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A Modern Basin-Floor Fan Imaged in the Makassar Straits, Indonesia: Insights from High-Resolution Multibeam Bathymetry and Backscatter, Sub-Bottom Profiler Data, and USBL-Navigated Cores

Daniel L. Orange1, John Decker2, Philip A. Teas3, Rhys D. Schneider3, Arthur H. Saller4, Adam Heffernan5, Norman Maher6, and Matt Levey6
1 AOA Geophysics Inc. and U.C. Santa Cruz, Moss Landing, CA
2 Unocal Indonesia Co, Jakarta, Indonesia
3 Unocal Indonesia, Balikpapan, Indonesia
4 Unocal Corp., Sugar Land, TX
5 Kerrr-McGee (formerly AOA Geophysics Inc.), Houston, TX
6 AOA Geophysics Inc, Moss Landing, CA

In this talk we will present very high resolution data (12 kHz multibeam bathymetry and backscatter, high resolution sub-bottom profiles, and piston cores) of a basin floor fan imaged in the very shallow section and expressed on the seafloor. A large (>2500 sq km), low relief basin floor fan occurs in water depths >2000 m, and was discovered on the basis of quantitative multibeam backscatter. The Makassar fan shows a systematically varying backscatter response where identifiable features of the fan can be mapped areally based upon similar backscatter patterns. Channels are straight, bifurcating, and show low sinuosity. Lobes are generally 1-2 km wide and 2 to 6 km long. The sediment lobes are more elongate in the along-channel direction (2 to 6 km long by 1-2 km wide) as compared to the typically equidimensional “textbook” lobes. Relic channels, presumably from lowstand times, can be seen almost to the distal limits of the fan and appear to feed the most distal sediment lobes. The channels incise restricted portions of the upper fan system, but do not appear to show significant levee formation adjacent to the channels, although low relief levees are expressed on the upper part of the fan. USBL-navigated cores, targeting the features identified on the backscatter data, show that the low backscatter channel axes are characterized by significant quantitities of sand, whereas the higher backscatter lobes are characterized by interbedded sand and mud. We interpret the backscatter signal to be dominated by volumetric scattering, and secondarily smoothness, rather than the more typical impedance.