Shale tectonics

Shale tectonics has been broadly defined as the deformation processes associated with shale or mudstone plasticity or mobility within a basin (Wood, 2010). The resultant structures include mud volcanoes and structures referred to variously as mud diapirs, shale diapirs, mobile shale/mud, or argillokinetic structures. The formation of some of these structures has been considered to be similar to the formation of analogous salt structures (Morley, 2003;

Rowan et al., 2004; Wood, 2004). Within the study area three types of shale-tectonic features are found: 1) mud volcanoes; 2) thickening of the shale into the core of the anticlines and diapir formation in the fold and thrust belt; and 3) lateral movement and shale withdrawal associated with sediment loading in the Bahia Basin.

Bathymetry data (Figure 4.12) and 2D seismic lines (Figs 4.13 and 4.14) clearly show the presence of large number of mud volcanoes. The volcanoes are identified by pyramidal/conical structures of circular to elliptical-base shape, with a maximum diameter of 2 km, a maximum flank slope of 30 degrees, and a maximum height of ca. 240 m above the seabed. The mud volcanoes are mostly located in a zone that encompasses the rear-most structures of the accretionary prism (i.e. structures 3, 4 and 5 – e.g. Figs 4.5 and 4.11) and extends south-eastwards into the Bahia Basin. A significant number of mud volcanoes are located close to, or immediately overlying, the steep faults behind the SCDB (i.e. thrust faults and Fault A – Figs 4.5 and 4.11) suggesting that the overpressured fluids that fed the mud volcanoes were released along these faults. Buried mud volcanoes that terminate within Miocene and Plio-Pleistocene sediments also occur indicating a long history of mud and fluid mobilisation in the area (e.g. Figs 4.7 and 4.10). Older phases of eruption at a paleo-seabed create typical “Christmas-trees” geometries (Stewart and Davies, 2006) in response to the changes in the relationship between the rate of sedimentation and the rate of mud extrusion; for example, Deville et al. (2006) describes that with constant rates of mud extrusion, at low sedimentation rates the mud extrude on the seabed creating mud volcanoes which are buried during stages of high sedimentation. When low sedimentation rates return mud can extrude again on the seabed creating stacked volcanoes, with serrated, or ‘Christmas tree’ style geometries. Figure 4.13 illustrates a comparison of the seismic

response described by Deville et al. (2006) and the geometries observed within the 2D seismic dataset in the Bahia Basin.

As previously indicated by different authors, mud volcanoes are likely sourced from deep overpressured-shale units rich in different kinds of fluids (i.e. water, gas, oil, or a combination of the above) and are connected to the surface along pipe-like or planar structures (Kopf, 2002; Kopf et al., 2003; Deville et al., 2006; Stewart and Davies, 2006; Evans et al., 2008; Bonini and Mazzarini, 2010; Bonini, 2012). Here, mud volcanoes (both on the seabed and covered by sedimentation) are fed by narrow conduits connected to a deeper source, which in this case corresponds to the overpressured-shale section of Oligocene to early Miocene age (Seismic Unit 2 – e.g. Figs 4.5, 4.6, 4.7, 4.10). The conduits are interpreted on seismic from discontinuities within the sedimentary section (Cartwright and Santamarina, 2015). Such discontinuities have been compared with pipes similar to those observed in igneous intrusions (e.g. Davies and Stewart, 2005), and typically occur along faults or at the crest of anticline structures (e.g. Figure 4.10).

Figure 4.12. Bathymetry and dip-attribute map of seabed the Bahia Basin. The area of the 3D seismic survey (light blue) and the coast line (purple) are shown for reference. Contour interval is 200 m.

Figure 4.13. A) Example of a mud volcano with a Christmas-tree geometry; from Deville et al., 2006. B) Section of seismic line L-1982-4900 showing examples of mud volcanoes extruding on the seabed. For location see Figure 4.12.

In the SCDB, the Oligocene to early Miocene shale unit frequently thickens within the core of the anticlines creating diapiric-like structures (e.g. structures 3 and 4 in Figure 4.5). In some cases volcanoes can be observed to be sourced from the thickened shales along thrusts and extrude within younger stratigraphy or at the present day sea floor. Within the Bahia Basin itself, mud volcanoes and shale diapirs are concentrated along the fault zone A on its north-western margin, but some volcanoes are also found elsewhere in the basin as seen in Figs 4.7 and 4.13 for example.

Apart from the shale movement and thickening within the core of the folds of the outer fold and thrust belt, examples of shale movement, associated with rapid thickness changes in both the shale unit itself and the overlying younger strata, are also found within the dataset.

Some of the geometries resemble salt-withdrawal structures found in salt provinces.

Examples of similar shale tectonic structures are known from other settings with mobile shale, such as the Niger Delta, the western Mediterranean Alboran basin, northern Venezuela, Makran accretionary prism, and NW Borneo (Morley and Guerin, 1996; Morley, 2003; Soto et al., 2010; Duerto and McClay, 2011), where local evacuation of mobile mud provides space for the accommodation of sediments. An example of this is seen very clearly

towards the NW occurs in response to the development of a down-building minibasin, ca. 5 km wide, during early to late Miocene times (Seismic Units 3 and 4). This process may also be present within some structures of the accretionary prism, such as structure 4 in Figure 4.5, where the thrusting and piggy-back basin formation at the rear of the thrust is accompanied by the process of shale movement into the core of the anticline, forming a diapir-like structure, and facilitating the down-building expansion of the piggy-back minibasin.

In summary, shale tectonics is manifest in a variety of ways; i.e. mud volcanism, thickening of shales into the core of anticlines, and shale evacuation under local depocentres. The overpressures required to allow the Oligocene muds to behave as a mobile material may have been generated by the deposition of the Magdalena Fan sediments over the underlying mud-rich Oligocene section. Regional seismic lines 5 and 6 show a some down-dip thickening of the Oligocene to early Miocene shale unit towards the area of the fold and thrust belt, and it appears regionally thinned under the Bahia Basin, which may indicate an overall down-dip flow of the shale unit, perhaps in response to the accumulation and loading of Magdalena Fan related sediments in the up-dip basins.