Middle Amazon Basin Research Articles

Lateral variation of crustal properties from aerogeophysical data in northern Brazil

A new method is proposed for the study of crustal azimuthal anisotropy, and the lateral variation of other crustal aerogeophysical parameters, using a quantitative approach. The processing of a large, newly acquired, high-resolution set of aerogeophysical data is considered using the existing Radon transform and the geostatistical analysis approaches. The data set includes an area of the Guyana shield, in northern Brazil, which was not included in previous surveys. The area is covered by dense rain forest vegetation and thick soil layers. Parameterization was performed considering the possible anisotropic character of the geophysical 2D data. Application of the newly proposed geostatistical data processing yielded high-resolution images of the lateral variation of quantitative geophysical parameters, which indicate good correlation with previously determined seismic anisotropy in the area. Average anisotropy as measured by the Radon transform and variogram analysis is scale dependent. At scales greater than [Formula: see text], the results seem to match those of the previous seismic studies. Images of the derived quantitative parameters from magnetic and radiometric data in the shield area indicate sharp changes that follow the known geologic changes observable in the survey area. Observed large-scale anisotropy in the area seems to be autocorrelated with two different geochronological provinces of the Amazonian craton. Application of the new technique to two other lower resolution aeromagnetic surveys located on the Amazon basin has produced two different results, including one successful data treatment of the aeromagnetic survey. The results obtained for this area indicate that the sedimentary cover seems to play almost no role in the observed magnetic anomaly anisotropy in the middle Amazon basin area. Most of the observed magnetic anomalies appear to be controlled by the continuing geologic features of the cratonic basement.

Caracterização e classificação de gleissolos da várzea do rio Solimões (Manacapuru e Iranduba), Amazonas, Brasil

As várzeas do rio Solimões são áreas inundáveis com predomínio de Gleissolos, onde se estabeleceu grande parte da população rural do Estado do Amazonas. Os objetivos deste trabalho foram caracterizar e classificar cinco perfis de Gleissolos, distribuídos em três áreas da bacia do Médio Amazonas, localizados nas várzeas dos municípios de Manacapuru e Iranduba. Após a descrição morfológica dos solos, coletaram-se amostras dos seus horizontes para caracterização física, química e mineralógica. Similarmente, todos os perfis apresentaram expressiva gleização na matriz do solo, com cores cinzenta a cinzenta-esverdeada, nos horizontes mais superficiais, e cinzenta-clara a cinzenta-escura, nos subsuperficiais, denotando a redução do ferro. Os teores mais elevados da fração areia em horizontes de subsuperfície indicaram presença de diferentes camadas de deposição fluvial, enquanto os elevados teores de silte evidenciaram a natureza sedimentar recente e o baixo grau de desenvolvimento pedogenético desses solos. Os cátions predominantes nos solos foram Ca2+ e Mg2+, que apresentaram porcentagens de sódio trocável (PST), com amplitude de variação entre 0,70 e 2,09. Os valores de carbono orgânico encontrados foram menores de 80 g kg-1, apresentando a natureza mineral dos sedimentos recém-depositados. A composição mineralógica da fração argila revelou presença significativa de argilominerais 2:1, mas sem grande variabilidade entre os perfis. Esses resultados refletiram-se em elevada soma e saturação por bases, caracterizando solos eutróficos e com argila de atividade alta.

A New Species of Freshwater Crab of The Genus Fredius Pretzmann, 1967 from The Middle Amazon River Basin, Brazil (Crustacea: Decapoda: Pseudothelphusidae)

A new species of pseudothelphusid crab, Fredius ykaa, from the Rio Marau drainage, in the middle Amazon basin, state of Amazonas, Brazil, is described and illustrated. The new species is characterized by a first gonopod with a straight border between the cephalic spine and the apex, as well as by a highly reduced mesial lobe relative to the size of the cephalic spine. The new species is compared with four other species of the genus Fredius Pretzmann, 1967, occurring in the Brazilian Amazon region and its geographical distribution is discussed.

