Evolution of the Magdalena Deepwater Fan in a Tectonically Active Setting, Offshore Colombia, in C

Romero-Otero, Gloria, Roger Slatt, and Carlos Pirmez, 2015, Evolution of the Magdalena Deepwater Fan in a Tectonically Active Setting, Offshore Colombia, in C. Bartolini and P. Mann, eds., Petroleum geology and potential of the 24 Colombian Caribbean Margin: AAPG Memoir 108, p. 675–708. Evolution of the Magdalena Deepwater Fan in a Tectonically Active Setting, Offshore Colombia

Gloria A. Romero-Otero Murphy Exploration and Production Corporation, 9805 Katy Fwy., Houston, Texas 77024, U.S.A. (e-mail: [email protected]) Roger M. Slatt The University of Oklahoma, Conoco-Phillips School of Geology and Geophysics, 100 East Boyd, St. Suite 810, Norman, Oklahoma 73072, U.S.A. (e-mail: [email protected]) Carlos Pirmez Shell Petroleum Development Company of Nigeria Ltd., Plot 461, Constitution Avenue, Central Business District, Abuja, Nigeria (e-mail: [email protected])

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.

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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.

Introduction and interpretation of its history. Seismic reflection data and detailed bathymetric coverage reveal the tempo- The Magdalena submarine fan is the main physi- ral and spatial evolution of the fan by examining the ographic feature comprising the present-day bathym- stratigraphic relationships between the various CLSs. etry of the modern seafloor morphology of offshore Based on the previous observations, the aims of this northwestern Colombia. It is one of the few deep-sea chapter are the following: fans with turbidity current activity today (Heezen, 1956; Munoz, 1966). The fan consists of a series of 1. To understand the evolution of the system from submarine channel–levee complexes (CLCs) and source to sink, by relating the evolution of the sub- mass transport deposits (MTDs), mainly formed marine fan to the Magdalena River drainage sys- by transport and deposition of sediments from the tem onshore, as outlined in previous studies. This Magdalena River, the main drainage system in Colom- approach allows for constraining the models for bia (Figure 1). The fan extends about 68,000 km2, with sedimentation–tectonic interactions and for plac- a volume of 180,000 km3 and extends to over 4000 m ing constraints on the timing of fan evolution. (13,123.3 ft) of water depth (Kolla and Buffler, 1984a, 2. To link the spatial and temporal evolution of the b; Wetzel, 1993; Reading and Richards, 1994). The sedimentary system to the patterns of tectonic de- Magdalena submarine fan forms a significant part of formation of the margin, including an analysis of the accretionary wedge complex formed by the col- the morphology of the various submarine CLSs, lision of the Caribbean and South American plates characterizing the thalweg profiles and variations (Duque-Caro, 1979; Breen, 1989). in sinuosity and gradient. Previous studies on the morphology and stratigra- 3. To study the effect of active deformation dur- phy of the Magdalena Fan (Kolla and Buffler, 1984a, b; ing the establishment of the channel systems and Ercilla et al., 2002a; Estrada et al., 2005a) showed that, post-abandonment, by characterizing the thalweg despite its active margin setting, the fan had features profiles, sinuosity, and slope changes. Most stud- that resemble the large fan systems encountered off ies of submarine fans are from passive margin set- major rivers on passive margins, such as sinuous tings. The Magdalena Fan is deposited in an active channel–levee systems (CLSs) and large MTDs. Previ- margin and reveals active deformation during the ous works by Hoover and Bebout (1985) and later by deposition of the channel systems, providing an Pirmez et al. (1990) addressed the link between migra- opportunity to study possible differences between tion of the Magdalena River course, regional tectonics, active and passive margin systems. and the deepwater fan deposits. 4. To discuss degradational processes to which the In this chapter we present new bathymetric and channels have been exposed after abandonment seismic data that complement previous studies and and the role of slope deformation on channel– allow for a more complete understanding of the fan levee morphology and knickpoint (KP) formation.

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Figure 1. Bathymetry map of the Magdalena Fan, southern Caribbean Sea. Location of the channel– levee complexes (CLCs) and active fan. Canyons: U (unamed), S (Sabanilla), M (Magdalena), D (delta front gullies), SB (shelf break). Regional cross sections X–X' and Y–Y' are shown in Figure 6, and locations are shown in Figures 9B, 10B, 12A, 17A, and 19A. Multi-beam bathymetry—slope of the bathymetry. Cities of Cartagena, Luruaco, and Barranquilla are shown as a reference. Thalweg sections for these channels are depicted in Figure 7. 5 km (3.1 mi)

Geological Setting offset in these two fault systems occurred during the last 10 Ma and is linked to the most important uplift The Magdalena submarine fan is an arcuate bathym- of the massif. The San Jacinto fold belt represents the etric feature, part of the accretionary wedge formed onshore portion of the accretionary complex com- by the subduction zone of the Caribbean–South posed of late Cretaceous to Pliocene sedimentary American plates (Duque-Caro, 1979; Kolla and Buffler, rocks. Deformation began during the early Paleogene 1984b; Breen, 1989). The Caribbean plate subducts and was reactivated during the late Miocene–Pliocene toward the east-southeast, at a low angle beneath the Andean compression (Ruiz et al., 2000). The Sinu South American plate and at a rate of 20 ± 2 mm/yr fold belt lies west of the San Jacinto fold belt and is (Trenkamp et al., 2002; Corredor, 2003). separated by the Sinu lineament (Duque-Caro, 1979) Sediment deposition along the Caribbean Mar- (Figure 2). Composed of Oligocene to Holocene sedi- gin of Colombia, and in the Magdalena Fan, began ments, the Sinu fold belt extends to the offshore area during late Cenozoic time (Kolla and Buffler, 1984b) represented by a series of imbricate structures, which (Figure 2). In the middle of the margin, the Magdalena become progressively younger toward the toe of the Fan forms a bathymetric bulge, separating two arcuate slope in an apparent break-forward sequence. The deformation fronts that delineate the fold-and-thrust decollement surface seems to occur in overpressured belts east and west of the Galerazamba shelf. The fan shales deposited during early Miocene (Vernette et al., appears largely undeformed (Breen, 1989), apparently 1992). Piggy-back basins have been preserved in the modifying the geometry of the margin. belt structures in the upper portions of the slope; they The main structural elements essential to the tectonic have been filled by mass transport complexes. Turbid- evolution of the margin are the Santa Marta massif, ity flows were later affected by collapse of some pre- the San Jacinto fold belt, and Sinu fold belt (Figure 2). existing compressional structures (normal faulting). The Santa Marta massif is an uplifted basement block, Sinu Belt structures are aligned to the Sinu lineament bounded by major strike-slip faults (Bucaramanga and were mostly formed during the Pliocene, although system to the west and Oca fault to the north). Kellog the prism is still active. Evidence for Pleistocene defor- and Bonini (1982) suggest that the majority of the mation occurs in some structures and is supported by

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Figure 2. Major structural elements. The Magdalena Fan is characterized by the presence of two deformation belts that correspond to the Sinu fold belt initiated during the Miocene. They are separated by the main fan deposition. (C) Canoas fault. SNSM denotes Sierra Nevada de Santa Marta (Santa Marta massif). 10 km (6.2 mi)

geodetic observations (Kellog and Vega, 1995). Shale fault systems. Later, Flinch et al. (2003) proposed an ridges and mud diapirism are important elements in accretionary prism for the area that extends from the the system. Diapirs located on the slope at the north- Uraba Basin in the south and joins the northern accre- eastern and southwestern deformation fronts, as well tionary wedge of Venezuela. High sediment supply as onshore (Totumo Volcano), are common in the to the offshore wedge induced a critical taper stage basin. Gas hydrates (identified on seismic profiles as (Davis et al., 1983) and subsequent collapse of the bottom-simulator reflectors [BSRs]) and gas seepage pre-existing compressional structures. Folding and are also present throughout the slope (Shepard, 1973; thrusting is less evident along the proto-Magdalena Shipley et al., 1979; Vernette et al., 1992). (Galerazamba area) because of a high sedimentation The tectonic history of the offshore accretionary rate during deformation. complex is still not completely resolved. Breen (1989) proposed that rapid Magdalena Fan deposition has had a structural effect on the geometry of the conver- Magdalena River History gent plate margin, creating an indentation and curvature in the accretionary wedge. As a consequence, The temporal variability of the Magdalena River is according to Breen (1989) the two arcuate deforma- intimately linked to the tectonic history of northern tion fronts were emplaced and deformation inboard of South America. Hoorn et al. (1995) indicate a change the Magdalena Fan increased, raising the tectonically in the northern South America drainage system dur- driven inland uplift (e.g., Santa Marta Block). Ruiz et ing the early Miocene. Initiation of the eastern cordil- al. (2000) presented a more complex scenario based lera uplift in the late–middle Miocene (between 12.9 on seismic interpretation and anomalies observed in and 11.8 Ma) generated a north and northeast flow of gravity and magnetics data. They divided northwest- the river system in addition to the existing east and ern Colombia into two zones separated by the Canoas southern flows. Part of the drainage was directed fault zone: (1) zone of accretion (south of the Canoas northward along the paleo-Orinoco River to a delta in fault) and (2) zone of transpression–transtension the Lake Maracaibo area. At 11.8 Ma, the current direc- (Figure 2) between the Canoas and Oca–Santa Marta tions shifted completely toward the north, changing

