We review geological evidence on the origin of the modern transcontinental Amazon River, and the paleogeographic history of riverine connections among the principal sedimentary basins of northern South America through the Neogene. Data are reviewed from new geochronological datasets using radiogenic and stable isotopes, and from traditional geochronological methods, including sedimentology, structural mapping, sonic and seismic logging, and biostratigraphy. The modern Amazon River and the continental-scale Amazon drainage basin were assembled during the late Miocene and Pliocene, via some of the largest purported river capture events in Earth history. Andean sediments are first recorded in the Amazon Fan at about 10.1-9.4 Ma, with a large increase in sedimentation at about 4.5 Ma. The transcontinental Amazon River therefore formed over a period of about 4.9-5.6 million years, by means of several river capture events. The origins of the modern Amazon River are hypothesized to be linked with that of mega-wetland landscapes of tropical South America (e.g. várzeas, pantanals, seasonally flooded savannahs). Mega-wetlands have persisted over about 10% northern South America under different configurations for >15 million years. Although the paleogeographic reconstructions presented are simplistic and coarse-grained, they are offered to inspire the collection and analysis of new sedimentological and geochronological datasets.

The temporal evolution of erosion over million-year timescales is key to understand the 26 development of mountain ranges and adjacent fold-and-thrust belts. While models of 27 orogenic wedge dynamics predict an instantaneous response of erosion to pulses of rock 28 uplift, stream-power based models predict that catchment-wide erosion maxima 29 significantly lag behind a pulse of rock uplift. Here, we explore the relationships between 30 rock uplift, erosion, and sediment deposition in the Argentine Precordillera fold-and-31 thrust belt at 30°S. Using a combination of 10 Be-derived paleo-erosion rates, constraints 32 on re-exposure using 26 Al/ 10 Be ratios, geomorphic observations and detrital zircon 33 provenance, we demonstrate that the attainment of maximum upland erosion rates lags 34 the maximum rate of deformation over million-year timescales. The magnitudes and 35 causes of the erosional delays shed new light on the catchment erosional response to 36 tectonic deformation and rock uplift in orogenic wedges. 37 38

Far from the continental margin, drainage basins in Central Amazonia should be in topographic steady state; but they are not. Abandoned remnant fluvial valleys up to hundreds of square kilometers in size are observed throughout Amazonia, and are evidence of significant landscape reorganization. While major Late Miocene drainage shifts occurred due to initiation of the transcontinental Amazon River, local landscape change has remained active until today. Driven either by dynamic topography, tectonism, and/or climatic fluctuations, drainage captures in Amazonia provide a natural experiment for assessing the geomorphic response of low‐slope basins to sudden, capture related base‐level falls. This paper evaluates the timing of geomorphic change by examining a drainage capture event across the Baependi fault scarp involving the Cuieiras and Tarumã‐Mirim River basins northwest of the city of Manaus in Brazil. A system of capture‐related knickpoints was generated by base‐level fall following drainage capture; through numerical modeling of their initiation and propagation, the capture event is inferred to have occurred between the middle and late Pleistocene, consistent with other studies of landscape change in surrounding areas. In low‐slope settings like the Amazon River basin, base‐level fall can increase erosion rates by more than an order of magnitude, and moderate to large river basins can respond to episodes of base‐level fall over timescales of tens to hundreds of thousands of years. Copyright © 2013 John Wiley & Sons, Ltd.

In the Miocene (23–5 Ma), a large wetland known as the Pebas System characterized western Amazonia. During the Middle Miocene Climatic Optimum (c. 17–15 Ma), this system reached its maximum extent and was episodically connected to the Caribbean Sea, while receiving sediment input from the Andes in the west, and the craton (continental core) in the east. Towards the late Miocene (c. 10 Ma) the wetland transitioned into a fluvial-dominated system. In biogeographic models, the Pebas System is often considered in two contexts: one describing the system as a cradle of speciation for aquatic or semi-aquatic taxa such as reptiles, molluscs and ostracods, and the other describing the system as a barrier for dispersal and gene flow for amphibians and terrestrial taxa such as plants, insects and mammals. Here we highlight a third scenario in which the Pebas System is a permeable biogeographical system. This model is inspired by the geological record of the mid-Miocene wetland, which indicates that sediment deposition was cyclic and controlled by orbital forcing and sea-level change, with environmental conditions repeatedly altered. This dynamic landscape favoured biotic exchange at the interface of (1) aquatic and terrestrial, (2) brackish and freshwater and (3) eutrophic to oligotrophic conditions. In addition, the intermittent connections between western Amazonia and the Caribbean Sea, the Andes and eastern Amazonia favoured two-way migrations. Therefore, biotic exchange and adaptation was probably the norm, not the exception, in the Pebas System. The myriad of environmental conditions contributed to the Miocene Amazonian wetland system being one of the most species-rich systems in geological history.