Abstract The Magdalena Fan is part of the Caribbean Basin of Colombia, an exploration frontier currently labelled as a gas-prone province. It is also one of the places with considerable chances of still finding billion barrel ( in situ ) hydrocarbon accumulations in Colombia. This chapter presents the elements that could assemble together to define an untested oil-play in the Magdalena Fan, which is described as Miocene turbidite lobes in frontal deformation anticlines, oil sourced from upper Cretaceous–Paleocene marine restricted shales. The play combines the following components: Upper Cretaceous–lower Paleocene marine oil-prone source rocks (interpreted to be equivalent to the Cansona and Arroyo-Seco Formations), which according to a three dimensional basin model, expelled the majority of the oil from approximately 5 my to the present day. Hydrocarbon migration along sandy middle Ecocene carrier beds and vertically through faults. Four-way and thrust-faulted anticlines in the frontal slab of the Southern Sinu fold belt that are associated with the late Miocene to Plio–Pleistocene uplift (beginning at ca 7.5 my). Middle Miocene deep water turbidite reservoir composed of frontal splays that developed during the lowstand systems tract. Top seal formed by a fine-grained succession deposited during the transgressive systems tract. There is still significant uncertainty related to the limited amount of data in the Magdalena Fan, especially in the deeper part of the stratigraphic section; nonetheless, the geologic model presented supports an exploration opportunity with the potential for unlocking considerable hydrocarbon resources.

Abstract The slope morphologies of the Magdalena deepwater fan exhibit a series of channel-levee complexes (CLCs), recording the evolution of the Magdalena delta. Detailed morphological analysis of the seafloor expression of the channels and their lateral relationship allows the reconstruction of the history of Pleistocene fan development. The Magdalena deepwater fan was deposited on the northern offshore Colombia accretionary wedge (Caribbean Sea), initiated during the late Miocene. Fan evolution is closely related to the Magdalena River delta migration and reflects control by tectonic processes occurring from Pliocene to present. Major delta shifts toward the southwest (Canal del Dique) and northeast (Cienaga de Santa Marta region) create a submarine fan that migrated with the river, becoming younger toward the southwest. The main fan was abandoned during the Holocene, focusing deposition on the Barranquilla region to the northeast with modern active sedimentation. The depositional processes in the active fan area are mainly dominated by turbidity currents, alternating with slumps/debris flows that generated large mass transport deposits (MTDs). Eight river delta phases were identified, linked to the onshore geology and their corresponded submarine fan expression, which is characterized by the presence of CLCs and MTDs. Seven CLCs were studied using multi-beam bathymetry and seismic profiles. The CLCs showed a big variation of sinuosity and gradient throughout the slope. The higher sinuosity values were encountered at areas of high gradients, suggesting that the channels attempt to reestablish its equilibrium profile by increasing sinuosity as a response of changes in the slope. Highly sinuous channels in the western fan suggest that sinuosity changes are controlled by changes on the slope associated with the deformation of the fold-and-thrust belt along the margin. In addition, channel’s forced migration, avulsions, convex-up profiles, and the presence of knickpoints (KPs) suggest ongoing deformation during western CLC deposition. Conversely, the northeastern section of the fan exhibits channel thalweg profiles with lower sinuosity values at deeper depths. Convex-up thalweg profiles in this area may represent disequilibrium profiles or post-abandonment deformation. Older CLCs are highly affected by degradational processes after the abandonment of the systems, increasing channel width and modifying levee walls. These processes should be considered when evaluating dimensions of buried deposits and reservoir quality prediction. A sequence of KPs in the western fan seems to connect sediment flows from the shelf break downslope through a series of steps in the slope, culminating with lobate unconfined deposits. Upstream KP migration in slope steps as a response to deformation may represent a key process to explain the initiation of deepwater channel systems on the Magdalena Fan, as well as channel systems deposited on other tectonically active basins. This study provides new understanding of the processes involved in the Magdalena deepwater fan and implications for channel systems characterization in areas with active deformation during deposition.

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.

Abstract Previous studies along the Andean subduction zones of South America have shown that forearc basins can develop over shallow-dipping the subduction zone dips horizontally or up to 15°, and that these shallow-dipping subduction zones can alternate with more steeply dipping (>30°) subduction zones over distances of 400–1500 km (249–932 mi). This study describes the Cenozoic structural and depositional history of the Lower Magdalena Basin (LMB)—an Oligocene to Recent forearc basin covering an area of 42,000 km 2 (16,216 mi 2 ) and overlying a zone of shallow subduction (the depth to the top of the Caribbean slab ranges from 30 km to 90 km [19 to 56 mi] beneath the LMB). Using 7000 km (4350 mi) of two-dimensional (2-D) seismic reflection lines tied to 33 wells, we describe the initial Oligocene subsidence of the forearc basin along a radial array of 70°- to 110°-striking normal faults that remained active until the early Miocene. During this period, the LMB was underfilled by 1–3 seconds two-way-time (TWT) (1500 m [4921 ft]) of shallow-marine and deep-marine facies. During middle Miocene the LMB remained underfilled with marine sediments deposited in water depths of 200–2600 m (656–8530 ft). An angular unconformity spanning the interval of 11–7 Ma marks a shortening and uplift affecting the Sinu accretionary prism west of the LMB that became emergent to form a prominent forearc high along the western edge of the LMB. The regional structure of the LMB is a broad syncline that folds all units older than early Miocene and produces an asymmetrical shape—in profile—with the western edge of the LMB (against the Sinu accretionary prism), steeper than the eastern edge of the LMB. After the late Miocene–Pliocene, the forearc high continued to elevate and separate the LMB from the outer Sinu accretionary prism. During this period, the LMB overfilled with terrigenous sediments of shallow marine facies that spilled offshore into the Caribbean Sea to form the proto-delta of the Magdalena Fan; these spilled sediments led to rapid tectonic accretion and growth of the offshore Sinu accretionary prism from 5 Ma to present. During the period of Oligocene to middle Miocene, different structural styles and subduction-related magmatic intrusions suggest that the Caribbean slab was subducting at an angle greater than 30° with a discontinuous volcanic arc. The decrease in the dip of the Caribbean slab to its modern dip angles of 4–8° occurred during the late Miocene and is interpreted as the entry of thicker Caribbean oceanic plateau crust into the subduction zone. Comparison of the segmented dip of the 400-km-long (249-mi-long) subducting Caribbean slab is consistent with the upper, 220-km-long (137-mi-long) shallow-dipping part subducting at rates of 2 cm/yr (0.78 in/yr) from 11 Ma (late middle Miocene) to Recent. We propose that this change from the steeper to shallower-dipping slab in the middle Miocene led to (1) increasing elevation of the forearc high of the Sinu prism along the eastern edge of the LMB; (2) the regional synclinal structure of the LMB in profile; and (3) the possible elevation of the entire LMB after 11 Ma as it changed from underfilled, deep-water marine environments to overfilled, shallow-water marine and fluvial environments.