Purus Arch Research Articles

New insights into the Cretaceous evolution of the Western Amazonian paleodrainage system

The geological history of the Amazon River is a complex story driven by geodynamic processes operating in the South American plate. In spite of recent research, the pre-Neogene structure of Amazon paleodrainage is still underconstrained. A recent study has suggested that the western Amazonian paleodrainage system was reversed, cratonic in origin during the Cretaceous. However, the extent and duration of this system are not well-constrained. To add new constraints, we investigated the sources of Cretaceous sedimentary rocks in the Brazilian Acre and Peruvian Madre de Dios basins, based on major and trace element concentrations, Sm–Nd isotopic compositions, and U–Pb detrital zircon dating. The late Cretaceous samples of the Acre Basin have εNd(0) values between −21.2 and −8.4 and zircon grains derived from terranes situated in the easternmost part of the Amazonian Craton: Ventuari–Tapajos (2.0–1.82 Ga) and Rio Negro–Juruena (1.82–1.54 Ga). In contrast, the Madre de Dios Basin samples have εNd(0) values ranging from −15.2 to −7.1 and a wide range of U–Pb zircon age distributions. The samples from the northern part of the basin (MD234 and MD238) are dominated by zircon ages between 0.9 and 1.3 Ga (Grenville/Sunsás, 30 %–43 %) and 0.5 and 0.7 Ga (Brazilian/Pampean, 26 %–16 %) with felsic provenance. The oldest MD234 sample also shows a dominant contribution from Paleozoic zircon (20 %). On the other hand, the samples from the southern part of the Madre de Dios Basin (samples MD-2019-4A; MD-2019-10; MD-2019-13; MD-2019-11D and MD-2019-12A) have U–Pb age distributions dominated by zircon ages between 0.9 and 1.3 Ga (Grenville/Sunsás, 30 %–45 %) and 1.54 and 1.3 Ga (Rondonia–San Ignácio, 16 %–43 %), pointing to a cratonic origin for these sedimentary rocks. To elucidate the distinction in provenance between the samples from the northern and southern parts of the Madre de Dios Basin, we suggest that a structural barrier (the Madidi Arch (?)) was active during the late Cretaceous. In this scenario, the northern part of the Madre de Dios Basin had similar sedimentary sources as the nearby basins like Huallaga, Bagua, Ucayali, and Marañón, whereas, in the southern part of the basin, sedimentation was influenced by local sources. Together with previously published studies, these data indicate that the continent-wide cratonic drainage developed between the Cenomanian and Maastrichtian in response to the Purus Arch uplift was probably driven by a geodynamic process related to the post-rift opening of the Atlantic Equatorial Ocean.

Diversification of Amazonian spiny tree rats in genus Makalata (Rodentia, Echimyidae): Cryptic diversity, geographic structure and drivers of speciation.

Amazonian mammal diversity is exceptionally high, yet new taxonomic discoveries continue to be made and many questions remain for understanding its diversification through time and space. Here we investigate the diversification of spiny rats in the genus Makalata, whose species are strongly associated with seasonally flooded forests, watercourses and flooded islands. We use a biogeographical approach based on a mitochondrial cytochrome b gene through divergence time estimation and reconstruction of ancestral areas and events. Our findings indicate an ancient origin of Makalata for the Guiana Shield and Eastern Amazonia as ancestral area. A first cladogenetic event led to a phylogeographic break into two broader clades of Makalata through dispersal, implying a pattern of western/Eastern Amazonian clades coinciding with the Purus Arch (middle Miocene). Most of subclades we infer originated between the late Pliocene to the early Pleistocene, with few recent exceptions in the early Pliocene through dispersal and vicariant events. The hypothesis of rivers as dispersal barriers is not corroborated for Makalata, as expected for mammalian species associated with seasonally flooded environments. We identify two key events for the expansion and diversification of Makalata species: the presence of geologically stable areas in the Guiana and Brazilian shields and the transition from lacustrine conditions in western Amazonia (Acre system) to a river system, with the establishment of the Amazon River transcontinental system and its tributaries. Our results are congruent with older geological scenarios for the Amazon basin formation (Miocene), but we do not discard the influence of recent dynamics on some speciation events and, mainly, on phylogeographic structuring processes.

