energy systems the actual fluvial styles in each location depend on paleoslope gradients and the position of the stratigraphic section relative to the orogenic front proximal sequences show transitions from braided to meandering systems whereas more distal sequences show changes from sand bed to fine grained meandering systems the average duration of the balfour stratigraphic cycles was 0 66 my i e six cycles during 4 my no climatic fluctuations are recorded during this time with the long term climatic background represented by temperate to humid conditions

climate change in response to orographic barrier uplift paleosol and stable isotope evidence from the late neogene santa maria basin northwestern argentina the late cenozoic history of the santa maria basin in the northeastern sierras pampeanas records the far reaching climatic consequences of the development of orographic barriers to atlantic derived moisture the evolution of climate and ecosystems is coupled with the uplift of basement blocks west and east of the basin to their present elevations of 4000 5500 m the climatic and ecological history of the basin for the past 12 m y was reconstructed from paleosol characteristics and carbon and oxygen isotopes of pedogenic material a fluvial environment succeeded the final marine regression after ca 12 ma paleosols with illuvial clay and calcic nodules indicate a seasonally dry climate between ca 9 and 7 ma paleosols formed under conditions of episodic waterlogging as shown by organic matter preservation and variegated colors after ca 7 ma pedogenesis occurred under enhanced drainage due to tectonic steepening of the eastern basin margin this can be related to westward thrusting of the eastern basin bounding sierra aconquija and cumbres calchaquies calcic and silicic rhizoliths and authigenic clay point to persistent wet dry seasonality carbon isotope values document the spread of c 4 plants between ca 5 and 3 ma carbon isotopes show an increase in c 3 plants this is likely a consequence of uplift of the sierra de quilmes to the west resulting in a transitory episode of orographic precipitation within the santa maria basin aridification of the valley occurred between ca 3 and 2 5 ma and was related to creation of high topography to the east development of thick calcretes suggests that the present semiarid conditions were established by the end of pliocene time carbon isotopes show that the quaternary vegetation was c 4 plant dominated and oxygen isotopes attest to intense soil water evaporation

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

late miocene climate variability and surface elevation in the central andes abstract temporal and spatial variations in topography and oxygen stable isotope ratios in precipitation in the central andes have stimulated widespread discussion about the competing roles of mantle and crustal processes and their feedbacks with global scale climatic change in uplifting and shaping the central andes in general one of the major obstacles in assessing the relative contributions of long term 10 5 10 6 a tectonic processes and precipitation as a proxy for climate to the uplift history of the andean orogen is the lack of integrated data sets that record late miocene patterns of uplift and climate radiogenic 87 sr 86 sr sedimentologic and stable isotope 18 o data from subandean foreland deposits of the chaco basin bolivia show a rapid 18 o and 87 sr 86 sr values at 8 5 ma that we interpret to reflect a change in precipitation patterns along the eastern cordillera and the subandean fold thrust belt in agreement with 13 c studies on paleosol carbonates we attribute this change to a southward deflection of the south american low level jet llj that currently exerts the dominant control over the seasonality and amount of precipitation along the eastern flanks of the andes deflection of the llj occurred most likely as the combined effects of readjustment of relief and topography within the eastern cordillera at 20 22 s and possibly associated surface uplift of the altiplano contemporaneous rapid positive shifts in 18 o and 87 sr 86 sr of pedogenic carbonate in fluvial foreland deposits are consistent with a transition to more seasonal precipitation conditions and critical threshold elevations being attained that affected south american atmospheric circulation patterns a four fold increase in sedimentation rates in the foreland together with a shift to strongly radiogenic 87 sr 86 sr ratios in paleo river water and sediment load as well as river incision into the well preserved san juan del oro paleo surface directly reflects a rapid change in moisture distribution along the eastern flank of the central andes at 8 5 ma and associated rearrangement of the drainage systems collectively the data presented here strongly suggest that between 12 and 8 ma the amount oxygen isotopic composition spatial distribution and seasonality of precipitation along the eastern flank of the bolivian andes changed dramatically which in turn require changes in trans andean moisture flux and isotopic composition of precipitation on the uplifting plateau region

anastomosing river deposits sedimentation rates and basin subsidence magdalena river northwestern colombia south america situated in a tectonically active foreland basin the magdalena river consists of vertically accreting levee confined channels and adjacent extensive wetlands which are interpreted as an anastomosing river sedimentary system equivalent rates of basin filling and subsidence average 3 8 mm yr 1 based on 18 14c dates from five bore holes drilled to depths of 55 m and sediment transport budgets from 35 years of measurement located in a savanna tropical climate anastomosing river deposits of the magdalena are remarkably similar to the anastomosing deposits of the upper columbia river in a temperate cold climate in western canada suggesting that climate is not a controlling factor of anastomosis the geometry of anastomosing channel fills in the magdalena consists of stratigraphically non uniform low sinuous narrow stringers of sand up to 30 m thick by 600 m wide a width depth ratio of 20 thin 1 2 m off channel crevasse splay sand sheets extend laterally up to 10 km distance when buried both sand deposits become encased by lacustrine or marsh mud to form stratigraphic traps while there are few modern anastomosing river systems as compared to braiding and meandering there may be a disproportionately large number of ancient anastomosed fluvial rock sequences due to the rapid rate of vertical accretion such a different depositional style and geometry of sand bodies have considerable significance in the interpretation of some ancient fluvial rock sequences because it provides an alternative to the meandered and braided river deposition models

