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Home , Huayquerian, Late Miocene, Montehermosan, Octomylodon, Phoberomys, Proterotheriidae

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 - of Southwestern Amazonia. The extensive Late Tertiary sediments of The Solimões Formation, outcropping in Southwestern Amazonia, were deposited by a complex megafan , originating in the high Peruvian . The megafan system was the sedimentological response to the Andean Quechua tec­ tonic phase of Tertiary , producing sediments that fdled the foreland basin of Southwestern Amazonia. Occurrences of varied vertebrate assemblages of the - age collected in these sediments support this interpretation. The fauna includes several genera and of fishes, , , 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, , 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— and . hermosan) of Southwestern Amazonia, on The vegetation of this area is the basis of vertebrate paleontology and primarely tropica] 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, .

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 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 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 / to with the Purus stuctural arch. Other archs Mio-Pliocene in age. like Iquitos and Carauari were overlapped The 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 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 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 sedi­ clayey sediments gradually. ments. The trough cross stratification be­ Our results show that this comes minor toward the top, changing to megafan system occupied the region clayey and silty sediments. south of the Solimòes river and ex­ These structures of cut and fill, and tended eastward at least to a longitude the typical channel sequence of fining of 672 W(Fig. 3). upward sediments, are indicative of ac­ This megafan system was the sedi­ tive and unstable channels. However, mentary response to the Andean tectonic non-repetitive scroll structures were rec­ movements in the Amazon foreland basin. ognized. It can infered that avulsion This tectonic phase was initiated in the have occured. Bolivian and Pemvian Andes Facies analysis of the Tertiary approximatelly 10 ± 2 millions ago units of Southwestern Amazonian in­ (data review in JORDAN et al, 1983). dicates that they were deposited as part of The Diaguita Upper Pliocene tec­ a megafan system with its headwater in tonic phase of the Andes (DUMONT the Pemvian Andes (IRIONDO, 1988; et al, 1989; 1991; JORDAN et al., LATRUBESSE, 1992; LATRUBESSE & 1983) created the present watershed of RAMONELL, 1992). the Ucayali and fluvial basins of

Southwestern Amazonia. Thus, tec­ isolated exposures. The fossiliferous tonic phase was responsible for the outcrop called Lula is situated along termination of the deposition of the both sides of a road embankment near Amazonia megafan system. Conse­ the city of Sena Madureira. This fos­ quently Southwestern Brasilian siliferous locality is located at the top Amazonia has become isolated from of one hill, and stratigraphically is situ­ the Andean Range as a source of sedi­ ated in the topmost levels of the ments since the Upper Pliocene. At this Solimões Formation. time, the present northward drainage Commonly the fossil bones are pattern of the Ucayali river was estab­ found in situ, and the bone beds are lished, beginning the development of the rich. Almost all of the bones and teeth present Purus and Jurua fluvial belts and collected, when found loose and out of the drainage network of Brasilian South­ the bonebed exposures, are broken, western Amazonia. These fluvial belts rolled and reworked. However, when have three Quaternary terrace levels. recovered in situ, some specimens are very well preserved. Vertebrate Paleontology and Late The majority of the specimens Miocene-Pliocene environments included in our revised list (Tab. 1) are In 1866, Chandless published the deposited at the Laboratory of Paleon­ first of many studies concerning fos­ tology of University Federal of Acre, sil vertebrate faunas of the Western in Rio Branco, Brazil. Amazon Region. RANCY (1985; Among the vertebrate fauna of 1991) provided historical reviews of Southwestern Amazonia, the fossil as­ previous research and emphasized the semblages of the distinct localities show involvement of the Laboratory of Pa­ some differences. These differences are leontology at the University Federal of qualitative (occurrence or absence of Acre, in Brazil. The last decade has determinate ) or quantitative (fre­ witnessed a marked increase in mate­ quency of a determinate taxon) in accor­ rial collected and many of specimens dance with the sedimentary variability. reported in this paper were gathered by The faunistic correlation was made the local staff and colleagues in the with occurrences of various taxa previously State of Acre. described from the Huayquerian- Material was recovered in the Montehennosan mammalian strata (Late Tertiary sediments from the banks of Miocene-Pliocene) of Argentina. These the upper Puras river and tributaries taxa are: the Potamarchus, (Talismã, Preventòrio, Niterói, Neoepiblema horridula, Cavalcante and Patos sites; Fig. 4) and burmeisteri and the toxodont ^Yat from the upper Jurua river (sites listed toxodont Gyrinodon found in the by PAULA COUTO, 1976). The fos- Urumaco Formation (Late Miocene- siliferous beds along the upper Jurua Pliocene) of is present in the and Purus sites are visible at low wa­ Solimões Formation as well. ter (roughly June through October) in In almost every paleontological

Table 1. Revised list of the Vertebrate Fauna referred to Late Miocene-Pliocene from Southwestem Amazonia. Taxa previously described from Huayquerian-Montehermosan of Argentina are labelled (*).