Evolution and biogeography of Anablepsoides killifishes shaped by Neotropical geological events (Cyprinodontiformes, Aplocheilidae)

AbstractAnablepsoides is a widely distributed Neotropical killifish genus found in shallow streams, in both dense forests and open areas, throughout northern and northeastern South America. The phylogenetic and biogeographic relationships of the genus are here analysed, based on two nuclear and four mitochondrial genes of 26 species and six out‐groups. The origin of Anablepsoides was recovered in Early Miocene in an area corresponding to the Paleo‐Amazon‐Orinoco system. The current analyses indicate that the initial diversification of the genus in two main clades was associated with marine transgressions related to the formation of the Pebas mega‐wetland isolating each MRCA of those main clades in the upper Amazon river basin and river basins of the Guiana Shield. The diversification of the genus and the colonisation of new areas may be associated with Miocene and Pliocene events such as changes in the sea level, formation and extinction of wetlands, rupture of the Purus arch and Amazon river assuming the current flow to the East. Also, the evolution of Anablepsoides could be associated with the diversification of several other Neotropical lineages, so that the present study leads to a better understanding of the evolution of the Neotropical freshwater biota and South American geological history.

Time-calibrated molecular phylogeny reveals a Miocene–Pliocene diversification in the Amazon miniature killifish genus Fluviphylax (Cyprinodontiformes: Cyprinodontoidei)

Species of Fluviphylax are widely distributed over the Amazon and Orinoco river drainages and are among the smallest fish in the neotropics, inhabiting areas near the margin of slow-flowing clear and black water streams and lakes. Here, we present the first multigene molecular phylogeny of Fluviphylax, including all five nominal species of Fluviphylax and three undescribed species. The analysis included fragments of one mitochondrial and five nuclear genes, totaling 5880 bp. The dataset was analyzed using maximum parsimony, maximum likelihood, and Bayesian inference approaches providing high-supported well-solved trees. A time-calibrated analysis was performed providing information on the origin and diversification of the miniature genus in the Amazon. We estimate that Fluviphylax lineage splits from its sister group, the Anablepidae and Poeciliidae (Poeciliinae sensu Parenti, 1981), during the Late Eocene, about 36.6 Mya; but lineage diversification started only in the Middle Miocene, about 16 Mya, during the formation of the Pebas system. Subsequent splits within Fluviphylax occurred in the Late Miocene–Pliocene, between 10 and 6 Mya and during the Pliocene, and were probably influenced by paleogeographical events such as the breaching of the Purus arch, the rise of the Vaupes arch, the uplift of the Fitzcarrald arch, and the capture of the Contigo and Uraricoera river drainages by the Branco River. The present time-calibrated analysis provides the first insight on the evolution of one of the smallest vertebrate taxa in the Amazon and Orinoco river drainages.

Sedimentology and Palynostratigraphy of a Pliocene-Pleistocene (Piacenzian to Gelasian) deposit in the lower Negro River: Implications for the establishment of large rivers in Central Amazonia

The Amazonas fluvial system originates in the Andes and runs ca. 6700 km to the Atlantic Ocean, having as the main affluent the Negro River (second largest in water volume). The Amazonas transcontinental system has been dated to the late Miocene, but the timing of origin and evolutionary processes of its tributaries are still poorly understood. Negro River alluvial deposits have been dated to the middle to late Pleistocene. Recently, we studied a number of boreholes drilled for the building of a bridge at the lower course of the Negro River. A thin (centimetric) sedimentary deposit was found, laterally continuous for about 1800 m, unconformably overlaying middle Miocene strata and unconformably overlain by younger Quaternary deposits. This deposit consists predominantly of brownish-gray sandstones cemented by siderite and with subordinate mudstone and conglomerate beds. Palynological, granulometric, textural and mineralogical data suggest that the initial Negro River aggradation took place in the deep incised valley under anoxic conditions and subsequently along the floodplain, with efficient transport of mixed origin particles (Andean and Amazonic). Angiosperm leaves, wood and pollen are indicative of a tropical continental palaeoenvironment. A well preserved palynoflora that includes Alnipollenites verus, Grimsdalea magnaclavata and Paleosantalaceaepites cingulatus suggests a late Pliocene to early Pleistocene (Piacenzian to Gelasian) age for this unit, which was an age yet unrecorded in the Amazon Basin. These results indicate that by the late Pliocene-early Pleistocene, large scale river activity was occurring in Central Amazonia linking this region with the Andean headwaters, and therefore incompatible with Central Amazonia barriers like the Purus arch.