composition and provenance history of late cenozoic sediments in southeastern bolivia implications for chaco foreland basin evolution and andean uplift abstract the chaco foreland basin marks the eastern edge of the andean orogenic system and contains up to 7 5 km of mostly nonmarine siliciclastics thus providing an opportunity to study the detrital record of late cenozoic andean geodynamics and paleogeography sandstones from the southern bolivian chaco foreland basin testify to variable contributions from the craton and the andean foreland fold and thrust belt in the context of an eastward advancing orogen sandstone and conglomerate petrography of the oligocene to recent strata exposed in the chaco basin show an overall trend towards reduced mineralogical maturity corresponding to diminishing transport distance increasing topographic gradient and changing depositional environment from arid continental plain oligocene to middle miocene petaca sandstone through marginal marine middle miocene to late miocene yecua sandstone and braided fluvial settings late miocene tariquia sandstone to semihumid alluvial fan environments late miocene to recent guandacay and emborozu formations paleocurrent patterns of pre tectonic sediments record westward transport the late miocene transitional marine yecua formation sediments show a complex shoreline related pattern syntectonic strata tariquia guandacay and emborozu formations filling the foreland basin indicate dominant east directed sediment transport from the advancing andean orographic front because the eastern cordillera provided a drainage barrier to altiplano erosion sandstones of the chaco basin do not directly record andean volcanism or altiplano uplift however climatic inferences from changing facies and petrographic composition suggest major uplift and formation of orographic barriers between 14 and 7 ma

shifting of the kosi river northern india the kosi river shifts laterally over the himalaya foreland plain by continual minor cutoffs and bank cutting and by episodic major shifts across watersheds by moving into and then out of preexisting adjacent less actively aggrading streams migration is unidirectional because after a channel is filled to instability floodwater will drain preferentially into a new adjacent low rather than across it to the next watershed or back to the last abandoned channel major shifts seem stochastic and autocyclic they do not correlate with the many severe quakes and floods that undoubtedly helped prime the system for shifts

chronology of miocene terrestrial deposits and fossil vertebrates from quebrada honda bolivia abstract at 3500 m above sea level in the eastern andean cordillera quebrada honda basin exposes more than 300 m of middle miocene sedimentary strata developed on distal alluvial fan sediments interstratified with volcanoclastic deposits overlying paleozoic basement these deposits are rich in exotic vertebrate fauna adapted to a wetter and warmer climate at lower paleoelevation as contrasted with the present high elevation cold and dry andean conditions this study provides new magnetostratigraphic and 40ar 39ar age control of the quebrada honda rio rosario and huayllajara fossiliferous areas demonstrating that they are 13 1 12 4 ma part of the succession evidences cyclical deposition of calcretes and reddish mudstones which may be a response to precessional orbital forcing climatic processes paleomagnetic measurements show a significant clockwise tectonic rotation related to postdepositional tectonism associated with evidence of an important uplift after the deposition of quebrada honda sediments this uplift of the andean eastern cordillera was associated with the formation of the subandean fold thrust belt and the development of the amazon and la plata drainage basins proposed uplift rates of 200 m ma are compatible with well established rates of uplift recorded in contemporaneous alpine sections

cenozoic evolution of the pamir plateau based on stratigraphy zircon provenance and stable isotopes of foreland basin sediments at oytag wuyitake in the tarim basin west china abstract the pamir salient is the western expression of mountain growth related to indo eurasian convergence though a rough framework has emerged describing the tectonic evolution of the pamir detailed knowledge of the spatial and temporal evolution of cenozoic deformation is necessary to determine how strain progressed through the orogenic belt here we present new stratigraphic zircon provenance and stable isotope data from jurassic to miocene strata along the pamir s northeastern margin near the town of oytag wuyitake in the tarim basin west china prominent 40 ma peaks in oligocene to early miocene detrital zircon grains record the erosion of an eocene belt of shoshonitic rocks in the central to southeastern pamir this is roughly coincident with an 4 shift in the oxygen isotopic composition 18o of carbonates during the eocene and or oligocene from an average of 8 7 to 12 6 suggesting a reorganization of atmospheric circulation during that time this could have been caused by uplift of tarim basin bounding ranges and or retreat of the paratethys sea a subsequent change from eocene to jurassic aged detrital zircon grains in the early to middle miocene indicates provenance shifted from source rocks in the central and or se pamir to the hanging wall of the main pamir thrust mpt coincident with prograding facies at that time this suggests deformation progressed outward toward the northeast margin of the pamir plateau in the early to middle miocene our results corroborate outward advancement of himalayan deformation affecting all margins of the tarim basin by the middle miocene

comparison of the bedform distribution and dynamics of modern and ancient sandy ephemeral flood deposits in the southwestern karoo region south africa abstract during january 1981 unusually high rainfall patterns in the semi arid southwestern karoo region of south africa gave rise to catastrophic floods in the area the most devastating of which was the ephemeral flash flood that partially destroyed the town of laingsburg flood waters at laingsburg spread from a dry river bed rising to a depth of 12 m within 7 h washing away houses and burying the town with up to 3 5 m of sand and silt facies sequences and bedforms in the laingsburg flood deposits are related to specific subenvironments and flow conditions during the flood event two large scale sedimentary units in erosive contact with each other were deposited by two major discharge pulses within the same flood event vertical and lateral variations in the bedform distributions reflect fluctuations in flow velocity and turbulence comparison is drawn between the laingsburg flood deposits and fluvial sandstones of the permian lower beaufort in the southwestern karoo the multistoried sandstone bodies composed mainly of a horizontally laminated lithofacies are interpreted as the products of a complex series of ephemeral flooding events individual storey units in the palaeochannel sequences are generally envisaged as the deposits of different floods however evidence from the modern flood shows that separate pulses of a single flooding episode can also produce a sequence of storey units bounded by erosive scours spectacular palaeosurfaces in the lower beaufort also have modern analogues in the laingsburg overbank deposits discussion on the preservation potential of delicate sedimentary structures and biogenic traces on these surfaces emphasizes the necessity for pulsatory deposition of bedload and suspended load material over short intervals of time by comparing facies profiles and surface bedforms the ephemeral fluvial facies model for the lower beaufort is critically appraised

