SEISMOSTRATIGRAPHIC ARCHITECTURE OF CENTRAL AMAZONIA: MESO-CENOZOIC LANDSCAPE EVOLUTION

In tropical South America, the interplay of tectonics, climate, and hydrology influences terrestrial sedimentary processes of modern rivers and their antecedents, leading to transport of large amounts of sediment through Amazonia. Ultimately, this sediment is either preserved as basin fill along the major axes of long-lived sedimentary basins or transported offshore. Because outcrops are rare and chronologies are not well established in Amazonia, little is known about the non-petroleum-bearing, shallow Meso-Cenozoic strata. Our basin-scale geophysical assessment aims to develop an overarching geologic framework for the Brazilian Amazon, including the Acre, Solimões, Amazonas, Marajó, and Foz do Amazonas basins, during the Cenozoic ─ a period that holds key information about the evolution of the neotropical rainforest and eventual establishment of eastward flowing, trans-continental Amazon drainage.

The age at which the Amazon effectively transmitted water and sediment from the Andes to the Atlantic Ocean is highly debated. In central Amazonia, Cretaceous strata of the Solimões and Amazonas basins are separated by the Purus Arch, apparent as a subsurface high onto which Cretaceous strata thin from both the west and the east. There is little evidence, however, of the thinning of Cenozoic strata over the arch. On its eastern flank (the Amazonas basin), a low-angle, erosional unconformity separates the Cretaceous Jazida da Fazendinha Fm. from the Paleocene to Miocene Alter do Chão Fm. (Caputo, 2011), which implies overtopping of the paleohigh, and provides evidence that the arch did not act as a barrier to either eastward or westward flow of the Amazon during most of the Cenozoic.

Integration of high-resolution seismic reflection profiles, borehole geophysical and stratigraphic logs, and structural lineament data will provide the framework for subsequent geochronologic and provenance studies. The latter will be undertaken as part of a trans-continental Amazon stratigraphic drilling project that aims to recover complete Amazonian Meso-Cenozoic sequences.