Eustatic and tectonic change effects in the reversion of the transcontinental Amazon River drainage system

ABSTRACT: The development of the transcontinental Amazon River System involved geological events in the Andes Chain; Vaupés, Purus and Gurupá arches; sedimentary basins of the region and sea level changes. The origin and age of this river have been discussed for decades, and many ideas have been proposed, including those pertaining to it having originated in the Holocene, Pleistocene, Pliocene, Late Miocene, or even earlier times. Under this context, the geology of the sedimentary basins of northern Brazil has been analyzed from the Mesozoic time on, and some clarifications are placed on its stratigraphy. Vaupés Arch, in Colombia, was uplifted together with the Andean Mountains in the Middle Miocene time. In the Cenozoic Era, the Purus Arch has not blocked this drainage system westward to marine basins of Western South America or eastward to the Atlantic Ocean. Also the Gurupá Arch remained high up to the end of Middle Miocene, directing this drainage system westward. With the late subsidence and breaching of the Gurupá Arch and a major fall in sea level, at the beginning of the Late Miocene, the Amazon River quickly opened its pathway to the west, from the Marajó Basin, through deep headward erosion, capturing a vast drainage network from cratonic and Andean areas, which had previously been diverted towards the Caribbean Sea. During this time, the large siliciclastic influx to the Amazon Mouth (Foz do Amazonas) Basin and its fan increased, due to erosion of large tracts of South America, linking the Amazon drainage network to that of the Marajó Basin. This extensive exposure originated the Late Miocene (Tortonian) unconformity, which marks the onset of the transcontinental Amazon River flowing into the Atlantic Ocean.

Post-rift subsidence of the French Guiana hyper-oblique margin: from rift-inherited subsidence to Amazon deposition effect

Abstract The recent surge of exploration activities over distal margins, with the acquisition of more and more high-quality and deep seismic data, has led to enhance concepts of the deformation and subsidence history of passive margins in general and sheared margins in particular. The French Guiana sheared margin is very narrow. The thinning of the upper crust is accommodated by few major faults relayed by well-expressed transfer zones, giving a general oblique trend to the margin. Another possible effect of the shear component during the rifting is the presence in the distal domain of a Moho high. Its exhumation is coeval with the emplacement of a deltaic system coming from the Demerara Plateau, evidencing a probably important early subsidence of the margin. This early subsidence in the late-rifting stage is increased during the early drifting, when the thinned crust reached its isostatic/thermal equilibrium in the Cenomano-Turonian before suffering an important Late Cretaceous sedimentation load. In the Palaeogene, starving of the margin and significant uplifts in the Guiana Craton are observed, possibly resulting from the rise of the Purus Arch (Andes fore-bulge?). Finally, the Amazon deposition by the Late Miocene–Pliocene provoked a large subsidence in the distal domain.

Mid-Late Pleistocene OSL chronology in western Amazonia and implications for the transcontinental Amazon pathway

