Microstructure of muddy contourites from the Gulf of Cadiz (IODP Exp. 339)

Friday, 19 December 2014
Naohisa Nishida, Geological Survey of Japan, Institute of Geology and Geoinformation, Tsukuba, Japan
In deep-sea environments, hemipelagites/pelagites and episodic-event deposits are formed from fall out of fine-grained suspensions and from sediment-gravity flows represented by turbidity currents, respectively. In addition, contourites are also formed from bottom currents along the slope. Bottom currents are one of the major processes for the sediment transportation and deposition in deep-sea environments, and are affected by local and global factors such as climate, sea level and tectonics. Therefore, it is important to discriminate the contourites from deep-sea sediments and to clarify their spatial and temporal distribution patterns to reconstruct fluctuations of bottom currents controlled under other factors. In general, major sedimentary facies contourite are characterized by bioturbated mottled facies and bi-gradational grading. However, it is difficult to discriminate muddy contourites from hemipelagites based only on the facies. Therefore, this study focuses on the microstructure of muddy contourites, which would be an additional characteristic/ diagnostic to identify muddy contourites. This study muddy contourites collected from 6 sites are analysed mainly in the Gulf of Cadiz by IODP Expedition 339 (Mediterranean Outflow). In addition, hemipelagites collected from off the west Iberian margin were also examined. Each studied sediments were sub-sampled from the core samples using 7 cc plastic cubes generally used for paleomagnetic analysis. Mean-grain size of the samples was 7.9–25.2 μm, and major mineral components were illite, calcite, and quartz. Most of the samples were wet condition, thus these were dried for SEM observations using a t-butyl alcohol replacing method, which allow us to prepare dried sample without deformation of microstructure. Then the microstructures of the samples were observed using a SEM (JEOL JSM-6390LV). As a result, it is recognized that hemipelagites are composed mainly by calcareous nannoplankton with random orientation of silt grains. In contrast, muddy contourites are characterized by silt grains with plate-like silt parallel or sub-parallel to the bedding plane, with less bioturbation than hemipelagites. These features in the muddy contourite are attributed to the effect of weak and steady bottom current.