ABSTRACT: Composite Volcano on the Manihiki Plateau: Lava Or Mud?
William T. Coulbourn, Peter J. Hill
A composite volcano was discovered on the northeastern edge of the Manihiki Plateau (10°20^primeS, 161°31^primeW) during the February 1987 CCOP/SOPAC cruise of the RV MOANA WAVE in the territorial waters of the Cook Islands. The sedimentary sequence on the plateau is about 1 km thick and it is known from DSDP 317. Individual cones each appear as accoustically transparent piercements on single-channel seismic reflection profiles collected during this and earlier expeditions to the area. For each of these piercement structures, SeaMARC II side-scan images reveal conical, steep-sided peaks of about 1 km diameter rising several hundred meters above the smooth and featureless plateau surface. Some of these cones are symmetric, but most are formed by multiple flow uni s; a few have summit depressions, and moats partially surround some of the cones.
Individual cones are more densely spaced about a center located near the northeastern edge of the Manihiki Plateau, about 50 km SSW of Rakahanga Island. At that location about 40 vents coalesce to form an edifice about 25 km in diameter, rising from the plateau depths of 3000 m to a summit at 1400 m depth. Patterns in the side-scan images suggest fluid sediment flow both on and to the northeast of this feature. Subbottom reflectors dip away from this feature in all directions. To the southwest, the sea-floor and subbottom reflectors are smooth and gently dipping, but to the northeast of this peak, an irregular sea floor truncates tilted subbottom reflectors.
Recent foraminiferal ooze was cored from a satellite cone. Eocene planktonic foraminifera are embedded in manganese-encrusted limestones and corralline fragments dredged from the summit of the composite edifice. Burrows in the limestone are lined with manganese.
Geopotential modeling indicates that the features are probably not entirely sedimentary, but that basement is involved in the formation of the edifice. A basement high under the composite cone probably serves to localize any emanations originating from within the sedimentary section.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990