The Cadiz Contourite Channel: Sandy Contourites, Bedforms and Dynamic Current Interaction
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Marine Geology 343 (2013) 99–114 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction D.A.V. Stow a, F.J. Hernández-Molina b,⁎, E. Llave c, M. Bruno d, M. García e, V. Díaz del Rio f, L. Somoza c, R.E. Brackenridge a a Institute of Petroleum Engineering, Heriot Watt University, Edinburgh EH14 4AS, UK b Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK c Instituto Geológico y Minero de España, Ríos Rosas, 23, 28003 Madrid, Spain d Dpto. Física Aplicada, Facultad de Ciencias del Mar, University of Cádiz, Puerto Real, Cádiz, Spain e Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Av de las Palmeras 4, 18200 Armilla, Granada, Spain f Instituto Español de Oceanografía, C/Puerto Pesquero s/n, 29640 Fuengirola, Spain article info abstract Article history: The Cadiz Contourite Channel is the largest and most prominent contourite channel in the middle slope of the Received 17 December 2010 Gulf of Cadiz, and is known to channelise the southern branch of the Lower Core of Mediterranean Outflow Received in revised form 28 May 2013 Water (MOW) as it flows westwards from the Gibraltar Gateway. The channel lies in water depth between Accepted 19 June 2013 650 and 1500 m, is 150 km long, 2–12 km wide, up to 120 m deep, and broadly s-shaped in plan view. It Available online 26 June 2013 has several associated subparallel marginal channels and shorter spillover channel segments. Its geometry fl Communicated by D.J.W. Piper is controlled by the interaction of a strong bottom current with the sea oor morphology, affected by neotectonic deformation and diapiric intrusion. Bottom photographs and dredge hauls reveal a channel Keywords: floor shaped by high-energy flow, in places with bare rock, boulders and gravel, and elsewhere covered Gulf of Cadiz with sandy contourites. The rocky substrate and derived clasts are formed of authigenic iron-rich carbonates, bottom currents testifying the high degree of fluid escape from adjacent diapiric ridges and mud volcanoes. The sandy sub- contourite channel strate shows a wide range of current-induced bedforms including small, straight-crested ripples, large sinu- sandy contourites ous sand waves and dunes (wavelength 3.5–5 m, height 0.3–0.9 m), weak surface lineation on sands, and internal tides/waves aligned gravel stringers and deep erosive scours around large boulders. Bedform orientation indicates flows directed to the south/south-west (main channel) and west (spillover channel), which can be related to MOW bottom currents, and current velocities that vary between about 0.2 and 0.8 m s−1, even in the same channel location. However, current vane orientation was clearly responding, at least in part, to tidal ef- fects and periodicity in the Gulf of Cadiz at the time the photographs were taken. Maximum current velocities are achieved by a combination of barotropic and internal tides (probably generated at the continental slope) that reinforce the normal MOW flow. In addition, meteorologically-induced internal waves with periods shorter than tidal ones may exert an even greater influence on current intensity, especially when they occur at times of sudden changes of meteorological forcing. This effect further influences MOW variability. In all cases, the funnelling effect of the Cadiz Channel amplifies tidal or meteorologically-induced bottom currents. © 2013 Elsevier B.V. All rights reserved. 1. Introduction (Maldonado et al., 1999; Gràcia et al., 2003). Continued neotectonic activity in the region is expressed by the occurrence of mud diapirs 1.1. Geological-oceanographic setting and diapiric ridges (Diaz-del-Río et al., 2003; Somoza et al., 2003; Fernández-Puga et al., 2007; Gonzalez et al., 2007; Martin-Puertas The Gulf of Cadiz straddles the diffuse boundary between the et al., 2007; Zitellini et al., 2009). Where mud diapirs break through Eurasian and African plates, immediately west of the Straits of Gibral- the seafloor they are known as mud volcanoes, and are commonly as- tar. Tectonic adjustments along this boundary led to opening of the sociated with gas escape. Tectonic setting and activity have been key Gibraltar oceanic gateway at the end of the Miocene, around 5.3 Ma long-term factors influencing seafloor morphological changes, which have in turn exerted strong control on the downslope and alongslope processes that have shaped the margin. ⁎ Corresponding author. E-mail addresses: [email protected] (D.A.V. Stow), [email protected] The present-day circulation pattern in the region is dominated by (F.J. Hernández-Molina). exchange between the Mediterranean Sea and the Atlantic Ocean of 0025-3227/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2013.06.013 100 D.A.V. Stow et