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from a meandering to an anastomosing fluvial sys- Deepwater Fan Deposits tem (Guerrero, 1993; Flynn et al., 1997). Bordine (1974) documented the paleo-geography for land deposits in Deepwater submarine fan deposition began as a result the lower Magdalena Valley (Figure 3). Late Miocene– of the migration and establishment of the Magdalena early Pliocene marginal and shallow marine deposits River northward during the Miocene. The proto- are the most prominent in the area. Link (1927) rec- Magdalena Fan was mainly fed by the Magdalena ognized a major ancient channel near Calamar, flow- River, but sediments of the Sinu River may also have ing northwestward near Luruaco, probably of late contributed to the fan (Pujos and Javelaud, 1991). The Pliocene age (based on planktonic foraminifera). The Magdalena Fan deposits were divided by Kolla and continuous northeast–southwest uplift and the pres- Buffler (1984b) into upper, middle, and lower fan, ence of the Pleistocene La Popa limestone near the based on sub-bottom profiles and piston core exami- ancient river mouth at Galerazamba indicate a forced nation. The more recent units reveal several peri- shift in the river occurred resulting in south-westward ods of incision and channel activity, reflecting uplift flow (Canal del Dique) (Figure 3). The reef build- in the sediment source region, changes in sea levels, ups were established on topographic highs created and delta shifts in space and time that can be related by shale diapirs. The uplift of the Atlantico-Turbaco to the Andean orogeny in the middle Pliocene (Kolla Hills across the river’s course caused a major east and and Butter, 1984b). Modifying the earliest division of northeast shift. Since then, the river has partially filled the area proposed by Ercilla et al. (2002a), the fan can its estuary and has built three small, submerged delta be divided into (1) deformed compressional belts and lobes across a narrow shelf (Hoover and Bebout, 1985). (2) main fan area. The deformed compressional belt It is important to mention that incipient deformation areas include the arcuate northeast and southwest has been observed in coastal deposits by deformation thrust belts, expressed on the seafloor as elongated of the Popa limestone along the coast line, particularly ridges with strike along the margin (Figures 1 and 4). at the Dique canal area (Martinez and Robertson, 1997; The main fan area is characterized by leveed channel Reyes et al., 2001). complexes, large-scale mass-flow deposits, canyons,

Figure 3. Magdalena River course shifts from the Pliocene to the present-day location. Relative order of delta phases: E, D, C, B, and A. Note the presence of the La Popa coralline limestone near Barranquilla that supports the southward shift of the drainage (Reyes, 2001). Cities of Cartagena, Luruaco, and Barranquilla are shown as a reference. 5 km (3.1 mi)

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Figure 4. Architectural elements of the Magdalena Fan (modified from Ercilla et al., 2002a). Northeast- ern and southwestern deformation belts are the boundaries of the channel-levee complex (CLC) studied. Also note the presence of mass transport deposits (MTDs) located at the inter-channel system lows. 5 km (3.1 mi)

Table 1. Sinuosity range. Spanish vessel Bio-Hesperides acquired approximately 32,500 km2 of bathymetry data (Ercilla et al., 2002a) 1.05–1.25 Low with the multibeam echosounder SimRad EM-12 S120. 1.25–1.5 Moderate Two surveys were acquired in 2002 on behalf of Eco- >1.5 High petrol (14,700 km2) and total E&P (11,400 km2). Data were collected using a hull-mounted, multi-beam Values of 1 correspond to straight channels. echosounder Reson SeaBat 8169 (50 kHz; for water depths between 100 m (328 ft) and 800 m [2624.6 ft]) and Simrad EM 12D (13 kHz; for water depths between and slump scars in the upper slope (Figure 3). The 800 m (2624.6 ft) and 3500 m [11,482.9 ft]). Additional CLSs are partially destroyed or buried by mass-flow bathymetry surveys that cover the shelf area and river deposits. Tectonic deformation in the main fan area is mouth were provided by the Centro de Investigaciones largely absent, but subtle evidence can be detected on Oceanograficas e Hidrograficas, Colombia (CIOH) the bathymetric and seismic data, particularly in the (6000 km2). Data were tide-corrected, processed by vicinity of the adjacent thrust belts. the contractor, and delivered in final GIS-compatible format. Proximity to the Magdalena River outflow area resulted in sounding errors because of freshwa- Data and Methods ter input, which altered sound velocity ranges, but did not appear to generate errors. This bathymetric dataset Data available for the study include high-resolution amounts to full coverage of the study area. bathymetry images of the northwest Caribbean off- Bathymetry interpretations and quantification of shore Colombia (Figure 1). The bathymetry covers a the dimensions of the architectural elements were major part of the Magdalena deepwater fan, approxi- made using ArcGis platform. Calculation of attributes mately 54,000 km2 of the seafloor (Figure 1). Four dif- such as slope and curvature was generated and used ferent surveys cover the area of the study. In 1997 the to enhance and facilitate the interpretation. Thalweg

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profiles were extracted directly from the bathymetry water depth of 20 m (65.6 ft) that seems to be related grid for each channel studied. Thalweg profiles for to a fault system on the platform. Beyond this step, each channel are referenced to the shelf break (60 m gradient is increasing from 0.2 to 1 until it reaches a [196.8 ft]) to allow a better comparison of the changes low gradient sector (20 m [65.6 ft] deeper). Step depth in gradient of the different systems. coincides with the shelf break for the areas outside the Quantification of channel parameters was done Galerazamba region (Figure 5A and B). The average measuring profiles every 5 km (3.1 mi). Channel width depth for the shelf break is 40 m (131.2 ft), but it can was measured from the levee crest to levee crest. occur at depths up to 70 m (229.6 ft) in some areas of Levee height was calculated from the channel thalweg the Galerazamba region. Continental slope gradient to the crest of the levee. Sinuosity and gradient were (Figure 5A) can be divided into an upper slope with measured by dividing the channel into segments for gradients ranging from 2.5° to 3.5°, a middle slope complete sinuous loops. Sinuosity is defined as the with gradients ranging from 1.5° to 2.5°, and a lower- ratio between the channel axis (thalweg) length and slope or continental rise with gradients < 1.5°. These the straight-down channel distance for a given section values exclude scarps and channel and canyon walls of the channel. Sinuosity ranges are shown in Table 1, that locally can reach gradients up to 50°. straight segments have values of 1, and sinuosity Slope profiles for the western, central, and eastern increases as the ratio increases. This concept has been areas show dramatic differences (Figure 5C) interpreted applied for fluvial and submarine channels. Wynn to reflect differential deformation on the fan. The central et al. (2007) and Clark et al. (1992) presented a compre- profile exhibits a concave-up morphology with gradi- hensive review in sinuous deepwater channels. ents diminished by the presence of MTDs. The western The two-dimensional (2–D) seismic lines shown in profile is located close to the toe thrust deformed area. this chapter illustrate the seismic expression of sub- It exhibits gentle gradients similar to the central profile, surface structure. They are part of a wider grid of seis- but with pronounced erosional features. In contrast, mic reflection data provided by Ecopetrol. Acquisition the eastern profile shows abrupt morphology variation parameters are industry standard, near zero phase with because of the compressional forces in the accretionary SEG normal polarity. Frequencies range from 20 Hz to wedge (steplike profile). Here, ridge-confined valleys 60 Hz around the level of interest. Seismic interpreta- or piggy-back basins operate as conduits and basins for tion was performed in SMT Kingdom Suite 8.1. The sediment transport and deposition. The thrust forelimb presence of water bottom multiples, gas hydrates, gas increases the gradient of the slope, leaving a marked chimney, and shale diapirs obscures the seismic signal break separating slope and continental rise. In addi- in places. In addition we used seismic data acquired tion, it is important to observe that the eastern section during RMS Charles Darwin expedition CD40a in 1987 is 300 m (984.2 ft) deeper (3500 m bsl) than the central (Figure 1). These data were available only as paper and possibly the western section (no bathymetry data copies and line interpretations (Pirmez et al., 1990). are available for the deep western sector).

Area Physiography Channel–levee Complexes

The continental shelf is generally narrow (2 km [1.2 A series of submarine CLCs are present on the modern mi]), with wider sections amplified by the sediment seafloor particularly in the central portion of the mar- discharge of the river mouth (e.g., 33 km (20.5 mi) in gin (Figure 1). Overlapping and compensational rela- the Galerazamba region, Figure 5C central profile), tionships allow us to establish the depositional order forming delta lobes (Figure 1). Sediment discharge for the complexes (Figure 6). Seven major complexes was therefore directly onto the continental slope dur- have been recognized, each separated by inter-channel ing the Plio-pleistocene, as is happening today (Kolla lows where MTDs and unconfined flows were depos- and Buffler, 1984b). Ercilla et al. (2002a) characterize ited. A summary of the most representative channel the central and eastern portion of the fan, dividing characteristics is presented in Table 2. These CLCs are the area into deformed and underformed zones. They discussed more thoroughly in the next subsections. refer to a bulge shape on the basin floor toward the Figure 7 depicts the thalweg profiles for the different north of the Galerazamba region, with the presence of channels using the shelf break as a reference point. It is large MTDs diminishing the slope gradient. important to notice that the older eastern systems are The shelf area is very smooth; gradients vary from found at deeper water depths and the younger west- 0° to 0.12°. However, a distinct step is present at a ern profiles generate gentle slopes at shallow depths,

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Figure 5. Slope variations of the Magdalena Fan. Upper left, division of the slope: upper slope (US), middle slope (MS), lower slope (LS). Upper right, slope map (0–5°, values >5 excluded). Lower, profiles for the western, central, and eastern areas. Shelf break (SB). Western profile exhibits very rough morphology and lower gradients. Central profile is very smooth because of the presence of an MTC at the inter-channel low. Eastern profile shows the modification of the slope by the thrust deformed belt, creating piggy-back basins (low gradients) separated by thrust-formed ridges. 5 km (3.1 mi)

closer to the shelf. Morphology of the main systems channels first described by Kolla and Buffler (1984b) on is summarized in Figure 8; the profiles are measured the Magdalena Fan. The channel levees aggrade (up to every 5 km (3.1 mi). The closeness of the profiles indi- 150 m [492.1 ft]) on the seafloor, forming a positive top- cates low gradients or higher sinuosity where the ver- ographic structure (Figure 9B). The thalweg profile of tical separation (depth axis) does not change much in the channels reveals a very rough morphology, except 5 km (3.1 mi). for IVc, which exhibits a smooth concave-up morphology (Figure 9A). The two younger channels IVd and IVe CLC-IV overlap and cannibalize the system on the upper slope. CLC-IV is the northernmost complex and is comprised Remnants of CLS-IVa occur at 2824-m (9265.0 ft) of three main CLSs: CLS-IVa, CLS-IVb, and CLS-IVc water depth (length of 30.2 km [18.7 mi]), with much (Figures 1 and 9) and two younger channels IVd and reworked levees and thalwegs (Figure 9A). The sinu- IVe. The main channel systems extend up to 120 km osity for this system is 1.35. The upper and lower (74.5 mi) from the shelf into water depths as deep section of the system is covered by younger channel as 3200 m bsl. This channel complex is among those systems.

