Gravimetria e estratigrafia cenozoica da porção onshore da Bacia do Marajó e Plataforma Bragantina, nordeste do Pará

Abstract

The Northern Brazilian coast was an exceptional depositional site for siliciclastic and carbonate deposits during Oligocene to Holocene distributed in several basins and continental platforms that include Foz do Amazonas, Pará-Maranhão, Marajó and Bragança-Vizeu basins adjacent to the Bragantina and Ilha de Santana platforms. Although these geotectonic compartments are regionally well-know, their tectonic limits are generally inferred in subsurface by geophysical data. Likewise, these surface limits are poorly defined due to the extensive Cenozoic sedimentary cover. This study allowed to obtain a new interpretation for the gravimetric field related to intracrustal or residual density contrast for the eastern portion of the Marajó Basin. The presence of a deep depression at the limit of this basin with the Bragantina Platform, the Vigia-Castanhal Trough, confirm previous geophysical interpretation, as well as, attested by subsurface and surface stratigraphic studies. Furthermore, the identification of low gravity values below the -10 mGal anomaly with high thickness of sediments suggests a strong subsidence not compatible with the previous interpretation of a platform between the Marajó Basin and the Bragantina Platform denominated “Pará Platform”. This segment is frankly subsiding with depressions below -30 mGal being included here as part of the eastern edge of the Marajó Basin. The platforms are here interpreted with anomalies above -10 mGAL exemplified by the Bragantina Platform. Following this tectonic proposal and using previous stratigraphy, the sedimentary filling of the eastern part of the Marajó Basin is represented by siliciclastic deposits from the Marajó and Barreiras formations, respectively from the Oligocene-Miocene and Middle Miocene. Coastal and marine carbonate deposits of the 120 m-thick of Middle Oligocene-Miocene Pirabas Formation fill the reduced accommodation space of the Bragantina Platform. This unit is overlaid by ~40 m of fluvio-coastal deposits of the Barreiras Formation. The Quaternary is represented by the Post-Barriers unit of the Pleistocene-Holocene and recent sediments that form the alluviums and coastal sandy strand plains. The structural limits of the Bragantina Platform are re-evaluated and the inland portions to the south are exposed Silurian sedimentary and crystalline rocks. These basement rocks were a geographic barrier to the Oligocene-lower Miocene transgression. The use of residual gravity anomaly based on crustal modeling combined with geological and stratigraphic data is an effective tool to assess the geotectonic compartments limits covered by Cenozoic sedimentation and provide a new evolutionary understanding for this part of Amazonia.