CHONDRICHTHYES SELACHII leucas (Valenciennes, 1841) PRISTIDAE Pristis sp. POTAMOTRYGONIDAE OSTEICHTHYES LEPIDOSIRENIDAE Lepidosiren megaios Silva Santos, 1987 OSTEOGLOSSIDAE DORADIDAE PIMELODIDAE Phractocephalus sp. Colossoma sp. ACREGOLIATHIDAE Acregoliath rancii Richter, 1989 REPTILIA CROCODYLIA ALLIGATORIDAE brasiliensis Barbosa Rodrigues, 1892 Caiman brevirostris Souza Filho, 1987 Caiman nov. sp. CROCODILIDAE Charactosuchus fieldsi Langston, 1965 Charactosuchus nov. sp. NETTOSUCHIDAE nativus (Gasparini, 1985) Mourasuchus amazonensis Price, 1964 Mourasuchus indet. GAVIALIDAE Gryposuchusjessei Gurich, 1912 Hesperogavialis cruxenti Bocquentin & Buffetaut, 1981 Hesperogavialis sp Brasilosuchus mendesi Souza Filho & Bocquentin, 1989 CHELONII colombiana Wood, 1976 Chelus /ew/s/Wood, 1976 bassleri Williams, 1956 sp. TESTUDINIDAE sp. SERPENTES indet. MAMMALIA Abothrodon pricei PaulaCouto, 1944 Gyrinodon quassus Hopwood, 1928 Gyrinodon sp. Trigodonops lopesi (Roxo, 1921) Toxodontherium listai Ameghino, 1983 Trigodon sp. Plesiotoxodon amazonensis Paula Couto, 1982 Neotrigodon utoquineae Spillmann, 1949 Neotoxodon pascuali Paula Couto, 1982 cont. Tabela 1. Mesenodon juruaensis Paula Couto, 1982 Mesotoxodon pricei Paula Couto, 1982 Minitoxodon acreensis PaulaCouto, 1982 EDENTATA TARDIGRADA OROPHODONTIDAE Octodontobradys puruensis Santos et al., 1993 Cf. Pliomorphus MEGALONYCHIDAE NOTHROTHERIINAE indet. Acremylodon campbelli (Frailey, 1986) Pseudoprepotherium sp. Cf. Ranculcus* Urumacotherium sp. CINGULATA GLYPTODONTIDAE Cf. Piohophorus Cf. Paraglyptodon AMPATHERIIDAE RODENTIA NEOEPIBLEMIDAE Neoepiblema horridula* Ameghino, 1889 Neoepiblema acreensis Bocquentin et al, 1989 Phoberomys burmeisteri* Ameghino, 1886 Phoberomys bordasi Patterson, 1942 NeoepiblemaCi. N. ambrosettianus* Ameghino, 1889 HYDROCHOERIDAE Kiyutherium orientalis* Francis & Mones, 1965 DINOMYIDAE Potamarchus murinus* Burmeister, 1885 Teiicomys * amazonensis Frailey, 1986 Tetrastylus * sp. Cf. Gyriabrus * CAVIDAE CARDIOMYINAE ERETHIZONTIDAE DASYPROCTIDAE ECHIMYIDAE HETEROPSOMYINAE ASTRAPOTHERIA ? Synastrapotherium SIRENIA TRICHECHIDAE ? CETACEA Plicodontinia mourai Miranda Ribeiro, 1938 Cf. Isquirorhynchus* Cf. Cullinia AVES ANHINGIDAE, Anhingasp. , PHORUSRHACINAE indet. locality of the region of Southwestern "...indicam áreas bajas, inundables y Amazonia, the majority of specimens cuerpos de agua estables, con collected, both in quantity and diver­ abundante vegetación y un clima tropi­ sity, are crocodilians and aquatic cal a subtropical...". Cartilagionous and . According to GASPARINI bony fishes, reptiles and river dolphins in­ (1981:195), the living crocodiles "...no habited fluvial, lacustre and palustre envi­ son 'termômetros geológicos' pero ronments. Like the living aquatic of pueden aportar interessantes datos al Amazonia, the Late Miocene-Pliocene conocimiento paleoclimàtico y fauna was subjected to great fluctuations in paleoambiental de una área y momento water level due to a marked difference in determinados. ...Con ciertas precipitation during the rainy and dry sea­ limitaciones se puede aplicar el sons and high-lower flood. We believe that actualismo a aquellas espécies y aún part of this fauna, isolated during the dry gêneros con representantes vivientes". seasons in swamps and lakes, would have The living small sized spectacled been able to circulate and mingle with river Caiman crocodilus of the Amazon are fauna, during rainy season as a result of river bank or swamp dwellers. They changes in the water level. At the present are carnivorous, piscivorus and crus­ time, many are able to avoid un­ tacean eaters (SIOLI, 1984). It is in­ favorable conditions by horizontal migra­ teresting to mention the occurrence at tion. Many families of Amazonian fishes the fossiliferous Lula site of many have air-breathing representatives. Many fragments of crabs.The living black have intestinal respiration. The caimans {Melanosuchus niger ) prefer swim-bladder of the pirarucu {Arapaima large, quiet, deep river water and are gigas) is modified to fulfill the function of stagnant pond dwellers. The large size a lung (SIOLI, 1984). There is no difficulty of this species enables to eat a wide va­ in comparing the huge Lepidosiren riety of animals (LANGSTON, 1965; megalos from the Miocene and the living SIOLI,1984). Presumably, the most recog­ Lepidosiren nizable counterpart of Melanosuchus niger paradoxa. The former is clearly related to in the Southwestern Amazon fossil fauna the latter, differing principally in its great is the big Purussaurus brasiliensis. It