the shape of mars and the topographic signature of the hemispheric dichotomy reanalysis of occultation data from the mariner 9 and viking orbiter spacecraft to determine the shape of mars indicated that the hemispheric dichotomy is not a fundamental feature of the shape of the planet it is a consequence of an approximately 3 kilometer offset between mars9s center of mass and center of figure and the boundary along most of its length consists of broad gradual surface slopes over distances of thousands of kilometers this result was supported by analysis of high spatial resolution earth based radar topographic profiles any successful model for the origin of the dichotomy must explain a planet with an ellipsoidal shape and a long wavelength gradual topographic transition between the northern and southern hemispheres

fractal analysis of venus topography in tinatin planitia and ovda regio we have applied a one dimensional fractal analysis to magellan altimetry data for two 20 20 equatorial regions on venus for the ovda regio highlands the power spectra of the altimetry data correlate well with power law fractal statistics over the full range of wavelengths considered 36 km 703 km the mean fractal dimension is dovda 1 64 for the tinatin planitia lowlands the power spectra consistently show a rollover at long wavelengths 200 km the long wavelength topography in this region either never developed or has subsequently relaxed the mean of the best fit fractal dimension for tinatin is dtinatin 1 41 the measure of roughness ra correlates well with variations in relief especially at short wavelengths 50 km our quantitative analysis of venus topography can be applied on a global scale the one dimensional method can be extended to two dimensional space yielding two dimensional maps of regional d and ra allowing a more complete use of regional two dimensional altimetry data

the rates of plate creation and consumption summary an alternative vector derivation of the expression for the rates of plate creation and consumption is given from which it can be seen that the rates are proportional to the scalar product of the edges of the geohedron and plate polyhedron corresponding to the same boundary the plate polyhedron is defined by the set of triple junction vectors this is used together with a recent determination of the angular velocity vectors and a number of auxiliary plate boundary vectors to revise the rates of plate creation and consumption along sections of plate boundaries all plates which have a substantial proportion of their boundary connected to a downgoing slab are found to be decreasing in area the total rate of plate generation is used in estimates of the area of the ocean floor and the total heat loss from the oceans the rate of consumption within a relatively small area around new guinea implies a large mass flux away from this region knowledge of the rates of plate creation and consumption is used in a number of different areas calculating the contribution of plate creation to the total heat loss of the earth calculating geochemical fluxes out of the mantle such as for the rare gases and other volatiles and return fluxes back into the mantle resulting from reactions between sea water and the ocean crust during hydrothermal circulation calculating the area of the ocean floor as a function of age mass fluxes related to the distribution of the rates of creation and destruction along the plate boundaries are important boundary conditions on the flow pattern in the mantle previous estimates of these rates deffeyes 1970 chase 1972 garfunkel 1975 have applied a simple expression derived for spreading ridges deffeyes 1970 using angular distances from the poles of rotation measured graphically on a globe an alternative derivation of the expression for rates of plate creation and consumption which is independent of assumptions about the nature of the boundary is given below in terms of the framework used to describe plate motions by mckenzie parker 1967 1974 this derivation emphasizes that the rates depend on two sets of vectors one the instantaneous rotation vectors defining the relative plate motions the other a set of triple junction and auxiliary vectors defining the plate boundaries

hypsometry of titan cassini radar topography data are used to evaluate titan s hypsometric profile and to make comparisons with other planetary bodies titan s hypsogram is unimodal and strikingly narrow compared with the terrestrial planets to investigate topographic extremes a novel variant on the classic hypsogram is introduced with a logarithmic abscissa to highlight mountainous terrain in such a plot the top of the terrestrial hypsogram is quite distinct from those of mars and venus due to the glacial buzz saw that clips terrestrial topography above the snowline in contrast to the positive skew seen in other hypsograms with a long tail of positive relief due to mountains there is an indication weak given the limited data for titan so far that the titan hypsogram appears slightly negatively skewed suggesting a significant population of unfilled depressions limited data permit only a simplistic comparison of titan topography with other icy satellites but we find that the standard deviation of terrain height albeit at different scales is similar to those of ganymede and europa