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Figure 6. (A) Seismic-line RMS Charles Darwin expedition CD40a in 1987 (lower fan). The vertical scale for this profile was missing from the original paper copy. (B) Bathymetry profile through the middle slope. The CLCs are older toward the west, with the exception of CLC-I that is the youngest complex in the fan. 10 km (6.2 mi)

Figure 7. Thalweg profiles for the Magdalena Fan. All the channels are referenced to the break of the slope at 60 m bsl to have a better comparison of the slope changes. Note that western thalwegs are deeper and with higher gradients at the upper slope. The profiles are ordered from young (upper) to old (lower). 100 m (328.1 ft) 50 km (31.1 mi)

Table 2. Morphometric measurements of main channel systems.

Measured Average Width Average Left Average Right Maximum Minimum Channel Depth Range Length (km) (m) Levee (m) Levee (m) Sinuosity Sinuosity I −1000 to 2720 110 1640 121 113 3.16 1.03 IIc5 −860 to −1920 39 1994 111 95 1.14 1.01 IIc4 −1380 to −2850 140 1390 62 67 4.08 1.14 IIId −1276 to 1810 19 1860 41 78 1.41 1.01 IIIc −1820 to 1350 156 1930 48 46 1.85 1.01 IVc −1960 to −3200 75 1990 37 44 Ivb1 −2760 72 2480 48 54 Iva1 −2958 30 1700 29 45

1Due to the advanced degradation stage, these values were collected at the most preserved interval.

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CLS-IVb occurs at 2167 m bsl, west of CLS-IVa. The channels IVd and IVe are shown in Figure 9C. The first morphology of the levees has been highly affected few milliseconds of the data contain very continuous by erosive processes (mainly the western levee). The reflections covering the area and partially filling chan- measured length of the CLS-IVb is 72.4 km (44.9 mi). nels IVe and IVd. Figure 9B shows evidence of more Sinuosity values increase downslope, from 1.10 until advanced erosional processes at work that have modi- an avulsion point at 2790 m bsl is reached, beyond fied the upper slope through transport of sediments which the sinuosity is up to 1.49. The thalweg pro- downslope via IVd and IVe. file is very irregular (Figure 9A) and characterized by Channels V-1 and V-2 occur downdip from IVc. higher gradients than CLS-IVa. These channels may represent remnants of older CLS-IVc is located at 1776 m bsl, west and parallel to channels not related to CLC-IV, based on the extreme CLS-IVb. The preserved section of CLS is 75.6 km (46.9 reworking of the thalwegs (Figures 1 and 9A). mi) long. It can be divided into two sections, a straight section (1.03 sinuosity) and a sinuous section (1.48 sin- CLC-III uosity) starting at 2780 m bsl. The change in sinuosity CLC-III is composed of at least three main channels coincides with the avulsion point of CLS-IVb. The thal- and four avulsions (Figure 1). This CLC occurs west of weg profile is very smooth and concave upwards and CLC-IV. It extends from the upper slope to the lower is very similar to CLS-IVb, but with higher gradients slope for a distance of 57 km (35.4 mi), at a water depth upslope. Profiles normal to the axis of CLS-IVc exhibit reaching 3668 m (12,034.1 ft). CLS-IIIa is the oldest and an open “U” shape (Figure 8). Present-day relief of the westernmost channel in the complex. The upper-slope levees is very smooth and reaches 70 m (229.6 ft) in section of IIIa (32.8 km [20.3 mi] length) imaged by the some areas, with an average of 40 m (131.2 ft). bathymetry exhibits 1.3 sinuosity. The system appears A younger channel system IVd cuts into the upper to be linked to the CLC-III (at 1370 m bsl), but it is slope, eroding a section of VIa. This is an abandoned completely buried by the levees of younger channels, aggradational channel that has been exposed to the as can be observed in the seismic profiles (Figure 10). erosional processes that created flows covering sec- The thalweg profile exhibits gradients very similar to tions of CLS-IVa (Figure 9B). Seismic expression of the upper-slope section of the complex (Figure 10A).

Figure 8. Channel profiles. Measured every 5 km (3.1 mi) of thalweg length. Vertical scale depth (below sea level). Note the change in the morphology of the conduit in the upper 1000 m bsl. Increase in sinuosity and lower slopes is shown by the decrease of space between the profiles in the deeper sections.

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Figure 9. (A) Thalweg profile of the remnants. Note the lower profile of IVa and the convex-up profile of IVb and IVc at 80 km (49.7 mi). The difference in thalweg depth corrobo- rates that the V remnant corresponds to an older system. (B) Main channel systems of IV in map view. Note how IVd and IVe are cannibalizing and covering the previous systems. Bathymetry map (curvature and slope) referenced to the general context (Figure 1). (C) A–A' seismic profile perpendicular to the flow direction. IVe and IVd thalwegs are covering older channel systems. Levees (yellow), channels (pink), and MTC (red). 5 km (3.1 mi) 20 m (65.6 ft)

Unfortunately, the lower-slope section of IIIa is not conduits west of this channel. This segment, named IIId covered by the bathymetric survey. (19.3 km [11.9 mi] long), was cannibalized by the mass The following conduit in the CLC sequence is IIIb. transport complex deposited between the inter-channel This system was later abandoned and replaced by IIIc. lows (Ercilla et al., 2002a; Estrada et al., 2005b) (Figures CLS-IIIb occurs at 2170 m bsl, with two eastward-migrat- 9C and 10C). The thalweg profiles show how the over- ing avulsion points at 3200 and 2800 m bsl (IIIb2 and all system becomes deeper toward the east. Figure 10C IIIb3) (Figure 1). This part of the system is topographi- shows the well-developed western levee, the filled thal- cally about 110 m (360.8 ft) higher than the younger east- weg of IIId, and the migration of the system toward the ern channel system (Figure 10A). Sinuosity increases for east. This complex corresponds to Channels I, II, and III IIIb, IIIb2, and IIIb3 at the avulsion point at 2800 m bsl of Ercilla et al. (2002a, 2005b). (forming IIIb3), with values up to 2.45, coinciding with the increase in sinuosity for CLC-IV. CLC-II CLS-IIIc, described by Estrada et al. (2005a), is the Toward the west on the fan, the next complex observed youngest of the complex. The sinuosity increases is CLC-II (Figure 1). This complex is a prominent fea- downslope up to 1.85, and the average width of the ture on the lower slope, as imaged on the CD40a seis- channel is 1930 m (6332.0 ft). An avulsion point is pre- mic line (Figure 6). The upper-slope section has been sent at 3160 m bsl, which resulted in an eastward shift cut and/or buried by several younger mass flows and and abandonment of CLS-IIc1. The levee relief decreases conduits (Figure 11A), which are covering the original downslope, and the channel becomes less entrenched, morphology in this area. Thus no morphologic meas- changing from a prominent “U” form to a shallower urements could be made in this complex. The height channel (Figure 8). The thalweg profile for this CLS is and extension of the complex appear to be similar to generally concave upwards with some convex areas. CLC-I on the lower slope (Figure 6), but smaller in size The higher-resolution bathymetry used for this study updip (Figure 11C). A prominent feature is the erosional allowed better definition of the avulsion points on the conduit that follows the channel course but becomes upper slope at 1588 and 1840 m bsl (Figure 10B), which diverted to the southwest, forming a lobate deposit in are cutting IIIc and depositing sediments over younger the inter-channel complex low (Figure 11A and B).

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A. B.

050100 150200 250300 -1400 IIIa Avulsion cuts

-1800 IIIb B’ IIIb2 IIIc -2200 IIIb3 IIIa

-2600 IIIc

Depth (m ) IIIc1 -3000 IIId IIId B

-3400

-3800 Distance (m) C. B B’ IIId IIIa Central MTD

Channels

Levees

5 km 0.5 s

Figure 10. (A) Thalweg profile of CLC-III. The westward migration and the deepening of the thalwegs as it is getting younger. IIIa, IIIb, IIIb2, IIIb3, IIIc, IIIc1, and IIId (youngest). Note the convex-up profile of IIIb and really steep gradients for the first 80 km (49.7 mi). (B) Main channel systems of III in map view. Bathymetry map (curvature and slope). Reference to the general context in Figure 1. The upper slope is characterized by avulsion points. Also note how the central mass transport complex (MTC) is cutting the eastern side of the channel systems. (C) B–B' seismic profile perpendicular to the flow direction. IIIa morphology is completely covered by later sedimentation. Eastern levee of IIId is completely modified by the erosion of central MTC. Levees (yellow), channels (pink), and MTC (red). 5 km (3.1 mi) 50 m (164 ft)

