Quaternary evolution of the contourite depositional system in the Gulf of Cadiz
E. Llave, F. J. Hernández-Molina, L. Somoza, D. A. V. Stow, V. Diaz Del Río, 2007. "Quaternary evolution of the contourite depositional system in the Gulf of Cadiz", Economic and Palaeoceanographic Significance of Contourite Deposits, A. R. Viana, M. Rebesco
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This paper provides for the first time a detailed vertical and spatial representation of Quaternary evolution of the contourite depositional system (CDS) in the Gulf of Cadiz, based on the results of careful morphological, structural and stratigraphic analyses using high-resolution seismic reflection profiles as well as oil company borehole data, and piston and gravity cores. Different drifts observed on the stratigraphic architecture allow us to propose a regional Quaternary evolution for the whole system, in which three major stages can be identified. (1) In the Early Pleistocene to Mid-Pleistocene, the CDS was mainly dominated by depositional processes, where the upper and lower cores of the Mediterranean Outflow Water (MOW) generated the mounded elongated Cadiz-Faro-Albufeira drift in the transition between the middle and upper slope, and the equivalent Huelva-Guadalquivir drift on the middle slope. During this stage the main erosive features were established close to the Strait of Gibraltar. (2) In the Mid-Pleistocene to Late Pleistocene, two important changes in the CDS took place. One occurred at the transition between the middle and upper slope, related to a change in the upper branch of the MOW, when a mixed drift began to develop, burying the eastern part of the Cadiz-Faro-Albufeira mounded elongated and separated drift. The second change is observed on the central area of the middle slope, related to the lower branch of the MOW, where a large contourite channel (the Guadalquivir channel) progressively eroded the western part of the mounded Huelva-Guadalquivir drift. Laterally an extensive sheeted drift buried the previous mounded deposits. (3) In the Late Pleistocene to Holocene, in the northern area of the CDS, a plastered drift started to be developed in the transitional zone between the upper and middle slope. On the middle slope, the mounded elongated Huelva-Guadalquivir drift was not developed and more erosive processes became dominant as the lower core of the MOW intensified. In the sector close to the Straits of Gibraltar, a field of broad seabed forms was generated. These three evolutionary stages have been controlled by tectonics, including recent diapiric movement, Guadalquivir Bank uplift, and reactivation along several fault systems and anticline-syncline structures. Tectonics has been a key factor in the sea-floor morphological changes, which has caused new pathways for the core and branches of the MOW, and consequently has produced the contourite stratigraphic and architectural changes. Superimposed on these tectonic changes, both climatic and eustatic changes during the Quaternary (but especially from the Mid-Pleistocene) have controlled the development of vertical contourite stratigraphy. The general conclusion of this study is that the contourite depositional system of the Gulf of Cadiz has changed from a dominantly depositional system to a dominantly erosive one during the Quaternary.
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There has lately been a growth in the number and level of studies of contourite deposits. Most recent studies of contourites have two major lines of interest. One, propelled by the oil industry's continuous move into increasingly deep waters, concerns their economic significance. The other involves the stratigraphic/palaeoceanographic record of ocean circulation changes imprinted on contourite deposits that can be a key to understanding better the climate-ocean connection. The application of many different theoretical, experimental and empirical resources provided by geophysics, sedimentology, geochemistry, petrology, scale modeling and field geology are used in the 16 papers of this volume, proposing answers to those two main aspects. The papers are subdivided into two major categories (economic interest and stratigraphic/palaeoceanographic significance), with case studies ranging from well-documented drifts to new examples of modern and fossil series, involving a large diversity of geographic and physiographic scenarios worldwide.