SEISMOSTRATIGRAPHIC ARCHITECTURE OF CENTRAL AMAZONIA: MESO-CENOZOIC LANDSCAPE EVOLUTION

In tropical South America, the interplay of tectonics, climate, and base-level influences terrestrial sedimentary processes of modern rivers and their antecedents, leading to transport of large volumes 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, little is known about the non-petroleum-bearing, shallow Meso-Cenozoic strata. Our seismostratigraphic analysis, the integration of high-resolution seismic reflection profiles, geophysical borehole logs, and gravimetric data, aimed to characterize basin style and develop an overarching geologic framework for the Brazilian Amazon, including the Acre, Solimões and Amazonas Basins, during the Meso-Cenozoic. This period holds key information about the tectonic and stratigraphic history, which led to the evolution of the neotropical rainforest and the eventual establishment of eastward flowing, trans-continental Amazon drainage.

Results suggest that sediments were deposited in an intracratonic rift basin that was significantly influenced by Meso-Cenozoic geodynamics (i.e., Penatecaua Magmatism, Juruá Transpression, and Andean orogeny) recorded in mapped structural features. 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 west and east. Based on available seismic and borehole data, we agree with Caputo (2011) that the Paleocene to Miocene Alter do Chão Fm. is, indeed, Cenozoic and that the Early Cretaceous Jazida da Fazendinha Fm. composes the covered Mesozoic sequence. This means that the thick axial-draining, fluvio-deltaic Cenozoic sequence is Andean-sourced, widespread and that the Purus arch did not act as a barrier to either eastward or westward flow of the Amazon during most of the Cenozoic.

Our seismostratigraphic analysis provides the necessary framework for subsequent geochronologic and provenance studies undertaken as part of a trans-continental, stratigraphic drilling expedition aiming to recover complete Amazonian Meso-Cenozoic sequences.