CLC-IIa and CLC-IIb Width of the conduit varies from 2.2 km (1.3 mi) in the As indicated in Figures 6 and 12A, younger deposi- upper slope to 1.3 km (0.8 mi) in the lowest part of the tion occurred to the west with CLC-IIa and CLC-IIb system. CLC-IIb is a very low sinuosity conduit when downslope from the western side of the Galezamba compared with the geometry of CLC-IIa (Figure 12A). shelf area. Down-cutting relationships on the seismic The higher sinuosity areas have values of 1.16 and 1.46. sections (Figure 12B) indicate that CLC-IIa at 1700 m Figure 12B clearly exhibits the relationship between bsl was deposited first. CLC-IIa developed a highly CLC-IIa and the younger CLC-IIb. CLC-IIb thalweg sinuous channel system (up to 3.34) for a distance of profile exhibits abrupt slope changes and some con- 106.6 km (66.2 mi) mainly to the west. The middle sec- vex-up sections (Figure 12C). Changes are particularly tion diverts toward the south and has an average width evident at the outer bends of CLC-IIa. Comparing the of 1.1 km (0.6 mi). Based on seismic interpretation, the channel profile for these channels, CLC-IIb has a much complex is partially buried by continuous reflectors and steeper gradient than Ch-IIa (Figure 12C). MTDs generated upslope. The morphology observed on the seafloor mimics the topographic highs at the CLC-IIc time the channel was active (Figure 12B). CLC-IIb can CLC-IIc corresponds to the westernmost channel be recognized upslope at 1284 m bsl. It is 45.8 km (28.4 with morphologic expression on the seafloor today mi) long and truncated by younger flows at 2281 m bsl. (Figure 1). The complex is recognized at the upper

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Figure 11. (A) CLC-II in map view. Bathymetry map (curvature and slope). Note how younger downslope flows are modifying the morphology of the system. (B) MTCs that are modifying the slope morphology (slope of bathymetry map). (C) C–C' seismic profile perpendicular to the flow direction. CLC-II buried morphology describes a probably sinuous channel system completely modified by post-abandonment flows. The size relationship is compared between CLC-I and CLC-II in Figure 6A. Levees (yellow) and channels (pink). 5 km (3.1 mi)

extension of the bathymetry survey (827 m bsl), reach- wide. CLS-IIc2 is found at 2408 m bsl. The upper sec- ing depths up to 3056 m bsl and covering an area of tion is 42.7 km (26.5 mi) long, and width varies from 2600 km2. The upper section of the slope is character- 0.7 to 0.5 km (0.4 to 0.3 mi) downslope. The lower sec- ized by erosional canyons–channels, up to 2 km (1.2 tion is 62 km (38.5 mi) long, and width is 0.7 km (0.4 mi) wide (Figure 8), which are controlled by the influ- mi) on average. Channels IIc1 and IIc2 have lower gra- ence of the deformation front. CLS-IIc1, CLS-IIc2, and dients (Figure 13) and higher sinuosity (up to 2.8 and CLS-IIc3 (Figure 12A) are remnants of the initial posi- 3.5, respectively) than IIc3 (Figure 12A). The thalweg tions of the complex in the lower section of the slope. profile for IIc1 probably is more affected by the lev- West of these CLS remnants is located CLS-IIc4, the ees of the neighboring channels (IIc3 and IIc4) (Figure most continuous channel in the complex (Figure 12A). 14). CLS-IIc2 thalweg profile is concave up with some CLS-IIc1 is found at 1995 m bsl. It is 40 km (24.8 mi) irregularities with an abrupt gradient change at 2440 m long (preserved segment) and 1.1 to 0.9 km (0.6 to 0.5 mi) bsl (Figure 13).

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Figure 12. (A) Location of CLC-IIa, CLC-IIb, and CLC-IIc. Bathymetry map (curvature and slope). Referenced to the general context in Figure 1. IIa increases sinuosity downslope, with marked bend toward the west. IIb lower sinuosity than older CLC-IIa. (B) D–D' seismic profile perpendicular to the flow direction. 020406080 100 120 140 160 Clearly depicts the relative age of the differ- -1000 ent systems, with IIc4 being the youngest of IIa the three. IIa and IIb thalwegs are filled, and -1400 morphology seems to be modified. Levees -1800 (yellow), channels (pink), MTC (red), and base of channels (blue). Arrow indicates the erosive Down flow Depth (m ) -2200 character of the flows going downslope. (C) Thalweg profile for IIa and IIb. Arrows in -2600 IIb IIa profile are indicating areas of the channel that have been modified by later flows. II-a II-b -3000 CLC-IIb profile exhibits a concave-up thalweg Distance (m) Down flow section that is indicated by the arrow on the map. 5 km (3.1 mi) 20 m (65.6 ft)

-600 CLS-IIc3 starts at 1960 m bsl as a fairly straight con- 050100 150 200250 duit (Figures 12A and 13). Increase in gradient and sin-

-1100 IIc1 uosity occurs at 2160 m bsl. The thalweg profile shows IIc2 these changes by convex-up sections (Figure 13). A sec- IIc3 -1600 IIc4 ond convex-up section is found at 2400 m bsl after which IIc5 the thalweg becomes straight. The upper straight section

Depth (m ) -2100 of CLS-IIc3 is parallel to the front limb toe of a thrust- fault ridge and is an erosional conduit (Figures 12A and -2600 14D). The channel is affected by deformation observed in Figure 14E, where it is part of the folded sequences. -3100 Distance (m) CLS-IIc4 is the most sinuous and therefore long- est thalweg measured on the complex (140 km [86.9 mi]) (Table 2). The upper section (1300 to 2000 m bsl) Figure 13. Thalweg profile of CLC-IIc, CLC-IIc1, CLC-IIc2, of IIc4 has characteristics of an erosional channel– CLC-IIc3, CLC-IIc4, and CLC-IIc5 (youngest) (location of the canyon with steep walls, U-shape profile, and channels; Figure 12A). IIc1 shows an abrupt change in the 1.4 km (0.8 mi) width on average (Figures 8 and profile because of the later establishment of IIc3. The remnants of IIc2 exhibit a convex-up profile where it encounters IIc3 14A, B, and C). Despite the erosional nature of the (black arrow), which is also evident at IIc3. Note the convex-up canyon–channel, sinuosity values are up to 1.7 profile of IIc4 (pink arrow) about 50 km (31.0 mi), which may (Figure 15). The lower section (2000 to 2800 m bsl, indicate deformation. Notice the fairly steep gradients of IIc5. 60 km [37.2 mi]) channel becomes aggradational 100 m (328.1 ft) 50 m (164 ft) with the development of levees (65 m [213.2 ft]

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Figure 14. Series of seismic profiles showing the changes in morphology of CLS-IIc4. Upper-slope erosional architecture (A to C) and middle to lower-slope aggradational morphology (D to F). High-amplitude reflectors (HARs) are observed on the channel thalweg (D and E). C and E show evidences of later slope deformation since the channel wedge is tilted. 5 km (3.1 mi)

in height from the thalweg) (Figure 14D, E, and bsl the profile is convex up, which corresponds to the F) and higher sinuosity (up to 4) (Figure 15) erosional section of the system (Figure 15A). High- with several cutoff loops (Figure 16). The deeper sec- amplitude reflections (HARs) are found at the chan- tion of the channel (3000 m bsl) broadens and appears nel thalwegs (Figure 14D). Channel wedges are tilted, to have migrated toward the north, abandoning indicating post-depositional deformation (Figure 14E). the main channel. This CLS is similar to the Pleisto- CLS-IIc5 is a younger avulsion of the system cene Borneo channel described by Posamentier et al. (Figure 12A). It is a 32.8-km (20.3 mi)-long ero- (2000). The thalweg profile is mainly concave up (Fig- sional channel–canyon that cuts IIc4 and that is cov- ure 15A), with some erosional cuts at the lower sec- ered by younger deposits downslope. It has a “U” tion and some bends of the channel. Around 1700 m shape with steep walls of 100 m (328.0 ft) height and

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A toward the southeast and south, probably generated -1200 020406080100 120 140160 180 by turbiditic flows overtopping the outer-bend levees -1400

-1600 by flowstripping (Piper and Normark 1983; Imran et

-1800 al., 1999; Posamentier and Kolla, 2003).

-2000 The northeastern overbank is covered by MTDs

pth (m) pth -2200 that fill the inter-channel lows (Figure 17A). The levee De -2400 height reaches up to 175 m (574.1 ft) (average values -2600 of 120 m [393.7 ft]). This is at least three times higher -2800 compared with the other levees in the fan. This system -3000 B 2.5 4.5 shows intra-channel terraces, which are more com- Aggradational Erosional 4 mon in the upper section of the channel (1000–2000 2 Gradient m bsl) (Figure 8). This is the only complex on the fan

) 3.5 Sinuosity es that is composed of a single system. The only possi-

re 1.5

3 y it eg ble avulsion point is located close to the edge of the (d nuos nt 2.5 Si ie 1 survey, although downslope avulsion beyond the area ad

Gr 2 mapped may be possible. 0.5 1.5

0 1 020406080100 120140 160180 Active Magdalena Fan Distance (km) In the active portion of the fan (eastern area) sedi- Figure 15. (A) Thalweg profile for IIc4; and (B) gradient and sinuosity values for IIc4 thalweg. The overall gradient ments are transported into the slope and abyssal tendency is to decrease downslope in the erosional upper plain through a series of canyons that are in commu- section. But the gradient changes drastically in the aggra- nication with the Magdalena River mouth (Canyons dational lower section. The sinuosity values are high at the U, S, M, D; Figures 1, 4, and 19A). The Magdalena steep segments of the channel. 20 km (12.4 mi) canyon is a prominent feature on the slope directly connected with the current Magdalena River. The canyon presents a maximum incision of 260 m (853.0 ft), is 2.5 km (1.5 mi) wide on average, and has a sinuos- approximately 2 km (1.2 mi) width (Figures 8 and 14A ity index of 1.22. The general form is a wide V-shaped and B). The conduit sinuosity reaches values of 1.14. canyon with some areas of higher confinement (Fig- The thalweg prolife is very steep and convex up in the ure 19B). The vertical profile shows more irregulari- lower section (Figure 13). ties in the first 10 km (6.2 mi) upslope and a smoother profile downslope (Figure 19C). Collapse scours are CLC-I common on the northeastern wall of the channel. CLC-I is the youngest levee complex on the modern The channel extends downslope about 30 km (18.6 seafloor based on the overlapping relationships on mi) before it reaches a step in the slope where it con- seismic and seafloor morphologies (Figures 6 and 17). verges with the U and Sabanilla canyons to continue It represents a prominent feature on the slope, with downslope (Figure 19B). The U canyon is located 10 levee heights up to 120 m (393.7 ft). The thalweg pro- km (6.2 mi) seaward of the shelf break and is not con- file is concave up, with steeper slopes on the bends of nected to any present drainage. It is a tributary net- the channel making the profile irregular (Figure 18A). work of small gullies, which develop a channel-like The shape of the channel is a “V” form on the upper feature at the change in slope. It is 1.2 km (0.7 mi) slope (900–1300 m bsl) and then broadens downslope wide with maximum incision of 80 m (262.4 ft). Saba- to a “U” form down to 2100 m bsl where it becomes nilla canyon is the westernmost canyon. It is a nar- narrower (Figure 8). Average width is 1.6 km (0.9 mi). rower feature (1.4–0.6 km [0.8–0.3 mi] wide) with 120 m Seismic profiles indicate a wider thalweg for younger (393.7 ft) of maximum incision. The head of the can- stages of channel growth, with the presence of yon is connected to the shelf break and extends 20 km HARs (Figure 17C). The overall gradient decreases (12.4 mi) seaward before it connects with the other downslope, but local highs correspond to high sinu- canyons. The “V” canyon geometry is lost once it osity values (Figure 18B). The average sinuosity is 1.4 reaches the step on the slope (Figure 19B). with values up to 3.6. The southwestern overbank of East of the river mouth a series of slope channels the complex exhibits sediment waves (Figure 17B). The or gullies are recognized (Figure 19A) (Posamentier azimuth map shows a conjugation of wave systems and Kolla, 2003), which connect downslope to the

