Abstract

A field of sand dunes was discovered by the side-looking sonars of a deep-towed geophysical instrument package at a water depth of 2.65 km on the north flank of the Carnegie Ridge in the eastern equatorial Pacific. The dunes are moving down the flat floor of an erosional valley that has been excavated through calcareous ooze to a resistant, manganese-encrusted chalk stratum. Transverse dunes (mean wave length 20 m) anc barchans (length 6 to 80 m) are arranged in long, narrow belts. Stereo photographs (875 oriented pairs) of the dunes and of the current-swept rock floor show that average dune height is —0.6 m, the steep (25° to 30°), lee side is generally smooth, and the upstream slope is covered with short-crested current ripples. A core from a transverse dune recovered moderately sorted sand, composed of broken fragments (70 percent) and intact tests of Quaternary age Foraminifera. Photographs and side-looking sonar records both indicate a northwestward, down-valley sediment transport. The dunes are thought to be formed and propelled by fast (>30 cm/sec) currents of dense water, which spill episodically over the Carnegie Ridge into the Panama Basin.

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