Microstructure of muddy contourites from the Gulf of Cádiz

In deep-sea sediments, contourites deposited from bottom currents mainly along slopes are associated with hemipelagites, pelagites, and sediment gravity-flow deposits. The major sedimentary facies of contourites are characterized by bioturbation and mottling and bigradational grading. However, it is difficult to discriminate muddy contourites from hemipelagites by using only these features of the lithofacies, particularly in core samples. Therefore, the present study focused on microstructure as an additional potentially diagnostic characteristic for identifying muddy contourites. Microstructures of muddy contourites, mainly from the Gulf of Cádiz, and of hemipelagites from off the West Iberian margin collected by IODP Expedition 339 were examined, along with their magnetic fabric and grain-size distribution. The results showed that the muddy contourites were composed mainly of silt grains, including plate-like silt particles that were oriented parallel or sub-parallel to the bedding plane. Some samples were composed of silt grains up to 100 μm in apparent size. In contrast, the hemipelagites were composed of calcareous nannoplankton or silt grains and randomly oriented clay particles. The magnetic fabric, as indicated by the anisotropy of magnetic susceptibility, and the grain-size distribution of the muddy contourites also differed from those of hemipelagite. On the basis of these features, the plate-like silt particles parallel or sub-parallel to the bedding plane characterizing the muddy contourites were interpreted to reflect rapid deposition from a low-concentration suspension of fine-grained particles in a weak bottom current. Although microstructural features used alone are insufficient, used in conjunction with magnetic fabric and grain-size distribution, they are expected to be a useful tool for discriminating muddy contourites from hemipelagites.