Continental slope stability evaluation through morphological analyses using 3D seismic data at the Brazilian Equatorial Margin

Deep-water infrastructures (e.g cables for telecommunication or oil and gas platforms) require the assessment of geohazards on continental shelves and slopes, particularly gravitational and tectonic instabilities. Here we focus on the Potiguar Basin a sedimentary basin located on the Brazilian Equatorial Margin (BEM) – a transform passive margin. BEM’s rift started in the Cretaceous and the predominant strike-slip motion is marked by Ocean Fracture Zones (OFZs) that are impinging the margin at angles minor then 45. Our analysis started with a detailed map of the seabed horizon (SBH) in a 3D high-resolution seismic cube of 1850 Km2 and well-logs, both provided by the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP). The SBH evidenced at least 15 canyons eroding the seabed with 9 of them forming erosive well-defined valleys like channels. The shelf-slope stability was evaluated through flow pathways by calculating the sinuosity index (SI) and by extracting the slope profiles of the canyons. In the eastern portion of the dataset where faults are not present, the flow pathways vary from rectilinear (SI equal 1.1) with one knick-point (steepness 7.6) located in the outer slope coincident with an abrupt discontinuity on the seafloor (interpreted as a mass-waste deposit), to sinuous (SI equal 1.3) with a smooth concave upward profile (average steepness 1.76), to meandering (SI equal 1.4) with profiles that vary from relatively smooth to broken by strong knick-points on the outer part of the slope (34). In this latter case, the knick-point coincides with a “meander cut-off”. Two erosive valleys are also present in the central portion of the area where several faults were mapped. They both have SI equal 1.2 indicating low sinuosity. The first presents a concave upward profile (steepness 2.36) until it reaches the outer part where a knick point is identified (steepness 11.29) coinciding with several mapped faults. The other erosive valley, despite presenting a very sharp curve that deviates the flow pathway abruptly to the west, does not show any knick-point (average total steepness 2.0). Our preliminary analysis shows that fault activity seems to have limited control of slope stability. Slope stability seems instead to be related to sediment dynamics with mass-waste deposits derived from canyon walls that cause migration of flow pathways, cut-off of meanders, and knick points initiation.