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Inheritance vs. neoformation of kaolinite during lateritic soil formation: A case study in the middle Amazon basin

AbstractThe tropical weathering of sedimentary kaolin deposits from the plateaux surrounding Manaus (Alter do Chao formation, Amazon basin, Brazil) leads to the in situ formation of thick kaolinitic soils. The structural changes of kaolinite have been investigated quantitatively by infrared spectroscopy and electron paramagnetic resonance. Both techniques consistently show that each sample contains two types of kaolinite in various proportions. The progressive decrease in kaolinite order from the bottom to the top of the profile results from the gradual replacement of an old population of well-ordered kaolinite, typical of the underlying sedimentary kaolin, by a more recent generation of poorly ordered soil kaolinite. The vertical pattern of kaolinite replacement differs from that of the transformation of Fe oxides and oxyhydroxides previously observed in the same profile. The inherited fraction of well-ordered kaolinite ranges from 60% at a depth of 9 m to 30% in the upper levels of the soil. The persistence of sedimentary kaolinite in the upper horizons suggests that the rate of kaolinite transformation is relatively slow at the time scale of lateritic soil formation. Kaolinite inheritance unlocks the lateritic record of past weathering conditions.

Systematic review of the neotropical shovelnose catfish genus Sorubim Cuvier (Siluriformes: Pimelodidae)

The genus Sorubim Cuvier, 1829, and two species, S. lima (Bloch & Schneider 1801) and S. trigonocephalus Miranda- Ribeiro, 1920, are redescribed based on the examination of type material and about 400 non-type specimens from ichthyological collections in Europe and North and South America. The five species of the genus are, in addition to S. lima and S. trigonocephalus, S. elongatus Littmann, Burr, Schmidt and Isern 2001, found in the Essequibo, Orinoco, and Amazon basins; S. cuspicaudus Littmann, Burr and Nass 2000, occurring in the Sinu, Cauca, and Magdalena rivers of Colombia and the Lago Maracaibo basin of Colombia and Venezuela; and S. maniradii Littmann, Burr and Buitrago-Suarez 2001, known from the upper and middle Amazon basin. Sorubim lima is the widest-ranging species, occurring in most of the major drainage basins of South America. Sorubim trigonocephalus is extremely rare in natural history collections and is currently known from only two major tributaries of the Amazon basin. Three species (S. maniradii, S. elongatus, and S. lima) occur syntopically. Species of Sorubim are diagnosed on the basis of body and head shape, differences in fin ray and gill-raker numbers, mental barbel position, and degree of pigmentation of the lateral stripe. At least two of the species (S. lima and S elongatus) make up part of the ornamental fish trade and are sold for food in local fish markets in South America. A key to adults of the five species is included.

Physico-Chemical Properties and Their Variation of Amazonian Dark Earths Distributed in Middle Amazon Basin, Brazil

Physico-Chemical Properties and Their Variation of Amazonian Dark Earths Distributed in Middle Amazon Basin, Brazil

Formation and evolution of lateritic profiles in the middle Amazon basin: Insights from radiation-induced defects in kaolinite

The content of radiation-induced defects (RIDs) in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle Amazon Basin (Brazil) is investigated using electron paramagnetic resonance. The paleodose registered by kaolinites ranges from 80 to 900 kGy. Present-day dose rates of radiation, determined from the whole-rock U and Th content, range between 4000 and 40,000 mGy/ka. In most samples, U and Th concentrations are correlated, suggesting that U has not been remobilized by lateritization. This observation is consistent with the fact that ∼80% of the total U content is incorporated in resistant minerals, such as zircon and Ti oxides. The heterogeneous distribution of U, observed by induced fission tracks mapping, makes it possible to neglect the α-radiation contribution of the U decay chains in the dose-rate calculation. The interpretation of the measured content of RIDs in kaolinite is then performed using the calculated present-day dose rate and assuming equilibrium in the radioactive decay chains. For the sedimentary samples, the amount of RIDs is broadly correlated to the dose rate and provides apparent absolute ages older than 20 Ma. The RID contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 Ma. The high RID content of these lateritic kaolinites shows that their chemical, isotopic, and crystallographic properties are not representative of present-day weathering conditions. Models assuming the “dynamical equilibrium” of kaolinites with local physical-chemical conditions prevailing in lateritic soils are thus questionable. Alternatively, our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo-climates.

