Long-range side-scan sonographs are used to map two distinct acoustic facies in the Amirante Trench, western Indian Ocean. A structural barrier formed by a seamount within the trench separates the two facies types. The acoustic facies developed S of the seamount is characterized by weak reflectivity returned from sediments deposited by the flow of the Deep Western Boundary Undercurrent through the channels of the Amirante Passage into the southern part of the Amirante Trench. N of the seamount the acoustic facies is characterized by high reflectivity and is confined to the trench axis: it arises from sediments episodically deposited by turbidity currents or debris flows possibly originated by slumping of reef talus from the atolls surmounting the adjacent Amirante Ridge.

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