Mass-transport complexes (MTCs) can significantly modify the seascape by eroding the substrate and depositing thick hetherolithic packages that can behave as hydrocarbon seals or reservoirs. MTC erosion can affect the integrity of underlying reservoir units, and affect subsequent sediment dispersal. Moreover, the irregular seabed profiles resulting from MTC erosion and emplacement can affect the distribution and architecture of subsequent sediments. This study uses a 1900 km2 3D seismic volume from the southern Magdalena Fan, offshore Colombia to investigate: i) the relationship between changes in the size, distribution and provenance of the MTCs and the evolution of tectonic structures; ii) the relationship between and the distribution of MTCs, the geometries of their basal erosion surfaces and their internal characteristics, with the morphology and composition of the seabed, and iii) the effects of MTC-related bathymetric irregularities on the architecture and development of channel-levee complex sets and avulsion lobes.
The size, distribution and provenance of MTCs changed through time with the oldest MTCs being smaller (9-100 km2 in area) and sourced and from local collapse of the growing anticlines. Younger MTCs are larger (more than 200-300 km2) and sourced from the shelf, postdating the main phase of folding and faulting in the study area. These changes were used to propose a model of the tectono-stratigraphic evolution of the study area, demonstrating that MTCs can be used to constrain the timing and style of tectonic deformation in time and space. Additionally, the local morphology of the MTC basal surfaces reflects compositional changes in the underlying units, with deeper erosion occurring above channel axes deposits and more subtle slope changes across different levee units. MTC dispersal was influenced by a combination of structural and depositional relief: channel-levee complex sets channelized, diverted or blocked the subsequent mass-flows depending on the orientation of the channel-levee complex sets with respect to the direction of the flow, and the height of the levees with respect to flow thickness. Within the largest MTC, the distribution of the seismic facies was influenced 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. Moreover, MTC erosion left behind an erosional remnant ridge upon which a younger channel-levee complex-set developed irregular levee geometries that led to levee collapse and channel avulsion. Map-view geometries and seismic-amplitude extractions suggest that the initial avulsion lobes were mud-prone and evolved to form sand-prone lobes. The distribution, morphology and evolution of the avulsion lobe complexes were influenced by megaclasts protruding on the MTC top surface.
This study demonstrates that: i) the architecture, geometries and distribution of MTCs, channel-levee complex sets and avulsion lobes are strongly influenced by bathymetric irregularities on the seabed at various scales; ii) flow-pathways, geometries, distribution and internal characteristics of MTCs can be affected by the properties of the substrate; iii) the stratigraphic evolution of the Magdalena Fan is characterised by the interaction between MTCs and channel-levee complex sets. The learnings from this study can be applied to deeper intervals that are less well imaged and to other margins dominated by MTCs and channel-levee complex sets.
