Paleoenvironmental Model for the Late Cenozoic of Southwestern Amazonia: Paleontology and Geology
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PALEOENVIRONMENTAL MODEL FOR THE LATE CENOZOIC OF SOUTHWESTERN AMAZONIA: PALEONTOLOGY AND GEOLOGY Edgardo Μ. LATRUBESSE1, Jean BOCQUENTIN1, José Carlos R. SANTOS', Carlos G. RAMONELL2 ABSTRACT — Our study provides paleontological and geological data substantiating a paleoenvironmental model for the upper Miocene-Pliocene of Southwestern Amazonia. The extensive Late Tertiary sediments of The Solimões Formation, outcropping in Southwestern Amazonia, were deposited by a complex megafan system, originating in the high Peruvian Andes. The megafan system was the sedimentological response to the Andean Quechua tec tonic phase of Tertiary age, producing sediments that fdled the foreland basin of Southwestern Amazonia. Occurrences of varied vertebrate fossil assemblages of the Huayquerian- Montehermosan Mammal age collected in these sediments support this interpretation. The fauna includes several genera and species of fishes, reptiles, birds, mammals and appears to be one that could have lived in or near a riverine habitat. In the Late Pliocene, the megafan system became inactive as a result of the influence of the Diaguita Tectonical Phase. Key-Words: Amazonia; Sedimentary model; Megafan; Vertebrate Paleontology; Mio-Pliocene. Modelo Paleoambiental da Amazônia Sul-Ocidental Durante o Cenozóico: Paleontologia e Geologia. RESUMO — O presente trabalho refere-se à paleontologia e geologia como elementos de sustentação para um modelo paleoambiental da Amazônia Sul-Ocidental durante o Mioceno Superior-Plioceno. Os sedimentos terciarios (Formação Solimòes) aflorantes nesta região fo ram depositados por um complexo sistema de megaleque originário dos Andes peruanos. Este sistema de megaleque representa uma resposta sedimentar à fase tectônica Quechua ocorrida nos Andes durante o Mioceno Medio/Superior. Os sedimentos foram acumulados na bacia sedimentar da Amazônia Sul-Ocidental. A ocorrência de sitios com grandes quantidades de vertebrados fósseis de Idade-Mamífero Huayqueriense-Montehermosense (Mioceno Superior- Plioceno) nos sedimentos da Formação Solimões corrobora essa interpretação. A fauna consta de vários gêneros e espécies de peixes, répteis, aves, mamíferos. No Plioceno superior, o sistema de megaleque ficou inativo devido à influência da fase tectônica Diaguita. Palavras-Chave: Amazônia; Modelo Sedimentar: Megaleque; Paleontologia de Vertebrados; Mio-Plioceno. INTRODUCTION State of Acre, Brazil, at sections ex posed along the Purus and Jurua river The objective of the present work basins and roadside outcrops. The State is to propose a new model of of Acre belongs to the Southwestern re paleoenvironmental conditions for the Late gion of Brazilian Amazonia and borders Miocene-Pliocene (Huayquerian-Monte— Peru and Bolivia. hermosan) of Southwestern Amazonia, on The vegetation of this area is the basis of vertebrate paleontology and primarely tropica] rainforest in which geology. The region considered includes rainfall average 2000mm annually. Al part of western Brazil, Peru and Boliv though moderately high, preci-pitation ian Amazonia (Fig. 1). is seasonal. Due to poor accessibility of Field work was carried out in the outcrops during the rainy season, field- 1 Universidade Federal do Acre, Campus Universitário, 69.915-900. Rio Branco, Acre, Brazil. 2 Universidad Nacional de Litoral, Casilla de Correo 495, (3000), Santa Fe, Argentina. ACTA AMAZONICA 27(2): 103-118. 1997. 10° Figure 1. Location map. work was carried out largely during the Geological Background period from June to August. The region under study is char The Geology of Amazonia is acterized by an undulating landscape dominated by four geotectonic elements: (maximum elevation 250m above sea Guyana and Brazilian Shields, both level) with ocasional flat topped hills, composed of Precambrian and Paleozoic which have a similar altitude. The rocks; the Amazon intracratonic basin maximum relative relief between the and the Andean Belt. tops of hills and the water level of the The sedimentary basin includes rivers is about 70m. different sub-basins in western Ama In the Upper Tertiary, Southwestern zon such as Maranón, Ucayali, Acre, Amazonia was an extensive aggradarional Madre de Dios and upper Amazon. plain. This plain was dissected between the Toward the east, structural arches di Late Pliocene and the Last Glacial Maxi vide the Amazon basin in Middle and mum resulting in the present badland land Lower Amazon. scape of Southwestern Amazonia observed Tertiary sediments cover almost in radar images (TRICART 1985; the totality of western Amazonia, West RADAMBRASIL, 1976). of Manaus. These sediments are assigned to the Solimòes Formation. The eastern lithostratigraphic