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Influence of Substrate and Bathymetry on the Emplacement of Mass-Transport Complexes: Insights From the Magdalena Fan, Offshore Colombia


The morphology and composition of the seabed can influence the pathways and dynamic evolution of mass flows, thus impacting flow runout distances and the characteristics of the resulting deposits. Therefore, understanding the influence of bathymetry and substrate can improve our ability to predict the presence, distribution and characteristics of mass-transport complexes (MTCs) and their ability to act as hydrocarbon reservoirs or seals. This study uses a 3D seismic volume located in the southern Magdalena Fan, offshore Colombia, to investigate the influence of bathymetric fluctuations and substrate heterogeneity on the distribution, morphology and internal characteristics of nine MTCs. The MTCs overlie six substrate units, comprising mainly channel-levee complexes. The local geometry of the MTC basal surfaces reflect changes in the underlying seismic units, with deeper erosion occurring above deposits associated with channel axes and more subtle slope changes across different levee units. MTC dispersal was controlled by a combination of structural relief associated with the Sinú deep-water fold-and-thrust-belt, and depositional relief associated with underlying channel-levee complexes. Channel-levee complexes confined, diverted or blocked the subsequent mass-flows. The style of interaction was largely controlled by the orientation of the channel complexes with respect to the direction of the incoming flow, and the height of the levees with respect to flow thickness. Within the largest MTC, the seismic facies distribution was affected by the underlying bathymetry, with internal contraction occurring updip of bathymetric highs, erosion and bypass above higher gradient slopes, and increased disaggregation towards the margins. Hence, bathymetric irregularities and substrate heterogeneity together influenced the pathways, geometries and internal characteristics of the MTCs. This study shows that the interaction between mass flows and the underlying seabed can influence mass-transport flow pathways and runout distances, the preservation of channel axes, the type of substrate over which the mass flows propagate and their compositional, textural and rheological evolution. This impacts the prediction of presence and continuity of underlying hydrocarbon reservoirs and the sealing or reservoir capacities of MTCs.