al. / Marine Geology 343 (2013) 99–114 water masses through the Straits of Gibraltar (Fig. 1). This exchange is erosional features, including several prominent contourite channels. driven by the near-bottom flow of highly saline and warm Mediterra- The system has been the subject of many previous studies (e.g. nean Outflow Water (MOW) and the turbulent flow of less saline, Melières, 1974; Gonthier et al., 1984; Faugères et al., 1985; Stow et cool-water mass of the Atlantic Inflow Water at the surface. The al., 1986, 2002b; Nelson et al., 1993, 1999; Schönfeld and Zahn, MOW forms a strong bottom current flowing towards the W and 2000; Weaver et al., 2000; Llave et al., 2001, 2004, 2006, 2007a, NW above North Atlantic Deep Water (Madelain, 1970; Melières, 2011; Mulder et al., 2002, 2003, 2006; Habgood et al., 2003; 1974; Zenk, 1975; Thorpe, 1976; Ambar and Howe, 1979; Zenk and Hernández-Molina et al., 2011; Stow et al., 2011a; Roque et al., Armi, 1990). After passing the Gibraltar seamount, the MOW forms 2012). It can be divided into five major morphosedimentary sectors: a turbulent overflow only some 150 to 200 m wide and reaches ve- 1) proximal scours and ribbons;2)overflow sedimentary lobes;3)chan- locities in excess of 2.7 m s−1. It then spreads westward into the nels and ridges;4)contourite depositional sector; and 5) submarine Gulf of Cadiz, veering north-westward due to the Coriolis force, and canyons. These different sectors are clearly related to the progressive progressively descends until the middle continental slope driven by deceleration of the MOW away from the Gibraltar Gateway, frictional gravity due to its excess density, eventually losing contact with the resistance of the seafloor, interaction with topography of the margin, seafloor at 1200–1400 m water depth, where it becomes neutrally and to Coriolis force (Hernández-Molina et al., 2006; Llave et al., buoyant (Baringer and Price, 1999). Due to interaction with the bot- 2007a; García et al., 2009). tom topography, the MOW splits into two principal branches This paper is principally concerned with the channels and ridges (Upper and Lower MOW). The upper branch follows roughly the sector, which is located in the central area of the middle slope be- boundary between the upper and proximal part of the middle slope tween the cities of Cadiz and Faro, lying at a water depth of between along the south Iberian margin, while the lower branch further splits 800 and 1600 m (Fig. 2A). It was first studied by Nelson et al. (1993, into three separate strands on the middle slope, in places flowing 1999) and Baraza et al. (1999) who considered it as the “Ridge and along and down segments of both contourite and turbidite channels Valley Province”, but it has been described in detail more recently by sev- (Hernández-Molina et al., 2006). eral authors (García, 2002; Mulder et al., 2002, 2003; Habgood et al., The interaction of MOW with the Gulf of Cadiz margin has resulted 2003; Hernández-Molina et al., 2003, 2006; Llave, 2003; Hanquiez in the development of a complex Contourite Depositional System et al., 2007; Llave et al., 2007a,b; García et al., 2009). According to (CDS) along the middle slope Hernández-Molina et al. (2003, 2006). these authors, the principal morphological elements of the channel This CDS represents a composite of both depositional drifts and and ridge sector of the Cadiz margin are due to a combination of the Fig. 1. Location sketch with main water-mass circulation along the margin (from Hernandez-Molina et al. 2003; Llave et al., 2011). D.A.V. Stow et al. / Marine Geology 343 (2013) 99–114 101 Fig. 2. A) Swath bathymetry of the central sector of the middle slope of the Gulf of Cadiz Contourite System (CDS) obtained using the Simrad EM12S-120 system, showing the main contourite channels, furrows and the main diapiric ridges, with NNE trend. B) Airgun deep monochannel seismic profile across the central sector. The Cadiz Contourite Channel is shown in A and B. strongly erosive action of bottom currents, as they descend the conti- and current flow from the main channel and adjacent channel seg- nental slope, and neotectonic activity. Up to nine separate contourite ments; (3) consider these data, together with previously published channels have been identified in this sector by García et al. (2009), work, in terms of bottom current velocity and its variability; and with five larger ones known as the Cadiz, Guadalquivir, Huelva, Diego (4) discuss evidence for the influence of other oceanographic controls Cao and Gusano channels (Fig. 2). on bottom current velocity and resultant deposition, including inter- nal tides and internal waves. 1.2. Aims and significance 2. Methods and track location The Cadiz Contourite Channel is the largest and best studied ero- sional feature of its type in the world.