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doi:10.2204/iodp.proc.339.108.2013 Site U13901Expedition 339 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1390 (36°19.110′N, 7°43.078′W, 992 m water depth) is located southeast of the Guadalquivir Bank and the Guadalquivir contourite channel (Fig. F1, F2) over the large sheeted drifts from the northwest part of Sector 3 of the contourite depositional system (CDS) (channel and ridge sector defined by Hernández-Molina et al., 2003, and Llave et al., 2007a). This site represents an opportunity for identifying the recent tectonic influence on the architecture and evolution of the CDS and how it affected Mediterranean Ocean Water (MOW) distribution. It was selected to recover an early Pleistocene to present sedimentary record as well as date several prominent unconformities (Fig. F2) that can be traced regionally across the dense seismic grid. This area, located in the central middle slope of the Gulf of Cádiz, has been affected by recent tectonic activity, which controlled the major morphostructural features and its recent evolution (Maldonado et al., 1999; Maestro et al., 2003; Terrinha et al., 2003, 2006, 2009; Medialdea et al., 2004, 2009; Fernández-Puga et al., 2007; Zitellini et al., 2009; Duarte et al., 2010; Roque et al., 2012) and determined a complex pattern of different erosive features, such as contourite moats, contourite channels, marginal valleys, and large isolated furrows (Hernández-Molina et al., 2003, 2006; Llave et al., 2007a; García et al., 2009) (see Fig. F4 in the “Site U1389” chapter [Expedition 339 Scientists, 2013g]). A general background about the area of Site U1390 has been included in “Background and objectives” in the “Site U1389” chapter (Expedition 339 Scientists, 2013g), although we summarize some new important local aspects here. Charting the chronology of neotectonic activity that has had significant effect on development and architecture of the CDS is essential to a more complete understanding of the margin system in this region. Margin evolution anywhere is controlled by complex interaction of many different forcing variables, most importantly sea level and climate, sediment supply, and tectonics. In the Gulf of Cádiz, it is theorized that by the end of the early Pliocene subsidence decreased and the continental margin was evolving toward its present, more stable conditions (Maldonado et al., 1999; Lopes et al., 2006; Medialdea et al., 2004; Roque et al., 2012). Nevertheless some neotectonic activity is also evident, as expressed by changes in the submarine topography, the occurrence of mud volcanoes and diapiric ridges (Somoza et al., 2003; Pinheiro et al., 2003; Terrinha et al., 2009; Fernández-Puga et al., 2007; Duarte et al., 2010), and fault reactivation (Alves et al., 2003; Medialdea et al., 2004, 2009; Zitellini et al., 2009; Terrinha et al., 2006, 2009). In addition, some studies have reported how the tectonics have exerted strong control on the pathways of MOW and, therefore, on the architecture of the CDS (Llave et al., 2001, 2007b, 2011; García et al., 2009; Roque et al., 2012). Llave et al. (2007b) and Stow et al. (2011) postulate that this tectonic activity has controlled the change from a tabular MOW flow regime to separate flows forming distinct cores of MOW in the Gulf of Cádiz. Although some tectonic unconformities were previously considered by these authors, no drilling or dating has been carried out. The location of Site U1390, close to the Guadalquivir Bank and Guadalquivir contourite channel, was planned in order to better determine the timing and effects of tectonic activity during the Quaternary. The Guadalquivir Bank represents a structural high in which Paleozoic and Mesozoic rocks of the Iberian margin have been uplifted (Gràcia et al., 2003; Medialdea et al., 2004, 2009; Terrinha et al., 2003, 2006; Roque et al., 2012) from the Neogene through the present. This morphostructure is associated with a cluster of earthquake epicenters showing a dominant reversed motion (Borges et al., 2001; Buforn et al., 2004). The Guadalquivir Bank has played an important role in this area, affecting the hydrodynamic system and accommodation space for the Pliocene–Quaternary sedimentation (Figs. F1, F2). The Guadalquivir contourite channel, crossing the southeastern boundary, is the second largest contourite channel in this sector being >90 km long (García et al., 2009). The Guadalquivir contourite channel is located between the Guadalquivir diapiric ridge and the Guadalquivir Bank and corresponds to Channel 3 identified by Kenyon and Belderson (1973). This channel has two almost parallel east-west–oriented branches that merge near the Guadalquivir Bank, the southern and northern branches of the Guadalquivir contourite channel. After merging, the unique channel is parallel to the Guadalquivir Bank. Widths range from 1 to 12 km, with the minimum values in the distal reaches. Vertical incision depths reach 130 m, and transverse profiles are asymmetrical, U- or V-shaped, with generally steeper northern flanks. The average axial gradient is 0.37° on the northern branch and 0.44° on the southern branch. The contourite channels were created by erosive processes. In their alongslope-trending zones, the present-day MOW has medium to low velocities of 10–50 cm/s in a southeast–northwest direction. In contrast, in the downslope-trending zones MOW shows velocities of 50–90 cm/s in a northeast–southwest direction (Madelain, 1970; Kenyon and Belderson, 1973; Melières, 1974; Zenk, 1975; Nelson et al., 1999; Hernández-Molina et al., 2006; García et al., 2009; Stow et al., in press). Hanquiez et al. (2007) suggest a similar intensification of the current velocity in the downslope-trending sectors, based on estimated transport velocity values and the orientation of bedforms. As a result of Coriolis effect, when the lower core of MOW reaches the Guadalquivir Bank, the lower section of the flow is affected by the topography, which increases the flow’s velocity and erosive capability while it is deflected toward the southwest and is constricted by the topography. Coriolis effect forces the trend of the flow toward the right, enhancing the erosive capability of the flow along the northwest flank. The flow in contourite channels is strong enough to erode both the northwest and southeast flanks, but no associated generation of mounded contourite drifts occurs. A similar effect of salt walls and diapirs has been interpreted to cause the acceleration of deep currents in the Brazilian margin (Viana, 2001; Viana et al., 2002). ObjectivesThe major objective for Site U1390 is to recover a sedimentary contourite record for the Quaternary (Stow et al., 2011), deposited under the influence of the lower core of MOW, and determine the synsedimentary neotectonic control on architecture and evolution of the CDS. A careful dating of the principal unconformities throughout the region is proposed, followed by correlation with the seismic reflection framework already established (see Fig. F19 in the “Expedition 339 summary” chapter [Expedition 339 Scientists, 2013a]). Specific objectives for Site U1390 include:
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