Reflection seismics and OFOS
After finishing hydroacoustic mapping of the south-eastern slope of the platform on October 6, we deployed the digital streamer and the acoustic sources and started a reflection seismic survey. This seismic survey aimed on linking the stratigraphy of ODP (Ocean Drilling Program) Sites 705/706, located south-east off the platform, with our seismic data from Saya the Malha Bank. However, already in reach of the sites, a technical problem emerged with the digital streamer, which finally needed to be recovered.
Nevertheless, seismic data acquisition during this expedition was very successful with 1750 km of multi-channel reflection seismic lines measured until now. This data provides insight into the sedimentological and structural architecture and the stratigraphic framework of the Saya de Malha carbonate platform. The slope is characterized by a very steep upper segment, with slope angles exceeding 20°. In its middle part, the cemented slope is buried under an 850 m thick wedge of peri-platform sediments. They overlay the crustal basement which could be identified in the seismic data due to its rough relief and the strong impedance contrast marked by high amplitude reflections. At ODP Site 706 this basement is composed of basaltic volcanics. The sediment wedge at the eastern slope of Saya de Malha is characterized by the alternation of well stratified sediment packages with intercalated mass transport deposits, especially in the uppermost part where slump deposits dominate. The edge of the platform is located at a water depth of ca. 240 m and is marked by a drowned fringing reef. Towards the interior of the platform, this old platform is overlain by a truncated sediment wedge. Data show that the platform margin stepped back with time; and that the producing areas constantly decreased as a result.
A longer sampling program started in the afternoon of October 8 and leads us from the south-western tip of the platform through the inner platform towards working area WA-4, located at the northwestern margin of Saya de Malha. Sedimentological sampling continued until October 11, two times interrupted by longer OFOS (Ocean Floor Observation System) transects. OFOS data will help us to interpret the sediment samples in a larger context, and to obtain a better image of sediment- and faunal distribution on the modern carbonate platform. Photos and videos of the sea floor are furthermore valuable for the interpretation of bottom current strength and direction, which could not be measured with the shipboard ADCPs.