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doi:10.2204/iodp.proc.339.107.2013 Site U13891Expedition 339 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1389 (36°25.515′N, 7°16.683′W; 644 m water depth) is located in Sector 3 of the contourite depositional system (CDS) (channel and ridge sectors defined by Hernández-Molina et al., 2003, and Llave et al., 2007a) on the northwest side of the Guadalquivir diapiric ridge (Figs. F1, F2). This is an important site for the recovery of a late Pliocene and Pleistocene succession under the influence of the lower core of Mediterranean Outflow Water (MOW). This sector is located in the central area of the middle slope between Cádiz and Faro, at 800–1600 meters below sea level (mbsl) (Fig. F3), and was first studied by Nelson et al. (1993, 1999) and Baraza et al. (1999), who originally described it as the “Ridge and Valley Province.” The sector has been described in more detail recently by several authors (Mulder et al., 2002, 2003; Habgood et al., 2003; Hernández-Molina et al., 2003, 2006; Hanquiez et al., 2007; Llave et al., 2007a; García et al., 2009). The sector includes some prominent morphostructural features, including the Guadalquivir Bank; the Doñana, Guadalquivir, and Cádiz diapiric ridges; and numerous isolated diapiric highs and mud volcanoes (Fig. F3) that are caused by recent tectonic activity (Maldonado et al., 1999; Maestro et al., 2003; Somoza et al., 2003; Medialdea et al., 2004, 2009; Fernández-Puga et al., 2007; Zitellini et al., 2009). Some of the diapiric ridges occur as a distinctly linear and segmented seafloor relief with a general northeast–southwest trend, whereas others are buried and show an undulating deformational pattern of topographic highs and lows. In this scenario, a complex pattern of erosive features occurs, composed of four types of submarine valleys: contourite moats, contourite channels, marginal valleys, and large isolated furrows (Hernández-Molina et al., 2003, 2006; Llave et al., 2007a; García et al., 2009). As many as nine contourite channels have been identified in this sector by García et al. (2009), with the five larger ones known as the Cádiz, Guadalquivir, Huelva, Diego Cão, and Gusano Channels (Figs. F3, F4). These channels are asymmetrical in cross section, with lengths ranging from 10 to >100 km, widths ranging from 1.5 to 10 km, and depths ranging from 10 to 350 m, and the incisions are generally greater on the northern flanks (Figs. F4, F5). Within the contourite channel, the floors are characterized by very high backscatter, representing scoured diapirs, outcrops of rock, and rubble partially covered by longitudinal bedforms and regularly spaced dunes (Hernández-Molina et al., 2006; Stow et al., 2009, in press). Nelson et al. (1993) identified gravel and shelf lags in the eastern channels and sand in western channels. Contourite channels are broadly S-shaped in plan view, with northwest-southeast–oriented alongslope segments veering to northeast-southwest–oriented downslope segments, caused by the interaction of bottom currents with the irregular morphology of the slope and diapiric ridges (Figs. F3, F4). Several marginal valleys with irregular morphology and a northeast–southwest orientation are noted behind the adjacent diapiric ridge flank, whereas other marginal valleys identified by García et al. (2009) have much more irregular trends. Erosive contourite channels and marginal valleys are present over a broad deformed sheeted drift. This drift represents the main depositional morphology in this sector (Figs. F4, F5), but some patch drifts can be also identified behind local topographic obstacles. The distribution and characteristics of these erosive features allowed García et al. (2009) to propose a new and more precise scheme for the MOW circulation pattern. This scheme includes a main alongslope circulation responsible for the excavation of the contourite moat and channels and a secondary downslope circulation responsible for the erosion of marginal valleys and isolated furrows. Based on seismic data through the Quaternary period, Llave et al. (2007a) and García et al. (2009) reported important depositional and oceanographic changes associated with tectonic events related to the aforementioned morphostructural features, coeval with the mid-Pleistocene revolution and marine isotope Stage 6. In these two periods, realignment of the diapiric ridges and their segmentation as the result of neotectonic effects led to the development of new local gateways. These affected the distribution of the different branches of the MOW, and hence, the distribution of associated contourite channels. As a result of these tectonic changes, some mounded, elongated, and separated drifts were buried (Figs. F6, F7) under the influence of the lower core of MOW. These are preserved in the sedimentary record (Llave et al., 2007b). ObjectivesThe major objective for Site U1389 was to recover a sedimentary contourite record through the late Pliocene and Pleistocene (see F17 in the “Expedition 339 summary” chapter [Expedition 339 Scientists, 2013a]) under the influence of the lower core of MOW (Stow et al., 2011). This record will allow us to investigate
Specific objectives for Site U1389 include
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