marine incursions and the influence of andean tectonics on the miocene depositional history of northwestern amazonia results of a palynostratigraphic study abstract new palynological and sedimentological data permit the age of neogene sediments in northwestern amazonia to be established more precisely and indicate that major environmental changes occurred in the area during this time based on a study of borehole samples the age of the solimoes formation solimoes basin northwestern brazil is determined as miocene and five pollen zones are distinguished which are correlated with existing zonations for northern south america in addition 30 new sporomorphs are described which belong to the following 12 genera psilamonocolpites retimonocolpites retitricolpites retibrevitricolpites psilatriporites nov gen bombacacidites psilatricolporites retitricolporites rugutricolporites psilastephanoporites nov gen psilastephanocolporoties and heterocolpites a correlation is made between the brazilian wells and some of the studied outcrops in colombian and peruvian amazonia based on palynological marker species the presence of coastal elements such as mangroves zonocostites indicates that during the miocene this area was influenced by marginal marine conditions caused by several marine incursions these incursions may be related to global sea level fluctuations miocene marine phases also are known from sites elsewhere in northern south america which at present like northwestern amazonia are entirely ruled by continental conditions sediment composition shows that during the early miocene the guyana shield was the major source area of sediment input in the basins of northwestern amazonia in the interval from early to middle miocene the andes became the major sediment source area the change in provenance is related to the uplift of the eastern cordillera this event caused a major change in palaeoenvironment and palaeogeography in northwestern amazonia which was characterized by the reverse of a northwestward directed fluvial system into an eastward directed fluvial lacustrine system with an estuarine character

biomarker stratigraphy of the lower cretaceous of espirito santo basin brazil abstract this study involves the use of molecular parameters steranes and terpanes and carbon isotopes as stratigraphic correlation markers in a lower cretaceous lacustrine sequence from the espirito santo basin southeast brazil the biomarker variations revealed changes in sedimentary environment that had not been detected by sedimentological paleontological and seismistratigraphic methods additionally the observed variations assumed to have been contemporary for the whole basin could also be used as chronostratigraphic markers the carbon isotope data led to the recognition of two sedimentary sequences the upper sequence 13 c around 26 begins with very low amounts of gammacerane and of a novel c24 tetracyclic alkane apparently with a des a hopane structure a sharp increase in concentration of the c24 compound divided the upper sequence into two stratigraphic subunits the second of which shows an increasing gammacerane concentration with depth the lower sequence 13 c around 28 starts with a distinct drop in abundance levels of the c24 tetracyclic alkane and of methyl steranes gammacerane concentrations attain a maximum at the top of this sequence showing subsequently a decreasing trend with depth but remaining reasonably high throughout the sequence however a second sharp decrease of the c24 tetracyclic alkane shows a modification in sedimentary environment downwards combined all these stratigraphic changes delimitate four depositional cycles which could be easily recognized in wells from the eastern part of the basin showing gradual erosion updip to the west such sedimentary changes could be correlated with independent geochemical information on neighbouring lacustrine sequences of the same age suggesting a dominant freshwater influence in the basal part of the sequence contrasted with a more saline environment in the upper levels the use of these markers has been of great value in redefining stratigraphic correlations in the espirito santo basin and may prove useful to resolve similar difficulties in vicinal areas

venus timing and rates of geologic activity we employ the accumulation of the impact crater population as a geochronometer to examine the observable record of the history of venus 1 b y analyzing age relationships between craters and the different geologic units we show that the earlier suite of units from heavily deformed tesserae through slightly deformed regional plains occurred during a time period an order of magnitude shorter than the subsequent period from the end of emplacement of the wrinkle ridge network until the present these results imply high global rates of endogenic volcanic activity during the first era comparable to that of mid oceanic ridge volcanism of earth and much lower global rates of endogenic activity by two orders of magnitude for the second period

coastal geomorphology of the martian northern plains the paper considers the question of the formation of the outflow channels and valley networks discovered on the martian northern plains during the mariner 9 mission parker and saunders 1987 and parker et al 1987 1989 data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains it is suggested that of the geological processes capable of producing similar morphologies on earth lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on mars

topographic comparisons of uplift features on venus and earth implications for venus tectonics abstract venus and earth display different hypsography we use topographic profiles to search for well understood terrestrial analogs to venusian features specifically by using cross correlation we correlate average profiles for terrestrial rifts slow and fast ultra slow incipient and inactive and also hotspots oceanic and continental with those for venusian chasmata and regiones to draw inferences as to the processes responsible for shaping venus surface correlations tend to improve with faster spreading rates venus correlations rank considerably lower than terrestrial ones suggesting that if chasmata are analogous to terrestrial spreading centers then spreading on venus barely attains ultra slow rates individual features normalized average profiles are correlated with profiles of other such features to establish the degree of similarity which in turn allows for the construction of a covariance matrix principal component analysis of this covariance matrix shows that yellowstone more strongly resembles atla beta and w eistla regiones than it does the terrestrial oceanic hotspots and that venusian chasmata especially ganis most closely resemble the ultra slow spreading arctic ridge