units, which could limit of the Tertiary basin fits approximately range from Paleocene /Oligocene to with the Purus stuctural arch. Other archs Mio-Pliocene in age. like Iquitos and Carauari were overlapped The Quaternary sediments are of by the Tertiary sediments. fluvial origin and are found in the al Clastic sediments formed principally luvial belts of the Southwestern Ama by sandstones, siltstones and claystones of zon rivers. In these belts there are three continental origin were assigned to the Ter terrace levels (LATRUBESSE & tiary and informally defined as RAMONELL, 1991; 1992) (Fig. 2). The lithostratigraphic units. upper terrace (UT) reaches 34 to 38m, CAPUTO et al. (1971) combined the the intermediate terrace (IT) 15 to 20m previously separately defined Puca and the lower (LT) 8 to 12m (relative (STEDNMANN, 1929), Solimòes (REGO, to the lowest dry season water level). The 1930) and Cruzeiro (OPPENHEIM, 1937) more exposed unit in the banks is the lower fonnations into a single formation, defined terrace level (Purus, Acre, Iaco and Jurua as the Solimòes Formation. rivers). In the Moa river, the terraces only The upper beds of the Solimòes attain heights of 20 to 25m, 9 to 12m and Formation have been correlated by 2 to 4m, respectivelly. FRAILEY et al. (1988) with the Inapari Formation (CAMPBELL & FRAILEY, The Solimòes Formation 1984), of Holocene age in Peru. Based on new geological and KRONBERG et al. (1990; 1991) as paleontological data we consider the signed a Late Pleistocene age for the Solimòes Fm. to be an Upper Tertiary Solimòes Fm. unit in the uppermost levels. The Ter These reports increased the con tiary sediments exposed along the fusion that existed in the literature on the banks of the Acre, Iaco, Purus, Jurua age of the Solimòes Formation because and Moa rivers, and in outcrops along previous reports from RADAMBRASIL the roads of this region, are differenti (1975; 1976) assigned a Plio-Pleistocene ated into two facies, a low energy and age to the formation. a high energy facies, within a fluvial- The maximum thickness of the out floodplain-lacustrine complex. The crops is approximately 70m. Sediment high energy facies (channel facies) is cores taken by the Brazilian Oil Company composed of sands, silty and clayey Petrobras (RADAMBRASIL, 1975; 1976; sands and some small gravels. This 1977) revealed a maximum thickness of facies is red brown to brown, and 1800m for the Solimòes Formation. shows cross-bedded stratification. The The sedimentary sequence de low energy facies is composed of scribed above has been recognized by green to gray-green clays and silty CAPUTO et al. (1971) to be Paleocene/ clays and belongs to flodplain-lacus- Pleistocene in age. Analysing the same trine-paludal environments. These data, we suspect that the thickness of sediments have planar lamination or 1800m includes at least two massive structure. In the two facies, o Figure 2. Schematic section showing the principal lanscape elements of Southwestern Brazilian Amazonia. -ι C (fi fossil bones, gypsum veins and calcar Megafans are typical widespread eous concretions are very common. geomorphologic features of the Qua Comparatively, floodplain-lacus- ternary plains of South America trine sediments are dominant and represen (IRIONDO, 1992) and avulsion a char tative in the outcrops. These sediments acteristic mechanism in large Western occur as extensive horizontal beds with Amazon rivers (DUMONT, 1994). massive structure which occasionally It has been proposed that a large shows laminar structure. Light varia magafan, together with smaller fans tions in color ranging between green- have created a very large aggradational grey green to light red can be attrib plain in Southwestern Amazonia in the uted to differences in the depth of the upper Tertiary (LATRUBESSE, 1992; level water and oxygenation. In some LATRUBESSE & RAMONELL, cases, the beds are wedged laterally 1992). At the time of the activities of in the form of large-scale pseudo-cross this megafan system the tropical cli stratification. We interpreted this ge mate could only have been character ometry of wedges of the beds in terms ized by well delimited dry and wet sea of paleochannel fill by clayey and silty sons producing a strong seasonal contrast sediments. The sediments are tapered in sediment load and discharge. During the in the direction of the paleochannel alluvial belts migration, new flooded ar border by differential compaction. eas were generated acompanied by the The high energy facies is indica abandonment of the existing swamps and tive of fluvial channels. These se lakes. These abandoned swamps and lakes quences are typically fining upward, were probably maintained by local rains or with trough cross beds in the bottom gen could have dried out and been filled with erally cutting the lower energy facies