Transformation of haematite and Al‐poor goethite to Al‐rich goethite and associated yellowing in a ferralitic clay soil profile of the middle Amazon Basin (Manaus, Brazil)

SummaryThe red and yellow colours of ferralitic soils in the tropics have for long intrigued pedologists. We have investigated the upward yellowing in a 10‐m thick profile representative of the Ferralsols of the plateaux of the Manaus region of Brazil. We determined changes in the nature and crystal chemistry of their Fe oxides by optical and Mössbauer spectroscopy as well as Rietveld refinement of X‐ray diffraction patterns. We attribute the upward yellowing of the soil to a progressive transformation of the Fe oxides at nearly invariant iron contents. Aluminium in contrast is strongly mobilized in the uppermost clay‐depleted topsoil where there is preferential dissolution of kaolinite and crystallization of gibbsite. Haematite decreases from 35 to 10% of the Fe oxides from the bottom to the top of the profile and the particles become smaller (75–10 nm). Its Al for Fe‐substitution remains almost unchanged (10–15 mol %). The average Al‐substitution rate of goethite increases from 25 to 33 mol %, and its mean crystal diameter remains in the range 20–40 nm. The proportion of Al‐rich goethite (33 mol %) increases at the expense of less Al‐substituted Fe oxides (haematite and goethite). This conversion with restricted transfer of iron means that the amount of Al stored in Fe oxides gradually increases. Kaolinite, haematite and Al‐poor goethite are thus witnesses of earlier stages of ferralitization of the soil. In contrast, Al‐rich goethite and gibbsite initiate the alitization (or bauxitization) of the soil. They correspond to the last generation of soil minerals, which most likely reflects the present‐day weathering conditions. The progressive replacement of kaolinite, haematite and Al‐poor goethite by new generations of Al‐rich goethite and gibbsite attests to greater activities of water and aluminium and smaller activity of aqueous silica in the topsoil than in the subsoil. We interpret this as a consequence of longer periods of wetting in the topsoil that could result from soil aging, more humid climate or both.

Stress orientations in Brazilian sedimentary basins from breakout analysis: implications for force models in the South American plate

Regional patterns of crustal stresses in Brazil were studied with a detailed breakout analysis performed in 541 wells distributed throughout the country: 481 from basins along the continental margin, and 60 from intracratonic basins. A total of about 591 km of cumulative well length, digitally sampled at 0.15 m intervals, was analysed. Most intervals (80 per cent) are confined between −400 and −4000 m. Only wells that deviate less than 10° from the vertical were considered. For a given well (1) the mean orientation and the standard deviation of each interval with breakouts were calculated, (2) all intervals with mean breakout orientations less than 10° from the orientation of the hole deviation were discarded, (3) all intervals with standard deviations less than 12° were selected, (4) the selected breakouts were weighted by their interval length to calculate the mean orientation and standard deviation of the whole well, and (5) the final results were classified according to the World Stress Map criteria. Thus, 16 wells were classified as A (a total of 7.2 km of breakouts), 42 as B (7.1 km), 78 as C (7.4 km), 205 as D (3.3 km), 184 as E (0.7 km) and 16 were discarded. In most basins, the breakout orientations from different wells were usually consistent for qualities A, B and C, allowing a good estimate of the regional maximum horizontal stress (SHmax); wells classified as quality D showed a large scatter. The regional SHmax determined from breakouts is generally in good agreement with the available nearby focal mechanisms. The main trends of SHmax are (1) NW-SE, parallel to the coast, along the equatorial marginal basins, (2) E-W in the Alagoas Basin, (3) NNE-SSW, parallel to the coast, in the Sergipe and Reconcavo basins, and (4) NW-SE in the Middle Amazon Basin. In the equatorial and eastern continental margins, north of 15° S, breakouts and focal mechanism measurements indicate that the SHmax orientation is remarkably parallel to the coastline, following a 90° bend of the coast in northeastern Brazil. Because the theoretically predicted intraplate stresses in northern Brazil trend about WNW to ENE (e.g. Meijer 1995; Coblentz & Richardson 1996), we interpret our observations as indicating that local sources of stress at the continental margins (e.g. flexural stresses and lateral density contrasts) dominate the plate-wide stresses and may have been underestimated in theoretical stress models of the South American plate. The new stress data (136 A-C quality out of a population of 541 wells) cover a region of mid-plate South America where only 23 data points had been compiled previously in the World Stress Map database. The more detailed observed patterns of the intraplate stress field should be helpful in better constraining future models of the forces driving the South American plate.

