Bahia Basin

The Bahia Basin is bounded by two main strike-slip deformation zones that accumulate normal displacement (Faults A and C in Figs 4.5, 4.6, 4.9 and 4.11). Fault A forms the main boundary with the frontal fold and thrust belt (i.e. SCDB) showing a dramatic change in the structural style. This fault zone, like the trend of the structures within the thrust belt, runs from SW to NE, almost parallel with the limits of the 3D seismic volume, and continuing the trend of the coast line and shelf break present towards the SW in the area of Barranquilla-1 well (Figs 4.3 and 4.11). As with most strike-slip faults, this fault zone might be formed by coalescing faults and can also be mapped by the alignment of several mud volcanoes that erupt on the seabed (Figs 4.3 and 4.11). Aside from the mud volcanoes, there is no further evidence of the presence of this fault on the seabed (i.e. folds, escarpments), which means there is no Recent activity along this fault zone. However, in the subsurface, this fault is defined by the thickening towards the south of the Plio-Pleistocene unit (i.e. Seismic Unit 5) within the Bahia Basin, and the normal offset of the dim and chaotic reflections that

characterises the overpressured-shale section of late Oligocene to early Miocene age (i.e.

Seismic Unit 2). In depth, the quality of the seismic image is poor, so the fault is interpreted to be almost vertical (perhaps dipping slightly to the south), crossing the late Cretaceous to middle Eocene unit down to the acoustic basement.

The second fault zone (i.e. the southern margin of the Bahia Basin, Fault C in Figs 4.5, 4.6, 4.9 and 4.11), in contrast to Fault A, strikes from west to east, with a similar trend to the Oca Fault System further to the east (Figs 4.1 and 4.3). Seismic lines (e.g. Figs 4.5 and 4.9) clearly show the normal displacement along this fault zone, which is very similar to the extension observed along the Oca Fault System (Ramirez, 2007; Vence, 2008; Restrepo-Correa and Ojeda, 2010; Cardona et al., 2013 – Figure 4.10). This extension is distributed across several small normal faults, and accumulated along a major strike-slip fault that also is interpreted to extend through the late Cretaceous to middle Eocene succession up to the boundary with the acoustic basement. Given, the similarities of this fault zone with the Oca Fault System, Fault C is interpreted to be a right-lateral transtensional fault that has formed in response to the continuous eastern movement of the Caribbean Plate in relation to South America (Duque-Caro, 1979; Audemard, 1996; Audemard, 2009; Montes et al., 2010).

Based on the analyses of the 2D seismic data, the Bahia Basin appears to have formed as a result of the concomitant activity along two dextral strike-slip zones (i.e. Faults A and C, Figure 4.11), which created a wide pull-apart basin system with the development of multiple depocentres (Figure 4.11). The depocentres are mainly expressed as the thickening in the Plio-Pleistocene unit (Seismic Unit 5), although there are also noticeable changes in the thickness of Oligocene, lower and upper Miocene sedimentary units (i.e. Seismic Units 2 to 4;

Figs 4.5, 4.6, 4.7, 4.9 and 4.10). These changes in thickness might be explained due to the migration of depocentres through time; this hypothesis, together with the structural evolution of the Bahia Basin, is discussed in detail in the following chapter (CHAPTER 5).

The Bahia Basin is also characterised by the presence of an important NE-SW trending fault in the centre of the basin (Fault B in Figs 4.5, 4.6, 4.7, 4.9 and 4.11). This fault is called the Bahia Fault and it is described in detail in the following chapter (CHAPTER 5). The Bahia Fault also accumulates significant strike-slip deformation and is the north-western boundary

of the main depocentre observed in the Bahia Basin. The footwall of the fault (north-western side), is deformed by a large number of closely-spaced, low-displacement normal faults that affect the Miocene unit (i.e. Seismic Units 3 and 4) and tend to disappear within the shale section of late Oligocene to early Miocene age (Seismic Unit 2).

On the hanging wall (south-eastern side) of the Bahia Fault, the depocentre deepens and widens from SW (Figure 4.5) to NE (Figure 4.9), forming a triangular shape that terminates abruptly along an N-S trending reverse fault to the east (Fault D) (Figure 4.6) and Fault C to the south. The reverse fault in the eastern boundary of the triangular depocentre (Fault D) may be the northern extension of the onshore Santa Marta-Bucaramanga Fault System (SMBFS in Figure 4.9, see also CHAPTER 2). This fault has a history of recent movement, as it deforms the Plio-Pleistocene unit (Seismic Unit 5) and even seems to offset Fault C on the southern margin of the basin (Figure 4.6).

The Santa Marta-Bucaramanga Fault System seems to act as an inversion structure of a pre-existing normal fault that extended southward, as the 2D seismic data shows a narrow continuation of the Bahia Basin towards the south (Fig. 4.6, 4.8 and 4.11). This is supported by Cienaga-1 well, which drilled a unit of Plio-Pleistocene sediments down to 384 m (1260 ft), and an undifferentiated Miocene section down to 3737 m (12260 ft) below sea level (Figure 4.8). This south-eastern part of the Bahia Basin is very complex, and may continue onshore in the basin between the Algarrobo and the Santa Marta-Bucaramanga faults, which form a pull-apart basin, on the western side of the Santa Marta Massif (SMM in Figure 4.11 – Reyes et al., 2000; Reyes-Harker et al., 2000; Ferreira et al., 2012; Bernal-Olaya, 2014), in response to the opening of the Lower Magdalena Basin and the rotation of the Santa Marta Massif (Duque-Caro, 1979; Reyes-Harker et al., 2000; Montes et al., 2010).

In general, the Bahia Basin shows a tendency to widen from the intersection between Faults A and C in the SW (Figure 4.11), to the Nazareth Basin towards the NE (Figure 4.10). The opening of the basin would have started in Oligocene to early Miocene times as a pull-apart basin between Fault A and the paleo-Oca Fault; in the latest stages the basin may been partially inverted along the Santa Marta-Bucaramanga Fault System (Fault D) in response to the northern expulsion of the Maracaibo Block.