Carbonate sedimentation on the Amazon shelf: depositional evolution and new age constraints on the burial of a tropical carbonate platform

The continental shelf offshore the present-day Amazon River is known to have hosted a mixed carbonate-siliciclastic platform from the Late Paleocene to the Late Miocene. However, character of this platform has not been properly described and the nature and timing of the cessation of carbonate sedimentation remains controversial. In the present work, we investigate the Neogene succession of the Offshore Amazon basin, based on the stratigraphic analysis of a grid of 2D/3D seismic reflection data, correlated to revised micropaleontological data from exploration wells. This allows us to propose a new model for the transition from carbonate platform to siliciclastic sedimentation, which is shown to have varied through time across three different sectors of the shelf. In the Central and SE shelves, carbonate gave way to terrigenous sedimentation at some point between 9.1-7.78 Ma (probably around 8 Ma), whereas on the NW shelf, carbonate production persisted until 4.0-3.7 Ma. Longer-lasting carbonate sedimentation in the latter area can be explained by a lesser influx of siliciclastic sediments, favored by the development of a ca.150-km wide embayment in the Central shelf that directed terrigenous sediments sourced from the paleo-Amazon River directly to the continental slope and deep ocean. Carbonate environments persisted across the NW shelf until 5.5 Ma, keeping up with base level oscillations by aggradation. From 5.5-3.7 Ma (Early Pliocene), increasing sediment supply from the paleo-Amazon river reached the NW shelf resulting in the progressive burial of inner-shelf carbonates beneath a prograding siliciclastic wedge up to 85 m thick. Around 3.7 Ma, the Central shelf embayment was completely infilled and lager volumes of sediments supplied by the paleo-Amazon river were transported to the NW shelf, which finally promoted the burial of carbonate-dominated environments everywhere on the basin. Thereafter, carbonate environments were restricted to reef-like features recognized on seismic data as bodies locally interbedded with muddy Plio-Quaternary succession, generally 1-3.5 km wide, but up to 55 km in extension near the present and previous paleoshelf breaks, attesting to reduced terrigenous influx to the outer shelf during interglacial marine transgressions. The end of the carbonate platform was favoured by a combination of sea-level lowering and increasing terrigenous sediment supply after ca. 8 Ma that is also linked to the rapid growth of the Amazon deep-sea fan.