The origin of the transcontinental Amazon drainage system remains unrevealed. Sedimentary deposits formed from the Neogene in the Amazonas and Solimões Basins constitute natural archives for reconstructing this event in space and time. However, paleoenvironmental and chronological analyses focusing on these deposits, or even their basic mapping, are still scarce to allow such investigation. In this context, primary interests are fluvial strata related to the lithostratigraphic Içá Formation, mapped over a widespread area in western Amazonian lowlands. Although long regarded as Plio-Pleistocene in age, this unit has not yet been dated and its overall depositional setting remains largely undescribed. The main goal of the present work is to contribute for improving facies analysis and chronology of these deposits, approaching an area in southwestern Amazonia and another in northern Amazonia, which are located more than 1000km apart. Despite this great distance, the sedimentological and chronological characteristics of deposits from these two areas are analogous. Hence, facies analysis revealed paleoenvironments including active channel, abandoned channel, point bar, crevasse splay and floodplain, which are altogether compatible with meandering fluvial systems. Similarly, optically stimulated luminescence (OSL) dating revealed thirty three ages ranging from 65.4±16.9 to 219.6±25.1ky (in addition to three outliners of 54.0±7.6, 337.3±36.9 and 346.6±48.6ky), and nine 97.1±9.9 to 254.8±23.8ky for the areas in southwestern and northern Amazonia, respectively. These data lead to establish that deposits mapped as Içá Formation over a vast area of western Brazilian Amazonia have a Mid-Late Pleistocene age, rather than the previously inferred Plio-Pleistocene age. It follows that if Plio-Pleistocene deposits exist in this region they remain to be dated and must be restricted to a narrow belt in western Amazonia, as well as isolated occurrences underlying the Mid-Late Pleistocene strata characterized herein. The combination of data from this work with previously published provenance studies supports main Andean sediment sources only in the Mid-Late Pleistocene. It is proposed that before this time, the Amazon River was restricted to eastern Amazonia, being separated from western Amazonian drainage basins due to the presence of the Purus Arch. Erosion and/or subsidence of this geological feature would have promoted the connection of these drainage systems, ultimately with the expressive record of the transcontinental Amazon pathway into the Atlantic Ocean in the Mid-Late Pleistocene.

Influence of Peruvian flat-subduction dynamics on the evolution of western Amazonia

Convection in the Earth's mantle is mainly driven by cold, dense subducting slabs, but relatively little is known about how 3D variations in slab morphology and buoyancy affect mantle flow or how the surface above deforms in response (i.e. dynamic topography). We investigate this problem by studying the dynamics of an active region of flat-slab subduction located in Peru in South America. Here the slab geometry is well known, based on the regional seismicity, and we have observations from the local geological record to validate our models. Of particular interest is the widespread subsidence and deposition of the Solimões Formation across western Amazonia that coincided with the development of the Peruvian flat-slab during the Mid-Late Miocene. This formation covers an extensive area from the foredeep to the Purus Arch located ∼2000km away from the trench. Close to the Andes the preservation of several kilometers of sedimentary thicknesses can be easily accounted for by flexure. Based on an estimate of the Andean loading we predict 2.8 to 3.6 km of accommodation space that spans 100 km. The spatial and temporal history of the Solimões Formation however, particularly the thick distal foreland accumulations up to 1.2 km deep, can only be matched with the addition of a longer-wavelength dynamic source of topography. Following the transition from normal to flat subduction, we predict over 1 km of dynamic subsidence (∼1500km wide) that propagates over 1000 km away from the trench, tracking the subduction leading edge. This is followed by a pulse of dynamic uplift over the flat segment behind it. We therefore propose that a combination of uplift, flexure and dynamic topography during slab flattening in Peru is responsible for the sedimentation history and landscape evolution of western Amazonia that eventually led to the configuration of the Amazon Drainage Basin we know today.

Neotropical Macrobrachium (Caridea: Palaemonidae): on the biology, origin, and radiation of freshwater-invading shrimp