simulating the thermochemical magmatic and tectonic evolution of venus s mantle and lithosphere two dimensional models 1 numerical convection models of the thermochemical evolution of venus are compared to present day topography and geoid and recent resurfacing history the models include melting magmatism decaying heat producing elements core cooling realistic temperature dependent viscosity and either stagnant lid or episodic lithospheric overturn in stagnant lid convection the dominant mode of heat loss is magmatic heat pipe which requires massive magmatism and produces very thick crust inconsistent with observations partitioning of heat producing elements into the crust helps but does not help enough episodic lid overturn interspersed by periods of quiescence effectively loses venus s heat while giving lower rates of volcanism and a thinner crust calculations predict 5 8 overturn events over venus s history each lasting 150 myr initiating in one place and then spreading globally during quiescent periods convection keeps the lithosphere thin magmatism keeps the mantle temperature constant over venus s history crustal recycling occurs by entrainment in stagnant lid convection and by lid overturn in episodic mode venus like amplitudes of topography and geoid can be produced in either stagnant or episodic modes with a viscosity profile that is earth like but shifted to higher values the basalt density inversion below the olivine perovskite transition causes compositional stratification around 730 km breakdown of this layering increases episodicity but far less than episodic lid overturn the classical stagnant lid mode with interior temperature rheological temperature scale lower than tcmb is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start core heat flow decreases with time possibly shutting off the dynamo particularly in episodic cases

d rossetti p mann de toledo a m goes new geological framework for western amazonia brazil and implications for biogeography and evolution quaternary research 63 2005 78 89 letter to the editor

provenance of quaternary and modern alluvial deposits of the amazonian floodplain brazil inferred from major and trace elements and pb nd sr isotopes abstract chemical and pb nd and sr isotope data from floodplain sedimentary deposits in the solimoes river northwest brazil constrain the variability of the deposits through the quaternary and help reconstruct the paleogeography of amazonia since the uplift of the andes compared to the modern alluvial deposits in the active channel the quaternary terrace deposits in the solimoes river have a higher chemical index of alteration cia higher sio2 al2o3 ratios higher concentrations of immobile elements higher radiogenic sr isotopic compositions more negative end t 0 values and older nd tdm ages these geochemical characteristics indicate that the quaternary sediment sources were composed by more felsic and or recycled old continental crust derived rock materials than the younger source areas which contributed major arc volcanic materials to the flood plain deposits this shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the solimoes river has occurred since the uplift of the andes

effects of crustal heterogeneity on the morphology of chasmata venus 1 the ix chel kuanja and vir ava chasmata ikvc are a 7000 km long collection of discontinuous topographic troughs and deformation suites collectively they account for approximately 12 5 of the 55 000 linear km of chasmata on venus however morphologically the ikvc differs significantly from the majority 75 of venusian chasmata because it lacks corona features coronae represent notable volumes of material both erupted onto the surface and intruded into the crust together with chasmata coronae may also be important indicators of venus s interior convective processes as well as significant contributors to venus s heat loss although ikvc lacks coronae detailed geologic and structural mapping along its length highlights previously unrecognized structural and volcanic relationships indicative of local uplift collapse and volcanism we call pseudocoronae in light of this mapping previous corona formation models theoretical and numerical are evaluated and an amended hypothesis diapir stagnation is presented for pseudocorona formation the diapir stagnation model proposes that variations of the crustal thickness perturb corona formation and thus affect their resultant surface morphology chasmata are accompanied by corona development when forming in regions of relatively thin or normal plains type crust 20 km but lack corona features when influenced by crustal heterogeneities mechanical and chemical at depth associated with thick crustal plateau roots 35 80 km e g ovda thetis and phoebe regios beneath crustal plateaus diapiric rise is inhibited by 1 the presence of a low density crustal keel that creates a deep seated density trap and 2 a composition dependent mechanically weak crust mantle boundary layer diapir stagnation can be characterized or accompanied by minimal broad doming of the surface formation of radial and concentric extension structures lateral dike propagation and collapse all of which may be accompanied by limited volume surface eruptions of the lowest density melt phases through time the crust mantle boundary layer may repeatedly lift and subside through conductive heating and viscous flow which may allow more voluminous melt migration and eruption however true coronae fail to develop

