Abstract

The active continental margin of northern Colombia has been affected by interactions between the Caribbean and South American plates since the early Miocene onset of the Andean orogeny. Since late Miocene time the Magdalena River has built a large submarine fan (Magdalena submarina fan, MSF) into the Caribbean Sea. Compression along the plate boundary has generated imbricated thrust zones in the eastern and western parts of the fan, straddling a relatively undeformed central area.

An evaluation of the grading stage (i.e., graded to out-of-grade continental margin [Ross et al., 1994, 1995]) was done for the east and west imbricated thrust zones, as well as for the undeformed central fan area by applying criteria developed by Pyles et al. (2011). The dataset used in this study includes side-scan sonar images of the modern seafloor, 12 two-dimensional seismic surveys, and piston core descriptions.

The east and west deformed wedges correspond to the early stage of out-of-grade submarine deposition with slope irregularities restricting sediment flow to tortuous corridors (i.e., piggy-back basins). Two types of sediment flows predominate: (1) relatively constant parallel-to-ridge flows originating from the shelf and (2) short-term perpendicular-to-ridge flows controlled by ridge instability and slumping. The upward stacking pattern near the canyon mouths of piggy-back basins consists of MTDs, lobe sands, and channel fills.

Deposition in the less deformed central fan corresponds to the late stage of an out-of-grade margin that is characterized by erosion and sediment bypass on the upper slope and deposition on the middle and lower slope. The angles of the upper, middle, and lower slopes range from 2.5°–3.5°, 1.5°–2.5°, and 0°–1.5°, respectively. There is no evidence of progradation beyond the central fan area shelf break and channelized features are mainly erosional. Channel-levee systems and mass transport deposits are common on the middle and lower slopes.

The MSF system is a special example of an out-of-grade fan that is in the process of becoming graded; therefore, systematic features are typical of an out-of-grade end member in transition to an “intermediate-grade” member, making this an interesting setting for stratigraphic studies on an active margin.