The regional intraplate stress field in South America

A compilation of lithospheric stress directions for continental South America and the inferred major patterns of the regional intraplate stress field are presented. Stress orientations are based primarily on earthquake focal mechanisms and Quaternary fault slip inversion published in the literature. Four new focal mechanisms based on short‐period P wave modeling, and selected centroid‐moment‐tensor solutions (published by U.S. Geological Survey) consistent with P wave first motions at South American and other World‐Wide Standard Seismograph Network stations are also included in the data base. The observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the South American plate. In the Andean plateau (altitudes greater than 3000 m), N‐S extensional stresses predominate. E‐W compressional stresses are observed in the sub‐Andean and platform regions extending up to about 1000 km east of the Andes. The maximum horizontal stress (SHmax) is uniformly oriented in the E‐W direction throughout western South America. Averages of the SHmax direction at grid points spaced 2.5° were taken as representing the “regional” field. The E‐W direction of this regional field is not affected by the change of strike of the Andean chain nor by the contact of a flat subducted slab underneath. The regional SHmax direction is oriented 15° more clockwise than the direction of the Nazca plate convergence. The difference between the regional SHmax orientations and the absolute plate motion may be only about 6°. This may indicate that contact forces with the Nazca plate may not be the only major contributor to the intraplate stresses in western South America. The eastern limit of this Andean stress province seems to coincide with aseismic regions in the Upper Amazon basin and in the Parana basin. In the central Amazonian region, seismicity and stress data suggest a seismic province with N‐S compressional stresses. The origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle Amazon basin. In northeastern Brazil, seismicity is characterized by upper crustal strike‐slip earthquakes bordering the Potiguar marginal basin. A model is proposed for this region in which the stress field is the result of a superposition of regional E‐W compressional stresses and local extensional stresses (oriented perpendicular to the continental margin) possibly related to density contrasts and sediment loading in the continental shelf.

Gravity anomalies and flexure of the lithosphere at the Middle Amazon Basin, Brazil

The Middle Amazon Basin is a large Paleozoic sedimentary basin on the Amazonian craton in South America. It contains up to 7 km of mainly shallow water sediments. A chain of Bouguer gravity highs of approximately +40 to +90 mGals transects the basin roughly coincident with the axis of maximum thickness of sediment. The gravity highs are flanked on either side by gravity lows of approximately −40 mGals. The observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide. Within this region, the continental crust has been intruded/replaced by more dense material to more than half its original thickness of 45–50 km. The much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere. The observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15–20 km. Although this value is lower than expected for a stable cratonic region of Early Proterozoic age, it is within the accepted range of effective elastic thicknesses for the earth. Rapid subsidence during the late Paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere. The high‐density zone in the lower crust, as delineated by gravity and flexural modeling, has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin. The linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust.

Oil and Gas Potential of Amazon Paleozoic Basins: ABSTRACT

The Paleozoic basins, covering an area of about 800,000 km2 (309,000 mi2) in the Amazon region, are elongate symmetrical intracratonic synclines filled with as much as 4,000 m (13,000 ft) of sediments, separated by basement uplifts or major arches and located in continental interior areas (as are the United States Illinois, Michigan, and Williston basins). These Amazon basins resulted from an initial crustal thinning followed by rifting with associated ultrabasic intrusions and, finally, cooling and subsidence. Gravity anomalies, coinciding with the axes of the synclines, support this genetic hypothesis. These synclines were filled during the Silurian-Devonian with one cycle of continental alluvial sediments grading upward to deltaic marine clastics and minor periglacial deposits. A regional unconformity separates the Devonian from the Permo-Carboniferous cycle when, following fluviodeltaic sedimentation, highly restricted marine conditions developed a sequence of evaporite deposits. Tectonics affected differentially these basins during the Triassic-Jurassic and Early Cretaceous, associated with widespread basic volcanism. A northeast-southwest thrust-fault system, branching southwest, characterizes a compressional orogenic province in the Upper Amazon basin. This compressional province, located in the Jurua River area, constitutes a major structural trend. Adjacent to those faults and extending for over 500 km (310 mi), large natural gas accumulations occur in several domal features. Sandstones of the Permian Monte Alegre Formation, sealed by evaporite strata, are the main reservoir rock. Geologic estimates of natural gas resources are presently rated at 120 billion m3 (4.237 tcf) and exploration follows the productive trend toward the west-southwest. The Middle Amazon basin, separated from the Upper Amazon by the Purus arch, was affected by lineament-block tectonics, also with associated volcanism and some local mild shearing. Minor domal features of Devonian periglacial Oriximina Formation sandstones comprise small subcommercial oil accumulations. In contrast with the Upper and Middle Amazon basins, the Lower Amazon basin has been the site of rifting since the Permo-Triassic. The rifting was associated with a nearby hot spot that uplifted the eastern part of the basin, forming the Gurupa arch. As a consequence of this uplift, a set of collapse grabens developed in the Lower Amazon basin. Potential reservoir rocks in Middle and Lower Amazon basins are Permian Monte Alegre and Devonian Oriximina sandstones. Major source rocks in al three basins are Devonian Barreirinha black shales. Organic geochemistry data indicate that both Upper and Lower Amazon basins are predominantly gas-prone, whereas the Middle Amazon basin shows potential for oil generation. Forecasts for the major exploratory trends in the Upper Amazon indicate a good possibility of extending the already discovered natural gas province. In the Lower Amazon basin, further exploration will consist in drilling well-defined structural features identified for the first time by seismic methods, with a possibility of discovering another gas province. Prospects in the Middle Amazon basin are for both oil and gas, but the main problem is identification of adequate structures, as well as stratigraphic traps. End_of_Article - Last_Page 1205------------

