South America
South America
Academic Project:
Neogene Stratigraphy and Depositional Environments of the Amazon shelf: the life and death of the Amapá Carbonate Platform (PhD A. Cruz)
The continental shelf offshore the present-day Amazon River is known to have hosted one of the world’s largest mixed carbonate-siliciclastic platforms from the late Paleocene to at least the late Miocene. The character of this platform remains poorly understood and the causes and timing of the cessation of carbonate sedimentation controversial. Here we examine the Neogene succession of the offshore Amazon Basin, based on the stratigraphic analysis of a grid of commercial 2D and 3D seismic reflection profiles correlated to revised micropaleontological data from exploration wells. Four main stages of distinct carbonate depositional patterns are defined and dated with reference to a new age model. The results provide improved constraints on the age of the transition from predominantly carbonate to siliciclastic sedimentation, which is shown to have varied through time across three different sectors of the shelf. On the Central and SE shelves, carbonate production gave way to terrigenous sedimentation between 9.1 and 7.78 Ma, whereas on the NW shelf carbonate production persisted until 5.5–3.7 Ma. Longer-lasting carbonate sedimentation on the NW shelf can be explained by a lesser influx of siliciclastic sediments, favored by the development of a 150-km wide embayment in the Central shelf. This embayment directed terrigenous sediments sourced from the paleo-Amazon River to the continental slope and deep ocean. As a result, carbonate production remained dominant across the NW shelf until 5.5 Ma, keeping up with base level oscillations. From 5.5 to 3.7 Ma (early Pliocene), sediment supply from the paleo-Amazon River promoted the progressive burial of carbonates on the inner NW shelf beneath a prograding siliciclastic wedge up to 85 m thick, coeval with the progressive filling of the central shelf embayment. Once the central embayment became completely filled, continuous sediment supply to the NW shelf resulted in the final transition from carbonate to siliciclastic-dominated environments on the offshore Amazon Basin.