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Analysis of all available seismic data from the Magdalena Fan in the southern Colombian basin, Caribbean Sea, allows subdivision of the sedimentary section into six seismic sequences (units). Although sediments were deposited in the present-day Magdalena Fan region since about Late Cretaceous, terrigenous sedimentation became significant only in the late Cenozoic during deposition of the upper three sequences associated with the uplifts of the Andes. These upper three sequences comprise the Magdalena Fan proper. The uppermost sequence probably represents the last main phase of sedimentation subsequent to the major uplift of the Andes in the Pliocene. The morphologic and shallow acoustic (3.5 kHz) characteristics of this fan unit are: upper fan, 1:60-1:110 gradients, chann ls having well-developed levees, and several subbottom reflectors in all areas except in channels; middle fan, 1:110-1:200 gradients, numerous channels with very subdued levees, and several to few subbottom reflectors; lower fan, <1:250 gradients, small channels, relatively smooth sea floor, and few or no subbottom reflectors. The decrease in number of subbottom reflectors as well as in subbottom penetration downfan apparently results from increasing amounts of coarse-grained sediments. Features in the form of regular hyperbolic echoes and sediment waves are very common in the upper, middle, and to some extent the lower fan, although their heights gradually decrease downfan. On multichannel seismic (MCS) records, the upper fan exhibits conspicuous channel-levee development and coalesc ng wedge-shaped reflection patterns indicative of levee deposits. The middle fan is characterized by the presence of chaotic and discontinuous reflection patterns, which may have resulted from the presence of numerous channels, and hyperbolic features and sediment waves of the type recorded on 3.5 kHz records. The lower fan region has relatively flat, continuous reflections. Within the topmost seismic unit, several episodes of sedimentation can be inferred from MCS records. These episodes are probably related to uplifts in the source region, lowered sea levels, or shifting of the Magdalena River delta in space and time, subsequent to the major orogeny in middle Pliocene.
A model of sedimentary processes proposed on the basis of the characteristics described visualizes a dominant role for channelized and overbank turbidity current deposition in the upper fan. The influence of these currents decreases and that of the unchannelized currents increases downfan. On the lower fan, deposition by unchannelized turbidity sheet flows forms the dominant mode of sedimentation. In addition, slumping on the back sides of oversteepened levees and along broad fronts of the continental slope, and other downslope mechanisms may also have influenced fan sedimentation significantly.
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