Conference:
International Meeting of Sedimentology, Toulouse, 10-12 October 2017, Page 212.
https://ims2017.sciencesconf.org
Authors:
Alberto Cruz, Jean-Pierre Suc, Tadeu dos Reis, Cleverson Guizan Silva, Didier Granjeon, Speranta-Maria Popescu, Christian Gorini.
Abstract:
This study addresses the Miocene to Recent continental shelf sedimentary record of the Amazon River Mouth Basin located off the Amazon River. A mixed carbonate-siliciclastic platform was established during the Late Paleocene in the basin mid-outer continental shelf, while siliciclastic sediments were trapped in the inner shelf. This scenario lasted until the Late Miocene when the mid-outer shelf witnessed a transition from predominantly carbonate sedimentation to an almost purely siliciclastic sedimentation. In the present work, we have refined the age model for the Cenozoic sedimentary succession of the offshore Amazon basin, based on the revision of the available micropaleontological data from industry wells and by comparing them with recently published astrochronologically-calibrated ages, considering the first and the last appearance of key planktonic species. Such work allowed us as well to establish a new age model for the carbonate sedimentation, including a more precise time for the carbonate production cessation in the basin, which was also tentatively correlated with eustatic sea-level curves. The resulting age model attests that the carbonate sedimentation ceased and was replaced by terrigenous sediments on the central and southeastern margin between 9.1 and 7.78 Ma (probably ~8 Ma). However, towards the northwestern margin, carbonate production persisted until around 4.0-3.8 Ma, pointing to the occurrence of a smaller flux of siliciclastic sediments into that area. Longer-lasting carbonate sedimentation on the NW carbonate platform can be probably explained by the presence of a ~150 km wide reentrant embayment formed in the margin’s central region during the Early-Middle Miocene, due to higher local subsidence rates, probably related to the existence of deep grabens-like structures inherited from the rifting phase. As a result, the shelf-edge backstepped in the central basin, forming a large embayment which connected the main source of terrigenous sediments from the paleo-Amazon river directly with the deep-basin. In such a scenario, the central basin became separated from the NW carbonate platform where local carbonate production was able to keep up with base level oscillations as the shelf-edge remained distally positioned located on a more stable region. Finally, during the Early Pliocene, a fraction of the paleo-Amazon river sediments was able to reach the NW region carried alongshore in the innermost shelf as indicated by a ~80 m thick prograding wedge. In the latest Early Pliocene most of the embayment between the central and northwestern shelf’s region became filled up by sediments delivered by the paleo-Amazon river when a combination of sea-level lowering and gradually higher volumes of terrigenous sedimentation rates finally promoted the burial of the previously carbonate-dominated environments across the entire basin. From the Late Pliocene onwards, only narrow reef-like features are locally recognized on the seismic data. These carbonates are usually 1 to 3.5 km wide but can reach more than 55 km in extension along the present and the paleoshelf break, attesting for reduced terrigenous influx into the outer shelf during interglacial marine transgressions.