--> Mechanisms of Erosion and Entrainment by Submarine Mass-Flows; Insights From the Santos Basin, Offshore Brazil

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Mechanisms of Erosion and Entrainment by Submarine Mass-Flows; Insights From the Santos Basin, Offshore Brazil

Abstract

Abstract

Mass-flows can modify the seascape by eroding the substrate and depositing thick, typically mudstone-dominated mass transport complexes (MTCs). Substrate entrainment can affect MTC sealing capacity by introducing spatial variations in the deposit texture and composition; associated erosion may modify the connectivity of underlying reservoirs. Although erosion and entrainment by MTCs are widely recognised, the processes through which they occur are poorly understood. This study uses a 13,000 km2 3D seismic reflection volume from the Santos Basin, offshore Brazil to investigate the mechanisms of substrate entrainment by MTC-related flows.

Three packages of low-amplitude, chaotic seismic reflections, interpreted as debrites, occur in the Oligocene-Recent stratigraphy. These packages contain large (up to 10 km2) coherent blocks interpreted as megaclasts. Continuous parallel reflections within the megaclasts resemble the surrounding stratigraphy, suggesting local entrainment. The basal shear surfaces (BSS) of the MTCs are generally parallel to the underlying stratigraphy, but locally cut across it forming ramps. The BSS commonly overlies a package of parallel yet discontinuous reflections, and sometimes, thicker packages of undulating parallel reflections. Typically, the lateral and frontal margins of the MTCs, defined as the limit between the chaotic seismic facies of the debrites and the parallel reflections of the surrounding stratigraphy, are sharp and well-defined. Locally, however, such boundaries are more gradational, being characterised by discontinuous, parallel reflections that become progressively more continuous away from the chaotic facies; this transition zone is 300 m to 3 km wide. The transition zones at both the base and margins of the MTCs are interpreted as substrate deformed by MTC passage and emplacement.

Our data show that the passage of a debris flow can deform, weaken and cause seismic-scale strain of the substrate, creating a basal shear zone that underlies the BSS. In addition, it appears that a similar shear zone can also develop beyond the lateral and frontal limits of the MTCs. Deformation and weakening of the substrate could facilitate entrainment and may explain the presence of kilometre-scale megaclasts and ramps observed along MTC BSSs, as well as the highly erosive nature of some MTCs, both of which can affect reservoir presence and seal capacity.