influence of the andes mountains on south american moisture transport convection and precipitation mountain ranges are known to have a first order control on mid latitude climate but previous studies have shown that the andes have little effect on the large scale circulation over south america we use a limited domain general circulation model regcm3 to evaluate the effect of the andes on regional scale atmospheric dynamics and precipitation we present experiments in which andean heights are specified at 250 m and 25 50 75 and 100 of their modern values our experiments indicate that the andes have a significant influence on moisture transport between the amazon basin and the central andes deep convective processes and precipitation over much of south america through mechanical forcing of the south american low level jet llj and topographic blocking of westerly flow from the pacific ocean when the andes are absent the llj is absent and moisture transport over the central andes is mainly northeastward as a result deep convection is suppressed and precipitation is low along the andes above 50 of the modern elevation a southward flowing llj develops along the eastern andean flanks and transports moisture from the tropics to the subtropics moisture drawn from the amazon basin provides the latent energy required to drive convection and precipitation along the andean front large northerly moisture flux and reduced low level convergence over the amazon basin leads to a reduction in precipitation over much of the basin our model results are largely consistent with proxy evidence of andean climate change and have implications for the timing and rate of andean surface uplift

sedimentology of coarse grained alluvial fans in the markham valley papua new guinea abstract in alluvial sediment sequences recognition of a hierarchy of bounding surfaces and the shapes and associated lithofacies of the sediment bodies they define provide an appropriate framework for understanding associations among depositional forms the processes responsible for them and their controls on system development this methodology architectural element analysis integrates principles from geomorphology and sedimentology miall 1985 1988 it is used here to analyse the evolution of the modern umi fan and the alluvial fan part of the pleistocene leron formation in the markham valley papua new guinea a series of terraces has developed as the umi river has incised into its fan detailed stratigraphic analysis of the lowest terrace in exposures up to 25 m high and kilometres long reveals that the fan is dominated by sheetflood deposits with minimal preservation of either debris flow or hyperconcentrated flood flow sediments channel fill elements make up a larger proportion of exposures in the proximal fan than elsewhere while 95 of distal fan exposures are composed of sheetflood sequences these depositional features likely result from massive sediment dispersal associated with the rapidly uplifting upland terrain abundant sediment availability and flashy discharge regime channel fill units along with slope related deposits from debris and hyperconcentrated flood flows are only likely to be preserved in the trench backfilling phase following fan entrenchment reworking of deposits plays a dominant role in preservation of sheetflood deposits at the expense of slope related deposits an hierarchical framework of lithosomes is developed which characterizes various scales and bounding surfaces of depositional units which make up the umi fan cf miall 1988 decelles et al 1991 smaller elements observed in terrace exposures are described as first to fourth order lithosomes they reflect geomorphic processes which are evident in and adjacent to the modern umi river deposits which infill the trench are interpreted as a fifth order lithosome constrained by trench geometry proximal distal relationships are remarkably similar in the umi fan and leron formation fan sequences in both instances debris flow deposits are seldom observed the largest sixth and seventh order lithosomes which were deposited within a time frame of approximately one hundred thousand years are interpreted by analysis of the leron formation in relation to its tectonic setting the umi and pleistocene leron formation fans exemplify fan development in a post collisional molasse basin under a tropical monsoonal climate carbonate concretions observed in distal fan facies may provide a possible diagnostic feature of these tropical savanna fans

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

alluvial fans and fluvial fans at the margins of continental sedimentary basins geomorphic and sedimentological distinction for geo energy exploration and development alluvial fans and fluvial fans are the most common depositional landforms along the margins of continental sedimentary basins their occurrence is determined by the area relief and hydrology of the catchment which ultimately control the relative ratios of sediment v flow discharge and runoff regimes fundamental morphological and process distinctions exist between these two kinds of deposystem which are seen as essentially different facies associations and internal architectures for the corresponding deposits alluvial fans commonly develop over short radial distances hundreds of metres to a few kilometres and are constructed by ephemeral flash flow events that lead to poor organization of the sedimentary facies and overall architecture of the corresponding deposits by contrast fluvial mega fans are fed by proper river systems which aggrade much larger volumes of clastic sediment over distances of up to a few hundred kilometres distinct channel belt and overbank domains are developed with a marked heterogeneity in the distribution of sedimentary facies represented by hierarchically well organized fluvial deposits the general properties of alluvial and fluvial fans reflect the different potentials for the corresponding successions to host economically attractive oil and gas resources and the different approaches required in exploration and prediction

basement involved blind thrusting in the eastern cordillera oriental southern bolivia evidence from cross sectional balancing gravimetric and magnetotelluric data abstract a neogene to recent thrust belt forms the subandean ranges and cordillera oriental along the eastern slope of the central andes in bolivia over much of its length there is an area of intermediate topographic height and structural level between the subandean zone and the cordillera oriental proper cross sectional balancing suggests that in southern bolivia the basal detachment of this tightly folded zone links up with a blind thrust that dips westward beneath the high standing eastern margin of the cordillera oriental and rides piggy back on a large buried thrust sheet which forms the backstop for the thin skinned subandean belt magnetotelluric and gravimetric data indicate that this thrust sheet is made up of basement rocks probably including intrusives and higher grade metamorphics such blind basement thrusts in a relatively external position may characterize much of the eastern andean thrust belt and their geometry is possibly related to preexisting structural patterns linked to the configuration of early palaeozoic basins and their margins

