Recognitions of contourites in deep-sea sediments
Speaker: Hsiao-Pei Chang
Graduate Institute of Geophysics, National Central University
Abstract
Contourite is defined for deposits at depths greater than 500 m and are formed by stable thermohaline-induced geostrophic bottom-currents. These currents commonly flow along bathymetric contours and travel at velocities between 2-20 cm/s, or up to 50 cm/s in rare cases.
Characteristics of contourites: (1) drift geometry: elongate mounds, sheets, channel-related drifts, confined drifts; (2) depositional unit: lenticular in shape and up-wardly convex geometry; (3) fining and sorting trends: perpendicular or parallel to the contours; (4) mineralogy and textural composition: regional pattern indicating transport perpendicular or parallel to contours; (5) sedimentary structures: contourites are bioturbated and contain irregular lag concentration of biogenic sand.
Reviews that have been proposed for distinguishing between the deposits of turbidity currents and bottom currents in deep-water sediments show no definite agreements. This talk utilizes two examples to demonstrate major mechanisms that shape and deposit contourites in deep-sea continental slopes. Rebesco et al. (2003) proposed that continental margins that border the Atlantic Ocean are influenced by strong contour currents. Sedimentary processes and structures across the continental rise in the western Weddell Sea (Antarctica) and Sao Tome deep-sea (Brazil) have been investigated using sediment acoustic and multichannel seismic data, integrated with multibeam depth sounding and core investigations. Gonthier et al. (2003) showed that a network of channels with associated along-channel ridges covers the upper continental slope of planktonic foraminifera; they probably come from the nearby beds outcropping along Soe Tome escarpment. It is important to compare fine-grained turbidites and contourites, to recognize that different turbidity current and bottom current mechanisms exist, and that their deposits may be closely interbedded in a continental rise environment.
Michels et al. (2001) shows that the unique characteristics of and high-latitude contourite systems that show interaction with turbidity current, debris flow; and the importance of local slope topography, developed as a result of syn-sedimentary tectonics and downslope mass movement, in focussing and shaping drift deposits.
According to seismic and echofacies data, we can see turbidites with strong reflection and clear bedding in echofacies. The bedding of the contourites are not clear, and sometimes there are internal reflection exist.Different scenarios illustrating the possible relationships between the slope trend, the current trend and the sediment levee pattern when either turbidity or contour current are involved in sediment deposition. The results demonstrate that sediment supply is an important, maybe the controlling factor of drift development on the continental rise.
References
Michels, K.H., Rogenhagen, J., Kuhn G. (2001) Recognition of contour-current influence in mixed contourite-turbidite sequences of the western Weddell Sea, Antarctica. Marine Geophysical Researches 22, 465-481.
Gonthier, E., Faugeres, J.-C., Viana, A., Figueiredo, A., Anschutz, P. (2003) Upper Quaternary deposits on the Sao Tome deep-sea channel levee system (South Brazilian Basin): Major turbidite versus contourite processes. Marine Geology 199, 159-180.