could size (SANTOS, 1987). L. paradoxa inhab­ be the largest terrestrial predator that ever its stragnant ponds of the Amazonian re­ lived, having became extinct during gion and is able to aestivate into a burrow the Late Tertiary interchange of terres­ during drier times. trial fauna between the Americas (BOCQUENTIN VTLLANUEVA, 1987). Among the mammals, the rodents The river turtles of the family Kiyutherium (Cardiatheriinae) were prob­ Pelomedusida e, a family endemic of ably herbivores like the present day South America, are presently widely dis­ capivara. The Kiyutherium inhabited areas tributed there. According to GASPARINI near bodies of water just like the capivara et al. (1986), the are good (PASCUAL & BONDESIO, 1985). Some indicators of paleoenvironments: of the herbivorous Toxodontidae may have had partly amphibious behavior. The protherotheres (litopterns) are Amazonia, were deposited by a mammals of more terrestrial habits and megafan complex system, whose have been reported only from the site headwater was situated in the high Patos (Alto Acre River). These Peruvian Andes. Some occurrence of litopterns developed into -like minor fans associated with the main types, and were probably browsing sytem is probable. The megafan forms living along forest edges sediments were deposited in re­ (ROMER, 1966; COLBERT, 1969). The sponse to the Quechua Andean Tec­ giant Synastrapotherìum was a big and tonic phase (Fig. 3). heavy astrapothere, clearly adapted for The paleovertebrate fauna recov­ pushing through the forests or wander­ ered in sediments of the Solimões For­ ing over the plains (COLBERT, 1969). mation, at the Purus and Jurua basins, Huge ground-sloths evolved during Mi­ are indicative of Huayquerian and ocene-Pliocene times, side by side with Montehermosan Mammals ages. The the and giant pampatheres. faunal assemblage occurs throughout the The large-bodied mylodonts surely were sedimentary sequence exposed. Accord­ not even semi-arboreal. They were, to­ ing to these data, the Solimões Forma­ gether with the Megatheriinae, essen­ tion has Late Miocene-Pliocene Age in tially grazing forms. The small bodied the uppermost levels. nothrotheres and megalonychides were The occurence of medium and probably browsing forms that lived large vertebrates suggest floodplain largely on leaves of trees and bushes and lacustrine-paludal environmental (KRAGLIEVICH, 1931; SCILLATO conditions associated with rivers, YANE, 1977). Occurrences of primates grasslands and gallery forests. These may indicate gallery forest. The present findings are in accordance with the day Platyrrhini rarely descend to the sedimentary megafan model. ground in their frequently flooded habi­ Fossil vertebrates are more abun­ tats and they evolved no specialized dant and diversified in the flodplain- form of terrestrial locomotion lacustrine sediments, where the remains (HERSHKOVITZ, 1977). of bony and cartilaginous fishes, river Therefore, the environmental ne­ dolphins, large crocodilians, turtles, ro­ cessities of the vertebrates that inhabited dents, toxodonts, ground-sloths, pri­ this region of Amazonia during the mates and birds were recovered. Huayquerian-Montehermosan mammals Taxonomical differences reported from ages, were grasslands and gallery forests distinct fossiliferous sites are related to along with rivers, swamps and lakes that different sub-environments in the main were subject to fluctuating seasons. system (high and low energy sedimen­ tary facies). CONCLUSION During the upper Pliocene, the The Cenozoic sediments of the megafan system was stopped by the ef­ Solimões Formation, widely distrib­ fects of Andean tectonical movements uted through Southwestern (Diaguita phase) which generated the formation of a watershed that separated the Vertebrates of the backboned animals the basin of Ucayali from the basins of through time. 2. ed. Wiley International Edition, John Wiley ; Sons, New York, southwestern Brazilian Amazonia rivers. 535 p. The current fluvial system originated DUMONT, J. F. 1994. Neotectonics and Riv­ after the "Diaguita Tectonical Phase" ers of the Amazon Headwaters. The vari­ during the Quaternary. ability of large Allluvial Rivers. Schumm and Winkley eds., ASCE Press, p. 103-113. The Quaternary sediments are restricted to the fluvial belts of the DUMONT, J.F.; DEZA,; GARCIA, F. 1991. Morphoestructural Provinces and present drainage systems, in the form Neotectonics in the Amazonian Low of terrace levels. 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Paleoenvironmental model for the late ... 117