The Tectono-Sedimentary Evolution of the Amazon Fan across the Central Transect (Foz do Amazonas Basin, Brazil)

Carrus, Emanuele ¹
(1)Scienze della Terra, Università "La Sapienza" Roma, Roma, Italy.

The problems of the tectono-sedimentary evolution of the Foz do Amazonas Basin, have been addressed by integrating on one regional seismic section a broad set of geophysical and well data.

Two different hypotheses were considered in the interpretation of gravitational tectonics affecting the Brazilian equatorial margin, exceptionally developed in this basin due to the thick sedimentary loading of the Amazon Fan. The first hypothesis contemplates thin skinned tectonics, while the second implies the presence and the activity of deep direct faults displacing the basement.

In order to discriminate which of the two hypotheses is the most reliable, the restoration of the central transect of the fan, 265 km long, was performed. The data on the depth of the basement, obtained by gravimetric and magnetic modeling, was also considered.

The first hypothesis envisages a deformation with loss of volume that, in order to be justified, requires removal of large amounts of sediments by erosion. The second hypothesis reduces the need of processes difficult to verify, therefore it is preferable.

Thanks to the high resolution of a proprietary 3D seismic survey, covering the critical area of the slope, it was possible to analyze in detail the syndepositional tectonic features of the most important sismostratigraphic units of the fan, in relation to the development of gravitational tectonics. It was identified the existence of at least two different levels of detachment. From seismic evidence and well data these levels can be interpreted as overpressured clays. These levels induced the detachment and deformation of the overlying sediments in at least two distinct tectonic phases, locally recorded by unconformities. In fact, fold and thrust belts, that are sealed by Tortonian unconformities, have been identified

The tectono-sedimentary evolution is characterized by a system of faults that is not related to a single event of gravitational collapse. This evolution differs from the reconstruction published in the literature for the possibility that the extension, due to a system of normal faults at the shelf break, is not completely transferred to the thrust in the shortening zone through the identified levels of detachment. It is hypothesized that a part of the extension observed may have been balanced during the sedimentation of the fan, from a temporary reactivation of deep crustal faults, originated during the rift continental margin of South America.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.