The contourite depositional system of the Gulf of Cádiz: A sedimentary model related to the bottom current activity of the Mediterranean outflow water and its interaction with the continental margin

Abstract

The present paper is mostly a synthesis of work conducted on the continental margin of the Gulf of Cádiz using a broad database collected by several cruises and projects supported by the Spanish Research Council and the NRL and Naval Oceanographic Office (USA), including: bathymetry, sidescan sonar imagery, seismic profiles, sediment cores, submarine photographs and physical oceanographic data. These data have enabled us to establish a detailed understanding of the morphologic development of the margin, its Pliocene and Quaternary stratigraphy, and a full characterization of the contourite depositional system (CDS) generated by the Mediterranean outflow water (MOW). The northern margin of the Gulf of Cádiz shows the following distinct features: (a) an active compressive framework where the “Cádiz Allocthonous Unit” provides an unstable substratum for Late Miocene, Pliocene and Quaternary sedimentation; (b) a relative lack of submarine canyons, except in the western area of the Algarve margin; (c) a very broad continental slope that lacks a marked continental rise; (d) a middle slope dominated by along-slope processes driven by the MOW, which has generated a complex CDS during the Pliocene and Quaternary; and (e) an irregular lower slope and abyssal plain region dominated by down-slope processes that is partly detached from an upper slope source region. The CDS is composed of both depositional and erosive features. The main depositional features are characterised by sedimentary wave fields, sedimentary lobes, mixed drifts, plastered drifts, elongated mounded, and separated drifts and sheeted drifts. The main erosive features are contourite channels, furrows, marginal valleys and moats. These various depositional and erosive features have a specific location along the margin, and their detailed distribution is essential to understand the present (and past) interaction of the MOW with the middle slope. Based on this distribution, five morphosedimentary sectors have been identified within the CDS, which from east to west are: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channels and ridges; (4) active contourite drifts; and (5) submarine canyons. The development of the CDS has been controlled in general by the Pliocene and Quaternary environmental and paleoceanographic changes and by the morphology of the margin, but in detail the development of each of these sectors is related to systematic deceleration of the MOW as it flows westwards, to the interaction with the margin bathymetry, and to the effects of Coriolis force. Our comprehensive sedimentary model for the CDS defines the Gulf of Cádiz margin as a mixed contourite-turbidite system with a detached combined drift-fan morphology. This is different from many other contourite influenced margins, where the contourite processes are dominant on the middle slope, and separated from the down-slope processes, which are characteristic on the lower slope and abyssal plains.