Cretaceous and possibly Paleogene strata, originally defined as the Alter do Chão Formation, are extensively exposed in the Amazonas Basin. While theoretical considerations suggest that the eastern Amazon has been draining into the Atlantic since pre-Miocene times, the limited published field data indicate a significant continental-scale reversal of drainage, transitioning from westward flow during the Cretaceous to the current eastward regional flow. This hypothesis has a significant impact on accepted paleogeographic reconstructions and has implications for the origin of the Amazon River and the associated biotic environments. Nevertheless, the observational database to support that hypothesis is still very scarce. To address this problem, the present study presents data from a basin-transverse profile where the entire stratigraphic succession of Cretaceous and Paleogene ages in the Amazonas Basin is exposed, namely the Jazida da Fazendinha and Alter do Chão formations. Facies analysis of cliff outcrops along the Tapajós River margins, including the type area of the units, reveals associations interpreted as fluvial deposits dominated by sandy bars. Paleocurrents were interpreted from more than 2000 individual measurements of fluvial cross-strata from various stratigraphic levels. The findings indicate that the regional flow direction during the deposition of both units was eastward across the entire basin transect. Integrating our dataset into the existing data on provenance reveals that, unlike the modern Andean-derived system, the primary sources for the Alter do Chão Formation were limited to the Amazon Craton. Our findings offer compelling empirical evidence to support the scenario proposed by previous numerical models. The findings also suggest that the rock record does not support the poorly documented and highly influential model of a significant westward-flowing river in Amazonia during the Cretaceous and the Paleogene.
Keywords
Alter do Chão Formation, Amazonas Basin, Amazonas paleocurrents, Eastern Amazonia, Amazonas paleogeography, Sanozama
