Abstract

A small area (10 × 16 km) on the northeast flank of the Line Islands archipelagic apron was studied in detail, using a deep-towed geophysical instrumentation system. Along the southern edge of the area, a moat several kilometres wide and 30 m deep lies at the base of an east-trending seamount cresting 1,500 m above the sediment apron. Both surface-ship 3.5-kHz and deep-tow 4-kHz reflection profiles show that much of the relief of this moat is the result of slower deposition of sediment (or the lack of deposition) in the upper part of the sediment apron.

In marked contrast, another moat in the northern portion of the area has been eroded 90 m below the smooth surface of the surrounding apron. This irregularly shaped moat is more than 7 km in length, although the adjacent seapeak (175 m high) is only 1 km in diameter. The deep-tow, high-resolution subbottom reflection profiles show that many flat-lying horizons in the sedimentary apron are truncated at the walls of the moat, and bottom photographs show outcropping ledges that parallel its contours. Current ripples were observed only in bottom photographs of terraces just above the moat floor. The sediments on the floor itself are suggestive of a coarse lag deposit. Short-term (up to 9 days) current-meter records showed that the prevalent current was about 2 to 3 cm/sec from the northeast, with small tidal perturbations. The morphology of this deep, irregular feature suggests that it may have formed during episodes of intensified bottom-water flow from the west-northwest, possibly as Pacific bottom water intermittently flowed through the area during the glacial intervals of the Pleistocene period.

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