• No results found

Tectono-sedimentary evolution of the Espírito Santo Basin

3. Geological setting

3.2 Geological setting of the Espírito Santo Basin

3.2.1 Tectono-sedimentary evolution of the Espírito Santo Basin

The Espírito Santo Basin is located on the continental margin of SE Brazil in the area between the Abrolhos Bank and the Campos Basin (Fig. 3.1). The basin covers approximately 41, 500 km2 in

area, of which 300 km2 are located onshore (França et al., 2007). The width of the Espírito Santo

Basin’s continental shelf increases from 50-60 km in the south of the study area, to 240 km on the Abrolhos Bank to the north (Bastos et al., 2015) (Fig. 3.1). The modern shelf break varies in depth from 40 to 80 m (Knoppers et al., 1999; Bastos et al., 2015).

The basement of the Espírito Santo Basin is part of the São Francisco Craton, which is composed of migmatites, granulites and granitoids (França et al., 2007). The development of the Espírito Santo Basin is closely related to the breakup of Gondwana supercontinent (Ojeda, 1982; Mohriak, 2008). Four tectono-sedimentary stages have been interpreted in the Espírito Santo Basin by Ojeda (1982) and Mohriak et al. (2008): pre-rift, syn-rift, transition and drift stages (Fig. 3.2). However, Chang et al. (1992) and França et al. (2007) did not recognized deposits of ‘pre-rift’ stage in the Espírito Santo Basin.

65

Figure 3.1. A) Regional map of the SE Brazilian margin showing the location of the studied 3D seismic volume from the Espírito Santo Basin. B) Contoured seafloor map of the study area generated from the interpreted seismic volume. It highlights the location of the modern channel relatively to salt diapirs D1 to D6.

66

Figure 3.2. Schematic diagram showing major tectonic stages of the Brazilian margin (redrawn from Ojeda, 1982). The location of the Espírito Santo Basin (ESB) is highlighted by the red box. A) Syn-rift phase dominated by continental environments. B) Transitional phase characterised by the deposition of evaporites. C) Early drift phase, with the formation of shallow-marine carbonate platforms. D) Late drift phase characterised by open marine sedimentation.

67

The ‘pre-rift’ stage spans the Late Jurassic to Early Cretaceous, when the breakup of Gondwana supercontinent started (Mohriak et al., 2008). This stage is marked by a small degree of asthenospheric uplift and lithospheric thinning. These processes resulted in normal faulting and the formation of local depocentres (Mohriak et al., 2008). A thin sedimentary sequence was deposited within these depocentres during this period. It is mainly located in the Barreirinhas and Sergipe- Alagoas basins offshore Brazil (Ojeda, 1982; Mohriak et al., 2008), but is likely absent in the Espírito Santo Basin (França et al., 2007). This thin sequence comprises fluvial and alluvial-fan deposits, together with lacustrine deposits (Ojeda, 1982).

The ‘syn-rift’ stage developed from the Late Berriasian to Early Aptian. This stage is characterised by an increased degree of asthenospheric uplift and lithospheric thinning (Mohriak et al., 2008). In the basins located in the south-southeastern part of offshore Brazil, such as the Espirito Santo Basin, the presence of Neocomian tholeiitic basalts overlying the Precambrian basement marks increased lithospheric stretching (Ojeda, 1982; Chang et al., 1992; Mohriak et al., 2008). This magmatic event was followed by intense normal faulting, and the formation of a series of half grabens and rift valleys (Mohriak et al., 2008). This stage records the widespread formation of rift sub-basins in the Espírito Santo Basin, in which fluvial-lacustrine sediments were accumulated (Ojeda, 1982; Chang et al., 1992) (Fig. 3.2). These fluvial-lacustrine sediments are interbedded with volcaniclastic intervals (Chang et al., 1992; França et al., 2007), comprising the lower-Cretaceous Cricaré Formation (França et al., 2007) (Fig. 3.3). The terrestrial deposits in this formation change from conglomerates and coarse sandstones in the proximal region to fine-grained mudstones and shales in more distal regions of the basin (França et al., 2007). The lacustrine shales of the Cricaré Formation are the main source rock of the basin (Estrella, 1984). The Cricaré Formation is overlain by the Mucuri Member, which changes from alluvial fan and fluvial deposits to lacustrine deposits and associated sabkha deposits (França et al., 2007) (Fig. 3.3).

By the end of this stage, a relative increase in lithospheric extension resulted in the reactivation of large faults and the formation of a regional breakup unconformity (Mohriak and Fainstein, 2012). This unconformity separates continental fluvial-lacustrine sediments from microbialites (pre-salt units) and overlying evaporates, and marks the beginning of the transitional stage, which spanned the Middle Aptian to Late Aptian/Early Albian. The deposition of thick evaporite sequences and

68

Figure 3.3. Stratigraphic column of the Espírito Santo Basin highlighting the main tectono- sedimentary stages and magmatic events in the basin (modified from França et al., 2007).

69

marine carbonates predominated at this stage (Ojeda, 1982) (Fig. 3.2). In the Espírito Santo Basin, the thick evaporite succession is named the Itaúnas Member, which belongs to the Mariricu Formation (França et al., 2007) (Fig. 3.3). The transitional stage is followed by a drift phase spanning from Late Aptian/Early Albian to the present day, and is characterised by the deposition of open marine strata (Ojeda, 1982; Chang et al., 1992) (Fig. 3.1). Strata deposited in this phase are divided into two megasequences: early-drift transgressive and late-drift regressive (Mohriak, 2003; Fiduk et al., 2004) (Fig. 3.3).

The transgressive megasequence consists of Albian carbonates and overlying muddy and sandy turbidites, and marks a general deepening-upwards trend towards the end of the Cretaceous (Davison, 1999; Fiduk et al., 2004; Alves et al., 2009). This megasequence started from the Eocene in the Espírito Santo Basin and is marked by the presence of calcareous marls (França et al., 2007). In the basin, the base of this megasequence comprises fan-delta clastics (São Mateus Member of the Barra Nova Formation) in the western proximal region, and shallow marine carbonates (Regência Member of the Barra Nova Formation) in distal regions of the basin (França et al., 2007) (Fig. 3.3). The carbonate platform is overlain by dark shales and turbidites (Urucutuca Formation) of Late Cretaceous age within the basin (França et al., 2007) (Fig. 3.3). Several submarine canyons were developed at this stage, e.g. the Regência and Fazenda Cedro Canyons, which are filled with turbidite and comprise important hydrocarbon reservoirs (e.g. Bruhn and Walker, 1997).

The transgressive megasequence is followed by the regressive megasequence, which started at the end of the Cretaceous and is related to the clastic progradation due to the uplift of the Serra do Mar and Serra da Mantiqueira ranges in southeast Brazil (Mohriak and Fainstein, 2012). The regressive megasequence marks the reactivation of rift structures and episodic magmatic activity (Demercian et al., 1993; Cobbold et al., 2001; Mohriak et al., 2008) (Fig. 3.3), and is associated with the development of the Abrolhos Bank to the north of the study area (Cordani, 1970). The development of the Abrolhos Bank changed local slope configuration by extending the continental shelf edge as far as 200 km eastwards (Fig. 3.1). The regressive megasequence is characterized by the deposition of recurrent MTDs (Omosanya and Alves 2013; Gamboa et al., 2010), together with turbiditic channels and lobes, which were sourced from the Abrolhos Bank and fluvial rivers (Bruhn and Walker, 1997; Davison, 1999; Alves et al., 2009; Gamboa and Alves, 2015).

70