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a -2,100 Break in the slope

-2,200

-2,300

-2,400

-2,500 KP2 0 5,000 10,000 15,000 20,000 Break in the slope IIc4 -2,100 -2,150 -2,200 -2,250 a -2,300 -2,350 -2,400 -2,450 IIc4 -2,500 0 10,000 20,000 30,000 40,000 b b -2,100 Break in the slope Break in the slope -2,150 -2,200 -2,250 -2,300 -2,350 -2,400 -2,450 -2,500

0 5,000 10,000 15,000 20,000

Figure 16. Channel system IIc4. The increase in sinuosity is observed to accommodate changes in the slope. A and B are showing the slope of nearby areas, and B is showing the thalweg profile. In addition, a series of cutoff loops are observed in this segment (yellow-dashed lines).

Magdalena canyon or a slump feature to the west. (IIc6) cutting a section of the slope covered by uncon- MTDs also occur. Numerous submarine cable breaks fined flows, gradients of 1.7° (Figure 18A). IIc6 (Fig- in the Magdalena River mouth area were reported in ure 12A) is a fairly young conduit that cut the slope the 1950s (Heezen, 1956), indicating active sediment until it encountered KP-1. The slope profile defines an gravity flows moving through the canyons. Detailed increase in the gradient and a convex-up morphology description of active deposition on the Magdalena Fan downslope (Figure 18A). is presented by Romero-Otero (2009). Downslope of KP-1 is a lower gradient area (1.13°) (Figure 18A), down to KP-2 at 2290 m bsl (at 16 km (9.9 mi) from KP-1) (Figure 20). KP-2 is 1.2 km (0.7 mi) Knickpoints wide, and 90 m (295.2 ft) high. An important charac- teristic of this KP is the presence of sinuous bends in The western upper slope (between IIc and IIb) the area of higher gradient (1.46°) (Figure 20). KP-3 (Figures 12A and 20) displays a series of KPs at areas is found at 2720 m bsl (23.4 km [14.5 mi] from KP-2 with a change in slope separated by lower-slope steps base), with a height of 60 m (196.8 ft) and variable (Figures 18A and 20). KP-1 is located 38 km (23.6 mi) width from 0.4 m (1.3 ft) up to 0.8 m (2.6 ft). This is a downslope from the shelf break at 1650 m bsl, where it less-entrenched feature with a minor gradient change intersects CLS-IIc5. KP-1 is an erosional feature 1.1 km to 0.85° (Figure 20). The role of active slope deforma- (0.6 mi) wide and at 130 m (426.5 ft) in height. Upslope tion on the distribution of these KPs is addressed later from the KP, it is possible to follow a channel or gullie on the discussion section.

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Figure 17. (A) CLC-I in map view. Bathymetry map (curvature and slope). Note how younger downslope flows are modifying the morphology of the northern levee/overbank. Referenced to the general context in Figure 1. (B) Azimuth map displaying the wave field at the southern levee/overbank of CLC-I. Arrows are parallel to the two main directions. (C) Seismic profiles showing the changes in morphology of CLS-I downslope. High-amplitude reflections (HARs) are observed on the channel thalweg. (Loca- tion of seismic profiles is shown in Figure 17A.) 2 km (1.2 mi)

Magdalena River Delta Phases—Submarine Fan Migration

The seafloor morphology and the apparent migration of the river course through time confirm a close relationship between the Magdalena River and the fan. Sedimentation rates increased during the last 2–4 Ma in many continental margins (Hay et al., 1988; Pelzhen et al., 2001) including the offshore Carib- bean sector (Bordine, 1974; Duque-Caro, 1984). The Magdalena Fan is mainly fed by the sediments transported in the Magdalena River load; therefore, the sediment depocenters shift laterally as the source of sediments and/or their feeder channels change course with time. In this chapter the nomenclature proposed by Pirmez et al. (1990) was used, which described the present-day seafloor expression of the CLCs with the associated river/delta phases (Table 3). Besides the evidence found in the outcropping deposits onshore, the shelf morphology reveals the past locations of the river mouth. The delta formed by the river cre- Figure 18. (A) Thalweg profile for I, overall concave up, but ates a series of lobes widening the shelf, such as at the with some local convex-up sections (e.g., 40 km [24.8 mi]). Galerazamba area (Figure 1). At least eight different It is also displaying the profile of the conduits joining the positions of the river mouth have been recognized for knickpoints (KP) in Figure 21. (B) Gradient and sinuosity the Plio–Pleistocene time interval (Table 3, Figure 3). values for I thalweg. The overall gradient tendency is to de- Late Miocene through Pliocene phases (Sucre and crease downslope, with some sections of steeper gradients. Plato and Phase E) are buried in the slope area but are The higher values in sinuosity coincide with these steep segments of the channel. 20 km (12.4 mi) the most prominent land features. The area south of

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Sabanilla U Magdalena Figure 19. (A) Active fan 0 0 0 A B -3000-1500 015003000 -3000 -1500 015003000 -3000 -1500 015003000 bathymetry map. Referenced -200 -200 -200 to the general context in

-400 -400 -400 Figure 1. (B) Canyon profiles measured every 5 km -600 -600 -600 (3.1 mi). Vertical scale -800 -800 -800 depth (below sea level). The Unnamed MTDs -1000 -1000 -1000 Sabanilla canyon changes its Gullies Magdalena -1200 -1200 -1200 morphology once it reaches the lower section of the -1400 -1400 -1400 slope (piggy-back basin). The

-1600 -1600 -1600 Sabanilla 200m Magdalena canyon describes -1800 -1800 -1800 a wider channel with an entrenched thalweg. Arcuate C scarps and creeping can be observed in the canyon walls. East of the Magdalena Magdalena Unnamed canyons channel/gullies and Sabanilla mass transport complexes are observed. (C) Canyon thalweg profiles. Active Magdalena canyon depicts a fairly smooth profile with some area where it is convex up (ridges sections). Sabanilla and the U canyon profiles are parallel. 200 m (656.2 ft)

Figure 20. Knickpoint (KP) sequence. Three- dimensional bathymetry of the western deformed belt area. A series of KPs on the slope are located in areas with abrupt changes (steps) followed by low gradient areas. Lobate shapes at the toe of the KPs. Some of these lobes present erosional cuts. IIc6 conduit seems to be connecting all the KP through the slope. Upper figures are the frontal view of the KPs (blue arrow). It is important to notice the sinuous morphology that KP-2 exhibits. The steps on the slope coincide with IIc4 higher sinuosity zones (yellow lines). Profile of the KP (IIc6) is shown in Figure 18A. 2 km (1.2 mi)

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Table 3. Summary of the evolution of the Magdalena Fan.

Phase River/delta Channels Time Migration A Barranquilla–Boca Vieja U, S, M, F Late Holocene–Present West B Early Holocene (to −5 kybp) East C North of Cartagena I Late Pleistocene East C North of Cartagena IIb Late Pleistocene(?) East C North of Cartagena IIa Late Pleistocene(?) East D South of Cartagena Mid-Pleistocene West Dique Canal North of Cartagena II (?) West Eb Galerazamba III Mid-Pleistocene(?) West Ea Puerto Colombia IV, V Early Pleistocene West E Luruaco >V (Buried) Late Pliocene(?) West Sucre and Plato Late Mio-Pliocene

the present Magdalena River is comprised of marginal This major shift is supported not only by the remnants shallow marine sediments (Figure 3) (Bordine, 1974). of a paleo-channel onshore but also by the establishment of the La Popa Formation coraline limestone, Phase Ea which would require low influx of terrigenous sedi- The early Pleistocene river mouth (Phase Ea) was ments in the northern coastal area (Bordine, 1974; located near Puerto Colombia, west of the present river Reyes et al., 2001). location. It generated deposits that correspond to CLS- IV and CLS-V. It is the oldest phase, which has an appar- Phase C ent expression on the seafloor morphology(Figure 21A). During the late Pleistocene, the river mouth switched north of Cartagena to develop Phase C, depositing Phase Eb fans that are overlapping and generating CLC-IIa, During the middle Pleistocene (Phase Eb) the river CLC-IIb, CLC-IIc, and CLC-I, from older to younger, mouth migrated to the southwest, toward the Gal- with CLC-I being the most recent on the entire sub- erazamba region, generating CLC-III (Figure 21A). marine fan (Figure 21C). This area of the fan presents Further westward, migration of the river mouth a dynamic interaction between deformation and sedi- resulted in CLC-II deposits. It is not possible with the mentation, which can be evidenced by the abrupt available information to define whether these depos- changes of orientation and sinuosity of the channels its correspond to a delta phase before the main shift and thalweg profiles. In CLC-IIc4, highly sinuous seg- to the southwest or the early stage of phase C (before ments and a series of cutoff loops are present where deposition of CLC-IIa). the slope has higher gradient (Figure 16). In response to the change in slope, the channel adjusted course, Phase D trying to maintain an equilibrium profile (Pirmez et Phase D is the product of continued migration of al., 2000; Deptuck et al., 2007). CLC-I presents higher the river toward the south to the Canal del Dique. gradients where the channel bends, and as a result Mid-Pleistocene sediments were deposited in the the sinuosity morphology is similar to the younger thrust belt area (Figure 21B), which corresponds to erosional cuts north of the channel at the upper slope the southernmost position reached by the river. This (Figures 12 and 16). phase generated deposits that were progressively deformed by the growth of the deformation front. It Phase B is possible that the nearby Sinu River delta exerted Due to the late stages of deformation during the strong influence on the deposits generated at this late Pleistocene, the river course was modified as a phase. No CLSs are recognized at the seafloor in this response to the Atlantico–Turbaco Hills uplift, caus- area in part because of the high input of recent sedi- ing a major depositional shift toward the east and ments by the Sinu River (Pujos and Javelaud, 1991). northeast (Hoover and Bebout, 1985) (Figure 21D).