the origin and fate of 4 methyl steroids ii dehydration of stanols and occurrence of c30 4 methyl steranes abstract certain features of the distributions of sedimentary 4 methyl steroid hydrocarbons have been investigated i competitive dehydration experiments with stanols adsorbed on a shale suggests a possible explanation for the low abundance of 4 methyl sterenes relative to their desmethyl counterparts in immature sediments ii a 4 methyl spirostene which is a minor product of the backbone rearrangement of 4 methylcholest 4 ene in the laboratory has been tentatively assigned its presence in a toarcian black shale with a second isomer provides further evidence that the sedimentary backbone rearrangement of 4 methyl sterenes is analogous to that of their desmethyl counterparts and iii the major c30 4 methyl steranes in semecourt shale toarcian paris basin differ from those in an eocene lacustrine shale from china this indicates the potential complexity of c30 steroidal alkanes both 4 methyl and desmethyl which can occur in sedimentary organic matter and shows that care must be taken in using m z 217 mass chromatograms alone for routine input and maturity assessment using steroidal alkanes

possible ancient oceans on mars evidence from mars orbiter laser altimeter data high resolution altimetric data define the detailed topography of the northern lowlands of mars and a range of data is consistent with the hypothesis that a lowland encircling geologic contact represents the ancient shoreline of a large standing body of water present in middle mars history the contact altitude is close to an equipotential line the topography is smoother at all scales below the contact than above it the volume enclosed by this contact is within the range of estimates of available water on mars and a series of extensive terraces parallel the contact in many places

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

correlation of venus surface features and geoid abstract the distributions of pairs of features on venus are compared using simple statistical tests we consider coronae types crater types and chasmata some of these are obviously associated and some are possibly associated coronae are grouped to represent evolutionary stages tectonic and volcanic modification of craters is considered and the chasmata are represented by five great circle arcs these arcs fit rift or extension zones at the 90 level totaling 55 000 km comparable to the length of spreading ridges on earth comparing these sets with venus geoid field we find that all the chasmata correlate positively with geoid highs and negatively with lows with the peaks in the geoid field occurring at the intersections of the arcs modified craters also exhibit this relation to the geoid the domal coronae the youngest and currently active may be sites of upwelling these correlate strongly with geoid highs and negatively with geoid lows whereas later stage coronae the majority do not show this correlation strong association of the youngest coronae with themis atla suggests that the most recent activity occurs along this arc

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

terrigenous sediment on ceara rise a cenozoic record of south american orogeny and erosion abstract ceara rise located east the amazon river mouth is covered with a thick blanket of pelagic carbonate and hemipelagic terrigenous sediment the terrigenous component has been extracted from 57 bulk sediment samples at ocean drilling program odp sites 925 and 929 on ceara rise to obtain a cenozoic record of riverine discharge from northern south america from the early eocene to early miocene 55 20 ma terrigenous accumulation was dominated by moderate amounts of generally large grained gray to green sediment especially depleted in elements that are enriched in post archaean shale e g cs th yb however pulsed inputs of relatively small grained gray to green terrigenous sediment less depleted in the above elements occurred in the late eocene and oligocene the accumulation of terrigenous sediment decreased significantly until 16 5 ma in the middle miocene 16 5 13 ma terrigenous accumulation was dominated by small amounts of small grained tan sediment notably depleted in na and heavy rare earth elements the accumulation rate of terrigenous sediment increased markedly from the latest miocene 10 ma to the present day a change characterized by deposition of gray green sediment enriched in elements that are enriched in post archaean shale observed changes in terrigenous sediment at ceara rise record tectonism and erosion in northern south america the brazil and guyana shields supplied sediment to the eastern south american margin until the middle miocene 20 16 5 ma when a period of thrusting shortening and uplift changed the source region probably first to highly weathered and proximal phanerozoic sediments by the late miocene 9 ma there was a transcontinental connection between the andes and eastern south america weathering products derived from the andes have increasingly dominated terrigenous deposition at ceara rise since the late miocene and especially since the late pliocene

