『Abstract
On the basis of paleontological content (vertebrates and palynology)
and facies analysis from river banks, road cuts, and three wells,
we have assigned the uppermost levels of the Solimoes（後のoの頭に〜）
Formation in western Amazonia, Brazil, to the Late Miocene. The
vertebrate fossil record from outcropping sediments is assigned
to the Huayquerian-Mesopotamian mammalian biozones, spanning 9-6.5
Ma. Additionally, we present results that demonstrate that deposits
in Peruvian Amazonia attributed to Miocene tidal environments
are actually fluvial sediments that have been misinterpreted (both
environmentally and chronologically) by several authors.
The entire Late Miocene sequence was deposited in a continental
environment within a subsiding basin. The facies analysis, fossil
fauna content, and palynological record indicate that the environment
of deposition was dominated by avulsive rivers associated with
megafan systems, and avulsive rivers in flood basins (swamps,
lakes, internal deltas, and splays). Soils developed on the flatter,
drier areas, which were dominated by grasslands and gallery forest
in a tropical to subtropical climate.
These Late Miocene sediments were deposited from westward of
the Purus arch up to the border of Brazil with Peru (Divisor Ranges)
and Bolivia (Pando block). Eastward of the liquitos structural
high, however, more detailed studies, including vertebrate paleontology,
need to be performed to calibrate with more precision the ages
of the uppermost levels of the Solimoes（後のoの頭に〜）
Formation.
The evolution of the basin during the late Miocene is mainly
related to the tectonic behavior of the Central Andes (〜3゜-15゜S).
At approximately 5 Ma, a segment of low angle of subduction was
well developed in the Nazca Plate, and the deformation in the
Subandean foreland produced the inland reactivation of the Divisor/Contamata
Ranges and tectonic arrangements in the Eastern Andes. During
the Pliocene southwestern Brazilian Amazonia ceased to be an effective
sedimentary basin, and became instead an erosional area that contributed
sediments to the Amazon fluvial system. At that time, the lowland
fluvial systems of southwestern Amazonia (the Purus, Jurua and
Javari（iの頭は´） basins) become isolated from
the Andes by the newly formed north-flowing Ucayali system and
south-east flowing Madre de Dios System. It was during the early
Pliocene that the Amazon fluvial system integrated regionally
and acquired its present appearance, and also when it started
to drain water and sediments on a large scale to the Atlantic
Ocean.
Keywords: Amazon basin; Amazon River; Late Miocene; Paleogeography;
Paleoecology; fossil vertebrates; Palinology; Solimoes（後のoの頭に〜）
Formation
Contents
1. Introduction
2. The Solimoes（後のoの頭に〜） Formation
2.1. Sedimentology revisited
2.2. Fossil fauna
2.2.1. Chronology
2.2.2. The vertebrate diversity
2.2.3. Summary of diet habits of mammals as environmental indication
2.2.4. Freshwater mollusks and ostracods
2.3. Palynology
2.3.1. Biostratigraphic data
2.3.1.1. Well IAS-32-AM
2.3.1.2. Well IAS-19-AM
2.3.1.3. Well IAS-27-AM
2.3.1.4. Outcrops samples
2.3.1.4.1. Patos, Upper Acre River
2.3.1.4.2. Barranco da Elizete, Upper Acre River
2.3.1.4.3. Murici, Upper Acre River
2.3.1.4.4. Niteroi（oの頭に´）, Lower Acre
2.3.2. Paleoenvironmental considerations
3. Amazonia during the Late Miocene
3.1. Correlation of the Solimoes（後のoの頭に〜）
Formation with others lithostratigraphic units of South America
3.2. The Amazon basin: evolutionary approach
4. Conclusions
Acknowledgments
References