Furrowed mud waves on the western Bermuda Rise

Abstract

A deep-tow sonar and photographic survey of 75 km 2 of the western Bermuda Rise, supported by limited coring and bottom-current measurements, mapped several types of bed forms molded by a northeasterly Antarctic Bottom Water current. Large-scale mud waves as much as 30 m high and 4 to 6 km apart are oriented 30° to the left of the mean current, and preferential deposition on their northwest faces has caused slow migration in that direction. Most of these faces have been dissected by straight parallel troughs or large furrows, as much as 50 m wide and 10 m deep, which form strips of roughened sea floor with anomalously high acoustic bottom loss. The “smooth” sea bed on the opposite faces of the mudwaves is lineated by lower amplitude, closely spaced “small furrows.” The distribution of these intermediate-scale, furrow bed forms may correlate with variations in sediment erodibility or in current strength over the surface of the large-scale mud waves. Some recent current ripples, tool marks, and current-smoothed sea bed indicate continued local current activity. However, the site is exposed to variable and generally weak bottom currents, which now may only maintain furrows that formed under a different flow regime.