Evolution of the Western Amazon Lowland Relief: impact of Andean foreland dynamics M.E. Rasanen*, J.S. Salo**, H. Jungnert & L. Romero Pittmantt *Department of Quaternary Geology; **Departmentof Biology, University of Turku, SF-20500 Turku, Finland; fRadiocarbon Datinx Laboratory, Unizwrsity of Helsinki, SF-001 70 Helsinki, Finland; tflnstituto Geolligica Mznero y Metalzirgico, Pablo Bernzudez 22 1, Lima 11, Peru has also been proposed for the origin of ABSTRACT the terra firme deposits in the southwest- The congruency in the depositional origin and age of the uppermost ern Amazon (Campbell and Frailey, sedimentary strata forming non-flooded rainforest ground (terra firme) 1984; Campbell et al., 1985). Based on in the western and central Amazon lowlands is a much debated the same deposits a more recent sugges- tion is that the terra firme is derived from subject. Here we conclude from the study of remote sensing imagery sedimentation in a late Pleistocene- that active Andean foreland dynamics have played a major role in the Holocene delta-lake environment, in- evolution of the Plio-Pleistocene fluvial landscape in the western itiated after 36,000 yr RP and with the Amazon. Foreland dynamics have resulted in a terra firme composed of latest lacustrine phases occurring as re- late Tertiary alluvium and younger alluvial terraces and plains. In cently as at 2800 yr BP (Frailey et al., Peru, thermoluminescence and I4C dating show local aggradation of 1988). This ’Lago Amazonas’ would this younger alluvium between 180 and 30 ka. documented high have covered approximately the same The area in central Amazon as its predeces- age heterogeneity of the terra firme has implications for considerations sor ’Belterra lake’. Clearly, with such of the biogeography of the Amazon forest. large discrepancies in the geohistory of the Amazon, there is no sound basis on Terra Nova, 2,320-332 which to make meaningful deductions about the factors behind Amazon biogeography. INTRODUCTION All previous concepts of the origin of portant for studies on the biogeography the surficial beds follow the ‘layer-cake’ The topmost sedimentary deposits of the Amazon basin. In many cases concept, according to which homogene- forming the present non-flooded rain- (Haffer, 1985; Haffer and Fitzpatrick, ous deposits were formed in a uniform forest ground (terra firme, Sioli, 1984) in 1985) the biogeographical tradition still environment. Little attention has been the western and central Amazon basin relies on the assumption that in large paid to the possible diachronism of the have been widely reported as a rela- areas of the lower and central Amazon deposits. The Amazon basin has a long tively thin (10-40 m thick) surficial, (east of the Iquitos arch, Fig. l),the terra fluvial history (Klammer, 1984; Irion, horizontal, sandy lithostratigraphic firme deposits result from sedimenta- 1984b; Rasanen et al., 1987) and despite unit (Oppenheim, 1946; Kummel, 1948; tion in a large inland ’Belterra lake‘ their lithostratigraphic similarity the Almeida, 1Y74; Projeto RADAM- during the high Calabrian (early Pleisto- surficial sequences may differ substan- BRASIL, 1977; ONERN, 1977, 1Y84; cene) sea-level (Sombroek, 1966). Thus tially in age. This supposition is sup- Khobzi et a/., 1980; Eden et al., 1982; biogeographers have regarded the ported by studies invoking Quaternary DNPM, 1984; Campbell and Frailey, Quaternary history of the lowland fluvial origins for some of the central 1984; Campbell ct nl., 1985; Frailey rt nl., Amazon as geodynamically relatively Amazon non-flooded terra firme, 1988). The extensive documentation of stable, especially in the central Brazilian formed when the sea levels were higher the surficial lithostratigraphic unifor- Amazon, which has long acted as a than they are during the present inter- mity of the terra firiiic has created an model for the palaeogeography of the glacial (Klammer, 1984; Irion, 198413). In illusion of the presence of a wide- whole basin. The ’Belterra lake’- the western Amazon (west of the spread ‘surficial mantle‘ in the Amazon hypothesis was originally based on clay Iquitos arch, Fig. l), though, the land- (Campbell and Frailey, 1984; Frailey et sediments found up to 180 m above sea scape is mostly unaffected by Quater- a/., 1988). This view has given rise to level in the eastern Amazon. The lake nary eustatic sea-level fluctuations. In several controversial hypotheses on the was supposed to have covered the Ecuador, in the Andean forelands, the genesis of these beds, the first being major part of the lowland Amazon basin surficial beds have been attributed to Agassiz’s erroneous proposal that they below this elevation. More recently the fluvial deposition following late- are glacigenous (Agassiz, 1865). existence of ’Belterra lake’ has little Pliocene Andean tectonism (Baldock, The sedimentary history of the beds support (Irion, 1984a). 