Comprising >240 extant species, Macrobrachium Bate, 1868 is the most speciose caridean genus in Palaemonidae. It is generally considered as a monophyletic clade that lives exclusively in limnic and brackish habitats. Thus, it may provide a suitable model for the reconstruction of evolutionary transitions of euryhaline shrimp from ancestral state of living in the sea towards invasions of freshwater environments. Reviewing patterns of the biology (adaptative physiological and reproductive traits) and modern biogeographic distribution of this clade, I propose here a scenario for its tentative origin and evolutionary invasion of limnic inland waters, especially in the Americas. Macrobrachium shows: 1) a world-wide tropical to subtropical distribution, with only few species occurring in temperate regions and none in cold waters at high latitudes; 2) a clear preference for low salinity conditions (based on strong osmoregulatory capacities); 3) larval export strategies in coastal species (with diadromous migrations and invariably an extended larval development in estuaries); and 4) almost exclusively an abbreviated and lecithotrophic mode of larval development in hololimnetic inland species. The extant patterns of geographic distribution of this genus are strikingly disjunct, with completely separate groups of species occurring in the Indo-Pacific region (where maximum diversity occurs), inWest Africa, and in the Americas. In a clade with common ancestry, this pattern can only be explained with a Tethyan origin and dispersal. Hence, I suggest an origin of ancestral Macrobrachium in theMesozoic. As a consequence of the breakup of Gondwana and fragmentation of the Tethys Sea, mainly during the Late Cretaceous and Palaeogene, Neotropical and West African clades became isolated from the remaining congeners and thus can be considered as Tethyan relicts. In the Neotropics, the closure of the Central American landbridge in the Late Pliocene was a significant vicariant event that caused diversification of coastal Macrobrachium in the Caribbean region, so that two separate groups of extant species live now in the Atlantic and Pacific drainage systems. The distribution of hololimnetic inland species can be explained by continental floodings occurring during the Miocene sea level highstand. Due to concomitant sub-Andean subsidence and impounded by the uplift of the Andes, the huge Pebas wetland system was formed at that time, covering the vast proto-Amazonas-Orinoco catchment area with sub-Andean lowlands reaching from Columbia to southern Bolivia. This allowed for incursions of marine-derived lineages of euryhaline shrimp and other coastal animals throughout northern and central South America, followed by adaptive radiations in freshwater. After the breaching of the continental divide (mainly the Purus Arch) in central Amazonia, probably in the Late Miocene, the modern eastward flow of the Amazon developed, draining to the Atlantic Ocean. Fluctuation but trending to a general decrease in global sea levels during the Pliocene and Pleistocene caused fragmentation of previously interconnected inland waters, promoting vicariant diversification of hololimnetic lineages. However, some hydrological connections between different catchment areas remained transitorily open, providing limited dispersal routes that also may have influenced speciation, e.g., through hybridisation. Thus, the modern distribution patterns of Neotropical Macrobrachium reflect interacting effects of tectonic events including Andean orogeny and the closure of the Panamaian landbridge, Miocene marine incursions, and possible cross-drainage dispersal through historical connections between river basins.

Palynostratigraphy and sedimentary facies of Middle Miocene fluvial deposits of the Amazonas Basin, Brazil

Palynostratigraphic and sedimentary facies analyses were made on sedimentary deposits from the left bank of the Solimões River, southwest of Manaus, State of Amazonas, Brazil. These provided the age-dating and subdivision of a post-Cretaceous stratigraphic succession in the Amazonas Basin. The Novo Remanso Formation is subdivided into upper and lower units, and delineated by discontinuous surfaces at its top and bottom. The formation consists primarily of sandstones and minor mudstones and conglomerates, reflecting fluvial channel, point bar and floodplain facies of a fluvial meandering paleosystem. Fairly well-preserved palynoflora was recovered from four palynologically productive samples collected in a local irregular concentration of gray clay deposits, rich in organic material and fossilized wood, at the top of the Novo Remanso Formation upper unit. The palynoflora is dominated by terrestrial spores and pollen grains, and is characterized by abundant angiosperm pollen grains (Tricolpites, Grimsdalea, Perisyncolporites, Tricolporites and Malvacearumpollis). Trilete spores are almost as abundant as the angiosperm pollen, and are represented mainly by the genera Deltoidospora, Verrutriletes, and Hamulatisporis. Gymnosperm pollen is scarce. The presence of the index species Grimsdalea magnaclavata Germeraad et al. (1968) indicates that these deposits belong to the Middle Miocene homonymous palynozone (Lorente, 1986; Hoorn, 1993; Jaramillo et al., 2011). Sedimentological characteristics (poorly sorted, angular to sub-angular, fine to very-coarse quartz sands facies) are typical of the Novo Remanso Formation upper part. These are associated with a paleoflow to the NE-E and SE-E, and with an entirely lowland-derived palinofloristic content with no Andean ferns and gymnosperms representatives. All together, this suggests a cratonic origin for this Middle Miocene fluvial paleosystem, which was probably born in the Purus Arch eastern flank and areas surrounding the crystalline. The palynological analysis results presented herein are the first direct and unequivocal evidence of the occurrence of Middle Miocene deposits in the central part of the Amazonas Basin. They also provide new perspectives for intra- and interbasin correlations, as well as paleogeographic and paleoenvironmental interpretations for the later deposition stages in the northern Brazilian sedimentary basins.

