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The Influence of Mass Transport Complexes on Sediment Dispersal Patterns in the Southern Magdalena Fan, Offshore Colombia


Mass transport complexes (MTCs) are important agents in the large-scale degradation and aggradation of continental margins; these flows can reshape the seascape quasi-instantaneously and influence sediment dispersal patterns. Examples from the Magdalena Fan, offshore Colombia, show how local deformation in the Sinú fold belt and uplift of the Andes have induced periods of instability and mass wasting, linked to MTCs that modify the bathymetry and influence long-term sediment pathways. This study is based on the detailed mapping and interpretation of 1600km2 of three dimensional seismic in the southern Magdalena Fan. Approximately 500m below mud line, an unconformity, which is a composite of three discrete erosion surfaces can be traced up-dip to different parts of the structural trend. It forms a ∼300km2 and 500m deep trough on the slope and is overlain by a 100m thick MTC that has an irregular top surface with protruding megaclasts. The MTC is onlapped by dim, parallel seismic reflectors. Subsequently, the southern external levee of a pre-existing NE-SW trending channel- levee system was breached causing a sustained avulsion into the adjacent bathymetric depression created by the MTC. A series of north- south trending bright amplitude crevasse lobes evolved into a new channel- levee system while the parent channel was filled by dim seismic facies. Above the avulsion complex, an unrelated series of eastward migrating NE-SW trending channel levee systems developed. The mapped three dimensional channel- levee surfaces illustrate the abrupt, down dip changes in the depositional architecture and geometry of the channel- levee systems. The lateral extent and geomorphology of the levees are affected by the erosional surfaces, the tectonic structures and the bathymetric anomalies created by older and coeval channel-levee systems. Also, the mud rich external levees form significant bathymetric features (more than 200m above the regional slope) that locally form mini-basins on the slope. The recent stratigraphy of the southern Magdalena Fan illustrates the importance of MTCs and mud-rich channel-levee systems in sculpting the seabed and changing sediment dispersal patterns. This has significant implications in the prediction of reservoir sands and stratigraphic trap potential in deeper exploration targets.