facies analysis and basin architecture of the neogene subandean synorogenic wedge southern bolivia foreland sedimentation in the subandean zone of south central bolivia spans from the upper oligocene to present it records sediment dispersal patterns in an initially distal and later proximal retroarc foreland basin and thereby contains stratigraphic information on the tectonic evolution of the adjacent andean fold thrust belt within the neogene orogenic wedge individual siliciclastic dominated depositional systems formed ahead of an eastward propagating deformation regime we defined described and interpreted eight architectural elements and 24 lithofacies from 15 outcrop locations representing the neogene foreland basin in the subandean zone and the chaco plain these are combined to interpret depositional settings the up to 7 5 km thick neogene wedge is subdivided in five stratigraphic units on the basis of facies associations and overall architecture 1 the basal oligocene miocene up to 250 m thick petaca formation consists dominantly of calcrete reworked conglomeratic pedogenic clasts and fluvial sandstone and mudstone this unit is interpreted to represent extensive pedogenesis under an arid to semiarid climate with subordinate braided fluvial processes 2 the overlying upper miocene up to 350 m thick yecua formation records numerous small scale transgressive regressive cycles of marginal marine tidal and shoreline facies of sandstone ooid limestones and varicoloured mudstone 3 the upper miocene up to 4500 m thick tariquia formation principally consists of sandstone with interbedded sandstone mudstone couplets representing frequent crevassing in anastomosing streams with an upsection increasing degree of connectedness 4 the up to 1500 m thick lower pliocene guandacay formation represents braided streams and consists principally of granule to cobble conglomerate interbedded with sandstone and sandy mudstone 5 the upper pliocene up to 2000 m thick emborozu formation consists predominantly of alluvial fandeposited cobble to boulder conglomerate interbedded with sandstone and sandy mudstone the coarsening and thickening upward pattern and eastward progradation coupled with the variable proportions of overbank facies channel size and degree of channel abandonment in the tariquia guandacay and emborozu formations reflect a distal through proximal fluvial megafan environment this long lived megafan grew by high sedimentation rates and a northeast through southeast radial paleoflow pattern on large coarse grained sediment lobes the marked overall

recognition of the sedimentary architecture of dryland anabranching anastomosing rivers abstract the application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions from stratigraphy are to be reliable from modern observations it appears that anabranching anastomosing rivers can be divided into separate humid and dryland classes each class has dissimilar characteristics to the other necessitating different criteria for their recognition in the rock record the criteria being used by sedimentologists to recognize deposits of anabranching rivers are derived solely from modern examples in humid regions heavily biased to a few specific cases in canada these criteria are dominated by the concept of ribbon sandstone bodies encased in fine grained floodplain sediment with an overall low sand to mud ratio in the succession there has been a general failure to appreciate that dryland anabranching rivers have characteristics significantly different from these and almost certainly leave a sedimentary record quite dissimilar to that previously expected avulsion or obtrusion the gradual formation of an alternative channel are the characteristic dynamic of anabranching rivers forming and maintaining coexisting multiple anabranch channels avulsion or obtrusion require stream banks to have a high resistance to erosion usually achieved by cohesive bank sediments or the reinforcement of banks by vegetative root mats in some drylands riparian vegetation can be sparse and bank resistance may depend in large part on a high clay content here channel fills are mud dominated and floodplains have a uniform fine grained character which is usually structureless because of intense pedoturbation and some bioturbation the evidence from such present day dryland anabranching rivers suggests that their sedimentary record is likely to be one of laterally extensive mud rich beds with weak or enigmatic fluvial indicators elsewhere dryland anabranching systems are sand dominated their banks reinforced with riparian gallery forests sustained by infrequent flow and a shallow water table though the stratigraphy of these has barely been investigated they are clearly different from humid region anabranching systems for their channels and islands are predominantly sandy anabranching rivers with prominent channel sandbodies separated by muddy islands have never been described from contemporary drylands the interpretation of a dryland origin for ancient stratigraphic successions with this form is made therefore without evidence that they can actually exist in the light of this such interpretations should be reevaluated

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

distinctive characteristics of a streamflow dominated alluvial fan deposit sanghori area kyongsang basin early cretaceous southeastern korea abstract in the northwestern part of the non marine kyongsang basin early cretaceous conglomeratic deposits occur at intervals along the northern basin margin and grade basinward into interbedded sandstone and mudstone one of these conglomeratic deposits referred to as the sanghori deposit is 150 m thick and characterized by amalgamated sheets of conglomerate and sandstone facies assemblages 1 and 2 alternating with interbedded sandstone and purple siltstone facies assemblage 3 facies assemblage 1 comprises clast supported disorganized and organized sheet conglomerates interbedded with massive to stratified sandstones this assemblage is interpreted as the product of proximal gravel bed stream deposition facies assemblage 2 consists of trough and planar cross stratified and horizontally stratified sandstones it represents sandy braided stream deposits in which sand bars and dunes were common sedimentary facies and architecture of the coarse grained facies assemblages suggest that the gravel bed and sandy braided streams were wide and shallow and rapidly aggraded during flash floods facies assemblage 3 consists of interbedded purple siltstone and tabular sandstone which show evidence of weak pedogenic modification and bioturbation these characteristics are interpreted as the result of floodplain processes sediments were supplied from the north and dispersed radially forming a streamflow dominated alluvial fan system the sanghori alluvial system was most probably fed from a large drainage basin resulting in the predominance of streamflow deposits and rounded clasts the alternation of the gravel bed stream and floodplain deposits was due to autocyclic switching of active fan sectors under the conditions of relatively slow and steady basin subsidence

transition from basement involved to thin skinned thrusting in the cordillera oriental of southern bolivia the eastern flank of the central andes in southern bolivia and northern argentina is underlain by a fold and thrust belt of neogene age it comprises an external thin skinned segment with basal detachment in silurian shales subandean ranges and an internal segment where units as deep as precambrian metasedimentary and igneous rocks are involved in thrusting eastern cordillera oriental between the cordillera oriental and the subandean ranges an intermediate segment interandean zone intervenes north of the argentinian bolivian border 22 s the structure of the interandean zone is characterized by folded and thrust faulted cambrian to triassic strata balanced cross sections across the western subandean ranges the interandean zone and the eastern cordillera oriental are based on previously available data and recent field work they suggest that the geological structure can be interpreted in terms of two major basement involved thrusts here referred to as interandean and subandean thrusts the more internal interandean thrust caused the development of broad basement anticlines in the eastern cordillera oriental however at shallower levels its displacement was accommodated by thin skinned folding and thrusting in the interandean zone in sequence activation of the subandean thrust uplifted the interandean zone on top of an imbricate basement sheet and caused the development of the thinskinned subandean ranges farther east published radiometric data and subsidence history of the subandean foreland basin suggest that the interandean thrust was active between about 10 ma and 5 ma the subandean thrust is still active southward displacement transfer from the subandean thrust onto the interandean thrust and a southward dipping lateral ramp of the interandean thrust result in the gradual transition from the interandean zone north of 22 s to the cordillera oriental of northern argentina a similar but larger scale transfer may be involved in the southern termination of the thin skinned subandean ranges of argentina at 23 s shortening in the western part of the thrust belt is of the order of 80 90 km total shortening in the eastern andean thrust belt from the undeformed foreland to the eastern margin of the cordillera oriental is 140 150 km