Patterns of diversification in the discus fishes ( Symphysodon spp. Cichlidae) of the Amazon basin

We carried out a phylogeograhic and population genetic analysis of fishes of the taxonomically contentious genus Symphysodon from the Amazon basin in order to test hypotheses of relationships among taxonomic units, and potential processes driving diversification within this genus. We sampled 334 individuals of the genus Symphysodon from 24 localities that span the complete geographic distribution of this genus. The sampling scheme included all known phenotypic groups, species and subspecies. Analyses were based on 474 bp of the mitochondrial control region and 1443 bp of the exon 3 of RAG1 gene. We observed 102 mtDNA haplotypes defined by 89 segregating sites, and 5 nuDNA alleles defined by three segregating sites. Maximum-likelihood, Bayesian-inference and statistical parsimony analyses revealed three well defined monophyletic groups. These clades corresponded to the ‘green’ and ‘blue’ groups of Symphysodon aequifasciatus, and to a previously morphologically unrecognized clade from the Xingu River drainage. These three clades were nested within a paraphyletic assemblage consisting of the ‘brown’ group of S. aequifasciatus and of both described subspecies of S. discus, the ‘Heckel’ and the ‘abacaxi’ discus. Nuclear allele sharing was observed among groups, but there were significant differences in frequencies. We inferred several processes including past fragmentation among groups, and restricted gene flow with isolation by distance within the paraphyletic ‘brown + Heckel + abacaxi’ groups, and suggest that differences among the ‘blue’, ‘Heckel’ and ‘brown’ groups are potentially maintained by differences in water chemistry preferences. We further inferred colonization of the western Amazon basin by an ancestor of the ‘green’ clade. The ‘green’ group was the only group with a pattern of haplotype distribution consistent of a demographic expansion, and the divergence of this clade from other groups of discus was consistent with recent geologic evidence on the breach of the Purus Arch which separates western Amazon from eastern Amazon. We further hypothesized that the differentiation of the ‘Xingu’ clade could be due to vicariance events resulting from Pleistocene sea level, and thus Amazon River level fluctuations. We discuss the bearings of our results on the current taxonomy of this group, and on the biological reality of the different forms, subspecies and species of Symphysodon concluding that we are probably observing a process of diversification, and therefore taxonomy will remain contentious.

Channel-floodplain geomorphology along the Solimões-Amazon River, Brazil

Across the cratonic landscape of Brazil the Solimoes-Amazon River transports to its delta plain 1240 Mt of suspended sediment derived from Andean erosion and reworks another 3200 Mt of floodplain sediments. Distribution of these sediments has resulted in a variable along-stream pattern of geomorphology. The upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel. Sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll-bar topography that forms the boundaries of hundreds of long, narrow lakes. In contrast, downstream reaches are characterized by channels restricted by stabilizing, long-term, levee building and floodplain construction dominated by overbank deposition. Overbank deposition buries the scroll-bar topography, resulting in a flat floodplain covered by a patchwork of large, more equant, shallow lakes. On the basis of estimated rates of recycling of floodplain sediments, the modern floodplain of the Brazilian Amazon could have been recycled in <5000 yr, and is recycled more rapidly in the upstream than the downstream reaches. The cratonic interior is interrupted by structural arches that bound intracratonic basins. Four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel-floodplain geomorphology at this spatial scale. Structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation, thus restricting channel movement. For example, as the river crosses the Purus arch, the valley narrows to <20 km compared to an average of ∪45 km, the water-surface gradient decreases, sediment is deposited, and yet the rate of channel migration is negligible. Hence, the effect of the arches is to create a landscape where, on the spatial scale of hundreds of kilometres, the river is confined and entrenched in its valley, is straight, and is relatively immobile. Local valley tilting apparently unrelated to the arch structures also imprints the geomorphology. In particular, a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large-scale, oxbow-type features on the Brazilian Amazon River floodplain.

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------------