Abstract

Bottom echoes recorded from the Northwest Pacific Basin on high-frequency (3.5 and 12 kHz) echograms can be classified into five general types; four types are related to sedimentation processes. The two most widely recorded echo types roughly correlate with the sediment type of the uppermost sub-sea floor: Type 1 (A and B) echoes (distinct with continuous parallel subbottoms) are generally recorded from the northwest portion of the basin, where sediments consist of siliceous ooze with inter-beds of volcanic ash. Type II echoes (distinct with continuous subbottoms in the top 10 to 20 m and semitransparent with discontinuous subbottoms below) characterize the southeastern portion of the basin, where brown (red) clays with interbeds of chert predominate.

Type III echoes are characterized by unconformable, migrating, or truncated subbottoms, occasionally with zones of regular, overlapping hyperbolas; they represent erosional-depositional bed forms created by the flow of thermohaline-induced bottom currents (that is, contour currents), presumably the Pacific Bottom Water (PBW). Most of the regions returning Type III echoes are of local extent and are at the bases of seamounts, ridges, and other topographic obstructions that locally accelerate current flow. A few zones of Type III echoes are of regional extent. One extensive Type III zone is along the southern boundary of the North Pacific Basin, where the PBW enters from the south through wide gaps in the topography. Another extensive zone is west of the Emperor Seamount Chain. The timing and duration of episodes of the sediment reworking indicated by the Type III echoes are uncertain. Hydrographic measurements and bottom photographs indicate only weak or sluggish PBW flow throughout the basin at present. Perhaps most of the sediment reworking occurred during Pleistocene and older glacial phases, when oceanic water masses may have circulated faster than at present.

Type IV echoes are prolonged, incoherent, or fuzzy echoes returned from small (< 5 to 25 km), localized slump–debris-flow deposits. These deposits often are observed at the bases of very gentle knolls (< 100 m relief). The majority of the deposits occur in areas of siliceous sediment (Type I echoes). Apparently, the physical properties of these sediments render them relatively unstable and, hence, more susceptible to failure than the brown clays.

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