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Figure 21. Evolution of the Magdalena Fan. (A) Early to middle Pleistocene produces phase Ea (CLC-IV) and Eb (CLC-III); (B) middle Pleistocene produces phase D; (C) late Pleistocene produces phase Ca, Cb, C; and (D) Holocene produces phases B and A (active fan). 10 km (6.2 mi)

This shift generated phase B, depositing sediments to its present position (Heezen, 1956; Bordine, 1974), in the Cienaga de Santa Marta area and creating an generating delta lobes in the shelf area (Figure 1). expansion of the continental shelf. The Sierra Nevada This late Pleistocene and Holocene Magdalena River de Santa Marta drainage system should have been an to the west did not build large leveed channels. important source of sediments for this area as well. Deposition was dominated by slumps/debris flow A major decrease in carbonate concentration in the that fills the slope valleys in the thrust-dominated Colombian Basin at 6000 year (Prell, 1978) may be region and overflow into the abyssal plain. Several related to the shift of the Magdalena River toward the canyons are driving the present-day sediment load east during this time. downslope (Heezen, 1956; Hoover and Bebout, 1985). With almost no development of a shelf, the sediment Phase A load is transported downslope through canyons and During the Holocene phase A, the river began to gullies and emplaced as gravity flow deposits fill the migrate westward once again (Figure 21D). During basins on the submarine fold-and-thrust belt. Addi- the last century the river has switched positions ini- tional sediments were remobilized and deposited tially to the Boca Vieja and Sabanilla canyon and then through this canyon by the longshore current that

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fluctuates NE–SW and SW–NE under the effect of the canyon downslope with parallel and similar sinuosity inter-tropical convergence zone (ITCZ) (Pujos et al., of CLC-I. 1986) associated with the closing of the Panama Modification of original levee morphology Isthmus 2.4 Ma. occurred in the older systems at the eastern part of the fan (Figure 8). Older channels are reworked by opportunistic mass transport flows traveling down Discussion the slope, taking advantage of the abandoned channel course and modifying the pre-existing morphol- Degradation Processes on the Channel Systems ogy. As a result of these events, the height of the levees varies significantly, and the thalweg profiles are very After abandonment of the river delta, submarine fan rough, as shown by CLC-II (Figure 8). Seismic profiles channel systems are exposed to degradational pro- (Figures 6A and 11C) exhibit well-developed levees cesses such as (1) erosion of the CLS by MTDs, (2) similar to CLC-III or CLC-I, with lateral migration of collapse of channel walls and levees, and (3) modifi- the thalweg (probably high sinuosity) at depth. But cation of the levee morphology. Erosion of a CLS by the seafloor morphology is very different, character- mass transport is a common process on the fan. Sev- ized by low sinuosity, remnants of levees, and loss of eral MTDs are generated on the upper slope, which channel character upslope, suggesting that later defor- erode the antecedent deposits while traveling down mation has occurred (Figure 11A). the slope and finally filling inter-channel lows. Some A similar process is observed in CLC-IIb. The of these events in the eastern fan (CLC-III and CLC- geometries of the channel bends seem to be modified IV) were identified by Ercilla et al. (2002a) and Estrada by younger flows that were channelized through the (2005b) (Figures 1 and 4). The western fan section abandoned course (Figure 12A, B, and C). As a result (CLC-II, CLC-IIa, CLC-IIb, CLC-IIc, and CLC-I) exhib- of these changes in the morphology, the channel could its MTDs at the inter-channel lows as well, but at a increase in dimensions or straighten, leading to erro- smaller scale (Figure 4). Collapse of the channel walls neous assumptions about the size and capacity of the and levees is an important process in some of the sys- flows if evaluating channels in the subsurface. On the tems. The channel displayed on Figure 22A depicts lower slope, modification of the morphologies could collapse scarps at both margins. Figure 22B indicates be associated with reworking of the channels by ocean collapse of the CLC-I levee walls, so as to form a bottom currents (Ercilla et al., 2002b).

Figure 22. Degradation of the channels. (A) Arcuate scarps at the walls of the channel (yellow). The channel was affected by the beheading of the thrust imbricates (parallel to the ridges) (blue arrow) and by the creation of a new channel course (red arrow). (B) Major scarp (yellow arrows) located at the northern levee of CLC-I. The scarp was connected to an older canyon downslope. 1 km (0.6 mi)

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Influence of Tectonics on the Magdalena 24A). Some of these structures are actively growing, Deepwater Fan affecting the morphology of the seafloor by generating steeper slope sections (Figures 20, 22, and 24A). Sedimentation vs. Structural Setting Forced migrations of the complete CLC-IIc to the east Slope gradient is one of the factors that regulates the as the deformation front was advancing are direct evi- channel morphology, as well as channel maturity and dence of active deformation in the southwestern area variation of flow characteristics such as current energy, during the Pleistocene (Figure 12A). As a result the flow volume, and sediment load (Pirmez et al., 2000; channel complex modified its course through increas- Babonneau et al., 2002; Kolla, 2007). Turbiditic systems ing sinuosity, becoming erosional or abandoning the in active tectonic settings evolve as the slope gradi- course at avulsion points. Abandonment of CLS-IIc2 ent is continuously modified by major compressional seems to be related to the growth of the fold at the events (Clark and Cartwright, 2011). Consequently, toe of the thrust front. CLS-IIc3 is controlled by the sedimentation style is modified as well. Compres- thrust front, becoming erosional and straight in some sional structures orthogonal to channels seem to segments. In addition, CLS-IIc 2 and CLS-IIc3 present cause large changes in the channel profiles as has been convex-up thalweg sections, which may indicate post- observed in the thrust front of the Barbados accretion- abandonment deformation (90 km [55.9 mi], Figure 13). ary prism (Huyghe et al., 2004) and the growth fold in The IIc4 thalweg does not present convex-up morphol- the western Niger delta (Heinio and Davies, 2007). ogy for the corresponding section on the slope; con- The western compressional belt structures on the versely it exhibits several cutoff loops (Figure 16). This Magdalena are almost orthogonal to the CLS axis suggests that deformation must have occurred con- (Figure 23A). Interaction of the deformation and the currently or immediately following channel system channels seems to be present during different phases IIc1-3 and must have slowed or ceased once channel of evolution of the fan. Some of the evidence can be IIc4 began to form. Nonetheless, the thalweg in IIc4 identified directly by changes in sinuosity and gradi- has a convex-up section at 50 km (31.0 mi) indicating ent of the channel systems (e.g., Figures 15 and 16) post-abandonment deformation. This is also observed or on the adjacent slope by the formation of KPs and on Figure 14C and E, where CLS-IIc3 and CLS-IIc4 are steps (Figures 12, 16, and 20). part of the folded sequence. Thrust imbricates and fold geometries with expres- Besides CLC-IIc, other complexes in the fan show sion on the seafloor extend into the slope, underlying convex-up thalwegs (CLC-IIb, CLC-I), which may be and deforming the fan sediments (Figures 23A and caused by (1) channel abandonment before reaching

Figure 23. Major fold axis and alignment of knickpoints (KP) and channel bends on the southwestern fan. Continuous black lines are folds associated with thrust imbricates with seafloor expression. Dashed black lines are deeper fold geometries. Red dotted lines are possible faults. Knickpoints (KP) are highlighted with blue arrows. 2 km (1.2 mi)

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Figure 24. (A) Western fan underlying deformation. Thrust faults and related fold and normal faults seem to be playing an important role in the development of high sinuosity channels. (B) Central fan—no major deformation is observed. Levees (yellow), channels (pink), canyons (or- ange), and channel–levee system base (blue). 10 km (6.2 mi)

the equilibrium profile or (2) deformation of the chan- There was no age control available for each sys- nel after abandonment (Figure 7). Based on the obser- tem to properly support variations in the sedimenta- vations and high sinuosity of the systems, it is more tion rates during the evolution of the submarine fan, likely that the channels have evolved over time and besides the relative ages provided by the correlations have reached some level of equilibrium with the pre- with the migration of the river on land. However, the existing valley, suggesting the geometry of the thal- lobate geometry of the whole fan and dimensions of wegs (convex up) is related to post-abandonment the channel systems (depth and width) are similar deformation. throughout the fan, which may indicate that sedi- The fan channel thalweg profiles in the northeast ment flows were steady through time and generated upper slope (upper 100 km [62.1 mi]) are consider- systems with similar dimensions. The rapid west- ably deeper (200 m [124.2 mi]) than those in the upper ward migration of the river after establishment of the slope on the southwestern side of the fan (Figure 7). Magdalena drainage system in the basin and relative Sinuosity values are considerably lower for the upper abundance of recent channel systems in the west may slope in the northeastern area. In addition, CLC-III explain the change in depths and sediment accumula- and CLC-IV channel thalwegs exhibit sections with tion on the western and eastern sides of the fan. convex-up profiles, indicating disequilibrium chan- Nonetheless, it is important to take into account the nels or post-abandonment deformation. This change active deformation of the upper slope in the south- in the basin depth could be due to (1) lower sediment western deformed belt that was taking place during discharges at the time of earlier delta-fan building, deposition of the western fan. Increments of sinuosity, (2) rapid migration of the river mouth toward the west forced migration of the channel systems, and convex- (which occurred in the latest Pleistocene–+Holocene), up thalweg profiles all indicate that deformation in or (3) higher deformation in the western fan, uplifting the west was active and extended on the western fan the continental slope. upper slope. Conversely, the upper slope at the eastern

