Crustal magnetization and sedimentation near two small seamounts west of the Juan de Fuca Ridge, northeast Pacific

Abstract

A saturation survey of 800 km2 of ridge flank topography was made near 47°N, 132°W. Echo sounding depths at 12 kHz and total-field magnetic intensity were measured from a surface vessel on a dense track line pattern. Thirteen long gravity cores, three seismic profiles, and two camera stations provided additional data. Two abyssal hills 700 meters high are topographically and magnetically delineated in considerable detail: one is smooth and associated with a negative magnetic anomaly; the other is of complex morphology and is associated with a positive anomaly. Neither anomaly can be due to simple bulk magnetization of the seamounts. The southeast seamount shows massive flows, cliffs, steep pseudomassive faces, and rubble slide, in sequence downward from the summit. Smooth sediment slopes on the skirt are broken by almost bare rock outcrops, with sharp rock-sediment contacts. The undulating basement plain in the area is covered by up to 70 meters of sediment containing internal acoustic reflectors that persist for 10 km or more. A corresponding correlation is observed in the layering of the surface cores. Most contain low carbonate gray clay layers that appear to be similar to Tufts Plain turbidite material but are not concentrated in topographic lows. Low-density turbid water may occasionally overflow from the abyssal plain and intersperse with the climatically controlled pelagic sequence.