late oligocene early miocene major tectonic crisis and related basins in bolivia recent advances in biochronology geochronology magnetostratigraphy structural geology and basin analysis make untenable the traditional correlation of the first major andeanage deformational episode in bolivia with the middle eocene incaic tectonic phase defined in central peru and instead demonstrate that the episode took place in late oligocene and early miocene time this major tectonic crisis resulted in contemporary development of the subandean external foreland basin and altiplano intermontane basin which were separated by the initiation of thrusting in the present cordillera oriental area the deformation suggests that the bolivian orocline began to develop at that time it is likely that this tectonic upheaval is genetically linked to the marked increase in rate of plate convergence produced by the contemporaneous breakup of the farallon plate

tectonic impact on the fluvial deposits of plio pleistocene himalayan foreland basin india abstract the himalayan foreland basin comprises a number of subbasins developed in various sedimentary environments during the plio pleistocene phases of himalayan orogeny this paper describes the evolutionary processes that led to the development of the himalayan foreland basin in the dehra dun and subathu subbasins the dehra dun subbasin essentially consists of conglomerate facies whereas subathu subbasin is primarily composed of sandstone mudstone intercalated with conglomerate the two subbasins were sites of braided river systems evidence from chronostratigraphy provenance and tectonic and climatic conditions suggests that differential tectonic movement sedimentation rate and basin subsidence coupled with climatic variation were responsible for the contrasting fluvial facies in these subbasins following the merger of the two subbasins at about 5 23 ma the conglomerate facies assemblage of the dehra dun subbasin passes laterally into a sandstone mudstone conglomerate facies assemblage of the subathu subbasin thickly bedded amalgamated sheet conglomerate of dehra dun subbasin has a high sedimentation rate 0 70 mm year between 5 23 and 4 8 ma with a slow rate of basin subsidence in contrast a slow sedimentation rate 0 39 mm year in coeval sediments of the subathu subbasin with relatively fast subsidence produced a succession of thick multistorey sandstone body with mature paleosols petrographic and clast composition data from dehra dun subbasin suggest that the detritus was derived from the pre tertiary lesser himalayan sequence lying in the north during an active tectonic phase of the main boundary thrust between 4 8 and 1 77 ma progradation of posttectonic conglomerate of the dehra dun subbasin continued during a quiescent phase of the main boundary thrust however as a consequence of partitioning of subathu subbasin along the intra foreland thrust interfingering of buff piedmont drainage and grey trunk drainage sandstones took place in contrast the syntectonic conglomerate of subathu subbasin originated from the uplifted proximal part of the foreland basin tertiary belt during the active phase of intra foreland thrust around 1 77 ma

sedimentary processes and facies of the waterlaid anvil spring canyon alluvial fan death valley california the anvil spring canyon fan of the panamint range piedmont in central death valley was built entirely by water flow processes as revealed by an analysis of widespread 2 to 12 m high stratigraphic cuts spanning the 9 7 km radial length of this 2 5 5 0 sloping fan two facies deposited from fan sheetfloods dominate the fan from apex to toe the main one 60 95 of cuts consists of sandy granular fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5 25 cm thick oriented parallel to the fan surface the other facies 0 25 of cuts comprises 10 to 60 cm thick wedge planar and wedge trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3 28 both facies are interpreted as resulting from rare sediment charged flash floods from the catchment and were deposited by supercritical standing waves of expanding sheetfloods on the fan standing waves were repeatedly initiated enlarged migrated and then terminated either by gradually rejoining the flood or by more violent breakage and washout the frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50 250 cm thick across the active depositional lobe of the fan erosional intensity during washout of the standing wave determined whether early phase backset bed deposits or washout phase sheetflood couplet deposits were selectively preserved in a given cycle two minor facies are also found in the anvil fan pebble cobble gravel lags 0 20 of cuts are present above erosional scours into the sheetflood couplet and backset deposits they consist of coarse gravel concentrated through fine fraction winnowing of the host sheetflood facies by sediment deficient water flows this reworking occurred during recessional flood stage or from non catastrophic discharge during the long intervals between major flash floods this facies is common at the surface giving rise to a braided stream appearance however it is stratigraphically limited present as thin continuous to discontinuous beds or lenses that bound 50 to 250 cm thick sheetflood sequences the other minor facies of the anvil fan consists of clast supported and imbricated thickly stratified pebbly cobbly boulder gravel present in narrow radially aligned ribbons nested within sheetflood deposits this facies is interpreted as representing deposition in the incised channel of the fan a subenvironment characterized by greater flow competence resulting from maintained depth from channel wall confinement and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel

oligocene miocene basin evolution in the northern altiplano bolivia implications for evolution of the central andean backthrust belt and high plateau the upper oligocene to lower miocene penas and aranjuez formations are exposed in north northwest trending outcrop belts of the central andean backthrust belt situated within the central andean plateau along the boundary between the northern altiplano and the eastern cordillera of bolivia sedimentary lithofacies analyses indicate that these coarse grained siliciclastic formations were deposited primarily in alluvial fan to braided fluvial environments an upsection change from principally fine grained sandstone to cobble conglomerate is consistent with increased proximity to the sediment source with time paleocurrent analyses reveal that flow was predominantly directed toward the west southwest away from cordillera real the elevated core of the eastern cordillera provenance data from conglomerate clast compositions and sandstone petrofacies suggest derivation from recycled quartz rich metasedimentary and sedimentary rocks from the paleozoic section in the eastern cordillera the paleoflow orientations sediment provenance and increased proximity of the sediment source suggest that deposition of the penas and aranjuez formations was related to surface uplift of the eastern cordillera relative to the altiplano growth strata observed in the aranjuez formation further indicate that shortening was synchronous with deposition probably in a hinterland basin new 40 ar 39 ar ages from a lowermost exposed igneous unit and interbedded ash fall tuff beds in the aranjuez and penas formations show that synorogenic sedimentation and fold thrust deformation in the frontal west southwestern zone of the central andean backthrust belt was concentrated during late oligocene early miocene time these age results are consistent with previous studies of east derived sedimentation in the altiplano and indicate regional uplift of the eastern cordillera at this time upsection trends in provenance data further suggest a progressively greater contribution from younger paleozoic strata possibly due to activation of new thrust faults during west southwestward propagation of the backthrust belt toward the altiplano such a progression of late oligocene early miocene shortening along the altiplano eastern cordillera boundary likely reflects significant crustal thickening potential isostatic uplift and initial topographic expression of the eastern margin of the central andean plateau

avulsions channel evolution and floodplain sedimentation rates of the anastomosing upper columbia river british columbia canada ages of channels of the anastomosing upper columbia river south eastern british columbia canada were investigated in a cross valley transect by c 14 dating of subsurface floodplain organic material from beneath levees the avulsion history within the transect was deduced from these data and morphological stages in channel development were recognized additionally floodplain sedimentation rates were established the new data demonstrate that the upper columbia river is a long lived dynamic anastomosing system results show that anastomosis at the study location has persisted since at least 2700 cal years bp with avulsions occurring frequently at least nine channels have formed in the studied cross valley transect within the past 3000 years channel lifetimes from formation to abandonment appear to be highly variable ranging from approximately 800 to 3000 years log jams provoking avulsions and or silting up of old channels are proposed as reasons for this variability long term average floodplain sedimentation rates appear to be significantly lower than previously proposed by smith 1983 int assoc sedimentol spec publ 6 155 168 a long term 4550 years average of 1 75 mm per year after compaction was based on c 14 dates whilea short term sedimentation rate of 0 8 mm was determined for a single relatively small seasonal flood in 1994 using sediment traps however short term sedimentation rates vary considerably over the floodplain with levees aggrading up to four times faster than floodbasins channels of the upper columbia river anastomosed reach follow a consistent pattern in their development with each stage being characterized by different morphology and processes channel evolution comprises the following succession 1 avulsion stage in which a crevasse splay channel deepens by scour and levee sedimentation 2 widening and deepening stage in which bank slumping and bed scouring dominates 3 infilling stage in which either channel narrowing bank accretion or channel shallowing bed accretion takes place and 4 abandonment stage in which the residual remnant channel is filled exclusively by silt clay and organic material vertical stacking super imposition of active channels on recent channel fill sand bodies is a notable feature of the upper columbia river which suggests that reoccupation of residual channels is a common process

controls on sedimentation in distal margin palaeovalleys in the early tertiary alpine foreland basin south eastern france the influence of palaeodrainage characteristics palaeogeography and tectonic setting are rarely considered as controls on stratigraphic organization in palaeovalley or incised valley systems this study is an examination of the influence of source region vs downstream base level controls on the sedimentary architecture of a set of bedrock confined palaeovalleys developed along the distal margin of the alpine foreland basin in south eastern france three distinct facies associations are observed within the palaeovalley fills fluvial facies association a is mainly dominated by poorly sorted highly disorganized clast to matrix supported cobble to boulder conglomerates that are interpreted as streamflood deposits facies association b comprises mainly yellow siltstones and is interpreted as recording deposition in an estuarine basin environment estuarine marine facies association c comprises interstratified estuarine siltstones and clean well sorted washover sandstones the sedimentary characteristics of the valley fill successions are related to the proximity of depositional sites to sediment source areas palaeovalleys located proximal to structurally controlled basement palaeohighs are entirely dominated by coarse fluvial streamflood deposits in contrast distal palaeovalley segments which are located several kilometres downstream contain successions showing upward transition from coarse fluvial facies into estuarine central basin fines and finally into estuarine marginal marine facies facies distributions suggest that the fluvial deposits form wedge shaped downstream thinning sediment bodies whereas the estuarine deposits form an upstream thinning wedge the vertical stacking of fluvial to estuarine to marginal marine depositional environments records the fluvial aggradation and subsequent transgression of relatively small bedrock confined river valleys which drained a rugged upland terrain facies geometries suggest that a fluvial sediment wedge initially prograded downvalley in response to high bed load sediment yields subsequently palaeovalleys became drowned during the passage of a marine transgression with the establishment of estuarine conditions initial fluvial aggradation and subsequent marine flooding of the palaeovalleys is a consequence of the interaction of high local rates of sediment supply and relative sea level rise driven by flexural subsidence of the basin

