EVIDENCE FOR A CONTINENT SCALE DRAINAGE INVERSION IN THE AMAZON BASIN SINCE THE LATE CRETACEOUS

New detrital zircon and sedimentologic data from deposits along the Amazon River reinforce interpretations that drainage patterns changed dramatically in the Amazon since the Cretaceous. It was proposed that the Amazon Basin was split into two basins before the mid-Miocene (e.g. Potter, 1997) by a paleo-divide (the Purus Arch?). Our data suggest this configuration was an intermediate step in a continent-scale drainage inversion.

The Late Cretaceous fluvial and alluvial Alter do Chão Formation is exposed along the eastern Amazon River. Formation thickness is maximum near the Rio Xingu mouth and thins westward. Marine facies were penetrated in the subsurface west of the Purus Arch suggesting a Cretaceous cross-continent connection. Paleocurrent measurements indicate west flow and Caputo (1984) suggested the sediment source was basement near the current Amazon mouth. Detrital zircon U-Pb age data from sandstone collected at Óbidos agree; ages are distributed unimodally around 2088 ± 7 Ma with a youngest grain age of 1648 ± 77 Ma, consistent with derivation from Maroni-Itacaiúnas basement of the easternmost Amazon craton.

Zircon age data were also collected for unnamed mid-to-late Miocene fluvial deposits at Manacapuru, on the east flank of the Purus Arch. Age spectra are distributed unimodally about 1904 ± 7 and 1910 ± 7 Ma with youngest grain ages of 1465 ± 51 and 1346 ± 49 Ma respectively. These ages are typical of the Ventuari-Tapajós province of the Amazon Craton and require no connection with the Andes. The results are consistent with the existence of a Miocene drainage divide and corroborate the presence of a Neomiocene lacustrine delta restricted to the western side of Purus Arch (Leguizamón et al., this meeting).

The data suggest that during the Cretaceous, cross-continent flow was east to west, possibly off an uplift related to Gondwana breakup. During the Miocene as western South America was affected by Andean orogeny and the eastern margin settled into its passive state, drainage shifted. As an intermediate step, the basin was split in two with an east draining eastern portion and an accompanying western basin that filled from the Andes to the west and the Purus-high to the east. When the western basin was sufficiently filled, probably after the late-Miocene, modern west directed Andes to Atlantic drainage begin.