Morphostructure and tectono-sedimentary control of a mixed contourite-turbidite system in a transpressive margin : Gulf of Cadiz, SW Iberia
Esentia, Ibimina Pepple
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A comprehensive analysis has been carried out of morphological features, morphometric characterisation and structural attributes of the northern Gulf of Cadiz slope, offshore Spain and Portugal. This study used an extensive database including high-resolution bathymetric mapping, 2D seismic reflection profiles and sediment cores recovered during IODP Expedition 339 in order to document the nature of a transpressive continental margin. The principal features include faults, channels, diapirs, fluid-escape features (pockmarks, mud volcanoes, mud mounds, chimneys), mass-transport deposits (slides, slumps, debrites), and erosionaldepositional forms (scarps, scours and sediment waves). A complete new morpho-sedimentary map of the region has been constructed, and a novel morphometric analysis carried out. This study shows that tectonism exerts the strongest control on sedimentation. Most structural features are aligned NE-SW, others are orientated E-W and NW-SE, and geomorphological features show parallel alignment. These morpho-tectonic features affect the physiography of the sea floor and hence hydrodynamics of currents. Diapiric ridges and large faults affect the seafloor gradient. Channels in the study area evolved, avulsed and changed direction in response to their interaction with geomorphological structures and current action. Furthermore, tectonism resulted in significant differences in the geomorphological characteristics and morphometrics (drainage density, basin shape, and stream numbers) between the northern and the southern half of the study area. This is linked to the presence of a large accretionary wedge complex in the south, resulting from large-scale plate tectonic interaction. The north-westwards advancement of the AWC, in addition to transpressive tectonism, is responsible for the number and dimension of morphological features in the south of the study area. The northern Gulf of Cadiz margin can be referred to as a mixed depositional system having a varied sedimentary assemblage including contourites, turbidites, bottom-current reworked turbidites, debrites, mass transport deposits, hemipelagites and pelagites. The different facies are, in part, spatially and temporally separated, and in part, closely interbedded. Contourites are the dominant facies in the Quaternary, with turbidites and associated downslope facies most evident in the older Pliocene-Miocene succession. Grainsize statistical analysis (over 1200 turbidite and over 600 contourite samples), together with mineralogical study, reveal distinctive features of standard turbidites and contourites, whereas reworked turbidites have some similarities to both. Bi-variate plots of grainsize statistical data help distinguish between turbidites and contourites. Especially valuable are the mean size vs skewness and the coarsest percentile vs median (CM) cross plots.