Current-Controlled Abyssal Sedimentation: Samoan Passage, Equatorial West Pacific

Abstract

A geological and geophysical study of the Samoan Passage in the west equatorial Pacific (168-170°W; 7-10°S) was carried out in 1971, utilizing underway seismic reflection profiles (3.5 kHz and sparker), bottom photographs, bottom current measurements, bottom water temperature profiles, and sediment cores. These data and those of previous expeditions were synthesized in this study of dynamic sedimentation processes. Specific sedimentation processes were found to be associated with surface (12 kHz) and subsurface (3.5 kHz) morphology and composition of the sea floor. (1) Parabolic echo returns, rugged topography, some local conformable subbottom penetration (< 10 m); process- pelagic draping of sediment, infrequent slumping (Passage walls). (2) Highly reflective, smooth topography, no subbottom penetration, manganese nodules on semi-indurated sediment; process- continued fast (20-50 cm/sec) bottom current flow causing erosion or non-deposition (Passage axis). (3) Moderately reflective bottom with up to 20 m of subbottom penetration lying over (2) above, compact unfossiliferous zeolitic and ash-rich clay containing some manganese nodules; process-slow deposition of compact sediment from bottom currents (5-10 cm/sec), the "wind slab effect" (lee portion of Passage and in certain areas north of the Passage). (4) Very weakly reflective bottom with up to 80 m of subbottom penetration, watery abyssal brown clays interbedded with reworked and cross bedded biogenic ooze; process-rapid accumulation of sediment from waning sediment-laden bottom currents (north of Passage). The sedimentary record in and around the Passage indicates that relatively intense current flow through the Passage was initiated at some time between the early Eocene and early Oligocene, and continued through most of the Tertiary. Lithologic data suggest significant periodic fluctuations in corrosive effects of bottom currents during the Quaternary.