This paper summarizes the geologic knowledge of the Foz do Amazonas basin in Northern Brazil, which has been investigated by Petrobras through geophysics and exploratory drilling. The basin covers an area of about 200,000 km2, including the present coastal zone, continental shelf, and. Amazon submarine fan. This potentially prospective basin has been filled from Albian to Recent; may contain 14 km of sediments along the present-day shelf edge; and consists of a central deep basin flanked by two marginal platforms.
The basin has passed through three tectonic stages since its beginning in the Albian; its main structural trends are related to fracture zones and ridges of the equatorial Atlantic Ocean.
This paper describes the structural and stratigraphic framework of the Foz do Amazonas basin and integrates it with the northern Brazilian continental margin.
The basin under the mouth of the Amazon River in northern Brazil, known as the Foz do Amazonas basin, comprises an area of about 200,000 km2, including a coastal zone, continental shelf, and a submarine fan. Presently two main rivers reach the basin, the Amazon and the Tocantins (Fig. 1). The channels of both rivers become shallow where they reach the coastline and here they have tidal ridges typical of tide-dominated deltas. Mud brought by these rivers is deflected northwestward by the strong Guiana current and is deposited on the inner shelf almost all the way to the Orinoco delta in northeastern Venezuela. The recent sedimentation does not reach the outer shelf and slope because of the recent high stand of sea level (Fig. 1).
Petroleo Brasileiro S.A. (Petrobras), the Brazilian national company for oil exploration, has investigated the basin with both geophysics and exploratory drilling. Seismic records of good quality permit the use of seismic stratigraphy for faciologic-structural mapping, identification of shelf margins, and the configuration of on and offlapping units.
The Foz do Amazonas basin has a sedimentary section over 5 km thick onshore and may be as thick as 14 km along the present-day shelf edge. The age of the sediments in the basin ranges from Middle Cretaceous (Albian?) to Quaternary.
The main structural provinces are two onshore grabens, Limoeiro and Mexiana, which extend to the continental shelf, and the "Amazon Deep" (Rezende1) which continues into the modern submarine fan (Amazon Cone) off the Amazon River (Fig. 2).
The Limoeiro graben extends southeastward into the Paleozoic Maranhao basin, while the Mexiana graben is aligned northeastward. The Mexiana graben continues to the continental shelf, where geophysics suggests that it bifurcates into the Mexiana West and Mexiana East grabens (Rezende1). Thus tectonics were very active during Upper Cretaceous, when terrigenous sedimentation prevailed in the basin. This tectonic activity was reduced during Paleogene times so that a wide carbonate platform developed over most of the modern continental shelf.