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

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

depositional subenvironments in an arid closed basin the wilkins peak member of the green river formation eocene wyoming u s a the wilkins peak member of the green river formation is a non marine dolomitic deposit that formed in an arid hydrologically closed basin a transect from the margin to the centre of the wilkins peak basin was examined in outcrop exposures in the south eastern corner of the basin and was supplemented with published core descriptions the rocks were divided into seven facies based upon lithology layering style and sedimentary fabrics 1 af facies quartzose conglomerates and sandstones representing sandy alluvial fans coming from the uinta mountains 2 sf facies quartzose peloidal dolomitic sandstones which represent sheet flood deposits extending basinward from the alluvial fans 3 dm facies irregularly laminated dolomitic mudstones with abundant mudcracks which represent sheet flood deposits on a subaerially exposed mudflat 4 el facies dolomitic mudstones with regular lamination and fewer but larger mudcracks representing sporadically exposed lake margins or shallow ponds 5 pl facies finely laminated oil shales and dolomitic mudstones with rare mudcracks which represent shallow perennial lakes 6 sm facies trona or halite deposits with dolomitic mudstone partings or dolomitic mudstones and sandstones disrupted by evaporites of varied morphology these represent a central salt pan and the surrounding saline mudflat 7 as facies arkosic sandstones siltstones and mudstones which represent braided river and more basinal deltaic plain deposits the five dolomitic facies form poorly defined cycles representing the intersection of sheet floods with a lake which transgressed and regressed in response to climatic variations there are at least forty 3 4 m thick cycles in the wilkins peak and one large scale cycle comprising the whole member which mimics the facies changes of the smaller cycles this is compatible with the transitions of the wilkins peak member to the tipton and laney members as climatic responses and not shifts in drainage the sheet flood sandstones actively aggrading subaerial mudflats and bedded evaporites of the wilkins peak are apparently unique within the green river formation they reflect the unusual provenance and hydrology of the wilkins peak and suggest chemical and sedimentary models established for the other green river members will not be applicable

late miocene northward propagation of the northeast pamir thrust system northwest china 2015 american geophysical union piggyback basins on the margins of growing orogens commonly serve as sensitive recorders of the onset of thrust deformation and changes in source areas the bieertuokuoyi piggyback basin located in the hanging wall of the pamir frontal thrust provides an unambiguous record of the outward growth of the northeast pamir margin in northwest china from the miocene through the quaternary to reconstruct the deformation along the margin we synthesized structural mapping stratigraphy magnetostratigraphy and cosmogenic burial dating of basin fill and growth strata the bieertuokuoyi basin records the initiation of the pamir frontal thrust and the takegai thrust 5 6ma as well as clast provenance and paleocurrent changes resulting from the pliocene to recent uplift and exhumation of the pamir to the south our results show that coeval deformation was accommodated on the major structures on the northeast pamir margin throughout the miocene to recent furthermore our data support a change in the regional kinematics around the miocene pliocene boundary 5 6ma rapid exhumation of ne pamir extensional domes coupled with cessation of the kashgar yecheng transfer system on the eastern margin of the pamir accelerated the outward propagation of the northeastern pamir margin and the southward propagation of the kashi atushi fold and thrust belt in the southern tian shan this coeval deformation signifies the coupling of the pamir and tarim blocks and the transfer of shortening north to the pamir frontal faults and across the quasi rigid tarim basin to the southern tian shan kashi atushi fold and thrust system

clastic deposition provenance and sequence of andean thrusting in the frontal eastern cordillera and llanos foreland basin of colombia sedimentological provenance and detrital thermochronological results for basin fill at the modern deformation front of the northern andes 6 n latitude provide a long term eocene to pliocene record of foreland basin sedimentation along the eastern cordillera llanos basin boundary in colombia lithofacies assemblages and paleocurrent orientations in the upward coarsening 5 km thick succession of the nunchia syncline reveal a systematic shift from craton derived shallow marine distal foreland back bulge accumulation in the mirador formation to orogen sourced deltaic and coastal influenced sedimentation of the distal to medial foreland foredeep in the carbonera and leon formations to anastomosing fluvial and distributive braided fluvial megafan systems of the proximal foreland foredeep to wedge top basin in the lower and upper guayabo formation these changes in depositional processes and sediment dispersal are supported by up section variations in detrital zircon u pb and u th he ages that record exhumation of evolving compartmentalized sediment source areas in the eastern cordillera the data are interpreted in terms of a progressive eastward advance in fold and thrust deformation with late eocene oligocene deformation in the axial zone of the eastern cordillera along the western edge of floresta basin soapaga thrust early miocene reactivation inversion of the eastern margin of the mesozoic rift system pajarito and guaicaramo thrusts and middle late miocene propagation of a footwall shortcut fault yopal thrust that created the nunchia syncline in a wedge top piggyback setting of the eastern foothills along the transition from the eastern cordillera to llanos foreland basin collectively the data presented here for the frontal eastern cordillera define a general in sequence pattern of eastward advancing fold and thrust deformation during cenozoic east west shortening in the colombian andes

distinguishing early groundwater alteration effects from pedogenesis in ancient alluvial basins examples from the palaeogene of southern portugal colour mottling and horizons of secondary carbonates are common in ancient alluvial sequences and are normally interpreted as pedogenic features they have been used to assess palaeoclimates soil drainage conditions and deposition rates palaeogene alluvial deposits in the sado and lisbon basins of portugal exhibit prominent colour variations and mottle patterns as well as carbonate accumulations both at the bases of fining upwards cyclothems and as thick units up to 20 m capping alluvial megasequences however these colour and carbonate features are interpreted as the products of shallow saline reducing groundwaters unrelated to pedogenesis such non pedogenic products are easily mistaken for soil formed ones and criteria for differentiating the two are reviewed to assist interpretations in other alluvial deposits key criteria are thickness gradational tops and bases absence of soil horizon features occurrence in coarser alluvium and prevalence of hydromorphic colour and mottling patterns