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fan (CLC-III and CLC-IV) has lower sinuosity chan- profiles provide information about the state of equi- nels than counterparts on the western side. Deforma- librium during the formation of the channel, assum- tion of the northeastern thrust belt seems not to have ing the channels are free to erode/deposit without affected CLC-IV and CLC-III at the time of deposition impediment (such as by abnormally lithified layers (Figures 23B and 24B). There is no expression of fault- underneath), but once abandoned, deformation and ing or deformation at the seafloor or at deeper levels. erosional processes can modify the profile. Mayall The northeastern deformation front was active before et al. (2006) discuss at least four processes that influ- the generation of CLC-IV. This is clearly evidenced ence the sinuosity of turbidite channels: initial erosive by the presence of a channel system (probably older base, lateral stacking, lateral accretion, and influence than CLC-IV) that could not keep up with deforma- of pre-existing seafloor topography. Even though lat- tion, leaving a beheaded hanging channel, and creat- eral stacking and lateral accretion are present in the ing a new course orthogonal to the deformation front Magdalena Fan complexes, there is a direct indication (Figure 22A). The deformation continued as is evi- of the relationship between seafloor topography and denced by the tilted position of the beheaded chan- sinuosity. nel, while the old channel course continues to focus The variation of gradient with distance shows in sediments downslope, generating a new pathway. The general a decreasing trend downslope, with local continuation of deformation of the northeastern thrust increases/decreases in gradient that mark departures belt was restricted to a few kilometers downslope of from the general trend (Figure 25A). The systems better the shelf break and was associated with the exten- fitting a concave curve are IIIc and IIIc1. The younger sion of the older thrust imbricates toward the shelf. systems I and IIc4 exhibit a more variable profile with Multiple erosional features at the upper slope in this some extreme high values. It is important to note that area (Figure 23B) and uplift of the shelf and very steep gradient values plotted represent today’s slope and gradients for the upper-slope thalweg on CLC-III and are affected by post-depositional modifications of the CLC-IV indicate uplift and active deformation of channel systems. This may represent an excess in gra- this area. dient for some of the values. The downslope distribu- In addition to the compressional tectonics, exten- tion of sinuosity (Figure 25B) does not show a distinct sional tectonics through normal faulting seems to also trend; the middle slope (60–120 km [37.2–74.5 mi]) have played an important role in the generation of exhibits larger variations. A comparison graph steep gradients on the slope. A slope overburden by between gradient and sinuosity for the entire fan is the high sedimentation rates may be subject to nor- shown in Figure 26. Sinuosity on the Magdalena Fan mal faults generating steep slopes (Figure 24A and (Figures 25B and 26) reaches high values up to 4, B). Normal faulting is common in the progradational which is higher than the values previously reported sequences of the deltas and has been identified as a for the Magdalena and other fans (Table 4). In addi- mechanism for equilibration of the slope in the area tion, IIc4’s high sinuosity segment (Figure 16) exhibits (Flinch et al., 2003). Also, normal faulting can be asso- several cutoff bends, which as mentioned previously ciated with the growth of the thrust faults, as a result correspond to a steep region of the slope. For the of forelimb collapse (Figure 24A). steeper gradients (>2°) the sinuosity is generally very low. Sinuosity reaches a maximum value where valley gradients reduce to about 1°. For gradients <1 degree, Sinuosity and Gradient sinuosity generally decreases with gradient (Figure 25; cf. Clark et al., 1992). Anomalous values are identi- Despite complex seafloor morphology, many subma- fied with very high sinuosity for any gradient. Those rine channels form concave-up profiles, constantly values were identified in Figure 26A, as points corre- adjusting their profiles toward equilibrium (Pirmez sponding to areas with gradients outside of the gen- et al., 2000). This is achieved by erosional and depo- eral trend. These values correspond mainly to the IIc4 sitional processes of turbidity currents, including channel system, indicating that high sinuosity values changes in channel sinuosity, channel incision/aggra- correspond to higher gradient sectors in the slope, dation, and development of distributary channels which are outside of the general profile for the fan. and aggradational sheets (Pirmez et al., 2000; Kneller, In the western Magdalena Fan, two processes can 2003; Adeogba et al., 2005). A good example of this be identified to accommodate the increase in slope adjustment is shown in Figure 14, where the sinuous gradient by the continuous deformation in the area: channel thalweg exhibits a smooth profile, while the (1) sinuosity increase in the channels and (2) genera- adjacent slope has a steeper gradient. The thalweg tion of KPs on the slope.

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Table 4. Published high sinuosity values.

System Sinuosity Reference Offshore Angola 3.3 Kolla et al. (2001) Almeria channel (Spain) 3.8 Cronin (1995) Amazon Fan 3 Pirmez et al. (1990 Offshore Trinidad and Tobago 2.4 Wood and Mize-Spansky (2009) Zaire 1.7 (10 km) Babonneau et al. (2002) Magdalena 4 This work

A 3 I IIc1 2.5 IIc2 IIc3 2 es) IIc4 re IIc5 eg

(d IIc6

1.5

nt IIIc ie IIIc1 ad 1

Gr IIId Outpoints 0.5 y = 2.801e–1E–05x R² = 0.7064 0 050100 150 200 B Distance (km) 4.5 I IIc1 4

IIc2 IIc3 Figure 25. (A) Gradient changes through the 3.5 y

slope measured for each channel system. All it 3 IIc4 IIc5 distances are referenced to the shelf break.

The dashed line indicates a decrease in the nuos 2.5

Si IIc6 IIIc gradient with distance (basinward), but there 2 are many points that are showing values IIIc1 IIId

higher than the trend line. The data labeled 1.5 “Outpoints” are referenced in Figure 26. (B) Sinuosity changes through the slope. 1 There is a big variability of sinuosity through 020406080100 120140 160180 200 the slope. 10 km (6.2 mi) Distance (km)

Knickpoints (KP) level, sediment flux, bedrock resistance, and/or tectonic deformation (Howard et al., 1994). Knickpoints Another mechanism for reaching the equilibrium pro- (KP) may migrate upstream, leaving cut terraces, file is the formation of KPs, a well-known process in or they may be smoothed out by slope replacement rivers, which has been gaining influence in deepwater (Gardner, 1983; Howard et al., 1994). systems architecture in regions with evolving topog- Increases in incision and flow velocity occur as raphy (Pirmez et al., 2000; Mitchell, 2006; Heinio and a result of increase of slope gradient (Pirmez et al., Davies, 2007). In fluvial systems KPs are defined as a 2000; Kneller, 2003). Channel width decreases toward steep gradient section between lower gradient sections the KP lip, defined as the break in slope where the along the river course, resulting from changes in base channel is over-steepened (Gardner, 1983). In areas of

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Gradient vs. Sinuosity 4.5 I 4 IIc1 IIc2 3.5 IIc3 IIc4 3 IIc5 IIc6 IIIc nuosity 2.5

Si IIId IIIc1 2

1.5

1 Figure 26. Gradient vs. sinuosity plot. For high 3.2 2.8 2.4 2 1.6 1.2 0.8 0.4 0 slopes the sinuosity is low. As the gradient Gradient decreases, the sinuosity increases, generating highly sinuous channels, until that point where the sinuosity starts decreasing downslope. The red box is highlighting the points that exhibit values with sinuosity higher than the normal distribution. These points correspond to areas where the gradient is higher (squared data in Down slope Figure 24), indicating irregularities on the slope.

low gradient (base of the KP), velocity reduction, flow In the western Magdalena Fan, west of CLC-IIc, spreading, and deposition occur (Pirmez et al., 2000; four main steps in the slope are connected by fairly Prather, 2003). narrow and well-developed KPs (Figure 20). They A KP begins as a small scour that grows at the are connecting areas of unconfined deposits (lobes) inflection point of the slope (edge of the step). Erosion (Figure 27A and B), truncating CLC-IIc5 and filling is enhanced at the KP lip by an increase of the slope low gradient sections of the slope (Figures 12A, 20, and at the KP toe by an increase in turbulence in the and 27A and B). KP-1 is located downslope of the steeper part of the slope, (hydraulic jumps; Komar, thrust fold (Figures 20 and 23A). KP-2 is located at 1971). Heinio and Davies (2007) proposed that “KPs the slope step where CLS-IIc4 increases its sinuosity, grow into larger features by positive feedback, in suggesting KP-2 was established after deposition of which steeper gradient enhances erosion and this IIc (Figure 16). KP-3 is very incipient but is aligned newly formed erosional scour promotes further ero- with a strait segment of CLS-IIc4, followed by a sion.” Once the KP is established, it may migrate change in the direction of the channel (Figures 20 and upstream, creating incised conduits in the low slope 23A). These KP are interpreted to have formed as a areas (Figure 20). As the turbidity flows continue result of uplift caused by continuous deformation of through the newly formed conduit, it shows a ten- the thrust belt (growth of folds). The location of the dency an equilibrium profile by increasing erosion and KP downslope of the thrust and fold axes, associated even generating some bends in the conduit. It has been with the increase in gradient of the nearby slope, proposed that enhanced deposition will occur down- suggests a structural control of their formation. The dip of the KP where slope decreases (e.g. Pirmez et al., downslope KP profile for IIc6 clearly depicts the 2000), perhaps even locally forming unconfined lobes slope changes (Figure 18A). (such as the perched slope fills of Beaubouef and Fried- Alternatively, the KP system could have been initi- mann, 2000) at the lower gradient steps of the slope by ated by IIc5 (youngest conduit of CLC-IIc) and later slope adjusted deceleration of the flows (Heinio and abandoned and fed by downslope flows traveling Davies, 2007). Preservation of these deposits depends through IIc6. However, both interpretations agree on upon the growth geometry of the folds and the accom- the formation of KPs as flow connectors that allow the modation space created on the slope. sediment distribution throughout the slope.

