Morfologia, distribuição e mecanismos condicionantes das feições de instabilidade de talude da Bacia da Foz do Amazonas, Margem Equatorial Brasileira

Abstract

In the present study we propose an up-to-date scenario for the occurrence of slope instability features that affect the Offshore Amazon basin. Interpretation of newly available high-resolution acoustic data (3.5 kHz and along-track multibeam profiles) integrated with new bathymetric digital models together with the re-interpretation of pre-existing 2D multichannel seismic lines revealed the occurrence of widespread recent submarine instabilities across large domains of the basin. Two main distinct areas where slope instabilities occur could thus be distinguished according to their morphological expression in terms of extension and size: the SE/NW and Central slopes: (i) the SE and NW slopes, located on the flanks of the Amazon Fan, exhibit large scar zones (up to 5.630 km2) and significative vertical reliefs (up to 1300m high), in areas with slope gradients up to 12°, reaching ~3250m depths. These scar zones would be the source to mostly cohesive MTDs observed downslope, with total lengths (total mappable length of the slide from the upslope shallower scar to the downslope limit of connected deposit) of ~285km; contrarily, (ii) the Central Sector – that coincides with the location of Amazon deep-sea fan - received higher sediment inputs, which is reflected by smoother slope gradients (<3°). Those slide scars occur between ~380 and 2850m, are normally smaller in area (<1600km2) cutting much shallower into the stratigraphy (scarps <180m high). Also in central sector, structurally-driven scars are found, as a result of gravity-tectonic structures that strongly impact seafloor, generating vertical reliefs >600m. They were predominantly associated to desintegrative MTDs, with impressive total lengths > 170Km downslope. While cohesive MTDs probably represent stronger rheologies and/or older sediments involved, desintegrative MTDs would consist in weaker and/or younger sediments remobilized – what would be expected due to the development of the Amazon Fan from Upper Miocene – Recent on central basin. Principal preconditioning factors include high sedimentation rates - variable along the distinct slope sectors - and the presence of anomalous pressures, as the overpressured unit that act as the most superior décollement level to the fan succession, promoting the gliding and deformation of strata located above. Comparing instability features and deposits from the present study with deeper sediment slide occurrences previously mapped within the Pliocene-Quaternary stratigraphy succession, we could conclude that: (i) both gravity-driven processes (exposed ones versus ancient) present similar areal distribution on the Central and on SE/NO sectors, showing that instability processes are quite recurrent; (ii) however, on NW and SE sectors, seafloor instabilities are less frequent, and together with normal upslope scarps retrogradation, lead to the development of much larger and deeper slide scars, as well as to higher volumes of sediment remobilizations, on the flanks of the Amazon fan. The identification of main geohazards on the Offshore Amazon basin is necessary since this basin represents a new frontier considering the hydrocarbon potential in the brazilian margin, and the results can subside future projects of hazard and risk assessment in the area