PETROLEUM GEOLOGY IN THE PALEOZOIC CLASTICS OF THE MIDDLE AMAZON BASIN, BRAZIL

Paleoenvironmental maps of all the stratigraphic units of the Middle Amazon Basin from Ordovician to Early Carboniferous are analysed in terms of their source‐rock and reservoir potentials. These maps were obtained by applying the following techniqes; detailed petrography of diamictites, sandstones and siltstones including measurements of index of clasticity; analysis of early and late diagenetic cementation processes; environmental interpretation of sedimentary structures; geochemistry of trace elements and lateral variations of clay mineral assemblages based on X‐ray diffraction data. Five distinct sedimentary models were recognized. Fluvio‐deltaic model consisting of fluvial, deltaic, distal deltaic and offshore subenvironments. Tidal flat model consisting of marshes, intertidal flat, shoreface and offshore subenvironments. Beach model consisting of dunes (inferred), beach, shoreforce and offshore subenvironments. Glacial model (Nhamundá time, Early Silurian), consisting of glacial (tillites), fluvial, beach, shoreface and offshore subenvironments. Glacial model (Curiri time, Late Devonian) consisting of glacial (tillites with subglacial sandstone channels and interglacial braided streams), shoreface and offshore subenvironments.From the association of these depositional models, the direction and intensity of the supply of coarse clastics was determined in a semi‐quantitative manner for each stratigraphic unit. The time andspace distribution of the detritalsupplies and of the areas of coarse clastic sedimentation indicates a continuous synsedimentary tectonic control of the depocenters by the differential subsidence of the underlying Precambrian basement in the shape of large blocks limited by a system of NW‐SE and NE‐SW lineaments. By using an index of synsedimentary tectonic activity it is possible to define three major episodes of coarse clastic sedimentation which have the best reservoir potential: Early Silurian fluvio‐glacial systems, Early Devonian progmding deltas and Late Devonian fluvio‐glacial systems. The tmps for hydrocarbons are mainly stmtigmphic but enhanced by the weak structures of the basin and result from the relationship between the coarse clastics and two intervening sequences of mature black shales.

Petroleum Geology of Paleozoic Clastic Rocks of Middle Amazon Basin, Brazil: ABSTRACT

Petroleum Geology of Paleozoic Clastic Rocks of Middle Amazon Basin, Brazil: ABSTRACT

Photogeologic Interpretation of Structure in the Amazon Basin: A Test Study

A photogeological test study of an area of about 1100 sq. mi. in the Middle Amazon Basin revealed evidence of structure in spite of low relief and heavy forest cover. The study suggests that a thin surficial formation rests with angular unconformity on a thick northward-dipping sedimentary sequence and that undulations in the mantle formation are structural warps. Active deformation or differential compaction of sediments over buried topographic highs arc believed to have caused these warps. Recent doming by igneous intrusion is not likely because the affected rocks are considerably younger than the last recorded episode of intrusion. The warping has nowhere caused a reversal of dip of the lower sediments but has affected the direction and magnitude of dip. Ironically, the rivers, which provide the easiest access to much of the interior and along which both field and geophysical surveys are run, may actually have been shunted off structure by the warping. Restriction of exploration to available routes of access, therefore, may be self-defeating as far as structural appraisal of at least some parts of the Amazon Basin is concerned.