1985). In Peru, some of the beds are forming the tcwa firmr has been very im- Catastrophic Holocene flashflooding postulated to be products of more re- 320 4' 0' 4 8 12 16 2 0' Fig. 1. Major continental andsubmarinestructures in northwestern South America (Unesco, 1975;Jordan et al., 1983;Cunha, 1988). Thelocations of the structural highs, the Andean retroarcand thepericratonicand intracratonic basins areshown in the Amazon sedimentary basin. Dotting depicts the relative thickness oftheir Tertiary-Quaternary deposits. The flat subducting segment of the Nazca plate is thrown, and the oceanic ridges are depicted according to their 3000 m bathymetric contours. 321 cent Pleistocene fluvial sedimentation from the convergence of the Nazca sub- tions forming the 'Planalto rebaixado da under the influence of.Sub-Andean tec- oceanic plate under the continental Amazonia' in Brazil (Almekia, 1974; tonics (Rasanen et al., 1987). In south- South American plate and giving rise to Projeto RADAMBRASIL, 1977; DNPM, eastern Colombia (Hoorn, 1988), north- the Andean orogeny slowly developing 1984) the 'Monte and Sierra Realms' in western Brazil (Almeida, 1974) and eastwards. Along the Andean strike, Bolivia (Campbell rt al., 1985) and the northern Peru (Rasanen et al., in prep.), the deformed Sub-Andean foreland belt Madre de Dios (Oppenheim, 1946), In- the alluvium forming the major terra is widest in the western Amazon in apari, Pagorene, Iquitos (Onern, 1977, firme has an eastern provenance. In Peru, where the tectonic stress is re- 1984; Campbell and Frailey, 1984) or Colombia, these sediments have a prop- leased in a thin-skinned thrust and fold Ucayali (Kummel, 1948) Formations in osed Oligo-Miocene age, and only the belt with reverse and thrust faults Peru. The unconformities exhibit ab- Quaternary terra firme terraces have dipping westwards. rupt changes in degree of consolidation obtained their material from the Andes The shallow subducting segment of and weathering and, at the marginal (Hoorn, 1988). the Nazca plate (Jordan et al., 1983) has parts of the foreland basins, even in the Thus it seems clear that over an area caused the present tectonic situation in dip of the strata (Oppenheim, 1975; of continental size such as the western the Peruvian foreland (Fig. 1). The flat Frailey, 1986). Unconformities are more and central Amazon, which has subduction is considered to have been parallel in the central parts of the basins, affinities with the Andes and the initiated only 10 to 5 Ma (Isacks, 1988, where differences in facies, degree cratonic shields, different parts of the Barberi rt a!., 1988) due to subduction of of weathering and consolidation are surficial deposits forming the terra firmr the Nazca aseismic ridge (Fig. 1).The in- usually less marked (Fig. 2). have aggraded diachronously. Sedi- itiation has been dated by the cessation In places, 0.2-2 m-thick conglomer- mentation has been controlled by Mio- of volcanism of the Andes along this ates occur above the basal unconformity, Pleistocene uplift and deformation of segment and by the reconstructed sub- with large-scale trough crossbedding in the Andes and their foreland and also duction geometry of the Nazca ridge the intercalated sands. These conglom- by eustatic sea-level changes in the (Pilger, 1984). This change in subduc- erates are frequently strongly cemented central Amazon. Although foreland de- tion geometry is among the most recent by iron oxides. The conglomerates are formation covers only a restricted area along the Cordilleran trench, and the the source beds for the majority of the of the western Amazon, it affects the low dip of the Benioff zone evidently Plio-Pleistocene vertebrate fossils and headwaters of the Amazon drainage effectively directs the compressional fossilized wood found in the area system, and hence Andean dynamics forces to the foreland crust (Suarez et al. , (Simpson and Paula Couto, 1981). Fre- have strongly modified both the 1983). The present high relative rate of quently documented in the western palaeohydrological conditions and sur- subduction, up to 11 cm yr-' at the Amazon, the conglomerates seem to face erosional patterns in the Amazon trench (Minster et al., 1974), further pro- phase out eastwards towards the lowlands. For this reason, the terra firme motes deformation (Jordan et d.,1983). central Amazon (Silva, 1988). The fauna forest beds are considerably more The foreland shortening is estimated to in the conglomerates seem to be some- heterogeneous in origin and age than have been recently in the range of 1-2 what younger (Simpson and Paula biogeographers have previously mm yr-' (Suarez et al., 1983). Deforma- Couto, 1981) than the fauna found in assumed. tion is further evidenced by modern the Oligo-Pliocene Pebas Formation Our evidence supporting this view is upper plate seismicity in the foreland, (Williams 1949) that is frequently based on analyses of side-scanning with the highest activity along the reported to underly the surficial forma- radar (SLAR) images of Peru and Brazil Cordilleras (Jordan et al., 1983; Suarez et tions in the western Amazon. and Landsat Planimetric Maps and on a/., 1983; Cisternas et af., 1988).