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

on the relation between transform fault resistance and plate motion a simple model for power dissipation due to friction along transform faults is used to examine the relations between relative velocity and power dissipation across transforms transform lengths and plate motions comparison is made between ridge transforms i e those that are part of spreading systems and major boundary transforms such as the san andreas fault power dissipation is calculated using velocity across the transform area above the 400 c oceanic or 350 c continental isotherm transform thermal structure is modeled by averaging the temperatures on either side of the transform whereas ridge transforms across which the relative plate velocity is slow can be of any length fast relative velocities only appear across short transforms when all transforms are used in this comparison a maximum speed as a function of transform length is suggested a minimum speed of 13 5 mm yr across ridge transforms was found in a case study of the pacific plate the transform lengths were found to have varied with time which has minimized the total power dissipation across the plate s transforms

holocene flooding and species diversity in southwestern amazonia abstract a thick blanket of holocene alluvium lies over southwestern lowland amazonia and may possibly occur throughout much of the amazon basin these deposits resulted from massive seasonal flooding from about 11 000 to about 5000 yr b p that was followed by two cycles of erosion and deposition interpretations based on these geologic data suggest that southwestern lowland amazonia is ecologically an island in a state of supersaturated disequilibrium as a result of colonization from pleistocene refugia on its perimeter and that habitats of highest diversity may be the most recent in origin conservation efforts and studies of amazonian biogeography soils and paleoethnography should be reevaluated in light of the geologic data

climate and tectonic influences on continental erosion of tropical south america 0 13 ma is tectonic uplift or climate change the primary driving force behind continent scale erosion here we examine these two hypotheses for tropical south america by comparing a high resolution record of terrigenous sediment accumulation rates from the western tropical atlantic ocean with continental tectonic reconstructions and both low latitude and high latitude climate indices that span the past 13 m y over the full record terrigenous sedimentation rates most closely follow lowland aridity as recorded by abundance ratios of terrigenous oxide minerals an indication that basin integrated climate is an important control of erosion step like transitions in the flux and composition of terrigenous material near 8 and 4 5 ma however follow dated tectonic events which suggests progressive expansion of the drainage basin into less humid or less vegetated regions that were more prone to erosion on long time scales high latitude climate variability and sea level as recorded by 1 8 o do not closely track terrigenous sedimentation we conclude that regional climate change provides a mechanism for long term erosion of tropical south america tectonics plays a significant but secondary role in this area perhaps by shaping the drainage basin to include regions in which the climate is more favorable to erosion

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

new geological framework for western amazonia brazil and implications for biogeography and evolution abstract although many of the current hypotheses to explain the origin and distribution of the amazon biodiversity has been based directly or indirectly on geological data the reconstruction of the geological history of the amazon region is still inadequate to analyze its relationship with the biodiversity this work has the main goal to characterize the sedimentary successions formed in the brazilian amazon in the neogene quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap radar image interpretation sedimentological studies and radiocarbon dating allowed the mapping of plio pleistocene to holocene units along the solimoes amazonas river brazil this integrated work led to the characterization of five sedimentary successions overlying miocene deposits of the solimoes pebas formation which include the following ica formation plio pleistocene deposits q1 37 400 43 700 14c yr b p deposits q2 27 200 14c yr b p deposits q3 6730 2480 14c yr b p and deposits q4 280 130 14c yr b p these deposits occur mostly to the west of manaus forming nw se elongated belts that are progressively younger from sw to ne indicating a subsiding basin with a depocenter that migrated to the ne the reconstruction of the depositional history is consistent with significant changes in the landscapes hence the closure of a large lake system at the end of the miocene gave rise to the development of a plio pleistocene fluvial system this was yet very distinct from the modern drainage with shallow energetic highly migrating braided to anastomosed channels having an overall northeast outlet this fluvial system formed probably under climatic conditions relatively drier than today s during the early pleistocene there was pronounced erosion followed by a renewed depositional phase ca 40 000 14c yr b p with the development of prograding lobes and or crevasse splays associated with a lake system i e fan delta and or fluvial flood plain areas after a period of erosion a fluvial system with eastward draining channels started to develop at around 27 000 14c yr b p the fluvial channels were overflooded in mid holocene time this flooding is attributed to an increased period of humidity with a peak between 5000 and 2500 14c yr b p the data presented herein support that rather than being a monotonous area the amazonia was a place with frequent changes in landscape throughout the neogene quaternary probably as a result of climatic and tectonic factors we hypothesize that these changes in the physical environment stressed the biota resulting in speciation and thus had a great impact on modern biodiversity