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P ’P Abandoned IIc6 Channel

P’ KP1 0.25s 2.5km

Q Q’ Onlap IIa Q’

Q KP2 0.25s 2.5km

Figure 27. Low-gradient sections of the slope allow the deposition of unconfined flows that heal the slope. (A) In deposits upslope of KP-1, the flows are filling an inter-channel low. (B) Lobate features downslope of KP-2. Even though the frequency of the seismic is low, it is possible to observe the onlap of the lobe against the channel overbank. 2.5 km (1.6 mi)

Initiation of Channel-levee Systems continuous deformation on the slope. The flows start to erode the slope at the KPs; subsequently Major deepwater fans are characterized by the pres- these KPs migrate and allow the flows to link other ence of a master canyon(s) feeding the continental KPs downslope, thus creating the initial course of slope, such as the Amazon Fan, Mississippi fan, Zaire a channel (Figure 28A). From the sequence of KPs Fan, and Indus Fan (Damuth and Kumar, 1975; Kas- described previously, it is important to notice that tens and Shor, 1985; McHargue and Webb, 1986; Droz the formation of the KPs is possible on low slopes et al., 1996; Normark and Carlson, 2003). In the pres- (0.08) as is observed in KP-3 (Figures 18A and 20). In ence of a constant source of sediments, what defines addition, some of the CLS bases are characterized by the initiation of a CLS in the Magdalena Fan without flat continuous reflections and basal channel scours the presence of a confined canyon? not deeply incised when observed on seismic pro- The initiation of a deepwater channel system files. A more complete dataset that allows us to fully has been linked to gullies, which by progressive understand the spatial and temporal relationships downslope enlargement by erosional process evolve will be needed to validate this idea. However, the into channels. Examples of the Fuji and Einstein sys- erosive nature and migration of KPs make it difficult tems in the eastern Gulf of Mexico have been reported to preserve them in the geologic record (Heinio and by Faulkenberry et al. (2005) and Sylvester et al. Davies, 2007). To evaluate the influence of KPs on the (2011). Megaflutes in the Ross Formation have been formation of channels, one may need to study fea- described as examples of possible features to initiate tures associated with the presence of “arrested” KPs CLSs, recording sediment bypass on an intra-slope in the geologic record, such as erosional notches on basin (Elliot, 2000). Several experimental efforts have slope deposits at the base of and adjacent to channel been completed to understand the processes involved systems. in the generation of CLSs (e.g., Metivier et al., 2005; Yu Continuous modification of the slope by active et al., 2006), but still there are no dynamic models to deformation will keep the slope above grade (Prather, explain the processes involved. 2003) and the channel systems out of the equilibrium A potential answer to the initiation of channels profile, inducing mechanisms such as migration, avul- in this tectonically active setting may be related to sion, KP formation, or abandonment of the system,

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Figure 28. Knickpoint (KP) evolution. Fault-related folds may create areas in the slope where erosional processes generate KP that migrate upslope and generate unconfined deposits downslope. Later these knickpoints (KP) are connected by an erosional conduit, which may explain the initiation of a channel–levee system. (B) The diagram depicts the same process of KP migration constrained within the inter-channel lows.

among others. Migration of KPs and subsequent part obscured and suppressed by the late Pleistocene establishment of channel systems have been used to dump of sediment shown at the eastern fan (CLC-III explain the interaction between the growth of mobile and CLC-IV). shale ridges and turbidite deposition in stepped slope Inter-channel lows also may play an important profiles, where at early stages, the low gradient section role in the establishment of new CLS (Figure 28B). of the slope is healed by unconfined deposits (lobes) Commonly, unconfined flows and MTDs fill the inter- (Figure 27A and B), with subsequent migration of KPs channel lows (Figures 4, 9, 10, 11, and 27A). Older lev- bypassing the previously healed section of the slope ees served as barriers to younger flows, increasing the (e.g., O’Byrne et al., 2004). The early Miocene shale sediment accumulation in these areas and facilitating sequence may play an important role in the deforma- the entrenchment and later connection of KPs (by heal- tion of the slope on the Magdalena Fan. This sequence ing the slope) (Figure 27A). In areas where changes in is the decollement surface for the thrust imbricates gradient are the product of deformation, the model of and the source for the mud diapirism onshore and KP formation and subsequent development of channels offshore (Duque-Caro, 1984; Vernette et al., 1992) and could be applied by the tendency of slope systems to seems to extend across the fan area. Even though the obtain a graded slope (Prather, 2003). deformation seems to be masked by the active sedimentation, the presence of highly sinuous bends and KP alignment on the slope and highly disrupted reflec- Significance for HydrocarbonE xploration tions may indicate continuous deformation on the Magdalena Fan area. The interaction between defor- Deepwater deposits are an important play for the oil mation and sedimentation (e.g., IIc4) (Figure 24) is in and gas industry (Weimer and Slatt, 2007). This study

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contains important findings that can impact hydrocar- piggy-back basins formed as a result of deforma- bon exploration in this and other tectonically active tion of the accretionary wedge. The shifts are cor- basins. Facies distribution and preservation of depos- roborated by a decrease in carbonate content of its along the slope will depend upon the interaction the Colombia Basin (6000 B.P.), growth of coralline of slope deformation and deepwater sedimentation. limestone at the coastal margin (Barranquilla), and As discussed earlier, slope deformation may enhance remnants of old river courses. the degradational processes on the channel systems, 3. In the older complexes, thalweg and levee mor- modifying the initial deposits. Prediction of reservoir phology have been affected by degradational pro- quality and extent would be problematical as these cesses after the abandonment of each system. They degradational processes are persistent on the slope. are ultimately buried, increasing channel width Changes in the slope control not only the CLC mor- and modifying the levees walls. Degradation of phology but also the distribution of coarser sediments. the channel should be considered when evaluating Preservation of deposits such as the initial upstream the dimensions of ancient deposits to obtain a bet- lobes at low gradient steps on the slope could be of ter estimation of the size of the expected deposits. importance for the tectonically active areas (O’Byrne Degradation processes associated with mass trans- et al., 2004; Adeogba et al., 2005; Heinio and Davis, port deposits also could be important elements of 2007), as it is in the western Niger delta. The constant sealing and stratigraphic trapping of potential res- modification of the slope will create steps in the slope, ervoirs in the underlying channel–levee deposits. creating accommodation, where unconfined flows can 4. Several CLSs in the fan show convex-up thalwegs, be deposited and preserved (Deptuck et al., 2013). indicating (1) the channel was abandoned before In addition the morphological parameters of the chan- reaching its equilibrium profile or (2) deformation nels could be used as an analog for reservoir charac- of the channel occurred after abandonment. terization for similar basins, enhancing the reservoir 5. The CLSs in the Magdalena Fan are highly sinuous. characterization for subsurface plays. Higher values of sinuosity (up to 4) correspond to areas of the slope with high gradients (out of the regional slope trend), suggesting that sinuosity is Conclusions controlled by changes in the slope. 6. There is evidence of multiple phases of deforma- 1. The seafloor morphology of the Magdalena deep- tion on the Magdalena Fan created by the deforma- water fan is characterized by the presence of seven tion of the larger accretionary wedge. Decrease in major CLCs separated by inter-channel lows bathymetric depths on the thalweg profiles for the where MTDs and unconfined flows are deposited. western side seems to support the idea of higher The older CLCs are labeled IV followed by III, II, deformation (compression) in this area. Alignment IIa, IIb, IIc, and I. of KPs, channel bends, and step profiles in the 2. Evolution of the fan is closely related to the western side is a clear indication of the deforma- Magdalena delta migration and the tectonic pro- tion that is active during and post-formation of the cesses that occurred in northern Colombia during channel facies. The presence of overpressure shales the Miocene to Present. The Plio-Pleistocene his- seems to play an important role in deformation of tory of the Magdalena River is represented by at the fan. least eight different phases, beginning at the north 7. A sequence of KPs seems to connect deposition of (west of the present river location) (CLC-IV–early sediments from the shelf break downslope through Pleistocene). Then, the river started migrating to- a series of steps, culminating in lobate unconfined ward the south (CLC-III and CLC-II), eventually deposits. Upstream KP migration in slope steps reaching the Canal del Dique (Phase D) during the as a response to deformation may represent a key middle Pleistocene. Later, the river shifted north of process to explain the initiation of deepwater chan- Cartagena (Phase C), forming CLC-IIa, CLC-IIb, nel systems in the Magdalena Fan, but further CLC-IIc, and CLC-I (youngest CLC of the entire research needs to be done to establish its importance. fan). A major northern shift of the river due to the In addition, the inter-channel lows could facilitate Atlantico–Turbaco uplift generated phase B, which the rapid confinement of the slope to initiate the KP focused sediments toward the Cienaga de Santa migration. Marta. The establishment of the present-day delta 8. The distribution of sediments in the Magdalena fan is very recent, switching positions between deepwater fan is highly controlled by the actively Boca Vieja and Sabanilla canyon before stabiliz- deforming slope, which will serve as an analog for ing at its present position. The fan is active today basins where slope deformation was active during with deposition of turbidite flows and MTDs in the deposition of deepwater sediments.

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