the late early miocene acipayam piggy back basin refining the last stages of lycian nappe emplacement in sw turkey abstract alluvial fan to shallow marine sedimentation in the acipayam piggy back basin took place in the hinterland zone of the se facing lycian nappe stack and constrains the age and style of late stage advance of these nappes syn orogenic sedimentation commenced in burdigalian time and is marked by coarse grained alluvial fan deposits that directly overlie mesozoic ophiolitic rocks palaeocurrent data obtained from these coarse clastic units suggest a south southeastward drainage the succession fines upwards and passes into fluvial deposits that become increasingly enriched in carbonate clasts derived from tectonically deeper seated nappes suggesting progressive unroofening of the nappe pile upwards the succession is transitional to a late burdigalian early langhian marginal marine succession consisting of off shore fines with coal seams that overly both the coarse clastic sediments and basement rocks clastic sedimentation ceased at the end of early langhian based on reef carbonates that cap the succession shallow marine deposits and overlying reef carbonates confirm a marine transgression that has been reported from elsewhere in the tauride belt detailed sedimentology of the acipayam basin fill succession its age and overall tectonic setting imply that during burdigalian time the succession was deposited in a piggy back basin located in the hinterland zone of actively moving thrust sheets we propose a model in which thick skinned thrusting invoked inversion of early extensional faults in the autochthon and may explain the generation of large open folds with thick sedimentary sequences in their core the formation of these large folds may have importantly influenced the frontal accretion such that active shortening moved into the hanging wall resulting in episodic nappe advanced of structurally higher nappes in a mode similar to thin skin thrusting the tectonosedimentary evolution of the acipayam piggy back is strongly dependent on its position relative to the hinge of the large open fold the absence of any compressional signatures in the basin fill succession indicates that these thrust translations ceased shortly after aquitanian time by serravallian the succession was interrupted and the region was uplifted to its present elevation probably by gravitationally isostatic orogenic rebound of the nappe stack after these episodes tectonic extension predominated and the succession became dissected by nappe front parallel normal faults that bound the present cameli basin

build up and depositional dynamics of an arc front volcaniclastic complex the miocene tepoztlan formation transmexican volcanic belt central mexico volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on earth ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment the early miocene tepoztlan formation at the southern edge of the transmexican volcanic belt belongs to the few remnants of this ancestral magmatic arc and therefore is thought to represent an example of the initial phase of evolution of the transmexican volcanic belt based on geological mapping detailed logging of lithostratigraphic sections palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility mapping of two dimensional panels from outcrop to field scale and geochronological data in an area of ca 1000 km 2 three periods in the evolution of the tepoztlan formation were distinguished which lasted around 4 myr and are representative of a volcanic cycle edifice growth phases followed by collapse in a magmatic arc setting the volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons similar to volcanic ring plains around at least three different stratovolcanoes these volcanoes resulted from various eruptions separated by repose periods during the first phase of the evolution of the tepoztlan formation 22ae 8t o 22 ae2 ma deposition was dominated by fluvial sediments in a braided river setting pyroclastic material from small andesitic dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes resulting in sediments ranging from cross bedded sand to an aggradational series of river gravels the second phase 22ae 2t o 21 ae3 ma was characterized by periods of strong volcanic activity resulting in voluminous accumulations of lava and tuff which temporarily overloaded and buried the original fluvial system with its detritus continuous build up of at least three major volcanic centres further accentuated the topography and in the third phase 21ae 3t o 18 ae8 ma mass flow processes represented by an increase of debris flow deposits became dominant marking a period of edifice destruction and flank failures

distribution of active faulting along orogenic wedges minimum work models and natural analogue abstract numerical 2 d models based on the principle of minimum work were used to examine the space time distribution of active faulting during the evolution of orogenic wedges a series of models focused on thin skinned thrusting illustrates the effects of arid conditions no erosion unsteady state conditions accretionary influx greater than erosional efflux and steady state conditions accretionary influx balances erosional efflux on the distribution of fault activity for arid settings a general forward accretion sequence prevails although a significant amount of internal deformation is registered the resulting fault pattern is a rather uniform spread along the profile under fixed erosional efficiency settings the frontal advance of the wedge front is inhibited reaching a steady state after a given forward propagation then the applied shortening is consumed by surface ruptures over a narrow frontal zone under a temporal increase in erosional efficiency i e transient non steady state mass balance conditions a narrowing of the synthetic wedge results a rather diffuse fault activity distribution is observed during the deformation front retreat once steady balanced conditions are reached a single long lived deformation front prevails fault activity distribution produced during the deformation front retreat of the latter scenario compares well with the structural evolution and hinterlandward deformation migration identified in southern bolivian subandes ssa from late miocene to present this analogy supports the notion that the ssa is not in steady state but is rather responding to an erosional efficiency increase since late miocene the results shed light on the impact of different mass balance conditions on the vastly different kinematics found in mountain ranges suggesting that those affected by growing erosion under a transient unbalanced mass flux condition tend to distribute deformation along both frontal and internal faults while others under balanced conditions would display focused deformation on a limited number of steady structures

late cenozoic uplift of the eastern cordillera bolivian andes abstract this paper analyses late cenozoic uplift in the bolivian andes using the morphology of well preserved regional paleosurfaces in the eastern cordillera that define three axially draining braided river catchments that formed between 12 and 9 ma rock uplift since the formation of the paleodrainage systems which has been quantified using four different methods is 1705 695 m with a mean erosion of 230 90 m as a consequence of entrenchment of the drainage systems the lack of faulting or tilting in the regions immediately farther west strongly suggest that rock uplift of the paleodrainage systems also extends to the western margin of the eastern cordillera balanced structural cross sections require crustal deformation at depth beneath the eastern cordillera in order to accommodate underthrusting of the brazilian shield beneath the thin skinned fold and thrust belt on the eastern margin of the bolivian andes in this case the observed uplift of the eastern cordillera is easily explained by sliding up a ramp in the major decollement dipping in the range 4 16 w as a consequence of the observed 60 110 km of shortening farther east in the subandean zone contemporaneous uplift of the western margin of the eastern cordillera is easily explained in terms of crustal thickening as a result of ductile squeezing in the lower crust accommodating at depth the subandean shortening it remains unclear how this uplift relates to that of the regions farther west in the altiplano and volcanic arc except that uplift in the eastern cordillera coincides with a phase of intense shortening in the northern altiplano commencing at 9 5 ma and continuing to 2 7 ma and possibly younger